Abolition by selective gastric vagotomy of the influence of water temperature on water intake: mediation via enhanced gastric clearance

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.

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.

Validation of tests of completeness of vagotomy in rats

In recent years there has been renewed interest in the role of the gastrointestinal tract and the abdominal organs in regulating body energy balance. In an attempt to characterize vagal influence, vagotomy has been utilized as a routine manipulation. Recent anatomical findings, behavioral observations and physiological data have led to a re-examination of the validity of the tests usually performed to assess completeness of the operation. In this review, a comprehensive profile of the vagotomized rat is drawn, standard indices are described and discussed and finally, new tests are suggested. It is concluded that, for most of the studies, a set of well-chosen criteria is necessary to ascertain the success of the surgery.

Water intake, pleasure and water temperature in humans

The influence of water temperature on intake and affective ratings was explored in human subjects. Dehydration whether by profuse sweating (body weight loss: 289 +/- 11 g, N = 20) or mountain climbing (body weight loss: 1660 +/- 58 g, N = 20) resulted in the same intake. Maximal intake was observed for water at 15 degrees C with respectively 199.0 +/- 17.0 ml and 222.7 +/- 17.4 ml. Colder and warmer water was ingested to a lesser extent. When 20 subjects were allowed to mix water to their preferred temperature, they chose 14.9 +/- 1 degree C and drunk 211.0 +/- 19.5 ml. Votes on a pleasure/displeasure scale increased from 50 degrees C to 0 degree C. Cold water was therefore both more pleasureable and less drunk. Dehydration resulted in a negative alliesthesia for warm water. Positive alliesthesia for cold water was probably the result of hyperthermia rather than dehydration.

Oro-gastro-intestinal inhibition of hypernatremia-induced drinking in the goat

Reduction of drinking by slow (5 ml/min) administration for 20 min of nearly body-warm (35 degrees C) and cold (15 degrees C) water into the mouth, the abomasum, or the duodenal bulb was studied in goats made thirsty by the simultaneous i.v. infusion of hypertonic (2 M) NaCl at 2 ml/min. During the control experiments the drinking response to corresponding infusion of 1.7 M NaCl was recorded. This in order to eliminate the possible influence on the results of a postabsorptive thirst inhibition which would occur if the administered water was completely absorbed already during the saline infusion. The entrance of warm water into the mouth or into the abomasum caused about 20%, and into the duodenal bulb about 30% reduction of drinking during the infusion of hypertonic NaCl. The corresponding reduction for cold water was when introduced into the mouth and duodenal bulb about 50% and into the abomasum about 60%. Cold water also considerably delayed the onset of drinking. The inhibition obtained during cold water administration was partially compensated for by increased post-infusional drinking. As regards the sensory input underlying preabsorptive inhibition of thirst, it is concluded that (regardless of distension, swallowing, and other mechanical factors) the entrance of pure water into various parts of the alimentary tract contributes to reduce the thirst drive. In addition, stimulation of oral, gastric, and duodenal cold receptors obviously diminish the urge to drink considerably.

Evidence for vagal involvement in the eating elicited by adrenergic stimulation of the paraventricular nucleus

We examined the role of the vagus nerves in mediating the eating and preprandial drinking seen after injection of norepinephrine (NE) into the region of the paraventricular hypothalamic nucleus of satiated rats. Complete subdiaphragmatic vagotomy (confirmed by gastric secretion tests) abolished the NE-elicited eating response, whether the diet used was lab chow, milk, or a milk-chow misture, and attenuated, by 38%, the NE-elicited drinking response. These effects occurred independently of changes in body weight or daily food intake imposed by vagal surgery. The vagotomized rats retained the capacity to rapidly increase eating in response to food deprivation or insulin injection challenges, indicating that the effect of vagotomy on NE-induced eating was not due to some non-specific impairment. Efferent vagal blockade of intact rats with systemic injections of atropine methyl nitrate (0.4 mg/kg) prior to central NE infusions yielded similar results. Finally-selective section of the coeliac branch of the vagus produced a 49% reduction of NE, elicited eating, as compared with a 29% reduction in water intake, while selective section of the gastric plus hepatic vagal branches, leaving only the coeliac branch intact, did not significantly affect either ingestive response. Both of these selectively vagotomized groups displayed an unimpaired capacity to increase food intake in response to systemic insulin injections. These results suggest participation of efferent vagal mechanisms in the adrenergic feeding, and, to a lesser extent, drinking phenomena and are consistent with a particular role for some function under coeliac vagal control (perhaps insulin secretion) in modulating the effects of NE on feeding behavior.

Effects of water temperature on short-term water intake and medullary neuronal responses in the hen

1. The daily water intake of adult Brown Leghorn hens was measured at water temperatures of 0, 10, 20, 30, 40 and 45 degrees C. The electrical activity of neurones in the nucleus ventro-lateralis anterior solitarii (NVLAS) of the medulla was also measured when the mouth was stimulated with water at the same temperatures. 2. There was a significant decrease in water intake when the temperature of the water was 10 or 45 degrees C. 3. There was an increase in activity of some of the NVLAS neurones with decreasing water temperature and this activity was in response to specific "cold" receptors. 4. Some NVLAS neurones were identified which showed an increase in activity at temperatures of 45 degrees C and more; it was concluded that these represented responses to thermal pain. 5. It is concluded that the effects of temperature on water intake may in part be explained by the behaviour of the oral receptors.