Intrinsic Cell-Class–Specific Modulation of Intracellular Chloride Levels and Inhibitory Function, in Cortical Networks, between Day and Night

Recent work showed unexpectedly large, daily modulation of intracellular chloride concentration ([Cl^-]_in) in cortical pyramidal cells, with consequences for GABAergic function and network excitability ( Alfonsa et al., 2023; Pracucci et al., 2023). One explanation for this [Cl^-]_in modulation is that it arises from variation in presynaptic drive. In that case, neuronal classes with similar synaptic inputs should show correlated changes in activity-dependent ionic redistribution. To examine this prediction, we performed in vivo, LSSm-ClopHensor imaging to measure [Cl^-]_in and pH_in in populations of parvalbumin- (PV) and somatostatin (SST)-expressing interneurons in neocortical Layer 2/3 of male and female adult mice. Imaging was performed at zeitgeber time (ZT)5 and ZT17, when pyramidal cell [Cl^-]_in shows maximal divergence ( Pracucci et al., 2023). Interestingly, PV interneurons also showed large physiological [Cl^-]_in modulation between these times but out-of-phase with that in pyramidal cells, being raised at ZT5 and lower at ZT17, and with a far higher mean [Cl^-]_in SST interneurons showed less modulation, with higher variance, and with a temporal dynamic resembling the pyramidal cell pattern. Notably, in vitro experimental assays of inhibition, involving these two classes of interneuron, differed markedly at ZT5 and ZT17. The persistence of these time-of-day effects in vitro and the difference in [Cl^-]_in dynamics between pyramidal cells and PV interneurons in vivo both point toward cell-intrinsic regulation being more important than activity-dependent effects in setting these slow, daily, physiological, ionic redistribution patterns. We discuss what other possible factors may influence variations in brain state through the day.