Sex differences in the endocannabinoid modulation of fear memory dynamics

Fear extinction is essential for the ability to recover from highly stressful and traumatic events. Impaired fear extinction is believed to contribute to the development and persistence of post-traumatic stress disorder (PTSD). Although women are more likely than men to develop PTSD, there is a paucity of understanding with respect to sex differences in fear extinction. The endocannabinoid (eCB) system - comprised of the cannabinoid type 1 receptor (CB1R), ligands anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), and their respective hydrolyzing enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) - is implicated in the process of fear extinction. We examined whether the eCB system may be involved in modulating fear memory expression and extinction in a sex-specific manner, in rats. We exposed adult Sprague-Dawley rats to an auditory fear conditioning and extinction paradigm and systemically administered the MAGL inhibitor MJN110, the FAAH inhibitor URB597, or vehicle prior to fear extinction in both sexes. Consistent with previous studies we found that males exhibited mostly passive fear responses (freezing), while a large proportion of females engaged in active fear responses (darting). Pharmacological manipulation of eCB signaling did not affect male fear expression. In females, MJN110 decreased freezing while URB597 increased freezing during late fear extinction learning. To determine whether these effects in females were CB1R-mediated, we systemically administered the CB1R antagonist AM251 prior to the enzyme inhibitors. Treatment with AM251 blocked the reduction in freezing induced by MJN110, indicating a CB1R-mediated mechanism. Injection of URB597+AM251, however, further elevated freezing throughout extinction learning as compared to rats treated with URB597 alone. In addition to CB1R, AEA is known to activate transient receptor potential vanilloid type-1 (TRPV1) channels. To determine whether the increase of freezing induced by URB597+AM251 was mediated by activation of TRPV1 receptors, the TRPV1 antagonist capsazepine (CPZ) was administered systemically together with AM251 and URB597. We found that CPZ rescued the augmentation of freezing induced by URB597+AM251 to levels comparable with controls during extinction learning and recall, suggesting that the URB597 enhancement of freezing was TRPV1- mediated. These results uncover contrasting roles of the eCB system in mediating fear extinction dynamics between sexes and will help develop sex-specific therapeutic strategies to treat trauma-related disorders that are more common in women than in men.