You cannot grow up without leaving your mother鈥檚 protection and learning to make your own way in the world. Now for rat pups, at least, we have a clear idea of the changes in the brain involved in this vital transition.
A team led by of New York University鈥檚 Langone Medical Center previously found that young pups will become attracted to odours 鈥� even when those odours are paired with electric shocks. Only when pups reach about 10 days old do they become capable of associating odours with negative stimuli. What enables this transition from blind attraction to the ability to learn about potential danger?
Sullivan found previously that odours associated with their mother suppress the release in rat pups of the stress hormone corticosterone. Now, together with of the Children鈥檚 Hospital of Philadelphia, her team has shown that corticosterone suppression in turn reduces levels of the neurotransmitter dopamine in the amygdala, a structure that acts as the brain鈥檚 鈥渇ear centre鈥�.
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Smelling a rat
The researchers found that in 8-day-old pups exposed to shock-paired odours, genes involved in the release of dopamine were relatively inactive. But these genes were much more active in 12-day-old pups, which learned to avoid the shock-linked odours.
What鈥檚 more, when the team injected 8-day-olds with corticosterone, the activity of the dopamine-associated genes rose, and the pups learned to avoid the shock-paired odours just like older rats. Directly infusing dopamine into the amygdala of the young pups had the same effect on behaviour. 鈥淒opamine was the only thing really correlated with what the pup was learning,鈥� says Sullivan.
Child abuse
These changes in brain physiology may be necessary in a species like rats, in which the young are born helpless. Initial attachment to the mother is vital for survival, but later, once the animals start to explore for themselves, it becomes important to replace blind attachment with the ability to learn about potential dangers.
If similar mechanisms operate in the brains of young monkeys and human infants, it may help explain why they remain strongly attached even to abusive mothers.
at the University of Chicago has studied this phenomenon in captive rhesus monkeys, where 5 to 10 per cent of mothers physically abuse their offspring.
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