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J Physiol. Jun 15, 2008; 586(Pt 12): 2903–2912.
Published online Apr 17, 2008. doi:  10.1113/jphysiol.2008.151233
PMCID: PMC2517205

Presynaptic (Type III) cells in mouse taste buds sense sour (acid) taste


Taste buds contain two types of cells that directly participate in taste transduction – receptor (Type II) cells and presynaptic (Type III) cells. Receptor cells respond to sweet, bitter and umami taste stimulation but until recently the identity of cells that respond directly to sour (acid) tastants has only been inferred from recordings in situ, from behavioural studies, and from immunostaining for putative sour transduction molecules. Using calcium imaging on single isolated taste cells and with biosensor cells to identify neurotransmitter release, we show that presynaptic (Type III) cells specifically respond to acid taste stimulation and release serotonin. By recording responses in cells isolated from taste buds and in taste cells in lingual slices to acetic acid titrated to different acid levels (pH), we also show that the active stimulus for acid taste is the membrane-permeant, uncharged acetic acid moiety (CH3COOH), not free protons (H+). That observation is consistent with the proximate stimulus for acid taste being intracellular acidification, not extracellular protons per se. These findings may also have implications for other sensory receptors that respond to acids, such as nociceptors.

Sour is a basic taste quality alongside sweet, bitter, salty and umami. Sourness is elicited by acids and is generally an aversive taste for humans and non-human animals. Presumably, sourness functions to signal rancidity in potential sources of food as well as to protect against acid/base unbalances that might follow consumption of excessive acid. Several molecular transduction mechanisms for sour taste have been put forth over the decades, including ASIC channels, HCN channels, and a matrix of KCNK channels in taste bud cells (reviewed by Roper, 2007). Recently, the TRP-like channels PKD2L1 and PKD1L3 have also been proposed as sour taste transducers. Taste cells express PKD2L1 and PKD1L3 (LopezJimenez et al. 2006) and when these channels are expressed in heterologous cells, they confer acid sensitivity (Ishimaru et al. 2006). Further, mutant mice lacking the taste cells that express PKD2L1 channels do not sense acid taste (Huang et al. 2006). PKD2L1 appears to be expressed selectively in one class of taste bud cells, Type III cells (Kataoka et al. 2008). Despite these findings, a definitive explanation for sour taste is lacking and there is no consensus yet regarding its underlying mechanism(s). In part, this is due to the lack of definitive information about which cells in the taste bud are the actual sour receptor cells and what the proximate stimulus for acid taste is.

Taste buds consist of several different categories of cells, generally classified as Types I, II, III and IV. Functional characterization of taste cells indicates that one of these categories, Type II, represents receptor cells (Clapp et al. 2004; DeFazio et al. 2006). These cells express G protein coupled taste receptors (T1Rs, T2Rs) and their downstream effectors, phospholipase C subtype β2 (PLC β2) and IP3 receptor subtype 3. Consequently, receptor cells are directly stimulated by sweet, bitter and umami taste compounds (Tomchik et al. 2007). In response to taste stimulation, receptor cells secrete ATP, a taste neurotransmitter, via an unconventional mechanism – gap junction hemichannels composed of the pannexin 1 gap junction protein (Huang et al. 2007; see Romanov et al. 2007). Receptor (Type II) cells, however, may not directly participate in sour taste. Other taste cells, possibly Type III cells, appear to be acid sensitive, although this has only been tested to date in situ (Richter et al. 2003; Tomchik et al. 2007). Type III cells form morphologically identifiable synapses with postsynaptic structures and have been shown to express molecules associated with vesicular exocytosis (Yee et al. 2001; DeFazio et al. 2006). Type III cells also express a candidate acid transducer channel, PKD2L1 (Kataoka et al. 2008). These cells have been termed presynaptic cells to underscore the observation that they possess synapses and express synapse-related proteins (DeFazio et al. 2006). When stimulated, presynaptic (Type III) cells release serotonin (5-HT) in a calcium-dependent manner, consistent with vesicular exocytosis at synapses (Huang et al. 2005, 2007).

The present work was undertaken (a) to explore which taste cell(s) are ultimately responsible for acid sensitivity, (b) to investigate how the cells respond to acid stimulation, and (c) to identify the neurotransmitter(s) involved in sour taste. By using single, isolated taste cells free of any indirect excitation that might occur in situ, the present findings confidently establish that only presynaptic (Type III) cells respond to acid stimulation with Ca2+ influx and transmitter (5-HT) secretion. Curiously, other taste cells, including receptor (Type II) also are affected by the presence of acids but in a fundamentally different manner from presynaptic cells, not involving Ca2+ influx and not associated with transmitter secretion.


Ethical approval

Mice were killed following National Institutes of Health guidelines, as approved by the University of Miami Animal Care and Use Committee. All experiments were conducted following the guidelines of these two regulatory bodies.


Adult C57BL/6J mice of both sexes were used in this study (n = 59). Mice were killed by exposure to 100% CO2 until they were unconscious, and remained in the chamber until clinical death was assured (~1–2 additional minutes). This procedure minimizes distress (NIH Office of Animal Care and Use, http://oacu.od.nih.gov/ARAC/EuthCO2.pdf). Cervical dislocation followed CO2 exposure and tongues were then removed for further dissection (next).

Isolated taste cells

We removed the lingual epithelium containing taste papillae from the tongue by injecting 1 mg ml−1 collagenase A (Roche), 2.5 mg ml−1 dispase II (Roche), and 1 mg ml−1 trypsin inhibitor (Sigma) directly under the epithelium surrounding taste papillae. The peeled epithelium was bathed in Ca2+-free Tyrode solution for 30 min at room temperature and isolated taste cells were drawn into fire-polished glass micropipettes with gentle suction. Taste cells were transferred to a shallow recording chamber having a glass coverslip base. The coverslip base was coated with Cell-Tak (BD Biosciences) to hold taste cells firmly attached. Taste cells were superfused with Tyrode solution (in mm: 140 NaCl, 5 KCl, 2 CaCl2, 1 MgCl2, 10 Hepes, 10 glucose, 10 sodium pyruvate, 5 NaHCO3, pH 7.4, 310–320 mosmol l−1). For nominally Ca2+-free Tyrode solution, MgCl2 was substituted for CaCl2 (in mm: 140 NaCl, 5 KCl, 3 MgCl2, 10 Hepes, 10 glucose, 10 sodium pyruvate, 5 NaHCO3, pH 7.4, 310–320 mosmol l−1).

Lingual slice preparation

We prepared lingual slices containing the vallate papilla and loaded taste cells with a calcium indicator dye as previously described (Caicedo et al. 2000, 2002; Tomchik et al. 2007). Briefly, Calcium Green-1 dextran (CaGD; 1 mm in H2O, molecular weight 3000 kDa; Invitrogen, Carlsbad, CA, USA) was injected iontophoretically through a fire-polished glass micropipette into the crypt surrounding the vallate papilla. Sections of 100 μm of the dye-loaded tissue were prepared with a vibrating microtome (VT1000S; Leica, Nussloch, Germany) and mounted in a recording chamber. Lingual sections were superfused with Tyrode solution (30°C) at a rate of 1 ml min−1. ‘Puffer’ pipettes (2 μm tip diameter) were used to deliver taste stimuli directly to the taste pores of taste buds in the lingual slice. Stimuli were ejected for 2 s with air pressure (1–5 p.s.i.) (Picospritzer; Medical Systems, Greenvale, NY, USA). Bathing solutions were as described above.

Ca2+ imaging

For isolated, Fura-2-loaded taste cells, sequential images were recorded at 40× with a band pass emission filter (510 ± 80 nm) when excited at 340 nm followed by 380 nm (e.g. Huang et al. 2007). Images were processed with Indec Workbench v5 software. Data shown are the ratios, F340/F380, indicating relative changes in [Ca2+]i. For lingual slices containing dye-loaded taste cells, taste buds were viewed with a scanning laser confocal microscope using argon and krypton lasers (Fluoview; Olympus Optical, Thornwood, NY, USA). We measured fluorometric signals as relative fluorescence change, ΔF/Fo, and corrected for photobleaching when necessary (Caicedo et al. 2000; Tomchik et al. 2007).


Isolated taste cells were stimulated by bath-perfusion of KCl (50 mm substituted equimolar for NaCl), taste mix (10 μm cycloheximide, 2 mm saccharin, 0.1 mm SC45647, 1 mm denatonium), sodium acetate (20 mm), or acetic acid (10 mm). All stimuli were made up in Tyrode solution and applied at pH 7.2 except for acetic acid. A series of acetic acid solutions with pH 5.0–7.0 was prepared by titrating 10 mm acetic acid with 1 n NaOH. HCl taste stimulus solutions of pH 1.5, 3 and 5 were prepared by titrating Tyrode buffer with 1 n HCl. In all cases, Na+ concentration in the external bath was kept constant between stimulus solutions by making appropriate ion substitutions (e.g. 20 mm sodium acetate substituted for 20 mm NaCl).

In lingual slices, acetic acid taste stimuli were delivered with focal pipettes positioned adjacent to the taste pore of taste buds. Acetic acid was titrated with NaOH or HCl to achieve specific concentrations of the protonated uncharged moiety (CH3COOH, hereafter, HOAc), according to the Henderson–Hasselbalch equation (using pKa of acetic acid of 4.76). For example, at pH 5.0, 37% of the aqueous solution of acetic acid consists of HOAc, and thus a solution of 10 mm acetic acid, titrated with NaOH to pH 5 will contain 3.7 mm HOAc (e.g. see Fig. 5A). To vary HOAc but keep pH constant, solutions of increasing initial acetic acid concentrations were titrated to pH 5.0. For example, 173 mm acetic acid titrated to pH 5.0 contains 64 mm HOAc (i.e. 37% of 173 mm, e.g. see Fig. 5B). NaCl was added to the acetic acid taste stimuli to yield equi-osmolar solutions (316 mosmol l−1). Lucifer yellow, 500 μm, was added to the stimulus solutions as a tracer to determine the distribution and dilution of stimulus at the taste pore (Richter et al. 2003). No further attempt was made to buffer the acetic acid taste stimuli. Acetic acid taste solutions were applied with a constant air pressure (‘puffer pipette’) for sufficient duration (2 s) to bathe the underlying taste pore entirely and uniformly with the stimulus concentration contained in the pipette.

Figure 5
The amplitude of acid-evoked Ca2+ responses varies with [HOAc] but not with [H+] (i.e. pH)

Biosensor cells

CHO cells coexpressing 5-HT2c receptors and P2x2/P2x3 receptors (hereafter, ‘dual biosensor cells’) were prepared and loaded with Fura-2 as described in Huang et al. (2007). An aliquot of suspended biosensor cells preloaded with Fura-2 was transferred to a recording chamber containing taste cells and tested for sensitivity to 5-HT (3 nm) or ATP (300 nm). Selected biosensor cells were drawn and held to a fire-polished glass micropipette with gentle suction for use in testing transmitter release from taste cells. In separate experiments we verified that biosensor cells do not respond to bath-applied KCl, taste mix (10 μm cycloheximide, 2 mm saccharin, 0.1 mm SC45647, 1 mm denatonium), acetic acid (10 mm; pH 5.0), or sodium acetate (20 mm; pH 7.2) (see Huang et al. 2005). To test for 5-HT secretion, dual biosensor cells were preincubated with 500 μm ATP for 30 min to desensitize purinoceptors for the duration of the experiment. Conversely, to test for ATP secretion, dual biosensor cells were preincubated for 30 min with 10 μm 5-HT, which rendered the serotonergic receptors refractory throughout the experiment.


Taste buds contain separate populations of sensory cells believed to participate directly in taste transduction – receptor (Type II) cells and presynaptic (Type III) cells. Only presynaptic cells form morphologically distinct synapses with gustatory afferent nerve terminals (Yee et al. 2001). Receptor cells specifically and selectively respond to sweet and bitter taste stimulation whereas presynaptic cells respond to KCl depolarization (DeFazio et al. 2006). We isolated individual taste cells, loaded them with Fura-2, and tested responses to a bath-applied mixture of sweet and bitter taste stimuli and to potassium chloride. This allowed us to reliably and unambiguously identify and distinguish receptor and presynaptic cells (Huang et al. 2007; Tomchik et al. 2007). Isolated receptor versus presynaptic cells, identified in this manner, were subjected to acid taste stimulation.

Receptor and presynaptic cells respond to acetic acid stimulation

When superfused with the sour taste stimulus acetic acid (10 mm, pH 5.0), isolated receptor and presynaptic cells alike exhibited pronounced Ca2+ transients. Surprisingly, applying a mineral acid, HCl, at the same or even more acidic pH, e.g. 10 mm HCl (pH 3), evoked small to negligible intracellular Ca2+ changes (Fig. 1A and B). (At pH 1.5, HCl did stimulate Ca2+ responses in taste cells. However, these responses were not repeatable and were likely to have represented cell damage.) To determine the source of Ca2+ in acid-evoked taste cell responses, we removed extracellular Ca2+ from the bath. Removing bath calcium significantly reduced acetic acid-induced Ca2+ responses in presynaptic (Type III) cells but had little effect on acid-evoked responses from receptor (Type II) cells (Fig. 1). These findings indicate that acetic acid elicits an increase in [Ca2+]i in presynaptic cells by Ca2+ influx, but in receptor cells presumably by release from intracellular Ca2+ stores. This conclusion was tested in greater detail (below).

Figure 1
Acetic acid taste stimulation evokes Ca2+ responses in receptor (Type II) cells and presynaptic (Type III) cells

The proximate stimulus for sour taste is believed to be intracellular acidification (Lyall et al. 2001, reviewed by Roper, 2007). Thus, we tested the effects of taste stimulation designed to lower intracellular pH while leaving extracellular pH unaffected. When cells are bathed in sodium acetate at neutral pH, this effectively acidifies the cytosol (e.g. Slotki et al. 1993; Speake & Elliott, 1998; see discussion in Roper, 2007). Further, intracellular acidification elicits IP3-mediated Ca2+ release from intracellular stores (Slotki et al. 1993). Accordingly, we tested whether sodium acetate stimulated intracellular Ca2+ release in isolated taste cells. Bathing Fura-2-loaded taste cells in 20 mm sodium acetate (substituted for NaCl, pH 7.2), a protocol that lowers cytosolic pH in other tissues by ~0.3 pH units (Speake & Elliott, 1998), evoked robust Ca2+ responses in receptor and presynaptic cells alike. Importantly, under these conditions, Ca2+ responses in both cell types were unaffected by removing Ca2+ from the bath (Fig. 2). This indicates that Ca2+ responses evoked by sodium acetate are produced by intracellular store release for receptor and presynaptic taste cells alike. (Note, Na+, or ‘salty’ taste, was not a stimulus in these experiments because sodium acetate was substituted equimolar for NaCl. Thus, there was no change in [Na+]o.)

Figure 2
Intracellular acidification by sodium acetate evokes Ca2+ responses in receptor cells and in presynaptic cells

Sodium acetate triggers Ca2+ release from intracellular stores

Intracellular Ca2+ store release in gustatory receptor (Type II) cells via a pathway involving phospholipase C subclass β2 (PLCβ2) and IP3 has been thoroughly documented for sweet, bitter and umami taste stimulation (e.g., Gilbertson et al. 2000; Tomchik et al. 2007). However, considerably less is known about Ca2+ store release in presynaptic (Type III) cells and how/whether it is triggered by intracellular acidification. Thus, we explored whether Ca2+ transients in presynaptic cells evoked by sodium acetate (i.e. by presumed intracellular acidification) were abolished by thapsigargin, a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase inhibitor that depletes intracellular Ca2+ stores. Ca2+ transients evoked by sodium acetate as well as Ca2+ signals produced by stimulating P2Y receptors were significantly reduced by incubating isolated taste cells with thapsigargin (1 μm, 5–10 min) (Fig. 3). In marked contrast, this treatment did not affect Ca2+ influx stimulated by KCl depolarization (not shown) or by acetic acid taste stimulation (10 mm, pH 5).

Figure 3
Intracellular Ca2+ release in presynaptic (Type III) cells is via a phospholipase C-mediated pathway

We also tested whether incubating taste cells with a broad spectrum PLC blocker affected Ca2+ transients evoked by sodium acetate. Indeed, U73122 (10 μm, 6 min) significantly reduced sodium acetate-evoked Ca2+ responses in presynaptic cells. In contrast, Ca2+ influx into presynaptic cells stimulated by 10 mm acetic acid (pH 5) was unaltered by U73122 (Fig. 3). Collectively, the findings suggest that intracellular acidification in presynaptic cells produced by sodium acetate stimulates PLC/IP3 release of Ca2+ from intracellular stores. This clearly differs from the frank Ca2+ influx in these cells stimulated by acetic acid at pH 5.

Threshold for acid-stimulated Ca2+ influx in presynaptic cells

Next, we investigated at which point Ca2+ responses make the transition from store release to Ca2+ influx in presynaptic cells as the acid stimulus strength increases. This threshold is important because transmitter (serotonin, 5-HT) secretion from presynaptic cells depends on Ca2+ influx, not intracellular Ca2+ release (Huang et al. 2005; see below). Establishing a threshold for Ca2+ influx might support a link between acid taste mechanisms and transmitter release. We applied 10 mm acetic acid titrated to different pH levels and measured Ca2+ responses in presynaptic cells in the presence and absence (replacement by Mg2+) of extracellular Ca2+. (In receptor, Type II, cells, acetic acid at all pH levels stimulated Ca2+ store release only.) Concentration–response curves showed that Ca2+ responses evoked by acetic acid titrated to pH 7 (i.e. sodium acetate) were solely generated by intracellular release. As the pH of the taste stimulus became more acidic, the Ca2+ response amplitude declined, but more importantly, a greater portion of the response was generated by Ca2+ influx (Fig. 4). The threshold for stimulating Ca2+ influx in isolated presynaptic cells appears to be between pH 6.0 and pH 6.5. By pH 5.0 there was negligible Ca2+ store release; nearly the entire acid-evoked Ca2+ signal was generated by Ca2+ influx.

Figure 4
Acetic acid taste stimulation of presynaptic (Type III) taste cells varies with pH

Intracellular acidification is the proximate stimulus for acid taste in taste buds in lingual slices

To further test the concept of intra- versus extracellular acidification as the proximate stimulus of sour taste, we recorded responses in intact taste buds in lingual slices of vallate papillae in response to stimuli with varying acidity. This preparation preserves sensory epithelial polarity and allows recordings under more physiologically relevant conditions (Caicedo et al. 2000, 2002; Richter et al. 2003; Tomchik et al. 2007). Taste cells were loaded with Calcium Green dextran and taste stimuli were delivered to the apical tips of taste buds via focal micropipettes. Concentrations of acid stimuli were measured at the taste pore by observing the dilution of a known concentration of the fluorescent tracer included in the taste stimulus.

Focal application of 20 mm sodium acetate, pH 7.2, did not evoke Ca2+ responses in taste cells in the lingual slice preparation (data not shown), unlike the situation when this stimulus was bath-applied to isolated taste cells. Presumably this is due to the healthier condition and more intact intracellular buffering capacity of taste cells in the lingual slice preparation. Furthermore, focal application of sodium acetate in the lingual slice preparation reaches considerably less exposed surface of taste cells – only the apical tips of the taste cells penetrate into the taste pore. However, as shown previously (Richter et al. 2003), titrating sodium acetate to pH 5 (i.e. stimulating with acetic acid) evokes robust responses in a subset of taste cells. Next, we tested whether acid-evoked taste cell responses varied with the concentration of extracellular proton in the stimulating solution (i.e. with pH) or with the concentration of protonated acetic acid (HOAc). We stimulated taste buds with solutions consisting of equal concentrations of the uncharged, protonated acetic acid moiety (HOAc) but varying in extracellular proton concentration (i.e. pH), and compared these findings with those when taste buds were stimulated with solutions of equal pH (i.e. equal proton concentration) but varying [HOAc]o (see Methods). The results clearly showed that the effective stimulus was the uncharged acetic acid moiety, not extracellular protons; responses varied with increasing [HOAc], not with pH (Fig. 5). The threshold for acid-evoked responses using focal taste stimulation in the lingual slice preparation appears to be ~30 mm HOAc. (This compares with the threshold of 2–5 mm HOAc when isolated presynaptic taste cells were bathed in acetic acid, i.e. 10 mm acetic acid at pH 6.0–6.5, Fig. 4.) These findings emphasize that the proximate stimulus for the acid-evoked responses is not extracellular H+ but intracellular acidification, consistent with the data from the isolated taste cells.

Acid taste stimulation elicits serotonin release from presynaptic cells

We previously reported that isolated taste buds secrete serotonin (5-HT) in response to acid taste stimulation (Huang et al. 2005) and subsequently that presynaptic cells specifically are the cells of origin for 5-HT secretion (Huang et al. 2007). Here we used biosensor cells to test whether acetic acid stimulated 5-HT release from isolated presynaptic cells. 5-HT biosensor cells were drawn onto a glass micropipette with gentle suction and positioned next to isolated presynaptic cells to measure acid-evoked release of the monoamine. We confirmed that the 5-HT biosensor itself was not directly stimulated by acetic acid at concentrations used in this report. Bath application of acetic acid (10 mm, pH 5.0) evoked Ca2+ transients in the presynaptic cell, as before, and also resulted in rapid and repeatable 5-HT biosensor responses, demonstrating stimulus-evoked 5-HT release from presynaptic cells (Fig. 6A).

Figure 6
Acid taste stimulation evokes 5-HT release from presynaptic (Type III) taste cells

We next tested whether 5-HT release evoked by acid stimulation was Ca2+ dependent. Mg2+ (3 mm) was substituted for Ca2+ (2 mm) in the bath, and isolated presynaptic cells were stimulated with acetic acid, as before. Replacing bath Ca2+ with Mg2+ rapidly and reversibly reduced or eliminated acid-evoked Ca2+ transients in presynaptic cells, consistent with acid-evoked Ca2+ influx into presynaptic cells as shown above (Fig. 1), and also reduced or eliminated 5-HT release (Fig. 6). Our findings strongly suggest that 5-HT release from presynaptic cells evoked by acetic acid stimulation is triggered by Ca2+ influx, consistent with vesicular exocytosis at synapses.

In contrast to presynaptic cells, which release 5-HT, gustatory receptor (Type II) cells secrete ATP in response to taste stimulation (Huang et al. 2007; Romanov et al. 2007). Thus we tested whether acetic acid also stimulates ATP secretion from receptor cells. As with 5-HT biosensors and presynaptic (Type III) cells, above, we positioned ATP biosensors against isolated receptor cells. We consistently recorded robust Ca2+ transients in receptor cells and ATP secretion in response to taste stimulation (with a mixture of 10 μm cycloheximide, 2 mm saccharin, 1 mm denatonium and 0.1 mm SC45647), confirming taste-evoked ATP secretion (Huang et al. 2007). Surprisingly, despite the presence of large, acid-evoked Ca2+ transients in receptor cells, in no case did we observe ATP secretion evoked by acetic acid stimulation (Fig. 7).

Figure 7
Acid taste stimulation does not evoke ATP secretion from receptor (Type II) cells

Because acetic acid stimulates Ca2+ release from intracellular stores in receptor (Type II) cells (Fig. 1), these data indicate store-released Ca2+per se is inadequate to trigger transmitter (ATP) secretion. This contrasts with acetic acid-evoked Ca2+ influx into, and subsequent 5-HT release from, presynaptic cells. Thus, we tested whether Ca2+ release from intracellular stores stimulated by intracellular acidification (with sodium acetate) could either trigger 5-HT release from presynaptic (Type III) taste cells or ATP secretion from receptor (Type II) cells. We bath-applied sodium acetate (20 mm, pH 7.2) to individual receptor cells and to presynaptic cells in parallel to produce intracellular acidification in these cells, as described in the previous experiments. However, in this case we also tested for transmitter secretion using 5-HT biosensors for presynaptic cells (Fig. 6B) and ATP biosensors for receptor cells (Fig. 7B). The results indicated that although sodium acetate triggers large Ca2+ transients due to store release of Ca2+ in receptor and presynaptic cells alike, this did not stimulate ATP or 5-HT secretion. This was the case even though the Ca2+ transients evoked by store release were as large if not larger than those produced by Ca2+ influx in presynaptic cells.

Taken together, the data indicate that mild intracellular acidification (e.g. bath-applied sodium acetate, pH 7) triggers intracellular Ca2+ release in receptor and presynaptic cells alike, but that this does not stimulate transmitter release in either cell type. Stronger acidic stimulation (10 mm acetic acid, pH 5) also elicits Ca2+ store release in receptor (Type II) cells but without transmitter secretion. In marked contrast, acid stimulation triggers Ca2+ influx into and 5-HT secretion from presynaptic cells. These data are summarized in Table 1.

Table 1
Summary of taste cell responses to sodium acetate and acetic acid stimulation


The present study was undertaken to investigate mechanisms underlying sour (acid) taste transduction in mouse taste buds, namely, to identify confidently which cells are directly sour responsive, to discover what is the proximate stimulus for acid taste stimulation, and to identify what neurotransmitter(s) sour-responsive taste cells release. The essential findings are that a specific sour-responsive subset of taste bud cells, namely presynaptic (Type III) taste bud cells, responds to intracellular acidification with Ca2+ influx and serotonin secretion. Only presynaptic cells show Ca2+influx and serotonin secretion with acid taste stimulation in the range that elicits sour taste in humans and aversive behaviour in rodents. These transduction mechanisms for acid taste differ fundamentally from taste transduction for sweet, bitter and umami, which involve GPCR activation, Ca2+ store release, and ATP secretion via pannexin hemichannels from receptor (Type II), not presynaptic (Type III), taste cells (Huang et al. 2007). Our findings reinforce and provide further clarification of the longstanding, though puzzling observation that organic acids such as acetic acid are more intensely sour than mineral acids such as HCl at the same pH (Harvey, 1920). Organic acids acidify the cytosol more readily than do mineral acids and thereby more effectively stimulate sour-responsive taste cells.

DeSimone and colleagues have postulated that the proximate stimulus for acid taste is intracellular acidification (Lyall et al. 2001, 2006). They applied acetic acid to the serosal or mucosal surfaces of isolated sheets of epithelium mounted in an Ussing chamber and recorded changes in intracellular pH, though without distinguishing taste cell types. Richter et al. (2003) extended these findings by showing that citric acid, applied to the mucosal surface of lingual slices, rapidly permeates the epithelium and acidifies the cytosol of all cells in the epithelium. However, only a subset of taste bud cells responded to cytosolic acidification with a transient influx of Ca2+. The present data identify these acid-responding cells as presynaptic (Type III) taste bud cells and show that the Ca2+ influx leads to the release of serotonin. Taste cells did not respond well to extracellular acidification alone, such as by bath applied HCl. Nor did taste cells show a concentration–response relationship for extracellular [H+]. Instead, taste cells responded in a concentration-dependent manner to the membrane-permeant, uncharged acetic acid moiety (HOAc), consistent with intracellular acidification. HOAc produces an intracellular acidification by diffusing into the cytosol. Once HOAc is inside the cell, it dissociates and delivers H+ to the cytosol, acidifying the intracellular milieu:

equation image

These findings are entirely consistent with the long-established psychophysical findings in human taste research that at equal pH values, organic acids such as acetic and citric acid are more effective taste stimuli (sour) than mineral acids such as HCl (Harvey, 1920). Indeed, counterintuitively, HCl is not nearly as sour as acetic (or citric) acid at equi-pH solutions; the sour threshold for solutions of HCl is much more acidic (Harvey, 1920). Of course, at sufficiently high enough concentration (i.e. low pH), HCl indeed evokes sour taste. The effectiveness of acetic and citric acids as sour tastants reflects the much higher membrane permeability of the protonated moieties of acetic and citric acids relative to protons, and thus the ability of the organic acids to deliver protons into the cell interior.

The implication of these findings for acid taste transduction mechanisms is that candidate sour taste transducer proteins such as ASIC channels (Ugawa et al. 1998; Richter et al. 2004), HCN channels (Stevens et al. 2001), or PKD2L1/PKD1L3 channels (LopezJimenez et al. 2006; Ishimaru et al. 2006; Huang et al. 2006) are likely to be gated by intracellular acidification instead of (or in addition to) extracellular protonation. Key proton-binding sites are likely to be in the intracellular domains of sour taste transduction molecules. To date, intracellular proton-binding sites have not been explored in detail with the proposed acid taste transduction channels. Parenthetically, a similar situation might hold for sensory transduction in inflammatory pain, where tissue acidification from lactic and carbonic acids is likely to generate local intracellular acidification in nociceptors.

It is interesting to note that intracellular acidification produced by sodium acetate triggers intracellular Ca2+ release in receptor and presynaptic cells alike. Intracellular Ca2+ release evoked by cytosolic acidification has been reported for other tissues (Slotki et al. 1993; Speake & Elliott, 1998). In presynaptic taste cells, further acidification (i.e. increased cytosolic acidification) ultimately triggers Ca2+ influx. This was not observed in taste receptor (Type II) cells. Furthermore, in presynaptic cells there even appears to be a concurrent suppression of intracellular Ca2+ release with increased cytosolic acidification (Figs 1C and and4).4). That is, there is a transition from Ca2+ store release to Ca2+ influx during increasingly stronger acid taste stimulation. Suppression of intracellular Ca2+ store release by sufficiently strong intracellular acidification may occur due to IP3 receptor inhibition at pH values ~6 and lower (Mourey et al. 1990; Lopez-Colome & Lee, 1996).

Our results show that sour taste stimulation elicits Ca2+ influx and 5-HT release from presynaptic (Type III) cells, but does not stimulate transmitter (ATP) secretion receptor (Type II) cells, despite the presence of robust Ca2+ signals. The lack of transmitter secretion from receptor cells may be explained by the fact that ATP secretion from these cells is via pannexin 1 (Px1) hemichannels (Huang et al. 2007). Px1 hemichannels are gated open by intracellular Ca2+, but intracellular acidification inhibits them (Locovei et al. 2006) and prevents ATP secretion (Huang et al. 2007). The blockage of gap junction hemichannels by intracellular acidification may similarly explain why the robust Ca2+ responses elicited by sodium acetate fail to trigger ATP secretion from receptor cells. It would be interesting to test whether sodium acetate or acid taste stimuli are able to reduce gustatory single fibre responses to sweet, bitter or umami tastants. To our knowledge, those experiments have not been conducted. Unlike its actions on gap junction hemichannels, intracellular acidification apparently has less effect on voltage-gated Ca2+ channels and thus upon depolarization-stimulated Ca2+ influx in presynaptic (Type III) cells. Hence, acid taste stimulation does not inhibit, but instead triggers, 5-HT release from presynaptic cells. A full explanation for these mechanisms awaits detailed intracellular pH measurements during sodium acetate and acetic acid taste stimulation under the conditions in our experiments.

Important next steps will include to resolve what is the role of 5-HT released by presynaptic cells when they are stimulated by acid tastants. Is 5-HT a synaptic transmitter onto sensory afferent fibres or a paracrine transmitter acting within taste buds (Kaya et al. 2004)? Recent studies indicate that taste thresholds are altered in human subjects when tissue 5-HT levels are manipulated by monoamine reuptake inhibitors (Heath et al. 2006) but precise sites and mechanisms of 5-HT actions in taste buds are not yet known.


This work was supported in part by grants NIH/NIDCD 3R01DC000374 (S.D.R.) and 5R01DC007630 (S.D.R.). We would like to thank Drs Seth Tomchik and Nirupa Chaudhari for their helpful comments during the experiments and writing.


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