Giant pyramidal neurons of the primary motor cortex express vasoactive intestinal polypeptide (VIP), a known marker of cortical interneurons

Vasoactive intestinal polypeptide (VIP) is known to be present in a subclass of cortical interneurons that are involved in the disinhibition of excitatory pyramidal neurons. Here, using three different antibodies, we demonstrate that VIP is also present in the giant layer 5 pyramidal (Betz) neurons which are characteristic of the limb and axial representations of the marmoset primary motor cortex (cytoarchitectural area 4ab). No VIP staining was observed in smaller layer 5 pyramidal cells present in the primary motor facial representation (cytoarchitectural area 4c), or in premotor cortex (e.g. the caudal subdivision of the dorsal premotor cortex, A6DC), indicating the selective expression of VIP in Betz cells. The most intense VIP staining occurred in the largest Betz cells, located in the medial part of A4ab. VIP in Betz cells was colocalized with neuronal specific marker (NeuN) and a calcium-binding protein parvalbumin (PV). PV also intensely labelled axon terminals surrounding Betz cell somata. Whereas Betz cells bodies were located in layer 5, VIP-positive (VIP+) interneurons were more abundant in the superficial cortical layers. They constituted about 5-7% of total cortical neuronal population, with the highest density observed in area 4c. Our results demonstrate the expression of VIP in the largest excitatory neurons of the primate cortex, which may offer new functional insights into the role of VIP in the brain, and also provide opportunities for genetic manipulation of Betz cells.