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PubMed1991

Temporal characteristics of capsaicin sensitization and desensitization on the tongue.

Previous work has demonstrated that repeated applications of capsaicin can sensitize the tongue to subsequent exposures to capsaicin. However, if stimulation is interrupted for a period of minutes, the contrasting phenomenon of desensitization occurs. The purpose of the present study was to investigate the effect of interstimulus interval (ISI) on the development of both sensitization and desensitization to sequential capsaicin stimuli. It was determined in the first experiment that the rate of sensitization varied inversely with ISI for intervals briefer than 3.5 min; when ISI was lengthened to 5.5 min, sensitization was replaced by a trend toward desensitization. A second experiment established that increasing ISI from 9.5 min to 14.5 min tended to reduce rather than to enhance desensitization. It was therefore concluded that the sensitization process can be optimized by decreasing the time between successive stimuli (ISIs less than 3.5 min), whereas the trend toward desensitization appears to be optimal when stimuli are separated by at least 5.5 min but by less than 14.5 min.

Adult↗
PubMed1991

Magnitude estimation of adaptation to salt using a flow chamber for stimulus delivery.

Adaptation course to NaCl was estimated using a flow chamber for stimulus delivery and changes in the perceived taste magnitude as the criterion. The adapting stimulus of different durations was applied to one side of the tongue. Magnitude estimates were made by comparing the intensity of taste at the end of each adaptation time with the perceived intensity of the same stimulus applied briefly to the unadapted side of the tongue. The course of adaptation followed a negatively accelerated decreasing function. The relationship between taste magnitude and the duration of the adapting stimulus can be approximated by a logarithmic equation and, somewhat less accurately, also by an exponential equation, which can be related to the Beidler theory of taste stimulation. Some advantages of the closed flow technique and comparison of taste magnitudes on the adapted and unadapted sides of the tongue are discussed.

Chorda Tympani Nerve↗
PubMed1991

Capsaicin desensitization and recovery on the human tongue.

The desensitization resulting from application of 10 or 100 ppm capsaicin was investigated, using daily testing of a capsaicin series (1-1000 ppm, in log steps). The series showed a significant decrement in perceived burn following desensitization with either concentration. Perceived burn of 100 and 1000 ppm did not recover from 100 ppm desensitization in six days, and perceived burn of 1-1000 ppm did not recover from 100 ppm desensitization in six days. When single capsaicin concentrations, rather than the series, were tested at one, two, four, or six days after desensitization, 10 ppm recovered from 10 ppm desensitization in one or two days, and 100 ppm recovered from 100 ppm desensitization between two and four days. This suggests that daily testing with the capsaicin series delayed recovery from desensitization. Nontasters of 6-n-propylthiouracil rated capsaicin burn lower than did tasters. The application method of rolling capsaicin onto the tongue with a swab was found to transiently inhibit burn. Implications for ingesting capsaicin products are discussed.

Capsaicin↗
PubMed1991

Taste reactivity in the hamster.

Taste reactivity, which was first described in the rat, consists of ingestive and aversive response components, the latter seen mostly to bitter-tasting stimuli. The present experiment characterized the hamster's taste reactivity to an array of stimuli (sugars: 1 M sucrose, d-fructose and d-glucose; sodium salts: 1 M NaCl, Na2SO4 and NaNO3; acids: 30 mM HCl, tartaric acid and citric acid; bitter-tasting stimuli: 100 mM quinine hydrochloride and nicotine sulfate and 10 mM denatonium benzoate). These 12 stimuli were chosen to represent 3 examples each of stimuli that taste sweet, salty, sour, or bitter to humans; they were presented in random order via an intraoral fistula, one stimulus each day per animal (n = 10). Infusions of 0.6 ml were delivered over a 1-min period from a syringe pump. Orofacial and somatic motor responses were recorded on videotape for later analysis and were also coded online into a computer. Ingestive responses included forward and lateral tongue protrusions and aversive responses included gaping, chin rubbing, forelimb flailing, fluid rejection, increased locomotion, and aversive posturing. Each stimulus group produced a characteristic pattern of these behaviors, with sugars eliciting only ingestive behaviors and the bitter stimuli evoking predominantly aversive responses. Both sodium salts and acids produced ingestive responses, as seen previously in the rat, although these stimuli also elicited aversive behaviors in the hamster, including apes. The patterns of responses were characterized using multivariate procedures; the stimuli fell into distinct groups that were separated primarily along an hedonic dimension.

Animals↗
PubMed1992

Amiloride reduces intensity responses of human fungiform papillae.

Microdrop stimulation of human fungiform papillae was used to confirm that the drug amiloride reportedly reduces the taste response to NaCl and sucrose. Amiloride caused magnitude reductions of saltiness in most subjects and of sweetness for some but not all subjects. More of the subjects showed a reduction in response to NaCl than a reduction to sucrose and there were individual differences in the strength of the reduction of magnitude.

Adult↗
PubMed1992

Human psychophysical studies of saltiness suppression by amiloride.

Two studies were performed to assess the effects of amiloride on the saltiness of NaCl. In the first study, four highly trained subjects were asked to report the first appearance of saltiness as NaCl microdrops were rapidly applied to an increasing number of fungiform papillae along the front, dorsal surface of the tongue. For two subjects, amiloride, a sodium channel blocker, caused a large and significant increase in the number of papillae needed to report a salty taste. The other two subjects were unaffected, as measured by this procedure. In the second study, four different subjects were trained in the same procedure as in the previous study. For testing, however, they were stimulated on a fixed number of papillae, which was two times the number self-selected during training as sufficient for perception of saltiness with NaCl, and sourness with citric acid. They reported the quality of the experience as salty, sour, or neither, with and without amiloride treatment of the papillae set. Two of the four subjects had a loss of saltiness of NaCl, but not sourness of citric acid, when the papillae set was treated with amiloride.

Adult↗
PubMed1992

Sweetness does not habituate during a sip-and-spit experiment.

In gravitational flow studies, the estimated saltiness of a NaCl/sucrose mixture increases after repetitive stimulations with sucrose. This increment is hypothesized to be the consequence of successive contrast and suppression release. According to the successive contrast hypothesis, the saltiness of the mixture percept is overestimated because its salty taste stands out against the preceding sweet tastes. The release of saltiness from suppression is supposed to originate from habituation to sweet stimuli. In the present study, the saltiness of a NaCl/sucrose mixture was judged after repetitive stimulations with sucrose using a sip-and-spit procedure. The increment in saltiness estimates after repeated stimulations with sucrose could be fully accounted for by successive contrast. No suppression release was observed. Differences in experimental procedure (the degree of experimental control, the subjects' arousal level, the variation in the proximal stimuli, the area of the tongue stimulated, and the subjective intensities of the habituating stimuli) that may affect the rate of habituation, are suggested as potential sources of the differences between the gravitational flow studies and the present one.

Adolescent↗
PubMed1992

Chorda tympani and lingual nerve responses to astringent compounds in rodents.

A wide variety of compounds in foods and beverages produce astringent sensations when introduced into the oral cavity. There is controversy, however, whether "astringency," with its associated puckering and drying sensations, is a fundamental taste quality or is a tactile sensation. To address this issue, electrophysiological recordings were made from the gerbil chorda tympani nerve and the rat lingual nerve. The chorda tympani nerve transmits taste information from the anterior 2/3 of the tongue, whereas the lingual nerve transmits tactile, thermal and pain sensations from the anterior 2/3 of the tongue. The astringent compounds tested were: tannic acid, tartaric acid, gallic acid, aluminum ammonium sulfate and aluminum potassium sulfate. Tannic acid, tartaric acid, and gallic acids were tested at concentrations up to 120 mM over a pH range from approximately 2 to 6. The aluminum salts were tested at concentrations up to 160 mM only at low pH's. All compounds rapidly (and at lower concentrations, reversibly) stimulate the chorda tympani nerve in a concentration-dependent manner at all pH's tested. The rapidity and reversibility of the chorda tympani responses suggest that astringent-tasting compounds interact directly with taste cells rather than indirectly by precipitating salivary proteins. At pH 6, tannic acid, tartaric acid, and gallic acid all elicit robust chorda tympani responses, implying that the ionized forms of these compounds produce taste sensations. None of these compounds stimulate lingual nerves over the same concentration and pH ranges used in the chorda tympani experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Lack of an order effect in brief contact taste tests with closely spaced test trials.

The number of licks during 30-s exposures to a range of concentrations of sucrose, glucose, fructose, maltose, and Polycose were recorded. A complete series of concentrations of each carbohydrate was offered in ascending, descending, or random order with 30-s intertrial intervals. The order of presentation differed on different days. For sucrose, glucose, fructose, and maltose the number of licks in the 30-s periods was an increasing monotonic function of concentration regardless of the order of stimulus presentation. For Polycose, the lick rate-concentration function was an inverted U-shaped function of concentration with a peak on the 0.1 M to 0.2 M range. In all cases the rate of licking declined during the 30-s exposure period, suggesting the development of gustatory sensory adaptation. The results indicate that it is possible to obtain a rapid assessment of the taste sensitivity of the rat to a variety of carbohydrate solutions on a single day with very brief intertrial intervals.

Animals↗
PubMed1993

Early onset of reduced morphine analgesia by ingestion of sweet solutions.

Morphine analgesia can be reduced by prior exposure to food and flavored fluids. The early onset of reduced morphine-induced analgesia (RMA) was studied in 82 male Wistar rats after allowing them access to either a dextrose-saccharin solution or unflavored tap water for 6 or 3 h (Experiment 1, n = 40) or for 3 h, 90, or 45 min (Experiment 2, n = 42). Morphine (4 mg/kg) was injected subcutaneously at the end of the drinking period, and after 25 min a series of tail flick tests was conducted. Morphine produced strong analgesia in all rats that drank unflavored tap water; however, in rats that drank the flavored solution, the analgesic effect of morphine was significantly attenuated following exposures of 6 or 3 h, but not following exposures of 90 or 45 min. Similar quantities of flavored fluid were consumed by groups at all exposure durations; thus, RMA was determined by duration of exposure and not amount consumed. No analgesia attributable to flavor consumption per se was observed. The results suggest that RMA is mediated by endogenous opioid activity in the gustatory and analgesic systems by a mechanism akin to tolerance that requires about 3 h to operate.

Animals↗
PubMed1993

Taste prestimulation increases the chorda tympani nerve response to menthol.

Electrophysiological recordings of the summated response of the chorda tympani nerve to menthol stimulation of the tongue were obtained from 15 adult Sprague-Dawley rats. The chorda tympani nerve response to menthol was of short duration, ending within 2.5 s after stimulus onset, leaving the receptors in a state of insensitivity to subsequent menthol stimulation. Rinse durations with deionized-distilled water up to 10 min failed to bring the receptors back to their original prestimulus state. Although stimulation with menthol prevented taste receptors from responding to subsequent presentations of menthol, the chorda tympani nerve would respond normally to NaCl, NH4Cl, KCl, sodium acetate, glucose, citric acid, and quinine-HCl solutions. Prior stimulation with one of these taste solutions resulted in the recovery of the menthol response. The magnitude of the recovered menthol response depended on the magnitude of the phasic response elicited by the preceding taste stimulus. A general explanation involving possible reception and transduction mechanisms was offered to account for menthol's unexpected stimulatory effects on the chorda tympani nerve.

Animals↗
PubMed1993

The effect of bilateral sectioning of the chorda tympani and glossopharyngeal nerves on the sweet taste in the mouse.

The chorda tympani nerve (CT) and the glossopharyngeal nerve (GL) have been considered important nerves for the sense of taste. We studied the effect of bilateral sectioning of the CT and/or GL on the sweet taste in the mouse. Before and after surgery we analyzed the daily drinking patterns, using the two-bottle preference test method. The normal mouse drank the low concentration sucrose solution (0.0125 M) more than distilled water. This report showed that the mouse who was bilateral sectioned, both CT and GL or bilateral sectioned CT, rejected drinking the low concentration of sucrose solution. In contrast, the mouse who was bilateral sectioned GL drank the low concentration sucrose solution like the normal mouse did. These phenomena suggested that the fungiform papilae play an important role to detect the low concentration of sucrose (0.0125 M) as a sweet favorable taste substance.

Animals↗
PubMed1993

Development of some early sensorimotor behaviors in sodium-restricted rats.

Rats exposed to low levels of dietary sodium throughout development exhibit reduced chorda tympani nerve taste responses to sodium stimuli during adulthood, apparently due to altered activity of some hormone(s) or growth factor(s) during early development. We were concerned that such an alternation in the activity of some humoral factor(s) could affect development globally. To test this possibility, we utilized a battery of morphological and behavioral measures in neonatal, sodium-restricted rats, expecting serious deficits to be reflected in altered onset and expression of these behaviors. As compared with control rat pups, preweanling sodium-restricted rat pups exhibited greatly diminished body weight gain and delayed acquisition of several morphological features. However, in terms of sensorimotor development, no significant differences between sodium-restricted and control rat pups were found. We interpret these results to indicate that despite significant somatic effects, sodium restriction may not influence the development of physical prowess or of early sensorimotor function in a global manner.

Animals↗
PubMed1993

Altered gustatory development in Na(+)-restricted rats is not explained by low Na+ levels in mothers' milk.

Placing pregnant rats on a sodium-deficient diet (0.03% NaCl) very early in gestation and then weaning the offspring (sodium-restricted rats) to the same diet precludes development of amiloride-sensitive sodium taste transduction pathways in the offspring. However, normal amiloride-sensitive sodium taste responses can be restored by permitting sodium ingestion by sodium-restricted rats. The present study tested the hypothesis that the concentration of sodium in sodium-restricted mothers' milk must be abnormally low in order to preserve altered gustatory function in the offspring. Other milk electrolyte and total protein concentrations were determined as well. Milk sodium was similar between sodium-restricted and control rat mothers at 10-13 and 16-20 days postpartum, as were levels of potassium and chloride. At 10-13 days postpartum, total protein was higher in milk from sodium-restricted mothers. Sodium-restricted mothers' milk calcium concentrations were higher versus controls at 16-20 days postpartum. These results indicate that the lack of gustatory amiloride sensitivity in sodium-restricted rats cannot be attributed to deficient dietary sodium levels during the suckling period.

Amiloride↗
PubMed1993

Rats discriminate between starch and other substances having a similar texture.

Studies examined the contribution of textural factors to the ability of rats to sense starch. If rats sense the abrasive effects of starch suspensions, conditioned aversions to starch should generalize to substances having a similar texture. Two substances having a texture similar to that of starch were examined, polymeric dialdehyde (a product derived from starch) and inulin (a polymer of fructose). Rats were trained to avoid dilute suspensions of rice starch, potato starch, polymeric dialdehyde, or dahlia inulin, by injecting them with lithium chloride after they drank the test substance. Generalization tests revealed that rats trained to avoid rice or potato starch subsequently avoided corn starch, whereas rats trained to avoid polymeric dialdehyde or dahlia inulin did not avoid corn starch. Rats trained to avoid polymeric dialdehyde avoided this substance to a greater degree than did rats trained to avoid rice or potato starch. Rats trained to avoid dahlia inulin avoided this substance to a greater degree than did the rats in the other groups, but avoidance of dahlia inulin did not generalize to chicory inulin. Untrained rats showed little or no preference for dilute suspensions of polymeric dialdehyde or inulin, but did prefer starch over vehicle. It is concluded that tactile sensitivity does not play a large role in the perception of dilute starch suspensions.

Animals↗
PubMed1993

Impairment of the gustatory engram by generalised seizure activity without associated loss of conditioned taste aversion.

Three experiments are reported concerning the disruption of conditioned taste aversion (CTA) by generalised seizure activity (GSA) induced by electroconvulsive shock (ECS). Rats were taught CTAs by pairing either strong or weak taste cues (10% or 2.5% sucrose solutions) with either strong (0.15 M) or weak (0.05 M) doses of LiCl. In Experiment 1 it is shown that a 2.5% sucrose cue combined with a weak dose of LiCl will reproduce the disruptive effect of GSA when the CTA is established using a 10% sucrose cue. Likewise, a CTA acquired using a 2.5% sucrose cue paired with a strong dose of LiCl will simulate the failure of GSA to disrupt a CTA which has been established with a 10% sucrose cue. These findings support the theory that GSA acts to disrupt CTA by weakening the gustatory engram and an apparent inability to disrupt CTA by GSA does not necessarily signify that the gustatory engram itself remains intact. In Experiment 2, a CTA was established to a 2.5% sucrose cue using the more toxic dose of LiCl. It is shown that GSA will cause a substantial learning loss irrespective of whether it is interpolated within the taste-illness interval, or after the aversion has been acquired. It is concluded that the gustatory engram continues to reside in an active labile state even after the CTA has been established. In Experiment 3 it was estimated that the memory of the 2.5% sucrose cue must have been reduced to just over half of its original strength in order to create the learning loss reported in Experiment 2.

Animals↗
PubMed1993

Effects of neuropeptide Y on ingestion of flavored solutions in nondeprived rats.

Recent evidence suggests that in addition to altering energy balance, neuropeptide Y (NPY) may stimulate ingestive behavior by modifying the orosensory quality of ingested substances. The present experiments investigated the effect of intracerebroventricular administration of NPY (5 micrograms/5 microliters) on ingestion of various flavored solutions in nondeprived rats. Experiment 1 examined the effects of NPY on ingestion of a range of concentrations of saline, sucrose, and saccharin solutions in single-bottle tests. Results indicated that NPY stimulates ingestion of both sucrose and saccharin solutions that are normally palatable. In Experiment 2, palatable sucrose solutions flavored with either orange or black cherry Kool-Aid for separate training groups were selectively associated with NPY injection during single-bottle training sessions. Subsequent two-bottle preference tests showed a significant shift in preference toward the flavor paired with NPY during training. The results of these experiments extend previous findings by showing that NPY can stimulate ingestion of sweet solutions regardless of caloric value and may potentiate sweet taste preference via an associative mechanism.

Animals↗
PubMed1993

Microstructural analysis of successive negative contrast in free-feeding and deprived rats.

Rats shifted from 1.0 M to 0.1 M sucrose lick at lower rates for the weaker solution than rats that have continual access to the 0.1 M sucrose solution only. This effect, referred to as successive negative contrast, has been investigated primarily in food-deprived rats and, in all cases, using total licks or total volume consumed as the dependent measure. The present experiment used a microstructural analysis of licking patterns to examine the changes in behavior that constitute the contrast effect in total licks in both deprived and free-feeding rats. Although the magnitude of the effect was similar, deprived rats recovered from contrast more rapidly than free-feeding rats. Furthermore, the patterns of licking behavior associated with contrast differed under the two deprivation conditions. Specifically, when compared with the unshifted controls, the contrast effect in deprived rats was accomplished through a decrease in the number of licks per burst, an increase in the number of bursts initiated, a brief increase in the length of the interburst intervals, and no change in length of the interlick intervals. In free-feeding animals, contrast was associated with a decrease in the number of licks per burst, a brief increase in the length of the interburst interval, and no changes in either the number of bursts initiated or in the length of the interlick intervals. Together, these data demonstrate that patterns of licking behavior are differentially affected by solution concentration, deprivation state, and relative aspects of reward value.

Animals↗