Possible involvement of undissociated acid molecules in the acid response of the chorda tympani nerve of the rat

To test whether undissociated acid is capable of exciting the chorda tympan i nerves in rats, we have used buffered acid solutions as taste stimuli. Th ese solutions were prepared by adding alkali to weak acids, such as acetic acid, so that the proportion of undissociated and dissociated acids was var ied whereas keeping the total acid concentration constant. When acetic acid solutions, adjusted to wide ranges of pH by NaOH, were applied to the tong ue, the response magnitude of the chorda tympani nerves was not varied syst ematically with pH changes. However, if the sodium effect was eliminated by amiloride or replacement of cation by potassium or Tris[hydroxymethyl]amin omethane; NH2C(CH2OH)(3) (Tris-base), the chorda tympani response was reduc ed systematically as pH increased. Similar results were obtained with citri c acid and ascorbic acid. This pH-dependent change in taste nerve response to acid cannot be solely attributed to the proton gradient because the resp onse magnitude induced by hydrogen itself, which was estimated from respons es to strong acids, was much smaller than that by equi-pH acetic acid (simi lar to 85%). Thus we cannot explain the pH-dependent responses of the chord a tympani nerves to weak acids unless effects of undissociated acid molecul es are postulated. It is therefore concluded that undissociated acids in we ak acid solutions can be a stimulant to taste receptor cells.