Hypothalamic dopamine and serotonin in the regulation of food intake

INGESTIVE BEHAVIOR AND OBESITY

Hypothalamic Dopamine and Serotonin in theRegulation of Food Intake

Michael M. Meguid, MD, PhD, Serguei O. Fetissov, MD, PhD, Madhu Varma, MD, MS,Tomoi Sato, MD, Lihua Zhang, MD, PhD, Alessandro Laviano, MD, and

Filippo Rossi-Fanelli, MDFrom the Neuroscience Program, Surgical Metabolism and Nutrition Laboratory,

Department of Surgery, University Hospital, SUNY Upstate Medical University, Syracuse,New York, USA; and the Obesity Unit, Department of Clinical Medicine, University of

“La Sapienza,” Rome, Italy

Because daily food intake is the product of the size of a meal and the frequency of meals ingested, thecharacteristic of meal size to meal number during a 24-h light–dark cycle constitutes an identifiablepattern specific to normal states and obesity and that occurs during early cancer anorexia. An under-standing of simultaneous changes in meal size and meal number (constituting a change in feedingpatterns) as opposed to an understanding of only food intake provides a more insightful dynamic picturereflecting integrated behavior. We have correlated this to simultaneous changes in dopamine and serotoninconcentrations and to their postsynaptic receptors, focusing simultaneously on two discrete hypothalamicfood-intake–related nuclei, in response to the ingestion of food. The relation between concentrations ofdopamine and serotonin limited to the lateral hypothalamic area (LHA) and the ventromedial nucleus(VMN) as they relate to the influence of meal size and meal number during the hyperphagia of obesityand anorexia of cancer as measured in our experiments are discussed. Based on these data, conceptualmodels are proposed concerning: 1) an “afferent–efferent neurotransmitter unit,” with facilitatory orinhibitory neuropeptide properties to generate an appropriate neuroendocrine and neuronal response thatultimately modifies food intake; 2) initiation and termination of a meal, thereby determining the numberand size of a meal under normal conditions; and 3) a schema integrating the onset mechanism of canceranorexia. Nicotine is used as a tool to further explore the relation of meal size to meal number, with afocus on simultaneous changes in dopamine and serotonin concentrations in the LHA and VMN with theonset of acute anorexia of nicotine infusion and acute hyperphagia of nicotine cessation. Data concerningthe role of sex-related hormones on dopamine and serotonin with regard to the LHA and VMN in relationto the modulation of food intake are also presented.Nutrition 2000;16:843–857. ©Elsevier Science Inc.2000

Key words: cancer anorexia, dopamine, serotonin, lateral hypothalamic area, ventromedial nucleus,cancer, obesity, nicotine

INTRODUCTION

Based on a body mass index (BMI) criterion of 25.0 to 29.9 kg/m2,approximately 25% to 33% of the U.S. population is overweightand another 30% is obese.1,2 Growing evidence links an increase inBMI with an increased incidence of coronary heart disease, ele-vated blood pressure, and diabetes.3,4 The higher prevalence ofthese conditions with obesity has focused public-health concern onthe risk of being overweight.5 There is increasing medical aware-ness of the physical, social, and economic costs of obesity in the

industrialized world6,7 that continues to be a major health problemwith no effective treatment in sight. As a last resort, the treatmentof morbid obesity has been relegated to the surgeon to perform avariety of surgical treatment manipulations (e.g., gastric stapling),but surgical treatment has a 20% long-term complication rate anda significant mortality risk. These procedures constitute a “fix” anddo not presume to be based on an understanding of the cause ofobesity.

At the other end of the spectrum, a fall in BMI to less than 20kg/m2 in association with ongoing weight loss commonly occurswith the anorexia of chronic wasting diseases such as cancer,inflammatory bowel disease, or human immunodeficiency virus.Here, too, the risk of adverse effects on health and general well-being increases. People with a BMI of less than 18.5 kg/m2 havea smaller muscle mass and their physical work capacity is re-duced.8,9 This has serious implications that are even more signif-icant especially during illness because both myocardial and respi-ratory function are compromised.10,11Furthermore, with a BMI ofless than 18.5 kg/m2, a deterioration in psychologic function ismanifest by apathy, lassitude, lack of self-help motivation, depres-sion, and anxiety.12 There is also evidence that the malnutritionsecondary to hypophagia and anorexia of disease compromises theimmune system, leading to impairment of cell-mediated immunity,

This work was supported in part by NIH grants DK43796 and CA70239,American Institute for Cancer Research grant 85A95, Syracuse UniversityHendricks Award, NATO grant CRG940731, and Smokeless TobaccoResearch Council Inc. grant 211-2506A. Dr. Laviano was supported bygrant AI93.00131.04 from the Italian National Research Council (CNR).

Correspondence to: Michael M. Meguid, MD, PhD, Department of Sur-gery, University Hospital, Upstate Medical University, 750 East AdamsStreet, Syracuse, NY 13210, USA. E-mail: [email protected]

Date accepted: July 5, 2000.

Nutrition 16:843–857, 2000 0899-9007/00/$20.00©Elsevier Science Inc., 2000. Printed in the United States. All rights reserved. PII S0899-9007(00)00449-4

phagocytosis, and antibody response.13 These changes in hostresistance increase susceptibility to infection.14 Consequently, pa-tients spend more time in the hospital,15 cannot tolerate full treat-ment regimens, experience a 40% to 60% greater frequency ofcomplications from surgical and medical treatment, take longer torecover from surgery, and have a two- to threefold increase indeath.16 Thus, the deviation beyond normal BMI range to the twoextremes causes major health problems associated with increasedphysical, social, and economic costs to the individual and tosociety.

Ultimately, an understanding of the neurobiochemical changesassociated with the hyperphagia or hypergluttony of obesity andthe hypophagia or anorexia of disease, i.e., an understanding of themechanisms that influence and control food intake and appetite, isessential to rationally treat these life-threatening conditions. Suchinformation will also advance our understanding of the normalcontrol of appetite and food intake, not only on earth but also in themicrogravity environment of space, where anorexia prevailsamong the astronauts.17,18

The long-term objectives of this laboratory continue to be theelucidation of the central and peripheral mechanisms that regulatefood intake and influence circadian rhythm in food intake. Westarted by focusing on the lateral hypothalamic area (LHA) andventromedial nucleus (VMN) and studying changes in dopamine(DA) and serotonin as measured by in vivo microdialysis. Thesemeasurements were related to simultaneous changes in meal sizeand meal number versus only food intake because this paradigmprovides a more meaningful approach to the understanding ofmechanism(s) in relation to food-intake control. From a pragmaticview point, we have approached the problem based on the thesisthat food intake (FI) is a function of the product of meal size (MZ)and meal number (MN), i.e.,FI 5 MZ 3 MN.

This simple but useful equation was initially proposed byothers.19,20 The concepts suggested by this equation are attractivebecause it provides several potential insights. 1) Meal size andmeal number are functionally interrelated, but to be separatelycontrolled they must be independently regulated, probably byseparate neuroanatomic sites. Under normal physiologic condi-tions in rats with ad libitum feeding, a change in one brings abouta compensatory change in the other, so as to maintain constancy ofdaily food intake.21 2) During human illness or in rat models ofdisease states such as hyperphagia of obesity22,23 or anorexia of

cancer,24,25 this reciprocal relation is disassociated. 3) Meal sizeand meal number are influenced by circadian rhythm: in normalrats, 70% is consumed during the dark cycle, whereas in hyperpha-gia of obesity an increase in light-phase food intake occurs,22 andin the anorexia of cancer a significant decrease in food intakeoccurs in the dark phase.25 4) Sex influences the relation of mealsize to meal number.26

Models Used to Study Hypothalamic Dopamine and Serotonin

We have used a variety of models that reflect dopamine andserotonin and leptin perturbations of food-intake controls. We haveused models of nicotine infusion to study acute anorexia and itscessation to gain insight into acute hyperphagia.27 To study theneurobiology of chronic anorexia we have used a rat cancermodel24,25; the study of the obese Zucker rat has provided insightinto the neurophysiology of chronic hyperphagia of obesity.28,29

Further insight was gained by studying the existence of sex dif-ferences.30,31 The integrated approach to investigating appetitecontrols involves the correlation and combination of several fac-tors, as outlined in Figure 1. These include 1) behavioral studies(measuring food intake and its constituents: meal size and mealnumber by a computerized rat eater meter developed in our labo-ratory32), 2) biochemical correlation to hypothalamic dopamineand serotonin via in vivo microdialysis, and 3) changes in hypo-thalamic receptor mRNA of dopamine and serotonin via reversetranscription–polymerase chain reaction, in situ hybridization, andimmunochemistry reflecting the coexpression with food-intake–related peptidergic neuromodulators. These studies have been fa-cilitated by using hypothalamically injected receptor subtype-specific agonists and antagonists to dopamine and serotonin.

Since the experiments by Hetherington and Ranson on bilaterallesion of the mediobasal hypothalamus including the VMN and thearcuate nuclei,33 the involvement of this area in food intake andbody-weight control has been generally accepted. Recently, itsneurochemical basis has gained even further support by the dis-covery that leptin brings about its action of reducing food intakeand body weight mostly through the neurons in the arcuate nucleusand the VMN.34 In addition to central anatomic areas, peripheralsites influencing the gastrointestinal tract and fat storage contributeto the regulation of food intake by changes in circulating nutrient

FIG. 1. Schema showing an afferent–efferent neurotransmitter unit, with a focus on dopamine and serotonin in relation to monoaminergic/peptidergic controlpoints and potential sites of their investigation in the study of food-intake regulation.

844 Meguid et al. Nutrition Volume 16, Number 10, 2000

concentrations and by related changes in neuroendocrinemediators.

Afferent vagal fibers from the gastrointestinal tract convergeonto the nucleus of tractus solitarius and from there project ontothe LHA. Because the study by Arees and Mayer35 demonstratedneuronal connections between the lateral and medial hypothala-mus, much additional anatomic, electrophysiologic, functional,neurochemical, and pathophysiologic data have consolidated theirrole in the reciprocal function in the regulation of food intake (seereferences in Table I). In this context, the LHA as the parasym-pathetic nervous system center and the ventromedial hypothalamusas the neuroendocrine sympathetic nervous system center are func-tionally integrated with other hypothalamic areas and nuclei arealso recognized in the control of food intake. These structures

include the paraventricular nucleus (PVN),36,37 the arcuate nucle-us,38,39 the dorsomedial hypothalamic nucleus,40 the VMN of thehypothalamus and the LHA,41–43, among others. Nevertheless,insufficient data exist to link together all these neuronal areas, sothat a comprehensive integrative functional picture of their inter-action in the regulation of food intake can emerge. However, theclose relation between two such areas, the LHA and VMN, hasbeen documented by data providing a relatively greater under-standing of not only their role in food-intake control but also theirinfluence on meal size and meal number as they relate to normaland diseased states, as summarized in Table I. This paradigmunderpins our present research in relation to the function of dopa-mine and serotonin.

LHA and VMN are involved reciprocally (Table I), beingcritical to food intake and energy-metabolism regulation.41–43En-hanced release of both norepinephrine and dopamine in the LHAis associated with inhibition of food intake44,45 but in the medialhypothalamic nuclei is associated with stimulation of food intake.Enhanced release of serotonin in the medial hypothalamic nuclei isassociated with inhibition of food intake.46 Thus, an effect ofmonoamine release seems to exist, which is specifically coordi-nated between lateral and medial hypothalamic areas in relation tofood intake, meal size, and meal number.

Rats, like humans, have a circadian rhythm in food intake. Inrats more than 70% of their food is consumed during the darkphase. The mechanism of this diurnal rhythm is not clear, but theretinohypothalamic projection to the suprachiasmatic nucleus(SCN), a pathway by which light information reaches the SCN, isinvolved in the generation of the circadian rhythm.47,48 The SCNsends monosynaptic efferents to the VMN. Electrophysiologicstudies have shown that the SCN exerts mainly an excitatoryinfluence on glucoreceptor neurons in the VMN. The SCN hasmultisynaptic inhibitory influence on the glucose-sensitive neuronsin the LHA. Thus, with light-on stimulus the SCN stimulates VMNand inhibits LHA, resulting in suppression of feeding. Further, thelight-on stimulus is a major regulatory factor of melatonin synthe-sis and secretion in the pineal gland. VMN neuronal activity isinfluenced by the pineal gland, which in turn is affected by theSCN.49 The SCN is involved in the entrainment and rhythmgeneration; the light–dark cycle being the primary synchronizingagent. Bilateral SCN lesions alter the circadian rhythms of feeding,whereas continuous light exposure suppresses total daily food andwater intake. In addition to the SCN light-entrained endogenouscircadian oscillator, the VMN50 and the LHA51 are entrained byfood intake as a second circadian oscillator. Glucose (i.e., carbo-hydrate) versus fat primarily affects the food-entrained ossilator.52

Neurons in VMN contain insulin, leptin, and estrogen receptorsand thus receive hormonal signals concerning small daily gains orlosses in calorie storage in peripheral fat deposits53 regulatingenergy metabolism.54,55 Electrolytic lesion of the VMN attenuatethe daily expression of several rhythms including feeding behaviorand corticosterone levels.56 Thus, the food-intake circadian systemconsists of at least two major pace-making systems, one entrainedby light (SCN) and the other entrained by food (the anatomicallyinterconnected and functionally interrelated VMN and LHA).

Broad Concepts of Food-Intake Regulation

The ingestion of food is a complex process that integrates sensoryinformation related to sight, smell, and taste of food, to previousingestion experience, and to satiety signals elicited by ingestionand hormonal signals related to energy balance. Conceptually,information concerning body energy and nutritional status (fromgut, liver, and fat stores) is continuously transmitted to the brain by1) the afferent autonomic neurons, 2) changes in concentrations ofcirculating nutrients and hormones, and 3) gastrointestinal andcentral neuroendocrine mediators. Relevant areas of the forebrainand midbrain further modulate this information. This “afferent

TABLE I.

RELATIONSHIP BETWEEN LHA AND VMN

Factor Comments

Anatomic Dendrites from both LHA and VMN are in closeproximity, enabling passage of LHA andVNM fiber trajectories interact with each other

Electrophysiologic VMN neuron frequency increase correlates witha decrease in LHA neuron frequency

Highly significant inverse relation of SUDSbetween VNM and LHA

Activity of LHA neurons affects gastricdistention

Physiologic LHA neuron stimulation activates feeding,whereas VMN neuron stimulation inhibitsfeeding

LHA DA increases in response to food intakeafter food deprivation

VMN DA decreases in response to food intakeafter food deprivation

LHA DA correlates with meal sizeVMN DA concentrations control the intermeal

interval and thus meal numberLeptin acts in both LHA and VMN

Functional–behavioral

Food stimulant (odor/sight) LHA EGG changedfrom high-amplitude, slow waves to low-amplitude fast waves; EGG of VMN showedthe reverse trend

Electrical stimulation of VMN and LHA causesreciprocal effects on glucose and glycogenlevels

LHA lesions increase intestinal transitNeurochemical Shows how neuropeptide CART acts on VMN to

cause anorexiaDA stimulation has opposite effects on the LHA

and VMNOrexin’s effect on LHA causes increase in food

intakeLHA controls short-term satiety and VMN

controls long-term satiationPathophysiologic VMN lesion reduces energy expenditure

Anorexia or cancer and IBD mediated by IL-1on VMN reduce meal number

LHA DA concentrations and meal size arebigger in obese than in lean Zucker rats

CART, cocaine and amphetamine-regulated transcript; DA, dopamine;EEG, electroencephalogram; IBD, inflammatory bowel disease; IL-1,interleukin-1; LHA, lateral hypothalamic area; SUDS, sudden unex-plained death syndrome; VMN, ventromedial nucleus.

Nutrition Volume 16, Number 10, 2000 845Dopamine and Serotonin in the Regulation of Food Intake

sensory” information is ultimately integrated into the hypotha-lamic food-intake–related nuclei by an “afferent–efferent neuro-transmitter unit” with facilitatory or inhibitory neuropeptide prop-erties to generate an adequate neuroendocrine and neural responsethat is manifested by changes in meal size and meal number (Fig.1).

We postulate that at this “neurotransmitter unit” the afferentinformation, which includes current metabolic status, is conveyedto the presynaptic monoaminergic system of the hypothalamus,where it is stored at the end of the presynaptic afferent neuron asprepackaged synaptosomes of neurotransmitters dopamine andserotonin. The presynaptic cell influences postsynaptic cells byrelease of 103 to 105 molecules of dopamine and serotonin bindingto postsynaptic receptors. Dopamine and serotonin may eitherfacilitate or inhibit second-messenger intracellular activity basedon the function of their receptor subtype to which they bind. Asoutlined schematically, this conveys information concerning mealsize, meal number, and food intake by activation of postsynapticneurons of the stimulatory food-intake neuropeptides (e.g.,melanin-concentrating hormone (MCH), neuropeptide Y (NPY),galanin, and orexin A/B) or by the inhibitory food-intake neu-ropeptides (e.g., melanocyte-stimulating hormone (MSH), cocaineand amphetamine-regulated transcript (CART), and corticotropin-releasing factor (CRF), among others), depending on the prevail-ing metabolic conditions, to effector organs to modulate foodintake. Leptin, insulin, and other peripheral peptides and steroidhormones modulate the synthesis and release of dopamine andserotonin. The concept of the “afferent–efferent neurotransmitterunit” (which “ reads” information from periphery and initiatescentrally mediated efferent response) provides a “cytomolecularworking diagram” to facilitate the investigation of normal anddisease-related food intake processes. Our laboratory has used theconcept shown in Figure 1 to formulate hypotheses to investigatekey control points upon which the data in this review are based.

The Neurotransmitters Dopamine and Serotonin

Dopamine and serotonin are two critical food-intake–related neu-rotransmitters. Among the monoamines, dopamine and serotoninare relevant brain messengers in food-intake regulation.44–46,57–62

Among other appetite-related brain messengers, dopamine is es-sential for feeding. The absence of dopamine production in theknockout mouse, which does not express tyrosine hydroxylase,44,63

does not inhibit motor capabilities to eat but causes an inability toinitiate feeding; this ability can be restored by gene delivery oftyrosine hydroxylase into the striatum.64 Dopamine release seemsto have a site-specific action in food-intake regulation, such as inthe nucleus accumbens, and it is associated with the reinforcementeffect in feeding.65 In the hypothalamus dopamine release is as-sociated with the duration of meal consumption, which is a factorin determining feeding pattern. Hence, dopamine is required toinitiate each meal and is thus associated with meal number andduration, i.e., meal size.41,42 Increased dopamine in the VMN, thearea of neuroendocrine and autonomic regulation of metabolism,accompanies food intake.66 Medial hypothalamic lesions produceboth hyperphagia and morbid obesity, suggesting that VMN do-pamine is involved in regulating both food intake and the body-weight setpoint.

Serotonin neurons of the midbrain and brainstem raphe nucleiare among the earliest neuronal phenotype to become differenti-ated during development of the central nervous system, and sero-tonin is thought to act as a regulatory and developmentalagent.67,68The more rostral nuclei (primarily the dorsal and medialraphe) form ascending projections, densely innervating such fore-brain areas as the hypothalamus, hippocampus, and cortex. Thus,the serotonergic system is involved in the regulation of suchdiverse functions as reproduction, mood, sleep, and cognition.Mice lacking serotonergic receptors display food-intake and body-

weight–related abnormalities.69 Serotonin release in the hypothal-amus is enhanced during feeding,70 is thought to promote satiation,and reflects carbohydrate ingestion.71

Numerous studies have characterized serotonin and the sero-toninergic system in the inhibitory role of food intake. The sug-gestion that serotonin acts to inhibit food intake is based onfindings that a variety of different directly and indirectly actingagonists at central serotonin synapses cause dramatic reduction infood intake.72 A body of evidence exists showing that dopaminerelease is regulated in part by serotonin-release.45,73,74

To function as compensatory mechanisms, it is likely that mealsize and meal number are independently regulated in different yetconnected anatomic sites of the brain. Using the automated com-puterized rat eater meter32 and the in vivo brain microdialysistechnique,75 we measured simultaneously the intra-VMN andintra-LHA concentrations of dopamine and serotonin as they re-lated to the control of meal size and meal number. Our data andthose of others indicate that, under normal conditions, the interac-tion between dopamine and serotonin within the LHA influencesmeal size42,70,76–78and that intra-VMN dopamine and serotonininfluence meal number.41,71,79

ROLE OF DOPAMINE AND SEROTONIN IN THEREGULATION OF FOOD INTAKE IN OBESITY

The feeding pattern of the obese Zucker rat versus its lean coun-terpart demonstrates a consistently larger meal size throughout the24-h light–dark cycle; thus, the obese Zucker rat consistentlyconsumes more food than its lean counterpart. The importantcontribution of dopamine to the regulation of food intake andfeeding pattern has been shown in the medial hypothalamus by thestudy of both pre- and postsynaptic dopaminergic systems, asmeasured by in vivo microdialysis. Dopamine release is rapidlybut transiently increased in the VMN during spontaneous eating inlean and obese rats, and this increase is more pronounced in obeserats, which consume larger meals. When rats are deprived of foodfor 24 h, food intake brings about an immediate decrease ofdopamine release to 65% of preeating levels in both lean and obeserats.29 These two findings shows that release of dopamine isassociated with both short-term (individual meals) and long-term(hunger) regulation of food intake. These data also suggest thatduring obesity the presynaptic dopaminergic system adequatelyresponds to the change of feeding status.

Extracellular concentration of VMN dopamine can be raisedphysiologically by grafting embryonic dopaminergic cells, as doc-umented by microdialysis. This grafting resulted in the increase offood intake by meal number and then by meal size.22 The VMNdopaminergic graft also resulted in changing the diurnal distribu-tion of food intake, so that food intake in the dark phase increasedthe change in the feeding pattern in obese rats.23 By measuringdopamine release every 20 min by microdialysis in the VMN for24 h, we found that the release has a diurnal rhythm influenced byfood deprivation.51 This change in feeding pattern that occurs inthe obese rat suggests an involvement of the diurnal rhythm inVMN dopamine release, resulting in abnormally high eating ac-tivity of obese rats during the light phase. A food-intake stimula-tory function for VMN dopamine was confirmed by a singlepharmacologic injection (10 nM) of dopamine into the VMN offreely moving lean and obese rats, which caused an increase ofmeal size with a compensatory decrease in meal number (unpub-lished observations).

Dopamine acts on its receptors postsynaptically to modulate theactivity of target neurons by stimulation or inhibition of adenylylcyclase activity by D1 and D5 or D2 to D4 subtypes of receptors,respectively. The finding that hypothalamic neurons express bothsubtypes of dopaminergic receptors provides evidence that dopa-mine may activate or inhibit its target neurons based on the 1)


function of the subtype and 2) level of the dopaminergic receptorexpressed for a given metabolic condition under normal or patho-logic conditions. Using reverse transcription–polymerase chainreaction, we found that, under ad libitum feeding conditions, thelevel of expression of D1- and D2-receptor mRNA measured whenthe feeding activity of rat is minimal (between 10 and 11AM) wasdifferent in the VMN of lean and obese rats (unpublished obser-vations). D1 receptor, which normally is not abundantly expressedin the hypothalamus, was found highly expressed in the VMN ofobese rats, whereas D2 receptor was detected in the lean rat but atan undetectable level in the VMN of the obese Zucker rat. Thedifferent levels of dopaminergic receptor expression in the twophenotypes may be related to the relatively low concentration ofligand (dopamine) in obese as opposed to lean rats,80 or it maydirectly depend on altered leptin signaling in postsynaptic neurons.

We tested the relevance of different levels of dopaminergicreceptor expression between obese and lean rats on food intake andfeeding pattern by using a single pharmacologic injection (10 nM)of specific receptor antagonists. A different response to food intakeand feeding pattern (relation of meal size to meal number) wasfound in the obese than in the lean rat. This finding allows us toconclude that the level of expression of different subtypes ofdopaminergic receptors in the VMN is directly involved in theregulation of meal size and meal number.

Model of Hypothalamic Dopaminergic and SerotonergicRegulation in Obesity

Based on our results,22,23,28,29,41,42,51,72,76–82we constructed amodel of dopaminergic and serotonergic regulation of food intake,as shown in Figure 2. This model is based on the followingpostulates. 1) Dopamine may act on three populations of neurons:neurons that influence meal number by prolonging intermeal in-tervals by the release of food-intake inhibitory neuropeptides suchasa-MSH and CART; neurons influencing meal size by startingand maintaining the meal by the release of food-intake stimulatory

neuropeptides such as NPY and Agouti-related protein (AgRP);and neurons influencing meal size by stopping the meal by therelease of food-intake inhibitory neurohormones and neuropep-tides such as vasopressin, oxytocine (OT), or corticotropin-releasing factor (CRF). 2) All three populations of neurons mayexpress both D1 and D2 receptors, which can be activated orinhibited by dopamine. 3) Dopamine first activates the D2 and thenthe D1 receptor, which have different thresholds of activation. Inthis way, dopamine can change its effect from inhibition to acti-vation according to its concentration at a specific subtype receptor.

According to our model (Fig. 2), the circadian or sensory-stimulated release of dopamine will inhibit food-intake inhibitoryneurons (2FI) and then activate food-intake stimulatory neurons(1FI) to start the next meal. The ingestion of food also stimulatesdopamine release through its oral and gastric components, whichmaintain the meal through1FI neurons until2FI neurosecretoryneurons stop the meal. Le Magnen and Devos’s postulate83 that ahighly significant correlation exists between meal size and elapsedtime after the onset of a new meal could be a reflection ofdopamine’s action. This action would stimulate inhibitory neu-ropeptide synthesis in quantities proportional to meal size.

This model is an example of how one neurotransmitter could beinvolved in the regulation of food intake. Other monoaminergicand peptidergic neurotransmitters exist that probably activate orinhibit either adenylyl cyclase or a different second messenger thatwould function synergistically or antagonistically with dopa-mine.74,84 However, having a simple model facilitates our under-standing of the involvement of dopamine in the complex regula-tion of food intake.

An example of the interaction of dopamine with another mono-amine is the release of serotonin in the hypothalamus. We inves-tigated this possibility by inserting double cannulas, one placedinto the ipsilateral LHA and the other into the contralateral VMN.Microdialysis was performed every 20 min simultaneously inawake, freely moving active rats during a complete 24-h light–dark cycle. Two paradigms were studied: the fed state and the

FIG. 2. Proposed model of hypothalamic dopamine and serotonin and neuropeptide interaction in the regulation of food intake, meal size, and meal numberbased on data derived from studies in the lateral and ventromedial hypothalamus.


fasted state. A well-defined rise in dopamine occurred in the LHAat late midday before the initiation of feeding. Dopamine rose to apeak with food intake. In contrast, in the fasted state, the initialdopamine rise was blunted in the absence of food. By applyingtime-series analysis to the data of simultaneous serotonin anddopamine release as measured by microdialysis every 20 min for24 h, we found that concurrent release of dopamine and serotoninwithin the VMN and the LHA is synchronized in the fed stateduring intermeal intervals.51 It was not synchronized during eatingor during fasting; food intake desynchronized dopamine and sero-tonin release within and between the ventromedial hypothalamusand the LHA.

These data point to a tight relation between dopaminergic andserotonergic systems, which is influenced by the feeding state of ananimal and which may be involved in maintaining the balancewithin and between sympathetic and parasympathetic nervoussystem centers. The data show that food intake is coupled unequiv-ocally to the release of dopamine and serotonin in the VMN andLHA, suggesting it as a mechanism of activation of postsynapticneurons associated with prevailing metabolic conditions.

By applying our model to the obese phenotype of leptin-resistant Zucker rats, we can explain their characteristic feedingpattern (Fig. 3): large meal size with low meal number by anelevated expression of D1 receptor in food-intake stimulatory andinhibitory neurons. The possibility to selectively target these neu-rons may open an opportunity to modulate feeding pattern toreduce or increase food intake for therapeutic use during obesity oranorexia.

Hypothalamic Dopamine and Serotonin in Clinical Obesity

As outlined in the previous section, a number of experimentalstudies have consistently indicated that, in leptin-resistant obesity,abnormal hypothalamic dopamine and serotonin activities contrib-ute to hyperphagia and body-weight gain and modulation of hy-pothalamic dopamine and serotonin levels may result in reducedfood intake and weight loss. When considered together, these datasuggest that brain neurotransmission may represent a common stepon which different appetite-related messengers converge. Moresimplistically, it can be hypothesized that most of the abnormalityexisting in the cascade of signaling pathways of the obese rat resultsin a disturbed hypothalamic monoaminergic neurotransmission.

From the clinician’s perspective, this hypothesis might yieldinteresting results in an attempt to develop effective therapeuticapproaches to disturbed eating behavior. Specifically, the inhibi-tory effect on food intake of hypothalamic serotonin85 may beexploited to reduce food intake and achieve weight loss in hy-perphagic obese patients. We recognize that serotonergic agentshave been used in the treatment of hyperphagia, but it must beemphasized that their side effects may limit their use.86 The recentdevelopment of sibutramine, a molecule that enhances serotoner-gic and dopaminergic activities, raises new enthusiasm and pros-pects for a pharmacologic therapeutic approach to the treatment ofhyperphagia and obesity because its effects on food intake do notappear to be diminished by severe side effects.87

A more physiologic and safer approach may be based on thedistinct characteristics of the enzymatic pathways transforming theamino acid tryptophan into serotonin. Tryptophan is readily trans-formed into serotonin, whose concentrations do not limit theenzyme activity; consequently, the more tryptophan that reachesthe brain, the more serotonin is produced.88 Thus, by providing thebrain with pharmacologic doses of the precursor tryptophan, moreserotonin should be produced, including within the hypothalamus;consequently, reduced food intake and body-weight loss should beachieved. To test this hypothesis, we used the direct precursor ofserotonin, 5-hydroxy-tryptophan, to avoid the possible occurrenceof the eosinophilia-myalgia syndrome associated with the use ofL-tryptophan.89 In a series of clinical, placebo-controlled, double-

blind studies, we consistently demonstrated that the oral adminis-tration of 5-hydroxy-tryptophan at a dose ranging from 750 to 900mg/d reduces food intake in dietary-unrestricted obese patients,90

enhances the adherence to a hypocaloric dietary regimen,91 andreduces carbohydrate craving in non–insulin-dependent diabeticpatients,92 supporting the proposed role of brain serotonin indetermining macronutrient selection.85

The clinical relevance of brain-neurotransmission modulation isalso emphasized by the promising results obtained in the oppositeclinical syndrome, i.e., anorexia associated with tumor growth (seefollowing section). In this setting, preliminary data have indicated thatthe reduction of the supply tryptophan in the brain achieved by themanipulation of the plasma amino-acid profile is associated withincreased food intake.93 In our opinion, these data support the hypoth-

FIG. 3. Comparison of food intake, meal size, and meal number betweenlean and obese Zucker rats.


esis that brain neurotransmission may represent a common step onwhich different appetite-related messengers converge.

ROLE OF SEROTONIN AND DOPAMINE IN THEREGULATION OF FOOD INTAKE IN ANOREXIA OFCANCER

Sex differences in feeding patterns exist in normal rats.26 They alsooccur in tumor-bearing (TB) rats.94 As outlined in the schema (Fig.4), the changes in the feeding patterns as observed in canceranorexia are a manifestation of a chain of biochemical eventspreceding and leading to abnormal hypothalamic monoaminergicactivity.

In male anorectic Fischer 344 rats with methyl cholanthrene(MCA) sarcoma, anorexia is observed to occur with tumor growth.As shown in Figure 5, in the early phase meal number progres-sively decreases, but food intake is maintained relatively constantthrough an apparently compensatory increase in meal size. Ap-proximately 1 to 2 d later, meal size significantly decreases andanorexia becomes profound and apparent.24,25,95

Anorexia also develops in female Fischer-344 rats. It is pre-dominantly due to a decrease in meal number; meal size remainsrelatively constant. Differences in feeding patterns between normaland TB female and male rats is dependent on the presence offemale sex hormone.94 This hypothesis was tested using ovariec-tomized female TB rats. The pattern of development of anorexia inovariectomized TB rats was the same as that in male rats: an initialdecrease in meal number followed by a decrease in meal size. Insham-operated TB female rats, meal size remained unchanged.The degree of anorexia in ovariectomized TB rats was significantlygreater, with a consequent lower food intake. This observationsuggests that female sex hormones, predominantly estradiol, playa significant role in feeding behavior and in immune functioncomparable to that observed in female septic patients.96

A similar pattern, i.e., a decrease in food intake through aninitial decrease in meal number followed by a decrease in mealsize, can be reproduced with systemic injection of interleukin(IL)–1a and/or tumor necrosis factor (TNF)–a.97,98Because at theonset of anorexia IL-1a in the cerebrospinal fluid is elevated in TB

rats99 and IL-1 receptors exist in the VMN,100 the role of cytokinesin cancer anorexia was determined by injecting IL-1 receptorantagonist in the VMN of anorectic TB rats.101 The results showedthat food intake increased only through an increase in meal num-ber. Furthermore, the systemic infusion of the TNF-receptor con-struct into anorectic TB rats increased food intake by both in-creases in both meal size and meal number.102 Based on the sumof these data, IL-1 and TNF-a play a pivotal role in the mechanismof cancer anorexia. Such cytokines act directly103and indirectly onthe hypothalamus by changes in monoamine release of dopamineand serotonin.104 Abnormalities of monoaminergic105,106and pep-tidergic107,108systems occur during cancer anorexia.

The concentration of NPY decreased in the PVN but increasedin the arcuate nucleus,108 both of which play a significant role inthe regulation of food intake. These data suggest that peptidergicdysfunction in the hypothalamic food-intake–related nuclei con-tributes to the mechanism(s) of cancer anorexia. Such mechanismsare well summarized elsewhere109; in this report, we focus ourinterest on hypothalamic dopaminergic and serotonergic dysfunc-tion in cancer anorexia.

The hypothalamic serotonergic system contributes to canceranorexia.105,110 Serotonin is a known satiating factor and sup-presses food intake when injected into the VMN of normal rats,111

where it plays a critical role in the cancer anorexia.95 Serotoninconcentrations are increased in the VMN in anorectic TB rats, asmeasured by in vivo microdialysis, and normalize after tumorresection.106We were also able to increase serotonin concentrationin the VMN by injection of IL-1a into the VMN.104

The synthesis of brain serotonin directly depends on the plasmaconcentration of its precursor, tryptophan.88,112In anorectic cancerpatients and TB rats, we measured an increase of tryptophanconcentration in plasma and cerebrospinal fluid24,113,114that pos-itively correlated with the degree of anorexia. Plasma-free trypto-phan is carried competitively across the blood–brain barrier by ashared transport system with other large neutral amino acids.115

Hence, the molar ratio of plasma-free tryptophan to large neutralamino acid is an important factor determining the brain availabilityof tryptophan and thus brain serotonin synthesis and activity, aspreviously reported.113 In addition, endotoxin or cytokines (IL-1)also facilitate plasma-free tryptophan synthesis through the sym-

FIG. 4. Schema indicating interaction of cytokines, monoamines, and neuropeptides with changes in meal size and meal number in cancer anorexia.


pathetic nervous system,116 indicating a link between cancer, cy-tokine, and serotonin (tryptophan). Furthermore, oral administra-tion of branched-chain amino acid, to decrease the ratio oftryptophan to large neutral amino acids, significantly increasedfood intake in cancer patients93; in patients with cancer plasma-free tryptophan decreased after curative tumor resection.117

In TB states hypothalamic dopaminergic system also appears tobe abnormal, which may be a consequence of serotoninergicsystem dysfunction, because of their interaction.73,118We reportedthat IL-1a facilitated VMN dopamine release in normal rats.104

However, in anorectic TB rats, we found low dopamine concen-trations in the VMN and a high turnover rate.106

We recently observed that dopaminergic receptor mRNA ex-pression also changed in anorectic TB rats. We measured foodintake, meal size, and meal number and dopaminergic D1- andD2-receptor mRNA expression in the hypothalamus in anorecticTB and non-TB normal rats. When anorexia developed, mani-fested by the characteristic feeding-pattern changes, D2 and D1receptors were highly expressed. D2 is postulated to influencesatiety119; D1 receptor is not abundantly expressed under normalconditions.120

To determine the function of the changes in the expression ofD1 and D2 receptor, we injected specific dopaminergic receptorantagonists into the VMN in anorectic TB and non-TB normal rats.The results showed that sulpiride (D2-receptor antagonist) signif-icantly increased food intake by an increase in meal number onlyin anorectic TB rats but did not significantly increase food intakein non-TB normal rats. SCH23390 (D1-receptor antagonist) de-creased food intake by a decrease in meal size in anorectic TB ratsbut failed to change food intake in non-TB normal rats. ObeseZucker rats have high D1- and low D2-receptor expression asso-ciated with huge meal size and increased food intake. Based onthese two sets of data, we deduce that, in anorectic TB rats, the

highly expressed D1 receptor in the hypothalamus facilitates foodintake and meal size, whereas the highly expressed D2 receptor inthe hypothalamus contributes to anorexia. Consequently, thesechanges in dopaminergic receptor expression in the hypothalamuscontribute to the feeding pattern that characteristically occurs incancer.

Leptin has anorectic properties when injected centrally andperipherally. Because cytokines such as TNF and IL-1 stimulateplasma leptin and leptin mRNA expression in the adipose tissue,121

determining role of leptin in cancer anorexia is of interest. Thus,we measured plasma leptin concentration at an early and a latephase of anorexia of cancer. We found that plasma leptin concen-tration at the early phase was not different between TB andnon-TB pair-fed rats. However, higher leptin concentrations wereobserved at the late phase of anorexia in TB as opposed to non-TBpair-fed rats,122 although the concentrations in both groups weresignificantly lower compared with those at the early phase, afinding that is in agreement with that reported by others.123,124

From this study, we conclude that leptin does not initiate anorexiaof cancer but is involved in the development and the maintenanceof cancer anorexia.

NPY-deficient mice have normal feeding behavior,125 whereasdopamine-deficient mice are aphagic,63 indicating that peptidergicsystem dysfunction can be compensated for by the monoaminergicsystem to initiate feeding. Consequently, manipulating the mono-aminergic system in cancer to compensate for peptidergic dysfunc-tion results in amelioration of cancer anorexia.

We injected specific monoaminergic receptor antagonists into thehypothalamus. Mianserin (serotonin 2C–receptor antagonist126) andsulpiride (D2-receptor antagonist) injected into the VMN or LHAincreased food intake in anorectic TB rats. However, enhancingendogenous hypothalamic dopamine production by implanting dopa-

FIG. 5. Changes in food intake with onset of anorexia. (a) Daily light- and dark-phase food intake, (b) meal number, and (c) meal size in anorectictumor-bearing rats. Day 0 is the day of onset of anorexia defined as a decrease in the rat’s food intake per 100 g of body weight, which is 1 g lower thanthe average in each rat before tumor inoculation for 3 consecutive d. Data from day23, the last day of normal food intake, to day12. Coefficient correlationswere applied to changes in food intake, meal number, and meal size for the period from day23 to day 0 to statistically describe the changes in the relationof meal number to meal size to determine the development of cancer anorexia. The mean linear correlation coefficient between food intake and meal numberwas 0.95 and that between food intake and meal size was 0.49, suggesting that the reduction of food intake at the onset of anorexia is due mainly to thedecrease in meal number. On day12, meal size decreased significantly.


minergic cells into the hypothalamus to facilitate physiologic dopa-mine release23 changed the feeding pattern but failed to changetemporally and quantitatively the development of anorexia in TB rats.

Hypothalamic Dopamine and Serotonin in Clinical CancerAnorexia

The sum of the evidence highlights the fact that, when the anorexiaof cancer is present, abnormal hypothalamic dopaminergic andserotonergic neurotransmission is contemporaneously detectable.Cytokines are intimately involved in the process of cancer an-orexia through different mechanisms including interference withmonoaminergic neurotransmission. Hence, pharmacologic manip-ulation to inhibit their synthesis usingv-3 fatty-acid–based phar-maconutrition or to suppress their function using pentoxifylline,for example, may provide a beneficial effect to counteract anorexiaand its detrimental metabolic sequelae by increasing food intake.In keeping with this reasoning, interesting and promising resultshave been obtained in anorectic TB rats receiving a dimeric,pegylated TNF receptor construct to block TNF activity.102 In thisstudy, we found that by blocking TNF activity a significant im-provement of food intake can be achieved. A similar drug isalready available on the market (Infliximab), but no report hasbeen published testing its effects on food intake in anorexia. Thus,it is extremely difficult to speculate whether this therapeutic strat-egy will result in clinically relevant benefits. Also, because themechanism(s) of cancer anorexia is multifactorial, it seems un-likely that blocking one of many pathways will have a significantand consistent effect.

In 1979, Krause et al. first demonstrated in an animal modelthat the decrease in food intake after tumor implantation is asso-ciated with a significant increase in brain tryptophan and serotonincontent.105 From these data, it was hypothesized that increasedbrain serotonin activity secondary to increased brain availability oftryptophan contributes to anorexia in cancer. Supporting this hy-pothesis, we confirmed these data in an experimental model ofanorexia,24 but we were also able to show that the increase of braintryptophan availability and content are common features in cancerpatients.113,114 Strengthening the relation between tumor, trypto-phan, and anorexia, we also showed that plasma tryptophan levelsnormalize after curative tumor resection in patients, resulting inimproved food intake.117Such data lend credence to the hypothesisthat serotonin is a critical anorectic agent.

In keeping with our hypothesis, the hypothalamic monoamin-ergic system may represent a major target on which differentappetite-related messengers converge; the strategy of interferingwith serotonin systems may produce encouraging results. Becausethe brain availability of tryptophan depends on the molar ratio ofplasma-free tryptophan to large neutral amino acid, which includethe branched-chain amino acids, we hypothesized that changes inplasma amino-acid concentration induced by oral administration ofbranched-chain amino acids, to decrease the ratio in cancer pa-tients, would decrease brain tryptophan availability and thus sero-tonin synthesis, leading to increased appetite and food intake. Inline with this reasoning, administration of branched-chain aminoacids to anorectic cancer patients resulted in the normalization ofthe ratio of plasma-free tryptophan to large neutral amino acid,decreased the incidence of anorexia in the study group, and sig-nificantly improved food intake.93

From the sum of these experimental and clinical data, wededuce that monoamines play a major role in the neural modula-tion of feeding behavior. Their manipulation appears to compen-sate for other abnormalities of food-intake–related systems. Fur-thermore, from the clinician’s perspective, the administration ofbranched-chain amino acids to modulate brain serotonin concen-tration appears to be safe and likely provides effective therapy toincrease food intake. Unfortunately, no available data currentlyexist indicating whether increased food intake in anorectic cancer

patients results in improved long-term endpoints, i.e., morbidityand mortality rates. However, while waiting for clinical reports toanswer these questions, it must be emphasized that restoring foodintake to normal or near normal significantly improves the qualityof life of the patient with anorexia, thus positively affecting psy-chological status, which is of great help in withstanding the det-rimental effects of aggressive antineoplastic regimens often usedin the treatment of cancer patients.

ROLE OF NEUROTRANSMITTERS AND NPY IN THEREGULATION OF FOOD INTAKE DURING NICOTINECONSUMPTION

Despite the fact that most people are aware of the health risksassociated with using tobacco products, approximately one-fourthof adults use them.127 Obviously, there must be some compellingreasons for its continued use. One frequently cited reason is thebelief that the use of tobacco products can suppress appetite andcontrol body-weight gain. There are also studies documentingweight gain after the cessation of smoking.128

Nicotine is the most pharmacologically active ingredient intobacco and is believed to be responsible for the tobacco-inducedinhibitory effect on eating, in addition to its psychostimulantproperties. Studies in rats have demonstrated that the chronicinfusion of nicotine decreases both food intake and body-weightgain and that cessation of nicotine reverses the process.129–131In asystemic infusion study, we administered nicotine subcutaneouslythrough mini-osmotic pump to male rats at 5 mg/kg of body weightper day for 7 d and then stopped the infusion. Control rats receivedsaline. As shown in Figure 6, nicotine decreased food intakepredominantly through a decrease in meal number, with an inhib-itory effect on meal size. The hyperphagia of nicotine cessation ledto increased food intake predominantly through an increase inmeal size only.

Because nicotine alters food-intake and feeding patterns, nico-tine seems to influence the VMN and LHA because these nucleiaffect meal number and meal size, respectively. Both food intake(by an increase in meal size only) and daily body weight returnedto normal several days after cessation of the nicotine administra-tion.27 When the same study was conducted in female rats, wherethe fluctuation of daily food intake corresponds to the estrouscycle, nicotine decreased food intake through meal number, pre-serving meal size. A concomitant reduction in daily body weightalso occurred. From these data we conclude that nicotine has aprimary effect on the VMN in the female.

To further assess the affect of nicotine on LHA dopamine andserotonin, microdialysis of the LHA was performed after 2 d ofnicotine infusion and 2 d after the nicotine infusion was stopped.During the nicotine infusion, both dopamine and serotonin con-centrations were significantly elevated relative to their concentra-tions in the saline-infusion control group. After cessation of thenicotine infusion, dopamine concentration fell below that of con-trol levels. The infusion of nicotine into the male rat did not raiseplasma leptin concentrations.132 However, changes in vagal affer-ent stimuli were recorded when nicotine was injected into theportal vein. Below 1 ng, a decrease in vagal afferent stimulioccurred; above 10 ng, an increase was recorded. These datasuggest that some of the anorectic effects of nicotine are mediatedthrough peripheral activity.133

In contrast to that long-term study, short-term experimentswere conducted in which overnight food-deprived male ratsunderwent nicotine administration directly into the LHA for 60min by using a microdialysis (push and pull) technique. LHAdopamine, dihydroxyphenylacetic acid, serotonin, and 5-hydroxy-indoleacetic acid were measured by microdialysis before, during,and after LHA nicotine administration. Food was made availablefor 40 min beginning immediately after cessation of nicotine


administration and the amount eaten was determined. Nicotineinfusion induced a consistent increase in dopamine and dihydroxy-phenylacetic acid during the first 40 min. A rise in serotonin alsooccurred, but the rise in serotonin persisted even after nicotineinfusion ceased. Also, eating induced a short-lasting increase indopamine and a long-lasting increase in serotonin. As expected,food intake during the infusion period was significantly lower inthe nicotine-treated group.134 Based on evidence showing thatserotonin facilitates the release of dopamine, we interpret thelong-lasting elevated LHA serotonin findings to be related tonicotine-induced hypophagia.

Both estradiol and nicotine are anorectic agents in decreasingfood intake and body weight. In their absence, such as duringmenopause, after ovariectomy, or after cessation of smoking, anincrease in food intake and body weight occurs. Both estradiol andnicotine individually influence hypothalamic dopamine and sero-tonin, whose interaction is also involved in food-intake and body-weight regulation. To explore their interrelation, we used ovariec-tomized (surgical menopause) and sham-operated rats thatreceived a continuous subcutaneous infusion of nicotine (5 mgzkg z d) through an osmotic minipump. On day 2 of nicotineinfusion, in vivo microdialysis using a double cannula, one in-serted into ipsilateral LHA and the other contralateral VMN, wasperformed to simultaneously measure dopamine and serotonin inawake, freely moving rats. Nicotine infusion decreased food intakeand body weight in both groups, with a greater decrease occurringin the ovariectomy group. An increase in LHA dopamine concen-tration occurred in ovariectomy group and an increase in VMNserotonin was observed in the sham-operated group. The data showthat a nicotine-induced decrease in food intake and body weightassociated with changes in hypothalamic monoamines in the pres-ence of estradiol (sham-operated rats) lowers food intake and bodyweight by an increase in serotonin concentration in the VMN; inovariectomized rats (menopause), nicotine lowers food intake andbody weight by an increase in dopamine concentration in theLHA.135

NPY is synthesized in arcuate neurons that project principallyonto the PVN. NPY injected into the PVN causes hyperphagia,reduced energy expenditure, and eventually induces obesity, andthese effects are opposed by nicotine. Nicotine (12 mgz kg z d) wasgiven to rats for either 1 or 12 d. One day of nicotine treatmentversus control significantly reduced food intake by 30% and sig-nificantly decreased body weight by 2%. NPY mRNA levels were

significantly reduced by 40% and NPY concentrations fell signif-icantly by 33% in the arcuate nucleus and PVN. Twelve days ofnicotine treatment significantly lowered cumulative food intake by8% and body weight by 10%. NPY and mRNA levels significantlyrose by 40%, whereas NPY levels again significantly decreased inthe arcuate nucleus and PVN. The results suggest that nicotinemay inhibit NPY synthesis in the hypothalamus, independentlyof any effects due to altered energy balance. Reduced activity ofNPYergic neurons in the projection from the arcuate nucleus tothe PVN contributes to the effects of nicotine on energybalance.136

A reasonable understanding of the mechanism underlying theregulation of meal size and meal number and thus food-intakecontrol has been achieved through studies of systemic and centralinfusion of nicotine into male and female rats. The mechanismincludes peripheral sensors, which provide intake feedback infor-mation to the central nervous system. In our studies the LHA andthe VMN modulate the production and release of dopamine andserotonin, which influences the motor effector systems to initiate,maintain, or terminate eating. Our findings indicate that chronicnicotine administration decreases food intake, which is mainlyachieved by a reduction in meal number, and this could be relatedto an increase in dopamine and serotonin activity in the LHA andthe VMN and by their interaction (Fig. 7).

ROLE OF DOPAMINE AND SEROTONIN IN RELATIONTO SEX AND FOOD INTAKE

Gonadal hormones are among the many factors affecting foodintake, feeding patterns, and body weight in rats. Hormonal effecton these measures are especially striking in females, in whom foodintake, body weight, and the indices of meal number and meal sizechange with the phase of the estrous cycle. Gonadal hormones alsoaffect the monoaminergic system. Ovarian hormones influencemonoamine release. In normal rats during estrous, serotonin re-lease was found to be decreased.137 Animal studies have demon-strated that estradiol and progesterone modulate functioning of thenigrostriatal dopaminergic system by raising the turnover of do-pamine by increasing the activity of tyrosine hydroxylase.138Ovar-ian hormones have a direct action on the hypothalamus by acti-vating the secretion of monoamines and of their release andturnover.139

FIG. 6. The effect of starting and stopping systematic nicotine infusion on food intake, meal number, and meal size.


Food intake in the mature male rat is relatively stable.26,30,140

Temporally there is an increase in meal size and a decrease in mealnumber. In the female rat, meal number and meal size changecyclically and reciprocally during the estrus cycle.26 Specifically,during the proestrous phase of the estrous cycle, when serumestradiol titers are rising and are approaching their peak concen-tration, meal number peaks, with a decrease in the correspondingmeal size. During the metestrus/diestrus phase, when serum estra-diol titers are lowest, meal number decreases with a correspondingincrease in meal size. Although food intake per 100 g of bodyweight was similar in males and females, weight gain in males wasapproximately seven times greater.26

After orchiectomy (surgical removal of the testes), food intakein the orchiectomized rat decreased significantly in dark-phasemeal number and was not compensated for by increases in dark-phase meal size.30 There was a shift in the circadian distribution offood intake such that the rat ate more during the light phase. Afterhormone replacement these changes were reversed. After ovariec-tomy the estrous cycling disappeared, the relation of meal numberto meal size was altered, and there was an increase in daily foodintake.31 After ovariectomy daily food intake increased signifi-cantly in light-phase food intake, achieved by significant increasesin meal size. Daily hormone replacement (not given cyclically) inthe ovariectomized rat, with estradiol alone or in combination withprogesterone, normalized food intake. Mean body weight rapidlyincreased after ovariectomy and decreased after orchiectomy.

These studies show that gonadal hormones influence foodintake and that there are clear sex differences in food intake andfeeding pattern. Absence of gonadal hormones attenuates sexdifferences in circadian distribution in food intake. Alterations ofthe internal hormonal milieu affect the circadian rhythm of feedingand disrupt the feeding patterns. Because both male and femalerats showed an increase in light-phase feeding and in meal size anda decrease in meal number after castration, similar mechanism(s)are operative in both sexes. The exact mechanisms by whichgonadal hormones affect the neuroendocrine and biochemicalprocesses involved in food regulation are not clear, but bothcentral and peripheral systems are affected. Testosterone ismetabolized to estradiol and dihydrotestosterone (DHT) at var-ious sites including the VMN, which is believed to be a neuralsite regulating food intake. It is believed that aromatization oftestosterone to estrogens may be responsible for the centraleffects of testosterone on food intake.140

Because specific changes were observed in meal number andmeal size, whose product equals food intake, after removing thegonads in both sexes and because these individual indices appearto be differentially regulated by different hypothalamic sites,namely the VMN and LHA, we examined the role that VMN playsin the relation of meal number to meal size by analyzing feedingpatterns in both sexes after specific VMN block. The VMN wasblocked in age-matched young adult male and female rats byinjecting colchicine intrahypothalamically. After the VMNblock141 with colchicine, food intake significantly increased by60% in both dark and light phases. This was achieved as a resultof a significant 70% increase in meal number in both dark and lightphases, with no significant change in meal size. The female rat alsoincreased its food intake in both dark and light phases, with themagnitude of light-phase increase being much more than that ofthe dark phase (84% versus 26%). This was achieved by anincrease in the meal size in both the dark and light phases.

These results show a sexual dimorphism between male andfemale rats after VMN inhibition. After castration or in the absenceor impairment of the neural target site (VMN block), sex differ-ences in food intake are attenuated, but differences in the feedingindex by which the male and female rat increase total food intake,i.e., meal size in the female rat, meal number in the male rat, isprobably sex related and indicates that the significance of gonadalhormones is not only the regulation of food intake but also themechanisms by which regulation is achieved. Neuroanatomic dif-ferences in the morphology in the rat central nervous system exist1) in the preoptic area, the amygdala and VMN, neuronal nuclearand nucleolar size, 2) in the arcuate nucleus in synaptic vesiclesand terminals, 3) in the preoptic area in synaptic organization inthe suprachiasmatic nucleus, 4) in dendritic branching patterns inthe preoptic area, and 5) in the lumbar spinal cord (fifth and sixthsegments) in gross nuclear volume. Thus, neurons sensitive togonadal steroids seem to occupy prominent positions within neuralcircuits. Steroid-sensitive neurons can increase neurotransmitterbiosynthesis, decrease uptake or degradation, or enhance receptorsensitivity to neurotransmitters.142

Using in vivo microdialysis, the intra-VMN and intra-LHAlevels of dopamine and serotonin were measured as they relate tothe control of meal size and meal number in normal male andfemale rats to examine their sex differences. After 12 h of fooddeprivation, normal intact male rats appeared to show higher LHAdopamine and serotonin levels than their normal intact femalecounterparts (Fig. 8). After ovariectomy, female rat VMN andLHA dopamine levels increased (Fig. 9) and the level of serotoninwas unchanged, although an increase was seen in the VMN.Whether sex differences exist in mRNA expression of the genesfor dopaminergic receptors in the VMN between male and femaleFischer rats in the normal state was also examined. In our intactnormal male rat, only the D2 receptor was expressed; in theproestrus female, both D1 and D2 receptors were expressed. Ourstudies show that there are sex differences in observed behaviorssuch as food intake because of sex differences in gene expression.In addition to morphologic differences in the hypothalamus, weshowed a functional difference, as documented by changes inhypothalamic dopamine and in their receptor expression. Thesechanges, influenced by gonadal hormones, translate into functionaldiversities characterized by different behaviors in each sex.

CONCLUSION

The control of food intake and energy expenditure is extremelycomplex and involves a number of different messengers.143 How-ever, different lines of investigations suggest that disturbances ofeating behavior are associated with abnormal hypothalamic neu-rotransmission, regardless of the signaling pathways involved intheir pathogenesis. This indicates that hypothalamic neurotrans-

FIG. 7. Schema showing interrelationship of the LHA and VMN, withnicotine and estrogen postulated to influence meal number and meal size.


mission and in particular dopaminergic and serotoninergic activitymay represent a step shared by different appetite-related brainmessengers including leptin and NPY. Based on this thesis, mod-ulation of dopaminergic and serotonergic activity within the hy-pothalamus is likely to contribute to reduced food intake and to

promoting weight loss in both animal models and humans. Werecognize that better understanding of the detailed mechanismsunderlying the pathogenesis of hyperphagia and of hypophagia isneeded to develop a more tailored therapeutic approach to distur-bances of eating behavior.

FIG. 8. Changes in overnight-fasted dopamine level in the lateral hypothalamic area between sexes, as measured with in vivo microdialysis.

FIG. 9. The effect of ovariectomy versus intact controls on dopamine and serotonin in the lateral hypothalamic area and ventromedial nucleus.


ACKNOWLEDGMENTS

Ms. Dawn Jensen-Nobile is thanked for her editorial assistance.

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Hypothalamic dopamine and serotonin in the regulation of food intake

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