The NMDA receptor modulator zelquistinel durably relieves behavioral deficits in three mouse models of autism spectrum disorder

Autism spectrum disorders (ASD) are complex neurodevelopmental disorders characterized by deficient social communication and interaction together with restricted, stereotyped behaviors. Currently approved treatments relieve comorbidities rather than core symptoms. Since excitation/inhibition balance and synaptic plasticity are disrupted in ASD, molecules targeting excitatory synaptic transmission appear as highly promising candidates to treat this pathology. Among glutamatergic receptors, the NMDA receptor has received particular attention through the last decade to develop novel allosteric modulators. Here, we show that positive NMDA receptor modulation by zelquistinel, a spirocyclic β-lactam platform chemical, relieves core symptoms in two genetic and one environmental mouse models of ASD. A single oral dose of zelquistinel rescued, in a dose-response manner, social deficits and stereotypic behavior in Shank3Δex13-16-/- mice while chronic intraperitoneal administration promoted a long-lasting relief of such autistic-like features in these mice. Subchronic oral mid-dose zelquistinel treatment demonstrated durable effects in Shank3Δex13-16-/-, Fmr1-/- and in utero valproate-exposed mice. Carry-over effects were best maintained in the Fmr1 null mouse model, with social parameters being still fully recovered two weeks after treatment withdrawal. Among recently developed NMDA receptor subunit modulators, zelquistinel displays a promising therapeutic potential to relieve core symptoms in ASD patients, with oral bioavailability and long-lasting effects boding well for clinical applications. Efficacy in three mouse models with different etiologies supports high translational value. Further, this compound represents an innovative pharmacological tool to investigate plasticity mechanisms underlying behavioral deficits in animal models of ASD.

Balance Between Projecting Neuronal Populations of the Nucleus Accumbens Controls Social Behavior in Mice

BACKGROUND

Deficient social interactions are a hallmark of major neuropsychiatric disorders, and accumulating evidence points to altered social reward and motivation as key underlying mechanisms of these pathologies. In the present study, we further explored the role of the balance of activity between D1 and D2 receptor-expressing striatal projection neurons (D1R- and D2R-SPNs) in the control of social behavior, challenging the hypothesis that excessive D2R-SPN activity, rather than deficient D1R-SPN activity, compromises social behavior.

METHODS

We selectively ablated D1R- and D2R-SPNs using an inducible diphtheria toxin receptor-mediated cell targeting strategy and assessed social behavior as well as repetitive/perseverative behavior, motor function, and anxiety levels. We tested the effects of optogenetic stimulation of D2R-SPNs in the nucleus accumbens (NAc) and pharmacological compounds repressing D2R-SPN.

RESULTS

Targeted deletion of D1R-SPNs in the NAc blunted social behavior in mice, facilitated motor skill learning, and increased anxiety levels. These behaviors were normalized by pharmacological inhibition of D2R-SPN, which also repressed transcription in the efferent nucleus, the ventral pallidum. Ablation of D1R-SPNs in the dorsal striatum had no impact on social behavior but impaired motor skill learning and decreased anxiety levels. Deletion of D2R-SPNs in the NAc produced motor stereotypies but facilitated social behavior and impaired motor skill learning. We mimicked excessive D2R-SPN activity by optically stimulating D2R-SPNs in the NAc and observed a severe deficit in social interaction that was prevented by D2R-SPN pharmacological inhibition.

CONCLUSIONS

Repressing D2R-SPN activity may represent a promising therapeutic strategy to relieve social deficits in neuropsychiatric disorders.

Chronic sodium bromide treatment relieves autistic-like behavioral deficits in three mouse models of autism

Autism Spectrum Disorders (ASD) are neurodevelopmental disorders whose diagnosis relies on deficient social interaction and communication together with repetitive behavior. To date, no pharmacological treatment has been approved that ameliorates social behavior in patients with ASD. Based on the excitation/inhibition imbalance theory of autism, we hypothesized that bromide ions, long used as an antiepileptic medication, could relieve core symptoms of ASD. We evaluated the effects of chronic sodium bromide (NaBr) administration on autistic-like symptoms in three genetic mouse models of autism: Oprm1^-/-, Fmr1^-/- and Shank3^Δex13-16-/- mice. We showed that chronic NaBr treatment relieved autistic-like behaviors in these three models. In Oprm1^-/- mice, these beneficial effects were superior to those of chronic bumetanide administration. At transcriptional level, chronic NaBr in Oprm1 null mice was associated with increased expression of genes coding for chloride ions transporters, GABA_A receptor subunits, oxytocin and mGlu4 receptor. Lastly, we uncovered synergistic alleviating effects of chronic NaBr and a positive allosteric modulator (PAM) of mGlu4 receptor on autistic-like behavior in Oprm1^-/- mice. We evidenced in heterologous cells that bromide ions behave as PAMs of mGlu4, providing a molecular mechanism for such synergy. Our data reveal the therapeutic potential of bromide ions, alone or in combination with a PAM of mGlu4 receptor, for the treatment of ASDs.

The microbial metabolite p-Cresol induces autistic-like behaviors in mice by remodeling the gut microbiota

BACKGROUND

Autism spectrum disorders (ASD) are associated with dysregulation of the microbiota-gut-brain axis, changes in microbiota composition as well as in the fecal, serum, and urine levels of microbial metabolites. Yet a causal relationship between dysregulation of the microbiota-gut-brain axis and ASD remains to be demonstrated. Here, we hypothesized that the microbial metabolite p-Cresol, which is more abundant in ASD patients compared to neurotypical individuals, could induce ASD-like behavior in mice.

RESULTS

Mice exposed to p-Cresol for 4 weeks in drinking water presented social behavior deficits, stereotypies, and perseverative behaviors, but no changes in anxiety, locomotion, or cognition. Abnormal social behavior induced by p-Cresol was associated with decreased activity of central dopamine neurons involved in the social reward circuit. Further, p-Cresol induced changes in microbiota composition and social behavior deficits could be transferred from p-Cresol-treated mice to control mice by fecal microbiota transplantation (FMT). We also showed that mice transplanted with the microbiota of p-Cresol-treated mice exhibited increased fecal p-Cresol excretion, compared to mice transplanted with the microbiota of control mice. In addition, we identified possible p-Cresol bacterial producers. Lastly, the microbiota of control mice rescued social interactions, dopamine neurons excitability, and fecal p-Cresol levels when transplanted to p-Cresol-treated mice.

CONCLUSIONS

The microbial metabolite p-Cresol induces selectively ASD core behavioral symptoms in mice. Social behavior deficits induced by p-Cresol are dependant on changes in microbiota composition. Our study paves the way for therapeutic interventions targeting the microbiota and p-Cresol production to treat patients with ASD. Video abstract.

The orphan receptor GPR88 blunts the signaling of opioid receptors and multiple striatal GPCRs

GPR88 is an orphan G protein-coupled receptor (GPCR) considered as a promising therapeutic target for neuropsychiatric disorders; its pharmacology, however, remains scarcely understood. Based on our previous report of increased delta opioid receptor activity in Gpr88 null mice, we investigated the impact of GPR88 co-expression on the signaling of opioid receptors in vitro and revealed that GPR88 inhibits the activation of both their G protein- and β-arrestin-dependent signaling pathways. In Gpr88 knockout mice, morphine-induced locomotor sensitization, withdrawal and supra-spinal analgesia were facilitated, consistent with a tonic inhibitory action of GPR88 on µOR signaling. We then explored GPR88 interactions with more striatal versus non-neuronal GPCRs, and revealed that GPR88 can decrease the G protein-dependent signaling of most receptors in close proximity, but impedes β-arrestin recruitment by all receptors tested. Our study unravels an unsuspected buffering role of GPR88 expression on GPCR signaling, with intriguing consequences for opioid and striatal functions.

The translational regulator FMRP controls lipid and glucose metabolism in mice and humans

Objectives

The Fragile X Mental Retardation Protein (FMRP) is a widely expressed RNA-binding protein involved in translation regulation. Since the absence of FMRP leads to Fragile X Syndrome (FXS) and autism, FMRP has been extensively studied in brain. The functions of FMRP in peripheral organs and on metabolic homeostasis remain elusive; therefore, we sought to investigate the systemic consequences of its absence.

Methods

Using metabolomics, in vivo metabolic phenotyping of the Fmr1-KO FXS mouse model and in vitro approaches, we show that the absence of FMRP induced a metabolic shift towards enhanced glucose tolerance and insulin sensitivity, reduced adiposity, and increased β-adrenergic-driven lipolysis and lipid utilization.

Results

Combining proteomics and cellular assays, we highlight that FMRP loss increased hepatic protein synthesis and impacted pathways notably linked to lipid metabolism. Mapping metabolomic and proteomic phenotypes onto a signaling and metabolic network, we predicted that the coordinated metabolic response to FMRP loss was mediated by dysregulation in the abundances of specific hepatic proteins. We experimentally validated these predictions, demonstrating that the translational regulator FMRP associates with a subset of mRNAs involved in lipid metabolism. Finally, we highlight that FXS patients mirror metabolic variations observed in Fmr1-KO mice with reduced circulating glucose and insulin and increased free fatty acids.

Conclusions

Loss of FMRP results in a widespread coordinated systemic response that notably involves upregulation of protein translation in the liver, increased utilization of lipids, and significant changes in metabolic homeostasis. Our study unravels metabolic phenotypes in FXS and further supports the importance of translational regulation in the homeostatic control of systemic metabolism.

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Loss of the translational regulator FMRP impacts glucose and lipid homeostasis in mouse and human.

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FMR1-deficiency modifies blood metabolic markers.

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Loss of FMRP enhances the insulin response and lipolysis.

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Loss of FMRP exaggerates hepatic protein synthesis.

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FMRP controls the translation of key hepatic proteins involved in lipid metabolism.

Back-translating behavioral intervention for autism spectrum disorders to mice with blunted reward restores social abilities

The mu opioid receptor (MOR) plays a critical role in modulating social behavior in humans and animals. Accordingly, MOR null mice display severe alterations in their social repertoire as well as multiple other behavioral deficits, recapitulating core and secondary symptoms of autism spectrum disorder (ASD). Such behavioral profile suggests that MOR dysfunction, and beyond this, altered reward processes may contribute to ASD etiopathology. Interestingly, the only treatments that proved efficacy in relieving core symptoms of ASD, early behavioral intervention programs, rely principally on positive reinforcement to ameliorate behavior. The neurobiological underpinnings of their beneficial effects, however, remain poorly understood. Here we back-translated applied behavior analysis (ABA)-based behavioral interventions to mice lacking the MOR (Oprm1^-/-), as a model of autism with blunted reward processing. By associating a positive reinforcement, palatable food reward, to daily encounter with a wild-type congener, we were able to rescue durably social interaction and preference in Oprm1^-/- mice. Along with behavioral improvements, the expression of marker genes of neuronal activity and plasticity as well as genes of the oxytocin/vasopressin system were remarkably normalized in the reward/social circuitry. Our study provides further evidence for a critical involvement of reward processes in driving social behavior and opens new perspectives regarding therapeutic intervention in ASD.

Long-Term Transcriptional Consequences of Drug Exposure as Cues to Understand Vulnerability to Relapse: Advances and Perspectives

Quitting drug abuse represents a true challenge for addicted individuals because of the highly persistent vulnerability to relapse. Identifying long-lasting, drug-induced alterations in the brain-including at the transcriptome level-that underlie such vulnerability appears invaluable to improve relapse prevention. Despite substantial technological developments and research effort, however, the picture of drug-induced adaptations provided by high-throughput transcriptomics remains frustratingly partial, notably because of methodological issues. Major advances were made, however, regarding the time course and specificity of long-term transcriptional consequences of drug exposure as well as the recruitment of small, noncoding mRNAs (or miRNAs [microRNAs]) that were previously undetectable. Most importantly, high-throughput studies have benefited from systems biology approaches and shifted their interest toward regulations within functional gene networks rather than individual changes. Such network-based gene discovery approaches have proven informative to delineate the physiological processes, cellular signaling pathways, and neuronal populations altered by drug exposure. Provided the high-throughput effort will be pursued, together with the development of adapted bioinformatics tools, addiction transcriptomics should progressively integrate data across multiple scales (from epigenome to protein), allowing a better understanding of the genetics of drug abuse and opening novel therapeutic trails.

Differential behavioral and molecular alterations upon protracted abstinence from cocaine versus morphine, nicotine, THC and alcohol

Unified theories of addiction are challenged by differing drug-seeking behaviors and neurobiological adaptations across drug classes, particularly for narcotics and psychostimulants. We previously showed that protracted abstinence to opiates leads to despair behavior and social withdrawal in mice, and we identified a transcriptional signature in the extended amygdala that was also present in animals abstinent from nicotine, Δ9-tetrahydrocannabinol (THC) and alcohol. Here we examined whether protracted abstinence to these four drugs would also share common behavioral features, and eventually differ from abstinence to the prototypic psychostimulant cocaine. We found similar reduced social recognition, increased motor stereotypies and increased anxiety with relevant c-fos response alterations in morphine, nicotine, THC and alcohol abstinent mice. Protracted abstinence to cocaine, however, led to strikingly distinct, mostly opposing adaptations at all levels, including behavioral responses, neuronal activation and gene expression. Together, these data further document the existence of common hallmarks for protracted abstinence to opiates, nicotine, THC and alcohol that develop within motivation/emotion brain circuits. In our model, however, these do not apply to cocaine, supporting the notion of unique mechanisms in psychostimulant abuse.

μ opioid receptor, social behaviour and autism spectrum disorder: reward matters

The endogenous opioid system is well known to relieve pain and underpin the rewarding properties of most drugs of abuse. Among opioid receptors, the μ receptor mediates most of the analgesic and rewarding properties of opioids. Based on striking similarities between social distress, physical pain and opiate withdrawal, μ receptors have been proposed to play a critical role in modulating social behaviour in humans and animals. This review summarizes experimental data demonstrating such role and proposes a novel model, the μ opioid receptor balance model, to account for the contribution of μ receptors to the subtle regulation of social behaviour. Interestingly, μ receptor null mice show behavioural deficits similar to those observed in patients with autism spectrum disorder (ASD), including severe impairment in social interactions. Therefore, after a brief summary of recent evidence for blunted (social) reward processes in subjects with ASD, we review here arguments for altered μ receptor function in this pathology. This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Fragile X Mental Retardation Protein (FMRP) controls diacylglycerol kinase activity in neurons

Fragile X syndrome (FXS) is caused by the absence of the Fragile X Mental Retardation Protein (FMRP) in neurons. In the mouse, the lack of FMRP is associated with an excessive translation of hundreds of neuronal proteins, notably including postsynaptic proteins. This local protein synthesis deregulation is proposed to underlie the observed defects of glutamatergic synapse maturation and function and to affect preferentially the hundreds of mRNA species that were reported to bind to FMRP. How FMRP impacts synaptic protein translation and which mRNAs are most important for the pathology remain unclear. Here we show by cross-linking immunoprecipitation in cortical neurons that FMRP is mostly associated with one unique mRNA: diacylglycerol kinase kappa (Dgkκ), a master regulator that controls the switch between diacylglycerol and phosphatidic acid signaling pathways. The absence of FMRP in neurons abolishes group 1 metabotropic glutamate receptor-dependent DGK activity combined with a loss of Dgkκ expression. The reduction of Dgkκ in neurons is sufficient to cause dendritic spine abnormalities, synaptic plasticity alterations, and behavior disorders similar to those observed in the FXS mouse model. Overexpression of Dgkκ in neurons is able to rescue the dendritic spine defects of the Fragile X Mental Retardation 1 gene KO neurons. Together, these data suggest that Dgkκ deregulation contributes to FXS pathology and support a model where FMRP, by controlling the translation of Dgkκ, indirectly controls synaptic proteins translation and membrane properties by impacting lipid signaling in dendritic spine.

Cues predicting drug or food reward restore morphine-induced place conditioning in mice lacking delta opioid receptors

RATIONALE

The exact role of delta opioid receptors in drug-induced conditioned place preference (CPP) remains debated. Under classical experimental conditions, morphine-induced CPP is decreased in mice lacking delta opioid receptors (Oprd1 (-/-)). Morphine self-administration, however, is maintained, suggesting that drug-context association rather than drug reward is deficient in these animals.

OBJECTIVES

This study further examined the role of delta opioid receptors in mediating drug-cue associations, which are necessary for the expression of morphine-induced CPP.

METHODS

We first identified experimental conditions under which Oprd1 (-/-) mice are able to express CPP to morphine (5, 10 or 20 mg/kg) in a drug-free state and observed that, in this paradigm, CPP was dependent on circadian time conditions. We then took advantage of this particularity to assess the ability of various cues (internal or discrete), predicting either drug or food reward, to restore CPP induced by morphine (10 mg/kg) in Oprd1 (-/-) mice in conditions under which they normally fail to express CPP.

RESULTS

We found that presentation of circadian, drug or auditory cues, predicting morphine or food reward, restored morphine CPP in Oprd1 (-/-) mice, which then performed as well as control mice.

CONCLUSIONS

This study reveals that, in contrast to spatial cues, internal or discrete morphine-predicting stimuli permit full expression of morphine CPP in Oprd1 (-/-) mice. Delta receptors, therefore, appear to play a crucial role in modulating spatial contextual cue-related responses. This activity may be critical when context gains control over behavior, as is the case for context-induced relapse in drug abuse.

Protracted abstinence from distinct drugs of abuse shows regulation of a common gene network

Addiction is a chronic brain disorder. Prolonged abstinence from drugs of abuse involves dysphoria, high stress responsiveness and craving. The neurobiology of drug abstinence, however, is poorly understood. We previously identified a unique set of hundred mu-opioid receptor-dependent genes in the extended amygdala, a key site for hedonic and stress processing in the brain. Here we examined these candidate genes either immediately after chronic morphine, nicotine, Δ9-tetrahydrocannabinol or alcohol, or following 4 weeks of abstinence. Regulation patterns strongly differed among chronic groups. In contrast, gene regulations strikingly converged in the abstinent groups and revealed unforeseen common adaptations within a novel huntingtin-centered molecular network previously unreported in addiction research. This study demonstrates that, regardless the drug, a specific set of transcriptional regulations develops in the abstinent brain, which possibly contributes to the negative affect characterizing protracted abstinence. This transcriptional signature may represent a hallmark of drug abstinence and a unitary adaptive molecular mechanism in substance abuse disorders.

Deletion of the δ opioid receptor gene impairs place conditioning but preserves morphine reinforcement

BACKGROUND

Converging experimental data indicate that δ opioid receptors contribute to mediate drug reinforcement processes. Whether their contribution reflects a role in the modulation of drug reward or an implication in conditioned learning, however, has not been explored. In the present study, we investigated the impact of δ receptor gene knockout on reinforced conditioned learning under several experimental paradigms.

METHODS

We assessed the ability of δ receptor knockout mice to form drug-context associations with either morphine (appetitive)- or lithium (aversive)-induced Pavlovian place conditioning. We also examined the efficiency of morphine to serve as a positive reinforcer in these mice and their motivation to gain drug injections, with operant intravenous self-administration under fixed and progressive ratio schedules and at two different doses.

RESULTS

Mutant mice showed impaired place conditioning in both appetitive and aversive conditions, indicating disrupted context-drug association. In contrast, mutant animals displayed intact acquisition of morphine self-administration and reached breaking-points comparable to control subjects. Thus, reinforcing effects of morphine and motivation to obtain the drug were maintained.

CONCLUSION

Collectively, the data suggest that δ receptor activity is not involved in morphine reinforcement but facilitates place conditioning. This study reveals a novel aspect of δ opioid receptor function in addiction-related behaviors.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Self-administration of the GABAA agonist muscimol into the medial septum: dependence on dopaminergic mechanisms

RATIONALE

Reinforcement in the medial septal division (MSDB) might involve local GABAergic mechanisms.

OBJECTIVES

We used intracranial self-administration to determine whether the GABAA agonist muscimol or antagonist bicuculline might have rewarding effects when infused into the MSDB. We assessed the anatomical specificity of muscimol intra-MSDB self-administration by injecting this molecule into the nucleus accumbens (NAc). Finally, we evaluated the involvement of dopaminergic mechanisms in muscimol self-administration.

MATERIALS AND METHODS

BALB/c mice were implanted with a guide cannula targeting the MSDB or the NAc. They were trained to discriminate between the two arms of a Y-maze, one arm being reinforced by muscimol or bicuculline injections. Another group of MSDB implanted mice was pre-treated intraperitoneally before muscimol self-administration with a D1 (SCH23390) or D2/D3 (sulpiride) receptor antagonist or vehicle. A last group of MSDB mice received additional bilateral guide cannulae targeting the ventral tegmental area (VTA) or a more dorsal region to assess the effects of intra-VTA injection of SCH23390 on intra-MSDB muscimol self-administration.

RESULTS

Mice self-administered intra-MSDB muscimol (0.6, 1.2, or 12 ng/50 nl), but not bicuculline (1.5 or 3 ng/50 nl). Systemic pre-treatment with SCH23390 (25 microg/kg) or sulpiride (50 mg/kg) or bilateral injection of SCH23390 (0.25 microg/0.1 microl) into the VTA prevented acquisition of intra-MSDB muscimol self-administration.

CONCLUSION

The activation of GABAA receptors in the MSDB supports self-administration, and dopamine release from the VTA may be involved in the acquisition of this behaviour. The MSDB could represent a common brain substrate for the rewarding properties of drugs facilitating GABAA tone.

Morphine self-administration into the lateral septum depends on dopaminergic mechanisms: Evidence from pharmacology and Fos neuroimaging

Mice self-administer morphine into the lateral septum (LS), but the neuronal connections underlying this behaviour remain unknown. The present study tested whether the acquisition of intra-LS morphine self-administration depends on dopaminergic mechanisms. Mice were allowed to self-inject morphine (5 or 20ng/50nl) or vehicle directly into the LS using a spatial discrimination Y-maze task. Fos imaging was used to evaluate neuronal activation in cerebral structures directly connected to the LS or belonging to the dopaminergic system. The involvement of dopaminergic and opioidergic mechanisms was assessed by pre-treating naive mice peripherally with the D1 antagonist SCH23390, the D2/D3 antagonist sulpiride or the opiate antagonist naloxone before daily self-administration sessions. Mice acquired self-administration behaviour for intra-LS morphine that was associated with increased Fos expression in the ventral tegmental area (VTA), dorsal and ventral striatum and prefrontal cortex. Pre-treating animals with naloxone, SCH23390 or sulpiride completely prevented them from acquiring intra-LS morphine self-administration. All three antagonists consistently blocked Fos expression in the prefrontal cortex, but not in the VTA and striatum. Taken together, our results show that morphine self-administration into the LS depends on dopaminergic (D1 and D2/D3) and opioidergic mechanisms. Furthermore, they suggest that opioid peptides released in the LS could participate in regulating the activity of mesotegmental dopaminergic neurons.

Food-induced behavioral sensitization, its cross-sensitization to cocaine and morphine, pharmacological blockade, and effect on food intake

Repeated administration of abused drugs sensitizes their stimulant effects and results in a drug-paired environment eliciting conditioned activity. We tested whether food induces similar effects. Food-deprived male mice were given novel food during 30 min tests in a runway (FR group) that measured locomotor activity. Whereas the activity of this group increased with repeated testing, that of a group exposed to the runways but that received the food in the home cage (FH group), or of a group satiated by prefeeding before testing (SAT group), decreased. When exposed to the runways in the absence of food, the paired group was more active than the other groups (conditioned activity); no activity differences were seen in an alternative, non-food-paired, apparatus. Conditioned activity survived a 3-week period without runway exposure. Conditioned activity was selectively reduced by the opiate antagonist naltrexone (10-20 mg/kg) and by the noncompetitive AMPA receptor antagonist GYKI 52466 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride] (5-10 mg/kg). The D1 antagonist SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride] (15-30 microg/kg) and D2 antagonist sulpiride (25-125 mg/kg) reduced activity nonspecifically. A single intraperitoneal dose of cocaine (10 mg/kg) or morphine (20 mg/kg) increased activity compared with saline, the stimulant effect being larger in the FR group, suggesting "cross-sensitization" to these drugs. However, pretreatment with GYKI 52466 or naltrexone at doses that suppressed conditioned activity in FR animals suppressed cross-sensitization to cocaine. When allowed ad libitum access to food in the runway, FR mice consumed more pellets in a time-limited test. Thus, many of the features of behavioral sensitization to drugs can be demonstrated using food reward and may contribute to excessive eating.

Previous experience of ethanol withdrawal increases withdrawal-induced c-fos expression in limbic areas, but not withdrawal-induced anxiety and prevents withdrawal-induced elevations in plasma corticosterone

RATIONALE

Increased anxiety is a characteristic of the acute ethanol withdrawal syndrome. Repeated exposure of rats to withdrawal from chronic ethanol increases sensitivity to seizures.

OBJECTIVES

We investigated whether repeated withdrawal experience increases withdrawal-induced anxiety and stress, and if it changes withdrawal-induced activation of related brain areas.

METHODS

Rats were chronically treated with an ethanol-containing liquid diet either for 24 days continuously (single withdrawal, SWD) or interspersed with 2x3-day withdrawal periods (repeated withdrawal, RWD), or with a control diet. Eight hours after ethanol withdrawal, anxiety-like behaviour was tested in the elevated plus-maze, blood corticosterone levels were measured, and expression level of markers of neuronal activity and plasticity, c-fos and zif268, was assessed.

RESULTS

Eight hours after ethanol withdrawal, SWD rats showed increased anxiety on the elevated plus-maze relative to control rats. Rats given previous withdrawal experiences did not show further increases in measures of anxiety. Corticosterone levels were elevated during withdrawal in SWD rats but not in RWD rats. RWD resulted in marked increases in c-fos expression in amygdala, hippocampus, nucleus accumbens and dorsolateral periaqueductal grey. In contrast, zif268 expression was not increased after RWD, and in central amygdala the marked increase in zif268 seen after SWD was absent after RWD.

CONCLUSIONS

The data suggest increased ability of withdrawal to activate neuronal circuits but reduced plasticity after RWD. We suggest parallels between the consequences of repeated ethanol withdrawal and repeated exposure to stress, and discuss implications of withdrawal for brain plasticity.

Modulation of anxiety by μ-opioid receptors of the lateral septal region in mice

Morphine and opiates are known to exert anxiolytic effects, probably by interacting with the GABAergic system. The lateral septum (LS), mainly constituted of GABA neurons, exhibits high densities of mu-opiate receptors and could thus represent one the brain sites where opiates interact with GABAergic transmission to modulate anxiety. We examined the effects of intra-LS morphine injections on measures of anxiety using the elevated plus-maze and hole-board tests. Fos imaging was used to identify neural circuits involved in anxiety modulation. Unilateral intra-LS morphine (100 or 500 ng/100 nl) decreased open-arm exploration in the plus-maze and reduced head-dipping frequency in the hole-board, an anxiogenic-like effect associated with decreased Fos expression in the ventral LS, the dorsal hippocampus and the anterior hypothalamus. Anatomical specificity was assessed by injecting morphine into the medial septum, which failed to produce anxiogenesis. Pre-injection of the mu-opioid receptor antagonist naloxonazine (100 ng/100 nl) into LS reversed morphine-induced anxiogenesis and the associated pattern of Fos expression, indicating a specific recruitment of mu-opioid receptors by morphine. Surprisingly, bilateral morphine injections (20 to 500 ng/100 nl) were not found anxiogenic, perhaps due to their stimulant effect. Taken together, these results suggest that LS mu-opioid receptors participate to the modulation of anxiety.

Effects of deletion of gria1 or gria2 genes encoding glutamatergic AMPA-receptor subunits on place preference conditioning in mice

RATIONALE

The conditioned place preference (CPP) paradigm has been used as a measure of the rewarding effects of a number of stimuli. Critically, this classical conditioning procedure requires the formation of associations between a rewarding stimulus and environmental cues, and the ability of these cues to direct subsequent behaviour.

OBJECTIVES

The purpose of the current experiments was to examine the role of glutamatergic transmission via subunit-specific populations of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the formation of stimulus--reward associations involving contextual stimuli.

METHODS

We investigated the ability of cocaine and food to induce a CPP in mice lacking either the GluR1 or GluR2 subunits of the AMPA receptor [gria1 or gria2 knockout (KO) mice]. In separate experiments, food pellets or cocaine (5--20 mg/kg IP) were paired with one compartment of the CPP apparatus, while no-food or vehicle was paired with an alternative compartment.

RESULTS

Following conditioning, gria1 KOs displayed a significant preference for the food or cocaine-paired compartment, and did not differ from wild-type (WT) controls. However, gria2 knockouts displayed a preference for a cocaine-paired compartment, but not a food-paired compartment, indicating a specific deficit in place preference conditioning to food.

CONCLUSIONS

These results obtained using knockout mice indicate that GluR2-containing AMPA receptors may be critical for learning about contextual stimuli relevant to food rewards, but not drug rewards. When the results are considered in relation to our previous findings with gria1 and gria2 knockout mice, they also raise questions about the CPP paradigm representing a model of conditioned reward over a conditioned approach interpretation. However, it would be important to confirm these findings with alternative approaches, should selective ligands become available.