Conditioned place preference in the corral: a procedure for measuring reinforcing properties of drugs

A novel conditioned place preference (CPP) method is described. The behavioral testing apparatus is a circular open field consisting of 4 uniform quadrants that are equally preferred by the rats prior to drug treatment. In an illustrative experiment, rats received an i.p. injection of either morphine (10 mg/kg), substance P (50 micrograms/kg) or vehicle (phosphate-buffered 0.01 M acetic acid in saline) on 3 consecutive days and were placed into their assigned treatment quadrant. Four animals were simultaneously treated with the aid of barriers, which restricted each rat to its treatment quadrant. On the test for CPPs, when provided a choice between the 4 quadrants, rats treated with morphine and substance P exhibited preferences for the quadrant which had been paired with the drugs, illustrating the usefulness of this procedure for assessing the reinforcing properties of the two drugs. Gross locomotor activity was not influenced by either treatment. The advantages of this version of the CPP method over the conventional shuttle-box procedures are discussed.