Role of peroxisome proliferator-activated receptors-alpha (PPARɑ) and the transient receptor potential vanilloid receptor (TRPV1) in mediating the anti-nociceptive effects of palmitoylethanolamine in rat

M. Alsalem1, A. Altarifi2, A. Imraish3. 1The University of Jordan, Amman, JORDAN, 2Jordan University for Science and Technology, Irbid, JORDAN, 3The University of Nottingham, Nottingham, UNITED KINGDOM.

Introduction: N-palmitoylethanolamine (PEA) is a ligand at peroxisome proliferator-activated receptors-alpha (PPARÉ‘), a nuclear receptor which has anti-inflammatory effects (1). Herein, Complete Freund’s adjuvant (CFA)-induced inflammatory pain model in rat and in vitro calcium imaging studies were used to evaluate the mechanisms that underlie the anti-nociceptive effects of PEA.

Methods: Adult male Sprague Dawley rats (180-200g) received subcutaneous injections of CFA (0.1 ml) or saline into the plantar surface of the left hindpaw. Von Frey filaments were applied to the mid-plantar aspect of the left hind paw using the “up-down” method to determine the paw withdrawal threshold (PWT). PEA (50 µg), WY14643 (50 µg, a selective PPARα agonist), PEA (50 µg) plus GW6471 (50 µg, a selective PPARα antagonist) or WY14643 (50 µg) plus GW6471 (50 µg) were injected into the plantar surface of the left hindpaw at day 7 post-CFA injection, then the behavioral tests were repeated 6 hour post-drug administration. DRG neurones were prepared for calcium imaging (2). Neurones were loaded with Fura 2AM. Capsaicin (15nM)-evoked changes in [Ca2+]i and the effects of 6 hours pre-incubation of 30µM PEA on capsaicin-evoked calcium responses were determined. Changes in [Ca2+]i were measured as ratios of peak florescence at excitation wavelengths of 340nm and 380nm and expressed as a percentage of the KCl (60mM) response. Statistical analysis used Student’s t test or one way ANOVA test followed by Dunnett’s post-hoc as appropriate.

Results: PWTs were significantly reduced 1 day after CFA injection. This reduction persisted for about three weeks (Fig 1). Both PEA and WY14643 significantly restored PWT (3.8 ± 0.24 vs 8.4 ± 1.2, n = 6 rats, P<0.01) and (3.81 ± 0.25 vs 8.42 ± 1.18, n = 6 rats, P<0.01) respectively in a PPARα-dependent fashion (8.4 ± 1.2 vs 5.33 ± 1.19, n = 6 rats, P<0.01) and (8.42 ± 1.18 vs 5.27 ±0.47, n = 6 rats, P<0.01). 15nM capsaicin produced 63.9 ± 13.4 % of KCl response (n = 68 neurons). Pre-incubation of DRG neurones with PEA 6 hours prior to stimulation with capsaicin, significantly reduce capsaicin-evoked calcium responses (42.9 ± 6.4 % of KCl response, n=54, P<0.001.

Conclusions: Modulating the activity of the TRPV1 channel could provide the mechanism that underlies the PPARα-mediated anti-nociceptive effects of PEA.

References

1. LoVerme J et al. (2006). J Pharmacol Exp Ther 319: 1051-1061

2. Millns PJ et al. (2001). Br J Pharmacol 132: 969-971