Neural plasticity in the gastrointestinal tract: chronic ...gastrointestinal (GI) tract and underlining the similarities between the reactions of peripheral autonomic nerves in different

REVIEW

Neural plasticity in the gastrointestinal tract: chronicinflammation, neurotrophic signals, and hypersensitivity

Ihsan Ekin Demir • Karl-Herbert Schafer •

Elke Tieftrunk • Helmut Friess • Guralp O. Ceyhan

Received: 11 September 2012 / Revised: 31 January 2013 / Accepted: 7 February 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Neural plasticity is not only the adaptive

response of the central nervous system to learning, struc-

tural damage or sensory deprivation, but also an

increasingly recognized common feature of the gastroin-

testinal (GI) nervous system during pathological states.

Indeed, nearly all chronic GI disorders exhibit a disease-

stage-dependent, structural and functional neuroplasticity.

At structural level, GI neuroplasticity usually comprises

local tissue hyperinnervation (neural sprouting, neural, and

ganglionic hypertrophy) next to hypoinnervated areas, a

switch in the neurochemical (neurotransmitter/neuropep-

tide) code toward preferential expression of neuropeptides

which are frequently present in nociceptive neurons (e.g.,

substance P/SP, calcitonin-gene-related-peptide/CGRP)

and of ion channels (TRPV1, TRPA1, PAR2), and con-

comitant activation of peripheral neural glia. The

functional counterpart of these structural alterations is

altered neuronal electric activity, leading to organ dys-

function (e.g., impaired motility and secretion), together

with reduced sensory thresholds, resulting in hypersensi-

tivity and pain. The present review underlines that neural

plasticity in all GI organs, starting from esophagus, stom-

ach, small and large intestine to liver, gallbladder, and

pancreas, actually exhibits common phenotypes and

mechanisms. Careful appraisal of these GI neuroplastic

alterations reveals that—no matter which etiology, i.e.,

inflammatory, infectious, neoplastic/malignant, or degen-

erative—neural plasticity in the GI tract primarily occurs in

the presence of chronic tissue- and neuro-inflammation. It

seems that studying the abundant trophic and activating

signals which are generated during this neuro-immune-

crosstalk represents the key to understand the remarkable

neuroplasticity of the GI tract.

Keywords Neural plasticity � Gastrointestinal tract �Enteric nervous system � Neuro-inflammation �Hypersensitivity � Pain

Introduction

In the peripheral nervous system, neuronal plasticity was

described more than 100 years ago in the pioneering works

by Cajal and Langley [12, 56] after nerve transsection and

neuro-regeneration. In the past two decades, plasticity was

also recognized as a striking feature of autonomic nerves in

the enteric nervous system (ENS) owing to intensifying

research in neurogastroenterology [34, 70, 110, 111].

Changes in innervation density, different neuropeptide

release patterns, and the entailing functional disturbances

are detected at the same fascinating extent in the ENS as in

the central nervous system (CNS) [110]. However, our

knowledge on the functional implications of visceral neu-

roplasticity is scarce.

The present review aims at illustrating the various

structural and functional neuroplastic alterations in the

gastrointestinal (GI) tract and underlining the similarities

between the reactions of peripheral autonomic nerves in

different GI organs as a response to insult. Furthermore, the

so far identified functional aspects of neural plasticity in

the GI tract are interpreted on the basis of their relevance

I. E. Demir (&) � E. Tieftrunk � H. Friess � G. O. Ceyhan

Department of Surgery, Klinikum rechts der Isar,

Technische Universitat Munchen, Ismaninger str. 22,

81675 Munich, Germany

e-mail: [email protected]

K.-H. Schafer

Department of Biotechnology, University of Applied Sciences

Kaiserslautern/Zweibrucken, Zweibrucken, Germany

123

Acta Neuropathol

DOI 10.1007/s00401-013-1099-4

Appendicitis:• Neuronal hyperplasia• Neural hypertrophy• Neuroma-like bodies• NGF, GAP-43• Mast cell infiltration

IBS:• Myenteric ganglionitis• Mast cell infiltration• NGF, BDNF and TRPV1• Decreased descending

pain inhibition

Chagas enteropathy:• Neuro-degeneration

Diverticulosis:• Decreased neuron and

glia content• Fewer ganglia

Chronic diverticulitis:• Increased neural

density• Neuro-inflammation

Colon cancer:• Hypo- and hyper-

innervation

Chronic gastritis:• Increased neural

density, peri-neuritis• DRG & nodose neuron

hyperexcitability

Gastroparesis:• Loss of neurons & ICC• Activation of ATP-

sensitive K+-channels

Gastric cancer:• Increase in

nociceptive fibers

Nutcracker esophagus:• Imbalance of neuro-

transmitters

Achalasia:• Loss of neurons and

ICC• Ganglionitis

GERD:• More nociceptive fibers• NGF, GDNF, NMDA• Throacic spinal neuron

hypersensitivity• Cross-sensitization in the

spinal cord• NR1 (NMDA) upregulation

Chronic pancreatitis & Pancreatic cancer:• Increased neural density &

hypertrophy• Neural remodelling• Pancretic neuritis• Enhanced excitability• Suppression of A-type

potassium currents • TRPV1 in DRG neurons

Chronic cholecystitis:• Increased neural

density & hypertrophy

Liver cirrhosis:• Hypoinnervation in

fibrotic areas• Hyperinnervation

along portal veins• Mast cell infiltration

Liver cancer (HCC, CCC):• Loss of innervation in

tumor-affected tissueareas

Central reorganization

Pain

Hypersensitivity

IBD:• Ganglionic hyperplasia• Ganglionic hypertrophy• Hyperplasia of glia• Myenteric plexitis• AH and S neuron

hyperexcitability

Acta Neuropathol

123

for the pathophysiology of pain and the progression of the

diseases with which they are associated. Finally, it aims to

emphasize that these similarities in neuroplastic reactions

of different organs seem to be generated upon a common

pathomechanistic background, i.e., their predominant

occurrence in states of chronic tissue inflammation, in the

presence of specific peripheral neuro-inflammation and the

corresponding neurotrophic signals.

Structural neuroplasticity in the GI tract: tissue

hyperinnervation and altered chemical coding

Owing to advances in histopathology and increased atten-

tion toward neural alterations, pathologists reported on

numerous structural plastic alterations in the GI tract, from

its very proximal (oral) end to its most distal parts, i.e.,

from esophagus to rectum. The following sections provide

an overview of the so far observed structural neuroplastic

alterations and associated chemical codes during the

pathogenesis of typical GI disorders.

Appendix

In contrast with its acute occurrence, the presence of

‘‘chronic appendicitis’’ remains an issue of debate. Histo-

morphological examination of specimens from patients

with ‘‘chronic appendicitis’’ or ‘‘neurogenic appendicopa-

thy’’ frequently yields intramucosal, finely vacuolated

nerve proliferations, and central neuromas, as well as

neuromuscular proliferations in the submucosa, with neu-

ropeptide-producing cells in the appendical stroma [43]. In

particular, these stromal cells which were shown to pro-

duce 5-hydroxytryptamine, somatostatin, and substance P

have long been considered as causal factors in the

generation of neurogenic appendicopathy [43]. In a

pathomorphological study on acute appendicitis, the num-

ber of Schwann cells, the number and size of ganglia, and

the immunoreactivity for the nerve growth factor/NGF

were all elevated when compared to normal appendix.

Furthermore, a significant correlation between NGF

expression and mast cell density was noted (Fig. 1) [19].

Interestingly, the density of mast cells strongly correlated

to the degree of neuronal hyperplasia and hypertrophy in

acute appendicitis [19]. Interconnecting nerve bundles in

the myenteric plexus were detected to be enlarged in 55 %

of patients who underwent appendectomy for acute

appendicitis and also in 41 % of patients with histologi-

cally normal appendix [72]. Therefore, neuroplastic

alterations in the appendix seem to result from repetitive

bouts of inflammation together with chronic luminal

obstruction [72]. Based on the extensive neural hypertro-

phy, elevated growth-associated-protein-43 (GAP-43)

content, increased presence of substance P (SP)-, vasoac-

tive intestinal peptide (VIP)-positive nerve fibers (Table 1)

and the concomitant perineural inflammatory cell infiltra-

tion, and non-acute appendicitis were suggested to be of

‘‘neuroimmune’’ origin [26].

Small and large intestine

The part of the GI tract in which visceral neuroplasticity

has probably been best studied is the intestine [110]. The

ENS which represents an independently functioning com-

ponent of the autonomic nervous system can undergo

profound structural plasticity in different contexts, e.g.,

during inflammation, infection, aging, and (congenital)

enteric neuropathies [110]. In this regard, inflammatory

bowel diseases (IBD), i.e., ulcerative colitis and Crohn’s

disease, harbor numerous alterations in different compo-

nents of the ENS [110]. In particular, both entities

frequently demonstrate hypertrophy and/or hyperplasia of

ENS ganglia, of extrinsic and intrinsic nerve bundles, and

hyperplasia of ENS glia cells in the colon and/or ileum

(Fig. 1) [110]. In Crohn’s disease, these plastic changes

were mostly found in the mucosa, submucosa, and myen-

teric plexus [101, 110]. Furthermore, there is a close

correlation between the extent of intestinal neuroplasticity

and the amount of perineural inflammatory cell infiltrates

around myenteric ganglia (Fig. 2) [65, 70, 110]. In addition

to these trophic alterations, ganglion/neural degeneration

and necrosis have been demonstrated in IBD both in

inflamed and non-inflamed intestinal areas [28].

Inflammation in the GI tract is often associated with the

preferential emergence of afferent (nociceptive) nerve

fibers which contain the pro-inflammatory (‘‘neurogenic

inflammation’’) neuropeptide substance P (SP) (Table 1)

[101, 110]. Especially in ulcerative colitis, SP-positive

Fig. 1 Structural and functional neural plasticity in the gastrointes-

tinal (GI) tract. Morphological analysis of nerves and intrinsic ganglia

within GI organs reveals that all parts of the GI tract, starting from

esophagus to the distal end of the large intestine, undergo in part very

similar structural alterations during disease processes. The upwardarrows denote upregulation of the preceding factors. In contrast to the

well-described pathomorphological alterations of GI nerves in disease

states, the concomitant functional alterations (yellow-colored text) of

peripheral GI neuronal networks have not yet been understood in

sufficient detail. The depicted alterations are all derived from

experimental models of the respective diseases. AH afterhyperpolar-

izing, DRG dorsal root ganglia, TRPV1 transient receptor potential

vanilloid 1, ATP adenosine triphosphate, cAMP cyclic adenosine

monophosphate, CREB cAMP response element binding protein,

HCC hepatocellular cancer, CCC cholangiocellular cancer, NMDAN-methyl-D-aspartate, NGF nerve growth factor, GDNF glial-cell-

derived neurotrophic factor, GAP-43 growth-associated-protein 43,

ICC interstitial cells of Cajal, IBD inflammatory bowel disease.

Please refer to the manuscript for the respective references

b

Acta Neuropathol

123

neurons of enteric ganglia are upregulated both in inflamed

and non-inflamed portions of the colon [101]. Such an

upregulation seems to be specific for SP, since cholinergic

neurons or vasoactive intestinal peptide (VIP)-containing

neurons are not subject to any major alteration in their

density in ulcerative colitis (Table 1) [74]. Intriguingly,

this remodeling process as observed in ulcerative colitis is

not equally encountered in Crohn’s disease, but similarly

Table 1 Neurochemical code alterations in diseases of the gastrointestinal (GI) tract

*

TH ChAT SP NKR CGRP VIP NOS NPK Galanin NPY PACAP

Appendix

Appendicitis

Intestine

Ulcerative colitis

Crohn’s disease

Trichinella spiralis-inf.

Chagas’ disease

Diverticulitis

Colon cancer

Stomach

Chronic gastritis

Gastric cancer

Esophagus

Nutcracker esophagus

Inef. esophageal motility

Achalasia

Tracheoesophageal fist.

Reflux esophagitis

Esophageal cancer

Liver

Cirrhosis

HCC

Gallbladder

Chronic cholecystitis

Pancreas

Chronic pancreatitis

Pancreatic cancer

Nearly all disorders of the GI tract demonstrate relative up- and downregulation of certain neuropeptides as part of the remodeling of the organ

innervation. In general, there is a tendency toward increased expression of the neuropeptides substance P (SP) and vasoactive intestinal peptide

(VIP), and suppression of the sympathetic innervation (tyrosine hydroxylase/TH- containing nerve fibers), particularly within inflammatory

disorders of the GI tract

ChAT choline acetyltransferase, CGRP calcitonin-gene-related-peptide, NOS nitric oxide synthase, NPK neuropeptide K, NPY neuropeptide Y,

PACAP pituitary adenylate cyclase-activating polypeptide, inf. infection, inef. esophageal motility ineffective esophageal motility, fist. fistula,

HCC hepatocellular cancer

* Indicates that SP and neurokinin receptors (NKR) are respectively up- and downregulated in the dilated chagasic colon, with contrasting

changes in the non-dilated parts [21]

Acta Neuropathol

123

observed in GI infections, e.g., during Trichinella spiralis

infection [110]. However, in such infection models, SP-

upregulation was seen to be limited to colon, whereas

jejunum exhibited decreased amounts of SP and VIP [77].

A similarity between ulcerative colitis and Crohn’s disease

in this context lies in the concomitant upregulation of SP

Co

lon

Liv

erP

ancr

eas

Normal Ulcer. Col. Ulcer. Col.

Colon-CA Colon-CA

Liver-CANormal

Normal PCa PCa

PCaPCa

a b c

d e

f g

h i j

k l

N

NNN

N

N

N

N

N

N

N

PCa N

m

Fig. 2 Histomorphology of neuroplasticity in the human gastroin-

testinal (GI) tract. Inflammatory and neuroplastic diseases of the

colon, liver, and pancreas harbor neuroplastic alterations when

compared to normal healthy state. a–b In ulcerative colitis (Ulcer.Col.), submucosal nerves (N) are enlarged and greater in number

when compared to normal colon. c In ulcerative colitis, enteric (here

myenteric) ganglia are frequently inflamed (‘‘ganglionitis’’). Arrowspoint to infiltrating inflammatory cells. d–e In colon cancer (Colon-CA), subserosal nerves are frequently invaded by cancer cells

(arrows) and increased in size. The endoneural invasion results in

disappearance of individual nerves fibers (arrowheads) between

invading cancer cells. f–g When compared to the small nerves in

periportal fields of the healty liver, liver cancer (Liver-CA) tissues

harbor enlarged nerves in fibrotic tissue areas. h In normal human

pancreas, intrapancreatic nerves are encountered in interlobular septae

in proximity of vessels and ducts. i–j In pancreatic cancer (PCa),

nerves (i) and ganglionic neurons (j, arrowheads) are frequently

infiltrated by inflammatory cells (arrows). k–l Nerves in PCa tissues

are enlarged, increased in number and are victims of neural invasion

by cancer cells (arrows). ‘‘N’’ denotes the immunostained nerve(s) on

each image. Immunostaining was performed against the pan-neural

marker protein gene product 9.5 (PGP9.5). The scale bars correspond

to 100 lm

Acta Neuropathol

123

receptors, i.e., neurokinin-1, -2, and -3 receptors (Table 1)

[86]. Moreover, a frequent finding is the parallel decrease

of the sympathetic, tyrosine hydroxylase-positive nerve

fibers in addition to SP-upregulation [101, 102]. Corre-

spondingly, in Crohn’s disease, the sympathetic repellant

factor semaphorin 3C is increased in colonic mucosal

crypts with a major deficiency of sympathetic nerve fibers

[102].

As one of the most common disorders of the GI tract,

diverticular disease similarly exhibits a prominent degree

of neuroplasticity (Fig. 1). Interestingly, colonic segments

with diverticulosis demonstrate decreased neural density

and diminished neuronal and glial cell numbers in myen-

teric ganglia, smaller numbers of submucous ganglia, and

an overall predominance of glia over neurons (‘‘higher glia

index’’) [115]. On the other hand, tissue specimens from

patients with acute or chronic diverticulitis were detected

to bear increased neural density when compared to patients

with non-inflamed colon (Fig. 1; Table 1). Furthermore,

patients with symptomatic diverticular disease, i.e., those

with recurrent attacks of abdominal pain and visceral

hypersensitivity, demonstrate upregulation of SP, NPK,

pituitary adenylate cyclase-activating peptide (PACAP),

VIP, and galanin (Table 1) [94]. Recently, these alterations

were demonstrated to be associated with ongoing local

inflammation, which can be clinically discrete (e.g., low-

level inflammation around colonic diverticula) or apparent

just like in experimental colitis (e.g., TNBS-colitis) [46].

Overall, these findings underscore that intestinal inflam-

mation can cause local neuromuscular disturbance which is

accompanied by upregulation of neuropeptides mediating

visceral hypersensitivity and local foci of tissue

hyperinnervation.

Irritable bowel syndrome (IBS) represents one of the

best studied diseases in terms of intrinsic neuroplasticity.

Histopathological assessment of jejunal biopsies from IBS

patients revealed frequently infiltrated myenteric ganglia

by lymphocytes [108]. Later studies ascribed a prominent

role to neuro-inflammation by infiltrating mast cells, since

IBS patients were found to have significantly elevated mast

cells counts in colonic mucosa and around intramural

nerves [7]. Interestingly, in diarrhea-predominant IBS,

mucosal mast cells counts correlated with the severity of

visceral hypersensitivity [79]. IBS-associated alterations in

the neuropeptide content of nerves have not yet been suf-

ficiently characterized. Nonetheless, in experimental IBS,

there was increased SP immunoreactivity in the ileocecal

junction, colon, the posterior horn of the spinal cord, and

the hypothalamus of rats [113].

Neoplastic diseases such as colon cancer have not yet

been subject to investigation in terms of neuroplasticity. In

the very few studies on this topic, Sitohy et al. showed an

increase in nerve fiber density in the muscularis propria in

experimental colon cancer, and Godlewski et al. demon-

strated that human colon cancer contained decreased

amounts of CGRP and NPY solely in tumor-affected nerve

fibers (Figs. 1, 2) [37, 97]. Importantly, neuritogenesis was

shown to correlate to worse prognosis and disease pro-

gression in colon cancer [3]. Interestingly, intracolonic

nerves which have greater NGF content were recently

reported to demonstrate more severe nerve invasion, and

thus be associated with worse prognosis of patients with

colon cancer [62].

Stomach

The presence of neuroplastic alterations has been subject to

morphological investigation in different disorders of the

stomach. Similar neurotrophic–neuroplastic alterations as

seen in the intestine or appendix are encountered in chronic

gastritis with Helicobacter pylori (H. pylori) infection [95].

Sipos et al. showed a clear increase in the density of SP,

VIP, and neuropeptide Y (NPY)-immunoreactive inflamed

nerve fibers in chronic gastritis mucosa (Table 1) [95, 96].

Immunoelectron-microscopic analysis of these immune

cells revealed a heterogenous cell population including

lymphocytes, NPY- and SP-containing mast cells, and

macrophages [95, 96]. The longevity of such neuroplastic

alterations could be elegantly demonstrated in a mouse

model of H. pylori infection where infected mice had

higher density of SP-, VIP-, and CGRP-immunoreactive

nerve fibers in the stomach (Table 1) [9]. This neuroplas-

ticity process was accompanied by declined cholinergic

signaling as evidenced by diminished acetylcholine release

after electric stimulation [9]. Strikingly, eradication of H.

pylori resulted in restoration of normal gastric acetylcho-

line release, but the high density of gastric SP- and CGRP-

immunoreactive nerves persisted [9]. Hence, these findings

demonstrate that chronic inflammation in the stomach can

also trigger neuroplasticity and altered neuropeptide

expression which can be long-lasting even after clearance

of the etiological agent.

The other major gastric disorder which harbors neuro-

plastic alterations is gastroparesis, which can be either due

to diabetes or idiopathic. Full-thickness gastric wall biop-

sies from patients with either diabetic or idiopathic

gastroparesis demonstrated a decrease in gastric innerva-

tion and neuronal cell bodies, and particularly a reduction

in the density of neuronal nitric oxide synthase (nNOS)-

containing neurons and tyrosine hydroxylase-immunor-

eactive sympathetic nerve fibers [29, 40]. Furthermore,

there was loss of the normal anatomic association between

ICCs and enteric nerve terminals [40]. This remodeling of

ICC networks was accompanied by upregulation of heter-

ogeneous inflammatory cell deposits and increase in local

connective tissue [40]. Overall, local (neuro-) inflammation

Acta Neuropathol

123

together with loss of ICC and neuronal networks were

suggested to play a causal role in the pathogenesis of

diabetic gastroparesis [29, 40]. Interestingly, recent com-

parative studies showed that diabetic and idiopathic

gastroparesis indeed exhibit major histological differences,

e.g., thickened basal lamina around smooth muscle cells

and nerves in diabetic gastroparesis and pronounced peri-

neural fibrosis in idiopathic gastroparesis [29, 40]. This

abnormal remodeling of gastric innervation is assumed to

result in delayed gastric emptying which is the leading

symptom in gastroparesis [29, 40].

The lack of studies on neuroplasticity in GI malignancies

is similarly reflected in the small number of studies which

investigated the innervation pattern of gastric cancer tissues.

While comparative studies with normal gastric tissue are not

present to date, some studies demonstrated increased pres-

ence of SP-containing nerve fibers in human gastric cancer

(Table 1; Fig. 1) [31, 87]. The presence of these fibers was

interpreted as potential accelerator of tumor growth due to

the presence of NK-1-receptor on gastric cancer cells and

the promotion of gastric cancer cell proliferation and

migration after SP-administration [31, 87].

Esophagus

Proper analysis of neuroplasticity in the esophagus necessi-

tates in-depth understanding of the complex esophageal

innervation in its striated and smooth muscle cell layers [73].

Imbalances in this complex circuit are assumed to result in

esophageal motility disorders such as achalasia and gastro-

esophageal reflux [73]. Similar to gastroparesis, ICCs were

shown to be lost to a major extent in the lower esophageal

sphincter high-pressure zone in achalasia, accompanied by a

similarly severe reduction in nNOS-containing neurons den-

sity (Fig. 1) [36]. Hence, achalasia was proposed to be related

to loss of inhibitory neurotransmission in the lower esopha-

geal sphincter. In common with several other disorders in the

GI tract, also achalasia patients revealed in part high amounts

of locally infiltrating T lymphocytes [52]. Interestingly,

patients who did not demonstrate such inflammatory cell

infiltration were found to bear an even more remarkable loss

of nerve fibers in the muscularis propria [52]. Looking at the

extent of these alterations, achalasia can in part be regarded as

a local inflammatory hypoinnervation, resulting in loss of

inhibitory neurotransmission [36, 52].

Similar to other GI organs, esophagus also exhibits a

disease-context-dependent neuroplasticity. In nutcracker

esophagus and ineffective esophageal motility, there is

impressive upregulation of cholinergic myenteric neurons

and nNOS-immunoreactive neurons (Fig. 1) [53], respec-

tively, suggesting a potential role for imbalance between

these neuronal subtypes in the hyper- and hypomotility

disorders of the esophagus [53].

Due to the frequently observed dysphagia following

surgical repair of esophageal atresia (EA) or tracheoe-

sophageal fistula (TEF), specimens from these patients

have been subject to structural analysis. Interestingly,

patients with EA demonstrated hyperplasia of myenteric

ganglia at both ends of the esophagus which was more

pronounced than in TEF (Fig. 1) [80]. Furthermore, TEF

was detected to harbor downregulation of SP-immunore-

active innervation and increase in the density of VIP- or

NOS-containing neurons (Table 1) [60]. Thus, altered

neuropeptide expression and ganglionic morphological

alterations were suggested to be pathomechanistically

involved in the esophageal dysfunction following repair of

EA or TEF [60, 80].

Coming to the most common esophageal disorder,

neuroplasticity and nociception in reflux esophagitis (RE)

have been studied extensively due to ‘‘heartburn’’ being the

chief complaint of these patients. Indeed, esophagus of

patients with RE possess a greater density of nerve fibers

(Fig. 1) [75] with a particular increase in VIP-containing

nerve fibers (Table 1) [75], and sensory nerve fibers with

the nociceptor-activating ion channel transient receptor

potential vanilloid 1 (TRPV1) [69]. Furthermore, experi-

mental models of RE showed a similar upregulation of

TRPV1-immunoreactive and SP-immunoreactive neurons

in the spinal cord, dorsal root ganglia, and nodose ganglia

[4]. More recently, this increase in the expression of

TRPV1 was demonstrated to correlate to intraesophageal

upregulation of NGF and glial-cell-derived neurotrophic

factor (GDNF) [92], highlighting the major pro-nociceptive

neuroplasticity associated with this painful disorder.

Liver

Liver with its most common diseases such as hepatitis and

cirrhosis has been studied for the structure of its intrinsic

innervation. In liver cirrhosis, intrinsic innervation was

reported to be reduced in the parenchyma in pre-cirrhotic

liver and nearly absent in the regenerating nodules in

established cirrhosis (Fig. 1) [58, 90]. Importantly, nerves

in pre-cirrhotic and cirrhotic livers were found to be pre-

served along the fibrous septae and in portal tracts [58, 90],

with predominance of SP- and NPY-positive nerve fibers

over CGRP-containing ones (Table 1) [100]. Interestingly,

in alcoholic hepatitis without overt clinicopathological

features of portal hypertension, or among patients with

psoriasis who received the potentially hepatotoxic drug

methotrexate, there was hyperinnervation around portal

vein branches [48]. In a large-scale study on 178 biopsy

specimens obtained from patients with primary biliary

cirrhosis, alcoholic liver disease, autoimmune hepatitis,

chronic hepatitis B, or chronic hepatitis C, Matsunaga et al.

[68] detected a parallel increase in the density of nerve

Acta Neuropathol

123

fibers and mast cells in cirrhotic livers, which were both

independent of the underlying etiology. Importantly, this

increase in inflammatory cells and nerve fibers correlated

with the degree of liver fibrosis in these patients [68].

Similar findings were observed in a rat model of cirrhosis,

in which nerve terminals were seen in close contact with

myofibroblasts in periseptal sinusoids [2]. In contrast with

cirrhosis which is characterized by widespread hypoinn-

ervation, toxic hepatic injury was shown to specifically

induce proliferation of portal nerve fibers [47].

In one of the very few studies on the innervation of pri-

mary liver tumors, Terada et al. [106] performed a

quantitative comparative analysis of nerve fibers in hepato-

cellular cancer (HCC) and intrahepatic cholangiocarcinoma

(CCC). There, the investigators found no nerve fibers in

tumoral areas and fibrous septa of the tumor, whereas some

nerve fibers were detectable near the liver capsule (Figs. 1, 2)

[106]. On the other hand, in CCC, tumor stroma contained a

small amount of fibers, and both entities showed nerve fibers

in non-affected tissue regions, particularly in pre-existing

portal tracts and sinusoids [106].

Gallbladder

Despite the high prevalence of its inflammatory diseases and

the associated pain syndrome, neuroplasticity of gallbladder

has not been subject to intensive investigation. In the single

large-scale study on gallbladder innervation in acute and

chronic cholecystitis, gallbladder tissue from patients with

chronic uncomplicated gallstone disease demonstrated a

prominent increase in nerve density and neural hypertrophy

in the whole tissue area, particularly in the gallbladder neck

(Fig. 1) [41]. In contrast, acute cholecystitis tissues showed

a major deficiency in the mean number and area of intra-

mural nerve fibers [41]. In accordance with these

observations, Gonda et al. [39] reported intramural hyper-

trophy of VIP-immunoreactive nerve fibers among patients

with chronic cholecystitis around hypertrophied muscle

bundles, Rokitansky Aschoff sinus, and in the gallbladder

mucosa (Table 1). However, tissues with acute or gangre-

nous cholecystitis were marked by reduction and

disappearance of VIP-containing nerve fibers in the gall-

bladder [39]. To date, further studies on neuroplasticity in

other gallbladder diseases such as cancer are lacking.

Pancreas

Among all organs of the GI tract, pancreas occupies the

special position of being the probably best studied one in

terms of its neuroplasticity in inflammation and cancer [24].

Chronic pancreatitis (CP) and pancreatic cancer (PCa) tis-

sues harbor a prominent degree of neural hypertrophy and

increased neural density (Figs. 1, 2), which correlate to the

abdominal pain severity of these patients [14, 24]. In these

enlarged nerves, the distribution of nerve fiber qualities is

marked by suppression of autonomic nerve fiber qualities

(especially sympathetic ones) and glial markers, collectively

termed ‘‘neural remodeling’’ [14]. This suppression is more

remarkable within nerves which demonstrate one of the two

further features of this ‘‘pancreatic neuropathy’’: pancreatic

neuritis and neural invasion by PCa cells [14]. Studies on the

nerve-infiltrating inflammatory cells in CP revealed that

these cells frequently contain interleukin 8 (IL-8), suggest-

ing a potential IL-8-mediated cross-signaling between

inflammatory cells and nerves which contain SP (Table 1)

[27]. Furthermore, there is a close correlation between the

expression of the neurotrophic factor artemin and the degree

of neuroplasticity in CP [13].

On the other hand, in PCa, much of the research on

neuroplasticity has been shaped by studies on neural

invasion [23]. Up to 100 % of patients with pancreatic

adenocarcinoma exhibit neural invasion (NI) [6]. Interest-

ingly, NI was reported to significantly correlate to

neuroplasticity in the pancreas [14]. Similar to CP, these

neuroplastic alterations are closely correlated with the

degree of pain sensation and worse survival in PCa [14].

These observations recently paved path for novel in vitro

and in vivo models of NI in PCa, which underlined a key

pathomechanistic role for neurotrophic factors, their

receptors, and neuronal chemokines in this process [23].

The study of neuroplasticity in PCa has recently been

further elaborated by development of a novel in vitro

model designed for this phenomenon [24]. Neurons derived

from dorsal root ganglia (DRG) and myenteric plexus (MP)

neurons, i.e., neurons representing extrinsic and intrinsic

pancreatic innervation, have been cultivated within tissue

extracts obtained from patients with PCa or CP and com-

pared to those grown in tissue extracts of normal pancreas

donors [24]. This model mimicked the key features of

pancreatic neuroplasticity at neuronal level owing to their

increased neurite density, complex axonal branching, and

perikaryonal hypertrophy [24]. The same model also

allowed the analysis of specific cell types from the intra-

pancreatic microenvironment, including PCa cells and

pancreatic stellate cells, or of molecular agents for their

capacity to induce neuroplasticity [16, 24].

Functional neuroplasticity in the GI tract:

inflammation and hyperexcitability

Small and large intestine

Investigating neuroplasticity in the human GI tract at

functional level represents a technically greater challenge

compared to structural alterations. Hence, our current

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knowledge on such functional alterations is mainly derived

from animal models. A common denominator for func-

tional neuroplasticity in the GI tract is inflammation-

induced hyperexcitability of certain neuronal sub-classes

[63, 65]. Among these, the so-called after hyperpolarizing

neuron (AH neuron) has been repeatedly demonstrated to

be hyperexcitable in experimental gut inflammation, in

particular, in TNBS-colitis and in Trichinella spiralis

(T. spiralis) infection of guinea pig small intestine [63, 65].

The mechanisms for this hyperexcitability seem to be

complex and dependent on the investigated layer of intes-

tinal wall. For T. spiralis infection, there is evidence for

decreased Ca2?-activated K? channel activity, enhanced

cAMP-pCREB signaling pathway activity, and more fre-

quent evoked fast EPSPs [17]. For TNBS-colitis,

submucosal neurons were measured to have shorter action

potential duration, whereas myenteric neurons exhibited

spontaneous activity and increased hyperpolarization-acti-

vated cation conductance [63, 70]. The other type of enteric

neuron which was reported to demonstrate altered activity

under inflammation is the S neuron which can be present as

a sensory, motor, or intermediate neuron [63, 65, 70]. For

TNBS-colitis, it is known that S neurons with ascending

projections exhibit increased excitability (Fig. 2) [17, 63,

65, 70]. At synaptic level, submucosal and myenteric

neurons demonstrate fast EPSPs with higher average

amplitudes in TNBS-colitis and enhanced neurotransmitter

release [63, 65, 70]. Interestingly, this is in contrast to

T. spiralis infection where during the phase of acute

inflammation, cells were detected to release smaller amounts

of neurotransmitters and a suppression of EPSPs [70].

Importantly, part of these alterations is detectable up to

several weeks after resolution of infection [70], suggesting

the persistent character of functional neuroplasticity in GI

inflammation.

One of the best studied examples of functional neuro-

plasticity in the GI tract is encountered in IBS. In this

disorder, visceral hypersensitivity as the hallmark of IBS

seems to be grounded on both peripheral and central neu-

roplastic alterations. First, mast cells which are found in

close spatial contact with intramural nerves in human IBS

specimens are assumed to activate sensory neurons via

increased mast cell tryptase and histamine secretion [8]. In

postinfectious IBS, persistent immune cell activity is con-

sidered responsible for sustained activation of sensory

nerves [45]. Here, there is a 3.9-fold increase in the amount

of TRPV1-expression sensory nerve fibers in IBS tissues

than in healthy controls [1]. More recent studies demon-

strated that IBS tissues contain a neurotrophic milieu,

characterized by increased tissue NGF and BDNF [116]. At

CNS level, IBS patients had decreased descending pain

inhibition in the spinal cord [82]. While neuroplastic

alterations should not be considered sufficient to explain

the pathogenesis of this multi-factorial disorder, they seem

to be imperative actors in the generation of IBS-associated

hypersensitivity.

At functional level, one of the most striking features of

enteric neuroplasticity are concomitant external (out-of-

organ) neuroplastic alterations, e.g., in unaffected/non-

inflamed organs and afferent nerve pathways. In TNBS-

induced ileitis, colonic enteric neurons and epithelial cells

were reported to undergo parallel alterations, including

hyperexcitability of submucosal AH neurons, increased

non-cholinergic synaptic transmission in S neurons, ele-

vated colonic prostaglandin E(2) content, greater number

of colonic 5-hydroxytryptamine (5-HT)-immunoreactive

enteroendocrine cells, and altered ion transport across

colonic epithelium [76]. Furthermore, acute TNBS-colitis

impaired gastric emptying via an extrinsic neural reflex

pathway over pelvic nerves [22]. Interestingly, functional

neuroplastic alterations as a result of intestinal inflamma-

tion can extend even beyond the GI tract. Experimental

TNBS-colitis was shown to be associated with bladder

dysfunction and detrusor instability due to hyperexcitabil-

ity of spinal bladder neurons [67], colonic TRPV1-induced

SP and CGRP release [78], and voltage-gated sodium

channel upregulation [59] in the bladder. Moreover, colo-

nic afferent nerve pathways and especially DRG neurons

have been extensively studied in terms of their contribution

to visceral hypersensitivity. In this context, TNBS-colitis

was shown to induce increased numbers of voltage-gated

sodium channels [54], increased expression of the receptor

tyrosine TrkA and of the glial-cell-line-derived neurotro-

phic factor GFRa3 [105], and decreased expression of the

mechanosensitive K(2P) receptor [55] in colon-specific

DRG neurons. Therefore, the extra-intestinal neuroplas-

ticity seems to contribute to intestinal dysfunction and

hypersensitivity at least as much as the intra-organ

neuroplasticity.

Stomach

For stomach, it was shown that gastric ulcer, particularly

kissing gastric ulcers, can sensitize vagal and spinal gastric

afferents (DRG and nodose neurons) (Fig. 1), as evidenced

by doubling of P2X receptor activity to its agonists [20].

Furthermore, Sugiura et al. [103] showed that gastric ulcers

are associated with altered acid-elicited currents in DRG

neurons, increasing their pH sensitivity, density, and

kinetics in gastric DRG neurons. In harmony with these

observations, Bielefeldt et al. [11] demonstrated that in rats

with acetic-acid induced gastric ulcer, sodium currents

recovered faster from inactivation in nodose and DRG

neurons than control animals (Fig. 1). In a later study, the

same group reported that NGF is upregulated during gas-

tritis and can induce increased tetrodotoxin-resistent

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sodium currents [11]. In gastroparesis, Zhou et al. [119]

showed that nodose ganglia undergo activation of their

ATP-sensitive K?-channels upon exposure to hyperglyce-

mia (as in diabetic gastroparesis, Fig. 1), resulting in

gastric smooth muscle relaxation.

Esophagus

Due to the common association of pain and heartburn with

disorders of esophagus, the innervation of this organ has

recently been subject to more intensive investigation. In

experimental acid-induced esophagitis, the four splice

variants of the NMDA receptor subunits were shown to be

differentially expressed, with significant upregulation of

the NR1 subunit in DRG, NG neurons, and esophageal

tissue (Fig. 2) [5]. In a study by Qin et al. [85], intra-

esophageal instillation of HCl, bradykinin, or capsaicin not

only increased the activity of thoracic spinal neurons, but

also elicited excitatory responses to esophageal distention

in an even greater number of spinal neurons (Fig. 1).

Thereby, they could provide evidence for cross-sensitiza-

tion of spinal neurons and for visceral hypersensitivity in

esophagitis (Fig. 1) [85]. In another study, Medda et al.

[71] showed that vagal afferent fibers, but not spinal neu-

rons, exhibited an increase in action potential firing upon

esophageal distension or acid-pepsin infusion, with similar

findings from Kang et al. [51] who detected increased

sensitivity of afferents after exposure to acid or thermal

stimuli such as heated saline. In a subsequent study, the

group demonstrated that the acid-induced sensitization of

esophageal neurons can be inhibited by TRPV1 antagonists

[81].

In one of the few studies on the neuro-immune crosstalk

in esophageal disorders, Gao et al. [35] showed that mast

cell activation led to increased phosphorylation of ERK1/2

in nodose neurons and enhanced mechano-excitability of

esophageal nodose C-fibers. A similar activating role was

also demonstrated for TRPA1, which can similarly be

induced by mast cells over a protease-activated-receptor-2

(PAR2)-dependent mechanism and result in vagal afferent

C-fiber hypersensitivity [118]. Therefore, mast cell-

induced vagal afferent C-fiber activation is considered a

key mechanism in esophageal perception and hypersensi-

tivity (Fig. 2).

Regarding myoelectric properties of human esophagus

during gastroesophageal reflux disease (GERD), Shafik

et al. [91] reported on three different patterns of electric

activity are encountered in GERD: First, reflecting a nor-

mal activity; the second, an irregular electric activity

suggesting decreased motility; and third, a silent stage

where no acid clearance occurred [91]. As ICCs are

assumed to be the electric potential generators in the

esophagus, they may be causal in the generation of acid

reflux and the associated electroesophagogram patterns

[91].

Important steps have, though, also been made in clinical

studies on humans in which acid-infusion-induced esoph-

ageal and also distal (i.e., gastric) hypersensitivity have

been repeatedly demonstrated [10, 42, 88]. A recent clin-

ical study addressed the effect of pregabalin, a centrally

acting voltage-sensitive-calcium-channel modulator, on

acid-induced esophageal hypersensitivity, and showed that

subjects pre-treated with pregabalin demonstrated

decreased esophageal hypersensitivity [18]. Thereby, the

investigators presented one of the first pharmacological

functional studies on acid-induced esophageal hypersensi-

tivity in humans [18].

Liver and gallbladder

Studies on the functional neuroplasticity of liver are totally

lacking. Nonetheless, gallbladder stasis was shown as a

potential contributor to gallstone formation [49]. Microin-

jection of prostaglandin E2 (PGE2) into the gallbladder

wall resulted in hyperpolarization, in reduction of ampli-

tudes of the fast and slow postsynaptic potentials, and thus

in the inhibition of intramural ganglionic neurons and

decreased gallbladder motility [49]. In experimental acute

cholecystitis, gallbladder contractions and NO-mediated

neurotransmission were significantly attenuated. Here,

emergence of contractions insensitive to tetrodotoxin and

sensitive to atropine and omega-conotoxin suggested that

intrinsic gallbladder innervation was the primarily affected

neural component [38]. The decreased neurotransmission

was also shown to be related to impaired calcium homeo-

stasis, diminished smooth muscle contractility, and the

inflammatory actions of reactive oxygen species and his-

tamine in the inflamed gallbladder [83].

Pancreas

Currently, no experimental model has yet been found to be

suitable for studying functional neuroplasticity in PCa, and

for CP, the only applied model remains CP as induced by

infusion of TNBS acid into the murine pancreatic duct [44,

117]. While this model has not yet been tested for the

presence of structural neuroplastic alterations, it was pre-

viously shown that mast cell-deficient mice had

significantly less pain sensation than wild-type mice during

TNBS-induced CP (Fig. 2) [44]. Using this model, Xu

et al. [117] demonstrated the enhanced excitability, sup-

pression of A-type potassium currents, and increased

expression of TRPV1 in pancreas-specific DRG neurons

(Fig. 2). In TNBS-induced CP in rats, Qian et al. [84]

showed that toll-like receptor 3 is upregulated in astrocytes

of the spinal cord, correlating to the degree of mechanical

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123

allodynia, and upregulation of inflammatory mediators

such as IL-1b, TNF-a, IL-6, and monocyte chemotactic

protein-1 (MCP-1). In another study, the same model was

demonstrated to induce astrocytic activation in the thoracic

spinal cord of rats, as evidenced by upregulation of the

glial fibrillary acidic protein (GFAP) and its reversibility

by the intrathecal administration of specific inhibitor

l-alpha-aminoadipate [30]. In this regard, an important role

was also demonstrated for spinal microglia which was seen

to be activated during TNBS-induced CP, to overexpress

phosphorylated p38 and to be further enhanced by

administration of the neuronal chemokine fractalkine [64].

These effects were reversible upon administration of the

microglia inhibitor minocycline [64].

Peripheral glia in the neuroplasticity of the GI tract:

glial activation in inflammation

The remarkable extent of structural and functional neuro-

plastic alterations in the GI tract seems in many cases to be

accompanied by reactive alterations in glial cells, particu-

larly in enteric glia. Overall, these alterations can be

considered to represent ‘‘glial activation’’, and like astro-

cytes of the central nervous system, enteric glia were

reported to increase their proliferation rate and expression

of GFAP as their major intermediate filament during

inflammation, e.g., in ulcerative colitis and Crohn’s disease

[112]. One can assume that activated glia cells may con-

tribute to the local reparative processes [89, 109], since the

selective depletion of GFAP-expressing enteric glia results

in disruption of mucosa and severe inflammation in the

small intestine, characterized by hemorrhagic necrosis

[109]. In line with these observations, enteric glia cells were

later found to modulate epithelial proliferation, differenti-

ation, and inflammation via S-nitrosoglutathione [89]. This

key role of enteric glia for mucosal integrity is somewhat

reflected in observations from human IBD, since patients

with Crohn’s disease were also reported to have smaller

amounts of enteric glial cells in the intestinal wall before the

onset of overt inflammation [112]. Their potential role in

local repair and structural maintenance is further supported

by the fact that glial cells are an important source of growth

factors such as pro-epidermal growth factor (proEGF),

NGF, GDNF, neurotrophin-3; of neurotransmitter precur-

sors; of major histocompatibility complex class II

molecules; and of pro-inflammatory cytokines such as IL-6

and IL-1b and can, thus, modulate the function of peripheral

inflammatory cells and neurons [110]. A recent study on the

state of intrapancreatic neural glia in PCa and CP showed

that the hypertrophic intrapancreatic nerves lose their nor-

mally high content of Sox10-expressing glia, together with

prominent upregulation of the glial intermediate filament

and neuroepithelial stem cell marker nestin, suggesting

potential de-differentiation of intrapancreatic glia as a result

of the activating environment [15].

Unfortunately, further structural and functional altera-

tions of glia cells in disease states of the GI tract are largely

absent. However, very recent intriguing results from

ontogenic studies on enteric glia revealed a novel role for

these cells, which may be crucial for understanding neu-

roplasticity in the GI tract. Tracing of embryonic and fetal

Sox10-expressing cells in the murine GI tract revealed that

these cells give rise to neurons and glia cells of myenteric

ganglia [57]. Postnatally, however, these cells ceased to

give rise to neurons [57]. Interestingly, glia-derived neu-

rons could be generated from enteric glia under in vitro

culture conditions or after chemical injury [50, 57]. Hence,

enteric glia cells rather seem to be multipotent cells with

both gliagenic and neurogenic potential and have recently

emerged as major actors in enteric neuroplasticity [50, 57].

Critical appraisal of GI neuroplasticity: the norm,

the bystander or the defining characteristic?

Despite the growing number of studies on neuroplastic

alterations in several GI organs, the existence of genuine

neuroplasticity in GI disorders is not undebated. This

controversy is in part reflected by studies on intestinal

neuronal dysplasia type B (IND-B), a submucous plexus

abnormality of infants characterized by giant enteric gan-

glia. Some studies, though, revealed that such giant ganglia

can similarly be encountered in normal colon and may,

therefore, be just part of the ‘‘normal variation’’ [66].

Looking at this controversy, one can consider three possi-

bilities for the true role of neuroplasticity in the GI tract:

1. Neuroplasticity as an ‘‘artificial entity’’: As in IND-B,

researchers should pay attention to the degree of

normal variations in GI neuromorphology. Do GI

organs in their normal state contain areas with greater

density of innervation or larger intrinsic ganglia? In the

human and murine pancreas, the innervation density in

the pancreatic head was reported to be greater than in

the body and tail [107]. Similarly, murine colon

naturally harbors hypoganglionic areas which are

surrounded by densely clustered enteric neurons [93].

In malignant GI tumors, there are possibly hypo- and

hyper-innervated areas in the same tissue [3].

2. Neuroplasticity as ‘‘adaptive response’’: The frequent

occurrence of neuroplastic alterations in inflammatory

GI disorders implicates that inflammatory cells may be

releasing neurotrophic agents and, thereby, causing

neural hypertrophy and neuro-sensitization. Such

enlarged and sensitized nerves may in turn signal

Acta Neuropathol

123

more frequently to the CNS as part of an evolutionary

protective response to signify disease at the periphery

[32]. On other hand, in IBD, neuroplasticity currently

seems to be a mere bystander, since an association

between symptom promotion and neuroplasticity has

not yet been convincingly reported. Still, in the

scenario of neuroplasticity as adaptive response, one

would expect the reversal of neuroplasticity after

clearance of the initiating tissue injury (e.g., epithelial

lesions). In this case, one would refer to this adaptive

response as ‘‘secondary neuroplasticity’’ (Table 2).

3. Neuroplasticity as ‘‘the defining characteristic’’: In this

third scenario, neuroplastic alterations can be consid-

ered to bear a crucial role in the pathophysiology of the

respective GI disease. In IBS, achalasia and nutcracker

esophagus, neuro-inflammation as well as degenerative

and regenerative neuro-alterations belong to the

defining histopathological characteristics of these dis-

orders. Hence, these neuroplastic alterations can be

assumed to sustain the actual disease, and their

reversal bears the potential of amending these

disorders.

Hyperinnervation(neurotrophic

factor -mediated )

Hypoinnervation( inflammatory damage ,

e.g. ROS- mediated )

GI INFLAMMATION

INTRA -ORGANINNERVATION

Fig. 3 Interaction of

neuroplasticity with

inflammation in the

gastrointestinal (GI) tract. In the

GI tract, inflammatory cells

such as mast cells, lymphocytes,

monocytes, and granulocytes

secrete a battery of neurotrophic

factors such as nerve growth

factor (NGF), neurotrophin-3

(NT-3) and neurotrophin-4

(NT-4) which sensitize

nociceptive nerve endings and

at the same time foster neural

growth. Mast cells additionally

secrete tryptase and histamine

which can also activate

nociceptive fibers. On the other

hand, immune cells can also

release neurotoxic agents such

as reactive oxygen species

(ROS) which can promote local

foci of tissue hypoinnervation.

Importantly, epithelial cells in

inflamed tissues were similarly

shown to secrete neurotrophic

factors such as NGF and brain-

derived-neurotrophic factor

(BDNF). Activated neurons not

only signal to the central

nervous system, but also release

the pro-inflammatory

neuropeptides substance P and

calcitonin-gene-related-peptide

(CGRP) in an antidromic

fashion into the inflamed tissue

and thereby aggravate local

inflammation. Overall,

inflammation-nerve interactions

in GI inflammation are

embedded in a vicious cycle

which promotes neuroplasticity

in conjunction with local

inflammation

Acta Neuropathol

123

But what criteria may help us identify the true role of

neuroplasticity in a given GI disorder? First of all, studies

on GI neuroplasticity should ascertain the representative

nature of the studied specimens. Samples from subjects

with matched demographic characteristics should be

obtained from several anatomic layers of the studied organ

and compared to the already known natural variation in

organ neuromorphology. In studies on tumor-associated

neuroplasticity, resected tumors should be examined in

their entirety, including intra-tumoral, peri-tumoral, and

extra-tumoral regions. Concomitant peri-tumoral inflam-

mation should be recorded and considered as confounder.

Once the observed neuroplasticity can be confirmed to

surpass the extent of the natural intra-organ and inter-

subject variation, it can be classified as ‘‘genuine

neuroplasticity’’.

Table 2 Visceral neuroplasticity: a bystander or the main actor?

Primary neuro-

inflammation

Secondary neuro-

inflammation

Associated with disease/symptom

progression?

Type of association with disease/

symptoms

Appendix

Appendicitis No Yes Yes Pain

‘‘Neurogenic

appendicitis’’

Yes No Yes Pain

Intestine

Ulcerative colitis No Yes Unclear Neurogenic inflammation ? Pain?

Dysmotility?

Crohn’s disease No Yes Unclear Neurogenic inflammation ? Pain?

Dysmotility?

Irritable bowel

syndrome

Yes No Yes Hypersensitivity, dysmotility

Trichinella spiralis-

inf.

No Yes Yes Hypersensitivity

Chagas’ disease Yes No Yes Megacolon

Diverticulitis No Yes Yes Hypersensitivity

Colon cancer No Unknown Unknown Unknown

Stomach

Chronic gastritis No Yes Yes Hypersensitivity/pain

Gastric cancer No Yes Yes Promotion of tumor growth

Esophagus

Nutcracker

esophagus

Yes No Yes Dysphagia/pain/dysmotility

Achalasia Yes No Yes Dysphagia/pain/dysmotility

Reflux esophagitis No Yes Yes Hypersensitivity

Esophageal cancer No Unknown Unknown Unknown

Liver

Cirrhosis No Yes Unknown Unknown

HCC No Unknown Unknown Unknown

Gallbladder

Chronic

cholecystitis

No Yes Yes Hypersensitivity/pain

Pancreas

Chronic

pancreatitis

No Yes Yes Increased inflammation and pain

Pancreatic cancer No Yes Yes Promotion of tumor growth and

pain

Numerous disorders of the gastrointestinal (GI) tract exhibit neuroplasticity in the presence of (chronic) inflammation. In some diseases,

inflammation is primarily directed to neurons or neural structures (‘‘primary’’ neuro-inflammation), whereas in the majority of other GI disorders,

intra-organ nerves are inflamed in the presence of general tissue inflammation (‘‘secondary neuro-inflammation’’). Here, it should be noted that

neuroplastic-neuroinflammatory alterations in the GI tract are frequently, but not unexceptionally, associated with disease progression or

symptoms. However, functional studies which could show alleviation of disease or symptoms after reversal of GI neuroplasticity are so far

lacking

Acta Neuropathol

123

Second, conclusions on the implications and the true

impact of GI neuroplasticity should be drawn primarily

from functional studies which target at selected compo-

nents of the complex network between inflammation,

nerves, glia, and epithelial cells. Here, genetically engi-

neered animal models allow by means of cell-specific

promoters the over-expression, knock-down, or knock-out

of defined molecular targets in each component of this

network. The reversibility or augmentation of neuroplas-

ticity in a certain GI disorder after modulation of disease-

associated molecular targets in non-neural cells would

allow the classification of neuroplasticity as ‘‘adaptive

response’’. Furthermore, if functional modulation of targets

in neural or glial cells results in clearance or amelioration

of the GI disease, neuroplasticity would qualify as ‘‘the

defining characteristic’’. As of today, representative func-

tional models are lacking for a large portion of the GI

disorders that are discussed in this review. In analogy with

Koch’s postulates for defining the causative role of

microbes in disease, we believe that these ‘‘postulates for

neuroplasticity’’ are going to be useful for future studies

trying to identify the true role and impact of neuroplasticity

in GI disorders.

Mechanisms of visceral neuroplasticity in GI

inflammation: a vicious cycle?

Despite the frequent presence of inflammation in GI neu-

roplasticity, the phenotypes of neuroplastic alterations are

very heterogeneous, e.g., tissue hyper- or hypoinnervation

(Fig. 1) and quite variable alterations in the neurochemical

code of visceral neurons (Table 1). The most plausible

explanation for this variability between different GI dis-

orders may be due to differences in the pattern of

inflammation between different GI disorders. In theory, one

can assume that, regardless of its specifics, inflammatory

conditions could be associated with or even create a neu-

rotrophic milieu. In abundance of neurotrophic factors such

as NGF, GDNF, and BDNF, typical traits of neuroplas-

ticity, i.e., tissue hyperinnervation, hyperexcitability, and

neurochemical code switch, may be induced. This

hypothesis is supported by the observation that colonic

mucosal cells of rats with TNBS-induced colitis secrete

increased amounts of NGF [99]. An association between

NGF expression and GI inflammation was so far reported

for esophagitis [92], appendicitis [19], colitis and IBD [25],

and chronic pancreatitis [33]. Moreover, in GI cancer-

associated neuroplasticity, neural hypertrophy is most

prominent in areas of perineural inflammation [14]. Inter-

estingly, another major source of such neurotrophic factors

are inflammatory cells per se. Mast cells (MC), B and T

lymphocytes, eosinophils, and basophils have all been

shown to produce considerable amounts of neurotrophic

factors (Fig. 3) [98]. Here, particularly MC were shown to

release large amounts of neurotrophins such as NGF, NT-3,

and NT-4 upon activation [98]. These cells and factors can

induce nerve hypertrophy (with increased presence of SP

and CGRP-containing nerve fibers) and specifically acti-

vate nociceptive signaling [104]. Furthermore, activated

nociceptive fibers which contain SP and CGRP can release

these neuropeptides and potentiate inflammation by the

neurogenic inflammatory property of these neuropeptides

[61]. On the other hand, inflammatory cells can at the same

time be responsible for nerve damage and, thereby, cause

local hypoinnervation [114]. Such cells can release cyto-

kines such as interleukin-6 (IL-6), IL-12, and TNF-alpha

and neuro-destructive agents such as reactive oxygen spe-

cies (ROS), phagocytose damaged neural components, and

co-operate with cytotoxic T lymphocytes in the clearance

of damaged neural structures [114]. It is conceivable that

during GI inflammation, such cells increasingly accumulate

around damaged neural structures and mediate their

clearance for the subsequent neuroplastic regeneration, as

known from Wallerian degeneration. Therefore, tissue

hypoinnervation and hyperinnervation can be present in the

same tissue, and the balance between the neuro-destructive

and neurotrophic actions of inflammatory cells may

determine the ultimate extent of neuroplasticity in GI

organs. Overall, there seems to be a vicious cycle com-

prising inflammation (e.g., mast cell activation), increased

neurotrophic/neurotoxic factor release from parenchymal

or inflammatory cells, hyper- and hypoinnervation, noci-

ceptive hyperexcitability, glial activation, and in turn

augmented inflammation (Fig. 3). This vicious cycle may

be central to understanding mechanisms of neuroplasticity

in the GI tract.

The specific subsets of inflammatory cells that compose

neuritis or ganglionitis lesions in the GI tract have not yet

been well characterized. The increased presence of MC in

IBS [7] and of lymphocytes, macrophages, and mast cells

in chronic gastritis [96] suggests that these cell types may

be involved in GI neuro-inflammation. Still, looking at all

disorders in which inflammation-associated GI neuroplas-

ticity was reported (e.g., reflux esophagitis, gastritis,

chronic pancreatitis, pancreratic cancer, IBD, IBS, liver

cirrhosis), it seems that GI neuroplastic alterations occur in

the presence of chronic rather than acute inflammation. In

this context, future studies should correlate the extent of GI

neuroplasticity with the severity of ongoing inflammation

(i.e., clinically apparent vs. silent) and both the timing and

the severity of acute episodes.

Currently, nearly the entire knowledge on GI neuro-

plasticity is derived from descriptive histomorphological

studies, and there is need for functional studies to elucidate

the actual pathomechanism behind GI neuroplasticity. In

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123

particular, we propose in vivo models in which the con-

tribution of neurotrophic factors such as NGF, GDNF, and

BDNF to neuroplasticity can be studied by driving their

expression under the control of cell-specific promoters

(e.g., villin for enterocytes, p48 for pancreatic acinar cells,

H?-K? ATPase for gastric parietal cells, albumin for

hepatocytes) with subsequent induction of inflammation

(e.g., TNBS-induced colitis or pancreatitis, H. pylori-

induced gastritis), and the study of organ neuroplasticity at

histological level and organ function by means of e.g.,

ex vivo motility assays. Tissues that are isolated from such

animals would also be accessible to detailed genomic and

proteomic analysis for identifying further co-activated

molecular pathways. In the same models, the contribution

of inflammatory cells and reversibility of neuroplasticity

can be studied, e.g., by depletion of specific inflammatory

cell subtypes, as previously performed in murine chronic

pancreatitis [44]. In addition, in vitro models in which

intrinsic GI neurons or glia cells can be cultured within

tissue extracts obtained from patients with different GI

disorders can be used to study the reaction of GI neurons to

disease microenvironment [24]. Neurons from such assays

can be subjected to gene microarray analysis for identify-

ing differentially regulated novel targets. We are convinced

that only such functional models bear the potential to shed

light on the individual pathomechanistic steps in the gen-

eration of GI neuroplasticity.

Summary and conclusion

Neuroplasticity, both at structural and functional level, is a

common feature of the entire GI nervous system during

pathological states, resulting in organ dysfunction, neuro-

pathic pain, and/or hypersensitivity. In many cases, this

visceral neuroplasticity comprises either a tissue hyperin-

nervation or partial loss of innervation, together with

preferential occurrence of certain neurotransmitter codes

over others. Interestingly, neuroplasticity in the GI tract is

most prominent in states of GI inflammation, and plastic

neurons, nerves, and ganglia are frequently found to be in

close spatial contact with specifically nerve/ganglion-

infiltrating inflammatory cells. These inflammatory cells

secrete activating molecular mediators with neurotrophic

and chemotactic features, leading to enhanced excitability,

decreased sensory thresholds, and neuropathic pain.

Importantly, this neuro-immune crosstalk—as also known

from the CNS—involves concomitant activation of glia

cells which are increasingly recognized to have a deter-

mining role in the GI tissue integrity, inflammation, and

neuroplasticity. Overall, looking at the entire GI tract,

visceral neuroplasticity not only involves similar structural

and functional alterations in different GI organs, but also a

disease-stage-dependent hypersensitivity and entailing

pain. Unsurprisingly, this recent recognition of GI neuro-

plasticity and the associated hypersensitivity and pain gave

birth to novel studies on the efficacy of neuropathic pain

medications for treating visceral pain and hypersensitivity.

The repeatedly demonstrated pain-relief upon administra-

tion of neuropathic pain medications to visceral pain

underscores the presence of neuroplastic-neuropathic pain

mechanisms in GI disease states. When neural plasticity is

considered as an adaptive response of the GI nervous

system to a pathological state, the majority of these adap-

tive responses seem to be generated upon the same

underlying condition, i.e., chronic tissue- and neuro-

inflammation, which can be encountered in either inflam-

matory, infectious, neoplastic/malignant, or degenerative

disorders of the GI tract. Consequently, the key to in-depth

understanding of neural plasticity in the GI tract seems to

lie at the intersection of chronic inflammation and the

associated trophic/excitatory signals. Future research shall,

therefore, elucidate the peripheral neuromodulatory prop-

erties of chronic inflammation in the GI tract.

Acknowledgments The authors are grateful to Ms. Martina Scholle

for her assistance in the generation of the figures.

Conflict of interest None.

References

1. Akbar A, Yiangou Y, Facer P, Brydon WG, Walters JR, Anand

P, Ghosh S (2010) Expression of the TRPV1 receptor differs in

quiescent inflammatory bowel disease with or without abdomi-

nal pain. Gut 59(6):767–774

2. Akiyoshi H, Terada T (1998) Mast cell, myofibroblast and nerve

terminal complexes in carbon tetrachloride-induced cirrhotic rat

livers. J Hepatol 29(1):112–119

3. Albo D, Akay CL, Marshall CL, Wilks JA, Verstovsek G, Liu H,

Agarwal N, Berger DH, Ayala GE (2011) Neurogenesis in

colorectal cancer is a marker of aggressive tumor behavior and

poor outcomes. Cancer 117(21):4834–4845

4. Banerjee B, Medda BK, Lazarova Z, Bansal N, Shaker R,

Sengupta JN (2007) Effect of reflux-induced inflammation on

transient receptor potential vanilloid one (TRPV1) expression in

primary sensory neurons innervating the oesophagus of rats.

Neurogastroenterol Motil 19(8):681–691

5. Banerjee B, Medda BK, Zheng Y, Miller H, Miranda A,

Sengupta JN, Shaker R (2009) Alterations in N-methyl-D-

aspartate receptor subunits in primary sensory neurons following

acid-induced esophagitis in cats. Am J Physiol Gastrointest

Liver Physiol 296(1):G66–G77

6. Bapat AA, Hostetter G, Von Hoff DD, Han H (2011) Perineural

invasion and associated pain in pancreatic cancer. Nat Rev

Cancer 11(10):695–707

7. Barbara G, Stanghellini V, De Giorgio R, Cremon C, Cottrell GS,

Santini D, Pasquinelli G, Morselli-Labate AM, Grady EF, Bun-

nett NW, Collins SM, Corinaldesi R (2004) Activated mast cells

in proximity to colonic nerves correlate with abdominal pain in

irritable bowel syndrome. Gastroenterology 126(3):693–702

Acta Neuropathol

123

8. Barbara G, Wang B, Stanghellini V, de Giorgio R, Cremon C,

Di Nardo G, Trevisani M, Campi B, Geppetti P, Tonini M,

Bunnett NW, Grundy D, Corinaldesi R (2007) Mast cell-

dependent excitation of visceral-nociceptive sensory neurons in

irritable bowel syndrome. Gastroenterology 132(1):26–37

9. Bercik P, De Giorgio R, Blennerhassett P, Verdu EF, Barbara G,

Collins SM (2002) Immune-mediated neural dysfunction in a

murine model of chronic Helicobacter pylori infection. Gastro-

enterology 123(4):1205–1215

10. Bhalla V, Liu J, Puckett JL, Mittal RK (2004) Symptom

hypersensitivity to acid infusion is associated with hypersensi-

tivity of esophageal contractility. Am J Physiol Gastrointest

Liver Physiol 287(1):G65–G71

11. Bielefeldt K, Ozaki N, Gebhart GF (2002) Mild gastritis alters

voltage-sensitive sodium currents in gastric sensory neurons in

rats. Gastroenterology 122(3):752–761

12. Cajal SRY (1928) Degeneration and regeneration of the nervous

system. Oxford University Press, Oxford

13. Ceyhan GO, Bergmann F, Kadihasanoglu M, Erkan M, Park W,

Hinz U, Giese T, Muller MW, Buchler MW, Giese NA, Friess H

(2007) The neurotrophic factor artemin influences the extent of

neural damage and growth in chronic pancreatitis. Gut 56(4):

534–544

14. Ceyhan GO, Bergmann F, Kadihasanoglu M, Altintas B, Demir

IE, Hinz U, Muller MW, Giese T, Buchler MW, Giese NA,

Friess H (2009) Pancreatic neuropathy and neuropathic pain—a

comprehensive pathomorphological study of 546 cases. Gas-

troenterology 136(1):177.e1–186.e1

15. Ceyhan GO, Demir IE, Rauch U, Bergmann F, Muller MW,

Buchler MW, Friess H, Schafer KH (2009) Pancreatic neurop-

athy results in ‘‘neural remodeling’’ and altered pancreatic

innervation in chronic pancreatitis and pancreatic cancer. Am J

Gastroenterol 104(10):2555–2565

16. Ceyhan GO, Schafer KH, Kerscher AG, Rauch U, Demir IE,

Kadihasanoglu M, Bohm C, Muller MW, Buchler MW, Giese

NA, Erkan M, Friess H (2010) Nerve growth factor and artemin

are paracrine mediators of pancreatic neuropathy in pancreatic

adenocarcinoma. Ann Surg 251(5):923–931

17. Chen Z, Suntres Z, Palmer J, Guzman J, Javed A, Xue J, Yu JG,

Cooke H, Awad H, Hassanain HH, Cardounel AJ, Christofi FL

(2007) Cyclic AMP signaling contributes to neural plasticity and

hyperexcitability in AH sensory neurons following intestinal

Trichinella spiralis-induced inflammation. Int J Parasitol

37(7):743–761

18. Chua YC, Ng KS, Sharma A, Jafari J, Surguy S, Yazaki E,

Knowles CH, Aziz Q (2012) Randomised clinical trial: pre-

gabalin attenuates the development of acid-induced oesophageal

hypersensitivity in healthy volunteers—a placebo-controlled

study. Aliment Pharmacol Ther 35(3):319–326

19. Coskun N, Sindel M, Elpek GO (2002) Mast cell density,

neuronal hypertrophy and nerve growth factor expression in

patients with acute appendicitis. Folia Morphol (Warsz) 61(4):

237–243

20. Dang K, Bielfeldt K, Lamb K, Gebhart GF (2005) Gastric ulcers

evoke hyperexcitability and enhance P2X receptor function in

rat gastric sensory neurons. J Neurophysiol 93(6):3112–3119

21. da Silveira AB, Freitas MA, de Oliveira EC, Neto SG, Luquetti

AO, Furness JB, Correa-Oliveira R, Reis D (2008) Substance P

and NK1 receptor expression in the enteric nervous system is

related to the development of chagasic megacolon. Trans R Soc

Trop Med Hyg 102(11):1154–1156

22. De Schepper HU, De Man JG, Van Nassauw L, Timmermans

JP, Herman AG, Pelckmans PA, De Winter BY (2007) Acute

distal colitis impairs gastric emptying in rats via an extrinsic

neuronal reflex pathway involving the pelvic nerve. Gut

56(2):195–202

23. Demir IE, Ceyhan GO, Liebl F, D‘Haese JG, Maak M, Friess H

(2010) Neural invasion in pancreatic cancer: the past, present

and future. Cancers 2(3):1513–1527

24. Demir IE, Ceyhan GO, Rauch U, Altintas B, Klotz M, Muller

MW, Buchler MW, Friess H, Schafer KH (2010) The microen-

vironment in chronic pancreatitis and pancreatic cancer induces

neuronal plasticity. Neurogastroenterol Motil 22(4):480–490

(e112–483)

25. di Mola FF, Friess H, Zhu ZW, Koliopanos A, Bley T, Di

Sebastiano P, Innocenti P, Zimmermann A, Buchler MW (2000)

Nerve growth factor and Trk high affinity receptor (TrkA) gene

expression in inflammatory bowel disease. Gut 46(5):670–679

26. Di Sebastiano P, Fink T, di Mola FF, Weihe E, Innocenti P,

Friess H, Buchler MW (1999) Neuroimmune appendicitis.

Lancet 354(9177):461–466

27. Di Sebastiano P, di Mola FF, Di Febbo C, Baccante G, Porreca

E, Innocenti P, Friess H, Buchler MW (2000) Expression of

interleukin 8 (IL-8) and substance P in human chronic pancre-

atitis. Gut 47(3):423–428

28. Dvorak AM, Osage JE, Monahan RA, Dickersin GR (1980)

Crohn’s disease: transmission electron microscopic studies. III.

Target tissues. Proliferation of and injury to smooth muscle and

the autonomic nervous system. Hum Pathol 11(6):620–634

29. Faussone-Pellegrini, Grover M, Pasricha P, Bernard C, Lurken

M, Smyrk T, Parkman H, Abell T, Snape W, Hasler W, Unalp-

Arida A, Nguyen L, Koch K, Calles J, Lee L, Tonascia J,

Hamilton F, Farrugia G, NIDDK gastroparesis clinical research

consortium (GpCRC) (2011) Ultrastructural differences between

diabetic and idiopathic gastroparesis. J Cell Mol Med

16(7):1573–1581

30. Feng QX, Wang W, Feng XY, Mei XP, Zhu C, Liu ZC, Li YQ,

Dou KF, Zhao QC (2010) Astrocytic activation in thoracic

spinal cord contributes to persistent pain in rat model of chronic

pancreatitis. Neuroscience 167(2):501–509

31. Feng F, Yang J, Tong L, Yuan S, Tian Y, Hong L, Wang W,

Zhang H (2011) Substance P immunoreactive nerve fibres are

related to gastric cancer differentiation status and could promote

proliferation and migration of gastric cancer cells. Cell Biol Int

35(6):623–629

32. Fregni F, Pascual-Leone A, Freedman SD (2007) Pain in chronic

pancreatitis: a salutogenic mechanism or a maladaptive brain

response? Pancreatology 7(5–6):411–422

33. Friess H, Zhu ZW, di Mola FF, Kulli C, Graber HU, Andren-

Sandberg A, Zimmermann A, Korc M, Reinshagen M, Buchler

MW (1999) Nerve growth factor and its high-affinity receptor in

chronic pancreatitis. Ann Surg 230(5):615–624

34. Galligan JJ, Furness JB, Costa M (1989) Migration of the

myoelectric complex after interruption of the myenteric plexus:

intestinal transection and regeneration of enteric nerves in the

guinea pig. Gastroenterology 97(5):1135–1146

35. Gao G, Ouyang A, Kaufman MP, Yu S (2011) ERK1/2 signaling

pathway in mast cell activation-induced sensitization of esoph-

ageal nodose C-fiber neurons. Dis Esophagus 24(3):194–203

36. Gockel I, Bohl JR, Eckardt VF, Junginger T (2008) Reduction of

interstitial cells of Cajal (ICC) associated with neuronal nitric

oxide synthase (n-NOS) in patients with achalasia. Am J Gas-

troenterol 103(4):856–864

37. Godlewski J (2010) Morphological changes in the enteric ner-

vous system caused by carcinoma of the human large intestine.

Folia Histochem Cytobiol 48(1):157–162

38. Gomez-Pinilla PJ, Camello PJ, Pozo MJ (2007) Effects of

melatonin on gallbladder neuromuscular function in acute cho-

lecystitis. J Pharmacol Exp Ther 323(1):138–146

39. Gonda T, Akiyoshi H, Ichihara K (1995) Hyperplastic inner-

vation of vasoactive intestinal peptide in human gallbladder with

cholelithiasis. Histol Histopathol 10(3):669–672

Acta Neuropathol

123

40. Grover M, Farrugia G, Lurken MS, Bernard CE, Faussone-

Pellegrini MS, Smyrk TC, Parkman HP, Abell TL, Snape WJ,

Hasler WL, Unalp-Arida A, Nguyen L, Koch KL, Calles J, Lee

L, Tonascia J, Hamilton FA, Pasricha PJ (2011) Cellular chan-

ges in diabetic and idiopathic gastroparesis. Gastroenterology

140(5):1575.e8–1585.e8

41. Hennig R, Zanli J, Osman T, Esposito I, Berhane T, Vetrhus M,

Sondenaa K, Buchler MW, Friess H (2007) Association between

gallstone-evoked pain, inflammation and proliferation of nerves

in the gallbladder: a possible explanation for clinical differ-

ences. Scand J Gastroenterol 42(7):878–884

42. Hobson AR, Khan RW, Sarkar S, Furlong PL, Aziz Q

(2004) Development of esophageal hypersensitivity following

experimental duodenal acidification. Am J Gastroenterol 99(5):

813–820

43. Hofler H, Heitz PU (1982) Neurogenic appendicopathy—an

immunocytochemical study. Wien Klin Wochenschr 94(21):

588–591

44. Hoogerwerf WA, Gondesen K, Xiao SY, Winston JH, Willis

WD, Pasricha PJ (2005) The role of mast cells in the patho-

genesis of pain in chronic pancreatitis. BMC Gastroenterol 5:8

45. Hughes PA, Brierley SM, Blackshaw LA (2009) Post-inflam-

matory modification of colonic afferent mechanosensitivity.

Clin Exp Pharmacol Physiol 36(10):1034–1040

46. Humes DJ, Simpson J, Smith J, Sutton P, Zaitoun A, Bush D,

Bennett A, Scholefield JH, Spiller RC (2012) Visceral hyper-

sensitivity in symptomatic diverticular disease and the role of

neuropeptides and low grade inflammation. Neurogastroenterol

Motil 24(4):e318–e163

47. Jaskiewicz K, Robson SC, Banach L (1993) Toxic hepatic injury

is associated with proliferation of portal nerve fibers. Pathol Res

Pract 189(10):1191–1194

48. Jaskiewicz K, Voigt MD, Robson SC (1994) Distribution of

hepatic nerve fibers in liver diseases. Digestion 55(4):247–252

49. Jennings LJ, Mawe GM (1998) PGE2 hyperpolarizes gallblad-

der neurons and inhibits synaptic potentials in gallbladder

ganglia. Am J Physiol 274(3 Pt 1):G493–G502

50. Joseph NM, He S, Quintana E, Kim YG, Nunez G, Morrison SJ

(2011) Enteric glia are multipotent in culture but primarily form

glia in the adult rodent gut. J Clin Invest 121(9):3398–3411

51. Kang YM, Bielefeldt K, Gebhart GF (2004) Sensitization of

mechanosensitive gastric vagal afferent fibers in the rat by

thermal and chemical stimuli and gastric ulcers. J Neurophysiol

91(5):1981–1989

52. Kilic A, Krasinskas AM, Owens SR, Luketich JD, Landreneau

RJ, Schuchert MJ (2007) Variations in inflammation and nerve

fiber loss reflect different subsets of achalasia patients. J Surg

Res 143(1):177–182

53. Kim HS, Park H, Lim JH, Choi SH, Park C, Lee SI, Conklin JL

(2008) Morphometric evaluation of oesophageal wall in patients

with nutcracker oesophagus and ineffective oesophageal motil-

ity. Neurogastroenterol Motil 20(8):869–876

54. King DE, Macleod RJ, Vanner SJ (2009) Trinitrobenzenesul-

phonic acid colitis alters Na 1.8 channel expression in mouse

dorsal root ganglia neurons. Neurogastroenterol Motil 21(8):

e880–e864

55. La JH, Gebhart GF (2011) Colitis decreases mechanosensitive

K2P channel expression and function in mouse colon sensory

neurons. Am J Physiol Gastrointest Liver Physiol 301(1):G165–

G174

56. Langley JN (1897) On the regeneration of preganglionic and of

postganglionic visceral nerve fibres. J Physiol 22:215–2130

57. Laranjeira C, Sandgren K, Kessaris N, Richardson W, Potocnik

A, Vanden Berghe P, Pachnis V (2011) Glial cells in the mouse

enteric nervous system can undergo neurogenesis in response to

injury. J Clin Invest 121(9):3412–3424

58. Lee JA, Ahmed Q, Hines JE, Burt AD (1992) Disappearance of

hepatic parenchymal nerves in human liver cirrhosis. Gut

33(1):87–91

59. Lei Q, Malykhina AP (2012) Colonic inflammation up-regulates

voltage-gated sodium channels in bladder sensory neurons via

activation of peripheral transient potential vanilloid 1 receptors.

Neurogastroenterol Motil 24(6):575–585 (e257)

60. Li K, Zheng S, Xiao X, Wang Q, Zhou Y, Chen L (2007) The

structural characteristics and expression of neuropeptides in the

esophagus of patients with congenital esophageal atresia and

tracheoesophageal fistula. J Pediatr Surg 42(8):1433–1438

61. Liddle RA, Nathan JD (2004) Neurogenic inflammation and

pancreatitis. Pancreatology 4(6):551–559 (discussion 559–560)

62. Liebl F, Demir IE, Rosenberg R, Boldis A, Yildiz E, Kujundzic

K, Kehl T, Dischl D, Schuster T, Maak M, Becker K, Langer R,

Laschinger M, Friess H, Ceyhan GO (2012) The severity of

neural invasion is associated with shortened survival in colon

cancer. Clin Cancer Res 19(1):50–61

63. Linden DR, Sharkey KA, Mawe GM (2003) Enhanced excit-

ability of myenteric AH neurones in the inflamed guinea-pig

distal colon. J Physiol 547(Pt 2):589–601

64. Liu PY, Lu CL, Wang CC, Lee IH, Hsieh JC, Chen CC, Lee HF,

Lin HC, Chang FY, Lee SD (2012) Spinal microglia initiate and

maintain hyperalgesia in a rat model of chronic pancreatitis.

Gastroenterology 142(1):165.e2–173.e2

65. Lomax AE, Fernandez E, Sharkey KA (2005) Plasticity of the

enteric nervous system during intestinal inflammation. Neuro-

gastroenterol Motil 17(1):4–15

66. Lumb PD, Moore L (1998) Are giant ganglia a reliable marker

of intestinal neuronal dysplasia type B (IND B)? Virchows Arch

432(2):103–106

67. Malykhina AP, Qin C, Greenwood-van Meerveld B, Foreman

RD, Lupu F, Akbarali HI (2006) Hyperexcitability of conver-

gent colon and bladder dorsal root ganglion neurons after

colonic inflammation: mechanism for pelvic organ cross-talk.

Neurogastroenterol Motil 18(10):936–948

68. Matsunaga Y, Kawasaki H, Terada T (1999) Stromal mast cells

and nerve fibers in various chronic liver diseases: relevance to

hepatic fibrosis. Am J Gastroenterol 94(7):1923–1932

69. Matthews PJ, Aziz Q, Facer P, Davis JB, Thompson DG, Anand

P (2004) Increased capsaicin receptor TRPV1 nerve fibres in the

inflamed human oesophagus. Eur J Gastroenterol Hepatol

16(9):897–902

70. Mawe GM, Strong DS, Sharkey KA (2009) Plasticity of enteric

nerve functions in the inflamed and postinflamed gut. Neuro-

gastroenterol Motil 21(5):481–491

71. Medda BK, Sengupta JN, Lang IM, Shaker R (2005) Response

properties of the brainstem neurons of the cat following intra-

esophageal acid-pepsin infusion. Neuroscience 135(4):1285–

1294

72. Nemeth L, Rolle U, Reen DJ, Puri P (2003) Nitrergic hyperin-

nervation in appendicitis and in appendices histologically

classified as normal. Arch Pathol Lab Med 127(5):573–578

73. Neuhuber WL, Raab M, Berthoud HR, Worl J (2006) Innerva-

tion of the mammalian esophagus. Adv Anat Embryol Cell Biol

185:1–73 back cover

74. Neunlist M, Aubert P, Toquet C, Oreshkova T, Barouk J, Lehur

PA, Schemann M, Galmiche JP (2003) Changes in chemical

coding of myenteric neurones in ulcerative colitis. Gut 52(1):

84–90

75. Newton M, Kamm MA, Soediono PO, Milner P, Burnham WR,

Burnstock G (1999) Oesophageal epithelial innervation in health

and reflux oesophagitis. Gut 44(3):317–322

76. O’Hara JR, Lomax AE, Mawe GM, Sharkey KA (2007) Ileitis

alters neuronal and enteroendocrine signalling in guinea pig

distal colon. Gut 56(2):186–194

Acta Neuropathol

123

77. Palmer JM, Greenwood B (1993) Regional content of enteric

substance P and vasoactive intestinal peptide during intestinal

inflammation in the parasitized ferret. Neuropeptides 25(2):

95–103

78. Pan XQ, Gonzalez JA, Chang S, Chacko S, Wein AJ, Malykhina

AP (2010) Experimental colitis triggers the release of substance

P and calcitonin gene-related peptide in the urinary bladder via

TRPV1 signaling pathways. Exp Neurol 225(2):262–273

79. Park JH, Rhee PL, Kim HS, Lee JH, Kim YH, Kim JJ, Rhee JC

(2006) Mucosal mast cell counts correlate with visceral hyper-

sensitivity in patients with diarrhea predominant irritable bowel

syndrome. J Gastroenterol Hepatol 21(1 Pt 1):71–78

80. Pederiva F, Burgos E, Francica I, Zuccarello B, Martinez L,

Tovar JA (2008) Intrinsic esophageal innervation in esophageal

atresia without fistula. Pediatr Surg Int 24(1):95–100

81. Peles S, Medda BK, Zhang Z, Banerjee B, Lehmann A, Shaker

R, Sengupta JN (2009) Differential effects of transient receptor

vanilloid one (TRPV1) antagonists in acid-induced excitation of

esophageal vagal afferent fibers of rats. Neuroscience 161(2):

515–525

82. Piche M, Bouin M, Arsenault M, Poitras P, Rainville P (2011)

Decreased pain inhibition in irritable bowel syndrome depends

on altered descending modulation and higher-order brain pro-

cesses. Neuroscience 195:166–175

83. Pozo MJ, Camello PJ, Mawe GM (2004) Chemical mediators of

gallbladder dysmotility. Curr Med Chem 11(13):1801–1812

84. Qian NS, Liao YH, Feng QX, Tang Y, Dou KF, Tao KS (2011)

Spinal toll like receptor 3 is involved in chronic pancreatitis-

induced mechanical allodynia of rat. Mol Pain 7:15

85. Qin C, Farber JP, Foreman RD (2008) Intraesophageal chemi-

cals enhance responsiveness of upper thoracic spinal neurons to

mechanical stimulation of esophagus in rats. Am J Physiol

Gastrointest Liver Physiol 294(3):G708–G716

86. Renzi D, Pellegrini B, Tonelli F, Surrenti C, Calabro A (2000)

Substance P (neurokinin-1) and neurokinin A (neurokinin-2)

receptor gene and protein expression in the healthy and inflamed

human intestine. Am J Pathol 157(5):1511–1522

87. Rosso M, Robles-Frias MJ, Covenas R, Salinas-Martin MV,

Munoz M (2008) The NK-1 receptor is expressed in human

primary gastric and colon adenocarcinomas and is involved in

the antitumor action of L-733,060 and the mitogenic action of

substance P on human gastrointestinal cancer cell lines. Tumour

Biol 29(4):245–254

88. Sarkar S, Thompson DG, Woolf CJ, Hobson AR, Millane T,

Aziz Q (2004) Patients with chest pain and occult gastro-

esophageal reflux demonstrate visceral pain hypersensitivity

which may be partially responsive to acid suppression. Am J

Gastroenterol 99(10):1998–2006

89. Savidge TC, Newman P, Pothoulakis C, Ruhl A, Neunlist M,

Bourreille A, Hurst R, Sofroniew MV (2007) Enteric glia reg-

ulate intestinal barrier function and inflammation via release of

S-nitrosoglutathione. Gastroenterology 132(4):1344–1358

90. Scoazec JY, Racine L, Couvelard A, Moreau A, Flejou JF,

Bernuau D, Feldmann G (1993) Parenchymal innervation of

normal and cirrhotic human liver: a light and electron micro-

scopic study using monoclonal antibodies against the neural

cell-adhesion molecule. J Histochem Cytochem 41(6):899–908

91. Shafik A, El-Sibai O, Shafik I, Shafik A (2004) Electroeso-

phagogram in gastroesophageal reflux disease with a new theory

on the pathogenesis of its electric changes. BMC Surg 4:13

92. Shieh KR, Yi CH, Liu TT, Tseng HL, Ho HC, Hsieh HT, Chen

CL (2010) Evidence for neurotrophic factors associating with

TRPV1 gene expression in the inflamed human esophagus.

Neurogastroenterol Motil 22(9):971–977 (e252)

93. Sibaev A, Franck H, Vanderwinden JM, Allescher HD, Storr M

(2003) Structural differences in the enteric neural network in

murine colon: impact on electrophysiology. Am J Physiol

Gastrointest Liver Physiol 285(6):G1325–G1334

94. Simpson J, Sundler F, Humes DJ, Jenkins D, Scholefield JH,

Spiller RC (2009) Post inflammatory damage to the enteric

nervous system in diverticular disease and its relationship to

symptoms. Neurogastroenterol Motil 21(8):e847–e858

95. Sipos G, Altdorfer K, Pongor E, Chen LP, Feher E (2006)

Neuroimmune link in the mucosa of chronic gastritis with

Helicobacter pylori infection. Dig Dis Sci 51(10):1810–1817

96. Sipos G, Sipos P, Altdorfer K, Pongor E, Feher E (2008) Cor-

relation and immunolocalization of substance P nerve fibers and

activated immune cells in human chronic gastritis. Anat Rec

(Hoboken) 291(9):1140–1148

97. Sitohy B, El-Salhy M (2002) Changes in the colonic enteric

nervous system in rats with chemically induced colon dysplasia

and carcinoma. Acta Oncol 41(6):543–549

98. Skaper SD, Pollock M, Facci L (2001) Mast cells differentially

express and release active high molecular weight neurotrophins.

Brain Res Mol Brain Res 97(2):177–185

99. Stanzel RD, Lourenssen S, Blennerhassett MG (2008) Inflam-

mation causes expression of NGF in epithelial cells of the rat

colon. Exp Neurol 211(1):203–213

100. Stoyanova II, Gulubova MV (1998) Peptidergic nerve fibres in

the human liver. Acta Histochem 100(3):245–256

101. Straub RH (2007) Autoimmune disease and innervation. Brain

Behav Immun 21(5):528–534

102. Straub RH, Grum F, Strauch U, Capellino S, Bataille F, Bleich

A, Falk W, Scholmerich J, Obermeier F (2008) Anti-inflam-

matory role of sympathetic nerves in chronic intestinal

inflammation. Gut 57(7):911–921

103. Sugiura T, Dang K, Lamb K, Bielefeldt K, Gebhart GF (2005)

Acid-sensing properties in rat gastric sensory neurons from

normal and ulcerated stomach. J Neurosci 25(10):2617–2627

104. Suzuki R, Furuno T, McKay DM, Wolvers D, Teshima R,

Nakanishi M, Bienenstock J (1999) Direct neurite-mast cell

communication in vitro occurs via the neuropeptide substance P.

J Immunol 163(5):2410–2415

105. Tanaka T, Shinoda M, Feng B, Albers KM, Gebhart GF (2011)

Modulation of visceral hypersensitivity by glial cell line-derived

neurotrophic factor family receptor and alpha;-3 in colorectal

afferents. Am J Physiol Gastrointest Liver Physiol 300(3):

G418–G424

106. Terada T, Matsunaga Y (2001) S-100-positive nerve fibers in

hepatocellular carcinoma and intrahepatic cholangiocarcinoma:

an immunohistochemical study. Pathol Int 51(2):89–93

107. Tiscornia OM, Martinez JL, Sarles H (1976) Some aspects of

human and canine macroscopic pancreas innervation. Am J

Gastroenterol 66(4):353–361

108. Tornblom H, Lindberg G, Nyberg B, Veress B (2002) Full-

thickness biopsy of the jejunum reveals inflammation and

enteric neuropathy in irritable bowel syndrome. Gastroenterol-

ogy 123(6):1972–1979

109. Van Landeghem L, Chevalier J, Mahe MM, Wedel T, Urvil P,

Derkinderen P, Savidge T, Neunlist M (2011) Enteric glia pro-

mote intestinal mucosal healing via activation of focal adhesion

kinase and release of proEGF. Am J Physiol Gastrointest Liver

Physiol 300(6):G976–G987

110. Vasina V, Barbara G, Talamonti L, Stanghellini V, Corinaldesi R,

Tonini M, De Ponti F, De Giorgio R (2006) Enteric neuroplas-

ticity evoked by inflammation. Auton Neurosci 126–127:264–272

111. von Boyen GB, Reinshagen M, Steinkamp M, Adler G, Kirsch J

(2002) Enteric nervous plasticity and development: dependence

on neurotrophic factors. J Gastroenterol 37(8):583–588

112. von Boyen GB, Schulte N, Pfluger C, Spaniol U, Hartmann C,

Steinkamp M (2011) Distribution of enteric glia and GDNF

during gut inflammation. BMC Gastroenterol 11:3

Acta Neuropathol

123

113. Wang WF, Yang YS, Peng LH, Sun G (2006) Alternation of

substance P-containing neural pathways in a rat model of irri-

table bowel syndrome with rectal distension. Chin J Dig Dis

7(4):211–218

114. Watkins LR, Maier SF (2002) Beyond neurons: evidence that

immune and glial cells contribute to pathological pain states.

Physiol Rev 82(4):981–1011

115. Wedel T, Busing V, Heinrichs G, Nohroudi K, Bruch HP,

Roblick UJ, Bottner M (2010) Diverticular disease is associated

with an enteric neuropathy as revealed by morphometric anal-

ysis. Neurogastroenterol Motil 22(4):407–414 e493–e404

116. Willot S, Gauthier C, Patey N, Faure C (2012) Nerve growth

factor content is increased in the rectal mucosa of children with

diarrhea-predominant irritable bowel syndrome. Neurogastro-

enterol Motil 24(8):734–739 (e347)

117. Xu GY, Winston JH, Shenoy M, Yin H, Pasricha PJ (2006)

Enhanced excitability and suppression of A-type K? current of

pancreas-specific afferent neurons in a rat model of chronic

pancreatitis. Am J Physiol Gastrointest Liver Physiol 291(3):

G424–G431

118. Yu S, Gao G, Peterson BZ, Ouyang A (2009) TRPA1 in mast

cell activation-induced long-lasting mechanical hypersensitivity

of vagal afferent C-fibers in guinea pig esophagus. Am J Physiol

Gastrointest Liver Physiol 297(1):G34–G42

119. Zhou SY, Lu Y, Song I, Owyang C (2011) Inhibition of gastric

motility by hyperglycemia is mediated by nodose ganglia KATP

channels. Am J Physiol Gastrointest Liver Physiol 300(3):

G394–G400

Acta Neuropathol

123