Effect of Environmental Enrichment on the Acquisition of Sign-tracking of an Ethanol Bottle in the Home Environment

Sign tracking is a Pavlovian conditioning procedure that has been used in preclinical drug-addiction research with rats to investigate individual differences in the attribution of incentive motivational value to reward cues that may lead to problematic behavior such as drug addiction. Animals demonstrate a tendency to interact with an object conditioned stimulus (CS) that signals the presentation or delivery of a reward, even though the delivery of the reward is independent of any interaction with the stimulus. Thus, it can be argued that sign tracking is a compulsive-like behavior, or an indication of compulsive “wanting” that is associated with increased vulnerability to drug use, abuse, and addiction. Whereas the dominant approach in the sign tracking literature is to use an arbitrary retractable lever CS and to emphasize the predictive validity of sign tracking of the lever CS on subsequent measures of addictive behavior, in the current study we were interested in employing a CS with greater affordance than an arbitrary lever CS to model compulsive ethanol consumption in Sprague Dawley rats as introduced by Tomie (2014). Moreover, in order to improve the face validity of the traditional sign tracking paradigm as a model of compulsive-like ethanol use and abuse, we raised half of our animals in a more complex or “natural” environment, typically referred to in the literature as environmental enrichment, consisting of small groups of rats in multilevel towers with access to objects and toys. Unlike previous studies, we also tested the animals in these enriched towers rather than in separate testing chambers. We found that rats in the environmental enrichment condition acquired sign-tracking behavior at a significantly faster rate and approached a higher asymptote than standard-caged rats. Testing within a home environment did not diminish sign-tracking activity, suggesting that sign tracking is not dependent on impoverished rearing and isolated test chambers and can emerge in a familiar setting. By testing rats within their home towers and extending the period of sign tracking beyond that of past studies, we have contributed to a limited body of pre-existing research on the influence of environmental enrichment on sign tracking and further validated Tomie’s model of sign-tracking of an ethanol bottle as an animal model of excessive and compulsive-like use and abuse