Effects of test conditions on the outcome of place conditioning with morphine and naltrexone in mice

Drug administration during test trials can increase the expression of place conditioning, offering an opportunity to determine the specificity of this enhanced response. Prior to training, Swiss-Webster mice spent similar dur ations in each of the distinctive compartments of a two-compartment box dur ing three 900-s tests. During a 4-day conditioning period, daily injections of morphine (5-20 mg/kg, SC) or vehicle were differentially paired with on e of two compartments of the box using an unbiased place conditioning proce dure. Post-conditioning tests were conducted 2 and 3 days after the last co nditioning day. Mice pre-treated during post-conditioning tests with vehicl e did not show significant preference for the morphine-paired compartment w hen conditioned with morphine. Pretreatment with morphine (2.5-30 mg/kg, SC ) led to a dose-dependent increase in time spent in the morphine-paired com partment. Post-conditioning tests in other groups of mice were conducted wi th heroin (0.1-3 mg/kg), fentanyl (0.01-0.3 mg/kg), cocaine (10-30 mg/kg) a nd pentobarbital (10-30 mg/kg), and results suggested that none of the test ed drugs facilitated the expression of the morphine-conditioned place prefe rence. In another experiment,, naltrexone (0.1-10 mg/kg, SC) was administer ed as the conditioning drug. When tested with naltrexone ((). 1-10 mg/kg), there was a dose-dependent avoidance of the naltrexone-paired compartment. Overall, the present data indicated that: (I)failure to exhibit place prefe rence or place aversion when tested in a drug-free state does not imply the failure of conditioning procedure; and (2) effects of the morphine cue rei nstatement during the post-conditioning tests appeared to be related to the unique pharmacological profile of the morphine stimulus.