Recent advances in the physiology of taste cells

Abstract

1. 1. The mean resting membrane potentials of taste cells in various species of vertebrates are −20 to −40 mV, the values being greatly dependent on the ionic environments to which the receptor membrane of a taste cell is adapted. 2. 2. Application of chemical stimulants of more than a threshold concentration to a taste cell, in general, produces a depolarizing receptor potential, but application of those of less than the threshold concentration produces no response or a hyperpolarizing receptor potential. On the other hand, some taste cells depolarize in response to lower stimulus concentrations and hyperpolarize in response to higher stimulus ones. Waveforms of taste cell responses are not always a sustained depolarization or hyperpolarization during stimulation, but are occasionally more complex, such as a depolarization preceded by a hyperpolarization or a depolarization followed an off-depolarization. Recently, an initial transient depolarization followed by a usual steady depolarization has been found in frog taste cells. 3. 3. Reversal potential for a depolarizing receptor potential elicited by NaCl and CaCl 2 is directed to more positive levels than the reference zero potential. Also, the reversal potential for a depolarizing response by sucrose is about −15 mV in rat taste cells. The reversal points for depolarizations induced by HCl differ between taste cells in rats and frogs, i.e. positive in rat and more negative than the resting potential in frog. The reversal point for Q-HCl-induced depolarizations is situated at a more negative level than the resting potential. 4. 4. The membrane conductance in taste cells increases during depolarizations of sufficient magnitude generated by various taste stimuli such as salts, acids and sugars, but it decreases during a depolarization by Q-HCl. The amount of an increase in conductance is most remarkable when monovalent salts depolarize the taste cell. 5. 5. Concerning mechanisms of the generation of receptor potential in taste cells, three main theories have been proposed as follows: (1) due to an increase in ionic permeability of taste cell membrane after adsorption of a taste stimulus onto the taste receptor membrane; (2) due to an increase in ionic permeability of taste receptor membrane during stimulation with monovalent salts; and (3) due to a change in a phase boundary potential appearing at the taste stimulus solution-taste receptor membrane interphase. Experimental evidence does not yet tell us which theory is most appropriate to account for the taste cell response elicited by a particular kind of stimulus. However, it may be safe to say that depolarizations evoked by salt stimuli result from a change in permeabilities of the taste receptor membrane and/or the cell membrane to Na +, K +, Ca 2+ ions, etc., since a remarkable increase in the membrane conductance and a positive reversal potential are observed and since the response magnitude depends on the amounts of Na + and Ca 2+ ions in the intercellular space. 6. 6. Most of the taste cells respond to more than one of the four basic taste stimuli with different magnitudes of depolarizations, indicating multiple sensitivity of individual taste cells. Statistically, it is concluded that receptor sites, situated on the taste receptor membrane, for the four basic stimuli are distributed randomly among the taste cells. 7. 7. After prolonged adaptation of taste cells to local anesthetics such as procaine and lidocaine, most of the taste cell responses induced by various kinds of taste stimuli are greatly reduced in amplitude. Prolonged treatment of taste cells with bitter substances such as Q-HCl, Q-H 2SO 4 and picric acid also cause the reduction of the taste cell responses generated by various taste stimuli. In contrast, a short period of adaptation to the bitter substances enhances an initial component of the taste cell responses to salts, acids and sugars.