Stress and the Developing Brain

 

 

We know that the early months and years of life are critical for brain development. But the question remains: just how do early influences act on the brain to promote or injure emotional development? Research has shown that many environmental influences such as infections or trauma have an effect on a young child’s brain development. Years of observation have shown that chronic stressors adversely affect brain development and now, animal studies are showing us in greater detail how this occurs.

 

One important line of research has focused on brain systems that control stress hormones--cortisol, for example. Cortisol and other stress hormones play an important role in emergencies: they help our bodies make energy available to enable effective responses, temporarily suppress the immune response, and sharpen attention. However, a number of studies conducted in people with depression indicate that excess cortisol released over a long time span may have many negative consequences for health. In the brain excess cortisol may cause shrinking of the hippocampus, a brain structure required for the formation of memories of persons, places, and events.

 

In experiments using animal models, scientists have shown that the first week of life is a critical period in the rat pup for its capacity to handle stress throughout life. In one set of experiments, rat pups were removed each day from their mothers for a period of 15 minutes and then returned. The maternal response of intensively licking and grooming the returned pup was shown to alter the brain chemistry of the pup in a positive way, making the animal less stress reactive. While these pups are able to mount an appropriate stress response in the face of threat, their response does not become excessive or inappropriate. Rat mothers who spontaneously lick and groom their pups with the same intensity even without human handling of the pups also produce pups that have a similarly stable, appropriate stress hormone response.

 

Striking differences were seen in rat pups removed from their mothers for periods of three hours a day, a model of neglect. In these instances, the mother rats tended to ignore the pups, at least initially, upon their return. In sharp contrast to those pups that were greeted attentively by their mothers after a short absence, the “neglected” pups were shown to have a more profound and excessive stress response in subsequent tests. This response appeared to last into adulthood. While the implications of these animal studies are worrisome, preliminary evidence now indicates that in rodents that have hypersensitive or dysregulated stress responses - for example, those “neglected” rat pups that were removed from their mothers for three hours a day - much of the damage can be repaired if the animals then are raised in an enriched environment.

 

Animal investigators are well aware of another kind of long-term change, again rooted in the first days of life. Laboratory rats are often raised in shoebox cages with few sources of stimulation. Scientists have compared these animals to rats raised in an enriched environment - characterized, for example, by a diverse and varied diet, a running wheel, mazes, and changes of toys - and found that the “privileged” rats consistently have a thicker cerebral cortex and denser networks of nerve cells than the “deprived” rats.

 

Another study recently reported that infant monkeys raised by mothers who experienced unpredictable conditions in obtaining food showed markedly high levels of cortiocotropin releasing factor (CRF), in their cerebrospinal fluid, and as adults, abnormally low levels of cerebrospinal fluid cortisol. This is a pattern often seen in humans with post-traumatic stress disorder and depression.

 

The distressed monkey mothers, uncertain about finding food, behaved inconsistently and sometimes neglectfully toward their offspring. The affected young monkeys were abnormally anxious when confronted with separations or new environments. They were also less social and more subordinate as adult animals.

 

It is far too early to draw firm conclusions from these animal studies about the extent to which early life experience produces a long-lived or permanent set point for stress responses or influences the development of the cerebral cortex. However, animal models that show the interactive effect of stress and brain development deserve serious consideration and continued study.

 

 

 

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