Influence of the Gonads on Cortisol Secretion in Female Rhesus Macaques*

Circadian and ultradian patterns of plasma cortisol were assessed in four intact female rhesus macaques during both the late follicular (days 9-10) and midluteal (days 21-22) phases of the menstrual cycle and compared to patterns in four ovariectomized macaques. Blood samples were drawn from a remote site at 15-min intervals for 24 h via an indwelling catheter. Measures of estrogen and progesterone were obtained for each subject. For purposes of data analyses, group cortisol measurements were collapsed across hourly intervals and submitted to analysis of variance. Pulsatile characteristics of cortisol release were determined for each subject using the PULSAR computer program. Circadian cortisol patterns, present in all three groups, were characterized by a progressive rise during early morning hours (0300-0600 h), followed by a decline of short duration. All groups then displayed an unexpected midday peak (0900-1400 h), at which time cortisol levels reached their daily zenith. In each of the three groups, cortisol levels reached a nadir during late afternoon hours shortly after the light phase ended. The amplitude of circadian changes and daily mean levels of cortisol were significantly reduced by ovariectomy, without alterations in pulsatile characteristics of cortisol secretion. Daily mean cortisol levels decreased from approximately 8 micrograms/100 ml in intact subjects to 4.5 micrograms/100 ml after ovariectomy. No significant differences in the circadian/ultradian periodicity of cortisol secretion were detected between the follicular and luteal groups. When data in the intact female groups were combined and compared to those previously obtained from gonadally intact adult male macaques, similar 24-h patterns of cortisol secretion were detected. Surprisingly, amplitude changes in cortisol concentrations after ovariectomy were temporally and quantitatively similar to those in orchidectomized males. In both male and female animals, circadian patterns of cortisol secretion were reduced by gonadectomy. These results are discussed in terms of the activational influence of gonadal steroids on hypothalamic-pituitary-adrenocortical function.