The effect of hunger on water intake in rats

Recovery of sweet taste preference in adult rats following bilateral chorda tympani nerve transection

Background

Numerous studies have noted the effect of chorda tympani (CT) nerve transection on taste sensitivity yet very few have directly observed its effects on taste receptor and taste signaling protein expressions in the tongue tissue.

Methods

In this study, bilateral CT nerve transection was performed in adult Sprague Dawley rats after establishing behavioral taste preference for sweet, bitter, and salty taste via short term two-bottle preference testing using a lickometer setup. Taste preference for all animals were subsequently monitored. The behavioral testing was paired with tissue sampling and protein expression analysis. Paired groups of CT nerve transected animals (CTX) and sham operated animals (SHAM) were sacrificed 7, 14, and 28 days post operation.

Results

Immunofluorescence staining of extracted tongue tissues shows that CT nerve transection resulted in micro-anatomical changes akin to previous investigations. Among the three taste qualities tested, only the preference for sweet taste was drastically affected. Subsequent results of the short-term two-bottle preference test indicated recovery of sweet taste preference over the course of 28 days. This recovery could possibly be due to maintenance of T1R3, GNAT3, and TRPM5 proteins allowing adaptable recovery of sweet taste preference despite down-regulation of both T1R2 and Sonic hedgehog proteins in CTX animals. This study is the first known attempt to correlate the disruption in taste preference with the altered expression of taste receptors and taste signaling proteins in the tongue brought about by CT nerve transection.

Interactions between the effects of food and water motivating operations on food- and water-reinforced responding in mice

Two experiments examined interactions between the effects of food and water motivating operations (MOs) on the food- and water-reinforced operant behavior of mice. In Experiment 1, mice responded for sucrose pellets and then water reinforcement under four different MOs: food deprivation, water deprivation, concurrent food and water deprivation, and no deprivation. The most responding for pellets occurred under food deprivation and the most responding for water occurred under water deprivation. Concurrent food and water deprivation decreased responding for both reinforcers. Nevertheless, water deprivation alone increased pellet-reinforced responding and food deprivation alone likewise increased water-reinforced responding relative to no deprivation. Experiment 2 demonstrated that presession food during concurrent food and water deprivation increased in-session responding for water relative to sessions where no presession food was provided. Conversely, presession water during concurrent food and water deprivation did not increase in-session responding for pellets. These results suggest that a) the reinforcing value of a single stimulus can be affected by multiple MOs, b) a single MO can affect the reinforcing value of multiple stimuli, and c) reinforcing events can also function as MOs. We consider implications for theory and practice and suggest strategies for further basic research on MOs.

Feeding conditions differentially affect the neurochemical and behavioral effects of dopaminergic drugs in male rats

The high co-morbidity of eating disorders and substance abuse suggests that nutritional status can impact vulnerability to drug abuse. These studies used rats to examine the effects of food restriction on dopamine clearance in striatum and on the behavioral effects of amphetamine (locomotion, conditioned place preference), the dopamine receptor agonist quinpirole (yawning), and the dopamine receptor antagonist raclopride (catalepsy). Amphetamine increased locomotion and produced conditioned place preference. Food restriction reduced dopamine clearance, which was restored by repeated treatment with amphetamine or by free feeding. Food restriction also decreased sensitivity to quinpirole-induced yawning and raclopride-induced catalepsy; normal sensitivity to both drugs was restored by free feeding. The same amphetamine treatment that normalized dopamine clearance, failed to restore normal sensitivity to quinpirole or raclopride, suggesting that in food-restricted rats the activity of dopamine transporters and dopamine receptors is differentially affected by pathways that are stimulated by amphetamine. These studies show that modest changes in nutritional status markedly alter dopamine neurotransmission and the behavioral effects of direct-acting dopamine receptor drugs (agonist and antagonist). These results underscore the potential importance of nutritional status (e.g., glucose and insulin) in modulating dopamine neurotransmission and in so doing they begin to establish a neurochemical link between the high co-morbidity of eating disorders and drug abuse.

Oxytocin, water intake, and food sodium availability in male rats

This study examined the effect of subcutaneous administration of the neurohormone oxytocin on water intake of ad lib-fed (with or without sodium availability in the diet) and food-deprived animals. Results of the first experiment showed that oxytocin increased water intake and urine excretion in food-deprived but not in ad lib-fed animals. However, oxytocin treatment did not modify the reduced water "balance" (fluid intake minus urine volume) resulting from food deprivation or the daily food intake (Experiment 1). The dose-dependent polydipsic effect of oxytocin on food-deprived rats was always preceded by an increase in sodium and fluid urine excretion (Experiment 2). Oxytocin also increased the water intake of animals fed ad lib with a low sodium diet (Experiment 3). These results suggest that the effect of oxytocin on water intake is dependent on the presence or absence of sodium in the diet and that the excretion of sodium is the main mechanism of oxytocinergic polydipsia in food-deprived male rats.

The relative-reinforcing value of food under differing levels of food deprivation and restriction

Food deprivation and restriction both increase food consumption. Food deprivation also increases the reinforcing value of food, but it is unknown if food restriction alone or combined with deprivation increases the reinforcing value of food. Forty, normal-weight, college-aged, unrestrained females were randomized to one of four conditions that crossed food deprivation (Dep) and restriction (Res): Dep/Res, Dep/No Res, No Dep/Res, No Dep/No Res. All participants arrived at least 13h food-deprived, and non-deprived participants consumed at least 365cal from a drink during the session. Restriction was manipulated by placing snack food in front of participants, without access for 15min during the session, or having no snack food placed in front of participants. Following the experimental manipulations, participants completed a computer choice task to determine the reinforcing value of food. Repeated measures analysis of variance found a significant main effect of deprivation (p<0.05) and trials (p<0.001) for food points earned. Deprived participants found food to be more reinforcing, and the reinforcing value of food decreased over time. This suggests that while short-term food deprivation increases the relative-reinforcing value of food, short-term food restriction has no effect on the relative-reinforcing value of food in unrestrained eaters.

Food and water intake as functions of resource consumption costs in a closed economy

In two experiments, rats living in a closed economy were offered continuous, concurrent access to four resources: food, water, a nest, and a running wheel. Costs of consuming food and water were imposed with bar-press requirements, and the price of either one or both resources was raised. As the consumption cost increased, less was consumed in each bout of resource use. Bout frequency increased, but not sufficiently to compensate for the fall in bout size, and total intake fell. Food and water tended to be complementary resources, in that as intake of one fell with its price, intake of the other also decreased. This interaction was accounted for by the defense of the ratio of body water to lean body mass. As amount consumed decreased, increases in feed efficiency (weight gain per unit of food ingested) and the use of stored calories compensated for the reduced energy intake. There was evidence of competition between feeding and drinking at the higher costs: When both commodities were expensive, the decline in the intake of each one was greater than when only one commodity was expensive. Although the time spent nesting, running, and in unmonitored activity was adjusted when feeding or drinking took more of the rat's day, there was no particular activity that was sacrificed.

The effect of food deprivation on the reinforcing value of food and smoking in bulimic and control women

The reinforcing value of both food and cigarettes was examined in female smokers with and without bulimia nervosa. Subjects underwent two food-deprivation (18 h) and two nondeprivation conditions and worked for either food vs. money or cigarettes vs. money on concurrent progressive variable ratio schedules. Schedules for food/cigarettes increased progressively while schedules for money remained the same. Results indicated no differences in the amount of time subjects spent working for food or the number of points earned for food in either bulimics or controls when examining the entire eight-game trial. Results of the initial two-game trial indicated marked differences, with the reinforcing value of food increasing after deprivation in control but not bulimic women and the total amount of time spent working for cigarettes increased after deprivation in controls. These results highlight that the effects of experimental food deprivation are most salient immediately after the deprivation and at low unit prices, and that bulimic and control women respond differently to a food-deprivation paradigm.

Food and water deprivation: changes in rat feeding, drinking, activity and body weight

Groups of male rats totally food deprived for periods up to 96 hr, water ad lib do not totally regain body weight lost through deprivation. Degree of body weight defence was estimated by fitting statistically a curve to the predeprivation baseline data and extrapolating the curve over the deprivation and postdeprivation period. From group data, body weight is not fully defended. However, marked individual differences in degree of body weight defence were noted. During food deprivation the daily drinking cycle persists for approximately 48 hr, then a reduction in nocturnal drinking appears. General activity increases but only in the light period. After food restitution, the main hyperphagia is confined to the first post-deprivation day. Additional, subtle long term effects of overeating in the light period are also found. In contrast to food deprived groups, water deprived groups accurately defend body weight. During water deprivation daily food intakes decrease immediately whereas nocturnal activity decreases after approximately 72 hr of deprivation has elapsed. After the termination of water deprivation the immediate response is gross hyperdipsia, but again subtle long term effects are apparent. Total daily food intake increases for many days but the increase is not confined to either light or dark periods. Water intake is similarly affected. These results are evaluated in relation to the notion of a body weight set-point regulating feeding behavior. Some limitations on the value of the findings in terms of species differences and lack of ecological validity are also discussed.

Persistence of rat nocturnal feeding and drinking during diurnal presentation of palatable diet

Male laboratory rats were fed a highly palatable diet during the light phase of a 12:12 LD cycle. Nocturnality of feeding was reduced from approximately 77% of the total daily intake to 56%. Increase in diurnal feeding was attributable to the ingestion of a large meal at the beginning of the L phase. Reduction of nocturnal feeding was attributable to decreased ingestion in the first 3 hr of the D phase. Outside these times the daily feeding pattern was unmodified. These findings do not represent a reversal of the feeding rhythm as previously reported, but a modification which is not consistent with interpretation as a phase-shift. Throughout the testing period total daily food intake remained unchanged and body weight was defended despite the increase in calories ingested. Daily drinking patterns and total daily water intake were not significantly altered by the experimental manipulation.

A chronometric approach to the study of feeding behavior

In many mammalian genera, the stimulus to feed is intimately associated with circadian rhythms. This stimulus arises from within the brain from biological time-keeping systems. Such a chronometric approach to feeding behavior follows from a consideration of the terrestrial mammal's space-time pattern within the ecological niche. The ecological niche is a division of time as well as space. The restriction of certain behaviors to certain times of day and the concomitant evolution of nocturnality or diurnality represent strong advantages for survival in the wild. Experimental data, primarily from studies on the rat, in support of the chronometric approach, include: the reinstatement of cyclic feeding patterns after food deprivation; the continuation of circadian pattern of wheel running and nocturnal drinking during food deprivation; consideration of the ontogeny of the feeding pattern; the phenomenon of anticipatory appetite--the experimental demonstration that time of day can act as a conditioned stimulus for feeding; the evaluation of rhythms in digestion, absorption and assimilatory biochemical processes; the realization that many of these rhythms are not simply a consequence of the presence of food in the gut; the realization that the brain exerts considerable control over the peripheral rhythmic nutritional processes via ANS and endocrinological systems; and the fact that within the brain the SCN and structures well known to be involved in nutritional regulation, such as the VMH, LHA and monoamine systems, may all be involved in the circadian pattern of feeding. Further, the function of these neurological structures may be understood better by consideration of data from temporal changes in feeding patterns.

Food deprivation increases oral and intravenous drug intake in rats

Rats given continuous access to etonitazene hydrochloride in their drinking water (5 micrograms per milliliter) more than doubled their drug intake while deprived of food. Another group of rats with implanted jugular catheters self-administered etonitazene (10 micrograms per kilogram) intravenously on a continuous reinforcement schedule, and the number of infusions increased significantly on days when they were deprived of food. These results suggest that feeding condition may be a powerful determinant of drug-reinforced behavior.

Effects of food deprivation on etonitazene consumption in rats

One group of free-feeding rats was given a 5 microgram/ml etonitazene HCl solution as their sole liquid. This group increased their drug intake by 100% when they were partially food-deprived during a 23-day period. Another group that remained food-satiated and received etonitazene for an equal number of days did not show similar increases in drug intake. However, this group drank greater volumes of the etonitazene solution than a food-satiated control group drank of water. These results are contrasted with a fourth group showing a 50% decrement in water intake during similar food-deprived conditions. The food-deprived group drinking etonitazene showed highly erratic drinking patterns compared to all the other groups. Daily liquid intake ranged from 30 to 250 ml in this group, and volumes oscillated from high to low on alternating days. When the food-deprived/food-satiated conditions were replicated in this experimental group, corresponding increases and decreases in drinking reliably occurred. However, during the second food-deprived phase, the large increases occurred almost immediately as contrasted with a gradual increase over 17 days during the first food-deprived phase. This would suggest a learning mechanism may be involved. Self-mutilation and other forms of stereotypy were noted only in food-deprived rats consuming etonitazene.

Inhibition of ad libitum feeding in rats by salt injections and water deprivation

Inhibition of ad libitum feeding in rats was induced by hypertonic NaCl injections. Though osmotic loads of sufficient size were capable of abolishing feeding completely for a time, the effect was not as large as had been predicted from a hypothesis of strictly linear subtractive inhibition. Feeding at a low level of hunger seems to be somewhat less affected by osmotic inhibition than feeding on a deprivation schedule. Inhibition of feeding was also produced by deprivation of water, and both the inhibition of food intake during deprivation, and the disinhibition by subsequent drinking indicated that the amount of inhibition of food intake is a non-linear (accelerating) function of water deficit. A model of the process indicating that the thirst signal undergoes a non-linear transformation before being subtracted from the signal corresponding to food demand is proposed.