--Mother-Child Attachments in Animals and Possible Parallels in People--
PHILADELPHIA, Oct. 26 /PRNewswire-USNewswire/ -- When do you first leave the
nest? Early in development infants of many species experience important
transitions -- such as learning when to leave the protective presence of their
mother to start exploring the wider world. Neuroscientists have now pinpointed
molecular events occurring in the brain during that turning point.
Based on animal studies, the findings may shed light on the strength of
attachments in many species -- including the conundrum of why human children
form strong attachments to even abusive caregivers.
"This is one of the few times we know what causes this type of early
transition," said psychologist Gordon A. Barr, Ph.D., of The Children's
Hospital of Philadelphia, co-author of a study that appeared online Sept. 27
in Nature Neuroscience. Barr performed the studies in rats with a longtime
collaborator, neuroscientist Regina M. Sullivan, Ph.D., of the Nathan Kline
Institute and New York University Langone Medical Center.
The youngest rats, called pups, first experience the mother's presence with
both positive and negative stimuli. Even if the mother does something
unpleasant, like stepping on or biting a pup, the baby rat stays close by the
mother, something called preference learning. "From an evolutionary
standpoint, this makes sense," said Barr. "The dependent baby has a better
chance of survival if it doesn't stray from the mother's side."
However, at about ten days of age, the rat pups experience a transition to
so-called aversion learning, in which they learn to avoid unpleasant stimuli.
Said Barr, "Once an animal is better able to move around, it needs to be able
to escape from stressful situations, again in the interests of its survival."
The maturing rat learns a type of safe behavior while away from parental
protection.
For neuroscientists, one puzzle has been how to understand the underlying
biological events in the changeover from preference learning to aversion
learning. In a series of studies reported in the current paper, the authors
focused on neurotransmitters in the brain, then manipulated those chemical
messages to mimic their natural effects in rats.
They conditioned the rat pups to associate a new odor with a negative event --
a mild electric shock. In adult rats, but not in immature rats, a shock
induces a telltale increase in levels of the stress hormone corticosterone.
Increased corticosterone, in turn, causes the amygdala, a learning center in
the brain, to have increased levels of the neurotransmitter dopamine.
Using microarrays (to detect changes in dopamine-related gene expression) and
microdialysis (to measure changes in dopamine levels), the study team
confirmed that changes in dopamine levels were linked to changes in learning
patterns.
On about their tenth day of life, rat pups start to make the transition from
preference learning to aversion learning. Based on their
corticosterone/dopamine findings, Barr and Sullivan were able to chemically
manipulate the learning transition. By injecting eight-day-old rat pups with
corticosterone, the scientists advanced the animals' learning behaviors -- the
young rats avoided the new (shock-associated) odor, just as older rats did.
Eight-day-old control rats did not show such avoidance behavior.
Injecting dopamine directly into an eight-day-old rat's amygdala had a similar
effect, switching their usual preference learning to aversion learning typical
of older animals. The researchers also toggled the switch in the other
direction. By blocking dopamine receptors in eight-day-old rats already
treated with corticosterone, the rats showed preference learning instead of
the aversion learning induced by corticosterone.
The neural mechanisms they found, said Barr, may also apply to infant behavior
in dogs, rats and people. "For humans," said Barr, "the findings may shed
light on the pathologically strong attachment that children are known to have
even for abusive caretakers." In addition, he said, the findings suggest that
scientists may detect neural mechanisms at the heart of other developmental
transitions, such as an infant's switch from breastfeeding to eating solid
food.
The National Institutes of Health provided grant support for this study, for
which Barr was the principal investigator and Sullivan was co-principal
investigator. Barr began the study while at Hunter College and New York State
Psychiatric Institute in New York City. Sullivan began the study at the
University of Oklahoma, in Norman, Okla.
About The Children's Hospital of Philadelphia: The Children's Hospital of
Philadelphia was founded in 1855 as the nation's first pediatric hospital.
Through its long-standing commitment to providing exceptional patient care,
training new generations of pediatric healthcare professionals and pioneering
major research initiatives, Children's Hospital has fostered many discoveries
that have benefited children worldwide. Its pediatric research program is
among the largest in the country, ranking second in National Institutes of
Health funding. In addition, its unique family-centered care and public
service programs have brought the 430-bed hospital recognition as a leading
advocate for children and adolescents. For more information, visit
http://www.chop.edu.
Contact: John Ascenzi
The Children's Hospital of Philadelphia
267-426-6055
Ascenzi@email.chop.edu
SOURCE The Children's Hospital of Philadelphia
John Ascenzi of The Children's Hospital of Philadelphia, +1-267-426-6055,
Ascenzi@email.chop.edu
