IMMUNOCYTOCHEMICAL EVIDENCE FOR A DIURNAL RHYTHM OF NEURONS SHOWING COLOCALIZATION OF VIP WITH GRP IN THE RAT SUPRACHIASMATIC NUCLEUS

The suprachiasmatic nucleus (SCN), which functions as a biological clo ck, contains several neuropeptides such as vasoactive intestinal pepti de (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing pe ptide (GRP). Studies from several laboratories have provided evidence for the coexistence of VIP with PHI and GRP, but reliable data about t he proportions of colocalization and a possible diurnal rhythmicity ar e lacking. In the present study, we therefore aimed at studying these aspects. To this end, rats were killed by perfusion fixation during th e middle of the day (Zeitgeber time [ZT] 7) and during the second part of the night (ZT 19). Coronal Vibratome sections through the SCN were double-immunolabeled for the presence of VIP and PHI or for VIP and G RP Analysis of the sections was done by semi-quantitative confocal las er scanning fluorescence microscopy. It turned out that, in keeping wi th previous literature data, VIP and PHI always coexist at the cellula r level. This was seen in all possible ratios, both during the day and at night. Part of these VIP/PHI-containing neurons (21%) and part of the GRP-containing neurons (33%) showed colocalization during the midd le of the day. During the second part of the night, these percentages increased significantly to 28% and 40%, respectively. This increase in percentages was due to a significant, nocturnal increase of the numbe r of profiles showing colocalization, in contrast to the number of pro files exclusively immunoreactive for VIP or GRP. (C) 1998 Wiley-Liss, Inc.