stress

It is well known 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 challenge the developmental process?

Research has suggested that both positive and negative experiences, chronic stressors, and various other environmental factors may affect a young child’s developing brain. And now, studies involving animals are revealing in greater detail how this may occur.
One important line of research has focused on brain systems that control stress hormones—cortisol, for example. 1,2 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.3-5 Excess cortisol may cause shrinking of the hippocampus, a brain structure required for the formation of certain types of memory.
In experiments with animals, scientists have shown that a well-defined period of early postnatal development may be without human handling of the pups an important determinant of the capac- also produce pups that have a similarly
ity to handle stress throughout life.2 In stable reaction, including an approprione set of studies, rat pups were ate stress hormone response.removed each day from their mothers for a period as brief as 15 minutes and Striking differences were seen in rat then returned. The natural maternal pups removed from their mothers for response of intensively licking and periods of 3 hours a day, a model of grooming the returned pup was shown maternal neglect, compared to pups to alter the brain chemistry of the pup that were not separated. After 3 hours, in a positive way, making the animal the mother rats tended to ignore the less reactive to stressful stimuli. While pups, at least initially, upon their these pups are able to mount an appro- return. In sharp contrast to those pups priate stress response in the face of that were greeted attentively by their threat, their response does not become mothers after a short absence, the
excessive or inappropriate. Rat mothers “neglected” pups were shown to have who spontaneously lick and groom a more profound and excessive stress their pups with the same intensity even response in subsequent tests. This response appeared to last into adulthood.

The implications of these animal studies are worrisome. However, research is in progress to determine the extent
to which the hypersensitive or dysregulated stress response of “neglected” rat pups can be reversed if, for example,
foster mothers are provided who will groom the pups more intensely, or if the animals are raised in an “enriched” environment following their separation. An enriched setting may include, for example, a diverse and varied diet, a running wheel, mazes, and changes of toys.
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 and found that the “privileged” rats consistently have
a thicker cerebral cortex and denser networks of nerve cells than the “deprived” rats.

9,10 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.

11 This is a pattern often seen in humans with post-traumatic stress disorder and depression.5 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 in humans. However, animal models that show the interactive effect of stress and brain development
deserve serious consideration and continued study.

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