Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.     

Age and sex effects on taste of sucrose, NaCl, citric acid and caffeine

A procedure combining forced choice discrimination with intensity scaling served to evaluate taste perception of sucrose, NaCl, citric acid and caffeine in 24 young and 24 geriatric subjects. Each group was divided equally by sex. No overall sex differences occurred for taste discrimination, and suprathreshold taste intensity scaling for sucrose and NaCl did not differ by sex or age. However, young adults generally discriminated lower concentrations of citric acid and caffeine from water blanks than did geriatric subjects. Younger subjects judged suprathreshold concentrations of caffeine significantly more intense, as did young females compared to young males; similarly, young females judged citric acid as stronger than did older males. The present results suggest that age and gender are major factors in sour and bitter perception.     

Age and sex effects on taste of sucrose, NaCl, citric acid and caffeine

A procedure combining forced choice discrimination with intensity scaling served to evaluate taste perception of sucrose, NaCl, citric acid and caffeine in 24 young and 24 geriatric subjects. Each group was divided equally by sex. No overall sex differences occurred for taste discrimination, and suprathreshold taste intensity scaling for sucrose and NaCl did not differ by sex or age. However, young adults generally discriminated lower concentrations of citric acid and caffeine from water blanks than did geriatric subjects. Younger subjects judged suprathreshold concentrations of caffeine significantly more intense, as did young females compared to young males; similarly, young females judged citric acid as stronger than did older males. The present results suggest that age and gender are major factors in sour and bitter perception.     

Electrophysiological actions of quinine on voltage-dependent currents in dissociated rat taste cells

 How taste receptor cells participate in encoding disparate compounds into distinct taste qualities represents a fundamental problem in the study of gustatory transduction mechanisms. Quinine is the most common stimulus employed to represent bitterness yet its electrophysiological consequences on voltage-dependent ion channels in the taste receptor cell have not been elucidated in detail. This study examines such effects on taste receptor cells dissociated from the foliate and circumvallate papillae of the rat. Outward potassium currents, which include transient, sustained and calcium-activated components, were reversibly inhibited by bath application of quinine, with an IC₅₀ of 5.1×10^–6 M. The time course of the current traces, along with voltage shifts in normalized conductance and inactivation curves, suggests that multiple mechanisms of inhibition may be occurring. Inwardly rectifying potassium currents were unaffected. Sodium currents, to somewhat higher concentrations of quinine (IC₅₀ = 6.4×10^–5 M), were also reduced in magnitude without noticeable effects on activation or reversal potential but with a shift in inactivation. Calcium currents, visualized with barium as a charge carrier, were enhanced in magnitude by the presence of low concentrations of quinine (10^–5 M) but were suppressed by higher concentrations (10^–4 M). Quinine broadened the waveform of the gustatory action potential and increased the input resistance. These data serve as genesis to future investigations of the signal transduction mechanism of quinine on voltage-dependent currents. Received: 18 November 1996 / Received after revision: 25 February 1997 / Accepted: 5 March 1997