Further studies on the role of the gonads in determining the ontogeny of gonadotropin secretion in the guinea pig (Cavia porcelus).
To further examine whether nongonadal restraint of gonadotropin secretion during prepubertal development plays a major role in the timing of puberty in the guinea pig, as it does in the monkey and other primates, two experiments were performed. In the first, neonatal male and female guinea pigs were either bilaterally gonadectomized or sham gonadectomized on day 2 of age, or left intact. Circulating LH concentrations were measured in blood samples collected by cardiac puncture every 4 days beginning on day 2 of age and continuing until the animals were 98 days of age. In the second experiment, the time course of the LH response to ovariectomy was examined in juvenile female guinea pigs that were castrated on day 10 of age and were either isolated from or allowed to remain with their mothers for the duration of the study. To place into perspective the time course of the LH response to prepubertal gonadectomy, the effect of castration on LH secretion in adult animals was also examined. Serum LH concentrations were determined by a RIA that employed GDN-15 as the first antibody. Orchidectomy on day 2 of age resulted in an immediate and progressive rise in serum LH concentrations, which plateaued by 30 days of age at levels indistinguishable from those of adult castrates. In contrast, ovariectomy on day 2 of age failed to elicit an immediate rise in serum LH. Instead, circulating LH concentrations in neonatally castrated female guinea pigs were not significantly different from those in intact controls until day 14 of age, at which time serum LH rose progressively to plateau by 30 days of age at levels comparable to those in adult castrates. The time courses of LH secretion after ovariectomy of 10-day-old guinea pigs that were either isolated from or allowed to remain with their mothers were not significantly different and were similar to that observed after ovariectomy at 2 days of age. For reasons presented in detail in Discussion, the foregoing findings fail to support the view that nongonadal restraint of gonadotropin secretion during prepubertal development plays a major role in the timing of sexual maturation in the guinea pig. This laboratory rodent, therefore, may be viewed as a poor paradigm for study of the neuroendocrine basis of puberty in primates.
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