Taste transduction mechanism: similar effects of various modifications of gustatory receptors on neural responses to chemical and electrical stimulation in the frog

Responses in the frog glossopharyngeal nerve induced by electrical stimulation of the tongue were compared with those induced by chemical stimuli under various conditions. (a) Anodal stimulation induced much larger responses than cathodal stimulation, and anodal stimulation of the tongue adapted to 5 mM MgCl2 produced much larger responses than stimulation with the tongue adapted to 10 mM NaCl at equal current intensities, as chemical stimulation with MgCl2 produced much larger responses than stimulation with NaCl at equal concentration. (b) The enhansive and suppressive effects of 8-anilino-1-naphthalenesulfonate, NiCl2, and uranyl acetate on the responses to anodal current were similar to those on the responses to chemical stimulation. (c) Anodal stimulation of the tongue adapted to 50 mM CaCl2 resulted in a large response, whereas application of 1 M CaCl2 to the tongue adapted to 50 mM CaCl2 produced only a small response. This, together with theoretical considerations, suggested that the accumulation of salts on the tongue surface is not the cause of the generation of the response to anodal current. (d) Cathodal current suppressed the responses induced by 1 mM CaCl2, 0.3 M ethanol, and distilled water. (e) The addition of EGTA or Ca-channel blockers (CdCl2 and verapamil) to the perfusing solution of the lingual artery reversibly suppressed both the responses to chemical stimulus (NaCl) and to anodal current with 10 mM NaCl. (f) We assume from the results obtained that electrical current from the microvillus membrane of a taste cell to the synaptic area supplied by anodal stimulation or induced by chemical stimulation activates the voltage-dependent Ca channel at the synaptic area.     

Responsiveness of neurons in the hamster parabrachial nuclei to taste mixtures

Responses from hamster parabrachial nuclei neurons to stimulation of the anterior tongue with sucrose, NaCl, HCl, quinine hydrochloride, and the six two-component mixtures of these stimuli were recorded. A cell's response to a mixture approached its response to the mixture's more effective component in the majority of cases, but was sometimes greater or smaller than this response. The best predictor of a neuron's response to a mixture, then, was its response to the mixture's more effective component. The single-component stimulus producing the maximum response was determined for each neuron and the response to this stimulus was compared with the responses evoked by the six mixtures. For 30% of the cells, a mixture elicited a response reliably, but only 1.1-2.1 times greater than the response to the best single-component stimulus. Thus, there were no neurons specialized to respond to these mixtures. The across-neuron patterns elicited by mixtures and the responses of best-stimulus classes to mixtures were studied for comparison with psychophysical data on taste mixtures. Mixtures were usually correlated with single-component stimuli in the mixture, but not with stimuli not in the mixture. In fact, five of the six mixtures fell directly between their components in a multidimensional scaling plot. In addition, a mixture was most effective in stimulating only those classes of neurons maximally stimulated by the mixture's components. These results correlate with psychophysical data suggesting that mixtures of taste stimuli evoke the same taste qualities as evoked by the mixture's components.     

A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex

The effects of different orientations of a Cadwell round magnetic coil (MC) were compared with each other and with surface electrical stimulation of motor cortex in monkeys anesthetized with pentobarbital or urethane. Recordings were made from within the lateral corticospinal tract, either from axonal populations or with a microelectrode from individual axons. A lateral-sagittally orientated MC directly excited corticospinal neurons at lower stimulus intensity than was required for indirect, i.e., transsynaptic excitation via inputs to corticospinal neurons. By contrast, in 2 out of 3 macaques tested, a vertex-tangential orientation could excite corticospinal neurons indirectly at lower intensities than were required for direct excitation; at higher intensities, direct excitation also occurred.The site of direct corticospinal excitation by a lateral-sagitally oriented MC was inferred by comparing the response variability and latency to MC and surface electrical stimuli. Cathodal stimuli elicited more variable corticospinal population responses and later individual axonal responses than were obtained with anodal stimuli. The variability in response is attributed to interaction between nearby, on-going synaptic bombardment and the stimulus, implying that surface cathodal stimuli directly activate corticospinal neurons at the spike trigger zone (presumably the initial segment). By contrast, the consistency and reduced latency of the corticospinal responses to surface anodal stimuli are attributed to the direct excitation of corticospinal fibers within the white matter. When the stimulus intensity is clearly above threshold, surface anodal and cathodal stimuli can activate corticospinal neurons both directly and indirectly.Direct corticospinal excitation by the MC can resemble the effects of either surface anodal or surface cathodal stimuli. We conclude that the MC can activate corticospinal neurons at the spike trigger zone or their fibers deeper in white matter. The findings in the monkey are used to interpret the effects of different MC orientations in the human.     

Virtual electrodes in cardiac tissue: a common mechanism for anodal and cathodal stimulation.

Traditional cable analyses cannot explain complex patterns of excitation in cardiac tissue with unipolar, extracellular anodal, or cathodal stimuli. Epifluorescence imaging of the transmembrane potential during and after stimulation of both refractory and excitable tissue shows distinctive regions of simultaneous depolarization and hyperpolarization during stimulation that act as virtual cathodes and anodes. The results confirm bidomain model predictions that the onset (make) of a stimulus induces propagation from the virtual cathode, whereas stimulus termination (break) induces it from the virtual anode. In make stimulation, the virtual anode can delay activation of the underlying tissue, whereas in break stimulation this occurs under the virtual cathode. Thus make and break stimulations in cardiac tissue have a common mechanism that is the result of differences in the electrical anisotropy of the intracellular and extracellular spaces and provides clear proof of the validity of the bidomain model.     

Are Participants Aware of the Type and Intensity of Transcranial Direct Current Stimulation?

Transcranial direct current stimulation (tDCS) is commonly used to alter cortical excitability but no experimental study has yet determined whether human participants are able to distinguish between the different types (anodal, cathodal, and sham) of stimulation. If they can then they are not blind to experimental conditions. We determined whether participants could identify different types of stimulation (anodal, cathodal, and sham) and current strengths after experiencing the sensations of stimulation during current onset and offset (which are associated with the most intense sensations) in Experiment 1 and also with a prolonged period of stimulation in Experiment 2. We first familiarized participants with anodal, cathodal, and sham stimulation at both 1 and 2 mA over either primary motor or visual cortex while their sensitivity to small changes in visual stimuli was assessed. The different stimulation types were then applied for a short (Experiment 1) or extended (Experiment 2) period with participants indicating the type and strength of the stimulation on the basis of the evoked sensations. Participants were able to identify the intensity of stimulation with shorter, but not longer periods, of stimulation at better than chance levels but identification of the different stimulation types was at chance levels. This result suggests that even after exposing participants to stimulation, and ensuring they are fully aware of the existence of a sham condition, they are unable to identify the type of stimulation from transient changes in stimulation intensity or from more prolonged stimulation. Thus participants are able to identify intensity of stimulation but not the type of stimulation.     

Examination of stimulation mechanism and strength-interval curve in cardiac tissue

Understanding the basic mechanisms of excitability through the cardiac cycle is critical to both the development of new implantable cardiac stimulators and improvement of the pacing protocol. Although numerous works have examined excitability in different phases of the cardiac cycle, no systematic experimental research has been conducted to elucidate the correlation among the virtual electrode polarization pattern, stimulation mechanism, and excitability under unipolar cathodal and anodal stimulation. We used a high-resolution imaging system to study the spatial and temporal stimulation patterns in 20 Langendorff-perfused rabbit hearts. The potential-sensitive dye di-4-ANEPPS was utilized to record the electrical activity using epifluorescence. We delivered S1-S2 unipolar point stimuli with durations of 2-20 ms. The anodal S-I curves displayed a more complex shape in comparison with the cathodal curves. The descent from refractoriness for anodal stimulation was extremely steep, and a local minimum was clearly observed. The subsequent ascending limb had either a dome-shaped maximum or was flattened, appearing as a plateau. The cathodal S-I curves were smoother, closer to a hyperbolic shape. The transition of the stimulation mechanism from break to make always coincided with the final descending phase of both anodal and cathodal S-I curves. The transition is attributed to the bidomain properties of cardiac tissue. The effective refractory period was longer when negative stimuli were delivered than for positive stimulation. Our spatial and temporal analyses of the stimulation patterns near refractoriness show always an excitation mechanism mediated by damped wave propagation after S2 termination.     

Improving Cycling Performance: Transcranial Direct Current Stimulation Increases Time to Exhaustion in Cycling

The central nervous system seems to have an important role in fatigue and exercise tolerance. Novel noninvasive techniques of neuromodulation can provide insights on the relationship between brain function and exercise performance. The purpose of this study was to determine the effects of transcranial direct current stimulation (tDCS) on physical performance and physiological and perceptual variables with regard to fatigue and exercise tolerance. Eleven physically active subjects participated in an incremental test on a cycle simulator to define peak power output. During 3 visits, the subjects experienced 3 stimulation conditions (anodal, cathodal, or sham tDCS—with an interval of at least 48 h between conditions) in a randomized, counterbalanced order to measure the effects of tDCS on time to exhaustion at 80% of peak power. Stimulation was administered before each test over 13 min at a current intensity of 2.0 mA. In each session, the Brunel Mood State questionnaire was given twice: after stimulation and after the time-to-exhaustion test. Further, during the tests, the electromyographic activity of the vastus lateralis and rectus femoris muscles, perceived exertion, and heart rate were recorded. RM-ANOVA showed that the subjects performed better during anodal primary motor cortex stimulation (491 ± 100 s) compared with cathodal stimulation (443 ± 11 s) and sham (407 ± 69 s). No significant difference was observed between the cathodal and sham conditions. The effect sizes confirmed the greater effect of anodal M1 tDCS (anodal x cathodal = 0.47; anodal x sham = 0.77; and cathodal x sham = 0.29). Magnitude-based inference suggested the anodal condition to be positive versus the cathodal and sham conditions. There were no differences among the three stimulation conditions in RPE (p = 0.07) or heart rate (p = 0.73). However, as hypothesized, RM- ANOVA revealed a main effect of time for the two variables (RPE and HR: p < 0.001). EMG activity also did not differ during the test accross the different conditions. We conclude that anodal tDCS increases exercise tolerance in a cycling-based, constant-load exercise test, performed at 80% of peak power. Performance was enhanced in the absence of changes in physiological and perceptual variables.     

Selective procaine inhibition of rat chorda tympani responses to electric taste stimulation

1. The lingual treatment of 1% procaine for 10 min selectively suppressed responses of the rat chorda tympani nerve to anodal current applied to the tongue with NaCl in the bathing medium to about 50% of control but the drug produced no significant suppression in responses to chemical taste stimuli. 2. The magnitude of suppression of response to anodal current varied with concentration of procaine and kind of bathing medium for the current stimulation (larger in the order of NaCl greater than KCl greater than CaCl2 greater than HCl). 3. Such ion specificity in procaine suppression suggests that responses of the chorda tympani nerve to anodal current are provoked through the taste cell (not direct action on the taste nerve), and that the receptor mechanisms for anodal current are at least partly different from that for chemical taste stimuli.     

Neurophysiological and biophysical evidence on the mechanism of electric taste

The phenomenon of electric taste was investigated by recording from the chorda tympani nerve of the rat in response to both electrical and chemical stimulations of the tongue with electrolytes in order to gain some insight into its mechanism on both a neurophysiological and biophysical basis. The maximum neural response levels were identical for an individual salt (LiCl, NaCl, KCl, or CaCl2), whether it was presented as a chemical solution or as an anodal stimulus through a subthreshold solution. These observations support the idea that stimulation occurs by iontophoresis of ions to the receptors at these current densities (less than 100 microA/cm2). Electric responses through dilute HCl were smaller than the chemically applied stimulations, but the integrated anodal responses appeared similar to chemical acid responses, as evidenced by an OFF response to both forms of stimuli. Hydrogen may be more permeant to the lingual epithelium and would thus be shunted away from the taste receptors during anodal stimulation. When the anion of electric taste was varied via subthreshold salt solutions, the response magnitude increased as the mobility of the anion decreased. The transport numbers of the salts involved adequately explains these differences. The physical aspects of ion migration occurring within the adapting fluid on the tongue are also discussed. Direct neural stimulation by the current appears to occur only at higher current densities (greater than 300 microA/cm2). If the taste cells of the tongue were inactivated with either iodoacetic acid (IAA) or N-ethyl maleimide (NEM), or removed with collagenase, then responses from the chorda tympani could be obtained only at these higher current densities. Latency measurements before and after IAA or NEM treatment corroborated these findings. The results are discussed in terms of several proposed mechanisms of electric taste and it is concluded that an ion accumulation mechanism can adequately explain the data.     

Taste qualities of solutions preferred by hamsters

Molecules of diverse chemical structure are sweet to humans and several lines of evidence (genetic, physiological, behavioral) suggest that there may be distinct sweet perceptual qualities. To address how many perceptual categories these molecules elicit in hamsters (Mesocricetus auratus), we studied patterns of generalization of conditioned taste aversions for seven sweeteners: 100 mM sucrose, 320 mM maltose, 32 mM D-phenylalanine, 3.2 mM sodium saccharin, 16 mM calcium cyclamate, 10 mM dulcin and 32 mM sodium m-nitrobenzene sulfonate. Each stimulus was preferred versus water in two-bottle intake tests and stimulated the chorda tympani nerve. For each of seven experimental groups the conditional stimulus (CS) was a sweetener and for the control group the CS was water. Apomorphine.HCl was injected i.p. after a CS was sampled and, after recovery, test stimuli (TS) were presented for 1 h daily. The intake (ml) of each TS consumed by experimental animals was compared with mean TS intake by the control group. Learned aversions for 18/21 stimulus pairs cross-generalized, resulting in a single cluster of generalization patterns for the seven stimuli. Cross-generalization failures (maltose-cyclamate, maltose-sucrose, cyclamate-NaNBS) may be the consequence of particular stimulus features (e.g. salience, cation taste), rather than the absence of a 'sucrose-like' quality. The results are consistent with a single hamster perceptual quality for a diverse set of chemical structures that are sweet to humans.     

Taste preference synergy between glutamate receptor agonists and inosine monophosphate in rats

Monosodium glutamate (MSG) elicits a taste called umami and interacts synergistically with nucleotide monophosphates such as 5'-inosine monophosphate (IMP) to potentiate this taste intensity. Indeed, the synergistic interaction of nucleotide monophosphates and MSG is a hallmark of umami. We examined interactions between MSG and other taste stimuli, including IMP, by measuring the lick rates of non-deprived rats during 30 s trials. To control for non-linear psychophysical functions, the concentration of one taste stimulus in a binary mixture was systematically increased while the concentration of the second taste stimulus was decreased (stimulus substitution method). Synergy between two stimuli was detected if the lick rate for a binary mixture exceeded that expected from the sum of the lick rates for each stimulus alone. In initial experiments, taste synergy was observed when rats were presented with mixtures of MSG and IMP but not with mixtures of MSG and sucrose. In subsequent experiments, glutamate receptor agonists other than MSG were presented with IMP to test for taste synergy. No evidence of synergy was seen when rats were presented with mixtures of IMP and kainic acid or IMP and N:-methyl-D-aspartate. However, taste synergy between IMP and L-AP4, a potent agonist at mGluR4 receptors, was observed. These results suggest that a metabotropic glutamate receptor similar to mGluR4 may be involved in the taste synergy that characterizes umami.     

Functional features of electroreceptors (small pit organs) in the catfish

Characteristics of responses of the small pit organs of the catfishIctalurus nebulosus to the action of electrical stimuli of varied polarity, intensity, and duration were studied. Single fibers of the lateral nerve innervating these organs possessed regular spontaneous activity with a frequency of 35–45/sec or grouped activity, coinciding with the rhythm of the animal's swimming movements. Threshold current densities varied from 10⁻¹¹ to 10⁻¹⁰ A/mm². Electrical stimuli evoked a phasic-tonic response of the receptor. The latent period was 10–50 msec for on-responses and 10–200 msec for off-responses. In the presence of strong electric fields the receptor responded to a cathodal stimulus by excitation, whereas under ordinary experimental conditions an anodal stimulus is excitatory. The properties of small pit organs are compared with the characteristics of other electroreceptors.     

Sensitivities of single nerve fibers in the hamster chorda tympani to mixtures of taste stimuli

Responses of three groups of neural fibers from the chorda tympani of the hamster to binary mixtures of taste stimuli applied to the tongue were analyzed. The groups displayed different sensitivities to six chemicals at concentrations that had approximately equal effects on the whole nerve. Sucrose-best fibers responded strongly only to sucrose and D-phenylalanine. NaCl-best and HCl-best fibers, responded to four electrolytes: equally to CaCl2 and nearly equally to HCl, but the former responded more to NaCl, and the latter responded more to NH4Cl. The groups of fibers dealt differently with binary mixtures. Sucrose- best fibers responded to a mixture of sucrose and D-phenylalanine as if one of the chemicals had been appropriately increased in concentration, but they responded to a mixture of either one and an electrolyte as if the concentration of sucrose or D-phenylalanine had been reduced. NaCl- best fibers responded to a mixture as if it were a "mixture" of two appropriate concentrations of one chemical, or somewhat less. But, responses of HCl-best fibers to mixtures were greater than that, approaching a sum of responses to components. These results explain effects on the whole nerve, suggest that the sensitivity of a mammalian taste receptor to one chemical can be affected by a second, which may or may not be a stimulus for that receptor, and suggest that some effects of taste mixtures in humans may be the result of peripheral processes.     

Neural coding of gustatory information.

The nervous system encodes information relating chemical stimuli to taste perception, beginning with transduction mechanisms at the receptor and ending in the representation of stimulus attributes by the activity of neurons in the brain. Recent studies have rekindled the long-standing debate about whether taste information is coded by the pattern of activity across afferent neurons or by specifically tuned 'labeled lines'. Taste neurons are broadly tuned to stimuli representing different qualities and are also responsive to stimulus intensity and often to touch and temperature. Their responsiveness is also modulated by a number of physiological factors. In addition to representing stimulus quality and intensity, activity in taste neurons must code information about the hedonic value of gustatory stimuli. These considerations suggest that individual gustatory neurons contribute to the coding of more than one stimulus parameter, making the response of any one cell meaningful only in the context of the activity of its neighbors.     

Ensemble cortical responses to rival visual stimuli: Effect of monocular transient

Binocular rivalry is a fascinating perceptual phenomenon that has been characterized extensively at the psychophysical level. However, the underlying neural mechanism remains poorly understood. In particular, the role of the early visual pathway remains controversial. In this study, we used voltage-sensitive dye imaging to measure the spatiotemporal activity patterns in cat area 18 evoked by dichoptic orthogonal grating stimuli. We found that after several seconds of monocular stimulation with an oriented grating, an orthogonal stimulus to the other eye evoked a reversal of the cortical response pattern, which may contribute to flash suppression in perception. Furthermore, after several seconds of rival binocular stimulation with unequal contrasts, transient increase in the contrast of the weak stimulus evoked a long-lasting cortical response. This transient-triggered response could contribute to the perceptual switch during binocular rivalry. Together, these results point to a significant contribution of early visual cortex to transient-triggered switch in perceptual dominance.     

Taste Responses to Binary Mixtures of Amino Acids in the sea catfish, Arius felis

In vivo electrophysiological recordings in the sea catfish,Arius felis, showed that the magnitude of the integrated facialtaste responses to binary mixtures of amino acids was predictablewith knowledge obtained from previous cross-adaptation studiesof the relative independence of the respective binding sitesof the component stimuli. Each component from which equal aliquotswere drawn to form the mixtures was adjusted in concentrationto provide for approximately equal reponse magnitudes. The magnitudeof the taste responses to binary mixtures whose component aminoacids showed minimal cross-adaptation was significantly greaterthan that to binary mixtures whose components exhibited considerablecross-reactivity. There was no evidence for mixture suppression.The relative magnitude of the taste responses in the sea catfishto stimulus mixtures is similar to that previously reportedfor olfactory receptor responses in the freshwater channel catfishand chorda tympani taste responses in the hamster. Chem. Senses21: 45–53, 1996.     

Paced monophasic and biphasic waveforms alter transmembrane potentials and metabolism of human fibroblasts

Resting transmembrane potential (TMP) of primary human fibroblast cells was altered in predictable directions by subjecting cell cultures to specific monophasic and biphasic waveforms. Cells electrically stimulated with an anodal pulse resulted in hyperpolarization while a cathodal waveform depolarized the TMP to below that of non-paced control cells. The biphasic waveform, consisting of an anodal pulse followed immediately by an inverse symmetric cathodal pulse, also lessened the TMP similar to that of the cathodal pulse. The effect of short-term pacing on the TMP can last up to 4 h before the potentials equilibrate back to baseline. While subjecting the cells to this electrical field stimulation did not appear to damage the integrity of the cells, the three paced electrical stimulation waves inhibited expansion of the cultures when compared to non-paced control cells. With longer pacing treatments, elongation of the cells and electrotaxis towards the anodal polarity were observed. Pacing the fibroblasts also resulted in modest, yet very statistically significant (and likely underestimated) changes to cellular adenosine-5'-triphosphate (ATP) levels, and cells undergoing anodal and biphasic (anodal/cathodal) stimulation also exhibited altered mitochondrial morphology. These observations indicate an active role of electrical currents, especially with anodal content, in affecting cellular metabolism and function, and help explain accumulating evidence of cellular alterations and clinical outcomes in pacing of the heart and other tissues in general.