Featured Finding Figure
Dopamine (DA) axons and receptors have recently been identified in the primate thalamus, including the mediodorsal thalamic nucleus (MD). In order to determine whether the DA innervation of the primate MD shares the anatomical features of the mesocortical or nigrostriatal DA projections, we performed tract-tracing [using the retrograde tracer cholera toxin B (CTb)] and immunocytochemistry studies to identify the location of the DA neurons that project to MD and immuno-electron microscopy to determine the distribution of the dopamine transporter (DAT) in axons within the MD. Similar to the mesocortical projection, retrogradely-labeled neurons (panel A) containing tyrosine hydroxylase (panel B), the rate-limiting enzyme in DA synthesis, were present in dorsal tier ventral mesencephalic nuclei, such as the ventral tegmental area and the dorsal portion of the substantia nigra pars compacta. In contrast, no dual-labeled neurons were present in the ventral tier nuclei, the primary origin of the nigrostriatal DA pathway. Similarly, none of the retrogradely-labeled neurons in the hypothalamus (panel.C) were immunoreactive for tyrosine hydroxylase (panel D). In addition, like the DA projection to the prefrontal cortex, DAT immunoreactivity was predominantly localized to the pre-terminal portion of axons in the MD, and was infrequently found in association with synaptic vesicles, in contrast to nigrostriatal DA axons. These findings indicate that the DA projection to the MD shares anatomical features with the mesocortical DA system, suggesting that the functional properties of DA neurotransmission in the MD might be more similar to those in the cortex than in the striatum.
Melchitzky DS, Erickson SL, Lewis DA: Dopamine innervation of the monkey mediodorsal thalamus: Location of projection neurons and ultrastructural characteristics of axon terminals. Neuroscience 143: 1021-1030, 2006.

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David A. Lewis, M.D. | Department of Psychiatry | University of Pittsburgh
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