ASCI 341 Ch. 5

Regulation of Reproduction: Nerves, Hormones and Targets

neural control

fast acting

reproductive system

regulated by interplay between nervous system and endocrine system

nervous system

Transduce external stimuli into neural signals which bring about a change in the reproductive. organs and tissues

simple neural reflexes

nerves release messages (neurotransmitters) directly onto target tissues

direct innervation of the target tissue by neuron (neurosecretory cell)

rapid effects on target tissues

neuroendocrine reflexes

a substance (message) released by a neuron (neurohormone) enters the blood and act on a target tissue

no direct innervation

rapid effects on target tissues

neuroendocrine reflex

sensory (afferent) nerves → interneurons in spinal cord → efferent nerves → hypothalamus → target tissue

utilize excitatory neurons

excitatory neurons

release excitatory neurotransmitters

increase possibility of nerve firing

inhibitory neurons

release inhibitory neurotransmitters

decrease possibility of nerve firing

“firing” of a nerve

postsynaptic action potential

Hypothalamus

neural control center for reproductive hormones

A complex portion of the brain consisting of clusters of nerve cell bodies (hypothalamic nuclei) - these surround a cavity in the center of the brain - 3rd ventricle

4 Fs - fighting, feeding, fleeing and mating

ventricular system filled with CSF that continually circulates through ventricles and subarachnoid spaces of the CNS sections, lateral, 3rd, 4th, and the central canal

3 centers

• surge center

• tonic center

• periventricular nucleus (PVN)

surge and tonic center

secrete gonadotropin-releasing hormone (GnRH)

axons from these centers extend into pituitary (infundibular) stalk - nerve endings terminate on a highly specialized capillary network -Hypothalamo-hypophyseal portal system

hormones move through vascular portal system to the AP

advantages of the H-AP portal system

hormones not diluted in circulation

• rapid response

• little dilution of peptide hormones

• have short ½ life

H secretes releasing factors inducing secretions of AP hormones

preovulatory LH surge controlled by GnRH from surge center (GnRH → LH, FSH)

Tonic LH release controlled by median eminence (ME)

Periventricular nucleus (PVN)

produce oxytocin

posterior pituitary → neural connection to hypothalamus, neurons release hormones into vasculature leaving PP

endocrine control

“slow acting”

hormone

a substance produced by a gland that acts on a remote tissue (target tissue) to bring about a change in that tissue

very small amounts can cause dramatic physiological changes

half-life

time required for ½ of a quantity of a hormone to disappear from the blood or from the body

increased by glycosylations and larger size

characteristics of reproductive hormones

act in minute quantities

short half-life

bind to specific receptors

regulate intracellular biochemical reactions

autocrine

acts on same cell as secreted itp

paracrine

acts on nearby cell

endocrine

enters circulation and acts on remote cell/tissue

hormones classified by 

source, mechanism of action, biochemistry 

types of biochemical classification

proteins, steroids, prostaglandins

peptide hormones

<20 aa

only have to break peptide bonds

in reproductive system mainly from the hypothalamus

GnRH (10aa), OT (9aa)

protein hormones

>20 aa

only peptide bonds to breakdown

mainly from AP

PRL (198aa), ACTH (200aa)

glycoprotein hormones

much larger, comprised of 2 protein hormone subunits - common alpha and distinct beta

aa have carbohydrate molecules attached to them

must breakdown hydrogen bonds, Van Der Waals forces, covalent bonds, and peptide bonds

increased amount of carbohydrate increases ½ life

degrade easily → cannot be taken orally

protein hormone half-life

minutes to hours

steroid hormone half-life

hours to days

prostaglandins half-life

seconds

steroid hormones

synthesized from cholesterol (C27)

composed of 4 acetic rings (A,B,C,D) derived from acetate

cyclopentanoperhydrophenanthrene nucleus

enzymes cleave cholesterol →

• progesterone (C21)

• testosterone (C19)

  • estrogen (C18)

prostaglandins

first believed to come from the prostate gland, first found in mammalian semen

one of the most ubiquitous, physiologically active substances in the human body

synthesized from arachidonic acid → derived from fatty acids and phospholipids

20-carbon unsaturated hydroxy fatty acids

produced by all tissues in the body, can have local effect on tissues, rapidly degraded in blood and lungs

PGF2a and PGE2

PGF2a

vasoconstriction

effects luteal cells of CL

regression of CL

involved with ovulation, parturition, sperm transport

PGE2

vasodilation

maintain CL

ovulation

embryo implantation

synthetic hormones

long half-life in circulation, not degraded in liver as quickly

given orally or as implants

strong affinity for receptors

agonist or antagonist for progesterone or estrogen

used for estrus synchronization

same core as steroids but different structure than endogenous

hormone receptors

have an affinity for a specific hormone

number of receptors regulates degree of cell stimulation and cellular response to a hormone

hormones can regulate whether there is an increase or decrease in number of receptors

testicular feminization

lack of androgen receptor in male

physiological activity of a hormone depends on

pattern and duration of secretion

half-life

receptor density

receptor-hormone affinity

protein hormone receptors

plasma membrane receptors

domains

• extracellular - hormone binding site

• transmembrane - changes configuration and activates other protein hormones (G-proteins) in response to hormone binding at the extracellular domain, activates 2nd messengers in response to hormone binding

• intracellular - unclear function, GnRH receptor does not have but works the same

steps of protein hormone action

hormone-receptor binding - affinity depends on hormone

adenylate cyclase activation -adenylate cyclase is mediated by a G-protein, causes conversion of ATP to cAMP

protein kinase activation - cAMP triggers activation of control enzymes, protein kinases, converts substrates into products, cAMP binds regulatory subunit and causes activation of catalytic subunit

synthesis of new products - products made by the cell are generally secreted by the cell

binding → cAMP → PK → product synthesis

steroid hormone receptor

passive transport through the plasma membrane and bind to nuclear receptors

steps of steroid hormone action

transport - not water soluble, must be attached to a water-soluble carrier protein for transport through the blood

movement through the cell membrane and cytoplasm - steroids dissociate with their carrier proteins and diffuse through the plasma membrane, diffuse through the cytoplasm and into the nucleus

binding to nuclear receptor - steroid receptor complex induces DNA transcription

mRNA and protein synthesis