L-glutamate as an excitatory transmitter at the neuromuscular junction of a beetle larva

The effects of L-glutamate and acetylcholine on the ventral muscle fibres of the larval mealworm Tenebrio molitor were studied by means of microelectrodes. Bath application of L-glutamate at concentrations higher than 1 × 10 ⁴M suppressed excitatory postsynaptic potentials (EPSPs) and evoked both a depolarisation and a reduction in the input resistance of the muscle fibre. In contrast, acetylcholine chloride (up to 1 mM) had no effect at all. Circumscribed spots could be detected on the fibre surface where iontophoretic applications of L-glutamate caused transient depolarizations (glutamate potentials). Focal extracellular recordings revealed that the glutamate sensitive spots were identical with synaptic sites. The reversal potentials of the EPSP and the L-glutamate potential were identical. These results are compatible with the hypothesis that L-glutamate is an excitatory transmitter at the neuromuscular junction.