A study of subunit selectivity, mechanism and site of action of the delta selective compound 2 (DS2) at human recombinant and rodent native GABAA receptors

Background and Purpose Most GABAA receptor subtypes comprise 2, 2 and 1 subunit, although for some isoforms, a replaces a -subunit. Extrasynaptic -GABAA receptors are important therapeutic targets, but there are few suitable pharmacological tools. We profiled DS2, the purported positive allosteric modulator (PAM) of -GABAA receptors to better understand subtype selectivity, mechanism/site of action and activity at native -GABAA receptors. Experimental Approach Subunit specificity of DS2 was determined using electrophysiological recordings of Xenopus laevis oocytes expressing human recombinant GABAA receptor isoforms. Effects of DS2 on GABA concentrationresponse curves were assessed to define mechanisms of action. Radioligand binding and electrophysiology utilising mutant receptors and pharmacology were used to define site of action. Using brain-slice electrophysiology, we assessed the influence of DS2 on thalamic inhibition in wild-type and 0/0 mice. Key Results Actions of DS2 were primarily determined by the -subunit but were additionally influenced by the , but not the , subunit (4/6x > 1x >> 2-GABAA receptors > 43). For -GABAA receptors, DS2 enhanced maximum responses to GABA, with minimal influence on GABA potency. (iii) DS2 did not act via the orthosteric, or known modulatory sites on GABAA receptors. (iv) DS2 enhanced tonic currents of thalamocortical neurones from wild-type but not 0/0 mice. Conclusions and Implications DS2 is the first PAM selective for 4/6x receptors, providing a novel tool to investigate extrasynaptic -GABAA receptors. The effects of DS2 are mediated by an unknown site leading to GABAA receptor isoform selectivity.