Selective inactivation of the dorsomedial prefrontal cortex and the basolateral amygdala attenuates conditioned-cued reinstatement of extinguished cocaine-seeking behavior in rats

Interactions of the basolateral amygdala with the dorsal hippocampus and dorsomedial prefrontal cortex regulate drug context-induced reinstatement of cocaine-seeking in rats

The basolateral amygdala (BLA), dorsomedial prefrontal cortex (dmPFC) and dorsal hippocampus (DH) are critical elements of the neurocircuitry of drug context-induced reinstatement of cocaine-seeking; however, little is known about functional interactions between these brain regions. The present study tested the hypothesis that serial information processing by the BLA and dmPFC mediates drug context-induced cocaine-seeking, whereas the BLA and DH independently control this behaviour. Rats were trained to self-administer cocaine in a distinct environment (cocaine-paired context) followed by extinction training in a different environment (extinction context). On the test days, rats received unilateral microinfusions of baclofen + muscimol or of vehicle into the BLA and either the contralateral or ipsilateral dmPFC or DH. Cocaine-seeking behaviour (i.e. nonreinforced presses on the cocaine-associated lever) was then assessed in the cocaine-paired and extinction contexts. Following vehicle pretreatment, exposure to the cocaine-paired context reinstated extinguished cocaine-seeking behaviour. BLA-dmPFC asymmetrical inactivation attenuated cocaine-seeking behaviour relative to vehicle treatment; however, this impairment equaled that produced by ipsilateral BLA-dmPFC inactivation. Furthermore, unilateral inactivation of the BLA or dmPFC did not alter this behaviour. BLA-DH asymmetrical inactivation selectively attenuated cocaine-seeking behaviour relative to vehicle treatment whereas ipsilateral or unilateral inactivation of the BLA and DH did not alter this behaviour. These findings indicate that the BLA and DH exhibit sequential information processing within the relapse circuitry. In contrast, interactions between the BLA and dmPFC are more complex and include parallel loops of information processing and/or necessary interhemispheric input from the dmPFC to the BLA, probably in addition to direct intrahemispheric interactions.

Medial prefrontal cortex infusions of bupivacaine or AP-5 block extinction of amphetamine conditioned place preference

The present experiments used reversible lesion techniques and intra-mPFC infusions of the n-methyl D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP-5) to examine the role of the mPFC in extinction of an amphetamine conditioned place preference (CPP). Following initial training and testing for an amphetamine (2 mg/kg) CPP, adult male Long-Evans rats were given extinction trials that were identical to training, except in the absence of peripheral amphetamine injections. Immediately prior to each extinction trial, rats received intra-mPFC infusions of the anesthetic drug bupivacaine (0.75% solution/0.5 microl), AP-5 (1.25, 2.5, 5.0 microg/0.5 microl), or saline. Following extinction training, rats were given a second CPP test session. Rats receiving intra-mPFC infusions of saline displayed extinction of CPP behavior. In contrast, intra-mPFC infusions of bupivacaine or AP-5 (2.5, 5.0 microg) blocked CPP extinction. The findings indicate (1) the mPFC mediates extinction of approach behavior to drug-associated environmental contexts, and (2) NMDA receptor blockade within the mPFC is sufficient to block extinction of amphetamine CPP behavior.

The role of dorsal vs ventral striatal pathways in cocaine-seeking behavior after prolonged abstinence in rats

RATIONALE

Recent studies have implicated an important role for the dorsal striatum during craving for cocaine and in cocaine-seeking after abstinence in rats.

OBJECTIVES

We compared the effects of pharmacological inactivation of mesencephalic dopamine (DA) cell body regions and dorsal vs ventral striatal terminal fields in an animal model of relapse after chronic cocaine self-administration.

MATERIALS AND METHODS

Rats self-administered cocaine for 2 h/day for ten sessions, followed by 2 weeks of abstinence (i.e., no extinction training). Immediately before being returned to the self-administration chamber, we assessed the effects of gamma-aminobutyric acid agonist inhibition of midbrain DA regions (substantia nigra [SN] and ventral tegmental area [VTA]) and striatum (dorsolateral caudate-putamen, nucleus accumbens core, and nucleus accumbens shell) on relapse to cocaine-seeking in the absence of reinforcement. Further testing examined daily extinction responding subsequent to the initial relapse test.

RESULTS

Inactivation of the dorsal caudate-putamen and midbrain regions attenuated cocaine seeking, while inactivation of the ventral striatum had no such effects. However, subsequent sessions under extinction conditions revealed a rebound in cocaine seeking in animals that had undergone inactivation in all regions except the dorsolateral caudate-putamen.

CONCLUSIONS

The dorsal but not ventral striatum plays a critical role in cocaine seeking immediately after abstinence. These data support the theory that chronic cocaine may shift activity from the ventral to dorsal striatum during drug seeking under certain conditions. While not necessary at the time of relapse, the ventral striatum appears to be involved in processing critical information of the relapse event.

Cortico-limbic circuitry for conditioned nicotine-seeking behavior in rats involves endocannabinoid signaling

RATIONALE

The endocannabinoid system plays an important role in conditioned drug seeking, but the neuronal mechanisms involved in this behavior are unclear.

OBJECTIVES

Here, we evaluate the role of endogenous cannabinoids in the cortico-limbic circuitry in cue-induced nicotine-seeking behavior in rats.

METHODS

Animals were first trained to self-administer nicotine (0.03 mg/kg/injection, IV) under conditions in which responding was reinforced jointly by response-contingent nicotine injections and audiovisual stimuli. During subsequent sessions, nicotine was withdrawn and responding was reinforced by contingent presentation of the stimuli only. One month after nicotine removal, the cannabinoid CB1 receptor antagonist, rimonabant, was injected bilaterally into the shell of the nucleus accumbens (ShNAcc, 0.3, 3, or 30 ng/0.5 microl), the basolateral amygdala (BLA, 30 ng/0.5 microl), or the prelimbic cortex (PLCx, 30 ng/0.5 microl).

RESULTS

Rimonabant injected into the ShNAcc dose-dependently reduced nicotine-seeking behavior without modifying spontaneous locomotor activity. Similar results were obtained when the drug (30 ng) was injected into the BLA or the PLCx. The anatomical specificity was confirmed in a control experiment using [(3)H]rimonabant. Fifteen minutes after drug injection, when the behavioral effects of rimonabant were already achieved, radioactivity was detected at the site of injection and had not diffused to adjacent regions.

CONCLUSIONS

These findings demonstrate that increased endocannabinoid transmission critically triggers conditioned nicotine-seeking behavior in key cortico-limbic regions.

Chronic cocaine reduces RGS4 mRNA in rat prefrontal cortex and dorsal striatum

Neuroadaptations affecting dopamine transmission within the prefrontal cortex and striatum are thought to underlie relapse to cocaine seeking after extended periods of abstinence. Regulator of G-protein signaling 4 (RGS4) is a forebrain-enriched protein known to be dynamically regulated by dopamine receptors in response to acute psychostimulant administration. In this report, chronic noncontingent (cocaine binge) or response-contingent (self-administration) delivery of cocaine followed by 2-3 weeks of abstinence resulted in a decrease of RGS4 mRNA in the dorsal striatum and prefrontal cortex. Furthermore, re-exposure to the cocaine-associated context after abstinence renewed the drug seeking and restored the levels of RGS4 mRNA to control values. Changes in RGS4 mRNA levels might signal abnormal receptor G-protein coupling that impacts cocaine seeking.

Role of the prelimbic cortex in the acquisition, re-acquisition or persistence of responding for a drug-paired conditioned reinforcer

The medial prefrontal cortex has been suggested to play a role in drug addiction due to its involvement in the reinstatement of drug seeking. In the present study, the role of the prelimbic cortex in persistent responding maintained by the earned presentations of a drug-paired conditioned reinforcer was studied. Temporary inactivation of the prelimbic, prefrontal cortex of rats had no effect on this persistent response, but did impair its initial acquisition, maintained by the drug-paired conditioned reinforcer. The lesion also impaired re-acquisition of this response after extinction by omission of the contingent conditioned reinforcer. These results suggest that the prelimbic cortex has a selective role in the acquisition, or re-acquisition, of instrumental responses for drug-paired conditioned reinforcers, that may be important in relapse to drug seeking. Anatomical controls with placements in the infralimbic cortex showed longer-lasting impairments in the acquisition of this response, consistent with the suggestion that the prelimbic and infralimbic cortices mediate different aspects of behavior, with the infralimbic being more specialized for habits. The implications of the present findings toward the understanding of drug seeking and relapse behaviors and the separate brain systems that may underlie them are discussed.

The effects of extinction training in reducing the reinstatement of drug-seeking behavior: involvement of NMDA receptors

Although the process of extinction has been well documented for various forms of behavioral responses, the effects of extinction on the reinstatement of drug-seeking behavior are relatively understudied. In this report, the effectiveness of an extinction training protocol to reduce primed reinstatement responses was compared with the effectiveness of an equivalent period of enforced abstinence. We found that extinction training performed in the drug taking environment significantly reduced reinstatement behavior subsequently primed by either contextual cues, conditioned cues, or cocaine infusion. The ability of extinction to reduce cocaine primed reinstatement was blocked by the systemic administration of the competitive NMDAR antagonist ((+/-)CPP, 5mg/kg i.p.) administered prior to each extinction training session. Interestingly, this pharmacological intervention had no impact on the effectiveness of extinction to reduce drug-seeking behavior primed by either contextual cues or conditioned cues. These results suggest that an extinction training experience involves multiple mechanisms that can be dissociated into nonNMDAR and NMDAR dependent components with respect to the type of reinstatement (i.e. context-, conditioned stimuli (CS)-, or drug-induced) being assessed.

A BDNF infusion into the medial prefrontal cortex suppresses cocaine seeking in rats

The medial prefrontal cortex (mPFC) is critical for reinstatement of cocaine seeking and is the main source of brain-derived neurotrophic factor (BDNF) to striatal regions of the brain relapse circuitry. To test the hypothesis that BDNF in the mPFC regulates cocaine-seeking behavior, rats were trained to press a lever for cocaine infusions (0.2 mg/inf, 2 h/day) paired with light+tone conditioned stimulus (CS) presentations on 10 consecutive days. After the last self-administration session, rats received a single infusion of BDNF (0.75 microg/0.5 microL/side) into the mPFC; this manipulation produced protracted effects on cocaine-seeking behavior (non-reinforced lever pressing). BDNF pretreatment administered after the last session attenuated cocaine seeking 22 h later and, remarkably, it also blocked cocaine-induced suppression of phospho-extracellular-regulated kinase and elevated BDNF immunoreactivity in the nucleus accumbens. The same pretreatment also suppressed cocaine-seeking behavior elicited by response-contingent CS presentations after 6 days of forced abstinence or extinction training, as well as a cocaine challenge injection (10 mg/kg, i.p.) after extinction training. However, BDNF infused into the mPFC had no effect on food-seeking behavior. Furthermore, BDNF infused on the sixth day of abstinence failed to alter responding, suggesting that the regulatory influence of BDNF is time limited. The suppressive effects of BDNF infused into the mPFC on cocaine seeking indicate that BDNF regulates cortical pathways implicated in relapse to drug seeking and that corticostriatal BDNF adaptations during early abstinence diminish compulsive drug seeking.

Involvement of AMPA/kainate, NMDA, and mGlu5 receptors in the nucleus accumbens core in cue-induced reinstatement of cocaine seeking in rats

RATIONALE

Nucleus accumbens glutamate transmission has been implicated in drug-seeking behavior, but the involvement of glutamate receptor subtypes in drug seeking maintained by drug-associated cues has not been fully investigated.

OBJECTIVE

This study examined the effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, N-methyl-D-aspartate (NMDA) and mGlu5 receptor blockade in the nucleus accumbens core on cue-induced reinstatement of cocaine seeking.

METHOD

Wistar rats were trained to self-administer cocaine and associate a compound stimulus (light and tone) with the drug under an FR4(FR5:S) second-order schedule of reinforcement. After extinction, during which neither cocaine nor the compound stimulus was available, responding was reinstated by contingent presentations of the compound stimulus. The effects of the intra-accumbal AMPA/kainate receptor antagonist 6-cyano-7-nitro-quinoxaline-2, 3-dione (CNQX; 0, 0.01, and 0.03 microg/side), the NMDA antagonist D-2-amino-5-phosphonopentanoate (D-AP5; 0, 1, and 2 microg/side), and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 0, 0.5, and 1 microg/side) on reinstatement were examined in a within-subjects design.

RESULTS

CNQX and D-AP5 attenuated cue-induced reinstatement of cocaine seeking dose-dependently. MPEP, however, decreased cocaine seeking only relative to baseline because also the saline vehicle included in the within-subjects series of injections decreased responding, possibly reflecting conditioned anhedonic effects of MPEP. In additional experiments, none of the antagonists attenuated locomotor activity or responding for sucrose pellets.

CONCLUSIONS

The results suggest that cue-induced reinstatement of cocaine seeking after a period of withdrawal from cocaine is sensitive to AMPA/kainate and NMDA receptor antagonism in the nucleus accumbens core and give further evidence for the role of the accumbal glutamate transmission in modulation of drug-seeking behavior.

Reinstatement of MDMA (ecstasy) seeking by exposure to discrete drug-conditioned cues

The widely used recreational drug MDMA (ecstasy) supports self-administration in animals, but it is not known whether MDMA-associated cues are able to reinstate drug seeking in a relapse model of drug addiction. To assess this possibility, drug-naïve rats were trained to press a lever for MDMA infusions (0.30 mg/kg/infusion, i.v.) paired with a compound cue (light and tone) in daily 2 h sessions. Responding was reinforced contingent on a modified fixed-ratio 5 schedule of reinforcement. Conditioned cue-induced reinstatement tests were conducted after lever pressing was extinguished in the absence of MDMA and the conditioned cues. Conditioned cues reinstated lever pressing after extinction, and the magnitude of reinstatement was positively correlated with the level of responding during MDMA self-administration. These results show for the first time that conditioned cues can trigger reinstatement of MDMA-seeking behavior in rats, and that individual differences in the pattern of MDMA self-administration can predict the magnitude of reinstatement responding.

Amygdala neurons differentially encode motivation and reinforcement

Lesion studies demonstrate that the basolateral amygdala complex (BLA) is important for assigning motivational significance to sensory stimuli, but little is known about how this information is encoded. We used in vivo electrophysiology procedures to investigate how the amygdala encodes motivating and reinforcing properties of cues that induce reinstatement of reward-seeking behavior. Two groups of rats were trained to respond to a sucrose reward. The "paired" group was trained with a reward-predictive cue, whereas the "unpaired" group was trained with a randomly presented cue. Both groups underwent identical extinction and reinstatement procedures during which the reward was withheld. The proportion of neurons that were phasically cue responsive during reinstatement was significantly higher in the paired group (46 of 100) than in the unpaired group (8 of 112). Cues that induce reward-seeking behavior can do so by acting as incentives or reinforcers. Distinct populations of neurons responded to the cue in trials in which the cue acted as an incentive, triggering a motivated reward-seeking state, or as a reinforcer, supporting continued instrumental responding. The incentive motivation-encoding population of neurons (34 of 46 cue-responsive neurons; 74%) extinguished in temporal agreement with a decrease in the rate of instrumental responding. The conditioned reinforcement-encoding population of neurons (12 of 46 cue-responsive neurons; 26%) maintained their response for the duration of cue-reinforced instrumental responding. These data demonstrate that separate populations of cue-responsive neurons in the BLA encode the motivating or reinforcing properties of a cue previously associated with a reward.

Cocaine-associated stimuli increase cocaine seeking and activate accumbens core neurons after abstinence

Electrophysiological recordings were completed in rats (n = 14) trained to self-administer cocaine to determine whether activation of nucleus accumbens (Acb) neurons (core vs shell) by cocaine-associated stimuli is enhanced after 1 month of cocaine abstinence. After self-administration training, 170 cells were recorded during a single test session conducted either the next day or 1 month later. The test session consisted of three phases during which (1) the cocaine cue was presented unexpectedly to rats, (2) rats responded for the same cue in the absence of the drug (extinction), and (3) the cocaine cue was presented randomly between cocaine-reinforced responding during resumption of self-administration. The cocaine stimulus significantly increased activation of Acb core (not shell) neurons after 1 month of cocaine abstinence (compared with 1 d); this finding occurred regardless of contingency of cue presentation or cocaine availability. Acb core activation was not observed in other rats (n = 7) presented with the same stimulus never paired with cocaine. The results reflect a cellular neuroadaptation in the Acb core related to cocaine-associated cues that is observed during initial cue exposure and sustained during extinction and resumption of self-administration after prolonged drug abstinence.

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

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

Distinct patterns of neural activation associated with ethanol seeking: effects of naltrexone

BACKGROUND

Alcoholism, like other substance abuse disorders, is a chronically relapsing condition. Compared with other abused drugs, however, little is known about the neural mechanisms mediating ethanol (EtOH)-craving and -seeking behavior leading to relapse. This study, therefore, was conducted to identify candidate brain regions that are recruited by an EtOH-associated contextual stimulus (S(+)). A secondary objective was to determine whether EtOH S(+)-elicited neural recruitment patterns are modified by the opiate antagonist naltrexone (NTX), a compound that reduces cue-induced craving in alcoholics and attenuates ethanol seeking in animal models of relapse.

METHODS

Rats were tested in a conditioned reinstatement model of relapse with subsequent examination of brain c-fos expression patterns elicited by an EtOH S(+) versus a cue associated with nonreward (S(-)). In addition, modification of these expression patterns by NTX was examined.

RESULTS

The EtOH S(+) reinstated extinguished responding and increased c-fos expression within the prefrontal cortex, hippocampus, nucleus accumbens, and hypothalamic paraventricular nucleus (PVN). Naltrexone suppressed the S(+)-induced reinstatement and attenuated hippocampal CA3 c-fos expression, while increasing neural activity in the extended amygdala and PVN.

CONCLUSIONS

Ethanol-associated contextual stimuli recruit key brain regions that regulate associative learning, goal-directed behavior, and Pavlovian conditioning of emotional significance to previously neutral stimuli. In addition, the data implicate the hippocampus, amygdala, and PVN as potential substrates for the inhibitory effects of NTX on conditioned reinstatement.

Experience-dependent effects of cocaine self-administration/conditioning on prefrontal and accumbens dopamine responses

Experiments were performed to examine the effects of cocaine self-administration and conditioning experience on operant behavior, locomotor activity, and nucleus accumbens (NAcc) and prefrontal cortex (PFC) dopamine (DA) responses. Sensory cues were paired with alternating cocaine and nonreinforcement during 12 (limited training) or 40 (long-term training) daily operant sessions. After limited training, NAcc DA responses to cocaine were significantly enhanced in the presence of cocaine-associated cues compared with nonreward cues and significantly depressed after cocaine-paired cues accompanied a nonreinforced lever response. PFC DA levels were generally nonresponsive to cues after the same training duration. However, after long-term training, cocaine-associated cues increased the magnitude of cocaine-stimulated PFC DA levels significantly over levels observed with nonreinforcement cues. Conversely, conditioned cues no longer influenced NAcc DA levels after long-term training. In addition, cocaine-stimulated locomotor activity was enhanced by cocaine-paired cues after long-term, but not after limited, training. Findings demonstrate that cue-induced cocaine expectation exerts a significant impact on dopaminergic and behavioral systems, progressing from mesolimbic to mesocortical regions and from latent to patent behaviors as cocaine and associative experiences escalate.

Differential impact of pavlovian drug conditioned stimuli on in vivo dopamine transmission in the rat accumbens shell and core and in the prefrontal cortex

RATIONALE

Conditioned stimuli (CSs) by pavlovian association with reinforcing drugs (US) are thought to play an important role in the acquisition, maintenance and relapse of drug dependence.

OBJECTIVE

The aim of this study was to investigate by microdialysis the impact of pavlovian drug CSs on behaviour and on basal and drug-stimulated dopamine (DA) in three terminal DA areas: nucleus accumbens shell, core and prefrontal cortex (PFCX).

METHODS

Conditioned rats were trained once a day for 3 days by presentation of Fonzies filled box (FFB, CS) for 10 min followed by administration of morphine (1 mg/kg), nicotine (0.4 mg/kg) or saline, respectively. Pseudo-conditioned rats were presented with the FFB 10 h after drug or saline administration. Rats were implanted with microdialysis probes in the shell, core and PFCX. The effect of stimuli conditioned with morphine and nicotine on DA and on DA response to drugs was studied.

RESULTS

Drug CSs elicited incentive reactions and released DA in the shell and PFCX but not in the core. Pre-exposure to morphine CS potentiated DA release to morphine challenge in the shell but not in the core and PFCX. This effect was related to the challenge dose of morphine and was stimulus-specific since a food CS did not potentiate the shell DA response to morphine. Pre-exposure to nicotine CS potentiated DA release in the shell and PFCX.

CONCLUSION

The results show that drug CSs stimulate DA release in the shell and medial PFCX and specifically potentiate the primary stimulant drug effects on DA transmission.

Fos and glutamate AMPA receptor subunit coexpression associated with cue-elicited cocaine-seeking behavior in abstinent rats

Cocaine-associated cues acquire incentive motivational effects that manifest as craving in humans and cocaine-seeking behavior in rats. We have reported an increase in neuronal activation in rats, measured by Fos protein expression, in various limbic and cortical regions following exposure to cocaine-associated cues. This study examined whether the conditioned neuronal activation involves glutamate AMPA receptors by measuring coexpression of Fos and AMPA glutamate receptor subunits (GluR1, GluR2/3, or GluR4). Rats trained to self-administer cocaine subsequently underwent 22 days of abstinence, during which they were exposed daily to either the self-administration environment with presentations of the light/tone cues previously paired with cocaine infusions (Extinction group) or an alternate environment (No Extinction group). All rats were then tested for cocaine-seeking behavior (i.e. responses without cocaine reinforcement) and Fos and AMPA glutamate receptor subunits were measured postmortem using immunocytochemistry. The No Extinction group exhibited increases in cocaine-seeking behavior and Fos expression in limbic and cortical regions relative to the Extinction group. A large number of Fos immunoreactive cells coexpressed GluR1, GluR2/3, and GluR4, suggesting that an action of glutamate at AMPA receptors may in part drive cue-elicited Fos expression. Importantly, there was an increase in the percentage of cells colabeled with Fos and GluR1 in the anterior cingulate and nucleus accumbens shell and cells colabeled with Fos and GluR4 in the infralimbic cortex, suggesting that within these regions, a greater, and perhaps even different, population of AMPA receptor subunit-expressing neurons is activated in rats engaged in cocaine-seeking behavior.

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

Evidence of stimulus attribute-specificity within the prefrontal cortex (PFC) suggests that different prefrontal subregions may contribute to cocaine addiction in functionally distinct ways. Thus, the present study examined the effects of lidocaine-induced inactivation of two distinct PFC subregions, the prelimbic (PL) or dorsal agranular insular (AId) cortices, on drug-seeking and drug-taking behaviors under cocaine maintenance and reinstatement testing conditions in rats trained to self-administer 1 mg/kg cocaine under a second-order schedule of drug delivery. Throughout maintenance and reinstatement phases, rats were exposed to conditioned light cues and contextual odor or sound cues. Results showed that PL inactivation during maintenance test sessions significantly reduced drug-seeking and drug-taking behaviors, and disrupted patterns of responding in rats exposed to light-sound, but not light-odor, cues. Moreover, lidocaine-induced inactivation of the PL significantly attenuated drug-seeking behavior during cue-induced and cocaine prime-induced reinstatement in rats exposed to light-sound cues only. In contrast, AId inactivation significantly attenuated cue-induced reinstatement of drug-seeking behavior in rats exposed to light-odor cues only. Drug-seeking and drug-taking behaviors in these rats were not disrupted during maintenance and cocaine prime-induced reinstatement testing regardless of the type of contextual cues used. Together, these data suggest that PL and AId subregions play separate yet overlapping roles in regulating cocaine addiction in rats in ways that are dependent on the presence or absence of cocaine and on the types of contextual cues present in the cocaine self-administration environment.

Inactivation of the ventromedial prefrontal cortex reduces expression of conditioned fear and impairs subsequent recall of extinction

Anxiety disorders are thought to reflect deficits in the regulation of fear expression. Evidence from rodent studies implicates the ventromedial prefrontal cortex (vmPFC) in the regulation of conditioned fear. Lesions of the vmPFC have had differing effects on the acquisition and expression of conditioned fear, as well as on recall of extinction. The use of permanent lesions, however, makes it difficult to assess the phase of training in which the vmPFC is acting and can trigger recruitment of other structures, thereby masking lesion deficits. To overcome these problems, we temporarily inactivated the vmPFC of rats with tetrodotoxin (10 ng in a 0.5-microl midline infusion) at one of four time points: prior to conditioning, prior to extinction, immediately after extinction or prior to recall of extinction. Consistent with lesion findings, inactivation of the vmPFC prior to acquisition had no effect but inactivation prior to extinction led to impaired recall of extinction the following day. In contrast to lesion findings, inactivation of the vmPFC decreased freezing at all time points, suggesting that some component of the vmPFC facilitates the expression of conditioned fear. These findings suggest that inactivation of the vmPFC can have opposite effects depending on the phase of training. The vmPFC appears to be involved both in stimulating the expression of conditioned fear and in serving as a site of extinction-related plasticity that inhibits fear during recall of extinction.

The psychogenetically selected Roman high- and low-avoidance rat lines: A model to study the individual vulnerability to drug addiction

The Roman high- (RHA) and low-avoidance (RLA) rat lines were selected for, respectively, rapid vs poor acquisition of two-way active avoidance in the shuttle-box. Here, we review experimental evidence indicating that, compared with their RLA counterparts, RHA rats display a robust sensation/novelty seeking profile, a marked preference and intake of natural or drug rewards, and more pronounced behavioral and neurochemical responses to the acute administration of morphine and psychostimulants. Moreover, we show that (i) the repeated administration of morphine and cocaine elicits behavioral sensitization in RHA, but not RLA, rats, (ii) in sensitized RHA rats, acute morphine and cocaine cause a larger increment in dopamine output in the core, and an attenuated dopaminergic response in the shell of the nucleus accumbens, as compared with RHA rats repeatedly treated with saline, and (iii) such neurochemical changes are not observed in the mesoaccumbens dopaminergic system of the sensitization-resistant RLA line. Behavioral sensitization plays a key role in several cardinal features of addiction, including drug craving, compulsive drug seeking and propensity to relapse following detoxification. Comparative studies in the Roman lines may therefore represent a valid approach to evaluate the contribution of the genotype on the neural substrates of drug sensitization and addiction.

Role of acetylcholine transmission in nucleus accumbens and ventral tegmental area in heroin-seeking induced by conditioned cues

The involvement of cholinergic transmission in heroin self-administration and the reinstatement of heroin-seeking was examined in rats trained to nose-poke for i.v. heroin. Systemic treatment with physostigmine, an inhibitor of acetylcholinesterase, modestly reduced the acquisition and rate of heroin self-administration, and this suppression of heroin intake was reversed by pretreatment with scopolamine but not by mecamylamine. Following 10-14 days of self-administration, rats were left in the home environment for 14 days. Subsequently, rats were evaluated for extinction of nose-pokes during the first hour after being returned to the self-administration apparatus. One hour later a conditioned stimulus (house light, light in the nose-poke hole, sound of the infusion pump) was presented to initiate cue-induced reinstatement. Physostigmine produced a dose-dependent inhibition of cue-induced reinstatement, but only the dose of 0.5 mg/kg significantly decreased nose-poke responding in the extinction test. Chronic treatment with physostigmine (0.1 mg/kg) did not impair performance during acquisition of heroin self-administration. However, during a subsequent reinstatement test conducted in the absence of physostigmine pretreatment, heroin seeking was significantly below that of rats chronically pretreated with saline. To evaluate brain regions mediating the effects of systemic drug treatment on reinstatement, physostigmine was microinjected into the nucleus accumbens (NAc) or ventral tegmental area (VTA). Microinjection of physostigmine into the NAc prior to presenting conditioned cues inhibited the reinstatement of heroin-seeking, without affecting extinction responding. In contrast, microinjection of physostigmine into the VTA augmented the reinstatement induced by conditioned cues and extinction responding. Inactivation of either NAc or VTA by microinjecting tetrodotoxin blocked both extinction responding and cue-induced reinstatement. These data demonstrate that cholinergic transmission influences heroin self-administration and reinstatement. Moreover, cue-induced reinstatement was inhibited by physostigmine in the NAc and potentiated by cholinergic stimulation in the VTA.

Chronic cocaine self-administration is associated with altered functional activity in the temporal lobes of non human primates

Previous studies utilizing a nonhuman primate model have shown that cocaine self-administration in its initial stages is accompanied by alterations in functional activity largely within the prefrontal cortex and ventral striatum. Continued cocaine exposure may considerably change this response. The purpose of the present investigation was to characterize the effects of reinforcing doses of cocaine on cerebral metabolism in a nonhuman primate model of cocaine self-administration, following an extended history of cocaine exposure, using the quantitative 2-[(14)C]deoxyglucose (2-DG) method. Rhesus monkeys were trained to self-administer 0.03 mg/kg/injection (n = 4) or 0.3 mg/kg/injection (n = 4) cocaine and compared to monkeys trained to respond under an identical schedule of food reinforcement (n = 6). Monkeys received 30 reinforcers per session for a total of 100 sessions. Metabolic mapping was conducted at the end of the final session. After this extended history, cocaine self-administration dose-dependently reduced glucose utilization throughout the striatum and prefrontal cortex similarly to the initial stages of self-administration. However, glucose utilization was also decreased in a dose-independent manner in large portions of the temporal lobe including the amygdala, hippocampus and surrounding neocortex. The recruitment of temporal structures indicates that the pattern of changes in functional activity has undergone significant expansion beyond limbic regions into association areas that mediate higher order cognitive and emotional processing. These data strongly contribute to converging evidence from human studies demonstrating structural and functional abnormalities in temporal and prefrontal areas of cocaine abusers, and suggest that substance abusers may undergo progressive cognitive decline with continued exposure to cocaine.

Neuronal substrates of relapse to cocaine-seeking behavior: role of prefrontal cortex

The return to drug seeking, even after prolonged periods of abstinence, is a defining feature of cocaine addiction. The neural circuitry underlying relapse has been identified in neuropharmacological studies of experimental animals, typically rats, and supported in brain imaging studies of human addicts. Although the nucleus accumbens (NAcc), which has long been implicated in goal-directed behavior, plays a critical role in this circuit, the prefrontal cortex (PFC) appears to process the events that directly trigger relapse: exposure to acute stress, cues previously associated with the drug, and the drug itself. In this paper, we review animal models of relapse and what they have revealed about the mechanisms underlying the involvement of the NAcc and PFC in cocaine-seeking behavior. We also present electrophysiological data from PFC illustrating how the hedonic, motor, motivational, and reinforcing effects of cocaine can be analyzed at the neuronal level. Our preliminary findings suggest a role for PFC in processing information related to cocaine seeking but not the hedonic effects of the drug. Further use of this recording technology can help dissect the functions of PFC and other components of the neural circuitry underlying relapse.

Exciting inhibition in psychostimulant addiction

Neuroplasticity induced in the nucleus accumbens by repeated psychostimulant administration is thought to underlie the vulnerability to relapse in addicts. Electrophysiological research presents a contradictory portrait of psychostimulant-induced neuroplasticity, reflecting both increases and decreases in excitatory transmission. Drug-induced adaptations of ionic conductances decrease the intrinsic excitability of individual nucleus accumbens spiny neurons but, in the context of the circuitry in which these neurons are embedded, such reduced intrinsic excitability increases the salience of excitatory drive that is elicited by drug-associated stimuli. Thus, we propose that reduced basal excitability, combined with enhanced excitatory drive by drug-associated stimuli, contributes to the two cardinal features of addiction: reduced responding to natural reward and enduring vulnerability to relapse.

Enhanced cortical and accumbal molecular reactivity associated with conditioned heroin, but not sucrose-seeking behaviour

Re-exposure to drug-related cues elicits drug-seeking behaviour and relapse in both humans and laboratory animals even after months of abstinence. Identifying neural and molecular substrates underlying conditioned heroin-seeking behaviour will be helpful in understanding mechanisms behind opiate relapse. In humans and animals, brain areas activated by natural reward-related stimuli (e.g. food, sex) do not show a complete overlap with those activated by stimuli associated with drugs of abuse, suggesting the involvement of different circuitry. To that end, we investigated neural reactivity by measuring immediate early gene (IEG) expression patterns in mesocorticolimbic system target areas following cue-induced reinstatement of heroin seeking and compared those IEG expression patterns to what was measured during natural reward (sucrose)-seeking behaviour. Animals were trained to administer heroin associated with a compound audio-visual cue. Re-exposure to the cue after 3 weeks of withdrawal reinstated heroin-seeking behaviour, which resulted in IEG expression of ania-3, MKP-1, c-fos and Nr4a3 in the medial prefrontal cortex (mPFC), and of ania-3 in the orbital frontal cortex (OFC) and nucleus accumbens core (NAC). The expression patterns for heroin-seeking behaviours did not generalize to sucrose-seeking behaviours, indicating that the two behaviours involve different connectivity pathways of neuronal signalling.

Different neural substrates mediate cocaine seeking after abstinence versus extinction training: a critical role for the dorsolateral caudate-putamen

Cue-induced reinstatement of extinguished drug seeking is a preclinical model of relapse. However, relapse typically occurs after abstinence rather than explicit extinction training. We show that inactivation of the dorsolateral caudate-putamen, but not other structures previously implicated in reinstatement, attenuates cocaine seeking after abstinence. This suggests that there is limited overlap in the substrates of cocaine seeking after abstinence versus extinction, and that habit learning exerts greater control over drug seeking than regions implicated in stimulus-reward associations.

Ionotropic and metabotropic glutamate receptor antagonism attenuates cue-induced cocaine seeking

Neuroanatomical and pharmacological evidence implicates glutamate transmission in drug-environment conditioning that partly controls drug seeking and relapse. Glutamate receptors could be targets for pharmacological attenuation of the motivational properties of drug-paired cues and for relapse prevention. The purpose of the present study was therefore to investigate the involvement of ionotropic and metabotropic glutamate receptor subtypes in cue-induced reinstatement of cocaine-seeking behavior. Rats were trained to self-administer cocaine using a second-order schedule of reinforcement (FR4(FR5:S)) under which a compound stimulus (light and tone) associated with cocaine infusions was presented contingently. Following extinction, the effects of the competitive NMDA receptor antagonist CGP 39551 (0, 2.5, 5, 10 mg/kg intraperitoneally (i.p.)), two competitive AMPA/kainate antagonists, CNQX (0, 0.75, 1.5, 3 mg/kg i.p.) and NBQX (0, 1.25, 2.5, 5 mg/kg i.p.), the NMDA/glycine site antagonist L-701,324 (0, 0.63, 1.25, 2.5 mg/kg i.p.), and the mGluR5 antagonist MPEP (0, 1.25, 2.5, 5 mg/kg i.p.) on cue-induced reinstatement of cocaine seeking were examined. The AMPA/kainate receptor antagonists CNQX and NBQX, the NMDA/glycine site antagonist L-701,324, and the mGluR5 antagonist MPEP attenuated significantly cue-induced reinstatement. The NMDA antagonist CGP 39551 failed to affect reinstatement. Additional control experiments indicated that attenuation of cue-induced reinstatement by CNQX, NBQX, L-701,324, and MPEP was not accompanied by significant suppression of spontaneous locomotor activity. These results suggest that conditioned influences on cocaine seeking depend on glutamate transmission. Accordingly, drugs with antagonist properties at various glutamate receptor subtypes could be useful in prevention of relapse induced by conditioned stimuli.

Involvement of the dorsal striatum in cue-controlled cocaine seeking

Through association with the interoceptive effects of drugs of abuse, neutral environmental stimuli can gain motivational properties themselves, becoming conditioned reinforcers that can evoke craving and relapse to drug seeking. Nucleus accumbens dopamine (DA) neurotransmission plays an important role in the reinforcing effect of cocaine itself, but, unlike nucleus accumbens glutamate, it seems not to mediate the conditioned reinforcing properties of cocaine-paired stimuli. Dorsal striatal DA transmission, in contrast, has been shown to be enhanced during cocaine seeking under a second-order schedule of reinforcement, which depends on the conditioned reinforcing properties of cocaine-associated stimuli. Therefore, the aim of the present study was to evaluate the role of DA and glutamate transmission in the dorsal striatum in cue-controlled cocaine seeking. Infusion of the DA receptor antagonist alpha-flupenthixol into the dorsal striatum decreased cocaine seeking under a second-order schedule of reinforcement. In addition, intradorsal striatal infusion of the AMPA/kainate (KA) receptor antagonist LY293558 (3SR, 4aRS, 6RS, 8aRS-6-[2-(iH-tetrazol-5-yl)ethyl]-1,2,3,4,4a,5,6,7,8,8a-decahydroiso-quinoline-3-carboxylic acid), but not the NMDA receptor antagonist AP-5, also decreased cue-controlled cocaine seeking. These data show that stimulation of DA and AMPA/KA receptors in the dorsal striatum is critical for well established drug seeking that depends on the reinforcing effects of cocaine-associated stimuli. In addition, given the importance of the dorsal striatum in stimulus-response habit learning, these data suggest that the habitual or compulsive quality of persistent drug seeking depends on dorsal striatal mechanisms.

Lesions of medial prefrontal cortex disrupt the acquisition but not the expression of goal-directed learning

Several studies have established that pretraining lesions of the medial prefrontal cortex (mPFC) render instrumental actions insensitive to devaluation of the instrumental outcome and degradation of the action-outcome contingency. Nevertheless, it remains to be assessed whether the involvement of the mPFC in goal-directed action is limited to the acquisition or to the expression of the action-outcome association in performance. The current series of experiments investigated this issue by comparing the effects of mPFC lesions made either before or after initial training using sensitivity to outcome devaluation as an assay of goal-directed performance. Whereas pretraining lesions left performance insensitive to outcome devaluation, posttraining lesions spared this effect. To determine whether the effect of mPFC lesions on outcome devaluation was the result of a more fundamental deficit in response selection, experiment 2 assessed the impact of pretraining and posttraining lesions on the ability of the instrumental outcome to selectively reinstate the performance of its associated action after a period of extinction. Although both lesions attenuated the magnitude of instrumental reinstatement generally, they left intact the ability of the instrumental outcome to influence response selection. Experiment 3 investigated the relationship between the outcome-selective devaluation and reinstatement effects and found evidence that these effects are both behaviorally and neurally dissociable at the level of the mPFC. These results indicate that the mPFC is selectively involved in the acquisition, but not the permanent storage or expression, of action-outcome associations in instrumental conditioning.

Effects of SCH-23390 infused into the amygdala or adjacent cortex and basal ganglia on cocaine seeking and self-administration in rats

Amygdala D1 receptors have been implicated in the motivating effects of cocaine-conditioned cues and cocaine itself, but the specific nucleus involved is unclear. Thus, we infused the D1 antagonist, SCH-23390, into the rostral basolateral amygdala (rBLA), caudal basolateral amygdala (cBLA), or central amygdala (CEA), and tested its effects on self-administration of cocaine, as well as reinstatement of extinguished cocaine-seeking behavior by cocaine-conditioned cues or cocaine itself. Two anatomical controls, the posterior regions of basal ganglia (BG) and somatosensory/insular cortices (CTX), were also examined. Cocaine self-administration was increased and cue and cocaine reinstatement were decreased by SCH-23390 infusion into every region when examined across the hour test session, with the exception that cBLA infusion did not alter cocaine reinstatement. In the first 20 min of the session, when SCH-23390 was more localized in the target sites, self-administration was increased by infusion into the CEA, cBLA, BG, and CTX, with lesser increases in the rBLA. Cocaine reinstatement was attenuated during the first 20 min only by infusion into the CEA, rBLA, and CTX. Cue reinstatement was not reliably observed in the first 20 min, but there was a trend for attenuation by infusion into the cBLA, and surprisingly, significant attenuations in the BG and CTX. The findings suggest that D1 receptors in subregions of the amygdala play differential roles in the reinforcing/motivational effects of cocaine, while the cue reinstatement effects are less clear. Further research is needed to examine the novel findings that neighboring regions of the BG and CTX may play a role in motivation for cocaine.

Differential involvement of the prelimbic cortex and striatum in conditioned heroin and sucrose seeking following long-term extinction

Relapse to drug taking is triggered by stimuli previously associated with consumption of drugs of misuse (cues) and involves brain systems controlling motivated behaviour towards natural reinforcers. In this study, we aimed to identify and compare neuronal pathways in corticostriatal systems that control conditioned heroin or natural reward (sucrose) seeking. To that end, rats were trained to self-administer heroin or sucrose in association with an identical compound cue. After more than 3 weeks of abstinence during extinction training, cue exposure robustly reinstated heroin and sucrose seeking, but induced distinct and even opposing changes in the expression of the neuronal activation marker zif268 in the prelimbic cortex and striatal complex, respectively. Because in the prelimbic area zif268 expression was enhanced during cue-induced heroin seeking but unaffected during sucrose seeking, a pharmacological intervention was aimed at this prefrontal region. Injection of a GABA agonist mixture within the prelimbic area enhanced conditioned heroin seeking, but had no effect on conditioned sucrose seeking. Our findings suggest a differential role of the prelimbic area and the striatum in the persistence of heroin vs. sucrose seeking following long-term extinction.

Neurobiology of relapse to heroin and cocaine seeking: an update and clinical implications

The central problem in the treatment of cocaine and heroin addiction is high rates of relapse to drug use after periods of forced or self-imposed abstinence. Relapse can be modeled in laboratory animals a reinstatement procedure in which responding for drug is extinguished and then reinstated by acute exposure to the drug, drug cues, or stress. In this review, we first summarize data from recent (2003-2005) studies on the neural substrates involved in reinstatement of heroin and cocaine seeking. We also discuss the neural mechanisms underlying the progressive increase in cocaine seeking after withdrawal (incubation of cocaine craving). Finally, we provide an update on several novel candidate medications for relapse prevention suggested by recent preclinical studies, and we discuss the translation of findings from nonhuman laboratory studies to the clinical phenomenon of relapse.

Dopamine D1 or D2 receptor antagonism within the basolateral amygdala differentially alters the acquisition of cocaine-cue associations necessary for cue-induced reinstatement of cocaine-seeking

The basolateral amygdala complex has been implicated in the formation and utilization of cocaine-cue associations in rat models of cue-induced reinstatement to cocaine-seeking behavior. We have previously demonstrated the importance of dopamine inputs to the basolateral amygdala complex in the reinstatement of cocaine-seeking behavior following chronic cocaine self-administration. Here we show that selective blockade of amygdalar dopamine D1 and D2 receptors during acquisition of cocaine-cue associations has distinctive effects on subsequent conditioned-cued cocaine-seeking behavior. Male, Sprague-Dawley rats were first trained to self-administer i.v. cocaine on a fixed ratio 1 schedule for 5 days. Subjects then received bilateral, intra-basolateral amygdala complex infusions of a dopamine D1 receptor antagonist (SCH23390, 0.25-2.0 microg/side; experiment 1), a dopamine D2 receptor antagonist (raclopride, 0.625-5.0 microg/side; experiment 2), or vehicle just prior to a single classical conditioning session, during which a light+tone cue was discretely paired with passive infusions of cocaine in the absence of lever responding. Following five additional days of cocaine self-administration and 7-10 days of extinction training, animals underwent multiple tests for cue-induced reinstatement. SCH23390 (2.0 microg/side), administered at the time of cocaine-cue association only, produced an attenuation of reinstatement to cue-induced cocaine-seeking behavior. In contrast, low doses of raclopride potentiated, while a higher dose of raclopride attenuated cue-induced reinstatement. These results demonstrate unique contributions of D1 vs. D2 receptors in mediating dopamine inputs within the basolateral amygdala complex during the formation of cocaine-stimulus associations that are critical for cue-induced reinstatement.

Ionotropic glutamate receptors in the ventral tegmental area regulate cocaine-seeking behavior in rats

Drug addiction is characterized by compulsive drug-seeking and drug-taking behavior and by a high rate of relapse even after long periods of abstinence. Although the mesocorticolimbic dopamine (DA) pathway is thought to play a critical role in drug craving and relapse, recent evidence also implicates glutamate, an amino acid known to activate DA neurons in the ventral tegmental area (VTA) via ionotropic receptors. To assess whether increased glutamate transmission in the VTA is involved in cocaine-primed drug-seeking behavior, we tested rats in a between-session reinstatement model. They were trained to press a lever for cocaine infusions (0.25 mg/infusion) accompanied by compound stimuli (light and tone) under a modified fixed-ratio 5 reinforcement schedule. Cocaine-primed reinstatement was conducted after lever pressing was extinguished in the absence of the conditioned stimuli. Blockade of ionotropic glutamate receptors in the VTA by local application of kynurenate (0.0, 1.0, 3.2, and 5.6 microg/side) dose-dependently decreased cocaine-primed reinstatement, whereas sucrose-primed reinstatement of sucrose-seeking behavior was unaffected. In addition, the minimum effective dose for decreasing cocaine-primed reinstatement was ineffective in the substantia nigra. Together, these data indicate that glutamatergic activation of the VTA is critical for cocaine-primed reinstatement. Because such activation can increase impulse flow in DA neurons and thus DA release in mesocorticolimbic targets, this glutamate-DA interaction in the VTA may underlie cocaine-primed relapse to cocaine-seeking behavior.

Effects of corticotropin-releasing hormone receptor antagonists on cocaine-induced dopamine overflow in the medial prefrontal cortex and nucleus accumbens of rats

Recent evidence suggests an important role for corticotropin-releasing hormone (CRH) and CRH receptors in cocaine reinforcement. CRH receptor antagonists reduce cocaine self-administration and attenuate the reinstatement of extinguished cocaine-seeking behavior, but little is known about the mechanisms involved. One possible mechanism for these effects may involve the cocaine-induced activation of CRH located in brain regions outside of the hypothalamus. CRH has been shown to increase dopaminergic transmission in regions relevant for cocaine reinforcement, such as the medial prefrontal cortex and the nucleus accumbens. Here, we report that CP-154,526, a CRH1-receptor antagonist, actually enhances cocaine-induced increases in dopamine overflow in the medial prefrontal cortex, measured using in vivo microdialysis. In contrast, the receptor antagonist did not alter cocaine-induced increases in dopamine in most of the nucleus accumbens, except for the most rostral part. These data suggest a surprising role for prefrontal cortex dopamine in the ability of CRH-receptor antagonists to attenuate cocaine seeking in rats.

Neural substrates of cocaine-cue associations that trigger relapse

Learned associations that occur during the process of repeated drug use in addiction can later manifest as trigger factors in relapse to renewed drug-seeking and drug-taking behavior. The process of conditioned-cued relapse of drug-seeking behavior has been successfully modeled in animals using the reinstatement procedure, in which chronic drug self-administration can be extinguished or withheld, and then reinstated using conditioned stimuli previously paired with the drug. Our laboratory has extensively studied the neural circuitry underlying conditioned-cued drug-seeking during the expression of reinstatement. In order to study the learning process of drug-cue pairings, we further developed a procedure whereby discrete cocaine-cue pairings can be conducted in a single pavlovian training session in animals previously trained to self-administer cocaine. Presentation of these cues during later reinstatement trials produces robust responding over extinction levels at levels similar to those seen when animals experience the cues on a daily basis. In a series of experiments, we have shown that reversible pharmacological inactivation of the basolateral complex of the amygdala just prior to acquisition of cocaine-cue associations blocks the ability of cocaine-paired stimuli to elicit conditioned-cued reinstatement. This learning process is mediated in part by muscarinic acetylcholine and dopaminergic inputs to the basolateral complex of the amygdala, as intra-amygdala infusion of selective receptor antagonists at the time of acquisition significantly affects reinstatement. We have also recently found that disruption of neural activity within the basolateral complex of the amygdala at the time of consolidation (just after cocaine-cue pairings) will disrupt reinstatement. Taken together, these results reveal the importance of the amygdala in the acquisition, consolidation, and expression of drug-stimulus learning that drives relapse to drug-seeking behavior.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Cannabinoid CB1 receptors control conditioned drug seeking

Recent developments have implicated cannabinoid CB1 receptors as a novel target for a new class of therapeutic agents used to treat drug addiction. CB1 receptors are expressed in the motivational circuitry of the brain and modulate drug seeking. Blockade of the CB1 receptor is particularly effective in reducing cue-induced reinstatement of drug seeking, an animal analogue of cue-induced relapse in human addicts. These relapse-preventing properties are observed with different classes of abused drug (i.e. psychostimulants, opiates, nicotine and alcohol). In addition, recent evidence indicates a more general role of CB1 receptors in reward-related memories, which is consistent with the proposed role of endocannabinoids in memory-related plasticity. Relapse-preventing actions and inhibitory effects on weight gain were confirmed recently in clinical trials with the CB1 antagonist rimonabant. Collectively, these clinical and preclinical studies suggest that antagonists of CB1 receptors offer a novel approach in the treatment of addictive behaviours.

Contextual cues associated with nicotine administration increase arc mRNA expression in corticolimbic areas of the rat brain

Conditioned responses to cues associated with the administration of drugs of misuse are an impediment to continued abstinence for drug-free addicted individuals. In order to study the neuroanatomical and cellular response of the brain to cues associated with nicotine administration, we conditioned Sprague-Dawley rats to receive an ascending dose regimen of nicotine over 14 days in two distinct non-home cage environments and assessed expression of the early response gene arc in corticolimbic areas in response to the nicotine-associated context. All of the rats received the same dose regimen of nicotine. Three days after the last training day, the rats were exposed to the test environment. The rats that had previously received nicotine exhibited increased motor activity compared with the rats that had received saline in the test environment. After 45 min in the test environment, brains were taken for Northern blotting and in situ hybridization analysis, which revealed an increase in levels of activity-regulated, dendritically localized mRNA for arc in a variety of brain regions (medial and lateral prefrontal cortices, cingulate cortex, primary sensory cortex, sensorimotor cortex, ventral striatum and amygdala). Plasma corticosterone levels were not different between the groups, suggesting that exposure to nicotine cues is insufficient to activate the hypothalamo-pituitary-adrenal axis. Given that Arc plays a direct role in neuronal plasticity and memory consolidation, its induction by nicotine-associated cues in brain regions critical for cognitive and emotional processing suggests that rats may be learning that these cues are no longer necessarily predictive of nicotine administration. Further work will be needed in order to assess the role of arc expression in the extinction of conditioned responses to drug-paired cues.

The Metabotropic Glutamate 5 Receptor Antagonist 3-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]-pyridine Reduces Ethanol Self-Administration in Multiple Strains of Alcohol-Preferring Rats and Regulates Olfactory Glutamatergic Systems

The metabotropic glutamate 5 receptor (mGlu5) receptor has been implicated as having a role in pain modulation, anxiety, and depression, as well as drug-seeking behavior. In the present study, we examined the effect of the selective mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on operant ethanol self-administration by two strains of rats, the Fawn-Hooded (FH) rat and the inbred alcohol-preferring (iP) rat. MTEP (2 mg/kg i.p.) caused a significant reduction in responding for ethanol by both strains of rats; however, in the iP rats, MTEP also induced apparent sedation at this dose, although still reduced alcohol responding at lower doses. Chronic MTEP (2 mg/kg/day) caused a significant reduction in ethanol consumption by FH rats in a two-bottle preference test; however, chronic treatment with this dose had no effect on anxiety-like behavior or depressive-like behavior in FH rats, suggesting the dose used was subthreshold for anxiolytic or antidepressive-like effects. Finally, repeated dosing with MTEP (2 mg/kg i.p.) caused significant reductions in expression of the mRNA encoding the NR1 subunit of the N-methyl-D-aspartate receptor and the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor in the cingulate cortex. A significant decrease in NR1 expression also occurred in the piriform cortex. Chronic MTEP also caused a significant decrease in mGlu5 gene expression and a significant increase in dopamine transporter and dopamine D(2)-like receptor binding within the olfactory tubercle. Collectively, these data suggest that MTEP can reduce alcohol-seeking behavior in different rodent models of alcoholism, and this effect is associated with regulation of cortical glutamate systems, particularly those in olfactory-related regions.

Intermittent cortical stimulation evokes sensitization to cocaine and enduring changes in matrix and striosome neuron responsiveness

Both the behavioral sensitization syndrome and the changes in the responsiveness of striatal neurons evoked by chronic cocaine exposure may be linked to enhanced neocortical activity, yet a direct demonstration of the effects of cortical stimulation on these parameters is lacking. We have found that repeated stimulation of the rat prelimbic cortex with picrotoxin (0.25 microg/0.25 microl, five injections on alternate days followed by 7-day withdrawal) contributed to increase c-Fos protein expression in the striosomes of the dorsolateral striatum, while producing the opposite effect in the matrix compartment, after a single exposure to cocaine (25 mg/kg). Moreover, rats exposed to cortical stimulation showed decreased locomotor activation but enhanced stereotypy following acute cocaine treatment. Thus, pulsatile stimulation of the prelimbic cortex facilitated modifications in striatal activity typically produced by chronic cocaine treatment and sensitized drug-naive animals to acute cocaine challenge. These results suggest that enhanced activation of the prelimbic cortex may contribute to the long-term adaptations induced by cocaine on neural activity and behavior.

Chronic ethanol ingestion modulates proanxiety factors expressed in rat central amygdala

Withdrawal anxiety following chronic ethanol exposure is often associated with relapse in recovering alcoholics. It is likely that brain regions regulating anxiety-like behaviors adapt during chronic ethanol exposure to ultimately regulate such behaviors. The central amygdala contains numerous neurotransmitter systems that have been implicated in the regulation of anxiety-like behavior, including corticotropin releasing factor (CRF) and NMDA-type glutamate receptors. Chronic ethanol exposure causes functional adaptations in both CRF and NMDA receptors that are likely to regulate anxiety-like behaviors expressed during withdrawal. However, the molecular mechanisms governing these adaptations remain unexplored. We therefore evaluated these neurotransmitter systems in Sprague-Dawley rats during chronic ingestion of an ethanol-containing liquid diet. Quantitative real-time reverse transcription-PCR demonstrated that preproCRF mRNA was significantly upregulated by chronic ethanol exposure, whereas mRNA expression of CRF binding protein did not change. There were also no significant changes observed in any of the NMDA subunit mRNAs, although there was a trend toward greater NR2A mRNA expression during chronic ethanol exposure. Using Western blotting analysis we measured NMDA receptor subunit protein expression. Chronic ethanol exposure did not affect protein levels of the NR1 and NR2B subunits. Like the mRNA measures, chronic ethanol exposure did influence NR2A protein levels but the effects were modest. Our results demonstrate that NMDA receptor subunit mRNA and protein expressions are not strongly influenced by exposure to chronic ethanol. This suggests that the functional NMDA receptor adaptations identified in previous studies [Roberto, M., Schweitzer, P., Madamba, S. G., Stouffer, D. G., Parsons, L. H., & Siggins, G. R. (2004). Acute and chronic ethanol exposure alter glutamatergic transmission in rat central amygdala: an in vitro and in vivo analysis. J Neurosci 24, 1594-1603] are likely to be mediated by post-translational events. In contrast, enhanced levels of CRF during/after chronic ethanol exposure are likely to be mediated by increased levels of prepro CRF mRNA. Together, our findings suggest that adaptations to chronic ethanol exposure by proanxiety factors expressed in the central nucleus appear to be mediated by distinct cellular and molecular mechanisms.

Altered Fos expression in neural pathways underlying cue-elicited drug seeking in the rat

Cocaine treatment paired with environmental cues establishes a conditioned place preference for that environment. Following expression of this preference, rats show elevated levels of immediate early genes (e.g. c-fos) in the prelimbic cortex (PrL), basolateral amygdala complex (BLC) and nucleus accumbens core (NAcc) compared to drug-unpaired controls. The PrL and BLC are reciprocally connected and both project to the NAcc. Together with the immediate early gene findings, these connections suggest the regions interact as a circuit contributing to cue-elicited drug seeking. To study this circuit, we iontophoresed Fluorogold (FG) into one brain region and assessed colocalization of FG with place preference-induced Fos in the others. Following FG iontophoresis in either the PrL or NAcc, more BLC cells double-labelled for Fos and FG were found in drug-paired than unpaired animals. Following FG iontophoresis in either the BLC or NAcc, no differences were found in the absolute number of PrL Fos/FG cells. This pattern of colocalization suggests that exposure to cocaine-associated cues leads to greater activation of the BLC's efferents to both the PrL and NAcc, while PrL output to the NAcc and BLC is unaffected in IEG expression. These results complement recent findings that suggested attenuated PrL output during place preference expression. Our findings support the view that the BLC, rather than the PrL, provides significant excitatory driving to the NAcc during cue-elicited drug seeking.

Differential involvement of orbitofrontal cortex subregions in conditioned cue-induced and cocaine-primed reinstatement of cocaine seeking in rats

Orbitofrontal cortex (OFC) damage elicits impulsivity and perseveration, and impairments in OFC function may underlie compulsive drug seeking in cocaine users. To test this hypothesis, we assessed the effects of fiber-sparing lesions or functional inactivation of OFC subregions on cocaine seeking in rats. Rats were trained to lever press for intravenous cocaine (0.20 mg/infusion) paired with the presentations of light plus tone stimuli. Responding was then allowed to extinguish. Rats received bilateral NMDA (0.1 M) or sham lesions of the lateral OFC (lOFC) or medial OFC (mOFC) before self-administration training (experiment 1) or muscimol plus baclofen (0.1 and 1.0 mM) or vehicle infusions into the lOFC or mOFC before reinstatement testing (experiment 2). The effects of these manipulations on reinstatement of cocaine seeking (i.e., responding on the previously cocaine-paired lever) were assessed in the presence of the light plus tone stimuli or after a cocaine priming injection (10 mg/kg, i.p.). Post-training lOFC inactivation impaired conditioned cue-induced reinstatement, whereas other manipulations failed to alter this behavior. This suggests that the lOFC plays a critical role in assessing the current motivational significance of cocaine-conditioned stimuli or in using this information to guide cocaine-seeking behavior if stimulus-reward learning takes place before lOFC damage. OFC inactivation failed to alter cocaine-primed reinstatement. However, lOFC lesions augmented cocaine-primed reinstatement in a perseverative manner, whereas mOFC lesions attenuated cocaine-primed reinstatement, suggesting that prolonged cell loss in OFC subregions may modulate the propensity for cocaine seeking in a subregion-specific manner.

Altered prelimbic cortex output during cue-elicited drug seeking

Cocaine treatment paired with environmental cues establishes a conditioned place preference (CPP) for that environment. After expression of this preference, rats show elevated levels of immediate early genes (IEGs; e.g. c-fos) in the prelimbic cortex (PrL), basolateral amygdala complex (BLC), and nucleus accumbens core (NAcc) compared with drug-unpaired controls. These findings, together with the known connections between these regions, suggest that they function as a circuit contributing to cue-elicited craving. To investigate the function of this circuit during drug-seeking, we characterized Fos immunoreactivity of particular neuron classes in each region. To distinguish between IEG activation of GABAergic and non-GABAergic (principally, excitatory projection) neurons, we combined Fos immunohistochemistry with immunohistochemistry for glutamic acid decarboxylase 67 (GAD67) or calcium/calmodulin-dependent protein kinase II (CAMKII) proteins. Within the BLC and NAcc of drug-paired and drug-unpaired animals tested for CPP, we observed no significant differences in the percentage of Fos-immunoreactive (IR) cells that were also GAD67-IR. We also observed no group difference in the degree of Fos/CAMKII overlap in the BLC. However, in PrL, the degree of Fos/GAD67 overlap in the drug-paired group was significantly higher than in the drug-unpaired group. Also, the Fos/CAMKII overlap in the entire PrL as well as just its layer V was significantly lower in the drug-paired animals compared with controls. These findings suggest that, during CPP expression in cocaine-paired animals, the PrL GABAergic interneurons are preferentially activated while PrL output is attenuated, perhaps through greater inhibition of layer V pyramidal neurons. These results suggest a shifting prefrontal cortex cell population response during cocaine-seeking.

The role of the dorsomedial prefrontal cortex, basolateral amygdala, and dorsal hippocampus in contextual reinstatement of cocaine seeking in rats

The present study tested the hypothesis that separate neural substrates mediate cocaine relapse elicited by drug-associated contextual stimuli vs explicit conditioned stimuli (CSs) and cocaine. Specifically, we investigated the involvement of the dorsal hippocampus (DH), basolateral amygdala (BLA), and dorsomedial prefrontal cortex (dmPFC) in contextual reinstatement of cocaine-seeking behavior and the involvement of the DH in explicit CS- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine in a distinct context or in the presence of CSs paired explicitly with cocaine infusions. Responding of context-trained rats was then extinguished in the previously cocaine-paired or an alternate context, whereas responding of explicit CS-trained rats was extinguished in the absence of the CSs. Subsequently, the target brain regions or anatomical control regions were functionally inactivated using tetrodotoxin (0 or 5 ng/side), and cocaine-seeking behavior (ie, nonreinforced responses) was assessed in the cocaine-paired context, in the alternate context, in the presence of the explicit CSs, or following cocaine priming (10 mg/kg, i.p.). DH inactivation abolished contextual, but failed to alter explicit CS- or cocaine-induced, reinstatement of cocaine-seeking behavior. BLA or dmPFC inactivation also abolished contextual reinstatement. Conversely, inactivation of the control brain regions failed to alter contextual reinstatement. In conclusion, the DH, BLA, and dmPFC play critical roles in contextual reinstatement. Previous findings suggest that the BLA is critical for explicit CS-induced, but not cocaine-primed, reinstatement and the dmPFC is critical for both explicit CS-induced and cocaine-primed reinstatement. Thus, distinct but partially overlapping neural substrates mediate context-induced, explicit CS-induced, and cocaine-primed reinstatement of extinguished cocaine-seeking behavior.

Extinction training regulates neuroadaptive responses to withdrawal from chronic cocaine self-administration

Cocaine produces multiple neuroadaptations with chronic repeated use. Many of these neuroadaptations can be reversed or normalized by extinction training during withdrawal from chronic cocaine self-administration in rats. This article reviews our past and present studies on extinction-induced modulation of the neuroadaptive response to chronic cocaine in the mesolimbic dopamine system, and the role of this modulation in addictive behavior in rats. Extinction training normalizes tyrosine hydroxylase levels in the nucleus accumbens (NAc) shell, an effect that could help ameliorate dysphoria and depression associated with withdrawal from chronic cocaine use. Extinction training also increases levels of GluR1 and GluR2/3 AMPA receptor subunits, while normalizing deficits in NR1 NMDA receptor subunits, in a manner consistent with long-term potentiation of excitatory synapses in the NAc shell. Our results suggest that extinction-induced increases in AMPA and NMDA receptors may restore deficits in cortico-accumbal neurotransmission in the NAc shell and facilitate inhibitory control over cocaine-seeking behavior. Other changes identified by gene expression profiling, including up-regulation in the AMPA receptor aggregating protein Narp, suggest that extinction training induces extensive synaptic reorganization. These studies highlight potential benefits for extinction training procedures in the treatment of drug addiction.

The role of prefrontal cortex D1-like and D2-like receptors in cocaine-seeking behavior in rats

RATIONALE

Evidence from preclinical and clinical studies indicates an important role for the mesocorticolimbic dopamine system in cocaine craving and relapse.

OBJECTIVES

To investigate the relative involvement of prefrontal cortex D1-like and D2-like dopamine receptors in cocaine-primed, drug-seeking behavior.

METHODS

Rats were trained to press a lever to self-administer cocaine (i.v., 0.25 mg per infusion) in daily 2-h sessions. Responding was reinforced, contingent on a modified fixed-ratio 5 schedule. Reinstatement tests began after lever-pressing behavior was extinguished in the absence of cocaine and conditioned cues (light and tone). Before each reinstatement test, rats received bilateral microinfusions of different doses of selective D1-like and D2-like antagonists, SCH 23390, and eticlopride, respectively, followed by intraperitoneal administration of 10 mg/kg cocaine; 3 min later the session started. Responding in the reinstatement test was reinforced only by the conditioned cues contingent on a fixed-ratio 5 schedule.

RESULTS

Both drugs dose dependently decreased cocaine-primed reinstatement without affecting operant behavior maintained by food. Eticlopride, but not SCH 23390, increased cocaine self-administration and decreased food-primed reinstatement at the dose found to decrease cocaine-primed reinstatement.

CONCLUSIONS

These data suggest that, although both D1-like and D2-like receptors in the prefrontal cortex are involved in cocaine-primed drug-seeking behavior, they may modulate different aspects of this process.