In male Siberian hamsters, administration of adult physiological levels of testosterone (T) and estrogen (E2) to juveniles inhibited pubertal onset by distinct pathways. It is presently unclear if T and E2 also exert an inhibitory effect on the reproductive function of sexually mature and sexually maturing hamsters. This study aims to determine if there is an age-dependent decline in the sensitivity of the hypothalamic-pituitary-gonadal (HPG) axis to these inhibitory steroids and if their actions remain distinct. Peripubertal and adult male Siberian hamsters were implanted with a silastic capsule containing T, E2, or cholesterol (Ch, control). Testosterone treatment significantly reduced testes mass and length and impaired spermatogenesis in both ages. In contrast, E2 treatment reduced testes mass only in peripubertal, but not adult, animals. In fact, E2 treatment significantly increased testes mass in adults without altering spermatogenesis. In addition, circulating E2 is very high immediately prior to pubertal onset and declines thereafter. Our results showed the inhibitory effects of T persist into adulthood whereas those of E2 subside as the animals become sexually mature. The decreased sensitivity of the HPG axis to the inhibitory effects of E2 in adult animals and the high level of circulating E2 immediately prior to pubertal onset suggest E2 may play an important role in the regulation of puberty in this species.
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