Quantitative fluorescence lifetime imaging uncovers a novel role for KCC2 chloride transport in dendritic microdomains

Intracellular chloride ion ([Cl]) homeostasis is critical for synaptic neurotransmission yet variations in subcellular domains are poorly understood owing to difficulties in obtaining quantitative, high-resolution measurements of dendritic [Cl]. We combined whole-cell patch clamp electrophysiology with simultaneous fluorescence lifetime imaging (FLIM) of the Cl dye MQAE to quantitatively map dendritic Cl levels in normal or pathological conditions. FLIM-based [Cl] estimates were corroborated by Rubi-GABA uncaging to measured E. Low baseline [Cl] in dendrites required Cl efflux via the K-Cl cotransporter KCC2 (). In contrast, pathological NMDA application generated spatially heterogeneous subdomains of high [Cl] that created dendritic blebs, a signature of ischemic stroke. These discrete regions of high [Cl] were caused by reversed KCC2 transport. Therefore monitoring [Cl] microdomains with a new high resolution FLIM-based technique identified novel roles for KCC2-dependent chloride transport to generate dendritic microdomains with implications for disease.