Activation of NMDA receptors linked to modulation of voltage-gated ion channels and functional implications

Catfish (Ictaluruspunctatus) cone horizontal cells containN-methyl-D-aspartate (NMDA) receptors, thefunction of which has yet to be determined. In the present study, wehave examined the effect of NMDA receptor activation on voltage-gatedion channel activity. NMDA receptor activation produced a long-termdownregulation of voltage-gated sodium and calcium currents but had noeffect on the delayed rectifying potassium current. NMDA'seffect was eliminated in the presence of AP-7. To determine whetherNMDA receptor activation had functional implications, isolated catfish cone horizontal cells were current clamped to mimic the cell's physiological response. When horizontal cells were depolarized, theyelicited a single depolarizing overshoot and maintained a depolarizedsteady state membrane potential. NMDA reduced the amplitude of thedepolarizing overshoot and increased the depolarized steady-statemembrane potential. Both effects of NMDA were eliminated in thepresence of AP-7. These results support the hypothesis that activationof NMDA receptors in catfish horizontal cells may affect the type ofvisual information conveyed through the distal retina.