Role of intercalated amygdala neurons in the extinction of conditioned fear

描述(由申请者提供)：对调节条件性恐惧反应获得的回路的研究构成了我们理解人类焦虑症的最大希望。通常用于研究这一过程的模型是经典的恐惧条件反射，即中性感觉刺激(CS)在与伤害性刺激配对后获得引发恐惧反应的能力。然而，从临床角度来看，或许更重要的是了解恐惧反应是如何消退的。在实验上，这种消退过程是通过重复呈现CS来模拟的，导致条件性恐惧下降到控制水平。这种方法类似于治疗人类恐惧症的方法，即在没有危险的情况下，向受试者展示令人恐惧的物体。众所周知，灭绝是由于杏仁核中发生的一种新的学习造成的，它与最初的恐惧记忆竞争，以阻止条件性恐惧的表达。然而，这种新的灭绝学习背后的机制仍然不清楚。这一提议检验了杏仁核(ITC)内嵌(ITC)神经元介导灭绝的假说。众所周知，条件性恐惧的获得涉及杏仁基底外侧核(BLA)CS输入的增强。反过来，BLA细胞刺激中央杏仁核(CE)中更多的神经元，这些神经元通过投射到脑干和下丘脑，引发恐惧反应。我们关注ITC神经元，因为它们可以控制BLA输入对CE神经元的影响，从而控制条件性恐惧的表达。事实上，ITC细胞是GABA能的，它们接受来自BLA的谷氨酸输入，并在CE中产生前馈抑制。此外，进入ITC神经元的BLA可以经历NMDA依赖的LTP。最后，ITC神经元接受来自下缘皮质的沉重投射，这一皮质区被认为在物种灭绝中起着关键作用。这导致我们假设，灭绝是由于向ITC神经元传递CS信息的BLA突触的NMDA依赖的增强，导致CE细胞对关于CS的BL输入的反应性降低。为了验证这一假说，我们将首先通过比较只接受恐惧条件化处理的大鼠和经历恐惧条件化和消退的大鼠脑片中用斑贴方法记录的ITC神经元中BLA诱发的反应的幅度，来检验消退是否与BLA输入到ITC细胞的增强有关。接下来，我们将在恐惧条件反射、消退训练和消退回忆过程中对ITC细胞进行细胞外记录，并询问ITC神经元是否像我们的模型预测的那样，由于消退训练而对CS产生更高的反应。最后，为了测试ITC细胞是否介导了边缘下皮质对消亡的影响，我们将研究细胞外记录的ITC神经元对边缘下刺激的反应，并测试诱发反应的性质、潜伏期和持续时间是否与ITC神经元产生IL刺激对CE神经元的抑制这一观点相一致。公共卫生相关性：尽管焦虑症影响了近13%的人口，但大多数可用的药物治疗效果有限，并带来了重要的副作用。因此，我们必须提高对焦虑症潜在机制的理解，以设计更好的治疗策略。如果得到支持，这里测试的假设将为焦虑症的治疗开辟新的策略。

DESCRIPTION (provided by applicant): Research on the circuits mediating the acquisition of conditioned fear responses constitutes our best hope of understanding human anxiety disorders. The model typically used to study this process is classical fear conditioning where a neutral sensory stimulus (CS) acquires the ability to elicit fear responses after pairing to a noxious stimulus. However, perhaps more important from a clinical perspective is to understand how fear responses subside. Experimentally, this extinction process is modeled with repetitive presentations of the CS alone, resulting in the decline of conditioned fear to control levels. This approach is similar to that used to treat human phobias where subjects are presented with the feared object in the absence of danger. Extinction is known to result from a new learning, which takes place in the amygdala, and competes with the original fear memory to prevent the expression of conditioned fear. However, the mechanisms underlying this new extinction learning remain unclear. This proposal tests the hypothesis that the intercalated (ITC) neurons of the amygdala mediate extinction. The acquisition of conditioned fear is known to involve a potentiation of CS inputs to the basolateral amygdala (BLA). In turn, BLA cells excite more neurons in the central amygdala (CE), which, via their projections to the brainstem and hypothalamus, evoke fear responses. We focus on ITC neurons because they can control the impact of BLA inputs on CE neurons and hence the expression of conditioned fear. Indeed, ITC cells are GABAergic, they receive glutamatergic inputs from BLA, and they generate feed-forward inhibition in CE. Moreover, BLA inputs to ITC neurons can undergo NMDA-dependent LTP. Last, ITC neurons receive a heavy projection from the infralimbic cortex, a cortical area thought to play a critical role in extinction. This leads us to hypothesize that extinction results from an NMDA-dependent potentiation of BLA synapses conveying CS information to ITC neurons, leading to a decreased responsiveness of CE cells to BL inputs about the CS. To test the hypothesis, we will first examine whether extinction is associated with a potentiation of BLA inputs to ITC cells by comparing the amplitude of BLA-evoked responses in ITC neurons recorded with the patch method in slices obtained from rats that underwent fear conditioning only vs. rats that underwent fear conditioning and extinction. Next, we will perform extracellular recordings of ITC cells during fear conditioning, extinction training, and extinction recall, and ask do ITC neurons become more responsive to the CS as a result of extinction training, as predicted by our model. Finally, to test whether ITC cells mediate the influence of the infralimbic cortex on extinction, we will study the responses of extracellularly recorded ITC neurons to infralimbic stimuli and test whether the nature, latency, and duration of evoked responses are compatible with the idea that ITC neurons generate the inhibition of CE neurons by IL stimuli. PUBLIC HEALTH RELEVANCE: Although anxiety disorders affect close to 13% of the population, most available pharmacological treatments have a limited efficacy and entail important side effects. It is thus imperative that we improve our understanding of the mechanisms underlying anxiety disorders to design better treatment strategies. If supported, the hypothesis tested here would open new strategies for the treatment of anxiety disorders.