Abstract
Changes in the effective membrane resistance of the abductor muscle of the dactylopodite of the crayfish were used to indicate changes in the GABA concentration in the synaptic cleft. Following bath application of GABA (10^β5^ to 5 Γ 10^β5^M), the muscle membrane resistance decreased and then increased slowly over the next few minutes. Renewing the solution or stirring the bath restored the GABA effect. Higher GABA concentrations produced a large stable decrease in membrane resistance. An active uptake system for GABA in the junctional region is suggested by the observation that the slow increase in membrane resistance following GABA application was decreased by cooling to 2Β°C or by the addition of known GABA uptake blockers such as LβDABA, Ξ²βguanidinopropionic acid, or nipecotic acid. The transport inhibitors, PCMBS and chlorpromazine, produced irreversible decreases in muscle membrane resistance, which precluded examining their effects on GABA inactivation. The decrease in GABA effect was not dependent on the external sodium concentration or on the degree of receptor activation. Nipecotic acid, which blocked GABA inactivation, did not affect the decay of the neurally evoked inhibitory junctional potential.