Displacement of extracellular chloride by immobile anionic constituents of the brain's extracellular matrix

Abstract

GABA is the primary inhibitory neurotransmitter. Membrane currents evoked by GABAA receptor activation have uniquely small driving forces: their reversal potential (EGABA) is very close to the resting membrane potential. As a consequence, GABAA currents can flow in either direction, depending on both the membrane potential and the local intra and extracellular concentrations of the primary permeant ion, chloride (Cl). Local cytoplasmic Cl concentrations vary widely due to displacement of mobile Cl ions by relatively immobile anions. Here we use new reporters of extracellular chloride (Cl-o) to demonstrate that Cl is displaced in the extracellular space by high and spatially heterogenous concentrations of immobile anions including sulfated glycosaminoglycans (sGAGs). Cl-o varies widely, and the mean Cl-o is only half the canonical concentration, i.e. the Cl concentration in the cerebrospinal fluid. These unexpectedly low and heterogenous Cl-o domains provide a mechanism to link the varied but highly stable distribution of sGAGs and other immobile anions in the brains extracellular space to neuronal signal processing via the effects on the amplitude and direction of GABAA transmembrane Cl currents. Key Points SummaryO_LIExtracellular chloride concentrations in the brain were measured using a new chloride-sensitive organic fluorophore and two photon fluorescence lifetime imaging. C_LIO_LIIn vivo, the extracellular chloride concentration was spatially heterogenous and only half of the CSF chloride concentration C_LIO_LIStable displacement of extracellular chloride by immobile extracellular anions was responsible for the low extracellular chloride concentration C_LIO_LIThe changes in extracellular chloride were of sufficient magnitude to alter the conductance and reversal potential of GABAA chloride currents C_LIO_LIThe stability of the extracellular matrix, the impact of the component immobile anions including sulfated glycosaminoglycans on extracellular chloride concentrations, and the consequent effect on GABAA signaling suggests a previously unappreciated mechanism for modulating GABAA signaling. C_LI