Pharmacological characterization of a novel cell line expressing human <em>α</em>₄<em>β</em>₃<em>δ</em> GABA_A receptors

The pharmacology of the stable cell line expressing human α₄β₃δ GABA_A receptor was investigated using whole-cell patch-clamp techniques.

α₄β₃δ receptors exhibited increased sensitivity to GABA when compared to α₄β₃γ₂ receptors, with EC₅₀'s of 0.50 (0.46, 0.53) μM and 2.6 (2.5, 2.6) μM 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 α₄β₃δ 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 α₄β₃δ and 2.5±0.2 s for α₄β₃γ₂. However, both P4S and THIP demonstrated similar levels of desensitization on both receptor subtypes suggesting this effect is agonist specific.

α₄β₃δ and α₄β₃γ₂ demonstrated equal sensitivity to inhibition by the cation zinc (2–3 μM IC₅₀). However, α₄β₃δ receptors demonstrated greater sensitivity to inhibition by lanthanum. The IC₅₀ for GABA antagonists SR-95531 and picrotoxin, was similar for α₄β₃δ and α₄β₃γ₂. Likewise, inhibition was observed on both subtypes at high and low pH.

α₄β₃δ receptors were insensitive to modulation by benzodiazepine ligands. In contrast Ro15-4513 and bretazenil potentiated GABA responses on α₄β₃γ₂ cells, and the inverse agonist DMCM showed allosteric inhibition of α₄β₃γ₂ receptors.

The efficacy of neurosteroids at α₄β₃δ receptors was greatly enhanced over that observed at α₄β₃γ₂ receptors. The greatest effect was observed using THDOC with 524±71.6% potentiation at α₄β₃δ and 297.9±49.7% at α₄β₃γ₂ 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 α₄β₃δ receptors versus α₄β₃γ₂ receptors.

We show that the α₄β₃δ 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.