Synchronization and maintenance of timekeeping in suprachiasmatic circadian clock cells by neuropeptidergic signaling

Circadian timekeeping in mammals is driven by transcriptional/posttranslational feedback loops [1] that are active within both peripheral tissues and the circadian pacemaker of the suprachiasmatic nuclei (SCN). Spontaneous synchronization of these molecular loops between SCN neurons is a primary requirement of its pacemaker role and distinguishes it from peripheral tissues, which require extrinsic, SCN-dependent cues to impose cellular synchrony [2, 3]. Vasoactive intestinal polypeptide (VIP) is an intrinsic SCN factor implicated in acute activation and electrical synchronization of SCN neurons [4, 5] and coordination of behavioral rhythms [6]. Using real-time imaging of cellular circadian gene expression across entire SCN slice cultures, we show for the first time that the Vipr2 gene encoding the VPAC(2) receptor for VIP is necessary both to maintain molecular timekeeping within individual SCN neurons and to synchronize molecular timekeeping between SCN neurons embedded within intact, organotypical circuits. Moreover, we demonstrate that both depolarization and a second SCN neuropeptide, gastrin-releasing peptide (GRP), can acutely enhance and synchronize molecular timekeeping in Vipr2-/- SCN neurons. Nevertheless, transiently activated and synchronized Vipr2-/- cells cannot sustain synchrony in the absence of VIP-ergic signaling. Hence, neuropeptidergic interneuronal signaling confers a canonical property upon the SCN: spontaneous synchronization of the intracellular molecular clockworks of individual neurons.