. Biology of Reproduction, lecture on Endocrinology

Biology of Reproduction

Fall 1998

text: Human Reproductive Biology 2nd Edition - R.E. Jones: Read pages 1-31 for this lecture

I. Endocrinology 			back to  Fall Syllabus

	A. Exocrine glands

		1. secrete products via a duct  to the surface

		2. e.g. sweat glands, digestive glands, uterine glands

	B. Endocrine (ductless) Gland

		1. no duct

		2. secretes hormone into blood

			a. Chemical Messenger

				i. chemical mediation

				ii. acting at target cell

	C. Paracrine/Autocrine/Neurotransmitter Secretion

		1. no blood vector

		2. local action

		3. secretory cells not organized into a gland

		4. target cells

			a. paracrines/neurotransmitters = cells nearby

			b. autocrines = same cells

II. Receptors

	A. Peptide or protein hormones  (short (peptide)
	    or long chains of amino acids)

		1. defined as a target cell because it has receptors
		    for the hormone

			a. receptors for peptide hormones are proteins
			   in the membrane (protrudes to surface) of the cell

				i. membrane receptors activate G proteins ®
				   2nd messenger systyems ®
				   activate enzymes ® cell response

		2. no target cell is responsive to just one hormone

			a. Antagonism    

				i. one hormone inhibits or blocks the effect of the other

			b. Synergism/facilitation

				i. the response of 2 hormones is greater than the sum

	B. Steroid Hormones (derived from cholesterol)

		1.  receptors (proteins) are in the cytoplasm or nucleus

			a. steroid-receptor complex binds to DNA

				i. stimulates transcription, and therefore
				   protein synthesis

			b. Steroids may also have, or bind to, membranal receptors

		2. steroid hormones may interact with other hormones

		3. receptors for thyroid hormones are also in the  nucleus

	C. Paracrines

		1. also have membranal receptors

III. Brain

	A. The brain regulates hormonal control of reproductive events

		1. Environmental cues (includes social input) are converted
		   by neural mechanisms into meaningful physiological
		  (including hormonal) responses

	B. Olfactory (gustatory?) cues are mediated by GnRH neurons
	   innervating the limbic (emotions, memory) areas of the brain
	   (especially amygdala and hippocampus) which in turn innervate
	   GnRH neurosecretory neurons in the hypothalamus

		1. GnRH acts both as a neurotransmitter/neuromodulator
		   and as a hormone

		2. visual cues via the optic nerve have effects through
		   the limbic system and the brain's rhythmic centers
		   (especially SCN = suprachiasmatic nucleus and pineal)

		3. tactile cues may travel via the spinal cord and be
		   enhanced by hormones (E2) and neuromodulators (GnRH)

	C. NE from the locus coeruleus (in brainstem) directly
	   stimulates hypothalamic GnRH

		1. Epi will also stimulate GnRH release

		2. excitatory amino acid transmitters (glutamate = Glu)
		   regulate preovulatory surges of GnRH and LH

		3. Opiates (endorphins/enkephalins) mediate the
		   timing of GnRH/LH surge

	D. Endocrine Axes

		1. Hypothalamus: Releasing Hormones (e.g. GnRH) stimulate

		2. Pituitary: Tropic Hormones (e.g. gonadotropins = LH, FSH)
			a. which in turn stimulate

		3. Hormones (e.g. E2 or T) from peripheral glands (e.g. ovary, testis)

IV. Hypothalamus

	A. Produces Releasing Hormones

		1. Hypophysiotropic area

			a. stimulates release of tropic
			   or hypophyseal (pituitary) hormones

			b. also Release-Inhibiting Hormones

			c. neurons which conduct electrical signals (slowly)

				i. release product into the blood instead of synapse

					(1) electrical signal traveling
					    along axon stimulates release

		2. secretion mediated by inputs from other areas of the brain

			a. e.g. locus coeruleus, amygdala, thalamus

				i. modulated by environmental stimuli

			b. and by feedback to the brain from released hormones

		3. neurons from the hypothalamus also project to
		   other areas of the brain

			a. may modify/regulate related behaviors

		4. RH's travel via the blood to the pituitary
			a. HTA

			b. Axons

			c. Median Eminance

			d. Primary capillary plexus

			e. Hypothalamo-hypophyseal Portal Veins

			f. Secondary Capillary Plexus

			g. Anterior Pituitary cell

	B. Releasing Hormones

		1. Gonadotropin-Releasing Hormone
		   (GnRH - also called LHRH)

			a. only releasing hormone that releases two hormones

				i. LH

				ii. FSH
			b. decapeptide

				i. > 14 different forms of GnRH exist

					(1) differences found in the 5th, 7th, & 8th amino acids

						(a) named for the animal first discovered in

				ii. mammals (inlcuding humans) have chicken II
				    and mammalian GnRH's

					(1) c2GnRH acts as a neuromodulator
					    and is found in most vertebrates

						(a) lizards have c2GnRH and s1GnRH

			c. GnRH is made in the arcuate and preoptic nuclei of the hypothalamus

					i. stimulation by NE from the locus ceruleus causes release of GnRH

		2. DA (PRIH) and TRH (PRH)

			a. Prolactin has releasing (TRH) and release inhibiting hormones (DA)

			b.  DA (PRIH) has stronger influence

				i. PRL is under tonic inhibitory control

			c. suckling stimulates PRL release facilitated by TRH

				i. neuroendocrine reflex		

			d. receptors for each releasing hormone

				i. separate receptors for RIH and RH

		3. some other hypothalamic hormones:
		   TRH (release of TSH), CRH (ACTH), GHRH and GHRIH (GH), 

V. Pituitary Gland - Hypophysis

	A. "Pituita" - slime or flem in Greek

		1. it was thought that the pituitary secreted saliva or flem

	B. Situated just below the brain (particularly the hypothalamus)

		1. Infundibulum = Stalk

		2. Anterior Pituitary = Adenohypophysis

			a.    Pars Distalis

			b.    Pars Tuberalis (ring around infundibulum)

			c.    Pars Intermedia (sparse or absent as distinct structure in adult humans)

			d. adeno = glandular 

				i. derived from Rathke's pouch

			e. hormones of the adenohypophysis (mostly pars distalis)

				i. basophils - mostly glycoproteins

					(1) gonadotropins:

						(a) Follicle-Stimulating Hormone   (FSH)

						(b) Luteinizing Hormone   		    (LH)

					(2) also TSH and  ACTH (a polypeptide)

				ii. acidophils - large proteins

					(1) Prolactin						(PRL)

					(2) GH

				iii. chromophobe - polypeptides

					(1) Endorphins and Enkephalins

					(2) a-MSH
		3. Posterior Pituitary = Neurohypophysis = Pars Nervosa

			a. like the HTA, neurohypophysis is made up of a
			   special kind of neurons = Neurosecretory Neurons

				i. they don't conduct electrical signals very well

				ii. synapses not with another neuron,
				    but with capillary bed

					(1) neurohemal organ

				iii. neural message causes the release of a hormone

			b. islands of neurosecretory cell bodies (perikarya)
			   in the hypothalamus = Neurosecretory Nuclei (SON, PVN)	

				i. axons and terminals compose neurohypophysis

			c. Oxytocin (Oxy)

				i. milk ejection

				ii. contractions of the uterus

			e. Arginine Vasopressin = Antidiuretic Hormone

				i. ­ blood pressure, water reabsorption

				ii. AVT combines effects of both AVP + OXY 

					(1) putative evolutionary precursor

					(2) found in fish, amphibians, reptiles and birds

					(3) other uterotonic forms include
					    MST, IST LVT, GLT, AST, VAT

VI. Homeostasis and Feedback