In vitro LHRH release from superfused hypothalamus as a function of the rat estrous cycle: effect of progesterone

The present study examines the effect of progesterone (P) on in vitro LHRH release from hypothalamic fragments from intact adult rats throughout the estrous cycle. Estrous cyclicity was monitored by daily vaginal smears, and animals which exhibited at least two consecutive 4-day estrous cycles were used. Animals were sacrificed between 10.00 and 13.00 h and the mediobasal hypothalamic-preoptic area-suprachiasmatic nucleus units were removed and transferred into superfusion chambers (one unit/chamber). Following a 2-hour control period, in which the spontaneous LHRH release was established, P (10 ng/ml) was infused in an intermittent mode (10 min-on, 20 min-off). Effluents were collected at 10-min intervals and LHRH concentrations were determined by RIA. The spontaneous LHRH release from control preparations was episodic throughout all stages of the estrous cycle with a significant low release rate and low LHRH amplitude only during estrus. Interestingly, intermittent infusion of P significantly stimulated LHRH release solely in hypothalamic fragments derived from proestrous rats. The P-stimulated LHRH release during the 1st-hour period after P infusion was significantly higher (p less than 0.05) than that observed in the control preparations during the same time period as well as from its own basal pre-P values (1-hour post-P: 4.26 +/- 0.96 vs. 1-hour control and pre-P: 2.34 +/- 0.38 and 2.32 +/- 0.57 pg/10 min, respectively). P administration did not stimulate in vitro LHRH release during the other stages of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)