Neurobiology, lecture on Neuroendocrine Regulation: Reproductive Axis

Summers
USD Department of Biology

Neurobiology

Spring 2000

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acronyms
text:
Fundamental Neuroscience

- Zigmond, et al.,:
Read pages 1127-1176 for this lecture

end

XXXIII.  Neuroendocrine Regulation  - Reproductive Axis	  back to XXXII. Limbic System and Hypothalamus


	A. Reproductive Axis


		1. GnRH (LHRH) from Hypothalamus stimulates secretion of:


		2. FSH from the adenohypophysis (stimulates follicle growth/spermatogenesis) and


		3. LH from the anterior pituitary (too) which stimulates the secretion of:


		4. T or E2 from the Gonads


			a. negative feedback: T, E2, LH, FSH, & GnRH
			   inhibit/control secretion of GnRH


			b. Positive Feedback in females (similar response in homosexual males)


				i. positive count of plasma [E2] in the hypothalamus


				ii. [threshold E2] triggers GnRH release ® LH surge ® ovulation


		5. other transmitters/modulators and hormones are involved in reproductive
		   and parental behaviors


			a. Oxy (AVT in reptiles and amphibians), GnRH, T, E2, DA, PRL


	B. Neural Control of GnRH and gonadotropin secretion (LH and FSH)


		1. GnRH is decapeptide cleaved from a 92 aa precursor molecule (preprohormone) 


			a. made in neurosecretory neurons of the arcuate mediobasal nuclei
			   and medial preoptic nuclei				


				i. arcuate means arch shaped, mediobasal cells are at the bottom
				   of the third ventricle


				ii. preoptic nuclei are among the anterior most cells of the hypothalamus


			b. axons extend to, and GnRH is secreted into the capillary beds of
			   the median eminence  


				i. receptors on gonadotropic cells of the pituitary


					(1) several 2nd messengers: Gs/AC/cAMP, IP3/DG/Ca++


						(a) stimulates the release of both LH and FSH


		2. neural-presynaptic input mediates GnRH release at
		   median eminence  terminals


			a. mamillary body


			b. arcuate DA neurons


			c. Suprachaismatic nucleus (SCN): rhythmic information           


		3. input from regions of the limbic system including the amygdala, septal
		    region, thalamus, hippocampus, and midbrain reticular formation modulate
		    synthesis and release of GnRH at the arcuate and preoptic nuclei


			a. emotional status affects endocrine secretion


		4. release is mediated directly by NE from the locus coeruleus	


			a. inhibited by  b-endorphin, also CRH, ACTH, B/F


			b. also direct innervation of arcuate and preoptic nuclei from the
			   goands via the vagus nerve


				i. these modulate NE activity		


		5. Sensory pathways from olfactory, tactile, and visual systems
		   indirectly innervate GnRH neurosecretory neurons
		   
		   
		   	a. via limbic system


	C. manner of feedback determines the manner of gonadotropin release


		1. negative - homeostasis


		2. positive


			a.  [E2] builds to threshold


				i. mediated by SCN (with AH and POA)


			b. preovulatory surge of GnRH stimulated by NE + Glu 


				i. [E2] ® H ® H1 ® IP3/Ca++ ® NOS ® NO ® Glu ® CO


				ii. timed/inhibited by  b-endorphin


		3. pulsatile release necessary for gonadotropin secretion


			a. autocrine +feedback


				i. SCN ® V-gated Ca++ ®+ GnRH ® IP3/Ca++ ®+ NOS ®+ NO ®+ GnRH


			b. autopriming ® LH/FSH release 4-6X higher on the second dosage of GnRH


			c. pulsatility maintains [GnRH-R]

XXXIV. Lordosis



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