Using strains of mice inbred to display either low or high anxiety-like
behavior, researchers in California are looking for new ways to think about
therapeutic strategies for anxiety disorders, reported Carrolee Barlow, M.D.,
Ph.D., of Brain Cells Inc. in San Diego, Calif., at a meeting of the American
Society for Genetic Therapy in Baltimore.
Panic disorder, obsessive-compulsive disorder, posttraumatic stress
disorder, social phobia, and other anxiety disorders are familiar diagnoses in
DSM-IV and in the caseloads of most psychiatrists. For the moment,
SSRIS, MAOIs, benzodiazepines, and beta-blockers are used to control symptoms
as neuroscientists search for brain regions and genetic switches to explain
and treat these conditions.
While the presence of true anxiety cannot be confirmed in mice, scientists
have found valid analogues.
"In animals, you have to rely on anxiety-like behavior because you
don't know if they have real anxiety," explained Barlow, who
collaborates with scientists at the Salk Institute for Biological Studies and
Mouse anxiety reveals itself in several measures of the animals' behavior.
For instance, the open-field test measures the time mice spend in the middle
of the test area compared with hugging the wall around the field. More-anxious
mice spend more time near the edge, presumably a more comforting, less-exposed
location, than do less-anxious mice, who spend more time in the open.
Barlow and her colleagues used microarray technology to assess the
expression levels of about 10,000 possible genes in seven brain areas.
Analysis of the results found a correlation between 17 candidate genes and
anxiety-related phenotypes across all the mouse strains tested.
"These genes were not just differentially expressed, but were
differentially expressed in a pattern matching anxiety behavior," said
Barlow. A statistically significant pattern also distinguished the most and
least anxious strains of mice.
Five of the 17 candidate genes expressed enzymes. Of those, glyoxalase 1
(Glo1) and glutathione reductase 1 (Gsr) had activity assays available and
correlated with anxiety-like behavior. They also matched the pattern found in
the microarrays and in subsequent reverse transcriptase quantitative
polymerase chain reaction analysis. The highest activities of these two
enzymes occurred in the most anxious strain and the lowest in the least
anxious. The next step was to see whether expression of these two enzymes
might cause anxiety.
Regulation of anxiety and fear occurs in the amygdala, periaqueductal gray,
pituitary gland, hypothalamus, hippocampus, cingulate cortex, and the bed
nucleus of the stria terminalis. The researchers chose the cingulate cortex as
their initial target.
Barlow and her colleagues attached the two relevant genes, Glo1 or Gsr, or
a control protein to lentiviruses and injected them into the cingulate cortex
of both anxious and nonanxious strains of mice, and in F1 crosses between the
two strains. Lentiviral vectors can enter central nervous systems cells and
don't cause cytotoxicity. The gene-bearing viral vectors caused overexpression
of the Glo1 or Gsr genes when they infect the brain. Both genes increased
anxiety behavior in both groups of mice.
The researchers next sought to decrease anxiety-like behavior by using a
small interfering RNA (siRNA), which inhibits Glo1 expression. Both an anxious
mouse strain and an intermediate strain spent significantly more time in the
middle of the test chamber than did control mice when an siRNA against Glo1
was injected into the cingulate cortex.
The two genes are part of a pathway that regulates oxidative stress, said
Barlow, suggesting that misregulation of oxidative stress may lead to anxiety.
Gsr regulates glutathione, a major antioxidant in the brain, and Glo1 uses
glutathione as a cofactor to detoxify cytotoxic methylglyoxal. Other studies
have suggested that oxidative stress may be involved in other neuropsychiatric
Barlow said it is too soon to argue for the effect of a single gene on a
complex behavior like anxiety, but "it's better to start with a very
selective pharmacological approach, rather than a systemic one, to better
tease out the effects of each gene."
These newly identified genes mark another step in understanding how mouse
(and eventually human) brains regulate normal and pathological anxiety, she
More information on this topic appeared in an article by the same
researchers in Nature in 2005. It is posted at<www.nature.com/nature/journal/v438/n7068/abs/nature04250.html;jsessionid=470C376688A618E7861367CEF4D4EEC3>.▪