The dopamine D3 receptor antagonist YQA14 that inhibits the expression and drug-primed reactivation of morphine-induced conditioned place preference in rats

Blockade of the Dopamine D3 Receptor Attenuates Opioids-Induced Addictive Behaviours Associated with Inhibiting the Mesolimbic Dopamine System

Opioid use disorder (OUD) has become a considerable global public health challenge; however, potential medications for the management of OUD that are effective, safe, and nonaddictive are not available. Accumulating preclinical evidence indicates that antagonists of the dopamine D3 receptor (D3R) have effects on addiction in different animal models. We have previously reported that YQA14, a D3R antagonist, exhibits very high affinity and selectivity for D3Rs over D2Rs, and is able to inhibit cocaine- or methamphetamine-induced reinforcement and reinstatement in self-administration tests. In the present study, our results illustrated that YQA14 dose-dependently reduced infusions under the fixed-ratio 2 procedure and lowered the breakpoint under the progressive-ratio procedure in heroin self-administered rats, also attenuated heroin-induced reinstatement of drug-seeking behavior. On the other hand, YQA14 not only reduced morphine-induced expression of conditioned place preference but also facilitated the extinguishing process in mice. Moreover, we elucidated that YQA14 attenuated opioid-induced reward or reinforcement mainly by inhibiting morphine-induced up-regulation of dopaminergic neuron activity in the ventral tegmental area and decreasing dopamine release in the nucleus accumbens with a fiber photometry recording system. These findings suggest that D3R might play a very important role in opioid addiction, and YQA14 may have pharmacotherapeutic potential in attenuating opioid-induced addictive behaviors dependent on the dopamine system.

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Ferulic acid-loaded nanostructure prevents morphine reinstatement: the involvement of dopamine system, NRF2, and ΔFosB in the striatum brain area of rats

Morphine is among the most powerful analgesics and pain-relieving agents. However, its addictive properties limit their medical use because patients may be susceptible to abuse and reinstatement. Morphine addiction occurs because of dopamine release in the mesolimbic brain area, implying in an increase in oxidative stress. Ferulic acid (FA), a phenolic phytochemical found in a variety of foods, has been reported to exert antioxidant and neuroprotective effects; however, its low bioavailability makes its nano-encapsulated form a promising alternative. This study aimed to evaluate the protective effects of a novel nanosystem with FA on morphine reinstatement and the consequent molecular neuroadaptations and oxidative status in the mesolimbic region. Rats previously exposed to morphine in conditioned place preference (CPP) paradigm were treated with ferulic acid-loaded nanocapsules (FA-Nc) or nonencapsulated FA during morphine-preference extinction. Following the treatments, animals were re-exposed to morphine to induce the reinstatement. While morphine-preference extinction was comparable among all experimental groups, FA-Nc treatment prevented morphine reinstatement. In the dorsal striatum, while morphine exposure increased lipid peroxidation (LP) and reactive species (RS), FA-Nc decreased LP and FA decreased RS levels. Morphine exposure increased the dopaminergic markers (D1R, D3R, DAT) and ΔFosB immunoreactivity in the ventral striatum; however, FA-Nc treatment decreased D1R, D3R, and ΔFosB and increased D2R, DAT, and NRF2. In conclusion, FA-Nc treatment prevented the morphine reinstatement, promoted antioxidant activity, and modified the dopaminergic neurotransmission, NRF2, and ΔFosB, what may indicate a neuroprotective and antioxidant role of this nanoformulation.

Knockout of Dopamine D3 Receptor Gene Blocked Methamphetamine-Induced Distinct Changes of Dopaminergic and Glutamatergic Synapse in the Nucleus Accumbens Shell of Mice

Structural plasticity changes in the brain are thought to underlie, at least partially, drug-induced persistent changes in behavior. Our previous study reported that increased synaptic density in the nucleus accumbens shell (NAcsh) correlates with and may contribute to behavioral sensitization induced by methamphetamine (METH). However, the distinct changes of dopaminergic and glutamatergic synapses and the modulating effects of dopamine D3 receptor remain unclear. In the current study, we used immunohistochemistry electron-microscopy and immunofluorescence to detect the changes of dopamine D1, D2, and glutamate NR2B-positive synapses and cells in the NAcsh of METH-sensitized wild type (WT) and knockout of dopamine D3 receptor gene (D3^–/–) mice. We found that METH induced long-term behavioral sensitization in WT mice, which was accompanied by an increased number and rate of dopamine D1 receptor-positive synapses and cells, as well as glutamate NR2B-positive synapses and cells. In contrast, the number and rate of dopamine D2 receptor-positive synapses and cells were significantly decreased in the NAcsh of METH-sensitized WT mice. D3^–/– mice exhibited attenuated acute locomotor responses and behavioral sensitization to METH compared with WT mice. Moreover, the knockout of dopamine D3 receptor gene inhibited METH-induced changes of dopaminergic and glutamatergic synapses in the NAcsh of METH-sensitized mice. Taken together, our results suggest that METH induced distinct changes of dopaminergic and glutamatergic synapses and cells in the NAcsh of mice, which was blocked by the knockout of dopamine D3 receptor gene, and may contribute to, at least partially, METH-induced behavior sensitization as well as the modulating effect of the dopamine D3 receptor.

Effects of the selective dopamine D3 receptor antagonist PG01037 on morphine-induced hyperactivity and antinociception in mice

Recent preclinical studies have reported that pretreatment with the novel and highly-selective dopamine D₃ receptor (D₃R) antagonists R-VK4-40 or VK4-116 attenuates the abuse-related behavioral effects of oxycodone while enhancing its analgesic properties. However, whether these observed effects are generalizable to the broad class of D₃R antagonists and/or extend to opioids other than oxycodone has not been extensively explored. The present study sought to assess the impact of pretreatment with another selective D₃R antagonist, PG01037, on several behavioral effects of morphine in mice. C57Bl/6 J mice were pretreated with PG01037 (0-10 mg/kg) and tested for 1) hyperlocomotion induced by acute morphine (5.6-56 mg/kg), 2) locomotor sensitization following repeated morphine (56 mg/kg), 3) antinociception following acute morphine (18 mg/kg), and 4) catalepsy following administration of PG01037 alone or in combination with morphine (56 mg/kg). PG01037 dose-dependently attenuated morphine-induced hyperlocomotion and morphine-induced antinociception at doses that did not alter basal locomotion or nociception alone, but did not prevent the induction of locomotor sensitization following repeated morphine administration. Moreover, PG01037 did not induce catalepsy either alone or in combination with morphine. These results suggest that attenuation of acute opioid-induced hyperactivity may be a behavioral effect shared among D₃R-selective antagonists, thus supporting continued investigations into their use as potential treatments for opioid use disorder. However, PG01037 is unlike newer, highly-selective D₃R antagonists in its capacity to reduce opioid-induced antinociception, indicating that modulation of opioid analgesia may vary across different D₃R antagonists.

Topiramate-chitosan nanoparticles prevent morphine reinstatement with no memory impairment: Dopaminergic and glutamatergic molecular aspects in rats

Besides their clinical application, chronic misuse of opioids has often been associated to drug addiction due to their addictive properties, underlying neuroadaptations of AMPA glutamate-receptor-dependent synaptic plasticity. Topiramate (TPM), an AMPAR antagonist, has been used to treat psychostimulants addiction, despite its harmful effects on memory. This study aimed to evaluate the effects of a novel topiramate nanosystem on molecular changes related to morphine reinstatement. Rats were previously exposed to morphine in conditioned place preference (CPP) paradigm and treated with topiramate-chitosan nanoparticles (TPM-CS-NP) or non-encapsulated topiramate in solution (S-TPM) during CPP extinction; following memory performance evaluation, they were re-exposed to morphine reinstatement. While morphine-CPP extinction was comparable among all experimental groups, TPM-CS-NP treatment prevented morphine reinstatement, preserving memory performance, which was impaired by both morphine-conditioning and S-TPM treatment. In the NAc, morphine increased D1R, D2R, D3R, DAT, GluA1 and MOR immunoreactivity. It also increased D1R, DAT, GluA1 and MOR in the dorsal hippocampus. TPM-CS-NP treatment decreased D1R, D3R and GluA1 and increased DAT in the NAc, decreasing GluA1 and increasing D2 and DAT in the dorsal hippocampus. Taken together, we may infer that TPM-CS-NP treatment was able to prevent the morphine reinstatement without memory impairment. Therefore, TPM-CS-NP may be considered an innovative therapeutic tool due to its property to prevent opioid reinstatement because it acts modifying both dopaminergic and glutamatergic neurotransmission, which are commonly related to morphine addiction.

Dopamine D3 receptor-based medication development for the treatment of opioid use disorder: Rationale, progress, and challenges

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the current national opioid crisis. Although several opioid-based pharmacotherapies are available (e.g., methadone, buprenorphine, naloxone), they show limited effectiveness in long-term relapse prevention. In response to the opioid crisis, the National Institute on Drug Abuse proposed a list of pharmacological targets of highest priority for medication development for the treatment of opioid use disorders (OUD). Among these are antagonists of dopamine D3 receptors (D3R). In this review, we first review recent progress in research of the dopamine hypothesis of opioid reward and abuse and then describe the rationale and recent development of D3R ligands for the treatment of OUD. Herein, an emphasis is placed on the effectiveness of newly developed D3R antagonists in the animal models of OUD. These new drug candidates may also potentiate the analgesic effects of clinically used opioids, making them attractive as adjunctive medications for pain management and treatment of OUD.

Newly Developed Dopamine D3 Receptor Antagonists, R-VK4-40 and R-VK4-116, Do Not Potentiate Cardiovascular Effects of Cocaine or Oxycodone in Rats

Opioid and cocaine abuse are major public health burdens. Existing medications for opioid use disorder are limited by abuse liability and side effects, whereas no treatments are currently approved in the United States for cocaine use disorder. Dopamine D3 receptor (D3R) antagonists have shown promise in attenuating opioid and cocaine reward and mitigating relapse in preclinical models. However, translation of D3R antagonists to the clinic has been hampered by reports that the D3R antagonists GSK598,809 (5-(5-((3-((1S,5R)-1-(2-fluoro-4-(trifluoromethyl)phenyl)-3-azabicyclo[3.1.0]hexan-3-yl)propyl)thio)-4-methyl-4H-1,2,4-triazol-3-yl)-4-methyloxazole) and SB-277,011A (2-(2-((1r,4r)-4-(2-oxo-2-(quinolin-4-yl)ethyl)cyclohexyl)ethyl)-1,2,3,4-tetrahydroisoquinoline-6-carbonitrile) have adverse cardiovascular effects in the presence of cocaine. Recently, we developed two structurally novel D3R antagonists, R-VK4-40 and R-VK4-116, which are highly selective for D3R and display translational potential for treatment of opioid use disorder. Here, we tested whether R-VK4-40 ((R)-N-(4-(4-(2-Chloro-3-ethylphenyl)piperazin-1-yl)-3-hydroxybutyl)-1H-indole-2-carboxamide) and R-VK4-116 ((R)-N-(4-(4-(3-Chloro-5-ethyl-2-methoxyphenyl)piperazin-1-yl)-3-hydroxybutyl)-1H-indole-2-carboxamide) have unwanted cardiovascular effects in the presence of oxycodone, a prescription opioid, or cocaine in freely moving rats fitted with surgically implanted telemetry transmitters. We also examined cardiovascular effects of the D3R antagonist, SB-277,011A, and L-741,626 (1-((1H-indol-3-yl)methyl)-4-(4-chlorophenyl)piperidin-4-ol), a dopamine D2 receptor-selective antagonist, for comparison. Consistent with prior reports, SB-277,011A increased blood pressure, heart rate, and locomotor activity alone and in the presence of cocaine. L-741,626 increased blood pressure and heart rate. In contrast, R-VK4-40 alone dose-dependently reduced blood pressure and heart rate and attenuated oxycodone-induced increases in blood pressure and oxycodone or cocaine-induced increases in heart rate. Similarly, R-VK4-116 alone dose-dependently reduced cocaine-induced increases in blood pressure and heart rate. These results highlight the safety of new D3R antagonists and support the continued development of R-VK4-40 and R-VK4-116 for the treatment of opioid and cocaine use disorders. SIGNIFICANCE STATEMENT: Opioid and cocaine abuse are major public health challenges and new treatments that do not adversely impact the cardiovascular system are needed. Here, we show that two structurally novel dopamine D3 receptor antagonists, R-VK4-40 and R-VK4-116, do not potentiate, and may even protect against, oxycodone- or cocaine-induced changes in blood pressure and heart rate, supporting their further development for the treatment of opioid and/or cocaine use disorders.

Selective dopamine D3 receptor antagonist YQA14 inhibits morphine-induced behavioral sensitization in wild type, but not in dopamine D3 receptor knockout mice

Increasing preclinical evidence demonstrates that dopamine D3 receptor (D3R) antagonists are a potential option for the treatment of drug addiction. The reinstatement of the addiction can be triggered by environmental stimuli that acquire motivational salience through repeated associations with the drug's effects. YQA14 is a novel D3R antagonist that has exhibited pharmacotherapeutic efficacy in reducing cocaine and amphetamine reward and relapse to drug seeking in mice. In this study we investigated the effects of YQA14 on morphine-induced context-specific locomotor sensitization in mice. We showed that repeated injection of YQA14 (6.25-25 mg/kg every day ip) prior to morphine (10 mg/kg every day sc) not only inhibited the acquisition, but also significantly attenuated the expression of morphine-induced locomotor sensitization. Furthermore, in the expression phase, one single injection of YQA14 (6.25-25 mg/kg, ip) dose-dependently inhibited the expression of morphine-induced behavioral sensitization. Moreover, YQA14 inhibited the expression of morphine-induced behavioral sensitization in wild mice (WT), but not in D3R knockout (D3R^-/-) mice in the expression phase. In addition, D3R^-/- mice also displayed the reduction in the expression phase compared with WT mice. In summary, this study demonstrates that blockade or knockout of the D3R inhibits morphine-induced behavior sensitization, suggesting that D3R plays an important role in the pathogenesis and etiology of morphine addiction, and it might be a potential target for clinical management of opioid addiction.

Role of dopamine D3 receptor in alleviating behavioural deficits in animal models of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a complicated psychiatric disorder, which occurs after exposure to a traumatic event. The main clinical manifestation of PTSD includes fear and stress dysregulation. In both animals and humans, dysregulation of dopamine function appears to be related to conditioned fear responses. Previous studies show that the dopamine D3 receptor (D3R) is involved in schizophrenia, autism, and substance use disorders and is related to emotional disorders. However, few studies have investigated the role of the D3R in the pathogenesis and aetiology of PTSD. In the current study, we have reported that D3R knockout (D3R^-/-) mice displayed decreased freezing time of contextual fearing and anxiolytic effects following training sessions consisting of exposure to inescapable electric foot-shocks. Similarly, highly selective blockade of D3Rs by YQA14, a novel D3R antagonist, significantly ameliorated freezing and anxiogenic-like behaviours in the single-prolonged stress (SPS) model of PTSD in rats. And more, YQA14 selectively alleviated the symptoms of PTSD in WT mice but not in D3R^-/- mice. In summary, this study demonstrates the anti-PTSD effects of blockade or knockout of the D3R, suggesting that the D3R might play an important role in the pathogenesis and aetiology of PTSD, and might be a potential target for the clinical management of PTSD.

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.

Dopamine D3/D2 Receptor Antagonist PF-4363467 Attenuates Opioid Drug-Seeking Behavior without Concomitant D2 Side Effects

Dopamine receptor antagonism is a compelling molecular target for the treatment of a range of psychiatric disorders, including substance use disorders. From our corporate compound file, we identified a structurally unique D3 receptor (D3R) antagonist scaffold, 1. Through a hybrid approach, we merged key pharmacophore elements from 1 and D3 agonist 2 to yield the novel D3R/D2R antagonist PF-4363467 (3). Compound 3 was designed to possess CNS drug-like properties as defined by its CNS MPO desirability score (≥4/6). In addition to good physicochemical properties, 3 exhibited low nanomolar affinity for the D3R (D3 K_i = 3.1 nM), good subtype selectivity over D2R (D2 K_i = 692 nM), and high selectivity for D3R versus other biogenic amine receptors. In vivo, 3 dose-dependently attenuated opioid self-administration and opioid drug-seeking behavior in a rat operant reinstatement model using animals trained to self-administer fentanyl. Further, traditional extrapyramidal symptoms (EPS), adverse side effects arising from D2R antagonism, were not observed despite high D2 receptor occupancy (RO) in rodents, suggesting that compound 3 has a unique in vivo profile. Collectively, our data support further investigation of dual D3R and D2R antagonists for the treatment of drug addiction.

P-Glycoprotein (ABCB1) limits the brain distribution of YQA-14, a novel dopamine D3 receptor antagonist

YQA-14 is a promising agent for treating addiction to cocaine and opioids. However, previous studies have showed there is marked contrast between the relatively small differences in pharmacological action in vivo and the large differences in their respective receptor binding properties in vitro. We hypothesized that the conflict between the in vivo and in vitro outcomes was attributable to poor brain exposure to YQA-14 caused by drug efflux transporters. To address this issue, we investigated the directional flux of YQA-14 across Caco-2 cells at 37°C or 4°C and the bidirectional transport in the presence and absence of transporter chemical inhibitors. These phenomena were further investigated by an in vivo determination of the brain and blood pharmacokinetics (PK) profile of YQA-14 following intraperitoneal administration with and without inhibitor. The efflux ratio of YQA-14 on Caco-2 cell monolayers was 2.39 and the efflux was temperature-dependent. When co-incubated with GF120918 or LY335979, the efflux of YQA-14 was markedly decreased. However, there was no significant difference in the permeability of YQA-14 when the cells were treated with Ko143. In vivo experiments showed that the brain-to-plasma ratio increased by more than 75-fold and 20-fold with co-administration of GF120918 and LY335979, respectively. Use of Ko143 did not change the brain-to-blood ratio of YQA-14. The results indicate that the brain distribution of YQA-14 was restricted because of active efflux transport at the blood brain barrier. In addition, P-glycoprotein (P-gp) played a dominant role in limiting the distribution of YQA-14 to the brain.

The dopamine D3 receptor antagonist, SR 21502, facilitates extinction of cocaine conditioned place preference

BACKGROUND

Pharmacotherapeutic agents that could facilitate extinction of cocaine cues would be useful in the treatment of cocaine addiction. We tested whether SR 21502, a selective dopamine (DA) D3 receptor antagonist, can facilitate extinction of cocaine conditioned place preference (CPP) in rats.

METHODS

In experiment 1, cocaine (10mg/kg) CPP was first established and then extinguished. During the extinction phase the rats were injected with SR 21502 and placed in the previously cocaine-paired compartment for four sessions and vehicle in the other compartment on four alternating sessions. The rats were then tested again for cocaine CPP. In experiment 2, different groups of rats were trained to associate SR 21502 with one compartment and saline with the other.

RESULTS

In experiment 1, the animals spent significantly more time in the cocaine-paired compartment after cocaine conditioning than they did before conditioning. Subsequently, the animals treated with SR 21502 during the extinction phase spent significantly less time in the cocaine-paired compartment than the vehicle group. In experiment 2, animals conditioned with SR 21502 preferred neither side of the CPP apparatus, indicating that SR 21502 produced no effects of its own.

CONCLUSIONS

These findings suggest that treatment with SR 21502, a DA D3 receptor antagonist, in the presence of cocaine cues can facilitate extinction of cocaine CPP and further suggest that this compound might be an effective cocaine addiction treatment.

A selective D3 receptor antagonist YQA14 attenuates methamphetamine-induced behavioral sensitization and conditioned place preference in mice

AIM

We have reported that a selective dopamine D3 receptor antagonist YQA14 attenuates cocaine reward and relapse to drug-seeking in mice. In the present study, we investigated whether YQA14 could inhibit methamphetamine (METH)-induced locomotor sensitization and conditioned place preference (CPP) in mice.

METHODS

Locomotor activity was monitored in mice treated with METH (1 mg/kg, ip) daily on d 4-13, followed by a challenge with METH (0.5 mg/kg) on d 21. CPP was examined in mice that were administered METH (1 mg/kg) or saline alternately on each other day for 8 days (METH conditioning). YQA14 was injected intraperitoneally 20 min prior to METH or saline.

RESULTS

Both repetitive (daily on d 4-13) and a single injection (on the day of challenge) of YQA14 (6.25, 12.5 and 25 mg/kg) dose-dependently inhibited the acquisition and expression of METH-induced locomotor sensitization. However, repetitive injection of YQA14 (daily during the METH conditioning) did not alter the acquisition of METH-induced CPP, whereas a single injection of YQA14 (prior to CPP test) dose-dependently attenuated the expression of METH-induced CPP. In addition, the repetitive injection of YQA14 dose-dependently facilitated the extinction and decreased the reinstatement of METH-induced CPP.

CONCLUSION

Brain D3 receptors are critically involved in the reward and psychomotor-stimulating effects of METH. Thus, YQA14 deserves further study as a potential medication for METH addiction.

The selective dopamine D3 receptor antagonist, SR 21502, reduces cue-induced reinstatement of heroin seeking and heroin conditioned place preference in rats

BACKGROUND

Because the role of dopamine (DA) D3 receptors has been investigated primarily in relation to cocaine-related behaviors little is known of the role of these receptors in heroin seeking.

PURPOSES

To investigate the effect of the selective DA D3 receptor antagonist, SR 21502, on cue-induced reinstatement of heroin seeking and heroin conditioned place preference (CPP).

METHODS

In experiment 1, rats were trained to self-administer intravenous heroin for 15 days followed by extinction. Following extinction animals were treated with one of several SR 21502 doses (0, 7.5, 10 or 15mg/kg) and a cue-induced reinstatement test was conducted. In experiment 2, animals were conditioned to experience heroin in one compartment of a CPP apparatus and saline in the other. On the test day animals were treated with 0, 3.75, 7.5, 10 or 15mg/kg of SR 21502 and tested for their CPP.

RESULTS

The results from experiment 1 showed a significant dose-related reduction in cue-induced reinstatement of active lever pressing in the 7.5 and 10mg groups and an absence of the reinstatement effect in the 15mg group. In experiment 2, animals treated with vehicle or 3.75mg of SR 21502 showed significant heroin place preferences but those treated with the higher doses showed no CPP.

CONCLUSIONS

Our findings suggest that DA D3 receptors play a significant role in heroin approach behaviors driven by conditioned stimuli. As such, we propose that SR 21502 holds potential as an effective pharmacotherapeutic agent for relapse prevention and should be studied further.

Blockage of acquisition and expression of morphine-induced conditioned place preference in rats due to activation of glutamate receptors type II/III in nucleus accumbens

Numerous studies have shown that glutamate in the nucleus accumbens (NAc) is an essential neurotransmitter for the extension of morphine-induced place preference. mGlu2/3 glutamate receptors in the NAc have important roles in the reward pathway. However, less is known about the role of this glutamate receptor subtype in morphine-induced conditioned place preference (CPP). In this study, we examined the effects of bilateral intra-accumbal administration of LY379268, an mGlu2/3 receptor agonist on the acquisition and expression of morphine-induced CPP in rats. Adult male Wistar rats (n=136; 220-250g) were evaluated in a CPP paradigm. Doses of LY379268 (0.3, 1 and 3μg/0.5μL saline per side) were administered into the NAc on both sides during the 3days of the conditioning (acquisition) or post-conditioning (expression) phase. The results show that bilateral intra-accumbal administration of LY379268 (0.3, 1 and 3μg) markedly decreased the acquisition of morphine-induced CPP in a dose-dependent manner. In a second series of experiments, we determined that injection of LY379268 into the NAc considerably attenuated the expression of morphine CPP only at the highest dose (3μg). Our findings suggest that activation of mGlu2/3 receptors in the NAc dose-dependently blocked both the establishment and the maintenance of morphine-induced CPP and confirmed the role of this system as a potential therapeutic target for addiction.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.