In Vitro Progesterone Stimulates the Release of Luteinizing Hormone-Releasing Hormone from Superfused Hypothalamic Tissue from Ovariectomized Estradiol-Primed Prepuberal Rats*

Publisher Website
Google Scholar
Abstract
This study was designed to determine if in vitro treatment with progesterone (P4) would activate the LHRH apparatus in hypothalmic fragments from ovariectomized, estradiol- primed rats. Immature, 28-day-old female rats were ovariectomized and implanted with Silastic capsules containing estradiol. At 30 days of age, the animals were decapitated, and the mediobasal hypothalamic-anterior hypothalamic-preoptic area fragments were removed and transferred to supervision chambers. These hypothalamic units received P4, delivered either in pulses (10 min on, 20 min off; 10 ng/ml) or in a continuous mode. LHRH was determined in perfusates and postsuperfusion tissues by RIA. The intermittent mode of P4 administration unequivocally stimulated episodic LHRH release. A maximal stimulatory effect of P4 (close to a 4-fold increment over pretreatment levels) was observed about 1 h after initial P4 administration. After a decline in LHRH release to approximately initial levels, it took about 1 h more for the intermittent P4 pulses to once again stimulate the LHRH secretory apparatus. In contrast to the intermittent mode, continuous P4 infusions were not effective in increasing LHRH release. Furthermore, the release profile was sustained at a stable low level without any pulsatile fluctuation. Intermittent superfusion of hypothalamic fragments with cholesterol did not stimulate LHRH release. A single P4 pulse of 10-min duration augmented the LHRH release rate (>150% increment over pretreatment levels), while that of 1-min duration and similar strength (10 ng/ml) failed to change the LHRH release rate. The stimulatory effectiveness of P4 on the LHRH release rate in vitro was reduced when hypothalamic units were superfused at 30 C instead of 37 C and was not present in hypothalamic fragments from intact prepuberal rats. These observations demonstrated for the first time that an in vitro intermittent infusion of P4, but not a continuous administration, can directly activate the neural LHRH secretory apparatus from hypothalamic fragments from ovariectomized, estradiol-primed prepuberal rats.