Pharmacological characterization of a novel cell line expressing human α4β3δ GABAA receptors

Abstract

The pharmacology of the stable cell line expressing human α 4 β 3 δ GABA A receptor was investigated using whole‐cell patch‐clamp techniques. α 4 β 3 δ receptors exhibited increased sensitivity to GABA when compared to α 4 β 3 γ 2 receptors, with EC 50's of 0.50 (0.46, 0.53) μ M and 2.6 (2.5, 2.6) μ M respectively. Additionally, the GABA partial agonists piperidine‐4‐sulphonate (P4S) and 4,5,6,7‐tetrahydroisothiazolo‐[5,4‐c]pyridin‐3‐ol (THIP) displayed markedly higher efficacy at α 4 β 3 δ receptors, indeed THIP demonstrated greater efficacy than GABA at these receptors. The δ subunit conferred slow desensitization to GABA, with rate constants of 4.8±0.5 s for α 4 β 3 δ and 2.5±0.2 s for α 4 β 3 γ 2. However, both P4S and THIP demonstrated similar levels of desensitization on both receptor subtypes suggesting this effect is agonist specific. α 4 β 3 δ and α 4 β 3 γ 2 demonstrated equal sensitivity to inhibition by the cation zinc (2–3 μ M IC 50). However, α 4 β 3 δ receptors demonstrated greater sensitivity to inhibition by lanthanum. The IC 50 for GABA antagonists SR‐95531 and picrotoxin, was similar for α 4 β 3 δ and α 4 β 3 γ 2. Likewise, inhibition was observed on both subtypes at high and low pH. α 4 β 3 δ receptors were insensitive to modulation by benzodiazepine ligands. In contrast Ro15‐4513 and bretazenil potentiated GABA responses on α 4 β 3 γ 2 cells, and the inverse agonist DMCM showed allosteric inhibition of α 4 β 3 γ 2 receptors. The efficacy of neurosteroids at α 4 β 3 δ receptors was greatly enhanced over that observed at α 4 β 3 γ 2 receptors. The greatest effect was observed using THDOC with 524±71.6% potentiation at α 4 β 3 δ and 297.9±49.7% at α 4 β 3 γ 2 receptors. Inhibition by the steroid pregnenolone sulphate however, showed no subtype selectivity. The efficacy of both pentobarbitone and propofol was slightly augmented and etomidate greatly enhanced at α 4 β 3 δ receptors versus α 4 β 3 γ 2 receptors. We show that the α 4 β 3 δ receptor has a distinct pharmacology and kinetic profile. With its restricted distribution within the brain and unique pharmacology this receptor may play an important role in the action of neurosteroids and anaesthetics. British Journal of Pharmacology (2002) 136, 965–974. doi:10.1038/sj.bjp.0704795