The regional density and laminar distribution of dopamine (DA) and serotonin (5-HT) afferents were investigated in the cerebral cortex of cynomolgus monkeys using a radioautographic technique that is based on the high affinity uptake capacity of these aminergic neurons. Large vibratome sections, 50 μm thick, were incubated with [3H] DA (0.2 μM) and desipramine (5 μM) or with unlabeled norepinephrine (5 μM) and [3H] 5-HT (0.6 μM), which allowed for the specific labeling of the DA and 5-HT innervations, respectively. After fixation, these sections were dried, defatted, and radioautographed by dipping. Semiquantitative data on the DA innervation also were provided by counting [3H] DA-labeled axonal varicosities in radioautographs from 4-μm-thick sections of the slices obtained after epon embedding.
The DA innervation was widespread and differed in density and laminar distribution in the agranular and granular cortices. DA afferents were densest in the anterior cingulate (area 24) and the motor areas (areas 4, 6, and supplementary motor area [SMA]). In the latter they displayed a trilaminar pattern of distribution, predominating in layers I, IIIa, and V-VI, with characteristic cluster-like formations in layer IIIa, especially in the medial part of motor areas. In the granular prefrontal (areas 46, 9, 10, 11, 12), parietal (areas 1, 2, 3, 5, 7), temporal (areas 21, 22), and posterior cingulate (area 23) cortices, DA afferents were less dense and showed a bilaminar pattern of distribution, predominating in the depth of layer I and in layers V-VI; density in layers II, III, and IV was only 20% of that in layer I. The lowest density was in the visual cortex, particularly in area 17, where the DA afferents were almost restricted to layer I.
The density of 5-HT innervation was generally greater than that of DA except in the motor areas and in the anterior cingulate cortex. Regionspecific laminar patterns characterized (1) motor areas where a lower density in layer III contrasted with the clusters of DA axons in the same layer; (2) the primary visual cortex (area 17), where two bands of higher density in layers III-IV and layer V outlined a poorly innervated zone in layer IVc-β (3) the peristriate area 18, where the 5-HT network was relatively loose but with a denser band in layer III.
Thus, DA innervation of the cerebral cortex displays major differences between rodents and primates, characterized by expanded cortical targets and by a highly differentiated laminar distribution. This suggests a greater functional importance and a direct role of DA at the cortical level in the control of motor and higher order brain functions.