gamma-Aminobutyric acid (GABA)- and barbiturate-mediated 36Cl- uptake in rat brain synaptoneurosomes: evidence for rapid desensitization of the GABA receptor-coupled chloride ion channel.

"Desensitization" of the gamma-aminobutyric acid (GABA) receptor-coupled chloride ion channel was studied using an in vitro method for measuring chloride (Cl-) permeability in brain vesicles (synaptoneurosomes). Muscimol, a GABA agonist, stimulated 36Cl- uptake in rat cerebral cortical synaptoneurosomes in a concentration-dependent manner (EC50 7.3 +/- 0.5 microM), whereas pentobarbital stimulated 36Cl- uptake in a biphasic manner, indicated by a bell-shaped concentration-response relationship, with a maximal response at 500 microM (EC50 271 +/- 17 microM). Higher concentrations of pentobarbital led to progressively smaller stimulation of 36Cl- uptake and blocked muscimol-stimulated 36Cl- uptake. Lower concentrations of pentobarbital (100-200 microM), when added with muscimol, produced an additive effect in stimulating 36Cl- uptake, whereas even lower (subthreshold) concentrations of pentobarbital (50 microM) potentiated muscimol-stimulated 36Cl- uptake. Following continuous exposure of synaptoneurosomes (up to 20 min) to muscimol (50 microM) or pentobarbital (500 microM), the 36Cl- uptake response diminished to a new steady state level with a t1/2 of approximately 6 sec and 30 sec, respectively. The decrement in response to these agonists was dependent on both concentration and length of exposure. No decrement was observed in the ability of subthreshold concentrations of pentobarbital to enhance muscimol-stimulated 36Cl- uptake following prolonged (20 min) incubation. "Heterologous desensitization" between muscimol and pentobarbital was observed in experiments where either muscimol or pentobarbital was added to the vesicles following pretreatment with the other. These findings suggest that "desensitization" of the GABA receptor/Cl- ion channel may involve both the GABA and barbiturate recognition sites or a common effector component such as the ionophore itself.