Amygdala Habituation Indirectly Links FAAH and CRHR1 Epistasis to Anxiety Disorders

Background: The endocannabinoid (eCB) and hypothalamic-pituitary-adrenal (HPA) axis systems play a pivotal role in stress and anxiety with increasing evidence that these effects may emerge, at least in part, due to their integrated modulatory effects on the amygdala. Rodent research suggests that stress-induced reductions in the eCB ligand anandamide (AEA) potentiate and sustain amygdala activity and anxiety. Such reductions in AEA are driven by corticotropin-releasing hormone receptor type 1 (CRHR1) signaling that increases the AEA catabolic enzyme, fatty acid amide hydrolase (FAAH). As such, genetic polymorphisms within the FAAH and CRHR1 genes may interact to impact amygdala function. Methods: Using data (n=661) from the ongoing Duke Neurogenetics Study (DNS), we examined whether the interaction of functionally-characterized polymorphisms within FAAH (rs324420) and CRHR1 (rs110402) is associated with individual differences in amygdala habituation (i.e., decreases) to threat-related stimuli measured using BOLD fMRI. Next, we examined whether amygdala habituation was predictive of DSM-IV-TR anxiety disorder diagnosis and followed up any significant associations with a moderated mediator to test if the FAAH x CRHR1 interaction was indirectly associated with the presence of an anxiety disorder through amygdala habituation. Results: Significant CRHR1 x FAAH genotype interactions emerged in bilateral basolateral and centromedial amygdala regions of interest. Individuals who were both FAAH A allele carriers (increased AEA signaling) and CRHR1 A allele homozygotes (increased cortisol response, n=84), had blunted threat-related amygdala habituation relative to individuals who were both FAAH A allele carriers (increased AEA signaling) and CRHR1 G allele carriers (reduced cortisol response, n=165) as well as individuals who were FAAH C allele homozygotes (reduced AEA signaling) and CRHR1 A allele homozygotes (increased cortisol response, n=162). Furthermore, reduced basolateral amygdala habituation predicted increased risk for anxiety disorder. Follow-up moderated mediation analyses demonstrated that the interaction between FAAH and CRHR1 genotypes was indirectly associated with increased anxiety disorder risk via the mediating effect of amygdala habituation. Specifically, blunted amygdala habituation within individuals who were both FAAH A allele carriers and CRHR1 A allele homozygotes (increased AEA and CRHR1 signaling), was associated with increased risk for an anxiety disorder. Conclusions: Consistent with rodent data, these findings suggest that potentiated CRHR1 signaling may counteract the effects of increased AEA signaling conferred by the FAAH A allele, resulting in blunted amygdala habituation to threat-related stimuli and increased risk for an anxiety disorder. While grounded within the rodent literature, these findings require replication in additional samples