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

Pavlovian or associative sensitization and its biological significance

Pavlovian conditioning is typically distinguished from sensitization but a Pavlovian conditional stimulus (CS) also results in sensitization. A Pavlovian CS can sensitize responding to a probe stimulus that is related to the unconditional stimulus (US) or to the US itself. Pavlovian sensitization has been studied in the defensive, sexual, and feeding systems. In Pavlovian sensitization, the focus is not on a conditional response (CR) directly elicited by the CS but on the response mode that is activated by the CS. Activation of a response mode increases the probability of particular responses and also increases reactivity to various stimuli. Pavlovian sensitization reflects this increased stimulus reactivity. Pavlovian sensitization helps uncover successful learning in situations where a conventional CR does not occur. Pavlovian sensitization also encourages broadening our conceptions of Pavlovian conditioning to include changes in afferent processes. Implications for biological fitness and for basic and translational research are discussed.

A laboratory model of canine search vigilance decrement, II: Noncontingent reward and Pavlovian appetitive stimuli

Detection dogs have demonstrated reduced performance in operational settings when required to search in an environment where few to no target odors are present. This study's purpose was to increase detection dog accuracy using noncontingent reward (NCR) and Pavlovian stimuli associated with reward. Eighteen dogs were randomly spilt into two groups and received four 40-trial sessions in an operational and training context at 90% odor prevalence (baseline). Following baseline, in the operational context (now at 10% odor prevalence), experimental dogs received an NCR schedule consisting of delivering food rewards at the end of 66% of trials. After the NCR Test, dogs returned to baseline. During baseline, the experimental dogs received 10 days of delayed Pavlovian conditioning to a tone. During the test phase, the conditioned stimulus (tone) was presented to experimental dogs on average every two trials for 30 s in the operational context (now at 10% odor prevalence). Overall, NCR showed a nonsignificant trend for increased responding in the experimental group but tended to increase false alerts; therefore, a permutation of an NCR-like reward schedule may maintain search. The Pavlovian conditioned stimulus didn't decrease timeouts or improve accuracy, but a within-session analysis indicated that the dogs were more likely to time out and less likely to false alert when the tone was on than when it was off.

Visual cues associated with sweet taste increase short-term eating and grab attention in healthy volunteers

Most studies that examine responses to food cues use images of actual foods as stimuli. Since foods are rewarding in multiple ways, it then becomes difficult to try and partial out the role of the importance of different aspects of food reward. Here we aimed to evaluate the impact of novel visual cues specifically associated with the immediate sensory reward from a liked sweet taste. In the training phase, one visual cue (CSsweet) was associated with the experience of sweet taste (10%sucrose) and a second, control cue (CSneutral) with a neutral taste (artificial saliva) using a disguised training procedure. In Experiment 1, participants (n = 45) were given an ad libitum snack intake test 30 min post-training, either labelled with CSsweet or CSneutral. Total caloric consumption was significantly higher in the CSsweet (650 ± 47 kcal) than CSneutral (477 ± 45 kcal) condition, but ratings of liking for the snacks did not differ significantly between conditions. In Experiment 2, participants (n = 80) exhibited an overall attentional bias (22.1 ± 9.9 ms) for the CSsweet relative to CSneutral cue (assessed using a dot-probe task), however rated liking for the CSsweet did not change significantly after cue-sweet training. Likewise, measures of expected satiety for drinks labelled with CSsweet did not differ significantly from CSneutral. Overall these two experiments provide evidence that associations between neutral visual cues and the experience of a liked sweet taste leads to cue-potentiated eating in the presence of the CSsweet cue. With no evidence that cue-sweet training altered rated liking for the visual cues, and in keeping with extant literature on the dissociation of hedonic and rewarding properties of food rewards, we propose this potentiation effect to reflect increased incentive salience.

Junk-food enhances conditioned food cup approach to a previously established food cue, but does not alter cue potentiated feeding; implications for the effects of palatable diets on incentive motivation

Efforts to stem the global rise in obesity have been minimally effective, perhaps in part because our understanding of the psychological and behavioral drivers of obesity is limited. It is well established that stimuli that are paired with palatable foods can powerfully influence food-seeking and feeding behaviors. However, how consumption of sugary, fatty "junk-foods" affects these motivational responses to food cues is poorly understood. Here, we determined the effects of short- and long-term "junk-food" consumption on the expression of cue potentiated feeding and conditioned food cup approach to Pavlovian conditioned stimuli (CS). Further, to determine the degree to which effects of "junk-food" were selective to Pavlovian motivational processes, we varied the predictive validity of the CS by including training groups conditioned with unique CS-US contingencies ranging from -1.0 to +1.0. "Junk-food" did not enhance cue potentiated feeding in any group, but expression of this potentiation effect varied with the CS-US contingency independent of diet. In contrast, "junk-food" consistently enhanced conditioned approach to the food cup; this effect was dependent on the previously established CS-US contingency. That is, consumption of "junk-food" following training enhanced approach to the food cup only in response to CSs with previously positive CS-US contingencies. This was accompanied by reduced motivation for the US itself. Together these data show that "junk-food" consumption selectively enhances incentive motivational responses to previously established food CSs, without altering cue potentiated feeding induced by these same CSs, and in the absence of enhanced motivation for food itself.

Examining the influence of CS duration and US density on cue-potentiated feeding through analyses of licking microstructure

In the current study, groups of mice were trained with either short (20 s) or long (120 s) conditioned stimulus (CS) durations associated with different rates of sucrose unconditioned stimulus (US) delivery, to examine whether different behavioral forms of cue-potentiated feeding in sated mice would be evoked. In training mice received presentations of an auditory CS for 20 s during which a sucrose US was delivered at a density of 1/9 s (Group-20-s). A second group of mice received an auditory CS for 120 s and a US density of 1/49 s (Group-120-s). During training, a shorter CS duration and higher rate of US delivery resulted in greater acquisition of food cup responding, and during the test stage Group-20-s mice also displayed higher CS evoked lick rates, though all mice showed cue-potentiated feeding. An analysis of licking microstructure also revealed that Group-120-s mice displayed CS evoked licking behavior that reflected an increase in the perceived palatability of the sucrose US. These findings are discussed with respect to the influence of CS interval and US density on associatively activated sensory and affective representations of a US, and contrast mediated effects resulting from presentation of excitatory and inhibitory conditioned stimuli.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Blaming the Brain for Obesity: Integration of Hedonic and Homeostatic Mechanisms

The brain plays a key role in the controls of energy intake and expenditure, and many genes associated with obesity are expressed in the central nervous system. Technological and conceptual advances in both basic and clinical neurosciences have expanded the traditional view of homeostatic regulation of body weight by mainly the hypothalamus to include hedonic controls of appetite by cortical and subcortical brain areas processing external sensory information, reward, cognition, and executive functions. Hedonic controls interact with homeostatic controls to regulate body weight in a flexible and adaptive manner that takes environmental conditions into account. This new conceptual framework has several important implications for the treatment of obesity. Because much of this interactive neural processing is outside awareness, cognitive restraint in a world of plenty is made difficult and prevention and treatment of obesity should be more rationally directed to the complex and often redundant mechanisms underlying this interaction.

Extended exposure to environmental cues, but not to sucrose, reduces sucrose cue reactivity in rats

In the present study, we examined the effects of extinction of sucrose-predictive contextual cues and/or sucrose satiation on the expression of sucrose cue reactivity in a rat model of relapse. Context extinction was imposed by housing rats in their home cage or in the operant conditioning chamber for 17 h prior to testing. For sucrose satiation, rats were allowed unlimited access to water or sucrose for 17 h prior to testing. Cue reactivity was assessed after either one (Day 1) or 30 (Day 30) days of forced abstinence from sucrose self-administration. An abstinence-dependent increase in sucrose cue reactivity was observed in all conditions ("incubation of craving"). Context extinction dramatically reduced lever responding on both Day 1 and Day 30. Sucrose satiation had no significant effect on cue reactivity in any condition. These results demonstrate that the context in which self-administration occurs maintains a powerful influence over cue reactivity, even after extended forced abstinence. In contrast, the primary reinforcer has little control over cue reactivity. These findings highlight the important role of conditioned contextual cues in driving relapse behavior.

The antagonism of ghrelin alters the appetitive response to learned cues associated with food

The rapid increase in obesity may be partly mediated by an increase in the exposure to cues for food. Food-paired cues play a role in food procurement and intake under conditions of satiety. The mechanism by which this occurs requires characterization, but may involve ghrelin. This orexigenic peptide alters the response to food-paired conditioned stimuli, and neural responses to food images in reward nuclei. Therefore, we tested whether a ghrelin receptor antagonist alters the influence of food-paired cues on the performance of instrumental responses that earn food and the consumption of food itself using tests of Pavlovian-to-instrumental transfer (PIT) and cue potentiated feeding (CPF), respectively. Food-deprived rats received Pavlovian conditioning where an auditory cue was paired with delivery of sucrose solution followed by instrumental conditioning to lever press for sucrose. Following training, rats were given ad libitum access to chow. On test day, rats were injected with the ghrelin receptor antagonist GHRP-6 [D-Lys3] and then tested for PIT or CPF. Disrupting ghrelin signaling enhanced expression of PIT. In addition, GHRP-6 [D-Lys3] impaired the initiation of feeding behavior in CPF without influencing overall intake of sucrose. Finally, in PIT tested rats, enhanced FOS immunoreactivity was revealed following the antagonist in regions thought to underlie PIT; however, the antagonist had no effect on FOS immunoreactivity in CPF tested rats.

Implications of learning theory for developing programs to decrease overeating

Childhood obesity is associated with medical and psychological comorbidities, and interventions targeting overeating could be pragmatic and have a significant impact on weight. Calorically dense foods are easily available, variable, and tasty which allows for effective opportunities to learn to associate behaviors and cues in the environment with food through fundamental conditioning processes, resulting in measurable psychological and physiological food cue reactivity in vulnerable children. Basic research suggests that initial learning is difficult to erase, and that it is vulnerable to a number of phenomena that will allow the original learning to re-emerge after it is suppressed or replaced. These processes may help explain why it may be difficult to change food cue reactivity and overeating over the long term. Extinction theory may be used to develop effective cue-exposure treatments to decrease food cue reactivity through inhibitory learning, although these processes are complex and require an integral understanding of the theory and individual differences. Additionally, learning theory can be used to develop other interventions that may prove to be useful. Through an integration of learning theory, basic and translational research, it may be possible to develop interventions that can decrease the urges to overeat, and improve the weight status of children.

Investigating habits: strategies, technologies and models

Understanding habits at a biological level requires a combination of behavioral observations and measures of ongoing neural activity. Theoretical frameworks as well as definitions of habitual behaviors emerging from classic behavioral research have been enriched by new approaches taking account of the identification of brain regions and circuits related to habitual behavior. Together, this combination of experimental and theoretical work has provided key insights into how brain circuits underlying action-learning and action-selection are organized, and how a balance between behavioral flexibility and fixity is achieved. New methods to monitor and manipulate neural activity in real time are allowing us to have a first look “under the hood” of a habit as it is formed and expressed. Here we discuss ideas emerging from such approaches. We pay special attention to the unexpected findings that have arisen from our own experiments suggesting that habitual behaviors likely require the simultaneous activity of multiple distinct components, or operators, seen as responsible for the contrasting dynamics of neural activity in both cortico-limbic and sensorimotor circuits recorded concurrently during different stages of habit learning. The neural dynamics identified thus far do not fully meet expectations derived from traditional models of the structure of habits, and the behavioral measures of habits that we have made also are not fully aligned with these models. We explore these new clues as opportunities to refine an understanding of habits.

Forebrain networks and the control of feeding by environmental learned cues

The motivation to eat is driven by a complex sum of physiological and non-physiological influences computed by the brain. Physiological signals that inform the brain about energy and nutrient needs are the primary drivers, but environmental signals unrelated to energy balance also control appetite and eating. The two components could act in concert to support the homeostatic regulation of food intake. Often, however, environmental influences rival physiological control and stimulate eating irrespective of satiety, or inhibit eating irrespective of hunger. If persistent, such maladaptive challenges to the physiological system could lead to dysregulated eating and ultimately to eating disorders. Nevertheless, the brain mechanisms underlying environmental contribution in the control of food intake are poorly understood. This paper provides an overview in recent advances in deciphering the critical brain systems using rodent models for environmental control by learned cues. These models use associative learning to compete with the physiological control, and in one preparation food cues stimulate a meal despite satiety, while in another preparation fear cues stop a meal despite hunger. Thus far, four forebrain regions have been identified as part of the essential cue induced feeding circuitry. These are telencephalic areas critical for associative learning, memory encoding, and decision making, the amygdala, hippocampus and prefrontal cortex and the lateral hypothalamus, which functions to integrate feeding, reward, and motivation. This circuitry also engages two orexigenic peptides, ghrelin and orexin. A parallel amygdalar circuitry supports fear cue cessation of feeding. These findings illuminate the brain mechanisms underlying environmental control of food intake and might be also relevant to aspects of human appetite and maladaptive overeating and undereating.

Learned food-cue stimulates persistent feeding in sated rats

Cues that predict food can stimulate appetite and feeding independent of physiological hunger. How long such effects might last is currently unknown. Here we began to characterize long-term effects in a rodent model of cue-potentiated feeding. Rats were conditioned to associate a tone with food pellets distinct from their regular laboratory chow, and then were tested along with controls for food consumption following tone presentations. In Experiment 1, rats were tested under sated or food-deprived conditions to determine whether fasting would augment cue-driven feeding. Rats in the control group regulated intake based on physiological state, while conditioned rats consumed similar large amounts of food regardless. Experiment 2 tested the durability of cue-potentiated feeding to repeated testing in sated rats. We observed robust cue-potentiated feeding during the first two tests, while in the third and fourth tests both groups ate similar large amounts of pellets. In both experiments the conditioned tone-cue induced binge-like consumption of the cued food and persistent feeding for the duration of 4-h tests. Rats then failed to adjust daily chow consumption to account for their increased intake post-cue. In summary, brief cue priming stimulated substantial intake in sated states that was behaviorally uncompensated for by homeostatic mechanisms.

Pavlovian to instrumental transfer of control in a human learning task

Pavlovian learning tasks have been widely used as tools to understand basic cognitive and emotional processes in humans. The present studies investigated one particular task, Pavlovian-to-instrumental transfer (PIT), with human participants in an effort to examine potential cognitive and emotional effects of Pavlovian cues upon instrumentally trained performance. In two experiments, subjects first learned two separate instrumental response-outcome relationships (i.e., R1-O1 and R2-O2) and then were exposed to various stimulus-outcome relationships (i.e., S1-O1, S2-O2, S3-O3, and S4-) before the effects of the Pavlovian stimuli on instrumental responding were assessed during a non-reinforced test. In Experiment 1, instrumental responding was established using a positive-reinforcement procedure, whereas in Experiment 2, a quasi-avoidance learning task was used. In both cases, the Pavlovian stimuli exerted selective control over instrumental responding, whereby S1 and S2 selectively elevated the instrumental response with which it shared an outcome. In addition, in Experiment 2, S3 exerted a nonselective transfer of control effect, whereby both responses were elevated over baseline levels. These data identify two ways, one specific and one general, in which Pavlovian processes can exert control over instrumental responding in human learning paradigms, suggesting that this method may serve as a useful tool in the study of basic cognitive and emotional processes in human learning.

Modeling appetitive Pavlovian-instrumental interactions in mice

In appetitive Pavlovian associative learning, a stimulus (conditioned stimulus, CS) that has been associated with the delivery of a reinforcing event (unconditioned stimulus, US; e.g., food) can subsequently elicit or modulate goal-directed instrumental behaviors. For example, a Pavlovian CS can serve to reinforce (novel) instrumental behavior (conditioned reinforcement or CRf), or it can energize and potentiate ongoing instrumental responses when presented non-contingently (Pavlovian-instrumental transfer or PIT). Notably, these different effects of a Pavlovian CS on instrumental behavior are mediated by dissociable psychological and neurobiological mechanisms. Given the critical role that Pavlovian-instrumental interactions play in regulating motivated behavior and maladaptive manifestations of motivation such as eating disorders and addictions, understanding the underlying psychological and neurobiological mechanisms will be important. This unit describes behavioral protocols that produce robust and reliable PIT and CRf in mice and that open the door for future studies using transgenic approaches into the molecular mechanisms underlying associative learning and motivation.

Learning and the motivation to eat: forebrain circuitry

Appetite and eating are not only controlled by energy needs, but also by extrinsic factors that are not directly related to energy balance. Environmental signals that acquire motivational properties through associative learning-learned cues-can override homeostatic signals and stimulate eating in sated states, or inhibit eating in states of hunger. Such influences are important, as environmental factors are believed to contribute to the increased susceptibility to overeating and the rise in obesity in the developed world. Similarly, environmental and social factors contribute to the onset and maintenance of anorexia nervosa and other eating disorders through interactions with the individual genetic background. Nevertheless, how learning enables environmental signals to control feeding, and the underlying brain mechanisms are poorly understood. We developed two rodent models to study how learned cues are integrated with homeostatic signals within functional forebrain networks, and how these networks are modulated by experience. In one model, a cue previously paired with food when an animal was hungry induces eating in sated rats. In the other model, food-deprived rats inhibit feeding when presented with a cue that signals danger, a tone previously paired with footshocks. Here evidence will be reviewed that the forebrain network formed by the amygdala, lateral hypothalamus and medial prefrontal cortex mediates cue-driven feeding, while a parallel amygdalar circuitry mediates suppression of eating by the fear cue. Findings from the animal models may be relevant for understanding aspects of human appetite and eating, and maladaptive mechanisms that could lead to overeating and anorexia.

Differential outcome effects in pavlovian biconditional and ambiguous occasion setting tasks

Three experiments with rats explored the differential outcome effect (DOE) using a pavlovian magazine approach conditioning preparation. Experiment 1 compared groups trained on a biconditional discrimination (AX+, AY-, BX-, BY+) with differential or nondifferential outcomes, and Experiment 2 examined this using an ambiguous occasion setting task (e.g., AX+, X-, Y+, AY-). In both experiments, subjects trained with differential outcomes learned the tasks better than subjects trained with nondifferential outcomes. Furthermore, subjects given differential outcome training learned the positive occasion setting component of the ambiguous task more efficiently than the negative occasion setting component, although both were enhanced by differential outcome training. Experiment 3 demonstrated that the ambiguous occasion setting task was reversed more readily when the target-outcome relations (as opposed to the modulator-outcome relations) were maintained during the reversal phase. These data suggest that an acquired distinctiveness effect may be responsible for the DOE in pavlovian learning.

Negative anticipatory contrast: does it involve anticipation of an impending reward?

Negative anticipatory contrast (NAC) corresponds to the suppression in consumption of a first rewarding substance (e.g., saccharin 0.15%) when it is followed daily by a second preferred substance (e.g., sucrose 32%). The NAC has been interpreted as resulting from anticipation of the impending preferred reward and its comparison with the currently available first reward [Flaherty, C.F., Rowan, G.A., 1985. Anticipatory contrast: within-subjects analysis. Anim. Learn. Behav. 13, 2-5]. In this context, one should expect that devaluation of the preferred substance after the establishment of the NAC would either reduce or abolish the contrast effect. However, contrary to this prediction, the results of the present study show that the NAC is insensitive to devaluation of the second, preferred, substance. This allows one to question that interpretation. The results reported in this study support the view that the NAC effect is controlled by memory of the relative value of the first solution, which is updated daily by means of both a gustatory and/or post-ingestive comparison of the first and second solutions, and memory of past pairings.

Forebrain circuits and control of feeding by learned cues

Professor Richard F. Thompson and his highly influential work on the brain substrates of associative learning and memory have critically shaped my research interests and scientific approach. I am tremendously grateful and thank Professor Thompson for the support and influence on my research and career. The focus of my research program is on associative learning and its role in the control of fundamental, motivated behaviors. My long-term research goal is to understand how learning enables environmental cues to control feeding behavior. We use a combination of behavioral studies and neural systems analysis approach in two well-defined rodent models to study how learned cues are integrated with homeostatic signals within functional forebrain networks, and how these networks are modulated by experience. Here, I will provide an overview of the two behavioral models and the critical neural network components mapped thus far, which include areas in the forebrain, the amygdala and prefrontal cortex, critical for associative learning and decision-making, and the lateral hypothalamus, which is an integrator for feeding, reward and motivation.

Pavlovian to instrumental transfer: a neurobehavioural perspective

Pavlovian-to-instrumental transfer (PIT) is a key concept in developing our understanding of cue-controlled behaviours. Here we have reviewed the literature on behavioural and neurobiological factors that influence PIT. Meta-analyses of the data for individual groups in PIT studies revealed that PIT is related to both the order and amounts of instrumental and Pavlovian training, and that it is critically determined by competition between instrumental and Pavlovian responses. We directly addressed the role of response competition in PIT in two experiments which showed that extensive Pavlovian conditioning produced more Pavlovian magazine visits and weaker PIT than moderate Pavlovian conditioning (Experiment 1); and that PIT lost after extensive Pavlovian conditioning was restored by Pavlovian extinction training (Experiment 2). These findings confirm that response competition is indeed an important determinant of PIT. This has significant implications for lesion and inactivation studies that assess the neurobiological substrates of PIT, as well as attempts to demonstrate PIT in the drug self-administration paradigm where the effect is yet to be reliably shown.

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.

Decreased firing of striatal neurons related to licking during acquisition and overtraining of a licking task

Neurons that fire in relation to licking, in the ventral part of the dorsolateral striatum (DLS), were studied during acquisition and performance of a licking task in rats for 14 sessions (2 h/d). Task learning was indicated by fewer errors of omission of licking and improved movement efficiency (i.e., shorter lick duration) over sessions. Number of licks did not change over sessions. Overtraining did not result in habit formation, as indicated by similar reductions of licking responses following devaluation by satiety in both early and late sessions. Twenty-nine lick neurons recorded and tracked over sessions exhibited a significant linear decrease in average firing rate across all neurons over sessions, correlating with concurrent declines in lick duration. Individually, most neurons (86%) exhibited decreased firing rates, while a small proportion (14%) exhibited increased firing rates, during lick movements that were matched over sessions. Reward manipulations did not alter firing patterns over sessions. Aside from the absence of habit formation, striatal processing during unconditioned movements (i.e., licking) was characterized by high activity of movement-related neurons during early performance and decreased activity of the same neurons during overtraining, similar to our previous report of head movement neurons during acquired, skilled, instrumental head movements that ultimately became habitual (Tang et al., 2007). Decreased activity in DLS neurons may reflect a common neural mechanism underlying improvement in movement efficiency with overtraining. Nonetheless, the decreased striatal firing in relation to a movement that did not become habitual demonstrates that not all DLS changes reflect habit formation.

Withdrawal from free-choice high-fat high-sugar diet induces craving only in obesity-prone animals

INTRODUCTION

Vulnerability for weight gain is an individual trait. Obese people undertake dieting, but permanent weight loss is difficult to attain due to repeated phases of relapse to excess consumption.

MATERIALS AND METHODS

In this study, male Wistar rats were trained to operantly self-administer pellets followed by free-choice access in the homecage to high-fat high-sugar (HFHS) diet consisting of 30% sucrose, lard, standard rodent chow and water. Animals were divided into obesity-prone (OP) and obesity-resistant (OR) groups based on relative weight gain compared to normally fed controls despite equal consumption of HFHS.

RESULTS AND DISCUSSION

After 4 weeks of HFHS access, OP and OR animals did not differ in motivation for food pellets in terms of progressive ratio break point, lever pressing or response rate. However, upon discontinuation of the HFHS diet, differences between the OP and OR groups were noted. OP animals increased their motivation (i.e. craving) during the second withdrawal week and reduced time spent in the centre of an open field (increased anxiety) compared to the OR animals. Both OP and OR animals consumed less of the standard rodent chow during the first week of withdrawal when compared to normally fed controls. But, while the OR animals quickly returned to control levels of food consumption, OP animals continued to consume less standard rodent chow.

CONCLUSION

The results show for the first time that withdrawal from free-choice HFHS induces craving that is specific to the OP animals and suggests that OP individuals may have withdrawal symptoms that are similar to those induced by addictive drugs.

Effects of cues associated with meal interruption on feeding behavior

Food consumption is controlled by both internal and external factors. Environmental signals associated with food may prepare an animal to forage, consume and digest more effectively. Furthermore, environmental cues that provide information about food availability enable animals to make predictions about future food resources and act upon that knowledge in appropriate fashion. For example, when exposed to a cue that signals the presence of food, animals can eat beyond their present needs to cope with predicted future famine. Interestingly, cues previously paired with meal interruption have a similar effect. In two experiments, food-deprived rats learned to associate one conditioned stimulus (CS+) with delivery of a food unconditioned stimulus (US), and another stimulus (IS) with an unexpected termination of CS-US trials. Subsequently, both CS+ and IS enhanced consumption of the US food by sated rats. The results of Experiment 2 indicated that IS's ability to potentiate feeding of sated rats in test depended more on its accompanying CS+ termination in training than on its signaling reductions in US frequency. These experiments may provide a novel animal model of binge-like behaviors in sated rats induced by external cues paired with meal interruption.

From operant learning to cognitive enrichment in farm animal housing: bases and applicability

This study has its basis in recent findings by our own and other laboratories and proposes a type of rewarded operant learning that seeks the detection of discriminatory cues as a cognitive enrichment in intensive husbandry systems. This type of cognitive enrichment has the ability to activate the intrinsically-rewarding mesolimbic brain axis when an animal acquires successful strategies to cope with environmental demands. It provides animals with the opportunity to develop positive affects through control of their environment and the anticipation of consummatory reward. If true animal welfare is considered more than simply the absence of stress and harm, provoking better affective conditions may be a suitable way of increasing the well-being of intensively-housed animals. Recent research with elaborated operant learning equipment, under experimental and quasi-commercial conditions, revealed better health, reduced boredom and less maladaptive behaviour as potentially practical advantages. A number of the issues regarding the transfer of this suggested form of cognitive enrichment to large scale, commercial farming are discussed.

Cognitive versus stimulus-response theories of learning

In his 1948 address to the Division of Theoretical-Experimental Psychology of the American Psychological Association, Kenneth W. Spence discussed six distinctions between cognitive and stimulus-response (S-R) theories of learning. In this article, I first review these six distinctions and then focus on two of them in the context of my own research. This research concerns the specification of stimulus-stimulus associations in associative learning and the characterization of the neural systems underlying those associations. In the course of describing Spence's views and my research, I hope to communicate some of the richness of Spence's S-R psychology and its currency within modern scientific analyses of behavior.

Sucrose taste but not Polycose taste conditions flavor preferences in rats

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This "poly" taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS-) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS- when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS- flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.

The influence of CS-US interval on several different indices of learning in appetitive conditioning

Four experiments examined the effects of varying the conditioned stimulus-unconditioned stimulus (CS-US) interval (and US density) on learning in an appetitive magazine approach task with rats. Learning was assessed with conditioned response (CR) measures, as well as measures of sensory-specific stimulus-outcome associations (Pavlovian-instrumental transfer, potentiated feeding, and US devaluation). The results from these studies indicate that there exists an inverse relation between CS-US interval and magazine approach CRs, but that sensory-specific stimulus-outcome associations are established over a wide range of relatively long, but not short, CS-US intervals. These data suggest that simple CR measures provide different information about what is learned than measures of the specific stimulus-outcome association, and that time is a more critical variable for the former than latter component of learning.

The influence of cost manipulation on water contrafreeloading induced by repeated exposure to quinpirole in the rat

RATIONALE

Quinpirole (QNP), a D2/D3 dopaminergic receptor agonist, was found to elicit an apparently antieconomical drinking behavior called contrafreeloading (CFL). The perseverative operant responding observed may represent a compulsive-like behavior prompted by sensitization to the effects of QNP.

OBJECTIVES

In the present study, we investigated the effect of different response costs on instrumental behavior and CFL in rats repeatedly treated with QNP (0.5 mg/kg i.p.). Moreover, we studied the consummatory components of ingestive behavior in no-choice paradigms and the role of learned operant conditioning in free drinking.

MATERIALS AND METHODS

In experiment 1, rats were trained to perform under three different fixed ratio schedules of reinforcement (FR1, FR3, and FR10) and were given a choice between operant and free access to water. In experiment 2, rats were divided into four groups, each one resembling experiment 1 in one or more features, with no choice available and water consumption measured at an interval of 0-60 min.

RESULTS

(a) Increasing FR significantly reduced CFL % in saline -- but not in QNP-injected groups; (b) under free-drinking conditions, QNP caused a progressive hypodipsic effect which was, however, contrasted by maintaining cues formerly contingent on operant access to water; and (c) under CFL conditions QNP-treated rats drank more than under free access conditions.

CONCLUSIONS

QNP confers rigidity in responding for water, impeding adaptation to different contingencies for access to the resource. In QNP-treated rats, CFL behavior appears adaptive as far as it allows animals to partially circumvent the hypodipsic effect of the drug.