What Is the Hippocampus?
The hippocampus is a part of the limbic system of the brain. The limbic system describes a group of brain structures that surround the brain stem. The brain structures that make up the limbic system play a major role in how one experiences certain emotions (fear and anger), motivations, and memory. The hippocampus is responsible for the ability to store and retrieve memories. People who have experienced some kind of damage to their hippocampus may have difficulties storing and recalling information. Along with other limbic structures, the hippocampus also plays a role in a person’s ability to overcome fear responses.
The Hippocampus’ Role in PTSD
Many people with PTSD experience memory-related difficulties. They may have difficulty recalling certain parts of their traumatic event. Alternatively, some memories may be vivid and always present for these individuals. People with PTSD may also have problems overcoming their fear response to thoughts, memories or situations that are reminiscent of their traumatic event. Due to the hippocampus’ role in memory and emotional experience, it is thought that some of the problems people with PTSD experience may lie in the hippocampus.
How Might PTSD Affect the Hippocampus?
Some studies suggest that constant stress may damage the hippocampus. When we experience stress, the body releases a hormone called cortisol, which is helpful in mobilizing the body to respond to a stressful event. Some animal studies, though, show that high levels of cortisol may play a role in damaging or destroying cells in the hippocampus. While cortisol is released in higher amounts when a person is under a great deal of stress, either chronically or acutely, this process is actually more complicated than just elevated cortisol. The increase in cortisol also signals the immune system, which releases inflammatory chemicals called cytokines, which in turn can activate cells called microglia. These in turn switch from production of serotonin to a higher production of glutamate, a very important excitatory neurotransmitter that, if present in excessive amounts, can lead to brain cell damage or death. Such a constant barrage of higher glutamate levels may be what damages the hippocampus. Antidepressants such as SSRI’s (such as Prozac) and SNRI’s (such as Cymbalta) help to block the transport of these inflammatory cytokines across the blood-brain barrier. Researchers have also looked at the size of the hippocampus in people with and without PTSD. They have found that people who have severe, chronic cases of PTSD have smaller hippocampi . This indicates that experiencing ongoing stress as a result of severe and chronic PTSD may ultimately damage the hippocampus, making it smaller.
Does the Hippocampus Play a Role in Determining PTSD Risk?
Not everyone who experiences a traumatic event develops PTSD. Therefore, researchers have also proposed that the hippocampus may play a role in determining who is at risk for developing PTSD. Specifically, it is possible that having a smaller hippocampus may be a sign that a person is vulnerable to developing a severe case of PTSD following a traumatic event. Some people may be born with a smaller hippocampus, which could interfere with their ability to recover from a traumatic experience, putting them at risk for developing PTSD. In twin studies that focused on identical twins, with one twin exposed to a traumatic event (combat) and the other unexposed, researchers are able to look at pre-existing vulnerabilities that may be present in both twins, as well as differences that may be due to trauma. Since twin participants share the same genes, studying identical twins can provide insight into the influence of genetics on developing certain conditions. For example, in this case, if the person who developed PTSD has a smaller hippocampus and has a non-trauma exposed twin who has a smaller hippocampus, it would suggest that a smaller hippocampus may be a sign of genetic vulnerability for developing PTSD following a traumatic experience. In fact, this is exactly what researchers have found. People with severe PTSD had a smaller hippocampus, and they also had a non-trauma exposed twin with a smaller hippocampus. Consequently, a smaller hippocampus may be a sign that a person is vulnerable or more likely to develop PTSD after a traumatic experience. Of course, it is important to remember that twins often share the same environment growing up, so it is difficult to tease apart the role nature versus nurture plays in the size of a person’s hippocampus. So, the verdict is still out on the true relationship between the hippocampus and PTSD.
How This Information Can Be Used
There is still much to learn about the role certain parts of the brain play in PTSD formation. Knowing how PTSD affects the brain (and vice versa), however, is very important to study. Understanding which parts of the brain may impact PTSD can lead to the development of more effective medications for treating the disorder. In addition, this information may also help us better identify who is at risk for developing PTSD following a traumatic event.
