Maximizing Electrochemical Information: A Perspective on Background-Inclusive Fast Voltammetry

This perspective encompasses a focused review of the literature leading to a tipping point in electroanalytical chemistry. We tie together the threads of a "revolution" quietly in the making for years through the work of many authors. Long-held misconceptions about the use of background subtraction in fast voltammetry are addressed. We lay out future advantages that accompany background-inclusive voltammetry, particularly when paired with modern machine-learning algorithms for data analysis.

Dopamine projections to the basolateral amygdala drive the encoding of identity-specific reward memories

To make adaptive decisions, we build an internal model of the associative relationships in an environment and use it to make predictions and inferences about specific available outcomes. Detailed, identity-specific cue-reward memories are a core feature of such cognitive maps. Here we used fiber photometry, cell-type and pathway-specific optogenetic manipulation, Pavlovian cue-reward conditioning and decision-making tests in male and female rats, to reveal that ventral tegmental area dopamine (VTADA) projections to the basolateral amygdala (BLA) drive the encoding of identity-specific cue-reward memories. Dopamine is released in the BLA during cue-reward pairing; VTADA→BLA activity is necessary and sufficient to link the identifying features of a reward to a predictive cue but does not assign general incentive properties to the cue or mediate reinforcement. These data reveal a dopaminergic pathway for the learning that supports adaptive decision-making and help explain how VTADA neurons achieve their emerging multifaceted role in learning.

Anterior cingulate and medial prefrontal cortex alcohol cue reactivity varies as a function of drink preference in alcohol use disorder

BACKGROUND

Functional MRI visual cue reactivity studies have not considered that brain responses to various alcohol-containing beverage types may vary as a function of an individual's drinking patterns and preferences. This study tested whether the brain's reward system responds differently to visual cues associated with an individuals' most commonly consumed ("preferred") alcohol beverage compared with less commonly consumed ("non-preferred") alcohol beverages in individuals with alcohol use disorder (AUD).

METHODS

Participants (N=70) with current AUD completed a standard visual alcohol cue reactivity procedure during fMRI and reported recent alcohol use through the Timeline Followback interview. Alcohol use patterns were used to infer drink preference. Repeated measure ANCOVAs were used to evaluate differences in subjective craving (alcohol urge) and neural reactivity to cues of individual's "preferred" versus "non-preferred" alcohol beverages.

RESULTS

Fifty-four (77%) participants were determined to have a "preferred" alcohol beverage, as defined by their pattern of alcohol use. These participants reported greater subjective alcohol urge (p=0.02) and activation in the anterior cingulate cortex (ACC) (p=0.005) and medial prefrontal cortex (mPFC) (p=0.001)) in response to visual cues associated with their "preferred" versus "non-preferred" alcohol beverage. Individuals with an alcohol preference did not differ from those with no alcohol preference on subjective or neural responses to their "preferred" and "non-preferred" alcohol cues.

DISCUSSION

Results suggest alcohol cue-elicited subjective and neural responses vary as a function of alcohol beverage preference in individuals with AUD and a behaviorally defined alcohol preference. Stronger ACC and mPFC activation may reflect greater subjective value of an individual's "preferred" alcohol beverage cue.

A dual-pathway architecture enables chronic stress to promote habit formation

Chronic stress can change how we learn and, thus, how we make decisions by promoting the formation of inflexible, potentially maladaptive, habits. Here we investigated the neuronal circuit mechanisms that enable this. Using a multifaceted approach in male and female mice, we reveal a dual pathway, amygdala-striatal, neuronal circuit architecture by which a recent history of chronic stress shapes learning to disrupt flexible goal-directed behavior in favor of inflexible habits. Chronic stress inhibits activity of basolateral amygdala projections to the dorsomedial striatum to impede the action-outcome learning that supports flexible, goal-directed decisions. Stress also increases activity in direct central amygdala projections to the dorsomedial striatum to promote the formation of rigid, inflexible habits. Thus, stress exerts opposing effects on two amygdala-striatal pathways to promote premature habit formation. These data provide neuronal circuit insights into how chronic stress shapes learning and decision making, and help understand how stress can lead to the disrupted decision making and pathological habits that characterize substance use disorders and other psychiatric conditions.

Dorsomedial prefrontal cortex activation disrupts Pavlovian incentive motivation

The dorsomedial prefrontal cortex (dmPFC) is known to make important contributions to flexible, reward-motivated behavior. However, it remains unclear if the dmPFC is involved in regulating the expression of Pavlovian incentive motivation, the process through which reward-paired cues promote instrumental reward-seeking behavior, which is modeled in rats using the Pavlovian-instrumental transfer (PIT) task. The current study examined this question using a bidirectional chemogenetic strategy in which inhibitory (hM4Di) or excitatory (hM3Dq) designer G-protein coupled receptors were virally expressed in dmPFC neurons, allowing us to later stimulate or inhibit this region by administering CNO prior to PIT testing. We found that dmPFC inhibition did not alter the tendency for a reward-paired cue to instigate instrumental reward-seeking behavior, whereas dmPFC stimulation disrupted the expression of this motivational influence. Neither treatment altered cue-elicited anticipatory activity at the reward-delivery port, indicating that dmPFC stimulation did not lead to more widespread motor suppression. A reporter-only control experiment indicated that our CNO treatment did not have non-specific behavioral effects. Thus, the dmPFC does not mediate the expression of Pavlovian incentive motivation but instead has the capacity to exert pronounced inhibitory control over this process, suggesting that it is involved in adaptively regulating cue-motivated behavior.

Amygdala-cortical collaboration in reward learning and decision making

Adaptive reward-related decision making requires accurate prospective consideration of the specific outcome of each option and its current desirability. These mental simulations are informed by stored memories of the associative relationships that exist within an environment. In this review, I discuss recent investigations of the function of circuitry between the basolateral amygdala (BLA) and lateral (lOFC) and medial (mOFC) orbitofrontal cortex in the learning and use of associative reward memories. I draw conclusions from data collected using sophisticated behavioral approaches to diagnose the content of appetitive memory in combination with modern circuit dissection tools. I propose that, via their direct bidirectional connections, the BLA and OFC collaborate to help us encode detailed, outcome-specific, state-dependent reward memories and to use those memories to enable the predictions and inferences that support adaptive decision making. Whereas lOFC→BLA projections mediate the encoding of outcome-specific reward memories, mOFC→BLA projections regulate the ability to use these memories to inform reward pursuit decisions. BLA projections to lOFC and mOFC both contribute to using reward memories to guide decision making. The BLA→lOFC pathway mediates the ability to represent the identity of a specific predicted reward and the BLA→mOFC pathway facilitates understanding of the value of predicted events. Thus, I outline a neuronal circuit architecture for reward learning and decision making and provide new testable hypotheses as well as implications for both adaptive and maladaptive decision making.

A bidirectional corticoamygdala circuit for the encoding and retrieval of detailed reward memories

Adaptive reward-related decision making often requires accurate and detailed representation of potential available rewards. Environmental reward-predictive stimuli can facilitate these representations, allowing one to infer which specific rewards might be available and choose accordingly. This process relies on encoded relationships between the cues and the sensory-specific details of the rewards they predict. Here, we interrogated the function of the basolateral amygdala (BLA) and its interaction with the lateral orbitofrontal cortex (lOFC) in the ability to learn such stimulus-outcome associations and use these memories to guide decision making. Using optical recording and inhibition approaches, Pavlovian cue-reward conditioning, and the outcome-selective Pavlovian-to-instrumental transfer (PIT) test in male rats, we found that the BLA is robustly activated at the time of stimulus-outcome learning and that this activity is necessary for sensory-specific stimulus-outcome memories to be encoded, so they can subsequently influence reward choices. Direct input from the lOFC was found to support the BLA in this function. Based on prior work, activity in BLA projections back to the lOFC was known to support the use of stimulus-outcome memories to influence decision making. By multiplexing optogenetic and chemogenetic inhibition we performed a serial circuit disconnection and found that the lOFC→BLA and BLA→lOFC pathways form a functional circuit regulating the encoding (lOFC→BLA) and subsequent use (BLA→lOFC) of the stimulus-dependent, sensory-specific reward memories that are critical for adaptive, appetitive decision making.

Nucleus Accumbens Cholinergic Interneurons Oppose Cue-Motivated Behavior

BACKGROUND

Environmental reward-predictive stimuli provide a major source of motivation for adaptive reward pursuit behavior. This cue-motivated behavior is known to be mediated by the nucleus accumbens (NAc) core. The cholinergic interneurons in the NAc are tonically active and densely arborized and thus well suited to modulate NAc function. However, their causal contribution to adaptive behavior remains unknown. Here we investigated the function of NAc cholinergic interneurons in cue-motivated behavior.

METHODS

We used chemogenetics, optogenetics, pharmacology, and a translationally analogous Pavlovian-to-instrumental transfer behavioral task designed to assess the motivating influence of a reward-predictive cue over reward-seeking actions in male and female rats.

RESULTS

The data show that NAc cholinergic interneuron activity critically opposes the motivating influence of appetitive cues. Chemogenetic inhibition of NAc cholinergic interneurons augmented cue-motivated behavior. Optical stimulation of acetylcholine release from NAc cholinergic interneurons prevented cues from invigorating reward-seeking behavior, an effect that was mediated by activation of β2-containing nicotinic acetylcholine receptors.

CONCLUSIONS

NAc cholinergic interneurons provide a critical regulatory influence over adaptive cue-motivated behavior and therefore are a potential therapeutic target for the maladaptive cue-motivated behavior that marks many psychiatric conditions, including addiction and depression.

Mesolimbic dopamine projections mediate cue-motivated reward seeking but not reward retrieval in rats

Efficient foraging requires an ability to coordinate discrete reward-seeking and reward-retrieval behaviors. We used pathway-specific chemogenetic inhibition to investigate how rats’ mesolimbic and mesocortical dopamine circuits contribute to the expression and modulation of reward seeking and retrieval. Inhibiting ventral tegmental area dopamine neurons disrupted the tendency for reward-paired cues to motivate reward seeking, but spared their ability to increase attempts to retrieve reward. Similar effects were produced by inhibiting dopamine inputs to nucleus accumbens, but not medial prefrontal cortex. Inhibiting dopamine neurons spared the suppressive effect of reward devaluation on reward seeking, an assay of goal-directed behavior. Attempts to retrieve reward persisted after devaluation, indicating they were habitually performed as part of a fixed action sequence. Our findings show that complete bouts of reward seeking and retrieval are behaviorally and neurally dissociable from bouts of reward seeking without retrieval. This dichotomy may prove useful for uncovering mechanisms of maladaptive behavior.

Distinct cortical-amygdala projections drive reward value encoding and retrieval

The value of an anticipated rewarding event is a crucial component of the decision to engage in its pursuit. But little is known of the networks responsible for encoding and retrieving this value. By using biosensors and pharmacological manipulations, we found that basolateral amygdala (BLA) glutamatergic activity tracks and mediates encoding and retrieval of the state-dependent incentive value of a palatable food reward. Projection-specific, bidirectional chemogenetic and optogenetic manipulations revealed that the orbitofrontal cortex (OFC) supports the BLA in these processes. Critically, the function of ventrolateral and medial OFC→BLA projections is doubly dissociable. Whereas lateral OFC→BLA projections are necessary and sufficient for encoding of the positive value of a reward, medial OFC→BLA projections are necessary and sufficient for retrieving this value from memory. These data reveal a new circuit for adaptive reward valuation and pursuit and provide insight into the dysfunction in these processes that characterizes myriad psychiatric diseases.

Habits Are Negatively Regulated by Histone Deacetylase 3 in the Dorsal Striatum

BACKGROUND

Optimal behavior and decision making result from a balance of control between two strategies, one cognitive/goal-directed and one habitual. These systems are known to rely on the anatomically distinct dorsomedial and dorsolateral striatum, respectively. However, the transcriptional regulatory mechanisms required to learn and transition between these strategies are unknown. Here we examined the role of one chromatin-based transcriptional regulator, histone modification via histone deacetylases (HDACs), in this process.

METHODS

We combined procedures that diagnose behavioral strategy in rats with pharmacological and viral-mediated HDAC manipulations, chromatin immunoprecipitation, and messenger RNA quantification.

RESULTS

The results indicate that dorsal striatal HDAC3 activity constrains habit formation. Systemic HDAC inhibition following instrumental (lever press → reward) conditioning increased histone acetylation throughout the dorsal striatum and accelerated habitual control of behavior. HDAC3 was removed from the promoters of key learning-related genes in the dorsal striatum as habits formed with overtraining and with posttraining HDAC inhibition. Decreasing HDAC3 function, either by selective pharmacological inhibition or by expression of dominant-negative mutated HDAC3, in either the dorsolateral striatum or the dorsomedial striatum accelerated habit formation, while HDAC3 overexpression in either region prevented habit.

CONCLUSIONS

These results challenge the strict dissociation between dorsomedial striatum and dorsolateral striatum function in goal-directed versus habitual behavioral control and identify dorsostriatal HDAC3 as a critical molecular directive of the transition to habit. Because this transition is disrupted in many neurodegenerative and psychiatric diseases, these data suggest a potential molecular mechanism for the negative behavioral symptoms of these conditions and a target for therapeutic intervention.

Regulation of habit formation in the dorsal striatum

Habits are an essential and pervasive component of our daily lives that allow us to efficiently perform routine tasks. But their disruption contributes to the symptoms that underlie many psychiatric diseases. Emerging data are revealing the cellular and molecular mechanisms of habit formation in the dorsal striatum. New data suggest that in both the dorsolateral and dorsomedial striatum histone deacetylase (HDAC) activity acts as a critical negative regulator of the transcriptional processes underlying habit formation. In this review, we discuss this recent work and draw conclusions relevant to the treatment of diseases marked by maladaptive habits.

Basolateral Amygdala to Orbitofrontal Cortex Projections Enable Cue-Triggered Reward Expectations

To make an appropriate decision, one must anticipate potential future rewarding events, even when they are not readily observable. These expectations are generated by using observable information (e.g., stimuli or available actions) to retrieve often quite detailed memories of available rewards. The basolateral amygdala (BLA) and orbitofrontal cortex (OFC) are two reciprocally connected key nodes in the circuitry supporting such outcome-guided behaviors. But there is much unknown about the contribution of this circuit to decision making, and almost nothing known about the whether any contribution is via direct, monosynaptic projections, or the direction of information transfer. Therefore, here we used designer receptor-mediated inactivation of OFC→BLA or BLA→OFC projections to evaluate their respective contributions to outcome-guided behaviors in rats. Inactivation of BLA terminals in the OFC, but not OFC terminals in the BLA, disrupted the selective motivating influence of cue-triggered reward representations over reward-seeking decisions as assayed by Pavlovian-to-instrumental transfer. BLA→OFC projections were also required when a cued reward representation was used to modify Pavlovian conditional goal-approach responses according to the reward's current value. These projections were not necessary when actions were guided by reward expectations generated based on learned action-reward contingencies, or when rewards themselves, rather than stored memories, directed action. These data demonstrate that BLA→OFC projections enable the cue-triggered reward expectations that can motivate the execution of specific action plans and allow adaptive conditional responding.SIGNIFICANCE STATEMENT Deficits anticipating potential future rewarding events are associated with many psychiatric diseases. Presently, we know little about the neural circuits supporting such reward expectation. Here we show that basolateral amygdala to orbitofrontal cortex projections are required for expectations of specific available rewards to influence reward seeking and decision making. The necessity of these projections was limited to situations in which expectations were elicited by reward-predictive cues. These projections therefore facilitate adaptive behavior by enabling the orbitofrontal cortex to use environmental stimuli to generate expectations of potential future rewarding events.

Amygdala mu-opioid receptors mediate the motivating influence of cue-triggered reward expectations

Environmental reward-predictive stimuli can retrieve from memory a specific reward expectation that allows them to motivate action and guide choice. This process requires the basolateral amygdala (BLA), but little is known about the signaling systems necessary within this structure. Here we examined the role of the neuromodulatory opioid receptor system in the BLA in such cue-directed action using the outcome-specific Pavlovian-to-instrumental transfer (PIT) test in rats. Inactivation of BLA mu-, but not delta-opioid receptors was found to dose-dependently attenuate the ability of a reward-predictive cue to selectively invigorate the performance of actions directed at the same unique predicted reward (i.e. to express outcome-specific PIT). BLA mu-opioid receptor inactivation did not affect the ability of a reward itself to similarly motivate action (outcome-specific reinstatement), suggesting a more selective role for the BLA mu-opioid receptor in the motivating influence of currently unobservable rewarding events. These data reveal a new role for BLA mu-opioid receptor activation in the cued recall of precise reward memories and the use of this information to motivate specific action plans.

Modulation of cue-triggered reward seeking by cholinergic signaling in the dorsomedial striatum

The dorsomedial striatum (DMS) has been strongly implicated in flexible, outcome-based decision making, including the outcome-specific Pavlovian-to-instrumental transfer effect (PIT), which measures the tendency for a reward-predictive cue to preferentially motivate actions that have been associated with the predicted reward over actions associated with different rewards. Although the neurochemical underpinnings of this effect are not well understood, there is growing evidence that striatal acetylcholine signaling may play an important role. This study investigated this hypothesis by assessing the effects of intra-DMS infusions of the nicotinic antagonist mecamylamine or the muscarinic antagonist scopolamine on expression of specific PIT in rats. These treatments produced dissociable behavioral effects. Mecamylamine infusions enhanced rats' tendency to use specific cue-elicited outcome expectations to select whichever action was trained with the predicted outcome, relative to their performance when tested after vehicle infusions. In contrast, scopolamine infusions appeared to render instrumental performance insensitive to this motivational influence of reward-paired cues. These drug treatments had no detectable effect on conditioned food cup approach behavior, indicating that they selectively perturbed cue-guided action selection without producing more wide-ranging alterations in behavioral control. Our findings reveal an important role for DMS acetylcholine signaling in modulating the impact of cue-evoked reward expectations on instrumental action selection.

Nucleus Accumbens Acetylcholine Receptors Modulate Dopamine and Motivation

Environmental reward-predictive cues can motivate reward-seeking behaviors. Although this influence is normally adaptive, it can become maladaptive in disordered states, such as addiction. Dopamine release in the nucleus accumbens core (NAc) is known to mediate the motivational impact of reward-predictive cues, but little is known about how other neuromodulatory systems contribute to cue-motivated behavior. Here, we examined the role of the NAc cholinergic receptor system in cue-motivated behavior using a Pavlovian-to-instrumental transfer task designed to assess the motivating influence of a reward-predictive cue over an independently-trained instrumental action. Disruption of NAc muscarinic acetylcholine receptor activity attenuated, whereas blockade of nicotinic receptors augmented cue-induced invigoration of reward seeking. We next examined a potential dopaminergic mechanism for this behavioral effect by combining fast-scan cyclic voltammetry with local pharmacological acetylcholine receptor manipulation. The data show evidence of opposing modulation of cue-evoked dopamine release, with muscarinic and nicotinic receptor antagonists causing suppression and augmentation, respectively, consistent with the behavioral effects of these manipulations. In addition to demonstrating cholinergic modulation of naturally-evoked and behaviorally-relevant dopamine signaling, these data suggest that NAc cholinergic receptors may gate the expression of cue-motivated behavior through modulation of phasic dopamine release.

Nucleus accumbens core dopamine signaling tracks the need-based motivational value of food-paired cues

Environmental reward-predictive stimuli provide a major source of motivation for instrumental reward-seeking activity and this has been linked to dopamine signaling in the nucleus accumbens (NAc) core. This cue-induced incentive motivation can be quite general, not restricted to instrumental actions that earn the same unique reward, and is also typically regulated by one's current need state, such that cues only motivate actions when this is adaptive. But it remains unknown whether cue-evoked dopamine signaling is similarly regulated by need state. Here, we used fast-scan cyclic voltammetry to monitor dopamine concentration changes in the NAc core of rats during a Pavlovian-to-instrumental transfer task in which the motivating influence of two cues, each signaling a distinct food reward (sucrose or food pellets), over an action earning a third unique food reward (polycose) was assessed in a state of hunger and of satiety. Both cues elicited a robust NAc dopamine response when hungry. The magnitude of the sucrose cue-evoked dopamine response correlated with the Pavlovian-to-instrumental transfer effect that was selectively induced by this stimulus. Satiety attenuated these cue-evoked dopamine responses and behavioral responding, even though rats had never experienced the specific food rewards in this state. These data demonstrate that cue-evoked NAc core responses are sensitive to current need state, one critical variable that determines the current adaptive utility of cue-motivated behavior. Food-predictive stimuli motivate food-seeking behavior. Here, we show that food cues evoke a robust nucleus accumbens core dopamine response when hungry that correlates with the cue's ability to invigorate general food seeking. This response is attenuated when sated, demonstrating that food cue-evoked accumbens dopamine responses are sensitive to the need state information that determines the current adaptive utility of cue-motivated action.

Inflated reward value in early opiate withdrawal

Through incentive learning, the emotional experience of a reward in a relevant need state (e.g. hunger for food) sets the incentive value that guides the performance of actions that earn that reward when the need state is encountered again. Opiate withdrawal has been proposed as a need state in which, through experience, opiate value can be increased, resulting in escalated opiate self-administration. Endogenous opioid transmission plays anatomically dissociable roles in the positive emotional experience of reward consumption and incentive learning. We, therefore, sought to determine if chronic opiate exposure and withdrawal produces a disruption in the fundamental incentive learning process such that reward seeking, even for non-opiate rewards, can become maladaptive, inconsistent with the emotional experience of reward consumption and irrespective of need. Rats trained to earn sucrose or water on a reward-seeking chain were treated with morphine (10-30 mg/kg, s.c.) daily for 11 days prior to testing in withdrawal. Opiate-withdrawn rats showed elevated reward-seeking actions, but only after they experienced the reward in withdrawal, an effect that was strongest in early (1-3 days), as opposed to late (14-16 days), withdrawal. This was sufficient to overcome a negative reward value change induced by sucrose experience in satiety and, in certain circumstances, was inconsistent with the emotional experience of reward consumption. Lastly, we found that early opiate withdrawal-induced inflation of reward value was blocked by inactivation of basolateral amygdala mu opioid receptors. These data suggest that in early opiate withdrawal, the incentive learning process is disrupted, resulting in maladaptive reward seeking.

Dynamic mesolimbic dopamine signaling during action sequence learning and expectation violation

Prolonged mesolimbic dopamine concentration changes have been detected during spatial navigation, but little is known about the conditions that engender this signaling profile or how it develops with learning. To address this, we monitored dopamine concentration changes in the nucleus accumbens core of rats throughout acquisition and performance of an instrumental action sequence task. Prolonged dopamine concentration changes were detected that ramped up as rats executed each action sequence and declined after earned reward collection. With learning, dopamine concentration began to rise increasingly earlier in the execution of the sequence and ultimately backpropagated away from stereotyped sequence actions, becoming only transiently elevated by the most distal and unexpected reward predictor. Action sequence-related dopamine signaling was reactivated in well-trained rats if they became disengaged in the task and in response to an unexpected change in the value, but not identity of the earned reward. Throughout training and test, dopamine signaling correlated with sequence performance. These results suggest that action sequences can engender a prolonged mode of dopamine signaling in the nucleus accumbens core and that such signaling relates to elements of the motivation underlying sequence execution and is dynamic with learning, overtraining and violations in reward expectation.

The Origins and Organization of Vertebrate Pavlovian Conditioning

Pavlovian conditioning is the process by which we learn relationships between stimuli and thus constitutes a basic building block for how the brain constructs representations of the world. We first review the major concepts of Pavlovian conditioning and point out many of the pervasive misunderstandings about just what conditioning is. This brings us to a modern redefinition of conditioning as the process whereby experience with a conditional relationship between stimuli bestows these stimuli with the ability to promote adaptive behavior patterns that did not occur before the experience. Working from this framework, we provide an in-depth analysis of two examples, fear conditioning and food-based appetitive conditioning, which include a description of the only partially overlapping neural circuitry of each. We also describe how these circuits promote the basic characteristics that define Pavlovian conditioning, such as error-correction-driven regulation of learning.

The basolateral amygdala in reward learning and addiction

Sophisticated behavioral paradigms partnered with the emergence of increasingly selective techniques to target the basolateral amygdala (BLA) have resulted in an enhanced understanding of the role of this nucleus in learning and using reward information. Due to the wide variety of behavioral approaches many questions remain on the circumscribed role of BLA in appetitive behavior. In this review, we integrate conclusions of BLA function in reward-related behavior using traditional interference techniques (lesion, pharmacological inactivation) with those using newer methodological approaches in experimental animals that allow in vivo manipulation of cell type-specific populations and neural recordings. Secondly, from a review of appetitive behavioral tasks in rodents and monkeys and recent computational models of reward procurement, we derive evidence for BLA as a neural integrator of reward value, history, and cost parameters. Taken together, BLA codes specific and temporally dynamic outcome representations in a distributed network to orchestrate adaptive responses. We provide evidence that experiences with opiates and psychostimulants alter these outcome representations in BLA, resulting in long-term modified action.

Basolateral amygdala rapid glutamate release encodes an outcome-specific representation vital for reward-predictive cues to selectively invigorate reward-seeking actions

Environmental stimuli have the ability to generate specific representations of the rewards they predict and in so doing alter the selection and performance of reward-seeking actions. The basolateral amygdala participates in this process, but precisely how is unknown. To rectify this, we monitored, in near-real time, basolateral amygdala glutamate concentration changes during a test of the ability of reward-predictive cues to influence reward-seeking actions (Pavlovian-instrumental transfer). Glutamate concentration was found to be transiently elevated around instrumental reward seeking. During the Pavlovian-instrumental transfer test these glutamate transients were time-locked to and correlated with only those actions invigorated by outcome-specific motivational information provided by the reward-predictive stimulus (i.e., actions earning the same specific outcome as predicted by the presented CS). In addition, basolateral amygdala AMPA, but not NMDA glutamate receptor inactivation abolished the selective excitatory influence of reward-predictive cues over reward seeking. These data support [corrected] the hypothesis that transient glutamate release in the BLA can encode the outcome-specific motivational information provided by reward-predictive stimuli.

Probing the neurochemical correlates of motivation and decision making

Online electrochemical detection techniques are the state-of-the-art for evaluating chemical communication in the brain underlying motivated behavior and decision making. In this Viewpoint, we discuss avenues for future technological development, as well as the requirement for increasingly sophisticated and interdisciplinary behavioral analysis.

Phasic mesolimbic dopamine release tracks reward seeking during expression of Pavlovian-to-instrumental transfer

BACKGROUND

Recent theories addressing mesolimbic dopamine's role in reward processing emphasize two apparently distinct functions, one in reinforcement learning (i.e., prediction error) and another in incentive motivation (i.e., the invigoration of reward seeking elicited by reward-paired cues). Here, we evaluate the latter.

METHODS

Using fast-scan cyclic voltammetry, we monitored, in real time, dopamine release in the nucleus accumbens core of rats (n = 9) during a Pavlovian-to-instrumental transfer task in which the effects of a reward-predictive cue on an independently trained instrumental action were assessed. Voltammetric data were parsed into slow and phasic components to determine whether these forms of dopamine signaling were differentially related to task performance.

RESULTS

We found that a reward-paired cue, which increased reward-seeking actions, induced an increase in phasic mesolimbic dopamine release and produced slower elevations in extracellular dopamine. Interestingly, phasic dopamine release was temporally related to and positively correlated with lever-press activity generally, while slow dopamine changes were not significantly related to such activity. Importantly, the propensity of the reward-paired cue to increase lever pressing was predicted by the amplitude of phasic dopamine release events, indicating a possible mechanism through which cues initiate reward-seeking actions.

CONCLUSIONS

These data suggest that those phasic mesolimbic dopamine release events thought to signal reward prediction error may also be related to the incentive motivational impact of reward-paired cues on reward-seeking actions.

Phasic mesolimbic dopamine signaling precedes and predicts performance of a self-initiated action sequence task

BACKGROUND

Sequential reward-seeking actions are readily learned despite the temporal gap between the earliest (distal) action in the sequence and the reward delivery. Fast dopamine signaling is hypothesized to mediate this form of learning by reporting errors in reward prediction. However, such a role for dopamine release in voluntarily initiated action sequences remains to be demonstrated.

METHODS

Using fast-scan cyclic voltammetry, we monitored phasic mesolimbic dopamine release, in real time, as rats performed a self-initiated sequence of lever presses to earn sucrose rewards. Before testing, rats received either 0 (n = 11), 5 (n = 11), or 10 (n = 8) days of action sequence training.

RESULTS

For rats acquiring the action sequence task at test, dopamine release was strongly elicited by response-contingent (but unexpected) rewards. With learning, a significant elevation in dopamine release preceded performance of the proximal action and subsequently came to precede the distal action. This predistal dopamine release response was also observed in rats previously trained on the action sequence task, and the amplitude of this signal predicted the latency with which rats completed the action sequence. Importantly, the dopamine response to contingent reward delivery was not observed in rats given extensive pretraining. Pharmacological analysis confirmed that task performance was dopamine-dependent.

CONCLUSIONS

These data suggest that phasic mesolimbic dopamine release mediates the influence that rewards exert over the performance of self-paced, sequentially-organized behavior and sheds light on how dopamine signaling abnormalities may contribute to disorders of behavioral control.

Differential dependence of Pavlovian incentive motivation and instrumental incentive learning processes on dopamine signaling

Here we attempted to clarify the role of dopamine signaling in reward seeking. In Experiment 1, we assessed the effects of the dopamine D(1)/D(2) receptor antagonist flupenthixol (0.5 mg/kg i.p.) on Pavlovian incentive motivation and found that flupenthixol blocked the ability of a conditioned stimulus to enhance both goal approach and instrumental performance (Pavlovian-to-instrumental transfer). In Experiment 2 we assessed the effects of flupenthixol on reward palatability during post-training noncontingent re-exposure to the sucrose reward in either a control 3-h or novel 23-h food-deprived state. Flupenthixol, although effective in blocking the Pavlovian goal approach, was without effect on palatability or the increase in reward palatability induced by the upshift in motivational state. This noncontingent re-exposure provided an opportunity for instrumental incentive learning, the process by which rats encode the value of a reward for use in updating reward-seeking actions. Flupenthixol administered prior to the instrumental incentive learning opportunity did not affect the increase in subsequent off-drug reward-seeking actions induced by that experience. These data suggest that although dopamine signaling is necessary for Pavlovian incentive motivation, it is not necessary for changes in reward experience, or for the instrumental incentive learning process that translates this experience into the incentive value used to drive reward-seeking actions, and provide further evidence that Pavlovian and instrumental incentive learning processes are dissociable.

Disruption of endogenous opioid activity during instrumental learning enhances habit acquisition

Considerable evidence suggests that in instrumental conditioning rats learn the relationship between actions and their consequences, or outcomes. Such goal-directed actions are sensitive to changes in outcome value. The present study assessed the role of the endogenous opioid system in goal-directed reward learning. In two experiments, rats were trained to lever press for food pellets either under vehicle or naloxone-induced opioid receptor blockade. Specific satiety procedures were used for outcome devaluation, and the effect of this devaluation on instrumental responding was then tested in extinction. In Experiment 1 outcome devaluation resulted in a reduction in lever pressing in rats that were trained after vehicle injections, indicating that actions in these rats were goal-directed. In contrast, actions in rats trained under naloxone were insensitive to outcome devaluation when tested off drug, suggesting that lever pressing had become habitual in these rats. Interestingly, in Experiment 2 naloxone-induced habitual behavior was shown to be specific to the context in which the training occurred under naloxone; rats showed normal sensitivity to outcome devaluation when tested in an alternate vehicle-trained context. Additionally, in Experiment 2 we found that the acute administration of naloxone on test had no effect in itself, indicating that opioid receptor-related processes contribute to the acquisition of goal-directed actions and not to their general performance. These data suggest that an intact endogenous opioid system is necessary for normal goal-directed learning and more importantly, reveal that a compromised endogenous opioid system during learning enhances the habitual control of actions.

Silicon Wafer-Based Platinum Microelectrode Array Biosensor for Near Real-Time Measurement of Glutamate in Vivo

Using Micro-Electro-Mechanical-Systems (MEMS) technologies, we have developed silicon wafer-based platinum microelectrode arrays (MEAs) modified with glutamate oxidase (GluOx) for electroenzymatic detection of glutamate in vivo. These MEAs were designed to have optimal spatial resolution for in vivo recordings. Selective detection of glutamate in the presence of the electroactive interferents, dopamine and ascorbic acid, was attained by deposition of polypyrrole and Nafion. The sensors responded to glutamate with a limit of detection under 1muM and a sub-1-second response time in solution. In addition to extensive in vitro characterization, the utility of these MEA glutamate biosensors was also established in vivo. In the anesthetized rat, these MEA glutamate biosensors were used for detection of cortically-evoked glutamate release in the ventral striatum. The MEA biosensors also were applied to the detection of stress-induced glutamate release in the dorsal striatum of the freely-moving rat.

Dopamine release is heterogeneous within microenvironments of the rat nucleus accumbens

Many individual neurons within the intact brain fire in stochastic patterns that arise from interactions with the neuronal circuits that they comprise. However, the chemical communication that is evoked by these firing patterns has not been characterized because sensors suitable to monitor subsecond chemical events in micron dimensions have only recently become available. Here we employ a voltammetric sensor technology coupled with principal component regression to examine the dynamics of dopamine concentrations in the nucleus accumbens (NAc) of awake and unrestrained rats. The sensor has submillimeter dimensions and provides high temporal (0.1 s) resolution. At select locations spontaneous dopamine transient concentration changes were detected, achieving instantaneous concentrations of approximately 50 nm. At other locations, transients were absent even though dopamine was available for release as shown by extracellular dopamine increases following electrical activation of dopaminergic neurons. At sites where dopamine concentration transients occur, uptake inhibition by cocaine enhances the frequency and magnitude of the rapid transients while also causing a more gradual increase in extracellular dopamine. These effects were largely absent from sites that did not support ongoing transient activity. These findings reveal an unanticipated spatial and temporal heterogeneity of dopamine transmission within the NAc that may depend upon the firing of specific subpopulations of dopamine neurons.

Phasic dopamine release evoked by abused substances requires cannabinoid receptor activation

Transient surges of dopamine in the nucleus accumbens are associated with drug seeking. Using a voltammetric sensor with high temporal and spatial resolution, we demonstrate differences in the temporal profile of dopamine concentration transients caused by acute doses of nicotine, ethanol, and cocaine in the nucleus accumbens shell of freely moving rats. Despite differential release dynamics, all drug effects are uniformly inhibited by administration of rimonabant, a cannabinoid receptor (CB1) antagonist, suggesting that an increase in endocannabinoid tone facilitates the effects of commonly abused drugs on subsecond dopamine release. These time-resolved chemical measurements provide unique insight into the neurobiological effectiveness of rimonabant in treating addictive disorders.

Cannabinoid modulation of electrically evoked pH and oxygen transients in the nucleus accumbens of awake rats

Cannabinoid receptors have been implicated in the regulation of blood flow in the cerebral vasculature. Because the nucleus accumbens (NAc) shows high levels of central cannabinoid receptor 1 (CB1) expression we examined the effects of cannabinoids on the local transient alkaline shifts and increases in extracellular oxygen induced by electrical stimulation of the medial forebrain bundle (MFB) in conscious animals. These changes result from increases in cerebral blood flow (CBF) and metabolism in the NAc that are evoked by the stimulation. Oxygen and pH changes were monitored using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in the NAc of freely moving rats. Administration of the cannabinoid receptor agonist WIN55,212-2 potently inhibited extracellular oxygen and pH changes, an effect that was reversed and prevented by pre-treatment with the CB1 receptor antagonists SR141716A and AM251. The effects on pH following WIN55,212-2 were similar to those following nimodipine, a recognized vasodilator. When AM251 was injected alone, the amplitude of electrically evoked pH shifts was unaffected. Administration of AM404 and VDM11, endocannabinoid transport inhibitors, partially inhibited pH transients in a CB1 receptor-dependent manner. The present findings suggest that CB1 receptor activation modulates changes in two well-established indices of local blood flow and metabolism resulting from electrically evoked activation of ascending fibers. Although endogenous cannabinoid tone alone is not sufficient to modify these responses, uptake blockade demonstrates that the system has the potential to exert CB1-specific effects similar to those of full agonists.

Real-time measurement of dopamine fluctuations after cocaine in the brain of behaving rats

Dopamine neurotransmission has been implicated in the modulation of many cognitive processes. Both rapid (phasic) and slower (tonic) changes in its extracellular concentration contribute to its complex actions. Fast in vivo electrochemical techniques can measure extracellular dopamine on a rapid time scale but without the selectivity afforded with slower techniques that use chemical separations. Cyclic voltammetry improves chemical resolution over other electrochemical methods, and it can resolve dopamine changes in the brains of behaving rodents over short epochs (<10 s). With this method, however, selective detection of slower dopamine changes is still elusive. Here we demonstrate that principal component regression of cyclic voltammetry data enables quantification of changes in dopamine and extracellular pH. Using this method, we show that cocaine modifies dopamine release in two ways: dopamine concentration transients increase in frequency and magnitude, whereas a gradual increase in steady-state dopamine concentration occurs over 90 s.

Cannabinoids enhance subsecond dopamine release in the nucleus accumbens of awake rats

Dopaminergic neurotransmission has been highly implicated in the reinforcing properties of many substances of abuse, including marijuana. Cannabinoids activate ventral tegmental area dopaminergic neurons, the main ascending projections of the mesocorticolimbic dopamine system, and change their spiking pattern by increasing the number of impulses in a burst and elevating the frequency of bursts. Although they also increase time-averaged striatal dopamine levels for extended periods of time, little is known about the temporal structure of this change. To elucidate this, fast-scan cyclic voltammetry was used to monitor extracellular dopamine in the nucleus accumbens of freely moving rats with subsecond timescale resolution. Intravenous administration of the central cannabinoid (CB1) receptor agonist, R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl) methanone mesylate, dose-dependently produced catalepsy, decreased locomotion, and reduced the amplitude of electrically evoked dopamine release while markedly increasing the frequency of detected (nonstimulated) dopamine concentration transients. The CB1 receptor antagonist [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide] reversed and prevented all agonist-induced effects but did not show effects on dopamine release when injected alone. These data demonstrate that doses of a cannabinoid agonist known to increase burst firing produce ongoing fluctuations in extracellular dopamine on a previously unrecognized temporal scale in the nucleus accumbens.