Estimation of the individual firing frequencies of two neurons recorded with a single electrode

When monitoring neurons with a single extracellular electrode, it is common to record action potentials from different neurons. A recurring problem with such recordings is to identify which neuron is active. Sorting spikes into separate classes is possible if each neuron discharge spikes differing by their shapes and sizes. However, this approach is not applicable when the spikes are indistinguishable. In this paper, we develop a method for estimating the respective firing frequencies of two neurons, producing indistinguishable spikes. It is based on the fact that, when a neuron fires a spike, there is an interval of time during which the probability of generating a second spike is very low. If a spike occurs during this 'silent period', it is likely to be generated from another neuron and the number of occurrences of such 'doublets' can be used to estimate the respective frequencies of two spike trains. We demonstrate here that a simple relation holds between the frequency of doublets d, the respective frequencies of the two neurons A and B, fA and fB, and a chosen value Delta shorter than the silent period, d=2fAfBDelta. This relation holds for a wide class of firing processes. We used this method to analyze responses from Drosophila taste sensilla. We first checked if the method was consistent with results obtained with stimuli that elicit responses of two taste neurons firing distinguishable spikes. We then applied this method to the study of a pair of taste neurons involved in the coding for salt taste in Drosophila melanogaster.     

Responses of Primate Taste Cortex Neurons to the Astringent Tastant Tannic Acid

In order to advance knowledge of the neural control of feeding,we investigated the cortical representation of the taste oftannic acid, which produces the taste of astringency. It isa dietary component of biological importance particularly toarboreal primates. Recordings were made from 74 taste responsiveneurons in the orbitofrontal cortex. Single neurons were foundthat were tuned to respond to 0.001 M tannic acid, and representeda subpopulation of neurons that was distinct from neurons responsiveto the tastes of glucose (sweet), NaCl (salty), HCI (sour),quinine (bitter) and monosodium glutamate (umami). In addition,across the population of 74 neurons, tannic acid was as wellrepresented as the tastes of NaCI, HCI quinine or monosodiumglutamate. Multidimensional scaling analysis of the neuronalresponses to the tastants indicates that tannic acid lies outsidethe boundaries of the four conventional taste qualities (sweet,sour, bitter and salty). Taken together these data indicatethat the astringent taste of tannic acid should be consideredas a distinct taste quality, which receives a separate representationfrom sweet, salt, bitter and sour in the primate cortical tasteareas. Chem. Senses 21: 135–145, 1996.     

Responsiveness of the cortical taste area neurons to a mixture of the four basic tastants in rats

The taste coding mechanism in the cortical taste area was investigated by analyzing the responses of 59 neurons in the cortical taste area of the anesthetized rat to a mixture of the four basic tastants in both absence and presence of bicuculline methiodide, a specific antagonist to the GABA(A) receptors. The mixture caused response suppression more frequently than response facilitation, both in the control state and during bicuculline application. Cluster analysis revealed that only a group of the neurons with the best response to both NaCl and HCl (group NH) showed the best response to the mixture in the control state, whereas during bicuculline application, in addition to group NH, two other groups of neurons responding to sucrose, or to HCl and quinine responded vigorously to the mixture. Multidimensional scaling located the mixture outside the space of the four basic tastants facing an NaCl-HCl line in both states. GABAergic inhibition caused the group NH to represent the taste of the mixture in the control state. Thus, the mixture probably tastes salty and sour to rats. No cortical neuron was found which specifically responded to the mixture.     

Bitter taste stimuli induce differential neural codes in mouse brain

A growing literature suggests taste stimuli commonly classified as "bitter" induce heterogeneous neural and perceptual responses. Here, the central processing of bitter stimuli was studied in mice with genetically controlled bitter taste profiles. Using these mice removed genetic heterogeneity as a factor influencing gustatory neural codes for bitter stimuli. Electrophysiological activity (spikes) was recorded from single neurons in the nucleus tractus solitarius during oral delivery of taste solutions (26 total), including concentration series of the bitter tastants quinine, denatonium benzoate, cycloheximide, and sucrose octaacetate (SOA), presented to the whole mouth for 5 s. Seventy-nine neurons were sampled; in many cases multiple cells (2 to 5) were recorded from a mouse. Results showed bitter stimuli induced variable gustatory activity. For example, although some neurons responded robustly to quinine and cycloheximide, others displayed concentration-dependent activity (p<0.05) to quinine but not cycloheximide. Differential activity to bitter stimuli was observed across multiple neurons recorded from one animal in several mice. Across all cells, quinine and denatonium induced correlated spatial responses that differed (p<0.05) from those to cycloheximide and SOA. Modeling spatiotemporal neural ensemble activity revealed responses to quinine/denatonium and cycloheximide/SOA diverged during only an early, at least 1 s wide period of the taste response. Our findings highlight how temporal features of sensory processing contribute differences among bitter taste codes and build on data suggesting heterogeneity among "bitter" stimuli, data that challenge a strict monoguesia model for the bitter quality.     

味觉刺激引起大鼠臂旁核神经元抑制性反应

本研究以轻度麻醉的大鼠为对象,应用细胞外微电极记录技术,观察并分析了脑桥臂旁核抑制性味觉神经元的自发活动及其对NaCl、HCl、盐酸奎宁（quinine HCl,QHCl））和蔗糖等四种基本味觉刺激的反应。共分析了18个具有自发活动的抑制性味觉神经元,自发放电频率分布在0．2～5．5Hz之间,平均放电频率（2．15±0．31）Hz。18个神经元中,1个神经元对单一味觉刺激呈抑制性反应,其余17个神经元对两种或两种以上的基本味觉刺激发生抑制性反应,且抑制具有潜伏期短、持续时间较长等特征。抑制持续时间5～80S,部分神经元表现为后抑制效应。根据神经元对四种基本味觉刺激呈抑制性反应的程度,将其分为NaCl优势神经元（n=8）,HCl优势神经元（n=3）,QHCl优势神经元n=3）和蔗糖优势神经元n=4）。其中NaCl优势神经元的反应谐宽最高（0．945）。这些神经元对欣快或厌恶刺激的区别能力较低。结果提示,在脑桥臂旁核存在对味觉刺激起抑制性反应的神经元,这些味觉神经元可能在味觉的调制及对欣快和厌恶刺激的编码中发挥重要的作用。     

Effect of concentration on taste-taste interactions in foods for elderly and young subjects

An increase in concentration of one of the tastants in a 'real food' might affect not only the perception of the taste quality of that manipulated tastant but also the other perceivable taste qualities. The influence of concentration increase of sodium or potassium chloride in tomato soup, sucrose or aspartame in iced tea, acetic or citric acid in mayonnaise, caffeine or quinine HCl in chocolate drink, monosodium glutamate (MSG) or inosine 5'-monophosphate (IMP) in broth on the other perceivable taste qualities in these foods was studied in 21 young subjects (19-33 years) and 21 older subjects (60-75 years). The results showed that for each of these tastants, except for the two acids, increasing the concentration provoked significant positive or negative interaction effects on the perception of one or more other taste qualities of the product. Especially in the young, olfaction plays a larger role in the assessment of taste intensity than has been hitherto assumed. The elderly are less able to discriminate between the taste qualities in a product, whereas the young are more able to do so.     

Temporal Characteristics of Gustatory Responses in Rat Parabrachial Neurons Vary by Stimulus and Chemosensitive Neuron Type

It has been demonstrated that temporal features of spike trains can increase the amount of information available for gustatory processing. However, the nature of these temporal characteristics and their relationship to different taste qualities and neuron types are not well-defined. The present study analyzed the time course of taste responses from parabrachial (PBN) neurons elicited by multiple applications of “sweet” (sucrose), “salty” (NaCl), “sour” (citric acid), and “bitter” (quinine and cycloheximide) stimuli in an acute preparation. Time course varied significantly by taste stimulus and best-stimulus classification. Across neurons, the ensemble code for the three electrolytes was similar initially but quinine diverged from NaCl and acid during the second 500ms of stimulation and all four qualities became distinct just after 1s. This temporal evolution was reflected in significantly broader tuning during the initial response. Metric space analyses of quality discrimination by individual neurons showed that increases in information (H) afforded by temporal factors was usually explained by differences in rate envelope, which had a greater impact during the initial 2s (22.5% increase in H) compared to the later response (9.5%). Moreover, timing had a differential impact according to cell type, with between-quality discrimination in neurons activated maximally by NaCl or citric acid most affected. Timing was also found to dramatically improve within-quality discrimination (80% increase in H) in neurons that responded optimally to bitter stimuli (B-best). Spikes from B-best neurons were also more likely to occur in bursts. These findings suggest that among PBN taste neurons, time-dependent increases in mutual information can arise from stimulus- and neuron-specific differences in response envelope during the initial dynamic period. A stable rate code predominates in later epochs.     

Stimulus-Specific Adaptation at the Synapse Level In Vitro

Stimulus-specific adaptation (SSA) is observed in many brain regions in humans and animals. SSA of cortical neurons has been proposed to accumulate through relays in ascending pathways. Here, we examined SSA at the synapse level using whole-cell patch-clamp recordings of primary cultured cortical neurons of the rat. First, we found that cultured neurons had high firing capability with 100-Hz current injection. However, neuron firing started to adapt to repeated electrically activated synaptic inputs at 10 Hz. Next, to activate different dendritic inputs, electrical stimulations were spatially separated. Cultured neurons showed similar SSA properties in the oddball stimulation paradigm compared to those reported in vivo. Single neurons responded preferentially to a deviant stimulus over repeated, standard stimuli considering both synapse-driven spikes and excitatory postsynaptic currents (EPSCs). Compared with two closely placed stimulating electrodes that activated highly overlapping dendritic fields, two separately placed electrodes that activated less overlapping dendritic fields elicited greater SSA. Finally, we used glutamate puffing to directly activate postsynaptic glutamate receptors. Neurons showed SSA to two separately placed puffs repeated at 10 Hz. Compared with EPSCs, GABAa receptor-mediated inhibitory postsynaptic currents showed weaker SSA. Heterogeneity of the synaptic inputs was critical for producing SSA, with glutamate receptor desensitization participating in the process. Our findings suggest that postsynaptic fatigue contributes largely to SSA at low frequencies.     

Hedonic taste in Drosophila revealed by olfactory receptors expressed in taste neurons

Taste and olfaction are each tuned to a unique set of chemicals in the outside world, and their corresponding sensory spaces are mapped in different areas in the brain. This dichotomy matches categories of receptors detecting molecules either in the gaseous or in the liquid phase in terrestrial animals. However, in Drosophila olfactory and gustatory neurons express receptors which belong to the same family of 7-transmembrane domain proteins. Striking overlaps exist in their sequence structure and in their expression pattern, suggesting that there might be some functional commonalities between them. In this work, we tested the assumption that Drosophila olfactory receptor proteins are compatible with taste neurons by ectopically expressing an olfactory receptor (OR22a and OR83b) for which ligands are known. Using electrophysiological recordings, we show that the transformed taste neurons are excited by odor ligands as by their cognate tastants. The wiring of these neurons to the brain seems unchanged and no additional connections to the antennal lobe were detected. The odor ligands detected by the olfactory receptor acquire a new hedonic value, inducing appetitive or aversive behaviors depending on the categories of taste neurons in which they are expressed i.e. sugar- or bitter-sensing cells expressing either Gr5a or Gr66a receptors. Taste neurons expressing ectopic olfactory receptors can sense odors at close range either in the aerial phase or by contact, in a lipophilic phase. The responses of the transformed taste neurons to the odorant are similar to those obtained with tastants. The hedonic value attributed to tastants is directly linked to the taste neurons in which their receptors are expressed.     

Multiple Human Taste Receptor Sites: A Molecular Modeling Approach

Numerous experimental data on the human peripheral taste systemsuggest the existence of multiple low-affinity and low-specificityreceptor sites which are responsible for the detection and thecomplete discrimination of a very large number of organic molecules.According to this hypothesis, a given molecule interacts withnumerous taste receptors and vice versa. Statistical analysisof taste intensities estimated by 58 human subjects for variousmolecules enables the calculation of taste intermolecular distances.For the present modeling study, we hypothesized that a shorttaste distance (i.e. taste similarity) between two distinctmolecules indicates that they bind with similar distributionsof affinities to the taste receptors, and hence display similarbinding motifs. In order to find common molecular binding motifsamong 14 selected organic tastants, hydrogen-bonding and hydrophobicinteraction properties were mapped onto their molecular surfaces.The 14 surfaces were then cut in 240 fragments, most of whichwere made up of 2–4 potentially interacting zones. A correspondenceindex was defined to measure the analogy between two optimallysuperimposed fragments. The 75 most representative fragmentswere all matched pairwise. Twelve distinct clusters of fragmentswere isolated from the 2775 calculated comparisons. These 12fragment types were used to calculate structural similaritydistances. We then performed a combinatorial analysis to identifywhich fragment combination best reconciled structural and tastedistances. We finally identified an optimal subset of sevenfragment types out of the 12, which significantly and best accountedfor the 91 pairwise taste distances between all 14 modeled tastants.These seven validated fragment types are therefore presentedas good candidates to be recognized by the same number of distincttaste receptor sites. Potential applications of these identifiedbinding motifs to tastant design are suggested. Chem. Senses21: 425–445, 1996.     

Do tastants have a smell?

The stimuli used in taste research are usually considered to be odourless. This was tested in two experiments with aqueous solutions of two representative compounds for each of the five taste qualities including umami. In the first experiment elderly and young subjects rated the intensity and pleasantness of three concentrations of the stimuli, while wearing or not wearing a noseclip. Saliva production was also measured. Blocking olfaction only influenced salivation for umami. It reduced taste intensity ratings, but as in an earlier experiment with the same compounds in food products, this effect was stronger in the young, who also liked the stimuli better wearing the noseclip. In the second experiment, another group of young people tried to detect the odours of the tastants dissolved in demineralized, double-distilled or Evian water. A considerable number of subjects could regularly detect seven of the ten tastants by olfaction and the extent to which they did correlated significantly with the reduction in taste intensity ratings for the different tastants found in the first experiment. We suggest that most tastants can be smelled and that this smell contributes to taste intensity ratings.     

Monosodium glutamate but not linoleic acid differentially activates gustatory neurons in the rat geniculate ganglion

To date, only one study has examined responses to monosodium glutamate (MSG) from gustatory neurons in the rat geniculate ganglion and none to free fatty acids. Accordingly, we recorded single-cell responses from geniculate ganglion gustatory neurons in anesthetized male rats to MSG and linoleic acid (LA), as well as to sucrose, NaCl, citric acid, and quinine hydrochloride. None of the 52 neurons responded to any LA concentration. In contrast, both narrowly tuned groups of gustatory neurons (sucrose specialists and NaCl specialists) responded to MSG, as did 2 of the broadly tuned groups (NaCl generalist(I) and acid generalists). NaCl-generalist(II) neurons responded only to the highest MSG concentration and only at low rates. No neuron type responded best to MSG; rather, responses to 0.1 M MSG were significantly less than those to NaCl for Na(+) -sensitive neurons and to sucrose for sucrose specialists. Interestingly, most Na(+) -sensitive neurons responded to 0.3 M MSG at levels comparable with those to 0.1 M NaCl, whereas sucrose specialists responded to 0.1 M MSG despite being unresponsive to NaCl. These results suggest that the stimulatory effect of MSG involves activation of sweet- or salt-sensitive receptors. We propose that glutamate underlies the MSG response of sucrose specialists, whereas Na(+) -sensitive neurons respond to the sodium cation. For the latter neuron groups, the large glutamate anion may reduce the driving force for sodium through epithelial channels on taste cell membranes. The observed concentration-dependent responses are consistent with this idea, as are cross-adaptation studies using 0.1 M concentrations of MSG and NaCl in subsets of these Na(+) -sensitive neurons.     

Inhibition of signal termination-related kinases by membrane-permeant bitter and sweet tastants: potential role in taste signal termination

Sweet and bitter taste sensations are believed to be initiated by the tastant-stimulated T1R and T2R G protein-coupled receptor (GPCR) subfamilies, respectively, which occur in taste cells. Although such tastants, with their significantly diverse chemical structures (e.g., sugar and nonsugar sweeteners), may share the same or similar T1Rs, some nonsugar sweeteners and many bitter tastants are amphipathic and produce a significant delay in taste termination (lingering aftertaste). We report that such tastants may permeate rat taste bud cells rapidly in vivo and inhibit known signal termination-related kinases in vitro, such as GPCR kinase (GRK)2, GRK5, and PKA. GRK5 and perhaps GRK2 and GRK6 are present in taste cells. A new hypothesis is proposed in which membrane-permeant tastants not only interact with taste GPCRs but also interact intracellularly with the receptors' downstream shutoff components to inhibit signal termination. amphipathic tastants; tastant permeation; desensitization; lingering aftertaste     

Rapid entry of bitter and sweet tastants into liposomes and taste cells: implications for signal transduction

Someamphipathic bitter tastants and non-sugar sweeteners are directactivators of G proteins and stimulate transduction pathways in cellsnot related to taste. We demonstrate that the amphipathic bittertastants quinine and cyclo(Leu-Trp) and the non-sugar sweetener saccharin translocate rapidly through multilamellar liposomes. Furthermore, when rat circumvallate (CV) taste buds were incubated withthe above tastants for 30 s, their intracellular concentrations increased by 3.5- to 7-fold relative to their extracellularconcentrations. The time course of this dramatic accumulation was alsomonitored in situ in rat single CV taste buds under a confocallaser-scanning microscope. Tastants were clearly localized to the tastecell cytosol. It is proposed that, due to their rapid permeation into taste cells, these amphipathic tastants may be available for activation of signal transduction components (e.g., G proteins) directly withinthe time course of taste sensation. Such activation may occur inaddition to the action of these tastants on putative G protein-coupledreceptors. This phenomenon may be related to the slow taste onset andlingering aftertaste typically produced by many bitter tastants andnon-sugar sweeteners.

     

Taste Responses in the Nucleus of the Solitary Tract in Saccharin-preferring and Saccharin-averse Rats

A minority of rats consistently reject the taste of sodium saccharinat concentrations that the majority find palatable. We choserats that selected either water (WP), or 0.03 M NaSaccharin(SP) in two-bottle preference tests and monitored single unitresponses to a range of taste qualities in the nucleus of thesolitary tract. WP rats gave significantly greater responsesto Na/Li salts and QHCI. Their responses to sugars were equalto those from SP rats. Total activity to NaSaccharin did notdiffer between the two groups, but its distribution across thethree identified neuron types did. The response was skewed fromone in which sugar (S) and sodium salt (N) participated nearlyequally (SP) to one dominated by the activity of N cells andnearly devoid of an S cell contribution (WP rats). Accordingly,the response profile for NaSaccharin was correlated nearly aswell with those of the sugars (+ 0.60) as with the Na/Li salts(+ 0.73) in SP rats, but was reshaped in WP rats to be nearlyidentical with those of the salts (+ 0.85) and unlike sugars(+ 0.30). In their heightened sensitivity to stimuli that humanscall salty and bitter, and in their rejection of the complextaste of NaSaccharin, WP rats showed many of the characteristicsof human tasters of PTC/PROP. Chem. Senses 21: 147–157,1996.     

The influence of taste on swallowing apnea, oral preparation time, and duration and amplitude of submental muscle contraction

Prior research has documented a modulating effect of taste on swallowing. We hypothesized that presentation of tastant stimuli would be a significant variable in swallowing-respiratory coordination, duration of oral bolus preparation, and submental muscle contraction. Twenty-three healthy females were presented with 1-cm(3) gelatin samples flavored with 4 tastants of increasing intensities. Visual analogue scale ratings of perceived intensity of each were used to identify relative equivalent concentrations across the 4 tastants. Data were then collected during ingestion of 5 trials of the 4 equivalent tastants using measurements of nasal airflow and submental surface electromyography (sEMG) to record biomechanical measures. Chi-square analysis failed to identify a statistically significant influence of taste on the phase location of swallowing apnea. Repeated measures analysis of variance demonstrated significant taste effects for oral preparation time, submental sEMG amplitude, and duration (P < 0.02). Sweet tastants were prepared for a shorter time when compared with bitter tastants. Swallow duration for sour, salty, and bitter tastants were longer than sweet and neutral tastants. Sour tastants resulted in the greatest amplitude of submental muscle contraction during swallowing. This study supports existing research that found that sour substances were swallowed with more effort when compared with other tastes.     

Quantitative Studies of Stimulus Coding in First-Order Vibrissa Afferents of Rats. 1. Receptive Field Properties and Threshold Distributions

We examined stimulus-response relationships of vibrissa-activated mechanosensory neurons of the rat's fifth (trigeminal) ganglion. Single-unit activity was recorded with tungsten microelectrodes. The vibrissae were deflected with a variety of parametrically controlled stimulus waveforms.

We found that the receptive field of each vibrissa-activated neuron consisted of a single vibrissa. Few, if any, unambiguous examples of spontaneous activity were observed in these neurons. Even if true spontaneous activity was present, its observed incidence was low, as were the measured discharge rates.

Thresholds of individual neurons were usually quite discrete; often a 1-2% increase in pulse magnitude (angular displacement) above a level to which the neuron did not respond caused it to discharge on every trial. The distribution of thresholds for the sample was continuous with a median of about 1° and a range of over three orders of magnitude. The most sensitive neurons responded to deflections of less than 0.1°. Many neurons responded to a single suprathreshold pulse with more than one spike. We found no consistent relationships among the thresholds of the additional evoked discharges of an individual neuron other than that the total number of evoked spikes either increased or stayed the same, but never decreased, as stimulus magnitude increased.

About one-third of the neurons examined had velocity thresholds below 3°/sec. Above that value, thresholds were distributed continuously throughout a range of over three orders of magnitude. The median velocity threshold was about 100°/sec. The broad and continuous distributions of both magnitude and velocity thresholds suggest that a population of vibrissa-activated neurons can code stimulus strength smoothly and continuously over a wide range, even though individual neurons may be poorly suited to do so.     

Role of microstructure of nerve impulse train in relation to transmission of neuronal activity and coding mechanism of neural information

Input-output relation were of giant neurons of a marine mollusc, Onchidium verruculatum, and a computer-simulated neuron investigated in terms of microstructure of nerve impulse train. The microstructure of input impulse train, the size of a unitary EPSP, and the extent of spontaneous firing activity of a single neuron had an important influence upon the effective summation of arriving synaptic inputs, the elicitation of output spikes, and intervals between succeeding output spikes. The neuron responded differently to respective input trains with different time structures, i.e. it discriminated input time pattern to various degrees. The manner in discrimination of input time pattern was dependent on the size of the unitary EPSP and the extent of the spontaneous firing activity, if it had. Some discussions were made with regard to possible coding systems of neural signal, assuming a frequency code and/or a pattern code.     

Changes in Taste Perception Following Mental or Physical Stress

Taste perception depends not only on the chemical and physicalproperties of tastants, but may also depend on the physiologicaland psychological conditions of those who do the tasting. Inthis study, the effects of mood state on taste sensitive wasevaluated in humans who were exposed to conditions of mentalor physical fatigue and tension. Taste responses to quininesulfate (bitter), citric acid (sour) and sucrose (sweet) weretested. The intensity of the taste sensations were recordedby a computerized time-intensity (TI) on-line system. Subjectsperformed mental tasks by personal computer or physical tasksby ergometer for 10–40 min. Before and after these sessions,the duration of the after-taste and the intensity of the sensationof taste were recorded by the TI system, and in addition, psychologicalmood states were evaluated with POMS (Profile of Mood State).TI evaluation showed that after the mental tasks, the perceivedduration of bitter, sour and sweet taste sensations was shortenedrelative to the control. Total amount of bitterness, sournessand sweetness was also significantly reduced. Furthermore, themaximum intensity of bitterness was significantly reduced. Therewere no significant differences in bitterness and sweetnesssensations following physical tasks. However, relative to beforethe physical task, the duration of the after-taste of sournesswas significantly shortened by the physical task. After thephysical task, the buffering capacity of saliva was significantlyincreased. Thus mental and physical tasks alter taste perceptionin different ways; the mechanisms underlying these changes remainto be determined. Chem. Senses 21: 195–200, 1996.     

Taste perception and coding in Drosophila

BACKGROUND: Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior. RESULTS: We have characterized in detail the expression of eight Gr genes in the labial palps, the fly's main taste organ. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities-the perception of bitter (caffeine) and sweet (trehalose) taste, respectively. CONCLUSION: Discrimination between two taste modalities-sweet and bitter-requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons.