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

Conditioned place preference (CPP) is a popular technique for assessing drug reward. 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 animals naturally prefer one chamber, data interpretation is compromised. Procedures for accommodating apparatus bias may be inadequate, therefore, eliminating bias is preferable. Bias in a standard black-and-white apparatus was assessed in 24 rats (Experiment 1). Subjects preferred the black chamber under bright lights, but exhibited no preference in the dark. Subjects then underwent a counterbalanced CPP regimen to 5 mg/kg morphine (n = 12, Experiment 2) or 20 mg/kg cocaine (n = 12, Experiment 3) 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. Bias-free CPP to morphine and cocaine with standard apparatus in rats is possible.