Available online at www.sciencedirect.comsubsequent extinction of the drug-reinforced responding.This phenomenological similarity or face validity has ledto a dramatic increase in the use of this model in theaddiction field (over 900 papers over the last decade,PubMed search). Face validity is of course not a sufficientcondition for a valid animal model of human diseases[11,12]. However, evidence is emerging for predictivevalidity of the reinstatement model for heroin and alcohol
cocaine craving; our studies also suggest a role of VTABDNF, and potentially NAc and amygdala BDNF, in thisincubation [24,28,29]. Below we summarize results fromseveral recent (since 2009) papers on mechanisms ofincubation of drug craving. We also discuss results fromselected studies in which cue-induced drug seeking inextinction tests was assessed after several weeks of with-drawal, but not early withdrawal.
www.sciencedirect.com Current Opinion in Neurobiology 2013, 23:675683Recent developments in animal Nathan J Marchant, Xuan Li and Yav
Drug craving and relapse to drug use during abstinence are
defining features of addiction. Evidence indicates that drug
craving and relapse in humans are often provoked by acute
exposure to the self-administered drug, drug-associated cues,
or stress. During the last two decades, this clinical scenario has
been primarily studied at the preclinical level using the classical
reinstatement model. However, a single preclinical model
cannot capture the complicated nature of human drug relapse.
Therefore, more recently, we and others have developed
several other models to study different facets of human drug
relapse. In this review, we introduce and discuss recent
findings from these other relapse models, including incubation
of drug craving, reacquisition and resurgence models, and
punishment-based and conflict-based relapse models.
Intramural Research Program, National Institute on Drug Abuse,
Baltimore, MD, USA
Corresponding authors: Marchant, Nathan J (email@example.com),
Shaham, Yavin (Yavin.firstname.lastname@example.org)
Current Opinion in Neurobiology 2013, 23:675683
This review comes from a themed issue on Addiction
Edited by Barry Everitt and Ulrike Heberlein
For a complete overview see the Issue and the Editorial
Available online 29th January 2013
0959-4388/$ see front matter, Published by Elsevier Ltd.
IntroductionA main problem for treatment of drug addiction is relapseto drug use after periods of abstinence . In drug addicts,drug craving and relapse during abstinence are oftentriggered by acute re-exposure to the drug , drug-associated cues , or stress . This clinical scenariohas been primarily studied over the last two decades usingthe classical reinstatement model . In this model,laboratory animals are tested for reinstatement of drugseeking induced by drug-priming , discrete cues ,discriminative cues , contextual cues , or stress[9,10], following drug self-administration training andodels of drug relapse Shaham
relapse: naltrexone, buprenorphine, or methadone, whichdecrease drug relapse in humans, decrease drug-primingor cue-induced reinstatement of drug seeking in rats .Additionally, potential medications that decrease stress-induced craving in humans (clonidine, lofexidine, orprazosin) decrease stress-induced reinstatement of drugseeking in rats .
The reinstatement model is currently the most commonlyused animal model of drug relapse. However, it is obviousthat a single preclinical model, which involves an operantextinction component, cannot capture the complicatednature of human drug relapse [13,14]. Therefore, morerecently, we and others have developed and used severalother models to study different facets of human drugrelapse. These include incubation of drug craving [15,16],reacquisition and resurgence models , and punish-ment-based and conflict-based relapse models[20,21,22]. Here, we introduce and discuss recent find-ings from these relapse models. Table 1 provides theexperimental phases in these models of relapse and thereinstatement model.
Animal models of drug relapseIncubation of drug craving
Incubation of drug craving refers to the time-dependentincreases in cue-induced drug seeking after withdrawalfrom cocaine, heroin, alcohol, methamphetamine, andnicotine [23,24]. This phenomenon has been demon-strated using established procedures used to assess cue-induced drug seeking , including extinction, cue-induced reinstatement, and acquisition of new responseprocedures [23,24]. To study the neuronal mechanisms ofincubation of drug craving, we and others have primarilyassessed cue-induced drug seeking in a single extinctiontest performed at different days after withdrawal from thedrug. During testing, rats are brought to the drug self-administration environment/context (operant chambers)and lever-presses (or nose-pokes) lead to contingentpresentations of discrete cues previously paired with druginfusions . In previous studies (published before2009), we and others have demonstrated a causal roleof ERK and glutamate in CeA, and calcium permeableGluA2-lacking AMPA receptors in NAc in incubation of
Experimental conditions in the different phases of the operant moKoya et al.  provided evidence for a role of ventral(infralmbic/ventral prelimbic), but not dorsal (dorsalprelimbic/anterior cingulate), mPFC in incubation ofcocaine craving. Time-dependent increases in extinctionresponding were associated with large (ventral mPFC) or
Name Phase 1 (training)Phase 2
Lever presses or nose-poke for drug infusions or delivery
Extinction of operant responding
induced bdrug cues
Incubation of drug craving
Same Home-cage Non-reinforespondinpresence associateddifferent dwithdrawa
Same Extinction of operant responding
Lever prespoke for ddelivery
Same Extinction of the original drug-reinforced responding; responding on a different lever or device is reinforced by a non-drug reward
Removal oreward indreinstatemreinforcedthe drug-a
Same Lever presses for drug infusions or delivery are suppressed by response-contingent shock
Resumptioreinforceddrug primidrug or no
Conflict-based relapse model
Same An electric barrier must be crossed to gain access to the drug-associated lever; shock intensity is gradually increased until cessation of drug self-administration occurs
Resumptioreinforcednon-continpresentaticues. Theremains Orelapse te
Current Opinion in Neurobiology 2013, 23:675683 s of drug relapsemodest (dorsal mPFC) increases in ERK phosphoryl-ation (used as an index of neuronal activation). After 30withdrawal days, reversible inactivation (using agonistsof GABAa and GABAb receptors) of ventral, but notdorsal, mPFC decreased extinction responding. After 1
ent of non- responding y drug priming, , or stress
-Reliable and reproducible behavioral effects-Evidence for predictive validity
-Abstinence (achieved by operant extinction) does not mimic the human condition
-Model does not capture negative consequences of drug use
rced extinction g in the of drug- cues at
ays after l
-Model increased craving for the drug after a period of abstinence/withdrawal
-Model does not capture negative consequences of drug use
ses or nose-rug infusions or
-Models the rapid resumption of drug use after a period of abstinence/extinction
-Abstinence achieved by operant extinction-The model does not capture negative
consequences of drug use-The presence of drug during testing
f the non-drug uces ent of non- responding on ssociated lever
-Models resumption of drug seeking after an alternative appetitive behavior is no longer reinforced
-Abstinence does not mimic the human condition
-The model does not capture negative consequences of drug use
n of non- responding by ng (or other n-drug stimuli)
-Models relapse after drug use is suppressed by a negative consequence of drug intake
-Individual differences in threshold for suppression of drug-reinforced responding
-Potential consequences of repeated shock exposure on behavior outside the context of punishment (e.g., non-specific behavioral suppression)
-It is unknown whether the model can detect stress-induce-relapse
n of non- responding by gent on of the drug electric barrier N during the st
-Models relapse after drug use is suppressed by a negative consequence of drug seeking (prior to drug taking)
-Individual differences in threshold for suppression of drug seeking
-Large and variable individual differences in response to cues in the relapse tests
-Model has not been shown to detect drug-priming- and stress-induced relapse
Animal models of relapse Marchant, Li and Shaham 677withdrawal day, reversible activation (using GABAa +GABAb receptor antagonists) of ventral, but not dorsal,mPFC injections increased extinction responding.Additional evidence for a role of mPFC in incubationof drug craving is the finding that weaker incubation ofheroin craving in adolescent rats is associated withblunted cue-induced Fos expression in this brain region;this effect was observed in both the infralimbic andprelimbic regions .
Another PFC region implicated in incubation of drug(heroin) craving is the lateral OFC. Fanous et al. found that reversible inactivation of this brain areadecreased extinction responding on withdrawal day 14but not day 1. These authors also used the Daun02inactivation procedure  to demonstrate a causal roleof selectively activated lateral OFC neurons (neuronalensembles) in incubation of heroin craving.
Pacchioni  reported that reversible inactivation ofdorsolateral striatum decreases incubated (high) extinc-tion responding after 60 days of withdrawal from cocaine.However, dorsolateral striatum inactivation alsodecreased non-incubated (low) extinction respondingon withdrawal day 1, suggesting a general role of thisbrain area in cue-induced cocaine seeking in extinctiontests  rather than a unique role in incubation ofcocaine craving. Xi et al.  reported that blockade ofD3 receptors in NAc core or shell or CeA, but not dorsalstriatum or BLA, decrease incubated cue-inducedcocaine seeking after 2128 withdrawal days. Whetherthis site-specific effect of the D3-receptor antagonist isselective for incubated (late withdrawal) versus non-incu-bated (early withdrawal) cocaine seeking is unknown,because systemic injections of the antagonist decreasedextinction responding in tests performed during bothearly (day 2) or late (day 30) withdrawal .
Gao et al.  reported that lesions of dorsolateral stria-tum and NAc shell, as well as blockade of D1-family, butnot D2-family, receptors decreases extinction respondingafter 3 weeks of withdrawal from morphine. On the basisof the results of Pacchioni et al. , and previous studieson the role of dorsolateral striatum and NAc shell dopa-mine in cue-induced drug seeking [38,39,40], it is likelythat the results of Gao et al.  reflect a general role ofD1-family receptors in cue-induced heroin seeking, inde-pendent of the withdrawal period. In a Fos-mappingstudy in mice, Madsen et al.  found that exposureto morphine cues during late withdrawal (21 days) extinc-tion tests increased Fos expression in NAc shell and core.These authors also found increased Fos expression inother brain areas, including anterior cingulate cortex,OFC, BNST, VTA, BLA. However, because Fos expres-
sion data are correlative, the role of these brain areas inincubation of morphine craving is a subject for futureresearch.
www.sciencedirect.com Lu et al.  and Airavaara et al.  provided evidencefor a role of VTA GDNF in incubation of cocaine, but notheroin, craving. Lu et al.  reported that viral over-expression of GDNF or exogenous GDNF injections inVTA potentiate incubation of cocaine craving. Moreimportant, chronic delivery of anti-GDNF antibodiesduring withdrawal days 114 prevented incubation ofcocaine craving, implicating endogenous GDNF in thisphenomenon. In contrast, Airavaara et al.  reported noeffect on incubation of heroin craving of either acuteexogenous VTA GDNF injections or chronic delivery ofanti-GDNF antibodies in VTA during the withdrawalperiod.
Li et al.  recently further studied the role of BDNF inNAc core and shell in incubation of cocaine craving. Theyconfirmed an earlier report  that after prolongedwithdrawal from cocaine NAc BDNF levels are increased,and further showed that after 90 withdrawal days, but not25 or 48 days, surface expression of phosphorylated TrkB(the preferential BDNF receptor) are increased. Analysisof BDNF protein levels in NAc sub-regions demon-strated increased BDNF levels after 45 (core but notshell) or 90 (both core and shell) withdrawal days. Finally,Li et al.  used a viral-vector approach to knockdownTrkB in NAc core and shell. Surprisingly, they found thatTrkB knockdown in NAc core, increased extinctionresponding on withdrawal day 1, but not days 30 or 90.In contrast, TrkB knockdown in NAc shell decreasedincubated extinction responding on withdrawal day 90,but not days 1 or 45. A potential interpretation of thiscomplicated data set is that during early withdrawal basalBDNF transmission in NAc core suppresses cue-inducedcocaine seeking, while after very prolonged withdrawalperiods elevated BDNF in NAc shell maintains highlevels of incubated cue-induced cocaine seeking. Thelatter idea is in agreement with the idea that whilemesolimbic BDNF plays a role in maintaining incubatedcue responding after prolonged withdrawal periods,BDNF does not mediate the development of incubationof cocaine craving .
Wolf and colleagues continued their mechanistic studieson the role of calcium-permeable GluA2-lacking AMPAreceptors in NAc in incubation of cocaine craving .This group and others replicated the initial findings ofaccumulation of these receptors in NAc after prolongedbut not early withdrawal from cocaine . They alsorecently identified a role of mGluR1 in accumulation ofGluA2-lacking AMPA receptors in NAc. In electrophysi-ology studies using cocaine-experienced rats after pro-longed withdrawal (>45 days), agonist activation ofmGluR1 eliminated the facilitation of glutamate trans-mission by GluA2-lacking AMPA receptors , indi-
cating the removal of GluA2-lacking AMPA receptorsfrom synapses. In follow-up studies, this group demon-strated the functional significance of this molecular
Current Opinion in Neurobiology 2013, 23:675683
678 Addictionmechanism in incubation of cocaine craving. Theyreported that NAc mGluR1 surface expression progress-ively decreases after withdrawal from cocaine .Additionally, preventing the decrease of cocaine-with-drawal-induced mGluR1 surface expression and increas-ing mGluR1 funct...