Inhibition of the amygdala and hippocampal calcium/calmodulin-dependent protein kinase II attenuates the dependence and relapse to morphine differently in rats

Blockade of mGluR5 in nucleus accumbens modulates calcium sensor proteins, facilitates extinction, and attenuates reinstated morphine place preference in rats

Numerous findings confirm that the metabotropic glutamate receptors (mGluRs) are involved in the conditioned place preference (CPP) induced by morphine. Here we focused on the role of mGluR5 in the nucleus accumbens (NAc) as a main site of glutamate action on the rewarding effects of morphine. Firstly, we investigated the effects of intra-NAc administrating mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl) ethynyl) pyridine hydrochloride (MTEP; 1, 3, and 10 μg/μl saline) on the extinction and the reinstatement phase of morphine CPP. Moreover, to determine the downstream signaling cascades of mGluR5 in morphine CPP, the protein levels of stromal interaction molecules (STIM1 and 2) in the NAc and hippocampus (HPC) were measured by western blotting. The behavioral data indicated that the mGluR5 blockade by MTEP at the high doses of 3 and 10 μg facilitated the extinction of morphine-induced CPP and attenuated the reinstatement to morphine in extinguished rats. Molecular results showed that the morphine led to increased levels of STIM proteins in the HPC and increased the level of STIM1 without affecting STIM2 in the NAc. Furthermore, intra-NAc microinjection of MTEP (10 μg) in the reinstatement phase decreased STIM1 in the NAc and HPC and reduced the STIM2 in the HPC. Collectively, our data show that morphine could facilitate brain reward function in part by increasing glutamate-mediated transmission through activation of mGluR5 and modulation of STIM proteins. Therefore, these results highlight the therapeutic potential of mGluR5 antagonists in morphine use disorder.

In vivo reduction of hippocampal Caveolin-1 by RNA interference alters morphine addiction and neuroplasticity changes in male mice

Prescription opioids are powerful pain-controlling medications that have both benefits and potentially serious risks. Morphine is one of the preferred analgesics that are widely used to treat chronic pain. However, chronic morphine exposure has been found to cause both functional and structural changes in several brain regions, including the medial prefrontal cortex (mPFC), ventral tegmental area (VTA), and hippocampus (HPC), which lead to addictive behavior. Caveolin-1 (Cav-1), a scaffolding protein of membrane lipid rafts (MLRs), has been shown to organize GPCRs and multiple synaptic signaling proteins within the MLRs to regulate synaptic signaling and neuroplasticity. Previously, we showed that in vitro morphine treatment significantly elevates Cav-1 expression and causes neuroplasticity changes. In this study, we confirmed that chronic morphine exposure can significantly increase Cav-1 expression (P < 0.05) and microtubule-associated protein (MAP-2)-positive neuronal dendritic growth in the hippocampus. Moreover, the rewarding effect and dendritic growth in the HPC induced by chronic morphine exposure were significantly inhibited by hippocampal Cav-1 knockdown. Together, these data suggest that Cav-1 in the hippocampus plays an essential role in the neuroplasticity changes that underlie morphine addiction behaviors.

Silver nanoparticles (Ag-NPs) in the central amygdala protect the rat conditioned by morphine from withdrawal attack due to naloxone via high-level nitric oxide

Repeated injection of morphine during conditioned place preference (CPP) leads to spatial craving due to high-level nitric oxide (NO) in the central nucleus of amygdala (CeA). Silver nanoparticles (Ag-NPs) can produce oxygen-free radicals that lead to NO formation. We aimed to show the Ag-NPs protective effect on naloxone (NLX)-induced morphine withdrawal in the conditioned rats. Wistar rats (300-350 g) were implanted with cannulae in the CeA. After recovery, they were randomly divided into experimental and saline groups. CPP was conducted by three-phase unbiased program. Morphine (0.5-7.5 mg/kg) was injected subcutaneously (s.c.) once/per day during the conditioning phase. Naloxone (NLX) (0.05-0.4 μg/rat) was given, intra-CeA, 10 min before the CPP test. Ag-NPs (0.0001-0.01 μg/rat) were administered alone or prior to the NLX effective dose (0.4 μg/rat), intra-CeA. Conditioning score and withdrawal signs (wet dog shaking and scratching) were obtained and compared with saline group data. All rats' brains were collected in formalin 10% and after 48-72 h stained with NADPH-diaphorase, the NO marker. All data were analyzed by one-way or two-way ANOVA. Morphine (2.5-7.5 mg/kg, s.c.) induced a significant CPP vs. saline (1 mL/kg, s.c.). The single Ag-NPs had no significant effect, whereas the NLX caused meaningful WDS and scratching. However, the NLX pre-treatment in combination with Ag-NPs eliminated these signs. Furthermore, the NO level increased in the CeA. The Ag-NPs may protect the morphine-conditioned rats against the NLX-induced withdrawal symptoms due to high-level NO in the CeA.

The role of calcium-calmodulin-dependent protein kinase II in modulation of spatial memory in morphine sensitized rats

It has been shown that drug addiction and memory system are related but the signaling cascades underlying this interaction is not completely revealed yet. It has been demonstrated that binding of Calcium-calmodulin-dependent protein kinase II (CaMKII) to NMDA receptor is important in the memory process. The main objective of the study was to evaluate the role of CaMKII on the spatial memory of rats which previously were sensitized by morphine. The effect of CaMKII inhibitor (KN-93) on memory changes was investigated by hippocampal microinjection of KN-93 on the morphine-sensitized rats. Also, the role of the NMDA receptor in memory retention by KN-93 on the morphine sensitized rat was investigated with NMDA agonist and antagonist. Sensitization was induced by morphine injection (once daily for 3 days) followed by 5 days free of the drug before the trial phase. For the evaluation of spatial memory, the Morris Water Maze test (MWM) was used. Results showed that pre-trial administration of morphine, induced amnesia in MWM (p < 0.05). Also, three days pretreatment with morphine (20 mg/kg) followed by five days washout period, caused to enhance memory retrieval in confront with a pre-trial challenging dose of morphine (5 mg/kg). In addition, KN-93 administration during induction phase in morphine sensitization phenomena facilitated morphine-induced memory retention. In addition, inhibition of the NMDA receptor and KN-93 during the induction phase did not improve memory. However; intra-CA1 co-administration of KN-93 and NMDA during the induction phase of morphine sensitization resulted in improving spatial memory. It can be concluded that the effect of CaMKII on memory retention in morphine-sensitized rats depends on NMDA receptor.

Neuronal RNA-binding protein HuD regulates addiction-related gene expression and behavior

The neuronal RNA-binding protein HuD is involved in synaptic plasticity and learning and memory mechanisms. These effects are thought to be due to HuD-mediated stabilization and translation of target mRNAs associated with plasticity. To investigate the potential role of HuD in drug addiction, we first used bioinformatics prediction algorithms together with microarray analyses to search for specific genes and functional networks upregulated within the forebrain of HuD overexpressing mice (HuD_OE ). When this set was further limited to genes in the knowledgebase of addiction-related genes database (KARG) that contains predicted HuD-binding sites in their 3' untranslated regions (3'UTRs), we found that HuD regulates networks that have been associated with addiction-like behavior. These genes included Bdnf and Camk2a, 2 previously validated HuD targets. Since addiction is hypothesized to be a disorder stemming from altered gene expression causing aberrant plasticity, we sought to test the role of HuD in cocaine conditioned placed preference (CPP), a model of addiction-related behaviors. HuD mRNA and protein were upregulated by CPP within the nucleus accumbens of wild-type C57BL/6J mice. These changes were associated with increased expression of Bdnf and Camk2a mRNA and protein. To test this further, we trained HuD_OE and wild-type mice in CPP and found that HuD_OE mice showed increased cocaine CPP compared with controls. This was also associated with elevated expression of HuD target mRNAs and proteins, CaMKIIα and BDNF. These findings suggest HuD involvement in addiction-related behaviors such as cocaine conditioning and seeking, through increased plasticity-related gene expression.

Inhibition of the lateral habenular CaMKⅡ abolishes naloxone-precipitated conditioned place aversion in morphine-dependent mice

Addictive substances mediate positive and negative states promoting compulsive drug use. However, substrates for aversive effects of drugs are not fully understood. We found that inactivation of the lateral habenula (LHb) by microinjection of tetrodotoxin (TTX) abolished naloxone-precipitated conditioned place aversion (CPA) in morphine-dependent mice. We also found that lateral habenular administration of KN-62, a specific inhibitor for calcium/calmodulin dependent protein kinase II (CaMKII), abolished naloxone-precipitated CPA in morphine-dependent mice. Furthermore, we found chronic morphine treatment induced overexpression of CaMKII in the LHb. In conclusion, our results suggest that the increased expression of CaMKII in the LHb is instrumental for morphine-driven aversive behaviors.

RACK1 promotes maintenance of morphine-associated memory via activation of an ERK-CREB dependent pathway in hippocampus

Existence of long-term drug-associated memories may be a crucial factor in drug cravings and relapse. RACK1 plays a critical role in morphine-induced reward. In the present study, we used conditioned place preference (CPP) to assess the acquisition and maintenance of morphine conditioned place preference memory. The hippocampal protein level of RACK1 and synaptic quantitation were evaluated by Western blotting, immunohistochemistry and electron microscopy, respectively. Additionally, shRACK1 (shGnb2l1) was used to silence RACK1 in vivo to evaluate the role and the underlying mechanism of RACK1 in maintenance of morphine CPP memory. We found that morphine induced CPP was maintained for at least 7 days after the last morphine treatment, which indicated a positive correlation with hippocampal RACK1 level, and was accompanied simultaneously by increases in the synapse density and hippocampal expression of synaptophysin (SYP), phosphorylation of extracellular signal-regulated kinase1/2 (pERK1/2) and the phosphorylation of cyclic adenosine monophosphate response element-binding (pCREB). ShGnb2l1 icv injection significantly suppressed the expression of all above proteins, decreased the synapse density in the hippocampus and attenuated the acquisition and maintenance of morphine CPP. Our present study highlights that RACK1 plays an important role in the maintenance of morphine CPP, likely via activation of ERK-CREB pathway in hippocampus.

Opiate Exposure State Controls a D2-CaMKIIα-Dependent Memory Switch in the Amygdala-Prefrontal Cortical Circuit

The mammalian basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) comprise a functionally interconnected circuit that is critical for processing opiate-related associative memories. In the opiate-naïve state, reward memory formation in the BLA involves a functional link between dopamine (DA) D1 receptor (D1R) and extracellular signal-related kinase 1/2 (ERK1/2) signaling substrates, but switches to a DA D2 (D2R)/Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα)-dependent memory substrate following chronic opiate exposure and spontaneous withdrawal. Using conditioned place preference (CPP) in rats paired with molecular analyses, we examined the role of intra-mPFC CaMKII, ERK and DAergic activity during the formation of opiate associative memories, and how opiate exposure state may regulate the functions of these molecular memory pathways. We report that the role of CaMKIIα signaling is functionally reversed within the BLA-mPFC pathway depending on opiate exposure state. Thus, in the opiate-naïve state, intra-mPFC but not intra-BLA blockade of CaMKII signaling prevents formation of opiate reward memory. However, following chronic opiate exposure and spontaneous withdrawal, the role of CaMKII signaling in the BLA-mPFC is functionally reversed. This behavioral memory switch corresponds to a selective increase in the expression of D2R and CaMKIIα, but not other calcium/calmodulin-related molecules, nor D1R expression levels within the mPFC.

CaM Kinases: From Memories to Addiction

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

Molecular and neuronal plasticity mechanisms in the amygdala-prefrontal cortical circuit: implications for opiate addiction memory formation

The persistence of associative memories linked to the rewarding properties of drugs of abuse is a core underlying feature of the addiction process. Opiate class drugs in particular, possess potent euphorigenic effects which, when linked to environmental cues, can produce drug-related "trigger" memories that may persist for lengthy periods of time, even during abstinence, in both humans, and other animals. Furthermore, the transitional switch from the drug-naïve, non-dependent state to states of dependence and withdrawal, represents a critical boundary between distinct neuronal and molecular substrates associated with opiate-reward memory formation. Identifying the functional molecular and neuronal mechanisms related to the acquisition, consolidation, recall, and extinction phases of opiate-related reward memories is critical for understanding, and potentially reversing, addiction-related memory plasticity characteristic of compulsive drug-seeking behaviors. The mammalian prefrontal cortex (PFC) and basolateral nucleus of the amygdala (BLA) share important functional and anatomical connections that are involved importantly in the processing of associative memories linked to drug reward. In addition, both regions share interconnections with the mesolimbic pathway's ventral tegmental area (VTA) and nucleus accumbens (NAc) and can modulate dopamine (DA) transmission and neuronal activity associated with drug-related DAergic signaling dynamics. In this review, we will summarize research from both human and animal modeling studies highlighting the importance of neuronal and molecular plasticity mechanisms within this circuitry during critical phases of opiate addiction-related learning and memory processing. Specifically, we will focus on two molecular signaling pathways known to be involved in both drug-related neuroadaptations and in memory-related plasticity mechanisms; the extracellular-signal-regulated kinase system (ERK) and the Ca(2+)/calmodulin-dependent protein kinases (CaMK). Evidence will be reviewed that points to the importance of critical molecular memory switches within the mammalian brain that might mediate the neuropathological adaptations resulting from chronic opiate exposure, dependence, and withdrawal.

Imbalance of HCN1 and HCN2 expression in hippocampal CA1 area impairs spatial learning and memory in rats with chronic morphine exposure

The hyperpolarization-activated cyclic-nucleotide-gated non-selective cation (HCN) channels play a vital role in the neurological basis underlying nervous system diseases. However, the role of HCN channels in drug addiction is not fully understood. In the present study, we investigated the expression of HCN1 and HCN2 subunits in hippocampal CA1 and the potential molecular mechanisms underlying the modulation of HCN channels in rats with chronic morphine exposure with approaches of electrophysiology, water maze, and Western blotting. We found that chronic morphine exposure (5 mg/kg, sc, for 7 days) caused an inhibition of long-term potentiation (LTP) and impairment of spatial learning and memory, which is associated with a decrease in HCN1, and an increase in HCN2 on cell membrane of hippocampal CA1 area. Additional experiments showed that the imbalance of cell membrane HCN1 and HCN2 expression under chronic morphine exposure was related to an increase in expression of TPR containing Rab8b interacting protein (TRIP8b) (1a-4) and TRIP8b (1b-2), and phosphorylation of protein kinase A (PKA) and adaptor protein 2 μ2 (AP2 μ2). Our results demonstrate the novel information that drug addiction-induced impairment of learning and memory is involved in the imbalance of HCN1 and HCN2 subunits, which is mediated by activation of TRIP8b (1a-4), TRIP8b (1b-2), PKA and AP2 μ2.

Neuropeptide trefoil factor 3 attenuates naloxone-precipitated withdrawal in morphine-dependent mice

RATIONALE

The persistence of physical dependence and craving in addicts is considered to contribute to relapse. Increasing evidence indicates that neuropeptide systems are associated with several phases of drug addiction, but little is known about whether the neuropeptide trefoil factor affects withdrawal symptoms.

OBJECTIVES

This study aims to investigate the potential effects of the neuropeptide trefoil factor 3 (TFF3) on naloxone-precipitated withdrawal symptoms in morphine-dependent mice.

RESULTS

Mice received increasing doses of morphine over 3 days. On day 4, the mice were injected with TFF3 (1.0 mg/kg, i.p.) 30 min after the last dose of morphine. Thirty minutes after TFF3 treatment, naloxone (1 mg/kg, i.p.) was injected, and body weight, jumping behavior, wet-dog shakes, and locomotor activity were assessed 30 min later. Naloxone caused significant weight loss and increased jumping behavior and wet-dog shakes in morphine-dependent mice. TFF3 (1.0 mg/kg) reversed these behavioral symptoms caused by morphine withdrawal, suggesting that TFF3 might ameliorate physical dependence associated with opiate addiction. Furthermore, TFF3 pretreatment significantly reduced morphine withdrawal-induced increases in plasma corticosterone and adrenocorticotropic hormone levels. The glucocorticoid receptor agonist RU486 blocked the behavioral effects of TFF3 on morphine withdrawal symptoms. Finally, Fos expression in the medial prefrontal cortex which was decreased during morphine withdrawal was increased by TFF3 pretreatment.

CONCLUSION

These findings indicate that TFF3 might be a potential therapeutic candidate for opiate addiction by regulating glucocorticoid secretion and neuronal activation in the prefrontal cortex.

Opiate exposure and withdrawal induces a molecular memory switch in the basolateral amygdala between ERK1/2 and CaMKIIα-dependent signaling substrates

Opiate reward memories are powerful triggers for compulsive opiate-seeking behaviors. The basolateral amygdala (BLA) is an important structure for the processing of opiate-related associative memories and is functionally linked to the mesolimbic dopamine (DA) pathway. Transmission through intra-BLA DA D1-like and D2-like receptors independently modulates the formation of opiate reward memories as a function of opiate-exposure state. Thus, in the opiate-naive state, intra-BLA D1 transmission is required for opiate-related memory formation. Once opiate dependence and withdrawal has developed, a functional switch to a DA D2-mediated memory mechanism takes place. However, the downstream molecular signaling events that control this functional switch between intra-BLA DA D1 versus D2 receptor transmission are not currently understood. Using an unbiased place conditioning procedure in rats combined with molecular analyses, we report that opiate reward memory acquisition requires intra-BLA ERK1/2 signaling only in the previously opiate-naive state. However, following chronic opiate exposure and withdrawal, intra-BLA reward memory processing switches to a CaMKIIα-dependent memory substrate. Furthermore, the ability of intra-BLA DA D1 or D2 receptor transmission to modulate the motivational salience of opiates similarly operates through a D1-mediated ERK-dependent mechanism in the opiate-naive state, but switches to a D2-mediated CaMKIIα-dependent mechanism in the dependent/withdrawn state. Protein analysis of BLA tissue revealed a downregulation of ERK1/2 phosphorylation and a dramatic reduction in both total and phosphorylated CaMKIIα signaling, specifically in the opiate-dependent/withdrawn state, demonstrating functional control of ERK1/2-dependent versus CaMKIIα-dependent memory mechanisms within the BLA, controlled by opiate-exposure state.

Role of amygdala in drug memory

Drug addiction is a chronic brain disorder with the hallmark of a high rate of relapse to compulsive drug seeking and drug taking even after long-term abstinence. Addiction has been considered as an aberrant memory that has been termed "addiction memory." Drug-related memory plays a critical role in the maintenance of learned addictive behaviors and emergence of relapse. Disrupting these long-lasting memories by administering amnestic agents or other manipulations during specific phases of drug memory is a promising strategy for relapse prevention. Recent studies on the processes of drug addiction and relapse have demonstrated that the amygdala is involved in associative drug addiction learning processes. In this review, we focus on preclinical studies that used conditioned place preference and self-administration models to investigate the differential roles of the amygdala in each phase of drug-related memory, including acquisition, consolidation, retrieval, reconsolidation, and extinction. These studies indicate that the amygdala plays a critical role in both cue-associative learning and the expression of cue-induced relapse to drug-seeking behavior.

Posttranslation modification of G protein-coupled receptor in relationship to biased agonism

Biased signaling has been reported with a series of G protein-coupled receptors (GPCRs), including β(2)-adrenergic receptor and μ-opioid receptor (OPRM1). The concept of biased signaling suggests that the agonists of one particular receptor may activate the downstream signaling pathways with different efficacies. Thus in an extreme case, agonists might activate different sets of signaling pathways, which provide a new route to develop drugs with increased efficacies and decreased side effects. Among the many factors, posttranslation modifications of receptor proteins have major roles in influencing the biased signaling. Take OPRM1, for example, the phosphorylation and palmitoylation of receptor can regulate the biased signaling induced by agonists. Thus, by modulating these posttranslation modifications, the biased signaling of GPCRs can be regulated. In addition, although it is not considered as posttranslation modification normally, the distribution of GPCRs on cell membrane, especially the distribution between lipid-raft and non-raft microdomains, also contributes to the biased signaling. Thus in this chapter, we described the methods used in our laboratory to study receptor phosphorylation, receptor palmitoylation, and membrane distribution of receptor by using OPRM1 as a model. A functional model was also provided on these posttranslational modifications at the last section of this chapter.

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

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

Differential modulation of drug-induced structural and functional plasticity of dendritic spines

Drug-induced plasticity of excitatory synapses has been proposed to be the cellular mechanism underlying the aberrant learning associated with addiction. Exposure to various drugs of abuse causes both morphological plasticity of dendritic spines and functional plasticity of excitatory synaptic transmission. Chronic activation of μ-opioid receptors (MOR) in cultured hippocampal neurons causes two forms of synaptic plasticity: loss of dendritic spines and loss of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. With use of live imaging, patch-clamp electrophysiology, and immunocytochemistry, the present study reveals that these two forms of synaptic plasticity are mediated by separate, but interactive, intracellular signaling cascades. The inhibition of Ca(2+)/calmodulin-dependent protein kinase II with 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62) blocks MOR-mediated structural plasticity of dendritic spines, but not MOR-mediated cellular redistribution of GluR1 and GluR2 AMPA receptor subunits. In contrast, the inhibition of calcineurin with tacrolimus (FK506) blocks both cellular processes. These findings support the idea that drug-induced structural and functional plasticity of dendritic spines is mediated by divergent, but interactive, signaling pathways.

Inhibition of CaMKII activity in the nucleus accumbens shell blocks the reinstatement of morphine-seeking behavior in rats

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) may be a core component in the common molecular pathways for drug addiction. Moreover, studies using animal models of drug addiction have demonstrated that changing CaMKII activity or expression influences animals' responses to the drugs of abuse. Here, we explored the roles of CaMKII in the nucleus accumbens (NAc) shell in the extinction and reinstatement of morphine-seeking behavior. Rats were trained to obtain intravenous morphine infusions through poking hole on a fixed-ratio one schedule. Selective CaMKII inhibitor myristoylated autocamtide-2-inhibitory peptide (myr-AIP) was injected into the NAc shell of rats after the acquisition of morphine self-administration (SA) or before the reinstatement test. The results demonstrated that injection of myr-AIP after acquisition of morphine SA did not influence morphine-seeking in the following extinction days and the number of days spent for reaching extinction criterion. However, pretreatment with myr-AIP before the reinstatement test blocked the reinstatement of morphine-seeking behavior induced by morphine-priming. Our results strongly indicate that CaMKII activity in the NAc shell is essential to the relapse to morphine-seeking.

Development and persistence of methamphetamine-conditioned hyperactivity in Swiss-Webster mice

These experiments examined the development and persistence of methamphetamine-conditioned hyperactivity in Swiss-Webster mice. Experiments 1 and 2 examined the development of conditioned hyperactivity, varying the methamphetamine dose (0.25-2.0 mg/kg), the temporal injection parameters (continuous; experiment 1 or intermittent; experiment 2), and the comparison control group (saline; experiment 1 or unpaired; experiment 2). Experiment 3 examined the persistence of methamphetamine-conditioned hyperactivity by comparing mice 1 (immediate) or 28 (delay) days after drug withdrawal. In each experiment, several behavioral measures (vertical counts, distance traveled, and velocity) were recorded and temporal analyses conducted to assess methamphetamine-conditioned hyperactivity. In experiments 1 and 2, it was found that methamphetamine-conditioned hyperactivity was (i) dose-dependent, (ii) detected early in the session, and (iii) detected by a behavioral measure indicative of general activity (i.e. distance traveled), and (iv) varied as a function of the number of conditioning sessions. In experiment 3, it was found that conditioned hyperactivity persisted for 28 days, though was weakened by nonassociative factors, following methamphetamine withdrawal. Collectively, these results suggest that conditioned hyperactivity to methamphetamine is robust and persists after prolonged periods of drug withdrawal in mice. Furthermore, these results are consistent with an excitatory classical conditioning interpretation of conditioned hyperactivity.

Cannabinoid CB1 receptor antagonist rimonabant disrupts nicotine reward-associated memory in rats

Exposure to cues previously associated with drug intake leads to relapse by activating previously acquired memories. Based on previous findings, in which cannabinoid CB(1) receptors were found to be critically involved in specific aspects of learning and memory, we investigated the role of CB(1) receptors in nicotine reward memory using a rat conditioned place preference (CPP) model. In Experiment 1, rats were trained for CPP with alternating injections of nicotine (0.5mg/kg, s.c.) and saline to acquire the nicotine-conditioned memory. To examine the effects of rimonabant on the reconsolidation of nicotine reward memory, rats were administered rimonabant (0, 0.3, and 3.0mg/kg, i.p.) immediately after reexposure to the drug-paired context. In Experiment 2, rats were trained for CPP similarly to Experiment 1. To examine the effects of rimonabant on the reinstatement of nicotine reward memory, rimonabant (0, 0.3, and 3.0mg/kg, i.p.) was administered before the test of nicotine-induced CPP reinstatement. In Experiment 3, to evaluate whether rimonabant itself produces a reward memory, rats were trained for CPP with alternating injections of different doses of rimonabant (0, 0.3, and 3.0mg/kg) and saline. Rimonabant at a dose of 3.0mg/kg significantly disrupted the reconsolidation of nicotine memory and significantly blocked the reinstatement of nicotine-induced CPP. However, rimonabant itself did not produce CPP. These findings provide clear evidence that CB(1) receptors play a role in nicotine reward memory, suggesting that CB(1) receptor antagonists may be a potential target for managing nicotine addiction.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Mu opioid receptor activation normalizes temporo-ammonic pathway driven inhibition in hippocampal CA1

The hippocampus of the mammalian brain is important for the formation of long-term memories. Hippocampal-dependent learning can be affected by a number of neurotransmitters including the activation of μ-opioid receptors (MOR). It has been shown that MOR activation can alter synaptic plasticity and network oscillations in the hippocampus, both of which are thought to be important for the encoding of information and formation of memories. One hippocampal oscillation that has been correlated with learning and memory formation is the 4-10 Hz theta rhythm. During theta rhythms, inputs to hippocampal CA1 from CA3 (Schaffer collaterals, SC) and the entorhinal cortex (perforant path) can integrate at different times within an individual theta cycle. Consequently, when excitatory inputs in the stratum lacunosum-moleculare (the temporo-ammonic pathway (TA), which includes the perforant path) are stimulated approximately one theta period before SC inputs, the TA can indirectly inhibit SC inputs. This inhibition is due to the activation of postsynaptic GABA(B) receptors on CA1 pyramidal neurons. Importantly, MOR activation has been shown to suppress GABA(B) inhibitory postsynaptic potentials in CA1 pyramidal neurons. Therefore, we examined how MOR activation affects the integration between TA inputs and SC inputs in hippocampal CA1. To do this we used voltage-sensitive dye imaging and whole cell patch clamping from acute hippocampal slices taken from young adult rats. Here we show that MOR activation has no effect on the integration between TA and SC inputs when activation of the TA precedes SC by less than one half of a theta cycle (<75 ms). However, MOR activation completely blocked the inhibitory action of TA on SC inputs when TA stimulation occurred approximately one theta cycle before SC activation (>150 ms). This MOR suppression of TA driven inhibition occurred in both the SC input layer of hippocampal CA1 (stratum radiatum) and the output layer of CA1 pyramidal neurons (stratum pyramidale). Thus MOR activation can have profound effects on the temporal integration between two primary excitatory pathways to hippocampal CA1 and subsequently the resultant output from CA1 pyramidal neurons. These data provide important information for understanding how acute or chronic MOR activation may affect the integration of activity within hippocampal CA1 during theta rhythm.

Changes in ensemble activity of hippocampus CA1 neurons induced by chronic morphine administration in freely behaving mice

The hippocampus plays an important role in the formation of new memories and spatial navigation. Recently, growing evidence supports the view that it is also involved in addiction to opiates and other drugs. Theoretical and experimental studies suggest that hippocampal neural-network oscillations at specific frequencies and unit firing patterns reflect information of learning and memory encoding. Here, using multichannel recordings from the hippocampal CA1 area in behaving mice, we investigated the phase correlations between the theta (4-10 Hz) and gamma (40-100 Hz) oscillations, and the timing of spikes modulated by these oscillations. Local field potentials and single unit recordings in the CA1 area of mice receiving chronic morphine treatment revealed that the power of the theta rhythm was strongly increased; at the same time, the theta frequency during different behavioral states shifted markedly, and the characteristic coupling of theta and gamma oscillations was altered. Surprisingly, though the gamma oscillation frequency changed, the power of gamma lacking theta did not. Moreover, the timing of pyramidal cell spikes relative to the theta rhythm and the timing of interneuron spikes relative to the gamma rhythm changed during chronic morphine administration. Furthermore, these responses were impaired by a selective D1/D5 receptor antagonist intra-hippocampus injection. These results indicate that chronic morphine administration induced the changes of ensemble activity in the CA1 area, and these changes were dependent on local dopamine receptor activation.

Mu-opioid receptors transiently activate the Akt-nNOS pathway to produce sustained potentiation of PKC-mediated NMDAR-CaMKII signaling

BACKGROUND

In periaqueductal grey (PAG) matter, cross-talk between the Mu-opioid receptor (MOR) and the glutamate N-methyl-D-Aspartate receptor (NMDAR)-CaMKII pathway supports the development of analgesic tolerance to morphine. In neurons, histidine triad nucleotide binding protein 1 (HINT1) connects the regulators of G protein signaling RGSZ1 and RGSZ2 to the C terminus of the MOR. In response to morphine, this HINT1-RGSZ complex binds PKCgamma, and afterwards, the interplay between PKCgamma, Src and Gz/Gi proteins leads to sustained potentiation of NMDAR-mediated glutamate responses.

METHODOLOGY/PRINCIPAL FINDINGS

Following an intracerebroventricular (icv) injection of 10 nmol morphine, Akt was recruited to the synaptosomal membrane and activated by Thr308 and Ser473 phosphorylation. The Akt activation was immediately transferred to neural Nitric Oxide Synthase (nNOS) Ser1417. Afterwards, nitric oxide (NO)-released zinc ions recruited PKCgamma to the MOR to promote the Src-mediated phosphorylation of the Tyr1325 NMDAR2A subunit. This action increased NMDAR calcium flux and CaMKII was activated in a calcium-calmodulin dependent manner. CaMKII then acted on nNOS Ser847 to produce a sustained reduction in NO levels. The activation of the Akt-nNOS pathway was also reduced by the binding of these proteins to the MOR-HINT1 complex where they remained inactive. Tolerance to acute morphine developed as a result of phosphorylation of MOR cytosolic residues, uncoupling from the regulated G proteins which are transferred to RGSZ2 proteins. The diminished effect of morphine was prevented by LNNA, an inhibitor of nNOS function, and naltrindole, a delta-opioid receptor antagonist that also inhibits Akt.

CONCLUSIONS/SIGNIFICANCE

Analysis of the regulatory phosphorylation of the proteins included in the study indicated that morphine produces a transient activation of the Akt/PKB-nNOS pathway. This activation occurs upstream of PKCgamma and Src mediated potentiation of NMDAR activity, ultimately leading to morphine tolerance. In summary, the Akt-nNOS pathway acts as a primer for morphine-triggered events which leads to the sustained potentiation of the NMDAR-CaMKII pathway and MOR inhibition.

Spinal matrix metalloproteinase-9 contributes to physical dependence on morphine in mice

Preventing and reversing opioid dependence continues to be a clinical challenge and underlying mechanisms of opioid actions remain elusive. We report that matrix metalloproteinase-9 (MMP-9) in the spinal cord contributes to development of physical dependence on morphine. Chronic morphine exposure and naloxone-precipitated withdrawal increase activity of spinal MMP-9. Spinal inhibition or targeted mutation of MMP-9 suppresses behavioral signs of morphine withdrawal and the associated neurochemical alterations. The increased MMP-9 activity is mainly distributed in the superficial dorsal horn and colocalized primarily with neurons and small numbers of astrocytes and microglia. Morphine exposure and withdrawal increase phosphorylation of NR1 and NR2B receptors, ERK1/2, calmodulin-dependent kinase II, and cAMP response element binding proteins; and such phosphorylation is suppressed by either spinal inhibition or targeted mutation of MMP-9. Further, spinal administration of exogenous MMP-9 induces morphine withdrawal-like behavioral signs and mechanical allodynia, activates NR1 and NR2 receptors, and downregulates integrin-beta1, while a function-neutralizing antibody against integrin-beta1 suppresses MMP-9-induced phosphorylation of NR1 and NR2B. Morphine withdrawal-induced MMP-9 activity is also reduced by an nNOS inhibitor. Thus, we hypothesize that spinal MMP-9 may contribute to the development of morphine dependence primarily through neuronal activation and interaction with NR1 and NR2B receptors via integrin-beta1 and NO pathways. The other gelatinase, MMP-2, is not involved in morphine dependence. Inhibiting spinal MMP-9 or MMP-2 reduces chronic and/or acute morphine tolerance. This study suggests a novel therapeutic approach for preventing, minimizing, or reversing opioid dependence and tolerance.

Chronic morphine treatment impaired hippocampal long-term potentiation and spatial memory via accumulation of extracellular adenosine acting on adenosine A1 receptors

Chronic exposure to opiates impairs hippocampal long-term potentiation (LTP) and spatial memory, but the underlying mechanisms remain to be elucidated. Given the well known effects of adenosine, an important neuromodulator, on hippocampal neuronal excitability and synaptic plasticity, we investigated the potential effect of changes in adenosine concentrations on chronic morphine treatment-induced impairment of hippocampal CA1 LTP and spatial memory. We found that chronic treatment in mice with either increasing doses (20-100 mg/kg) of morphine for 7 d or equal daily dose (20 mg/kg) of morphine for 12 d led to a significant increase of hippocampal extracellular adenosine concentrations. Importantly, we found that accumulated adenosine contributed to the inhibition of the hippocampal CA1 LTP and impairment of spatial memory retrieval measured in the Morris water maze. Adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine significantly reversed chronic morphine-induced impairment of hippocampal CA1 LTP and spatial memory. Likewise, adenosine deaminase, which converts adenosine into the inactive metabolite inosine, restored impaired hippocampal CA1 LTP. We further found that adenosine accumulation was attributable to the alteration of adenosine uptake but not adenosine metabolisms. Bidirectional nucleoside transporters (ENT2) appeared to play a key role in the reduction of adenosine uptake. Changes in PKC-alpha/beta activity were correlated with the attenuation of the ENT2 function in the short-term (2 h) but not in the long-term (7 d) period after the termination of morphine treatment. This study reveals a potential mechanism by which chronic exposure to morphine leads to impairment of both hippocampal LTP and spatial memory.

Nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala are involved in ethanol-induced conditioned place preference

The purpose of this study was to evaluate whether nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala (BLA) can potentiate ethanol response in the conditioned place preference (CPP) paradigm. I.p. administration of different doses of ethanol (0.25-1 g/kg) did not induce CPP. However, the higher dose of the drug (1.5 g/kg i.p.) induced place aversion. Furthermore, microinjection of nicotine (0.5-1 microg/rat) into both CA1 regions (intra-CA1) and the BLA (intra-BLA) did not produce a significant CPP. Interestingly, intra-CA1 or -BLA administration of nicotine plus ethanol (0.5 g/kg) during conditioning phase significantly induced a strong CPP. Microinjection of mecamylamine, the nicotinic acetylcholine receptor antagonist, into the CA1 regions or into the BLA did not alter CPP. However, intra-CA1 or -BLA microinjection of mecamylamine (1-4 microg/rat) reversed the response induced by the microinjection of nicotine (1 microg/rat, intra-CA1 or -BLA) plus ethanol (0.5 g/kg i.p.) in the CPP paradigm. On the other hand, the microinjection of nicotine (0.5-1.5 microg/rat) into the BLA, but not into the CA1 regions before the testing phase potentiated the response of ethanol on the expression of conditioned place preference. Moreover, intra-CA1 administration of nicotine plus ethanol increased the locomotor activity on the test day which was reversed by pretreatment with mecamylamine, while other treatments had no effect on locomotor activity. It can be concluded that the activation of nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala can potentiate the ethanol response in the CPP paradigm.

Chronic morphine administration induces over-expression of aldolase C with reduction of CREB phosphorylation in the mouse hippocampus

In recent studies, alterations in the activity and expression of metabolic enzymes, such as those involved in glycolysis, have been detected in morphine-dependent patients and animals. Increasing evidence demonstrates that the hippocampus is an important brain region associated with morphine dependence, but the molecular events occurring in the hippocampus following chronic exposure to morphine are poorly understood. Aldolase C is the brain-specific isoform of fructose-1, 6-bisphosphate aldolase which is a glycolytic enzyme catalyzing reactions in the glycolytic, gluconeogenic, and fructose metabolic pathways. Using Western blot and immunofluorescence assays, we found the expression of aldolase C was markedly increased in the mouse hippocampus following chronic morphine treatment. Naloxone pretreatment before morphine administration suppressed withdrawal jumping, weight loss, and overexpression of aldolase C. CREB is a transcription factor regulated through phosphorylation on Ser133, which is known to play a key role in the mechanism of morphine dependence. When detecting the expression of phosphorylated CREB (p-CREB) in the mouse hippocampus using Western blot and immunohistochemistry, we found CREB phosphorylation was clearly decreased following chronic morphine treatment. Interestingly, laser-confocal microscopy showed that overexpression of aldolase C in mouse hippocampal neurons was concomitant with the decreased immunoreactivity of p-CREB. The results suggest potential links between the morphine-induced alteration of aldolase C and the regulation of CREB phosphorylation, a possible mechanism of morphine dependence.

Targeted mutation of EphB1 receptor prevents development of neuropathic hyperalgesia and physical dependence on morphine in mice

EphB receptor tyrosine kinases, which play important roles in synaptic connection and plasticity during development and in matured nervous system, have recently been implicated in processing of pain after nerve injury and morphine dependence. Subtypes of the EphB receptors that may contribute to the neuropathic pain and morphine dependence have not been identified. Here we demonstrate that the subtype EphB1 receptor is necessary for development of neuropathic pain and physical dependence on morphine. The results showed that peripheral nerve injury produced thermal hyperalgesia in wild-type (EphB1+/+) control littermate mice, but not in EphB1 receptor homozygous knockout (EphB1-/-) and heterozygous knockdown (EphB1+/-) mice. Hyperalgesia in the wild-type mice was inhibited by intrathecal administration of an EphB receptor blocking reagent EphB2-Fc (2 microg). Intrathecal administration of an EphB receptor activator ephrinB1-Fc (1 microg) evoked thermal hyperalgesia in EphB1+/+, but not EphB1-/- and EphB1+/- mice. Cellularly, nerve injury-induced hyperexcitability of the medium-sized dorsal root ganglion neurons was prevented in EphB1-/- and EphB1+/- mice. In chronically morphine-treated mice, most of the behavioral signs and the overall score of naloxone-precipitated withdrawal were largely diminished in EphB1-/- mice compared to those in the wild-type. These findings indicate that the EphB1 receptor is necessary for development of neuropathic pain and physical dependence on morphine and suggest that the EphB1 receptor is a potential target for preventing, minimizing, or reversing the development of neuropathic pain and opiate dependence.

Roles of hippocampal NMDA receptors and nucleus accumbens D1 receptors in the amphetamine-produced conditioned place preference in rats

There are glutamatergic projections from the hippocampus to the nucleus accumbens (NAc), which regulate DA transmission in this structure. To be precise, the ventral hippocampal (VH) glutamatergic neurons project to the nucleus accumbens shell region (NAcSh), whereas the dorsal hippocampus (DH) sends glutamatergic projections to the nucleus accumbens core region (NAcC). This study investigates the roles of hippocampal N-methyl-D-aspartate (NMDA) glutamate receptors and NAc type 1 dopamine receptor (D1) in amphetamine-produced conditioned place preference (AMPH-CPP) in rats. Our earlier reports showed that AMPH-CPP results in the enhancement of hippocampal CaMKII activity and it can be impaired by NMDA antagonist (AP5). In this study AMPH-CPP did not alter the NAc CaMKII activity, although AMPH-CPP was impaired by a blockade of D1 receptors (SCH23390) during conditioning. Moreover, inactivation of hippocampal area (dorsal hippocampus or ventral hippocampus) impaired AMPH-CPP, but its effect was diminished by the activation of D1 receptors in accumbal region (NAc core or NAc shell). By inactivating both DH and NAc core resulted in the disruption of rat's CPP expression. However, the impaired CPP expression was recovered during the next testing session, suggesting the disruption of CPP expression was a short term effect. Moreover, the disruption of CPP expression was not exhibited if NAc core was not inactivated. Interestingly, the rats that received activation in VH but an inactivation in NAc shell before testing show impaired CPP expression compared to those received inactivation in both VH and NAc shell. DH activation plus an inactivation in NAc core before testing show a significantly higher rate of the weakening of AMPH-CPP expression. Similarly, an activation of VH plus an inactivation of NAc shell before testing also show a statistically significant lower CPP score on tests 3 and 4. These results, taken together, indicate that NMDA receptor activation in DH and VH have different enhancing effects on the AMPH-CPP as their innervations onto the different NAc regions are essential for AMPH-CPP establishment. If the deterioration of AMPH-CPP expression (or extinction process) resembles the formation of new learning, then this active process might have been facilitated by the hippocampal NMDA receptor activations during testing.

Protein kinases and addiction

Although drugs of abuse have different chemical structures and interact with different protein targets, all appear to usurp common neuronal systems that regulate reward and motivation. Addiction is a complex disease that is thought to involve drug-induced changes in synaptic plasticity due to alterations in cell signaling, gene transcription, and protein synthesis. Recent evidence suggests that drugs of abuse interact with and change a common network of signaling pathways that include a subset of specific protein kinases. The best studied of these kinases are reviewed here and include extracellular signal-regulated kinase, cAMP-dependent protein kinase, cyclin-dependent protein kinase 5, protein kinase C, calcium/calmodulin-dependent protein kinase II, and Fyn tyrosine kinase. These kinases have been implicated in various aspects of drug addiction including acute drug effects, drug self-administration, withdrawal, reinforcement, sensitization, and tolerance. Identifying protein kinase substrates and signaling pathways that contribute to the addicted state may provide novel approaches for new pharmacotherapies to treat drug addiction.

EphB receptor signaling in mouse spinal cord contributes to physical dependence on morphine

Cellular and molecular mechanisms underlying opioid tolerance and dependence remain elusive. We investigated roles of EphB receptor tyrosine kinases--which play important roles in synaptic connection and plasticity during development and in the matured nervous system--in development and maintenance of physical dependence on morphine in the mouse spinal cord (SC). Spinal administration of an EphB receptor blocking reagent EphB2-Fc prevents and/or suppresses behavioral responses to morphine withdrawal and associated induction of c-Fos and depletion of calcitonin gene-related peptide. Western blotting and immunohistochemical fluorescence staining demonstrates that EphB1 receptor protein is significantly up-regulated in the spinal dorsal horn following escalating morphine treatment. Chronic morphine exposure and withdrawal significantly increased phosphorylation of N-methyl-D-aspartate receptor subunit NR2B as well as the activated forms of extracellular signal-regulated kinase and the cAMP response element binding protein in SC. The increased levels of phosphorylation of these molecules, however, are significantly inhibited by the EphB receptor blocker. These findings indicate that EphB receptor signaling, probably by interacting with NR2B in SC, contributes to the development of opioid physical dependence and withdrawal effects. This novel role for EphB receptor signaling suggests that these molecules may be useful therapeutic targets for preventing, minimizing, or reversing the development of opiate dependence.

The differential effect of morphine and beta-endorphin administered intracerebroventricularly on pERK and pCaMK-II expression induced by various nociceptive stimuli in mice brains

The present study was performed to characterize the differential molecular mechanisms of morphine and beta-endorphin which are injected intracerebroventiricularly in mice. In the immunoblot assay, the increases of phosphorylated extracellular signal-regulated protein kinase (pERK) as well as phosphorylated calcium/calmodulin-dependent protein kinase IIalpha (pCaMK-IIalpha) expression induced by noxious stimuli were attenuated by intracerebroventricular (i.c.v.) beta-endorphin pretreatment in the hypothalamus, but not by i.c.v. morphine pretreatment. In addition to these immunoblot results, immunohistochemical study also showed that the attenuation of pERK or pCaMK-IIalpha immunoreactivity elicited by i.c.v. pretreatment of beta-endorphin mainly occurred in the paraventricular nucleus of the hypothalamus (PVN). We also investigated the effect of morphine and beta-endorphin on pERK and pCaMK-IIalpha expression in the locus coeruleus (LC). I.c.v. injection of morphine significantly increased pERK as well as pCaMK-IIalpha expression in the locus coeruleus, while beta-endorphin increased only pCaMK-IIalpha in the LC. In addition, beta-endorphin significantly attenuated pERK expression induced by SP i.t. injection. These results suggest that the antinociceptive effects of supraspinally administered morphine and beta-endorphin are involved with differentially intracellular signal transduction molecules-pERK, pCaMK-IIalpha in the PVN and the LC.

Expression changes of hippocampal energy metabolism enzymes contribute to behavioural abnormalities during chronic morphine treatment

Dependence and impairment of learning and memory are two well-established features caused by abused drugs such as opioids. The hippocampus is an important region associated with both drug dependence and learning and memory. However, the molecular events in hippocampus following exposure to abused drugs such as opioids are not well understood. Here we examined the effect of chronic morphine treatment on hippocampal protein expression by proteomic analyses. We found that chronic exposure of mice to morphine for 10 days produced robust morphine withdrawal jumping and memory impairment, and also resulted in a significant downregulation of hippocampal protein levels of three metabolic enzymes, including Fe-S protein 1 of NADH dehydrogenase, dihydrolipoamide acetyltransferase or E2 component of the pyruvate dehydrogenase complex and lactate dehydrogenase 2. Further real-time quantitative PCR analyses confirmed that the levels of the corresponding mRNAs were also remarkably reduced. Consistent with these findings, lower ATP levels and an impaired ability to convert glucose into ATP were also observed in the hippocampus of chronically treated mice. Opioid antagonist naltrexone administrated concomitantly with morphine significantly suppressed morphine withdrawal jumping and reversed the downregulation of these proteins. Acute exposure to morphine also produced robust morphine withdrawal jumping and significant memory impairment, but failed to decrease the expression of these three proteins. Intrahippocampal injection of D-glucose before morphine administration significantly enhanced ATP levels and suppressed morphine withdrawal jumping and memory impairment in acute morphine-treated but not in chronic morphine-treated mice. Intraperitoneal injection of high dose of D-glucose shows a similar effect on morphine-induced withdrawal jumping as the central treatment. Taken together, our results suggest that reduced expression of the three metabolic enzymes in the hippocampus as a result of chronic morphine treatment contributes to the development of drug-induced symptoms such as morphine withdrawal jumping and memory impairment.

Roles of hippocampal N-methyl-D-aspartate receptors and calcium/calmodulin-dependent protein kinase II in amphetamine-produced conditioned place preference in rats

This study investigates the roles of hippocampal N-methyl-D-aspartate (NMDA) glutamate receptors and CaMKII (calcium/calmodulin-dependent protein kinase II) in amphetamine-produced conditioned place preference (AMPH-CPP) in rats. An earlier report showed that AMPH-CPP resulted in the enhancement of hippocampal CaMKII activity. In this study, AMPH-CPP significantly increased hippocampal GluR1 receptors, though AMPH-CPP was impaired by either blockade of NMDA receptors (AP5) or inhibition of CaMKII (KN-93) during conditioning. These treatments also impaired CPP if administered before testing, but CPP recovered during the next testing session. Therefore, these treatments had no effect on the extinction of CPP. If the conditioned rats were, however, reexposed to AMPH-CPP after a hippocampal-infusion of AP5 or KN-93, the extinction of the original CPP was greater than that seen in the controls. The hippocampal-infusion of D-cycloserine before CPP testing enhanced the extinction of CPP. These results, taken together, indicate that NMDA receptor activation and CaMKII activity are essential for the AMPH-CPP. AMPH-CPP reexposure is similar to the memory reconsolidation process, being disrupted by either a blockade of the NMDA receptor or an inhibition of CaMKII. Furthermore, the extinction of CPP resembles new learning, which is an active process and is facilitated by a partial NMDA agonist.

Calcium/calmodulin-dependent protein kinase II supports morphine antinociceptive tolerance by phosphorylation of glycosylated phosducin-like protein

The long isoform of the phosducin-like protein (PhLPl) is widely expressed in the brain and it is thought to influence G-protein signalling by regulating the activity of Gbetagamma dimers. We show that in the mature nervous system, PhLPl exists as both a 38kDa non-glycosylated isoform and as glycosylated isoforms of about 45, 100 and 150kDa. Additionally, neural PhLPl is subject to serine phosphorylation, which augments upon the activation of Mu-opioid receptors (MORs), as does its association with Gbetagamma subunits and 14-3-3 proteins. While the intracerebroventricular (icv) administration of morphine to mice rapidly reduced the association of MORs with G proteins, it increased the serine phosphorylation of these receptors. Moreover, activated Ca2+/calmodulin-dependent protein kinase II (CaMKII) accumulated in the MOR environment and phosphorylated PhLPl was seen to co-precipitate with these opioid receptors. Opioid-induced phosphorylation of PhLPl was impaired by inhibiting the activity of CaMKII and, in these circumstances, the association of PhLPl with Gbetagamma dimers and 14-3-3 proteins was diminished. Furthermore, these events were coupled with the recovery of G protein regulation by the MORs, while there was a decrease in serine phosphorylation of these receptors and morphine antinociceptive tolerance diminished. It seems that CaMKII phosphorylation of PhLPl stabilizes the PhLPl.Gbetagamma complex by promoting its binding to 14-3-3 proteins. When this complex fails to bind to 14-3-3 proteins, the association of PhLPl with Gbetagamma is probably disrupted by GalphaGDP subunits and the MORs recover control on G proteins.

Effects of scopolamine on morphine-induced conditioned place preference in mice

It is well known that the cholinergic system plays a crucial role in learning and memory. Psychopharmacological studies in humans and animals have shown that a systemic cholinergic blockade may induce deficits in learning and memory. Accumulated studies have indicated that learning and memory play an important role in drug addition. In the present study, in order to get a further understanding about the functions of the cholinergic system in drug-related learning and memory, we examined the effects of scopolamine (0.5, 1.0 and 2.0 mg/kg) on morphine-induced conditioned place preference (CPP). Two kinds of morphine exposure durations (4 days and 12 days) were used. The main finding was that all doses of scopolamine enhanced the extinction of morphine-induced CPP in mice treated with morphine for 12 days. However, in mice treated with morphine for 4 days, all doses of scopolamine did not inhibit morphine-induced CPP. The highest dose (2.0 mg/kg) of scopolamine even significantly delayed the extinction of morphine-induced CPP. Our results suggest that the effects of a systemic cholinergic blockade on morphine-induced CPP depend on the morphine exposure time.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

RGS14 prevents morphine from internalizing Mu-opioid receptors in periaqueductal gray neurons

Opioid agonists display different capacities to stimulate mu-opioid receptor (MOR) endocytosis, which is related to their ability to provoke the phosphorylation of specific cytosolic residues in the MORs. Generally, opioids that efficiently promote MOR endocytosis and recycling produce little tolerance, as is the case for [D-Ala(2), N-MePhe(4),Gly-ol(5)] encephalin (DAMGO). However, morphine produces rapid and profound antinociceptive desensitization in the adult mouse brain associated with little MOR internalization. The regulator of G-protein signaling, the RGS14 protein, associates with MORs in periaqueductal gray matter (PAG) neurons, and when RGS14 is silenced morphine increased the serine 375 phosphorylation in the C terminus of the MOR, a GRK substrate. Subsequently, these receptors were internalized and recycled back to the membrane where they accumulated on cessation of antinociception. These mice now exhibited a resensitized response to morphine and little tolerance developed. Thus, in morphine-activated MORs the RGS14 prevents GRKs from phosphorylating those residues required for beta-arresting-mediated endocytosis. Moreover morphine but not DAMGO triggered a process involving calcium/calmodulin-dependent kinase II (CaMKII) in naïve mice, which contributes to MOR desensitization in the plasma membrane. In RGS14 knockdown mice morphine failed to activate this kinase. It therefore appears that phosphorylation and internalization of MORs disrupts the CaMKII-mediated negative regulation of these opioid receptors.

Adult neurogenesis and the memories of drug addiction

Evidence accumulated in the last decade indicating that psychoactive substances negatively influence neurogenesis in the adult hippocampus has provided new insights into the neurobiology of drug addiction. Using a variety of experimental approaches and treatments, drugs such as psychomotor stimulants, opioids, alcohol and psychedelic compounds have been shown to alter one or several aspects of adult neurogenesis, including the rate of progenitor proliferation, the survival of newly generated cells, and the maturation and acquisition of cellular phenotype. This evidence, though critical from a neurotoxicological standpoint, cannot be linked unambiguously to the process of drug dependence at this stage of research. Drug addiction is a complex recurrent process involving the acquisition and maintenance of drug taking, followed by detoxification, abstinence and eventual relapse to drug seeking. The specific contribution of adult hippocampal neurogenesis to each of these processes is yet to be determined. This notwithstanding, the suggested role of the hippocampus in the storage and retrieval of declarative and contextual memories on the one hand, and in the regulation of mood and affect on the other, provides a fertile ground for further exploring the mutual relationships between postnatal hippocampal neurogenesis and addictive behaviours.

Involvement of glucose and ATP-sensitive potassium (K+) channels on morphine-induced conditioned place preference

In the present study, the effects of glucose and ATP-sensitive K+ channel compounds on the acquisition of morphine-induced place preference in male mice were investigated. Subcutaneous administration of different doses of morphine (2.5-7.5 mg/kg) produced a dose-dependent conditioned place preference. With a 3-day conditioning schedule, it was found that glucose (100, 200, 500 and 1000 mg/kg), diazoxide (15, 30 and 60 mg/kg) or glibenclamide (3, 6 and 12 mg/kg) did not produce significant place preference or place aversion. Intraperitoneal administration of the glucose (1000 mg/kg) or glibenclamide (6 and 12 mg/kg) with a lower dose of morphine (0.5 mg/kg) elicited the significant conditioned place preference. The response of glibenclamide (6 mg/kg) was reversed by diazoxide (15, 30 and 60 mg/kg). Drug injections had no effects on locomotor activity during the test sessions. It is concluded that glucose and the ATP-sensitive K+ channel may play an active role in morphine reward.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.