MK801-induced antagonism of NMDA-preferring excitatory amino acid receptors in horizontal cells of the turtle retina.

Intracellular recordings were made from axon terminals of L-type horizontal cells in the turtle (Pseudemys scripta elegans) retina. Superfusion with Ringer's solution containing 3.0 mM N-methyl-D-aspartate (NMDA) or 0.2 mM kainic acid (KA) induced depolarization and reduction in the hyperpolarizing light responses of horizontal cells, consistent with an agonist effect of these excitatory amino acid (EAA) analogs on postsynaptic receptors. Delivery of 0.1 mM MK801, a selective blocker of NMDA-type EAA receptors, had no apparent effect on membrane potential or photoresponses, nor did it change the KA depolarization. Exposure of the retina to 3.0 mM NMDA following 0.1 mM MK801 always caused hyperpolarization of the horizontal cell and loss of light responses. Because MK801 is specific for NMDA-preferring receptors, we suggest that the reversal of the NMDA response to one of antagonism following MK801 is strong evidence for the presence of NMDA-preferring EAA receptors in turtle horizontal cells.