Water temperature preference shifts during hydration

Reduced menthol sensitivity in a prodromal Parkinson's disease model induced by intranasal rotenone treatment

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor symptoms, and it is associated with several prodromal non-motor symptoms, including an impaired sense of smell, taste and touch. We previously reported that bitter taste impairments occur independently of olfactory impairments in an early-stage PD animal model using short-term intranasal rotenone-treated mice. Cool temperatures also affect bitter taste perception, but it remains unclear whether or not bitter taste impairments result from an altered sensitivity for intraoral cool stimuli. We examined disturbances in the intraoral menthol sensitivity, such as coolness at low concentrations of menthol, using a brief-access test. Once a day, one solution from the 7-concentration series of (-)-menthol (0-2.3 mM) or the bitter taste quinine-HCl (0.3 mM) was randomly presented 20 times for 10 s to water-deprived mice before and 1 week after rotenone treatment. The total number of licks within 20 times was significantly decreased with the presentation of 2.3 mM menthol and quinine-HCl, compared to distilled water in untreated mice, but not in rotenone-treated mice. The correlation between the licks for quinine-HCl and that for menthol was increased after rotenone treatment. In contrast, the 2-bottle choice test for 48 h clarified that menthol sensitivity was increased after rotenone treatment. Furthermore, a thermal place preference test revealed that seeking behavior toward a cold-floored room was increased in the rotenone-treated mice despite the unchanged plantar cutaneous cold sensitivity. These results suggest that taste impairments in this model mice are at least partly due to intraoral somatosensory impairments, accompanied by peripheral/central malfunction.

Temperature Is Sufficient to Condition a Flavor Preference for a Cold-Paired Solution in Rats

Increasing evidence suggests that stimulus temperature modifies taste signaling. However, understanding how temperature modifies taste-driven behavior is difficult to separate as we must first understand how temperature alone modifies behavior. Previous work has suggested that cold water is more rewarding and "satiating" than warm water, and water above orolingual temperature is avoided in brief-access testing. We explored the strength of cold water preference and warm water avoidance by asking: (1) if cold temperature alone was sufficient to condition a flavor preference and (2) if avoidance of warm stimuli is driven by novelty. We addressed these questions using custom-designed equipment that allows us to monitor and maintain solution temperatures. We conducted two-bottle preference tests, after pairing Kool-Aid flavors with 10 or 40 °C. Rats preferred the flavor paired with cold temperature, both while it was cold and for 1 day while solutions were presented at 22 °C. We then examined the role of novelty in avoidance of 40 °C. Rats were maintained on 10, 22, or 40 °C water in their home cage to increase familiarity with the temperatures. Rats were then subject to a series of brief-access taste tests to water or sucrose at 10 to 40 °C. Rats that had 40 °C experience licked more to 40 °C water, but not sucrose, during brief-access testing. In a series of two-bottle preference tests, rats maintained on 40 °C water had a decreased preference for 10 °C water when paired opposite 40 °C water. Together, these data contribute to our understanding of orosensory-driven behavior with water at different temperatures.

Modulation of taste processing by temperature

Taste stimuli have a temperature that can stimulate thermosensitive neural machinery in the mouth during gustatory experience. Although taste and oral temperature are sometimes discussed as different oral sensory modalities, there is a body of literature that demonstrates temperature is an important component and modulator of the intensity of gustatory neural and perceptual responses. Available data indicate that the influence of temperature on taste, herein referred to as "thermogustation," can vary across taste qualities, can also vary among stimuli presumed to share a common taste quality, and is conditioned on taste stimulus concentration, with neuronal and psychophysical data revealing larger modulatory effects of temperature on gustatory responding to weakened taste solutions compared with concentrated. What is more, thermogustation is evidenced to involve interplay between mouth and stimulus temperature. Given these and other dependencies, identifying principles by which thermal input affects gustatory information flow in the nervous system may be important for ultimately unravelling the organization of neural circuits for taste and defining their involvement with multisensory processing related to flavor. Yet thermal effects are relatively understudied in gustatory neuroscience. Major gaps in our understanding of the mechanisms and consequences of thermogustation include delineating supporting receptors, the potential involvement of oral thermal and somatosensory trigeminal neurons in thermogustatory interactions, and the broader operational roles of temperature in gustatory processing. This review will discuss these and other issues in the context of the literature relevant to understanding thermogustation.

Water restriction and fluid temperature alter preference for water and sucrose solutions

The role of diet temperature in ingestive behavior is poorly understood. We examined the importance of stimulus temperature and water-restriction state on the preference for and intake of water and sucrose. Using custom-designed equipment that allows us to monitor and maintain solution temperatures during testing (±0.1 °C), we conducted a series of 2-bottle preference tests (10 °C water vs. sucrose 10-40 °C) and brief access tests (10-40 °C water and sucrose). Water-restricted rats preferred cold water over any sucrose concentration (0.0-1.0 M) if the sucrose was 30 or 40 °C, whereas the same rats preferred sucrose at all concentrations and temperatures when unrestricted suggesting that the water-restriction state interacts with temperature preference. In a series of brief-access tests using a Davis Rig (MS-180), rats reduced licking to cold sucrose compared with 20 °C sucrose, suggesting that unlike water, cold temperature reduced the palatability of sucrose.

Interactions of temperature and taste in conditioned aversions

The influence of temperature on taste cues and the ability to discriminate and learn about different temperatures of foods are important factors regulating ingestion. The goal of this research was to demonstrate that thermal orosensory input can serve as a salient stimulus to guide ingestive behavior in the rat, and also that it interacts with gustatory input during choice and conditioned aversion experiments. A novel apparatus with Peltier refrigerators was used to control the temperature of solutions in 10-min, 2-bottle tests. It was determined that naive rats preferred cold water (10 degrees C) to warm water (40 degrees ). When cold water was paired with a toxic LiCl injection, rats avoided cold water and drank warm water, thus demonstrating that cold water could serve as the conditioned stimulus in a conditioned temperature aversion. Rats conditioned against cold water could discriminate 10 degrees C water from 16 degrees C water, but not from 13 degrees C water, thus showing an ability to discriminate orosensory thermal cues to within 3-6 degrees C. Rats also generalized conditioned aversions from cold water to cold saccharin and cold sucrose solutions. However, if rats were conditioned against a compound taste and thermal stimulus (10 degrees C, 0.125% saccharin), the rats could distinguish and avoid each component individually, i.e., by avoiding cold water or warm saccharin. Finally, daily 2-bottle extinction tests were used to assess the strength of aversions conditioned against a taste cue (0.25 M sucrose), a thermal cue (10 degrees C water), or the combination. Aversions to taste or temperature alone persisted for 7-14 days of extinction testing, but the combined taste-temperature aversion was stronger and did not extinguish after 20 days of extinction testing. These results demonstrate that temperature can serve as a salient cue in conditioned aversions that affect ingestion independent of taste cues or by potentiating taste cues.

Oral hydration, parotid salivation and the perceived pleasantness of small water volumes

Previous studies have suggested that the preference for drinking cold water is increased when the drinker has a dry mouth. In a first experiment, we investigated whether a positive shift in preference would occur for small water volumes (0.75 ml and 1.5 ml) at 8, 16 or 25 degrees C, delivered into a mouth that had been dried using a warmed airflow, versus a normally hydrated mouth. Subjects rated the perceived wetness (or dryness) of their mouth, and the perceived pleasantness (or unpleasantness) of the water samples, using a labeled magnitude scale. Cooler water samples were preferred, and consistent with previous research, this preference was slightly enhanced when the subject's mouth was dried. The coldest water sample led to significantly wetter mouthfeel than the other two less cold samples, consistent with the possibility that the coldest water increased the rate of salivation. However, a second experiment found that although the rate of parotid salivation was increased if the mouth had been dried using a warm airflow, the different water temperatures did not induce different rates of parotid salivation. This indicates that enhanced preference for cold water when the mouth is dry is not invariably based in the reward gained from mouth rewetting via increased parotid saliva flow.

Effects of mouth dryness on drinking behavior and beverage acceptability

In humans, the association between mouth dryness and thirst has been examined in a variety of contexts. Typically, drinking behavior produces a concomitant reduction in unpleasant dry mouth sensations. Evidence is reviewed for a mechanism that influences the termination of drinking behavior by metering this change. Drinking behavior causes a progressive increase in parotid saliva flow. Thus, one possibility is that satiety results from a decrease in the reward associated with mouth wetting during a drinking episode. Beverages can differ in their satiating ability. This variability may be related to their mouth-wetting characteristic, and may be reflected in a shift in their acceptability when the mouth becomes dry. Physically drying the mouth appears to increase the acceptability of beverages that are either cold or acidic. It may be significant that two important determinants of mouth wetting are temperature and acidity. Cold or acidic beverages are also likely to be regarded as 'thirst-quenching.' Thus, shifts in acceptability, 'thirst quenching' and satiety may all be related to the mouth-wetting properties of a beverage. The extent to which this coincidence is meaningful warrants further investigation. However, if a common underlying process exists, then this may help to elucidate reasons for voluntary dehydration and aberrant drinking behavior in the elderly.

Effects of temperature and volume on measures of mouth dryness, thirst and stomach fullness in males and females

The immediate post-ingestive effects of temperature on thirst reduction were explored. Thirst was assessed by both ratings and a volume selection task. Supplementary ratings assessed mouth dryness and stomach fullness. All measures were taken before and 0, 2.5 and 5 min after drinking one of four water samples (150 ml/5 degrees C, 400 ml/5 degrees C, 150 ml/22 degrees C or 400 ml/22 degrees C). After 2.5 min, the cold sample produced greater reductions than the warmer sample in (i) the thirst ratings of males who received 400-ml samples, and (ii) the volume selections of females who received 150-ml samples. It is proposed that temperature influenced thirst reduction because of its differential effects on the post-ingestive state of the mouth.

Mouth-state dependent changes in the judged pleasantness of water at different temperatures

Dehydration increases the pleasantness of cold (0 degrees C) water (Boulze et al., 1983, Physiol. Behav. 30:97-102, 1983). The hypothesis that the mouth dryness induced by dehydration mediates this hedonic shift was investigated. Hydrated assessors (n = 16) judged 3 degrees C water as more pleasant after artificial mouth drying than did controls (n = 16). Mouth drying failed to influence similar judgements of water 13 degrees C, 23 degrees C, and 33 degrees C. We propose that preference shifts depend on temperature because cold water offers more rewarding relief from the sensations resulting from mouth dryness. Measures on saliva production were consistent with this proposal. Assessors swilled with water (3 degrees C, 13 degrees C, 23 degrees C, and 33 degrees C) for 5 s and then emptied their mouths. Measures of subsequent saliva flow confirmed that cold (3 degrees C) water induces an elevated rate of saliva flow and consequently leaves the mouth in a wetter state.

Temperature preference of drinking water in rats

The water temperature preference of adult male albino rats was studied. Deprived and normally hydrated animals were exposed at peak hours of consumption to a choice between 12 degrees C and 30 degrees C water or to 12 degrees C and 34 degrees C water. Food intake was not restricted. There was a clear preference for the warmer water. It is concluded that oropharyngeal temperature cues influence water intake.