GABA responses and their partial occlusion by glycine in cultured rat medullary neurons

Whole-cell current responses to bath application of GABA and glycine were studied in medullary neurons cultured from embryonic rats. Two current components were seen in the responses to bath application of GABA, one component which desensitized and another which did not. These two current components have different dose-response characteristics for GABA, with the nondesensitizing component being activated more effectively and reaching its peak amplitude at lower agonist concentrations than the desensitizing one. The agonist concentrations producing half of the maximum responses are 2.8 ± 0.3 (±S.E.M., n = 9) and 14.7 ± 2.7 ( n = 5) μM for the nondesensitizing and desensitizing components, respectively. The two current components for GABA are differentially affected by the antagonists, picrotoxin and bicuculline. The antagonist concentrations which block 50% of the control desensitizing and nondesensitizing responses to GABA are 33 and 320 μM for picrotoxin, and 3 and 50 μM for bicuculline, respectively. Thus, the characteristics of the GABA responses are analagous to those described previously for glycine in that there are two components which are differentially sensitive to agonist concentration [Lewis et al. (1991) J. Neurophysiol. 40, 1178–1187]. We now find there is occlusion between the responses to GABA and glycine, indicating that they share a population of receptors or channels. The occlusion was incomplete (