Two pharmacologically distinct receptors mediate two postsynaptic potential (PSP) components in the afferent synapse...

In the Plotosus electroreceptor, strong brief excitatory pulses to the ampulla induced bi-phasic postsynaptic potential (PSP), externally recorded in the afferent nerves, a fast PSP during the pulse and a later slow PSP. Both PSPs increased in size on increasing stimulus intensity, with a lower saturation level for the slow PSP. Time to peak for the fast PSP gradedly decreased to below 10 ms on stronger stimuli, while in contrast that for the slow PSP increased to over 100 ms. The two PSPs were also readily distinguished pharmacologically. Some Glu antagonists, kynurenic acid and gamma-DGG, suppressed the slow PSP, but had little effect on the fast PSP. The suppression was dose-dependent and reversible over the entire range of input-output relations. In addition, L-aspartate at 100-300 microM markedly prolonged the slow PSP, but had little effect on the fast PSP. These results indicate two distinct populations of postsynaptic receptors: one for the slow PSP pharmacologically corresponding to a Glu receptor, and the other for the fast PSP of still unknown property. The two PSPs, though discriminated under specified stimulus conditions, seem to be involved also in the synaptic responses of longer duration in general.