Cerebral dopamine and noradrenaline in mice withdrawn from repeated morphine treatment and development of tolerance to a test dose of morphine.

1. In mice withdrawn from 3 days of morphine, dopamine depletion was significantly (P < 0.01) retarded in the "rest of the forebrain and midbrain" and tended to be retarded in the striatum. 2. In control mice withdrawn from 3 days of saline treatment a 10 mg/kg test dose of morphine significantly enhanced noradrenaline depletion in all selected brain areas. This did not occur in test mice withdrawn from 1-3 days of morphine, indicating development of tolerance. 3. These results suggest that dopamine depletion is retarded in the "rest of the forebrain and midbrain" of mice withdrawn from 3 days of repeated morphine and that noradrenaline depletion is unchanged in all selected brain areas of mice withdrawn from morphine. 4. Tolerance develops to the enhancing effect of morphine on noradrenaline depletion in mice withdrawn from 3 days of repeated morphine but not in those withdrawn from 1 day of treatment only.