A research team found that when baby rhesus monkeys endured high rates of maternal rejection and mild abuse in their first month of life, their brains often produced less serotonin, a chemical that transmits impulses in the brain. Low levels of serotonin are associated with anxiety and depression and impulsive aggression in both humans and monkeys.
Abused females who became abusive mothers in adulthood had lower serotonin in their brains than abused females who did not become abusive parents, the research showed.
Because the biological make up of humans and monkeys is quite similar, the findings from the monkey research could be valuable in understanding human child abuse, said Dario Maestripieri, Associate Professor in Comparative Human Development at the University of Chicago.
"This research could have important implications for humans because we do not fully understand why some abused children become abusive parents and others don't," Maestripieri said. The research suggests that treatments with drugs that increase brain serotonin early in an abused child's life could reduce the likelihood that the child will grow up to become abusive, Maestripieri said.
Maestripieri is lead author of a paper reporting the research, "Early Maternal Rejection Affects the Development of Monoaminergic Systems and Adult Abusive Parenting in Rhesus Macaques" published in the current issue of Behavioral Neuroscience.
The study is the first to show that naturally occurring individual variation in maternal behavior in monkeys can affect the brain development of the offspring, and the first to show a link between serotonin and infant abuse in primates. Other research using rats as animal models has shown that serotonin production is influenced by a mother's treatment of her off spring early in their lives and affects the offsprings's responses to stress in adulthood.
In earlier research on abuse of infant monkeys, Maestripieri and his colleagues studied females that were switched at birth between abusive and non-abusive mothers. That research showed that the offspring of non-abusive mothers were likely to become abusive themselves if they were raised by abusive mothers. That finding showed that the intergenerational transmission of abusive parenting was the result of early experience and not of genetic similarities between mothers and daughters.
"We didn't know if the reason that the babies grew up to be abusive was that they watched how their mothers parented and repeated their behavior or whether their early experience produced long-term changes in brain processes that regulate emotions. We also noticed that about half of the babies exposed to abuse became abusive parents, whereas the other half did not. This is also the case in humans as not all abused children grow up to be abusers."
In order to study the possible effects of early abuse on brain development, the team watched the mothers as they parented to note hitting and other negative behaviors toward their infants, and took samples of their cerebrospinal fluid to measure the metabolite concentrations of brain neurotransmitters such as serotonin every six months. They followed the infants until they became mothers themselves and studied their maternal behavior to note any abusive behaviors toward their infants.
The team found that infants who became abusive as adults had about 10 to 20 percent less serotonin than did infants who did not become abusive as parents or infants who were not exposed to maternal abuse. The reduced level of serotonin remained constant into adulthood.
Monkeys who were rejected more by their mothers early in life had lower serotonin in their brains than monkeys reared by less rejecting mothers. The effects of ma ternal behavior on brain serotonin in the offspring were observed in both infants that were reared by their genetic mothers and infants reared by foster mothers.