Most people see DNA as the most significant factor in genetics, but when it comes to behavioral differences—even those as complex as mothers' affection—researchers say we shouldn't overlook other biochemical factors. Biologists have recently begun looking harder at epigenetics—the chemical modification to DNA that can change genes' activity—to explain things that basic DNA transcription can't. This year's Neal E. Miller lecturer, Michael Meaney, PhD, explained why it's important to psychology at APA's 2009 Annual Convention.
It's long been known that certain early experiences, such as abuse, family strife and emotional neglect can increase one's chances for depression, drug abuse, anxiety, heart disease and other conditions, but the mechanism behind this has been unclear, said Meaney, a brain researcher at McGill University.
"The question becomes, can we define the pathways by which the social environment might come to predict these particular health outcomes?" he said.
A popular theory has it that early experiences somehow alter people's neural and endocrine responses to stress, which more directly influences mental health and behavior. That could happen if a chemical alteration gets in the way of genes' expressing their normal responses to stress.
"DNA is just a molecule like any other molecule," he said. "It's subject to modification."
For example, methyl groups are biochemical marks that can modify DNA by binding to DNA and preventing other proteins known as a transcription factor from turning the gene on. That, effectively, silences the gene's expression.
But can that actually influence behavior over a lifetime? Meaney looked to rat models to find out. In his lab, he and his colleagues studied the licking and grooming behavior of mother rats toward their pups and divided them into consistently high-licking and low-licking groups. They found that pups reared by low-licking mothers carried the methyl mark on genes that normally inhibit stress responses. As adults, these animals showed a greater stress response than animals reared by high-licking mothers.
They also found those traits tended to carry on to the next generation: High-licking moms had daughters that went on to lick and groom their own litters more frequently than the daughters of low-lickers. But interestingly, when they took the offspring of a low-licking mother and raised it with a high-licking one, the rat adopts the high-licking behavior when it has babies of its own.
That suggests that the behavior isn't strictly genetic, but potentially influenced by epigenetics, too, Meaney said.
In rats, one source of significant stress is parental neglect. That stress in turn increases the amount of methyl groups in the body, making it more likely that they'll bind to susceptible spots in the genome. One such spot is on an area of the genome that promotes production of receptors for estrogen, which is associated with maternal care. Fewer estrogen receptors mean less oxytocin and lower maternal investment, which feeds back into increased stress for their offspring, triggering a feedback loop.
"What we've come to suppose is that at least within the rat, differences in methylation driven by maternal care are in fact responsible for changes in gene expression," Meaney said.
It's difficult to directly correlate these results to humans, he emphasized. But looking at autopsy reports of people who died either via suicide or highway accidents, and looking at their reported life histories, Meaney and his colleagues have found a correlation between a history of abuse and increased methylation.
"So we're starting to get some kind of a picture on how the quality of early family environment might be explaining, in part, variations in health outcomes," he said.
HAPPY BIRTHDAY, NEAL
This year commemorates the 100th birthday of Neal E. Miller, PhD (1909–2002), the renowned neuroscientist who was a driving force in the development of biofeedback. His research found that classical conditioning could alter the way the autonomic nervous system functions.
The Neal E. Miller Lecture Program is an annual APA Science Directorate event developed by the association's Board of Scientific Affairs. Each year, the board selects an eminent scientist to give a lecture on neuroscience and animal research at the APA Annual Convention. The program honors Miller's outstanding contributions to neuroscience and animal research in psychology. Miller delivered the first lecture in 1994.