NR2B-containing NMDA receptor is required for morphine-but not stress-induced reinstatement

A locus coeruleus to dorsal hippocampus pathway mediates cue-induced reinstatement of opioid self-administration in male and female rats

Opioid use disorder is a chronic relapsing disorder encompassing misuse, dependence, and addiction to opioid drugs. Long term maintenance of associations between the reinforcing effects of the drug and the cues associated with its intake are a leading cause of relapse. Indeed, exposure to the salient drug-associated cues can lead to drug cravings and drug seeking behavior. The dorsal hippocampus (dHPC) and locus coeruleus (LC) have emerged as important structures for linking the subjective rewarding effects of opioids with environmental cues. However, their role in cue-induced reinstatement of opioid use remains to be further elucidated. In this study, we showed that chemogenetic inhibition of excitatory dHPC neurons during re-exposure to drug-associated cues significantly attenuates cue-induced reinstatement of morphine-seeking behavior. In addition, the same manipulation reduced reinstatement of sucrose-seeking behavior but failed to alter memory recall in the object location task. Finally, intact activity of tyrosine hydroxylase (TH) LC-dHPCTh afferents is necessary to drive cue induced reinstatement of morphine-seeking as inhibition of this pathway blunts cue-induced drug-seeking behavior. Altogether, these studies show an important role of the dHPC and LC-dHPCTh pathway in mediating cue-induced reinstatement of opioid seeking.

A novel dopamine D2 receptor-NR2B protein complex might contribute to morphine use disorders

Dopamine receptors can form heteromeric interactions with other receptors, including glutamate receptors, and present a novel pharmacological target because it contribute to dopamine-dysregulated brain disorders such as addiction and other motor-related diseases. In addition, dopamine receptors D2 (D2Rs) and glutamate NMDA receptors subtype-NR2B have been implicated in morphine use disorders; however, the molecular mechanism underlying the heteromeric complex of these two receptors in morphine use disorders is unclear. Herein, we focus on interactions between D2R and NR2B in morphine-induced conditioned place preference (CPP) and hyperlocomotion mice models. We found that the D2R-NR2B complex significantly increases in morphine-induced mice models, accompanied by ERK signaling impairment, implying the complex could contribute to the morphine addiction pathophysiological process. Further, we design a brain-penetrant interfering peptide (TAT-D2-KT), which could disrupt interactions of D2R-NR2B and decrease addictive-like behaviors concurrent to ERK signaling improvement. In summary, our data provided the first evidence for a D2R-NMDAR complex formation in morphine use disorders and its underlying mechanism of ERK signaling, which could present a novel therapeutic target with direct implications for morphine acquisition and relapse treatment.

The Pleiotropic Face of CREB Family Transcription Factors

cAMP responsive element-binding protein (CREB) is one of the most intensively studied phosphorylation-dependent transcription factors that provide evolutionarily conserved mechanisms of differential gene expression in vertebrates and invertebrates. Many cellular protein kinases that function downstream of distinct cell surface receptors are responsible for the activation of CREB. Upon functional dimerization of the activated CREB to cis-acting cAMP responsive elements within the promoters of target genes, it facilitates signal-dependent gene expression. From the discovery of CREB, which is ubiquitously expressed, it has been proven to be involved in a variety of cellular processes that include cell proliferation, adaptation, survival, differentiation, and physiology, through the control of target gene expression. In this review, we highlight the essential roles of CREB proteins in the nervous system, the immune system, cancer development, hepatic physiology, and cardiovascular function and further discuss a wide range of CREB-associated diseases and molecular mechanisms underlying the pathogenesis of these diseases.

Evaluation of the effect of mesenchymal stem cells injection in the nucleus accumbens on the morphine reinstatement behavior in a conditioned place preference model in Wistar rat: Expression changes of NMDA receptor subunits and NT-3

Mesenchymal stem cells (MSCs) have been recently shown to improve functional recovery in animal models of CNS disorders and are currently being examined in clinical studies for sclerosis, stroke, and CNS lesions. The activation of endogenous CNS protection and repair mechanisms is unclear. MSC-based approaches are considered a new potential target for neurodegenerative disorders. This study was designed to discover the effect of MSCs injection in the nucleus accumbens (NAc) on the reinstatement of behavior in morphine-induced conditioned place preference (CPP) in male rats. The CPP was induced via intra-peritoneal (i.p.) morphine injection (5 mg/kg) for three consecutive days. After being tested for CPP induction, animals received MSCs or culture medium (DMEM F-12) in their NAc using stereotaxic surgery. Following extinction, a priming dose of morphine (2 mg/kg) was administered to induce reinstatement. Expression of GluN1, GluN2A, and GluN2B subunits of the NMDA receptor and the NT-3 gene in the NAc was assessed on the last day of extinction and following CPP reinstatement. The results showed that local injection of MSCs attenuated reinstatement after receiving a priming dose of morphine, and also shortened the period of CPP extinction. The mRNA expression of the NT-3 gene in the group receiving MSCs was increased compared to control animals, as was observed for GluN1 and GluN2B, but not GluN2A. It is concluded that intra-NAc injection of MSCs may facilitate morphine extinction and alleviate reinstatement behavior which may be via expression changes in NMDA receptor subunits and NT-3 gene.

Enhancing GluN2A-type NMDA receptors impairs long-term synaptic plasticity and learning and memory

N-methyl-D-aspartic acid type glutamate receptors (NMDARs) play critical roles in synaptic transmission and plasticity, the dysregulation of which leads to cognitive defects. Here, we identified a rare variant in the NMDAR subunit GluN2A (K879R) in a patient with intellectual disability. The K879R mutation enhanced receptor expression on the cell surface by disrupting a KKK motif that we demonstrated to be an endoplasmic reticulum retention signal. Expression of GluN2A_K879R in mouse hippocampal CA1 neurons enhanced the excitatory postsynaptic currents mediated by GluN2A-NMDAR but suppressed those mediated by GluN2B-NMDAR and the AMPA receptor. GluN2A_K879R knock-in mice showed similar defects in synaptic transmission and exhibited impaired learning and memory. Furthermore, both LTP and LTD were severely impaired in the KI mice, likely explaining their learning and memory defects. Therefore, our study reveals a new mechanism by which elevated synaptic GluN2A-NMDAR impairs long-term synaptic plasticity as well as learning and memory.

Inhibition of reinstatement of alcohol-induced conditioned place preference in mice by Lonicera japonica polysaccharide

Alcohol is one of the most commonly used addictive substances. Addiction memory is a necessary part of the mechanism underlying drug addiction. Lonicera japonica and its extract can mitigate organ damage caused by alcohol. In this study, Lonicera japonica polysaccharide (LJP) was extracted, and its effect on alcohol addiction memory was investigated. LJP inhibited the cue-induced reinstatement of conditioned place preference and elevated Tuj1 and DCX levels in the hippocampus to suppress neuronal damage. LJP also reduced the hippocampal glutamate (Glu) levels, alcohol-induced abnormal enhancement of the addiction memory, and decreased VPS34 phosphorylation and hyper activation of autophagy pathways in the hippocampus of alcohol-dependent mice. Our results suggest that LJP is a potential functional food to treat or ameliorate alcohol-induced addiction and that VPS34 is a potential target for modulating alcohol cravings.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Prolonged Withdrawal From Escalated Oxycodone Is Associated With Increased Expression of Glutamate Receptors in the Rat Hippocampus

People suffering from opioid use disorder (OUD) exhibit cognitive dysfunctions. Here, we investigated potential changes in the expression of glutamate receptors in rat hippocampi at 2 h and 31 days after the last session of oxycodone self-administration (SA). RNA extracted from the hippocampus was used in quantitative polymerase chain reaction analyses. Rats, given long-access (9 h per day) to oxycodone (LgA), took significantly more drug than rats exposed to short-access (3 h per day) (ShA). In addition, LgA rats could be further divided into higher oxycodone taking (LgA-H) or lower oxycodone taking (LgA-L) groups, based on a cut-off of 50 infusions per day. LgA rats, but not ShA, rats exhibited incubation of oxycodone craving. In addition, LgA rats showed increased mRNA expression of GluA1-3 and GluN2a-c subunits as well as Grm3, Grm5, Grm6, and Grm8 subtypes of glutamate receptors after 31 days but not after 2 h of stopping the SA experiment. Changes in GluA1-3, Grm6, and Grm8 mRNA levels also correlated with increased lever pressing (incubation) after long periods of withdrawal from oxycodone. More studies are needed to elucidate the molecular mechanisms involved in altering the expression of these receptors during withdrawal from oxycodone and/or incubation of drug seeking.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Direct administration of ifenprodil and citalopram into the nucleus accumbens inhibits cue-induced nicotine seeking and associated glutamatergic plasticity

Nicotine use disorder has been associated with glutamatergic alterations within the basal ganglia that might contribute to relapse. Specifically, initiation of cue-induced nicotine seeking produces rapid, transient synaptic potentiation (t-SP) in nucleus accumbens core (NAcore) medium spiny neurons (MSNs), defined as increases in spine head diameter and AMPA to NMDA current ratios (A/N). Ifenprodil, which inhibits nicotine reinstatement when administered systemically, antagonizes GluN2B-containing NMDA receptors, has affinity for serotonin receptors, and blocks serotonin transporters (SERT). The mechanisms underlying its therapeutic efficacy, however, remain unknown. Using pharmacological and genetic approaches, the current study examined the role of NAcore GluN2B receptors as well as SERT in mediating cue-induced nicotine seeking and associated MSN structure and physiology. Prior to reinstatement, rats received intra-NAcore injections of either ifenprodil, citalopram or artificial cerebral spinal fluid (15 min prior), or GluN2B or control siRNAs (3 consecutive days prior). Rats were sacrificed after a 15-min cue-induced reinstatement session for dendritic spine analysis, western blotting or whole-cell electrophysiology. Intra-NAcore ifenprodil blocked nicotine-seeking behavior and promoted a higher frequency of shorter spines on MSN dendrites. However, a decrease in membrane-bound GluN2B receptor expression did not prevent cue-induced nicotine seeking or associated MSN cell physiology. Interestingly, intra-NAcore citalopram, an SSRI, prevented cue-induced nicotine seeking. Together, these results indicate that the therapeutic effects of ifenprodil on cue-induced nicotine seeking may, in part, be due to its actions at SERT rather than GluN2B, which may be specific to nicotine-seeking as opposed to other drugs of abuse.

Spermidine, a positive modulator of the NMDA receptor, facilitates extinction and prevents the reinstatement of morphine-induced conditioned place preference in mice

RATIONALE

Individuals with opioid use disorders often relapse into drug-seeking behavior after recalling memories linked to the drug use experience. Improving extinction efficacy has been used as a strategy to treat substance use disorders and suppress relapse. Although N-methyl-D-aspartate receptor (NMDAr) agonists facilitate acquisition, consolidation, and extinction, no study has addressed whether spermidine (SPD), a natural polyamine ligand of the NMDA receptor, facilitates the extinction and reinstatement of morphine-induced conditioned place preference (CPP).

OBJECTIVES AND METHODS

The aim of the present study was to investigate the effect of SPD, an NMDAr agonist, on the extinction and reinstatement of morphine-induced CPP in mice. Adult male albino Swiss mice received saline (0.9% NaCl) or morphine (5 mg/kg) intraperitoneally (i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. SPD (10-30 mg/kg, i.p.) or ifenprodil (NMDAr antagonist, 0.1-1 mg/kg, i.p.) were injected 15 min before extinction training.

RESULTS

SPD and ifenprodil facilitated the extinction of morphine-induced CPP. SPD treatment during the extinction period impaired reinstatement induced by a priming dose of morphine (1.25 mg/kg). Ifenprodil (0.1 mg/kg) prevented the facilitatory effect of spermidine on the extinction of morphine-induced CPP but did not prevent reinstatement induced by morphine.

CONCLUSIONS

These results suggest that SPD facilitated the extinction of morphine-induced CPP by modulating the polyamine binding site of the NMDA receptor. Our findings reveal important effects of SPD and ifenprodil on the re-exposure-induced decrease in morphine-induced CPP, which may be promising for developing novel pharmacological strategies to treat opioid use disorder.

Geranylgeranylacetone blocks the reinstatement of morphine-conditioned place preference

Morphine is widely used for clinical pain management and induces the dependence. Addiction to morphine is a major public health issue. Geranylgeranylacetone (GGA) is widely used in clinic for treating ulcer. GGA induces expression of thioredoxin-1 (Trx-1) extensively. Trx-1 is a redox regulating protein and plays protecting roles in nervous system. GGA prevents mice against morphine-induced hyperlocomotion, rewarding effect, and withdrawal syndrome. However, whether GGA blocks morphine-conditioned place preference (CPP) reinstatement is still unknown. In the present study, we found that GGA administration blocked the reinstatement of morphine-CPP. The expressions of Trx-1, N-methyl d-aspartate receptor 2B subunit (NR2B), phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (p-CaMKII), phosphorylated extracellular signaling regulated kinases (p-ERK), and phosphorylated cAMP-response element binding protein (p-CREB) were induced in nucleus accumbens (NAc) and hippocampus by morphine or GGA, whereas these proteins were not changed by morphine in GGA-treated mice. Our results indicate that GGA may prevent the reinstatement of morphine-CPP through strengthening the expression of Trx-1 and regulating NR2B/ERK pathway. Thus, we suggest that GGA may be a promising therapeutic candidate for morphine-induced relapse.

Phosphorylated SNAP25 in the CA1 regulates morphine-associated contextual memory retrieval via increasing GluN2B-NMDAR surface localization

Although our previous studies have demonstrated both protein kinase C (PKC) and GluN2B-containing N-methyl-d-aspartate receptor (GluN2B-NMDAR) play crucial roles in morphine-associated learning and memory, the relationship between them remains unexplored. In this study, we validated the enhanced PKC and membrane GluN2B protein expression in the hippocampal CA1 after morphine conditioned place preference (CPP) expression in rats. Interestingly, we also found that phosphorylation of SNAP25 at Ser187 (pSer187-SNAP25), a PKC-activated target, was significantly increased following morphine CPP expression. Blocking the pSer187-SNAP25 by intra-CA1 injection of an interfering peptide impaired morphine CPP expression and accompanied by the reduced ratio of GluN2B membrane/total in the CA1. In addition, intra-CA1 blockade of pSer187-SNAP25 did not affect natural learning and memory process as evidenced by intact sucrose-induced CPP expression and normal locomotor activity in rats. Therefore, our results reveal that enhanced pSer187-SNAP25 by PKC recruits GluN2B-NMDAR to the membrane surface in the hippocampal CA1 and mediates context-induced addiction memory retrieval. Our findings in this study fill in the missing link and provide better understanding of the molecular mechanisms involved in morphine-associated contextual memory retrieval.

What does the Fos say? Using Fos-based approaches to understand the contribution of stress to substance use disorders

Despite extensive research efforts, drug addiction persists as a largely unmet medical need. Perhaps the biggest challenge for treating addiction is the high rate of recidivism. While many factors can promote relapse in abstinent drug users, the contribution of stress is particularly problematic, as stress is uncontrollable and pervasive in the lives of those struggling with addiction. Thus, understanding the neurocircuitry that underlies the influence of stress on drug seeking is critical for guiding treatment. Preclinical research aimed at defining this neurocircuitry has, in part, relied upon the use of experimental approaches that allow visualization of cellular and circuit activity that corresponds to stressor-induced drug seeking in rodent relapse models. Much of what we have learned about the mechanisms that mediate stressor-induced relapse has been informed by studies that have used the expression of the immediate early gene, cfos, or its protein product, Fos, as post-mortem activity markers. In this review we provide an overview of the rodent models used to study stressor-induced relapse and briefly summarize what is known about the underlying neurocircuitry before describing the use of cfos/Fos-based approaches. In addition to reviewing findings obtained using this approach, its advantages and limitations are considered. Moreover, new techniques that leverage the expression profile of cfos to tag and manipulate cells based on their activity patterns are discussed. The intent of the review is to guide the interpretation of old and design of new studies that utilize cfos/Fos-based strategies to study the neurocircuitry that contributes to stress-related drug use.

Re-exposure to morphine-associated context facilitated long-term potentiation in the vSUB-NAc glutamatergic pathway via GluN2B-containing receptor activation

The glutamatergic projection from the ventral subiculum of the hippocampus (vSUB) to the nucleus accumbens (NAc) shell has been reported to play a key role in drug-related behavior. The GluN2B subunit of N-methyl-D-aspartate receptors (NMDARs) in the NAc can be selectively elevated after the retrieval of drug-conditioned memory. However, whether the increased GluN2B-containing NMDARs (GluN2B-NMDARs) are able to alter the synaptic plasticity of the vSUB-NAc glutamatergic pathway remains unclear. Here, we found that the long-term potentiation (LTP) in the vSUB-NAc pathway was facilitated and the GluN2B subunit protein level was elevated in synaptoneurosomes of the NAc shell, but not in the core, following morphine-induced conditioned place preference (CPP) expression in rats. The facilitated LTP was prevented by the GluN2B-NMDAR antagonist RO25-6981. Also, a neurochemical disconnection following microinjection of RO25-6981 into the NAc shell, plus microinfusion of GABA agonist baclofen and muscimol into the contralateral vSUB prevented the expression of morphine-induced CPP. These findings suggest that the retrieval of drug-associated memory potentiated synaptic plasticity in the vSUB-NAc pathway, which was dependent on GluN2B-NMDAR activation in the NAc shell. These findings provide a new explanation for the mechanisms that underlie the morphine-associated-context memory. The GluN2B-NMDARs may be regarded as a potential target for erasing morphine-related memory.

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.

NMDA antagonist MK 801 in nucleus accumbens core but not shell disrupts the restraint stress-induced reinstatement of extinguished cocaine-conditioned place preference in rats

Relapse is a common feature of cocaine addiction. In rodents, it can be elicited by cues, stress or the drug. Restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP) is a useful model to study the mechanisms involved in stress-induced relapse of drug-seeking behavior. There is evidence that the glutamate NMDA receptors are critically involved in drug- and cue-induced reinstatement of seeking behavior and drug-CPP responses. The aim of this study was to investigate the contribution of NMDA receptors within core vs. shell nucleus accumbens (NAc) subregions to restraint stress-induced reinstatement of extinguished cocaine-CPP. After extinction of cocaine-conditioned preference, animals were administered MK 801 systemically or directly into intra-core or intra-shell, and restrained for 30min or left undisturbed in their home-cages. First, we demonstrated that restraint stress-induced reinstatement of extinguished cocaine-CPP depends on the duration of restraint as well as on the context in which it is applied. Second, this effect was blocked by systemic MK 801 administration either before or after restraint. Third, intra-core but not intra-shell administration abrogated the restraint stress-induced reinstatement. These findings show that NMDA receptors within NAc core, but not shell, play a critical role in restraint stress-induced reinstatement of cocaine-CPP.

A Nonrewarding NMDA Receptor Antagonist Impairs the Acquisition, Consolidation, and Expression of Morphine Conditioned Place Preference in Mice

N-methyl-D-aspartate (NMDA) receptor antagonists block morphine-induced conditioned place preference (CPP). Although polyamines are endogenous modulators of the NMDA receptor, it is not known whether polyaminergic agents induce CPP or modulate morphine-induced CPP. Here, we examined whether polyamine ligands modify morphine CPP acquisition, consolidation, and expression. Adult male albino Swiss mice received saline (0.9 % NaCl, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. The effect of arcaine (3 mg/kg, i.p.) or spermidine (30 mg/kg, i.p.), respectively, an antagonist and an agonist of the polyamine-binding site at the NMDA receptor, on the acquisition, consolidation, and expression of morphine CPP was studied. In those experiments designed to investigate whether spermidine prevented or reversed the effect of arcaine, spermidine (30 mg/kg, i.p.) was administered 15 min before or 15 min after arcaine, respectively. Arcaine and spermidine did not induce CPP or aversion per se. Arcaine (3 mg/kg, i.p.) impaired the acquisition, consolidation, and expression of morphine CPP. Spermidine prevented the impairing effect of arcaine on the acquisition of morphine CPP but not the impairing effect of arcaine on consolidation or expression of morphine CPP. These results suggest that arcaine may impair morphine CPP acquisition by modulating the polyamine-binding site at the NMDA receptor. However, the arcaine-induced impairment of consolidation and expression of morphine CPP seems to involve other mechanisms.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Synaptic Cytoskeletal Plasticity in the Prefrontal Cortex Following Psychostimulant Exposure

Addiction is characterized by maladaptive decision-making, a loss of control over drug consumption and habit-like drug seeking despite adverse consequences. These cognitive changes may reflect the effects of drugs of abuse on prefrontal cortical neurobiology. Here, we review evidence that amphetamine and cocaine fundamentally remodel the structure of excitatory neurons in the prefrontal cortex. We summarize evidence in particular that these psychostimulants have opposing effects in the medial and orbital prefrontal cortices ('mPFC' and 'oPFC', respectively). For example, amphetamine and cocaine increase dendrite length and spine density in the mPFC, while dendrites are impoverished and dendritic spines are eliminated in the oPFC. We will discuss evidence that certain cytoskeletal regulatory proteins expressed in the oPFC and implicated in postnatal (adolescent) neural development also regulate behavioral sensitivity to cocaine. These findings potentially open a window of opportunity for the identification of novel pharmacotherapeutic targets in the treatment of drug abuse disorders in adults, as well as in drug-vulnerable adolescent populations. Finally, we will discuss the behavioral implications of drug-related dendritic spine elimination in the oPFC, with regard to reversal learning tasks and tasks that assess the development of reward-seeking habits, both used to model aspects of addiction in rodents.

[³H]Ifenprodil binding in post-mortem brains of Cloninger type 1 and 2 alcoholics: a whole-hemisphere autoradiography study

The glutamate N-methyl-d-aspartate (NMDA) receptor NR2B subunits are sensitive to ethanol and are found in brain areas related to ethanol addiction, dependence, development of alcohol tolerance, and alcohol withdrawal syndrome. Previous studies indicate that early-onset Cloninger type 2 alcoholics have an intact, responsive, dopaminergic system in the nucleus accumbens (NAC), whereas type 1 alcoholics have dopaminergic defects. NR2B-containing NMDA receptors in the NAC are involved in both non-opioid and opioid receptor-mediated reward. Our aim was to evaluate the putative [(3)H]ifenprodil binding alterations of NR2B receptors in limbic, hippocampal, and cortical brain areas of type 1 alcoholics (n=8), type 2 alcoholics (n=8), and control subjects (n=10) by postmortem whole hemisphere autoradiography. We found significantly different binding levels among these three subject groups, and the main difference was localized in the decreased binding in type 2 alcoholics and controls in the nucleus accumbens. Although preliminary and from relatively small diagnostic groups, these results suggest pathological alterations in the NR2B-mediated reward system of type 2 alcoholics.

Blocking TRPV1 in nucleus accumbens inhibits persistent morphine conditioned place preference expression in rats

The function of TRPV1 (transient receptor potential vanilloid subfamily, member 1) in the central nervous system is gradually elucidated. It has been recently proved to be expressed in nucleus accumbens (NAc), a region playing an essential role in mediating opioid craving and taking behaviors. Based on the general role of TRPV1 antagonist in blocking neural over-excitability by both pre- and post-synaptic mechanisms, TRPV1 antagonist capsazepine (CPZ) was tested for its ability to prohibit persistent opioid craving in rats. In the present study, we assessed the expression of TRPV1 in nucleus accumbens and investigated the effect of CPZ in bilateral nucleus accumbens on persistent morphine conditioned place preference (mCPP) in rats. We also evaluated the side-effect of CPZ on activity by comparing cross-beam times between groups. We found that morphine conditioned place preference increased the TRPV1 expression and CPZ attenuated morphine conditioned place preference in a dose-dependent and target-specific manner after both short- and long-term spontaneous withdrawal, reflected by the reduction of the increased time in morphine-paired side. CPZ (10 nM) could induce prolonged and stable inhibition of morphine conditioned place preference expression. More importantly, CPZ did not cause dysfunction of activity in the subjects tested, which indicates the inhibitory effect was not obtained at the sacrifice of regular movement. Collectively, these results indicated that injection of TRPV1 antagonist in nucleus accumbens is capable of attenuating persistent morphine conditioned place preference without affecting normal activity. Thus, TRPV1 antagonist is one of the promising therapeutic drugs for the treatment of opioid addiction.

Synaptic glutamate spillover due to impaired glutamate uptake mediates heroin relapse

Reducing the enduring vulnerability to relapse is a therapeutic goal in treating drug addiction. Studies with animal models of drug addiction show a marked increase in extrasynaptic glutamate in the core subcompartment of the nucleus accumbens (NAcore) during reinstated drug seeking. However, the synaptic mechanisms linking drug-induced changes in extrasynaptic glutamate to relapse are poorly understood. Here, we discovered impaired glutamate elimination in rats extinguished from heroin self-administration that leads to spillover of synaptically released glutamate into the nonsynaptic extracellular space in NAcore and investigated whether restoration of glutamate transport prevented reinstated heroin seeking. Through multiple functional assays of glutamate uptake and analyzing NMDA receptor-mediated currents, we show that heroin self-administration produced long-lasting downregulation of glutamate uptake and surface expression of the transporter GLT-1. This downregulation was associated with spillover of synaptic glutamate to extrasynaptic NMDA receptors within the NAcore. Ceftriaxone restored glutamate uptake and prevented synaptic glutamate spillover and cue-induced heroin seeking. Ceftriaxone-induced inhibition of reinstated heroin seeking was blocked by morpholino-antisense targeting GLT-1 synthesis. These data reveal that the synaptic glutamate spillover in the NAcore results from reduced glutamate transport and is a critical pathophysiological mechanism underling reinstated drug seeking in rats extinguished from heroin self-administration.

Hippocampal long-term potentiation is disrupted during expression and extinction but is restored after reinstatement of morphine place preference

Learned associations between environmental cues and morphine use play an important role in the maintenance and/or relapse of opioid addiction. Although previous studies suggest that context-dependent morphine treatment alters glutamatergic transmission and synaptic plasticity in the hippocampus, their role in morphine conditioned place preference (CPP) and reinstatement remains unknown. We investigated changes in synaptic plasticity and NMDAR expression in the hippocampus after the expression, extinction, and reinstatement of morphine CPP. Here we report that morphine CPP is associated with increased basal synaptic transmission, impaired hippocampal long-term potentiation (LTP), and increased synaptic expression of the NR1 and NR2b NMDAR subunits. Changes in synaptic plasticity, synaptic NR1 and NR2b expression, and morphine CPP were absent when morphine was not paired with a specific context. Furthermore, hippocampal LTP was impaired and synaptic NR2b expression was increased after extinction of morphine CPP, indicating that these alterations in plasticity may be involved in the mechanisms underlying the learning of drug-environment associations. After extinction of morphine CPP, a priming dose of morphine was sufficient to reinstate morphine CPP and was associated with LTP that was indistinguishable from saline control groups. In contrast, morphine CPP extinguished mice that received a saline priming dose did not show CPP and had disrupted hippocampal LTP. Finally, we found that reinstatement of morphine CPP was prevented by the selective blockade of the NR2b subunit in the hippocampus. Together, these data suggest that alterations in synaptic plasticity and glutamatergic transmission play an important role in the reinstatement of morphine CPP.

Activation of mGluR2/3 receptors in the ventro-rostral prefrontal cortex reverses sensorimotor gating deficits induced by systemic NMDA receptor antagonists

Prepulse inhibition (PPI) of acoustic startle is an operational measure of sensorimotor gating, which is disrupted in schizophrenia. NMDA receptor (NMDAR) antagonist induced PPI disruption has become an important pharmacological model for schizophrenia; however, knowledge of the underlying mechanism remains incomplete. This study examines the role of NMDAR in the caudal pontine reticular nucleus (PnC) and the medial prefrontal cortex (mPFC) in NMDARs antagonist induced PPI deficits, as well as the NMDA receptor subtypes involved. We administered the NMDA antagonist MK-801 locally into the caudal pontine reticular formation (PnC), where the PPI mediating pathway converges with the primary startle pathway, and into the mPFC prior to behavioural testing. PnC microinjections had no effect on startle and PPI, whereas injections into the ventro-rostral part, but not into the dorso-caudal part of the mPFC, disrupted PPI. These effects could be mimicked by local injection of the NR2B subunit specific antagonist ifenprodil, whereas co-application of MK-801 and the mGluR2/3 agonist LY354740 had no effect on PPI. Moreover, PPI disruptions by systemically administered MK-801 could be reversed by local injections of LY354740 into the ventro-rostral mPFC, but not into the dorso-caudal mPFC. Our results indicate that NR2B subunit containing NMDARs in a specific subregion of the mPFC play a major role in PPI disruptions by systemic NMDAR antagonism. Our results further support the hypothesis that glutamate hyper-function in the mPFC is a main mechanism involved in sensory gating deficits induced by systemic MK-801, supporting the notion that this is an important mechanism in schizophrenia pathology.

Region-specific expression of brain-derived neurotrophic factor splice variants in morphine conditioned place preference in mice

It is well established that brain-derived neurotrophic factor (BDNF) plays a pivotal role in brain plasticity-related processes, such as learning, memory and drug addiction. However, changes in expression of BDNF splice variants after acquisition, extinction and reinstatement of cue-elicited morphine seeking behavior have not yet been investigated. Real-time PCR was used to assess BDNF splice variants (I, II, IV and VI) in various brain regions during acquisition, extinction and reinstatement of morphine-conditioned place preference (CPP) in mice. Repeated morphine injections (10mg/kg, i.p.) increased expression of BDNF splice variants II, IV and VI in the hippocampus, caudate putamen (CPu) and nucleus accumbens (NAcc). Levels of BDNF splice variants decreased after extinction training and continued to decrease during reinstatement induced by a morphine priming injection (10mg/kg, i.p.). However, after reinstatement induced by exposure to 6 min of forced swimming (FS), expression of BDNF splice variants II, IV and VI was increased in the hippocampus, CPu, NAcc and prefrontal cortex (PFC). After reinstatement induced by 40 min of restraint, expression of BDNF splice variants was increased in PFC. These results show that exposure to either morphine or acute stress can induce reinstatement of drug-seeking, but expression of BDNF splice variants is differentially affected by chronic morphine and acute stress. Furthermore, BDNF splice variants II, IV and VI may play a role in learning and memory for morphine addiction in the hippocampus, CPu and NAcc.

Combined action of MK-801 and ceftriaxone impairs the acquisition and reinstatement of morphine-induced conditioned place preference, and delays morphine extinction in rats

OBJECTIVE

It is well established that glutamate and its receptors, particularly the N-methyl-D-aspartate receptor (NMDAR), play a significant role in addiction and that the inhibition of glutamatergic hyperfunction reduces addictive behaviors in experimental animals. Specifically, NMDAR antagonists such as MK-801, and an inducer of the expression of glutamate transporter subtype-1 (GLT-1) (ceftriaxone) are known to inhibit addictive behavior. The purpose of this study was to determine whether the combined action of a low dose of MK-801 and a low dose of ceftriaxone provides better inhibition of the acquisition, extinction, and reinstatement of morphine-induced conditioned place preference (CPP) than either compound alone.

METHODS

A morphine-paired CPP experiment was used to study the effects of low doses of MK-801, ceftriaxone and a combination of both on reward-related memory (acquisition, extinction, and reinstatement of morphine preference) in rats.

RESULTS

A low dose of neither MK-801 (0.05 mg/kg, i.p.) nor ceftriaxone (25 mg/kg, i.p.) alone effectively impaired CPP behaviors. However, when applied in combination, they reduced the acquisition of morphine-induced CPP and completely prevented morphine reinstatement. Their combination also notably impaired the extinction of morphine-induced CPP.

CONCLUSION

The combined action of a low dose of an NMDAR antagonist (MK-801) and GLT-1 activation by ceftriaxone effectively changed different phases of CPP behavior.

Glutamate mechanisms underlying opiate memories

As the major excitatory neurotransmitter in the brain, glutamate plays an undisputable integral role in opiate addiction. This relates, in part, to the fact that addiction is a disorder of learning and memory, and glutamate is required for most types of memory formation. As opiate addiction develops, the addict becomes conditioned to engage in addictive behaviors, and these behaviors can be triggered by opiate-associated cues during abstinence, resulting in relapse. Some medications for opiate addiction exert their therapeutic effects at glutamate receptors, especially the NMDA receptor. Understanding the neural circuits controlling opiate addiction, and the locus of glutamate's actions within these circuits, will help guide the development of targeted pharmacotherapeutics for relapse.

Essential role of NR2B-containing NMDA receptor-ERK pathway in nucleus accumbens shell in morphine-associated contextual memory

Learned associations between the rewarding effect of addictive drugs and drug-paired contexts resist extinction and contribute to the high rate of relapse observed in drug addicts. Although it has been shown that extracellular signal-regulated kinase 1/2 (ERK1/2) activity in the nucleus accumbens (NAc) is modulated by the primary rewarding effect of opiates, little is known as to its role in the morphine-associated contextual memory. In the present study, we investigated the ERK1/2 activity indicated by phosphorylated ERK1/2 (pERK1/2) levels in rats using a morphine-induced conditioned place preference (CPP) procedure. Our results showed that, in rats that had undergone morphine conditioning, after testing (expression phase) pERK1/2 in the NAc shell but not the NAc core or the adjacent caudate putamen was specifically increased. pERK1/2 levels in several other parts of the brain involved in drug-seeking, such as the medial prefrontal cortex, dorsal hippocampus, and basolateral amygdala, showed no significant changes. A significant positive correlation was observed between the elevated pERK1/2 level in the NAc shell and the degree of conditioned preference for morphine-associated contexts. Bilateral injection of an inhibitor of ERK activation into the NAc shell attenuated ERK1/2 phosphorylation and prevented the expression of morphine CPP, but injections into the core did not. Selective inhibition of NR2B-containing NMDA receptor in the NAc shell by ifenprodil prevented CPP expression and down-regulated local ERK1/2 phosphorylation. These findings collectively suggest that recall of morphine-associated contextual memory depends specifically upon ERK1/2 activation in the NAc shell and that ERK1/2 phosphorylation is regulated by the upstream NR2B-containing NMDA receptor.

Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Gentiopicroside attenuates morphine rewarding effect through downregulation of GluN2B receptors in nucleus accumbens

AIMS

Gentiopicroside (Gent) is one of the secoiridoid compound isolated from Gentiana lutea. This compound exhibits analgesic activities and inhibits the expression of GluN2B-containing N-methyl-D-aspartate (NMDA) receptors in the anterior cingulate cortex in mice. Nucleus accumbens (NAc) is a forebrain structure known for its role in drug addiction. However, little is known about the role of Gent on morphine dependence and synaptic transmission changes in the NAc.

METHODS

Conditioned place preference (CPP) test and behavioral sensitization of locomotor activity were used to investigate drug-seeking related behaviors. Brain slices containing NAc were prepared, and whole-cell patch-clamp recordings were performed to record the excitatory postsynaptic currents (EPSCs). Expression of proteins was detected by Western blot analysis.

RESULTS

Systemic administration of Gent attenuated the CPP effect induced by morphine, but had no effect on morphine-induced behavioral sensitization. Gent significantly reversed overexpression of GluN2B-containing NMDA receptors and dopamine D2 receptors in NAc during the first week of morphine withdrawal. However, the compound did not affect the overexpression of GluN2A-containing NMDA receptors, GluA1, and dopamine D1 receptors. Lastly, Gent significantly reduced NMDA receptors-mediated EPSCs in the NAc.

CONCLUSION

Our study provides strong evidence that Gent inhibits morphine dependence through downregulation of GluN2B-containing NMDA receptors in the NAc.

Citalopram alleviates chronic stress induced depression-like behaviors in rats by activating GSK3β signaling in dorsal hippocampus

Aside from monoamine disturbances, recent evidence has implicated particular intracellular pathways, including Wnt signaling, in the pathogenesis of major depressive disorder. In the present study, we investigated the role of Wingless (Wnt)-Dishevelled (DVL)-glycogen synthase kinase 3β (GSK3β) signaling in the depression-like behaviors exhibited by rats exposed to chronic forced swim stress. We found that the rats subjected to forced swim stress for 14 consecutive days exhibited obvious depression-like behaviors and showed decreased levels of phosphorylated GSK3β and β-catenin in the hippocampus. Chronic citalopram treatment alleviated the depression-like behaviors and reversed the disruptions of the phosphorylated GSK3β and β-catenin in stressed rats. Furthermore, when the stressed rats with citalopram treatment received bilateral, dorsal hippocampus infusions of a DVL inhibitor, sulindac, the depression-like effects induced by chronic stress reappeared. These findings suggest that the Wnt-DVL-GSK3β signaling in the hippocampus is markedly involved in the pathophysiology of depression induced by chronic stress. The Wnt-DVL-GSK3β pathway may mediate the therapeutic action of citalopram, and the manipulation of DVL could be a target for novel antidepressants.

The NMDA-NR1 receptor subunit and the mu-opioid receptor are expressed in somatodendritic compartments of central nucleus of the amygdala neurons projecting to the bed nucleus of the stria terminalis

The pathway between the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) is emerging as a critical mediator of stress-related affective processes. Evidence also indicates that exposure to drugs of abuse, like opioids, is associated with NMDA-type glutamate receptor-dependent plasticity in the CeA and BNST. However, there is little evidence that NMDA receptors are expressed in CeA neurons projecting to the BNST, or are required for opioid-induced BNST neural activation. Immunoelectron microscopy, tract tracing, and conditional gene deletion technology were used to investigate the synaptic organization of the NMDA receptor and the mu-opioid receptor (μOR) in the CeA-BNST pathway. By dual labeling electron microscopy, numerous CeA-BNST projection neurons expressed the NMDA-NR1 receptor subunit (NR1) or μOR. By triple labeling, it was also found that NR1 and μOR were co-expressed in some CeA-BNST projection neurons. Despite being colocalized in somato-dendritic compartments of CeA neurons, NR1 and μOR were rarely expressed in their axonal terminations in the BNST. Deleting the NR1 gene in CeA neurons resulted in a reduction of morphine-induced Fos protein labeling in the ventral BNST. In summary, NR1 and μOR are coexpressed in somatodendritic sites of CeA neurons, including those projecting to the BNST. In addition, expression of the NR1 gene in CeA neurons is required for morphine-induced BNST neural activation. Thus, postsynaptic NMDA receptors and μORs are positioned for the co-modulation of CeA projection neurons to the BNST, which may provide a synaptic substrate for stress-induced emotional processes critically involved in opioid addictive behaviors.

Essential role of protein kinase C in morphine-induced rewarding memory

Protein kinase C (PKC) is involved in intra-cellular signal transduction in various physiological and pathological processes including substance abuse. In the present study, the role of PKC in morphine-induced rewarding memory was investigated using the conditioned place preference (CPP) model. We found a significant translocation of PKCs from cytosol to membrane component in nucleus accumbens (NAc) of morphine-conditioned rats in a dose-dependent manner. The translocation was reduced gradually with the maintenance of morphine-induced CPP. Specifically, the protein level of PKCγ in membrane of the NAc was increased in morphine CPP rats, and decreased during the attenuation of morphine-induced CPP, while the protein level of PKCγ in cytosol of the NAc showed an opposite change. Furthermore, the PKC translocation inhibitor γV5-3 impaired the morphine-induced CPP when microinjected into the NAc. These findings indicated that PKC, especially the γ isoform, is essential for the acquisition and maintenance of morphine-associated reward memory. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Variations in the GRIN2B gene are associated with risky decision-making

The dopaminergic system is known to modulate decision-making. As N-methyl-D-aspartate (NMDA) receptors strongly influence dopaminergic function, it is conceivable that the glutamatergic system is also involved in decision-making. We examined whether polymorphisms in the N-methyl-d-aspartate receptor 2B subunit gene (GRIN2B) influence decision-making using the Iowa Gambling Task (IGT). In total, 245 (n = 245, 127 female) healthy German students were included in the analysis. Two synonymous SNPs in exon 13, rs1806191 (H1178H) and rs1806201 (T888T) showed the strongest association with aspects of IGT performance. Females with a CC allele in rs1806201 made less use both of a win-stay strategy and demonstrated more exploratory behaviour during task execution. For rs1806191, we found a strong additive effect in usage of a win-stay strategy. This, partly sex-dependent, correlation of the win-stay/lose-shift behaviour with GRIN2B genotypes suggests that healthy individuals with certain GRIN2B variations respond differently to ambiguous conditions, possibly by altered perception of wins and losses. These findings underline the necessity to integrate the glutamatergic system when examining decision-making processes.

Effect of glutamate receptor antagonists microinjected into the nucleus accumbens on place aversion induced by naloxone in single-dose, morphine-treated rats

It is well established that acute morphine withdrawal can be observed following opioid receptor antagonism in rodents. Glutamate receptor antagonists can attenuate the conditioning place aversion (CPA) induced by naloxone in single-dose, morphine-treated rats. Anatomically, the nucleus accumbens appears to be involved in opiate dependence. In the present study, we examined the effects of various glutamate receptor antagonists in the nucleus accumbens on naloxone-induced CPA in rats. MK-801 (an NMDA receptor antagonist), GYKI52466 (an AMPA receptor antagonist), and MCPG (a metabotropic glutamate receptor antagonist) significantly attenuated naloxone-induced CPA following microinjection into the accumbens. In contrast, none of the agents showed place conditioning ability on their own in either morphine-exposed or naïve rats. The present study suggests that glutamate receptors in the nucleus accumbens play a key role in the motivational component of withdrawal during acute morphine dependence.

Cyclic adenosine monophosphate-independent tyrosine phosphorylation of NR2B mediates cocaine-induced extracellular signal-regulated kinase activation

BACKGROUND

Activation of the extracellular signal-regulated kinase (ERK) in the striatum is crucial for long-term behavioral alterations induced by drugs of abuse. In response to cocaine, ERK phosphorylation (i.e., activation) is restricted to medium-sized spiny neurons expressing dopamine D1 receptor (D1R) and depends on a concomitant stimulation of D1R and glutamate N-methyl-D-aspartate receptor (NMDAR). However, the mechanisms responsible for this activation, especially the respective contribution of D1R and NMDAR, remain unknown.

METHODS

We studied striatal neurons in culture stimulated with D1R agonist and/or glutamate and wild-type or genetically modified mice treated with cocaine. Biochemical, immunohistochemical, and imaging studies were performed. Mice were also subjected to behavioral experiments.

RESULTS

Stimulation of D1R cannot activate ERK by itself but potentiates glutamate-mediated calcium influx through NMDAR that is responsible for ERK activation. Potentiation of NMDAR by D1R depends on a cyclic adenosine monophosphate-independent signaling pathway, which involves tyrosine phosphorylation of the NR2B subunit of NMDAR by Src family kinases. We also demonstrate that the D1R/Src family kinases/NR2B pathway is responsible for ERK activation by cocaine in vivo. Inhibition of this pathway abrogates cocaine-induced locomotor sensitization and conditioned place preference.

CONCLUSIONS

Our results show that potentiation of NR2B-containing NMDAR by D1R is necessary and sufficient to trigger cocaine-induced ERK activation. They highlight a new cyclic adenosine monophosphate-independent pathway responsible for the integration of dopamine and glutamate signals by the ERK cascade in the striatum and for long-term behavioral alterations induced by cocaine.

Cocaine-induced metaplasticity in the nucleus accumbens: silent synapse and beyond

The neuroadaptation theory of addiction suggests that, similar to the development of most memories, exposure to drugs of abuse induces adaptive molecular and cellular changes in the brain which likely mediate addiction-related memories or the addictive state. Compared to other types of memories, addiction-related memories develop fast and last extremely long, suggesting that the cellular and molecular processes that mediate addiction-related memories are exceptionally adept and efficient. We recently demonstrated that repeated exposure to cocaine generated a large portion of "silent" glutamatergic synapses within the nucleus accumbens (NAc). Silent glutamatergic synapses are synaptic connections in which only N-methyl-D-aspartic acid receptor (NMDAR)-mediated responses are readily detected whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are absent or highly labile. Extensive experimental evidence suggests that silent synapses are conspicuously efficient plasticity sites at which long-lasting plastic changes can be more easily induced and maintained. Thus, generation of silent synapses can be regarded as a process of metaplasticity, which primes the NAc for subsequent durable and robust plasticity for addiction-related memories. Focusing on silent synapse-based metaplasticity, this review discusses how key brain regions, such as the NAc, utilize the metaplasticity mechanism to optimize the plasticity machineries to achieve fast and durable plastic changes following exposure to cocaine. A summary of recent related results suggests that upon cocaine exposure, newly generated silent synapses may prime excitatory synapses within the NAc for long-term potentiation (LTP), thus setting the direction of future plasticity. Furthermore, because cocaine-generated silent synapses are enriched in NMDARs containing the NR2B subunit, the enhanced NR2B-signaling may set up a selective recruitment of certain types of AMPARs. Thus, silent synapse-based metaplasticity may lead to not only quantitative but also qualitative alterations in excitatory synapses within the NAc. This review is one of the first systematic analyses regarding the hypothesis that drugs of abuse induce metaplasticity, which regulates the susceptibility, the direction, and the molecular details of subsequent plastic changes. Taken together, metaplasticity ultimately serves as a key step in mediating cascades of addiction-related plastic alterations.

Advances in animal models of drug addiction

Drug addiction is a syndrome of impaired response inhibition and salience attribution, which involves a complex neurocircuitry underlying drug reinforcement, drug craving, and compulsive drug-seeking and drug-taking behaviors despite adverse consequences. The concept of disease stages with transitions from acute rewarding effects to early- and end-stage addiction has had an important impact on the design of nonclinical animal models. This chapter reviews the main advances in nonclinical paradigms that aim to at model (1) positive and negative reinforcing effects of addictive drugs; (2) relapse to drug-seeking behavior; (3) reconsolidation of drug cue memories, and (4) compulsive/impulsive drug intake. In addition, recent small animal neuroimaging studies and invertebrate models will be briefly discussed (see also Bifone and Gozzi, Animal models of ADHD, 2011). Continuous improvement in modeling drug intake, craving, withdrawal symptoms, relapse, and comorbid psychiatric associations is a necessary step to better understand the etiology of the disease and to ultimately foster the discovery, validation and optimization of new efficacious pharmacotherapeutic approaches. The modeling of specific subprocesses or constructs that address clinically defined criteria will ultimately increase our understanding of the disease as a whole. Future research will have to address the questions of whether some of these constructs can be reliably used as outcome measures to assess the effects of a treatment in clinical settings, whether changes in those measures can be a target of therapeutic efforts, and whether they relate to biological markers of traits such as impulsivity, which contribute to increased drug-seeking and may predict binge-like patterns of drug intake.

Morphine induces AMPA receptor internalization in primary hippocampal neurons via calcineurin-dependent dephosphorylation of GluR1 subunits

Chronic morphine treatment resulting in the alteration of postsynaptic levels of AMPA receptors, thereby modulating synaptic strength, has been reported. However, the mechanism underlying such drug-induced synaptic modification has not been resolved. By monitoring the GluR1 trafficking in primary hippocampal neurons using the pHluorin-GluR1 imaging and biotinylation studies, we observed that prolonged morphine exposure significantly induced loss of synaptic and extrasynaptic GluR1 by internalization. The morphine-induced GluR1 endocytosis was independent of neural network activities or NMDA receptor activities, as neither blocking the sodium channels with tetrodotoxin nor NMDA receptors with dl-APV altered the effects of morphine. Instead, morphine-induced GluR1 endocytosis is attributed to a change in the phosphorylation state of the GluR1 at Ser(845) as morphine significantly decreased the dephosphorylation of GluR1 at this site. Such changes in Ser(845) phosphorylation required morphine-induced activation of calcineurin, based on the observations that a calcineurin inhibitor, FK506, completely abrogated the dephosphorylation, and morphine treatment led to an increase in calcineurin enzymatic activity, even in the presence of dl-APV. Importantly, pretreatment with FK506 and overexpression of the GluR1 mutants, S845D (phospho-mimic) or S845A (phospho-blocking) attenuated the morphine-induced GluR1 endocytosis. Therefore, the calcineurin-mediated GluR1-S845 dephosphorylation is critical for the morphine-induced changes in the postsynaptic AMPA receptor level. Together, these findings reveal a novel molecular mechanism for opioid-induced neuronal adaptation and/or synaptic impairment.

Effects of ifenprodil on morphine-induced conditioned place preference and spatial learning and memory in rats

Drug addiction, as well as learning and memory, share common mechanisms in terms of neural circuits and intracellular signaling pathways. In the present study, the role of N-methyl-D-aspartate (NMDA) receptors, particularly those containing NR2B subunits, in morphine-induced conditioned place preference (CPP) and Morris water maze (MWM) learning and memory task was investigated. CPP was used as a paradigm for assessing the rewarding effect of morphine, and MWM was used to measure spatial learning and memory in male Sprague-Dawley rats. We found that ifenprodil, an antagonist highly selective for NR2B-containing NMDA receptors, dose-dependently blocked the development, maintenance and reinstatement of morphine-induced CPP, without evident impairment of the acquisition and retrieval of spatial memory in the MWM task. However, the consolidation of spatial memory was disrupted by a high dose (10 mg/kg) of ifenprodil. These results clearly demonstrate that NR2B-containing NMDA receptors are actively involved in addiction memory induced by morphine conditioning, but not in the acquisition and retrieval of spatial learning and memory. In conclusion, NR2B-containing NMDA receptors can be considered potential targets for the treatment of opiate addiction.

Allosteric modulation of the NMDA receptor by neurosteroids in rat brain and the impact of long term morphine administration

This study examined the allosteric modulation of the NMDA receptor by nanomolar concentrations of neurosteroids in rats treated long term with morphine. The neurosteroids dehydroepiandrosterone sulfate (DHEAS), pregnenolone sulfate (PS) and pregnanolone sulfate (3α5βS) are important mediators in the central nervous system. They induce rapid responses by non-classical steroidal mechanisms, e.g. via interaction with the N-methyl-D-aspartate (NMDA) receptor, and are known to modify the binding of ifenprodil to the NMDA receptor subunit NR2B. The NMDA receptor is involved in several processes, including memory, learning, synaptic plasticity and neuronal development. Morphine, a μ-opioid receptor agonist, has an important role in the clinical treatment of pain. The main drawback of morphine treatment is the associated development of dependence and tolerance. The mechanisms behind these phenomena are still to be elucidated, but several reports suggest the involvement of the NMDA receptor. The results of the present study indicate that the allosteric modulation induced by the neurosteroids DHEAS, PS and 3α5βS was similar in all tested brain regions. This suggests that the NR2B receptor subunit behaves independently of its site of expression. Moreover, the NR2B subunit was up-regulated in the frontal cortex but not in the hippocampus or hypothalamus. It is concluded that morphine does not affect the neurosteroid modulatory effect on ifenprodil binding in the rat hippocampus or hypothalamus but does significantly affect both the expression of the NR2B subunit and the 3α5βS modulatory effect on ifenprodil binding in the frontal cortex. It is suggested that the observed effect of long term morphine on the properties of NR2B in the frontal cortex may be associated with the mechanism underlying the development of opiate dependence.