March 31, 2000
La Jolla, CA – Mice are not usually noted for their stalwart natures, but a new Salk Institute study shows that the loss of a single gene can render them especially anxious. The resulting "neurotic" mice approach new situations tentatively and appear to experience stress more acutely than normal mice.
The nervous rodents, according to the study's authors, suggest potential new targets for anti-anxiety and anti-depressant drugs.
"The stress response has many components," said Kuo-Fen Lee, Salk assistant professor and senior author of the study, which appears in the current Nature Genetics.
"There are likely to be a number of points in the pathway that could serve as drug targets," he added. "The key will be identifying appropriate therapies for different individuals."
As reported in the study, the Salk-led team created the anxious mice by removing a gene called CRHR2 (for corticotropin releasing hormone receptor 2). This receptor – a molecule that can detect and respond to hormones – normally resides in the brains of mice, and in humans as well, where it can recognize the hormone CRH, known to play a central role in stress responses.
Without this receptor, mice react far stronger and quicker to stress.
For example, the "neurotic mice" show heightened sensitivity to having their movements restricted. Their levels of the stress hormone ACTH (adrenocorticotropic hormone) peaked after only two minutes of restraint, whereas it took 10 minutes for normal mice to respond vigorously. Also, the mutant mice produced seven times more of another stress hormone, corticosterone, than the control mice.
"This tells you the response is extremely accelerated," said Tracy Bale, first author on the study and a postdoctoral fellow in Salk Professor Wylie Vale's laboratory. She added that it is relatively easy for mice to release ACTH rapidly, since it is held in reserve in the pituitary gland, "ready to be dumped into the bloodstream."
Release of corticosterone, however, is dependent upon ACTH traveling through the bloodstream to the adrenal glands, where it switches on genes needed to manufacture corticosterone.
"That this can all happen inside two minutes indicates that these animals are devoting a lot of resources to reacting to stress," said Bale.
The "neurotic" mice show heightened anxiety in behavioral tests, huddling in enclosed and protected spaces to a greater extent than their counterparts and displaying a reluctance to explore new and open areas. The anxious mice also differ in their abilities to regain appetite when food is withheld for a time; their blood pressure also takes longer to drop in response to urocortin (a hormone related to CRH; both hormones were discovered at Salk). All animals in the study were genetically matched except for the CRHR2 gene.
The brains of the mutant mice appear to develop normally, but the investigators found differences in gene activity.
"We found elevated levels of the stress hormone CRH in the amygdala," said Lee. "This is particularly intriguing because we know that this part of the brain is very much involved in coordinating fear and stress responses. It suggests that these mice may have emotionally heightened responses to stressful elements of their environment."
The current study builds on earlier work by Salk scientists with a stress-resistant or "mellow" mouse created by deleting the CRHR1 receptor, which also binds the hormone CRH.
"Now we think that although CRHR2 can bind CRH weakly, a more important partner is urocortin, which interacts with CRHR2 in a self-regulating loop to promote and limit the body's responses to stress," Vale said. "It underscores how complex the whole system really is."
The human CRHR1 receptor, isolated in Vale's laboratory at Salk, is currently being investigated by several pharmaceutical companies as a potential drug target for treatment of depressive and anxiety disorders. The current work suggests that CRHR2 may be another candidate for drug development.
Salk co-authors include Professor Paul Sawchenko and postdoctoral fellows Raymond Chan and George Smith; collaborators at The Scripps Research Institute include Professor George Koob, Assistant Professor Lisa Gold and postdoctoral fellow Angelo Contarino. The study, titled "Mice deficient for corticotropin-releasing hormone receptor-2 display anxiety-like behaviour and are hypersensitive to stress," was supported by the National Institutes of Health, the Robert J. and Helen C. Kleberg Foundation, the Ludwick Family Foundation and The Foundation for Research.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation.