Chemical parcellation of the anterior thalamic nuclei in the human brain

The anterior thalamic nuclei (ATN) encompass a large region of the anteromedial aspect of the human thalamus. Three ATN have been classically described: anteroventral (AV), anteromedial (AM) and anterodorsal (AD). The present study has carried out histochemical and immunohistochemical procedures in the ATN of normal individuals to analyze whether these nuclei are chemically distinct. The markers used in this study were acetylcholinesterase (AChE), limbic system-associated membrane protein (LAMP), the calcium binding proteins calbindin D-28k (CB), parvalbumin (PV), and calretinin (CR), and the neuropeptides substance P (SP) and enkephalin (ENK). Other cytoarchitectural and myeloarchitectural techniques, specifically Nissl and Gallyas stainings, were used to delineate the boundaries of the ATN. The main findings of this study are: 1) AChE was very abundant in the AD and was irregular or heterogeneously distributed in the AV and AM; 2) LAMP immunoreactive (ir) neuropil was present throughout the ATN and its distribution was heterogeneous in the AV and AM; 3) the ATN harbored CB-, PV- and CR-ir neurons and neuropil; and, 4) the neuropeptide analysis revealed numerous SP positive varicose fibers scattered throughout the ATN in contrast to very few ENK-ir varicose fibers. These morphological findings describe a heterogeneous chemical anatomy in the human ATN which may reflect regional differences in the functional organization of the ATN with respect to the other thalamic nuclei and the cerebral cortex.