Conditioned object preference: an alternative approach to measuring reward learning in rats

Pavlovian conditioned approach behavior can be directed as much toward discrete cues as it is toward the environmental contexts in which those cues are encountered. The current experiments characterized a tendency of rats to approach object cues whose prior exposure had been paired with reward (conditioned object preference, COP). To demonstrate the phenomenon, rats were conditioned to associate cocaine or saline with two different objects. Rats acquired a preference, assessed using investigation times directed toward each object, for the cocaine-paired object following conditioning. Furthermore, high levels of object investigation during cocaine conditioning predicted stronger preferences for the cocaine-paired object in the test phase. Conditioned approach diminished across extinction but was reinstated through a priming injection of cocaine. To determine whether preferences are affected by reward value, rats were conditioned using three objects paired with 0, 5, or 20 mg/kg of cocaine. This produced object preferences in the post-test that scaled with cocaine dose used for conditioning. Finally, we explored whether contextual cues modulate expression of COP by testing rats for renewal of cocaine seeking. When conditioning was conducted in one context and extinction training in a second context, COP was renewed when the rats were retested in the original context. Thus, conditioned object preferences are readily acquired, easily measured, and amenable to a number of standard Pavlovian conditioning manipulations. This task promises to become a valuable addition to the panoply of behavioral tools available to test mechanisms underlying adaptive and maladaptive reward processing.