GIRK Channels Mediate the Nonphotic Effects of Exogenous Melatonin.

Melatonin supplementation has been used as a therapeutic agent for several diseases, yet little is known about the underlying mechanisms by which melatonin synchronizes circadian rhythms. G-protein signaling plays a large role in melatonin-induced phase shifts of locomotor behavior and melatonin receptors activate G-protein-coupled inwardly rectifying potassium (GIRK) channels in Xenopus oocytes. The present study tested the hypothesis that melatonin influences circadian phase and electrical activity within the central clock in the suprachiasmatic nucleus (SCN) through GIRK channel activation. Unlike wild-type littermates, GIRK2 knock-out (KO) mice failed to phase advance wheel-running behavior in response to 3 d subcutaneous injections of melatonin in the late day. Moreover, in vitro phase resetting of the SCN circadian clock by melatonin was blocked by coadministration of a GIRK channel antagonist tertiapin-q (TPQ). Loose-patch electrophysiological recordings of SCN neurons revealed a significant reduction in the average action potential rate in response to melatonin. This effect was lost in SCN slices treated with TPQ and SCN slices from GIRK2 KO mice. The melatonin-induced suppression of firing rate corresponded with an increased inward current that was blocked by TPQ. Finally, application of ramelteon, a potent melatonin receptor agonist, significantly decreased firing rate and increased inward current within SCN neurons in a GIRK-dependent manner. These results are the first to show that GIRK channels are necessary for the effects of melatonin and ramelteon within the SCN. This study suggests that GIRK channels may be an alternative therapeutic target for diseases with evidence of circadian disruption, including aberrant melatonin signaling. SIGNIFICANCE STATEMENT: Despite the widespread use of melatonin supplementation for the treatment of sleep disruption and other neurological diseases such as epilepsy and depression, no studies have elucidated the molecular mechanisms linking melatonin-induced changes in neuronal activity to its therapeutic effects. Here, we used behavioral and electrophysiological techniques to address this scientific gap. Our results show that melatonin and ramelteon, a potent and clinically relevant melatonin receptor agonist, significantly affect the neurophysiological function of suprachiasmatic nucleus neurons through activation of G-protein-coupled inwardly rectifying potassium (GIRK) channels. Given the importance of GIRK channels for neuronal excitability (with >600 publications on these channels to date), our study should generate broad interest from neuroscientists in fields such as epilepsy, addiction, and cognition.