The opioid receptors as targets for drug abuse medication

Psychotherapies in opioid use disorder: toward a step-care model

Opioid use disorder (OUD) is characterized by a lack of control in opioid use, resulting in psychological distress and deficits in interpersonal and social functioning. OUD is often associated with psychiatric comorbidities that increase the severity of the disorder. The consequences of OUD are dramatic in terms of increased morbi-mortality. Specific medications and psychotherapies are essential tools not only in the treatment of OUD but also in the prevention of suicide and overdoses. In our review, we assess the different types of psychotherapies (counseling, motivational interviewing, contingency management, cognitive-behavioral therapy, and dialectical-behavior therapy) that are delivered to opioid users, either associated or un-associated with OUD medications and/or medications for psychiatric disabilities. We describe the application of these therapies first to adult opioid users and then to adolescents. This work led us to propose a stepped-care model of psychotherapies for OUD which provided information to assist clinicians in decision-making regarding the selection of psychotherapeutic strategies according to patients' OUD severity.

HPA-Axis Activity and Nutritional Status Correlation in Individuals with Alcohol Use Disorder

Impaired activity of the hypothalamic-pituitary-adrenal axis (HPA-axis) is evident in alcohol use disorder (AUD), and may be implicated in various nutritional and metabolic alterations often seen in individuals with this disorder. The present study examined a possible correlation between HPA-axis activity and nutritional status components in individuals with AUD. Fourteen AUD and fourteen non-AUD males participated; anthropometric and body composition measurements were made, and fasting blood samples were analysed for plasma adrenal corticotropic hormone (ACTH), catecholamines, cortisol and beta-endorphin. Nutrient intake was estimated via a three-day diet record. Waist circumference and waist-to-hip ratio were increased in the AUD group. Thiamine and folic intake were lower in AUD group, although only folic acid intake was insufficient in both AUD and non-AUD groups. Increased epinephrine and norepinephrine were also observed in AUD group compared to non-AUD group. No clear correlation between HPA-axis activity and nutritional status components was found. This study showed that nutrient intake, body composition, and HPA-axis activity were different among AUD and non-AUD individuals. More research on the correlation between nutritional status and HPA-axis activity in AUD individuals should be conducted.

Tramadol Treatment Induces Change in Phospho-Cyclic Adenosine Monophosphate Response Element-Binding Protein and Delta and Mu Opioid Receptors within Hippocampus and Amygdala Areas of Rat Brain

BACKGROUND

Tramadol induces its unique effects through opioid pathways, but the exact mechanism is not known. The study aims to evaluate changes in the level of mu-opioid receptor (µOR), delta-opioid receptor (δOR), and phosphorylated cyclic adenosine monophosphate (cAMP) response element-binding protein (p-CREB) in the hippocampus (HPC) and amygdala (AL) areas of tramadol-treated rats.

METHODS

For this purpose, a total of 36 male rats were divided into two main groups for chronic or acute tramadol exposure. The animals were then exposed to 5 mg.kg-1 of tramadol, 10 mg.kg-1 of tramadol, and normal saline. The HPC and AL areas of the animals were dissected upon completion of the period. The levels of p-CREB and µOR were quantified using the western blotting technique. The data were subjected to analysis of variance (ANOVA) followed by Tukey's post-hoc analysis. The differences with the P-value lower than 0.05 were considered as significant.

FINDINGS

In the HPC and AL areas of the brain, the level of µOR was decreased by acute tramadol exposure, while no significant difference was observed by chronic tramadol exposure. Moreover, results showed that the level of p-CREB dose-dependently increased by acute and chronic tramadol exposure.

CONCLUSION

HPC and AL are essential in the control of tramadol abuse. Tramadol abuse affects gene expression and transcription factors such as CREB. With acute drug tramadol treatments, the level of cAMP response element-binding protein (CREB) rapidly increases, while by chronic tramadol treatment, "peak and trough pattern is observing". The activation of the rewarding mechanism is a precise instance of addictive behavior in tramadol-treated individuals.

Oxycodone self-administration activates the mitogen-activated protein kinase/ mitogen- and stress-activated protein kinase (MAPK-MSK) signaling pathway in the rat dorsal striatum

To identify signaling pathways activated by oxycodone self-administration (SA), Sprague–Dawley rats self-administered oxycodone for 20 days using short—(ShA, 3 h) and long-access (LgA, 9 h) paradigms. Animals were euthanized 2 h after SA cessation and dorsal striata were used in post-mortem molecular analyses. LgA rats escalated their oxycodone intake and separated into lower (LgA-L) or higher (LgA-H) oxycodone takers. LgA-H rats showed increased striatal protein phosphorylation of ERK1/2 and MSK1/2. Histone H3, phosphorylated at serine 10 and acetylated at lysine 14 (H3S10pK14Ac), a MSK1/2 target, showed increased abundance only in LgA-H rats. RT-qPCR analyses revealed increased AMPA receptor subunits, GluA2 and GluA3 mRNAs, in the LgA-H rats. GluA3, but not GluA2, mRNA expression correlated positively with changes in pMSK1/2 and H3S10pK14Ac. These findings suggest that escalated oxycodone SA results in MSK1/2-dependent histone phosphorylation and increases in striatal gene expression. These observations offer potential avenues for interventions against oxycodone addiction.

Nicotine increases alcohol self-administration in male rats via a μ-opioid mechanism within the mesolimbic pathway

Background and Purpose

Alcohol and nicotine use disorders are commonly comorbid. Both alcohol and nicotine can activate opioid systems in reward‐related brain regions, leading to adaptive changes in opioid signalling upon chronic exposure. The potential role of these adaptations for comorbidity is presently unknown. Here, we examined the contribution of μ and κ‐opioid receptors to nicotine‐induced escalation of alcohol self‐administration in rats.

Experimental Approach

Chronic nicotine was tested on alcohol self‐administration and motivation to obtain alcohol. We then tested the effect of the κ antagonist CERC‐501 and the preferential μ receptor antagonist naltrexone on basal and nicotine‐escalated alcohol self‐administration. To probe μ or κ receptor adaptations, receptor binding and G‐protein coupling assays were performed in reward‐related brain regions. Finally, dopaminergic activity in response to alcohol was examined, using phosphorylation of DARPP‐32 in nucleus accumbens as a biomarker.

Key Results

Nicotine robustly induced escalation of alcohol self‐administration and motivation to obtain alcohol. This was blocked by naltrexone but not by CERC‐501. Escalation of alcohol self‐administration was associated with decreased DAMGO‐stimulated μ receptor signalling in the ventral tegmental area (VTA) and decreased pDARPP‐32 in the nucleus accumbens shell in response to alcohol.

Conclusions and Implications

Collectively, these results suggest that nicotine contributes to escalate alcohol self‐administration through a dysregulation of μ receptor activity in the VTA. These data imply that targeting μ rather than κ receptors may be the preferred pharmacotherapeutic approach for the treatment of alcohol use disorder when nicotine use contributes to alcohol consumption.

GPCR and Alcohol-Related Behaviors in Genetically Modified Mice

G protein-coupled receptors (GPCRs) constitute the largest class of cell surface signaling receptors and regulate major neurobiological processes. Accordingly, GPCRs represent primary targets for the treatment of brain disorders. Several human genetic polymorphisms affecting GPCRs have been associated to different components of alcohol use disorder (AUD). Moreover, GPCRs have been reported to contribute to several features of alcohol-related behaviors in animal models. Besides traditional pharmacological tools, genetic-based approaches mostly aimed at deleting GPCR genes provided substantial information on how key GPCRs drive alcohol-related behaviors. In this review, we summarize the alcohol phenotypes that ensue from genetic manipulation, in particular gene deletion, of key GPCRs in rodents. We focused on GPCRs that belong to fundamental neuronal systems that have been shown as potential targets for the development of AUD treatment. Data are reviewed with particular emphasis on alcohol reward, seeking, and consumption which are behaviors that capture essential aspects of AUD. Literature survey indicates that in most cases, there is still a gap in defining the intracellular transducers and the functional crosstalk of GPCRs as well as the neuronal populations in which their signaling regulates alcohol actions. Further, the implication of only a few orphan GPCRs has been so far investigated in animal models. Combining advanced pharmacological technologies with more specific genetically modified animals and behavioral preclinical models is likely necessary to deepen our understanding in how GPCR signaling contributes to AUD and for drug discovery.

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Dopamine synthesis capacity correlates with µ-opioid receptor availability in the human basal ganglia: A triple-tracer PET study

Animal studies have suggested that dopamine and opioid neurotransmitter systems interact in brain regions that are relevant for reward functions, but data in humans are very limited. The interaction is potentially important in disorders affecting these neurotransmitter systems, such as addiction. Here, we investigated whether subcortical μ-opioid receptor (MOR) availability and presynaptic dopamine synthesis capacity are correlated in the healthy human brain or in pathological gamblers (PGs) using positron emission tomography with 6-[¹⁸F]fluoro-l-dopa and [¹¹C]carfentanil. The specificity of the findings was further investigated by including a serotonin transporter ligand, [¹¹C]MADAM, as a negative control. Thirteen PG patients and 15 age-, sex- and weight-matched controls underwent the scans. In both groups, presynaptic dopamine synthesis capacity was associated with MOR availability in the putamen, caudate nucleus and globus pallidus. No similar associations were observed between dopamine synthesis capacity and [¹¹C]MADAM binding, supporting a specific interplay between presynaptic dopamine neurotransmission and opioid receptor function in the basal ganglia. Correlations were similar between the groups, suggesting that the dopamine-opioid link is general and unaffected by behavioral addiction. The results provide in vivo human evidence of a connection between endogenous opioid and dopamine signaling in the brain.

Opioid and nondopamine reward circuitry and state-dependent mechanisms

A common notion is that essentially all addictive drugs, including opioids, activate dopaminergic pathways in the brain reward system, and the inappropriate use of such drugs induces drug dependence. However, an opioid reward response is reportedly still observed in several models of dopamine depletion, including in animals that are treated with dopamine blockers, animals that are subjected to dopaminergic neuron lesions, and dopamine-deficient mice. The intracranial self-stimulation response is enhanced by stimulants but reduced by morphine. These findings suggest that dopaminergic neurotransmission may not always be required for opioid reward responses. Previous findings also indicate the possibility that dopamine-independent opioid reward may be observed in opioid-naive states but not in opioid-dependent states. Therefore, a history of opioid use should be considered when evaluating the dopamine dependency of opioid reward.

Exercise training reduces alcohol consumption but does not affect HPA-axis activity in heavy drinkers

BACKGROUND

It has been suggested that physical exercise could have potential beneficial effects in substance abusers, which are based on both physiological and psychological theories. Although a few studies have examined the effect of exercise on alcohol intake and fitness in individuals with alcohol use disorders (AUDs), there is a gap in the literature concerning the physiological and biochemical mechanisms that could be affected by physical exercise in this population.

PURPOSE

The purpose of the present study was to examine physiological and biochemical responses to exercise after an 8-week supervised exercise training (ET) intervention in heavy drinkers. The investigation was mainly focused on the relationship among exercise, opioids, the hypothalamic-pituitary-adrenal axis (HPA) activity and heavy alcohol drinking.

METHODS

Eleven (Age: 30.3±3.5yrs; Body Mass Index: 28.4±0.86kg/m²) male heavy drinkers volunteered to participated in an 8-week supervised intervention of moderate intensity exercise (50-60% of Heart Rate Reserve). All participants were exhibiting low physical activity and used to drink heavily. Before intervention, the participants were asked to record their daily alcohol intake without changing their physical activity levels for 4weeks (control condition). During the 8-week supervised ET intervention, participants were recording their daily alcohol intake and were motivated to increase gradually the duration and frequency of ET. Blood samples were collected prior to and after 4weeks of the control condition, the day before the beginning of the ET intervention, and at the end of the 4th and 8th week of ET intervention. Blood samples were analyzed for β-E, epinephrine, norepinephrine, adrenocorticotropin, cortisol, gamma-glutamyl transferase (γ-GT), aspartate transaminase and alanine transaminase. Physiological and alcohol-related indices were also examined.

RESULTS

The 8-week supervised ET intervention resulted in reduced alcohol consumption, reduced γ-GT levels, and fitness improvement in heavy drinker. ET intervention did not significantly change the hormonal responses.

CONCLUSION

The results indicate that physical exercise could act as a healthy habit that can help individuals with AUDs reduce alcohol intake and improve health status; however, this is not related with changes in hormones associated with the HPA-axis.

Chronic nicotine-induced changes in gene expression of delta and kappa-opioid receptors and their endogenous ligands in the mesocorticolimbic system of the rat

Delta and kappa opioid receptors (DOR and KOR, respectively) and their endogenous ligands, proenkephalin (PENK) and prodynorphin (PDYN)-derived opioid peptides are proposed as important mediators of nicotine reward. This study investigated the regulatory effect of chronic nicotine treatment on the gene expression of DOR, KOR, PENK and PDYN in the mesocorticolimbic system. Three groups of rats were injected subcutaneously with nicotine at doses of 0.2, 0.4, or 0.6 mg/kg/day for 6 days. Rats were decapitated 1 hr after the last dose on day six, as this timing coincides with increased dopamine release in the mesocorticolimbic system. mRNA levels in the ventral tegmental area (VTA), lateral hypothalamic area (LHA), amygdala (AMG), dorsal striatum (DST), nucleus accumbens, and medial prefrontal cortex were measured by quantitative real-time PCR. Our results showed that nicotine upregulated DOR mRNA in the VTA at all of the doses employed, in the AMG at the 0.4 and 0.6 mg/kg doses, and in the DST at the 0.4 mg/kg dose. Conversely, PDYN mRNA was reduced in the LHA with 0.6 mg/kg nicotine and in the AMG with 0.4 mg/kg nicotine. KOR mRNA was also decreased in the DST with 0.6 mg/kg nicotine. Nicotine did not regulate PENK mRNA in any brain region studied.

Pharmacological treatments for methamphetamine addiction: current status and future directions

INTRODUCTION

Methamphetamine (MA) abuse remains a global health challenge despite intense research interest in the development of pharmacological treatments. This review provides a summary of clinical trials and human studies on the pharmacotherapy of methamphetamine use disorder (MUD). Areas covered: We summarize published clinical trials that tested candidate medications for MUD and also conducted PubMed and Google Scholar searches to identify recently completed clinical trials using the keywords 'methamphetamine' 'addiction' 'pharmacotherapy' and 'clinical trial.' To determine the status of ongoing clinical trials targeting MUD, we also searched the ClinicalTrials.gov online database. We conclude this review with a discussion of current research gaps and future directions. Expert commentary: Clinical trials examining the potential for pharmacotherapies of MUD have largely been negative. Future studies need to address several limitations to reduce the possibility of Type II errors: small sample sizes, high dropout rates or multiple comorbidities. Additionally, new treatment targets, such as MA-induced disruptions in cognition and in the neuroimmune system, merit trials with agents that selectively modulate these processes.

Neurochemical substrates of the rewarding effects of MDMA: implications for the development of pharmacotherapies to MDMA dependence

In recent years, studies with animal models of reward, such as the intracranial self-stimulation, self-administration, and conditioned place preference paradigms, have increased our knowledge on the neurochemical substrates of the rewarding effects of 3,4-methylenedioxymetamphetamine (MDMA) in rodents. However, pharmacological and neuroimaging studies with human participants are scarce. Serotonin [5-hydroxytryptamine (5-HT)], dopamine (DA), endocannabinoids, and endogenous opiates are the main neurotransmitter systems involved in the rewarding effects of MDMA in rodents, but other neurotransmitters such as glutamate, acetylcholine, adenosine, and neurotensin are also involved. The most important finding of recent research is the demonstration of differential involvement of specific neurotransmitter receptor subtypes (5-HT2, 5-HT3, DA D1, DA D2, CB1, μ and δ opioid, etc.) and extracellular proteins (DA and 5-HT transporters) in the acquisition, expression, extinction, and reinstatement of MDMA self-administration and conditioned place preference. It is important to extend the research on the effects of different compounds acting on these receptors/transporters in animal models of reward, especially in priming-induced, cue-induced, and stress-induced reinstatement. Increase in knowledge of the neurochemical substrates of the rewarding effects of MDMA may contribute to the design of new pharmacological treatments for individuals who develop MDMA dependence.

Tightrope or Slackline? The Neuroscience of Psychoactive Substances

Novel psychoactive substances flood worldwide markets faster than they can be banned. Legislators struggle to find a balance between free availability, prescription systems, and criminalisation, while physicians try to balance risks and benefits of drug treatment and identify drug abuse - a tightrope walk. Classification of psychoactive substances is central to these decision-making processes but existing classifications rely on unrelated, inconsistent, and shifting guidelines that categorise drugs by chemical structure, toxicity, or addictive potency. We propose that a new categorisation of drugs based on neurobiological mechanisms of action may help to simplify the regulation of drug use, delivers a neurobiological context, and streamlines classification and future regulatory directions. We provide guidelines to distinguish between drug abuse and treatment and to navigate the controversies over legalising or banning drugs. Finally, we comment on the role neuroscientific research can play in the future to solve imminent problems in this highly important field.

[What brings neurobiology to addictions?]

Addictions are multifactorial, and there are no experimental models replicating all aspects of this pathology. The development of animal models reproducing the clinical symptoms of addictions allows significant advances in the knowledge of the neurobiological processes involved in addiction. Preclinical data highlight different neuroadaptations according to the routes of administration, speeds of injection and frequencies of exposure to drugs of abuse. The neuroadaptations induced by an exposure to drugs of abuse follow dynamic processes in time. Despite significant progresses in the knowledge of neurobiology of addictions allowing to propose new therapeutic targets, the passage of new drugs in clinical is often disappointing. The lack of treatment efficacy reported in clinical trials is probably due to a very important heterogeneity of patients with distinct biological and genetic factors, but also with different patterns of consumption that can lead to different neuroadaptations, as clearly observed in preclinical studies.