Age and seasonal differences in the synthesis and metabolism of testosterone by testicular tissue and pineal HIOMT activity of Uinta ground squirrels ( Spermophilus armatus)

Abstract

Male Uinta ground squirrels ( Spermophilus armatus) were sacrificed from a free-living population (a) during the breeding season, which immediately followed emergence from hibernation; (b) after the reproductive season; (c) and just prior to aestivation/hibernation. HIOMT activity of the pineal gland was assayed and related to the ability of the gonads to synthesize and metabolize testosterone. Older squirrels had higher HIOMT activity than did the younger animals. The activity of this enzyme was lowest in squirrels during the breeding season. HIOMT activity increased after the breeding season to its highest level just before the squirrels enter aestivation/hibernation. At this time, testicular weight increased concomitant with an apparent increase in HIOMT activity. Testicular size and weight were largest at the time of emergence of the animals from hibernation. Androgen synthesis was also greatest during the breeding season. As would be expected, both decreased rapidly there-after. The testes formed little 17α,20α-dihydroxyprogesterone during or after the breeding season, contrary to what has been reported for rats and house sparrows. The older squirrels demonstrated a greater capacity for testosterone metabolism during the breeding season than did the younger animals.