EPSP amplitudes at crayfish neuromuscular junctions are sensitive to methods of increasing glutamate concentrations in the synaptic cleft

Abstract

Glutamate is the primary excitatory neurotransmitter in crayfish, and theoretically, increasing glutamate concentrations in the synaptic cleft should have similar excitatory results regardless of the method causing the increase. To test this hypothesis, we conducted three different experimental trials using crayfish neuromuscular junctions: increasing glutamate by high frequency stimulation, submersion in 2mM glutamate bath, and submersion in a glutamate uptake inhibitor, 1-aminocyclobutane-trans-1, 3-dicarboxylic acid. We found that high frequency stimulation caused depression rather than excitation, addition of glutamate to the bath caused no change in EPSP amplitudes, and inhibition of glutamate uptake caused greater depolarization in the postsynaptic cell. The results carry significant implications regarding the sensitivity of glutamatergic synapses, and demonstrate that for healthy excitation, glutamate must be both present in the cleft and able to diffuse away from the cleft. The only trial resulting in an increase in EPSP amplitude was the uptake inhibitor trial, possibly because this was the only condition that met the sensitive needs of the glutamatergic synapse for excitation.