Effects of excitotoxic lesions of the basolateral amygdala on conditional discrimination learning with primary and conditioned reinforcement

Drug memory reconsolidation: from molecular mechanisms to the clinical context

Since its rediscovery at the beginning of the 21st Century, memory reconsolidation has been proposed to be a therapeutic target for reducing the impact of emotional memories that can go awry in mental health disorders such as drug addiction (substance use disorder, SUD). Addiction can be conceptualised as a disorder of learning and memory, in which both pavlovian and instrumental learning systems become hijacked into supporting drug-seeking and drug-taking behaviours. The past two decades of research have characterised the details of the molecular pathways supporting the reconsolidation of pavlovian cue-drug memories, with more recent work indicating that the reconsolidation of instrumental drug-seeking memories also relies upon similar mechanisms. This narrative review considers what is known about the mechanisms underlying the reconsolidation of pavlovian and instrumental memories associated with drug use, how these approaches have translated to experimental medicine studies, and the challenges and opportunities for the clinical use of reconsolidation-based therapies.

Basolateral amygdala and orbitofrontal cortex, but not dorsal hippocampus, are necessary for the control of reward-seeking by occasion setters

Reward-seeking in the world is driven by cues that can have ambiguous predictive and motivational value. To produce adaptive, flexible reward-seeking, it is necessary to exploit occasion setters, other distinct features in the environment, to resolve the ambiguity of Pavlovian reward-paired cues. Despite this, very little research has investigated the neurobiological underpinnings of occasion setting, and as a result little is known about which brain regions are critical for occasion setting. To address this, we exploited a recently developed task that was amenable to neurobiological inquiry where a conditioned stimulus is only predictive of reward delivery if preceded in time by the non-overlapping presentation of a separate cue-an occasion setter. This task required male rats to maintain and link cue-triggered expectations across time to produce adaptive reward-seeking. We interrogated the contributions of the basolateral amygdala and orbitofrontal cortex to occasion setting as these regions are thought to be critical for the computation and exploitation of state value, respectively. Reversible inactivation of either structure prior to the occasion-setting task resulted in a profound inability of rats to use the occasion setter to guide reward-seeking. In contrast, inactivation of the dorsal hippocampus, a region fundamental for context-specific responding was without effect nor did inactivation of the basolateral amygdala or orbitofrontal cortex in a standard Pavlovian conditioning preparation affect conditioned responding. We conclude that neural activity within the orbitofrontal cortex and basolateral amygdala circuit is necessary to update and resolve ambiguity in the environment to promote cue-driven reward-seeking.

Serotonergic modulation of basolateral amygdala nucleus in the extinction of reward-driven learning: The role of 5-HT bioavailability and 5-HT1A receptor

Serotonin (5-HT) neurotransmission has been associated with reward-related behaviour. Moreover, the serotonergic system modulates the basolateral amygdala (BLA), a structure involved in reward encoding, and reward prediction error. However, the role played by 5-HT on BLA during a reward-driven task has not been fully elucidated. In this paper, we investigated whether serotonergic modulation of the BLA is involved in reward-driven learning. To this end, we trained Long Evans rats in an operant conditioning task, and examined the effects of fluoxetine treatment (a selective serotonin reuptake inhibitor, 10 mg/kg) in combination with BLA lesions with NMDA (20 mg/mL) on extinction learning. We also investigated whether intra-BLA injection of the serotonergic 5-HT_1A receptor agonist 8-OH DPAT, or antagonist WAY-100635, alters extinction performance. We found that fluoxetine treatment strongly accelerated extinction learning, while BLA lesions partially reverted this effect and slightly impaired consolidation of extinction. Stimulation and inhibition of 5-HT_1A receptors in BLA induced opposite effects to those of fluoxetine, impairing or accelerating extinction performance, respectively. Our findings suggest that 5-HT modulates reward-driven learning, and 5-HT_1A receptors located in the BLA are relevant for extinction.

Basolateral amygdala - nucleus accumbens circuitry regulates optimal cue-guided risk/reward decision making

Maladaptive decision making is a characteristic feature of substance use disorder and pathological gambling. Studies in humans and animals have implicated neural circuits that include the basolateral amygdala (BLA) and nucleus accumbens (NAc) in facilitating risk/reward decision making. However, the preclinical literature has focussed primarily on situations where animals use internally-generated information to adapt to changes in reward likelihood, whereas many real-life situations require the use of external stimuli to facilitate context-appropriate behavior. We recently developed the "Blackjack" task, to measure cued risk/reward decision making requiring rats to chose between Small/Certain and Large/Risky rewards, with auditory cues at the start of each trial explicitly informing that the probability of obtaining a large reward was either good (50%) or poor (12.5%). Here we investigated the contribution of the BLA and its interaction with the NAc in guiding these types of decisions. In well-trained male rats, bilateral inactivation of the BLA induced suboptimal decision making, primarily by reducing risky choice on good-odds trials. In comparison, pharmacological disconnection of the BLA and NAc-shell also induced suboptimal decision making, diverting choice from more preferred option by reducing or increasing risky choice on good vs. poor odds trials respectively. Together, these results suggest that the BLA-NAc circuitry plays a crucial role in integrating information provided by discriminative stimuli. Furthermore, this circuitry may aid in guiding action selection of advantageous options in situations to maximize rewards. Finally, they suggest that perturbations in optimal decision making observed in substance abuse and gambling disorders may be driven in part by dysfunction within this circuitry.

Occasion setting

Occasion setting refers to the ability of 1 stimulus, an occasion setter, to modulate the efficacy of the association between another, conditioned stimulus (CS) and an unconditioned stimulus (US) or reinforcer. Occasion setters and simple CSs are readily distinguished. For example, occasion setters are relatively immune to extinction and counterconditioning, and their combination and transfer functions differ substantially from those of simple CSs. Similarly, the acquisition of occasion setting is favored when stimuli are separated by longer intervals, by empty trace intervals, and are of different modalities, whereas the opposite conditions typically favor the acquisition of simple associations. Furthermore, the simple conditioning and occasion setting properties of a single stimulus can be independent, for example, that stimulus may simultaneously predict the occurrence of a reinforcer and indicate that another stimulus will not be reinforced. Many behavioral phenomena that are intractable to simple associative analysis are better understood within an occasion setting framework. Besides capturing the distinction between direct and modulatory control common to many arenas in neuroscience, occasion setting provides a model for the hierarchical organization of memory for events and event relations, and for contextual control more broadly. Although early lesion studies further differentiated between occasion setting and simple conditioning functions, little is known about the neurobiology of occasion setting. Modern techniques for precise manipulation and monitoring of neuronal activity in multiple brain regions are ideally suited for disentangling contributions of simple conditioning and occasion setting in associative learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Surviving threats: neural circuit and computational implications of a new taxonomy of defensive behaviour

Research on defensive behaviour in mammals has in recent years focused on elicited reactions; however, organisms also make active choices when responding to danger. We propose a hierarchical taxonomy of defensive behaviour on the basis of known psychological processes. Included are three categories of reactions (reflexes, fixed reactions and habits) and three categories of goal-directed actions (direct action-outcome behaviours and actions based on implicit or explicit forecasting of outcomes). We then use this taxonomy to guide a summary of findings regarding the underlying neural circuits.

Differential Contributions of Nucleus Accumbens Subregions to Cue-Guided Risk/Reward Decision Making and Implementation of Conditional Rules

The nucleus accumbens (NAc) is a key node within corticolimbic circuitry for guiding action selection and cost/benefit decision making in situations involving reward uncertainty. Preclinical studies have typically assessed risk/reward decision making using assays where decisions are guided by internally generated representations of choice-outcome contingencies. Yet, real-life decisions are often influenced by external stimuli that inform about likelihoods of obtaining rewards. How different subregions of the NAc mediate decision making in such situations is unclear. Here, we used a novel assay colloquially termed the "Blackjack" task that models these types of situations. Male Long-Evans rats were trained to choose between one lever that always delivered a one-pellet reward and another that delivered four pellets with different probabilities [either 50% (good-odds) or 12.5% (poor-odds)], which were signaled by one of two auditory cues. Under control conditions, rats selected the large/risky option more often on good-odds versus poor-odds trials. Inactivation of the NAc core caused indiscriminate choice patterns. In contrast, NAc shell inactivation increased risky choice, more prominently on poor-odds trials. Additional experiments revealed that both subregions contribute to auditory conditional discrimination. NAc core or shell inactivation reduced Pavlovian approach elicited by an auditory CS+, yet shell inactivation also increased responding during presentation of a CS-. These data highlight distinct contributions for NAc subregions in decision making and reward seeking guided by discriminative stimuli. The core is crucial for implementation of conditional rules, whereas the shell refines reward seeking by mitigating the allure of larger, unlikely rewards and reducing expression of inappropriate or non-rewarded actions.SIGNIFICANCE STATEMENT Using external cues to guide decision making is crucial for adaptive behavior. Deficits in cue-guided behavior have been associated with neuropsychiatric disorders, such as attention deficit hyperactivity disorder and schizophrenia, which in turn has been linked to aberrant processing in the nucleus accumbens. However, many preclinical studies have often assessed risk/reward decision making in the absence of explicit cues. The current study fills that gap by using a novel task that allows for the assessment of cue-guided risk/reward decision making in rodents. Our findings identified distinct yet complementary roles for the medial versus lateral portions of this nucleus that provide a broader understanding of the differential contributions it makes to decision making and reward seeking guided by discriminative stimuli.

NMDA receptors in the avian amygdala and the premotor arcopallium mediate distinct aspects of appetitive extinction learning

Extinction learning is an essential mechanism that enables constant adaptation to ever-changing environmental conditions. The underlying neural circuit is mostly studied with rodent models using auditory cued fear conditioning. In order to uncover the variant and the invariant neural properties of extinction learning, we adopted pigeons as an animal model in an appetitive sign-tracking paradigm. The animals firstly learned to respond to two conditioned stimuli in two different contexts (CS-1 in context A and CS-2 in context B), before conditioned responses to the stimuli were extinguished in the opposite contexts (CS-1 in context B and CS-2 in context A). Subsequently, responding to both stimuli was tested in both contexts. Prior to extinction training, we locally injected the N-methyl-d-aspartate receptor (NMDAR) antagonist 2-Amino-5-phosphonovaleric acid (APV) in either the amygdala or the (pre)motor arcopallium to investigate their involvement in extinction learning. Our findings suggest that the encoding of extinction memory required the activation of amygdala, as visible by an impairment of extinction acquisition by concurrent inactivation of local NMDARs. In contrast, consolidation and subsequent retrieval of extinction memory recruited the (pre)motor arcopallium. Also, the inactivation of arcopallial NMDARs induced a general motoric slowing during extinction training. Thus, our results reveal a double dissociation between arcopallium and amygdala with respect to acquisition and consolidation of extinction, respectively. Our study therefore provides new insights on the two key components of the avian extinction network and their resemblance to the data obtained from mammals, possibly indicating a shared neural mechanism underlying extinction learning shaped by evolution.

Acoustic startle response in rats predicts inter-individual variation in fear extinction

Although a large portion of the population is exposed to a traumatic event at some point, only a small percentage of the population develops post-traumatic stress disorder (PTSD), suggesting the presence of predisposing factors. Abnormal acoustic startle response (ASR) has been shown to be associated with PTSD, implicating it as a potential predictor of the development of PTSD-like behavior. Since poor extinction and retention of extinction learning are characteristic of PTSD patients, it is of interest to determine if abnormal ASR is predictive of development of such deficits. To determine whether baseline ASR has utility in predicting the development of PTSD-like behavior, the relationship between baseline ASR and freezing behavior following Pavlovian fear conditioning was examined in a group of adult, male Sprague-Dawley rats. Baseline acoustic startle response (ASR) was assessed preceding exposure to a Pavlovian fear conditioning paradigm where freezing behavior was measured during fear conditioning, extinction training, and extinction testing. Although there was no relationship between baseline ASR and fear memory following conditioning, rats with low baseline ASR had significantly lower magnitude of retention of the extinction memory than rats with high baseline ASR. The results suggest that baseline ASR has value as a predictive index of the development of a PTSD-like phenotype.

Amygdala Dopamine Receptors Are Required for the Destabilization of a Reconsolidating Appetitive Memory

Disrupting maladaptive memories may provide a novel form of treatment for neuropsychiatric disorders, but little is known about the neurochemical mechanisms underlying the induction of lability, or destabilization, of a retrieved consolidated memory. Destabilization has been theoretically linked to the violation of expectations during memory retrieval, which, in turn, has been suggested to correlate with prediction error (PE). It is well-established that PE correlates with dopaminergic signaling in limbic forebrain structures that are critical for emotional learning. The basolateral amygdala is a key neural substrate for the reconsolidation of pavlovian reward-related memories, but the involvement of dopaminergic mechanisms in inducing lability of amygdala-dependent memories has not been investigated. Therefore, we tested the hypothesis that dopaminergic signaling within the basolateral amygdala is required for the destabilization of appetitive pavlovian memories by investigating the effects dopaminergic and protein synthesis manipulations on appetitive memory reconsolidation in rats. Intra-amygdala administration of either the D1-selective dopamine receptor antagonist SCH23390 or the D2-selective dopamine receptor antagonist raclopride prevented memory destabilization at retrieval, thereby protecting the memory from the effects of an amnestic agent, the protein synthesis inhibitor anisomycin. These data show that dopaminergic transmission within the basolateral amygdala is required for memory labilization during appetitive memory reconsolidation.

The amygdala: securing pleasure and avoiding pain

The amygdala has traditionally been associated with fear, mediating the impact of negative emotions on memory. However, this view does not fully encapsulate the function of the amygdala, nor the impact that processing in this structure has on the motivational limbic corticostriatal circuitry of which it is an important structure. Here we discuss the interactions between different amygdala nuclei with cortical and striatal regions involved in motivation; interconnections and parallel circuitries that have become increasingly understood in recent years. We review the evidence that the amygdala stores memories that allow initially motivationally neutral stimuli to become associated through pavlovian conditioning with motivationally relevant outcomes which, importantly, can be either appetitive (e.g. food) or aversive (e.g. electric shock). We also consider how different psychological processes supported by the amygdala such as conditioned reinforcement and punishment, conditioned motivation and suppression, and conditioned approach and avoidance behavior, are not only psychologically but also neurobiologically dissociable, being mediated by distinct yet overlapping neural circuits within the limbic corticostriatal circuitry. Clearly the role of the amygdala goes beyond encoding aversive stimuli to also encode the appetitive, requiring an appreciation of the amygdala's mediation of both appetitive and fearful behavior through diverse psychological processes.

Contribution of the amygdala, but not orbitofrontal or medial prefrontal cortices, to the expression of flavour preferences in marmoset monkeys

The development of food preferences contributes to a balanced diet, and involves both innate and learnt factors. By associating flavour cues with the reinforcing properties of the food (i.e. postingestive nutrient cues and innately preferred tastes, such as sweetness), animals acquire individual preferences. How the brain codes and guides selection when the subject has to choose between different palatable foods is little understood. To investigate this issue, we trained common marmoset monkeys (Callithrix jacchus) to respond to abstract visual patterns on a touch-sensitive computer screen to gain access to four different flavoured juices. After preferences were stable, animals received excitotoxic lesions of either the amygdala, the orbitofrontal cortex or the medial prefrontal cortex. Neither the orbitofrontal nor the medial prefrontal cortex lesions affected pre-surgery-expressed flavour preferences or the expression of preferences for novel flavours post-surgery. In contrast, amygdala lesions caused a shift in the preferences for juices expressed pre-surgery such that, post-surgery, juices were chosen according to their overall carbohydrate (simple sugars) content or 'sweetness'. Subsequent tests revealed that amygdala-lesioned animals only expressed juice preferences if they differed in 'sweetness'. Unlike controls, orbitofrontal cortex-lesioned and medial prefrontal cortex-lesioned animals, they were unable to display preferences between juices matched for 'sweetness' i.e. 5% sucrose solutions aromatised with different essential oils. The most parsimonious explanation is that the amygdala contributes to the expression of food preferences based on learnt cues but not those based on an innate preference for sweetness.

Basolateral amygdala modulates terminal dopamine release in the nucleus accumbens and conditioned responding

BACKGROUND

Dopamine signaling in the nucleus accumbens (NAc) is essential for goal-directed behaviors and primarily arises from burst firing of ventral tegmental area neurons. However, the role of associative neural substrates such as the basolateral amygdala (BLA) in regulating phasic dopamine release in the NAc, particularly during reward seeking, remains unknown.

METHODS

Male Sprague-Dawley rats learned to discriminate two cues: a discriminative stimulus (DS) that predicted sucrose reinforcement contingent upon a lever press and a nonassociated stimulus (NS) that predicted a second lever never reinforced with sucrose. Following training, a test session was completed in which NAc dopamine was measured using fast-scan cyclic voltammetry in conjunction with inactivation of the ipsilateral BLA (gamma-aminobutyric acid agonists; baclofen/muscimol) to determine the contribution of BLA activity to dopamine release in the NAc core during the task.

RESULTS

Under vehicle conditions, DS and NS presentation elicited dopamine release within the NAc core. The DS evoked significantly more dopamine than the NS. Inactivation of the BLA selectively attenuated the magnitude of DS-evoked dopamine release, concurrent with an attenuation of DS-evoked conditioned approaches. Other behavioral responses (e.g., lever pressing) and dopamine release concomitant with those events were unaltered by BLA inactivation. Furthermore, neither ventral tegmental area electrically stimulated dopamine release nor the probability of high concentration dopamine release events was altered following BLA inactivation.

CONCLUSIONS

These results demonstrate that the BLA terminally modulates dopamine signals within the NAc core under specific, behaviorally relevant conditions, illustrating a functional mechanism by which the BLA selectively facilitates responding to motivationally salient environmental stimuli.

Effects of repeated MDMA administration on the motivation for palatable food and extinction of operant responding in mice

RATIONALE

Repeated administration of 3,4-methylenedioxymethamphetamine (MDMA) produces mainly dopaminergic neurotoxicity in mice. However, the consequences of this exposure on the behavioural responses related to natural reinforcing stimuli are still largely unknown.

OBJECTIVES

We examined whether repeated treatment with neurotoxic and non-neurotoxic doses of MDMA could exert acute and long-lasting effects on the motivation of mice to obtain a highly palatable food and on the extinction and reinstatement of food-seeking behaviour. Food-deprived mice were first trained to acquire stable responding on fixed ratio (FR) schedules of reinforcement and then treated twice daily with saline, 3 or 30 mg/kg MDMA during four consecutive days.

RESULTS

The high dose of MDMA impaired instrumental responding on the first and third day of treatment, whilst no residual effects were apparent on FR5 responding at any of the doses studied 24 h after treatment withdrawal. Breaking points were decreased in mice treated with both doses of MDMA. This decrease in motivation for palatable food was not due to unspecific locomotor or coordination deficits. A resistance to extinction was observed only with the highest dose of MDMA, whilst all mice showed similar reinstatement of palatable food-seeking behaviour irrespective of previous treatment. Autoradiography of [3H]-mazindol binding revealed a decrease in striatal dopamine transporter binding only in mice treated with the highest dose of MDMA.

CONCLUSIONS

This study demonstrates that repeated treatment with MDMA decreases the incentive motivation for a palatable food reward and that long-lasting MDMA-induced dopaminergic neurotoxicity increases the resistance to extinction of responding in the absence of reward.

Lesions of the medial striatum in monkeys produce perseverative impairments during reversal learning similar to those produced by lesions of the orbitofrontal cortex

The ability to switch responding between two visual stimuli based on their changing relationship with reward is dependent on the orbitofrontal cortex (OFC). OFC lesions in humans, monkeys, and rats disrupt performance on a common test of this ability, the visual serial discrimination reversal task. This finding is of particular significance to our understanding of psychiatric disorders such as obsessive-compulsive disorder (OCD) and schizophrenia, in which behavioral inflexibility is a prominent symptom. Although OFC dysfunction can occur in these disorders, there is considerable evidence for more widespread dysfunction within frontostriatal and frontoamygdalar circuitry. Because the contribution of these subcortical structures to behavioral flexibility is poorly understood, the present study compared the effects of excitotoxic lesions of the medial striatum (MS), amygdala, and OFC in the marmoset monkey on performance of the serial reversal task. All monkeys were able to learn a novel stimulus-reward association but, compared with both control and amygdala-lesioned monkeys, those with MS or OFC lesions showed a perseverative impairment in their ability to reverse this association. However, whereas both MS and OFC groups showed insensitivity to negative feedback, only OFC-lesioned monkeys showed insensitivity to positive feedback. These findings suggest that, for different reasons, both the MS and OFC support behavioral flexibility after changes in reward contingencies, and are consistent with the hypothesis that striatal and OFC dysfunction can contribute to pathological perseveration.

Basolateral amygdala lesions and sensitivity to reinforcer magnitude in concurrent chains schedules

Previous studies show that the basolateral amygdala (BLA) is required for behavior to adjust when the value of a reinforcer decreases after satiation or pairing with gastric distress. This study evaluated the effect of pre- or post-training excitotoxic lesions of the BLA on changes in preference with another type of contingency change, reinforcer magnitude reversal. Rats were trained to press left and right levers during a variable-interval choice phase for 50 microl or 150 microl sucrose delivered to consistent locations after a 16-s delay. Tones were presented during the first and last 2s of the delay to reinforcement. The tone frequency predicted the magnitude of sucrose reinforcement in baseline conditions. All groups acquired stable preference for the lever on the large (150 microl) reinforcer side. However, nose poking during the delay to large reinforcement was highly accurate (i.e., to the reinforced side) for all groups except the rats with BLA lesions induced before training, suggesting impaired control of behavior by the tone. After the acquisition of stable preference, the locations of the reinforcer magnitudes were unpredictably reversed for a single session. Pre-training lesions blunted changes in preference when the reinforcer magnitudes were reversed. Lesions induced after stable preference was acquired, but prior to reversal, did not disrupt changes in preference. The data suggest that the BLA contributes to the adaptation of choice behavior following changes in reinforcer magnitude. Impaired learning about the tone-reinforcer magnitude relationships may have disrupted discrimination of the reinforcer magnitude reversal.

Mental fatigue: costs and benefits

A framework for mental fatigue is proposed, that involves an integrated evaluation of both expected rewards and energetical costs associated with continued performance. Adequate evaluation of predicted rewards and potential risks of actions is essential for successful adaptive behaviour. However, while both rewards and punishments can motivate to engage in activities, both types of motivated behaviour are associated with energetical costs. We will review findings that suggest that the nucleus accumbens, orbitofrontal cortex, amygdala, insula and anterior cingulate cortex are involved evaluating both the potential rewards associated with performing a task, as well as assessing the energetical demands involved in task performance. Behaviour will only proceed if this evaluation turns out favourably towards spending (additional) energy. We propose that this evaluation of predicted rewards and energetical costs is central to the phenomenon of mental fatigue: people will no longer be motivated to engage in task performance when energetical costs are perceived to outweigh predicted rewards.

Differential involvement of the basolateral amygdala and mediodorsal thalamus in instrumental action selection

Although it has been shown that the basolateral amygdala (BLA) and the mediodorsal thalamus (MD) are critical for goal-directed instrumental performance, much remains unknown about the respective contributions of these structures to action selection. The current study assessed the effects of post-training BLA and MD lesions on several tests of instrumental action selection. We found that MD damage disrupted the influence of pavlovian cues over action selection but left intact rats' ability to select actions based on either the expected value or the discriminative stimulus properties of the outcome. In contrast, BLA lesions impaired performance on all three tests of action selection. Because both lesion types disrupted the influence of cues that signal reward over instrumental performance, we then investigated the involvement of these structures in pavlovian contingency learning using a task in which the predictive status of one of two cues is degraded by delivering its outcome noncontingently during the intertrial interval. As expected, the sham group selectively suppressed their conditioned approach performance to the cue that no longer signaled its outcome but continued to respond to the control stimulus. In contrast, both lesioned groups were impaired on this task. Interestingly, whereas the MD group displayed a nonspecific reduction in responding to both cues, the BLA group continued to show high levels of responding to both cues as if their performance was completely insensitive to this contingency manipulation. These findings demonstrate that the BLA and MD make important yet distinct contributions to instrumental action selection.

Individual differences in amphetamine self-administration: the role of the central nucleus of the amygdala

Rats categorized as high responder (HR), based on their activity in an inescapable novel environment, self-administer more amphetamine than low responder (LR) rats. The current study examined if the central nucleus of the amygdala (ACe) contributes to the elevated response for amphetamine in HR rats. Male Sprague-Dawley rats were classified as HR and LR rats based on their activity in inescapable novelty and novelty place preference, and then were trained to self-administer amphetamine (0.1 mg/kg/infusion). Once stable responding was achieved, rats received microinfusions of the GABA(A) agonist muscimol (0.5 microg/0.5 microl) or phosphate-buffered saline into the ACe immediately before self-administration of amphetamine (0.1, 0.03, 0.01, or 0.001 mg/kg/infusion) or saline. An additional group of rats was trained to lever press for sucrose rather than amphetamine. Based on the inescapable novelty test, HR rats self-administered more amphetamine than LR rats at the 0.03 and 0.01 mg/kg/infusion unit doses; there were no significant individual differences in amphetamine self-administration based on the novelty place preference test. Inactivation of the ACe with muscimol decreased self-administration at the 0.03 and 0.01 mg/kg/infusion unit doses in HR rats, but had no effect on LR rats. ACe inactivation had no reliable effect on inactive lever responding and appeared to be region specific based on anatomical controls. In addition, while inactivation of the ACe decreased responding for sucrose, inactivation did not differentially affect HR and LR rats. These results suggest that the ACe contributes to the elevated rate of amphetamine self-administration in HR rats.

Neural mechanisms of extinction learning and retrieval

Emotional learning is necessary for individuals to survive and prosper. Once acquired, however, emotional associations are not always expressed. Indeed, the regulation of emotional expression under varying environmental conditions is essential for mental health. The simplest form of emotional regulation is extinction, in which conditioned responding to a stimulus decreases when the reinforcer is omitted. Two decades of research on the neural mechanisms of fear conditioning have laid the groundwork for understanding extinction. In this review, we summarize recent work on the neural mechanisms of extinction learning. Like other forms of learning, extinction occurs in three phases: acquisition, consolidation, and retrieval, each of which depends on specific structures (amygdala, prefrontal cortex, hippocampus) and molecular mechanisms (receptors and signaling pathways). Pharmacological methods to facilitate consolidation and retrieval of extinction, for both aversive and appetitive conditioning, are setting the stage for novel treatments for anxiety disorders and addictions.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Correlates of intellectual ability with morphology of the hippocampus and amygdala in healthy adults

Several prior imaging studies of healthy adults have correlated volumes of the hippocampus and amygdala with measures of general intelligence (IQ), with variable results. In this study, we assessed correlations between volumes of the hippocampus and amygdala and full-scale IQ scores (FSIQ) using a method of image analysis that permits detailed regional mapping of this correlation throughout the surface contour of these brain structures. We delineated the hippocampus and amygdala in high-resolution magnetic resonance images of the brain from 34 healthy individuals. We then correlated FSIQ with overall volumes and with the surface morphologies of each of these structures. Hippocampus volumes correlated significantly and inversely with FSIQ independently of gender, age, socioeconomic status, and whole brain volume. Left and right hippocampus volumes correlated respectively with verbal and performance IQ subscales. Higher IQs were significantly associated with large inward deformations of the surface of the anterior hippocampus bilaterally. These findings suggest that a smaller anterior hippocampus contributes to an increased efficiency of neural processing that subserves overall intelligence.

The role of different subregions of the basolateral amygdala in cue-induced reinstatement and extinction of food-seeking behavior

Reinstatement of previously extinguished instrumental responding for drug-related cues has been used as an animal model for relapse of drug abuse, and is disrupted by inactivation of the basolateral amygdala (BLA). However, the role that the BLA plays in reinstatement induced by cues associated with natural rewards is unclear. The present study assessed the effects of inactivation of different regions of the BLA in cue-induced reinstatement of food-seeking behavior and in the extinction of instrumental responding for food. In experiment 1, rats acquired a lever pressing response for food reward paired with a light/tone conditioned stimulus (CS). They were then subjected to extinction training, where both food and the CS were withheld. Reinstatement of extinguished responding was measured during response-contingent presentations of the CS alone. Following saline infusions into the caudal or rostral BLA, rats displayed a significant increase in lever pressing during reinstatement sessions. Inactivation of these subregions with bupivacaine did not attenuate responding for the CS in the absence of food delivery. In fact, inactivation of the caudal BLA potentiated responding relative to vehicle treatments. Analysis of within-session responding revealed that caudal BLA inactivation retarded extinction of lever pressing in response to the CS. In experiment 2, inactivation of the caudal BLA on the first or second day of extinction training significantly retarded the acquisition of extinction learning on the following day. These data indicate that that the caudal BLA may play a specific role in the extinction of appetitive conditioned responses, by monitoring changes in the reinforcing value of pavlovian conditioned stimuli linked to action-outcome associations once these associations have been formed. Moreover, these findings support a growing body of evidence indicating that separate neural circuits incorporating the BLA may play different roles in mediating reinstatement of reward-seeking behaviors induced by either drug or food related stimuli.

Instrumental learning and relearning in individuals with psychopathy and in patients with lesions involving the amygdala or orbitofrontal cortex

Previous work has shown that individuals with psychopathy are impaired on some forms of associative learning, particularly stimulus-reinforcement learning (Blair et al., 2004; Newman & Kosson, 1986). Animal work suggests that the acquisition of stimulus-reinforcement associations requires the amygdala (Baxter & Murray, 2002). Individuals with psychopathy also show impoverished reversal learning (Mitchell, Colledge, Leonard, & Blair, 2002). Reversal learning is supported by the ventrolateral and orbitofrontal cortex (Rolls, 2004). In this paper we present experiments investigating stimulus-reinforcement learning and relearning in patients with lesions of the orbitofrontal cortex or amygdala, and individuals with developmental psychopathy without known trauma. The results are interpreted with reference to current neurocognitive models of stimulus-reinforcement learning, relearning, and developmental psychopathy.

Impaired outcome-specific devaluation of instrumental responding in mice with a targeted deletion of the AMPA receptor glutamate receptor 1 subunit

The present study evaluated the proposal that mice with a targeted deletion of the glutamate receptor 1 (GluR1) subunit of the AMPA receptor are impaired in using an instrumental or pavlovian signal to gain access to a representation of the sensory-specific motivational properties of a primary reward. In experiment 1, mice were trained to approach two goal boxes in a plus-maze; each goal box contained a different reward (sucrose solution vs food pellet). After acquisition, one of the rewards was devalued by an outcome-specific satiety procedure. Subsequent test trials performed in extinction showed an increase in the latency to enter the devalued goal arm, relative to the nondevalued goal arm in control but not GluR1-/- mice. In experiment 2, a similar outcome-specific satiety procedure was used to examine the effects of reward devaluation on an instrumental nose-poke response. During testing, control but not GluR1-/- mice decreased their rate of responding on a nose poke associated with a devalued reward. A subsequent choice test showed that GluR1-/- mice were able to discriminate between the devalued and nondevalued outcomes used in both experiments. These deficits mirror those seen after lesions of the basolateral amygdala and suggests that GluR1-mediated neurotransmission in this region contributes to encoding the relationship between sensory-specific aspects of reward and their incentive value.

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a simple discrimination task

In the present experiment, the effects of neurotoxic lesions (quinolinic acid) of the dorsolateral or dorsomedial striatum were investigated on a simple instrumental discrimination task (CS+/CS-). Rats with lesions of the dorsolateral striatum were found to be impaired in the acquisition of this task, as compared to rats with either dorsomedial striatal or sham lesions. Furthermore, dorsolateral striatal lesioned animals had significantly lower levels of responding across the course of discrimination training, as assessed both by overall rate of response during CS+ presentations and number of CS+ trials without a response, despite having shown levels of responding during variable interval training that did not differ from that of sham lesioned animals. In contrast, animals with lesions of the dorsomedial striatum did not show an impairment in acquisition of the present task, but had slightly higher rates of responding during CS- presentations. It is argued that the poor acquisition and low response rates observed in animals with dorsolateral striatal lesions reflect a failure in stimulus-response learning, while the performance of animals with dorsomedial striatal lesions may have been the result of an increase in overall activity rate.

Neural substrates of olfactory discrimination learning with auditory secondary reinforcement. I. Contributions of the basolateral amygdaloid complex and orbitofrontal cortex

The basolateral amygdaloid complex (BLA) and orbitofrontal cortex (OFC) share extensive reciprocal connections, and interactions between these regions likely contribute to both mnemonic and affective processes. The present study examined the potential differential contributions of the BLA and OFC to performance of an olfactory discrimination task that incorporates auditory conditioned reinforcement and to expression of immediate post-shock freezing behavior. Damage to the BLA had little effect on performance of the conditioned reinforcement task but abolished immediate post-shock freezing behavior. In contrast, damage to OFC resulted in both a mild but significant performance decrement in the conditioned reinforcement task and a significant attenuation of immediate post-shock freezing behavior. These findings suggest that immediate post-shock freezing behavior is likely critically dependent upon interactions between the BLA and OFC. However, although mnemonic processes underlying accurate performance of the conditioned reinforcement task might be supported by OFC in part, such processes are independent of either the BLA or interactions between these two regions.

The amygdalar circuit that acquires taste aversion memory differs from the circuit that extinguishes it

Experimental extinction is the decline in the frequency or intensity of a conditioned behaviour resulting from repetitive performance of the behaviour in the absence of the unconditioned stimulus or reinforcer (Pavlov, 1927). Ample behavioural evidence indicates that experimental extinction does not reflect unlearning of the original trace, but rather a relearning process, in which the new association of the conditioned stimulus with the absence of the original reinforcer comes to control behaviour (Rescorla, 1996). If experimental extinction is indeed learning rather than forgetting, are the neuronal circuits that subserve learning and extinction identical? We address this question by double dissociation analysis of the role of the central (CeA) and the basolateral (BLA) nuclei of the rat's amygdala in the acquisition and extinction, respectively, of conditioned taste aversion (CTA). Whereas local blockade of protein synthesis or beta-adrenergic receptors in the CeA blocks acquisition but not extinction of CTA, a similar intervention in the BLA blocks extinction but not acquisition. Hence, the amygdalar circuit that acquires taste aversion memory differs functionally from the circuit that extinguishes it.

Artificial rearing causes changes in maternal behavior and c-fos expression in juvenile female rats

This study investigated the effects of early-rearing experiences on responsiveness to pups and on the pattern of c-fos activation in the brain of juvenile female rats. From Days 4 to 20, littermate females were reared with their mothers (MR) or artificially (AR). AR rats received minimal licking-like tactile stimulation (AR-min) or maximal stimulation (AR-max). On Day 20, rats were exposed to pups for 4 or 8 days, exposed to a playmate for 4 or 8 days, or left in isolation for 4 or 8 days. Compared with MR rats, pup-exposed AR rats engaged in less pup licking, and all AR rats showed significant reductions in c-fos immunoreactivity in the medial preoptic area and the parietal and piriform cortices. The AR-min group showed the greatest difference in Fos-lir compared with the MR groups. Possible mechanisms that mediate the effects of rearing on the development of neural circuits underlying maternal behavior are discussed.

Appetitive conditioning-induced plasticity is expressed during paradoxical sleep in the medial geniculate, but not in the lateral amygdala

This study examined whether neurons in the medial division of the medial geniculate (MGm) and the dorsal part of the lateral amygdala (LAd) express learning-induced plasticity in paradoxical sleep (PS) after appetitive conditioning, as they do in PS after fear conditioning. Rats received tone-food pairings in 3 sessions. After each session, the tone was presented at a nonawakening intensity during PS. Multiunit activity was simultaneously recorded in MGm and LAd. During waking, increases in tone-evoked discharges developed in MGm and LAd; however, as training continued, they lessened in LAd, but not in MGm. During PS, conditioned tone responses were expressed in MGm, but not in LAd. Thus, these results demonstrate dissociation of MGm and LAd plasticity. Moreover, compared with fear conditioning results, they suggest that expression of amygdalar plasticity in PS depends on the emotional salience of the stimulus.

Emotion and motivation: the role of the amygdala, ventral striatum, and prefrontal cortex

Emotions are multifaceted, but a key aspect of emotion involves the assessment of the value of environmental stimuli. This article reviews the many psychological representations, including representations of stimulus value, which are formed in the brain during Pavlovian and instrumental conditioning tasks. These representations may be related directly to the functions of cortical and subcortical neural structures. The basolateral amygdala (BLA) appears to be required for a Pavlovian conditioned stimulus (CS) to gain access to the current value of the specific unconditioned stimulus (US) that it predicts, while the central nucleus of the amygdala acts as a controller of brainstem arousal and response systems, and subserves some forms of stimulus-response Pavlovian conditioning. The nucleus accumbens, which appears not to be required for knowledge of the contingency between instrumental actions and their outcomes, nevertheless influences instrumental behaviour strongly by allowing Pavlovian CSs to affect the level of instrumental responding (Pavlovian-instrumental transfer), and is required for the normal ability of animals to choose rewards that are delayed. The prelimbic cortex is required for the detection of instrumental action-outcome contingencies, while insular cortex may allow rats to retrieve the values of specific foods via their sensory properties. The orbitofrontal cortex, like the BLA, may represent aspects of reinforcer value that govern instrumental choice behaviour. Finally, the anterior cingulate cortex, implicated in human disorders of emotion and attention, may have multiple roles in responding to the emotional significance of stimuli and to errors in performance, preventing responding to inappropriate stimuli.

Effects of excitotoxic lesions of the basolateral amygdala on cocaine-seeking behavior and cocaine conditioned place preference in rats

Incentive motivation for cocaine, elicited by cocaine-associated stimuli, is thought to be involved in craving and relapse. To examine the role of the basolateral amygdala complex (BLC) in this phenomenon, we assessed the effects of post-training BLC lesions on extinction of cocaine-seeking behavior and cocaine-conditioned place preference (CPP) and the effects of pre-training BLC lesions on acquisition of cocaine-CPP. In Experiment 1, rats were first trained to self-administer cocaine and then received bilateral infusions of the excitotoxin, N-methyl-D-aspartic acid (NMDA, 0.12 M; 0.3 microl/side), or vehicle into the BLC. They were then tested repeatedly for extinction of cocaine-seeking behavior (i.e. nonreinforced responses in the presence of cocaine-paired stimuli). Subsequently, they were trained and tested for acquisition of cocaine-CPP (i.e. increased time spent in a previously cocaine-paired, relative to a saline-paired, environment). Locomotion and compartment entries were also measured. In Experiment 2, rats were first trained and tested for cocaine-CPP, and then received NMDA or vehicle infusions into the BLC. Subsequently, they were tested repeatedly for extinction of cocaine-CPP. Post-training BLC lesions retarded extinction of cocaine-seeking behavior and cocaine-CPP, whereas pre-training lesions disrupted acquisition of cocaine-CPP. These effects did not appear to be related to changes in general activity. We suggest that pre-training BLC lesions disrupted acquisition of cocaine-CPP by impairing assignment of incentive value to cocaine-paired stimuli, whereas post-training BLC lesions disrupted extinction of cocaine-conditioned behaviors by impairing the assessment of the current incentive value of cocaine-paired stimuli.

Lesions of the basolateral amygdala disrupt selective aspects of reinforcer representation in rats

The amygdala is known to play a role in learning about motivationally significant events. We investigated this role further by examining the effects of excitotoxic lesions of the basolateral amygdala on the ability of rats to use instrumental outcomes to direct responding (the differential outcomes effect) and on the ability of Pavlovian cues to modulate instrumental performance based on shared outcomes (reinforcer-selective Pavlovian-to-instrumental transfer). We found that basolateral amygdala (BLA) lesions did not affect the ability of rats to learn a basic instrumental conditional discrimination, but did disrupt the ability of differential outcomes to facilitate acquisition. In Pavlovian-to-instrumental transfer, BLA lesions did not disrupt the basic enhancement of instrumental performance but did abolish the reinforcer specificity of that enhancement. These results suggest that the BLA is involved in the representation of the sensory aspects of motivationally significant events.

The role of the primate amygdala in conditioned reinforcement

Conditioned reinforcement refers to the capacity of a conditioned stimulus to support instrumental behavior by acquiring affective properties of the primary reinforcer with which it is associated. Conditioned reinforcers maintain behavior over protracted periods of time in the absence of, and potentially in conflict with, primary reinforcers and as such may play a fundamental role in complex social behavior. A relatively large body of evidence supports the view that the amygdala (and in particular the basolateral area) contributes to conditioned reinforcement by maintaining a representation of the affective value of conditioned stimuli. However, a recent study in primates (Malkova et al., 1997), using a second-order visual discrimination task, suggests that the amygdala is not critical for the conditioned reinforcement process. In the present study, excitotoxic lesions of the amygdala in a new world primate, the common marmoset, resulted in a progressive impairment in responding under a second-order schedule of food reinforcement. In addition, the responding of amygdala-lesioned animals was insensitive to the omission of the conditioned reinforcer, unlike that of control animals, for which responding was markedly reduced. In contrast, lesioned animals were unimpaired when responding on a progression of fixed-ratio schedules of primary reinforcement. These data confirm that the amygdala is critical for the conditioned reinforcement process in primates, and taken together with other recent work in monkeys, these results suggest that the contribution of the amygdala is to provide the affective value of specific reinforcers as accessed by associated conditioned stimuli.

The neuropsychological basis of addictive behaviour

The argument advanced in this review is that drug addiction can be understood in terms of normal learning and memory systems of the brain which, through the actions of chronically self-administered drugs, are pathologically subverted, thereby leading to the establishment of compulsive drug-seeking habits, strengthened by the motivational impact of drug-associated stimuli and occurring at the expense of other sources of reinforcement. We review data from our studies that have utilized procedures which reveal the various influences of pavlovian stimuli on goal-directed behaviour, namely discriminated approach, pavlovian-to-instrumental transfer and conditioned reinforcement, in order to demonstrate their overlapping and also unique neural bases. These fundamental studies are also reviewed in the context of the neural and psychological mechanisms underlying drug-seeking behaviour that is under the control of drug-associated environmental stimuli. The ways in which such drug-seeking behaviour becomes compulsive and habitual, as well as the propensity for relapse to drug-seeking even after long periods of relapse, are discussed in terms of the aberrant learning set in train by the effects of self-administered drugs on plastic processes in limbic cortical-ventral striatal systems.

Involvement of the central nucleus of the amygdala and nucleus accumbens core in mediating Pavlovian influences on instrumental behaviour

Pavlovian conditioned cues exert a powerful influence on instrumental actions directed towards a common reward, this is known as Pavlovian-to-instrumental transfer (PIT). The nucleus accumbens (NAcc) has been hypothesized to function as an interface between limbic cortical structures required for associative conditioning, like the amygdala, and response mechanisms through which instrumental behaviour can be selected and performed. Here we have used selective excitotoxic lesions to investigate the involvement of subnuclei of the amygdala as well as the core and shell regions of the nucleus accumbens on PIT in rats. Within the amygdala, selective lesions of the central nucleus (CeN), but not of the basolateral nucleus (BLA), abolished the PIT effect. In addition, selective lesions of the NAcc core, but not the NAcc shell, also abolished PIT. None of the lesions impaired the acquisition of Pavlovian food cup approaches or instrumental responding itself. These data demonstrate that the CeN and NAcc core are central components of the neural system mediating the impact of Pavlovian cues on instrumental responding. We suggest that this effect may depend upon the regulation of the dopaminergic innervation of the NAcc core by projections from the CeN to the ventral tegmental area.

Recovery of associative function following early amygdala lesions in rats

Adult rats with amygdala lesions made at either Postnatal Day (PND) 10 or PND40 were tested on a series of reversal tasks that tap the ability to form stimulus-reward associations. PND40 rats were significantly impaired relative to both controls and PND10 rats on learning rate of the original discrimination and subsequent reversals. Analyses of discrete learning phases revealed that the impairment was specific to the postchance phase. The PND10 group was not impaired relative to controls on any measure. These results confirm prior findings that amygdala lesions sustained in adulthood impair the formation of stimulus-reward associations. They also demonstrate that substantial sparing or recovery of function is possible when the lesion is made during early development. Furthermore, the findings support the view that behavioral recovery may be more likely if the lesion is sustained near the time of peak synaptogenesis.