Differential alteration of the HPG axis by gonadal steroid hormones in the male Siberian hamster

Puberty involves the complex integration of a variety of neural and endocrine factors. The efficacy of gonadal steroid hormones, testosterone (T) and 17β-estradiol (E₂), to exert inhibitory effects gradually declines during puberty. The importance of diminished hormonal sensitivity to the correct timing of pubertal onset remains unclear. The focus of this dissertation was to characterize the effects of T and E₂ during the processes of puberty and seasonal gonadal recrudescence in the male Siberian hamster.; Chapters 1 and 2 showed chronic administration of T induced gonadal regression in prepubertal, peripubertal and adult hamsters. By contrast, E₂ had opposing effects in juveniles (inhibitory) and adults (stimulatory). Also, T and E₂ acted differentially on the hypothalamo-pituitary-gonadal axis with T having a greater inhibitory effect on spermatogenesis than E ₂. Collectively, these results suggested the site of E₂ action changes during the process of puberty and that E₂ might play an important role in normal male reproduction.; In Chapter 3, an in vitro static pituitary incubation system was used to determine basal and gonadotropin-releasing hormone-stimulated gonadotropin secretion in T- and E₂-treated animals. Testosterone and E₂ differentially regulated pituitary gonadotropin secretion in juveniles, but not adults. Further, both hormones caused the juvenile pituitary to assume an adult-like pattern of gonadotropin secretion, with significantly reduced basal secretion levels. These results further supported the hypothesis that the site of E₂ action changes following puberty.; Siberian hamsters are seasonal breeders that undergo annual cycles of reproductive quiescence followed by spontaneous gonadal recrudescence. Consistent with the stimulatory effects of E₂ shown in reproductively active adults, Chapter 4 showed E₂ treatment accelerated recrudescence. These results suggested the opposing effects of E₂ were due to age and not reproductive status.; Chapter 5 demonstrated that the inhibitory effects of T and E₂ during puberty are not consistent for all rodent species. House mice, non-seasonal breeders, are less affected by T and E₂ treatment than Siberian and Syrian hamsters. This suggests seasonal breeders might be more sensitive than non-seasonal breeders to disruptions in the timing of pubertal onset by external negative perturbations.