Neurobiological constraints on behavioral models of motivation

The "Seven Deadly Sins" of Neophobia Experimental Design

Neophobia, an aversive response to novelty, is a behavior with critical ecological and evolutionary relevance for wild populations because it directly influences animals' ability to adapt to new environments and exploit novel resources. Neophobia has been described in a wide variety of different animal species from arachnids to zebra finches. Because of this widespread prevalence and ecological importance, the number of neophobia studies has continued to increase over time. However, many neophobia studies (as well as many animal behavior studies more generally) suffer from one or more of what we have deemed the "seven deadly sins" of neophobia experimental design. These "sins" include: (1) animals that are not habituated to the testing environment, (2) problems with novel stimulus selection, (3) non-standardized motivation, (4) pseudoreplication, (5) lack of sufficient controls, (6) fixed treatment order, and (7) using arbitrary thresholds for data analysis. We discuss each of these potential issues in turn and make recommendations for how to avoid them in future behavior research. More consistency in how neophobia studies are designed would facilitate comparisons across different populations and species and allow researchers to better understand whether neophobia can help explain animals' responses to human-altered landscapes and the ability to survive in the Anthropocene.

"Wanting" versus "needing" related value: An fMRI meta-analysis

Consumption and its excesses are sometimes explained by imbalance of need or lack of control over "wanting." "Wanting" assigns value to cues that predict rewards, whereas "needing" assigns value to biologically significant stimuli that one is deprived of. Here we aimed at studying how the brain activation patterns related to value of "wanted" stimuli differs from that of "needed" stimuli using activation likelihood estimation neuroimaging meta-analysis approaches. We used the perception of a cue predicting a reward for "wanting" related value and the perception of food stimuli in a hungry state as a model for "needing" related value. We carried out separate, contrasts, and conjunction meta-analyses to identify differences and similarities between "wanting" and "needing" values. Our overall results for "wanting" related value show consistent activation of the ventral tegmental area, striatum, and pallidum, regions that both activate behavior and direct choice, while for "needing" related value, we found an overall consistent activation of the middle insula and to some extent the caudal-ventral putamen, regions that only direct choice. Our study suggests that wanting has more control on consumption and behavioral activation.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Norepinephrine in the Medial Pre-frontal Cortex Supports Accumbens Shell Responses to a Novel Palatable Food in Food-Restricted Mice Only

Previous findings from this laboratory demonstrate: (1) that different classes of addictive drugs require intact norepinephrine (NE) transmission in the medial pre Frontal Cortex (mpFC) to promote conditioned place preference and to increase dopamine (DA) tone in the nucleus accumbens shell (NAc Shell); (2) that only food-restricted mice require intact NE transmission in the mpFC to develop conditioned preference for a context associated with milk chocolate; and (3) that food-restricted mice show a significantly larger increase of mpFC NE outflow then free fed mice when experiencing the palatable food for the first time. In the present study we tested the hypothesis that only the high levels of frontal cortical NE elicited by the natural reward in food restricted mice stimulate mesoaccumbens DA transmission. To this aim we investigated the ability of a first experience with milk chocolate to increase DA outflow in the accumbens Shell and c-fos expression in striatal and limbic areas of food–restricted and ad-libitum fed mice. Moreover, we tested the effects of a selective depletion of frontal cortical NE on both responses in either feeding group. Only in food-restricted mice milk chocolate induced an increase of DA outflow beyond baseline in the accumbens Shell and a c-fos expression larger than that promoted by a novel inedible object in the nucleus accumbens. Moreover, depletion of frontal cortical NE selectively prevented both the increase of DA outflow and the large expression of c-fos promoted by milk chocolate in the NAc Shell of food-restricted mice. These findings support the conclusion that in food-restricted mice a novel palatable food activates the motivational circuit engaged by addictive drugs and support the development of noradrenergic pharmacology of motivational disturbances.

Opioid-induced rewards, locomotion, and dopamine activation: A proposed model for control by mesopontine and rostromedial tegmental neurons

Opioids, such as morphine or heroin, increase forebrain dopamine (DA) release and locomotion, and support the acquisition of conditioned place preference (CPP) or self-administration. The most sensitive sites for these opioid effects in rodents are in the ventral tegmental area (VTA) and rostromedial tegmental nucleus (RMTg). Opioid inhibition of GABA neurons in these sites is hypothesized to lead to arousing and rewarding effects through disinhibition of VTA DA neurons. We review findings that the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei, which each contain cholinergic, GABAergic, and glutamatergic cells, are important for these effects. LDTg and/or PPTg cholinergic inputs to VTA mediate opioid-induced locomotion and DA activation via VTA M5 muscarinic receptors. LDTg and/or PPTg cholinergic inputs to RMTg also modulate opioid-induced locomotion. Lesions or inhibition of LDTg or PPTg neurons reduce morphine-induced increases in forebrain DA release, acquisition of morphine CPP or self-administration. We propose a circuit model that links VTA and RMTg GABA with LDTg and PPTg neurons critical for DA-dependent opioid effects in drug-naïve rodents.

Further investigation of phenotypes and confounding factors of progressive ratio performance and feeding behavior in the BACHD rat model of Huntington disease

Huntington disease is an inherited neurodegenerative disorder characterized by motor, cognitive, psychiatric and metabolic symptoms. We recently published a study describing that the BACHD rat model of HD shows an obesity phenotype, which might affect their motivation to perform food-based behavioral tests. Further, we argued that using a food restriction protocol based on matching BACHD and wild type rats' food consumption rates might resolve these motivational differences. In the current study, we followed up on these ideas in a longitudinal study of the rats' performance in a progressive ratio test. We also investigated the phenotype of reduced food consumption rate, which is typically seen in food-restricted BACHD rats, in greater detail. In line with our previous study, the BACHD rats were less motivated to perform the progressive ratio test compared to their wild type littermates, although the phenotype was no longer present when the rats' food consumption rates had been matched. However, video analysis of food consumption tests suggested that the reduced consumption rate found in the BACHD rats was not entirely based on differences in hunger, but likely involved motoric impairments. Thus, restriction protocols based on food consumption rates are not appropriate when working with BACHD rats. As an alternative, we suggest that studies where BACHD rats are used should investigate how the readouts of interest are affected by motivational differences, and use appropriate control tests to avoid misleading results. In addition, we show that BACHD rats display distinct behavioral changes in their progressive ratio performance, which might be indicative of striatal dysfunction.

The role of orgasm in the development and shaping of partner preferences

BACKGROUND

The effect of orgasm on the development and shaping of partner preferences may involve a catalysis of the neurochemical mechanisms of bonding. Therefore, understanding such process is relevant for neuroscience and psychology.

METHODS

A systematic review was carried out using the terms Orgasm, Sexual Reward, Partner Preference, Pair Bonding, Brain, Learning, Sex, Copulation.

RESULTS

In humans, concentrations of arousing neurotransmitters and potential bonding neurotransmitters increase during orgasm in the cerebrospinal fluid and the bloodstream. Similarly, studies in animals indicate that those neurotransmitters (noradrenaline, oxytocin, prolactin) and others (e.g. dopamine, opioids, serotonin) modulate the appetitive and consummatory phases of sexual behavior and reward. This suggests a link between the experience of orgasm/sexual reward and the neurochemical mechanisms of pair bonding. Orgasm/reward functions as an unconditioned stimulus (UCS). Some areas in the nervous system function as UCS-detection centers, which become activated during orgasm. Partner-related cues function as conditioned stimuli (CS) and are processed in CS-detector centers.

CONCLUSIONS

Throughout the article, we discuss how UCS- and CS-detection centers must interact to facilitate memory consolidation and produce recognition and motivation during future social encounters.

Epigenetic Mechanisms in Psychiatric Diseases and Epigenetic Therapy

Preclinical Research Epigenetic mechanisms refer covalent modification of DNA and histone proteins that control transcriptional regulation of gene expression. Epigenetic regulation is involved in the development of the nervous system and plays an important role in the pathophysiology of psychiatric disorders, including depression, bipolar disorder, and schizophrenia. Epigenetic drugs, including histone deacetylation and DNA methylation inhibitors have received increased attention for the management of psychiatric diseases. The purpose of this review is to discuss the potential of epigenetic drugs to treat these disorders and to clarify the mechanisms by which they regulate the dysfunctional genes in the brain. Drug Dev Res 77 : 407-413, 2016. © 2016 Wiley Periodicals, Inc.

GLP-1 influences food and drug reward

Natural rewards, including food, water, sleep and social interactions, are required to sustain life. The neural substrates that regulate the reinforcing effects of these behaviors are also the same neurobiological mechanisms mediating mood, motivation and the rewarding effects of pharmacological stimuli. That the neuropeptide glucagon-like peptide-1 (GLP-1) is under investigation for both the homeostatic and hedonic controls of feeding is not surprising or novel. However, if the neural substrates that underline food reward are shared with other reward-related behaviors generally, then future research should investigate and embrace the likelihood that endogenous and exogenous GLP-1 receptor activation may influence multiple reward-related behaviors. Indeed, studies of the neurobiological mechanisms underlying motivated feeding behavior have informed much of the basic research investigating neural substrates of drug addiction. An emerging literature demonstrates a role for the GLP-1 system in modulating maladaptive reward behaviors, including drug and alcohol consumption. Thus, if GLP-1-based pharmacotherapies are to be used to treat drug addiction and other diseases associated with maladaptive reward behaviors (e.g. obesity and eating disorders), the neuroscience field must conduct systematic, mechanistic neuropharmacological and behavioral studies of each GLP-1 receptor-expressing nucleus within the brain. It is possible that behavioral selectivity may result from these studies, which could inform future approaches to targeting GLP-1R signaling in discrete brain nuclei to treat motivated behaviors. Equally as likely, non-selective effects on natural reward and maladaptive reward behaviors may be observed for GLP-1-based pharmacotherapies. In this case, a better understanding of the effects of increased central GLP-1R activation on motivated behaviors will aid in clinical approaches toward treating aberrant feeding behaviors and/or drug dependence.

Dopamine in the Brain: Hypothesizing Surfeit or Deficit Links to Reward and Addiction

Recently there has been debate concerning the role of brain dopamine in reward and addiction. David Nutt and associates eloquently proposed that dopamine (DA) may be central to psycho stimulant dependence and some what important for alcohol, but not important for opiates, nicotine or even cannabis. Others have also argued that surfeit theories can explain for example cocaine seeking behavior as well as non-substance-related addictive behaviors. It seems prudent to distinguish between what constitutes “surfeit” compared to” deficit” in terms of short-term (acute) and long-term (chronic) brain reward circuitry responsivity. In an attempt to resolve controversy regarding the contributions of mesolimbic DA systems to reward, we review the three main competing explanatory categories: “liking”, “learning”, and “wanting”. They are (a) the hedonic impact -liking reward, (b) the ability to predict rewarding effects-learning and (c) the incentive salience of reward-related stimuli -wanting. In terms of acute effects, most of the evidence seems to favor the “surfeit theory”. Due to preferential dopamine release at mesolimbic-VTA-caudate-accumbens loci most drugs of abuse and Reward Deficiency Syndrome (RDS) behaviors have been linked to heightened feelings of well-being and hyperdopaminergic states.The “dopamine hypotheses” originally thought to be simple, is now believed to be quite complex and involves encoding the set point of hedonic tone, encoding attention, reward expectancy, and incentive motivation. Importantly, Willuhn et al. shows that in a self-administration paradigm, (chronic) excessive use of cocaine is caused by decreased phasic dopamine signaling in the striatum. In terms of chronic addictions, others have shown a blunted responsivity at brain reward sites with food, nicotine, and even gambling behavior. Finally, we are cognizant of the differences in dopaminergic function as addiction progresses and argue that relapse may be tied to dopamine deficiency. Vulnerability to addiction and relapse may be the result of the cumulative effects of dopaminergic and other neurotransmitter genetic variants and elevated stress levels. We therefore propose that dopamine homeostasis may be a preferred goal to combat relapse.

Early-life adversity and adolescent depression: mechanisms involving the ventral striatum

Early-life adversity is a well-established risk factor for the development of depression later in life. Here we discuss the relationship between early-life adversity and depression, focusing specifically on effects of early-life caregiver deprivation on alterations in the neural and behavioral substrates of reward-processing. We also examine vulnerability to depression within the context of sensitive periods of neural development and the timing of adverse exposure. We further review the development of the ventral striatum, a limbic structure implicated in reward processing, and its role in depressive outcomes following early-life adversity. Finally, we suggest a potential neurobiological mechanism linking early-life adversity and altered ventral striatal development. Together these findings may help provide further insight into the role of reward circuitry dysfunction in psychopathological outcomes in both clinical and developmental populations.

BDNF signaling in the VTA links the drug-dependent state to drug withdrawal aversions

Drug administration to avoid unpleasant drug withdrawal symptoms has been hypothesized to be a crucial factor that leads to compulsive drug-taking behavior. However, the neural relationship between the aversive motivational state produced by drug withdrawal and the development of the drug-dependent state still remains elusive. It has been observed that chronic exposure to drugs of abuse increases brain-derived neurotrophic factor (BDNF) levels in ventral tegmental area (VTA) neurons. In particular, BDNF expression is dramatically increased during drug withdrawal, which would suggest a direct connection between the aversive state of withdrawal and BDNF-induced neuronal plasticity. Using lentivirus-mediated gene transfer to locally knock down the expression of the BDNF receptor tropomyosin-receptor-kinase type B in rats and mice, we observed that chronic opiate administration activates BDNF-related neuronal plasticity in the VTA that is necessary for both the establishment of an opiate-dependent state and aversive withdrawal motivation. Our findings highlight the importance of a bivalent, plastic mechanism that drives the negative reinforcement underlying addiction.

Principles of motivation revealed by the diverse functions of neuropharmacological and neuroanatomical substrates underlying feeding behavior

Circuits that participate in specific subcomponents of feeding (e.g., gustatory perception, peripheral feedback relevant to satiety and energy balance, reward coding, etc.) are found at all levels of the neural axis. Further complexity is conferred by the wide variety of feeding-modulatory neurotransmitters and neuropeptides that act within these circuits. An ongoing challenge has been to refine the understanding of the functional specificity of these neurotransmitters and circuits, and there have been exciting advances in recent years. We focus here on foundational work of Dr. Ann Kelley that identified distinguishable actions of striatal opioid peptide modulation and dopamine transmission in subcomponents of reward processing. We also discuss her work in overlaying these neuropharmacological effects upon anatomical pathways that link the telencephalon (cortex and basal ganglia) with feeding-control circuits in the hypothalamus. Using these seminal contributions as a starting point, we will discuss new findings that expand our understanding of (1) the specific, differentiable motivational processes that are governed by central dopamine and opioid transmission, (2) the manner in which other striatal neuromodulators, specifically acetylcholine, endocannabinoids and adenosine, modulate these motivational processes (including via interactions with opioid systems), and (3) the organization of the cortical-subcortical network that subserves opioid-driven feeding. The findings discussed here strengthen the view that incentive-motivational properties of food are coded by substrates and neural circuits that are distinguishable from those that mediate the acute hedonic experience of food reward. Striatal opioid transmission modulates reward processing by engaging frontotemporal circuits, possibly via a hypothalamic-thalamic axis, that ultimately impinges upon hypothalamic modules dedicated to autonomic function and motor pattern control. We will conclude by discussing implications for understanding disorders of "non-homeostatic" feeding.

Reward uncertainty enhances incentive salience attribution as sign-tracking

Conditioned stimuli (CSs) come to act as motivational magnets following repeated association with unconditioned stimuli (UCSs) such as sucrose rewards. By traditional views, the more reliably predictive a Pavlovian CS-UCS association, the more the CS becomes attractive. However, in some cases, less predictability might equal more motivation. Here we examined the effect of introducing uncertainty in CS-UCS association on CS strength as an attractive motivation magnet. In the present study, Experiment 1 assessed the effects of Pavlovian predictability versus uncertainty about reward probability and/or reward magnitude on the acquisition and expression of sign-tracking (ST) and goal-tracking (GT) responses in an autoshaping procedure. Results suggested that uncertainty produced strongest incentive salience expressed as sign-tracking. Experiment 2 examined whether a within-individual temporal shift from certainty to uncertainty conditions could produce a stronger CS motivational magnet when uncertainty began, and found that sign-tracking still increased after the shift. Overall, our results support earlier reports that ST responses become more pronounced in the presence of uncertainty regarding CS-UCS associations, especially when uncertainty combines both probability and magnitude. These results suggest that Pavlovian uncertainty, although diluting predictability, is still able to enhance the incentive motivational power of particular CSs.

Valproate alters dopamine signaling in association with induction of Par-4 protein expression

Chromatin remodeling through histone modifications has emerged as a key mechanism in the pathophysiology of psychiatric disorders. Valproate (VPA), a first-line medication for bipolar disorder, is known to have histone deacetylase (HDAC) inhibitor activity, but the relationship between its efficacy as a mood stabilizer and HDAC inhibitory activity is unclear. Here we provide evidence that prostate apoptosis response-4 (Par-4), an intracellular binding partner of dopamine D2 receptors (DRD2), plays a role in mediating the effectiveness of VPA. We found that chronic VPA treatment enhanced the expression of Par-4 in cultured neurons and adult mouse brains. This Par-4 induction phenomenon occurred at the transcriptional level and was correlated with an increase in histone H3 and H4 acetylation of the Par-4 promoter regions. Furthermore, chronic VPA treatment potentiated the suppression of the cAMP signaling cascade upon dopamine stimulation, which was blocked by sulpiride treatment. These results indicate that VPA potentiates DRD2 activity by enhancing Par-4 expression via a chromatin remodeling mechanism.

Prefrontal/accumbal catecholamine system processes high motivational salience

Motivational salience regulates the strength of goal seeking, the amount of risk taken, and the energy invested from mild to extreme. Highly motivational experiences promote highly persistent memories. Although this phenomenon is adaptive in normal conditions, experiences with extremely high levels of motivational salience can promote development of memories that can be re-experienced intrusively for long time resulting in maladaptive outcomes. Neural mechanisms mediating motivational salience attribution are, therefore, very important for individual and species survival and for well-being. However, these neural mechanisms could be implicated in attribution of abnormal motivational salience to different stimuli leading to maladaptive compulsive seeking or avoidance. We have offered the first evidence that prefrontal cortical norepinephrine (NE) transmission is a necessary condition for motivational salience attribution to highly salient stimuli, through modulation of dopamine (DA) in the nucleus accumbens (NAc), a brain area involved in all motivated behaviors. Moreover, we have shown that prefrontal-accumbal catecholamine (CA) system determines approach or avoidance responses to both reward- and aversion-related stimuli only when the salience of the unconditioned stimulus (UCS) is high enough to induce sustained CA activation, thus affirming that this system processes motivational salience attribution selectively to highly salient events.

[Decision-making and schizophrenia]

Abnormalities involving the prefrontal cortex (PFC) have long been postulated to underpin the pathophysiology of schizophrenia. Investigations of PFC integrity have focused mainly on the dorsolateral PFC (DLPFC) and abnormalities in this region have been extensively documented. However, defects in schizophrenia may extend to other prefrontal regions, including the ventromedial PFC (VMPFC), and evidence of VMPFC abnormalities comes from neuropathological, structural and functional studies. Patients with acquired brain injury to the VMPFC display profound disruption of social behaviour and poor judgment in their personal lives. The Iowa Gambling Task (IGT) was developed to assess decision-making in these neurological cases : it presents a series of 100 choices from four card decks that differ in the distribution of rewarding and punishing outcomes. Whilst healthy volunteers gradually develop a preference for the two "safe" decks over the course of the task, patients with VMPFC lesions maintain a preference for the two "risky" decks which are associated with high reinforcement in the short term, but significant long-term debt. Interestingly, damage to VMPFC may cause both poor performance on the IGT and lack of insight concerning the acquired personality modification. Recently, our group reported a trait-related decisionmaking impairment in the three phases of bipolar disorder. In a PET study, VMPFC dysfunction was shown in bipolar manic patients impaired on a decision-making task and an association between decision-making cognition and lack of insight was described in mania. A quantitative association between grey matter volume of VMPFC and memory impairment was previously reported in schizophrenia. Research suggests that lack of insight is a prevalent feature in schizophrenia patients, like auditory hallucinations, paranoid or bizarre delusions, and disorganized speech and thinking. Because schizophrenia is associated with significant social or occupational dysfunction, previous research assessed decision-making function but indicates conflicting results. Thirteen studies have reported impaired IGT performance in patients with schizophrenia and, in seven reports, no significant differences in IGT performance between patient and healthy control groups were found. Those discrepancies may relate to multiple factors. First, most of the studies included small sample size and negative findings may be due to the large variance of net scores. Second, as suggested by Rodríguez-Sánchez et al., there is a wide disparity in performance by control subjects across studies. Third, intelligence quotient (IQ) score and level of education may be correlated with IGT performance, which may explain IGT performance differences in studies that did not control for educational or IQ score. Fourth, only two studies have systematically controlled for substance use disorder, a potential confounder. Fifth, only two studies assessed the impact of antipsychotic (AP) class on performance. Sixth, to our knowledge, no study assessed the impact of AP dosage on decision-making ability, while AP dose-reduction and dopamine increase, might lead to improvements, in cognitive functions in schizophrenia and in IGT performance in bipolar disorder, respectively. Finally, discrepancies between studies may be related to the heterogeneity of diagnostic groups. Two of the negative studies included schizophrenia and schizoaffective disorder while positive studies have generally included only patients with schizophrenia. Nevertheless, some studies that included only patients with schizophrenia failed to find differences between groups. Thus, further research should assess decision-making in schizophrenia by testing a large group of patients with homogeneity of diagnostic, in comparison with a large group of control subjects. Authors should control for IQ or level of education, substance use disorder and smoking status. While it is now accepted that DLPFC defects in schizophrenia may extend to VMPFC, future investigations should test for an association between memory, insight ability and IGT performance and assess the impact of antipsychotic dosage upon performance.

"Liking" and "wanting" linked to Reward Deficiency Syndrome (RDS): hypothesizing differential responsivity in brain reward circuitry

In an attempt to resolve controversy regarding the causal contributions of mesolimbic dopamine (DA) systems to reward, we evaluate the three main competing explanatory categories: "liking,""learning," and "wanting" [1]. That is, DA may mediate (a) the hedonic impact of reward (liking), (b) learned predictions about rewarding effects (learning), or (c) the pursuit of rewards by attributing incentive salience to reward-related stimuli (wanting). We evaluate these hypotheses, especially as they relate to the Reward Deficiency Syndrome (RDS), and we find that the incentive salience or "wanting" hypothesis of DA function is supported by a majority of the evidence. Neuroimaging studies have shown that drugs of abuse, palatable foods, and anticipated behaviors such as sex and gaming affect brain regions involving reward circuitry, and may not be unidirectional. Drugs of abuse enhance DA signaling and sensitize mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Addictive drugs have in common that they are voluntarily selfadministered, they enhance (directly or indirectly) dopaminergic synaptic function in the nucleus accumbens (NAC), and they stimulate the functioning of brain reward circuitry (producing the "high" that drug users seek). Although originally believed simply to encode the set point of hedonic tone, these circuits now are believed to be functionally more complex, also encoding attention, reward expectancy, disconfirmation of reward expectancy, and incentive motivation. Elevated stress levels, together with polymorphisms of dopaminergic genes and other neurotransmitter genetic variants, may have a cumulative effect on vulnerability to addiction. The RDS model of etiology holds very well for a variety of chemical and behavioral addictions.

Apathy After Traumatic Brain Injury: An Overview of the Current State of Play

Apathy is a decrease in behavioural, cognitive and emotional components of goal-directed behaviour. Clinically, it is characterised by diminished initiation, reduced concern, and decreased activity. Apathy is a common occurrence following traumatic brain injury (TBI), occurring in around 60% of people. Consequences are widespread, negatively impacting independence, social integration, rehabilitation outcome, vocational outcome, coping and caregiver burden. The current knowledge base on apathy following TBI is presented, with implications for clinical practice. This includes a review of clinical presentations, neuroanatomical and neurochemical substrates associated with apathy, and differential diagnoses. Instruments to measure apathy are presented, highlighting those with demonstrated reliability and validity for the TBI population. Current evidence for pharmacological and non-pharmacological methods of treatment is described, with a model for non-pharmacological interventions provided and discussion of challenges faced by clinicians when treating the patient with apathy. In the TBI arena, greater understanding of apathy and methods of treatment is pivotal given the frequency of occurrence and widespread negative consequences.

d-Amphetamine stimulates unconditioned exploration/approach behaviors in crayfish: towards a conserved evolutionary function of ancestral drug reward

In mammals, rewarding properties of drugs depend on their capacity to activate a dopamine-mediated appetitive motivational seeking state--a system that allows animals to pursue and find all kinds of objects and events needed for survival. With such states strongly conserved in evolution, invertebrates have recently been developed into a powerful model in addiction research, where a shared ancestral brain system for the acquisition of reward can mediate drug addiction in many species. A conditioned place preference paradigm has illustrated that crayfish seek out environments that had previously been paired with psychostimulant and opioid administration. The present work demonstrates that the administration of D-amphetamine stimulates active explorative behaviors in crayfish through the action of the drug within their head ganglion. Crayfish, with a modularly organized and experimentally accessible, ganglionic nervous system offers a unique model to investigate (1) the fundamental, biological mechanisms of addictive drug reward; (2) how an appetitive/seeking disposition is implemented in a simple neural system, and (3) how it mediates the rewarding actions of major drugs of abuse.

Emergence of dormant conditioned incentive approach by conditioned withdrawal in nicotine addiction

BACKGROUND

Nicotine is one of the determinants for the development of persistent smoking, and this maladaptive behavior is characterized by many symptoms, including withdrawal and nicotine seeking. The process by which withdrawal affects nicotine seeking is poorly understood.

METHOD

The impact of a withdrawal-associated cue on nicotine (.2 mg/kg)-conditioned place preference was assessed in male C57BL/6J mice (n = 8-17/group). To establish a cue selectively associated with withdrawal distinct from those associated with nicotine, a tone was paired with withdrawal in their home cages; mice were chronically exposed to nicotine (200 μg/mL for 15 days) from drinking water in their home cages and received the nicotinic acetylcholine receptor antagonist mecamylamine (2.5 mg/kg) to precipitate withdrawal in the presence of a tone. The effect of the withdrawal-associated tone on nicotine-conditioned place preference was then evaluated in the place-conditioning apparatus after a delay, when nicotine-conditioned place preference spontaneously disappeared.

RESULTS

A cue associated with precipitated withdrawal reactivated the dormant effect of nicotine-associated cues on conditioned place preference. This effect occurred during continuous exposure to nicotine but not during abstinence.

CONCLUSIONS

A conditioned withdrawal cue could directly amplify the incentive properties of cues associated with nicotine. This observation extends the contemporary incentive account of the role of withdrawal in addiction to cue-cue interaction.

Individual aggressiveness in the crab Chasmagnathus: Influence in fight outcome and modulation by serotonin and octopamine

In a previous work we found that size-matched Chasmagnathus crabs establish winner-loser relationships that were stable over successive encounters but no evidence of escalation was revealed through fights. Here, we evaluated the hypothesis that size-matched fights between these crabs would be resolved according to the contestants' level of aggressiveness. Moreover, we aim at analysing the proximate roots of aggression, addressing the influence of the biogenic amines serotonin (5HT) and octopamine (OA) in crab's agonistic behaviour. To achieve these purposes, the following experiments were carried out. First, we performed successive fight encounters between the same opponents, varying the number of encounters and the interval between them, to assess the stability and progression of the winner-loser relationship. Then, we analysed dominance relationships in groups of three crabs, evaluating the emergence of linearity. Thirdly, we examined the effects of 5HT and OA injections over the fight dynamics and its result. Our findings show that contest outcome is persistent even through four encounters separated by 24h, but a comparison between encounters does not reveal any saving in fight time or increase in the opponent disparity. Within a group of crabs, a rank-order of dominance is revealed which is reflected in their fight dynamics. Interestingly, these results would not be due to winner or loser effects, suggesting that fight outcome could be mainly explained as resulting from differences in the level of aggressiveness of each opponent. Moreover, this individual aggressiveness can be modulated in opposite directions by the biogenic amines 5HT and OA, being increased by 5HT and decreased by OA.

The impact of cigarette deprivation and cigarette availability on cue-reactivity in smokers

AIMS

This experiment was conducted to determine the impact of cigarette deprivation and cigarette availability on reactivity measures to cigarette cues.

PARTICIPANTS

Smokers were recruited who were 18 years of age or older, not attempting to quit or cut down on their smoking, smoked at least 20 cigarettes daily, had been smoking regularly for past year and had an expired carbon monoxide level of at least 10 parts per million.

DESIGN

Smokers were assigned randomly to abstain from smoking for 24 hours (n = 51) or continue smoking their regular amount (n = 50). Twenty-four hours later, they were exposed to trials of either a lit cigarette or a glass of water with a 0, 50 or 100% probability of being able to sample the cue on each trial. Craving, mood, heart rate, skin conductance, puff topography and latency to access door to sample the cue were measured.

FINDINGS

Both exposure to cigarette cues and increasing availability of those cues produced higher levels of craving to smoke. Deprivation produced a generalized increase in craving. There was no consistent evidence, however, that even under conditions of high cigarette availability, deprived smokers were sensitized selectively to presentations of cigarette cues.

CONCLUSIONS

The data suggest that, even under conditions of immediate cigarette availability, deprivation and cue presentations have independent, additive effects on self-reported craving levels in smokers.

The uncertainty processing theory of motivation

Most theories describe motivation using basic terminology (drive, 'wanting', goal, pleasure, etc.) that fails to inform well about the psychological mechanisms controlling its expression. This leads to a conception of motivation as a mere psychological state 'emerging' from neurophysiological substrates. However, the involvement of motivation in a large number of behavioural parameters (triggering, intensity, duration, and directedness) and cognitive abilities (learning, memory, decision, etc.) suggest that it should be viewed as an information processing system. The uncertainty processing theory (UPT) presented here suggests that motivation is the set of cognitive processes allowing organisms to extract information from the environment by reducing uncertainty about the occurrence of psychologically significant events. This processing of information is shown to naturally result in the highlighting of specific stimuli. The UPT attempts to solve three major problems: (i) how motivations can affect behaviour and cognition so widely, (ii) how motivational specificity for objects and events can result from nonspecific neuropharmacological causal factors (such as mesolimbic dopamine), and (iii) how motivational interactions can be conceived in psychological terms, irrespective of their biological correlates. The UPT is in keeping with the conceptual tradition of the incentive salience hypothesis while trying to overcome the shortcomings inherent to this view.

'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.

Human insula activation reflects risk prediction errors as well as risk

Understanding how organisms deal with probabilistic stimulus-reward associations has been advanced by a convergence between reinforcement learning models and primate physiology, which demonstrated that the brain encodes a reward prediction error signal. However, organisms must also predict the level of risk associated with reward forecasts, monitor the errors in those risk predictions, and update these in light of new information. Risk prediction serves a dual purpose: (1) to guide choice in risk-sensitive organisms and (2) to modulate learning of uncertain rewards. To date, it is not known whether or how the brain accomplishes risk prediction. Using functional imaging during a simple gambling task in which we constantly changed risk, we show that an early-onset activation in the human insula correlates significantly with risk prediction error and that its time course is consistent with a role in rapid updating. Additionally, we show that activation previously associated with general uncertainty emerges with a delay consistent with a role in risk prediction. The activations correlating with risk prediction and risk prediction errors are the analogy for risk of activations correlating with reward prediction and reward prediction errors for reward expectation. As such, our findings indicate that our understanding of the neural basis of reward anticipation under uncertainty needs to be expanded to include risk prediction.

Reward pathways in Parkinson's disease: clinical and theoretical implications

AIMS

The mesolimbic and mesocortical circuits are particularly involved in reward-related behavior in humans. Because these systems may be in some way altered in Parkinson's disease (PD), it is likely that some psychiatric manifestations of PD, such as hedonistic homeostatic dysregulation and pathological gambling, as well as impulsive decision making, may be ascribed to their involvement. The aim of the current article was to review recent literature on this topic in order to analyze whether these disturbances share a common ground and whether actual theoretical frameworks on addiction prove a useful tool for their interpretation.

METHODS

Data were identified on searches of MEDLINE/PubMed databases from relevant articles published up until March 2007.

RESULTS

All clinical manifestations (hedonistic homeostatic dysregulation, pathological gambling and impulsive decision making) seem to share a common multifaceted ground in which factors related to antiparkinsonian therapy, premorbid personality and progression of disease interact. Theoretical interpretations and conclusions drawn from experimental studies may help to shed light on the underlying pathological mechanisms.

CONCLUSIONS

Further studies are needed to analyze why, despite a common ground, only some patients develop those neuropsychiatric complications described here.

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.

Expression of brain derived neurotrophic factor, activity-regulated cytoskeleton protein mRNA, and enhancement of adult hippocampal neurogenesis in rats after sub-chronic and chronic treatment with the triple monoamine re-uptake inhibitor tesofensine

The changes of gene expression resulting from long-term exposure to monoamine antidepressant drugs in experimental animals are key to understanding the mechanisms of action of this class of drugs in man. Many of these genes and their products are either relevant biomarkers or directly involved in structural changes that are perhaps necessary for the antidepressant effect. Tesofensine is a novel triple monoamine reuptake inhibitor that acts to increase noradrenaline, serotonin, and dopamine neurotransmission. This study was undertaken to examine the effect of sub-chronic (5 days) and chronic (14 days) administration of Tesofensine on the expression of brain derived neurotrophic factor (BDNF) and activity-regulated cytoskeleton protein (Arc) in the rat hippocampus. Furthermore, hippocampi from the same animals were used to investigate the effect on cell proliferation by means of Ki-67- and NeuroD-immunoreactivity. We find that chronic, but not sub-chronic treatment with Tesofensine increases BDNF mRNA in the CA3 region of the hippocampus (35%), and Arc mRNA in the CA1 of the hippocampus (65%). Furthermore, the number of Ki-67- and neuroD-positive cells increased after chronic, but not sub-chronic treatment. This study shows that Tesofensine enhances hippocampal gene expression and new cell formation indicative for an antidepressant potential of this novel drug substance.

Polyvagal Theory and developmental psychopathology: emotion dysregulation and conduct problems from preschool to adolescence

In science, theories lend coherence to vast amounts of descriptive information. However, current diagnostic approaches in psychopathology are primarily atheoretical, emphasizing description over etiological mechanisms. We describe the importance of Polyvagal Theory toward understanding the etiology of emotion dysregulation, a hallmark of psychopathology. When combined with theories of social reinforcement and motivation, Polyvagal Theory specifies etiological mechanisms through which distinct patterns of psychopathology emerge. In this paper, we summarize three studies evaluating autonomic nervous system functioning in children with conduct problems, ages 4-18. At all age ranges, these children exhibit attenuated sympathetic nervous system responses to reward, suggesting deficiencies in approach motivation. By middle school, this reward insensitivity is met with inadequate vagal modulation of cardiac output, suggesting additional deficiencies in emotion regulation. We propose a biosocial developmental model of conduct problems in which inherited impulsivity is amplified through social reinforcement of emotional lability. Implications for early intervention are discussed.

Reward-aversion circuitry in analgesia and pain: implications for psychiatric disorders

Sensory and emotional systems normally interact in a manner that optimizes an organism's ability to survive using conscious and unconscious processing. Pain and analgesia are interpreted by the nervous system as aversive and rewarding processes that trigger specific behavioral responses. Under normal physiological conditions these processes are adaptive. However, under chronic pain conditions, functional alterations of the central nervous system frequently result in maladaptive behaviors. In this review, we examine: (a) the interactions between sensory and emotional systems involved in processing pain and analgesia in the physiological state; (b) the role of reward/aversion circuitry in pain and analgesia; and (c) the role of alterations in reward/aversion circuitry in the development of chronic pain and co-morbid psychiatric disorders. These underlying features have implications for understanding the neurobiology of functional illnesses such as depression and anxiety and for the development and evaluation of novel therapeutic interventions.

Various aspects of feeding behavior can be partially dissociated in the rat by the incentive properties of food and the physiological state

The authors investigated the role of food incentive properties and homeostatic state on the motivational, anticipatory, and consummatory aspects of feeding. Behavioral tests were carried out on food-sated and food-restricted rats that were presented with 2 kinds of food differing in their palatability level. Both food-sated and food-restricted rats consumed large quantities and were highly motivated when presented with very palatable food. In contrast, only food-restricted rats developed anticipatory responses, regardless of the kind of food presented. These data suggest that food incentive properties play a key role in the control of consummatory and motivational components of feeding but seem less involved in the regulation of anticipatory behavior.

Decision making, impulse control and loss of willpower to resist drugs: a neurocognitive perspective

Here I argue that addicted people become unable to make drug-use choices on the basis of long-term outcome, and I propose a neural framework that explains this myopia for future consequences. I suggest that addiction is the product of an imbalance between two separate, but interacting, neural systems that control decision making: an impulsive, amygdala system for signaling pain or pleasure of immediate prospects, and a reflective, prefrontal cortex system for signaling pain or pleasure of future prospects. After an individual learns social rules, the reflective system controls the impulsive system via several mechanisms. However, this control is not absolute; hyperactivity within the impulsive system can override the reflective system. I propose that drugs can trigger bottom-up, involuntary signals originating from the amygdala that modulate, bias or even hijack the goal-driven cognitive resources that are needed for the normal operation of the reflective system and for exercising the willpower to resist drugs.

Apomorphine effects on emotional modulation of the startle reflex in rats

RATIONALE

Emotional modulation of the startle reflex in the rat may be used to assess whether activation of dopamine receptors specifically increases hedonia, incentive, fear or arousal.

OBJECTIVES

The objective of the study is to determine the effects of apomorphine (0.8 mg/kg s.c.) on the startle reflex of rats (72 male Sprague-Dawley rats) exposed to one of three affective conditions. These conditions were negative affective stimulus (exposure to cat smell), positive affective stimulus (availability of a 20% sucrose solution), neutral stimulus (no additional affective stimulus) and one of two appetitive "drive" states (food deprived or non-food deprived).

METHODS

The startle response (whole-body flinch response) was measured after presentation of a range of intensities of acoustic stimuli (65-120 dB, 40-ms duration white noise). The resulting sigmoidal stimulus intensity-response magnitude (SIRM) curves were fitted using a logistic regression procedure, and features of these functions were abstracted for analysis.

RESULTS

Maximal startle amplitudes were increased by the negative affect (fear) stimulus in non-food-deprived rats and decreased by the positive affect stimulus in food-deprived rats. Apomorphine mimicked the effects of food deprivation under both affect conditions, but also produced an effect in food-deprived rats similar to that of the positive affect condition.

CONCLUSIONS

The results are consistent with both a positive incentive effect and a direct hedonic action of apomorphine, but inconsistent with a role in general arousal. In addition, a method of analysing SIRM functions with logistic regressions is introduced as a useful means of standardising startle reflex measurements.

Par-4 Links Dopamine Signaling and Depression

Prostate apoptosis response 4 (Par-4) is a leucine zipper containing protein that plays a role in apoptosis. Although Par-4 is expressed in neurons, its physiological role in the nervous system is unknown. Here we identify Par-4 as a regulatory component in dopamine signaling. Par-4 directly interacts with the dopamine D2 receptor (D2DR) via the calmodulin binding motif in the third cytoplasmic loop. Calmodulin can effectively compete with Par-4 binding in a Ca2+-dependent manner, providing a route for Ca2+-mediated downregulation of D2DR efficacy. To examine the importance of the Par-4/D2DR interaction in dopamine signaling in vivo, we used a mutant mouse lacking the D2DR interaction domain of Par-4, Par-4DeltaLZ. Primary neurons from Par-4DeltaLZ embryos exhibit an enhanced dopamine-cAMP-CREB signaling pathway, indicating an impairment in dopamine signaling in these cells. Remarkably, Par-4DeltaLZ mice display significantly increased depression-like behaviors. Collectively, these results provide evidence that Par-4 constitutes a molecular link between impaired dopamine signaling and depression.

Amines and motivated behaviors: a simpler systems approach to complex behavioral phenomena

Recent investigations in invertebrate neurobiology have opened up new lines of research into the basic roles of behavioral, neurochemical, and physiological effects in complex behavioral phenomena, such as aggression and drug-sensitive reward. This review summarizes a body of quantitative work, which identifies biogenic amines as a pharmacological substrate for motivated behaviors in the crayfish, Orconectes rusticus. Specifically, this paper details progress that has (1) explored links between serotonin and an individual's aggressive state, and (2) demonstrated the existence of crayfish reward systems that are sensitive to human drugs of abuse, such as psychostimulants. First, we summarize a set of experimental approaches that explore aggression in crayfish and the significance of aminergic systems in its control. Agonistic behavior in crustaceans can be characterized within a quantitative framework; different types of behavioral plasticity in aggressive behavior are in need of physiological explanation, and pharmacological intervention involving serotonergic systems bring about characteristic changes in behavior. A second set of experiments demonstrates that psychostimulants (cocaine and D: -amphetamine) serve as rewards when an intra-circulatory infusion is coupled to a distinct visual environment. Work in novel model systems such as crayfish constitutes a useful comparative approach to the study of aggression and drug addiction.

Imaginary Relish and Exquisite Torture: The Elaborated Intrusion Theory of Desire

The authors argue that human desire involves conscious cognition that has strong affective connotation and is potentially involved in the determination of appetitive behavior rather than being epiphenomenal to it. Intrusive thoughts about appetitive targets are triggered automatically by external or physiological cues and by cognitive associates. When intrusions elicit significant pleasure or relief, cognitive elaboration usually ensues. Elaboration competes with concurrent cognitive tasks through retrieval of target-related information and its retention in working memory. Sensory images are especially important products of intrusion and elaboration because they simulate the sensory and emotional qualities of target acquisition. Desire images are momentarily rewarding but amplify awareness of somatic and emotional deficits. Effects of desires on behavior are moderated by competing incentives, target availability, and skills. The theory provides a coherent account of existing data and suggests new directions for research and treatment.

Nucleus accumbens dopamine release is necessary and sufficient to promote the behavioral response to reward-predictive cues

The nucleus accumbens is part of the neural circuit that controls reward-seeking in response to reward-predictive cues. Dopamine release in the accumbens is essential for the normal functioning of this circuit. Previous studies have shown that injection of dopamine receptor antagonists into the accumbens severely impairs an animal's ability to perform operant behaviors specified by predictive cues. Furthermore, excitations and inhibitions of accumbens neurons evoked by such cues are abolished by inactivation of the ventral tegmental area, the major dopaminergic input to the accumbens. These results indicate that dopamine is necessary to elicit neural activity in the accumbens that drives the behavioral response to cues. Here we show that accumbens dopamine release is causal to the rats' reward-seeking behavioral response by demonstrating that dopamine in this structure is both necessary and sufficient to promote the appropriate behavioral response to reward-predictive cues.

Dopamine modulates the plasticity of mechanosensory responses in Caenorhabditis elegans

Dopamine-modulated behaviors, including information processing and reward, are subject to behavioral plasticity. Disruption of these behaviors is thought to support drug addictions and psychoses. The plasticity of dopamine-mediated behaviors, for example, habituation and sensitization, are not well understood at the molecular level. We show that in the nematode Caenorhabditis elegans, a D1-like dopamine receptor gene (dop-1) modulates the plasticity of mechanosensory behaviors in which dopamine had not been implicated previously. A mutant of dop-1 displayed faster habituation to nonlocalized mechanical stimulation. This phenotype was rescued by the introduction of a wild-type copy of the gene. The dop-1 gene is expressed in mechanosensory neurons, particularly the ALM and PLM neurons. Selective expression of the dop-1 gene in mechanosensory neurons using the mec-7 promoter rescues the mechanosensory deficit in dop-1 mutant animals. The tyrosine hydroxylase-deficient C. elegans mutant (cat-2) also displays these specific behavioral deficits. These observations provide genetic evidence that dopamine signaling modulates behavioral plasticity in C. elegans.