The transduction channel TRPM5 is gated by intracellular calcium in taste cells

Thermal taste, PROP responsiveness, and perception of oral sensations

Differences between 6-n-propylthiouracil (PROP) taster groups have long been the focus of studies on individual variation in perception of oral sensation. Recently, "thermal taste" was described, the phenomenon whereby some individuals perceive "phantom" taste sensations after thermal stimulation of small areas of the tongue. As with PROP taster status (PTS), thermal taster status (TTS) has been proposed as a proxy for general responsiveness to oral stimuli. Here we examined the influence of PTS and TTS, independently, on the perceived intensity of sweet, sour, salty, bitter, astringent, and metallic stimuli, and temperature on heating or cooling the tongue. Interactions between PTS and TTS were also examined, and fungiform papillae (FP) density and salivary flow rate (SFR) were determined. Both PTS and TTS were associated with perceived stimulus intensities. PROP super-tasters (pSTs) rated all oral stimuli as more intense than PROP non-tasters (pNTs). Thermal tasters (TTs) gave higher logged ratings than thermal non-tasters (TnTs) for all oral sensations including temperature, with the exception of metallic flavour (at low concentration) and PROP. Examination of ETA-squared values showed that PTS had a greater effect on perceived intensities than did TTS for most sensations. No PTSTTS interaction was found for any oral stimuli. In contrast with PTS, TTS was not associated with FP density, and neither PTS nor TTS were associated with SFR. We conclude that pSTs and TTs possess greater responsiveness across a range of taste and trigeminal stimuli and concentrations.

The taste of sugars

Sugars evoke a distinctive perceptual quality ("sweetness" in humans) and are generally highly preferred. The neural basis for these phenomena is reviewed for rodents, in which detailed electrophysiological measurements have been made. A receptor has been identified that binds sweeteners and activates G-protein-mediated signaling in taste receptor cells, which leads to changes in neural firing rates in the brain, where perceptions of taste quality, intensity, and palatability are generated. Most cells in gustatory nuclei are broadly tuned, so quality perception presumably arises from patterns of activity across neural populations. However, some manipulations affect only the most sugar-oriented cells, making it useful to consider them as a distinct neural subtype. Quality perception may also arise partly due to temporal patterns of activity to sugars, especially within sugar-oriented cells that give large but delayed responses. Non-specific gustatory neurons that are excited by both sugars and unpalatable stimuli project to ventral forebrain areas, where neural responses provide a closer match with behavioral preferences. This transition likely involves opposing excitatory and inhibitory influences by different subgroups of gustatory cells. Sweeteners are generally preferred over water, but the strength of this preference can vary across time or between individuals, and higher preferences for sugars are often associated with larger taste-evoked responses.

Intestinal glucose sensing and regulation of intestinal glucose absorption

SGLT1 (Na(+)/glucose co-transporter 1) transports the dietary sugars, D-glucose and D-galactose, from the lumen of the intestine into enterocytes. SGLT1 regulation has important consequences for the provision of glucose to the respiring tissues and is therefore essential for maintaining glucose homoeostasis. SGLT1 expression is directly regulated in response to changes in the sugar content of the diet. To monitor these variations, there is a requirement for a glucose-sensing system located on the luminal membrane of gut cells. This short review focuses on recent findings on intestinal sugar sensing and the downstream mechanisms responsible for enhancement in SGLT1 expression.

The taste transduction channel TRPM5 is a locus for bitter-sweet taste interactions

Ordinary gustatory experiences, which are usually evoked by taste mixtures, are determined by multiple interactions between different taste stimuli. The most studied model for these gustatory interactions is the suppression of the responses to sweeteners by the prototype bitter compound quinine. Here we report that TRPM5, a cation channel involved in sweet taste transduction, is inhibited by quinine (EC(50)=50 microM at -50 mV) owing to a decrease in the maximal whole-cell TRPM5 conductance and an acceleration of channel closure. Notably, quinine inhibits the gustatory responses of sweet-sensitive gustatory nerves in wild-type (EC(50)= approximately 1.6 mM) but not in Trpm5 knockout mice. Quinine induces a dose- and time-dependent inhibition of TRPM5-dependent responses of single sweet-sensitive fibers to sucrose, according to the restricted diffusion of the drug into the taste tissue. Quinidine, the stereoisomer of quinine, has similar effects on TRPM5 currents and on sweet-induced gustatory responses. In contrast, the chemically unrelated bitter compound denatonium benzoate has an approximately 100-fold weaker effect on TRPM5 currents and, accordingly, at 10 mM it does not alter gustatory responses to sucrose. The inhibition of TRPM5 by bitter compounds constitutes the molecular basis of a novel mechanism of taste interactions, whereby the bitter tastant inhibits directly the sweet transduction pathway.

Abnormal taste perception in mice lacking the type 3 inositol 1,4,5-trisphosphate receptor

Inositol 1,4,5-trisphosphate receptor (IP3R) is one of the important calcium channels expressed in the endoplasmic reticulum and has been shown to play crucial roles in various physiological phenomena. Type 3 IP3R is expressed in taste cells, but the physiological relevance of this receptor in taste perception in vivo is still unknown. Here, we show that mice lacking IP3R3 show abnormal behavioral and electrophysiological responses to sweet, umami, and bitter substances that trigger G-protein-coupled receptor activation. In contrast, responses to salty and acid tastes are largely normal in the mutant mice. We conclude that IP3R3 is a principal mediator of sweet, bitter, and umami taste perception and would be a missing molecule linking phospholipase C beta2 to TRPM5 activation.