Ontogeny of chorda tympani nerve responses to gustatory stimuli in the rat

Electrical activity (whole-nerve responses) of the chorda tympani nerve (CT) in rats at 2-110 days of age was recorded in response to chemical (0.5 M NaCl, 0.5 M LiCl, 0.5 M NH4Cl, 0.1 M citric acid) stimuli applied to the dorsal surface of the tongue. The CT of rats as young as two days of age was differentially responsive to the various chemical stimuli, and the responsiveness of the CT to 0.5 M NaCl and 0.5 M LiCl relative to 0.5 M NH4Cl increased between 20 and 34 days of age and again between 35 and 90 days of age. The responsiveness of the CT to 0.1 M citric acid relative to 0.5 M NH4Cl increased from 6--19 days of age, then decreased thereafter to adulthood. A concentration series of NH4Cl solutions (0.1 M, 0.25 M, 0.5 M and 1.0 M) revealed that the CT of rats 6--110 days of age displayed increased response magnitudes as the concentration of NH4Cl increased. The CT was responsive to mechanical and thermal stimulation of the tongue, and this responsiveness was essentially unchanged throughout development. These results show that the CT of rats as young as two days of age is differentially responsive to chemical stimuli applied to the tongue, and that CT responsiveness to chemical stimuli changes during development.