A pro-nociceptive phenotype unmasked in mice lacking fatty-acid amide hydrolase

Abstract

Fatty-acid amide hydrolase (FAAH) is the major enzyme responsible for degradation of anandamide, an endocannabinoid. Pharmacological inhibition or genetic deletion of FAAH (FAAH KO) produces antinociception in preclinical pain models that is largely attributed to anandamide-induced activation of cannabinoid receptors. However, FAAH metabolizes a wide range of structurally related, biologically active lipid signaling molecules whose functions remain largely unknown. Some of these endogenous lipids, including anandamide itself, may exert pro-nociceptive effects under certain conditions. In our study, FAAH KO mice exhibited a characteristic analgesic phenotype in the tail flick test and in both formalin and carrageenan models of inflammatory nociception. Nonetheless, intradermal injection of the transient receptor potential channel V1 (TRPV1) agonist capsaicin increased nocifensive behavior as well as mechanical and heat hypersensitivity in FAAH KO relative to wild-type mice. This pro-nociceptive phenotype was accompanied by increases in capsaicin-evoked Fos-like immunoreactive (FLI) cells in spinal dorsal horn regions implicated in nociceptive processing and was attenuated by CB1 (AM251) and TRPV1 (AMG9810) antagonists. When central sensitization was established, FAAH KO mice displayed elevated levels of anandamide, other fatty-acid amides, and endogenous TRPV1 agonists in both paw skin and lumbar spinal cord relative to wild-type mice. Capsaicin decreased spinal cord 2-AG levels and increased arachidonic acid and prostaglandin E2 levels in both spinal cord and paw skin irrespective of genotype. Our studies identify a previously unrecognized pro-nociceptive phenotype in FAAH KO mice that was unmasked by capsaicin challenge. The heightened nociceptive response was mediated by CB1 and TRPV1 receptors and accompanied by enhanced spinal neuronal activation. Moreover, genetic deletion of FAAH has a profound impact on the peripheral and central lipidome. Thus, genetic deletion of FAAH may predispose animals to increased sensitivity to certain types of pain. More work is necessary to determine whether such changes could explain the lack of efficacy of FAAH inhibitors in clinical trials.

Small molecule inhibitors of PSD95–nNOS protein–protein interactions suppress formalin-evoked Fos protein expression and nociceptive behavior in rats

“…The subdivisions used were the superficial laminae (laminae I and II), the nucleus proprius (laminae III and IV), the neck of the dorsal horn (laminae V and VI) and the ventral horn (laminae VII, VIII, IX, and X) (Presley et al, 1990). Intraplantar injection of saline in lieu of formalin (n =3) did not induce appreciable expression of Fos-like immunoreactivity, consistent with the results of our previously published studies (Tsou et al, 1996, Nackley et al, 2003a, Nackley et al, 2003b, Carey et al, 2016) (data not shown).…”

“…Immunohistochemical experiments were conducted as previously described (Tsou et al, 1996, Nackley et al, 2003a, Nackley et al, 2003b, Carey et al, 2016). Transverse sections (3μm) of the L4-L5 lumbar spinal cord were cut on a cryostat and maintained in an antifreeze solution (50% sucrose in ethylene glycol and 0.1 M PBS) prior to immunostaining.…”

“…The number of FLI cells was counted in each subdivision of the spinal cord as defined by (Presley et al, 1990) and averaged to produce a single mean for each spinal cord region for each animal. Statistical analyses were performed to compare the number of FLI cells in each spinal cord region (averaged across animals, not across sections) as described in our previously published work ((Tsou et al, 1996, Nackley et al, 2003a, Nackley et al, 2003b, Carey et al, 2016); see also Fig 1D). The subdivisions used were the superficial laminae (laminae I and II), the nucleus proprius (laminae III and IV), the neck of the dorsal horn (laminae V and VI) and the ventral horn (laminae VII, VIII, IX, and X) (Presley et al, 1990).…”

Positive Allosteric Modulation of Cannabinoid Receptor Type 1 Suppresses Pathological Pain Without Producing Tolerance or Dependence

“…By contrast, lipoxin A4 preferentially enhances anandamide but not 2-AG signaling(28). However, FAAH and MGL also degrade lipid mediators that do not bind to cannabinoid receptors(8, 12, 44). Sustained inhibition of MGL does not desensitize CB1 receptors in brain regions relevant to catalepsy (i.e.…”

Broad impact of deleting endogenous cannabinoid hydrolyzing enzymes and the CB1 cannabinoid receptor on the endogenous cannabinoid-related lipidome in eight regions of the mouse brain

“…Showing that other types of pain sensitivity are also reduced in FAAH KO mice, pain behavior was attenuated in the formalin test [27]. Consequently, FAAH became a therapeutic target for chronic pain; however, limited analgesic efficacy of a FAAH inhibitor was found in a human clinical trial [45], which may be explained by the observation that FAAH deletion actually enhances some forms of pain [46]. Similarly to FAAH, MAGL is a potential drug target to treat pain.…”