Rats usually learn quickly to fear and avoid an odor that predicts a painful electrical shock. But rat pups learn to prefer an odor that is associated with electrical shock when their mother is present during the experiment. A study published this week in Nature Neuroscience shows that this may be due to the low activity of the neurotransmitter dopamine within the amygdala of rat pups.
The amygdala, a small region of the brain, is required for normal fear learning. In infant rats the amygdala might be too immature to function properly, which could explain why the pups do not learn to fear the shock-associated odor.
Gordon Barr, Regina Sullivan, and colleagues compared the biochemical responses in the amygdala of both 8 and 12 day rat pups to paired odor-shock stimuli while their mother was present. In the older pups, which had learned to avoid the odor, the scientists found elevated levels of dopamine. In the younger pups, which did not learn to avoid the odor, no such increase of dopamine was seen. When amygdala dopamine levels were artificially elevated, the young pups also learned to avoid the odor.
The infants of many mammalian species develop a strong attachment to their mother, or other primary caregiver, during the initial helpless postnatal phase of their lives. This attachment, regardless of the quality of care given, is thought to be an innate mechanism for survival. The infant rats' paradoxical odor-shock preference is therefore considered to be an animal model of abusive human caregiver-child relationships. As such, the work by Barr and colleagues suggests one mechanism that may foster infant attachment even to inadequate primary caregivers.