The diurnal modulation of hormonal responses in the rat varies with different stimuli.

The circadian clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus not only controls the basal daily temporal organization of many neuroendocrine functions, but also its responsiveness. We studied the time-of-day influence on plasma changes in adrenocorticotropic hormone (ACTH), corticosterone, glucagon and leptin concentrations elicited by an insulin-induced hypoglycaemic event. Male Wistar rats were exposed to an insulin challenge at six different times during the light/dark cycle. The time-of-day of exposure markedly affected the responses of all four hormones studied. Generally, the magnitude of the different hormone responses correlated with their basal daily release pattern (i.e. the responses of ACTH and corticosterone were largest around lights off, and glucagon and leptin responses were most pronounced during the dark period). With regard to the hormones of the hypothalamic-pituitary-adrenal axis, the presently reported time-of-day dependent modulation is completely opposite to that previously reported for novelty or restraint. Therefore, these findings provide further support for the existence of at least two different neural pathways that are able to activate the hypothalamic-pituitary-adrenal axis, and provide different substrates for modulation by the biological clock. This observation warrants a thorough examination of possible functional explanations for the observed differences.