Effects of competitive NMDA receptor antagonists on excitatory amino acid-evoked currents in mouse spinal cord neurones

The effects of CGP 37849 [DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoate] and its ethylester CGP 39551 on whole-cell currents evoked by the endogeno us excitatory amjno acids, L-glutamate and L-aspartate, were studied in cul tured mouse spinal cord neurones. Although CGP 37849 was the more potent co mpound, both antagonists inhibited 20 mu M L aspartate or 2 mu M L-glutamat e currents concentration-dependently and reversibly. We calculated IC50 val ues of 370 +/- 180 nM for CGP 37849 and 2200 +/- 140 nM for CGP 39551 (inhi bition of L-aspartate current), and 210 +/- 25 nM for CGP 37849 and 6000 +/ - 4700 nM for CGP 39551 (inhibition of L-glutamate current). Both CGP 37849 and CGP 39551 selectively blocked N-methyl-D-aspartate (NMDA)-evoked inwar d current. Current evoked by 5 mu M kainate or 5 mu M alpha-amino-3-hydroxy -5-methyl-4-isoxazolepropionate (AMPA) was unaffected by 10 mu M CGP 39551. Current evoked by NMDA was concentration-dependently blocked by CGP 39551 with an IC50 of 2100 +/- 220 nM. After application of 10 mu M CGP 37849, 17 +/- 6% of the current evoked by 5 mu M L-glutamate remained. This residual current was due to non-NMDA receptor activation since application of 25 mu M 2-amino-5-phosphonovalerate (APV) together with CGP 37849 did not signif icantly alter the residual current, whereas application of 6-cyano7-nitroqu inoxaline-2,3-dione (CNQX) with CGP 37849 did significantly inhibit this cu rrent. Clamping cells at potentials ranging from -80 to +60 mV showed a lin ear potential-current relationship for the 20 mu M Laspartate-evoked curren t with reversal potential around 0 mV. The proportion of the L-aspartate cu rrent antagonized by CGP 37849 or CGP 39551 appeared to be independent of c lamping potential. The concentration-current relationship of L-aspartate in the absence of the antagonists showed an EC50 of 49 +/- 14 mu M. Upon appl ication of 1 mu M CGP 37849 and 10 mu M CGP 39551, the L-aspartate concentr ation-curret curve shifted to higher concentrations, and resulted in a 5- a nd 13-fold increase in the EC50 of L-aspartate, respectively, whereas I-max was not changed by application of the antagonists. Thus, the potent NMDA a ntagonists CGP 37849 and CGP 39551 were shown to inhibit excitatory amino a cid responses specifically by competitive binding to the neurotransmitter r ecognition site of the NMDA receptor. Selective, competitive antagonism of L-glutamate- and L-aspartate-evoked NMDA receptor responses probably underl ies the effects of CGP 37849 and CGP 39551 such as their rtnticonvulsant, n europrotectant and antidepressant actions. (C) Elsevier, Paris.