Using SuperClomeleon to measure changes in intracellular chloride during development and after early life stress.

Intraneuronal chloride concentrations ([Cl - ] i ) decrease during development resulting in a shift from depolarizing to hyperpolarizing γ-aminobutyric acid (GABA) responses via chloride-permeable GABA A receptors. This GABA shift plays a pivotal role in postnatal brain development, and can be strongly influenced by early life experience. Here, we assessed the applicability of the recently developed fluorescent SuperClomeleon (SClm) sensor to examine changes in [Cl - ] i using two-photon microscopy in brain slices. We used SClm mice of both sexes to monitor the developmental decrease in neuronal chloride levels in organotypic hippocampal cultures. We could discern a clear reduction in [Cl - ] i between DIV3 and DIV9 (equivalent to the second postnatal week in vivo ) and a further decrease in some cells until DIV22. In addition, we assessed alterations in [Cl - ] i in the medial prefrontal cortex (mPFC) of P9 male SClm mouse pups after early life stress (ELS). ELS was induced by limiting nesting material between P2 and P9. ELS induced a shift towards higher (i.e. immature) chloride levels in layer 2/3 cells in the mPFC. Although conversion from SClm fluorescence to absolute chloride concentrations proved difficult, our study underscores that the SClm sensor is a powerful tool to measure physiological changes in [Cl - ] i in brain slices. Significance Statement The reduction of intraneuronal chloride concentrations is crucial for brain development, as it ensures a shift from the initial excitatory action of the neurotransmitter GABA in immature neurons to the inhibitory GABA signaling in the adult brain. Despite the significance of chloride maturation, it has been difficult to study this phenomenon in experiments. Recent development of chloride sensors enable direct imaging of intracellular chloride signaling in neurons. Here we assessed the applicability of the SuperClomeleon chloride sensor to measure physiologically relevant changes in chloride levels using two-photon microscopy in cultured and acute brain slices. Although we also point out some limitations, we conclude that the SuperClomeleon sensor is a powerful tool to measure physiological changes in intracellular chloride.