No evidence for a diurnal vasoactive intestinal polypeptide (VIP) rhythm in the human suprachiasmatic nucleus.

The mammalian suprachiasmatic nucleus (SCN) is implicated in the temporal organization of circadian rhythms in a variety of physiological, endocrine and behavioral processes. Since the environmental light-dark cycle is the main zeitgeber for many of these rhythms, photic information may have a synchronizing effect on the endogenous clock of the SCN by inducing periodic changes in the activity of certain groups of neurons. The present study was performed to investigate the diurnal profile of the vasoactive intestinal polypeptide (VIP)-producing neurons in the SCN of humans. No significant diurnal variations were found in the volume of the VIP subdivision of the SCN nor in the number of VIP-producing neurons. In contrast with the VIP cell population, the subdivision of the human SCN containing vasopressin-producing neurons has previously been reported to exhibit a distinct diurnal rhythm, with low values during the night and peak values during the early morning. These findings suggest that the expression of vasopressin, but not that of VIP, in the human SCN exhibits an endogenous circadian rhythm.