GABAB receptors within the ventral tegmental area are involved in the expression and acquisition of morphine-induced place preference in morphine-sensitized rats

Effect of ASP8062 on morphine self-administration and morphine-induced respiratory suppression in monkeys

ASP8062 is an orally available GABA_B receptor positive allosteric modulator (PAM). This study assessed the potential of ASP8062 for treating opioid use disorder (OUD). Three rhesus monkeys were pretreated with ASP8062 (0.3, 1 or 3 mg/kg) by oral administration 1 h prior to a 2-h morphine self-administration session (0.03 mg/kg, iv, per injection) under a fixed-ratio 5 schedule. We further examined the potential worsening of morphine-induced respiratory suppression by ASP8062 after coadministration of morphine (10 mg/kg, sc) and ASP8062 (10 mg/kg, po) in cynomolgus monkeys using a custom-made whole-body plethysmograph. Plasma concentrations of ASP8062 (3 or 10 mg/kg, po) were assessed in cynomolgus monkeys using liquid chromatography-tandem mass spectroscopy (LC-MS/MS). ASP8062 at 3 mg/kg, po decreased the morphine self-administrations with significant differences from the vehicle-treated group (IC₅₀ = 0.97 ± 0.36 mg/kg). Exposure levels at 3 mg/kg observed in monkeys were comparable to the clinical exposure levels which positive pharmacodynamic effects were previously shown. Further, ASP8062 did not potentiate morphine-induced respiratory suppression up to exposure levels higher than the clinically relevant dose. ASP8062 may reduce opioid use in OUD patients without affecting respiratory system, providing justification for further ASP8062 development as a potential treatment option for OUD.

Blockade of the GABAB receptor suppressed alcohol self-administration in rats: an effect similar to that produced by GABAB receptor activation

Literature data suggest that activation and blockade of the GABAB receptor may produce similar effects on several reward-related behaviours. Accordingly, the present study was designed to investigate whether treatment with the GABAB receptor antagonist, SCH 50911, reproduced the suppressing effect of the GABAB receptor agonist, baclofen, and several positive allosteric modulators of the GABAB receptor on operant oral alcohol self-administration in rats. To this end, Sardinian alcohol-preferring (sP) rats were trained to lever-respond for alcohol (15% v/v) under the fixed ratio (FR) 4 (FR4) schedule of reinforcement. Once lever-responding had stabilized, rats were exposed to test sessions preceded by treatment with SCH 50911 (0, 25, 50, and 100 mg/kg; i.p.). Two independent experiments were conducted, differing solely in the set of rats used. Selectivity of SCH 50911 effect on alcohol self-administration was assessed by evaluating the effect of SCH 50911 (0, 25, 50, and 100 mg/kg; i.p.) on self-administration of a sucrose solution (0.7% w/v) in sP rats exposed to the FR4 schedule. In both 'alcohol' experiments, treatment with SCH 50911 reduced lever-responding for alcohol and amount of self-administered alcohol. SCH 50911 effect was characterized by large interindividual variability, with several instances of dose-unrelated reductions, and frequent occurrence of complete suppression of lever-responding for alcohol. Similar data were collected in the 'sucrose' experiment. These results extend to alcohol self-administration with the notion that activation and blockade of GABAB receptor may produce unidirectional effects on reward-related behaviours; these similarities are discussed in terms of differential contribution of pre- and postsynaptic GABAB receptors.

Potentiation of morphine-induced antinociception by harmaline: involvement of μ-opioid and ventral tegmental area NMDA receptors

RATIONAL

Morphine is one of the most well-known and potent analgesic agents; however, it can also induce various side effects. Thus, finding drugs and mechanisms which can potentiate the analgesic effects of low doses of morphine will be a good strategy for pain management.

OBJECTIVE

The involvement of μ-opioid receptors and ventral tegmental area (VTA) glutamatergic system in harmaline and morphine combination on the nociceptive response were investigated. Also, we examined reward efficacy and tolerance expression following the drugs.

METHODS

Animals were bilaterally cannulated in the VTA by stereotaxic instrument. A tail-flick (TF) apparatus and conditioned place preference (CPP) paradigm were used to measure nociceptive response and rewarding effects in male NMRI mice respectively.

RESULTS

Morphine (2 mg/kg, i.p.) had no effect in TF test. Also, harmaline (1.25 and 5 mg/kg, i.p.) could not change pain threshold. Combination of a non-effective dose of harmaline (5 mg/kg) and morphine (2 mg/kg) produced antinociception and also prevented morphine tolerance but had no effect on the acquisition of CPP. Systemic administration of naloxone (0.5 and 1 mg/kg) and intra-VTA microinjection of NMDA (0.06 and 0.1 μg/mouse) before harmaline (5 mg/kg) plus morphine (2 mg/kg) prevented antinociception induced by the drugs. D-AP5 (0.5 and 1 μg/mouse, intra-VTA) potentiated the effect of low-dose harmaline (1.25 mg/kg) and morphine (2 mg/kg) and induced antinociception. Microinjection of the same doses of NMDA or D-AP5 into the VTA alone had no effect on pain threshold.

CONCLUSION

The findings showed that harmaline potentiated the analgesic effect of morphine and reduced morphine tolerance. Glutamatergic and μ-opioidergic system interactions in the VTA seem to have a modulatory role in harmaline plus morphine-induced analgesia.

GABAB receptors within the central nucleus of amygdala may involve in the morphine-induced incentive tolerance in female rats

Objective(s):

Central nucleus of amygdala (CeA) is the most important region for morphine-induced reward, and GABAergic system plays an important role on morphine reinforcement. The influence of CeA administration of GABA_B receptor agonist and antagonist on the expression and acquisition of morphine-induced incentive tolerance using conditioned place preference (CPP) paradigm was investigated in the present study. Our purpose was to evaluate the role of CeA GABA_B receptors in morphine tolerance.

Materials and Methods:

Seven days after surgery and cannulation, the experiments were begun. Subcutaneous (SC) injections of morphine induced CPP. Administration of one daily dose of morphine (12.5 mg/kg) for 3 days in order to develop tolerance to the drug reduced the conditioning induced by morphine (7.5 mg/kg, SC). GABA_B receptor agonist, baclofen (1.5, 6 and 12 µg/rat) or GABA_B receptor antagonist, CGP35348 (1.5, 6 and 12 µg/rat) were injected into the CeA 5 min before the experiments in the test day (expression of tolerance) or 5 min before each injection of morphine (12.5 mg/kg) (acquisition of tolerance).

Results:

It was shown that injections of baclofen (1.5 and 12 µg/rat) reduced acquisition, whereas the dose of 6 µg/rat of the drug exacerbated the acquisition of morphine tolerance. Baclofen at all doses significantly increased the expression of tolerance to morphine. Administration of CGP35348 (1.5, 6 and 12 µg/rat) reduced the acquisition and expression of morphine tolerance.

Conclusion:

These results confirmed the importance of GABA_B receptors with in the CeA in morphine tolerance in female rats.

Neurobiology of Kratom and its main alkaloid mitragynine

Kratom or its main alkaloid, mitragynine is derived from the plant Mitragyna speciosa Korth which is indigenous to Southeast Asian countries. This substance has become widely available in other countries like Europe and United States due to its opium- and coca-like effects. In this article, we have reviewed available reports on mitragynine and other M. speciosa extracts. M. speciosa has been proven to have a rewarding effect and is effective in alleviating the morphine and ethanol withdrawal effects. However, studies in human revealed that prolonged consumption of this plant led to dependence and tolerance while cessation caused a series of aversive withdrawal symptoms. Findings also showed that M. speciosa extracts possess antinociceptive, anti-inflammatory, anti-depressant, and muscle relaxant properties. Available evidence further supports the adverse effects of M. speciosa preparations, mitragynine on cognition. Pharmacological activities are mainly mediated via opioid receptors as well as neuronal Ca²⁺ channels, expression of cAMP and CREB protein and via descending monoaminergic system. Physicochemical properties of mitragynine have been documented which may further explain the variation in pharmacological responses. In summary, current researchs on its main indole alkaloid, mitragynine suggest both therapeutic and addictive potential but further research on its molecular effects is needed.

Effect of Transient Inactivation of Ventral Tegmental Area on the Expression and Acquisition of Nicotine-Induced Conditioned Place Preference in Rats

Background:

Nicotine can activate dopaminergic neurons within the ventral tegmental area (VTA). However, there is no evidence about complete inhibition of VTA on nicotine reinforcement.

Methods:

in the present study, we used conditioned-place preference (CPP) method to study the effect of transient inhibition of left and/or right side of the VTA by lidocaine on nicotine reward properties. Male Wistar rats seven days after recovery from surgery and cannulation were conditioned to nicotine (1.5 mg/kg) in an unbiased designed CPP apparatus. Five min before each nicotine injection in conditioning phase, lidocaine (2%) was administered either uni- or bi-laterally into the VTA (0.5µl/rat).

Results:

results revealed that lidocaine administration into the left but not right side of the VTA reduced nicotine CPP significantly. The reduction was potentiated when lidocaine injected in to both sides of the VTA. In addition, the number of compartment crossing was reduced when lidocaine injected in both side of VTA as well as left side. On the other hand, rearing was reduced when lidocaine injected to the right but not left side of VTA. At last, sniffing was reduced only in the group in which received lidocaine in both side of VTA. Sniffing and rearing increased in the group in which received lidocaine in right side.

Conclusion:

It is concluded that the right and left side of VTA play different role in nicotine-induced activity and reward.

Targeting GABAB receptors for anti-abuse drug discovery

INTRODUCTION

There is increasing evidence encouraging the development of drugs that positively modulate the γ-aminobutyric acid type B (GABA(B)) receptor for combating addiction. Compounds that target GABA(B) receptors are unique as anti-abuse therapies because of their impact against multiple addictive drugs.

AREAS COVERED

The authors present the basic information concerning the drug actions of GABA and GABA(B) receptor orthosteric agonists and positive allosteric modulators (PAM). Furthermore, they discuss several recent excellent reviews and newer results pertaining to GABA(B) receptor drug effects on responses to and self-administration of: alcohol (ethanol), nicotine, cocaine, (meth)amphetamine, and opioids. Preclinical and clinical data are considered.

EXPERT OPINION

Clinical data exist only for baclofen and mostly for alcohol use disorders. Additional trials are needed, but effects are promising. Whether PAMs, given alone or in combination with a direct GABA(B) receptor agonist, will be clinically effective and have fewer side effects requires investigation. The sedative effects of baclofen, a Food and Drug Administration (FDA)-approved drug, become less severe over time. Based on existing data, baclofen is well-tolerated. However, genetic and physiological differences are likely to contribute to individual responses to different therapeutic agents. The more immediate development of baclofen as a therapeutic for alcohol use disorders may be of significant benefit to some individuals.

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.

Transient inactivation of the nucleus accumbens reduces both the expression and acquisition of morphine-induced conditioned place preference in rats

In the present study, the effects of transient inhibition of the shell and core parts of the nucleus accumbens by lidocaine on the expression and acquisition of morphine-induced conditioned place preference in male Wistar rats were investigated. In addition, the number of bouts of sniffing, rearing, and compartment crossing was scored. Lidocaine hydrochloride was injected into different parts of the nucleus accumbens 5 min before each morphine session for the transient inhibition of particular anatomical regions. Subcutaneous (s.c.) injection of morphine (2.5 and 5mg/kg) induced place preference. Transient inhibition of the left and/or right side of the shell part of nucleus accumbens reduced morphine place conditioning. However, when both sides of the nucleus were inhibited, inhibition was weaker when compared to the results when only one side was inhibited. Also, the number of compartment crossings in these animals reduced significantly. Nevertheless, the number of rearing occurrences was reduced only when both sides of the shell part of the nucleus accumbens were inhibited. In contrast, the number of sniffing bouts increased in all three groups. The results for the core part of the nucleus accumbens also indicated that place preference was inhibited after transient inhibition of the left, right, and both sides. However, although the number of total compartment crossings was reduced in all experimental groups, the reduction was not statistically significant. The data obtained was similar to the number of rearings, yet the number of sniffing bouts increased in the experimental groups compared to the control. In conclusion, these results confirmed the involvement of the left and right sides and core and shell parts of the nucleus accumbens in morphine place conditioning.

The role of nucleus accumbens shell GABA receptors on ventral tegmental area intracranial self-stimulation and a potential role for the 5-HT(2C) receptor

Brain γ-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT)(2C) receptors are implicated in the neuronal regulation of reward- and aversion-related behaviour. Within the mesocorticolimbic pathways of the brain, relationships between GABA containing neurons and 5-HT(2C) receptor activity may be important in this context. The primary aim of this study was to investigate the role of NAc shell GABA receptors on ventral tegmental area intracranial self-stimulation (ICSS) and to examine the systemic effects of GABAergic ligands in this context. The second aim was to investigate the relationship between GABA receptor- and 5-HT(2C) receptor-related ICSS behaviour, using systemic administration of the selective agonist WAY 161503. Locomotor activity was assessed to compare the potential motor effects of drugs; feeding behaviour and intra-NAc injections of amphetamine (1.0 µg/side) were used as positive controls. When administered systemically the GABA(A) receptor agonist muscimol and antagonist picrotoxin did not selectively change ICSS reward thresholds, although the 5-HT(2C) receptor agonist WAY 161503 (1.0 mg/kg) decreased reward measures. Intra-NAc shell administration of muscimol (225 ng/side) and picrotoxin (125 ng/side), respectively, decreased and increased measures of reward. Intra-NAc shell baclofen (0-225 ng/side; GABA(B) receptor agonist) did not affect any ICSS measures although it increased feeding. Combining picrotoxin and WAY 161503 attenuated the effects of each. These results suggest that a 5-HT(2C) and GABA(A) receptor-mediated neuronal relationship in the NAc shell may be relevant for the regulation of brain reward pathways.

Both GABA(B) receptor activation and blockade exacerbated anhedonic aspects of nicotine withdrawal in rats

Nicotine dependence is maintained by the aversive, depression-like effects of nicotine withdrawal and the rewarding effects of acute nicotine. GABA(B) receptor antagonists exhibit antidepressant-like effects in rodents, whereas GABA(B) receptor agonists attenuate the rewarding effects of nicotine. Recent studies with GABA(B) receptor positive modulators showed that these compounds represent potentially improved medications for the treatment of nicotine dependence because of fewer side-effects than GABA(B) receptor agonists. Thus, GABA(B) receptor agonists and antagonists, and GABA(B) receptor positive modulators may have efficacy as smoking cessation aids by targeting different aspects of nicotine dependence and withdrawal. The present study assessed the effects of the GABA(B) receptor agonist CGP44532, the GABA(B) receptor antagonist CGP56433A, and the GABA(B) receptor positive modulator BHF177 on the anhedonic aspects of nicotine withdrawal. Rats were prepared with stimulating electrodes in the posterior lateral hypothalamus. After establishing stable intracranial self-stimulation (ICSS) thresholds, rats were prepared with subcutaneous osmotic minipumps delivering either nicotine or saline for 7 or 14days. ICSS thresholds were assessed 6h post-pump removal. Thirty hours after pump removal, CGP44532, CGP56433A, and BHF177 were administered 30min prior to ICSS testing. Both GABA(B) receptor activation (CGP44532 and BHF177) and blockade (CGP56433A) elevated ICSS thresholds in all groups, resulting in exacerbated effects of nicotine withdrawal in the nicotine-treated groups. These similar effects of GABA(B) receptor activation and blockade on the anhedonic depression-like aspects of nicotine withdrawal were surprising and perhaps reflect differential efficacy of these compounds at presynaptic hetero- and autoreceptors, as well as postsynaptic, GABA(B) receptors.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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 neurologic 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).

GABAB receptors in reward processes

gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the brain which acts through different receptor subtypes. Metabotropic GABA(B) receptors are widely distributed throughout the brain. Alterations in GABA signaling through pharmacological activation or deactivation of the GABA(B) receptor regulate behavior and brain reward processes. GABA(B) receptor agonists and, most recently, positive modulators have been found to inhibit the reinforcing effects of drugs of abuse, such as cocaine, amphetamine, nicotine, ethanol, and opiates. This converging evidence of the effects of GABA(B) compounds on the reinforcing properties of addictive drugs is based on behavioral studies that used a variety of procedures with relevance to reward processes and drug abuse liability, including intracranial self-stimulation, intravenous self-administration under both fixed- and progressive-ratio schedules of reinforcement, reinstatement, and conditioned place preference. GABA(B) receptor agonists and positive modulators block the reinforcing effects of drugs of abuse in these animal models. However, GABA(B) receptor agonists also have undesirable side-effects. GABA(B) receptor modulators have potential advantages as medications for drug addiction. These compounds have a better side-effect profile than GABA(B) agonists because they are devoid of intrinsic agonistic activity in the absence of GABA. They only exert their modulatory actions in concert with endogenous GABAergic activity. Thus, GABA(B) receptor positive modulators are promising therapeutics for the treatment of various aspects of dependence (e.g., initiation, maintenance, and relapse) on various drugs of abuse, such as cocaine, nicotine, heroin, and alcohol.

Preclinical evidence of new opioid modulators for the treatment of addiction

IMPORTANCE OF THE FIELD

Addiction to opiates is one of the most severe forms of substance dependence, and despite a variety of pharmacological approaches to treat it, relapse is observed in a high percentage of subjects. New pharmacological compounds are necessary to improve the outcome of treatments and reduce adverse side effects. Moreover, drugs that act on the opioid system can also be of benefit in the treatment of alcohol or cocaine addiction. AREA COVERED BY THIS REVIEW: Recent preclinical studies of pharmacological agents for the treatment of opiate addiction (2008 to the present date).

WHAT THE READER WILL GAIN

The reader will be informed of the latest drugs shown in animal models to modify dependence on opiates and the reinforcing effects of these drugs. In addition, reports of the latest studies to test these compounds in models of other drug addictions are reviewed.

TAKE HOME MESSAGE

The classic clinical pharmacotherapy for opiate dependence, involving mu-opioid receptor agonists or antagonists, has not yielded a high success rate in humans. In pharmacotherapy for opioid dependence, new options are emerging and different pharmacological strategies are now being tested.