Consequences of Nigrostriatal Denervation on the Functioning of the Basal Ganglia in Human and Nonhuman Primates: An In Situ Hybridization Study of Cytochrome Oxidase Subunit I mRNA

To examine the consequences of nigrostriatal denervation and chronic levodopa (l-DOPA) treatment on functional activity of the basal ganglia, we analyzed, using in situhybridization, the cellular expression of the mRNA encoding for cytochrome oxidase subunit I (COI mRNA), a molecular marker for functional neuronal activity, in the basal ganglia. This analysis was performed in monkeys rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication, some of which had been receiving l-DOPA, and in patients with Parkinson’s disease (PD). In MPTP-intoxicated monkeys compared with control animals, COI mRNA expression was increased in the subthalamic nucleus (STN) and in the output nuclei of the basal ganglia, i.e., the internal segment of the globus pallidus and the substantia nigra pars reticulata. This increase was partially reversed by l-DOPA treatment. COI mRNA expression remained unchanged in the external segment of the globus pallidus (GPe). In PD patients, all of whom had been treated chronically by l-DOPA, COI mRNA expression in the analyzed basal ganglia structures was similar to that in control subjects. These results are in agreement with the accepted model of basal ganglia organization, to the extent that the output nuclei of the basal ganglia are considered to be overactive after nigrostriatal denervation, partly because of increased activity of excitatory afferents from the STN. Yet, our results would also seem to contradict this model, because the overactivity of the STN does not seem to be attributable to a hypoactivation of the GPe.