Activation of human alpha 1 and alpha 2 homomeric glycine receptors by taurine and GABA

1. Two ligand binding a subunits, alphat and alpha2, of the human (H) glyci ne receptor (GlyR) are involved at inhibitory synapses in the adult and neo natal spinal cord, respectively. The ability of homomeric alpha H1 and alph a H2 GlyRs to be activated by glycine, taurine and GABA was studied in Xeno pus oocytes or in the human embryonic kidney HEK-293 cell line. 2. In outside-out patches from HEK cells, glycine, taurine and GABA activat ed both GlyRs with the same main unitary conductance, i.e. 85 +/- 3 pS (n = 6) for alpha H1, and 95 +/- 5 pS (n = 4) for alpha H2. 3. The sensitivity of both alpha H1 and alpha H2 GlyRs to glycine was highl y variable. In Xenopus oocytes the EC50 for glycine (EC50gly) was between 2 5 and 280 mum for alpha H1 (n = 44) and between 46 and 541 mum for alpha H2 (n = 52). For both receptors, the highest EC50gly values were found on cel ls with 3 low maximal glycine responses. 4. The actions of taurine and GABA were dependent on the EC50gly: (i) their EC50 values were linearly correlated to EC50gly, with EC50tau approximate to 10 EC50gly and EC50GABA approximate to 500-800 EC50gly; (ii) they could act either as full or weak agonists depending on the EC50gly. 5. The Hill coefficient (n(H)) of glycine remained stable regardless of the EC50gly whereas n(H) for taurine decreased with increasing EC50tau. 6. The degree of desensitization, evaluated by fast application of saturati ng concentrations of n agonist on outside-out patches from Xenopus oocytes, was similar for glycine and taurine on both GlyRs and did not exceed 50 %. 7. Our data concerning the variations of EC50gly and the subsequent behavio ur of taurine and GABA could be qualitatively described by the simple del C astillo-Katz scheme, assuming that the agonist gating constant varies where as the binding constants are stable. However, the stability of the Hill coe fficient for glycine, was not explained by this model, suggesting that othe r mechanisms are involved in the modulation of EC50.