Abstract

Parkinsonism, a common unwanted side effect of typical antipsychotic (neuroleptic) drugs, is induced by the blockade of striatal dopamine D2 receptors. In rats with hemi-parkinsonism induced by unilateral lesion of dopaminergic nigrostriatal neurons with 6-hydroxydopamine, D2 antagonists inhibit contralateral turning induced by D2 agonists and augment the levels of neurotensin mRNA in dopaminergically intact striatum. By contrast, D1 agonists induce contralateral turning and augment neurotensin mRNA levels in dopamine-depleted striatum. These effects could be inhibited by D1 but not by D2 antagonists. Here we used a hemi-parkinsonian model to investigate the effects of putative D1 agonist/D2 antagonist LEK-8829 (9,10-didehydro-N-methyl-(2-propynyl)-6-methyl-8-aminomethylergoline bimaleinate), an experimental antipsychotic, on turning behavior and the expression of striatal neurotensin, preprotachykinin and neurotransmitter-induced early gene protein 4 (ania-4) mRNAs. We found that LEK-8829 inhibited contralateral turning induced by D2 agonist quinpirole, but only if the rats were cotreated with D1 antagonist SCH-23390. In situ hybridization showed that LEK-8829 induced the expression of neurotensin and ania-4 mRNAs in dopamine-intact striatum that could be completely blocked only by the combined treatment with SCH-23390 and quinpirole. In addition, LEK-8829 augmented the expression of neurotensin, preprotachykinin and ania-4 mRNAs in dopamine-depleted striatum that could be completely blocked by SCH-23390. This study clearly demonstrates that in hemi-parkinsonian rats D1 agonistic activity of LEK-8829 confers its anti-parkinsonian drug-like properties and modulates its neuroleptic drug-like properties, which are dependent on the blockade of dopamine D2 receptors. These findings imply that atypical antipsychotics with D1 intrinsic activity might have a reduced propensity for the induction of extrapyramidal syndrome.