Ethanol-conditioned flavor preferences compared with sugar- and fat-conditioned preferences in rats

Ackroff, K., Rozental, D., Scalfani, A. (2004). Ethanol-conditioned flavor preferences compared with sugar- and fat-conditioned preferences in rats. Physiology & Behavior, 81, 699-713.

Past studies in rats have suggested that the postingestive effects of various nutrients can condition strong flavor preferences. Research has been conducted on ethanol using oral conditioning (where the ethanol is mixed in with the flavoring, thus adding its own flavor to the mixture) and intragastic conditioning (where the ethanol is injected into the rat’s stomach after it ingests the flavoring, thus eliminating the ethanol’s flavor as a conditioning factor). However, the findings of these studies have been somewhat contradictory. Mehiel and Bolles (1988) found that rats equally preferred a flavor paired with ethanol and a flavor paired with sucrose, while Sherman et al. (1983) found that rats preferred a flavor paired with glucose to a flavor paired with ethanol. These studies different in terms of design, route of administration, and sugar used, but it is unclear which of these factors are responsible for the confliction. The present study examined this question in a series of four related experiments.

In the first experiment, sucrose and ethanol were compared to each other as well as to a control of water. These nutrients were each paired with a different flavor and administered through intragastric infusion. Unlimited access to alternating flavor/nutrient combinations was provided during a training period and then the amount of each combination consumed was measuring during a test in which the rats could choose between different combinations. The results were that (1) the rats preferred both sucrose- and ethanol-paired flavors to the water-paired flavor, (2) the rats strongly preferred the sucrose-paired flavor to the ethanol-paired flavor, and (3) the rats ingested more sucrose mixture than ethanol mixture during the training period.

The second experiment attempts to control for finding (3) in experiment one by adding the additional constraint that sucrose mixture consumption was limited to the previous day’s ethanol mixture consumption. Findings (1) and (2) did not change as a result.

In the third experiment, the oral conditioning method was used in place of intragastric infusion for nutrient administration. Once again, the findings did not change.

Finally, in the fourth experiment, ethanol was compared to fructose and corn oil using the intragastic infusions. Similar to the sucrose findings, the rats preferred these new sugars to ethanol, while still preferring ethanol to water.

This study supports the findings of Sherman et al. (1983) and eliminates training intakes, route of administration, and specific sugar/fat as explanations of Mehiel and Bolles’ (1988) contradictory findings. Something other than energy concentration affects the efficacy of ethanol as a reinforcer, making it less powerful than other sugars/fats.