Inhibition of the Daily LH Release Mechanism by Progesterone Acting at the Hypothalamus1

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Abstract
Simulation of proestrus-like serum levels of progesterone (P) limited daily LH [luteinizing hormone] release in the estrogen primed, ovariectomized rat to a single day and abolished delayed LH release in the pentobarbital (PB) blocked proestrous rat. This study describes sites responsive to inhibition of LH release by implantation of P. Pentobarbital (35 mg/kg) given at 1330 h on proestrus postponed the LH and P surges for 24 h. Unilateral or bilateral implantation of P directly into the ventral hippocampus from 1400 h proestrus to 0800 h on the day of estrus in PB blocked rats did not prevent the LH surge on the day of estrus. Implantation of P into the preoptic-anterior hypothalamic area (POA-AHA) of the PB blocked rat for the same interval prevented the LH surge during estrus or diestrus-1. Repetitive daily LH surges induced by chronic s.c. estradiol implantation in acutely ovariectomized rats was prevented by P implantation in the POA-AHA. Subsequent removal of the P allowed daily LH surges to resume. P also decreased the response of the pituitary gland to exogenous LHRH [LH releasing hormone]. This effect was overcome by priming with LHRH 60 min prior to the LHRH challenge. Thus P, secreted on proestrus, prevents the daily neural signal for LH release by acting on both the hypothalamus and pituitary gland.