Characteristics of "reverse tolerance" to amphetamine-induced locomotor stimulation in mice.

The characteristics of chronically administered amphetamine on the locomotor and anticonvulsant effects were studied in adult CF-1 mice. The influence of dose of the drug and interdose interval on the development of "reverse tolerance" to the locomotor stimulation was investigated, in addition to the selectivity of the response and the persistence of the change in pharmacodynamics. Once-daily treatment with 6 mg/kg amphetamine for 4 weeks resulted in a 2-3 fold increase in locomotor activity. The increase in responsiveness, however, was limited to the first period of 2 weeks and there was no subsequent change in pharmacodynamics during the last 2 weeks of treatment. After 36 days of withdrawal, the response had not returned to that of control, illustrating the persistence of the effect. The results of varying the interdose interval indicated that "reverse tolerance" occurred even when the interval was as long as 14 days. These results represent additional evidence of the persistence of the phenomenon. Selectivity of the changes in the CNS was illustrated by the cross-reactivity with a motor-stimulant dose of cocaine but not with that of morphine. Selectivity was also demonstrated by the failure of "reverse tolerance" to develop to the anticonvulsant effects of amphetamine, which also appear to be mediated dopaminergically.