EFFECT OF CHRONIC STRESS IN ADULT LIFE.

In recent years, behavioral neuroscientists have debated the
meaning and significance of a plethora of independently conducted
experiments seeking to establish the impact of chronic, early-life
stress upon behavior - both at the time that stress is experienced,
and upon the same individuals later in life, during adulthood.

These experiments, typically conducted in rodents, have on the
one hand clearly indicated a link between certain kinds of early
stress and dysfunction in the neuroendocrine system, particularly
in the so-called HPA axis (hypothalamic-pituitary-adrenal), which
regulates the endocrine glands and stress hormones including
corticotropin and glucocorticoid.
Yet the evidence is by no means unequivocal. Stress studies in
rodents have also clearly identified a native capacity, stronger in
some individuals than others, and seemingly weak or absent in
still others, to bounce back from chronic early-life stress. Some
rodents subjected to early life stress have no apparent behavioral
consequences in adulthood - they are disposed neither to anxiety
nor depression , the classic pathologies understood to be induced
by stress in certain individuals.
Experiments designed to assess the
impacts of social stress upon adolescent mice, both at the time
they are experienced and during adulthood. Involving many
different kinds of stress tests and means of measuring their
impacts, the research indicates that a "hostile environment in
adolescence disturbs psychoemotional state and social behaviors
of animals in adult life," the team says.
The tests began with 1-month-old male mice - the equivalent, in
human terms of adolescents - each placed for 2 weeks in a cage
shared with an aggressive adult male. The animals were
separated by a transparent perforated partition, but the young
males were exposed daily to short attacks by the adult males.
This kind of chronic activity produces what neurobiologists call
social-defeat stress in the young mice. These mice were then
studied in a range of behavioral tests.
"The tests assessed levels of anxiety, depression, and capacity to
socialize and communicate with an unfamiliar partner,". These experiments showed that in young mice chronic
social defeat induced high levels of anxiety helplessness,
diminished social interaction, and diminished ability to
communicate with other young animals. Stressed mice also had
less new nerve-cell growth (neurogenesis) in a portion of the
hippocampus known to be affected in depression: the subgranular
zone of the dentate gyrus.
Another group of young mice was also exposed to social stress,
but was then placed for several weeks in an unstressful
environment. Following this "rest" period, these mice, now old
enough to be considered adults, were tested in the same manner
as the other cohort.
In this second, now-adult group, most of the behaviors impacted
by social defeat returned to normal, as did neurogenesis, which
retuned to a level seen in healthy controls. "This shows that young
mice, exposed to adult aggressors, were largely resilient
biologically and behaviorally,".
However, in these resilient mice, the team measured two latent
impacts on behavior. As adults they were abnormally anxious, and
were observed to be more aggressive in their social interactions.
"The exposure to a hostile environment during their adolescence
had profound consequences in terms of emotional state and the
ability to interact with peers,"