Some previous proposals for the role of depression in evolution have
focused on how it affects behavior in a social context. A pair of
psychiatrists addresses this puzzle in a different way, tying together
depression and resistance to infection. They propose that genetic
variations that promote depression arose during evolution because they
helped our ancestors fight infection.
Could depression be an evolutionary byproduct of the ability to fight infection?
An outline of their proposal appears online in the journal Molecular Psychiatry.
The co-authors are Andrew Miller, MD, William P. Timmie professor of
psychiatry and behavioral sciences at Emory and director of psychiatric
oncology at Winship Cancer Institute, and Charles Raison, MD, previously
at Emory and now at the University of Arizona.
For several years, researchers have seen links between depression and
inflammation, or over-activation of the immune system. People with
depression tend to have higher levels of inflammation, even if they're
not fighting an infection.
"Most of the genetic variations that have been linked to depression
turn out to affect the function of the immune system," Miller says.
"This led us to rethink why depression seems to stay embedded in the
genome."
"The basic idea is that depression and the genes that promote it were
very adaptive for helping people -- especially young children -- not
die of infection in the ancestral environment, even if those same
behaviors are not helpful in our relationships with other people,"
Raison says.
Infection was the major cause of death in humans' early history, so
surviving infection was a key determinant in whether someone was able to
pass on his or her genes. The authors propose that evolution and
genetics have bound together depressive symptoms and physiological
responses that were selected on the basis of reducing mortality from
infection. Fever, fatigue/inactivity, social avoidance and anorexia can
all be seen as adaptive behaviors in light of the need to contain
infection, they write.
The theory provides a new explanation for why stress is a risk factor
for depression. The link between stress and depression can be seen as
the byproduct of a process that preactivates the immune system in
anticipation of a wound, they write.
Similarly, a disruption of sleep patterns can be seen in both mood
disorders and when the immune system is activated. This may come from
our ancestors' need to stay on alert to fend off predators after injury,
Miller says.
Miller and Raison's theory could also guide future research on
depression. In particular, the presence of biomarkers for inflammation
may be able to predict whether someone will respond to various
treatments for depression.
Miller and Raison are involved in ongoing research on whether certain
medications, which are normally used to treat auto-immune diseases, can
be effective with treatment-resistant depression.
From sciencedaily
0 comments:
Post a Comment