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Previously identified differences in serotonin innervation have been proposed to underlie differences in behavior, such as personality style and sociability. Contrasting serotonergic fiber densities have been found in the amygdala of chimpanzees versus bonobos, and humans and apes are known to have more serotonin than monkeys in the dorsal and medial caudate nucleus and dorsal putamen. Our present work builds on earlier results by examining serotonergic axon innervation density in the nucleus accumbens and ventral pallidum, two important nodes in the reward system. The present sample included humans (n = 6; NIH NeuroBioBank), pigtailed macaque monkeys (n = 5; National Primate Research Center, University of Washington), and capuchin monkeys (n = 6; Alpha Genesis). All individuals were adult and free of neuropathological alterations. Brain sections were immunohistochemically processed for serotonin transporter (SERT) (Millipore, MAB 5618), and stereological methods (SpaceBalls probe, MBF Bioscience) were used to quantify the length density of SERT-immunoreactive axons and neuron densities from adjacent Nissl-stained sections. Repeated measures ANOVA was used to evaluate differences of SERT-immunoreactive axon densities and neuron densities among species. The main effect of brain region was significant (F 1,2 = 12.25, p = 0.004) with greater SERT innervation in ventral pallidum compared to the nucleus accumbens in all species. The main effect of species and the interaction of species x brain region were not significant. Based on these results, the serotonergic system in the nucleus accumbens and ventral pallidum appears to be evolutionarily conserved in the amount of innervation supplied to neurons among human and other anthropoid primates.
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This content will become publicly available on August 1, 2026
Distribution of Serotonergic Transporter Innervation in the Nucleus Accumbens and Ventral Pallidum Is Highly Conserved Among Primates
ABSTRACT 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.
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- Award ID(s):
- 2021785
- PAR ID:
- 10644321
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Comparative Neurology
- Volume:
- 533
- Issue:
- 8
- ISSN:
- 0021-9967
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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