Brain hemodynamic changes mediated by dopamine receptors: Role of the cerebral microvasculature in dopamine-mediated neurovascular coupling.

The coupling between neurotransmitter-induced changes in neuronal activity and the resultant hemodynamic response is central to the interpretation of neuroimaging techniques. In the present study, MRI experiments showed that dopamine transporter blockers such as cocaine and dopamine releasers such as amphetamine and D1 receptor agonists induced large positive increases in relative cerebral blood volume (rCBV) that were not sensitive to nitric oxide synthase inhibition. However, D1/D5 receptor antagonism with SCH-23390 prevented or blocked the hemodynamic response without any concomitant effect on dopamine release. Dopamine D2/D3 receptor agonists, in contrast, induced negative changes in rCBV in brain regions corresponding largely to those endowed with these receptors. D1 and D5 receptor mRNAs were expressed in microvessels of responsive brain areas, while D2 and D3 receptors were not consistently associated with the microvascular bed. D3 receptors had an astroglial localization. Together, these experiments show that direct effects of dopamine upon the vasculature cannot be ignored in measuring the hemodynamic coupling associated with dopaminergic drugs. These results further suggest that this coupling is partially mediated through D1/D5 receptors on the microvasculature leading to increased rCBV and through astroglial D3 receptors leading to decreased rCBV. These data provide additional support for the role of local post-synaptic events in neurovascular coupling and emphasize that the interpretation of fMRI signals exclusively in terms of neuronal activity may be incomplete.