Depressed Mice Reveal Critical Chemical Pathway For Treatment

Serotonin, a chemical that helps brain cells communicate with one another, is the target of the most successful anti-depressant medications. Low levels of serotonin are implicated in depression and many other psychiatric disorders, including increased anxiety, aggression and obsessive-compulsive disorder.

The Duke team created mice with a mutation in the gene for tryptophan hydroxylase 2 (Tph2), which helps make serotonin in the brain. An equivalent human mutation has been identified in some people with unipolar major depression. These patients often show resistance to treatment with SSRI antidepressant drugs.

Mice with the mutation had 80 percent less serotonin in their brains than normal mice and exhibited behavioral changes that mirror the symptoms of humans with low serotonin.

However, the study revealed a possible means for alleviating these symptoms. The drop in serotonin levels was accompanied by an increase in the activity of another enzyme, called glycogen synthase kinase 3 (GSK-3), which helps a cell respond to chemical signals, including serotonin.

Communication between cells operates much like a string of medieval signaling towers – a fire lighted in one tower alerts the next in the chain, quickly transmitting a message across far distances. The Duke researchers discovered that blocking one of these signaling towers, GSK-3, restored normal behavior in the mutant mice.

The findings appear in the January 29, 2008 edition of the Proceedings of the National Academy of Sciences. The study was funded by the National Institutes of Health, the Lennon Family Foundation, NARSAD and the Canadian Institutes of Health Research.

GSK-3 is well known in the pharmaceutical industry – many different psychiatric drugs block the enzyme, including lithium, selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants and monoamine oxidase inhibitors (MAOIs).

The researchers tested the SSRI drug fluoxetine (Prozac) in the mutant mice, finding that short-term treatment relieved the animal's depressive symptoms and inhibited GSK-3 activity in the brain. The team is now evaluating the effects of long-term treatment with SSRI drugs.

They also prevented depression from developing by breeding mice with a mutation in the gene for GSK-3. "That GSK-3 is involved was expected. But the fact that removing one version of the GSK-3B gene reversed the behavior was quite surprising to us," said lead author Jean-Martin Beaulieu, Ph.D., now at Université Laval in Quebec. "This suggests that serotonin's effects on mood and aggression may be partly controlled through regulation of GSK-3 activity."

The dramatic drop in serotonin seen in the mice is caused by a single-letter difference in the spelling of a gene that has 200,000 letters of DNA code. This one-letter change is called a single nucleotide polymorphism, or SNP – a site where the DNA sequence of individuals differs by just one of four nucleotides (A, T, C or G). For example, some people may have G at a particular site, while others have an A. The SNP studied by the Duke researchers affects the Tph2 gene, built of some 100,000 nucleotide pairs.

The study also confirms that the Tph2 enzyme is critical for making brain serotonin, said Xiaodong Zhang, Ph.D., study co-author and an assistant professor at the Duke-NUS Graduate Medical School Singapore. The results imply that humans with this mutation may have serious deficits in brain serotonin, he said.

In addition to revealing new clues to serotonin signaling in the brain, the Tph2-mutant mice could also serve as an animal model of drug-resistant depression. The Duke researchers have patented the strain of mice used in the study, said senior study author Marc Caron, Ph.D., James B. Duke professor of cell biology.

"These animals may be one of the better models for preclinical studies," Caron said. "We now have an animal model that mimics many of the things you would expect of people that are depressed."

Collaborators on the study include Ramona Rodriguiz, Tatyana Sotnikova, Michael Cools, William Wetsel and Raul Gainetdinov, all of Duke.