Neuropeptide Y Blocks Serotonergic Phase Shifts of the Suprachiasmatic Circadian Clock In Vitro

The mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) can be reset in vitro by various neurochemical stimuli. This study investigated the phase-shifting properties of neuropeptide Y (NPY) and serotonin (5-HT) agonists when applied alone, as well as their combined effects on clock resetting. These neurotransmitters have both been shown to advance the SCN clock in vitro when applied during the daytime. By monitoring the SCN neuronal activity rhythm in vitro, I first confirm that the 5HT_1A/5HT₇ agonist (+)DPAT maximally advances the SCN clock when applied at zeitgeber time 6 (ZT6). Conversely, NPY only phase advances the neuronal activity rhythm when applied at ZT 10. This effect occurs through stimulation of Y₂ receptors. NPY, again acting through Y₂ receptors, blocks (+)DPAT-induced phase shifts at ZT 6, while neither (+)DPAT nor 5-HT affect NPY-induced phase shifts at ZT 10. NPY appears to block (+)DPAT-induced phase shifts by preventing increases in cyclic AMP. These data are the first to demonstrate in vitro interactions between daytime resetting stimuli in the rat, and provide critical insights into mechanisms controlling circadian clock phase.