Regulation of the suprachiasmatic circadian pacemaker by serotonin and neuropeptide Y in vitro

The suprachiasmatic nuclei (SCN) of the hypothalamus contain a circadian pacemaker which persists in vitro, with a period of $\sim$24 hours. Isolated in a hypothalamic slice the SCN continue to show a circadian rhythm in neuronal activity, for up to 3 days. In this study we investigated the role of serotonin and neuropeptide Y (NPY) in the mammalian circadian system, utilizing this in vitro approach.Serotonin and NPY are believed to provide nonphotic stimuli which can regulate the phase of the pacemaker in vivo. We tested the ability of these agents to reset the phase of the pacemaker when directly and briefly applied to the ventrolateral SCN (region of serotonergic and NPY input sites) by microdrop technique, at different phases of the circadian cycle on day 1. The effect of exposure to serotonin or NPY was assessed by extracellularly recording the time-of-peak in the rhythm of SCN neuronal activity on days 2 and 3, and comparing this peak-time to control peak-times.Serotonin was found to reset the rhythm of electrical activity during the subjective daytime between circadian time (CT) 3-11, with greatest effect at CT 7. This effect of serotonin was mimicked by agonists specific for 5HT$\sb{\rm 1A}$ and 5HT$\sb7$ receptors. Regulation of the pacemaker by serotonin was found to be Gi-like protein-dependent, as pertussis toxin completely blocked serotonin phase-shifts. Investigation of NPY's effects on the SCN phase indicated two periods of sensitivity, between CT 5-9 and CT 21-24, during which NPY induced phase-advances in the electrical activity rhythm.Integration of nonphotic stimuli at the level of SCN was investigated by simultaneously exposing the SCN to serotonin and NPY, at CT 7 and CT 22. Combined treatment at CT 7 resulted in phase-shifts which were found to be modulated by NPY: increasing the NPY dose inhibited serotonin's effect on pacemaker phase. At CT 22 the effect of combined treatment was not observably different from that of NPY alone.This study demonstrates a direct role for serotonin and NPY as nonphotic regulators of the SCN pacemaker, and establishes NPY as a signal which can influence serotonin mediated inputs to the SCN.U of I OnlyETDs are only available to UIUC Users without author permissio