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Click on the blue word to pay attention to us, keenly perceiving threats and making timely defensive behaviors are the basic skills for animal survival
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The hypothalamus plays a key role in regulating body temperature, eating, attacking, defense and many other physiological behaviors
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The hypothalamus is mainly composed of the paraventricular area, medial and lateral areas.
The medial hypothalamic area closely related to defensive behavior is further subdivided into the prehypothalamic nucleus (AHN), the ventromedial dorsal medial (VMHdm), and nipple Prebody nucleus (PMD)
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Studies have shown that a large number of neurons in the AHN brain region of rats and mice are activated when exposed to the presence of live snakes
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This indicates that this brain area is involved in defensive behavior, but the specific mechanism is unknown
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On January 3, 2022, the research team of Cao Peng from the Beijing Institute of Life Sciences revealed in Nature Neuroscience how inhibitory neurons in the AHN brain perceive threat information and make loop mechanisms for defensive behavior
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Figure 1: Defensive aggression behavior In order to be able to simulate defensive aggression behaviors, the research team exposed mice to the environment of plastic snakes coated with snake secretions or plastic snakes equipped with alligator clips (Figure 1).
Plastic snakes cannot induce defensive and aggressive behaviors of mice biting, but can increase their stiffness, risk assessment, escape and other defensive behaviors, and giving a certain strength to the tail causes the mice to bite the plastic snakes, showing defensive and aggressive behaviors.

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Replacing the plastic snake with a plastic cover or wood and giving heavy pressure to the tail can also induce defensive and aggressive behavior in mice
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These results indicate that harmful mechanical stimuli can induce defensive and aggressive behavior in mice
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Figure 2: Optical fiber calcium imaging records mechanical stimulation to activate inhibitory neurons in the AHN brain area.
Fluorescence in situ hybridization experiments found that about 92% of the neurons in the AHN brain area are inhibitory neurons and 8% are excitatory neurons
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The optical fiber calcium imaging recording system found that the calcium ion activity of inhibitory neurons in the AHN brain region of mice was significantly increased after heavy-pressure tail stimulation (Figure 2), which indicates that the mechanical stimulation that causes defensive behavior can activate the inhibitory neurons in the AHN brain region
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Figure 3: Photoactivation of inhibitory neurons in the AHN brain area promotes defensive and aggressive behavior.
Optogenetic inhibition of inhibitory neurons in the AHN brain area can quickly prevent the defensive and aggressive behavior in mice, and inhibit the excitatory nerves in this brain area.
Yuan has little effect on defensive attack behavior
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Further light activation of inhibitory neurons in the AHN brain region can enhance the defensive behavior of the plastic snake biting plastic snakes caused by mechanical stimulation.
However, when the fake snake is replaced with a real snake, the activation of the above neurons can also promote the tearing between the mouse and the real snake.
Biting (Figure 3, facing the danger, the last line of defensive behavior), but weakens other defensive behaviors such as risk assessment, avoidance, and rigidity
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So after activating inhibitory neurons in the AHN brain area, it triggers specific defensive and aggressive behaviors, or is it a broad sense of aggressive behavior? After the mice face the invasion of their peers, they will initiate social aggressive behaviors.
When the inhibitory neurons in the AHN brain area are activated, the ongoing social aggressive behaviors will be inhibited
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What is interesting is that when the mice and their peers did not engage in social aggressive behavior, activating this type of neuron would not cause aggressive behavior, but it would increase social exploratory behavior
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When mice live in the same environment with their companions and real snakes, before the inhibitory neurons in the AHN brain area are not activated, the mice tend to engage in social exploratory behaviors with their companions and hardly engage in defensive aggressive behaviors, but they are activating this type After neurons, mice tend to bite with real snakes, and their original social exploratory behaviors are significantly weakened
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This indicates that the activation of inhibitory neurons in the AHN brain area induces specific defensive and aggressive behaviors
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Figure 4: The input structure of retrograde tracer virus labeling inhibitory neurons in the AHN brain region.
Retrograde tracer virus reveals that inhibitory neurons in the AHN brain region receive compaction from the lateral parabrachial nucleus (LPB) and thalamic paraventricular nucleus (PVT) The single synaptic input also accepts sparse synaptic input from the ventromedial hypothalamus (VMH) and lateral septal nucleus (LS) and medial amygdala (Figure 4)
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The anterograde tracer virus found that the inhibitory neurons in the AHN brain area projected to the medial preoptic area, LS, VMH, PMD, and ventrolateral periapical gray matter (vlPAG)
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LS and vlPAG brain areas are closely related to aggressive behavior
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After the research members light activated AHN inhibitory neurons to project to LS (abbreviated as AHN→LS inhibitory loop) and vlPAG (abbreviated as AHN→vlPAG inhibitory loop), only activation of AHN→vlPAG inhibitory loop promoted mice Defensive bite behavior
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After inhibiting the activity of neurons in the brain region of vlPAG, it can block the defensive and aggressive behaviors caused by the light activation of the neurons
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In addition, acute photoinhibition or chronic inhibition of the AHN→vlPAG inhibitory loop can prevent mechanical stimulation from inducing defensive aggressive behavior
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This article reveals that inhibitory neurons in the AHN brain area encode threat-related sensory information, and specifically regulate defensive behavior through the loop projected to the gray matter around the ventrolateral aqueduct
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[References] 1.
https://doi.
org/10.
1038/s41593-021-00985-4 The pictures in the article are from the references