stress associated with mental arithmetic calculations
or simulated public speaking, results in
greater changes in plasma cortisol levels than
most reported differences between the values in
patients with depression and those in controls.38
It is possible that chronic mild elevations of cortisol,
especially at night, when cortisol levels in
normal subjects are very low, have a pathogenic
role in depression. It is also possible that peripheral
cortisol elevations are only a reflection of
central disturbances in CRH signaling, which mediate
the effects of environmental stress on mood.59
A major liability of the hypothalamic–pituitary–
adrenal axis theory of depression is the difficulty
of defining the relationship of stress to depression.
Some patients have a single lifetime depressive
episode, whereas a larger proportion have a
recurrent or even chronic course. Various types
of acute stress, early childhood trauma, or longterm
psychosocial problems may be involved and
may lead to different responses of the stress system.
Stress may be causative in some cases and
secondary to depressed mood in others.
Severe stress in rodents does not necessarily
model the common stresses of childhood. The
association of abuse in childhood with psychopathologic
disorders, including depression, in
adulthood could be due to common factors linking
family perpetrators of abuse and their victims,
including not only shared genes but also a shared
environment of poverty, poor nutrition, and poor
prenatal care. Depression is not uncommon in
people with no psychosocial risk factors. Most
patients treated for depression have no evidence
of hypothalamic–pituitary–adrenal dysfunction,
just as most such patients have no direct evidence
of brain monoamine deficiency.
The classic teaching is that neurons do not divide
in the adult mammalian brain, but studies
have shown that neurogenesis occurs in several
areas of the brain, especially the hippocampus.
Neurogenesis is more prominent in rodents than
in primates,60 and some have questioned whether
it occurs in the human cortex.61 Elevated levels of
glucocorticoids can reduce neurogenesis, and this
has been suggested as a mechanism for the decreased
size of the hippocampus on magnetic
resonance images of the brain in many patients
with depression.62 In postmortem studies of patients
with depression, cell loss in the subgenual
prefrontal cortex, atrophy in the dorsolateral prefrontal
cortex and the orbitofrontal cortex, and