SEASONAL REGULATION OF MELATONIN RECEPTORS IN RODENT PARS TUBERALIS -CORRELATION WITH REPRODUCTIVE STATE

Using quantitative autoradiography, we have studied the relationship b etween melatonin receptor density in the pars tuberalis (PT) and photo periodic changes in sexual activity in a photoperiodic (Syrian hamster ), and a non photoperiodic (rat) rodent. Syrian hamsters exposed to sh ort photoperiod (SP) for 13 weeks or kept in long photoperiod (LP) wit h daily melatonin injections for 8 weeks, had both gonadal regression and a significant decrease in 2-I-125- melatonin binding site density in the PT when compared to controls. In contrast, when the animals wer e sexually active, photorefractory Syrian hamsters exposed to SP for 2 7 weeks, or rats kept for 13 weeks in SP, the PT melatonin receptor de nsity was similar to that of control animals kept in LP. These results show clearly that a correlation exists between gonadal status and den sity of melatonin receptors in the PT and suggest that the PT could be the site where melatonin mediates its effects on seasonal function.