Activity of castrated male voles: rhythms of responses to testosterone replacement.

Male Microtus montanus are reported to decrease nocturnality in the absence o testosterone (T). Castrated male voles, housed in activity wheels under LD 16:8, received 21 daily injections of either 0, 30, 60, or 120 micrograms T at either 2 hours after lights on midlight, 2 hours before lights off, or middark. Significant increase in both nocturnality and total activity occurred only in response to T administration at midlight and 2 hours before lights off. For increasing nocturnality, animals were more sensitive to T at midlight; for increasing total activity, animal were more sensitive to T2 hours before lights off. Although the rhythms for these responses differed, the most sensitive time for each occurred when plasma T levels were elevated in intact male voles under LD 16:8. Thus, increases in nocturnality and total activity appear to be mediated by separate pathways, each posessing diel rhythmicity.