Strain differences in the rewarding and dopamine-releasing effects of morphine in rats

Effects of fentanyl and the adulterant levamisole on the rewarding and locomotor effects of methamphetamine in rats

BACKGROUND

People who use psychostimulant substances can be exposed to unknown adulterants, such as the synthetic opioid fentanyl (FEN) and the anthelmintic cholinergic agent levamisole (LEV). This work explores the rewarding and locomotor effects of methamphetamine (METH) in combination with FEN or LEV.

METHODS

We used adult male Wistar rats in the conditioned-place preference (CPP) paradigm (conditioning, extinction, and reinstatement phases) and in the open field test to study effective doses of METH, FEN, or LEV, or ineffective doses of METH+FEN or METH+LEV in combination.

RESULTS

METH and LEV, at 1mg/kg METH each, and 30µg/kg FEN produced CPP. Extinction to METH- or LEV-induced CPP occurred after eight saline injections, but it took 8-26 sessions to extinguish FEN-induced CPP. A challenge dose of 0.5mg/kg METH reinstated CPP. The same occurred with 15µg/kg FEN but not with 0.5 or 1mg/kg LEV. Training animals with ineffective doses of METH (0.01mg/kg) combined with either FEN (0.3µg/kg) or LEV (0.01mg/kg) produced CPP. Sub-effective doses of METH or FEN alone did not induce reinstatement after extinction. However, animals challenged with LEV, METH+FEN, or METH+LEV mixtures did it. Combining FEN (3µg/kg) with 0.1mg/kg METH increased locomotor activity.

CONCLUSION

Ineffective FEN and LEV doses mixed with METH produce effects larger than would be expected based on the effects of either drug alone. This outcome suggests a supra-additive interaction, which could increase the risk of developing a METH use disorder.

Molecular chaperone heat shock protein 70 inhibitors suppress conditioned place preference induced by morphine exposure in male rats

Previous studies have indicated a role for molecular chaperone heat shock protein 70 (Hsp70) in the development of behavioural sensitization to morphine in rodents, suggesting that Hsp70 expression following morphine exposure is involved in molecular changes that may underlie addiction vulnerability. The current study was carried out to investigate the role of Hsp70 in the positive reinforcing properties of morphine using conditioned place preference (CPP) in male rats. An unbiased CPP procedure of three phases (pre-conditioning: d1-d3; conditioning: d4-d6; and testing: d7) was used. During the conditioning phase, morphine injections (5 mg/kg, subcutaneously) were administered to induce significant place preference. To explore the effect of Hsp70 on the development and expression of morphine CPP, Hsp70 inhibitors (PES, KNK437 and methylene blue) were administered into the lateral ventricle prior to either morphine conditioning sessions or a morphine challenge on the test day. Furthermore, Hsp70 expression within the mesocorticolimbic system was measured after the treatment with KNK437, a transcriptional inhibitor. We found that PES and KNK437, respectively, injected intracerebroventricularly dose-dependently attenuated both the development and expression of morphine CPP. Methylene blue treatment demonstrated an attenuation of the development, but had no effect on the expression of morphine CPP. Following KNK437 treatment, Hsp70 expression was significantly inhibited in the shell of nucleus accumbens (NAc) during both the development and expression of morphine CPP. The findings suggest that Hsp70 in the NAc shell plays an important role in the reinforcing effects of morphine and may be involved in the development of morphine dependence.

Prenatal opioid exposure and vulnerability to future substance use disorders in offspring

The heightened incidence of opioid use during pregnancy has resulted in unprecedented rates of neonates prenatally exposed to opioids. Prenatal opioid exposure (POE) results in significantly adverse medical, developmental, and behavioral outcomes in offspring. Of growing interest is whether POE contributes to future vulnerability to substance use disorders. The effects of POE on brain development is difficult to assess in humans, as the timing, dose, and route of drug exposure together with complex genetic and environmental factors affect susceptibility to addiction. Preclinical models of POE have allowed us to avoid methodological difficulties and confounding factors of POE in humans. Here, we review the effects of maternal opioid exposure on the developing brain with an emphasis on the neurobiological basis of drug addiction and on preclinical models of POE and their limitations. These studies have indicated that POE increases self-administration of drugs, reward-driven behaviors in the conditioned place paradigm, and locomotor sensitization. While addiction is multifaceted and vulnerability to drug addiction is still inconclusive in human studies of prenatally exposed infants, animal studies do provide a noteworthy corroboration of negative behavioral outcomes.

Early Life Stress and Risks for Opioid Misuse: Review of Data Supporting Neurobiological Underpinnings

A robust body of research has shown that traumatic experiences occurring during critical developmental periods of childhood when neuronal plasticity is high increase risks for a spectrum of physical and mental health problems in adulthood, including substance use disorders. However, until recently, relatively few studies had specifically examined the relationships between early life stress (ELS) and opioid use disorder (OUD). Associations with opioid use initiation, injection drug use, overdose, and poor treatment outcome have now been demonstrated. In rodents, ELS has also been shown to increase the euphoric and decrease antinociceptive effects of opioids, but little is known about these processes in humans or about the neurobiological mechanisms that may underlie these relationships. This review aims to establish a theoretical model that highlights the mechanisms by which ELS may alter opioid sensitivity, thereby contributing to future risks for OUD. Alterations induced by ELS in mesocorticolimbic brain circuits, and endogenous opioid and dopamine neurotransmitter systems are described. The limited but provocative evidence linking these alterations with opioid sensitivity and risks for OUD is presented. Overall, the findings suggest that better understanding of these mechanisms holds promise for reducing vulnerability, improving prevention strategies, and prescribing guidelines for high-risk individuals.

Conditioned Place Preference (CPP) in Rats: From Conditioning to Reinstatement Test

Opioid addiction in humans is a chronically relapsing disorder characterized by discontinuous periods of drug use and abstinence resulting in dependence. With time, the probability of falling into renewed drug consumption becomes particularly high and constitutes a considerable problem in the management of opioid addicts. Opioid addiction represents an important health concern and animal models have been crucial in understanding the neurobiology and pathophysiology of this complex disease. Although animal models of addiction do not fully reproduce the human condition, they do permit investigation of specific elements of the process as well as identification of potential therapeutic targets. In this chapter, we provide a step-by-step description of the morphine-conditioned place preference (CPP) model that represents a useful preclinical animal model extensively used to study the rewarding/aversive effect of drugs.

Endogenous Opioids at the Intersection of Opioid Addiction, Pain, and Depression: The Search for a Precision Medicine Approach

Opioid addiction and overdose are at record levels in the United States. This is driven, in part, by their widespread prescription for the treatment of pain, which also increased opportunity for diversion by sensation-seeking users. Despite considerable research on the neurobiology of addiction, treatment options for opioid abuse remain limited. Mood disorders, particularly depression, are often comorbid with both pain disorders and opioid abuse. The endogenous opioid system, a complex neuromodulatory system, sits at the neurobiological convergence point of these three comorbid disease states. We review evidence for dysregulation of the endogenous opioid system as a mechanism for the development of opioid addiction and/or mood disorder. Specifically, individual differences in opioid system function may underlie differences in vulnerability to opioid addiction and mood disorders. We also review novel research, which promises to provide more detailed understanding of individual differences in endogenous opioid neurobiology and its contribution to opioid addiction susceptibility.

Inter-individual and inter-strain differences in cognitive and social abilities of Dark Agouti and Wistar Han rats

Healthy animals displaying extreme behaviours that resemble human psychiatric symptoms are relevant models to study the natural psychobiological processes of maladapted behaviours. Using a Rat Gambling Task, healthy individuals spontaneously making poor decisions (PDMs) were found to co-express a combination of other cognitive and reward-based characteristics similar to symptoms observed in human patients with impulse-control disorders. The main goals of this study were to 1) confirm the existence of PDMs and their unique behavioural phenotypes in Dark Agouti (DA) and Wistar Han (WH) rats, 2) to extend the behavioural profile of the PDMs to probability-based decision-making and social behaviours and 3) to extract key discriminative traits between DA and WH strains, relevant for biomedical research. We have compared cognitive abilities, natural behaviours and physiological responses in DA and WH rats at the strain and at the individual level. Here we found that the naturally occurring PDM's profile was consistent between both rat lines. Then, although the PDM individuals did not take more risks in probability discounting task, they seemed to be of higher social ranks. Finally and despite their similarities in performance, WH and DA lines differed in degree of reward sensitivity, impulsivity, locomotor activity and open space-occupation. The reproducibility and conservation of the complex phenotypes of PDMs and GDMs (good decision makers) in these two genetically different strains support their translational potential. Both strains, present large phenotypic variation in behaviours pertinent for the study of the underlying mechanisms of poor decision making and associated disorders.

Changes of EEG band oscillations to tonic cold pain and the behavioral inhibition and fight-flight-freeze systems

Using electroencephalography (EEG) power measures within conventional delta, theta, alpha, beta, and gamma bands, the aims of the current study were to highlight cortical correlates of subjective perception of cold pain (CP) and the associations of these measures with behavioral inhibition system (BIS), fight-flight-freeze system (FFFS), and behavioral approach system personality traits. EEG was recorded in 55 healthy right-handed women under (i) a white noise interruption detection condition (Baseline); (ii) enduring CP induced by the cold cup test. CP and Baseline EEG band power scores within conventional frequency bands served for covariance analyses. We found that: (1) higher Pain scorers had higher EEG beta power changes at left frontal, midline central, posterior temporal leads; (2) higher BIS was associated with greater EEG delta activity changes at parietal scalp regions; (3) higher FFFS was associated with higher EEG delta activity changes at temporal and left-parietal regions, and with lower EEG gamma activity changes at right parietal regions. High FFFS, compared to Low FFFS scorers, also showed a lower gamma power across the midline, posterior temporal, and parietal regions. Results suggest a functional role of higher EEG beta activity in the subjective perception of tonic pain. EEG delta activity underpins conflict resolution system responsible for passive avoidance control of pain, while higher EEG delta and lower EEG gamma activity changes, taken together, underpin active avoidance system responsible for pain escape behavior.

The influence of reward sensitivity, heart rate dynamics and EEG-delta activity on placebo analgesia

Personality traits have been shown to interact with environmental cues to modulate biological responses including treatment responses, and potentially having a role in the formation of placebo effects. Here we used the Reinforcement Sensitivity Theory Personality Questionnaire (RST-PQ) to identify personality traits that predict placebo analgesic responding. Cardiac inter-beat (RR) time series and electroencephalographic (EEG) band oscillations were recorded from healthy women in a cold-pain (Pain) and placebo analgesia (PA) condition. The measures of Hypnotizability, and self-reported ratings of Hypnotic Depth, Motivation, Pain Expectation, Involuntariness in PA responding, Pain and Distress intensity were obtained. Separate principal components factor analyses with varimax rotation were performed on summarized heart rate variability (HRV) measures of time, frequency, nonlinear Complexity, and EEG-band activity. Both analyses yielded a similar three-factor solution including Frequency HRV (factor-1), Complexity HRV dynamics (factor-2), and time HRV & EEG-delta activity (factor-3). Reward Interest sub-trait of the Behavioral Approach System (BAS-RI), Pain Expectation, Involuntariness in PA responding, and Hypnotic Depth were positively associated, whereas negative changes in time-HRV & EEG-delta scores were associated with Pain Reduction. Multiple mediation analyses disclosed that BAS-RI, potentially served by the dopaminergic system, through Involuntariness in PA responding can alter placebo responding to laboratory pain. Our results also show that a linear compound of HR slowing and higher EEG delta activity during PA explains a substantial proportion of the variance in placebo analgesic responses. Future studies should examine the potential role that these individual difference measures may play in patient responsiveness to treatments for clinical pain.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Modest Amounts of Voluntary Exercise Reduce Pain- and Stress-Related Outcomes in a Rat Model of Persistent Hind Limb Inflammation

Aerobic exercise improves outcomes in a variety of chronic health conditions, yet the support for exercise-induced effects on chronic pain in humans is mixed. Although many rodent studies have examined the effects of exercise on persistent hypersensitivity, the most used forced exercise paradigms that are known to be highly stressful. Because stress can also produce analgesic effects, we studied how voluntary exercise, known to reduce stress in healthy subjects, alters hypersensitivity, stress, and swelling in a rat model of persistent hind paw inflammation. Our data indicate that voluntary exercise rapidly and effectively reduces hypersensitivity as well as stress-related outcomes without altering swelling. Moreover, the level of exercise is unrelated to the analgesic and stress-reducing effects, suggesting that even modest amounts of exercise may impart significant benefit in persistent inflammatory pain states.

PERSPECTIVE

Modest levels of voluntary exercise reduce pain- and stress-related outcomes in a rat model of persistent inflammatory pain, independently of the amount of exercise. As such, consistent, self-regulated activity levels may be more relevant to health improvement in persistent pain states than standardized exercise goals.

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

Drugs of abuse cause persistent alterations in synaptic plasticity that may underlie addiction behaviors. Evidence suggests glial cells have an essential and underappreciated role in the development and maintenance of drug abuse by influencing neuronal and synaptic functions in multifaceted ways. Microglia and astrocytes perform critical functions in synapse formation and refinement in the developing brain, and there is growing evidence that disruptions in glial function may be implicated in numerous neurological disorders throughout the lifespan. Linking evidence of function in health and under pathological conditions, this review will outline the glial and neuroimmune mechanisms that may contribute to drug-abuse liability, exploring evidence from opioids, alcohol, and psychostimulants. Drugs of abuse can activate microglia and astrocytes through signaling at innate immune receptors, which in turn influence neuronal function not only through secretion of soluble factors (eg, cytokines and chemokines) but also potentially through direct remodeling of the synapses. In sum, this review will argue that neural-glial interactions represent an important avenue for advancing our understanding of substance abuse disorders.

Role of nucleus accumbens μ opioid receptors in the effects of morphine on ERK1/2 phosphorylation

RATIONALE

Despite the critical role attributed to phosphorylated extracellular signal regulated kinase (pERK1/2) in the nucleus accumbens (Acb) in the actions of addictive drugs, the effects of morphine on ERK1/2 phosphorylation in this area are still controversial.

OBJECTIVES

In order to investigate further this issue, we studied (1) the ability of morphine to affect ERK1/2 phosphorylation in the shell (AcbSh) and core (AcbC) of Sprague-Dawley and Wistar rats and of CD-1 and C57BL/6J mice and (2) the role of dopamine D1 and μ-opioid receptors in Sprague-Dawley rats and CD-1 mice.

METHODS

The pERK1/2 expression was assessed by immunohistochemistry.

RESULTS

In rats, morphine decreased AcbSh and AcbC pERK1/2 expression, whereas in mice, increased it preferentially in the AcbSh compared with the AcbC. Systemic SCH 39166 decreased pERK1/2 expression on its own in the AcbSh and AcbC of Sprague-Dawley rats and CD-1 mice; furthermore, in rats, SCH 39166 disclosed the ability of morphine to stimulate pERK1/2 expression. Systemic (rats and mice) and intra-Acb (rats) naltrexone prevented both decreases, in rats, and increases, in mice.

CONCLUSIONS

These findings confirm the differential effects of morphine in rats and mice Acb and that D1 receptors exert a facilitatory role on ERK1/2 phosphorylation; furthermore, they indicate that, in rats, removal of the D1-dependent pERK1/2 expression discloses the stimulatory influence of morphine on ERK1/2 phosphorylation and that the morphine's ability to decrease pERK1/2 expression is mediated by Acb μ-opioid receptors. Future experiments may disentangle the psychopharmacological significance of the effects of morphine on pERK1/2 in the Acb.

Olfactory bulbectomy increases reinstatement of methamphetamine seeking after a forced abstinence in rats

Drug addiction is commonly associated with depression and comorbid patients also suffer from higher cravings and increased relapse rate. To address this issue preclinically we combined the olfactory bulbectomy (OBX) model of depression and intravenous methamphetamine self-administration procedure in rats to assess differences in relapse-like behavior. Male Sprague-Dawley rats were divided randomly into two groups; in one group the bilateral olfactory bulbectomy (OBX) was performed while the other group was sham operated. After recovery, intracardiac catheter was implanted. Intravenous self-administration procedure was conducted in operant boxes using nose-poke operandi (Coulbourn Instruments, Inc., USA) under fixed ratio 1 schedule of reinforcement. Methamphetamine was available at dose 0.08 mg/kg/infusion. After stable methamphetamine intake was maintained, a period of forced abstinence was initiated and rats were kept in their home-cages for 14 days. Finally, one reinstatement session was conducted in operant boxes with no drug delivery. In the reinstatement session the mean of 138.4 active nose-pokes was performed by the OBX group, while the sham group displayed 41 responses, i.e. 140 % and 48 % of basal nose-poking during maintenance phase in OBX and sham operated group respectively. OBX group also showed significantly more passive nose-pokes indicating hyperactive behavioral traits in bulbectomized rats. However, the % of active operandum preference was equal in both groups. Olfactory bulbectomy model significantly increased reinstatement of methamphetamine seeking behavior. This paradigm can be used to evaluate potential drugs that are able to suppress the drug-seeking behavior.

Social play behavior, ultrasonic vocalizations and their modulation by morphine and amphetamine in Wistar and Sprague-Dawley rats

RATIONALE

Social play behavior is the most characteristic social behavior in young mammals. It is highly rewarding and crucial for proper neurobehavioral development. Despite the importance of genetic factors in normal and pathological social behaviors, little information is available about strain influences on social play.

OBJECTIVE AND METHODS

The aim of this study was to investigate differences in social play behavior, 50-kHz ultrasonic vocalizations (USVs) and their modulation by acute morphine and amphetamine administration in two rat strains widely used in behavioral pharmacology studies, i.e., Wistar and Sprague-Dawley rats.

RESULTS

Sprague-Dawley rats showed higher levels of social play than Wistar rats. In both strains, no correlation was found between the performance of social behaviors and the emission of 50-kHz USVs. In Wistar and Sprague-Dawley rats, morphine increased and amphetamine decreased social play. The effects of morphine, however, were more pronounced in Wistar than Sprague-Dawley animals. In both strains, morphine did not affect USV emission, while amphetamine increased it during cage exploration. In Sprague-Dawley rats only, amphetamine decreased USVs during social interaction.

CONCLUSIONS

Wistar and Sprague-Dawley rats differ in their absolute levels of social play behavior and 50-kHz USVs, and quantitative differences exist in their response to pharmacological manipulations of social play. The emission of 50-kHz USVs and the behavioral parameters thought to reflect rewarding social interactions in adolescent rats are dissociable.

Vendor differences in alcohol consumption and the contribution of dopamine receptors to Pavlovian-conditioned alcohol-seeking in Long-Evans rats

RATIONALE

Drug-associated environmental stimuli elicit craving in humans and drug-seeking in animals.

OBJECTIVES

We tested the hypothesis that Pavlovian-conditioned alcohol-seeking is mediated by dopamine, using rats from two vendors.

METHODS

Male, Long-Evans rats (220-240 g) from Charles River (St-Constant, QC, Canada) and Harlan Laboratories (Indianapolis, IN, USA) received 21 sessions of intermittent, 24-h access to ethanol (15 %, v/v) and water in the home-cage. Subsequently, rats were trained to discriminate between one conditioned stimulus (CS+) that was paired with ethanol (0.2 ml per CS+) and a second stimulus (CS-) that was not. Entries into a fluid port where ethanol was delivered were recorded. Next, rats were exposed to a different context where cues and ethanol were withheld. At test, responding to the CS+ and CS- without ethanol was assessed in the second, non-alcohol context. Injections (1 ml/kg; s.c.) of the dopamine D1-receptor antagonist SCH 23390 (0, 3.33, and 10 μg/kg) or dopamine D2-receptor antagonist eticlopride (0, 5, and 10 μg/kg) were administered before test.

RESULTS

Home-cage alcohol consumption was higher in Harlan rats than Charles River rats. At test, saline-treated rats responded more to the alcohol-predictive CS+ than the CS-. While SCH 23390 attenuated CS+ responding in rats from both vendors, eticlopride reduced CS+ responding in Harlan rats only. Subsequently, SCH 23390 but not eticlopride attenuated CS+ responding when the CS+ was again paired with ethanol.

CONCLUSIONS

These results indicate important differences in alcohol consumption in Long-Evans rats from different suppliers, and highlight a novel role for dopamine in Pavlovian-conditioned alcohol-seeking.

Role of mu, delta and kappa opioid receptors in ethanol-reinforced operant responding in infant rats

We recently observed that naloxone, a non-specific opioid antagonist, attenuated operant responding to ethanol in infant rats. Through the use of an operant conditioning technique, we aimed to analyze the specific participation of mu, delta, and kappa opioid receptors on ethanol reinforcement during the second postnatal week. In Experiment 1, infant rats (PDs 14-17) were trained to obtain 5, 7.5, 10, or 15% ethanol, by operant nose-poking. Experiment 2 tested blood ethanol levels (BELs) attained by operant behavior. In Experiment 3, at PDs 16-18, rats received CTOP (mu antagonist: 0.1 or 1.0 mg/kg), naltrindole (delta antagonist: 1.0 or 5.0 mg/kg) or saline before training. In Experiment 4, rats received nor-binaltorphimine (kappa antagonist: 10.0 or 30.0 mg/kg, a single injection after completion of PD 15 operant training), spiradoline mesylate (kappa agonist: 1.0 or 5.0 mg/kg; at PDs 16-18) or saline (PDs 16-18), before the conditioning. Experiments 5 and 6 assessed possible side effects of opioid drugs in locomotor activity (LA) and conditioned taste aversion (CTA). Ethanol at 7.5 and 10% promoted the highest levels of operant responding. BELs were 12-15 mg/dl. In Experiment 3 naltrindole (dose-response effect) and CTOP (the lowest dose) were effective in decreasing operant responding. Nor-binaltorphimine at 10.0 mg/kg and spiradoline at 5.0 mg/kg also blocked ethanol responding. The effects of opioid drugs on ethanol reinforcement cannot be explained by effects on LA or CTA. Even though particular aspects of each opioid receptor require further testing, a fully functional opioid system seems to be necessary for ethanol reinforcement, during early ontogeny.

Targeting dynorphin/kappa opioid receptor systems to treat alcohol abuse and dependence

This review represents the focus of a symposium that was presented at the "Alcoholism and Stress: A Framework for Future Treatment Strategies" conference in Volterra, Italy on May 3-6, 2011 and organized/chaired by Dr. Brendan M. Walker. The primary goal of the symposium was to evaluate and disseminate contemporary findings regarding the emerging role of kappa-opioid receptors (KORs) and their endogenous ligands dynorphins (DYNs) in the regulation of escalated alcohol consumption, negative affect and cognitive dysfunction associated with alcohol dependence, as well as DYN/KOR mediation of the effects of chronic stress on alcohol reward and seeking behaviors. Dr. Glenn Valdez described a role for KORs in the anxiogenic effects of alcohol withdrawal. Dr. Jay McLaughlin focused on the role of KORs in repeated stress-induced potentiation of alcohol reward and increased alcohol consumption. Dr. Brendan Walker presented data characterizing the effects of KOR antagonism within the extended amygdala on withdrawal-induced escalation of alcohol self-administration in dependent animals. Dr. Georgy Bakalkin concluded with data indicative of altered DYNs and KORs in the prefrontal cortex of alcohol dependent humans that could underlie diminished cognitive performance. Collectively, the data presented within this symposium identified the multifaceted contribution of KORs to the characteristics of acute and chronic alcohol-induced behavioral dysregulation and provided a foundation for the development of pharmacotherapeutic strategies to treat certain aspects of alcohol use disorders.

Decreased reward sensitivity in rats from the Fischer344 strain compared to Wistar rats is paralleled by differences in endocannabinoid signaling

Background

The aim of the present study was to examine if differences in the endocannabinoid (ECB) system might be linked to strain specific variations in reward-related behavior in Fischer344 (Fischer) and Wistar rats.

Methodology/Principal Findings

Two rat strains, the Fischer and the Wistar strain, were tested for different aspects of reward sensitivity for a palatable food reward (sweetened condensed milk, SCM) in a limited-access intake test, a progressive ratio (PR) schedule and the pleasure-attenuated startle (PAS) paradigm. Additionally, basic differences in the ECB system and cannabinoid pharmacology were examined in both rat strains. Fischer rats were found to express lower reward sensitivity towards SCM compared to Wistar rats. These differences were observed for consummatory, motivational and hedonic aspects of the palatable food reward. Western blot analysis for the CB1 receptor and the ECB degrading enzyme fatty acid amide hydrolase (FAAH) revealed a lower expression of both proteins in the hippocampus (HPC) of Fischer rats compared to the Wistar strain. Furthermore, increased cannabinoid-stimulated extracellular-regulated kinase (ERK) phosphorylation was detected in Wistar rats compared to the Fischer strain, indicating alterations in ECB signaling. These findings were further supported by the pharmacological results, where Fischer rats were found to be less sensitive towards the effects of the CB1 receptor antagonist/inverse agonist SR141716 and the cannabinoid agonist WIN 55,212-2.

Conclusions/Significance

Our present findings indicate differences in the expression of the CB1 receptor and FAAH, as well as the activation of ECB signaling pathways between Fischer and Wistar rats. These basic differences in the ECB system might contribute to the pronounced differences observed in reward sensitivity between both rat strains.

Chronic morphine alters the presynaptic protein profile: identification of novel molecular targets using proteomics and network analysis

Opiates produce significant and persistent changes in synaptic transmission; knowledge of the proteins involved in these changes may help to understand the molecular mechanisms underlying opiate dependence. Using an integrated quantitative proteomics and systems biology approach, we explored changes in the presynaptic protein profile following a paradigm of chronic morphine administration that leads to the development of dependence. For this, we isolated presynaptic fractions from the striata of rats treated with saline or escalating doses of morphine, and analyzed the proteins in these fractions using differential isotopic labeling. We identified 30 proteins that were significantly altered by morphine and integrated them into a protein-protein interaction (PPI) network representing potential morphine-regulated protein complexes. Graph theory-based analysis of this network revealed clusters of densely connected and functionally related morphine-regulated clusters of proteins. One of the clusters contained molecular chaperones thought to be involved in regulation of neurotransmission. Within this cluster, cysteine-string protein (CSP) and the heat shock protein Hsc70 were downregulated by morphine. Interestingly, Hsp90, a heat shock protein that normally interacts with CSP and Hsc70, was upregulated by morphine. Moreover, treatment with the selective Hsp90 inhibitor, geldanamycin, decreased the somatic signs of naloxone-precipitated morphine withdrawal, suggesting that Hsp90 upregulation at the presynapse plays a role in the expression of morphine dependence. Thus, integration of proteomics, network analysis, and behavioral studies has provided a greater understanding of morphine-induced alterations in synaptic composition, and identified a potential novel therapeutic target for opiate dependence.

Plasma concentrations of corticosterone and buprenorphine in rats subjected to jugular vein catheterization

The present study investigated the postoperative plasma concentrations of corticosterone and buprenorphine in male Wistar and Sprague-Dawley rats, treated with buprenorphine administered either through subcutaneous (SC) injection or through voluntary ingestion (VI). The animals were treated with buprenorphine for pre-emptive analgesia prior to surgical placement of a jugular catheter, followed by automated blood sampling during 96 h. Buprenorphine was administered on a regular basis throughout the experiment, and blood was collected on selected time points. Body weight was measured before and 96 h after surgery. It was found that the two rat stocks responded in a similar manner to both buprenorphine treatments, with the exception of body weight change in Wistar rats, in which body weight was reduced after SC treatment. The plasma concentration of corticosterone was significantly higher in the SC-treated animals than in the VI-treated animals during the first 18 h of the study, while plasma buprenorphine concentration was at least as high and more even over time after VI treatment. The present study shows that buprenorphine administration through VI is suitable for both Wistar and Sprague-Dawley rats, with lower stress response and higher plasma concentrations of buprenorphine than after the traditional SC route of administration.

Peri, pre and postnatal morphine exposure: exposure-induced effects and sex differences in the behavioural consequences in rat offspring

This study investigated the behavioural consequences of peri, pre and postnatal morphine (MO) exposure in rats. From gestational day 1 dams were treated with either saline or MO subcutaneously once a day (5 mg/kg on the first 2 days, 10 mg/kg subsequently). Spontaneous locomotor activity in a new environment (habituation) and antinociceptive effects of MO were measured separately in male and female pups after weaning and also in late adolescence or adulthood. The rewarding effect of MO was assessed by conditioned place preference in adult animals. Both exposure-induced and sex differences were observed. A significant delay in habituation to a new environment and decreased sensitivity to the antinociceptive effect of MO were found in male offspring of MO-treated dams. In contrast, the place preference induced by MO was enhanced in the MO-exposed adult animals and this effect was more marked in females. Prenatal exposure to MO resulted in more marked changes than the postnatal exposure through maternal milk. The results indicate that a medium MO dose administered once-daily results in long-term consequences in offspring and may make them more vulnerable to MO abuse in adulthood.

Ethanol exposure selectively alters beta-endorphin content but not [3H]-DAMGO binding in discrete regions of the rat brain

The dopaminergic mesocorticolimbic system plays an important role in the reinforcing effects of ethanol. Opioid peptides modulate the activity of this system and have been suggested to mediate, at least in part, the reinforcing properties of ethanol. Thus, beta-endorphin (beta-END) could participate in the development of ethanol reinforcement and addiction. The aim of this work was to investigate the acute and chronic ethanol effects on beta-END content in regions of the mesolimbic system and to examine if chronic ethanol treatment alters ligand binding to mu opioid receptor (muOR). Male Wistar rats received a single acute ethanol dose of 2.5 g/kg or water by intra-gastric administration. For chronic ethanol treatment experiments, one group of rats was given ethanol (10% v/v solution) to drink, two groups were given equivalent volumes of sucrose (14.14% isocaloric solution) or water, respectively, and a fourth group had ad libitum access to food and water. Treatment was followed for 4 weeks. Beta-endorphin content in brain regions was quantified by radioimmunoassay and ligand binding studies to muOR were performed by quantitative autoradiography using 8 nM [(3)H]-DAMGO as radioligand. Acute ethanol decreased beta-END content in the hypothalamus (26%) 1h after administration. No ethanol effects were observed in the midbrain, ventral tegmental area, substantia nigra, nucleus accumbens, nucleus accumbens-septum and prefrontal cortex. Chronic ethanol treatment neither changed beta-END levels nor [(3)H]-DAMGO binding to mu opioid receptors in any of the regions studied. However, beta-END levels in the sucrose group were significantly increased in the nucleus accumbens and substantia nigra, in comparison to all other groups. These findings suggest that different neural mechanisms and specific brain regions may be involved in the reinforcing effects of ethanol and sucrose.

Acute systemic blockade of D2 receptors does not accelerate the extinction of cocaine-associated place preference

Facilitation of extinction can be used as a therapeutic tool in treatment of both post-traumatic stress disorder and drug addiction. The present study examined whether the blockade of D2 receptors before each extinction trial would accelerate the extinction of cocaine-induced place preference. Male Wistar rats were initially conditioned and tested for a cocaine-associated place-preference (20 mg/kg). On the following day after the initial test, the animals were submitted to extinction training. This training consisted of daily sessions in which the subjects were alternatively confined during 30 min in the saline and cocaine-associated environment. However, 30 min before each extinction trial the animals received a systemic injection of D2 antagonist sulpiride. While one group was treated with a dose of 50 mg/kg (ip), the other group was treated with a dose of 100 mg/kg. An additional control group received injections of saline during extinction trials. Twenty-four hours after the last extinction trial, the animals were tested again for their preferences to cocaine and saline associated environments. Since one round of extinction trial was not sufficient to produce extinction of cocaine associated place preference, the animals were submitted to a second cycle of extinction trials and test. The systemic administration of the two doses of sulpiride (50 and 100 mg/kg) 30 min before each conditioning did not enhance the extinction of cocaine-associated place preference. This finding suggests that the D2 receptors are not involved in a acute protocol of extinction of cocaine-induced place preference.

Acute effects of morphine on distinct forms of impulsive behavior in rats

RATIONALE

Disturbances in impulse control are key features of substance abuse disorders, and conversely, many drugs of abuse are known to elicit impulsive behavior both clinically and preclinically. To date, little is known with respect to the involvement of the opioid system in impulsive behavior, although recent findings have demonstrated its involvement in delay discounting processes. The aim of the present study was to further investigate the role of the opioid system in varieties of impulsivity.

MATERIALS AND METHODS

To this end, groups of rats were trained in the five-choice serial reaction time task (5-CSRTT) and stop-signal task (SST), operant paradigms that provide measures of inhibitory control and response inhibition, respectively. In addition, another group of rats was trained in the delayed reward paradigm, which measures the sensitivity towards delay of gratification and as such assesses impulsive choice.

RESULTS AND DISCUSSION

Results demonstrated that morphine, a selective micro-opioid receptor agonist, primarily impaired inhibitory control in the 5-CSRTT by increasing premature responding. In addition, in keeping with previous data, morphine decreased the preference for the large over small reward in the delayed reward paradigm. The effects of morphine on measures of impulsivity in both the 5-CSRTT and delayed reward paradigm were blocked by naloxone, a micro-opioid receptor antagonist. Naloxone by itself did not alter impulsive behavior, suggesting limited involvement of an endogenous opioid tone in impulsivity. Response inhibition measured in the SST was neither altered by morphine nor naloxone, although some baseline-dependent effects of morphine on response inhibition were observed.

CONCLUSION

In conclusion, the present data demonstrate that acute challenges with morphine modulate distinct forms of impulsive behavior, thereby suggesting a role for the opioid system in impulsivity.

The anatomy of the mesolimbic reward system: a link between personality and the placebo analgesic response

The anticipation of clinical benefit, a crucial component of placebo analgesia, has been suggested to be a special case of reward anticipation. Since reward processing is closely linked to the ventral striatum and the neurotransmitter dopamine, we examined the relationships between brain gray matter, placebo analgesic response, and personality traits associated with dopaminergic neurotransmission. We report that dopamine-related traits predict a substantial portion of the pain relief an individual gains from a sham treatment. Voxel-based morphometry of magnetic resonance images shows that the magnitude of placebo analgesia is related to gray matter density (GMD) in several brain regions, including the ventral striatum, insula, and prefrontal cortex. Similarly, GMD in ventral striatum and prefrontal cortex is related to dopamine-related personality traits. Our findings highlight the relationship between placebo and reward and potentially offer ways of identifying subjects who are likely to show large placebo analgesic responses.

Behavioural and biochemical responses to morphine associated with its motivational properties are altered in adenosine A(2A) receptor knockout mice

BACKGROUND AND PURPOSE

The purinergic system through the A(2A) adenosine receptor regulates addiction induced by different drugs of abuse. The aim of the present study was to investigate the specific role of A(2A) adenosine receptors (A(2A)Rs) in the behavioural and neurochemical responses to morphine associated with its motivational properties.

EXPERIMENTAL APPROACH

Mice lacking A(2A)Rs (A(2A) knockout (KO) mice) and wild-type littermates were used to evaluate behavioural responses induced by morphine. Antinociception was assessed using the tail-immersion and the hot-plate tests. Place-conditioning paradigms were used to evaluate the rewarding effects of morphine and the dysphoric responses of morphine withdrawal. Microdialysis studies were carried out to evaluate changes in the extracellular levels of dopamine in the nucleus accumbens of A(2A) KO mice after morphine administration.

KEY RESULTS

The acute administration of morphine induced a similar enhancement of locomotor activity and antinociceptive responses in both genotypes. However, the rewarding effects induced by morphine were completely blocked in A(2A) KO mice. Also, naloxone did not induce place aversion in animals lacking the A(2A)Rs.

CONCLUSIONS AND IMPLICATIONS

Our findings demonstrate that the rewarding and aversive effects associated with morphine abstinence were abolished in A(2A) KO mice, supporting a differential role of the A(2A) adenosine receptor in the somatic and motivational effects of morphine addiction. This study provides evidence for the role of A(2A)Rs as general modulators of the motivational properties of drugs of abuse. Pharmacological manipulation of these receptors may represent a new target in the management of drug addiction.

Morphine sex-dependently induced place conditioning in adult Wistar rats

The present study was conducted to investigate the potential sex-differences in morphine-induced conditioned place preference. A 3-day unbiased conditioning procedure was used to establish conditioned place preference in adult male and female Wistar rats (weighing 200-250 g). The effect of morphine on locomotor activity of subjects was also studied. Naloxone (0.5-2 mg/kg, i.p.), a selective antagonist of mu-opioid receptor or sulpiride (0.5-2 mg/kg, s.c.), a selective antagonist of dopamine D(2) receptor was administered, during conditioning, to indicate the receptor-mediated mechanisms governing upon possible sex-differences to the opioid response. Results show that morphine (0.5-10 mg/kg, s.c.) differently produced a significant place preference in female and male Wistar rats. Although, the opioid maximum response in both sexes was observed at 7.5 mg/kg, but, it was found that female rats acquired conditioned place preference at a lower dose (0.5 mg/kg, s.c.) of morphine compared to male rats. Moreover, the increase in morphine-induced response at higher doses (5-10 mg/kg, s.c.) was more pronounced in females than the males, indicating that female Wistar rats are more sensitive to the place conditioning induced by morphine. Also, the females were more sensitive to locomotor activation induced by morphine at least at one dose (7.5 mg/kg). Animals' body-weight at 10 mg/kg of opioid was increased, the effect that was not dependent to sex. The results also demonstrate that naloxone (1 and 2 mg/kg, i.p.) induced a significant place preference in two sexes with no significant effect on animals' locomotor activity. The antagonist in males but not in females showed a significant effect on animals' body-weight. Naloxone (0.5-2 mg/kg, i.p.) prior-administration to morphine, during conditioning, attenuated the opioid response in two sexes. The attenuation of the morphine response was more pronounced in males than the other sex at the higher dose (2 mg/kg) of the antagonist. In addition, the preadministration of naloxone, during morphine conditioning, both attenuated the drug-induced hyperactivity in females and decreased the animals' body-weight, albeit more effectively in females than the males. Sulpiride injections (1 and 2 mg/kg s.c.), during the conditioning period, induced a significant aversion in males but not in females with no significant effect either on locomotor activity or body-weight in both sexes. When sulpiride (0.5-2 mg/kg, s.c.), during conditioning, was morphine pre-injected, the antagonist at higher doses significantly attenuated the opioid response in males, reflecting the involvement of dopamine D(2) receptor in sex-dependent morphine-conditioned place preference. Prior-injections of sulpiride to morphine produced a significant effect on locomotor activity of females. The effect of the antagonist preinjections on body-weight was also observed in males. Present results indicate sex-differences both in reinforcing and locomotor activity effects of morphine in Wistar rats.

Repeated administration of dopaminergic agents in the nucleus accumbens and morphine-induced place preference

In the present study, the effects of repeated intra nucleus accumbens (intra-NAc) injections of dopamine receptor agents on morphine-induced conditioned place preference (CPP) in rats were investigated by using an unbiased 3-days schedule of place conditioning design. The animals receiving once daily subcutaneous (s.c.) injections of morphine (0.5-7.5mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5mg/kg of the opioid. Three days intra-NAc injections of apomorphine (0.5 and 1 microg/rat) followed by 5 days free of the drug, increased or decreased, respectively CPP induced by the lower dose of morphine (0.5mg/kg, s.c.). Morphine-induced CPP was also significantly increased in the animals that had previously received the 3-days intra-NAc injections of SKF 38393 (4 and 8 microg/rat) or quinpirole (2 and 4 microg/rat, intra-NAc). The CPP induced by a higher dose of morphine (5mg/kg, s.c.) was significantly decreased in the animals that had previously received the 3-days SCH 23390 (0.005 and 0.01 microg/rat; intra-NAc). On the other hand, the CPP induced by morphine (5mg/kg, s.c.) was significantly increased in the animals that had previously received the 3-days sulpiride administration (5 microg/rat, intra-NAc). The 3-days administration of apomorphine, SKF 38393 or quinpirole, but not SCH 23390 and sulpiride reduced the locomotor activity in the test session. It is concluded that repeated injections of dopamine receptors agents followed by 5 days free of the drugs in the NAc can affect morphine reward.

Individual differences in male rat ejaculatory behaviour: searching for models to study ejaculation disorders

In addition to investigating sexual function in rats that display normal ejaculatory behaviour, studying rats that are either 'hyposexual' or 'hypersexual' may provide important insights into the aetiology of ejaculatory dysfunctions in men, such as premature and retarded ejaculation. To this end, rats were matched into groups of 'sluggish', 'normal' and 'rapid' ejaculators based on their ejaculation frequencies displayed in a series of weekly sexual behaviour tests. Selecting rats on this parameter revealed large and stable differences in other parameters of sexual behaviour as well, including ejaculation latency and mount frequency but not intromission frequency and mount latency, putative indices of sexual motivation. Neuroanatomically, Fos immunoreactivity as a measure of neuronal activation was increased in rapid ejaculators compared with sluggish ejaculators in ejaculation-related brain areas, presumably associated with the differences in ejaculatory behaviour. Although the total number of oxytocin neurones within subregions of the hypothalamus did not differ between groups, in the supraoptic nucleus of the hypothalamus more oxytocin neurones were activated in rapid ejaculators compared with the other groups. Apart from the differences observed in ejaculatory behaviour, groups did not differ with respect to their locomotor activity and approach-avoidance behaviour as measured in the elevated plus-maze. Finally, apomorphine-induced stereotypy was similar in sluggish and rapid ejaculators, suggesting no large differences in dopamine susceptibility. Altogether, the present results suggest stable differences in male rat ejaculatory behaviour. Further exploring the neurobiological mechanisms underlying these differences may be a promising approach to gain insights into the aetiology of sexual dysfunctions such as premature, retarded or an-ejaculation.

Enhanced rewarding properties of morphine, but not cocaine, in beta(arrestin)-2 knock-out mice

The reinforcing and psychomotor effects of morphine involve opiate stimulation of the dopaminergic system via activation of mu-opioid receptors (muOR). Both mu-opioid and dopamine receptors are members of the G-protein-coupled receptor (GPCR) family of proteins. GPCRs are known to undergo desensitization involving phosphorylation of the receptor and the subsequent binding of beta(arrestins), which prevents further receptor-G-protein coupling. Mice lacking beta(arrestin)-2 (beta(arr2)) display enhanced sensitivity to morphine in tests of pain perception attributable to impaired desensitization of muOR. However, whether abrogating muOR desensitization affects the reinforcing and psychomotor properties of morphine has remained unexplored. In the present study, we examined this question by assessing the effects of morphine and cocaine on locomotor activity, behavioral sensitization, conditioned place preference, and striatal dopamine release in beta(arr2) knock-out (beta(arr2)-KO) mice and their wild-type (WT) controls. Cocaine treatment resulted in very similar neurochemical and behavioral responses between the genotypes. However, in the beta(arr2)-KO mice, morphine induced more pronounced increases in striatal extracellular dopamine than in WT mice. Moreover, the rewarding properties of morphine in the conditioned place preference test were greater in the beta(arr2)-KO mice when compared with the WT mice. Thus, beta(arr2) appears to play a more important role in the dopaminergic effects mediated by morphine than those induced by cocaine.

Repeated morphine injections induce behavioural sensitization in Roman high- but not in Roman low-avoidance rats

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs poor active avoidance acquisition has resulted in two phenotypes that differ in their behavioural and neurochemical responses to addictive drugs, including morphine. To compare the ability of these lines to develop behavioural sensitization to morphine, female RHA and RLA rats were treated twice daily with either saline or escalating doses of morphine (5, 10, and 20 mg/kg, s.c. on the 1st, 2nd, and 3rd day of treatment, respectively), and were challenged with morphine (0.5 or 2 mg/kg, s.c.) 1 day before and 3 weeks after repeated morphine administration. The locomotor activation produced by either challenge dose of morphine was more pronounced in RHA rats repeatedly treated with morphine vs the respective saline-treated controls, whereas no significant change in locomotor activity was observed in RLA rats. The results show that behavioral sensitization to morphine was induced in RHA but not in RLA rats.

Gonadal hormones differentially modulate cocaine-induced conditioned place preference in male and female rats

There is accumulating evidence that suggests there are sex differences in behavioral and subjective responses to cocaine. However, it is not known whether differences in cocaine reward contribute to sex differences in these responses or whether gonadal hormones affect the rewarding properties of cocaine. In the present study, conditioned place preference (CPP), a measure of non-contingent reward, was used to determine the effects of endogenous gonadal hormones and of estrogen and progesterone replacement on cocaine reward. Neurochemical measurements were also taken to identify monoaminergic substrates which underlie the behavioral phenotype. Although both intact and gonadectomized male and female rats showed a significant CPP for cocaine, ovariectomy attenuated the magnitude of CPP. These alterations coincided with a decrease in serum levels of corticosterone. In ovariectomized rats, pretreatment with progesterone inhibited cocaine CPP while estrogen plus progesterone potentiated the magnitude of CPP. Additionally, gonadectomy and ovarian hormone replacement in female rats affected serotonin/dopamine levels and turnover ratios in the ventral tegmental area and nucleus accumbens shell. While no effects of castration were observed, ovariectomy decreased levels of dopamine and serotonin in the ventral tegmental area. In females, progesterone replacement increased levels of serotonin and dopamine in the ventral tegmental area, while estrogen plus progesterone replacement increased dopamine levels in the nucleus accumbens. Collectively, these results indicate that ovarian hormones may influence cocaine reward by altering monoaminergic systems, which, in turn, may contribute to the current sex disparities in overall cocaine use.

Differential distribution of CREB in the mesolimbic dopamine reward pathway

The transcription factor cAMP response element binding protein (CREB) has been implicated in the long-term neuronal plasticity associated with addiction. While CREB is expressed in many cells throughout the brain, very little is known about the relative concentrations of CREB protein in various brain regions. Studies in which CREB levels have been altered, either constitutively throughout the brain via gene targeting or transiently in specific brain regions, demonstrate variable roles for this protein in mediating reinforcing properties of drugs of abuse. To investigate the complex nature of CREB function in addiction, we examined the distribution of CREB protein in the nucleus accumbens (NAc) and ventral tegmental area (VTA), two brain regions that are part of the well-defined mesolimbic dopamine pathway involved in reward processing. Our data demonstrate significantly more CRE binding activity and CREB protein in the NAc compared to levels present in the VTA of wild-type mice. Phospho-CREB levels are increased in the NAc of both wild-type and CREBalphaDelta mutant animals after cocaine. However, morphine-induced increases of phospho-CREB levels are seen in the VTA of wild-type mice but not CREBalphaDelta mutant mice. Consequently, the 90% reduction of CREB in CREBalphaDelta mutant mice differentially affects CREB phosphorylation and induction of downstream targets of CREB in the NAc and VTA.

Effect of strain and sex on mu opioid receptor-mediated G-protein activation in rat brain

Strain and sex differences in mu opioid-mediated antinociception have been reported in rodents. The present studies evaluated mu opioid receptor-mediated G-protein activation in Lewis and Fischer 344 (F344) male and female rats using agonist-stimulated [35S]GTPgammaS binding. Compared to Lewis rats, F344 rats exhibited a 35% higher level of net DAMGO-stimulated [35S]GTPgammaS binding in striatum. Basal [35S]GTPgammaS binding was approximately 30% lower in thalamus of Lewis than F344 rats. Female Lewis rats also exhibited slightly ( approximately 15%) lower basal [35S]GTPgammaS binding in cingulate cortex relative to F344 rats of either sex. The relative efficacies of the mu partial agonists, morphine and buprenorphine, were also examined. Buprenorphine exhibited approximately 40% lower relative efficacy in the periaqueductal gray in Lewis compared to F344 rats, but no other relative efficacy differences were found between strains or sexes. Moreover, regional differences in the relative efficacy of buprenorphine were also detected in Lewis but not F344 rats. In contrast to these results, the only difference found between sexes was the 13% lower basal [35S]GTPgammaS binding in the cingulate cortex of female compared to male Lewis rats. These results suggest that differences in mu opioid receptor-mediated G-protein activation may contribute to strain differences in opioid antinociception, whereas sex differences may result predominantly from other mechanisms.

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.

Involvement of dopamine D1 receptors of the central amygdala on the acquisition and expression of morphine-induced place preference in rat

In the present study, the effects of intra-central amygdala (CeA) injection of dopamine D1 receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (0.5-10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intra-CeA administration of the dopamine D1 receptor agonist, SKF 38393 (2 and 4 micro g/rat) with an ineffective dose of morphine (0.5 mg/kg), elicited a significant conditioned place preference. On the other hand, a single dose of SKF 38393 (2 micro g/rat, intra-CeA) in combination with the lower doses (0.5 and 2.5 mg/kg), but not with the higher doses of morphine potentiated morphine-induced CPP. Furthermore, intra-CeA administration of the dopamine D1 receptor antagonist, SCH 23390 (0.5-1 micro g/rat) decreased the acquisition of conditioned place preference induced by morphine (7.5 mg/kg). The response of SKF 38393 was decreased by SCH 23390 (0.75 micro g/rat). SKF 38393 or SCH 23390 by themselves did not elicit any effect on place conditioning. On the other hand, intra-CeA administration of SKF 38393 or SCH 23390 significantly decreased the expression of morphine (7.5 mg/kg)-induced place preference. SKF 38393 or SCH 23390 injections into the CeA had no effects on the locomotor activity on the test sessions. The results indicate that the dopamine D1 receptors in the CeA may be involved in the acquisition and expression of morphine-induced place preference.

Involvement of dopamine D2 receptors of the central amygdala on the acquisition and expression of morphine-induced place preference in rat

In the present study, the effects of intra-central amygdala (CeA) injections of dopamine (DA) D2-like receptor agonist and antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a dose-dependent conditioned place preference (CPP). Using a 3-day schedule of conditioning, it was found that the DA D2/D3 receptor agonist, quinpirole (0.3-3 microg/rat), or the DA D2 receptor antagonist, sulpiride (0.04-5 microg/rat), did not produce a significant place preference or place aversion. Intra-CeA administration of quinpirole (0.3 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. On the other hand, quinpirole (0.3 microg/rat) injection into the CeA induced CPP in combination with the lower doses of morphine (0.5 and 2.5 mg/kg), but decreased the response of higher dose (7.5 mg/kg) of morphine. This response of quinpirole was attenuated by sulpiride (0.2 microg/rat). Sulpiride by itself (0.04-5 microg/rat) reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of the higher dose of sulpiride (1 and 5 microg/rat) or the higher dose of quinpirole (3 microg/rat) during acquisition decreased the locomotor activity of the animals on the testing days. The injection of the low dose of quinpirole (0.3 microg/rat) on the test day reduced the expression of morphine-induced CPP, but the high dose of quinpirole (3 microg/rat) potentiated this expression. The administration of sulpiride (5 microg/rat) attenuated the quinpirole response. The injection of sulpiride (1 and 5 microg/rat) abolished the expression of morphine-induced CPP. It is concluded that the CeA DA D2-like receptors may play an active role in morphine reward.

Effects of cocaine administration on VTA cell activity in response to prefrontal cortex stimulation

The repeated use of psychostimulants in humans has been associated with progressive enhancement of anxiety, panic attacks, and eventually paranoid psychosis. The appearance of such behaviors has been termed behavioral sensitization, which forms part of the basic pathological mechanisms involved in drug addiction. Psychostimulants act via a circuit involving the ventral tegmental area (VTA), prefrontal cortex (PFC), and nucleus accumbens. The PFC sends glutamatergic projections that activate dopaminergic neurons in the VTA. These projections provide an extremely important excitatory drive necessary for the development of sensitization. The effects of cocaine administration on the response of dopaminergic VTA cells to activation of the PFC have not been reported. Here the effects of acute cocaine administration on VTA cell response to PFC stimulation are examined. Statistical analysis of the changes in spontaneous activity and evoked response revealed a significant decrease in spontaneous activity at 1.0 mg/kg i.v. after cocaine treatment compared to baseline levels. The net effect was an increase in signal-to-noise ratio. Treatment with MK-801 at a dose of 2 mg/kg showed that the excitatory response was, at least partially, NMDA-mediated. Prazosin pretreatment (0.5 mg/kg i.p.) did not prevent a significant decrease in spontaneous activity brought about by cocaine (15 mg/kg, i.p.). Nonetheless, prazosin alone induced a significant decrease in the response to PFC stimulation when compared to baseline. In addition, iontophoretic application of norepinephrine (NE) onto VTA cells revealed that NE potentiated (19.2%), enhanced (26.9%), or suppressed (46.2%) the glutamate-evoked response in VTA cells. The results suggest that a possible role of cocaine in the process of sensitization might be to amplify the PFC-induced excitation at the VTA. Since the iontophoretic release of NE in almost half of the sampled cells produced similar effects to those of cocaine it may suggest a possible NE-mediated mechanism for cocaine actions.

Sensitization to cocaine after a single intra-cerebral injection of orphanin FQ/nociceptin

Orphanin FQ/nociceptin (OFQ/N) has been shown to modulate mesolimbic dopaminergic neurotransmission. Repeated administration of OFQ/N into the ventral tegmental area results in a sensitized locomotor response to subsequent peripheral cocaine administration. The aim of the present study was to examine the potential for OFQ/N to produce a sensitized locomotor response to cocaine after a single intra-VTA administration and to determine if this effect of OFQ/N extrapolates to other points along the mesolimbic or nigrostriatal dopaminergic axes. Bilateral administration of OFQ/N (30 microg/side) into the VTA on day 1 to male Sprague--Dawley rats resulted in an enhanced locomotor response to cocaine (10 mg/kg i.p) administered on day 2. However, OFQ/N (3, 10 and 30 microg per side) administered on day 2, 5 mins prior to the administration of cocaine (10 mg/kg i.p), in animals treated with aCSF or OFQ/N on day 1, similarly blocked the action of cocaine, suggesting that the sensitized response was not due to tolerance to the effect of endogenously released OFQ/N. The administration of OFQ/N into the substantia nigra or nucleus accumbens failed to produce a significant sensitized response to a cocaine challenge 24 h later. A significant increase in cocaine stimulated locomotor response on day 2 was observed after injection of OFQ/N into the striatum on day 1. These results demonstrate the ability of a single intra-VTA or intra-striatal administration of OFQ/N to produce increases in the sensitivity to cocaine and may indicate a role for endogenous OFQ/N systems in regulating responses to psychostimulant drugs.

Biphasic dose-related effects of morphine on dopamine release

The effects of morphine on extracellular dopamine levels in brain have never been studied over a wide range of doses within a single study. This has made it difficult to make definitive interpretations of drug interactions with morphine. An inhibition of morphine-induced increases in dopamine could be interpreted as either antagonism or potentiation depending the shape of the morphine dose-response curve. Accordingly, the aim of the present study was to determine the effects of a wide range of morphine doses (0, 5, 10, 20 and 30 mg/kg, i.p.) on extracellular dopamine, DOPAC and HVA levels in the nucleus accumbens and striatum of awake and freely moving female Sprague-Dawley rats. The results show that, in both brain regions, the dose-response curve for morphine-induced increases in dopamine is non-monotonic while the dose-response curve for morphine-induced increases in DOPAC and HVA is monotonic in the nucleus accumbens. The results of this study are discussed in terms of their implications for interpreting drug interactions with morphine and with relationship to morphine's mode of action at mu and kappa opioid receptors.

A comparison of morphine-induced locomotor activity and mesolimbic dopamine release in C57BL6, 129Sv and DBA2 mice

Inbred mouse strains show marked variations in morphine-induced locomotion and reward behaviors. As increases in mesolimbic dopamine release and locomotion have been implicated as being critical aspects of drug-seeking and reward-related behaviors, the present study sought to determine the relationship between morphine-induced changes in locomotion and mesolimbic dopamine release. Freely moving microdialysis of the ventral striatum was performed in mouse strains chosen on the basis of their documented differences in locomotor and reward response to morphine (C57BL6 and DBA2) and use in the production of genetically modified mice (129Sv). Both C57BL6 and 129Sv mice showed significant increases in locomotion and ventral striatal extracellular dopamine levels following subcutaneous morphine administration (3 mg/kg), with the former strain showing the largest increase in both parameters. Ventral striatal extracellular DA levels increased in DBA2 mice to a similar extent as 129Sv mice following morphine administration, despite this strain showing no locomotor response. Intra-strain analysis found no correlation between morphine-induced locomotion and mesolimbic dopamine release in any of the strains studied. Thus, no universal relationship between morphine-induced mesolimbic dopamine release and locomotion exists between, and particularly within, inbred mouse strains. Furthermore, morphine-induced increases in mesolimbic activity correlate negatively with the rewarding potential of morphine described in previously reported conditioned place preference studies.

Spatial learning and morphine-rewarded place preference negatively correlates in mice

Accumulating evidence has indicated that there might exist some correlation between opiate reward and certain kinds of learning and memory processes. The present study attempted to investigate the correlation between individual differences in morphine reward and capacities in spatial learning and spontaneous alternation. In the present studies, good-response (GR) and poor-response (PR) mice were respectively selected according to their performance in a spatial learning test involving the Morris water maze or in a spontaneous alternation task using the Y-maze. In a place preference conditioning procedure, morphine (3.0 mg/kg) produced significant conditioned place preference (CPP) in both GR and PR mice selected by using either the Morris water maze or the Y-maze. The PR mice selected with the Morris water maze showed significantly more CPP induced by morphine than the GR mice. However, no detectable difference was observed in morphine-induced CPP between the GR and PR mice selected with the Y-maze. These results suggested that the variation in morphine-induced CPP in mice is somehow differentially related to that of spatial learning but unlikely to that of spontaneous alternation.

Attenuation of the reinforcing efficacy of morphine by 18-methoxycoronaridine

In previous studies, 18-methoxycoronaridine, a novel iboga alkaloid congener, has been reported to decrease the self-administration of morphine, cocaine, ethanol and nicotine, and to attenuate naltrexone-precipitated signs of morphine withdrawal. In the present study, the nature of the interaction between 18-methoxycoronaridine and morphine was further investigated. Using in vivo microdialysis, 18-methoxycoronaridine pretreatment (40 mg/kg i.p., 19 h beforehand) was found to markedly inhibit morphine-induced (5 mg/kg, i.p.) dopamine release in the nucleus accumbens and striatum; 18-methoxycoronaridine also enhanced morphine-induced increases in extracellular levels of dopamine's metabolites. These effects, which were more prominent in the nucleus accumbens than in the striatum, suggest that 18-methoxycoronaridine selectively interferes with morphine-induced dopamine release, without altering morphine-induced stimulation of dopamine synthesis. In intravenous morphine self-administration experiments, the effects of acute 18-methoxycoronaridine treatment (40 mg/kg, p.o.) were assessed in rats responding for one of several different unit infusion dosages of morphine (0.01-0.16 mg/kg/infusion). 18-Methoxycoronaridine produced a downward shift in the entire morphine dose-response curve without any displacement to the left or right. These results suggest that 18-methoxycoronaridine reduced the reinforcing efficacy of morphine without altering its apparent potency. Together, the microdialysis and self-administration data suggest that 18-methoxycoronaridine profoundly alters mechanisms crucial to the development and maintenance of opioid addiction.

The effects of quinine and 4-aminopyridine on conditioned place preference and changes in motor activity induced by morphine in rats

1. The effects of two unselective potassium (K(+)-) channel blockers, quinine (12.5, 25 and 50 mg/kg) and 4-aminopyridine (1 and 2 mg/kg), on conditioned place preference and biphasic changes in motor activity induced by morphine (10 mg/kg) were tested in Wistar rats. Quinine is known to block voltage-, calcium- and ATP-sensitive K(+)-channels while 4-aminopyridine is known to block voltage-sensitive K(+)-channels. 2. In the counterbalanced method, quinine attenuated morphine-induced place preference, whereas 4-aminopyridine was ineffective. In the motor activity test measured with an Animex-activity meter neither of the K(+)-channel blockers affected morphine-induced hypoactivity, but both K(+)-channel blockers prevented morphine-induced secondary hyperactivity. 3. These results suggest the involvement of quinine-sensitive but not 4-aminopyridine-sensitive K(+)-channels in morphine reward. It is also suggested that the blockade of K(+)-channels sensitive to these blockers is not sufficient to prevent morphine-induced hypoactivity whereas morphine-induced hyperactivity seems to be connected to both quinine- and 4-aminopyridine-sensitive K(+)-channels.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.