Upregulation of Anandamide Hydrolysis in the Basolateral Complex of Amygdala Reduces Fear Memory Expression and Indices of Stress and Anxiety

Abstract

Increased anandamide (AEA) signaling through inhibition of its catabolic enzyme fatty acid amide hydrolase (FAAH) in the basolateral complex of amygdala (BLA) is thought to buffer against the effects of stress and reduces behavioral signs of anxiety and fear. However, examining the role of AEA signaling in stress, anxiety, and fear through pharmacological depletion has been challenging due to the redundant complexity of its biosynthesis and the lack of a pharmacological synthesis inhibitor. We developed a herpes simplex viral vector to rapidly yet transiently overexpress FAAH specifically within the BLA to assess the impact of suppressing AEA signaling on stress, fear, and anxiety in male rats. Surprisingly, FAAH overexpression in BLA dampened stress-induced corticosterone release, reduced anxiety-like behaviors, and decreased conditioned fear expression. Interestingly, depleting AEA signaling in the BLA did not prevent fear conditioning itself or fear reinstatement. These effects were specific to the overexpression of FAAH because they were reversed by intra-BLA administration of an FAAH inhibitor. Moreover, the fear-suppressive effects of FAAH overexpression were also mitigated by intra-BLA administration of a low dose of a GABA_A receptor antagonist, but not an NMDA/AMPA/kainate receptor antagonist, suggesting that they were mediated by an increase in GABAergic neurotransmission. Our data suggest that a permissive AEA tone within the BLA might gate GABA release and that loss of this tone through elevated AEA hydrolysis increases inhibition in the BLA, which in turn reduces stress, anxiety, and fear. These data provide new insights on the mechanisms by which amygdalar endocannabinoid signaling regulates emotional behavior.

SIGNIFICANCE STATEMENT Amygdala endocannabinoid signaling is involved in the regulation of stress, anxiety, and fear. Our data indicate that viral-mediated augmentation of anandamide hydrolysis within the basolateral amygdala reduces behavioral indices of stress, anxiety, and conditioned fear expression. These same effects have been previously documented with inhibition of anandamide hydrolysis in the same brain region. Our results indicate that the ability of anandamide signaling to regulate emotional behavior is nonlinear and may involve actions at distinct neuronal populations, which could be influenced by the basal level of anandamide. Modulation of anandamide signaling is a current clinical therapeutic target for stress-related psychiatric illnesses, so these data underscore the importance of fully understanding the mechanisms by which anandamide signaling regulates amygdala-dependent changes in emotionality.