Age-related changes in the feedback regulation of gonadotrophin secretion in the immature and adult male rabbit.

Male rabbits were castrated at infantile (30 days), peripubertal (60 days) and adult (7-8 months) stages. Two different doses of testosterone were injected 10 days after castration (5 injections at 12 h intervals). Plasma LH and FSH were determined by RIA 1, 5 and 10 days after castration and 1 h after the last injection of testosterone. The response of both gonadotrophins to castration was age-dependent. In 30 day old castrated males LH was not significantly modified and FSH had increased only 10 days after castration. In 60 day old and adult males FSH and LH levels were increased 24 h after castration and continued to rise as time progressed. For both gonadotrophins, the response of adult males to castration was higher than that of immature animals. At all stages studied, the highest dose of testosterone (250 micrograms/kg body weight) depressed post-castration LH and FSH levels. Twenty-five micrograms of testosterone per kg body weight was effective to depress LH levels only in 30 day old males, suggesting a change in the hypothalamic-pituitary unit to te negative feedback of androgens. These findings suggest that there are marked changes in the hypothalamic-pituitary unit around the beginning of the peripubertal state. These changes could play a determinant role in the onset of puberty.