The contribution of excitatory amino acids to central sensitization and persistent nociception after formalin-induced tissue injury.

The contribution of excitatory amino acids (EAAs) to the development of central sensitization and persistent nociception in response to tissue injury in rats was examined following the subcutaneous injection of formalin into the hindpaw. Formalin-induced nociceptive behaviors were enhanced by intrathecal pretreatment with the EAAs L-glutamate and L-aspartate. An enhancement of the formalin nociceptive response was also produced by intrathecal pretreatment with the receptor-selective EAA agonists NMDA and trans-(+/- )-1-amino-1,3-cyclopentane dicarboxylic acid (ACPD), but not (R,S)-alpha-amino-3-hydroxy-5-methylisozazole-4-propionic acid hydrobromide (AMPA). The effect of NMDA was enhanced by a combined administration with AMPA or APCD. Formalin nociceptive responses were dose-dependently reduced by intrathecal pretreatment with the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) and (+)-MK-801 hydrogen maleate, but not the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione or the selective metabotropic EAA receptor antagonist 2-amino-3-phosphonopropionic acid. The results suggest that EAAs acting at the NMDA receptor contribute to central sensitization and persistent nociception following subcutaneous formalin injection.