A newly detailed brain region provides the continuous alarm signal needed to keep animals on high alert during a threat. The research, published in Neuron, indicates that the amygdalostriatal transition zone bridges the gap between learning about a danger and sustaining a defensive response.
The amygdalostriatal transition zone, or ASt, is a small cluster of brain cells located between the amygdala, a center for emotion processing, and the striatum, an area involved in controlling movements. When a threat appears, the brain must rapidly detect it and maintain a defensive state until the danger passes.
A 1995 study of rats demonstrated that fear-learning neurons in the amygdala respond to threat sounds in a very brief, rapid burst lasting only milliseconds. Later, a 2002 laboratory study showed that warning signals travel rapidly from the amygdala directly into the neighboring ASt.
The disconnect between brief brain signals and long-lasting fear responses became a major question, as a 2009 study of rodents highlighted the puzzle that amygdala signals fade almost immediately even though defensive behaviors persist for tens of seconds.
The new study explores this fast pathway to determine whether the ASt provides the continuous alarm signal needed to maintain defensive behaviors over longer periods. The research was led by first author Fergil Mills, an assistant professor at the University of Utah who conducted the study as a postdoctoral fellow in the laboratory of senior author Kay M. Tye at the Salk Institute for Biological Studies.
“It felt as though an important part of the neural circuitry had yet to be found,” Mills told PsyPost. “In this study, we characterized the ‘amygdalostriatal transition zone’ (ASt), a largely unexplored brain region, and discovered that it maintained responses to threat cues for the full duration that they were present.”
“This was a previously unknown role for the structure, and our work identifies the ASt as a ‘missing piece’ of the neural circuits for learning and behavior,” he added.
The scientists began by examining the genetic makeup of the ASt and surrounding brain structures. They extracted and analyzed the genetic material of 97,434 individual cell nuclei from the brains of mice. The analysis indicated that the ASt has a distinct genetic identity.
Specifically, the ASt contains an unusually high concentration of a specific type of brain cell called a Drd2-positive neuron, which carries a specific receptor for the chemical messenger dopamine. About 71 percent of the neurons in the ASt were of this type, compared to just 26 percent for a related cell type known as Drd1a-positive neurons.
Next, the team measured the electrical activity of ASt neurons in 15 live mice. The mice were trained in an environment where a 20-second auditory tone predicted a mild foot shock, while a different 20-second tone predicted a sweet liquid reward. The researchers found that ASt neurons responded strongly to the shock-predicting tone.
Unlike the brief bursts seen in the amygdala, the ASt neurons fired continuously for the entire 20-second duration of the threat cue. Trials where the mice showed the highest levels of defensive behavior, such as freezing in place, were accompanied by ASt firing rates that were over three times higher than in trials with low defensive behavior.
To see if this brain activity actually produced the defensive behavior, the researchers used optogenetics, a technique that allows scientists to control specific neurons using light. They tested 18 mice that were genetically engineered so their ASt neurons could be activated by blue light lasers. When the researchers shined light into the ASt, the mice immediately began to freeze.
In a separate test where one side of an enclosure triggered the laser, the mice actively avoided that side. The team then isolated the effects of the two different cell types by testing 35 additional mice. They found that activating only the Drd2-positive neurons produced the same freezing and avoidance behaviors, while activating the Drd1a-positive neurons had no effect on these behaviors.
A motor coordination test using a rotating rod showed that the freezing was a defensive response, not a general inability to move. The researchers also tracked the activity of these specific cell types during the tone task using miniature microscopes attached to the heads of 15 mice. By observing calcium levels, which rise when neurons are active, they saw that the Drd2-positive neurons maintained high, sustained levels of activity throughout the threat-predicting tones.
Finally, the team tested whether the ASt is strictly necessary for fear responses. They used a different optogenetic tool to silence the Drd2-positive ASt neurons in a group of 21 mice. During the tone task, blocking these neurons caused the mice to reduce their defensive behaviors, such as freezing or sudden darting, by nearly 50 percent. Blocking these cells did not change the animals’ responses to reward cues, nor did it alter their general movement in an open arena.
“The neural and behavioral results that we observed were striking,” Mills said. “Inhibiting a subpopulation of ASt neurons caused a nearly 50% reduction in fear responses – a major decrease for a structure never before established as part of the fear circuit.”
“This tells us that the ASt is not just correlated with fear responses, but makes a meaningful causal contribution to them,” he noted.
As with all research, there are a few things to keep in mind. The experiments used only a single type of threat, which was a foot shock, and a single type of reward, which was a sweet drink. It is not completely settled whether the ASt specifically processes the negative nature of an experience, or if it might also respond to other intense stimuli.
Additionally, the mice did not show ASt activation when placed in a context or environment they associated with fear, only when they heard the specific warning tone.
“An interesting detail of our study was that the ASt maintains responses to specific threat cues, but not to locations where negative experiences occurred,” Mills explained. “This suggests that the ASt has a specialized role in fear triggered by sensory cues, rather than the broader environment we are in – a key new insight into how the brain organizes different kinds of fear responses.”
“Importantly, we believe that our findings complement rather than contradict the established amygdala circuits for fear learning,” he said. “We think the ASt is a key part of a much larger circuit, and that the ASt and amygdala likely work together to orchestrate responses to threats. Also, these experiments were performed in mice, so a critical next step will be to determine how these findings translate to humans.”
Scientists hope to investigate whether this pathway plays a role in conditions like anxiety or post-traumatic stress disorder, where responses to external stimuli are disrupted.
“Understanding the circuits for fear learning is critical to human health, because maladaptive fear responses are a hallmark of neuropsychiatric disorders such as PTSD and anxiety,” Mills pointed out. “In these disorders, fear responses can become persistent, generalized, or spontaneous, leading to debilitating health outcomes.”
“In order to develop new treatments for these disorders, we have to understand the fundamental circuitry that the brain uses for fear learning,” he added. “That’s what makes the discovery of the ASt’s role in fear so exciting – it’s part of a larger puzzle that we need to solve to improve human health.”
Although the amygdala has been heavily researched in the context of psychiatric conditions, the ASt represents a new frontier for understanding how the brain responds to danger. Future studies will need to explore exactly how the ASt communicates with the amygdala during the initial learning of a threat.
“Now that we know the ASt has a role in fear responses, it is also a new site of interest for disorders where fear responses are disrupted,” Mills explained. “It is easy to imagine that if ASt circuits mediate sustained responses to threats, then dysfunction of these circuits could give rise to the maladaptive fear responses we see in different disorders.”
“We are now collaborating with other researchers to study the ASt’s function in humans, establishing a new translational direction for a brain region that has been almost entirely overlooked in mental health research,” he said. “I want to thank Kay Tye for her incredible mentorship, and all the members of the Tye lab and Mills lab who contributed to the study. Exploring the ASt was an amazing adventure, and we are looking forward to the new science that will stem from these discoveries.”
The study, “Amygdalostriatal transition zone neurons encode sustained cue responses to guide defensive behaviors,” was authored by Fergil Mills, Christopher R. Lee, James R. Howe, Hao Li, Maria N. Keisler, Shan Shao, Felix H. Taschbach, Mackenzie E. Lemieux, Faith Aloboudi, Jesse White, May G. Chan, Matilde Borio, Laurel R. Keyes, Hannah S. Chen, Fabiha Bushra, Gates P. Schneider, Dani P. Lemmon, Kyung J. Lee, Alexa L. Gross, Kanha Batra, Reesha R. Patel, Meenakshi M. Asokan, Jeremy Delahanty, Christian Cazares, Christopher R. Heyman, Nicholas B. Poll, Liezl Maree, Romy Wichmann, Talmo D. Pereira, Marcus K. Benna, Cory M. Root, and Kay M. Tye.