The suprachiasmatic nucleus (SCN), the central circadian clock in mammals, generates robust yet adaptable circadian rhythms through electrically mediated coordination among heterogeneous peptidergic neuronal populations with presumed cell type-specific roles. Previous studies have proposed that circadian changes in membrane excitability of individual SCN neurons arise from time-of-day-dependent shifts in the relative balance of subthreshold Na⁺ and K⁺ conductances. Although multiple channels have been implicated in these processes, how nocturnally dominant K⁺ conductances are implemented in a cell type-specific manner remains poorly understood. Prokineticin 2 (Prok2) has been identified as a SCN signaling peptide essential for behavioral circadian regulation; however, the electrophysiological properties of Prok2-expressing neurons and the mechanisms underlying their diurnal rhythmicity remain largely unexplored. Here, using electrophysiological approaches in mice of either sex, we show that Prok2 neurons exhibit diurnal variations in electrical properties, with higher excitability during the day and reduced excitability at night, and that G protein-coupled inwardly rectifying potassium (GIRK) channel-mediated basal current contributes to nighttime hyperpolarization. Immunofluorescence and single-cell RT-PCR analyses revealed that GIRK1 and GIRK3 are the predominant GIRK subunits expressed in Prok2 neurons. Moreover, Prok2 neuron-specific deletion of GIRK3 using in vivo genome editing resulted in significant nocturnal depolarization and induced abnormalities in behavioral rhythms, including delayed activity onset and circadian period lengthening, with altered SCN network activity. Together, these findings suggest that tonic, G protein-dependent regulation of GIRK channels provides a night-specific inhibitory mechanism that contributes to intrinsic diurnal neuronal excitability in Prok2 neurons and supports the regulation of behavioral circadian rhythms.Significance Statement In the mammalian suprachiasmatic nucleus, neuronal populations expressing distinct peptides interact through coordinated electrical activity to generate circadian rhythms. However, the ion channel mechanisms underlying diurnal rhythmicity of electrical activity in functionally distinct cell types remain incompletely understood. Here, using a comprehensive approach, we identify G protein-coupled inwardly rectifying potassium (GIRK) channels as a key determinant of nocturnal hyperpolarization in prokineticin 2 (Prok2)-expressing neurons, regulatory nodes involved in behavioral circadian rhythms. We further demonstrate that Prok2 neuron-specific loss of the GIRK3 subunit alters behavioral rhythms. These findings suggest that G protein-dependent regulation of GIRK channels is a key mechanism underlying diurnally patterned intrinsic electrical activity in Prok2 neurons and contributes to behavioral circadian rhythms.
