N-methyl-D-aspartate receptors coexist with kainate and quisqualate receptors on single isolated catfish horizontal cells.

Horizontal cells enzymatically isolated from catfish retina were exposed to the putative neurotransmitters aspartate (Asp) or N-methyl-D-aspartate (NMDA). Under voltage clamp conditions, inward currents were recorded when the holding potential was more negative than zero and outward currents were recorded when the membrane potential was more positive than zero. The current voltage curve was highly non-linear in the range of membrane potential between -30 and -100 mV. This non-linearity was largely removed in zero magnesium solution. 2-Amino-phosphonovaleric acid selectively blocked Asp and NMDA responses. These response characteristics are consistent with the presence of NMDA receptors in these cells.