Differential Regulation of Striatal Preproenkephalin and Preprotachykinin mRNA Levels in MPTP-Lesioned Monkeys Chronically Treated with Dopamine D1 or D2 Receptor Agonists

Abstract : Studies in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in parkinsonian patients show elevated preproenkephalin (PPE) mRNA levels, unaltered by chronic l -DOPA therapy, whereas preprotachykinin (PPT) mRNA levels are decreased by the lesion and corrected by l -DOPA. The relative contributions of the dopamine D₁ and D₂ receptors for PPE mRNA regulation were investigated in the present study and compared with those for PPT mRNA. In situ hybridization was used to measure peptide mRNA levels in the striatum of MPTP cynomolgus monkeys after chronic 1-month treatment with the D₁ agonist SKF-82958, administered subcutaneously in pulsatile or continuous mode, compared with the long-acting D₂ agonist cabergoline. Normal as well as untreated MPTP animals were also studied. PPE mRNA levels were elevated in the caudate nucleus and putamen of untreated MPTP monkeys compared with control animals with a more pronounced increase in the lateral as compared with the medial part of both structures. PPT mRNA levels showed a rostrocaudal gradient, with higher values in the middle of the caudate-putamen and more so in the medial versus the lateral parts. PPT mRNA levels were decreased in the caudate and putamen of untreated MPTP monkeys compared with control animals, and this was observed in the middle and posterior parts of these brain areas. Elevated PPE and decreased PPT mRNA levels observed after MPTP exposure were corrected after treatment with cabergoline (0.25 mg/kg, every other day), a dose that had antiparkinsonian effects and did not give sustained dyskinesia. In contrast, elevated PPE mRNA levels observed in untreated MPTP monkeys were markedly increased by pulsatile administration of SKF-82958 (1 mg/kg, three times daily) in two monkeys in which the parkinsonian symptoms were improved and dyskinesias developed, whereas it remained close to control values in a third one that did not display dyskinesias despite a sustained improvement in disability ; a shorter duration of motor benefit (wearing off) over time was observed in these three animals. By contrast, pulsatile administration of SKF-82958 corrected the decreased PPT level observed in untreated MPTP monkeys. Continuous treatment with SKF-82958 (equivalent daily dose) produced no clear antiparkinsonian and dyskinetic responses and did not alter the denervation-induced elevation of PPE or decrease of PPT mRNA levels. The present data suggest an opposite contribution of the dopamine D₁ receptors (stimulatory) as compared with the dopamine D₂ receptors (inhibitory) on PPE mRNA, whereas a similar stimulatory contribution of D₁ or D₂ receptors is observed for PPT mRNA. An increase in PPE expression could be involved in the induction of dyskinesias and wearing off, whereas our data do not support this link for PPT. The antiparkinsonian response was associated with a correction of the lesion-induced decrease of PPT.