Time Courses of Concentrations of Circulating Gonadotropin, Prolactin, Testosterone, and Cortisol in Adult Male Rhesus Monkeys (Macaca mulatta) Throughout the 24 h Light-Dark Cycle1

Abstract

Attempts were made to characterize the time courses of concentrations of circulating LH, FSH, PRL, testosterone (T), and cortisol throughout the light-dark cycle (lights on 0600–1800 h) in intact adult male rhesus monkeys. Blood samples were withdrawn from five animals every 20 min with a remote sampling device that permitted continuous access to the venous circulation with minimal restraint of the animal. Mean concentrations of plasma T were highest during darkness and declined abruptly shortly after the lights came on to reach a nadir ∿3 h later at 0900 h. Mean T levels then increased progressively and in an apparently rhythmic fluctuating fashion throughout the remainder of the light phase of the 24 h cycle. Individual animals exhibited very striking moment-to-moment changes in plasma T concentration composed of 7–12 discrete peaks during the 24 h cycle. The frequency of these episodes of testicular T secretion throughout the light phase of the cycle, and particularly during the initial 6 h of illumination (approximately one plasma T peak every 5 h), was noticeably slower than that during darkness (approximately one peak every 2 h). The amplitude of plasma T peaks, however, was similar throughout the entire light-dark cycle. Essentially every peak in concentration of circulating T was preceded by a brief, but distinct, elevation in plasma LH, and most discharges of LH (89%) were followed by an episode of T secretion. These findings suggest that in the male rhesus monkey the characteristic diurnal variation in circulating T is determined primarily by changes in the frequency of episodic testicular T secretion which, in turn, appears to be occasioned by a corresponding pattern of intermittent release of LH. Plasma prolactin levels were lowest at ∿0800 h, and concentrations of circulating cortisol exhibited, as expected, a diurnal variation typical of this plasma steroid. The time course of FSH was not forthcoming because concentrations of this plasma gonadotropin in the circulation were usually immeasureable by the RIA employed.