Childhood Trauma and the Brain – Part 3

I’ll start this last part by describing some fascinating research by Montreal neuroscientist Michael Meaney that shows how early childhood experiences can have positive effects that can be measured in later adulthood. Paradoxically enough, this involved giving infant rats (pups) a stressful experience. Infant rats were removed from their mother for 15 minutes each day from post-natal day two to day 14. These were referred to as the “handled” rats.

When these rats reached adulthood, they showed that they could respond better to stress than non-deprived rats. This was determined by measuring the time it took for the rat to recover to normal stress hormone levels and their overall reactivity to the stress.

This finding intrigued Meaney who did follow-up studies. He initially thought that it was the handling outside the cage that produced this effect. However, he later determined that when the pup was returned to the mother she licked it vigorously and this promoted the changes in the infant rat’s nervous system. This led to additional studies in which his graduate students simulated this licking effect by stroking the rat pups with soft paintbrushes.

Meaney determined that this licking stimulated developmental changes in the rats that made them more effective in controlling their stress responses. It’s intriguing that this increased level of licking and grooming had such a big effect in the handled rats. What about in humans?

There are many reports that the best way to make premature infants thrive is to handle an stroke them repeatedly. While it’s crucial to provide them sustenance and warmth, stroking them has very positive effects. While this is important knowledge in its own right, it’s also important to recognize that this effect is similar in rats and human infants.

There was another aspect of Meaney’s research that is important. Meaney also had a group of pups that were removed daily from the mother for three hours at a time. In this case, the mothers didn’t lick them as much when they were returned to her and the experience for the pups was quite stressful. As adults, these rats had a reduced ability to recover from stress and were more reactive to stress.

This result clearly showed that the infant stress experienced from the maternal deprivation had significant negative effects. In contrast, the brief separation of the handled pups was not so much stressful as an opportunity for the mother to enhance her care of the pups. As such, the result was positive for the handled pups, but negative for the separated pups.

Recall from part one that unpredictability makes stresses more intense. Researchers have developed an ingenious way to provide an unpredictable experience for rat pups. The control pups were raised in a standard cage environment which had adequate food, water and nesting material. This allowed the mother to care for the pups in a regular and predictable way.

In contrast, the experimental group had a cage with insufficient nesting material. This meant that the mother could only nest with one or two pups at a time. Thus, the pups never knew when they would receive maternal care or for how long. In this condition the pups showed continual high levels of stress hormone and even some damage to the gland that produces the stress hormone. The result of this is that, as adults, these stressed pups showed memory deficits and difficulty learning about reward availability.

Under these conditions the mother also experiences chronic stress and her maternal behaviour was only intermittent. Thus, while temporary stress can led to better control of the stress response in adult rats, chronic stress can produce long-lasting learning deficits.