title: High-dose methadone maintenance in rats: effects on cocaine self-administration and behavioral side effects.
abstract: It has been demonstrated that high-dose methadone maintenance is efficacious in reducing cocaine abuse in opioid-dependent individuals, but it is not clear whether this is caused by an action of methadone on the direct reinforcing properties of cocaine or on cocaine seeking. Also, it is not clear whether high-dose methadone maintenance may induce behavioral side effects, which could limit its clinical use. Here, we report that high-dose methadone maintenance (20-40 mg/kg/day) does not reduce, and even enhances cocaine (10-30 mg/kg, i.p.)-induced elevation in dopamine concentration in the ventral striatum measured by in vivo microdialysis. In parallel, however, rats maintained on high-dose methadone (30 mg/kg/day) seek and consume significantly less cocaine than controls when tested for intravenous cocaine (0.5 mg/kg/infusion) self-administration on a progressive ratio schedule of reinforcement. This reduction in cocaine self-administration does not result from impaired sensory-motor functioning as rats maintained on high-dose methadone show normal locomotor activity. Furthermore, the reduction in responding for cocaine does not seem to result from general behavioral deficits as male rats maintained on high methadone doses respond normally to palatable food and thermal pain, although their sexual responses to receptive females are greatly suppressed. Taken together, these results from studies in rats support the usefulness of larger doses of methadone to reduce severe cocaine abuse in opioid-dependent individuals and possibly in the management of pure-cocaine addiction.
authors.affiliation: Department of Psychology, University of Guelph, Guelph, ON, Canada NIG 2WI. fleri@uoguelph.ca
authors.forename: Francesco
authors.initials: F
authors.lastname: Leri
authors.forename: Robert E
authors.initials: RE
authors.lastname: Sorge
authors.forename: Erin
authors.initials: E
authors.lastname: Cummins
authors.forename: David
authors.initials: D
authors.lastname: Woehrling
authors.forename: James G
authors.initials: JG
authors.lastname: Pfaus
authors.forename: Jane
authors.initials: J
authors.lastname: Stewart
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D003042:Cocaine / Q000008:administration & dosage*; D003216:Conditioning, Operant / Q000187:drug effects; D018765:Dopamine Uptake Inhibitors / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D004435:Eating / Q000187:drug effects; D005260:Female; D008297:Male; D008691:Methadone / Q000494:pharmacology*; D017551:Microdialysis / Q000379:methods; D009043:Motor Activity / Q000187:drug effects; D009294:Narcotics / Q000494:pharmacology*; D010052:Ovariectomy / Q000379:methods; D051381:Rats; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D012646:Self Administration; D012737:Sex Factors; D012726:Sexual Behavior, Animal / Q000187:drug effects
chemical_list: D018765:Dopamine Uptake Inhibitors; D009294:Narcotics; D003042:Cocaine; D008691:Methadone