Effects of estrogen and progesterone on LHRH release from a hypothalamic synaptosomal fraction of ovariectomized rats

Mitochondrial-synaptosomal fractions (P2) from the basomedial hypothalamus of adult ovariectomized rats were employed to study the effects of estradiol benzoate (EB) and progesterone (P) on the release of luteinizing hormone-releasing hormone (LHRH). Treatment of ovariectomized rats with 5 or 50 micrograms of EB significantly reduced the total LHRH released from P2 under both control and K+-stimulated conditions. Furthermore, rats given 50 microgram EB demonstrated cyclic variations in the magnitude of inhibition of LHRH release. Comparison of LHRH release from P2 of rats sacrificed at 0900 hr with that from those sacrificed at 1500 hr revealed a small but persistent facilitation of LHRH release each afternoon. This facilitation, associated with an increase in the soluble component of LHRH release, was absent when rats also received 5 mg of P. No effects on LHRH release were observed when 17 beta-estradiol alone or when P was applied to P2 in vitro. The data show that the regulatory effects of estrogen and progesterone given in vivo on LHRH secretion can be observed in a subcellular fraction of the hypothalamus containing neurosecretory cell terminals.