Dopamine depletion and subsequent treatment with l‐DOPA, but not the long‐lived dopamine agonist pergolide, enhances activity of the Akt pathway in the rat striatum

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Abstract
Dysregulation of signaling pathways is believed to contribute to Parkinson’s disease pathology and l‐DOPA‐induced motor complications. Long‐lived dopamine (DA) agonists are less likely to cause motor complications by virtue of continuous stimulation of DA receptors. In this study, we compared the effects of the unilateral 6‐hydroxydopamine lesion and subsequent treatment with l‐DOPA and DA agonist pergolide on signaling pathways in rats. Pergolide caused less pronounced behavioral sensitization than l‐DOPA (25 mg/kg, i.p., 10 days), particularly at lower dose (0.5 and 0.25 mg/kg, i.p.). Pergolide, but not l‐DOPA, reversed lesion‐induced up‐regulation of preproenkephalin and did not up‐regulate preprodynorphine or DA D3 receptor in the lesioned hemisphere. Pergolide was as effective as l‐DOPA in reversing the lesion‐induced elevation of ERK2 phosphorylation in response to acute apomorphine administration (0.05 mg/kg, s.c.). Chronic l‐DOPA significantly elevated the level of Akt phosphorylation at both Thr308 and Ser473 and concentration of phosphorylated GSK3α, whereas pergolide suppressed the lesion‐ and/or challenge‐induced supersensitive Akt responses. The data indicate that l‐DOPA, unlike pergolide, exacerbates imbalances in the Akt pathway caused by the loss of DA. The results support the hypothesis that the Akt pathway is involved in long‐term actions of l‐DOPA and may be linked to l‐DOPA‐induced dyskinesia.

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