A comparison of similar ionic responses to gamma-aminobutyric acid and acetylcholine

1. Fast Na+-, Cl-, and K+-Conductance increase responses to gamma-aminobutyric acid (GABA) show times to peak similar to the comparable ionic responses to acetylcholine (ACh). 2. On some identified neurons, both putative transmitters elicit responses due to the same conductance change. For example, in cell R2 both substances cause an increase in Cl- conductance. Receptors for GABA and ACh on R2 do not cross desensitize and therefore are distinct. The ACh but not the GABA response is blocked by alpha-bungarotoxin and strychnine. 3. In R2 both responses reverse at -58 mV, and the Cl- ionophore (for both responses) appears to be partially permeant to propionate and isethionate, but impermeant to acetate, sulfate, and methylsulfate. 4. The Cl- responses but not the Na+ responses to both ACh and GABA are blocked by both picrotoxin and bicuculline, the classical GABA antagonists. 5. These results are compatible with the hypothesis that the ionophores associated with receptors to different neurotransmitters but mediating the same ionic conductance change have many common properties and may, in fact, be identical. Bicuculline and picrotoxin may be specific blockers of the Cl- ionophore, not the GABA receptor.