Suppression of spontaneous LH surges in estrogen-treated ovariectomized rats by microimplants of antiestrogens into...

Studies by others have shown that parenteral administration of antiestrogens blocks the positive feedback effect of estrogen on the luteinizing hormone (LH) surge mechanism. Since all estrogen-accumulating cells could be affected by this treatment, it is difficult to identify the site(s) at which this steroid acts to affect LH surges. In the present study we attempted to deprive specific hypothalamic neurons of estrogen by stereotaxically implanting antiestrogen-containing microcannulae into the brains of ovariectomized (OVX) rats which, otherwise, were completely estrogenized. The animal model used in these studies was the 14-day OVX rat into which 2 estradiol-containing Silastic capsules were inserted s.c. on day 14 (day 0). Microcannulae were placed into either the medial or lateral preoptic nuclei (MPN, LPN) on day 0 and the effects on LH release were examined 2 days later (day 2). When empty cannulae were placed into the MPN or LPN, 6 of 7 and 8 of 8 rats, respectively, had normal spontaneous LH surges. In contrast, when cannulae containing either CI-628, LY 10074 or Keoxifene were implanted into MPN only 33.3, 0, and 14.3% of the rats, respectively, had LH surges by 16.00 h on day 2 (time of LH peak). When antiestrogen-containing cannulae were placed into the LPN, all rats displayed normal LH patterns of release and concentrations. The antiestrogens did not prevent estrogen from suppressing elevated high post-ovariectomy plasma LH concentrations (negative feedback). To evaluate whether Keoxifene affected releasable luteinizing hormone-releasing hormone (LH-RH), we examined the effects of MPN-Keoxifene implants on LH secretion evoked by electrochemical stimulation (ECS) of the MPN or the medial basal hypothalamus (MBH). In ketamine-anesthetized rats with empty cannulae, plasma LH increased significantly to reach peak concentrations 30-45 min after ECS. Similar LH concentrations and release patterns occurred in rats with the antiestrogen implant. Other studies examined the effects of MPN-Keoxifene implants on norepinephrine (NE) concentrations and rate constants following administration of alpha-methyl-p-tyrosine. NE concentrations and rate constants in the MPN and median eminence did not differ significantly in rats which had received empty versus Keoxifene-containing microcannulae. In the final series of studies we examined the response of LH-RH neurons to an intracerebroventricular (i.c.v.) infusion of norepinephrine (20 micrograms). Plasma LH peaked within 10 min after i.c.v. NE and, thereafter, declined towards baseline. Keoxifene did not affect LH-RH neuronal responsiveness to i.c.v. NE.(ABSTRACT TRUNCATED AT 400 WORDS)