Effects of bath-applied L-glutamate and related chemicals on the afferent synapse of the Plotosus electroreceptor.

Tonic electroreceptors of the marine catfish Plotosus were isolated, and effects of chemicals applied in the bath were examined in terms of firing rate (F) responses in single unit afferent nerve. L-Glutamate (L-Glu) and agonists caused marked F increase in the spontaneous discharge. Their potencies, estimated from concentrations for 50% of max F increase, were in the order of quisqualate (2 microM), kainate (7 microM), L-Glu (0.4 mM), L-homocysteate (0.4 mM), D-Glu (3 mM) and L-aspartate (L-Asp, greater than 10 mM). N-Methyl-D,L-aspartate (10 mM) had no effect. L-Glu induced F increase also in the receptors fully suppressed either by cathodal pulses or by high Mg (15 mM), which indicated the postsynaptic action. The synaptic responses were often affected differently in the fast and slow phases, here termed as the peak F and the adapted F, respectively. L-Asp potentiated only the adapted F. Kynurenic acid (Kyn) suppressed only the adapted F, but incompletely and rather dose-independently. Kyn, however, competitively antagonized the amino acid-induced responses. The present results suggest the presence of two distinct postsynaptic receptors, one a Kyn-sensitive Glu receptor that is responsible for part of the adapted F, and the other still undetermined that is responsible for most of the synaptic responses.