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.

Assessing midbrain neuromelanin and its relationship to reward learning in anorexia nervosa: Stage 1 of a registered report.

INTRODUCTION: Anorexia nervosa (AN) is a debilitating and potentially chronic eating disorder, characterized by low hedonic drive toward food, which has been linked with perturbations in both reward processing and dopaminergic activity. Neuromelanin-sensitive magnetic resonance imaging (MRI) is an emerging method to index midbrain neuromelanin-a by-product of dopaminergic synthesis. The assessment of midbrain neuromelanin, and its association with AN psychopathology and reward-related processes, may provide critical insights into reward circuit function in AN. METHODS: This study will incorporate neuromelanin-sensitive MRI into an existing study of appetitive conditioning in those with AN. Specifically, those with acute and underweight AN (N = 30), those with weight-restored AN (N = 30), and age-matched healthy controls (N = 30) will undergo clinical assessment of current and previous psychopathology, in addition to structural neuromelanin-sensitive MRI, diffusion MRI, and functional MRI (fMRI) during appetitive conditioning. CONCLUSION: This study will be among the first to interrogate midbrain neuromelanin in AN-a disorder characterized by altered dopaminergic activity. Results will help establish whether abnormalities in the midbrain synthesis of dopamine are evident in those with AN and are associated with symptomatic behavior and reduced ability to experience pleasure and reward.

Relationship between energy balance and reward system gene polymorphisms and appetitive traits in young Mexican subjects.

Introduction: Appetitive traits are influenced by the interplay between genetic and environmental factors. This study aimed to explore the relationship between gene polymorphisms involved in the regulation of energy balance and food reward and appetitive traits in young Mexican subjects. Methods: This cross-sectional study involved 118 university freshman undergraduates who completed the Adult Eating Behaviour Questionnaire for Spanish speakers (AEBQ-Esp) to assess their appetitive traits. A real-time PCR system was employed to determine gene polymorphisms involved in energy balance (LEP rs7799039, MC4R rs17782313, FTO rs9939609, GHRL rs696217), and reward system (DRD2/ANKK1 Taq1A rs1800497 and COMT rs4680). Results: The mean age of participants was 20.14 ± 3.95 years, 71.2% were women and their mean BMI was 23.52 ± 4.05 kg/m2. COMT Met allele carriers presented a significantly higher "Emotional overeating" mean score than Val allele carriers (2.63 ± 0.70 vs. 2.23 ± 0.70, p = 0.028). The MC4R CC + CT genotype correlated positively with "Emotional overeating" (Phi = 0.308, p = 0.01). The COMT MetMet+MetVal genotype correlated with higher "Emotional overeating" (r = 0.257, p = 0.028; Phi = 0.249, p = 0.033). The protective genotype FTO TT correlated positively with "Emotional undereating" (Phi = 0.298, p = 0.012). Carriers of the risk genotype MC4R CC + CT presented a higher risk of "Emotional overeating" than TT carriers (OR = 2.4, 95% CI 1.3-4.8, p = 0.034). Carriers of the risk genotype COMT MetMet+MetVal (OR = 3.4, 95% CI 1.1-10.3, p = 0.033), were associated with a higher risk of "Emotional overeating" than ValVal carriers. The protective FTO genotype TT was associated with "Emotional undereating" (OR = 1.8, 95% CI 1.1-9.1, p = 0.014). Discussion: The study found a relationship between the protective genotypes of FTO TT and "Emotional undereating" and risk genotypes of COMT Met/Met+Met/Val and MC4R CC + CT with "Emotional overeating." These genetic factors may increase weight gain by enhancing hedonic food consumption and reducing satiety control. Future studies should focus on replication studies in ethnically diverse young adults and life stages to explore the relationship between polymorphisms and appetitive traits and weight. This will help tailor personalized nutrigenetic strategies to counteract disordered eating patterns leading to obesity and associated co-morbidities.

Birth weight, slowness in eating and feeding practices as independent determinants of rapid weight gain.

AIM: Our aim was to identify independent determinants of rapid weight gain in infants at 3-4, 6, and 12 months of age. METHODS: A cohort study was conducted on Mexican term infants in public and private settings between March 2021 and May 2023. Rapid weight gain was defined as a ≥0.67 SD change in weight-for-age-Z-score from birth to 3-4, 6, and 12 months of age. Maternal and infant characteristics were described, and infant feeding practices, appetitive traits, weight, and length were analysed at 3-4, 6, and 12 months of age. Rapid weight gain predictors were determined using generalised linear regression models. RESULTS: In total, 168 infants were recruited (55% boys). Small-for-gestational-age status increased rapid weight gain risk 1.5 times, whereas large-for-gestational-age status represented a 20%-30% decrease. Slowness in eating decreased the risk by 10%. Protective factors were older maternal age and higher educational level, whereas formula feeding, early complementary feeding, greater food enjoyment, and satiety responsiveness increased the risk. CONCLUSIONS: Small for gestational age, slowness in eating, and feeding practices can be rapid weight gain predictors across the first year of life.

Sugar consumption induces the consummatory suppression of sugary ethanol: Differential effects of sugar restriction according to sex and age.

Sweet foods activate the reward system that is essential in processing natural reinforcers. Maturation changes in this system during adolescence are linked to heightened impulsivity and risk-seeking behavior, including the use of drugs like ethanol. This usually starts with the consumption of sugary mixtures. However, the influence of sugar exposure on ethanol consumption remains inconclusive. The present research examines the effect of long-term sugar exposure on sugary ethanol (S-EtOH) preference and net intake, exploring the implications of sex, age, accessor restriction of sugar, and its effect during the transition into adulthood. Wistar rats of both sexes were given 24-hour access to a sugar solution for 21 days during adolescence or adulthood. Subsequently, four preference tests of S-EtOH vs. water were carried out every other day, with or without sugar access between each preference test. Our results demonstrate that continuous acute and long-term sugar access induces a consummatory suppression effect on S-EtOH intake, particularly in adult rats, irrespective of sex. This effect becomes more pronounced with more extended periods of exposure to sugar, leading to a higher prevalence of low consumers. Notably, when sugar access was restricted after high familiarization, the suppression effect in adolescent male rats was reduced. Under these conditions, the rats appeared to be more susceptible to developing a preference for S-EtOH consumption. Furthermore, our longitudinal observations reveal that sugar access or restriction conditions during the transition from adolescence to adulthood play a crucial role in shaping S-EtOH consumption patterns in adulthood.

PACE: a Novel Eating Behavior Phenotype to Assess Risk for Obesity in Middle Childhood.

BACKGROUND: Behavioral phenotypes that predict future weight gain are needed to identify children susceptible to obesity. OBJECTIVES: This prospective study developed an eating behavior risk score to predict change in adiposity over 1 y in children. METHODS: Data from 6 baseline visits (Time 1, T1) and a 1-y follow-up visit (Time 2, T2) were collected from 76, 7- to 8-y-old healthy children recruited from Central Pennsylvania. At T1, children had body mass index (BMI) percentiles <90 and were classified with either high (n = 33; maternal BMI ≥30 kg/m2) or low (n = 43; maternal BMI ≤25 kg/m2) familial risk for obesity. Appetitive traits and eating behaviors were assessed at T1. Adiposity was measured at T1 and T2 using dual-energy x-ray absorptiometry, with a main outcome of fat mass index (FMI; total body fat mass divided by height in meters squared). Hierarchical linear regressions determined which eating measures improved prediction of T2 FMI after adjustment for covariates in the baseline model (T1 FMI, sex, income, familial risk, and Tanner stage). RESULTS: Four eating measures-Portion susceptibility, Appetitive traits, loss of control eating, and eating rate-were combined into a standardized summary score called PACE. PACE improved the baseline model to predict 80% variance in T2 FMI. PACE was positively associated with the increase in FMI in children from T1 to T2, independent of familial risk (r = 0.58, P < 0.001). Although PACE was higher in girls than boys (P < 0.05), it did not differ by familial risk, income, or education. CONCLUSIONS: PACE represents a cumulative eating behavior risk score that predicts adiposity gain over 1 y in middle childhood. If PACE similarly predicts adiposity gain in a cohort with greater racial and socioeconomic diversity, it will inform the development of interventions to prevent obesity. This trial was registered at clinicaltrials.gov as NCT03341247.

Appetitive conditioning with pornographic stimuli elicits stronger activation in reward regions than monetary and gaming-related stimuli.

Appetitive conditioning plays an important role in the development and maintenance of pornography-use and gaming disorders. It is assumed that primary and secondary reinforcers are involved in these processes. Despite the common use of pornography and gaming in the general population appetitive conditioning processes in this context are still not well studied. This study aims to compare appetitive conditioning processes using primary (pornographic) and secondary (monetary and gaming-related) rewards as unconditioned stimuli (UCS) in the general population. Additionally, it investigates the conditioning processes with gaming-related stimuli as this type of UCS was not used in previous studies. Thirty-one subjects participated in a differential conditioning procedure in which four geometric symbols were paired with either pornographic, monetary, or gaming-related rewards or with nothing to become conditioned stimuli (CS + porn, CS + game, CS + money, and CS-) in an functional magnetic resonance imaging study. We observed elevated arousal and valence ratings as well as skin conductance responses for each CS+ condition compared to the CS-. On the neural level, we found activations during the presentation of the CS + porn in the bilateral nucleus accumbens, right medial orbitofrontal cortex, and the right ventral anterior cingulate cortex compared to the CS-, but no significant activations during CS + money and CS + game compared to the CS-. These results indicate that different processes emerge depending on whether primary and secondary rewards are presented separately or together in the same experimental paradigm. Additionally, monetary and gaming-related stimuli seem to have a lower appetitive value than pornographic rewards.

Acute nicotine administration reduces the efficacy of punishment in curbing remifentanil consumption in a seeking-taking chain schedule of reinforcement.

RATIONALE: Nicotine dependence is highly comorbid with opioid use disorders (OUDs). The use of nicotine-containing products increases the propensity to misuse prescription opioids and addressing both nicotine and opioid use simultaneously is more efficacious for treatment of OUDs than treating opioid use alone. OBJECTIVES: Given this extreme comorbidity, further elucidation of the effects of nicotine as a factor in promoting vulnerability to development of OUDs is needed. Here, we sought to further explore the effects of nicotine administration on operant self-administration of remifentanil (RMF), a fast-acting synthetic µ-opioid receptor agonist, using a heterogenous seeking-taking chain schedule of reinforcement in unpunished and punished conditions. METHODS: Male and female rats received nicotine (0.4 mg/kg) or saline prior to operant self-administration sessions. These sessions consisted of pressing a 'seeking' lever to gain access to a 'taking' lever that could be pressed for delivery of 3.2 µg/kg RMF. After acquisition, continued drug seeking/taking was punished through contingent delivery of foot-shock. RESULTS: Nicotine, relative to saline, increased RMF consumption. Furthermore, nicotine treatment resulted in significantly higher seeking responses and cycles completed, and this effect became more pronounced during punished sessions as nicotine-treated rats suppressed RMF seeking significantly less than controls. Nicotine treatment functionally reduced the efficacy of foot-shock punishment as a deterrent of opioid-seeking. CONCLUSIONS: Nicotine administration enhanced both appetitive and consummatory responding for RMF and engendered a punishment-insensitive phenotype for RMF that was less impacted by contingent administration of foot-shock punishment. These findings provide further support for the hypothesis that nicotine augments vulnerability for addiction-like behaviors for opioids.

Parental perceptions of body weight and appetite in infants and toddlers with cystic fibrosis.

Nutritional status has clinical relevance and is a target of guidance to parents of children with cystic fibrosis (CF). Growth is routinely monitored in CF clinics but there is no standardized way of assessing appetitive behaviors or parents' perceptions of their children's appetite. Greater understanding of these factors could improve clinical guidance regarding parent feeding behaviors. We therefore aimed to assess parent perceptions of child weight, and parent reports of child appetite using the Baby Eating Behavior Questionnaire (BEBQ), in a sample of infants and toddlers with CF, compared with a community sample. We additionally assessed relationships of parent perceptions of child weight with parent feeding behaviors in the sample with CF. Anthropometric and questionnaire data were collected for 32 infants and toddlers with CF, as well as 193 infants and toddlers drawn from RESONANCE, a community cohort study. Parents perceived children with CF to be lower in weight than their actual weight, to a greater extent than was evident in the community sample. Parents who perceived their children with CF to be underweight vs. right weight reported greater slowness in eating on the BEBQ. Parents perceived children with CF to have greater slowness in eating and lower enjoyment of food, compared to parents of children in the community sample, independent of sample differences in child weight, age, and sex. Our results demonstrate the potential utility of the BEBQ in a clinical sample and suggest it may be helpful for clinicians to assess parents' perceptions of their child's weight and appetite to promote a fuller understanding of the child's nutritional status, facilitate appropriate feeding behaviors and alleviate unnecessary concerns.

Keeper effect: Animals are more active in the presence of their caretakers.

Zookeepers are the primary caretakers of animals, providing daily care through frequent and close interactions. From the animal's perspective, most of these daily interactions are likely to have positive outcomes. With consistent and reliable interactions, a human-animal relationship is expected to develop. Our aim of this study was to investigate if the presence of the primary keeper in the public viewing area of zoo exhibits impacts the behavior of animals. We observed the behavior of 15 individuals of six species in the presence and absence of their primary keeper. Overall, we observed animals being more active in their keeper's presence than when the keeper was not present. When we considered if the keeper was nearby around times of offered opportunities to animals (e.g., feeding, enrichment, and training), our results showed that animals were equally as active when the keeper was present before an opportunity and when no opportunity was offered. These equal activity levels imply that the keeper is a cue for a forthcoming event to the animals, which reflects anticipatory behavior. Overall, we demonstrate that keeper presence is an environmental context in which animals behave differently than in keeper absence.

Dual roles of dopaminergic pathways in olfactory learning and memory in the oriental fruit fly, Bactrocera dorsalis.

Dopamine (DA) is a key regulator of associative learning and memory in both vertebrates and invertebrates, and it is widely believed that DA plays a key role in aversive conditioning in invertebrates. However, the idea that DA is involved only in aversive conditioning has been challenged in recent studies on the fruit fly (Drosophila melanogaster), ants and crabs, suggesting diverse functions of DA modulation on associative plasticity. Here, we present the results of DA modulation in aversive olfactory conditioning with DEET punishment and appetitive olfactory conditioning with sucrose reward in the oriental fruit fly, Bactrocera dorsalis. Injection of DA receptor antagonist fluphenazine or chlorpromazine into these flies led to impaired aversive learning, but had no effect on the appetitive learning. DA receptor antagonists impaired both aversive and appetitive long-term memory retention. Interestingly, the impairment on appetitive memory was rescued not only by DA but also by octopamine (OA). Blocking the OA receptors also impaired the appetitive memory retention, but this impairment could only be rescued by OA, not by DA. Thus, we conclude that in B. dorsalis, OA and DA pathways mediate independently the appetitive and aversive learning, respectively. These two pathways, however, are organized in series in mediating appetitive memory retrieval with DA pathway being at upstream. Thus, OA and DA play dual roles in associative learning and memory retrieval, but their pathways are organized differently in these two cognitive processes - parallel organization for learning acquisition and serial organization for memory retrieval.

Appetitively motivated tasks in the IntelliCage reveal a higher motivational cost of spatial learning in male than female mice.

The IntelliCage (IC) permits the assessment of the behavior and learning abilities of mice in a social home cage context. To overcome water deprivation as an aversive driver of learning, we developed protocols in which spatial learning is motivated appetitively by the preference of mice for sweetened over plain water. While plain water is available at all times, only correct task responses give access to sweetened water rewards. Under these conditions, C57BL/6J mice successfully mastered a corner preference task with the reversal and also learned a more difficult time-place task with reversal. However, the rate of responding to sweetened water decreased strongly with increasing task difficulty, indicating that learning challenges and reduced success in obtaining rewards decreased the motivation of the animals to seek sweetened water. While C57BL/6J mice of both sexes showed similar initial taste preferences and learned similarly well in simple learning tasks, the rate of responding to sweetened water and performance dropped more rapidly in male than in female mice in response to increasing learning challenges. Taken together, our data indicate that male mice can have a disadvantage relative to females in mastering difficult, appetitively motivated learning tasks, likely due to sex differences in value-based decision-making.

Validation of the Adult Eating Behavior Questionnaire in an Italian Community Sample.

(1) Background: Appetitive traits in adults can be measured through the Adult Eating Behavior Questionnaire (AEBQ), a questionnaire adapted from the Child Eating Behavior Questionnaire (CEBQ). The AEBQ has been validated in several countries. The aim of the present study was to explore and validate the factor structure of the Italian version of the AEBQ. Furthermore, convergent validity and correlations between factors and BMI were explored to assess its criterion validity. (2) Methods: Participants (N = 624, mean age of 32.08 ± 14.94 years) completed the AEBQ, the Eating Attitude Test (EAT-40), and the Dutch Eating Behavior Questionnaire (DEBQ). They also self-reported demographic and anthropometric data. A Confirmatory Factor Analysis (CFA) was used to test three different alternative models that emerged in previous validations. (3) Results: The CFA revealed a good model fit (RMSEA = 0.0634, TLI = 0.894, CFI = 0.907) for the 7-factor structure, without the Hunger items, showing a valid and reliable (Cronbach's α > 0.7) structure. Convergent and divergent validity of the AEBQ yielded favorable results, and relationships between the AEBQ and BMI factors revealed that the Food Approach traits were positively associated with BMI. (4) Conclusions: Finally, this study provides initial support for the use of the AEBQ as a valid and reliable tool to measure a wide range of appetitive traits in the adult Italian population.

The effects of a retrieval cue on renewal of conditioned responses in human appetitive conditioning.

Contextual renewal of reward anticipation may be one potential mechanism underlying relapse in eating and substance use disorders. We therefore tested retrieval cues, a method derived from an inhibitory retrieval-based model of extinction learning to attenuate contextual renewal using an appetitive conditioning paradigm. A pilot study was carried out in Experiment 1 to validate a differential chocolate conditioning paradigm, in which a specific tray was set up as a conditioned stimulus (CS) for eating chocolate (unconditioned stimulus, US). Using an ABA renewal design in Experiment 2, half of the participants were presented with a retrieval cue in the acquisition phase (group AC) and the other half in the extinction phase (group EC). Presentation of the retrieval cue in the EC was associated with reduced renewal of US-expectancy, while there was a clear renewal effect for US-expectancy in the AC. One limitation was the difference in cue presentations between both groups due to the number of trials in acquisition and extinction. Experiment 3 therefore aimed at replicating the results of Experiment 2, but with fewer cue presentations for the EC to match the AC. No significant group differences were observed indicating no effect of the retrieval cue. Theoretical and clinical implications in light of the differing results are discussed.

Exploring relationships among appetitive traits, negative affect, and binge eating in adults with overweight or obesity.

Binge eating (BE) is a significant public health concern due to its prevalence and impact on mental and physical health. While research has suggested both negative affect and appetitive traits are associated with BE, few studies have investigated these constructs concurrently. Structural equation modeling (SEM) evaluated relationships between negative affect, reward-related appetitive traits, and BE among 293 adults with overweight or obesity (OW/OB) seeking treatment for BE, overeating, and weight management (m age = 46.6; m body mass index[BMI] = 34.5; 81.2 % female; 20.1 % Latinx, 60.8 % White non-Latinx). BE was related to negative affect (ß = 0.53; p < 0.01) and appetitive traits (ß = 1.53; p < 0.001). Negative affect and appetitive traits were related to one another (r = 0.42; p < 0.001), and the full model accounted for 77 % of the variance in BE. In an exploratory follow-up analysis, multigroup SEM evaluated the above relationships in models stratified by sex. Exploratory findings demonstrated both negative affect and appetitive traits were related to BE across sex, particularly when examining BE cognitions and behaviors. However, relationships in men depended upon BE assessment tool. These findings highlight that both negative affect and appetitive traits are related to BE, and jointly may represent significant risk and maintenance factors, particularly in adults with OW/OB. Our findings also highlight the importance of future investigation of sex differences in BE and the potential impact of assessment method.

Associations of Appetitive Traits and Parental Feeding Style With Diet Quality During Early Childhood.

BACKGROUND: Appetitive traits and parent feeding styles are associated with body mass index in children, yet their associations with child diet quality are unclear. OBJECTIVE: The objective was to examine relations of appetitive traits and parental feeding style with diet quality in 3.5-year-old children. DESIGN: The study was a secondary, cross-sectional analysis of data from Sprouts, a follow-up study of the Pregnancy Eating Attributes Study (PEAS). Birthing parents completed the Child Eating Behavior Questionnaire, Caregiver's Feeding Styles Questionnaire, and proxy 24-hour dietary recalls for their children from February 2019 to December 2020. PARTICIPANTS/SETTING: Participants were 162 birthing parents (early pregnancy BMI ≥ 18.5 and absence of preexisting diabetes, any medical condition contraindicating study participation, self-reported eating disorder, or medications that could affect diet or weight) and their children living in North Carolina. MAIN OUTCOME MEASURES: Healthy Eating Index-2015 (HEI-2015) total scores were calculated. STATISTICAL ANALYSES PERFORMED: Path modeling was conducted using PROC CALIS with full information maximum likelihood (FIML) to account for missing data (< 2% of all data in dataset). Associations of child appetitive traits and parental feeding style with child HEI-2015 scores, adjusting for exclusive breastfeeding duration and household income-poverty ratio, were examined. Tests of simple effects were conducted in subsamples split by parental feeding style. Hypotheses were formulated during data collection. RESULTS: A 1-standard deviation (SD) greater food fussiness was associated with a 2.4-point lower HEI-2015 total score (P = .02; 95% confidence interval [CI] [-4.32, -0.48]) in children. When parental feeding style was authoritarian, a 1-SD greater food responsiveness was associated with a 4.1-point higher HEI-2015 total score (P = .007; 95% CI [1.12, 7.01]) in children. When parental feeding style was authoritative, a 1-SD greater slowness in eating was associated with a 5.8-point lower HEI-2015 total score (P = .01; 95% CI [-10.26, -1.33]) in children. CONCLUSIONS: Parental feeding style may modify the association of appetitive traits with diet quality in young children. Future research could determine whether matching parent feeding styles to child appetitive trait profiles improves child diet quality.

The Effects of Sleeve Gastrectomy on the Appetitive Domain of Taste Using the Progressive Ratio Task.

INTRODUCTION: Sleeve gastrectomy (SG) is an effective treatment for obesity in adolescents. The underlying weight loss mechanism may impact the peripheral and central gustatory system along with reward circuits in the brain. This study aims to assess changes in appetitive behavior in short-, medium-, and long-term follow-up. METHODS: In this prospective observational study, a total of 8 adolescents with obesity who underwent SG and 9 comparator unoperated participants were studied. Appetitive behaviour towards fat and sweet taste stimuli was assessed using the Progressive Ratio Task (PRT) over a 6 year period. RESULTS: Mean body mass index (BMI) of the surgical patients dropped from 51.5 ± 2.8 kg/m2 to 31.4 ± 1.9 and 30.9 ± 2.3 kg/m2 at 1 and 6 years follow-up, respectively. (p < 0.001). The median (interquartile range) total rewards earned during the PRT was 6 (5-7) pre-surgery, 5 (3-6) after one year and 4 (2-4) after six years from surgery (p = 0.007). CONCLUSION: SG reduced appetitive behaviour at 1 year with maintained the benefit over 6 years as measured by the progressive ratio task.

Altered reward sensitivity to sucrose outcomes prior to drug exposure in alcohol preferring rats.

Addiction involves key impairments in reward sensitivity (RS). The current study explored impaired RS to natural reward as a predisposing factor to addictive-like behavior. Alcohol preferring (P) rats are selectively bred based on significantly greater ethanol consumption and preference and offer the ability to inspect differences in subjects with a positive family history of addictive-like behavior. P rat's RS was compared to RS in the well-used Sprague-Dawley (SD) strain. To assess RS in a novel manner, instrumental incentive contrast, discrimination and consumption of sucrose solution were examined. Animals performed in a free operant situation for different sucrose concentration solutions using a block of 'mixed' trials with alternating outcome concentrations (e.g., 5 and 10 % sucrose) to change outcome value in a predictable manner. Animals also performed for reward in blocks of single outcome trials (5 or 10 or 20 or 40 % sucrose daily exposure) surrounding the mixed block. RS (e.g., reward discrimination and contrast effects between and within-sessions) was measured by changes in trials completed, instrumental response latency and consumption. P rats expressed an altered profile of RS with a greater tendency toward equivalent responding to different outcomes within the same session and an absence of incentive contrast from diverse reward comparisons. In contrast, SD animals expressed within-session reward discrimination and a subset of incentive contrast effects. These effects were moderated by food deprivation more consistently in SD compared to P rats. P rat alterations in processing natural rewards could predispose them to addictive-like behaviors including greater alcohol consumption and preference.

An examination of the effects of nucleus accumbens core nociceptin on appetitive and consummatory motivation for food.

The nucleus accumbens (NAc) is a critical region for regulating the appetitive and consummatory aspects of motivated behavior. Previous work has shown differential effects of NAc µ-, δ-, and κ- receptor stimulation on food intake and for shifting motivation within an effort-based choice (EBC) task. However, the motivational role of the nociceptin opioid peptide (NOP) receptor, a fourth member of the opioid receptor family, is less well understood. These experiments therefore characterized the effect of NAc injections of nociceptin, the endogenous ligand for the NOP receptor, on consummatory and appetitive motivation. Three groups of male Sprague-Dawley rats received nociceptin injections into the NAc core prior to testing in a progressive ratio lever pressing task, an EBC task, or a palatable feeding assay. In the feeding experiment, 10 nmol of nociceptin increased consumption in the first 30 min, but this increase was not sustained through the end of the 2-hr session. Additionally, nociceptin injections did not alter breakpoint in the progressive ratio task. However, in the EBC task, nociceptin significantly decreased breakpoint for sugar pellets without affecting consumption of rat chow. These data suggest that NAc NOP receptor stimulation transiently increases consummatory motivation toward palatable diets and inhibits appetitive motivation when alternate food options are freely available. This pattern of effects contrasts with those obtained following NAc stimulation of other opioid receptors, suggesting that the four opioid receptor classes each serve unique roles in modulating food-directed motivation within the NAc core.