Hyperactivity induced by the dopamine D2/D3 receptor agonist quinpirole is attenuated by inhibitors of endocannabinoid degradation in mice

Effect of Positive Allosteric Modulation and Orthosteric Agonism of Dopamine D2-like Receptors on Respiration in Mouse Models of Rett Syndrome

Rett syndrome (RTT) is an autism spectrum disorder caused by loss-of-function mutations in the methyl-CPG-binding protein 2 (Mecp2) gene. Frequent apneas and irregular breathing are prevalent in RTT, and also occur in rodent models of the disorder, including Mecp2Bird and Mecp2R168X mice. Sarizotan, a serotonin 5-HT1a and dopamine D2-like receptor agonist, reduces the incidence of apneas and irregular breathing in mouse models of RTT (Abdala et al., 2014). Targeting the 5HT1a receptor alone also improves respiration in RTT mice (Levitt et al., 2013). However, the contribution of D2-like receptors in correcting these respiratory disturbances remains untested. PAOPA, a dopamine D2-like receptor positive allosteric modulator, and quinpirole, a dopamine D2-like receptor orthosteric agonist, were used in conjunction with whole-body plethysmography to evaluate whether activation of D2-like receptors is sufficient to improve breathing disturbances in female heterozygous Mecp2Bird/+ and Mecp2R168X/+ mice. PAOPA did not significantly change apnea incidence or irregularity score in RTT mice. PAOPA also had no effect on the ventilatory response to hypercapnia (7% CO2). In contrast, quinpirole reduced apnea incidence and irregularity scores and improved the hypercapnic ventilatory response in Mecp2R168X/+ and Mecp2Bird/+ mice, while also reducing respiratory rate. These results suggest that D2-like receptors could contribute to the positive effects of sarizotan in the correction of respiratory abnormalities in Rett syndrome. However, positive allosteric modulation of D2-like receptors alone was not sufficient to evoke these effects.

Effects of L-Dopa, SKF-38393, and quinpirole on exploratory, anxiety- and depressive-like behaviors in pubertal female and male mice

Adolescence is a phase of substantial changes in the brain, characterized by maturational remodeling of many systems. This remodeling allows functional plasticity to adapt to a changing environment. The dopaminergic system is under morphological and physiological changes during this phase. In the present study, we investigated if changes in the dopaminergic tone alter mice behavior in a receptor and sex-specific manner, specifically at the beginning of the puberty period. We administered L-Dopa, SKF-38393 (D1 dopamine receptor agonist), and Quinpirole (D2 dopamine receptor agonist) and tested male and female mice's motor, anxiety- and depressive-like behavior. While females displayed an impaired exploratory drive, males presented an intense depressive-like response. Our results provide insights into the function of dopaminergic development in adolescent behavior and highlight the importance of studies in this time window with male and female subjects.

CHL1 depletion affects dopamine receptor D2-dependent modulation of mouse behavior

Introduction

The dopaminergic system plays a key role in the appropriate functioning of the central nervous system, where it is essential for emotional balance, arousal, reward, and motor control. The cell adhesion molecule close homolog of L1 (CHL1) contributes to dopaminergic system development, and CHL1 and the dopamine receptor D2 (D2R) are associated with mental disorders like schizophrenia, addiction, autism spectrum disorder and depression.

Methods

Here, we investigated how the interplay between CHL1 and D2R affects the behavior of young adult male and female wild-type (CHL+/+) and CHL1-deficient (CHL1-/-) mice, when D2R agonist quinpirole and antagonist sulpiride are applied.

Results

Low doses of quinpirole (0.02 mg/kg body weight) induced hypolocomotion of CHL1+/+ and CHL1-/- males and females, but led to a delayed response in CHL1-/- mice. Sulpiride (1 mg/kg body weight) affected locomotion of CHL1-/- females and social interaction of CHL1+/+ females as well as social interactions of CHL1-/- and CHL1+/+ males. Quinpirole increased novelty-seeking behavior of CHL1-/- males compared to CHL1+/+ males. Vehicle-treated CHL1-/- males and females showed enhanced working memory and reduced stress-related behavior.

Discussion

We propose that CHL1 regulates D2R-dependent functions in vivo. Deficiency of CHL1 leads to abnormal locomotor activity and emotionality, and to sex-dependent behavioral differences.

N-Oleoyl Glycine and Its Derivatives Attenuate the Acquisition and Expression of Cocaine-Induced Behaviors

Introduction: The endocannabinoid system (ECS) plays a key modulatory role during synaptic plasticity and homeostatic processes in the brain and plays an important role in the neurobiological processes underlying drug addiction. Impaired endocannabinoid (eCB) signaling contributes to dysregulated synaptic plasticity, increased stress responsivity, and craving that propel addiction. Therefore, we hypothesized that boosting the ECS by exogenous administration of selective eCBs will attenuate cocaine-induced behaviors. Materials and Methods: The behavioral paradigms included psychomotor sensitization (PS) and conditioned place preference (CPP). Liquid chromatography-mass spectrometry analysis was used for quantitative profiling of eCBs in mouse brain. Results: We first measured the levels of eCBs in different brain areas of the reward system following chronic cocaine treatment. We found that following daily administration of cocaine, the levels of N-oleoyl glycine (OlGly) were significantly elevated in the nucleus accumbens (NAc) in a region-specific manner. We next tested whether administration of OlGly will attenuate cocaine-induced behaviors. We found that administration of OlGly during withdrawal, but not during acquisition of PS, attenuated the expression of cocaine sensitization. In addition, the administration of OlGly during the acquisition of cocaine CPP, but not during withdrawal, attenuated the expression of cocaine-conditioned reward. To enhance the stability of OlGly and its duration of action, two methylated derivatives of OlGly were synthesized, the monomethylated OlGly (HU-595) and dimethylated OlGly (HU-596). We found that the effect of administration of HU-595 or HU-596 during cocaine conditioning did not differ from the OlGly-induced decrease in the expression of CPP. Conclusion: Our findings suggest that the ECS is involved in the common neurobiological mechanisms underlying the development and expression of cocaine reward and drug-seeking. Boosting the ECS exogenously has beneficial effects against cocaine-induced behaviors.

The potent α2-adrenoceptor antagonist RS 79948 also inhibits dopamine D2 -receptors: Comparison with atipamezole and raclopride

Neurochemical, electrophysiological and behavioral evidence indicate that the potent α₂-adrenoceptor antagonist RS 79948 is also a dopamine (DA) D₂ receptor antagonist. Thus, results from ligand binding and adenylate cyclase activity indicate that RS 79948 binds to D₂ receptors and antagonized D₂ receptor-mediated inhibition of cAMP synthesis at nanomolar concentrations. RESULTS: from microdialysis indicated that RS 79948 shared with the selective α₂-adrenergic antagonist atipamezole the ability to increase the co-release of DA and norepinephrine (NE) from noradrenergic terminals in the medial prefrontal cortex (mPFC), except that RS 79948-induced DA release persisted after noradrenergic denervation, unlike atipamezole effect, indicating that RS 79948 releases DA from dopaminergic terminals as well. Similarly to the D₂ antagonist raclopride, but unlike atipamezole, RS 79948 increased extracellular DA and DOPAC in the caudate nucleus. Electrophysiological results indicate that RS 79948 shared with raclopride the ability to activate the firing of ventral tegmental area (VTA) DA neurons, while atipamezole was ineffective. RESULTS: from behavioral studies indicated that RS 79948 exerted effects mediated by independent, cooperative and contrasting inhibition of α₂-and D₂ receptors. Thus, RS 79948, but not atipamezole, prevented D₂-autoreceptor mediated hypomotility produced by a small dose of quinpirole. RS 79948 potentiated, more effectively than atipamezole, quinpirole-induced motor stimulation. RS 79948 antagonized, less effectively than atipamezole, raclopride-induced catalepsy. Future studies should clarify if the dual α₂-adrenoceptor- and D₂-receptor antagonistic action might endow RS 79948 with potential therapeutic relevance in the treatment of schizophrenia, drug dependence, depression and Parkinson's disease.

The mouse at the popcorn stage of development

In mice, rats, and rabbits vigorous jumping and hyperexcitability occur at the popcorn stage of postnatal development. In view of subcortical structures appearing before cortical ones, the trait is deemed to occur at the maturation time of ascending excitatory projections from the brainstem and to disappear at the maturation time of descending inhibitory projections from the forebrain. There is evidence that the popcorn stage may be due in part to the lack of a cholinergic influence on dopamine systems. Based mostly on results found in adult mice and rats, there may also be a role for cortico-subcortical systems that include the cerebellum and basal ganglia requiring the influence of biogenic amines, glutamate, and endocannabinoids.

Distinct patterns of dyskinetic and dystonic features following D1 or D2 receptor stimulation in a mouse model of parkinsonism

L-DOPA-induced dyskinesia (LID) is a significant complication of dopamine replacement therapy in Parkinson's disease (PD), and the specific role of different dopamine receptors in this disorder is poorly understood. We set out to compare patterns of dyskinetic behaviours induced by the systemic administration of L-DOPA and D1 or D2 receptor (D1R, D2R) agonists in mice with unilateral 6-hydroxydopamine lesions. Mice were divided in four groups to receive increasing doses of L-DOPA, a D1R agonist (SKF38393), a D2/3 agonist (quinpirole), or a selective D2R agonist (sumanirole). Axial, limb and orofacial abnormal involuntary movements (AIMs) were rated using a well-established method, while dystonic features were quantified in different body segments using a new rating scale. Measures of abnormal limb and trunk posturing were extracted from high-speed videos using a software for markerless pose estimation (DeepLabCut). While L-DOPA induced the full spectrum of dyskinesias already described in this mouse model, SKF38393 induced mostly orofacial and limb AIMs. By contrast, both of the D2-class agonists (quinpirole, sumanirole) induced predominantly axial AIMs. Dystonia ratings revealed that these agonists elicited marked dystonic features in trunk/neck, forelimbs, and hindlimbs, which were overall more severe in sumanirole-treated mice. Accordingly, sumanirole induced pronounced axial bending and hindlimb divergence in the automated video analysis. In animals treated with SKF38393, the only appreciable dystonic-like reaction consisted in sustained tail dorsiflexion and stiffness. We next compared the effects of D1R or D2R selective antagonists in L-DOPA-treated mice, where only the D2R antagonist had a significant effect on dystonic features. Taken together these results indicate that the dystonic components of LID are predominantly mediated by the D2R.

Effects of Fatty Acid Amide Hydrolase Inhibitors Acute Administration on the Positive and Cognitive Symptoms of Schizophrenia in Mice

The connection between the endocannabinoid system (ECS) and schizophrenia is supported by a large body of research. The ECS is composed of two types cannabinoid (CB: CB1 and CB2) receptors and their endogenous ligands, endocannabinoids. The best-known endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), are intracellularly degraded by fatty acid hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. Thus, the function of ECS might be modulated in a direct way, through CB receptor ligands or indirectly by FAAH and MAGL inhibitors. We evaluated that the direct influence of ECS, using FAAH (URB 597) and MAGL (JZL 184) inhibitors, on the schizophrenia-like effects in mice. The behavioral schizophrenia-like symptoms were obtained in animals by using N-methyl D-aspartate (NMDA) receptor antagonists, MK-801. An acute administration of MK-801 (0.3 and 0.6 mg/kg) induced psychotic symptoms in rodents, manifested as the increase in locomotor activity, measured in actimeters, as well as the memory impairment, assessed in the passive avoidance (PA) task. We revealed that an acute administration of URB 597, at the dose of 0.3 mg/kg, attenuated MK-801 (0.6 mg/kg)-induced memory impairment. In turn, an acute administration of URB 597 at a higher dose (1 mg/kg) potentiated MK-801 (0.3 mg/kg)-induced memory impairment. Similarly, an acute administration of JZL 184 (20 and 40 mg/kg) intensified an amnestic effect of MK-801 (0.3 mg/kg). Moreover, an acute injection of JZL 184 (1 mg/kg) potentiated hyperlocomotion is provoked by MK-801 (0.3 and 0.6 mg/kg) administration. The present findings clearly indicate that ECS, through an indirect manner, modulates a variety of schizophrenia-like responses in mice.

Targeting Dopamine D2, Adenosine A2A, and Glutamate mGlu5 Receptors to Reduce Repetitive Behaviors in Deer Mice

Repetitive behaviors are seemingly purposeless patterns of behavior that vary little in form and are characteristic of many neurodevelopmental, psychiatric, and neurologic disorders. Our work has identified an association between hypofunctioning of the indirect basal ganglia pathway and the expression of repetitive behavior in the deer mouse model. In this study, we targeted indirect pathway cells of the striatum with single drugs and drug combinations that bind to dopamine D₂, adenosine A_2A, and glutamate mGlu₅ receptors. These receptors function both individually and as receptor heteromers. We found that only the triple drug cocktail (L-741,626+CGS21680+CDPPB) that was designed to increase striatal indirect basal ganglia pathway cell function reduced repetitive behavior in adult male deer mice. No single drug or double drug combinations were effective at selectively reducing repetitive behavior. We found this triple drug cocktail reduced repetitive behavior in both short-acting and long-acting formulations and was effective throughout 7 days of daily administration. Conversely, another triple drug cocktail (quinpirole+SCH58261+MTEP) that was designed to further reduce striatal indirect basal ganglia pathway cell function caused a significant increase in repetitive behavior. Significant and behaviorally selective effects on repetitive behavior were only achieved with the triple drug cocktails that included doses of L-741,626 and quinpirole that have off-target effects (e.g., dopamine D₃ receptors). These data further a role for decreased indirect basal ganglia pathway activation in repetitive behavior and suggest that targeting these receptors and/or heteromeric complexes on the indirect pathway neurons of the striatum may offer pharmacotherapeutic benefit for individuals with repetitive behavior disorders.

Cannabinoid dependence induces sustained changes in GABA release in the globus pallidus without affecting dopamine release in the dorsal striatum: A dual microdialysis probe study

A dual probe microdialysis study was designed to characterize GABA and dopamine (DA) release in the basal ganglia of cannabinoid-dependent Wistar rats. Whereas chronic administration of the cannabinoid receptor agonist WIN55,212 (WIN) resulted in increased basal GABA release, the D2 agonist receptor-mediated control of GABA and DA release elicited by quinpirole was similar in both cannabinoid-dependent and non dependent animals. However, quinpirole did induce a greater number of more stereotypies in cannabinoid-dependent animals, indicating a dysregulated behavioral response.

Opposing roles of CB1 and CB2 cannabinoid receptors in the stimulant and rewarding effects of cocaine

BACKGROUND AND PURPOSE

The endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) bind to CB₁ and CB₂ cannabinoid receptors in the brain and modulate the mesolimbic dopaminergic pathway. This neurocircuitry is engaged by psychostimulant drugs, including cocaine. Although CB₁ receptor antagonism and CB₂ receptor activation are known to inhibit certain effects of cocaine, they have been investigated separately. Here, we tested the hypothesis that there is a reciprocal interaction between CB₁ receptor blockade and CB₂ receptor activation in modulating behavioural responses to cocaine.

EXPERIMENTAL APPROACH

Male Swiss mice received i.p. injections of cannabinoid-related drugs followed by cocaine, and were then tested for cocaine-induced hyperlocomotion, c-Fos expression in the nucleus accumbens and conditioned place preference. Levels of endocannabinoids after cocaine injections were also analysed.

KEY RESULTS

The CB₁ receptor antagonist, rimonabant, and the CB₂ receptor agonist, JWH133, prevented cocaine-induced hyperlocomotion. The same results were obtained by combining sub-effective doses of both compounds. The CB₂ receptor antagonist, AM630, reversed the inhibitory effects of rimonabant in cocaine-induced hyperlocomotion and c-Fos expression in the nucleus accumbens. Selective inhibitors of anandamide and 2-AG hydrolysis (URB597 and JZL184, respectively) failed to modify this response. However, JZL184 prevented cocaine-induced hyperlocomotion when given after a sub-effective dose of rimonabant. Cocaine did not change brain endocannabinoid levels. Finally, CB₂ receptor blockade reversed the inhibitory effect of rimonabant in the acquisition of cocaine-induced conditioned place preference.

CONCLUSION AND IMPLICATIONS

The present data support the hypothesis that CB₁ and CB₂ receptors work in concert with opposing functions to modulate certain addiction-related effects of cocaine.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

The CB1 receptor is required for the establishment of the hyperlocomotor phenotype in developmentally-induced hypothyroidism in mice

Alterations in motor functions are well-characterized features observed in humans and experimental animals with thyroid hormone dysfunctions during development. We have previously suggested the implication of the endocannabinoid system in the hyperlocomotor phenotype observed in developmentally induced hypothyroidism in rats. In this work we have further analyzed the implication of endocannabinoids in the effect of hypothyroidism on locomotor activity. To this end, we evaluated the locomotor activity in adult mice lacking the cannabinoid receptor type 1 (CB1R^-/-) and in their wild type littermates (CB1R^+/+), whose hypothyroidism was induced in day 12 of gestation and maintained during the experimental period. Our results show that hypothyroidism induced a hyperlocomotor phenotype only in CB1R^+/+, but not in CB1R^-/- mice. In contrast with our previous results in rats, the expression of CB1R in striatum and the motor response to the cannabinoid agonist HU210 was unaltered in hypothyroid CB1R^+/+ mice suggesting that the cannabinoid system is not altered by hypothyroidism. Also, no effect of HU210 was observed in locomotion of CB1R^-/- mice. Finally, since the dopaminergic system plays a major role in the control of locomotor activity we studied its function in hypothyroid wild type and knockout animals. Our results show no alteration in the behavioral response induced by the dopamine D1 receptor agonist SKF38393. However we observed a decreased response to the dopamine D2 receptor antagonist haloperidol only in hypothyroid CB1R^+/+ mice, which might indicate potential alterations in D2R signaling in these animals. In conclusion, our data suggest that the cannabinoid system is necessary for the induction of hyperlocomotor phenotype in mice with developmentally induced hypothyroidism.

Interaction between interleukin-1β and type-1 cannabinoid receptor is involved in anxiety-like behavior in experimental autoimmune encephalomyelitis

Background

Mood disorders, including anxiety and depression, are frequently diagnosed in multiple sclerosis (MS) patients, even independently of the disabling symptoms associated with the disease. Anatomical, biochemical, and pharmacological evidence indicates that type-1 cannabinoid receptor (CB1R) is implicated in the control of emotional behavior and is modulated during inflammatory neurodegenerative diseases such as MS and experimental autoimmune encephalomyelitis (EAE).

Methods

We investigated whether CB1R could exert a role in anxiety-like behavior in mice with EAE. We performed behavioral, pharmacological, and electrophysiological experiments to explore the link between central inflammation, mood, and CB1R function in EAE.

Results

We observed that EAE-induced anxiety was associated with the downregulation of CB1R-mediated control of striatal GABA synaptic transmission and was exacerbated in mice lacking CB1R (CB1R-KO mice). Central blockade of interleukin-1β (IL-1β) reversed the anxiety-like phenotype of EAE mice, an effect associated with the concomitant rescue of dopamine (DA)-regulated spontaneous behavior, and DA-CB1R neurotransmission, leading to the rescue of striatal CB1R sensitivity.

Conclusions

Overall, results of the present investigation indicate that synaptic dysfunction linked to CB1R is involved in EAE-related anxiety and motivation-based behavior and contribute to clarify the complex neurobiological mechanisms underlying mood disorders associated to MS.

Targeting anandamide metabolism rescues core and associated autistic-like symptoms in rats prenatally exposed to valproic acid

Autism spectrum disorders (ASD) are characterized by altered sociability, compromised communication and stereotyped/repetitive behaviors, for which no specific treatments are currently available. Prenatal exposure to valproic acid (VPA) is a known, although still underestimated, environmental risk factor for ASD. Altered endocannabinoid activity has been observed in autistic patients, and endocannabinoids are known to modulate behavioral traits that are typically affected in ASD. On this basis, we tested the hypothesis that changes in the endocannabinoid tone contribute to the altered phenotype induced by prenatal VPA exposure in rats, with focus on behavioral features that resemble the core and associated symptoms of ASD. In the course of development, VPA-exposed rats showed early deficits in social communication and discrimination, compromised sociability and social play behavior, stereotypies and increased anxiety, thus providing preclinical proof of the long-lasting deleterious effects induced by prenatal VPA exposure. At the neurochemical level, VPA-exposed rats displayed altered phosphorylation of CB1 cannabinoid receptors in different brain areas, associated with changes in anandamide metabolism from infancy to adulthood. Interestingly, enhancing anandamide signaling through inhibition of its degradation rescued the behavioral deficits displayed by VPA-exposed rats at infancy, adolescence and adulthood. This study therefore shows that abnormalities in anandamide activity may underlie the deleterious impact of environmental risk factors on ASD-relevant behaviors and that the endocannabinoid system may represent a therapeutic target for the core and associated symptoms displayed by autistic patients.

Antipsychotic-induced sensitization and tolerance: Behavioral characteristics, developmental impacts, and neurobiological mechanisms

Antipsychotic sensitization and tolerance refer to the increased and decreased drug effects due to past drug use, respectively. Both effects reflect the long-term impacts of antipsychotic treatment on the brain and result from the brain's adaptive response to the foreign property of the drug. In this review, clinical evidence of the behavioral aspect of antipsychotic sensitization and tolerance is selectively reviewed, followed by an overview of preclinical literature that examines these behavioral characteristics and the related pharmacological and nonpharmacological factors. Next, recent work on the developmental impacts of adolescent antipsychotic sensitization and tolerance is presented and recent research that delineates the neurobiological mechanisms of antipsychotic sensitization and tolerance is summarized. A theoretical framework based on "drug learning and memory" principles is proposed to account for the phenomena of antipsychotic sensitization and tolerance. It is maintained that antipsychotic sensitization and tolerance follow basic principles of learning or acquisition ("induction") and memory ("expression"). The induction and expression of both effects reflect the consequences of associative and nonassociative processing and are strongly influenced by various pharmacological, environmental, and behavioral factors. Drug-induced neuroplasticity, such as functional changes of striatal dopamine D2 and prefrontal serotonin (5-HT)2A receptors and their mediated signaling pathways, in principle, is responsible for antipsychotic sensitization and tolerance. Understanding the behavioral characteristics and neurobiological underpinnings of antipsychotic sensitization and tolerance has greatly enhanced our understanding of mechanisms of antipsychotic action, and may have important implications for future drug discovery and clinical practice.

Cannabidiol effects in the prepulse inhibition disruption induced by amphetamine

RATIONALE

The information processing appears to be deficient in schizophrenia. Prepulse inhibition (PPI), which measures the inhibition of a motor response by a weak sensory event, is considered particularly useful to understand the biology of information processing in schizophrenia patients. Drugs that facilitate dopaminergic neurotransmission such as amphetamine induce PPI disruption in human and rodents. Clinical and neurobiological findings suggest that the endocannabinoid system and cannabinoids may be implicated in the pathophysiology and treatment of schizophrenia. Cannabidiol (CBD), a non-psychotomimetic constituent of the Cannabis sativa plant, has also been reported to have potential as an antipsychotic.

OBJECTIVE

Our aim was to investigate if CBD pretreatment was able to prevent PPI disruption induced by amphetamine. Since one possible mechanism of CBD action is the facilitation of endocannabinoid-mediated neurotransmission through anandamide, we tested the effects of an anandamide hydrolysis inhibitor (URB597) in the amphetamine-induced PPI disruption.

METHODS

Male Swiss mice were treated with CBD systemic or intra-accumbens, or URB597 (systemic) prior to amphetamine and were exposed to PPI test.

RESULTS

Amphetamine (10 mg/kg) disrupted PPI while CBD (15-60 mg/kg) or URB597 (0.1-1 mg/kg) administered alone had no effect. Pretreatment with CBD attenuated the amphetamine-disruptive effects on PPI test after systemic or intra-accumbens administration. Similar effects were also found with the inhibitor of anandamide hydrolysis.

CONCLUSION

These results corroborate findings indicating that CBD induces antipsychotic-like effects. In addition, they pointed to the nucleus accumbens as a possible site of these effects. The increase of anandamide availability may be enrolled in the CBD effects.

Repeated administration of aripiprazole produces a sensitization effect in the suppression of avoidance responding and phencyclidine-induced hyperlocomotion and increases D2 receptor-mediated behavioral function

The present study investigated how repeated administration of aripiprazole (a novel antipsychotic drug) alters its behavioral effects in two behavioral tests of antipsychotic activity and whether this alteration is correlated with an increase in dopamine D2 receptor function. Male adult Sprague-Dawley rats were first repeatedly tested with aripiprazole (3, 10 and 30 mg/kg, subcutaneously (sc)) or vehicle in a conditioned avoidance response (CAR) test or a phencyclidine (PCP) (3.20 mg/kg, sc)-induced hyperlocomotion test daily for five consecutive days. After 2-3 days of drug-free retraining or resting, all rats were then challenged with aripiprazole (1.5 or 3.0 mg/kg, sc). Repeated administration of aripiprazole progressively increased its inhibition of avoidance responding and PCP-induced hyperlocomotion. More importantly, rats previously treated with aripiprazole showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle in the challenge tests. An increased sensitivity to quinpirole (a selective D2/3 agonist) in prior aripiprazole-treated rats was also found in the quinpirole-induced hyperlocomotion test, suggesting an enhanced D2/3-mediated function. These findings suggest that aripiprazole, despite its distinct receptor mechanisms of action, induces a sensitization effect similar to those induced by other antipsychotic drugs and this effect may be partially mediated by brain plasticity involving D2/3 receptor systems.

Cocaine-induced endocannabinoid release modulates behavioral and neurochemical sensitization in mice

The endocannabinoid system has been implicated in the development of synaptic plasticity induced by several drugs abused by humans, including cocaine. However, there remains some debate about the involvement of cannabinoid receptors/ligands in cocaine-induced plasticity and corresponding behavioral actions. Here, we show that a single cocaine injection in Swiss-Webster mice produces behavioral and neurochemical alterations that are under the control of the endocannabinoid system. This plasticity may be the initial basis for changes in brain processes leading from recreational use of cocaine to its abuse and ultimately to dependence. Locomotor activity was monitored with photobeam cell detectors, and accumbens shell/core microdialysate dopamine levels were monitored by high-performance liquid chromatography with electrochemical detection. Development of single-trial cocaine-induced behavioral sensitization, measured as increased distance traveled in sensitized mice compared to control mice, was paralleled by a larger stimulation of extracellular dopamine levels in the core but not the shell of the nucleus accumbens. Both the behavioral and neurochemical effects were reversed by CB1 receptor blockade produced by rimonabant pre-treatments. Further, both behavioral and neurochemical cocaine sensitization were facilitated by pharmacological blockade of endocannabinoid metabolism, achieved by inhibiting the fatty acid amide hydrolase enzyme. In conclusion, our results suggest that a single unconditioned exposure to cocaine produces sensitization through neuronal alterations that require regionally specific release of endocannabinoids. Further, the present results suggest that endocannabinoids play a primary role from the earliest stage of cocaine use, mediating the inception of long-term brain-adaptive responses, shaping central pathways and likely increasing vulnerability to stimulant abuse disorders.

Cocaine-induced behavioral sensitization is associated with changes in the expression of endocannabinoid and glutamatergic signaling systems in the mouse prefrontal cortex

BACKGROUND

Endocannabinoids modulate the glutamatergic excitatory transmission by acting as retrograde messengers. A growing body of studies has reported that both signaling systems in the mesocorticolimbic neural circuitry are involved in the neurobiological mechanisms underlying drug addiction.

METHODS

We investigated whether the expression of both endocannabinoid and glutamatergic systems in the prefrontal cortex (PFC) were altered by an acute and/or repeated cocaine administration schedule that resulted in behavioral sensitization. We measured the protein and mRNA expression of the main endocannabinoid metabolic enzymes and the cannabinoid receptor type 1 (CB1). We also analyzed the mRNA expression of relevant components of the glutamate-signaling system, including glutamate-synthesizing enzymes, metabotropic receptors, and ionotropic receptors.

RESULTS

Although acute cocaine (10 mg/kg) produced no significant changes in the endocannabinoid-related proteins, repeated cocaine administration (20 mg/kg daily) induced a pronounced increase in the CB1 receptor expression. In addition, acute cocaine administration (10 mg/kg) in cocaine-sensitized mice (referred to as cocaine priming) induced a selective increase in the endocannabinoid-degrading enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). These protein changes were accompanied by an overall decrease in the ratios of endocannabinoid synthesis/degradation, especially the N-acyl phosphatidylethanolamine phospholipase D/FAAH and diacylglycerol lipase alpha/MAGL ratios. Regarding mRNA expression, while acute cocaine administration produced a decrease in CB1 receptors and N-acyl phosphatidylethanolamine phospholipase D, repeated cocaine treatment enhanced CB1 receptor expression. Cocaine-sensitized mice that were administered priming injections of cocaine mainly displayed an increased FAAH expression. These endocannabinoid changes were associated with modifications in glutamatergic transmission-related genes. An overall decrease was observed in the mRNA expression of the glutamate-synthesizing gene kidney-type glutaminase (KGA), the metabotropic glutamate receptors (mGluR3 and GluR), and subunits of NMDA ionotropic receptors (NR1, NR2A, NR2B and NR2C) after acute cocaine administration, while mice repeatedly exposed to cocaine only displayed an increase in NR2C. However, in cocaine-sensitized mice primed with cocaine, this inhibition was reversed and a strong increase was detected in the mGluR5, NR2 subunits, and both GluR1 and GluR3.

CONCLUSIONS

These findings indicate that cocaine sensitization is associated with an endocannabinoid downregulation and a hyperglutamatergic state in the PFC that, overall, contribute to an enhanced glutamatergic input into PFC-projecting areas.

Neuroinflammation as a possible link between cannabinoids and addiction

OBJECTIVE

Substance dependence disorder is a chronically relapsing condition characterised by neurobiological changes leading to loss of control in restricting a substance intake, compulsion and withdrawal syndrome. In the past few years, (endo)cannabinoids have been raised as a possible target in the aetiology of drug addiction. On the other hand, although the exact mechanisms of the genesis of addiction remain poorly understood, it is possible that neuroinflammation might also play a role in the pathophysiology of this condition. Studies demonstrated that (endo)cannabinoids act as immunomodulators by inhibiting cytokines production and microglial cell activation. Thus, in the present review, we explore the possible role of neuroinflammation on the therapeutic effects of cannabinoids on drug addiction.

METHODS

We conducted an evidence-based review of the literature in order to assess the role of cannabinoids on the neuroinflammatory hypothesis of addiction (terms: addiction, cannabinoids and inflammation). We searched PubMed and BioMedCentral databases up to April 2014 with no date restrictions.

RESULTS

In all, 165 eligible articles were included in the present review. Existing evidence suggests that disruption in cannabinoid signalling during the drug addiction process leads to microglial activation and neuroinflammation.

CONCLUSION

The literature showed that inflammation and changes in endocannabinod signalling occur in drug abuse; however, it remains uncertain whether these changes are causally or coincidentally associated with addiction. Additional studies, therefore, are needed to elucidate the contribution of neuroinflammation on the behavioural and neuroprotective effects of cannabinoids on drug addiction.

Differential effects of intermittent versus continuous haloperidol treatment throughout adolescence on haloperidol sensitization and social behavior in adulthood

Animal work on the behavioral effects of antipsychotic treatment suggests that different dosing regimens could affect drug sensitivity differently, with an intermittent treatment regimen tending to cause a sensitization effect, while a continuous treatment causing a tolerance. In this study, we explored how haloperidol (HAL) sensitization induced throughout adolescence and tested in adulthood was differentially impacted by these two dosing regimens in the conditioned avoidance response (CAR) test. We also examined how these two dosing regiments affected social interaction and social memory in adulthood. Male adolescent Sprague-Dawley rats were treated with HAL via either osmotic minipump (HAL-0.25 CONT; 0.25 mgkg(-1)day(-1), n = 14) or daily injection (HAL-0.05 INT; 0.05 mgkg(-1)day(-1) injection, sc, n = 14), or sterile water (n = 14) from postnatal days (PND) 44 to 71. HAL sensitization was assessed in a challenge test in which all rats were injected with HAL (0.025 and 0.05 mg/kg, sc) on PND 80 and PND 82. Two days later, half of the rats from each group (n = 7/group) were assayed in two 4-trial social interaction tests in which a subject rat was given four 5-min social encounters with a familiar or novel juvenile rat (PND 35-40) at 10 min intervals. Another half were tested in a quinpirole-induced hyperlocomotion assay to assess the potential HAL-induced change in D2-mediated function. Results show that only the intermittent dosing group under the HAL 0.05 mg/kg challenge showed a robust sensitization effect as rats in this group made significantly fewer avoidance responses than those in the vehicle and HAL-0.25 CONT groups. Adolescent HAL treatment did not affect social behavior and social memory, as rats from all 3 groups exhibited a similar level of social interaction and showed a similar level of sensitivity to the change of social stimuli. Similarly, adolescent HAL treatment also did not produce a long-lasting change in D2 function, as all 3 groups exhibited a similar level of increase in motor activity under quinpirole challenge. These findings suggest that HAL sensitization is a dosing-specific phenomenon. It is more likely to be seen under an intermittent dosing regimen than under a continuous dosing one. The findings that the intermittent HAL treatment did not impair social functioning and did not alter D2 function suggest a dissociation between drug-induced alterations in drug sensitivity and those in social function and neuroreceptors.

Changes in endocannabinoid and N-acylethanolamine levels in rat brain structures following cocaine self-administration and extinction training

Preclinical investigations have demonstrated that drugs of abuse alter the levels of lipid-based signalling molecules, including endocannabinoids (eCBs) and N-acylethanolamines (NAEs), in the rodent brain. In addition, several drugs targeting eCBs and/or NAEs are implicated in reward and/or seeking behaviours related to the stimulation of dopamine systems in the brain. In our study, the brain levels of eCBs (anandamide (AEA) and 2-arachidonoylglycerol (2-AG)) and NAEs (oleoylethanolamide (OEA) and palmitoylethanolamide (PEA)) were analyzed via an LC-MS/MS method in selected brain structures of rats during cocaine self-administration and after extinction training according to the "yoked" control procedure. Repeated (14days) cocaine (0.5mg/kg/infusion) self-administration and yoked drug delivery resulted in a significant decrease (ca. 52%) in AEA levels in the cerebellum, whereas levels of 2-AG increased in the frontal cortex, the hippocampus and the cerebellum and decreased in the hippocampus and the dorsal striatum. In addition, we detected increases (>150%) in the levels of OEA and PEA in the limbic areas in both cocaine treated groups, as well as an increase in the tissue levels of OEA in the dorsal striatum in only the yoked cocaine group and increases in the tissue levels of PEA in the dorsal striatum (both cocaine groups) and the nucleus accumbens (yoked cocaine group only). Compared to the yoked saline control group, extinction training (10days) resulted in a potent reduction in AEA levels in the frontal cortex, the hippocampus and the nucleus accumbens and in 2-AG levels in the hippocampus, the dorsal striatum and the cerebellum. The decreases in the limbic and subcortical areas were more apparent for rats that self-administered cocaine. Following extinction, there was a region-specific change in the levels of NAEs in rats previously injected with cocaine; a potent increase (ca. 100%) in the levels of OEA and PEA was detected in the prefrontal cortex and the hippocampus, whilst a drop was noted in the striatal areas versus yoked saline yoked animals. Our findings support the previous pharmacological evidence that the eCB system and NAEs are involved in reinforcement and extinction of positively reinforced behaviours and that these lipid-derived molecules may represent promising targets for the development of new treatments for drug addiction.

Long-lasting sensitization induced by repeated risperidone treatment in adolescent Sprague-Dawley rats: a possible D2 receptor mediated phenomenon?

RATIONALE

Risperidone use in children and adolescents for the treatment of various neuropsychiatric disorders (e.g., schizophrenia, autism, disruptive behavior, etc.) has increased substantially in recent decades. However, its long-term effect on the brain and behavioral functions is not well understood.

OBJECTIVE

The present study investigated how a short-term risperidone treatment in adolescence impacts antipsychotic response in adulthood in the conditioned avoidance response and phencyclidine (PCP)-induced hyperlocomotion tests.

METHODS

Male adolescent Sprague-Dawley rats (postnatal days [P] 40-44 or 43-48) were first treated with risperidone (0.3, 0.5, or 1.0 mg/kg, subcutaneously (sc)) and tested in the conditioned avoidance or PCP (3.2 mg/kg, sc)-induced hyperlocomotion model daily for five consecutive days. After they became adults (~P 76-80), they were challenged with risperidone (0.3 mg/kg, sc) to assess their sensitivity to risperidone reexposure. A quinpirole (a D2/3 receptor agonist, 1.0 mg/kg, sc)-induced hyperlocomotion test was later conducted to assess the risperidone-induced functional changes in D2 receptor.

RESULTS

In the risperidone challenge test in adulthood, adult rats previously treated with risperidone in adolescence made significantly fewer avoidance responses and exhibited significantly lower PCP-induced hyperlocomotion than those previously treated with vehicle. They also appeared to be more hyperactive than the vehicle-pretreated ones in the quinpirole-induced hyperlocomotion test. Prepulse inhibition of acoustic startle or fear-induced 22 kHz ultrasonic vocalizations in adulthood was not altered by adolescence risperidone treatment.

CONCLUSIONS

Adolescent risperidone exposure induces a long-term increase in behavioral sensitivity to risperidone that persists into adulthood. This long-lasting change might be due to functional upregulation of D2-mediated neurotransmission.

Influence of dopamine D2-type receptors on motor behaviors in the green tree frog, Hyla cinerea

Dopamine modulates a range of behaviors that include motor processes, learning, and incentive motivation. Research supports anatomical conservation of dopaminergic populations in the midbrain across vertebrate species, however, less evidence is available for dopamine receptor distributions. In order to test the behavioral role of dopamine in an anatomically conserved dopaminergic system, the effects of D2-type receptor manipulation on motor behaviors were examined in the anuran amphibian green tree frog, Hyla cinerea. In two different within-subject experiments, frogs were treated with a control treatment, and a high and low dose of either a D2 receptor-specific agonist, quinpirole, or antagonist, haloperidol, then exposed to a testing session to measure changes in swimming and climbing motor behaviors. No treatments resulted in complete immobility or catalepsy, however treatment-specific effects on certain motor behaviors were present. The high quinpirole dose (1mg/kg bw) generally inhibited motor behaviors associated with exiting water and jumping, while both haloperidol treatments (0.12mg/kg bw and 1.2mg/kg bw) generally stimulated motor behaviors associated with exiting water, as predicted based on receptor mechanisms. Performance improvement also appeared in frogs in each experiment, suggesting that the D2 receptor is not involved in the motor learning mechanism in this species. Overall, the results support general conservation of D2 receptors in motor processes in vertebrate species.

Time-dependence of risperidone and asenapine sensitization and associated D2 receptor mechanism

When an antipsychotic drug is given repeatedly and intermittently, there is often a long-term increase in its behavioral efficacy, termed antipsychotic sensitization. With the passage of time, the magnitude of antipsychotic sensitization may increase or decrease based on the principle of time-dependent sensitization (TDS) or memory decay, respectively. In the present study, we examined the time-dependent feature and possible dopamine D2 receptor mechanism of sensitization induced by risperidone and asenapine in the conditioned avoidance response test. Well-trained male adult Sprague-Dawley rats were first repeatedly treated with risperidone (1.0mg/kg) or asenapine (0.2mg/kg) and tested for avoidance response daily for 5 consecutive days. Eight, 18 or 38 days after the 5th drug treatment, all rats were retested drug-free to assess the long-term impact of prior risperidone or asenapine treatment. Drug-pretreated rats had significantly lower avoidance than vehicle-pretreated ones on this test, and the group differences increased with the passage of time. In the subsequent drug challenge test at 10, 20 or 40 days after the 5th drug treatment, all rats were injected with a low dose of risperidone (0.3mg/kg) or asenapine (0.1mg/kg). Drug-pretreated rats again made significantly fewer avoidances than controls, confirming the antipsychotic sensitization effect. Finally, in the quinpirole (a D2/3 receptor agonist, 1.0mg/kg, sc)-induced hyperlocomotion test, risperidone-pretreated rats exhibited a significantly higher level of motor activity than the vehicle-pretreated ones. These findings suggest that risperidone and asenapine sensitization is long-lasting, follows the TDS principle, and is likely mediated by D2 receptor supersensitivity.