Regulation of dopamine receptor and neuropeptide expression in the basal ganglia of monkeys treated with MPTP

Abstract

In Parkinson's disease (PD), striatal dopamine deficiency has been associated with complex changes in the functional and neurochemical anatomy of the basal ganglia. In this study, we simultaneously analyzed the regulation of D1 and D2 dopamine receptors and levels of the neuropeptides, substance P, and enkephalin (ENK) in various basal ganglia nuclei following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic denervation of striatum in nonhuman primates. Both unilateral and bilateral lesioned animals were used for this study. Striatal dopamine deficiency resulted in distinct alterations in D1, D2, substance P, and enkephalin levels and distribution: (1) Both D1 and D2 protein levels were significantly up-regulated in striatum. (2) There was an overall up-regulation of striatal substance P expression following dopamine denervation. (3) Substance P distribution was 'reversed' in dopamine depleted striatum: striosomes, which normally express higher levels of substance P, showed decreased expression, whereas substance P expression was up-regulated in the matrix. (4) Substance P expression was up-regulated in the internal segment of the globus pallidus (GPi), but remained unchanged in substantia nigra (SN). (5) Enkephalin levels were increased in striatum and the external segment of the globus pallidus (GPe), but not in substantia nigra. All the changes were more pronounced in the bilateral lesioned monkeys, though the data represent a pooled statistical evaluation of unilateral and bilateral lesioned monkeys. Our studies indicate that D1 and D2 dopamine receptors and substance P and enkephalin undergo regulatory changes in response to nigrostriatal dopamine denervation. Simultaneous study of the alterations in these various components of the 'direct' and 'indirect' pathways in the same animals will enable better understanding of the pathophysiology of PD and its therapeutic targets.