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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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Hedonic eating, obesity, and addiction result from increased neuropeptide Y in the nucleus accumbens during human brain evolution
The nucleus accumbens (NAc) is central to motivation and action, exhibiting one of the highest densities of neuropeptide Y (NPY) in the brain. Within the NAc, NPY plays a role in reward and is involved in emotional behavior and in increasing alcohol and drug addiction and fat intake. Here, we examined NPY innervation and neurons of the NAc in humans and other anthropoid primates in order to determine whether there are differences among these various species that would correspond to behavioral or life history variables. We quantified NPY-immunoreactive axons and neurons in the NAc of 13 primate species, including humans, great apes, and monkeys. Our data show that the human brain is unique among primates in having denser NPY innervation within the NAc, as measured by axon length density to neuron density, even after accounting for brain size. Combined with our previous finding of increased dopaminergic innervation in the same region, our results suggest that the neurochemical profile of the human NAc appears to have rendered our species uniquely susceptible to neurophysiological conditions such as addiction. The increase in NPY specific to the NAc may represent an adaptation that favors fat intake and contributes to an increased vulnerability to eating disorders, obesity, as well as alcohol and drug dependence. Along with our findings for dopamine, these deeply rooted structural attributes of the human brain are likely to have emerged early in the human clade, laying the groundwork for later brain expansion and the development of cognitive and behavioral specializations.
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- PAR ID:
- 10469121
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 120
- Issue:
- 38
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2311118120
