Serotonin transporter gene polymorphism, differential early rearing, and behavior in rhesus monkey neonates

A polymorphism in the serotonin (5-HT) transporter gene regulatory region (5-HTTLPR) is associated with measures of 5-HT transporter (5-HTT) expression and 5-HT-mediated behaviors in humans. An analogous length variation of the 5-HTTLPR has been reported in rhesus monkeys (rh5-HTTLPR). A retrospective association study was conducted on 115 rhesus macaque infants either homozygous for the long 5HTTLPR variant (I/I) or heterozygous for the short and long form (I/s). To assess contributions of genotype and early rearing environment, 36 mother-reared monkeys (I/I = 26, I/s = 10) and 79 nursery-reared monkeys (I/I = 54, I/s = 25) were assessed on days 7, 14, 21, and 30 of life on a standardized primate neurobehavioral test designed to measure orienting, motor maturity, reflex functioning, and temperament. Both mother-reared and nursery-reared heterozygote animals demonstrated increased affective responding relative to I/I homozygotes. Nursery-reared, but not mother-reared, IN infants exhibited lower orientation scores than their I/I counterparts. These results demonstrate the contributions of rearing environment and genetic background, and their interaction, in a nonhuman primate model of behavioral development.