Effect of Methylphenidate on Methadone-Induced Analagesia in Rat

Background: A known problem of chronic administration of opioid is tolerance to its analgesic effect. Co-administration of a non-opioid analgesic may help reducing the opiate dose and consequently tolerance and dependency. It is reported that repeated use of methylphenidate, one of amphetamines, increased the expression of µ opioid receptor and had antinociception. Therefore, this study aimed to evaluate the effect of methylphenidate on methadone-induced analgesia. Methods: Nine groups of 8 male rats weighting 270 ± 20 g were randomly selected; control group received 1 ml/kg oral normal saline, metylphenidate groups received 5, 10 and 15 mg/kg oral metylphenidate, methadone groups received 5, 10, 20 mg/kg oral methadone, and combined groups received 5 mg/kg methadone +5 or 15 mg/kg methylphenidate orally. Pain was measured using plantar test apparatus. Findings: Only, 15 mg/kg methylphenidate produced significant analgesic effect. In addition, co-administration of non-effective doses of methylphenidate and 5 mg/ kg methadone significantly increased the analgesic effects of methadone at 60 minutes after gavage. Conclusion: We found that methylphenidate potentiate the analgesic effects of methadone in rats.