CREB modulates the functional output of nucleus accumbens neurons: a critical role of N-methyl-D-aspartate glutamate receptor (NMDAR) receptors

CRF regulates pain sensation by enhancement of corticoaccumbal excitatory synaptic transmission

Both peripheral and central corticotropin-releasing factor (CRF) systems have been implicated in regulating pain sensation. However, compared with the peripheral, the mechanisms underlying central CRF system in pain modulation have not yet been elucidated, especially at the neural circuit level. The corticoaccumbal circuit, a structure rich in CRF receptors and CRF-positive neurons, plays an important role in behavioral responses to stressors including nociceptive stimuli. The present study was designed to investigate whether and how CRF signaling in this circuit regulated pain sensation under physiological and pathological pain conditions. Our studies employed the viral tracing and circuit-, and cell-specific electrophysiological methods to label the CRF-containing circuit from the medial prefrontal cortex to the nucleus accumbens shell (mPFCCRF-NAcS) and record its neuronal propriety. Combining optogenetic and chemogenetic manipulation, neuropharmacological methods, and behavioral tests, we were able to precisely manipulate this circuit and depict its role in regulation of pain sensation. The current study found that the CRF signaling in the NAc shell (NAcS), but not NAc core, was necessary and sufficient for the regulation of pain sensation under physiological and pathological pain conditions. This process was involved in the CRF-mediated enhancement of excitatory synaptic transmission in the NAcS. Furthermore, we demonstrated that the mPFCCRF neurons monosynaptically connected with the NAcS neurons. Chronic pain increased the protein level of CRF in NAcS, and then maintained the persistent NAcS neuronal hyperactivity through enhancement of this monosynaptic excitatory connection, and thus sustained chronic pain behavior. These findings reveal a novel cell- and circuit-based mechanistic link between chronic pain and the mPFCCRF → NAcS circuit and provide a potential new therapeutic target for chronic pain.

Synaptic turnover promotes efficient learning in bio-realistic spiking neural networks

While artificial machine learning systems achieve superhuman performance in specific tasks such as language processing, image and video recognition, they do so use extremely large datasets and huge amounts of power. On the other hand, the brain remains superior in several cognitively challenging tasks while operating with the energy of a small lightbulb. We use a biologically constrained spiking neural network model to explore how the neural tissue achieves such high efficiency and assess its learning capacity on discrimination tasks. We found that synaptic turnover, a form of structural plasticity, which is the ability of the brain to form and eliminate synapses continuously, increases both the speed and the performance of our network on all tasks tested. Moreover, it allows accurate learning using a smaller number of examples. Importantly, these improvements are most significant under conditions of resource scarcity, such as when the number of trainable parameters is halved and when the task difficulty is increased. Our findings provide new insights into the mechanisms that underlie efficient learning in the brain and can inspire the development of more efficient and flexible machine learning algorithms.

Key transcription factors mediating cocaine-induced plasticity in the nucleus accumbens

Repeated cocaine use induces coordinated changes in gene expression that drive plasticity in the nucleus accumbens (NAc), an important component of the brain's reward circuitry, and promote the development of maladaptive, addiction-like behaviors. Studies on the molecular basis of cocaine action identify transcription factors, a class of proteins that bind to specific DNA sequences and regulate transcription, as critical mediators of this cocaine-induced plasticity. Early methods to identify and study transcription factors involved in addiction pathophysiology primarily relied on quantifying the expression of candidate genes in bulk brain tissue after chronic cocaine treatment, as well as conventional overexpression and knockdown techniques. More recently, advances in next generation sequencing, bioinformatics, cell-type-specific targeting, and locus-specific neuroepigenomic editing offer a more powerful, unbiased toolbox to identify the most important transcription factors that drive drug-induced plasticity and to causally define their downstream molecular mechanisms. Here, we synthesize the literature on transcription factors mediating cocaine action in the NAc, discuss the advancements and remaining limitations of current experimental approaches, and emphasize recent work leveraging bioinformatic tools and neuroepigenomic editing to study transcription factors involved in cocaine addiction.

Regulation of Phosphorylated State of NMDA Receptor by STEP61 Phosphatase after Mild-Traumatic Brain Injury: Role of Oxidative Stress

Traumatic Brain Injury (TBI) mediates neuronal death through several events involving many molecular pathways, including the glutamate-mediated excitotoxicity for excessive stimulation of N-methyl-D-aspartate receptors (NMDARs), producing activation of death signaling pathways. However, the contribution of NMDARs (distribution and signaling-associated to the distribution) remains incompletely understood. We propose a critical role of STEP₆₁ (Striatal-Enriched protein tyrosine phosphatase) in TBI; this phosphatase regulates the dephosphorylated state of the GluN2B subunit through two pathways: by direct dephosphorylation of tyrosine-1472 and indirectly via dephosphorylation and inactivation of Fyn kinase. We previously demonstrated oxidative stress’s contribution to NMDAR signaling and distribution using SOD2^+/− mice such a model. We performed TBI protocol using a controlled frontal impact device using C57BL/6 mice and SOD2^+/− animals. After TBI, we found alterations in cognitive performance, NMDAR-dependent synaptic function (decreased synaptic form of NMDARs and decreased synaptic current NMDAR-dependent), and increased STEP₆₁ activity. These changes are reduced partially with the STEP₆₁-inhibitor TC-2153 treatment in mice subjected to TBI protocol. This study contributes with evidence about the role of STEP₆₁ in the neuropathological progression after TBI and also the alteration in their activity, such as an early biomarker of synaptic damage in traumatic lesions.

Potential evidence of peripheral learning and memory in the arms of dwarf cuttlefish, Sepia bandensis

CREB (cAMP response element-binding) transcription factors are conserved markers of memory formation in the brain and peripheral circuits. We provide immunohistochemical evidence of CREB phosphorylation in the dwarf cuttlefish, Sepia bandensis, following the inaccessible prey (IP) memory experiment. During the IP experiment, cuttlefish are shown prey enclosed in a transparent tube, and tentacle strikes against the tube decrease over time as the cuttlefish learns the prey is inaccessible. The cues driving IP learning are unclear but may include sensory inputs from arms touching the tube. The neural activity marker, anti-phospho-CREB (anti-pCREB) was used to determine whether IP training stimulated cuttlefish arm sensory neurons. pCREB immunoreactivity occurred along the oral surface of the arms, including the suckers and epithelial folds surrounding the suckers. pCREB increased in the epithelial folds and suckers of trained cuttlefish. We found differential pCREB immunoreactivity along the distal-proximal axis of trained arms, with pCREB concentrated distally. Unequal CREB phosphorylation occurred among the 4 trained arm pairs, with arm pairs 1 and 2 containing more pCREB. The resulting patterns of pCREB in trained arms suggest that the arms obtain cues that may be salient for learning and memory of the IP experiment.

Unraveling the molecular mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages

Alcohol interferes with foetal development and prenatal alcohol exposure can lead to adverse effects known as foetal alcohol spectrum disorders. We aimed to assess the underlying neurobiological mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages, in discrete brain areas. Pregnant C57BL/6 female mice were exposed to binge alcohol drinking from gestation to weaning. Subsequently, alcohol seeking and taking behaviour were evaluated in male adolescent offspring, as assessed in the two-bottle choice and oral self-administration paradigms. Brain area samples were analysed to quantify AMPAR subunits GluR1/2 and pCREB/CREB expression following alcohol self-administration. We measured the expression of mu and kappa opioid receptors both during acute alcohol withdrawal (assessing anxiety alterations by the EPM test) and following reinstatement in the two-bottle choice paradigm. In addition, alcohol metabolism was analysed by measuring blood alcohol concentrations under an acute dose of 3 g/kg alcohol. Our findings demonstrate that developmental alcohol exposure enhances alcohol intake during adolescence, which is associated with a decrease in the pCREB/CREB ratio in the hippocampus, prefrontal cortex and striatum, while the GluR1/GluR2 ratio showed a decrease in the hippocampus. Moreover, PLAE mice showed behavioural alterations, such as increased anxiety-like responses during acute alcohol withdrawal, and higher BAC levels. No significant changes were identified for mu and kappa opioid receptors mRNA expression. The current study highlights that early alcohol exposed mice increased alcohol consumption during late adolescence. Furthermore, a diminished CREB signalling and glutamatergic neuroplasticity are proposed as underpinning neurobiological mechanisms involved in the sensitivity to alcohol reinforcing properties.

The potential for one drug, administered at the earliest preclinical stage, to prevent the subsequent decline of cognition that eventuates in dementia

In the process that eventuates in mild cognitive impairment (MCI) and ultimately in Alzheimer's dementia, the earliest identifiable change is in the function of synapses. If started at that early point in time, when there is subjective but not objective memory loss plus abnormal brain imaging with fluorodeoxyglucose and Pittsburgh compound B, treatment with a single drug directed at synaptic dysfunction might prevent development of cognitive impairment. Each of four drugs, dantrolene, lithium, minocycline, and piracetam, benefits synaptic impairment. This presentation has two sections. In the first, evidence is discussed at length, for abnormality in the axo-spinous synapse as being the earliest change before objective cognitive decline. The second section explains the benefits to synapses provided by the four mentioned drugs. Dantrolene and lithium perhaps have the strongest supporting data for use as single agents: their efficacy should be subjected to clinical trial.

Plasticity at Thalamo-amygdala Synapses Regulates Cocaine-Cue Memory Formation and Extinction

Repeated drug use has long-lasting effects on plasticity throughout the brain's reward and memory systems. Environmental cues that are associated with drugs of abuse can elicit craving and relapse, but the neural circuits responsible for driving drug-cue-related behaviors have not been well delineated, creating a hurdle for the development of effective relapse prevention therapies. In this study, we used a cocaine+cue self-administration paradigm followed by cue re-exposure to establish that the strength of the drug cue association corresponds to the strength of synapses between the medial geniculate nucleus (MGN) of the thalamus and the lateral amygdala (LA). Furthermore, we demonstrate, via optogenetically induced LTD of MGN-LA synapses, that reversing cocaine-induced potentiation of this pathway is sufficient to inhibit cue-induced relapse-like behavior.

Calcineurin Promotes Neuroplastic Changes in the Amygdala Associated with Weakened Cocaine-Cue Memories

Interfering with memory reconsolidation or inducing memory extinction are two approaches for weakening maladaptive memories in disorders such as addiction and post-traumatic stress disorder. Both extinction and reconsolidation are regulated by intracellular protein kinases and phosphatases, and interfering with these signaling molecules can alter memory strength. The calcium-dependent protein phosphatase, calcineurin (CaN), has been implicated in both the consolidation and extinction of fear memories. However, the role of CaN in regulating drug-cue associative memories has not been investigated. Prior studies have demonstrated that plasticity at thalamo-lateral amygdala (T-LA) synapses is critically involved in the regulation of cocaine-cue memories. Therefore, in the present study, we tested the effects of LA administration of an activator of CaN, chlorogenic acid (CGA), on behavioral and electrophysiological indices of cocaine cue memory reconsolidation and extinction. Male, Sprague-Dawley rats were trained to self-administer cocaine paired with an audiovisual cue. The cue memory was then either briefly reactivated, extinguished, or not manipulated, followed immediately by LA infusion of CGA. Rats were tested 24 h later for cue-induced reinstatement, or LA slices were prepared for electrophysiological recordings. We found that intra-LA infusions of CGA following cue extinction or reconsolidation reduced cue-induced reinstatement, which was blocked by co-infusion of the CaN inhibitor, FK-506. Similarly, CGA infusions following cue re-exposure significantly attenuated EPSC amplitude at T-LA synapses, suggesting that CaN affects cocaine-cue memory reconsolidation and extinction by altering T-LA synaptic strength. Therefore, CaN signaling in the LA may represent a novel target for disrupting cocaine-associated memories to reduce relapse.SIGNIFICANCE STATEMENT Repetitive drug use induces synaptic plasticity that underlies the formation of long-lasting associative memories for environmental cues paired with the drug. We previously identified thalamo-amygdala synapses (T-LA) that project via the interal capsule, as an important locus for the regulation of cocaine-cue memories. These synapses are strengthened by repeated cocaine-cue pairings, but this is reversed by extinction training or by optogenetic induction of in vivo long-term depression (LTD). Here, we demonstrate that activating calcineurin, a calcium-dependent phosphatase, following the reactivation or extinction of a cocaine-cue memory, induces LTD-like changes at T-LA synapses, and a corresponding decrease in cue-induced reinstatement, suggesting that calcineurin may be a potential therapeutic target for relapse prevention.

Epigenetic Mechanisms of Opioid Addiction

Opioid use kills tens of thousands of Americans each year, devastates families and entire communities, and cripples the health care system. Exposure to opioids causes long-term changes to brain regions involved in reward processing and motivation, leading vulnerable individuals to engage in pathological drug seeking and drug taking that can remain a lifelong struggle. The persistence of these neuroadaptations is mediated in part by epigenetic remodeling of gene expression programs in discrete brain regions. Although the majority of work examining how epigenetic modifications contribute to addiction has focused on psychostimulants such as cocaine, research into opioid-induced changes to the epigenetic landscape is emerging. This review summarizes our knowledge of opioid-induced epigenetic modifications and their consequential changes to gene expression. Current evidence points toward opioids promoting higher levels of permissive histone acetylation and lower levels of repressive histone methylation as well as alterations to DNA methylation patterns and noncoding RNA expression throughout the brain's reward circuitry. Additionally, studies manipulating epigenetic enzymes in specific brain regions are beginning to build causal links between these epigenetic modifications and changes in addiction-related behavior. Moving forward, studies must leverage advanced chromatin analysis and next-generation sequencing approaches combined with bioinformatics pipelines to identify novel gene networks regulated by particular epigenetic modifications. Improved translational relevance also requires increased focus on volitional drug-intake models and standardization of opioid exposure paradigms. Such work will significantly advance our understanding of how opioids cause persistent changes to brain function and will provide a platform on which to develop interventions for treating opioid addiction.

Sex-Specific Effects of Stress on Mood-Related Gene Expression

Women are twice as likely as men to be diagnosed with major depressive disorder (MDD). Recent studies report distinct molecular changes in depressed men and women across mesocorticolimbic brain regions. However, it is unclear which sex-related factors drive distinct MDD-associated pathology. The goal of this study was to use mouse experimental systems to investigate sex-specific mechanisms underlying the distinct molecular profiles of MDD in men and women. We used unpredictable chronic mild stress to induce an elevated anxiety-/depressive-like state and "four core genotypes" (FCG) mice to probe for sex-specific mechanisms. As predicted, based on previous implications in mood, stress impacted the expression of several dopamine-, GABA-, and glutamate-related genes. Some of these effects, specifically in the prefrontal cortex, were genetic sex-specific, with effects in XX mice but not in XY mice. Stress also impacted gene expression differently across the mesocorticolimbic circuit, with increased expression of mood-related genes in the prefrontal cortex and nucleus accumbens, but decreased expression in basolateral amygdala. Our results suggest that females are sensitive to the effects of chronic stress, partly due to their genetic sex, independent of gonadal hormones. Furthermore, these results point to the prefrontal cortex as the node in the mesocorticolimbic circuitry with the strongest female-specific effects.

Cascades of Homeostatic Dysregulation Promote Incubation of Cocaine Craving

In human drug users, cue-induced drug craving progressively intensifies after drug abstinence, promoting drug relapse. This time-dependent progression of drug craving is recapitulated in rodent models, in which rats exhibit progressive intensification of cue-induced drug seeking after withdrawal from drug self-administration, a phenomenon termed incubation of drug craving. Although recent results suggest that functional alterations of the nucleus accumbens (NAc) contribute to incubation of drug craving, it remains poorly understood how NAc function evolves after drug withdrawal to progressively intensify drug seeking. The functional output of NAc relies on how the membrane excitability of its principal medium spiny neurons (MSNs) translates excitatory synaptic inputs into action potential firing. Here, we report a synapse-membrane homeostatic crosstalk (SMHC) in male rats, through which an increase or decrease in the excitatory synaptic strength induces a homeostatic decrease or increase in the intrinsic membrane excitability of NAc MSNs, and vice versa. After short-term withdrawal from cocaine self-administration, despite no actual change in the AMPA receptor-mediated excitatory synaptic strength, GluN2B NMDA receptors, the SMHC sensors of synaptic strength, are upregulated. This may create false SMHC signals, leading to a decrease in the membrane excitability of NAc MSNs. The decreased membrane excitability subsequently induces another round of SMHC, leading to synaptic accumulation of calcium-permeable AMPA receptors and upregulation of excitatory synaptic strength after long-term withdrawal from cocaine. Disrupting SMHC-based dysregulation cascades after cocaine exposure prevents incubation of cocaine craving. Thus, cocaine triggers cascades of SMHC-based dysregulation in NAc MSNs, promoting incubated cocaine seeking after drug withdrawal.SIGNIFICANCE STATEMENT Here, we report a bidirectional homeostatic plasticity between the excitatory synaptic input and membrane excitability of nucleus accumbens (NAc) medium spiny neurons (MSNs), through which an increase or decrease in the excitatory synaptic strength induces a homeostatic decrease or increase in the membrane excitability, and vice versa. Cocaine self-administration creates a false homeostatic signal that engages this synapse-membrane homeostatic crosstalk mechanism, and produces cascades of alterations in excitatory synapses and membrane properties of NAc MSNs after withdrawal from cocaine. Experimentally preventing this homeostatic dysregulation cascade prevents the progressive intensification of cocaine seeking after drug withdrawal. These results provide a novel mechanism through which drug-induced homeostatic dysregulation cascades progressively alter the functional output of NAc MSNs and promote drug relapse.

Age-related NMDA signaling alterations in SOD2 deficient mice

Oxidative stress affects the survival and function of neurons. Hence, they have a complex and highly regulated machinery to handle oxidative changes. The dysregulation of this antioxidant machinery is associated with a wide range of neurodegenerative conditions. Therefore, we evaluated signaling alterations, synaptic properties and behavioral performance in 2 and 6-month-old heterozygous manganese superoxide dismutase knockout mice (SOD2^+/- mice). We found that their low antioxidant capacity generated direct oxidative damage in proteins, lipids, and DNA. However, only 6-month-old heterozygous knockout mice presented behavioral impairments. On the other hand, synaptic plasticity, synaptic strength and NMDA receptor (NMDAR) dependent postsynaptic potentials were decreased in an age-dependent manner. We also analyzed the phosphorylation state of the NMDAR subunit GluN2B. We found that while the levels of GluN2B phosphorylated on tyrosine 1472 (synaptic form) remain unchanged, we detected increased levels of GluN2B phosphorylated on tyrosine 1336 (extrasynaptic form), establishing alterations in the synaptic/extrasynaptic ratio of GluN2B. Additionally, we found increased levels of two phosphatases associated with dephosphorylation of p-1472: striatal-enriched protein tyrosine phosphatase (STEP) and phosphatase and tensin homolog deleted on chromosome Ten (PTEN). Moreover, we found decreased levels of p-CREB, a master transcription factor activated by synaptic stimulation. In summary, we describe mechanisms by which glutamatergic synapses are altered under oxidative stress conditions. Our results uncovered new putative therapeutic targets for conditions where NMDAR downstream signaling is altered. This work also contributes to our understanding of processes such as synapse formation, learning, and memory in neuropathological conditions.

Specific rescue by ortho-hydroxy atorvastatin of cortical GABAergic neurons from previous oxygen/glucose deprivation: role of pCREB

The statin atorvastatin (ATV) given as a post-treatment has been reported beneficial in stroke, although the mechanisms involved are not well understood so far. Here, we investigated in vitro the effect of post-treatment with ATV and its main bioactive metabolite ortho-hydroxy ATV (o-ATV) on neuroprotection after oxygen and glucose deprivation (OGD), and the role of the pro-survival cAMP response element-binding protein (CREB). Post-OGD treatment of primary cultures of rat cortical neurons with o-ATV, but not ATV, provided neuroprotection to a specific subset of cortical neurons that were large and positive for glutamic acid decarboxylase (large-GAD⁽⁺⁾ neurons, GABAergic). Significantly, only these GABAergic neurons showed an increase in phosphorylated CREB (pCREB) early after neuronal cultures were treated post-OGD with o-ATV. We found that o-ATV, but not ATV, increased the neuronal uptake of glutamate from the medium; this provides a rationale for the specific effect of o-ATV on pCREB in large-GABAergic neurons, which have a higher ratio of synaptic (pCREB-promoting) vs extrasynaptic (pCREB-reducing) N-methyl-D-aspartate (NMDA) receptors (NMDAR) than that of small-non-GABAergic neurons. When we pharmacologically increased pCREB levels post-OGD in non-GABAergic neurons, through the selective activation of synaptic NMDAR, we observed as well long-lasting neuronal survival. We propose that the statin metabolite o-ATV given post-OGD boosts the intrinsic pro-survival factor pCREB in large-GABAergic cortical neurons in vitro, this contributing to protect them from OGD.

Morphine-induced inhibition of Ca2+ -dependent d-serine release from astrocytes suppresses excitability of GABAergic neurons in the nucleus accumbens

The nucleus accumbens (NAc) plays a critical role in addictive drug-induced behavioral changes. d-serine is present at high levels in the brain and is involved in the regulation of N-methyl-d-aspartate glutamate (NMDA)-dependent synaptic activity. In this study, we aimed to examine the involvement of d-serine in morphine addiction. Morphine decreased the NMDA receptor-mediated excitatory postsynaptic currents and excitability of GABAergic neurons in the NAc, while exogenous d-serine alleviated the effects of morphine. Morphine reduced extracellular d-serine levels in rat NAc or in primary culture of astrocytes through inhibition of intracellular Ca²⁺ signals and blockade of d-serine release from cell vesicles. Morphine induced robust internalization of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate acid receptor (AMPAR) in primary cultured astrocytes. Moreover, administration of exogenous d-serine to rats inhibited the development of locomotor sensitization to morphine, attenuated the morphine-induced potentiation on conditioned place preference and suppressed the morphine-enhanced expression of p-CREB and ΔFosB in the NAc. Overall, our results showed that morphine inhibited d-serine release from astrocytes through modulation of AMPAR-mediated Ca²⁺ influx, and led to the inhibition of postsynaptic excitability of GABAergic neurons in the NAc. This work may provide a new insight into the underlying mechanisms of morphine addiction.

Do specific NMDA receptor subunits act as gateways for addictive behaviors?

Addiction to alcohol and drugs is a major social and economic problem, and there is considerable interest in understanding the molecular mechanisms that promote addictive drives. A number of proteins have been identified that contribute to expression of addictive behaviors. NMDA receptors (NMDARs), a subclass of ionotropic glutamate receptors, have been of particular interest because their physiological properties make them an attractive candidate for gating induction of synaptic plasticity, a molecular change thought to mediate learning and memory. NMDARs are generally inactive at the hyperpolarized resting potentials of many neurons. However, given sufficient depolarization, NMDARs are activated and exhibit long-lasting currents with significant calcium permeability. Also, in addition to stimulating neurons by direct depolarization, NMDARs and their calcium signaling can allow strong and/or synchronized inputs to produce long-term changes in other molecules (such as AMPA-type glutamate receptors) which can last from days to years, binding internal and external stimuli in a long-term memory trace. Such memories could allow salient drug-related stimuli to exert strong control over future behaviors and thus promote addictive drives. Finally, NMDARs may themselves undergo plasticity, which can alter subsequent neuronal stimulation and/or the ability to induce plasticity. This review will address recent and past findings suggesting that NMDAR activity promotes drug- and alcohol-related behaviors, with a particular focus on GluN2B subunits as possible central regulators of many addictive behaviors, as well as newer studies examining the importance of non-canonical NMDAR subunits and endogenous NMDAR cofactors.

Cocaine-Induced Synaptic Alterations in Thalamus to Nucleus Accumbens Projection

Exposure to cocaine induces addiction-associated behaviors partially through remodeling neurocircuits in the nucleus accumbens (NAc). The paraventricular nucleus of thalamus (PVT), which projects to the NAc monosynaptically, is activated by cocaine exposure and has been implicated in several cocaine-induced emotional and motivational states. Here we show that disrupting synaptic transmission of select PVT neurons with tetanus toxin activated via retrograde trans-synaptic transport of cre from NAc efferents decreased cocaine self-administration in rats. This projection underwent complex adaptations after self-administration of cocaine (0.75 mg/kg/infusion; 2 h/d × 5 d, 1d overnight training). Specifically, 1d after cocaine self-administration, we observed increased levels of AMPA receptor (AMPAR)-silent glutamatergic synapses in this projection, accompanied by a decreased ratio of AMPAR-to-NMDA receptor (NMDAR)-mediated EPSCs. Furthermore, the decay kinetics of NMDAR EPSCs was significantly prolonged, suggesting insertion of new GluN2B-containing NMDARs to PVT-to-NAc synapses. After 45-d withdrawal, silent synapses within this projection returned to the basal levels, accompanied by a return of the AMPAR/NMDAR ratio and NMDAR decay kinetics to the basal levels. In amygdala and infralimbic prefrontal cortical projections to the NAc, a portion of cocaine-generated silent synapses becomes unsilenced by recruiting calcium-permeable AMPARs (CP-AMPARs) after drug withdrawal. However, the sensitivity of PVT-to-NAc synapses to CP-AMPAR-selective antagonists was not changed after withdrawal, suggesting that CP-AMPAR trafficking is not involved in the evolution of cocaine-generated silent synapses within this projection. Meanwhile, the release probability of PVT-to-NAc synapses was increased after short- and long-term cocaine withdrawal. These results reveal complex and profound alterations at PVT-to-NAc synapses after cocaine exposure and withdrawal.

Etiological theories of addiction: A comprehensive update on neurobiological, genetic and behavioural vulnerability

Currently, about 246 million people around the world have used an illicit drug. The reasons for this use are multiple: e.g. to augment the sensation of pleasure or to reduce the withdrawal and other aversive effects of a given substance. This raises the problem of addiction, which remains a disease of modern society. This review offers a comprehensive update of the different theories about the etiology of addictive behaviors with emphasis on the neurobiological, environmental, psychopathological, behavioural and genetic aspects of addictions, discussed from an evolutionary perspective. The main conclusion of this review is that vulnerability to drug addiction suggests an interaction between many brain systems (including the reward, decision-making, serotonergic, oxytocin, interoceptive insula, CRF, norepinephrine, dynorphin/KOR, orexin and vasopressin systems), genetic predisposition, sociocultural context, impulsivity and drugs types. Further advances in biological and psychological science are needed to address the problems of addiction at its roots.

The microRNA miR-124 suppresses seizure activity and regulates CREB1 activity

miR-124, a brain-specific microRNA, was originally considered as a key regulator in neuronal differentiation and the development of the nervous system. Here we showed that miR-124 expression was suppressed in patients with epilepsy and rats after drug induced-seizures. Intrahippocampal administration of a miR-124 duplex led to alleviated seizure severity and prolonged onset latency in two rat models (pentylenetetrazole- and pilocarpine-induced seizures), while miR-124 inhibitor led to shortened onset latency in pilocarpine-induced seizure rat models. Moreover, the result of local field potentials (LFPs) records further demonstrated miR-124 may have anti-epilepsy function. Inhibition of neuronal firing by miR-124 was associated with the suppression of mEPSC, AMPAR- and NMDAR-mediated currents, which were accompanied by decreased surface expression of NMDAR. In addition, miR-124 injection resulted in decreased activity and expression of cAMP-response element-binding protein1 (CREB1). a key regulator in epileptogenesis. A dual-luciferase reporter assay was used to confirm that miR-124 targeted directly the 3'UTR of CREB1 gene and repressed the CREB1 expression in HEK293T cells. Immunoprecipitation studies confirmed that the CREB1 antibody effectively precipitated CREB1 and NMDAR1 but not GLUR1 from rat brain hippocampus. These results revealed a previously unknown function of miR-124 in neuronal excitability and provided a new insight into molecular mechanisms underlying epilepsy.

Role of the Brain's Reward Circuitry in Depression: Transcriptional Mechanisms

Increasing evidence supports an important role for the brain's reward circuitry in controlling mood under normal conditions and contributing importantly to the pathophysiology and symptomatology of a range of mood disorders, such as depression. Here we focus on the nucleus accumbens (NAc), a critical component of the brain's reward circuitry, in depression and other stress-related disorders. The prominence of anhedonia, reduced motivation, and decreased energy level in most individuals with depression supports the involvement of the NAc in these conditions. We concentrate on several transcription factors (CREB, ΔFosB, SRF, NFκB, and β-catenin), which are altered in the NAc in rodent depression models--and in some cases in the NAc of depressed humans, and which produce robust depression- or antidepressant-like effects when manipulated in the NAc in animal models. These studies of the NAc have established novel approaches toward modeling key symptoms of depression in animals and could enable the development of antidepressant medications with fundamentally new mechanisms of action.

Role of DNA methylation in the nucleus accumbens in incubation of cocaine craving

One of the major challenges of cocaine addiction is the high rate of relapse to drug use after periods of withdrawal. During the first few weeks of withdrawal, cue-induced cocaine craving intensifies, or "incubates," and persists over extended periods of time. Although several brain regions and molecular mechanisms were found to be involved in this process, the underlying epigenetic mechanisms are still unknown. Herein, we used a rat model of incubation of cocaine craving, in which rats were trained to self-administer cocaine (0.75 mg/kg, 6 h/d, 10 d), and cue-induced cocaine-seeking was examined in an extinction test after 1 or 30 d of withdrawal. We show that the withdrawal periods, as well as cue-induced cocaine seeking, are associated with broad, time-dependent enhancement of DNA methylation alterations in the nucleus accumbens (NAc). These gene methylation alterations were partly negatively correlated with gene expression changes. Furthermore, intra-NAc injections of a DNA methyltransferase inhibitor (RG108, 100 μm) abolished cue-induced cocaine seeking on day 30, an effect that persisted 1 month, whereas the methyl donor S-adenosylmethionine (500 μm) had an opposite effect on cocaine seeking. We then targeted two proteins whose genes were demethylated by RG108-estrogen receptor 1 (ESR1) and cyclin-dependent kinase 5 (CDK5). Treatment with an intra-NAc injection of the ESR1 agonist propyl pyrazole triol (10 nm) or the CDK5 inhibitor roscovitine (28 μm) on day 30 of withdrawal significantly decreased cue-induced cocaine seeking. These results demonstrate a role for NAc DNA methylation, and downstream targets of DNA demethylation, in incubation of cocaine craving.

The Use of Herpes Simplex Virus in Ex Vivo Slice Culture

Herpes simplex virus (HSV) can be used for a wide range of genetic manipulations in ex vivo slices of central nervous system tissue from both young and adult rodents. The fast expression of the HSV viral-mediated gene transfer, which can be engineered to produce cell-type specificity, can be utilized in slice cultures for a variety of purposes over a 1- to 4-day period with spatial and temporal specificity. This protocol exploits the rapid expression of HSV viral vectors by utilizing slice culture for electrophysiological recordings, avoiding the need to do intracranial viral injections. Brain slice cultures maintain many aspects of in vivo biology, including functional local synaptic circuitry with preserved brain architecture, while allowing good experimental access and precise control of the extracellular environment, making them ideal platforms for quick access to evaluate expression effects of HSV viral-mediated gene transfer on the molecular and cellular properties of specific neurons. This protocol provides an easy way to study neuronal function following viral expression of a gene of interest.

Effects of increasing CREB-dependent transcription on the storage and recall processes in a hippocampal CA1 microcircuit

The involvement of the hippocampus in learning processes and major brain diseases makes it an ideal candidate to investigate possible ways to devise effective therapies for memory-related pathologies like Alzheimer's Disease (AD). It has been previously reported that augmenting CREB activity increases the synaptic Long-Term Potentiation (LTP) magnitude in CA1 pyramidal neurons and their intrinsic excitability in healthy rodents. It has also been suggested that hippocampal CREB signaling is likely to be down-regulated during AD, possibly degrading memory functions. Therefore, the concept of CREB-based memory enhancers, i.e. drugs that would boost memory by activation of CREB, has emerged. Here, using a model of a CA1 microcircuit, we investigate whether hippocampal CA1 pyramidal neuron properties altered by increasing CREB activity may contribute to improve memory storage and recall. With a set of patterns presented to a network, we find that the pattern recall quality under AD-like conditions is significantly better when boosting CREB function with respect to control. The results are robust and consistent upon increasing the synaptic damage expected by AD progression, supporting the idea that the use of CREB-based therapies could provide a new approach to treat AD.

The role of D-serine as co-agonist of NMDA receptors in the nucleus accumbens: relevance to cocaine addiction

Cocaine addiction is characterized by compulsive drug use despite adverse consequences and high rate of relapse during periods of abstinence. Increasing consensus suggests that addiction to drugs of abuse usurps learning and memory mechanisms normally related to natural rewards, ultimately producing long-lasting neuroadaptations in the mesocorticolimbic system. This system, formed in part by the ventral tegmental area and nucleus accumbens (NAc), has a central role in the development and expression of addictive behaviors. In addition to a broad spectrum of changes that affect morphology and function of NAc excitatory circuits in cocaine–treated animals, impaired N-methyl-D-aspartate receptor (NMDAR)-dependent synaptic plasticity is a typical feature. D-serine, a D-amino acid that has been found at high levels in mammalian brain, binds with high affinity the co-agonist site of NMDAR and mediates, along with glutamate, several important processes including synaptic plasticity. Here we review recent literature focusing on cocaine-induced impairment in synaptic plasticity mechanisms in the NAc and on the fundamental role of D-serine as co-agonist of NMDAR in functional and dysfunctional synaptic plasticity within this nucleus. The emerging picture is that reduced D-serine levels play a crucial role in synaptic plasticity relevant to cocaine addiction. This finding opens new perspectives for therapeutic approaches to treat this addictive state.

The neural rejuvenation hypothesis of cocaine addiction

A leading hypothesis guiding current molecular and cellular research into drug addiction conceptualizes key aspects of addiction as a form of memory in which common neuroplasticity mechanisms that mediate normal learning and memory processes are 'hijacked' by exposure to drugs of abuse to produce pathologic addiction-related memories. Such addiction-related memories are particularly robust and long-lasting and once formed are less amenable to updating. Here we propose a neural rejuvenation hypothesis of cocaine addiction. According to this hypothesis, repeated exposure to drugs of abuse induces some plasticity mechanisms normally associated with brain development within the reward circuitry that mediate the highly efficient and unusually stable memory abnormalities that characterize addiction.

Intrathecal administration of cyclic AMP response element-binding protein-antisense oligonucleotide attenuates neuropathic pain after peripheral nerve injury and decreases the expression of N-methyl-D-aspartic receptors in mice

The aim of the present study was to determine whether the cAMP response element binding protein (CREB) contributes to neuropathic pain during development stage. Adult (7-8 weeks old) male C57BL/6 mice weighing 20-25 g were used. Intrathecal catheter implantation and chronic constriction of the sciatic nerve of the animals were performed. Western blotting and reverse transcription PCR experiments were carried out. Our study demonstrated that the expression of spinal NMDAR after peripheral nerve injury was modulated by central CREB. Chronic constriction nerve injury (CCI) in mice induced thermal hyperalgesia and mechanical allodynia. The increase of NR1 and NR2B subunits of the NMDAR was significantly diminished by intrathecal administration of the CREB antisense oligonucleotide against CREB and pCREB. Additionally, nociceptive behavior induced by CCI was attenuated by intrathecal administration of the CREB antisense oligonucleotide during the period of injection, and the above effects of relieving pain lasted at least 12 days following the last injection. Our results suggested that central functional pCREB may contribute to the development of neuropathic pain and regulate the expression of the NR1 and NR2B subunits of the NMDAR in the process.

Reduced D-serine levels in the nucleus accumbens of cocaine-treated rats hinder the induction of NMDA receptor-dependent synaptic plasticity

Cocaine seeking behaviour and relapse have been linked to impaired potentiation and depression at excitatory synapses in the nucleus accumbens, but the mechanism underlying this process is poorly understood. We show that, in the rat nucleus accumbens core, D-serine is the endogenous coagonist of N-methyl-D-aspartate receptors, and its presence is essential for N-methyl-D-aspartate receptor-dependent potentiation and depression of synaptic transmission. Nucleus accumbens core slices obtained from cocaine-treated rats after 1 day of abstinence presented significantly reduced D-serine concentrations, increased expression of the D-serine degrading enzyme, D-amino acid oxidase, and downregulated expression of serine racemase, the enzyme responsible for D-serine synthesis. The D-serine deficit was associated with impairment of potentiation and depression of glutamatergic synaptic transmission, which was restored by slice perfusion with exogenous D-serine. Furthermore, in vivo administration of D-serine directly into the nucleus accumbens core blocked behavioural sensitization to cocaine. These results provide evidence for a critical role of D-serine signalling in synaptic plasticity relevant to cocaine addiction.

Roles of nucleus accumbens CREB and dynorphin in dysregulation of motivation

Psychostimulants such as amphetamine and cocaine are believed to produce dependence by causing rapid, supraphysiological elevations in synaptic dopamine (DA) within the nucleus accumbens (NAc) (Volkow et al. 2009, Neuropharmacology 56: 3-8). These changes in forebrain DA transmission are similar to those evoked by natural reinforcers (Louilot et al. 1991, Brain Res 553: 313-317; Roitman et al. 2004, J Neurosci 24: 1265-1271), but are of greater magnitude and longer duration. Repeated drug exposure causes compensatory neuroadaptations in neurons of the NAc, some of which may modulate excess DA in a homeostatic fashion. One such adaptation is the activation of the transcription factor CREB (cAMP response element-binding protein) within neurons of the NAc. Although elevated levels of transcriptionally active CREB appear to attenuate DA transmission by increasing expression of the endogenous κ opioid receptor (KOR) ligand dynorphin, increased dynorphin transmission may ultimately have undesirable effects that contribute to drug withdrawal states as well as comorbid psychiatric illnesses such as depression. This state may prompt a return to drug use to mitigate the adverse effects of withdrawal. This article summarizes our current understanding of how CREB and dynorphin contribute to the dysregulation of motivation and describes novel therapeutic strategies that derive from preclinical research in this area.

Transcriptional mechanisms of drug addiction

Regulation of gene expression is considered a plausible mechanism of drug addiction given the stability of behavioral abnormalities that define an addicted state. Numerous transcription factors, proteins that bind to regulatory regions of specific genes and thereby control levels of their expression, have been implicated in the addiction process over the past decade or two. Here we review the growing evidence for the role played by several prominent transcription factors, including a Fos family protein (ΔFosB), cAMP response element binding protein (CREB), and nuclear factor kappa B (NFκB), among several others, in drug addiction. As will be seen, each factor displays very different regulation by drugs of abuse within the brain's reward circuitry, and in turn mediates distinct aspects of the addiction phenotype. Current efforts are geared toward understanding the range of target genes through which these transcription factors produce their functional effects and the underlying molecular mechanisms involved. This work promises to reveal fundamentally new insight into the molecular basis of addiction, which will contribute to improved diagnostic tests and therapeutics for addictive disorders.

Searching for presynaptic NMDA receptors in the nucleus accumbens

The nucleus accumbens shell (NAc) is a key brain region mediating emotional and motivational learning. In rodent models, dynamic alterations have been observed in synaptic NMDA receptors (NMDARs) within the NAc following incentive stimuli, and some of these alterations are critical for acquiring new emotional/motivational states. NMDARs are prominent molecular devices for controlling neural plasticity and memory formation. Although synaptic NMDARs are predominately located postsynaptically, recent evidence suggests that they may also exist at presynaptic terminals and reshape excitatory synaptic transmission by regulating presynaptic glutamate release. However, it remains unknown whether presynaptic NMDARs exist in the NAc and contribute to emotional and motivational learning. In an attempt to identify presynaptically located NMDARs in the NAc, the present study uses slice electrophysiology combined with pharmacological and genetic tools to examine the physiological role of the putative presynaptic NMDARs in rats. Our results show that application of glycine, the glycine-site agonist of NMDARs, potentiated presynaptic release of glutamate at excitatory synapses on NAc neurons, whereas application of 5,7-dichlorokynurenic acid or 7-chlorokynurenic acid, the glycine-site antagonists of NMDARs, produced the opposite effect. However, these seemingly presynaptic NMDAR-mediated effects could not be prevented by application of d-APV, the glutamate-site NMDAR antagonist, and were still present in the mice in which NMDAR NR1 or NR3 subunits were genetically deleted. Thus, rather than suggesting the existence of presynaptic NMDARs, our results support the idea that an unidentified type of glycine-activated substrate may account for the presynaptic effects appearing to be mediated by NMDARs.

Overexpression of CREB in the nucleus accumbens shell increases cocaine reinforcement in self-administering rats

Chronic exposure to addictive drugs enhances cAMP response element binding protein (CREB)-regulated gene expression in nucleus accumbens (NAc), and these effects are thought to reduce the positive hedonic effects of passive cocaine administration. Here, we used viral-mediated gene transfer to produce short- and long-term regulation of CREB activity in NAc shell of rats engaging in volitional cocaine self-administration. Increasing CREB expression in NAc shell markedly enhanced cocaine reinforcement of self-administration behavior, as indicated by leftward (long-term) and upward (short-term) shifts in fixed ratio dose-response curves. CREB also increased the effort exerted by rats to obtain cocaine on more demanding progressive ratio schedules, an effect highly correlated with viral-induced modulation of BDNF protein in the NAc shell. CREB enhanced cocaine reinforcement when expressed either throughout acquisition of self-administration or when expression was limited to postacquisition tests, indicating a direct effect of CREB independent of reinforcement-related learning. Downregulating endogenous CREB in NAc shell by expressing a short hairpin RNA reduced cocaine reinforcement in similar tests, while overexpression of a dominant-negative CREB(S133A) mutant had no significant effect on cocaine self-administration. Finally, increasing CREB expression after withdrawal from self-administration enhanced cocaine-primed relapse, while reducing CREB levels facilitated extinction of cocaine seeking, but neither altered relapse induced by cocaine cues or footshock stress. Together, these findings indicate that CREB activity in NAc shell increases the motivation for cocaine during active self-administration or after withdrawal from cocaine. Our results also highlight that volitional and passive drug administration can lead to substantially different behavioral outcomes.

Distinct Changes in CREB Phosphorylation in Frontal Cortex and Striatum During Contingent and Non-Contingent Performance of a Visual Attention Task

The cyclic-adenosine monophosphate response element-binding protein (CREB) family of transcription factors has been implicated in numerous forms of behavioral plasticity. We investigated CREB phosphorylation along some nodes of corticostriatal circuitry such as frontal cortex (FC) and dorsal (caudate-putamen, CPu) and ventral (nucleus accumbens, NAC) striatum in response to the contingent or non-contingent performance of the five-choice serial reaction time task (5-CSRTT) used to assess visuospatial attention. Three experimental manipulations were used; an attentional performance group (contingent, "master"), a group trained previously on the task but for whom the instrumental contingency coupling responding with stimulus detection and reward was abolished (non-contingent, "yoked") and a control group matched for food deprivation and exposure to the test apparatus (untrained). Rats trained on the 5-CSRTT (both master and yoked) had higher levels of CREB protein in the FC, CPu, and NAC compared to untrained controls. Despite the divergent behavior of "master" and "yoked" rats CREB activity in the FC was not substantially different. In rats performing the 5-CSRTT ("master"), CREB activity was completely abolished in the CPu whereas in the NAC it remained unchanged. In contrast, CREB phosphorylation in CPu and NAC increased only when the contingency changed from goal-dependent to goal-independent reinforcement ("yoked"). The present results indicate that up-regulation of CREB protein expression across cortical and striatal regions possibly reflects the extensive instrumental learning and performance whereas increased CREB activity in striatal regions may signal the unexpected change in the relationship between instrumental action and reinforcement.

Transcriptional and epigenetic mechanisms of addiction

Investigations of long-term changes in brain structure and function that accompany chronic exposure to drugs of abuse suggest that alterations in gene regulation contribute substantially to the addictive phenotype. Here, we review multiple mechanisms by which drugs alter the transcriptional potential of genes. These mechanisms range from the mobilization or repression of the transcriptional machinery - including the transcription factors ΔFOSB, cyclic AMP-responsive element binding protein (CREB) and nuclear factor-κB (NF-κB) - to epigenetics - including alterations in the accessibility of genes within their native chromatin structure induced by histone tail modifications and DNA methylation, and the regulation of gene expression by non-coding RNAs. Increasing evidence implicates these various mechanisms of gene regulation in the lasting changes that drugs of abuse induce in the brain, and offers novel inroads for addiction therapy.

Genetic approaches to investigate the role of CREB in neuronal plasticity and memory

In neurons, the convergence of multiple intracellular signaling cascades leading to cAMP-responsive element-binding protein (CREB) activation suggests that this transcription factor plays a critical role in integrating different inputs and mediating appropriate neuronal responses. The nature of this transcriptional response depends on both the type and strength of the stimulus and the cellular context. CREB-dependent gene expression has been involved in many different aspects of nervous system function, from embryonic development to neuronal survival, and synaptic, structural, and intrinsic plasticity. Here, we first review the different methodological approaches used to genetically manipulate CREB activity and levels in neurons in vivo in the adult brain, including recombinant viral vectors, mouse transgenesis, and gene-targeting techniques. We then discuss the impact of these approaches on our understanding of CREB's roles in neuronal plasticity and memory in rodents. Studies combining these genetic approaches with electrophysiology and behavior provide strong evidence that CREB is critically involved in the regulation of synaptic plasticity, intrinsic excitability, and long-term memory formation. These findings pave the way for the development of novel therapeutic strategies to treat memory disorders.

Virogenetic and optogenetic mechanisms to define potential therapeutic targets in psychiatric disorders

A continuously increasing body of knowledge shows that the brain is an extremely complex neural network and single neurons possess their own complicated interactive signaling pathways. Such complexity of the nervous system makes it increasingly difficult to investigate the functions of specific neural components such as genes, proteins, transcription factors, neurons and nuclei in the brain. Technically, it has been even more of a significant challenge to identify the molecular and cellular adaptations that are both sufficient and necessary to underlie behavioral functions in health and disease states. Defining such neural adaptations is a critical step to identify the potential therapeutic targets within the complex neural network that are beneficial to treat psychiatric disorders. Recently, the new development and extensive application of in vivo viral-mediated gene transfer (virogenetics) and optical manipulation of specific neurons or selective neural circuits in freely-moving animals (optogenetics) make it feasible, through loss- and gain-of-function approaches, to reliably define sufficient and necessary neuroadaptations in the behavioral models of psychiatric disorders, including drug addiction, depression, anxiety and bipolar disorders. In this article, we focus on recent studies that successfully employ these advanced virogenetic and optogenetic techniques as a powerful tool to identify potential targets in the brain, and to provide highly useful information in the development of novel therapeutic strategies for psychiatric disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Molecular substrates of action control in cortico-striatal circuits

The purpose of this review is to describe the molecular mechanisms in the striatum that mediate reward-based learning and action control during instrumental conditioning. Experiments assessing the neural bases of instrumental conditioning have uncovered functional circuits in the striatum, including dorsal and ventral striatal sub-regions, involved in action-outcome learning, stimulus-response learning, and the motivational control of action by reward-associated cues. Integration of dopamine (DA) and glutamate neurotransmission within these striatal sub-regions is hypothesized to enable learning and action control through its role in shaping synaptic plasticity and cellular excitability. The extracellular signal regulated kinase (ERK) appears to be particularly important for reward-based learning and action control due to its sensitivity to combined DA and glutamate receptor activation and its involvement in a range of cellular functions. ERK activation in striatal neurons is proposed to have a dual role in both the learning and performance factors that contribute to instrumental conditioning through its regulation of plasticity-related transcription factors and its modulation of intrinsic cellular excitability. Furthermore, perturbation of ERK activation by drugs of abuse may give rise to behavioral disorders such as addiction.

A silent synapse-based mechanism for cocaine-induced locomotor sensitization

Locomotor sensitization is a common and robust behavioral alteration in rodents whereby following exposure to abused drugs such as cocaine, the animal becomes significantly more hyperactive in response to an acute drug challenge. Here, we further analyzed the role of cocaine-induced silent synapses in the nucleus accumbens (NAc) shell and their contribution to the development of locomotor sensitization. Using a combination of viral vector-mediated genetic manipulations, biochemistry, and electrophysiology in a locomotor sensitization paradigm with repeated, daily, noncontingent cocaine (15 mg/kg) injections, we show that dominant-negative cAMP-element binding protein (CREB) prevents cocaine-induced generation of silent synapses of young (30 d old) rats, whereas constitutively active CREB is sufficient to increase the number of NR2B-containing NMDA receptors (NMDARs) at synapses and to generate silent synapses. We further show that occupancy of CREB at the NR2B promoter increases and is causally related to the increase in synaptic NR2B levels. Blockade of NR2B-containing NMDARs by administration of the NR2B-selective antagonist Ro256981 directly into the NAc, under conditions that inhibit cocaine-induced silent synapses, prevents the development of cocaine-elicited locomotor sensitization. Our data are consistent with a cellular cascade whereby cocaine-induced activation of CREB promotes CREB-dependent transcription of NR2B and synaptic incorporation of NR2B-containing NMDARs, which generates new silent synapses within the NAc. We propose that cocaine-induced activation of CREB and generation of new silent synapses may serve as key cellular events mediating cocaine-induced locomotor sensitization. These findings provide a novel cellular mechanism that may contribute to cocaine-induced behavioral alterations.

Viral-mediated expression of a constitutively active form of cAMP response element binding protein in the dentate gyrus increases long term synaptic plasticity

The transcription factor cAMP response element binding protein (CREB) is a key player in synaptic plasticity and learning. Phosphorylation of CREB induced by neuronal activity leads to gene transcription, a process thought to contribute to memory formation. We have previously reported that increasing CREB activity in glutamatergic CA1 pyramidal neurons or in dentate gyrus (DG) granule cells is sufficient to enhance hippocampal-dependent memory formation. This enhancement correlates with an increase in CA1 glutamatergic synaptic plasticity. However, the effects of increasing CREB activity on DG glutamatergic plasticity have not been investigated. To address this issue, we boosted CREB-dependent transcription in DG granule cells in vivo via viral mediated expression of a constitutively active form of CREB (CREBCA). Using in vitro extracellular field recordings of infected slices, we observed an increase in long-term potentiation (LTP) while short-term plasticity and basic synaptic transmission remained unaffected. These data demonstrate that boosting CREB activity in DG granule cells is sufficient to enhance LTP and suggest that this enhancement participates in the formation of better memories.

Viral-mediated expression of a constitutively active form of CREB in the dentate gyrus does not induce abnormally enduring fear memory

Increasing CREB-dependent transcription in dentate gyrus (DG) granule cells in vivo using viral-mediated expression of a constitutively active form of CREB (CREBCA) is sufficient to enhance contextual fear memory but whether this treatment renders memory abnormally enduring is unknown. Here we confirm that over-expressing CREBCA in the DG increases retention of contextual fear conditioning (CFC) and show that this memory decays normally. Specifically, the retention scores of CREBCA mice are significantly higher than those of GFP-infected controls 24h after the conditioning, but match them after a longer exposure session and are still in the same range 48 h later. Our findings provide evidence that boosting selectively CREB activity in the DG promotes the formation of a stronger memory trace but does not increase its resistance to extinguish.

Exposure to cocaine alters dynorphin-mediated regulation of excitatory synaptic transmission in nucleus accumbens neurons

BACKGROUND

Dysregulation of excitatory synaptic input to nucleus accumbens (NAc) medium spiny neurons (MSNs) underlies a key pathophysiology of drug addiction and addiction-associated emotional and motivational alterations. Dynorphin peptides, which exhibit higher affinity to κ type opioid receptors, are upregulated within the NAc upon exposure to cocaine administration, and the increased dynorphin-signaling in the NAc has been critically implicated in negative mood observed in cocaine- or stress-exposed animals. Despite such apparent behavioral significance of the NAc dynorphins, the understanding of how dynorphins regulate excitatory synaptic transmission in the NAc remains incomplete.

METHODS

We used electrophysiological recording in brain slices to examine the effects of dynorphins on excitatory synaptic transmission in the NAc.

RESULTS

We focused on two key dynorphins, dynorphin A and B. Our current results show that dynorphin A and B differentially regulated excitatory postsynaptic currents (EPSCs) in NAc MSNs. Whereas perfusions of both dynorphin A and B to NAc slices decreased EPSCs in MSNs, the effect of dynorphin A but not dynorphin B was completely reversed by the κ receptor-selective antagonist nor-binaltorphimine. These results implicate κ receptor-independent mechanisms in dynorphin B-mediated synaptic effects in the NAc. Furthermore, repeated exposure to cocaine (15 mg/kg/day via intraperitoneal injection for 5 days, with 1, 2, or 14 days withdrawal) completely abolished dynorphin A-mediated modulation of EPSCs in NAc MSNs, whereas the effect of dynorphin B remained largely unchanged.

CONCLUSIONS

Given the quantitatively higher abundance of dynorphin B in the NAc, our present results suggest that the dynorphin B-mediated, κ receptor-independent pathways predominate in the overall effect of dynorphins in cocaine-pretreated animals and potentially in cocaine-induced alterations in mood.

Cocaine- and morphine-induced synaptic plasticity in the nucleus accumbens

The critical brain areas and molecular mechanisms involved in drug abuse and dependence have been extensively studied. Drug-induced persistent behaviors such as sensitization, tolerance, or relapse, however, far outlast any previously reported mechanisms. A challenge in the field of addiction, therefore, has been to identify drug-induced changes in brain circuitry that may subserve long-lasting changes in behavior. This study examined behavioral changes and electron microscopic evidence of altered synaptic connectivity within the nucleus accumbens (NAc) following repeated administration of cocaine or morphine. The unbiased quantitative stereological physical disector method was used to estimate the number of synapses per neuron. Increases in the synapse-to-neuron ratio were found in the NAc shell of cocaine-treated (49.1%) and morphine-treated (55.1%) rats and in the NAc core of cocaine-treated animals (49.1%). This study provides direct ultrastructural evidence of drug-induced synaptic plasticity and identifies synaptic remodeling as a potential neural substrate underlying drug-induced behavioral sensitization.

MeCP2 controls BDNF expression and cocaine intake through homeostatic interactions with microRNA-212

The X-linked transcriptional repressor methyl CpG binding protein 2 (MeCP2), known for its role in the neurodevelopmental disorder Rett syndrome, is emerging as an important regulator of neuroplasticity in postmitotic neurons. Cocaine addiction is commonly viewed as a disorder of neuroplasticity, but the potential involvement of MeCP2 has not been explored. Here we identify a key role for MeCP2 in the dorsal striatum in the escalating cocaine intake seen in rats with extended access to the drug, a process that mimics the increasingly uncontrolled cocaine use seen in addicted humans. MeCP2 regulates cocaine intake through homeostatic interactions with microRNA-212 (miR-212) to control the effects of cocaine on striatal brain-derived neurotrophic factor (BDNF) levels. These data suggest that homeostatic interactions between MeCP2 and miR-212 in dorsal striatum may be important in regulating vulnerability to cocaine addiction.

Cocaine-induced homeostatic regulation and dysregulation of nucleus accumbens neurons

Homeostatic response is an endowed self-correcting/maintaining property for living units, ranging from subcellular domains, single cells, and organs to the whole organism. Homeostatic responses maintain stable function through the ever-changing internal and external environments. In central neurons, several forms of homeostatic regulation have been identified, all of which tend to stabilize the functional output of neurons toward their prior "set-point." Medium spiny neurons (MSNs) within the forebrain region the nucleus accumbens (NAc) play a central role in gating/regulating emotional and motivational behaviors including craving and seeking drugs of abuse. Exposure to highly salient stimuli such as cocaine administration not only acutely activates a certain population of NAc MSNs, but also induces long-lasting changes in these neurons. It is these long-lasting cellular alterations that are speculated to mediate the increasingly strong cocaine-craving and cocaine-seeking behaviors. Why do the potentially powerful homeostatic mechanisms fail to correct or compensate for these drug-induced maladaptations in neurons? Based on recent experimental results, this review proposes a hypothesis of homeostatic dysregulation induced by exposure to cocaine. Specifically, we hypothesize that exposure to cocaine generates false molecular signals which misleads the homeostatic regulation process, resulting in maladaptive changes in NAc MSNs. Thus, many molecular and cellular alterations observed in the addicted brain may indeed result from homeostatic dysregulation. This review is among the first to introduce the concept of homeostatic neuroplasticity to understanding the molecular and cellular maladaptations following exposure to drugs of abuse.

Continuing the search for the engram: examining the mechanism of fear memories

The goal of my research is to gain insight using rodent models into the fundamental molecular, cellular and systems that make up the base of memory formation. My work focuses on fear memories. Aberrant fear and/or anxiety may be at the heart of many psychiatric disorders. In this article, I review the results of my research group; these results show that particular neurons in the lateral amygdala, a brain region important for fear, are specifically involved in particular fear memories. We started by showing that the transcription factor CREB (cAMP/Ca(2+) response element binding protein) plays a key role in the formation of fear memories. Next, we used viral vectors to overexpress CREB in a subset of lateral amygdala neurons. This not only facilitated fear memory formation but also "drove" the memory into the neurons with relatively increased CREB function. Finally, we showed that selective ablation of the neurons overexpressing CREB in the lateral amygdala selectively erased the fear memory. These findings are the first to show disruption of a specific memory by disrupting select neurons within a distributed network.

Exposure to cocaine dynamically regulates the intrinsic membrane excitability of nucleus accumbens neurons

Drug-induced malfunction of nucleus accumbens (NAc) neurons underlies a key pathophysiology of drug addiction. Drug-induced changes in intrinsic membrane excitability of NAc neurons are thought to be critical for producing behavioral alterations. Previous studies demonstrate that, after short-term (2 d) or long-term (21 d) withdrawal from noncontingent cocaine injection, the intrinsic membrane excitability of NAc shell (NAcSh) neurons is decreased, and decreased membrane excitability of NAcSh neurons increases the acute locomotor response to cocaine. However, animals exhibit distinct cellular and behavioral alterations at different stages of cocaine exposure, suggesting that the decreased membrane excitability of NAc neurons may not be a persistent change. Here, we demonstrate that the membrane excitability of NAcSh neurons is differentially regulated depending on whether cocaine is administered contingently or noncontingently. Specifically, the membrane excitability of NAcSh medium spiny neurons (MSNs) was decreased at 2 d after withdrawal from either 5 d intraperitoneal injections (15 mg/kg) or cocaine self-administration (SA). At 21 d of withdrawal, the membrane excitability of NAcSh MSNs, which remained low in intraperitoneally pretreated rats, returned to a normal level in SA-pretreated rats. Furthermore, after a reexposure to cocaine after long-term withdrawal, the membrane excitability of NAcSh MSNs instantly returned to a normal level in intraperitoneally pretreated rats. Conversely, in SA-pretreated rats, the reexposure elevated the membrane excitability of NAcSh MSMs beyond the normal level. These results suggest that the dynamic alterations in membrane excitability of NAcSh MSNs, together with the dynamic changes in synaptic input, contribute differentially to the behavioral consequences of contingent and noncontingent cocaine administration.

A pause in nucleus accumbens neuron firing is required to initiate and maintain feeding

Nucleus accumbens (NAc) inactivation increases food intake, indicating that NAc neurons exert ongoing inhibition of feeding. We previously described a subpopulation of NAc neurons that pause during sucrose licking and proposed that the pause permits consumption. We tested this hypothesis by first recording NAc neurons during sucrose consumption, and then electrically stimulating through the same electrodes. A large proportion of NAc shell and core neurons were inhibited during sucrose consumption, and local electrical stimulation abruptly interrupted licking. Effective stimulation sites were more anterior than ineffective sites in NAc. At low stimulus intensities, licking resumed immediately on stimulation offset. The latency to lick resumption from NAc neuron inhibition onset ( approximately 460 ms) was very similar to that after electrical stimulation offset ( approximately 440 ms). These results directly support the hypothesis that a significant subpopulation of NAc neurons inhibit palatable food consumption and that a pause in their firing is required to initiate and maintain consumption.

Synaptic adaptations of CA1 pyramidal neurons induced by a highly effective combinational antidepressant therapy

BACKGROUND

Antidepressants (AD) need to be chronically administered (weeks to months) to provide beneficial effects. Evidence suggests that combined administration of inhibitors of monoamine reuptake and phosphodiesterase type 4 allows a highly effective therapeutic action. Also, this coadministration more rapidly boosts the cyclic adenosine monophosphate (cAMP) pathway, which is normally activated during chronic treatment of single compounds. Little is known, however, about how this augmentation therapy affects the core mechanism of glutamatergic plasticity. We therefore investigated how in vivo combinational subchronic rolipram and imipramine (scRI) treatment affects depressive behavior, cAMP-dependent transcription, and glutamatergic transmission in the hippocampus, a region critically implicated in depression.

METHODS

Antidepressant properties of scRI were investigated through the forced swim test. Changes in cAMP-dependent transcription and synaptic transmission of CA1 pyramidal cells were explored with green fluorescent protein, enzyme-linked immunosorbent assay, electrophysiology recordings, and Golgi-Cox staining.

RESULTS

We demonstrate that scRI displays robust antidepressant properties compared with single-drug treatments and increases hippocampal c-Fos expression and brain-derived neurotrophic factor protein levels. These effects are accompanied by a specific increase in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in already existing synapses. Finally, both acute and subchronic treatments led to enhancement of long-term potentiation but differently affected spine density and morphology, with scRI administration specifically resulting in a large increase in stubby spines.

CONCLUSIONS

We conclude that scRI is highly effective in providing antidepressive effects, including the hippocampal transcriptional alterations normally observed with longer single-drug treatments. Furthermore, we identified scRI-induced modifications in glutamatergic transmission that probably underlie the beneficial action of this combinational therapy.