Intra-hippocampal inhibition of protein kinase AII attenuates morphine-induced conditioned place preference

Activation of Epac in the BLA disrupts reconsolidation and attenuates heroin-seeking behaviour

The susceptibility to drug cravings evoked by stimuli poses a formidable hurdle in the treatment of addiction and the prevention of relapse. Pharmacological interventions targeting drug-associated memories hold promise for curbing relapse by impeding the process of memory reconsolidation, predominantly governed by cAMP signalling. Exchange Protein Activated by cAMP (Epac) serves as a distinctive mediator of cAMP signalling, which has been implicated in reinforcing the effects of cocaine and facilitating the acquisition. Nonetheless, the role of Epac in heroin-related memory and the subsequent seeking behaviour remains enigmatic. In this study, we explored the impact of Epac activation on the reconsolidation process of drug-related memories associated with heroin self-administration. Over the course of 10 consecutive days, rats underwent training, wherein they acquired the behaviour of nose poking to obtain heroin accompanied by a tone + light cue. This nose-poking behaviour was subsequently extinguished when heroin infusion and cue presentation were discontinued. Subsequently, we administered 8-pCPT-cAMP (8-CPT), an Epac-specific activator, into the basolateral amygdala at various time points, either in the presence or absence of a conditioned stimulus. Our findings demonstrate that administering 8-CPT immediately after memory retrieval effectively reduces cue- and heroin-induced reinstatement, with the observed effects persisting significantly for a minimum of 28 days. However, infusion of 8-CPT for a duration of 6 h following the memory retrieval trial, or without it altogether, had no discernible impact. Thus, these findings strongly suggest that Epac activation can disrupt the reconsolidation of heroin-associated memory, thereby diminishing the reinstatement of heroin-seeking behaviour.

Disrupting reconsolidation by PKA inhibitor in BLA reduces heroin-seeking behavior

Drug abuse is considered a maladaptive pathology of emotional memory and is associated with craving and relapse induced by drug-associated stimuli or drugs. Reconsolidation is an independent memory process with a strict time window followed by the reactivation of drug-associated stimulus depending on the basolateral amygdala (BLA). Pharmacology or behavior treatment that disrupts the reconsolidation can effectively attenuate drug-seeking in addicts. Here, we hypothesized that heroin-memory reconsolidation requires cAMP-dependent protein kinase A (PKA) of BLA based on the fundamental effect of PKA in synaptic plasticity and memory process. After 10 days of acquisition, the rats underwent 11 days of extinction training and then received the intra-BLA infusions of the PKA inhibitor Rp-cAMPS at different time windows with/without a reactivation session. The results show that PKA inhibitor treatment in the reconsolidation time window disrupts the reconsolidation and consequently reduces cue-induced reinstatement, heroin-induced reinstatement, and spontaneous recovery of heroin-seeking behavior in the rats. In contrast, there was no effect on cue-induced reinstatement in the intra-BLA infusion of PKA inhibitor 6 h after reactivation or without reactivation. These data suggest that PKA inhibition disrupts the reconsolidation of heroin-associated memory, reduces subsequent drug seeking, and prevents relapse, which is retrieval-dependent, time-limited, and BLA-dependent.

Oxytocin signaling in the treatment of drug addiction: Therapeutic opportunities and challenges

Drug addiction is one of the leading causes of mortality worldwide. Despite great advances were achieved in understanding the neurobiology of drug addiction, the therapeutic options are severely limited, with poor effectiveness and serious side effects. The neuropeptide oxytocin (OXT) is well known for its effects on uterine contraction, sexual/maternal behaviors, social affiliation, stress and learning/memory by interacting with the OXT receptor and other neuromodulators. Emerging evidence suggests that the acute or chronic exposure to drugs can affect the OXT system. Additionally, OXT administration can ameliorate a wide range of abused drug-induced neurobehavioral changes. Overall, OXT not only suppresses drug reward in the binge stage of drug addiction, but also reduces stress responses and social impairments during the withdrawal stage and, finally, prevents drug/cue/stress-induced reinstatement. More importantly, clinical studies have also shown that OXT can exert beneficial effects on reducing substance use disorders of a series of drugs, such as heroin, cocaine, alcohol, cannabis and nicotine. Thus, the present review focuses on the role of OXT in treating drug addiction, including the preclinical and clinical therapeutic potential of OXT and its analogs on the neurobiological perspectives of drugs, to provide a better insight of the efficacy of OXT as a clinical addiction therapeutic agent.

Scabronine G Methyl Ester Improves Memory-Related Behavior and Enhances Hippocampal Cell Proliferation and Long-Term Potentiation via the BDNF-CREB Pathway in Olfactory Bulbectomized Mice

A previous study reported that scabronine G methyl ester (SG-ME) potentially enhances the in vitro secretion of neurotrophic factors such as nerve growth factor via the protein kinase C (PKC)-ζ pathway. However, it remains unknown whether SG-ME can improve cognitive dysfunctions in olfactory bulbectomized (OBX) mice. To address this question, we evaluated SG-ME-treated and untreated OBX mice in a passive avoidance test. We also investigated potential effects of SG-ME on several parameters: cell proliferation and cAMP response element-binding protein (CREB) phosphorylation in the hippocampal dentate gyrus by immunohistochemistry, brain-derived neurotrophic factor (BDNF) levels in the hippocampus by Western blotting, p-CREB levels in the hippocampus by MapAnalyzer, and long-term potentiation (LTP) by electrophysiology. On the 14th day after surgery OBX mice showed altered passive avoidance and decreases in both cell proliferation and long-term potentiation in the hippocampus, while these changes were reversed by SG-ME (20 μg/mouse) 24 h after the treatment. The improvement in memory deficits was prevented when SG-ME was co-administeredwith either zeta inhibitory peptide (PKC-ζ inhibitor), anti-BDNF antibody, ANA-12 (TrkB antagonist), U0126 (MEK inhibitor), H-89 (PKA inhibitor), LY294002 (PI3K inhibitor) or KN-93 (CaMKII inhibitor). We found that SG-ME enhanced brain-derived neurotrophic factor and p-CREB levels in the hippocampus while p-CREB was localized in neurons, but not in astrocytes nor microglial cells. These findings revealed the potential of SG-ME in improving memory impairments by enhancing cell proliferation and LTP via activation of the BDNF/CREB signaling pathway in neurons.

TRPV1 modulates morphine self-administration via activation of the CaMKII-CREB pathway in the nucleus accumbens

Opioid addiction is a growing problem for public health, and opioids have correspondingly become more heavily regulated over time. We have previously shown that TRPV1 plays a critical role in morphine addiction using a self-administration paradigm in rats, and the current study evaluates the effects of the TRPV1 signaling pathway on morphine self-administration (SA). We found that treatment with a selective TRPV1 antagonist, SB366791, significantly decreased the morphine SA-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII), Akt and the cAMP response element binding protein (CREB) in the nucleus accumbens (NAc). In addition, phospho-PKA and phospho-PKC expression levels were significantly increased in the NAc of the morphine-SA groups, regardless of SB366791 treatment. Finally, local microinjection of SB366791 into the NAc significantly suppressed the maintenance of morphine SA. Taken together, our findings highlight that TRPV1 plays an important role in morphine addiction, likely via activation of the CaMKII-CREB pathway in the NAc.

Oprm1 A112G, a single nucleotide polymorphism, alters expression of stress-responsive genes in multiple brain regions in male and female mice

BACKGROUND

OPRM1 A118G, a functional human mu-opioid receptor (MOR) polymorphism, is associated with drug dependence and altered stress responsivity in humans as well as altered MOR signaling. MOR signaling can regulate many cellular processes, including gene expression, and many of the long-term, stable effects of drugs and stress may stem from changes in gene expression in diverse brain regions. A mouse model bearing an equivalent polymorphism (Oprm1 A112G) was previously generated and studied. Mice homozygous for the G112 allele show differences in opioid- and stress-related phenotypes.

APPROACH

The current study examines the expression of 24 genes related to drug and stress responsivity in the caudoputamen, nucleus accumbens, hypothalamus, hippocampus, and amygdala of drug-naïve, stress-minimized, male and female mice homozygous for either the G112 variant allele or the wild-type A112 allele.

RESULTS

We detected nominal genotype-dependent changes in gene expression of multiple genes. We also detected nominal sex-dependent as well as sex-by-genotype interaction effects on gene expression. Of these, four genotype-dependent differences survived correction for multiple testing: Avp and Gal in the hypothalamus and Oprl1 and Cnr1 in the hippocampus.

CONCLUSIONS

Changes in the regulation of these genes by mu-opioid receptors encoded by the G112 allele may be involved in some of the behavioral and molecular consequences of this polymorphism observed in mice.

5-HT2A Serotonin Receptor Density in Adult Male Rats' Hippocampus after Morphine-based Conditioned Place Preference

INTRODUCTION

A close interaction exists between the brain opioid and serotonin (5-HT) neurotransmitter systems. Brain neurotransmitter 5-HT plays an important role in the regulation of reward-related processing. However, a few studies have investigated the potential role of 5-HT2A receptors in this behavior. Therefore, the aim of the present study was to assess the influence of morphine and Conditioned Place Preference (CPP) on the density of 5-HT2A receptor in neurons of rat hippocampal formation.

METHODS

Morphine (10 mg/kg, IP) was injected in male Wistar rats for 7 consecutive days (intervention group), but control rats received just normal saline (1 mL/kg, IP). We used a hotplate test of analgesia to assess induction of tolerance to analgesic effects of morphine on days 1 and 8 of injections. Later, two groups of rats were sacrificed one day after 7 days of injections, their whole brains removed, and the striatum and PFC immediately dissected. Then, the NR1 gene expression was examined with a semi-quantitative RT-PCR method.

RESULTS

Our data showed that the maximum response was obtained with 2.5 mg/kg of morphine. The density of 5-HT2A receptor in different areas of the hippocampus increased significantly at sham-morphine and CPP groups (P<0.05). On the other hand, the CPP groups had more 5-HT2A receptors than sham-morphine groups and also the sham-morphine groups had more 5-HT2A receptors than the control groups.

CONCLUSION

We concluded that the phenomenon of conditioned place preference induced by morphine can cause a significant increase in the number of serotonin 5-HT2A receptors in neurons of all areas of hippocampus.

Disrupting the memory of places induced by drugs of abuse weakens motivational withdrawal in a context-dependent manner

Addicts repeatedly relapse to drug seeking even after years of abstinence, and this behavior is frequently induced by the recall of memories of the rewarding effects of the drug. Established memories, including those induced by drugs of abuse, can become transiently fragile if reactivated, and during this labile phase, known as reconsolidation, can be persistently disrupted. Here we show that, in rats, a morphine-induced place preference (mCPP) memory is linked to context-dependent withdrawal as disrupting the reconsolidation of the memory leads to a significant reduction of withdrawal evoked in the same context. Moreover, the hippocampus plays a critical role in linking the place preference memory with the context-conditioned withdrawal, as disrupting hippocampal protein synthesis and cAMP-dependent-protein kinase A after the reactivation of mCPP significantly weakens the withdrawal. Hence, targeting memories induced by drugs may represent an important strategy for attenuating context-conditioned withdrawal and therefore subsequent relapse in opiate addicts.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

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.

Effects of scopolamine and ketamine on reconsolidation of morphine conditioned place preference in rats

Persistent memory associated with addictive drugs contributes to the relapse of drug abuse. The current study was conducted to examine the effects of scopolamine and ketamine on reconsolidation of morphine-induced conditioned place preference (CPP). In experiment 1, after morphine CPP was acquired, rats were injected with ketamine (60 mg/kg, intraperitoneally) and scopolamine (2 mg/kg, intraperitoneally), respectively, after reexposure to an earlier morphine-paired context or in their home cages. The CPP was reassessed 24 and 48 h after reexposure. An additional group of rats received saline following reexposure to the earlier morphine-paired context. In experiment 2, two groups of rats were only given saline during the CPP training and subsequent administration of ketamine or scopolamine during the reexposure. In experiment 1, rats failed to exhibit morphine CPP when ketamine and scopolamine were administered only after reexposure to a morphine-paired context. CPP was not abolished by ketamine or scopolamine administration in the animals' home cages. Also, the animals receiving only saline injections showed strong morphine CPP 24 h after a short exposure to the morphine-paired context. In experiment 2, ketamine or scopolamine treatment alone did not induce CPP or aversion. Administration of scopolamine and ketamine, after reexposure to a drug-paired context, resulted in the disruption of morphine CPP, suggesting the potential effects of scopolamine and ketamine in disrupting memory associated with environmental cues and addictive drugs.

Adolescent rats find repeated Delta(9)-THC less aversive than adult rats but display greater residual cognitive deficits and changes in hippocampal protein expression following exposure

The current study examined whether adolescent rats are more vulnerable than adult rats to the lasting adverse effects of cannabinoid exposure on brain and behavior. Male Wistar rats were repeatedly exposed to Delta-9-tetrahydrocannabinol (Delta(9)-THC, 5 mg/kg i.p.) in a place-conditioning paradigm during either the adolescent (post-natal day 28+) or adult (post-natal day 60+) developmental stages. Adult rats avoided a Delta(9)-THC-paired environment after either four or eight pairings and this avoidance persisted for at least 16 days following the final Delta(9)-THC injection. In contrast, adolescent rats showed no significant place aversion. Adult Delta(9)-THC-treated rats produced more vocalizations than adolescent rats when handled during the intoxicated state, also suggesting greater drug-induced aversion. After a 10-15 day washout, both adult and adolescent Delta(9)-THC pretreated rats showed decreased social interaction, while only Delta(9)-THC pretreated adolescent rats showed significantly impaired object recognition memory. Seventeen days following their last Delta(9)-THC injection, rats were euthanased and hippocampal tissue processed using two-dimensional gel electrophoresis proteomics. There was no evidence of residual Delta(9)-THC being present in blood at this time. Proteomic analysis uncovered 27 proteins, many involved in regulating oxidative stress/mitochondrial functioning and cytoarchitecture, which were differentially expressed in adolescent Delta(9)-THC pretreated rats relative to adolescent controls. In adults, only 10 hippocampal proteins were differentially expressed in Delta(9)-THC compared to vehicle-pretreated controls. Overall these findings suggest that adolescent rats find repeated Delta(9)-THC exposure less aversive than adults, but that cannabinoid exposure causes greater lasting memory deficits and hippocampal alterations in adolescent than adult rats.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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.