Downregulation of j-Opioid Receptors inBasolateral Amygdala and Septum of RatsWithdrawn for 14 Days From an Escalating

Dose ‘‘Binge’’ Cocaine AdministrationParadigm

ALEXIS BAILEY,* ROBERT GIANOTTI, ANN HO, AND MARY J. KREEKThe Laboratory of the Biology of Addictive Diseases, The Rockefeller University, New York, New York 10021

KEY WORDS opioid receptor; escalating ‘‘binge’’ cocaine administration; with-drawal

ABSTRACT There is evidence showing that the opioid systems play an importantrole in cocaine addiction; fewer studies have examined their roles in cocaine with-drawal. This study was conducted to determine whether cocaine or chronic withdrawalfrom cocaine alters the receptor component of the k-opioid system. Male Fischer ratswere injected with saline or cocaine (3 � 15 mg/kg/day for 4 days, 3 � 20 mg/kg/dayfor 4 days, 3 � 25 mg/kg/day for 4 days, and 3 � 30 mg/kg/day for 2 days), three timesdaily at 1-h intervals in an escalating dose paradigm for 14 days. Identically treatedrats were withdrawn from cocaine or saline for 14 days. We performed quantitativeautoradiographic mapping of k-opioid receptors (KOP-r) in the brains of rats treatedwith this escalating dose ‘‘binge’’ cocaine administration paradigm and of rats with-drawn from cocaine for 14 days. A significant condition (chronic/withdrawal) effectwas shown across all regions analyzed. A significant increase in [3H]CI-977 binding toKOP-r was detected in the septum of rats treated with an escalating dose binge co-caine administration paradigm and killed 30 min after the last cocaine injection. Incontrast, there was a decrease in KOP-r binding in the septum and the basolateralamygdala of rats withdrawn for 14 days from chronic escalating dose binge cocaineadministration, compared to rats at the end of 14 days chronic escalating dose cocaineadministration. These results reconfirm and extend that KOP-r undergoes upregula-tion in response to chronic binge cocaine administration here, with an escalating dose.The observed lowering in KOP-r binding, which was shown in two brain regions of co-caine withdrawn animals, might contribute to the persistent dysphoria reported along time after the discontinuation of the drug. Synapse 61:820–826, 2007.VVC 2007 Wiley-Liss, Inc.

INTRODUCTION

Cocaine, one of the most commonly used psychosti-mulants, induces an elevation of extracellular dopa-mine (DA), which becomes attenuated after chronictreatment in the striatum (Carboni et al., 1989; Mai-sonneuve et al., 1995; Maisonneuve and Kreek, 1994;Wise, 1996). Repeated cocaine administration has beenfound to induce changes in many G-protein coupled re-ceptor systems in the brain, including opioid, adeno-sine, glutamate, and GABA receptor systems (Sharpeet al., 2000; Toda et al., 2003; Turchan et al., 1999;Unterwald et al., 1992; Xi et al., 2002, 2003).

There is a large body of evidence indicating thatopioid systems are affected by cocaine treatment(Kreek et al., 2002). An increase in preprodynorphin(ppdyn) mRNA was observed in the caudate putamen

Contract grant sponsor: Smart Foundation; Contract grant sponsor: NIH/NIDA; Contract grant numbers: DA-P60-05130, DA-00049

*Correspondence to: Alexis Bailey, School of Biomedical and Molecular Scien-ces, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom.E-mail: a.bailey@surrey.ac.uk

Received 8 February 2007; Accepted 11 April 2007

DOI 10.1002/syn.20436

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2007 WILEY-LISS, INC.

SYNAPSE 61:820–826 (2007)

of rodents, nonhuman primates, and humans after co-caine administration (Adams et al., 2003; Hurd et al.,1992; Hurd and Herkenham, 1993; Rosin et al., 2003;Spangler et al., 1993; Turchan et al., 1998; Wermeet al., 2000). An increase in dynorphin immunoreactiv-ity was also reported in rats which received cocainesubacutely (Sivam, 1989; Smiley et al., 1990). In addi-tion, chronic cocaine exposure was shown to affectexpression of opioid receptors. An increase in m-(MOP-r) and k-opioid receptor (KOP-r) density has beenreported in several brain regions following steady dose14-day ‘‘binge’’ pattern cocaine administration, includ-ing the nucleus accumbens, the caudate putamen, andthe cingulate cortex (Collins et al., 2002; Unterwaldet al., 1992, 1994). Moreover, behavioral data support afunctional interaction between cocaine and the endoge-nous k-opioid system. KOP-r agonists U-50488 and R-84760 were shown to decrease cocaine-induced locomo-tor activity (Crawford et al., 1995; Zhang et al., 2004b),self administration (Kuzmin et al., 1997), and condi-tioned place preference (Crawford et al., 1995; Zhanget al., 2004b), which may result from the attenuation ofincreased DA levels by KOP-r agonists.

A common feature of human cocaine abuse is thedevelopment, over time, of tolerance or adaptation,which results in administration of increasing doses ofcocaine. In order to more closely model this pattern ofchronic cocaine abuse, we have modified our bingepattern of cocaine administration to an escalation ofcocaine administered in a binge pattern during thecourse of the 14-day study. Further, as there havebeen a limited number of studies observing neuro-chemical changes that occur after withdrawal periodsof 10 or more days, we designed a study including a14-day withdrawal time point after chronic cocaineadministration. In order to retest the hypothesis thatthe k-opioidergic system is affected by chronic cocaineadministration, and to test the hypothesis that thechanges in the k-opioidergic system persist after long-term withdrawal from chronic cocaine administration,we examined by quantitative autoradiographywhether there are any changes in KOP-r in thebrains of rats chronically treated, in this case, withan escalating dose binge chronic cocaine administra-tion paradigm and in rats withdrawn for 14 days fol-lowing escalating dose binge cocaine.

MATERIALS AND METHODSAnimals and treatment

Male Fischer rats (Charles River Laboratories,Kingston, NY), 210–240 g, were studied. Rats werehoused individually in a temperature-controlled roomwith a 12-h light/dark schedule. Food and water wereavailable ad libitum. All animals were weighed dailythroughout the study. Following 7 days of acclimation,i.p. injections of either saline (1 ml/kg) or cocaine were

then administered in a escalating dose binge paradigmto mimic a common pattern of self-administration inhuman cocaine abusers. Animals in the escalating dosegroup received 3 � 15 mg/kg/injection on days 1–4, 3 �20 mg/kg/injection on days 5–8, 3 � 25 mg/kg/injectionon days 9–12, and 3 � 30 mg/kg/injection on days 13and 14. Three daily i.p. injections (1 h apart) wereadministered for a maximum daily cocaine total dose of90 mg/kg, with the first injection 30 min after the startof the light cycle. Animals were randomized into fourgroups. The first two groups received 14 days of eithersaline or cocaine injections. One animal died on day 13after the third cocaine injection from seizure. No seiz-ures were observed in any other animals. The other twogroups were withdrawn from saline or cocaine for14 days after the last injection of the chronic binge co-caine administration described earlier. Thirty minutesor 14 days after the last injection, animals were killedby decapitation after a 15-s exposure to CO2. Brainswere rapidly removed and frozen by immersion for 30 sin isopentane cooled to �358C. The study was conductedin conformity with an animal protocol approved by theAnimal Care and Use Committee of The LaboratoryAnimal Research Center of the Rockefeller University.

Autoradiographic procedure

General procedures for quantitative autoradiogra-phy were performed as detailed previously (Baileyet al., 2002b; Kitchen et al., 1997). Adjacent frozencoronal sections (20-mm thick) were cut at 300-mmintervals throughout the brains of chronic saline- andcocaine-treated and from withdrawn chronic saline-and cocaine-treated animals for the determination oftotal and nonspecific binding of [3H]((�)-N-methyl-N-[7-(1-pyrrodinyl)-1-oxospirol[4,5]dec-8-yl]-4-benzofura-nacetamide (CI-977), to KOP-r. Sections from chronicsaline- and cocaine-treated animals and from animalswithdrawn from chronic saline- and cocaine-treat-ment were processed together. Ligand concentrationwas � 3–4 times Kd with [3H]CI-977 used at a concen-tration of 2.5 nM. Nonspecific binding was determinedin the presence of 1 lM naloxone. The incubationperiod was 1 h, and following washing, the slides wereapposed to [3H]Hyperfilm (Amersham; New Jersey)for a period of 6 weeks (Bailey et al., 2002; Kitchenet al., 1997). Films were developed using 50% KodakD19 developer.

Quantitative analysis and statisticalprocedures

Quantitative analysis of brain sections was per-formed as detailed previously (Kitchen et al., 1997)using an MCID image analyzer (Imaging Research,Canada). Brain structures were identified by referenceto the rat atlas of Paxinos and Watson (1986). For sta-tistical analysis of the effect of chronic escalating dose

821DECREASED KOP-r IN COCAINE WITHDRAWAL

Synapse DOI 10.1002/syn

binge cocaine administration, the regions analyzedwere arranged in descending rank order of KOP-r den-sity of the saline-treated animals. The five top regionsof the greatest rank order of KOP-r density in the con-trol condition (i.e., dorsal endopiriform nucleus, thebasolateral amygdala, the nucleus accumbens shell,nucleus accumbens core, and the septum) were theregions used for further statistical analysis. In theremaining five regions, there was low KOP-r densityand/or greater variance in the saline-treated animals.Therefore, three-way ANOVA for drug treatment(saline/cocaine), condition (chronic/withdrawal), andregion with repeated measures on the third factor wasperformed in the five regions with the greatest KOP-rdensity to compare quantitative measurements of[3H]CI-977 autoradiographic binding in the brains ofrats at the end of a chronic binge escalating dose co-caine or saline treatment and in brains of rats with-drawn for 14 days from a chronic escalating dose bingecocaine or saline. Then, for each of the five regions, atwo-way ANOVA was conducted for drug treat-ment (saline/cocaine), condition (chronic/withdrawal)followed by Newman Keuls post hoc tests.

Materials

[3H]CI-977 (41 Ci/mmol) was purchased from Amer-sham Biosciences. Naloxone was purchased fromSigma-Aldrich (St. Louis, MO).

RESULTSKOP-r density in 10 brain regions of

saline-treated control rats

The rank order of KOP-r density in the brains ofchronically saline-treated rats is (from highest to low-est): endopiriform nucleus > basolateral amygdala >nucleus accumbens shell > nucleus accumbens core >septum > hypothalamus > caudate putamen > cingu-late cortex > frontal cortex > thalamus (Table I).

KOP-r density in rats treated for 14 dayswith chronic escalating dose binge cocaineadministration and in rats withdrawn for

14 days from chronic binge cocaine

The pattern of distribution of KOP-r was similar inall animals (Fig. 1). The KOP-r density in the brainsof rats at the end of chronic escalating dose binge co-caine administration is shown with their appropriatecontrols in Figure 2A, with an apparent increase inall regions.

Three-way ANOVA showed a significant main effectof condition (chronic/withdrawal), F(1,19) ¼ 6.32, P <0.05, and of region F(4.76) ¼ 121.85, P < 0.000001),with no main effect of treatment (saline/cocaine) (F< 0.00). The treatment 3 condition interaction wasF(1,19) ¼ 4.36, P ¼ 0.054. Newman–Keuls post hocanalysis showed a significant decrease in [3H]CI-977binding to KOP-r across the regions in withdrawn co-caine group compared to the chronic cocaine one (P <0.05) (Table I, Figs. 2A and 2B). In the dorsal endo-piriform nucleus, two-way ANOVA showed a with-drawal effect which did not reach significance(F(1,21) ¼ 4.24, P ¼ 0.052). In the basolateral amyg-dala, two-way ANOVA also showed a main effect ofwithdrawal which did not reach significance (F(1,20)¼ 4.34, P ¼ 0.053) and a significant treatment 3 con-dition interaction (F(1,20) ¼ 6.89, P < 0.02). Post hocNeuwan–Keuls test showed a decrease in cocainewithdrawal compared to chronic cocaine administra-tion (P < 0.02). In neither the nucleus accumbensshell nor core did we find any significant treatmentor condition effect. In the septum, two-way ANOVAalso showed a main effect of withdrawal F(1,21) ¼10.62, P ¼ 0.01) and a significant treatment 3 condi-tion interaction F(1,21) ¼ 6.29, P < 0.05. Post hocNewman–Keuls test showed a increase in KOP-rbinding in chronic cocaine-treated animals comparedto saline (P < 0.05) and a decrease in cocaine with-drawal compared to chronic cocaine administration (P< 0.01).

TABLE I. Quantitative autoradiography of k-opioid receptors in the brains of chronic saline, cocaine, and withdrawn animals

RegionBregma coordinates

(mm)

[3H]CI-977 specific binding

Saline,n ¼ 8

Cocaine,n ¼ 7

Saline withdrawal,n ¼ 8

Cocaine withdrawal,n ¼ 8

Dorsal endopiriform nucleus 1.00 97.4 6 8.9 117 6 7.8 86.1 6 11.5 87.6 6 10.8Basolateral amygdala �2.56 49.7 6 5.5 62.6 6 6.0 54.3 6 8.5 23.3 6 12.5a

Nucleus accumbens shell 2.20 36.3 6 5.3 48.4 6 8.1 36.2 6 5.8 41.9 6 3.3Core 2.20 28.2 6 4.4 33.6 6 5.0 26.3 6 8.2 27.9 6 8.0Septum 0.20 27.6 6 2.7 40.9 6 3.7c 24.2 6 4.8 15.2 6 6.3b

Hypothalamus �2.56 21.1 6 5.4 29.2 6 2.0 30.8 6 9.5 7.0 6 3.0Caudate–putamen 1.00 19.5 6 3.4 27.7 6 4.8 17.4 6 4.0 12.7 6 6.4Cingulate cortex (layer 1) 3.70 17.3 6 3.1 22.7 6 7.5 17.8 6 8.3 17.7 6 8.3Frontal cortex 3.70 14.2 6 1.6 22.6 6 7.3 21.8 6 9.7 19.6 6 7.6Thalamus �2.56 7.3 6 2.3 15.8 6 7.0 16.1 6 7.5 2.0 6 1.3

aP < 0.05 vs. chronic cocaine.bP < 0.01 vs. chronic cocaine.cP < 0.05 vs. saline control.

822 A. BAILEY ET AL.

Synapse DOI 10.1002/syn

DISCUSSION

The qualitative and quantitative distribution ofKOP-r in coronal sections of brains of rats at the endof a chronic escalating dose binge cocaine and salinetreatment and in rats withdrawn for 14 days fromthat regimen (Fig. 1) was quite similar to thatreported by Unterwald et al. (1994) in the same Fi-scher rat strain using [3H] Bremazocine in the pres-ence of MOP-r and DOP-r blocking ligands, and to pre-vious studies reported by Kitchen et al. (1997) andBailey et al. (2002) using [3H]CI977 in the mouse. Inthe present study, [3H]CI977 was used instead of [3H]bremazocine, as it has been shown to be highly selec-tive to j-1 sites (Simonin et al., 2001) with low affinityto MOP-r, DOP-r and j-2 sites. [3H] bremazocine, onthe other hand, in the presence of MOP-r, DOP-r andj-1 blocking ligands have been thought to bind to j-2sites (Simonin et al., 2001). The presence, however, ofputative j-2 sites is under dispute as Simonin et al.(2001) showed a complete absence of [3H] bremazocinebinding in triple MOP-r/DOP-r/j-1 deficient mice, sug-gesting that the putative j-2 sites are in fact a mixedpopulation of KOP-r, DOP-r, and predominantly MOP-r gene product or indeed heterodimers.

This paradigm of escalating dose binge chronic co-caine administration was developed to mimic humancocaine abusers who tend to escalate their dose of co-caine early during the onset of abuse, but cease thisescalation because of monetary considerations and dueto intolerable adverse effects. The apparent increase

in KOP-r binding observed in all the brain regionsanalyzed, including dopaminergic regions, nucleusaccumbens, and basolateral amygdala, as well as non-dopaminergic regions, the endopiriform nucleus, andthe septum, in animals treated with chronic escalat-ing dose binge cocaine, is in agreement both withstudies from our laboratory with 14 days steady dose(45 mg/kg/day) chronic binge cocaine administration(Unterwald et al., 1994) and with a study using con-tinuous (minipump) 14 day cocaine (50 mg/kg/day)administration (Collins et al., 2002), in which theselective KOP-r ligand [3H]U59488 was used. As co-caine results in an increase of DA in the striatumand in an increase in dopaminergic activity, it islikely that KOP-r is regulated by alterations of thedopaminergic activity induced by cocaine. The precisemechanism of this regulation is unknown. Activationof the KOP-r has been shown to decrease extracellu-lar DA release in the nucleus accumbens and stria-tum (Di-Chiara and Imperato, 1988; Spanagel et al.,1992; Zhang et al., 2004a,b), whereas KOP-r antago-nists were shown to increase DA in the same regions(Spanagel et al., 1992). Moreover, the KOP-r agonistU-50488 was shown to decrease cocaine-induced loco-motor activity (Crawford et al., 1995), self administra-tion (Kuzmin, 1997: p 177), and conditioned placepreference (Crawford et al., 1995; Zhang et al.,2004a,b). As a result, the increase of KOP-r densityin brain regions of chronically binge cocaine-treatedanimals, with both steady dose and escalating dose

Fig. 1. Computer-enhanced autoradiograms of coronal brain sec-tions from brains of rats at the end of 14-day binge cocaine adminis-tration and of rats withdrawn for 14 days from a chronic escalatingdose binge cocaine treatment. k-Opioid receptors were labeled with[3H]CI-977 (2.5 nM). The adjacent sections shown are from the level

of the caudate (Bregma 1.00 mm) and the amygdala (Bregma –2.56mm). The color bar represents a pseudocolor interpretation of rela-tive black and white film images in fmol/mg tissue equivalent. Sec-tions cut from chronic saline, cocaine, and withdrawn brains wereprocessed in parallel.

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Synapse DOI 10.1002/syn

paradigms, suggests that chronic administration ofcocaine results in a compensatory increase in the ac-tivity of the j-opioid system, given that cocaine andthe endogenous KOP-r agonist dynorphin exert oppos-ing effects on DA levels. Interestingly, chronic escalat-

ing dose binge cocaine administration increased KOP-r density significantly in the septum, which is ahighly serotonergic region (Battaglia et al., 1991;Pomperiani et al., 1994). Indeed, cocaine blocks theserotonergic transporter as well as the DA trans-

Fig. 2. Quantitative autoradiography of KOP-r binding in coro-nal brain sections from rats at the end of 14-day binge cocaineadministration and of rats withdrawn for 14 days from a chronicescalating dose binge cocaine treatment. The data are expressed asmean specific binding 6SEM of [3H]CI-977 (fmol/mg) in brainregions of (A) chronic saline- and cocaine-treated rats and (B) 14day withdrawn from chronic saline and cocaine. The regions wereplaced in descending rank order by KOP-r density in saline-treatedanimals. They were grouped into regions of higher KOP-r expres-sion (line) for further analysis and into those regions of lower KOP-

r expression where the coefficient of variance was not acceptable forfurther statistical analysis. Regional determinations were madefrom both left and right sides of the sections that were 300 lmapart. The labeling was carried out on sections from saline, cocaine-treated, and withdrawn rats in a completely paired protocol. Specificbinding was >50% in regions of high binding. Abbreviations—En:dorsal endopiriform nucleus; BLA: basolateral amygdala; AcS: nu-cleus accumbens shell; AcC: nucleus accumbens core; SPT: septum;Hyp: hypothalamus; CPu: caudate putamen; Cg: cingulate cortex;FCx: frontal cortex; Thal: thalamus.

824 A. BAILEY ET AL.

porter, and the septum is a region with high levels ofserotonin receptors (Battaglia et al., 1991; Pomperianiet al., 1994). On the basis of our results, we suggestthat serotonin may play an important role in theupregulation of KOP-r by cocaine.

A significant decrease in KOP-r density was ob-served in our study in the basolateral amygdalaand septum of rats withdrawn for 14 days fromchronic binge cocaine compared with rats at theend of the 14-day chronic cocaine administration.This is the first study to investigate KOP-r bindingin long-term withdrawal from chronic cocaineadministration. This is of importance as it providesevidence of the duration of neurochemical alterationsafter the cessation of cocaine administration, whichmight be involved in relapse. A persistent increase inMOP-r binding in the frontal cortex, the cingulatecortex, and the caudate putamen was observed 14days after the last dose of cocaine administrationfrom the same 14-day binge escalating dose cocainein the same animals used in this study (Bailey et al.,2005).

The decrease in KOP-r binding in long-term with-drawn animals compared with the chronically cocainetreated ones is in agreement with the subacute find-ings of Turchan et al. (1998), who showed a decreasein KOP-r binding 48 h after the last cocaine adminis-tration from a 5-day binge cocaine (60 mg/kg/day)administration paradigm. The mechanism of this down-regulation is unknown. It is likely, however, that thedecrease of KOP-r density is related to the alterationsof extracellular dynorphin levels, since preprodynor-phin mRNA levels have been shown to be consistentlyincreased in the nucleus accumbens 3 h, 24 h, andeven 48 h following 5-day binge cocaine treatment(Turchan et al., 1998). In addition, Turchan et al.(1998) showed an increase in the preprodynorphin-derived peptide levels and release in the nucleusaccumbens and the caudate putamen measured at 48 h(but not at 3 h) after withdrawal from 5 days binge co-caine administration, pointing to an enhancement ofthe function of preprodynorphin neurons. As a result,the decrease in KOP-r density in the basolateral amyg-dala and the septum of 14 day cocaine-withdrawn ani-mals in this study might be due to a agonist-mediateddownregulation of KOP-r. Indeed, it has been shownthat chronic treatment with an exogenous KOP-r ago-nist leads to a decrease of the receptor target of thisagonist, a phenomenon called agonist-mediated recep-tor desensitization (Liu-Chen, 2004). However, thereare no studies investigating whether preprodynorphinelevation persists after 14-day cocaine withdrawal and,indeed, no studies investigating the effects of chroniccocaine treatment and prolonged withdrawal on prepro-dynorphin levels in the septum and the basolateralamygdala.

MOP-r activation results in elevation of DA levels inthe striatum and is thought to be linked with the neu-ronal mechanism partly mediating euphoria (Hurdand Herkenham, 1993). On the other hand, KOP-ractivation by dynorphin decreases DA release in thestriatum, which is thought to heighten the responsive-ness of the neuronal mechanisms mediating dyspho-ria. As a result, the lowering of KOP-r, coupled withnormal or elevated dynorphin levels, which was ob-served in the septum and basolateral amygdala 2 weeksafter the discontinuation of cocaine might contribute topersistent dysphoria a long time after the discontinua-tion of the drug in humans (Gawin, 1991). This is ofinterest since we have shown a persistent increase inMOP-r, 14 days after the discontinuation of cocaine, fol-lowing the same 14-day binge escalating dose cocaineadministration paradigm, which coupled with the lackof increase in b-endorphin or enkephalin release,observed during cocaine abstinence, yields a relative en-dogenous MOP-r deficiency (Schluger et al., 2001), sug-gesting an important role of the m-opioid system in co-caine craving during and after administration followedby relapse (Bailey et al., 2005). Such a role of the MOP-r system has also been suggested by Becker et al. (2002)in cocaine reward, who reported an attenuation ofcocaine self administration in MOP-r knockout mice.The decrease of KOP-r density after 14 days with-drawal from chronic cocaine is found in a brain regioninvolved in mood, emotions, and stress/anxiety (baso-lateral amygdala), rather than regions thought to beinvolved in reward (nucleus accumbens, frontal cortex,and caudate putamen). The KOP-r system has beenshown to be involved in mediating stress-induced co-caine responses. Antagonism of KOP-r was shown toblock stress-induced behavioral responses in mice(McLaughlin et al., 2003). The basolateral amygdalamay be partly involved in mediating stress-inducedresponses to cocaine. Indeed, Zhou et al. (2005) re-cently reported an increase in the stress hormone argi-nine–vasopressin mRNA levels in the amygdala of 3-hwithdrawn rats treated with chronic binge cocaine. Asa result, the decrease in KOP-r in the basolateralamygdala coupled with the possible normalization oreven increase in preprodynorphin release in the basao-lateral amygdala during long-term withdrawal fromcocaine, which would activate the KOP-r system, mightcontribute to the increase in stress and anxiety follow-ing cessation of cocaine administration. It also suggestsan important role for the KOP-r in the basolateralamygdala in cocaine craving, after administration fol-lowed by relapse (stress induced relapse). The decreaseof KOP-r in the septum might be linked with thedecrease in metabolic activity observed in that regionduring withdrawal from cocaine. Indeed, Hammeret al. (1993) has shown a decrease in [14C]-2-deoxyglu-cose 6 and 72 h following cocaine self-administration.

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ACKNOWLEDGMENTS

We thank Matthew Swift Yan Zhou and Susan Russofor comments and proof-reading the manuscript.

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Synapse DOI 10.1002/syn