Involvement of dopamine receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats

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

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

The dopamine D1 receptor antagonist SCH-23390 blocks the acquisition, but not expression of mitragynine-induced conditioned place preference in rats

Mitragynine (MG) is the primary active constituent of Mitragyna speciosa Korth (kratom), a psychoactive Southeast Asian plant with potential therapeutic use. Numerous studies support roles of dopaminergic system in drug reward. However, the involvement of the dopaminergic system in mediating MG reward and drug-seeking is poorly understood. Using conditioned place preference (CPP) paradigm, the present study aims to evaluate the roles of the dopamine (DA) D1 receptor in the acquisition and expression of MG-induced CPP in rats. The effects of SCH-23390, a selective DA D1 receptor antagonist, on the acquisition of MG-induced CPP were first investigated. Rats were pre-treated systemically with SCH-23390 (0, 0.1 and 0.3 mg/kg, i.p.) prior to MG (10 mg/kg) conditioning sessions. Next, we tested the effects of the DA D1 receptor antagonist on the expression of MG-induced CPP. Furthermore, the effects of a MG-priming dose (5 mg/kg) on the reinstatement of extinguished CPP were tested. The results showed that SCH-23390 dose-dependently suppressed the acquisition of a MG-induced CPP. In contrast, SCH-23390 had no effect on the expression of a MG-induced CPP. The findings of this study suggested a crucial role of the DA D1 receptor in the acquisition, but not the expression of the rewarding effects of MG in a CPP test. Furthermore, blockade of the D1-like receptor during conditioning did not prevent MG priming effects on CPP reinstatement test, suggesting no role for the DA D1 receptor in reinstatement sensitivity.

Anterior cingulate cortex and its projections to the ventral tegmental area regulate opioid withdrawal, the formation of opioid context associations and context-induced drug seeking

Clinical evidence suggests that there are correlations between activity within the anterior cingulate cortex (ACC) following re-exposure to drug-associated contexts and drug craving. However, there are limited data contributing to our understanding of ACC function at the cellular level during re-exposure to drug-context associations as well as whether the ACC is directly related to context-induced drug seeking. Here, we addressed this issue by employing our novel behavioral procedure capable of measuring the formation of drug-context associations as well as context-induced drug-seeking behavior in male mice (8–12 weeks of age) that orally self-administered oxycodone. We found that mice escalated oxycodone intake during the long-access training sessions and that conditioning with oxycodone was sufficient to evoke conditioned place preference (CPP) and drug-seeking behaviors. Additionally, we found that thick-tufted, but not thin-tufted pyramidal neurons (PyNs) in the ACC as well as ventral tegmental area (VTA)-projecting ACC neurons had increased intrinsic membrane excitability in mice that self-administered oxycodone compared to controls. Moreover, we found that global inhibition of the ACC or inhibition of VTA-projecting ACC neurons was sufficient to significantly reduce oxycodone-induced CPP, drug seeking, and spontaneous opioid withdrawal. These results demonstrate a direct role of ACC activity in mediating context-induced opioid seeking among other behaviors, including withdrawal, that are associated with the DSM-V criteria of opioid use disorder.

Quetiapine attenuates the acquisition of morphine-induced conditioned place preference and reduces ERK phosphorylation in the hippocampus and cerebral cortex

Background: Quetiapine is an atypical antipsychotic that antagonizes dopamine and serotonin receptors. It has been suggested that quetiapine can be used to treat substance use disorders, including opioid use disorder. Opioids modulate dopaminergic functions associated with conditioned reinforcement and these effects can be measured via the conditioned place preference (CPP) paradigm. Opioids' unconditioned effects are regulated by several proteins, including extracellular signal-regulated kinase (ERK) and cAMP-responsive element-binding (CREB).Objective: To assess the effect of quetiapine on morphine-induced CPP and motor activity levels, and on the levels of ERK and CREB proteins in the hippocampus and cerebral cortex.Methods: 42 male rats were exposed to a CPP protocol, in which they underwent a conditioning paradigm with saline, quetiapine (40 mg/kg), morphine (10 mg/kg), morphine plus quetiapine (10, 20, or 40 mg/kg), or morphine plus memantine (7.5 mg/kg, a positive control drug) (n = 6 per group). The rats were tested for CPP and exploratory activity. Levels of ERK and CREB proteins in the hippocampus and cerebral cortex were also measured.Results: Quetiapine co-administered with morphine inhibited morphine-induced CPP [F (6, 70) = 11.67, p < .001] and morphine's effects on motor activity (p < .001). Morphine enhanced ERK phosphorylation in the hippocampus (p < .001) and cerebral cortex (p < .001), an effect inhibited by quetiapine.Conclusion: Quetiapine attenuates morphine-induced CPP and locomotion and these effects are associated with a reduction of ERK phosphorylation in the hippocampus and cerebral cortex. These results suggest that quetiapine should be further explored as a potential treatment for opioid use disorder.

Dopamine receptors in the anterior cingulate cortex implicate in nicotine enhanced morphine analgesia

RATIONALE

It is well known that the anterior cingulate cortex (ACC) plays an important role in acute pain perception.

OBJECTIVES

In the present study, we aimed to investigate the possible involvement of the ACC dopamine D1 and D2 receptors in nicotine plus morphine-induced analgesia.

METHODS

The ACC's of adult male Wistar rats were bilaterally cannulated by stereotaxic instrument and the tail-flick test was used to measure the thermal pain threshold.

RESULTS

The results indicated that subcutaneous (s.c.) injection of nicotine (0.3 mg/kg) potentiated the analgesic response of intraperitoneal (i.p.) administration of morphine (3 mg/kg). Systemic administration of the same doses of nicotine or morphine alone had no effect on tail-flick latency. Intra-ACC administration of apomorphine (0.3-0.9 μg/rat), the non-selective D1/D2 receptors agonist, plus ineffective doses of nicotine (0.1 mg/kg, s.c.) plus morphine (3 mg/kg, i.p) induced analgesia in rats. In addition, the analgesia induced with co-administration of nicotine and morphine was inhibited via intra-ACC administration of SCH23390 (0.5-1 μg/rat) or sulpiride (0.5-2 μg/rat), the selective antagonists of D1 or D2 receptors, respectively. The intra-ACC microinjection of the same doses of drugs alone had no effect on tail-flick latency. Cubic interpolation analysis also confirmed that activation or inactivation of the ACC D1 and D2 receptors by different doses of drugs can modulate the nicotine-morphine analgesic response.

CONCLUSIONS

The findings suggest that the ACC has an important role in acute thermal pain perception and modulates the analgesia induced by nicotine plus morphine via dopaminergic receptors.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

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

Pentylenetetrazol and Morphine Interaction in a State-dependent Memory Model: Role of CREB Signaling

Introduction:

State-dependent (STD) memory is a process, in which the learned information can be optimally retrieved only when the subject is in the state similar to the encoding phase. This phenomenon has been widely studied with morphine. Several studies have reported that Pentylenetetrazole (PTZ) impairs memory in experimental animal models. Due to certain mechanistic interactions between morphine and PTZ, it is hypothesized that PTZ may interfere with the morphine-STD. The cyclic adenosine monophosphate Response Element-Binding (CREB) is considered as the main downstream marker for long-term memory. This study was designed to determine the possible interaction between PTZ and morphine STD and the presumable changes in CREB mRNA.

Methods:

In an Inhibitory Avoidance (IA) model, posttraining morphine (2.5, 5, and 7.5 mg/ kg-i.p.) was used. The pre-test morphine was evaluated for morphine-induced STD memory. Moreover, the effect of a pre-test PTZ (60 mg/kg-i.p.) was studied along with morphine STD. Locomotion testing was carried out using open-field. Eventually, using real-time-PCR, the CREB mRNA changes in the hippocampus were evaluated.

Results:

Posttraining MOR (7.5 mg/kg-i.p.) impaired IA memory (P<0.001). The pre-test injection of similar doses of morphine recovered the morphine-induced memory impairment (P<0.001). The pre-test PTZ impaired the IA memory recall (P<0.001); however, the pre-test PTZ along with morphine STD potentiated the morphine-induced STD (P<0.001). Alterations in CREB mRNA were observed in all groups. No difference was seen in the locomotor activity.

Conclusion:

Presumably, the certain interactive effect of PTZ on morphine-induced STD is mediated through gamma-aminobutyric acid and opioid systems via CREB signaling.

Oxytocin Attenuates Expression, but Not Acquisition, of Sucrose Conditioned Place Preference in Rats

Maladaptation of reward processing for natural rewards, such as sucrose or sugar, may play a role in the development of diseases such as obesity and diabetes. Furthermore, uncovering mechanisms to disrupt or reverse maladaptation of reward-seeking behaviors for natural reinforcers can provide insight into treatment of such diseases, as well as disorders such as addiction. As such, studying the effects of potential pharmacotherapeutics on maladaptive sugar-seeking behavior offers valuable clinical significance. Sucrose conditioned place preference (CPP) paradigms can offer insight into aspects of reward processes as it provides a way to assess acquisition and expression of context-reward associations. The present study examined the effect of peripheral oxytocin injections on sucrose CPP in rats. Oxytocin, when administered prior to CPP test, attenuated expression of sucrose CPP. However, oxytocin, when administered during sucrose conditioning, did not affect subsequent place preference. These findings suggest oxytocin sufficiently attenuates expression of sucrose-associated place preference.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

Place preferences induced by electrical stimulation of the external lateral parabrachial subnucleus in a sequential learning task: Place preferences induced by NLPBe stimulation

It is known that electrical stimulation of the external lateral parabrachial nucleus (NLPBe) can sustain concurrent taste and place learning. Place preferences can be learned through different procedures. Previous studies demonstrated that electrical stimulation of the PBNLe can generate aversive and preference place learning using concurrent procedures. In the concurrent procedure, the animals can move freely in the maze, and intracranial electrical stimulation is associated with their voluntary stay in one of the two maze compartments. However, the rewarding properties of most stimuli, whether natural or drugs of abuse, have usually been investigated using the sequential procedure, in which animals are confined while receiving the unconditioned stimulus and then undergo a choice test without stimulation in a later phase. This study examined whether this stimulation can sustain place preference learning in sequential tasks. Results demonstrated that place preferences can also be induced by the electrical stimulation of the NLBe using sequential procedures. These findings suggest that the NLPBe may form part of a brain reward axis that shares certain characteristics with those observed in the processing of natural rewarding agents and especially of drugs of abuse.

Blockade of dopamine D1 receptors in male rats disrupts morphine reward in pain naïve but not in chronic pain states

The rewarding effect of opiates is mediated through dissociable neural systems in drug naïve and drug-dependent states. Neuroadaptations associated with chronic drug use are similar to those produced by chronic pain, suggesting that opiate reward could also involve distinct mechanisms in chronic pain and pain-naïve states. We tested this hypothesis by examining the effect of dopamine (DA) antagonism on morphine reward in a rat model of neuropathic pain.Neuropathic pain was induced in male Sprague-Dawley rats through chronic constriction (CCI) of the sciatic nerve; reward was assessed in the conditioned place preference (CPP) paradigm in separate groups at early (4-8 days post-surgery) and late (11-15 days post-surgery) phases of neuropathic pain. Minimal effective doses of morphine that produced a CPP in early and late phases of neuropathic pain were 6 mg/kg and 2 mg/kg respectively. The DA D1 receptor antagonist, SCH23390, blocked a morphine CPP in sham, but not CCI, rats at a higher dose (0.5 mg/kg), but had no effect at a lower dose (0.1 mg/kg). The DA D2 receptor antagonist, eticlopride (0.1 and 0.5 mg/kg), had no effect on a morphine CPP in sham or CCI rats, either in early or late phases of neuropathic pain. In the CPP paradigm, morphine reward involves DA D1 mechanisms in pain-naïve but not chronic pain states. This could reflect increased sensitivity to drug effects in pain versus no pain conditions and/or differential mediation of opiate reward in these two states.

A Mini-Review of the Role of Glutamate Transporter in Drug Addiction

Goals: The development of new treatment for drug abuse requires identification of targetable molecular mechanisms. The pathology of glutamate neurotransmission system in the brain reward circuit is related to the relapse of multiple drugs. Glutamate transporter regulates glutamate signaling by removing excess glutamate from the synapse. And the mechanisms between glutamate transporter and drug addiction are still unclear.

Methods: A systematic review of the literature searched in Pubmed and reporting drug addiction in relation to glutamate transporter. Studies were screened by title, abstract, and full text.

Results: This review is to highlight the effects of drug addiction on glutamate transporter and glutamate uptake, and targeting glutamate transporter as an addictive drug addiction treatment. We focus on the roles of glutamate transporter in different brain regions in drug addiction. More importantly, we suggest the functional roles of glutamate transporter may prove beneficial in the treatment of drug addiction.

Conclusion: Overall, understanding how glutamate transporter impacts central nervous system may provide a new insight for treatment of drug addiction.

State-Dependent Memory: Neurobiological Advances and Prospects for Translation to Dissociative Amnesia

In susceptible individuals, overwhelming traumatic stress often results in severe abnormalities of memory processing, manifested either as the uncontrollable emergence of memories (flashbacks) or as an inability to remember events (dissociative amnesia, DA) that are usually, but not necessarily, related to the stressful experience. These memory abnormalities are often the source of debilitating psychopathologies such as anxiety, depression and social dysfunction. The question of why memory for some traumatic experiences is compromised while other comparably traumatic experiences are remembered perfectly well, both within and across individuals, has puzzled clinicians for decades. In this article, we present clinical, cognitive, and neurobiological perspectives on memory research relevant to DA. In particular, we examine the role of state dependent memory (wherein memories are difficult to recall unless the conditions at encoding and recall are similar), and discuss how advances in the neurobiology of state-dependent memory (SDM) gleaned from animal studies might be translated to humans.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

Dissociative role for dorsal hippocampus in mediating heroin self-administration and relapse through CDK5 and RhoB signaling revealed by proteomic analysis

Addiction is characterized by drug craving, compulsive drug taking and relapse, which is attributed to aberrant neuroadaptation in brain regions implicated in drug addiction, induced by changes in gene and protein expression in these regions after chronic drug exposure. Accumulating evidence suggests that the dorsal hippocampus (DH) plays an important role in mediating drug-seeking and drug-taking behavior and relapse. However, the molecular mechanisms underlying these effects of the DH are unclear. In the present study, we employed a label-free quantitative proteomic approach to analyze the proteins altered in the DH of heroin self-administering rats. A total of 4015 proteins were quantified with high confidence, and 361 proteins showed significant differences compared with the saline control group. Among them, cyclin-dependent kinase 5 (CDK5) and ras homolog family member B (RhoB) were up-regulated in rats with a history of extended access to heroin. Functionally, inhibition of CDK5 in the DH enhanced heroin self-administration, indicating that CDK5 signaling in the DH acts as a homeostatic compensatory mechanism to limit heroin-taking behavior, whereas blockade of the Rho-Rho kinase (ROCK) pathway attenuated context-induced heroin relapse, indicating that RhoB signaling in the DH is required for the retrieval (recall) of addiction memory. Our findings suggest that manipulation of CDK5 signaling in the DH may be essential in determining vulnerability to opiate taking, whereas manipulation of RhoB signaling in the DH may be essential in determining vulnerability to relapse. Overall, the present study suggests that the DH can exert dissociative effects on heroin addiction through CDK5 and RhoB signaling.

Hippocampal neural activity reflects the economy of choices during goal-directed navigation

Distinguishing spatial contexts is likely essential for the well-known role of the hippocampus in episodic memory. We studied whether types of hippocampal neural organization thought to underlie context discrimination are impacted by learned economic considerations of choice behavior. Hippocampal place cells and theta activity were recorded as rats performed a maze-based probability discounting task that involved choosing between a small certain reward or a large probabilistic reward. Different spatial distributions of place fields were observed in response to changes in probability, the outcome of the rats' choice, and whether or not rats were free to make that choice. The degree to which the reward location was represented by place cells scaled with the expected probability of rewards. Theta power increased around the goal location also in proportion to the expected probability of signaled rewards. Furthermore, theta power dynamically varied as specific econometric information was obtained "on the fly" during task performance. Such an economic perspective of memory processing by hippocampal place cells expands our view of the nature of context memories retrieved by hippocampus during adaptive navigation.

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.

The hippocampal NMDA receptors may be involved in acquisition, but not expression of ACPA-induced place preference

Numerous studies have investigated the functional interactions between the endocannabinoid and glutamate systems in the hippocampus. The present study was made to test whether N-methyl-D-aspartate (NMDA) receptors of the CA1 region of the dorsal hippocampus (CA1) are implicated in ACPA (a selective cannabinoid CB1 receptor agonist)-induced place preference. Using a 3-day schedule of conditioning, it was found that intraperitoneal (i.p.) administration of ACPA (0.02mg/kg) caused a significant conditioned place preference (CPP) in male albino NMRI mice. Intra-CA1 microinjection of the NMDA or D-[1]-2-amino-7-Phosphonoheptanoic acid (D-AP7, NMDA receptor antagonist), failed to induce CPP or CPA (condition place aversion), while NMDA (0.5μg/mouse) potentiated the ACPA (0.01mg/kg)-induced CPP; and D-AP7 (a specific NMDA receptor antagonist; 0.5 and 1μg/mouse) reversed the ACPA (0.02mg/kg)-induced CPP. Moreover, microinjection of different doses of glutamatergic agents on the testing day did not alter the expression of ACPA-induced place preference. None of the treatments, with the exception of ACPA (0.04mg/kg), had an effect on locomotor activity. In conclusion, these observations provide evidence that glutamate NMDA receptors of the CA1 may be involved in the potentiation of ACPA rewarding properties in the acquisition, but not expression, of CPP in mice.

CP-154,526 Modifies CREB Phosphorylation and Thioredoxin-1 Expression in the Dentate Gyrus following Morphine-Induced Conditioned Place Preference

Corticotropin-releasing factor (CRF) acts as neuro-regulator of the behavioral and emotional integration of environmental and endogenous stimuli associated with drug dependence. Thioredoxin-1 (Trx-1) is a functional protein controlling the redox status of several proteins, which is involved in addictive processes. In the present study, we have evaluated the role of CRF1 receptor (CRF1R) in the rewarding properties of morphine by using the conditioned place preference (CPP) paradigm. We also investigate the effects of the CRF1R antagonist, CP-154,526, on the morphine CPP-induced activation of CRF neurons, CREB phosphorylation and Trx expression in paraventricular nucleus (PVN) and dentate gyrus (DG) of the mice brain. CP-154,526 abolished the acquisition of morphine CPP and the increase of CRF/pCREB positive neurons in PVN. Moreover, this CRF1R antagonist prevented morphine-induced CRF-immunoreactive fibers in DG, as well as the increase in pCREB expression in both the PVN and DG. In addition, morphine exposure induced an increase in Trx-1 expression in DG without any alterations in PVN. We also observed that the majority of pCREB positive neurons in DG co-expressed Trx-1, suggesting that Trx-1 could activate CREB in the DG, a brain region involved in memory consolidation. Altogether, these results support the idea that CRF1R antagonist blocked Trx-1 expression and pCREB/Trx-1 co-localization, indicating a critical role of CRF, through CRF1R, in molecular changes involved in morphine associated behaviors.

Histone acetylation of the htr3a gene in the prefrontal cortex of Wistar rats regulates ethanol-seeking behavior

Previous reports showed that decreased histone deacetylase activity significantly potentiated the rewarding effects of psychostimulants, and that encoding of the 5-HT3 receptor by the htr3a gene was related to ethanol-seeking behavior. However, the effects of a histone deacetylase inhibitor on ethanol-seeking behavior and epigenetic regulation of htr3a mRNA expression after chronic ethanol exposure are not fully understood. Using quantitative reverse transcription-polymerase chain reaction and chromatin immunoprecipitation analysis, we investigated the effects of chronic ethanol exposure and its interaction with a histone deacetylase inhibitor on histone-acetylation-mediated changes in htr3a mRNA expression in the htr3a promoter region. The conditioned place preference procedure was used to evaluate ethanol-seeking behavior. Chronic exposure to ethanol effectively elicited place conditioning. In the prefrontal cortex, the acetylation of H3K9 and htr3a mRNA expression in the htr3a promoter region were significantly higher in the ethanol group than in the saline group. The histone deacetylase inhibitor sodium butyrate potentiated the effects of ethanol on htr3a mRNA expression and enhanced ethanol-induced conditioned place preferences. These results suggest that ethanol upregulates htr3a levels through mechanisms involving H3K9 acetylation, and that histone acetylation may be a therapeutic target for treating ethanol abuse.

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.

The oxytocin receptor impairs ethanol reward in mice

It is well established that oxytocin, and its receptor (OxtR), play a crucial role in addiction and that the stimulation of oxytocin neurotransmission reduces addictive behaviors to ethanol in laboratory animals. However, the impact of OxtR modulation on acquisition, extinction and reinstatement of drug-elicited ethanol-conditioned place preference (EtOH-CPP) has not yet been investigated. In this study, we evaluated the effects of OxtR pharmacological modulation, using the oxytocin analog Carbetocin, and genetic overexpression in the nucleus accumbens (NAcc), using lentiviral-mediated gene transfer technology, of the OxtR on acquisition, extinction and reinstatement of drug-elicited EtOH-CPP in mice. In the first experiment, results showed that Carbetocin administration and NAcc OxtR-overexpression (LV-OxtR) reduced EtOH-CPP establishment. In the second experiment, systemic Carbetocin treatment and OxtR overexpression resulted in decreased time spent in the ethanol-paired compartment following completion of a 7-day extinction protocol. Finally, the third experiment showed that Carbetocin and LV-OxtR suppressed primed reinstatement of EtOH-CPP. It is concluded that pharmacological and genetic modulation of the OxtR can modulate the acquisition, extinction, and reinstatement of conditioned reinforcing effects of ethanol. Taken together, the current findings add to the growing literature on oxytocin neurotransmission modulation in the pharmacotherapy of ethanol addiction and alcoholism.

A Scutellaria baicalensis radix water extract inhibits morphine-induced conditioned place preference

CONTEXT

Scutellaria baicalensis Georgi (Lamiaceae) has been used as a traditional herbal preparation for the treatment of neuropsychiatric disorders in Asian countries for centuries.

OBJECTIVE

To evaluate the effects of S. baicalensis on morphine-induced drug dependence in rats.

MATERIALS AND METHODS

In order to evaluate the effect of S. baicalensis and baicalin on morphine-induced dependence-like behavior, a water extract of S. baicalensis [500 mg/kg, intraperitoneally (i.p.)] or baicalin (50 mg/kg, i.p., a flavonoid found in S. baicalensis) was administered prior to morphine injection [5 and 2.5 mg/kg, respectively, subcutaneously (s.c.)] to rats for 8 and 4 d, respectively. Morphine-induced conditioned place preference was assessed by measuring the time spent in a drug-paired chamber. The effect of S. baicalensis on dopamine receptor supersensitivity (locomotor activity) and dopamine agonist-induced climbing behavior due to a single apomorphine treatment (2 mg/kg, s.c.) was also measured.

RESULTS

At 50 mg/kg, a water extract of S. baicalensis decreased morphine (5 mg/kg)-induced conditioned place preference by 86% in rats. Apomorphine (2 mg/kg)-induced locomotor activity (dopamine receptor supersensitivity) in rats and climbing behavior in mice were attenuated after pretreatment with 500 mg/kg of S. baicalensis water extract by 41% and 56%, respectively. In addition, baicalin-reduced morphine-induced conditioned places preference by 86% in rats at 50 mg/kg.

DISCUSSION AND CONCLUSION

These results suggest that S. baicalensis can ameliorate drug addiction-related behavior through functional regulation of dopamine receptors.

Cocaine seeking and taking: role of hippocampal dopamine D1-like receptors

Despite the well-documented involvement of dopamine D1-like receptor stimulation in cocaine-induced goal-directed behaviours, little is known about the specific contribution of D1-like receptor populations in the dorsal hippocampus (DH) to drug context-induced cocaine-seeking or drug-reinforced instrumental behaviours. To investigate this question, rats were trained to lever press for un-signalled cocaine infusions in a distinct context followed by extinction training in a different context. Cocaine-seeking behaviour (non-reinforced lever responding) was then assessed in the previously cocaine-paired and extinction contexts. SCH23390-induced D1-like receptor antagonism in the DH, but not the overlying trunk region of the somatosensory cortex, dose-dependently inhibited drug context-induced cocaine-seeking behaviour, without altering cocaine-reinforced instrumental responding, cocaine intake, food-reinforced instrumental responding, or general motor activity, relative to vehicle treatment. These findings suggest that D1-like receptor stimulation in the DH is critical for the incentive motivational effects and/or memory of cocaine-paired contextual stimuli that contribute to drug-seeking behaviour.

NMDA receptors in the midbrain play a critical role in dopamine-mediated hippocampal synaptic potentiation caused by morphine

A single exposure to drugs of abuse produces an NMDAR (N-methyl-D-aspartate receptor)-dependent synaptic potentiation at excitatory synapses of dopamine (DA) neurons in the ventral tegmental area (VTA) of the midbrain. All addictive drugs can increase DA concentrations in projection areas of the midbrain, including the hippocampus. Hippocampal DA release subsequently modulates hippocampal plasticity and drug-associated memories. Using in vivo electrophysiological recording techniques in anesthetized rats, we show that systemic injection of morphine induced hippocampal synaptic potentiation in a dose-dependent manner. Intra-VTA but not intra-hippocampus injection of morphine evoked this potentiation. Local hippocampal dopamine D1 receptors (D1R) are required in the morphine-induced synaptic potentiation and conditioned place preference (CPP). Moreover, both NMDAR activation in the VTA and VTA/hippocampus dopaminergic connections are essential for the morphine-evoked potentiation and CPP. These findings suggest that NMDAR signalings in the midbrain play a key role in regulating dopamine-mediated hippocampal synaptic plasticity underlying drug-induced associative memory.

Cannabinoid receptors in the basolateral amygdala are involved in the potentiation of morphine rewarding properties in the acquisition, but not expression of conditioned place preference in rats

Several studies show the role of the basolateral amygdala (BLA) in drug-seeking, relapse and the brain׳s emotional systems. Several lines of evidence indicate a functional interaction between opioid and endogenous cannabinoid systems. In the present study, we investigated the role of intra-BLA cannabinoid CB1 receptors in the potentiation, acquisition and expression of morphine-induced conditioned place preference (CPP). One-hundred and forty-two adult male Wistar rats weighing 230-280g were bilaterally implanted by two separate cannulae into the BLA. The CPP paradigm was done, and conditioning score and locomotor activity were recorded by Ethovision software. Results showed that intra-BLA administration of different doses of WIN55,212-2 (1, 2 and 4mmol/0.3µl DMSO) as a cannabinoid receptor agonist during the conditioning phase induced place preference in animals that received the ineffective (2mg/kg) dose of morphine compared to respective control group in saline-treated animals. On the other hand, intra-BLA injection of the cannabinoid CB1 receptor antagonist AM251 (45 and 90µmol/0.3µl DMSO) during the 3-day conditioning phase reduced morphine-induced CPP. Furthermore, microinjection of both AM251 (15, 45 and 90µmol) and WIN55,212-2 (1-4mmol), into the BLA had no effect on the expression of morphine (5mg/kg)-induced CPP. Our findings suggest that cannabinoid CB1 receptors in the BLA are involved in the development of reward-related behaviors and they can potentiate the rewarding effects of morphine. It seems that the glutamatergic projection from the BLA to the nucleus accumbens and reward-related learning in the hippocampus may be involved in the acquisition and expression of opioid reward-related behaviors in rats.

Role of dorsal hippocampal orexin-1 receptors in associating morphine reward with contextual stimuli

In this study, we evaluated the role of orexin receptors in the dorsal hippocampus (dHPC) in the development of morphine-induced conditioned place preference (CPP) and modification of hippocampal c-Fos and cyclic AMP response element-binding protein (CREB) levels. Orexin-A (0.5, 5, and 50 pmol) and the orexin-1 receptor antagonist, SB334867 (10, 20, and 40 nmol), were bilaterally infused into the dHPC immediately before conditioning with morphine (0.5 or 7.5 mg/kg) using the CPP task. Western blotting was then used to measure the protein levels of c-Fos, total CREB, and phosphorylated CREB (pCREB) in the hippocampus. Orexin did not enhance the rewarding efficacy of morphine (0.5 mg/kg), but caused a reduction in hippocampal c-Fos. Successful conditioning with morphine (7.5 mg/kg) was associated with increased levels of hippocampal c-Fos and CREB, but with decreased CREB phosphorylation. Intrahippocampal administration of SB334867 before conditioning sessions disrupted the rewarding effect of morphine (7.5 mg/kg) and blocked morphine-induced increases in hippocampal CREB protein levels. The results suggest that orexin signaling within the dHPC is necessary for the development of morphine CPP. Morphine reward is related to altered levels of hippocampal c-Fos and CREB. Inhibition of morphine-induced increases in CREB levels might be the underlying mechanism for the disruption of morphine CPP.

Withania somnifera prevents acquisition and expression of morphine-elicited conditioned place preference

Previous studies have reported that some of the central effects of morphine are counteracted by the administration of the methanolic extract of the root of Indian ginseng, Withania somnifera Dunal (WSE). The present study sought to determine whether WSE affects acquisition and expression of morphine-elicited conditioned place preference (CPP) in CD-1 mice. In CPP acquisition experiments, WSE (0, 25, 50, and 100 mg/kg) was administered, during conditioning, 30 min before morphine (10 mg/kg), whereas in expression experiments, WSE (0, 25, 50, and 100 mg/kg) was administered 30 min before the postconditioning test. The results demonstrate (i) that WSE was devoid of motivational properties; (ii) that WSE (100 mg/kg) was devoid of effects on spontaneous and morphine-stimulated motor activity and on spatial memory; and (iii) that WSE (50 and 100 mg/kg) significantly prevented the acquisition and expression of CPP. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for µ-opioid and γ-aminobutyric acid B receptors. These experiments revealed a higher affinity of WSE for γ-aminobutyric acid B than for µ-opioid receptors. Overall, these results point to WSE as an interesting alternative tool, worthy of further investigation, to study opiate addiction.

Synergistic effects between CA1 mu opioid and dopamine D1-like receptors in impaired passive avoidance performance induced by hepatic encephalopathy in mice

BACKGROUND AND AIM

Numerous investigations have indicated that hepatic encephalopathy (HE) alters the levels of various neurotransmitters. However, comprehensive data regarding the effects of CA1 opioidergic and dopaminergic (DAergic) systems on HE-induced amnesia are still lacking.

METHODS

Following intra-dorsal hippocampal (CA1) injection of mu opioid and dopamine D1- and D2-like receptors antagonists in male mice, one-trial step-down and hole-board paradigms were used to assess memory and exploratory behaviors, respectively.

RESULTS

Our data demonstrated that HE impairs memory 24 days after bile duct ligation (BDL). Furthermore, while the higher dose of DA D1-like receptor antagonist (SCH23390, 0.5 μg/mouse) induced amnesia and anxiogenic-like behaviors, mu receptor antagonist (naloxone: 0.0125, 0.025 and 0.05 μg/mouse) and DA D2-like receptor antagonist (sulpiride: 0.0625, 0.125 and 0.25 μg/mouse) by themselves, could not exert an effect on memory performance in passive avoidance task. On the other hand, pre-test injection of all drugs reversed the HE-induced amnesia 24 days after BDL, while having no effect on exploratory behaviors. Pre-test co-administration of the subthreshold dose SCH23390 (0.25 μg/mouse) and sulpiride (0.0625 μg/mouse) or naloxone (0.0125 μg/mouse) could likewise reverse the BDL-induced amnesia. However, when the subthreshold sulpiride plus naloxone were co-administered, BDL-induced amnesia was not blocked.

CONCLUSIONS

Memory performance is impaired 24 days post BDL and CA1 mu opioid and DA D1-like receptors antagonist synergistic effects are likely involved in this phenomenon.

Glutamate mechanisms underlying opiate memories

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

Up-regulation of dopamine D1 receptor in the hippocampus after establishment of conditioned place preference by cocaine

The hippocampus plays an important role in the formation of contextual memory between the environment and the rewarding effect of abused drugs. The dopaminergic neural transmission in the hippocampus seems to be critical for such memory. Using conditioned place preference in rats, we found that the protein level of the dopamine D(1) receptor and its prerequisite mRNA in the hippocampus increased in animals that showed a clear preference for the environment paired with cocaine. The increase was not a simple reflection of the repeated administration of cocaine. Instead, it is attributable to conditioning, because systematic contingency between drug administration and exposure to a particular environment was necessary for the increase. Furthermore, we found that the mRNA of the dopamine D(1) receptors increased in the granule cell layer of the dentate gyrus. These results suggest that the alteration of dopamine D(1) receptor in the hippocampus, especially in the dentate gyrus, is related to the induction of drug-induced contextual memory. The finding implicates the relevance of the dopaminergic signal transduction in the hippocampus to drug dependence.

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.

Activation of phosphatidylinositol 3-kinase/Akt-mammalian target of Rapamycin signaling pathway in the hippocampus is essential for the acquisition of morphine-induced place preference in rats

Hippocampus is a critical structure for the acquisition of morphine-induced conditioned place preference (CPP), which is a usual learning paradigm for assessing drug reward. However, the precise mechanisms remain largely unknown. Phosphatidylinositol 3-kinase (PI3K) and its downstream targets, including Akt, mammalian target of Rapamycin (mTOR) and 70-kDa ribosomal S6 kinase (p70S6K), are critical molecules implicated in learning and memory. Here, we tested the role of PI3K/Akt-mTOR-p70S6K signaling pathway in morphine-induced CPP in the hippocampus. Our results showed that the acquisition of morphine CPP increased phosphorylation of Akt in the hippocampal CA3, but not in the nucleus accumbens (NAc), the ventral tegmental area (VTA) or the CA1. Moreover, the phosphorylated Akt exclusively expressed in the CA3 neurons. Likewise, levels of phosphorylated mTOR and p70S6K were significantly enhanced in the CA3 following morphine CPP. The alterations of these phosphorylated proteins are positively correlated with the acquisition of morphine CPP. More importantly, microinjection of PI3K inhibitor (LY294002) or mTOR inhibitor (Rapamycin) into the CA3 prevented the acquisition of CPP and inhibited the activation of PI3K-Akt signaling pathway. In addition, pre-infusion of β-FNA (β-funaltrexamine hydrochloride), a selective irreversible μ opioid receptor antagonist, into CA3 significantly prevented the acquisition of CPP and impaired Akt phosphorylation. All these results strongly implied that the PI3K-Akt signaling pathway activated by μ opioid receptor in hippocampal CA3 plays an important role in acquisition of morphine-induced CPP.

Effect of 5-aza-2-deoxycytidine microinjecting into hippocampus and prelimbic cortex on acquisition and retrieval of cocaine-induced place preference in C57BL/6 mice

The long lasting addiction-related abnormal memory is one of the most important foundations for relapse. DNA methylation may be a possible mechanism for persistence of such memory. Here we injected the DNA methyltransferases (DNMTs) inhibitor, 5-aza-2-deoxycytidine (5-aza) into hippocampus CA1 area and prelimbic cortex during the stages of acquisition and expression of cocaine-induced place preference in C57BL/6 mice. Results showed that in CA1 DNA methylation inhibitors could restrain acquisition but had no impact on expression of the cocaine-induced conditioned place preference (CPP). On the contrary, in prelimbic cortex, 5-aza had no effect on acquisition but blocked expression. Our results indicated that DNA methylation in hippocampus is required for learning; while DNA methylation in prelimbic cortex is necessary for memory retrieval. The present finding is consistent with the role of the hippocampus as a structure contributing to cocaine-induced memory acquisition, and prelimbic cortex, a part of prefrontal cortex as an area responsible for cocaine-induced memory retrieval. In conclusion, DNA methylation does play an important role in drug-induced learning and memory although the detailed effect still calls for further research.