Kaempferol Facilitated Extinction Learning in Contextual Fear Conditioned Rats via Inhibition of Fatty-Acid Amide Hydrolase

Abstract

Background: Fear, stress, and anxiety-like behaviors originate from traumatic events in life. Stress response is managed by endocannabinoids in the body by limiting the uncontrolled retrieval of aversive memories. Pharmacotherapy-modulating endocannabinoids, especially anandamide, presents a promising tool for treating anxiety disorders. Here, we investigated the effect of kaempferol, a flavonoid, in the extinction of fear related memories and associated anxiety-like behavior. Methods: The ability of kaempferol to inhibit fatty-acid amide hydrolase (FAAH, the enzyme that catabolizes anandamide) was assessed in vitro using an enzyme-linked immunosorbent assay (ELISA) kit. For animal studies (in vivo), the extinction learning was evaluated using contextual fear conditioning (CFC, a behavioral paradigm based on ability to learn and remember aversive stimuli). Furthermore, an elevated plus-maze (EPM) model was used for measuring anxiety-like behavior, while serum corticosterone served as a biochemical indicator of anxiety. Lastly, the interaction of kaempferol with FAAH enzyme was also assessed in silico (computational study). Results: Our data showed that kaempferol inhibited the FAAH enzyme with an IC50 value of 1 µM. In CFC, it reduced freezing behavior in rats. EPM data demonstrated anxiolytic activity as exhibited by enhanced number of entries and time spent in the open arm. No change in blood corticosterone levels was noted. Our computational study showed that Kaempferol interacted with the catalytic amino acids (SER241, PHE192, PHE381, and THR377) of FAAH enzyme Conclusion: Our study demonstrate that kaempferol facilitated the extinction of aversive memories along with a reduction of anxiety. The effect is mediated through the augmentation of endocannabinoids via the inhibition of FAAH enzyme.