Bsc4905 chapter5ppt

Chapter 5:Regulation of Reproduction

Nerves, Hormones and Target Tissues

Adapted from Senger, Pathways to Pregnancy and Parturition

Take Home Message

Hormones originate from endocrine glands or nerves. They enter the blood, target specific receptors in cells to produce new products or new hormones. Protein hormones act via plasma membrane receptors and exert effects in the cytoplasm. Steroid hormones act through nuclear receptors and cause transcription and translation, resulting in new proteins. They both cause changes to the function of their target cells.

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Reproduction is regulated by both the nervous system and the endocrine system.

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Fundamental pathways of nervous involvement:

◦ Simple neural reflex

◦ Neuroendocrine reflex

Sensory neurons (afferent, signals to the spinal cord)

The spinal cord

Efferent neurons (signals leaving the spinal cord)

Target tissues (organs responding to specific stimuli)

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Main difference between two pathways:◦ Simple neural reflex employs nerves to release

neurotransmitters directly to the target tissue.

◦ Neuroendocrine reflex requires a neurohormone enter the

blood and act on a remote target tissue.

Interneuron: neurons of the CNS between the afferent

and efferent neurons. Either excitatory or inhibitory.

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Neurotransmitter: substance of small molecular weight

released from the terminals of nerves that causes other

nerves to fire or causes contraction of smooth muscle (like

that surrounding reproductive tract).

Simple neural reflex example:

◦ Stimulus in glans penis recognized by sensory neurons, transmitted to

the spinal cord where they synapse with efferent neurons. Lead to

contractions resulting in ejaculation.

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Neuroendocrine reflex begins with sensory neurons that

synapse with interneurons of the spinal cord. Efferent

neurons then synapse with other neurons in the

hypothalamus, releasing small molecular weight

neurohormones into the blood. They then travel to a

target tissue.

◦ Suckling reflex: sensory nerves detect tactile stimulus,

synapsing with the hypothalamus and releasing oxytocin from

the pituitary.

The Major Pathways and Steps of a Simple Neural Reflex and a Neuroendocrine Reflex

Stimulus

Afferent Neurons

transmission

Neurotransmitter

Target Tissue

Response

Hypothalamus

Neurohormone

Blood

Target Tissue

Response

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Inhibitory neuron:

inhibits other neurons.◦ Distinguished from excitatory

neurotransmitter because instead

of increasing the probability of a

postsynaptic action potential, it

decreases it.

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Hypothalamus consists of clusters of nerve cell

bodies called hypothalamic nuclei.

◦ Neurons in the tonic center and surge center produce

gonadotropin releasing hormone (GnRH)

◦ Neurons of the paraventricular nucleus produce oxytocin

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

Capillary network where nerve endings from the pituitary

terminate known as hypothalamo-hypophyseal portal

system.

Blood enters capillary system from superior hypophyseal

artery, of small arterial capillaries. Enables small quantities

(picograms) of hormones to be deposited into capillary

plexis.

Anatomy of the Hypothalamus and Pituitary Gland

Neural Reflexes and Neuroendorcine Reflexes Cause Rapid Changes in Target Tissues

The posterior lobe of the

pituitary does not have a

portal system.

Neurohormones are

deposited directly into

capillaries in the posterior

lobe of the pituitary.

Endocrine Control is Generally Slower, but Longer Lasting than Neural Control

Endocrine system relies on hormones to cause

responses.

◦ Substance produced by a gland that acts on a target

tissue, to bring about functional change in that target

tissue.

Act in blood from nanograms (108) to picograms

(10-12) per mL of blood!!

Positive and Negative Feedback are the Major “Controllers” of Reproductive Hormones

Negative feedback control = suppression of GnRH

neurons

◦ Progesterone inhibits GnRH, allowing only some follicular

development. Females on progesterone do not cycle.

Positive feedback control = stimulation of GnRH

neurons

◦ Induced ovulation with stimulation/production of GnRH

P4 GnRH FSH & LH little follicular development

Reproductive hormones:

Act in minute quantities

Have short half-lives

Bind to specific receptors

Regulate intracellular biochemical reactions

For hormones to cause a response they must first (1) bind

to a tissue that has receptors for the hormone.

Hormone classification:

Source

Mode of action

Biochemical classification

Glandular Origin Constitutes One Method of Hormonal Classification

Hypothalamic hormones produced by neurons in

the hypothalamus, causing release of other

hormones from the anterior lobe of the pituitary.

Gonadotropin releasing hormone (GnRH) is the

primary releasing hormone of reproduction.

◦ Neuropeptide are small molecules generally less than 20

AA. Synthesized and released from neurons.

Glandular Origin Constitutes One Method of Hormonal Classification

Pituitary hormones are released from the anterior and

posterior lobes into the blood.

◦ Anterior lobe primary hormone is follicle stimulating hormone

(FSH), lutenizing hormone (LH) and prolactin.

◦ Posterior lobe releases oxytocin.

Gonadal hormones originate from the gonads and affect

the hypothalamus, anterior lobe of the pituitary and tissues

of the reproductive tract.

◦ Mediate sex characteristics that cause “maleness” or “femaleness”

Glandular Origin Constitutes One Method of Hormonal Classification

Uterine and placental hormones govern

cyclicity and maintenance of pregnancy.◦ Prostaglangin F2a

Placental hormones include progesterone,

estrogen, equine chorionic gonadotropin (eCG)

and human chorionic gonadotropin (hCG).

Mode of Action is Another Method of Hormonal Classification

Neurohormones are synthesized by neurons and release

directly into the blood.

Releasing hormones are synthesized in the

hypothalamus and cause release of other hormones from

the anterior lobe (pituitary).

Gonadotropins are hormones released from the

gonadotroph cells, stimulating the gonads.

◦ “tropin” means an affinity for or nourish.

Gonadotropins!

Luteinizing hormone (LH) causes ovulation and

simulates the corpus luteum to produce progesterone.

Follicle stimulating hormone (FSH) causes follicular

growth in the ovary of the female and stimulates Sertoli

cells in the male.

Sexual promoters (estrogen, progesteron,

testosterone) are produced by gonads. Development of

secondary sex characteristics.

Mode of Action is Another Method of Hormonal Classification

Human chorionic gonadotropin (hCG) and equine

chorionic gonadotropin (eCG) are produced by the early

embryo, causing stimulation of the maternal ovary.

General metabolic hormones promote well-being…

◦ Thyroxin from thyroid gland

◦ Adrenal corticoids from adrenal cortex

◦ Growth hormone (somatotropin) from anterior lobe of the pituitary

Luteolytic hormones lead to destruction of CL

◦ “lytic” similar to lysis

Hormonal Biochemical Structure Constitutes Another Classification Method

Peptides are small molecules joined by peptide bonds GnRH

Glycoproteins are polypeptides that containe carbohydrate

moieties are range in molecular weight.

◦ Composed of two polypeptide chains of alpha and beta subunits. Most

have same alpha-subunit, but are distinguishable from their beta-

subunits..

◦ FSH, LH and thyroid stimulating hormone

◦ Inhibin: suppresses FSH secretion

◦ Prolactin is a single polypeptide.

Hormonal Biochemical Structure Constitutes Another Classification Method

Steroid hormones have a common molecular

nucleus: cyclopentanopherhydrophenanthrene nucleus.

Prostaglandins were first found in the seminal

plasma, believed to be from the prostate gland.

◦ Most ubiquitous and physiologically active substances in the

body.

◦ Lipids of 20-carbon unsaturated hydrozy fatty acids derived

from arachidonic acid.

Pheramones are Another Class of Substances that Cause Remote Effects

Pheromones also directly

influence reprodutive

processes.

◦ Substances secreted to

outside of the body.

◦ Generally volatiles and

detected by olfactory

system by members of the

same species.

Pheromones are Another Class of Substances that Cause Remote Effects

Endocrine glands are composed of cells that synthesize

and secrete specific hormone molecules.

Hormone actions require presence of specific receptors on

target cells.

◦ Integral part of the plasma membrane containing distinct regions:

receptor domains.

Extracellular domain

Transmembrane domain

Intracellular domain

Pheromones are Another Class of Substances that Cause Remote Effects

Protein hormones utilize plasma

membrane receptors…

… steroid hormones diffuse

into the cell and attach to specific nuclear

receptors.

Protein Hormones: Steps of Action

1. Hormone-receptor

binding

2. Adenylate cyclase

activation

3. Protein kinase

activation

4. Synthesis of new

products

Protein Hormones: Steps of Action

Steroid Hormones: Steps of Action

1. Steroid transport

2. Movement through cell

membrane and cytoplasm

3. Binding to nuclear

receptor

4. mRNA synthesis and

protein synthesis

Steroid Hormones: Steps of Action

Half-life of a Hormone Determines How Long it Will Act

Hormonal potency is dependent on..

◦ Receptor density

◦ Hormone receptor affinity

The density of a target tissue receptor varies as a

function of the cell type, and to the degree at which

hormones promote (up-regulate) or inhibit (down-

regulate) the synthesis of receptors.

Half-life of a Hormone Determines How Long it Will Act

Hormone agonists are analogs (similar

molecular structure) that bind to specific

receptors and initially cause the same biologic

effect as the native hormone.

◦ Promote affinity!

◦ Antagonists have greater affinity for receptor, but

promote weaker biologic activity than native hormone.

Steroids are Metabolized by the Liver and Excreted in the Urine and Feces

Liver inactivates steroid molecules:

1. Saturates double bonds

2. Sulfate or glucuronide residue attached

Creating water-soluble steroid, excreted in urine

Steroids also eliminated in feces… enter gut

through bile duct as glucuronide or sulfate;

bacteria modify them prior to defecation.

Protein Hormones are Degraded in the Liver and Kidneys

Half-life of pituitary gonadotropins is

between 20 and 120 minutes…

◦Removal of polysaccharide chains dramtically

reduces their half-life

Gonadotropins that have lost their

glycosylation sites bind to liver cells and

are degraded.

Protein Hormones are Degraded in the Liver and Kidneys

Hormones can be detected in physiologic fluids

(blood, saliva, urine, lymph, tears, feces) using

radioimmunoassay (RIA) and enzyme-linked

immunosorbent assay (ELISA) technology.

◦ ELISA testing is beneficial because no radioactive

isotopes are needed, it can be conducted on-site with

little training, is has no health hazards and is

inexpensive.