The role of long chain fatty acids and their epoxide metabolites in nociceptive signaling

Abstract

Lipid derived mediators contribute to inflammation and the sensing of pain. The contributions of omega-6 derived prostanoids in enhancing inflammation and pain sensation are well known. Less well explored are the opposing anti-inflammatory and analgesic effects of the omega-6 derived epoxyeicosatrienoic acids. Far less has been described about the epoxidized metabolites derived from omega-3 long chain fatty acids. The epoxide metabolites are turned over rapidly with enzymatic hydrolysis by the soluble epoxide hydrolase being the major elimination pathway. Despite this, the overall understanding of the role of lipid mediators in the pathology of chronic pain is growing. Here, we review the role of long chain fatty acids and their metabolites in alleviating both acute and chronic pain conditions. We focus specifically on the epoxidized metabolites of omega-6 and omega-3 long chain fatty acids as well as a novel strategy to modulate their activity in vivo.

Keywords: EpDPEs); Epoxy fatty acids (EpFAs); Epoxydocosapentanoic acids (EDP; Epoxyeicosatrienoic acids (EETs); Omega-3 fatty acids; Pain; Soluble epoxide hydrolase (sEH).

Figure 1

Omega-6 and omega-3 long chain PUFAs share a similar metabolic fate. The fatty acids are designated based on the position (3 or 6) of the first double bond from the methyl (omega, ω) end of the molecule. Both omega-6 arachidonic acid (ARA) and omega-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are metabolized by cytochrome P450 oxidases. Several epoxy fatty acid regioisomer metabolites the epoxyeicosatrienoic acids (EETs), epoxyeicosatetraenoic acids (EEQs) and epoxydocosapentaenoic acids (EDPs) respectively, and their enantiomers are possible for each of the substrates based on which double bond is epoxidized. Arachidonic acid, for example, may form the 5–6, 7–8, 11–12, or 14–15 EET (pictured). The epoxidized fatty acids are subject to further metabolism by the soluble epoxide hydrolase which converts them to their vicinal diols termed the dihydroxyeicosatrienoic acids (DHETs), dihydroxyeicosatetraenoic acids (DHEQs) and dihydroxydocosapentaenoic acids (DiDPAs) respectively. The EpFAs have multiple beneficial biological effects including mediating analgesia while the diols are thought largely to lack these effects.

Figure 2

DHA dietary supplementation improves pain thresholds in neuropathic rats. In a type I diabetic neuropathy model, rats fed an enriched DHA diet (1% DHA custom made diet with Nu-Chek Prep Inc. 99% purity oil) one week prior to diabetes induction with streptozocin [117] and maintained on the diet for several weeks (pictured day 9–25) showed improved pain thresholds (von Frey assay) compared to rats on a omega-6 control diet (1% Oleic oil custom made diet with Nu-Chek Prep Inc. 99% purity oil) (Two Way Analysis of Variance, Holm Sidak post hoc, ‡ p<0.001 groups, *p ≤ 0.030 time points). When both groups were switched to normal chow the improved scores were maintained for up to 10 days (pictured day 25–35) but then these effects dissipated. Thus, dietary supplementation with the omega-3 PUFA DHA improved nociceptive outcomes in a chronic pain model.

Figure 3

EpFAs block chronic neuropathic pain in type I diabetic mice. To test the hypothesis that EpFA metabolites exert analgesic effects the metabolites were administered to diabetic neuropathic mice. Type I diabetic neuropathy was induced with streptozocin [118] and mice were assessed for neuropathic pain (allodynia) before treatment using the von Frey assay. EDP and EET regioisomeric mixtures were both dosed at 1 mg/kg administered via intraperitoneal injection (i.p.) and pain thresholds were assessed over the time course. Both EpFA mixtures significantly increased pain thresholds indicating pain relief (One Way Analysis of Variance, Holm Sidak post hoc, *p ≤ 0.007 EDPs, # p ≤ 0.003 EETs). The EDP regioisomer mixture was more effective than the EETs (Mann-Whitney Rank Sum Test, t=256.0, ‡ p ≤ 0.048) but the effects of both treatments were short lived due to their suspected rapid degradation by the soluble epoxide hydrolase enzyme. However, exogenous administration of the EpFA metabolites demonstrates their efficacy in chronic pain conditions.

Figure 4

The sEHI TPPU improves pain thresholds with enhanced efficacy in omega-3 supplemented neuropathic rats. Similar to the effects of parent PUFA and EpFA administration, sEHI mediate analgesia. A low 0.3 mg/kg dose of the sEH inhibitor TPPU (structure pictured) demonstrated increased efficacy when administered to rats fed a omega-3 DHA enriched diet compared to an omega-6 oleic control diet in the diabetic neuropathy model (Two Way Analysis of Variance, Holm Sidak post hoc, ‡p ≤ 0.001 groups, *p ≤ 0.050 time points). The DHA fed rats had higher baseline threshold before subcutaneous (s.c.) TPPU administration, nevertheless their thresholds improved over 24% of rats fed an omega-6 control diet (insert). Thus, the use of sEH inhibition with diet supplementation is potential strategy for increasing efficacy while dose limiting inhibitors for long term dosing in chronic pain conditions.

Figure 5

sEHI block the chronic pain of equine laminitis. Laminitis is severe inflammation of the hoof leading to intense pain, tissue destruction, and morbid hypertension in horses. The condition can be crippling and often fatal. A horse presenting with laminitis in both front hooves was treated with NSAIDs but suffered refractory pain (Start of Treatment). Therefore the sEHI t-TUCB 0.1 mg/kg i.v. was administered once daily for 9 days in addition to the standard of care. After this treatment the horse was both standing well (Post 9 Days of Treatment) and able to walk around the stall. Treatment of equine laminitis has continued with success and no observed adverse reactions to date.