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

Intracellular chloride ion ([Cl-]i) 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-]i. 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-]i estimates were corroborated by Rubi-GABA uncaging to measured EGABA. Low baseline [Cl-]i in dendrites required Cl- efflux via the K+-Cl- cotransporter KCC2 (SLC12A5). In contrast, pathological NMDA application generated spatially heterogeneous subdomains of high [Cl-]i that created dendritic blebs, a signature of ischemic stroke. These discrete regions of high [Cl-]i were caused by reversed KCC2 transport. Therefore monitoring [Cl-]i 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.