Facilitation or Inhibition of the Estradiol‐Induced Gonadotropin Surge in the Immature Rat by Progesterone: Regulation of GnRH and LH Messenger RNAs and Activation of GnRH Neurons

Abstract

1) suppression of LH and free alpha-subunit secretion by P can be accounted for at least partly by suppression of alpha-subunit mRNA levels; 2) P facilitation is not associated with changes in LH subunit or GnRH mRNA levels; 3) the large proportion of cFos-positive GnRH neurons in P-facilitated rats closely parallels increases in serum LH concentrations but is not accompanied by changes in GnRH mRNA levels. It is likely, therefore, that P acts in the facilitation model to trigger release of pre-existing GnRH stores by altering synthesis or activity of neuro-transmitters/neuropeptides involved in GnRH regulation and/or release of LH stores by altering, for example, pituitary responsiveness to GnRH (including self-priming) and components of the LH secretory apparatus. Similar possibilities may also obtain for the blockade of the gonadotropin surge in the inhibition model.

The Negative Feedback Action of Progesterone on Luteinizing Hormone Release is not Associated with Changes in GnRH mRNA Expression in the Ewe1

“…Despite the unambiguous action of progesterone on GnRH/LH release there was no associated change in the cellular content of GnRH mRNA and, thus, our data do not support the original hypotheses. Studies from two other laboratories, using immature (27) and adult (28) rats, also failed to find an association between GnRH gene expression and the actions of progesterone on GnRH release. However, the results of these studies are not directly comparable with ours as their primary focus is on the stimulatory actions of progesterone on GnRH release while ours is on the inhibitory actions.…”

Progesterone Treatment That either Blocks or Augments the Estradiol-Induced Gonadotropin-Releasing Hormone Surge Is Associated with Different Patterns of Hypothalamic Neural Activation

“…In the rat, this effect has been shown to be due to effects of progesterone which increase pituitary responsiveness to GnRH [50, 51]and stimulate increased release of GnRH from the hypothalamus [52]. The source of this increased GnRH release, however, remains controversial, as while the proportion of activated GnRH neurons in such progesterone-treated animals is increased [53], GnRH mRNA levels do not appear to be enhanced [54, 55]. Fos expression in these animals is thus likely to reflect the activation of cellular processes that precede the release of preformed GnRH [54].…”

“…The source of this increased GnRH release, however, remains controversial, as while the proportion of activated GnRH neurons in such progesterone-treated animals is increased [53], GnRH mRNA levels do not appear to be enhanced [54, 55]. Fos expression in these animals is thus likely to reflect the activation of cellular processes that precede the release of preformed GnRH [54]. This apparent difference between rats and sheep in terms of the effects of progesterone upon the patterns of Fos expression during the surge, which is accompanied by increased GnRH secretion in both species, must therefore reflect either different mechanisms of action that accompany the different progesterone treatment paradigms or fundamental differences in the mechanisms by which progesterone achieves this effect in the two species.…”

Glucocorticoids in Mate Choice

“…Activation of the hypothalamic-pituitary-adrenal axis in response to stress suppresses the hypothalamic-pituitary-gonadal axis which is responsible for mediating sexual behaviour [58], resulting in reduced expression of sexual behaviours. One mechanism, for example, by which stress suppresses reproductive behaviour is via inhibition of the gonadotropin protein hormones which regulate reproductive function by glucocorticoids [59,60]. It is well known that chronically elevated stress suppresses reproduction by, for example, reducing sex drive, courtship behaviour and fertility(see for example [61 -63]).…”