Background

Propofol can inhibit fatty acid amidohydrolase, the enzyme responsible for the metabolism of anandamide (an endocannabinoid). To study the potential antinociceptive effect of propofol, we administered different doses (0.005, 0.05, 0.5, 5, and 500 microg) of the anesthetic in the hind paw of animals to determine an ED50. To further investigate the mechanisms by which propofol produced its antinociceptive effect, we used specific antagonists for the cannabinoid CB1 (AM251) and CB2 (AM630) receptors and measured fatty-acid amide/endocannabinoid (anandamide, 2-arachidonylglycerol, and palmitoylethanolamide) concentrations in skin paw tissues.

Methods

Formalin tests were performed on 65 Wistar rats allocated to six different groups: 1) control (Intralipidtrade mark 10%); 2) propofol (ED50 dose); 3) AM251; 4) AM251 + propofol; 5) AM630; 6) AM630 + propofol. Drugs were injected subcutaneously in the dorsal surface of the hind paw (50 microL) 15 min before 2.5% formalin injection into the same paw. Fatty-acid amide/endocannabinoid levels were measured by high performance liquid chromatography/mass spectrometry analysis.

Results

Propofol produced a dose-dependent antinociceptive effect for the early and late phases of the formalin test with an ED50 of 0.08 +/- 0.061 microg for the latter phase. This effect was antagonized by AM251 and AM630. It was locally mediated, since a higher dose of propofol given in the contralateral paw was not antinociceptive. Finally, only paw concentrations of palmitoylethanolamide were significantly increased.

Conclusion

In a test of inflammatory pain, locally injected propofol decreased pain behavior in a dose-dependent manner. This antinociceptive effect was mediated, in part, by CB1 and CB2 receptors.

BACKGROUND AND PURPOSE The endocannabinoid 2-arachidonoylglycerol (2-AG) is degraded primarily by monoacylglycerol lipase (MGL). We compared peripheral antinociceptive effects of JZL184, a novel irreversible MGL inhibitor, with the reversible MGL-preferring inhibitor URB602 and exogenous 2-AG in rats. EXPERIMENTAL APPROACH Nociception in the formalin test was assessed in groups receiving dorsal paw injections of vehicle, JZL184 (0.001-300 µg), URB602 (0.001-600 µg), 2-AG (ED(50)), 2-AG + JZL184 (at their ED(50)), 2-AG + URB602 (at their ED(50)), AM251 (80 µg), AM251 + JZL184 (10 µg), AM630 (25 µg) or AM630 + JZL184 (10 µg). Effects of MGL inhibitors on endocannabinoid accumulation and on activities of endocannabinoid-metabolizing enzymes were assessed. KEY RESULTS Intra-paw administration of JZL184, URB602 and 2-AG suppressed early and late phases of formalin pain. JZL184 and URB602 acted through a common mechanism. JZL184 (ED(50) Phase 1: 0.06 ± 0.028; Phase 2: 0.03 ± 0.011 µg) produced greater antinociception than URB602 (ED(50) Phase 1: 120 ± 51.3; Phase 2: 66 ± 23.9 µg) or 2-AG. Both MGL inhibitors produced additive antinociceptive effects when combined with 2-AG. Antinociceptive effects of JZL184, like those of URB602, were blocked by cannabinoid receptor 1 (CB(1)) and cannabinoid receptor 2 (CB(2)) antagonists. JZL184 suppressed MGL but not fatty-acid amide hydrolase or N-arachidonoyl-phosphatidylethanolamine phospholipase D activities ex vivo. URB602 increased hind paw 2-AG without altering anandamide levels. CONCLUSIONS AND IMPLICATIONS MGL inhibitors suppressed formalin-induced pain through peripheral CB(1) and CB(2) receptor mechanisms. MGL inhibition increased paw skin 2-AG accumulation to mediate these effects. MGL represents a target for the treatment of inflammatory pain.

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit fatty-acid amide hydrolase (FAAH), the enzyme responsible for the metabolism of anandamide, an endocannabinoid. It has been suggested that the mechanisms of action of NSAIDs could be due to inhibition of cyclooxygenase (COX) and also to an increase in endocannabinoid concentrations. In a previous study we have demonstrated that the local analgesic interaction between anandamide and ibuprofen (a non-specific COX inhibitor) was synergistic for the acute and inflammatory phases of the formalin test. To test this hypothesis further, we repeated similar experiments with rofecoxib (a selective COX-2 inhibitor) and also measured the local concentrations of anandamide, and of two fatty-acid amides, oleoylethanolamide and palmitoylethanolamide. We established the ED(50) for anandamide (34.52 pmol+/-17.26) and rofecoxib (381.72 pmol+/-190.86) and showed that the analgesic effect of the combination was synergistic. We also found that paw tissue levels of anandamide, oleoylethanolamide and palmitoylethanolamide were significantly higher when anandamide was combined with NSAIDs and that this effect was greater with rofecoxib. In conclusion, local injection of anandamide or rofecoxib was antinociceptive in a test of acute and inflammatory pain and the combination of anandamide with rofecoxib was synergistic. Finally, locally injected anandamide with either NSAID (ibuprofen or rofecoxib) generates higher amount of fatty-acid ethanolamides. The exact comprehension of the mechanisms involved needs further investigation.