Interactions between the ventral hippocampus and amygdala during renewal of fear

焦虑症是一个重大的公共卫生问题。消除经典的条件性恐惧是治疗焦虑症的有效干预模式，例如暴露疗法。有趣的是，在消除恐惧的过程中学习到的记忆是与背景相关的，这对焦虑的临床干预提出了相当大的挑战。例如，在消亡背景之外呈现一个消退的条件刺激(CS)会产生新的条件性恐惧。最近的研究表明，消亡记忆编码在杏仁核中，海马体对于灭绝后对消失的CS重新产生恐惧是必不可少的。因此，我们假设，海马体和杏仁核之间的直接轴突连接对于消除恐惧反应的上下文相关表达是必不可少的。为了验证这一假设，我们将首先检验功能性切断腹侧海马体(VH)和杏仁核基底外侧核(BA)在恐惧消退后更新的后果。将GABA激动剂Muscimol微量注射到一侧大脑半球的VH和对侧大脑半球的BA中，然后进行提取测试，在提取测试中，在灭绝环境或另一种环境中呈现熄灭的CS。我们预测，当CS出现在灭绝背景之外时，VH和BA的药理学分离将损害恐惧的更新。为了进一步表征VH-BA投射对更新的贡献，我们将研究VH-BA断开对杏仁核内c-fos表达的影响。C-fos和GAD67的双重免疫组织化学将使我们能够阐明BA内的投射神经元或抑制神经元在恐惧或消退记忆的提取过程中是由海马输入驱动的。我们预计，恐惧的重现将增加基底动脉内c-fos阳性投射神经元的数量，而VH-BA的断开将限制这种表达。最后，我们将使用c-fos免疫组织化学和向BA注射逆行示踪剂(霍乱毒素b，CTB)来确定投射到BA的VH神经元是否参与了恐惧的更新。我们预测，与表达消亡的大鼠相比，在BA投射的VH神经元中，恐惧的重新出现将与c-fos的表达有关。总而言之，这些结果将提供对调节灭绝后恐惧记忆表达的神经回路机制的洞察。我们希望，这项工作将为那些患有焦虑和恐惧病理(如创伤后应激障碍)的人提供有效的治疗干预措施。

Anxiety disorders are a significant public health problem. Extinction of classically conditioned fear is a useful model of therapeutic interventions, such as exposure therapy, for anxiety disorders. Interestingly, the memories learned during the extinction of fear are context-dependent, and this presents a considerable challenge to clinical interventions for anxiety. For example, presenting an extinguished conditional stimulus (CS) outside of the extinction context produces a renewal of conditioned fear. Recent work suggests that extinction memories are encoded in the amygdala, and that the hippocampus is essential for the renewal of fear to an extinguished CS after extinction. Accordingly, we hypothesize that direct axonal connections between the hippocampus and amygdala are essential for the context-dependent expression of an extinguished fear response. To test this hypothesis, we will first examine the consequences of functionally disconnecting the ventral hippocampus (VH) and basolateral nucleus of the amygdala (BA) on the renewal of fear after extinction. Microinfusions of the GABA agonist muscimol will be made into the VH in one hemisphere and the BA in the contralateral hemisphere prior to a retrieval test in which an extinguished CS is presented in either the extinction context or another context. We predict that pharmacological disconnection of the VH and BA will impair fear renewal of fear when the CS is presented outside the extinction context. To further characterize the contribution of the VH-BA projection to renewal, we will examine the effect of VH-BA disconnections on the expression of c-fos within the amygdala. Dual immunohistochemistry for c-fos and GAD67 will allow us to elucidate whether projection neurons or inhibitory neurons in the BA are driven by hippocampal input during the retrieval of fear or extinction memories. We expect that the renewal of fear will increase the number of c-fos positive projection neurons in the BA, and that VH-BA disconnections will limit this expression. Lastly, we will use c-fos immunohistochemistry and injection of retrograde tracers (cholera toxin b, CTb) into the BA to determine whether VH neurons projecting to the BA are engaged during the renewal of fear. We predict that the renewal of fear will be associated with c-fos expression in a greater proportion of BA-projecting VH neurons than in rats expressing extinction. Collectively, these results will provide insight into the neural circuit mechanisms for regulating the expression of fear memories after extinction. It is our hope that this work will inform effective therapeutic interventions for those individuals who suffer from anxiety and fear pathologies, such as post-traumatic stress disorder.