Comparison of glycine and GABA actions on the zebrafish homomeric glycine receptor.

1. Glycine and GABA can be co-released from the same presynaptic terminals and in lower vertebrates they can activate the same glycine receptors (GlyRs). Thus we examined the effects of these two inhibitory transmitters on the homomeric GlyRs formed by the alphaZ1 subunit, of the zebrafish using two expression systems: Xenopus oocytes and the human BOSC 23 cell line. 2. The apparent affinity (EC50) of alphaZ1 for these neurotransmitters was highly variable. In Xenopus oocytes the EC50 ranged from 37 to 360 microM (mean +/- s. d. EC50 116 +/- 75 microM, n = 83) for glycine and from 8 to 120 mM (mean EC50 40 +/- 30 mM, n = 37) for GABA. 3. In BOSC cells the EC50 varied from 9 to 92 microM (mean EC50 33 +/- 17 microM, n = 19) and from 0.7 to 19.1 mM (mean EC50 4.9 +/- 4.7 mM, n = 29) for glycine and GABA, respectively. 4. GABA activated alphaZ1 GlyRs either as a weak or full agonist: its efficacy (defined as Imax,GABA/Imax,Gly) was related to EC50 by an exponential relationship. A linear relationship was observed between EC50 values for GABA and glycine. 5. In outside-out patches, GABA and glycine activated alphaZ1 with identical single-channel conductances (85-100 pS), but with different kinetics and marked effect of concentration on burst duration for glycine only. 6. In outside-out patches deactivation time constants were concentration dependent for glycine, but not for GABA. 7. Our data demonstrate that the kinetics of glycine and GABA interactions with alphaZ1 are different and that they determine the properties of these neurotransmitter actions on the GlyR.