A new study takes a close look at the brain region that suppresses our fear.
According to recent estimates, 3.6 percent of the adult population in the United States have experienced post-traumatic stress disorder (PTSD) in the past year, while almost 7 percent have had the condition at some point in their lives.
Although almost half of all U.S. adults experience at least one traumatic event during their lifetime, not all of them develop PTSD symptoms, which include frightening flashbacks of the traumatic event, sleep problems, and difficulty fighting off fear.
Current treatments for PTSD include medications and various forms of therapy, including exposure therapy and talking therapy. However, most PTSD drugs target all of the neurons in the brain indiscriminately, while behavioral therapy does not entirely prevent relapse.
New research, however, may bring scientists closer to developing PTSD therapies that are more targeted, effective, and long-lasting.
Stephen Maren, the University Distinguished Professor of psychological and brain sciences at Texas A&M University in College Station, led a team of researchers who found a new area in the brain's thalamus that controls our response to fear.
Although the study was in rodents, the findings help illuminate the human brain's response to fear, as well as potential new clinical strategies for treating PTSD.
Nucleus reuniens 'critical' for fear extinction
Prof. Maren and his colleagues used c-Fos expression imaging to track the neuronal activity of rats that they exposed to fear-inducing scenarios. They began by pairing five audible tones with mild electric shocks that they delivered to the rodents' feet. This triggered the rodents' fear and created a Pavlovian response in them.
Then, Prof. Maren's team used the equivalent of exposure therapy on the rodents, gradually exposing them to the five tones for extended periods.
In this fear extinction context, the neurons in the rodents' nucleus reuniens were more active and fired more in anticipation of the painful stimulus, indicating the suppression of fear.
The researchers also used pharmacogenetic tools called designer receptors exclusively activated by designer drugs to inhibit projection neurons in the rodents' prefrontal cortex.
These neurons projected to the nucleus reuniens, and the experiments revealed that inhibiting them made the rodents unable to suppress fear.
As the authors explain in their paper, scientists already knew that the "thalamic nucleus reuniens [...] receives dense projections from the medial prefrontal cortex [...] and may serve a pivotal role in regulating emotional learning and memory."
However, the new results show that the neurons in this area "are critical for the extinction of Pavlovian fear memories in rats."
The study's lead investigator comments on the findings, saying, "It's interesting because we know that the prefrontal cortex plays an emotion regulation role, and so there has been a lot of interest in how it accomplishes that."
"So [our] research, identifying this particular projection from the prefrontal cortex to the nucleus reuniens in the thalamus, points us to parts of the brain that are important for the inhibitory function of fear, which could be an avenue to new drugs, therapies, and interventions for psychiatric disorders."
Prof. Stephen Maren