Menthol Induces Surgical Anesthesia and Rapid Movement in Fishes

To determine whether fishes respond to menthol, Japanese medaka Oryzias latipes, goldfish Carassius auratus, and zebrafish Danio rerio 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. There were no differences in either percentage or latency of the response in surgical anesthesia among dl-, d-, and ltypes of menthol. A high (3.0 mM) concentration of any of the three types of menthol induced rapid movement followed by the 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 -aminobutyric acid (GABA) system in menthol-induced surgical anesthesia, the effect of the receptors antagonist for the GABAA was tested. Pretreatment with a specific GABAA receptor antagonist prolonged the latency of the anesthetic response to menthol, but not to cold-water stimulation, in medaka and goldfish. These results demonstrate that menthol can play a role in the induction of surgical anesthesia in fishes, related at least in part to the activation of GABAA receptors, and of rapid movement possibly via cold nociceptors.