Illicit dopamine transients: Reconcilingactions of abused drugsDan P. Covey1, Mitchell F. Roitman2, and Paul A. Garris1

1 School of Biological Sciences, Illinois State University, Normal, IL 61790-4120, USA2 Department of Psychology, University of Illinois at Chicago, Chicago, IL 60607-7137, USA

Opinion

Glossary

Dopamine transporter (DAT): DAT is a plasma membrane protein that is a

member of the SLC6 (solute carrier 6) gene family of Na+/Cl–-dependent

transporters and terminates dopamine signaling by clearing extracellular

dopamine released by exocytosis [104]. Uptake via DAT also contributes to

recycling dopamine because newly re-uptaken dopamine is repackaged into

vesicles by the vesicular monoamine transporter for re-release. DAT-Is target

DAT and have been traditionally thought to exert their effects on drug

reinforcement by inhibiting dopamine uptake.

Psychostimulants: a class of drugs that increase psychomotor activity and

exhibit antidepressant effects, and act by increasing monoamine (e.g.,

dopamine, norepinephrine, and serotonin) levels in the brain [105]. Popular

psychostimulants, such as cocaine, amphetamine, methamphetamine, and

ecstasy, produce feelings of euphoria, relief from fatigue, improved perfor-

mance on some simple tasks, and anorexia.

Drug reinforcement: in general, reinforcement (positive or negative) refers to

an increase in behavior directed toward a particular outcome [106]. Similarly to

natural rewards, abused drugs promote positive reinforcement. Drug reinfor-

cement is demonstrated by the paradigm of drug self-administration, in which

delivery of abused drugs contingent on an instrumental response (e.g., lever

pressing or nose poking) increases the likelihood that the response is made.

Conditioned stimulus: a conditioned stimulus (e.g., a tone or light) is

predictively and temporally associated with an unconditioned stimulus (e.g.,

natural reward or abused drug). Although initially neutral and evoking no

innate response, after associative learning the conditioned stimulus evokes a

conditioned response similar to that evoked by the unconditioned stimulus.

Drug-conditioned stimuli come to elicit approach behavior and support the

maintenance of drug taking. They also can act as conditioned reinforcers

driving instrumental responding [20] and are crucial for the reinstatement of

drug seeking and taking even after long periods of abstinence [6].

Fast-scan cyclic voltammetry (FSCV): an electrochemical, microsensor-based

approach for temporally, spatially, and chemically resolving neurochemicals in

situ [10]. The typical microsensor is the carbon-fiber microelectrode (CFM). For

dopamine measurements using FSCV in awake animals, the potential of the

Phasic increases in brain dopamine are required for cue-directed reward seeking. Although compelling withinthe framework of appetitive behavior, the view that illicitdrugs hijack reward circuits by hyperactivating thesedopamine transients is inconsistent with establishedpsychostimulant pharmacology. However, recent workreclassifying amphetamine (AMPH), cocaine, and otheraddictive dopamine-transporter inhibitors (DAT-Is) sup-ports transient hyperactivation as a unifying hypothesisof abused drugs. We argue here that reclassification alsoidentifies generating burst firing by dopamine neuronsas a keystone action. Unlike natural rewards, which areprocessed by sensory systems, drugs act directly on thebrain. Consequently, to mimic natural rewards and ex-ploit reward circuits, dopamine transients must be eli-cited de novo. Of available drug targets, only burst firingachieves this essential outcome.

Dopamine, psychostimulants, and reinforcementA long-held tenet in the pharmacology of abused drugs isthat, despite marked differences in cellular targets, allclasses of these substances increase brain levels of extra-cellular dopamine [1]. Drug-induced dopamine elevationsoccur to the greatest extent in the nucleus accumbens(NAc), a brain region that is crucial for translating moti-vational input into behavioral output [2,3]. This sharedoutcome of a hyper-dopamine state is thought to mediatethe initial reinforcing properties of abused drugs (Box 1),the general focus of this Opinion article. Not unexpectedly,extensive work has been directed at refining this generalscheme, and abused drugs have been classified on the basisof specific mechanisms for targeting dopamine neurons[4,5]. Moreover, there is an emergent hypothesis thatabused drugs hijack reward circuits by hyperactivatingextracellular phasic (�1–2 s) signals called dopamine tran-sients [6,7]. Although attractive with regard to the proces-sing of natural rewards by phasic dopamine signaling inappetitive behavior, this hypothesis is inconsistent withcurrently accepted mechanisms for how addictive DAT-Is,including AMPH, methamphetamine, cocaine, and meth-ylphenidate (Ritalin1), act on dopamine neurons.

0166-2236/$ – see front matter

� 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tins.2014.02.002

Corresponding author: Garris, P.A. (pagarri@ilstu.edu).

200 Trends in Neurosciences, April 2014, Vol. 37, No. 4

In this Opinion we highlight recent work calling forreclassifying these psychostimulants (see Glossary). Weargue that this reclassification reconciles dopamine theo-ries of appetitive behavior and the hijacking of rewardcircuits by abused drugs with a mechanistic understandingof psychostimulant action on dopamine neurons. We beginby summarizing the role of phasic dopamine signaling inappetitive behavior, the emergent hypothesis that abuseddrugs usurp this process, and the traditional view of drugaction on dopamine neurons. On the basis of reclassifyingDAT-Is, and by virtue of eliciting dopamine transients denovo, we then argue that generating burst firing by dopa-mine neurons is the keystone action by which abused drugshijack reward circuits.

CFM is linearly scanned from –0.4 to +1.3 V and back at regular 10 Hz intervals.

FSCV is so named because the potential sweep is cyclical and made at high

rates (e.g., 400 V/s). Dopamine is oxidized to dopamine-o-quinone at �+0.65 V

during the positive sweep, and this is reduced back to dopamine at �–0.2 V

during the negative sweep. The relationship between applied potential and

measured current, called a voltammogram, serves as a chemical signature to

identify the detected species. Chemical specificity of FSCV is improved by

chemometrics called principle component regression.

Box 1. Drug addiction

Drug addiction is ultimately characterized by compulsive drug

seeking and taking despite negative consequences and relapse

following periods of abstinence [6,86,87]. The transition to addiction

begins with goal-directed drug use that is reinforced by rewarding,

often hedonic, drug effects. Later stages in the transition to addiction

are characterized by an escalation in drug use and difficulty limiting

drug intake (i.e., drug abuse). Such behaviors progress to compul-

sive drug seeking and taking in a subset of susceptible individuals

following extended drug abuse [86]. Because relapse is prone to

occur even following extended periods of drug abstinence, and long

after withdrawal symptoms have subsided, addiction is hypothe-

sized to represent a disorder of learning and memory [6] arising from

drug-induced neuroadaptations in brain circuits controlling moti-

vated behavior [88]. Drug-induced alterations in the dopamine

reward circuit are crucial for the transition through each stage of

the addiction process. The initial reinforcing effects of abused drugs

are dependent on these substances targeting midbrain dopamine

neurons originating in the ventral tegmental area (VTA) and

projecting to the nucleus accumbens (NAc) [88,89]. Acute drug

exposure acts preferentially on dopamine neurons innervating the

medial shell of the NAc [28], and this increases extracellular

dopamine here to a greater extent than in the NAc core and dorsal

striatum [1,28,90]. Drug-induced dopamine elevations in the NAc

shell support the behavior-invigorating or motivational effects of

abused drugs, particularly the psychostimulants [91,92]. Acute drug

exposure also elicits long-term potentiation at glutamatergic sy-

napses onto dopamine neurons [79,80,93]. This drug-induced

strengthening of excitatory input may increase the incidence of

burst firing [94] and support the progressive manifestation of

synaptic plasticity in striatal regions that occurs following repeated

drug exposure and which acts to strengthen drug-seeking behaviors

[95,96]. Reciprocal feedback between the striatum and midbrain

dopamine neurons [97] also results in a ventromedial- to dorsolat-

eral-directed progression in the primary striatal region controlling

behavior following chronic drug exposure [91,98,99]. This process

begins with the initial activation of dopamine neurons projecting to

the NAc shell, which projects back to and recruits dopamine neurons

innervating the NAc core. Dopamine input to the NAc core is

particularly important for associating drug rewards with discrete

cues and for these cues to motivate drug seeking [87,100]. These

cue–drug associations are also crucial for the maintenance and

escalation of drug intake, and for driving relapse [87,101]. This

‘spiraling’ feedback loop continues following prolonged drug intake

such that dopamine neurons projecting to the dorsolateral striatum

gain greater control, which supports drug seeking and taking

transitioning from a behavior that is goal-directed to one that is

ultimately habitual and compulsive [87,91]. It should be noted that,

although dopamine input to striatal regions is crucial to the addiction

process, numerous other brain regions and neurotransmitter

systems are clearly necessary for addiction to manifest [86,102,103].

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

Phasic dopamine signaling plays a crucial role inappetitive behaviorIntrinsic properties coupled with converging input fromnumerous excitatory and inhibitory afferents enable dopa-mine neurons to signal in two general modes: tonic andphasic [8–11]. During tonic dopamine signaling, slow andirregular firing contributes to a low ambient level of extra-cellular dopamine that binds to high-affinity D2 dopaminereceptors and supports movement, cognition, and motiva-tion. By contrast, during phasic dopamine signaling, rapidand synchronous burst firing elicits dopamine concentra-tion spikes called transients (Figure 1A,B) that activatelow-affinity D1 dopamine receptors. These transients aremonitored with fast-scan cyclic voltammetry (FSCV) andare faithfully reproduced by electrical stimulation, whichfosters analysis of underlying mechanisms for dopaminerelease and uptake (Box 2). Similarly to burst firing bydopamine neurons, natural rewards evoke dopamine tran-sients that are transferred to predictive cues followingassociative learning (Figure 1C,D). The conditionedtransfer highlights the functional link between thesetwo components of phasic dopamine signaling, wheresomatodendritic burst firing elicits dopamine transientsin terminal fields. Dopamine transients also occur ‘spon-taneously’, in other words in the absence of overt environ-mental stimuli (Figure 1E, left), and are pharmacologicallyevoked by abused drugs (Figure 1E, right), the specificfocus of this Opinion article.

Compelling evidence obtained from monitoring burstfiring by dopamine neurons [12,13] and dopamine transi-ents [14,15] supports a crucial role for phasic dopaminesignaling in appetitive behavior by encoding key attributesof natural rewards, such as timing, cost, magnitude, prob-ability, and uncertainty. Dopamine transients also exhibitthe requisite temporal precision and amplitude to promotethe plasticity of corticostriatal synapses that is associatedwith reward learning [16,17]. At least two general, not

necessarily mutually exclusive, conceptual models haveemerged to integrate these phenomena. First, phasic do-pamine signaling serves a teaching function in reinforce-ment learning by providing a ‘reward prediction error’describing the difference between expected and receivedreward [11]. In this manner, unexpected or greater thanexpected rewards phasically increase dopamine and rein-force behavior, expected rewards cause no change in dopa-mine and behavior, and absent or worse than expectedrewards phasically decrease dopamine and suppress be-havior (but see [18]). Second, phasic dopamine signalingattributes ‘incentive salience’ or ‘wanting’ to reward pre-dicting cues, which underlies their ability to motivatebehaviors directed toward obtaining rewards and to actas conditioned reinforcers [19,20]. Consistent with boththeories, recent work using transgenic and optogeneticapproaches for selectively manipulating neuronal activityindicate that phasic dopamine signaling is necessary andsufficient for forming cue–reward associations and for cue-directed reward seeking [21–24].

Abused drugs hijack reward circuits by hyperactivatingdopamine transientsAn emergent hypothesis is that abused drugs activate do-pamine transients to a greater degree than natural rewards,leading to overvaluation of cues predicting drug availability[6,7]. Indeed, abused drugs from broad classes, includingethanol, cocaine, nicotine, and cannabinoids, have now beendemonstrated to augment dopamine transients (Figure 1E,right) [25–28]. Although drug-evoked dopamine transientsresemble those occurring naturally [29], abused drugs evokea quantitatively greater response. The robust nature of thisactivation is strikingly demonstrated during drug self-ad-ministration, which emulates voluntary drug taking byhumans. Indeed, transient frequency is increased �10-foldfor the duration of repeated cocaine injections [30,31]. Theseeffects are considered to be pharmacological in nature and

201

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TRENDS in Neurosciences

Figure 1. Phasic dopamine signaling. (A) Electrophysiological recoding of an identified dopamine neuron in vivo. (Left) The recorded neuron was labeled with a neurobiotin

tracer (blue) and identified with a green fluorescent Nissl stain (green). The neurochemical phenotype was confirmed by labeling with an antibody against tyrosine

hydroxylase (TH, red). (Right) The dopamine neuron fired in a bursting pattern (outlined box). (B) Extracellular phasic dopamine signals recorded with FSCV at a CFM. (Left)

Dopamine transients evoked by an unpredicted food reward (‘Pellet’) at time 0 s. (Right) Transient-like signal evoked by brief (0.4 s) electrical stimulation (‘Stim’) applied to

dopamine axons at time 0 s. (Top) Color plots display sequential voltammograms indicating that dopamine is evoked by the stimulation and food reward (measured current

in color, z axis; applied voltage, y axis; time, x axis). (Bottom) Current measured by the CFM at the peak oxidation potential for dopamine (i.e., dopamine current) versus

time. (Inset) Individual voltammograms also identify the signal evoked by stimulation and food reward as dopamine. (C) (Left) Burst firing by dopamine neurons in

response to an unpredicted juice reward. (Right) Burst firing by dopamine neurons transfers to the reward-predicting conditioned stimulus once the cue–reward

contingency is learned. Each panel shows the peri-event time histogram (top) and raster plot (bottom) of neuronal activity from the same neuron. (D) Dopamine transients

measured by FSCV in response to food reward (unconditioned stimulus) and a predictive cue (conditioned stimulus) during Pavlovian conditioning. Heat map shows the

transfer of dopamine transients elicited by the food reward to the conditioned stimulus. (E) Drug-induced activation of dopamine transients measured by FSCV in awake

animals recorded by FSCV at a CFM. Recordings reflect fluctuations in dopamine concentration versus time. Dopamine transients (identified by asterisks) recorded before

(left) and after (right) administration of ethanol (2 g/kg, i.v.), cocaine (0.33 mg, i.v.), or AMPH (1 mg/kg, i.p.). Reproduced with permission from [76] (A), [36] (B), [107] (C),

[108] (D), and [27,31,42] (E). Abbreviations: Eapp(V), applied voltage; AMPH, amphetamine; CFM, carbon-fiber microelectrode; CS, conditioned stimulus; FSCV, fast-scan

cyclic voltammetry; i.p., intraperitoneal; i.v, intravenous; R, reward.

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

mediated by central drug actions [32] (but see [33]). Thus,unlike natural rewards, which are processed by sensorysystems and afferent input to dopamine neurons, and whoseneuronal responses are subject to modification during asso-ciative learning, abused drugs act directly on the brain[7,11]. However, cues predicting cocaine delivery also elicitdopamine transients [30,34,35] in a similar manner to cuespredicting food reward [19,36], which reflects learned asso-ciations and non-pharmacological effects. Thus, althoughnatural rewards and abused drugs both activate phasicdopamine signaling, qualitative and quantitative aspectsof this activation differ.

Distinct actions of abused drugs on phasic dopaminesignaling are thought to drive aberrant learning of cue–drug associations, leading to the hijacking of reward cir-cuits. For example, the sheer number of pharmacologically

202

evoked dopamine transients should increase the probabili-ty of learned associations between drug taking and envi-ronmental stimuli [7]. The robust drug-induced increase inphasic dopamine signaling should also confer to abuseddrugs a higher reward magnitude compared to naturalrewards, resulting in cue-evoked dopamine transients withcorrespondingly greater amplitude [7,13,14]. Additionally,persistent positive prediction errors should be produced byabused drugs directly targeting the brain and reliably androbustly eliciting dopamine transients even if drug deliv-ery is expected [37,38]. Consistent with aberrant rewardlearning, drug-paired cues maintain cocaine seeking in theabsence of cocaine delivery for up to 1 year after only asingle session of cocaine self-administration, which con-trasts sharply with responding to cues previously pairedwith a highly palatable food reward – that extinguishes

Box 2. Analysis of dopamine transients

Dopamine transients are characterized by amplitude, duration (i.e.,

width at half amplitude), and frequency (i.e., inverse of inter-transient

interval or ITI) (Figure I, left) [10]. These descriptive measures are not

fundamental, but rather reflect burst firing of dopamine neurons,

vesicular dopamine release, and dopamine uptake. Unfortunately,

interactions between these three neural mechanisms preclude

definitive assignment to changes in dopamine transients. However,

insight into the mechanism is provided by independent assessment

of burst firing monitored by electrophysiology (see Figure 1A in main

text), and by dopamine release and uptake from electrically evoked

phasic dopamine signals monitored by FSCV (Figure I, right). This

latter analysis resolves the respective contributions of dopamine

release and uptake by fitting evoked phasic signals to equations that

describe the rising phase as a balance between release and uptake,

and ascribe the falling phase to uptake [10]. As described in the text,

considerable evidence suggests that the drug-induced increases in

burst firing by dopamine neurons and the frequency of dopamine

transients are tightly associated. Moreover, direct comparisons of

electrically evoked phasic signals and dopamine transients suggest

that transient amplitude is relatively insensitive to dopamine uptake

but highly dependent on dopamine release, although transient

duration is more sensitive to dopamine uptake than to release [109].

In excellent agreement, cocaine- and AMPH-induced increases in the

amplitude of electrically evoked phasic signals better correlate with

upregulated dopamine release than with inhibited dopamine uptake

[42,56,57,60].

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TRENDS in Neurosciences

Figure I. Quantifying dopamine signaling with FSCV. Spontaneous dopamine transients (Left) and electrically-evoked dopamine (Right) recorded with FSCV in the NAc

of an awake rat. Color plots below each recording display sequential voltammograms identifying dopamine as the measured analyte (See Figure 1 for details of the

color plot). Abbreviations: E(V), applied potential in volts.

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

within 3 months [39]. Although hyperactivation of dopa-mine transients usurping reward circuits thus fits wellwith dopamine theory of appetitive behavior, this hypoth-esis is not supported by established psychostimulant phar-macology. In the next section we summarize the traditionalview of drug action on dopamine neurons and identify keydiscrepancies for addictive DAT-Is.

Actions of abused drugs on dopamine neurons:traditional viewAbused drugs have traditionally been classified on the basisof three functional targets on dopamine neurons: firing ofaction potentials, vesicular dopamine release, and dopa-mine uptake [4,5]. Activation of each target is thought toincrease brain levels of extracellular dopamine [1]. In gen-eral, (i) ethanol, nicotine, cannabinoids, and opiates increaseburst firing by dopamine neurons; (ii) nicotine and opiatesupregulate vesicular dopamine release; (iii) cocaine- andAMPH-like psychostimulants inhibit dopamine uptake(Figure 2A; for details of these mechanisms see the figure

legend and Box 3). However, important mechanistic differ-ences exist for these subclasses of addictive DAT-Is. Forexample, cocaine-like blockers bind to and allostericallyinhibit DAT. By contrast, AMPH-like releasers are sub-strates of DAT and reverse its function, releasing intracel-lular dopamine into the extracellular space independently ofaction potentials. This reverse dopamine transport or effluxis driven by AMPH redistributing dopamine from vesicularto cytosolic compartments, which also disrupts exocytoticdopamine release. Both cocaine- and AMPH-like DAT-Isadditionally suppress dopamine cell firing by elevatingextracellular dopamine that activates somatodendritic D2dopamine autoreceptors.

Although consistent with elevated brain dopaminelevels as a shared action of abused drugs, the traditionalview of drug action does not account for the effects ofaddictive DAT-Is on phasic dopamine signaling. For exam-ple, cocaine augments the frequency, amplitude, and du-ration of dopamine transients [10,30,31,40]. Inhibition ofuptake should mediate increased transient duration.

203

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TRENDS in Neurosciences

Figure 2. Actions of abused drugs on dopamine neurons. This figure summarizes the actions of abused drugs. It is important to note that these actions may differ across

heterogenous subsets of midbrain dopamine neurons (see Boxes 3 and 4). Facilitation and inhibition are indicated by ‘+’ and ‘–’, respectively. (A) Traditional model. (1)

Nicotine and ethanol enhance burst firing by dopamine neurons via enhancing excitatory glutamatergic drive [75,78]. (2) Nicotine [75] and ethanol [77] share with opiates

and cannabinoids [110] the ability to disinhibit firing by reducing GABAergic input. Nicotine also activates firing directly via nicotinic acetylcholine receptors on dopamine

neurons [74] (not shown). (3) By contrast, both AMPH- and cocaine-like DAT-Is suppress firing by elevating extracellular dopamine that activates somatodendritic D2

dopamine autoreceptors [4]. (4) At dopamine terminals, nicotine and opiates upregulate vesicular dopamine release. Nicotine mobilizes the reserve pool of dopamine

vesicles to the readily releasable pool [111] and shares with opiates the ability to increase the amplitude of phasic relative to tonic dopamine signals [112,113]. (5) Cocaine

inhibits dopamine uptake by blocking DAT [104]. (6) As a DAT substrate, AMPH enters the dopamine terminal to deplete vesicular dopamine stores and promote DAT-

mediated reverse dopamine transport [5]. (B) New model. The new model of drug action on dopamine neurons extends the old model described in (A) by reclassifying DAT-

Is. Actions proposed for other abused drugs and for DAT-Is inhibiting dopamine uptake are thus not changed in the new model and are shaded. The new classification of

DAT-Is is only briefly described here. Details and supporting references are given in the text. (1) Cocaine and AMPH directly and indirectly activate burst firing by dopamine

neurons by enhancing noradrenergic input. Cocaine increases burst firing by (2) enhancing glutamatergic input via presynaptic D1 dopamine receptors and (3) inhibiting

GABAergic input. (4) By acting as DAT substrates, AMPH and its analog methamphetamine directly depolarize dopamine neurons. (5) AMPH and cocaine upregulate

vesicular dopamine release. (6) AMPH-induced dopamine efflux is modest, suggesting that this action potential-independent mechanism is not the primary AMPH target for

activating dopamine signaling. Abbreviations: AMPH, amphetamine; DA, dopamine; DAT, dopamine transporter; GLUT, glutamate; NE, norepinephrine.

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

However, it is difficult to reconcile the autoreceptor-medi-ated suppression of dopamine cell firing with robustincreases in transient frequency [30,31,40]. Moreover,increases in transient amplitude suggest actions besidesinhibition of dopamine uptake [28,32]. An even more promi-nent discrepancy exists for AMPH. This psychostimulant

204

should disrupt phasic dopamine signaling by depletingvesicular dopamine stores and impairing action potential-dependent exocytotic dopamine release according to itshistoric mechanism. Nevertheless, genetic disruption ofnorepinephrine synthesis supports AMPH-induced afferentactivation of dopamine neurons [41], and recent work with

Box 3. Generation of burst firing: nicotine and ethanol

Nicotine and ethanol, which unlike the cocaine- and AMPH-like

psychostimulants do not inhibit dopamine uptake, have been

extensively investigated for their ability to generate burst firing by

dopamine neurons. Indeed, pharmacological activation of burst

firing is essential for nicotine [76,114] and ethanol [77] to exert their

reinforcing properties. Nicotine activates dopamine cell bodies via

nicotinic acetylcholine receptors (nAChRs) directly [74,76,114], and

indirectly via glutamatergic [75] and GABAergic [76] inputs, result-

ing in an overall facilitation of burst firing. Similarly to nicotine,

ethanol elicits burst firing by activating nAChRs on dopamine cell

bodies [115], although this occurs indirectly via facilitation of

presynaptic cholinergic input. Ethanol also increases burst firing

by elevating excitatory glutamatergic drive [77] via actions on

presynaptic D1 dopamine receptors [78], and by decreasing

inhibitory GABAergic input [77] via actions on presynaptic opioid

receptors [78]. Several brain regions provide afferent control of

dopamine neurons to regulate drug seeking and taking

[110,116,117]. Well-established excitatory inputs originate from the

lateral dorsal tegmentum and pedunculopontine nucleus, which

contribute both glutamatergic and cholinergic input, and the medial

prefrontal cortex and lateral hypothalamus, which predominantly

contribute glutamatergic input. Crucial GABAergic inputs arise from

the ventral pallidum, lateral habenula, bed nucleus of the stria

terminalis, rostromedial tegmental nucleus, and from local inter-

neurons. It should be noted, however, that the number of afferent

regions regulating dopamine neurons appears to be much greater

than previously thought [118]. Moreover, midbrain dopamine

neurons are heterogeneous in terms of firing rate, autoregulatory

control, and projection target [119,120]. Functional heterogeneity is

also apparent in that anatomically distinct populations of dopamine

neurons appear to encode either rewarding stimuli, aversive stimuli,

or both [93,121]. It therefore appears that, for abused drugs to

reinforce behavior by generating burst firing of dopamine neurons,

these substances must selectively activate subpopulations of

dopamine neurons – specifically, the reward-encoding versus

aversion-encoding neurons. Although this appears to be the case

at least for cocaine [93], the neural mechanisms that mediate this

selective activation remain to be determined.

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

FSCV demonstrates that AMPH acts like cocaine and ro-bustly increases both the frequency and amplitude of dopa-mine transients (Figure 1E) [42]. Although bringing AMPHinto the fold further supports hijacking of reward circuits byhyperactivating dopamine transients as a unifying hypoth-esis of abused drugs, it is clear that established psychosti-mulant pharmacology is inconsistent with this hypothesis.It is thus important to revisit the traditional view of drugaction from the perspective of phasic dopamine signaling. Aswe describe in the next section, new evidence calls for a

Table 1. Reclassifying addictive DAT-Is: targets on dopamineneurons in addition to dopamine uptake

Targets DAT-I Refs

Exocytotic dopamine release Cocaine

AMPH

Methylphenidate

[42,53–60]

Trafficking of dopamine vesicles Cocaine

AMPH

Methamphetamine

Methylphenidate

[63–67]

Cell firing Cocaine

AMPH

Methamphetamine

Methylphenidate

[43–45,52]

Cell excitability AMPH

Methamphetamine

[47,51,52]

reclassification of addictive DAT-Is that is congruent withactivation of dopamine transients (Figure 2B; Table 1).

Actions of abused drugs on dopamine neurons: newview(i) Abused drugs generate burst firing by dopamine

neuronsWe argue here that, similarly to other abused drugs,addictive DAT-Is generate burst firing by dopamineneurons. This postulate is supported by recent evidencedemonstrating that cocaine activates burst firing bydopamine neurons in awake animals but suppressesfiring in anesthetized animals [43]. Thus, suppressionof dopamine cell firing does not appear to be thedominate action of addictive DAT-Is in awake animals,indicating that other drug effects overcome inhibitionby somatodendritic autoreceptors. In excellent agree-ment, several addictive DAT-Is, including cocaine,methylphenidate, AMPH, and methamphetamine,robustly enhance bursting firing by dopamine neuronsin anesthetized animals when administered in thepresence of raclopride to block dopamine autoreceptors[44,45]. Coadministration of cocaine and raclopridealso increases the frequency of dopamine transients inanesthetized animals [46], further linking theseextracellular phasic signals to burst firing.Diverse mechanisms potentially underlie the activa-tion of burst firing by addictive DAT-Is. For example,cocaine and AMPH increase noradrenergic input,which activates dopamine neurons directly [47] orindirectly via glutamatergic afferents [44]. Addition-ally, cocaine-induced elevations in extracellulardopamine acting on D1 dopamine receptors maydepolarize dopamine neurons directly [48], or indi-rectly by exciting glutamatergic [49] or inhibitingGABAergic [50] inputs. As DAT substrates, AMPHand methamphetamine could depolarize dopamineneurons directly during uptake [51,52]. Regardless ofthe cellular mechanism, that cocaine and AMPHgenerate burst firing by dopamine neurons is consis-tent with these psychostimulants increasing thefrequency of dopamine transients.

(ii) Subclasses of abused drugs, including addictive DAT-Is, upregulate vesicular dopamine releaseWe argue here that, similarly to opiates and nicotine,addictive DAT-Is upregulate vesicular dopaminerelease. This mechanism is consistent with thesepsychostimulants increasing the amplitude of dopa-mine transients and could also increase apparenttransient frequency by raising transient amplitudeabove detection thresholds. Our postulate is supportedby a large body of evidence encompassing severaladdictive DAT-Is, although cocaine is perhaps the beststudied. Indeed, cocaine has been found to upregulatevesicular dopamine release in several preparations,including brain-slice [53,54], anesthetized [55–57], andawake [58]. Upregulation of dopamine release hasmore recently been extended to methylphenidate,a cocaine-like DAT-I [59] and, surprisingly, evenAMPH [42,55,56,60]. It should be emphasized thatthe evidence for upregulated dopamine release by

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addictive DAT-Is is typically based on studies using asingle dose administered non-contingently. Thus, thisline of inquiry should be extended to repetitive dosingparadigms such as self-administration, especially forAMPH and methamphetamine, which have beendemonstrated in brain slices to deplete vesiculardopamine stores in a dose-dependent fashion [61].Further complicating this endeavor, however, is thatextended-access self-administration of methamphet-amine is associated with neurotoxic loss of markers fordopamine neurons [62].Diverse mechanisms also potentially mediate theupregulation of vesicular dopamine release by addic-tive DAT-Is. Cocaine and methylphenidate maymobilize the reserve dopamine pool through actionson synaptic proteins [54,57,59] and enhance bothvesicular dopamine uptake and trafficking [63–67].AMPH may similarly promote mobilization of thereserve pool [60] and vesicular trafficking [66], butalso upregulate vesicular dopamine release bydistinct mechanisms. These include: (i) elevation ofcytosolic dopamine levels by enhancing dopaminesynthesis and inhibiting dopamine degradation [60],and selectively depleting the reserve pool [56]; (ii)liberation of intracellular Ca2+ stores [68]; (iii)enhancing presynaptic membrane excitability as aDAT substrate [51]. Similar mechanisms may applyto AMPH analogs, such as methamphetamine, whichalso enhances membrane excitability [52] and altersvesicular dopamine trafficking [64].

(iii) Inhibition of dopamine uptake is not the definingmechanism for addictive DAT-Is to activate phasicdopamine signalingCocaine augmentation of the frequency, amplitude,and duration of dopamine transients was originallyattributed to this psychostimulant inhibiting dopa-mine uptake [10,30,31,40]. By contrast, we argue herethat inhibition of dopamine uptake is not the definingaction for addictive DAT-Is to activate phasic dopa-mine signaling. This postulate is based on two lines ofreasoning. First, as discussed above, addictive DAT-Isincrease the frequency and amplitude of dopaminetransients by actions independent of inhibiting dopa-mine uptake [28,32]. In this regard, addictive DAT-Isresemble nicotine and ethanol, which increase both thefrequency and amplitude of dopamine transients, butwhich do not inhibit dopamine uptake or prolongtransient duration [26,27]. Second, although inhibitingdopamine uptake prolongs transient duration owing tothe slowed extracellular clearance of dopamine,transient amplitude is relatively insensitive to uptakeinhibition (Box 2). Thus, uptake inhibition may notnecessarily lead to an increase in transient amplitudeand, by virtue of surpassing detection thresholds,apparent frequency. In excellent agreement, the CB1cannabinoid receptor antagonist, rimonabant, pre-vents the cocaine-induced increase in transientamplitude and frequency without altering the increasein transient duration due to uptake inhibition [26].Inhibiting dopamine uptake is further questioned as adefining action by the demonstration that several

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DAT-Is with high affinity for DAT do not exhibitreinforcing properties [69,70].

Abused drugs augment extant dopamine transients andelicit dopamine transients de novo

The new view of drug mechanism proposed herein identifiestwo shared actions of abused drugs. This first common actionis to augment extant dopamine transients. These ‘ongoing’transients are evoked by natural rewards and their predic-tive cues or occur spontaneously. All three functional targetsof abused drugs should contribute to the augmentation ofextant dopamine transients. For example, upregulation ofvesicular dopamine release and inhibition of dopamineuptake would increase the amplitude and prolong the dura-tion of dopamine transients, respectively. In addition, be-cause ethanol and cannabinoids increase transientamplitude, without upregulating vesicular dopamine re-lease or inhibiting dopamine uptake [25–27,71], drug-in-duced alterations in intra-burst properties (e.g., increase innumber or frequency of action potentials) would also in-crease amplitude. Because larger dopamine transientsevoked by food-predicting cues enhance the ability of thesecues to promote food seeking [19], abused drugs augmentingextant dopamine transients should similarly drive ongoingappetitive behavior. Indeed, low-dose AMPH increases theamplitude and duration of dopamine transients evoked bycues predicting food reward [42] and enhances cue-drivenfood seeking [72]. It is interesting to speculate that theseactions may also contribute to the efficacy of addictive DAT-Is as cognitive enhancers (Box 4).

The second common action of abused drugs is to elicitdopamine transients de novo. As opposed to modifyingextant transients, this drug action creates new transients.Considerable evidence supports the conclusion that, of thethree functional targets, only generating burst firing bydopamine neurons elicits dopamine transients de novo. Forexample, genetic disruption of the NMDA receptor impairsboth burst firing and dopamine transients [21], and selec-tive optogenetic stimulation of dopamine neurons withburst patterns evokes transient-like signals [22]. More-over, pharmacological disruption of burst firing preventsthe ability of cocaine, nicotine, ethanol, and cannabinoidsto increase the frequency of dopamine transients[25,26,28,73]. Finally, ethanol and cannabinoids augmentburst firing and dopamine transients without upregulatingdopamine release or inhibiting dopamine uptake [25–27,71,74–78]. Once elicited, other actions of abused drugswould enhance these now ‘extant’ dopamine transients asdescribed above, thereby producing an even more exagger-ated drug response. Indeed, hyperactivation of dopaminetransients by high-dose AMPH is so intense that it pro-duces an effective pharmacological ‘deafferentation’,decoupling previously acquired cue–food reward associa-tions and abolishing ongoing appetitive behavior [42].

Generating burst firing by dopamine neurons is thekeystone action of abused drugsWe now bring forward and integrate key ideas developed inpreceding sections to argue that generating burst firing isthe keystone action of abused drugs. To begin, dopamine

Box 4. Outstanding Questions

� What mechanisms mediate the clinical efficacy of DAT-Is?

DAT-Is are prescribed as cognitive enhancers for attention deficit

hyperactivity disorder, traumatic brain injury, and drug abuse

[105,122–124]. Whether activation of phasic dopamine signaling,

as is described herein for cocaine and AMPH, contributes to clinical

efficacy of DAT-Is is not known. The ability of AMPH to enhance

associative learning [72] and augment dopamine transients [42]

supports this possibility.

� What is the role of tonic dopamine signaling in the actions of

addictive DAT-Is?

Addictive DAT-Is robustly increase extracellular dopamine levels

measured by microdialysis [1]. These results have been interpreted

to reflect enhanced tonic dopamine signaling, and this could be

mediated by addictive DAT-Is acting on vesicular dopamine release

and dopamine uptake similar to phasic dopamine signaling,

uniquely enhancing tonic firing by dopamine neurons. However,

probe implantation damage limits quantifying these drug-induced

increases [125], and other mechanisms besides tonic firing by

dopamine neurons, such as glutamatergic input and drug-induced

dopamine transients, may prominently contribute to basal dopa-

mine levels [42,126,127].

� Does DAT function as a dopamine ‘receptor’ in drug reinforcement?

DAT-Is can induce conformational changes in DAT that are

capable of triggering distinct downstream signaling events via

several DAT-interacting proteins [70,128]. Similarly to a transmem-

brane receptor, these actions may promote alterations in scaffold-

ing proteins and intracellular second messenger pathways. It is not

known whether actions of addictive DAT-Is other than inhibiting

dopamine uptake, such as upregulating vesicular dopamine release

and activating burst firing of dopamine neurons, are mediated by

DAT functioning as a transmembrane receptor.

� What is the relationship between dopamine transients and synaptic

plasticity?

Pulsatile changes in extracellular dopamine, such as the

dynamics exhibited by dopamine transients, are thought to be

crucial for synaptic plasticity mediated by D1 dopamine receptors

during reward learning [16,17]. However, precise relationships

between attributes (e.g., frequency, amplitude, duration, and

pattern) of dopamine transients and synaptic plasticity, and

between drug-induced activation of these phasic dopamine signals

and enhanced synaptic plasticity, have not been established.

� What is the significance of addictive DAT-Is uniquely activating

dopamine signaling?

Of all classes of abused drugs, only addictive DAT-Is activate

phasic dopamine signaling by acting on all three functional drug

targets of dopamine neurons: burst firing, vesicular dopamine

release, and dopamine uptake. Moreover, DAT substrates, such as

AMPH and methamphetamine, uniquely increase tonic dopamine

signaling via action potential-independent dopamine efflux. How-

ever, it is not known what this unique activation of dopamine

signaling confers to drug reinforcement.

� How do subpopulations of dopamine neurons respond to acute

drug exposure?

Figure 2 in main text presents a summary of drug actions on

dopamine neurons. However, in recent years it has become

increasingly clear that midbrain dopamine neurons are a hetero-

geneous group of cells [129,130] that show diversity in terms of their

electrophysiological properties and behavioral functions depending

on their respective afferent inputs [9] and projection targets [119,120].

How abused drugs differentially affect these subpopulations of

dopamine neurons, and how cell specific actions support their acute

and long-term behavioral effects, remains to be elucidated.

Opinion Trends in Neurosciences April 2014, Vol. 37, No. 4

transients arise from burst firing by dopamine neurons,and are necessary and sufficient for predictive cues to formcue–reward associations and to promote reward seekingduring appetitive behavior. To hijack this process, abuseddrugs must act robustly on dopamine neurons. For cues topromote drug seeking, abused drugs must also act similar-ly to natural rewards and elicit a dopamine transient thatcan transfer to the predictive cue. However, unlike naturalrewards that are processed by sensory systems and affer-ent input to generate burst firing and elicit dopaminetransients, abused drugs act centrally to activate dopa-mine neurons. Moreover, their effects are ultimately me-diated by three functional targets on dopamine neurons:firing of action potentials, vesicular dopamine release, anddopamine uptake. Nevertheless, to mimic natural rewardsand provide a dopamine transient for transferring to thepredictive cue, abused drugs must elicit dopamine transi-ents de novo. Of available functional targets on dopamineneurons, only burst firing achieves this essential outcome.

The theoretical argument that generating burst firingby dopamine neurons is the keystone action of abuseddrugs is thus surprisingly straightforward. This actionalso appears to meet principal empirical criteria to bedeemed essential:(i) Necessary. All abused drugs generate burst firing by

dopamine neurons.(ii) Sufficient. Ethanol and cannabinoids generate burst

firing by dopamine neurons but do not upregulatedopamine release or inhibit dopamine uptake.

(iii) Robust. Abused drugs intensely increase burst firingby dopamine neurons and the frequency of dopaminetransients. Additional effects of abused drugs to

increase transient amplitude by upregulating vesicu-lar dopamine release and prolonging transientduration by inhibiting dopamine uptake are similarlyrobust and would further contribute to the augmen-tation of newly elicited dopamine transients. Therobust activation of dopamine transients is thusconsistent with a higher reward magnitude conferredto abused drugs compared to natural rewards, andshould result in cue-evoked transients with corre-spondingly greater amplitude [7,13,14]. Exaggeratedcue-evoked dopamine transients would in turnincrease the relative value ascribed to drug-associat-ed cues, and may mediate the powerful ability ofconditioned stimuli to reinstate drug seeking andtaking [39].

(iv) Reliable. Generating burst firing by dopamine neu-rons faithfully elicits dopamine transients de novo. Bycontrast, upregulating vesicular dopamine releaseand inhibiting dopamine uptake, although robust, arenot reliable because these actions modify extantdopamine transients, which must be elicited inde-pendently. The reliable activation of dopaminetransients even after the establishment of drugpredicting cues as conditioned stimuli would beinterpreted as a persistent positive prediction errorthat, when coupled to robust activation, may act to‘hyper-reinforce’ behaviors preceding drug delivery[37,38].

The well-established ability of abused drugs to elicitlong-term potentiation at excitatory glutamatergic synap-ses on dopamine neurons [79,80] may serve to enhance

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their ability to generate burst firing and thereby increaseboth the robustness and reliability by which dopaminetransients are elicited.

Concluding remarksOn the basis of reclassifying addictive DAT-Is with anemphasis on phasic dopamine signaling, we have arguedthat generating burst firing of dopamine neurons is thekeystone action of abused drugs. The essential outcome ofthis action is eliciting dopamine transients de novo. Reclas-sifying DAT-Is thus reconciles dopamine theories of appe-titive behavior with a mechanistic understanding of howabused drugs hijack reward circuits, leading to an over-learning of cues predicting drug availability. Identifyingthis keystone action of abused drugs also targets burstfiring by dopamine neurons as a potential therapeuticintervention. In support of this strategy, the CB1 cannabi-noid receptor antagonist, rimonabant, which suppressesdrug- and cue-evoked activation of dopamine transients viadisrupting burst firing [26,81], shows promise in treatingdrug abuse [82,83]. We readily acknowledge substantivecaveats in our argument. In particular, activation of dopa-mine transients has not been confirmed for all abuseddrugs, and particular attention should be directed at otherDAT-Is besides cocaine and AMPH, and the opiates, whichcan act independently of dopamine signaling [84]. More-over, generating burst firing by dopamine neurons has alsonot been confirmed in awake animals for all abused drugs,and there is crucial need to establish this mechanism forthe addictive DAT-Is. This is a not a simple task, however,because of difficulties with in vivo identification of dopa-mine units [85]. Therefore, FSCV and refined electrophys-iological approaches will be instrumental in the future forfurther characterizing the actions of abused drugs ondopamine neurons.

AcknowledgmentsThe authors gratefully thank the National Institute of Drug Abuse(grants DA021770 and DA024036 to P.A.G. and DA025634 to M.F.R.) andNational Science Foundation (grant DBI0754615 to P.A.G.) for fundingtheir research on amphetamine.

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