The heterogeneity in GABAA receptor-mediated IPSC kinetics reflects heterogeneity of subunit composition among inhibitory and excitatory interneurons in spinal lamina II

GABAergic inhibition displays rich functional diversity throughout the CNS, which arises from variations in the nature of inputs, subunit composition, subcellular localization of receptors and synapse geometry, or reuptake mechanisms. In the spinal dorsal horn (SDH), GABA(A) and glycine receptors play a major role in the control of excitability and accuracy of nociceptive processing. Identifying which components shape the properties of the inhibitory synapses in different cell types is necessary to understand how nociceptive information is integrated. To address this, we used transgenic mice where inhibitory interneurons express GAD65-EGFP. We found that GABA(A), but not glycine receptor-mediated evoked IPSCs displayed slower kinetics in EGFP+ vs. EGFP- interneurons. GABA(A) miniature IPSC decay kinetics showed a large variability in both populations, however the distribution of decays differed between EGFP+ and EGFP- interneurons. The range of mIPSC decay kinetics observed was replicated in experiments using rapid application of GABA on outside-out patches taken from SDH neurons in slices. Furthermore, GABA(A) decay kinetics were not affected by uptake blockers and were not different in mice lacking delta or alpha 5 subunits, indicating that intrinsic channel properties likely underlie the heterogeneity. To identify whether other a subunits shape the various kinetic properties observed we took advantage of knock-in mice carrying point mutations in either the alpha 1, alpha 2, or alpha 3 subunits rendering Ro 15-4513 a selective agonist at the benzodiazepine modulatory site. We found that alpha 1 and alpha 2 subunit underlie the fast decaying component of IPSCs while the slow component is determined by the alpha 3 subunit. The differential distribution of GABA(A) subunits at inhibitory synapses thus sculpts the heterogeneity of the SDH inhibitory circuitry. This diversity of inhibitory elements can be harnessed to selectively modulate different components of the spinal nociceptive circuitry for therapeutic interventions.