Micro-opioid receptor alkylation in the ventral pallidum and ventral tegmental area, but not in the nucleus accumbens, attenuates the effects of heroin on cocaine self-administration in rats

Impact of OPRM1 (Mu-opioid Receptor Gene) A112G Polymorphism on Dual Oxycodone and Cocaine Self-administration Behavior in a Mouse Model

The use of mu-opioid receptor (MOP-r) agonists such as oxycodone together with cocaine is prevalent, and deaths attributed to using these combinations have increased.

RATIONALE

It is unknown if functional single nucleotide polymorphisms (SNPs), such as the OPRM1 (MOP-r gene) SNP A118G, can predispose individuals to more dual opioid and psychostimulant intake. The dual self-administration (SA) of MOP-r agonists and cocaine has not been thoroughly examined, especially with regard to neurobiological changes.

OBJECTIVES

We examined oxycodone SA and subsequent dual oxycodone and cocaine SA in male and female A112G (A/G and G/G, heterozygote and homozygote, respectively) mice, models of human A118G carriers, versus wild-type (A/A) mice.

METHODS

Adult male and female A/G, G/G and A/A mice self-administered oxycodone (0.25 mg/kg/infusion, 4hr/session, FR 1.) for 10 consecutive days (sessions 1-10). Mice then self-administered cocaine (2 hr) following oxycodone SA (4 hr, as above) in each session for a further 10 consecutive days (sessions 11-20). Message RNA transcripts of 24 reward-related genes were examined in the dorsal striatum.

RESULTS

Male and female A/G and G/G mice had greater oxycodone SA than A/A mice did in the initial 10 days and in the last 10 sessions. Further, A/G and G/G mice showed greater cocaine intake than A/A mice. Dorsal striatal mRNA levels of Pdyn, Fkbp5, Oprk1, and Oprm1 were altered following oxycodone and cocaine SA.

CONCLUSIONS

These studies demonstrated that this functional genetic variation in Oprm1 affected dual opioid and cocaine SA and altered specific gene expression in the striatum.

Interactions between opioids and stimulants: Behavioral pharmacology of abuse-related effects

Opioid abuse continues to be a significant public health challenge, with rates of opioid-related overdose deaths increasing continuously over the last two decades. There also has been a sharp increase in overdose deaths involving stimulant drugs, primarily cocaine and methamphetamine. Recent estimates indicate a high prevalence of co-use of opioids and stimulants, which is a particularly complex problem. Behavioral pharmacology research over the last few decades has characterized interactions between opioids and stimulants as well as evaluated potential treatments. This chapter describes interactions between opioids and stimulants, with a focus on pre-clinical studies of abuse-related behavioral effects using self-administration, reinstatement, drug discrimination, place conditioning, and intracranial self-stimulation paradigms in laboratory animals. In general, the literature provides substantial evidence of mutual enhancement between opioids and stimulants for abuse-related effects, although such results are not ubiquitous. Enhanced abuse-related effects could manifest in many ways including engaging in drug seeking and taking behaviors with greater persistence, effort, and motivation and/or increased likelihood of relapse. Moreover, studies on opioid/stimulant combinations set the stage for evaluating potential treatments for polysubstance use. Behavioral pharmacology research has proven invaluable for elucidating these relationships using rigorous experimental designs and quantitative analyses of pharmacological and behavioral data.

Behavioral and Biochemical Impact of Chronic Unpredictable Mild Stress on the Acquisition of Nicotine Conditioned Place Preference in Rats

Addiction is a chronic psychiatric disease which represents a global problem, and stress can increase drug addiction and relapse. Taking into account frequent concomitance of nicotine dependence and stress, the purpose of the present study was to assess behavioral and biochemical effects of chronic unpredictable mild stress (CUMS) exposure on nicotine reward in rats measured in the conditioned place preference (CPP) paradigm. Rats were submitted to the CUMS for 3 weeks and conditioned with nicotine (0.175 mg/kg) for 2 or 3 days. Our results revealed that only CUMS-exposed animals exhibited the CPP after 2 days of conditioning indicating that stressed rats were more sensitive to the rewarding properties of nicotine and that chronic stress exacerbates nicotine preference. Administration of metyrapone (50 mg/kg), a glucocorticosteroid antagonist, and imipramine (15 mg/kg), an antidepressant, abolished nicotine CPP in stressed rats after 2 days of conditioning. The biochemical experiments showed increased markers of oxidative stress after nicotine conditioning for 2 and 3 days, while the CUMS further potentiated pro-oxidative effects of nicotine. Moreover, metyrapone reversed oxidative changes caused by stress and nicotine, while imipramine was not able to overwhelm nicotine- and stress-induced oxidative damages; however, it could exert antioxidant effect if administered repeatedly. The results suggest that recent exposure to a stressor may augment the rewarding effects of nicotine through anhedonia- and stress-related mechanisms. Our study contributes to the understanding of behavioral and biochemical stress-induced modification of the rewarding effects of nicotine on the basis of the development of nicotine dependence.

Glutamate and Opioid Antagonists Modulate Dopamine Levels Evoked by Innately Attractive Male Chemosignals in the Nucleus Accumbens of Female Rats

Sexual chemosignals detected by vomeronasal and olfactory systems mediate intersexual attraction in rodents, and act as a natural reinforcer to them. The mesolimbic pathway processes natural rewards, and the nucleus accumbens receives olfactory information via glutamatergic projections from the amygdala. Thus, the aim of this study was to investigate the involvement of the mesolimbic pathway in the attraction toward sexual chemosignals. Our data show that female rats with no previous experience with males or their chemosignals display an innate preference for male-soiled bedding. Focal administration of the opioid antagonist β-funaltrexamine into the posterior ventral tegmental area does not affect preference for male chemosignals. Nevertheless, exposure to male-soiled bedding elicits an increase in dopamine efflux in the nucleus accumbens shell and core, measured by microdialysis. Infusion of the opioid antagonist naltrexone in the accumbens core does not significantly affect dopamine efflux during exposure to male chemosignals, although it enhances dopamine levels 40 min after withdrawal of the stimuli. By contrast, infusion of the glutamate antagonist kynurenic acid in the accumbens shell inhibits the release of dopamine and reduces the time that females spend investigating male-soiled bedding. These data are in agreement with previous reports in male rats showing that exposure to opposite-sex odors elicits dopamine release in the accumbens, and with data in female mice showing that the behavioral preference for male chemosignals is not affected by opioidergic antagonists. We hypothesize that glutamatergic projections from the amygdala into the accumbens might be important to modulate the neurochemical and behavioral responses elicited by sexual chemosignals in rats.

Involvement of μ- and δ-opioid receptor function in the rewarding effect of (±)-pentazocine

Most opioid receptor agonists have abuse potential, and the rewarding effects of opioids can be reduced in the presence of pain. While each of the enantiomers of pentazocine has a differential pharmacologic profile, (±)-pentazocine has been used clinically for the treatment of pain. However, little information is available regarding which components of pentazocine are associated with its rewarding effects, and whether the (±)-pentazocine-induced rewarding effects can be suppressed under pain. Therefore, the present study was performed to investigate the effects of pain on the acquisition of the rewarding effects of (±)-pentazocine, and to examine the mechanism of the rewarding effects of (±)-pentazocine using the conditioned place preference paradigm. (±)-Pentazocine and (-)-pentazocine, but not (+)-pentazocine, produced significant rewarding effects. Even though the rewarding effects induced by (±)-pentazocine were significantly suppressed under pain induced by formalin, accompanied by increase of preprodynorphin mRNA levels in the nucleus accumbens, a high dose of (±)-pentazocine produced significant rewarding effects under pain. In the normal condition, (±)-pentazocine-induced rewarding effects were blocked by a low dose of naloxone, whereas the rewarding effects induced by high doses of pentazocine under pain were suppressed by naltrindole (a δ-opioid receptor antagonist). Interestingly, (±)-pentazocine did not significantly affect dopamine levels in the nucleus accumbens. These findings suggest that the rewarding effects of (-)-pentazocine may contribute to the abuse potential of (±)-pentazocine through μ- as well as δ-opioid receptors, without robust activation of the mesolimbic dopaminergic system. We also found that neural adaptations can reduce the abuse potential of (±)-pentazocine under pain.

Effect of baclofen on morphine-induced conditioned place preference, extinction, and stress-induced reinstatement in chronically stressed mice

RATIONALE AND OBJECTIVE

A stress-induced increase in excitability can result from a reduction in inhibitory neurotransmission. Modulation of gamma-aminobutyric acid (GABA)ergic transmission is an effective treatment for drug seeking and relapse. This study investigated whether baclofen, a GABA(B) receptor agonist, had an impact on morphine-induced conditioned place preference (CPP), extinction, and stress-induced relapse in chronically stressed mice.

METHODS

Chronic stress was induced by restraining mice for 2 h for seven consecutive days. We first investigated whether chronic stress influenced morphine-induced CPP, extinction, and stress-induced relapse in the stressed mice. Next, we investigated whether three different doses of baclofen influenced chronic stress as measured by the expression of morphine-induced CPP. We chose the most effective dose for subsequent extinction and reinstatement experiments. Reinstatement of morphine-induced CPP was induced by a 6-min forced swim stress. Locomotor activity was also measured for each test.

RESULTS

Chronic stress facilitated the expression of morphine-induced CPP and prolonged extinction time. Forced swim stress primed the reinstatement of morphine-induced CPP in mice. Baclofen treatment affected the impact of chronic stress on different phases of morphine-induced CPP.

CONCLUSIONS

Our results showed that baclofen antagonized the effects of chronic stress on morphine-induced CPP. These findings suggest the potential clinical utility of GABA(B) receptor-positive modulators as an anti-addiction agent in people suffering from chronic stress.

The rostral subcommissural ventral pallidum is a mix of ventral pallidal neurons and neurons from adjacent areas: an electrophysiological study

The ventral pallidum (VP) is a part of the ventral striatopallidal system and is involved in reward-related behaviors. The VP is composed of a ventromedial (VPvm) and a dorsolateral (VPdl) subregion, and some rostral-caudal differences are reported. Study of the VP often focuses on the subcommissural VP, typically considered homogenous in spite of known subdivisions. In this work, we used slice electrophysiology combined with immunohistochemistry for marker neuropeptides to test whether the subcommissural VP is functionally homogenous. Using sagittal slices, we show that more lateral levels (2.40 mm) of the subcommissural VP are homogenous but that a more medial slice (1.90 mm) contains two types of neurons. One type, located more caudally, resembles neurons in the lateral subcommissural VP, with long aspiny dendrites, primarily GABAergic input, and characteristic electrophysiological properties, such as depolarized membrane potential and spontaneous action potential discharge. The second type of neuron, located mostly in the rostral subcommissural VP, shows properties that are akin to medium spiny neurons of adjacent regions, including spiny dendrites, major glutamatergic input, hyperpolarized membrane potential, and no spontaneous action potentials. The two types of neurons were present in both the VPvm and VPdl, implying that the mix is not a characteristic of histologically defined subregions. We conclude that at medial levels the rostral subcommissural VP contains a mix of typical ventral pallidal neurons and spiny neurons similar to those in adjacent regions. This observation needs to be considered when interpreting past experiments and designing future experiments in the subcommissural VP.

Stress and addiction: contribution of the corticotropin releasing factor (CRF) system in neuroplasticity

Corticotropin releasing factor (CRF) has been shown to induce various behavioral changes related to adaptation to stress. Dysregulation of the CRF system at any point can lead to a variety of psychiatric disorders, including substance use disorders (SUDs). CRF has been associated with stress-induced drug reinforcement. Extensive literature has identified CRF to play an important role in the molecular mechanisms that lead to an increase in susceptibility that precipitates relapse to SUDs. The CRF system has a heterogeneous role in SUDs. It enhances the acute effects of drugs of abuse and is also responsible for the potentiation of drug-induced neuroplasticity evoked during the withdrawal period. We present in this review the brain regions and circuitries where CRF is expressed and may participate in stress-induced drug abuse. Finally, we attempt to evaluate the role of modulating the CRF system as a possible therapeutic strategy for treating the dysregulation of emotional behaviors that result from the acute positive reinforcement of substances of abuse as well as the negative reinforcement produced by withdrawal.

Locomotor stimulant effects of acute and repeated intrategmental injections of salsolinol in rats: role of mu-opioid receptors

RATIONALE

Microinjections of ethanol and acetaldehyde into ventral tegmental area (VTA) produce locomotor activation in rats through mechanisms dependent on the mu-opioid receptors. However, it is not clear how these drugs can interact with these receptors. It has been hypothesized that salsolinol could be the responsible for this interaction.

OBJECTIVES

The aim of the study was to investigate the ability of salsolinol to induce both motor activation and motor sensitization in rats after repeated intra-VTA administration.

MATERIALS

Rats received one microinjection into the posterior VTA of artificial cerebrospinal fluid (aCSF; 200 nL), salsolinol (0.3-3,000.0 pmol/200 nL), or salsolinol (30.0 pmol/200 nL) with either naltrexone (13.2 nmol/200 nL) or with the antagonist of the mu-opioid receptors, beta-funaltrexamine (beta-FNA; 2.5 nmol/300 nL). In the sensitization experiments, four microinjections of salsolinol (30.0 pmol/200 nL) or aCSF (200 nL) were performed over a 2-week period. This period was followed by a single challenge session, in which 0.3 pmol of salsolinol was microinjected to rats. Spontaneous activity was always monitored postinjection.

RESULTS

Intra-VTA salsolinol administration induces an increase of the spontaneous motor activity of the rats with the maximal effect at the dose of 30.0 pmol/200 nL. Salsolinol effects were blocked by the treatment with naltrexone or beta-FNA. Moreover, repeated injections of salsolinol produced locomotor sensitization.

CONCLUSIONS

Salsolinol induces locomotor activity and motor sensitization after intra-VTA administration. Moreover, the implication of the mu-opioid receptors was shown since the treatment with naltrexone or beta-FNA was able to suppress the salsolinol effects.

Reward processing by the opioid system in the brain

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

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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).

Ventral pallidum roles in reward and motivation

In recent years the ventral pallidum has become a focus of great research interest as a mechanism of reward and incentive motivation. As a major output for limbic signals, the ventral pallidum was once associated primarily with motor functions rather than regarded as a reward structure in its own right. However, ample evidence now suggests that ventral pallidum function is a major mechanism of reward in the brain. We review data indicating that (1) an intact ventral pallidum is necessary for normal reward and motivation, (2) stimulated activation of ventral pallidum is sufficient to cause reward and motivation enhancements, and (3) activation patterns in ventral pallidum neurons specifically encode reward and motivation signals via phasic bursts of excitation to incentive and hedonic stimuli. We conclude that the ventral pallidum may serve as an important 'limbic final common pathway' for mesocorticolimbic processing of many rewards.