Abstract

Binding of γ-amino[¹⁴C]butyric acid (GABA) to crayfish muscle particulate fractions was measured by equilibrium dialysis at 0° in Tris-buffered Van Harreveld's solution containing β-mercaptoethanol. Binding was found to saturate at about 7 pmoles/mg of particulate protein (200 pmoles/g of wet tissue), and the concentration of GABA giving halfmaximal binding was 1.3 ± 0.5 µM. A single class of binding sites exhibiting no cooperativity was apparently involved. GABA binding was insensitive to several treatments which would break cell membranes. It was partially inhibited by Na⁺-free conditions, but was less sensitive in this regard than membrane transport of GABA in either unhomogenized crayfish muscle or mouse brain particles. Binding, however, like transport, was inhibited by sulfhydryl reagents. Picrotoxin did not inhibit binding at 0.3 mM, but bicuculline was a weak inhibitor (K_[unknown] = 350 µM). Chlorpromazine and imipramine were also weak inhibitors. The quantity, affinity, and tissue location of the binding sites were thus consistent with either synaptic receptor sites or transport binding sites. However, the K_m value for GABA transport showed significantly lower affinity, 22 µM. Furthermore, comparison of inhibition potency for several GABA structural analogues revealed significant differences between binding and transport; in particular D-glutamic acid and imidazoleacetic acid inhibited binding but not transport; nipecotic acid inhibited transport but not binding. Reasonably good agreement in specificity was found for compounds active on GABA synaptic receptors and in inhibition of GABA binding. Although the concentration of GABA giving half-maximal binding is lower than the concentrations usually reported to give half-maximal synaptic responses, various explanations for this discrepancy are available. We conclude that these cell-free binding studies probably measure physiological GABA receptor sites, although high-affinity binding to sites involved in membrane transport or other biological activity of GABA could also contribute. Because of the lack of totally selective blocking agents, these sites cannot be unamibiguously distinguished at this time.