Ay. Bespalov et al., Effects of test conditions on the outcome of place conditioning with morphine and naltrexone in mice, PSYCHOPHAR, 141(2), 1999, pp. 118-122
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.