Function of NMDA receptors and persistent sodium channels in a feedback pathway of the electrosensory system

Voltage-dependent amplification of ionotropic glutamatergic excitatory postsynaptic potentials (EPSPs) can, in many vertebrate neurons, be due either to the intrinsic voltage dependence of N-methyl-D-aspartate (NMDA) receptors, or voltage-dependent persistent sodium channels expressed on postsynaptic dendrites or somata. In the electrosensory lateral line lobe (ELL) of the gymnotiform fish Apteronotus leptorhynchus, glutamatergic inputs onto pyramidal cell apical dendrites provide a system where both amplification mechanisms are possible. We have now examined the roles for both NMDA receptors and sodium channels in the control of EPSP amplitude at these synapses. An antibody specific for the A. leptorhynchus NR1 subunit reacted strongly with ELL pyramidal cells and were particularly abundant in the spines of pyramidal cell apical dendrites. We have also shown that NMDA receptors contributed strongly to the late phase of EPSPs evoked by stimulation of the feedback fibers terminating on the apical dendritic spines; further, these EPSPs were voltage dependent. Blockade of NMDA receptors did not, however, eliminate the voltage dependence of these EPSPs. Blockade of somatic sodium channels by local somatic ejection of tetrodotoxin (TTX), or inclusion of QX314 (an intracellular sodium channel blocker) in the recording pipette, reduced the evoked EPSPs and completely eliminated their voltage dependence. We therefore conclude that, in the subthreshold range, persistent sodium currents are the main contributor to voltage-dependent boosting of EPSPs, even when they have a large NMDA receptor component.