Increased elasticity of sucrose demand during hyperdopaminergic states in rats.

RATIONALE: Deficits in cost-benefit decision-making are a core feature of several psychiatric disorders, including substance addiction, eating disorders and bipolar disorder. Mesocorticolimbic dopamine signalling has been implicated in various processes related to cognition and reward, but its precise role in reward valuation and cost-benefit trade-off decisions remains incompletely understood. OBJECTIVES: We assessed the role of mesocorticolimbic dopamine signalling in the relationship between price and consumption of sucrose, to better understand its role in cost-benefit decisions. METHODS: Dopamine neurons in the ventral tegmental area (VTA) were chemogenetically activated in rats, and a behavioural economics approach was used to quantify the relationship between price and consumption of sucrose. Motivation for sucrose was also assessed under a progressive ratio (PR) schedule of reinforcement. To further gauge the role of dopamine in cost-benefit trade-offs for sucrose, the effects of treatment with D-amphetamine and the dopamine receptor antagonist alpha-flupentixol were assessed. RESULTS: Chemogenetic activation of VTA dopamine neurons increased demand elasticity, while responding for sucrose under a PR schedule of reinforcement was augmented upon stimulation of VTA dopamine neurons. Treatment with amphetamine partially replicated the effects of chemogenetic dopamine neuron activation, whereas treatment with alpha-flupentixol reduced free consumption of sucrose and had mixed effects on demand elasticity. CONCLUSIONS: Stimulation of mesocorticolimbic dopaminergic neurotransmission altered cost-benefit trade-offs in a complex manner. It reduced the essential value of palatable food, increased incentive motivation and left free consumption unaltered. Together, these findings imply that mesocorticolimbic dopamine signalling differentially influences distinct components of cost expenditure processes aimed at obtaining rewards.

Individual differences in voluntary alcohol intake in rats: relationship with impulsivity, decision making and Pavlovian conditioned approach.

RATIONALE: Alcohol use disorder (AUD) has been associated with suboptimal decision making, exaggerated impulsivity, and aberrant responses to reward-paired cues, but the relationship between AUD and these behaviors is incompletely understood. OBJECTIVES: This study aims to assess decision making, impulsivity, and Pavlovian-conditioned approach in rats that voluntarily consume low (LD) or high (HD) amounts of alcohol. METHODS: LD and HD were tested in the rat gambling task (rGT) or the delayed reward task (DRT). Next, the effect of alcohol (0-1.0 g/kg) was tested in these tasks. Pavlovian-conditioned approach (PCA) was assessed both prior to and after intermittent alcohol access (IAA). Principal component analyses were performed to identify relationships between the most important behavioral parameters. RESULTS: HD showed more optimal decision making in the rGT. In the DRT, HD transiently showed reduced impulsive choice. In both LD and HD, alcohol treatment increased optimal decision making in the rGT and increased impulsive choice in the DRT. PCA prior to and after IAA was comparable for LD and HD. When PCA was tested after IAA only, HD showed a more sign-tracking behavior. The principal component analyses indicated dimensional relationships between alcohol intake, impulsivity, and sign-tracking behavior in the PCA task after IAA. CONCLUSIONS: HD showed a more efficient performance in the rGT and DRT. Moreover, alcohol consumption enhanced approach behavior to reward-predictive cues, but sign-tracking did not predict the level of alcohol consumption. Taken together, these findings suggest that high levels of voluntary alcohol intake are associated with enhanced cue- and reward-driven behavior.

Loss of control over alcohol seeking in rats depends on individual vulnerability and duration of alcohol consumption experience.

Alcohol use disorder (AUD) is characterized by excessive alcohol use and persistent alcohol seeking despite knowledge of its negative consequences. Importantly, AUD typically develops after chronic excessive alcohol use in a subgroup of individuals who drink alcohol, suggesting that AUD results from an interaction between individual vulnerability and prolonged alcohol exposure. The present study assessed the contribution of prolonged exposure to alcohol and individual levels of alcohol intake to the development of loss of control over alcohol seeking in a conditioned suppression model. To investigate the impact of prolonged alcohol exposure, conditioned suppression of alcohol seeking was assessed after 2 and 4 months of intermittent alcohol access (IAA) in a subgroup of rats drinking moderate amounts of alcohol. We observed that suppression of alcohol seeking was reduced after 4 months compared with 2 months of IAA. The influence of individual levels of alcohol intake on loss of control over alcohol seeking was subsequently determined by assessing conditioned suppression in subgroups of low and high alcohol drinking rats. Unlike the low alcohol drinking rats, the high alcohol drinking rats showed aversion-resistant alcohol seeking after 2 months of IAA, although both groups showed comparable levels of conditioned freezing. These findings show that the development of loss of control over alcohol seeking, a key characteristic of AUD in humans, is dependent on both the extent of alcohol exposure and the individual's propensity to consume alcohol.

Dopaminergic neurotransmission in ventral and dorsal striatum differentially modulates alcohol reinforcement.

Dopaminergic neurotransmission in the striatum has been widely implicated in the reinforcing properties of substances of abuse. However, the striatum is functionally heterogeneous, and previous work has mostly focused on psychostimulant drugs. Therefore, we investigated how dopamine within striatal subregions modulates alcohol-directed behaviour in rats. We assessed the effects of infusion of the dopamine receptor antagonist alpha-flupenthixol into the shell and core of the nucleus accumbens (NAcc) and the dorsolateral striatum (DLS) on responding for alcohol under fixed ratio 1 (FR1) and progressive ratio (PR) schedules of reinforcement. Bilateral infusion of alpha-flupenthixol into the NAcc shell reduced responding for alcohol under both the FR1 (15 µg/side) and the PR schedule (3.75-15 µg/side) of reinforcement. Infusion of alpha-flupenthixol into the NAcc core (7.5-15 µg/side) also decreased responding for alcohol under both schedules. By contrast, alpha-flupenthixol infusion into the DLS did not affect FR1 responding, but reduced responding under the PR schedule (15 µg/side). The decreases in responding were related to earlier termination of responding during the session, whereas the onset and rate of responding remained largely unaffected. Together, these data suggest that dopamine in the NAcc shell is involved in the incentive motivation for alcohol, whereas DLS dopamine comes into play when obtaining alcohol requires high levels of effort. In contrast, NAcc core dopamine appears to play a more general role in alcohol reinforcement. In conclusion, dopaminergic neurotransmission acts in concert in subregions of the striatum to modulate different aspects of alcohol-directed behaviour.

Dopamine receptor agonists modulate voluntary alcohol intake independently of individual levels of alcohol intake in rats.

RATIONALE: Individual susceptibility to alcohol use disorder has been related to functional changes in dopaminergic neurotransmission. OBJECTIVES: The aim of the current work was to assess the effects of selective dopamine D1 and D2 receptor agonists and antagonists on alcohol consumption in rats that differ in individual levels of alcohol intake. METHODS: The effects of the dopamine D1 receptor agonist SKF 82958, the dopamine D1 receptor antagonist SCH 23390, the dopamine D2 receptor agonist sumanirole and the dopamine D2 receptor antagonist L741,626 on alcohol consumption and preference were assessed at different time points after treatment in subgroups of low and high alcohol drinking rats (LD and HD) using an intermittent alcohol access paradigm. RESULTS: SKF 82958 decreased alcohol intake and alcohol preference throughout the 24-h session. Sumanirole decreased alcohol intake during the first 2 h, but increased alcohol intake during the remainder of the session. The effects of SKF 82958 and sumanirole on alcohol intake and alcohol preference were comparable in LD and HD. By contrast, the dopamine receptor antagonists SCH 23390 and L741,626 did not alter alcohol consumption in either group at any time point. CONCLUSIONS: These data indicate that stimulation of dopamine D1 receptors reduces alcohol intake, but that endogenous dopamine does not play a primary role in alcohol consumption. Moreover, the difference in alcohol consumption between LD and HD does not involve altered dopamine signaling.

Early social isolation augments alcohol consumption in rats.

There is a considerable degree of individual vulnerability for alcohol use disorder (AUD) as only a subpopulation of individuals who regularly consume alcohol develop AUD. It is therefore very important to understand the factors and mechanisms that contribute towards the individual risk for AUD. In this respect, social influences, in particular during development, may be relevant for AUD as disruptions in early social experiences are associated with an increased risk for AUD. Social play, the most prominent form of social behaviour shown by young mammals, is rewarding and considered to be important for social, emotional and cognitive development. Recent studies suggest that early social isolation, effectively depriving animals from social play, increases the risk for addictive behaviour. The aim of this study was therefore to explore the long-term consequences of early social isolation on alcohol consumption and motivation for alcohol. To this end, rats were socially isolated from postnatal days 21-42, followed by 4 weeks of social housing, and voluntary alcohol consumption and operant responding for alcohol were determined in adulthood. We observed enhanced levels of alcohol consumption in adulthood in previously isolated rats, whereas operant responding for alcohol was not altered. The impact of early social isolation was independent of the individual variation in alcohol consumption. These data indicate that social isolation, during a developmental period when social play is highly abundant, enhances the propensity to consume alcohol in adulthood. This implies that early social experience may be a protective factor against excessive alcohol use.

Altered performance in a rat gambling task after acute and repeated alcohol exposure.

RATIONALE: A bidirectional relationship between alcohol use disorder (AUD) and deficits in impulse control and decision making has been suggested. However, the mechanisms by which neurocognitive impairments predispose to, or result from AUD remain incompletely understood. OBJECTIVES: The aim of this study is to gain more insight in the effects of alcohol exposure on decision making and impulse control. We used two modified versions of the rat gambling task (rGT) that differ in the net gain and the punishment magnitude associated with the different response options. METHODS: In experiment 1, we assessed the effects of acute alcohol treatment (0-0.8 g/kg) on rGT performance. In experiment 2, we determined the effects of alcohol on rGT acquisition (15 sessions, 0.6 g/kg). Next, these animals were challenged with alcohol (0-1.0 g/kg) prior to rGT sessions. RESULTS: Acute alcohol treatment suppressed baseline performance in both rGT versions but only modestly altered decision making. Treatment with alcohol during acquisition increased risky choices in the rGT version that involved larger punishment and blunted the reduction in win-shift behavior during acquisition in both rGT versions. Moreover, rats treated with alcohol during acquisition showed an increase in premature and perseverative responding upon subsequent alcohol challenges (0-1.0 g/kg) and were less sensitive to the behavioral suppressant effects of alcohol. CONCLUSIONS: Our results show that repeated alcohol exposure alters decision making during rGT acquisition and reduces the ability to adjust choice behavior on the basis of feedback. In addition, repeated alcohol exposure unmasks its behavioral disinhibitory effects in the rGT. Impaired responsiveness to choice feedback and behavioral disinhibition may contribute to the development of AUD.

Individual Variation in Alcohol Intake Predicts Reinforcement, Motivation, and Compulsive Alcohol Use in Rats.

BACKGROUND: Alcohol is one of the most commonly used psychoactive substances. Prolonged alcohol use can result in alcohol use disorder (AUD), characterized by excessive and compulsive alcohol consumption. Importantly, however, the development of AUD only happens in a minority of individuals who consume alcohol. To understand the individual vulnerability for AUD, models that capture both the individual variability in alcohol consumption and the transition from casual to compulsive alcohol use are essential. METHODS: Individual variability in voluntary alcohol intake and the preference for alcohol were assessed under continuous alcohol access (CAA) and intermittent-every-other-day alcohol access (IAA) schedules in the home cage using outbred Lister Hooded rats. Subsequently, the reinforcing properties of alcohol were tested in an operant setting. In subsequent experiments, we performed a quinine adulteration experiment to assess inflexible alcohol consumption and blood alcohol levels (BALs) were assessed after voluntary alcohol consumption. RESULTS: We found marked individual differences in alcohol consumption and preference under both access schedules, whereby subgroups of high- and low-alcohol-drinking rats (HD and LD) could be identified. HD with IAA increased their alcohol intake over days in the first month, whereas LD did not. Moreover, when alcohol access time was extended from 7 to 24 h/d for rats with IAA, alcohol intake profoundly increased in HD with IAA, whereas LD with IAA maintained low levels of alcohol intake. Furthermore, HD earned more alcohol than LD under both fixed ratio and progressive ratio schedules of reinforcement. We further found that HD continued their intake of a quinine-adulterated alcohol solution to a larger extent than LD and HD showed higher BALs after 30 minutes of alcohol consumption. CONCLUSIONS: These profound individual differences in alcohol intake, reinforcement, motivation, and AUD-like behavior provide a promising tool to unravel the neurobehavioral underpinnings of individual vulnerability for AUD.

Nociception and conditioned fear in rats: strains matter.

When using rats in pain research, strain-related differences in outcomes of tests for pain and nociception are acknowledged. However, very little is known about the specific characteristics of these strain differences. In this study four phylogenetically distant inbred rat strains, i.e. Wistar Kyoto (WKY), Fawn Hooded (FH), Brown Norway (BN) and Lewis (LE), were investigated in different tests related to pain and nociception. During Pavlovian fear conditioning, the LE and WKY showed a significantly longer duration of freezing behaviour than the FH and BN. Additionally, differences in c-Fos expression in subregions of the prefrontal cortex and amygdala between rat strains during retrieval and expression of conditioned fear were found. For example, the BN did not show recruitment of the basolateral amygdala, whereas the WKY, FH and LE did. During the hot plate test, the WKY and LE showed a lower thermal threshold compared to the BN and FH. In a follow-up experiment, the two most contrasting strains regarding behaviour during the hot plate test and Pavlovian fear conditioning (i.e. FH and WKY) were selected and the hot plate test, Von Frey test and somatosensory-evoked potential (SEP) were investigated. During the Von Frey test, the WKY showed a lower mechanical threshold compared to the FH. When measuring the SEP, the FH appeared to be less reactive to increasing stimulus intensities when considering both peak amplitudes and latencies. Altogether, the combined results indicate various differences between rat strains in Pavlovian fear conditioning, nociception related behaviours and nociceptive processing. These findings demonstrate the necessity of using multiple rat strains when using tests including noxious stimuli and suggest that the choice of rat strains should be considered. When selecting a strain for a particular study it should be considered how this strain behaves during the tests used in that study.

Predictability of painful stimulation modulates the somatosensory-evoked potential in the rat.

Somatosensory-evoked potentials (SEPs) are used in humans and animals to increase knowledge about nociception and pain. Since the SEP in humans increases when noxious stimuli are administered unpredictably, predictability potentially influences the SEP in animals as well. To assess the effect of predictability on the SEP in animals, classical fear conditioning was applied to compare SEPs between rats receiving SEP-evoking electrical stimuli either predictably or unpredictably. As in humans, the rat's SEP increased when SEP-evoking stimuli were administered unpredictably. These data support the hypothesis that the predictability of noxious stimuli plays a distinctive role in the processing of these stimuli in animals. The influence of predictability should be considered when studying nociception and pain in animals. Additionally, this finding suggests that animals confronted with (un)predictable noxious stimuli can be used to investigate the mechanisms underlying the influence of predictability on central processing of noxious stimuli.

Individual variations in maternal care early in life correlate with later life decision-making and c-fos expression in prefrontal subregions of rats.

Early life adversity affects hypothalamus-pituitary-adrenal axis activity, alters cognitive functioning and in humans is thought to increase the vulnerability to psychopathology--e.g. depression, anxiety and schizophrenia--later in life. Here we investigated whether subtle natural variations among individual rat pups in the amount of maternal care received, i.e. differences in the amount of licking and grooming (LG), correlate with anxiety and prefrontal cortex-dependent behavior in young adulthood. Therefore, we examined the correlation between LG received during the first postnatal week and later behavior in the elevated plus maze and in decision-making processes using a rodent version of the Iowa Gambling Task (rIGT). In our cohort of male and female animals a high degree of LG correlated with less anxiety in the elevated plus maze and more advantageous choices during the last 10 trials of the rIGT. In tissue collected 2 hrs after completion of the task, the correlation between LG and c-fos expression (a marker of neuronal activity) was established in structures important for IGT performance. Negative correlations existed between rIGT performance and c-fos expression in the lateral orbitofrontal cortex, prelimbic cortex, infralimbic cortex and insular cortex. The insular cortex correlations between c-fos expression and decision-making performance depended on LG background; this was also true for the lateral orbitofrontal cortex in female rats. Dendritic complexity of insular or infralimbic pyramidal neurons did not or weakly correlate with LG background. We conclude that natural variations in maternal care received by pups may significantly contribute to later-life decision-making and activity of underlying brain structures.

Transient inactivation of the medial prefrontal cortex affects both anxiety and decision-making in male wistar rats.

In both humans and rats high levels of anxiety impair decision-making in the Iowa gambling task (IGT) in male subjects. Expression of the immediate early gene c-fos as marker of neural activity in rat studies indicated a role of the medial prefrontal cortex (prelimbic and infralimbic region; mPFC) in mediating the relationship between anxiety and decision-making. To delineate this relationship further and assess the underlying neurobiology in more detail, we inactivated in the present study the mPFC in male rats using a mixture of the GABA-receptor agonists muscimol and baclofen. Rats were exposed to the elevated plus maze (EPM) to measure effects on anxiety and to the rodent version of the IGT (r-IGT). Inactivation led to increased levels of anxiety on the EPM, while not affecting general activity. The effect in the r-IGT (trials 61-120) was dependent on levels of performance prior to inactivation (trial 41-60): inactivation of the mPFC hampered task performance in rats, which already showed a preference for the advantageous option, but not in rats which were still choosing in a random manner. These data suggest that the mPFC becomes more strongly involved as rats have learned task-contingencies, i.e., choose for the best long-term option. Furthermore they suggest, along with the data of our earlier study, that both anxiety and decision-making in rats are mediated through a neural circuitry including at least the mPFC. The data are discussed in relation to recent data of rodent studies on the neural circuitry underlying decision-making.

Mu-opioid receptor knockout mice show diminished food-anticipatory activity.

We have previously suggested that during or prior to activation of anticipatory behaviour to a coming reward, mu-opioid receptors are activated. To test this hypothesis schedule induced food-anticipatory activity in mu-opioid receptor knockout mice was measured using running wheels. We hypothesized that mu-knockout mice show little food-anticipatory activity. In wildtype mice we observed that food-anticipatory activity increased proportional to reduced food intake levels during daily scheduled food access, and thus reflects the animal's physiological need for food. mu-Knockout mice do not adjust their schedule induced running wheel behaviour prior to and during feeding time in the same way as wildtype mice; rather than showing more running wheel activity before than during feeding, they showed an equal amount of activity before and during feeding. As food-anticipatory activity is dependent on the mesolimbic dopamine system and mu-opioid receptors regulate dopaminergic activity, these data suggest a change in the dopamine system's activity in mu-knockout mice. As we observed that mu-knockout mice tended to show a stronger locomotor activity response than wildtype mice to the indirect dopamine agonist d-amphetamine, it appears that the dopaminergic system per se is intact and sensitive to activation. We found no differences in the expression of pro-opiomelanocortin, a precursor of endogenous endorphin, in the arcuate nucleus between mu-knockout mice and wildtype mice during restricted feeding, showing that the mu-opioid receptor does not regulate endogenous endorphin levels. These data overall suggest a role for mu-opioid receptors in adapting reward related behaviour to the requirements of the environment.

Is it possible to replace stimulus animals by scent-filled cups in the social discrimination test?

A study in which the rat social discrimination test was refined is described. This test measures social memory by using, in general, juvenile rats as stimulus animals. Rats are offered a first juvenile to investigate (learning trial), and after a specified interval, the rats are offered the same rat and a second juvenile rat to investigate again (retrieval trial). When the rats sniff the second juvenile in the retrieval trial more than the first, social memory for the second juvenile is said to be present. This test is mainly based on scents from the juvenile. Attempts were made to refine the test to reduce the number of animals used, to enhance the scope of the test, and to improve its validity. Firstly, the stimulus animals were replaced by the scent of juveniles, in the form of cups filled with sawdust taken from cages of juvenile rats. Similar results to those in the original test were obtained when using these scents. Furthermore, male and female scents were tested, and showed the same results as for the juvenile scents. Secondly, rats were also given two cups (one scent-filled and one filled with plain sawdust) in the learning trial, to determine which allowed a more-precise delineation of motivational, discriminatory and memory components. Overall, it is possible to replace stimulus animals by scent-filled cups in the social discrimination test, to enhance the scope of the test, and to draw more-valid conclusions with respect to social memory.