Genetic determinants of morphine activity and thermal responses in 15 inbred mouse strains

Preconception opioids interact with mouse strain to alter morphine withdrawal in the next generation

RATIONALE

Transgenerational effects of preconception morphine exposure in female rats have been reported which suggest that epigenetic modifications triggered by female opioid exposure, even when that exposure ends several weeks prior to pregnancy, has significant ramifications for their future offspring.

OBJECTIVE

The current study compares two mouse strains with well-established genetic variation in their response to mu opioid receptor agonists, C57BL/6J (BL6) and 129S1/svlmJ (129) to determine whether genetic background modifies the impact of preconception opioid exposure.

METHODS

Adolescent females from both strains were injected daily with morphine for a total of 10 days using an increasing dosing regimen with controls receiving saline. Several weeks after their final injection, aged-matched BL6 and 129 morphine (Mor-F0) or saline (Sal-F0) females were mated with drug naïve males to generate Mor-F1 and Sal-F1 offspring, respectively. As adults, F1 mice were made morphine dependent using thrice daily morphine injections for 4 days. On day 5, mice were administered either saline or morphine followed 3 h later by naloxone. Behavioral and physiological signs of withdrawal were then measured.

RESULTS

Regardless of strain or sex, morphine-dependent Mor-F1 mice had significantly lower levels of withdrawal-induced corticosterone but significantly higher glucose levels when compared to Sal-F1 controls. In contrast, both strain- and preconception opioid exposure effects on physical signs of morphine dependence were observed.

Neuroimaging of opioid exposure: a review of preclinical animal models to inform addiction research

Opioid use results in thousands of overdose deaths each year. To address this crisis, we need a better understanding of the neurobiological mechanisms that drive opioid abuse. The noninvasive imaging tools positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and manganese-enhanced magnetic resonance imaging (MEMRI) can be used to identify how brain activity responds to acute opioid exposure and adapts to chronic drug treatment. These techniques can be performed in humans and animal models, and brain networks identified in animals closely map to the human brain. Animal models have the advantage of being able to systematically examine the independent effects of opioid exposure in a controlled environment accounting for the complex factors that drive opioid misuse in humans. This review synthesizes literature that utilized noninvasive neuroimaging tools (PET, fMRI, and MEMRI) measuring brain activity correlates in animals to understand the neurobiological consequences of exposure to abused opioids. A PubMed search in September 2023 identified 25 publications. These manuscripts were divided into 4 categories based on the route and duration of drug exposure (acute/chronic, active/passive administration). Within each category, the results were generally consistent across drug and imaging protocols. These papers cover a 20-year range and highlight the advancements in neuroimaging methodology during that time. These advances have enabled researchers to achieve greater resolution of brain regions altered by opioid exposure and to identify patterns of brain activation across regions (i.e., functional connectivity) and within subregions of structures. After describing the existing literature, we suggest areas where additional research is needed.

Profiling the Effects of Repetitive Morphine Administration on Motor Behavior in Rats

Efficient repetitive clinical use of morphine is limited by its numerous side effects, whereas analgesic tolerance necessitates subsequent increases in morphine dose to achieve adequate levels of analgesia. While many studies focused on analgesic tolerance, the effect of morphine dosing on non-analgesic effects has been overlooked. This study aimed to characterize morphine-induced behavior and the development and progression of morphine-induced behavioral tolerance. Adult male Sprague-Dawley rats were repetitively treated with subcutaneous morphine for 14 days in two dose groups (A: 5 mg/kg/day (b.i.d.) → 10 mg/kg/day; B: 10 mg/kg/day (b.i.d.) → 20 mg/kg/day). Motor behavior was assessed daily (distance traveled, speed, moving time, rearing, rotation) in an open-field arena, before and 30 min post-injections. Antinociception was measured using tail-flick and hot-plate assays. All measured parameters were highly suppressed in both dosing groups on the first treatment day, followed by a gradual manifestation of behavioral tolerance as the treatment progressed. Animals in the high-dose group showed increased locomotor activity after 10 days of morphine treatment. This excitatory phase converted to an inhibition of behavior when a higher morphine dose was introduced. We suggest that the excitatory locomotor effects of repetitive high-dose morphine exposure represent a signature of its behavioral and antinociceptive tolerance.

Adolescent Stress Reduces Adult Morphine-Induced Behavioral Sensitization in C57BL/6J Mice

Deaths related to opioid use have skyrocketed in the United States, leading to a public health epidemic. Research has shown that both biological (genes) and environmental (stress) precursors are linked to opioid use. In particular, stress during adolescence-a critical period of frontal lobe development-influences the likelihood of abusing drugs. However, little is known about the biological mechanisms through which adolescent stress leads to long-term risk of opioid use, or whether genetic background moderates this response. Male and female C57BL/6J and BALB/cJ mice were exposed to chronic variable social stress (CVSS) or control conditions throughout adolescence and then tested for morphine locomotor sensitization or morphine consumption in adulthood. To examine possible mechanisms that underlie stress-induced changes in morphine behaviors, we assessed physiological changes in response to acute stress exposure and prefrontal cortex (PFC) miRNA gene expression. Adolescent stress did not influence morphine sensitization or consumption in BALB/cJ animals, and there was limited evidence of stress effects in female C57BL/6J mice. In contrast, male C57BL/6J mice exposed to adolescent CVSS had blunted morphine sensitization compared to control animals; no differences were observed in the acute locomotor response to morphine administration or morphine consumption. Physiologically, C57BL/6J mice exposed to CVSS had an attenuated corticosterone recovery following an acute stressor and downregulation of twelve miRNA in the PFC compared to control mice. The specificity of the effects for C57BL/6J vs. BALB/cJ mice provides evidence of a gene-environment interaction influencing opioid behaviors. However, this conclusion is dampened by limited locomotor sensitization observed in BALB/cJ mice. It remains possible that results may differ to other doses of morphine or other behavioral responses. Long-term differences in stress reactivity or miRNA expression in C57BL/6J mice suggests two possible biological mechanisms to evaluate in future research.

Effects of Three Consecutive Days of Morphine or Methadone Administration on Analgesia and Open-Field Activity in Mice with Ehrlich Carcinoma

This study assessed the exploratory behavioral responses in BALB/c mice inoculated with Ehrlich ascitic carcinoma after 3 consecutive days of treatment with morphine or methadone. Fifty-three female mice, 60 ± 10 d old, were used. Seven days after intraperitoneal tumor inoculation (2 × 10⁶ cells), the animals were randomized into 7 groups: morphine 5 mg/kg (MO₅), morphine 7.5 mg/kg (MO_7.5), morphine 10 mg/kg (MO₁₀), methadone 2.85 mg/kg (ME_2.85), methadone 4.3 mg/kg (ME_4.3), methadone 5.7 mg/kg (ME_5.7), and 0.9% NaCl (Saline) (n = 7). Drug treatments were administered subcutaneously every 6 h for 3 d. The animals were evaluated for analgesia using the mouse grimace scale (MGS) and for general activity using the open field test. The MGS was performed before tumor inoculation (day 0), on day 7 at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 at 40, 150, 240, and 360 min after drug injection. The open field test was performed before tumor inoculation (day 0), on day 7 after inoculation at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 after inoculation at 40, 150, and 360 min after drug injection. MGS results indicated that administration of morphine promoted analgesia for up to 240 min. Conversely, methadone reduced MGS scores only at 40 min. All tested doses promoted a significant dose-dependent increase in the total distance traveled and the average speed, and increase that was markedly pronounced on days 8 and 9 as compared with day 7. The frequencies of rearing and self-grooming decreased significantly after morphine or methadone administration. Despite the difference in analgesia, both drugs increased locomotion and reduced the frequency of rearing and self-grooming as compared with the untreated control animals.

Opioid withdrawal behavior in spiny mice: A novel preclinical model of neonatal opioid withdrawal syndrome (NOWS)

As the opioid epidemic continues to grow, opioid use among pregnant women is increasing significantly. This has led to a steady rise in the number of infants born with neonatal opioid withdrawal syndrome (NOWS). Although short-term withdrawal symptoms associated with NOWS are well characterized, there are many gaps in our understanding of the short and long-term effects of prenatal opioid exposure. Current animal models of NOWS are limited by shortened gestational periods, large litter sizes, and primary organogenesis occurring after birth. This often leads to postnatal treatment to mimic drug exposure during third-trimester development. Using the unique rodent species Acomys cahirinus, more commonly known as spiny mice, which have an extended 40-day gestation period, small litter sizes, and increased in utero organogenesis we aim to study the short-term effects of prenatal morphine exposure by assessing withdrawal behavior. To model maternal opioid use, dams were treated daily with morphine (10 and 30 mg/kg S.C.) beginning on gestation day 19 until the day of birth; this resulted in a cumulative exposure of 19-21 days. Withdrawal behaviors for each pup were recorded daily between postnatal days 0-7 (PND 0-7). Our study found that prenatal morphine exposure in spiny mice led to an increase in withdrawal behavior throughout the early postnatal period and validated the use of this species as a novel pre-clinical model of NOWS. We are hopeful this rodent model will further our understanding of the short and long-term consequences of prenatal opioid exposure on neurodevelopment and behavior.

The bivalent ligand, MMG22, reduces neuropathic pain after nerve injury without the side effects of traditional opioids

ABSTRACT

Functional interactions between the mu opioid receptor (MOR) and the metabotropic glutamate receptor 5 (mGluR5) in pain and analgesia have been well established. MMG22 is a bivalent ligand containing MOR agonist (oxymorphamine) and mGluR5 antagonist (MPEP) pharmacophores tethered by a 22-atom linker. MMG22 has been shown to produce potent analgesia in several models of chronic inflammatory and neuropathic pain (NP). This study assessed the efficacy of systemic administration of MMG22 at reducing pain behavior in the spared nerve injury (SNI) model of NP in mice, as well as its side-effect profile and abuse potential. MMG22 reduced mechanical hyperalgesia and spontaneous ongoing pain after SNI, with greater potency early (10 days) as compared to late (30 days) after injury. Systemic administration of MMG22 did not induce place preference in naive animals, suggesting absence of abuse liability when compared to traditional opioids. MMG22 also lacked the central locomotor, respiratory, and anxiolytic side effects of its monomeric pharmacophores. Evaluation of mRNA expression showed the transcripts for both receptors were colocalized in cells in the dorsal horn of the lumbar spinal cord and dorsal root ganglia. Thus, MMG22 reduces hyperalgesia after injury in the SNI model of NP without the typical centrally mediated side effects associated with traditional opioids.

Differential genetic risk for methamphetamine intake confers differential sensitivity to the temperature-altering effects of other addictive drugs

Mice selectively bred for high methamphetamine (MA) drinking (MAHDR), compared with mice bred for low MA drinking (MALDR), exhibit greater sensitivity to MA reward and insensitivity to aversive and hypothermic effects of MA. Previous work identified the trace amine-associated receptor 1 gene (Taar1) as a quantitative trait gene for MA intake that also impacts thermal response to MA. All MAHDR mice are homozygous for the mutant Taar1 m1J allele, whereas all MALDR mice possess at least one copy of the reference Taar1 + allele. To determine if their differential sensitivity to MA-induced hypothermia extends to drugs of similar and different classes, we examined sensitivity to the hypothermic effect of the stimulant cocaine, the amphetamine-like substance 3,4-methylenedioxymethamphetamine (MDMA), and the opioid morphine in these lines. The lines did not differ in thermal response to cocaine, only MALDR mice exhibited a hypothermic response to MDMA, and MAHDR mice were more sensitive to the hypothermic effect of morphine than MALDR mice. We speculated that the μ-opioid receptor gene (Oprm1) impacts morphine response, and genotyped the mice tested for morphine-induced hypothermia. We report genetic linkage between Taar1 and Oprm1; MAHDR mice more often inherit the Oprm1 D2 allele and MALDR mice more often inherit the Oprm1 B6 allele. Data from a family of recombinant inbred mouse strains support the influence of Oprm1 genotype, but not Taar1 genotype, on thermal response to morphine. These results nominate Oprm1 as a genetic risk factor for morphine-induced hypothermia, and provide additional evidence for a connection between drug preference and drug thermal response.

In vitro and in vivo pharmacological characterization of the synthetic opioid MT-45

MT-45 is a synthetic opioid that was developed in the 1970s as an analgesic compound. However, in recent years MT-45 has been associated with multiple deaths in Europe and has been included in the class of novel psychoactive substances known as novel synthetic opioids (NSOs). Little is known about the pharmaco-toxicological effects of MT-45. Therefore, we used a dynamic mass redistribution (DMR) assay to investigate the pharmacodynamic profile of this NSO in vitro compared with morphine. We then used in vivo studies to investigate the effect of the acute systemic administration of MT-45 (0.01-15 mg/kg i.p.) on motor and sensorimotor (visual, acoustic and tactile) responses, mechanical and thermal analgesia, muscle strength and body temperature in CD-1 male mice. Higher doses of MT-45 (6-30 mg/kg i.p.) were used to investigate cardiorespiratory changes (heart rate, respiratory rate, SpO₂ saturation and pulse distention). All effects of MT-45 were compared with those of morphine. In vitro DMR assay results demonstrated that at human recombinant opioid receptors MT-45 behaves as a potent selective mu agonist with a slightly higher efficacy than morphine. In vivo results showed that MT-45 progressively induces tail elevation at the lowest dose tested (0.01 mg/kg), increased mechanical and thermal antinociception (starting from 1 to 6 mg/kg), decreased visual sensorimotor responses (starting from 3 to 6 mg/kg) and reduced tactile responses, modulated motor performance and induced muscle rigidity at higher doses (15 mg/kg). In addition, at higher doses (15-30 mg/kg) MT-45 impaired the cardiorespiratory functions. All effects were prevented by the administration of the opioid receptor antagonist naloxone. These findings reveal the risks associated with the ingestion of opioids and the importance of studying these drugs and undertaking more clinical studies of the current molecules to better understand possible therapeutic interventions in the case of toxicity.

The Behavioral Effects of the Antidepressant Tianeptine Require the Mu-Opioid Receptor

Depression is a debilitating chronic illness that affects around 350 million people worldwide. Current treatments, such as selective serotonin reuptake inhibitors, are not ideal because only a fraction of patients achieve remission. Tianeptine is an effective antidepressant with a previously unknown mechanism of action. We recently reported that tianeptine is a full agonist at the mu opioid receptor (MOR). Here we demonstrate that the acute and chronic antidepressant-like behavioral effects of tianeptine in mice require MOR. Interestingly, while tianeptine also produces many opiate-like behavioral effects such as analgesia and reward, it does not lead to tolerance or withdrawal. Furthermore, the primary metabolite of tianeptine (MC5), which has a longer half-life, mimics the behavioral effects of tianeptine in a MOR-dependent fashion. These results point to the possibility that MOR and its downstream signaling cascades may be novel targets for antidepressant drug development.

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.

Differential expressions of the alternatively spliced variant mRNAs of the µ opioid receptor gene, OPRM1, in brain regions of four inbred mouse strains

The µ opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing in rodents and humans, with dozens of alternatively spliced variants of the OPRM1 gene. The present studies establish a SYBR green quantitative PCR (qPCR) assay to more accurately quantify mouse OPRM1 splice variant mRNAs. Using these qPCR assays, we examined the expression of OPRM1 splice variant mRNAs in selected brain regions of four inbred mouse strains displaying differences in µ opioid-induced tolerance and physical dependence: C56BL/6J, 129P3/J, SJL/J and SWR/J. The complete mRNA expression profiles of the OPRM1 splice variants reveal marked differences of the variant mRNA expression among the brain regions in each mouse strain, suggesting region-specific alternative splicing of the OPRM1 gene. The expression of many variants was also strain-specific, implying a genetic influence on OPRM1 alternative splicing. The expression levels of a number of the variant mRNAs in certain brain regions appear to correlate with strain sensitivities to morphine analgesia, tolerance and physical dependence in four mouse strains.

The heritability of oxycodone reward and concomitant phenotypes in a LG/J × SM/J mouse advanced intercross line

The rewarding property of opioids likely contributes to their abuse potential. Therefore, determining the genetic basis of opioid reward could aid in understanding the neurobiological mechanisms of opioid addiction, provided that it is a heritable trait. Here, we characterized the rewarding property of the widely abused prescription opioid oxycodone (OXY) in the conditioned place preference (CPP) assay using LG/J and SM/J parental inbred mouse strains and 17 parent-offspring families of a LG/J × SM/J F47 /F48 advanced intercross line (AIL). Following OXY training (5 mg/kg, i.p.), SM/J mice and AIL mice, but not LG/J mice, showed an increase in preference for the OXY-paired side, suggesting a genetic basis for OXY-CPP. SM/J mice showed greater locomotor activity than LG/J mice in response to both saline and OXY. LG/J, SM/J, and AIL mice all exhibited robust OXY-induced locomotor sensitization. Narrow-sense heritability (h(2) ) estimates of the phenotypes using linear regression and maximum likelihood estimation showed good agreement (r = 0.91). OXY-CPP was clearly not a heritable trait whereas drug-free- and OXY-induced locomotor activity and sensitization were significantly and sometimes highly heritable (h(2)  = 0.30-0.84). Interestingly, the number of transitions between the saline- and OXY-paired sides emerged as a reliably heritable trait following OXY training (h(2)  = 0.46-0.66) and could represent a genetic component of drug-seeking behavior. Thus, although OXY-CPP does not appear to be amenable to genome-wide quantitative trait locus mapping, this protocol will be useful for mapping other traits potentially relevant to opioid abuse.

Juvenile ethanol exposure increases rewarding properties of cocaine and morphine in adult DBA/2J mice

Convergent data showed that ethanol exposure during adolescence can alter durably ethanol-related behaviour at adulthood. However, the consequences of juvenile ethanol exposure on the reinforcing effects of other drugs of abuse remain unclear. In the present work, we evaluated in adult male DBA/2J mice the effects of early ethanol exposure on the sensitivity to the incentive effects of cocaine and morphine, and on extracellular signal-regulated kinase (ERK) activation in response to cocaine. Juvenile male mice received intragastric administration of ethanol (2×2.5g/kg/day) or water for 5 days starting on postnatal day 28. When reaching adult age (10 week-old), animals were subjected to an unbiased procedure to assess conditioned place preference (CPP) to cocaine or morphine. In addition, activation of ERK in response to an acute injection of cocaine was investigated using immunoblotting in the striatum and the nucleus accumbens. Mice that have been subjected to early ethanol exposure developed CPP to doses of cocaine (5mg/kg) or morphine (10mg/kg) below the threshold doses to induce CPP in water pre-exposed mice. In addition, early ethanol administration significantly increased striatal ERK phosphorylation normally induced by acute cocaine (10 and 20mg/kg) in adult mice. These results show that, in DBA/2J mice, early exposure to ethanol enhanced the perception of the incentive effects of cocaine and morphine. Ethanol pre-exposure also induced a positive modulation of striatal ERK signalling, in line with the inference that juvenile ethanol intake may contribute to the development of addictive behaviour at adult age.

The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K⁺ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.

GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

Behavioural and anatomical characterization of mutant mice with targeted deletion of D1 dopamine receptor-expressing cells: response to acute morphine

Considerable topographic overlap exists between brain opioidergic and dopaminergic neurons. Pharmacological blockade of the dopamine D(1) receptor (Drd1a) reverses several behavioural phenomena elicited by opioids. The present study examines the effects of morphine in adult mutant (MUT) mice expressing the attenuated diphtheria toxin-176 gene in Drd1a-expressing cells, a mutant line shown previously to undergo post-natal striatal atrophy and loss of Drd1a-expression. MUT and wild-type mice were assessed behaviourally following acute administration of 10 mg/kg morphine. Treatment with morphine reduced locomotion and rearing similarly in both genotypes but reduced total grooming only in MUT mice. Morphine-induced Straub tail and stillness were heightened in MUT mice. Chewing and sifting were decreased in MUT mice and these effects were not modified by morphine. Loss of striatal Drd1-positive cells and up-regulated D(2)-expression, as reflected in down-regulated D(1)-like and up-regulated D(2)-like binding, respectively, is not uniform along the cranio-caudal extent in this model but appears to be greater in the caudal striatum. Preferential caudal loss of µ-opioid-expression, a marker for the striosomal compartment, was seen. These data indicate that Drd1a-positive cell loss modifies the exploratory behavioural response elicited by morphine, unmasking novel morphine-induced MUT-specific behaviours and generating a hypersensitivity to morphine for others.

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.

Social influences on morphine-conditioned place preference in adolescent BALB/cJ and C57BL/6J mice

RATIONALE

Among human adolescents, drug use is substantially influenced by the attitudes and behaviors of peers. Social factors also affect the drug-seeking behaviors of laboratory animals. Conditioned place preference (CPP) experiments indicate that social context can influence the degree to which rodents derive a rewarding experience from drugs of abuse. However, the precise manner by which social factors alter drug reward in adolescent rodents remains unknown.

OBJECTIVES

We employed the relatively asocial BALB/cJ (BALB) mouse strain and the more prosocial C57BL/6J (B6) strain to explore whether "low" or "high" motivation to be with peers influences the effects of social context on morphine CPP (MCPP).

METHODS

Adolescent mice were conditioned by subcutaneous injections of morphine sulfate (0.25, 1.0, or 5.0 mg/kg). During the MCPP procedure, mice were housed in either isolation (Ih) or within a social group (Sh). Similarly, following injection, mice were conditioned either alone (Ic) or within a social group (Sc).

RESULTS

Adolescent B6 mice expressed a robust MCPP response except when subjected to Ih-Sc, which indicates that, following isolation, mice with high levels of social motivation are less susceptible to the rewarding properties of morphine when they are conditioned in a social group. By contrast, MCPP responses of BALB mice were most sensitive to morphine conditioning when subjects experienced a change in their social environment between housing and conditioning (Ih-Sc or Sh-Ic).

CONCLUSIONS

Our findings demonstrate that susceptibility to morphine-induced reward in adolescent mice is moderated by a complex interaction between social context and heritable differences in social motivation.

Morphine-induced motor stimulation, motor incoordination, and hypothermia in adolescent and adult mice

RATIONALE

Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the motor stimulating, ataxic, and hypothermic effects of morphine in adolescent, late adolescent, and adult mice. Plasma and brain levels of morphine were assessed to examine possible pharmacokinetic differences among the age groups.

METHODS

Locomotion was measured as occlusions of horizontal infrared light beams, ataxia as failing the horizontal wire test, body temperature by rectal probe, and morphine levels by HPLC-UV.

RESULTS

Morphine (3.2-56 mg/kg, i.p.) increased locomotion along an inverted U-shaped dose-response curve in adolescent, late adolescent, and adult male C57BL/6J mice. Its potency to stimulate locomotion was similar in all age groups. However, maximal stimulation was higher in adolescents than in late adolescents, and higher in late adolescents than in adults. In contrast, adolescents showed less ataxia than adults when given morphine (5.6-100 mg/kg, i.p.). The hypothermic effects of morphine did not differ among the age groups. Morphine levels, which peaked in plasma at 15 min and in brain at 45 min after i.p. injection, did not show age-related differences.

CONCLUSIONS

The finding that adolescents are not generally more sensitive to morphine than adults, but differ in their sensitivity to effects involving nigrostriatal/mesolimbic dopamine systems, is consistent with evidence of overactivity of these dopamine systems during adolescence relative to adulthood. The age-related differences observed here are unlikely due to pharmacokinetic factors.

Naloxone-precipitated morphine withdrawal behavior and brain IL-1β expression: comparison of different mouse strains

The development of opioid dependence involves classical neuronal opioid receptor activation and is due in part to engagement of glia causing a proinflammatory response. Such opioid-induced glial activation occurs, at least in part, through a non-classical opioid mechanism involving Toll-like-receptor 4 (TLR4). Among the immune factors released following the opioid-glia-TLR4 interaction, interleukin-1β (IL-1β) plays a prominent role. Previous animal behavioral studies have demonstrated significant heterogeneity of chronic morphine-induced tolerance and dependence between different mouse strains. The aim of this study was to investigate whether the heterogeneity of chronic opioid-induced IL-1β expression contributes to differences in opioid tolerance and withdrawal behaviors. Chronic morphine-induced tolerance and dependence were assessed in 3 inbred wild-type mouse strains (Balb/c, CBA, and C57BL/6) and 2 knockout strains (TLR4 and MyD88). Analysis of brain nuclei (medial prefrontal cortex, cortex, brain stem, hippocampus, and midbrain and diencephalon regions combined) revealed that, of inbred wild-type mice, there are significant main effects of morphine treatment on IL-1β expression in the brain regions analyzed (p<0.02 for all regions analyzed). A significant increase in hippocampal IL-1β expression was found in C57BL/6 mice after morphine treatment, whilst, a significant decrease was found in the mPFC region of wild-type Balb/c mice. Furthermore, the results of wild-type inbred strains demonstrated that the elevated hippocampal IL-1β expression is associated with withdrawal jumping behavior. Interestingly, knockout of TLR4, but not MyD88 protected against the development of analgesic tolerance. Gene sequence differences of IL - 1β and TLR4 genes alone did not explain the heterogeneity of dependence behavior between mouse strains. Together, these data further support the involvement of opioid-induced CNS immune signaling in dependence development. Moreover, this study demonstrated the advantages of utilizing multiple mouse strains and indicates that appropriate choice of mouse strains could enhance future research outcomes.

Age-dependent and strain-dependent influences of morphine on mouse social investigation behavior

Opioid-coded neural circuits play a substantial role in how individuals respond to drugs of abuse. Most individuals begin using such drugs during adolescence and within a social context. Several studies indicate that adolescent mice exhibit a heightened sensitivity to the effects of morphine, a prototypical opiate drug, but it is unclear whether these developmental differences are related to aspects of motivated behavior. Moreover, exposure to opioids within the rodent brain can alter the expression of social behavior, yet little is known about whether this relationship changes as a function of development or genetic variation. In this study, we conducted a series of experiments to characterize the relationship between genetic background, adolescent development and morphine-induced changes in mouse social investigation (SI). At two time points during adolescent development [postnatal days (PD) 25 and 45], social interactions of test mice of the gregarious C57BL/6J (B6) strain were more tolerant to the suppressive effects of morphine [effective dose 50 (ED50)=0.97 mg/kg and 2.17 mg/kg morphine, respectively] than test mice from the less social BALB/cJ (BALB) strain (ED50=0.61 mg/kg and 0.91 mg/kg morphine, respectively). By contrast, this strain-dependent difference was not evident among adult mice on PD 90 (ED50=1.07 mg/kg and 1.41 mg/kg morphine for BALB and B6 mice, respectively). An additional experiment showed that the ability of morphine to alter social responsiveness was not directly related to drug-induced changes in locomotor behavior. Finally, administration of morphine to stimulus mice on PD 25 reduced social investigation of test mice only when individuals were from the B6 genetic background. Overall, these results indicate that alterations in endogenous opioid systems are related to changes in SI that occur during adolescence, and that morphine administration may mimic rewarding aspects of social encounter.

Prepulse inhibition of the startle reflex and response to antipsychotic treatments in two outbred mouse strains in comparison to the inbred DBA/2 mouse

RATIONALE

Naturally low prepulse inhibition (PPI) in DBA/2 mice is increased by marketed antipsychotics and compounds acting at novel targets relevant to schizophrenia. Whether other mouse strains with naturally low PPI respond similarly and could be translational models of schizophrenia is unknown.

OBJECTIVE

Baseline levels of PPI were determined in outbred CF-1 and Black Swiss mice. CF-1 and Black Swiss mice were then compared to DBA/2 mice for their responses to typical (haloperidol) and atypical (clozapine) antipsychotics and to compounds with potential antipsychotic activity, a histamine H(3) receptor antagonist (thioperamide) and a glycine transporter-1 inhibitor (SSR504734).

RESULTS

CF-1 and Black Swiss mice had naturally low PPI, similar to the level in C57BL/6 mice, but higher than that in DBA/2 mice. Haloperidol (0.3-1 mg/kg) increased PPI in DBA/2, CF-1, and Black Swiss mice. Clozapine (3 mg/kg) increased PPI in DBA/2 and CF-1 mice, but not in Black Swiss mice. Thioperamide (10-30 mg/kg) and SSR504734 (30 mg/kg) increased PPI only in DBA/2 mice. Strain differences in PPI responsiveness were not due to differences in brain concentrations of the tested compounds.

CONCLUSIONS

CF-1 mice with naturally low PPI may be useful for testing typical and atypical antipsychotics while Black Swiss mice only responded to a typical antipsychotic. DBA/2 mice remain the only strain with naturally low PPI that responds to marketed antipsychotics, as well as to compounds with novel mechanisms of action. Thus, DBA/2 mice may be the strain of choice for screening novel chemical entities for their ability to increase PPI.

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

There is growing agreement that genetic factors play an important role in the risk to develop heroin addiction, and comparisons of heroin addiction vulnerability in inbred strains of mice could provide useful information on the question of individual vulnerability to heroin addiction. This study examined the rewarding and locomotor-stimulating effects of heroin in male C57BL/6J and DBA/2J mice. Heroin induced locomotion and sensitisation in C57BL/6J but not in DBA/2J mice. C57BL/6J mice developed conditioned place preference (CPP) to the highest doses of heroin, while DBA/2J showed CPP to only the lowest heroin doses, indicating a higher sensitivity of DBA/2J mice to the rewarding properties of heroin vs C57BL/6J mice. In order to investigate the neurobiological substrate underlying some of these differences, the effect of chronic 'intermittent' escalating dose heroin administration on the opioid, dopaminergic and stress systems was explored. Twofold higher mu-opioid receptor (MOP-r)-stimulated [35S]GTPgammaS binding was observed in the nucleus accumbens and caudate of saline-treated C57BL/6J mice compared with DBA/2J. Heroin decreased MOP-r density in brain regions of C57BL/6J mice, but not in DBA/2J. A higher density of dopamine transporters (DAT) was observed in nucleus accumbens shell and caudate of heroin-treated DBA/2J mice compared with heroin-treated C57BL/6J. There were no effects on D1 and D2 binding. Chronic heroin administration decreased corticosterone levels in both strains with no effect of strain. These results suggest that genetic differences in MOP-r activation and DAT expression may be responsible for individual differences in vulnerability to heroin addiction.

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.

Forebrain PENK and PDYN gene expression levels in three inbred strains of mice and their relationship to genotype-dependent morphine reward sensitivity

RATIONALE

Vulnerability to drug abuse disorders is determined not only by environmental but also by genetic factors. A body of evidence suggests that endogenous opioid peptide systems may influence rewarding effects of addictive substances, and thus, their individual expression levels may contribute to drug abuse liability.

OBJECTIVES

The aim of our study was to assess whether basal genotype-dependent brain expression of opioid propeptides genes can influence sensitivity to morphine reward.

METHODS

Experiments were performed on inbred mouse strains C57BL/6J, DBA/2J, and SWR/J, which differ markedly in responses to morphine administration: DBA/2J and SWR/J show low and C57BL/6J high sensitivity to opioid reward. Proenkephalin (PENK) and prodynorphin (PDYN) gene expression was measured by in situ hybridization in brain regions implicated in addiction. The influence of the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI), which attenuates effects of endogenous PDYN-derived peptides, on rewarding actions of morphine was studied using the conditioned place preference (CPP) paradigm.

RESULTS

DBA/2J and SWR/J mice showed higher levels of PDYN and lower levels of PENK messenger RNA in the nucleus accumbens than the C57BL/6J strain. Pretreatment with nor-BNI enhanced morphine-induced CPP in the opioid-insensitive DBA/2J and SWR/J strains.

CONCLUSIONS

Our results demonstrate that inter-strain differences in PENK and PDYN genes expression in the nucleus accumbens parallel sensitivity of the selected mouse strains to rewarding effects of morphine. They suggest that high expression of PDYN may protect against drug abuse by limiting drug-produced reward, which may be due to dynorphin-mediated modulation of dopamine release in the nucleus accumbens.

Motivational effects of opiates in conditioned place preference and aversion paradigm--a study in three inbred strains of mice

RATIONALE

Interstrain differences in the motivational properties of morphine and heroin have been previously reported in mice, suggesting the involvement of a genotype-dependent modulation of the rewarding effects of opiates. Yet, interstrain differences in the motivational effects of naloxone have not been described.

OBJECTIVES

The aim of our study was to examine genotype modulation of the motivational effects of opiates in inbred stains of mice with known, distinct, opiate-induced phenotypes, and morphine-induced striatal transcriptional responses.

METHODS

We studied the rewarding properties of morphine (5, 10, and 20 mg/kg i.p.) and heroin (1, 5, and 10 mg/kg i.p.) in conditioned place preference (CPP) as well as the aversive properties of naloxone (1, 10, and 20 mg/kg i.p.) in the conditioned place aversion (CPA) paradigm in C57Bl/6J (C57), DBA/2J (DBA), and SWR/J (SWR) inbred strains of mice.

RESULTS

Our results show that morphine and heroin as well as naloxone induce CPP and CPA, respectively, in a genotype- and dose-dependent manner in these studied inbred strains of mice. Interestingly, C57 mice are the most sensitive in the case of the rewarding properties of morphine and heroin but are the least sensitive to the aversive effects of naloxone, whereas the DBA strain exhibit the opposite behavioral effects.

CONCLUSIONS

We suggest that motivational homeostasis can be modulated by mu opioid receptors in mice, with the C57 mice representing a genotype that is more sensitive to processes related to rewards, whereas the genotype of DBA is more sensitive to aversion.

Genetic correlates of morphine withdrawal in 14 inbred mouse strains

Male mice from 14 standard inbred strains were exposed to morphine in a sustained released preparation injected subcutaneously. Five hours later withdrawal was precipitated by intraperitoneal injection of naloxone. Mice were tested from 0 to 15 min after naloxone for withdrawal jumping behavior, and then from minute 15-16 for other signs, including boli count, presence of soft stool, lacrimation, "wet dog" shakes, and air chewing. They were also assessed for change in body temperature 17 min after naloxone. Strains differed markedly in the severity of withdrawal for jumping, change in body temperature, and number of fecal boli. Strains also differed in percentage of animals displaying soft stool and air chewing behavior. The other two signs were seen at too low frequency for analysis. Correlations of strain mean withdrawal severity with other responses to morphine and other abused drugs showed that high morphine withdrawal jumping and low change in body temperature were both genetically related to high morphine consumption, but not generally to other measures of morphine withdrawal or morphine sensitivity.

Multivariate analyses reveal common and drug-specific genetic influences on responses to four drugs of abuse

Vulnerability to abused drugs is influenced by multiple genes unique to each drug and to risk genes for polydrug abuse. If several inbred mouse strains respond to different drugs similarly, this implies the action of a common group of genes. Simultaneous analysis of multiple responses to multiple drugs has been attempted infrequently. We performed multivariate analyses of published strain responses to four drugs. Genetic similarity in responses did not simply track pharmacological class. Withdrawal severity and preference for ethanol and diazepam were affected by many genes in common, although inversely. We focused on behavioral responses, but there is a growing archival database of physiological, pharmacological and biochemical strain traits. The genomics community is increasingly focusing on single-nucleotide polymorphism and haplotype-based gene mapping approaches, for which inbred strain data are also useful. Thus, similar analyses should be applicable to other laboratories, traits and genotypes.

Morphine effects on striatal transcriptome in mice

Global transcriptional analysis of mouse striata following acute and chronic exposure to morphine reveals multiple physiological factors which may affect opioid-related phenotypes and implicates a number of gene networks, including glucocorticoid receptor regulated genes, in the response to this opioid.

Background

Chronic opiate use produces molecular and cellular adaptations in the nervous system that lead to tolerance, physical dependence, and addiction. Genome-wide comparison of morphine-induced changes in brain transcription of mouse strains with different opioid-related phenotypes provides an opportunity to discover the relationship between gene expression and behavioral response to the drug.

Results

Here, we analyzed the effects of single and repeated morphine administrations in selected inbred mouse strains (129P3/J, DBA/2J, C57BL/6J, and SWR/J). Using microarray-based gene expression profiling in striatum, we found 618 (false discovery rate < 1%) morphine-responsive transcripts. Through ontologic classification, we linked particular sets of genes to biologic functions, including metabolism, transmission of nerve impulse, and cell-cell signaling. We identified numerous novel morphine-regulated genes (for instance, Olig2 and Camk1g), and a number of transcripts with strain-specific changes in expression (for instance, Hspa1a and Fzd2). Moreover, transcriptional activation of a pattern of co-expressed genes (for instance, Tsc22d3 and Nfkbia) was identified as being mediated via the glucocorticoid receptor (GR). Further studies revealed that blockade of the GR altered morphine-induced locomotor activity and development of physical dependence.

Conclusion

Our results indicate that there are differences between strains in the magnitude of transcriptional response to acute morphine treatment and in the degree of tolerance in gene expression observed after chronic morphine treatment. Using whole-genome transcriptional analysis of morphine effects in the striatum, we were able to reveal multiple physiological factors that may influence opioid-related phenotypes and to relate particular gene networks to this complex trait. The results also suggest the possible involvement of GR-regulated genes in mediating behavioral response to morphine.

Effects of mu- and kappa-2 opioid receptor agonists on pain and rearing behaviors

BACKGROUND

Management of pain involves a balance between inhibition of pain and minimization of side effects; therefore, in developing new analgesic compounds, one must consider the effects of treatment on both pain processing and behavior. The purpose of this study was to evaluate the effects of the mu and kappa-2 opioid receptor agonists on general and pain behavioral outcomes.

METHODS

As a general behavioral assessment, we modified the cylinder rearing assay and recorded the number and duration of rearing events. Thermal sensitivity was evaluated using either a reflexive measure of hindpaw withdrawal latency to a radiant heat source or using an orofacial operant thermal assay. Acetic acid-induced visceral pain and capsaicin-induced neurogenic inflammatory pain were used as painful stimuli. The mu-opioid receptor agonist, morphine or the kappa-2 receptor agonist GR89696 was administered 30 min prior to testing. A general linear model repeated measures analysis was completed for baseline session comparisons and an analysis of variance was used to evaluate the effects of treatment on each outcome measure (SPSS Inc). When significant differences were found, post-hoc comparisons were made using the Tukey honestly significant difference test. *P < 0.05 was considered significant in all instances.

RESULTS

We found that morphine and GR89,696 dose-dependently decreased the number of reaching events and rearing duration. Rearing behavior was not affected at 0.5 mg/kg for morphine, 1.25 x 10-4 mg/kg for GR89,696. Hindpaw thermal sensitivity was significantly increased only at the highest doses for each drug. At the highest dose that did not significantly influence rearing behavior, we found that visceral and neurogenic inflammatory pain was not affected following GR89,696 administration and morphine was only partially effective for blocking visceral pain.

CONCLUSION

This study demonstrated that high levels of the opioids produced significant untoward effects and made distinguishing an analgesic versus a more general effect more difficult. Quantification of rearing behavior in conjunction with standard analgesic assays can help in gaining a better appreciation of true analgesic efficacy of experimental drugs.

Supersensitivity to amphetamine in protein kinase-C interacting protein/HINT1 knockout mice

Protein kinase C interacting protein/histidine triad nucleotide binding protein 1 (PKCI/HINT1) is a member of the histidine triad protein family. Although this protein is widely expressed in the mammalian brain including mesocorticolimbic and mesostriatal regions, its physiological function in CNS remains unknown. Recent microarray studies reported decreased mRNA expression of PKCI/HINT1 in the frontal cortex of individuals with schizophrenia, suggesting the possible involvement of this protein in the pathophysiology of the disease. In view of the documented link between dopamine (DA) transmission and schizophrenia, the present study used behavioral and neurochemical approaches to examine the influence of constitutive PKCI/HINT1 deletion upon: (i) basal and amphetamine (AMPH)-evoked locomotor activity; (ii) DA dynamics in the dorsal striatum, and (iii) postsynaptic DA receptor function. PKCI/HINT1(-/-) (KO) mice displayed lower spontaneous locomotion relative to wild-type (WT) controls. Acute AMPH administration significantly increased locomotor activity in WT mice; nonetheless, the effect was enhanced in KO mice. Quantitative microdialysis studies revealed no alteration in basal DA dynamics in the striatum or nucleus accumbens of KO mice. The ability of acute AMPH to increase DA levels was unaltered indicating that function in presynaptic DA neurotransmission in these regions do not underlie the behavioral phenotype of KO mice. In contrast to WT mice, systemic administration of the direct-acting DA receptor agonist apomorphine (10 mg/kg) significantly increased locomotor activity in KO mice suggesting that postsynaptic DA function is altered in these animals. These results demonstrate an important role of PKCI/HINT1 in modulating the behavioral response to AMPH. Furthermore, they indicate that the absence of this protein may be associated with dysregulation of postsynaptic DA transmission.

Transcriptional profiling of C57 and DBA strains of mice in the absence and presence of morphine

Background

The mouse C57BL/6 (C57) and DBA/2J (DBA) inbred strains differ substantially in many aspects of their response to drugs of abuse. The development of microarray analyses represents a genome-wide method for measuring differences across strains, focusing on expression differences. In the current study, we carried out microarray analysis in C57 and DBA mice in the nucleus accumbens of drug-naïve and morphine-treated animals.

Results

We identified mRNAs with altered expression between the two strains. We validated the mRNA expression changes of several such mRNAs, including Gnb1, which has been observed to be regulated by several drugs of abuse. In addition, we validated alterations in the enzyme activity of one mRNA product, catechol-O-methyltransferase (Comt). Data mining of expression and behavioral data indicates that both Gnb1 and Comt expression correlate with aspects of drug response in C57/DBA recombinant inbred strains. Pathway analysis was carried out to identify pathways showing significant alterations as a result of treatment and/or due to strain differences. These analyses identified axon guidance genes, particularly the semaphorins, as showing altered expression in the presence of morphine, and plasticity genes as showing altered expression across strains. Pathway analysis of genes showing strain by treatment interaction suggest that the phosphatidylinositol signaling pathway may represent an important difference between the strains as related to morphine exposure.

Conclusion

mRNAs with differing expression between the two strains could potentially contribute to strain-specific responses to drugs of abuse. One such mRNA is Comt and we hypothesize that altered expression of Comt may represent a potential mechanism for regulating the effect of, and response to, multiple substances of abuse. Similarly, a role for Gnb1 in responses to multiple drugs of abuse is supported by expression data from our study and from other studies. Finally, the data support a role for semaphorin signaling in morphine effects, and indicate that altered expression of genes involved in phosphatidylinositol signaling and plasticity might also affect the altered drug responses in the two strains.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Effects of intracerebroventricularly-injected morphine on anxiety, memory retrieval and locomotor activity in rats: involvement of vasopressinergic system and nitric oxide pathway

Morphine has been shown to alter several behavioural processes. We aimed to investigate the effects of intracerebroventricular (i.c.v.) morphine on anxiety, memory retrieval and locomotor activity in rats and to elucidate the possible involvement of the vasopressinergic system and the nitric oxide (NO) pathway in these effects. Rats were pretreated with morphine (0.5, 5, 50 microg/5 microl; i.c.v.) or saline (5 microl; i.c.v.) 30 min before the elevated plus maze test, the probe trial of the Morris water maze and the open field test. Morphine (5 microg/5 microl; i.c.v.) induced significant anxiolytic effects in the elevated plus maze. None of the doses of morphine produced any effects in the probe trial of the Morris water maze and the open field. Pretreatment with an arginine vasopressin (AVP) V(1) receptor antagonist (25, 125 ng/5 microl; i.c.v.), an AVP V(2) receptor antagonist (25, 125 ng/5 microl; i.c.v.), or L-NAME, an NO synthase inhibitor (5, 25 microg/5 microl; i.c.v.) 30 min before morphine significantly prevented the anxiolytic effects of morphine. These results suggest that i.c.v. morphine has significant anxiolytic effects, probably mediated by both vasopressinergic system and NO pathway, but has no effect on memory retrieval or locomotor activity, at least at the applied doses.

Effects of morphine on circadian rhythms of motor activity and body temperature in pig-tailed macaques

Previous studies of the effects of opiates on motor activity and body temperature in nonhuman primates have been limited in scope and typically only conducted with restrained animals. The present study used radio-telemetry devices to continuously measure activity and temperature in unrestrained pig-tailed macaques for 24 h following morphine administration. Two dose-response functions (0.56 to 5.6 mg/kg, i.m.) were determined, one with morphine administered at 9 a.m. and one with morphine administrated at 3 p.m. Under both the 9 a.m. or 3 p.m. administration schedules, body temperature and activity were increased acutely. Activity was also reduced the following morning after morphine administered at either time. In other regards, morphine's effects on both temperature and activity differed between 9 a.m. and 3 p.m. injection, including periods of decreased activity immediately after the acute increases after 9 a.m. but not 3 p.m. administration. Surprisingly, motor activity also increased 9-12 h post-injection following morphine administered at 9 a.m., but not at 3 p.m. These results clearly show an interaction between timing of morphine administration and effects on temperature and activity. These results also underscore the fact that single injections of drugs may have multiple and delayed effects on circadian rhythms in macaques.

Gene expression profiling in the striatum of inbred mouse strains with distinct opioid-related phenotypes

Background

Mouse strains with a contrasting response to morphine provide a unique model for studying the genetically determined diversity of sensitivity to opioid reward, tolerance and dependence. Four inbred strains selected for this study exhibit the most distinct opioid-related phenotypes. C57BL/6J and DBA/2J mice show remarkable differences in morphine-induced antinociception, self-administration and locomotor activity. 129P3/J mice display low morphine tolerance and dependence in contrast to high sensitivity to precipitated withdrawal observed in SWR/J and C57BL/6J strains. In this study, we attempted to investigate the relationships between genetic background and basal gene expression profile in the striatum, a brain region involved in the mechanism of opioid action.

Results

Gene expression was studied by Affymetrix Mouse Genome 430v2.0 arrays with probes for over 39.000 transcripts. Analysis of variance with the control for false discovery rate (q < 0.01) revealed inter-strain variation in the expression of ~3% of the analyzed transcripts. A combination of three methods of array pre-processing was used to compile a list of ranked transcripts covered by 1528 probe-sets significantly different between the mouse strains under comparison. Using Gene Ontology analysis, over-represented patterns of genes associated with cytoskeleton and involved in synaptic transmission were identified. Differential expression of several genes with relevant neurobiological function (e.g. GABA-A receptor alpha subunits) was validated by quantitative RT-PCR. Analysis of correlations between gene expression and behavioural data revealed connection between the level of mRNA for K homology domain containing, RNA binding, signal transduction associated 1 (Khdrbs1) and ATPase Na+/K+ alpha2 subunit (Atp1a2) with morphine self-administration and analgesic effects, respectively. Finally, the examination of transcript structure demonstrated a possible inter-strain variability of expressed mRNA forms as for example the catechol-O-methyltransferase (Comt) gene.

Conclusion

The presented study led to the recognition of differences in the gene expression that may account for distinct phenotypes. Moreover, results indicate strong contribution of genetic background to differences in gene transcription in the mouse striatum. The genes identified in this work constitute promising candidates for further animal studies and for translational genetic studies in the field of addictive and analgesic properties of opioids.

Alcohol withdrawal severity in inbred mouse (Mus musculus) strains

Male mice (Mus musculus) from 15 standard inbred strains were exposed to a nearly constant concentration of ethanol (EtOH) vapor for 72 hr, averaging 1.59 +/- 0.03 mg EtOH/mL blood at withdrawal. EtOH- and air-exposed groups were tested hourly for handling-induced convulsions for 10 hr and at Hours 24 and 25. Strains differed markedly in the severity of withdrawal (after subtraction of control values), and by design these differences were independent of strain differences in EtOH metabolism. Correlation of strain mean withdrawal severity with other responses to EtOH supported previously reported genetic relationships of high EtOH withdrawal with low drinking, high conditioned taste aversion, low tolerance to EtOH-induced hypothermia, and high stimulated activity after low-dose EtOH. Also supported were the positive genetic correlations among EtOH, barbiturate, and benzodiazepine withdrawal. Sensitivity of naive mice to several chemical convulsant-induced seizures was also correlated with EtOH withdrawal.

Pancreatic cancer pain and its correlation with changes in tumor vasculature, macrophage infiltration, neuronal innervation, body weight and disease progression

To begin to understand the relationship between disease progression and pain in pancreatic cancer, transgenic mice that develop pancreatic cancer due to the expression of the simian virus 40 large T antigen under control of the rat elastase-1 promoter were examined. In these mice precancerous cellular changes were evident at 6 weeks and these included an increase in: microvascular density, macrophages that express nerve growth factor and the density of sensory and sympathetic fibers that innervate the pancreas, with all of these changes increasing with tumor growth. In somatic tissue such as skin, the above changes would be accompanied by significant pain; however, in mice with pancreatic cancer, changes in pain-related behaviors, such as morphine-reversible severe hunching and vocalization only became evident at 16 weeks of age, by which time the pancreatic cancer was highly advanced. These data suggest that in mice as well as humans, there is a stereotypic set of pathological changes that occur as pancreatic cancer develops, and while weight loss generally tracks disease progression, there is a significant lag between disease progression and behaviors indicative of pancreatic cancer pain. Defining the mechanisms that mask this pain in early and mid-stage disease and drive the pain in late-stage disease may aid in earlier diagnosis, survival, and increased quality of life of patients with pancreatic cancer.

Behavioral characterization of morphine effects on motor activity in mice

A biphasic effect of morphine on locomotion has been extensively described. Nevertheless, the effects of this opioid on other behavioral parameters have been overlooked. The aim of the present study was to verify the effects of different doses of morphine on motor behaviors observed in an open-field. Adult female mice were injected with saline or morphine (10, 15 and 20 mg/kg, i.p.) and observed in an open-field for quantification of locomotor and rearing frequencies as well as duration of immobility and grooming. The lowest dose of morphine decreased locomotion (and increased immobility duration) while the highest dose increased it. All doses tested decreased rearing and grooming. Thus, the effects of morphine on locomotion do not parallel to its effects on rearing and grooming. Our results indicate that locomotion not always reflects the effect of drugs on motor activity, which can be better investigated when other behavioral parameters are concomitantly taken into account.

HS014, a selective melanocortin-4 (MC4) receptor antagonist, modulates the behavioral effects of morphine in mice

RATIONALE

Melanocortin and opioid systems regulate feeding as well as other behaviors; however, the relationship between the two systems is not yet defined. Since agonist-induced stimulation of melanocortin receptors blocks the behavioral effects of mu opioid receptor agonists, and melanocortin-4 (MC4) receptors and mu opioid receptors share a similar anatomical distribution in the central nervous system, MC4 receptor blockade may increase opioid responsiveness.

OBJECTIVES

The goal of this study was to test the hypothesis that blockade of MC4 receptors increases the behavioral effects of morphine.

METHODS

The effects of HS014 (0.0032, 0.032, and 1 nmol, i.c.v.), a selective MC4 antagonist, on morphine-induced (3.2, 10, and 32 mg/kg, i.p.) locomotor activity (measured in the open field for 15 min) and antinociception (measured in the hot plate at 55 degrees C) were assessed in C57Bl/6 mice. In addition, the effects of morphine were evaluated in A(y) mice, a genetic model for MC4 receptor blockade.

RESULTS

The dose-effect curve of morphine for locomotor activity was shifted downwards in C57Bl/6 mice pretreated with HS014 and in A(y) mice. The dose-effect curve of morphine for antinociception was shifted two- and threefold to the left in C57Bl/6 mice pretreated with HS014 and in A(y) mice, respectively.

CONCLUSIONS

These results indicate that blockade of MC4 receptors increases the antinociceptive effects of morphine without changing the potency of morphine for locomotor activity, suggesting that MC4 receptor antagonists may be candidate drugs that can be clinically used for the treatment of pain.

The use of norms of reaction to analyze genotypic and environmental influences on behavior in mice and rats

Norms of reaction (NoRs) represent the phenotypic values of genotypes as functions of environmental parameters and permit the visualization of differences in phenotypic response of different genotypes. NoR graphs can be used to analyze interactions between genotypic and environmental factors during development to produce phenotypes in inbred strains of rats and mice. We describe the main features of NoRs, the history of their use in this context, and discuss several applications in behavioral neuroscience. In addition, we give a test for determining whether distinct strains have different NoRs.

Alterations in morphine-induced reward, locomotor activity, and thermoregulation in CREB-deficient mice

Previous studies in our lab have shown a robust decrease in the rewarding properties of morphine in CREB(alphaDelta) mutant mice. To determine whether the genetic effects of the global CREB(alphaDelta) mutation are specific to reward or generalizable, we examined a variety of morphine-induced behaviors regulated by different neural circuitry. At low doses of morphine (5 and 10 mg/kg), CREB(alphaDelta) mutant mice show a reduction in reward yet similar locomotor activity in response to morphine compared to wild type littermates. However, at a high dose (20 mg/kg), CREB(alphaDelta) mutant mice show an increase in reward and locomotor activity. Morphine-induced thermoregulation is attenuated in CREB(alphaDelta) mutant mice at high doses of morphine compared to wild type animals. The behavioral differences in response to morphine seen in CREB(alphaDelta) mutant mice are not due to changes in mu opioid receptor (MOR) mRNA expression, as the CREB deletion has no effect on baseline MOR mRNA in three of the brain regions involved in mediating these behaviors: the ventral tegmental area (VTA), nucleus accumbens (NAc), and hypothalamus. These data demonstrate that at low doses, deficits in morphine-induced changes in CREB deficient mice are limited to reward and thermoregulation. However, at higher doses, CREB mutant mice actually find morphine more rewarding and exhibit increased locomotor activity compared to their wild type littermates. Together, these results indicate that the role of CREB in dose-dependent changes in behaviors induced by morphine is different depending on the brain regions involved in mediating the behavior.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

Use of A Standard Strain for External Calibration in Behavioral Phenotyping

The present paper evaluates the inclusion of a standard strain or outbred stock in multi-strain behavioral phenotyping protocols to perform the same role as the external standard in biochemical assay procedures. As potential standards, the F344 inbred strain and an outbred stock of Long Evans were tested with three other inbred strains. To evaluate the influence of rearing conditions on phenotype stability, one group of F344s was born at the University of Tsukuba, another, bred elsewhere and delivered to Tsukuba at 4 weeks of age. All animals were tested in open-field (OF), runway emergence (RE) and digging tests as adults. The results showed no influence of breeding or transportation history on OF and RE behavior of the two F344 groups, while there was evidence that digging behavior may be affected by the different rearing experience. The inclusion of a 'standard strain or stock' in phenotyping protocols involving multiple inbred strains or lines of rats, mice and flies has obvious advantages by providing a reference point for inter-laboratory comparisons. The properties of inbred strains and outbred stocks favorable to their use as standards are discussed.

Congenic C57BL/6 mu opiate receptor (MOR) knockout mice: baseline and opiate effects

Homozygous mu-opioid receptor (MOR) knockout (KO) mice developed on a chimeric C57B6/129SV background lack morphine-induced antinociception, locomotion and reward. Therefore it appears that MOR largely mediates these morphine actions. However, one factor that could affect the extent of knockout deficits in morphine-induced behavior is the genetic background against which the gene deletion is expressed. To examine the effect of genetic background chimeric C57B6/129SV MOR knockout mice from the 15th generation of those developed in our laboratory were backcrossed for 10 successive generations with C57BL/6 mice, a strain which is more sensitive to many of the properties of morphine, to produce congenic MOR (con-MOR) KO mice. Heterozygote conMOR KO mice display attenuated morphine locomotion and reduced morphine analgesia compared to wild-type mice. Homozygote con-MOR KO mice display baseline hyperalgesia, no morphine place preference, no morphine analgesia and no morphine locomotion. These results are not qualitatively different from those observed in the MOR KO strain with a chimeric C57B6/129SV background, and suggest that although the strain has separate influences on these functions, it does not substantially interact with deletion of the mu opiate receptor gene.

Hyperalgesia and blunted morphine analgesia in G protein-gated potassium channel subunit knockout mice

Our aim was to determine whether G protein-gated potassium (Kir3) channels contribute to thermonociception and morphine analgesia. Western blotting was used to probe for the presence of Kir3.1, Kir3.2, Kir3.3, and Kir3.4 subunits in the mouse brain and spinal cord. Hot-plate paw-lick latencies for wild-type, Kir3.2 knockout, Kir3.3 knockout, and Kir3.4 knockout mice were measured at 52 degrees C and 55 degrees C, following the s.c. injection of either saline or 10 mg/kg morphine. Paw-lick latencies for Kir3.4 knockout mice were similar to those of wild-type mice, consistent with the restricted expression pattern of Kir3.4 subunit in the mouse brain. In contrast, Kir3.2 knockout and Kir3.3 knockout mice displayed hyperalgesia at both temperatures tested, and both Kir3.2 knockout and Kir3.3 knockout mice displayed shorter paw-lick latencies following morphine administration, with Kir3.2 knockout mice exhibiting the more dramatic phenotype. Kir3.2/Kir3.3 double knockout mice displayed a greater degree of hyperalgesia than either the Kir3.2 knockout or Kir3.3 knockout mice, while performing similarly to Kir3.2 knockout mice following morphine administration. We conclude that G protein-gated potassium channels containing Kir3.2 and/or Kir3.3 play a significant role in responses to moderate thermal stimuli. Furthermore, the activation of Kir3 channels containing the Kir3.2 subunit contributes to the analgesia evoked by a moderate dose of morphine. As such, receptor-independent Kir3 channel agonists may represent a novel and selective class of analgesic agent.

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

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

Quantitative genetics and mouse behavior

Quantitative differences are observed for most complex behavioral and pharmacological traits within any population. Both environmental and genetic influences regulate such individual differences. The mouse has proven to be a superb model in which to investigate the genetic basis for quantitative differences in complex behaviors. Genetically defined populations of mice, including inbred strains, heterogeneous stocks, and selected lines, have been used effectively to document these genetic differences. Recently, quantitative trait loci methods have been applied to map the chromosomal regions that regulate variation with the goal of eventually identifying the gene polymorphisms that reside in these regions.