Probing α4βδ GABAA Receptor Heterogeneity: Differential Regional Effects of a Functionally Selective α4 β1 δ/α4β3δ Receptor Agonist on Tonic and Phasic Inhibition in Rat Brain

In the present study, the orthosteric GABA(A) receptor (GABA(A)R) ligand 4,5,6,7-tetrahydroisothiazolo[5,4-c] pyridin-3-ol (Thio-THIP) was found to possess a highly interesting functional profile at recombinant human GABA(A)Rs and native rat GABAARs. Whereas Thio-THIP displayed weak antagonist activity at alpha(1,2,5)beta(2,3),gamma(2 delta) and (rho 1) GABA(A)Rs and partial agonism at alpha(6)beta(2,3)delta GABA(A)Rs expressed in Xenopus oocytes, the pronounced agonism exhibited by the compound at alpha(4)beta(1)delta and alpha(4)beta(3)delta GABA(A)Rs was contrasted by its negligible activity at the alpha(1)beta(2)delta subtype. To elucidate to which extent this in vitro profile translated into functionality at native GABAARs, we assessed the effects of 100 mu m Thio-THIP at synaptic and extrasynaptic receptors in principal cells of four different brain regions by slice electrophysiology. In concordance with its alpha beta(2,3)delta agonism, Thio-THIP evoked robust currents through extrasynaptic GABAARs in cerebellar granule cells. In contrast, the compound did not elicit significant currents in dentate gyrus granule cells or in striatal medium spiny neurons (MSNs), indicating predominant expression of extrasynaptic alpha(4)beta(2)delta receptors in these cells. Interestingly, Thio-THIP evoked differential degrees of currents in ventrobasal thalamus neurons, a diversity that could arise from differential expression of extrasynaptic alpha(4)beta delta subtypes in the cells. Finally, whereas 100 mu M Thio-THIP did not affect the synaptic currents in ventrobasal thalamus neurons or striatal MSNs, it reduced the current amplitudes recorded from dentate gyrus granule cells, most likely by targeting perisynaptic alpha(4)beta delta receptors expressed at distal dendrites of these cells. Being the first published ligand capable of discriminating between beta(2)- and beta(3)- containing receptor subtypes, Thio-THIP could be a valuable tool in explorations of native alpha(4) beta delta GABA(A)Rs.