Dopamine transporter-immunoreactive axons in the mediodorsal thalamic nucleus of the macaque monkey.

The reciprocal connections between the mediodorsal thalamic nucleus and the prefrontal cortex participate in a circuit that is essential to a number of higher cognitive processes. Projections from the dopamine-containing cells of the ventral mesencephalon to the prefrontal cortex are also critical for these cognitive abilities. It is unclear, however, whether dopamine axons innervate the mediodorsal thalamic nucleus in primates. In order to address this question, we examined the distribution of dopamine transporter-immunoreactive axons in the mediodorsal thalamic nucleus of macaque monkeys. Labeled axons were distributed quite heterogeneously in this nucleus, and did not strictly follow cytoarchitectonic subdivision boundaries. The ventral and lateral portions of the mediodorsal thalamic nucleus, which include parts of the parvicellular and multiform subdivisions, had the highest density of dopamine transporter-immunoreactive axons. In contrast, the dorsomedial portion, which included primarily the magnocellular subdivision, had the lowest density of labeled axons. In both lightly and densely innervated portions of the nucleus, small, dense clusters of dopamine transporter-immunoreactive axons were present. Axons immunoreactive for tyrosine hydroxylase were distributed in a pattern very similar to that of dopamine transporter-labeled axons. In contrast, noradrenergic axons, as revealed by dopamine beta-hydroxylase immunoreactivity, were present in higher density and were more evenly distributed throughout the mediodorsal thalamic nucleus. This dopamine innervation of the mediodorsal thalamic nucleus reveals another possible anatomical substrate through which dopamine may influence the cognitive functions mediated by thalamo-prefrontal circuitry.