Alterations in GAD67, dynorphin and enkephalin mRNA in striatal output neurons following priming in the 6-OHDA model of Parkinson's disease.

In the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease, administration of a dopaminergic agonist sensitizes rats to a subsequent administration of dopaminergic drugs given days apart (priming). In situ hybridization was used to evaluate changes on striatal gene expression of rats primed three days previously with either L-dopa, SKF38393 or quinpirole. Double labeling was used to identify the neuronal population in which such alterations occurred. GAD67 and enkephalin mRNA were increased by the lesion whereas dynorphin mRNA was decreased as compared to the intact striatum. Priming with L-dopa and SKF38393 significantly increased GAD67 mRNA in the lesioned striatum and reversed dynorphin mRNA reduction, as compared to drug-naive rats, whereas quinpirole failed to produce any effect. Enkephalin mRNA was not affected by priming. Results suggest that 6-OHDA lesion-induced adaptive changes on striatal gene expression are modified by priming. Priming brings striatal output neurons to a higher level of activity, which may explain the sensitized behavioral response observed following a dopaminergic agonist challenge. These changes are in relation to the different types of dopamine agonists utilized and suggest that modifications in gene expression induced by priming might be predictive of the dyskinetic potential of a drug.