Press Release November 06, 2002
Mimicking Brain’s “All Clear” Quells Fear in Rats
Rats normally freeze with fear when they hear a tone they have been conditioned to associate with an electric shock. Dr. Gregory Quirk and Mohammed Milad, Ponce School of Medicine, Puerto Rico, have now demonstrated that stimulating a site in the front part of the brain, the prefrontal cortex, extinguishes this fear response by mimicking the brain's own "safety signal." They report on their findings in the November 7, 2002 Nature.
"Repeated exposure to traumatic reminders without any adverse consequences causes fear responses to gradually disappear," explained Quirk. "Such reduction of fear appears to be an active rather than passive process. It doesn't erase the fear association from memory, but generates a new memory for safety."
The researchers recorded electrical activity of neurons in the prefrontal cortex as rats were fear-conditioned—taught to fear a tone by repeatedly pairing it with a shock. Then they abolished this conditioned fear by presenting the tone without the shock; the animals no longer froze when they heard the tone.
Although inactive during both procedures, neurons near the middle of the prefrontal cortex, the infralimbic area, fired conspicuously when the tone was sounded on the following day. This activity proved to be the brain's way of signaling that the tone no longer presaged a shock. The more the cells fired—i.e., the louder this safety signal—the less the rats froze. Animals that showed the most infralimbic activity behaved as if they had never been fear conditioned at all.
The researchers then electrically stimulated the infralimbic area in rats that had been fear conditioned but not extinguished—in effect simulating the safety signal, while pairing it with the tone. Remarkably, the rats showed little freezing. Later, the rats continued to be unafraid of the tone even without the stimulation, suggesting that memory for extinction was strengthened by experimentally mimicking the safety signal.
Since the prefrontal cortex is known to project to the amygdala, a hub of fear memory deep in the brain, the researchers propose that increased activity of infralimbic neurons in the prefrontal cortex strengthens memory of safety by inhibiting the amygdala's memory of fear. They speculate that stimulating parts of the prefrontal cortex in anxiety disorder patients, using an experimental technique called transcranial magnetic stimulation, might help them control fear.
The mission of the NIMH is to transform the understanding and treatment of mental illnesses through basic and clinical research, paving the way for prevention, recovery and cure. For more information, visit the NIMH website.
About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit the NIH website.
NIMH Press Office
More Science News about:
- Mental Health Information
- Statistics on Mental Disorders
- Summaries of Scientific Meetings
- Information about NIMH
- RePORTER: Research Portfolio Online Reporting Tool Expenditures and Results
- PubMed Central: An Archive of Life Sciences Journals
- Recommendations for Reporting on Suicide
- News from the FieldExternal Link: Please review our disclaimer.
News From the Field
NIMH-Funded Science on EurekAlert
- Out of Sync With the World: Body Clocks of Depressed People Are Altered at Cell LevelExternal Link: Please review our disclaimer.
- Nerve Stimulation for Severe Depression Changes Brain FunctionExternal Link: Please review our disclaimer.
- Nearly 20 Percent of Suicidal Youths Have Guns in Their HomeExternal Link: Please review our disclaimer.