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Posted on May 10, 2018, 3 a.m.
Two adjacent clusters of nerve cells in the brain whose activation levels upon seeing a visual threat spelt the difference between a bold or timid response have been identified by researchers at Stanford University, as published in Nature.
The clusters are located in the middle of the brain sending signals to different brain areas, igniting opposite behaviors when presented with visual threat. Selective alteration of activity levels of the 2 nuclei researchers were able to dispose model mice to either aggressively stand ground, freeze, or run to a hiding space when approached by a simulated predator. Human brains most likely possess equivalent circuitry, findings ways in which to noninvasively shift balance between signaling strengths of the nuclei in certain situations people find as threatening may help patients with excessive phobias, post-traumatic stress, or anxiety disorders lead more normal lives.
Looking for brain regions that were more active, mice were exposed to looming predator threats, which enabled researchers to identify the ventral midline thalamus structure. Inputs and outputs of the structure was mapped that registered internal brain states. The ventral midline thalamus receives broad inputs, in contrast output destinations are remarkably selective. Outputs were traced to 2 main destinations: the medial prefrontal cortex, and the basolateral amygdala. Prior studies have tied the amygdala to processing threat detection and fear, while the medial prefrontal cortex is associated with anxiety and high level executive functions. Nerve tract leading to the basolateral amygdala emanates from the xiphoid nucleus nerve cell cluster in the ventral midline thalamus. The tract leading to the medial prefrontal cortex comes from the reuniens cluster which envelopes the xiphoid nucleus.
Specific sets of nerve cells in the mice brains were selectively modified to be inhibited or stimulated in these 2 nerve tracts. Xiphoid activity was exclusively stimulated which increased propensity to freeze in place; boosting activity in the tract running from the reuniens to medial prefrontal cortex increased boldness and the mice stood their ground, which is ordinarily associated with aggression and is a response seldom seen in the species towards predators.
The bold courageous behavior was clear, loud, and unmistakable. Tails were thumping on the ground and against the side if the chamber, which is equivalent of beating your chest saying let’s fight. Mice which had the reuniens nucleus stimulated ran around more in the open as opposed to running to hide, in absence of simulated looming predator mice relaxed.
Stimulating reuniens nucleus for 30 seconds before exposure to looming predator induced the same bold courageous behavior in other experiments suggesting stimulation of nerve cells leading from reunions nucleus to prefrontal cortex induces shifts in the brain internal state which predisposes mice to act more boldly.
Human brains harbor the equivalent structure to the ventral midline thalamus, it is speculated that individuals with PTSD, phobias, or anxiety, malfunctioning circuitry may prevent the ventral midline thalamus signaling from dropping off with exposure to stress inducing situations. The team is exploring efficacy of techniques in adjusting the arousal states of individuals suffering from such problems, thinking that reducing ventral midline thalamus signaling or altering balance of signal strength may increase ability to cope with stress.
Materials provided by Stanford Medicine.
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Journal Reference:
Lindsey D. Salay, Nao Ishiko, Andrew D. Huberman. A midline thalamic circuit determines reactions to visual threat. Nature, 2018; DOI: 10.1038/s41586-018-0078-2
