Levodopa- and Neuroleptic-Induced Dyskinesias : Studies on Pharmacological Modification and Processing of Opioid Neuropeptides

Sammanfattning: Dyskinesias or abnormal involuntary movements are a debilitating complication of long-term levodopa treatment of Parkinson’s disease (PD) that is widely experienced and may compromise the efficacy of the drug therapy. Tardive dyskinesia is another important adverse effect seen with antipsychotic drug treatment. The neural mechanisms underlying levodopa- and neuroleptic-induced dyskinesia are not clear and involvement of the endogenous opioid neuropeptide system has been implicated. In this thesis, the role of the opioid system is investigated in models of dyskinesia and PD using behavioral, neurochemical and advanced analytical chemistry techniques. In addition, the motor effects of a new partial dopamine agonist with normalizing properties on both reduced and elevated dopamine transmission are studied and a new model for tardive dyskinesia is presented.Using microdialysis in combination with micro-electrospray mass spectrometry, the in vivo processing of the opioid neuropeptide dynorphin A(1-17) was studied and 32 metabolites were detected in the striatum. Altered in vivo metabolism of the peptide was found in a model of PD with more metabolites formed in the dopamine-depleted striatum. Moreover, dynorphin A(1-17) was differently processed in levodopa-, bromocriptine and saline-treated animals. Levodopa treatment caused an increase in the mRNA expression of the precursor of dynorphin, preproenkephalin-B as well as the precursor of enkephalin, preproenkephalin-A, in all sub-regions of the dopamine-depleted striatum. A non-selective opioid receptor antagonist, naloxone, was found to reduce levodopa-induced dyskinesia with maintained antiparkinsonian response and a normalization of hyperkinesia. Moreover, the new drug GMC1111 showed dopamine stabilizing properties in models of levodopa-induced dyskinesia and PD. This might prove useful in the treatment of PD.Altogether, these results suggest that the endogenous opioid system is involved in the pathophysiology of levodopa-induced dyskinesia.