Inhibitory effects of some heavy metal ions on taste nerve responses in mice

Effects of treating the tongue with 7 heavy metal salts on chorda tympani nerve responses to sweet, salty, sour, and bitter stimuli were studied in ddy mice. CuCl2 and ZnCl2 at 10(-5) M inhibited responses to sucrose and Na saccharin without affecting responses to the other taste stimuli. Inhibition of responses to sweet stimuli was reversible, but recovery was very slow. No such selective inhibition of sweet responses was observed with FeSO4, MnCl2, CoCl2, NiCl2 or CdCl2. FeSO4 (1 mM) had a very small effect on all responses. CdCl2 (1 mM) depressed responses to all the stimuli to some extent. MnCl2, CoCl2, and NiCl2 (1 mM) slightly inhibited responses to NaCl and quinine hydrochloride, but not HCl. Responses to sweet stimuli were slightly inhibited by 1 mM MnCl2 or CoCl2. Kinetic analyses of the inhibition of sucrose responses by CuCl2 indicate that it is a competitive inhibition. Based on the results it is hypothesized that Cu2+ and possibly Zn2+ also compete with sweet-tasting molecules for receptor proteins while other heavy metal ions do not possess a strong ability to bind to the proteins.