The human brain, an extraordinary organ, governs how we perceive, act, and remember. Yet, its functions often exceed our understanding. Among its myriad roles, the brain processes stress and memory in intricate ways, revealing both its resilience and vulnerability.
Researchers at The Hospital for Sick Children (SickKids) have made groundbreaking discoveries linking stress, memory, and the brain’s biological systems. Their work sheds light on the profound influence stress has on how memories are created, stored, and retrieved — and offers hope for individuals struggling with conditions like post-traumatic stress disorder (PTSD).
Stress can mess with how memories are made in the brain, creating confusion that makes people feel threatened even in harmless situations. This might explain why someone who burns their hand on a hot pan might avoid hot pans and the entire kitchen. Such generalized fear is common in people with conditions like post-traumatic stress disorder (PTSD) and anxiety.
Memories are stored in clusters of brain cells called engrams.
Imagine giving a presentation that went poorly. The memory of that failure can trigger stress in similar scenarios in the future. This is a simple example of how stress ties itself to memory. But in more severe cases, such as trauma or anxiety disorders, the relationship between stress and memory becomes significantly more complex.
Traumatic stress can cause stress-induced aversive memory generalization, where unrelated stimuli trigger frightening memories with no clear connection to the original event. This phenomenon can be debilitating for those with PTSD, where safe environments evoke fearful responses.
Dr. Sheena Josselyn and Dr. Paul Frankland from SickKids, alongside Dr. Matthew Hill from the University of Calgary, have uncovered how stress affects memory specificity. Their research focuses on the endocannabinoid system, a key player in memory formation and emotional regulation.
To study this, the team ran a three-step experiment with mice:
Inducing Stress: The stress was induced in mice by giving them a stress hormone or placing them in a small tube for 30 minutes.
Memory Testing: The mice were exposed to two different sounds in a chamber. The first sound (neutral) had no consequences, while the second sound was paired with a mild foot shock (fearful event).
Memory Recall: Later, the mice were placed in a new setting, and the two sounds were played again to see how they reacted.
The unstressed mice froze only when they heard the second, fearful sound. But the stressed mice froze to both sounds, showing they couldn’t separate the safe experience from the scary one.
Normally, "gatekeeper" interneurons in the amygdala help constrain memory engrams (the physical representation of memories) to ensure specificity. However, under stress, excessive release of endocannabinoids disrupts these interneurons, causing memory engrams to grow larger and less specific.
Dr. Josselyn compares this process to a velvet rope at an exclusive club: “When stress induces the release of too many endocannabinoids, the velvet rope falls, allowing more generalized aversive memories to form.”
By blocking endocannabinoid receptors on these specific interneurons, the researchers demonstrated a way to reduce this generalization, potentially alleviating a core symptom of PTSD.
Their findings align with earlier research published in Science (2023), which revealed that larger, more generalized memory engrams are typical in developing brains. This similarity between developing and stress-induced memory engrams offers new avenues for understanding how stress influences memory across the lifespan.
Remarkably, the research team created a preclinical model to mirror PTSD symptoms. By exposing subjects to mild stress before a distressing event, they replicated the generalized fear responses seen in PTSD, allowing them to study the biological underpinnings of these memories.
Dr. Josselyn and her team are now exploring whether stressed memories can be adjusted after they’ve already formed. They hope to uncover new ways to reduce the negative effects of stress on memory, paving the way for better treatments for PTSD and anxiety.
This research highlights how stress alters memory formation in the brain and offers hope for understanding and managing its impact.
With continued efforts, researchers are paving the way for innovative treatments that could restore memory specificity and reduce the harmful effects of stress-induced conditions like PTSD.
Reference:
Mallapaty, S. (2024). Stress can disrupt memory and lead to needless anxiety — here’s how. Nature. https://doi.org/10.1038/d41586-024-03724-4.
Gajbhiye, S. (2024, November 18). Stress changes how we recall memories, making them seem frightening. Earth.com. https://www.earth.com/news/stress-changes-how-we-recall-memories-making-them-seem-frightening/
(Input from various sources)
(Rehash/Josna Lewis/MSM)
