Gastrin-releasing peptide phase-shifts suprachiasmatic nuclei neuronal rhythms in vitro

The main mammalian circadian pacemaker is located in the suprachiasmatic nu clei (SCN) of the hypothalamus. Gastrin-releasing peptide (GRP) and its rec eptor (BB2) are synthesized by rodent SCN neurons, but the role of GRP in c ircadian rhythm processes is unknown. In this study, we examined the phase- resetting actions of GRP on the electrical activity rhythms of hamster and rat SCN neurons in vitro. In both rat and hamster SCN slices, GRP treatment during the day did not alter the time of peak SCN firing. In contrast, GRP application early in the subjective night phase-delayed, whereas similar t reatment later in the subjective night phase-advanced the firing rate rhyth m in rat and hamster SCN slices. These phase shifts were completely blocked by the selective BB2 receptor antagonist, [D-Phe(6), Des-Met(14)]-bombesin 6-14 ethylamide. We also investigated the temporal changes in the expressi on of genes for the BB1 and BB2 receptors in the rat SCN using a quantitati ve competitive RT-PCR protocol. The expression of the genes for both recept ors was easily detected, but their expression did not vary over the diurnal cycle. These data show that GRP phase-dependently phase resets the rodent SCN circadian pacemaker in vitro apparently via the BB2 receptor. Because t his pattern of phase shifting resembles that of light on rodent behavioral rhythms, these results support the contention that GRP participates in the photic entrainment of the rodent SCN circadian pacemaker.