GABAA-receptors-mediated anesthetic effects by menthol in fishes

Masanori Kasai, Ryotaro Okayama, Hirotoshi Kawano, Akiteru Tokunaga

Dept. of Chem.&BioSci., Faculty of Sci., Kagoshima Univ., Kagoshima, Japan

Menthol that is widely used in food and pharmaceutical industry is well known to be able to induced cold and pain sensations in subjects. To determine whether fishes respond to menthol, Japanese medaka, goldfish and zebrafish were exposed to various types of menthol receptors agonists and the behavioral responses to these drugs were observed. Waterbone application of dl-menthol (0.5 mM) induced surgical anesthesia in 100% of medaka, 90% of goldfish, and 100% of zebrafish. The percentage of response increased dose-dependently from 0.2 mM to 0.5 mM. Sedation (motion loss) was observed more than 0.2 mM in goldfish by both d-, and l- types of menthol in goldfish. There were no differences in either percentage or the response in the anesthesia among dl-, d-, and l- types of menthol. A high (3.0 mM) concentration of any of the three types of menthol induced rapid movement followed by the surgical anesthetic response. Rapid movement was observed with allyl isothiocyanate, a cold nociceptor agonist, but not with icilin, a cold receptor agonist, in medaka and goldfish. Both allyl isothiocyanate and icilin failed to induce surgical anesthesia. To determine the involvement of γ-aminobutyric acid (GABA) system in menthol-induced anesthesia, the effect of the receptors antagonist was tested. Pretreatment with a specific GABAA receptor antagonist prolonged the latency of the surgical anesthetic responses and completely attenuated the sedation to menthol in goldfish. These results demonstrate that menthol can play a role in the induction of anesthesia in fishes, related at least in part to the activation of GABAA receptors, and of rapid movement possibly via cold nociceptors.