iGABASnFR Imaging Reveals Diffusion-Driven GABA Clearance in the Cerebral Cortex

GABAergic signaling consists of presynaptic release, synaptic and extra-synaptic receptor activation, and signal termination by one of several mechanisms including receptor desensitization, diffusion of GABA, and active clearance by GABA transporters (GATs). Understanding how long GABA is free in the extracellular space is key to understanding how inhibition controls activity but has been technically challenging. Estimates of GABA's persistence from GABA _A and GABA _B recordings range from tens of milliseconds to multiple seconds, but may reflect receptor properties rather than extracellular GABA dynamics. Using the fluorescent GABA sensor, iGABASnFr, in the mouse cerebral cortex, we show that GABA rapidly disperses (tens of milliseconds) from sites of release sites via diffusion rather than GATs. This GABA then accumulates in the extracellular space, where GATs require hundreds of milliseconds to remove extracellular GABA following its release, and even longer when the local density of GABA release is elevated. This extracellular summation of GABA can act hetero-synaptically by activating and/or desensitizing both extra-synaptic and neighboring synaptic receptors. Together these findings reveal a stark disconnect between phasic IPSC signals, which rapidly depress, and extracellular GABA, which strongly accumulates, raising new questions about how GABAergic inhibition works to shape network function.