d-Methadone is antinociceptive in the rat formalin test

Area of Science:

• Pharmacology
• Neuroscience
• Pain Research

Background:

• Methadone's l-isomer has opioid activity, while the d-isomer is a weak opioid.
• Both isomers bind to the N-methyl-D-aspartate (NMDA) receptor.
• The functional NMDA receptor antagonist activity of d-methadone in vivo remains to be fully elucidated.

Purpose of the Study:

• To investigate the in vivo N-methyl-D-aspartate (NMDA) receptor antagonist activity of d-methadone.
• To evaluate the antinociceptive effects of d-methadone in established pain models.
• To differentiate the mechanisms of antinociception between d-methadone and l-methadone.

Main Methods:

• Utilized rat tail-flick and formalin tests to assess antinociception.
• Administered intrathecal (spinal) doses of d- and l-methadone.
• Evaluated the effect of naloxone on d-methadone's antinociceptive activity.
• Assessed the impact of d-methadone on N-methyl-D-aspartate (NMDA) receptor-mediated nociceptive behaviors.

Main Results:

• Intrathecal l-methadone showed significant antinociception (ED50 = 15.6 microg/rat) in the tail-flick test.
• Spinal d-methadone did not produce antinociception in the tail-flick test up to 460 microg/rat.
• d-Methadone dose-dependently reduced formalin-induced flinching in phase 2, but not phase 1.
• Naloxone did not block the antinociceptive effects of d-methadone.
• d-Methadone antagonized N-methyl-D-aspartate (NMDA) receptor-mediated behaviors in a dose-dependent manner.

Conclusions:

• d-Methadone possesses functional, in vivo N-methyl-D-aspartate (NMDA) receptor antagonist activity.
• The antinociceptive effects of d-methadone are mediated via NMDA receptor antagonism, not opioid receptors.
• These findings suggest a distinct therapeutic potential for d-methadone in pain management beyond its opioid activity.