紧张性抑制电流下调在青春期小鼠恐惧消退能力弱化中的作用及机制

The ability to extinguish learned fear is markedly down-regulated upon entering adolescence, which is generally known to be closely associated with higher emotional reactivity and higher prevalence of emotional diseases in this period. The tonic inhibitory currents mediated by activation of extrasynaptic GABAA receptors is critical for the modulation of neuronal activity. However, the brain mechanisms underlying the weakening of fear extinction remains largely unknown. We have recently observed that the tonic inhibitory current, which is mediated by GABAA receptors situated extrasynaptically, is predominantly expressed in the interneurons but not projection neurons of amygdala. Activation of this current results in clear disinhibition of amygdala with consequent enhancement in the retrieval of learned fear. Further evidence shows that the tonic inhibitory current experiences a dramatic reduction in the projection neurons but not their proximal interneurons when the mice spans from childhood to adolescence. We thus hypothesize that such a reduction may further contribute to the disinhibition of amygdala and lead to the weakening of fear extinction. In this project, we will explore the specific subtype of GABAARs mediating the decline of tonic current during the transition to adolescence and investigate the role of such reduction in modulating the activity as well as the morphology of amygdala projection neurons. Finally, we will test the contribution of the reduced tonic inhibition in the weakened fear extinction during this development period. This project will help to clarify the mechanism for and the function of the reduced tonic inhibition in amygdala, and may provide experimental and theoretical reference for further prevention and treatment of mental illness occurring in adolescence.

进入青春期后，大脑对恐惧记忆的消退能力明显弱化，这与青春期个体的强烈情感反应和精神疾患高发密切相关，但其产生的具体机制尚不明确。突触外GABAA受体激活后产生的紧张性抑制电流是调控神经元活动的重要方式。我们最近研究表明，在青春期小鼠杏仁核内，紧张性抑制电流主要表达在中间神经元上，它通过引发杏仁核去抑制增强恐惧记忆。进一步发现，步入青春期后，小鼠杏仁核内的紧张性抑制电流在投射神经元（而非中间神经元）上显著下调。据此推测，这种下调将加剧杏仁核去抑制，从而弱化青春期小鼠的恐惧消退能力。在本项目中，我们拟探查介导上述电流下调的具体GABAA受体亚型；分析这种下调对杏仁核投射神经元活动状态及突触构成的影响；阐明其在青春期小鼠恐惧消退能力弱化中的作用。本项目的完成有助于明晰杏仁核投射神经元紧张性抑制电流下调在弱化青春期恐惧消退能力中的作用及机制，为防治青春期频发的精神疾患提供实验证据和理论参考。

青春期后大脑对恐惧记忆的消退能力明显减弱，这与青春期个体的强烈情感反应和精神疾患高发密切相关，但其产生的具体机制仍不清楚。突触外GABAA受体激活后产生的紧张性抑制电流是调控神经元活动的重要方式。我们最近研究表明，在青春期小鼠外侧杏仁核（LA）内，紧张性抑制电流主要表达在中间神经元上，它主要通过引起杏仁核的去抑制而增强恐惧记忆。然而，进入青春期后引起的紧张性抑制电流变化是否与小鼠恐惧消退能力弱化有关并不明确。为此，本项目针对该问题进行了深入探索，我们发现：1)进入青春期后，小鼠LA内神经元的紧张性抑制电流在投射神经元（而非中间神经元）上显著下调，而微小突触后抑制性电流mIPSCs无明显变化；此外，海马CA1区和前额叶皮层（第5层）投射神经元紧张性抑制电流均无明显影响；2）利用药理学工具以筛选介导紧张性抑制电流的亚基在LA内的表达格局变化，我们发现：含δ亚基（而不是含α3和α5）的GABAAR可能是介导青春期LA投射神经元紧张性电流下调的重要受体；3）进一步对紧张性抑制电流调控不同发育期小鼠LA神经元的兴奋性研究发现，10 μM GABA灌流对处于青春期前后小鼠神经元的兴奋性具有明显下调作用，但这种作用在青春期前显著强于青春期后。这进一步验证了紧张性抑制电流下调可能参与调控青春期发育过程中杏仁核的功能变化；4）为进一步明确紧张性抑制电流的潜在病理意义。利用慢性束缚应激引发过度焦虑的小鼠模型，我们发现，青春期应激应激引发的小鼠焦虑行为的同时，伴随杏仁核神经元上的紧张性抑制电流的下调，同时削弱了GABA对神经元兴奋性的抑制作用。本项目的完成有助于明晰杏仁核投射神经元紧张性抑制电流下调在弱化青春期恐惧消退能力中的作用及机制，为防治青春期频发的精神疾患提供实验证据和理论参考。