The alpha2-adrenergic receptor antagonist idazoxan reduces dyskinesia and enhances anti-parkinsonian actions of L-dopa in the MPTP-lesioned primate model of Parkinson's disease

Pathophysiology of levodopa-induced dyskinesia: potential for new therapies

Involuntary movements--or dyskinesias--are a debilitating complication of levodopa therapy for Parkinson's disease, and is experienced in most patients. Despite the importance of this problem, little was known about the cause of dyskinesia until recently; however, this situation has changed significantly in the past few years. Our increased understanding of levodopa-induced dyskinesia is not only valuable for improving patient care, but also in providing us with new insights into the functional organization of the basal ganglia and motor systems.

Nonhuman primates: a critical role in current disease research

This review article emphasizes the critical role of nonhuman primates (NHPs) in biomedical research. It focuses on the most recent contributions that NHPs have made to the understanding, treatment, and prevention of important infectious diseases (e.g., acquired immunodeficiency syndrome, hepatitis, malaria) and chronic degenerative disorders of the central nervous system (e.g., Parkinson's and Alzheimer's diseases). The close phylogenetic relation of NHPs to humans not only opens avenues for testing the safety and efficacy of new drugs and vaccines but also offers promise for evaluating the potential of new gene-based treatments for human infectious and genetic diseases.

Neural mechanisms underlying peak-dose dyskinesia induced by levodopa and apomorphine are distinct: evidence from the effects of the alpha(2) adrenoceptor antagonist idazoxan

Dyskinesia, secondary to dopamine replacement therapy, is the major complication of currently available therapies for Parkinson's disease. Alpha(2) adrenoceptor antagonists, such as idazoxan, can significantly reduce levodopa-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, nonhuman primate model of Parkinson's disease and in human. This action of adrenoceptor antagonists may involve blockade of the actions of noradrenaline synthesised from levodopa. We hypothesise that, because dopamine receptor agonists, such as apomorphine, cannot be metabolised to produce noradrenaline, activation of adrenoceptors may not be involved in dyskinesia produced by such agents. If this were the case, idazoxan would not be expected to reduce apomorphine-induced dyskinesia. MPTP-lesioned marmosets with stable dyskinesia induced by prolonged levodopa therapy were given an acute challenge with apomorphine (0.3 mg/kg subcutaneously) or levodopa (8.0 mg/kg orally), these doses produced equivalent peak-dose dyskinesia. Idazoxan (2.5 mg/kg p.o.), or vehicle, was then administered with either apomorphine or levodopa. Idazoxan abolished levodopa-induced dyskinesia but did not affect apomorphine-induced dyskinesia (P < 0.05 and P > 0.05, respectively, Wilcoxon matched pairs test). Idazoxan also extended the anti-parkinsonian actions of levodopa but did not affect those of apomorphine. The pharmacological characteristics of the neural mechanisms underlying levodopa-induced dyskinesia and apomorphine-induced dyskinesia in parkinsonism thus appear to be distinct, at least with respect to the involvement of alpha(2) adrenoceptors. Specifically, levodopa, but not apomorphine-induced dyskinesia, involves activation of adrenoceptors. This finding may have major implications for understanding dyskinesia and should be borne in mind when designing clinical studies in which levodopa or dopamine receptor agonist challenges are employed to assess potential anti-dyskinetic properties of drugs.

Idazoxan, an alpha-2 antagonist, and L-DOPA-induced dyskinesias in patients with Parkinson's disease

Dyskinesia is a frequent and disabling side effect in patients with Parkinson's disease treated with chronic dopa-therapy. Preclinical data in the 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP) monkey suggest that alpha-2 antagonists may reduce dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. We assessed, in a pilot randomised placebo-controlled study, the effects of single oral doses (10 mg, 20 mg, and 40 mg) of idazoxan, an alpha-2 antagonist, on motor parkinsonian disability and L-DOPA-induced dyskinesia following an acute oral challenge of L-DOPA in 18 patients with Parkinson's disease. The severity of L-DOPA-induced dyskinesia improved after 20 mg idazoxan pretreatment, while there was no concommittant deterioration in the antiparkinsonian response to L-DOPA. These results suggest that blocking alpha-2 receptors in patients with Parkinson's disease might improve L-DOPA-induced dyskinesia without the cost of a return of parkinsonian symptomatology. Further studies are required to assess whether this property could have potential therapeutic applications in the long-term management of dyskinetic patients with Parkinson's disease.