Figure 6.

Model of GRP mediation of photic phase resetting in the SCN. During photic phase resetting, we propose that GRP cells and GRP are involved in the following ways. “Transduction of the photic signal in the SCN,” Ventral SCN cells in the night phase expressing GRP and/or VIP (top cell) show a persistent increase in the frequency of spontaneous action potentials (far right, top), as well as the induction of Per1 in the presence of light (represented by the light bulb) (Kuhlman et al., 2003). “Communication of the intra-SCN photic signal,” GRP is released from ventral cells and activates BB₂ receptors (BB2R) on cells concentrated in the dorsal and medial SCN regions (Karatsoreos et al., 2006). One should note that other neurotransmitters and peptides such as VIP may also be released and involved in this process. “Initiation of SCN phase resetting,” MAPK, ERK, and CREB intracellular pathways are activated (Butcher et al., 2002; Antle et al., 2005; this study), resulting in activation of Per1 that is necessary for a persistent increase in the frequency of spontaneous action potentials to be induced (this study). “Phase shift of SCN rhythms and behavior,” Phase advance of the electrical activity rhythm, behavior, and gene expression.