The many faces of CREB

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The many faces of CREB William A. Carlezon Jr1, Ronald S. Duman2 and Eric J. Nestler3 1

Department of Psychiatry, Harvard Medical School and McLean Hospital, Belmont MA 02478, USA Department of Psychiatry, Yale University School of Medicine, New Haven CT 06508, USA 3 Department of Psychiatry and Center for Basic Neuroscience, The University of Texas Southwestern Medical Center, Dallas TX 75390-9070, USA 2

The transcription factor CREB is best known for its involvement in learning and memory. However, emerging evidence suggests that CREB activity has very different roles – sometimes beneficial, sometimes detrimental – depending on the brain region involved. Induction of CREB in the hippocampus by antidepressant treatments could contribute to their therapeutic efficacy. By contrast, activation of CREB in the nucleus accumbens and several other regions by drugs of abuse or stress mediates certain aspects of drug addiction, and depressive and anxiety-like behaviors. These complexities suggest that strategies that exploit regional differences in upstream factors or that target specific CREB-regulated genes, rather than CREB itself, could make a promising contribution to the treatment of neuropsychiatric conditions. Introduction cAMP-response-element-binding protein (CREB) is expressed in all cells in the brain and is a member of a family of proteins that function as transcription factors. Localized within the nucleus, transcription factors such as CREB are crucial for stimulus–transcription coupling: the transmission of events that occur at cell membranes into alterations in gene expression. In turn, altering gene expression, by regulating the expression of virtually all types of neuronal proteins, can ultimately affect the function of individual neurons and entire neuronal circuits. There are several comprehensive reviews that describe the molecular mechanisms involved in CREBmediated gene transcription [1–5]. The purpose of this article is to present a short overview of these mechanisms, and then to focus on how CREB might be relevant to the development and expression of neuropsychiatric disorders, and whether treatments that affect CREB directly might have potential as therapies for these conditions. Overview of CREB-mediated gene transcription Numerous intracellular signaling pathways are involved in transmitting information initiated by membrane receptor-mediated actions to the cell nucleus, where they interact with CREB to trigger processes that culminate in gene transcription. The effects of signaling pathways involving adenylyl cyclase (AC) and cAMP [6], Ca2C[6,7] and mitogen activated protein kinase (MAPK) [8] on CREB and CREB-regulated gene transcription have been Corresponding author: Carlezon, W.A. ([email protected]). Available online 27 June 2005

the focus of much research (Figure 1). The key steps involved in CREB-mediated gene transcription include dimerization, binding at response elements in DNA, and phosphorylation [3,4]. CREB is one of several transcription factors that bind as dimers to the cAMP-response element (CRE), a specialized stretch of DNA that contains the consensus nucleotide sequence TGACGTCA. CRE sites are found within the regulatory (promoter or enhancer) region of numerous genes; if a promoter contains CREs, then it could be subject to regulation by CREB, depending on several tissue-specific factors, including the conformation of the nearby chromatin (as will be discussed later). Traditionally, it has been thought that CREB dimers are bound to their CRE sites under basal conditions, but are inactive. According to this view, events at the neuronal membrane, which stimulate intracellular signaling cascades, cause the phosphorylation of both members of the CREB dimer and trigger its transcriptional activity [4]. However, recent work raises the question of whether CREB is constitutively bound to its CRE sites in all cases or, instead, whether CREB phosphorylation sometimes initiates this interaction [9]. These factors are likely to contribute to tissue-specific differences in CREB function, as will be discussed. The ability of signaling pathways triggered by G-protein-coupled receptor stimulation of AC to activate CREB has been characterized most thoroughly. For example, stimulation of dopamine D1-like receptors activates excitatory Gs-coupled receptors, which stimulates AC, increases accumulation of cAMP, and causes liberation of the catalytic subunits of cAMP-dependent protein kinase (PKA). The free catalytic subunits then enter the cell nucleus where they phosphorylate CREB. Ca2C-associated intracellular messengers and MAPKactivated kinases can similarly increase phosphorylation of CREB [6–8,10]. In turn, phosphorylation of CREB activates a cascade of events that involves recruitment of associated proteins such as CREB-binding protein (CBP) and assembly of a larger transcriptional complex (Figure 2a). This transcriptional complex promotes processes such as histone acetylation, which alter the conformation of the nearby chromatin [11–13] and enable the synthesis of RNA by RNA polymerase II. There are numerous phosphorylation sites on the CREB protein that differentially regulate CREB activity. PKA, Ca2C/calmodulin-dependent kinase (CaMK) IV and MAPK-activated ribosomal S6 kinases (RSKs) each phosphorylate CREB at serine 133, which stimulates the

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Figure 1. Some of the cellular events involved in regulation of CREB. Neurotransmitters and neurotrophins act at membrane receptors (e.g. TrkB, AMPA receptors, NMDA receptors and G-protein-coupled receptors) to trigger intracellular signaling cascades that culminate in phosphorylation (P) of CREB. Phosphorylation of CREB at serine 133 activates CREB-mediated gene transcription. The pathways depicted in the figure grossly simplify the many types of interactions that exist among the different intracellular signaling pathways, some of which are shown by the broken lines. As just one example, activation of a Gi-linked receptor might be expected to reduce CREB phosphorylation via inhibition of adenylyl cyclase, but in fact such events have been observed to induce CREB phosphorylation in some cell types, possibly via activation of MAPK cascades. The protein products of numerous representative CREB target genes are depicted in green. Expression of these CREB targets might be region specific; each protein is not necessarily expressed in all areas of the brain. Black arrows signify activation; gray arrows signify inhibition. Abbreviations not defined in the main text: CaM, calmodulin; GluR1Rs, glutamate receptor subunit GluR1 homomeric AMPA receptors; PDE, phosphodiesterase; PLC, phospholipase C; TrkB, neurotrophin tyrosine kinase receptor type 2.

recruitment of CBP and thereby leads to activation of gene transcription [14]. By contrast, CaMKII phosphorylates CREB at serine 142, which promotes the dissociation of the CREB dimer and thus reduces CREB-mediated gene transcription [15,16]. As such, the regulation of CREB-mediated gene transcription is exceedingly complex, and several distinct pathways have roles in translating the effects of diverse extracellular stimuli into alterations in the expression of CRE-containing genes. By extension, several pathways are potential targets for treatment strategies that enable control over CREB and CREB-mediated gene transcription. CREB-mediated gene transcription can be directly altered experimentally by several molecular tools. As one example, engineered mutations of CREB have enabled the development of dominant-negative forms of the protein that act as CREB antagonists. CREBM1 – also known as mCREB – contains a point mutation at serine 133 (serine is replaced by alanine) that prevents both phosphorylation at this site and transactivation of genes www.sciencedirect.com

by CREB [14,17,18]. This mutation does not affect the ability of mCREB to dimerize with endogenous (wild-type) CREB or another mCREB molecule, nor does it affect binding at CRE sites (Figure 2b). As such, transfer of mCREB into a neuron can reduce or block CREBmediated gene transcription either by occluding CRE sites as mCREB homodimers or by heterodimerizing with wild-type CREB; such heterodimers can bind to available CRE sites, but cannot recruit CBP because only one subunit of the dimer can be phosphorylated at serine 133. KCREB is a distinct dominant-negative mutant: it contains a point mutation in the DNA-binding domain that prevents it from binding to CREs. It dimerizes with endogenous CREB, and the resulting heterodimers are unable to bind to DNA-binding sites [19]. Conversely, CREB-VP16 is a constitutively active form of CREB; it can homodimerize, or heterodimerize with wild-type CREB, and activate transcription – via its VP16 viral transcription-activation domain – even in the absence of serine 133 phosphorylation. Use of these various mutants and

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Figure 2. Simplified schematic of the events leading to CREB-mediated gene transcription. (a) Phosphorylation of both members of the CREB dimer at serine 133 activates a cascade of events that involves recruitment of associated proteins including CBP/p300-interacting transactivator and assembly of a larger transcriptional complex. This complex promotes chromatin remodeling via histone acetyl transferase (HAT) and several other proteins, which alters the interaction between the chromatin and the histones (blue cylinders). The red lines represent N-terminal tails of the histones, which undergo acetylation (A) and several other modifications. These processes enable the synthesis of RNA by RNA polymerase II (Pol-II). (b) When mCREB – containing a dominant-negative mutation that prevents phosphorylation at serine 133 – dimerizes with a wild-type CREB molecule and binds at a cAMP-response elements (CRE), CREB-mediated gene transcription is disrupted because CBP/p300 is not recruited.

genetic deletion of CREB have increased our understanding of the molecular mechanisms of gene transcription, and enabled unique insights into how CREB might ultimately affect complex behavior in diverse species. The ability of CREB to affect the function of neurons, brain circuits and complex behavior depends on its ability to alter the expression of target genes. Efforts have been made recently, using chromatin immunoprecipitation and related techniques, to identify the CREB ‘transcriptome’: all those genes regulated by CREB [9,20,21]. A few interesting lessons have been learned. First, studies in cultured cells suggest that CREB can bind to a few thousand gene promoters in a given cell type, although it remains unknown what fraction of these genes are functionally regulated upon this binding. Second, not all genes that contain CRE sites are functional CREB targets, presumably because certain sites might not be accessible within the chromatin. Third, it is clear that the CREB transcriptome differs dramatically from cell type to cell type [9], which means that CREB regulates partially distinct subsets of genes within each brain region. These differences are believed to result from diverse factors such as the requirement of other transcription factors and cofactors and the conformation of the chromatin, which can affect the availability of CRE sites and the assembly of the mature transcriptional complex [4]. Differential expression of proteins that modulate intracellular signaling pathways [e.g. phosphodiesterase type IV (PDE4), www.sciencedirect.com

which degrades cAMP] can similarly cause differences in CREB regulation among distinct cell types, Because of these differences, the consequences of a change in CREB function, or in an upstream pathway, in various brain regions are likely to be multifaceted and difficult to predict a priori. It is technically more difficult to identify CREB targets in the brain than in cultured cells, although CREB regulation of several genes within particular brain regions has been implicated in various phenomena relevant to neuropsychiatric disorders [22]. Some examples include genes that encode other transcription factors (e.g. cFos) [23], transcriptional repressors [e.g. cAMP response element modulator (CREM)] [24], intracellular messengers (e.g. AC-VIII) [25], neurotransmitter synthetic enzymes [e.g. tyrosine hydroxylase (TH)] [26], peptide transmitters [e.g. corticotropin releasing factor (CRF) and somatostatin] [4,27], growth factors [e.g. brain-derived neurotrophic factor (BDNF)] [28], opioid peptides (dynorphin and enkephalin) [29–32] and neurotransmitter receptor subunits (e.g. GluR1) [33,34]. However, these are likely to represent a small subset of the genes whose regulation by CREB in the brain contributes to control of complex behavior under normal and pathophysiological conditions. Recent studies have used DNA microarray techniques to identify the CREB transcriptome within particular brain regions [22], but much more work is needed to validate these targets and understand their roles in mediating

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aspects of CREB function under physiological and pathological conditions. Roles of CREB in complex behaviors CREB is best known for its roles in learning and memory, which have been the focus of much research. CREB affects learning and memory processes because it is a crucial mediator of experience-based neuroadaptations. Experience can take many forms, such as exposure to a maze, to a physical or emotional stressor, or to an addictive substance. Each of these stimuli can affect the intracellular pathways that converge upon CREB. In turn, alterations in CREB activity fundamentally affect the way that cells function in isolation, within groups, or within larger circuits. Because CREB is found within all neural circuits and influences the expression of such diverse genes, it is not surprising that sometimes the consequences of boosting CREB-mediated gene regulation are beneficial, whereas at other times they seem detrimental. CREB and memory One of the first indications that CREB is involved in memory processes was the result of studies involving Aplysia. Serotonin [also known as 5-hydroxytryptamine (5-HT)], which causes long-term facilitation of the gillwithdrawal reflex in Aplysia, was shown to activate transcription of a reporter gene artificially driven by CREs in a CREB-dependent fashion [35]. The connection between CREB and long-term memory was further established in several organisms, using methods that enabled disruption of CREB function. Induction of a dominant-negative mutation or deletion of key CREB isoforms blocked long-term memory in Drosophila [36] and in mice [18,37]. Methods that enabled increases in CREB function then demonstrated enhancements of memory. For example, expression of a CREB activator isoform facilitated long-term memory in Drosophila [38]. Moreover, viral-vector-mediated increases in CREB expression within the rat amygdala increased long-term memory in a fear conditioning paradigm, making the effects of massed training resemble those normally seen after spaced training [39]. The mechanisms that underlie the ability of CREB to facilitate memory are not completely understood – they could involve processes such as the induction of long-term potentiation or depression of synaptic strength [9,40], the growth and formation of new synaptic connections [40,41], or protein synthesis-dependent processes involved in the retrieval and reconsolidation of memory [42]. Efforts are still underway to identify the specific target genes of CREB that mediate its facilitation of learning and memory within the various brain regions and organisms studied. Regardless of these mechanisms, the connection between CREB and enhanced memory has opened the door to new and potentially revolutionary methods with which to improve deficits in cognitive function that accompany certain disease states. It has also raised the possibility that even normal memory and cognitive function could be improved [43,44]. The development of such memory enhancers is cause for excitement and has the potential to benefit almost everyone. However, the use of www.sciencedirect.com

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therapeutics that target CREB directly or indirectly involves several important assumptions which, at least at present, are unproven. Foremost is the assumption that CREB function can be precisely controlled in vivo. Here, the most obvious complication is that CREB is involved in so many crucial cellular functions that it might be difficult to control specific processes [43] and target specific brain regions using therapeutics. This complication is illustrated by studies involving aged monkeys, which suggest that the relationship between CREB and memory is not invariably ‘more is better’. When agents that stimulate PKA activity – which lead to phosphorylation and activation of CREB – were injected into the prefrontal cortex of aged monkeys, there was a substantial decline in cognitive function [45]. By contrast, inhibitors of PKA boosted cognitive-function performance in this test group. The explanation for these seemingly paradoxical findings is that the cAMP–PKA signal transduction pathway might become disinhibited within the PFC with advancing age, and thus further stimulation of this CREB-regulating pathway produces additional deficits. Such data provide strong support for the notion that, at least within the prefrontal cortex, cognitive function requires an optimal range of dopamine D1 receptor–cAMP stimulation [46]: stimulation of these pathways can facilitate cognitive function if activity is low, but is disruptive if activity is normal or already high. Although these studies do not directly establish a role for CREB in the disruption of cognitive performance, they implicate signaling pathways known to regulate CREB activity. Another assumption is that memory enhancement is always beneficial. This assumption is complicated by the possibility that treatments designed to induce episodic or sustained memory enhancements could have the unintended side effect of preventing unimportant – or traumatic – information from being forgotten. An experimental treatment for post-traumatic stress disorder, for example, is propranolol, a b-adrenoceptor antagonist, which inhibits the cAMP–PKA pathway and disrupts memory [47]. Both of these examples illustrate that the benefits of enhancing CREB function and CREB-related neuroplasticity throughout the brain might not always outweigh the risks. As will now be described, recent work designed to establish causal connections between CREB and the biological basis of psychiatric disorders similarly suggests that generalized enhancement of CREB function would have a substantial impact on many facets of complex behavior, sometimes beneficial and at other times detrimental. CREB and addiction Although learning and memory have important roles in aspects of addiction [48–51], CREB-mediated gene expression has been implicated in addiction via brain regions not normally involved in traditional formulations of learning and memory processes. For example, morphine inhibits CREB phosphorylation in the locus coeruleus [52], a brain region important for opiate physical dependence and withdrawal [53,54]. This inhibition undergoes tolerance with chronic morphine exposure,

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due to induction of CREB expression in this region, and precipitation of opiate withdrawal by removing the inhibitory effect of morphine causes dramatic increases in CREB phosphorylation [52,55]. Blockade of CREB function within the locus coeruleus by antisense oligonucleotides or virus-mediated overexpression of dominantnegative mCREB reduces expression of CREB-regulated target genes, and reduces the electrophysiological and behavioral markers of opiate physical dependence and withdrawal [25,56]. CREB might exert a similar influence on morphine physical dependence and withdrawal at the level of several other brainstem nuclei, such as the periaqueductal gray [57]. Accordingly, mice lacking certain forms of CREB show attenuated opiate physical withdrawal [58]. These studies provided early evidence that increased CREB function within brain addiction circuits could be part of the molecular processes that lead to opiate dependence [59]. Alterations in CREB function within the striatum are also involved in addiction processes. Repeated exposure to opiates or stimulant drugs increases activity of the cAMP–PKA pathway within the nucleus accumbens (NAc) [60], which is the ventral component of the striatum and a key neural substrate for the rewarding actions of many drugs of abuse [61,62]. As would be expected, these drugs also activate CREB-mediated transcription within this brain region [30,63–65]. Direct activation of PKA activity – which increases CREB phosphorylation – within the NAc reduces the rewarding effects of cocaine, whereas PKA inhibition has the opposite effect [66]. These findings suggest that CREB activity in this region can regulate the addictive qualities of drugs of abuse. Indeed, direct elevations of CREB levels within the rat NAc by virusmediated gene transfer reduce the rewarding effects of cocaine and make low doses aversive [31]. Conversely, expression of mCREB produces the opposite pattern of results, causing increases in the rewarding effects of cocaine. Viral-vector-mediated alterations in CREB function have similar effects on the rewarding effects of morphine [67]. Similar effects on cocaine reward were seen in transgenic mice with mutations that increase [22] or decrease [68] CREB function. Finally, precipitated opiate withdrawal in primary cultures of NAc neurons increases CREB phosphorylation and the expression of CREB-related target genes [69]. Considering recent observations that NAc function can also contribute to physical signs of opiate withdrawal, it is conceivable that CREB activity in the NAc also influences this aspect of addiction, although this has not yet been studied. Viral-vector-mediated alterations in CREB function in the ventral tegmental area (VTA) of the midbrain, another brain region implicated in the rewarding effects of opiates [70] and cocaine [71], also affect addictive behaviors [34]. Interestingly, within this structure, the consequences of alterations in CREB function appear to be particularly dependent on the anatomical localization of the gene transfer: increased CREB function in rostral portions of the VTA increases the rewarding effects of cocaine and morphine, whereas similar changes in caudal portions have the opposite effects. Such data could indicate within-VTA differences in neuronal input and outputs, or www.sciencedirect.com

they could reflect region-specific signal discrimination through CREB [4]. Studies of CREB-knockdown mice further support a role of CREB in regulation of morphine reward via the VTA [72]. Considered together, these studies reinforce the notion that CREB is a key regulator of experience-dependent changes in complex behavior. When experience takes the form of exposure to drugs of abuse, increased CREB function appears to cause tolerance and dependence – adaptations commonly associated with the development and maintenance of addictive behaviors. Although the CREB-regulated target genes that contribute to these effects are not known, one particular candidate gene within the NAc is dynorphin. Increased dynorphin expression mediated by CREB appears to be associated with aversive or depressive-like effects such as those that often accompany drug withdrawal. Many additional CREB targets have been identified in the NAc [73], and the consequences of increased expression of these molecules might be different to those caused by increased expression of dynorphin. Further work is needed to relate each change and its behavioral consequence (e.g. tolerance to drug reward) to specific processes that contribute to the development and maintenance of addictive behaviors. In addition, recent research has demonstrated that CREB activation (via virus-mediated expression of the constitutively active CREB-VP16 mutant) increases the electrical excitability of NAc neurons, whereas inhibition of CREB (via expression of mCREB) has the opposite effect [74]. An important goal of current research is to identify the target genes of CREB that regulate NAc neuronal excitability, and to relate this regulation to behavioral aspects of addiction. CREB and depression The relationship between CREB and depressive behaviors is another good illustration of how the consequences of elevated CREB activity can differ throughout the brain. Depending on the brain region under study, elevated CREB function can either reduce or produce depressivelike behaviors in laboratory animals. In the hippocampus, CREB appears to be a crucial mediator of antidepressant effects. A wide variety of standard antidepressant treatments (e.g. noradrenaline-reuptake inhibitors, selective serotonin-reuptake inhibitors and electroconvulsive seizures) increase CREB activity within the hippocampus [75–77]. In addition, direct elevation of CREB protein levels using virus-mediated gene transfer has antidepressant-like effects in rodents [78]. Although the mechanisms responsible for these effects remain the focus of intense study, accumulating evidence suggests roles for CREB-regulated expression of neural growth factors [79,80]. For example, many treatments with antidepressant effects in humans also increase the expression of BDNF, a CREB-regulated target gene, in the hippocampus [81]. Such findings raise the possibility that antidepressant treatments have the common effect of stimulating growth factor activity within this region, which is known to be particularly susceptible to damage by stress [82,83]. Increases in growth factor activity within the hippocampus could stimulate regenerative

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processes such as dendritic sprouting and neurogenesis. Each of these processes is seen after sustained regimens of treatments that have antidepressant effects in humans [84]. These processes could contribute to the restoration of normal mood by repairing stress-related damage, and help to explain why the therapeutic effects of antidepressant treatments often take weeks or months to become apparent. Although the hippocampus is certainly not the only brain region involved in depression, it is a prominent limbic structure where abnormalities have been reported in human patients [84,85]. Systemic administration of drugs that act upstream of CREB to stimulate cAMP signaling (PDE4 inhibitors) also have antidepressant effects in models of depression [86] and in early clinical trials [87]. Although the brain regions in which these effects are mediated are not known, the hippocampus is one possible site of action [86]. These drugs are also beginning to receive consideration as cognitive enhancers [43,88]. As such, these findings highlight the potential utility of targeting upstream signaling molecules that lead to selective regulation of CREB [43]. By contrast, elevated CREB activity in the NAc produces various depressive-like effects in rodents. CREB activity in the NAc is elevated by exposure to drugs of abuse (as already stated) or stress [67,89]. Each of these stimuli has been widely implicated in the development and expression of depressive disorders in laboratory animals and in humans. In rats, viral-vector-mediated elevations of CREB levels within the NAc reduce the rewarding effects of cocaine [31,89] and sucrose [67]. These data indicate that a sustained elevation of CREB activity in the NAc produces anhedonia, a hallmark symptom of depression characterized by a diminished ability to experience rewarding stimuli as rewarding. In fact, this CREB-related phenotype appears to reflect a generalized numbing of behavioral responses to emotional stimuli, because animals with increased CREB function in the NAc also show reduced responses to a wide range of aversive conditions [67]. Elevations of CREB levels within the NAc also make low doses of cocaine aversive (a putative sign of dysphoria or negative affect) and increase immobility behavior in the forced-swim test (a putative sign of ‘behavioral despair’) [89]. Similarly, overexpression of CREB in the NAc of inducible transgenic mice produces a depressive-like phenotype [90] and reduces the rewarding effects of cocaine [22]. Finally, early developmental exposure to methylphenidate – a manipulation that causes sustained increases in CREB expression within the NAc [91] – produces these same signs of anhedonia, dysphoria and behavioral despair [92–94]. Conversely, reductions in CREB activity in the NAc, through viral-vector-mediated expression of mCREB, increase the rewarding effects of cocaine and morphine [31,67] and produce antidepressant-like effects in the forced-swim test [88] and the learned helplessness paradigm [89]. Likewise, expression of mCREB in the NAc of inducible transgenic mice produces antidepressant-like effects [89]. Targeted deletion of key CREB isoforms in mutant mice also produces antidepressant-like effects [95]. Regulation of depression-like behavior by changes in CREB activity within the NAc appears to be mediated www.sciencedirect.com

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partly by dynorphin, an endogenous ligand of k opioid receptors [31]. Administration of k-receptor-selective agonists produces effects that are qualitatively similar to those produced by increased CREB function in the NAc, including increased immobility in the forced-swim test [96] and signs of anhedonia in reward models [97]. By contrast, k-receptor-selective antagonists attenuate the prodepressive-like consequences of elevated CREB expression within the NAc [31,89] and mimic the mCREB phenotype. Although the mechanisms by which k-receptor-selective antagonists produce antidepressantlike effects in rats [89,96–98] are unclear, one possibility is that these agents block k opioid receptors that normally inhibit neurotransmitter release from mesolimbic dopamine neurons [99–101]. Indeed, evidence suggests that the actions of a k-receptor-selective antagonist within the NAc itself are sufficient to cause an antidepressant-like effect in the learned helplessness paradigm [90]. Considered together, these data raise the possibility that CREB-mediated transcription of dynorphin within the NAc decreases dopamine function, which triggers certain features of depression. Until recently, the NAc has not been considered a likely site for the pathophysiology of depression, although it makes intuitive sense that symptoms of anhedonia, reduced energy and reduced motivation – which are prominent in many depressed patients – involve this brain reward region [85,89]. Within the amygdala, the consequences of alterations in CREB function in models of depression appear to be state-dependent. Virus-mediated expression of CREB in the amygdala before training in the learned helplessness paradigm causes prodepressive-like effects, whereas expression after training results in antidepressant-like effects [102]. One explanation for these results is that when CREB is expressed before training it enhances learning of the helplessness conditioning, but when expressed after training it enhances the ability of the animal to overcome the helplessness training, or relearning. In either case, these findings provide further evidence that the actions of CREB are regionally and temporally specific. Together, this research strongly suggests that sustained, increased CREB function within the hippocampus produces antidepressant-like effects that correlate with elevated expression of growth factors such as BNDF, whereas the same increases in CREB function within the NAc produce many depressive-like signs that correlate with increased dynorphin expression and stimulation of k opioid receptors. Such observations highlight the fact that CREB functions generally to regulate plasticity, a process that is not inherently good or bad; it could be adaptive, maladaptive, or both simultaneously. In the case of depression, the seemingly paradoxical requirements for antidepressant efficacy – elevations in CREB activity in one region and reductions in another – could detract from the therapeutic actions of treatment regimens that produce global influences on CREB function in brain. CREB and anxiety Increasing evidence indicates that CREB function in the extended amygdala – a macrostructure that includes

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components of the amygdala, bed nucleus of the stria terminalis (BNST) and NAc [103] – can regulate anxietylike behaviors in rats. Data from studies in which CREB was modulated directly using virus-mediated gene transfer indicate that disruption of CREB function within the NAc produces anxiety-like effects [67,104], whereas induction of CREB function within the amygdala produces similar behavioral effects [105]. Studies in which other methods were used to alter CREB function (e.g. exposure to drugs or knockout of CREB isoforms) also suggest a role for CREB in anxiety disorders, although the conclusions are somewhat different [105,106], perhaps owing to differences in overall patterns of CREB expression throughout the brain. Preliminary data suggest an inverse relationship between CREB activation within the BNST and levels of fear and anxiety-like behaviors, as measured using the fear-potentiated startle paradigm [107]. Additional research on the relationship among CREB, CREB-regulated target genes (e.g. neuropeptide Y [105]) and anxiety behaviors might ultimately reveal potential benefits of altering CREB function in the treatment of anxiety-related disorders. The notion that elevated CREB function in NAc causes certain depressive-like symptoms, whereas reduced CREB function in this region causes anxiety-like behavior, might seem paradoxical but can be understood within the role CREB could have in this reward circuit under normal conditions. Our hypothesis is that CREB is a key regulator of the reactivity of brain reward circuits and thereby regulates individual sensitivity to emotional stimuli. Short-term increases in CREB activity in NAc, induced by normal rewarding or aversive stimuli, would dampen responses to subsequent stimuli and facilitate the ability to actively deal with the situation at hand (e.g. consumption of reward or escape from danger). Under more pathological conditions, however, larger and more sustained increases in CREB activity, induced by drugs of abuse or excessive stress, would lead to an excessive dampening of emotional reactivity and to the behavioral phenotype

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already outlined. Conversely, sustained reductions in CREB activity, which are seen under conditions of social isolation [104], would heighten emotional reactivity and in the extreme be associated with a state of anxiety. This work highlights the notion that extreme increases or decreases in CREB function in NAc might be detrimental and argues for further caution in designing CREB-based therapeutics. Other important roles of CREB CREB also has important effects on processes not directly related to complex motivated behavior. For example, CREB within the ocular cortex has a key role in establishing ocular dominance [108,109] and is involved in the loss (but not recovery) of binocular vision [110]. It is conceivable that these processes – or others in the periphery – would be subject to unintentional regulation by CREBtargeting agents that are intended to produce therapeutic effects in the treatment of other CREB-related disorders. The future of CREB-related therapeutics Clearly, increases in CREB function can enhance memory under certain circumstances in diverse species. These observations have provided dramatic improvements in our understanding of both the molecular mechanisms of CREB function and the biological basis of learning and memory. They have justifiably generated a great deal of enthusiasm in the development of therapeutic agents that could improve memory deficits that are associated with conditions characterized by cognitive decline, such as Alzheimer’s disease. Safe and effective cognitive enhancers that would be effective in people who are not afflicted by disease states would also have a great deal of appeal in cultures that are increasingly preoccupied with productivity and self-improvement. However, increases in CREB function can also disrupt cognitive performance under some circumstances. The brain regions in which CREB acts to enhance or disrupt memory could be different, and it remains to be determined which effect

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Figure 3. Activation of CREB-mediated gene transcription in specific brain regions can produce effects that seem either beneficial (green) or detrimental (red). The specific functions of CREB assigned to each brain region are of course simplifications, because many of the regions depicted interact at the level of neural circuits and mediate many complex behavioral outputs. In most cases, the specific CREB-regulated target genes that produce these behavioral adaptations are not understood. Future work could identify region-specific upstream regulators or downstream targets of CREB that would enable precise control over gene transcription within these regions. Abbreviations and representative references: Amg, amygdala [39,102]; BNST, bed nucleus of the stria terminalis [107]; Hip, hippocampus [37,79]; LC, locus coeruleus [25]; NAc, nucleus accumbens [60,89]; OC, occipital cortex [107]; PAG, periaqueductal gray [57]; PFC, prefrontal cortex [45]; VTA, ventral tegmental area [34]. www.sciencedirect.com

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would predominate if it were possible to enhance CREB function throughout the brain. Importantly, CREB also influences other experience-dependent processes that can affect complex motivated behavior. Certain examples of these processes seem beneficial, whereas others seem detrimental. That these effects can often be linked, at least in part, to particular brain regions (Figure 3) highlights the fact that general alterations in CREB function cannot be expected to produce uniform effects throughout the brain. However, the links between effects and regions might be crucial if the promise of CREB-related therapeutic drugs is to be realized in the future. The brainregion-specific functions of CREB illustrated in Figure 3 no doubt represent an over-simplification but, given the complex interactions and redundancies among neural circuits in the brain, there is also no doubt that different brain regions and neural circuits can have distinct behavioral functions. The molecular elements that cause CREB to have specialized roles in each brain region and neural circuit could potentially be utilized and exploited by therapeutics that alter the activity of this transcription factor. Future work could reveal that certain signal transduction pathways are more important in some brain regions than in others. Another fruitful approach might be to focus on regulation of downstream targets of CREB [43]. Therapeutics that act directly on specific CREB target genes, which themselves are expressed more narrowly within the affected brain regions, might enable better control over the proteins that cause disease states or their symptoms, while sparing other crucial functions. Regardless, research on CREB is improving our understanding of the molecular mechanisms controlling both normal complex behavior and the abnormal behavior associated with prominent psychiatric and neurological conditions, and it could one day facilitate discovery of therapies for these disorders. Acknowledgements Supported by grants from the National Institute of Mental Health and National Institute on Drug Abuse.

References 1 Yin, J.C. and Tully, T. (1996) CREB and the formation of long-term memory. Curr. Opin. Neurobiol. 6, 264–268 2 Mayford, M. and Kandel, E.R. (1999) Genetic approaches to memory storage. Trends Genet. 15, 463–470 3 Shaywitz, A.J. and Greenberg, M.E. (1999) CREB: a stimulusinduced transcription factor activated by a diverse array of extracellular signals. Annu. Rev. Biochem. 68, 821–861 4 Mayr, B. and Montminy, M. (2001) Transcriptional regulation by the phosphorylation-dependent factor CREB. Nat. Rev. Mol. Cell Biol. 2, 599–609 5 Josselyn, S.A. et al. (2004) Inducible repression of CREB function disrupts amygdala-dependent memory. Neurobiol. Learn. Mem. 82, 159–163 6 Dash, P.K. et al. (1991) cAMP response element-binding protein is activated by Ca2C/calmodulin- as well as cAMP-dependent protein kinase. Proc. Natl. Acad. Sci. U. S. A. 88, 5061–5065 7 Sheng, M. et al. (1991) CREB: a Ca2C-regulated transcription factor phosphorylated by calmodulin-dependent kinases. Science 252, 1427–1430 8 Xing, J. et al. (1996) Coupling of the RAS–MAPK pathway to gene activation by RSK2, a growth factor-regulated CREB kinase. Science 273, 959–963 www.sciencedirect.com

443

9 Cha-Molstad, H. et al. (2004) Cell-type-specific binding of the transcription factor CREB to the cAMP-response element. Proc. Natl. Acad. Sci. U. S. A. 101, 13572–13577 10 Deisseroth, K. et al. (1996) Signaling from synapse to nucleus: postsynaptic CREB phosphorylation during multiple forms of hippocampal synaptic plasticity. Neuron 16, 89–101 11 Bannister, A.J. et al. (2002) Histone methylation: dynamic or static? Cell 109, 801–806 12 Horn, P.J. and Peterson, C.L. (2002) Chromatin higher order folding – wrapping up transcription. Science 297, 1824–1827 13 Sassone-Corsi, P. (2002) Unique chromatin remodeling and transcriptional regulation in spermatogenesis. Science 296, 2176–2178 14 Gonzalez, G.A. and Montminy, M.R. (1989) Cyclic AMP stimulates somatostatin gene transcription by phosphorylation of CREB at serine 133. Cell 59, 675–680 15 Matthews, R.P. et al. (1994) Calcium/calmodulin-dependent protein kinase types II and IV differentially regulate CREB-dependent gene expression. Mol. Cell. Biol. 14, 6107–6116 16 Wu, X. and McMurray, C.T. (2001) Calmodulin kinase II attenuation of gene transcription by preventing cAMP response element-binding protein (CREB) dimerization and binding of the CREB-binding protein. J. Biol. Chem. 276, 1735–1741 17 Chrivia, J.C. et al. (1993) Phosphorylated CREB binds specifically to the nuclear protein CBP. Nature 365, 855–859 18 Kwok, R.P. et al. (1994) Nuclear protein CBP is a coactivator for the transcription factor CREB. Nature 370, 223–226 19 Pittenger, C. et al. (2002) Reversible inhibition of CREB/ATF transcription factors in region CA1 of the dorsal hippocampus disrupts hippocampus-dependent spatial memory. Neuron 34, 447–462 20 Impey, S. et al. (2004) Defining the CREB regulon: a genome-wide analysis of transcription factor regulator regions. Cell 119, 1041–1054 21 Zhang, X. et al. Genome-wide analysis of cAMP-response element binding protein occupancy, phosphorylation, and target gene activation in human tissues. Proc. Natl. Acad. Sci. U. S. A. (in press) 22 McClung, C.A. and Nestler, E.J. (2003) Regulation of gene expression and cocaine reward by CREB and DFosB. Nat. Neurosci. 6, 1208–1215 23 Sheng, M. et al. (1990) Membrane depolarization and calcium induce c-fos transcription via phosphorylation of transcription factor CREB. Neuron 4, 571–582 24 Foulkes, N.S. et al. (1991) CREM gene: use of alternative DNA-binding domains generates multiple antagonists of cAMP-induced transcription. Cell 64, 739–749 25 Lane-Ladd, S.B. et al. (1997) CREB (cAMP response element-binding protein) in the locus coeruleus: biochemical, physiological, and behavioral evidence for a role in opiate dependence. J. Neurosci. 17, 7890–7901 26 Kim, K.S. et al. (1993) Both the basal and inducible transcription of the tyrosine hydroxylase gene are dependent upon a cAMP response element. J. Biol. Chem. 268, 15689–15695 27 Itoi, K. et al. (1996) Major role of 3 0 ,5 0 -cyclic adenosine monophosphatedependent protein kinase A pathway in corticotropin-releasing factor gene expression in the rat hypothalamus in vivo. Endocrinology 137, 2389–2396 28 Finkbeiner, S. et al. (1997) CREB: a major mediator of neuronal neurotrophin responses. Neuron 19, 1031–1047 29 Douglass, J. et al. (1994) Identification of multiple DNA elements regulating basal and protein kinase A-induced transcriptional expression of the rat prodynorphin gene. Mol. Endocrinol. 8, 333–344 30 Cole, R.L. et al. (1995) Neuronal adaptation to amphetamine and dopamine: molecular mechanisms of prodynorphin gene regulation in rat striatum. Neuron 14, 813–823 31 Carlezon, W.A., Jr. et al. (1998) Regulation of cocaine reward by CREB. Science 282, 2272–2275 32 Sakai, N. et al. (2002) Inducible and brain region specific CREB transgenic mice. Mol. Pharmacol. 61, 1453–1464 33 Borges, K. and Dingledine, R. (2001) Functional organization of the GluR1 glutamate receptor promoter. J. Biol. Chem. 276, 25929–25938 34 Olson, V.G. et al. (2005) Regulation of drug reward by CREB: Evidence for two functionally distinct subregions of the ventral tegmental area. J. Neurosci. 25, 5553–5562

444

Review

TRENDS in Neurosciences Vol.28 No.8 August 2005

35 Kaang, B.K. et al. (1993) Activation of cAMP-responsive genes by stimuli that produce long-term facilitation in Aplysia sensory neurons. Neuron 10, 427–435 36 Yin, J.C. et al. (1994) Induction of a dominant negative CREB transgene specifically blocks long-term memory in Drosophila. Cell 79, 49–58 37 Bourtchuladze, R. et al. (1994) Deficient long-term memory in mice with a targeted mutation of the cAMP-responsive element-binding protein. Cell 79, 59–68 38 Yin, J.C. et al. (1995) CREB as a memory modulator: induced expression of a dCREB2 activator isoform enhances long-term memory in Drosophila. Cell 81, 107–115 39 Josselyn, S. et al. (2001) Long-term memory is facilitated by cAMP response element-binding protein overexpression in the amygdala. J. Neurosci. 21, 2404–2412 40 Marie, H. et al. (2005) Generation of silent synapses by acute in vivo expression of CaMKIV and CREB. Neuron 45, 741–752 41 Martin, K.C. et al. (1997) Synapse-specific, long-term facilitation of Aplysia sensory to motor synapses: a function for local protein synthesis in memory storage. Cell 91, 927–938 42 Kida, S. et al. (2002) CREB required for the stability of new and reactivated fear memories. Nat. Neurosci. 5, 348–355 43 Barco, A. et al. (2003) CREB, memory enhancement and the treatment of memory disorders: promises, pitfalls and prospects. Expert Opin. Ther. Targets 7, 101–114 44 Tully, T. et al. (2003) Targeting the CREB pathway for memory enhancers. Nat. Rev. Drug Discov. 2, 267–277 45 Ramos, B.P. et al. (2003) Dysregulation of protein kinase a signaling in the aged prefrontal cortex: new strategy for treating age-related cognitive decline. Neuron 40, 835–845 46 Goldman-Rakic, P.S. et al. (2000) D1 receptors in prefrontal cells and circuits. Brain Res. Rev. 31, 295–301 47 McGaugh, J.L. (2004) The amygdala modulates the consolidation of memories of emotionally arousing experiences. Annu. Rev. Neurosci. 27, 1–28 48 Berke, J.D. and Hyman, S.E. (2000) Addiction, dopamine, and the molecular mechanisms of memory. Neuron 25, 515–532 49 Nestler, E.J. (2001) Total recall – the memory of addiction. Science 292, 2266–2267 50 Carlezon, W.A., Jr. and Wise, R.A. (2003) Unmet expectations: the brain minds. Nat. Med. 9, 15–16 51 Kelley, A.E. (2004) Memory and addiction: shared neural circuitry and molecular mechanisms. Neuron 44, 161–179 52 Guitart, X. et al. (1992) Regulation of cyclic AMP response elementbinding protein (CREB) phosphorylation by acute and chronic morphine in the rat locus coeruleus. J. Neurochem. 58, 1168–1171 53 Aghajanian, G.K. (1978) Tolerance of locus coeruleus neurones to morphine and suppression of withdrawal response by clonidine. Nature 276, 186–188 54 Koob, G.F. et al. (1992) Neural substrates of opiate withdrawal. Trends Neurosci. 15, 186–191 55 Widnell, K.L. et al. (1994) Regulation of cAMP response element binding protein in the locus coeruleus in vivo and in a locus coeruleus-like (CATH.a) cell line in vitro. Proc. Natl. Acad. Sci. U. S. A. 91, 10947–10951 56 Han, M.H. et al. (2004) Regulation by CREB of locus coeruleus neuronal firing in slice cultures. Program number 577.1. In 2004 Abstract Viewer and Itinerary Planner. Society for Neuroscience, online (http://apu.sfn.org/) 57 Punch, L.J. et al. (1997) Opposite modulation of opiate withdrawal behaviors on microinfusion of a protein kinase A inhibitor versus activator into the locus coeruleus or periaqueductal gray. J. Neurosci. 17, 8520–8527 58 Maldonado, R. et al. (1996) Reduction of morphine abstinence in mice with a mutation in the gene encoding CREB. Science 273, 657–659 59 Nestler, E.J. (2004) Historical review: Molecular and cellular mechanisms of opiate and cocaine addiction. Trends Pharmacol. Sci. 25, 210–218 60 Terwilliger, R. et al. (1991) A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function. Brain Res. 548, 100–110 61 Koob, G.F. et al. (1998) Neuroscience of addiction. Neuron 21, 467–476 www.sciencedirect.com

62 Wise, R.A. (2004) Dopamine, learning and motivation. Nat. Rev. Neurosci. 5, 483–494 63 Turgeon, S.M. et al. (1997) Enhanced CREB phosphorylation and changes in c-Fos and FRA expression in striatum accompany amphetamine sensitization. Brain Res. 749, 120–126 64 Shaw-Lutchman, T.Z. et al. (2002) Regional and cellular mapping of CRE-mediated transcription during naltrexone-precipitated morphine withdrawal. J. Neurosci. 22, 3663–3672 65 Shaw-Lutchman, T.Z. et al. (2003) Regulation of CRE-mediated transcription in mouse brain by amphetamine. Synapse 48, 10–17 66 Self, D.W. et al. (1998) Involvement of cAMP-dependent protein kinase in the nucleus accumbens in cocaine self-administration and relapse of cocaine-seeking behavior. J. Neurosci. 18, 1848–1859 67 Barrot, M. et al. (2002) CREB activity in the nucleus accumbens shell controls gating of behavioral responses to emotional stimuli. Proc. Natl. Acad. Sci. U. S. A. 99, 11435–11440 68 Walters, C.L. and Blendy, J.A. (2001) Different requirements for cAMP response element binding protein in positive and negative reinforcing properties of drugs of abuse. J. Neurosci. 21, 9438–9444 69 Chartoff, E.H. et al. (2003) Dopamine-dependent increases in phosphorylation of cAMP response element binding protein (CREB) during precipitated morphine withdrawal in primary cultures of rat striatum. J. Neurochem. 87, 107–118 70 Bozarth, M.A. and Wise, R.A. (1981) Intracranial self-administration of morphine into the ventral tegmental area in rats. Life Sci. 28, 551–555 71 Neumaier, J.F. et al. (2002) Elevated expression of 5-HT1B receptors in nucleus accumbens efferents sensitizes animals to cocaine. J. Neurosci. 22, 10856–10863 72 Walters, C.L. et al. (2003) Differential distribution of CREB in the mesolimbic dopamine reward pathway. J. Neurochem. 87, 1237–1244 73 McClung, C.A. et al. (2004) DFosB: a molecular switch for long-term adaptation in the brain. Mol. Brain Res. 132, 146–154 74 Dong, Y. et al. (2004) CREB-mediated regulation of excitability and synaptic transmission in nucleus accumbens neurons. Program number 577.2. In 2004 Abstract Viewer and Itinerary Planner. Society for Neuroscience, online (http://apu.sfn.org/) 75 Nibuya, M. et al. (1996) Chronic antidepressant administration increases the expression of cAMP response element binding protein (CREB) in rat hippocampus. J. Neurosci. 16, 2365–2372 76 Jeon, S.H. et al. (1997) Electroconvulsive shock increases the phosphorylation of cyclic AMP response element binding protein at Ser-133 in rat hippocampus but not in cerebellum. Neuropharmacology 36, 411–414 77 Thome, J. et al. (2000) cAMP response element-mediated gene transcription is upregulated by chronic antidepressant treatment. J. Neurosci. 20, 4030–4036 78 Chen, A.C. et al. (2001) Expression of the cAMP response element binding protein (CREB) in hippocampus produces an antidepressant effect. Biol. Psychiatry 49, 753–762 79 Duman, R.S. et al. (1997) A molecular and cellular hypothesis of depression. Arch. Gen. Psychiatry 54, 597–606 80 Newton, S.S. et al. (2003) Gene profile of electroconvulsive seizures: induction of neurotrophic and angiogenic factors. J. Neurosci. 23, 10841–10851 81 Nibuya, M. et al. (1995) Regulation of BDNF and trkB mRNA in rat brain by chronic electroconvulsive seizure and antidepressant drug treatments. J. Neurosci. 15, 7539–7547 82 Sapolsky, R.M. et al. (1985) Prolonged glucocorticoid exposure reduces hippocampal neuron number: implications for aging. J. Neurosci. 5, 1222–1227 83 McEwen, B.S. and Magarinos, A.M. (2001) Stress and hippocampal plasticity: implications for the pathophysiology of affective disorders. Hum. Psychopharmacol. 16, S7–S19 84 Duman, R.S. (2004) Depression: a case of neuronal life and death? Biol. Psychiatry 56, 140–145 85 Nestler, E.J. et al. (2002) Neurobiology of depression. Neuron 34, 13–25 86 O’Donnell, J. and Zhang, H-T. (2004) Antidepressant effects of inhibitors of cAMP phosphodiesterase (PDE4). Trends Pharmacol. Sci. 25, 158–163 87 Fleischhacker, W.W. et al. (1992) A multicenter double-blind study of

Review

88

89

90

91

92

93 94

95

96

97

98

99

TRENDS in Neurosciences Vol.28 No.8 August 2005

three different doses of the new cAMP-phosphodiesterase inhibitor rolipram in patients with major depresive disorder. Neuropsychobiology 26, 59–64 Barad, M. et al. (1998) Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory. Proc. Natl. Acad. Sci. U. S. A. 95, 15020–15025 Pliakas, A.M. et al. (2001) Altered responsiveness to cocaine and increased immobility in the forced swim test associated with elevated cAMP response element binding protein expression in nucleus accumbens. J. Neurosci. 21, 7397–7403 Newton, S.S. et al. (2002) Inhibition of cAMP response elementbinding protein or dynorphin in the nucleus accumbens produces an antidepressant-like effect. J. Neurosci. 22, 10883–10890 Andersen, S.L. et al. (2002) Altered responsiveness to cocaine in rats exposed to methylphenidate during development. Nat. Neurosci. 5, 13–14 Bolan˜os, C.A. et al. (2003) Methylphenidate treatment during preand periadolescence alters behavioral responses to emotional stimuli at adulthood. Biol. Psychiatry 54, 1317–1329 Carlezon, W.A., Jr. et al. (2003) Enduring behavioral effects of early exposure to methylphenidate in rats. Biol. Psychiatry 54, 1330–1337 Mague, S.D. et al. (2005) Early developmental exposure to methylphenidate reduces cocaine-induced potentiation of brain stimulation reward in rats. Biol. Psychiatry 57, 120–125 Conti, A.C. et al. (2002) cAMP response element-binding protein is essential for the upregulation of brain-derived neurotrophic factor transcription, but not the behavioral or endocrine responses to antidepressant drugs. J. Neurosci. 22, 3262–3268 Mague, S.D. et al. (2003) Antidepressant-like effects of k-opioid receptor antagonists in the forced swim test in rats. J. Pharmacol. Exp. Ther. 305, 323–330 Todtenkopf, M.S. et al. (2004) Effects of k-opioid receptor ligands on intracranial self-stimulation in rats. Psychopharmacology (Berl.) 172, 463–470 McLaughlin, J.P. et al. (2003) Kappa opioid receptor antagonism and prodynorphin gene disruption block stress-induced behavioral responses. J. Neurosci. 23, 5674–5683 Di Chiara, G. and Imperato, A. (1988) Drugs abused by humans

100

101

102

103 104

105

106

107

108

109

110

445

preferentially increase synaptic dopamine concentrations in the mesolimbic system of freely moving rats. Proc. Natl. Acad. Sci. U. S. A. 85, 5274–5278 Shippenberg, T.S. and Rea, W. (1997) Sensitization to the behavioral effects of cocaine: modulation by dynorphin and k-opioid receptor agonists. Pharmacol. Biochem. Behav. 57, 449–455 Svingos, A.L. et al. (1999) Cellular sites for dynorphin activation of k-opioid receptors in the rat nucleus accumbens shell. J. Neurosci. 19, 1804–1813 Wallace, T.L. et al. (2004) Effects of cyclic adenosine monophosphate response element binding protein overexpression in the basolateral amygdala on behavioral models of depression and anxiety. Biol. Psychiatry 56, 151–160 Heimer, L. et al. (1997) The accumbens: beyond the core-shell dichotomy. J. Neuropsychiatry Clin. Neurosci. 9, 354–381 Barrot, M. et al. (2005) Regulation of anxiety and initiation of sexual behavior by CREB in the nucleus accumbens. Proc. Natl. Acad. Sci. U. S. A. 102, 8357–8362 Pandey, S.C. et al. (2003) The decreased phosphorylation of cyclic adenosine monophosphate (cAMP) response element binding (CREB) protein in the central amygdala acts as a molecular substrate for anxiety related to ethanol withdrawal in rats. Alcohol. Clin. Exp. Res. 27, 396–409 Valverde, O. et al. (2004) Modulation of anxiety-like behavior and morphine dependence in CREB-deficient mice. Neuropsychopharmacology 29, 1122–1133 Meloni, E.G. et al. (2003) Differential expression of phosphorylated cAMP response element binding protein (PCREB) in brain nuclei that mediate fear and anxiety in the rat. Program number 344.6. In 2003 Abstract Viewer and Itinerary Planner. Society for Neuroscience, online (http://apu.sfn.org/) Mower, A.F. et al. (2002) cAMP/Ca2C response element-binding protein function is essential for ocular dominance plasticity. J. Neurosci. 22, 2237–2245 Pham, T.A. et al. (2004) A semi-persistent adult ocular dominance plasticity in visual cortex is stabilized by activated CREB. Learn. Mem. 11, 738–747 Liao, D.S. et al. (2002) Different mechanisms for loss and recovery of binocularity in the visual cortex. J. Neurosci. 22, 9015–9023

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