Cerebral Cortex Advance Access originally published online on June 22, 2007
Cerebral Cortex 2008 18(3):584-597; doi:10.1093/cercor/bhm089
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Differences in Cortical Serotonergic Innervation among Humans, Chimpanzees, and Macaque Monkeys: A Comparative Study
1 Department of Anthropology and School of Biomedical Sciences, Kent State University, Kent, OH, USA, 2 Department of Anthropology, The George Washington University, Washington, DC, USA, 3 Biological Sciences, Kent State University, Kent, OH, USA, 4 Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, USA, 5 Department of Neuroscience, Mount Sinai School of Medicine, New York, NY, USA, 6 The New York Consortium in Evolutionary Primatology, New York, NY, USA
Address correspondence to Mary Ann Raghanti, Department of Anthropology, Lowry Hall, Kent State University, Kent, OH 44242, USA. Email: mraghant{at}kent.edu.
In this study, we assess the possibility that the evolution of human intellectual capacities was supported by changes in the supply of serotonin to the frontal cortex. To this end, quantitative comparative analyses were performed among humans, chimpanzees, and macaques. Immunohistochemical methods were used to visualize serotonin transporter–immunoreactive (SERT-ir) axons within the cerebral cortex. Areas 9 and 32 were chosen for evaluation due to their roles in working memory and theory of mind, respectively. Primary motor cortex was also evaluated because it is not associated with higher cognitive functions. The findings revealed that humans do not display a quantitative increase in serotonin innervation. However, the results indicated region- and layer-specific differences among species in serotonergic innervation pattern. Compared with macaques, humans and chimpanzees together displayed a greater density of SERT-ir axons relative to neuron density in layers V/VI. This change was detected in cortical areas 9 and 32, but not in primary motor cortex. Further, morphological specializations, coils of axons, were observed in humans and chimpanzees that were absent in macaques. These features may represent a greater capacity for cortical plasticity exclusive to hominoids. Taken together, these results indicate a significant reorganization of cortical serotonergic transmission in humans and chimpanzees.
Key Words: area 4 • area 9 • area 32 • human evolution • prefrontal cortex • serotonin transporter