Psychopharmacology (2003) 168:3–20DOI 10.1007/s00213-002-1224-x

R E V I E W

Yavin Shaham · Uri Shalev · Lin Lu · Harriet de Wit ·Jane Stewart

The reinstatement model of drug relapse:history, methodology and major findings

Received: 2 May 2002 / Accepted: 10 July 2002 / Published online: 26 October 2002� Springer-Verlag 2002

Abstract Rational and objectives: The reinstatementmodel is currently used in many laboratories to investi-gate mechanisms underlying relapse to drug seeking.Here, we review briefly the history of the model anddescribe the different procedures that have been used tostudy the phenomenon of reinstatement of drug seeking.The results from studies using pharmacological andneuroanatomical techniques to determine the neuronalevents that mediate reinstatement of heroin, cocaine andalcohol seeking by acute priming injections of drugs,drug-associated cues and environmental stressors aresummarized. In addition, several issues are discussed,including (1) the concordance between the neuronalmechanisms involved in drug-induced reinstatement andthose involved in drug reward and discrimination, (2) therole of drug withdrawal states and periods in reinstate-ment of drug seeking, (3) the role of neuronal adaptationsinduced by exposure to drugs in relapse, and (4) thedegree to which the rat reinstatement model provides asuitable preclinical model of relapse to drug taking.Conclusions: The data derived from studies using thereinstatement model suggest that the neuronal events thatmediate drug-, cue- and stress-induced reinstatement ofdrug seeking are not identical, that the mechanismsunderlying drug-induced reinstatement are to some degreedifferent from those mediating drug discrimination orreward, and that the duration of the withdrawal periodfollowing cocaine and heroin self-administration has a

profound effect on reinstatement induced by drug cuesand stress. Finally, there appears to be a good correspon-dence between the events that induce reinstatement inlaboratory animals and those that provoke relapse inhumans.

Keywords Alcohol · Cocaine · Conditioned drug cues ·Extinction · Heroin · Reinstatement · Relapse · Review ·Reward · Stress

Introduction and brief history

In 1981 de Wit and Stewart (1981) reported that non-contingent priming injections of cocaine or re-exposure tococaine-paired cues reinstates lever-pressing behaviorfollowing extinction of the drug-reinforced behavior.Based on these data, and those from earlier studies(Stretch et al. 1971; Davis and Smith 1976), de Wit andStewart suggested that their “reinstatement model” couldbe used to study factors involved in relapse to drugs.Broadly defined, “reinstatement of drug seeking” in thismodel refers to the resumption of a previously drug-reinforced behavior by non-contingent exposure to drugor non-drug stimuli after extinction (Stewart and de Wit1987).

The intuitive appeal of the reinstatement model forbasic scientists and clinicians is due to the fact that factorsreported to reinstate drug seeking in laboratory animalsalso provoke relapse and craving in humans. Thesefactors include re-exposure to drug or drug-associatedcues (Ludwig et al. 1974; Jaffe et al. 1989; O’Brien et al.1992) and exposure to certain stressors (Brown et al.1995; Sinha 2001). Both drugs and stressors can reinstatedrug seeking in laboratory animals even followingprolonged withdrawal periods (Shalev et al. 2002a). Inthe sections below, we will describe the history, metho-dology, major findings of the reinstatement model, andselected issues arising from these findings.

The phenomenon of reinstatement of learned behaviorsafter extinction is described in the early classical and

Y. Shaham ()) · U. Shalev · L. LuBehavioral Neuroscience Branch, NIDA/IRP,5500 Nathan Shock Drive, Baltimore, MD 21224, USAe-mail: yshaham@intra.nida.nih.govTel.: +1-410-5501755Fax: +1-410-5501612

H. de WitDepartment of Psychiatry, University of Chicago, Chicago, Ill.,USA

J. StewartCenter for Studies in Behavioral Neurobiology,Department of Psychology, Concordia University, Montreal,Quebec, Canada

operant conditioning studies of Pavlov and Skinner. Forexample, Pavlov described the “priming effect” of re-exposure to the unconditioned stimulus after extinction indogs trained in a classical conditioning study, and wrotethat “the restoration of an extinguished reflex is greatlyaccelerated by a fresh application of the unconditionedstimulus” (Pavlov 1927, p. 60). Using an operantparadigm, Skinner and others reported reinstatement oflever pressing in rats after extinction training by non-contingent presentations of food or water (Skinner 1938).Over the years, the priming effect has been demonstratedwith a range of reinforcing stimuli (de Wit 1996). Earlystudies by Deutsch and Howarth (1962) and Brimer(1970) also demonstrated that exposure to “stressful”stimuli (footshock, loud noise, bright light) reinstateslever-pressing behavior previously reinforced by electri-cal brain stimulation or food.

Stretch and Gerber were the first to demonstrate thatnon-contingent priming injections of self-administereddrugs reinstate amphetamine or cocaine seeking afterextinction (Stretch et al. 1971; Gerber and Stretch 1975).Subsequently, Davis and Smith (1976) and de Wit andStewart (1981, 1983) demonstrated that priming injec-tions of drugs reinstate opioid and stimulant drug seekingin rats. Stewart and Vezina were the first to demonstratethat intracranial drug injections of morphine or amphet-amine into the ventral tegmental area or the nucleusaccumbens reinstate heroin or cocaine seeking, respec-tively (Stewart 1984; Stewart and Vezina 1988). Schuster,Goldberg and colleagues (Schuster and Woods 1968;Goldberg 1976) and Davis and Smith (1976) found thatconditioned cues previously paired with drug infusionsreinstate morphine or cocaine seeking in rhesus monkeysor rats when extinction of lever-pressing was conducted inthe absence of these cues. More recently it was found thattwo stressors, acute exposure to intermittent footshock(Shaham and Stewart 1995a) and acute (1-day) fooddeprivation (Shalev et al. 2000c), reinstate heroin seekingin rats. The latter report extends an earlier study byCarroll (1985), who found that acute food restriction (30–40% of daily free-feeding amount) reinstated cocaineseeking in rats that had been food restricted during self-administration training. Since its introduction to the drugabuse field by Stretch and Gerber in 1971, the reinstate-ment model has been used sporadically until 1995 (19papers were published on reinstatement of drug seekingduring this time period; Medline search). Since 1996, thenumber of published papers on reinstatement was in-creased sharply (127 reports during the last 6 years),reflecting a renewed interest in the phenomenon ofrelapse and in recognition of its importance in drugaddiction.

Methodology

Reinstatement of drug seeking in laboratory animals witha history of drug self-administration has been studiedusing the between-session, within-session and between-

within-session variations of the reinstatement model(Fig. 1) (Shalev et al. 2002a). In the between-sessionprocedure, training for drug self-administration, extinc-tion of the drug- reinforced behavior and tests forreinstatement are conducted during sequential dailysessions (Stretch et al. 1971). In the within-sessionprocedure, tests for reinstatement are carried out in dailysessions consisting of 1–2 h of drug self-administration,followed by 3–4 h of extinction of the drug-reinforcedbehavior, and then after responding has ceased, a test forreinstatement (de Wit and Stewart 1981). In the between-within variation, rats are initially trained for drug self-administration. Subsequently, extinction training and testsfor reinstatement are conducted on the same day afterdifferent days of drug withdrawal (Tran-Nguyen et al.1998). In all of the above procedural variations, trainingcontinues until rats reach stable drug-taking behavior(typically less than 15–20% variation from the meannumber of infusions over several consecutive sessions)and extinction training continues until rats reach apredetermined extinction criterion (e.g. 20% or lessresponding during the last extinction session as comparedwith the first extinction session). In subsequent testsessions, reinstatement of lever-pressing behavior isdefined as significantly higher responding on the leverpreviously paired with drug infusions (typically referredto as the active lever) following exposure to the exper-imental manipulations (drug priming, drug cues, stress) ascompared with the control manipulations.

Most recently, several laboratories have developed areinstatement procedure that is based on the conditionedplace preference (CPP) model. In this procedure, rats areinitially trained to associate one compartment of a choiceapparatus with drug injections and a second compartmentwith injections of the drug vehicle. Following training,rats are given a choice between the two compartments ona drug-free test day, and typically spend more time in thedrug-paired environment (by definition, a CPP). Then,during an extinction phase, the acquired preference for thedrug-paired context is extinguished by pairing injectionsof saline with both compartments (drug-associated and

Fig. 1 A schematic diagram of the timeline of within-session,between-session and between-within-session reinstatement proce-dures. See text for a description of these procedures. From Fig. 2 inShalev U, Grimm JW, Shaham Y (2002a), with permission fromASPET

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vehicle-associated) or by allowing rats to explore thedrug- and vehicle-associated compartments during dailysessions in the absence of the drug. Following a test forextinction, tests for reinstatement of the CPP are givenafter a drug injection or exposure to non-drug stimuli(Mueller and Stewart 2000; Sanchez and Sorg 2001;Mueller et al. 2002). The CPP model also has been usedto study “reactivation” of acquired drug preference that isno longer observed following several drug-free weeks(Wang et al. 2000; Lu et al. 2000a).

In another variant of the reinstatement model, Etten-berg and colleagues have used a runway to study the roleof discriminative drug cues on reinstatement (McFarlandand Ettenberg 1997). In this method, the dependentmeasure is the run-time from a start box to a goal box,where a drug infusion is administered. During training,rats are presented with a discriminative cue (e.g. specificodor) in the start box, runway and the goal box thatpredicts a drug injection in the goal box, and a differentodor that predicts a saline injection in the goal box.During this phase, run times decrease in the presence ofthe drug-predictive cue, but not the saline cue. Rats arethen given extinction sessions in the absence of both thediscriminative odor cues and the drug. Over these trials,run times increase progressively. On tests for reinstate-ment by cues, exposure to the drug-associated cue in thestart box and runway decreases the time to reach the goalbox (reinstatement by discriminative cues). On tests forreinstatement by drugs, a drug injection given in the goalbox decreases the run times on the subsequent test day(Ettenberg et al. 1996; McFarland and Ettenberg 1997).Thus, several experimental procedures are being used tostudy reinstatement of drug seeking in animals, each withits advantages and disadvantages (see Shalev et al. 2002afor a review). Shalev et al. (2002a) also discussed in detail

methodological issues that should be considered whentrying to interpret data derived from the above proceduralvariations of the reinstatement model. These include thepotential confounds associated with training for foodreinforcement before drug self-administration training aswell as the administration of non-contingent priminginjections during training; the impact of response ratesand total drug intake during training on the magnitude ofresponding during tests for reinstatement; and issuesassociated with selectivity of the effects of pharmacolog-ical and brain manipulations on reinstatement of drugseeking versus their potential non-selective effects (e.g.locomotor activation/suppression). In the next section,major findings from studies in which these procedureshave been used to determine the neurochemical pathwaysunderlying reinstatement of drug seeking are outlined.Because most of the studies were conducted in cocaine-trained rats, the discussion will center on findings fromthese studies, but relevant data from studies with heroin-or alcohol-trained rats also will be mentioned.

Major findings

Table 1 summarizes the data from studies on reinstate-ment of cocaine seeking by drug priming, drug cues andstressors in which pharmacological agents, injectedsystemically, and other methods were used to identifyneurotransmitter systems involved. Figure 2 summarizeswhat has been learned from these and neuroanatomicalstudies about the brain sites and circuits that may underliereinstatement of cocaine (and possibly heroin) seekinginduced by these events.

Table 1 Effects of certain pharmacological (systemic injections),immunological and surgical manipulations on reinstatement ofcocaine seeking induced by cocaine priming, cocaine cues orstressors in rats. Blank cells, not tested. References: Comer et al.1993; Self et al. 1996, 2000; Erb et al. 1998, 2000; Shaham et al.

1998; Campbell et al. 1999; Mantsch and Goeders 1999a, 1999b;Schenk et al. 1999; Bespalov et al. 2000; Carrera et al. 2000;Grottick et al. 2000; Schenk 2000; Ciccocioppo et al. 2001; DeVries et al. 2001; Weiss et al. 2001; Alleweireldt et al. 2002;Crombag et al. 2002; Shalev et al. 2002b

Pharmacological agent Cocainepriming

Cocaine cues Stress

Discretecues

Contextual/discriminative cues

Intermittentfootshock

Fooddeprivation

Cannabinoid antagonist (SR 141716A) Attenuation Attenuation No effectCorticosterone synthesis inhibitor (ketoconazole) No effect AttenuationAdrenalectomy No effect Attenuation AttenuationCRF receptor antagonist (d-Phe CRF) No effect AttenuationCRF1 receptor antagonist (CP-154,526) AttenuationD1-like antagonists (SCH 23390, SCH 39166) Attenuation Attenuation AttenuationD1-like agonists (SKF 81297, ABT-431) Attenuation AttenuationD2-like antagonists (raclopride, nafadotride) AttenuationGABAB agonist (baclofen) AttenuationNMDA antagonist (D-CPPene) Attenuationa2-adrenoceptor agonists (clonidine, lofexidine) No effect AttenuationKappa opioid agonist (U69593) AttenuationOpioid partial agonist (buprenorphine) Attenuation5-HT2C agonist (Ro 60-0175) Attenuation5-HT1A antagonist (WAY 100653) AttenuationCocaine immunogen (GNC-KLH) Attenuation

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Drug priming-induced reinstatement

There is evidence that dopamine (DA) and glutamatewithin the cell body and terminal regions of the meso-corticolimbic DA system are involved in reinstatementinduced by priming injections of cocaine and possiblyheroin as well. Systemic injections of indirect DAagonists (e.g. amphetamine, DA reuptake blockers) orD2-like receptor agonists mimic the effect of cocaine orheroin priming on reinstatement (Self and Nestler 1998;Spealman et al. 1999; De Vries and Shippenberg 2002). Inaddition, D1- and D2-like receptor antagonists attenuateheroin- or cocaine-induced reinstatement (Shaham andStewart 1996; Khroyan et al. 2000). Interestingly, D1- andD2-like receptor agonists have opposite effects oncocaine-induced reinstatement, the former attenuatingthe effect of cocaine priming, while the latter potentiatingit (Self et al. 1996; Khroyan et al. 2000).

Neuroanatomical studies indicate that the cell bodyregion of the mesocorticolimbic DA system in the ventraltegmental area (VTA) plays a critical role in drug-inducedreinstatement. Activation of these midbrain DA neuronsby local morphine (Di Chiara and North 1992) or NMDA(Karreman et al. 1996) infusions reinstates heroin andcocaine seeking (Stewart 1984; Vorel et al. 2001).Infusions of amphetamine, DA and cocaine into the

terminal regions of the nucleus accumbens (NAc) or themedial prefrontal cortex (mPFC) also reinstate drugseeking (Stewart and Vezina 1988; Cornish and Kalivas2000; McFarland and Kalivas 2001; Park et al. 2002). Incontrast, reversible inactivation of the DA neurons in theVTA by GABAergic agonists blocks cocaine-inducedreinstatement (McFarland and Kalivas 2001). In addition,D1-like and non-selective DA receptor antagonists blockcocaine-induced reinstatement when injected into thecingulate and prelimbic regions of the mPFC (McFarlandand Kalivas 2001; Capriles et al. 2002; Park et al. 2002),but not into the NAc (Cornish and Kalivas 2000).Recently, however, selective D1- and D2-like receptorantagonists were reported to block cocaine-inducedreinstatement when injected into the NAc shell (C.Pierce, unpublished data). On the other hand, McFarlandand Kalivas (2001) found that reversible inactivation ofthe NAc core, but not shell, blocks cocaine-inducedreinstatement. The reasons for these contradictory resultsare not clear.

Cocaine-induced reinstatement is also modulated byglutamate within the NAc. Blockade of AMPA, but notNMDA, receptors in the NAc reduces the priming effectsof systemic or intra-mPFC cocaine, whereas activation ofAMPA receptors in the NAc reinstates cocaine seeking(Cornish et al. 1999; Cornish and Kalivas 2000; Park et al.

Fig. 2 Diagrammatical summary of our present knowledge of thesystems of the brain critical for the induction of reinstatement ofdrug seeking by cues, drugs, and footshock stressor, respectively.Most of the data described below is from studies with cocaine-trained rats. In each case, fat arrows indicate pathways that may beinvolved in reinstatement; thin arrows indicate some of the existingdirect anatomical connections and dashed arrows indicate indirectconnections. Cue-induced reinstatement: the effect of cocaine cueson reinstatement is blocked by systemic and intra-BLA injectionsof D1-like receptor antagonists and by reversible inactivation of theBLA and the dorsal mPFC. Drug-induced reinstatement: the effectof cocaine priming on reinstatement is blocked by systemic, intra-mPFC and possibly by intra-NAc shell injections of DA receptorantagonists. This effect of cocaine priming is also blocked byreversible inactivation of the dorsal mPFC, NAc core, VP and theVTA. In contrast, the effect of cocaine on reinstatement ismimicked by systemic injections of D2-like receptor agonists, byactivation of VTA DA neurons with morphine or excitatory aminoacid agonists, and by infusions of cocaine, DA and amphetamine

into the NAc and mPFC. Footshock stress-induced reinstatement:the effect of footshock on reinstatement is blocked by systemic andintra-BNST, but not intra-amygdala, injections of a CRF receptorantagonist. This effect of footshock stress is also blocked bysystemic and ventricular, but not intra-LC, injections of a2-adrenoceptor agonists (which reduce NA cell firing and release),by intra-BNST and intra-amygdala infusions of a2-adrenoceptorantagonists, and by 6-OHDA lesions of the VNAB projectionsarising from the LTg. In addition, the effect of footshock onreinstatement is attenuated by disrupting the CRF projections fromthe CeA to the BNST by infusions of a CRF antagonist into theBNST and TTX into the contralateral CeA. See text for moredetails and for references. 6-OHDA 6-hydroxydopamine, A8 andA10 dopamine (DA) cell groups, BLA basolateral amygdala, BNSTbed nucleus of the stria terminalis, CeA central amygdala, CRFcorticotropin-releasing factor, LC locus coeruleus, LTg noradren-aline (NA) cell groups of the lateral tegmental nuclei, mPFC medialprefrontal cortex, NAc nucleus accumbens, TTX tetrodotoxin, VTAventral tegmental area, VP ventral pallidum

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2002). On the other hand, NMDA receptor antagonists,injected systemically (De Vries et al. 1998b) or intra-NAc(Park et al. 2002), potently reinstate cocaine seeking.These data raise the intriguing possibility that AMPA andNMDA receptors in the NAc play opposing roles in drugseeking (Park et al. 2002).

At present, little is known about the role of DA orglutamate in brain areas other than the VTA, NAc andmPFC in cocaine- or heroin-induced reinstatement. Mc-Farland and Kalivas (2001) found that inactivation of theventral pallidum attenuates cocaine-induced reinstate-ment, whereas inactivation of the substantia nigra, centraland basolateral nuclei of the amygdala (CeA and BLA)and the mediodorsal thalamus has no effect. On the otherhand, Fuchs and See (2002) reported that inactivation ofthe BLA does attenuate heroin-induced reinstatement.

Finally, there is evidence that several other neuro-transmitter systems are involved in drug-induced rein-statement. Systemic injections of mu opioid receptorantagonists block reinstatement induced by priminginjections of heroin (Shaham and Stewart 1996) andalcohol (Le et al. 1999), but not cocaine (Comer et al.1993), and kappa opioid agonists attenuate cocaine-induced reinstatement (Schenk et al. 1999). Agonists of5-HT2C (Grottick et al. 2000) and GABAB (Campbell etal. 1999) receptors also attenuate cocaine-induced rein-statement. Interestingly, a synthetic cannabinoid agonistcan also reinstate cocaine seeking and a CB1 receptorantagonist attenuates cocaine-induced reinstatement (DeVries et al. 2001). It is likely that some of the abovepharmacological manipulations attenuate cocaine-inducedreinstatement via inhibition of drug priming-inducedmesocorticolimbic DA release (see Shalev et al. 2002a).Based on evidence for a cannabinoid-dopamine interac-tion in the striatum (Giuffrida et al. 1999), however, deVries et al. (2001) suggested that activation of endoge-nous endocannabinoid systems downstream from the DAsynapse may contribute to cocaine seeking.

Cue-induced reinstatement

In recent years, several laboratories have been investigat-ing reinstatement induced by different types of drug-associated cues. These include “discrete cues” that arepaired with drug infusions during self-administrationtraining (See 2002); “discriminative cues” that, followingdiscrimination training, become predictors of drug avail-ability during the acquisition of the drug self-administra-tion behavior (Weiss et al. 2000); and “contextual cues”or diffuse “background” stimuli (e.g. operant chamberfan, time of day) that become associated with druginjections, as in CPP experiments, or with the availabilityand the effects of the self-administered drug (Crombagand Shaham 2002).

Studies in rats with a history of cocaine self-admin-istration point to critical roles for the BLA and D1-likereceptors in cue-induced reinstatement. Systemic injec-tions of D1-like receptor antagonists attenuate reinstate-

ment induced by discrete (Alleweireldt et al. 2002),discriminative (Ciccocioppo et al. 2001) and contextual(Crombag et al. 2002) cues previously associated withcocaine. Furthermore, intra-BLA injections of a D1-like,but not D2-like, receptor antagonist block discrete cue-induced reinstatement of cocaine seeking (See et al.2001). In addition, exposure to discriminative cuesfollowing periods of withdrawal induces DA release andFos immunoreactivity in the BLA (Neisewander et al.2000; Weiss et al. 2000), and D1-like receptor antagonistsblock the effect of these cues on Fos-induction (Cicco-cioppo et al. 2001). There is also evidence from lesionstudies that the BLA is involved in reinstatement ofcocaine (Meil and See 1997) and heroin (Fuchs and See2002) seeking by discrete cues. In addition, it has beenfound that inactivation of the CeA and the mPFC byinfusions of tetrodotoxin (TTX) can attenuate reinstate-ment of cocaine seeking by discrete cues (See 2002),whereas infusions into the NAc have no effect (Grimmand See 2000).

There is also evidence that D2-like receptors may beinvolved in reinstatement of cocaine seeking by discrim-inative or contextual cues. Systemic injections of D2-likereceptor antagonists have been found to attenuate theeffect of these cues on reinstatement (Weiss et al. 2001;Crombag et al. 2002). Finally, studies using NMDA andCB1 receptor antagonists, injected systemically, point to apotential role of glutamate and endocannabinoids indiscrete cue-induced reinstatement of cocaine seeking(Bespalov et al. 2000; De Vries et al. 2001).

Much less is known about the pharmacology ofreinstatement by drug cues in rats with a history ofheroin or alcohol self-administration. In the runwaymodel, the preferential D2-like receptor antagonist, halo-peridol, has no effect on reinstatement of heroin seekingby discriminative cues (McFarland and Ettenberg 1997).In the self-administration model, however, both D1- andD2-like receptor antagonists were reported to attenuatereinstatement of alcohol seeking induced by alcohol cues(Liu and Weiss 2002). It is not known whether thesedifferent findings are related to the specific antagonistsselected for testing, the doses of the antagonists, ordifferences in the neural processes involved in reinstate-ment induced by alcohol- and heroin-associated cues.

Stress-induced reinstatement

Several stressors, including a brief presentation of inter-mittent footshock, acute food deprivation and the centralnervous system administration of the stress hormonecorticotropin-releasing factor (CRF) reinstate drug seek-ing in rats (Le and Shaham 2002; Shaham et al. 2000a). Inaddition, inactivation of the medial septum with TTXreinstates heroin seeking (Highfield et al. 2000). Septallesions mimic certain physiological and psychologicalresponses to stressors (Gray 1987). Using the CPPreinstatement procedure, Sanchez and Sorg (2001) re-ported that a conditioned fear state (induced by stimuli

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paired with shock) reinstates cocaine CPP. Conditionedfear stimuli, however, did not reinstate drug seeking inrats with a history of either cocaine or heroin self-administration (Shaham et al. 2000a). Another finding isthat the effect of intermittent footshock stress onreinstatement, documented in rats with a history ofheroin, cocaine, nicotine and alcohol self-administration,does not generalize to rats previously trained to self-administer non-drug reinforcers such as sucrose (Le andShaham 2002; Shaham et al. 2000a). Potential reasons forthis selective effect of footshock stress on reinstatementof drug seeking are discussed elsewhere (Shaham et al.2000a).

Studies on the neuronal mechanisms underlying foot-shock-induced reinstatement have identified two neuro-transmitter systems and two brain structures that arecritically involved in footshock stress-induced, but not indrug-induced, reinstatement, namely, brain CRF andnoradrenaline (NA) systems and the CeA and the bednucleus of the stria terminalis (BNST) (Erb et al. 2001b;Shaham et al. 2000a). Selective CRF1 and non-selectiveCRF receptor antagonists attenuate footshock-inducedreinstatement in heroin-, cocaine- and alcohol-trained rats(Shaham et al. 1997; Erb et al. 1998; Le et al. 2000).These receptor antagonists, but not a CRF2 receptorantagonist, also attenuate footshock-induced reactivationof morphine and cocaine CPP (Lu et al. 2001, 2000a). Inaddition, a2-adrenoceptor agonists, which decrease NAcell firing and release, attenuate footshock-induced rein-statement of heroin, cocaine and speedball (a heroin-cocaine combination) seeking (Erb et al. 2000a; Highfieldet al. 2001; Shaham et al. 2000b).

Studies using 6-hydroxydopamine lesions, intracranialinfusions, and reversible inactivation by TTX have furtheridentified two neuronal systems that are involved infootshock-induced reinstatement. One is an NA systemwith axons in the ventral NA bundle (VNAB) thatoriginates in the lateral tegmental NA cell groups andinnervates the CeA and BNST (Fritschy and Grzanna1991; Aston-Jones et al. 1999). The dorsal NA bundleoriginating from the locus coeruleus does not appear to bedirectly involved in the effect of footshock on reinstate-ment (Wang et al. 2001; Shaham et al. 2000b). Thesecond involves the CRF system of the BNST, andappears to include a CRF containing projection from CeAto the BNST (Erb et al. 2001a; Shaham et al. 2000b).Antagonism of CRF receptors in the BNST (but not in theCeA) and blockade of postsynaptic beta-adrenoceptors inboth the BNST and CeA attenuate footshock stress-induced reinstatement of cocaine seeking, suggesting aninteraction between these two systems (Erb and Stewart1999; Leri et al. 2002).

Studies using tracing methods or DSP-4 injections(which selectively destroy neurons of the locus coeruleus,but not the lateral tegmental NA nuclei) have shown thatthe ventrolateral BNST and the CeA are two of the maintargets of the lateral tegmental NA neurons (Fritschy andGrzanna 1991; Delfs et al. 2000). In the ventral BNST,NA neurons also form synaptic contacts with CRF-

containing neurons (Phelix et al. 1994). Footshock stressinduces release of NA in the ventrolateral BNST that canbe blocked by an a2-adrenoceptor agonist given eithersystemically or directly into the region of the A2 NA cellgroup, which is part of the lateral tegmental NA neurons(J. Stewart, unpublished observations). However, ICVinfusions of the CRF receptor antagonist, d-Phe CRF, thatblock footshock induced reinstatement do not alterfootshock stress-induced NA release in the BNST (Erbet al. 2001b). Together, the data from all of these studieson the role of CRF and NA systems suggest thatactivation of VNAB NA neurons leads to the activationof CRF systems within the BNST and/or CeA. Finally,TTX inactivation of the mPFC (prelimbic area) or theorbitofrontal cortex attenuates footshock-induced rein-statement of cocaine seeking, pointing to a potentialinvolvement of these brain sites in footshock stress-induced reinstatement (Capriles et al. 2002).

Neither selective D1- or D2-like receptor antagonistsnor the opioid receptor antagonist, naltrexone, affectfootshock-induced reinstatement of heroin seeking; how-ever, systemic injections of a long-lasting non-selectiveDA receptor antagonist do attenuate this effect (Shahamand Stewart 1996). These data suggest that DA plays anindirect/modulatory role in footshock-induced reinstate-ment. Additional evidence for a role for DA in footshock-induced reinstatement comes from the recent observationthat intra-mPFC (prelimbic area) or intra-orbitofrontalinfusions of a D1-like receptor antagonist attenuatefootshock-induced reinstatement of cocaine seeking(Capriles et al. 2002).

In alcohol-trained rats, the 5-HT reuptake blocker,fluoxetine, but not naltrexone, attenuates footshock-induced reinstatement (Le et al. 1999). In a follow-upstudy, Le et al. (2002) reported that infusions of 8-OH-DPAT (a 5-HT1A agonist that decreases 5-HT cell firingand release) into the 5-HT cell body region of the medianraphe nucleus (MRN) reinstate alcohol seeking. Intra-MRN infusions of low doses of CRF (3–10 ng) alsoreinstated alcohol seeking and intra-MRN infusions of aCRF receptor antagonist blocked the effect of intermittentfootshock on reinstatement. These data suggest that aninteraction between CRF and 5-HT neurons within theMRN is involved in footshock stress-induced reinstate-ment of alcohol seeking.

Another finding is that adrenalectomy (ADX), whichremoves circulating corticosterone, though it has no effecton footshock-induced reinstatement of heroin or alcoholseeking (Shaham et al. 1997; Le et al. 2000), attenuatescocaine seeking induced by this stressor (Erb et al. 1998).In cocaine-trained rats, however, footshock is an effectivestimulus for reinstatement in ADX rats given exogenouscorticosterone via pellets to provide constant low diurnallevels of the hormone. These data suggest that althoughbasal levels of corticosterone are necessary for themanifestation of footshock-induced reinstatement of co-caine seeking, the stressor-induced rise in corticosteronedoes not mediate its effect on reinstatement (Erb et al.1998). Potential reasons for the putative “selective” role

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of corticosterone in footshock-induced reinstatement ofcocaine, but not heroin and alcohol, seeking are discussedelsewhere (Shaham et al. 2000a). In addition, Martin-Fardon et al. (2000) reported that nociceptin selectivelydecreases footshock-induced reinstatement of alcohol, butnot cocaine seeking. Thus, it appears that both nociceptinand corticosterone play a drug-specific modulatory role infootshock-induced reinstatement.

Much less is known about the neuronal mechanismsunderlying reinstatement of drug seeking by acute fooddeprivation. In heroin-trained rats, ventricular infusions ofleptin, a hormone involved in energy balance and bodyweight regulation (Friedman and Halaas 1998), attenuatereinstatement induced by food deprivation, but not byheroin priming or footshock (Shalev et al. 2002c). Inaddition, as was found for footshock in cocaine-trainedrats (Erb et al. 1998), ADX attenuates food deprivation-induced reinstatement of cocaine seeking, but this effectcan be reversed by replacement of basal levels of thehormone (Shalev et al. 2002b). Interestingly, and alsosimilar to the findings with footshock (Shaham et al.1997), ADX had no effect on food deprivation-inducedreinstatement of heroin seeking (U. Shalev, unpublishedobservations).

Summary and future directions

Studies of drug- and cue-induced reinstatement of drugseeking indicate that the mesocorticolimbic DA system iscritically involved via its various projections (Fig. 2). Incocaine-trained rats, the brain regions implicated in drug-induced reinstatement are the mPFC and the NAc (bothcore and shell), whereas the critical sites for cue-inducedreinstatement are the BLA and mPFC. Several unresolvedquestions for future research are (1) the role of DAreceptors in the NAc core and shell in drug-inducedreinstatement, (2) the degree of involvement of these sub-divisions of the NAc in cue-induced reinstatement, (3) thedegree to which common neuronal circuits mediate drug-or cue-induced reinstatement in rats with a history ofcocaine, heroin and alcohol self-administration, and (4)the degree to which common circuits mediate reinstate-ment by discrete versus contextual cues. The latter twoissues are interesting for two reasons. First, manipulationsthat block cocaine-taking behavior controlled by discretecues in a second order schedule procedure (e.g. a D3receptor partial agonist, BLA lesions) have no effect onheroin-taking behavior under this schedule (Everitt andRobbins 2000). Second, there is evidence in the generallearning literature showing that the neuronal circuitsmediating the behavioral effects of discrete and contex-tual cues are not identical (LeDoux 2000).

CRF and NA within the BNST and the CeA playcritical roles in footshock-induced reinstatement (Fig. 2).The mPFC also plays an important role, as it does inreinstatement induced by cocaine priming and discretecocaine cues. Thus, it appears that regions within themPFC may serve as a “final common pathway” for the

mediation of relapse behavior (Neisewander et al. 2000;See 2002). Reinstatement induced by acute food depri-vation appears to be leptin-dependent, but little is knownabout the brain sites and neurotransmitters that areinvolved. A direction for future research will be todetermine the degree of overlap between the neuronalmechanisms mediating reinstatement induced by foot-shock and by food deprivation.

Another general issue will be to determine theconcordance between the systems mediating reinstate-ment in the drug self-administration and the CPPvariations of the reinstatement model. For example, wedo not know what accounts for the fact that conditionedfear stimuli can reinstate CPP (Sanchez and Sorg 2001),but not drug self-administration behavior (Shaham et al.2000a).

Discussion

In the sections below we discuss several issues that ariseconcerning the interpretation of reinstatement of drugself-administration behavior, as observed in studies usinglaboratory animals, and the relevance of such findings forrelapse to drug taking in humans.

Relationships between drug priming-inducedreinstatement, drug discrimination, and drug reward

It has been suggested from time to time that priminginjections of drugs reinstate drug seeking after extinctionbecause they activate brain systems involved in theirdiscriminative stimulus properties (Stretch et al. 1971;Stolerman 1992; Di Chiara 1995) or their rewardingeffects (Stewart et al. 1984). Pharmacological and neu-roanatomical data indicate, however, that it may bepossible to dissociate, at least to some degree, themechanisms mediating the effects of drug priming fromthose mediating the discriminative or rewarding effects ofdrugs. For example, D1-like receptor agonists that substi-tute for cocaine in the drug discrimination procedure donot reinstate cocaine seeking (Spealman et al. 1999)(Fig. 3). Furthermore, although a main conclusion fromdrug discrimination studies is that stimulus generalizationis typically observed within a given drug class (Stolermanet al. 1995), heroin or morphine seeking can be reinstatedby indirect DA agonists and D2-like agonists (Davis andSmith 1976; De Vries et al. 1999). Finally, intra-VTAinjections of morphine reinstate heroin seeking (Stewart1984), but do not substitute for heroin in a drugdiscrimination procedure (Shaham and Stewart 1995a;Jaeger and van der Kooy 1996).

There is also evidence that the neuronal eventsunderlying cocaine-induced reinstatement are to somedegree different from those involved in the acutereinforcing effects of the drug (Shalev et al. 2002a).Thus, D1-like receptor agonists are self-administeredunder certain conditions by rats and monkeys, but do

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not reinstate cocaine seeking (Self and Nestler 1998;Spealman et al. 1999). Manipulations of corticosteronesecretion affect cocaine self-administration behavior(Piazza and Le Moal 1996; Goeders 1997), but haveminimal effects on cocaine-induced reinstatement (Erb etal. 1998; Mantsch and Goeders 1999b). A CB1 receptorantagonist attenuates cocaine-induced reinstatement, buthas no effect on cocaine self-administration behavior (DeVries et al. 2001). NMDA and AMPA agonists haveminimal effects on cocaine self-administration behaviorbut they reinstate cocaine seeking (Cornish et al. 1999).Finally, DA receptor antagonists do not have consistenteffects on heroin self-administration (Ettenberg et al.1982; Mello and Negus 1996), but they attenuate heroin-

induced reinstatement (Shaham and Stewart 1996;McFarland and Ettenberg 1997). In the majority of thesecases, however, the differences between the effect ofthese various manipulations on reinstatement induced bypriming drug injections and on self-administration ofdrugs may arise from the fact that the effects of priminginjections are studied under extinction conditions whereno further drug is available, whereas during self-admin-istration the animal is free to take a considerable amountof drug over time. Thus, whether the differences observedin the effects of the manipulations reflect, in all cases,dissociation between the neural mechanisms mediatingthe reinforcing effects and the reinstating effects ofcocaine is still an issue for future research.

Role of drug withdrawal in reinstatement of drug seeking

The events that induce reinstatement of drug seeking inanimals, drug priming, drug cues and stressors have allbeen reported to provoke relapse in humans. Anothercondition thought to play an important role in relapse toheroin and alcohol, and possibly to other drugs in humans,is the drug withdrawal syndrome (Dackis and Gold 1985;O’Brien et al. 1986). Compulsive drug use and drugrelapse are thought to occur because the addict is seekingdrugs to alleviate the aversive symptoms of drug with-drawal (negative reinforcement); symptoms that can alsobe elicited by stimuli previously paired with the drugwithdrawal syndrome (Himmelsbach 1943; Wikler 1973;Siegel 1989).

There are no published studies reporting that a state ofacute withdrawal promotes the reinstatement of respond-ing after cocaine, alcohol or nicotine self-administration.In the case of heroin self-administration, acute opioidwithdrawal, precipitated by injections of opioid antago-nists, does not reinstate heroin seeking following extinc-tion (Shaham and Stewart 1995b; Shaham et al. 1996). Onthe other hand, rats that had gone through extinctiontraining while heroin-containing minipumps were im-planted, resumed heroin seeking in tests given 24 h afterpump removal. It is not clear, however, whether thiseffect is due to the motivational effects arising from theopioid withdrawal state or due to the fact that extinctionwas carried out in the heroin state whereas reinstatementwas not; that is, was a state-dependent phenomenon(Shaham et al. 1996).

Recent evidence from studies of reinstatement aftertermination of drug taking demonstrates that the intervalelapsed since a drug was last self-administered (or thedrug withdrawal period) affects drug seeking in a mannerthat is not readily predicted from negative reinforcementtheories (Shalev et al. 2002a). In these studies, rats weretrained to self-administer heroin or cocaine for 6–9 h/day,leading to levels of drug intake sufficient to inducewithdrawal symptoms early after drug withdrawal (1–2days) (Shaham et al. 1996; Sarnyai et al. 2001). Theywere then tested using the between-within session proce-dure (see Fig. 1) in which they remained undisturbed for

Fig. 3 Comparison of cocaine-like priming effects and cocaine-likediscriminative stimulus effects of DA receptor agonists. Top panelshows maximum reinstatement of extinguished cocaine-seekingbehavior. Bottom panel shows maximum cocaine-lever respondingin monkeys trained to discriminate cocaine from vehicle. Data arefrom Fig. 7 in Spealman et al. (1999), with permission fromElsevier Science

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varying periods of time (1 day to 8–9 weeks) beforetesting. Both cue-induced reinstatement (Grimm et al.2001) and footshock-induced reinstatement (Shalev et al.2001) were maximal at time points that are well beyondthe acute withdrawal phase and, surprisingly, the leastamount of lever pressing occurred on day 1 of withdrawalfrom cocaine (Fig. 4) or heroin (Fig. 5). These data are inagreement with those from the second order scheduleprocedure, which is used to study drug seeking motivatedby drug-associated cues. Arroyo et al. (1998) found adecrease in lever-pressing behavior that was maintainedby cocaine cue during early withdrawal from “binge”cocaine self-administration.

Finally, there is some evidence that the drug with-drawal period also affects the magnitude of reinstatementinduced by drug priming. Tran-Nguyen et al. (1998)found that cocaine priming injections (15 mg/kg, IP)produced higher rates of lever pressing after 1 month ofabstinence compared to 1 day or 1 week without cocaine.On the other hand, De Vries et al. (2002) found that a D2-like receptor agonist, quinpirole, would reinstate heroin

seeking after a short (within 1 week), but not after aprolonged (3 weeks) period of withdrawal. In a recentstudy, however, Grimm et al. (2002) found that theresponsiveness to cocaine priming (5 and 15 mg/kg, IP)did not differ in tests given at 1, 30 and 90 days afterwithdrawal from cocaine self-administration (6 h/day for10 days), whereas lever pressing in tests for extinctionbehavior and cue-induced reinstatement were higher 30and 90 days after withdrawal than on day 1. The reasonsfor the difference between these data and those of Tran-Nguyen et al. (1998) are not clear, but they may be relatedto the fact that daily cocaine intake in the study of Grimmet al. (2002) was 6–7 times higher than in the earlierstudy.

Taken together, the evidence suggests that drugwithdrawal syndromes or states do not play a criticalrole in relapse to drug seeking in rats as measured in the

Fig. 5A,B Time-dependent changes in extinction behavior andfootshock stress-induced reinstatement following withdrawal fromheroin. A Mean (€SEM) number of non-reinforced responses on thepreviously active lever during the first five 60-min sessions ofextinction. B Mean number of non-reinforced responses on thepreviously active lever during the test for reinstatement of heroinseeking. Baseline: the last 60-min extinction session on which therats reached the extinction criterion prior to exposure to footshock.Post-footshock: 60-min session after exposure to footshock. Post-Post-Shock: subsequent 60-min session. Extinction training startedafter 1, 6, 12, 25, or 66 days of withdrawal from heroin. *Differentfrom day 1 withdrawal (P<0.05). Data are from Shalev et al.(2001), with permission from Springer-Verlag

Fig. 4A,B Time-dependent changes in extinction behavior andcue-induced reinstatement following withdrawal from cocaine. AMean (€SEM) number of non-reinforced responses on the leverpreviously associated with cocaine, averaged across six extinctionsessions in the presence of the house light and lever that hadpreviously been associated with cocaine availability. B Mean(€SEM) number of non-reinforced responses on the lever previ-ously associated with cocaine in the subsequent presence of thelight-tone signal (conditioned reinforcer) that had previouslyassociated with earned cocaine injections. Baseline data are fromthe previous extinction session. *Different from day 1 (P<0.01).Data are from Fig. 1 in Grimm et al. (2001), with permission fromNature

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reinstatement model. Several recent studies show, how-ever, that reinstatement of drug seeking is profoundlyaffected by the duration of the withdrawal periodfollowing termination of heroin and cocaine self-admin-istration; in general, less responding is observed aftershort periods of withdrawal than after long periods (weeksand months). An important question for future researchwill be to identify the mechanisms underlying these time-dependent changes in reinstatement of drug seekingfollowing the termination of drug-self administration.

Drug-induced neuronal adaptations and reinstatementof heroin and cocaine seeking

The major hypothesis that guides current neurobiologicalresearch on drug addiction is that chronic drug exposurecauses long-lasting, neuroadaptive changes in the brain atthe molecular and cellular levels that may contribute tocompulsive drug use and relapse (Nestler and Aghajanian1997; Nestler 2001). Although there is no direct evidencefor this idea, numerous studies have shown that chronicexposure to opioid and stimulant drugs alters geneexpression (Hope et al. 1994; Nestler 2000), electrophys-iology (White and Kalivas 1998) and neurochemisty(Rossetti et al. 1992; Vanderschuren and Kalivas 2000).Studies using knockout mice and virally mediated over-expression of certain target genes in the NAc and VTAsuch as delta-fosB, cyclic-AMP-response-element-bind-ing (CREB) protein, cyclin-dependent kinase 5 (cdk5),and GluR1 and GluR2 (AMPA glutamate receptorsubunits) also have shown that the sensitivity for therewarding and locomotor activating effects of opioid andstimulant drugs can be affected by these manipulations(Carlezon et al. 1997, 1998; Kelz et al. 1999; Bibb et al.2001). In one study, which was inspired by the “neu-roadaptation hypothesis,” and by the knowledge that thecAMP system in the NAc is upregulated followingchronic cocaine exposure (Nestler 2001), Self et al.(1998) reported that acute inhibition of the cAMP systemin the NAc reinstated cocaine seeking. The relevance ofthis observation to relapse behavior following prolongedwithdrawal periods is not obvious, however, becausealterations in PKA function are observed following acute(1 day), but not prolonged withdrawal from cocaine (7 or21 days) (B.T. Hope, unpublished data).

At present, it is not known which of the manymolecular alterations that are observed following chronicexposure to drugs are associated with the propensity torelapse and, more specifically, with the time dependentincreases in drug seeking following withdrawal. There isreason to believe, however, that at least some of thesemolecular/neurochemical alterations are associated withreinstatement of drug seeking. First, studies on theneuronal mechanisms underlying locomotor sensitizationindicate that certain molecular and neurochemical alter-ations within the mesocorticolimbic DA system (e.g.enhancement of cocaine-induced DA release in the NAc,D1 receptor super-sensitivity in this area) are manifested

at time points beyond the acute withdrawal phase andpersist for at least several weeks (White and Kalivas1998). Second, drug-induced reinstatement of heroin andcocaine seeking following long-term abstinence is asso-ciated with sensitized locomotor responses to the drugs(De Vries et al. 1998a, 1999). Third, the time course ofthe expression of sensitized locomotor responses to D2-like receptor agonists is correlated with the time-course oftheir effect on reinstatement (De Vries et al. 2002).Fourth, in rats sensitized to amphetamine a priminginjection of the drug potentiates both reinstatementresponding and DA release (Vezina et al. 2002). Fifth,activation of AMPA receptors in the NAc, known toaccompany the expression of the sensitized locomotorresponse to cocaine (Vanderschuren and Kalivas 2000),induces reinstatement of cocaine seeking (Cornish andKalivas 2000). Together, these data suggest that theneuronal alterations involved in enduring changes in thesensitivity to the locomotor activating effects of drugsmay also be involved in reinstatement induced by drugpriming. It should be pointed out, however, that theavailable data are correlational and it has not beestablished, for example, that “reversal” of cocainelocomotor sensitization (see Li et al. 2000) has an effecton cocaine-induced reinstatement.

Based on data on cross-sensitization of locomotoractivity between drug and stressors (Kalivas and Stewart1991), it has also been suggested that sensitization of thestimulant effects of drugs may be of relevance to theunderstanding of relapse induced by stressors (Robinsonand Berridge 1993; Shaham and Stewart 1995b). Thisidea, however, is challenged by pharmacological andneuroanatomical studies suggesting that different neuro-nal systems are involved in drug priming- and footshockstress-induced relapse (Stewart 2000; Shalev et al.2002a). Repeated exposure to drugs may, however,induce neuronal adaptations in brain systems involvedin stress responses (Kreek and Koob 1998; Sarnyai et al.2001). For example, former opioid users show increasedautonomic responses to a physical stressor (Himmelsbach1941), and increased reactivity to environmental stressorsis a common feature of protracted drug withdrawal (Jaffe1990). Two recent studies support the possibility thatdrug-induced neuronal changes alter reinstatement in-duced by stressors. Ahmed et al. (2000) found that ratstrained to lever press for heroin for 11 h/day demonstratehigher rates of responding during tests for footshock-induced reinstatement than rats trained for 1 h/day. Inaddition, as mentioned above, Shalev et al. (2001)reported time-dependent changes in the effect of foot-shock stress on reinstatement of heroin seeking followingwithdrawal. Footshock reinstated heroin seeking after 6,12, 25 or 66 days of withdrawal, but not after 1 day(Fig. 5). These data are in agreement with the idea thatdrugs induce long-term neuroadaptive changes that taketime to develop following drug withdrawal, are long-lasting, and are dependent on the amount of drugexposure (Pierce and Kalivas 1997; Stewart 2002).

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Other studies have shown that pre-exposure to psy-chostimulant drugs enhances stress- and drug-inducedactivation of the hypothalamic-pituitary adrenal axis(Schmidt et al. 1999; Barr et al. 2002). The relevance ofthese data to stress-induced relapse, however, is notknown because our data demonstrate that increases incorticosterone release by stressors is not involved in theireffect on reinstatement (Shaham et al. 2000a; Shalev et al.2002b). In addition, levels of extracellular CRF, asmeasured in the amygdala using microdialysis, areincreased in the period immediately after withdrawalfrom alcohol (Pich et al. 1995) and cocaine (Richter andWeiss 1999). Olive (2002) recently reported that extra-cellular CRF levels also are increased in the lateral BNSTduring early withdrawal from alcohol. Furthermore,Zorrilla et al. (2001) showed that rats withdrawn fromalcohol or cocaine initially exhibit reduced tissue contentof CRF-like immunoreactivity in the amygdala (see alsoSarnyai et al. 1995), followed by a progressive increase,that culminates in elevated levels 6 weeks post-with-drawal. These authors also reported time-dependentchanges in CRF content in other brain areas followingdrug withdrawal. The initial decrease in peptide contentmay be due to increased CRF release, resulting in tissuecontent depletion, whereas the long-term increase may bedue to increased CRF synthesis (Sarnyai et al. 2001).

It is not known whether pre-exposure to such drugsalter the response of these CRF systems to stressorspresented long after withdrawal. Shalev et al. (2001)recently found, however, that footshock stress increasesCRF mRNA in the CeA and the dorsal BNST following 1or 6 days of withdrawal from heroin, but not sucrose, self-administration, suggesting that prior drug exposureenhances the response of extrahypothalamic CRF systemsto stress. In support of this idea, Slawecki et al. (1999)reported increased neuronal sensitivity in the cortex toCRF 10–15 weeks after withdrawal from alcohol. Thus,neuronal adaptations in extrahypothalamic CRF, mostlikely within the extended amygdala systems, may lead toincreased sensitivity to relapse to drug seeking induced byenvironmental stressors (Koob 1999; Sarnyai et al. 2001).

Finally, it has been suggested that the studies of theneurobiology of memory may provide clues on themechanisms underlying compulsive drug use and relapse(Wise 1987; White 1996; Everitt et al. 2001), or that drugaddiction is an aberrant form of learning, mediated bymaladaptive recruitment of certain memory systems in thebrain (Di Chiara 1999; Berke and Hyman 2000; Lu et al.2000b). In fact, some of the neuroadaptive changes inresponse to drugs (Wolf 1998; Flores and Stewart 2000;Nestler 2001) involve molecular substrates known to beinvolved in memory formation and retrieval. Interesting-ly, recent studies have shown that cellular and synapticchanges such as long-term potentiation and depression(LTP and LTD), involving alterations in the activity ofcAMP pathways, CREB, growth factors, and glutamate(Berke and Hyman 2000; LeDoux 2000), take place inregions of the brain that mediate some of the long-lastingeffects of exposure to drugs. For example, cocaine can

induce LTP in the VTA (Ungless et al. 2001) and LTD inthe NAc (Thomas et al. 2001). Much has yet to belearned, however, about the relevance of these in vitrodata for relapse phenomena. Perhaps more relevant forour understanding of the long-lasting effects of drugexposure on susceptibility to relapse is the evidence thatexposure to cocaine or amphetamine leads to morpho-logical changes in neurons within the mesocorticolimbicDA terminal regions, such as increases in dendriticbranching and density of dendritic spines on mediumspiny neurons in the NAc shell and on pyramidal cells inthe prefrontal cortex that are evident several weeks afterwithdrawal (Robinson and Kolb 1999; Robinson et al.2001). In support of the idea that exposure to drugs andsubsequent withdrawal can initiate processes that promoteneural plasticity, profound increases in the levels of brain-derived neurotrophic factor (BDNF), known to beinvolved in synaptic plasticity and connectivity (Thoenen1995), have been found in the VTA, NAc and amygdalafollowing 30 and 90 days, but not 1 day, of withdrawalfrom cocaine, but not from sucrose, self-administration(Lu et al. 2002). Furthermore, these alterations parallel tosome degree the time-dependent changes in extinctionbehavior and cue-induced reinstatement of cocaine seek-ing discussed above.

Taken together, numerous studies have reported thatchronic drug exposure can lead to molecular, neurochem-ical, and structural alterations in the brain, many of whichare involved in the synaptic plasticity underlying learningand memory. Much work is necessary, however, todetermine which of these many changes mediate the time-dependent changes in the propensity to relapse followingwithdrawal from heroin and cocaine. In our view, thehypothesis with the most intuitive appeal at this time isthat it is the long-lasting modifications of neuronalmorphology and connectivity within those systems medi-ating drug reward and reinstatement that are critical; andthat these changes are brought about by the induction ofneurotrophic factors in response to intensive drug expo-sure.

Is the reinstatement model a valid preclinical modelof drug relapse?

A key question in interpreting the data from preclinicalstudies using the reinstatement model is whether thesedata are of relevance to the understanding of drug relapsein humans. A primary goal of the studies with laboratoryanimals is to understand the human relapse process, andultimately to develop treatments that reduce the incidenceof relapse. For this goal, it is critical that the modelsparallel the relapse process in humans. The validity of themodel can be assessed in at least three ways: (1) byexamining the commonalities and differences between theexperimental conditions in the reinstatement model andthe naturally occurring conditions in human relapse; (2)by comparing the findings of studies with laboratoryanimals with laboratory-based studies of reinstatement

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with humans; and (3) by examining clinical reports fromhuman drug users who relapse. Any comparisons betweennon-humans and humans are complicated by manyfactors, including differences in the subjects’ expectan-cies and histories, their ability to communicate verbally,and the type of dependent measures used (e.g. subjectivereports or overt drug-seeking, and the doses and durationsof drug use). Nevertheless, it is important to examine thedegree of concordance between human and non-humandata.

Similarities and differences between the reinstatementphenomenon and human relapse

In many respects, the reinstatement phenomena seen inlaboratory animals using drug, cues and stress correspondclosely with relapse in humans. In both cases, theindividuals have a history of drug self-administration,are currently drug-free and not actively seeking the drugbefore the reinstatement or relapse event occurs. In bothcases, drug-seeking responses return after the precipitat-ing event. In both cases, administration of the drug,exposure to drug-related cues, and exposure to stressfulevents appear to increase drug seeking. There are,however, differences between the laboratory animalmodel and the human situation. For example, animalsundergo a period of extinction during which they performthe drug-seeking responses but these are not followed bydrug administration. In contrast, humans rarely undergothis form of extinction, i.e. performing the drug-seekingresponse without reinforcement. Instead, humans ceaseusing drugs for other reasons, which may includepunishment, availability of alternative reinforcers or lackof drug availability. It can be argued, however, that duringthe period when drugs are no longer taken, the effective-ness of cues previously associated with drugs and drugtaking will diminish if they are repeatedly encounteredwithout drug administration. Thus, even in the humansituation there is an opportunity for extinction ofresponses to conditioned cues.

Another difference between the procedures used tomeasure “relapse” in laboratory animals and the humansituation is that, in the animal studies, responses madeduring the test for reinstatement are not reinforced,whereas in humans a drug-seeking response usuallyresults in drug delivery. Thus, in the animal studies whatis being assessed during the test is the vigor of respondingin the presence of drug-related cues, i.e. drug seeking (oras in the CPP studies, the amount of time the animalspends in the presence of drug-related cues) rather thanthe resumption of drug taking. There is no doubt,however, that if the animals were given access to thedrug, they would rapidly resume drug taking.

Another important difference between the animalreinstatement model and human relapse is the level ofconflict, or the degree of control over the behavior byother rewards and punishments. Whereas animals in self-administration procedures are typically given free access

to the drug (i.e. there are no negative consequences todrug use), human drug users usually abstain from drugseither because of negative consequences of use (e.g. jobloss or social disapproval) or because of strong alternativeincentives (e.g. staying out of jail). In animals, respondingis suppressed only by the lack of drug availability duringextinction. Whether these differences between the factorscontrolling abstinence in human drug users and the factorscontrolling extinction responding in animals affect thevalidity of the reinstatement model remains to bedetermined. In a recent study, Leri and Stewart (2002)have tried to determine in rats which of the events (e.g.drug availability, re-exposure to cues, exposure to stress)that take place during an initial “lapse” to drug seeking ortaking are critical for relapse to drug seeking on asubsequent day. These studies though preliminary, mayprovide a way for animal studies to model more closelyconditions that affect relapse in human addicts.

Laboratory studies of reinstatement in humans

Several recent laboratory-based studies with humansprovide support for the validity of the animal reinstate-ment model with drugs, cues and stress. For example,administration of cocaine to abstinent cocaine usersincreased ratings of craving and wanting more cocaine(Jaffe et al. 1989; Breiter et al. 1997; Haney et al. 2001).In other studies, it has been shown that low doses ofalcohol increase the desire for more alcohol in normalsocial drinkers as well as in alcoholics (Ludwig et al.1974; de Wit 1996; Kirk and de Wit 2000). Interestingly,this “priming” effect of alcohol appears to be dampenedby naltrexone, the opioid antagonist approved for thetreatment of alcoholism (O’Malley et al. 1992). Clinical-ly, patients treated with naltrexone are less likely to drink,and it has been suggested that naltrexone dampens thepriming effect of alcohol-related cues in the environment(O’Malley 1996). Even when the naltrexone-treatedpatients do drink they report less enjoyment from thealcohol effects and they are less likely to progress to a fullrelapse (Davidson et al. 1996; O’Malley 1996). Thesefindings suggest that the opioid system may be involvedin drug-induced and cue-induced craving for alcohol andrelapse to drinking in humans and are in agreement withpreclinical reports that naltrexone attenuates both drug-and cue-induced reinstatement of alcohol seeking in rats(Katner et al. 1999; Le et al. 1999). Interestingly, there isalso a report that naltrexone reduces the appetizer effectof food in humans (Yeomans and Gray 1997), suggestingthat opioid systems may be involved in priming effects ofnon-drug reinforcers as well.

As in laboratory animals, there is evidence fromstudies in abstinent drug users that both drug-related cuesand stress increase subjective ratings of desire for drug(O’Brien et al. 1992; Sinha 2001). For example, in onestudy, cocaine users were exposed to an imagery proce-dure involving stressful images or drug-related images.Both types of cues increased craving for drugs, as well as

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increasing anxiety, heart rate and cortisol levels (Sinha etal. 2000).

Clinical reports of relapse episodes

Another valuable source of information about precipitat-ing factors for relapse comes from the description ofevents immediately preceding relapse episodes in humandrug users. For example, Shiffman and colleagues haveinvestigated the events and mood states surroundingrelapse to smoking in recently abstinent smokers. In anearly study examining antecedents of relapse in ex-smokers who called a relapse counseling hotline, Shiff-man (1982) reported that about a third of relapse criseswere associated with positive affective states. In a laterstudy, Shiffman et al. (1996) studied the events thatimmediately preceded instances of relapse in smokers,using real-life monitoring of feelings and activities. Theyfound that positive affective states such as “good mood”and “relaxing” accounted again for about a third of therelapse episodes. On the other hand, Shiffman (1982) alsofound that about a third of abstinent smokers reported thatnegative affect (anger, anxiety and depression) elicited astrong urge to smoke, and in the more recent study theyfound that “bad mood” and stress preceded more than athird of relapse episodes (Shiffman et al. 1996; but seeHall et al. 1990 for a negative report). The positiveaffective states may increase drug taking by the sameprocess as priming doses of drugs. The negative affectivestates, however, may increase relapse by increasingimpulsivity and impairing decision-making capacity. Thisidea derives some support from economic decision-making studies, which show that stress and negativeaffect increase impulsive decisions (Leith and Baumeister1996; Gray 1999; Tice et al. 2001). In agreement with theidea that processes underlying impulsivity or responseinhibition may be involved in relapse, it was reported thatinactivation of the median raphe or the medial septum,which disrupts response inhibition as measured in otherlearning tasks, reinstates alcohol (Le et al. 2002) andheroin (Highfield et al. 2000) seeking, respectively.

Together, research with human subjects and patientslends support for the validity of the reinstatement modelof drug relapse. Despite the procedural differencesbetween the laboratory models of the behavior and thereal-life situation among drug users, there is a goodcorrespondence between humans and non-humans in thetypes of events that restore responding after a drug-freeperiod. A final issue concerns the validity of thereinstatement model used to study the relapse phenom-enon in humans in the laboratory. In studies in laboratoryanimals, one measures the behavior of the animal (leverpressing or spending time in the presence of drug-relatedcues), whereas in the majority of the laboratory-basedexperimental studies in humans on the effect of drug, cuesand stressors, the dependent measure is subjective self-report of craving. An unresolved clinical issue is thedegree to which drug craving is associated with drug

relapse and there are reports that the correlation betweenthe two measures is modest (Tiffany 1990; Pickens andJohanson 1992; Tiffany and Conklin 2000). This state ofaffairs calls for the development of laboratory-basedexperimental procedures to study drug-taking behavior inaddition to subjective craving in humans.

Concluding remarks

The reinstatement model was introduced to the drugaddiction field more than 30 years ago, and has been usedsporadically until 6 years ago. Recently, the model hasbeen used in many laboratories to study neuronalmechanisms underlying reinstatement of drug seekinginduced by drug re-exposure, drug cues and stressors. Itappears that the neuronal mechanisms that mediate theseevents are to some degree dissociable (Fig. 2) and that themechanisms underlying drug-induced reinstatement arenot identical to those mediating drug discrimination orreward. Perhaps the most interesting recent neuroanatom-ical finding is that inactivation of the mPFC attenuatesdrug-, cue-, and footshock stress-induced reinstatement ofcocaine seeking, suggesting a critical role of this brain sitein drug relapse. Another potentially significant recentfinding is that the drug withdrawal period can profoundlymodulate reinstatement of drug seeking. Based on datafrom studies on neuronal adaptations following with-drawal from chronic drug exposure, we speculate thatprocesses involved in synaptic plasticity such as neuronalre-organization and growth factors activity may con-tribute to relapse vulnerability following prolongedabstinence. Finally, the limited data in humans indicatethat there is a reasonable correspondence between rein-statement in laboratory animals and relapse and craving inhumans. In the case of drug relapse, however, the abovestatement is primarily based on clinical impressions orcorrelational studies. It is important, therefore, fromtheoretical and practical (medication development) per-spectives to develop experimental, laboratory-based,human models of drug seeking to complement theexisting procedures that are used to measure drug craving.

Acknowledgement We thank Dr. Roger Spealman for providingFig. 3.

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