From receptors to the brain: Psychophysical clues to taste physiology

Abstract

To understand human taste requires not only physiological studies ranging from receptor mechanisms to brain circuitry, but also psychophysical studies that quantitatively describe the perceptual output of the system. As obvious as this requirement is, differences in research approaches, methodologies, and objectives complicate the ability to meet it. Discussed here is an example of how the discovery two decades ago of a perceptual taste illusion (thermal taste) has led to physiological and psychophysical research on both peripheral and central mechanisms of taste, including most recently a psychophysical study of the heat sensitivity of the human sweet taste receptor TAS1R2/T1R3, and an fMRI study of a possible central gain mechanism that may underlie, in part, differences in human taste sensitivity. In addition to the new data and hypotheses these studies have generated, they illustrate instances of research on taste motivated by evidence derived from different approaches and levels of analysis.

Introduction

For more than a century researchers in taste physiology and taste psychophysics have benefitted from a mutual symbiotic relationship: discoveries about the physiology of taste have informed and guided psychophysical investigations of taste perception, and psychophysical findings have motivated research into the mechanisms of taste. However, because psychophysical measurements engage the entire sensory system--from receptors to cognition—it can be challenging to draw meaningful inferences about specific taste mechanisms. But careful control of stimulus conditions combined with rigorous psychophysical methods has the potential to provide tests of hypotheses about sensory, perceptual, and cognitive processes, and unexpected findings can lead to discovery of new phenomena that generate hypotheses about the mechanisms that produce them. This paper focuses on one such finding two decades ago, namely that tastes can be evoked by temperature alone. Described as thermal taste [1], the phenomenon showed that heating or cooling gustatory areas of the tongue can evoke sensations of taste. Specifically, warming from ≤20°C toward normal tongue temperature at ~1°C/s often evoked sweetness while temperature was rising, while cooling the tongue to ≤20°C less often evoked sourness or saltiness that persisted at cold temperatures. Two recent studies will be discussed that were motivated in whole or in part by thermal taste which have led to new evidence of potential mechanisms of taste perception: one yielded evidence that the human sweet taste receptor TAS1R2/TAS1R3 can be activated by heating, and the other provides support for the existence of a “gain” mechanism in the brain that may contribute to inter- and intra-individual variability in the responsiveness to all taste stimuli.

Evidence the Sweet Taste Receptor can be Activated by Heating

The phenomenon of thermal taste [1] resolved the longstanding question in taste physiology of whether thermal stimulation of the taste nerves in animals [2–5] and humans [6] is encoded as taste. Soon after its discovery, two studies were published that described possible receptor mechanisms of thermal taste. Askwith et al. [7] reported that members of the DEG/ENaC family of cation channels--candidate receptors for salt and sour taste in rodents--are activated by cooling, providing a possible explanation for salty and sour thermal tastes when the tongue is cooled [1]. However, ENaC is not considered to play a major role in human salt taste [8, 9]. A more promising receptor for sour taste was recently identified in mice [10], but its thermal sensitivity has not yet been studied. A second study by Talavera et al. [11] proved more relevant to sweet taste in humans. Those authors showed that activation of TRPM5, a calcium-gated, non-specific cation channel in the transduction cascade of the human sweet taste receptor, TAS1R2/TAS1R3, increased with temperature, which they proposed could explain both sweet thermal taste and the heat sensitivity of sweet taste in general [12, 13].

While the temperature sensitivity of TRPM5 appears to play an important role in the heat sensitivity of sweet taste, its ability to explain sweet thermal taste is less clear. First, while sweet thermal taste is perceived only while the tongue is being heated from below normal oral temperature [14], the activation of TRPM5 appears to depend directly on temperature [11]. Second, bitter and umami thermal tastes are not reported nearly as frequently as sweetness [1, 14] even though TRPM5 is a downstream effector for all G-protein coupled receptors (GPCRs) [15, 16], including TAS2R bitter receptors and the TAS1R1-TAS1R3 umami receptor [17]. In addition, studies have shown differential effects of temperature on the perception of sweeteners [18, 19] and on the magnitude of response of neurons in the nucleus of the solitary tract (NTS) that respond to sweet, bitter and umami stimuli [20]. The latter evidence suggests that at least one other mechanism in addition to TRPM5 may underlie the heat sensitivity of sweet taste. After finding superadditive effects of heating in mouse NTS neurons responsive to sweet and umami stimuli, Wilson & Lemon [20] speculated that heat may also produce conformational changes in the T1R2/T1R3 sweet receptor and T1R1/T1R3 umami receptor. Similarly, based upon the psychophysical evidence of differential effects of heat on the sensitivity and adaptation to various sweeteners [18, 19, 21], Green & Nachtigal [19] proposed that heating may produce conformational changes that affect the agonist binding in the N-terminal and transmembrane domains of the TAS1R2/TAS1R3 sweet taste receptor.

Since the papers of Askwith et al. [7] and Talavera et al. [11], no additional studies of the temperature sensitivity of rodent or human taste receptors have been published. However, research on receptors that mediate temperature sensitivity have shown that thermoreceptors can serve a wide variety of sensory functions [22], including taste [23]. Most relevant to sweet thermal taste is the finding that in addition to sensing gustatory stimuli, the Drosophila gustatory receptor GR28B(D) guides the navigational response to heat gradients encountered in the environment [24]. Although it is not yet clear whether GR28B(D) is an ionotropic receptor or a 7-transmembrane receptor like the mammalian sweet taste receptor, the existence in flies of a heat-sensitive receptor that can also function as a taste receptor supported the possibility that heating alone may be able to activate TAS1R2/TAS1R3. Consistent with this hypothesis are data from prior studies showing that the N-terminal domain of GPCRs regulates basal signaling activity in the absence of ligands [25–27]. This finding means that a change in protein conformation produced by heating could activate G-proteins and the downstream effectors that ultimately increase intracellular calcium, which would in turn increase activation of TRPM5.

We recently investigated this hypothesis in humans using the sweet taste blocker lactisole [14]. Our reasoning was that because lactisole is thought to prevent activation of TAS1R2/TAS1R3 by locking the receptor in an inactive state [28], blockade of sweet thermal taste by lactisole would demonstrate that heating can activate the receptor itself. To test this hypothesis we conducted a psychophysical experiment in which subjects rated the intensity of taste sensations experienced while dipping the tongue tip into either pure flowing water or water with lactisole added as the water was heated from 20° to 35°C at 1°C/s [14]. The results supported the hypothesis: sweet thermal taste wa completely blocked by lactisole in a concentration that also blocked the sweetness of sucrose. The results also lend support to the hypothesis of Talavera et al. [11] that TRPM5 functions as a “coincidence detector” of increases in heat and intracellular calcium. We therefore propose that sweet thermal taste occurs when heating simultaneously increases the sensitivity of TRPM5 to calcium while intracellular calcium levels rise due to a change in receptor conformation (Fig. 1).

Figure 1:

Representation of the proposed mechanism of sweet thermal taste.Left: At 20°C neither the sweet taste receptor TAS1R2/TAS1R3 nor the heat-sensitive channel TRPM5 are activated and intracellular calcium (Ca) is at or below baseline levels. Center: At 35°C the taste receptor remains inactive while the sensitivity of TRPM5 to calcium is increased, allowing sodium (Na) to enter the taste receptor cell (blue arrow); however because intracellular calcium remains at basal levels, influx of sodium through TRPM5 is submaximal. Right: Heating from 20° to 35°C at ~1–2°C/s induces a conformational change in the N-terminal domain (N) of the receptor which activates G-proteins and other downstream effectors resulting in a rise in intracellular calcium. The combination of high calcium levels and heat-sensitization of TRPM5 leads to an influx of sodium sufficient to depolarize the receptor cell. Note that conformational change could also potentially occur in the 7-transmembrane domain of one or both monomers (green areas) or their linkage (stippled gray areas).

The heat sensitivity of TAS1R2/TAS1R3 indicates that like GR28B(D) in Drosophila [24], TAS1R2/TAS1R3 possesses the properties of both a gustatory receptor and a thermoreceptor. However, unlike GR28B(D) there is no evidence TAS1R2/TAS1R3 functions as a temperature receptor. It is also unclear what advantage its heat sensitivity may provide for sweet taste perception. One possibility is that heating increases the sensitivity to sugars in foods consumed at cool or cold ambient temperatures as the food warms in the mouth during ingestion. If so, the advantage may be limited to the detection of low concentrations, as we recently found no significant difference in the reported sweetness of suprathreshold concentrations of sucrose [14].

Although the hypothesis of receptor activation by heating can in theory explain sweet thermal taste, it leaves unanswered why umami thermal taste is so rarely reported [14] even though the umami receptor TAS1R1/TAS1R3 is also a 7-transmembrane heterodimer [29]. However the different protein structure of TAS1R1/TAS1R3, particularly the TAS1R1 monomer and its linkage with TAS1R3, may make it less vulnerable to conformational change during heating. Protein structure may also explain why bitter thermal taste is reported infrequently ([1, 14]; see, however [30]), since the TAS2R family of bitter receptors has low homology with other GPCRS [31], including a much smaller N-terminal domain [29, 32]. Significant differences in structure seem a plausible explanation for differences in thermal taste in as much as protein structure plays a role in allosteric interactions, which are a key property of GPCRs [33, 34]. Thus the present evidence supporting the hypothesis that temperature can modulate activation of TAS1R2/TAS1R3 may be important for understanding not only the mechanisms of thermal sweet taste, but also the effects that temperature may have on the activity of other 7-transmembrane receptors.

Variability in Thermal Taste: Clues about Central Mechanisms of Taste Perception

A striking characteristic of thermal taste is its variability: not everyone perceives it, and among those who do its perception can vary from day to day and even from trial to trial. Individual differences were evident in the first published study of the phenomenon [1]: while 21 of 24 subjects who were initially tested perceived at least one taste when the tongue was warmed or cooled between 5° and 35°C, only 19 reported perceiving at least 2 taste qualities. Of those 19, 16 reported sweetness during warming trials whereas 6 reported sourness and 6 reported saltiness during cooling trials. To investigate the source of these differences, a second study tested the hypothesis that individuals who experience sweet thermal taste are more sensitive to sweet taste in general [35]. Although the results showed that sweet thermal taste perceivers did rate the sweetness of both sucrose and saccharin significantly higher than non-perceivers, they also gave higher ratings to salty, sour, bitter, and umami stimuli. Additional testing showed that while perceivers also rated the sweet odor of vanillin stronger than non-perceivers, there was no difference between groups in the perceived intensity of warm and cold stimulation on the lip or hand. These findings led the authors to propose that perception of sweet thermal taste depends in part on individual differences in a “central gain mechanism” that is specific to taste and possibly flavor perception [35]. Additional support for this hypothesis came from a subsequent psychophysical study [36] which again found an association between the sensitivity to thermal taste and chemical taste, but not chemesthesis.

Although brain mechanisms associated with individual differences in perception have been investigated in other sensory modalities, most notably the sense of pain [37–40], the possibility that a central mechanism plays a role in individual differences in taste has drawn little research attention. Most studies of individual differences in taste have continued to focus on possible peripheral sensory factors. While major advances in the genetics and biology of taste receptors have led to discovery of associations between receptor expression and taste sensitivity [41, 42], in recent decades psychophysical studies of individual differences have been dominated by the controversial hypothesis that sensitivity to the bitter taste stimulus 6-n-propylthiouracil (PROP), which is mediated by alleles of the TAST2R38 bitter taste receptor, is also predictive of suprathreshold perception of human taste in toto [e.g., 43, 44, 45].

However, a recent study by Veldhuizen et al. [46] that combined fMRI and psychophysical approaches provided the first evidence of a possible central gain mechanism (CGM) in human taste. Consistent with prior data indicating that resection of the amygdala increases the perception of taste [47, 48], the authors found that activation of the central amygdala was correlated with psychophysical ratings of the taste intensity of sweet, sour, salty and bitter stimuli. Direct causal modeling (DCM) further identified inhibitory inputs from the amygdala to ventral posterior medial (VPM) thalamus, which also predicted individual differences in mean taste response (see Fig. 2).

Figure 2:

Shown are 5 brain areas proposed to be involved in a Central Gain Mechanism of taste (vpm/pul = ventroposterior/pulvinar; md = medial). In an fMRI study, Dynamic Causal Modeling (DCM) determined that the BOLD response to sweet, sour and salty stimuli in these areas was significantly associated with log-mean ratings of perceived taste intensity. Arrows indicate the direction and valence of influences between brain regions (blue = excitatory; red = inhibitory), and arrow thickness reflects Bayesian modeling estimates of connectivity strengths. (Adapted from Veldhuizen, et al. 2020)

While the Veldhuizen et al. [46] study provides plausible evidence for a CGM, additional work will be necessary to uncover the details of the proposed circuitry and how it may interact with other perceptual mechanisms, such as those that underlie changes in arousal and attention [49–52]. These interactions, which can be triggered by both endogenous and exogeneous factors, may contribute to the variability across experimental trials and sessions that complicates classification of individuals as thermal “tasters” vs. “non-tasters” [14, 30].

However, the primary value of further study of the proposed CGM will not be a greater understanding of the thermal taste illusion, but rather a greater understanding of the genetic and brain mechanisms that underlie variations in taste perception in general. Similarly, future studies of the temperature sensitivity of taste receptors, their downstream effectors, and the interactions among taste receptor cells will be needed to clarify the peripheral mechanisms that underlie thermal modulation in taste signaling. Ideally these studies will continue to be informed by findings acquired at all levels of analysis that contribute to our understanding of human taste physiology.