Distribution of the dopamine innervation in the macaque and human thalamus.

We recently defined the thalamic dopaminergic system in primates; it arises from numerous dopaminergic cell groups and selectively targets numerous thalamic nuclei. Given the central position of the thalamus in subcortical and cortical interplay, and the functional relevance of dopamine neuromodulation in the brain, detailing dopamine distribution in the thalamus should supply important information. To this end we performed immunohistochemistry for dopamine and the dopamine transporter in the thalamus of macaque monkeys and humans to generate maps, in the stereotaxic coronal plane, of the distribution of dopaminergic axons. The dopamine innervation of the thalamus follows the same pattern in both species and is most dense in midline limbic nuclei, the mediodorsal and lateral posterior association nuclei, and in the ventral lateral and ventral anterior motor nuclei. This distribution suggests that thalamic dopamine has a prominent role in emotion, attention, cognition and complex somatosensory and visual processing, as well as in motor control. Most thalamic dopaminergic axons are thin and varicose and target both the neuropil and small blood vessels, suggesting that, besides neuronal modulation, thalamic dopamine may have a direct influence on microcirculation. The maps provided here should be a useful reference in future experimental and neuroimaging studies aiming at clarifying the role of the thalamic dopaminergic system in health and in conditions involving brain dopamine, including Parkinson's disease, drug addiction and schizophrenia.