by Melvi Agolli | Edited by Maryam Kamel
Stress manifests itself in many different ways and in academic, social, and cultural contexts. For instance, it’s no secret that the COVID-19 pandemic caused a general increase in anxiety for all of our loved ones, exacerbated social isolation and loneliness, and made academic learning much more difficult for students of all ages. Recent disturbing global events, such as Russia’s invasion of Ukraine, escalating tensions and fighting between Israel and Palestine, and the protests in Iran against the regime, have also contributed to a rise in global anxiety through creating uncertainty about the future of our planet. But could these large-scale socio-political events affect our mental health at the individual level? If so, how?
A recent study published in the journal Trends in Neurosciences provides a new perspective on how stress changes the functions and structures of the hippocampus in rodents, which plays a significant role in learning and memory. More specifically, the study focused on place cells, a specialized cell type found in the CA1 region of the hippocampus. This type of cell encodes spatial information by firing when the rodent is in a particular physical setting, also called the cell’s receptive field. In other words, place cells only fire when the rodent enters a specific location within its environment and remain silent in any other location. Because of this, place cells can be thought of as biological markers that allow an organism to physically locate itself in its environment.
Whenever there’s a modification to our familiar surroundings, place cells either fire more or less quickly in response to the change. If an organism experiences significant changes in their surroundings, place cells alter their firing rate, as well as their receptive fields — a process called spatial remapping. This enables the rodent to recognize minor changes in their familiar environment, in addition to encoding and remembering spatial information about the novel environments it finds itself in.
What does this all have to do with stress in humans? The study found that stress significantly impacts how rodent place cells react in familiar and new environments. Normally, place cells form receptive fields of particular locations within an animal’s familiar environment. When the animal is put in a new environment, place cells remap their receptive fields to encode the new spatial information. In times of chronic stress, the study found that place cells are less active in familiar environments; in other words, the animals become less receptive to familiar situations. In new environments, place cells are not able to remap their receptive fields as efficiently, meaning that the stressed animal has difficulty adapting to and remembering details about new situations. Though the study addresses hippocampal mechanisms in rodents, similar mechanisms are present in humans, which has some interesting implications.
By interfering with the normal functioning of place cells, chronic stress impairs our ability to interact with our current environment. We remember and notice less and less about what makes our environment so unique, since place cells do not encode as much information about changes to our familiar environments during periods of stress. Thus, we may become less receptive to changes in our physical surroundings, and the people in them. By becoming more “numb” to our surroundings, we may feel less inspired to take action, connect with people, or otherwise interact with our environment. Similarly, the fact that place cells do not react as much to new environments during periods of chronic stress may impair our ability to adapt and thrive in new situations. Ultimately, our willingness to try new challenges and our curiosity about exploring new environments and ideas may decline.
On a similar note, the study describes how hippocampal neural activity during relaxed states begins to resemble that of stressed states after periods of chronic stress. In other words, the more stress a rodent is exposed to, the more its neural circuits begin to show stress-like responses even when the animal is outside of stressful situations. This is important because relaxed states are crucial for consolidating memories and learning, as they enable synaptic plasticity (i.e., strengthening of neural pathways). This suggests that losing the ability to achieve relaxed states due to chronic stress could reduce one’s ability to learn and encode new information. Ultimately, prolonged periods of stress can harm our ability to learn and grow even when the stressor is no longer present in our surrounding environment.
The way we perceive and remember our environment has an enormous impact on our ability to interact with objects and people in it, as well as our tendency to learn and grow from our experiences. By affecting the functions of spatial memory, chronic stress from apparently distant global crises can lead to real changes in our mood, growth, and learning. That’s why, now more than ever, it’s so important to be in touch with our feelings and devise coping strategies, as well as seek help, when we need it.