How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

Muscarinic and NMDA Receptors in the Substantia Nigra Play a Role in Reward-Related Learning

BACKGROUND

Reward-related learning, where animals form associations between rewards and stimuli (i.e., conditioned stimuli [CS]) that predict or accompany those rewards, is an essential adaptive function for survival.

METHODS

In this study, we investigated the mechanisms underlying the acquisition and performance of conditioned approach learning with a focus on the role of muscarinic acetylcholine (mACh) and NMDA glutamate receptors in the substantia nigra (SN), a brain region implicated in reward and motor processes.

RESULTS

Using RNAscope in situ hybridization assays, we found that dopamine neurons of the SN express muscarinic (mACh5), NMDA2a, NMDA2b, and NMDA2d receptor mRNA but not mACh4. NMDA, but not mACh5, receptor mRNA was also found on SN GABA neurons. In a conditioned approach paradigm, rats were exposed to 3 or 7 conditioning sessions during which light/tone (CS) presentations were paired with delivery of food pellets, followed by a test session with CS-only presentations. Intra-SN microinjections of scopolamine (a mACh receptor antagonist) or AP-5 (a NMDA receptor antagonist) were made either prior to each conditioning session (to test their effects on acquisition) or prior to the CS-only test (to test their effects on expression of the learned response). Scopolamine and AP-5 produced dose-dependent significant reductions in the acquisition, but not performance, of conditioned approach.

CONCLUSIONS

These results suggest that SN mACh and NMDA receptors are key players in the acquisition, but not the expression, of reward-related learning. Importantly, these findings redefine the role of the SN, which has traditionally been known for its involvement in motor processes, and suggest that the SN possesses attributes consistent with a function as a hub of integration of primary reward and CS signals.

Brain areas modulation in consciousness during sevoflurane anesthesia

Sevoflurane is presently one of the most used inhaled anesthetics worldwide. However, the mechanisms through which sevoflurane acts and the areas of the brain associated with changes in consciousness during anesthesia remain important and complex research questions. Sevoflurane is generally regarded as a volatile anesthetic that blindly targets neuronal (and sometimes astrocyte) GABAA receptors. This review focuses on the brain areas of sevoflurane action and their relation to changes in consciousness during anesthesia. We cover 20 years of history, from the bench to the bedside, and include perspectives on functional magnetic resonance, electroencephalogram, and pharmacological experiments. We review the interactions and neurotransmitters involved in brain circuits during sevoflurane anesthesia, improving the effectiveness and accuracy of sevoflurane's future application and shedding light on the mechanisms behind human consciousness.

Neural Vulnerability Factors That Predict Future Weight Gain

PURPOSE OF REVIEW

The current article discusses five neural vulnerability theories for weight gain and reviews evidence from prospective studies using imaging and behavioral measures reflecting neural function, as well as randomized experiments with humans and animals that are consistent or inconsistent with these theories.

RECENT FINDINGS

Recent prospective imaging studies examining predictors of weight gain and response to obesity treatment, and repeated-measures imaging studies before and after weight gain and loss have advanced knowledge of etiologic processes and neural plasticity resulting from weight change. Overall, data provide strong support for the incentive sensitization theory of obesity and moderate support for the reward surfeit theory, inhibitory control deficit theory, and dynamic vulnerability model of obesity, which attempted to synthesize the former theories into a single etiologic model. Data provide little support for the reward deficit theory. Important directions for future studies are delineated.

Effective connectivity underlying reward-based executive control

Motivational influences on cognitive control play an important role in shaping human behavior. Cognitive facilitation through motivators such as prospective reward or punishment is thought to depend on regions from the dopaminergic mesocortical network, primarily the ventral tegmental area (VTA), inferior frontal junction (IFJ), and anterior cingulate cortex (ACC). However, how interactions between these regions relate to motivated control remains elusive. In the present functional magnetic resonance imaging study, we used dynamic causal modeling (DCM) to investigate effective connectivity between left IFJ, ACC, and VTA in a task-switching paradigm comprising three distinct motivational conditions (prospective monetary reward or punishment and a control condition). We found that while prospective punishment significantly facilitated switching between tasks on a behavioral level, interactions between IFJ, ACC, and VTA were characterized by modulations through prospective reward but not punishment. Our DCM results show that IFJ and VTA modulate ACC activity in parallel rather than by interaction to serve task demands in reward-based cognitive control. Our findings further demonstrate that prospective reward and punishment differentially affect neural control mechanisms to initiate decision-making.

Oxytocin and Addiction: Potential Glutamatergic Mechanisms

Recently, oxytocin (OXT) has been investigated for its potential therapeutic role in addiction. OXT has been found to diminish various drug-seeking and drug-induced behaviors. Although its behavioral effects are well-established, there is not much consensus on how this neuropeptide exerts its effects. Previous research has given thought to how dopamine (DA) may be involved in oxytocinergic mechanisms, but there has not been as strong of a focus on the role that glutamate (Glu) has. The glutamatergic system is critical for the processing of rewards and the disruption of glutamatergic projections produces the behaviors seen in drug addicts. We introduce the idea that OXT has direct effects on Glu transmission within the reward processing pathway. Thus, OXT may reduce addictive behaviors by restoring abnormal drug-induced changes in the glutamatergic system and in its interactions with other neurotransmitters. This review offers insight into the mechanisms through which a potentially viable therapeutic target, OXT, could be used to reduce addiction-related behaviors.

Neurobiology of reward-related learning

A major goal in psychology is to understand how environmental stimuli associated with primary rewards come to function as conditioned stimuli, acquiring the capacity to elicit similar responses to those elicited by primary rewards. Our neurobiological model is predicated on the Hebbian idea that concurrent synaptic activity on the primary reward neural substrate-proposed to be ventral tegmental area (VTA) dopamine (DA) neurons-strengthens the synapses involved. We propose that VTA DA neurons receive both a strong unconditioned stimulus signal (acetylcholine stimulation of DA cells) from the primary reward capable of unconditionally activating DA cells and a weak stimulus signal (glutamate stimulation of DA cells) from the neutral stimulus. Through joint stimulation the weak signal is potentiated and capable of activating the VTA DA cells, eliciting a conditioned response. The learning occurs when this joint stimulation initiates intracellular second-messenger cascades resulting in enhanced glutamate-DA synapses. In this review we present evidence that led us to propose this model and the most recent evidence supporting it.

CaMKII antagonism in the ventral tegmental area impairs acquisition of conditioned approach learning in rats

This study investigated the role of calcium2+/calmodulin-dependent protein kinase II (CaMKII), a protein in the second messenger pathway of NMDA receptors, in the ventral tegmental area (VTA) in the acquisition and performance of conditioned approach learning. Male Long-Evans rats (N = 79) were exposed to 3 (to test acquisition) or 7 (to test performance) conditioning sessions in which they received 30 paired presentations of a light stimulus (CS) and a food pellet (US) on a random time schedule. These conditioning sessions were then followed by one 30-min session without the CS or US and lastly by a CS-only test session, where only the light stimulus was presented (without food) according to the same schedule as the conditioning sessions. Bilateral intra-VTA injections of KN93 (vehicle, 3.0, 4.5 or 6.0 μg/0.5 μL), a CaMKII inhibitor, were administered prior to each conditioning session to test effects on the acquisition of conditioned approach or prior to the CS-only test session to test effects on the performance of conditioned approach. KN93, when given prior to conditioning sessions, significantly reduced the number of conditioned approach responses emitted during CS presentations in the CS-only test. When KN93 was given prior to the CS-only test it had no effect. These results suggest that CaMKII activation in the VTA is necessary for the acquisition, but not the performance, of reward-related learning.

Aberrations in Incentive Learning and Responding to Heroin in Male Rats After Adolescent or Adult Chronic Binge-Like Alcohol Exposure

BACKGROUND AND PURPOSE

Binge drinking is a serious problem among adolescents and young adults despite its adverse consequences on the brain and behavior. One area that remains poorly understood concerns the impact of chronic intermittent ethanol (CIE) exposure on incentive learning.

METHODS

Here, we examined the effects of CIE exposure during different developmental stages on conditioned approach and conditioned reward learning in rats experiencing acute or protracted withdrawal from alcohol. Two or 21 days after adolescent or adult CIE exposure, male rats were exposed to pairings of a light stimulus (CS) and food pellets for 3 consecutive daily sessions (30 CS-food pellet pairings per session). This was followed by conditioned approach testing measuring responses (food trough head entries) to the CS-only presentations and by conditioned reward testing measuring responses on a lever producing the CS and on another producing a tone. We then measured behavioral sensitization to repeated injections of heroin (2 mg/kg/d for 9 days).

RESULTS

Adolescent and adult alcohol-treated rats showed significantly impaired conditioned reward learning regardless of withdrawal period (acute or prolonged). We found no evidence of changes to conditioned approach learning after adolescent or adult exposure to CIE. Finally, in addition to producing long-term impairments in incentive learning, CIE exposure enhanced locomotor activity in response to heroin and had no effect on behavioral sensitization to heroin regardless of age and withdrawal period.

CONCLUSIONS

Our work sets a framework for identifying CIE-induced alterations in incentive learning and inducing susceptibility to subsequent opioid effects.

The effect of intrinsic and extrinsic motivation on memory formation: insight from behavioral and imaging study

Motivation can be generated intrinsically or extrinsically, and both kinds of motivation show similar facilitatory effects on memory. However, effects of extrinsic and intrinsic motivation on memory formation have not been studied in combination and thus, it is unknown whether they interact and how such interplay is neurally implemented. In the present study, both extrinsic monetary reward and intrinsic curiosity enhanced memory performance, without evidence for an interaction. Functional magnetic resonance imaging revealed that curiosity-driven activity in the ventral striatal reward network appears to work cooperatively with the fronto-parietal attention network, while enhancing memory formation. In contrast, the monetary reward-modulated subsequent memory effect revealed deactivation in parietal midline regions. Thus, curiosity might enhance memory performance by allocation of attentional resources and reward-related processes; while, monetary reward does so by suppression of task-irrelevant processing.

Positive emotions have a unique capacity to capture attention

Most of the previous research in the area of cognitive psychology or cognitive neuroscience focused on studying negative emotions and argued that the negative emotional stimuli capture attention involuntarily as compared to neutral stimuli. However, in the last decades, researchers started paying attention in studying positive emotions also as positive emotions have evolutionary significance and are essential for many aspects of our life. The theme of this chapter is to present an overview of research in the area of positive emotions, and make a case that positively-valenced stimulus is prioritized over others. Primarily, when attentional resources are not constrained, many studies have shown that similar to negative stimuli, positive stimuli also capture attention automatically irrespective of whether they are relevant or irrelevant to the primary task. It suggests a fundamental prioritization of these stimuli by the cognitive/motivational system. However, when attentional resources are constrained, only positive or high rewarding stimuli win the competition for attentional resources compared to negative or stimuli associated with high punishment. Positive or high rewarding stimuli also receive priority in temporal selection, when attention is constrained. Theoretical implications of these results have been discussed. Possible cognitive and neural mechanisms have been proposed underlying these effects.

Neural vulnerability factors for obesity

Multiple theories identify neural vulnerability factors that may increase risk for overeating and weight gain. Early cross-sectional neuroimaging studies were unable to determine whether aberrant neural responsivity was a risk factor for or a consequence of overeating. More recent obesity risk, prospective, repeated-measures, and experimental neuroimaging studies with humans have advanced knowledge of etiologic processes and neural plasticity resulting from overeating. Herein, we review evidence from these more rigorous human neuroimaging studies, in conjunction with behavioral measures reflecting neural function, as well as experiments with animals that investigated neural vulnerability theories for overeating. Findings provide support for the reward surfeit theory that posits that individuals at risk for obesity initially show hyper-responsivity of reward circuitry to high-calorie food tastes, which theoretically drives elevated intake of such foods. However, findings provide little support for the reward deficit theory that postulates that individuals at risk for obesity show an initial hypo-responsivity of reward circuitry that motives overeating. Further, results provide support for the incentive sensitization and dynamic vulnerability theories that propose that overconsumption of high-calorie foods results in increased reward and attention region responsivity to cues that are associated with hedonic reward from intake of these high-calorie foods via conditioning, as well as a simultaneous decrease in reward region responsivity to high-calorie food tastes. However, there is little evidence that this induced reduction in reward region response to high-calorie food tastes drives an escalation in overeating. Finally, results provide support for the theory that an initial deficit in inhibitory control and a bias for immediate reward contribute to overconsumption of high-calorie foods. Findings imply that interventions that reduce reward and attention region responsivity to food cues and increase inhibitory control should reduce overeating and excessive weight gain, an intervention theory that is receiving support in randomized trials.

The strength of reward-related learning depends on the degree of activation of ventral tegmental area dopamine neurons

We tested whether (1) the capacity of a reward-associated conditioned stimulus (CS) to cause conditioned activation of ventral tegmental area (VTA) dopamine (DA) neurons is associated with its capacity to elicit conditioned approach responses and (2) whether the acquisition of these capacities by a CS requires N-methyl-D-aspartate (NMDA) and muscarinic acetylcholine (mACh) receptor stimulation. Rats were trained to emit a conditioned approach response to a light CS that was previously paired with food and were treated systemically with scopolamine (a mACh receptor antagonist) or MK-801 (an NMDA receptor antagonist) either prior to each conditioning session (during which animals experienced paired CS and food presentations) or prior to the conditioned approach (CS-only) test. Brains were harvested after the CS-only test and processed for tyrosine hydroxylase (TH) (DA cells) and c-fos in the VTA. When animals received scopolamine or MK-801 treatment prior to conditioning sessions we observed significantly fewer TH-labeled (i.e., DA) cells in the VTA that expressed c-fos and significantly less conditioned approach responding during the CS-only test. Further analysis showed a correlation between the number of VTA DA cells activated and the number of conditioned approach responses. Treatments made prior to the CS-only test did not affect responding. Altogether these results suggest that the degree to which a CS elicits conditioned approach depends partially on the degree to which the CS activates VTA DA cells and that the acquisition of both of these capacities by a CS requires mACh and NMDA receptor stimulation.

Motor skill learning and reward consumption differentially affect VTA activation

Dopamine release from the ventral tegmental area (VTA) terminals in the primary motor cortex (M1) enables motor skill acquisition. Here, we test the hypothesis that dopaminergic VTA neurons projecting to M1 are activated when rewards are obtained during motor skill acquisition, but not during task execution at plateau performance, or by rewards obtained without performing skilled movements. Rats were trained to perform a skilled reaching task for 3 days (acquisition) or 7 days (plateau). In combination with retrograde labelling of VTA-to-M1 projection neurons, double immunofluorescence for c-fos and tyrosine hydroxylase (TH) was used to assess activation of dopaminergic and non-dopaminergic VTA neurons. Dopaminergic VTA-to-M1 projection neurons were indeed activated during successful motor skill acquisition, but not when rats failed to learn or had reached plateau performance, nor by food rewards alone. By contrast, dopaminergic VTA neurons that did not project to M1 were activated by both skilled reaching and food rewards. Non-dopaminergic neurons were found to be activated by motor task performance at plateau, but not during skill acquisition. These results indicate that distinct populations of VTA neurons are activated by motor skill acquisition and task performance. Moreover, this activation is not merely related to consumption of food rewards.

Sexual odor preference and dopamine release in the nucleus accumbens by estrous olfactory cues in sexually naïve and experienced male rats

Sexual behavior is a natural reward that activates mesolimbic dopaminergic system. Microdialysis studies have shown that extracellular level of dopamine (DA) in the nucleus accumbens (NAcc) significantly increases during copulation in male rats. The NAcc DA level is also known to be increased during the presentation of a sexually receptive female before mating. This rise in DA was probably associated with sexual motivation elicited by incentive stimuli from the receptive female. These microdialysis studies, however, did not thoroughly investigated if olfactory stimuli from estrous females could significantly increase the extracellular DA in the NAcc of male rats. The present study was designed to examine systematically the relationship between the expression of preference for the olfactory stimuli from estrous females and the effects of these stimuli on the extracellular DA levels in the NAcc measured by in vivo microdialysis in male Long-Evans (LE) rats. We used two types of olfactory stimuli, either airborne odors (volatile stimuli) or soiled bedding (volatile plus nonvolatile stimuli). The sexually experienced male rats, which experienced six ejaculations, significantly preferred both of these olfactory stimuli from estrous females as opposed to males. Exposure to these female olfactory stimuli gradually increased extracellular DA in the NAcc, which reached significantly higher level above baseline during the period following the removal of the stimuli although not during the 15-min stimulus presentation period. The sexually naïve male rats, on the other hand, showed neither preference for olfactory stimuli from estrous females nor increase in the NAcc DA after exposure to these stimuli. These data suggest that in male LE rats olfactory stimuli from estrous females in and of themselves can be conditional cues that induce both incentive motivation and a significant increase in the NAcc DA probably as a result of being associated with sexual reward through copulatory experience.

The Development of Severe and Chronic Violence Among Youth: The Role of Psychopathic Traits and Reward Processing

Psychopathic traits are a manifestation of a personality pathology that comprises a core affective-interpersonal dysfunction (callous-unemotional traits) and an impulsive-antisocial behavioral component. Of particular importance, psychopathic traits are associated with the perpetration of some of the most severe acts of violence, and they appear to indicate a subset of youth at risk for earlier onset, greater frequency, and persistence of violent offending. Although these youth represent a minority of the population, they commit a significant proportion of the violence in the general community. In our review, we highlight evidence of a unique neurobiological predisposition that underlies the core affective deficits and describe contemporary accounts for the developmental processes leading to the antisocial behavior associated with psychopathy. Current evidence suggests that, for this subset of youth, the structure and function of neural circuitry supporting emotion processing, reward learning, decision making, and the development of emotion related to empathy may be crucial to understanding why they are at risk for violence. In particular, a reward dominant pattern of neurobehavioral conditioning may explain how these youth progress to some of the most severe and persistent forms of violence. However, this pattern of conditioning may also be essential to the primary prevention of such deleterious behavior. We suspect that effective strategies to prevent such violence may ultimately be informed by understanding these affective and motivational mechanisms.

Blockade of NMDA receptors blocks the acquisition of cocaine conditioned approach in rats

Conditioned stimuli (CSs) exert motivational effects on both adaptive and pathological reward-related behaviors, including drug taking and seeking. We developed a paradigm that allows us to investigate the neuropharmacology by which previously neutral stimuli acquire the capacity to function as CSs and elicit (intravenous) cocaine conditioned approach and used this paradigm to test the role of NMDA receptor stimulation in the acquisition of cocaine conditioned approach. Rats were injected systemically with the NMDA receptor antagonist, MK-801, before the start of 4 consecutive conditioning sessions, each of which consisted of 20 randomly presented light/tone (CS) presentations paired with cocaine infusion contingent upon nose pokes. Rats later were subjected to a CS-only test. To test the role of NMDA receptor stimulation in the already established conditioned approach, rats were injected with MK-801 prior to the CS-only test that occurred after 18 CS-cocaine conditioning sessions. Blockade of NMDA receptors significantly impaired the acquisition of cocaine-conditioned approach as indicated by the emission of significantly fewer nose pokes and significantly longer latencies to nose poke during CS presentations. When MK-801 treatment was applied after the acquisition of conditioned approach responding it had no effect on these measures. These results suggest that NMDA receptor stimulation plays an important role in the acquisition of reward-related conditioned responses driven by intravenous cocaine-associated CSs.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Blockade of muscarinic acetylcholine receptors in the ventral tegmental area blocks the acquisition of reward-related learning

In the present study we investigated whether stimulation of muscarinic acetylcholine (mACh) receptors in the ventral tegmental area (VTA) plays a role in the acquisition of food-based conditioned approach learning. Rats were exposed to 3 (in Experiment 1) or 7 (in Experiment 2) conditioning sessions in which 30, randomly presented light (CS) presentations were paired with delivery of food pellets (US), followed by one session with no light or food and finally one CS-only test session with only light stimulus presentations. Bilateral microinjections of scopolamine (a mACh receptor antagonist) were made either prior to each conditioning session (Experiment 1; to test effects on acquisition) or prior to the CS-only test (Experiment 2; to test effects on performance of the learned response). Scopolamine produced a dose-related significant reduction in the acquisition of conditioned approach but had no effect on its performance. These results suggest that mACh receptor stimulation in the VTA plays a necessary role in the acquisition of reward-related learning.

A Decade of Orexin/Hypocretin and Addiction: Where Are We Now?

One decade ago, our laboratory provided the first direct evidence linking orexin/hypocretin signaling with drug seeking by showing that activation of these neurons promotes conditioned morphine-seeking behavior. In the years since, contributions from many investigators have revealed roles for orexins in addiction for all drugs of abuse tested, but only under select circumstances. We recently proposed that orexins play a fundamentally unified role in coordinating "motivational activation" under numerous behavioral conditions, and here we unpack this hypothesis as it applies to drug addiction. We describe evidence collected over the past 10 years that elaborates the role of orexin in drug seeking under circumstances where high levels of effort are required to obtain the drug, or when motivation for drug reward is augmented by the presence of external stimuli like drug-associated cues/contexts or stressors. Evidence from studies using traditional self-administration and reinstatement models, as well as behavioral economic analyses of drug demand elasticity, clearly delineates a role for orexin in modulating motivational, rather than the primary reinforcing aspects of drug reward. We also discuss the anatomical interconnectedness of the orexin system with wider motivation and reward circuits, with a particular focus on how orexin modulates prefrontal and other glutamatergic inputs onto ventral tegmental area dopamine neurons. Last, we look ahead to the next decade of the research in this area, highlighting the recent FDA approval of the dual orexin receptor antagonist suvorexant (Belsomra®) for the treatment of insomnia as a promising sign of the potential clinical utility of orexin-based therapies for the treatment of addiction.

Picture Novelty Influences Response Selection and Inhibition: The Role of the In-Group Bias and Task-Difficulty

The human visual system prioritizes processing of novel information, leading to faster detection of novel stimuli. Novelty facilitates conflict resolution through the enhanced early perceptual processing. However, the role of novel information processing during the conflict-related response selection and inhibition remains unclear. Here, we used a face-gender classification version of the Simon task and manipulated task-difficulty and novelty of task-relevant information. The novel quality of stimuli was made task-irrelevant, and an in-group bias was tightly controlled by manipulation of a gender of picture stimuli. We found that the in-group bias modulated the role of novelty in executive control. Novel opposite-sex stimuli facilitated response inhibition only when the task was not demanding. By contrast, novelty enhanced response selection irrespective of the in-group factor when task-difficulty was increased. These findings support the in-group bias mechanism of visual processing, in cases when attentional resources are not limited by a demanding task. The results are further discussed along the lines of the attentional load theory and neural mechanisms of response-inhibition and locomotor activity. In conclusion, our data showed that processing of novel information may enhance executive control through facilitated response selection and inhibition.

Pontomesencephalic Tegmental Afferents to VTA Non-dopamine Neurons Are Necessary for Appetitive Pavlovian Learning

The ventral tegmental area (VTA) receives phenotypically distinct innervations from the pedunculopontine tegmental nucleus (PPTg). While PPTg-to-VTA inputs are thought to play a critical role in stimulus-reward learning, direct evidence linking PPTg-to-VTA phenotypically distinct inputs in the learning process remains lacking. Here, we used optogenetic approaches to investigate the functional contribution of PPTg excitatory and inhibitory inputs to the VTA in appetitive Pavlovian conditioning. We show that photoinhibition of PPTg-to-VTA cholinergic or glutamatergic inputs during cue presentation dampens the development of anticipatory approach responding to the food receptacle during the cue. Furthermore, we employed in vivo optetrode recordings to show that photoinhibition of PPTg cholinergic or glutamatergic inputs significantly decreases VTA non-dopamine (non-DA) neural activity. Consistently, photoinhibition of VTA non-DA neurons disrupts the development of cue-elicited anticipatory approach responding. Taken together, our study reveals a crucial regulatory mechanism by PPTg excitatory inputs onto VTA non-DA neurons during appetitive Pavlovian conditioning.

Brain regions involved in ingestive behavior and related psychological constructs in people undergoing calorie restriction

Human food intake is regulated by physiological energy homeostatic mechanisms and hedonic mechanisms. These are affected by both very short-term and longer-term calorie restriction (CR). To date, there are parallel discussions in the literature that fail to integrate across these disciplines and topics. First, much of the available neuroimaging research focusses on specific functional paradigms (e.g. reward, energy homeostasis). These paradigms often fail to consider more complex and inclusive models that examine how potential brain regions of interest interact to influence ingestion. Second, the paradigms used focus primarily on short-term CR (fasting) which has limited generalizability to clinical application. Finally, the behavioral literature, while frequently examining longer-term CR and related psychological constructs in the context of weight management (e.g. hedonic restraint, 'liking', 'wanting' and food craving), fails to adequately tie these phenomena to underlying neural mechanisms. The result is a less than complete picture of the brain's role in the complexity of the human experience of ingestion. This disconnect highlights a major limitation in the CR literature, where attempts are persistently made to exert behavioral control over ingestion, without fully understanding the complex bio behavioral systems involved. In this review we attempt to summarize all potential brain regions important for human ingestion, present a broad conceptual overview of the brain's multifaceted role in ingestive behavior, the human (psychological) experiences related to ingestion and to examine how these factors differ according to three forms of CR. These include short-term fasting, extended CR, and restrained eating. We aim to bring together the neuroimaging literature with the behavioral literature within a conceptual framework that may inform future translational research.

Neural vulnerability factors that increase risk for future weight gain

Theorists have proposed several neural vulnerability factors that may increase overeating and consequent weight gain. Early cross-sectional imaging studies could not determine whether aberrant neural responsivity was a precursor or consequence of overeating. However, recent prospective imaging studies examining predictors of future weight gain and response to obesity treatment, and repeated-measures imaging studies before and after weight gain and loss have advanced knowledge of etiologic processes and neural plasticity resulting from weight change. The present article reviews evidence from prospective studies using imaging and behavioral measures reflecting neural function, as well as randomized experiments with humans and animals that are consistent or inconsistent with 5 neural vulnerability theories for excessive weight gain. Extant data provide strong support for the incentive sensitization theory of obesity and moderate support for the reward surfeit theory, inhibitory control deficit theory, and dynamic vulnerability model of obesity, which attempted to synthesize the former theories into a single etiologic model. However, existing data provide only minimal support for the reward deficit theory. Findings are synthesized into a new working etiologic model that is based on current scientific knowledge. Important directions for future studies, which have the potential to support or refute this working etiologic model, are delineated. (PsycINFO Database Record

Concurrent antagonism of NMDA and AMPA receptors in the ventral tegmental area reduces the expression of conditioned approach learning in rats

Conditioned stimuli (CSs) come to function as CSs by acquiring the capacity to activate the same mesocorticolimbic dopamine (DA) neurons activated by primary rewards, producing conditioned activation of these neurons and their associated motivational states. This model stipulates that CSs activate mesocorticolimbic DA systems through the activation of glutamate receptors on DA neurons in the ventral tegmental area (VTA). We tested the hypothesis that glutamate receptor stimulation in the VTA is necessary for the expression of conditioned approach. Rats were tested in a conditioned approach protocol that consisted of 7 consecutive conditioning sessions (light presentations and food were paired), one session with no light or food and one test session with only light stimulus (CS-only) presentations. The number of head entries during the CS and pre-CS (baseline) periods was used to calculate difference scores. Bilateral VTA microinjections of glutamate receptor antagonists were made prior to the CS-only session. Kynurenic acid (ionotropic glutamate receptor antagonist; 1.125-4.5 μg/0.5 μl) significantly reduced difference scores compared to vehicle (0 μg), whereas MCPG (metabotropic glutamate receptor antagonist; 1.875-7.5 μg), AP-5 (NMDA antagonist; 0.03125-2.0 μg), and NBQX (AMPA antagonist; 0.5-4.0 μg) had no effects. When AP-5 and NBQX were administered simultaneously at doses of 0.25/4.0 and 2.0/4.0 μg, respectively, the combination significantly reduced the difference scores compared to 0/0 μg, indicating a reduction in the expression of conditioned approach. These findings indicate that expression of conditioned approach learning requires NMDA or AMPA receptor stimulation in the VTA.

Muscarinic, but not nicotinic, acetylcholine receptor blockade in the ventral tegmental area attenuates cue-induced sucrose-seeking

The mesolimbic dopamine (DA) system is known to play a role in cue-mediated reward-seeking for natural rewards and drugs of abuse. Specifically, cholinergic and glutamatergic receptors in the ventral tegmental area (VTA) have been shown to regulate cue-induced drug-seeking. However, the potential role of these VTA receptors in regulating cue-induced reward seeking for natural rewards is unknown. Here, we examined whether blockade of VTA acetylcholine receptors (AChRs) and N-methyl-d-aspartate receptors (NMDARs) would alter cue-induced sucrose seeking in male Sprague-Dawley rats. Subjects underwent 10 days of sucrose self-administration training (fixed ratio 1 schedule) followed by 7 days of forced abstinence. On withdrawal day 7, rats received bilateral VTA infusion of vehicle, the muscarinic AChR antagonist scopolamine (2.4 or 24 μg/side), the nicotinic AChR antagonist mecamylamine (3 or 30 μg/side), or the NMDAR antagonist AP-5 (0.1 or 1 μg/side) immediately prior to examination of cue-induced sucrose-seeking. Scopolamine infusion led to robust attenuation, but did not completely block, sucrose-seeking behavior. In contrast, VTA administration of mecamylamine or AP-5 did not alter cue-induced sucrose-seeking. Together, the data suggest that VTA muscarinic AChRs, but not nicotinic AChRs nor NMDARs, facilitate the ability of food-associated cues to drive seeking behavior for a food reward.

Why psychopathy matters: Implications for public health and violence prevention

Psychopathy is an early-appearing risk factor for severe and chronic violence. The violence largely attributable to psychopathy constitutes a substantial portion of the societal burden to the public health and criminal justice systems, and thus necessitates significant attention from prevention experts. Yet, despite a vast base of research in psychology and criminology, the public health approach to violence has generally neglected to consider this key variable. Fundamentally, the public health approach to violence prevention is focused on achieving change at the population level to provide the most benefit to the maximum number of people. Increasing attention to the individual-level factor of psychopathy in public health could improve our ability to reduce violence at the community and societal levels. We conclude that the research literature on psychopathy points to a pressing need for a broad-based public health approach with a focus on primary prevention. Further, we consider how measuring psychopathy in public health research may benefit violence prevention, and ultimately society, in general.

Selective lesions of the cholinergic neurons within the posterior pedunculopontine do not alter operant learning or nicotine sensitization

Cholinergic neurons within the pedunculopontine tegmental nucleus have been implicated in a range of functions, including behavioral state control, attention, and modulation of midbrain and basal ganglia systems. Previous experiments with excitotoxic lesions have found persistent learning impairment and altered response to nicotine following lesion of the posterior component of the PPTg (pPPTg). These effects have been attributed to disrupted input to midbrain dopamine systems, particularly the ventral tegmental area. The pPPTg contains a dense collection of cholinergic neurons and also large numbers of glutamatergic and GABAergic neurons. Because these interdigitated populations of neurons are all susceptible to excitotoxins, the effects of such lesions cannot be attributed to one neuronal population. We wished to assess whether the learning impairments and altered responses to nicotine in excitotoxic PPTg-lesioned rats were due to loss of cholinergic neurons within the pPPTg. Selective depletion of cholinergic pPPTg neurons is achievable with the fusion toxin Dtx-UII, which targets UII receptors expressed only by cholinergic neurons in this region. Rats bearing bilateral lesions of cholinergic pPPTg neurons (>90% ChAT+ neuronal loss) displayed no deficits in the learning or performance of fixed and variable ratio schedules of reinforcement for pellet reward. Separate rats with the same lesions had a normal locomotor response to nicotine and furthermore sensitized to repeated administration of nicotine at the same rate as sham controls. Previously seen changes in these behaviors following excitotoxic pPPTg lesions cannot be attributed solely to loss of cholinergic neurons. These findings indicate that non-cholinergic neurons within the pPPTg are responsible for the learning deficits and altered responses to nicotine seen after excitotoxic lesions. The functions of cholinergic neurons may be related to behavioral state control and attention rather than learning.

Microinjections of a dopamine D1 receptor antagonist into the ventral tegmental area block the expression of cocaine conditioned place preference in rats

Stimulation of dopamine (DA) D1 receptors in the ventral tegmental area (VTA) is involved in primary rewards. In the current study we investigated whether VTA D1 receptor stimulation likewise plays a role in mediating the rewarding effects of cocaine-associated stimuli, using the cocaine conditioned place preference (CPP) paradigm. Rats were prepared with cannulae so as to allow microinjections in the VTA and later conditioned to a cocaine-associated environment using the CPP paradigm. Prior to each conditioning session rats were injected with either saline or cocaine (10mg/kg, intraperitoneally) and then placed in one of the two sides of the CPP apparatus. Sessions lasted 30min a day over a period of eight days, such that rats alternated daily between consistently experiencing cocaine in one side and saline in the other. On the test day, which was conducted one day after conditioning, rats were given bilateral microinjections of one of four doses of the D1 antagonist, SCH 23390, (0, 2, 4 or 8μg/0.5μl) directly into the VTA and allowed free access to both sides of the apparatus. Preference for either side was measured as time spent in each side and compared to the same measures taken before conditioning. The D1 antagonist produced a dose-related, significant reduction in the preference for the cocaine-paired side compared to vehicle. These data suggest that the expression of cocaine conditioned place preference requires stimulation of VTA D1 receptors and, as such, are the first to suggest a role for VTA dendritically released DA in cocaine-, or other reward-, related learning.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.

Possible contributions of a novel form of synaptic plasticity in Aplysia to reward, memory, and their dysfunctions in mammalian brain

Recent studies in Aplysia have identified a new variation of synaptic plasticity in which modulatory transmitters enhance spontaneous release of glutamate, which then acts on postsynaptic receptors to recruit mechanisms of intermediate- and long-term plasticity. In this review I suggest the hypothesis that similar plasticity occurs in mammals, where it may contribute to reward, memory, and their dysfunctions in several psychiatric disorders. In Aplysia, spontaneous release is enhanced by activation of presynaptic serotonin receptors, but presynaptic D1 dopamine receptors or nicotinic acetylcholine receptors could play a similar role in mammals. Those receptors enhance spontaneous release of glutamate in hippocampus, entorhinal cortex, prefrontal cortex, ventral tegmental area, and nucleus accumbens. In all of those brain areas, glutamate can activate postsynaptic receptors to elevate Ca²⁺ and engage mechanisms of early-phase long-term potentiation (LTP), including AMPA receptor insertion, and of late-phase LTP, including protein synthesis and growth. Thus, presynaptic receptors and spontaneous release may contribute to postsynaptic mechanisms of plasticity in brain regions involved in reward and memory, and could play roles in disorders that affect plasticity in those regions, including addiction, Alzheimer’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD).

Differential effects on natural reward processing in rats after repeated heroin

RATIONALE

Heroin users report reward deficits as well as reward enhancements (to drug stimuli). To better understand the causal relation between chronic heroin and alterations in natural reward processing, we used experimental techniques in animal models.

METHODS

Separate groups of rats were trained in several food reward paradigms: conditioned place preference (CPP), food-reinforced lever pressing under a progressive ratio schedule of reinforcement, free feeding, and lever pressing with conditioned reinforcement. After training, the rats were subjected to 10 daily heroin (2 mg/kg) or saline vehicle injections and tested at 3, 15, and 30 days post-treatment.

RESULTS

Repeated heroin treatment abolished the CPP and significantly reduced break points for food reward at 3, 15, and 30 days post-treatment. Repeated heroin did not affect free feeding. Finally, repeated heroin significantly enhanced responding for a food-based conditioned reinforcer.

CONCLUSIONS

Repeated heroin decreases the attractiveness of food-associated cues and reduces motivation to work for natural reward. However, it appears to enhance natural conditioned reward processes that involve the acquisition of novel responding. Thus, repeated heroin appears to produce differential effects on natural reward processing depending on the nature of the reward-directed behavior.

On the nature of extraversion: variation in conditioned contextual activation of dopamine-facilitated affective, cognitive, and motor processes

Research supports an association between extraversion and dopamine (DA) functioning. DA facilitates incentive motivation and the conditioning and incentive encoding of contexts that predict reward. Therefore, we assessed whether extraversion is related to the efficacy of acquiring conditioned contextual facilitation of three processes that are dependent on DA: motor velocity, positive affect, and visuospatial working memory. We exposed high and low extraverts to three days of association of drug reward (methylphenidate, MP) with a particular laboratory context (Paired group), a test day of conditioning, and three days of extinction in the same laboratory. A Placebo group and an Unpaired group (that had MP in a different laboratory context) served as controls. Conditioned contextual facilitation was assessed by (i) presenting video clips that varied in their pairing with drug and laboratory context and in inherent incentive value, and (ii) measuring increases from day 1 to Test day on the three processes above. Results showed acquisition of conditioned contextual facilitation across all measures to video clips that had been paired with drug and laboratory context in the Paired high extraverts, but no conditioning in the Paired low extraverts (nor in either of the control groups). Increases in the Paired high extraverts were correlated across the three measures. Also, conditioned facilitation was evident on the first day of extinction in Paired high extraverts, despite the absence of the unconditioned effects of MP. By the last day of extinction, responding returned to day 1 levels. The findings suggest that extraversion is associated with variation in the acquisition of contexts that predict reward. Over time, this variation may lead to differences in the breadth of networks of conditioned contexts. Thus, individual differences in extraversion may be maintained by activation of differentially encoded central representations of incentive contexts that predict reward.

Interactions between VTA orexin and glutamate in cue-induced reinstatement of cocaine seeking in rats

RATIONALE

Glutamate and orexin/hypocretin systems are involved in Pavlovian cue-triggered drug seeking.

OBJECTIVES

Here, we asked whether orexin and glutamate interact within ventral tegmental area (VTA) to promote reinstatement of extinguished cocaine seeking in a rat self-administration paradigm.

METHODS/RESULTS

We first found that bilateral VTA microinjections of the orexin 1 receptor (OX1R) antagonist SB-334867 (SB) or a cocktail of the AMPA and NMDA glutamate receptor antagonists CNQX/AP-5 reduced reinstatement of cocaine seeking elicited by cues. In contrast, neither of these microinjections nor systemic SB reduced cocaine-primed reinstatement. Additionally, unilateral VTA OX1R blockade combined with contralateral VTA glutamate blockade attenuated cue-induced reinstatement, indicating that VTA orexin and glutamate are simultaneously necessary for cue-induced reinstatement. We further probed the receptor specificity of glutamate actions in VTA, finding that CNQX, but not AP-5, dose-dependently attenuated cue-induced reinstatement, indicating that AMPA but not NMDA receptor transmission is required for this type of cocaine seeking. Given the necessary roles of both OX1 and AMPA receptors in VTA for cue-induced cocaine seeking, we hypothesized that these signaling pathways interact during this behavior. We found that PEPA, a positive allosteric modulator of AMPA receptors, completely reversed the SB-induced attenuation of reinstatement behavior. Intra-VTA PEPA alone did not alter cue-induced reinstatement, indicating that potentiating AMPA activity with this drug specifically compensates for OX1R blockade, rather than simply inducing or enhancing reinstatement itself.

CONCLUSIONS

These findings show that cue-induced, but not cocaine-primed, reinstatement of cocaine seeking is dependent upon orexin and AMPA receptor interactions in VTA.

Evaluation of reward from pain relief

The human experience of pain is multidimensional and comprises sensory, affective, and cognitive dimensions. Preclinical assessment of pain has been largely focused on the sensory features that contribute to nociception. The affective (aversive) qualities of pain are clinically significant but have received relatively less mechanistic investigation in preclinical models. Recently, operant behaviors such as conditioned place preference, avoidance, escape from noxious stimulus, and analgesic drug self-administration have been used in rodents to evaluate affective aspects of pain. An important advance of such operant behaviors is that these approaches may allow the detection and mechanistic investigation of spontaneous neuropathic or ongoing inflammatory/nociceptive (i.e., nonevoked) pain that is otherwise difficult to assess in nonverbal animals. Operant measures may allow the identification of mechanisms that contribute differentially to reflexive hypersensitivity or to pain affect and may inform the decision to progress novel mechanisms to clinical trials for pain therapy. Additionally, operant behaviors may allow investigation of the poorly understood mechanisms and neural circuits underlying motivational aspects of pain and the reward of pain relief.

Pain relief produces negative reinforcement through activation of mesolimbic reward-valuation circuitry

Relief of pain is rewarding. Using a model of experimental postsurgical pain we show that blockade of afferent input from the injury with local anesthetic elicits conditioned place preference, activates ventral tegmental dopaminergic cells, and increases dopamine release in the nucleus accumbens. Importantly, place preference is associated with increased activity in midbrain dopaminergic neurons and blocked by dopamine antagonists injected into the nucleus accumbens. The data directly support the hypothesis that relief of pain produces negative reinforcement through activation of the mesolimbic reward-valuation circuitry.

Cocaine-induced adaptations in metabotropic inhibitory signaling in the mesocorticolimbic system

The addictive properties of psychostimulants such as cocaine are rooted in their ability to activate the mesocorticolimbic dopamine (DA) system. This system consists primarily of dopaminergic projections arising from the ventral tegmental area (VTA) and projecting to the limbic and cortical brain regions, such as the nucleus accumbens (NAc) and prefrontal cortex (PFC). While the basic anatomy and functional relevance of the mesocorticolimbic DA system is relatively well-established, a key challenge remaining in addiction research is to understand where and how molecular adaptations and corresponding changes in function of this system facilitate a pathological desire to seek and take drugs. Several lines of evidence indicate that inhibitory signaling, particularly signaling mediated by the Gi/o class of heterotrimeric GTP-binding proteins (G proteins), plays a key role in the acute and persistent effects of drugs of abuse. Moreover, recent evidence argues that these signaling pathways are targets of drug-induced adaptations. In this review we discuss inhibitory signaling pathways involving DA and the inhibitory neurotransmitter GABA in two brain regions - the VTA and PFC - that are central to the effects of acute and repeated cocaine exposure and represent sites of adaptations linked to addiction-related behaviors including sensitization, craving, and relapse.

Presynaptic inhibition of glutamate transmission by α2 receptors in the VTA

The ventral tegmental area (VTA) forms part of the mesocorticolimbic system and plays a pivotal role in reward and reinforcing actions of drugs of abuse. Glutamate transmission within the VTA controls important aspects of goal-directed behavior and motivation. Noradrenergic receptors also present in the VTA have important functions in the modulation of neuronal activity. Here we studied the effects of α2 noradrenergic receptor activation in the alteration of glutamate neurotransmission in VTA dopaminergic neurons from male Sprague-Dawley rats. We used whole-cell patch-clamp recordings from putative VTA dopaminergic neurons and measured excitatory postsynaptic currents. Clonidine (40 μm) and UK 14,304 (40 μm), both α2 receptor agonists, reduced (approximately 40%) the amplitude of glutamate-induced excitatory postsynaptic currents. After clonidine administration, there was a dose-dependent reduction over the concentration range of 15-40 μm. Using yohimbine (20 μm) and two other α2 adrenergic receptor antagonists, idaxozan (40 μm) and atipemazole (20 μm), we demonstrated that the inhibitory action is specifically mediated by α2 receptors. Moreover, by inhibiting protein kinases with H-7 (75 μm), Rp-adenosine 3',5'-cyclic (11 μm) and chelerythrine (1 μm) it was shown that the clonidine-induced inhibition seems to involve a selective activation of the protein kinase C intracellular pathway. Increased paired-pulse ratios and changes in spontaneous and miniature excitatory postsynaptic current frequencies but not amplitudes indicated that the effect of the α2 agonist was presynaptically mediated. It is suggested that the suppression of glutamate excitatory inputs onto VTA dopaminergic neurons might be relevant in the regulation of reward and drug-seeking behaviors.

A food-associated CS activates c-Fos in VTA DA neurons and elicits conditioned approach

Neutral stimuli associated with unconditioned stimuli (USs) acquire the ability to act as conditioned stimuli (CSs), which can elicit behaviors similar to the US with which they are associated. The neural mechanisms by which this occurs are not fully known. We have previously proposed a model stipulating CSs function as such because they acquire the capacity to activate dopamine (DA) neurons at the level of the ventral tegmental area (VTA). In the present experiments we hypothesized that a food-associated CS (light), which demonstrably functions as such by eliciting conditioned responses (CRs), comes to acquire the capacity to activate VTA DA neurons. In Experiment 1, rats were allowed to eat or not eat food (food being the US). In Experiment 2, rats were trained to retrieve food pellets after light presentations (the CS) and then tested for the expression of the food checking response (the CR) with only CS presentations. In Experiment 1, eating food (exposure to the US) caused a significantly greater number of VTA DA (TH-labeled) cells to express c-Fos than not eating. In Experiment 2, CS (light) presentations caused a significant amount of conditioned approach and a significantly greater number of VTA TH-labeled (DA) cells to express c-Fos. These findings support our model stipulating that conditioned approach learning occurs when CSs acquire the capacity to cause conditioned activation of VTA DA neurons.

Extinction reveals that primary sensory cortex predicts reinforcement outcome

Primary sensory cortices are traditionally regarded as stimulus analysers. However, studies of associative learning-induced plasticity in the primary auditory cortex (A1) indicate involvement in learning, memory and other cognitive processes. For example, the area of representation of a tone becomes larger for stronger auditory memories and the magnitude of area gain is proportional to the degree that a tone becomes behaviorally important. Here, we used extinction to investigate whether 'behavioral importance' specifically reflects a sound's ability to predict reinforcement (reward or punishment) vs. to predict any significant change in the meaning of a sound. If the former, then extinction should reverse area gains as the signal no longer predicts reinforcement. Rats (n = 11) were trained to bar-press to a signal tone (5.0 kHz) for water-rewards, to induce signal-specific area gains in A1. After subsequent withdrawal of reward, A1 was mapped to determine representational areas. Signal-specific area gains, estimated from a previously established brain-behavior quantitative function, were reversed, supporting the 'reinforcement prediction' hypothesis. Area loss was specific to the signal tone vs. test tones, further indicating that withdrawal of reinforcement, rather than unreinforced tone presentation per se, was responsible for area loss. Importantly, the amount of area loss was correlated with the amount of extinction (r = 0.82, P < 0.01). These findings show that primary sensory cortical representation can encode behavioral importance as a signal's value to predict reinforcement, and that the number of cells tuned to a stimulus can dictate its ability to command behavior.

Multiple roles for orexin/hypocretin in addiction

Orexins/hypocretins are hypothalamic peptides involved in arousal and wakefulness, but also play a critical role in drug addiction and reward-related behaviors. Here, we review the roles played by orexins in a variety of animal models of drug addiction, emphasizing both commonalities and differences for orexin's involvement in seeking of the major classes of abused drugs, as well as food. One common theme that emerges is an involvement of orexins in drug seeking triggered by external stimuli (e.g., cues, contexts or stressors). We also discuss the functional neuronal circuits in which orexins are embedded, and how these circuits mediate addiction-related behaviors, with particular focus on the role of orexin and glutamate interactions within the ventral tegmental area. Finally, we attempt to contextualize the role of orexins in reward by discussing ways in which these peptides, expressed in only a few thousand neurons in the brain, can have such wide-ranging effects on behavior.

A food predictive cue must be attributed with incentive salience for it to induce c-fos mRNA expression in cortico-striatal-thalamic brain regions

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS-US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of "connectivity" between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems-the cue must also be attributed with incentive salience.

FoxP2 brainstem neurons project to sodium appetite regulatory sites

The transcription factor Forkhead box protein 2 (FoxP2) is expressed in two cell groups of the brainstem that have been implicated in sodium appetite regulation: the pre-locus coeruleus (pre-LC) and parabrachial nucleus--external lateral-inner subdivision (PBel-inner). Because the connections of these two groups are unknown, neuroanatomical tracing methods were used to define their central projections. The pre-LC outputs were first analyzed using an anterograde axonal tracer--Phaseolus vulgaris leucoagglutinin (PHAL) to construct a brain map. Next, we examined whether the FoxP2 immunoreactive (FoxP2+) neurons of the pre-LC contribute to these projections using a retrograde neuronal tracer--cholera toxin β-subunit (CTb). CTb was injected into selected brain regions identified in the anterograde tracing study. One week later the rats were killed, and brainstem sections were processed by a double immunohistochemical procedure to determine whether the FoxP2+ neurons in the pre-LC and/or PBel-inner contained CTb. FoxP2+ pre-LC neurons project to: (1) ventral pallidum; (2) substantia innominata and bed nucleus of the stria terminalis; (3) paraventricular, central medial, parafascicular, and subparafascicular parvicellular thalamic nuclei; (4) paraventricular (PVH), lateral, perifornical, dorsomedial (DMH), and parasubthalamic hypothalamic nuclei; and (5) ventral tegmental area (VTA), periaqueductal gray matter (PAG), dorsal and central linear raphe nuclei. FoxP2+ PBel-inner neurons project to the PVH and DMH, with weaker connections to the LHA, VTA, and PAG. Both the pre-LC and PBel-inner project to central sites implicated in sodium appetite, and related issues, including foraging behavior, hedonic responses to salt intake, sodium balance, and cardiovascular regulation, are discussed.

Novelty increases the mesolimbic functional connectivity of the substantia nigra/ventral tegmental area (SN/VTA) during reward anticipation: Evidence from high-resolution fMRI

Reward and novelty are potent learning signals that critically rely on dopaminergic midbrain responses. Recent findings suggest that although reward and novelty are likely to interact, both functions may be subserved by distinct neuronal clusters. We used high-resolution functional magnetic resonance imaging (fMRI) to isolate neural responses to reward and novelty within the human substantia nigra/ventral tegmental area (SN/VTA) complex to investigate the spatial delineation and integration of reward- and novelty-related activity clusters. We demonstrate that distinct clusters within the caudal portion of the medial SN/VTA and the lateral portion of the right SN are predominantly modulated by the anticipation of reward, while a more rostral part of the medial SN/VTA was exclusively modulated by novelty. In addition, the caudal medial SN/VTA cluster embodied an interaction between novelty and reward where novelty selectively increased reward-anticipation responses. This interaction, in turn, was paralleled by differences in the functional-connectivity patterns of these SN/VTA regions. Specifically, novel as compared to familiar reward-predictive stimuli increased the functional connectivity of the medial SN/VTA with mesolimbic regions, including the nucleus accumbens and the hippocampus, as well as with the primary visual cortex. This functional correlation may highlight how afferents of the medial SN/VTA provide integrative information about novelty and reward, or, alternatively, how medial SN/VTA activity may modulate memory processes for novel events associated with rewards.

Biphasic dopamine regulation in mesoaccumbens pathway in response to non-contingent binge and escalating methamphetamine regimens in the Wistar rat

RATIONALE

Methamphetamine (MA) increases extracellular dopamine (DA) and at chronic high doses induces toxicity as indicated by decreased expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT). Notably, rats will self-administer MA in escalating quantities producing such toxicity. However, the impact of MA at sub-toxic doses on DA regulation is not well established.

OBJECTIVE

The temporal dynamics of DA regulation following cessation of sub-toxic escalating and binge doses of non-contingent MA were investigated as changes therein may be associated with escalation of MA intake.

MATERIALS AND METHODS

MA was administered 3×/day using an established 14-day escalating-dose regimen (0.1-4.0 mg/kg) or a single-day binge-style administration (3 × 4 mg/kg). DA tissue content, DA turnover, TH protein, TH phosphorylation, DAT, and vesicular monoamine transporter 2 were measured in nigrostriatal and mesoaccumbens pathways 48 h and 2 weeks after MA cessation.

RESULTS

Changes in striatal DA regulation were limited to increased DA turnover. However, in the mesoaccumbens pathway, escalating MA had biphasic effects. DA was increased in ventral tegmental area (VTA) and decreased in nucleus accumbens at 48 h post-MA while the reverse was seen at 2 weeks. These changes were matched by similar changes in TH protein and, in the VTA, by changes in DAT.

CONCLUSION

Escalation of MA intake produces both transient and long-lasting effects upon DA, TH, and DAT in the mesoaccumbens pathway. The eventual decrease of DA in the VTA is speculated to contribute to craving for MA and, thus, may be associated with MA escalation and resulting dopaminergic toxicity.

The influence of reward associations on conflict processing in the Stroop task

Performance in a behavioral task can be facilitated by associating stimulus properties with reward. In contrast, conflicting information is known to impede task performance. Here we investigated how reward associations influence the within-trial processing of conflicting information using a color-naming Stroop task in which a subset of ink colors (task-relevant dimension) was associated with monetary incentives. We found that color-naming performance was enhanced on trials with potential-reward versus those without. Moreover, in potential-reward trials, typical conflict-induced performance decrements were attenuated if the incongruent word (task-irrelevant dimension) was unrelated to reward. In contrast, incongruent words that were semantically related to reward-predicting ink colors interfered with performance in potential-reward trials and even more so in no-reward trials, despite the semantic meaning being entirely task-irrelevant. These observations imply that the prospect of reward enhances the processing of task-relevant stimulus information, whereas incongruent reward-related information in a task-irrelevant dimension can impede task performance.

The effects of VTA NMDA receptor antagonism on reward-related learning and associated c-fos expression in forebrain

The mechanisms whereby reward-associated stimuli come to function as conditioned stimuli and acquire the capacity to activate the same neural regions activated by primary rewards (i.e., dopamine terminal regions) is not fully understood. We hypothesized that NMDA receptor stimulation in the VTA is necessary for the acquisition by a CS to both produce conditioned approach and activate dopamine terminal regions. Rats were tested in a conditioned approach protocol that consisted of light stimulus-food conditioning sessions (30 randomly presented light stimulus-food pellet pairings), a session with no stimuli or food and 1 session with only light stimulus (CS-only) presentations. Food trough head entries during the CS and just prior to the CS were recorded and a CS/pre-CS ratio indicating the conditioned approach response was calculated. Brain tissue was harvested after the CS-only session and processed for c-fos expression in prefrontal cortex area 2, nucleus accumbens core and shell and medial and lateral caudate. When bilateral intra-VTA microinjections of AP-5 (0, 0.25 or 0.5 μg) were made prior to each of the conditioning sessions the 0.5 μg AP-5 dose prevented the acquisition of conditioned approach; when 0.5 μg AP-5 injections were made prior to the CS-only test they failed to affect expression of the response. Also, 0.5 μg AP-5 prior to conditioning significantly reduced c-fos expression in response to the CS in nucleus accumbens core. These results suggest that VTA NMDA receptor stimulation is necessary for both the acquisition of reward-related learning and acquisition by the CS to activate dopamine terminal regions.