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Chimpanzees exhibited significant genetic and neuroanatomical sex-related differences while scratching, a common indicator of anxiety, according to a new study from  Georgia State University. This reveals their promise as a model of human mental illness.

In people, as well as primates, self-grooming actions like head scratching or beard stroking can indicate anxiety, uncertainty, social tension, and impending danger. For the current study, researchers assessed scratching behaviors in 76 chimpanzees that were socially housed at the Yerkes National Primate Research Center. Half of the chimpanzees exhibited DupA, one form of the vasopressin V1a receptor gene, which is responsible for complex social behavior. The other half of the group exhibited a different form of the gene, DupB. To stimulate scratching, the non-human primates were shown a 30 minute video including scenes and vocalizations of unfamiliar chimpanzees negotiating possession and sharing of a watermelon.

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The researchers took MRIs of each chimpanzee in order to determine how this behavior affected brain activity. "We then examined whether variation in a gene associated with the neuropeptide vasopressin, a neuropeptide known to be associated with social and affiliative behavior, was associated with scratching and if it this association varied by sex (i.e., nature of the association was different for males and females); it was associated with scratching and in a sex-specific manner. Using structural brain scans (magnetic resonance images), we then examined whether variation in this same gene was associated with differences in gray matter. After identifying which brain regions were associated with variation in this gene (which were largely located in the frontal cortex), we then went back to the behavior asking whether these regions then, in turn, were associated with scratching and, like the genetics data, whether these associations varied by sex," Robert D. Latzman, lead author of the study and assistant professor in the Department of Psychology at Georgia State told ALN exclusively.

Analysis revealed significant differences between male and female behaviors. DupB+/- males showed higher rates of scratching than DupB-/-, while the opposite was seen in females. Significant brain differences were seen between the two groups as well. "Findings suggest that the expression of negative emotionality or anxiety (as indexed by scratching behaviors in our study) is influenced by genetically-specific neuroanatomical variation in a sex-specific way (stronger effects of vasopressin for males than females)," he continued.

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This research could lead to a new model for studying metal illnesses in people. "Broadly, this study demonstrates the exceptional and unique promise that chimpanzees hold for research aimed at advancing our understanding of the relationship among genes, brain circuits, and psychopathology-relevant behaviors in service of better understanding psychiatric conditions in humans," Latzman concluded.

The research was published in the journal Pyschophysiology.

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