Distribution of Serotonergic Transporter Innervation in the Nucleus Accumbens and Ventral Pallidum Is Highly Conserved Among Primates.

The nucleus accumbens (NAcc) and ventral pallidum (VP) are key nodes in the mesolimbic reward pathway that facilitate stimulus salience, including the regulation of social motivation and attachment. Primate species display variation in social behaviors, including different levels of impulsivity, bonding, and aggression. Previous research has implicated neuromodulation of the reward pathway in the differential expression of various social behaviors, suggesting that differences in neurotransmitter innervation may play a role in species-specific patterns. To explore this, we examined serotonergic innervation in the NAcc and VP among primates. We used stereology to quantify serotonin transporter-immunoreactive (SERT-ir) axon length density in the NAcc and VP of 13 primate species, including humans, great apes, and cercopithecid and platyrrhine monkeys. Our data show that serotonergic innervation density within both the NAcc and VP is highly conserved among species. This finding contrasts with our previous findings of higher levels of SERT-ir axons in the dorsal striatum of humans and great apes relative to monkeys, a human-specific increase in dopaminergic innervation within the NAcc and VP, and a human-specific increase of neuropeptide Y in the NAcc, highlighting the mosaic nature of innervation patterns among species.