The importance of A9 dopaminergic neurons in mediating the functional benefits of fetal ventral mesencephalon transplants and levodopa-induced dyskinesias.

Intrastriatal transplantation of fetal ventral mesencephalon (VM) tissue provides the potential to alleviate motor symptoms of Parkinson's disease (PD) and levodopa-induced dyskinesia (LID). However, the degree of recovery varies among individuals with an incidence of "off-phase", graft-induced dyskinesia (GID) in some patients. We hypothesised that this variability is due to the heterogeneous nature of dopaminergic neurons in the transplant. We therefore investigated this in the unilateral 6-hydroxydopamine-lesioned rat model of PD. These animals were primed to develop LID and then transplanted with fetal VM into the caudal aspects of the striatum. No GID was observed but in a significant number of animals the transplants ameliorated LID. There was a correlation between the degree of behavioural and LID recovery with the number of A9 dopaminergic neurons in the transplant, based on their expression of a G-protein-coupled inward rectifying current potassium channel (Girk2). Furthermore, we showed that LID development is related to an abnormal expression profile of cyclin-dependent kinase 5 (Cdk5) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) in the striatum and that intrastriatal VM transplants normalised both Cdk5 expression and DARPP-32 phosphorylation in animals exhibiting functional improvement. These results suggest that an A9 dopaminergic neuron-enriched transplant may be the key to an effective PD cell replacement therapy through normalisation of the altered striatal expression of Cdk5/DARPP-32.