Acute and chronic morphine treatment and the hydroxylation of (1-14C)-L-tyrosine in the mouse brain

Tyrosine hydroxylase activity in slices of caudate nucleus was increased by morphine (100 mg/kg i.p.) administered to naive mice. During chronic treatment with morphine tolerance developed to this effect and 36 h after the final chronic morphine injection there was a decrease in enzyme activity in this area. There was no change in tyrosine hydroxylase activity in slices of diencephalon, brainstem or parietal cortex from either naive or morphine tolerant mice. There- fore, changes in tyrosine hydroxylase activity, measured in vitro, could account for the changes in dopamine synthesis, but not noradrenaline synthesis, produced by morphine in vivo.