- 1. The cytosolic concentration of a neurotransmitter is believed to be an important factor determining its release. The effects of 3-mercaptopropionic acid (MP) and aminooxyacetic acid (AOAA), glutamate decarboxylase (GAD) blockers, on GABAergic postsynaptic and presynaptic inhibitory neurotransmission were examined in the crayfish (Procambarus clarkii) opener neuromuscular synapses. 2. Intracellular recordings of evoked excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) as well as loose macropatch clamp measurements of excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) were used to evaluate the effects of the drugs, which were applied exclusively to the nerve bundle. 3. Under normal conditions, a stimulus train to the inhibitor preceding the excitor stimulation elicited a large reduction in EPSP amplitude in a time interval-dependent manner. This inhibition is effected by postsynaptic as well as presynaptic processes. 4. Treatment with MP or AOAA decreased the IPSP amplitude and its altered conductance but had no effect on the IPSP reversal potential or the resting potential of the cell. They did, however, slightly increase the Rin of the fiber. 5. Quantal analysis of single IPSCs revealed that GAD blockers increased the number of failures and thus reduced quantal content (m), diminished the probability of release (p), but did not affect the quantum current (q) or the statistical parameter (n), believed to be the number of available active zones. 6. Quantal analysis of EPSCs, released after interaction with the inhibitor, revealed a reduction in m without any effect on q. GAD blockers greatly reduced the efficacy of this inhibition without affecting the EPSC q. 7. GAD blockers increased the output of the excitor release sites by the following mechanisms: 1) increased EPSC, 2) increased EPSC facilitation, or 3) enhancement of spontaneous activity (miniature EPSCs). 8. Short time incubation with picrotoxin and CGP-35348 eliminated IPSCs and evoked inhibition. However, longer exposure (90 min) increased the excitor responses, similarly to the effects of GAD blockers. 9. Baclofen, a gamma-aminobutyric acid-B (GABAB) agonist, antagonized AOAA effects on evoked inhibition. 10. These results demonstrate that GAD blockers decrease postsynaptic and presynaptic inhibition by reducing both tonic and evoked release, most likely by diminishing p. 11. The reduction in GABA synthesis and release revealed a complex mechanism for GABAergic metabotropic regulation of inhibition efficacy and the release from the excitor glutamatergic terminals.