AMPAR-independent effect of striatal αCaMKII promotes the sensitization of cocaine reward

Single-nucleus RNA-sequencing of orbitofrontal cortex in rat model of methamphetamine-induced sensitization

The behavioral sensitization, characterized by escalated behavioral responses triggered by recurrent exposure to psychostimulants, involves neurobiological mechanisms that are brain-region and cell-type specific. Enduring neuroadaptive changes have been observed in response to methamphetamine (METH) within the orbitofrontal cortex (OFC), the cell-type specific transcriptional alterations in response to METH sensitization remain understudied. In this study, we utilized Single-nucleus RNA-sequencing (snRNA-seq) to profile the gene expression changes in the OFC of a rat METH sensitization model. The analyses of differentially expressed genes (DEGs) unveiled cell-type specific transcriptional reactions associated with METH sensitization, with the most significant alterations documented in microglial cells. Bioinformatic investigations revealed that distinct functional and signaling pathways enriched in microglia-specific DEGs majorly involved in macroautophagy processes and the activation of N-methyl-D-aspartate ionotropic glutamate receptors (NMDAR). To validate the translational relevance of our findings, we analyzed our snRNA-seq data in conjunction with a transcriptomic study of individuals with opioid use disorder (OUD) and a large-scale Genome-Wide Association Studies (GWAS) from multiple externalizing phenotypes related to drug addiction. The validation analysis confirmed the consistent expression changes of key microglial DEGs in human METH addiction. Moreover, the integration with GWAS data revealed associations between addiction risk genes and the DEGs observed in specific cell types, particularly microglia and excitatory neurons. Our study highlights the importance of cell-type specific transcriptional alterations in the OFC in the context of METH sensitization and their potential translational relevance to human drug addiction.

Physiological acetic acid concentrations from ethanol metabolism stimulate accumbens shell medium spiny neurons via NMDAR activation in a sex-dependent manner

Recent studies have implicated the ethanol metabolite, acetic acid, as neuroactive, perhaps even more so than ethanol itself. In this study, we investigated sex-specific metabolism of ethanol (1, 2, and 4 g/kg) to acetic acid in vivo to guide electrophysiology experiments in the accumbens shell (NAcSh), a key node in the mammalian reward circuit. There was a sex-dependent difference in serum acetate production, quantified via ion chromatography only at the lowest dose of ethanol (males > females). Ex vivo electrophysiology recordings of NAcSh medium spiny neurons (MSN) in brain slices demonstrated that physiological concentrations of acetic acid (2 mM and 4 mM) increased NAcSh MSN excitability in both sexes. N-methyl-D-aspartate receptor (NMDAR) antagonists, AP5 and memantine, robustly attenuated the acetic acid-induced increase in excitability. Acetic acid-induced NMDAR-dependent inward currents were greater in females compared to males and were not estrous cycle dependent. These findings suggest a novel NMDAR-dependent mechanism by which the ethanol metabolite, acetic acid, may influence neurophysiological effects in a key reward circuit in the brain from ethanol consumption. Furthermore, these findings also highlight a specific sex-dependent sensitivity in females to acetic acid-NMDAR interactions. This may underlie their more rapid advancement to alcohol use disorder and increased risk of alcohol related neurodegeneration compared to males.

Physiological acetic acid concentrations from ethanol metabolism stimulate accumbens shell neurons via NMDAR activation in a sex-dependent manner

Recent studies have implicated the ethanol metabolite, acetic acid, as neuroactive, perhaps even more so than ethanol itself. In this study, we investigated sex-specific metabolism of ethanol (1, 2, and 4g/kg) to acetic acid in vivo to guide electrophysiology experiments in the accumbens shell (NAcSh), a key node in the mammalian reward circuit. There was a sex-dependent difference in serum acetate production, quantified via ion chromatography only at the lowest dose of ethanol (males>females). Ex vivo electrophysiology recordings of NAcSh neurons in brain slices demonstrated that physiological concentrations of acetic acid (2 mM and 4 mM) increased NAcSh neuronal excitability in both sexes. N -methyl- D -aspartate receptor (NMDAR) antagonists, AP5, and memantine robustly attenuated the acetic acid-induced increase in excitability. Acetic acid-induced NMDAR-dependent inward currents were greater in females compared to males. These findings suggest a novel NMDAR-dependent mechanism by which the ethanol metabolite, acetic acid, may influence neurophysiological effects in a key reward circuit in the brain.

Nucleus accumbens in the pathogenesis of major depressive disorder: A brief review

Major depressive disorder (MDD) is the most prevalent mental disorder characterized by anhedonia, loss of motivation, avolition, behavioral despair and cognitive abnormalities. Despite substantial advancements in the pathophysiology of MDD in recent years, the pathogenesis of this disorder is not fully understood. Meanwhile,the treatment of MDD with currently available antidepressants is inadequate, highlighting the urgent need for clarifying the pathophysiology of MDD and developing novel therapeutics. Extensive studies have demonstrated the involvement of nuclei such as the prefrontal cortex (PFC), hippocampus (HIP), nucleus accumbens (NAc), hypothalamus, etc., in MDD. NAc,a region critical for reward and motivation,dysregulation of its activity seems to be a hallmark of this mood disorder. In this paper, we present a review of NAc related circuits, cellular and molecular mechanisms underlying MDD and share an analysis of the gaps in current research and possible future research directions.

Cav1.3 L-type Ca2+ channel-activated CaMKII/ERK2 pathway in the ventral tegmental area is required for cocaine conditioned place preference

We have previously demonstrated that pharmacological blockade of ventral tegmental area (VTA) Cav1.3 L-type calcium channels (LTCCs) using Cav1.2 dihydropyridine insensitive (Cav1.2DHP-/-) mutant mice attenuates cocaine conditioned place preference (CPP). However, the molecular mechanisms by which Cav1.3 channels mediate the effects of cocaine in the VTA remain largely unknown. In this study using Cav1.2DHP-/- male mice, we find that cocaine place preference increases CaM kinase IIα, ERK2, and CREB phosphorylation in the VTA, proteins strongly linked to cocaine behaviors. To further explore the causal role of these intracellular signaling proteins in cocaine preference, the CaM kinase II inhibitor, KN93 was directly injected into the VTA of male mice before each cocaine conditioning session. We found that KN93 attenuates conditioned preference for cocaine compared to vehicle treated mice and decreased VTA ERK2 and CREB phosphorylation. Additionally, blockade of the ERK pathway with the MEK inhibitor, U0126 or knockdown of ERK2 using siRNA, attenuated cocaine preference and VTA CREB phosphorylation but not CaMKIIα phosphorylation, suggesting that ERK is activated downstream of CaMKIIα. Examination of postsynaptic density (PSD) GluA1 subunit of AMPA receptors in the nucleus accumbens (NAc) that we have previously shown to be upregulated following long withdrawal periods, was blunted by KN93, U0126 and ERK2 siRNA when examined 30 days following cocaine CPP. Taken together, these findings demonstrate that Cav1.3 channels in the VTA are required for cocaine reward behavior and activation of the CaMKIIα/ERK/CREB signaling pathway in the VTA is necessary for long-lasting changes in the NAc. This article is part of the Special Issue on 'L-type calcium channel mechanisms in neuropsychiatric disorders'.

A missense mutation in Kcnc3 causes hippocampal learning deficits in mice

Although a wide variety of genetic tools has been developed to study learning and memory, the molecular basis of memory encoding remains incompletely understood. Here, we undertook an unbiased approach to identify novel genes critical for memory encoding. From a large-scale, in vivo mutagenesis screen using contextual fear conditioning, we isolated in mice a mutant, named Clueless, with spatial learning deficits. A causative missense mutation (G434V) was found in the voltage-gated potassium channel, subfamily C member 3 (Kcnc3) gene in a region that encodes a transmembrane voltage sensor. Generation of a Kcnc3^G434V CRISPR mutant mouse confirmed this mutation as the cause of the learning defects. While G434V had no effect on transcription, translation, or trafficking of the channel, electrophysiological analysis of the G434V mutant channel revealed a complete loss of voltage-gated conductance, a broadening of the action potential, and decreased neuronal firing. Together, our findings have revealed a role for Kcnc3 in learning and memory.

Acute and protracted abstinence from methamphetamine bidirectionally changes intrinsic excitability of indirect pathway spiny projection neurons in the dorsomedial striatum

Methamphetamine (meth) is an addictive psychostimulant and illicit use presents significant personal and socioeconomic harm. Behavioral studies support the involvement of the dorsal striatum in drug-seeking but stimulant induced dysfunction in this region is understudied. The dorsal striatum can be subdivided into the dorsomedial (DMS) and dorsolateral (DLS) striatum with the DMS implicated in goal-directed and DLS in habitual behaviors; both regions are primarily composed of GABAergic direct (dSPNs) and indirect pathway (iSPNs) spiny projection neurons. To examine the effect of repeated meth on SPNs, mice were administered meth (2 mg/kg) for ten consecutive days and intrinsic excitability, dendritic excitability, and spine density were examined. DMS iSPN intrinsic excitability was increased at 1 day but decreased at 21 days of abstinence. In contrast, DMS dSPN intrinsic excitability was unchanged at either timepoint. Dendritic excitability and spine densities were unaltered in DMS iSPNs and dSPNs at 1 and 21 days of abstinence. The effect of repeated meth on iSPN excitability was specific to the DMS; DLS iSPN intrinsic excitability, dendritic excitability, and spine density were unchanged at 1 and 21 days of abstinence. These findings point toward DMS iSPN dysfunction in meth use disorders with differential dysfunction dependent on abstinence duration.

Rewarding Social Interaction in Rats Increases CaMKII in the Nucleus Accumbens

Calcium/calmodulin-dependent protein kinase II (CaMKII) is known to be involved in the sensitized locomotor responses and drug-seeking behavior to psychostimulants. However, little is known about the contribution of CaMKII signaling in the nucleus accumbens (NAc) in natural rewards such as social interaction. The present experiments explored the implication of CaMKII signaling in drug versus natural reward. In the NAc of rats expressing cocaine or social interaction conditioned place preference (CPP), αCaMKII activation was induced in those expressing social interaction but not cocaine CPP. In order to investigate the role of NAc CaMKII in the expression of reward-related learning of drug versus non-drug stimuli, we inhibited CaMKII through an infusion of KN-93, a CaMKII inhibitor, directly into the NAc shell or core, before the CPP test in a concurrent paradigm in which social interaction was made available in the compartment alternative to the one associated with cocaine during conditioning. Whereas vehicle infusions led to equal preference to both stimuli, inhibition of CaMKII by a KN-93 infusion before the CPP test in the shell but not the core of the NAc shifted the rats' preference toward the cocaine-associated compartment. Altogether, these results suggest that social interaction reward engages CaMKII in the NAc.

Caveolin-1 regulates medium spiny neuron structural and functional plasticity

RATIONALE

Caveolin-1 (CAV1) is a structural protein critical for spatial organization of neuronal signaling molecules. Whether CAV1 is required for long-lasting neuronal plasticity remains unknown.

OBJECTIVE AND METHODS

We sought to examine the effects of CAV1 knockout (KO) on functional plasticity and hypothesized that CAV1 deficiency would impact drug-induced long-term plasticity in the nucleus accumbens (NAc). We first examined cell morphology of NAc medium spiny neurons in a striatal/cortical co-culture system before moving in vivo to study effects of CAV1 KO on cocaine-induced plasticity. Whole-cell patch-clamp recordings were performed to determine effects of chronic cocaine (15 mg/kg) on medium spiny neuron excitability. To test for deficits in behavioral plasticity, we examined the effect of CAV1 KO on locomotor sensitization.

RESULTS

Disruption of CAV1 expression leads to baseline differences in medium spiny neuron (MSN) structural morphology, such that MSNs derived from CAV1 KO animals have increased dendritic arborization when cultured with cortical neurons. The effect was dependent on phospholipase C and cell-type intrinsic loss of CAV1. Slice recordings of nucleus accumbens shell MSNs revealed that CAV1 deficiency produces a loss of neuronal plasticity. Specifically, cocaine-induced firing rate depression was absent in CAV1 KO animals, whereas baseline electrophysiological properties were similar. This was reflected by a loss of cocaine-mediated behavioral sensitization in CAV1 KO animals, with unaffected baseline locomotor responsiveness.

CONCLUSIONS

This study highlights a critical role for nucleus accumbens CAV1 in plasticity related to the administration of drugs of abuse.

Cocaine engages a non-canonical, dopamine-independent, mechanism that controls neuronal excitability in the nucleus accumbens

Drug-induced enhanced dopamine (DA) signaling in the brain is a canonical mechanism that initiates addiction processes. However, indirect evidence suggests that cocaine also triggers non-canonical, DA-independent, mechanisms that contribute to behavioral responses to cocaine, including psychomotor sensitization and cocaine self-administration. Identifying these mechanisms and determining how they are initiated is fundamental to further our understanding of addiction processes. Using physiologically relevant in vitro tractable models, we found that cocaine-induced hypoactivity of nucleus accumbens shell (NAcSh) medium spiny neurons (MSNs), one hallmark of cocaine addiction, is independent of DA signaling. Combining brain slice studies and site-directed mutagenesis in HEK293T cells, we found that cocaine binding to intracellular sigma-1 receptor (σ1) initiates this mechanism. Subsequently, σ1 binds to Kv1.2 potassium channels, followed by accumulation of Kv1.2 in the plasma membrane, thereby depressing NAcSh MSNs firing. This mechanism is specific to D1 receptor-expressing MSNs. Our study uncovers a mechanism for cocaine that bypasses DA signaling and leads to addiction-relevant neuroadaptations, thereby providing combinatorial strategies for treating stimulant abuse.

The Sigma-1 Receptor: When Adaptive Regulation of Cell Electrical Activity Contributes to Stimulant Addiction and Cancer

The sigma-1 receptor (σ1R) is an endoplasmic reticulum (ER)-resident chaperone protein that acts like an inter-organelle signaling modulator. Among its several functions such as ER lipid metabolisms/transports and indirect regulation of genes transcription, one of its most intriguing feature is the ability to regulate the function and trafficking of a variety of functional proteins. To date, and directly relevant to the present review, σ1R has been found to regulate both voltage-gated ion channels (VGICs) belonging to distinct superfamilies (i.e., sodium, Na⁺; potassium, K⁺; and calcium, Ca²⁺ channels) and non-voltage-gated ion channels. This regulatory function endows σ1R with a powerful capability to fine tune cells’ electrical activity and calcium homeostasis—a regulatory power that appears to favor cell survival in pathological contexts such as stroke or neurodegenerative diseases. In this review, we present the current state of knowledge on σ1R’s role in the regulation of cellular electrical activity, and how this seemingly adaptive function can shift cell homeostasis and contribute to the development of very distinct chronic pathologies such as psychostimulant abuse and tumor cell growth in cancers.

Synaptic Depotentiation and mGluR5 Activity in the Nucleus Accumbens Drive Cocaine-Primed Reinstatement of Place Preference

Understanding the neurobiological processes that incite drug craving and drive relapse has the potential to help target efforts to treat addiction. The NAc serves as a critical substrate for reward and motivated behavior, in part due to alterations in excitatory synaptic strength within cortical-accumbens pathways. The present studies investigated a causal link between cocaine-induced reinstatement of conditioned place preference and rapid reductions of cocaine-dependent increases in NAc shell synaptic strength in male mice. Cocaine-conditioned place preference behavior and ex vivo whole-cell electrophysiology showed that cocaine-primed reinstatement and synaptic depotentiation were disrupted by inhibiting AMPAR internalization via intra-NAc shell infusion of a Tat-GluA23Y peptide. Furthermore, reinstatement was driven by an mGluR5-dependent reduction in AMPAR signaling. Intra-NAc shell infusion of the mGluR5 antagonist MTEP blocked cocaine-primed reinstatement and corresponding depotentiation, whereas infusion of the mGluR5 agonist CHPG itself promoted reinstatement and depotentiated synaptic strength in the NAc shell. Optogenetic examination of circuit-specific plasticity showed that inhibition of infralimbic cortical input to the NAc shell blocked cocaine-primed reinstatement, whereas low-frequency stimulation (10 Hz) of this pathway in the absence of cocaine triggered a reduction in synaptic strength akin to that observed with cocaine, and was sufficient to promote reinstatement in the absence of a cocaine challenge. These data support a model in which mGluR5-mediated reduction in GluA2-containing AMPARs at NAc shell synapses receiving input from the infralimbic cortex is a critical factor in triggering reinstatement of cocaine-primed conditioned approach behavior.SIGNIFICANCE STATEMENT These studies identified a sequence of neural events whereby reexposure to cocaine activates a signaling cascade that alters synaptic strength in the NAc shell and triggers a behavioral response driven by a drug-associated memory.

Reduced ΔFosB expression in the rat nucleus accumbens has causal role in the neuropathic pain phenotype

The neuropathic pain phenotype is the consequence of functional and morphological reorganization of the PNS and CNS. This reorganization includes DRGs and the spinal cord, and extends to multiple supraspinal areas including the limbic and reward systems. Several recent papers show that acute manipulation of cortical and subcortical brain areas causally correlates with the cognitive, emotional and sensory components of neuropathic pain, yet mechanisms responsible for pain chronification remain largely unknown. Here we show that nucleus accumbens expression of ΔFos-B, a transcription factor that plays a critical role in addiction and in the brain response to stress, is reduced long term following peripheral neuropathic injury. Conversely, boosting ΔFos-B expression in the nucleus accumbens by viral transfection causes a significant and long-lasting improvement of the neuropathic allodynia. We suggest that ΔFos-B in the nucleus accumbens is a key modulator of long term gene expression leading to pain chronification.

In silico identification and in vivo validation of miR-495 as a novel regulator of motivation for cocaine that targets multiple addiction-related networks in the nucleus accumbens

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression and are implicated in the etiology of several neuropsychiatric disorders, including substance use disorders (SUDs). Using in silico genome-wide sequence analyses, we identified miR-495 as a miRNA whose predicted targets are significantly enriched in the Knowledgebase for Addiction Related Genes (ARG) database (KARG; http://karg.cbi.pku.edu.cn). This small non-coding RNA is also highly expressed within the nucleus accumbens (NAc), a pivotal brain region underlying reward and motivation. Using luciferase reporter assays, we found that miR-495 directly targeted the 3'UTRs of Bdnf, Camk2a and Arc. Furthermore, we measured miR-495 expression in response to acute cocaine in mice and found that it is downregulated rapidly and selectively in the NAc, along with concomitant increases in ARG expression. Lentiviral-mediated miR-495 overexpression in the NAc shell (NAcsh) not only reversed these cocaine-induced effects but also downregulated multiple ARG mRNAs in specific SUD-related biological pathways, including those that regulate synaptic plasticity. miR-495 expression was also downregulated in the NAcsh of rats following cocaine self-administration. Most importantly, we found that NAcsh miR-495 overexpression suppressed the motivation to self-administer and seek cocaine across progressive ratio, extinction and reinstatement testing, but had no effect on food reinforcement, suggesting that miR-495 selectively affects addiction-related behaviors. Overall, our in silico search for post-transcriptional regulators identified miR-495 as a novel regulator of multiple ARGs that have a role in modulating motivation for cocaine.

Autophosphorylation of alpha isoform of calcium/calmodulin-dependent kinase II regulates alcohol addiction-related behaviors

The development of addiction is associated with a dysregulation of glutamatergic transmission in the brain reward circuit. α isoform of calcium/calmodulin-dependent kinase II (αCaMKII) is one of the key proteins that regulates structural and functional plasticity of glutamatergic synapses. αCaMKII activity can be controlled by the autophosphorylation of threonine 286. The role of this autophosphorylation in the regulation of addiction-related behaviors has been proposed but is still poorly understood. Here, using αCaMKII autophosphorylation-deficient mutant mice (T286A), we show that, in comparison with wild-type animals, they are less resistant to high doses of alcohol and do not show psychostimulant response neither to alcohol injections nor during voluntary alcohol drinking. T286A mutants are also less prone to develop alcohol addiction-related behaviors including an increased motivation for alcohol, persistent alcohol seeking during withdrawal and alcohol consumption on relapse. Finally, we demonstrate that αCaMKII autophosphorylation regulates also alcohol-induced remodeling of glutamatergic synapses in the hippocampus and amygdala. In conclusion, our data suggest that αCaMKII autophosphorylation-dependent remodeling of glutamatergic synapses is a plausible mechanism for the regulation of the alcohol addiction-related behaviors.

Associative Learning Drives the Formation of Silent Synapses in Neuronal Ensembles of the Nucleus Accumbens

BACKGROUND

Learned associations between environmental stimuli and rewards play a critical role in addiction. Associative learning requires alterations in sparsely distributed populations of strongly activated neurons, or neuronal ensembles. Until recently, assessment of functional alterations underlying learned behavior was restricted to global neuroadaptations in a particular brain area or cell type, rendering it impossible to identify neuronal ensembles critically involved in learned behavior.

METHODS

We used Fos-GFP transgenic mice that contained a transgene with a Fos promoter driving expression of green fluorescent protein (GFP) to detect neurons that were strongly activated during associative learning, in this case, context-independent and context-specific cocaine-induced locomotor sensitization. Whole-cell electrophysiological recordings were used to assess synaptic alterations in specifically activated GFP-positive (GFP+) neurons compared with surrounding nonactivated GFP-negative (GFP-) neurons 90 min after the sensitized locomotor response.

RESULTS

After context-independent cocaine sensitization, cocaine-induced locomotion was equally sensitized by repeated cocaine injections in two different sensitization contexts. Correspondingly, silent synapses in these mice were induced in GFP+ neurons, but not GFP- neurons, after sensitization in both of these contexts. After context-specific cocaine sensitization, cocaine-induced locomotion was sensitized exclusively in mice trained and tested in the same context (paired group), but not in mice that were trained in one context and then tested in a different context (unpaired group). Silent synapses increased in GFP+ neurons, but not in GFP- neurons from mice in the paired group, but not from mice in the unpaired group.

CONCLUSIONS

Our results indicate that silent synapses are formed only in neuronal ensembles of the nucleus accumbens shell that are related to associative learning.

Whole-cell Patch-clamp Recordings in Brain Slices

Whole-cell patch-clamp recording is an electrophysiological technique that allows the study of the electrical properties of a substantial part of the neuron. In this configuration, the micropipette is in tight contact with the cell membrane, which prevents current leakage and thereby provides more accurate ionic current measurements than the previously used intracellular sharp electrode recording method. Classically, whole-cell recording can be performed on neurons in various types of preparations, including cell culture models, dissociated neurons, neurons in brain slices, and in intact anesthetized or awake animals. In summary, this technique has immensely contributed to the understanding of passive and active biophysical properties of excitable cells. A major advantage of this technique is that it provides information on how specific manipulations (e.g., pharmacological, experimenter-induced plasticity) may alter specific neuronal functions or channels in real-time. Additionally, significant opening of the plasma membrane allows the internal pipette solution to freely diffuse into the cytoplasm, providing means for introducing drugs, e.g., agonists or antagonists of specific intracellular proteins, and manipulating these targets without altering their functions in neighboring cells. This article will focus on whole-cell recording performed on neurons in brain slices, a preparation that has the advantage of recording neurons in relatively well preserved brain circuits, i.e., in a physiologically relevant context. In particular, when combined with appropriate pharmacology, this technique is a powerful tool allowing identification of specific neuroadaptations that occurred following any type of experiences, such as learning, exposure to drugs of abuse, and stress. In summary, whole-cell patch-clamp recordings in brain slices provide means to measure in ex vivo preparation long-lasting changes in neuronal functions that have developed in intact awake animals.

Cocaine and Amphetamine Induce Overlapping but Distinct Patterns of AMPAR Plasticity in Nucleus Accumbens Medium Spiny Neurons

Repeated exposure to psychostimulant drugs such as cocaine or amphetamine can promote drug-seeking and -taking behavior. In rodent addiction models, persistent changes in excitatory glutamatergic neurotransmission in the nucleus accumbens (NAc) appear to drive this drug-induced behavioral plasticity. To study whether changes in glutamatergic signaling are shared between or exclusive to specific psychostimulant drugs, we examined synaptic transmission from mice following repeated amphetamine or cocaine administration. Synaptic transmission mediated by AMPA-type glutamate receptors was potentiated in the NAc shell 10-14 days following repeated amphetamine or cocaine treatment. This synaptic enhancement was depotentiated by re-exposure to amphetamine or cocaine. By contrast, in the NAc core only repeated cocaine exposure enhanced synaptic transmission, which was subsequently depotentiated by an additional cocaine but not amphetamine injection during drug abstinence. To better understand the drug-induced depotentiation, we replicated these in vivo findings using an ex vivo model termed 'challenge in the bath,' and showed that drug-induced decreases in synaptic strength occur rapidly (within 30 min) and require activation of metabotropic glutamate receptor 5 (mGluR5) and protein synthesis in the NAc shell, but not NAc core. Overall, these data demonstrate the specificity of neuronal circuit changes induced by amphetamine, introduce a novel method for studying drug challenge-induced plasticity, and define NAc shell medium spiny neurons as a primary site of persistent AMPA-type glutamate receptor plasticity by two widely used psychostimulant drugs.

CaM Kinases: From Memories to Addiction

Drug addiction is a major psychiatric disorder with a neurobiological basis that is still insufficiently understood. Initially, non-addicted, controlled drug consumption and drug instrumentalization are established. They comprise highly systematic behaviours acquired by learning and the establishment of drug memories. Ca(2+)/calmodulin-dependent protein kinases (CaMKs) are important Ca(2+) sensors translating glutamatergic activation into synaptic plasticity during learning and memory formation. Here we review the role of CaMKs in the establishment of drug-related behaviours in animal models and in humans. Converging evidence now shows that CaMKs are a crucial mechanism of how addictive drugs induce synaptic plasticity and establish various types of drug memories. Thereby, CaMKs are not only molecular relays for glutamatergic activity but they also directly control dopaminergic and serotonergic activity in the mesolimbic reward system. They can now be considered as major molecular pathways translating normal memory formation into establishment of drug memories and possibly transition to drug addiction.

Opposing role for Egr3 in nucleus accumbens cell subtypes in cocaine action

An imbalance in molecular signaling cascades and transcriptional regulation in nucleus accumbens (NAc) medium spiny neuron (MSN) subtypes, those enriched in dopamine D1 versus D2 receptors, is implicated in the behavioral responses to psychostimulants. To provide further insight into the molecular mechanisms occurring in MSN subtypes by cocaine, we examined the transcription factor early growth response 3 (Egr3). We evaluated Egr3 because it is a target of critical cocaine-mediated signaling pathways and because Egr3-binding sites are found on promoters of key cocaine-associated molecules. We first used a RiboTag approach to obtain ribosome-associated transcriptomes from each MSN subtype and found that repeated cocaine administration induced Egr3 ribosome-associated mRNA in NAc D1-MSNs while reducing Egr3 in D2-MSNs. Using Cre-inducible adeno-associated viruses combined with D1-Cre and D2-Cre mouse lines, we observed that Egr3 overexpression in D1-MSNs enhances rewarding and locomotor responses to cocaine, whereas overexpression in D2-MSNs blunts these behaviors. miRNA knock-down of Egr3 in MSN subtypes produced opposite behavioral responses from those observed with overexpression. Finally, we found that repeated cocaine administration altered Egr3 binding to promoters of genes that are important for cocaine-mediated cellular and behavioral plasticity. Genes with increased Egr3 binding to promoters, Camk2α, CREB, FosB, Nr4a2, and Sirt1, displayed increased mRNA in D1-MSNs and, in some cases, a reduction in D2-MSNs. Histone and the DNA methylation enzymes G9a and Dnmt3a displayed reduced Egr3 binding to their promoters and reduced mRNA in D1-MSNs. Our study provides novel insight into an opposing role of Egr3 in select NAc MSN subtypes in cocaine action.

Intrinsic plasticity: an emerging player in addiction

Exposure to drugs of abuse, such as cocaine, leads to plastic changes in the activity of brain circuits, and a prevailing view is that these changes play a part in drug addiction. Notably, there has been intense focus on drug-induced changes in synaptic excitability and much less attention on intrinsic excitability factors (that is, excitability factors that are remote from the synapse). Accumulating evidence now suggests that intrinsic factors such as K+ channels are not only altered by cocaine but may also contribute to the shaping of the addiction phenotype.

αCaMKII controls the establishment of cocaine's reinforcing effects in mice and humans

Although addiction develops in a considerable number of regular cocaine users, molecular risk factors for cocaine dependence are still unknown. It was proposed that establishing drug use and memory formation might share molecular and anatomical pathways. Alpha-Ca(2+)/calmodulin-dependent protein kinase-II (αCaMKII) is a key mediator of learning and memory also involved in drug-related plasticity. The autophosphorylation of αCaMKII was shown to accelerate learning. Thus, we investigated the role of αCaMKII autophosphorylation in the time course of establishing cocaine use-related behavior in mice. We found that αCaMKII autophosphorylation-deficient αCaMKII(T286A) mice show delayed establishment of conditioned place preference, but no changes in acute behavioral activation, sensitization or conditioned hyperlocomotion to cocaine (20 mg kg(-1), intraperitoneal). In vivo microdialysis revealed that αCaMKII(T286A) mice have blunted dopamine (DA) and blocked serotonin (5-HT) responses in the nucleus accumbens (NAcc) and prefrontal cortex after acute cocaine administration (20 mg kg(-1), intraperitoneal), whereas noradrenaline responses were preserved. Under cocaine, the attenuated DA and 5-HT activation in αCaMKII(T286A) mice was followed by impaired c-Fos activation in the NAcc. To translate the rodent findings to human conditions, several CAMK2A gene polymorphisms were tested regarding their risk for a fast establishment of cocaine dependence in two independent samples of regular cocaine users from Brazil (n=688) and Switzerland (n=141). A meta-analysis across both samples confirmed that CAMK2A rs3776823 TT-allele carriers display a faster transition to severe cocaine use than C-allele carriers. Together, these data suggest that αCaMKII controls the speed for the establishment of cocaine's reinforcing effects.

Cellular basis of memory for addiction

Despite the importance of numerous psychosocial factors, at its core, drug addiction involves a biological process: the ability of repeated exposure to a drug of abuse to induce changes in a vulnerable brain that drive the compulsive seeking and taking of drugs, and loss of control over drug use, that define a state of addiction. Here, we review the types of molecular and cellular adaptations that occur in specific brain regions to mediate addiction-associated behavioral abnormalities. These include alterations in gene expression achieved in part via epigenetic mechanisms, plasticity in the neurophysiological functioning of neurons and synapses, and associated plasticity in neuronal and synaptic morphology mediated in part by altered neurotrophic factor signaling. Each of these types of drug-induced modifications can be viewed as a form of “cellular or molecular memory.” Moreover, it is striking that most addiction-related forms of plasticity are very similar to the types of plasticity that have been associated with more classic forms of “behavioral memory,” perhaps reflecting the finite repertoire of adaptive mechanisms available to neurons when faced with environmental challenges. Finally, addiction-related molecular and cellular adaptations involve most of the same brain regions that mediate more classic forms of memory, consistent with the view that abnormal memories are important drivers of addiction syndromes. The goal of these studies which aim to explicate the molecular and cellular basis of drug addiction is to eventually develop biologically based diagnostic tests, as well as more effective treatments for addiction disorders.

Nucleus accumbens shell excitability is decreased by methamphetamine self-administration and increased by 5-HT2C receptor inverse agonism and agonism

Methamphetamine profoundly increases brain monoamines and is a widely abused psychostimulant. The effects of methamphetamine self-administration on neuron function are not known for the nucleus accumbens, a brain region involved in addictive behaviors, including drug-seeking. One therapeutic target showing preclinical promise at attenuating psychostimulant-seeking is 5-HT2C receptors; however, the effects of 5-HT2C receptor ligands on neuronal physiology are unclear. 5-HT2C receptor agonism decreases psychostimulant-mediated behaviors, and the putative 5-HT2C receptor inverse agonist, SB 206553, attenuates methamphetamine-seeking in rats. To ascertain the effects of methamphetamine, and 5-HT2C receptor inverse agonism and agonism, on neuronal function in the nucleus accumbens, we evaluated methamphetamine, SB 206553, and the 5-HT2C receptor agonist and Ro 60-0175, on neuronal excitability within the accumbens shell subregion using whole-cell current-clamp recordings in forebrain slices ex vivo. We reveal that methamphetamine self-administration decreased generation of evoked action potentials. In contrast, SB 206553 and Ro 60-0175 increased evoked spiking, effects that were prevented by the 5-HT2C receptor antagonist, SB 242084. We also assessed signaling mechanisms engaged by 5-HT2C receptors, and determined that accumbal 5-HT2C receptors stimulated Gq, but not Gi/o. These findings demonstrate that methamphetamine-induced decreases in excitability of neurons within the nucleus accumbens shell were abrogated by both 5-HT2C inverse agonism and agonism, and this effect likely involved activation of Gq-mediated signaling pathways.

Emerging role of CaMKII in neuropsychiatric disease

Although it has been known for decades that hippocampal calcium/calmodulin (CaM)-dependent protein kinase II (CaMKII) plays an essential role in learning and memory consolidation, the roles of CaMKII in other brain regions are only recently being explored in depth. A series of recent studies suggest that CaMKII dysfunction throughout the brain may underlie myriad neuropsychiatric disorders, including drug addiction, schizophrenia, depression, epilepsy, and multiple neurodevelopmental disorders, perhaps through maladaptations in glutamate signaling and neuroplasticity. I review here the structure, function, subcellular localization, and expression patterns of CaMKII isoforms, as well as recent advances demonstrating that disturbances in these properties may contribute to psychiatric disorders.

Interactions between N-ethylmaleimide-sensitive factor and GluR2 in the nucleus accumbens contribute to the expression of locomotor sensitization to cocaine

Many studies have reported a withdrawal-dependent increase in synaptic AMPA receptor (AMPAR) levels in the nucleus accumbens (NAc) of cocaine-sensitized rats; however, the exact relationship between the expression of sensitization and altered AMPAR surface expression in the NAc has not yet been investigated. We demonstrated that the expression of behavioral sensitization was negatively controlled by N-ethylmaleimide-sensitive factor (NSF)-GluR2 interactions in the NAc. The upregulation of NSF-GluR2 interactions, which may be resulted by the increase in NSF S-nitrosylation after withdrawal from cocaine, was associated with the changes in the expression of behavioral sensitization. Disruption of NSF-GluR2 interactions in the NAc with a specific peptide, TAT-pep-R845A, increased the locomotor response of rats to cocaine by decreasing GluR2 surface insertion. In contrast, prevention of GluR2-containing AMPARs removal from synapses with Pep2-EVKI attenuated the expression of behavioral sensitization. Similarly, treatment with the nitric oxide donor, S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the expression of locomotor sensitization by promoting GluR2 surface expression. This effect was mediated by the binding of S-nitrosylated NSF to GluR2, which promoted the surface expression of AMPARs. Noticeably, exogenous injection of SNAP into NAc also attenuated the expression of cocaine-induced conditioned place preference. Thus, these results indicate that increased NSF-GluR2 interactions in the NAc after withdrawal from cocaine attenuated the expression of behavioral sensitization and serves as a negative regulatory mechanism in drug-exposed individuals.

Fluoxetine epigenetically alters the CaMKIIα promoter in nucleus accumbens to regulate ΔFosB binding and antidepressant effects

Chronic social defeat stress in mice produces a susceptible phenotype characterized by several behavioral abnormalities consistent with human depression that are reversed by chronic but not acute exposure to antidepressant medications. Recent work in addiction models demonstrates that the transcription factor ΔFosB and protein kinase calmodulin-dependent protein kinase II (CaMKII) are co-regulated in nucleus accumbens (NAc), a brain reward region implicated in both addiction and depression models including social defeat. Previous work has also demonstrated that ΔFosB is induced in NAc after chronic social defeat stress or after chronic antidepressant treatment, wherein it mediates a pro-resilience or antidepressant-like phenotype. Here, using chromatin immunoprecipitation assays, we found that ΔFosB binds the CaMKIIα gene promoter in NAc and that this binding increases after mice are exposed to chronic social defeat stress. Paradoxically, chronic exposure to the antidepressant fluoxetine reduces binding of ΔFosB to the CaMKIIα promoter and reduces CaMKII expression in NAc, despite the fact that ΔFosB is induced under these conditions. These data suggest a novel epigenetic mechanism of antidepressant action, whereby fluoxetine induces some chromatin change at the CaMKIIα promoter, which blocks the ΔFosB binding. Indeed, chronic fluoxetine reduces acetylation and increases lysine-9 dimethylation of histone H3 at the CaMKIIα promoter in NAc, effects also seen in depressed humans exposed to antidepressants. Overexpression of CaMKII in NAc blocks fluoxetine's antidepressant effects in the chronic social defeat paradigm, whereas inhibition of CaMKII activity in NAc mimics fluoxetine exposure. These findings suggest that epigenetic suppression of CaMKIIα expression in NAc is behaviorally relevant and offer a novel pathway for possible therapeutic intervention in depression and related syndromes.

CaMKII activity in the ventral tegmental area gates cocaine-induced synaptic plasticity in the nucleus accumbens

Addictive drugs such as cocaine induce synaptic plasticity in discrete regions of the reward circuit. The aim of the present study is to investigate whether cocaine-evoked synaptic plasticity in the ventral tegmental area (VTA) and nucleus accumbens (NAc) is causally linked. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a central regulator of long-term synaptic plasticity, learning, and drug addiction. We examined whether blocking CaMKII activity in the VTA affected cocaine conditioned place preference (CPP) and cocaine-evoked synaptic plasticity in its target brain region, the NAc. TatCN21 is a CaMKII inhibitory peptide that blocks both stimulated and autonomous CaMKII activity with high selectivity. We report that intra-VTA microinjections of tatCN21 before cocaine conditioning blocked the acquisition of cocaine CPP, whereas intra-VTA microinjections of tatCN21 before saline conditioning did not significantly affect cocaine CPP, suggesting that the CaMKII inhibitor blocks cocaine CPP through selective disruption of cocaine-cue-associated learning. Intra-VTA tatCN21 before cocaine conditioning blocked cocaine-evoked depression of excitatory synaptic transmission in the shell of the NAc slices ex vivo. In contrast, intra-VTA microinjection of tatCN21 just before the CPP test did not affect the expression of cocaine CPP and cocaine-induced synaptic plasticity in the NAc shell. These results suggest that CaMKII activity in the VTA governs cocaine-evoked synaptic plasticity in the NAc during the time window of cocaine conditioning.

Biological substrates of addiction

This review is an introduction to addiction, the reward circuitry, and laboratory addiction models. Addiction is a chronic disease hallmarked by a state of compulsive drug seeking that persists despite negative consequences. Most of the advances in addiction research have centered on the canonical and contemporary drugs of abuse; however, addictions to other activities and stimuli also exist. Substances of abuse have the potential to induce long-lasting changes in the brain at the behavioral, circuit, and synaptic levels. Addiction-related behavioral changes involve initiation, escalation, and obsession to drug seeking and much of the current research is focused on mapping these manifestations to specific neural pathways. Drug abuse is well known to recruit components of the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. In addition, altered function of a wide variety of brain regions is tightly associated with specific manifestations of drug abuse. These regions peripheral to the mesolimbic pathway likely play a role in specific observed comorbidities and endophenotypes that can facilitate, or be caused by, substance abuse. Alterations in synaptic structure, function, and connectivity, as well as epigenetic and genetic mechanisms are thought to underlie the pathologies of addiction. In preclinical models, these persistent changes are studied at the levels of molecular pharmacology and biochemistry, ex vivo and in vivo electrophysiology, radiography, and behavior. Coordinating research efforts across these disciplines and examining cell type- and circuit-specific phenomena are crucial components for translating preclinical findings to viable medical interventions that effectively treat addiction and related disorders. WIREs Cogn Sci 2014, 5:151-171. doi: 10.1002/wcs.1273 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Dopamine D1 receptor gene variation modulates opioid dependence risk by affecting transition to addiction

Dopamine D1 receptor (DRD1) modulates opioid reinforcement, reward, and opioid-induced neuroadaptation. We propose that DRD1 polymorphism affects susceptibility to opioid dependence (OD), the efficiency of transition to OD, and opioid-induced pleasure response. We analyzed potential association between seven DRD1 polymorphisms with the following traits: duration of transition from the first use to dependence (DTFUD), subjective pleasure responses to opioid on first use and post-dependence use, and OD risk in 425 Chinese with OD and 514 healthy controls. DTFUD and level of pleasure responses were examined using a semi-structured interview. The DTFUD of opioid addicts ranged from 5 days to 11 years. Most addicts (64.0%) reported non-comfortable response upon first opioid use, while after dependence, most addicts (53.0%) felt strong opioid-induced pleasure. Survival analysis revealed a correlation of prolonged DTFUD with the minor allele-carrying genotypes of DRD1 rs4532 (hazard ratios (HR) = 0.694; p = 0.001) and rs686 (HR = 0.681, p = 0.0003). Binary logistic regression indicated that rs10063995 GT genotype (vs. GG+TT, OR = 0.261) could predict decreased pleasure response to first-time use and the minor alleles of rs686 (OR = 0.535) and rs4532 (OR = 0.537) could predict decreased post-dependence pleasure. Moreover, rs686 minor allele was associated with a decreased risk for rapid transition from initial use to dependence (DTFUD≤30 days; OR = 0.603) or post-dependence euphoria (OR = 0.603) relative to major allele. In conclusion, DRD1 rs686 minor allele decreases the OD risk by prolonging the transition to dependence and attenuating opioid-induced pleasure in Chinese.

αCaMKII autophosphorylation controls the establishment of alcohol drinking behavior

The α-Ca(2+)/calmodulin-dependent protein kinase II (αCaMKII) is a crucial enzyme controlling plasticity in the brain. The autophosphorylation of αCaMKII works as a 'molecular memory' for a transient calcium activation, thereby accelerating learning. We investigated the role of αCaMKII autophosphorylation in the establishment of alcohol drinking as an addiction-related behavior in mice. We found that alcohol drinking was initially diminished in αCaMKII autophosphorylation-deficient αCaMKII(T286A) mice, but could be established at wild-type level after repeated withdrawals. The locomotor activating effects of a low-dose alcohol (2 g/kg) were absent in αCaMKII(T286A) mice, whereas the sedating effects of high-dose (3.5 g/kg) were preserved after acute and subchronic administration. The in vivo microdialysis revealed that αCaMKII(T286A) mice showed no dopamine (DA) response in the nucleus accumbens to acute or subchronic alcohol administration, but enhanced serotonin (5-HT) responses in the prefrontal cortex. The attenuated DA response in αCaMKII(T286A) mice was in line with altered c-Fos activation in the ventral tegmental area after acute and subchronic alcohol administration. In order to compare findings in mice with the human condition, we tested 23 single-nucleotide polymorphisms (SNPs) in the CAMK2A gene for their association with alcohol dependence in a population of 1333 male patients with severe alcohol dependence and 939 controls. We found seven significant associations between CAMK2A SNPs and alcohol dependence, one of which in an autophosphorylation-related area of the gene. Together, our data suggest αCaMKII autophosphorylation as a facilitating mechanism in the establishment of alcohol drinking behavior with changing the DA-5-HT balance as a putative mechanism.

Behavioral and structural responses to chronic cocaine require a feedforward loop involving ΔFosB and calcium/calmodulin-dependent protein kinase II in the nucleus accumbens shell

The transcription factor ΔFosB and the brain-enriched calcium/calmodulin-dependent protein kinase II (CaMKIIα) are induced in the nucleus accumbens (NAc) by chronic exposure to cocaine or other psychostimulant drugs of abuse, in which the two proteins mediate sensitized drug responses. Although ΔFosB and CaMKIIα both regulate AMPA glutamate receptor expression and function in NAc, dendritic spine formation on NAc medium spiny neurons (MSNs), and locomotor sensitization to cocaine, no direct link between these molecules has to date been explored. Here, we demonstrate that ΔFosB is phosphorylated by CaMKIIα at the protein-stabilizing Ser27 and that CaMKII is required for the cocaine-mediated accumulation of ΔFosB in rat NAc. Conversely, we show that ΔFosB is both necessary and sufficient for cocaine induction of CaMKIIα gene expression in vivo, an effect selective for D1-type MSNs in the NAc shell subregion. Furthermore, induction of dendritic spines on NAc MSNs and increased behavioral responsiveness to cocaine after NAc overexpression of ΔFosB are both CaMKII dependent. Importantly, we demonstrate for the first time induction of ΔFosB and CaMKII in the NAc of human cocaine addicts, suggesting possible targets for future therapeutic intervention. These data establish that ΔFosB and CaMKII engage in a cell-type- and brain-region-specific positive feedforward loop as a key mechanism for regulating the reward circuitry of the brain in response to chronic cocaine.

Amphetamine-induced sensitization has little effect on multiple learning paradigms and fails to rescue mice with a striatal learning defect

Behavioral sensitization to psychostimulants such as amphetamine (AMPH) is associated with synaptic modifications that are thought to underlie learning and memory. Because AMPH enhances extracellular dopamine in the striatum where dopamine and glutamate signaling are essential for learning, one might expect that the molecular and morphological changes that occur in the striatum in response to AMPH, including changes in synaptic plasticity, would affect learning. To ascertain whether AMPH sensitization affects learning, we tested wild-type mice and mice lacking NMDA receptor signaling in striatal medium spiny neurons in several different learning tests (motor learning, Pavlovian association, U-maze escape test with strategy shifting) with or without prior sensitization to AMPH. Prior sensitization had minimal effect on learning in any of these paradigms in wild-type mice and failed to restore learning in mutant mice, despite the fact that the mutant mice became sensitized by the AMPH treatment. We conclude that the changes in synaptic plasticity and many other signaling events that occur in response to AMPH sensitization are dissociable from those involved in learning the tasks used in our experiments.

Persistent reversal of enhanced amphetamine intake by transient CaMKII inhibition

Amphetamine exposure transiently increases Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) α expression in the nucleus accumbens (NAcc) shell and this persistently increases local GluA1 S831 phosphorylation and enhances behavioral responding to the drug. Here we assessed whether transiently interfering with CaMKII signaling using a dominant-negative CaMKIIα mutant delivered to the NAcc shell with herpes simplex viral vectors could reverse these long-lasting biochemical and behavioral effects observed following exposure to amphetamine. As expected, transient expression of CaMKIIα K42M in the NAcc shell produced a corresponding transient increase in CaMKIIα and decrease in pCaMKIIα (T286) protein levels in this site. Remarkably, this transient inhibition of CaMKII activity produced a long-lasting reversal of the increased GluA1 S831 phosphorylation levels in NAcc shell and persistently blocked the enhanced locomotor response to and self-administration of amphetamine normally observed in rats previously exposed to the drug. Together, these results indicate that even transient interference with CaMKII signaling may confer long-lasting benefits in drug-sensitized individuals and point to CaMKII and its downstream pathways as attractive therapeutic targets for the treatment of stimulant addiction.

Psychostimulant-induced neuroadaptations in nucleus accumbens AMPA receptor transmission

Medium spiny neurons of the nucleus accumbens serve as the interface between corticolimbic regions that elicit and modulate motivated behaviors, including those related to drugs of abuse, and motor regions responsible for their execution. Medium spiny neurons are excited primarily by AMPA-type glutamate receptors, making AMPA receptor transmission in the accumbens a key regulatory point for addictive behaviors. In animal models of cocaine addiction, changes in the strength of AMPA receptor transmission onto accumbens medium spiny neurons have been shown to underlie cocaine-induced behavioral adaptations related to cocaine seeking. Here we review changes in AMPA receptor levels and subunit composition that occur after discontinuing different types of cocaine exposure, as well as changes elicited by cocaine reexposure following abstinence or extinction. Signaling pathways that regulate these cocaine-induced adaptations will also be considered, as they represent potential targets for addiction pharmacotherapies.

Dynamic interaction between sigma-1 receptor and Kv1.2 shapes neuronal and behavioral responses to cocaine

The sigma-1 receptor (Sig-1R), an endoplasmic reticulum (ER) chaperone protein, is an interorganelle signaling modulator that potentially plays a role in drug-seeking behaviors. However, the brain site of action and underlying cellular mechanisms remain unidentified. We found that cocaine exposure triggers a Sig-1R-dependent upregulation of D-type K(+) current in the nucleus accumbens (NAc) that results in neuronal hypoactivity and thereby enhances behavioral cocaine response. Combining ex vivo and in vitro studies, we demonstrated that this neuroadaptation is caused by a persistent protein-protein association between Sig-1Rs and Kv1.2 channels, a phenomenon that is associated to a redistribution of both proteins from intracellular compartments to the plasma membrane. In conclusion, the dynamic Sig-1R-Kv1.2 complex represents a mechanism that shapes neuronal and behavioral response to cocaine. Functional consequences of Sig-1R binding to K(+) channels may have implications for other chronic diseases where maladaptive intrinsic plasticity and Sig-1Rs are engaged.

Genetic inhibition of CaMKII in dorsal striatal medium spiny neurons reduces functional excitatory synapses and enhances intrinsic excitability

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is abundant in striatal medium spiny neurons (MSNs). CaMKII is dynamically regulated by changes in dopamine signaling, as occurs in Parkinson's disease as well as addiction. Although CaMKII has been extensively studied in the hippocampus where it regulates excitatory synaptic transmission, relatively little is known about how it modulates neuronal function in the striatum. Therefore, we examined the impact of selectively overexpressing an EGFP-fused CaMKII inhibitory peptide (EAC3I) in striatal medium spiny neurons (MSNs) using a novel transgenic mouse model. EAC3I-expressing cells exhibited markedly decreased excitatory transmission, indicated by a decrease in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs). This decrease was not accompanied by changes in the probability of release, levels of glutamate at the synapse, or changes in dendritic spine density. CaMKII regulation of the AMPA receptor subunit GluA1 is a major means by which the kinase regulates neuronal function in the hippocampus. We found that the decrease in striatal excitatory transmission seen in the EAC3I mice is mimicked by deletion of GluA1. Further, while CaMKII inhibition decreased excitatory transmission onto MSNs, it increased their intrinsic excitability. These data suggest that CaMKII plays a critical role in setting the excitability rheostat of striatal MSNs by coordinating excitatory synaptic drive and the resulting depolarization response.

The region-specific activation of Ca2+/calmodulin dependent protein kinase II and extracellular signal-regulated kinases in hippocampus following chronic alcohol exposure

Previous studies suggest that hippocampal CA1, CA3, and DG regions may have distinct roles in alcohol dependence. Extracellular signal-regulated kinases (ERKs) and Ca(2+)/calmodulin dependent protein kinase II (CaMKII) have been shown to contribute to the molecular mechanism underlying drug dependence and relapse, and there may be an interaction between the activation of ERKs and CaMKII. However, little is known regarding the mechanisms underlying the effects of alcohol exposure, withdrawal, and relapse, particularly with regard to the interaction between CaMKII and ERK1/2 signaling in hippocampal subregions. In the present study, rats were provided water containing 6% alcohol as their only drinking source. We found that alcohol exerted locomotor stimulant and anxiolytic effects on rats in open field behaviors. Following chronic alcohol exposure, phospho-ERK1/2 was significantly decreased in the DG. Alcohol withdrawal was associated with an increase of phospho-ERK1/2 in the CA1 and DG, while alcohol re-exposure induced a decrease of phospho-ERK1/2 in the CA1, CA3, and DG. The activation of CaMKII (Thr286) correlated with the effects of alcohol on phospho-ERK1/2. Our results indicate that region-specific activation CaMKII-ERK1/2 signaling in the hippocampal CA1 and DG may play an important role in alcohol dependence.

Calcium-permeable AMPA receptors in the VTA and nucleus accumbens after cocaine exposure: when, how, and why?

In animal models of drug addiction, cocaine exposure has been shown to increase levels of calcium-permeable AMPA receptors (CP-AMPARs) in two brain regions that are critical for motivation and reward-the ventral tegmental area (VTA) and the nucleus accumbens (NAc). This review compares CP-AMPAR plasticity in the two brain regions and addresses its functional significance. In VTA dopamine neurons, cocaine exposure results in synaptic insertion of high conductance CP-AMPARs in exchange for lower conductance calcium-impermeable AMPARs (CI-AMPARs). This plasticity is rapid in onset (hours), GluA2-dependent, and can be observed with a single cocaine injection. Whereas it is short-lived after experimenter-administered cocaine, it persists for months after cocaine self-administration. In addition to strengthening synapses and altering Ca(2+) signaling, CP-AMPAR insertion alters subsequent induction of plasticity at VTA synapses. However, CP-AMPAR insertion is unlikely to mediate the increased DA cell activity that occurs during early withdrawal from cocaine exposure. Metabotropic glutamate receptor 1 (mGluR1) exerts a negative influence on CP-AMPAR accumulation in the VTA. Acutely, mGluR1 stimulation elicits a form of LTD resulting from CP-AMPAR removal and CI-AMPAR insertion. In medium spiny neurons (MSNs) of the NAc, extended access cocaine self-administration is required to increase CP-AMPAR levels. This is first detected after approximately a month of withdrawal and then persists. Once present in NAc synapses, CP-AMPARs mediate the expression of incubation of cue-induced cocaine craving. The mechanism of their accumulation may be GluA1-dependent, which differs from that observed in the VTA. However, similar to VTA, mGluR1 stimulation removes CP-AMPARs from MSN synapses. Loss of mGluR1 tone during cocaine withdrawal may contribute to CP-AMPAR accumulation in the NAc. Thus, results in both brain regions point to the possibility of using positive modulators of mGluR1 as treatments for cocaine addiction.