Place conditioning in a two- or three-conditioning compartment apparatus: a comparative study with morphine and U-50,488.

We have comparatively studied the effects of two opioids in the rat place conditioning paradigm in identical experimental conditions (including double drug/saline conditioning daily sessions for 3 days), with the only exception of using either a two- or three-conditioning compartment apparatus. Morphine-induced place preference appeared to be similar with two- and three-conditioning compartments, but U-50,488-induced place aversion was consistently more prominent when a two-conditioning compartment apparatus was used. It is suggested that, when the results of conditioning are being tested, the presence of neutral environments decreases the sensitivity of the procedure to quantify place aversions.