Taste receptor cells respond to gustatory stimuli using a complex arrangement of receptor molecules, signaling cascades, and ion channels. When stimulated, these cells produce action potentials that result in the release of neurotransmitter onto an afferent nerve fiber that in turn relays the identity and intensity of the gustatory stimuli to the brain. A variety of mechanisms are used in transducing the four primary tastes. Direct interaction of the stimuli with ion channels appears to be of particular importance in transducing stimuli reported as salty or sour, whereas the second messenger systems cyclic AMP and inositol trisphosphate are important in transducing bitter and sweet stimuli. In addition to the four basic tastes, specific mechanisms exist for the amino acid glutamate, which is sometimes termed the fifth primary taste, and for fatty acids, a so-called nonconventional taste stimulus. The emerging picture is that not only do individual taste qualities use more than one mechanism, but multiple pathways are available for individual tastants as well.

Cellular mechanisms of taste transduction

The activity of taste cells maintained in the intact hamster tongue was monitored in response to acid stimulation by recording action currents from taste receptor cells with an extracellular "macro" patch pipette: a glass pipette was pressed over the taste pore of fungiform papillae and perfused with citric acid, hydrochloric acid, or NaCl. Because this technique restricted stimulus application to the small surface area of the apical membranes of the taste cells, many nonspecific, and potentially detrimental, effects of acid stimulation could be avoided. Acid stimulation reliably elicited fast transient currents (action currents of average amplitude, 9 pA) which were consistently smaller than those elicited by NaCl (29 pA). The frequency of action currents elicited by acid stimuli increased in a dose-dependent manner with decreasing pH from a threshold of about pH 5.0. Acid-elicited responses were independent of K+, Na+, Cl-, or Ca2+ at physiological (salivary) concentrations, and were unaffected by anthracene-9-carboxylic acid, tetraethylammonium bromide, diisothiocyanate-stilbene-2,2'-disulfonic acid, vanadate, or Cd2+. In contrast, amiloride (< or = 30 microM) fully and reversibly suppressed acid-evoked action currents. At submaximal amiloride concentrations, the frequency and amplitude of the action currents were reduced, indicating a reduction of the taste cell apical conductance concomitant with a decrease in cell excitation. Exposure to low pH elicited, in addition to transient currents, an amiloride-sensitive sustained d.c. current. This current is apparently carried by protons instead of Na+ through amiloride-sensitive channels. When citric acid was applied while the taste bud was stimulated by NaCl, the action currents became smaller and the response resembled that produced by acid alone. Because of the strong interdependence of the acid and salt (NaCl) responses when both stimuli are applied simultaneously, and because of the similarity in the concentration dependence of amiloride block, we conclude that amiloride-sensitive Na+ channels on hamster taste receptor cells are permeable to protons and may play a role in acid (sour) taste.

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Proton currents through amiloride-sensitive Na channels in hamster taste cells. Role in acid transduction.

https://www.cell.com/neuron/pdf/0896-6273(93)90208-9.pdf

Proton currents through amiloride-sensitive Na+ channels in isolated hamster taste cells: Enhancement by vasopressin and CAMP

The breadth of responsiveness of rat taste cells to the four basic taste stimuli was studied using the entropy measure (H) proposed by Smith and Travers. H values range from 0.0 for narrow tuning to 1.0 for broad tuning. Based on the responses of depolarizing receptor potentials of 26 rat taste cells to the four basic taste stimuli, taste cells were classified into nine NaCl-best, four Q-HCl (quinine-HCl)-best, 10 HCl-best and three sucrose-best cells. NaCl-best cells were narrowly tuned to the four basic taste stimuli, but the other three stimuli-best cells were broadly tuned to the stimuli. In all, 85% of the taste cells responded to more than one of four basic taste stimuli. The mean H values for NaCl-best, Q-HCl-best, HCl-best and sucrose-best cells were 0.285, 0.832, 0.781 and 0.796 respectively. The mean H value for all 26 taste cells was 0.621. This was larger than H in rat gustatory fibers. Transformation of large H values in taste cells into small H values in taste fibers may be due to a non-random interaction between taste cells and taste fibers during the synaptic formation. Broad tuning properties of rat taste cells suggest that the across-taste cell response pattern may play an important role in taste quality coding mechanisms.

/pdf/broad-tuning-of-rat-taste-cells-for-four-basic-taste-stimuli-3pq9zul3yj.pdf

Sato Toshihide

Papers

Vasopressin increases frog gustatory neural responses elicited by NaCl and HCl.

Topographical difference in taste organ density and its sensitivity of frog tongue

Dye-coupling among frog (Rana catesbeiana) taste disk cells

Aldosterone increases gustatory neural response to NaCl in frog.

The adaptation of the frog tongue to bitter solutions: the effect on gustatory neural responses to acid, sugar and bitter stimuli