Effect of morphine on dopaminergic neurons in the rat basal forebrain and striatum

Administration of morphine (20 mg/kg i.p.) resulted in an increase in the rate of dopamine (DA) loss by ca. 40% in the forebrain cortex of rats, when the drug was administered 30 min after treatment with the synthesis inhibitor alpha-methyl-para-tyrosine methylester (alpha-MpT). This effect was specific as it was antagonized by naloxone. In the same rats the rate of DA loss in the striatum was unaltered. It was shown that this differential effect was not due to a delayed inhibition of DA synthesis in the forebrain cortex when compared with the striatum. When morphine, on the other hand, was administered before inhibition of DA biosynthesis (30 min before alpha-MpT) an increase in the rate of DA loss was also observed in the striatum and a more marked increase was seen in the forebrain cortex. Sub-dissection of the forebrain cortex showed that most of the DA (80%) was present in basal parts of the forebrain and not in the frontal or cingulate cortex. The results suggest, therefore, that morphine administration results in DA release in basal parts of the forebrain, but not in the striatum.