Daily Rhythmicity of Serum Testosterone Concentration in the Male Laboratory Rat*

Abstract

Daily rhythmicity of serum testosterone concentration in the mature male laboratory rat was examined under various lighting schedules. In rats living in a standard light cycle (12-h light, 12-h dark; lights on at 0600 h), a trimodal rhythm was predominant, with elevations near 0200, 1200, and 1800 h. This pattern was reasonably stable in seven different studies, despite differences in experimental design, method of blood collection, anesthesia, and whether individual rats were sampled once or repeatedly, and was found both in groups of animals and in individuals, including a study using 40-day-old rats. In constant illumination, the pattern was disrupted, but in constant darkness the trimodal pattern was maintained, indicating that the rhythm is endogenous. In a reversed light cycle (12-h dark, 12-h light; lights on at 1800 h), the "midday" elevation was reversed; in an altered light cycle (12-h dark, 12-h light; lights on at 2300 h), the time of the "midday" elevation was shifted. Serum testosterone concentration was higher during the light phase than the dark phase, and was higher in constant light than in constant darkness. A seasonal shift in the daily rhythmicity of serum testosterone concentration is suggested. The trimodal rhythmicity contrasts with the circadian rhythmicity of other hormones. Its functional role in the life of the animal is unknown.

Regulation of Human CYP2C18 and CYP2C19 in Transgenic Mice: Influence of Castration, Testosterone, and Growth Hormone

“…In naive males, testosterone is not secreted at constant levels but rather in an episodic manner (Mock et al, 1978), which was not the case in our study. The lack of pulsatility may be the reason for the limited response in testosterone-treated females.…”

Differential effects of dexamethasone treatment on lipopolysaccharide-induced testicular inflammation and reproductive hormone inhibition in adult rats

“…Intraperitoneal injections of LPS at 0·1 mg/kg body weight in endotoxin-free saline and DEX at 50 µg/kg body weight were given to the appropriate rats at 1200 h, and rats were accordingly killed at 1800 h (6 h group), 0600 h (18 h group) and 1200 h (72 h group). Control groups received injections of saline and were included at each time-point because of significant circadian rhythms in testosterone and LH (Mock et al 1978, O'Bryan et al 2000. The dose of DEX used in this study was able to prevent death in adrenalectomised rats injected with 0·1 mg/kg LPS (our unpublished data), and was consistent with doses used to protect against the inflammatory actions of LPS in rats in previous studies (Nakano et al 1987, Kapcala et al 1995.…”

The disruption of circadian clockwork in differentiating cells from rat reproductive tissues as identified by in vitro real-time monitoring system

“…Remarkably, after exposure to potent resetting cues, no obvious synchronized Per2 oscillation could be observed in these differentiating Leydig cells, which is in dramatic contrast to the highamplitude of Per2 oscillation in the adhesive TIC population mainly consisting of nondifferentiating fibroblast-like (mesenchymal) cells, macrophages, and endothelial cells (Alvarez & Sehgal 2005). However, the secretion of testosterone from Leydig cells has long been known to have a low-amplitude of diurnal rhythm (Goodman et al 1974, Mock et al 1978. The underlying mechanism is not yet clear.…”