Molecular Neurobiology of Addiction

Molecular Neurobiology of Addiction

Molecular Neurobiology of Addiction E J Nestler, Mount Sinai School of Medicine, New York, NY, USA ª 2010 Elsevier Ltd. All rights reserved. Glossary...

282KB Sizes 0 Downloads 122 Views

Molecular Neurobiology of Addiction E J Nestler, Mount Sinai School of Medicine, New York, NY, USA ª 2010 Elsevier Ltd. All rights reserved.

Glossary Chromatin – Material in the cell nucleus composed of DNA, histones, and non-histone proteins. The structure of chromatin (whether its open or closed) around a gene determines the rate at which that gene is transcribed. Chromatin immunoprecipitation (ChIP) – Experimental procedure in which lightly fixed chromatin is fragmented and then immunoprecipitated with an antibody to a histone modification, transcription factor, or some other nuclear regulatory protein; the level of a given gene in the immunoprecipitate is then quantified. Dominant-negative antagonist – A protein, which by itself is inactive biologically, inhibits the activity of another endogenous protein. Gene expression – The process controlling the types and amounts of individual genes expressed (transcribed into mRNA and protein) in a tissue. Histone – The major protein constitutent of chromatin. Covalent modification of histones (e.g., their acetylation, methylation, and phosphorylation) controls the degree to which chromatin is active or inactive. Histone acetyl transferase – Enzymes that add acetyl groups to histones and thereby generally promote gene expression. Histone deacetylase – Enzymes that remove acetyl groups to histones and thereby generally inhibit gene expression. Nucleosome – Unit of chromatin consisting of the DNA double helix wound around an octomeric complex of eight histone proteins. Protein phosphorylation – A process by which phosphor groups are added to other proteins by enzymes called protein kinases. Phospho groups, because of their large size and charge, alter the activities of proteins, for example, they can activate TFs. Second messenger – Signals in the cells that are induced by neurotransmitter–receptor interactions and mediate the effect of those interations on virtually all aspects of a nerve cell’s function, including the regulation of gene expression. Transcription – The process by which a gene is transcribed into its encoded mRNA (messenger RNA). mRNA is then translated into protein. Transcription factor – A class of protein that binds to specific receptor sites present in target genes and thereby increases or decreases the extent to which that gene is expressed.

250

The Synapse as the Target of Drugs of Abuse All drugs of abuse initially affect the brain by influencing the amount of a neurotransmitter present at the synapse or by interacting with specific neurotransmitter receptors. Table 1 lists examples of such acute pharmacological actions of some commonly used drugs of abuse. The fact that drugs of abuse initially influence different neurotransmitter and receptor systems in the brain explains the very different actions produced by these drugs acutely. For example, the presence of high levels of opioid receptors in the brainstem and spinal cord explains why opiates can exert such profound effects on respiration, level of consciousness, and nociception. In contrast, the importance of noradrenergic mechanisms in the regulation of cardiac function explains why cocaine can exert potent cardiotoxic effects. In contrast to the many disparate acute actions of drugs of abuse, the drugs exert some common behavioral effects: they are all positively reinforcing after shortterm exposure and cause a similar behavioral syndrome (addiction) in vulnerable individuals following longterm exposure. This suggests that there are certain regions of the brain where the distinct, acute, pharmacological actions of these drugs converge. Indeed, we now know that activation of opioid receptors (by opiates), inhibition of monoamine reuptake (by cocaine), or facilitation of -aminobutyric acid (GABA)-ergic and inhibition of N-methyl-D-aspartate (NMDA) glutamatergic neurotransmission (by ethanol) elicit some common neurobiological responses – in the brain’s reward circuitry – that mediate their reinforcing and addicting properties. The best-established components of the reward circuitry, as discussed elsewhere in this encyclopedia, include dopamine (DA)ergic neurons in the ventral tegmental area (VTA) of the midbrain and their projections to the nucleus accumbens (NAcc) and several other limbic forebrain regions.

Transcriptional Mechanisms of Addiction The acute pharmacological actions of a drug of abuse per se do not explain the long-term effects of repeated drug exposure. To understand such long-term effects, it is

Molecular Neurobiology of Addiction Table 1 Examples of acute pharmacologic actions of drugs of abuse Drug

Action

Opiates Cocaine Amphetamine Ethanol

Agonist at , , and  opioid receptorsa Inhibits monoamine reuptake transporters Stimulates monoamine release Facilitates GABAA receptor function and inhibits NMDA glutamate receptor functionb Agonist at nicotinic acetylcholine receptors Agonist at CB1 cannabinoid receptorsc Partial agonist at 5HT2A serotonin receptors Antagonist at NMDA glutamate receptors

Nicotine Cannabinoids Hallucinogens Phencyclidine (PCP) a

Activity at  and  receptors is thought to mediate the reinforcing actions of opiates. b The mechanism by which ethanol produces these effects has not been established. In addition, ethanol affects many other neurotransmitter systems in brain. c Several lipid-related molecules have been implicated as endogenous ligands for this receptor, such as anandamide. GABAA, -aminobutyric acid A; NMDA, N-methyl-D-aspartate; 5HT2, 5-hydroxytryptamine (serotonin) 2.

necessary to understand adaptive mechanisms in neurons, which involve the ability of neurotransmitter–receptor interactions to regulate virtually every process in a neuron over a longer timescale. Such effects are mediated by altering the functional activity of proteins that are already present in the neuron or by regulating the amount of the proteins through the regulation of gene expression. The remainder of this article provides an overview of the mechanisms by which repeated exposure to a drug of abuse regulates gene expression in brain reward regions. The activation or repression of specific transcriptional patterns is part of a neuron’s response to virtually every cellular signal. Accordingly, transcriptional regulation can be viewed as the ultimate target of signal transduction cascades, and is a critical mediator of a wide range of neuroadaptations in neural structure, connectivity, and function, which occur in response to environmental stimuli, such as drugs of abuse. Considering the importance placed on alterations in gene expression in addiction, it is surprising that only a small number of transcription factors have, to date, been directly implicated in addictive disorders. Moreover, even less is known about the target genes that are regulated by these transcription factors to mediate stable behavioral change. This article focuses on those transcription factors which have been shown to regulate the activity of the brain’s reward pathways in animal models of drug addiction.

CREB–CREM–ATF Family The cyclic adenosine monophosphate (cAMP) response-element-binding protein (CREB) family of transcription factors, in addition to CREB itself, include cAMP

251

response-element modulator (CREM) and activators of transcription (ATFs). These proteins are bZip transcription factors; bZip refers to the basic domain (b) of the proteins that binds a specific consensus sequence (cAMP response element (CRE)) in gene promoter regions, and the leucine zipper domain (Zip) which allows homo- or heterodimerization of two bZip transcription factors necessary for the transcriptional activity of these proteins. Although most proteins of this family heterodimerize selectively with other CREB–CREM–ATF transcription factors, some family members (specifically ATF2) can dimerize with bZips of other families and act on distinct response elements. To complicate matters further, there are many splice variants of these transcription factors that can act as potent repressors of transcription due to the absence of activation domains in the proteins. An example is inducible cAMP early repressor (ICER) – a product of the CREM gene, which represses CRE-mediated transcription. Most evidence for a role of the CREB–CREM–ATF family in brain reward regions in addiction models has focused on CREB itself. CREB is activated both in the NAcc and VTA in response to acute and chronic administration of certain drugs of abuse (cocaine, amphetamine, opiates, etc.). This has been shown by direct demonstration of increases in the phosphorylation of CREB at ser133, which is required for its transcriptional activation. It has also been demonstrated by use of CRE-LacZ transgenic mice, in which CRE transcriptional activity is induced in brain reward regions under these conditions. There is now a large body of evidence to support the view that CREB activation in the NAcc by drugs of abuse mediates a form of tolerance and dependence to drug exposure. Viral-mediated overexpression of CREB in the NAcc decreases an animal’s sensitivity to the rewarding effects of several drugs of abuse, whereas blockade of CREB function in this region, via overexpression of a dominant-negative antagonist of CREB, causes the opposite effect. Studies of bitransgenic mice – in which CREB or its dominant negative is inducibly expressed in the NAcc of adult animals – as well as studies of CREB knockdown mice, generally support these conclusions. Interestingly, increased CREB activity in the NAcc also reduces an animal’s sensitivity to natural rewards and induces depression-like behavior in several rodent assays. Thus, CREB activation in the NAcc by drugs of abuse could mediate some of the negative emotional symptoms seen in many drug addicts during early phases of withdrawal. CREB activity in the VTA is more complex. Here, CREB can either decrease or increase an animal’s sensitivity to a drug of abuse depending on the subregion of the VTA involved. Further work is needed to better understand these actions of CREB, as well as the influence of several other members of the CREB– CREM–ATF family in regulating responses to drugs of

252 Molecular Neurobiology of Addiction

abuse. There is recent evidence, for example, that drugs of abuse – in addiction to activating CREB – also activate ICER as well as ATF2, ATF3, and ATF4 in the NAcc. Consequently, the net effect of drug exposure on the expression of CRE-containing target genes is likely to be very complex. For example, activation of ICER – a transcriptional repressor – may serve to dampen the functional consequences of CREB (and of some of the ATFs) induced during the course of drug exposure. The effects of CREB and related transcription factors on NAcc function and behavior are mediated in part by the opioid peptide dynorphin, which is increased by CREB and decreased upon inactivation of CREB. Dynorphin acts on -opioid receptors within the NAcc and VTA to produce aversive effects by reducing DA release from presynaptic dopaminergic terminals (Figure 1). Thus, activation of CREB, and the resulting induction of dynorphin, in response to long-term drug exposure represents a mechanism of tolerance to drug reward as well as dysphoria during drug withdrawal (dependence).

AP1 Family Fos and Jun proteins, like CREB proteins, are bZip transcription factors. Fos family members dimerize with Jun family members to form active AP1 complexes, which then bind to AP1 sites in gene promoters. Genes encoding Fos and Jun family proteins are immediate early genes, which means that they are induced very rapidly in

response to an acute stimulus. Accordingly, acute administration of virtually any drug of abuse induces many Fos and Jun family members in the NAcc. Not only is the induction very rapid, but it is also very transient, because the Fos and Jun mRNA’s and proteins are all highly unstable. FosB is a C-terminal truncated splice variant of the FosB gene. It, too, is induced rapidly in response to acute drug exposures. However, unlike all other Fos and Jun family proteins, FosB is a relatively stable protein. This stability is mediated partly by the loss of the C-terminus – which contains degron domains that are conserved in all other Fos family proteins – and by its phosphorylation. As a result, in response to repeated drug administration, levels of FosB accumulate and eventually become the predominant Fos family protein in NAcc neurons. This pattern of induction is illustrated in Figure 2. Induction of FosB occurs as a common response to many classes of addictive drugs, including cocaine, amphetamine, methamphetamine, opiates, nicotine, phencyclidine (PCP), cannabinoids, and alcohol. FosB is also induced in the NAcc after repeated consumption of natural rewards, like wheel running, sucrose intake, or sexual behavior. Studies with inducible bitransgenic mice and viral vectors show that induction of FosB in the NAcc increases an animal’s sensitivity to the rewarding effects of drugs of abuse, and also seems to increase incentive motivation, or drive, for the drugs. In addition, FosB increases an animal’s drive for natural rewards. Thus, FosB may function as a sustained molecular switch,

Figure 1 Feedback between the NAc and VTA via CREB activation. Cocaine and amphetamine have been shown to activate prodynorphin gene expression in the NAc via D1 dopamine receptor stimulation, the cyclic adenosine monophosphate (cAMP) pathway, and the phosphorylation of CREB. The resulting dynorphin peptides are transported to presynaptic terminals including terminals that feed back on VTA dopaminergic neurons. Dynorphin peptides are agonists at inhibitory -opioid receptors, resulting in decreased dopamine release. From Nestler EJ (2001) Molecular basis of long-term plasticity underlying addiction. Nature Reviews Neuroscience 2: 119–128.

Molecular Neurobiology of Addiction

253

Figure 2 Regulation of FosB by drugs of abuse. The figure shows the several waves of induction of Fos family proteins in the NAc after a single exposure to a drug of abuse. These proteins include c-Fos and several other Fos family proteins (FosB, Fra-1, Fra-2, etc.). Unlike all of these Fos family proteins, which are highly unstable, isoforms of FosB are highly stable and, therefore, persist in the brain long after drug exposure. Because of this stability, FosB accumulates uniquely with repeated drug exposures. From Nestler EJ (2001) Molecular basis of long-term plasticity underlying addiction. Nature Reviews Neuroscience 2: 119–128.

which first helps initiate and then maintains a state of addiction for many types of rewards for a relatively prolonged period of time. There is early evidence that induction of FosB in the NAcc may mediate the ability of a drug of abuse to increase the dendritic branching, and the number of spines on distal dendrites, in NAcc neurons. Although the precise functional role played by these morphological changes is not known, the changes have been shown to persist for at least several months after the last drug exposure, and are hypothesized to mediate the nearpermanent sensitization in drug responsiveness seen in certain animal models of addiction. FosB is one mediator of these drug-induced changes in dendritic structure via its regulation of several target genes. For example, chronic cocaine administration induces cyclin-dependent kinase 5 (Cdk5) and nuclear factor B (NFB) in the NAcc, effects mediated via FosB, and infusion of an inhibitor of either Cdk5 or NFB into the NAcc prevents the ability of cocaine to increase dendritic spine density in this region. The implication of these findings is that structural changes caused by repeated cocaine administration may be mediated, in part, via induction of FosB and may persist long after the FosB signal itself

dissipates. While FosB is the longest-lasting known molecular change in the brain seen in the context of drug exposure, and perhaps to any other perturbation of the adult brain, it nevertheless undergoes proteolysis at some finite rate: FosB dissipates to normal levels within a month or two of drug withdrawal. This means that FosB cannot per se mediate the extremely long-lived changes in brain and behavior associated with addiction and depression. One possibility is that FosB causes other changes in the brain, such as the changes described in dendritic structure, which themselves are more permanent.

Other Transcription Factors that Regulate Brain Reward Pathways Recent work has shown that several other transcription factors are also regulated in brain reward regions by drugs of abuse, although in general much less is known about the behavioral consequences of their induction compared with CREB and FosB. As stated above, the transcription factor, NFB, has recently been shown to be induced in the NAcc by chronic cocaine. NFB has been the subject

254 Molecular Neurobiology of Addiction

of intense investigation for its widespread involvement in a multitude of disease states and for its inducible pattern of gene expression. Several types of cellular stress, including cytokines, growth factors, viruses, and environmental hazards, induce NFB in peripheral cells. Under normal conditions, NFB, which is composed of a dimer of any of several subunits, termed p50, p52, p65, c-Rel, and RelB (most abundantly p50 and p65), remains sequestered in the cytoplasm by inhibitory-B (IB). Upon phosphorylation by I- kinase (IK), IB releases the inactive NFB dimer, which can then be phosphorylated and transported to the nucleus where it undergoes further posttranslational modifications to initiate transcription of a range of genes involved in cell survival. In the brain, NFB has received considerably less attention, however, it has been implicated in certain forms of learning-related plasticity. Although the involvement of NFB in addiction and depression models is not well studied, NFB has been implicated in the neurotoxic effects of amphetamine and amphetamine-like drugs. Furthermore, chronic cocaine administration upregulates the NFB subunits p65, p105 (precursor to p50), and IB and thereby increases NFB-dependent transcription in the NAcc where it serves to enhance dendritic arborizations. As already mentioned, cocaine induction of the NFB subunits is mediated by FosB. Studies are underway to better understand the target genes downstream of NFB upregulation in drug-addiction models. Clock and other members of the basic helix–loop–helix– PAS (PER-ARNT-SIM) transcription factor family are best known for their control of circadian rhythms. The first evidence that this gene family may be involved in reward came from studies of Drosophila, where loss of Clock dramatically altered the flies’ responses to cocaine. More recent work has demonstrated a role for Clock and other circadian genes in mammalian models of addiction. Thus, mice lacking Clock show dramatically increased activity of VTA DA neurons and increased behavioral responses to cocaine. Mice lacking the Clock homolog, NPAS2, or the Clock-NPAS2 target genes, Per (period), also show abnormal responses to the rewarding effects of cocaine. Current research is focused on defining the target genes through which these circadian genes act to regulate the VTA–NAcc pathway in addiction models. Several drugs of abuse induce the release of systemic glucocorticoids, which activate glucocorticoid receptors expressed throughout the brain. These receptors, which are members of the nuclear receptor family of transcription factors, are expressed in the VTA and NAcc. As glucocorticoids are released in response to stress, they may provide one important mechanism by which stress regulates the reward pathway and addiction behavior. Estrogen and progesterone receptors, members of the nuclear receptor family as well, are also expressed in the

VTA–NAcc pathway and have been found to regulate drug-reward mechanisms. Such effects may contribute to gender differences observed in behavioral responses to drugs of abuse and stress. However, the target genes for glucocorticoid, estrogen, and progesterone receptors in the VTA and NAcc, through which steroid hormones regulate reward mechanisms, have not yet been established.

Regulation of Chromatin Remodeling in Brain Reward Pathways Binding of transcription factors to their target genes regulates transcription via chromatin-remodeling events, which are becoming increasingly well understood. Interest in chromatin remodeling in the context of drug addiction comes from two sources. First, studies of chromatin remodeling make it possible to delineate the detailed molecular events by which drug-regulated transcription factors activate or repress target genes in the VTA and NAcc in vivo. Second, such changes in chromatin structure provide a novel mechanism by which exposure to a drug of abuse may cause lasting changes in gene expression that outlive changes in the transcription factors themselves. The rate of expression of a particular gene is controlled by its location within nucleosomes and by the activity of the cell’s transcriptional machinery. A nucleosome is a short span of DNA that is wound around a complex of histones and other nuclear proteins. Transcription of a gene requires the unwinding of a nucleosome, which makes the gene accessible to the basal transcription complex, comprised of RNA polymerase (which transcribes the new RNA strand) and numerous regulatory proteins (which unwind the nucleosomes). Transcription factors act by enhancing (or inhibiting) the activity of the basal transcription complex; this is achieved by altering nucleosomal structure through changes in histone aceylation, effects mediated by histone acetyltransferases (HAT) or histone deacetylases (HDAC), as well as through many other modifications of histones or the DNA directly. Transcription factors also recruit other regulatory proteins to the complex, which further modify chromatin structure. For example, chromatin remodeling involves enzymes (e.g., SWItch/ Sucrose NonFermentable (SWI–SNF; mating switching and sucrose nonfermenting) complex) that reposition nucleosomes, in an adenosine triphosphate (ATP)-dependent manner, and thereby further make genes accessible for transcription. Recent evidence has demonstrated the relevance of chromatin remodeling to drug-induced neuroadaptations within the brain’s reward pathway. This work is made possible by use of chromatin immunoprecipitation (ChIP)

Molecular Neurobiology of Addiction

assays, in which brain tissue (under control or drug-treated conditions) is lightly fixed in formaldehyde to crosslink DNA to nearby histones and other proteins. The fixed chromatin is then sheared into smaller fragments, immunoprecipitated using an antibody directed against a protein of interest, the immunoprecipitate is un-crosslinked, and then individual genes of interest are analyzed by quantitative real-time polymerase chain reaction (PCR). Alternatively, the immunoprecipitated DNA is amplified and analyzed on a promoter chip, or sequenced, to obtain a genome-wide assessment of regulated genes. This approach makes it possible to identify changes that occur genome-wide within the brain reward circuit as a consequence of drug exposure. Using ChIP assays, differential chromatin-remodeling events (such as histone acetylation and deacetylation) have been observed at various cocaine-regulated genes. For example, it was found that chronic, but not acute, cocaine administration induces histone acetylation at the Cdk5 gene promoter in the NAcc. This is consistent with earlier observations that cocaine induces Cdk5 expression in this brain region, but provides the first direct evidence that this induction is mediated via activation of the Cdk5 gene per se. Moreover, after chronic cocaine use, ChIP assays revealed increased binding of FosB to the Cdk5

Ac

Ac

gene promoter, while inducible overexpression of FosB in the NAcc of adult bitransgenic mice, which is sufficient for induction of Cdk5 expression in vivo, caused increased binding of FosB to the Cdk5 gene. Chronic cocaine administration, or inducible overexpression of FosB, also causes increased binding of Brg1 (a component of the SWI–SNF complex) to the Cdk5 gene. Together, these results support a scheme whereby the gradual accumulation of FosB, in response to chronic cocaine administration, recruits chromatin-remodeling factors, such as specific HATs to induce histone acetylation as well as Brg1-containing chromatin-remodeling complexes, to its target genes such as Cdk5 (Figure 3). Importantly, recent work has established the behavioral relevance of histone acetylation in the NAcc. First, viralmediated overexpression of particular HDACs in the NAcc dramatically blocked the rewarding effects of cocaine. Conversely, treatment of animals with structurally distinct HDAC inhibitors caused the opposite effect and increased an animal’s sensitivity to the behavioral effects of cocaine. These HDAC inhibitors were also found to interact synergistically with cocaine to induce histone acetylation at responsive gene promoters. One of the important HDACs involved in these phenomena is HDAC5, whose function is downregulated in the NAcc Deacetylated histone tails

HDAC

Me

255

Ac

Me

Ac

Rep

(a) Acetylated histone tails Ac

HAT

Ac JunD

Ac

Ac TFIID

Ac

RNA Pol II

Ac

ΔFosB SWI-SNF (b)

Cdk5

Activator recruits HAT and remodeler

Figure 3 Scheme of proposed chromatin-remodeling events at a cocaine-activated gene. (a) shows the repressed state of chromatin, where a site-specific repressor (Rep) recruits an HDAC complex, which removes acetyl groups (Ac) from histone N-terminal tails. Gene inactivation likely involves other modifications, such as methylation (Me) of histone tails. (b) shows the active state of chromatin around a cocaine-activated gene (e.g., Cdk5), where a cocaine-induced transcriptional activator (e.g., FosB-JunD) recruits a HAT and chromatin-remodeling complex (SWI–SNF), which induce acetylation (and presumably demethylation and other modifications) of histone tails and repositioning of nucleosomes. These actions facilitate the binding of general transcription factors and the basal transcriptional apparatus (e.g., TFIID and RNA polymerase II (PolII)) to the promoter. From Kumar A, Green TA, Russo SJ, Renthal W, and Nestler EJ (2009) Transcription and reward systems. In: Squire LR (ed.) Encyclopedia of Neuroscience, ch 195, vol. 9, pp. 1063–1070. Oxford: Academic Press.

256 Molecular Neurobiology of Addiction

by chronic cocaine and which normally serves to dampen cocaine’s effects on gene expression and behavior. These findings demonstrate directly that histone acetylation can modify both biochemical and behavioral responses to cocaine, and implicate chromatin remodeling as a key mechanism underlying the molecular control of the brain’s reward regions by drugs of abuse.

See also: Animal Models of Bipolar Disorder; Cellular Plasticity in Cocaine and Alcohol Addiction; Drug Addiction; Neural Systems of Motivation; Neurobiology of Opioid Addiction; Neurophysiology of Drug Reward; Psychostimulants; Rewarding Brain Stimulation.

Further Reading Future Research Progress has been made in identifying transcription factors, for example, CREB, FosB, NFB, Clock, and steroid hormone receptors, among others, that mediate the effects of drugs of abuse on reward circuits in brain. Future research is needed to better understand how drugs, presumably via regulation of DA and other receptors and of downstream intracellular signaling cascades in the reward circuitry, lead to the regulation of these transcription factors, and how these factors induce chromatin-remodeling events at particular target genes. ChIP assays now make it possible for the first time to identify the precise steps by which different transcriptional complexes are formed at each of the genes regulated in addiction models, and ChIP-chip or ChIP-seq (sequence) assays will help identify the complex gene-regulatory networks affected by these transcription factors to regulate the reward circuitry. This new dimension of molecular research will dramatically expand our understanding of the regulatory control of brain reward regions, and provide fundamentally new paths for the discovery of novel treatments for drug addiction and related conditions.

Acknowledgments Preparation of this article was supported by the National Institute on Drug Abuse. This article is adapted with permission of the publisher from Kumar et al. (2009).

Carlezon WA, Jr., Duman RS, and Nestler EJ (2005) The many faces of CREB. Trends in Neuroscience 28: 436–445. Green TA, Alibhai IN, Unterberg S, et al. (2008) Induction of activating transcription factors ATF2, ATF3, and ATF4 in the nucleus accumbens and their regulation of emotional behavior. Journal of Neuroscience 28: 2025–2032. Kreibich AS and Blendy JA (2005) The role of cAMP response elementbinding proteins in mediating stress-induced vulnerability to drug abuse. International Review in Neurobiology 65: 147–178. Kumar A, Choi K-H, Renthal W, et al. (2005) Chromatin remodeling is a key mechanism underlying cocaine induced plasticity in striatum. Neuron 48: 303–314. Kumar A, Green TA, Russo SJ, Renthal W, and Nestler EJ (2009) Transcription and reward systems. In: Squire LR (ed.) Encyclopedia of Neuroscience, ch 195, vol. 9, pp. 1063–1070. Oxford: Academic Press. McClung CA and Nestler EJ (2003) Regulation of gene expression and cocaine reward by CREB and FosB. Nature Neuroscience 6: 1208–1215. McClung CA and Nestler EJ (2008). Neuroplasticity mediated by altered gene expression. Neuropsychopharmacology 33: 3–17. Nestler EJ (2001) Molecular basis of long-term plasticity underlying addiction. Nature Reviews Neuroscience 2: 119–128. Nestler EJ (2008) Transcriptional mechanisms of addiction: Role of delta FosB. Philosophical Transactions of the Royal Society B: Biological Sciences 363: 3245–3255. Renthal W and Nestler EJ (2008) Epigenetic mechanisms in drug addiction. Trends in Molecular Medicine 14: 341–350. Renthal W, Kumar A, Xiao G-H, et al. (2009) Genome wide analysis of chromatin regulation by cocaine reveals a novel role for sirtuins. Neuron 62(3): 335–348. Ron D and Jurd R (2005) The ups and downs of signaling cascades in addiction. Science STKE 2005(319): 1–16. Russo SJ, Wilkinson MB, Mazei-Robison MS, et al. (2009) Nuclear factor B signaling regulates neuronal morphology and cocaine reward. Journal of Neuroscience 29: 3529–3537. Uz T, Ahmed R, Akhisaroglu M, et al. (2005) Effect of fluoxetine and cocaine on the expression of clock genes in the mouse hippocampus and striatum. Neuroscience 134: 1309–1316. Yao WD, Gainetdinov RR, Arbuckle MI, et al. (2004). Identification of PSD-95 as a regulator of dopamine-mediated synaptic and behavioral plasticity. Neuron 41: 625–638. Yuferov V, Nielsen D, Butelman E, and Kreek MJ (2005) Microarray studies of psychostimulant-induced changes in gene expression. Addiction of Biology 10: 101–118.