Effects of Age on Photo-induced Testicular Regression, Recrudescence, and Refractoriness in the Short-tailed Field Vole Microtus agrestis

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Abstract
Male voles initially raised on 16L:8D cycles were taken at 35, 55 or 80 days of age and exposed to the following photoperiods: 1) 16L:8D for up to 6 months; 2) 8L:16D for up to 6 months; or 3) 8L:16D for 1, 2, or 3 months followed by 16L:8D for 1, 2, or 3 months. Animals on treatments 1 and 2 were sampled at monthly or bimonthly intervals. In 16L:8D, animals 35 or 55 days old showed some initial increased sexual development as measured by testicular and seminal vesicle weights, seminiferous tubular diameters, and state of spermatogenesis. Once sexually mature, they and animals initially 80 days old showed little or no reduction in sexual development throughout the experiment. In 8L:16D, maximum regression of the testes occurred after 3 months in 35-day-old animals and after 4 months in 55- and 80-day-old animals. In all cases recrudescence of the testes had occurred after 6 months exposure to 8L:l6D. In 8L:16D (1 to 3 months) followed by 16L:8D (1 to 3 months), a rapid increase occurred in all parameters measured such that the majority of animals became sexually mature. The results indicate no effect of age on the response to short photoperiods, and spontaneous recrudescence and the onset of photorefractoriness occur after the same length of time. Similarly, the response to long photoperiods does not depend on age or the length of previous exposure to short photoperiods. The relevance of these results to the natural breeding cycle is discussed.