Physiology and function of amygdalo-cortical endocannabinoid signaling

Vanderbilt University, Nashville TN

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Abstract

PROJECT SUMMARY Stress exposure is ubiquitous risk factor for the development of negative emotional states and neuropsychiatric disorders such as anxiety, depression, and post-traumatic stress disorder (PTSD). However, a causal link between stress-induced neuropathologies and resultant pathological behavioral states remains elusive. There is significant evidence to suggest that stress induces a wide variety of morphological and neurochemical adaptions in the prelimbic component of the medial prefrontal cortex (plPFC), a key nodal structure that is highly implicated in the top down regulation of emotional behavior. Indeed, susceptibility to the depressive effects of traumatic stress exposure is correlated with synaptic potentiation within this region. Interestingly, the anxiolytic effects of cannabinoid compounds are primarily mediated by activation of CB1 receptors on glutamatergic axon terminals within the mPFC. Stress itself dynamically alters endocannabinoid (eCB) signaling within the mPFC, as stress exposure induces a dramatic increase in levels of the EC ligand 2- AG in the mPFC, which is purported to represent an adaptive coping response that buffers against the negative physiological effects induced by stress exposure. However, it is completely unknown what neural pathways are modulated by eCBs to elicit this buffering effect on stress-induced behavioral pathologies. In our studies, we are looking at the projections to the plPFC from the Basolateral Amygdala (BLA), a brain region that has been highly implicated in the pathophysiology of affective disorders and PTSD. The BLA sends glutamatergic projections to the plPFC, and also receives reciprocal glutamatergic input from the plPFC. Using optogenetic projection targeting techniques combined with retrograde labeling of plPFC cells, we gain both specificity of input to the plPFC but also specificity of output of plPFC cells projecting to the BLA. In order to understand the role that eCB signaling plays in buffering against stress induced psychopathologies in the BLA- plPFC circuit, we have developed a behavioral model that separates mice into stress-susceptible and stress- resilient groups. The current proposal will aim to answer two critical open questions: Does eCB signaling in the BLA-plPFC circuit represent a resiliency factor that blunts stress-induced behavioral pathology? Can we experimentally induce stress susceptibility by impairing eCB signaling within BLA-plPFC reciprocal circuits? Using state of the art viral mediated gene transfer approaches and conditional gene knockout animals, we aim to understand the role of eCB signaling within the BLA-plPFC circuits in conferring resiliency to stress. These experiments represent the first investigation of eCB signaling in a reciprocal glutamatergic circuit. This will enhance are understanding of the pathophysiology of anxiety and mood disorders, as well as reveal a putative target for the anxiolytic effects of cannabinoid agonists. This proposal also constitutes a rigorous technical training plan, in which I will learn and refine skills related to stereotaxic surgery, whole-cell electrophysiology, immunohistochemistry, and animal behavior.