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 clock, contains several neuropeptides such as vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP). Studies from several laboratories have provided evidence for the coexistence of VIP with PHI and GRP, but reliable data about the proportions of colocalization and a possible diurnal rhythmicity are lacking. In the present study, we therefore aimed at studying these aspects. To this end, rats were killed by perfusion fixation during the 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 GRP. Analysis of the sections was done by semi-quantitative confocal laser scanning fluorescence microscopy. It turned out that, in keeping with previous literature data, VIP and PHI always coexist at the cellular 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 middle 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 number of profiles showing colocalization, in contrast to the number of profiles exclusively immunoreactive for VIP or GRP.