Background and aims: Menthol is considered a cold-sensitive thermoTRP channel TRPM8 agonist and it induces a nociceptive behavior when applied to the lips of animals. Here we investigated if the orofacial antinociception promoted by menthol would be related to TRPA1 or TRPV1 antagonism.
Methods: Acute nociception was induced by menthol (1.2 µM), cinnamaldehyde (0.33 µM) or capsaicin (4.93 µM) applied into in the upper lip (5.0 µL) of adult wild zebrafish. Then, animals (n=6) were individually placed in a glass Petri dish, divided into quadrants, and the nociceptive response was registered after menthol (0 - 10'), cinnamaldehyde (0 - 5') or capsaicin (10 - 20') application. Zebrafish were pretreated by intraperitoneal injection (20 µL) with vehicle (Control;) or menthol (0.1; 0.3 or 1.0 mg/mL) 30 min before induction. The effect of menthol on zebrafish locomotor behavior was evaluated with the open field test (0 - 5´). Naive groups (n=6) were included in all tests. The experimental protocols followed the ethical guidelines of CONCEA and were approved by the UECE Animal Research Ethics Committee (#7210149/2016).
Results: Pre-treatment with menthol was associated with a reduction in nociceptive behavior induced by menthol (0.1 mg/mL; *p< 0.05 vs Control) and cinnamaldehyde (0.1 and 0.3 mg/mL; ****p< 0.001 vs Control), but not by capsaicin. Menthol (0.1 mg/mL) reduced the locomotor activity of zebrafish (*p< 0.05 vs Control).
Conclusion: The results indicate the potential clinical application of menthol as inhibitor of orofacial nociception and that this effect may be due to the modulation of TRPA1 channel.

MENTHOL, A TRPM8 AGONIST, PROMOTES OROFACIAL ANTINOCICEPTION IN ADULT ZEBRAFISH THROUGH TRPA1 ANTAGONISM