Role of TRPA1 in the pharmacological effect triggered by the topical application of trans-anethole in mice.

PURPOSE

This study investigated the pharmacological effects of topical trans-anethole, a natural compound found in anise, star anise, and fennel essential oils, and its relationship with the transient receptor potential of ankyrin 1 (TRPA1).

METHODS

The effects of topical anethole were assessed by eye wiping, nociceptive behaviour, and ear oedema in mice. Histological evaluations were performed on the ears of the animals topically treated with anethole.

RESULTS

Anethole caused less eye irritation than capsaicin (a TRPV1 agonist) and allyl isothiocyanate (a TRPA1 agonist). Anethole (250 and 500 nmol/20 µL/paw) promoted neurogenic nociception in the paw (20.89 ± 3.53 s and 47.56 ± 8.46 s, respectively) compared with the vehicle (0.88 ± 0.38 s). HC030031 (56.1 nmol/20 µL/paw), a TRPA1 antagonist, abolished this nociceptive response. Anethole (4, 10, and 20 µmol/20 µL/ear) induced ear oedema (30.25 ± 4.78 μm, 78.00 ± 3.74 μm, and 127.50 ± 27.19 μm, respectively) compared with the vehicle (5.00 ± 0.5 μm). HC030031 (56.1 nmol/20 µL/ear) inhibited the oedema induced by anethole (10 µmol/20 µL/ear). Ears pre-treated with anethole or allyl isothiocyanate on the first day and re-exposed to these compounds on the third day showed a reduction in oedema (68.16 ± 6.04% and 38.81 ± 8.98.9%, respectively). Cross-desensitisation between anethole and allyl isothiocyanate was observed. Histological analysis confirmed the beneficial effects of anethol.

CONCLUSION

As repeated topical applications of anethole induce the desensitisation of TRPA1, we suggest its clinical application as a topical formulation for treating skin diseases or managing pain associated with this receptor. Anethole may also have advantages over capsaicin and allyl isothiocyanate because of its low pungency and pleasant aroma.