Abstract:  

Palmitoylethanolamine (PEA) is a ligand at peroxisome proliferator-activated receptors-? (PPAR?), a nuclear receptor that has anti-inflammatory effects. Herein, complete Freund?s adjuvant (CFA)-induced inflammatory pain model in rats and in-vitro calcium imaging studies were used to evaluate the mechanisms that underlie the antinociceptive effects of PEA, including modulating the activity of the transient receptor potential vanilloid receptor 1, which is a key receptor involved in the development of inflammatory pain. Adult male Sprague-Dawley rats (180?250?g) received subcutaneous injections of CFA (0.1?ml) into the plantar surface of the left hind paw. Von Frey filaments were used to determine the paw withdrawal threshold. PEA (50??g), WY14643 (50??g, a selective PPAR? agonist) were injected into the plantar surface of the left hind paw at day 7 after CFA injection, and behavioral tests were repeated 6?h after drug administration. Rats were killed and dorsal root ganglia neurons were dissected and prepared for calcium imaging. Neurons were loaded with the calcium-sensitive ratiometric dye Fura-2AM. Changes in [Ca2+]i were measured as ratios of peak florescence at excitation wavelengths of 340 and 380?nm and expressed as a percentage of the KCl (60?mM) response. Both PEA and WY14643 significantly restored the paw withdrawal threshold in a PPAR?-dependent fashion (P<0.01). Capsaicin of 15?nM produced 63.9?13.4% of KCl response. Preincubation of dorsal root ganglia neurons with PEA 6?h before stimulation with capsaicin, significantly reduce capsaicin-evoked calcium responses (42.9?6.4% of KCl response, n=54, P<0.001). In conclusion, modulating transient receptor potential vanilloid receptor 1 activity could provide the mechanism that underlies PEA antinociceptive effects observed in vivo.