Is glutamate the transmitter of crustacean motoneurons?

1. Bath-application of L-glutamate to crayfish opener muscle causes depolarization and resistance changes which both increase with falling temperature. At temperatures above 15 degrees C there is usually a resistance increase, at lower temperatures the resistance is decreased. 2. Meso-gamma . gamma'-diaminosuberic acid-dihydrochloride (meso-di-GABA) and dl-diamino-nonanedicarboxylic acid dihydrochloride (C-9) were newly synthesized as potential glutamate blockers. 3. Meso-di-GABA (10(-4) to 10(-3)M) usually caused a significant increase (15 degrees C) or decrease (7 degrees C) of membrane resistance and slight depolarization. Excitatory junction potentials (ejps) were reversibly depressed or blocked while the effects of glutamate were potentiated. The depression or block of neuromuscular transmission was not prevented by picrotoxin or by concanavaline A. 4. C-9 (3 x 10(-4) M) depressed or blocked the effect of applied glutamate with little or no effect on ejps. 5. The results are best explained by assuming that bath-applied glutamate acts mainly on extrasynaptic receptors. Meso-di-GABA is assumed to block synaptic receptors and to activate non-synaptic receptors while C-9 seems to act mainly as a blocker of glutamate action on non-synaptic receptors.