SEASONAL-VARIATION OF MELATONIN BINDING-SITES IN THE PARS TUBERALIS OF THE MALE MINK (MUSTELA-VISON)

The seasonal changes of 2-[I-125]iodomelatonin binding were studied us ing quantitative autoradiography in the pars tuberalis (PT) of the min k, a short-day breeder, kept out of doors. Studies were performed at 7 times of the year (July, September, October, January, February, and M ay), corresponding to different stales of responsiveness of the gonada l system to the photoperiod, Melatonin binding was observed in the PT and on the ventral border of the pars distalis, Histological staining revealed that the binding on the border of the pars distalis correspon ded to the zona tuberalis, a ventral extension of the PT. The binding was specific and saturable. The density of melatonin binding varied si gnificantly with the time of year. The lowest density of binding was f ound in July, when animals experienced a long daylength and sexual res t, increased from July to reach a maximum in October, when animals exp erienced decreasing daylength and the hypothalamo-pituitary activity r esumed, then slightly decreased and remained constant from November to May. The saturation study demonstrated that the decrease in melatonin binding density between October and February resulted from a change i n the number (Bmax: October 70.6 +/- 4.0 vs February 49.6 +/- 2.8 fmol /mg protein; P < 0.01) but not in the affinity (Kd: October 33.6 +/- 7 .1 vs February 20.8 +/- 5.1 pM; P>0.05) of the binding sites. These re sults are discussed according to the different phases of mink reproduc tive cycle and to reported data on the sites of action of melatonin on seasonal reproduction and prolactin secretion.