ACETYLCHOLINESTERASE HISTOCHEMISTRY IN THE MACAQUE THALAMUS REVEALS TERRITORIES SELECTIVELY CONNECTED TO FRONTAL, PARIETAL AND TEMPORAL ASSOCIATION CORTICES

The patterns of histochemical staining for acetylcholinesterase (AChE) activity in the macaque thalamus were analyzed and compared with the distribution of cells and terminals labeled from injections of axonal tracers in the dorsolateral and orbital prefrontal cortex, in area 7a of the posterior parietal cortex and in the polysensory cortex of the superior temporal sulcus. AChE histochemistry is very useful in deline ating the thalamic nuclei connected with the association cortex and in uncovering thalamic subdivisions that are barely evident on cytoarchi tectonic grounds. Moreover, AChE activity reveals previously unrecogni zed heterogeneities within several thalamic nuclei, like the ventral a nterior (VA), where a new ventromedial subdivision (VAvm) is described , the medial pulvinar (PulM) or the mediodorsal nucleus (MD). In this nucleus three distinct chemical domains are present: the medial, ventr al and lateral sectors characterized by low, moderate and high AChE ac tivities, respectively. The staining pattern of the lateral sector is markedly heterogeneous with patches of intense AChE activity surrounde d by a moderately stained matrix. The MD medial sector is connected wi th the orbitofrontal cortex, whereas the AChE-rich patches in the late ral sector are selectively connected with the dorsolateral prefrontal, parietal and temporal association cortices. In the PulM, a dorsomedia l AChE-rich patch is selectively connected with the orbitofrontal cort ex, whereas the surrounding territory, which shows moderate AChE activ ity, is preferentially connected with the parietal and temporal cortic es. Chemically specific domains in the anterior, ventral anterior, mid line, and intralaminar thalamic nuclei are also connected with the exa mined association cortices. These findings indicate that the topograph ic patterns of the thalamo-cortical connections of primate association areas conform to the chemical architecture of the thalamus. This impl ies that because each cortical area is connected to a particular set o f thalamic regions, the influence of the thalamus on cortical function is exclusive for each area, highly diverse among the various associat ion areas, and subject to a wide range of modulation at the thalamic l evel.