Asymmetrical neural cross-adaptation of hamster chorda tympani responses to sodium and chloride salts

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Abstract
Cross-adaptation has occurred when exposure to an adapting chemical stimulus (A) reduces the response to a subsequent test stimulus (B). The degree of cross-adaptation between two stimuli is thought to reflect the overlap of their ''neural activation processes''. We measured self-(A .fwdarw. A) and reciprocal cross-adaptation (A .fwdarw. B, B .fwdarw. A) of the response of the hamster chorda tympani nerve with lingual presentations of stimuli eliciting equal unadapted transient responses. Adapting and test stimuli were 0.1 M NaCl, 0.1 M NaNO3, 0.1 M NaBr, 0.4 M Na acetate (NaAc), 0.09 M LiCl and 0.4 M NH4Cl. Nearly complete and symmetrical cross-adaptation was seen for NaCl, NaNO3 and NaBr. Those Na salts paired with LiCl showed strong but asymmetrical cross-adaptation. Exposure to sodium completely eliminated the response to LiCl but not vice versa, suggesting that lithium and sodium are not completely interchangeable taste stimuli for the hamster chorda tympani. Relatively little cross-adaptation between NH4Cl and other salts suggests relatively separate neural activation processes. Strongly asymmetrical cross-adaptation was found between NaAc and the other sodium salts. Responses to NaCl, NaNO3 or NaBr were eliminated after adaptation to NaAc whereas the response to NaAc during the reciprocal cross was strong. Asymmetries are discussed in reference to sensitivities of single nerve fibers for the chorda tympani, effects of adaptation and the concept of anion inhibition.