Brn3b acts via Tac2 to regulate defensive responses to visual threat

Defensive responses to visually threatening stimuli represent an essential fear-related survival instinct, widely detected across species. The neural circuitry mediating visually triggered defensive responses has been delineated in the midbrain. However, the molecular mechanisms regulating the development and function of these circuits remain unresolved. Here we show that midbrain-specific deletion of the transcription factor Brn3b causes a loss of neurons projecting to the lateral posterior nucleus of the thalamus. Brn3b deletion also downregulates the expression of the neuropeptide tachykinin 2 (Tac2). Furthermore, Brn3b mutant mice display impaired defensive freezing responses to visual threat precipitated by social isolation. This behavioral phenotype could be ameliorated by overexpressing Tac2, suggesting that Tac2 acts downstream of Brn3b in regulating defensive responses to threat. Together, our experiments identify specific genetic components critical for the functional organization of midbrain fear-related visual circuits. Similar mechanisms may contribute to the development and function of additional long-range brain circuits underlying fear-associated behavior. To investigate the role of Brn3b on the development of mouse midbrain, we utilized the conditional knockout strategy to delete Brn3b in the midbrain only. We then conducted RNA sequencing and compared gene expression between heterozygotes and homozygotes of Brn3b at neonatal stage (P2-P4).针对视觉威胁刺激的防御反应是一类与恐惧相关的核心生存本能，已在众多物种中被广泛检出。介导视觉诱发防御反应的神经环路已在中脑层面得到阐明，但调控此类环路发育与功能的分子机制仍未明确。本研究发现，中脑特异性敲除转录因子Brn3b会导致投射至丘脑外侧后核的神经元丢失；同时，Brn3b敲除还会下调神经肽速激肽2（Tac2）的表达。此外，Brn3b突变小鼠在社交隔离诱发的视觉威胁下，其防御性僵住反应出现损伤。过表达Tac2可改善该行为表型，提示Tac2作为Brn3b的下游效应分子参与调控针对威胁的防御反应。综上，本实验明确了调控中脑恐惧相关视觉环路功能组织的特异性遗传元件。类似机制或可参与调控恐惧相关行为所依赖的其他长程脑环路的发育与功能。为探究Brn3b对小鼠中脑发育的调控作用，本研究采用条件性敲除策略，仅在小鼠中脑内敲除Brn3b。随后通过RNA测序技术，比较了新生阶段（出生后第2~4天，P2-P4）Brn3b杂合子与纯合子小鼠的基因表达差异。