The anhedonia hypothesis: Mark III

Anticipation: An Essential Feature of Anhedonia

The following essay addresses the evolution of the term "anhedonia" as a key construct in biological psychiatry, especially as it pertains to positive emotional and motivational states central to mental health and well-being. In its strictest definition, anhedonia was intended to convey an inability to experience "pleasure" derived from ingestion of sweet tastes or the experience of pleasant odors and tactile sensations, among a host of positive sensations. However, this definition has proved to be too restrictive to capture the complexity of key psychological factors linked to major depression, schizophrenia, and substance use disorders it was originally intended to address. Despite the appeal of the elegant simplicity of the term anhedonia, its limitations soon became apparent when used to explain psychological constructs including aspects of learning, memory, and incentive motivation that are major determinants of success in securing the necessities of life. Accordingly, the definition of anhedonia has morphed into a much broader term that includes key roles in the disturbance of motivation in the form of anergia, impaired incentive motivation, along with deficits in associative learning and key aspects of memory, on which the ability to predict the consequences of one's actions are based. Here we argue that it is this latter capacity, namely predicting the likely consequences of motivated behavior, which can be termed "anticipation," that is especially important in the key deficits implied by the general term anhedonia in the context of neuropsychiatric conditions.

Oxycodone in the Opioid Epidemic: High 'Liking', 'Wanting', and Abuse Liability

It is estimated that nearly a third of people who abuse drugs started with prescription opioid medicines. Approximately, 11.5 million Americans used prescription drugs recreationally in 2016, and in 2018, 46,802 Americans died as the result of an opioid overdose, including prescription opioids, heroin, and illicitly manufactured fentanyl (National Institutes on Drug Abuse (2020) Opioid Overdose Crisis. https://www.drugabuse.gov/drugs-abuse/opioids/opioid-overdose-crisis . Accessed 06 June 2020). Yet physicians will continue to prescribe oral opioids for moderate-to-severe pain in the absence of alternative therapeutics, underscoring the importance in understanding how drug choice can influence detrimental outcomes. One of the opioid prescription medications that led to this crisis is oxycodone, where misuse of this drug has been rampant. Being one of the most highly prescribed opioid medications for treating moderate-to-severe pain as reflected in the skyrocketed increase in retail sales of 866% between 1997 and 2007, oxycodone was initially suggested to be less addictive than morphine. The false-claimed non-addictive formulation of oxycodone, OxyContin, further contributed to the opioid crisis. Abuse was often carried out by crushing the pills for immediate burst release, typically by nasal insufflation, or by liquefying the pills for intravenous injection. Here, we review oxycodone pharmacology and abuse liability as well as present the hypothesis that oxycodone may exhibit a unique pharmacology that contributes to its high likability and abuse susceptibility. We will discuss various mechanisms that likely contribute to the high abuse rate of oxycodone including clinical drug likability, pharmacokinetics, pharmacodynamics, differences in its actions within mesolimbic reward circuity compared to other opioids, and the possibility of differential molecular and cellular receptor interactions that contribute to its selective effects. We will also discuss marketing strategies and drug difference that likely contributes to the oxycodone opioid use disorders and addiction.

Affective valence in the brain: modules or modes?

How do brain systems evaluate the affective valence of a stimulus - that is, its quality of being good or bad? One possibility is that a neural subsystem, or 'module' (such as a subregion of the brain, a projection pathway, a neuronal population or an individual neuron), is permanently dedicated to mediate only one affective function, or at least only one specific valence - an idea that is termed here the 'affective modules' hypothesis. An alternative possibility is that a given neural module can exist in multiple neurobiological states that give it different affective functions - an idea termed here the 'affective modes' hypothesis. This suggests that the affective function or valence mediated by a neural module need not remain permanently stable but rather can change dynamically across different situations. An evaluation of evidence for the 'affective modules' versus 'affective modes' hypotheses may be useful for advancing understanding of the affective organization of limbic circuitry.

Shaping consumption propensity through the emotional response evoked by nutritional labels: Evidence from an fMRI study

The study aims to investigate a) the ability of nutritional labels in pictorial and textual form to guide consumption choices applying the dual-coding theory; b) the effectiveness of a new type of pictorial label (i.e. the body label) that appeals to the self-congruence theory and to positive emotional response. The research hypotheses were tested trough a 2x4x2 fMRI experimental design with 2 levels of product version (regular vs light), 4 levels of label type (text vs traffic light vs star rating vs body) and 2 levels of group of people (normal weight vs overweight). The body light label generates more brain activation in areas involved in the reward circuit compared to the body regular one, and compared to all the other types of labels for both versions, only in the overweight subject group. Furthermore, the star rating label has the worst performance in orienting healthy food choices as it requires more cognitive effort. The results are of interest to policy maker's strategies and to out-of-store and in-store communication strategies with the aim to contract the excessive body-weight phenomenon.

Optogenetic self-stimulation in the nucleus accumbens: D1 reward versus D2 ambivalence

The nucleus accumbens (NAc) contains multiple subpopulations of medium spiny neurons (MSNs). One subpopulation expresses D1-type dopamine receptors, another expresses D2-type receptors, and a third expresses both. The relative roles in NAc of D1 neurons versus D2 neurons in appetitive motivation were assessed here. Specifically, we asked whether D1-Cre mice would instrumentally seek optogenetic self-stimulation specifically targeted at D1 MSNs in NAc, and similarly if D2-Cre mice would self-stimulate D2 neurons in NAc. Mice were implanted with Cre-targeted channelrhodopsin (ChR2) virus and optic fibers in NAc. Subsequently, mice could earn brief NAc laser illuminations by actively touching a metal spout in one task, or by going to a particular location in a separate task. Results indicated that D1 neuronal excitation in NAc supported intense self-stimulation in both tasks. D1-Cre mice earned hundreds to thousands of spout-touches per half-hour session, and also sought out locations that delivered NAc laser to excite D1 MSNs. By comparison, D2 ChR2 mice showed lower but still positive levels of self-stimulation in the spout-touch task, earning dozens to hundreds of NAc laser illuminations. However, in the location task, D2 mice failed to show positive self-stimulation. If anything, a few D2 individuals gradually avoided the laser location. Brain-wide measures indicated that D1 and D2 stimulations in NAc recruited heavily overlapping patterns of Fos activation in distant limbic structures. These results confirm that excitation of D1 MSNs in NAc supports strong incentive motivation to self-stimulate. They also suggest that excitation of D2 neurons in NAc supports self-stimulation under some conditions, but fails under others and possibly may even shift to negative avoidance.

Abundant collateralization of temporal lobe projections to the accumbens, bed nucleus of stria terminalis, central amygdala and lateral septum

Behavioral flexibility is subserved in part by outputs from the cerebral cortex to telencephalic subcortical structures. In our earlier evaluation of the organization of the cortical-subcortical output system (Reynolds and Zahm, J Neurosci 25:11757-11767, 2005), retrograde double-labeling was evaluated in the prefrontal cortex following tracer injections into pairs of the following subcortical telencephalic structures: caudate-putamen, core and shell of the accumbens (Acb), bed nucleus of stria terminalis (BST) and central nucleus of the amygdala (CeA). The present study was done to assess patterns of retrograde labeling in the temporal lobe after similar paired tracer injections into most of the same telencephalic structures plus the lateral septum (LS). In contrast to the modest double-labeling observed in the prefrontal cortex in the previous study, up to 60-80 % of neurons in the basal and accessory basal amygdaloid nuclei and amygdalopiriform transition area exhibited double-labeling in the present study. The most abundant double-labeling was generated by paired injections into structures affiliated with the extended amygdala, including the CeA, BST and Acb shell. Injections pairing the Acb core with the BST or CeA produced significantly fewer double-labeled neurons. The ventral subiculum exhibited modest amounts of double-labeling associated with paired injections into the Acb, BST, CeA and LS. The results raise the issue of how an extraordinarily collateralized output from the temporal lobe may contribute to behavioral flexibility.

Low putamen activity associated with poor reward sensitivity in childhood chronic fatigue syndrome

Motivational signals influence a wide variety of cognitive processes and components of behavioral performance. Cognitive dysfunction in patients with childhood chronic fatigue syndrome (CCFS) may be closely associated with a low motivation to learn induced by impaired neural reward processing. However, the extent to which reward processing is impaired in CCFS patients is unclear. The aim of the present functional magnetic resonance imaging (fMRI) study was to determine whether brain activity in regions related to reward sensitivity is impaired in CCFS patients. fMRI data were collected from 13 CCFS patients (mean age, 13.6 ± 1.0 years) and 13 healthy children and adolescents (HCA) (mean age, 13.7 ± 1.3 years) performing a monetary reward task. Neural activity in high- and low-monetary-reward conditions was compared between CCFS and HCA groups. Severity of fatigue and the reward obtained from learning in daily life were evaluated by questionnaires. Activity of the putamen was lower in the CCFS group than in the HCA group in the low-reward condition, but not in the high-reward condition. Activity of the putamen in the low-reward condition in CCFS patients was negatively and positively correlated with severity of fatigue and the reward from learning in daily life, respectively. We previously revealed that motivation to learn was correlated with striatal activity, particularly the neural activity in the putamen. This suggests that in CCFS patients low putamen activity, associated with altered dopaminergic function, decreases reward sensitivity and lowers motivation to learn.

Gene expression profiling of midbrain dopamine neurons upon gestational nicotine exposure

Maternal smoking during pregnancy is associated with low birth weight, increased risk of stillbirth, conduct disorder, attention-deficit/hyperactivity disorder and neurocognitive deficits. Ventral tegmental area dopamine (DA) neurons in the mesocorticolimbic pathway were suggested to play a critical role in these pathological mechanisms induced by nicotine. Nicotine-mediated changes in genetic expression during pregnancy are of great interest for current researchers. We used patch clamp methods to identify and harvest DA and non-DA neurons separately and assayed them using oligonucleotide arrays to elucidate the alterations in gene expressions in these cells upon gestational nicotine exposure. Microarray analysis identified a set of 135 genes as significantly differentially expressed between DA and non-DA neurons. Some of the genes were found to be related to neurological disease pathways, such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Significantly up-/down-regulated genes found in DA neurons were mostly related to G-protein-coupled protein receptor signaling and developmental processes. These alterations in gene expressions may explain, partially at least, the possible pathological mechanisms for the diseases induced by maternal smoking.

Multivariate pattern analysis strategies in detection of remitted major depressive disorder using resting state functional connectivity

Understanding abnormal resting-state functional connectivity of distributed brain networks may aid in probing and targeting mechanisms involved in major depressive disorder (MDD). To date, few studies have used resting state functional magnetic resonance imaging (rs-fMRI) to attempt to discriminate individuals with MDD from individuals without MDD, and to our knowledge no investigations have examined a remitted (r) population. In this study, we examined the efficiency of support vector machine (SVM) classifier to successfully discriminate rMDD individuals from healthy controls (HCs) in a narrow early-adult age range. We empirically evaluated four feature selection methods including multivariate Least Absolute Shrinkage and Selection Operator (LASSO) and Elastic Net feature selection algorithms. Our results showed that SVM classification with Elastic Net feature selection achieved the highest classification accuracy of 76.1% (sensitivity of 81.5% and specificity of 68.9%) by leave-one-out cross-validation across subjects from a dataset consisting of 38 rMDD individuals and 29 healthy controls. The highest discriminating functional connections were between the left amygdala, left posterior cingulate cortex, bilateral dorso-lateral prefrontal cortex, and right ventral striatum. These appear to be key nodes in the etiopathophysiology of MDD, within and between default mode, salience and cognitive control networks. This technique demonstrates early promise for using rs-fMRI connectivity as a putative neurobiological marker capable of distinguishing between individuals with and without rMDD. These methods may be extended to periods of risk prior to illness onset, thereby allowing for earlier diagnosis, prevention, and intervention.

Reprint of: Locus coeruleus neuronal activity determines proclivity to consume alcohol in a selectively-bred line of rats that readily consumes alcohol

Sprague-Dawley rats selectively-bred for susceptibility to stress in our laboratory (Susceptible, or SUS rats) voluntarily consume large amounts of alcohol, and amounts that have, as shown here, pharmacological effects, which normal rats will not do. In this paper, we explore neural events in the brain that underlie this propensity to readily consume alcohol. Activity of locus coeruleus neurons (LC), the major noradrenergic cell body concentration in the brain, influences firing of ventral tegmentum dopaminergic cell bodies of the mesocorticolimbic system (VTA-DA neurons), which mediate rewarding aspects of alcohol. We tested the hypothesis that in SUS rats alcohol potently suppresses LC activity to markedly diminish LC-mediated inhibition of VTA-DA neurons, which permits alcohol to greatly increase VTA-DA activity and rewarding aspects of alcohol. Electrophysiological single-unit recording of LC and VTA-DA activity showed that in SUS rats alcohol decreased LC burst firing much more than in normal rats and as a result markedly increased VTA-DA activity in SUS rats while having no such effect in normal rats. Consistent with this, in a behavioral test for reward using conditioned place preference (CPP), SUS rats showed alcohol, given by intraperitoneal (i.p.) injection, to be rewarding. Next, manipulation of LC activity by microinfusion of drugs into the LC region of SUS rats showed that (a) decreasing LC activity increased alcohol intake and increasing LC activity decreased alcohol intake in accord with the formulation described above, and (b) increasing LC activity blocked both the rewarding effect of alcohol in the CPP test and the usual alcohol-induced increase in VTA-DA single-unit activity seen in SUS rats. An important ancillary finding in the CPP test was that an increase in LC activity was rewarding by itself, while a decrease in LC activity was aversive; consequently, effects of LC manipulations on alcohol-related reward in the CPP test were perhaps even larger than evident in the test. Finally, when increased LC activity was associated with (i.e., conditioned to) i.p. alcohol, subsequent alcohol consumption by SUS rats was markedly reduced, indicating that SUS rats consume large amounts of alcohol because of rewarding physiological consequences requiring increased VTA-DA activity. The findings reported here are consistent with the view that the influence of alcohol on LC activity leading to changes in VTA-DA activity strongly affects alcohol-mediated reward, and may well be the basis of the proclivity of SUS rats to avidly consume alcohol.

Influences of Chronic Mild Stress Exposure on Motor, Non-Motor Impairments and Neurochemical Variables in Specific Brain Areas of MPTP/Probenecid Induced Neurotoxicity in Mice

Parkinson's disease (PD) is regarded as a movement disorder mainly affecting the elderly population and occurs due to progressive loss of dopaminergic (DAergic) neurons in nigrostriatal pathway. Patients suffer from non-motor symptoms (NMS) such as depression, anxiety, fatigue and sleep disorders, which are not well focussed in PD research. Depression in PD is a predominant /complex symptom and its pathology lies exterior to the nigrostriatal system. The main aim of this study is to explore the causative or progressive effect of chronic mild stress (CMS), a paradigm developed as an animal model of depression in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (25 mg/kg. body wt.) with probenecid (250 mg/kg, s.c.) (MPTP/p) induced mice model of PD. After ten i.p. injections (once in 3.5 days for 5 weeks) of MPTP/p or exposure to CMS for 4 weeks, the behavioural (motor and non-motor) impairments, levels and expressions of dopamine (DA), serotonin (5-HT), DAergic markers such as tyrosine hydroxylase (TH), dopamine transporter (DAT), vesicular monoamine transporters-2 (VMAT 2) and α-synuclein in nigrostriatal (striatum (ST) and substantia nigra (SN)) and extra-nigrostriatal (hippocampus, cortex and cerebellum) tissues were analysed. Significantly decreased DA and 5-HT levels, TH, DAT and VMAT 2 expressions and increased motor deficits, anhedonia-like behaviour and α-synuclein expression were found in MPTP/p treated mice. Pre and/or post exposure of CMS to MPTP/p mice further enhanced the MPTP/p induced DA and 5-HT depletion, behaviour abnormalities and protein expressions. Our results could strongly confirm that the exposure of stress after MPTP/p injections worsens the symptoms and neurochemicals status of PD.

Locus coeruleus neuronal activity determines proclivity to consume alcohol in a selectively-bred line of rats that readily consumes alcohol

Sprague-Dawley rats selectively-bred for susceptibility to stress in our laboratory (Susceptible, or SUS rats) voluntarily consume large amounts of alcohol, and amounts that have, as shown here, pharmacological effects, which normal rats will not do. In this paper, we explore neural events in the brain that underlie this propensity to readily consume alcohol. Activity of locus coeruleus neurons (LC), the major noradrenergic cell body concentration in the brain, influences firing of ventral tegmentum dopaminergic cell bodies of the mesocorticolimbic system (VTA-DA neurons), which mediate rewarding aspects of alcohol. We tested the hypothesis that in SUS rats alcohol potently suppresses LC activity to markedly diminish LC-mediated inhibition of VTA-DA neurons, which permits alcohol to greatly increase VTA-DA activity and rewarding aspects of alcohol. Electrophysiological single-unit recording of LC and VTA-DA activity showed that in SUS rats alcohol decreased LC burst firing much more than in normal rats and as a result markedly increased VTA-DA activity in SUS rats while having no such effect in normal rats. Consistent with this, in a behavioral test for reward using conditioned place preference (CPP), SUS rats showed alcohol, given by intraperitoneal (i.p.) injection, to be rewarding. Next, manipulation of LC activity by microinfusion of drugs into the LC region of SUS rats showed that (a) decreasing LC activity increased alcohol intake and increasing LC activity decreased alcohol intake in accord with the formulation described above, and (b) increasing LC activity blocked both the rewarding effect of alcohol in the CPP test and the usual alcohol-induced increase in VTA-DA single-unit activity seen in SUS rats. An important ancillary finding in the CPP test was that an increase in LC activity was rewarding by itself, while a decrease in LC activity was aversive; consequently, effects of LC manipulations on alcohol-related reward in the CPP test were perhaps even larger than evident in the test. Finally, when increased LC activity was associated with (i.e., conditioned to) i.p. alcohol, subsequent alcohol consumption by SUS rats was markedly reduced, indicating that SUS rats consume large amounts of alcohol because of rewarding physiological consequences requiring increased VTA-DA activity. The findings reported here are consistent with the view that the influence of alcohol on LC activity leading to changes in VTA-DA activity strongly affects alcohol-mediated reward, and may well be the basis of the proclivity of SUS rats to avidly consume alcohol.

Dopamine receptors play distinct roles in sexual behavior expression of rats with a different sexual motivational tone

Dopamine (DA) plays a central role in the expression of male sexual behavior. The effects of DA-enhancing drugs on copulation seem to vary depending on the dose of the agonist used, the type of DA receptor activated, and the sexual condition of the animals. The aim of the present study was to carry out a systematic analysis of the effects of dopaminergic agonists on the expression of male sexual behavior by sexually competent rats in different sexual motivational states, that is when sexually active (sexually experienced) and when temporarily inhibited (sexually exhausted). To this end, the same doses of the nonselective DA receptor agonist apomorphine, the selective D2-like DA receptor agonist quinpirole, and the selective D1-like DA receptor agonist SKF38393 were injected intraperitoneally to sexually experienced or sexually exhausted male rats and their sexual behavior was recorded. Low apomorphine doses induced expression of sexual behavior in sexually satiated rats, but only reduced the intromission latency of sexually experienced rats. SKF38393 facilitated the expression of sexual behavior by sexually exhausted rats, but not that of sexually experienced males and quinpirole did not exert an effect in both types of animal. In line with these results, the apomorphine-induced reversal of sexual exhaustion was blocked by the D1-like receptor antagonist SCH23390. The data suggest that DA receptors play distinct roles in the expression of sexual behavior by male rats depending on their motivational state and that activation of D1-like receptors promotes the expression of sexual behavior in satiated rats.

From prediction error to incentive salience: mesolimbic computation of reward motivation

Reward contains separable psychological components of learning, incentive motivation and pleasure. Most computational models have focused only on the learning component of reward, but the motivational component is equally important in reward circuitry, and even more directly controls behavior. Modeling the motivational component requires recognition of additional control factors besides learning. Here I discuss how mesocorticolimbic mechanisms generate the motivation component of incentive salience. Incentive salience takes Pavlovian learning and memory as one input and as an equally important input takes neurobiological state factors (e.g. drug states, appetite states, satiety states) that can vary independently of learning. Neurobiological state changes can produce unlearned fluctuations or even reversals in the ability of a previously learned reward cue to trigger motivation. Such fluctuations in cue-triggered motivation can dramatically depart from all previously learned values about the associated reward outcome. Thus, one consequence of the difference between incentive salience and learning can be to decouple cue-triggered motivation of the moment from previously learned values of how good the associated reward has been in the past. Another consequence can be to produce irrationally strong motivation urges that are not justified by any memories of previous reward values (and without distorting associative predictions of future reward value). Such irrationally strong motivation may be especially problematic in addiction. To understand these phenomena, future models of mesocorticolimbic reward function should address the neurobiological state factors that participate to control generation of incentive salience.

Effects of SKF-83566 and haloperidol on performance on progressive ratio schedules maintained by sucrose and corn oil reinforcement: quantitative analysis using a new model derived from the Mathematical Principles of Reinforcement (MPR)

RATIONALE

Mathematical models can assist the interpretation of the effects of interventions on schedule-controlled behaviour and help to differentiate between processes that may be confounded in traditional performance measures such as response rate and the breakpoint in progressive ratio (PR) schedules.

OBJECTIVE

The effects of a D1-like dopamine receptor antagonist, 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrobromide (SKF-83566), and a D2-like receptor antagonist, haloperidol, on rats' performance on PR schedules maintained by sucrose and corn oil reinforcers were assessed using a new model derived from Killeen's (Behav Brain Sci 17:105-172, 1994) Mathematical Principles of Reinforcement.

METHOD

Separate groups of rats were trained under a PR schedule using sucrose or corn oil reinforcers. SKF-83566 (0.015 and 0.03 mg kg(-1)) and haloperidol (0.05 and 0.1 mg kg(-1)) were administered intraperitoneally (five administrations of each treatment). Running and overall response rates in successive ratios were analysed using the new model, and estimates of the model's parameters were compared between treatments.

RESULTS

Haloperidol reduced a (the parameter expressing incentive value) in the case of both reinforcers, but did not affect the parameters related to response time and post-reinforcement pausing. SKF-83566 reduced a and k (the parameter expressing sensitivity of post-reinforcement pausing to the prior inter-reinforcement interval) in the case of sucrose, but did not affect any of the parameters in the case of corn oil.

CONCLUSIONS

The results are consistent with the hypothesis that blockade of both D1-like and D2-like receptors reduces the incentive value of sucrose, whereas the incentive value of corn oil is more sensitive to blockade of D2-like than D1-like receptors.

Appetitive and consummative responding for liquid sucrose in the spontaneously hypertensive rat model of attention deficit hyperactivity disorder

The spontaneously hypertensive rat (SHR) is one proposed animal model of attention deficit hyperactivity disorder (ADHD) argued to show strong face validity on the basis of behavioural characteristics. However, SHR may have fundamental alterations to the sensitivity of fluid reward due to altered renal function that has the potential to affect performance in complex reinforced behavioural tests. This could particularly confound determination of operant motivational alterations in the SHR. We assessed baseline bodyweight, home cage lab chow and water intake in the SHR and their typical control strains: Wistar and Wistar Kyoto. We also assessed sucrose preference, and appetitive and consummative positive and negative contrast for sucrose (4% versus 20%) on a motivational runway. As expected, SHR showed enhanced water intake compared to Wistar and Wistar Kyotos but comparable lab chow intake at baseline. SHR exhibited sucrose preference for 4% and 20%, as did both control strains, but the preference for 4% was enhanced in the SHR. SHR showed significant negative and positive contrast in sucrose consumption on the runway, as did Wistar Kyotos. Wistars exhibited neither. Appetitive contrast was not measurable in the SHR due to a robust locomotor velocity increase at the age of testing. The enhanced fluid intake found in the SHR argues against using fluid reinforcers in behavioural tests. We suggest the presence of both forms of contrast in the SHR is unusual for rats tested in ad lib. food conditions while the contrast pattern in Wistars indicate abnormalities in reward sensitivity in this control strain.

Contributions of Philip Teitelbaum to affective neuroscience

As part of a festschrift issue for Philip Teitelbaum, I offer here the thesis that Teitelbaum deserves to be viewed as an important forefather to the contemporary field of affective neuroscience (which studies motivation, emotion and affect in the brain). Teitelbaum's groundbreaking analyses of motivation deficits induced by lateral hypothalamic damage, of roles of food palatability in revealing residual function, and of recovery of 'lost' functions helped shape modern understanding of how motivation circuits operate within the brain. His redefinition of the minimum requirement for identifying motivation raised the conceptual bar for thinking about the topic among behavioral neuroscientists. His meticulous analyses of patterned stages induced by brain manipulations, life development and clinical disorders added new dimensions to our appreciation of how brain systems work. His steadfast highlighting of integrative functions and behavioral complexity helped provide a healthy functionalist counterbalance to reductionist trends in science of the late 20th century. In short, Philip Teitelbaum can be seen to have made remarkable contributions to several domains of psychology and neuroscience, including affective neuroscience.

Individual differences in the neurophysiology of reward and the obesity epidemic

The obesity epidemic has unfolded in a matter of decades, not millennia, and therefore cannot be attributed to a drift in the genome. Rather, the temporal characteristics of the epidemic more closely track environmental and lifestyle changes, such as reduced physical activity, increased availability of palatable and energy-dense foods and drinks, and increased acceptance of eating outside of meal time (among others). One important observation is that not everyone is becoming obese. This suggests that individual factors interact with recent environmental changes to predispose some to overeat. One hypothesis that has been gaining traction in the neuroscience community is that individual differences in the neural encoding of foods may predispose some to overeat in the presence of a surplus of energy-dense, palatable foods and drinks. The aim of this review is to highlight several possible ways by which individual differences in the neurophysiology of food reward may lead to overeating.

The tempted brain eats: pleasure and desire circuits in obesity and eating disorders

What we eat, when and how much, all are influenced by brain reward mechanisms that generate "liking" and "wanting" for foods. As a corollary, dysfunction in reward circuits might contribute to the recent rise of obesity and eating disorders. Here we assess brain mechanisms known to generate "liking" and "wanting" for foods and evaluate their interaction with regulatory mechanisms of hunger and satiety, relevant to clinical issues. "Liking" mechanisms include hedonic circuits that connect together cubic-millimeter hotspots in forebrain limbic structures such as nucleus accumbens and ventral pallidum (where opioid/endocannabinoid/orexin signals can amplify sensory pleasure). "Wanting" mechanisms include larger opioid networks in nucleus accumbens, striatum, and amygdala that extend beyond the hedonic hotspots, as well as mesolimbic dopamine systems, and corticolimbic glutamate signals that interact with those systems. We focus on ways in which these brain reward circuits might participate in obesity or in eating disorders.

A neural computational model of incentive salience

Incentive salience is a motivational property with 'magnet-like' qualities. When attributed to reward-predicting stimuli (cues), incentive salience triggers a pulse of 'wanting' and an individual is pulled toward the cues and reward. A key computational question is how incentive salience is generated during a cue re-encounter, which combines both learning and the state of limbic brain mechanisms. Learning processes, such as temporal-difference models, provide one way for stimuli to acquire cached predictive values of rewards. However, empirical data show that subsequent incentive values are also modulated on the fly by dynamic fluctuation in physiological states, altering cached values in ways requiring additional motivation mechanisms. Dynamic modulation of incentive salience for a Pavlovian conditioned stimulus (CS or cue) occurs during certain states, without necessarily requiring (re)learning about the cue. In some cases, dynamic modulation of cue value occurs during states that are quite novel, never having been experienced before, and even prior to experience of the associated unconditioned reward in the new state. Such cases can include novel drug-induced mesolimbic activation and addictive incentive-sensitization, as well as natural appetite states such as salt appetite. Dynamic enhancement specifically raises the incentive salience of an appropriate CS, without necessarily changing that of other CSs. Here we suggest a new computational model that modulates incentive salience by integrating changing physiological states with prior learning. We support the model with behavioral and neurobiological data from empirical tests that demonstrate dynamic elevations in cue-triggered motivation (involving natural salt appetite, and drug-induced intoxication and sensitization). Our data call for a dynamic model of incentive salience, such as presented here. Computational models can adequately capture fluctuations in cue-triggered 'wanting' only by incorporating modulation of previously learned values by natural appetite and addiction-related states.

Affective status in relation to impulsive, motor and motivational symptoms: personality, development and physical exercise

The contributions of impulsive and risk-taking behaviour in depressive and bipolar disorders, motivational and motor behaviours in anhedonic and substance addictive states, and the factors, particularly distress and trauma, underlying the development of neuropathology in affective status are described from clinical, epidemiological and laboratory perspectives. In order to distinguish one case factor for biopsychological substrates of health, an array of self-reported characteristics, e.g., positive or negative affect, stress or energy, optimism, etc., that may be predictive or counterpredictive for the propensity for physical exercise and activity were analysed using a linear regression in twelve different studies. Several individual characteristics were found to be markedly and significantly predictive of the exercise propensity, i.e., positive affect, energy, health-seeking behaviour and character, while optimism was of lesser, though significant, importance. Several individual characteristics were found to be significantly counterpredictive: expression of BDI- and HAD-depression, major sleep problems and lack/negligence of health-seeking behaviour. The consequences of physical activity and exercise for both affective well-being, cognitive mobility and neurogenesis is noted, particularly with regard to developmental assets for younger individuals. Affective disorder states may be studied through analyses of personal characteristics that unfold predispositions for symptoms-profiles and biomarkers derived from properties of dysfunction, such as impulsiveness, temperament dimensions, anhedonia and 'over-sensitivity', whether interpersonal or to reward.

'Liking' and 'wanting' food rewards: brain substrates and roles in eating disorders

What brain reward systems mediate motivational 'wanting' and hedonic 'liking' for food rewards? And what roles do those systems play in eating disorders? This article surveys recent findings regarding brain mechanisms of hedonic 'liking', such as the existence of cubic-millimeter hedonic hotspots in nucleus accumbens and ventral pallidum for opioid amplification of sensory pleasure. It also considers brain 'wanting' or incentive salience systems important to appetite, such as mesolimbic dopamine systems and opioid motivation circuits that extend beyond the hedonic hotspots. Finally, it considers some potential ways in which 'wanting' and 'liking' might relate to eating disorders.

Dopamine and reward: the anhedonia hypothesis 30 years on

The anhedonia hypothesis--that brain dopamine plays a critical role in the subjective pleasure associated with positive rewards--was intended to draw the attention of psychiatrists to the growing evidence that dopamine plays a critical role in the objective reinforcement and incentive motivation associated with food and water, brain stimulation reward, and psychomotor stimulant and opiate reward. The hypothesis called to attention the apparent paradox that neuroleptics, drugs used to treat a condition involving anhedonia (schizophrenia), attenuated in laboratory animals the positive reinforcement that we normally associate with pleasure. The hypothesis held only brief interest for psychiatrists, who pointed out that the animal studies reflected acute actions of neuroleptics whereas the treatment of schizophrenia appears to result from neuroadaptations to chronic neuroleptic administration, and that it is the positive symptoms of schizophrenia that neuroleptics alleviate, rather than the negative symptoms that include anhedonia. Perhaps for these reasons, the hypothesis has had minimal impact in the psychiatric literature. Despite its limited heuristic value for the understanding of schizophrenia, however, the anhedonia hypothesis has had major impact on biological theories of reinforcement, motivation, and addiction. Brain dopamine plays a very important role in reinforcement of response habits, conditioned preferences, and synaptic plasticity in cellular models of learning and memory. The notion that dopamine plays a dominant role in reinforcement is fundamental to the psychomotor stimulant theory of addiction, to most neuroadaptation theories of addiction, and to current theories of conditioned reinforcement and reward prediction. Properly understood, it is also fundamental to recent theories of incentive motivation.

Chronic social stress, hedonism and vulnerability to obesity: lessons from rodents

Obesity is a current health pandemia. Determinants of this pathology are rather complex and include genetic, developmental and environmental factors only partially disclosed. Stress related neuroendocrine dysregulation and overconsumption of high palatable high caloric food and are likely to contribute to this modern health threats. Despite the evidence that psychosocial stress is one of the main sources of stress in humans and may play an important role in the development of the stress disorders, including obesity and metabolic syndrome, animal models focusing on the relationship between chronic stress and energy homeostasis are scattered and most of them encompasses physical rather than psychosocial stress. Aim of the present paper is to review rodent studies on the effect of psychosocial stress throughout life on body weight and food intake regulation. In the second part of the review special focus will be given on the mechanisms linking stress and the reward system.

The neural basis of academic achievement motivation

We have used functional magnetic resonance imaging to study the neural correlates of motivation, concentrating on the motivation to learn and gain monetary rewards. We compared the activation in the brain obtained during reported high states of motivation for learning, with the ones observed when the motivation was based on monetary reward. Our results show that motivation to learn correlates with bilateral activity in the putamen, and that the higher the reported motivation, as derived from a questionnaire that each subject filled prior to scanning, the greater the change in the BOLD signals within the putamen. Monetary motivation also activated the putamen bilaterally, though the intensity of activity was not related to the monetary reward. We conclude that the putamen is critical for motivation in different domains and the extent of activity of the putamen may be pivotal to the motivation that drives academic achievement and thus academic successes.

Effort-related functions of nucleus accumbens dopamine and associated forebrain circuits

BACKGROUND

Over the last several years, it has become apparent that there are critical problems with the hypothesis that brain dopamine (DA) systems, particularly in the nucleus accumbens, directly mediate the rewarding or primary motivational characteristics of natural stimuli such as food. Hypotheses related to DA function are undergoing a substantial restructuring, such that the classic emphasis on hedonia and primary reward is giving way to diverse lines of research that focus on aspects of instrumental learning, reward prediction, incentive motivation, and behavioral activation.

OBJECTIVE

The present review discusses dopaminergic involvement in behavioral activation and, in particular, emphasizes the effort-related functions of nucleus accumbens DA and associated forebrain circuitry.

RESULTS

The effects of accumbens DA depletions on food-seeking behavior are critically dependent upon the work requirements of the task. Lever pressing schedules that have minimal work requirements are largely unaffected by accumbens DA depletions, whereas reinforcement schedules that have high work (e.g., ratio) requirements are substantially impaired by accumbens DA depletions. Moreover, interference with accumbens DA transmission exerts a powerful influence over effort-related decision making. Rats with accumbens DA depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead, these rats select a less-effortful type of food-seeking behavior.

CONCLUSIONS

Along with prefrontal cortex and the amygdala, nucleus accumbens is a component of the brain circuitry regulating effort-related functions. Studies of the brain systems regulating effort-based processes may have implications for understanding drug abuse, as well as energy-related disorders such as psychomotor slowing, fatigue, or anergia in depression.

The debate over dopamine's role in reward: the case for incentive salience

INTRODUCTION

Debate continues over the precise causal contribution made by mesolimbic dopamine systems to reward. There are three competing explanatory categories: 'liking', learning, and 'wanting'. Does dopamine mostly mediate the hedonic impact of reward ('liking')? Does it instead mediate learned predictions of future reward, prediction error teaching signals and stamp in associative links (learning)? Or does dopamine motivate the pursuit of rewards by attributing incentive salience to reward-related stimuli ('wanting')? Each hypothesis is evaluated here, and it is suggested that the incentive salience or 'wanting' hypothesis of dopamine function may be consistent with more evidence than either learning or 'liking'. In brief, recent evidence indicates that dopamine is neither necessary nor sufficient to mediate changes in hedonic 'liking' for sensory pleasures. Other recent evidence indicates that dopamine is not needed for new learning, and not sufficient to directly mediate learning by causing teaching or prediction signals. By contrast, growing evidence indicates that dopamine does contribute causally to incentive salience. Dopamine appears necessary for normal 'wanting', and dopamine activation can be sufficient to enhance cue-triggered incentive salience. Drugs of abuse that promote dopamine signals short circuit and sensitize dynamic mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Such drugs interact with incentive salience integrations of Pavlovian associative information with physiological state signals. That interaction sets the stage to cause compulsive 'wanting' in addiction, but also provides opportunities for experiments to disentangle 'wanting', 'liking', and learning hypotheses. Results from studies that exploited those opportunities are described here.

CONCLUSION

In short, dopamine's contribution appears to be chiefly to cause 'wanting' for hedonic rewards, more than 'liking' or learning for those rewards.

Discrete neurochemical coding of distinguishable motivational processes: insights from nucleus accumbens control of feeding

BACKGROUND AND OBJECTIVES

The idea that nucleus accumbens (Acb) dopamine transmission contributes to the neural mediation of reward, at least in a general sense, has achieved wide acceptance. Nevertheless, debate remains over the precise nature of dopamine's role in reward and even over the nature of reward itself. In the present article, evidence is reviewed from studies of food intake, feeding microstructure, instrumental responding for food reinforcement, and dopamine efflux associated with feeding, which suggests that reward processing in the Acb is best understood as an interaction among distinct processes coded by discrete neurotransmitter systems.

RESULTS

In agreement with several theories of Acb dopamine function, it is proposed here that allocation of motor effort in seeking food or food-associated conditioned stimuli can be dissociated from computations relevant to the hedonic evaluation of food during the consummatory act. The former appears to depend upon Acb dopamine transmission and the latter upon striatal opioid peptide release. Moreover, dopamine transmission may play a role in 'stamping in' associations between motor acts and goal attainment and perhaps also neural representations corresponding to rewarding outcomes. Finally, evidence is reviewed that amino acid transmission specifically in the Acb shell acts as a central 'circuit breaker' to flexibly enable or terminate the consummatory act, via descending connections to hypothalamic feeding control systems.

CONCLUSIONS

The heuristic framework outlined above may help explain why dopamine-compromising manipulations that strongly diminish instrumental goal-seeking behaviors leave consummatory activity relatively unaffected.

Haloperidol, dynamics of choice, and the parameters of the matching law

The idea that dopamine mediates the reinforcing effects of stimuli persists in the field of neurosciences. The present study shows that haloperidol, a dopamine antagonist, does not eliminate the reinforcing value of food reinforcers. The ratio of reinforcers changed seven times across two levers within sessions, modeling a dynamic environment. The magnitude of the reinforcer was manipulated independently of the reinforcer ratio. Four doses of intraperitoneal haloperidol were assessed over periods of 12 daily sessions. Haloperidol did not impair the discrimination that the rats established between rich and lean levers; the response distributions favored the lever associated with the higher probability of reinforcement and the larger pellets. The parameters of the generalized matching law (bias and sensitivity) were used to estimate effects of haloperidol upon the motor system and upon the rats' motivation for food reinforcers.

Hedonic hot spots in the brain

Hedonic "liking" for sensory pleasures is an important aspect of reward, and excessive 'liking' of particular rewards might contribute to excessive consumption and to disorders such as obesity. The present review aims to summarize recent advances in the identification of brain substrates for food 'liking' with a focus on opioid hot spots in the nucleus accumbens and ventral pallidum. Drug microinjection studies have shown that opioids in both areas amplify the 'liking' of sweet taste rewards. Modern neuroscience tools such as Fos plume mapping have further identified hedonic hot spots within the accumbens and pallidum, where opioids are especially tuned to magnify 'liking' of food rewards. Hedonic hot spots in different brain structures may interact with each other within the larger functional circuitry that interconnects them. Better understanding of how brain hedonic hot spots increase the positive affective impact of natural sensory pleasures will help characterize the neural mechanisms potentially involved in 'liking' for many rewards.

Stress, depression, and anhedonia: caveats concerning animal models

Numerous animal models of depression have been advanced, each having multiple attributes and some limitations. This review provides caveats concerning etiologically valid animal models of depression, focusing on characteristics of the depressive subtype being examined (e.g. typical vs atypical major depression, dysthymia, melancholia), and factors that contribute to the interindividual behavioral variability frequently evident in stressor-related behavioral paradigms. These include the stressor type (processive vs systemic stressors), and characteristics of the stressor (controllability, predictability, ambiguity, chronicity, intermittence), as well as organismic variables (genetic, age, sex), experiential variables (stressor history, early life events) and psychosocial and personality factors that moderate stressor reactivity. Finally, a model of depression is reviewed that evaluates the effects of stressors on hedonic processes, reflected by responding for rewarding brain stimulation. Anhedonia is a fundamental feature of depression, and assessment of stressor-related reductions in the rewarding value of brain stimulation, especially when coupled with other potential symptoms of depression, provides considerable face, construct and predictive validity. Stressful events markedly impact rewarding brain stimulation, and this effect varies across strains of mice differentially reactive to stressors, is modifiable by antidepressant treatments, and allows for analyses of the contribution of different brain regions to anhedonic processes. The paradigm is sensitive to several factors known to acts as moderators of stress responses, but analyses remain to be conducted with regard to several such variables.

Espresso reward learning, hold the dopamine: theoretical comment on Robinson et al. (2005)

QUESTION

Is dopamine needed for reward learning?

ANSWER

No--at least, not in the brain of a caffeinated dopamine-deficient (DD) mutant mouse. That is the conclusion of an important paper in this issue by S. Robinson, S. M. Sandstrom, V. H. Denenberg, and R. D. Palmiter (see record 2005-01705-001). Those authors demonstrate that reward learning can proceed normally in the brains of DD mice, even though they contain no dopamine at the time of learning, if the mice are given caffeine just before learning. Caffeine activates the DD mice by a nondopaminergic mechanism, allowing them to learn where to obtain food reward in a T-maze runway. Their reward-learning-without-dopamine is revealed on a subsequent test day, when dopamine function is restored by L-dopa administration. Robinson et al. conclude that dopamine is not needed for normal learning about rewards, or for hedonic "liking" of rewards during learning, but rather specifically for a motivational "wanting" component of reward, such as incentive salience.

Dynamic increases in dopamine during paced copulation in the female rat

The role of dopamine in the rewarding aspects of sexual behaviour in female rats was investigated. This is a unique model because sexual behaviour is only rewarding when copulatory stimuli are experienced at the preferred rate of copulation for a female rat. In addition, increases in dopamine in the nucleus accumbens occur during sexual behaviour only when the female achieves this preferred rate of copulation. In this study, minute-by-minute changes in nucleus accumbens dopamine were monitored using in vivo microdialysis. We report here that extracellular dopamine in the nucleus accumbens increases before coital stimulation only when sexual behaviour is occurring under conditions that are rewarding to the female rat. We conclude that increases in dopamine in the nucleus accumbens are involved in anticipation, not consummation, of sexual behaviour in the female rat.

The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition?

Psychopharmacological studies have implicated the dopaminergic innervation of the nucleus accumbens (NAC) in reward and reinforcement, in the actions of addictive drugs, and in the control of the symptoms of schizophrenia. Recent developments in in vivo dialysis, and other in vivo neurochemical techniques have permitted a more direct analysis of the behavioural correlates of increased dopamine release in rats, and have largely confirmed these findings in relation to reward, and drugs of abuse potential. However, dopamine release has also been found to be increased by many other stimuli/situations including aversive stimuli, stimuli conditioned to aversive stimuli, complex novel stimuli, and in the process of conditioning itself. These results contrast with electrophysiological data obtained in the behaving monkey, where rewarding stimuli, or stimuli predictive of reward are associated with increased firing of presumptive dopamine neurones projecting to the NAC (and indeed to the striatum), but mild aversive stimuli are not, leading to the suggestion that this system subserves a more purely reward function, or indeed that it provides a reward error signal. Further exploration of these issues will depend upon a comparison of increased dopamine cell firing and increased dopamine release, and an analysis of the behavioural effects of blocking these increases in dopamine transmission. One suggestion, deriving from work on latent inhibition, is that the significance of dopamine release by salient stimuli is to allow learning about stimuli which would otherwise be excluded on the basis of familiarity. This suggests that in addition to a role in some types of learning about salient stimuli, dopamine release in NAC may have a role in controlling the attention paid to familiar stimuli. Since it is difficult to see a connection between simple learning about rewards, and the symptoms of schizophrenia, this provides a more convincing link between the dopamine theory of schizophrenia, and the attentional difficulties held by many theorists to underlie schizophrenic symptoms.

Compulsive use of dopamine replacement therapy in Parkinson's disease: reward systems gone awry?

Dopamine replacement therapy (DRT) is the most effective treatment for Parkinson's disease (PD); it provides substantial benefit for most patients, extends independence, and increases survival. A few patients with PD, however, take increasing quantities of medication far beyond those required to treat their motor disabilities. These patients demand rapid drug escalation and continue to request more DRT despite the emergence of increasingly severe drug-induced motor complications and harmful behavioural consequences. In this article we detail the features of compulsive DRT-seeking and intake in PD, in relation to theories of compulsive drug use.

Pleasures of the brain

How does the brain cause positive affective reactions to sensory pleasure? An answer to pleasure causation requires knowing not only which brain systems are activated by pleasant stimuli, but also which systems actually cause their positive affective properties. This paper focuses on brain causation of behavioral positive affective reactions to pleasant sensations, such as sweet tastes. Its goal is to understand how brain systems generate 'liking,' the core process that underlies sensory pleasure and causes positive affective reactions. Evidence suggests activity in a subcortical network involving portions of the nucleus accumbens shell, ventral pallidum, and brainstem causes 'liking' and positive affective reactions to sweet tastes. Lesions of ventral pallidum also impair normal sensory pleasure. Recent findings regarding this subcortical network's causation of core 'liking' reactions help clarify how the essence of a pleasure gloss gets added to mere sensation. The same subcortical 'liking' network, via connection to brain systems involved in explicit cognitive representations, may also in turn cause conscious experiences of sensory pleasure.

Clozapine, quetiapine and olanzapine among addicted schizophrenic patients: towards testable hypotheses

Although life prevalence of substance use disorders among patients with schizophrenia is close to 50%, few studies have been carried out to date to identify an integrated pharmacological treatment for this comorbidity. So far, the most promising results, that we report here, have been obtained with clozapine. To a lesser extent, quetiapine and olanzapine, both clozapine analogues, have also shown promising results. Further to these observations, the present paper critically reviews the advantages associated with clozapine, quetiapine and olanzapine, and their relevance to the treatment of addiction among schizophrenic patients. Six characteristics seem to distinguish clozapine, quetiapine and olanzapine from the first-generation antipsychotics: (1) acting preferentially on the reward system, these second-generation antipsychotics (mainly clozapine and quetiapine) induce almost no extrapyramidal symptoms; (2) quickly dissociating from D(2), theses drugs (mainly clozapine and quetiapine) seem not to induce dysphoria, unlike conventional antipsychotics like haloperidol;(3) these drugs (mainly clozapine) seem more effective in the treatment of negative symptoms than conventional antipsychotics; (4) because of a diversified activity on several serotoninergic and noradrenergic receptors, these drugs positively alter mood, which does not seem to be the case with conventional antipsychotics, except for flupenthixol; (5) these drugs have a positive impact on cognition, which is not the case with the first-generation antipsychotics; (6) unlike conventional antipsychotics, these drugs seem to have a moderate affinity for 5-HT(3), the receptor on which ondansetron, an anti-craving medication, acts.

Extending mathematical principles of reinforcement into the domain of behavioral pharmacology

Mathematical principles of reinforcement (MPR) attempts to integrate the empirical laws of reinforcement schedules that have accumulated over the decades. MPR is based on three principles: incentives excite behavior; there are temporal constraints on responding; and coupling of responses to reinforcers strengthens behavior [Behav. Brain Sci. 17 (1994) 105]. In the present paper MPR is extended into the domain of behavioral pharmacology, specifically to model the effects of D-amphetamine on operant behavior. In Experiment 1a, a five-component multiple fixed-ratio schedule was designed to generate behavioral baselines that were subsequently used to assess drug effect. In Experiments 1b and 1c, the quality and quantity of reinforcer were manipulated. The data generated by the three experiments were consistent with MPR. In Experiment 2, MPR was used to model the effects of D-amphetamine on rats responding under the five-component multiple fixed-ratio schedule. According to the model, the rate-decreasing effects of D-amphetamine were due primarily to motor disruption and secondarily to increased impulsivity; at the highest dosages, D-amphetamine also may have decreased the incentive value of food.

Motivational views of reinforcement: implications for understanding the behavioral functions of nucleus accumbens dopamine

Although the Skinnerian 'Empirical Law of Effect' does not directly consider the fundamental properties of stimuli that enable them to act as reinforcers, such considerations are critical for determining if nucleus accumbens dopamine systems mediate reinforcement processes. Researchers who have attempted to identify the critical characteristics of reinforcing stimuli or activities have generally arrived at an emphasis upon motivational factors. A thorough review of the behavioral literature indicates that, across several different investigators offering a multitude of theoretical approaches, motivation is seen by many as being fundamental to the process of reinforcement. The reinforcer has been described as a goal, a commodity, an incentive, or a stimulus that is being approached, self-administered, attained or preserved. Reinforcers also have been described as activities that are preferred, deprived or in some way being regulated. It is evident that this 'motivational' or 'regulatory' view of reinforcement has had enormous influence over the hypothesis that DA directly mediates 'reward' or 'reinforcement' processes. Indeed, proponents of the DA/reward hypothesis regularly cite motivational theorists and employ their language. Nevertheless, considerable evidence indicates that low/moderate doses of DA antagonists, and depletions of DA in nucleus accumbens, can suppress instrumental responding for food while, at the same time, these conditions leave fundamental aspects of reinforcement (i.e. primary or unconditioned reinforcement; primary motivation or primary incentive properties of natural reinforcers) intact. Several complex features of the literature on dopaminergic involvement in reinforcement are examined below, and it is argued that the assertions that DA mediates 'reward' or 'reinforcement' are inaccurate and grossly oversimplified. Thus, it appears as though it is no longer tenable to assert that drugs of abuse are simply turning on the brain's natural 'reward system'. In relation to the hypothesis that DA systems are involved in 'wanting', but not 'liking', it is suggested in the present review that 'wanting' has both directional aspects (e.g. appetite to consume food) and activational aspects (e.g. activation for initiating and sustaining instrumental actions; tendency to work for food). The present paper reviews findings in support of the hypothesis that low doses of DA antagonists and accumbens DA depletions do not impair appetite to consume food, but do impair activational aspects of motivation. This suggestion is consistent with the studies showing that low doses of DA antagonists and accumbens DA depletions alter the relative allocation of instrumental responses, making the animals less likely to engage in instrumental responses that have a high degree of work-related response costs. In addition, this observation is consistent with studies demonstrating that accumbens DA depletions make rats highly sensitive to ratio requirements on operant schedules. Although accumbens DA is not seen as directly mediating appetite to consume food, principles of behavioral economics indicate that accumbens DA could be involved in the elasticity of demand for food in terms of the tendency to pay work-related response costs. Future research must focus upon how specific aspects of task requirements (i.e. ratio requirements, intermittence of reinforcement, temporal features of response requirements, dependence upon conditioned stimuli) interact with the effects of accumbens DA depletions, and which particular factors determine sensitivity to the effects of DA antagonism or depletion.

Neural responses during anticipation of a primary taste reward

The aim of this study was to determine the brain regions involved in anticipation of a primary taste reward and to compare these regions to those responding to the receipt of a taste reward. Using fMRI, we scanned human subjects who were presented with visual cues that signaled subsequent reinforcement with a pleasant sweet taste (1 M glucose), a moderately unpleasant salt taste (0.2 M saline), or a neutral taste. Expectation of a pleasant taste produced activation in dopaminergic midbrain, posterior dorsal amygdala, striatum, and orbitofrontal cortex (OFC). Apart from OFC, these regions were not activated by reward receipt. The findings indicate that when rewards are predictable, brain regions recruited during expectation are, in part, dissociable from areas responding to reward receipt.

Behaviors induced or disrupted by complex partial seizures

We reviewed the neural mechanisms underlying some postictal behaviors that are induced or disrupted by temporal lobe seizures in humans and animals. It is proposed that the psychomotor behaviors and automatisms induced by temporal lobe seizures are mediated by the nucleus accumbens. A non-convulsive hippocampal afterdischarge in rats induced an increase in locomotor activity, which was suppressed by the injection of dopamine D(2) receptor antagonist in the nucleus accumbens, and blocked by inactivation of the medial septum. In contrast, a convulsive hippocampal or amygdala seizure induced behavioral hypoactivity, perhaps by the spread of the seizure into the frontal cortex and opiate-mediated postictal depression. Mechanisms underlying postictal psychosis, memory disruption and other long-term behavioral alterations after temporal lobe seizures, are discussed. In conclusion, many of the changes of postictal behaviors observed after temporal lobe seizures in humans may be found in animals, and the basis of the behavioral change may be explained as a change in neural processing in the temporal lobe and the connecting subcortical structures.

DARPP-32 knockout mice exhibit impaired reversal learning in a discriminated operant task

The current study was conducted to examine the performance of mice with a targeted deletion of the gene for DARPP-32 in a discriminated operant task using food reinforcement. DARPP-32 plays a central role in regulating the efficacy of dopaminergic neurotransmission. Initially, wild-type and DARPP-32 knockout mice were trained to nose-poke for food on a continuous reinforcement schedule. The minimum response requirement was increased every 5 days until the animals were responding on an FR-15 schedule of reinforcement. At the completion of extensive operant training, reversal learning was assessed. Wild-type and DARPP-32 knockout mice exhibited equivalent performance during acquisition of this task, with both groups increasing operant responding as the schedule of reinforcement was raised. However, significant differences in discrimination learning were observed during the reversal phase, with DARPP-32 knockout mice requiring significantly more trials to reach criterion than wild-type controls. These results provide evidence for a functional role of DARPP-32 in the mediation of processes underlying learning and memory.

Dysthymia: a review of pharmacological and behavioral factors

Although dysthymia, a chronic, low-grade form of depression, has a morbidity rate as high as that of major depression, and increases the risk for major depressive disorder, limited information is available concerning the etiology of this illness. In the present report we review literature concerning the biological and characterological features of dysthymia, the effectiveness of antidepressant treatments, the influence of stressors in the precipitation and maintenance of the disorder, and both quality of life and psychosocial correlates of the illness. We also provisionally suggest that dysthymia may stem from disturbances of neuroendocrine and neurotransmitter functioning (eg, corticotropin releasing hormone and arginine vasopressin within the hypothalamus, or alternatively monoamine variations within several extrahypothalamic sites), and may also involve cytokine activation. The central disturbances may reflect phenotypic variations of neuroendocrine processes or sensitization of such mechanisms. It is suggested that chronic stressor experiences or stressors encountered early in life lead to the phenotypic neurochemical alterations, which then favor the development of the dysthymic state. Owing to the persistence of the neurochemical disturbances, vulnerability to double depression is increased, and in this instance treatment with antidepressants may attenuate the symptoms of major depression but not those of the basal dysthymic state. Moreover, the residual features of depression following treatment may be indicative of underlying neurochemical disturbances, and may also serve to increase the probability of illness recurrence or relapse.