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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. 

Endogenous and exogenous cannabinoids signal through the cannabinoid 1 receptor (CB1R) to modulate various aspects of social behavior, including aggression and anxiety. In rodents and primates, CB1R expression in the basolateral amygdala is dense and cannabinoid signaling in this region has been reported to influence social behavior. Little is known about how endocannabinoid signaling in the amygdala contributes to primate social diversity. The behaviorally diverse and species-rich cercopithecoid monkey genus Macaca is an ideal model for addressing this topic. Japanese (Macaca fuscata) and rhesus macaques (M. mulatta) display similar social styles in some respects; however, there is evidence to suggest they differ in their stress response, amygdala structure, and monoaminergic signaling. To further assess the molecular basis of social style in Japanese and rhesus macaques, we used immunohistochemistry and stereological methods to compare CB1R-immunoreactive (CB1R-ir) axon density in the basolateral amygdala, which is comprised of the lateral, basal, and accessory basal nuclei. Our study sample included 6 Japanese and 5 rhesus macaques. Repeated-measures ANOVAs were used to evaluate species differences, with amygdala region as the within-subjects measure and species as the between-subjects factor. This revealed significant main effects for species and area (p values < 0.05) with no interaction. Post hoc tests revealed higher CB1R-ir axon density in the basal and accessory basal nuclei of rhesus macaques compared to Japanese macaques. Our results suggest that CB1R-mediated signaling in the lateral nucleus of the amygdala is comparable between the two species, while the differences we observed in the basal and accessory basal nuclei may contribute to the nuanced behavioral differences observed between them.