Castration affects brain iodomelatonin binding in hamsters maintained in long but not short days

The effects of castration on 2-[125I]iodomelatonin ([125I]melatonin) binding sites in discrete brain areas were investigated in male Syrian hamsters exposed to long and short days. In hamsters maintained in long days (14 h light: 10 h darkness), castration produced a marked decrease in [125I]melatonin binding in the brain, particularly in the medulla-pons hypothalamus and hippocampus. Maximal response in the medulla-pons and hypothalamus was observed at 3 days; specific [125I]melatonin binding subsequently increased to reach control levels within 30 days after castration. In the hippocampus, the decrease in [125I]melatonin binding was still evident at 90 days after castration and could be reversed by testosterone. Exposure to short days (8 h light: 16 h darkness) did not affect [125I]melatonin binding in the various brain areas of the intact hamsters; even after 90 days when circulating testosterone decreased to castrated levels, the binding remained as in intact, long-day-housed controls. Moreover, [125I]melatonin binding in the various brain areas of hamsters exposed to short days was unaffected by castration. The results clearly indicate that the regulation by testosterone of melatonin receptors in the medulla-pons, hypothalamus and hippocampus of the male hamster depends on the prevailing photoperiod.