Context-dependent prefrontal cortex regulation of cocaine self-administration and reinstatement behaviors in rats

tRNA epitranscriptomic alterations associated with opioid-induced reward-seeking and long-term opioid withdrawal in male mice

DNA cytosine methylation has been documented as a potential epigenetic mechanism of transcriptional regulation underlying opioid use disorder. However, methylation of RNA cytosine residues, which would drive another level of biological influence as an epitranscriptomic mechanism of gene and protein regulation has not been studied in the context of addiction. Here, we probed whether chronic morphine exposure could alter tRNA cytosine methylation (m5C) and resulting expression levels in the medial prefrontal cortex (mPFC), a brain region crucial for reward processing and executive function that exhibits opioid-induced molecular restructuring. We identified dynamic changes in glycine tRNA (tRNAGlyGCC) cytosine methylation, corresponding to altered expression levels of this tRNA at multiple timepoints following 15 days of daily morphine. Additionally, a robust increase in methylation, coupled with decreased expression, was present after 30 days of withdrawal, suggesting that repeated opioid administration produces changes to the tRNA regulome long after discontinuation. Furthermore, forebrain-wide knockout of neuronal Nsun2, a tRNA methyltransferase, was associated with disruption of opioid conditioned place preference, and this effect was recapitulated by regional mPFC Nsun2 knockout. Taken together, these studies provide a foundational link between the regulation of tRNA cytosine methylation and opioid reward and highlight the tRNA machinery as a potential therapeutic target in addiction.

The anterior insula and its projection to amygdala nuclei modulate the abstinence-exacerbated expression of conditioned place preference

RATIONALE

Relapse into substance use is often triggered by exposure to drug-related environmental cues. The magnitude of drug seeking depends on the duration of abstinence, a phenomenon known as the incubation of drug craving. Clinical and preclinical research shows that the insular cortex is involved in substance use disorders and cue-induced drug seeking. However, the role of the insula on memory retrieval and motivational integration for cue-elicited drug seeking remains to be determined.

OBJECTIVES

We investigated the role of the anterior insular cortex (aIC) and its glutamatergic projection to amygdala nuclei (aIC-AMY) on the expression of conditioned place preference (CPP) during early and late abstinence.

METHODS

Male adult C57BL/6J mice underwent amphetamine-induced CPP, and their preference was tested following 1 or 14 days of abstinence. aIC and aIC-AMY functional role in CPP expression was assessed at both abstinence periods by employing optogenetic silencing and behavioral pharmacology.

RESULTS

Compared to a single day, an exacerbated preference for the amphetamine-paired context was observed after 14 days of abstinence. Photoinhibition of either aIC or aIC-AMY projection reduced CPP expression following late but not early abstinence. Similarly, the antagonism of aIC NMDA receptors reduced CPP expression after 14 days of abstinence but not 1 day.

CONCLUSIONS

These results suggest that aIC and its glutamatergic output to amygdala nuclei constitute critical neurobiological substrates mediating enhanced motivational cue reactivity during the incubation of amphetamine craving rather than contextual memory recall. Moreover, cortical NMDA receptor signaling may become sensitized during abstinence, ultimately modulating disproportioned drug seeking.

Differential Roles of Oxytocin Receptors in the Prefrontal Cortex and Nucleus Accumbens on Cocaine Self-Administration and Reinstatement of Cued Cocaine Seeking in Male Rats

BACKGROUND

Little is known about the specific roles of cortical and accumbal oxytocin receptors in drug use disorders. To better understand the importance of the endogenous oxytocin system in cocaine relapse behavior, we developed an adeno-associated viral vector-expressing short hairpin (sh) RNAs to selectively degrade the rat oxytocin receptor (OxyR) mRNA in vivo.

METHODS

Male (Sprague-Dawley) rats received bilateral infusions of the shRNA for the oxytocin receptor (shOxyR) or an shRNA control virus into the prefrontal cortex (PFC) or the nucleus accumbens core (NAc). Rats self-administered cocaine on an escalating FR ratio for 14 days, lever responding was extinguished, and rats were tested for cued and cocaine-primed reinstatement of drug seeking.

RESULTS

OxyR knockdown in the PFC delayed the acquisition of lever pressing on an fixed ratio 1 schedule of reinforcement. All rats eventually acquired the same level of lever pressing and discrimination, and there were no differences in extinction. OxyR knockdown in the NAc had no effect during acquisition. In both the PFC and NAc, the shOxyR decreased cued reinstatement relative to shRNA control virus but was without effect during drug-primed reinstatement. OxyR knockdown in the PFC increased chamber activity during a social interaction task.

CONCLUSIONS

This study provides critical new information about how endogenous OxyRs function to affect drug seeking in response to different precipitators of relapse. The tool developed to knockdown OxyRs in rat could provide important new insights that aid development of oxytocin-based therapeutics to reduce return-to-use episodes in people with substance use disorder and other neuropsychiatric disorders.

Still a "hidden island"? The rodent insular cortex in drug seeking, reward, and risk

The insular cortex (IC) is implicated in risky decision making and drug-seeking behaviors, in a manner dissociable from natural reward seeking. However, evidence from rodent studies of motivated behaviors suggests that the role of the IC is not always consistent across procedures. Moreover, there is evidence of dissociation of function between posterior (pIC) and anterior (aIC) subregions in these behaviors. Under which circumstances, and by which mechanisms, these IC subregions are recruited to regulate motivated behaviors remains unclear. Here, we discuss evidence of rodent pIC and aIC function across drug-related behaviors, natural reward seeking, and decision making under risk and highlight procedural differences that may account for seemingly conflicting findings. Although gaps in the literature persist, we hypothesize that IC activity is broadly important for selection of appropriate behaviors based on learned action-outcome contingencies and that associated risk is sufficient, but not necessary, to recruit the aIC in reward seeking without involving the pIC.

Using lickometry to infer differential contributions of salience network regions during compulsion-like alcohol drinking

Alcohol use disorder extracts substantial personal, social and clinical costs, and continued intake despite negative consequences (compulsion-like consumption) can contribute strongly. Here we discuss lickometry, a simple method where lick times are determined across a session, while analysis across many aspects of licking can offer important insights into underlying psychological and action strategies, including their brain mechanisms. We first describe studies implicating anterior insula (AIC) and dorsal medial prefrontal cortex (dMPF) in compulsion-like responding for alcohol, then review work suggesting that AIC/ventral frontal cortex versus dMPF regulate different aspects of behavior (oral control and overall response strategy, versus moment-to-moment action organization). We then detail our lickometer work comparing alcohol-only drinking (AOD) and compulsion-like drinking under moderate- or higher-challenge (ModChD or HiChD, using quinine-alcohol). Many studies have suggested utilization of one of two main strategies, with higher motivation indicated by more bouts, and greater palatability suggested by longer, faster bouts. Instead, ModChD shows decreased variability in many lick measures, which is unexpected but consistent with the suggested importance of automaticity for addiction. Also surprising is that HiChD retains several behavior changes seen with ModChD, reduced tongue variability and earlier bout start, even though intake is otherwise disrupted. Since AIC-related measures are retained under both moderate- and higher-challenge, we propose a novel hypothesis that AIC sustains overall commitment regardless of challenge level, while disordered licking during HiChD mirrors the effects of dMPF inhibition. Thus, while AIC provides overall drive despite challenge, the ability to act is ultimately determined within the dMPF.

Role of Piriform Cortex and Its Afferent Projections in Relapse to Fentanyl Seeking after Food Choice-Induced Voluntary Abstinence

We previously demonstrated a role of piriform cortex (Pir) in relapse to fentanyl seeking after food choice-induced voluntary abstinence. Here, we used this model to further study the role of Pir and its afferent projections in fentanyl relapse. We trained male and female rats to self-administer palatable food pellets for 6 d (6 h/day) and fentanyl (2.5 µg/kg/infusion, i.v.) for 12 d (6 h/day). We assessed relapse to fentanyl seeking after 12 voluntary abstinence sessions, achieved through a discrete choice procedure between fentanyl and palatable food (20 trials/session). We determined projection-specific activation of Pir afferents during fentanyl relapse with Fos plus the retrograde tracer cholera toxin B (injected into Pir). Fentanyl relapse was associated with increased Fos expression in anterior insular cortex (AI) and prelimbic cortex (PL) neurons projecting to Pir. We next used an anatomical disconnection procedure to determine the causal role of these two projections (AI→Pir and PL→Pir) in fentanyl relapse. Contralateral but not ipsilateral disconnection of AI→Pir projections decreased fentanyl relapse but not reacquisition of fentanyl self-administration. In contrast, contralateral but not ipsilateral disconnection of PL→Pir projections modestly decreased reacquisition but not relapse. Fluorescence-activated cell sorting and quantitative PCR data showed molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse. Finally, we found minimal or no sex differences in fentanyl self-administration, fentanyl versus food choice, and fentanyl relapse. Our results indicate that AI→Pir and PL→Pir projections play dissociable roles in nonreinforced relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after food choice-induced voluntary abstinence.SIGNIFICANCE STATEMENT We previously showed a role of Pir in fentanyl relapse after food choice-induced voluntary abstinence in rats, a procedure mimicking human abstinence or a significant reduction in drug self-administration because of the availability of alternative nondrug rewards. Here, we aimed to further characterize the role of Pir in fentanyl relapse by investigating the role of Pir afferent projections and analyzing molecular changes in relapse-activated Pir neurons. We identified dissociable roles of two Pir afferent projections (AI→Pir and PL→Pir) in relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after voluntary abstinence. We also characterized molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse.

Distinct suppressing effects of deep brain stimulation in the orbitofrontal cortex on the development, extinction, and reinstatement of methamphetamine-seeking behaviors

AIMS

The orbitofrontal cortex (OFC) is implicated in compulsive drug-seeking and relapse, the characteristics that result in addiction treatment failure. Structural and functional impairments within the OFC have been detected in many substance use disorders (SUDs). Deep brain stimulation (DBS) is proposed as a promising therapeutic option in treating SUDs. Therefore, the present study aimed to investigate the potential efficacy of DBS application on the various stages of the methamphetamine-conditioned place preference (CPP) paradigm in rats.

MAIN METHODS

Electrodes were implanted unilaterally in the rat's right OFC. DBS in the form of high- or low-frequency stimulation (HFS: 130 Hz, LFS: 13 Hz) was applied during the 5-day conditioning phase (a daily 30-min session) or extinction period (30-min session, daily, ten days) of methamphetamine-induced CPP in two separate sets of experiments. Following extinction, place preference was reinstated by injecting a priming dose of methamphetamine (0.25 mg/kg).

KEY FINDINGS

The HFS and LFS significantly decreased the methamphetamine place preference when applied over the conditioning period. In the extinction experiment, only HFS could remarkably accelerate the extinction of reward-context associations and even reduce the methamphetamine-induced reinstatement of seeking behaviors.

SIGNIFICANCE

Conclusively, DBS administration in the OFC demonstrated some positive results, including suppressing effects on the development, maintenance, and relapse of methamphetamine-seeking behavior. These findings encourage conducting more preclinical studies to strongly suggest a wide range of DBS applications in cortical areas such as OFC as an efficient treatment modality for psychostimulant use disorder.

The roles of rat medial prefrontal and orbitofrontal cortices in relapse to cocaine-seeking: A comparison across methods for identifying neurocircuits

A large body of research supports the notion that regions of the rodent frontal cortex regulate reinstatement of cocaine seeking after cessation of intravenous cocaine self-administration. However, earlier studies identifying the roles of medial (mPFC) and orbital prefrontal cortices (OFC) in reinstatement relied on pharmacological inactivation methods, which indiscriminately inhibited cells within a target region. Here, we first review the anatomical borders and pathways of the rat mPFC and OFC. Next, we compare and contrast findings from more recent cocaine seeking and reinstatement studies that used chemogenetics, optogenetics, or advanced tracing to manipulate specific local cell types or input/output projections of the mPFC and OFC subregions. We found that these studies largely corroborated the roles for mPFC subregions as ascribed by pharmacological inactivation studies. Namely, the prelimbic cortex generally drives cocaine seeking behaviors while the infralimbic cortex is recruited to inhibit cocaine seeking by extinction training but may contribute to seeking after prolonged abstinence. While the OFC remains understudied, we suggest it should not be overlooked, and, as with prelimbic and infralimbic cortices, we identify specific pathways of interest for future studies.

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.

Kappa opioid receptor modulation of excitatory drive onto nucleus accumbens fast-spiking interneurons

The dynorphin/kappa opioid receptor (KOR) system within the nucleus accumbens (NAc) contributes to affective states. Parvalbumin fast-spiking interneurons (PV-FSIs), a key component of feedforward inhibition, participate in integration of excitatory inputs to the NAc by robustly inhibiting select populations of medium spiny output neurons, therefore greatly influencing NAc dependent behavior. How the dynorphin/KOR system regulates feedforward inhibition in the NAc remains unknown. Here, we elucidate the molecular mechanisms of KOR inhibition of excitatory transmission onto NAc PV-FSIs using a combination of whole-cell patch-clamp electrophysiology, optogenetics, pharmacology, and a parvalbumin reporter mouse. We find that postsynaptic KOR stimulation induces long-term depression (LTD) of excitatory synapses onto PV-FSI by stimulating the endocytosis of AMPARs via a PKA and calcineurin-dependent mechanism. Furthermore, KOR regulation of PV-FSI synapses are input specific, inhibiting thalamic but not cortical inputs. Finally, following acute stress, a protocol known to elevate dynorphin/KOR signaling in the NAc, KOR agonists no longer inhibit excitatory transmission onto PV-FSI. In conclusion, we delineate pathway-specific mechanisms mediating KOR control of feedforward inhibitory circuits in the NAc and provide evidence for the recruitment of this system in response to stress.

Better living through understanding the insula: Why subregions can make all the difference

Insula function is considered critical for many motivated behaviors, with proposed functions ranging from attention, behavioral control, emotional regulation, goal-directed and aversion-resistant responding. Further, the insula is implicated in many neuropsychiatric conditions including substance abuse. More recently, multiple insula subregions have been distinguished based on anatomy, connectivity, and functional contributions. Generally, posterior insula is thought to encode more somatosensory inputs, which integrate with limbic/emotional information in middle insula, that in turn integrate with cognitive processes in anterior insula. Together, these regions provide rapid interoceptive information about the current or predicted situation, facilitating autonomic recruitment and quick, flexible action. Here, we seek to create a robust foundation from which to understand potential subregion differences, and provide direction for future studies. We address subregion differences across humans and rodents, so that the latter's mechanistic interventions can best mesh with clinical relevance of human conditions. We first consider the insula's suggested roles in humans, then compare subregional studies, and finally describe rodent work. One primary goal is to encourage precision in describing insula subregions, since imprecision (e.g. including both posterior and anterior studies when describing insula work) does a disservice to a larger understanding of insula contributions. Additionally, we note that specific task details can greatly impact recruitment of various subregions, requiring care and nuance in design and interpretation of studies. Nonetheless, the central ethological importance of the insula makes continued research to uncover mechanistic, mood, and behavioral contributions of paramount importance and interest. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Effects of food restriction on the conditioned reinforcing properties of an opioid-associated stimulus

Food restriction promotes drug self-administration; however, the effects of food restriction on the conditioned reinforcing properties of drug-associated stimuli are less clear. We tested the extent to which food restriction modified the conditioned reinforcing properties of a remifentanil-associated stimulus following conditioning with 3.2 or 1.0 μg/kg/infusion of remifentanil. First, we provided restricted (20 g/day standard chow) or ad libitum access to standard chow to rats. Second, within each feeding condition, we exposed rats to 20 intravenous infusions of remifentanil and 20 stimulus presentations that were delivered response independently each day for 5 days. For the experimental group (paired Pavlovian), the remifentanil infusions and stimulus presentations were delivered concurrently. The control group (random control) received the same number of infusions and stimulus presentations, but were not paired. For 28 sessions, we tested the extent to which the stimulus functioned as a conditioned reinforcer by allowing rats to freely respond for presentations of the remifentanil-associated stimulus. Following conditioning with 3.2 μg/kg/infusion of remifentanil, we found that rats that in the Paired Pavlovian group responded for the remifentanil-associated stimulus significantly more than rats in the Random control group, regardless of feeding condition. Following conditioning with 1.0 μg/kg/infusion of remifentanil, the remifentanil-associated stimulus was not associated with conditioned reinforcing properties, regardless of feeding condition. These findings confirm previous research demonstrating that a remifentanil-associated stimulus takes on conditioned reinforcing properties in a dose-dependent manner.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

The Projection From Ventral CA1, Not Prefrontal Cortex, to Nucleus Accumbens Core Mediates Recent Memory Retrieval of Cocaine-Conditioned Place Preference

Drug-paired cues inducing memory retrieval by expressing drug-seeking behaviors present a major challenge to drug abstinence. How neural circuits coordinate for drug memory retrieval remains unclear. Here, we report that exposure of the training chamber where cocaine-conditioned place preference (CPP) was performed increased neuronal activity in the core of nucleus accumbens (AcbC), ventral CA1 (vCA1), and medial prefrontal cortex (mPFC), as shown by elevated pERK and c-Fos levels. Chemogenetic inhibition of neuronal activity in the vCA1 and AcbC, but not mPFC, reduced the time spent in the cocaine-paired compartment, suggesting that the vCA1 and AcbC are required for the retrieval of cocaine-CPP memory and are key nodes recruited for cocaine memory storage. Furthermore, chemogenetic inhibition of the AcbC-projecting vCA1 neurons, but not the AcbC-projecting mPFC neurons, decreased the expression of cocaine-CPP. Optogenetic inhibition of the vCA1–AcbC projection, but not the mPFC–AcbC projection, also reduced the preference for the cocaine-paired compartment. Taken together, the cue-induced natural recall of cocaine memory depends on vCA1–AcbC circuits. The connectivity from the vCA1 to the AcbC may store the information of the cue–cocaine reward association critically required for memory retrieval. These data thus provide insights into the neural circuit basis of retrieval of drug-related memory.

Infralimbic Cortex Biases Preference Decision Making for Offspring over Competing Cocaine-Associated Stimuli in New Mother Rats

In the context of drug abuse, converging evidence suggests that cocaine use in new mothers is significantly reduced by the competing motivation related to child rearing. Given the demonstrated importance of the medial prefrontal cortex (mPFC) in decision-making processes, we investigated the contribution of distinct regions of the mPFC [anterior cingulate (Cg1), prelimbic (PrL), infralimbic (IL)] to decision making in new mother rats performing a concurrent pup/cocaine choice conditioned place preference (CPP) task. When given a choice, inactivation of IL cortex significantly biased decision making of mother rats toward cocaine-associated cues, highly contrasting the distribution of preferences by control groups. In contrast, inactivation of PrL cortex had the opposite effect, significantly increasing offspring bias in the decision making, such that none of the mothers chose the cocaine-associated alternative. Cg1 inactivation was without effect. Functional inactivation of these same mPFC subregions had no effect in a non-conflict CPP task in which context-induced cocaine or pup seeking were examined separately. Notably, inactivation of the IL cortex also interfered with maternal behavior. Taken together, we have identified a specific role of the IL cortex in the prioritization of offspring over drug competing alternatives, thus promoting resistance to drug use in new mothers.

Deep brain stimulation of the orbitofrontal cortex prevents the development and reinstatement of morphine place preference

The orbitofrontal cortex (OFC) is involved in compulsive drug seeking and drug relapse. Its involvement in cue-, context-, and stress-induced reinstatement of drug seeking has also been confirmed in animal models. Deep brain stimulation (DBS) was proposed to be an effective intervention for patients with treatment-refractory addiction. Therefore, in the present study, we investigated the potential efficacy of DBS in the OFC for controlling addictive-like behaviors in rats. Rats were bilaterally implanted with electrodes in the OFC and trained to the morphine conditioned place preference (CPP; 3, 5, and 7 mg/kg). High-frequency (HF; 130 Hz) or low-frequency (LF; 13 Hz) DBS-like stimulation was applied during the conditioning (40 minutes, once daily, 3 days) or extinction (20 minutes, once daily, 6-10 days) trials. Following the extinction, morphine preference was reinstated by a priming dose of morphine (2 mg/kg). When applied during the conditioning phase, HF-DBS significantly decreased preference for the morphine-associated context. HF-DBS during the extinction phase of morphine CPP reduced the number of days to full extinction of morphine preference and prevented morphine priming-induced recurrence of morphine preference. LF-DBS did not change any of these addictive behaviors. HF-DBS had no significant effect on novel object recognition memory. In conclusion, HF-DBS of the OFC prevented morphine preference, facilitated extinction of morphine preference, and blocked drug priming-induced reinstatement of morphine seeking. These findings may indicate a potential applicability of DBS in the treatment of relapse to drug use. Further studies will be necessary to assess the translatability of these findings to the clinic.

Optogenetic inhibition of the medial prefrontal cortex reduces methamphetamine-primed reinstatement in male and female rats

Preclinical findings suggest sex-differences exist in drug-seeking behavior following methamphetamine (METH) self-administration. The medial prefrontal cortex (mPFC), is thought to contribute to the reinstatement of drug-seeking in males. Glutamatergic neurons project from the prelimbic portion of the mPFC to various brain regions modulating activity including the nucleus accumbens; thus the prelimbic region of the mPFC is thought to contribute to drug-seeking behaviors. Although studied in males, little research has investigated the role of the mPFC in females. The purpose of this study was to investigate if the prelimbic portion of the mPFC plays a role in METH-seeking behavior in both male and female rats. Animals were allowed to self-administer METH, and underwent extinction and two reinstatement sessions. Reinstatement sessions were counterbalanced such that optogenetic inhibition targeting the prelimbic cortex of the mPFC occurred only during one reinstatement session. Results revealed an increase in METH consumption during self-administration in male and female animals. During extinction, lever-pressing behavior decreased as training progressed. Under sham conditions, female rats exhibited significantly higher drug-seeking behavior during reinstatement. However, when optogenetic inhibition was applied, both male and female animals significantly decreased drug-seeking. In both males and females, the prelimbic portion of the mPFC plays an important role in drug-seeking behavior as related to METH-seeking.

The Insula: A Brain Stimulation Target for the Treatment of Addiction

Substance use disorders (SUDs) are a growing public health concern with only a limited number of approved treatments. However, even approved treatments are subject to limited efficacy with high long-term relapse rates. Current treatment approaches are typically a combination of pharmacotherapies and behavioral counselling. Growing evidence and technological advances suggest the potential of brain stimulation techniques for the treatment of SUDs. There are three main brain stimulation techniques that are outlined in this review: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), and deep brain stimulation (DBS). The insula, a region of the cerebral cortex, is known to be involved in critical aspects underlying SUDs, such as interoception, decision making, anxiety, pain perception, cognition, mood, threat recognition, and conscious urges. This review focuses on both the preclinical and clinical evidence demonstrating the role of the insula in addiction, thereby demonstrating its promise as a target for brain stimulation. Future research should evaluate the optimal parameters for brain stimulation of the insula, through the use of relevant biomarkers and clinical outcomes for SUDs.

Prelimbic Cortical Neurons Track Preferred Reward Value and Reflect Impulsive Choice during Delay Discounting Behavior

In delay discounting, individuals discount the value of a reward based on the delay to its receipt. The prelimbic cortex (PrL) is heavily interconnected with several brain regions implicated in delay discounting, but the specific contributions of the PrL to delay discounting are unknown. Here, we used multineuron electrophysiological recording methods in Long-Evans male (n = 10) and female (n = 9) rats to characterize the firing dynamics of PrL neurons during discrete cue and lever press events in a delay discounting task. Rats' initial preference for the large reward decreased as delays for that outcome increased across blocks, reflecting classic discounting behavior. Electrophysiological recordings revealed that subgroups of neurons exhibited phasic responses to cue presentations and lever presses. These phasic neurons were found to respond to either large/delay, small/immediate, or both trial types and the percentage of these neurons shifted across blocks as the expected value of the reward changed. Critically, this shift was only seen during trials in which animals could choose their preferred option (free choice trials) and not during trials where animals could choose only one option (forced choice trials). Further, this shift was dependent on rats' inherent impulsivity because high impulsive rats demonstrated a greater percentage of small/immediate-responsive neurons as the task progressed. Collectively, these findings suggest a unique role for the PrL in encoding reward value during delay discounting that is influenced by individual differences in impulsivity.SIGNIFICANCE STATEMENT In delay discounting, individuals discount the value of a reward based on the delay to its receipt. Here, we used electrophysiology to investigate the role of the prelimbic cortex (PrL) in this process. We found that subsets of neurons shifted activity as a function of the changing expected delay and reward magnitude, but this shift was only evident during trials in which animals could choose their preferred option. Further, this dynamic neural activity depended on rats' inherent impulsivity, with impulsive rats exhibiting a stronger neural shift toward the immediate reward as the task progressed. These findings suggest a role for the PrL in encoding reward value during delay discounting that is influenced by goal-directed context and individual differences in impulsivity.

Prelimbic and infralimbic cortical inactivations attenuate contextually driven discriminative responding for reward

The infralimbic (IL) and prelimbic (PL) cortices of the medial prefrontal cortex (mPFC) have been shown to differentially control context-dependent behavior, with the PL implicated in the expression of contextually conditioned fear and drug-seeking, and the IL in the suppression of these behaviors. However, the roles of these subregions in contextually driven natural reward-seeking remain relatively underexplored. The present study further examined the functional dichotomy within the mPFC in the contextual control over cued reward-seeking, using a contextual biconditional discrimination (CBD) task. Rats were first trained to emit a nose poke response to the presentation of an auditory stimulus (e.g., X) for the delivery of sucrose reward, and to withhold a nose poke response to the presentation of another auditory stimulus (e.g., Y) in a context-specific manner (e.g. Context A: X+, Y−; Context B: X−, Y+). Following acquisition, rats received bilateral microinjections of GABA receptor agonists (muscimol and baclofen), or saline into the IL or PL, prior to a CBD training session and a probe test (under extinction conditions). Both IL and PL inactivation resulted in robust impairment in CBD performance, indicating that both subregions are involved in the processing of appetitively motivated contextual memories in reward-seeking.

Common neurocircuitry mediating drug and fear relapse in preclinical models

BACKGROUND

Comorbidity of anxiety disorders, stressor- and trauma-related disorders, and substance use disorders is extremely common. Moreover, therapies that reduce pathological fear and anxiety on the one hand, and drug-seeking on the other, often prove short-lived and are susceptible to relapse. Considerable advances have been made in the study of the neurobiology of both aversive and appetitive extinction, and this work reveals shared neural circuits that contribute to both the suppression and relapse of conditioned responses associated with trauma or drug use.

OBJECTIVES

The goal of this review is to identify common neural circuits and mechanisms underlying relapse across domains of addiction biology and aversive learning in preclinical animal models. We focus primarily on neural circuits engaged during the expression of relapse.

KEY FINDINGS

After extinction, brain circuits involving the medial prefrontal cortex and hippocampus come to regulate the expression of conditioned responses by the amygdala, bed nucleus of the stria terminalis, and nucleus accumbens. During relapse, hippocampal projections to the prefrontal cortex inhibit the retrieval of extinction memories resulting in a loss of inhibitory control over fear- and drug-associated conditional responding.

CONCLUSIONS

The overlapping brain systems for both fear and drug memories may explain the co-occurrence of fear and drug-seeking behaviors.

Role of Glutamatergic Projections from the Ventral CA1 to Infralimbic Cortex in Context-Induced Reinstatement of Heroin Seeking

The prelimbic cortex (PL) and infralimbic cortex (IL) play a role in context-induced reinstatement of heroin seeking in an animal model of drug relapse. Both the PL and IL receive direct glutamatergic projections from the ventral CA1 (vCA1), which is also involved in context-induced reinstatement of cocaine and heroin seeking. Here we studied the role of vCA1-PL and vCA1-IL projections in context-induced reinstatement of heroin seeking by using electrophysiological, neuropharmacological, chemogenetic, and molecular methods. We showed that context-induced reinstatement of heroin seeking caused selective activation of the vCA1-IL but not vCA1-PL glutamatergic projections, decreased synaptosomal GluA2 expression in the IL, impaired basal synaptic transmission, and facilitation of long-term depression (LTD) in the vCA1-IL pathway. Additionally, chemogenetic inactivation of the vCA1-IL but not vCA1-PL pathway decreased context-induced reinstatement of heroin seeking. Inactivation of the vCA1-IL pathway also reversed synaptosomal GluA2 downregulation and basal transmission reduction, and blocked LTD induction. Taken together, our results demonstrate a critical role of the vCA1-IL glutamatergic projection in context-induced reinstatement of heroin seeking in a rat model of drug relapse.

Regionally Specific Effects of Oxytocin on Reinstatement of Cocaine Seeking in Male and Female Rats

BACKGROUND

Oxytocin reduces cued reinstatement of cocaine seeking in male and female rats, but the underlying neurobiology has not been uncovered. The majority of effort on this task has focused on oxytocin and dopamine interactions in the nucleus accumbens core. The nucleus accumbens core is a key neural substrate in relapse, and oxytocin administration in the nucleus accumbens core reduces reinstatement to methamphetamine cues. Further, the nucleus accumbens core has strong glutamatergic innervation from numerous regions including the prefrontal cortex. Thus, we hypothesize that oxytocin regulates presynaptic glutamate terminals in the nucleus accumbens core, thereby affecting reinstatement.

METHODS

To begin to evaluate this hypothesis, we examined the effects of intra-nucleus accumbens core oxytocin on extracellular glutamate levels in this region. We next determined if direct infusion of oxytocin into the nucleus accumbens core could attenuate cued reinstatement of cocaine seeking in a manner dependent on metabotropic glutamate 2/3 receptors. Finally, we tested if site-specific application of oxytocin in the prefrontal cortex reduced cued reinstatement of cocaine seeking.

RESULTS

We found an increase in nucleus accumbens core extracellular glutamate for several minutes following reverse dialysis of oxytocin. In male and female rats with a history of cocaine self-administration, site-specific application of oxytocin in the nucleus accumbens core and prefrontal cortex had opposing effects, decreasing and increasing cued reinstatement, respectively. The mGlu2/3 antagonist LY-341495 reversed oxytocin's ability to attenuate cued reinstatement.

CONCLUSIONS

While the precise mechanism by which oxytocin increases nucleus accumbens core glutamate is yet to be determined, the present results clearly support oxytocin mediation of glutamate neurotransmission in the nucleus accumbens core that impacts cued cocaine seeking.

Exposure to the Abused Inhalant Toluene Alters Medial Prefrontal Cortex Physiology

Inhalants, including toluene, target the addiction neurocircuitry and are often one of the first drugs of abuse tried by adolescents. The medial prefrontal cortex (mPFC) is involved in regulating goal-directed/reward-motivated behaviors and different mPFC sub-regions have been proposed to promote (prelimbic, PRL) or inhibit (infralimbic, IL) these behaviors. While this dichotomy has been studied in the context of other drugs of abuse, it is not known whether toluene exposure differentially affects neurons within PRL and IL regions. To address this question, we used whole-cell electrophysiology and determined the intrinsic excitability of PRL and IL pyramidal neurons in adolescent rats 24 h following a brief exposure to air or toluene vapor (10 500 p.p.m.). Prior to exposure, fluorescent retrobeads were injected into the NAc core (NAcc) or shell (NAcs) sub-regions to identify projection-specific mPFC neurons. In toluene treated adolescent rats, layer 5/6 NAcc projecting PRL (PRL5/6) neurons fired fewer action potentials and this was associated with increased rheobase, increased spike duration, and reductions in membrane resistance and amplitude of the I_h current. No changes in excitability were observed in layer 2/3 NAcc projecting PRL (PRL2/3) neurons. In contrast to PRL neurons, layer 5 IL (IL5) and layer 2/3 (IL2/3) NAcc projecting neurons showed enhanced firing in toluene-exposed animals and in IL5 neurons, this was associated with a reduction in rheobase and AHP. For NAcs projecting neurons, toluene exposure significantly decreased firing of IL5 neurons and this was accompanied by an increased rheobase, increased spike duration, and reduced I_h amplitude. The intrinsic excitability of PRL5, PRL2/3, and IL2/3 neurons projecting to the NAcs was not affected by exposure to toluene. The changes in excitability observed 24 h after toluene exposure were not observed when recordings were performed 7 days after the exposure. Finally, there were no changes in intrinsic excitability of any region in rats exposed to toluene as adults. These findings demonstrate that specific projections of the reward circuitry are uniquely susceptible to the effects of toluene during adolescence supporting the idea that adolescence is a critical period of the development that is vulnerable to drugs of abuse.

Functional role for cortical-striatal circuitry in modulating alcohol self-administration

The cortical-striatal brain circuitry is heavily implicated in drug-use. As such, the present study investigated the functional role of cortical-striatal circuitry in modulating alcohol self-administration. Given that a functional role for the nucleus accumbens core (AcbC) in modulating alcohol-reinforced responding has been established, we sought to test the role of cortical brain regions with afferent projections to the AcbC: the medial prefrontal cortex (mPFC) and the insular cortex (IC). Long-Evans rats were trained to self-administer alcohol (15% alcohol (v/v)+2% sucrose (w/v)) during 30 min sessions. To test the functional role of the mPFC or IC, we utilized a chemogenetic technique (hM4D_i-Designer Receptors Activation by Designer Drugs) to silence neuronal activity prior to an alcohol self-administration session. Additionally, we chemogenetically silenced mPFC→AcbC or IC→AcbC projections, to investigate the role of cortical-striatal circuitry in modulating alcohol self-administration. Chemogenetically silencing the mPFC decreased alcohol self-administration, while silencing the IC increased alcohol self-administration, an effect absent in mCherry-Controls. Interestingly, silencing mPFC→AcbC projections had no effect on alcohol self-administration. In contrast, silencing IC→AcbC projections decreased alcohol self-administration, in a reinforcer-specific manner as there was no effect in rats trained to self-administer sucrose (0.8%, w/v). Additionally, no change in self-administration was observed in the mCherry-Controls. Together these data demonstrate the complex role of the cortical-striatal circuitry while implicating a role for the insula-striatal circuit in modulating ongoing alcohol self-administration.

Cued Reinstatement of Cocaine but Not Sucrose Seeking Is Dependent on Dopamine Signaling in Prelimbic Cortex and Is Associated with Recruitment of Prelimbic Neurons That Project to Contralateral Nucleus Accumbens Core

BACKGROUND

Drug cues recruit prelimbic cortex neurons that project to ipsilateral nucleus accumbens core. However, it is not known if the same is true for prelimbic cortex projections that decussate to innervate contralateral nucleus accumbens core. Further, a role for prelimbic cortex dopamine signaling in cued reinstatement of cocaine seeking has not been shown.

METHODS

We assessed Fos expression in prelimbic cortex neurons that project to contralateral nucleus accumbens core following cued reinstatement of cocaine or sucrose seeking. We also tested the effect of intra-prelimbic cortex infusions of the D1/D2 antagonist fluphenazine on cued cocaine and sucrose seeking.

RESULTS

Prelimbic cortex-contralateral nucleus accumbens core projections were activated by cocaine cues but not sucrose cues, and this activation correlated with reinstatement behavior. Blockade of prelimbic cortex dopamine signaling prevented cued reinstatement of cocaine- but not sucrose-seeking behavior.

CONCLUSIONS

Cued cocaine seeking is associated with activation of the prelimbic cortex-contralateral nucleus accumbens core pathway. Prelimbic cortex dopamine signaling is necessary for cues to reinstate drug-seeking behavior.

The infralimbic and prelimbic cortices contribute to the inhibitory control of cocaine-seeking behavior during a discriminative stimulus task in rats

The infralimbic and prelimbic (IL and PL, respectively) regions of the medial prefrontal cortex regulate the control of drug-seeking behavior. However, their roles in cocaine seeking in a discriminative stimulus (DS)-based self-administration task are unclear. To address this issue, male Sprague Dawley rats were trained on a DS task in which, on a trial-by-trial basis, a DS+ indicated that a lever press would produce a cocaine infusion, whereas a distinct DS- indicated that a lever press would produce nothing. IL and PL inactivation via GABA receptor activation decreased performance accuracy and disinhibited behavioral responding on DS- trials, resulting in greater lever pressing during the DS- presentation. This was accompanied by a decrease in cocaine infusions obtained, a finding confirmed in a subsequent experiment using a standard FR1 cocaine self-administration paradigm. We repeated the DS study using a food reward and found that inactivation of each region decreased performance accuracy but had no effect on the total number of food pellets earned. Additional experiments with the cocaine DS task found that dopamine receptor blockade in the IL, but not PL, reduced performance accuracy and disinhibited behavioral responding on DS- trials, whereas AMPA receptor blockade in the IL and PL had no effect on performance accuracy. These findings strongly suggest that, in a DS-based self-administration task in which rats must actively decide whether to engage in lever pressing (DS+) or withhold lever pressing (DS-) on a trial-by-trial basis, both the IL and PL contribute to the inhibitory control of drug-seeking behavior.

Interplay of prefrontal cortex and amygdala during extinction of drug seeking

Extinction of Pavlovian conditioning is a complex process that involves brain regions such as the medial prefrontal cortex (mPFC), the amygdala and the locus coeruleus. In particular, noradrenaline (NA) coming from the locus coeruleus has been recently shown to play a different role in two subregions of the mPFC, the prelimbic (PL) and the infralimbic (IL) regions. How these regions interact in conditioning and subsequent extinction is an open issue. We studied these processes using two approaches: computational modelling and NA manipulation in a conditioned place preference paradigm (CPP) in mice. In the computational model, NA in PL and IL causes inputs arriving to these regions to be amplified, thus allowing them to modulate learning processes in amygdala. The model reproduces results from studies involving depletion of NA from PL, IL, or both in CPP. In addition, we simulated new experiments of NA manipulations in mPFC, making predictions on the possible results. We searched the parameters of the model and tested the robustness of the predictions by performing a sensitivity analysis. We also present an empirical experiment where, in accord with the model, a double depletion of NA from both PL and IL in CPP with amphetamine impairs extinction. Overall the proposed model, supported by anatomical, physiological, and behavioural data, explains the differential role of NA in PL and IL and opens up the possibility to understand extinction mechanisms more in depth and hence to aid the development of treatments for disorders such as addiction.

Endogenous glutamate within the prelimbic and infralimbic cortices regulates the incubation of cocaine-seeking in rats

The incubation of cue-reinforced cocaine-seeking coincides with increased extracellular glutamate within the ventromedial prefrontal cortex (vmPFC). The vmPFC is comprised of two subregions that oppositely regulate drug-seeking, with infralimbic (IL) activity inhibiting, and prelimibic (PL) activity facilitating, drug-seeking. Thus, we hypothesized that increasing and decreasing endogenous glutamate within the IL would attenuate and potentiate, respectively, cue-reinforced drug-seeking behavior, with the converse effects observed upon manipulations of endogenous glutamate within the PL. Male Sprague-Dawley rats were trained to self-administer cocaine (0.25 mg/infusion; 6 h/day X 10 days), the delivery of which was signaled by a tone-light cue. Rats were then subdivided into 3 or 30 day withdrawal groups. For testing, rats were microinjected with vehicle, 20 mM of the mGlu2/3 agonist LY379268 (to lower endogenous glutamate), or 300 μM of the excitatory amino acid transporter inhibitor threo-β-benzyloxyaspartate (TBOA; to raise endogenous glutamate) into either the IL or PL (0.5 μl/side) and then given a 30-min test for cue-reinforced drug-seeking. Vehicle-infused rats exhibited incubated responding on the cocaine-associated lever. Neither LY379268 nor TBOA altered behavior at 3 days withdrawal, indicating that glutamate within neither subregion regulates cue-reinforced drug-seeking during early withdrawal. At 30 days withdrawal, intra-PL LY379268 microinjection significantly decreased drug-seeking behavior, while the effect was more modest when infused intra-IL. Interestingly, intra-IL TBOA attenuated incubated drug-seeking during protracted withdrawal, but did not affect behavior when infused intra-PL. These results argue that glutamate release within the PL in response to drug-seeking likely drives the manifestation of incubated cocaine-seeking during protracted withdrawal.

Blockade of α2-adrenergic receptors in prelimbic cortex: impact on cocaine self-administration in adult spontaneously hypertensive rats following adolescent atomoxetine treatment

RATIONALE

Research with the spontaneously hypertensive rat (SHR) model of attention deficit/hyperactivity disorder demonstrated that chronic methylphenidate treatment during adolescence increased cocaine self-administration established during adulthood under a progressive ratio (PR) schedule. Compared to vehicle, chronic atomoxetine treatment during adolescence failed to increase cocaine self-administration under a PR schedule in adult SHR.

OBJECTIVES

We determined if enhanced noradrenergic transmission at α2-adrenergic receptors within prefrontal cortex contributes to this neutral effect of adolescent atomoxetine treatment in adult SHR.

METHODS

Following treatment from postnatal days 28-55 with atomoxetine (0.3 mg/kg) or vehicle, adult male SHR and control rats from Wistar-Kyoto (WKY) and Wistar (WIS) strains were trained to self-administer 0.3 mg/kg cocaine. Self-administration performance was evaluated under a PR schedule of cocaine delivery following infusion of the α2-adrenergic receptor antagonist idazoxan (0 and 10-56 μg/side) directly into prelimbic cortex.

RESULTS

Adult SHR attained higher PR break points and had greater numbers of active lever responses and infusions than WKY and WIS. Idazoxan dose-dependently increased PR break points and active lever responses in SHR following adolescent atomoxetine vs. vehicle treatment. Behavioral changes were negligible after idazoxan pretreatment in SHR following adolescent vehicle or in WKY and WIS following adolescent atomoxetine or vehicle.

CONCLUSIONS

α2-Adrenergic receptor blockade in prelimbic cortex of SHR masked the expected neutral effect of adolescent atomoxetine on adult cocaine self-administration behavior. Moreover, greater efficacy of acute idazoxan challenge in adult SHR after adolescent atomoxetine relative to vehicle is consistent with the idea that chronic atomoxetine may downregulate presynaptic α2A-adrenergic autoreceptors in SHR.

Role of the agranular insular cortex in contextual control over cocaine-seeking behavior in rats

RATIONALE

Environmental stimulus control over drug relapse requires the retrieval of context-response-cocaine associations, maintained in long-term memory through active reconsolidation processes. Identifying the neural substrates of these phenomena is important from a drug addiction treatment perspective.

OBJECTIVES

The present study evaluated whether the agranular insular cortex (AI) plays a role in drug context-induced cocaine-seeking behavior and cocaine memory reconsolidation.

METHODS

Rats were trained to lever press for cocaine infusions in a distinctive context, followed by extinction training in a different context. Rats in experiment 1 received bilateral microinfusions of vehicle or a GABA agonist cocktail (baclofen and muscimol (BM)) into the AI or the overlying somatosensory cortex (SSJ, anatomical control region) immediately before a test of drug-seeking behavior (i.e., non-reinforced lever presses) in the previously cocaine-paired context. The effects of these manipulations on locomotor activity were also assessed in a novel context. Rats in experiment 2 received vehicle or BM into the AI after a 15-min reexposure to the cocaine-paired context, intended to reactivate context-response-cocaine memories and initiate their reconsolidation. The effects of these manipulations on drug context-induced cocaine-seeking behavior were assessed 72 h later.

RESULTS

BM-induced pharmacological inactivation of the AI, but not the SSJ, attenuated drug context-induced reinstatement of cocaine-seeking behavior without altering locomotor activity. Conversely, AI inactivation after memory reactivation failed to impair subsequent drug-seeking behavior and thus cocaine memory reconsolidation.

CONCLUSIONS

These findings suggest that the AI is a critical element of the neural circuitry that mediates contextual control over cocaine-seeking behavior.

Neural systems mediating the inhibition of cocaine-seeking behaviors

Over the past decades, research has targeted the neurobiology regulating cocaine-seeking behaviors, largely in the hopes of identifying potential targets for the treatment of cocaine addiction. Although much of this work has focused on those systems driving cocaine seeking, recently, studies examining the inhibition of cocaine-related behaviors have made significant progress in uncovering the neural systems that attenuate cocaine seeking. Such systems include the infralimbic cortex, nucleus accumbens shell, and hypothalamus. Research in this field has focused largely on the infralimbic cortex, as activity in this region appears to attenuate cocaine seeking during reinstatement and contribute to extinction learning. However, an overarching theory of function for this region that includes its role in other types of reward seeking and learning remains to be determined. Furthermore, the precise relationship between other regions involved in attenuating cocaine-seeking behavior and the infralimbic cortex remains unclear. Recent advances in the use of viral vectors combined with optogenetics, chemogenetics, and other approaches have greatly affected our capacity to investigate those systems inhibiting behavior dependent on cocaine-associated memories. This review will present current understanding regarding the neurobiology underlying the inhibition of such behaviors, especially focusing on the extinction of such memories, and explore how viral-vector targeting of specific brain circuits has begun to alter, and will continue to enrich, our knowledge regarding this issue.

Role of TAAR1 within the Subregions of the Mesocorticolimbic Dopaminergic System in Cocaine-Seeking Behavior

A novel G-protein coupled receptor, trace amine-associated receptor 1 (TAAR1), has been shown to be a promising target to prevent stimulant relapse. Our recent studies showed that systemic administration of TAAR1 agonists decreased abuse-related behaviors of cocaine. However, the role of TAAR1 in specific subregions of the reward system in drug addiction is unknown. Here, using a local pharmacological activation method, we assessed the role of TAAR1 within the subregions of the mesocorticolimbic system: that is, the VTA, the prelimbic cortex (PrL), and infralimbic cortex of medial prefrontal cortex, the core and shell of NAc, BLA, and CeA, on cue- and drug-induced cocaine-seeking in the rat cocaine reinstatement model. We first showed that TAAR1 mRNA was expressed throughout these brain regions. Rats underwent cocaine self-administration, followed by extinction training. RO5166017 (1.5 or 5.0 μg/side) or vehicle was microinjected into each brain region immediately before cue- and drug-induced reinstatement of cocaine-seeking. The results showed that microinjection of RO5166017 into the VTA and PrL decreased both cue- and drug priming-induced cocaine-seeking. Microinjection of RO5166017 into the NAc core and shell inhibited cue- and drug-induced cocaine-seeking, respectively. Locomotor activity or food reinforced operant responding was unaffected by microinjection of RO5166017 into these brain regions. Cocaine-seeking behaviors were not affected by RO5166017 when microinjected into the substantia nigra, infralimbic cortex, BLA, and CeA. Together, these results indicate that TAAR1 in different subregions of the mesocorticolimbic system distinctly contributes to cue- and drug-induced reinstatement of cocaine-seeking behavior.

SIGNIFICANCE STATEMENT

TAAR1 has been indicated as a modulator of the dopaminergic system. Previous research showed that systemic administration of TAAR1 agonists could attenuate cocaine-related behaviors, suggesting that TAAR1 may be a promising drug target for the treatment of cocaine addiction. However, the specific role of TAAR1 in subregions of the mesocorticolimbic system in drug addiction is unknown. Here, we first showed that TAAR1 mRNA is expressed throughout the subregions of the mesocorticolimbic system. Then, by using a local pharmacological activation method, we demonstrated that TAAR1 in different subregions of the mesocorticolimbic system distinctly contributes to cue- and drug-induced reinstatement of cocaine-seeking behavior.

The paraventricular nucleus of the thalamus is differentially recruited by stimuli conditioned to the availability of cocaine versus palatable food

The paraventricular nucleus of the thalamus (PVT) is not traditionally considered part of the brain addiction neurocircuitry but has received growing attention with regard to a role in the modulation of drug-seeking behavior. This study sought to establish the pattern of neural activation induced by a response-reinstating discriminative stimulus (SD ) conditioned to either cocaine (COC) or a conventional reinforcer using a palatable food substance, sweetened condensed milk (SCM). Male Wistar rats were trained to associate one SD (S+ ; COC or SCM availability) and a distinctly different SD (S- ; non-reward; i.e. the availability of saline or the absence of SCM). Following extinction of COC- and SCM-reinforced responding, rats were presented with the respective S+ or S- alone and tested for the reinstatement of reward seeking. The COC S+ and SCM S+ elicited identical reinstatement, whereas the non-reward S- was behaviorally ineffective. PVT sections were obtained following completion of the reinstatement tests and labeled for Fos. The number of Fos+ neurons was compared among rats that were presented with the COC S+ , SCM S+ or S- . Rats that were presented with the COC S+ exhibited a significant increase in Fos expression compared with rats that were presented with the S- . Moreover, Fos expression was significantly correlated with the number of reinstatement responses that were induced by the COC S+ . In contrast, the SCM S+ and S- produced identical increases in Fos expression, without behaviorally relevant correlations. The findings implicate the PVT as an important site that is selectively recruited during COC-seeking behavior.

Deficits in the extinction of ethanol-seeking behavior following chronic intermittent ethanol exposure are attenuated with positive allosteric modulation of mGlu5

Alcoholism is a chronic relapsing disorder characterized by periods of heavy alcohol consumption and unsuccessful attempts at abstinence. Relapse is one of the most problematic aspects in the treatment of alcoholism and is triggered by ethanol-associated cues. Extinction-based cue exposure therapies have proven ineffective in the treatment of alcoholism. However, positive allosteric modulation of mGlu5 with CDPPB enhances the extinction learning of alcohol-seeking behavior. The current study investigated the impact of chronic alcohol exposure on the extinction of ethanol-seeking behavior. Adult Wistar rats were trained to self-administer alcohol with a light/tone stimulus serving as the alcohol cue. After training, one group of rats was exposed to chronic intermittent ethanol (CIE) daily for a period of 2 weeks to induce ethanol dependence. Control rats were exposed to air for the same period of time. Both groups were then retrained to self-administer ethanol and subsequently tested for changes in extinction learning. CIE exposed rats consumed more ethanol compared to their pre-CIE levels and to control rats. During extinction training, CIE rats responded significantly more on the previously active lever and required more sessions to reach extinction criteria compared to control rats. Treatment with CDPPB facilitated extinction in control rats and attenuated the increased resistance to extinction in CIE-exposed rats. These results demonstrate that chronic ethanol exposure not only alters ethanol intake, but also the extinction of ethanol-seeking behaviors. The ability to attenuate deficits through modulation of mGlu5 provides a potential target for pharmacological manipulation that could ultimately reduce relapse in alcoholics.

Going and stopping: Dichotomies in behavioral control by the prefrontal cortex

The rodent dorsal medial prefrontal cortex (PFC), specifically the prelimbic cortex (PL), regulates the expression of conditioned fear and behaviors interpreted as reward-seeking. Meanwhile, the ventral medial PFC, namely the infralimbic cortex (IL), is essential to extinction conditioning in both appetitive and aversive domains. Here we review evidence that supports, or refutes, this "PL-go/IL-stop" dichotomy. We focus on the extinction of conditioned fear and the extinction and reinstatement of cocaine- or heroin-reinforced responding. We then synthesize evidence that the PL is essential for developing goal-directed response strategies, while the IL supports habit behavior. Finally, we propose that some functions of the orbital PFC parallel those of the medial PFC in the regulation of response selection. Integration of these discoveries may provide points of intervention for inhibiting untethered drug seeking in drug use disorders, failures in extinction in Post-traumatic Stress Disorder, or co-morbidities between the two.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Persistent variations in neuronal DNA methylation following cocaine self-administration and protracted abstinence in mice

Continued vulnerability to relapse during abstinence is characteristic of cocaine addiction and suggests that drug-induced neuroadaptations persist during abstinence. However, the precise cellular and molecular attributes of these adaptations remain equivocal. One possibility is that cocaine self-administration leads to enduring changes in DNA methylation. To address this possibility, we isolated neurons from medial prefrontal cortex and performed high throughput DNA sequencing to examine changes in DNA methylation following cocaine self-administration. Twenty-nine genomic regions became persistently differentially methylated during cocaine self-administration, and an additional 28 regions became selectively differentially methylated during abstinence. Altered DNA methylation was associated with isoform-specific changes in the expression of co-localizing genes. These results provide the first neuron-specific, genome-wide profile of changes in DNA methylation induced by cocaine self-administration and protracted abstinence. Moreover, our findings suggest that altered DNA methylation facilitates long-term behavioral adaptation in a manner that extends beyond the perpetuation of altered transcriptional states.

The Dorsal Agranular Insular Cortex Regulates the Cued Reinstatement of Cocaine-Seeking, but not Food-Seeking, Behavior in Rats

Prior studies suggest that the insular cortex (IC), and particularly its posterior region (the PIc), is involved in nicotine craving and relapse in humans and rodents. The present experiments were conducted to determine whether the IC and its different subregions regulate relapse to cocaine-seeking behavior in rats. To address this issue, male Sprague-Dawley rats underwent cocaine self-administration followed by extinction training and reinstatement tests. Before each reinstatement, the PIc or the more anterior dorsal agranular IC (AId) was inactivated to determine their roles in the reinstatement to cocaine seeking. In contrast to the nicotine findings, PIc inactivation had no effect on cue-induced reinstatement for cocaine seeking. However, AId inactivation reduced cued reinstatement while having no effect on cocaine-prime reinstatement. AId inactivation had no effect on reinstatement of food-seeking behavior induced by cues, a food-prime, or cues+food-prime. Based on previous work hypothesizing a role for corticotropin-releasing factor (CRF) in the IC during craving and relapse, a subsequent experiment found that CRF receptor-1 (CRF1) blockade in the AId similarly reduced cued reinstatement. Our results suggest that the AId, along with CRF1 receptors in this region, regulates reinstatement to cocaine seeking, but not food seeking, depending on the type of reinstatement, whereas PIc activity does not influence cue-induced reinstatement.

Anisomycin in the medial prefrontal cortex reduces reconsolidation of cocaine-associated memories in the rat self-administration model

We tested the hypothesis that infusion of anisomycin into the medial prefrontal cortex (mPFC) disrupts the reconsolidation of a cocaine-associated memory in the rat cocaine self-administration model. Male Sprague-Dawley rats were trained to lever press for cocaine self-administration (0.5 mg/kg/infusion) along with a cue light presentation on an FR1 followed by an FR3 schedule of reinforcement for 2 h/day. Rats were then given extinction sessions or an equivalent forced abstinence period followed by a 5 min memory reactivation session during which time they received an ip cocaine injection (10 mg/kg, ip) and were allowed to press for contingent cue light presentation. Immediately after reactivation, they were administered an intra-mPFC infusion of vehicle or anisomycin. Two additional control groups received extinction and either no memory reactivation and intra-mPFC infusions as above or intra-mPFC infusions 6 h after memory reactivation. A fourth group received forced abstinence and intra-mPFC infusions immediately after memory reactivation. Combined cocaine + cue-induced reinstatement was given 2-3 days (early) and 8-12 days (late) later. Rats given anisomycin in the Extinction + Reactivation demonstrated decreased reinstatement, while anisomycin treatment did not alter behavior in any of the other three groups. These results suggest that extinction training may recruit the mPFC such that it renders the memory susceptible to disruption by anisomycin. These findings have implications for using extinction training prior to or in conjunction with other therapies, including reconsolidation disruption, to enhance prefrontal control over drug-seeking behavior.

Involvement of the rostral agranular insular cortex in nicotine self-administration in rats

Our prior work demonstrated the involvement of the caudal granular subregion of the insular cortex in a rat model of nicotine self-administration. Recent studies in various animal models of addiction for nicotine and other drugs have identified a role for the rostral agranular subregion (RAIC). The current research was undertaken to examine the involvement of the RAIC in a rat model of nicotine self-administration. We investigated the inactivating effects of local infusions of a γ-aminobutyric acid agonist mixture (baclofen/muscimol) into the RAIC on nicotine self-administration under a fixed-ratio 5 (FR-5) schedule and on reinstatement of nicotine seeking induced by nicotine-associated cues in rats. We also evaluated the effects of RAIC inactivation on food self-administration under an FR5 schedule as a control. Inactivation of the RAIC decreased nicotine, but not food, self-administration. RAIC inactivation also prevented the reinstatement, after extinction, of nicotine seeking induced by nicotine-associated cues. Our study indicates that the RAIC is involved in nicotine-taking and nicotine-seeking in rats. Modulating insular cortex function appears to be a promising approach for nicotine dependence treatment.

Cortico-striatal circuits: Novel therapeutic targets for substance use disorders

It is widely believed that substance use disorder (SUD) results from both pre-alterations (vulnerability) and/or post-alterations (drug effects) on cortico-striatal circuits. These circuits are essential for cognitive control, motivation, reward dependent learning, and emotional processing. As such, dysfunctions in cortico-striatal circuits are thought to relate to the core features of SUD, which include compulsive drug use, loss of the ability to control drug intake, and the emergence of negative emotional states (Koob and Volkow, 2010. Neuropsychopharmacology 35(1), 217-238). While the brain circuits underlying SUD have been studied in human patients largely through imaging studies, experiments in animals have allowed researchers to examine the specific cell-types within these circuits to reveal their role in behavior relevant to SUD. Here, we will review imaging studies on cortico-striatal systems that are altered in SUD, and describe animal experiments that relate SUD to specific neural projections and cell types within this circuitry. We will end with a discussion of novel clinical approaches such as deep brain stimulation (DBS), repeated transcranial magnetic stimulation (rTMS), and pharmacological targeting of G protein-coupled receptor (GPCR) heteromers that may provide promising avenues for modulating these circuits to combat SUD in humans.

Dorsal medial prefrontal cortex (MPFC) circuitry in rodent models of cocaine use: implications for drug addiction therapies

Although the importance of the medial prefrontal cortex (MPFC) in cocaine addiction is well established, its precise contribution to cocaine seeking, taking and relapse remains incompletely understood. In particular, across two different models of cocaine self-administration, pharmacological or optogenetic activation of the dorsal MPFC has been reported to sometimes promote and sometimes inhibit cocaine seeking. We highlight important methodological differences between the two experimental paradigms and propose a framework to potentially reconcile the apparent discrepancy. We also draw parallels between these pre-clinical models of cocaine self-administration and human neuro-imaging studies in cocaine users, and argue that both lines of evidence point to dynamic interactions between cue-reactivity processes and control processes within the dorsal MPFC circuitry. From a translational perspective, these findings underscore the importance of interventions and therapeutics targeting not just a brain region, but a specific computational process within that brain region, and may have implications for the design and implementation of more effective treatments for human cocaine addiction.

Alcohol sensory processing and its relevance for ingestion

Alcohol possesses complex sensory attributes that are first detected by the body via sensory receptors and afferent fibers that promptly transmit signals to brain areas involved in mediating ingestive motivation, reinforcement, and addictive behavior. Given that the chemosensory cues accompanying alcohol consumption are among the most intimate, consistent, and immediate predictors of alcohol's postabsorptive effects, with experience these stimuli also gain powerful associative incentive value to elicit craving and related physiologic changes, maintenance of ongoing alcohol use, and reinstatement of drug seeking after periods of abstinence. Despite the above, preclinical research has traditionally dichotomized alcohol's taste and postingestive influences as independent regulators of motivation to drink. The present review summarizes current evidence regarding alcohol's ability to directly activate peripheral and central oral chemosensory circuits, relevance for intake of the drug, and provides a framework for moving beyond a dissociation between the sensory and postabsorptive effects of alcohol to understand their neurobiological integration and significance for alcohol addiction.

Enhancement of extinction learning attenuates ethanol-seeking behavior and alters plasticity in the prefrontal cortex

Addiction is a chronic relapsing disorder in which relapse is often initiated by exposure to drug-related cues. The present study examined the effects of mGluR5 activation on extinction of ethanol-cue-maintained responding, relapse-like behavior, and neuronal plasticity. Rats were trained to self-administer ethanol and then exposed to extinction training during which they were administered either vehicle or the mGluR5 positive allosteric modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) or CDPPB. CDPPB treatment reduced active lever responding during extinction, decreased the total number of extinction sessions required to meet criteria, and attenuated cue-induced reinstatement of ethanol seeking. CDPPB facilitation of extinction was blocked by the local infusion of the mGluR5 antagonist 3-((2-methyl-4-thiazolyl)ethynyl) pyridine into the infralimbic (IfL) cortex, but had no effect when infused into the prelimbic (PrL) cortex. Analysis of dendritic spines revealed alterations in structural plasticity, whereas electrophysiological recordings demonstrated differential alterations in glutamatergic neurotransmission in the PrL and IfL cortex. Extinction was associated with increased amplitude of evoked synaptic PrL and IfL NMDA currents but reduced amplitude of PrL AMPA currents. Treatment with CDPPB prevented the extinction-induced enhancement of NMDA currents in PrL without affecting NMDA currents in the IfL. Whereas CDPPB treatment did not alter the amplitude of PrL or IfL AMPA currents, it did promote the expression of IfL calcium-permeable GluR2-lacking receptors in both abstinence- and extinction-trained rats, but had no effect in ethanol-naive rats. These results confirm changes in the PrL and IfL cortex in glutamatergic neurotransmission during extinction learning and demonstrate that manipulation of mGluR5 facilitates extinction of ethanol cues in association with neuronal plasticity.

A potential role for the paraventricular nucleus of the thalamus in mediating individual variation in Pavlovian conditioned responses

There is ample evidence to suggest that the paraventricular nucleus of the thalamus (PVT) mediates cue-reward learning, especially as it relates to drug-seeking behavior. However, its exact role in these complex processes remains unknown. Here we will present and discuss data from our own laboratory which suggests that the PVT plays a role in multiple forms of stimulus-reward learning, and does so via distinct neurobiological systems. Using an animal model that captures individual variation in response to reward-associated cues, we are able to parse the incentive from the predictive properties of reward cues and to elucidate the neural circuitry underlying these different forms of cue-reward learning. When rats are exposed to a classical Pavlovian conditioning paradigm, wherein a cue predicts food reward, some rats, termed sign-trackers, approach and manipulate the cue upon its presentation. This behavior is indicative of attributing incentive salience to the cue. That is, the cue gains excessive control over behavior for sign-trackers. In contrast, other rats, termed goal-trackers, treat the cue as a mere predictor, and upon its presentation go to the location of reward delivery. Based on our own data utilizing this model, we hypothesize that the PVT represents a common node, but differentially regulates the sign- vs. goal-tracking response. We postulate that the PVT regulates sign-tracking behavior, or the attribution of incentive salience, via subcortical, dopamine-dependent mechanisms. In contrast, we propose that goal-tracking behavior, or the attribution of predictive value, is the product of “top-down” glutamatergic processing between the prelimbic cortex (PrL) and the PVT. Together, data from our laboratory and others support a role for the PVT in cue-motivated behaviors and suggest that it may be an important locus within the neural circuitry that goes awry in addiction and related disorders.

Adolescent atomoxetine treatment in a rodent model of ADHD: effects on cocaine self-administration and dopamine transporters in frontostriatal regions

Cocaine abuse and attention deficit/hyperactivity disorder (ADHD) are often comorbid. Preclinical research indicates that medial prefrontal (mPFC) and orbitofrontal (OFC) cortices are important neural substrates for both disorders. Using the spontaneously hypertensive rat (SHR) model of ADHD, we reported that adolescent treatment with the stimulant methylphenidate, a dopamine (DAT) and norepinephrine (NET) transporter inhibitor, enhanced cocaine self-administration during adulthood, and was associated with increased DAT function in mPFC. This study investigates the effects of atomoxetine ((R)-N-methyl-γ-(2-methylphenoxy)-benzenepropanamine hydrochloride) treatment, a selective NET inhibitor, during adolescence on cocaine self-administration and on DAT function and cell-surface expression in mPFC and OFC during adulthood. SHR acquired cocaine self-administration faster than Wistar-Kyoto and Wistar. Across cocaine doses, SHR earned more cocaine infusions and had higher progressive-ratio breakpoints than Wistar-Kyoto and Wistar, demonstrating that the SHR phenotype models comorbid ADHD and cocaine abuse. Prior atomoxetine treatment did not augment cocaine self-administration in SHR, but acquisition was enhanced in Wistar-Kyoto. No strain differences were found for DAT kinetic parameters or cellular localization in the vehicle controls. Atomoxetine did not alter DAT kinetic parameters or localization in SHR mPFC. Rather, atomoxetine decreased V(max) and DAT cell surface expression in SHR OFC, indicating that inhibition of NET by atomoxetine treatment during adolescence indirectly reduced DAT function and trafficking to the cell surface in OFC, specifically in the ADHD model. Thus, atomoxetine, unlike methylphenidate, does not enhance vulnerability to cocaine abuse in SHR and may represent an important alternative for teens with ADHD when drug addiction is a concern.

Cortical activation of accumbens hyperpolarization-active NMDARs mediates aversion-resistant alcohol intake

Compulsive drinking despite serious adverse medical, social and economic consequences is a characteristic of alcohol use disorders in humans. Although frontal cortical areas have been implicated in alcohol use disorders, little is known about the molecular mechanisms and pathways that sustain aversion-resistant intake. Here, we show that nucleus accumbens core (NAcore) NMDA-type glutamate receptors and medial prefrontal (mPFC) and insula glutamatergic inputs to the NAcore are necessary for aversion-resistant alcohol consumption in rats. Aversion-resistant intake was associated with a new type of NMDA receptor adaptation, in which hyperpolarization-active NMDA receptors were present at mPFC and insula but not amygdalar inputs in the NAcore. Accordingly, inhibition of Grin2c NMDA receptor subunits in the NAcore reduced aversion-resistant alcohol intake. None of these manipulations altered intake when alcohol was not paired with an aversive consequence. Our results identify a mechanism by which hyperpolarization-active NMDA receptors under mPFC- and insula-to-NAcore inputs sustain aversion-resistant alcohol intake.

Systems level neuroplasticity in drug addiction

Drug addiction is a chronic relapsing disorder for which research has been dedicated to understand the various factors that contribute to development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide a broad overview of various theories of addiction, drugs of abuse, and the neurobiology involved across the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the involvement of these pathways and associated circuits in mediating conditioned responses, drug craving, and loss of behavioral control thought to underlie withdrawal and relapse. With a better understanding of the neurobiological factors that underlie drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.