Augmenting glutamatergic, but not dopaminergic, activity in the nucleus accumbens shell disrupts responding to a discrete alcohol cue in an alcohol context.

Discrete alcohol cues and contexts are relapse triggers for people with alcohol use disorder exerting particularly powerful control over behaviour when they co-occur. Here, we investigated the neural substrates subserving the capacity for alcohol-associated contexts to elevate responding to an alcohol-predictive conditioned stimulus (CS). Specifically, rats were trained in a distinct 'alcohol context' to respond by entering a fluid port during a discrete auditory CS that predicted the delivery of alcohol and were familiarized with a 'neutral context' wherein alcohol was never available. When conditioned CS responding was tested by presenting the CS without alcohol, we found that augmenting glutamatergic activity in the nucleus accumbens (NAc) shell by microinfusing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) reduced responding to an alcohol CS in an alcohol, but not neutral, context. Further, AMPA microinfusion robustly affected behaviour, attenuating the number, duration and latency of CS responses selectively in the alcohol context. Although dopaminergic inputs to the NAc shell were previously shown to be necessary for CS responding in an alcohol context, here, chemogenetic excitation of ventral tegmental area (VTA) dopamine neurons and their inputs to the NAc shell did not affect CS responding. Critically, chemogenetic excitation of VTA dopamine neurons affected feeding behaviour and elevated c-fos immunoreactivity in the VTA and NAc shell, validating the chemogenetic approach. These findings enrich our understanding of the substrates underlying Pavlovian responding for alcohol and reveal that the capacity for contexts to modulate responding to discrete alcohol cues is delicately underpinned by the NAc shell.

Corticostriatal Suppression of Appetitive Pavlovian Conditioned Responding.

The capacity to suppress learned responses is essential for animals to adapt in dynamic environments. Extinction is a process by which animals learn to suppress conditioned responding when an expected outcome is omitted. The infralimbic (IL) cortex to nucleus accumbens shell (NAcS) neural circuit is implicated in suppressing conditioned responding after extinction, especially in the context of operant cocaine-seeking behavior. However, the role of the IL-to-NAcS neural circuit in the extinction of responding to appetitive Pavlovian cues is unknown, and the psychological mechanisms involved in response suppression following extinction are unclear. We trained male Long Evans rats to associate a 10 s auditory conditioned stimulus (CS; 14 trials per session) with a sucrose unconditioned stimulus (US; 0.2 ml per CS) in a specific context, and then following extinction in a different context, precipitated a renewal of CS responding by presenting the CS alone in the original Pavlovian conditioning context. Unilateral, optogenetic stimulation of the IL-to-NAcS circuit selectively during CS trials suppressed renewal. In a separate experiment, IL-to-NAcS stimulation suppressed CS responding regardless of prior extinction and impaired extinction retrieval. Finally, IL-to-NAcS stimulation during the CS did not suppress the acquisition of Pavlovian conditioning but was required for the subsequent expression of CS responding. These results are consistent with multiple studies showing that the IL-to-NAcS neural circuit is involved in the suppression of operant cocaine-seeking, extending these findings to appetitive Pavlovian cues. The suppression of appetitive Pavlovian responding following IL-to-NAcS circuit stimulation, however, does not appear to be an extinction-dependent process.SIGNIFICANCE STATEMENT Extinction is a form of inhibitory learning through which animals learn to suppress conditioned responding in the face of nonreinforcement. We investigated the role of the IL cortex inputs to the NAcS in the extinction of responding to appetitive Pavlovian cues and the psychological mechanisms involved in response suppression following extinction. Using in vivo optogenetics, we found that stimulating the IL-to-NAcS neural circuit suppressed context-induced renewal of conditioned responding after extinction. In a separate experiment, stimulating the IL-to-NAcS circuit suppressed conditioned responding in an extinction-independent manner. These findings can be used by future research aimed at understanding how corticostriatal circuits contribute to behavioral flexibility and mental disorders that involve the suppression of learned behaviors.

Optogenetic Activation of the Infralimbic Cortex Suppresses the Return of Appetitive Pavlovian-Conditioned Responding Following Extinction.

The infralimbic medial prefrontal cortex (IL) is important for suppressing learned behavior after extinction, but whether this function extends to responses acquired through appetitive Pavlovian conditioning is unclear. We trained male, Long-Evans rats to associate a white-noise conditional stimulus (CS; 10 s; 14 presentations per session) with 10% liquid sucrose (0.2 mL per CS presentation), and recorded entries into the fluid port during the CS. The CS was presented without sucrose in subsequent extinction and test sessions. Increasing IL activity with pretest microinfusions of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 0, 0.3 nmol; 0.3 µl/side) reduced the reinstatement of CS-elicited port entries. The same result was obtained when IL neurons that expressed Channelrhodopsin-2 (ChR2) were optically stimulated during CS presentations at test (473 nm, 5 ms pulses at 20 Hz for 10.2 s, unilateral). Optical stimulation of ChR2-expressing IL neurons during CS presentations also reduced spontaneous recovery and context-induced renewal. Furthermore, optical stimulation (1) during intertrial intervals had no impact on renewal, (2) depolarized ChR2-expressing IL pyramidal neurons in vitro, and (3) preferentially increased Fos in ChR2-expressing neurons. These novel converging data highlight a critical role for the IL in suppressing the return of appetitive Pavlovian-conditioned responding following extinction.

Neural correlates of recall and extinction in a rat model of appetitive Pavlovian conditioning.

Extinction is a fundamental form of inhibitory learning that is important for adapting to changing environmental contingencies. While numerous studies have investigated the neural correlates of extinction using Pavlovian fear conditioning and appetitive operant reward-seeking procedures, less is known about the neural circuitry mediating the extinction of appetitive Pavlovian responding. Here, we aimed to generate an extensive brain activation map of extinction learning in a rat model of appetitive Pavlovian conditioning. Male Long-Evans rats were trained to associate a conditioned stimulus (CS; 20 s white noise) with the delivery of a 10% sucrose unconditioned stimulus (US; 0.3 ml/CS) to a fluid port. Control groups also received CS presentations, but sucrose was delivered either during the inter-trial interval or in the home-cage. After conditioning, 1 or 6 extinction sessions were conducted in which the CS was presented but sucrose was withheld. We performed Fos immunohistochemistry and network connectivity analyses on a set of cortical, striatal, thalamic, and amygdalar brain regions. Neural activity in the prelimbic cortex, ventral orbitofrontal cortex, nucleus accumbens core, and paraventricular nucleus of the thalamus was greater during recall relative to extinction. Conversely, prolonged extinction following 6 sessions induced increased neural activity in the infralimbic cortex, medial orbitofrontal cortex, and nucleus accumbens shell compared to home-cage controls. All these structures were similarly recruited during recall on the first extinction session. These findings provide novel evidence for the contribution of brain areas and neural networks that are differentially involved in the recall versus extinction of appetitive Pavlovian conditioned responding.

Learning processes in relapse to alcohol use: lessons from animal models.

RATIONALE: Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use. OBJECTIVE: We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies. KEY FINDINGS: Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats. CONCLUSION: Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

The rodent medial prefrontal cortex and associated circuits in orchestrating adaptive behavior under variable demands.

Emerging evidence implicates rodent medial prefrontal cortex (mPFC) in tasks requiring adaptation of behavior to changing information from external and internal sources. However, the computations within mPFC and subsequent outputs that determine behavior are incompletely understood. We review the involvement of mPFC subregions, and their projections to the striatum and amygdala in two broad types of tasks in rodents: 1) appetitive and aversive Pavlovian and operant conditioning tasks that engage mPFC-striatum and mPFC-amygdala circuits, and 2) foraging-based tasks that require decision making to optimize reward. We find support for region-specific function of the mPFC, with dorsal mPFC and its projections to the dorsomedial striatum supporting action control with higher cognitive demands, and ventral mPFC engagement in translating affective signals into behavior via discrete projections to the ventral striatum and amygdala. However, we also propose that defined mPFC subdivisions operate as a functional continuum rather than segregated functional units, with crosstalk that allows distinct subregion-specific inputs (e.g., internal, affective) to influence adaptive behavior supported by other subregions.

Ventral Hippocampal CA1 and CA3 Differentially Mediate Learned Approach-Avoidance Conflict Processing.

Approach-avoidance conflict arises when an animal encounters a stimulus that is associated simultaneously with positive and negative valences [1]. The effective resolution of approach-avoidance conflict is critical for survival and is believed to go awry in a number of mental disorders, such as anxiety and addiction. An accumulation of evidence from both rodents and humans suggests that the ventral hippocampus (anterior in humans) plays a key role in approach-avoidance conflict processing [2-8], with one influential model proposing that this structure modulates behavioral inhibition in the face of conflicting goals by increasing the influence of negative valences [9]. Very little is known, however, about the contributions of specific hippocampal subregions to this process-an important issue given the functional and anatomical heterogeneity of this structure. Using a non-spatial cue-based paradigm in rats, we found that transient pharmacological inactivation of ventral CA1 produced an avoidance of a conflict cue imbued with both learned positive and learned negative outcomes, whereas inactivation of the ventral CA3 resulted in the opposite pattern of behavior, with significant preference for the conflict cue. In contrast, dorsal CA1- and CA3-inactivated rats showed no change in conflict behavior, and furthermore, additional behavioral tasks confirmed that the observed pattern of approach-avoidance findings could not be explained by other factors, such as differential alterations in novelty detection or locomotor activity. Our data demonstrate that ventral CA1 and CA3 subserve distinct and opposing roles in approach-avoidance conflict processing and provide important insight into the functions and circuitry of the ventral hippocampus.

Alcohol-seeking and relapse: A focus on incentive salience and contextual conditioning.

Environmental stimuli that reliably accompany alcohol intake can become associated with the pharmacological effects of alcohol through classical (Pavlovian) conditioning. Of growing interest to addiction researchers is whether or not this process results in the attribution of incentive salience to alcohol-predictive cues, which could motivate alcohol-seeking behavior and relapse. To evaluate this question, we present a review of rodent behavioral studies that examined the capacity of alcohol-predictive cues to (i) support sign-tracking behavior, (ii) serve as conditioned reinforcers, and (iii) produce Pavlovian-to-instrumental transfer. A second, emerging area of research is focused on delineating the role of context in alcohol-seeking behavior and relapse. Here, we review studies showing that alcohol-associated contexts (i) support conditioned place preference, (ii) renew extinguished alcohol-seeking behavior, and (iii) modulate alcohol-seeking responses elicited by discrete alcohol-predictive cues. These behavioral effects may be mediated by unique psychological processes, and have important implications for cue-reactivity studies and neurobiological research.

Individual Differences in the Attribution of Incentive Salience to a Pavlovian Alcohol Cue.

Individual differences exist in the attribution of incentive salience to conditioned stimuli associated with food. Here, we investigated whether individual differences also manifested with a Pavlovian alcohol conditioned stimulus (CS). We compiled data from five experiments that used a Pavlovian autoshaping paradigm and tests of conditioned reinforcement. In all experiments, male, Long-Evans rats with unrestricted access to food and water were acclimated to 15% ethanol. Next, rats received Pavlovian autoshaping training, in which a 10 s presentation of a retractable lever served as the CS and 0.2 mL of 15% ethanol served as the unconditioned stimulus (US). Finally, rats underwent conditioned reinforcement tests in which nose-pokes to an active aperture led to brief presentations of the lever-CS, but nose-pokes to an inactive aperture had no consequence. Rats were categorized as sign-trackers, goal-trackers and intermediates based on a response bias score that reflected their tendencies to sign-track or goal-track at different times during training. We found that distinct groups of rats either consistently interacted with the lever-CS ("sign-trackers") or routinely approached the port during the lever-CS ("goal-trackers") across a majority of the training sessions. However, some individuals ("shifted sign-trackers") with an early tendency to goal-track later shifted to comparable asymptotic levels of sign-tracking as the group identified as sign-trackers. The lever-CS functioned as a conditioned reinforcer for sign-trackers and shifted sign-trackers, but not for goal-trackers. These results provide evidence of robust individual differences in the extent to which a Pavlovian alcohol cue gains incentive salience and functions as a conditioned reinforcer.