Endocrine Abstracts

Two-way interactions between the hypothalamo–pituitary–adrenal (HPA) axis and the hypothalamo–pituitary–testicular (HPT) axis are well recognised. For example, in rats disruption of the HPT axis alters the circadian secretion of corticosterone. Conversely, stress conditions have varying effects on the level of testosterone secretion in both rats and humans. In contrast to humans, where several pulses of testosterone secretion can be detected across the 24 h period with the largest pulse occurring in the waking hours, rats appear to exhibit a diurnal rhythm of testosterone secretion with two major secretory episodes, one in the light phase and one in the dark phase.

We have used our automated blood sampling system to investigate the true circadian pattern of testosterone secretion under basal conditions and have investigated how this responds to changes in levels of circulating corticosteroids. Analysis of plasma testosterone revealed a notable bimodal pattern of testosterone secretion. The two secretory episodes were 13.17 h±48 min apart with one in the light phase and the other in the dark phase of the cycle. Interestingly, the circadian rise in plasma corticosterone levels falls between the two testosterone secretory episodes. Treatment of rats with methylprednisolone totally abolished the normal bimodal profile of testosterone secretion. These rats show ultradian pulses of testosterone throughout the 24 h with no bimodal pattern detected. Adrenalectomised rats maintain their bimodal circadian pattern although some dysregulation of the time of onset of the dark phase-associated rise is apparent. Our findings suggest that the endogenous HPT rhythm is not dependent on an intact HPA axis but that increased activity of the HPA axis can inhibit normal HPT regulation.