d-Methadone is antinociceptive in the rat formalin test.

The l-isomer of methadone possesses opioid activity, whereas the d-isomer is weak or inactive as an opioid. Both d- and l-methadone have been shown to bind to the N-methyl-D-aspartate (NMDA) receptor. To determine whether d-methadone has functional, in vivo NMDA receptor antagonist activity, the antinociceptive effects of d-methadone were evaluated in the rat tail-flick and formalin tests. Cumulative dose-response analysis in the tail-flick test revealed an ED50 value for intrathecal (spinal) l-methadone of 15.6 microg/rat. In contrast, spinal d-methadone produced no antinociception at a cumulative dose of 460 microg/rat. d-Methadone in a dose range from 32 to 320 microg/rat dose-dependently reduced formalin-induced flinching behavior during phase 2 but not during phase 1 of the formalin test. These antinociceptive effects of d-methadone were not blocked by a spinal dose of naloxone that effectively antagonized an antinociceptive (tail-flick test) dose of l-methadone. d-Methadone at an intrathecal dose of 250 microg shifted the ED50 value for NMDA-induced nociceptive behaviors more than 3-fold to the right, which indicates an antagonism of these NMDA receptor-mediated effects. These results indicate that d-methadone is antinociceptive as a result of its NMDA receptor antagonist activity.