Effects of morphine on axonal transport in nigrostriatal neurons.

Axonal transport in the nigrostriatal tract of protein labeled with ³H]-leucine (10 h and 5 days) or of ³H]dopamine (10 h) was not significantly different from controls in rats treated with morphine by acute injection 20 mg/kg, intraperitoneal (ip)] or chronically implanted with 75-mg pellets. This applied both to the amount of label accumulated in the striatum and to the gel electrophoretic protein patterns in both the striatum and the substantia nigra. Rats chronically treated with morphine gave more variable results in all tests than the controls but the results did not seem to correlate with the observable behavioral effects. The difference between our results and the significant increase in protein transport reported by others in morphine-dependent animals probably depends on the lower doses of morphine used in the present experiments. The animals, however, seemed dependent as judged by their response to a naloxone challenge. There was also no significant difference between morphine-treated and control rats in the activities of tyrosine hydroxylase, acetylcholinesterase, monoamine oxidase, dopa decarboxylase, or glutamic decarboxylase in the striatum or of glutamic decarboxylase or tyrosine hydroxylase in the substantia nigra. Striatal choline acetyltransferase was elevated in the rats chronically treated with morphine as previously reported by others.