|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.