Comparative analysis of cytoarchitectonic and neurochemical aspects of the globus pallidus in the rock cavy ( <i>Kerodon rupestris</i> ) and the common marmoset ( <i>Callithrix jacchus</i> )

The globus pallidus (GP), a subcortical structure within the basal ganglia, plays a crucial role in modulating cortico-subcortical circuits related to motor control and associative/limbic functions. However, the cytoarchitectonic and neurochemical organization of this structure remains poorly characterized in naturally existing wild-type species, particularly in the South American rock cavy (Kerodon rupestris) and the neotropical common marmoset (Callithrix jacchus). In this study, using samples from K. rupestris (n = 5) and C. jacchus (n = 7), we employed immunohistochemistry and morphometric analyses to characterize the expression of calbindin (CB) and parvalbumin (PV) in the GP. In K. rupestris, although external (GPe) and internal (GPi) segments were identifiable, neither the lateral nor medial medullary laminae were observed. While the neuropil showed diffuse immunoreactivity, no CB immuno-positive neuronal labelling was identified. In contrast, PV immuno-positive neuronal labelling was also broadly present in this species. In C. jacchus, CB immuno-positive neuronal labelling was restricted to the medial medullary lamina, whereas PV immuno-positive neuronal labelling was observed across both pallidal subdivisions. These findings reveal important structural and molecular differences between rodents and primates, suggesting distinct evolutionary trajectories in GP organization. Understanding such differences contributes toward broadening our knowledge of the functional nuances of the basal ganglia.