NO EVIDENCE FOR A DIURNAL VASOACTIVE INTESTINAL POLYPEPTIDE (VIP) RHYTHM IN THE HUMAN SUPRACHIASMATIC NUCLEUS

The mammalian suprachiasmatic nucleus (SCN) is implicated in the tempo ral organization of circadian rhythms in a variety of physiological, e ndocrine and behavioral processes. Since the environmental light-dark cycle is the main zeityeber for many of these rhythms, photic informat ion may have a synchronizing effect on the endogenous clock of the SCN by inducing periodic changes in the activity of certain groups of neu rons. The present study was performed to investigate the diurnal profi le of the vasoactive intestinal polypeptide (VIP)-producine neurons in the SCN of humans. No significant diurnal variations were found in th e volume of the VIP subdivision of the SCN nor in the number of VIP-pr oducing neurons. In contrast with the VIP cell population. the subdivi sion of the human SCN containing vasopressin-producing neurons has pre viously been reported to exhibit a distinct diurnal rhythm, with low v alues 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.