Regulation of cortical microcircuits by unitary GABAergic volume transmission

Abstract

Gamma-aminobutyric acid (GABA) is predominantly released by local interneurons in the cerebral cortex to particular subcellular domains of the target cells 1, 2. This suggests that compartmentalized, synapse specific action of GABA is required in cortical networks for phasic inhibition 2– 4. However, GABA released at the synaptic cleft diffuses to receptors outside the postsynaptic density and thus tonically activates extrasynaptic GABAA and GABAB receptors, which include subtypes of both receptor families especially sensitive to low concentrations of GABA 3– 7. The synaptic and extrasynaptic action of GABA is in line with idea that neurons of the brain use synaptic (or wiring) transmission and nonsynaptic (or volume) transmission for communication 8, 9. However, reuptake mechanisms restrict the spatial extent of extrasynaptic GABAergic effects 10, 11 and it was proposed that concerted action of several presynaptic interneurons or sustained firing of individual cells or increased release site density is required to reach ambient GABA levels sufficient to activate extrasynaptic receptors 4, 9, 11– 13. Here we show that individual neurogliaform cells release GABA sufficient for volume transmission within the axonal cloud and thus neurogliaform cells do not require synapses to produce inhibitory responses in the overwhelming majority of nearby neurons. Neurogliaform cells suppress connections between other neurons acting on presynaptic terminals which do not receive synapses at all in the cerebral cortex and, moreover, reach extrasynaptic, δ subunit containing GABAA (GABAA _δ) receptors responsible for tonic inhibition. We reveal that GABAA _δ receptors are localized to neurogliaform cells preferentially among cortical interneurons. Neurosteroids, which are modulators of GABAA _δ receptors, alter unitary GABAergic effects between neurogliaform cells. In contrast to the specifically placed synapses formed by other interneurons, the output of neurosteroid sensitive neurogliaform cells represents the ultimate form of spatial unspecificity in GABAergic systems leading to long lasting network hyperpolarization combined with widespread suppression of communication in the local circuit.