Accelerated development of tolerance during repeated cycles of ethanol exposure.

Adult male rats were subjected to 1--4 cycles of daily gastric intubation with ethanol (6 g/kg) for 16 days, separated by 17-day alcohol-free periods. Tolerance produced by this treatment (designated 'physiological tolerance') was measured by change in effect of a 2.2 g/kg i.p. dose of ethanol on the moving-belt test. It occurred in each cycle, disappeared completely in the drug-free periods, and developed more rapidly in the second and later cycles than in the first. Tolerance produced by the 'behavioral augmentation' technique (daily test practice under the influence of ethanol) also developed more rapidly on a second than on a first cycle. The progression from within-session to between-session tolerance was still evident, but accelerated. With 25-day alcohol cycles, separated by a one-month drug-free period, the 'carry-over' effect (i.e., more rapid acquisition of tolerance in the second cycle) applied equally, regardless of whether or not tolerance was produced by the same technique in both cycles, or by a crossover in either direction between the two techniques.