Neural substrates of cocaine-cue associations that trigger relapse

Opposing roles for the ventral prefrontal cortex and the basolateral amygdala on the spontaneous recovery of cocaine-seeking in rats

RATIONALE

The neural circuitry subserving cocaine-seeking after extinction vs abstinence alone requires different constituent brain structures. Spontaneous recovery of cocaine-seeking, a model, which incorporates both extinction and abstinence, depends on an unknown neural circuit.

OBJECTIVES

The present study examined the hypothesis that the spontaneous recovery of cocaine-seeking would require overlapping but distinct neural circuits compared to models that incorporate either extinction or abstinence alone.

MATERIAL AND METHODS

Rats were trained to self-administer cocaine (0.2 mg/inf), then responding on the cocaine-paired lever was extinguished, followed by an additional period of abstinence in the home cage. Finally, rats were returned to the self-administration context for a test of spontaneous recovery (SR TEST). Just before the SR TEST, discrete brain regions were inactivated with a GABA agonist cocktail (1 mM baclofen + 0.1mM muscimol) to determine the relative importance of these brain regions in the spontaneous recovery of cocaine-seeking.

RESULTS

The inactivation of the ventromedial prefrontal cortex (vPFC) enhanced cocaine-seeking, whereas the inactivation of the basolateral amygdala (BLA) attenuated spontaneous recovery. Inactivation of the nucleus accumbens core (Core) resembled the effects of BLA inactivation, but these results were confounded by an inhibitory effect of the vehicle treatment. Finally, the spontaneous recovery of cocaine-seeking was unaltered by manipulations of the dorsomedial prefrontal cortex (dPFC) and the nucleus accumbens shell (Shell).

CONCLUSIONS

The neural circuitry subserving cocaine-seeking behavior in a spontaneous recovery model requires the BLA and possibly the Core, like extinction models. In addition, this behavior is subject to regulation by vPFC, in a manner functionally opposite to that of the BLA.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

The neural circuitry underlying reinstatement of heroin-seeking behavior in an animal model of relapse

Reinstatement of extinguished drug-seeking has been utilized in the study of the neural substrates of relapse to drugs of abuse, particularly cocaine. However, limited studies have examined the circuitry that drives the reinstatement of heroin-seeking behavior in the presence of conditioned cues, or by heroin itself. In order to test the hypothesis that the circuitry underlying reinstatement in heroin-experienced animals would show overlapping, yet distinct differences from cocaine-experienced animals, we used transient inhibition of several cortical, striatal, and limbic brain regions during reinstatement of heroin-seeking produced by heroin-paired cues, or by a single priming dose of heroin. Rats lever pressed for i.v. heroin discretely paired with a conditioned stimulus (CS) during daily 3-h sessions for a period of 2 weeks, followed by daily extinction of lever responding. Subsequent reinstatement of heroin-seeking was measured as lever responding in the absence of heroin reinforcement. The first set of reinstatement tests involved response-contingent CS presentations following bilateral intracranial infusion of either a combination of GABA receptor agonists (baclofen-muscimol, B/M) or vehicle (saline) into one of 13 different brain regions. The second set of reinstatement tests involved a single heroin injection (0.25 mg/kg, s.c.) following either B/M or vehicle infusions. Our results showed that vehicle-infused animals reinstated to both CS presentations and a priming injection of heroin, while B/M inactivation of several areas known to be important for the reinstatement of cocaine-seeking also attenuated heroin-seeking in response to CS presentations and/or a priming dose of heroin. However, as predicted, inactivation of areas previously shown to not affect cocaine-seeking significantly attenuated heroin-seeking, supporting the hypothesis that the circuitry underlying the reinstatement of heroin-seeking is more diffusely distributed than that for cocaine.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

NMDA receptor blockade in the basolateral amygdala disrupts consolidation of stimulus-reward memory and extinction learning during reinstatement of cocaine-seeking in an animal model of relapse

Previous research from our laboratory has implicated the basolateral amygdala (BLA) complex in the acquisition and consolidation of cue-cocaine associations, as well as extinction learning, which may regulate the long-lasting control of conditioned stimuli (CS) over drug-seeking behavior. Given the well established role of NMDA glutamate receptor activation in other forms of amygdalar-based learning, we predicted that BLA-mediated drug-cue associative learning would be NMDA receptor dependent. To test this hypothesis, male Sprague-Dawley rats self-administered i.v. cocaine (0.6 mg/kg/infusion) in the absence of explicit CS pairings (2-h sessions, 5 days), followed by a single 1-h classical conditioning (CC) session, during which they received passive infusions of cocaine discretely paired with a light+tone stimulus complex. Following additional cocaine self-administration sessions in the absence of the CS (2-h sessions, 5 days) and extinction training sessions (no cocaine or CS presentation, 2-h sessions, 7 days), the ability of the CS to reinstate cocaine-seeking on three test days was assessed. Rats received bilateral intra-BLA infusions (0.5 microl/hemisphere) of vehicle or the selective NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP-5), immediately prior to the CC session (acquisition), immediately following the CC session (consolidation), or immediately following reinstatement testing (consolidation of conditioned-cued extinction learning). AP-5 administered before or after CC attenuated subsequent CS-induced reinstatement, whereas AP-5 administered immediately following the first two reinstatement tests impaired the extinction of cocaine-seeking behavior. These results suggest that NMDA receptor-mediated mechanisms within the BLA play a crucial role in the consolidation of drug-CS associations into long-term memories that, in turn, drive cocaine-seeking during relapse.

Peptide YY3-36 decreases reinstatement of high-fat food seeking during dieting in a rat relapse model

A major problem in treating obesity is high rates of relapse to maladaptive food-taking habits during dieting. This relapse is often provoked by acute re-exposure to palatable food, food-associated cues, or stress. We used a reinstatement model, commonly used to study relapse to abused drugs, to explore the effect of peptide YY3-36 (PYY3-36) on reinstatement of high-fat (35%, 45 mg pellets) food seeking induced by acute exposure to the pellets (pellet priming), a cue previously associated with pellet delivery (pellet cue), or yohimbine (2 mg/kg, a pharmacological stressor). Rats were placed on a restricted diet (16 g of chow per day) and lever-pressed for the pellets for 9-12 sessions (6 h/d, every 48 h); pellet delivery was paired with a tone-light cue. They were then given 10-20 extinction sessions wherein lever presses were not reinforced with the pellets and subsequently tested for reinstatement of food seeking. Systemic PYY3-36 injections (100-200 microg/kg) decreased pellet priming- and pellet cue-induced reinstatement of food seeking but not yohimbine-induced reinstatement. Arcuate nucleus (Arc) injections of PYY3-36 (0.4 microg per side) decreased pellet priming-induced reinstatement. The attenuation of pellet priming-induced reinstatement by systemic PYY3-36 was reversed by systemic (2 mg/kg) but not Arc (0.5 microg per side) injections of the Y2 receptor antagonist BIIE0246. Arc PYY3-36 injections did not decrease pellet cue-induced reinstatement. Finally, systemic PYY3-36 injections had minimal effects on ongoing food self-administration or heroin priming- or heroin cue-induced reinstatement of heroin seeking. These data identify an effect of systemic PYY3-36 on relapse to food seeking that is independent of Y2 receptor activation in Arc and suggest that PYY3-36 should be considered for the treatment of relapse to maladaptive food-taking habits during dieting.

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.

Effects of GABAB receptor agents on cocaine priming, discrete contextual cue and food induced relapses

In the present study we investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phosphinic acid (SKF 97541), and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl)-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) on cocaine seeking behavior. The effects of the above drugs on the reinstatement of responding induced by natural reinforcer (food) were also studied. Male Wistar rats were trained to self-administer either cocaine (0.5 mg/kg/infusion) or food (sweet milk) and responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p.), a discrete contextual cue, or a contingent presentation of food. SCH 50911 (3-10 mg/kg) dose-dependently attenuated responding on the previously cocaine-paired lever during both reinstatement conditions, with slightly greater efficacy at reducing conditioned cue reinstatement. At the same time, it failed to alter reinstatement of food-seeking behavior. Baclofen (1.25-5 mg/kg) and SKF 97541 (0.03-0.3 mg/kg) attenuated cocaine- or food-seeking behavior; the effect of the drug appeared more effective for cocaine-seeking than food-seeking. CGP 7930 (10-30 mg/kg) reduced cocaine seeking without affecting food-induced reinstatement on reward seeking. Our results indicate that tonic activation of GABA(B) receptors is required for cocaine seeking behavior in rats. Moreover, the GABA(B) receptor antagonist SCH 50911 was effective in reducing relapse to cocaine at doses that failed to alter reinstatement of food-seeking behavior (present study), basal locomotor activity, cocaine and food self-administration (Filip et al., submitted for publication), suggesting its selective effects on motivated drug-seeking behavior. The potent inhibitory responses on cocaine seeking behavior were also seen following the GABA(B) receptor agonists or the allosteric positive modulator, however, doses of baclofen and SKF 97541 that inhibited cocaine-seeking were only threefold lower of those that inhibited food-seeking. In addition, the direct GABA(B) receptor agonists and the allosteric positive modulator cause decreases in cocaine or food self-administration (Filip et al., submitted for publication), indicating their nonspecific effects on relapse to drug-seeking and drug-taking behavior. In conclusion, the GABA(B) receptor antagonist SCH 50911 seems to be viable treatment for reducing cocaine craving and preventing relapse, while the GABA(B) receptor allosteric positive modulator CGP 7930 may hold the highest promise for attenuating cue-evoked relapses to cocaine as well as the direct rewarding properties of cocaine.

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.

Neural correlates of inflexible behavior in the orbitofrontal-amygdalar circuit after cocaine exposure

Addiction is characterized by compulsive or inflexible behavior, observed both in the context of drug-seeking and in contexts unrelated to drugs. One possible contributor to these inflexible behaviors may be drug-induced dysfunction within circuits that support behavioral flexibility, including the basolateral amygdala (ABL) and the orbitofrontal cortex (OFC). Here we describe data demonstrating that chronic cocaine exposure causes long-lasting changes in encoding properties in the ABL and the OFC during learning and reversal in an odor-guided task. In particular, these data suggest that inflexible encoding in ABL neurons may be the proximal cause of cocaine-induced behavioral inflexibility, and that a loss of outcome-expectant encoding in OFC neurons could be a more distal contributor to this impairment. A similar mechanism of drug-induced orbitofrontal-amygdalar dysfunction may cause inflexible behavior when animals and addicts are exposed to drug-associated cues and contexts.

A conflict rat model of cue-induced relapse to cocaine seeking

RATIONALE AND OBJECTIVE

Relapse to drug use in humans can be induced by exposure to drug-associated cues. The ability of drug cues to provoke 'relapse' has been studied in laboratory animals using a reinstatement model in which resumption of drug seeking is assessed after extinction of drug-reinforced responding. In this model, there are no adverse consequences of drug-seeking behavior. However, in humans, abstinence is often self-imposed, and relapse episodes likely involve making a choice between the desire for the drug and the negative consequences of pursuing it (a conflict situation). In this paper, we describe a conflict model of cue-induced relapse in rats that approximate the human condition.

MATERIALS AND METHODS

Rats were trained to lever press for cocaine; infusions were paired with a discrete light cue. An 'electric barrier' was then introduced by electrifying the floor area near the levers. Responding decreased over days with increasing shock intensities, until the rats did not approach the levers for 3 days. Subsequently, the effect of intermittent noncontingent light-cue presentations on resumption of lever responding (relapse) was assessed in extinction tests, with the electric barrier remaining activated; during testing, lever presses led to contingent light-cue presentations.

RESULTS

Noncontingent cue exposure led to resumption of lever presses during the relapse tests in 14 of the 24 rats. Surprisingly, 24 h later, 11 of the 24 rats resumed lever responding in a subsequent post-noncontingent cue test under similar extinction conditions. Large individual differences in responding were observed during both tests.

CONCLUSIONS

At its current stage of development, the conflict relapse model appears particularly suitable for studying individual differences in cue-induced relapse to cocaine seeking or factors that promote this relapse.

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.

Microinjection of the delta-opioid receptor selective antagonist naltrindole 5'-isothiocyanate site specifically affects cocaine self-administration in rats responding under a progressive ratio schedule of reinforcement

Whether the delta-opioid receptor (DOR) system can modulate behavioral effects of cocaine remains equivocal. We examined whether site- and subtype-selective blockade of DORs within the rat mesocorticolimbic system affects cocaine self-administration. The DOR antagonist naltrindole 5'-isothiocyanate (5'-NTII; 5nmol) was microinjected into the nucleus accumbens (NAcc), ventral tegmental area (VTA), or amygdala (AMYG) in rats self-administering 1.5mg/kg cocaine under a progressive ratio (PR) schedule. Intra-NAcc 5'-NTII significantly decreased cocaine self-administration, while 5'-NTII administration into the VTA significantly increased cocaine-maintained responding. 5'-NTII administration into the AMYG produced no effect. These data support a site-specific role of DORs in cocaine's behavioral effects.

Conditioned effects of heroin on the expression of inducible nitric oxide synthase in the rat are susceptible to extinction and latent inhibition

RATIONALE

The administration of heroin has been shown to inhibit the induction of nitric oxide, a molecule known to play a critical role in immune function. Previous research has shown that this alteration can be conditioned to environmental stimuli that have been associated with drug administration. However, it remains unknown whether the conditioned effects of heroin on nitric oxide formation follow accepted principles of learning.

OBJECTIVE

This study sought to determine whether manipulations that induce extinction and latent inhibition, two learning paradigms known to reduce the expression of conditioned responses, would alter heroin's conditioned effects on the expression of inducible nitric oxide synthase (iNOS).

MATERIALS AND METHODS

The conditioning procedure involved repeated pairing of heroin administration with placement into a standard conditioning chamber. Rats were repeatedly exposed to the chambers without heroin reinforcement to determine whether the conditioned response would extinguish. To induce latent inhibition, rats received repeated exposure to the chamber before the start of conditioning to inhibit the acquisition of the conditioned response. Ten days after the final conditioning session, all rats were injected with lipopolysaccharide (LPS) to induce iNOS expression. Spleen and liver tissue were removed to determine iNOS expression using reverse transcriptase polymerase chain reaction. Blood was collected to determine the concentration of nitrite/nitrate.

RESULTS

The results showed that both extinction and latent inhibition reduced the conditioned effects of heroin on the production of nitric oxide.

CONCLUSION

This study provides the first evidence that the conditioned effects of heroin on nitric oxide production follow accepted principles of learning.

Long-term ethanol self-administration by the nonhuman primate, Macaca fascicularis, decreases the benzodiazepine sensitivity of amygdala GABA(A) receptors

BACKGROUND

Rodent models of chronic alcohol exposure are typically constrained to relatively short periods of forced ethanol due to the lifespan of these animals. Nonhuman primate models, particularly those employing long-term self-administration, are conceptually more similar to human alcoholic individuals.

METHODS

We performed whole-cell patch clamp recordings on acutely dissociated amygdala neurons isolated from cynomolgus macaque coronal temporal lobe slices. Slices were prepared from control monkeys or monkeys allowed to self-administer oral ethanol for 18 months. Flunitrazepam and acute ethanol modulation of currents gated by exogenous gamma-aminobutyric acid (GABA) application was assessed in these isolated neurons. Complementary experiments were performed on amygdala total RNA using quantitative real-time reverse transcription/polymerase chain reaction to understand potential ethanol-dependent adaptations to subunit composition.

RESULTS

Gamma-aminobutyric acid-gated currents from ethanol-exposed macaque amygdala neurons exhibited reduced modulation by flunitrazepam compared with control neurons. However, this was specific for benzodiazepines as the modest inhibition of GABA-gated currents by acute ethanol was not affected by the chronic ethanol consumption. We also measured mRNA expression levels for the beta, gamma, and delta subunits in total amygdala RNA isolated from control and ethanol-drinking animals. beta1 and gamma2 expression was significantly reduced in samples from ethanol-exposed amygdala.

CONCLUSIONS

Our findings demonstrate that chronic ethanol self-administration reduces the benzodiazepine sensitivity of amygdala GABA(A) receptors. This reduced sensitivity may be the result of decreased expression of an amygdala gamma subunit. These findings suggest that, while rodent and nonhuman primate models of chronic ethanol exposure share many characteristics, the specific molecular adaptations associated with the amygdala GABAergic system may not be identical.

Black agouti (ACI) rats show greater drug- and cue-induced reinstatement of methamphetamine-seeking behavior than Fischer 344 and Lewis rats

Fischer 344 (F344) and Lewis (LEW) rats differ in methamphetamine self-administration (SA) and methamphetamine-induced reinstatement of previously extinguished behavior. We sought to determine whether genetic background also influences methamphetamine reinforcement efficacy, conditioned reinstatement, and methamphetamine-primed reinstatement of responding in F344, LEW, and Black Agouti (ACI) rats. We implanted rats with jugular catheters and trained them to self-administer methamphetamine (0.06 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement during daily 2-h SA sessions. A compound stimulus (light+tone; LT) was paired with each infusion. Dose-dependent intake was determined for each rat. Rats then entered the extinction phase of the experiment where responding resulted in no programmed consequences. Following extinction sessions, rats underwent conditioned reinstatement testing. For conditioned reinstatement, rats received response-contingent presentations of the LT and no methamphetamine. Last, methamphetamine-primed reinstatement test sessions where conducted where subjects received experimenter delivered infusions of methamphetamine (0.06, 0.12, or 0.24 mg/kg). The strains did not differ in PR responding across the doses tested. The ACI rats demonstrated the highest behavioral output during extinction training, conditioned- and methamphetamine-primed reinstatement of previously extinguished behavior compared to the other strains. These data suggest that genetic background differentially influences extinction, conditioned reinstatement and methamphetamine-primed reinstatement in rats.

Selective inactivation of the ventral hippocampus attenuates cue-induced and cocaine-primed reinstatement of drug-seeking in rats

Recent evidence suggests that the hippocampus may have a functional role in mediating relapse to cocaine-seeking behavior. Based on the importance of the ventral CA subfields in mediating reward, the present experiment determined the effects of temporary inactivation of the ventral hippocampus on reinstatement of cocaine-seeking in a rodent model of relapse. Male, Sprague-Dawley rats self-administered i.v. cocaine (0.6 mg/kg/infusion) in the presence of discrete conditioned cues (tone+light) in daily 2-h sessions for ten days. Following seven days of extinction sessions in which neither cues nor drug were available, rats underwent four reinstatement tests in a counterbalanced, within-subjects design. Bilateral microinjections of GABA receptor agonists (baclofen/muscimol (B/M; 1.0 mM/0.1 mM) [corrected] into the ventral hippocampus significantly attenuated cue-induced and cocaine-primed reinstatement compared with vehicle microinjections in the same rats. In contrast, injections just outside the ventral hippocampus did not block either form of reinstatement. Furthermore, inactivation failed to affect responding for food reinforcement, baseline extinction responding, or locomotor activity. These data indicate that the ventral hippocampus plays an important role in the relapse to cocaine-seeking behavior and may interact with key limbic structures previously implicated in cocaine addiction.

Does response-contingent access to cocaine reinstate previously extinguished cocaine-seeking behavior in C57BL/6J mice?

Inbred strains of mice are valuable tools for determining the impact of genes and the environment on behavior. However, use of mice in intravenous (iv) cocaine self-administration (SA) extinction/reinstatement paradigms has yielded mixed results. Mice do demonstrate significant conditioned reinstatement but do not significantly reinstate previously extinguished cocaine-seeking behavior when passively primed with cocaine. We tested the hypothesis that C57BL/6J (B6) mice would reinstate previously extinguished cocaine-seeking behavior when provided with response-contingent access to conditioned cues and fixed doses of cocaine. Male B6 mice were implanted with jugular catheters and trained to lever press for cocaine infusions. Each infusion was paired with a compound stimulus (light and tone; LT). Following 14 days of SA, subjects underwent extinction training--responding resulted in no programmed consequences. After at least 5 extinction sessions, cue-primed reinstatement was tested (LT-test). For the LT-test, mice received response-contingent presentations of the LT. After the LT-test, subjects returned to extinction training. Once responding decreased to extinction criteria, cocaine priming began. During cocaine priming, mice had response-contingent access to saline, 1.05, 3.5, or 17.5 mg/kg cocaine. Response-contingent presentations of the LT significantly reinstated cocaine-seeking behavior in the mice. Response-contingent access to cocaine dose-dependently reinstated responding. Our results suggest that response-contingent access to cocaine is a robust method for modeling cocaine craving and relapse in mice.

Activation in extended amygdala corresponds to altered hedonic processing during protracted morphine withdrawal

Previously we reported that during protracted morphine abstinence rats show reduced conditioned place preferences (CPP) for food-associated environments, compared to non-dependent subjects. To determine the brain regions involved in this altered reward behavior, we examined neural activation (as indexed by Fos-like proteins) induced by a preference test for a food-associated environment in 5-week morphine-abstinent versus non-dependent animals. The results indicate that elevated Fos expression in the anterior cingulate cortex (Cg) and basolateral amygdala (BLA) correlated positively with preference behavior in all groups. In contrast, Fos expression in stress-associated brain areas, including the ventral lateral bed nucleus of the stria terminalis (VL-BNST), central nucleus of the amygdala (CE), and noradrenergic (A2) neurons in the nucleus tractus solitarius (NTS) was significantly elevated only in morphine-abstinent animals. Furthermore, the number of Fos positive neurons in these areas was found to correlate negatively with food preference in abstinent animals. These results indicate that the altered hedonic processing during protracted morphine withdrawal leading to decreased preference for cues associated with natural rewards may involve heightened activity in stress-related brain areas of the extended amygdala and their medullary noradrenergic inputs.

Postretrieval propranolol disrupts a cocaine conditioned place preference

The current study examined whether a postretrieval drug memory could be disrupted by the beta-adrenoceptor antagonist propranolol, administered following reactivation in a cocaine-mediated conditioned place preference paradigm. Following cocaine conditioning, rats were given a test of conditioned place preference, followed immediately by intraperitoneal administration of propranolol or saline. Rats that received propranolol following the preference test showed no preference for the cocaine-paired floor during a subsequent test, while vehicle-treated rats continued to express a preference for the cocaine-paired floor. These deficits in behavior were specific to retrieval of the cocaine-mediated memory, suggesting that postretrieval propranolol induced an impairment of drug-seeking behavior that is consistent with the disruption of a reconsolidation phase following retrieval.

Toward a model of drug relapse: an assessment of the validity of the reinstatement procedure

BACKGROUND AND RATIONALE

The reinstatement model is widely used to study relapse to drug addiction. However, the model's validity is open to question.

OBJECTIVE

We assess the reinstatement model in terms of criterion and construct validity.

RESEARCH HIGHLIGHTS AND CONCLUSIONS

We find that the reinstatement model has adequate criterion validity in the broad sense of the term, as evidenced by the fact that reinstatement in laboratory animals is induced by conditions reported to provoke relapse in humans. The model's criterion validity in the narrower sense, as a medication screen, seems promising for relapse to heroin, nicotine, and alcohol. For relapse to cocaine, criterion validity has not yet been established primarily because clinical studies have examined medication's effects on reductions in cocaine intake rather than relapse during abstinence. The model's construct validity faces more substantial challenges and is yet to be established, but we argue that some of the criticisms of the model in this regard may have been overstated.

Drug addiction: the neurobiology of disrupted self-control

The nature of addiction is often debated along moral versus biological lines. However, recent advances in neuroscience offer insights that might help bridge the gap between these opposing views. Current evidence shows that most drugs of abuse exert their initial reinforcing effects by inducing dopamine surges in limbic regions, affecting other neurotransmitter systems and leading to characteristic plastic adaptations. Importantly, there seem to be intimate relationships between the circuits disrupted by abused drugs and those that underlie self-control. Significant changes can be detected in circuits implicated in reward, motivation and/or drive, salience attribution, inhibitory control and memory consolidation. Therefore, addiction treatments should attempt to reduce the rewarding properties of drugs while enhancing those of alternative reinforcers, inhibit conditioned memories and strengthen cognitive control. We posit that the time has come to recognize that the process of addiction erodes the same neural scaffolds that enable self-control and appropriate decision making.