PHOTOPERIODIC EFFECTS ON PUBERTY AND SPECIFIC 2-[I-125]IODOMELATONIN BINDING-SITES IN SIBERIAN HAMSTERS

When juvenile male Siberian hamsters are transferred from a long photo period to a short photoperiod, sexual maturation is greatly delayed by a pineal-dependent process. We hypothesized that the eventual onset o f puberty during short photoperiod exposure may be caused by a loss of receptors for the pineal hormone, melatonin. This study quantitated s pecific 2-[I-125]iodomelatonin binding sites in the suprachiasmatic nu clei and pars tuberalis of Siberian hamsters exposed to short photoper iod (10 h light per day) for either 12 or 30 weeks and in hamsters exp osed to long photoperiod (16 h light per day) for the same time interv als. Photoperiodic exposure significantly affected testes weight. The hamsters exposed to long photoperiod for either 12 or 30 weeks had mea n testes weights > 700 mg, in contrast to hamsters in short photoperio d for 12 weeks (mean testes weights < 30 mg) or 30 weeks (mean testes weights approximately 350 mg). The affinity of specific 2-[I-125]iodom elatonin binding sites in both regions was significantly lower in hams ters exposed to short photoperiod as compared to hamster exposed to lo ng photoperiod, at either 12 or 30 weeks. In contrast, there were no e ffects of photoperiod or duration of exposure on the density of specif ic 2-[I-125]iodomelatonin binding sites in either the suprachiasmatic nuclei or the pars tuberalis. Furthermore, a change in the affinity of the specific 2-[I-125]iodomelatonin binding sites in the suprachiasma tic nuclei was observed between the hamsters housed in short photoperi od for 12 weeks (sexually immature) and the hamsters housed in short p hotoperiod for 30 weeks (undergoing puberty). These results demonstrat e that although the onset of puberty after long-term exposure to short photoperiod does not involve a loss of specific 2-[I-125]iodomelatoni n binding sites in the suprachiasmatic nuclei or pars tuberalis, it is associated with a decrease in the affinity of specific 2-[I-125]iodom elatonin binding sites in these regions.