October 19, 2021
Written by: Marissa Maroni
The Halloween season is in full swing and with it comes bright orange and yellow leaves, warm apple cider, and haunted houses. Haunted houses are known for their ominous appearance, the presumable jump scares, and strobe lights leading many people to leave them shaking in fear. Even when waiting in line, one may feel their heart rate increase as they start to anticipate the potential spooky rooms to come. This is a common experience, but have you ever wondered what’s happening in your brain that causes you to recognize this haunted house as a fearful place? What could be happening to cause you to react defensively and run from the haunts in the house? Recent work aimed at answering these questions found that specific brain regions simultaneously respond to a fearful place leading to defensive behaviors.
The Robogator simulates a predator interaction
Using rats as their model organism, researchers from the University of Washington created a fearful scenario using a simulated predator, called the Robogator, to test brain activity and behavior in response to fear (figure 1). If anyone was looking for a terrifyingly unique Halloween costume, I think the Robogator could be it. In the fear inducing scenario, rats who were hungry after being deprived of food were placed in an arena where they had to travel from their nest to retrieve a food pellet that was placed in front of the Robogator. Once the rat got close enough to the robot, it would lunge forward towards the rat inducing a defensive fear response during which the rat would retreat to its nest. While this defensive behavior was occurring, the researchers were also using electrodes to record the activity of two brain regions, the basal amygdala (BA) and dorsal hippocampus (dHPC). They recorded these regions specifically as they are important for fear and location signaling, respectively. Along with this fear test, the rats underwent additional tests prior to and after encountering the robot to provide baseline brain activity before and after introducing the predator. This system gave the researchers the power to be able to assess how the BA and dHPC respond to a fearful location.
The BA and dHPC have specific coordinated activity
Using the simulated-predator fear test, scientists were able to compare the activity of the BA and the dHPC. Scientists found that there was a synchronization in activity between these two brain regions before and after the robot lunged forward. This gave the first clue to the scientists that these brain regions could be communicating to the rat that this is a dangerous situation and to return to their nest. Looking closer at the data, the researchers were able to categorize cells in the BA (fear-related) and in the dHPC (location-related). They could identify neurons in the BA that specifically responded to the Robogator and referred to these as “robot cells.” Further during the training scenario when the robot was not present, they could categorize their cells in the dHPC into two groups: one group representing neurons that fire when the rats are close to the nest, and the other group representing neurons that fire when they are far from the nest (near the food pellet). They referred to the latter group as “distal cells” because they fire when the rat is in a location distal or far from the nest. They found that while under normal conditions the distal cells only fired when the rat was far away from the nest, during the simulated-predator test the cells were active regardless of proximity to the nest. This suggested to the authors that these distal cells are remapping and firing at locations different from the control scenario. Further, they found that the location-related dPHC cells that were paired to robot cells remapped more so in comparison to non-robot cells during the simulated-predator session. This suggests that these distal cells that are paired to robot cells are altering their activity in response to the fear scenario.
Activating the BA can cause defensive behaviors and dHPC activity instability
Although the previous experiments suggest a relationship between BA and dHPC there could be other variables contributing to the altered activity in these two brain regions. The researchers wanted to see if they could better link this change in activity to the defensive behavior of retreating to the nest. The researchers were able to activate the BA using optogenetics, a technique that can be used to stimulate neurons in living organisms using light. Interestingly, they found that when they activated the BA, the rats would retreat to their nest even though there was no predator! This suggested that the fleeing fear behavior may be directly caused by BA activity. They were then able to show that activating the BA caused the distal cells in the dHPC to become destabilized, similar to the patterns they observed while recording neuronal activity during the simulated-predator fear test. This study suggests that during a fearful scenario neuronal activity in these two brain regions may be connected and code for the recognition of a fearful location and lead to subsequent fleeing behaviors.
What could this study mean? This research gives scientists insight into how different regions of your brain communicate and code behaviors in response to a fearful location. This spooky season while waiting in line for a haunted house, make sure to mention to your friends that if they’re scared it could be their BA signaling to their dHPC!
1. Kong, M. S., Kim, E. J., Park, S., Zweifel, L. S., Huh, Y., Cho, J., & Kim, J. J. (2021). ‘Fearful-place’coding in the amygdala-hippocampal network. Elife, 10, e72040.
Figure 1 image from reference 1 Kong et al.