Hippocampal long-term potentiation is reduced by chronic opiate treatment and can be restored by re-exposure to opiates

Persistent effects of repeated adolescent and adult heroin vapor inhalation in female Wistar rats

Adolescent drug exposure has been associated with more severe mental health outcomes related to substance abuse and anxiety disorders. The aim of the present study was to contrast the long-term effects of repeated heroin vapor inhalation during adolescence with similar heroin exposure in adulthood. Groups of female Wistar rats underwent twice daily 30-minute sessions of heroin or propylene glycol (control) vapor inhalation from postnatal days (PND) 36-45 or PND 85-94, respectively. Nociception was assessed after vapor inhalation sessions and forty days later, for the Adolescent-Exposed and Adult-Exposed groups. Anxiety-like behavior was assessed with an elevated plus-maze (EPM) and spatial learning was assessed with a Barnes maze. Acute effects of naloxone (0.3 mg/kg, i.p.) and heroin (0.5 and 1.0 mg/kg, s.c.) on thermal nociception were determined on PND 140/189 and PND 149/198, respectively. Repeated heroin vapor inhalation produced anti-nociceptive tolerance across sessions in both adolescent and adult rats, with the adolescents exhibiting more complete tolerance. Heroin vapor inhalation produced anxiolytic effects, regardless of age of exposure. There were no effects of heroin on spatial learning. Naloxone produced acute hyperalgesia in all but the Adolescent-Exposed heroin group, and heroin anti-nociception was blunted in both heroin-exposed groups at the highest heroin dose. Repeated heroin vapor inhalation can produce lasting effects on nociception and anxiety-like behavior that persist for months after the exposure. Importantly, these findings suggest that adolescent exposure to heroin vapor produces specific effects on nociception that are not observed when exposure occurs in adulthood.

Repeated Morphine Exposure Alters Temporoamonic-CA1 Synaptic Plasticity in Male Rat Hippocampus

In this study, the electrophysiological and biochemical consequences of repeated exposure to morphine in male rats on glutamatergic synaptic transmission, synaptic plasticity, the expression of GABA receptors and glutamate receptors at the temporoammonic-CA1 synapse along the longitudinal axis of the hippocampus (dorsal, intermediate, ventral, DH, IH, VH, respectively) were investigated. Slice electrophysiological methods, qRT-PCR, and western blotting techniques were used to characterize synaptic plasticity properties. We showed that repeated morphine exposure (RME) reduced excitatory synaptic transmission and ability for long-term potentiation (LTP) in the VH as well as eliminated the dorsoventral difference in paired-pulse responses. A decreased expression of NR2B subunit in the VH and an increased expression GABAA receptor of α1 and α5 subunits in the DH were observed following RME. Furthermore, RME did not affect the expression of NR2A, AMPA receptor subunits, and γ2GABAA and GABAB receptors in either segment of the hippocampus. In sum, the impact of morphine may differ depending on the region of the hippocampus studied. A distinct change in the short- and long-term synaptic plasticity along the hippocampus long axis due to repeated morphine exposure, partially mediated by a change in the expression profile of glutamatergic receptor subunits. These findings can be useful in further understanding the cellular mechanism underlying deficits in information storage and, more generally, cognitive processes resulting from chronic opioid abuse.

Divergent opioid-mediated suppression of inhibition between hippocampus and neocortex across species and development

Opioid receptors within the CNS regulate pain sensation and mood and are key targets for drugs of abuse. Within the adult rodent hippocampus (HPC), μ-opioid receptor agonists suppress inhibitory parvalbumin-expressing interneurons (PV-INs), thus disinhibiting the circuit. However, it is uncertain if this disinhibitory motif is conserved in other cortical regions, species, or across development. We observed that PV-IN mediated inhibition is robustly suppressed by opioids in HPC but not neocortex in mice and nonhuman primates, with spontaneous inhibitory tone in resected human tissue also following a consistent dichotomy. This hippocampal disinhibitory motif was established in early development when immature PV-INs and opioids already influence primordial network rhythmogenesis. Acute opioid-mediated modulation was partially occluded with morphine pretreatment, with implications for the effects of opioids on hippocampal network activity during circuit maturation as well as learning and memory. Together, these findings demonstrate that PV-INs exhibit a divergence in opioid sensitivity across brain regions that is remarkably conserved across evolution and highlights the underappreciated role of opioids acting through immature PV-INs in shaping hippocampal development.

Addictive drugs modify neurogenesis, synaptogenesis and synaptic plasticity to impair memory formation through neurotransmitter imbalances and signaling dysfunction

Drug abuse changes neurophysiological functions at multiple cellular and molecular levels in the addicted brain. Well-supported scientific evidence suggests that drugs negatively affect memory formation, decision-making and inhibition, and emotional and cognitive behaviors. The mesocorticolimbic brain regions are involved in reward-related learning and habitual drug-seeking/taking behaviors to develop physiological and psychological dependence on the drugs. This review highlights the importance of specific drug-induced chemical imbalances resulting in memory impairment through various neurotransmitter receptor-mediated signaling pathways. The mesocorticolimbic modifications in the expression levels of brain-derived neurotrophic factor (BDNF) and the cAMP-response element binding protein (CREB) impair reward-related memory formation following drug abuse. The contributions of protein kinases and microRNAs (miRNAs), along with the transcriptional and epigenetic regulation have also been considered in memory impairment underlying drug addiction. Overall, we integrate the research on various types of drug-induced memory impairment in distinguished brain regions and provide a comprehensive review with clinical implications addressing the upcoming studies.

Adolescent morphine exposure impairs dark avoidance memory and synaptic potentiation of ventral hippocampal CA1 during adulthood in rats

Adolescence is a neurobiological critical period for neurodevelopmental processes. Adolescent opioid exposure can affect cognitive abilities via regional-specific lasting changes in brain structure and function. The current study was therefore designed to assess the long-term effects of adolescent morphine exposure on dark avoidance memory and synaptic plasticity of the ventral hippocampal CA1. Adolescent Wistar rats received escalating doses of morphine for 10 days. Morphine injections were started with an incremental dose of 2.5 mg/kg to reach a dose of 25 mg/kg. 30 days after the last injection, inhibitory memory and in vitro field potential recording were evaluated. Also, the weight of the animals was measured during drug and post-drug exposure. We found that adolescent morphine exposure decreased weight gain during morphine and post-morphine exposure. Passive avoidance memory was impaired in the morphine group. Moreover, adolescent morphine exposure caused an increase in baseline synaptic responsiveness and failed long-term potentiation (LTP) in the ventral hippocampal CA1 during adulthood. In the morphine group, the mean values of the field excitatory postsynaptic potential (fEPSP) slopes required to elicit a half-maximal population spike (PS) amplitude were significantly greater than that of the saline group. Therefore, adolescent morphine exposure has a durable effect on memory functions, synaptic activity, and plasticity of ventral hippocampal CA1. Adults with adolescent morphine exposures may experience maladaptive behaviors and cognitive disabilities.

A Novel CaMKII Inhibitory Peptide Blocks Relapse to Morphine Seeking by Influencing Synaptic Plasticity in the Nucleus Accumbens Shell

Drugs of abuse cause enduring functional disorders in the brain reward circuits, leading to cravings and compulsive behavior. Although people may rehabilitate by detoxification, there is a high risk of relapse. Therefore, it is crucial to illuminate the mechanisms of relapse and explore the therapeutic strategies for prevention. In this research, by using an animal model of morphine self-administration in rats and a whole-cell patch–clamp in brain slices, we found changes in synaptic plasticity in the nucleus accumbens (NAc) shell were involved in the relapse to morphine-seeking behavior. Compared to the controls, the amplitude of long-term depression (LTD) induced in the medium spiny neurons increased after morphine self-administration was established, recovered after the behavior was extinguished, and increased again during the relapse induced by morphine priming. Intravenous injection of MA, a new peptide obtained by modifying Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor “myr-AIP”, decreased CaMKII activity in the NAc shell and blocked the reinstatement of morphine-seeking behavior without influence on the locomotor activity. Moreover, LTD was absent in the NAc shell of the MA-pretreated rats, whereas it was robust in the saline controls in which morphine-seeking behavior was reinstated. These results indicate that CaMKII regulates morphine-seeking behavior through its involvement in the change of synaptic plasticity in the NAc shell during the relapse, and MA may be of great value in the clinical treatment of relapse to opioid seeking.

Differential Gene Expression in the Hippocampi of Nonhuman Primates Chronically Exposed to Methamphetamine, Cocaine, or Heroin

OBJECTIVE

Methamphetamine (MA), cocaine, and heroin cause severe public health problems as well as impairments in neural plasticity and cognitive function in the hippocampus. This study aimed to identify the genes differentially expressed in the hippocampi of cynomolgus monkeys in response to these drugs.

METHODS

After the monkeys were chronically exposed to MA, cocaine, and heroin, we performed large-scale gene expression profiling of the hippocampus using RNA-Seq technology and functional annotation of genes differentially expressed. Some genes selected from RNA-Seq analysis data were validated with reverse transcription-quantitative polymerase chain reaction (RT-qPCR). And the expression changes of ADAM10 protein were assessed using immunohistochemistry.

RESULTS

The changes in genes related to axonal guidance (PTPRP and KAL1), the cell cycle (TLK2), and the regulation of potassium ions (DPP10) in the drug-treated groups compared to the control group were confirmed using RT-qPCR. Comparative analysis of all groups showed that among genes related to synaptic long-term potentiation, CREBBP and GRIN3A were downregulated in both the MA- and heroin-treated groups compared to the control group. In particular, the mRNA and protein expression levels of ADAM10 were decreased in the MA-treated group but increased in the cocaine-treated group compared to the control group.

CONCLUSION

These results provide insights into the genes that are upregulated and downregulated in the hippocampus by the chronic administration of MA, cocaine, or heroin and basic information for developing novel drugs for the treatment of hippocampal impairments caused by drug abuse.

Impairment of spatial memory and dorsal hippocampal synaptic plasticity in adulthood due to adolescent morphine exposure

Opioid exposure during adolescence, a crucial period of neurodevelopment, has lasting neurological and behavioral consequences and affects the cognitive functions in adulthood. This study investigated the effects of adolescent morphine exposure in spatial learning and memory and synaptic plasticity of the CA1 area of the dorsal hippocampus. Adolescent Wistar rats received increasing doses of morphine for 1, 5, and 10 days. Acute morphine group was injected 2.5 mg/kg morphine for 1 day, subchronic morphine group for 5 days, with an increasing dose of 2.5 mg/kg and reached to the dose of 12.5 mg/kg and chronic morphine group for 10 days that began with an increasing dose of 2.5 mg/kg and reached to the dose of 25 mg/kg. Then after 25 days and reaching adulthood, spatial learning and memory were evaluated via the Morris water maze (MWM) test. Moreover, we test the electrophysiological properties of dorsal hippocampal plasticity in adult rats by in vitro field potential recordings. Subchronic and chronic adolescent morphine exposure impaired spatial learning and memory in the MWM test. Baseline synaptic responses in the chronic morphine group were increased and long-term potentiation (LTP) impaired in the CA1 area in subchronic and chronic morphine groups. In adulthood, the slope of the field excitatory postsynaptic potential (fEPSP) required to elicit a half-maximal population spike (PS) amplitude was significantly larger in subchronic and chronic adolescent morphine exposure compared to the saline group. Therefore, subchronic and chronic adolescent morphine exposure altered synaptic transmission and plasticity in addition to learning and memory. Long-term morphine exposure during adolescence can interfere with neurodevelopment, making a persistent impression on plasticity and cognitive capability in adulthood.

Effects of different intensities of treadmill exercise on cued fear extinction failure, hippocampal BDNF decline, and Bax/Bcl-2 ratio alteration in chronic-morphine treated male rats

Chronic morphine impairs cued fear extinction, which may contribute to the high prevalence of anxiety disorders and the replase of opiate addiction. This work investigated the effects of forced exercise with different intensities on cued fear extinction impairment and alternations of hippocampal BDNF and apoptotic proteins induced by chronic morphine. Rats were injected with bi-daily doses of morphine or saline for ten days and then received a cued or contextual fear conditioning training, which was followed by fear extinction training for four consecutive days. Cued, but the not contextual fear response was impaired in morphine-treated rats. Then, different saline or morphine-treated rats underwent forced exercise for 4-weeks with light, moderate or high intensities. Subsequently, rats received a cued fear conditioning followed by four days of extinction training, and the expression of hippocampal BDNF and apoptotic proteins was determined. A relatively long time after the last injection of morphine (35 days), rats again showed cued fear extinction failure and reduced hippocampal BDNF, which recovered by light and moderate, but not high exercise. Light and moderate, but not high-intensity treadmill exercise enhanced the up-regulation of Bcl-2 and down-regulation of the Bax proteins in both saline- and morphine-treated rats, which shifted the balance between pro-apoptotic and anti-apoptotic factors in favor of cell survival. These findings highlight the impact of exercise up to moderate intensity in the recovery of cued extinction failure, more likely via BDNF in addicted individuals.

Combined molecular, structural and memory data unravel the destructive effect of tramadol on hippocampus

Tramadol is a synthetic analogue of codeine and stimulates neurodegeneration in several parts of the brain that leads to various behavioral impairments. Despite the leading role of hippocampus in learning and memory as well as decreased function of them under influence of tramadol, there are few studies analyzing the effect of tramadol administration on gene expression profiling and structural consequences in hippocampus region. Thus, we sought to determine the effect of tramadol on both PC12 cell line and hippocampal tissue, from gene expression changes to structural alterations. In this respect, we investigated genome-wide mRNA expression using high throughput RNA-seq technology and confirmatory quantitative real-time PCR, accompanied by stereological analysis of hippocampus and behavioral assessment following tramadol exposure. At the cellular level, PC12 cells were exposed to 600μM tramadol for 48 hrs, followed by the assessments of ROS amount and gene expression levels of neurotoxicity associated with neurodegenerative pathways such as apoptosis and autophagy. Moreover, the structural and functional alteration of the hippocampus under chronic exposure to tramadol was also evaluated. In this regard, rats were treated with tramadol at doses of 50 mg/kg for three consecutive weeks. In vitro data revealed that tramadol provoked ROS production and caused the increase in the expression of autophagic and apoptotic genes in PC12 cells. Furthermore, in-vivo results demonstrated that tramadol not only did induce hippocampal atrophy, but it also triggered microgliosis and microglial activation, causing upregulation of apoptotic and inflammatory markers as well as over-activation of neurodegeneration. Tramadol also interrupted spatial learning and memory function along with long-term potentiation (LTP). Taken all together, our data disclosed the neurotoxic effects of tramadol on both in vitro and in-vivo. Moreover, we proposed a potential correlation between disrupted biochemical cascades and memory deficit under tramadol administration.

Plasticity in the Hippocampus, Neurogenesis and Drugs of Abuse

Synaptic plasticity in the hippocampus assists with consolidation and storage of long-lasting memories. Decades of research has provided substantial information on the cellular and molecular mechanisms underlying synaptic plasticity in the hippocampus, and this review discusses these mechanisms in brief. Addiction is a chronic relapsing disorder with loss of control over drug taking and drug seeking that is caused by long-lasting memories of drug experience. Relapse to drug use is caused by exposure to context and cues associated with the drug experience, and is a major clinical problem that contributes to the persistence of addiction. This review also briefly discusses some evidence that drugs of abuse alter plasticity in the hippocampus, and that development of novel treatment strategies that reverse or prevent drug-induced synaptic alterations in the hippocampus may reduce relapse behaviors associated with addiction.

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.

Gene expression in the striatum of cynomolgus monkeys after chronic administration of cocaine and heroin

Cocaine and heroin cause impairment of neural plasticity in the brain including striatum. This study aimed to identify genes differentially expressed in the striatum of cynomolgus monkeys in response to cocaine and heroin. After chronic administration of cocaine and heroin in the monkeys, we performed large-scale transcriptome profiling in the striatum using RNA-Seq technology and analysed functional annotation. We found that 547 and 1238 transcripts were more than 1.5-fold up- or down-regulated in cocaine- and heroin-treated groups, respectively, compared to the control group, and 3432 transcripts exhibited differential expression between cocaine- and heroin-treated groups. Functional annotation analysis indicated that genes associated with nervous system development (NAGLU, MOBP and TTL7) and stress granule disassembly (KIF5B and KLC1) were differentially expressed in the cocaine-treated group compared to the control group, whereas gene associated with neuron apoptotic process (ERBB3) was differentially expressed in the heroin-treated group. In addition, IPA network analysis indicated that genes (TRAF6 and TRAF3IP2) associated with inflammation were increased by the chronic administration of cocaine and heroin. These results provide insight into the correlated molecular mechanisms as well as the upregulation and down-regulation of genes in the striatum after chronic exposure to cocaine and heroin.

Signaling mechanisms of μ-opioid receptor (MOR) in the hippocampus: disinhibition versus astrocytic glutamate regulation

μ-opioid receptor (MOR) is a class of opioid receptors that is critical for analgesia, reward, and euphoria. MOR is distributed in various brain regions, including the hippocampus, where traditionally, it is believed to be localized mainly at the presynaptic terminals of the GABAergic inhibitory interneurons to exert a strong disinhibitory effect on excitatory pyramidal neurons. However, recent intensive research has uncovered the existence of MOR in hippocampal astrocytes, shedding light on how astrocytic MOR participates in opioid signaling via glia-neuron interaction in the hippocampus. Activation of astrocytic MOR has shown to cause glutamate release from hippocampal astrocytes and increase the excitability of presynaptic axon fibers to enhance the release of glutamate at the Schaffer Collateral-CA1 synapses, thereby, intensifying the synaptic strength and plasticity. This novel mechanism involving astrocytic MOR has been shown to participate in hippocampus-dependent conditioned place preference. Furthermore, the signaling of hippocampal MOR, whose action is sexually dimorphic, is engaged in adult neurogenesis, seizure, and stress-induced memory impairment. In this review, we focus on the two profoundly different hippocampal opioid signaling pathways through either GABAergic interneuronal or astrocytic MOR. We further compare and contrast their molecular and cellular mechanisms and their possible roles in opioid-associated conditioned place preference and other hippocampus-dependent behaviors.

Modeling SARS-CoV-2 infection in individuals with opioid use disorder with brain organoids

The COVID-19 pandemic has aggravated a preexisting epidemic: the opioid crisis. Much literature has shown that the circumstances imposed by COVID-19, such as social distancing regulations, medical and financial instability, and increased mental health issues, have been detrimental to those with opioid use disorder (OUD). In addition, unexpected neurological sequelae in COVID-19 patients suggest that COVID-19 compromises neuroimmunity, induces hypoxia, and causes respiratory depression, provoking similar effects as those caused by opioid exposure. Combined conditions of COVID-19 and OUD could lead to exacerbated complications. With limited human in vivo options to study these complications, we suggest that iPSC-derived brain organoid models may serve as a useful platform to investigate the physiological connection between COVID-19 and OUD. This mini-review highlights the advances of brain organoids in other neuropsychiatric and infectious diseases and suggests their potential utility for investigating OUD and COVID-19, respectively.

Habenula kisspeptin retrieves morphine impaired fear memory in zebrafish

The habenula is an evolutionarily conserved brain structure, which has recently been implicated in fear memory. In the zebrafish, kisspeptin (Kiss1) is predominantly expressed in the habenula, which has been implicated as a modulator of fear response. Hence, in the present study, we questioned whether Kiss1 has a role in fear memory and morphine-induced fear memory impairment using an odorant cue (alarm substances, AS)-induced fear avoidance paradigm in adult zebrafish, whereby the fear-conditioned memory can be assessed by a change of basal place preference (= avoidance) of fish due to AS-induced fear experience. Subsequently, to examine the possible role of Kiss1 neurons-serotonergic pathway, kiss1 mRNA and serotonin levels were measured. AS exposure triggered fear episodes and fear-conditioned place avoidance. Morphine treatment followed by AS exposure, significantly impaired fear memory with increased time-spent in AS-paired compartment. However, fish administered with Kiss1 (10^–21 mol/fish) after morphine treatment had significantly lower kiss1 mRNA levels but retained fear memory. In addition, the total brain serotonin levels were significantly increased in AS- and Kiss1-treated groups as compared to control and morphine treated group. These results suggest that habenular Kiss1 might be involved in consolidation or retrieval of fear memory through the serotonin system.

Proteomic analysis of protein composition of rat hippocampus exposed to morphine for 10 days; comparison with animals after 20 days of morphine withdrawal

Opioid addiction is recognized as a chronic relapsing brain disease resulting from repeated exposure to opioid drugs. Cellular and molecular mechanisms underlying the ability of organism to return back to the physiological norm after cessation of drug supply are not fully understood. The aim of this work was to extend our previous studies of morphine-induced alteration of rat forebrain cortex protein composition to the hippocampus. Rats were exposed to morphine for 10 days and sacrificed 24 h (groups +M10 and −M10) or 20 days after the last dose of morphine (groups +M10/−M20 and −M10/−M20). The six altered proteins (≥2-fold) were identified in group (+M10) when compared with group (−M10) by two-dimensional fluorescence difference gel electrophoresis (2D-DIGE). The number of differentially expressed proteins was increased to thirteen after 20 days of the drug withdrawal. Noticeably, the altered level of α-synuclein, β-synuclein, α-enolase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was also determined in both (±M10) and (±M10/−M20) samples of hippocampus. Immunoblot analysis of 2D gels by specific antibodies oriented against α/β-synucleins and GAPDH confirmed the data obtained by 2D-DIGE analysis. Label-free quantification identified nineteen differentially expressed proteins in group (+M10) when compared with group (−M10). After 20 days of morphine withdrawal (±M10/−M20), the number of altered proteins was increased to twenty. We conclude that the morphine-induced alteration of protein composition in rat hippocampus after cessation of drug supply proceeds in a different manner when compared with the forebrain cortex. In forebrain cortex, the total number of altered proteins was decreased after 20 days without morphine, whilst in hippocampus, it was increased.

The neuroprotective effect of NeuroAid on morphine-induced amnesia with respect to the expression of TFAM, PGC-1α, ΔfosB and CART genes in the hippocampus of male Wistar rats

Morphine is a natural alkaloid which derived from the opium poppy Papaver somniferum. Many studies have reported the effect of morphine on learning, memory and gene expression. CART (cocaine-amphetamine regulated transcript)is an important neuropeptide which has a critical role in physiological processes including drug dependence and antioxidant activity. ΔfosB is a transcription factor which modulates synaptic plasticity and affects learning and memory. TFAM (the mitochondrial transcription factor A) and PGC-1α (Peroxisome proliferator-activated receptor γ coactivator-1α) are critically involved in mitochondrial biogenesis and antioxidant pathways. NeuroAid is a Chinese medicine that induces neuroprotective and anti-apoptotic effects. In this research, we aimed to investigate the effect of NeuroAid on morphine-induced amnesia with respect to the expression of TFAM, PGC-1α, ΔfosB and CART in the rat's hippocampus. In this study, Morphine sulfate (at increasing doses), Naloxone hydrochloride (2.5 mg/kg) and NeuroAid (2.5 mg/kg) were administered intraperitoneal and real-time PCR reactions were done to assess gene expression. The results showed, morphine impaired memory of step-through passive avoidance, while NeuroAid had no effect. NeuroAid attenuated (but not reversed) morphine-induced memory impairment in morphine-addicted rats. Morphine increased the expression of PGC-1α and decreased the expression of CART. However, NeuroAid increased the expression of TFAM, PGC-1α, ΔfosB and CART. NeuroAid restored the effect of morphine on the expression of CART and PGC-1α. In conclusion, morphine impaired memory of step-through passive avoidance and NeuroAid attenuated this effect. The effect of NeuroAid on morphine-induced memory impairment/gene expression may be related to its anti-apoptotic and neuroprotective effects.

Deciphering functional brain circuitry during morphine withdrawal with dynamic manganese-enhanced MRI

Withdrawal plays a key role in the development of addiction, and several brain regions, such as the extended amygdala, are functional during this stage. Manganese-enhanced magnetic resonance imaging (MEMRI) is a promising method for directly studying neural activity during morphine withdrawal, versus functional MRI, which is based on hemodynamic alterations. The functional brain circuitry associated with morphine withdrawal has not been thoroughly investigated, and there are very few longitudinal studies utilizing MEMRI to explore brain activity during this stage. In our experiments, we revealed essential brain regions involved in morphine withdrawal by application of a novel dynamic MEMRI approach, and demonstrated dynamic alterations of functional brain activities in these associated brain regions. Our results demonstrate that the dynamic MEMRI approach is an effective method that may be applied to reveal dynamic alterations in functional brain activity during morphine withdrawal.

Impact of different intensities of forced exercise on deficits of spatial and aversive memory, anxiety-like behavior, and hippocampal BDNF during morphine abstinence period in male rats

Forced exercise can alleviate cognitive-behavioral deficits in an experimental model of addiction. However, the effects of different intensities of forced exercise in improving behavioral, cognitive and biochemical deficits during morphine abstinence period are not well investigated. Thus, the current work examined the effects of different loads of forced exercise on cognition functions, anxiety behavior and BDNF changes in the hippocampus, and prefrontal cortex (PFC), and also serum levels of BDNF and corticosterone during the abstinent period in male rats. Animals received morphine injections (10 mg/kg, twice a day) for 10 consecutive days. Then, the animals were exposed to a 4-week forced exercise training program under low, moderate or high intensities (30 min per session on 5 days a week), which accompanied by behavioral and biochemical tests. In Experiment 1, anxiety-like behaviors using elevated plus maze (EPM), and light/dark box (L/D box) were examined. In Experiment 2, cognitive functions using T-maze alteration and passive avoidance tasks were tested, which accompanied by BDNF measurements in the hippocampus and PFC. In Experiment 3, serum levels of BDNF and corticosterone following the termination of forced exercise regimen were measured. Morphine-abstinent animals exhibited anxiogenic -like behaviors in the EPM, but not L/D box. They also exhibited impaired T-maze alternation performance and passive avoidance memory, and a decline in hippocampal BDNF, but not PFC. Forced exercise at a moderate intensity alleviated anxiety, cognitive and BDNF defects in morphine-abstinent animals. The high load exercise enhanced serum levels of corticosterone in both saline and morphine groups. Thus, regular moderate forced exercise may be beneficial in preserving cognitive and mood functions in male addicts during the abstinent period and drug rehabilitation.

Indices of dentate gyrus neurogenesis are unaffected immediately after or following withdrawal from morphine self-administration compared to saline self-administering control male rats

Opiates - including morphine - are powerful analgesics with high abuse potential. In rodents, chronic opiate exposure or self-administration negatively impacts hippocampal-dependent function, an effect perhaps due in part to the well-documented opiate-induced inhibition of dentate gyrus (DG) precursor proliferation and neurogenesis. Recently, however, intravenous (i.v.) morphine self-administration (MSA) was reported to enhance the survival of new rat DG neurons. To reconcile these disparate results, we used rat i.v. MSA to assess 1) whether a slightly-higher dose MSA paradigm also increases new DG neuron survival; 2) how MSA influences cells in different stages of DG neurogenesis, particularly maturation and survival; and 3) if MSA-induced changes in DG neurogenesis persist through a period of abstinence. To label basal levels of proliferation, rats received the S-phase marker bromodeoxyuridine (BrdU, i.p.) 24 -h prior to 21 days (D) of i.v. MSA or saline self-administration (SSA). Either immediately after SA (0-D) or after 4 weeks in the home cage (28-D withdrawal), stereology was used to quantify DG proliferating precursors (or cells in cell cycle; Ki67+ cells), neuroblast/immature neurons (DCX+ cells), and surviving DG granule cells (BrdU+ cells). Analysis revealed the number of DG cells immunopositive for these neurogenesis-relevant markers was similar between MSA and SSA rats at the 0-D or 28-D timepoints. These negative data highlight the impact experimental parameters, timepoint selection, and quantification approach have on neurogenesis results, and are discussed in the context of the large literature showing the negative impact of opiates on DG neurogenesis.

Circular RNA expression profile in the spinal cord of morphine tolerated rats and screen of putative key circRNAs

Morphine tolerance developed after repeated or continuous morphine treatment is a global health concern hindering the control of chronic pain. In our previous research, we have reported that the expression of lncRNAs and microRNAs have been greatly modified in the spinal cord of morphine tolerated rats, and the modulating role of miR-873a-5p, miR-219-5p and miR-365 have already been confirmed. However, whether circular RNAs, another essential kind of non-coding RNA, are involved in the pathogenesis of morphine tolerance is still beyond our knowledge. In this study, we conducted microarray analysis for circRNA profile and found a large number of circRNAs changed greatly in the spinal cord by morphine treatment. Among them, we selected nine circRNAs for validation, and seven circRNAs are confirmed. Gene Ontology/Kyoto Encyclopedia of Genes and Genomes (GO/KEGG) analysis were used for functional annotation. Besides, we confirmed the modified expression of seven circRNAs after validation by real-time PCR, selected 3 most prominently modulated ones among them and predicted their downstream miRNA-mRNA network and analyzed their putative function via circRNA-miRNA-mRNA pathway. Finally, we enrolled the differentially expressed mRNAs derived from the identical spinal cord, these validated circRNAs and their putative miRNA targets for ceRNA analysis and screened a promising circRNA-miRNA-mRNA pathway in the development of morphine tolerance. This study, for the first time, provided valuable information on circRNA profile and gave clues for further study on the circRNA mechanism of morphine tolerance.

Delay-Dependent Impairments in Memory and Motor Functions After Acute Methadone Overdose in Rats

Methadone is used as a substitution drug for the treatment of opioid dependence and chronic pain. Despite its widespread use and availability, there is a serious concern with respect to the relative safety of methadone. The purpose of this study was to characterize how acute methadone overdose affects the cognitive and motor performance of naïve healthy rats. The methadone overdose was induced by administering an acute toxic dose of methadone (15 mg/kg; ip; the equivalent dose of 80% of LD50) to adolescent rats. Resuscitation using a ventilator pump along with a single dose of naloxone (2 mg/kg; ip) was administered following the occurrence of apnea. The animals which were successfully resuscitated divided randomly into three apnea groups that evaluated either on day 1, 5, or 10 post-resuscitation (M/N-Day 1, M/N-Day 5, and M/N-Day 10 groups) in the Y-maze and novel object memory recognition tasks as well as pole and rotarod tests. The data revealed that a single toxic dose of methadone had an adverse effect on spontaneous behavior. In addition, Recognition memory impairment was observed in the M/N-Day 1, 5, and 10 groups after methadone-induced apnea. Further, descending time in the M/N-Day 5 group increased significantly in comparison with its respective Saline control group. The overall results indicate that acute methadone-overdose-induced apnea produced delay-dependent cognitive and motor impairment. We suggest that methadone poisoning should be considered as a possible cause of delayed neurological disorders, which might be transient, in some types of memory or motor performance in naïve healthy rats.

Effects of rhynchophylline on the hippocampal miRNA expression profile in ketamine-addicted rats

In the past few years, ketamine, a noncompetitive NMDA antagonist, has been widely abused worldwide as a new type of synthetic drug, severely affecting the physical and mental health of ketamine abusers. Previous studies have suggested that rhynchophylline can alleviate drug abuse and reverse the conditioned place preference caused by the abuse. MicroRNAs (miRNAs) are important factors regulating gene expression and are involved in the drug addiction process. The hippocampus is a critical area in the brain involved in causing drug addiction. However, the hippocampal miRNA expression profile and the effects of rhynchophylline on miRNA expression during ketamine abuse have not been reported. Thus, this study analyzed the hippocampal miRNA expression profile during ketamine-dependence formation and the effects of rhynchophylline on the differential expression of miRNAs induced by ketamine. The results of microarray analysis suggested that the expression levels of miR-331-5p were significantly different among three groups (the control, ketamine, and ketamine + rhynchophylline groups). miR-331-5p levels were significantly decreased in the ketamine model group and were upregulated in the ketamine + rhynchophylline group. Bioinformatics analysis of miR-331-5p and the 3' UTR of nuclear receptor related 1 protein (Nurr1) identified binding sites and showed downregulation, and the overexpression of miR-331-5p in hippocampal tissues showed that miR-331-5p is a negative transcription regulatory factor of Nurr1. Interestingly, we found that the downstream protein of Nurr1, brain-derived neurotrophic factor (BDNF), showed identical expression trends in the hippocampus as Nurr1. However, the transcription of the protein upstream of Nurr1, cyclic adenosine monophosphate response element-binding protein (CREB), did not show any significant differences between the ketamine group and the ketamine + rhynchophylline group. However, after rhynchophylline intervention, p-CREB showed significant differences between the ketamine and the ketamine + rhynchophylline groups. In summary, miR-331-5p is a key regulatory factor of Nurr1, and rhynchophylline can participate in the process of resistance to ketamine addiction through the miR-331-5p/Nurr1/BDNF pathway or inhibition of CREB phosphorylation.

Cross state-dependency of learning between tramadol and MK-801 in the mouse dorsal hippocampus: involvement of nitric oxide (NO) signaling pathway

RATIONALE

Tramadol, an atypical μ-opioid receptor agonist, as a psychoactive drug, is frequently abused by human beings. Understanding the neurobiological mechanisms of drug-associated learning and memory formation may help prevent drug addiction and relapse. Previous study revealed that dorsal hippocampus (CA1) plays a crucial role in the retrieval of tramadol-associated memory and that its role depends on the expression of CA1 N-methyl-D-aspartate (NMDA) receptors (Jafari-Sabet et al. Can J Physiol Pharmacol 96:45-50, 2018).

OBJECTIVE

To clarify the exact mechanisms involved, the activation of CA1 nitric oxide (NO) signaling pathway by L-arginine (a nitric oxide precursor) on the interaction between tramadol and MK-801 in memory retrieval was examined. The dorsal hippocampal CA1 regions of adult male NMRI mice were bilaterally cannulated and a single-trial step-down inhibitory avoidance apparatus was used for the assessment of memory retrieval.

RESULTS

Post-training and/or pre-test microinjection of tramadol (0.5 and 1 μg/mouse) and/or a non-competitive NMDA receptor antagonist, MK-801 (0.25 and 0.5 μg/mouse), induced amnesia which were reversed when the same doses of the drugs were administered 24 h later in a pre-test session, suggesting tramadol state-dependent learning (SDL) and MK-801 SDL. The amnesia induced by post-training microinjection of tramadol (1 μg/mouse) was reversed by pre-test microinjection of MK-801 (0.25 and 0.5 μg/mouse). Pre-test microinjection of MK-801 (0.125 and 0.25 μg/mouse) with an ineffective dose of tramadol (0.25 μg/mouse) potentiated tramadol SDL. The amnesia induced by post-training microinjection of MK-801 (0.5 μg/mouse) was reversed by pre-test microinjection of tramadol (0.5 and 1 μg/mouse). Pre-test microinjection of tramadol (0.25 and 0.5 μg/mouse) with an ineffective dose of MK-801 (0.125 μg/mouse) potentiated MK-801 SDL. Pre-test microinjection of ineffective doses of L-arginine (0.125, 025, and 0.5 μg/mouse) improved amnesia induced by the co-administration of tramadol and MK-801. Pre-test microinjection of L-arginine (0.125, 025, and 0.5 μg/mouse) could not reverse amnesia induced by post-training microinjection of tramadol while same doses of L-arginine improved MK-801 response on tramadol SDL.

CONCLUSION

The results strongly propose that activation of CA1 NO signaling pathway has a pivotal role in cross SDL among tramadol and MK-801.

Dentate gyrus neurogenesis ablation via cranial irradiation enhances morphine self-administration and locomotor sensitization

Adult dentate gyrus (DG) neurogenesis is important for hippocampal-dependent learning and memory, but the role of new neurons in addiction-relevant learning and memory is unclear. To test the hypothesis that neurogenesis is involved in the vulnerability to morphine addiction, we ablated adult DG neurogenesis and examined morphine self-administration (MSA) and locomotor sensitization. Male Sprague-Dawley rats underwent hippocampal-focused, image-guided X-ray irradiation (IRR) to eliminate new DG neurons or sham treatment (Sham). Six weeks later, rats underwent either MSA (Sham = 16, IRR = 15) or locomotor sensitization (Sham = 12, IRR = 12). Over 21 days of MSA, IRR rats self-administered ~70 percent more morphine than Sham rats. After 28 days of withdrawal, IRR rats pressed the active lever 40 percent more than Sham during extinction. This was not a general enhancement of learning or locomotion, as IRR and Sham groups had similar operant learning and inactive lever presses. For locomotor sensitization, both IRR and Sham rats sensitized, but IRR rats sensitized faster and to a greater extent. Furthermore, dose-response revealed that IRR rats were more sensitive at a lower dose. Importantly, these increases in locomotor activity were not apparent after acute morphine administration and were not a byproduct of irradiation or post-irradiation recovery time. Therefore, these data, along with other previously published data, indicate that reduced hippocampal neurogenesis confers vulnerability for multiple classes of drugs. Thus, therapeutics to specifically increase or stabilize hippocampal neurogenesis could aid in preventing initial addiction as well as future relapse.

NMDA receptors in the dorsal hippocampal area are involved in tramadol state-dependent memory of passive avoidance learning in mice

The precise neurobiological mechanisms of tramadol abuse underlying the cognitive function are still unknown. The aim of the present study was to examine the possible effects of intra-CA1 injections of N-methyl-d-aspartate (NMDA), a glutamate NMDA receptor (NMDAR) agonist, and d,l-2-amino-5-phosphonopentanoic acid (DL-AP5), a competitive NMDAR antagonist, on tramadol state-dependent memory. A single-trial step-down passive avoidance task was used for the assessment of memory retrieval in adult male NMRI mice. Post-training i.p. administration of an atypical μ-opioid receptor agonist, tramadol (2.5 and 5 mg/kg), dose-dependently induced impairment of memory retention. Pre-test injection of tramadol (2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under post-training administration of tramadol (5 mg/kg) influence. Pre-test intra-CA1 injection of NMDA (10−5 and 10−4 μg/mouse) 5 min before the administration of tramadol (5 mg/kg, i.p.) dose-dependently inhibited tramadol state-dependent memory. Pre-test intra-CA1 injection of DL-AP5 (0.25 and 0.5 μg/mouse) reversed the memory impairment induced by post-training administration of tramadol (5 mg/kg). Pre-test administration of DL-AP5 (0.25 and 0.5 μg/mouse) with an ineffective dose of tramadol (1.25 mg/kg) restored the retrieval and induced tramadol state-dependent memory. It can be concluded that dorsal hippocampal NMDAR mechanisms play an important role in the modulation of tramadol state-dependent memory.

Does Prenatal Methamphetamine Exposure Induce Sensitization to Drugs in Adulthood?

Behavioral sensitization is defined as augmented psychomotor activity, which can be observed after drug re-administration following withdrawal of repeated drug exposure. It has been shown that abuse of one drug can lead to increased sensitivity to certain other drugs. This effect of developed general drug sensitivity is called cross-sensitization and has been reported between drugs with similar as well as different mechanisms of action. There is growing evidence that exposure to drugs in utero not only causes birth defects and delays in infant development, but also impairs the neural reward pathways, in the brains of developing offspring, in such a way that it can increase the tendency for drug addiction later in life. This review summarizes the results of preclinical studies that focused on testing behavioral cross-sensitization, after prenatal methamphetamine exposure, to drugs administered in adulthood, with both similar and different mechanisms of action. Traditionally, behavioral sensitization has been examined using the Open field or the Laboras Test to record locomotor activity, and the Conditioned Place Preference and Self-administration test to examine drug-seeking behavior. However, it seems that prenatal drug exposure can sensitize animals not only to the locomotor-stimulating and conditioning effects of drugs, but may also be responsible for modified responses to various drug effects.

Mitochondrial impairments contribute to spatial learning and memory dysfunction induced by chronic tramadol administration in rat: Protective effect of physical exercise

Despite the worldwide use of tramadol, few studies have been conducted about its effects on memory and mitochondrial function, and controversial results have been reported. Recently, there has been an increasing interest in physical exercise as a protective approach to neuronal and cognitive impairments. Therefore, the aim of this study was to investigate the effects of physical exercise on spatial learning and memory and brain mitochondrial function in tramadol-treated rats. After completion of 2-week (short-term) and 4-week (long-term) treadmill exercise regimens, male Wistar rats received tramadol (20, 40, 80mg/kg/day) intraperitoneally for 30days. Then spatial learning and memory was assessed by Morris water maze test (MWM). Moreover, brain mitochondrial function was evaluated by determination of mitochondrial reactive oxygen species (ROS) level, mitochondrial membrane potential (MMP), mitochondrial swelling and cytochrome c release from mitochondria. Chronic administration of tramadol impaired spatial learning and memory as well as brain mitochondrial function as indicated by increased ROS level, MMP collapse, increased mitochondrial swelling and cytochrome c release from mitochondria. Conversely, treadmill exercise significantly attenuated the impairments of spatial learning and memory and brain mitochondrial dysfunction induced by tramadol. The results revealed that chronic tramadol treatment caused memory impairments through induction of brain mitochondrial dysfunction. Furthermore, pre-exposure to physical exercise markedly mitigated these impairments through its positive effects on brain mitochondrial function.

Sex differences in the strategies of spatial learning in prenatally-exposed rats treated with various drugs in adulthood

In the present study we investigated the sex differences in the effect of adult long-term drug treatment on cognitive functions of Wistar rats, which were prenatally exposed to MA (5mg/kg) or saline. Cognitive functions were tested as an ability of spatial learning in the Morris Water Maze (MWM), which consisted of three types of tests: "Place Navigation Test"; "Probe Test", and "Memory Recall Test". Adult animals were injected daily, after completion of the last trial, either with saline or cocaine (COC; 5mg/kg), MDMA (3,4-methylenedioxy-methamphetamine; 5mg/kg), morphine (MOR; 5mg/kg), or delta-9-tetrahydrocannabinol (THC; 2mg/kg). Results revealed worsened MWM performance in female rats after drug treatment in adulthood. Not only were traditionally investigated parameters affected by drug treatment (latency of platform acquisition, search strategy, distance traveled), but also strategies used by animals (thigmotaxis, scanning). Analyses of search strategies observed in the Place Navigation Test, as well as in the Memory Recall Test, demonstrated variations in the percentage of time spent in thigmotaxis and scanning in females after treatment with COC, MDMA, MOR, and THC. Although we did not see a sensitizing effect of prenatal MA, in some cases the effect of drug treatment in adulthood differed depending on the prenatal drug exposure. The data presented in this study demonstrates that exposure to drugs with various mechanisms of action alters spatial abilities of female rats in the MWM. Alterations in the effect of adult drug treatment with reference to prenatal drug exposure were also found in the present study.

Effects of environmental enrichment on behavioral and spatial cognitive deficits in morphine-dependent and -withdrawn rats

This study was designed to examine the effect of environmental enrichment during morphine dependence and withdrawal on morphine-induced behavioral and spatial cognitive disorders in morphine-withdrawn rats. Adult male Wistar rats (190 ± 20 g) were injected with bi-daily doses (10 mg/kg, 12 h intervals) of morphine for 14 days. Rats were reared in SE or EE during the development of dependence on morphine and withdrawal. Then, rats were tested for spatial learning and memory (the water maze), spontaneous withdrawal signs, and grooming behavior. We found that the EE blocked chronic morphine-induced partial impairments of spatial memory retention. Moreover, the EE diminished the occurrence of spontaneous morphine withdrawal signs as mild and the self-grooming behavior. Our findings showed that EE ameliorates chronic morphine-induced partial deficits of spatial cognition, obsessive-like behavior, and the overall severity of the morphine withdrawal. Thus, environmental enrichment may be a potential therapeutic strategy for spatial memory and behavioral deficits in morphine-dependent individuals.

The role of the ubiquitin proteasome system in the memory process

Quite intuitive is the notion that memory formation and consolidation is orchestrated by protein synthesis because of the synaptic plasticity necessary for those processes. Nevertheless, recent advances have begun accumulating evidences of a high requirement for protein degradation on the molecular mechanisms of the memory process in the mammalian brain. Because degradation determines protein half-life, degradation has been increasingly recognized as an important intracellular regulatory mechanism. The proteasome is the main player in the degradation of intracellular proteins. Proteasomal substrates are mainly degraded after a post-translational modification by a poly-ubiquitin chain. Latter process, namely poly-ubiquitination, is highly regulated at the step of the ubiquitin molecule transferring to the protein substrate mediated by a set of proteins whose genes represent almost 2% of the human genome. Understanding the role of polyubiquitin-mediated protein degradation has challenging researchers in many fields of investigation as a new source of targets for therapeutic intervention, e.g. E3 ligases that transfer ubiquitin moieties to the substrate. The goal of present work was to uncover mechanisms underlying memory processes regarding the role of the ubiquitin-proteasome system (UPS). For that purpose, preceded of a short review on UPS and memory processes a top-down systems biology approach was applied to establish central proteins involved in memory formation and consolidation highlighting their cross-talking with the UPS. According to that approach, the pattern of expression of several elements of the UPS were found overexpressed in regions of the brain involved in processing cortical inputs.

Glucocorticoid Homeostasis in the Dentate Gyrus Is Essential for Opiate Withdrawal-Associated Memories

Drug-withdrawal-associated aversive memories might trigger relapse to drug-seeking behavior. However, changes in structural and synaptic plasticity, as well as epigenetic mechanisms, which may be critical for long-term aversive memory, have yet to be elucidated. We used male Wistar rats and performed conditioned-place aversion (CPA) paradigm to uncover the role of glucocorticoids (GCs) on plasticity-related processes that occur within the dentate gyrus (DG) during opiate-withdrawal conditioning (memory formation-consolidation) and after reactivation by re-exposure to the conditioned environment (memory retrieval). Rats subjected to conditioned morphine-withdrawal robustly expressed CPA, while adrenalectomy impaired naloxone-induced CPA. Importantly, while activity-regulated cytoskeletal-associated protein (Arc) expression was induced in sham- and ADX-dependent animals during the conditioning phase, Arc and early growth response 1 (Egr-1) induction was restricted to sham-dependent rats following memory retrieval. Moreover, we found a correlation between Arc induction and CPA score, and Arc was selectively expressed in the granular zone of the DG in dopaminoceptive, glutamatergic and GABAergic neurons. We further found that brain-derived neurotrophic factor was regulated in the opposite way during the test phase. Our results also suggest a role for epigenetic regulation on the expression of glucocorticoid receptors and Arc following memory retrieval. Our data provide the first evidence that GC homeostasis is important for the expression of long-term morphine-withdrawal memories. Moreover, our results support the idea that targeting Arc and Egr-1 in the DG may provide important insights into the role of these signaling cascades in withdrawal-context memory re-consolidation. Together, disrupting these processes in the DG might lead to effective treatments in drug addiction thereby rapidly and persistently reducing invasive memories and subsequent drug seeking.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

Fatty acid amide hydrolase inhibitor URB597 prevented tolerance and cognitive deficits induced by chronic morphine administration in rats

Inhibitors of the endocannabinoid metabolic enzyme fatty acid amide hydrolase exert therapeutic effects, but might also be associated with some of the adverse effects of cannabis. However, at least one fatty acid amide hydrolase inhibitor, URB597, has beneficial effects without signs of abuse or dependence. Although previous investigations have evaluated URB597-morphine interactions, the effects of URB597 on morphine tolerance and cognition deficits have not been studied previously. Rats were rendered tolerant to or dependent on morphine by an injection of morphine (10 mg/kg, subcutaneous) twice daily, respectively, for 7 or 10 days. URB597 (1 mg/kg, intraperitoneal) was administered before morphine. The tail-flick and passive avoidance learning tests were used to evaluate tolerance and cognition. Chronic morphine injection led to significant tolerance to the antinociceptive effect on days 5 and 7. URB597 completely prevented the development of morphine tolerance. URB597 also enhanced memory acquisition in the passive avoidance learning test, and although morphine impaired memory, URB597 alleviated this effect. These data show that URB597 protects against tolerance and memory deficits in chronic usage of morphine and suggests URB597 as a promising candidate for the treatment of adverse effects of opioids.

Brain Reward Circuits in Morphine Addiction

Morphine is the most potent analgesic for chronic pain, but its clinical use has been limited by the opiate's innate tendency to produce tolerance, severe withdrawal symptoms and rewarding properties with a high risk of relapse. To understand the addictive properties of morphine, past studies have focused on relevant molecular and cellular changes in the brain, highlighting the functional roles of reward-related brain regions. Given the accumulated findings, a recent, emerging trend in morphine research is that of examining the dynamics of neuronal interactions in brain reward circuits under the influence of morphine action. In this review, we highlight recent findings on the roles of several reward circuits involved in morphine addiction based on pharmacological, molecular and physiological evidences.

Impaired hippocampal synaptic plasticity and NR2A/2B expression ratio in remifentanil withdrawal rats

Remifentanil is a kind of synthetic opioid which has gained wide clinical acceptance by anesthesiologists. In this study, we attempted to test whether withdrawal effects on learning mechanisms can be triggered by repeated low-dose remifentanil treatment. Male Sprague-Dawley (SD) rats were subjected to remifentanil (50μg/kgs.c.) twice per day at 12h intervals for 15 days. When the animals of remifentanil group were withdrawn from remifentanil at 10h after the last injection, changes in open field test, Morris water maze test (MWM) and synaptic efficacy were examined in each group. We demonstrated that repeated exposure to 50μg/kg remifentanil produced enhanced locomotor activity indicating that a remifentanil addiction animal model in rats was established. MWM results showed that exposure to remifentanil had no influence on the spatial cognition. After withdrawal of remifentanil rats showed impaired spatial cognition. In electrophysiology test, remifentanil group rats showed a trend for a rightward shift of input/output relationship and significant deficits in maintenance of STP and LTP. Immunohistochemistry results demonstrated increased NR2A/NR2B ratio that should be included depression of LTP. In the whole-cell patch-clamp recording, after elimination from remifentanil incubation, mEPSC frequency was down regulated in hippocampal CA1 neurons, indicating that basal synaptic transmission were affected by remifentanil withdrawal. Taken together, the current findings demonstrate that the remifentanil withdrawn rats exhibit obvious impairment of hippocampus-dependent memory and synaptic plasticity. Increased hippocampal NR2A/NR2B expression ratio and the changes of basal synaptic transmission may participate in the impairment of LTP.