Mouse strain differences in locomotor, sensitisation and rewarding effect of heroin; association with alterations in MOP-r activation and dopamine transporter binding

Resveratrol by elevating the SIRT1 BDNF, GDNF and PSD95 levels reduce heroin addiction related behaviors

OBJECTIVE

To study the effects of resveratrol on heroin addiction-related behaviors and to preliminarily explore the possible intervention mechanism of resveratrol in heroin dependence.

METHODS

The effects of resveratrol on heroin withdrawal symptoms were observed by naloxone; The effect of resveratrol on heroin reward memory acquisition was detected by CPP paradigm; The effect of resveratrol on the mental excitability of heroin was tested by open field experiment; The effect of resveratrol on heroin spatial learning and memory was tested by water maze test. Western blot was used to detect Sirtuin 1 (SIRT1) Expression of brain-derived neurotrophic factor (BDNF), glial cell derived neurotrophic factor (GDNF), and postsynaptic density protein (PSD95).

RESULTS

The behavioral results showed that the withdrawal behavior of the resveratrol intervention group was reduced compared with the heroin chronic dependence group (P<0.05), and the shift score of the conditioned place preference test of the resveratrol intervention group was reduced compared with the heroin chronic dependence group (P<0.05) The spatial learning and memory ability of the water maze in the resveratrol intervention group was improved compared with the heroin chronic dependence group (P<0.05), and the mental excitability of the resveratrol intervention group was lower than that of the heroin chronic dependence group (P<0.05), but higher than that of the saline group (P<0.05); SIRT1 The expression levels of BDNF, GDNF and PSD95 protein were significantly increased (P<0.05).

CONCLUSION

The behavioral results of this study suggest that resveratrol can be used as a potential drug to treat heroin dependence. At the same time, SIRT1 The expression of BDNF, GDNF, and PSD95 increased; SIRT1, BDNF, GDNF, and PSD95 play an essential role in heroin addiction.

Inhibition of SIRT1 in the nucleus accumbens attenuates heroin addiction-related behavior by decreasing D1 neuronal autophagy

This study aimed to investigate the effects of SIRT1 modulation on heroin addiction-like behavior and its possible biological mechanisms. Wild-type C57BL/6J and Sirt1loxp/loxp D1-Cre mice were used in this experiment, and Sirt1 loxp/loxp D1-Cre(-) mice were used as a control for conditional knockout mice. Mice were divided into saline control and heroin-dependent groups. Behavioral methods were used to record the withdrawal response, conditioned place preference (CPP) changes, and open field test results. Transmission electron microscopy (TEM) was used to observe the structure of autophagosomes in nucleus accumbens (NAc) neurons. The expression of SIRT1 and autophagy-related proteins and genes, such as LC3Ⅱ, ATG5 , and ATG7 , was detected in the NAc of each mouse group via western blot, real-time quantitative PCR (qPCR) analyzes, and immunofluorescence. The results of this experiment showed that compared with the saline group, mice in the wild-type heroin-dependent group showed marked withdrawal symptoms, with more autophagosomes observed in NAc via TEM. Compared with wild-type and Sirt1loxp/loxp D1-Cre(-) heroin-dependent groups, CPP formation was found to be reduced in the conditional knockout mouse group, with a significant decrease in spontaneous activity. Western blot, qPCR, and immunofluorescence results indicated that the expression of LC3Ⅱ, ATG-5, and ATG-7 was significantly reduced in the NAc of the Sirt1loxp/loxp D1-Cre(+) group. It was still, however, higher than that in the saline control group. These results suggest that inhibition of Sirt1 expression may prevent heroin-induced addiction-related behaviors via reducing D1 neuronal autophagy.

The effect of chronic stress on the immunogenicity and immunoprotection of the M6-TT vaccine in female mice

Active vaccination is an effective therapeutic option to reduce the reinforcing effects of opioids. Several studies showed that chronic stress affects the immune system decreasing the efficiency of some vaccines. Heroin withdrawal is a stressor and it is a stage in which the patient who abuses heroin is vulnerable to stress affects the immune response and consequently its immunoprotective capacity, then, the objective was to determine the effect of heroin-withdrawal and heroin-withdrawal plus immobilization, on the immune (immunogenicity) and protective response (behavioral response) of morphine-6-hemisuccinate-tetanus toxoid (M6-TT) vaccine in animals of two inbred mice strains with different sensitivity to drug-opioid and stress. Female BALB/c and C57Bl/6 inbred mice were immunized with the M6-TT. A solid-phase antibody-capture ELISA was used to monitor antibody titer responses after each booster dose in vaccinated animals. During the vaccination period, the animals were subjected to two different stress conditions: drug-withdrawal (DW) and immobilization (IMM). The study used tail-flick testing to evaluate the heroin-induced antinociceptive effects. Additionally, heroin-induced locomotor activity was evaluated. Stress decreased the heroin-specific antibody titer generated by the M6-TT vaccine in the two inbred mouse strains evaluated. In the two stress conditions, the antibody titer was not able to decrease the heroin-induced antinociceptive effects and locomotor activity. These findings suggest that stress decreases the production of antibodies and the immunoprotective capacity of the M6-TT vaccine. This observation is important to determine the efficacy of active vaccination as a potential therapy for patients with opioid drug use disorder, since these patients during drug-withdrawal present stress disorders, which could affect the efficacy of therapy such as active vaccination.

P-glycoprotein (MDR1/ABCB1) Restricts Brain Penetration of the Main Active Heroin Metabolites 6-monoacetylmorphine (6-MAM) and Morphine in Mice

BACKGROUND & PURPOSE

Heroin (diacetylmorphine; diamorphine) is a highly addictive opioid prodrug. Heroin prescription is possible in some countries for chronic, treatment-refractory opioid-dependent patients and as a potent analgesic for specific indications. We aimed to study the pharmacokinetic interactions of heroin and its main pharmacodynamically active metabolites, 6-monoacetylmorphine (6-MAM) and morphine, with the multidrug efflux transporters P-glycoprotein/ABCB1 and BCRP/ABCG2 using wild-type, Abcb1a/1b and Abcb1a/1b;Abcg2 knockout mice.

METHODS & RESULTS

Upon subcutaneous (s.c.) heroin administration, its blood levels decreased quickly, making it challenging to detect heroin even shortly after dosing. 6-MAM was the predominant active metabolite present in blood and most tissues. At 10 and 30 min after heroin administration, 6-MAM and morphine brain accumulation were increased about 2-fold when mouse (m)Abcb1a/1b and mAbcg2 were ablated. Fifteen minutes after direct s.c. administration of an equimolar dose of 6-MAM, we observed good intrinsic brain penetration of 6-MAM in wild-type mice. Still, mAbcb1 limited brain accumulation of 6-MAM and morphine without affecting their blood exposure, and possibly mediated their direct intestinal excretion. A minor contribution of mAbcg2 to these effects could not be excluded.

CONCLUSIONS

We show that mAbcb1a/1b can limit 6-MAM and morphine brain exposure. Pharmacodynamic behavioral/postural observations, while non-quantitative, supported moderately increased brain levels of 6-MAM and morphine in the knockout mouse strains. Variation in ABCB1 activity due to genetic polymorphisms or environmental factors (e.g., drug interactions) might affect 6-MAM/morphine exposure in individuals, but only to a limited extent.

Hyperactivity in Mice Induced by Opioid Agonists with Partial Intrinsic Efficacy and Biased Agonism Administered Alone and in Combination with Morphine

Opioid analgesics such as morphine and fentanyl induce mu-opioid receptor (MOR)-mediated hyperactivity in mice. Herein, we show that morphine, fentanyl, SR-17018, and oliceridine have submaximal intrinsic efficacy in the mouse striatum using ³⁵S-GTPγS binding assays. While all of the agonists act as partial agonists for stimulating G protein coupling in striatum, morphine, fentanyl, and oliceridine are fully efficacious in stimulating locomotor activity; meanwhile, the noncompetitive biased agonists SR-17018 and SR-15099 produce submaximal hyperactivity. Moreover, the combination of SR-17018 and morphine attenuates hyperactivity while antinociceptive efficacy is increased. The combination of oliceridine with morphine increases hyperactivity, which is maintained over time. These findings provide evidence that noncompetitive agonists at MOR can be used to suppress morphine-induced hyperactivity while enhancing antinociceptive efficacy; moreover, they demonstrate that intrinsic efficacy measured at the receptor level is not directly proportional to drug efficacy in the locomotor activity assay.

Role of Efficacy as a Determinant of Locomotor Activation by Mu Opioid Receptor Ligands in Female and Male Mice

Mu opioid receptor (MOR) agonists produce locomotor hyperactivity in mice as one sign of opioid-induced motor disruption. The goal of this study was to evaluate the degree of MOR efficacy required to produce this hyperactivity. Full dose-effect curves were determined for locomotor activation produced in male and female ICR mice by (1) eight different single-molecule opioids with high to low MOR efficacy, and (2) a series of fixed-proportion fentanyl/naltrexone mixtures with high to low fentanyl proportions. Data from the mixtures were used to quantify the efficacy requirement for MOR agonist-induced hyperactivity relative to efficacy requirements determined previously for other MOR agonist effects. Specifically, efficacy requirement was quantified as the EP₅₀ value, which is the "Effective Proportion" of fentanyl in a fentanyl/naltrexone mixture that produces a maximal effect equal to 50% of the maximal effect of fentanyl alone. Maximal hyperactivity produced by each drug and mixture in the present study correlated with previously published data for maximal stimulation of GTPɣS binding in MOR-expressing Chinese hamster ovary cells as an in vitro measure of relative efficacy. Additionally, the EP₅₀ value for hyperactivity induced by fentanyl/naltrexone mixtures indicated that opioid-induced hyperactivity in mice has a relatively high efficacy requirement in comparison to some other MOR agonist effects, and in particular is higher than the efficacy requirement for thermal antinociception in mice or fentanyl discrimination in rats. Taken together, these data show that MOR agonist-induced hyperactivity in mice is efficacy dependent and requires relatively high levels of MOR agonist efficacy for its full expression. Significance Statement Mu opioid receptor (MOR) agonist-induced hyperlocomotion in mice is dependent on the MOR efficacy of the agonist and requires a relatively high degree of efficacy for its full expression.

The role of 6-acetylmorphine in heroin-induced reward and locomotor sensitization in mice

We have previously demonstrated that heroin's first metabolite, 6-acetylmorphine (6-AM), is an important mediator of heroin's acute effects. However, the significance of 6-AM to the rewarding properties of heroin still remains unknown. The present study therefore aimed to examine the contribution of 6-AM to heroin-induced reward and locomotor sensitization. Mice were tested for conditioned place preference (CPP) induced by equimolar doses of heroin or 6-AM (1.25-5 μmol/kg). Psychomotor activity was recorded during the CPP conditioning sessions for assessment of drug-induced locomotor sensitization. The contribution of 6-AM to heroin reward and locomotor sensitization was further examined by pretreating mice with a 6-AM specific antibody (anti-6-AM mAb) 24 hours prior to the CPP procedure. Both heroin and 6-AM induced CPP in mice, but heroin generated twice as high CPP scores compared with 6-AM. Locomotor sensitization was expressed after repeated exposure to 2.5 and 5 μmol/kg heroin or 6-AM, but not after 1.25 μmol/kg, and we found no correlation between the expression of CPP and the magnitude of locomotor sensitization for either opioid. Pretreatment with anti-6-AM mAb suppressed both heroin-induced and 6-AM-induced CPP and locomotor sensitization. These findings provide evidence that 6-AM is essential for the rewarding and sensitizing properties of heroin; however, heroin caused stronger reward compared with 6-AM. This may be explained by the higher lipophilicity of heroin, providing more efficient drug transfer to the brain, ensuring rapid increase in the brain 6-AM concentration.

Human Self-Domestication and the Extended Evolutionary Synthesis of Addiction: How Humans Evolved a Unique Vulnerability

Humans are more vulnerable to addiction in comparison to all other mammals, including nonhuman primates, yet there is a lack of research addressing this. This paper reviews the field of comparative addiction neuroscience, highlighting the significant inter-species variation in the mesocortical dopaminergic and other neuromodulatory systems involved in addiction. Artificial selection gives rise to significant changes in neuroanatomy, neurophysiology and behaviour as shown in certain rodent strains and other domesticated animals. These changes occur over a few generations, relatively short periods of time in evolutionary terms, and demonstrate how dynamic these neuromodulatory systems are in response to the environment. During the course of human evolution, traits crucial to our survival, expansion and domination (traits such as the ability to innovate, adapt to different environments and thrive in a civilization) have been positively selected for, yet also predispose humans to addiction. This is evident in our unique neurochemistry and receptor-drug activation potencies. Examples of these are provided as possible targets for precision medicine.

Opioid Neurobiology, Neurogenetics and Neuropharmacology in Zebrafish

Despite the high prevalence of medicinal use and abuse of opioids, their neurobiology and mechanisms of action are not fully understood. Experimental (animal) models are critical for improving our understanding of opioid effects in vivo. As zebrafish (Danio rerio) are increasingly utilized as a powerful model organism in neuroscience research, mounting evidence suggests these fish as a useful tool to study opioid neurobiology. Here, we discuss the zebrafish opioid system with specific focus on opioid gene expression, existing genetic models, as well as its pharmacological and developmental regulation. As many human brain diseases involve pain and aberrant reward, we also summarize zebrafish models relevant to opioid regulation of pain and addiction, including evidence of functional interplay between the opioid system and central dopaminergic and other neurotransmitter mechanisms. Additionally, we critically evaluate the limitations of zebrafish models for translational opioid research and emphasize their developing utility for improving our understanding of evolutionarily conserved mechanisms of pain-related, addictive, affective and other behaviors, as well as for fostering opioid-related drug discovery.

The role of striatal Gαq/11 protein in methamphetamine-induced behavioral sensitization in mice

Gα_q/11 protein transduces signals from neurotransmitter receptors and has been implicated in several functions of the central nervous system. In this study, the role of Gα_q/11 protein in methamphetamine (METH)-induced behavioral sensitization was investigated using neurochemical and behavioral approaches. Repeated treatment with METH (2mg/kg, intraperitoneally) significantly increased behavioral sensitization as well as Gα_q/11 protein expression and Gα protein activity in the striata of mice, while a single treatment of METH at the same dose did not affect these parameters. Repeated intrastriatal injections of a Gα_q/11 inhibitor, [D-Trp7,9,10]-substance P, significantly reduced behavioral sensitization and striatal dopamine (DA) level in response to METH, with no effect on striatal tyrosine hydroxylase expression. These results suggest that Gα_q/11 protein facilitates METH-induced behavioral sensitization by modulating DA release in the mouse striatum.

Mu Opioid Receptors in Gamma-Aminobutyric Acidergic Forebrain Neurons Moderate Motivation for Heroin and Palatable Food

BACKGROUND

Mu opioid receptors (MORs) are central to pain control, drug reward, and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate the contribution of MORs expressed in gamma-aminobutyric acidergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.

METHODS

We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in gamma-aminobutyric acidergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology, and microdialysis; probed neuronal activation by c-Fos immunohistochemistry and resting-state functional magnetic resonance imaging; and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.

RESULTS

Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area, local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, and neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.

CONCLUSIONS

Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus, beyond a well-established role in reward processing, operating at the level of local ventral tegmental area neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.

Locomotor activity: A distinctive index in morphine self-administration in rats

Self-administration of addictive drugs is a widely used tool for studying behavioral, neurobiological, and genetic factors in addiction. However, how locomotor activity is affected during self-administration of addictive drugs has not been extensively studied. In our present study, we tested the locomotor activity levels during acquisition, extinction and reinstatement of morphine self-administration in rats. We found that compared with saline self-administration (SA), rats that trained with morphine SA had higher locomotor activity. Rats that successfully acquired SA also showed higher locomotor activity than rats that failed in acquiring SA. Moreover, locomotor activity was correlated with the number of drug infusions but not with the number of inactive pokes. We also tested the locomotor activity in the extinction and the morphine-primed reinstatement session. Interestingly, we found that in the first extinction session, although the number of active pokes did not change, the locomotor activity was significantly lower than in the last acquisition session, and this decrease can be maintained for at least six days. Finally, morphine priming enhanced the locomotor activity during the reinstatement test, regardless of if the active pokes were significantly increased or not. Our results clearly suggest that locomotor activity, which may reflect the pharmacological effects of morphine, is different from drug seeking behavior and is a distinctive index in drug self-administration.

A critical role of striatal A2A R-mGlu5 R interactions in modulating the psychomotor and drug-seeking effects of methamphetamine

Addiction to psychostimulants is a major public health problem with no available treatment. Adenosine A2A receptors (A2A R) co-localize with metabotropic glutamate 5 receptors (mGlu5 R) in the striatum and functionally interact to modulate behaviours induced by addictive substances, such as alcohol. Using genetic and pharmacological antagonism of A2A R in mice, we investigated whether A2A R-mGlu5 R interaction can regulate the locomotor, stereotypic and drug-seeking effect of methamphetamine and cocaine, two drugs that exhibit distinct mechanism of action. Genetic deletion of A2A R, as well as combined administration of sub-threshold doses of the selective A2A R antagonist (SCH 58261, 0.01 mg/kg, i.p.) with the mGlu5 R antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (0.01 mg/kg, i.p.), prevented methamphetamine- but not cocaine-induced hyperactivity and stereotypic rearing behaviour. This drug combination also prevented methamphetamine-rewarding effects in a conditioned-place preference paradigm. Moreover, mGlu5 R binding was reduced in the nucleus accumbens core of A2A R knockout (KO) mice supporting an interaction between these receptors in a brain region crucial in mediating addiction processes. Chronic methamphetamine, but not cocaine administration, resulted in a significant increase in striatal mGlu5 R binding in wild-type mice, which was absent in the A2A R KO mice. These data are in support of a critical role of striatal A2A R-mGlu5 R functional interaction in mediating the ambulatory, stereotypic and reinforcing effects of methamphetamine but not cocaine-induced hyperlocomotion or stereotypy. The present study highlights a distinct and selective mechanistic role for this receptor interaction in regulating methamphetamine-induced behaviours and suggests that combined antagonism of A2A R and mGlu5 R may represent a novel therapy for methamphetamine addiction.

Dissociation of heroin-induced emotional dysfunction from psychomotor activation and physical dependence among inbred mouse strains

RATIONALE

Opiate addiction is a brain disorder emerging through repeated intoxication and withdrawal episodes. Epidemiological studies also indicate that chronic exposure to opiates may lead in susceptible individuals to the emergence of depressive symptoms, strongly contributing to the severity and chronicity of addiction. We recently established a mouse model of heroin abstinence, characterized by the development of depressive-like behaviors following chronic heroin exposure.

OBJECTIVES

While genetic factors regulating immediate behavioral responses to opiates have been largely investigated, little is known about their contribution to long-term emotional regulation during abstinence. Here, we compared locomotor stimulation and physical dependence induced by heroin exposure, as well as emotional dysfunction following abstinence, across mice strains with distinct genetic backgrounds.

METHODS

Mice from three inbred strains (C57BL/6J, Balb/cByJ, and 129S2/SvPas) were exposed to an escalating chronic heroin regimen (10-50 mg/kg). Independent cohorts were used to assess drug-induced locomotor activity during chronic treatment, naloxone-precipitated withdrawal at the end of chronic treatment, and emotional-like responses after a 4-week abstinence period.

RESULTS

Distinct behavioral profiles were observed across strains during heroin treatment, with no physical dependence and low locomotor stimulation in 129S2/SvPas. In addition, different behavioral impairments developed during abstinence across the three strains, with increased despair-like behavior in 129S2/SvPas and Balb/cByJ, and low sociability in 129S2/SvPas and C57BL/6J.

CONCLUSIONS

Our results indicate that depressive-like behaviors emerge during heroin abstinence, whatever the severity of immediate behavioral responses to the drug. In addition, inbred mouse strains will allow studying several aspects of mood-related deficits associated with addiction.

Fentanyl effects on breath generation in C57BL/6J and A/J mouse strains

We examined the effect of fentanyl on chemoresponsiveness in mouse strains divergent in the expression of spontaneous and post-hypoxic pauses. Frequency and tidal volume were recorded with plethysmography in A/J and C57BL/6J (B6) male mice. Mice selected at random received an intraperitoneal (IP) injection of either saline, low dose fentanyl (LDF = 0.04 mg/kg), or high dose fentanyl (HDF = 0.4 mg/kg) under hypoxia (8% O2) or hyperoxia (100%O2). LDF produced a decrease in frequency during hypoxia in B6, but not A/J, mice. HDF significantly decreased frequency and tidal volume in both strains under hypoxia and hyperoxia (p<0.01); naloxone, an opioid antagonist, reversed this response. The acute administration of fentanyl at any dose did not promote apneas in strains of mice exhibiting regular or irregular respiratory patterns. However, higher doses depressed respiratory frequency in both strains. The B6 mice responded with a depressive response to hypoxia that did not recover with reoxygenation, but did recover with time or naloxone.

Temporal and anatomic patterns of immediate-early gene expression in the forebrain of C57BL/6 and DBA/2 mice after morphine administration

Although morphine was previously reported to produce an instant induction of c-fos in the striatum, our recent studies have demonstrated that the expression of numerous immediate early genes (IEGs) is significantly elevated at delayed time-points (several hours) after morphine administration. To better dissect the time-course of opioid-produced IEG induction, we used in situ hybridization to examine the expression of the IEGs c-fos, zif268 and arc in the mouse forebrain at several time-points after acute morphine injection. To link drug-produced behavioral changes with the activity of specific neuronal complexes, this study was performed comparatively in the C57BL/6 and DBA/2 mouse strains, which differ markedly in their locomotor responses to opioids and opioid reward. Our study demonstrates that morphine produces two episodes of IEG induction, which are separate in time (30 min vs. 4-6 h) and which have different neuroanatomic distribution. At 30 min, one or more IEGs were induced in circumscribed subregions of the dorsal striatum (dStr) and of the nucleus accumbens (NAc) shell, as well as in the lateral septum. The observed inter-strain differences in IEG expression at 30 min support earlier proposals that activation of the dorsomedial striatum may mediate morphine-elicited locomotor stimulation (both effects were present only in the C57BL/6 strain). In contrast, NAc shell activation does not appear to be linked to morphine-elicited changes in locomotor behavior. The second IEG induction (of arc and of zif268) was more widespread, involving most of the dStr and the cortex. The second IEG induction peaked earlier in the DBA/2 mice than in the C57BL/6 mice (4 h compared with 6 h) and displayed no apparent relation to locomotor behavior. This delayed episode of IEG activation, which has largely been overlooked thus far, may contribute to the development of long-term effects of opioids such as tolerance, dependence and/or addiction.

Epigenetically modified nucleotides in chronic heroin and cocaine treated mice

Epigenetic changes include the addition of a methyl group to the 5' carbon of the cytosine ring, known as DNA methylation, which results in the generation of the fifth DNA base, namely 5-methylcytosine. During active or passive demethylation, an intermediate modified base is formed, 5-hydroxymethylcytosine. We have currently quantified 5-methylcytosine and 5-hydroxymethylcytosine in the liver and brain of mice treated with cocaine or heroin, using liquid chromatography/tandem mass spectrometry (LC-MS/MS). Our results show that global 5-methylcytosine levels are not affected by heroin or cocaine administration, neither in the liver nor in the brain. However, 5-hydroxymethylcytosine levels are reduced in the liver following cocaine administration, while they are not affected by cocaine in the brain or by heroin administration in the liver and the brain. Elucidation of the epigenetic phenomena that takes place with respect to drug abuse and addiction, via quantitative analysis of different modified bases, may enable a better understanding of the underlying mechanisms and may lead to more personalized and effective treatment options.

The oxytocin analogue carbetocin prevents emotional impairment and stress-induced reinstatement of opioid-seeking in morphine-abstinent mice

The main challenge in treating opioid addicts is to maintain abstinence due to the affective consequences associated with withdrawal which may trigger relapse. Emerging evidence suggests a role of the neurohypophysial peptide oxytocin (OT) in the modulation of mood disorders as well as drug addiction. However, its involvement in the emotional consequences of drug abstinence remains unclear. We investigated the effect of 7-day opioid abstinence on the oxytocinergic system and assessed the effect of the OT analogue carbetocin (CBT) on the emotional consequences of opioid abstinence, as well as relapse. Male C57BL/6J mice were treated with a chronic escalating-dose morphine regimen (20-100 mg/kg/day, i.p.). Seven days withdrawal from this administration paradigm induced a decrease of hypothalamic OT levels and a concomitant increase of oxytocin receptor (OTR) binding in the lateral septum and amygdala. Although no physical withdrawal symptoms or alterations in the plasma corticosterone levels were observed after 7 days of abstinence, mice exhibited increased anxiety-like and depressive-like behaviors and impaired sociability. CBT (6.4 mg/kg, i.p.) attenuated the observed negative emotional consequences of opioid withdrawal. Furthermore, in the conditioned place preference paradigm with 10 mg/kg morphine conditioning, CBT (6.4 mg/kg, i.p.) was able to prevent the stress-induced reinstatement to morphine-seeking following extinction. Overall, our results suggest that alterations of the oxytocinergic system contribute to the mechanisms underlying anxiety, depression, and social deficits observed during opioid abstinence. This study also highlights the oxytocinergic system as a target for developing pharmacotherapy for the treatment of emotional impairment associated with abstinence and thereby prevention of relapse.

The behavioral effects of acute Δ⁹-tetrahydrocannabinol and heroin (diacetylmorphine) exposure in adult zebrafish

The use of psychotropic drugs in clinical and translational brain research continues to grow, and the need for novel experimental models and screens is becoming widely recognized. Mounting evidence supports the utility of zebrafish (Danio rerio) for studying various pharmacological manipulations, as an alternative model complementing the existing rodent paradigms in this field. Here, we explore the effects of acute 20-min exposure to two commonly abused psychotropic compounds, Δ(9)-tetrahydrocannabinol (THC) and heroin, on adult zebrafish behavior in the novel tank test. Overall, THC administration (30 and 50 mg/L) produces an anxiogenic-like reduction of top swimming, paralleled with a slower, continuous bottom swimming. In contrast, heroin exposure (15 and 25 mg/L) evoked a hyperlocomotor response (with rapid bouts of bottom swimming and frequent 'bouncing' motions) without altering anxiety-sensitive top/bottom endpoints. The behavioral effects of these two compounds in zebrafish seem to parallel the respective rodent and human findings. Collectively, this emphasizes the growing significance of novel emerging aquatic models in translational drug abuse research and small molecule screening.

Tolerance and sensitization to chronic escalating dose heroin following extended withdrawal in Fischer rats: possible role of mu-opioid receptors

RATIONALE/OBJECTIVES

Heroin addiction is characterized by recurrent cycles of drug use, abstinence, and relapse. It is likely that neurobiological changes during chronic heroin exposure persist across withdrawal and impact behavioral responses to re-exposure. We hypothesized that, after extended withdrawal, heroin-withdrawn rats would express behavioral tolerance and/or sensitization in response to heroin re-exposure and that these responses might be associated with altered mu-opioid receptor (MOPr) activity.

METHODS

Male Fischer rats were exposed chronically to escalating doses of heroin (7.5-75 mg/kg/day), experienced acute spontaneous withdrawal and extended (10-day) abstinence, and were re-exposed chronically to heroin. Homecage behaviors and locomotor activity in response to heroin, as well as somatic withdrawal signs, were recorded. Separate groups of rats were sacrificed after extended abstinence and MOPr expression and G-protein coupling were analyzed using [(3)H]DAMGO and [(35)S]GTPγS assays.

RESULTS

The depth of behavioral stupor was lower during the initial days of heroin re-exposure compared to the initial days of the first exposure period. Behavioral responses (e.g., stereotypy) and locomotion were elevated in response to heroin re-exposure at low doses. Rats conditioned for heroin place preference during the chronic re-exposure period expressed heroin preference during acute withdrawal; this preference was stronger than rats conditioned during chronic heroin exposure that followed chronic saline and injection-free periods. Extended withdrawal was associated with increased MOPr expression in the caudate-putamen and frontal and cingulate cortices. No changes in G-protein coupling were identified.

CONCLUSIONS

Aspects of tolerance/sensitization to heroin are present even after extended abstinence and may be associated with altered MOPr density.

The endogenous opioid system in cocaine addiction: what lessons have opioid peptide and receptor knockout mice taught us?

Cocaine addiction has become a major concern in the UK as Britain tops the European 'league table' for cocaine abuse. Despite its devastating health and socio-economic consequences, no effective pharmacotherapy for treating cocaine addiction is available. Identifying neurochemical changes induced by repeated drug exposure is critical not only for understanding the transition from recreational drug use towards compulsive drug abuse but also for the development of novel targets for the treatment of the disease and especially for relapse prevention. This article focuses on the effects of chronic cocaine exposure and withdrawal on each of the endogenous opioid peptides and receptors in rodent models. In addition, we review the studies that utilized opioid peptide or receptor knockout mice in order to identify and/or clarify the role of different components of the opioid system in cocaine-addictive behaviours and in cocaine-induced alterations of brain neurochemistry. The review of these studies indicates a region-specific activation of the µ-opioid receptor system following chronic cocaine exposure, which may contribute towards the rewarding effect of the drug and possibly towards cocaine craving during withdrawal followed by relapse. Cocaine also causes a region-specific activation of the κ-opioid receptor/dynorphin system, which may antagonize the rewarding effect of the drug, and at the same time, contribute to the stress-inducing properties of the drug and the triggering of relapse. These conclusions have important implications for the development of effective pharmacotherapy for the treatment of cocaine addiction and the prevention of relapse.

Csnk1e is a genetic regulator of sensitivity to psychostimulants and opioids

Csnk1e, the gene encoding casein kinase 1-epsilon, has been implicated in sensitivity to amphetamines. Additionally, a polymorphism in CSNK1E was associated with heroin addiction, suggesting that this gene may also affect opioid sensitivity. In this study, we first conducted genome-wide quantitative trait locus (QTL) mapping of methamphetamine (MA)-induced locomotor activity in C57BL/6J (B6) × DBA/2J (D2)-F(2) mice and a more highly recombinant F(8) advanced intercross line. We identified a QTL on chromosome 15 that contained Csnk1e (63-86 Mb; Csnk1e=79.25 Mb). We replicated this result and further narrowed the locus using B6.D2(Csnk1e) and D2.B6(Csnk1e) reciprocal congenic lines (78-86.8 and 78.7-81.6 Mb, respectively). This locus also affected sensitivity to the μ-opioid receptor agonist fentanyl. Next, we directly tested the hypothesis that Csnk1e is a genetic regulator of sensitivity to psychostimulants and opioids. Mice harboring a null allele of Csnk1e showed an increase in locomotor activity following MA administration. Consistent with this result, coadministration of a selective pharmacological inhibitor of Csnk1e (PF-4800567) increased the locomotor stimulant response to both MA and fentanyl. These results show that a narrow genetic locus that contains Csnk1e is associated with differences in sensitivity to MA and fentanyl. Furthermore, gene knockout and selective pharmacological inhibition of Csnk1e define its role as a negative regulator of sensitivity to psychostimulants and opioids.

Comparison of the performance of DBA/2 and C57BL/6 mice in transitive inference and foreground and background contextual fear conditioning

DBA/2 mice have altered hippocampal structure and perform poorly in several hippocampus-dependent contextual/spatial learning tasks. The performance of this strain in higher cognitive tasks is less studied. Transitive inference is a hippocampus-dependent task that requires an abstraction to be made from prior rules to form a new decision matrix; performance of DBA/2 mice in this task is unknown, whereas contextual fear conditioning is a hippocampus-dependent task in which DBA/2 mice have deficits. The present study compared DBA/2J and C57BL/6J inbred mice in two different contextual fear conditioning paradigms and transitive inference to test whether similar deficits are seen across these hippocampus-dependent tasks. For background fear conditioning, mice were trained with two paired presentations of an auditory conditioned stimulus (CS, 30 seconds, 85 dB white noise) paired with an unconditioned stimulus (US, 2 seconds, 0.57 mA footshock), the context was a continuous background CS. Mice were tested for contextual learning 24 hours later. Foreground fear conditioning differed in that no auditory CS was presented. For transitive inference, separate mice were trained to acquire a series of overlapping odor discrimination problems and tested with novel odor pairings that either did or did not require the use of transitive inference. DBA/2 mice performed significantly worse than the C57BL/6 in both foreground and background fear conditioning and transitive inference. These results demonstrate that the DBA/2 mice have deficits in higher-cognitive processes and suggest that similar substrates may underlie deficits in contextual learning and transitive inference.

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.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 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 (Section 16); and immunological responses (Section 17).

[(35)S]GTPγS binding and opioid tolerance and efficacy in mouse spinal cord

The present study examined efficacy of a series of opioid agonists and then using chronic in vivo treatment protocols, determined tolerance to opioid agonist stimulated [(35)S]GTPγS (guanosine 5'-O-(3-[(35)S] thio)triphosphate) binding in mouse spinal cord membranes and compared it directly to spinal analgesic tolerance. The [(35)S]GTPγS binding assay was used to estimate efficacy (E(max) and τ; Operational Model of Agonism) of a series of opioid agonists for G-protein activation in mouse spinal cord. The rank order of opioid agonist efficacy determined in the [(35)S]GTPγS assay using the Operational Model and E(max) was similar. These efficacy estimates correlated with historical analgesic efficacy estimates. For tolerance studies, mice were continuously treated s.c. for 7days with morphine, oxycodone, hydromorphone, etorphine or fentanyl and [(35)S]GTPγS studies were conducted in spinal cord membranes. Other mice were tested in i.t. analgesia dose response studies (tailflick). Tolerance to DAMGO ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin) or morphine stimulated [(35)S]GTPγS binding (decrease in E(max)) was observed following etorphine and fentanyl treatment only. These treatment protocols downregulate μ-opioid receptor density whereas morphine, oxycodone and hydromorphone do not. Spinal analgesic tolerance was observed following all treatment protocols examined (morphine, oxycodone and etorphine). Opioid antagonist treatment that specifically upregulates (chronic naltrexone) or downregulates (clocinnamox) μ-opioid receptor density produced a corresponding change in opioid agonist stimulated [(35)S]GTPγS binding. Although receptor downregulation and G-protein uncoupling are among potential mechanisms of opioid tolerance, the present results suggest that uncoupling in mouse spinal cord plays a minor role and that the [(35)S]GTPγS assay is particularly responsive to changes in μ-opioid receptor density.

Regional mRNA expression of the endogenous opioid and dopaminergic systems in brains of C57BL/6J and 129P3/J mice: strain and heroin effects

We have previously shown strain and dose differences in heroin-induced behavior, reward and regional expression of somatostatin receptor mRNAs in C57BL/6J and 129P3/J mice. Using Real Time PCR we examined the effects of five doses of heroin on the levels of the transcripts of endogenous opioid peptides and their receptors and dopaminergic receptors in the mesocorticolimbic and nigrostriatal pathways in these same mice. Compared to C57BL/6J animals, 129P3/J mice had higher mRNA levels of Oprk1 in the nucleus accumbens and of Oprd1 in the nucleus accumbens and a region containing both the substantia nigra and ventral tegmental area (SN/VTA). In the cortex of 129P3/J mice, lower levels of both Oprk1 and Oprd1 mRNAs were observed. Pdyn mRNA was also lower in the caudate putamen of 129P3/J mice. Strain differences were not found in the levels of Oprm1, Penk or Pomc mRNAs in any region examined. Within strains, complex patterns of heroin dose-dependent changes in the levels of Oprm1, Oprk1 and Oprd1 mRNAs were observed in the SN/VTA. Additionally, Oprd1 mRNA was dose-dependently elevated in the hypothalamus. Also in the hypothalamus, we found higher levels of Drd1a mRNA in C57BL/6J mice than in 129P3/J animals and higher levels of DAT (Slc6a3) mRNA in the caudate putamen of C57BL/6J animals than in 129P3/J counterparts. Heroin had dose-related effects on Drd1a mRNA in the hypothalamus and on Drd2 mRNA in the caudate putamen.