Noradrenergic modulation of stress-induced deficits in fear extinction

Texas A&M University, College Station TX

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Abstract

Project Summary/Abstract The overall goal of this project is to understand the neural circuits underlying stress-induced impairments in extinction learning. Early intervention strategies (e.g., psychological debriefing and exposure therapy) are thought to rely on extinction-like mechanisms to reduce pathological fear. These therapies attempt to reduce or prevent the development of stress- and trauma-related disorders such as posttraumatic stress disorder (PTSD). Despite this, disorders of fear and anxiety are prevalent in our society, due in part to a lack of empirically-driven treatment options. For example, both animal models and human data suggest early interventions after a traumatic event may actually undermine long-term recovery. In the laboratory, Pavlovian fear conditioning procedures in rats have provided fundamental knowledge regarding the brain circuits mediating learned fear. However, less is known about the mechanisms of extinction learning, a process intended to reduce conditional fear, which is highly sensitive to stress. We have previously shown extinction training delivered soon after fear conditioning fails to reduce long-term fear in rats [the ?immediate extinction deficit? (IED)]. This may be due to elevated levels of the stress neurotransmitter norepinephrine (NE), which is released from synaptic terminals originating in the locus coeruleus (LC). The LC densely innervates the medial prefrontal cortex (mPFC) and basolateral complex of the amygdala (BLA), two key brain structures subserving learned fear and its subsequent extinction. How stress impairs extinction learning at the circuit level remains poorly understood. Here we propose LC-NE enhances BLA activity soon after fear conditioning which ultimately suppresses mPFC signaling. As such, mPFC feedback to the BLA (which crucially underlies extinction learning) is impaired, leading to poor long-term extinction. We will test this hypothesis in two specific aims. Aim 1 will use a circuit-mapping approach to examine if LC projections to the mPFC and BLA are differentially engaged following immediate (Expt 1) and delayed (Expt 2) extinction. To address this question, we will use functional neuroanatomical techniques combining fluorescently labeled retrograde tracers and Fos immunohistochemistry to examine the functional relevance of the LC?mPFC and LC?BLA pathways. Aim 2 is focused on parsing the neural circuitry underlying the IED. We will use a combination of LC specific DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) and in vivo electrophysiology in awake, behaving animals to examine the contribution of LC-NE to mPFC and BLA single-unit activity during and soon after fear conditioning (Expt 3). Lastly, we will use LC-DREADD ?disconnections? to selectively inactivate LC?BLA projections to determine if this direct pathway underlies the IED (Expt 4). In summary, this project will provide both basic and clinically relevant information regarding how NE and stress impair extinction and potentiate fear.