Brain Signal Discovered that Turns Off Fear

The study was reported in the November 7, 2002, issue of Nature.

Normally, rats freeze with fear when hearing a tone they have been conditioned to associate with an electric shock. Researchers have detected a brain signal that when stimulated eliminates a fear response by mimicking the brain's own "safety signal.” They say this discovery could lead to new physiologic treatments for runaway fear responses such as those seen in anxiety disorders.

The researchers recorded electrical activity of neurons in the prefrontal cortex as rats were fear-conditioned. Then they abolished this conditioning by presenting the tone without the shock and the animals no longer froze when they heard the tone. Although inactive during both procedures, neurons near the middle of the prefrontal cortex 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, the less the rats froze.

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. 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 anxiety.

"Repeated exposure to traumatic reminders without any adverse consequences causes fear responses to gradually disappear,” explained Dr. Gregory Quirk, of the Ponce School of Medicine (Ponce, Puerto Rico), who led the research. "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 research was funded by the US National Institute of Mental Health (NIMH)




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National Institute of Mental Health (NIMH)