Cortico-basal ganglia circuits involved in different motivation disorders in non-human primates

The ventral striatum (VS) is of particular interest in the study of neuropsychiatric disorders. In this study, performed on non-human primates, we associated local perturbation with monosynaptic axonal tracer injection into medial, central and lateral VS to characterize anatomo-functional circuits underlying the respective expression of sexual manifestations, stereotyped behaviors and hypoactive state associated with loss of food motivation. For the three behavioral effects, we demonstrated the existence of three distinct cortico-basal ganglia (BG) circuits that were topographically organized and overlapping at some cortical (orbitofrontal cortex, anterior cingulate cortex) and subcortical (caudal levels of BG) levels, suggesting interactions between motivation domains. Briefly, erection was associated with a circuit involving the orbitofrontal cortex, medial prefrontal cortex (areas 10, 11) and limbic parts of BG, i.e. medial parts of the pallidal complex and the substantia nigra pars reticulata (SNr). Stereotyped behavior was linked to a circuit involving the lateral orbitofrontal cortex (area 12/47) and limbic parts of the pallidal complex and of the SNr, while the apathetic state was underlined by a circuit involving not only the orbital and medial prefrontal cortex but also the lateral prefrontal cortex (area 8, 45), the anterior insula and the lateral parts of the medial pallidal complex and of the ventro-medial SNr. For the three behavioral effects, the cortico-BG circuits mainly involved limbic regions of the external and internal pallidum, as well as the limbic part of the substantia nigra pars reticulata (SNr), suggesting the involvement of both direct and indirect striatal pathways and both output BG structures. As these motivation disorders could still be induced in dopamine (DA)-depleted monkeys, we suggest that DA issued from the substantia nigra pars compacta (SNc) modulates their expression rather than causes them. Finally, this study may give some insights into the structure to target to achieve therapeutic benefits from deep brain stimulation in motivation disorders.