Spinal cord injury (SCI) is the result of an initial physical trauma followed by a secondary degenerative process. SCI leads to the destruction of ascending and descending axonal tracts that control motor, sensory and autonomic functions. The level of SCI is an important factor because the consequences are devastating. Patients suffer from permanent, often lifelong motor and sensory disabilities below the site of injury, combined with impaired basic vital functions. Functional recovery is restricted because axons in the central nervous system (CNS) regenerate poorly. Post-traumatic inflammatory reaction may play an important role in the secondary injury processes after SCI [1, 2]. In particular, the secondary damage is sustained by a large number of cellular, molecular, and biochemical cascades and a large body of data suggests the presence of a local inflammatory response, which amplifies the secondary damage .
Nuclear receptors are ligand-dependent transcription factors whose activation affects genes controlling vital processes. Among them, the peroxisome proliferator-activated receptors (PPARs) have emerged as links between lipids, metabolic diseases, and innate immunity . PPARs are activated by fatty acids and their derivatives, many of which also signal through membrane receptors, thereby creating a lipid signaling network between the cell surface and the nucleus. There are three known isoforms of PPARs – PPAR-α, PPAR-γ, and PPAR-β/δ – each with different tissue specificity and physiological function . All three isoforms share common molecular structure and functional domains similar to other nuclear receptor superfamilies. Upon ligand binding, PPARs forms a heterodimer with the retinoic acid receptor (RXR) and control the expression of genes that have PPAR response elements. PPARs exhibit complex ligand binding modes. PPAR C-terminal ligand binding domains are 60 to 70% homologous with large Y-shaped ligand binding pockets composed of three sub-arms that display significant homology between the subtypes. All three PPARs bind a variety of natural and synthetic ligands, none of which completely fills the ligand binding pocket, and PPAR ligands can adopt different binding modes. Many agonists, however, conform to a standard pharmacophoric model in which ligands comprise a hydrophilic head group that binds arm I and a hydrophobic tail that binds arm II and/or arm III . PPAR-γ, or NRIC3, plays an important role in glucose and lipid homeostasis, inflammation, and adipocyte differentiation . This transcription factor is further regulated by commonly known coactivator proteins such as CBP/p300, the SRC family, TRAP 220 , and co-repressors such as SMART, NCoR, and RIP140 . The identification of an endogenous physiological ligand for PPAR-γ has been problematic. Most of the studies determining the binding efficiency have been performed in either cell-free or cell-based systems. The specificity of compounds to act as ligands for PPAR-γ was determined by a lack of response when cells were either pretreated with a known antagonist of PPAR-γ or with constructs that lacked PPAR ligand binding domain. However, in cell-based systems it is conceivable that a metabolite of the parent compound, not the parent compound itself, might be mediating the response through interactions with PPAR-γ. PPAR-δ activation improves the overall metabolic profile . While no PPAR-δ agonists are yet approved for human use, they have been shown to enhance fatty acid oxidation in skeletal muscle, reduce serum triglycerides, increase serum high-density lipoprotein cholesterol and stimulate aspects of reverse cholesterol transport, improve glucose homeostasis, and trigger thermogenesis and weight loss .
Several studies have clearly demonstrated that PPAR-α and PPAR-γ agonists exert beneficial effects in several experimental models of CNS injury and disease, such as amyotrophic lateral sclerosis [12, 13], Parkinson’s disease [14, 15], cerebral ischemia or hemorrhage [16, 17], and experimental autoimmune encephalomyelitis [18, 19]. Recently, the presence of PPAR-α and PPAR-γ in discrete areas of the brain and spinal cord has been suggested [20, 21].
Indeed, we have recently demonstrated using PPAR-α knockout (PPAR-αKO) mice that endogenous PPAR-α activity reduces the degree of development of inflammation and tissue injury events associated with spinal cord trauma in mice, suggesting the existence of an intrinsic anti-inflammatory mechanism mediated by PPAR-α . In addition, an endogenous ligand for PPAR-γ, the cyclopentanone prostaglandin 15-deoxy-Δ12,14PGJ2, which is a metabolite of the prostaglandin D2, reduces the development of inflammation and tissue injury associated with spinal cord trauma .
In vivo administration of the saturated N-acylethanolamine palmitoylethanolamide (PEA), which explicates several effects on CNS such as modulating the pentobarbital-evoked hypnotic effect , controlling inflammatory pain , protecting against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity  and learning and memory impairment in mice , reduces inflammation and tissue injury associated with SCI  and promotes the initiation of neurotrophic substance after SCI . While the biological effects of PEA are well documented, the molecular mechanisms and site of action remain under debate. In vitro, PEA behaves as an endogenous agonist for a cannabinoid receptor (CB)2-like receptor subtype on mast cells , can inhibit gap junction formation, and modulates GABA and 5-HT receptors. In vivo, the analgesic effect of PEA is reversed by the CB2 antagonist SR144528 , while PPAR-α mediates the anti-inflammatory effects of PEA in carrageenan-induced paw edema and phorbol ester-induced ear edema, suggesting that this fatty-acid ethanolamide may serve, like its analog OEA, as an endogenous ligand for PPAR-α . There is, as yet, little direct evidence for possible interactions of PEA with PPAR-δ or PPAR-γ in the spinal cord . Our study will investigate whether the protective and anti-inflammatory effects of PEA observed in a compression model of SCI are partially mediated by other PPAR isotypes, in addition to PPAR-α. In fact, while we have recently demonstrated that PPAR-α modulates the anti-inflammatory property of PEA in a mouse model of inflammatory pain , on the contrary Benetti and colleagues have shown that CB1, PPAR-γ and TRPV1 receptors mediated the antinociception induced by PEA in mice with chronic constriction injury of the sciatic nerve .
To characterize the role of PPAR-α, PPAR-γ and PPAR-δ in PEA-mediated anti-inflammatory and neuroprotective activities, we performed spinal cord trauma in mice and studied the involvement of all of PPAR isotypes in mediating acute effects of PEA on the spinal cord.