Cyclic AMP Signaling Control of Action Potential Firing Rate and Molecular Circadian Pacemaking in the Suprachiasmatic Nucleus

Abstract Circadian pacemaking in suprachiasmatic nucleus (SCN) neurons revolves around transcriptional/posttranslational feedback loops, driven by pro-tein products of “clock ” genes. These loops are synchronized and sustained by intercellular signaling, involving vasoactive intestinal peptide (VIP) via its VPAC2 receptor, which positively regulates cAMP synthesis. In turn, SCN cells commu-nicate circadian time to the brain via a daily rhythm in electrophysiological activ-ity. To investigate the mechanisms whereby VIP/VPAC2/cAMP signaling controls SCN molecular and electrical pacemaking, we combined bioluminescent imaging of circadian gene expression and whole-cell electrophysiology in organotypic SCN slices. As a potential direct target of cAMP, we focused on hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. Mutations of VIP-ergic signal-ing compromised the SCN molecular pacemaker, diminishing the amplitude and intercellular synchrony of circadian gene expression. These deficits were transiently reversed by elevation of cAMP. Similarly, cellular synchrony in electrical firing rates was lost in SCN slices lacking the VPAC2 receptor for VIP. Whole-cell current