They conditioned the rat pups to associate a new odor with a negative eventa 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 behaviorsthe 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 abusiv
|Contact: John Ascenzi|
Children's Hospital of Philadelphia