Electrophysiological demonstration of N-methyl- d-aspartate receptors at the afferent synapse of catfish electroreceptor organs

An excitatory amino acid, most probably l-glutamate, acts as a neurotransmitter at the receptor cell–afferent fibre synapses in the ampullary electroreceptor organs of the freshwater catfish Ictalurus nebulosus. In the present study, we have used an electrophysiological approach to investigate the presence of N-methyl- d-aspartate receptors at this level. N-Methyl- d-aspartate, dissolved in an Mg 2+-containing (normal) solution, had no effect on afferent activity, not even at 5 mM. However, addition of 5 mM N-methyl- d-aspartate to an Mg 2+-free solution evoked an enduring increase in firing rate. The application of N-methyl- d-aspartate combined with electrical sine wave stimulation produced a firing increase in the primary afferents, even in the presence of Mg 2+ (1.5 mM). Glycine (0.01–0.001 mM) significantly potentiated the N-methyl- d-aspartate responses. Addition of antagonists of the actions of N-methyl- d-aspartate, 7-chlorokynurenic acid, dl-2-amino-5-phosphonovaleric acid and ketamine in concentrations of 0.5–2.0 mM led to a decrease in resting and stimulus-evoked activity. 7-Chlorokynurenic acid also blocked the responses to application of N-methyl- d-aspartate. The glycine agonist d-serine (0.01 mM) prevented the 7-chlorokynurenic inhibitory effect. These results suggest the involvement of N-methyl- d-aspartate receptors in mediating the actions of l-glutamate at the afferent synapses of the electroreceptor organs of the catfish.