The action of ovarian steroids on the hypothalamic LHRH pulse generator

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 5β,3β 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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physiology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2011

Author dc:creator

• Park, Ok-Kyong

Contributors dc:contributor

• Ramirez, Victor D.

• Biology, Neuroscience
• Biology, Animal Physiology

Statement dc:rights

• Copyright 1989 Park, Ok-Kyong

Language dc:language

eng

Identifier

AAI8916293
(UMI)AAI8916293

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/19613