Mechanisms of conditioned meal initiation

Transcranial Magnetic Stimulation for Reducing the Relative Reinforcing Value of Food in Adult Patients With Obesity Pursuing Metabolic and Bariatric Surgery: Protocol for a Pilot, Within-Participants, Sham-Controlled Trial

BACKGROUND

Metabolic and bariatric surgery (MBS) is the most effective and durable obesity treatment. However, there is heterogeneity in weight outcomes, which is partially attributed to variability in appetite and eating regulation. Patients with a strong desire to eat in response to the reward of palatable foods are more likely to overeat and experience suboptimal outcomes. This subgroup, classified as at risk, may benefit from repetitive transcranial magnetic stimulation (rTMS), a noninvasive brain stimulation technique that shows promise for reducing cravings and consumption of addictive drugs and food; no study has evaluated how rTMS affects the reinforcing value of food and brain reward processing in the context of MBS.

OBJECTIVE

The goal of the Transcranial Magnetic Stimulation to Reduce the Relative Reinforcing Value of Food (RESTRAIN) study is to perform an initial rTMS test on the relative reinforcing value (RRV) of food (the reinforcing value of palatable food compared with money) among adult patients who are pursuing MBS and report high food reinforcement. Using a within-participants sham-controlled crossover design, we will compare the active and sham rTMS conditions on pre- to posttest changes in the RRV of food (primary objective) and the neural modulation of reward, measured via electroencephalography (EEG; secondary objective). We hypothesize that participants will show larger decreases in food reinforcement and increases in brain reward processing after active versus sham rTMS.

METHODS

Participants (n=10) will attend 2 study sessions separated by a washout period. They will be randomized to active rTMS on 1 day and sham rTMS on the other day using a counterbalanced schedule. For both sessions, participants will arrive fasted in the morning and consume a standardized breakfast before being assessed on the RRV of food and reward tasks via EEG before and after rTMS of the left dorsolateral prefrontal cortex.

RESULTS

Recruitment and data collection began in December 2022. As of October 2023, overall, 52 patients have been screened; 36 (69%) screened eligible, and 17 (47%) were enrolled. Of these 17 patients, 3 (18%) were excluded before rTMS, 5 (29%) withdrew, 4 (24%) are in the process of completing the protocol, and 5 (29%) completed the protocol.

CONCLUSIONS

The RESTRAIN study is the first to test whether rTMS can target neural reward circuits to reduce behavioral (RRV) and neural (EEG) measures of food reward in patients who are pursuing MBS. If successful, the results would provide a rationale for a fully powered trial to examine whether rTMS-related changes in food reinforcement translate into healthier eating patterns and improved MBS outcomes. If the results do not support our hypotheses, we will continue this line of research to evaluate whether additional rTMS sessions and pulses as well as different stimulation locations produce clinically meaningful changes in food reinforcement.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05522803; https://clinicaltrials.gov/study/NCT05522803.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/50714.

The sensory properties and metabolic impact of natural and synthetic sweeteners

The global rise in obesity, type II diabetes, and other metabolic disorders in recent years has been attributed in part to the overconsumption of added sugars. Sugar reduction strategies often rely on synthetic and naturally occurring sweetening compounds to achieve their goals, with popular synthetic sweeteners including saccharin, cyclamate, acesulfame potassium, aspartame, sucralose, neotame, alitame, and advantame. Natural sweeteners can be further partitioned into nutritive, including polyols, rare sugars, honey, maple syrup, and agave, and nonnutritive, which include steviol glycosides and rebaudiosides, luo han guo (monk fruit), and thaumatin. We choose the foods we consume largely on their sensory properties, an area in which these sugar substitutes often fall short. Here, we discuss the most popular synthetic and natural sweeteners, with the goal of providing an understanding of differences in the sensory profiles of these sweeteners versus sucrose, that they are designed to replace, essential for the effectiveness of sugar reduction strategies. In addition, we break down the influence of these sweeteners on metabolism, and present results from a large survey of consumers' opinions on these sweeteners. Consumer interest in clean label foods has driven a move toward natural sweeteners; however, neither natural nor synthetic sweeteners are metabolically inert. Identifying sugar replacements that not only closely imitate the sensory profile of sucrose but also exert advantageous effects on body weight and metabolism is critical in successfully the ultimate goals of reducing added sugar in the average consumer's diet. With so many options for sucrose replacement available, consumer opinion and cost, which vary widely with suagr replacements, will also play a vital role in which sweeteners are successful in widespread adoption.

Contributions of Pavlovian incentive motivation to cue-potentiated feeding

Cues signaling the availability of palatable food acquire the ability to potentiate food seeking and consumption. The current study employed a combination of behavioral, pharmacological, and analytical techniques to probe the role of Pavlovian incentive motivation in cue-potentiated feeding. We show that a cue paired with sucrose solution (CS+) can transfer its control over feeding to stimulate sucrose consumption at a new receptacle, and that this effect depends on activation of D1 dopamine receptors, which is known to modulate other forms of cue-motivated behavior but not taste palatability. Microstructural analyses of sucrose-licking behavior revealed that the CS+ tended to increase the frequency with which rats engaged in active bouts of licking behavior without having a reliable effect on the duration of those licking bouts, a measure that was instead associated with sucrose palatability. Furthermore, we found that individual differences in CS+ elicited increases in bout frequency were associated with total sucrose intake at test, supporting the view that this process was related to meaningful dysregulation of eating behavior. The current study, therefore, (1) demonstrates that a dopamine-dependent Pavlovian incentive motivational process can mediate cue-potentiated feeding, and (2) lays out an experimental and analytical approach for parsing this aspect of behavior.

Neural circuits of eating behaviour: Opportunities for therapeutic development

Understanding of the neural and physiological substrates of hunger and satiety has increased rapidly over the last three decades, and pharmacological targets have already been identified for the treatment of obesity that has moved from pre-clinical screening to therapies approved by regulatory authorities. Initially, this review describes the way in which physiological signals of energy availability interact with hedonic and rewarding properties of food to modulate the neural circuitry that supports eating behaviour. This is followed by a brief account of current and promising targets for drug development and a review of the wide range of preclinical paradigms that model important influences on human eating behaviour, and can be used to guide early stages of the drug development process.

Ghrelin signaling in the ventral hippocampus stimulates learned and motivational aspects of feeding via PI3K-Akt signaling

BACKGROUND

The stomach-derived hormone ghrelin drives higher-order feeding processes related to food reward and food seeking via central nervous system signaling at its receptor (GHSR1A). The specific nuclei mediating these effects are only partially understood. Here, we use a rat model to examine whether ghrelin signaling in the ventral subregion of the hippocampus (VHPC), a brain substrate of recent interest in energy balance control, affects learned and motivational aspects of feeding behavior.

METHODS

The effects of VHPC ghrelin administration were examined on feeding-relevant behavioral paradigms, including meal pattern analysis, operant lever pressing for sucrose, and conditioned stimulus-induced feeding. The intracellular signaling and downstream neuronal pathways stimulated by VHPC GHSR1A activation were assessed with immunoblot analysis and behavioral pharmacology.

RESULTS

Ghrelin delivery to the VHPC but not the dorsal hippocampus increased food intake primarily by increasing meal frequency. Intra-VHPC ghrelin delivery also increased willingness to work for sucrose and increased spontaneous meal initiation in nondeprived rats after the presentation of a conditioned stimulus that previously signaled meal access when the rats were food-restricted. The food intake enhancing effects of VHPC ghrelin were blocked by co-administration of a phosphoinositide 3-kinase (PI3K) inhibitor (LY294002). Immunoblot analyses provided complementary support for ghrelin activated PI3K-Akt signaling in the VHPC and revealed that this activation is blunted with high-fat diet consumption. Other immunoblot results show that VHPC GHSR1A signaling activates downstream dopaminergic activity in the nucleus accumbens.

CONCLUSIONS

These findings illuminate novel neuronal and behavioral mechanisms mediating ghrelinergic control of cognitive aspects of feeding control.

Principles of motivation revealed by the diverse functions of neuropharmacological and neuroanatomical substrates underlying feeding behavior

Circuits that participate in specific subcomponents of feeding (e.g., gustatory perception, peripheral feedback relevant to satiety and energy balance, reward coding, etc.) are found at all levels of the neural axis. Further complexity is conferred by the wide variety of feeding-modulatory neurotransmitters and neuropeptides that act within these circuits. An ongoing challenge has been to refine the understanding of the functional specificity of these neurotransmitters and circuits, and there have been exciting advances in recent years. We focus here on foundational work of Dr. Ann Kelley that identified distinguishable actions of striatal opioid peptide modulation and dopamine transmission in subcomponents of reward processing. We also discuss her work in overlaying these neuropharmacological effects upon anatomical pathways that link the telencephalon (cortex and basal ganglia) with feeding-control circuits in the hypothalamus. Using these seminal contributions as a starting point, we will discuss new findings that expand our understanding of (1) the specific, differentiable motivational processes that are governed by central dopamine and opioid transmission, (2) the manner in which other striatal neuromodulators, specifically acetylcholine, endocannabinoids and adenosine, modulate these motivational processes (including via interactions with opioid systems), and (3) the organization of the cortical-subcortical network that subserves opioid-driven feeding. The findings discussed here strengthen the view that incentive-motivational properties of food are coded by substrates and neural circuits that are distinguishable from those that mediate the acute hedonic experience of food reward. Striatal opioid transmission modulates reward processing by engaging frontotemporal circuits, possibly via a hypothalamic-thalamic axis, that ultimately impinges upon hypothalamic modules dedicated to autonomic function and motor pattern control. We will conclude by discussing implications for understanding disorders of "non-homeostatic" feeding.

Promoting children's healthy eating in obesogenic environments: Lessons learned from the rat

Current statistics on children's eating patterns and obesity rates are consistent with the idea that genetic taste predispositions, traditional feeding practices, and the obesogenic environment combine to increase the likelihood of unhealthy outcomes in many individuals. In this paper, we focus on one particular level of analysis through which this unhealthy combination of factors may begin to be disassembled: children's learning about food and flavors. Much of the research on children's learning about food and flavors has been inspired by the animal literature, which has a long history of carefully controlled studies elucidating the mechanisms through which rats and other animals learn to prefer and avoid foods and flavors. This literature provides many clues as to the processes by which learning paradigms may be used to encourage the intake of healthy foods, altering the implicit learning of obesogenic eating patterns that is likely to occur without intervention in the current environment. Overall, the implications of the literature are that children should be repeatedly exposed to a variety of flavors early in life, and that new flavors should be paired with already-liked flavors and positive contexts. This message is consistent with recent research results from our laboratory, showing that familiarization and associative learning paradigms may be used to increase young children's acceptance of, preference for, and intake of previously-unfamiliar, healthy foods.

The basolateral amygdala mediates the effects of cues associated with meal interruption on feeding behavior

Considerable evidence shows that environmental cues that signal food delivery when rats are food-deprived can substantially potentiate feeding later when rats are food-sated. Similarly, cues associated with meal interruption, food removal or impending food scarcity may also induce increased eating. For example, after learning the association between a discrete "interruption" stimulus and the unexpected termination of food trials, sated rats show enhanced food consumption when exposed to that stimulus. In Experiment 1, unlike sham-lesioned controls, rats with bilateral excitotoxic lesions of the basolateral amygdala (BLA) failed to display such cue-potentiated feeding. In Experiment 2, potentiation of feeding by an interruption signal was found to be food-specific. That is, a stimulus that signaled interruption of trials with one food but not trials with a second food later only facilitated consumption of the first food. These studies extend our knowledge of the psychological and neural processes underlying cue-induced feeding. Understanding these mechanisms may contribute our understanding of the etiology and treatment of binge eating disorders.

Involvement of dopamine and opioids in the motivation to eat: influence of palatability, homeostatic state, and behavioral paradigms

RATIONALE

Motivation for food depends on several variables including food palatability, the homeostatic state of the organism, and the nature of the behavior required to obtain the reward. However, few studies to date have tried to evaluate motivation for food considering all these variables at the same time. Since dopamine and opioids have been deeply involved in the regulation of feeding, it is of interest to investigate their role considering all the mentioned variables.

OBJECTIVES

In this study, we evaluated the involvement of dopamine and endogenous opioids on food consumption and food motivation using behavioral paradigms that differ in the motor requirement to gain access to the reward, when food palatability and homeostatic state were taken into account.

MATERIALS AND METHODS

Pellets differentiated on palatability were offered to sated and restricted rats in consummatory tests and in different behavioral paradigms measuring motivational state, but requiring different motor outputs (runway and an operant progressive ratio 3 task). Peripheral injections of naloxone or flupenthixol were administered when these tasks were learned and stable.

RESULTS

Naloxone decreased food intake when pellets were palatable, while flupenthixol was without any effect. When considering motivation, naloxone decreased performances in both the runway and progressive ratio tests while flupenthixol was only effective in the progressive ratio test.

CONCLUSIONS

Impairing the opioid neurotransmission diminishes motivation to obtain food, possibly through a decrease in the perceived palatability of the food reward. The dopaminergic system appears to be more involved in the modulation of motivation to obtain food in a cost/benefit-related manner.

Reinforcer-specificity of appetitive and consummatory behavior of rats after Pavlovian conditioning with food reinforcers

We examined the reinforcer-specificity of Pavlovian conditioning in the control of appetitive and consummatory behaviors in Pavlovian-to-instrumental transfer, cue-potentiated eating, and devaluation procedures. Rats received pairings of one conditioned stimulus with sucrose and another conditioned stimulus with maltodextrin. In Experiment 1, rats were also trained to earn sucrose for one instrumental response and maltodextrin for another. In a transfer test, the Pavlovian cues enhanced the rate of instrumental responding more when the food reinforcer predicted by the instrumental response and the Pavlovian cue were consistent than when they were inconsistent, but both cues enhanced both responses. In Experiment 2, sated rats' consumption of each food was potentiated in the presence of a cue for that food, but not in the presence of a cue for the other food. In Experiment 3, one food was devalued by pairing it with lithium chloride, prior to testing food consumption and food-cup directed behaviors. The food cues selectively controlled food-cup related behaviors, regardless of the presence of the devalued or nondevalued foods in the food cup. Together, these results are consistent with the view that conditioned cues modulate appetitive and consummatory behaviors with increasing levels of specificity. The closer an action comes to ingestion, the more it is controlled by sensory properties conveyed by learned cues. These data are discussed in the context of allostatic regulation of food foraging and intake.

The debate over dopamine's role in reward: the case for incentive salience

INTRODUCTION

Debate continues over the precise causal contribution made by mesolimbic dopamine systems to reward. There are three competing explanatory categories: 'liking', learning, and 'wanting'. Does dopamine mostly mediate the hedonic impact of reward ('liking')? Does it instead mediate learned predictions of future reward, prediction error teaching signals and stamp in associative links (learning)? Or does dopamine motivate the pursuit of rewards by attributing incentive salience to reward-related stimuli ('wanting')? Each hypothesis is evaluated here, and it is suggested that the incentive salience or 'wanting' hypothesis of dopamine function may be consistent with more evidence than either learning or 'liking'. In brief, recent evidence indicates that dopamine is neither necessary nor sufficient to mediate changes in hedonic 'liking' for sensory pleasures. Other recent evidence indicates that dopamine is not needed for new learning, and not sufficient to directly mediate learning by causing teaching or prediction signals. By contrast, growing evidence indicates that dopamine does contribute causally to incentive salience. Dopamine appears necessary for normal 'wanting', and dopamine activation can be sufficient to enhance cue-triggered incentive salience. Drugs of abuse that promote dopamine signals short circuit and sensitize dynamic mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Such drugs interact with incentive salience integrations of Pavlovian associative information with physiological state signals. That interaction sets the stage to cause compulsive 'wanting' in addiction, but also provides opportunities for experiments to disentangle 'wanting', 'liking', and learning hypotheses. Results from studies that exploited those opportunities are described here.

CONCLUSION

In short, dopamine's contribution appears to be chiefly to cause 'wanting' for hedonic rewards, more than 'liking' or learning for those rewards.

Opioids for hedonic experience and dopamine to get ready for it

BACKGROUND AND RATIONALE

More than two decades ago, Wise proposed his "anhedonia hypothesis" to explain the role of dopamine in motivated behaviors. The hypothesis posits that dopamine mediates the pleasure experienced by reward obtainment. However, some experimental findings have contested this hypothesis and several authors have proposed alternative functions for dopamine with regard to motivation. Brain dopamine has been suggested to rather code for the preparatory aspects of behavior, while brain opioids seem to mediate the perception of the hedonic properties of rewards.

OBJECTIVES

The main goal of this review is to reexamine dopamine and opioids involvement in feeding when different aspects such as the anticipatory, motivational and consummatory components of this behavior are taken into account, but also when the physiologic state of the organism and the palatability of the food are considered.

RESULTS AND CONCLUSIONS

Altogether, the data presented point out for an implication of dopamine in the anticipatory/preparatory aspects of feeding more than on the motivational and consummatory aspects. However, dopamine involvement in the anticipatory/preparatory component of feeding seems specifically related to very relevant stimuli, such as highly palatable foods. On the other hand, our data, as well as those present in the literature, strongly suggest a role for opioids in food intake through their modulation of the hedonic perception of food. As a consequence, opioids are involved in those aspects of motivation driven by food palatability rather than by food homeostatic need.

Differential regulation of the consummatory, motivational and anticipatory aspects of feeding behavior by dopaminergic and opioidergic drugs

Various aspects of feeding behavior (eg consumption, motivation and anticipation) are regulated by homeostatic and hedonic systems, and are modulated by dopaminergic and opioid brain systems. Here, we have studied the modulation of these aspects of feeding behavior by opioid and dopaminergic neurotransmission while taking into account food palatability and homeostatic state. Foods that varied in palatability were presented to either food sated or food restricted rats following injections of different doses of naloxone, an opioid receptor antagonist, or flupenthixol, a dopaminergic receptor antagonist, in behavioral paradigms that measured different aspects of feeding. Naloxone decreased food intake in a dose-dependent manner in sated rats given access to palatable food, without modifying food intake in food restricted rats. Flupenthixol did not have any effect on food intake. With regard to motivation, which was tested in a straight alley, naloxone increased the latency to reach the food only in sated rats presented with palatable food. Flupenthixol did not modify the latency of any group. Conditioned locomotor activity to repeated food presentation, a measure of anticipation, is expressed only in food restricted rats. Naloxone did not modify anticipatory activity, whereas flupenthixol decreased it only in food restricted rats presented with palatable food. These results reinforce the idea that the opioid system regulates feeding through the modulation of the perceived palatability of food. The dopaminergic system seems to be more important for the regulation of anticipatory activity related to motivationally relevant stimuli.

Nucleus accumbens opioid, GABaergic, and dopaminergic modulation of palatable food motivation: contrasting effects revealed by a progressive ratio study in the rat

The current studies were designed to evaluate whether incentive motivation for palatable food is altered after manipulations of opioid, GABAergic, and dopaminergic transmission within the nucleus accumbens. A progressive ratio schedule was used to measure lever-pressing for sugar pellets after microinfusion of drugs into the nucleus accumbens in non-food-deprived rats. The mu opioid agonist D-Ala2, NMe-Phe4, Glyo15-enkephalin and the indirect dopamine agonist amphetamine induced a marked increase in break point and correct lever-presses; the GABA(A) agonist muscimol did not affect breakpoint or lever-presses. The data suggest that opioid, dopaminergic, and GABAergic systems within the accumbens differentially modulate food-seeking behavior through mechanisms related to hedonic evaluation of food, incentive salience, and control of motor feeding circuits, respectively.

Blood glucose dynamics and control of meal initiation: a pattern detection and recognition theory

A new framework for understanding the control of feeding behavior, with special emphasis on the evolution of hunger, the initiation of feeding, and its dependence on patterns of blood glucose, is the subject of this review. A perspective on the current status and future directions of this search for a more complete understanding of the regulation of feeding behavior in laboratory rats and humans is presented including theoretical and experimental components. First, a historical perspective on the role of blood glucose in the control of feeding is presented. Next, the theoretical approaches that have been applied to the control of feeding and had a strong influence on experimental feeding research are summarized. This is followed by a statement and overview of a current theory that has emerged from studies of the role of transient declines in blood glucose in the control of meal initiation. The current working hypothesis that transient declines in blood glucose are endogenous metabolic patterns that are detected and recognized by the central nervous system and are mapped into meal initiation in rats and are correlated with meal requests in humans are then presented. Then, the experimental studies on meal initiation and its dependence on patterns of blood glucose, first in rats and then in humans, are reviewed in detail. Finally, the future directions of the work, limitations, and the implications for the understanding of the control of feeding behavior and the regulation of energy balance are discussed.

The effects of amygdala lesions on conditioned stimulus-potentiated eating in rats

Both control rats and rats with neurotoxic lesions of the amygdala central nucleus ate more food during presentations of a conditioned stimulus (CS) previously paired with food than during an unpaired CS. This potentiation occurred regardless of whether the food was presented in its usual place or in a different location. By contrast, rats with neurotoxic lesions of basolateral amygdala showed no evidence for conditioned potentiation of eating. These results are considered in the context of anatomical projections from these amygdalar areas to other brain regions involved in feeding, and the role of amygdala subregions in the acquisition of motivational value in conditioning.

Enhanced conditioned approach responses in transgenic mice with impaired glucocorticoid receptor function

The long-term consequences of impaired glucocorticoid receptor (GR) function on reward-related learning were studied in transgenic mice with impaired GR function in a series of experiments taxing conditioned and unconditioned approach responses to stimuli predictive of food. There was a double-dissociation in that transgenic mice with impaired GR activity showed enhanced conditioned exploration in situations when stimuli predicted reward, while free-feeding food consumption over 24 h was reduced. Previous experiments have shown altered accumbens dopaminergic activity in these animals. In line with these findings, we observed an enhanced behavioural stimulation of transgenic mice following administration of d-amphetamine (2 mg/kg). This suggests that the increase in preparatory responses in transgenic mice may be mediated via an enhanced accumbens dopaminergic activity, possibly secondary to alterations in other brain systems.

Haloperidol does not attenuate conditioned place preferences or locomotor activation produced by food- or heroin-predictive discriminative cues

The present study examined whether a discriminative cue previously predictive of food or heroin reinforcement could activate and direct behavior in an environment that had never been paired with primary reinforcement. Olfactory cues, predicting the availability (S+) or unavailability (S-) of either heroin (0.1 mg/kg i.v.) or food (45 mg Noyes food pellets) reinforcement in the goal box of a straight-arm runway, were later tested in a separate environment for their ability to elicit locomotion (activate behavior) or induce a conditioned place preference (direct behavior). Presentation of the S+, but not the S-, resulted in a reliable increase in spontaneous locomotor activity that was not blocked by pretreatment with the dopamine receptor antagonist, haloperidol. Similarly, subjects displayed a preference for a novel location in which the S+, but not the S-, was placed. This preference was also unaltered by pretreatment with haloperidol. These data suggest that discriminative cues can profoundly affect behavior, even in environments that have themselves never been associated with primary reinforcement. Additionally, the conditioned motivational quality of these cues is unaltered by treatment with the same dopamine receptor antagonist shown in previous work to attenuate the primary reinforcing properties of heroin and food.

Presentation of an ethanol-paired stimulus complex alters response patterns during extinction

It has been hypothesized that environmental stimuli previously paired with ethanol consumption play a role in excessive ethanol intake. This study examined the ability of orally self-administered ethanol to establish a tone-light stimulus complex as a conditioned reinforcer (CSR). Male Long-Evans rats were trained to orally self-administer 10% ethanol (10E) using the sucrose-substitution procedure. During training, a tone-light stimulus complex was paired with ethanol presentation in a stimulus complex paired (SC-paired) group but not in a control group. Responding during extinction in the presence and absence of the stimulus complex was then examined. Following the initiation of ethanol self-administration, 10E maintained greater responding in the SC-paired group compared to the control group. When the stimulus complex was presented contingent on responding during extinction, the rate of extinction was slightly attenuated in the SC-paired group but not in the control group. The altered rate of extinction in the SC-paired group was characterized by: 1) a slight decrease in total session responding over successive days of extinction and 2) a transient attenuation of extinction burst response rate during the first extinction session. These data suggest the stimulus complex could function as a weak CS(R), but overall its ability to maintain lever pressing was minimal.

Evidence that oral and nutrient reinforcers differentially condition appetitive and consummatory responses to flavors

Rats tend to increase their intake of a flavor that has previously been paired with either sweet taste or with caloric repletion. However, it is unclear whether such a change in intake is caused by changes in appetitive behaviors such as orienting and approach, or changes in consummatory behaviors and oral responsiveness. Also, it is unclear whether oral reinforcers (sweetness) and postingestive reinforcers (nutrients) lead to the same kinds of behavioral change. In the current experiments, weanling rats with oral and gastric cannulas repeatedly experienced a flavor paired with either sweetness, high caloric density, or neither. Rats were then tested for differences in appetitive olfactory orienting and consummatory oral responsiveness elicited by the flavor. Results suggest that oral reinforcement (sweetness) produces conditioning of appetitive responding to the flavor, while postingestive reinforcement produces conditioning of consummatory responding. A second experiment indicates that these behavioral changes are specific increases in responsiveness conditioned by flavor + unconditioned stimulus (US) pairing, and are unlikely to be nonspecific effects of daily unconditioned stimulus exposure.

Metabolic and hormonal controls of food intake: highlights of the last 25 years--1972-1997

The six major research advances in metabolic and hormonal controls of food intake that have altered the direction or have broadened the scope of the field in the last 25 years are discussed. The advances selected are: (1) GI processes and meal termination-the CCK pathway; (2) Brain insulin hypothesis; (3) Glucose-dependent processes in periphery, plasma, and brain including the transient declines in blood glucose signaling meal initiation; (4) Fatty acid oxidation in the liver; (5) Behavioral and metabolic patterns; and (6) New pathways from molecular genetics and molecular biology-the OB protein pathway.

Locating reward cue at response manipulandum (CAM) induces symptoms of drug abuse

Appetitive instrumental discrimination learning procedures provide for CAM (cue and manipulandum) when the reward cue (discriminative stimulus positively correlated with positive reinforcement) is located at the response manipulandum (object that when contacted or manipulated defines the performance of the instrumental response). Evidence reviewed shows that CAM induces excessive and compulsive instrumental responding relative to otherwise comparable non-CAM control procedures. In humans, symptoms of drug abuse are particularly likely when the drug-taking implement (response manipulandum at which instrumental drug-taking is directed) is also predictive of the drug's rewarding effects (reward cue). Evidence that the predictive relationship between a drug-taking implement and drug reward relates to drug abuse is reviewed, and implications for treatment and prevention are considered. CAM is related to neurobiological models of drug abuse that emphasize the role of the neurotransmitter dopamine (DA). CAM produces convergence of DA-mediated responding for conditioned reinforcement with DA mediation of psychomotor activation and incentive-motivational processes to yield reflexive cue-directed responding not observed in non-CAM controls.

Procurement time as a determinant of meal frequency and meal duration

Foraging involves the expenditure of both time and effort in the acquisition of food; animals typically modify their meal patterns so as to reduce these expenditures or costs. The contribution of time, as compared with effort, to the overall cost perceived by an animal is not known. We investigated the effect of foraging time as a cost independent of effort by measuring the meal patterns of rats living in a laboratory foraging simulation in which they earned all their daily intake. They pressed a bar once to initiate an interval (procurement interval) leading to the presentation of a large cup of food from which they could eat a meal of any size. As the length of the interval increased from 1 s to 46 hr, meal frequency decreased regularly. Meal size increased in a compensatory fashion, and total daily intake was conserved through an interval of 23 hr. The changes in meal frequency occurred because of changes in the rat's latency to bar press after each meal. The functions relating meal frequency and size to the procurement interval were of the same shape as those seen when cost is the completion of a bar-press requirement, which entails the expenditure of both effort and time. When the bar-press requirement was increased to 10, meal frequency was reduced, but time and effort did not appear to simply add together in the rat's perception of cost. These data reveal that time is preceived to be a cost by rats foraging in this laboratory environment. These results suggest that the time parameters of foraging are different from those of consumption.

Dopamine functions in appetitive and defensive behaviours

The data reviewed here are compatible with the hypothesis that telencephalic dopamine activity is elicited by motivationally significant stimuli which in turn creates a neural state in which animals are more prepared to respond to significant stimuli in the environment. This analysis may be viewed as extensions of both the sensorimotor hypothesis, which depicts dopamine as potentiating the ability of stimuli to elicit responses (Clody and Carlton, 1980; Marshall et al., 1974; White, 1986) and of the incentive motivational hypothesis, which emphasizes the importance of dopamine in responding to stimuli that serve as signals of biologically significant events (Blackburn et al., 1989a; Crow, 1973; Mogenson and Phillips, 1976). In addition, we have sought to emphasize that not all responses are equally dependent upon the integrity of forebrain dopamine activity. Some responses, such as ingestion of standard foods by hungry animals, copulation, and escape, are relatively impervious to dopamine disruption. Further, once other behaviours, such as avoidance or appetitive operant responses, have been acquired, they can be maintained at an initially high rate despite perturbation of dopamine systems, although performance deteriorates with repeated testing. This analysis has emerged from the joint consideration of how both appetitive and defensive behaviours are influenced by dopamine antagonists, along with an examination of dopamine release during sequences of behaviour. The data reviewed suggest that dopamine is involved in fundamental psychological processes through which environmental stimuli come to exert control over certain aspects of behaviour. In the future, as knowledge in this field advances, there will have to be an integration of the literature on dopamine and motivation with the literature on dopamine and motor systems. We expect that dopamine release will be seen as a mechanism by which important environmental cues, of innate or learned significance, lead to a general enhancement of motor skeletal responses directed towards distal cues. We conclude with a caveat: Caution must be exercised when attempting to infer a general role of any neurotransmitter in motivated behaviour based on the study of a limited number of motivational systems. Although neurotransmitter pathways may figure prominently in the control of certain behaviours, it is incorrect to think of neurotransmitters as having a single role in behaviour. However, when comparative analyses reveal a common thread among different motivational systems, as is becoming apparent for the general role of mesotelencephalic dopamine pathways in behaviour, then the goal of generating coherent and comprehensive theory concerning a neurotransmitter's function in behaviour will begin to be realised.(ABSTRACT TRUNCATED AT 400 WORDS)