Imagine a mouse, tentatively exploring its surroundings, its senses alert to any sign of danger. In one part of its environment, the mouse has experienced something deeply unpleasant—a mild but startling shock. As it roams, it pauses near the spot where the shock occurred, its behavior changing noticeably. It stretches out, sniffs the air, and hesitates to step back into the area where it was hurt. What’s happening in the mouse’s brain during these moments of heightened awareness? And how does it remember where the danger lies, ensuring it avoids a repeat encounter?
A recent study published in the journal Current Biology provides some answers. Researchers have mapped a brain circuit responsible not only for detecting immediate threats but also for creating lasting memories of those threats. These findings could help us better understand how animals—and by extension, humans—process fear and protect themselves from danger.
The study was led by Newton Sabino Canteras, a professor in the Department of Anatomy at the University of São Paulo’s Biomedical Sciences Institute in Brazil. The research team set out to identify the brain regions involved in signaling fear and how these regions help animals recognize environments associated with physical or predatory threats. The idea was to understand how the brain encodes memories of these dangerous locations, ensuring that the animal can avoid them in the future.
“We were interested in locating a brain region associated with fear signaling and finding out how it could identify environments previously related to physical or predatory threats, such as a place where the individual underwent an aversive physical stimulus,” Canteras said.
In the wild, animals must constantly navigate a world full of potential threats. They need to detect danger, respond defensively, and remember where these dangers are so they can avoid them in the future. This ability to learn from past experiences is crucial for survival. For example, if an animal encounters a predator in a certain area, it’s vital that it remembers to steer clear of that location in the future.
Previous studies have used fear conditioning in animals—essentially training them to associate a specific environment with an unpleasant stimulus, such as a mild electric shock. However, these studies typically confined the animals to a small chamber where they had no choice but to face the threat. This setup doesn’t fully replicate real-life situations, where animals are free to move and can choose to avoid danger. The researchers wanted to explore how animals behave when they have the freedom to escape a dangerous environment and what brain circuits are involved in this process.
In the new study, the experiments were conducted in an apparatus that consisted of a “safe” cage connected to a “conditioning” cage. The conditioning cage was where the mice received mild foot shocks, simulating a threatening environment. The mice were given the freedom to explore the entire apparatus, mimicking more natural conditions where animals encounter and avoid threats in their environment.
This setup enabled the researchers to compare the behavior and brain activity of mice confined to the conditioning cage with those that were free to explore, thereby identifying the brain circuits involved in detecting threats and forming fear memories.
To observe and manipulate brain activity, the researchers used fiber photometry, a technique that measures neural activity in specific brain regions by detecting changes in fluorescence from calcium-sensitive indicators. They focused on the dorsal premammillary nucleus (PMd), a brain region implicated in threat detection.
One of the study’s key findings was that the PMd acts as a critical “threat detector.” When a mouse approached the area where it had previously received shocks, the PMd became highly active. This activity dropped when the mouse moved away from the dangerous area or turned its back on it.
“It’s a very clear threat detector and interacts dynamically with the source. If the mouse turns its back on the source, the PMd isn’t activated, but if it looks at the source or moves close to it, the PMd ‘sounds the alarm,’” Canteras explained.
To further investigate the PMd’s role, the researchers employed chemogenetic silencing, a technique where specific receptors in the PMd were inactivated by introducing a virus and administering a particular drug. When the PMd was silenced, the mice’s behavior changed dramatically. Instead of avoiding the shock-associated box, the mice entered it without hesitation, as if they no longer perceived it as dangerous. This finding suggested that the PMd is not only crucial for detecting threats but also essential for forming memories of those threats.
The researchers also explored how the PMd interacts with other parts of the brain, particularly the septo-hippocampal-hypothalamic circuit, which integrates environmental information, and the periaqueductal gray (PAG) in the brainstem and the ventral anteromedial thalamus (AMv) in the thalamus, regions known for their roles in processing fear and coordinating defensive responses. By using optogenetic silencing, which involves inactivating specific neural pathways with light, they selectively targeted the pathways from the PMd to these two areas.
Interestingly, inactivating the pathway to the PAG reduced the mouse’s immediate defensive behavior—it was less likely to avoid the shock-associated box—but didn’t seem to affect its long-term fear memory. This suggests that the PMd>PAG pathway is primarily involved in organizing immediate defensive behaviors, such as freezing or fleeing, which are critical for survival in the face of imminent danger.
On the other hand, inactivating the pathway to the AMv didn’t change the mouse’s behavior right away but significantly impacted its fear memory. The mice seemed to “forget” that the box was dangerous, showing that this pathway plays a key role in consolidating fear memories. This distinction underscores that while the PMd>PAG pathway is crucial for immediate responses to threats, the PMd>AMv pathway is more involved in the long-term processing and reconsolidation of fear memories.
Additionally, the study highlighted the broader circuit involving the septo-hippocampal-hypothalamic pathway, which integrates upstream contextual information and interacts with the PMd to influence both immediate defensive responses and the formation of fear memories. The PMd, therefore, does not work in isolation but as part of a larger network that is essential for both detecting threats and updating memories to reflect changes in the environment.
The implications of these findings extend beyond basic neuroscience. Understanding how these circuits work could inform new treatments for anxiety and fear-related disorders, such as post-traumatic stress disorder (PTSD), where patients experience exaggerated fear responses and difficulty in managing fear memories. By targeting specific pathways within this circuit, it might be possible to develop therapies that help patients better manage their responses to fear and reduce the impact of traumatic memories.
The study, “A subiculum-hypothalamic pathway functions in dynamic threat detection and memory updating,” was authored by Juliette M.A. Viellard, Fernando F. Melleu, Alicia M. Tamais, Alisson P. de Almeida, Carolina Zerbini, Juliane M. Ikebara, Karolina Domingues, Miguel A.X. de Lima, Fernando A. Oliveira, Simone C. Motta, and Newton S. Canteras
