Dopa-induced blood flow responses in nonhuman primates.
Initially, treatment with the dopamine precursor levodopa provides substantial symptomatic relief for patients with Parkinson's disease (PD). However, as the disease progresses, side effects such as involuntary movements or psychosis may accompany the response to medication. The mechanisms underlying these actions of levodopa remain unclear. To develop methodology for longitudinal studies of the effects of PD and levodopa treatment in living nonhuman primates, we first studied the effects of an acute dose of levodopa on regional brain activity in sedated baboons using positron emission tomography. We found that levodopa significantly decreased regional cerebral blood flow (rCBF) bilaterally in putamen and right cingulate and increased rCBF in right lateral temporal cortex and bilateral frontal cortex. We then performed similar studies on a nemestrina in awake and sedated states to determine whether these responses were affected by sedation. Interestingly, the directions of the rCBF responses in the putamen and temporal cortex were reversed depending on the presence or absence of sedation. Specifically, responses were decreased in sedated animals, but increased dose-dependently in the awake nemestrina. These findings have important implications for the interpretation of studies that use anesthesia. The responses in the awake nemestrina were most similar to those reported in humans and thus may be the most useful model system. Future imaging studies using selective dopaminergic agents in awake animals may permit the identification of relatively specific agonist-mediated pathways and may help separate the mechanisms that mediate levodopa's benefit from those that produce its unwanted side effects.