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Effects of cathodal current, which draws cations away from the tongue and drives anions toward the tongue, depend on the ionic content of electrolytes through which the current is passed. To address the role of cations and anions in human salt tastes, cathodal currents of −40 to −80 µA were applied to human subjects’ tongues through supra-threshold salt solutions. The salts were sodium chloride, sodium bromide, potassium chloride, ammonium chloride, calcium chloride, sodium nitrate, sodium sulfate, sodium saccharin, sodium acetate and sodium benzoate, which taken together encompass salty, bitter, sour and sweet taste qualities. The taste of NaCl, the salty and bitter tastes of the other chloride salts and the taste of NaNO3 were inhibited, suggesting the current displaced stimulatory cations from salty and bitter receptors. However, bitter tastes of non-halide sodium salts were not inhibited, likely because other bitter receptors respond to anions. A discharge current at cathode-off ubiquitously evoked a metallic taste reminiscent of anodal taste used in clinical electrogustometry. Analogous effects on ambient NaCl responses were recorded from the hamster chorda tympani nerve. Increases in tastes of the saccharin and benzoate anions were not evoked during current flow, suggesting that cathodal current does not carry stimulatory anions to sweet receptors. Cathodal current may selectively inhibit salty and bitter-salty tastes for which proximal stimuli are cations.
Although there is general acceptance of Tas2R and Tas1R G-protein coupled receptors for bitter and sweet tastes in humans and other mammals [1, 2, 3, 4], a consensus has not emerged for receptors involved in human ionic tastes. Acid taste is associated with intracellular acidification and transient receptor (TRP) channel PDK2 [5, 6, 7]; but, species diversity has made the human salty and bitter tastes of salts more difficult to study. Rodent salt tastes include a prominent, specific Na+–Li+ component that depends on a functional epithelial Na+ channel, ENaC [8, 9]; but, ENaC plays little role in human salty taste transduction [10, 11, 12]. Moreover, rodent “bitter” contains perceptually distinct taste qualities, one for ionic stimuli that are bitter to humans and others for non-ionic stimuli that are also bitter to humans [13, 14].
There are no human salty-taste inhibitors other than chlorhexidine, a substantive bitter bis-biguanide that may disrupt lingual epithelial ion flow [15, 16, 17]. Another way to disrupt ion flow is to apply electric currents to the tongue. Perceptions evoked by weak electric currents, “electric tastes” , depend on the direction of ion flow; that is, whether the anode (+) or cathode (−) contacts the tongue. Two questions still arise in the electric-taste literature. One is whether it is the current or electrolysis products, acids and bases, at anode and cathode (respectively) that are the actual electric-taste stimuli . The other is whether the electrically evoked perception is a pure taste or combined activation of sensory nerves for taste (e.g., chorda tympani) and other sensory modalities (e.g., trigeminal) .
Localized clinical testing is conveniently done with the anode [21, 22]. Short pulses of weak anodal current evoke a sour-metallic, battery taste whether current is applied through metal electrodes  or passed through a salty NaCl bridging solution [24, 25]. Bridging solutions between electrode and tongue prevent accumulation of H+ on the tongue surface, and revealed that current flow, not electrolysis products, generates battery taste. However, the ion non-specificity and ubiquitous battery taste quality of anodal current suggest that the current either non-specifically activates multiple receptors-channels and/or directly activates taste nerve endings.
Weak anodal current may stimulate the trigeminal nerve [20, 26], which richly innervates taste-bud containing fungiform papillae on the tip of the tongue [27, 28]; and a brief, anode-like, metallic taste appears at cathode-off when the current is only slightly higher . The reaction to cathode-off is likely generated by a brief “anodal” capacitive discharge current . The cathode itself evokes tastes only at much higher currents, making trigeminal involvement even more of a concern . Remarkably, the possibility that weak cathodal currents would inhibit ongoing tastes of salt solutions was overlooked. Here, we present the results of three human studies on the ion-specificity of cathodal inhibition of salts with salty, sour, bitter and sweet taste qualities [30, 31] to identify cation-dependent human tastes.
We also present, for comparison, the effects of weak electric currents applied through an ambient stimulating NaCl solution on neural activity of the hamster chorda tympani nerve. Rodent chorda tympani nerves contain mostly taste fibers and neural activity may be affected by application of current through sub-threshold bridging solutions; invariably, after release of cathodal current, an excitatory neural burst occurs [32, 33, 34, 35].
Thirty-four subjects, 12 women and 22 men, reporting normal taste function and mean age of 26 (S.D. 6) years, volunteered to participate. Subjects were recruited from students and staff of the University of Connecticut Health Center. The research protocol was approved by the Institutional Review Board of the University of Connecticut Health Center. Subjects gave informed consent.
Stimulus chambers were used to contain ionic solutions in three experiments (A, B and C). Experiment A addressed cation specificity by having subjects simultaneously rate total intensity and identify NaCl, KCl, NH4Cl, and CaCl2 quality (Table 1). Experiments B and C addressed anion specificity with different psychophysical methods. In experiment B, subjects separately rated salty, sour, sweet and bitter intensities of NaCl, NaNO3, Na+-acetate and Na+-saccharin (Table 2). In Experiment C, subjects separately rated total intensity and identified 0.5 M NaCl, NaBr, Na2SO4 and Na+ benzoate quality (Table 3).
A modified, portable, battery-operated electrogustometer (RION TR-06) provided 2-seconds of direct electrical current of −40 µA and −80 µA. The two currents were used in Experiment A, −80 µA was used in Experiment B and −40 µA was used in Experiment C. Experiment A showed that the 2 currents had indistinguishable effects. The −80 µA was used in Experiment B to compensate for lower current density, given the larger area stimulated [21, 36].
In Experiments A and C, salt solutions were presented in a 5 mL plastic tube with a 12-mm diameter opening to10 subjects (mean age: 26.4 yrs, 7 men and 3 women) and 12 subjects (mean age: 27.3 yrs, 6 men and 6 women), respectively. In Experiment B, salt solutions were presented in a 28 mL plastic cup with a 4-cm diameter opening to 12 subjects (mean age: 24.1 yrs, 9 men and 3 women). The cathode contacted solutions via a steel screw inserted through the bottom of the vessels. The anode was held in the hand or skin-tact® to the wrist. The electric circuit was completed when the subject’s tongue tip was placed into the solution. On each trial, subjects made 3 judgments within a short time––before, when the tongue tip was inserted into the solution, during the 2-second current application, and immediately after current termination. The experimenter vocally specified the time point for each decision. Stimuli and current levels were presented in random order in each session.
Experiment A, Chloride Salts. Without training, subjects simultaneously rated total intensity and identified quality [Figure 1] on each trial. Subjects, instructed to choose all appropriate qualities on the list, selected one quality 74%, two qualities 21% and three qualities 5% of the time. Two replicates for two salts were obtained in one session and 2 replicates for the other two salts in a second session. Subjects indicated choices and the experimenter recorded responses at the end of each trial.
Experiment B, Sodium Salts. After presentation of a reference solution (0.1 M NaCl for salty, 3.2 mM citric acid for sour, 0.1 M sucrose for sweet and 0.1 mM quinine·HCl for bitter), subjects rated salty, sour, sweet and bitter intensities separately for all four salts [Table 2] in each of 2 sessions. Subjects indicated choices and the experimenter recorded responses at the end of a trial.
Experiment C, 0.5 M Sodium Salts. After training with 0.3 M and 2.5 mM NaCl, subjects rated 2 replicates of total intensity [Figure 1] in one session. After presentation of reference solutions (0.3 M NaCl for salty, 10 mM citric acid for sour, 0.5 M sucrose for sweet and 1 mM quinine·HCl for bitter), subjects separately identified 2 replicates of each quality [Figure 1 list minus burn] in a second session. Subjects, instructed to choose all appropriate qualities, selected one quality 89%, two qualities 10% and three qualities 1% of the time. Subjects recorded choices on data sheets during a trial.
For each experiment, intensity data were analyzed with repeated-measures analysis of variance (ANOVA) and, if appropriate, followed by ANOVA for data subsets. For Experiment A––time, current, compound, concentration and replicate were the 5 factors. For Experiment B––time, current, compound, quality and replicate were the 5 factors. For Experiment C––time, current, compound, and replicate were the 4 factors. A significant interaction between time and current revealed that intensity ratings depended on current. A significant interaction between the factors of time, current and compound (or quality) revealed that the 4 compounds (or qualities) were affected differently by current. Post-hoc comparisons, α = 0.05, used Neuman-Keuls (NK) tests.
Quality identities during and after current were compared to the same control time epochs; identities during and after current were also compared. Numbers of subjects who identified each individual quality (whether alone or in a doublet or triplet) for each stimulus replicate were counted. The salty identity was expected to decrease and, correspondingly, no-taste expected to increase during current. Metallic was expected to increase after cathode-off. Salty, expected to decrease during current, was expected to rebound with current release. Average numbers of subjects identifying qualities chosen by > 10% of the subjects were compared with within-subjects t tests. The α level, 0.05, was adjusted for multiple comparisons with a Bonferroni correction .
Using an animal protocol approved by the Animal Care Committee of the University of Connecticut Health Center, electrophysiological recordings were obtained from the whole chorda tympani nerve (CT) of the golden hamster (Mesocricetus auratus) . Male hamsters (>100 grams) obtained from Charles River Laboratories (Wilmington MA) were deeply anesthetized with sodium pentobarbital (7.5 mg/100 g, initially, plus 25% supplements). The nerve, approached from beneath the mandible, was dissected free and looped over a nichrome-wire recording electrode positioned above musculature contacted by the reference electrode. Rectified, squared, integrated records of differentially amplified neural responses were displayed on a strip chart recorder. Solutions were delivered to the anterior portion of the tongue, enclosed in a glass chamber, from an overhead funnel at ~1.5 mL/sec. Constant current was applied across a nichrome-wire electrode inserted into the chamber, above but not in contact with the tongue, and a stainless steel pin inserted into rear leg musculature . Water and 0.1 M NaCl, a bridging solution, were alternately flowed through the tongue chamber, covering tongue and electrode. Five-second epochs of anodal (+9 µA) or cathodal (−9 µA) current were applied during ongoing NaCl-evoked CT neural activity, which has an initial transient and, notably, a lengthy tonic component in rodents .
Before current was applied, high concentrations of the four chloride salts had a medium average intensity of 5.8 ±.5 and low concentrations had a weak average intensity of 2.6 ±.5, F (1,9) = 159.4, p < .000001 (Figure 1, Table 1). As the initial 5-way ANOVA revealed no difference between −40µA and −80µA, data for the two currents were combined. Two 4-way ANOVA were used to separately evaluate data for high and low stimulus concentrations because of the interaction between concentration, compound, time and current (F (6, 54) = 3.98, p = .002).
Although intensities differed before current application (Table 1), F(3, 27) = 4.84, p = .008 (0-sec time points), cathodal current similarly affected the intensity of high concentrations of the 4 salts (F(2,18) = 35.08, p =.000001). During current (2-sec time points), average intensity ratings decreased from a control value of 4.6 (gray line) to 3.3 (p =.0002) and, after current was released (4-sec time points), average intensity increased above the control of 5.0 to 5.4 (p =.01).
Intensities differed before current application, F(3, 27) = 4.28, p = .01, (Table 1) and average intensity was affected by current (4,36) = 10.62, p <.00001), decreasing (p =.01) during current from 2.0 to 1.6 (2-sec time points) and increasing (p <.0008) above the 2.2 control to 2.9 after current was released (4-sec time points). Current affected low concentrations of the 4 compounds somewhat differently, F(6,54) = 3.90, p = .003); all four stimuli increased in intensity after current release, but only KCl (2.4 to 1.8) and NaCl (2.2 to 1.6) decreased significantly in intensity during current. Note individual significant comparisons are encircled by gray ovals
Counts of quality identifications by the 10 subjects for the 8 salts revealed that salty, bitter, no-taste, and tingle identities were chosen by more than 10 % of the subjects (per presentation) in the during current (0, −40 or −80 µA) epoch. Each common identity was affected during current. Average numbers of subjects identifying either salty or bitter decreased t(7) = 6.09, p = 0.0005; and no taste, t(7) = 7.23, p = .0002, and tingle, t(7) = 8.62, p=.0003, increased compared to the 0-µA control (Figure 3). Also, no taste (4.0 ± .37 vs. 2.1 ± .22, t(7) = 3.72, p=.007) and tingle (5.2 ± .28 vs. 4.0 ± .34, t(7) =2.81, p = .03) were identified more often for low than high concentrations.
The average number of subjects identifying metallic, 0.81 during current, increased from a 0.88 control to 2.94 subjects after current release, t(7) = 6.19, p=.0005. There was no change in the subjects’ use of salty, bitter, and tingle in the after epoch. Neither sweet, sour nor burn was used by more than 10 % of the subjects after or during current.
Thus, effects of current on chloride salts did not depend on the counter cation. During cathodal current, total intensity of the salty-bitter NaCl, KCl, NH4Cl, and CaCl2 decreased by 25%, salty-bitter was identified less often, and no-taste and tingle were identified more often, especially at low concentrations. After current release, intensity increased by 16% and metallic was identified more often.
The sodium salts had distinct quality profiles before current was applied; saccharin was moderately sweet and weakly bitter; the other stimuli were salty, sour and bitter. NaCl had a medium-strong salty intensity, higher than the salty intensity of either NaNO3 or Na acetate (p = .0001) (Table 2). The initial 5-way ANOVA revealed that compound and quality interacted with time and current (F (18,198) = 3.11, p = .00005); therefore, two sets of 4, 3-way repeated measures ANOVA were run. One set separately evaluated the 4 taste qualities. The 2nd set separately evaluated the 4 sodium salts.
The effect of current on salty, F(2,22) = 14.8, p = .00009, depended on the salt, F(6,66) = 9.19, p = .000001 (Figure 4). During current (2-sec time points), NaCl, Na acetate and NaNO3 were less salty (p < .01), and, after current release (4-sec time points), NaCl was significantly more salty (p = .03) than respective 0-µA controls. The four salts average sour intensity was reduced from 2.3 to 1.8 during current, F(2,22) = 5.30, p = .01; whereas, the four salts average bitter intensity increased after current release from 2.5 to 3.0, F(2,22) = 5.62, p = .01. Average sweet intensity was unaffected.
The effect of current on NaCl, F(2,22) = 12.9, p = .0002 (Figure 5A), depended on the quality, F(6,66) = 7.52, p = .000004. The strong salty taste of NaCl was decreased most during current and rebounded after current release (Figure 4). During current, weaker sour (from 2.9 to 1.6, p =.0001), sweet (from 1.4 to 0.6, p = .0005), and bitter (from 2.6 to 2.2, p=.02) tastes of NaCl also decreased. The effect of current on NaNO3, F(2,22) = 3.94, p = .03, did not depend on quality; the average four-quality taste of NaNO3 decreased during current but did not rebound after current release (Figure 5A). In contrast, unaffected during current, the four-quality intensities of Na-acetate, F(2,22) = 6.84, p = .005, and Na-saccharin, F(2,22) = 4.01, p = .03, increased after current release (Figure 5B).
Thus, the effects of cathodal current on sodium salts depended on the counter anion. Combined, NaCl and NaNO3 taste intensity decrements during current were salty, sour and sweet; whereas, combined Na-acetate and Na-saccharin taste intensity increments after current release were bitter, sour and salty.
Before current was applied, NaCl and NaBr had medium average intensities, Na2SO4 and Na-benzoate had weak average intensities, F (3,33) = 14.4, p < .000004; and subjects most often identified NaCl, NaBr and Na2SO4 as salty, and Na-benzoate as sweet (Table 3).
Salt intensity depended on current (F(2,22) = 10.85, p =.0005); but the individual salts were affected differently (F(6,66) = 2.48, p = .03). Average halide intensity decreased from 5.1 to 3.5 during current (p =.001) (Figure 6A); but average non-halide intensity, unaffected during current, increased above the 2.9 control to 4.6 (p =.0002) after current release (Figure 6B).
A control average 7.8 subjects identified NaCl, NaBr and NaSO4 as salty (Table 3); during current this was reduced to 4.0 subjects (t(5) = 4.8, p = .005); salty identities rebounded to 6.5 subjects after current release (t (5) = 7.3, p = .0007) (Figure 7A). Tasteless identities rose to an average 3.5 ±.2 subjects during current compared to control 1.3 ±.3 subjects, t(5) = 7.05, p = .0009. Bitter identities, used by more than 10 % of the subjects before current for NaBr, Na2SO4 and Na-benzoate, were unaffected by current. And, neither metallic nor tingle was used by more than 10 % of the subjects before or during current. The metallic identity appeared after current release; it was used by 2.8 subjects on average, compared to the average control 0.4 subjects (t(7) = 9.0, p = .00004 (Figure 7B).
Thus, effects of current on 0.5 M sodium salts depended on the counter anion. During cathodal current, total intensity of NaCl and NaBr decreased 39%, salty was identified less often, and no-taste was identified more often. After current release, salty identifications rebounded. The intensity of Na2SO4 and Na-benzoate were unaffected during current, but after current release, increased by 59% when a metallic identity appeared.
A near duplicate of the protocol that we used in humans was applied to the hamster taste nerve, with the highly effective salt solution serving as the current bridge. Figure 8 presents the recording of 2.4 minutes of neural activity elicited in the hamster CT by 0.1 M NaCl, upon which 5-sec epochs of anodal (+9 µA) or cathodal (−9 µA) current were superimposed. Especially note the neural activity decrements during cathodal current: 2, 5-sec. inhibitions to baseline (green dotted line) for the 2, 5-sec. −9 µA applications. Thus, inhibition of hamster neural taste activity occurs during cathodal current application as did perceptual inhibition in humans (Figure 2A, Figure 5A, Figure 6A). Also note the transient increases in CT neural activity, above the tonic response to NaCl (red dotted line), occurring after current was turned off. The neural off-response transiently overshot the 0-current, tonic ambient response of the nerve. Likewise, in humans, the perceptual intensity of NaCl overshoots the 0-current control (Figure 6A). The epoch after current release is associated with emergence of metallic taste and return of salty taste (Figure 4, Figure 7) in humans. The 5-second, +9 µA anodal current through 0.1 M NaCl elicits additional initial transient and tonic neural responses superimposed on NaCl elicited tonic activity, anodal responses quite similar to neural responses elicited without tonic activity evoked by a bridging solution .
This is the first study of the inhibition of ongoing ionic tastes by cathodal current. Cathodal inhibition is stimulus-specific, primarily affecting electrolytes with a salty taste; and after current release, a rebound in salty taste identity is accompanied by a transient metallic taste, likely due to an “anodal,” tissue-capacitance discharge current . Cathodal effects are unlike the singular, metallic identity elicited with anodal stimulation [18, 23] used to map regional taste dysfunction in humans [22, 36]. Furthermore, despite species differences, surprisingly similar effects were for the first time illustrated in ambient responses of the hamster chorda tympani nerve (CT). Human CT NaCl responses to continuous stimulation adapt much more rapidly toward baseline  than hamster CT responses (Figure 8); and humans do not share with rodents a strong Na+/Li+-specific taste where hamsters equate the tastes of NaCl and NaNO3, which are quite distinct to humans [15, 41, 42].
Application of cathodal current inhibited the human strong salty taste epitomized by NaCl and NaBr. The salty identity and weak salty intensity of NaNO3 and Na-acetate; and the strong intensity and salty-bitter identity of KCl, NH4Cl and CaCl2 were also consistently inhibited. Yet, the weak salty-bitter taste of NaSO4; sour tastes, generally; and sweet tastes of Na-saccharin and Na-benzoate were unchanged. Thus, cathodal current inhibits a subset of the multiple salt tastes experienced by humans when sampling ionic stimuli.
Rodent CT nerve recordings of weak cathodal current applied through subliminal NaCl and Na-acetate show marginal inhibitory effects (33, 39); cathodal current likewise has only a marginal effect on weak-tasting chloride salts (Figure 2). Cathodal current imposed on subliminal Na-saccharin and Na-m-nitrobenzene sulfonate [34, 35, 39], however, may elicit rodent CT neural excitation. To hamsters, Na-saccharin and Na-m-nitrobenzene-sulfonate taste like sucrose [43, 44, 45]. Also, −10-µA imposed on 1 mM saccharin elicited bursting activity in hamster single CT nerve fibers specific to sugar and saccharin . Bursting CT responses are associated with sweet stimuli in hamsters . We saw no increment in intensity or sweet identity of supra-threshold Na-saccharin or Na-benzoate during weak cathodal current in humans. Either the supra-threshold sweet chemicals’ tastes obscured cathode-evoked increments or the difference is due to species variation, which is not likely to be as strong for sweet receptors as for bitter receptors .
Besides humans not having the Na+-specific taste that hamsters and other rodents have, we do not make distinctions among the bitter tastes of ionic and non-ionic compounds that hamsters do . Perhaps, distinguishing sodium salts from all other salts and distinguishing among ionic and non-ionic bitters are more important for survival in granivorous hamster than in the omnivorous human.
The tissue-capacitance discharge current at cathode-off evoked a metallic taste like the non-specific taste at anode-on . This cathode-off effect, long known for humans  and occurring in the response of rodent CT nerves [32, 33, Figure 8] was ubiquitous, observable with stronger stimuli but especially clear for weaker chemical stimuli (Figure 2), and was present whether or not significant inhibition of the stimulus occurred during current flow. Metallic taste elicited at cathode-off, like anode-on [24, 25] is independent of the solution through which current flows. Its prominence with weaker ambient stimuli may relate to its purity, given the little inhibition and, as a result, potential recovery from inhibition, for weak salt stimuli. The “cathode-off” phenomenon helps explain why humans perceive similar tastes with 1-sec. electric stimuli of either polarity; subjects may be detecting metallic with anode at “on” or with cathode at “off” .
In addition to a metallic taste to the cathode-off current of opposite polarity, inhibited tastes would be expected to recover after release of inhibition . For NaCl, salty intensity (Figure 4, Figure 6) and identity (Figure 7) increases accompanied the emergent metallic identity. Return of inhibited cationic tastes after current release were not seen in other cases. Perhaps the post-current increments were weak compared to the sharp metallic burst, or metallic and returning cationic tastes may have been confused. In experiment B, in which intensities of sweet, salty, sour and bitter taste qualities were rated and metallic was not an option, subjects identified the sensation at cathode-off as bitter-sour-salty (19%,14% and 7% increments, respectively), identities also chosen for anodal taste, albeit less frequently than metallic .
Both anode and cathode likely exert their effects by conduction of ions in the vicinity of the receptors. Nevertheless, there is a fundamental difference between anode and cathode effects in electric taste. The anode is always stimulatory while the cathode has no effect or inhibits responses to stimulatory cations. The anode is stimulatory without chemical stimuli present, while the cathode inhibits ongoing tastes of specific salts. The non-specific positive anode taste, which is not due to electrolysis products, may directly depolarize taste receptor cells (TRC) or activate nerve endings. The negative cathode draws effective cations away from receptors located on TRC apical membranes and, at cathode-off, TRC are depolarized by a capacitive positive current surge, identical to the anode.
Cathode inhibition established NaCl, NaBr, KCl, CaCl, NH4Cl2 and NaNO3 as adequate cationic taste stimuli that likely utilize cation channels for transduction. Other salts, Na-acetate, Na-saccharin, Na-benzoate and Na2SO4, hardly inhibited by the cathode, may stimulate Tas1R or Tas2R G-protein coupled receptors, or both [3, 4, 49]. To accommodate multiple perceptual qualities, several mechanisms likely serve as human salt taste receptors, which are likely distinct from one or another of the several candidate rodent receptors [6, 50, 51].
Inhibition of taste responses during cathodal current identifies cationic salt tastes; whereas, the metallic quality evoked by anode-on and cathode-off is likely a bypassing of normal taste mechanisms that is associated with gustatory dysfunction .
We gratefully acknowledge Tuyen D. Nguyen for help with our stimulus chamber design, outstanding dental-student researchers Du-Tran Do, Hung Su and Ricardo Abakah, and Janneane F. Gent for developing our statistical approach, and each of our volunteer human subjects.
This work was supported by NIH grant DC004849.
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We presented this work in 2 posters at the Association for Chemoreception Sciences Meeting in Sarasota FL on April 12, 2003 and April 26, 2009.
Dr. Thomas P. Hettinger, Center for Chemosensory Sciences, Oral Health and Diagnostic Sciences, University of Connecticut Health Center, Farmington, CT 06030-1718, USA, Telephone: 860-679-3354, Email: ude.chcu.noruen@gnitteht.
Dr. Marion E. Frank, Center for Chemosensory Sciences, Oral Health and Diagnostic Sciences, University of Connecticut Health Center, Farmington, CT 06030-1715, USA, Telephone: 860-679-3354, Email: ude.chcu.noruen@knarfm.