Concussions have gained a significant amount of media attention in the past year, specifically since Sidney Crosby, Canada’s poster-boy for our national sport of hockey suffered a concussion which is threatening his future career.  Concussions are considered a mild traumatic brain injury (mTBI) in which the brain bounces off of the inside of the skull (usually due to a forceful blow to the head) and results in a very short loss in brain function.  Demonstrated symptoms of a concussion are usually only psychological, as there is typically no obvious physiological damage to the brain.  One category of individuals who have suffered concussions and does not get significant media attention for is those who have been involved in combat.  It has been noticed that many of these individuals tend to develop an anxiety disorder known as posttraumatic stress disorder (PTSD) in which they encounter a terrifying event which may have caused or threatened serious harm to themselves or others.

In this particular study the research group of Reger et al. (2012) produced a model involving 105 male rats in which they investigated a potential relation of concussions and PTSD on a neurobiological level.  More specifically, they were interested in examining if induced concussion-like events would cause an increase in the rats’ abilities to learn to fear an unpleasant stimulus (an electrical shock).  The researchers hypothesized that the male rats would become more easily affected by a stressful situation after they had endured trauma of the brain similar to a concussion.  To induce a concussion-like event the researchers performed a systematic surgery on the rats.  The rats would be anaesthetized before there was an incision made in the middle of the head, allowing the researchers to remove a piece of the skull, therefore exposing the brain.  Upon exposure of the brain a plastic cap was sealed over the opening and connected to a device which would then cause a pulsation of a saline liquid that would result in brain trauma.  Two days after each rat’s surgery it would begin a training phase in which it would be exposed to one of five conditioning tasks.  Each of the tasks administered electric shocks to the rats, and varied on the strength of shocks, time between shocks, and properties of a sound which was paired with the shock.  The act of stopping what they were doing in fear (freezing) was recorded whenever the sound cue was then presented to the rodents.  Seventeen days after the surgery one of the groups was euthanized, allowing for the researchers to examine any potential changes in the brain.  The researchers were specifically interested in examining two portions of the brain; the hippocampus and the amygdala.  The hippocampus has been found to be involved with circuitry of the brain when processing whether or not an object is to be feared, while the amygdala is involved in the context and formulation of fear caused by a specific cue (i.e. the sound paired with shocks).

Upon completion of the experiments of the study, the researchers found that there had been a significant increase of a receptor in the brain known as NMDA receptors, which are involved in the excitation level of portions of the brain involved in fear and learning fear.  Also it was found that all forms of training showed an increase in fear shown towards the sound cue which had been paired with the shocks.  Interestingly, the researchers found that the injured animals had an increase in the act of generalizing their fear towards a stimulus which may not have been involved with the traumatic experience.  Although there may be more complex interactions for humans, it seems this finding could suggest reasoning for why individuals who have suffered a concussion and develop PTSD may show severe reactions of stress towards situations which had little or nothing to do with the traumatic event in which they endured.  Overall, the data found in this research suggests that individuals who suffer concussions may become more susceptible to an enhancement of fear learning.  The increase of NMDA receptors suggest that a an increase in the activity of the amygdala (the structure involved in forming fear) may explain the relation between concussions and formation of PTSD.

Source: Reger, M. L., Poulos, A. M., Buen, F., Giza, C. C., Hovda, D. A., Fanselow, M. S., & Mechanisms of Compromised Stress Resilience During Development and Aging. (February 15, 2012). Concussive Brain Injury Enhances Fear Learning and Excitatory Processes in the Amygdala. Biological Psychiatry, 71, 4, 335-343.