Effects of aging on photoperiodic responsiveness and specific 2-[125I]-iodomelatonin binding sites in the pars tuberalis and suprachiasmatic nuclei of Siberian hamsters.

Siberian hamsters undergo reproductive quiescence during exposure to a short day photoperiod, but this response appears to diminish with age. This study investigated whether age-related losses in photoperiodic responsiveness may be related to decreases in specific 2-[125I]-iodomelatonin binding sites in the suprachiasmatic nuclei or pars tuberalis. Adult male Siberian hamsters (group 1: 3-6 months of age and group 2: 9-12 months of age) were exposed to short photoperiod (10 hr of light/day) for 10 weeks. Profound testicular regression was evident in the majority of the hamsters in group 1, but in only a few of the hamsters in group 2. There were no significant differences between the groups in the density or affinity of the specific 2-[125I]-iodomelatonin binding sites in either the suprachiasmatic nuclei or the pars tuberalis. These findings suggest that the failure of older hamsters to respond to short photoperiod is not caused by a loss of specific 2-[125I]-iodomelatonin binding sites in the suprachiasmatic nuclei or pars tuberalis.