psycho p harm 163

7/28/2019 Psycho p Harm 163

1/10

Psychopharmacology (2002) 163:352-361DOI 10.1007/s00213-002-1207-y

R E V I E W

Leonard L. Howell Kristin M. Wilcox

Functional imaging and neurochemical correlatesof stimulant self-administration in primates

Received: 29 March 2002 / Accepted: 8 July 2002 / Published online: 29 August 2002 Springer-Verlag 2002

Abstract Rationale: Recent advances in neuroimagingand in vivo neurochemistry have documented drug-induced functional changes in brain activity under

physiologically relevant conditions. These approacheshave significant strengths and important limitations thatshould be considered. Objectives: The present reviewdescribes current in vivo approaches to characterize drugeffects as they relate to behavior, and highlights keycontributions derived from each approach in the contextof stimulant self-administration in primates. Meth-ods: Techniques relating in vitro neurochemistry tobehavioral pharmacology are compared to several in vivoapproaches, including microdialysis, positron emissiontomography (PET) neuroimaging and functional magneticresonance imaging (fMRI). Results: In vitro neurochem-ical correlates of behavioral pharmacology established a

significant relationship between dopamine and the rein-forcing effects of abused stimulants. Subsequent in vivomicrodialysis studies in behaving animals supported acritical role for nucleus accumbens dopamine in thereinforcing effects of stimulants. PET neuroimaging inmonkeys and humans documented a close relationshipbetween dopamine transporter (DAT) occupancy in vivoand the reinforcing effects of stimulants. The effective-ness of selective DAT inhibitors to reduce cocaine self-administration also was linked to DAT occupancy in vivo.Lastly, the measurement of cerebral blood flow andmetabolism with PET and fMRI has begun to define theneuronal circuitry influenced by acute and chronicstimulant exposure. Conclusions: Collectively, in vivoneurochemistry and functional imaging have comple-

mented in vitro approaches and have enhanced our currentunderstanding of the neurobiology of abused stimulants.

Keywords Neuroimaging Neurochemistry Stimulants Nonhuman primates Drug self-administration Dopamine

Introduction

There has been a long-standing interest in physiologicaland neurochemical concomitants linked to drug effects onbehavior. Among the earlier seminal investigations werestudies that documented orderly relationships between thebehavioral and cardiovascular effects of drugs (e.g. Herdet al. 1969; Kelleher et al. 1972; Dews and Herd 1974). In

parallel, other investigators focused more directly onneurochemical correlates of drug-behavior interactions(e.g. Seiden 1975). More recent approaches directedtoward mechanistic accounts of drug-induced changes inbehavior represent a logical extension of these pioneeringresearch advances. Efforts focusing on in vitro neuro-chemical correlates of behavioral pharmacology haveidentified important neurochemical mechanisms relevantto drug effects on behavior. The extension of these effortsto in vivo analyses has provided direct measures ofneurochemistry under physiologically relevant conditionsto complement in vitro determinations. Progress inneuroimaging technology has further allowed for nonin-vasive, functional assessments of brain chemistry andphysiology with direct application to human investiga-tions of drug abuse. Each of these approaches hassignificant strengths and important limitations that shouldbe considered before a given methodology is applied to aspecific research question. The present review highlightssome of the key contributions that have derived from theuse of each approach to study psychomotor stimulant(hereafter referred to as stimulant) self-administrationbehavior in nonhuman primates and humans. Collective-ly, notable advances have been made in our currentunderstanding of the neurobiology of abused stimulants

L.L. Howell ()) K.M. WilcoxYerkes National Primate Research Center, Emory University,954 Gatewood Road, NE, Atlanta, GA 30329, USAe-mail: [email protected].: +1-404-7277786Fax: +1-404-7271266

L.L. HowellDepartment of Psychiatry and Behavioral Sciences,and Pharmacology, Emory University, Atlanta, GA 30322, USA


2/10

that may have important implications for medicationsdevelopment to treat stimulant abuse.

In vitro neurochemical correlatesof behavioral pharmacology

In a seminal paper, Ritz et al. (1987) reported a significant

correlation between the potencies of stimulants in self-administration studies and their binding affinity at thedopamine transporter (DAT). The analysis was based ondata derived from a variety of published reports andincluded stimulants from distinct chemical classes thatwere studied in rodent and nonhuman primate species. Insubsequent studies in nonhuman primates, additionalevidence was obtained to link the behavioral effects ofcocaine and related drugs to their actions at specificcocaine recognition sites associated with the dopamineuptake system. Specific binding sites for [3H]cocainewere identified in caudate-putamen membranes preparedfrom cynomolgus and squirrel monkey brains (Madras et

al. 1989). [3

H]Cocaine was displaced stereoselectivelyfrom these sites by enantiomers and diastereoisomers ofcocaine, a phenyltropane analog, and by several mono-amine uptake inhibitors structurally unrelated to cocaine.In behavioral studies, squirrel monkeys were trained torespond under a fixed-interval schedule of stimulus-shocktermination, and dose-effect curves were established forcocaine and the other drugs studied in neurochemicalexperiments (Spealman et al. 1989). The results demon-strated a strong association between the potencies of 15different drugs for producing cocaine-like behavioral-stimulant effects and for displacing specifically-bound[3H]cocaine in caudate-putamen.

In related studies, there was a close correspondencebetween the relative potencies of cocaine and relateddrugs for increasing rates of schedule-controlled respond-ing and for maintaining IV self-administration under asecond-order fixed-interval schedule (Bergman et al.1989). These results suggest that similar neurochemicalmechanisms may mediate the behavioral-stimulant andreinforcing effects of cocaine-like drugs in nonhumanprimates. Also, the potency relations for self-administereddrugs, except GBR 12909, generally corresponded withtheir relative potencies for displacing [3H]cocaine fromstriatal binding sites. GBR 12909 was less potent thancocaine in maintaining self-administration behavior, eventhough it was two-fold more potent than cocaine indisplacing [3H]cocaine from monkey caudate-putamen.The authors suggested that the discrepancy in relativepotency may have been due to pharmacokinetic factors,and highlighted the importance of in vivo characterizationof GBR 12909 bioavailability to determine drug concen-tration at its central site of action.

Recent studies have used a similar approach toinvestigate the effects of other drugs that, like cocaine,have local anesthetic properties. These local anesthetics,e.g. procaine, also have been shown to bind to the DATand inhibit dopamine uptake. Accordingly, it is not

surprising that they can exhibit reinforcing effects. Whenthe binding affinities of cocaine and several localanesthetics at the DAT in rhesus monkey brain werecompared to their potencies in i.v. self-administrationstudies, a significant correlation was obtained (Wilcox etal. 1999). In contrast, there was no relationship betweentheir sodium channel affinities, presumed to mediate localanesthetic properties, and self-administration potencies.

Autoradiographic techniques have been used in vitroand ex vivo to map the regional distribution of cocainebinding sites labeled with the cocaine analog [3H]WIN35,428 in squirrel monkey brain (Canfield et al. 1990;Kaufman and Madras 1992). High densities of [3H]WIN35,428 binding sites were observed in the caudate-putamen and nucleus accumbens. In all regions, bindingwas significantly reduced by co-incubation with ()-cocaine. These results indicated that cocaine binding siteslabeled by [3H]WIN 35,428 were localized primarily indopamine-rich brain regions linked to the behavioraleffects of cocaine. In a related study, several additionalbrain regions exhibited intermediate densities of [3H]WIN

35,428 binding, including the substantia nigra, amygdalaand hypothalamus (Kaufman et al. 1991). The latterresults raise the possibility that these brain regions, inaddition to caudate-putamen and nucleus accumbens, maycontribute to the behavioral effects of cocaine. Similarly,neuroanatomical mapping of cocaine distribution in brainafter IV administration of [3H]cocaine resulted in thehighest accumulation in dopamine-rich brain regionsincluding caudate-putamen and nucleus accumbens (Ma-dras and Kaufman 1994). However, the locus coeruleus,hippocampus and amygdala also accumulated significantquantities of labeled cocaine, consistent with the idea thatother prominent targets of cocaine may contribute to its

behavioral pharmacology.Collectively, in vitro neurochemical correlates ofbehavioral pharmacology have identified important neu-rochemical mechanisms relevant to the neurobiology ofstimulant abuse. A substantial literature has established asignificant relationship between dopamine and the rein-forcing effects of cocaine and related stimulants. Specificbrain regions that may contribute to the behavioral effectsof cocaine have been identified, and long-term neuro-chemical changes associated with chronic stimulantexposure have begun to be characterized. This researchhas established the foundation of our current knowledgeof neurochemical mechanisms underlying the behavioraleffects of stimulants. However, it is important to keep inmind that in vitro investigations are necessarily restrictedin scope because they cannot mimic physiologicalconditions in intact animals. For example, ease of passageinto the brain or metabolic activity may dictate whether orhow much of a drug reaches its sites of action. Longi-tudinal studies are difficult, and data are often obtained inthe absence of functional information. Moreover, becauseit is difficult to predict the in vitro concentrations thathave relevance to actions in vivo, potency relationshipsestablished in vitro do not necessarily correspond to thosedetermined in behavioral studies. The development of in

353


3/10

vivo approaches to neurochemical investigations hasbegun to provide important alternative methodologiesfor avoiding or resolving these types of problems.

In vivo neurochemistry

In vivo microdialysis has become a widely used technique

for sampling extracellular neurochemicals in discretebrain structures. The technique overcomes many of thelimitations of in vitro receptor binding and tissuehomogenate preparations by providing real-time func-tional measures of neurochemistry in behaving animals.The approach is well suited for pharmacological studiesthat characterize dose-response relationships and drugtime course of action. Initial studies in rats trained to self-administer cocaine provided convincing evidence that thereinforcing properties of cocaine were mediated primarilyby dopamine release in the nucleus accumbens (Pettit andJustice 1989, 1991). Intravenous self-administration ofcocaine was shown to be linked to episodic fluctuations in

extracellular dopamine. More recent studies have shown asimilar relationship between the timing of amphetamineself-administration and dopamine neurochemistry (Ranal-di et al. 1999). Moreover, the context in which cocaine isadministered can significantly alter the neurochemicalresponse to equivalent brain concentrations of cocaine, asevidenced by greater increases in nucleus accumbensdopamine when cocaine is self-administered compared toyoked, non-contingent administration (Hemby et al.1997).

While in vivo microdialysis has been used extensivelyin rodents to examine the neurochemical effects ofcocaine, only recently has the technique been extended

into behavioral studies in nonhuman primates. In view ofthe expense of this type of work in primates, such studiesgenerally are undertaken as long-term neurochemicalstudies involving repeated experiments in the samesubject. Also, verification of accurate probe placementis obtained with MRI rather than histology, obviating theneed to sacrifice valuable subjects (Bradberry et al. 2000;Czoty et al. 2000). The ability to utilize a repeated-measures, within-subjects design has been proven crucialto the success of this line of research (Iyer et al. 1995;Czoty et al. 2000, 2002). For example, recent studies wereconducted to document longitudinal changes in neuro-chemical responses to self-administered cocaine in rhesusmonkeys (Bradberry 2000). Subjects were trained underfixed-ratio schedules of IV cocaine delivery and allowedto receive two injections per session. During individualcocaine self-administration sessions, acute tolerance de-veloped to cocaine-induced elevations in extracellulardopamine in the striatum. However, long-term exposureto cocaine self-administration enhanced the neurochem-ical response to a fixed dose of cocaine in a time-dependent manner. Hence, in vivo microdialysis has beenused effectively in nonhuman primates to document bothacute tolerance and long-term sensitization to self-administered cocaine.

In vivo microdialysis protocols also have been imple-mented in conscious squirrel monkeys to identify neuro-chemical correlates of drug interactions during cocaineself-administration behavior (Czoty et al. 2002). In thesestudies, a serotonin uptake inhibitor, alaproclate, and aserotonin direct agonist, quipazine, were used to suppresscocaine self-administration under a second-order scheduleof IV cocaine delivery. These effects were produced by

doses of the serotonergic agents that did not havenonspecific behavioral suppressant actions, as evident incompanion studies of behavior maintained by an identicalschedule of stimulus termination. Importantly, the samepretreatment doses that decreased cocaine self-adminis-tration significantly attenuated cocaine-induced increasesin extracellular dopamine in a separate group of awakemonkeys used for in vivo microdialysis. Hence, in vivomeasures effectively identified ongoing neurochemicalactions on extracellular dopamine that likely were relateddirectly to the behavioral effects of the serotonergicagonists.

In vivo microdialysis techniques have provided a

means to measure ongoing neurochemical activity, aug-menting other types of in vivo approaches that can beused to characterize neurochemical mechanisms underly-ing drug effects on behavior and their roles in drugaddiction. The electrophysiological studies of Schultz andcolleagues are particularly noteworthy and have elegantlyilluminated the role of dopamine neuronal activity inreward processing in behaving nonhuman primates(Schultz et al. 1997; Martin-Soelch et all 2001; Watanabeet al. 2001). Nevertheless, technical considerations stilllimit the scope of investigation for these methodologies.They involve invasive surgical preparations that requiresignificant effort to maintain effectively over extended

periods, and they are restricted to limited target structuresdefined a priori as being relevant to the research question.For microdialysis, the technical features are uniquelychallenging: analyses are limited to small molecules thatcan effectively diffuse across the dialysis membrane;absolute quantitation of basal levels of neurochemicalsdepends on a problematic correspondence between invitro and in vivo probe recovery; and temporal resolutionis on the order of minutes rather than moment to moment.

Functional neuroimaging

Functional imaging provides an alternative, noninvasiveapproach toward understanding CNS function and neuralmechanisms underlying drug-induced changes in behav-ior. Current technologies include positron emissiontomography (PET), single-photon emission computedtomography (SPECT) and functional magnetic resonanceimaging (fMRI). In PET, ligands of interest are radiola-beled with unstable atoms that emit positrons. Whenpositrons collide with electrons, dual photons are emittedwhich are recognized by a detector array in the tomo-graph. A computer algorithm then uses this information tomap the source and concentration of the radiotracer.

354


4/10

SPECT imaging uses different radiotracers that emit asingle photon. Due to methodological differences insingle versus dual photon detection, SPECT imaging haslower sensitivity and resolution compared to PET imag-ing. A number of radiotracers have been developed foruse in PET and SPECT imaging that enable the in vivomeasurement of brain neurochemistry and physiology.Since radiotracers can be used to label compounds

without influencing their pharmacology, functional im-aging can also measure drug distribution and pharmaco-kinetics in brain. In contrast to PET imaging and SPECTimaging, fMRI does not require the use of radiotracers.Instead, the subject is placed into a homogeneousmagnetic field where presentations of radiofrequencypulses cause transient energy changes. Current fMRIstudies rely on changes in signal intensity due to bloodoxygenation levels, thereby quantifying hemodynamicresponses associated with neuronal activity. Collectively,these neuroimaging techniques have been used to identifythe neural substrates and target systems that mediate thereinforcing effects of stimulants.

Currently, functional imaging techniques are beingused in nonhuman primates and human subjects to studythe neurochemical actions of cocaine and cocaine-likedrugs that can be related to their behavioral effects. Forexample, recent PET imaging studies to characterize theinteraction of cocaine and candidate medications wereconducted with the new DAT radioligand 8-(2-[18F]flur-oethyl)2b-carbomethoxy-3b(4-chlorophenyl) nortropane([18F]FECNT) (Goodman et al. 2000). DAT occupancyby cocaine was determined by displacement of FECNTusing a reference tissue method of kinetic modeling inrhesus monkeys (Votaw et al. 2002). Doses of 0.1 and1.0 mg/kg cocaine occupied 53% and 87% of the

transporters, respectively. These results confirmed thatFECNT labels a cocaine-sensitive binding site, and thathigh levels of DAT occupancy are associated withbehaviorally active doses of cocaine.

In a related study, the reinforcing effects of cocainewere compared to its phenyltropane analog, RTI-113, inrhesus monkeys responding under a second-order sched-ule of IV drug self-administration (Wilcox et al. 2002).Both drugs reliably and equipotently maintained self-administration behavior, with DAT occupancies of 6576% and 9499% for optimum doses of cocaine and RTI-113, respectively. When administered as a pretreatment,RTI-113 dose-dependently reduced responding main-tained by cocaine, with DAT occupancies rangingbetween 72 and 84% for pretreatment doses of RTI-113that effectively suppressed cocaine self-administration.The relationship between DAT occupancy and theeffectiveness of selective DAT inhibitors to reducecocaine self-administration extends to the phenylpiper-azine, GBR 12909. GBR 12909 is a high-affinity ligandthat has a cocaine-like profile of behavioral effects(Rothman 1990; Rothman et al. 1992). In rhesusmonkeys, IV administration of GBR 12909 was foundto suppress cocaine self-administration dose-dependentlyunder multiple fixed-ratio schedules of cocaine and food

delivery (Glowa et al. 1995). The same doses of GBR12909 that decreased cocaine self-administration inrhesus monkeys were subsequently tested in baboons toquantify DAT occupancy during PET neuroimaging with[11C]WIN 35,428 (Villemagne et al. 1999). The resultsindicated that, like RTI-113, doses of GBR 12909 thatdecrease cocaine self-administration in nonhuman pri-mates also occupy a substantial fraction (>50%) of

dopamine transporters.With regard to pharmacokinetic profile, PET neu-

roimaging studies with [11C]WIN 35,428 have confirmedthat, compared to cocaine, GBR 12909 has a slower onsetand longer duration of DAT occupancy in consciousrhesus monkeys (Tsukada et al. 2000). Of interest, thetime course of effects for DAT occupancy in these studiesclosely paralleled drug-induced increases in extracellulardopamine measured with in vivo microdialysis in rhesusmonkeys or, in separate studies, squirrel monkeys (Czotyet al. 2000). Moreover, the time-course and potencydifferences in neurochemical effects paralleled differ-ences in the behavioral-stimulant effects of these drugs in

monkeys performing under fixed-interval schedules ofstimulus termination (Howell et al. 1997, 2000). Takentogether, these data offer compelling evidence that DAToccupancy measures obtained with PET neuroimaging areclosely linked to functional changes in dopamine neuro-chemistry and behavior.

PET neuroimaging with [11C]cocaine has been used todescribe the distribution and pharmacokinetics of cocainein humans. Studies with [11C]cocaine at tracer doses inthe human brain yielded results similar to those reportedin baboons, with a rapid uptake in the striatum and aclearance half-life of about 20 min (Fowler et al. 1989).Subsequently, a direct relationship was established be-

tween self-reports of high induced by cocaine and thetime course for striatal uptake (Volkow et al. 1997). Thedescription of cocaines rapid uptake and reversiblekinetics in the human brain illustrated how neuroimagingtechniques in human subjects could be expanded toquantify DAT availability, and DAT occupancy by drugsthat target the dopamine transporter (Fowler et al. 2001).Indeed, functional imaging studies in human drug usershave begun to relate the acute neurochemical effects ofstimulants to their reinforcing effects, and the resultsobtained are in close agreement with preclinical studiesconducted in nonhuman primates (Table 1). A goodexample is provided by recent studies with methylpheni-date.

Methylphenidate has affinity for the DAT comparableto cocaine, and behaviorally-relevant doses of methyl-phenidate can block the uptake of [11C]cocaine. Likewise,doses of cocaine that induce euphoria can block theuptake of [11C]methylphenidate. In human cocaineabusers, subjective ratings of high have been correlatedwith percent DAT occupancy measured with PETneuroimaging and [11C]cocaine following acute adminis-tration of cocaine (Volkow et al. 1997) or methylpheni-date (Volkow et al. 1999b). Approximately 50%occupancy of striatal DAT was required for subjects to

355


5/10


6/10

Strickland et al. 1993; Levin et al. 1994). Local perfusiondeficits have been linked closely to changes in cerebralmetabolism. Measures of brain glucose metabolism withfluorodeoxyglucose (FDG) in chronic cocaine usersshowed transient increases in metabolic activity in brainregions associated with dopaminergic systems duringcocaine withdrawal (Volkow et al. 1991) and persistentdecreases in frontal brain metabolism after months of

detoxification (Volkow et al. 1992). The same pattern ofdecreased glucose metabolism (Reivich et al. 1985) andperfusion deficit (Volkow et al. 1988) was observed in theprefrontal cortex in a subset of cocaine users who wereimaged on multiple occasions.

Chronic exposure to stimulant drugs in humans mayalso lead to significant reductions in neuronal markers ofdopaminergic function. For example, dual-tracer PETneuroimaging with FDG and [11C]raclopride to measurebrain metabolism and D2 receptor binding, respectively,has documented both reduced frontal metabolism anddecreased dopamine D2 receptor availability in cocaine ormethamphetamine abusers (Volkow et al. 1993, 2001a).

Moreover, D2 receptor availability was associated withmetabolic rate in the orbitofrontal cortex. Based on suchfindings, the authors speculated that D2 receptor-mediateddysregulation of the orbitofrontal cortex could underliecompulsive drug taking. This is an intriguing suggestion,but it awaits further experimental evidence. In other PETneuroimaging studies with [11C]WIN-35,428 as theligand, reduced DAT density in nucleus accumbens,striatum and prefrontal cortex was documented in meth-amphetamine users (Sekine et al. 2001). Importantly, thereduction in DAT binding in these studies was signifi-cantly correlated with the duration of drug use and theseverity of persistent psychiatric symptoms. Subsequent

PET studies with [

11

C]d-threo-methylphenidate as theDAT ligand found partial recovery of DAT binding inmethamphetamine abusers during protracted abstinence(Volkow et al. 2001b). However, neuropsychologicalfunction did not improve to the same extent, leading theauthors to suggest that recovery of DAT density was notsufficient for complete recovery.

It is becoming increasingly accepted that behavior,brain chemistry and neuronal function can be readilyinfluenced by environmental conditions as well as bypharmacological challenge. Neuroimaging techniqueshave proven especially useful in studying dynamicchanges in neuronal activity that may be associated withenvironmental variables. For example, differences inhousing conditions and the dominance rank amongsocially housed nonhuman primates recently have beenassociated with differential levels of dopamine D2 recep-tors. An initial study using PET neuroimaging with[18F]FCP in socially-housed female cynomolgus monkeysdocumented reduced availability of D2 receptors insubordinate monkeys compared to the dominant groupmembers (Grant et al. 1998). However, it was unclearwhether the observed differences in D2 binding reflected apredisposition that determined dominance rank or neuro-chemical alterations in response to dominance rank. In a

subsequent series of experiments in cynomolgus mon-keys, subjects were first scanned using PET neuroimagingwith [18F]FCP while individually-housed and again afterthey were placed in social groups and allowed to establisha stable social hierarchy (Morgan et al. 2002). Themonkeys did not differ in the availability of D2 receptorsduring individual housing. However, social housingincreased the availability of D2 receptors in dominant

monkeys without producing any changes in subordinategroup members. The alterations in dopaminergic functionwere apparently a consequence of change in dominancerank and related changes in social interactions. Impor-tantly, the neurochemical changes had a significantinfluence on vulnerability to cocaine use. Intravenouscocaine delivery reliably functioned as a reinforcer insubordinate subjects but failed to maintain self-adminis-tration in dominant group members. Subordinate monkeysreliably self-administered cocaine across a range of doses,and the shape of the dose-effect curve was characterizedas an inverted U-shaped function typical of stimulant self-administration in nonhuman primates. In contrast, cocaine

failed to maintain rates of responding higher than salinerates in dominant monkeys, indicating that cocaine didnot function as a reinforcer in these subjects. During self-administration sessions, subordinate monkeys also hadsignificantly higher cocaine intakes compared to domi-nant monkeys. These provocative findings obtained withrepeated PET neuroimaging in individual subjects docu-ment rapid neurochemical changes in response to envi-ronmental conditions, and subsequent alterations inpropensity to self-administer cocaine.

Analogous relationships between dopamine receptordensities and the behavioral effects of stimulants alsohave been reported in studies with human subjects. PET

neuroimaging with [

11

C]raclopride was used to measuredopamine D2 receptor occupancy as an indirect measureof stimulant-induced elevations in extracellular dopamine(Volkow et al. 1999c). Parallel measures for self-reportedhigh were obtained and related to methylphenidate-induced changes in brain dopamine. The intensity ofhigh was significantly correlated with estimated levelsof released dopamine. In a related study, dopamine D2receptor levels were determined in healthy men who hadno history of drug abuse (Volkow et al. 1999d). Subjectswho reported liking the effects of methylphenidate hadsignificantly lower D2 receptor levels in striatum com-pared to subjects who disliked the drug. In addition, therewas a direct relationship between the intensity ofunpleasant effects and D2 receptor levels. The resultsindicated that subjective responses to stimulants inhumans may be correlated with D2 receptor levels, andthat low levels of D2 receptors may contribute tostimulant abuse. Note the parallels between these findingsand those in nonhuman primates described earlier (Mor-gan et al. 2002). The ability to manipulate D 2 receptordensity environmentally in nonhuman primates providesespecially strong support for orderly relationships be-tween such variables and the reinforcing effects ofcocaine.

357


7/10

The noninvasive measurement of cerebral blood flowwith PET neuroimaging and [15O] water provides anotherin vivo functional measure to characterize drug-inducedchanges in brain activity. However, the high incidence ofpolydrug use among human cocaine users complicates theidentification of functional changes in CNS activity duespecifically to the direct pharmacological effects of

cocaine. As with other types of neuroimaging techniques,the use of nonhuman primates can provide the experi-mental control necessary to obtain and document theeffects of cocaine exposure independent of major con-founding variables. Initial studies have been conducted toexamine this possibility. Functional changes in cerebralblood flow measured by PET were determined inconscious, drug-naive rhesus monkeys following acuteIV administration of cocaine (Howell et al. 2001, 2002).Repeated baseline determinations of cerebral blood flowprior to drug administration were stable. In contrast,cocaine had significant dose-related effects on cerebralblood flow at 5 min postinjection that diminished

markedly by 15 min postinjection. Brain activation mapsnormalized to global flow showed prominent cocaine-induced activation of prefrontal cortex localized primarilyto dorsolateral regions. The brain activation effects wereblocked by pretreatment with the selective serotoninuptake inhibitor, alaproclate. Importantly, the same doseof alaproclate that blocked cocaine-induced brain activa-tion was effective in attenuating cocaine self-administra-tion and cocaine-induced elevations in extracellulardopamine in squirrel monkeys (Czoty et al. 2002). Hence,there appeared to be a close concordance among in vivomeasures of behavior, neurochemistry and functionalimaging.

The acute effects of stimulants on cerebral blood flowand metabolism have been examined more frequently inhuman subjects and especially in studies to evaluate theneuronal basis of drug-induced euphoria. The acute IVadministration of cocaine in human users resulted insignificant blood flow decreases in selected frontal andbasal ganglia regions as measured by SPECT (Pearlson etal. 1993; Wallace et al. 1996; Johnson et al. 1998).Cocaine-induced euphoria following acute IV adminis-tration also has been associated with regional decreases incerebral metabolism (London et al. 1990). Other inves-

tigators have reported cocaine-induced increases in bloodflow mainly in the frontal and parietal regions after acutedrug administration in humans (Mathew et al. 1996).Similarly, a study using fMRI reported dynamic patternsof brain activation following cocaine administration incocaine-dependent subjects (Breiter et al. 1997). Someregions showed short duration of activation that was

correlated with ratings of rush, whereas other regionsshowed sustained activation associated with measures ofcraving.

The transient pattern of brain activation induced bycocaine in humans is consistent with that reported inconscious rhesus monkeys imaged 5 min postinjection(Howell et al. 2001, 2002). It is noteworthy that themajority of studies have measured drug effects at latertime points (up to 45 min postinjection) and have foundcocaine-induced decreases in cerebral blood flow andmetabolism in chronic cocaine users. Not surprisingly, thetime of measurement appears to be a major determinant ofexperimental outcome, especially with a rapid-acting drug

like cocaine. Notwithstanding such differences in exper-imental procedures and outcomes, however, enoughcollective evidence has accumulated to consider acutedrug effects on cerebral blood flow and metabolism as ameans to characterize the functional neuroanatomyunderlying the etiology of stimulant abuse (Table 2).

In summary, functional neuroimaging offers severalnotable advantages for drug abuse research. The proce-dures are noninvasive and, thus, well suited for humanstudies. The capability to conduct similar studies innonhuman primates and human subjects provides apowerful tool for linking findings in human and labora-tory animal research efforts. Longitudinal designs can be

implemented to characterize the consequences of chronicdrug exposure and potential recovery of brain function.Hence, analyses are not restricted to between-subjectcomparisons at single time point determinations. Thisconsideration is particularly important in human studieswhere terminal endpoints are not an option, and innonhuman primate studies where significant resources arecommitted to individual subjects. Functional imaging canbe used to study drug distribution and pharmacokineticsof binding to relevant substrates in vivo. The informationthat can be derived from such studies is especially

Table 2 Acute effects of cocaine on cerebral metabolism and blood flow

Decreases in cerebral metabolism

London et al. 1990 Human polydrug abusers; PET imaging [18F]FDG; 40 mg (IV) cocaine

Decreases in cerebral blood flow

Johnson et al. 1998 Human cocaine-dependent; SPECT technetium-99-m-bicisate; 0.325 and 0.650 mg/kg (IV) cocainePearlson et al. 1993 Human cocaine-dependent; SPECT technetium-99-m-exametazine; 48 mg (IV) cocaineWallace et al. 1996 Human cocaine-dependent; SPECT technetium-99-m-exametazine; 40 mg (IV) cocaine

Transient regional increases in cerebral blood flow

Breiter et al. 1997 Human cocaine-dependent; fMRI (BOLD); 0.6 mg/kg (IV) cocaineMathew et al. 1996 Human cocaine-dependent; laser Doppler 133 xenon; 0.3 mg/kg (IV) cocaineHowell et al. 2002 Rhesus monkeys; PET [15O]water; 0.3 and 1.0 mg/kg (IV) cocaine

358


8/10

valuable for the determination of dosing regimens wherebioavailability is a significant concern. Lastly, the mea-surement of cerebral blood flow changes coupled tocerebral metabolism is beginning to establish the inte-grated neuronal circuitry underlying drug effects onbehavior in real time. Despite the many advantages,however, there currently are important limitations to theuse of imaging technology. It is very expensive and time-

consuming, and there are limited resources with highdemand for use. There also are limits to anatomical andtemporal resolution that dictate the appropriate applica-tion of imaging technology. Lastly, functional imaging innonhuman primates requires anesthetic agents that alterbrain function or, alternatively, the implementation ofextensive behavioral training protocols to restrain awakesubjects. The potential impact of the technology isconsiderable, but the resources required can be pro-hibitive for many research applications.

Summary

Recent advances in neuroimaging and in vivo neuro-chemistry have documented drug-induced functionalchanges in brain activity under physiologically relevantconditions. Current in vivo approaches allow for simul-taneous measures of behavioral activity and brain func-tion, providing an analysis of integrated systems in theintact organism that are relevant to drug abuse. Neu-roimaging of cerebral blood flow changes coupled tocerebral metabolism measured with PET and fMRI isparticularly well suited to define the neuronal circuitryunderlying drug effects on behavior. The ability to studydrug interactions with specific neurochemical targets in

vivo also can be used to identify pharmacokineticconsiderations that are critical in medications develop-ment. Lastly, the ability to conduct within-subject,longitudinal assessments of neurochemistry and brainfunction could greatly enhance our ability to documentlong-term changes due to chronic drug exposure andpotential recovery during prolonged abstinence or duringtreatment interventions. A review of the literature showsclose concordance among functional measures of behav-ior, neurochemistry and neuroimaging. Moreover, theclinical relevance of information derived from nonhumanprimates has been established in the outcome of func-tional imaging studies in humans. These complementaryand integrative approaches should have important impli-cations for medications development to treat stimulantabuse.

Acknowledgements The authors gratefully acknowledge the tech-nical assistance of Peggy Plant. This work was supported in part byUS Public Health Service grant RR00165 (Division of ResearchResources, National Institutes of Health).

References

Bergman J, Madras BK, Johnson SE, Spealman RD (1989) Effectsof cocaine and related drugs in nonhuman primates. III. Self-administration by squirrel monkeys. J Pharmacol Exp Ther251:150155

Bradberry CW (2000) Acute and chronic dopamine dynamics in anonhuman primate model of recreational cocaine use. J Neuro-sci 20:71091775

Bradberry CW, Barrett-Larimore RL, Jatlow P, Rubino SR (2000)Impact of self-administered cocaine and cocaine cues onextracellular dopamine in mesolimbic and sensorimotor stria-tum in rhesus monkeys. J Neurosci 20:38743883

Breiter HC, Gollub RL, Weisskoff RM, Kennedy DN, Makris N,Berke JD, Goodman JM, Kantor HL, Gastfriend DR, Rior-den JP, Mathew RT, Rosen BR, Hyman SE (1997) Acuteeffects of cocaine on human brain activity and emotion. Neuron19:591611

Canfield DR, Spealman RD, Kaufman MJ, Madras BK (1990)Autoradiographic localization of cocaine binding sites by[3H]CFT ([3H]WIN 35,428) in the monkey brain. Synapse6:189195

Czoty PW, Justice JB Jr, Howell LL (2000) Cocaine-inducedchanges in extracellular dopamine determined by microdialysisin awake squirrel monkeys. Psychopharmacology 148:299306

Czoty PW, Ginsburg BC, Howell LL (2002) Serotonergic attenu-

ation of the reinforcing and neurochemical effects of cocaine insquirrel monkeys. J Pharmacol Exp Ther 300: 831837

Dewey SL, Smith GS, Logan J, Brodie JD, Fowler JS, Wolf AP(1993) Striatal binding of the PET ligand 11C-raclopride isaltered by drugs that modify synaptic dopamine levels. Synapse13:350356

Dews PB, Herd JA (1974) Behavioral activities and cardiovascularfunctions: effects of hexamethonium on cardiovascular changesduring strong sustained static work in rhesus monkeys.J Pharmacol Exp Ther 189:1223

Fowler JS, Volkow ND, Wolf AP, Dewey SL, Schlyer DJ,MacGregor RR, Hitzemann R, Logan J, Bendriem B, Gatley SJ,Christman D (1989) Mapping cocaine binding in human andbaboon brain in vivo. Synapse 4:371377

Fowler JS, Volkow ND, Wang G-J, Gatley SJ, Logan J (2001)[11C]Cocaine: PET studies of cocaine pharmacokinetics, dopa-

mine transporter availability and dopamine transporter occu-pancy. Nucl Med Biol 28:561572

Glowa JR, Wojnicki FHE, Matecka D, Bacher JD, Mansbach RS,Balster RL, Rice KC (1995) Effects of dopamine reuptakeinhibitors on food- and cocaine-maintained responding. I.Dependence on unit dose of cocaine. Exp Clin Psychopharma-col 3:219231

Goodman MM, Kilts CD, Keil R, Shi B, Martarello L, Xing D,Votaw J, Ely TD, Lambert P, Owens MJ, Camp, VM,Malveaux E, Hoffman JM (2000) 18F-Labeled FECNT: aselective radioligand for PET imaging of brain dopaminetransporters. Nucl Med Biol 27:112

Grant KA, Shively CA, Nader MA, Ehrenkaufer RL, Line SW,Morton TE, Gage HD, Mach RH (1998) Effect of social statuson striatal dopamine D2 receptor binding characteristics incynomolgus monkeys assessed with positron emission tomo-

graphy. Synapse 29:8083Hemby SE, Co C, Koves TR, Smith JE, Dworkin SI (1997)

Differences in extracellular dopamine concentrations in thenucleus accumbens during response-dependent and response-independent cocaine administration in the rat. Psychopharma-cology 133:716

Herd JA, Morse WH, Kelleher RT, Jones LG (1969) Arterialhypertension in the squirrel monkey during behavioral exper-iments. Am J Physiol 217:2429

Holman BL, Carvalho PA, Mendelson J, Teoh SK, Nardin R,Hallgring E, Hebben N, Johnson KA (1991) Brain perfusion isabnormal in cocaine-dependent polydrug users: a study usingtechnetium-99m-HMPAO and ASPECT. J Nucl Med 32:12061210

359


9/10

Holman BL, Mendelson J, Garada B, Teoh SK, Hallgring E,Johnson KA, Mello NK (1993) Regional cerebral blood flowimproves with treatment in chronic cocaine polydrug users.J Nucl Med 34:723727

Howell LL, Byrd LD (1991) Characterization of the effects ofcocaine and GBR 12909, a dopamine uptake inhibitor, onbehavior in the squirrel monkey. J Pharmacol Exp Ther258:178185

Howell LL, Czoty PW, Byrd LD (1997) Pharmacological interac-tions between serotonin and dopamine on behavior in the

squirrel monkey. Psychopharmacology 131:4048Howell LL, Czoty PW, Kuhar MJ, Carroll FI (2000) Comparative

behavioral pharmacology of cocaine and the selective dopa-mine uptake inhibitor RTI-113 in the squirrel monkey. J Phar-macol Exp Ther 292:521529

Howell LL, Hoffman JM, Votaw JR, Landrum AM, Jordan JF(2001) An apparatus and behavioral training protocol toconduct positron emission tomography (PET) neuroimagingin conscious rhesus monkeys. J Neurosci Meth 106:161169

Howell LL, Hoffman JM, Votaw JR, Landrum AM, Wilcox KM,Lindsey KP (2002) Cocaine-induced brain activation deter-mined by positron emission tomography neuroimaging inconscious rhesus monkeys. Psychopharmacology 159:154160

Iyer RN, Nobiletti JB, Jatlow PI, Bradberry CW (1995) Cocaineand cocaethylene: effects on extracellular dopamine in theprimate. Psychopharmacology 120:150155

Johnson B, Lamki L, Fang B, Barron B, Wagner L, Wells L,Kenny P, Overton D, Dhother S, Abramson D, Chen R,Kramer L (1998) Demonstration of dose-dependent global andregional cocaine-induced reductions in brain blood flow using anovel approach to quantitative single photon emission comput-erized tomography. Neuropsychopharmacology 18:377384

Kaufman MJ, Madras BK (1992) distribution of cocaine recogni-tion sites in monkey brain: II. Ex vivo autoradiography with[3H]CFT and [125]RTI-55. Synapse 12:99111

Kaufman MJ, Spealman RD, Madras BK (1991) Distribution ofcocaine recognition sites in monkey brain: I. In vitro autora-diography with [3H]CFT. Synapse 9:177187

Kelleher RT, Morse WH, Herd JA (1972) Effects of propranolol,phentolamine and methyl atropine on cardiovascular functionin the squirrel monkey during behavioral experiments. J Phar-macol Exp Ther 182:204217

Laruelle M, Iyer RN, Al-Tikriti MS, Zea-Ponce Y, Malison R,Zoghbi SS, Baldwin RM, Kung HF, Charney DS, Hoffer PB,Innis RB, Bradberry CW (1997) Microdialysis and SPECTmeasurements of amphetamine-induced dopamine release innonhuman primates. Synapse 25:114

Levin JM, Holman BL, Mendelson JH, Teoh SK, Garada B,Johnson KA, Springer S (1994) Gender differences in cerebralperfusion in cocaine abuse: technetium-99m-HMPAO SPECTstudy of drug-abusing women. J Nucl Med 35:19021909

London ED, Cascella NG, Wong DF, Phillips RL, Dannals RF,Links JM, Herning R, Grayson R, Jaffe JH, Wagner HN Jr(1990) Cocaine-induced reduction of glucose utilization inhuman brain. A study using positron emission tomography and[fluorine 18]-fluorodeoxyglucose. Arch Gen Psychiatry47:567574

Mach RH, Nader MA, Ehrenkaufer LE, Line SW, Smith CR,Gage HD, Morton TE (1997) Use of positron emissiontomography to study the dynamics of psychostimulant-induceddopamine release. Pharmacol Biochem Behav 57:477486

Madras BK, Kaufman MJ (1994) Cocaine accumulates in dopa-mine-rich regions of primate brain after IV administration:comparison with mazindol distribution. Synapse 18:261275

Madras BK, Fahey MA, Bergman J, Canfield DRD, Spealman RD(1989) Effects of cocaine and related drugs in nonhumanprimates. I. [3H]cocaine binding sites in caudate-putamen.J Pharmacol Exp Ther 251:131141

Martin-Soelch C, Leenders KL, Chevalley A-F, Missimer J,Knig G, Magyar S, Mino A, Schultz W (2001) Rewardmechanisms in the brain and their role in dependence: evidence

from neurophysiological and neuroimaging studies. Brain ResRev 36:139149

Mathew RJ, Wilson WH, Lowe JV, Humphries D (1996) Acutechanges in cranial blood flow after cocaine hydrochloride. BiolPsychiatry 40:609616

Morgan D, Grant KA, Gage HD, Mach RH, Kaplan JR, Prioleau O,Nader SH, Buchheimer N, Ehrenkaufer R, Nader MA (2002)Social dominance in monkeys: dopamine D2 receptors andcocaine self-administration. Nat Neurosci 5:169174

Nader MA, Grant KA, Davies HM, Mach RH, Childers SR (1997)

The reinforcing and discriminative stimulus effects of the novelcocaine analog 2b-propanoyl-3b-(4-tolyl)-tropane in rhesusmonkeys. J Pharmacol Exp Ther 280:541550

Pearlson GD, Jeffery PJ, Harris GJ, Ross CA, Fischman MW,Camargo EE (1993) Correlation of acute cocaine-inducedchanges in local cerebral blood flow with subjective effects.Am J Psychiatry 150:495497

Pettit HO, Justice JB Jr (1989) Dopamine in the nucleus accumbensduring cocaine self-administration as studied by in vivomicrodialysis. Pharmacol Biochem Behav 34:899904

Pettit HO, Justice JB Jr (1991) Effect of dose on cocaine self-administration behavior and dopamine levels in the nucleusaccumbens. Brain Res 539:94102

Ranaldi R, Pocock D, Zereik R, Wise RA (1999) Dopaminefluctuations in the nucleus accumbens during maintenance,extinction, and reinstatement of intravenous d-amphetamine

self-administration. J Neurosci 19:41024109Reivich M, Alavi A, Wolf A, Fowler J, Russell J, Arnett C,MacGregor RR, Shiue CY, Atkins H, Anand A, Dann R,Greenberg J (1985) Glucose metabolic rate kinetic modelparameter determination in humans: the lumped constants andrate constants for 18F-fluorodeoxyglucose and 11C-deoxyglu-cose. J Cereb Blood Flow Metab 5:179192

Ritz MC, Lamb RJ, Goldberg SR, Kuhar MJ (1987) Cocainereceptors on dopamine transporters are related to self-admin-istration of cocaine. Science 237:12191223

Rothman RB (1990) High affinity dopamine reuptake blockers aspotential cocaine antagonists: a strategy for drug development.Life Sci 46:PL17PL21

Rothman RB, Grieg N, Kim A, de Costa BR, Rice KC, Carroll FI,Pert A (1992) Cocaine and GBR 12909 produce equivalentmotoric responses at different occupancy of the dopamine

transporter. Pharmaol Biochem Behav 43:11351142Schultz W, Dayan P, Montague PR (1997) A neural substrate ofprediction and reward. Science 275:15931599

Seiden LS (1975) Brain monoamines and behavior. Psychophar-macol Bull 11:6061

Sekine Y, Iyo M, Ouchi Y, Matsunaga T, Tsukada H, Okada H,Yoshikawa E, Futatsubashi M, Takei N, Mori N (2001)Methamphetamine-related psychiatric symptoms and reducedbrain dopamine transporters studied with PET. Am J Psychiatry158:12061214

Spealman RD, Madras BK, Bergman J (1989) Effects of cocaineand related drugs in nonhuman primates. II. Stimulant effectson schedule-controlled behavior. J Pharmacol Exp Ther251:142149

Strickland TL, Mena I, Villanueva-Meyer J, Miller BL, Cum-mings J, Mehringer CM, Satz P, Myers H (1993) Cerebralperfusion and neuropsychological consequences of chroniccocaine use. J Neuropsychiatr Clin Neurosci 5:419427

Tsukada H, Harada N, Nishiyama S, Ohba H, Kakiuchi T (2000)Dose-response and duration effects of acute administrations ofcocaine and GBR 12909 on dopamine synthesis and transporterin the conscious monkey brain: PET studies combined withmicrodialysis. Brain Res 860:141148

Villemagne VL, Rothman RB, Yokoi F, Rice KC, Matecka D,Dannals RF, Wong DF (1999) Doses of GBR12909 thatsuppress cocaine self-administration in non-human primatessubstantially occupy dopamine transporters as measured by[11C]WIN35,428 PET scans. Synapse 32:4450

360


10/10

Volkow ND, Mullani N, Gould KL, Adler S, Krajewski K (1988)Cerebral blood flow in chronic cocaine users: a study withpositron emission tomography. Br J Psychiatry 151:641648

Volkow ND, Fowler JS, Wolf AP, Hitzemann R, Dewey S,Bendriem B, Alpert R, Hoff A (1991) Changes in brain glucosemetabolism in cocaine dependence and withdrawal. AmJ Psychiatry 148:621626

Volkow ND, Hitzemann R, Wang GJ, Fowler JS, Wolf AP,Dewey SL, Handlesman L (1992) Long-term frontal brainmetabolic changes in cocaine abusers. Synapse 11:184190

Volkow ND, Fowler JS, Wang GJ, Hitzemann R, Logan J,Schlyer DJ, Dewey SL, Wolf AP (1993) Decreased dopamineD2 receptor availability is associated with reduced frontalmetabolism in cocaine abusers. Synapse 14:169177

Volkow ND, Wang G-J, Fischman M, Fowler JS, Foltin R,Abumrad NN, Vitkun S, Logan J, Gatley SJ, Pappas N,Hitzeman RR, Shea C (1997) Relationship between thesubjective effects of cocaine and dopamine transporter occu-pancy. Nature 386:827830

Volkow ND, Fowler JS, Gatley SJ, Dewey SL, Wang G-J, Logan J,Ding Y-S, Franceschi D, Gifford A, Morgan A, Pappas N,King P (1999a) Comparable changes in synaptic dopamineinduced by methylphenidate and by cocaine in the baboonbrain. Synapse 31:5966

Volkow ND, Wang G-J, Fowler JS, Gatley SJ, Logan J, Ding Y-S,Dewey SL, Hitzemann R, Gifford AN, Pappas NR (1999b)

Blockade of striatal dopamine transporters by intravenousmethylphenidate is not sufficient to induce self-reports ofhigh. J Pharmacol Exp Ther 288:1420

Volkow ND, Wang G-J, Fowler JS, Logan J, Gatley SJ, Wong C,Hitzemann R, Pappas NR (1999c) Reinforcing effects ofpsychostimulants in humans are associated with increases inbrain dopamine and occupancy of D2 receptors. J PharmacolExp Ther 291:409415

Volkow ND, Wang G-J, Fowler JS, Logan J, Gatley SJ, Gifford A,Hitzemann R, Ding Y-S, Pappas N (1999d) Prediction ofreinforcing responses to psychostimulants in humans by braindopamine D2 receptor levels. Am J Psychiatry 156:14401443

Volkow ND, Wang G-J, Fischman MW, Foltin R, Fowler JS,Franceschi D, Francerschi M, Logan J, Gatley SJ, Wong C,Ding Y-S, Hitzemann R, Pappas N (2000) Effects of route ofadministration on cocaine induced dopamine transporter block-ade in the human brain. Life Sci 67:15071515

Volkow ND, Chang L, Wang G-J, Fowler JS, Ding Y-S, Sedler M,Logan J, Franceschi D, Gatley J, Hitzemann R, Gifford A,Wong C, Pappas N (2001a) Low level of brain dopamine D2receptors in methamphetamine abusers: association with me-tabolism in the orbitofrontal cortex. Am J Psychiatry 158:2015

2021Volkow ND, Chang L, Wang G-J, Fowler JS, Franceschi D,

Sedler M, Gatley SJ, Miller E, Hitzemann R, Ding Y-S, Logan J(2001b) Loss of dopamine transporters in methamphetamineabusers recovers with protracted abstinence. J Neurosci21:94149418

Votaw JR, Howell LL, Martarello L, Hoffman JM, Kilts CD,Lindsey KP, Goodman MM (2002) Measurement of dopaminetransporter occupancy for multiple injections of cocaine using asingle injection of [F-18]FECNT. Synapse 44:203210

Wallace EA, Wisniewski G, Zubal G, van Dyck CH, Pfau SE,Smith EO, Rosen MI, Sullivan MC, Woods SW, Kosten TR(1996) Acute cocaine effects on absolute cerebral blood flow.Psychopharmacology 128:1720

Watanabe M, Cromwell HC, Tremblay L, Hollerman JR, Hikosa-ka K, Schultz W (2001) Behavioral reactions reflecting

differential reward expectations in monkeys. Exp Brain Res140:511518Wilcox KM, Paul IA, Woolverton WL (1999) Comparison between

dopamine transporter affinity and self-administration potencyof local anesthetics in rhesus monkeys. Eur J Pharmacol367:175181

Wilcox KM, Lindsey KP, Votaw JR, Goodman MM, Martarello L,Carroll FI, Howell LL (2002) Self-administration of cocaineand the cocaine analog RTI-113: relationship to dopaminetransporter occupancy determined by PET neuroimaging inrhesus monkeys. Synapse 43:7885

361

psycho p harm 163

Documents