γ‐AMINOBUTYRIC ACID RECEPTOR BINDING and UPTAKE IN MEMBRANE FRACTIONS OF CRAYFISH MUSCLE

The uptake and binding of [ 3 H]GABA and the binding of [ 3 H]muscimol were measured in cell‐free fractions of crayfish muscle. The uptake of GABA was saturable, of high affinity ( K m = 0.5μ m ), and inhibited by low concentrations of compounds believed to block GABA uptake specifically, such as nipecotic acid and 2,4,diaminobutyric acid. The GABA uptake activity was localized to sucrose gradient fractions enriched in sarcolemma as demonstrated by marker enzymes and electron microscopy. The binding of the potent GABAergic agonist muscimol was also localized to the sarcolemma. The binding was saturable, of high affinity (K D = 9 n m ), and inhibited by GABA (K 1 = 125 n m ) and by low concentrations of receptor‐specific GABA analogues, such as isoguvacine, imidazole acetic acid, and 3‐aminopropane sulfonic acid. The rank order for inhibition by GABA analogues of [ 3 H]muscimol binding sites correlated very well with activity on GABA synapses in invertebrates, consistent with specific postsynaptic receptor labeling.