Hypothalamic Prolactin Stimulates the Release of Luteinizing Hormone-Releasing Hormone from Male Rat Hypothalamus*

Abstract

Previous works from our laboratory and others have shown that there is a PRL-like immunoreactive protein with immunological, chromatographic, and biological characteristics identical to those of pituitary PRL, and this is widely distributed in the rat central nervous system. Since pituitary PRL is important in controlling hypothalamic LHRH release, we have hypothesized that hypothalamic PRL-like immunoreactive protein might serve a similar role, that of an endogenous neuromodulator influencing hypothalamic LHRH release. To this end, we have examined the effect of PRL antiserum and normal rabbit serum on the release of immunoreactive LHRH from rat hypothalamic fragments cultured in vitro. In the first experiment, LHRH release from hypothalami of intact rats, bathed in PRL antiserum (1:200 in Krebs-Ringer bicarbonate buffer), was significantly lower than that from hypothalami bathed in normal rabbit serum (1:200 in Krebs-Ringer bicarbonate buffer) for 90 min of incubation. It was, however, possible that the PRL, immunoneutralized in the first experiment, was material that represented contamination from pituitary PRL. Therefore, we repeated the experiment using hypothalami from animals that had been hypophysectomized 2 weeks before death. Again, PRL antibody significantly inhibited the release of LHRH compared with that by hypothalami incubated in normal rabbit serum. Since testosterone is important to LHRH synthesis, a third experiment was carried out using hypothalami from hypophysectomized male rats that had been implanted sc with testosterone-containing capsules 72 h before death. By 72 h serum testosterone levels had normalized. PRL antibody added to medium containing hypothalamic explants from these animals substantially inhibited in vitro LHRH release, a pattern essentially similar to that seen in intact and hypophysectomized animals without testosterone replacement. From these studies we have concluded that hypothalamic PRL is an important neuromodulator that promotes the release of LHRH from the hypothalamus. Testosterone, at least under the experimental conditions employed, appears not to be essential in this hypothalamic PRL-LHRH interaction.