Evidence for activation of the central nervous system-pituitary mechanism for gonadotropin secretion at the time of puberty in the male rat.

During sexual development in the male rat, serum testosterone (T) levels increase markedly at 45-60 days of age. At the time of the pubertal rise in T levels, activation of the hypothalamic-pituitary axis is difficult to demonstrate, since there is little change in serum LH levels and a decrease in serum FSH levels. We determined whether experimental maintenance of stable pubertal T levels in these animals as they passed through the normal age of puberty would allow demonstration of a major increase in serum gonadotropin levels. At 14-15 days of age, male rats were castrated and outfitted with either T-containing or empty Silastic capsules. Another group of rats was left intact and outfitted with empty capsules. At various times between 29 and 58 days of age, blood was drawn for measurement of serum LH, FSH, and T levels. In the T-implanted castrated rats, serum T levels were comparable to those in midpubertal intact rats, without significant differences among age groups. In this setting of stable T levels, serum LH and FSH were suppressed to levels at or below those in pubertal intact rats until 51 days of age, when they increased significantly into the untreated castrate range. In contrast, untreated castrate animals demonstrated markedly reduced serum T and elevated LH and FSH levels that did not change significantly throughout the entire study. In intact rats, serum T levels were stable until 58 days of age, when they increased over 2-fold; serum LH levels did not change significantly with age, and serum FSH levels decreased significantly by 54 days of age. A separate group of rats was castrated and outfitted with T-containing Silastic capsules at 21 days of age. In these animals, there were significant increases in hypothalamic LHRH, norepinephrine (NE), and dopamine levels and NE turnover rate at 56 compared to 36 days of age. We conclude that stable pubertal levels of T are able to suppress gonadotropin levels in castrated rats until the normal age of puberty, at which time LH and FSH levels increase markedly. This decrease in sensitivity of the hypothalamic-pituitary axis to T negative feedback at puberty is accompanied by increases in hypothalamic LHRH, NE, and dopamine levels and NE turnover rate. These results provide direct evidence for activation of the central nervous system-pituitary mechanism regulating gonadotropin secretion at puberty in the male rat.(ABSTRACT TRUNCATED AT 400 WORDS)