Differences in drug-induced place conditioning between BALB/c and C57Bl/6 mice

Western diet consumption does not impact the rewarding and aversive effects of morphine in male Sprague-Dawley rats

The impacts of high-fat and/or high-sugar diets on opioid-induced effects are well documented; however, little is known about the effect of such diet on the affective responses to opiates. To address this issue, in the present experiment male Sprague-Dawley rats were given ad libitum access to a western-style diet (high in saturated fat and sugar) or a standard laboratory chow diet beginning in adolescence and continuing into adulthood at which point they were trained in a combined conditioned taste avoidance (CTA)/conditioned place preference (CPP) procedure to assess the aversive and rewarding effects of morphine, respectively. On four conditioning cycles, animals were given access to a novel saccharin solution, injected with morphine (1 mg/kg or 5 mg/kg), and then placed on one side of a place preference chamber. Animals were then tested for place preference and saccharin preference. All subjects injected with morphine displayed significant avoidance of the morphine-associated solution (CTA) and preferred the side associated with the drug (CPP). Furthermore, there were no differences between the two diet groups, indicating that chronic exposure to the western diet had no impact on the affective properties of morphine (despite increasing caloric intake, body weight, body fat and lean body mass). Given previously reported increases in drug self-administration in animals with a history of western-diet consumption, this study suggests that western-diet exposure may increase drug intake via mechanisms other than changes in the rewarding or aversive effects of the drug.

Mirtazapine attenuates the cocaine-induced locomotor sensitization in male and female C57BL/6J and BALBA/cJ mouse

BACKGROUND

Clinical studies have described the efficacy of various therapeutic approaches. Results are inconsistent and clinical application is limited. Clinical trials have suggested that individual variability in the response to pharmacological therapies and sex affects the efficacy of some antidepressant drugs. Mouse strain-dependent variability influenced the response to antidepressant drugs. Some mouse strains respond faster and better to antidepressants than other mouse strains. We recently reported a series of preclinical studies that showed that dosing of mirtazapine, a noradrenergic and serotonergic antidepressant, in male and female Wistar rats decreased cocaine-induced locomotor activity and attenuated the induction and expression of cocaine-induced locomotor sensitization. Therefore, the aim of this study was to evaluate the mirtazapine effects, on cocaine-induced locomotor activity and cocaine-induced locomotor sensitization in male and female mice of the C57BL/6J and BALB/cJ strains, which differ in sensitivity to the cocaine motor effects and response to antidepressant drugs.

METHODS

Male and female BALB/cJ and C57BL/6J inbred mice (20-25 g) were daily dosed with 10 mg/kg of cocaine during the induction and expression of locomotor sensitization. During drug withdrawal, cocaine was withdrawn, and the groups received daily mirtazapine (30 mg/kg, i.p.) or saline. Mirtazapine was administered 30 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min in transparent Plexiglass activity chambers.

RESULTS

Cocaine-induced locomotor activity were greater in C57BL/6J strain mice than BALB/cJ strain mice during the induction and expression phase of locomotor sensitization. The female mice of both strains showed a higher cocaine locomotor response than males and mirtazapine significantly decreased cocaine-induced locomotor activity, as well as the induction and expression of locomotor sensitization, regardless of mouse strain or sex.

CONCLUSION

The results suggest mirtazapine may be considered an effective therapeutic option to treat cocaine use disorder in men and women with very diverse genetic backgrounds.

Role of Organic Cation Transporter 3 and Plasma Membrane Monoamine Transporter in the Rewarding Properties and Locomotor Sensitizing Effects of Amphetamine in Male andFemale Mice

A lack of effective treatment and sex-based disparities in psychostimulant addiction and overdose warrant further investigation into mechanisms underlying the abuse-related effects of amphetamine-like stimulants. Uptake-2 transporters such as organic cation transporter 3 (OCT3) and plasma membrane monoamine transporter (PMAT), lesser studied potential targets for the actions of stimulant drugs, are known to play a role in monoaminergic neurotransmission. Our goal was to examine the roles of OCT3 and PMAT in mediating amphetamine (1 mg/kg)-induced conditioned place preference (CPP) and sensitization to its locomotor stimulant effects, in males and females, using pharmacological, decynium-22 (D22; 0.1 mg/kg, a blocker of OCT3 and PMAT) and genetic (constitutive OCT3 and PMAT knockout (−/−) mice) approaches. Our results show that OCT3 is necessary for the development of CPP to amphetamine in males, whereas in females, PMAT is necessary for the ability of D22 to prevent the development of CPP to amphetamine. Both OCT3 and PMAT appear to be important for development of sensitization to the locomotor stimulant effect of amphetamine in females, and PMAT in males. Taken together, these findings support an important, sex-dependent role of OCT3 and PMAT in the rewarding and locomotor stimulant effects of amphetamine.

The morphine/heroin vaccine decreased the heroin-induced antinociceptive and reinforcing effects in three inbred strains mouse

Clinical trials have indicated that a vaccine must be immunogenic in genetically diverse human populations and that immunogenicity and protective efficacy in animal models are two key indices required for the approval of a new vaccine. Additionally, the immune response (immunogenicity) and immunoprotection are dependent on the mouse strain. Therefore, the objective of the present study was to determine the immune response (immunogenicity) and the protective efficacy (behavioral response) in three inbred mouse strains immunized with the M₆TT vaccine. Female BALB/c, C57Bl/6, and DBA/2 inbred mice were immunized with the M₆-TT vaccine. A solid-phase antibody-capture ELISA was used to monitor antibody titer responses after each booster dose in vaccinated animals. The study used tail-flick testing to evaluate the antinociceptive effects induced by heroin. Additionally, heroin-induced locomotor activity and place preference were evaluated. The M₆-TT vaccine was able to generate a specific antibody titer in the three inbred mouse strains evaluated. The antibodies reduced the antinociceptive effect of different doses of heroin. In addition, they decreased the heroin-induced locomotor activity and place preference. These findings suggest that the M₆-TT vaccine generates a powerful immunogenic response capable of reducing the antinociceptive and reinforcing effects of heroin in different inbred mouse strains, which supports its possible future use in clinical trials in genetically diverse human populations.

Mapping the living mouse brain neural architecture: strain-specific patterns of brain structural and functional connectivity

Mapping brain structural and functional connectivity (FC) became an essential approach in neuroscience as network properties can underlie behavioral phenotypes. In mouse models, revealing strain-related patterns of brain wiring is crucial, since these animals are used to answer questions related to neurological or neuropsychiatric disorders. C57BL/6 and BALB/cJ strains are two of the primary "genetic backgrounds" for modeling brain disease and testing therapeutic approaches. However, extensive literature describes basal differences in the behavioral, neuroanatomical and neurochemical profiles of the two strains, which raises questions on whether the observed effects are pathology specific or depend on the genetic background of each strain. Here, we performed a systematic comparative exploration of brain structure and function of C57BL/6 and BALB/cJ mice using Magnetic Resonance Imaging (MRI). We combined deformation-based morphometry (DBM), diffusion MRI and high-resolution fiber mapping (hrFM) along with resting-state functional MRI (rs-fMRI) and demonstrated brain-wide differences in the morphology and "connectome" features of the two strains. Essential inter-strain differences were depicted regarding the size and the fiber density (FD) within frontal cortices, along cortico-striatal, thalamic and midbrain pathways as well as genu and splenium of corpus callosum. Structural dissimilarities were accompanied by specific FC patterns, emphasizing strain differences in frontal and basal forebrain functional networks as well as hubness characteristics. Rs-fMRI data further indicated differences of reward-aversion circuitry and default mode network (DMN) patterns. The inter-hemispherical FC showed flexibility and strain-specific adjustment of their patterns in agreement with the structural characteristics.

Oral Monosodium Glutamate Differentially Affects Open-Field Behaviours, Behavioural Despair and Place Preference in Male and Female Mice

Background:

Monosodium glutamate (MSG) is a flavour enhancer which induces behavioural changes in animals. However the influence of sex on the behavioural response to MSG has not been investigated.

Objective:

The sex-differential effects of MSG on open-field behaviours, anxiety-related behaviour, behavioural despair, place-preference, and plasma/brain glutamate levels in adult mice were assessed.

Methods:

Mice were assigned to three groups (1-3), based on the models used to assess behaviours. Animals in group 1 were for the elevated-plus maze and tail-suspension paradigms, group 2 for the open-field and forced-swim paradigms, while mice in group 3 were for observation in the conditioned place preference paradigm. Mice in all groups were further assigned into five subgroups (10 males and 10 females), and administered vehicle (distilled water at 10 ml/kg) or one of four doses of MSG (20, 40, 80 and 160 mg/kg) daily for 6 weeks, following which they were exposed to the behavioural paradigms. At the end of the behavioural tests, the animals were sacrificed, and blood was taken for estimation of glutamate levels. The brains were also homogenised for estimation of glutamate levels.

Results:

MSG was associated with a reduction in locomotion in males and females (except at 160 mg/kg, male), an anxiolytic response in females, an anxiogenic response in males, and decreased behavioural despair in both sexes (females more responsive). Postconditioning MSG-associated place-preference was significantly higher in females. Plasma/ brain glutamate was not significantly different between sexes.

Conclusion:

Repeated MSG administration alters a range of behaviours in a sex-dependent manner in mice.

Loss of CELF6 RNA binding protein impairs cocaine conditioned place preference and contextual fear conditioning

In addition to gene expression differences in distinct cell types, there is substantial post-transcriptional regulation driven in part by RNA binding proteins (RBPs). Loss-of-function RBP mutations have been associated with neurodevelopmental disorders, such as Fragile-X syndrome and syndromic autism. Work performed in animal models to elucidate the influence of neurodevelopmental disorder-associated RBPs on distinct behaviors has showed a connection between normal post-transcriptional regulation and conditioned learning. We previously reported cognitive inflexibility in a mouse model null for the RBP CUG-BP, Elav-like factor 6 (CELF6), which we also found to be associated with human autism. Specifically, these mice failed to potentiate exploratory hole-poking behavior in response to familiarization to a rewarding stimuli. Characterization of Celf6 gene expression showed high levels in monoaminergic populations such as the dopaminergic midbrain populations. To better understand the underlying behavioral disruption mediating the resistance to change exploratory behavior in the holeboard task, we tested three hypotheses: Does Celf6 loss lead to global restricted patterns of behavior, failure of immediate response to reward or failure to alter behavior in response to reward (conditioning). We found the acute response to reward was intact, yet Celf6 mutant mice exhibited impaired conditioned learning to both reward and aversive stimuli. Thus, we found that the resistance to change by the Celf6 mutant in the holeboard was most parsimoniously explained as a failure of conditioning, as the mice had blunted responses even to potent rewarding stimuli such as cocaine. These findings further support the role of RBPs in conditioned learning.

Mouse-Derived Isograft (MDI) In Vivo Tumor Models II. Carcinogen-Induced cMDI Models: Characterization and Cancer Therapeutic Approaches

In this second study, we established syngeneic in vivo models named carcinogen-induced mouse-derived isografts (cMDIs). Carcinogen-induced tumors were obtained during short-term observation (3–9 months) of CBA/J mice treated with various administration routes with 3-methylcholanthrene (MCA) or N-methyl-N-nitrosourea (MNU) as carcinogens. During necropsy, primary tumors and suspicious tissues were assessed macroscopically and re-transplanted (in PDX-like manner) into sex-matched syngeneic animals. Outgrowing tumors were histologically characterized as either spinocellular carcinoma (1/8) or various differentiated sarcomas (7/8). Growth curves of four sarcomas showed striking heterogeneity. These cMDIs were further characterized by flow cytometry, RNA sequencing, or efficacy studies. A variable invasion of immune cells into the tumors, as well as varying expression of tyrosine kinase receptor, IFN-γ signature, or immune cell population marker genes could be observed. Immune checkpoint inhibitor treatment (anti-mPD-1, anti-mCTLA-4, or a combination thereof) showed different responses in the various cMDI models. In general, cMDI models are carcinogen-induced tumors of low passage number that were propagated as tissue pieces in mice without any tissue culturing. Therefore, the tumors contained conserved tumor characteristics and intratumoral immune cell populations. In contrast to the previously described spontaneous MDI, carcinogen induction resulted in a greater number of individual but histologically related tumors, which were preferentially sarcomas.

Mouse-Derived Isograft (MDI) In Vivo Tumor Models I. Spontaneous sMDI Models: Characterization and Cancer Therapeutic Approaches

Syngeneic in vivo tumor models are valuable for the development and investigation of immune-modulating anti-cancer drugs. In the present study, we established a novel syngeneic in vivo model type named mouse-derived isografts (MDIs). Spontaneous MDIs (sMDIs) were obtained during a long-term observation period (more than one to two years) of naïve and untreated animals of various mouse strains (C3H/HeJ, CBA/J, DBA/2N, BALB/c, and C57BL/6N). Primary tumors or suspicious tissues were assessed macroscopically and re-transplanted in a PDX-like manner as small tumor pieces into sex-matched syngeneic animals. Nine outgrowing primary tumors were histologically characterized either as adenocarcinomas, histiocytic carcinomas, or lymphomas. Growth of the tumor pieces after re-transplantation displayed model heterogeneity. The adenocarcinoma sMDI model JA-0009 was further characterized by flow cytometry, RNA-sequencing, and efficacy studies. M2 macrophages were found to be the main tumor infiltrating leukocyte population, whereas only a few T cells were observed. JA-0009 showed limited sensitivity when treated with antibodies against inhibitory checkpoint molecules (anti-mPD-1 and anti-mCTLA-4), but high sensitivity to gemcitabine treatment. The generated sMDI are spontaneously occurring tumors of low passage number, propagated as tissue pieces in mice without any tissue culturing, and thus conserving the original tumor characteristics and intratumoral immune cell populations.

Alpha-PVP induces the rewarding effect via activating dopaminergic neuron

A synthetic cathinone, 1-phenyl-2-(1-pyrrolidinyl)-1-pentanone (α-PVP), was occasionally found in the "bath salt" type of designer drugs, as an active ingredient. It has been reported that drivers who consumed α-PVP were in an excited state and incapable of controlling their behavior, causing traffic accidents. Despite its acute excitatory effects, there is no information on the psychological dependency elicited by α-PVP use. The purpose of the present study was to clarify whether the reward pathway is activated with repeated doses of α-PVP in experimental animals. Treatment of male C57BL/6j mice with α-PVP (25 mg/kg, i.p.), once a day, for 3 days significantly increased the conditioned place preference scores. Therefore, repeated doses of α-PVP were shown to induce palatability in mice. α-PVP increases extracellular dopamine levels in the nucleus accumbens shell immediately after administration. The number of cells immunopositive for phosphorylated cAMP-regulatory element binding protein (CREB) was significantly increased in the α-PVP-treated mice in our study. These results indicate that the administration of α-PVP activates the phosphorylation of CREB in the nucleus accumbens shell. Our results suggest that α-PVP stimulates the reward pathway by increasing the extracellular dopamine levels and CREB phosphorylation in the nucleus accumbens shell, eventually causing positive reinforcement in mice.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

Resting-state functional MRI and [18F]-FDG PET demonstrate differences in neuronal activity between commonly used mouse strains

The existence of numerous interesting mouse models of neurological disorders enables the investigation of causal relations between pathological events and the effect of treatment regimes. However, mouse models of a specific neurological disease are often generated using different background strains, which raises the question whether the observed effects are specific to pathology or depend on the used strain. This study used two independent in vivo functional imaging techniques to evaluate whether mouse strain differences exist in functional connectivity (FC) and brain glucose metabolism i.e. indirect measures of neuronal activity. For this purpose, C57BL/6, BALB/C and SJL mice (N=15/group, male) were evaluated using resting-state functional MRI (rsfMRI) and static [18F]-fluorodeoxyglucose Positron Emission Tomography ([18F]-FDG PET). RsfMRI and [18F]-FDG PET data were analyzed with independent component analysis (ICA). FC was quantified by calculating the mean network-specific FC strength and [18F]-FDG uptake was quantified by calculating the mean network-specific standard uptake value corrected for plasma glucose levels and body weight (SUVglu). The ICA results showed spatially similar neurological components in the rsfMRI and [18F]-FDG PET data, suggesting that patterns of metabolic covariance in the mouse brain reflect FC networks. Comparing FC and [18F]-FDG data showed that strain-dependent differences in brain activity exist for several brain networks i.e. the frontal, cingulate, (hypo)thalamus, striatum, and sensorimotor networks. The results of this study have implications for the interpretation of in vivo functional imaging data in mouse models of neurological disorders generated on different background strains.

Sex differences in morphine-induced behavioral sensitization and social behaviors in ICR mice

Gender and genetic strain are two prominent variants that influence drug abuse. Although certain sex-related behavioral responses have been previously characterized in ICR mice, little is known about the effects of sex on morphine-induced behavioral responses in this outbred strain. Therefore, in this study, we investigated the sex differences of morphine-induced locomotion, anxiety-like and social behaviors in ICR mice. After morphine or saline exposure for four consecutive days (twice daily), increased locomotion, more time spent in the central area, as well as attenuated rearing and self-grooming behaviors were found in morphine-treated females in an open field; no differences were found in locomotion and the time spent in the central area between male and female controls. When interacting with the same-sex individuals, female controls were engaged in more social investigation, following, body contacting and self-grooming behaviors than controls; morphine exposure reduced contacting and self-grooming behaviors in females; in contrast, these effects were not found in males. These results indicate that female ICR mice are more prosocial and are more susceptible to morphine exposure than males.

Differences in cocaine-induced place preference persistence, locomotion and social behaviors between C57BL/6J and BALB/cJ mice

C57BL/6J and BALB/cJ mice display significant differences in sociability and response to drugs, but the phenotypic variability of their susceptibility to cocaine is still not well known. In this study, the differences between these two mice strains in the persistence of cocaine-induced conditioned place preference (CPP), as well as the locomotion and social behaviors after the 24-hour withdrawal from a four-day cocaine (20 mg/kg/day) administration were investigated. The results showed that the cocaine-induced CPP persisted over two weeks in C57BL/6J mice, while it diminished within one week among BALB/cJ mice. After 24-hours of cocaine withdrawal, high levels of locomotion as well as low levels of social interaction and aggressive behavior were found in C57BL/6J mice, but no significant changes were found in BALB/cJ mice, indicating that cocaine-induced CPP persistence, locomotion and social behavior are not consistent between these two strains, and that overall C57BL/6J mice are more susceptible to cocaine than BALB/cJ mice at the tested doses.

Expression of HIV gp120 protein increases sensitivity to the rewarding properties of methamphetamine in mice

Methamphetamine abuse and human immunodeficiency virus (HIV) infection induce neuropathological changes in corticolimbic brain areas involved in reward and cognitive function. Little is known about the combined effects of methamphetamine and HIV infection on cognitive and reward processes. The HIV/gp120 protein induces neurodegeneration in mice, similar to HIV-induced pathology in humans. We investigated the effects of gp120 expression on associative learning, preference for methamphetamine and non-drug reinforcers, and sensitivity to the conditioned rewarding properties of methamphetamine in transgenic (tg) mice expressing HIV/gp120 protein (gp120-tg). gp120-tg mice learned the operant response for food at the same rate as non-tg mice. In the two-bottle choice procedure with restricted access to drugs, gp120-tg mice exhibited greater preference for methamphetamine and saccharin than non-tg mice, whereas preference for quinine was similar between genotypes. Under conditions of unrestricted access to methamphetamine, the mice exhibited a decreased preference for increasing methamphetamine concentrations. However, male gp120-tg mice showed a decreased preference for methamphetamine at lower concentrations than non-tg male mice. gp120-tg mice developed methamphetamine-induced conditioned place preference at lower methamphetamine doses compared with non-tg mice. No differences in methamphetamine pharmacokinetics were found between genotypes. These results indicate that gp120-tg mice exhibit no deficits in associative learning or reward/motivational function for a natural reinforcer. Interestingly, gp120 expression resulted in increased preference for methamphetamine and a highly palatable non-drug reinforcer (saccharin) and increased sensitivity to methamphetamine-induced conditioned reward. These data suggest that HIV-positive individuals may have increased sensitivity to methamphetamine, leading to high methamphetamine abuse potential in this population.

A history of chronic morphine exposure during adolescence increases despair-like behaviour and strain-dependently promotes sociability in abstinent adult mice

A crucial issue in treating opiate addiction, a chronic relapsing disorder, is to maintain a drug-free abstinent state. Prolonged abstinence associates with mood disorders, strongly contributing to relapse. In particular, substance use disorders occurring during adolescence predispose to depression later in adulthood. Using our established mouse model of opiate abstinence, we characterized emotional consequences into adulthood of morphine exposure during adolescence. Our results indicate that morphine treatment in adolescent mice has no effect on anxiety-like behaviours in adult mice, after abstinence. In contrast, morphine treatment during adolescence increases behavioural despair in adult mice. We also show that morphine exposure strain-dependently enhances sociability in adult mice. Additional research will be required to understand where and how morphine acts during brain maturation to affect emotional and social behaviours into adulthood.

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.

Psychomotor stimulant effects of cocaine in rats and 15 mouse strains

Relative to intravenous drug self-administration, locomotor activity is easier to measure with high throughput, particularly in mice. Therefore its potential to predict differences in self-administration between genotypes (e.g., targeted mutations, recombinant inbred strains) is appealing, but such predictive value is unverified. The main goal of this study was to evaluate the utility of the locomotor assay for accurately predicting differences in cocaine self-administration. A second goal was to evaluate any correlation between activity in a novel environment, and cocaine-induced hyperactivity, between strains. We evaluated locomotor activity in male and female Sprague-Dawley rats and 15 mouse strains (129S1/SvImJ, 129S6/SvEvTac, 129X1/SvJ, A/J, BALB/cByJ, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, SJL/J, SPRET/EiJ, and outbred Swiss Webster and CD-1/ICR), as well as cocaine self-administration in BALB substrains. All but BALB/cJ mice showed locomotor habituation and significant cocaine-induced hyperactivity. BALB/cJ mice also failed to self-administer cocaine. BALB/cByJ mice showed modest locomotor habituation, cocaine-induced locomotion, and cocaine self-administration. As previously reported, female rats showed greater cocaine-induced locomotion than males, but this was only observed in one of 15 mouse strains (FVB/NJ), and the reverse was observed in two strains (129X1/SvJ, BALB/cByJ). The intriguing phenotype of the BALB/cJ strain may indicate some correlation between all-or-none locomotion in a novel environment, and stimulant and reinforcing effects of cocaine. However, neither novelty- nor cocaine-induced activity offered a clear prediction of relative reinforcing effects among strains. Additionally, these results should aid in selecting mouse strains for future studies in which relative locomotor responsiveness to psychostimulants is a necessary consideration.

Cocaine locomotor activation, sensitization and place preference in six inbred strains of mice

BACKGROUND

The expanding set of genomics tools available for inbred mouse strains has renewed interest in phenotyping larger sets of strains. The present study aims to explore phenotypic variability among six commonly-used inbred mouse strains to both the rewarding and locomotor stimulating effects of cocaine in a place conditioning task, including several strains or substrains that have not yet been characterized for some or all of these behaviors.

METHODS

C57BL/6J (B6), BALB/cJ (BALB), C3H/HeJ (C3H), DBA/2J (D2), FVB/NJ (FVB) and 129S1/SvImJ (129) mice were tested for conditioned place preference to 20 mg/kg cocaine.

RESULTS

Place preference was observed in most strains with the exception of D2 and 129. All strains showed a marked increase in locomotor activity in response to cocaine. In BALB mice, however, locomotor activation was context-dependent. Locomotor sensitization to repeated exposure to cocaine was most significant in 129 and D2 mice but was absent in FVB mice.

CONCLUSIONS

Genetic correlations suggest that no significant correlation between conditioned place preference, acute locomotor activation, and locomotor sensitization exists among these strains indicating that separate mechanisms underlie the psychomotor and rewarding effects of cocaine.

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.

Automation of the free-exploratory paradigm

The free-exploratory paradigm (FEP) is currently the only proposed animal model of trait anxiety, making it highly valuable to behavioural neuroscience. However, FEP has not yet been automated, so its results depend on human scoring, which can be quite imprecise. The aim of this study was, therefore, to validate an automated version of FEP, using a commercially available video-tracking system (ANY-maze(©) - Stoelting Co., USA). To achieve this, two experiments were performed. The first one evaluated the reliability of the video-based automation of FEP, and the second, assessed whether the zeolites, used as a bedding material in the first experiment to facilitate video-tracking, influenced the animals' behaviour in FEP. In experiment I, 15 drug-naive, adult, male rats were tested in FEP, while their behaviour was simultaneously evaluated by ANY-maze(©) and two human observers. Subsequently, the intraclass correlation coefficient (ICC) was calculated for the automated and manual results of the parameters "percentage of time in the novel side" (%TNS) and "total units visited" (TUV). The analysis resulted in high, significant values of ICC (%TNS: 0.9962 and TUV: 0.9453). In experiment II, 18 drug-naive, adult, male rats were allocated to two different groups: (1) tested in FEP with zeolites; and (2) tested in FEP with sawdust. The data obtained were analysed using the Student's t-test, which revealed no significant differences between the groups for the parameters %TNS and TUV. In conclusion, the data presented here show that automation of FEP, using a video-based tracking system, is not only possible, but also highly reliable.

Novel approach to data analysis in cocaine-conditioned place preference

Only a subgroup of human drug users progress from initial drug taking to drug addiction. The learned associations between the effects of the drug and the environment in which it is experienced is an important aspect of the progression to continued drug taking and drug seeking. These associations can be modeled using the conditioned place preference (CPP) paradigm, although no current method of CPP analysis allows for the identification of within-group variability among subjects. In this study, we adapted a 'criterion' method of analysis to separate 'CPP expressing' from 'non-CPP expressing' rats to study more directly within-group variability in the CPP paradigm. Male Sprague-Dawley rats were conditioned with cocaine (5, 10, 20 mg/kg) or saline in an unbiased three-chamber CPP apparatus in either a single-trial or four-trial CPP procedure. A classification and regression tree analysis of time spent in the cocaine-paired chamber established a time of 324 s spent in the cocaine-paired chamber as the criterion for cocaine CPP expression. This criterion effectively discriminated control from cocaine-conditioned rats and was reliable for rats trained in both single trial and four-trial CPP procedures. The criterion method showed an enhanced ability to detect effective doses of cocaine in the single-trial CPP procedure and a blockade of CPP expression by MK 212 (0.125 mg/kg) treatment in a subgroup of rats. These data support the utility of the criterion analysis as an adjunct to traditional methods that compare group averages in CPP.

Conditioned place preference and locomotor activity in response to methylphenidate, amphetamine and cocaine in mice lacking dopamine D4 receptors

Methylphenidate (MP) and amphetamine (AMPH) are the most frequently prescribed medications for the treatment of attention-deficit/hyperactivity disorder (ADHD). Both drugs are believed to derive their therapeutic benefit by virtue of their dopamine (DA)-enhancing effects, yet an explanation for the observation that some patients with ADHD respond well to one medication but not to the other remains elusive. The dopaminergic effects of MP and AMPH are also thought to underlie their reinforcing properties and ultimately their abuse. Polymorphisms in the human gene that codes for the DA D4 receptor (D4R) have been repeatedly associated with ADHD and may correlate with the therapeutic as well as the reinforcing effects of responses to these psychostimulant medications. Conditioned place preference (CPP) for MP, AMPH and cocaine were evaluated in wild-type (WT) mice and their genetically engineered littermates, congenic on the C57Bl/6J background, that completely lack D4Rs (knockout or KO). In addition, the locomotor activity in these mice during the conditioning phase of CPP was tested in the CPP chambers. D4 receptor KO and WT mice showed CPP and increased locomotor activity in response to each of the three psychostimulants tested. D4R differentially modulates the CPP responses to MP, AMPH and cocaine. While the D4R genotype affected CPP responses to MP (high dose only) and AMPH (low dose only) it had no effects on cocaine. Inasmuch as CPP is considered an indicator of sensitivity to reinforcing responses to drugs these data suggest a significant but limited role of D4Rs in modulating conditioning responses to MP and AMPH. In the locomotor test, D4 receptor KO mice displayed attenuated increases in AMPH-induced locomotor activity whereas responses to cocaine and MP did not differ. These results suggest distinct mechanisms for D4 receptor modulation of the reinforcing (perhaps via attenuating dopaminergic signalling) and locomotor properties of these stimulant drugs. Thus, individuals with D4 receptor polymorphisms might show enhanced reinforcing responses to MP and AMPH and attenuated locomotor response to AMPH.

Restraint stress-induced brain activation patterns in two strains of mice differing in their anxiety behaviour

Genetically identical inbred mouse strains are one of the most useful tools in dissecting the genetic basis of complex disorders. C57BL/6 and BALB/c mice differ markedly in emotionality. In particular, BALB/c mice are more stress-sensitive and have been proposed to be a model of pathological anxiety. There is a paucity of studies investigating whether brain activation in response to a stressor is different in these two strains. To this end, having confirmed that the strains differ in anxiety responses in a light-dark box test, we then examined if restraint stress induced increases in c-Fos protein expression in selective regions of the mouse brain. The areas of interest analysed were the paraventricular nucleus (PVN) of the hypothalamus, prefrontal cortex (PFC), the paraventricular thalamic nucleus (PV) and the hippocampus. These areas were chosen due to their known involvement in stress response. Our data demonstrate that BALB/c showed a similar cellular activation pattern to stress, with respect to c-Fos expression, in the PVN, PV and in the hippocampus. On the other hand, BALB/c showed markedly blunted stress-induced brain activation compared with stressed C57BL/6 mice in both the CG1 and CG2 regions of the PFC. The lower levels of stress-induced activity in high anxiety BALB/c mice, possibly indicate a circuit dysregulation at the cortico-limbic level in response to stress.

Acquisition of and withdrawal from cocaine self-administration regulates 5-HT mRNA expression in rat striatum

This study investigated how different stages of cocaine self-administration in rats affect the expression of two serotonin receptors in dorsal and ventral striatum, the 5-HT(1B) and 5-HT(6) subtypes, which have both been implicated in mediating some aspects of cocaine-related behaviors. In the first experiment, rats were trained to work for saccharin (oral) or cocaine (i.v.) reinforcers. We found that continuous access to cocaine for 23 days did not change the level of 5-HT(1B) mRNA expression compared to control animals receiving saccharin. However, a single cocaine session, given either by self-administration or non-contingently, increased 5-HT(1B) mRNA in dorsal striatum, whereas forced abstinence for two weeks after cocaine reduced 5-HT(1B) mRNA expression in the same subregion. 5-HT(6) mRNA was not changed by any of these treatments. A follow-up experiment investigated the effects of limited versus extended access to cocaine as well as forced abstinence, and we found that 14 days of forced abstinence significantly reduced 5-HT(1B) mRNA throughout the dorsal and ventral striatum compared to no withdrawal. These results suggest that the influence of 5-HT(1B) receptors in striatal projection neurons may be increased during cocaine acquisition and reduced after forced abstinence and may therefore be targets for pharmacological intervention in addiction.

Individual housing of mice--impact on behaviour and stress responses

The replicability of results derived from studies in rodents might be influenced by stress caused by inappropriate housing conditions. Here we compared the experimental behaviour and stress response (circulating corticosterone level and adrenal tyrosine hydroxylase activity) of individually-housed male and female inbred mice with that of animals housed in social groups. All mice were behaviourally tested in the modified hole board test (mHB). Male C57BL/6, BALB/c and A mice housed in groups of 3 were compared with individually-housed mice. In a subsequent experiment female C57BL/6 and BALB/c mice were housed under similar conditions. To exclude the possible effects of within-cage order of testing, only one individual per group was behaviourally tested. Neither male nor female mice housed individually showed stronger signs of stress than their socially-housed counterparts. However, we observed a within-cage order effect on the hormonal stress response (corticosterone) in socially-housed female C57BL/6 mice. No effects of individual housing on behaviour in the mHB were found.

Complicated interaction between psychostimulants and morphine in expression of phenotype of behavior in the dopaminergic system of BALB/c mice

It is believed that BALB/c mice appear to be less sensitive to the locomotor effects of abused drugs compared to other strains, and several behaviors induced by abused drugs depend on genetic factors. The present study was designed to investigate the effects of the interaction between psychostimulants and morphine on behavior in BALB/c mice. Morphine and cocaine induced hyperlocomotion and hypolocomotion, respectively, while methamphetamine did not affect locomotor activity and high doses of methamphetamine significantly increased self-injurious behavior. Cocaine or methamphetamine increased the effects of morphine on locomotor behavior. Haloperidol (a dopamine-receptor antagonist) attenuated the hyperlocomotion induced by the combination of cocaine or methamphetamine plus morphine. These results indicate that the synergistic effects of methamphetamine or cocaine and morphine on locomotor activity are mediated through enhancement of the dopaminergic system and that combinations of psychostimulants and morphine enhance the locomotor activity in BALB/c mice. On the other hand, morphine completely attenuated methamphetamine-induced self-injurious behavior. Furthermore, a low dose (0.01 mg/kg) of haloperidol significantly increased the effects of methamphetamine and morphine on the locomotor activity. Hyperlocomotion induced by psychostimulants is mediated by the mesolimbic dopaminergic system, whereas stereotyped behaviors is mediated by the nigrostriatal dopaminergic system. Our findings suggest that balances of the activation of dopaminergic neurons (between mesolimbic and nigrostriatal systems) may play an important role to engender corresponding behavioral outcomes in BALB/c mice.

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

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

Oxidative stress in methamphetamine-induced self-injurious behavior in mice

Previous studies have shown that N-methyl-D-aspartate, the formation of free radicals and poly(ADP-ribose) polymerase are related to methamphetamine-induced neurotoxicity. This study was designed to investigate the involvement of oxidative stress in methamphetamine-induced self-injurious behavior in mice. In this study, methamphetamine (20 mg/kg) induced continuous self-injurious behavior in six of seven mice. N-methyl-D-aspartate-receptor antagonists (MK801 and 3-((R)-2-carboxypiperazin-4-yl) propyl-1-phosphonic acid) significantly attenuated this methamphetamine-induced self-injurious behavior. These results suggest that the activation of N-methyl-D-aspartate receptors is involved in methamphetamine-induced self-injurious behavior. Furthermore, we found that the nonselective nitric oxide synthase inhibitor l-N-nitro-L-arginine methyl ester hydrochloride and the neuronal nitric oxide synthase inhibitor 7-nitroindazole, but not the inducible nitric oxide synthase inhibitor aminoguanidine, the free-radical inhibitors fullerene and 3-methyl-1-phenyl-2-pyrazolin-5-one-186, or the poly(ADP-ribose) polymerase inhibitor benzamide, significantly attenuated methamphetamine-induced self-injurious behavior. The present results show that oxidative stress, which is mediated by the activation of neuronal nitric oxide synthase, is associated with methamphetamine-induced self-injurious behavior. These findings may help us to better understand the clinical phenomenon of self-injurious behavior.

Involvement of glucose and ATP-sensitive potassium (K+) channels on morphine-induced conditioned place preference

In the present study, the effects of glucose and ATP-sensitive K+ channel compounds on the acquisition of morphine-induced place preference in male mice were investigated. Subcutaneous administration of different doses of morphine (2.5-7.5 mg/kg) produced a dose-dependent conditioned place preference. With a 3-day conditioning schedule, it was found that glucose (100, 200, 500 and 1000 mg/kg), diazoxide (15, 30 and 60 mg/kg) or glibenclamide (3, 6 and 12 mg/kg) did not produce significant place preference or place aversion. Intraperitoneal administration of the glucose (1000 mg/kg) or glibenclamide (6 and 12 mg/kg) with a lower dose of morphine (0.5 mg/kg) elicited the significant conditioned place preference. The response of glibenclamide (6 mg/kg) was reversed by diazoxide (15, 30 and 60 mg/kg). Drug injections had no effects on locomotor activity during the test sessions. It is concluded that glucose and the ATP-sensitive K+ channel may play an active role in morphine reward.

NMDA receptors of dorsal hippocampus are involved in the acquisition, but not in the expression of morphine-induced place preference

In the present study, involvement of the N-methyl-d-aspartate (NMDA) receptors of the CA1 region of dorsal hippocampus (intra-CA1) in the acquisition or expression of morphine-induced conditioned place preference in rats was studied. Male Wistar rats were used in these experiments. NMDA-receptor agonist (NMDA) and antagonist (MK-801) were injected into the CA1 region of the dorsal hippocampus (intra-CA1) and morphine was injected subcutaneously. An unbiased conditioned place preference paradigm was used to study the effect of these agents. In the first set of experiments, the drugs were used during the development of conditioned place preference by morphine or they were used alone in order to see if they induce conditioned place preference or conditioned place aversion. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (2.5-10 mg/kg) induced conditioned place preference in rat. NMDA (0.1-1 microg/rat) or MK-801 (1-4 microg/rat) did not induce conditioned place preference or conditioned place aversion. Intra-CA1 administration of different doses of NMDA (0.1-1 microg/rat) increased, while MK-801 (1-4 microg/rat) decreased morphine-induced place preference. MK-801 reversed the effect of NMDA on morphine response. In the second set of experiments, when the drugs were used before testing on Day 5, in order to test their effects on the expression of morphine (7.5 mg/kg)-induced place preference, intra-CA1 administration of NMDA or MK-801 did not alter the morphine response. None of the drugs influenced locomotion. It is concluded that NMDA receptor of the CA1 region of hippocampus are involved in the acquisition but not expression of morphine-induced place preference.

Morphine-induced place preference: Involvement of cholinergic receptors of the ventral tegmental area

In the present study, the effects of intra-ventral tegmental area injections of cholinergic agents on morphine-induced conditioned place preference were investigated by using an unbiased 3-day schedule of place conditioning design in rats. The conditioning treatments with subcutaneous injections of morphine (0.5-7.5 mg/kg) induced a significant dose-dependent conditioned place preference for the drug-associated place. Intra-ventral tegmental area injection of an anticholinesterase, physostigmine (2.5 and 5 microg/rat) or nicotinic acetylcholine receptor agonist, nicotine (0.5 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant conditioned place preference. Furthermore, intra-ventral tegmental area administration of muscarinic acetylcholine receptor antagonist, atropine (1-4 microg/rat) or nicotinic acetylcholine receptor antagonist, mecamylamine (5 and 7.5 microg/rat) dose-dependently inhibited the morphine (5 mg/kg)-induced place preference. Atropine or mecamylamine reversed the effect of physostigmine or nicotine on morphine response respectively. The injection of physostigmine, but not atropine, nicotine or mecamylamine, into the ventral tegmental area alone produced a significant place aversion. Moreover, intra-ventral tegmental area administration of the higher doses of physostigmine or atropine, but not nicotine or mecamylamine decreased the locomotor activity. We conclude that muscarinic and nicotinic acetylcholine receptors in the ventral tegmental area may critically mediate the rewarding effects of morphine.

Melatonin enhances the rewarding properties of morphine: involvement of the nitric oxidergic pathway

Melatonin has different interactions with opioids including the enhancement of the analgesic effects of morphine and also reversal of tolerance and dependence to morphine. The present study assessed the effect of melatonin on morphine reward in mice using a conditioned place preference (CPP) paradigm. Our data showed that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal (i.p.) administration of melatonin (1-40 mg/kg) alone did not induce either CPP or conditioned place aversion (CPA), while the combination of melatonin (5-20 mg/kg) and sub-effective dose of morphine (0.5 mg/kg) led to rewarding effect. We further investigated the involvement of the nitric oxidergic pathway in the enhancing effect of melatonin on morphine CPP, by a general nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). L-NAME (1 and 5 mg/kg, i.p.) alone or in combination with morphine (0.5 mg/kg) did not show any significant CPP or CPA. Co-administration of L-NAME (5 mg/kg) with an ineffective combination of melatonin (1 mg/kg) plus morphine (0.5 mg/kg) produced significant CPP that may imply the similarity of action of melatonin and L-NAME and involvement of the nitric oxidergic pathway in this regard. Our results indicate that pretreatment of animals with melatonin enhances the rewarding properties of morphine via a mechanism which may involve the nitric oxidergic pathway.

Influence of paternal genotypes on F1 behaviors: Lessons from several mouse strains

F1 and F2 mouse hybrids derived from different parental strains are becoming a useful tool in behavioral research, underlining the importance of their in-depth behavioral phenotyping. 129S1/SvImJ (S1), C57BL/6 (B6), NMRI (N) and BALB/c (BC) mice are commonly used in behavioral neuroscience, demonstrating marked behavioral differences. Here, we assess behavioral phenotypes of male mice of S1 and several hybrid strains (S1B6, S1N, S1BC) in a battery of behavioral tests, including the open field, novel odor exposure, novelty-induced grooming, horizontal rod (Suok) and the elevated plus maze tests. In addition, we assessed aggression and social barbering in these strains. Overall, the substantial differences observed here between these strains allow us to determine the influence of different genetic backgrounds on mouse behaviors, and more fully understand how different strain-specific behaviors overlap in the F1 progeny. Our results imply complex interplay between parental genotypes in anxiety, activity, grooming, aggression and barbering of their F1 progeny, further confirming the utility of F1 hybrids in behavioral neurogenetics.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.

Rewarding effects of 3,4-methylenedioxymethamphetamine ("Ecstasy") in dominant and subordinate OF-1 mice in the place preference conditioning paradigm

We tested the ability of 3,4-methylenedioxymethamphetamine (MDMA) to induce conditioned place preference (CPP) in dominant and subordinate OF-1 mice subjected to cohabitation and repeated sessions of agonistic confrontation, as well as in non-confronted mice. We selected doses of MDMA (2, 6, 10 mg/kg) previously reported to induce CPP in mice and we measured expression of c-Fos evoked by the treatments in non-confronted mice. MDMA induced c-Fos protein in several corticolimbic regions involved in drug-induced reward. Mice were exposed to brief sessions of agonistic confrontation on 5 consecutive days. Determinations of circulating hormones and drug conditioning tests were carried out on completion of the encounters. The results of hormone assays indicated that dominant mice had higher serum concentrations of testosterone, but lower levels of corticosterone, than submissive mice. Post-conditioning tests after drug conditioning (4 injections of MDMA or saline on alternate days) showed that MDMA significantly produced CPP at doses of 2 and 6 mg/kg, but not at 10 mg/kg, an inverted U-shaped pattern of conditioning that was invariable in non-confronted, dominant and subordinate mice. These results demonstrate that the endocrine and behavioural correlates linked to social status and social stress in mice are not paralleled by significant changes in the rewarding efficacy of MDMA in the CPP paradigm under the specific conditions tested.

GABA(A) receptors of hippocampal CA1 regions are involved in the acquisition and expression of morphine-induced place preference

In the present study, the effects of bilateral intra-hippocampal CA1 (intra-CA1) injections of GABA(A) receptor agonist and/or antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. The conditioning treatments with subcutaneous (s.c.) injections of different doses of morphine (0.5-7.5 mg/kg) induced a conditioned place preference (CPP) for the drug-associated place in a dose-dependent manner. Intra-CA1 administration of the GABA(A) receptor agonist, muscimol (0.25, 0.5 and 1 microg/rat) significantly inhibited the morphine (5 mg/kg, s.c.)-induced CPP. Intra-CA1 injections of different doses of the GABA(A) receptor antagonist, bicuculline (0.25, 0.5 and 1 microg/rat), in combination with an ineffective dose of morphine (0.5 mg/kg, s.c.) elicited a significant CPP. However, muscimol or bicuculline by themselves did not elicit any effect on place conditioning. Furthermore, the muscimol-induced inhibition of morphine response was reversed by bicuculline (1 microg/rat, intra-CA1) administration. On the other hand, the bilateral intra-CA1 injections of muscimol (0.25, 0.5 and 1 microg/rat) or bicuculline (0.5, 1 and 2 microg/rat) significantly decreased the expression of morphine-induced CPP. Intra-CA1 administration of different doses of muscimol or bicuculline had no effect on locomotor activity in the testing phase. Our data indicated that the GABA(A) receptors of the hippocampal CA1 regions may play an important role in the acquisition and expression of morphine-induced place preference.

Intra-hippocampal inhibition of protein kinase AII attenuates morphine-induced conditioned place preference

Morphine and other drugs of abuse modulate protein kinase A (PKA) signaling within the mesolimbic reward pathway. Using a balanced conditioned place preference (CPP) paradigm, we studied the possible involvement of protein kinase AII (PKA II) on the acquisition, expression and consolidation of morphine place conditioning in male Wistar rats. Subcutaneous administration of various doses of morphine sulfate (1-9 mg/kg) induced CPP in a dose-dependent manner. H-89, a selective PKA II inhibitor, was administered into CA1 region of the hippocampus at 1, 2.5 and 5 microM/rat. Using a 3-day schedule of conditioning, it was found that the H-89 did not produce a significant place preference or place aversion. H-89 (1, 2.5 and 5 microM/rat) significantly reduced the time spent by rats in the morphine compartment when given immediately after each conditioning session (consolidation), whereas it had no effect when administered before morphine during the conditioning phase (acquisition) or before testing for place preference in the absence of morphine (expression). It is concluded that the PKA II may play an active role in the consolidation of reward-related memory of morphine in CA1 region of the hippocampus.

Feeling strained? Influence of genetic background on depression-related behavior in mice: a review

Depression is a growing pandemic in developed societies. The use of inbred mouse strains in pre-clinical psychiatric research has proven to be a valuable resource. Firstly, they provide the background for genetic manipulations that aid in the discovery of molecular pathways that may be involved in major depression. Further, inbred mouse strains are also being used in the determination of genetic and environmental influences that may pre-dispose or trigger depression-related behavior. This review aims to highlight the utility of inbred mouse strains in depression research, while providing an overview of the current state of research into behavioral differences between strains in paradigms commonly used in the field. Neurochemical differences that may underlie strain differences are examined, and some caveats and cautions associated with the use of inbred strains are highlighted.

Histaminergic receptors of medial septum and conditioned place preference: D1 dopamine receptor mechanism

In the present study, the effects of intra-medial septum injections of histamine and/or the histamine H1 or H2 receptor antagonists on the acquisition of conditioned place preference (CPP) in male Wistar rats have been investigated. Our data showed that the conditioning treatments with intra-medial septum injection of different doses of histamine (0.5-15 microg/rat) induced a significant CPP for the drug-associated place. Using a 3-day schedule of conditioning, it was found that the histamine H1 receptor antagonist, pyrilamine (10 and 15 microg/rat, intra-medial septum) also induced a significant place preference. In addition, pyrilamine inhibited the histamine (7.5 microg/rat)-induced place preference. Intra-medial septum administration of the histamine H2 receptor antagonist, ranitidine (5-15 microg/rat) alone or in combination with histamine did not produce a significant place preference or place aversion. On the other hand, intra-medial septum administration of the dopamine D1 receptor antagonist, SCH 233390 (0.5, 0.75 and 1 microg/rat) inhibited the histamine (7.5 microg/rat) or pyrilamine (15 microg/rat)-induced place preference in a dose-dependent manner, but no effect was observed for the dopamine D2 receptor antagonist, sulpiride on the histamine or pyrilamine response. The administration of histamine (2.5-15 microg/rat) or pyrilamine (10 and 15 microg/rat) during acquisition increased locomotor activity of the animals on the testing days. The results suggest that histaminergic receptors of the medial septum may be involved in CPP and thus it is postulated that dopamine D1 receptors may play an important role in this effect.

Agmatine potentiates morphine-induced conditioned place preference in mice: modulation by alpha2-adrenoceptors

The effects of agmatine, an endogenous polyamine metabolite formed by decarboxylation of L-arginine, and its combination with morphine on conditioned place preference (CPP) has been investigated in male mice. Our data show that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increases the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal administration of agmatine (1-40 mg/kg) alone does not induce either CPP or conditioned place aversion, while combination of agmatine and subeffective doses of morphine leads to potent rewarding effects. Lower doses of morphine (0.1, 0.05, and 0.01 mg/kg) are able to induce CPP in mice pretreated with agmatine 1, 5, and 10 mg/kg, respectively. Concomitant intraperitoneal administration of UK 14 304 (0.5 mg/kg), a highly selective alpha2-agonist, with per se noneffective dose of morphine (0.5 mg/kg) and also its combination with noneffective doses of agmatine (1 mg/kg) plus morphine (0.05 mg/kg) produces significant CPP. UK 14 304 (0.05, 0.5 mg/kg) alone, or in combination with agmatine (1, 5 mg/kg) have had no effect. We have further investigated the possible involvement of the alpha2-adrenoceptors in the potentiating effect of agmatine on morphine-induced place preference. Selective alpha2-antagonists, yohimbine (0.005 mg/kg) and RX821002 (0.1, 0.5 mg/kg), block the CPP induced by concomitant administration of agmatine (5 mg/kg) and morphine (0.05 mg/kg). Yohimbine (0.001-0.05 mg/kg) or RX821002 (0.05-0.5 mg/kg) alone or in combination with morphine (0.05 mg/kg) or agmatine (5 mg/kg) fail to show any significant place preference or aversion. Our results indicate that pretreatment of animals with agmatine enhances the rewarding properties of morphine via a mechanism which may involve alpha2-adrenergic receptors.

Rewarding properties of sildenafil citrate in mice: role of the nitric oxide-cyclic GMP pathway

RATIONALE

Sildenafil citrate is widely prescribed for erectile dysfunction and acts by inhibiting phosphodiesterase type-5, resulting in accumulation of cyclic-guanosine monophosphate (cGMP) via activation of nitric oxide synthase (NOS). The nitric oxide (NO) system is relevant to the rewarding effects of various drugs of abuse. Several epidemiologic studies indicate that sildenafil is abused in a recreational fashion.

OBJECTIVES

In the present study, the rewarding properties of sildenafil and probable involvement of the NO-cGMP pathway were investigated in adult male NMRI mice.

METHODS

The ability of sildenafil citrate (1-40 mg/kg) to produce conditioned place preference (CPP) was studied in an unbiased CPP paradigm. The effects of NO precursor L-arginine, nonselective NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), and the inhibitor of guanylyl cyclase methylene blue (MB) on sildenafil-induced CPP were assessed.

RESULTS

Mice that received sildenafil (20 and 40 mg/kg) in one environment during conditioning phase displayed a preference for this environment. Both L-NAME (5 mg/kg) and MB (1 mg/kg) in combination with sildenafil (20 mg/kg) suppressed the acquisition of sildenafil-induced place preference. Lower and per se noneffective dose of sildenafil (10 mg/kg) and L-arginine (60 mg/kg), when coadministered, exerted a significant place conditioning.

CONCLUSIONS

Sildenafil shows rewarding properties that may involve the NO-cGMP pathway.

Cocaine- and Amphetamine-Regulated Transcript (CART) Peptides Modulate the Locomotor and Motivational Properties of Psychostimulants

Drug addiction results from a subversion of neural circuits that control motivation. Although the hedonic and addictive properties of psychostimulants and drugs of abuse are predominantly attributed to dopamine and glutamate, it is appreciated that other signaling molecules in the brain are important. This study suggests that cocaine- and amphetamine-regulated transcript (CART) peptides modulate the locomotor and motivational properties of psychostimulants. The behavioral effects of cocaine and amphetamine were examined in Carttm1Amgen knockout (Cart KO) and wild-type (WT) mice. Acute amphetamine administration increased in locomotor activity in WT mice, but this response was attenuated in Cart KO mice. Repeated amphetamine produced locomotor sensitization in WT mice but hardly any in Cart KO mice. Amphetamine elicited conditioned place preference in both genotypes, but amphetamine's potency was reduced in the Cart KO mice. Intravenous cocaine self-administration was observed in both genotypes, but Cart KO mice consumed less cocaine and responded less for cocaine than WT mice. The behavioral effects of psychostimulants were reduced in the mutant Cart KO mice. By contrast, open field activity and sucrose preference of drug-naive mice WT and Cart KO mice were not significantly different. The attenuated effects of amphetamine and cocaine in Cart KO mice suggest a positive neuromodulatory role for CART peptides in the locomotor and motivational properties of psychostimulants and implicate CART peptides in psychostimulant addiction.

The effects of SB 224289 on anxiety and cocaine-related behaviors in a novel object task

Cocaine facilitates dopamine transmission from ventral tegmental area (VTA) neurons that project to nucleus accumbens (NAcc), and previous experiments suggest that serotonin-1B (5-HT1B) receptors are involved in this effect. Specifically, activation of 5-HT1B receptors in VTA during cocaine exposure increases dopamine release in NAcc and enhances cocaine-induced locomotor activity, reward, and reinforcement. Thus, it is reasonable to hypothesize that blocking 5-HT1B activity may have the opposite effect. To investigate this hypothesis, SB 224289, a highly selective 5-HT1B antagonist, was used to block this receptor. In an open field/novel object exploration test, SB 224289 reduced cocaine-induced locomotion. However, SB 224289 also increased anxiety-like behavior, both alone and in combination with cocaine. This experiment gives evidence that 5-HT1B antagonists may reduce some of the behavioral effects of cocaine, but may have negative effects on anxiety as well.

Contrasting grooming phenotypes in three mouse strains markedly different in anxiety and activity (129S1, BALB/c and NMRI)

129S1/SvImJ (129S1), NMRI and BALB/c mice are widely used in behavioural research, demonstrating marked strain differences in their behavioural phenotypes. Grooming is a complex and essential ritual in the rodent behavioural repertoire with a general cephalocaudal progression (forepaws-nose-face-body-legs-tail and genitals). Various stressors as well as genetic manipulations have been reported to alter mouse grooming and its patterning, underlying the importance of analysis of grooming behaviours. Although strain differences between these mice have been assessed in many studies, no comparative analyses of their grooming have been performed. Here we show strain differences in spontaneous (novelty-induced) grooming between 129S1, NMRI and BALB/c mice. Overall, 129S1 mice demonstrated lower grooming activity and impaired microstructure (more interrupted bouts and incorrect transitions contrary to the cephalocaudal rule), accompanied by lower vertical exploration. In contrast, BALB/c and NMRI mice showed high vertical activity and unimpaired grooming microstructure, also exhibiting different grooming levels (BALB/c>NMRI). Our study suggests that contrasting grooming phenotypes in these mice may not be due to the strain differences in their sensory abilities, general activity levels, brain anatomy or aggressiveness, but rather reflect a complex interplay between anxiety, motor and displacement activity in these strains (hypoactive anxious phenotype in 129S1 mice, active anxious phenotype in BALB/c and non-anxious high displacement phenotype in NMRI mice). We suggest that ethological analysis of mouse grooming, such as that reported here, may be a useful tool in neurobehavioural research.

Effects of dopamine- and serotonin-related compounds on methamphetamine-induced self-injurious behavior in mice

Methamphetamine induces hyperlocomotion, and high doses of methamphetamine induce self-injurious behavior (SIB) in rodents. It is well known that the monoaminergic system is involved in methamphetamine-induced behavior. However, the effects of dopamine- and serotonin (5-HT)-related compounds on high-dose methamphetamine-induced behavior have not been sufficiently clarified. Therefore, the present study was designed to investigate the effects of dopamine receptor antagonists and indirect 5-HT receptor agonists on high-dose methamphetamine-induced behavior in mice. Methamphetamine (20 mg/kg) initially increased locomotor activity. As the dosage increased, continuous SIB accompanied by a reduction in locomotor activity was observed. The hyperlocomotion and SIB induced by 20 mg/kg of methamphetamine was abolished by high doses of SCH23390 and haloperidol, indicating that the hyperlocomotion and SIB induced by high doses of methamphetamine are mediated by the activation of D1- and D2-receptors. Furthermore, haloperidol (0.1 mg/kg) potently increased locomotor activity in combination with 20 mg/kg methamphetamine. These results suggest that excess dopaminergic activation, especially activation of dopamine D2-receptors, may be involved in the decrease in locomotor activity induced by a high dose of methamphetamine. On the other hand, indirect 5-HT receptor agonists attenuated methamphetamine-induced SIB, suggesting that the stimulation of 5-HT receptors plays an important role in high-dose methamphetamine-induced SIB in mice.