Substantia nigra pars compacta is critical to both the acquisition and expression of learned orienting of rats

Evidence for incentive salience sensitization as a pathway to alcohol use disorder

The incentive salience sensitization (ISS) theory of addiction holds that addictive behavior stems from the ability of drugs to progressively sensitize the brain circuitry that mediates attribution of incentive salience (IS) to reward-predictive cues and its behavioral manifestations. In this article, we establish the plausibility of ISS as an etiological pathway to alcohol use disorder (AUD). We provide a comprehensive and critical review of evidence for: (1) the ability of alcohol to sensitize the brain circuitry of IS attribution and expression; and (2) attribution of IS to alcohol-predictive cues and its sensitization in humans and non-human animals. We point out gaps in the literature and how these might be addressed. We also highlight how individuals with different alcohol subjective response phenotypes may differ in susceptibility to ISS as a pathway to AUD. Finally, we discuss important implications of this neuropsychological mechanism in AUD for psychological and pharmacological interventions attempting to attenuate alcohol craving and cue reactivity.

The amygdalo-nigrostriatal network is critical for an optimal temporal performance

The amygdalo-nigrostriatal (ANS) network plays an essential role in enhanced attention to significant events. Interval timing requires attention to temporal cues. We assessed rats having a disconnected ANS network, due to contralateral lesions of the medial central nucleus of the amygdala (CEm) and dopaminergic afferents to the lateral striatum, as compared to controls (sham and ipsilateral lesions of CEm and dopaminergic afferents to LS) in a temporal bisection task. ANS disconnection induced poorer temporal precision and increased response latencies to a short duration. The present results reveal a role of the ANS network in temporal processing.

Dorsolateral striatum is critical for the expression of surprise-induced enhancements in cue associability

The dorsolateral striatum (DLS) is frequently implicated in sensory-motor integration, including the performance of sensory orienting responses (ORs) and learned stimulus-response habits. Our laboratory previously identified a role for the DLS in rats' performance of conditioned ORs to Pavlovian cues for food delivery. Here, we considered whether DLS is also critical to another aspect of attention in associative learning, the surprise-induced enhancement of cue associability. A large behavioral literature shows that a cue present when an expected event is omitted enters into new associations more rapidly when that cue is subsequently paired with food. Research from our laboratory has shown that both cue associability enhancements and conditioned ORs depend on the function of a circuit that includes the amygdala central nucleus and the substantia nigra pars compacta. In three experiments, we explored the involvement of DLS in surprise-induced associability enhancements, using a three-stage serial prediction task that permitted separation of DLS function in registering surprise (prediction error) and enhancing cue associability, and in using that increased associability to learn more rapidly about that cue later. The results showed that DLS is critical to the expression, but not the establishment, of the enhanced cue associability normally produced by surprise in this task. They extend the role of DLS and the amygdalo-nigro-striatal circuit underlying learned orienting to more subtle aspects of attention in associative learning, but are consistent with the general notion that DLS is more important in the expression of previously acquired tendencies than in their acquisition.

Amygdalo-striatal interaction in the enhancement of stimulus salience in associative learning

Function of the central nucleus of the amygdala (CeA) is critical to 2 aspects of attention in associative learning: the conditioning of orienting responses (ORs) to cues paired with food, and the enhancement of cue salience by the surprising omission of expected events. Such salience enhancements have been found to depend on interactions within a circuit that includes CeA, the substantia nigra pars compacta (SNc), the substantia innominata (SI), and the posterior parietal cortex (PPC). The acquisition and expression of conditioned ORs requires interactions among CeA, SNc, and the dorsal lateral striatum (DLS), but not SI or PPC. Here, we considered whether CeA-DLS interactions are also important in surprise-induced salience enhancements in a serial prediction task. Rats received unilateral lesions of CeA and DLS, either contralaterally, which disrupted interactions between those structures, or ipsilaterally, which produced comparable damage to each structure but permitted interactions between them in 1 hemisphere. Rats with ipsilateral lesions of CeA and DLS showed the salience enhancements normally observed in this task, but rats with contralateral lesions of those structures did not. Thus, convergence of information processing by CeA and DLS is essential for surprise-induced salience enhancements, as well as for conditioned ORs.

Impulsivity, risk-taking, and distractibility in rats exhibiting robust conditioned orienting behaviors

When a neutral cue is followed by a significant event such as food delivery, some animals become engaged with the cue itself and acquire cue-directed behaviors. One type of cue-directed behavior is observed following insertion of a lever used as a conditioned stimulus (CS). Rats showing robust approach behavior to the lever also display impulsivity and altered attention, as compared to rats showing behavior directed toward the reward delivery location. The current study used a light CS to categorize rats' propensity for cue-directed behavior, and assessed whether individual differences in impulsivity and related behaviors still emerged. During the light-food pairings, some rats displayed enhanced rearing or orienting to the light (Orienters) prior to showing food cup approach behavior, while other rats only showed food cup approach behavior (Nonorienters). Our results showed that Orienters made more impulsive and risky decisions in two different choice tasks, and were quicker to leave a familiar dark environment to enter a novel bright field. Orienters also showed less accurate target detection when a visual distractor was introduced during an attentional challenge. Our current study suggests that light CS-induced rearing/orienting behavior might not necessarily share an identical mechanism with lever CS-approach behavior in predicting impulsivity-related behaviors.

Posterior parietal cortex is critical for the encoding, consolidation, and retrieval of a memory that guides attention for learning

Within most contemporary learning theories, reinforcement prediction error, the difference between the obtained and expected reinforcer value, critically influences associative learning. In some theories, this prediction error determines the momentary effectiveness of the reinforcer itself, such that the same physical event produces more learning when its presentation is surprising than when it is expected. In other theories, prediction error enhances attention to potential cues for that reinforcer by adjusting cue-specific associability parameters, biasing the processing of those stimuli so that they more readily enter into new associations in the future. A unique feature of these latter theories is that such alterations in stimulus associability must be represented in memory in an enduring fashion. Indeed, considerable data indicate that altered associability may be expressed days after its induction. Previous research from our laboratory identified brain circuit elements critical to the enhancement of stimulus associability by the omission of an expected event, and to the subsequent expression of that altered associability in more rapid learning. Here, for the first time, we identified a brain region, the posterior parietal cortex, as a potential site for a memorial representation of altered stimulus associability. In three experiments using rats and a serial prediction task, we found that intact posterior parietal cortex function was essential during the encoding, consolidation, and retrieval of an associability memory enhanced by surprising omissions. We discuss these new results in the context of our previous findings and additional plausible frontoparietal and subcortical networks.

Updating appetitive memory during reconsolidation window: critical role of cue-directed behavior and amygdala central nucleus

When presented with a light cue followed by food, some rats simply approach the foodcup (Nonorienters), while others first orient to the light in addition to displaying the food-cup approach behavior (Orienters). Cue-directed orienting may reflect enhanced attentional and/or emotional processing of the cue, suggesting divergent natures of cue-information processing in Orienters and Nonorienters. The current studies investigate how differences in cue processing might manifest in appetitive memory retrieval and updating using a paradigm developed to persistently attenuate fear responses (Retrieval-extinction paradigm; Monfils et al., 2009). First, we examined whether the retrieval-extinction paradigm could attenuate appetitive responses in Orienters and Nonorienters. Next, we investigated if the appetitive memory could be updated using reversal learning (fear conditioning) during the reconsolidation window (as opposed to repeated unreinforced trials, i.e., extinction). Both extinction and new fear learning given within the reconsolidation window were effective at persistently updating the initial appetitive memory in the Orienters, but not the Nonorienters. Since conditioned orienting is mediated by the amygdala central nucleus (CeA), our final experiment examined the CeA's role in the retrieval-extinction process. Bilateral CeA lesions interfered with the retrieval-extinction paradigm-did not prevent spontaneous recovery of food-cup approach. Together, our studies demonstrate the critical role of conditioned orienting behavior and the CeA in updating appetitive memory during the reconsolidation window.

Amygdala central nucleus interacts with dorsolateral striatum to regulate the acquisition of habits

The role of the amygdala central nucleus (CeN) in habit learning was assessed in two experiments. First, we examined the effects of bilateral lesions of the anterior CeN on an overtraining-induced lever press habit evaluated using an outcome devaluation protocol. Overtraining generated habitual performance and rendered sham lesioned rats insensitive to outcome devaluation, an effect that was also found in rats given control lesions of the posterior CeN. In contrast, rats with lesions of the anterior CeN did not show normal habit acquisition and their performance remained goal-directed and sensitive to outcome devaluation. Nevertheless, lesions of either the posterior or the anterior CeN abolished the general excitatory influence of a Pavlovian conditioned stimulus on instrumental performance. Second, we assessed the functional interaction between the CeN and dorsolateral striatum (DLS), a region previously implicated in the acquisition of habits, using asymmetrical lesions to disconnect these structures. Rats were given a unilateral lesion of anterior CeN and a unilateral lesion of the DLS, made either ipsilateral (control) or contralateral (disconnection) to the CeN lesion, and given overtraining followed by outcome devaluation. Although the ipsilateral lesioned rats were insensitive to devaluation, the contralateral CeN-DLS lesion impaired habit acquisition, rendering performance sensitive to the devaluation treatment. These results are the first to implicate the CeN and its connection with a circuit involving DLS in habit learning. They imply that, in instrumental conditioning, regions of amygdala parse the instrumental outcome into the reward and reinforcement signals mediating goal-directed and habitual actions, respectively.

Effects of lesions of the amygdala central nucleus on autoshaped lever pressing

Neutral cues paired with rewards often appear to acquire motivational significance, as if the incentive motivational value of the reward is transferred to the cue. Such cues have been reported to modulate the performance of instrumental action (Pavlovian-instrumental transfer, PIT), serve as conditioned reinforcers in the establishment of new learning, and be the targets of approach and other cue-directed behaviors. Here we examined the effects of lesions of the amygdala central nucleus (CeA) on the acquisition of discriminative autoshaped lever-pressing. Insertion of one lever into the experimental chamber was reinforced by sucrose delivery, but insertion of another lever was not reinforced. Although sucrose delivery was not contingent on lever pressing, both CeA- and sham-lesioned rats rapidly came to press the reinforced but not the nonreinforced lever. Despite their showing little evidence of impairments in autoshaped lever pressing, these same CeA-lesioned rats showed significant deficits in the expression of PIT in a subsequent phase of the experiment. The lack of impaired autoshaping in CeA-lesioned rats contrasts with effects previously reported for conditioned orienting responses (ORs) and for other putative measures of incentive learning including PIT and conditioned approach to visual cues.

The central amygdala projection to the substantia nigra reflects prediction error information in appetitive conditioning

The central amygdala nucleus (CeA) plays a critical role in cognitive processes beyond fear conditioning. For example, intact CeA function is essential for enhancing attention to conditioned stimuli (CSs). Furthermore, this enhanced attention depends on the CeA's connections to the nigrostriatal system. In the current study, we examined the role of the CeA's connections to two midbrain dopamine regions, the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA), in processing CS information when predictions of reward or nonreward were confirmed or disconfirmed. Initially, two different retrograde tracers were injected into the SNc and the VTA of rats, to label CeA cells. Different groups of rats then received a visual CS either paired or unpaired with food. Finally, Fos induction was assessed after a test session in which rats were exposed to the visual CS alone or paired with food. Colabeling of Fos and the retrograde tracer(s) showed that CeA neurons projecting to the SNc, but not to the VTA, were engaged in processing CS information when the training and testing conditions differed. These results suggest that the CeA-nigral pathway represents prediction error information during appetitive conditioning.

The integrative function of the basal ganglia in instrumental conditioning

Recent research in instrumental conditioning has focused on the striatum, particularly the role of the dorsal striatum in the learning processes that contribute to instrumental performance in rats. This research has found evidence of what appear to be parallel, functionally and anatomically distinct circuits involving dorsomedial striatum (DMS) and dorsolateral striatum (DLS) that contribute to two independent instrumental learning processes. Evidence suggests that the formation of the critical action-outcome associations mediating goal-directed action are localized to the dorsomedial striatum, whereas the sensorimotor connections that control the performance of habitual actions are localized to the dorsolateral striatum. In addition to the dorsal striatum, these learning processes appear to engage distinct cortico-striatal networks and to be embedded in a complex of converging and partially segregated loops that constitute the cortico-striatal thalamo-cortical feedback circuit. As the entry point for the basal ganglia, cortical circuits involving the dorsal striatum are clearly in a position to control a variety of motor functions but, as recent studies of various neurodegenerative disorders have made clear, they are also involved in a number of cognitive and executive functions including action selection, planning, and decision-making.

Temporally limited role of substantia nigra-central amygdala connections in surprise-induced enhancement of learning

Prediction error plays an important role in modern associative learning theories. For example, the omission of an expected event (surprise) can enhance attention to cues that accompany those omissions, such that subsequent new learning about those cues is more rapid. Many studies from our laboratories have demonstrated that circuitry that includes the amygdala central nucleus (CeA), the cholinergic neurons in the substantia innominata/nucleus basalis region and their innervation of the posterior parietal cortex is critical for this surprise-induced enhancement of attention in learning. We recently showed that midbrain dopamine neurons, known to code prediction error, are also important for surprise-induced enhancement of learning through their interaction with CeA. The present study examined whether in rats the communication between the substantia nigra pars compacta (SNc) and CeA is critical only at the time of surprise, for example to detect prediction error information, or is also needed to maintain and later express that information as enhanced learning. All animals received unilateral CeA lesions and unilateral cannula implants targeting the SNc located contralateral to the lesioned CeA. As the SNc-CeA connections are mainly ipsilateral, inactivating SNc contralateral to the lesioned CeA provided transient blockage of SNc and CeA communication. The results show that SNc-CeA communication is critical for processing prediction error information at the time of surprise, but neither SNc nor SNc-CeA communication is necessary to express that information as enhanced learning later.

Dissociable effects of disconnecting amygdala central nucleus from the ventral tegmental area or substantia nigra on learned orienting and incentive motivation

Recent evidence suggests that the amygdala central nucleus (CeA) and midbrain-striatal dopamine systems are critically involved in the alteration of attentional and emotional processing of initially neutral stimuli by associative learning. In rats, the acquisition of learned orienting responses (ORs) to visual cues paired with food is impaired by lesions of the CeA, and by lesions that disconnect CeA from the dorsolateral striatum (DLS), a region traditionally implicated in elevated responsiveness to sensory stimuli. Similarly, the acquisition of emotional significance to cues paired with food also depends on the function of CeA and of the ventral striatal nucleus accumbens (ACB), a region often considered crucial to acquired reward and motivation. For example, the ability of a cue previously paired with food to increase the rate of food-reinforced instrumental responding (Pavlovian-instrumental transfer, PIT) is eliminated by lesions of the CeA or the accumbens core. In this experiment, we found that lesions that functionally disconnected CeA from the substantia nigra pars compacta (SNc) impaired the acquisition of conditioned orienting to auditory cues paired with food, but had no effect on their ability to enhance instrumental responding, relative to the effects of unilateral lesions of that region. By contrast, lesions that disconnected CeA from the ventral tegmental area (VTA) had no effect on the acquisition of conditioned orienting, but facilitated Pavlovian-instrumental transfer relative to unilateral midbrain lesions, rescuing that function to sham-lesion control levels. Otherwise, unilateral lesions of either midbrain region impaired transfer. Implications of these results for circuit models of amygdalo-striatal interactions in associative learning are discussed.

Dissociation of attention in learning and action: effects of lesions of the amygdala central nucleus, medial prefrontal cortex, and posterior parietal cortex

Many associative learning theories assert that the predictive accuracy of events affects the allocation of attention to them. More reliable predictors of future events are usually more likely to control action based on past learning, but less reliable predictors are often more likely to capture attention when new information is acquired. Previous studies showed that a circuit including the amygdala central nucleus (CEA) and the cholinergic substantia innominata/nucleus basalis magnocellularis (SI/nBM) is important for both sustained attention guiding action in a five-choice serial reaction time (5CSRT) task and for enhanced new learning about less predictive cues in a serial conditioning task. In this study, the authors found that lesions of the cholinergic afferents of the medial prefrontal cortex interfered with 5CSRT performance but not with surprise-induced enhancement of learning, whereas lesions of cholinergic afferents of posterior parietal cortex impaired the latter effects but did not affect 5CSRT performance. CEA lesions impaired performance in both tasks. These results are consistent with the view that CEA affects these distinct aspects of attention by influencing the activity of separate, specialized cortical regions via modulation of SI/nBM.

The nucleus accumbens and Pavlovian reward learning

The ability to form associations between predictive environmental events and rewarding outcomes is a fundamental aspect of learned behavior. This apparently simple ability likely requires complex neural processing evolved to identify, seek, and use natural rewards and redirect these activities based on updated sensory information. Emerging evidence from both animal and human research suggests that this type of processing is mediated in part by the nucleus accumbens (NAc) and a closely associated network of brain structures. The NAc is required for a number of reward-related behaviors and processes specific information about reward availability, value, and context. In addition, this structure is critical for the acquisition and expression of most Pavlovian stimulus-reward relationships, and cues that predict rewards produce robust changes in neural activity in the NAc. Although processing within the NAc may enable or promote Pavlovian reward learning in natural situations, it has also been implicated in aspects of human drug addiction, including the ability of drug-paired cues to control behavior. This article provides a critical review of the existing animal and human literature concerning the role of the NAc in Pavlovian learning with nondrug rewards and considers some clinical ture concerning the role of the NAc in Pavlovian learning with nondrug implications of these findings.

Role of substantia nigra-amygdala connections in surprise-induced enhancement of attention

Coding of prediction error by midbrain dopamine neurons has been examined extensively in the framework of associative learning theory. Most of this research has focused on the role of prediction error in determining the reinforcement value of unconditioned stimuli: poorly predicted ("surprising") outcomes are more effective reinforcers and produce a greater dopamine response than well predicted outcomes. However, surprise also enhances attention to cues that signal poorly predicted outcomes. Previous reports from our laboratories demonstrated that circuitry, including the amygdala central nucleus (CeA), the cholinergic neurons of the substantia innominata/nucleus basalis region, and their innervation of the posterior parietal cortex, is critical to these surprise-induced enhancements of attention in associative learning. The present study considered the origin of prediction error information important for the operation of this system by examining the effects of disrupting communication between the midbrain substantia nigra pars compacta (SNc) and the CeA. Rats received unilateral lesions of the SNc and lesions of the CeA in either the contralateral or ipsilateral hemisphere. Contralateral lesions eliminated the surprise-induced enhancement of attention and learning that was displayed by rats with ipsilateral control lesions. These results show that SNc-CeA communication is critical to mechanisms by which the coding of prediction error by midbrain dopamine neurons is translated into enhancement of attention and learning modulated by the cholinergic system.