Multimodal responses of taste neurons in the frog nucleus tractus solitarius

The responses of 216 neurons in the nucleus tractus solitarius (NTS) of the American bullfrog were recorded following taste, temperature, and tactile stimulation. Cells were classified on the basis of their responses to 5 taste stimuli: 0.5 M NaCl, 0.0005 M quinine-HCl (QHCl), 0.01 M acetic acid, 0.5 M sucrose, and deionized water (water). Neurons showing excitatory responses to 1, 2, 3, or 4 of the 5 kinds of taste stimuli were named Type I, II, III, or IV, respectively. Cells whose spontaneous rate was inhibited by taste and/or tactile stimulation of the tongue were termed Type V. Type VI neurons were excited by tactile stimulation alone. Of the 216 cells, 115 were excited or inhibited by taste stimuli (Types I-V), with 35 being Type I, 34 Type II, 40 Type III, 2 Type IV and 4 Type V. The remaining 101 cells were responsive only to tactile stimulation (Type VI). Of those 111 cells excited by taste stimulation (Types I-IV), 106 (95%) responded to NaCl, 66 (59%) to acetic acid, 44 (40%) to QHCl, 10 (9%) to water, and 9 (8%) to warming. No cells responded to sucrose. Of the 111 cells of Types I-IV, 76 (68%) were also sensitive to mechanical stimulation of the tongue. There was some differential distribution of these neuron types within the NTS, with more narrowly tuned cells (Type I) being located more dorsally in the nucleus than the more broadly tuned (Type III) neurons. Cells responding exclusively to touch (Type VI) were also more dorsally situated than those responding to two or more taste stimuli (Types II and III).     

Gustation as a factor in the ingestion of sweet and fat emulsions by the rat

This paper was written to pay honor to Professor Gerard P. Smith because of his strong influence on me to study the ingestion of sweet and fat mixtures. Three experiments are reported here, in which the laboratory rat was given an emulsion of a glucose+saccharin mixture with corn oil. In the first experiment, a two-bottle, 24-h test was given comparing the emulsion with water. Over 6 weeks, the concentration of the corn oil was gradually increased. When given only food and water, or the glucose/saccharin solution, the rats regulated their caloric intake and grew at a normal rate. In contrast, when the corn oil was present, the rats significantly increased their caloric intake, resulting in a marked increase in body weight. In the second experiment, a detailed analysis of the ingestion revealed that the rate of licking the emulsion during drinking bouts increased in a linear manner as the concentration of the corn oil was increased. In the third experiment, a conditioned taste aversion to the sweet/fat emulsion generalized to the fat more than to the sweet solutions. The implications for a gustatory input are discussed.     

Assessment of gustatory function by means of tasting tablets

OBJECTIVE: To develop a simple test for the screening of gustatory function in clinical settings. STUDY DESIGN: We tested 101 healthy volunteers (44 male and 57 female volunteers; mean age, 47 y) with the following gustatory test: the substances sucrose (sweet), citric acid (sour), sodium chloride (salty), and caffeine (bitter) were presented as tablets (diameter 4 mm) similar to common sweetener tablets. For quantitative assessment of whole-mouth gustatory function we used six different dosages with dilutions of each tastant in 50% steps. The highest dosage could be easily detected (sucrose, 30 mg; citric acid, 3 mg; sodium chloride, 2 mg; caffeine, 2 mg), and the lowest concentration was within threshold range. METHODS: Twenty-eight tablets (six different dosages of the four basic tastes plus four tasteless tablets) were tried in a randomized order. The entire test required 15 to 20 minutes. To evaluate the within-subject test-retest reliability, sessions were repeated after 1 week. Results were compared with those obtained by means of a conventional three-drop, forced-choice procedure using the method of ascending limits. RESULTS: Results of the new gustatory test were significantly correlated with those obtained using the three-drop, forced-choice procedure (correlation coefficient [r] = 0.66, P<.001). In general, women performed better than men. Furthermore, younger subjects exhibited a significantly higher gustatory sensitivity in both tests compared with older subjects. CONCLUSIONS: This quantitative test of whole-mouth gustatory function is easy to use, can be self-administered, requires little time, and has a long shelf-life. It appears to be suited for routine clinical assessment of gustatory function.     

Cortical unit responses to chemical stimulation of the oral cavity in the rat

The gustatory sensitivity of individual cortical neurons in the rat was examined using a wide range of chemical stimuli. Several subsets of cortical taste cells were identified based on similarities in their response profiles. Although adjacent cortical neurons can differ in their responsiveness to individual tastants, some overlap in taste sensitivities is usually observed.     

A study of olfaction and gustatory senses in sheep after olfactory bulbectomy

The importance of taste and smell in discrimination of sodium salts was examined in normal and anosmic sheep. To test for anosmia, faeces of pig, calf and sheep were used as noxious odours. Intact sheep, and sheep with one olfactory bulb removed avoided the aversive stimulus whereas following total bulbectomy, the faecal odour was no longer a deterrent. Olfactory bulbectomy did not affect food intake but changes in fluid intake, urinary loss and electrolyte excretion were apparent. In two-choice preference tests, normal sheep showed a marked aversion for 0.48 M NaHCO3 and 0.51 M NaCl when compared to water. Following either unilateral or bilateral olfactory bulbectomy the aversion for sodium salts persisted but was less extreme. This suggests that in the final discrimination of sodium salts the sense of taste is dominant but olfaction or the olfactory bulb may have a minor role.     

Electrophysiological actions of quinine on voltage-dependent currents in dissociated rat taste cells

 How taste receptor cells participate in encoding disparate compounds into distinct taste qualities represents a fundamental problem in the study of gustatory transduction mechanisms. Quinine is the most common stimulus employed to represent bitterness yet its electrophysiological consequences on voltage-dependent ion channels in the taste receptor cell have not been elucidated in detail. This study examines such effects on taste receptor cells dissociated from the foliate and circumvallate papillae of the rat. Outward potassium currents, which include transient, sustained and calcium-activated components, were reversibly inhibited by bath application of quinine, with an IC₅₀ of 5.1×10^–6 M. The time course of the current traces, along with voltage shifts in normalized conductance and inactivation curves, suggests that multiple mechanisms of inhibition may be occurring. Inwardly rectifying potassium currents were unaffected. Sodium currents, to somewhat higher concentrations of quinine (IC₅₀ = 6.4×10^–5 M), were also reduced in magnitude without noticeable effects on activation or reversal potential but with a shift in inactivation. Calcium currents, visualized with barium as a charge carrier, were enhanced in magnitude by the presence of low concentrations of quinine (10^–5 M) but were suppressed by higher concentrations (10^–4 M). Quinine broadened the waveform of the gustatory action potential and increased the input resistance. These data serve as genesis to future investigations of the signal transduction mechanism of quinine on voltage-dependent currents. Received: 18 November 1996 / Received after revision: 25 February 1997 / Accepted: 5 March 1997     

论中药煎剂掩味研究的必要性、现状与对策

目的:为开展中药煎剂掩味研究提供思路和方法。方法:分析中药煎剂掩味研究的必要性和现状,提出中药煎剂掩味研究的对策。结果:在把握中药呈味规律的前提下,借鉴食品与化学制药领域相关成果,以苦味中药中生物碱类成分的掩味为切入点,采用"抑苦"为主、"增甜"、"加香"为辅的手段,并与体内、外口感评价相结合的方法,是目前中药煎剂掩味研究相对可行、合理的途径之一。结论:随着中药煎剂掩味研究的不断深入开展,将在一定程度上有利于中药煎剂使用比例的提高和中医药现代化进程的推进。     

Effects of olfactory and gustatory stimuli on the biomechanics of swallowing

We have previously documented increased amplitude of motor-evoked potentials (MEPs) from the submental muscles during volitional swallowing following simultaneous odor and tastant stimulation. The MEP denotes neural excitability from the motor cortex to the target muscle(s). However, it is unknown if changes in the MEP transfer to the swallowing muscles to facilitate improved swallowing. Thus, we sought to evaluate changes in the biomechanics of swallowing following stimulation protocols that are known to influence neural excitability. Sixteen healthy participants were exposed to low and high concentrations of lemon odor and tastant. The odor and tastant concentrations which produced the highest amplitude of submental electromyography (EMG) were then combined for simultaneous stimuli presentation. Outcome measures included EMG from the submental muscles, as well as lingual and pharyngeal manometry. Poststimulation results showed decreased midglossopalatal pressure at 30 min and decreased duration at anterior and midglossopalatal pressure and increased EMG duration at 60 min. This study strengthens the justification for the use of flavor in managing patients with dysphagia as long-term changes were present in the poststimulation period.     

The Effect of Taste and Palatability on Lingual Swallowing Pressure

There is evidence that a strong, unpalatable, sour bolus improves swallowing in neurogenic dysphagia. It is not known whether other tastes may alter swallowing physiology. This study investigated the effect of moderate versus high taste concentrations (sweet, sour, salty, bitter) and barium taste samples on lingual swallowing pressure in ten healthy young adults, using a three-bulb lingual pressure array secured to the hard palate. Palatability of the samples was analyzed using the nine-point hedonic scale. Results showed that moderate sucrose, high salt, and high citric acid elicited significantly higher lingual swallowing pressures compared with the pressures generated by water. Pressures in the anterior bulb were significantly higher than those recorded from the middle or posterior bulb. There was no significant effect of palatability on lingual swallowing pressures. High salt and citric acid are known to elicit chemesthesis mediated by the trigeminal nerve. These results suggest that chemesthesis may play a crucial role in swallowing physiology. If true, dysphagia diet recommendations that include trigeminal irritants such as carbonation may be beneficial to individuals with dysphagia. However, before this recommendation more research is needed to examine how food properties and their perception affect swallowing in individuals with and without dysphagia.This research was supported by Syracuse University.