Route of administration influences substitution patterns in rats trained to discriminate methadone vs. vehicle

Replacement therapy with the synthetic mu-opioid agonist methadone is an efficacious treatment for opioid abuse. While much is known about methadone's pharmacology, its discriminative stimulus properties remain largely unexplored. The present study sought to establish methadone discrimination in rats. Moreover, some research suggests that route of administration alters the discriminative stimulus of methadone. Thus, the present study also compared intraperitoneal (i.p.) and subcutaneous (s.c.) routes of administration. Male Sprague-Dawley rats were trained to discriminate 3.0mg/kg methadone (i.p.) from vehicle in a two-lever discrimination procedure. Generalization tests were conducted with a variety of compounds administered i.p. and s.c. Methadone fully substituted for itself, yielding ED(50)s of 1.5mg/kg (i.p.) and 0.2mg/kg (s.c.). Naltrexone (i.p.), an opioid antagonist produced a dose-dependent reduction in methadone-appropriate responding. The methadone stereoisomers fully substituted for methadone when given s.c.; however, when administered i.p., (+) and (-) methadone produced partial and no substitution, respectively. Heroin fully generalized to methadone regardless of administration route, while morphine fully substituted when given s.c., but not i.p. The kappa-agonist U50-488 failed to generalize to methadone with either route of administration. These results demonstrated that methadone's discriminative stimulus is mediated through mu-opioid receptor activity and is similar to that of commonly abused opioids (heroin, morphine). Additionally, route of administration produced differential results for many of the drugs tested, suggesting decreased drug bioavailability following i.p. administration due to hepatic first pass metabolism. Taken together, these results suggest that methadone's shared subjective effects with abused opioids, as well as its unique metabolic properties contribute to its efficacy in opioid maintenance therapy.