Aspartate and glutamate as possible transmitters at the ‘slow’ and ‘fast’ neuromuscular junctions of the body wall muscles ofMusca larvae

1. Body wall muscles 6A and 7A ofMusca larvae are each dually innervated by a single fast and a single slow axon. Perfusion of 1 mM E-glutamate abolished the neurally evoked fast EPSP but not the slow EPSP, while 1 mM aspartate abolished the slow EPSP but not the fast EPSP. 2. Iontophoresis of L-glutamate or aspartate from single or multibarreled micropipettes produced potentials when ejected at localised sites on the muscle surface. These sites were identified as synaptic terminals by recording extracellular synaptic currents and the ability to evoke antidromic nerve impulses and EPSPs upon focal stimulation. 3. The site of action of iontophoretically applied E-glutamate corresponded to the fast neuromuscular junction, while that of aspartate corresponded to the slow neuromuscular junction. The conduction velocity of antidromic impulses evoked by focal stimulation was lower in slow axons than in fast axons. Sensitive sites could therefore be unequivocally identified as fast or slow synapses. 4. The two types of receptor (glutamate-preferring and aspartate-preferring) were agonist specific, although each agonist would act as an antagonist of the other receptor type. 5. The ionic fluxes of the fast EPSP were the same as for L-glutamate potentials, and the ionic fluxes of the slow EPSP were the same as for aspartate potentials. In conditions where comparisons were possible, there was no significant difference between the reversal potentials of iontophoretically applied L-glutamate and the neurally evoked fast EPSP. Similarly, there was no significant difference between iontophoretically applied aspartate and the neurally evoked slow EPSP. The reversal potentials of iontophoretically applied L-glutamate and aspartate were significantly different. 6. When axonal conduction was blocked with tetrodotoxin (TTX), graded focal stimulation of nerve terminals produced graded postsynaptic potentials. At identified fast terminals, the evoked graded potentials were similar in time course to iontophoretically applied L-glutamate potentials while at slow terminals, evoked potentials were similar to aspartate potentials. 7. When blunt iontophoretic micropipettes were positioned over fast synapses in the presence of TTX, the postsynaptic membrane became desensitised to both ejected L-glutamate and the natural transmitter. Dose-response curves of iontophoretically applied L-glutamate in the presence of the natural transmitter at the same terminal region indicated competitive interaction between L-glutamate and the natural transmitter. 8. Iontophoretic potentials could not be obtained in the presence of either 6 mM calcium or 20 mM magnesium, although large EPSPs could be recorded under these conditions. Iontophoretic potentials could be evoked in these conditions if 10 mM bicarbonate ions were present. 9. It is concluded the L-glutamate is a specific agonist at the fast transmitter receptor, while aspartate is a specific agonist at the slow transmitter receptor. However, in view of the different effects of calcium on the neurally evoked potential and the iontophoretic potentials, a better understanding of these synapses is needed before claiming a transmitter role for these amino acids.