INHIBITION OF HAMSTER CHORDA TYMPANI NEURAL RESPONSE BY COPPER CHLORIDE

Copper chloride was evaluated as a specific inhibitor of neural respon ses to sweet taste stimuli in the golden hamster (Mesocricetus auratus ). The chorda tympani whole-nerve response to taste stimuli was record ed before and after the tongue was treated 30 S with 0.01, 0.1 and 1 m M CuCl2. Sweet stimuli [sucrose, fructose, saccharin (calcium salt), D -phenylalanine], which primarily stimulate chorda tympani S fibers, an d non-sweet stimuli (NaCl, NH4Cl) were used. At 0.01 mM, copper chlori de had little effect. At 0.10 mM it partially inhibited responses to s ucrose and saccharin, but had little effect on responses to D-Phe, fru ctose, NaCl, NH4Cl, or a mixture of sucrose plus L-Phe. L-Phe, which h as the same chelating properties as D-Phe, is not an S-fiber stimulus and likely reduced sucrose inhibiton by chelating the cupric ion. Anal ysis of concentration - response functions revealed that 0.1 mM copper chloride inhibited the neural response to low concentrations of sucro se by about 25%, but did not significantly inhibit high concentrations of sucrose, suggesting competitive inhibition. In contrast, 0.1 mM Cu Cl2 reduced saccharin responses by 25% throughout the effective range, suggesting non-competitive inhibition. Occupation of a saccharide rec eptor site by copper may interfere with dimer but not monomer receptio n and distort the saccharin receptor site. At 1 mM, CuCl2 non-competit ively 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 establi sh because its effects are weak at concentrations where they are speci fic.