Let’s suppose that you are strolling through a park on a Saturday afternoon. All goes well until a mountain lion pounces out of the bushes and stares you right in the eye as it salivates for some human flesh. What would you do?
Naturally, when you encounter danger, it’s only natural to feel afraid. In this instance, you are very afraid of the mountain lion standing 8 feet away from you, and in response to this fear, your body induces a “fight or flight” response as it prepares to defend itself against danger or avoid it. When this “fight or flight” response takes effect, you decide to slowly inch for way toward the grocery store just across the street, hoping that the cougar will remain calm. Luckily, a park patrol comes around in his scooter and tranquilizes the animal before it is able to react, and you are able to avoid the danger posed by the presence of this mountain lion in the park.
"Fight or Flight" Response
After this encounter, you become slightly weary of walks in the park or you panic whenever you hear the bushes rustling near you. You begin to have this new awareness toward your surroundings while at the park because your body is able to develop fear memories to avoid this danger and increase your chance of survival in the future. In this case, your body develops a fear memory of mountain lions at the park, and this fear memory can be retrieved throughout your lifetime. For some time, you’ll most likely avoid spending your Saturday afternoons at the park, but over time, your body will become conditioned to this fear and you would take your walks as usual.
In this, we see the relevance of fear memory development, retrieval, and conditioning as we experience everyday life. Interestingly, fear memory can also play a significant in anxiety disorders, including PTSD (post-traumatic stress disorder). PTSD is characterized by changes or damage to the body’s “fight or fight” response that causes an individual to feel stressed or frightened even when they are not in danger. While scientists have a general understanding of neuroanatomy and the effects of PTSD on mental health, the impaired mechanisms behind the disease are still yet unknown.
Our understanding of neuroanatomy allows us to identify particular brain structures that are involved in fear memory, and ultimately PTSD. The amygdala, for example, is a brain structure known for its role in emotion, learning, and memory, and appears to be active in fear acquisition. It is also involved in learning not to fear, as it works in conjunction with the prefrontal cortex, which is involved with decision-making, problem-solving, and judgment.
To better understand the mechanisms of fear memory retrieval and conditioning in the amygdala and PFC, Do-Monte et al. looked specifically at fear memory retrieval circuits. They began by administering a mild shock on rats to induce fear, and tested the rats for fear memory retrieval circuits with respect to time. When the shock was administered, a chemical response was projected from the pre-limbic prefrontal cortex (PL) to the basolateral amygdala (BLA) and central nucleus of the amygdala (CeA).
In order to analyze temporal changes in fear memory retrieval circuits, they studied neural activity using a marker, which determined the activation of each key structure throughout the retrieval circuits. The marker, c-Fos, showed consistent activation in the PL, suggesting that it is necessary for both early and late memory retrieval. Conversely, after 24 hours there was a gradual shift in BLA activation from the PL to the increased activation of the paraventricular region of the thalamus (PVT) from the PL. Ultimately, memory retrieval circuit shifts from a PL-BLA circuit to PL-PVT-CeA over time. This indicates that the PVT is gradually recruited for fear retrieval in a time-dependent manner, where this pathway may be necessary for fear memory retrieval at late points in an animal’s lifetime.
Neural Pathways involved in early and late memory retrieval
The investigation on the role of PVT activation continued in the work done by Penzo and his colleagues. Specifically with respect to the PL-PVT-CeA retrieval circuit, we see that the PVT is involved in fear processing and conditioning by activating the CeA. This means that PVT is involved in allowing for you to return to your routine of taking walks in the park, because your body recognizes that the source of danger is no longer present at the park.
This process of activation can be impaired, however, due to a deletion of a brain-derived neurotrophic factor (BDNF), which binds to a TrkB receptor to activate the memory-storing CeA neurons. The consequent of this deletion, in turn, induces constant fearful responses as a result of the storage of fearful memories, which are not conditioned.
“While our memories feel constant across time, the neural pathways supporting them actually change with time,” explains Do-Monte. The neural pathways supporting our memories allow for us to develop, retrieve, and condition our fear memories, in order for us to survive. With time, however, we see that dysregulation of retrieval circuits can result in inappropriate fearful memory storage, which may contribute to exacerbated fear responses in PTSD that occur long after a traumatic event occurs.
Do-Monte et al. and Penzo et al. provide us with insight regarding the mechanisms of fear memory retrieval in rats, which is applicable to humans. Their work has identified new pathways for old memories, which shows that fearful events may occur months or years prior to the onset of symptoms for patients with PTSD. This informs our understanding of PTSD and ways in which neural pathways can be targeted for therapy.
1) Do-Monte, Fabricio H., Quiñones-Laracuente, and Quirk, Gregory J. A temporal shift in the circuits mediating retrieval of fear memory. Nature. 2015 Jan 19 doi: 10.1039/nature14030 Retrieved March 11, 2015. http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14030.html
2) New Brain Pathways for Old Memories- NIH Research Matters- National Institutes of Health (2015, Feb 5) Retrieved March 11, 2015 from http://www.nih.gov/researchmatters/february2015/02022015memories.htm
3) Penzo MA, Robert V, Tucciarone J, De Bundel D, Wang M, Wan Aelst L, Darvas M, Parda LF, Palmiter RD, he M, Huang ZJ, Li B. The paraventricular thalamus controls a central amygdala fear circuit. Nature (2015) Retrieved March 11, 2015. http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13978.html