Effects of selective opioid receptor antagonists on alcohol-induced and nicotine-induced antinociception

Differences in the association between kappa opioid receptors and pain among Black and White adults with alcohol use disorders

BACKGROUND

The relationship between alcohol and pain is complex. Associations between pain and alcohol use disorder (AUD) vary by race, but the underlying biological basis is not understood. We examined the association of the kappa opioid receptor (KOR) with responses to the cold-pressor test (CPT), before and after treatment with the opioid antagonist naltrexone, among individuals with AUD who self-identified as Black or White.

METHODS

Thirty-seven individuals (12 Black, 24 White, and 1 Multiracial) with AUD participated in two CPTs, separated by 1 week during which they received naltrexone 100 mg daily. During each CPT, pain reporting threshold (PRT), average pain increase rate (APIR), relative pain recovery (RPR), and alcohol craving were recorded. KOR availability was measured using [¹¹ C]-LY2795050 positron emission tomography (PET) prior to treatment with naltrexone.

RESULTS

Black participants reported higher PRT and APIR than White participants during the CPT before, but not after, naltrexone treatment. Among Black participants, KOR availability was positively associated with PRT and APIR before, but not after naltrexone. Greater KOR availability was associated with faster RPR for White, but not Black, participants. The CPT induced more alcohol craving in Black than White participants, particularly in individuals with low KOR availability, an effect that was not attenuated by naltrexone.

CONCLUSIONS

KOR involvement and naltrexone effects on responses to the CPT were different between Black and White participants. These preliminary findings suggest that further exploration of the differences in the opioid system and pain among Black and White individuals with AUD and their relationship with naltrexone's effects is warranted.

Targeting the Opioid Receptors: A Promising Therapeutic Avenue for Treatment in “Heavy Drinking Smokers”

Aims

Despite a general decline in tobacco use in the last decades, the prevalence of tobacco smoking in individuals with alcohol use disorder (AUD) remains substantial (45–50%). Importantly, the co-use of both substances potentiates the adverse effects, making it a significant public health problem. Substantial evidence suggests that AUD and Tobacco use disorder (TUD) may share common mechanisms. Targeting these mechanisms may therefore provide more effective therapy. Numerous studies describe a potential role of the endogenous opioid system in both AUD and TUD. Reviewing this literature, we aim to evaluate the efficacy of molecules that target the opioid system as promising therapeutic interventions for treating alcohol and tobacco co-use disorders.

Methods

We provide a synthesis of the current epidemiological knowledge of alcohol and tobacco co-use disorders. We evaluate clinical and preclinical research that focuses on the regulation of the endogenous opioid system in alcohol, nicotine, and their interactions.

Results

The epidemiological data confirm that smoking stimulates heavy drinking and facilitates alcohol craving. Pharmacological findings suggest that treatments that are efficacious in the dual addiction provide a beneficial treatment outcome in comorbid AUD and TUD. In this regard, MOP, DOP and NOP-receptor antagonists show promising results, while the findings prompt caution when considering KOP-receptor antagonists as a treatment option in alcohol and tobacco co-use disorders.

Conclusions

Existing literature suggests a role of the opioid system in sustaining the high comorbidity rates of AUD and TUD. Molecules targeting opioid receptors may therefore represent promising therapeutic interventions in ‘heavy drinking smokers.’

Neurobiological aspects of pain in the context of alcohol use disorder

Alcohol is an effective and widely utilized analgesic. However, the chronic use of alcohol can actually facilitate nociceptive sensitivity over time, a condition known as hyperalgesia. Excessive and uncontrollable alcohol drinking is also a hallmark feature of alcohol use disorder (AUD). Both AUD and chronic pain are typically accompanied by negative affective states that may underlie reinforcement mechanisms contributing to AUD maintenance or progression. Frequent utilization of alcohol to relieve pain in individuals suffering from AUD or other chronic pain conditions may thus represent a powerful negative reinforcement construct. This chapter will describe ties between alcohol-mediated pain relief and potential exacerbation of AUD. We describe neurobiological systems engaged in alcohol analgesia as well as systems recruited in the development and maintenance of AUD and hyperalgesia. Although few effective therapies exist for either chronic pain or AUD, the common interaction of these conditions will likely lead the way for promising new discoveries of more effective and even simultaneous treatment of AUD and co-morbid hyperalgesia. An abundance of neurobiological findings from multiple laboratories has implicated a potentiation of central amygdala (CeA) signaling in both pain and AUD, and these data also suggest that attenuation of stress-related systems (including corticotropin-releasing factor, vasopressin, and glucocorticoid receptor activity) would be particularly effective and comprehensive therapeutic strategies targeting the critical intersection of somatic and motivational mechanisms driving AUD, including alcohol-induced hyperalgesia.

The Association Between Alcohol Consumption and Pain Interference in a Nationally Representative Sample: The Moderating Roles of Gender and Alcohol Use Disorder Symptomatology

BACKGROUND

Prior research on alcohol consumption and pain has yielded inconsistent results regarding the directionality of effects for both consumption-to-pain and pain-to-consumption relations. The present study sought to examine directionality of these relations by testing bidirectional longitudinal associations between consumption and pain interference, a crucial aspect of pain that captures pain-related disability and has been regarded as a valuable measure of treatment outcome. In addition, this study explored possible moderation of these bidirectional longitudinal associations by gender and alcohol use disorder (AUD) symptomatology.

METHODS

Analyses included 29,989 current/former drinkers who were interviewed at both waves (2001 and 2004) of the U.S. National Epidemiological Survey on Alcohol and Related Conditions (NESARC). Analyses used self-report data from both waves on past-year average daily volume of alcohol consumed and past-month pain interference (1 item from the Medical Outcomes Study 12-item Short-Form Health Survey [MOS-SF-12]). AUDADIS-IV data from Wave 1 were used to index baseline AUD symptomatology (i.e., symptom count). Cross-lagged panel modeling and multigroup analyses were employed.

RESULTS

Regarding the consumption-to-pain-interference relation, in general, higher baseline alcohol consumption was associated with lower subsequent pain interference at follow-up. However, among men with higher AUD-symptom counts, the opposite pattern emerged, with higher baseline alcohol consumption being significantly related to higher subsequent pain interference at follow-up. Regarding the pain-interference-to-consumption relation, higher baseline pain interference was significantly associated with lower subsequent alcohol consumption at follow-up, and no moderating effects were observed.

CONCLUSIONS

The distinctive patterns of the consumption-to-pain-interference relation observed among men with elevated AUD symptomatology suggest that this relation might be driven by different mechanisms across different groups of individuals. Specifically, the detrimental effect of alcohol on pain interference might emerge at relatively advanced stages of AUD among men, consistent with Koob's Dark Side of Alcohol Addiction theory in human research.

Pharmacological mechanisms of alcohol analgesic-like properties in mouse models of acute and chronic pain

Alcohol use and chronic pain are highly comorbid. Acute alcohol use typically produces an analgesic effect. However, chronic use can worsen the progression of chronic pain. In rodent models, acute models of pain have primarily been used to investigate the relationship between alcohol and pain analgesia. Here, we use two models of chronic pain, chronic inflammatory and peripheral neuropathic pain, to investigate acute alcohol's antinociceptive and analgesic properties. We hypothesize that acute ethanol is acting through opioid receptors to create an analgesic-like effect in both reflexive and affective dimensions of pain. Using male and female C57BL/6J mice, oral ethanol administration (0-1.25 g/kg) showed a dose-dependent reversal of mechanical hypersensitivity in both Complete Freund's Adjuvant (CFA) and chronic constriction injury (CCI) models of chronic inflammatory and neuropathic pain. No sex differences were observed. Using the conditioned place preference (CPP) task to assess the subjective responses to ethanol's anti-nociceptive properties, CCI-injured animals showed a preference for the ethanol-paired side, suggesting a reduction in an aversive and pain-like state produced by nerve injury. These effects are likely mediated through the kappa and possibly the mu opioid systems, since ethanol-induced anti-nociception following CCI was fully reversed by pretreatment with the kappa selective antagonist, nor-BNI, or high doses of naltrexone. These data show that ethanol possesses analgesic-like properties in chronic inflammatory and neuropathic pain models in mice and provide new insight into ethanol as it relates to chronic pain.

The association between frequent alcohol drinking and opioid consumption after abdominal surgery: A retrospective analysis

Aims

It is perceived that patients with a history of frequent alcohol consumption require more opioids for postoperative pain control and experience less postoperative nausea and vomiting than patients without such a history. However, there is scarce evidence supporting this notion. The aim of this study was to assess association between frequent alcohol consumption and opioid requirement for postoperative pain control and occurrence of postoperative nausea and vomiting.

Methods

The medical records for 4143 patients using intravenous patient-control analgesia with opioids after abdominal surgery between January 2010 and September 2013 were obtained, and associations were sought between the cumulative opioid consumption (in intravenous morphine equivalence) per body weight (mg/kg) in the first 2 days after abdominal operation and several demographic and clinical variables by multiple regression analysis. The association between the occurrence of postoperative nausea and vomiting and several demographic and clinical variables was also sought by multiple logistic regression analysis.

Results

Frequent alcohol drinking, among other previously reported factors, was associated with increased opioid consumption for postoperative pain control (p < 0.001). The estimate effect of frequent alcohol drinking was 0.117 mg/kg. Frequent alcohol drinking was also associated with decreased risks of postoperative nausea (odds ratio = 0.59, p = 0.003) and vomiting (odds ratio = 0.49, p = 0.026).

Conclusions

Frequent alcohol drinking was associated with increased opioid consumption for postoperative pain control and decreased risks of postoperative nausea and vomiting after abdominal surgery.

Acute analgesic effects of nicotine and tobacco in humans: a meta-analysis

Although animal models have consistently demonstrated acute pain inhibitory effects of nicotine and tobacco, human experimental studies have yielded mixed results. The main goal of this meta-analysis was to quantify the effects of nicotine/tobacco administration on human experimental pain threshold and tolerance ratings. A search of PubMed and PsycINFO online databases identified 13 eligible articles, including k = 21 tests of pain tolerance (N = 393) and k = 15 tests of pain threshold (N = 339). Meta-analytic integration for both threshold and tolerance outcomes revealed that nicotine administered through tobacco smoke and other delivery systems (eg, patch, nasal spray) produced acute analgesic effects that may be characterized as small to medium in magnitude (Hedges g = 0.35, 95% confidence interval = 0.21-0.50). Publication bias-corrected estimates remained significant and indicated that these effects may be closer to small. Sex composition was observed to be a significant moderator, such that pain threshold effects were more robust among samples that included more men than women. These results help to clarify a mixed literature and may ultimately help to inform the treatment of both pain and nicotine dependence. Pain and tobacco smoking are both highly prevalent and comorbid conditions. Current smoking has been associated with more severe chronic pain and physical impairment. Acute nicotine-induced analgesia could make smoking more rewarding and harder to give up. Future research should use dynamic measures of experimental pain reactivity and further explore biopsychosocial mechanisms of action.

Critical needs in drug discovery for cessation of alcohol and nicotine polysubstance abuse

Polysubstance abuse of alcohol and nicotine has been overlooked in our understanding of the neurobiology of addiction and especially in the development of novel therapeutics for its treatment. Estimates show that as many as 92% of people with alcohol use disorders also smoke tobacco. The health risks associated with both excessive alcohol consumption and tobacco smoking create an urgent biomedical need for the discovery of effective cessation treatments, as opposed to current approaches that attempt to independently treat each abused agent. The lack of treatment approaches for alcohol and nicotine abuse/dependence mirrors a similar lack of research in the neurobiology of polysubstance abuse. This review discusses three critical needs in medications development for alcohol and nicotine co-abuse: (1) the need for a better understanding of the clinical condition (i.e. alcohol and nicotine polysubstance abuse), (2) the need to better understand how these drugs interact in order to identify new targets for therapeutic development and (3) the need for animal models that better mimic this human condition. Current and emerging treatments available for the cessation of each drug and their mechanisms of action are discussed within this context followed by what is known about the pharmacological interactions of alcohol and nicotine. Much has been and will continue to be gained from studying comorbid alcohol and nicotine exposure.

Delta Opioid Pharmacology in Relation to Alcohol Behaviors

Delta opioid receptors (DORs) are heavily involved in alcohol-mediated processes in the brain. In this chapter we provide an overview of studies investigating how alcohol directly impacts DOR pharmacology and of early studies indicating DOR modulation of alcohol behavior. We will offer a brief summary of the different animal species used in alcohol studies investigating DORs followed by a broader overview of the types of alcohol behaviors modulated by DORs. We will highlight a small set of studies investigating the relationship between alcohol and DORs in analgesia. We will then provide an anatomical overview linking DOR expression in specific brain regions to different alcohol behaviors. In this section, we will provide two models that try to explain how endogenous opioids acting at DORs may influence alcohol behaviors. Next, we will provide an overview of studies investigating certain new aspects of DOR pharmacology, including the formation of heteromers and biased signaling. Finally, we provide a short overview of the genetics of the DORs in relation to alcohol use disorders (AUDs) and a short statement on the potential of using DOR-based therapeutics for treatment of AUDs.

Interrelations between pain and alcohol: An integrative review

Pain and alcohol use are both highly prevalent in the general population, and pain-alcohol interrelations are of increasing empirical interest. Previous research has identified associations between pain and alcohol dependence, and the current review provides novel contributions to this emerging domain by incorporating studies that have tested relations between pain and low-to-moderate alcohol consumption, and by identifying potential psychosocial mechanisms of action. Specifically, we sought to integrate evidence of pain-alcohol relations derived from two directions of empirical inquiry (i.e., effects of alcohol on pain and effects of pain on alcohol use) across psychological, social, and biological literatures. We observed converging evidence that associations between alcohol consumption and pain may be curvilinear in nature. Whereas moderate alcohol use was observed to be associated with positive pain-related outcomes (e.g., greater quality of life), excessive drinking and alcohol use disorder appear to be associated with deleterious pain-related outcomes (e.g., greater pain severity). We also observed evidence that alcohol administration confers acute pain-inhibitory effects, and that situational pain may motivate alcohol consumption (e.g., drinking for pain-coping). Future research can inform theoretical and clinical applications through examination of temporal relations between pain and alcohol consumption, tests of hypothesized mechanisms, and the development of novel interventions.

Clinical and pharmacokinetic considerations of novel formulations of fentanyl for breakthrough cancer pain

SUMMARY 

This review will provide an overview of pharmacokinetics and clinical practice considerations of the novel formulations of fentanyl for the treatment of breakthrough cancer pain. First, we provide a brief description of the physicochemical properties of fentanyl. Second, we describe the basic pharmacokinetics of fentanyl, the specifics of various formulations and how they affect pharmacokinetics of fentanyl. Finally, we offer the perspectives on clinical practices in the proper uses of these products. Advancements in the formulations and delivery of fentanyl have provided a safer and more effective treatment for breakthrough cancer pain. These dosage forms offer overlapping yet distinct pharmacokinetic advantages to allow more choices for physicians and patients in the management of breakthrough cancer pain.

Diabetes-causing gene, kruppel-like factor 11, modulates the antinociceptive response of chronic ethanol intake

BACKGROUND

Alcohol (EtOH [ethanol]) is an antinociceptive agent, working in part, by reducing sensitivity to painful stimuli. The transcription factor Kruppel-like factor 11 (KLF11), a human diabetes-causing gene that also regulates the neurotransmitter metabolic enzymes monoamine oxidase (MAO), has recently been identified as an EtOH-inducible gene. However, its role in antinociception remains unknown. Consequently, we investigated the function of KLF11 in chronic EtOH-induced antinociception using a genetically engineered knockout mouse model.

METHODS

Wild-type (Klf11(+/+) ) and KLF11 knockout (Klf11(-/-) ) mice were fed a liquid diet containing EtOH for 28 days with increasing amounts of EtOH from 0% up to a final concentration of 6.4%, representing a final diet containing 36% of calories primarily from EtOH. Control mice from both genotypes were fed liquid diet without EtOH for 28 days. The EtOH-induced antinociceptive effect was determined using the tail-flick test before and after EtOH exposure (on day 29). In addition, the enzyme activity and mRNA levels of MAO A and MAO B were measured by real-time RT-PCR and enzyme assays, respectively.

RESULTS

EtOH produced an antinociceptive response to thermal pain in Klf11(+/+) mice, as expected. In contrast, deletion of KLF11 in the Klf11(-/-) mice abolished the EtOH-induced antinociceptive effect. The mRNA and protein levels of KLF11 were significantly increased in the brain prefrontal cortex of Klf11(+/+) mice exposed to EtOH compared with control Klf11(+/+) mice. Furthermore, MAO enzyme activities were affected differently in Klf11 wild-type versus Klf11 knockout mice exposed to chronic EtOH. Chronic EtOH intake significantly increased MAO B activity in Klf11(+/+) mice.

CONCLUSIONS

The data show KLF11 modulation of EtOH-induced antinociception. The KLF11-targeted MAO B enzyme may contribute more significantly to EtOH-induced antinociception. Thus, this study revealed a new role for the KLF11 gene in the mechanisms underlying the antinociceptive effects of chronic EtOH exposure.

Participation of central imidazoline binding sites in antinociceptive effect of ethanol and nicotine in rats

Despite synergistic morbidity and mortality, concomitant consumption of alcohol and tobacco is increasing, and their antinociceptive effect has been linked with co-abuse. Present study was designed to investigate the role of imidazoline binding sites in the antinociceptive effect of nicotine, ethanol, and their combination. Separate group of male Sprague-Dawley rats (200-250 g) were treated with different doses of alcohol (0.50-2 g/kg, i.p.) or nicotine (0.25-1 mg/kg, i.p.), and their combination evaluated in tail flick test. Influence of endogenous imidazoline binding site ligands, agonist, and antagonists were determined by their prior treatment with effective or subeffective doses of either ethanol or nicotine. Ethanol, nicotine, or their subeffective dose combination exhibited significant antinociceptive effects in dose-dependent manner. Antinociceptive effect of ethanol and nicotine was significantly augmented by intracerebroventricular (i.c.v.) administration of endogenous imidazoline receptor ligands, harmane (25 μg/rat, i.c.v.) and agmatine (10 μg/rat, i.c.v.), as well as imidazoline I1 /α2 adrenergic receptor agonist, clonidine (2 μg/rat, i.c.v.), I1 agonist moxonidine (25 μg/rat, i.c.v.), and imidazoline I2 agonist, 2-BFI (10 μg/rat, i.c.v.). Conversely, antinociception elicited by ethanol or nicotine or their subeffective dose combination was antagonized by pretreatment with imidazoline I1 antagonist, efaroxan (10 μg/rat, i.c.v.), and I2 antagonist, idazoxan (4 μg/rat, i.c.v.), at their per se ineffective doses. These findings project imidazoline binding ligands as important therapeutic molecules for central antinociceptive activity as well as may reduce the co-abuse potential of alcohol and nicotine.

Alcohol dependence as a chronic pain disorder

Dysregulation of pain neurocircuitry and neurochemistry has been increasingly recognized as playing a critical role in a diverse spectrum of diseases including migraine, fibromyalgia, depression, and PTSD. Evidence presented here supports the hypothesis that alcohol dependence is among the pathologies arising from aberrant neurobiological substrates of pain. In this review, we explore the possible influence of alcohol analgesia and hyperalgesia in promoting alcohol misuse and dependence. We examine evidence that neuroanatomical sites involved in the negative emotional states of alcohol dependence also play an important role in pain transmission and may be functionally altered under chronic pain conditions. We also consider possible genetic links between pain transmission and alcohol dependence. We propose an allostatic load model in which episodes of alcohol intoxication and withdrawal, traumatic stressors, and injury are each capable of dysregulating an overlapping set of neural substrates to engender sensory and affective pain states that are integral to alcohol dependence and comorbid conditions such as anxiety, depression, and chronic pain.

Influence of δ-opioid receptors in the behavioral effects of nicotine

Multiple studies in animal models and humans suggest that the endogenous opioid system is an important neurobiological substrate for nicotine addictive properties. In this study, we evaluated the participation of δ-opioid receptors in different behavioral responses of nicotine by using δ-opioid receptor knockout mice. Acute nicotine administration induced hypolocomotion and antinociception in wild-type mice, which were similar in knockout animals. The development of tolerance to nicotine-induced antinociception was also similar in both genotypes. In agreement, the expression and functional activity of δ-opioid receptors were not modified in the different layers of the spinal cord and brain areas evaluated after chronic nicotine treatment. The somatic manifestation of the nicotine withdrawal syndrome precipitated by mecamylamine was also similar in wild-type and δ-opioid receptor knockout mice. In contrast, nicotine induced a conditioned place preference in wild-type animals that was abolished in knockout mice. Moreover, a lower percentage of acquisition of intravenous nicotine self-administration was observed in mice lacking δ-opioid receptors as well as in wild-type mice treated with the selective δ-opioid receptor antagonist naltrindole. Accordingly, in-vivo microdialysis studies revealed that the enhancement in dopamine extracellular levels induced by nicotine in the nucleus accumbens was reduced in mutant mice. In summary, the present results show that δ-opioid receptors are involved in the modulation of nicotine rewarding effects. However, this opioid receptor does not participate either in several acute effects of nicotine or in the development of tolerance and physical dependence induced by chronic nicotine administration.

Pain, nicotine, and smoking: research findings and mechanistic considerations

Tobacco addiction and chronic pain represent 2 highly prevalent and comorbid conditions that engender substantial burdens upon individuals and systems. Interrelations between pain and smoking have been of clinical and empirical interest for decades, and research in this area has increased dramatically over the past 5 years. We conceptualize the interaction of pain and smoking as a prototypical example of the biopsychosocial model. Accordingly, we extrapolated from behavioral, cognitive, affective, biomedical, and social perspectives to propose causal mechanisms that may contribute to the observed comorbidity between these 2 conditions. The extant literature was 1st dichotomized into investigations of either effects of smoking on pain or effects of pain on smoking. We then integrated these findings to present a reciprocal model of pain and smoking that is hypothesized to interact in the manner of a positive feedback loop, resulting in greater pain and increased smoking. Finally, we proposed directions for future research and discussed clinical implications for smokers with comorbid pain disorders. We observed modest evidence that smoking may be a risk factor in the multifactorial etiology of some chronically painful conditions and that pain may come to serve as a potent motivator of smoking. We also found that whereas animal studies yielded consistent support for direct pain-inhibitory effects of nicotine and tobacco, results from human studies were much less consistent. Future research in the emerging area of pain and smoking has the potential to inform theoretical and clinical applications with respect to tobacco smoking, chronic pain, and their comorbid presentation. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Chronic ethanol consumption in rats produces opioid antinociceptive tolerance through inhibition of mu opioid receptor endocytosis

It is well known that the mu-opioid receptor (MOR) plays an important role in the rewarding properties of ethanol. However, it is less clear how chronic ethanol consumption affects MOR signaling. Here, we demonstrate that rats with prolonged voluntary ethanol consumption develop antinociceptive tolerance to opioids. Signaling through the MOR is controlled at many levels, including via the process of endocytosis. Importantly, agonists at the MOR that promote receptor endocytosis, such as the endogenous peptides enkephalin and β-endorphin, show a reduced propensity to promote antinociceptive tolerance than do agonists, like morphine, which do not promote receptor endocytosis. These observations led us to examine whether chronic ethanol consumption produced opioid tolerance by interfering with MOR endocytosis. Indeed, here we show that chronic ethanol consumption inhibits the endocytosis of MOR in response to opioid peptide. This loss of endocytosis was accompanied by a dramatic decrease in G protein coupled receptor kinase 2 (GRK2) protein levels after chronic drinking, suggesting that loss of this component of the trafficking machinery could be a mechanism by which endocytosis is lost. We also found that MOR coupling to G-protein was decreased in ethanol-drinking rats, providing a functional explanation for loss of opioid antinociception. Together, these results suggest that chronic ethanol drinking alters the ability of MOR to endocytose in response to opioid peptides, and consequently, promotes tolerance to the effects of opioids.

Activation of the opioid μ1, but not δ or κ, receptors is required for nicotine reinforcement in a rat model of drug self-administration

There has long been an interest in examining the involvement of opioid neurotransmission in nicotine rewarding process and addiction to nicotine. Over the past 3 decades, however, clinical effort to test the effectiveness of nonselective opioid antagonists (mainly naloxone and naltrexone) for smoking cessation has yielded equivocal results. In light of the fact that there are three distinctive types of receptors mediating actions of the endogenous opioid peptides, this study, using a rat model of nicotine self-administration, examined involvement of different opioid receptors in the reinforcement of nicotine by selective blockade of the μ1, the δ, and the κ opioid receptors. Male Sprague-Dawley rats were trained in daily 1h sessions to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. After establishment of stable nicotine self-administration behavior, the effects of the opioid antagonists were tested. Separate groups of rats were used to test the effects of naloxanazine (selective for μ1 receptors, 0, 5 and 15 mg/kg), naltrindole (selective for δ receptors, 0, 0.5 and 5mg/kg), and 5'-guanidinonaltrindole (GNTI, selective for κ receptors, 0, 0.25 and 1mg/kg). In each individual drug group, the 3 drug doses were tested by using a within-subject and Latin-Square design. The effects of these antagonists on food self-administering behavior were also examined in the same rats in each respective drug group after retrained for food self-administration. Pretreatment with naloxonazine, but not naltrindole or GNTI, significantly reduced responses on the active lever and correspondingly the number of nicotine infusions. None of these antagonists changed lever-pressing behavior for food reinforcement. These results indicate that activation of the opioid μ1, but not the δ or the κ, receptors is required for the reinforcement of nicotine and suggest that opioid neurotransmission via the μ1 receptors would be a promising target for the development of opioid ligands for smoking cessation.

Effect of the selective kappa-opioid receptor antagonist JDTic on nicotine antinociception, reward, and withdrawal in the mouse

RATIONALE

Several lines of evidence support a role for the endogenous opioid system in mediating behaviors associated with drug dependence. Specifically, recent findings suggest that the kappa-opioid receptor (KOR) may play a role in aspects of nicotine dependence, which contribute to relapse and continued tobacco smoking.

OBJECTIVE

The objective of this study is to determine the involvement of the KOR in the initial behavioral responses of nicotine, nicotine reward, and nicotine withdrawal using the highly selective KOR antagonist JDTic. JDTic doses of 1, 4, 8, or 16 mg/kg were administered subcutaneously (s.c.) 18 h prior to nicotine treatment.

RESULTS

JDTic dose-dependently blocked acute nicotine-induced antinociception in the tail-flick but not the hot-plate test and did not significantly attenuate morphine's antinociceptive effect in either the tail-flick or hot-plate test. Furthermore, JDTic (8 and 16 mg/kg, s.c.) failed to block the expression of nicotine reward as measured by the conditioned place preference model. In contrast, JDTic and the KOR antagonist norBNI attenuated the expression of both the physical (somatic signs and hyperalgesia) and affective (anxiety-related behavior and conditioned place aversion) nicotine withdrawal signs.

CONCLUSIONS

Our findings clearly show that the KOR is involved in mediating the withdrawal aspects of nicotine dependence. The results from this study suggest that blockade of the KOR by selective KOR antagonists may be useful smoking cessation pharmacotherapies.

Differential role of mu, delta and kappa opioid receptors in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

The opioid system modulates ethanol intake and reinforcement in adult and preweanling rodents. While adult heterogeneous rats normally do not show ethanol-mediated locomotor stimulation, preweanling rats show it quite clearly. We recently observed that naloxone, a non-specific opioid antagonist, attenuated ethanol-induced locomotor activation in preweanling rats. In the present study we tested the role of specific opioid receptors (mu, delta and kappa) in ethanol-mediated locomotor stimulation and ethanol intake. In Experiment 1 13-day-old rats received naloxonazine (mu antagonist: 0, 7.5 or 15 mg/kg), naltrindole (delta antagonist: 0, 2 or 4 mg/kg) or nor-binaltorphimine (kappa antagonist: 0, 2, 4 or 8 mg/kg) before an intragastric administration of ethanol (0 or 2.5 g/kg), and subsequent locomotor activity assessment. In Experiment 2, the same opioid antagonists were administered on postnatal days 13 and 14 before consumption of ethanol (6%), saccharin (0.05%) or distilled water. In Experiment 1 only naloxonazine reduced ethanol-mediated locomotor stimulation. None of the opioid antagonists affected locomotor activity in water controls. In Experiment 2 naloxonazine and naltrindole suppressed ingestion of all the solutions tested. Similar to what has been reported in adult rodents, mu-opioid receptors seem to modulate ethanol-activating effects during early ontogeny. Hence, there seems to be a partial overlap of neurochemical mechanisms involved in the rewarding and stimulating effects of ethanol in preweanling rats. Mu-receptor antagonists reduced both ethanol-induced activity and ethanol intake, but it is unclear whether the latter effect is specific to ethanol or only a reflection of an effect on consummatory behavior generally, since mu and delta receptor antagonists also suppressed ingestion of water and saccharin.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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.

The effect of formalin pretreatment on nicotine-induced antinociceptive effect: the role of mu-opioid receptor in the hippocampus

Nicotine is attractive as an analgesic component despite that its antinociceptive mechanism is not well known until now. In the present study, we examined the antinociceptive effect of nicotine administered supra-spinally on acetic acid-induced visceral pain induction (writhing test), and found that the antinociceptive effect of nicotine was abolished by mu-, delta-, and kappa-opioid receptor antagonist administered i.c.v. In addition, s.c. 5% formalin pretreatment at 5 h, 20 h, 40 h, and 1 week prior to i.c.v. nicotine injection abolished the antinociceptive effect of nicotine in the writhing test, suggesting that s.c. formalin pretreatment induced tolerance to the antinociceptive effect of nicotine in the supra-spinal region. Furthermore, neuronal loss of the hippocampal cornus ammonis (CA) 3 region reduced nicotine-induced an antinociceptive effect in the writhing test. In Western blot assay, we examined s.c. formalin injection down-regulated mu-opioid receptor in the hippocampus after 40 h, and its effect was maintained for 1 week. However, various acetylcholine receptor subunits and delta-, and kappa-opioid receptors were not altered. These results suggest that s.c. formalin pretreatment can contribute to induce tolerance on nicotine-induced antinociception as down-regulating mu-opioid receptor in the hippocampus, especially 40 h after s.c. formalin injection.