Mediation of the short-loop negative feedback of luteinizing hormone (LH) on LH-releasing hormone release by melatonin-induced inhibition of LH release from the pars tuberalis.

The pineal hormone melatonin is thought to mediate the effects of the pineal gland on seasonal reproduction by altering the release of gonadotropins. The mechanism by which melatonin controls gonadotropin secretion has been obscure. Recently, labeled 2-iodomelatonin was used to localize melatonin receptors in brain by radioautography. The highest concentration of melatonin receptors was found in the pars tuberalis of the pituitary gland of mammals. Pituitary hormones, in particular luteinizing hormone (LH), have been localized in cells of the pars tuberalis. Consequently, we hypothesized that melatonin might act on its receptors in the pars tuberalis to alter the release of LH. It would then be possible for this LH to diffuse into the overlying median eminence, there to alter the release of LH-releasing hormone (LHRH) from the axons of the LHRH neurons. To evaluate this hypothesis, we incubated median eminence-pars tuberalis tissue from male rats in vitro. After preincubation in Krebs-Ringer bicarbonate buffer for 30 min, test substances were added to fresh medium and the incubation was continued for 30 min. LHRH or LH released into the medium was measured by radioimmunoassay. Melatonin induced a dose-related release of LHRH with the maximum response at the greatest concentration tested (1 microM). This concentration of melatonin also significantly reduced the release of LH into the medium. The increased release of LHRH induced by melatonin (10 microM) was completely blocked by the addition of LH (50 ng/ml), which by itself had no significant effect on LHRH release. Rat LH antiserum (final dilution, 1:1800) significantly elevated LHRH output, whereas normal rabbit serum at a similar dilution had no effect. Finally, LHRH (0.1 microM) induced a significant release of LH from median eminence-pars tuberalis tissue that was completely blocked by melatonin (10 microM). The results support the hypothesis that LH released from the pars tuberalis diffuses to the LHRH terminals in the median eminence to suppress LHRH release. Melatonin acts on its receptors in the pars tuberalis to inhibit LH release, thereby stimulating the release of LHRH from its terminals in the median eminence. The negative short-loop feedback of LH inhibits basal LHRH release in vitro since antiserum against LH increased LHRH release. The results suggest a concept concerning the mechanism by which melatonin can affect the release of pituitary hormones from the pars tuberalis. It is likely that these pituitary hormones diffuse into the median eminence to modify the release of hypothalamic releasing and inhibiting peptides, thereby altering plasma pituitary hormone concentrations.