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Aging, Reproduction, and the Melatonin Rhythm in the Siberian Hamster

Department of Neurobiology and Physiology, Northwestern University, Evanston, IL 60208, USA

Steven M. Yellon

Center for Perinatal Biology, Department of Physiology, Loma Linda University School of Medicine, Loma Linda CA 92350, USA

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 failing 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 appropriate with respect to photoperiod in 25-day-old juveniles, adult (5 months), and aged (17 months) hamsters. Neither advanced age nor timed melatonin treatments affected GnRH neuron numbers or distribution. Fertility was maintained in aged and adult males to a comparable extent with respect to latency to first litter and number of pups per litter; reproductive success was dramatically reduced in aged compared to adult females. Because melatonin rhythms accurately reflect day length information throughout the continuum from puberty to advanced age, the present evidence suggests that limitations in testis regression in response to short days or exogenous melatonin in a subset of aged males result from a reduced ability to respond to melatonin. In the wild, failure to undergo testicular regression in the presence of shortening day lengths may extend the breeding season of aged males.

Key Words: photoperiod • pineal • circadian • individual variation • Phodopus • gonadotropin releasing hormone • fertility

Journal of Biological Rhythms, Vol. 16, No. 3, 243-253 (2001)
DOI: 10.1177/074873040101600307


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