Marmoset superior colliculus: neuronal expression of somatostatin but not vasoactive intestinal peptide or neuropeptide Y.

The superior colliculus (SC) is a layered midbrain structure that plays a crucial role in integrating sensory information toward functions such as directing eye and head movements. Despite significant literature on its anatomical structure and connections, there are still important gaps in our knowledge of the diversity of cell types, particularly in primates. Here, using immunostaining, we examined the expression of three different neuropeptides [somatostatin (SST), vasoactive intestinal peptide (VIP), and neuropeptide Y (NPY)] in the SC of adult marmoset monkeys (Callithrix jacchus). We found neurons expressing SST (SST-positive, SST+) across all cellular layers of the SC, which corresponded to approximately 3-5% of the total neuronal population in this structure. SST+ neuronal density as estimated by stereological sampling methods was about 3,140/mm3 in the top layer, stratum griseum superficiale (SGS) and decreased across the dorsoventral axis, roughly in line with the overall neuronal density estimated from NeuN stained nuclei. Co-staining of SST with gamma-aminobutyric acid (GABA), confirmed the inhibitory nature of these cells. However, we found no evidence of VIP- or NPY-positive neurons in the marmoset SC, despite the presence of clearly stained neurons in other structures, in the same sections. Our data adds to the understanding of neuronal diversity of SC in primates and provides quantitative estimates of SST+ neurons in this structure that is essential for better understanding of its function and phylogeny.