Localization of melatonin binding sites in the pars tuberalis of the mink at three times during the seasonal testicular cycle

Abstract

The strict photoperiodic dependence of gonadotropic function observed in mink provides an excellent physiological model for studying the activity of GnRH hypothalamic neurons. In mink, exposure to more than 10 h light inhibits the activity of this neurohormonal system. Melatonin plays an essential role in this type of regulation of gonadotropic function. In mink, contrary to results in many other photosensitive species, melatonin was observed to mediate the gonadostimulating effect of short days. However, the binding sites and action of this substance have not yet been defined. We thus attempted to identify melatonin binding sites in the mink brain. The study was carried out at three times during the seasonal testicular cycle in male minks maintained under natural environmental conditions. Coronal sections were taken from the whole brain and pituitary gland. Using 2-[ 125I]iodomelatonin, we observed a strong concentration of binding sites in the pars tuberalis and not in the part of the median eminence surrounded by the pars tuberalis. This concentrated localization of melatonin binding sites brings up the problem of defining the action of this substance in photoregulating the activity of the GnRH neurohormonal system.