γ-Aminobutyric acid (GABA)- and barbiturate-mediated36
Cl-uptake in rat brain synaptoneurosomes: Evidence for rapid desensitization of the GABA receptor-coupled chloride ion channel
'Desensitization' of the γ-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, stimulated36Cl-uptake in rat cerebral cortical synaptoneurosomes in a concentration-dependent manner (EC507.3 ± 0.5 μM), whereas pentobarbital stimulated36Cl-uptake in a biphasic manner, indicated by a bell-shaped concentration-response relationship, with a maximal response at 500 μM (EC50271 ± 17 μM). Higher concentrations of pentobarbital led to progressively smaller stimulation of36Cl-uptake and blocked muscimol-stimulated36Cl-uptake. Lower concentrations of pentobarbital (100-200 μM), when added with muscimol, produced an additive effect in stimulating36Cl-uptake, whereas even lower (subthreshold) concentrations of pentobarbital (50 μM) potentiated muscimol-stimulated36Cl-uptake. Following continuous exposure of synaptoneurosomes (up to 20 min) to muscimol (50 μM) or pentobarbital (500 μM), the36Cl-uptake response diminished to a new steady state level with a t1 of ~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-stimulated36Cl-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.
Schwartz, RD; Suzdak, PD; Paul, SM
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