Species difference in sensitivity to ethanol's hedonic effects.

Recent research suggests that rats and mice differ in their sensitivity to ethanol's rewarding effect in the place conditioning paradigm. However, these species have not previously been examined in a comparable manner. The present study compared genetically heterogeneous rats and mice using an identical place conditioning procedure. Each animal received four pairings of a distinctive tactile floor stimulus with injection of ethanol (1.5 g/kg); a different tactile stimulus was paired with saline injection. Ethanol suppressed activity in rats but elevated activity in mice. As in most previous studies with drug-naive animals, rats showed aversion whereas mice showed preference for the ethanol-paired stimulus. This difference cannot be attributed to differences in housing conditions, apparatus, stimuli, or temporal parameters. Rather, it appears to represent a species difference in initial sensitivity to ethanol's hedonic effects. If one assumes that ethanol is both rewarding and aversive, this outcome might be explained by a species difference in tolerance/sensitization, the time-course of the biphasic hedonic response to a single ethanol exposure, or selective association. Together with other recent studies from this laboratory, the present findings suggest the mouse may well be the species of choice for studying preferences conditioned by ethanol.