JOURNAL OF BIOLOGICAL RHYTHMS / June 2001Horton and Yellon / AGING, EPRODUCTION, AND MELATONIN Aging, Reproduction, and the Melatonin Rhythm in the Siberian Hamster

Abstract The present study tested the hypothesis that responsiveness to melatonin, the presence of the melatonin rhythm in circulation, and parameters of the GnRH neuron system are sustained across the aging continuum in Siberian hamsters. Afternoon melatonin injections induced testicular atrophy in 42 % of aged males compared to 100 % of adult males. The proportion of aged males fail-ing to respond to the melatonin injections was similar to the proportion that failed to undergo testicular regression upon exposure to short days. Exposure to short days induced testicular atrophy in juvenile and adult hamsters; however, regression was incomplete or absent in 43 % of aged males. The nocturnal rise in melatonin was similar with regard to duration and peak amplitude, and appro-priate with respect to photoperiod in 25-day-old juveniles, adult (5 months), and aged (17 months) hamsters. Neither advanced age nor timed melatonin treat-ments affected GnRH neuron numbers or distribution. Fertility was maintained in aged and adult males to a comparable extent with respect to latency to first lit-ter and number of pups per litter; reproductive success was dramaticall