The pharmacology of the stable cell line expressing human α4β3δ GABAA 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.
British Journal of Pharmacology (2002) 136, 965–974. doi:10.1038/sj.bjp.0704795