NMDA receptors on adult frog spinal motoneurons in culture.

Whole-cell membrane currents induced by superfusion of NMDA were examined in cultured motoneurons from the spinal cord of the adult frog in Mg(2+)-free Ringer solution containing 10 microM glycine. The amplitude of the response to 100 microM NMDA was 280 +/- 37 pA (mean +/- S.D.; n = 24) with a reversal potential +6.1 +/- 3.0 mV (mean +/- S.D.; n = 6). At a membrane potential of -60 mV, the response to 100 microM NMDA was blocked by 0.1 mM Mg2+ or 100 microM AP5. From the dose-response curve, the estimated EC50 was 77 microM and the calculated Hill coefficient was 1.6. NMDA receptors on frog motoneurons appear to have properties similar to those of mammals but may be expressed at lower density.