Interactions between the ventral hippocampus and amygdala during renewal of fear

University Of Michigan At Ann Arbor, Ann Arbor MI

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Abstract

DESCRIPTION (provided by applicant): 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. PUBLIC HEALTH RELEVANCE: The work proposed in this grant will help elucidate the neural circuits involved in the recovery of fear, which is critical in understanding the pathology involved in post-traumatic stress disorder (PTSD). Though exposure therapy is commonly used to treat PTSD and anxiety disorders, one problem that clinicians regularly encounter is the persistence of fear even in the face of extensive therapy. The research in this grant will try to explain what neural mechanisms account for this phenomenon with the hope that further understanding will result in better treatment and therapy.