Age-Related Differences in the Effect of Castration upon Hypothalamic LHRH Content in Male Rats

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Abstract
Male rats aged between 15 and 75 days were orchidectomized or only anesthetized. After various periods from 2 h to 42 days, 8 animals of both groups were sacrificed. Serum concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) as well as the hypothalamic luteinizing hormone-releasing hormone (LHRH) content were determined by radioimmunoassay. At the times studied within 15 days of castration, no significant change in hypothalamic LHRH content was observed in rats orchidectomized at 21 days of age. However, when studied 3–6 weeks after castration, those animals showed a reduced hypothalamic LHRH content. No differences were observed between anesthetized and orchidectomized rats aged 15,18 or 21 days and studied 1 week later. In contrast, castration of 75-day-old rats resulted in a significant reduction of the hypothalamic LHRH content after 2–15 days. A significant decrease was also observed 1 week after orchidectomy of rats aged 24–50 days. 1 day after orchidectomy, LH and FSH serum levels were markedly increased in 21- as well as 75-day-old rats. In the latter, serum LH concentrations did not change any further whereas in the former a plateau was only seen after 1 week. In rats orchidectomized at various ages ranging from 15 to 50 days, no consistent differences appeared in serum gonadotropin concentrations evaluated 7 days later. According to these data, variations in hypothalamic LHRH content after orchidectomy differ according to age and maturity. In 15- to 21-day-old rats, the age-related increase in hypothalamic LHRH content was not immediately affected by castration as it was in older animals. This might suggest that (1) changes in sensitivity to gonadal factors according to age are primarily localized in the hypothalamus, (2) some maturational processes in the hypothalamus are not dependent on the presence of gonadal factors.