Previous studies in our laboratory [Endocrinology 114:1605–1612 (1984); Neuroendocrinology 41:252–257 (1985)] examined the influence of ovarian steroids on pulsatile luteinizing hormone (LH) release, and involved immediate replacement following ovariectomy (OVX) of estradiol (E2) and progesterone (P) for a 24-hour period within the physiological context of the estrous cycle. The present study investigated the effects of replacing E2 and/or P 1 week after OVX, and therefore examined whether the time elapsed following OVX influences the effects of ovarian steroids on pulsatile LH release. Immediately after jugular venous cannulation, rats were implanted with either empty silastic capsules or capsules capable of restoring physiological levels of E2 and/or P comparable to those found in intact rats between the intervals of diestrus 1 (Dl) and diestrus 2 (D2), or D2 and proestrous morning. 24 h later, these rats were bled continuously at a rate of 50 µl whole blood/6 min for 3 h for analysis of pulsatile LH secretion. Rats with empty capsules had decreased levels of E2 and P and elevated mean blood LH levels, pulse amplitudes and frequencies. Two groups of animals with E2 capsules had plasma E2 levels comparable to those seen either in the D1-D2 or D2-proestrous intervals, decreased levels of P, and in both cases significant decreases in LH pulse amplitude, but no change in LH pulse frequency or basal LH secretion. Since mean blood LH levels in 8-day ovariectomized rats are determined by LH pulse amplitude, frequency and basal LH secretion [Neuroendocrinology 37:421–426 (1983)], the E2-induced effects on LH pulse amplitude were not sufficient to decrease mean blood LH levels. Rats with P capsules had physiological D1-D2 plasma levels of P and decreased levels of E2, but no change in any parameter of pulsatile LH release. Rats implanted with both E2 and P capsules had physiological D1-D2 levels of both steroids which caused a significant decrease in mean blood LH levels, due to decreases in LH pulse amplitude and pulse frequency. There was no significant difference in the extent of the decrease in pulse amplitude produced by E2 (low) or E2 (low) + P. This indicates that the decreased LH pulse amplitude following E2 (low) + P is due to the E2 (low) alone. The suppressive effects on LH pulse amplitude produced by E2 (low or high) were mediated centrally, since the in vitro responses to LHRH of anterior pituitary (AP) fragments from E2-implanted rats were greater than those of AP fragments from rats implanted with empty capsules. When compared with our previous studies, the present results demonstrate that some of the responses of the pulsatile LH secretory system to the negative feedback effects of ovarian steroids remain the same, while others are altered when these same levels of steroids are replaced for 24 h 1 week after OVX rather than immediately after OVX. D2-proestrous levels of E2 retain their suppressive action on LH pulse amplitude, D1-D2 levels of E2 + P combined remain effective in decreasing pulse frequency and D1-D2 levels of E2 are again ineffective in altering pulse frequency. In contrast, D2-proestrous E2 levels lose their suppressive action on LH pulse frequency, D1-D2 levels of P lose their action in decreasing pulse amplitude and D1-D2 levels of E2 gain the ability to suppress LH pulse amplitude.

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