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Feedback action of estradiol on GnRH secretion during the follicular phase of the estrus cycle

100%

Karsch F. J., Evans N. P.

Acta Neurobiologiae Experimentalis

1996

vol. 56

issue 3

715-725

The pattern of GnRH secretion during the follicular phase of the estrus cycle of sheep is characterized by an initial marked in episodic secretion followed by a massive and sustained discharge - the preovulatory GnRH surge.Studies emploing a physiological model for the follicular phase have revealed that estradiol has profound and complex feedback effects on GnRH release during the preovulatory period.These include both quantitative efects on pulses and qualitative effects, in addition to inducting a preovulatory GnRH surge.In stimulating the surge, estradiol causes a highly characteristic change in the minute-to-minute pattern of GnRH in hypophyseal portal blood.Initially, a strictly episodic pattern gives way to one in which GnRH is consistently elevated between pulses.Then, following enhancement of both pulsatile and interpulse components, GnRH becomes extramely high and variable for the majority of the surge.From this point, a regular and well organized pulse pattern is not apparent.The characteristic time course of GnRH at surge onset provides insight into possible mechanistic changes in the GnRH neurosecretory system.Such changes include quantitative and qualitative alternations in the pulse generating mechanism, recruiment of a surge specific population of GnRH neurones, morphologic alternations in GnRH neurones , morphologic alternations in GnRH neurones and neighboring cells, and changes in efficiency or route of delivery of GnRH from its siet of release to the portal vasculature.These possibilities, while untested and speculative, provide a conceptual framework for future research.

Neural systems mediating the negative feedback actions of estradiol and progesterone in the ewe

84%

Goodman R. L.

Acta Neurobiologiae Experimentalis

1996

vol. 56

issue 3

727-741

The ewe shows a marked seasonal variation in the effect of ovarian steroids on pulsatile GnRH secretion.In the breeding season progesterone inhibits GnRH pulse frequency, while estradiol suppresses puilse amplitude.In anestrus, both steriods inhibit pulse frequency.The effects of progestrone in both seasons are mediated by endogenous opioid peptides (EOP) that act in the preoptic area (POA) and medial basal hypothalmus (MBH).However, knife cut studies indicate that actions in the MBH are most important.Moreover, blockade of EOP receptors activates GnRH perikarya in the MBH, but not those in the POA.Thus interactions between EOP and GnRH neurons within the MBH may be critical for progesterone negative feedback.The neural systems mediating estradiol suppression of GnRH pulse amplitude in the breeding season are largely unknown, although alpha-adrenergic neurons nay be involved.The seasonal variation in inhibition of GnRH pulse frequency by estradiol is postulated to be mediated by a group of dopaminergic (DA) neurons that have three important properties: (1)they inhibit GnRH pulse frequency; (2) their activity is stimulated by estradiol; and (3) they are functional in anestrus, but not the breeding season.Recent work examining the effects of lesions of DA neyrone and the ability of estradiol to induce Fos inDA cells srongly suggest that DA neurons in the retrochiasmatic area (A15) and POA (A14) have all three characteristics.We thus propose that these DA neurons are responsible for the seasonal variation in the ability of estradiol to inhibit GnRH pulse frequency.

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2 Acta Neurobiologiae Experimentalis

2 1996

1 Evans N. P.

1 Goodman R. L.

1 Karsch F. J.

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Feedback action of estradiol on GnRH secretion during the follicular phase of the estrus cycle

Neural systems mediating the negative feedback actions of estradiol and progesterone in the ewe