A team of researchers has identified a small brain region—the amygdalostriatal transition zone, or ASt—as the brain’s continuous alarm signal that keeps animals on high alert during threat. The work, published in Neuron, shows the ASt bridges learning about danger with the maintenance of defensive responses. Lead author Fergil Mills, an assistant professor at the University of Utah, conducted the study as part of Kay M. Tye’s laboratory at the Salk Institute for Biological Studies.
The ASt lies in the narrow gap between the amygdala, a centre for emotion processing, and the striatum, which helps govern movement. When a threat appears, the brain must rapidly detect it and sustain a defensive state until the danger passes, a task the ASt appears to service in this study.
To probe its identity, the team sequenced genetic material from 97,434 individual cell nuclei from mouse brains and found the ASt has a distinctive genetic profile. It contains a markedly high proportion of Drd2-positive neurons, bearing the dopamine receptor Drd2, accounting for about 71 per cent of cells in the ASt, compared with roughly 26 per cent Drd1a-positive neurons.
Electrical activity within the ASt was then recorded in 15 live mice trained to associate a 20-second tone with a mild foot shock or with a sweet reward. The researchers observed that ASt neurons fired continuously for the full 20-second threat cue, in contrast to the brief bursts seen in the amygdala. Trials with strong defensive behaviour, such as freezing, showed ASt firing rates more than three times higher than trials with weaker responses.
To test causality, the team used optogenetics to activate ASt neurons in 18 mice. When blue light was delivered to the ASt, the mice immediately froze or avoided the lit side of an enclosure. In a separate set of tests isolating cell types, 35 additional mice showed that activating only Drd2-positive neurons produced the same freezing and avoidance, while activating Drd1a-positive neurons had no effect on these behaviours.
A motor-coordination test using a rotating rod demonstrated that the freezing observed was a defensive response rather than a general motor impairment. In addition, 15 mice were fitted with miniature microscopes to monitor calcium levels in specific cell types during the task, revealing sustained activity of Drd2-positive neurons throughout threat cues.
Finally, the researchers asked whether the ASt is strictly necessary for fear responses. They silenced the Drd2-positive ASt neurons in 21 mice while they performed the tone task; the animals showed a nearly 50 per cent reduction in defensive behaviours such as freezing or sudden darts. The silencing did not alter responses to reward cues or 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.
“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 emphasised that the findings remain preliminary. The experiments used a single threat type (foot shock) and a single reward (a sweet drink). It remains to be determined whether the ASt responds specifically to the negative nature of an experience or to other intense stimuli, and the mice did not show ASt activation when placed in a fear-associated context, only when the warning tone was heard.
“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.”
He added that the work complements the established amygdala circuits for fear learning and that the ASt probably forms part of a larger fear-response circuit alongside the amygdala. The team is pursuing human studies to translate these findings, establishing a new translational direction for a brain region that has been largely overlooked in mental health research.
“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,” Mills 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,” lists Fergil Mills and colleagues, including 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.
