Photoperiodic Regulation of SK Channels in Dorsal Raphe Serotonin Neurons

ABSTRACT

Day length, or seasonal photoperiod, shapes mood and affective behaviors, but the neural mechanisms underlying these effects are still being defined. Serotonin neurons of the dorsal raphe nucleus (DRN) are critical regulators of affective behaviors, and photoperiod modulates their excitability and ongoing activity. Here, we investigated the influence of seasonal photoperiod on the function and expression of small conductance calcium‐activated potassium (SK) channels, which mediate the after‐hyperpolarizing potential (AHP) in dorsal raphe serotonin neurons. Building on previous work demonstrating that long summer‐like photoperiod increases serotonergic neuron excitability and decreases AHP amplitude, we hypothesized that day length influences SK channel activity, thereby contributing to differences in neuronal excitability observed between Long, Equinox, and Short photoperiod conditions. Using multielectrode array recordings from DRN slices, we found a significant, dose‐dependent increase in spike rate upon application of the SK channel inhibitor apamin, indicating that SK channels indeed influence the spike rate of dorsal raphe serotonin neurons. In addition, whole‐cell recordings from DRN neurons in slices demonstrated that SK channel–mediated AHP currents were smaller in amplitude in Long photoperiod mice compared to Short and Equinox photoperiod mice. However, we found no significant differences in mRNA levels of the SK3 subunit ( Kcnn3 ) in DRN serotonin neurons across photoperiod conditions as determined by single‐molecule fluorescence in situ hybridization. Overall, these findings indicate that photoperiod modulates SK channel function in DRN serotonin neurons, possibly through posttranscriptional mechanisms, contributing to the increase in serotonin neuron excitability observed in long summer‐like photoperiods.