The research, published in the October issue of The Journal of Clinical Investigation, also showed that blocking the enzyme reduced levels of stress hormones. The same issue of the journal includes a commentary on the research (Anxiolytic Drug Targets: Beyond the Usual Suspects) by Joshua A. Gordon of Columbia University's Center for Neurobiology and Behavior.
Anti-anxiety drugs such as Valium often leave people sedated, less capable at mental tasks, and can become addicting, whereas drugs without these side effects -- such as some antidepressants -- may not reduce anxiety as well, said Robert Messing, MD, UCSF professor of neurology at the Gallo Research Center and senior author on the paper.
But in the new study, mice lacking the enzyme showed reduced anxiety while maintaining normal levels of alertness and learning abilities, the researchers reported.
The enzyme, known as protein kinase C epsilon (PKC epsilon), is present in many neurons in the brain, but its role is not well known. Its ability to affect anxiety but not sedation may stem from indirect, rather than direct action, the researchers found.
When the neurotransmitter GABA binds to proteins on the surface of many neurons, known as GABA-A receptors, the neurons become less active, which tends to reduce anxiety. Drugs such as Valium act by increasing the action of GABA at the GABA-A receptors. A group of brain molecules derived from progesterone, known as neurosteroids, also act to increase the action at the GABA-A receptor, and thereby reduce anxiety. But PKC epsilon appears to make the GABA-A receptor less sensitive to the neurosteroids, increasing anxiety. Experiments using brain membranes and studies of mouse behavior showed that knocking out PKC epsilon increases GABA-A receptor sensitivity to neurosteroids, and thereby decreases anxiety.
"PKC modulates the modulator," Messing said. Because it is an intermediary, it is a promising focus for drugs that could decouple the biochemical pathway that leads to anxiety from the pathway that leads to sedation, he said.
The researchers focused on physiological and behavioral effects of knocking out the PKC gene. Messing and his colleagues developed the strain of PKC knockout mice in 1998 and have been studying the neurological effects.
In tests that measure the tendency of mice to avoid heights and exposed environments, the research showed that PKC epsilon-knockout mice expressed less anxiety-like behavior than wild-type mice. The mice also had lower levels of the stress hormone corticosterone. When GABA-A receptor activity was chemically blocked, anxiety and stress hormone levels increased to normal.
Other studies by Messing and colleagues have shown that knocking out the PKC epsilon gene reduces pain related to inflammation and decreases consumption of alcohol in mice. Work by others indicates that knocking out PKC epsilon reduces fertility of female mice by weakening their immune function. But a drug designed to inhibit PKC epsilon might well avoid this side effect, since it would not knock out all enzyme function, Messing noted.
"Although we don't yet know precisely how PKC epsilon normally affects GABA-A receptors, the research clearly shows its effect on anxiety-related behavior," Messing said. "A drug based on this finding may well treat anxiety without compromising alertness or cognitive abilities."
UCSF and the Gallo Clinic and Research Center have applied for a patent for the use of PKC epsilon inhibitors to treat anxiety.
First author on the paper is Clyde W. Hodge, PhD, formerly at UCSF's Gallo Clinic and Research Center and now an associate professor in the psychiatry department at University of North Carolina.
Co-authors are Jacob Raber, assistant professor of behavioral neuroscience and neurology at Oregon Health and Science University; Thomas McMahon, staff research associate; postdoctoral scientists Helen Walter, Ana Maria Sanchez-Perez, and Kristin Mehmert, and M. Foster Olive, senior scientist, all the Gallo Clinic and Research Center; and A. Leslie Morrow, professor in the pharmacology and psychiatry department of the University of North Carolina.
The research is funded by funds to UCSF from the State of California for medical research on alcohol and substance abuse, and by the National Institutes of Health.
Journal
Journal of Clinical Investigation