Essay

Leading Edge

Review

The Brain on Drugs: From Reward to Addiction

Nora D. Volkow1,* and Marisela Morales1 1National Institute on Drug Abuse, National Institutes of Health, Bethesda, MD 20892, USA

*Correspondence: [email protected]

http://dx.doi.org/10.1016/j.cell.2015.07.046

Advances in neuroscience identified addiction as a chronic brain disease with strong genetic, neu- rodevelopmental, and sociocultural components. We here discuss the circuit- and cell-level mech- anisms of this condition and its co-option of pathways regulating reward, self-control, and affect. Drugs of abuse exert their initial reinforcing effects by triggering supraphysiologic surges of dopa- mine in the nucleus accumbens that activate the direct striatal pathway via D1 receptors and inhibit the indirect striato-cortical pathway via D2 receptors. Repeated drug administration triggers neuro- plastic changes in glutamatergic inputs to the striatum and midbrain dopamine neurons, enhancing the brain’s reactivity to drug cues, reducing the sensitivity to non-drug rewards, weakening self- regulation, and increasing the sensitivity to stressful stimuli and dysphoria. Drug-induced impair- ments are long lasting; thus, interventions designed to mitigate or even reverse them would be beneficial for the treatment of addiction.

The nature of addiction is frequently debated as either a per-

sonal ‘‘lifestyle choice’’ or a ‘‘biological vulnerability.’’ Current

evidence shows that most drugs of abuse exert their initial

reinforcing effects by activating reward circuits in the brain

and that, while initial drug experimentation is largely a voluntary

behavior, continued drug use impairs brain function by inter-

fering with the capacity to exert self-control over drug-taking

behaviors and rendering the brain more sensitive to stress

and negative moods. Indeed, individuals with genetic vulnera-

bilities, exposed to chronic stress, or suffering from comorbid

psychiatric conditions, as well as those who abused drugs

during early adolescence, are at greater risk of transitioning

into the automatic and compulsive behaviors that characterize

addiction.

Drugs modulate the expression of genes involved in neuro-

plasticity through epigenetic and possibly RNA modifications,

ultimately perturbing intracellular signaling cascades and

the neuronal circuits whose dysfunction have been implicated

in the long-lasting changes associated with addiction.

Here, we highlight some of the most significant and recent

findings in drug reward and addiction, describing the circuit,

behavioral, and synaptic mechanisms underlying this process.

Space limitations do not allow us to review the intracellular

signaling cascades and epigenetic modifications associated

with addiction; thus, we refer readers to recent reviews on

these topics (Heller et al., 2014; Nestler, 2012; Pascoli et al.,

2014a).

Drug Reward Signaling in Brain Dopamine (DA) neurons located in the ventral tegmental area

(VTA) and projecting to the nucleus accumbens (NAc) play a

key role in the processing of reward-related stimuli, including

those associated with drugs of abuse (Wise, 2008). Drugs

712 Cell 162, August 13, 2015 ª2015 Elsevier Inc.

of abuse, through their different pharmacological effects, in-

crease the release of DA in the shell subregion of the NAc

(Di Chiara, 2002), mimicking the phasic DA neuronal firing

that leads to very fast DA increases (Owesson-White et al.,

2009) and thus the mechanism through which the brain signals

reward (Box 1). The large DA increases triggered by phasic

DA cell firing are necessary to stimulate D1 receptors (D1R)

in the NAc.

DA neurons in the VTA fire in either a tonic (1–8 Hz) or a tran-

sient (<500 ms) high-frequency phasic mode (>15 Hz), with the

phasic mode resulting in larger DA increases than the tonic

mode. Though it was initially believed that DA signaling in the

brain encoded for reward, more recent findings have revealed

that it encodes for a reward prediction signal. Specifically,

these studies have shown that phasic DA firing is time locked

to unexpected or novel reward but is also triggered by cues

that predict reward. Moreover, the firing frequency of DA neu-

rons triggered by cues is associated with the expected reward

value and its probability of delivery, but if the expected reward

does not materialize, DA cell firing is inhibited (Schultz, 2002).

Changes in the response patterns of DA cell firing are modu-

lated by more distinct projections for tonic than for phasic

firing (Box 1). Changes in phasic DA firing patterns modify

the strength of cortico-striatal glutamatergic synapses, thus

altering signaling in D1R- and D2R-expressing GABAergic me-

dium spiny neurons (MSNs) (Paladini and Roeper, 2014). This is

distinct from DA signaling in the NAc driven by release from

tonic DA neuron firing, which results in lower DA increases

than from phasic firing but that are sufficient to stimulate D2R

signaling and have been mostly associated with motivational

drive (Dreyer et al., 2010; Trifilieff et al., 2013). Though most

studies link drug-induced neuroplasticity with the fast and large

transient DA changes triggered by drugs, the contribution from

Box 1. Modulation of VTA DA Neuronal Firing

Recent pseudorabies virus-based methods for monosynaptic network

tracing have shown that neurons from many brain areas synapse on

distinct VTA DA neuron subpopulations (Lammel et al., 2014) and

that neurons from the dorsal raphe (DR) provide the majority of mono-

synaptic inputs (Ogawa et al., 2014). Studies of the influence of these

projections on DA neurons have been limited to a few brain structures

(Paladini and Roeper, 2014). For instance, the control of tonic firing of

VTA DA neurons involves the stria terminals and the ventral pallidum

(Georges and Aston-Jones, 2001; Mahler et al., 2014), whereas the

control of phasic firing of VTA DA neurons involves the pedunculo

pontine tegmentum (PPT), the subthalamic nucleus (STN), and the lat-

erodorsal tegmentum (Floresco et al., 2003; Lodge and Grace, 2006).

VTA DA neurons receive GABAergic innervation from local GABAegic

neurons, the NAc, globus pallidus, and rostromedial tegmental nu-

cleus, among others. These GABAergic projections are implicated in

the control of burst timing (Paladini and Roeper, 2014). It is likely that

phasic and tonic changes in DA neuronal firing triggered by repeated

drug administration, reflect neuroplastic changes in these regions

and on inputs that relay to them. For example, the lateral habenula

(LHb) indirectly inhibits VTA DA neurons via its inputs to GABA neurons

in rostromedial tegmental nucleus (Ji and Shepard, 2007), eliciting

aversion (Lammel et al., 2012), and these inputs are modified by

repeated cocaine administration (Meye et al., 2015). Thus, future

studies will be able to assess their contribution to the dysphoria and

enhanced stress reactivity in addiction.

We recently showed abundant glutamatergic projections from the DR

to VTA DA neurons that innervate the NAc, whose activation induced

DA release in NAc and evoked reward (Qi et al., 2014). The DR is

best known as a serotonergic structure that regulates emotional be-

haviors. However, findings on the role of DR serotonergic neurons in

reward have been inconsistent (Cohen et al., 2015; Fonseca et al.,

2015; Liu et al., 2014; McDevitt et al., 2014; Miyazaki et al., 2014),

which is likely to reflect, in part, the functional diversity of these neu-

rons. In this regard, cellular recordings from DR serotonergic neurons

in behaving mice have revealed that they convey reward information

through tonic as well as phasic firing and that they signal reward and

punishment on multiple timescales (Cohen et al., 2015). The DR also

has glutamatergic and GABAergic neurons, some of which co-release

serotonin, and thus future studies are necessary to tease apart the spe-

cific targets of the diverse serotonergic neurons and of their neigh-

boring GABAergic and glutamatergic neurons (Liu et al., 2014; McDe-

vitt et al., 2014; Qi et al., 2014). In this regard, we recently showed that,

within the VTA, DR neurons expressing the vesicular glutamate trans-

port (VGluT3) preferentially establish synapses on DA neurons (Qi et al.,

2014). These DR-VGluT3 neurons provide a major glutamatergic input

to VTA DA neurons, including those that innervate the NAc. Selective

activation of these DR-VGluT3 fibers results in VTA glutamate release,

NAc DA release, and reward (Qi et al., 2014). Notably, these DR

VGluT3-glutamatergic neurons (some of which may co-release seroto-

nin) are highly interactive with the serotonergic system (Commons,

2009). Thus, a better understanding of the function and connections

of the diverse DR neurons will help us determine whether they serve

as a link between reward and mood regulation and whether they

contribute to the high co-morbidity between drug use and depression.

the longer-lasting stimulation of D2R (also D3R and D4R) has

been much less investigated.

VTA DA neurons project predominantly to the NAc, where DA

interacts with D1R, D2R, and D3R, which are mainly expressed

in MSNs. Stimulatory striatal MSNs that express D1R (D1R-

MSNs) signal through the direct striatal pathway, whereas those

that express D2R (D2R-MSNs) signal through the striatal indirect

pathway and act in an inhibitory manner. D3R mostly co-localize

with D1R-MSNs, with which they heteromerize, potentiating their

function (Marcellino et al., 2008). The ventral striatal direct and in-

direct pathways have distinct roles in modulating reward and

motivation. The direct pathway is associated with reward,

whereas the indirect one is associated with punishment (Hikida

et al., 2010; Kravitz et al., 2012). Thus, DA receptor stimulation

of the direct pathway directly mediates reward, whereas DA-re-

ceptor-mediated inhibition of the indirect pathway opposes

aversive responses. This could explain why maximal drug

reward is obtained when DA binds to both D1R and D2R. How-

ever, in contrast to the situation in the dorsal striatum, where the

direct and indirect pathways are fully segregated, in the NAc,

both D1R- and D2R-expressing MSNs project into the ventral

globus pallidum (Smith et al., 2013b). To be reinforcing, drug-

induced DA increases need to be fast and sufficiently large to

stimulate low-affinity D1R in addition to D2R, leading to the acti-

vation of the direct pathway and the inhibition of the indirect

pathway. D1R stimulation in the NAc by itself is sufficient to pro-

duce drug reward (Caine et al., 2007), whereas D2R stimulation

is not (Caine et al., 2002; Durieux et al., 2009; Norman et al.,

2011), and maximal reward occurs when both D1R and D2R

are activated (Steinberg et al., 2014; Welter et al., 2007). Indeed,

brain imaging studies in humans have documented that fast DA

increases triggered by drugs are associated with the ‘‘high’’

associated with drug abuses, whereas stable DA increases are

not (Volkow et al., 2008). Specifically, when large DA increases

triggered by stimulant drugs were achieved over a short time

period (<10 min), they were associated with reward, whereas

DA increases achieved over 60 min were not. The rate depen-

dency for a drug’s rewarding effects might explain why the

time course of the subjective ‘‘high’’ is much shorter than the

longer-lasting DA increases triggered by drugs such as cocaine

and more notable methylphenidate (Figure 1). Presumably, stim-

ulation of D1 and D2R only occurs when drugs achieve fast peak

concentrations, whereas as the concentration of DA starts to

decrease, D2R are predominantly stimulated (Luo et al., 2011).

This may also explain why routes of administration that achieve

faster and higher drug levels in the brain, such as smoking and

intravenous injection, are more rewarding and addictive than

routes of administration that result in slow brain uptake, like

oral administration.

DA increases that are sufficiently large to activate D1R, such

as those induced by drugs in the NAc, can induce associative

learning, also referred to as conditioning (Zweifel et al., 2009).

Stimuli (including contextual or environmental) associated with

the drug become conditioned and, with repeated co-exposure,

will trigger phasic DA neuronal firing in the VTA, resulting in

fast, large, and short-lasting DA increases in the NAc. The DA in-

creases triggered by these conditioned stimuli (CS) are believed

to reflect the expectation of receiving a reward. Glutamatergic

projections into D1R-expressing MSNs coming from the amyg-

dala (involved in emotional reactivity), hippocampus (involved

in memory), and ventral PFC (involved in salience attribution)

mediate these conditioned responses. The increased dopami-

nergic signaling that follows exposure to the CS ensures that

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Figure 1. Fast Brain Uptake and Fast DA Increases Triggered by

Drugs Are Necessary for Reward (A) Pharmacokinetics of cocaine and methylphenidate (MPH) in the human brain and relationship to the drug-induced ‘‘high.’’ Upper panels show axial brain images of the distribution of [11C]cocaine and [11C]methylphenidate at different time points (in minutes) following their intravenous administration. Lower panels show time activity curves for the concentration of [11C]cocaine and [11C]methylphenidate in the striatum in conjunction with the temporal course of the ‘‘high’’ experienced after intravenous administration of the drug. These findings suggest that the ‘‘high’’ is associated with the initial fast rate of uptake of the drug in brain and presumably the associated fast DA increases they trigger. (B) Regression slopes between MPH-induced DA increase (assessed as the reduction in the specific binding of [11C]raclopride to D2R and D3R) and the self-reports of ‘‘high’’ when MPH was administered intravenously, which re- sults in very fast drug delivery in the brain (measures initiated at 1 min post MPH) and when MPH was administered orally, which results in very slow brain uptake (measures initiated at 60 min post MPH). MPH-induced DA increases resulted in a ‘‘high’’ only when it was given intravenously, but not when given orally, consistent with fast DA increases being necessary for drug reward. Figure modified from Volkow et al., 1995, 1999, 2001b.

Box 2. Opioid Regulation of the Mesolimbic DA Pathway

Endogenous opioids modulate DA neuronal firing in midbrain (Margolis

et al., 2014) and striatal MSNs (Gianoulakis, 2009), where dynorphin

co-localizes in D1R-MSNs and enkephalin in D2R-MSNs (Gerfen

et al., 1990). Endogenous opioids are implicated in hedonic responses

to natural and drug rewards (Le Merrer et al., 2009) and also in the ad-

aptations that follow repeated drug exposures and drug relapse (Koob

et al., 2014).

Microdialysis studies measuring the effects of various drug classes

(including stimulants, opiates, THC, and alcohol) have found in-

creases in extracellular levels of endogenous opioids in the NAc

and VTA (reviewed in Murphy, 2015). The roles of endogenous opioids

in drug reward are particularly well established for alcohol and opi-

ates, whose intake is reduced in mu opioid receptor (MOR) knockout

mice and decreased by administration of opioid receptor antagonists

(reviewed in Tseng et al., 2013). Moreover, naltrexone, an opiate

antagonist, is an FDA-approved medication for the treatment of alco-

holism and opiate use disorders. In animal models, MOR antagonists

interfere with cocaine and nicotine reward (Berrendero et al., 2010;

Giuliano et al., 2013); however, clinical trials with naltrexone have

failed to show therapeutic benefit for cocaine or nicotine use

disorders.

The endogenous opioid system also contributes to the effects of

stress on drug consumption. Specifically, dynorphin, through its acti-

vation of kappa receptors (KOR), is implicated in the stress-induced

potentiation of drug reward (reviewed in Ehrich et al., 2014). Activation

of KOR on DA terminals inhibits DA release in the NAc, which is impli-

cated in the dysphoria that follows drug withdrawal (Tejeda et al.,

2012). These findings have generated interest in KOR antagonists

or partial agonists as medications to prevent relapse in addiction

(Al-Hasani et al., 2013; Butelman et al., 2012; Grosshans et al., 2015;

Schlosburg et al., 2013; Smith et al., 2013a).

The effects of drugs on the opioid system in the human brain have

been investigated with positron emission tomography (PET) using

[11C]carfentanil to assess MOR and their occupancy by enkephalins.

These studies have shown that acute alcohol, but not intravenous

amphetamine, increases enkephalins (Guterstam et al., 2013; Mitchell

et al., 2012). Increases in enkephalins after cigarette smoking have

been inconsistent, which is likely to reflect, in part, the influence of

MOR gene variants in these responses. More specifically, increases

were observed only in smokers with the AA variant of the MOR

A118G polymorphism (Domino et al., 2015).

Brain-imaging studies of MOR in alcoholics and cocaine abusers have

reported increased [11C]carfentanil binding, which has been inter-

preted to reflect reduced levels of endogenous enkephalins (hence,

decreased competition for ligand binding) though they could also

reflect MOR upregulation (Weerts et al., 2011; Zubieta et al., 1996).

In cocaine abusers, the increases in [11C]carfentanil binding have

been associated with worse clinical outcomes (reviewed in Volkow,

2010). In contrast, studies in smokers have shown no changes in

[11C]carfentanil binding (Kuwabara et al., 2014).

Brain-imaging studies of delta opioid receptors in alcoholics

(measured with PET and [11C]methylnaltrindole) showed no differences

when compared to controls (Weerts et al., 2011). To our knowledge, no

PET studies have been done in substance abusers using kappa recep-

tor ligands.

the individual will have the motivation to engage in the behaviors

necessary to procure the reinforcer, be it natural or pharmaco-

logical. Because D1R have a lower affinity for DA than D2R,

cue exposure or drug intoxication will lead to D1R occupancy

only when peak DA levels are present; all the while, DA binding

to D2R will be longer-lasting and persist even after peak levels

have subsided (Luo et al., 2011). Thus, while DA stimulation of

D1R-MSNs in the direct pathway signals the expectation for

the reward, DA stimulation of D2R-MSNs in the indirect pathway

is more likely to sustain the motivation needed to procure and

consume the reinforcer.

For natural reinforcers such as food or sex, the DA signals

triggered by the CS drive the motivation to get the reward since

with their repeated delivery the DA cells stop firing in response

to their consumption (Schultz et al., 1997). This is in sharp

contrast to the response to drugs of abuse, which due to their

pharmacological properties, continue increasing DA release

during their consumption. Thus, DA in the NAc will increase

upon exposure to drug cues, which will trigger the desire to

take the drug (craving) also during their consumption, which

will sustain the motivation to continue consuming them. This

may explain why drugs are more likely to result in compulsive

patterns of administration than natural reinforcers. However,

714 Cell 162, August 13, 2015 ª2015 Elsevier Inc.

the DA increases triggered by cocaine, and presumably other

drugs, activate D2R auto-receptors inhibiting DA cell firing

and DA release (Bello et al., 2011), which is perhaps why the

intensity of the cocaine ‘‘high’’ is reduced with subsequent

Figure 2. Drug-Induced Synaptic Neuro-

plasticity in Brain Reward Circuitry Mesocorticolimbic brain areas where there is evi- dence of drug-induced neuroplasticity along with the associated synaptic modifications and their behavioral consequences. AMG, amygdala; NAc, nucleus accumbens; (m)PFC, (medial) prefrontal cortex; VTA, ventral tegmental area; CRFR, corti- cotropin releasing factor. Figure modified from van Huijstee and Mansvelder (2014) and work from Krishnan et al. (2010) and Lee et al. (2013).

administrations, whereas the motivation to continue to take the

drug continues unabated.

Endogenous opioids and cannabinoids have been also impli-

cated in drug reward responses, in part through the opioid regu-

lation of the mesolimbic DA pathway (Box 2) and through studies

of the role of cannabinoids in adaptations that occur with

repeated drug exposures (for reviews, see Covey et al., 2014;

Panagis et al., 2014).

Transition into Addiction The ability of drugs to increase DA in the NAc and trigger condi-

tioned responses in both naive and addicted individuals indi-

cates that changes in DA levels alone cannot account for the

addiction phenotype. Importantly, addiction seems to emerge

gradually, although the rate of this transition varies as a func-

tion of several factors, including the type of drug (i.e., faster

for methamphetamine and slower for cannabinoids), the

pattern of exposure (greater for regular than occasional use),

and the developmental stage (faster in adolescence than in

adulthood) (Robins and Przybeck, 1985; Schramm-Sapyta

et al., 2009). The transition from controlled to compulsive

drug taking has been associated with a shift in the involvement

of striatal subregions (NAc), implicated in the rewarding

response to drugs, to the dorsal striatum that is associated

with habit formation (Everitt and Robbins, 2013). The speed

with which addiction emerges is influenced by genetics, with

some individuals transitioning faster than others due to genetic

vulnerabilities (Kessler et al., 2007). To properly emulate the

addiction phenotype, animal models of human addiction must

feature compulsive drug consumption that occurs in spite of

adverse consequences (Piazza and Deroche-Gamonet, 2013).

This is because a characteristic of addiction is the failure

of the individual to control his/her drug consumption despite

catastrophic adverse consequences (i.e., incarceration or

loss of job or child custody). Interestingly, in such models,

only 10% of experimental animals will develop an addiction

phenotype, which is similar to the estimated percentage of

drug-exposed individuals who become addicted (Seedall and

Anthony, 2013).

Cell 162

The pharmacological mechanisms of

action of various drugs types influence

the nature of the molecular and cellular

changes associated with their repeated

consumption. For example, alpha 6 and

beta 3 nicotinic receptors are upregu-

lated in DA neurons with repeated nico-

tine exposure (Visanji et al., 2006), dopamine transporters are

downregulated in the striatum with repeated methamphetamine

exposure (Groman et al., 2012), and cannabinoid receptors 1 are

downregulated in striatum with repeated delta9-tetrahydrocan-

nabinol (THC) exposure (Romero et al., 1997). These changes

are likely to contribute to the emergence of tolerance to the

drug’s effects and the need to use increasingly larger doses in

an attempt to achieve the same behavioral effect. In turn, expo-

sure to higher drug doses facilitates the neuroplastic changes

that ultimately change the reactivity of brain DA pathways to

drugs and drug cues.

Drug-Induced Neuroplasticity in DA Pathways Drug-induced DA increases trigger various forms of synaptic

plasticity that can result in strengthening or weakening of synap-

tic connectivity in various brain reward regions (Figure 2 and

Box 3) (Grueter et al., 2012). These drug-induced neuroplastic

changes largely hinge on the epigenetic enhancement or

silencing of gene expression and on epitranscriptomic (RNA edit-

ing) modulation of translation, two mechanisms whose details are

beginning to be uncovered (Kenny, 2014; Robison and Nestler,

2011; Satterlee et al., 2014). Among the transcription factors

implicated in the long-lasting neuroplastic changes that follow

repeated drug exposure is DFosB (Maze et al., 2010). Drug-

induced neuroplasticity evokes the same types of molecular pro-

cesses involved in long-term potentiation (LTP) and long-term

depression (LTD) that underlie learning and memory. The

changes in synaptic strength that occur as a result of LTP are

associated with larger synapses and dendritic spines, while

those that follow LTD involve smaller synapses and dendritic

spines (De Roo et al., 2008). These synaptic modifications

generate a long-lasting molecular memory for the drug’s

rewarding and conditioning effects that will modify subsequent

behaviors (Hyman, 2005).

Most of the studies of neuroplasticity have investigated the

effects of chronic cocaine in the NAc (Sesack and Grace,

2010). Chronic cocaine increases dendritic spine density in

MSNs (Russo et al., 2010), and DFosB is implicated in their gen-

eration (Maze et al., 2010). The structural changes observed in

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Figure 3. Drug-Induced Reductions in Striatal D2R Are Associated with Decreased Activity in Prefrontal Cortex (A) Schematic representation of the indirect pathways in which DA neurons from the ventral tegmental area (VTA) and the substantia nigra compacta (SN) provide DA inputs to the striatal GABA neurons expressing D2R (D2R-MSNs). These striatal neurons target GABA cells in the globus pallidum external (GPe), which provide inhibition to glutamatergic neurons within the subthalamic nu- cleus (STN). The STN glutamatergic neurons provide an excitatory input to GABA neurons present in the substantia nigra reticulata (SNR) and the globus pallidum internal (Gpi), which inhibit glutamate neurons of the thalamus innervating the frontal cortex. Drug-induced reductions in D2R within the striatum impair the inhibition of this indirect pathway by DA, resulting in reduced thalamo-cortical stimulation and consequently reduced activity in the frontal cortex. (B) Relationship between D2R in striatum and brain glucose metabolism in frontal regions of drug abusers tested both with [11C]raclopride and FDG. Images next to the y axis show axial brain metabolic images at the level of the orbitofrontal cortex, and images below the x axis show axial images of D2R striatal availability for a control and a cocaine abuser. Regression slopes correspond to the association between metabolism in the orbitofrontal cortex (OFC) and D2R availability in striatum in cocaine-addicted and in metham- phetamine-addicted subjects. Figure modified from Volkow et al., 2011.

Box 3. Synaptic Plasticity in DA Circuits

Many drugs of abuse, including cocaine, but also morphine, nicotine,

and ethanol, can evoke synaptic plasticity in VTA DA neurons (Bowers

et al., 2010; Lüscher and Malenka, 2011). Because VTA DA neurons are

heterogeneous in their synaptic connectivity, molecular composition,

and electrophysiological and signaling properties (Lammel et al.,

2014), it is conceivable that drugs differentially affect their subpopula-

tions. For instance, VTA DA neurons that lack DAT or D2R (Li et al.,

2013) innervate the medial PFC, but not the NAc (Bannon and Roth,

1983; Lammel et al., 2012; Sesack and Grace, 2010). Moreover,

some of the VTA DA neurons innervating the NAc have axons with mi-

cro-domains for either DA or glutamate signaling, further emphasizing

their diversity (Zhang et al., 2015).

DA regulates excitatory synaptic plasticity both by increasing and

decreasing synaptic strength through LTP and LTD. Synaptic strength

is controlled by the insertion or removal of AMPAR or NMDAR and by

changes in the subunit composition of AMPA receptors. Specifically,

the insertion of high-calcium permeable AMPAR (GluR2 subunit) con-

tributes to the drug-induced increases in AMPAR-to-NMDAR ratios

associated with LTP in models of addiction (Boudreau et al., 2007;

Conrad et al., 2008; Kourrich et al., 2007). These AMPAR have higher

single-channel conductance than GluA2-containing receptors (Guire

et al., 2008; Liu and Cull-Candy, 2000), and their upregulation in-

creases the responsiveness of MSNs in the NAc to glutamate released

by cortical and limbic terminals when exposed to drugs or drug cues

(Wolf and Ferrario, 2010). Drug-induced neuroplastic changes have

been uncovered in glutamatergic inputs to the NAc from PFC, basolat-

eral amygdala, and ventral hippocampus (Figure 2) (Di Forti et al., 2014;

Lee and Dong, 2011; MacAskill et al., 2014; Pascoli et al., 2014b).

Recently, the use of genetic cellular tagging has enabled researchers

to identify the clusters of neurons within the PFC that trigger excitatory

signals into NAc with exposure to cocaine cues (Cruz et al., 2014).

Though not as extensively investigated as the NAc, the dorsal striatum

also undergoes neuroplastic changes with repeated cocaine expo-

sure; these are implicated in habit learning and in the automatic

cocaine consumption triggered by repeated cocaine exposures (Ever-

itt et al., 2008; Hearing et al., 2011; Lavaur et al., 2009; Parikh et al.,

2014).

dendritic spines involve cytoskeletal and actin-myosin rear-

rangements and other structural proteins that regulate spine

morphology and dendritic arborization (Toda et al., 2006). In

addition, the formation of new spines is preceded by the gener-

ation of ‘‘silent’’ synapses containing NMDARs, but not

AMPARs (Huang et al., 2009; Malenka and Nicoll, 1997), which

are subsequently unsilenced through the insertion of AMPAR

lacking GluA2 (Box 3)(Conrad et al., 2008; Dobi et al., 2011).

Role of Dopamine in Addiction Repeated exposure to different types of drugs has been associ-

ated with downregulation of D2R in striatum (Nader et al., 2006;

Thanos et al., 2001; Volkow et al., 2001a). Specifically, studies in

rodents and non-human primates have found reduced levels of

D2R in the striatum, including in the NAc, upon chronic drug ex-

posures, as well as in animals with a propensity to self-admin-

ister drugs (Everitt et al., 2008). In rodents, low levels of D2R in

striatum are associated with impulsivity and predict escalating

and compulsive administration of cocaine (Everitt et al., 2008).

Similarly, human brain-imaging studies of addicted individuals

716 Cell 162, August 13, 2015 ª2015 Elsevier Inc.

have shown reductions in D2R availability in ventral and dorsal

striatum for most of the drugs, except for marijuana (reviewed

in Volkow and Baler, 2014). Low levels of D2R in the striatum

will result in reduced DA inhibition of the indirect pathway. D2R

have high affinity for DA, so they are stimulated by the relatively

low DA levels achieved through tonic DA cell firing. Reduced

D2R-mediated DA inhibition of the indirect pathway will lead to

reduced thalamo-cortical stimulation and consequently reduced

activity in PFC brain regions (Figure 3) (Black et al., 2010).

Indeed, the reductions in striatal D2R (dorsal and ventral) in

drug abusers have been associated with decreased activity in

the PFC, including anterior cingulate (ACC) and orbitofrontal

(OFC) cortical regions. The ACC and OFC are necessary for

self-control and for processing salience attribution, and their

disruption is associated with a propensity for impulsive and

compulsive behaviors (Volkow and Fowler, 2000). Thus, low

levels of D2R in striatum may mediate the risk for compulsive

drug taking in part by impairing PFC regions that inhibit prepo-

tent responses and enable flexibility of behavioral choices as a

function of changing environments (Volkow et al., 2006a).

Figure 4. Stimulant-Induced Dopamine Increases Are Blunted in

Active Cocaine Users (A) Brain maps of normal controls (left) and active cocaine abusers (right) showing DA increases in the striatum after methylphenidate (MPH) adminis- tration reveal a robust response in controls, but not in cocaine abusers. (B) Correlation between MPH-induced DA increases (measured as changes in non-displaceable binding potentials or BPND) in ventral striatum (where NAc is located) and the changes in craving scores (cocaine craving questionnaire [CCS]). Figure modified from Volkow et al., 2014a.

Indeed, in rodents, optogenetic stimulation of the PFC prevented

cocaine relapse (Chen et al., 2013). In contrast to the findings of

low striatal D2R in addicted individuals, which in laboratory ani-

mals is associated with vulnerability for compulsive cocaine

intake, the chemogenetic stimulation of D2R-MSNs (perhaps

somewhat akin to lack of DA inhibition through D2R) has been

shown to inhibit cocaine intake in mice (Bock et al., 2013). This

seemingly paradoxical finding suggests that the low D2R levels

in addicted individuals may reflect reduced postsynaptic and

presynaptic receptors and that the firing of D2R-MSNs is not

only modulated by D2R, but also by NMDAR, AMPAR, GABAR,

A2AR, and CB1R among other receptors. Thus, low D2R levels

are likely to unbalance DA’s modulation of the indirect pathway,

whereas the chemogenetic inhibition of D2R-MSNs interrupts

the signaling in this circuit. In this regard, it is interesting to

note that D2R agonists tend to decrease cocaine intake rather

than increase it, as seen with the chemogenetic inhibition of

D2R-MSNs.

Though, theoretically, enhanced signaling through D1R and its

activation of the direct pathway would be consistent with facilita-

tion of drug reward (Gore and Zweifel, 2013), the findings related

to the consequences of repeated drug exposure on D1R have

not been consistent. Thus, whereas some studies have shown

that chronic cocaine potentiated D1R signaling (Pascoli et al.,

2012), others have reported decreased D1R excitability (Kim

et al., 2011), and while in nonhuman primates repeated cocaine

exposure was associated with reductions in D1R in striatum

(Moore et al., 1998), human studies failed to observe this effect

(Martinez et al., 2009).

D1R-MSNs and D2R-MSNs do not fire independently of each

other, and during drug exposure, enhanced DA signaling will

stimulate one population but inhibit the other. Thus, it is likely

that the balance between striatal signaling through the direct

(D1R-mediated) and the indirect (D2R-mediated) pathways un-

derlies drug responses and that an imbalance between these

pathways may ultimately underlie the behavioral changes

observed in addiction. In fact, we have recently shown that, dur-

ing acute cocaine intoxication, signaling through both D1R and

D2R in the striatum was markedly reduced in mice previously

exposed to chronic cocaine, although the attenuation was

greater for D2R than for D1R (Park et al., 2013). As a result, DA

signaling though D1R versus D2R was biased in favor of DA-

mediated D1R signaling during the state of intoxication (Park

et al., 2013). Since DA stimulation of D1R is associated with

enhanced sensitivity to drug reward, a higher D1R-to-D2R

signaling ratio during drug intoxication could contribute to

compulsive drug taking. The D3Rs, which are highly expressed

in the mesolimbic DA system, have also been implicated in the

transition to addiction, and D3R antagonists have been pro-

posed as promising targets for the development of addiction

treatments (Heidbreder and Newman, 2010). D3R in the NAc

are upregulated by chronic cocaine (Conrad et al., 2010),

whereas D3R blockade interferes with cocaine reward (Heid-

breder and Newman, 2010). In humans, imaging and postmor-

tem studies have found an upregulation of D3R in the NAc of

cocaine abusers (Payer et al., 2014; Staley and Mash, 1996).

However, findings in mice expressing no D3R (D3R KO mice)

have been equivocal, with one study showing that D3R KO ani-

mals display enhanced motivation for cocaine-seeking behavior

(Song et al., 2012) and another showing no effect on motivation

(Caine et al., 2012).

A powerful approach for investigating changes in DA signaling

in addiction is to compare the DA responses triggered by drugs

in addicted versus non-addicted individuals. PET studies have

made it possible to measure drug-induced DA increases in

humans (Volkow et al., 1994)—for instance, in studies that

compared the effects of drug intoxication between cocaine

abusers and controls, where stimulant drugs such as methylphe-

nidate and amphetamine were used as pharmacological

challenges. These studies have consistently shown that the DA

increases triggered by stimulants are markedly attenuated in

cocaine abusers, both in dorsal and ventral striatal regions

(reviewed in Volkow et al., 2014a). These blunted responses

are consistent with preclinical findings showing reduced DA

signaling during cocaine intoxication in mice chronically

exposed to cocaine (Park et al., 2013). Despite these markedly

blunted DA responses, the DA increases in NAc were associated

with drug-induced craving (Volkow et al., 2014a) (Figure 4). In

cocaine abusers (Martinez et al., 2011) and in methamphetamine

abusers undergoing substance abuse treatment (Wang et al.,

2012), the blunted DA responses to stimulant drugs have been

also associated with worse clinical outcomes. Studies in alco-

holics have further documented blunted DA increases upon

challenge with a stimulant drug, which are consistent with

reduced DA cell activity (Martinez et al., 2005; Volkow et al.,

2007), but have also uncovered increased brain reactivity to

the DA increases, which suggests impaired downstream modu-

lation (Martinez et al., 2005; Volkow et al., 2007, 2013). In

contrast, in marijuana abusers, while stimulant-induced DA

increases in striatum did not differ from controls (Urban et al.,

2012; Volkow et al., 2014b), the brain reactivity to DA stimulation

was blunted, an effect that was associated with negative

emotionality (Volkow et al., 2014b). Similarly, a PET study

using 3,4-dihydroxy-6-[18F]-fluoro-l-phenylalanine ([18F]-DOPA)

reported that the reduced striatal DA synthesis capacity

Cell 162, August 13, 2015 ª2015 Elsevier Inc. 717

Box 4. The ‘‘Dark Side of Addiction’’

The ‘‘dark side of addiction’’ involves allostatic changes that lead to

drug use as a means to counteract the dysphoria and distress associ-

ated with drug withdrawal and discontinuation. Studies implicate the

extended amygdala (central nucleus of the amygdala, bed nucleus of

the stria terminalis, and a transition area in the NAc shell), cortico-

tropin-releasing factor (CRF), and norepinephrine in these allostatic

responses (Koob, 2013). Upregulation of the dynorphin/kappa opioid

receptor system has also been associated with the dysphoria and

the increased sensitivity to stress during drug discontinuation (see

also Box 2). In parallel, downregulation of signaling though neurotrans-

mitters associated with positive rewards, including DA, enkephalins

(Zubieta et al., 1996), endocannabinoids (Serrano and Parsons,

2011), and the reduced DA inhibition through D2R of the indirect

pathway, which signals aversive responses (Danjo et al., 2014), might

also contribute to dysphoria in addiction.

Recently, studies have identified the lateral habenula (LHb) as a brain

region disrupted by drugs that might also contribute to the dark side of

addiction (Velasquez et al., 2014). The LHb, through its projection to

the rostromedial tegmental nucleus, can inhibit DA cell firing (Ji and

Shepard, 2007). The LHb is activated upon exposure to unrequited

expectation and aversive stimuli, which could therefore also contribute

to the enhanced sensitivity to stress in addiction.

Humans display activation in both the VTA and the habenula in

response to aversive events (Hennigan et al., 2015). We had found

that diverse VTA cellular phenotypes, which may be involved in

different behaviors, synapse on LHb neurons (Root et al., 2014b).

This synaptic complexity is reflected in behavioral studies showing

that LHb photo-activation of some fibers from the VTA evokes reward

(Stamatakis et al., 2013), while activation of others evokes aversion

(Hennigan et al., 2015; Root et al., 2014a). The LHb synaptic

complexity provided by VTA is not surprising, as multiple streams of in-

formation may be encoded and decoded by the different types of VTA

cells and their corresponding efferent to downstream brain targets.

observed in marijuana abusers was associated with apathy and

amotivation (Bloomfield et al., 2014).

The regional brain activation responses to a stimulant drug

also differ between controls and cocaine abusers in ventral pre-

frontal regions. In control subjects, intravenous stimulant admin-

istration decreased the activity of ventral medial frontal regions

(OFC and ventral ACC), whereas in cocaine abusers, it activated

these regions, which are involved in salience attribution and con-

ditioning (Dosenbach et al., 2006; O’Doherty et al., 2001; Shack-

man et al., 2011). Activation of the OFC in cocaine abusers was

associated with craving (Volkow et al., 2005). In contrast, activity

in the right inferior frontal region Ba 44, a key brain region

involved in inhibitory control (Aron et al., 2004), was associated

with the deactivation of the NAc and ventral PFC upon success-

ful control of cocaine craving (Volkow et al., 2010). This pattern of

responses uncovers distinct contributions of PFC regions to

addiction on the basis of their striatal projections: dlPFC and

inferior frontal regions that project to the dorsal caudate facilitate

self-control, whereas ventral PFC regions projecting to NAc facil-

itate drug taking (Goldstein and Volkow, 2011). This is also

consistent with preclinical findings that identified distinct contri-

butions of prelimbic mPFC (PL) and infralimbic mPFC (IL) to

cocaine seeking in rats (review in Bossert et al., 2013). Studies

using the reinstatement model of relapse found that, after extinc-

718 Cell 162, August 13, 2015 ª2015 Elsevier Inc.

tion of cocaine self-administration, PL activity promoted cocaine

seeking while IL activity inhibited it (Peters et al., 2008). Impor-

tantly, in the incubation of the cocaine-craving model (response

to cocaine cues progressively increases with time after with-

drawal), reversible inactivation of IL, but not PL, decreased

‘‘incubated’’ cue-induced cocaine seeking after prolonged with-

drawal, while pharmacological activation of IL, but not PL,

increased cocaine seeking during early withdrawal (Koya et al.,

2009). However, in the same animal model, optogenetic inhibi-

tion of PL neurons (projecting to NAc core) that previously under-

went a specific form of cocaine-induced synaptic plasticity

(recruitment of silent synapse) decreased incubation of craving,

while the opposite effect was observed following inhibition of the

IL projection to NAc shell (Di Forti et al., 2014). In clear contrast,

in a punishment-induced suppression model of ‘‘compulsive’’

cocaine seeking, in which most rats suppressed cocaine self-

administration by shock punishment while a few did not (punish-

ment-resistant ‘‘compulsive’’ rats), optogenetic stimulation of PL

inhibited cocaine seeking in punishment-resistant rats while op-

togenetic inhibition increased it (Chen et al., 2013). However, in

the same model, excitotoxic lesions of the PL or IL had no

detectable effect on cocaine seeking in punishment-resistant

rats (Pelloux et al., 2013). Taken together, the PL and IL appear

to play different and complex roles in cocaine-seeking behaviors

in rat addiction models, which are highly dependent on the

particular behavior being assessed and the experimental proce-

dure used to manipulate local neuronal activity. The results un-

derscore the complexity of the neuroplasticity within the mPFC

circuitry, a multimodal brain structure involved in the orchestra-

tion of diverse behaviors.

The regional brain responses to drug-associated cues have

also been investigated with neuroimaging. These studies have

shown that, in cocaine abusers, exposure to cocaine cues trig-

gers DA release in the dorsal striatum and that this effect was

associated with craving (Volkow et al., 2006b; Wong et al.,

2006). Interestingly, similar increases were not observed in to-

bacco smokers upon exposure to nicotine cues (Chiuccariello

et al., 2013), whereas exposure to alcohol cues in healthy con-

trols resulted in DA decreases (Yoder et al., 2009). Studies with

fMRI have shown, more or less consistently, that exposure to

cues in substance abusers is associated with increased activa-

tion of NAc and VTA (Goudriaan et al., 2013), which most likely

reflects not only cue-induced DA increases, but also excitatory

stimulation stemming from glutamatergic terminals into NAc

and midbrain. These studies have also identified several other

regions that are co-activated during cue exposure, including

the PFC, cerebellum, limbic regions, and insula (Jasinska et al.,

2014). Activation of the insula is noteworthy since this region is

involved in interoceptive awareness, contributes to the

conscious awareness of drug craving, and is part of the default

mode network (DMN) that enables internal mind wandering,

perhaps facilitating rumination about drug use in addicted sub-

jects (Naqvi and Bechara, 2010).

Neuronal Circuitry in Addiction The use of imaging tools to study changes in the brains of in-

dividuals suffering from addictions has helped to identify the

brain regions and associated circuits that are disrupted and

Figure 5. Neuronal Circuitry of Addiction Proposed neuronal network of interacting brain regions and associated cir- cuits that are disrupted in addicted individuals. Changes occur in reward (NAc and VTA), conditioning/memory (amygdala and medial OFC for emotional and salience attribution; hippocampus and dorsal striatum for memories and habits), executive control (ACC, inferior PFC, dlPFC, and lateral OFC), moti- vation/drive (medial OFC for attribution of salience, ventral ACC, VTA, dorsal striatum, NAc), interoception (insula and ACC), and aversion/avoidance (habenula). This model proposes that, during addiction, the enhanced expectation value of the drug in the reward, motivation, and memory circuits overrides the control circuits, favoring a positive-feedback loop initiated by consumption of the drug and perpetuated by the enhanced activation of the motivation/drive and memory circuits. These circuits also interact with those involved in mood regulation, including stress reactivity (which includes participation of the extended amygdala, hypothalamus, and habenula) and interoception (which includes participation of the insula, ACC, and the default mode network [DMN] and contributes to a heightened awareness of craving). NAc, nucleus accumbens; VTA, ventral tegmental area; PFC, prefrontal cortex; OFC, orbitofrontal cortex; ACC, anterior cingulate cortex; dlPFC, dorsolateral PFC.

Box 5. Translational Opportunities

d Enhance tonic dopaminergic D2R signaling through the indirect

pathway to improve control. Pharmacologically, this is challenging

because currently available D2R agonists (or partial agonists) also

bind to D3R, and stimulation of D3R has been associated with

impaired impulse-control disorders (Seeman, 2015).

d Enhance function of prefrontal regions involved in executive func-

tion, including self-control via transcranial magnetic or electrical

stimulation, mindfulness, or other behavioral interventions, and

through medications that increase DA signaling in prefrontal re-

gions (i.e., tomoxetine, oral stimulants, modafinil).

d Decrease the reactivity of stress-associated circuits (extended

amygdala, habenula) through the use of biofeedback or medica-

tions (CRF or kappa antagonists)

d Decrease the motivation value of conditioned responses to drug

cues (by targeting PFC, amygdala, hippocampus) through the

use of behavioral extinction interventions, including coupling inter-

ventions with medications (i.e., d-cycloserine).

d Reduce dysphoria and enhance hedonic responses to non-drug

rewards during withdrawal and drug discontinuation though the

use of cognitive behavioral interventions or medications.

understand how these changes influence behaviors associated

with the addiction phenotype. These studies have revealed

changes in reward and motivation, resulting in increased moti-

vation toward drugs and drug cues and in decreased motiva-

tion for non-drug reward and cues, executive control, resulting

in reduced ability to control the urge to take the drug, triggered

by cues, emotional states, or an impaired ability to delay grat-

ification, as well as mood and interoceptive circuits, resulting

in an enhanced sensitivity to stressors and dysphoria or the

so-called ‘‘dark side of addiction’’ (Box 4 and Figure 5; Koob,

2013).

In this model of addiction, the motivation to take the drug is not

only driven by conditioned responses to cues, but also by nega-

tive emotional states. The exposure to drug cues results in the

activation of glutamatergic projections from the ventral PFC,

the ventral hippocampus, and the amygdala (and presumably

medial thalamus) to striatal projections that increase DA

signaling and release in the NAc and dorsal striatum. The

enhanced craving and desire for drug taking will eventually

lead to drug consumption, and although the drug-induced DA in-

creases are markedly attenuated in the NAc, in particular of

cocaine abusers and alcoholics, they are sufficient to enhance

the craving and to sustain the drive to continue taking the drug

(Figure 4), perhaps by stimulation of upregulated D3R that facil-

itate transmission through the D1R direct pathway (Fiorentini

et al., 2010). The ventromedial PFC (including OFC and ventral

ACC) in drug-addicted individuals, which in the absence of

drug or drug cues is hypofunctional, becomes hyperactive

when exposed to drugs or cues, enhancing reward salience

calculation through its involvement in the processing of the

outcome value of that reward (Volkow et al., 1996).

The control circuit, which relies on the PFC, including ACC,

lateral OFC, dlPFC, and the inferior frontal cortex (BA 44), is dis-

rupted in addicted individuals. The reduction in D2R signaling in

the striatum leads to reduced activity in these PFC regions

(Figure 3), which is necessary for proper control, planning, and

flexibility of behaviors and for delaying gratification (Volkow

and Baler, 2015). Specifically, the OFC is critically involved in

salience attribution; its main contribution is to offer predictive in-

formation about alternative options and outcomes, which is

essential for shifting behaviors when the reward is no longer re-

inforcing. Meanwhile, the ACC enables inhibitory learning by

keeping tabs on conflicts between predicted and actual out-

comes and by conveying that information to inhibitory circuits

orchestrated by the dlPFC, the inferior frontal regions (BA 44),

and the dorsal caudate. This inhibitory arm of the decision-mak-

ing process is facilitated, in turn, by tonic DA signaling sustaining

activity in ACC, dlPFC, and inferior PFC. Together with the OFC,

the insular cortex and the ACC also allow the circuit to estimate

the level of uncertainty involved in choosing among alternative

options. Finally, the extended amygdala, insula, and lateral ha-

benula, which provide information about emotional salience,

interoceptive awareness, and pertinent reward omission events,

respectively, contribute to dysphoria, anhedonia, and the

enhanced stress reactivity that follows drug withdrawal (Koob

and Le Moal, 2005).

In consequence, the addicted individual experiences

enhanced reactivity to drug cues and to stressful stimuli, the

reactivity to natural reward is decreased, and there is loss of flex-

ibility to adjust the saliency value of reward as a function of their

context. Although the neuronal adaptations that follow repeated

Cell 162, August 13, 2015 ª2015 Elsevier Inc. 719

Box 6. Perspectives in Reward Circuitry and Addiction

Identification of DA neuron subtypes in VTA and SN and character-

ization of their projections, inputs, and function.

Investigation of interactions between the nuclei and circuits medi-

ating reward and those involved with mood regulation, including

those with the dorsal raphe.

Investigation of the dynamic interactions between coordinated

pathways (for instance, the direct and indirect MSNs pathways)

in drug reward and in compulsive drug taking.

Investigation of the neurocircuitry that drives ‘‘antireward’’ and the

dark side of addiction (Koob et al., 2014), including the role of the

habenula and kappa/dynorphin signaling.

Investigation of how genes influence the molecular biology and

neurocircuitry that underlies individual heterogeneity in vulnera-

bility and resilience to addiction.

Identification of biomarkers that can be used to personalize pre-

vention and therapeutic interventions in substance use disorders.

drug exposures are not fully understood, the picture that is

emerging of the drug-induced neuronal impairments is already

helping us to think about interventions that could remediate

them through the development of medications/immunother-

apies, magnetic or electrical stimulation strategies, and/or

behavioral approaches (Box 5). The development of such inter-

ventions will benefit from an understanding of the specific cir-

cuitry or functional processes being targeted, rather than using

abstinence as the only beneficial outcome in addiction treat-

ment. For example, interventions designed to counteract

dysphoria or strengthen executive control, even if not resulting

in complete abstinence, may improve long-term success and

recovery from addiction. In parallel, these neurobiological ad-

vances have begun to reveal the molecular and neuronal bases

underlying the heterogeneity of the clinical presentation and

the outcomes of addicted individuals (Box 6). In the near future,

these advances might enable the development of tailored thera-

peutic interventions on the basis of the specific molecular targets

and/or circuits disrupted in a given individual.

In conclusion, uncovering the neurobiology underlying drug

abuse has led to the recognition of addiction as a chronic dis-

ease of the brain. At the same time, these advances have

revealed potential targets for interventions that could usher in

a new era of more effective and personalized addiction

treatments.

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