Scientists discover that brain region acts like an “anxiety meter,” scaling activity to match threat level

Scientists have discovered that a specific area of the brain, called the ventral hippocampus, functions like an “anxiety meter.” This brain region increases its activity in proportion to the level of anxiety experienced. Using a specially designed maze for mice, the scientists showed in new research published in The Journal of Neuroscience that as the animals encountered increasingly anxiety-provoking situations, the activity of neurons in their ventral hippocampus rose accordingly.

Anxiety is a fundamental emotion that helps animals and humans survive by preparing them to face danger. It’s a natural response to threats, prompting vigilance and action. However, when anxiety becomes excessive or persistent, it can lead to significant problems, such as chronic anxiety disorders. These conditions affect millions worldwide and can severely impact quality of life. While some treatments for anxiety exist, they are not always effective for everyone, highlighting the urgent need to better understand the brain mechanisms behind anxiety.

“Anxiety is a deeply personal yet broadly experienced state. While some level of anxiety is normal and even beneficial, excessive or persistent anxiety can be debilitating, making everyday life challenging,” said study author Carlo Cerquetella, a postdoctoral researcher in Stéphane Ciocchi’s Lab at the University of Bern.

“Society often downplays anxiety as mere overthinking or nervousness, but for those affected, it can be overwhelming, affecting decision-making, social interactions and overall well-being. Despite its prevalence, we still don’t fully understand how anxiety is processed in the brain. This is why I am passionate about uncovering how anxiety is encoded and how different situations are represented at the neural level – knowledge that could pave the way for better treatments and interventions.”

Previous research has indicated that several brain areas, including the amygdala, prefrontal cortex, and hippocampus, play roles in processing emotions. However, it remained unclear how the brain represents and responds to different levels of anxiety, rather than just anxiety itself. The researchers suspected that the hippocampus, a brain structure known for its role in learning and memory, might be involved in this process. The hippocampus helps us make sense of our surroundings by comparing new information with past experiences. Within the hippocampus, the ventral part has been particularly linked to processing threats and anxiety.

They hypothesized that this ventral hippocampus could be the key to understanding how the brain scales its response to varying levels of anxiety. Existing methods for studying anxiety in animals, like the elevated plus maze, often involve situations where anxiety levels are somewhat fixed and difficult to precisely control or quantify. To overcome these limitations, the researchers developed a new, adaptable maze that could create a range of anxiety-inducing scenarios within a controlled environment.

This maze could be configured in six different ways to create varying levels of anxiety. In the “no anxiety” setting, the maze was fully enclosed with walls, and at ground level, so the mice could not see any heights. In the “very low anxiety” setting, one half of the maze was opened up, exposing the mice to a height of 20 centimeters. The “low anxiety” setting increased the height to 70 centimeters, and the “moderate anxiety” setting raised it further to 120 centimeters. The “high anxiety” setting kept the 120-centimeter height but also narrowed the width of the open part of the maze.

Finally, the “very high anxiety” setting elevated the maze to 170 centimeters, maintaining the narrower width. Small food rewards were placed at the end of the open part of the maze to encourage the mice to explore. An automatic door at the beginning of the maze allowed for breaks between trials and created a safe starting area.

The study involved male mice aged 4 to 6 months. Some mice underwent a procedure to make specific neurons in their ventral hippocampus sensitive to light. This technique, called optogenetics, allows researchers to temporarily control neuron activity using light. These mice had tiny optical fibers implanted in their ventral hippocampus. Another group of mice had electrodes implanted in the same brain region to record the electrical activity of individual neurons.

During experiments, the mice were placed in the adjustable maze, and their behavior was recorded. For the optogenetics experiments, light was shone into the ventral hippocampus of some mice as they moved from the closed to the open part of the maze, temporarily reducing the activity of those neurons. For the electrophysiology experiments, the activity of neurons in the ventral hippocampus was recorded as the mice navigated the different anxiety levels in the maze.

The researchers measured how the mice behaved in each anxiety setting, specifically looking at the number of times they ventured into the open part of the maze and the amount of time they spent there. They found that as the anxiety level of the maze increased, the mice completed fewer trials and spent less time in the open area. This confirmed that the maze was indeed successfully inducing different levels of anxiety in the mice.

Importantly, when the researchers used light to reduce the activity of neurons in the ventral hippocampus, they observed that the mice showed less anxiety-related behavior. These mice were more likely to enter the open parts of the maze and spend more time there, even in the high anxiety settings.

Reducing ventral hippocampus activity disrupted the normal scaling of anxiety. Normally, as anxiety levels increased, the mice’s behavior changed accordingly. But when the ventral hippocampus was inhibited, this scaling effect was weakened; the mice’s behavior did not change as much across the different anxiety levels. This suggested that the ventral hippocampus is essential for properly perceiving and responding to different levels of anxiety.

“I was genuinely surprised by the entire study’s results, and this is what had the biggest impact in driving and pushing my work forward,” Cerquetella told PsyPost. “However, if I had to highlight one specific finding, it would be the optogenetic part. The strength of the optogenetic inhibition of hippocampal cells in altering the animal’s anxiety state, especially in the most anxiogenic context, was remarkable. It truly underscored the powerful role of the hippocampus in anxiety regulation.”

By recording the activity of individual neurons in the ventral hippocampus, the researchers discovered that the overall activity of this brain region increased progressively as the anxiety levels in the maze went up. This scaling activity was observed specifically in the open, anxiety-inducing parts of the maze, but not in the safe, closed part. This increase in activity was not simply due to the mice moving more or differently in the maze.

Further analysis revealed two key mechanisms at the single neuron level contributing to this scaling. First, some neurons showed “tuning,” meaning their firing rate increased in a graded manner as anxiety levels rose. Second, there was “neuronal recruitment.” At each increasing level of anxiety, new neurons became active in the ventral hippocampus, and these neurons remained active at even higher anxiety levels. Both of these processes – enhanced tuning of existing neurons and the recruitment of new neurons – contributed to the overall scaling of activity in the ventral hippocampus as anxiety increased.

To ensure that these changes in ventral hippocampus activity were specifically related to anxiety and not just to the novelty of the maze configurations, the researchers tested a “novel” maze setting. In this setting, the open part of the maze was changed to look and feel different, but the height and openness remained similar to the “no anxiety” control. They found that while the mice explored this novel section, it did not cause the same increase in ventral hippocampus activity as the anxiety-inducing maze configurations. This suggested that the changes they observed were primarily driven by anxiety, not just general novelty.

Finally, the researchers used a computer algorithm, called a linear classifier, to see if they could predict the anxiety level based on the recorded neuron activity in the ventral hippocampus. The classifier was able to accurately identify the anxiety level from the neural activity, further supporting the idea that the ventral hippocampus encodes information about anxiety levels in a scalable way. When the classifier was trained to distinguish only between the lowest and highest anxiety levels, it still showed a gradual scaling in its predictions for the intermediate anxiety levels, reinforcing the concept of the ventral hippocampus as an “anxiety meter.”

“Our study highlights the crucial role of the hippocampus in representing different intensities of anxiety,” Cerquetella explained. “Rather than simply signaling the presence or absence of an anxiogenic situation, the hippocampus also provides insight into how intense the anxiety-inducing experience is.”

The researchers acknowledge that this study was conducted in mice, and it remains to be seen if the same mechanisms are at play in humans. “A major caveat of this study is the challenge of translating findings from rodents to humans, as well as the fact that we used only one experimental setup,” Cerquetella noted. “The choice to focus on one model was due to time and resource limitations.”

“Regarding the translational aspect, it is inherently difficult to fully interpret what an animal feels and how it perceives an anxiogenic situation in a human framework. However, anxiety is a highly conserved state among all mammals.”

“Moreover, the hippocampus, which plays a central role in representing anxiety and anxiety-related behavior, is functionally conserved across rodents and humans,” Cerquetella said. “This suggests that similar circuit mechanisms may underlie both normal and pathological emotional behaviors, such as anxiety.”

Understanding this mechanism could offer new avenues for developing more effective treatments for anxiety disorders and other conditions.

“My long-term goal is to further investigate the neural circuits underlying anxiety and other mood-related disorders, such as depression,” Cerquetella said. “These conditions affect countless individuals and can be deeply debilitating. Understanding how they are encoded in the brain is crucial for developing better treatments and finding ways to help people cope more effectively with these devastating diseases.”

The study, “Scaling of ventral hippocampal activity during anxiety,” was authored by Carlo Cerquetella Camille Gontier, Thomas Forro, Jean-Pascal Pfister, and Stéphane Ciocchi.