Long-term exposure of hypothalamic explants to melatonin alters the release of gonadotrophin releasing hormone and the density of melatonin binding sites in the pars tuberalis of the male mink (Mustela vison)

To investigate the action of melatonin on the reproductive system, the effe ct of prolonged versus short-term exposure to melatonin on the release of g onadotrophin releasing hormone (GnRH) was examined in hypothalamic explants of male mink sacrificed in July, September or November. Mediobasal hypotha lamic (MBH) explants including the pars tuberalis (PT) were incubated for 1 night with or without melatonin (10(-8) M) for 8 hr or 16 hr and the relea se of GnRH was then measured. The next day, the explants were incubated fur ther but in a melatonin free buffer, and the release of GnRH was measured w ith increasing time. Half of the July and September explants had melatonin binding sites quantified by autoradiography. In November, a 16-hr exposure to melatonin induced a significant increase in the release of GnRH during t he night, compared with control or 8-hr melatonin exposure. This increase p ersisted for at least 45 min after the withdrawal of melatonin, suggesting a stimulatory effect of melatonin on the synthesis of GnRH; this effect was apparent in July, September and November. In September, the density of mel atonin binding in the PT was significantly lower in the explants incubated for 16 hr with melatonin, compared with those incubated for 8 hr. Thus, in vitro, a long exposure to melatonin, mimicking a single long night, stimula tes the release and synthesis of GnRH in parallel with a decrease in the de nsity of melatonin binding in the PT. These effects seem to depend heavily on the duration of exposure to melatonin.