METABOTROPIC GLUTAMATE mGluR5 RECEPTOR BLOCKADE OPPOSES ABNORMAL INVOLUNTARY MOVEMENTS AND THE INCREASES IN GLUTAMIC ACID DECARBOXYLASE mRNA LEVELS INDUCED BY I-DOPA IN STRIATAL NEURONS OF 6-HYDROXYDOPAMINE-LESIONED RATS

The present study examined the effect of a subchronic systemic administration of the glutamate metabotropic mGluR5 receptor antagonist MPEP on I-DOPA-induced dyskinesias and striatal gene expression in adult rats with a unilateral 6-OHDA lesion of dopamine neurons. The daily systemic administration of I-DOPA for 2 weeks induced a gradual increase in limb dyskinesia and axial dystonia. The subchronic systemic co-administration of MPEP reduced the severity of limb dyskinesia and axial dystonia over the whole duration of I-DOPA treatment. Subchronic I-DOPA administration was paralleled by a significant increase in mRNA levels of the two isoforms of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD67 and GAD65) and preprodynorphin (PPD). Single cell analysis on emulsion radioautographs indicated that I-DOPA-induced increases in GAD67 occurred predominantly in preproenkephalin-unlabeled striatonigral and, to a lesser extent, in preproenkephalin-labeled striatopallidal neurons. MPEP completely reversed the effects of I-DOPA on GAD67 and reduced the increases in GAD65 and PPD mRNA levels in striatonigral neurons. MPEP also reversed the small I-DOPA-induced increase in GAD67 mRNA levels in striatopallidal neurons. Altogether, the findings support the idea that the relative efficacy of mGluR5 receptor antagonists to oppose I-DOPA-induced abnormal involuntary movements involves an ability to oppose increases in GAD gene expression and GABA-mediated signaling in striatonigral and striatopallidal neurons. The results also confirm the potential usefulness of antagonists of mGluR5 receptors as adjuncts in the treatment of I-DOPA-induced dyskinesia in patients with Parkinson's disease. (C) 2009 IBRO. Published by Elsevier Ltd. All rights reserved.