Spatiotemporal distribution of the VIP receptor, VPAC2R, in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals, is located in the hypothalamus. SCN neurons must synchronize their circadian rhythms to coordinate daily rhythms in physiology and behavior. Vasoactive intestinal polypeptide (VIP) is a candidate synchronizing factor implicated in the maintenance of circadian rhythms and entrainment to external cues. SCN neurons release VIP in a circadian manner and both VIP and its cognate receptor, vasoactive intestinal polypeptide receptor 2 (VPAC2R), are necessary to maintain synchrony among SCN neurons. We found that daily VIP application to organotypic SCN slices entrains Period2 expression with a phase response curve that accurately predicts the time and rate of entrainment. Based on previous reports of diurnal variation in Vipr2 mRNA levels, we hypothesized that the time-dependent differential response to VIP was due to changes in VPAC2R expression. We, therefore, sought to characterize the spatiotemporal expression of VPAC2R protein in the brain. We immunolabeled coronal sections of C57BL/6 mice brains with a new antibody specific to VPAC2R and observed low VPAC2R expression throughout the brain and high expression in the SCN. VPAC2R was detected throughout the rostral to caudal extent of the SCN with higher intensity staining in the dorsal regions. VPAC2R was primarily expressed on the cell body and dendrites of neurons and colocalized with a variety of cell types, including arginine vasopressin and VIP neurons, in the SCN. The receptor abundance and regional localization in the SCN did not change, however, with time of day in a 12 hour-12 hour light-dark cycle or in constant darkness. These results suggest that VPAC2R is broadly expressed among SCN neurons throughout the day so that it is well positioned to mediate circadian synchrony among SCN neurons