Weaning or Withdrawal?
Bonding with mom is an important part of a monkey’s development. But just like with people, some kids are easy to wean, while others cling. Now a study of monkeys links these differences in attachment to a gene that’s known to be important in addiction.
Christina Barr and colleagues at the National Institute on Alcoholism & Alcohol Abuse study the mu-opioid receptor gene because it tells brain cells to make receptors that respond to opium-like molecules, including the body’s natural painkillers, and other pleasure chemicals, but alsolike those in narcotics, alcohol and nicotine. Some people have a version of the gene that is much more sensitive to the rewarding effects of these chemicals than people without this version.
“It’s thought that individuals who experience more of a high following taking a drink are going to regularly consume alcohol and therefore be more prone to developing alcohol addiction,” Barr explains. “We’ve actually been studying this gene in rhesus macaques who have a similar alternative form of the gene to the one that’s present in humans. And what really intrigued me about this was the fact that you had these very similar patterns of variation in two unrelated species, because it suggested to me the possibility that there was some kind of benefit to having a more sensitive mu-opioid receptor.”
“Maybe early in the evolutionary history of humans, [these gene variations] could have conferred some kind of benefit, but then now in modern society, where we have access to drugs and alcohol, [the variations] may make an individual more at risk for developing addiction,” she explains.
Barr says a natural place to look was at in the realm of emotional attachments, since personal and social attachments are also mediated by the brain’s reward system. There’s also an obvious way that increased attachment to mom could have been beneficial.
“The development of attachment of an infant to its mother or caregiver is critical for that infant to survive, because she provides not only nourishment, but also protection from predation and injury and other kinds of dangers,” says Barr.
The researchers collaborated with researchers at the National Institute of Child Health and Human Development who had collected observations of differences in attachment behaviors in colonies of rhesus monkeys. They also genotyped the monkeys and could compare their behavior with their DNA.
They published their findings in the Proceedings of the National Academy of Sciences.
“These infants who have this alternative form of the gene which makes them more sensitive appeared to be more clingy with their mothers,” she says. “So even earlier in infancy, they spent more time in social contact with their mothers.”
And at six months of age, those monkeys are also more resistant to weaning. “This is the time at which most infants are becoming more independent, they’re exploring their environments, they’re interacting and playing with peers. And what we found was that animals that had this alternative form of the mu-opioid receptor gene were not exhibiting these behaviors.”
She says it’s striking how these behaviors resemble addiction. “In a sense it’s very similar to effects that you would see during periods of intoxication and withdrawal.”
She says the next step is to find out if this link is true in people. If so, it might help in both prevention and treatment of addictions.
Barr and senior author Markus Heilig say, “If it’s the case that in humans there is a common genetic link between early attachment differences and later alcohol use, this could actually provide us with strategies for early personalized prevention of alcohol use disorders.”
As for treatment, other studies have shown a drug that acts on the receptor, naltrexone, appears to be more effective in alcoholics with the more sensitive form of the gene. “If an individual has a history of having an attachment disorder, or has a current attachment disorder, you might be more likely to recommend trying this kind of drug for treatment of their alcohol problem.”
But the researchers add that anyone can get addicted if they use drugs or alcohol. This gene is just one factor that has been found to increase people’s’ risk.
The research is also a contribution to attachment research. Disorders such as anxious attachment, are generally attributed to abuse, neglect or otherwise poor parenting in early childhood, and not as having a genetic basis.
If some differences in attachment are due to genetic variations, it could clear up something plenty of moms like Barr herself wonder about.
“This might inform us of why we might have one child that we treat one way and it exhibits one type of behavioral response to our leaving, and then on the other hand, we might have another child who, even if we treat them in a very similar way, could respond differently,” she says. “Because there are not only these environmental variables and behavior of the mother that influence the development of mother-infant attachment, but also potentially a number of different genetic variables as well.”
This research was published in PNAS Early online edition the week of March 31, 2008, and funded by the intramural research programs of the National Institute on Alcoholism and Alcohol Abuse and the National Institute of Child Health and Human Development.
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