Effect of levodopa priming on dopamine neuron transplant efficacy and induction of abnormal involuntary movements in parkinsonian rats.

Clinical trials of neural grafting for Parkinson's disease (PD) have produced variable, but overall disappointing, results. One particular disappointment has been the development of aberrant motor complications following dopamine (DA) neuron grafting. Despite a lack of consistent benefit, the utility of dopamine neuron replacement remains supported by clinical and basic data. In a continued effort to elucidate factors that might improve this therapy, we used a parkinsonian rat model to examine whether pregraft chronic levodopa affected graft efficacy and/or graft-induced dyskinesia (GID) induction. Indeed, all grafted PD patients to date have had a pregraft history of long-term levodopa. It is well established that long-term levodopa results in a plethora of long-lasting neurochemical alterations and genomic changes indicative of altered structural and synaptic plasticity. Thus, therapeutic dopamine terminal replacement in a striatal environment complicated by such changes could be expected to lead to abnormal or inappropriate connections between graft and host brain and to contribute to suboptimal efficacy and/or postgraft GID behaviors. To investigate the effect of pregraft levodopa, one group of parkinsonian rats received levodopa for 4 weeks prior to grafting. A second levodopa-naïve group was grafted, and the grafts were allowed to mature for 9 weeks prior to introducing chronic levodopa. We report here that, in parkinsonian rats, preexposure to chronic levodopa significantly reduces behavioral and neurochemical efficacy of embryonic dopamine grafts. Furthermore, dopamine terminal replacement prior to introduction of chronic levodopa is highly effective at preventing development of levodopa-induced dyskinesias, and GID-like behaviors occur regardless of pregraft levodopa status.