SEASONAL-CHANGES IN MELATONIN BINDING-SITES IN THE PARS TUBERALIS OF MALE EUROPEAN HAMSTERS AND THE EFFECT OF TESTOSTERONE MANIPULATION

Seasonal changes in specific 2-[I-125]iodomelatonin binding were assessed in the pars tuberalis of male European hamsters (Cricetus cricetus) kept under natural environmental conditions. Saturation studies were performed on pars tuberalis membrane preparations every 5-7 weeks for 13 months. The maximum number of melatonin binding sites occurred in spring and summer (B(max), 4.0 +/- 0.3 fmol/mg protein, n = 9) with significantly reduced binding in winter (B(max), 1.2 +/- 0.2 fmol/mg protein, n = 2). As an assay control, simultaneous saturation studies were performed on rat pars tuberalis membranes. No summer-winter difference in binding site density was observed in the rat studies. In both hamsters and rats no seasonal changes in binding affinity were found. Although an apparent correlation between B(max) values and paired testes weight of the hamsters was noted, in early spring the increase in the number of melatonin binding sites preceded the elevation in testicular weight. Castration during the period of sexual activity or sexual inactivity also failed to reduce the elevated number of binding sites observed in the spring and summer. In addition, testosterone implanted sc 5 weeks before had no effect on the low 2-[I-125]iodomelatonin binding site density found in winter (November). These results suggest that the observed seasonal variation in the number of melatonin binding sites is independent of seasonal changes in circulating testosterone concentrations.