The addicted brain craves newneurons: putative role for adult-bornprogenitors in promoting recoveryChitra D. Mandyam and George F. Koob

Committee on the Neurobiology of Addictive Disorders, The Scripps Research Institute, La Jolla, CA, USA

Review

Glossary

Addiction: chronic relapsing disorder characterized by a compulsion to seek

and take drugs, loss of control in limiting intake and emergence of a negative

emotional state during withdrawal.

Compulsivity: behaviorally defined as the persistent re-initiation of habitual

acts in responding in the face of incorrect responses in choice situations or

preservation of responding in the face of adverse consequences.

Drug craving: is the desire for the previously experienced effects of a

psychoactive substance. This desire can become compelling and can increase

in the presence of both internal and external cues, particularly with perceived

substance availability.

Drug dependence: in animal models, manifestation of a withdrawal syndrome.

Drug relapse: reinstatement of drug seeking in previously detoxified indivi-

duals. This can occur in individuals after detoxification and long periods of

abstinence, despite sincere efforts to refrain; behavior can be provoked by

stress, cues or contexts previously associated with drug use.

Impulsivity: behaviorally defined as a predisposition toward rapid, unplanned

reactions to internal and external stimuli without regard for negative

consequences.

Negative reinforcement: process by which removal of an aversive stimulus

(e.g. negative emotional state of drug withdrawal or a footshock) increases the

probability of a response.

Place conditioning: a procedure whereby the effects of drugs are paired with

distinct environments, and the animal changes its subsequent preference for

that distinct environment.

Positive reinforcement: a process by which presentation of a stimulus (usually

pleasant; e.g. pleasurable effects of a drug) increases the probability of a

response.

Reinstatement of drug seeking: animal model that investigates relapse to drug

seeking. Learned self-administration behavior is extinguished by explicit non-

reward, and subjects are later tested for their ability to reinstate drug-seeking

behavior (e.g. lever pressing in the operant chamber) in response to a priming

stimulus (i.e. drug, cue, context or stressor).

Addiction is a chronic relapsing disorder associated withcompulsive drug taking, drug seeking and a loss of controlin limiting intake, reflected in three stages of a recurrentcycle: binge/intoxication, withdrawal/negative affect,and preoccupation/anticipation (‘‘craving’’). This reviewdiscusses the role of adult-born neural and glial progeni-tors in drug seeking associated with the different stagesof the addiction cycle. A review of the current literaturesuggests that the loss of newly born progenitors, partic-ularly in hippocampal and cortical regions, plays a role indetermining vulnerability to relapse in rodent models ofdrug addiction. The normalization of drug-impaired neu-rogenesis or gliogenesis may help reverse neuroplasticityduring abstinence and, thus, may help reduce the vulner-ability to relapse and aid recovery.

IntroductionAddiction (see Glossary) to drugs of abuse has emotionaland financial tolls on society, cutting across ages, races,ethnicities and genders, with increases in mortality, mor-bidity and economic costs. Broadly defined, addiction is achronic relapsing disorder characterized by a compulsionto seek and take drugs, a loss of control in limiting intakeand emergence of a negative emotional state during with-drawal [1]. The addiction cycle involves elements of bothimpulsivity and compulsivity and is composed of threestages: (i) binge/intoxication; (ii) withdrawal/negativeaffect; and (iii) preoccupation/anticipation (craving). Thestudy of the neurobiological bases of addiction and relapsehas significantly progressed, but to date few treatmentsare known to reverse the drug-induced neuroplasticitychanges that convey the vulnerability to relapse (reviewedin [2]). Understanding the neuroplastic changes that un-derlie the relapse stage of addiction can help generatebetter treatment options for addiction.

The ability of the brain to generate new progenitorscontinuously throughout adulthood may have importantimplications for addiction. Broadly defined, progenitorsare the progeny of stem cells characterized by limitedself-renewal and can survive and mature into differentiat-ing cells, such as neurons and glia, in the brain. There aretwo main neurogenic areas in the adult brain: the subven-tricular zone (SVZ) that lines the lateral ventricles in theforebrain, which contain progenitor cells that give rise to

Corresponding author: Mandyam, C.D. (cmandyam@scripps.edu).Keywords: Prefrontal cortex; Gliogenesis; Hippocampus; Neurogenesis; Addiction.

250 0166-2236/$ – see front matter � 2011 Elsevier Ltd. All rights res

neurons in the olfactory bulb, and the subgranular zone(SGZ) in the dentate gyrus (DG) of the hippocampus thatgives rise to granule cell neurons (Figure 1). Neurogenesishas also been shown to occur in the neocortex [medialprefrontal cortex (mPFC); Figure 2] [3–5]. Emerging evi-dence suggests that alterations in the rate of adult neuro-genesis and gliogenesis in these brain regions contribute tothe regulation of drug taking and drug seeking, particularlyin the hippocampus and cortex. Therefore, this reviewfocuses on altered plasticity in these regions that resultsfrom drug self-administration, and considers recent findingsthat implicate alterations in neural and glial progenitors inthe phenomenology of drug abuse.

Hippocampus and mPFC: roles in drug taking andseekingAnimal models have been developed that parallel the threestages of the addiction cycle and include various paradigms

Self-administration: arbitrary instrumental action, such as lever pressing, to

gain access to positive reinforcers, such as food or drugs of abuse.

erved. doi:10.1016/j.tins.2011.12.005 Trends in Neurosciences, April 2012, Vol. 35, No. 4

(b)

(a)

Proliferating progenitor

SGZ

GCL

Mol

type3Immature neurontype2a

Mature neuron

Granule neuron type2btype1

radial glia-like

Ki-67

DCX/PSA-NCAM/NeuroD1

Sox2

GFAP

SGZ

GCL Key:

H

PPmf

SchMol

Bregma –3.6

BrdU 2 h 2–6 d 10 d 28 d

TRENDS in Neurosciences

Figure 1. Schematic of a coronal section through the adult rat hippocampus and distinct developmental stages of adult-born hippocampal progenitors. (a) Coronal section

through the adult rat brain at bregma –3.6 highlighting the hippocampal dentate gyrus (DG; yellow with black- and gray-shaded regions). The hippocampal trisynaptic

pathway is indicated: perforant path (PP) connections in violet, mossy fiber (mf) connections in pink and Schaffer collaterals (Sch) connections in green. The DG is

subdivided into the molecular layer (Mol), granule cell layer (GCL, gray) and hilus (H, yellow). The subgranular zone (SGZ) is indicated as a black hatched area between the

granule cell layer and the hilus of the DG. A granule cell neuron is indicated in the GCL, which is magnified in (b) to depict the various developmental milestones of

hippocampal neural stem cells. (b) Schematic of the hippocampal GCL demonstrating the sequence of preneuronal, early neuronal and postmitotic cell types during

postnatal neurogenesis. Cells are born as type-1 radial glia-like stem cells and slowly divide to produce type-2 cells. Rapidly dividing type-2 cells differentiate into immature

neuron type-3 cells and, finally, into a mature granule cell neuron. Various endogenous markers of proliferation [e.g. sex-determining region Y-box 2 (Sox2) and Ki-67] and

differentiation [e.g. doublecortin (DCX), polysialic acid-neural cell adhesion molecule (PSA-NCAM) and neurogenic differentiation factor 1 (NeuroD1)] can be used in

combination with the astrocytic maker glial fibrillary acidic protein (GFAP) to determine the cell type of proliferating cells.

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

of drug self-administration for the binge/intoxicationstage [6], motivational elements of withdrawal for thewithdrawal/negative affect stage, and drug-, cue-, context-and stress-induced reinstatement for the preoccupation-anticipation stage [7]. These models have been extensivelyused to uncover the key brain regions, brain circuitry,neurotransmitters and neuromodulators associated withdrug-taking and -seeking behavior [8,9]. The ventralstriatum [i.e. a brain region that includes the nucleusaccumbens (NAc) core and shell and some nuclei of theolfactory tubercle], a terminal projection of the neuralconnections from the ventral tegmental area (VTA) and

PFC, is considered a focal point for the reward and rein-statement associated with drug-seeking behavior [1]. Therelease of the neurotransmitter dopamine in these regionsis considered to be significantly modulated by variousdrugs of abuse, particularly psychostimulants, such ascocaine, methamphetamine and nicotine, to produce theirrewarding effects. Furthermore, evidence indicates thatneurotransmitters other than dopamine may also play asignificant role in the rewarding effects of drugs of abuse,including opioid peptides. The NAc is also tactically situ-ated in the brain such that it receives inputs from severalother brain regions, including the mPFC, basolateral

251

Cg1

PrL

IL

LV

Proliferatingprogenitor

Neuron

NG2-glia

GFAP-glia

(40–60%)

(5–15%)

(1–3%)

Matura

t io

n

Bregma 2.7

(a)

(c)

(b)

Endothelial cell (15–30%)

Ref [4] [5] [88] [89] [90] [91] [92] [94] 0

20

40

60

80

100RECA+Key:Rip+PLP/O4+NG2+GFAP/S100β+DCX+NeuN+

Per

cen

t o

f co

rtic

al m

atu

re B

rdU

-IR

cel

ls

TRENDS in Neurosciences

Figure 2. Schematic of a coronal section through the adult rat prefrontal cortex and distinct cell types generated by cortical progenitors. (a) The adult rat medial prefrontal

cortex (mPFC), which is equivalent to the human dorsolateral PFC, spans a 3 mm3 area bilaterally along the rostral–caudal levels of the rodent brain [18]. Anatomically, the

mPFC is clearly distinguishable from other cortical areas in bregma regions 3.7–2.2 of the adult rat brain [92]. A coronal section through the adult rat brain at bregma 2.7

highlights the mPFC in tan color. The mPFC is further divided into the anterior cingulate cortex (Cg1), prelimbic cortex (PrL) and infralimbic cortex (IL) subregions [130]. (LV,

lateral ventricle). (b) The mPFC contains proliferating cells (solid-black circles; one cell in a rectangle box is enlarged and is a cycling cell in yellow) that mostly mature into

glia (oligodendrocytes in blue and astrocytes in green) and endothelial cells (blue cells in blood vessels) or neurons (red) to a lesser extent. The percentage of mature

phenotypes reflect data pooled from various reports (see below) of 20-day or older BrdU-IR cells that were co-labeled with various markers for immature (NG2; Rip) or

mature [proteolipid protein (PLP); oligodendrocyte marker clone O4 (O4)] oligodendrocytes, mature astrocytes [glial fibrillary acidic protein (GFAP); S100 calcium binding

protein B (S100b)], endothelial cells (RECA, pan-endothelium marker), or mature neurons [neuronal nuclease (NeuN)]. (c) Twenty-day-old or older BrdU-immunoreactive

cells were co-labeled with various markers for immature (NG2, Rip) or mature (PLP, O4) oligodendrocytes, mature astrocytes (GFAP, S100b), endothelial cells (RECA),

immature neurons [doublecortin (DCX)] or mature neurons (NeuN). The ratio of labeled phenotypes (y-axis) from various reports [4,5,88–92,94] using adult rat brain tissue

(x-axis) is indicated. Such findings indicate that there is a large variability between studies, although most studies found that NG2+ cells constituted the majority of BrdU-

labeled cells, whereas NeuN+ cells were relatively rare.

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

amygdala, insula and hippocampal regions, and theseinputs are hypothesized to play a key role in cue- andcontext-specific associations with drugs [1]. Activation ofdopamine, glutamate and corticotropin-releasing factorsystems in these key brain regions are associated withdrug-, cue- and stress-induced reinstatement, respectively[10,11].

The hippocampus and mPFC are implicated in themodulation of the reinforcing actions of drugs of abuseand play a key role in the reinstatement of drug-seekingbehavior [1,8]. Hippocampal integrity may be importantfor drug-context memories associated with drug reward[12] and is fundamental to the formation of context-specific memories associated with the reinstatementof drug seeking [13–16]. This hypothesis is supported bythe observation that activation and inactivation of the

252

hippocampal-subicular and hippocampal-VTA pathways,respectively, enhances and blocks drug-seeking behaviorin rodents [14,16]. Thus, it appears that the hippocampalneural plasticity that underlies learning and memoryfunction also contributes to the modulation of rewardpathways in drug addiction [17].

The prelimbic and infralimbic cortices of the mPFC havewidespread connections to the basal forebrain, amygdalaand hypothalamus that mediate diverse functions, includ-ing attentional processing, goal-directed behavior andworking memory [18]. These functions that depend onthe mPFC play a prominent role in the fundamentalpathway that underlies drug reinstatement triggered bydrug priming, conditioned stimuli and external stress [8].Pharmacologically, excitatory inputs from the hippocam-pus and mPFC increase dopaminergic activity in the

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

VTA, which have been hypothesized to contribute to drugseeking in response to cues and context following extinc-tion [16,19–21]. Altogether, the hippocampus and PFCappear to play critical roles in the modulation of thereinforcing effects of drugs, and the release of neurotrans-mitters from key brain regions associated with cues andcontext are hypothesized to be essential components of thehuman condition of craving.

Additionally, it is well established that the neuralconnections from the CA1 and subiculum of the hippocam-pus that terminate in the prelimbic cortex of the mPFCfunction to facilitate the acquisition, maintenance andindependent storage and consolidation of declarative,spatial and associative long-term memories [22]. ThemPFC and hippocampus interact under a variety of func-tional demands, including drug taking. Such interactionsmay depend on both memory processes and content [1,23]and may be critical for optimal performance [24]. There-fore, excessive drug use during abuse and addiction couldcompromise normal learning and memory systems and, inaddition to activation of reward pathways associated withcraving, may also disrupt executive control pathways,thus contributing to the impulsivity and impairment ofdecision making that is characteristic of individuals withaddiction [25].

Table 1. Hippocampal neurogenesis is altered after self-administraanimal models of drug addiction

Stage Source of reinforcement Animal models

Binge/Intoxication Positive reinforcement Conditioned place prefer

� MDMA

� Cocaine

Drug self-administration

� Methamphetamine

- Intermittent access

- Limited access (1h

� Cocaine

- Limited access (1h

- Extended access (>

� Nicotine

- Limited access (1h

� Heroin

- Extended access (6

� Alcohol

- Nondependent drin

Withdrawal/

Negative affect

Negative reinforcement Conditioned place avers

Increased self-administra

during dependence

� Methamphetamine

- Extended access (6

� Cocaine

- Extended access (>

� Alcohol

- Excessive drinking d

Relapse Conditioned positive and

negative reinforcement

Cue-induced reinstateme

Stress-induced reinstate

Protracted abstinence/w

- Extended access co

- Extended access co

a–, no change; ", increase; #, decrease; ND, not determined.

Hippocampal neurogenesis, drug taking, and drugseekingOne possible contributor to hippocampal neural plasticity isadult neurogenesis in the SGZ of the DG. The process ofneurogenesis involves stem-like precursor cells that prolif-erate into preneuronal progenitors, which in turn differen-tiate into immature neurons and eventually mature intogranule cell neurons [26] (Figure 1). It is widely acknowl-edged that a large proportion (>80%) of hippocampal pro-genitors migrate a short distance to become granule neuronsin the DG [27]. Furthermore, there is sufficient evidence thatsupports the functional incorporation of the newly bornneurons in the DG [28–30], although the hypothesis con-cerning the role of new DG neurons on sparse coding (i.e.facilitation of the formation of new hippocampus-dependentmemories) awaits confirmation [31–33]. New DG neuronsare also involved in certain aspects of addiction. Reinforcingdoses of drugs self-administered by rodents decrease DGneurogenesis [34–43] (Table 1). It has been hypothesizedthat new DG neurons may block memories associated withthe contextual reinstatement of drug seeking or enhanceextinction learning [44,45]. Thus, the reduction in sponta-neous neurogenesis (i.e. a reduction in neuronal turnover)that is observed after self-administration of various drugs ofabuse may result in a more robust and long-lasting memory

tion of various drugs of abuse and after withdrawal/relapse in

Hippocampal neurogenesisa Refs

Proliferation Immature

neurons

Mature

neurons

ence

– ND – [40]

– ND – [41]

(1h/d, 2d/w) " " – [35]

/d, 5d/w) # # # [35]

/d, 7d/w) # ND # [43]

4h/d, 5d/w) # " – [39]

/d, 5-7d/w) # # ND [36]

h/d, 7d/w) # ND ND [36]

king (30m/d, 5d/w) # # # [37,48]

ion No data

tion

h/d, 5d/w) # # # [35]

4h/d, 5d/w) – – ND [60]

uring dependence # # # [37,50]

nt No data

ment No data

ithdrawal

caine (14d) # # ND [60]

caine (28d) – ", – –, ND [39,60]

253

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

of drug taking and seeking or decrease extinction learning.However, additional experiments are needed before theprecise function of DG neurogenesis in drug taking andseeking is determined [14,46]

The developmental stages of DG progenitors and thelineage for adult-generated DG neurons have been deter-mined by utilizing transgenic mice that express nestin (atype VI intermediate filament protein that is highlyexpressed in neural progenitor cells) under the control ofa GFP promoter (Figure 1b). Data from mouse studiesindicate that proliferating cells in the postnatal DG arenot homogeneous and that the process of postnatal neuro-genesis is an uncoordinated cluster of developmental stagesthat progress in parallel, including actively dividing cellsthat are radial glia-like (type1), preneuronal (type2a), inter-mediate (type2b) and early neuronal (type3) [47] (Figure 1b).Importantly, pools of slowly dividing type1 cells appear to bethe precursors of adult-generated DG neurons. Further-more, the distinct cell types have been individually labeledusing combinations of exogenous [5-bromo-20-deoxyuridine(BrdU]); Box 1; Figure 2b] and endogenous [Ki-67, prolifer-ating cell nuclear antigen, phosphorylated histone-H3,sex-determining region Y-box 2 (Sox2)] markers of cellproliferation and cell differentiation [doublecortin (DCX),neurogenic differentiation factor 1 (NeuroD1) and polysialicacid-neural cell adhesion molecule (PSA-NCAM)] [47,48].By incorporating these exogenous and endogenous markersof proliferation and differentiation, critical information hasbeen obtained in adult rat and nonhuman primate models todemonstrate a similar developmental profile of DG progeni-tors compared with mouse models.

Recent studies have focused on how drugs of abuse alterthe process of DG neurogenesis by modifying the develop-mental stages of newly born adult DG progenitors andtheir pathway to attain a neuronal phenotype [47](Table 1). For example, limited-access nicotine self-admin-istration decreases the proliferation and differentiation of

Box 1. Detection of adult born progenitors, adult

neurogenesis and gliogenesis

Actively dividing progenitors in the adult mammalian brain are

usually labeled with exogenously administered mitotic markers,

such as [3H]thymidine, 5-bromo-20-deoxyuridine (BrdU), 5-iodo-20-

deoxyuridine (IdU), 5-chloro-20-deoxyuridine, and 5-ethynyl-20-deox-

yuridine (EdU) [123–126]. Exogenous mitotic markers are incorpo-

rated into DNA during the synthesis-(S)-phase of the cell cycle of

actively dividing progenitors in the brain, thereby assisting with

birth dating the cells. The time (hours to days to months) of

euthanasia after a pulse of the mitotic marker (usually administered

intraperitoneally or intravenously) determines the age of the

progenitor cell when analyzed in post-mortem tissue. For example,

a BrdU pulse minutes to hours before euthanasia will label

proliferating cells, and a BrdU pulse days to months before

euthanasia will label surviving cells, thereby allowing the character-

ization of the kinetics, dynamics and phenotype acquisition of newly

born progenitors (Figures 1b and 2b, main text). More recently,

endogenous markers of cell proliferation and cell differentiation,

such as Ki-67, proliferating cell nuclear antigen (PCNA), phosphory-

lated histone-H3, Sox2, DCX, NeuroD1, PSA-NCAM and several

others, have been used to label progenitors at distinct stages of

maturation [127–129]. Combinatorial labeling with exogenous and

endogenous markers has provided critical information about the

morphological and functional development of progenitors in the

adult brain.

254

DG progenitors [36]. Extended-access heroin self-adminis-tration, which results in compulsive drug seeking, decreasesthe proliferation of DG progenitors [38]. Limited- and ex-tended-access cocaine self-administration decreases the pro-liferation of DG progenitors [39,43]. Furthermore, extendedaccess to cocaine increases differentiation without alteringthe survival of progenitors [39]. Limited- and extended-access methamphetamine self-administration decreasesthe proliferation, differentiation and survival of DG progeni-tors [35]. Surprisingly, intermittent-access methamphet-amine self-administration (which provided less total drugthan limited access or extended access) increases the prolif-eration and differentiation of DG progenitors, but such anincrease in the immature neuronal population was notassociated with an increase in neurogenesis [35]. Nondepen-dent ethanol self-administration studied in both rodent andnonhuman primate models decreased the proliferation,differentiation and survival of DG progenitors [37,48,49].Excessive drinking during alcohol dependence was alsofound to decrease all aspects of DG neurogenesis [37,50].Altogether, certain neuromodulatory effects on neurogen-esis appear to be particularly sensitive to the amount of dailydrug intake and a higher amount of drug intake producesmore pronounced effects on DG neurogenesis.

Role of DG neurogenesis in drug taking and seekingMechanistic approaches to address potential roles foradult-generated DG neurons in drug taking and seekingare currently being undertaken. For example, proceduressuch as low doses of irradiation have been used to ablateproliferating progenitors in the DG, and changes in theenvironment of the animal (e.g. voluntary wheel running inthe home cage) have been used to enhance the proliferationand maturation of hippocampal progenitors to provide afunctional link between hippocampal neurogenesis andhippocampal-dependent memory [51]. Similarly, irradia-tion and wheel running have been used to examine therelationship between hippocampal neurogenesis and drugtaking and seeking. Studies in rats that have used irradi-ation to ablate hippocampal neurogenesis before any co-caine experience have demonstrated that irradiated ratshad enhanced cocaine-taking behavior, reflected by in-creased self-administration on a fixed-ratio schedule ofreinforcement in an extended-access model compared withnon-irradiated rats [46]. In other studies, voluntary wheelrunning before and during cocaine and methamphetamineexperience reduced the maintenance of drug-reinforcedbehavior [52–55] and positive-reinforcing effects of cocainecompared with sedentary animals [56]. Voluntary wheelrunning before and during ethanol experience reducedethanol self-administration [57] and diminished intoxicat-ed behavioral responses to heavy binge ethanol adminis-tration compared with sedentary controls [58]. Suchstudies did not measure neurogenesis in the hippocampus,which would be needed to correlate DG neurogenesis withthe drug-induced behavioral outcomes. However, some ofthe effects of wheel running on drug taking may be attrib-utable to the neuromodulatory effects of running on DGneurogenesis [59]. Thus, increased hippocampal neurogen-esis may be beneficial in diminishing the motivationalimpact of drugs after chronic exposure to drugs of abuse.

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

The effects on DG proliferation, differentiation and neu-rogenesis after drug withdrawal or reinstatement to drugseeking after abstinence have been less well studied. Thestudies that have addressed this issue have found thatwithdrawal from cocaine self-administration decreasedthe proliferation and enhanced the differentiation and mat-uration of DG progenitors compared with control animals[39,60]. Although more work is required to support thehypothesis of the enhanced survival of DG progenitorsduring withdrawal from drug exposure, one could proposethat the abnormal survival of progenitors during withdraw-al from the drug could be a part of the recovery process.Consistent with this hypothesis, other studies suggestedthat the degree of neurogenesis in the DG affects behavioralresponses after drug withdrawal. Ablation of DG progeni-tors by irradiation during withdrawal from cocaine self-administration delayed the extinction of cocaine-seekingbehavior in rats [46]. Furthermore, voluntary wheel run-ning before and during extinction reduced drug-primed[61,62] and cue-induced [62,63] cocaine-seeking behaviorin rats after a period of forced abstinence. The levels ofneurogenesis in the hippocampus were not measured inthese studies, but, as discussed above, such findings suggestthat enhanced neurogenesis after wheel running is an un-derlying mechanism for such a reduction in drug-inducedresponses after withdrawal. Other studies have also indi-cated a beneficial role for hippocampal neurogenesis inreducing the vulnerability to relapse. For example, ratsgenetically inbred for high novelty-seeking behavior andrats exposed to early environmental stress (e.g. prenatalstress) exhibited reduced DG neurogenesis [42,64,65] andwere prone to developing addictive-like behaviors [66–68].Although more studies are needed, adult DG neurogenesisappears to be important for the maintenance of hippocampalneuroplasticity, such that reducing spontaneous DG neuro-genesis during abstinence may enhance the vulnerability torelapse, and enhancing DG neurogenesis during abstinencemay help reduce the vulnerability to relapse.

Role of olfactory bulb neurogenesis in addictionNeurogenesis in the olfactory bulb contributes to olfactoryfunction [69], and deficits in olfactory function and sensi-tivity are evident in drug- and alcohol-dependent humans[70,71]; these behavioral deficits may also predict thepropensity to relapse [72]. Recent studies have demon-strated that chronic cocaine self-administration and expo-sure to chronic ethanol vapors in rodents (models of drugdependence) reduce the proliferation of neural progenitorsin the SVZ, a source of adult-generated olfactory neurons[39,50]. However, protracted withdrawal from chronic drugexposure normalizes proliferation in the SVZ, albeit pro-ducing permanent changes in the SVZ neurogenic niche[39,50]. Therefore, future studies should address whetherdrug-induced decreases in SVZ proliferation and conse-quential decreases in olfactory bulb neurogenesis producedrug-induced deficits in olfaction.

Role for glia in modulating neuroplasticity responses inthe brainGlial fibrillary acidic protein (GFAP)- and non-GFAP-pos-itive glia have received some attention in the past decade

with respect to their possible roles in addiction. GFAP-gliain the mPFC contain cystine/glutamate antiporters [73]that maintain extracellular nonsynaptic glutamate levels[74] and provide physical support to neurons by regulatingextracellular potassium and the uptake of glutamate atsynapses [75]. The glutamate released by GFAP-positiveglia antiporters is known to modulate neuronal metabo-tropic glutamate receptors and extracellular glutamateand dopamine levels in areas other than the mPFC [74].Thus, this is one way that GFAP-positive glia can influencelocal synaptic activity [76]. Although neuroadaptations inmPFC GFAP-glia have not yet been clearly indicated in thereinstatement of drug seeking [74], it is a hypothesis thatmerits testing.

NG2 chondroitin sulphate proteoglycan (NG2)-positiveglia (non-GFAP-positive glia) express AMPA receptors[77], making them antigenically distinct from GFAP-glia[75,78]. These cells are known to play important roles inthe nervous system and may, in certain enriched environ-ments, support the neurogenesis of cortical progenitors[79–81]. NG2-glia are involved in the induction and expres-sion of long-term potentiation (LTP) at neuron–glia synap-ses [77]. Moreover, NG2-positive glia contain voltage-gatedion channels [82] that assist with maintaining the homeo-static function of surrounding neurons in the hippocampus[77]. Other important roles of NG2-glia include drainingexcess ions and neurotransmitters from the extracellularspace, activity that may be important for maintaining theproliferative environment and activity of surrounding neu-rons [83]. Therefore, NG2-glia are likely to play a moreprominent role than GFAP-glia in maintaining both syn-aptic neuromodulatory responses and neuroplasticresponses in the adult brain after brain insults, such asaddiction and relapse.

Role of DG gliogenesis in addictionIn addition to the neuronal network that mediates most ofthe neuromodulatory effects associated with drug takingand seeking, glia-mediated nonsynaptic events, includingsupportive-maintenance roles, also appear to be impor-tant. In the DG, >10% of adult-born progenitors matureinto glia, mostly into astroglia that express GFAP [84].Notably, drugs that are reinforcing in vivo do not alter thenumber or proportion of DG progenitors that mature intoGFAP-positive glia [35–39]. However, newly born DGmicroglia may play a role in drug withdrawal [85,86].Recent studies have demonstrated that withdrawal fromethanol produces immediate exaggerated microglial pro-liferation in the DG and several other hippocampal regions[85,86]. These studies suggest that the hippocampus inalcohol-dependent animals is susceptible to inflammation,and such pro-inflammatory events (i.e. microglial prolifer-ation) following withdrawal may, in turn, affect DG neu-rogenesis and the optimal function of the hippocampus[87].

Role of mPFC gliogenesis in addictionUsing combinatorial labeling techniques, it has been dem-onstrated that a large proportion of progenitors in the adultmammalian mPFC, including the human PFC, mature intoa glial phenotype [Figure 2a,b; regulated intramembrane

255

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

proteolysis-positive (RIP+) oligodendrocyte, NG2+ oligoden-drocyte precursor, and/or GFAP+ astrocyte] and a smallproportion mature into interneurons [3–5]. The distributionof progenitors in the mPFC is suggested to be uniform, andmost of the cells in the mPFC (>50%) mature into oligoden-drocyte precursors [4,5,88–90] (Figure 2c). External factorshave been shown to both positively (e.g. antidepressants andwheel running) and negatively (e.g. stress, drugs of abuseand ethanol) regulate cell birth and cell maturity in themPFC [4,37,88,90–94].

Intravenous self-administration of methamphetaminedecreased (with limited and extended access) and in-creased (with intermittent access where the animal re-ceived even less methamphetamine than the limited accessgroup) the birth and survival of mPFC progenitors [4](Table 2), whereas intermittent, limited and extendedaccess to methamphetamine increased cell death in themPFC [4]. Such findings support altered mPFC plasticityafter methamphetamine exposure. Moreover, they indicatethat the effects of methamphetamine on cell death (whichincludes both neurons and glia) are distinct compared withthe changes observed with cell birth (i.e. neurogenesis isobserved to a much lesser extent in the mPFC comparedwith gliogenesis). The progenitors reduced by limited andextended access were GFAP-positive and NG2-positive gliain the mPFC, whereas most of the progenitors induced byintermittent access were phenotyped as NG2-positive glia[4]. Although evidence suggests that increases in GFAP-positive glia occur in most injury types restricted to sites ofneuronal loss [95], normal levels of GFAP-positive glia andenhanced NG2-positive glial levels after intermittent ac-cess to methamphetamine could indicate otherwise. Forexample, an enhanced NG2-glial response could indicate aprotective mechanism [i.e. characteristic central nervoussystem (CNS]) gliosis] against methamphetamine-inducedbrain insult [96]. The rapid response of increased NG2-gliaproliferation after intermittent methamphetamine insultcould be attributable to altered glutamate release locallyby glutamatergic neurons following methamphetamineexposure. This is because NG2-glial differentiation and

Table 2. Gliogenesis in the mPFC is altered after self-administratimodels of drug addictiona

Stage Source of reinforcement Animal models

Binge/ Intoxication Positive reinforcement Conditioned place preferen

Drug self-administration

� Methamphetamine

- Intermittent access (1

- Limited access (1h/d,

� Alcohol

- Nondependent drinkin

Withdrawal/

Negative affect

Negative reinforcement Conditioned place aversion

Increased self-administratio

during dependence

� Methamphetamine

- Extended access (6h/

� Alcohol

- Excessive drinking du

aThe effects of relapse on gliogenesis in the mPFC have not yet been studied.

b–, no change; ", increase; #, decrease; ND, not determined.

256

the response to CNS insult are mediated by glutamatereleased by neurons and GFAP-positive glia [97]. An in-crease in the number of NG2-positive glia that expressAMPA receptors may further produce alterations in extra-cellular glutamate in the mPFC, which could contribute tothe neuromodulatory effects that occur via the mPFC inthe other brain regions (e.g. the NAc and VTA) associatedwith the binge/intoxication stage of the addiction cycle. Thedecrease in GFAP-positive and NG2-positive glia afterprolonged limited and extended access to methamphet-amine could be indicative of the neurotoxic effects of meth-amphetamine.

Exposure to ethanol vapors after excessive drinking tothe point of dependence (i.e. operant ethanol self-adminis-tration in rodents followed by 14 hours per day, every dayexposure to ethanol vapors over several weeks) resulted ina decrease in the birth and survival of mPFC progenitors[37]. Nondependent ethanol-drinking rats did not exhibitsignificant changes in cell birth and cell survival in themPFC. Cell death was differentially regulated in bothnondependent ethanol-drinking and alcohol-dependentrats, and both groups showed significantly decreased apo-ptosis [44], indicating a compensatory state in the mPFC inresponse to excessive ethanol exposure.

What can be gleaned from these two studies is thatpsychostimulants and ethanol alter the local homeostasisof the proliferative environment in the mPFC by decreas-ing the birth of newly born cells and increasing the death ofexisting older cells. Therefore, the involvement of the PFCin several aspects of an addiction phenotype, including anassociation with classical conditioning to drug exposure[98] (a phenomenon associated with relapse to drug seek-ing), and the response inhibition behavior maintained bythe PFC may involve a key role of newly generated pro-genitors.

In addition to these correlative studies, a few studieshave demonstrated that an altered local microenvironmentin the PFC contributes to the neuroadaptations associatedwith the reinstatement of drug seeking. Recent studieshave implicated reduced brain-derived neurotrophic factor

on of various drugs of abuse and after withdrawal in animal

mPFC gliogenesisb Refs

Proliferation Survival GFAP-glia NG2-glia

ce No data

h/d, 2d/w) " " – " [4]

5d/w) # # # # [4]

g (30m/d, 5d/w) – – ND ND [37]

No data

n

d, 5d/w) # # # # [4]

ring dependence # # ND ND [37]

Box 2. Outstanding questions

� Do epigenetics play a role in mediating alterations in hippocampal

neurogenesis induced by drugs of abuse?

� Do drugs of abuse distinctly alter intrinsic signals that maintain

neurogenesis versus gliogenesis?

� Can imaging human neurogenesis become feasible with im-

proved technology?

� Does reduced hippocampal neurogenesis by drugs of abuse lead

to cognitive impairments in human addicts?

� Do current therapeutic strategies in humans that incorporate

physical activity during abstinence alter neurogenesis and

gliogenesis to reduce craving?

� Do current or novel therapeutic strategies in humans that alter

neurogenesis and gliogenesis reduce craving?

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

(BDNF) levels [99] and increased phosphorylated extracel-lular signal-regulated kinase (pERK) [100] in the mPFC incocaine craving and cocaine-associated memories thatactivate the neuronal circuitry associated with relapse.Particularly notable, wheel running during forced absti-nence prevented some of the mPFC neuroadaptations,such as increases in pERK in the mPFC, and reducedcocaine craving and the reinstatement of cocaine seeking[63]. An independent study demonstrated that wheel run-ning increased NG2- and GFAP-positive glia in the mPFC[4], although such regulation has yet to be demonstratedduring forced abstinence after drug taking. Altogether,adult cortical gliogenesis appears to be altered after expo-sure to drugs of abuse; therefore, normalizing gliogenesisin the mPFC during abstinence may help restore some ofthe maladaptive neuroplastic alterations in response todrug addiction.

Therapeutic implications: can enhancing neurogenesispromote functional recovery in the addicted brain?Voluntary exercise appears to decrease relapse to drugseeking in some cases. The impact of voluntary exercise onabstinence from drug use has been studied in humans withregard to nicotine [101–103] and alcohol [104] addiction,with some of these studies indicating a beneficial effect.With regard to animal models of addiction, only a fewstudies have assessed whether exercise after the cessationof drug use can reduce the reinstatement of drug-seekingbehavior [61–63]. However, as discussed above, none ofthese studies addressed whether the decrease in the rein-statement of drug-seeking behavior after wheel runningduring forced abstinence was caused by a reversal of drug-induced neuroplastic events, such as increased neurogen-esis and gliogenesis. Although one could reasonably hy-pothesize that this could be the mechanism, mechanisticstudies that prove otherwise are currently lacking.

As discussed in the previous sections, despite the reduc-tion of neurogenesis and gliogenesis in animal models ofaddiction that may contribute to the addiction process, theaddicted brain responds to environmental factors thatstimulate the neurogenic-gliogenic niche to reduce drugseeking [63,105–108]. These studies also suggest that thehippocampus and mPFC of the addicted brain retainmicroenvironmental elements that revive the normal pro-liferation and survival of neural and glial progenitorsduring withdrawal. Therefore, knowledge about the cellu-lar and molecular mechanisms that maintain the neuro-genic niche in vivo should be beneficial for the design of newtherapeutic strategies to augment endogenous neural pro-liferation during abstinence.

Recent work in this field has also demonstrated how theneurogenic niche can be the source of extrinsic factors thatpromote the maturation and survival of newly born DGprogenitors (reviewed in [109]). Particularly interesting isthe fact that monoamines (e.g. dopamine) play an impor-tant role in maintaining and enhancing the proliferationand maturation of hippocampal progenitors [110,111]. Thissuggests that the drug-induced consequences of long-termdecreases in dopamine function associated with addictionmay play a significant role in drug-induced alterations inhippocampal neurogenesis. Other mechanistic studies

(reviewed in [109]) have shown that certain transcriptionfactors intrinsic to progenitors could undergo chromatinremodeling, including histone modifications. Such epige-netic modifications could control certain aspects of adultneurogenesis, such as the transition from proliferation todifferentiation. Furthermore, radial glia-like stem cell-de-rived intrinsic factors in the neurogenic niche have beendemonstrated to have nourishing and instructive effectsthat promote certain stages of adult neurogenesis [112–

114]. Functional studies, such as the knockdown of suchintrinsic factors, including wingless-type MMTV integra-tion site family3 protein (WNT3) in the DG, have showndecreased neurogenesis and impaired hippocampal-depen-dent memory [115], supporting a role for the neurogenicniche in hippocampal function.

Other intrinsic factors, such as Notch [116–118], dis-rupted-in-schizophrenia1 (DISC1) [119,120] and cyclin-dependent kinase (Cdk5) [121,122], are also known toregulate the stem cell pool and affect the differentiation-maturation of hippocampal progenitors. However, itremains to be determined whether the efficacy of neuronaldevelopment can be augmented by manipulating the hip-pocampal microenvironment, such as by enhancing theexpression of the molecular targets that underlie themaintenance of the neurogenic niche in the addicted brain.Therefore, the goal of such a therapeutic approach wouldbe to first determine whether drugs of abuse produceepigenetic changes in mature neurons or induce molecularchanges in progenitor cells to inhibit neurogenesis [109].Follow-up studies could then determine whether normal-izing the neurogenic and gliogenic niche by enhancing-reducing intrinsic molecular signals during abstinencecontributes to enhanced endogenous neurogenesis andreduced drug seeking. Recruitment of the endogenousprogenitor cell population during the protracted absti-nence-withdrawal negative affect stage may then assistin reversing the altered neuroplasticity that occurs notonly in neurogenic regions, but also throughout the manyother brain regions that are known to be affected afteraddiction to drugs of abuse.

Concluding remarksThe identification of adult neurogenesis in the hippocam-pus, cortex and olfactory bulb over the past two decadeshas shed new light on the neuroplasticity of these brainregions and, more recently, the plasticity events that areassociated with drug and alcohol addiction. Future

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studies aimed at understanding the potential link be-tween correlative decreases in neurogenesis and thefunction of these brain regions will allow researchers todetermine whether decreases in neuro- and gliogenesisby drugs of abuse is behaviorally relevant to the processof addiction. Demonstrating correlative changes inhippocampal-prefrontal cell genesis associated with thereward and relapse phases of addiction in humans willsupport the hypothesis that adult neurogenesis is a vul-nerability factor for addiction. This may pave the way forfuture therapeutic possibilities for treating drug addic-tion disorders that involve enhancing neural stem cellproduction and the functional incorporation of new neu-rons into affected neural circuits (Box 2).

AcknowledgmentsPreparation of this review was supported by funds from the NationalInstitute on Drug Abuse (DA022473 to C.D.M. and DA004398, DA023597,and DA010072 to G.F.K.), National Institute on Alcohol Abuse andAlcoholism (AA008459 and AA006420 to G.F.K) and Pearson Center forAlcoholism and Addiction Research (to G.F.K.). We appreciate theeditorial assistance of Michael Arends. This is publication number21090 from The Scripps Research Institute.

References1 Koob, G.F. and Volkow, N.D. (2010) Neurocircuitry of addiction.

Neuropsychopharmacology 35, 217–2382 Kalivas, P.W. and Volkow, N.D. (2011) New medications for drug

addiction hiding in glutamatergic neuroplasticity. Mol. Psychiatry 16,974–986

3 Gould, E. et al. (1999) Neurogenesis in the neocortex of adult primates.Science 286, 548–552

4 Mandyam, C.D. et al. (2007) Methamphetamine self-administrationand voluntary exercise have opposing effects on medial prefrontalcortex gliogenesis. J. Neurosci. 27, 11442–11450

5 Dayer, A.G. et al. (2005) New GABAergic interneurons in the adultneocortex and striatum are generated from different precursors. J.Cell Biol. 168, 415–427

6 Caine, S.B. et al. (1993) Intravenous Drug-self AdministrationTechniques in Animals, Oxford University Press

7 Rossi, N.A. and Reid, L.D. (1976) Affective states associated withmorphine injections. Physiol. Psychol. 4, 269–274

8 Shaham, Y. et al. (2003) The reinstatement model of drug relapse:history, methodology and major findings. Psychopharmacology 168,3–20

9 McFarland, K. and Kalivas, P.W. (2001) The circuitry mediatingcocaine-induced reinstatement of drug-seeking behavior. J. Neurosci.21, 8655–8663

10 Koob, G.F. (1999) Stress, corticotropin-releasing factor, and drugaddiction. Ann. N. Y. Acad. Sci. 897, 27–45

11 Knackstedt, L.A. and Kalivas, P.W. (2009) Glutamate andreinstatement. Curr. Opin. Pharmacol. 9, 59–64

12 Black, Y.D. et al. (2004) Hippocampal memory system function andthe regulation of cocaine self-administration behavior in rats. Behav.Brain Res. 151, 225–238

13 Fuchs, R.A. et al. (2005) The role of the dorsomedial prefrontal cortex,basolateral amygdala, and dorsal hippocampus in contextualreinstatement of cocaine seeking in rats. Neuropsychopharmacology30, 296–309

14 Vorel, S.R. et al. (2001) Relapse to cocaine-seeking after hippocampaltheta burst stimulation. Science 292, 1175–1178

15 Hernandez-Rabaza, V. et al. (2008) The hippocampal dentate gyrus isessential for generating contextual memories of fear and drug-induced reward. Neurobiol. Learn. Mem. 90, 553–559

16 Luo, A.H. et al. (2011) Linking context with reward: a functionalcircuit from hippocampal CA3 to ventral tegmental area. Science 333,353–357

17 Hyman, S.E. et al. (2006) Neural mechanisms of addiction: the role ofreward-related learning and memory. Annu. Rev. Neurosci. 29, 565–

598

258

18 Gabbott, P.L. et al. (2005) Prefrontal cortex in the rat: projections tosubcortical autonomic, motor, and limbic centers. J. Comp. Neurol.492, 145–177

19 Floresco, S.B. et al. (2001) Glutamatergic afferents from thehippocampus to the nucleus accumbens regulate activity of ventraltegmental area dopamine neurons. J. Neurosci. 21, 4915–4922

20 Taepavarapruk, P. et al. (2000) Hyperlocomotion and increaseddopamine efflux in the rat nucleus accumbens evoked by electricalstimulation of the ventral subiculum: role of ionotropic glutamate anddopamine D1 receptors. Psychopharmacology 151, 242–251

21 Hiranita, T. et al. (2006) Suppression of methamphetamine-seekingbehavior by nicotinic agonists. Proc. Natl. Acad. Sci. U.S.A. 103, 8523–

852722 McDonald, R.J. and White, N.M. (1993) A triple dissociation of

memory systems: hippocampus, amygdala, and dorsal striatum.Behav. Neurosci. 107, 3–22

23 Simons, J.S. and Spiers, H.J. (2003) Prefrontal and medial temporallobe interactions in long-term memory. Nat. Rev. Neurosci. 4, 637–

64824 Churchwell, J.C. et al. (2010) Prefrontal and hippocampal

contributions to encoding and retrieval of spatial memory.Neurobiol. Learn. Mem. 93, 415–421

25 Koob, G.F. and Le Moal, M. (1997) Drug abuse: hedonic homeostaticdysregulation. Science 278, 52–58

26 Abrous, D.N. et al. (2005) Adult neurogenesis: from precursors tonetwork and physiology. Physiol. Rev. 85, 523–569

27 Kaplan, M.S. and Hinds, J.W. (1977) Neurogenesis in the adult rat:electron microscopic analysis of light radioautographs. Science 197,1092–1094

28 Gould, E. et al. (1999) Learning enhances adult neurogenesis in thehippocampal formation. Nat. Neurosci. 2, 260–265

29 Shors, T.J. et al. (2002) Neurogenesis may relate to some but not alltypes of hippocampal-dependent learning. Hippocampus 12, 578–584

30 Aimone, J.B. et al. (2006) Potential role for adult neurogenesis in theencoding of time in new memories. Nat. Neurosci. 9, 723–727

31 Deng, W. et al. (2010) New neurons and new memories: how does adulthippocampal neurogenesis affect learning and memory? Nat. Rev.Neurosci. 11, 339–350

32 Aimone, J.B. et al. (2011) Resolving new memories: a critical look atthe dentate gyrus, adult neurogenesis, and pattern separation.Neuron 70, 589–596

33 Castilla-Ortega, E. et al. (2011) When is adult hippocampalneurogenesis necessary for learning? Evidence from animalresearch. Rev. Neurosci. 22, 267–283

34 Eisch, A.J. and Harburg, G.C. (2006) Opiates, psychostimulants, andadult hippocampal neurogenesis: insights for addiction and stem cellbiology. Hippocampus 16, 271–286

35 Mandyam, C.D. et al. (2008) Varied access to intravenousmethamphetamine self-administration differentially alters adulthippocampal neurogenesis. Biol. Psychiatry 64, 958–965

36 Abrous, D.N. et al. (2002) Nicotine self-administration impairshippocampal plasticity. J. Neurosci. 22, 3656–3662

37 Richardson, H.N. et al. (2009) Permanent impairment of birth andsurvival of cortical and hippocampal proliferating cells followingexcessive drinking during alcohol dependence. Neurobiol. Dis. 36,1–10

38 Eisch, A.J. et al. (2000) Opiates inhibit neurogenesis in the adult rathippocampus. Proc. Natl. Acad. Sci. U.S.A. 97, 7579–7584

39 Noonan, M.A. et al. (2008) Withdrawal from cocaine self-administration normalizes deficits in proliferation and enhancesmaturity of adult-generated hippocampal neurons. J. Neurosci. 28,2516–2526

40 Catlow, B.J. et al. (2010) Effects of MDMA (‘ecstasy’) duringadolescence on place conditioning and hippocampal neurogenesis.Eur. J. Pharmacol. 628, 96–103

41 Brown, T.E. et al. (2010) Reducing hippocampal cell proliferation inthe adult rat does not prevent the acquisition of cocaine-inducedconditioned place preference. Neurosci. Lett. 481, 41–46

42 Garcia-Fuster, M.J. et al. (2010) Impact of cocaine on adulthippocampal neurogenesis in an animal model of differentialpropensity to drug abuse. Eur. J. Neurosci. 31, 79–89

43 Sudai, E. et al. (2011) High cocaine dosage decreases neurogenesis inthe hippocampus and impairs working memory. Addict. Biol. 16,251–260

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

44 Canales, J.J. (2007) Adult neurogenesis and the memories of drugaddiction. Eur. Arch. Psychiatry Clin. Neurosci. 257, 261–270

45 Canales, J.J. (2010) Comparative neuroscience of stimulant-inducedmemory dysfunction: role for neurogenesis in the adult hippocampus.Behav. Pharmacol. 21, 379–393

46 Noonan, M.A. et al. (2010) Reduction of adult hippocampalneurogenesis confers vulnerability in an animal model of cocaineaddiction. J. Neurosci. 30, 304–315

47 Steiner, B. et al. (2006) Type-2 cells as link between glial and neuronallineage in adult hippocampal neurogenesis. Glia 54, 805–814

48 Taffe, M.A. et al. (2010) Long-lasting reduction in hippocampalneurogenesis by alcohol consumption in adolescent nonhumanprimates. Proc. Natl. Acad. Sci. U.S.A. 107, 11104–11109

49 Nixon, K. (2006) Alcohol and adult neurogenesis: roles inneurodegeneration and recovery in chronic alcoholism. Hippocampus16, 287–295

50 Hansson, A.C. et al. (2010) Long-term suppression of forebrainneurogenesis and loss of neuronal progenitor cells followingprolonged alcohol dependence in rats. Int. J. Neuropsychopharmacol.13, 583–593

51 Kim, W.R. et al. (2011) Time-dependent involvement of adult-borndentate granule cells in behavior. Behav. Brain Res. DOI: S0166-4328(11)00528-6

52 Cosgrove, K.P. et al. (2002) Wheel-running attenuates intravenouscocaine self-administration in rats: sex differences. Pharmacol.Biochem. Behav. 73, 663–671

53 Smith, M.A. and Pitts, E.G. (2011) Access to a running wheel inhibitsthe acquisition of cocaine self-administration. Pharmacol. Biochem.Behav. 100, 237–243

54 Smith, M.A. et al. (2011) The effects of aerobic exercise on cocaine self-administration in male and female rats. Psychopharmacology 218,357–369

55 Miller, M.L. et al. (2011) Reciprocal inhibitory effects of intravenous d-methamphetamine self-administration and wheel activity in rats.Drug Alcohol Depend. DOI: S0376-8716(11)00358-9

56 Smith, M.A. et al. (2008) Aerobic exercise decreases the positive-reinforcing effects of cocaine. Drug Alcohol Depend. 98, 129–135

57 McMillan, D.E. et al. (1995) Effects of access to a running wheel onfood, water and ethanol intake in rats bred to accept ethanol. DrugAlcohol Depend. 40, 1–7

58 Leasure, J.L. and Nixon, K. (2010) Exercise neuroprotection in a ratmodel of binge alcohol consumption. Alcohol. Clin. Exp. Res. 34, 404–

41459 van Praag, H. (2008) Neurogenesis and exercise: past and future

directions. Neuromolecular Med. 10, 128–14060 Garcia-Fuster, M.J. et al. (2011) Decreased proliferation of adult

hippocampal stem cells during cocaine withdrawal: possible role ofthe cell fate regulator FADD. Neuropsychopharmacology 36, 2303–

231761 Zlebnik, N.E. et al. (2010) Reduction of extinction and reinstatement of

cocaine seeking by wheel running in female rats. Psychopharmacology209, 113–125

62 Smith, M.A. et al. (2011) Access to a running wheel decreases cocaine-primed and cue-induced reinstatement in male and female rats. DrugAlcohol Depend. DOI: S0376-8716(11)00350-4

63 Lynch, W.J. et al. (2010) Aerobic exercise attenuates reinstatement ofcocaine-seeking behavior and associated neuroadaptations in theprefrontal cortex. Biol. Psychiatry 68, 774–777

64 Lemaire, V. et al. (2000) Prenatal stress produces learning deficitsassociated with an inhibition of neurogenesis in the hippocampus.Proc. Natl. Acad. Sci. U.S.A. 97, 11032–11037

65 Mandyam, C.D. et al. (2008) Stress experienced in utero reducessexual dichotomies in neurogenesis, microenvironment, and celldeath in the adult rat hippocampus. Dev. Neurobiol. 68, 575–589

66 Davis, B.A. et al. (2008) The effects of novelty-seeking phenotypes andsex differences on acquisition of cocaine self-administration inselectively bred High-Responder and Low-Responder rats.Pharmacol. Biochem. Behav. 90, 331–338

67 Kippin, T.E. et al. (2008) Prenatal stress enhances responsiveness tococaine. Neuropsychopharmacology 33, 769–782

68 Flagel, S.B. et al. (2010) An animal model of genetic vulnerability tobehavioral disinhibition and responsiveness to reward-related cues:implications for addiction. Neuropsychopharmacology 35, 388–400

69 Alvarez-Buylla, A. et al. (2000) The subventricular zone: source ofneuronal precursors for brain repair. Prog. Brain Res. 127, 1–11

70 Gordon, A.S. et al. (1990) The effect of chronic cocaine abuse on humanolfaction. Arch. Otolaryngol. Head Neck Surg. 116, 1415–1418

71 Potter, H. and Butters, N. (1979) Continuities in the olfactory deficitsof chronic alcoholics and alcoholics with the Korsakoff syndrome.Curr. Alcohol 7, 261–271

72 Rupp, C.I. et al. (2006) Executive function and memory in relation toolfactory deficits in alcohol-dependent patients. Alcohol. Clin. Exp.Res. 30, 1355–1362

73 Pow, D.V. (2001) Visualising the activity of the cystine-glutamateantiporter in glial cells using antibodies to aminoadipic acid, aselectively transported substrate. Glia 34, 27–38

74 Baker, D.A. et al. (2003) Neuroadaptations in cystine-glutamateexchange underlie cocaine relapse. Nat. Neurosci. 6, 743–749

75 Wigley, R. et al. (2007) Morphological and physiological interactions ofNG2-glia with astrocytes and neurons. J. Anat. 210, 661–670

76 Chung, W.S. and Barres, B.A. (2011) The role of glial cells in synapseelimination. Curr. Opin. Neurobiol. DOI: S0959-4388(11)00158-9

77 Ge, W.P. et al. (2006) Long-term potentiation of neuron-glia synapsesmediated by Ca2+-permeable AMPA receptors. Science 312, 1533–

153778 Nishiyama, A. et al. (2005) Astrocytes and NG2-glia: what’s in a

name? J. Anat. 207, 687–69379 Benarroch, E.E. (2005) Neuron-astrocyte interactions: partnership for

normal function and disease in the central nervous system. MayoClin. Proc. 80, 1326–1338

80 Alvarez-Buylla, A. and Lim, D.A. (2004) For the long run: maintaininggerminal niches in the adult brain. Neuron 41, 683–686

81 Sanai, N. et al. (2004) Unique astrocyte ribbon in adult human braincontains neural stem cells but lacks chain migration. Nature 427, 740–

74482 Sontheimer, H. and Waxman, S.G. (1993) Expression of voltage-

activated ion channels by astrocytes and oligodendrocytes in thehippocampal slice. J. Neurophysiol. 70, 1863–1873

83 D’Ambrosio, R. et al. (1998) Functional specialization and topographicsegregation of hippocampal astrocytes. J. Neurosci. 18, 4425–4438

84 Cameron, H.A. et al. (1993) Differentiation of newly born neurons andglia in the dentate gyrus of the adult rat. Neuroscience 56, 337–344

85 Nixon, K. and Crews, F.T. (2004) Temporally specific burst in cellproliferation increases hippocampal neurogenesis in protractedabstinence from alcohol. J. Neurosci. 24, 9714–9722

86 Nixon, K. et al. (2008) Distinct cell proliferation events duringabstinence after alcohol dependence: microglia proliferationprecedes neurogenesis. Neurobiol. Dis. 31, 218–229

87 Crews, F.T. and Nixon, K. (2009) Mechanisms of neurodegenerationand regeneration in alcoholism. Alcohol Alcohol. 44, 115–127

88 Czeh, B. et al. (2007) Chronic social stress inhibits cell proliferation inthe adult medial prefrontal cortex: hemispheric asymmetry andreversal by fluoxetine treatment. Neuropsychopharmacology 32,1490–1503

89 Banasr, M. et al. (2007) Chronic unpredictable stress decreases cellproliferation in the cerebral cortex of the adult rat. Biol. Psychiatry 62,496–504

90 Ongur, D. et al. (2007) Electroconvulsive seizures stimulate glialproliferation and reduce expression of Sprouty2 within theprefrontal cortex of rats. Biol. Psychiatry 62, 505–512

91 Madsen, T.M. et al. (2005) Electroconvulsive seizure treatment increasescell proliferation in rat frontal cortex. Neuropsychopharmacology 30, 27–

3492 Kodama, M. et al. (2004) Chronic olanzapine or fluoxetine

administration increases cell proliferation in hippocampus andprefrontal cortex of adult rat. Biol. Psychiatry 56, 570–580

93 Magavi, S.S. et al. (2000) Induction of neurogenesis in the neocortex ofadult mice. Nature 405, 951–955

94 Hoehn, B.D. et al. (2005) Neurogenesis in rats after focal cerebralischemia is enhanced by indomethacin. Stroke 36, 2718–2724

95 Miyake, T. et al. (1992) Reactive proliferation of astrocytes studied byimmunohistochemistry for proliferating cell nuclear antigen. BrainRes. 590, 300–302

96 Burns, K.A. et al. (2009) Developmental and post-injury corticalgliogenesis: a genetic fate-mapping study with Nestin-CreER mice.Glia 57, 1115–1129

259

Review Trends in Neurosciences April 2012, Vol. 35, No. 4

97 Yuan, X. et al. (1998) A role for glutamate and its receptors in theregulation of oligodendrocyte development in cerebellar tissue slices.Development 125, 2901–2914

98 Rhodes, J.S. et al. (2005) Patterns of brain activation associated withcontextual conditioning to methamphetamine in mice. Behav.Neurosci. 119, 759–771

99 Berglind, W.J. et al. (2007) A BDNF infusion into the medialprefrontal cortex suppresses cocaine seeking in rats. Eur. J.Neurosci. 26, 757–766

100 Koya, E. et al. (2009) Role of ventral medial prefrontal cortex inincubation of cocaine craving. Neuropharmacology 56 (Suppl. 1),177–185

101 Taylor, A.H. et al. (2007) The acute effects of exercise on cigarettecravings, withdrawal symptoms, affect and smoking behaviour: asystematic review. Addiction 102, 534–543

102 Taylor, A.H. et al. (2005) Acute effects of self-paced walking on urges tosmoke during temporary smoking abstinence. Psychopharmacology181, 1–7

103 Ussher, M.H. et al. (2008) Exercise interventions for smokingcessation. Cochrane Database Syst. Rev. CD002295

104 Brown, R.A. et al. (2009) Aerobic exercise for alcohol recovery:rationale, program description, and preliminary findings. Behav.Modif. 33, 220–249

105 Thiel, K.J. et al. (2009) Anti-craving effects of environmentalenrichment. Int. J. Neuropsychopharmacol. 12, 1151–1156

106 Burmeister, J.J. et al. (2003) Effects of fluoxetine and d-fenfluramineon cocaine-seeking behavior in rats. Psychopharmacology 168, 146–

154107 Crews, F.T. et al. (2004) Exercise reverses ethanol inhibition of neural

stem cell proliferation. Alcohol 33, 63–71108 Herrera, D.G. et al. (2003) Selective impairment of hippocampal

neurogenesis by chronic alcoholism: protective effects of anantioxidant. Proc. Natl. Acad. Sci. U.S.A. 100, 7919–7924

109 Hsieh, J. and Eisch, A.J. (2010) Epigenetics, hippocampalneurogenesis, and neuropsychiatric disorders: unraveling thegenome to understand the mind. Neurobiol. Dis. 39, 73–84

110 Hoglinger, G.U. et al. (2004) Dopamine depletion impairs precursorcell proliferation in Parkinson disease. Nat. Neurosci. 7, 726–735

111 Mu, Y. et al. (2011) Dopaminergic modulation of cortical inputs duringmaturation of adult-born dentate granule cells. J. Neurosci. 31, 4113–

4123112 Song, H. et al. (2002) Astroglia induce neurogenesis from adult neural

stem cells. Nature 417, 39–44113 Barkho, B.Z. et al. (2006) Identification of astrocyte-expressed factors

that modulate neural stem/progenitor cell differentiation. Stem CellsDev. 15, 407–421

114 Lie, D.C. et al. (2005) Wnt signalling regulates adult hippocampalneurogenesis. Nature 437, 1370–1375

260

115 Jessberger, S. et al. (2009) Dentate gyrus-specific knockdown of adultneurogenesis impairs spatial and object recognition memory in adultrats. Learn. Mem. 16, 147–154

116 Ables, J.L. et al. (2010) Notch1 is required for maintenance of thereservoir of adult hippocampal stem cells. J. Neurosci. 30, 10484–

10492117 Alexson, T.O. et al. (2006) Notch signaling is required to maintain all

neural stem cell populations – irrespective of spatial or temporalniche. Dev. Neurosci. 28, 34–48

118 Breunig, J.J. et al. (2007) Notch regulates cell fate and dendritemorphology of newborn neurons in the postnatal dentate gyrus.Proc. Natl. Acad. Sci. U.S.A. 104, 20558–20563

119 Duan, X. et al. (2007) Disrupted-In-Schizophrenia 1 regulatesintegration of newly generated neurons in the adult brain. Cell130, 1146–1158

120 Enomoto, A. et al. (2009) Roles of disrupted-in-schizophrenia 1-interacting protein girdin in postnatal development of the dentategyrus. Neuron 63, 774–787

121 Lagace, D.C. et al. (2008) Cdk5 is essential for adulthippocampal neurogenesis. Proc. Natl. Acad. Sci. U.S.A. 105,18567–18571

122 Jessberger, S. et al. (2008) Cdk5 regulates accurate maturation ofnewborn granule cells in the adult hippocampus. PLoS Biol. 6,e272

123 Altman, J. (1969) Autoradiographic and histological studies ofpostnatal neurogenesis. 3. Dating the time of production and onsetof differentiation of cerebellar microneurons in rats. J. Comp. Neurol.136, 269–293

124 Cameron, H.A. and Gould, E. (1996) Distinct populations of cells inthe adult dentate gyrus undergo mitosis or apoptosis in response toadrenalectomy. J. Comp. Neurol. 369, 56–63

125 Maslov, A.Y. et al. (2004) Neural stem cell detection, characterization,and age-related changes in the subventricular zone of mice. J.Neurosci. 24, 1726–1733

126 Chehrehasa, F. et al. (2009) EdU, a new thymidine analogue forlabelling proliferating cells in the nervous system. J. Neurosci.Methods 177, 122–130

127 Eisch, A.J. and Mandyam, C.D. (2007) Adult neurogenesis: cananalysis of cell cycle proteins move us ‘Beyond BrdU’? Curr.Pharm. Biotechnol. 8, 147–165

128 Johnson, M.A. et al. (2009) Cell-intrinsic signals that regulate adultneurogenesis in vivo: insights from inducible approaches. BMB Rep.42, 245–259

129 Kempermann, G. et al. (2004) Milestones of neuronal development inthe adult hippocampus. Trends Neurosci. 27, 447–452

130 Stewart, C.V. and Plenz, D. (2006) Inverted-U profile of dopamine-NMDA-mediated spontaneous avalanche recurrence in superficiallayers of rat prefrontal cortex. J. Neurosci. 26, 8148–8159