Methadone and morphine have equivalent analgesic but not reward potency in female mice.

Background: Morphine is a potent opioid analgesic but carries high abuse potential. Some evidence indicates that the opioid methadone, used in maintenance therapy for opioid dependence, also provides analgesia with lower misuse liability, but direct behavioral comparisons of morphine and methadone under matched conditions remain limited. Methods: Dose responses curves for morphine and methadone (0.3-10 mg/kg) were generated in adult female C57BL6/J mice using paradigms to measure reward (conditioned place preference) and analgesia (von Frey and hot plate). Results: Morphine supported a conditioned place preference at all doses tested, while methadone induced a significant place preference at 1 and 10 mg/kg, but not at 0.3 mg/kg. Both opioids increased mechanical and thermal antinociceptive thresholds only at the highest dose tested (10 mg/kg). Conclusions: These findings demonstrate that methadone has a lower reward, but similar analgesic potency compared to morphine in female mice.