Apparatus bias and the use of light and texture in place conditioning

In a typical conditioned place preference (CPP) preparation, animals alternately experience drug and vehicle effects in distinct chambers of an apparatus, spending more time in the drug-paired chamber post-conditioning. However, if all animals prefer the same chamber before conditioning, data interpretation may be compromised. Unbiased apparatus has been systematically validated with ethanol in mice ([Cunningham, C.L., Feree, N.K., Howard, M.A. Apparatus bias and place conditioning with ethanol in mice. Psychopharmacology (Berl) 2003;170:409-422]); the present study sought to identify and eliminate bias in a standard black-and-white apparatus and validate that apparatus for CPP with morphine and cocaine in rats. Apparatus bias was assessed in 24 adult female Sprague-Dawley rats. Subjects preferred the black chamber under bright lighting conditions, with no preference in the dark. Subjects then underwent a counterbalanced CPP regimen to 5 mg/kg SC morphine (n = 12) or 20 mg/kg IP cocaine (n = 12) using only tactile conditioned stimuli. Significant absolute preferences for the drug-paired chamber were produced by both drugs, with no effect of drug-paired chamber assignment on CPP expression; vehicle-treated controls (n = 12) showed no preferences. Bias-free CPP to morphine and cocaine using standard apparatus in rats is possible. Implications for place conditioning are discussed, including the potential value of systematically exploiting apparatus bias in addition to eliminating it.