Age, Photoperiodic Responses, and Pineal Function in Meadow Voles, Microtus pennsylvanicus

We tested whether juvenile males of Microtus pennsylvanicus were more sensitive than adults to the suppressive effects of short photoperiods. Voles were transferred to short photoperiods (10L:14D) at 20 or 80 d of age, and 60 d later (i.e., at 80 or 140 d) the animals were killed at intervals throughout the day and night. Pineal glands were collected for measurement of melatonin, and the testes were weighed. There were no differences in paired testicular weights of 80 and 140 d old animals held on long days (median testicular weights: 1,953 and 1,843 mg). In contrast, median testicular weights of voles held on short days were 504 and 1,112 mg, respectively, at 80 and 140 d of age; the testicular weights of both groups were significantly different from their age‐matched controls (P .001, two‐sample t‐tests on log transformed data). The responses of the two age groups were compared by normalizing the individual values by the mean and variance of the respective long‐day controls. This comparison suggests that the responsiveness to photoperiod decreases as the animals age (t‐test, P= .01). Duration and amplitude of the nocturnal rise in pineal melatonin content were similar in differently aged animals. In two experiments, voles were injected daily with melatonin from 20 to 80 or 80 to 140 d of age. Melatonin‐injected animals had smaller testes than did saline‐injected controls (ANOVA: P= .01), and injections were more effective in the afternoon than in the morning (P= .01). Comparison of the effectiveness of short day and melatonin injections in juvenile and adult voles suggests that while short days inhibited testicular development of young animals more than it induced regression of adults, this decrease in responsiveness may involve factors other than alterations in the nocturnal pattern of melatonin production.