Conditioned food aversions and satiety signals

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Evidence that carbohydrate-to-fat ratio and taste, but not energy density or NOVA level of processing, are determinants of food liking and food reward

This virtual (online) study tested the common but largely untested assumptions that food energy density, level of processing (NOVA categories), and carbohydrate-to-fat (CF) ratio are key determinants of food reward. Individual participants (224 women and men, mean age 35 y, 53% with healthy weight, 43% with overweight or obesity) were randomised to one of three, within-subjects, study arms: energy density (32 foods), or level of processing (24 foods), or CF ratio (24 foods). They rated the foods for taste pleasantness (liking), desire to eat (food reward), and sweetness, saltiness, and flavour intensity (for analysis averaged as taste intensity). Against our hypotheses, there was not a positive relationship between liking or food reward and either energy density or level of processing. As hypothesised, foods combining more equal energy amounts of carbohydrate and fat (combo foods), and foods tasting more intense, scored higher on both liking and food reward. Further results were that CF ratio, taste intensity, and food fibre content (negatively), independent of energy density, accounted for 56% and 43% of the variance in liking and food reward, respectively. We interpret the results for CF ratio and fibre in terms of food energy-to-satiety ratio (ESR), where ESR for combo foods is high, and ESR for high-fibre foods is low. We suggest that the metric of ESR should be considered when designing future studies of effects of food composition on food reward, preference, and intake.

Further evidence for sensitivity to energy density and a two-component model of meal size: Analysis of meal calorie intakes in Argentina and Malaysia

Previously, we demonstrated a non-linear association between meal caloric intake and meal energy density (ED, kcal/g) in data from a controlled trial in the US and from free-living participants in the UK [1]. In both datasets, meal caloric intake increased with ED in lower energy-dense meals (below ∼1.75 kcal/g) and decreased in higher energy-dense meals (above ∼1.75 kcal/g). In the current study, we sought to explore whether this pattern extends to data from free-living participants in Argentina (N = 2738 meals) and Malaysia (N = 4658 meals). Again, a significant breakpoint was found in both the Argentinean (2.04 kcal/g (SE = 0.06)) and Malaysian (2.17 kcal/g (SE = 0.06)) datasets with mean centered meal caloric intake increasing with ED below the breakpoint and decreasing above the breakpoint. These results lend further support for our two-component theoretical model of meal size (g) in which a volume signal is dominant in lower energy-dense meals and a calorie-content signal is dominant in higher energy-dense meals. Together, our research adds to evidence supporting human sensitivity to calories and exposes a complexity in the correspondence between meal energy content and meal size in everyday (non-manipulated) meals. Further research is needed to provide causal evidence for this sensitivity and whether individual variation impacts meal size and energy balance.

CCK-58 elicits both satiety and satiation in rats while CCK-8 elicits only satiation

Reduction of food intake by exogenous cholecystokinin (CCK) has been demonstrated primarily for its short molecular form, CCK-8. Mounting evidence, however, implicates CCK-58 as a major physiologically active CCK form, with different neural and exocrine response profiles than CCK-8. In three studies, we compared meal-pattern effects of intraperitoneal injections CCK-8 vs. CCK-58 in undeprived male Sprague-Dawley rats consuming sweetened condensed milk. In study 1, rats (N=10) received CCK-8, CCK-58 (0.45, 0.9, 1.8 and 3.6 nmol/kg) or vehicle before a 4-h test-food presentation. At most doses, both CCK-8 and CCK-58 similarly reduced meal size relative to vehicle. Meal-size reduction prompted a compensatory shortening of the intermeal interval (IMI) after CCK-8, but not after CCK-58, which uniquely increased the satiety ratio (IMI/size of the preceding meal). In the second study, lick patterns were monitored after administration of 0.9 nmol/kg CCK-58, CCK-8 or vehicle. Lick cluster size, lick efficiency and interlick-interval distribution remained unaltered compared to vehicle, implying natural satiation, rather than illness, following both CCK forms. In study 3, threshold satiating doses of the two CCK forms were given at 5 and 30 min after meal termination, respectively. CCK 58, but not CCK-8 increased the intermeal interval and satiety ratio compared to vehicle. In conclusion, while CCK 58 and CCK-8 both stimulate satiation, thereby reducing meal size, CCK-58 consistently exerts a satiety effect, prolonging IMI. Given the physiological prominence of CCK-58, these results suggest that CCK's role in food intake regulation may require re-examination.

Dose-dependent effects of peptide YY(3-36) on conditioned taste aversion in rats

We used a conditioned taste aversion test to assess whether PYY(3-36) reduces food intake by producing malaise. Two-hour IV infusion of PYY(3-36) (8, 15, and 30 pmol/kg/min) at dark onset in non-food-deprived rats produced a dose-dependent inhibition of feeding and a conditioned aversion to the flavored chow paired with PYY(3-36) infusion. In food-deprived rats, PYY(3-36) at 2 and 4 pmol/kg/min inhibited intake of a flavored saccharin solution without producing conditioned taste aversion, whereas higher doses (8 and 15 pmol/kg/min) inhibited saccharin intake and produced taste aversion. These results suggest that anorexic doses of PYY(3-36) may produce a dose-dependent malaise in rats, which is similar to that reported for PYY(3-36) infusion in humans. Previous studies have shown that PYY(3-36) potently inhibits gastric emptying, and that gut distention can produce a conditioned taste aversion. Thus, PYY(3-36) may produce conditioned taste aversion in part by slowing gastric emptying.

On Gerard P. Smith's scientific character and thought

A fundamental tenet of the intellectual history of science is that the examination of the intellectual qualities of individual eminent scientists is a fruitful exercise for scholars who come after. I therefore review some salient aspects of the scientific character and thought of Gerard P. Smith (GPS), as they have been expressed in his writings. The first section considers some biographical details in respect to the sort of qualities that may prepare a mind for scientific achievement. The second section considers some logical and rhetorical qualities evident in the scientific writing of GPS, especially in the period of his career before the crucial tests of the CCK satiation hypothesis. The third section then considers the character of his thought as revealed in the formal statement, the operationalization, and the critical testing of the CCK satiation hypothesis. The qualities of mind discussed certainly contributed decisively to the scientific achievements of GPS, and their emulation may well contribute to the success of others.

The relationship between food reward and satiation revisited

The postingestive satiating action of food is often viewed as producing a positive affective state that rewards eating. However, in an early test of this idea, Van Vort and Smith [Physiol. Behav. 30 (1983) 279] reported that rats did not learn to prefer a food that was "real-fed" and satiating over a food that was "sham-fed" and not satiating. Subsequent investigators obtained similar findings with concentrated nutrient sources. With dilute nutrient sources, however, rats learned to prefer the real-fed to the sham-fed food. These and other findings demonstrate that nutrients have rewarding postingestive effects that enhance food preferences via a conditioning process. These reward effects appear separate from the satiating actions of nutrients, which may actually reduce food reward. Food intake and preference are controlled by a complex interaction of positive and negative signals generated by nutrients in the mouth and at postingestive sites.

Cholecystokinin and satiety: current perspectives

In the almost 30 years since the ability of peripheral administration of the brain/gut peptide cholecystokinin (CCK) to inhibit food intake was first demonstrated, significant progress in our overall understanding of the role of CCK in ingestive behavior has been made. A physiologic role for endogenous CCK in the control of meal size has been demonstrated and sites and mechanisms of action for CCK in food intake have been investigated. Recent work has uncovered roles for the CCK satiety pathway in the mediation of the feeding modulatory actions of estradiol, insulin, and leptin. The availability of the Otsuka Long Evans Tokushima Fatty (OLETF) rat, a strain lacking CCK(A) receptors, provides a unique model for the study of how deficits in a within-meals satiety signaling pathway may result in long-term changes in food intake and body weight.

Angiotensin II reduces alcohol intake and choice in water- or food-restricted rats

Two experiments were conducted to assess the effects of administration of the neuropeptide and hormone angiotensin II (AII) on ethanol intake and choice. First, 18 male Wistar rats were water deprived for 23 h and given access to 5% w/v ethanol for 30 min, followed by 30 min of access to water; food was ad lib. Following adaptation to this schedule, rats were randomly assigned to receive an IP injection of 0, 100, or 200 micrograms/kg of AII at either -30 or 0 min prior to ethanol access. Each AII injection decreased ethanol intake only if injected immediately before access; water and food intake were unaffected. Secondly, rats were given food daily at 2% of body weight with ad lib water and randomly assigned to receive either only water or 4% w/v ethanol ad lib on alternate days. Following adaptation, rats were randomly assigned to receive IP saline or 200 micrograms/kg of AII prior to presentation of a choice of ethanol or water for 1 h. AII reduced ethanol intake and increased water intake at 0-30 min after injection. Results confirm previous reports of inhibition of alcohol consumption by peripheral AII, and indicate a temporal constraint on AII's effect, which is consistent with a role as a short-term satiety factor.

Exogenous cholecystokinin octapeptide in broiler chickens: satiety, conditioned colour aversion, and vagal mediation

Intraperitoneal injections of 3.5, 7.0, 14.0, and 28.0 micrograms/kg of CCK-8 into free-feeding broiler chickens significantly reduced food intake and delayed feeding (p < 0.05). To determine whether CCK can condition preference or aversion and to investigate the latency and the reversal of the effect, a low (2 micrograms/kg) and a high (14 micrograms/kg) dose of CCK-8 were administered using the coloured food paradigm. One colour, the conditioning stimulus (CS+), was paired with injections of CCK-8; the other colour was paired with injections of saline (CS-). The 2 micrograms/kg dose of CCK-8 neither reduced food intake nor conditioned a colour aversion. The 14 micrograms/kg dose significantly reduced food intake and conditioned a colour aversion (p < 0.05). When vagotomy was performed, the 14 micrograms/kg dose of CCK suppressed feeding in sham-operated birds (p < 0.05) but not in vagotomized birds (p > 0.05). A significant aversion for the food paired with CCK was obtained in sham-operated birds (p < 0.001) but not in vagotomized birds (p > 0.05). It was concluded that IP injections of CCK-8 reduce food intake in broiler chickens and that chicks can learn to associate the colour of the food with injections of CCK, developing an aversion. It was also shown that the vagus nerve mediates the CCK satiety effects and that aversion conditioning to CCK is dependent upon intact vagal innervation of the viscera.

Effects of bombesin on temporal patterns of ingestion in the rat

Bombesin, an analog to gastrin releasing peptide, has previously been shown to inhibit food intake in the rat. In order to further characterize the effects of bombesin on ingestive behavior, the present study examined licking patterns of rats drinking sweetened condensed milk following bombesin (4 micrograms/kg, IP) or vehicle injection under two levels of food deprivation (0 and 24 h). Both bombesin treatment and satiety (reducing food deprivation from 24 to 0 h) significantly decreased total milk consumption during a 1-h test. Analysis of licking patterns suggested that bombesin and satiety operate by similar but not identical mechanisms. Deprivation reduction tended to decrease ingestion by reducing the rate of drinking and size of the first meal. Bombesin, by contrast, reduced the total duration of drinking and the number of meals taken, but had no effect on lick rate. Cumulative intake records suggested that bombesin reduces the volume threshold for termination of drinking such that intake continues at a normal rate below this threshold but ceases above it. This bombesin-imposed threshold for cessation of drinking was between about 5 and 7 ml for individual animals under both food-deprived and nondeprived testing conditions.

Effect of enterostatin given intravenously and intracerebroventricularly on high-fat feeding in rats

The effect of enterostatin, the amino-terminal pentapeptide of pancreatic procolipase, on high-fat food intake has been investigated after intracerebroventricular as well as after intravenous injection. After an overnight fast enterostatin given i.c.v. at doses of 167 pmol and 333 pmol produced a significant and dose-dependent reduction in high-fat food intake, while a higher dose of 667 pmol had no effect. Following intravenous injection of enterostatin the intake of high-fat food was suppressed at doses of 8.3 nmol and 16.7 nmol, while no effect was observed at higher doses. The inhibition of feeding started 3 h after the initiation of feeding and persisted to the end of the test period (6 h). Enterostatin at a dose of 16.7 nmol gave no sign of aversion in an aversion test comparing the effect of enterostatin, lithium chloride and saline on liquid intake. The data suggest that enterostatin may exert its satiety effect on high-fat feeding by being absorbed into the bloodstream.

Bombesin reduces alcohol choice in nutritive expectancy and limited-access procedures

Bombesin is a bioactive tetradecapeptide found in nerves of the brain and gut and previously shown to inhibit intake of ethanol in forced-choice, one-bottle tests in water-deprived rats. In the present experiments, intraperitoneal bombesin (4-100 micrograms/kg) reduced selection of alcohol in two-bottle choice tests with water. In an application of the "nutritive expectancy" procedure, weight-reduced rats received access to 4% w/v ethanol and water. Bombesin injection suppressed the intake of ethanol, but not of water, in rats with prior ethanol experience. In an application of the "limited access" procedure, nondeprived rats received access to 6% w/v ethanol and water in nonhome cages during either the light or the dark phase of a 12:12 h lighting cycle. Bombesin injection lowered the intake of ethanol, but not of water, in both phases of the lighting cycle and in both sexes. Water deprivation is not necessary for bombesin to inhibit alcohol intake and this effect cannot be explained by hypodipsia, nonspecific debilitation, or conditioned aversion. Endogenous bombesin-like neuropeptides may specifically reduce choice of alcohol by signaling satiation with ethanol.

Effects of peripheral neurotensin on behavior of the rat

Neurotensin (NT), a tridecapeptide found in brain and gut neurons, inhibits feeding and grooming, increases drinking, and enhances ethanol-induced sedation in rats after central injection. We tested the behavioral effects of IP injection of NT (0.1-100 micrograms/kg) in water-deprived rats given access to 5 or 10% ethanol for 30 min, followed by 30-min access to water. Behaviors during alcohol access were quantified with an instantaneous time-sampling observational technique. Food intake and observed feeding and grooming behaviors were significantly inhibited by large doses of NT (10-100 micrograms/kg) and water intake and resting behavior were increased. When the "limited access procedure" was used to induce ethanol selection in nondeprived rats, NT did not affect ethanol or water intake. Peripheral NT affects intake of food and water and observed feeding, grooming, and resting after peripheral injection in deprived rats, but does not affect ethanol consumption. These actions suggest physiological roles for endogenous neurotensin and its receptors in regulation of specific behaviors.

Effects of cholecystokinin on morphine-elicited hyperactivity in hamsters

The effects of the octapeptide cholecystokinin (CCK) on hamster locomotor activity were investigated in three experiments. In Experiment 1, the effect of CCK (25, 50, 75 micrograms/kg) on morphine (2.5 mg/kg)-elicited hyperactivity was studied. Results indicated that CCK antagonized morphine-elicited hyperactivity and that CCK alone elicited hypoactivity. There were no effects of dose of CCK. In Experiment 2, the effects of intraperitoneal (IP) and subcutaneous (SC) routes of administration of CCK (25 micrograms/kg) on locomotor activity were studied. Compared to saline controls, CCK induced hypoactivity that was of greater magnitude and of longer duration when administered IP than SC. Experiment 3 was designed to replicate the route of administration effect observed in Experiment 2 and to determine whether sensitization to CCK-induced hypoactivity develops over the course of a few injections. Results indicated that CCK-induced hypoactivity was greater after IP than SC administration but that sensitization was not detectable. It is concluded that CCK antagonizes morphine-elicited hyperactivity in the hamster by acting, in part, independently of morphine to produce opposite behavioral effects.

Cholecystokinin-induced satiation with ethanol: effects of lighting cycle and limited access procedures

Cholecystokinin (CCK) is a brain-gut neuropeptide and hormone previously shown to inhibit alcohol intake in water- or food-deprived rats. The effects of CCK and the phase of lighting cycle on alcohol intake in rats were investigated in a comparison of limited access and water-restriction procedures. The limited access procedure (LAP) is a recently developed technique for inducing free-choice alcohol consumption in nondeprived animals. Two groups of 12 male rats each were maintained in either normal or reversed 12:12 L:D lighting cycles and simultaneously given 40 minutes' access to 6% w/v ethanol and water in nonhome cages. After adaptation to this procedure, CCK octapeptide (0.5-16 micrograms/kg) was injected IP prior to access to fluids. During LAP, CCK reduced alcohol intake and increased water intake more potently in the dark phase. These effects of CCK were more reliable when the design was replicated, which suggests the importance of acquired expectancies for the development of CCK's actions. CCK more effectively reduced alcohol intake in LAP, than in a 23.3-h water-deprivation procedure for inducing alcohol intake in a 2-bottle choice test with water. However, CCK was less so effective in LAP, than in the water-deprivation procedure when alcohol was presented alone in a 1-bottle test. The alcohol satiation effect of CCK is independent of prior deprivation and not an artifact of thirst reduction, debilitation, or conditioned aversion, because CCK strongly increased water intake in the limited access procedure, and ethanol preference remained robust after experience with CCK. CCK may operate endogenously as a specific factor in satiation with ethanol.

Cholecystokinin and satiation with alcohol

Release of the brain-gut peptide cholecystokinin (CCK) is stimulated by intragastric instillation of ethanol, and peripheral administration of CCK inhibits ethanol consumption. To assess the temporal specificity of the inhibitory effect of CCK on alcohol intake, water-deprived rats were given 5% ethanol at 20, 10 or 0 min after intraperitoneal injections of CCK octapeptide. Delaying access to ethanol for 20 min prevented a significant effect of CCK on intake. CCK's temporally constrained inhibitory action on alcohol consumption is consistent with an ethanol satiation effect. To test the motivational specificity of CCK's effect on fluid intake, rats were allowed a 2-bottle choice of 2% ethanol and water after CCK injections. Ethanol solution intake was suppressed by CCK, and total water intake was unaffected. The putative alcohol satiation action of CCK is appropriately specific to ethanol solution in free-choice tests. Hungry, but not fluid-deprived rats that were either ethanol experienced or naive received a 2-bottle choice of 4% ethanol or water after CCK or saline injections. CCK again specifically inhibited ethanol intake, but this effect required prior ethanol experience. Doses of CCK and naloxone, an opioid receptor blocker, combined to inhibit ethanol intake in an infra-dose-additive manner in water-deprived rats. CCK may act endogenously, in part on opioid receptor-mediated processes, as a preabsorptive satiety signal of ethanol. The full expression of this action appears to depend on prior conditioning of nutritive expectancy of the postingestive effects of alcohol.

Effect of bombesin on behaviors associated with ethanol satiation and blood ethanol levels

The behavioral specificity and physiological significance of bombesin-induced inhibition of ethanol intake were assessed in water-deprived rats. The behavioral display accompanying suppression of 5% ethanol intake by bombesin tetradecapeptide (BBS-14, 1-4 micrograms/kg) was measured with an instantaneous time-sampling technique. Blood ethanol levels were measured after peripheral BBS-14 and bombesin nonapeptide (BBS-9) administration, and after either oral self-administration or peripheral injection of ethanol. The display accompanying BBS-14-reduced ethanol consumption differed from control in that less drinking and feeding behaviors were observed and resting increased, dose-dependently. The typical behavioral sequence of ethanol satiation was observed in all conditions. Both BBS-14 and -9 reduced blood ethanol levels when oral intake was suppressed, and BBS-14 did not affect blood ethanol levels or elimination rate when ethanol was injected. The results are compatible with an hypothesis of a functional role for endogenous bombesin-like peptides and receptors in a neuropeptide control of ethanol intake and energy balance.

Cholecystokinin

1. CCK-peptides are distributed throughout the whole brain with the exception of the cerebellum. 2. There is strong evidence that they act as neuromodulators on the noradrenergic, opioid and mainly dopaminergic system. 3. CCK reduces food-intake. However, tolerance occurs, when chronically given. Thus, potential benefits in the treatment of obesity seem unlikely. 4. CCK increases threshold and tolerance to electrically and thermally induced cutaneous pain. CCK yields relief of pain in colic and ischaemic pain. 5. To date, results about CCK-content in CSF and post-mortem-brain in various psychiatric and neurological diseases related to the dopaminergic system are equivocal. 6. Treatment studies do not provide evidence for beneficial effects of CCK-peptides in schizophrenia.

Specific neuroanatomical and neurochemical correlates of grooming and satiety effects of bombesin

Experiments characterized the behavioral effects of bombesin (BN) administered systemically (1-8 micrograms/kg; IP) or centrally at the nucleus tractus solitarius (NTS) and the nucleus accumbens (NA) (0.0001-1.0 microgram/0.5 microliter). The role of dopaminergic system(s) in the mediation of BN-induced behavioral effects in rats was also investigated. In the "satiety" paradigm, grooming and eating behaviors were monitored in 5-hr-food-deprived rats trained to take a portion of their daily food intake over the 20 min observation period. The elicitation of grooming and satiety appeared to be specific to BN administration at the NTS but not the NA. The dopaminergic system(s) did not appear to mediate the behavior effects of BN intra NTS. The BN-induced grooming profile appeared to be paradigm independent although the baseline grooming profile was not. At the NTS, a dissociation of BN-induced grooming and satiety was evident on the basis of time-course and dose-effect. The similarity of the time-course of satiety induced by BN injected at the NTS and IP suggested that BN may mediate a physiological satiety signal at the NTS. Furthermore, since BN induced satiety 10(4) more potently and 20% more efficaciously intra NTS than IP, the NTS may be a critical site for the action of BN on satiety.

Characteristics of bombesin-induced inhibition of intake of ethanol

The temporal and neural dependencies of the inhibitory effect of the administration of bombesin tetradecapeptide (BBS) on the intake of ethanol were assessed in the water-deprived rat. Variation of the intraperitoneal (i.p.) injection of neuropeptide--5% ethanol access interval (0-20 min), revealed that suppression induced by bombesin (0.5-4.0 micrograms/kg) was significantly greater and more potent at shorter intervals. The intake of ethanol was less in rats with subdiaphragmatic vagotomies, but bombesin equivalently suppressed the intake. Intracerebroventricular injection of bombesin more potently and completely inhibited the intake of ethanol but bombesin injected intraventricularly, unlike that given intraperitoneally, elicited excessive grooming and scratching behavior. The suppressant effect of bombesin, given intraperitoneally, requires close temporal contiguity of administration and caloric solution access, which is consistent with a satiety action of a neuropeptide. This satiation effect to ethanol of peripherally administered bombesin appears to reflect a non-vagal, extra-ventricular neural action.

Oxytocin secretion in response to cholecystokinin and food: differentiation of nausea from satiety

Administration of cholecystokinin (CCK) to rats caused a dose-dependent increase in plasma levels of the neurohypophyseal hormone oxytocin (OT). The OT secretion was comparable to that found in response to nausea-producing chemical agents that cause learned taste aversions. The effect of CCK on OT secretion was blunted after gastric vagotomy, as was the inhibition of food intake induced by CCK. Food ingestion also led to elevated plasma OT in rats, but CCK and aversive agents caused even greater OT stimulation. Thus, after administration of large doses of CCK, vagally mediated activation of central nausea pathways seems to be predominantly responsible for the subsequent decrease in food intake. Despite their dissimilar affective states, both nausea and satiety may activate a common hypothalamic oxytocinergic pathway that controls the inhibition of ingestion.