α₁β₂δ, a silent GABA_A receptor: recruitment by tracazolate and neurosteroids

Abstract

Background and purpose: This study investigated the a 1 b 2 d isoform of the GABA A receptor that is presumably expressed in the forebrain. The functional and pharmacological properties of this receptor combination are largely unknown. Experimental approach: We expressed a 1 b 2 d GABA A receptors in Xenopus laevis oocytes. GABA-activated currents, in the presence and absence of modulators, were recorded using the two-electrode voltage clamp technique. Key results: The a 1 b 2 d isoform of the GABA A receptor exhibited an extremely small GABA-mediated current. Tracazolate increased the current amplitude evoked by a half-maximal concentration (EC 50 ) of GABA by 59-fold. The maximum current was increased 23-fold in the presence of a saturating GABA concentration. Concomitant with the increase in the maximum, was a 4-fold decrease in the EC 50 . Finally, a mutation in the second transmembrane domain of the d subunit that increases receptor efficacy (L286S), eliminated the increase in the maximum GABA-activated current. The endogenous neurosteroid, tetrahydrodeoxycorticosterone (THDOC), also decreased the EC 50 and increased the maximum current amplitude, although to a lesser degree than that of tracazolate. Conclusions and implications: Taken all together, these findings indicate that the small GABA-mediated currents in the absence of the modulator are due to a low efficacy for activation. In the absence of modulators, a 1 b 2 d GABA receptors would be effectively silent and therefore contribute little to inhibition in the CNS. In the presence of tracazolate or endogenous neurosteroids however, this particular receptor isoform could exert a profound inhibitory influence on neuronal activity.

Estrous Cycle Regulation of Extrasynaptic δ-Containing GABAA Receptor-Mediated Tonic Inhibition and Limbic Epileptogenesis

“…a1bd receptors possess low desensitization kinetics and neurosteroid sensitivity similar to a4bd (Bianchi and Macdonald, 2003;Zheleznova et al, 2008). The a1 could plausibly assemble with d extrasynaptically.…”

Neurosteroid Structure-Activity Relationships for Functional Activation of Extrasynaptic GABAA Receptors

“…Molecular layer (ML) interneurons express the a1bd isoform extrasynaptically, but the contribution to network tonic conductance is not very well understood (Sun et al, 2004;Glykys et al, 2007). The a1bd receptor isoform exhibits very little GABA-mediated current, but it can be significantly potentiated by neurosteroids (Zheleznova et al, 2008(Zheleznova et al, , 2009). To better understand neurosteroid sensitivity of d-containing extrasynaptic receptors, we compared tonic currents between WT DGGCs and ML interneurons in the slice (Fig.…”

Diversity of Structure and Function of α1α6β3δ GABAA Receptors

“…5 summarizes the effects of co-application of 7.5, 15, 30, or 45 mM ethanol and a 30-s preapplication of 45 mM ethanol before the addition of GABA on the functional receptors. All measurements were carried out at a GABA concentration eliciting EC 20 . However, we observed that none of the four receptors ␤ 3 -␣ 1 -␦/␣ 6 -␤ 3 (R12), ␤ 3 -␣ 6 -␦/␣ 1 -␤ 3 (R13), ␣ 6 -␤ 3 -␣ 1 /␤ 3 -␦ (R23), and ␤ 3 -␣ 1 -␦/ ␤ 3 -␣ 6 (R52) was modulated by 30 mM ethanol, a concentration corresponding to 1.38‰ (w/v).…”