An arthropod NMDA receptor.

Identified crayfish visual interneurons respond to illumination with a compound EPSP of up to 40 mV. L-glutamate, quisqualate, and kainate mimic the depolarizing action of the natural transmitter. In reduced Mg2+, N-methyl-D-aspartate (NMDA) elicits a depolarization with a reversal potential (Erev) = -60 mV. Erev is independent of extracellular calcium but shifts to +4 mV if potassium conductances are blocked by intracellular CS+. The results suggest that NMDA may gate more than one class of ionic channel. The NMDA-elicited response is enhanced and prolonged by glycine, and kynurenate competitively blocks the action of glycine. The NMDA antagonist, D-AP7, selectively blocks the NMDA response while enhancing the EPSP. The actions of NMDA are consistent with a role in the neural mechanisms of visual adaptation. This is the first description of an NMDA receptor in an invertebrate.