Different responsiveness of the chorda tympani and glossopharyngeal nerves to L-lysine in mice.

Differential taste responsiveness and functional role of the two taste nerves, the chorda tympani (CT) and the glossopharyngeal (GL), were studied in mice by examining neural and behavioral responses to an essential amino acid, L-lysine (Lys). Relative responses to Lys were larger in the GL than in the CT nerve. The neural threshold for the Lys response was about 2.5 log units lower in the GL (about 1.0 microM) than in the CT nerve (about 300 microM). An analysis of concentration-response relationships suggests a possibility that there are two different receptors (high and low affinity types) for Lys showing different dissociation constants. The posterior tongue region possesses both types, while the anterior region possesses only the low affinity type. Behavioral aversion threshold for Lys in intact mice, measured by use of a single bottle test, was about 1.0 microM. This threshold was the same as its neural threshold in the GL nerve. Animals whose bilateral GL nerves were sectioned showed a higher aversion threshold (about 300 microM) which was the same as the neural threshold in the CT nerve. An aversion conditioned to Lys significantly generalized to L-arginine in the intact and CT-denervated mice, and L-arginine and L-histidine in the GL-denervated mice, but the generalization pattern across various taste stimuli including the four basic taste stimuli (NaCl, HCl, quinine HCl and sucrose) did not prominently differ among the intact, the GL-denervated and CT-denervated mice. These results suggest that taste sensitivity to Lys is higher in the GL than in the CT nerve, but taste quality information for Lys conveyed by two taste nerves is not largely different.