Self-administration of intravenous amphetamine is predicted by individual differences in sucrose feeding in rats

Individual Differences in Frustrative Nonreward Behavior for Sucrose in Rats Predict Motivation for Fentanyl under Progressive Ratio

Frustrative nonreward (FN) is a construct in the Negative Valence Systems domain of the Research Domain Criteria (RDoC) from the National Institute of Mental Health. An organism’s response to frustrating situations (e.g., inability to obtain an expected reward) has broad implications for a variety of neuropsychiatric conditions, including substance use disorders. The current project developed a first of its kind rat operant behavioral model of FN based loosely on the human Point Subtraction Aggression Paradigm (PSAP). The current study shows that individual differences in FN for sucrose pellets are consistent across sessions at baseline and that the task is sensitive to reward size in male rats. More importantly, high FN behavior for sucrose predicts early “breaking” for intravenous fentanyl self-administration under a progressive ratio (PR) schedule. These results solidify frustration/ FN as an important factor for substance use disorders in addition to craving, impulsivity, and habit.

Individual differences in rat sensitivity to CO2

Feelings of fear, anxiety, dyspnea and panic when inhaling carbon dioxide (CO2) are variable among humans, in part due to differences in CO2 sensitivity. Rat aversion to CO2 consistently varies between individuals; this variation in aversion may reflect CO2 sensitivity, but other personality traits could also account for individual differences in aversion. The aims of this study were to 1) assess the stability of individual differences in rat aversion to CO2, 2) determine if individual differences in sweet reward motivation are associated with variation in aversion to CO2, and 3) assess whether variation in aversion to CO2 is related to individual differences in motivation to approach gains (promotion focus) or maintain safety (prevention focus). Twelve female Sprague Dawley rats were exposed multiple times at three different ages (3, 9 and 16 months old) to CO2 in approach-avoidance testing to assess motivation to avoid CO2 against motivation to gain sweet rewards. Rats were also tested for motivation to find hidden sweet rewards, and for their motivation to approach rewards or darkness. Tolerance to CO2 increased with repeated exposures and was higher at older ages. Individual differences in aversion to CO2 were highly repeatable but unrelated to motivation for sweet rewards or the strength of promotion and prevention focus. These results indicate that individual differences in aversion to CO2 reflect variation in CO2 sensitivity.

A breeding strategy to identify modifiers of high genetic risk for methamphetamine intake

Trace amine-associated receptor 1 (Taar1) impacts methamphetamine (MA) intake. A mutant allele (Taar1^m1J ) derived from the DBA/2J mouse strain codes for a non-functional receptor, and Taar1^m1J/m1J mice consume more MA than mice possessing the reference Taar1⁺ allele. To study the impact of this mutation in a genetically diverse population, heterogeneous stock-collaborative cross (HS-CC) mice, the product of an eight-way cross of standard and wild-derived strains, were tested for MA intake. HS-CC had low MA intake, so an HS-CC by DBA/2J strain F2 intercross was created to transfer the mutant allele onto the diverse background, and used for selective breeding. To study residual variation in MA intake existing in Taar1^m1J/m1J mice, selective breeding for higher (MAH) vs lower (MAL) MA intake was initiated from Taar1^m1J/m1J F2 individuals; a control line of Taar1^+/+ individuals (MAC) was retained. The lines were also examined for MA-induced locomotor and thermal responses, and fluid and tastant consumption. Taar1^m1J/m1J F2 mice consumed significantly more MA than Taar1^+/+ F2 mice. Response to selection was significant by generation 2 and there were corresponding differences in fluid consumed. Fluid consumption was not different in non-MA drinking studies. Taar1^m1J/m1J genotype (MAL or MAH vs MAC mice) was associated with heighted MA locomotor and reduced hypothermic responses. MAL mice exhibited greater sensitization than MAH mice, but the selected lines did not consistently differ for thermal or tastant phenotypes. Residual variation among high-risk Taar1^m1J/m1J mice appears to involve mechanisms associated with neuroadaptation to MA, but not sensitivity to hypothermic effects of MA.

Effect of chronic variable stress on sensitization to amphetamine in high and low sucrose-consuming rats

BACKGROUND

Individual vulnerability to stress manifests in the interaction of innate properties and environment. There is a growing interest in the individual variability in vulnerability to stress and how it contributes to the development of psychiatric disorders. Intake of palatable substances is often measured in animal models. We have previously demonstrated that the consumption of sucrose solution is a stable trait in rats.

AIMS

The present study aimed to compare the sensitivity of rats with high vs low liquid sucrose consumption to chronic variable stress and the stress effect on behavioural sensitization to amphetamine.

METHODS

Male Wistar rats were subjected to a chronic stress regimen and subsequent repeated treatment with amphetamine (1 mg/kg, intraperitoneally). Fifty-kHz ultrasonic vocalizations, locomotor activity and stereotypies were measured.

RESULTS

In no-stress baseline conditions, the behavioural response to acute amphetamine was similar in rats with high vs low sucrose consumption. Prior chronic stress potentiated the effect of amphetamine only in rats with high sucrose consumption. Behavioural sensitization to repeated administration of amphetamine was observed in non-stressed rats with lower sucrose preference, but not in the respective stressed group that had increased monoamine turnover in the nucleus accumbens. In contrast, in rats with high sucrose preference the amphetamine sensitization effect was prevalent in stressed rats, but not in non-stressed animals.

INTERPRETATION

Chronic stress can change the psychostimulant effect but this depends on the inherent reward sensitivity of the animal. Trait-wise, sucrose intake reflects vulnerability to chronic stress and may interact with the development of addiction.

Behavioural characterisation of chronic unpredictable stress based on ethologically relevant paradigms in rats

The chronic unpredictable stress (CUS) paradigm is extensively used in preclinical research. However, CUS exhibits translational inconsistencies, some of them resulting from the use of adult rodents, despite the evidence that vulnerability for many psychiatric disorders accumulates during early life. Here, we assessed the validity of the CUS model by including ethologically-relevant paradigms in juvenile rats. Thus, socially-isolated (SI) rats were submitted to CUS and compared with SI (experiment 1) and group-housed controls (experiment 1 and 2). We found that lower body-weight gain and hyperlocomotion, instead of sucrose consumption and preference, were the best parameters to monitor the progression of CUS, which also affected gene expression and neurotransmitter contents associated with that CUS-related phenotype. The behavioural characterisation after CUS placed locomotion and exploratory activity as the best stress predictors. By employing the exploratory factor analysis, we reduced each behavioural paradigm to few latent variables which clustered into two general domains that strongly predicted the CUS condition: (1) hyper-responsivity to novelty and mild threats, and (2) anxiety/depressive-like response. Altogether, the analyses of observable and latent variables indicate that early-life stress impairs the arousal-inhibition system leading to augmented and persistent responses towards novel, rewarding, and mildly-threatening stimuli, accompanied by lower body-weight gain.

The influence of central interleukin-6 on behavioral changes associated with acute alcohol intoxication in adult male rats

Recent studies have demonstrated brain cytokine fluctuations associated with acute ethanol intoxication (increased IL-6) and withdrawal (increased IL-1β and TNFα). The purpose of the present studies was to examine the potential functional role of increased central interleukin-6 (IL-6). We utilized two tests of ethanol sensitivity to establish a potential role for IL-6 after high (3.5-4.0 g/kg, intraperitoneally [i.p.]) or moderate (2.0 g/kg, i.p.) doses of ethanol: loss of righting reflex (LORR) and conditioned taste aversion (CTA), respectively. Briefly, guide cannulae were implanted into the third ventricle of adult male Sprague-Dawley rats. In the first experiments, rats were infused with 25, 50, 100, or 200 ng of IL-6; or 0.3, 3.0, or 9.0 μg of the JAK/STAT inhibitor AG490 30 min prior to a high-dose ethanol challenge. Although sleep time was not affected by exogenous IL-6, infusion of AG490 increased latency to lose the righting reflex relative to vehicle-infused rats. Next, we assessed whether IL-6 was sufficient to produce a CTA. Moderately water-deprived rats received intracerebroventricular (i.c.v.) infusions of 25, 50, or 100 ng IL-6 immediately after 60-min access to 5% sucrose solution. Forty-eight hours later, rats were returned to the context and given 60-min access to sucrose solution. IL-6 infusion had no significant effect on sucrose intake when all rats were considered together. However, a median split revealed that low sucrose-consuming rats significantly increased their drinking on test day, an effect that was not seen in rats that received 50 or 100 ng of IL-6. In the last study, AG490 had no effect on ethanol-induced CTA (2 g/kg). Overall, these studies suggest that IL-6 had only a minor influence on ethanol-induced behavioral changes, yet phenotypic differences in sensitivity to IL-6 were apparent. These studies are among the first to examine a potential functional role for IL-6 in ethanol-related behaviors, and may have important implications for understanding the relationship between acute ethanol intoxication and its associated behavioral alterations.

Individual behavioural predictors of amphetamine-induced emission of 50 kHz vocalization in rats

Measurement of ultrasonic vocalizations (USVs) produced by adult rats represents a highly useful index of emotional arousal. The associations found between 50 kHz USV production and a variety of behavioural and pharmacological protocols increasingly suggests they serve as a marker of positive motivational states. This study used a powerful within-subjects design to investigate the relationships among individual differences in approach to a sweet-food reward, predisposition to emit 50 kHz USVs spontaneously, and 50 kHz USVs emission following acute systemic administration of amphetamine. Both approach motivation and predisposition to call were found to not correlate with each other but did predict 50 kHz USV response to acute amphetamine. These two behavioural phenotypes appear to represent dissociable predictors of acute amphetamine-induced emission of 50 kHz USVs in a non-sensitization paradigm. In contrast to that, a measure of sucrose preference was not found to predict 50 kHz USV emission following amphetamine. Acute amphetamine was also found to increase average sound frequency of emitted USVs and selectively increase the proportion of Trill subtype 50 kHz USVs. Together, these data demonstrate that acute amphetamine-induced 50 kHz USVs in the adult rat represent more than just a univariate motivational state and may represent the product of dissociable subsystems of emotional behavior.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

An animal model of differential genetic risk for methamphetamine intake

The question of whether genetic factors contribute to risk for methamphetamine (MA) use and dependence has not been intensively investigated. Compared to human populations, genetic animal models offer the advantages of control over genetic family history and drug exposure. Using selective breeding, we created lines of mice that differ in genetic risk for voluntary MA intake and identified the chromosomal addresses of contributory genes. A quantitative trait locus was identified on chromosome 10 that accounts for more than 50% of the genetic variance in MA intake in the selected mouse lines. In addition, behavioral and physiological screening identified differences corresponding with risk for MA intake that have generated hypotheses that are testable in humans. Heightened sensitivity to aversive and certain physiological effects of MA, such as MA-induced reduction in body temperature, are hallmarks of mice bred for low MA intake. Furthermore, unlike MA-avoiding mice, MA-preferring mice are sensitive to rewarding and reinforcing MA effects, and to MA-induced increases in brain extracellular dopamine levels. Gene expression analyses implicate the importance of a network enriched in transcription factor genes, some of which regulate the mu opioid receptor gene, Oprm1, in risk for MA use. Neuroimmune factors appear to play a role in differential response to MA between the mice bred for high and low intake. In addition, chromosome 10 candidate gene studies provide strong support for a trace amine-associated receptor 1 gene, Taar1, polymorphism in risk for MA intake. MA is a trace amine-associated receptor 1 (TAAR1) agonist, and a non-functional Taar1 allele segregates with high MA consumption. Thus, reduced TAAR1 function has the potential to increase risk for MA use. Overall, existing findings support the MA drinking lines as a powerful model for identifying genetic factors involved in determining risk for harmful MA use. Future directions include the development of a binge model of MA intake, examining the effect of withdrawal from chronic MA on MA intake, and studying potential Taar1 gene × gene and gene × environment interactions. These and other studies are intended to improve our genetic model with regard to its translational value to human addiction.

Central oxytocin and food intake: focus on macronutrient-driven reward

Centrally acting oxytocin (OT) is known to terminate food consumption in response to excessive stomach distension, increase in salt loading, and presence of toxins. Hypothalamic-hindbrain OT pathways facilitate these aspects of OT-induced hypophagia. However, recent discoveries have implicated OT in modifications of feeding via reward circuits: OT has been found to differentially affect consumption of individual macronutrients in choice and no-choice paradigms. In this mini-review, we focus on presenting and interpreting evidence that defines OT as a key component of mechanisms that reduce eating for pleasure and shape macronutrient preferences. We also provide remarks on challenges in integrating the knowledge on physiological and pathophysiological states in which both OT activity and macronutrient preferences are affected.

The contribution of brain reward circuits to the obesity epidemic

One of the defining characteristics of the research of Ann E. Kelley was her recognition that the neuroscience underlying basic learning and motivation processes also shed significant light upon mechanisms underlying drug addiction and maladaptive eating patterns. In this review, we examine the parallels that exist in the neural pathways that process both food and drug reward, as determined by recent studies in animal models and human neuroimaging experiments. We discuss contemporary research that suggests that hyperphagia leading to obesity is associated with substantial neurochemical changes in the brain. These findings verify the relevance of reward pathways for promoting consumption of palatable, calorically dense foods, and lead to the important question of whether changes in reward circuitry in response to intake of such foods serve a causal role in the development and maintenance of some cases of obesity. Finally, we discuss the potential value for future studies at the intersection of the obesity epidemic and the neuroscience of motivation, as well as the potential concerns that arise from viewing excessive food intake as an "addiction". We suggest that it might be more useful to focus on overeating that results in frank obesity, and multiple health, interpersonal, and occupational negative consequences as a form of food "abuse".

Feed-forward mechanisms: addiction-like behavioral and molecular adaptations in overeating

Food reward, not hunger, is the main driving force behind eating in the modern obesogenic environment. Palatable foods, generally calorie-dense and rich in sugar/fat, are thus readily overconsumed despite the resulting health consequences. Important advances have been made to explain mechanisms underlying excessive consumption as an immediate response to presentation of rewarding tastants. However, our understanding of long-term neural adaptations to food reward that oftentimes persist during even a prolonged absence of palatable food and contribute to the reinstatement of compulsive overeating of high-fat high-sugar diets, is much more limited. Here we discuss the evidence from animal and human studies for neural and molecular adaptations in both homeostatic and non-homeostatic appetite regulation that may underlie the formation of a "feed-forward" system, sensitive to palatable food and propelling the individual from a basic preference for palatable diets to food craving and compulsive, addiction-like eating behavior.

A history of bingeing on fat enhances cocaine seeking and taking

Binge eating and substance dependence are disorders characterized by a loss of control over consummatory behaviors. Given the common characteristics of these two types of disorders, it is not surprising that the comorbidity between eating disorders and substance abuse disorders is high (20-40%; Conason et al., 2006). It is unknown, however, whether loss of control in one disorder predisposes an individual to loss of control in the other. The present study, therefore, used a rodent model to test whether a history of binge eating would augment subsequent responding for cocaine. Using the limited access protocol described by Corwin et al. (1998), 45 adult male Sprague-Dawley rats were maintained on one of four dietary protocols for a period of six weeks: chow only (Chow; n = 9), continuous access to an optional source of dietary fat (Ad Lib; n = 12), 1-h access to an optional source of dietary fat daily (Daily; n = 12), or 1-h access to an optional source of dietary fat on Monday, Wednesday, and Friday (MWF; n = 12). All four groups also had unrestricted access to a nutritionally complete diet of chow and water. Fat-bingeing behaviors developed in the MWF rats, the group with the most restricted access to the optional fat. Thereafter, cocaine-seeking and -taking behaviors were assessed in all rats using a self-administration protocol modified from that described by Deroche-Gamonet et al. (2004), which focused on the motivation for and preoccupation with obtaining and consuming drug (assessed using a progressive ratio [PR] schedule of reinforcement) and persistence in responding for drug during periods of signaled drug non-availability (SNA). Rats with the MWF history tended to take more cocaine late in fixed ratio (FR) training, they persisted in their efforts to obtain cocaine in the face of signaled non-availability, worked harder for cocaine on a PR schedule of reinforcement, and exhibited more goal-directed behavior toward the cocaine-associated operandum. These results demonstrate a link between binge-type intake of fat and the development of drug-seeking and -taking behaviors, suggesting that a history of fat bingeing may predispose individuals to exhibit more robust "addiction-like" behaviors toward a substance of abuse. Thus, it appears that conditions promoting excessive behavior toward one substance (e.g., a palatable fatty food) beget excessive behavior toward another (e.g., cocaine).

A multivariate assessment of individual differences in sensation seeking and impulsivity as predictors of amphetamine self-administration and prefrontal dopamine function in rats

Drug abuse vulnerability has been linked to sensation seeking (behaviors likely to produce rewards) and impulsivity (behaviors occurring without foresight). Since previous preclinical work has been limited primarily to using single tasks as predictor variables, the present study determined if measuring multiple tasks of sensation seeking and impulsivity would be useful in predicting amphetamine self-administration in rats. Multiple tasks were also used as predictor variables of dopamine transporter function in the medial prefrontal and orbitofrontal cortexes, as these neural systems have been implicated in sensation seeking and impulsivity. Rats were tested on six behavioral tasks as predictor variables to evaluate sensation seeking (locomotor activity, novelty place preference, and sucrose reinforcement on a progressive ratio schedule) and impulsivity (delay discounting, cued go/no-go, and passive avoidance), followed by d-amphetamine self-administration (0.0056-0.1 mg/kg infusion) and kinetic analysis of dopamine transporter function as outcome variables. The combination of these predictor variables into a multivariate approach failed to yield any clear relationship among predictor and outcome measures. Using multivariate approaches to understand the relation between individual predictor and outcome variables in preclinical models may be hindered by alterations in behavior due to training and thus, the relation between various individual differences in behavior and drug self-administration may be better assessed using a univariate approach in which a only a single task is used as the predictor variable.

Consumption of palatable food decreases the anorectic effects of serotonergic, but not dopaminergic drugs in baboons

We examined the effects of periodic access to a palatable, high sugar content food (candy) in 8 male baboons on the anorectic response to d-amphetamine, which increases dopamine, and dexfenfluramine, which increases serotonin. During candy access, up to 200 candies containing 75% of energy as sugar were available during the morning on Mondays, Wednesdays and Fridays; food pellets (19% of energy as sugar) were available in the afternoon and throughout the remaining days of the week. During candy access, baboons consumed a mean of 177 pieces of candy containing 696 kcal (2.91 MJ) in the morning compared to 44 food pellets and 150 kcal (0.63 MJ) in the morning on non-candy days. Food pellet intake was lower during candy access. Complete dose-response functions for the effects of the drugs on food pellet intake on days that candy was not available were determined before, during, and after the period of access to candy. Dexfenfluramine and amphetamine produced dose-dependent decreases in food pellet intake and increases in latency to eat food pellets before, during, and after candy access. During access to candy, the dose-response function for dexfenfluramine was shifted to the right indicating the development of tolerance, while that for amphetamine was shifted to the left indicating sensitization. Only the dose-response function for dexfenfluramine returned to baseline after candy access suggesting that the difference was specific to concurrent palatable food consumption. We hypothesize that tolerance to the effects of dexfenfluramine reflects a decrease in the satiating effect of serotonin release due to repeatedly eating large amounts of palatable food.

Reward systems and food intake: role of opioids

Humans eat for many reasons, including the rewarding qualities of foods. A host of neurotransmitters have been shown to influence eating behavior and some of these appear to be involved in reward-induced eating. Endogenous opioid peptides and their receptors were first reported more than 30 years ago, and studies suggesting a role of opioids in the regulation of food intake date back nearly as far. Opioid agonists and antagonists have corresponding stimulatory and inhibitory effects on feeding. In addition to studies aimed at identifying the relevant receptor subtypes and sites of action within the brain, there has been a continuing interest in the role of opioids on diet/taste preferences, food reward, and the overlap of food reward with others types of reward. Data exist that suggest a role for opioids in the control of appetite for specific macronutrients, but there is also evidence for their role in the stimulation of intake based on already-existing diet or taste preferences and in controlling intake motivated by hedonics rather than by energy needs. Finally, various types of studies indicate an overlap between mechanisms mediating drug reward and palatable food reward. Preference or consumption of sweet substances often parallels the self-administration of several drugs of abuse, and under certain conditions, the termination of intermittent access to sweet substances produces symptoms that resemble those observed during opiate withdrawal. The overconsumption of readily available and highly palatable foods likely contributes to the growing rates of obesity worldwide. An understanding of the role of opioids in mediating food reward and promoting the overconsumption of palatable foods may provide insights into new approaches for preventing obesity.

Early life protein restriction alters dopamine circuitry

Adverse prenatal environment, such as intrauterine growth retardation (IUGR), increases the risk for negative neurobehavioral outcomes. IUGR, affecting approximately 10% of all US infants, is a known risk factor for attention deficit hyperactivity disorder (ADHD), schizophrenia spectrum disorders and addiction. Mouse dams were fed a protein deficient (8.5% protein) or isocaloric control (18% protein) diet through pregnancy and lactation (a well validated rodent model of IUGR). Dopamine-related gene expression, dopamine content and behavior were examined in adult offspring. IUGR offspring have six to eightfold over-expression of dopamine (DA)-related genes (tyrosine hydroxylase (TH) and dopamine transporter) in brain regions related to reward processing (ventral tegmental area (VTA), nucleus accumbens, prefrontal cortex (PFC)) and homeostatic control (hypothalamus), as well as increased number of TH-ir neurons in the VTA and increased dopamine in the PFC. Cyclin-dependent kinase inhibitor 1C (Cdkn1c) is critical for dopaminergic neuron development. Methylation of the promoter region of Cdkn1c was decreased by half and there was a resultant two to sevenfold increase in Cdkn1c mRNA expression across brain regions. IUGR animals demonstrated alterations in dopamine-dependent behaviors, including altered reward-processing, hyperactivity and exaggerated locomotor response to cocaine. These data describe significant dopamine-related molecular and behavioral abnormalities in a mouse model of IUGR. This animal model, with both face validity (behavior) and construct validity (link to IUGR and dopamine dysfunction) may prove useful in identifying underlying mechanisms linking IUGR and adverse neurobehavioral outcomes such as ADHD.

Effect of complete maternal and littermate deprivation on morphine-induced Fos-immunoreactivity in the adult male rat brain

Previous research has demonstrated that rats reared in isolation from their dam and littermates show altered behavioral responsiveness to both natural and drug-mediated rewards. This study examined the effects of complete maternal deprivation through the use of artificial rearing on neural activation after acute morphine exposure in adulthood. Male rats were either artificially reared (AR) or maternally reared (MR) from postnatal day 5 to 21. In adulthood (4 mo old), rats received a single injection of morphine sulfate (10 mg/kg) or equivolume saline 2 h before perfusion and brain extraction. Neural activation was quantified using Fos immunohistochemistry. Analyses of several brain regions revealed a consistent pattern of differences between AR and MR rats. Specifically, relative to MR rats, AR rats showed significantly greater morphine-induced Fos-immunoreactivity in brain regions associated with the mesocorticolimbic "reward" pathway. These results support the hypothesis that functional activity in reward neurocircuitry can be altered by early life experience.

Operant responding for sucrose by rats bred for high or low saccharin consumption

The use of rats differing in the intake of sweet substances has highlighted some interesting parallels between taste preferences and drug self-administration. For example, rats selectively bred to consume high (HiS) or low (LoS) amounts of a 0.1% saccharin solution (when compared to water consumption), show corresponding differences across several measures of cocaine self-administration (HiS>LoS). In this study, we measured whether the two strains also differ when response requirements are imposed for obtaining a sucrose reinforcer. Male HiS and LoS rats were measured for operant responding for sucrose pellets under fixed-ratio (FR) schedules of 1, 3, 5 and 10 and under a progressive-ratio (PR) schedule, during which the response requirement for each successive pellet increased exponentially. The effect of systemic naltrexone (0.3, 1 and 3mg/kg) on PR responding for sucrose pellets was also tested. Under all FR and PR schedules, the number of pellets obtained by the LoS rats were significantly lower than those obtained by the HiS rats. Although the LoS weighed more than the HiS rats, this difference does not appear to explain differences in operant behavior. No strain differences in the effect of naltrexone were observed; the 3mg/kg dose reduced the number of pellets obtained in both strains. Measures of locomotor activity taken prior to operant trials suggest that the differences in responding were not due to differences in general activity levels. These studies provide further characterization of the HiS and LoS rat lines by demonstrating that motivation to consume sucrose is greater in HiS than in LoS rats.

Motivation for sucrose in sated rats is predicted by low anxiety-like behavior

Anxiety has been implicated in obesity and in the overconsumption of highly palatable foods such as those high in fat, sugar, or both. Also, the novelty-seeking trait has been associated with failure in weight-loss programs. The aim of this study was to investigate the associations of experimental anxiety and the self-administration of sucrose and high fat pellets in non-food deprived rats across different operant schedules. Male Wistar rats were subjected to the elevated plus-maze test (EPM) of anxiety-like behavior. The rats were tested for fixed ratio 5 (FR5) and progressive ratio (PR) operant responding for 50% sucrose, 95% sucrose, and high-fat pellets. PR active lever press response for 95% sucrose, but not the other pellet types, was correlated to % time spent on open arms (P=0.019) in the EPM. On the FR5 schedule, activity (closed arm entries) was correlated to the self-administration of 50% sucrose (P=0.027) and high-fat (P=0.002). This indicates an association of novelty-induced activity and self-administration of palatable food in sated rats, as well as a specific association of PR lever press response for 95% sucrose and low anxiety-like behavior. It has been argued that such active lever press response on PR may be interpreted as craving for the reinforcer; thus, our findings indicate an inverse relationship of experimental anxiety and craving for sucrose. This connection may have implications for human situations, since anxiety and novelty-seeking have been associated with obesity and failure in weight-loss programs.

Effects of environmental enrichment and social isolation on sucrose consumption and preference: associations with depressive-like behavior and ventral striatum dopamine

Little attention has been directed towards environmental control of sensitivity to natural reward and its possible relationship with other motivated behaviors, besides the well-known effects of environmental enrichment and social isolation on drug self-administration and locomotor sensitization to psychostimulants. Here, we investigate the effects of these rearing conditions on sucrose consumption and preference, and tissue levels of striatal dopamine. The possible relationship among sucrose intake, immobility behavior in the forced swimming test, and dopamine concentration was explored through correlation and regression analyses. Even though all animals preferred sucrose over water, we found, that during postnatal period, isolated rats consumed more sucrose than control or enriched littermates. In isolated rats sucrose intake correlated positively with ventral but not with dorsal striatum dopamine, even when striatal dopamine did not differ among groups. Especially in isolated animals immobility behavior was positively predicted by differences in sucrose intake. The dopamine concentration did not correlate with immobility behavior. Taken together, the present data support previous findings regarding the effects of early life events upon reward-sensitivity and depressive-like behavior, and also provide further evidence about the relationship between these motivated behaviors and the likely role of ventral striatum dopamine in regulating them.

Liquid sucrose bingeing in rats depends on the access schedule, concentration and delivery system

Previous studies have reported binge-type consumption of solid vegetable shortening in non-food deprived rats maintained on schedules of limited shortening access. The current study determined if limited access would promote binge-type consumption of sucrose solutions. Adult male rats (6 groups, n = 10 each) were provided with one of three different sucrose concentrations (3.2%, 10%, 32% w/v) for 2 h either everyday (Daily) or Monday, Wednesday, and Friday (Intermittent). A 'binge' during the 2-h access periods was operationally defined as Intermittent intakes significantly greater than Daily intakes. Sucrose initially was provided in a 100 ml glass tube equipped with a stainless-steel drinking spout. Under these conditions, there were no differences in sucrose intake between Daily and Intermittent groups at any of the concentrations. In contrast, when sucrose was provided in a modified 60 ml plastic syringe with the same drinking spout, intakes of the Intermittent groups consuming 3.2% and 10% sucrose were greater than those of the respective Daily groups, indicating that binge-type consumption of sucrose occurred. These results demonstrate that brief, intermittent access to low and moderate concentrations of sucrose can promote binge-type behavior, and the characteristics of the drinking apparatus can affect sucrose intake.

Individual differences in elevated plus-maze exploration predicted progressive-ratio cocaine self-administration break points in Wistar rats

RATIONALE

There are considerable individual differences in vulnerability to drug addiction, but the mechanisms underlying such differences are poorly understood. Cocaine has potent reinforcing effects that support operant responding. However, cocaine also elicits aversive reactions and produces an approach-avoidance conflict in rats. We hypothesized that preexisting individual differences in open arm exploration on the elevated plus-maze, a well-known model for the study of clinically effective anxiolytic drugs, would predict individual differences in cocaine-motivated behavior.

OBJECTIVES

To assess whether individual differences in sensitivity to anxiety-like behavior on the plus-maze predict motivation to self-administer intravenous (i.v.) cocaine.

MATERIALS AND METHODS

Rats were assessed drug-free for individual differences in open arm exploration on the elevated plus-maze, and later trained to perform an operant response for i.v. cocaine (0, 0.1, 0.3, 0.6, 0.9, 1.2, and 1.5 mg kg(-1) infusion(-1)) on a progressive-ratio reinforcement schedule. Rats were split at the median into low and high open arm explorers based on time spent in the open arms of the plus-maze. Self-administration levels were compared across groups.

RESULTS

Rats identified as high open arm explorers on the elevated plus-maze attained higher levels of operant responding for cocaine. Open arm times and break points were significantly correlated at the highest cocaine doses (1.2 and 1.5 mg kg(-1) infusion(-1)).

CONCLUSIONS

These results indicate that individual differences in anxiety-like behavior on the elevated plus-maze predict motivation to self-administer cocaine, and suggest the possibility that reduced sensitivity to aversive stimuli may be associated with increased vulnerability to the rewarding properties of cocaine.

Neonatal stress and litter composition alter sucrose intake in both rat dam and offspring

The early postnatal environment can have long lasting effects on the physiology and behavior of both mother and offspring. A great deal of evidence indicates that stress during this time period is a risk factor for the future development of a multitude of disorders including substance abuse. The maternal separation paradigm is used to model such stress in rats. The current study evaluated the effects of maternal separation and litter composition on sucrose consumption, a non-drug measure of reward, in both male Long-Evans rat offspring and mother. On postnatal day 2, rats were cross-fostered, placed in single-sex or mixed-sex litters, and subsequently stressed by daily dam-pup separations during the first two postnatal weeks. The length of the separation (15 min, 1 h, or 3 h) was randomly assigned to each day. A two-bottle choice test was given to maternally separated and nonhandled offspring when they were adults, and to dams 2-4 weeks after weaning. Intake of a 10% sucrose solution or water was compared for 1 h daily across five consecutive days. Rearing condition had a profound effect on total fluid intake and sucrose solution intake by both offspring and dam, with the separated offspring and dams generally consuming a greater amount of total fluid and 10% sucrose solution than their nonhandled counterparts. Litter composition also affected consumption, with the offspring from maternally separated mixed-sex litters consuming more total fluid and 10% sucrose solution than offspring from maternally separated single-sex litters. These results indicate that early postnatal and postpartum stress can lead to changes in sucrose consumption, a non-drug measure of reward, indicating that such stress may alter the underlying brain reward mechanisms.

Individual differences in sucrose intake and preference in the rat: Circadian variation and association with dopamine D2 receptor function in striatum and nucleus accumbens

Measurement of sucrose intake or preference is currently in widespread use in preclinical psychopharmacology, and used for predicting sensitivity to rewards, but limited information is available about the consistency of individual sucrose intake or preference. In the present study, individual differences in sucrose intake and preference in free-feeding rats were studied during the dark and light phases, and associations of these measures with the function of D(2) receptors in the striatum and nucleus accumbens were characterized. Altogether eight two-bottle tests were carried out intermittently during light and dark phase. Ten days after the last test, animals were sacrificed. Intake, and to a lesser degree preference of sucrose during the dark phase were higher as compared to the light phase, and sucrose intake, but not preference was individually very consistent across different tests, especially during the dark phase. The average dark phase sucrose intake and preference correlated positively with dopamine-dependent [(35)S]GTPgammaS binding in nucleus accumbens. Dopamine-dependent [(35)S]GTPgammaS binding in striatum correlated negatively with sucrose preference in the first test. This study has demonstrated that sucrose intake is an individually stable trait, especially when measured during the dark phase, and persistent individual differences in sucrose consumption and possibly reward sensitivity in general are related to dopamine D(2) receptor function in the nucleus accumbens. Individual differences in D(2) receptor function in the striatum may influence behaviour of rats in novel situations.

Artificial rearing alters the response of rats to natural and drug-mediated rewards

Artificial rearing (AR) of infant rats permits precise control over key features of the early environment without maternal influence. The present study examined the behavioral response of AR rats towards natural and drug-mediated rewards, as well as their exploratory and affective behaviors. Adolescent AR rats showed increased preference for sucrose consumption relative to chow and demonstrated greater activity in the open field and in the elevated plus-maze compared to maternally reared (MR) rats. With respect to measures of emotionality, AR rats showed enhanced avoidance of the open arms of the plus-maze, indicating increased anxiety, but they did not differ from MR rats in exploring the center of the open field. Adult AR rats displayed a stronger conditioned response to morphine in a place preference test. These findings support the potential of the AR model to contribute to understanding the role of early experience in the development of behavioral motivation.

Bingeing rats: a model of intermittent excessive behavior?

Intermittent excessive behaviors (IEB) characterize a variety human disorders including binge eating, drug abuse, alcoholism, aberrant sexual conduct, and compulsive gambling. Clinical co-morbidity exists among IEB, and limited treatment options are available. The use of behavioral models of bingeing and other feeding protocols is beginning to clarify neural similarities and differences that exist between IEB directed toward obtaining and consuming food and IEB directed toward obtaining and consuming drugs of abuse. Research from this laboratory using a limited access binge-type eating protocol may provide new insight into IEB.

Wheel running as a predictor of cocaine self-administration and reinstatement in female rats

Avidity for behaviors mediated by nondrug rewards, such as novelty seeking or intake of sweets or fats, is predictive of enhanced vulnerability to the locomotor-activating and rewarding effects of drugs of abuse. The purpose of the present study was to determine whether avidity for wheel running was predictive of subsequent cocaine-induced locomotor activity, cocaine self-administration, and cocaine-seeking behavior in rats. Rats with high (HiR) and low (LoR) levels of wheel running were selected from an outbred sample of Wistar rats. These rats were first tested for their locomotor response to an acute injection of cocaine (10 mg/kg, i.p.). Subsequently, a multi-phase self-administration procedure was used to examine the effect of wheel running on the maintenance, extinction, and cocaine-induced reinstatement of cocaine-seeking behavior in HiR and LoR rats. The results indicate no significant differences between HiR and LoR rats in the cocaine-induced stimulation of locomotor activity. During maintenance, HiR rats self-administered more cocaine than LoR rats. While there were no group differences in saline self-administration behavior during extinction, HiR rats showed higher cocaine-induced reinstatement than LoR rats. Rats that were previously high responders to novelty (day 1 in locomotor track) also showed significantly higher reinstatement than low novelty responders. These results suggest that a propensity for wheel running is associated with increased vulnerability for cocaine self-administration and reinstatement and that HiR rats are more motivated than LoR rats to seek cocaine.

Novelty seeking and drug use: contribution of an animal model

Although sensation seeking or novelty seeking is a reliable predictor of drug use in humans, individual differences in free-choice novelty seeking in animal models have generally failed to predict drug use. In the current article, hierarchical multiple regression analyses were used on data collected from a large sample of rats. Rats were screened on measures of inescapable and free-choice novelty tests and then were trained to lever press for sucrose or intravenous amphetamine. Although scores from the inescapable novelty test weakly predicted responding for amphetamine, the addition of free-choice novelty preference scores into the regression analyses significantly improved the predictive models. These results indicate that, similar to evidence in humans, individual differences in novelty seeking may be able to predict drug use in rats.

Brain stimulation reward performance and sucrose maintained behaviors in alcohol-preferring and -nonpreferring rats

BACKGROUND

The relation between ethanol (EtOH) preference and sensitivity to brain stimulation reward (BSR) was examined under multiples schedules of reinforcement in the current study. For comparison, the study also examined the relation between EtOH preference and motivation for a sweet, palatable sucrose solution under similar schedules of reinforcement.

METHODS

To investigate sensitivity to BSR performance, alcohol-preferring and -nonpreferring rats were tested using the curve-shift (rate-frequency) paradigm under several intensity levels during a 20-min session. Animals were first trained under an optimal current intensity, which produced maximal responding (i.e., 100%) across a series of descending frequencies (i.e., 300-20 Hz). BSR was then evaluated at 100%, 75%, and 50% of the optimal current. The sensitivity of the curve-shift method was further evaluated under the animal's optimal current using the FR1, FR6, and FR12 schedules. To examine responding for the sucrose solution, a separate group of alcohol-preferring and -nonpreferring rats was initially stabilized on an FR1 schedule and then subsequently on FR6 and FR12 schedules.

RESULTS

The results demonstrated that reducing the reinforcing efficacy of BSR via reduction in current intensity/reinforcement schedule produced marked orderly rightward shifts in the rate-frequency curves relating responding to stimulation frequency in both rat lines. However, no differences were found between the lines with either manipulation. Specifically, both lines demonstrated orderly reductions in response rate and increases in BSR threshold parameters (i.e., half maximal frequency/responding, minimum and maximum frequencies). In contrast to BSR, genetic selection for EtOH preference was highly associated with responding for the sweet, palatable sucrose solution. The association was even more salient as the reinforcement schedule increased (i.e., reward cost).

CONCLUSION

The results demonstrate that responding for BSR is not associated with EtOH preference, insofar as alcohol-preferring and -nonpreferring rats respond similarly under an array of reinforcement schedules and current intensities. In contrast, genetic selection for EtOH preference is highly associated with responding for a palatable sucrose reward, and the relation increases as the reward cost for the sucrose increases. These findings suggest that similar/overlapping mechanisms of action regulate the reinforcing properties of EtOH and sucrose but that overlapping yet distinct neuronal mechanism may modulate the reward characteristics of BSR and EtOH preference.

Impulsivity (delay discounting) as a predictor of acquisition of IV cocaine self-administration in female rats

RATIONALE

Previous research in humans suggests a relationship between drug abuse and impulsivity as shown by selection of a smaller immediate reward over a larger delayed reward. However, it is not clear whether impulsivity precedes drug abuse or drug abuse influences impulsivity.

OBJECTIVE

The hypothesis of the present experiment was that rats selected for choosing smaller, immediate over larger, delayed food would acquire IV cocaine self-administration faster than those choosing larger, delayed food rewards.

METHODS

Female rats were screened for locomotor activity and trained on a delay discounting procedure that allowed them access to two response levers and a food pellet dispenser. Under a fixed-ratio (FR) 1 schedule, responding on one lever resulted in immediate delivery of one 45 mg pellet, while responding on the other lever resulted in delivery of three 45 mg pellets after a variable delay that increased after responses on the delay lever and decreased after responses on the immediate lever. For each rat, a mean adjusted delay (MAD) was calculated for each daily session, and stability was defined as MADs varying less than 5 s across 5 days. Based on their average MADs, rats were separated into low impulsive (LoI) and high impulsive (HiI) groups, implanted with an indwelling IV catheter, and trained to lever press for cocaine (0.2 mg/kg) under an FR1 schedule.

RESULTS

There were no differences in locomotor activity between the LoI and HiI groups; however, a greater percentage of the HiI group acquired cocaine self-administration, and they did so at a significantly faster rate than the LoI rats.

CONCLUSIONS

Performance on the delay discounting model of impulsivity predicted vulnerability to subsequent acquisition of cocaine self-administration.

Individual differences in cocaine- and amphetamine-induced activation of male Sprague-Dawley rats: contribution of the dopamine transporter

Previously we found that outbred male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively), based on their open-field locomotor response to acute cocaine (COC; 10 mg/kg, i.p.). Here, we extended this analysis to amphetamine (AMPH; 0.5, 1, and 5 mg/kg, i.p.) and found that the individual differences in behavioral activation were not as pronounced as with COC. This was confirmed with observational analysis of behaviors. Differences in drug-induced activation could involve differential dopamine transporter (DAT) function/trafficking. To address this possibility, we measured [3H]DA uptake into dorsal striatal synaptosomes prepared from rats injected 30 min earlier with saline, COC, or AMPH to determine DAT activity, and radioligand binding to determine the total number of DATs. Striatal [3H]DA uptake in COC-treated HCRs was significantly higher than in LCRs. Furthermore, regardless of LCR/HCR classification, uptake in individual COC-treated rats was significantly correlated with their locomotor behavior in the 30 min after drug administration. In contrast, AMPH-treated rats did not differ in uptake, nor were uptake and locomotor activity correlated. DAT number did not differ between LCRs or HCRs, or between AMPH-treated rats. In addition, when individual differences in COC-induced behavior were no longer detected in LCRs and HCRs 1 week after initial classification, uptake was also similar. Together, these results suggest that a difference in expression of functional DATs on the cell surface contributes to the individual differences observed in COC-induced, but not AMPH-induced, behavioral activation of rats.

Individual differences in cocaine-induced locomotor activity in rats: behavioral characteristics, cocaine pharmacokinetics, and the dopamine transporter

Outbred male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response to acute cocaine. Concomitant measurement of dopamine clearance in these rats revealed that the differential behavioral responses are associated with the magnitude of dopamine transporter (DAT) inhibition by cocaine. Here, we investigated several factors that might contribute to cocaine-induced behavioral variability and its association with differential inhibition of DAT function. In rats classified as LCRs or HCRs after 10 mg/kg cocaine injection, we found no differences in (1) novelty-induced locomotion, (2) cocaine levels in dorsal striatum or nucleus accumbens (NAc), (3) DAT number or affinity in NAc, or (4) DAT affinity for cocaine in NAc. In rats given 20 mg/kg cocaine, behavior was more uniform across individuals, but still warranted separation into LCR/HCR categories. Additionally, we analyzed the stability of the LCR/HCR classification made during the first test with 10 or 20 mg/kg cocaine by retesting rats 7 days later with saline or cocaine (10 or 20 mg/kg). Before injection, HCRs were more active relative to LCRs and to their own behavior on the first test day. Following cocaine, LCRs and HCRs exhibited similar drug-induced changes in locomotion, but there were unique effects that depended on the cocaine dose given on the first and second test days. Our results argue against several likely explanations for individual differences in cocaine-induced behavior and highlight the influence of a single cocaine exposure on subsequent behavioral responses to the drug.

A diet promoting sugar dependency causes behavioral cross-sensitization to a low dose of amphetamine

Previous research in this laboratory has shown that a diet of intermittent excessive sugar consumption produces a state with neurochemical and behavioral similarities to drug dependency. The present study examined whether female rats on various regimens of sugar access would show behavioral cross-sensitization to a low dose of amphetamine. After a 30-min baseline measure of locomotor activity (day 0), animals were maintained on a cyclic diet of 12-h deprivation followed by 12-h access to 10% sucrose solution and chow pellets (12 h access starting 4 h after onset of the dark period) for 21 days. Locomotor activity was measured again for 30 min at the beginning of days 1 and 21 of sugar access. Beginning on day 22, all rats were maintained on ad libitum chow. Nine days later locomotor activity was measured in response to a single low dose of amphetamine (0.5 mg/kg). The animals that had experienced cyclic sucrose and chow were hyperactive in response to amphetamine compared with four control groups (ad libitum 10% sucrose and chow followed by amphetamine injection, cyclic chow followed by amphetamine injection, ad libitum chow with amphetamine, or cyclic 10% sucrose and chow with a saline injection). These results suggest that a diet comprised of alternating deprivation and access to a sugar solution and chow produces bingeing on sugar that leads to a long lasting state of increased sensitivity to amphetamine, possibly due to a lasting alteration in the dopamine system.

Sugars: hedonic aspects, neuroregulation, and energy balance

The prevalence of obesity has increased dramatically in recent years in the United States, with similar patterns seen in several other countries. Although there are several potential explanations for this dramatic increase in obesity, dietary influences are a contributing factor. An inverse correlation between dietary sugar intake and body mass index has been reported, suggesting beneficial effects of carbohydrate intake on body mass index. In this review we discuss how sugars interact with regulatory neurochemicals in the brain to affect both energy intake and energy expenditure. These neurochemicals appear to be involved in dietary selection, and sugars and palatable substances affect neurochemical changes in the brain. For example, rats that drink sucrose solutions for 3 wk have major changes in neuronal activity in the limbic area of the brain, a region involved in pleasure and other emotions. We also investigate the relations between sucrose (and other sweet substances), drugs of abuse, and the mesolimbic dopaminergic system. The presence of sucrose in an animal's cage can affect the animals desire to self-administer drugs of abuse. Also, an animal's level of sucrose preference can predict its desire to self-administer cocaine. Such data suggest a relation between sweet taste and drug reward, although the relevance to humans is unclear. Finally, we address the influence of sugar on body weight control. For example, sucrose feeding for 2 wk decreases the efficiency of energy utilization and increases gene expression of uncoupling protein 3 in muscle, suggesting that sucrose may influence uncoupling protein 3 activity and contribute to changes in metabolic efficiency and thus regulation of body weight.

Chronic access to a sucrose solution enhances the development of conditioned place preferences for fentanyl and amphetamine in male Long-Evans rats

Consumption of palatable food and fluids alters the behavioral consequences of psychoactive drugs. To further investigate the effects of intake of palatable nutrients on the rewarding properties of these drugs, the effects of chronic intake of a sweet sucrose solution on the development of conditioned place preferences (CPP) to a mu-opioid agonist, fentanyl, and to a stimulant drug, amphetamine, were examined. Male Long-Evans rats consumed laboratory chow and water or chow, water, and a 32% sucrose solution. CPP testing was conducted in a three-chamber apparatus. In Experiment 1 (over four conditioning days), rats received saline, 0.004, or 0.016 mg/kg sc fentanyl citrate before being placed on the nonpreferred side of the apparatus and saline (subcutaneously) before being placed on the preferred side during a separate session on the same day. When given access to all three chambers, rats injected with 0.016 mg/kg fentanyl spent significantly more time on the drug-paired side than rats injected with saline. Furthermore, sucrose-fed rats displayed a significantly greater CPP than chow-fed rats. After conditioning, rats were tested for fentanyl-induced antinociception using the tail-flick test. Using a cumulative dose procedure, fentanyl (0.003, 0.010, 0.030, and 0.100 mg/kg sc) led to dose-dependent increases in tail-flick latencies. Rats fed with sucrose displayed significantly greater responses to fentanyl than those in the chow group. In Experiment 2, rats spent significantly more time on the drug-paired side of the CPP apparatus following injections of 0.33 or 1.0 mg/kg amphetamine than after saline injections. Additionally, following injection of 0.33 mg/kg amphetamine, sucrose-fed rats spent significantly more time on the drug-paired side of the chamber than chow-fed rats.

Cholecystokinin modulation of mesolimbic dopamine function: regulation of motivated behaviour

This article reviews evidence and presents a hypothesis regarding the effects of stress on motivated behaviour, and in particular the observation that stress can have both motivationally inhibitory and motivationally facilitatory effects. This issue will be addressed with regard to psychostimulant self-administration, and the role that the neurobiological mechanisms underlying motivated behaviour are thought to be involved in the evolution of addictions. Evidence from animal studies shows that stress and stress-related hormones such as corticosterone can facilitate mesolimbic dopamine function and the behavioural effects of psychostimulants, particularly at lower levels of stress. Conversely, higher levels of stress can inhibit motivated behaviour, and evidence is presented that this may occur in part through the effects of the neuropeptide cholecystokinin (CCK), acting through CCK-B receptors in the nucleus accumbens. Individual differences in endogenous CCK and dopamine systems are hypothesized to be important modulators of individual differences in motivated behaviour.

Taste function in methadone-maintained opioid-dependent men

It has been shown repeatedly that opioid dependence is associated with increased consumption of refined sugars. It is possible that this association results from altered taste reactivity in opioid-dependent subjects. Thus, in the present study, we compared taste responses to sweet, bitter, sour and salty solutions in methadone-maintained opioid-dependent men and healthy control subjects. The two groups did not differ in terms of rated intensity or pleasantness of sucrose (1-30%), quinine (0.001-0.005%), citric acid (0.02-0.1%) and sodium chloride (0.18-0.9%) solutions. Proportions of 'sweet-likers', i.e. subjects rating a 30% sucrose (0.88 M) solution as the most pleasant, were also similar in both groups. In line with the previous findings, the methadone-maintained subjects reported adding more table sugar to caffeinated beverages. The results of the present study suggest that changes in taste reactivity may not be responsible for altered dietary choices in opioid addicts.