Selective responding of nucleus accumbens core and shell dopamine to aversively conditioned contextual and discrete stimuli

Acute ethanol disrupts conditioned inhibition in the male rat

RATIONALE

Alcohol can disrupt conditioned sexual inhibition (CSI) established by first-order conditioning in male rats. CSI can also be induced using second-order conditioning, during which male rats are trained to associate a neutral odor with a nonreceptive female. As a result, when given access to two receptive females (one scented and one unscented) during a copulatory preference test, they display CSI toward the scented female.

OBJECTIVE

The present study examined the effect of low-to-moderate doses of alcohol on CSI and brain activation following exposure to alcohol and the olfactory cue alone.

METHODS

Sexually-naïve Long-Evans rats received alternate conditioning sessions with unscented receptive or scented (almond extract) non-receptive females. Following the conditioning phase, males were injected with saline, alcohol 0.5 g/kg or 1 g/kg, 45 min before a copulatory test with two receptive females, with one bearing the olfactory cue. Fos activation was later assessed, following exposure to alcohol and the olfactory cue alone, in several brain regions involved in the expression and regulation of male sexual behavior.

RESULTS

While males in the saline group displayed sexual avoidance towards the scented female, those injected with alcohol before the copulatory test, regardless of the dose, copulated indiscriminately with both females. Subsequent exposure to alcohol and the olfactory cue alone induced different Fos expression between groups in several brain regions.

CONCLUSIONS

Low to moderate doses of alcohol disrupt conditioned sexual inhibition in male rats and induce a differential pattern of neural activation, particularly in regions involved in the expression and regulation of sexual behavior.

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Involvement of D2-like dopaminergic receptors in contextual fear conditioning in female rats: influence of estrous cycle

Introduction: Dopamine has been increasingly recognized as a key neurotransmitter regulating fear/anxiety states. Nevertheless, the influence of sex and estrous cycle differences on the role of dopamine in fear responses needs further investigation. We aimed to evaluate the effects of sulpiride (a dopaminergic D2-like receptor antagonist) on contextual fear conditioning in females while exploring the influence of the estrous cycle. Methods: First, using a contextual fear conditioning paradigm, we assessed potential differences in acquisition, expression, and extinction of the conditioned freezing response in male and female (split in proestrus/estrus and metestrus/diestrus) Wistar rats. In a second cohort, we evaluated the effects of sulpiride (20 and 40 mg/kg) on contextual conditioned fear in females during proestrus/estrus and metestrus/diestrus. Potential nonspecific effects were assessed in motor activity assays (catalepsy and open-field tests). Results: No sex differences nor estrous cycle effects on freezing behavior were observed during the fear conditioning phases. Sulpiride reduced freezing expression in female rats. Moreover, females during the proestrus/estrus phases of the estrous cycle were more sensitive to the effects of sulpiride than females in metestrus/diestrus. Sulpiride did not cause motor impairments. Discussion: Although no sex or estrous cycle differences were observed in basal conditioned fear expression and extinction, the estrous cycle seems to influence the effects of D2-like antagonists on contextual fear conditioning.

Motivational competition and the paraventricular thalamus

Although significant progress has been made in understanding the behavioral and brain mechanisms for motivational systems, much less is known about competition between motivational states or motivational conflict (e.g., approach - avoidance conflict). Despite being produced under diverse conditions, behavior during motivational competition has two signatures: bistability and metastability. These signatures reveal the operation of positive feedback mechanisms in behavioral selection. Different neuronal architectures can instantiate this selection to achieve bistability and metastability in behavior, but each relies on circuit-level inhibition to achieve rapid and stable selection between competing tendencies. Paraventricular thalamus (PVT) is identified as critical to this circuit level inhibition, resolving motivational competition via its extensive projections to local inhibitory networks in the ventral striatum and extended amygdala, enabling adaptive responding under motivational conflict.

c-Fos expression following context conditioning and deep brain stimulation in the bed nucleus of the stria terminalis in rats

Deep brain stimulation (DBS) in the bed nucleus of the stria terminalis (BST), a region implicated in the expression of anxiety, shows promise in psychiatric patients, but its effects throughout the limbic system are largely unknown. In male Wistar rats, we first evaluated the neural signature of contextual fear (N = 16) and next, of the anxiolytic effects of high-frequency electrical stimulation in the BST (N = 31), by means of c-Fos protein expression. In non-operated animals, we found that the left medial anterior BST displayed increased c-Fos expression in anxious (i.e., context-conditioned) versus control subjects. Moreover, control rats showed asymmetric expression in the basolateral amygdala (BLA) (i.e., higher intensities in the right hemisphere), which was absent in anxious animals. The predominant finding in rats receiving bilateral BST stimulation was a striking increase in c-Fos expression throughout much of the left hemisphere, which was not confined to the predefined regions of interest. To conclude, we found evidence for lateralized c-Fos expression during the expression of contextual fear and anxiolytic high-frequency electrical stimulation of the BST, particularly in the medial anterior BST and BLA. In addition, we observed an extensive and unexpected left-sided c-Fos spread following bilateral stimulation in the BST.

Evaluation of electrochemical methods for tonic dopamine detection in vivo

Dysfunction in dopaminergic neuronal systems underlie a number of neurologic and psychiatric disorders such as Parkinson's disease, drug addiction, and schizophrenia. Dopamine systems communicate via two mechanisms, a fast "phasic" release (sub-second to second) that is related to salient stimuli and a slower "tonic" release (minutes to hours) that regulates receptor tone. Alterations in tonic levels are thought to be more critically important in enabling normal motor, cognitive, and motivational functions, and dysregulation in tonic dopamine levels are associated with neuropsychiatric disorders. Therefore, development of neurochemical recording techniques that enable rapid, selective, and quantitative measurements of changes in tonic extracellular levels are essential in determining the role of dopamine in both normal and disease states. Here, we review state-of-the-art advanced analytical techniques for in vivo detection of tonic levels, with special focus on electrochemical techniques for detection in humans.

Dopamine D1-like receptors in the dorsomedial prefrontal cortex regulate contextual fear conditioning

RATIONALE

Dopamine D1 receptor (D1R) signalling is involved in contextual fear conditioning. The D1R antagonist SCH23390 impairs the acquisition of contextual fear when administered systemically or infused locally into the dorsal hippocampus or basolateral amygdala.

OBJECTIVES

We determined if state dependency may account for the impairment in contextual fear conditioning caused by systemic SCH23390 administration. We also examined if the dorsomedial prefrontal cortex (dmPFC), nucleus accumbens (NAc), and ventral hippocampus (VH) are involved in mediating the effect of systemic SCH23390 treatment on contextual fear conditioning.

METHODS

In experiment 1, SCH23390 (0.1 mg/kg) or vehicle was given before contextual fear conditioning and/or retrieval. In experiment 2, SCH23390 (2.5 μg/0.5 uL) or vehicle was infused locally into dmPFC, NAc, or VH before contextual fear conditioning, and retrieval was tested drug-free. Freezing was quantified as a measure of contextual fear.

RESULTS

In experiment 1, SCH23390 given before conditioning or before both conditioning and retrieval decreased freezing at retrieval, whereas SCH23390 given only before retrieval had no effect. In experiment 2, SCH23390 infused into dmPFC before conditioning decreased freezing at retrieval, while infusion of SCH23390 into NAc or VH had no effect.

CONCLUSIONS

The results of experiment 1 confirm those of previous studies indicating that D1Rs are required for the acquisition but not retrieval of contextual fear and rule out state dependency as an explanation for these findings. Moreover, the results of experiment 2 provide evidence that dmPFC is also part of the neural circuitry through which D1R signalling regulates contextual fear conditioning.

A dopaminergic switch for fear to safety transitions

Overcoming aversive emotional memories requires neural systems that detect when fear responses are no longer appropriate so that they can be extinguished. The midbrain ventral tegmental area (VTA) dopamine system has been implicated in reward and more broadly in signaling when a better-than-expected outcome has occurred. This suggests that it may be important in guiding fear to safety transitions. We report that when an expected aversive outcome does not occur, activity in midbrain dopamine neurons is necessary to extinguish behavioral fear responses and engage molecular signaling events in extinction learning circuits. Furthermore, a specific dopamine projection to the nucleus accumbens medial shell is partially responsible for this effect. In contrast, a separate dopamine projection to the medial prefrontal cortex opposes extinction learning. This demonstrates a novel function for the canonical VTA-dopamine reward system and reveals opposing behavioral roles for different dopamine neuron projections in fear extinction learning.

Fear memories are overcome only when it is ascertained that fearful responses are not appropriate. Here the authors demonstrate that activity in dopamine neurons is necessary to extinguish fear responses and two distinct dopamine neuron projections exert opposing effects on extinction learning.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Interaction between DRD2 variation and sound environment on mood and emotion-related brain activity

Sounds, like music and noise, are capable of reliably affecting individuals' mood and emotions. However, these effects are highly variable across individuals. A putative source of variability is genetic background. Here we explored the interaction between a functional polymorphism of the dopamine D2 receptor gene (DRD2 rs1076560, G>T, previously associated with the relative expression of D2S/L isoforms) and sound environment on mood and emotion-related brain activity. Thirty-eight healthy subjects were genotyped for DRD2 rs1076560 (G/G=26; G/T=12) and underwent functional magnetic resonance imaging (fMRI) during performance of an implicit emotion-processing task while listening to music or noise. Individual variation in mood induction was assessed before and after the task. Results showed mood improvement after music exposure in DRD2GG subjects and mood deterioration after noise exposure in GT subjects. Moreover, the music, as opposed to noise environment, decreased the striatal activity of GT subjects as well as the prefrontal activity of GG subjects while processing emotional faces. These findings suggest that genetic variability of dopamine receptors affects sound environment modulations of mood and emotion processing.

A role for phasic dopamine release within the nucleus accumbens in encoding aversion: a review of the neurochemical literature

Survival is dictated by an organism's fitness in approaching positive stimuli and avoiding harm. While a rich literature outlines a role for mesolimbic dopamine in reward and appetitive behaviors, dopamine's involvement in aversion and avoidance behaviors remains controversial. Debate surrounding dopamine's function in the processing of negative stimuli likely stems from conflicting results reported by single-unit electrophysiological studies. Indeed, a number of studies suggest that midbrain dopaminergic cells are inhibited by the presentation of negative or fearful stimuli, while others report no change, or even an increase, in their activity. These disparate results may be due to population heterogeneity. Recent evidence demonstrates that midbrain dopamine neurons are heterogeneous in their projection targets, responses to environmental stimuli, pharmacology, and influences on motivated behavior. Thus, in order to assemble an accurate account of dopamine function during aversive stimulus experience and related behavior, it is necessary to examine the functional output of dopamine neural activity at mesolimbic terminal regions. This Review presents a growing body of evidence that dopamine release in the nucleus accumbens encodes not only reward, but also aversion. For example, our laboratory recently utilized fast-scan cyclic voltammetry to show that real-time changes in accumbal dopamine release are detected when animals are presented with predictors of aversion and its avoidance. These data, along with other reports, support a considerably more nuanced view of dopamine neuron function, wherein accumbal dopamine release is differentially modulated by positive and negative affective stimuli to promote adaptive behaviors.

Involvement of the dopaminergic system in the consolidation of fear conditioning in hippocampal CA3 subregion

The hippocampus, the primary brain structure related to learning and memory, receives sparse but comprehensive dopamine innervations and contains dopamine D1 and D2 receptors. Systematic hippocampal dopaminergic dysfunction can cause deficits in spatial working memory and impair consolidation of contextual fear memories. CA3 is involved in the rapid acquisition of new memories and has extensive nerve fibre connections with other brain structures such as CA1, the amygdala, and the medial prefrontal cortex (mPFC). A bidirectional fibrous connection between CA3 and the amygdala reflects the importance of CA3 in fear conditioning. The present study evaluated the effects of a 6-OHDA lesion in CA3 on the acquisition and expression of conditioned fear. The results showed CA3 involvement in the expression but not the acquisition of conditioned fear. Injection of SCH23390 and quinpirole into the bilateral CA3 attenuated a conditioned fear-related freezing response, whereas SKF38393 and sulpiride were not associated with this effect. The present study found that a 6-OHDA lesion in CA3 up-regulated the expression of GluR1 in BLA and down-regulated NR2B in CA1 and the basolateral amygdala (BLA). Our data suggest that dopamine depletion in hippocampal subdivision CA3 may not be necessary for the acquisition of conditioned fear, but the expression of conditioned fear is likely dependent on the integrity of mesohippocampal dopaminergic connections. It is probable that both D1 and D2 dopaminergic receptors modulate the expression of conditioned fear. Changes in the expression of NR2B and GluR1 indicate that CA3 may modulate the activities of other brain structures.

Free-operant avoidance behavior by rats after reinforcer revaluation using opioid agonists and D-amphetamine

The associative processes that support free-operant instrumental avoidance behavior are still unknown. We used a revaluation procedure to determine whether the performance of an avoidance response is sensitive to the current value of the aversive, negative reinforcer. Rats were trained on an unsignaled, free-operant lever press avoidance paradigm in which each response avoided or escaped shock and produced a 5 s feedback stimulus. The revaluation procedure consisted of noncontingent presentations of the shock in the absence of the lever either paired or unpaired with systemic morphine and in a different cohort with systemic d-amphetamine. Rats were then tested drug free during an extinction test. In both the d-amphetamine and morphine groups, pairing of the drug and shock decreased subsequent avoidance responding during the extinction test, suggesting that avoidance behavior was sensitive to the current incentive value of the aversive negative reinforcer. Experiment 2 used central infusions of D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), a mu-opioid receptor agonist, in the periacqueductal gray and nucleus accumbens shell to revalue the shock. Infusions of DAMGO in both regions replicated the effects seen with systemic morphine. These results are the first to demonstrate the impact of revaluation of an aversive reinforcer on avoidance behavior using pharmacological agents, thereby providing potential therapeutic targets for the treatment of avoidance behavior symptomatic of anxiety disorders.

The role of the nucleus accumbens shell in the mediation of the reinforcing properties of a safety signal in free-operant avoidance: dopamine-dependent inhibitory effects of d-amphetamine

Safety signals (SSs) have been shown to reinforce instrumental avoidance behavior due to their ability to signal the absence of an aversive event; however, little is known of their neural mediation. This study investigated whether infusions of d-amphetamine in the nucleus accumbens (Nac), previously shown to potentiate responding for appetitive conditioned reinforcers (CRfs), also regulate avoidance responding for a SS. Rats were trained on a free-operant task in which lever-press responses avoided shock and were reinforced with an auditory SS. Rats were then cannulated in the Nac core (NacC) or shell (NacS) and infused with d-amphetamine and, in separate NacS groups, other drugs, before extinction sessions with the SS present or absent following responding. Selective effects of d-amphetamine were found in the NacS, but not in the NacC, when the SS was present in the session. A significant increase in response rate during the presentation of the SS reflected a disruption of its fear-inhibiting properties. In parallel, a decrease in avoidance response rate reflected the reduced influence of the SS as a CRf. Inactivation of the NacS reduced avoidance responding only when the SS was present in the session, whereas the D1-D2 DA receptor antagonist α-flupenthixol reduced responding both before and during the SS regardless of the presence of the SS. Atomoxetine (ATO), a selective noradrenaline reuptake inhibitor, had no effect on responding. These results indicate a role for the NacS in the mediation of the conditioned reinforcing properties of a SS. These effects appear to be modulated by dopaminergic mechanisms but seem distinct from those previously reported with food-related CRfs.

Striatal dopamine D1 receptor is essential for contextual fear conditioning

Fear memory is critical for animals to trigger behavioural adaptive responses to potentially threatening stimuli, while too much or inappropriate fear may cause psychiatric problems. Numerous studies have shown that the amygdala, hippocampus and medial prefrontal cortex play important roles in Pavlovian fear conditioning. Recently, we showed that striatal neurons are required for the formation of the auditory fear memory when the unconditioned stimulus is weak. Here, we found that selective ablation of striatal neurons strongly diminished contextual fear conditioning irrespective of the intensity of footshock. Furthermore, contextual fear conditioning was strongly reduced in striatum-specific dopamine D1 receptor knockout mice. On the other hand, striatum-specific dopamine D2 receptor knockout mice showed freezing responses comparable to those of control mice. These results suggest that striatal D1 receptor is essential for contextual fear conditioning.

Dopamine and extinction: a convergence of theory with fear and reward circuitry

Research on dopamine lies at the intersection of sophisticated theoretical and neurobiological approaches to learning and memory. Dopamine has been shown to be critical for many processes that drive learning and memory, including motivation, prediction error, incentive salience, memory consolidation, and response output. Theories of dopamine's function in these processes have, for the most part, been developed from behavioral approaches that examine learning mechanisms in reward-related tasks. A parallel and growing literature indicates that dopamine is involved in fear conditioning and extinction. These studies are consistent with long-standing ideas about appetitive-aversive interactions in learning theory and they speak to the general nature of cellular and molecular processes that underlie behavior. We review the behavioral and neurobiological literature showing a role for dopamine in fear conditioning and extinction. At a cellular level, we review dopamine signaling and receptor pharmacology, cellular and molecular events that follow dopamine receptor activation, and brain systems in which dopamine functions. At a behavioral level, we describe theories of learning and dopamine function that could describe the fundamental rules underlying how dopamine modulates different aspects of learning and memory processes.

Dopamine D2 receptors modulate the expression of contextual conditioned fear: role of the ventral tegmental area and the basolateral amygdala

Although dopaminergic systems are more commonly associated with the reinforcing effects of various stimuli, numerous reports have demonstrated a relationship between changes in dopaminergic transmission and aversive situations. In the present study, we examined the involvement of D1-like and D2-like receptors in the expression of conditioned freezing using the context as the conditioned stimulus. Intraperitoneal injections of the D1 agonist SKF38393 or the D1 antagonist SCH23390 did not change the conditioned freezing in rats subjected to the contextual fear paradigm. In contrast, intraperitoneal injections of the D2 agonist quinpirole and the D2 antagonist sulpiride caused a significant dose-dependent reduction in the expression of contextual conditioned freezing. As these data may reflect that the systemic manipulations acted on dopaminergic receptors in different brain areas, the effects of administration of quinpirole and sulpiride into the ventral tegmental area (VTA) and the basolateral amygdala complex (BLA) on the expression of contextual conditioned freezing were also evaluated. Intra-VTA quinpirole and intra-BLA sulpiride injections reduced the conditioned freezing response; intra-VTA sulpiride and intra-BLA quinpirole injections had no significant effects. These data suggest that D2-like receptors, but not D1-like receptors, play an important role in the expression of contextual conditioned freezing. Quinpirole may act at D2 presynaptic receptors located in the VTA, decreasing dopamine levels in the terminal fields of the mesolimbic pathway. The effects of sulpiride, in contrast, appear to be triggered by an action on postsynaptic dopaminergic receptors located in the BLA. However, it cannot be totally excluded that the injected solutions did not also affect neighboring amygdalar regions. Together with previous findings, the present data suggest the need to consider dopaminergic mechanisms in the mesolimbic circuit as novel targets for the pharmacological treatment of fear-related disorders, especially post-traumatic stress disorder.

Individual variation in resisting temptation: implications for addiction

When exposed to the sights, sounds, smells and/or places that have been associated with rewards, such as food or drugs, some individuals have difficulty resisting the temptation to seek out and consume them. Others have less difficulty restraining themselves. Thus, Pavlovian reward cues may motivate maladaptive patterns of behavior to a greater extent in some individuals than in others. We are just beginning to understand the factors underlying individual differences in the extent to which reward cues acquire powerful motivational properties, and therefore, the ability to act as incentive stimuli. Here we review converging evidence from studies in both human and non-human animals suggesting that a subset of individuals are more "cue reactive", in that certain reward cues are more likely to attract these individuals to them and motivate actions to get them. We suggest that those individuals for whom Pavlovian reward cues become especially powerful incentives may be more vulnerable to impulse control disorders, such as binge eating and addiction.

Heads for learning, tails for memory: reward, reinforcement and a role of dopamine in determining behavioral relevance across multiple timescales

Dopamine has long been tightly associated with aspects of reinforcement learning and motivation in simple situations where there are a limited number of stimuli to guide behavior and constrained range of outcomes. In naturalistic situations, however, there are many potential cues and foraging strategies that could be adopted, and it is critical that animals determine what might be behaviorally relevant in such complex environments. This requires not only detecting discrepancies with what they have recently experienced, but also identifying similarities with past experiences stored in memory. Here, we review what role dopamine might play in determining how and when to learn about the world, and how to develop choice policies appropriate to the situation faced. We discuss evidence that dopamine is shaped by motivation and memory and in turn shapes reward-based memory formation. In particular, we suggest that hippocampal-striatal-dopamine networks may interact to determine how surprising the world is and to either inhibit or promote actions at time of behavioral uncertainty.

Aversive stimuli differentially modulate real-time dopamine transmission dynamics within the nucleus accumbens core and shell

Although fear directs adaptive behavioral responses, how aversive cues recruit motivational neural circuitry is poorly understood. Specifically, while it is known that dopamine (DA) transmission within the nucleus accumbens (NAc) is imperative for mediating appetitive motivated behaviors, its role in aversive behavior is controversial. It has been proposed that divergent phasic DA transmission following aversive events may correspond to segregated mesolimbic dopamine pathways; however, this prediction has never been tested. Here, we used fast-scan cyclic voltammetry to examine real-time DA transmission within NAc core and shell projection systems in response to a fear-evoking cue. In male Sprague Dawley rats, we first demonstrate that a fear cue results in decreased DA transmission within the NAc core, but increased transmission within the NAc shell. We examined whether these changes in DA transmission could be attributed to modulation of phasic transmission evoked by cue presentation. We found that cue presentation decreased the probability of phasic DA release in the core, while the same cue enhanced the amplitude of release events in the NAc shell. We further characterized the relationship between freezing and both changes in DA as well as local pH. Although we found that both analytes were significantly correlated with freezing in the NAc across the session, changes in DA were not strictly associated with freezing while basic pH shifts in the core more consistently followed behavioral expression. Together, these results provide the first real-time neurochemical evidence that aversive cues differentially modulate distinct DA projection systems.

Leptin regulates dopamine responses to sustained stress in humans

Neural systems that identify and respond to salient stimuli are critical for survival in a complex and changing environment. In addition, interindividual differences, including genetic variation and hormonal and metabolic status likely influence the behavioral strategies and neuronal responses to environmental challenges. Here, we examined the relationship between leptin allelic variation and plasma leptin levels with DAD2/3R availability in vivo as measured with [(11)C]raclopride PET at baseline and during a standardized pain stress challenge. Allelic variation in the leptin gene was associated with varying levels of dopamine release in response to the pain stressor, but not with baseline D2/3 receptor availability. Circulating leptin was also positively associated with stress-induced dopamine release. These results show that leptin serves as a regulator of neuronal function in humans and provides an etiological mechanism for differences in dopamine neurotransmission in response to salient stimuli as related to metabolic function. The capacity for leptin to influence stress-induced dopaminergic function is of importance for pathological states where dopamine is thought to play an integral role, such as mood, substance-use disorders, eating disorders, and obesity.

Subsecond dopamine release in the nucleus accumbens predicts conditioned punishment and its successful avoidance

The mesolimbic dopamine system is believed to be a pathway that processes rewarding information. While previous studies have also implicated a general role for dopamine in punishment and its avoidance, the precise nature of subsecond dopamine release during these phenomena remains unknown. Here, we used fast-scan cyclic voltammetry to investigate whether subsecond dopamine release events in the nucleus accumbens encode cues predicting the avoidance of punishment during behavior maintained in a signaled footshock avoidance procedure. In this task, rats could initiate an avoidance response by pressing a lever within a warning period, preventing footshock. Alternatively, once footshocks commenced, animals could initiate an escape response by pressing the lever, terminating footshock. This design allowed us to assess subsecond dopamine release events during the presentation of a warning signal, safety periods, and two distinct behavioral responses. We found that release consistently increased upon presentation of the warning signal in a manner that reliably predicted successful punishment avoidance. We also observed subsecond dopamine release during the safety period, as occurs following the receipt of reward. Conversely, we observed a decrease in release at the warning signal during escape responses. Because of this finding, we next assessed dopamine release in a conditioned fear model. As seen during escape responses, we observed a time-locked decrease in dopamine release upon presentation of a cue conditioned to inescapable footshock. Together, these data show that subsecond fluctuations in mesolimbic dopamine release predict when rats will successfully avoid punishment and differentially encode cues related to aversive outcomes.

Enhanced striatal sensitivity to aversive reinforcement in adolescents versus adults

Neurodevelopmental changes in mesolimbic regions are associated with adolescent risk-taking behavior. Numerous studies have shown exaggerated activation in the striatum in adolescents compared with children and adults during reward processing. However, striatal sensitivity to aversion remains elusive. Given the important role of the striatum in tracking both appetitive and aversive events, addressing this question is critical to understanding adolescent decision-making, as both positive and negative factors contribute to this behavior. In this study, human adult and adolescent participants performed a task in which they received squirts of appetitive or aversive liquid while undergoing fMRI, a novel approach in human adolescents. Compared with adults, adolescents showed greater behavioral and striatal sensitivity to both appetitive and aversive stimuli, an effect that was exaggerated in response to delivery of the aversive stimulus. Collectively, these findings contribute to understanding how neural responses to positive and negative outcomes differ between adolescents and adults and how they may influence adolescent behavior.

Age-associated changes in the hippocampal-ventral striatum-ventral tegmental loop that impact learning, prediction, and context discrimination

Studies of the neural mechanisms of navigation and context discrimination have generated a powerful heuristic for understanding how neural codes, circuits, and computations contribute to accurate behavior as animals traverse and learn about spatially extended environments. It is assumed that memories are updated as a result of spatial experience. The mechanism, however, for such a process is not clear. Here we suggest that one revealing approach to study this issue is to integrate our knowledge about limbic system mediated navigation and context discrimination with knowledge about how midbrain neural circuitry mediates decision-making. This perspective should lead to new and specific neural theories about how choices that we make during navigation determine what information is ultimately learned and remembered. This same circuitry may be involved when past experiences come to bias future spatial perceptions and response selection. With old age come not only important changes in limbic system operations, but also significant decline in the function of midbrain regions that underlie accurate and efficient decisions. Thus, suboptimal accuracy of spatial context-based decision-making may be, at least in part, responsible for the common observation of spatial memory decline in old age.

Action dominates valence in anticipatory representations in the human striatum and dopaminergic midbrain

The acquisition of reward and the avoidance of punishment could logically be contingent on either emitting or withholding particular actions. However, the separate pathways in the striatum for go and no-go appear to violate this independence, instead coupling affect and effect. Respect for this interdependence has biased many studies of reward and punishment, so potential action-outcome valence interactions during anticipatory phases remain unexplored. In a functional magnetic resonance imaging study with healthy human volunteers, we manipulated subjects' requirement to emit or withhold an action independent from subsequent receipt of reward or avoidance of punishment. During anticipation, in the striatum and a lateral region within the substantia nigra/ventral tegmental area (SN/VTA), action representations dominated over valence representations. Moreover, we did not observe any representation associated with different state values through accumulation of outcomes, challenging a conventional and dominant association between these areas and state value representations. In contrast, a more medial sector of the SN/VTA responded preferentially to valence, with opposite signs depending on whether action was anticipated to be emitted or withheld. This dominant influence of action requires an enriched notion of opponency between reward and punishment.

Alterations in cognitive function and behavioral response to amphetamine induced by prenatal inflammation are dependent on the stage of pregnancy

Maternal infection during human pregnancy has been associated with the development of schizophrenia in the adult offspring. The stage of development and the maternal inflammatory response to infection, which undergoes quantitative and qualitative changes throughout gestation, are thought to determine critical windows of vulnerability for the developing brain. In order to investigate how these two factors may contribute to the outcome in the offspring, we studied the inflammatory response to turpentine (TURP) injection (100 μl/dam) and its consequences in the adult offspring, in pregnant rats at gestational day (GD) 15 or 18, which correspond to late first and early second trimester of human pregnancy, respectively. Maternal inflammatory response to TURP was different between the two GDs, with fever and circulating levels of the pro-inflammatory interleukin (IL)-6 significantly attenuated at GD 18, compared to GD 15. In the adult offspring, TURP challenge at GD 15 induced a significant decrease in pre-pulse inhibition (PPI) of acoustic startle, increased latency in the cued task of the Morris-water maze, prolonged conditioned fear response and enhanced locomotor effect of amphetamine. In contrast, the same immune challenge at GD 18 induced only a prolonged conditioned fear response. These results suggest a window of vulnerability at GD 15, at which TURP seems to affect several behaviors that are strongly modulated by dopamine. This was supported by increased tyrosine hydroxylase expression in the nucleus accumbens of the adult offspring of mothers treated at GD 15.

Long-term memory for pavlovian fear conditioning requires dopamine in the nucleus accumbens and basolateral amygdala

The neurotransmitter dopamine (DA) is essential for learning in a Pavlovian fear conditioning paradigm known as fear-potentiated startle (FPS). Mice lacking the ability to synthesize DA fail to learn the association between the conditioned stimulus and the fear-inducing footshock. Previously, we demonstrated that restoration of DA synthesis to neurons of the ventral tegmental area (VTA) was sufficient to restore FPS. Here, we used a target-selective viral restoration approach to determine which mesocorticolimbic brain regions receiving DA signaling from the VTA require DA for FPS. We demonstrate that restoration of DA synthesis to both the basolateral amygdala (BLA) and nucleus accumbens (NAc) is required for long-term memory of FPS. These data provide crucial insight into the dopamine-dependent circuitry involved in the formation of fear-related memory.

Adenosine A1 and A2A receptors are not upstream of caffeine's dopamine D2 receptor-dependent aversive effects and dopamine-independent rewarding effects

Caffeine is widely consumed throughout the world, but little is known about the mechanisms underlying its rewarding and aversive properties. We show that pharmacological antagonism of dopamine not only blocks conditioned place aversion to caffeine, but also reveals dopamine blockade-induced conditioned place preferences. These aversive effects are mediated by the dopamine D(2) receptor, as knockout mice showed conditioned place preferences in response to doses of caffeine that C57Bl/6 mice found aversive. Furthermore, these aversive responses appear to be centrally mediated, as a quaternary analog of caffeine failed to produce conditioned place aversion. Although the adenosine A(2A) receptor is important for caffeine's physiological effects, this receptor seems only to modulate the appetitive and aversive effects of caffeine. A(2A) receptor knockout mice showed stronger dopamine-dependent aversive responses to caffeine than did C57Bl/6 mice, which partially obscured the dopamine-independent and A(2A) receptor-independent preferences. Additionally, the A(1) receptor, alone or in combination with the A(2A) receptor, does not seem to be important for caffeine's rewarding or aversive effects. Finally, excitotoxic lesions of the tegmental pedunculopontine nucleus revealed that this brain region is not involved in dopamine blockade-induced caffeine reward. These data provide surprising new information on the mechanism of action of caffeine, indicating that adenosine receptors do not mediate caffeine's appetitive and aversive effects. We show that caffeine has an atypical reward mechanism, independent of the dopaminergic system and the tegmental pedunculopontine nucleus, and provide additional evidence in support of a role for the dopaminergic system in aversive learning.

"Sex, drugs and the brain": the interaction between drugs of abuse and sexual behavior in the female rat

Preclinical and clinical research investigating female sexual motivation has lagged behind research on male sexual function. The present review summarizes recent advances in our understanding of the specific roles of various brain areas, as well as our understanding of the role of dopaminergic neurotransmission in sexual motivation of the female rat. A number of behavioral paradigms that can be used to thoroughly evaluate sexual behavior in the female rat are first discussed. Although traditional assessment of the reflexive, lordosis posture has been useful in understanding the neuroanatomical and neurochemical systems that contribute to copulatory behavior, the additional behavioral paradigms described in this review have helped us expand our understanding of appetitive and consumatory behavioral patterns that better assess sexual motivation - the equivalent of "desire" in humans. A summary of numerous lesion studies indicates that different areas of the brain, including forebrain and midbrain structures, work together to produce the complex repertoire of female sexual behavior. In addition, by investigating the effects of commonly addictive drugs, we are beginning to elucidate the role of dopaminergic neurotransmission in female sexual motivation. Consequently, research in this area may contribute to meaningful advances in the treatment of human female sexual dysfunction.

Ability of predator odour exposure to elicit conditioned versus sensitised post traumatic stress disorder-like behaviours, and forebrain deltaFosB expression, in rats

At present, exposure of a rodent to the odour of a predator is one of the most common animal models of post traumatic stress disorder (PTSD). Despite this, the model remains incompletely characterized, particularly in regard to within subject assessment of major PTSD-like behaviours. In an attempt to redress this situation, we have extensively characterized the two broad categories of behaviour that are considered to characterize PTSD, that is sensitized behaviours such as social withdrawal and hypervigilance and conditioned behaviours such as avoidance of trauma linked cues. Specifically, we determined the presence and duration of both conditioned and sensitized behaviours, in the same cohort of animals, after three exposures to predator odour. Conditioned fear was assessed on the basis of inhibition of locomotor activity upon return to context 2, 7, 14, 21, and 28 days after the last odour exposure session. To assess the impact on sensitization behaviours, we monitored acoustic startle responses and social interaction behaviour 4, 9, 16, 23, and 30 days after the last exposure session. In addition to examining the behavioural consequences associated with odour exposure, we also determined the key brain regions that were activated using DeltaFosB immunohistochemistry. Our results show that the two groups of behaviours thought to characterize PTSD (conditioned and sensitized) do not travel together in the predator odour model, with clear evidence of enduring changes in conditioned fear but little evidence of changes in social interaction or acoustic startle. With regard to associated patterns of activity in the brain, we observed that odour-exposed animals exhibited significantly higher numbers of FosB-positive nuclei in only the medial prefrontal cortex (mPFC), a finding that can be viewed as being consistent with the observed behavioural changes.

The D2 dopamine receptor gene variant C957T affects human fear conditioning and aversive priming

Polymorphisms of DRD2 and ANKK1 have been associated with psychiatric syndromes where there is believed to be an underlying learning process deficit such as addiction, post-traumatic stress disorder and psychopathy. We investigated the effects of the DRD2 C957T and ANKK1 TaqIA single nucleotide polymorphism (SNP), which have been associated with psychopathic traits in alcoholic patients, on fear conditioning and aversive priming in healthy volunteers. We found that the DRD2 C957T SNP, but not the ANKK1 TaqIA SNP, was associated with both differential conditioning of the skin conductance response and the aversive priming effect. There were no differences between the genotype groups with respect to the extinction of the skin-conductance conditioned response. These results suggest that the C957T SNP could be related to learning differences associated with the risk of developing psychiatric disorders in individuals that are carriers of the C homozygous genotype. Our genetic data raise the possibility that the dopaminergic system functional variations determined by this SNP could affect fear learning.

Reciprocal patterns of c-Fos expression in the medial prefrontal cortex and amygdala after extinction and renewal of conditioned fear

After extinction of conditioned fear, memory for the conditioning and extinction experiences becomes context dependent. Fear is suppressed in the extinction context, but renews in other contexts. This study characterizes the neural circuitry underlying the context-dependent retrieval of extinguished fear memories using c-Fos immunohistochemistry. After fear conditioning and extinction to an auditory conditioned stimulus (CS), rats were presented with the extinguished CS in either the extinction context or a second context, and then sacrificed. Presentation of the CS in the extinction context yielded low levels of conditioned freezing and induced c-Fos expression in the infralimbic division of the medial prefrontal cortex, the intercalated nuclei of the amygdala, and the dentate gyrus (DG). In contrast, presentation of the CS outside of the extinction context yielded high levels of conditioned freezing and induced c-Fos expression in the prelimbic division of the medial prefrontal cortex, the lateral and basolateral nuclei of the amygdala, and the medial division of the central nucleus of the amygdala. Hippocampal areas CA1 and CA3 exhibited c-Fos expression when the CS was presented in either context. These data suggest that the context specificity of extinction is mediated by prefrontal modulation of amygdala activity, and that the hippocampus has a fundamental role in contextual memory retrieval.

Activation of the septohippocampal system differentiates anxiety from fear in startle paradigms

It has been suggested that different brain areas are involved in the modulation and expression of fear and anxiety. In the present study we investigated these potential differences by using the fear-potentiated-startle (FPS) and light-enhanced-startle (LES) paradigms to differentiate between fear and anxiety, respectively. Male Wistar rats were tested in the FPS and LES paradigm and perfused 1 h after the test session. Fos immunoreactivity (IR) was quantified in 21 brain areas and compared between FPS, LES and four control conditions. Both FPS and LES procedures significantly enhanced the acoustic startle response. A principal component analysis of Fos-IR-data showed that 70% of the changes in Fos-IR could be explained by three independent components: an arousal-component, identifying brain areas known to be activated under conditions of vigilance, arousal and stress, a LES- and an FPS-component. The LES component comprised the septohippocampal system and functionally interrelated areas including nucleus accumbens, anterior cingulate cortex, lateral habenula and supramammillary areas, but not the dorsolateral part of the bed nucleus of the stria terminalis. The central amygdaloid nucleus and the dorsolateral part of the bed nucleus of the stria terminalis loaded exclusively on the FPS component. Analysis of the separate brain areas revealed significantly higher Fos-IR in LES relative to FPS in the anterior cingulate cortex, nucleus accumbens shell, lateral septum, lateral habenula and area postrema. We conclude that the neural circuitry activated during FPS and LES shows clear differences. In anxiety as induced by LES, activation of the septohippocampal system and related areas seems to play a major role. In fear as induced by FPS, the central amygdaloid nucleus and the dorsolateral part of the bed nucleus of the stria terminalis loaded on the same component, but Fos-IR observed in these brain regions did not differentiate between anxiety and fear. Furthermore, principal-component analysis appears a useful tool in detecting and describing correlated changes in patterns of neuronal activity.

Involvement of dopaminergic mechanisms in the nucleus accumbens core and shell subregions in the expression of fear conditioning

The involvement of dopamine (DA) mechanisms in the nucleus accumbens (NAC) in fear conditioning has been proposed by many studies that have challenged the view that the NAC is solely involved in the modulation of appetitive processes. However, the role of the core and shell subregions of the NAC in aversive conditioning remains unclear. The present study examined DA release in these NAC subregions using microdialysis during the expression of fear memory. Guide cannulae were implanted in rats in the NAC core and shell. Five days later, the animals received 10 footshocks (0.6 mA, 1 s duration) in a distinctive cage A (same context). On the next day, dialysis probes were inserted through the guide cannulae into the NAC core and shell subregions, and the animals were behaviorally tested for fear behavior either in the same context (cage A) or in a novel context (cage B). Dialysates were collected every 5 min for 90 min and analyzed by high-performance liquid chromatography. The rats exhibited a significant fear response in cage A but not in cage B. Moreover, increased DA levels in both NAC subregions were observed 5-25 min after the beginning of the test when the animals were tested in the same context compared with accumbal DA levels from rats tested in the different context. These findings suggest that DA mechanisms in both the NAC core and shell may play an important role in the expression of contextual fear memory.

The role of the striatum in aversive learning and aversive prediction errors

Neuroeconomic studies of decision making have emphasized reward learning as critical in the representation of value-driven choice behaviour. However, it is readily apparent that punishment and aversive learning are also significant factors in motivating decisions and actions. In this paper, we review the role of the striatum and amygdala in affective learning and the coding of aversive prediction errors (PEs). We present neuroimaging results showing aversive PE-related signals in the striatum in fear conditioning paradigms with both primary (shock) and secondary (monetary loss) reinforcers. These results and others point to the general role for the striatum in coding PEs across a broad range of learning paradigms and reinforcer types.

Selectively increased trace conditioning under the neurotensin agonist PD 149163 in an aversive procedure in which SR 142948A was without intrinsic effect

There is evidence to suggest that neurotensin (NT) may enhance cognitive function. The present study, therefore, examined the role of NT in associative learning between a conditioned stimulus (CS) and an unconditioned stimulus (UCS). This was tested in a trace procedure using conditioned suppression of drinking with a noise CS and foot shock UCS. We compared the effects of an NT agonist (PD 149163, 0.25 and 1 mg/kg) with those of an NT antagonist (SR 142948A, 0.01 and 0.1 mg/kg). Conditioning after drug treatment was followed by drug-free tests of associative learning. At 0.25 but not 1 mg/kg, PD 149163 selectively increased conditioning over the trace interval: there was no such increased conditioning in the 0s group. This increased conditioning over the trace is an effect that is reliably produced by dopamine (DA) agonists in the same procedure. However, dissimilar to the effects of DA agonists, conditioning to box context, was reduced under PD 149163. Doses of SR 142948A, selected on the basis of their effects in similar aversively motivated tests of latent inhibition, were without intrinsic effect in the present procedure. The dose-related dissociation between trace and contextual conditioning effects under PD 149163 is considered as cognitive enhancement.

Low tryptophan diet increases stress-sensitivity, but does not affect habituation in rats

Cerebral dysfunction of 5-HT (serotonin) has been associated with stress response and with affective disorders. Stress alone is insufficient to induce depression, since only a minor proportion of subjects that have experienced stressful life events develop depressive episodes. We investigated whether long-term brain 5-HT depletion induced in rats by a diet with low content of its precursor tryptophan affects stress-responsiveness in rats. Stress-sensitivity was measured through various physiological parameters and by measuring the rats' response to acoustic stimuli. One group of rats was subjected to daily acoustic stimulus sessions for 5 days. Other groups received both immobilization stress and acoustic stimulus sessions daily for either 9 days (chronic experiment) or 1 day (acute experiment). A low tryptophan diet led to decreases in plasma tryptophan levels, low ratio of tryptophan/large neutral amino acid, whole blood 5-HT, and neuronal 5-HT content in the Dorsal and Median Raphe Nuclei, as well as altered c-fos expression in the brain. Without concomitant immobilization, the diet alone did not affect reactivity and habituation to acoustic stimuli, although plasma corticosterone levels, but not the adrenal weights, were increased on day 5. Low tryptophan and chronic immobilization stress together with the acoustic testing procedure increased adrenal weight, plasma corticosterone levels and reactivity to the acoustic stimuli, but not the rate of habituation to acoustic stimuli. These results show that cerebral dysfunction of serotonin achieved through a low tryptophan diet, increases the sensitivity of rats to external and stressful stimuli, but does not impair the capacity to adapt to these stimuli. Accordingly, brain-serotonin modulates reactivity to stress, but not stress coping.

Neuroimmune mechanisms of opioid-mediated conditioned immunomodulation

Morphine administration elicits pronounced effects on the immune system, including decreases in natural killer (NK) cell activity and lymphocyte mitogenic responsiveness. These immune alterations can become conditioned to environmental stimuli that predict morphine as a result of Pavlovian conditioning processes. Prior work in our laboratory has shown that acute morphine exposure produces dopamine-dependent reductions of NK cell activity that are mediated peripherally by neuropeptide Y Y1 receptors. The present study examined the involvement of dopamine D1 and neuropeptide Y Y1 receptors in the conditioned immunomodulatory effects of morphine. Rats received two conditioning sessions during which an injection of morphine was paired with a distinctive environment which served as the conditioned stimulus (CS). The results show that systemic administration of the D1 antagonist SCH-23390 prior to CS re-exposure prevented the conditioned suppression of splenic NK activity but did not alter conditioned decreases in mitogen-induced lymphocyte proliferation. Furthermore, bilateral microinjections of SCH-23390 directly into the nucleus accumbens shell fully blocked conditioned changes in NK activity. In a subsequent manipulation, subcutaneous injection of the Y1 receptor antagonist BIBP3226 prior to CS re-exposure was also shown to prevent conditioned effects on NK activity. Collectively, these findings provide evidence that the nucleus accumbens shell plays an important role in conditioned immunomodulation and further suggest that the conditioned and unconditioned immunomodulatory effects of opioids involve similar receptor mechanisms.

Extracellular signal-regulated kinase 1/2 involvement in the enhancement of contextual fear conditioning by nicotine

Contextual fear conditioning is enhanced by nicotine, but the cellular mechanisms underlying this effect are unknown. Extracellular signal regulated kinase 1/2 (ERK 1/2) has been shown to play an integral role in the formation of contextual fear memories. As such, it is possible that ERK 1/2 is involved in the enhancement of contextual fear conditioning by nicotine. To determine whether ERK 1/2 plays a role in this enhancement, a dose of SL327 (a selective, systemic ERK 1/2 inhibitor) that is subthreshold for inhibiting contextual fear conditioning was coadministered with nicotine prior to training, testing, or both training and testing of contextual fear conditioning in C57BL/6 mice. When administered prior to training, this subthreshold dose of SL327 attenuated the enhancement of contextual fear conditioning by nicotine to levels similar to those of vehicle-treated animals. When administered prior to testing, the subthreshold dose of SL327 did not significantly alter conditioning. These results suggest that activation of ERK 1/2 by nicotine during acquisition leads to an enhancement of contextual fear conditioning.

An intact dopaminergic system is required for context-conditioned release of 5-HT in the nucleus accumbens of postweaning isolation-reared rats

We investigated the effect of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT) on extracellular dopamine and 5-HT levels in the nucleus accumbens of group- and isolation-reared rats. Microdialysis with high-performance liquid chromatography-electrochemical detection was used to quantify dopamine and 5-HT efflux in the nucleus accumbens following foot shock and in association with a conditioned emotional response (CER). Isolation- and group-reared rats received i.p. injections of either saline (0.9%) or AMPT (200 mg/kg) 15 h and 2 h prior to sampling. There was no significant difference between saline-treated isolation- or group-reared rats for basal efflux of dopamine or 5-HT, however as expected, AMPT-treatment significantly reduced dopamine efflux in both groups to an equivalent level (50-55% saline-treated controls). Exposure to mild foot shock stimulated basal dopamine efflux in saline-treated groups only, although the effect was significantly greater in isolation-reared rats. In AMPT-treated rats, foot shock did not affect basal dopamine efflux in either rearing group. Foot shock evoked a prolonged increase in 5-HT efflux in both isolation- and group-reared saline-treated rats but had no effect on 5-HT efflux in AMPT-treated rats. In response to CER, isolation-rearing was associated with significantly greater efflux of both dopamine and 5-HT in saline-treated rats, compared to saline-treated, group-reared controls. However in AMPT-treated rats, efflux of dopamine or 5-HT did not change in response to CER. These data suggest that unconditioned or conditioned stress-induced changes in 5-HT release of the nucleus accumbens are dependent upon intact catecholaminergic neurotransmission. Furthermore, as the contribution of noradrenaline to catecholamine efflux in the nucleus accumbens is relatively minor compared to dopamine, our findings suggest that dopamine efflux in the nucleus accumbens is important for the local regulation of 5-HT release in this region. Finally, these findings implicate the isolation-enhanced presynaptic dopamine function in the accumbens with the augmented ventral striatal 5-HT neurotransmission characterized by isolation-reared rats.

Multiple Dopamine Functions at Different Time Courses

Many lesion studies report an amazing variety of deficits in behavioral functions that cannot possibly be encoded in great detail by the relatively small number of midbrain dopamine neurons. Although hoping to unravel a single dopamine function underlying these phenomena, electrophysiological and neurochemical studies still give a confusing, mutually exclusive, and partly contradictory account of dopamine's role in behavior. However, the speed of observed phasic dopamine changes varies several thousand fold, which offers a means to differentiate the behavioral relationships according to their time courses. Thus dopamine is involved in mediating the reactivity of the organism to the environment at different time scales, from fast impulse responses related to reward via slower changes with uncertainty, punishment, and possibly movement to the tonic enabling of postsynaptic motor, cognitive, and motivational systems deficient in Parkinson's disease.

Effects of dorsal and ventral hippocampal NMDA stimulation on nucleus accumbens core and shell dopamine release

This study has analysed the effects of infusing N-methyl-D-aspartate (NMDA) into either the ventral or dorsal hippocampus on dopamine (DA) transmission in the nucleus accumbens (NAC) core or shell for the first time. Dopamine was measured using in vivo microdialysis with high performance liquid chromatography with electrochemical detection (HPLC-EC). Unilateral NMDA infusion (0.5 microg) into the ventral hippocampus (VH) increased extracellular DA levels in NAC shell during the first 30 min following infusion compared to saline (SAL) infused animals. In contrast, NAC core DA levels were unaffected. NMDA infusion into the dorsal hippocampus (DH) led to a decrease in NAC core DA levels; this effect was not observed in the SAL-infused group. DA levels in NAC shell remained unaltered. At the end of the experiments, we examined the response to a systemic amphetamine (AMPH) injection of 1mg/kg on extracellular DA levels of the NAC core and shell. Interestingly, on2ly animals previously infused with NMDA into the VH exhibited a sensitized DA response in the NAC shell in response to the AMPH injection. We can conclude that VH activation has an acute stimulatory effect on DA release in the shell and that DH activation has a suppressive effect on extracellular DA levels in the core.