The action of ovarian steroids on the hypothalamic LHRH pulse generator
Park, Ok-Kyong
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Permalink
https://hdl.handle.net/2142/19613
Description
Title
The action of ovarian steroids on the hypothalamic LHRH pulse generator
Author(s)
Park, Ok-Kyong
Issue Date
1989
Doctoral Committee Chair(s)
Ramirez, Victor D.
Department of Study
Molecular and Integrative Physiology
Discipline
Physiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Neuroscience
Biology, Animal Physiology
Language
eng
Abstract
In order to investigate the action of ovarian steroids upon the hypothalamic LHRH pulse generator in the female rat, I have examined the in vivo activity of the hypothalamic LHRH pulse generator in the different endocrine conditions which was followed by the study on the action of progestins on LHRH release from ovariectomized-estrogen primed (OVX + E$\sb2$) rats. The present study reveals that the rat pituitary receives LHRH in larger quanta but at a fixed frequency (about 1 pulse/h) during proestrus than other cycling phases of the estrous cycle. Surprisingly, removal of ovarian steroids did not significantly alter the activity of the hypothalamic LHRH pulse generator as a function of castration interval, suggesting that post-OVX LH changes are mainly due to removal of steroidal inhibition on the pituitary gland. Estrogen replacement (both an E$\sb2$ implant and EB injections) in the OVX rat was able to reduce elevated LH levels, confirming that estrogen exerts the direct negative feedback on the pituitary gland. Both estrogen replacement therapies were able to induce a LH surge which was accompanied by different LHRH profiles (only EB injection induced LHRH hypersecretion associated with a LH surge), suggesting that these different regiments activate different neuroendocrine mechanisms. A subcutaneous injection of progesterone in OVX + E$\sb2$ rats simulated the spontaneous activity of the hypothalamic LHRH pulse generator during proestrus, suggesting that steroidal interaction as achieved in OVX + E$\sb2$ + P$\sb4$ regiment would be a possible model for experimental investigation of the role of progesterone on LHRH hypersecretion. It is well documented that progesterone stimulates LHRH release from the OVX + E$\sb2$ rat hypothalamus only when infused in a pulsatile mode whose physiological significance during the rat estrous cycle is confirmed by the present study. It appears that a 5$\beta$,3$\beta$ progesterone metabolite (pregnanolone) shares the stimulatory action upon the hypothalamic LHRH neural apparatus with its mother molecule and that this stimulatory action of progestins is through noradrenergic neurotransmission.
Therefore, it is likely that progesterone may act as a final neurosubstance (however, it requires the priming action of estrogen) responsible for LHRH hypersecretion leading to proestrous LH surge in the female rat.
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