Ethanol regulated preference in rats

A series of experiments evaluated the determinants of preference for mixtures of ethanol plus sucrose relative to sucrose in rats. One dipper served 10% ethanol mixed with 10% sucrose, and the second dipper served 10% sucrose. Lever presses operated each dipper according to a variable-interval 5-s schedule. In three experiments the subjects were given pre-session meals of sucrose (2.5–20 ml) or sucrose (20 ml) plus chow (5 or 10 g). Pre-session meals decreased responding maintained by sucrose but not responding maintained by ethanol mixture. In two experiments body weight was varied from 85% to 125% of the initial free-feeding values. Increases in body weight, like pre-session meals, decreased responding reinforced by sucrose, but typically did not decrease responding reinforced by ethanol mixture. Throughout most of the study, ethanol consumption remained at about 1.25 ml per half hour session (3–4 g/kg per 30 min). For example, pre-session access to ethanol mixture decreased within-session ethanol consumption, but total consumption, counting both sources, remained about 1.25 ml/session. The within-session patterns of responding also differed. Responding reinforced by ethanol mix decreased as a function of ethanol consumption, whereas responding reinforced by sucrose was relatively constant throughout the session. The simplest explanation of the results is that ethanol's pharmacological consequences regulated preference.     

Preference for saccharin-sweetened alcohol relative to isocaloric sucrose

 This experiment tested the reinforcing efficacy of a saccharin-sweetened alcohol solution relative to an isocaloric sucrose drink in rats. One dipper served 10% alcohol plus 0.25% saccharin, and a second, concurrently available, dipper served 14.2% sucrose. During the course of the experiment, access to the two drinks was challenged by increasing the schedule requirement (variable-interval) that determined when a lever press would operate the dipper. There were two main findings. First, the rats continued to consume significant amounts of alcohol despite access to the isocaloric sucrose solution. Second, schedule-requirement increases that decreased sucrose-reinforced responding failed to decrease saccharin-sweetened alcohol reinforced responding. These results extend and replicate earlier findings from studies in which alcohol was mixed with sucrose, and the alcohol mixtures held a caloric advantage over the competing sucrose solutions. The experiment also included controls for differences in baseline response rates and for the influence of saccharin on preference. In the baseline response-rate control conditions, the two reinforcers were 10% sucrose and a mixture of 10% sucrose-plus-quinine. The results showed that the persistence of sweetened-alcohol reinforced responding could not be explained by differences in baseline response rates or the reinforcing properties of saccharin. Rather, the findings were consistent with the idea that the rats were defending baseline levels of alcohol-plus-saccharin consumption. Received: 15 June 1996 / Final version: 13 August 1996