Researchers have mapped a brain circuit responsible for instantly detecting threats and forming memories of fear.
“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,”
said the last author of the study1, Newton Sabino Canteras, a professor in the Department of Anatomy at the University of São Paulo’s Biomedical Sciences Institute (ICB-USP).
The hippocampus is known to be involved in spatial navigation and orientation. This brain region also detects threats from the environment.
Canteras noted that anything dangerous leaves a mark there, allowing for the specific location of the threat. The subiculum is an important component that is closely associated with the hippocampus.
“The subiculum is basically the star of the show. It transfers information linked to environmental threats to the hypothalamus. We set out to investigate how it behaves when the animal faces an environment previously associated with an aversive stimulus,”
he said.
Avoidance Behavior
The researchers used fiber photometry to observe activity in the subiculum. They inserted a virus that encodes a calcium-sensitive protein capable of detecting cell activity. It produces fluorescence in response to biological activity.
Their behavioural paradigm consisted of habituating a mouse to an apparatus with two boxes connected by a corridor. Initially, the mouse was restricted to one box and received aversive physical stimuli (electric shocks to its paws). On the following day, it was placed in the apparatus to avoid the aversive box.
“In this type of test, the animal stretches and sniffs, moves one way and another, but doesn’t go back into the box where its paws were shocked. It displays what we call avoidance behavior,”
Canteras said.
Dorsal Premammillary Nucleus
The dorsal premammillary nucleus (PMd) was the focus of photometry analysis because it is an important part of the studied neural circuit. The PMd becomes very active when the animal gets close to the source of danger and then runs away from 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.” That’s one of the key findings of the study,”
he said.
The researchers then used a widely used chemogenetic technique known as DREADDs (designer receptors exclusively activated by designer drugs) to deactivate the PMd.
Fear Response and Memory Reconsolidation
They injected a virus that expressed a receptor [the protein hM4Di] which silenced the PMd in the presence of a specific drug. The drug inhibited the cells from firing. They recovered normal function as soon as the medication was removed.
“When the PMd was silenced in this way, the animal drastically reduced its defensive response. Instead of avoiding the threatening box, it went back in as if it were a harmless environment — as if nothing had happened,”
he said.
They concluded that PMd inactivation influenced both contextual fear responses and reconsolidation of fear memory. Thus, after PMd inactivation, the animal considers the environment safe and navigates it as if there were no risks on the following day.
Inactivating PAG Projection Pathway
Next the team investigated the functional roles of the PMd’s main targets in the nervous system: the periaqueductal gray or PAG (in the brainstem) and the ventral anteromedial thalamus (in the thalamus).
“There’s a technique whereby I can specifically inactivate the projection that goes to the brainstem or to the thalamus. I insert into the PMd a virus that expresses a protein acting as a light-sensitive chlorine channel. When I light up the terminal fields, these terminations are silenced and I can manipulate a specific projection pathway,”
Canteras said.
The researchers discovered that while inactivating the PAG projection pathway reduced the animal’s defensive behaviour, it still responded well to fear the next day, indicating that fear memory was unaffected.
“Expression of behavior is affected, but not fear memory reconsolidation. On the other hand, inactivation of the thalamus pathway has practically no immediate effect but significantly affects fear memory consolidation,”
he said, adding that both events are mediated primarily by different nucleus projection sites.
- Juliette M.A. Viellard et al. A subiculum-hypothalamic pathway functions in dynamic threat detection and memory updating. Current Biology (2024). DOI: 10.1016/j.cub.2024.05.006
Image credit: Current Biology (2024). DOI: 10.1016/j.cub.2024.05.006