Inhibition of hamster chorda tympani neural response by copper chloride

Abstract

Copper chloride was evaluated as a specific inhibitor of neural responses to sweet taste stimuli in the goldern hamster (Mesocricetus auratus). The chorda tympani whole-nerve response to taste stimuli was recorded before and after the tongue was treated for 30 s with 0.01, 0.1 and 1 mM CuCl2. Sweet stimuli [sucrose, fructose, saccharin (calcium salt), D-phenylalanine], which primarily stimulate chorda tympani S fibers, and non-sweet stimuli (NaCl, NH4Cl) were used. At 0.01 mM, copper chloride had little effect. At 0.10 mM it partially inhibited responses to sucrose and saccharin, but had little effect on responses to D-Phe, fructose, NaCl, NH4Cl, or a mixture of sucrose plus L-Phe. L-Phe, which has the same chelating properties as D-Phe, is not an S-fiber stimulus and likely reduced sucrose inhibiton by chelating the cupricion. Analysis of concentration–response functions revealed that 0.1 mM copper chloride inhibited the neural response to low concentrations of sucrose by about 25%, but did not significantly inhibit high concentrations of surcrose, suggesting competitive inhibition. In contrast, 0.1 mM CuCl2 reduced saccharin responses by 25% throughtout the effective range, suggesting non-competitive inhibition. Occupation of a saccharide receptor site by copper may interfere with dimer but not monomer reception and distort the saccharin receptor site. At 1 mM, CuCl2 non-competitively inhibited responses to sucrose, fructose, saccharin and the non-sweet NaCl (an N-fiber stimulus), but not NH4Cl (an H-fiber stimulus). The mechanisms of copper chloride inhibition are difficult to establish because its effects are weak at concentrations where they are specific.