Excitatory amino acid receptors have been implicated in mediating pain. 3-((±)-2-Carboxypiperazin4-yl)-propyl-1-phosphonic acid (CPP), a competitive N-methyl-d-aspartate (NMDA) antagonist and MK-801, a phencyclidine (PCP) ligand and non-competitive NMDA antagonist, were injected intrathecally in mice alone or in combination with 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA antagonist. When tested in the formalin model of pain, antinociception following CPP plus DNQX was greater than that after MK-801 plus DNQX in both the acute and tonic phases. These dissimilarities are not consistent with activity of CPP and MK-801 at the same sites in the spinal cord.
- 3-((±)-2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid