Pak, T.R.*; Lynch, G.R.; Tsai, P.-S.: TESTOSTERONE AND ESTROGEN ACT VIA DIFFERENT PATHWAYS TO INHIBIT PUBERTY IN MALE SIBERIAN HAMSTERS

Testosterone (T), dihydrotestosterone (DHT), and 17b-estradiol (E) induce testicular regression in peripubertal, male, Siberian hamsters, Phodopus sungorus. Since T can be converted to DHT or E, whether T acts directly or through its metabolites to inhibit gonadal development is not known. The purpose of this study was: 1) to determine if T, DHT, and E have differential effects on the hypothalamic-pituitary-gonadal axis (HPG); and 2) to determine if peripubertal hamsters show a change in sensitivity to T inhibition following the pubertal transition. In Experiment 1, pre-pubertal hamsters were implanted with a capsule containing crystalline T, DHT, E, or cholesterol (Ch). After 15 days, blood, hypothalami, pituitaries, and testes were collected. In experiment 2, pre-pubertal animals were implanted with T or Ch, castrated at 35-days of age, and implant was removed at 45-days of age. The implant was replaced 5 (Group A) or 10 (Group B) days post implant removal. Results from Experiment 1 showed E significantly reduced pituitary FSH content but not T or DHT. Plasma FSH content was undetectable in T and DHT groups but E-treated animals were similar to controls. Experiment 2 showed that plasma FSH levels were undetectable in T-treated animals until implant removal. FSH levels rise significantly as early as 1-day post-implant removal. Plasma FSH was undetectable following re-implantation in Group A, whereas they remained significantly higher in Group B. Taken together, our data showed that T and E act via different pathways to inhibit the HPG axis during pubertal maturation in the male Siberian hamster. Further, a decrease in the sensitivity to T-induced HPG inhibition occurred during a defined period immediately after the peri-pubertal transition. Supported by NSF grant IBN-9996398