DIFFERENTIAL ACTIVATION OF 2 DISTINCT MECHANISMS FOR PRESYNAPTIC INHIBITION BY A SINGLE INHIBITORY AXON

1. Presynaptic inhibition of excitatory transmitter release evoked by inhibitory axon stimulation was studied at individual release boutons of the crayfish opener neuromuscular junction. 2. Presynaptic inhibiti on was maximal (similar to 30%) when a single inhibitory action potent ial preceded the excitatory test action potential by 1-2 ms. This inhi bition lasted at most 5 ms. It was blocked by 50 mu M picrotoxin, and is probably mediated mainly by gamma-aminobutyric acid-A (GABA(A)) rec eptors. 3. Presynaptic inhibition produced by a brief train of inhibit ory action potentials (5 pulses at 100 Hz) was maximal (similar to 60% ) when the last inhibitory action potential (of the train) preceded th e excitatory test action potential by 10 ms. This inhibition lasted up to 50 ms. It seems that in this case GABA(B) receptors were activated as well, because the combined action of picrotoxin (50 mu M) and 20H- Saclofen (100 mu M) was required to block the inhibition. 4. We thus s how that one and the same inhibitory release bouton can differentially activate two distinct mechanisms for presynaptic inhibition by activa ting GABA(A) and GABA(B) receptors.