Background: In animal studies, the ability of ethanol to function as a reinforcer has been described as weak to moderate. This is in contrast with the human condition, where the consumption of alcoholic beverages can result in a variety of unwanted drinking behaviors. However, when the ethanol self-administration pattern is examined, animal studies over the last several years indicate that the ability of ethanol presentation to maintain behavior may be greater than originally assumed.
Methods: We reevaluated the ability of ethanol to function as a reinforcing stimulus in two paradigms by using an analysis of drinking bout characteristics. Data from previous studies that employed two self-administration models were analyzed. With the "dipper" model, small amounts of ethanol are presented after each completion of a response requirement; with the "sipper" model, the animal is allowed access to a drinking tube that contains ethanol for an extended period after completing a single response requirement. For both models, the consumption pattern could be characterized as occurring in a bout. Each drinking bout was divided into runs within the bout, and run rates and size were analyzed. As well, in the sipper model, the data on response requirement size were reviewed to demonstrate the ability of ethanol presentation to maintain high levels of responding in this model.
Results: From this assessment, we suggest that ethanol presentation in non-food- or non-water-restricted rats is as reinforcing as many other stimuli generally considered to be strong reinforcers (i.e., food in food-restricted rats). Using run size, we demonstrated that intake control appears to be regulated by shifts in run size during the bout and not run rate.
Conclusions: Assessment of the pattern of ethanol consummatory bouts and the behaviors that precede them is critical in understanding how ethanol functions as a reinforcer. By using a drinking pattern analysis, the shifts in the momentary salience of the ethanol stimulus can be evaluated in these animal models. In addition, the separation of responding required to gain access to ethanol from consumption of ethanol demonstrated that ethanol presentation in this procedure can be a strong reinforcer for rats.