Pharmacology of GABA A receptors exhibiting different levels of spontaneous activity

Abstract

The present study examines the spontaneous channel activity of GABA A receptors and the pharmacology of various full agonists (γ-aminobutyric acid (GABA), isoguvacine), partial agonists (4,5,6,7-tetrahydroisoxazolo-[5,4- c]pyridin-3-ol (THIP), piperidine-4-sulphonic acid (P4S), imidazole-4-acetic acid), competitive antagonists (bicuculline, 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide (SR95531)) and non-competitive antagonists (picrotoxinin, zinc). Experiments were performed on oocytes separately expressing human α 1β 2γ 2S, α 1β 3ε and α 1β 2(L259S)γ 2S receptors using two-electrode voltage clamp electrophysiology. Quantifying spontaneous channel activity showed this varied significantly between the α 1β 2γ 2S (0.2±0.07%), α 1β 3ε (20±3%) and α 1β 2(L259S)γ 2S (83±4%) receptors. A direct correlation was found between the relative agonist potencies and the level of spontaneous activity of the GABA A receptors. Furthermore, the maximum responses for partial agonists were increased as a function of increased levels of spontaneous activity. There was no relationship between the potency/efficacy of competitive antagonists and the degree of spontaneous activity. However, the non-competitive allosteric inhibitor picrotoxinin showed an opposite dependence on spontaneous activity compared to that seen for agonists, whereas zinc showed a more complex dependence on the receptor subunit composition. These novel findings indicate that the potency and efficacy of ligands acting on GABA A receptors are highly dependent on the level of spontaneous activity of the receptor.