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Neurophysiological and biophysical evidence on the mechanism of electric taste
Abstract:
The phenomenon of electric taste was investigated by recording from the chorda tympani nerve of the rat in response to both electrical and chemical stimulations of the tongue with electrolytes in order to gain some insight into its mechanism on both a neurophysiological and biophysical basis. The maximum neural response levels were identical for an individual salt (LiCl, NaCl, KCl, or CaCl2), whether it was presented as a chemical solution or as an anodal stimulus through a subthreshold solution. These observations support the idea that stimulation occurs by iontophoresis of ions to the receptors at these current densities (less than 100 microA/cm2). Electric responses through dilute HCl were smaller than the chemically applied stimulations, but the integrated anodal responses appeared similar to chemical acid responses, as evidenced by an OFF response to both forms of stimuli. Hydrogen may be more permeant to the lingual epithelium and would thus be shunted away from the taste receptors during anodal stimulation. When the anion of electric taste was varied via subthreshold salt solutions, the response magnitude increased as the mobility of the anion decreased. The transport numbers of the salts involved adequately explains these differences. The physical aspects of ion migration occurring within the adapting fluid on the tongue are also discussed. Direct neural stimulation by the current appears to occur only at higher current densities (greater than 300 microA/cm2). If the taste cells of the tongue were inactivated with either iodoacetic acid (IAA) or N-ethyl maleimide (NEM), or removed with collagenase, then responses from the chorda tympani could be obtained only at these higher current densities. Latency measurements before and after IAA or NEM treatment corroborated these findings. The results are discussed in terms of several proposed mechanisms of electric taste and it is concluded that an ion accumulation mechanism can adequately explain the data.