Effect of Aripiprazole on 5-HT2 Receptor–mediated Wet-Dog Shake Responses and Disruption of Prepulse Inhibition in Rats

The Role of Zebrafish and Laboratory Rodents in Schizophrenia Research

Schizophrenia is a severe disorder characterized by positive, negative and cognitive symptoms, which are still not fully understood. The development of efficient antipsychotics requires animal models of a strong validity, therefore the aims of the article were to summarize the construct, face and predictive validity of schizophrenia models based on rodents and zebrafish, to compare the advantages and disadvantages of these models, and to propose future directions in schizophrenia modeling and indicate when it is reasonable to combine these models. The advantages of rodent models stem primarily from the high homology between rodent and human physiology, neurochemistry, brain morphology and circuitry. The advantages of zebrafish models stem in the high fecundity, fast development and transparency of the embryo. Disadvantages of both models originate in behavioral repertoires not allowing specific symptoms to be modeled, even when the models are combined. Especially modeling the verbal component of certain positive, negative and cognitive symptoms is currently impossible.

Lactobacillus plantarum PS128 ameliorates 2,5-Dimethoxy-4-iodoamphetamine-induced tic-like behaviors via its influences on the microbiota-gut-brain-axis

We previously reported a novel psychobiotic strain of Lactobacillus plantarum PS128 (PS128) which could ameliorate anxiety-like& depression-like behaviors and modulate cerebral dopamine (DA) and serotonin (5-HT) in mice. Here, we examine the possibility of using PS128 administration to improve tic-like behaviors by using a 5-HT_2A and 5-HT_2C receptor agonist 2,5-Dimethoxy-4-iodoamphetamine (DOI). PS128 was orally administered to male Wistar rat for 2 weeks before two daily DOI injections. We recorded the behaviors immediately after the second DOI injection and compared the results with control and haloperidol treatment groups. PS128 significantly reduced tic-like behaviors and pre-pulse inhibition deficit in a threshold-dose of 10⁹ CFU per day. Brain tissue analysis showed that DOI induced abnormal DA efflux in the striatum and prefrontal cortex, while PS128 ingestion improved DA metabolism and increased norepinephrine (NE) levels in these two regions. In addition, PS128 ingestion increased DA transporter and β-arrestin expressions and decreased DOI-induced phosphorylation of DA and cAMP regulated phosphoprotein of molecular weight 32 kDa (DARPP-32) at Thr34 and extracellular regulated protein kinases (ERK). PS128 ingestion also modulated peripheral 5-HT levels and shaped the cecal microbiota composition, which helps to alleviate DOI-induced dysbiosis. These results suggested that PS128 ameliorated DOI-induced tic-like hyper-active behaviors via stabilizing cerebral dopaminergic pathways through its modulation of host's microbiota-gut-brain axis. Thus, we believe there are potentials for utilizing psychobiotics to improve syndromes caused by DA dysregulation in DA-related neurological disorders and movement disorders such as Tourette syndrome.

2-Bromoterguride-a potential atypical antipsychotic drug without metabolic effects in rats

RATIONALE

Recently, we showed that 2-bromoterguride acted as a dopamine D2 receptor partial agonist, a serotonin 5-HT2A and α2C-adrenergic receptor antagonist, and exhibited antidopaminergic efficacy in amphetamine-induced locomotion (AIL) in rats without inducing catalepsy.

OBJECTIVE

To extend our knowledge on the antipsychotic effects of 2-bromoterguride, we used convergent preclinical animal models and tests; i.e., conditioned avoidance response (CAR), predictive of antipsychotic-like effects; Fos protein expression, a molecular marker for (atypical) antipsychotic activity; wet dog shake behavior, a test for the in vivo effects of drugs acting on central 5-HT2A receptors; and investigated metabolic changes as a common side effect of atypical antipsychotic drugs (APDs).

RESULTS

Acute treatment with 2-bromoterguride (0.1 and 0.3 mg/kg) decreased the CAR at 30, 90, and 270 min post-injection in rats without inducing escape failures at any time. Fos protein expression, as shown by Western blotting, was enhanced by 2-bromoterguride in the nucleus accumbens (NAc), the dorsolateral striatum (dStr), and the medial prefrontal cortex (mPFC). (±)-2,5-Dimethoxy-4-iodoamphetamine (DOI)-induced wet dog shakes in rats were reduced by 2-bromoterguride. Chronic treatment with 2-bromoterguride did not affect metabolic parameters such as body weight development and body fat composition as well as behavioral parameters such as food intake and locomotor activity.

CONCLUSIONS

Our data suggest that 2-bromoterguride is a promising candidate in the treatment of schizophrenia due to its atypical antipsychotic-like activity and its inability to induce weight gain.

Pretreatment or Posttreatment with Aripiprazole Attenuates Methamphetamine-induced Stereotyped Behavior in Mice

Aripiprazole is a third-generation atypical antipsychotic and a dopamine D₂ receptor partial agonist. In the present study, we investigated whether a single administration of aripiprazole to mice, either as a pretreatment or as a posttreatment, would affect stereotypy induced by methamphetamine (METH). Pretreatment of male ICR mice with aripiprazole (1 or 10 mg/kg, i.p.) attenuated the incidence of METH-induced stereotypical behavior in a dose-dependent manner. Pretreatment of mice with 1 mg/kg aripiprazole produced an increase in the locomotor activity in mice treated with METH compared with mice treated with vehicle plus METH and with 10 mg/kg aripiprazole plus METH. This increase in locomotion is indicative of a rightward shift in the dose–response curve for METH, consistent with a shift in the type of stereotypical behavior observed from biting to sniffing. Aripiprazole posttreatment, after METH-induced stereotypical behavior, was fully expressed and also significantly attenuated overall stereotypy in an aripiprazole dose-dependent manner. These data suggest that the antagonism of METH effects by aripiprazole should be investigated as a potential treatment for acute METH overdose.

Investigation of the mechanisms mediating MDMA "Ecstasy"-induced increases in cerebro-cortical perfusion determined by btASL MRI

RATIONALE

Acute administration of the recreational drug of abuse 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) has previously been shown to increase cerebro-cortical perfusion as determined by bolus-tracking arterial spin labelling (btASL) MRI.

OBJECTIVES

The purpose of the current study was to assess the mechanisms mediating these changes following systemic administration of MDMA to rats.

METHODS

Pharmacological manipulation of serotonergic, dopaminergic and nitrergic transmission was carried out to determine the mechanism of action of MDMA-induced increases in cortical perfusion using btASL MRI.

RESULTS

Fenfluramine (10 mg/kg), like MDMA (20 mg/kg), increased cortical perfusion. Increased cortical perfusion was not obtained with the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodophenyl-aminopropane hydrochloride (DOI) (1 mg/kg). Depletion of central 5-HT following systemic administration of the tryptophan hydroxylase inhibitor para-chlorophenylalanine (pCPA) produced effects similar to those observed with MDMA. Pre-treatment with the 5-HT receptor antagonist metergoline (4 mg/kg) or with the 5-HT reuptake inhibitor citalopram (30 mg/kg), however, failed to produce any effect alone or influence the response to MDMA. Pre-treatment with the dopamine D1 receptor antagonist SCH 23390 (1 mg/kg) failed to influence the changes in cortical perfusion obtained with MDMA. Treatment with the neuronal nitric oxide (NO) synthase inhibitor 7-nitroindazole (7-NI) (25 mg/kg) provoked no change in cerebral perfusion alone yet attenuated the MDMA-related increase in cortical perfusion.

CONCLUSIONS

Cortical 5-HT depletion is associated with increases in perfusion although this mechanism alone does not account for MDMA-related changes. A role for NO, a key regulator of cerebrovascular perfusion, is implicated in MDMA-induced increases in cortical perfusion.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Animal models of tic disorders: a translational perspective

Tics are repetitive, sudden movements and/or vocalizations, typically enacted as maladaptive responses to intrusive premonitory urges. The most severe tic disorder, Tourette syndrome (TS), is a childhood-onset condition featuring multiple motor and at least one phonic tic for a duration longer than 1 year. The pharmacological treatment of TS is mainly based on antipsychotic agents; while these drugs are often effective in reducing tic severity and frequency, their therapeutic compliance is limited by serious motor and cognitive side effects. The identification of novel therapeutic targets and development of better treatments for tic disorders is conditional on the development of animal models with high translational validity. In addition, these experimental tools can prove extremely useful to test hypotheses on the etiology and neurobiological bases of TS and related conditions. In recent years, the translational value of these animal models has been enhanced, thanks to a significant re-organization of our conceptual framework of neuropsychiatric disorders, with a greater focus on endophenotypes and quantitative indices, rather than qualitative descriptors. Given the complex and multifactorial nature of TS and other tic disorders, the selection of animal models that can appropriately capture specific symptomatic aspects of these conditions can pose significant theoretical and methodological challenges. In this article, we will review the state of the art on the available animal models of tic disorders, based on genetic mutations, environmental interventions as well as pharmacological manipulations. Furthermore, we will outline emerging lines of translational research showing how some of these experimental preparations have led to significant progress in the identification of novel therapeutic targets for tic disorders.

Serotonergic hallucinogens as translational models relevant to schizophrenia

One of the oldest models of schizophrenia is based on the effects of serotonergic hallucinogens such as mescaline, psilocybin, and (+)-lysergic acid diethylamide (LSD), which act through the serotonin 5-HT(2A) receptor. These compounds produce a 'model psychosis' in normal individuals that resembles at least some of the positive symptoms of schizophrenia. Based on these similarities, and because evidence has emerged that the serotonergic system plays a role in the pathogenesis of schizophrenia in some patients, animal models relevant to schizophrenia have been developed based on hallucinogen effects. Here we review the behavioural effects of hallucinogens in four of those models, the receptor and neurochemical mechanisms for the effects and their translational relevance. Despite the difficulty of modelling hallucinogen effects in nonverbal species, animal models of schizophrenia based on hallucinogens have yielded important insights into the linkage between 5-HT and schizophrenia and have helped to identify receptor targets and interactions that could be exploited in the development of new therapeutic agents.

Mutual independence of 5-HT(2) and α1 noradrenergic receptors in mediating deficits in sensorimotor gating

RATIONALE

Prepulse inhibition (PPI), a preattentional information-filtering mechanism, is disrupted by serotonin (5-HT) or norepinephrine (NE) agonists to model deficits seen in schizophrenia, but whether this effect occurs through interactions between these systems is not known.

OBJECTIVES

These studies investigated whether PPI/activity changes induced by agonists of one system were dependent on neurotransmission within the other.

METHODS

Male Sprague-Dawley rats received the 5-HT(2) receptor agonist DOI (1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane) (0, 0.3 mg/kg), with or without antagonists for α1 (prazosin:0, 0.3, or 1 mg/kg) or β (timolol:0, 3, or 10 mg/kg) receptors or their combination (0 or 0.3 mg/kg prazosin + 3 mg/kg timolol), or the 5-HT(2) antagonist ritanserin (0, 2 mg/kg). Separately, the α1-adrenergic receptor agonist cirazoline (0, 0.68 mg/kg) was given with and without ritanserin (0, 0.5, or 2 mg/kg) or the NE antagonists (0 or 0.3 mg/kg prazosin + 3 mg/kg timolol). Finally, combinations of subthreshold doses of DOI (0, 0.01, 0.025 mg/kg) and cirazoline (0, 0.1, 0.25 mg/kg) were tested for their ability to disrupt PPI, and concomitant administration of all three antagonists (0 vs. 0.3 mg/kg prazosin + 3 mg/kg timolol + 2 mg/kg ritanserin) was assessed for its ability to modify PPI. Locomotion was assessed in an additional set of experiments.

RESULTS

Doses/combinations of prazosin and timolol that reversed cirazoline-induced effects did not alter DOI-induced effects, and ritanserin did not affect cirazoline at doses that blocked DOI-mediated effects. Concomitant antagonism of α1 + β + 5-HT(2) receptors did not modify PPI, nor did combinations of subthreshold doses of cirazoline and DOI.

CONCLUSIONS

5-HT(2) receptors and α1 and β NE receptors may act through independent mechanisms to modulate sensorimotor gating and locomotor activity.

Pharmacological blockade of dopamine D2 receptors by aripiprazole is not associated with striatal sensitization

The partial agonist profile of novel antipsychotics such as aripiprazole has hardly been demonstrated in biochemical assays on animal tissues. As it is established that responses induced by dopamine D₂ receptor agonists are increased in models of dopaminergic sensitization, this paradigm was used in order to facilitate the detection of the partial agonist properties of aripiprazole. At variance with all other partial and full agonists tested, the partial agonist properties of aripiprazole were not revealed in guanosine 5′-O-(γ-[³⁵S]thiotriphosphate ([³⁵S]GTPγS) binding assays on striatal membranes from haloperidol-treated rats. Hence,aripiprazole behaved as an antagonist, efficiently inhibiting the functional response to dopamine. Similarly, in behavioural assays, aripiprazole dose-dependently inhibited the stereotypies elicited by apomorphine. However, at variance with haloperidol, repeated administrations of aripiprazole(3 weeks) at the doses of 10 and 30 mg/kg did not induce any up-regulation or hyperfunctionality of the dopamine D₂ receptors in the striatum. These data highlight the putative involvement of other pharmacological targets for aripiprazole that would support in the prevention of secondary effects commonly associated with the blockade of striatal dopamine D₂ receptors. Hence, in additional experiments, aripiprazole was found to efficiently promote [³⁵S]GTPγS binding in hippocampal membranes through the activation of 5-HT(₁A) receptors. Further experiments investigating the second messenger cascades should be performed so as to establish the functional properties of aripiprazole and understand the mechanism underlying the prevention of dopamine receptor regulation in spite of the observed antagonism.

5-HT(2A) and 5-HT(2C) receptor stimulation are differentially involved in the cortical dopamine efflux-Studied in 5-HT(2A) and 5-HT(2C) genetic mutant mice

Both 5-HT(2A) and 5-HT(2C) receptors modulate cortical dopamine efflux, but in opposite directions. We have now compared the ability of the three 5-HT(2A/2C) receptor agonists, DOI (R(-)-2,5-dimethoxy-4-iodoamphetamine), mCPP (meta-chlorophenylpiperazine) and MK-212 (6-Chloro-2-(piperazinyl) pyrazine), to modulate cortical dopamine efflux in 5-HT(2A) and 5-HT(2C) genetic mutant mice. In the 5-HT(2A) mice, the preferential 5-HT(2A) receptor agonist DOI (2.5mg/kg, s.c.) induced a slight but significant increase in cortical dopamine efflux only in the wild type (WT) mice; MK-212 (2.5mg/kg) reduced dopamine efflux in both WT and receptor knockout (KO) mice; moreover, MCPP, 2.5mg/kg, had no effect in either types. In 5-HT(2C) mice, DOI increased dopamine efflux in both types; while MK-212 decreased dopamine efflux in the WT, but not the receptor KO mice. These results provide new evidence that 5-HT(2A) receptor stimulation enhances and 5-HT(2C) receptor stimulation inhibits cortical dopamine efflux, and suggest the effects of DOI, MK-212 and mCPP on the cortical dopamine efflux are due to their different abilities on 5-HT(2A) and 5-HT(2C) receptors stimulation. Of these three agents, only DOI, the more selective 5-HT(2A) receptor agonist, is hallucinogenic. The absence of hallucinations with mCPP may be due to its relatively more potent 5-HT(2C) receptor agonist effect, inhibiting the ability of mCPP to enhance dopamine efflux in cortical and perhaps limbic regions as well. The present data provide additional evidence that hallucinations are due, in part, to 5-HT(2A) rather than 5-HT(2C) receptor stimulation. These findings suggest that 5-HT(2C) receptor agonists may be useful as antipsychotics, consistent with previous suggestions.

Comparison of the mGlu(5) receptor positive allosteric modulator ADX47273 and the mGlu(2/3) receptor agonist LY354740 in tests for antipsychotic-like activity

Recently, it has been proposed that activation of either metabotropic glutamate receptors e.g. mGlu(5) by positive allosteric modulators or stimulation of mGluR(2/3) receptors by agonists may offer new strategy in schizophrenia treatment. The aim of the present study was to compare the effect of mGlu(5) receptor positive allosteric modulator, ADX47273 (S-(4-Fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidin-1-yl}-methanone), mGluR(2/3) agonist, LY354740 ((1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate) and selected neuroleptics in animal models for positive schizophrenia symptoms. ADX47273 (3 and 10mg/kgi.p.), the typical antipsychotic haloperidol (0.1 and 0.2mg/kgi.p.), the atypical antipsychotics aripiprazole (1.25-5mg/kgi.p.) and olanzapine (2.5 and 5mg/kgi.p.) all reduced amphetamine-induced hyperlocomotion in Sprague-Dawley rats, unlike the mGlu(2/3) receptor agonist LY354740 (1-10mg/kgi.p.). Interestingly, haloperidol (0.1 and 0.2mg/kgi.p.), aripiprazole (1.25-5mg/kgi.p.) and olanzapine (1.25-5mg/kgi.p.), but not ADX47273 (1-10mg/kgi.p.), all reduced spontaneous locomotion and rearings at doses effective against amphetamine-induced hyperlocomotion. This indicates that the effect of ADX47273 in combination with amphetamine may be specific, and also suggests a lack of sedative side effects. Moreover, ADX47273 (30mg/kgi.p.), haloperidol (0.1 and 0.2mg/kgi.p.) and aripiprazole (5 and 10mg/kgi.p.) reversed apomorphine (0.5mg/kgs.c.)-induced deficits of prepulse inhibition, whereas neither LY354740 (1-10mg/kgi.p.) nor olanzapine (1.25-5mg/kgi.p.) produced this effect. Lack of effect of olanzapine was unexpected and at present no convincing explanation can be provided. In conclusion, in selected rodent models for positive schizophrenia symptoms, ADX47273 showed better efficacy than LY354740.