GABA pharmacology of the glycine receptor

Glycine receptors (GlyR) belong to the ligand-gated ion channel superfamily. GlyR is a pentamer made of two glycosylated integral membrane proteins: alpha (48kDa) and beta (58kDa). Four alpha subunits have been discovered and each can bind glycine and produce a chloride current. The beta subunit alone does not form functional GlyRs.; Glycinergic and GABAergic inhibition are juxtaposed at one synaptic layer in the vertebrate retina. We explored the specificity of GABA antagonists when used at commonly employed concentrations: 50 muM picrotoxinin, 100 muM bicuculline, 20 muM SR95531 (gabazine), 100 muM imidazole-4-acetic acid (I4AA) or 50 muM TPMPA (1, 2, 5, 6-tetrahydropyridine-4-yl) methylphosphinic acid).; In retinal neurons, several GABA antagonists were competitive glycine receptor antagonists. Glycine EC50 (39 muM) was shifted by bicuculline (to 81 muM) > SR95531 (to 72 muM) ≈ picrotoxinin (to 71 muM). Picrotoxinin had the largest effect due to an additional non-competitive inhibition. The response to 100 muM glycine was reduced by picrotoxinin (46%) > bicuculline (38%) > SR95531 (29%). GABAC receptor antagonists, TPMPA and 14AA, had little effect on the glycine current in retina.; These antagonists were also tested on the responses of homomeric receptors formed of alpha1 or alpha2 glycine subunits expressed in HEK293 cells. All antagonists were competitive antagonists. At al glycine receptors, the glycine EC50 (60 muM) was shifted by picrotoxinin (to 108 muM) > SR95531 (to 80 muM) > I4AA (to 68 muM). TPMPA and bicuculline were ineffective. At alpha2 glycine receptors, the glycine EC50 (111 muM) was shifted by picrotoxinin (to 357 muM) > bicuculline (to 174 muM) > I4AA (to 160 muM) > SR95531 (to 151 muM). TPMPA was ineffective. GABA receptor antagonists were more effective at homomeric alpha2 receptors, bicuculline had the biggest alpha1/alpha2 differential effect.; Two amino acids in the second transmembrane region (M2) of GABA receptors, 2' serine and 6' threonine, have been linked with picrotoxinin inhibition. Though M2 regions in GABA and glycine receptors are highly homologous, neither 2' serine or 6' threonine is essential for picrotoxinin inhibition in glycine receptors. Thus, several antagonists act at both GABA and glycine receptors, although the mechanisms of inhibition differ.