Circadian/ultradian patterns of plasma cortisol were assessed in four intact and four gonadectomized male rhesus macaques. Concomitant measures of testosterone (T) were analyzed in the intact animals. Blood samples were drawn every 15 min for 28 h via an indwelling catheter. Plasma concentrations of cortisol and T were determined by RIA. Inverse diurnal patterns of cortisol and T secretion were documented in the control group. The circadian pattern of plasma cortisol was characterized by a progressive rise during the early morning hours (0200-0800 h), followed by a gradual decline until lowest levels were reached at 1600-1900 h. T levels were lowest from 0800-1200 h (2-4 ng/ml) and reached a zenith from 2200–0200 h (8-10ng/ml). Surprisingly, circadian fluctuations in cortisol were absent after orchidectomy. Cortisol levels in castrates were less [P < 0.05) than those during peak levels of cortisol secretion in the intact animals (6.25 vs. 10.1 Mg/100 ml) and elevated (P < 0.01) compared to concentrations in the intact animals at their nadir (6.4 vs. 2.5 Mg/100 ml). The loss of circadian fluctuation was not associated with a significant change in frequency of pulsatile cortisol release or a change in the daily mean level. These results are the first in nonhuman primates to demonstrate that the testes modulate circadian activity in the hypothalamic-pituitary-adrenocortical axis. The data differ from previous findings in rodents suggesting that castration increases the synthesis and release of ACTH/corticosterone.
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