The actions of L-glutamate and its agonists on the ampullary electroreceptor organs of the catfish Ictalurus nebulosus

Abstract

Abstract 1. Ampullary electroreceptors of the freshwater catfish, Ictalurus nebulosus, were examined for the effects of bath-applied l -glutamate ( l -GLU) and its agonists quisqualate (Q), kainate (KA) and n -methyl- d -aspartate (NMDA). Resting discharge rate and stimulus-evoked activity in single afferent fibers were recorded in an in vitro preparation. 2. l -GLU (0.1–1 mM), Q (0.8–40 μM) and KA (10–250 μM) strongly increased both resting activity and stimulus-evoked activity in the afferent fibres. NMDA had no effect, even at a concentration of 5.0 mM. 3. The potencies of l -GLU and its agonists, arbitrary defined as the concentrations which gave 50% of maximal frequency increase, were of the order of 7 μM (Q), 25 μM (KA) and 0.6 mM ( l -GLU). 4. The excitatory effects of l -GLU persisted in receptors suppressed by high Mg2+, indicating that l -GLU was acting at the postsynaptic site. 5. The data presented are consistent with our current concept that the action of l -GLU in Ictalurus electroreceptors is mediated via Q/AMPA- and KA-types, but not the NMDA-type, of membrane receptors.