Effects of betahistine metabolites on frog ampullar receptors

Previous studies have demonstrated that betahistine, an histamine-like substance used widely as an anti-vertigo drug, can decrease ampullar receptor resting discharge without affecting their mechanically evoked responses. Pharmacokinetic studies have shown that this drug is transformed, mainly at the hepatic level, into aminoethylpyridine (M1), hydroxyethylpyridine (M2), then excreted with the urine as pyridylacetic acid (M3). The goal of the present study was to investigate whether betahistine metabolites are also able to affect vestibular receptor activity. Results demonstrated that, in the range tested (10(-7)-10(-2) M), M2 and M3 exerted no effect, whereas M1, at concentrations higher than 10(-6) M, was able to reduce the resting discharge of ampullar receptors without affecting the evoked responses. M1 therefore exerts effects similar to those of betahistine on ampullar receptors. This might be of some clinical interest. On the basis of our data, the hypothesis may be put forward that the anti-vertigo action of betahistine is at first achieved by betahistine itself and then sustained by M1.