Lack of ovarian steroid negative feedback on pulsatile luteinizing hormone release between estrus and diestrous day 1 in the rat estrous cycle.

The object of this study was to determine the roles of ovarian estradiol (E2) and progesterone (P) in regulating pulsatile LH release between estrus and diestrous day 1 (D1) in the rat estrous cycle. Three groups of rats were bled at rates of 75 or 100 microliter whole blood/6 or 8 min, respectively, between 0930-1230 h on estrus or 24 h after either sham ovariectomy (OVX) on estrus (i.e. on D1) or OVX on estrus. There were no differences in plasma E2 and P levels in rats between estrus and early D1. However, after OVX on estrus, plasma levels of both steroids declined and were significantly lower 24 h later than values in D1 controls, indicating an active ovarian secretion of both hormones in this interval of the rat cycle. A significant increase in mean blood LH levels occurred between estrus and D1 due to an increase in LH pulse amplitude and frequency. After OVX on estrus, all parameters of pulsatile LH release also increased within 24 h, but mean blood LH levels as well as LH pulse amplitude and frequency were virtually identical to values in D1 controls, despite the decline in plasma E2 and P levels. Thus, OVX did not augment the increases in LH pulse amplitude and frequency that occur between estrus and D1. This demonstrates that the increase in pulsatile LH release from estrus to early D1 occurs in the absence of ovarian steroid negative feedback; the increases in LH pulse amplitude and frequency are not under negative feedback control by the low plasma levels of E2 and P present at this time. These data stand in direct contrast to the presence of prominent ovarian steroid negative feedback systems operative between D1 and diestrous day 2, and diestrous day 2 and proestrus.