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.

Chronic exposure to cafeteria-style diet in rats alters sweet taste preference and reduces motivation for, but not 'liking' of sucrose.

Obesity is associated with changes to taste perception and brain reward circuitry. It is important to understand how these effects alter the preference for palatable foods and drinks, given that these are widely consumed, and leading risk factors for obesity. This study examined the effects of diet-induced obesity on sweet taste preference by analysing the microstructure of licking for sugar solutions and assessing pERK expression in the nucleus accumbens shell and insula. Adult male Sprague-Dawley rats were fed standard chow (Control; n = 16) or a varied, palatable cafeteria diet (Caf; n = 16) for 12 weeks. Two-choice preference tests between 2%, 8% and 32% sucrose solutions were conducted at baseline and in weeks 11-12 of the diet. Rats in the Caf group trebled energy intake and doubled weight gain relative to controls. In tests held under water restriction after 11 weeks of diet, the Control group reliably preferred higher sucrose concentrations (i.e., 32% > 8% > 2%). Relative to controls, the Caf group showed a stronger preference for 32% vs. 2% sucrose, lower preference for 32% vs. 8% sucrose, and were indifferent to 8% vs. 2% sucrose. Testing without water restriction increased preference for higher sucrose concentrations in both groups. Chronic Caf diet increased the latency to lick, decreased total licks and reduced alternations between spouts, but did not alter lick cluster size, a measure of hedonic appraisal, on any test. Following a final exposure to a novel sucrose concentration, neuronal activity (pERK) in the insula and nucleus accumbens shell was significantly reduced in the Caf group. Results indicate that differences in 'liking' do not underlie obesity-induced changes to sweet taste preference.

Effort-reward balance and work motivation in rats: Effects of context and order of experience.

Being motivated means exerting effort toward a goal. The 'law of least work' emphasizes a preference for exerting relatively less effort. The law crosses boundaries among species and between physical and mental work. Organisms should be highly sensitive to shifts in effort-reward balance (ERB) in order to make optimal choices. We used a free operant-foraging task to investigate changes in ERB on choice between options requiring more or less effort. Results showed a consistent preference for the option with less effort and insensitivity to shifts in ERB. A second aim explored the influence of order of experience on effort choice. Choice for the more effortful option significantly increased after experiencing an equal effort-reward relationship during the initial free operant-foraging session. This relative increase in choice for the effortful option persisted even after effort-reward imbalance. The findings highlight the importance of contextual factors such as order of experience when examining the impact of shifting effort-reward associations. Instead of ignoring or reducing order effects, the sequence of experience (e.g. for shifts in ERB) could be manipulated to enhance or reduce value of outcomes or effort itself.

An investigation of variety effects during operant responding in the rat utilizing different reward flavors.

Humans and nonhuman animals respond to food diversity by increasing intake and appetitive behaviors, reflecting enhanced valuation for items presented in the context of variety. Previous work on food variety effects has posited two main explanatory mechanisms. Variety could slow habituation processes by decreasing exposure to a single food item or could elicit contrast effects in which comparisons between items impact relative valuation. This study used three flavors of sucrose rewards to investigate rats' responses to qualitative reward variety in different variety contexts: low (2 flavors) and high (3 flavors) conditions. Control sessions used only a single flavored pellet (no variety). Animals were tested in low (10 trials), moderate (20 trials) and high consumption (30 trials) sessions. A trial within each session was defined as completion of the operant response and acquisition of the reward pellet. Cues associated with flavors were used to examine predictability and between-trial ('micro') variety. Indicators of a variety effect were found including faster responding for rewards during the variety context compared to an initial control (no variety) context. This decrease in response latency continued to be observed for some measures in post-variety control contexts. The most robust statistical finding of variety effects was found using trial-by-trial analysis, with shorter response latencies obtained for trials with outcomes differing from the preceding trial compared to successive trials with identical outcomes. These results have implications for understanding how a general reward context like variety impacts behavior, and for informing clinical approaches focusing on motivation and eating disorders.

A possible social relative reward effect: Influences of outcome inequity between rats during operant responding.

Social interactions/situations have dramatic influences on motivation. Creating animal models examining these influences promotes a better understanding of the psychological and biological underpinnings of social motivation. Rodents are sensitive to social history/experience during associative conditioning and food-sharing tasks. Would reward-oriented operant behavior be sensitive to social influences by showing a negative contrast-like effect when another organism obtains a greater value outcome? We used a side-by-side arrangement of operant response chambers wherein one animal obtained consistently high reward signaled by a discrete cue. The neighboring, experimental rat experienced different combinations of high and low reward trial sequences. Control conditions included distraction from a conspecific in the neighboring chamber (rat distractor) or cue/food dispenser operating without a conspecific (program distractor) in addition to testing subjects alone. Results support an influence of the other animal actively performing the task on the experimental subject's behavior. Primarily, responding was significantly slower for the low reward trials while the neighboring rat was receiving the higher magnitude reward. The lever-press and not food-cup retrieval latency was significantly slower during exposure to a conspecific neighbor performing the operant task. The effect was not obtained in all session sequences and was more pronounced using longer series of consecutive low reward trials. The slowing effect was also obtained with the program-distractor experience in a different trial sequence. These findings suggest a social-induced negative incentive contrast effect in rats possibly mediated by an outcome inequity process that could have key similarities to complex situational-affective effects on motivation involving frustration or jealously.

Neural encoding of choice during a delayed response task in primate striatum and orbitofrontal cortex.

Reward outcomes are available in many diverse situations and all involve choice. If there are multiple outcomes each rewarding, then decisions regarding relative value lead to choosing one over another. Important factors related to choice context should be encoded and utilized for this form of adaptive choosing. These factors can include the number of alternatives, the pacing of choice behavior and the possibility to reverse one's choice. An essential step in understanding if the context of choice is encoded is to directly compare choice with a context in which choice is absent. Neural activity in orbitofrontal cortex and striatum encodes potential value parameters related to reward quality and quantity as well as relative preference. We examined how neural activations in these brain regions are sensitive to choice situations and potentially involved in a prediction for the upcoming outcome selection. Neural activity was recorded and compared between a two-choice spatial delayed response task and an imperative 'one-option' task. Neural activity was obtained that extended from the instruction cue to the movement similar to previous work utilizing the identical imperative task. Orbitofrontal and striatal neural responses depended upon the decision about the choice of which reward to collect. Moreover, signals to predictive instruction cues that precede choice were selective for the choice situation. These neural responses could reflect chosen value with greater information on relative value of individual options as well as encode choice context itself embedded in the task as a part of the post-decision variable.

The effects of ethanol on diverse components of choice in the rat: reward discrimination, preference and relative valuation.

Alcohol consumption impairs judgment and choice. How alcohol alters these crucial processes is primarily unknown. Choice can be fractionated into different components including reward discrimination, preference and relative valuation that can function together or in isolation depending upon diverse factors including choice context. We examined the diverse components and contextual effects by analyzing the effects of alcohol drinking on choice behavior in a task with a reduced level of temporal and spatial constraints. Rats were trained to drink 10% ethanol during 6 weeks of behavior testing using a combined sucrose-fade and two-bottle free-choice procedure. Two different sucrose pellet outcomes (e.g., constant vs. variable) were presented each week to examine the impact of voluntary drinking on reward-based decision-making. Behavioral contexts of single option, free choice and extinction were examined for each outcome set. Comparisons were made between alcohol and control groups and within the alcohol group over time to inspect choice profiles. Between-group results showed alcohol drinking animals expressed altered place preference and modified sucrose reward approach latencies. The within-group profile showed that alcohol drinking animals can express adequate reward discrimination, preference and incentive contrast during free choice. All of these components were significantly reduced during the context of extinction. Control animals were also impacted by extinction but not as severely. The findings point to a need for a greater focus on the context and the diverse components of choice when examining external and internal factors influencing decision-making during alcohol or other substance of abuse exposure.

Effects of anandamide administration on components of reward processing during free choice.

Previous research has implicated the positive modulation of anandamide, an endocannabinoid neurotransmitter, on feeding behavior. Anandamide is particularly noteworthy as it acts as an endogenous ligand of the CB1 receptor, the same receptor that is activated by tetrahydrocannabinol, the primary psychoactive component in Cannabis sativa. Cannabis legalization in North America has presented with a need to study endocannabinoid agonists and their effects on behavior. Much has yet to be determined in terms of the role of the endocannabinoid system in decision-making scenarios. The research presented here tested the hypothesis that anandamide would augment motivation and reward processing via appetitive and consummatory measures during an operant, foraging task. A three-box design was used in order to provide the animals with a free choice, exploratory foraging environment. Discrimination, preference, and incentive contrast were analyzed as discrete measures of decision-making in the three-box paradigm. Anandamide administration (1mg/kg) was found to significantly increase motivation for the optimal foraging outcome and alter basic processing of reward information involved in discrimination and relative valuation. The positive effects of anandamide on eating behavior and motivation have implications toward possible treatment modalities for patient populations presenting with disorders of motivation. These findings suggest the need for continued investigation of the endocannabinoid system as a central component of motivated behavior.

Striatal Activity and Reward Relativity: Neural Signals Encoding Dynamic Outcome Valuation.

The striatum is a key brain region involved in reward processing. Striatal activity has been linked to encoding reward magnitude and integrating diverse reward outcome information. Recent work has supported the involvement of striatum in the valuation of outcomes. The present work extends this idea by examining striatal activity during dynamic shifts in value that include different levels and directions of magnitude disparity. A novel task was used to produce diverse relative reward effects on a chain of instrumental action. Rats (Rattus norvegicus) were trained to respond to cues associated with specific outcomes varying by food pellet magnitude. Animals were exposed to single-outcome sessions followed by mixed-outcome sessions, and neural activity was compared among identical outcome trials from the different behavioral contexts. Results recording striatal activity show that neural responses to different task elements reflect incentive contrast as well as other relative effects that involve generalization between outcomes or possible influences of outcome variety. The activity that was most prevalent was linked to food consumption and post-food consumption periods. Relative encoding was sensitive to magnitude disparity. A within-session analysis showed strong contrast effects that were dependent upon the outcome received in the immediately preceding trial. Significantly higher numbers of responses were found in ventral striatum linked to relative outcome effects. Our results support the idea that relative value can incorporate diverse relationships, including comparisons from specific individual outcomes to general behavioral contexts. The striatum contains these diverse relative processes, possibly enabling both a higher information yield concerning value shifts and a greater behavioral flexibility.

Effects of striatal lesions on components of choice: Reward discrimination, preference, and relative valuation.

The striatum is a key structure involved in reward processing and choice. Recently, we have developed a paradigm to explore how components of reward processing work together or independently during choice behavior. These components include reward discrimination, preference and relative valuation, and the goal of the present study was to determine how the striatum is involved in these dissociable components during this novel free choice paradigm. We tested choice utilizing two different outcome series with one being a more straightforward single-option discrimination anchored by a 0 reward outcome, and the other as a multi-option outcome discrimination of greater difficulty. We compared the free choice reward task to a sequential reward task and an extinction task. Striatal lesions impaired responding only in the free choice version with alterations in both appetitive and consummatory measures. Ventral striatal lesions had greater impact altering discrimination, preference and relative valuation in both the single and multi-option week studies. A major factor involved in these deficits was a significant aversion to the multi-option that contained a larger outcome option but with a longer delay to reward. Dorsal striatal lesions caused less impairment even leading to enhanced choice behavior compared to control animals during the more difficult multi-option free choice series. Overall, the results suggest that the context of action is crucial when linking striatal function to choice behavior and its diverse components. The implications include the idea that striatal involvement in decision-making is increased when responses are self-paced and diverse in a more naturalistic environment.

Function of the centromedial amygdala in reward devaluation and open-field activity.

The present research aimed at determining the role played by the amygdala in reward devaluation using transient inactivation induced by lidocaine microinfusions into the centromedial region. Two situations involving reward devaluation were tested in rats: consummatory successive negative contrast (cSNC) and anticipatory negative contrast (ANC). In cSNC, rats exposed to a downshift from 32% to 4% sucrose consume less 4% sucrose than rats always exposed to 4% sucrose. Extensive evidence suggests that reward devaluation in the cSNC situation is accompanied by negative emotion. In ANC, rats consume less 4% sucrose when each session is closely followed by access to 32% sucrose rather than by 4% sucrose. Evidence suggests that reward devaluation in the ANC situation does not involve negative emotions; rather, ANC appears to involve Pavlovian anticipation of the higher value solution. To test the effects of lidocaine microinfusions in a situation known to induce negative emotion, but unrelated to reward devaluation, animals were also exposed to a lighted open field. Centromedial amygdala inactivation reduced the cSNC effect and increased exploratory behavior in the open field, both effects consistent with a reduction in negative emotional state. However, no detectable effects of amygdala inactivation were observed in the ANC situation. These results suggest that, first, the function of the amygdala is not unique to reward devaluation and, second, it is concerned with tagging the devaluation experience with aversive valence.

Emotion and relative reward processing: an investigation on instrumental successive negative contrast and ultrasonic vocalizations in the rat.

Incentive contrast effects include changes in behavioral responses after a reward upshift (positive contrast) or downshift (negative contrast). Proposed influences on these behavioral changes are emotional state reactions after experiencing or anticipating a change in reward outcome. Rat ultrasonic vocalizations have been shown to be indicators of emotional state during behavior and anticipatory periods. The objective of the present study was to monitor rodent ultrasounds during incentive contrast using a classical runway procedure called instrumental successive negative contrast. The procedure is one that has been used often to examine incentive relativity because of its reliability in measuring negative contrast effects. Rats were trained to run in the alleyway to receive a high (12 pellets) or low magnitude (1 pellet) outcome. The high magnitude was then shifted to the low and running speeds in the alleyway for the reward and USV emission were compared. Replicating previous work, a negative contrast effect was observed with postshift running speeds significantly slower in the shifted group compared to the unshifted group. USVs did not follow the same pattern with an apparent lack of significant differences between the groups following the reward downshift. We also tested another group of animals using a visual predictive cue in the same runway test. When visual cues predicted high or low magnitude outcome, no incentive contrast was found for the running speeds following an outcome downshift, but a weak contrast effect was observed for the USV emission. These results demonstrate a separation between USVs and behavioral indicators of incentive contrast suggesting that concomitant shifts in negative affect may not be necessary for anticipatory relative reward processes.

Psicofísica comparada: Efectos de la pérdida de incentivos en ratas normales e inyectadas con etanol / Comparative Psychophysic: Loss of incentive effects in normal rats injected with ethanol

Un problema de la Psicofísica particularmente relevante para el estudio de procesos de condicionamiento es el de la relación entre las propiedades de refuerzo y físicas de los es tímulos. Papini y Pellegrini (2006) observaron que el nivel de respuestas de consumo en ratas expuestas a una disminución sor presiva en la con centración de soluciones azucaradas resulta similar, si la razón de cambio de las soluciones (concentración post-cambio / pre-cambio) es la misma. Por ejemplo, una disminución de 32 a 4% y una de 16 a 2% produjeron niveles de consumo similares. Estos resultados son acordes a la Ley de Weber y podrían implicar el desarrollo de procesos emocionales; algo que se ha establecido mediante el estudio de drogas ansiolíticas. En situaciones experimentales análogas en las que se estudió el efecto de contraste negativo de incentivo (con una disminución de 32 a 4%) se demostró que la administración de etanol en el segundo ensayo post-cambio produce una recuperación más rápida de las respuestas deprimidas debido a una disminución del refuerzo (Becker & Flaherty, 1982), en comparación con controles que recibieron salina. En el presente trabajo se informa un experimento que involucró ocho grupos de ratas y que buscó replicar ambos fenómenos así como estudiar sus posibles interacciones. Se hallaron resultados compatibles con los datos previos, pero no se pudo concluir si el etanol afecta de manera determinante los niveles de respuesta proporcionales observados durante disminuciones de incentivo. Los resultados se discuten en términos de su implicancia para el estudio de la Psicofísica Comparada.

A psychophysical problem that might be of particular interest in current research on conditioning phenomena, and that exceeds the area of sensation and perception, concerns the reinforcing properties of stimuli and their relationships to the physical measures of the stimuli. Stimuli acting as reinforcers are operationally defined as those which have the property of changing the probability of response output of an organism if they are presented in an ordered fashion in relation to other stimuli or responses. From a biopsychological point of view the scale in which this property is measured is referred to as the stimulus incentive value. Papini and Pellegrini (2006) observed that the level of consummatory responses in rats exposed to surprising reductions in sucrose concentrations is similar if the shift ratio of the solutions (preshift / postshift concentration) is the same. For example, a 32 to 4% downshift and a 16 to 2% downshift produced similar levels of consummatory responding. Pellegrini and Papini (2007) and Pellegrini, Lopez Seal, and Papini (2008) extended this observations to measures of anticipatory behaviours of rats in studies implementing runway, autoshaping, solid food and within subjects design. These results are in agreement with the Weber´s Law. The incentive downshift situations as observed in spaced-trials conditions are interesting in psychological terms because they might imply the development of emotional processes (Amsel, 1992; Flaherty, 1996; Papini, 2002, 2003). Therefore, the aforementioned proportionality in incentive downshift situations might also imply proportional emotional responses. The evidence favoring an emotional interpretation of behavioral regulation during incentive downshifts arise mostly from studies which used drugs with anxiolitic-like effects (for example, clordiazepoxide and ethanol). For example, in similar experimental conditions it has been shown that the administration of ethanol before the second postshift trial reduces the level of response depression due to incentive downshifts (Becker & Flaherty, 1982). Here we present one experiment with eight groups of rats designed to replicate both of these findings and to study their potential interactions. Groups differed in the concentration of preshift sucrose solutions (either 32 or 16%) and postshift solution (16, 8, 4 or 2%), which implied two different downshift ratios (.125 and .25); and in the drug condition [vehicle (S), or an ip. injection of .75g/kg ethanol (E) administered 10 min. before the second postshift trial]. In short, the eight groups were labeled as follows: 32-8S, 32-8E, 32-4S, 32-4E, 16-4S, 16-4E, 16-2S, and 16-2E. Results are in agreement with previous data, indicating, first, a strong significant effect of downshift ratio F(1, 34) = 19,15, p < .0001; and second, a small but significant effect of ethanol administration on the recovery from incentive downshift F(1, 34) = 5,09, p < .031. However, these results are not conclusive regarding the possibility that ethanol administration can modify the proportionality observed in consummatory responding in incentive downshift situations. Also important is the observation that the 32-4 and 16-4 conditions differed respecting its postshift consummatory performance (32-4 < 16-4); the difference was significant [F(1, 18) = 6,71; p < .018] and is interpreted as a special case of the successive negative incentive contrast effect. In his classic writings on Adaptation Level Theory, Helson (1964, pp. 448-449), argued that "The evidence shows that reinforcing agents behave like psychophysical stimuli being scalable on continua having neutral or indifferent regions and in being subject to both series and anchor effects". Present results are in agreement with this statement and their implications for studies on Comparative Psychophysics are discussed. It is suggested that further research with alternative anxiolitic drugs and experimental designs might be of interest.