Involvement of mu- and delta-opioid receptors in the ethanol-associated place preference in rats exposed to foot shock stress

Role of the Dynorphin/Kappa Opioid Receptor System in the Motivational Effects of Ethanol

Evidence has demonstrated that dynorphin (DYN) and the kappa opioid receptor (KOR) system contribute to various psychiatric disorders, including anxiety, depression, and addiction. More recently, this endogenous opioid system has received increased attention as a potential therapeutic target for treating alcohol use disorders. In this review, we provide an overview and synthesis of preclinical studies examining the influence of alcohol (ethanol [EtOH]) exposure on DYN/KOR expression and function, as well as studies examining the effects of DYN/KOR manipulation on EtOH's rewarding and aversive properties. We then describe work that has characterized effects of KOR activation and blockade on EtOH self-administration and EtOH dependence/withdrawal-related behaviors. Finally, we address how the DYN/KOR system may contribute to stress-EtOH interactions. Despite an apparent role for the DYN/KOR system in motivational effects of EtOH, support comes from relatively few studies. Nevertheless, review of this literature reveals several common themes: (i) rodent strains genetically predisposed to consume more EtOH generally appear to have reduced DYN/KOR tone in brain reward circuitry; (ii) acute and chronic EtOH exposure typically up-regulate the DYN/KOR system; (iii) KOR antagonists reduce behavioral indices of negative affect associated with stress and chronic EtOH exposure/withdrawal; and (iv) KOR antagonists are effective in reducing EtOH consumption, but are often more efficacious under conditions that engender high levels of consumption, such as dependence or stress exposure. These results support the contention that the DYN/KOR system plays a significant role in contributing to dependence- and stress-induced elevation in EtOH consumption. Overall, more comprehensive analyses (on both behavioral and mechanistic levels) are needed to provide additional insight into how the DYN/KOR system is engaged and adapts to influence the motivation effects of EtOH. This information will be critical for the development of new pharmacological agents targeting KORs as promising novel therapeutics for alcohol use disorders and comorbid affective disorders.

Ethanol drives aversive conditioning through dopamine 1 receptor and glutamate receptor-mediated activation of lateral habenula neurons

There has been increasing interest in the lateral habenula (LHb) given its potent regulatory role in many aversion-related behaviors. Interestingly, ethanol can be rewarding as well as aversive; we therefore investigated whether ethanol exposure alters pacemaker firing or glutamate receptor signaling in LHb neurons in vitro and also whether LHb activity in vivo might contribute to the acquisition of conditioned place aversion to ethanol. Surprisingly, in epithalamic slices, low doses of ethanol (1.4 mM) strongly accelerated LHb neuron firing (by ~60%), and ethanol's effects were much reduced by blocking glutamate receptors. Ethanol increased presynaptic glutamate release, and about half of this effect was mediated by dopamine subtype 1 receptors (D1Rs) and cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. In agreement with these findings, c-Fos immunoreactivity in LHb regions was enhanced after a single administration of a low dose of ethanol (0.25 g/kg i.p.). Importantly, the same dose of ethanol in vivo also produced strong conditioned place aversion, and this was prevented by inhibiting D1Rs or neuronal activity within the LHb. By contrast, a higher dose (2 g/kg) led to ethanol conditioned place preference, which was enhanced by inhibiting neuronal activity or D1Rs within the LHb and suppressed by infusing aminomethylphosphonic acid or the D1R agonist SKF38393 within the LHb. Our in vitro and in vivo observations show, for the first time, that ethanol increases LHb excitation, mediated by D1R and glutamate receptors, and may underlie a LHb aversive signal that contributes to ethanol-related aversion.

Switch from excitatory to inhibitory actions of ethanol on dopamine levels after chronic exposure: Role of kappa opioid receptors

Acute ethanol exposure is known to stimulate the dopamine system; however, chronic exposure has been shown to downregulate the dopamine system. In rodents, chronic intermittent exposure (CIE) to ethanol also increases negative affect during withdrawal, such as, increases in anxiety- and depressive-like behavior. Moreover, CIE exposure results in increased ethanol drinking and preference during withdrawal. Previous literature documents reductions in CIE-induced anxiety-, depressive-like behaviors and ethanol intake in response to kappa opioid receptor (KOR) blockade. KORs are located on presynaptic dopamine terminals in the nucleus accumbens (NAc) and inhibit release, an effect which has been linked to negative affective behaviors. Previous reports show an upregulation in KOR function following extended CIE exposure; however it is not clear whether there is a direct link between KOR upregulation and dopamine downregulation during withdrawal from CIE. This study aimed to examine the effects of KOR modulation on dopamine responses to ethanol of behaving mice exposed to air or ethanol vapor in a repeated intermittent pattern. First, we showed that KORs have a greater response to an agonist after moderate CIE compared to air exposed mice using ex vivo fast scan cyclic voltammetry. Second, using in vivo microdialysis, we showed that, in contrast to the expected increase in extracellular levels of dopamine following an acute ethanol challenge in air exposed mice, CIE exposed mice exhibited a robust decrease in dopamine levels. Third, we showed that blockade of KORs reversed the aberrant inhibitory dopamine response to ethanol in CIE exposed mice while not affecting the air exposed mice demonstrating that inhibition of KORs "rescued" dopamine responses in CIE exposed mice. Taken together, these findings indicate that augmentation of dynorphin/KOR system activity drives the reduction in stimulated (electrical and ethanol) dopamine release in the NAc. Thus, blockade of KORs is a promising avenue for developing pharmacotherapies for alcoholism.

The delta opioid receptor tool box

In recent years, the delta opioid receptor has attracted increasing interest as a target for the treatment of chronic pain and emotional disorders. Due to their therapeutic potential, numerous tools have been developed to study the delta opioid receptor from both a molecular and a functional perspective. This review summarizes the most commonly available tools, with an emphasis on their use and limitations. Here, we describe (1) the cell-based assays used to study the delta opioid receptor. (2) The features of several delta opioid receptor ligands, including peptide and non-peptide drugs. (3) The existing approaches to detect delta opioid receptors in fixed tissue, and debates that surround these techniques. (4) Behavioral assays used to study the in vivo effects of delta opioid receptor agonists; including locomotor stimulation and convulsions that are induced by some ligands, but not others. (5) The characterization of genetically modified mice used specifically to study the delta opioid receptor. Overall, this review aims to provide a guideline for the use of these tools with the final goal of increasing our understanding of delta opioid receptor physiology.

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.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

Effect of chronic ethanol treatment on μ-opioid receptor function, interacting proteins and morphine-induced place preference

RATIONALE

Both the acute and chronic consumption of ethanol have been reported to modify several molecular events in the central nervous system, and the endogenous μ-opioid receptor system is involved in the reinforcing/rewarding effects of ethanol.

OBJECTIVES

The present study was designed to clarify the effects of chronic ethanol treatment on cellular processes involving μ-opioid receptor and the development of morphine-induced rewarding effects.

METHODS

Male C57BL/6J mice were continuously treated with a liquid diet containing 3.0 w/v ethanol. The direct involvement of μ-opioid receptor functions in the activation of G-proteins and changes in protein levels in the lower midbrain of mice after chronic treatment with ethanol were investigated by a [(35)S] GTPγS binding assay and Western blotting, respectively. The rewarding effects of morphine (5 mg/kg) under treatment with ethanol were measured by the conditioned place preference paradigm.

RESULTS

The function of μ-opioid receptor was increased by treatment with ethanol in the lower midbrain using [(35)S] GTPγS binding assay. Furthermore, the GRK2 protein level was significantly increased by treatment with ethanol. Chronic treatment with ethanol enhanced the rewarding effects of morphine. On the other hand, this enhancement of the rewarding effects of morphine by ethanol treatment was significantly inhibited by the GRK2 inhibitor β-adrenergic receptor kinase 1 inhibitor.

CONCLUSIONS

The present study demonstrated that chronic treatment with ethanol enhanced the rewarding effects of morphine by up-regulating functional changes in μ-opioid receptor, mediated by GRK2.

Effects of acute stress on acquisition of nicotine conditioned place preference in adolescent rats: a role for corticotropin-releasing factor 1 receptors

RATIONALE

Studies indicate that adolescence is a time of increased sensitivity to the rewarding effects of nicotine, and that stress is associated with an increased risk for smoking initiation in this age group. It is possible that stress leads to increased nicotine use in adolescence by augmenting its rewarding properties. Corticotropin-releasing factor type 1 receptors (CRF-R1) mediate physiological and behavioral stress responses. They may also mediate stress-induced potentiation of activity in multiple neural substrates implicated in nicotine reward.

OBJECTIVES

The aim of the present study was to determine the effect of acute stressor exposure on single trial nicotine conditioned place preference (CPP) in adolescent male rats using a biased CPP procedure and the role of CRF-R1 in this effect.

RESULTS

A single episode of intermittent footshock administered 24 h before the start of place conditioning dose-dependently facilitated acquisition of CPP to nicotine (0.2, 0.4, and 0.6 mg/kg). Pretreatment with CP-154,526 (20 mg/kg), a selective CRF-R1 antagonist, 30 min before footshock exposure significantly attenuated the effect of prior stress to facilitate nicotine CPP acquisition. CP-154,526 pretreatment had no effect in animals conditioned with a nicotine dose that produced CPP under non-stress conditions, suggesting a specific role for CRF-R1 following stress.

CONCLUSIONS

Taken together, the results suggest that during adolescence, nicotine reward is enhanced by recent stressor exposure in a manner that involves signaling at CRF-R1. Information from studies such as this may be used to inform efforts to prevent and treat adolescent nicotine dependence.

The role of the dynorphin-kappa opioid system in the reinforcing effects of drugs of abuse

BACKGROUND

Initial hypotheses regarding the role of the kappa opioid system in drug addiction suggested that kappa receptor stimulation had anti-addictive effects. However, recent research suggests that kappa receptor antagonists may reverse motivational aspects of dependence. In the present review, we revisit the studies that measured the effects of kappa receptor ligands on the reinforcing and rewarding effects of drugs and postulate underlying neurobiological mechanisms for these effects to elaborate a more complex view of the role of kappa receptor ligands in drug addiction.

RESULTS

The review of studies indicates that kappa receptor stimulation generally antagonizes the acute reinforcing/rewarding effects of drugs whereas kappa receptor blockade has no consistent effect. However, in a drug dependent-like state, kappa receptor blockade was effective in reducing increased drug intake. In animal models of reinstatement, kappa receptor stimulation can induce reinstatement via a stress-like mechanism. Results in conditioned place preference/aversion and intracranial self-stimulation indicate that kappa receptor agonists produce, respectively, aversive-like and dysphoric-like effects. Additionally, preclinical and postmortem studies show that administration or self-administration of cocaine, ethanol, and heroin activate the kappa opioid system.

CONCLUSION

kappa receptor agonists antagonize the reinforcing/rewarding effects of drugs possibly through punishing/aversive-like effects and reinstate drug seeking through stress-like effects. Evidence suggests that abused drugs activate the kappa opioid system, which may play a key role in motivational aspects of dependence. Kappa opioid systems may have an important role in driving compulsive drug intake.

Mechanisms of respiratory insufficiency induced by methadone overdose in rats

Methadone may cause respiratory depression. We aimed to understand methadone-related effects on ventilation as well as each opioid-receptor (OR) role. We studied the respiratory effects of intraperitoneal methadone at 1.5, 5, and 15 mg/kg (corresponding to 80% of the lethal dose-50%) in rats using arterial blood gases and plethysmography. OR antagonists, including intravenous 10 mg/kg-naloxonazine at 5 minutes (mu-OR antagonist), subcutaneous 30 mg/kg-naloxonazine at 24 hours (micro1-OR antagonist), 3 mg/kg-naltrindole at 45 minutes (delta-OR antagonist) and 5 mg/kg-Nor-binaltorphimine at 6 hours (kappa-OR antagonist) were pre-administered. Plasma concentrations of methadone enantiomers were measured using high-performance liquid chromatography coupled to mass-spectrometry. Methadone dose-dependent inspiratory time (T(I)) increase tended to be linear. Respiratory depression was observed only at 15 mg/kg and characterized by an increase in expiratory time (T(E)) resulting in hypoxemia and respiratory acidosis. Intravenous naloxonazine completely reversed all methadone-related effects on ventilation, while subcutaneous naloxonazine reduced its effects on pH (P < 0.05), PaCO(2) (P < 0.01) and T(E) (P < 0.001) but only partially on T(I) (P < 0.001). Naltrindole reduced methadone-related effects on T(E) (P < 0.001). Nor-binaltorphimine increased methadone-related effects on pH and PaO(2) (P < 0.05) Respiratory effects as a function of plasma R-methadone concentrations showed a decrease in PaO(2) (EC(50): 1.14 microg/ml) at lower concentrations than those necessary for PaCO(2) increase (EC(50): 3.35 microg/ml). Similarly, increased T(I) (EC(50): 0.501 microg/ml) was obtained at lower concentrations than those for T(E) (EC(50): 4.83 microg/ml). Methadone-induced hypoxemia is caused by mu-ORs and modulated by kappa-ORs. Additionally, methadone-induced increase in T(E) is caused by mu1- and delta-opioid receptors while increase in T(I) is caused by mu-ORs.

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.

Opioid antagonists block the acquisition of ethanol-mediated conditioned tactile preference in infant rats

It has been difficult to find conditioned preference for tactile cues paired with ethanol intoxication in rats. Toward understanding the ontogeny of ethanol reinforcement, we aimed at establishing a simple and reliable procedure for (1) assessing primary appetitive conditioning to ethanol in infant rats and (2) discerning the role the opioid system plays in ethanol-mediated conditioning at this age. Experiment 1 determined the parameters (i.e., dose, interval of conditioning) for assessing ethanol-mediated conditioning. Pups were then trained with differential Pavlovian conditioning (Experiments 2 and 3) in which ethanol intoxication (1.0-2.0 g/kg, intragastrically or intraperitoneally delivered) was paired with a tactile stimulus (sandpaper) while an alternative texture signaled the absence of ethanol's effects. Unpaired control conditions were also used. Tactile preferences were assessed after two conditioning sessions. Paired rats spent significantly more time on sandpaper than unpaired controls, an effect that was greater after intragastric administration of 1.0 than 2.0 g/kg ethanol. This effect was replicated in Experiments 4a and 4c and found to be inhibited by pretreatment with general (naloxone [NAL]) or specific (d-Pen-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 [CTOP] and naltrindole) opioid antagonists. Blood ethanol levels at conditioning were not altered by NAL (Experiment 4b). The study outlines a procedure that reveals appetitive conditioning to ethanol by infant rats. The results are discussed in terms of a potential ethanol-induced activation of the endogenous opioid system during the onset of the intoxication process.

Blockade of ethanol reward by the kappa opioid receptor agonist U50,488H

Alcoholism is a pervasive social problem, and thus understanding factors that regulate alcohol (ethanol) reward is important for designing effective therapies. One putative regulatory system includes the kappa opioid receptor (KOR) and its endogenous ligand, dynorphin. Previously, we demonstrated that acute ethanol increased preprodynorphin expression via brain-derived neurotrophic factor (BDNF) in striatal neurons, and that blockade of the KOR attenuated decreases in ethanol intake observed following increased expression of BDNF. As high doses of KOR agonists can generate an aversive state, we hypothesized that endogenous dynorphin may regulate ethanol intake by interfering with the rewarding properties of ethanol. We found that low, nonaversive doses of the KOR agonist U50,488H blocked the rewarding properties of ethanol during conditioning, thus impairing the acquisition of conditioned place preference. Importantly, we demonstrate that U50,488H also inhibited the conditioned increase in locomotor activation normally observed in the ethanol-paired chamber on test day. Taken together, these data indicate that the KOR/dynorphin system may acutely regulate ethanol intake via inhibition of the rewarding properties of ethanol.

Central reinforcing effects of ethanol are blocked by catalase inhibition

Recent studies have systematically indicated that newborn rats are highly sensitive to ethanol's positive reinforcing effects. Central administrations of ethanol (25-200mg %) associated with an olfactory conditioned stimulus (CS) promote subsequent conditioned approach to the CS as evaluated through the newborn's response to a surrogate nipple scented with the CS. It has been shown that ethanol's first metabolite, acetaldehyde, exerts significant reinforcing effects in the central nervous system. A significant amount of acetaldehyde is derived from ethanol metabolism via the catalase system. In newborn rats, catalase levels are particularly high in several brain structures. The present study tested the effect of catalase inhibition on central ethanol reinforcement. In the first experiment, pups experienced lemon odor either paired or unpaired with intracisternal (IC) administrations of 100mg% ethanol. Half of the animals corresponding to each learning condition were pretreated with IC administrations of either physiological saline or a catalase inhibitor (sodium-azide). Catalase inhibition completely suppressed ethanol reinforcement in paired groups without affecting responsiveness to the CS during conditioning or responding by unpaired control groups. A second experiment tested whether these effects were specific to ethanol reinforcement or due instead to general impairment in learning and expression capabilities. Central administration of an endogenous kappa opioid receptor agonist (dynorphin A-13) was used as an alternative source of reinforcement. Inhibition of the catalase system had no effect on the reinforcing properties of dynorphin. The present results support the hypothesis that ethanol metabolism regulated by the catalase system plays a critical role in determination of ethanol reinforcement in newborn rats.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

The effects of a single exposure to uncontrollable stress on the subsequent conditioned place preference responses to oxycodone, cocaine, and ethanol in rats

RATIONALE

Acute stress has been shown to facilitate the rewarding effects of a number of commonly abused drugs, although the stressor typically must be administered either immediately before or during drug administration and often in the same environment. We have previously reported that a single session of an uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stressor can enhance the conditioned place preference (CPP) response to morphine, even when stressor and drug administration are separated temporally and spatially. However, this persistent, trans-situational enhancement did not occur to amphetamine CPP.

OBJECTIVES

The following experiments were conducted to determine whether the long-term effects of IS on drug reward are specific to opioids.

MATERIALS AND METHODS

Adult, male Sprague-Dawley rats were exposed to a single session of IS or remained in their home cages (HC). Twenty-four hours later, using an unbiased procedure, CPP conditioning was conducted with either oxycodone (0, 2, or 5 mg/kg, sc), cocaine (0, 1, 5, or 10 mg/kg, ip), or ethanol (0.3, 1, or 2 g/kg, ip). Another group of rats were exposed to IS, ES, or HC treatment and conditioned with oxycodone (5 mg/kg, sc) 24 h later.

RESULTS

IS enhanced the subsequent CPP response to oxycodone, but not cocaine or ethanol. This enhancement was dependent on the controllability of the stressor, as ES did not affect oxycodone CPP.

CONCLUSIONS

These results indicate that the long-term, trans-situational enhancing effect of uncontrollable stress on drug reward is specific to opioids.

Reinforcing properties of ethanol in neonatal rats: involvement of the opioid system

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanol's gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanol's reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

Influence of age at drinking onset on long-term ethanol self-administration with deprivation and stress phases

BACKGROUND

Onset of alcohol use during adolescence has potentially long-lasting consequences, e.g., prospective alcohol dependence. To obtain new insight into the effects of early chronic ethanol consumption, we compared the drinking behavior of two adult male Wistar rat groups: one that initiated alcohol consumption during adolescence (adolescent group) and the other that initiated their drinking during adulthood (adult group) in a model of long-term alcohol self-administration. We investigated the magnitude of the effects of deprivation and stress on alcohol intake and the influence of these events on the alcohol drinking behavior across time.

METHODS

Heterogeneous Wistar rats aged 31 days (adolescents) and 71 days (adults) were given ad libitum access to water, as well as 5% and 20% ethanol solutions during an observation period of 30 wk. A deprivation phase of 14 days was instituted after eight wk of access to alcohol. After 16 and 26 wk of alcohol access, all animals were subjected for three consecutive days to forced swimming and electric foot shocks, respectively.

RESULTS

At the onset of drinking, adolescent animals consumed less alcohol and showed lower preference than adults. The deprivation phase was followed by increased intake of highly concentrated ethanol solution without appreciable differences between age groups. Repeated swim stress produced a slight increase in ethanol consumption in both animal groups; however, alcohol intake was not significantly different between groups, whereas the foot shock stress-induced increase in alcohol intake was significantly higher in the animal group that initiated alcohol consumption during adolescence. After swim stress, the drinking behavior of the adolescent group resembled that of the adult group. In particular, the adolescent group increased their preference for 20% ethanol solution for the remainder of the experiment.

CONCLUSIONS

Age of voluntary alcohol drinking onset does not appear to be a strong predictor for prospective alcohol intake and relapse-like drinking behavior under the present experimental conditions. However, male Wistar rats that initiated alcohol consumption during adolescence seem to be more susceptible to acute stressor-specific effects in terms of alcohol consumption.

Influence of stressors on the rewarding effects of alcohol in Wistar rats: studies with alcohol deprivation and place conditioning

RATIONALE AND OBJECTIVES

Studies on laboratory animals have provided conflicting results regarding the actions of stressors on the rewarding effects of alcohol. In the present study, we first examined the effects of footshock or social defeat, given during deprivation, on the alcohol deprivation effect (ADE). We then tested the effects of stressors on place conditioning to alcohol, another technique used to measure drug reward.

METHODS

Male Wistar rats were trained to drink 10% alcohol in a 24 h access, free-choice design and received intermittent footshock or defeat 5 times during a 2-week alcohol deprivation period, followed by 2 weeks of free access to alcohol. There were three such cycles. In the place conditioning studies, animals received footshock, defeat, or no stress immediately prior to conditioning sessions where they received alcohol (0.6 or 1.0 g/kg, i.p.) or vehicle injections.

RESULTS

Alcohol intake of footshock-treated animals was significantly higher than that of controls following the first and second, but not the third period of alcohol deprivation and stress exposure. Defeat caused a smaller increase in alcohol intake that was significant only after the first deprivation and stress cycle. In the place conditioning studies, we found that either stressor blocked the place aversion induced by 1.0 g/kg alcohol.

CONCLUSIONS

These results demonstrate that stressors can modify the rewarding and aversive properties of alcohol, measured using two different paradigms. Footshock and defeat produced transient, but significant increases in the magnitude of ADE, while exposure to either stressor reduced the aversive effects of a high dose of alcohol measured using the place conditioning paradigm.

Micro1-opioid antagonist naloxonazine alters ethanol discrimination and consumption

The endogenous opioid system is implicated in excessive ethanol-drinking behavior. However, the role of individual opioid receptor subtypes in the mechanism underlying excessive ethanol-drinking behavior is not yet well understood. Therefore, we investigated the ability of a selective micro1-opioid antagonist, naloxonazine, to modulate ethanol-drinking behavior and ethanol discrimination in a rat model with the use of ethanol self-administration and drug discrimination paradigms. The effects of naloxonazine (0.001-10 mg/kg) on ethanol intake were examined in Sprague-Dawley rats under conditions of limited access to 10% (wt./vol.) ethanol and ad libitum access to food and water. Pretreatment with high doses of naloxonazine (1-10 mg/kg) significantly reduced ethanol consumption. When the effects of naloxonazine on food intake in free-feeding male rats were examined, naloxonazine (1.8-10 mg/kg) significantly suppressed 24-h food intake. Another group of rats was trained to discriminate ethanol (1.25 g/kg, i.p.) from saline on a fixed-ratio schedule (FR 10), and ethanol dose-response tests were conducted once rats had acquired ethanol-saline discrimination. Injections were given 15 min before ethanol dose-response tests were conducted, and after characterization of the ethanol dose-response curve, the effects of naloxonazine on ethanol discrimination were assessed by administering naloxonazine (0.001-10 mg/kg, i.p.) 15 min before ethanol administration. Treatment with naloxonazine (0.001-1.8 mg/kg, i.p.) before the ED(100) dose of ethanol partially antagonized the discriminative stimulus of ethanol without having any effect on the response rate. The results support the suggestion of involvement of micro1-opioid receptors in the discriminative effects of ethanol and ethanol-drinking behavior.

Suppression of ethanol responding by centrally administered CTOP and naltrindole in AA and Wistar rats

BACKGROUND

Both mu- and delta-opioid receptors have been implicated in the reinforcing actions of ethanol. However, selective opioid receptor antagonists have not altered ethanol intake in all rodent strains consistently, which suggests that genotype may modulate their suppressive effects. Therefore, we tested the effects of the selective mu-antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and the selective delta-antagonist naltrindole in both high-drinking AA (Alko, Alcohol) and heterogeneous Wistar rats.

METHODS

AA and Wistar rats were trained to respond for ethanol (10% w/v) in a two-lever operant condition by using a saccharin fading procedure. After stable baseline responding was established, rats were implanted stereotaxically either with a guide cannula above the lateral ventricle or with bilateral cannulas above the nucleus accumbens, basolateral amygdala, or ventral tegmental area. After postoperative recovery, AA and Wistar animals were tested after intracerebroventricular microinjections of either CTOP (0-3 microg) or naltrindole (0-30 microg) or subcutaneous injections of naloxone (0-1 g/kg), which was used as a reference antagonist. Effects of intracerebral microinjections of CTOP and naltrindole (both 0-500 ng) were tested only in Wistar rats.

RESULTS

Subcutaneous naloxone and intracerebroventricular CTOP and naltrindole suppressed ethanol self-administration in a similar manner in AA and Wistar rats. Cumulative response patterns indicated that naloxone and naltrindole had no effect on the initiation of responding but suppressed it later during the session, whereas CTOP also affected initiation. In Wistar rats, naltrindole microinjections into both the nucleus accumbens and basolateral amygdala decreased ethanol responding, whereas CTOP was effective only in the amygdala. Injections of these antagonists into the ventral tegmental area had little effect on ethanol intake.

CONCLUSIONS

The results confirm previous results which showed that both mu- and delta-opioid receptors are involved in the regulation of ethanol self-administration and indicate that genetic differences between AA and Wistar rats produced by selection do not modify the effects of opioid antagonists. The nucleus accumbens and the basolateral amygdala may be important central sites for the mediation of their suppressive effects.

Assessment of SNC 80 and naltrindole within a conditioned taste aversion design

Although compounds with relative selectivity for the mu and kappa opiate receptors subtypes have been reported to condition taste aversions, it is not known whether systemically administered delta compounds have the ability to produce aversions. To that end, female Long-Evans rats were adapted to water deprivation and were given pairings of a novel saccharin solution and various doses of the selective delta agonist SNC 80 (0.32-10.0 mg/kg; Experiment 1) or the selective delta antagonist naltrindole (1.0-18.0 mg/kg; Experiment 2). For comparison, the relatively selective mu agonist morphine (Experiment 1) and mu antagonist naloxone (Experiment 2) were assessed under identical conditions. Both SNC 80 (Experiment 1) and naltrindole (Experiment 2) were effective as unconditioned stimuli within this design, inducing dose-dependent taste aversions with repeated conditioning trials. Although at no dose did animals injected with SNC 80 differ from those injected with morphine, aversions induced by SNC 80 were acquired at a faster rate than those induced by morphine. Subjects injected with naloxone drank significantly less than those injected with naltrindole at the 10 mg/kg dose, and aversions induced by naloxone at 5.6 and 10 mg/kg were acquired at a faster rate than those induced by naltrindole. Although the basis for opioid agonist- and antagonist-induced taste aversions is not known, the differences between aversions induced by SNC 80 and naltrindole and those induced by morphine and naloxone, respectively, may be a function of their relative selectivity for specific opiate receptor subtypes.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Roles of 5-HT3 and opioid receptors in the ethanol-induced place preference in rats exposed to conditioned fear stress

The effect of the selective 5-HT3 receptor antagonist ondansetron on the ethanol-induced place preference in rats exposed to conditioned fear stress, which stimulates the release of endogenous opioid peptides (beta-endorphin and enkephalins), was investigated using the conditioned place preference paradigm. In addition, we also examined the effect of ondansetron on the ethanol-induced place preference enhanced by the administration of mu- and delta-opioid receptor agonists (exogenous opioids). The administration of ethanol (300 mg/kg, i.p.) induced a significant place preference in rats exposed to conditioned fear stress. Pretreatment with ondansetron (0.01 and 0.1 mg/kg, s.c.) effectively attenuated this ethanol-induced place preference. When the mu-opioid receptor agonist morphine (0.1 mg/kg, s.c.) or the selective delta-opioid receptor agonist 2-methyl-4a(alpha)-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a(alpha)-octah ydroquinolino [2,3,3-g] isoquinoline (TAN-67; 20 mg/kg, s.c.) was administered in combination with 75 mg/kg ethanol (which tended to produce a place preference), the ethanol-induced place preference was significantly enhanced. The selective mu-opioid receptor antagonist beta-funaltrexamine at a dose of 10 mg/kg significantly attenuated the enhancement of the ethanol-induced place preference produced by morphine. Ondansetron (0.1 mg/kg, s.c.) also significantly attenuated the enhancement of the ethanol-induced place preference produced by morphine. Furthermore, the selective delta-opioid receptor antagonist naltrindole at a dose of 3 mg/kg significantly attenuated the enhancement of the ethanol-induced place preference produced by TAN-67. Ondansetron (0.1 mg/kg, s.c.) slightly, but significantly, attenuated the enhancement of the ethanol-induced place preference produced by TAN-67. These results suggest that 5-HT3 receptors may be involved in the rewarding mechanism of ethanol under psychological stress, and may play an important role in the rewarding effect of ethanol through the activation of mu- and delta-opioid receptors.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.