Withdrawal from acute morphine dependence is accompanied by increased anxiety-like behavior in the elevated plus maze

Effects of xylazine on naloxone-precipitated fentanyl withdrawal in male and female rats

PURPOSE

The combination of fentanyl and xylazine (i.e., "tranq-dope") was recently declared an emerging national health threat in the United States. Given the recency of this development, very little is known regarding the behavioral pharmacology of fentanyl-xylazine combinations. The purpose of this study was to characterize the somatic and affective withdrawal symptoms of this drug combination.

METHODS

Male and female Long Evans rats were given twice daily (08:00 and 20:00) subcutaneous injections of fentanyl, xylazine, or combined fentanyl-xylazine for five days. On the sixth (testing) day, rats were given a final injection at 08:00. Four hours later, rats were injected intraperitoneally with either saline or a naloxone challenge before behavioral observation. Somatic withdrawal was examined using the Gellert-Holtzman scale and anxiety-like behavior was examined using the elevated plus maze.

RESULTS

Naloxone administration did not induce somatic or affective symptoms in rats treated with fentanyl alone, a low dose of xylazine alone, or a high dose of xylazine alone. Naloxone induced somatic but not affective withdrawal symptoms in rats treated with both fentanyl and xylazine.

CONCLUSION

Chronic co-exposure to fentanyl and xylazine produces an opioid-like somatic withdrawal syndrome at doses that are not apparent with either drug alone. These results corroborate clinical reports that xylazine worsens fentanyl withdrawal and suggest that novel interventions may be required to treat withdrawal from fentanyl-xylazine combinations in humans.

Model of negative affect induced by withdrawal from acute and chronic morphine administration in male mice

Opioid use disorder (OUD) is a chronic relapsing disorder that is a major burden for the lives of affected individuals, and society as a whole. Opioid withdrawal is characterized by strong physical symptoms, along with signs of negative affect. Negative affect due to opioid withdrawal is a major obstacle to recovery and relapse prevention. The mechanisms behind negative affect due to either spontaneous or antagonist-precipitated opioid withdrawal are not well known, and more animal models need be developed. Here, we present behavioral models of negative affect upon naloxone-precipitated morphine withdrawal in adult male mice. Social, anxiety, and despair-like deficits were investigated following naloxone administration in mice receiving morphine under three dosing regimens; acute, chronic constant dose and chronic escalating doses. Social behaviour in the three-chamber social preference test was decreased following withdrawal from chronic and escalating but not acute morphine. Anxiety-like behaviour in the open field was increased for all three treatments. Despair-like behaviour was increased following withdrawal from chronic and escalating but not acute morphine. Altogether, these animal models will contribute to study behavioural and neuronal circuitries involved in the several negative affective signs characterizing OUD.

Ketamine metabolite alleviates morphine withdrawal-induced anxiety via modulating nucleus accumbens parvalbumin neurons in male mice

Opioid withdrawal generates extremely unpleasant physical symptoms and negative affective states. A rapid relief of opioid withdrawal-induced anxiety has obvious clinical relevance but has been rarely reported. We have shown that injection of ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) leads to a rapid alleviation of anxiety-like behaviors in male mice undergoing chronic morphine withdrawal. Here we investigated the contribution of nucleus accumbens shell (sNAc) parvalbumin (PV)-neurons to this process. Chronic morphine withdrawal was associated with higher intrinsic excitability of sNAc PV-neurons via reduced voltage-dependent potassium currents. Chemogenetic inhibition of sNAc PV-neurons reversed the enhanced excitability of PV-neurons and anxiety-like behaviors in these morphine withdrawal male mice, while activation of sNAc PV-neurons induced anxiety-like behaviors in naive male mice. (2R,6R)-HNK reversed the altered potassium currents and intrinsic excitability of sNAc PV-neurons. Our findings demonstrate an important contribution of sNAc PV-neurons to modulating morphine withdrawal-induced anxiety-like behaviors and rapid relief of anxiety-like behaviors by (2R,6R)-HNK, this newly identified target may have therapeutic potentials in treating opioid addiction and anxiety disorders.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Paternal morphine exposure enhances morphine self-administration and induces region-specific neural adaptations in reward-related brain regions of male offspring

Background

A growing body of preclinical studies report that preconceptional experiences can have a profound and long-lasting impact on adult offspring behavior and physiology. However, less is known about paternal drug exposure and its effects on reward sensitivity in the next generation.

Methods

Adult male rats self-administered morphine for 65 days; controls received saline. Sires were bred to drug-naïve dams to produce first-generation (F1) offspring. Morphine, cocaine, and nicotine self-administration were measured in adult F1 progeny. Molecular correlates of addiction-like behaviors were measured in reward-related brain regions of drug naïve F1 offspring.

Results

Male, but not female offspring produced by morphine-exposed sires exhibited dose-dependent increased morphine self-administration and increased motivation to earn morphine infusions under a progressive ratio schedule of reinforcement. This phenotype was drug-specific as self-administration of cocaine, nicotine, and sucrose were not altered by paternal morphine history. The male offspring of morphine-exposed sires also had increased expression of mu-opioid receptors in the ventral tegmental area but not in the nucleus accumbens.

Conclusions

Paternal morphine exposure increased morphine addiction-like behavioral vulnerability in male but not female progeny. This phenotype is likely driven by long-lasting neural adaptations within the reward neural brain pathways.

Hippocampal and amygdalar increased BDNF expression in the extinction of opioid-induced place preference

The opioid crisis was exacerbated during the COVID-19 pandemic in the United States with alarming statistics about overdose-related deaths. Current treatment options, such as medication assisted treatments, have been unable to prevent relapse in many patients, whereas cue-based exposure therapy have had mixed results in human trials. To improve patient outcomes, it is imperative to develop animal models of addiction to understand molecular mechanisms and identify potential therapeutic targets. We previously found increased brain derived neurotrophic factor (bdnf) transcript in the ventral striatum/nucleus accumbens (VS/NAc) of rats that extinguished morphine-induced place preference. Here, we expand our study to determine whether BDNF protein expression was modulated in mesolimbic brain regions of the reward system in animals exposed to extinction training. Drug conditioning and extinction sessions were followed by Western blots for BDNF in the hippocampus (HPC), amygdala (AMY) and VS/NAc. Rears, as a measure of withdrawal-induced anxiety were also measured to determine their impact on extinction. Results showed that animals who received extinction training and successfully extinguished morphine CPP significantly increased BDNF in the HPC when compared to animals deprived of extinction training (sham-extinction). This increase was not significant in animals who failed to extinguish (extinction-resistant). In AMY, all extinction-trained animals showed increased BDNF, regardless of behavior phenotype. No BDNF modulation was observed in the VS/NAc. Finally, extinction-trained animals showed no difference in rears regardless of extinction outcome, suggesting that anxiety elicited by drug withdrawal did not significantly impact extinction of morphine CPP. Our results suggest that BDNF expression in brain regions of the mesolimbic reward system could play a key role in extinction of opioid-induced maladaptive behaviors and represents a potential therapeutic target for future combined pharmacological and extinction-based therapies.

The long-term effects of repeated heroin vapor inhalation during adolescence on measures of nociception and anxiety-like behavior in adult Wistar rats

RATIONALE

Adolescents represent a vulnerable group due to increased experimentation with illicit substances that is often associated with the adolescent period, and because adolescent drug use can result in long-term effects that differ from those caused by drug use initiated during adulthood.

OBJECTIVES

The purpose of the present study was to determine the effects of repeated heroin vapor inhalation during adolescence on measures of nociception, and anxiety-like behavior during adulthood in female and male Wistar rats.

METHODS

Rats were exposed twice daily to 30 min of heroin vapor from post-natal day (PND) 36 to PND 45. At 12 weeks of age, baseline thermal nociception was assessed across a range of temperatures with a warm-water tail-withdrawal assay. Anxiety-like behavior was assessed in an elevated plus-maze (EPM) and activity was measured in an open-field arena. Starting at 23 weeks of age, baseline thermal nociception was re-assessed, nociception was determined after acute heroin or naloxone injection, and anxiety-like behavior was redetermined in the EPM.

RESULTS

Adolescent heroin inhalation altered baseline thermal nociception in female rats at 12 weeks of age and in both female and male rats at ~ 23 weeks. Heroin-treated animals exhibited anxiety-like behavior when tested in the elevated plus-maze, showed blunted heroin-induced analgesia, but exhibited no effect on naloxone-induced hyperalgesia.

CONCLUSIONS

The present study demonstrates that heroin vapor inhalation during adolescence produces behavioral and physiological consequences in rats that persist well into adulthood.

Goofballing of Opioid and Methamphetamine: The Science Behind the Deadly Cocktail

Globally, millions of people suffer from various substance use disorders (SUD), including mono-and polydrug use of opioids and methamphetamine. Brain regions such as the cingulate cortex, infralimbic cortex, dorsal striatum, nucleus accumbens, basolateral and central amygdala have been shown to play important roles in addiction-related behavioral changes. Clinical and pre-clinical studies have characterized these brain regions and their corresponding neurochemical changes in numerous phases of drug dependence such as acute drug use, intoxication, craving, withdrawal, and relapse. At present, many studies have reported the individual effects of opioids and methamphetamine. However, little is known about their combined effects. Co-use of these drugs produces effects greater than either drug alone, where one decreases the side effects of the other, and the combination produces a prolonged intoxication period or a more desirable intoxication effect. An increasing number of studies have associated polydrug abuse with poorer treatment outcomes, drug-related deaths, and more severe psychopathologies. To date, the pharmacological treatment efficacy for polydrug abuse is vague, and still at the experimental stage. This present review discusses the human and animal behavioral, neuroanatomical, and neurochemical changes underlying both morphine and methamphetamine dependence separately, as well as its combination. This narrative review also delineates the recent advances in the pharmacotherapy of mono- and poly drug-use of opioids and methamphetamine at clinical and preclinical stages.

Paradoxical anxiolytic effect of the 'bath salt' synthetic cathinone MDPV during early abstinence is inhibited by a chemokine CXCR4 or CCR5 receptor antagonist

Chemokine CXCR4 and CCR5 receptors are best known as HIV co-entry receptors, but evidence that CXCR4 or CCR5 blockade reduces rewarding and locomotor-stimulant effects of psychostimulants in rats suggests a role in psychostimulant use disorders. We investigated the impact of CXCR4 or CCR5 receptor antagonism on anxiety-related effects of the synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV) in the elevated zero-maze (EZM) assay. Rats exposed to a 4-day MDPV binge dosing paradigm and tested 24 or 72 h post-treatment spent more time in the open compartment at the 24-h time point but less time at the 72-h post-binge time point. Daily administration of AMD 3100, a CXCR4 antagonist (10 mg/kg), or maraviroc, a CCR5 antagonist (2.5 mg/kg), during MDPV treatment inhibited the MDPV-induced increase in time spent in the open compartment. Neither antagonist affected the MDPV-induced reduction in time spent in the open compartment at the 72-h post-binge time point. Cocaine, administered in the same paradigm as MDPV, did not increase time spent in the open compartment 24-h post-binge, suggesting specificity to MDPV. The present results identify a surprising anxiolytic-like effect of MDPV 24 h after cessation of repeated exposure that is sensitive to chemokine CXCR4 and CCR5 receptor activity.

Escalating morphine dosage fails to elicit conditioned analgesia in a preclinical chronic neuropathic pain model

Many people with chronic pain escalate their opioid dosage to counteract tolerance effects. A treatment regimen consisting of placebos admixed with opioids has been suggested as a possible therapeutic option that could reduce the harm of long-term opioid use. However, the analgesic efficacy of such a regimen requires further investigation before widespread adoption. We have recently reported that a 4-day pharmacological conditioning procedure, which paired morphine (6 mg/kg) with contextual cues, elicited placebo analgesia in subpopulations of male (35%) and female (25%) rats with sciatic nerve chronic constriction injury (CCI). Here, we investigated how an escalating morphine dosage during conditioning affects the incidence and strength of placebo analgesia. Forty-four male, Sprague-Dawley rats received CCI. Thirty-eight (86%) rats developed strong cold allodynia by day 6 post-surgery, as measured by hind paw withdrawal (HPW) behaviour on a 5°C cold plate (120 s). In this experiment, pharmacological conditioning consisted of an escalating morphine dose over 4 days (8/9/10/12 mg/kg). This dosing regimen produced strong reductions in HPW behaviour and counteracted the effects of morphine tolerance during conditioning. However, none of the rats given the placebo treatment (n = 12) demonstrated reductions in HPW behaviour when morphine was substituted for saline (i.e. placebo analgesia), but instead showed a strong behavioural response (rearing). These results demonstrate that a high, escalating dose of morphine failed to produce conditioned placebo analgesia in rats with CCI. It is possible that admixing placebos with opioids may be similarly ineffective in chronic pain patients when the opioids regimen is high or escalating.

Synergistic effect between citalopram and citicoline on anxiolytic effect in non-sensitized and morphine-sensitized mice: An isobologram analysis

In the present study, the effects of intraperitoneal (i.p.) injections of citalopram and citicoline on morphine-induced anxiolytic effects were investigated in non-sensitized and morphine-sensitized mice using elevated plus-maze (EPM). Subcutaneous (s.c.) administration morphine (5 mg/kg) increased the percentage of open arm time (%OAT, in morphine-sensitized mice), and open arm entries (%OAE, in non-sensitized mice), but not a locomotor activity, indicating an anxiolytic response to morphine. On the other hand, i.p. administration of naloxone decreased %OAT (morphine-sensitized mice), and %OAE (non-sensitized and morphine-sensitized mice), but not a locomotor activity, showing an anxiogenic effect to naloxone. Moreover, i.p.co-administration of citalopram (5 and 10 mg/kg) and citicoline (75 mg/kg) induced the anxiolytic effect. Interestingly, i.p. co-administration of low doses of citalopram (0.5, 1 and 2.5 mg/kg) and citicoline (25 mg/kg) significantly increased %OAT and %OAE in non-sensitized as well as %OAT in morphine-sensitized mice, indicating an anxiolytic effect. An isobolographic analysis of data was performed, presenting a synergistic interaction between citalopram and citicoline upon the production of anxiolytic effect in non-sensitized and morphine-sensitized mice. In conclusion, it seems that (1) morphine sensitization affects the anxiety behavior in the EPM, (2) μ-opioid receptors play an important role in morphine anxiolytic effect, (3) citalopram and citicoline induced anti-anxiety effect, (4) a synergistic effect of citalopram and citicoline upon induction of anti-anxiety behavior in non-sensitized and morphine-sensitized mice.

Mechanisms of Shared Vulnerability to Post-traumatic Stress Disorder and Substance Use Disorders

Psychoactive substance use is a nearly universal human behavior, but a significant minority of people who use addictive substances will go on to develop an addictive disorder. Similarly, though ~90% of people experience traumatic events in their lifetime, only ~10% ever develop post-traumatic stress disorder (PTSD). Substance use disorders (SUD) and PTSD are highly comorbid, occurring in the same individual far more often than would be predicted by chance given the respective prevalence of each disorder. Some possible reasons that have been proposed for the relationship between PTSD and SUD are self-medication of anxiety with drugs or alcohol, increased exposure to traumatic events due to activities involved in acquiring illegal substances, or addictive substances altering the brain's stress response systems to make users more vulnerable to PTSD. Yet another possibility is that some people have an intrinsic vulnerability that predisposes them to both PTSD and SUD. In this review, we integrate clinical and animal data to explore these possible etiological links between SUD and PTSD, with an emphasis on interactions between dopaminergic, adrenocorticotropic, GABAergic, and glutamatergic neurobehavioral mechanisms that underlie different emotional learning styles.

Transgenerational influence of parental morphine exposure on pain perception, anxiety-like behavior and passive avoidance memory among male and female offspring of Wistar rats

Accumulating evidence suggests that epigenetic mechanisms play an important role in the formation and maintenance of memory within the brain. Moreover, the effect of parental drug-exposure before gestation on behavioral state of offspring has been little studied. The main objective of the current study is to evaluate the effect of parental morphine exposure on avoidance memory, morphine preference and anxiety-like behavior of offspring. The total of 32 males and 32 females were used for mating. The animals were treated with morphine. The offspring according to their parental morphine treatment was divided into four groups (n=16) including paternally treated, maternally treated, both of parents treated and naïve animals. The pain perception, anxiety-like behavior, and avoidance memory were evaluated in the offspring. In the current study, the total of 256 offspring was used for the experiments (4 tasks × 4 groups of offspring × 8 female offspring × 8 male offspring). The finding revealed that the avoidance memory and visceral pain were reduced significantly in male and female offspring with at least one morphine-treated parent. Moreover, anxiety-like behavior was reduced significantly in the male offspring with at least one morphine-treated parent. While anxiety-like behavior was increased significantly in female offspring that were treated by morphine either maternally or both of parents. The data revealed that the endogenous opioid system may be altered in the offspring of morphine-treated parent(s), and epigenetic role could be important. However, analysis of variance signified the important role of maternal inheritance.

The Mechanisms Involved in Morphine Addiction: An Overview

Opioid use disorder is classified as a chronic recurrent disease of the central nervous system (CNS) which leads to personality disorders, co-morbidities and premature death. It develops as a result of long-term administration of various abused substances, along with morphine. The pharmacological action of morphine is associated with its stimulation of opioid receptors. Opioid receptors are a group of G protein-coupled receptors and activation of these receptors by ligands induces significant molecular changes inside the cell, such as an inhibition of adenylate cyclase activity, activation of potassium channels and reductions of calcium conductance. Recent data indicate that other signalling pathways also may be involved in morphine activity. Among these are phospholipase C, mitogen-activated kinases (MAP kinases) or β-arrestin. The present review focuses on major mechanisms which currently are considered as essential in morphine activity and dependence and may be important for further studies.

Impairment of opiate-mediated behaviors by the selective TRPV1 antagonist SB366791

Transient receptor potential vanilloid type 1 (TRPV1), the archetypal member of the vanilloid TRP family, was initially identified as the receptor for capsaicin, the pungent ingredient in hot chili peppers. We previously demonstrated that TRPV1 in the dorsal striatum significantly contributes to morphine reward by using the conditioned place preference paradigm in mice; however, it is unknown whether TRPV1 has the same effect in other reward models. In this study, we investigated the role of TRPV1 in morphine reward by using a self-administration paradigm in rats. We found that treatment with a selective TRPV1 antagonist, SB366791, significantly decreased morphine self-administration on a fixed-ratio 1 schedule or a progressive ratio schedule of reinforcement. In addition, treatment with another selective TRPV1 antagonist, AMG9810, not only significantly prevented morphine self-administration but also prevented morphine-induced c-fos expression in the nucleus accumbens. Furthermore, administration of SB366791 decreased an anxiolytic-like effect during the morphine abstinence period. Moreover, treatment with SB366791 significantly decreased morphine-priming reinstatement. Taken together, our findings suggest that blockade of TRPV1 receptors could provide an approach to limiting morphine addiction.

Effect of prenatal restraint stress and morphine co-administration on plasma vasopressin concentration and anxiety behaviors in adult rat offspring

Stressful events and exposure to opiates during gestation have important effects on the later mental health of the offspring. Anxiety is among the most common mental disorders. The present study aimed to identify effects of prenatal restraint stress and morphine co-administration on plasma vasopressin concentration (PVC) and anxiety behaviors in rats. Pregnant rats were divided into four groups (n = 6, each): saline, morphine, stress + saline and stress + morphine treatment. The stress procedure consisted of restraint twice per day, two hours per session, for three consecutive days starting on day 15 of pregnancy. Rats in the saline and morphine groups received either 0.9% saline or morphine intraperitoneally on the same days. In the morphine/saline + stress groups, rats were exposed to restraint stress and received either morphine or saline intraperitoneally. All offspring were tested in an elevated plus maze (EPM) on postnatal day 90 (n = 6, each sex), and anxiety behaviors of each rat were recorded. Finally, blood samples were collected to determine PVC. Prenatal morphine exposure reduced anxiety-like behaviors. Co-administration of prenatal stress and morphine increased locomotor activity (LA) and PVC. PVC was significantly lower in female offspring of the morphine and morphine + stress groups compared with males in the same group, but the opposite was seen in the saline + stress group. These data emphasize the impact of prenatal stress and morphine on fetal neuroendocrine development, with long-term changes in anxiety-like behaviors and vasopressin secretion. These changes are sex specific, indicating differential impact of prenatal stress and morphine on fetal neuroendocrine system development. Lay Summary Pregnant women are sometimes exposed to stressful and painful conditions which may lead to poor outcomes for offspring. Opiates may provide pain and stress relief to these mothers. In this study, we used an experimental model of maternal exposure to stress and morphine in pregnant rats. The findings indicated that maternal stress increased anxiety in offspring while morphine decreased such effects, but had negative effects on the levels of a hormone controlling blood pressure, and activity of offspring. Hence morphine should not be used in pregnancy for pain and stress relief.

Effects of naltrexone are influenced by childhood adversity during negative emotional processing in addiction recovery

Naltrexone is an opioid receptor antagonist used in the management of alcohol dependence. Although the endogenous opioid system has been implicated in emotion regulation, the effects of mu-opioid receptor blockade on brain systems underlying negative emotional processing are not clear in addiction. Individuals meeting criteria for alcohol dependence alone (n=18, alcohol) and in combination with cocaine and/or opioid dependence (n=21, alcohol/drugs) and healthy individuals without a history of alcohol or drug dependence (n=21) were recruited. Participants were alcohol and drug abstinent before entered into this double-blind, placebo-controlled, randomized, crossover study. Functional magnetic resonance imaging was used to investigate brain response while viewing aversive and neutral images relative to baseline on 50 mg of naltrexone and placebo. We found that naltrexone modulated task-related activation in the medial prefrontal cortex and functional connectivity between the anterior cingulate cortex and the hippocampus as a function of childhood adversity (for aversive versus neutral images) in all groups. Furthermore, there was a group-by-treatment-by-condition interaction in the right amygdala, which was mainly driven by a normalization of response for aversive relative to neutral images under naltrexone in the alcohol/drugs group. We conclude that early childhood adversity is one environmental factor that influences pharmacological response to naltrexone. Pharmacotherapy with naltrexone may also have some ameliorative effects on negative emotional processing in combined alcohol and drug dependence, possibly due to alterations in endogenous opioid transmission or the kappa-opioid receptor antagonist actions of naltrexone.

Study of the effects of controlled morphine administration for treatment of anxiety, depression and cognition impairment in morphine-addicted rats

Background:

Morphine dependency usually results in undesired outcomes such as anxiety, depression, and cognitive alterations. In this study, morphine was used to manage morphine dependence-induced anxiety, depression, and learning and memory disturbances.

Materials and Methods:

Forty rats were divided equally into five groups. Group 1 received saline for 21 days. Groups 2–5 were dependent by increasing administration of morphine (15–45 mg/kg) for 7 days. For the next 14 days, morphine was administered as the following regimen: Group 2: once daily; 45 mg/kg (positive controls), Group 3: the same dose with an increasing interval (6 h longer than the previous intervals each time), Group 4: the same dose with an irregular intervals (12, 24, 36 h intervals interchangeably), and Group 5: decreasing doses once daily (every time 2.5 mg/kg less than the former dosage). On days 22–26, elevated plus maze (EPM), open field test (OFT), forced swim test (FST), and tail suspension test (TST) were performed to investigate anxiety level and depression in animals. Between 17^th and 21^st days, Morris water maze (MWM) was used to evaluate the spatial learning and memory.

Results:

Chronic morphine administration caused depression and anxiety as observed by FST, EPM, and TST and decreased motor activity in OFT and caused impairment in learning and memory performance in MWM. Treatment with our protocol as increasing interval, irregular interval, and decreasing dosage of morphine caused marked reduction in depression, anxiety, and improved cognition performance compared with positive control group; and attenuated motor deficits in morphine-dependent rats, remarkably.

Conclusions:

Change in dosage regimens of morphine can reduce morphine-induced anxiety, depression, and cognitive impairments.

Chronic morphine and tramadol re-exposure induced an anti-anxiety effect in prepubertal rats exposed neonatally to the same drugs

Anxiety disorders are among the most common mental disorders. Drugs that are often administered to manage medical problems cause rebound anxiety. The use of morphine and tramadol has increased in recent decades. In the present study, the effects of morphine and tramadol exposure during the neonatal and prepubertal periods on anxiety-like behaviours in prepubertal rats were investigated. Male neonate rats were injected subcutaneously with saline, morphine or tramadol (3-21 mg/kg) on a daily basis from postnatal Day (P) 8 to P14. On P22, rats were divided into seven groups (saline/saline, saline/tramadol, saline/morphine, tramadol/saline, tramadol/tramadol, morphine/saline and morphine/morphine) and were injected with saline, tramadol or morphine for seven consecutive days. All rats were tested in an elevated plus maze (EPM) on P24 (acute effects), P27 (chronic effects) and P29. Locomotor activity was increased by the second and third exposure to the EPM. Re-exposure to chronic morphine and tramadol resulted in increased locomotor activity, whereas acute and chronic administration of these drugs induced no notable difference. Anxiety decreased markedly after re-exposure to tramadol and this anxiolytic-like behaviour was more dominant in EPM re-exposure in rats that had received higher doses of tramadol. Re-exposure to tramadol elicited a stronger anxiolytic-like behaviour than re-exposure to morphine. It can be concluded that repeated morphine and tramadol administration during the neonatal period followed by re-exposure to these drugs at an immature stage produces considerable anxiolytic-like behaviour. Exposure to chronic morphine and tramadol during the neonatal period may affect the developing brain, which may induce long-term changes in the opioid response.

Chronic CRF1 receptor blockade reduces heroin intake escalation and dependence-induced hyperalgesia

Opioids represent effective drugs for the relief of pain, yet chronic opioid use often leads to a state of increased sensitivity to pain that is exacerbated during withdrawal. A sensitization of pain-related negative affect has been hypothesized to closely interact with addiction mechanisms. Neuro-adaptive changes occur as a consequence of excessive opioid exposure, including a recruitment of corticotropin-releasing factor (CRF) and norepinephrine (NE) brain stress systems. To better understand the mechanisms underlying the transition to dependence, we determined the effects of functional antagonism within these two systems on hyperalgesia-like behavior during heroin withdrawal utilizing models of both acute and chronic dependence. We found that passive or self-administered heroin produced a significant mechanical hypersensitivity. During acute opioid dependence, systemic administration of the CRF1 receptor antagonist MPZP (20 mg/kg) alleviated withdrawal-induced mechanical hypersensitivity. In contrast, several functional adrenergic system antagonists (clonidine, prazosin, propranolol) failed to alter mechanical hypersensitivity in this state. We then determined the effects of chronic MPZP or clonidine treatment on extended access heroin self-administration and found that MPZP, but not clonidine, attenuated escalation of heroin intake, whereas both drugs alleviated chronic dependence-associated hyperalgesia. These findings suggest that an early potentiation of CRF signaling occurs following opioid exposure that begins to drive both opioid-induced hyperalgesia and eventually intake escalation.

Differential effects of endocannabinoid catabolic inhibitors on morphine withdrawal in mice

BACKGROUND

Inhibition of endocannabinoid catabolic enzymes fatty acid amide hydrolase (FAAH) and/or monoacylglycerol lipase (MAGL) reduces somatic morphine withdrawal signs, but its effects on aversive aspects of withdrawal are unknown. The present study investigated whether Δ(9)-tetrahydrocannabinol (THC), the MAGL inhibitor JZL184, the FAAH inhibitor PF-3845, or the dual FAAH/MAGL inhibitor SA-57 would reduce acquisition of morphine withdrawal-induced conditioned place avoidance (CPA) and jumping.

METHODS

Mice were implanted with placebo or 75 mg morphine pellets, 48 h later injected with naloxone or saline and placed in the conditioning apparatus, and assessed for CPA at 72 h. Subjects were also observed for jumping behavior following naloxone challenge.

RESULTS

Naloxone (0.056 mg/kg) produced robust CPA in morphine-pelleted, but not placebo-pelleted, mice. Morphine pretreatment prevented the occurrence of withdrawal CPA and withdrawal jumping, while clonidine (an α2 adrenergic receptor agonist) only blocked withdrawal CPA. THC, JZL184, and SA-57 significantly reduced the percentage of mice that jumped during the conditioning session, but did not affect acquisition of withdrawal CPA. PF-3845 did not reduce morphine withdrawal CPA or jumping. Finally, neither THC nor the endocannabinoid catabolic enzyme inhibitors in non-dependent mice elicited a conditioned place preference or aversion.

CONCLUSIONS

These findings suggest that inhibiting endocannabinoid catabolic enzymes reduces somatic morphine withdrawal signs, but not aversive aspects as inferred in the CPA paradigm. The observation that non-dependent mice administered inhibitors of endocannabinoid degradation did not display place preferences is consistent with the idea that that endocannabinoid catabolic enzymes might be targeted therapeutically, with reduced risk of abuse.

BU08073 a buprenorphine analogue with partial agonist activity at μ-receptors in vitro but long-lasting opioid antagonist activity in vivo in mice

BACKGROUND AND PURPOSE

Buprenorphine is a potent analgesic with high affinity at μ, δ and κ and moderate affinity at nociceptin opioid (NOP) receptors. Nevertheless, NOP receptor activation modulates the in vivo activity of buprenorphine. Structure activity studies were conducted to design buprenorphine analogues with high affinity at each of these receptors and to characterize them in in vitro and in vivo assays.

EXPERIMENTAL APPROACH

Compounds were tested for binding affinity and functional activity using [(35) S]GTPγS binding at each receptor and a whole-cell fluorescent assay at μ receptors. BU08073 was evaluated for antinociceptive agonist and antagonist activity and for its effects on anxiety in mice.

KEY RESULTS

BU08073 bound with high affinity to all opioid receptors. It had virtually no efficacy at δ, κ and NOP receptors, whereas at μ receptors, BU08073 has similar efficacy as buprenorphine in both functional assays. Alone, BU08073 has anxiogenic activity and produces very little antinociception. However, BU08073 blocks morphine and U50,488-mediated antinociception. This blockade was not evident at 1 h post-treatment, but is present at 6 h and remains for up to 3-6 days.

CONCLUSIONS AND IMPLICATIONS

These studies provide structural requirements for synthesis of 'universal' opioid ligands. BU08073 had high affinity for all the opioid receptors, with moderate efficacy at μ receptors and reduced efficacy at NOP receptors, a profile suggesting potential analgesic activity. However, in vivo, BU08073 had long-lasting antagonist activity, indicating that its pharmacokinetics determined both the time course of its effects and what receptor-mediated effects were observed.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Affective and neuroendocrine effects of withdrawal from chronic, long-acting opiate administration

Although the long-acting opiate methadone is commonly used to treat drug addiction, relatively little is known about the effects of withdrawal from this drug in preclinical models. The current study examined affective, neuroendocrine, and somatic signs of withdrawal from the longer-acting methadone derivative l-alpha-acetylmethydol (LAAM) in rats. Anxiety-like behavior during both spontaneous and antagonist-precipitated withdrawal was measured by potentiation of the startle reflex. Withdrawal elevated corticosterone and somatic signs and blunted circadian variations in baseline startle responding. In addition, fear to an explicit, Pavlovian conditioned stimulus (fear-potentiated startle) was enhanced. These data suggest that anxiety-like behavior as measured using potentiated startle responding does not emerge spontaneously during withdrawal from chronic opiate exposure - in contrast to withdrawal from acute drug exposure - but rather is manifested as exaggerated fear in response to explicit threat cues.

Corticotropin-releasing factor (CRF) and α 2 adrenergic receptors mediate heroin withdrawal-potentiated startle in rats

Anxiety is one of the early symptoms of opioid withdrawal and contributes to continued drug use and relapse. The acoustic startle response (ASR) is a component of anxiety that has been shown to increase during opioid withdrawal in both humans and animals. We investigated the role of corticotropin-releasing factor (CRF) and norepinephrine (NE), two key mediators of the brain stress system, on acute heroin withdrawal-potentiated ASR. Rats injected with heroin (2 mg/kg s.c.) displayed an increased ASR when tested 4 h after heroin treatment. A similar increase in ASR was found in rats 10-20 h into withdrawal from extended access (12 h) to i.v. heroin self-administration, a model that captures several aspects of heroin addiction in humans. Both the α 2 adrenergic receptor agonist clonidine (10 μg/kg s.c.) and CRF1 receptor antagonist N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo[1,5-a] pyrimidin-7-amine (MPZP; 20 mg/kg s.c.) blocked heroin withdrawal-potentiated startle. To investigate the relationship between CRF1 and α 2 adrenergic receptors in the potentiation of the ASR, we tested the effect of MPZP on yohimbine (1.25 mg/kg s.c.)-potentiated startle and clonidine on CRF (2 μg i.c.v.)-potentiated startle. Clonidine blocked CRF-potentiated startle, whereas MPZP partially attenuated but did not reverse yohimbine-potentiated startle, suggesting that CRF may drive NE release to potentiate startle. These results suggest that CRF1 and α 2 receptors play an important role in the heightened anxiety-like behaviour observed during acute withdrawal from heroin, possibly via CRF inducing the release of NE in stress-related brain regions.

Desmopressin accelerates the rate of urinary morphine excretion and attenuates withdrawal symptoms in rats

AIM

The aim of this study was to examine the effects of desmopressin on morphine withdrawal symptoms and vasopressin level in morphine-dependent subjects.

METHODS

Wistar male rats were injected s.c. with morphine once per day for 5 consecutive days to induce morphine dependence. After morphine use ceased on day 5, an equal number of rats were assigned to one of four groups for either saline or desmopressin by either intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) injection. From days 5 to 10, urine was collected daily and tested for the presence of morphine, and withdrawal symptoms were monitored to assess the effects of desmopressin.

RESULTS

Significant weight loss occurred among all morphine-addicted rats during the withdrawal period. With both methods (i.p. and i.c.v.), the period of urinary morphine excretion was shorter for the two groups that were given desmopressin (experimental groups) than the two groups that were not given desmopressin (control groups), and no significant difference in urinary morphine excretion was found between the two experimental groups. During the early stage of withdrawal, the severity of the withdrawal symptoms in the experimental groups was significantly lower than that in the control groups.

CONCLUSION

Desmopressin decreases the extent of morphine withdrawal symptoms, indicating that this agent might be appropriate for treating morphine addiction. Desmopressin appears to reduce withdrawal symptoms not by exerting an anti-diuretic effect but rather by exerting an effect on the central nervous system.

A polyamine-deficient diet opposes hyperalgesia, tolerance and the increased anxiety-like behaviour associated with heroin withdrawal in rats

In humans, hyperalgesia, tolerance and anxiety disorders are common symptoms during heroin withdrawal syndrome. Significant evidence supports a role of NMDA receptors in these phenomena. Because polyamines may positively modulate the functioning of NMDA receptors and mainly originate from dietary intake, one hypothesis is that a polyamine deficient diet (PD diet) may reduce withdrawal symptoms. To address this question, we investigated the ability of a PD diet to prevent or to alleviate some symptoms of withdrawal syndrome as hyperalgesia, and increased anxiety-like behaviour in rats receiving 14 once daily subcutaneous heroin injections. Here, we show that a PD diet has both preventive and curative properties for reducing certain signs of withdrawal such as hyperalgesia, tolerance and increased anxiety-like behaviour observed in rats fed with a standard diet. Moreover, in heroin-withdrawn rats which were returned to basal pain sensitivity level, hyperalgesia following acute analgesia induced by a single heroin dose was observed in heroin-treated rats fed with standard diet, not in rats fed with a PD diet. Similarly, a stress-induced hyperalgesia induced by a non-nociceptive environmental stress session was observed in heroin-treated rats fed with standard diet. In contrast, a stress-induced analgesia was observed in heroin-treated rats fed with a PD diet, as it was observed in non heroin-treated rats. Since a PD diet for several weeks did not induce appreciable side-effects in rats, these preclinical results suggest that a PD diet could be an effective strategy for improving the relief of certain negative emotional states of heroin withdrawal syndrome and to allow reducing other medications generally used, such as opioid maintenance drugs.

Effects of exogenous cholecystokinin octapeptide on acquisition of naloxone precipitated withdrawal induced conditioned place aversion in rats

Cholecystokinin octapeptide (CCK-8), a gut-brain peptide, regulates a variety of physiological behavioral processes. Previously, we reported that exogenous CCK-8 attenuated morphine-induced conditioned place preference, but the possible effects of CCK-8 on aversively motivated drug seeking remained unclear. To investigate the effects of endogenous and exogenous CCK on negative components of morphine withdrawal, we evaluated the effects of CCK receptor antagonists and CCK-8 on the naloxone-precipitated withdrawal-induced conditioned place aversion (CPA). The results showed that CCK2 receptor antagonist (LY-288,513, 10 µg, i.c.v.), but not CCK1 receptor antagonist (L-364,718, 10 µg, i.c.v.), inhibited the acquisition of CPA when given prior to naloxone (0.3 mg/kg) administration in morphine-dependent rats. Similarly, CCK-8 (0.1–1 µg, i.c.v.) significantly attenuated naloxone-precipitated withdrawal-induced CPA, and this inhibitory function was blocked by co-injection with L-364,718. Microinjection of L-364,718, LY-288,513 or CCK-8 to saline pretreated rats produced neither a conditioned preference nor aversion, and the induction of CPA by CCK-8 itself after morphine pretreatments was not significant. Our study identifies a different role of CCK1 and CCK2 receptors in negative affective components of morphine abstinence and an inhibitory effect of exogenous CCK-8 on naloxone-precipitated withdrawal-induced CPA via CCK1 receptor.

Protracted manifestations of acute dependence after a single morphine exposure

RATIONALE

Acute opiate exposure produces a state of dependence in humans and animals, which is revealed by signs and symptoms of withdrawal precipitated by opioid receptor antagonists. The physiological changes that underlie this state of acute dependence develop rapidly and can persist long after the end of chronic opiate exposure.

OBJECTIVES

The purpose of this investigation was to determine the persistence of acute dependence after a single morphine exposure in rodents, focusing on changes in behavior thought to reflect the negative emotional consequences of withdrawal.

METHODS

The acoustic startle reflex and conditioned place aversion were measured following naloxone administration at different time points after a single morphine exposure.

RESULTS

Naloxone administration produced significant potentiation of acoustic startle-a form of anxiety-like behavior-for at least 80 days after one exposure to morphine. In contrast, naloxone produced a conditioned place aversion 24 h but not 20 days after one morphine exposure.

CONCLUSIONS

Together with existing literature, these results suggest acute as well as chronic opiate exposure leave rodents persistently vulnerable to express anxiety-like behavior in response to opioid receptor antagonists or stressful experience. The adaptations in brain function that underlie this protracted state of dependence may provide a foundation for the escalation of withdrawal severity that develops over repeated opiate exposure, and increase the likelihood of progression from casual drug use to compulsive drug abuse.

The effect of pituitary adenylate cyclase-activating polypeptide on elevated plus maze behavior and hypothermia induced by morphine withdrawal

The aim of the present investigation was to study the effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on morphine withdrawal-induced behavioral changes and hypothermia in male CFLP mice. Elevated plus maze (EPM) and jump tests were used to assess naloxone-precipitated morphine withdrawal-induced behavior responses. Different doses of subcutaneous (s.c.) naloxone, (0.1 and 0.2 mg/kg, respectively) were used to precipitate the emotional and psychical aspects of withdrawal on EPM and 1 mg/kg (s.c.) was used to induce the somatic withdrawal signs such as jumping, and the changes in body temperature. In our EPM studies, naloxone proved to be anxiolytic in mice treated with morphine. Chronic intracerebroventricular (i.c.v.) administration of PACAP alone had no significant effect on withdrawal-induced anxiolysis and total activity at doses of 500 ng and 1 μg. At dose of 500 ng, however, PACAP significantly counteracted the reduced motor activity in the EPM test in mice treated with morphine and diminished the hypothermia and shortened jump latency induced by naloxone in mice treated with morphine. These findings indicate that anxiolytic-like behavior may be mediated via a PACAP-involved pathway and PACAP may play an important role in chronic morphine withdrawal-induced hypothermia as well.

Measuring the incentive value of escalating doses of heroin in heroin-dependent Fischer rats during acute spontaneous withdrawal

RATIONALE/OBJECTIVES

Although continued heroin use and relapse are thought to be motivated, in part, by the positive incentive-motivational value attributed to heroin, little is understood about heroin's incentive value during the relapse-prone state of withdrawal. This study uses place preference to measure the incentive value attributed to escalating-dose heroin in the context of heroin dependence.

METHODS

Male Fischer rats were exposed chronically to escalating doses of heroin in the homecage and during place preference conditioning sessions. Conditioned preference for the context paired with escalating-dose heroin was tested after homecage exposure was discontinued and rats entered acute spontaneous withdrawal. Individuals' behavioral and locomotor responses to heroin and somatic withdrawal signs were recorded.

RESULTS

Conditioned preference for the heroin-paired context was strong in rats that received chronic homecage exposure to escalating-dose heroin and were tested in acute withdrawal. Behavioral responses to heroin (e.g., stereotypy) varied widely across individuals, with rats that expressed stronger heroin preference also expressing stronger behavioral activation in response to heroin. Individual differences in preference were also related to locomotor responses to heroin but not to overt somatic withdrawal signs.

CONCLUSIONS

Escalating doses of heroin evoked place preference in rats, suggesting that positive incentive-motivational value is attributed to this clinically relevant pattern of drug exposure. This study offers an improved preclinical model for studying dependence and withdrawal and provides insight into individual vulnerabilities to addiction-like behavior.

Opioids and anxiety

The opioid system plays a crucial role in the neural modulation of anxiety. The involvement of opioid ligands and receptors in physiological and dysfunctional forms of anxiety is supported by findings from a wide range of preclinical and clinical studies, including clinical trials, experimental research, and neuroimaging, genetic, and epidemiological data. In this review we provide a summary of studies from a variety of research disciplines to elucidate the role of the opioid system in the neurobiology of anxiety. First, we report data from preclinical studies using animal models to examine the modulatory role of central opioid system on defensive responses conducive to fear and anxiety. Second, we summarize the human literature providing evidence that clinical and experimental human studies are consistent with preclinical models. The implication of these data is that activation of the opioid system leads to anxiolytic responses both in healthy subjects and in patients suffering from anxiety disorders. The role of opioids in suppressing anxiety may serve as an adaptive mechanism, collocated in the general framework of opioid neurotransmission blunting acute negative and distressing affective responses.

Anxiety profile in morphine-dependent and withdrawn rats: effect of voluntary exercise

Withdrawal from chronic opiates is associated with an increase in anxiogenic-like behaviours, but the anxiety profile in the morphine-dependent animals is not clear. Thus, one of the aims of the present study was to examine whether morphine-dependent rats would increase the expression of anxiogenic-like behaviours in novel and stressful conditions. Additionally, recent studies have shown that voluntary exercise can reduce anxiety levels in rodents. Therefore, another aim of this study was to examine the effect of voluntary exercise on the anxiety profile in both morphine-dependent animals and animals experiencing withdrawal. Rats were injected with bi-daily doses (10 mg/kg, at 12 h intervals) of morphine over a period of 10 days in which they were also allowed voluntary exercise. Following these injections, anxiety-like behaviours were tested in the elevated plus-maze (EPM) model and the light/dark (L/D) box. We found reductions in time spent in, and entries into, the EPM open arms and reductions in time spent in the lit side of the L/D box for both sedentary morphine-dependent and withdrawn rats as compared to the sedentary control groups. The exercising morphine-dependent and withdrawn rats exhibited an increase in EPM open arm time and entries and L/D box lit side time as compared with the sedentary control groups. We conclude that voluntary exercise decreases the severity of the anxiogenic-like behaviours in both morphine-dependent and withdrawn rats. Thus, voluntary exercise could be a potential natural method to ameliorate some of the deleterious behavioural consequences of opiate abuse.

Pharmacological modulation of anxiety-like phenotypes in adult zebrafish behavioral models

Zebrafish (Danio rerio) are becoming increasingly popular in neurobehavioral research. Here, we summarize recent data on behavioral responses of adult zebrafish to a wide spectrum of putative anxiolytic and anxiogenic agents. Using the novel tank test as a sensitive and efficient behavioral assay, zebrafish anxiety-like behavior can be bi-directionally modulated by drugs affecting the gamma-aminobutyric acid, monoaminergic, cholinergic, glutamatergic and opioidergic systems. Complementing human and rodent data, zebrafish drug-evoked phenotypes obtained in this test support this species as a useful model for neurobehavioral and psychopharmacological research.

Evaluation of antinociceptive effect of Aegiceras corniculatum stems extracts and its possible mechanism of action in rodents

ETHNOPHARMACOLOGICAL RELEVANCE

Aegiceras corniculatum (Linn.) Blanco is used in various traditional medicinal system(s) for the treatment of rheumatism, painful arthritis and inflammation. Therefore, the pharmacological studies of its antinociceptive effect was undertaken to validate its traditional use.

MATERIALS AND METHODS

n-Hexane, ethyl acetate and methanol extract(s) derived from Aegiceras corniculatum (stems) were studied using various nociceptive model(s) induced chemically or thermally in mice including acetic acid-induced writhing, formalin-induced paw licking and hot plate test.

RESULTS

In acetic acid-induced writhing test, plant extracts dose dependently decreased the writhing numbers. The methanolic extract (1-10mg/kg, i.p. in mice) of the plant was more potent than acetaminophen and acetyl salicylic acid, with an IC(50) of 4.2 ± 0.99 mg/kg. Moreover, the time of nociceptive behaviors induced by intraplantar formalin injection was also suppressed during 1st and 2nd phases in the presence of ethyl acetate extract whereas, n-hexane and methanolic extracts inhibited the paw licking in mice during the 1st (IC(50) 12 ± 0.76 mg/kg) and 2nd phases (IC(50) 3.8 ± 0.55 mg/kg). Naloxone, β-funaltrexamine, and naltrindole antagonized the n-hexane extract-induced antinociception in the first phase of formalin test indicating its non-selective analgesic response via opioid receptor(s). However, ethyl acetate extract was devoid of any opioid action. Additionally, these extracts significantly inhibited the pain stimulation in hot plate test. Withdrawal syndrome of morphine dependence was also diminished in the presence of plant extracts via potentiation of GABAergic system.

CONCLUSION

These results suggested that Aegiceras corniculatum extract(s) possesses analgesic properties and acts on the central nervous system, thereby suppressing the inflammatory pain justifying its folklore use.

Withdrawal from repeated administration of morphine alters histamine-induced anxiogenic effects produced by intra-ventral hippocampal microinjection

In the present study, the influence of withdrawal from repeated administration of morphine on intra-ventral hippocampal microinjection of histamine-induced anxiety-like behavior was investigated in male Wistar rats. Three days subcutaneous administration of morphine (5-10 mg/kg) followed by five days free of the drug decreased the percentage open arm time and the percentage open arm entries. Intra-ventral hippocampal administration of histamine (2.5-7.5 microg/rat) decreased percentage open arm time and percentage open arm entries. Intra-ventral hippocampal histamine-induced anxiogenic effect was reversed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal administration of pyrilamine (5-20 microg/rat) or ranitidine (10-40 microg/rat) decreased percentage open arm time and percentage open arm entries. Pyrilamine- or ranitidine-induced anxiogenic effect was not changed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal injections of clobenpropit increased percentage open arm time. The percentage open arm time and percentage open arm entries were decreased in the morphine-treated animals compared with non-morphine-treated controls. Percentage open arm entries and locomotor activity was reduced with some doses of clobenpropit. It can be concluded that the histamine system is involved in anxiety-like behavior, and repeated injections of morphine followed by five days free of the drugs interact with histamine receptor mechanism.

Morphine-induced stereotyped thigmotaxis could appear as enhanced fear and anxiety in some behavioural tests

This study questions whether the classical interpretation for unconditional fear/anxiety tests is valid when animals are under the influence of some drugs of abuse. We used a modified version of the trimethylthiazoline (TMT)-avoidance task, a measure of unconditional fear. Halfway into a corridor maze we placed a 3-cm-high barrier. This provided a wall in the middle of the corridor, one that the mice can easily climb over. Saline- and morphine-treated mice were randomly placed in the 'safe' or 'unsafe' (TMT) side and observed for 10 min. As expected, saline-injected mice spent only about 25% of the time in the TMT side, regardless of the side they were initially placed into. In contrast, morphine-treated mice did not cross the barrier even once, regardless of their initial placement. Specifically, morphine-treated mice initially placed in the TMT side appeared to exhibit the expected reduction in unconditional fear, that is, spending the entire time in the TMT side, a significant increase over the controls. Yet, morphine-treated mice placed in the safe side never even entered the TMT side; thus, these mice appeared to exhibit a behavioural response that is classically interpreted as increased fear, that is, spending significantly less time in the TMT side versus the controls. In summary, this study demonstrates that the classical interpretation of some unconditioned fear or anxiety tests could be misleading when animals are under the influence of drugs that might induce other competing behaviours.

Surgical incision induces anxiety-like behavior and amygdala sensitization: effects of morphine and gabapentin

The role of affective dimension in the postoperative pain is still poorly understood. The present study investigated the development of anxiety-like behavior and amygdala sensitization in incisional pain. Using hind-paw incision model in rats, we showed that surgical incision induced the anxiety-like behavior as determined by elevated plus-maze and open-field tests. Intraperitoneal (IP) morphine administration reversed mechanical allodynia and anxiety-like behavior in a dose-dependent manner. Gabapentin also partially reduced incision-evoked mechanical allodynia and anxiety-like behavior in a dose-dependent manner. After incision, the expression of phosphorylated cAMP response elements (CRE-) binding protein (p-CREB) was transiently upregulated in the central and basolateral nuclei in the bilateral amygdala. The upregulation of p-CREB was inhibited by morphine and gabapentin. The present study suggested that surgical incision could induce anxiety and amygdala sensitization that can be inhibited by morphine and gabapentin. Thus treatment of surgery-induced affective disturbances by morphine and gabapentin may be a potential important adjunct therapy in the postoperative pain management.

Buprenorphine modulates methamphetamine-induced dopamine dynamics in the rat caudate nucleus

Methamphetamine (METH) abuse and addiction present a major problem in the United States and globally. Oxidative stress associated with exposure to METH mediates to the large extent METH-evoked neurotoxicity. While there are currently no medications approved for treating METH addiction, its pharmacology provides opportunities for potential pharmacotherapeutic adjuncts to behavioral therapy in the treatment of METH addiction. Opioid receptor agonists can modulate the activity of dopamine neurons and could, therefore, modify the pharmacodynamic effects of METH in the dopaminergic system. Efficacy of the adjunctive medication with buprenorphine has been demonstrated in the treatment of cocaine addiction extending beyond opiate addiction. We investigated the interactions of morphine (10 mg/kg) and buprenorphine (0.01 and 10 mg/kg) with METH (2 mg/kg) affecting striatal dopaminergic transmission. The extracellular concentration of dopamine (DA) and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were determined using brain microdialysis coupled with high performance liquid chromatography with electrochemical detection (HPLC-ED) in the caudate nucleus of adult, awake, male Sprague-Dawley rats. Compared to METH alone, extracellular DA release was prolonged for 140 min without changes in DA peak-effect by combined treatment with morphine/METH. Morphine did not change DOPAC efflux evoked by METH. On the other hand, both buprenorphine doses attenuated the METH-induced DA peak-effect. However, whereas high buprenorphine dose extended DA outflow for 190 min, the low-dose abbreviated DA release. High buprenorphine dose also shortened METH-induced decrease in DOPAC efflux. Data confirm that opiates modulate dopaminergic neurotransmission evoked by METH. Alteration of dopaminergic response to METH challenge under buprenorphine may suggest effectiveness of buprenorphine treatment in METH addiction.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Dansyl-PQRamide, a putative antagonist of NPFF receptors, reduces anxiety-like behavior of ethanol withdrawal in a plus-maze test in rats

Much evidence indicates that endogenous opioid peptides are involved in effects caused by ethanol. The aim of the present study was to determine whether dansyl-PQR amide, a putative antagonist of receptors for an anti-opioid peptide-neuropeptide FF (NPFF) could affect anxiety-like behavior measured during withdrawal from acute-, and chronic ethanol administration in the elevated plus maze test in rats. Our study indicated that intracerebroventricular (i.c.v.) administration of dansyl-PQRamide (2.4 and 4.8 nmol) reversed anxiety-like behavior measured as a percent time spent in the open arms, and a percent open arm entries onto the open arms in the elevated plus-maze test in rats. These effects were inhibited by NPFF (10 and/or 20 nmol, i.c.v.) in the experiments performed during withdrawal from acute- and chronic ethanol administration. During withdrawal from acute ethanol, naloxone (1mg/kg, i.p.), a nonselective opioid receptor antagonist, attenuated only an increased percent time spent in the open arms induced by dansyl-PQR amide (4.8 nmol). Dansyl-PQR amide, NPFF and naloxone given alone to naive rats did not have influence on spontaneous locomotor activity of animals. Furthermore, NPFF potentiated anxiety-like behavior during withdrawal from chronic, but not acute, ethanol administration in rats. Our data suggest that NPFF system is involved in regulation of affective symptoms of ethanol withdrawal. It seems that involvement of the NPFF system in ethanol withdrawal anxiety-like behavior is associated with regulation of the opioid system activity.

GABA and opioid mechanisms of the central amygdala underlie the withdrawal-potentiated startle from acute morphine

Anxiety is an affective symptom common to withdrawal from acute or chronic opiate treatment. Although the potentiation of the acoustic startle reflex has been proposed as an index of increased anxiety, there are variable effects of the opiate withdrawal on the startle reflex in chronic dependence models. On the other hand, withdrawal from acute morphine treatment consistently potentiates the acoustic startle reflex, a response that seems to be mediated by the central nucleus of the amygdala (CeA). However, the underlying neurochemical mechanisms have not been elucidated yet. In the present study, we firstly made a comparison between the effects of the withdrawal from both acute and chronic treatments with morphine on the motor activity and the anxiety-like behavior of rats tested in two experimental models, the acoustic startle reflex and the open-field tests. Our second objective was to investigate the role of GABAergic and opioid mechanisms of the CeA in the modulation of the withdrawal-potentiated startle as a measure of anxiety induced by morphine withdrawal. For the production of chronic dependence, rats received morphine injections (10 mg/kg; s.c.) twice daily during 10 days. Forty-eight hours after the interruption of this treatment, independent groups were probed in the startle reflex and open-field tests. For the acute dependence model, groups of rats were tested in the open field and startle tests under control conditions and under withdrawal from a single injection of morphine (10 mg/kg; s.c.) precipitated by naltrexone injections (0.1 mg/kg; s.c.). The results obtained showed that withdrawal from chronic and acute morphine treatments produced anxiety-like behavior in the open field test, although the anxiogenic-like effects could not be dissociated from the motor effects in the acute dependence model. On the other hand, only the withdrawal from acute morphine treatment significantly potentiated the startle response. Next, we examined the effects of intra-CeA microinjections of muscimol-a GABA(A) receptors agonist-and DAMGO-a mu-opioid receptors agonist-on the potentiated startle induced by acute morphine withdrawal. The results obtained showed that intra-CeA injections of muscimol (1 nmol) and DAMGO (0.5 and 1 nmol) significantly inhibited this response. These findings suggest that the acute dependence model is more suitable to study the aversive effects of morphine withdrawal on the acoustic startle response than the chronic opiate dependence model. Besides, mechanisms mediated by mu- and GABA(A)-receptors in the CeA appear to exert an inhibitory influence on the anxiety-like behavior induced by withdrawal from acute morphine treatment.