Aripiprazole increases dopamine but not noradrenaline and serotonin levels in the mouse prefrontal cortex

F15063, a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties: influence on neuronal firing and neurotransmitter release

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)-ethyl]-(3-cyclopenten-1-yl-benzyl)-amine) is a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties. Herein, we compared its effects on rat ventral tegmental area dopamine and dorsal raphe serotonin electrical activity with those of the dopamine D2 receptor partial agonist/5-HT1A receptor agonist, SSR181507. Further, we investigated the modulation of extracellular dopamine and noradrenaline in the medial prefrontal cortex and serotonin in the hippocampus of freely moving rats by F15063 using in vivo microdialysis. In the ventral tegmental area, F15063 (200-700 microg/kg, i.v.) did not alter the electrical activity of dopamine neurons whereas SSR181507 (250-1000 microg/kg, i.v.) partially inhibited it, consistent with dopamine D2 receptor partial agonism. Both compounds reduced the inhibition of firing rate induced by the full agonist apomorphine. In the dorsal raphe, both ligands suppressed firing activity, consistent with agonism at 5-HT1A autoreceptors, although SSR181507 (25-75 microg/kg, i.v.) was more potent than F15063 (100-300 microg/kg, i.v.). F15063 (0.63-40 mg/kg, i.p.) dose-dependently increased dopamine levels in the prefrontal cortex and decreased hippocampal 5-HT. These effects were reversed by the selective 5-HT1A receptor antagonist WAY100635 (0.16 mg/kg, s.c.), indicating that they were mediated by 5-HT1A receptors (at post- and pre-synaptic levels, respectively). In the medial prefrontal cortex, noradrenaline levels were moderately but significantly increased by F15063 at 2.5 mg/kg. In conclusion, whereas SSR181507 exhibits (partial) agonism at dopamine D2 and 5-HT1A receptors, F15063 blocks dopamine D2-like receptors whilst activating 5-HT1A receptors. Such a profile distinguishes F15063 from SSR181507 and currently available antipsychotic drugs.

Aripiprazole blocks reinstatement but not expression of morphine conditioned place preference in rats

Aripiprazole is an atypical antipsychotic drug primarily characterized by partial agonist activity at dopamine(DA) D2 receptors and serotonin-1A (5-hydroxytryptamine, 5-HT1A) receptors and minimal side effects.Based on its pharmacological profile, including stabilization of mesocorticolimbic DA activity (a pathway implicated in drug addiction), we investigated the effects of aripiprazole on relapse to morphine seeking in rats. In experiment 1, rats underwent morphine-induced conditioned place preference (CPP) training with alternate injections of morphine (5 mg/kg, s.c.) and saline (1 ml/kg, s.c.) for 8 consecutive days. To examine the effect of aripiprazole on the expression of morphine-induced CPP, rats received aripiprazole (0, 0.03, 0.1,and 0.3 mg/kg, i.p.) 30 min before testing for the expression of CPP. In experiment 2, rats underwent the same CPP training as in experiment 1 and subsequent extinction training. To examine the effect of aripiprazole on reinstatement of morphine-induced CPP, rats received aripiprazole 30 min before testing for reinstatement of CPP. In experiment 3, to assess the effects of aripiprazole on locomotor activity, aripiprazole was administered 30 min before testing for locomotor activity. Aripiprazole significantly decreased the reinstatement of CPP induced by a priming injection of morphine but had no effect on the expression of morphine-induced CPP or locomotor activity. The D2 and 5-HT1A partial agonist and 5-HT2A antagonist properties of aripiprazole likely account for the blockade of relapse to drug seeking. These findings suggest that aripiprazole may have therapeutic value for reducing craving and preventing relapse to drug seeking.

Clozapine-induced ERK1 and ERK2 signaling in prefrontal cortex is mediated by the EGF receptor

The atypical antipsychotic drug clozapine is effective in treatment-refractory schizophrenia. The intracellular signaling pathways that mediate clozapine action remain unknown. A potential candidate is the mitogen-activated protein kinase extracellular signal-regulated kinase (MAPK-ERK) cascade that links G-protein-coupled receptor and ErbB growth factor signaling systems, thereby regulating synaptic plasticity and connectivity, processes impaired in schizophrenia. Here, we examined how clozapine differentially modulated phosphorylation of the MAPK isoforms, ERK1/ERK2 in primary murine prefrontal cortical neurons compared to the typical antipsychotic drug haloperidol. While clozapine and haloperidol acutely decreased cortical pERK1 activation, only clozapine but not haloperidol stimulated pERK1 and pERK2 with continued drug exposure. This delayed ERK increase however, did not occur via the canonical dopamine D(2)-Gi/o-PKA or serotonin 5HT(2A)-Gq-phospholipase-C-linked signaling pathways. Rather, epidermal growth factor (EGF) receptor signaling mediated clozapine-induced ERK activation, given dose-dependent reduction of pERK1 and pERK2 stimulation with the EGF receptor inhibitor, AG1478. Immunocytochemical studies indicated that clozapine treatment increased EGF receptor (Tyr1068) phosphorylation. In vivo mouse treatment studies supported the in vitro findings with initial blockade, subsequent activation, and normalization of the cortical ERK response over 24 h. Furthermore, in vivo clozapine-induced ERK activation was significantly reduced by AG1478. This is the first report that clozapine action on prefrontal cortical neurons involves the EGF signaling system. Since EGF receptor signaling has not been previously linked to antipsychotic drug action, our findings may implicate the EGF system as a molecular substrate in treatment-resistant schizophrenia.

Effects of bifeprunox and aripiprazole on rat serotonin and dopamine neuronal activity and anxiolytic behaviour

The atypical antipsychotic bifeprunox is a partial dopamine D(2) and 5-HT(1A) receptor agonist. Using in-vivo electrophysiological and behavioural paradigms in the rat, the effects of bifeprunox and aripiprazole were assessed on ventral tegmental area (VTA) dopamine and dorsal raphe serotonin (5-HT) cell activity and on foot shock-induced ultrasonic vocalisation (USV). In VTA, bifeprunox and aripiprazole decreased (by 20-50%) firing of dopamine neurons. Interestingly, bursting activity was markedly reduced (by 70-100%), bursting being associated with a larger synaptic dopamine release than single spike firing. Both ligands reduced inhibition of firing rate induced by the full dopamine receptor agonist apomorphine, whereas the D(2) receptor antagonist haloperidol prevented these inhibitory effects, confirming partial D(2)-like agonistic properties. On 5-HT neurons, bifeprunox was more potent than aripiprazole to suppress firing activity. The 5-HT(1A) receptor antagonist WAY-100,635 prevented their effects. In the USV test of anxiolytic-like activity, bifeprunox had higher potency than aripiprazole to reduce vocalisations. Both WAY-100,635 and haloperidol reversed the effects of both agonists. The present in-vivo study shows that bifeprunox is a potent partial D(2)-like and 5-HT(1A) receptor agonist reducing preferentially the phasic activity of dopamine neurons. Thus, bifeprunox would be expected to be an effective compound against positive and negative symptoms of schizophrenia.

Antipsychotic drugs: comparison in animal models of efficacy, neurotransmitter regulation, and neuroprotection

Various lines of evidence indicate the presence of progressive pathophysiological processes occurring within the brains of patients with schizophrenia. By modulating chemical neurotransmission, antipsychotic drugs may influence a variety of functions regulating neuronal resilience and viability and have the potential for neuroprotection. This article reviews the current literature describing preclinical and clinical studies that evaluate the efficacy of antipsychotic drugs, their mechanism of action and the potential of first- and second-generation antipsychotic drugs to exert effects on cellular processes that may be neuroprotective in schizophrenia. The evidence to date suggests that although all antipsychotic drugs have the ability to reduce psychotic symptoms via D(2) receptor antagonism, some antipsychotics may differ in other pharmacological properties and their capacities to mitigate and possibly reverse cellular processes that may underlie the pathophysiology of schizophrenia.

Aripiprazole ameliorates phencyclidine-induced impairment of recognition memory through dopamine D1 and serotonin 5-HT1A receptors

RATIONALE

Cognitive deficits, including memory impairment, are regarded as a core feature of schizophrenia. Aripiprazole, an atypical antipsychotic drug, has been shown to improve disruption of prepulse inhibition and social interaction in an animal model of schizophrenia induced by phencyclidine (PCP); however, the effects of aripiprazole on recognition memory remain to be investigated.

OBJECTIVES

In this study, we examined the effect of aripiprazole on cognitive impairment in mice treated with PCP repeatedly.

MATERIALS AND METHODS

Mice were repeatedly administered PCP at a dose of 10 mg/kg for 14 days, and their cognitive function was assessed using a novel-object recognition task. We investigated the therapeutic effects of aripiprazole (0.01-1.0 mg/kg) and haloperidol (0.3 and 1.0 mg/kg) on cognitive impairment in mice treated with PCP repeatedly.

RESULTS

Single (1.0 mg/kg) and repeated (0.03 and 0.1 mg/kg, for 7 days) treatment with aripiprazole ameliorated PCP-induced impairment of recognition memory, although single treatment significantly decreased the total exploration time during the training session. In contrast, both single and repeated treatment with haloperidol (0.3 and 1.0 mg/kg) failed to attenuate PCP-induced cognitive impairment. The ameliorating effect of aripiprazole on recognition memory in PCP-treated mice was blocked by co-treatment with a dopamine D1 receptor antagonist, SCH23390, and a serotonin 5-HT1A receptor antagonist, WAY100635; however, co-treatment with a D2 receptor antagonist raclopride had no effect on the ameliorating effect of aripiprazole.

CONCLUSIONS

These results suggest that the ameliorative effect of aripiprazole on PCP-induced memory impairment is associated with dopamine D1 and serotonin 5-HT1A receptors.

Potentiation of the antidepressant-like effect of fluoxetine by aripiprazole in the mouse tail suspension test

We examined the effect of the novel atypical antipsychotic drug aripiprazole alone or in combination with the selective serotonin reuptake inhibitor fluoxetine in the mouse tail suspension test. We also investigated the effect of aripiprazole on glucose metabolism. Combined treatment with aripiprazole and a sub-effective dose of fluoxetine significantly decreased the duration of immobility in the tail suspension test. Aripiprazole by itself did not affect the duration of immobility. While olanzapine significantly increased blood glucose level in the glucose tolerance test, aripiprazole did not affect glucose metabolism. We suggest that aripiprazole augments the antidepressant-like effect of fluoxetine without affecting glucose metabolism.

Aripiprazole in the treatment of depressive and anxiety disorders: a review of current evidence

Despite the availability of different classes of drugs for the treatment of depressive and anxiety disorders, there are a number of clinically significant unmet needs, such as a high prevalence of treatment resistance, partial response, subsyndromal symptomatology, recurrence and relapse. With the approval of atypical antipsychotics, which are associated with a lower adverse effect burden than typical antipsychotics, consideration of their off-label use for the treatment of affective disorders and various other psychiatric disorders has become a viable option. However, consideration should be given to the US FDA black box warning indicating that atypical antipsychotics may increase mortality risk, particularly in the elderly population with dementia-related psychosis. There has been much conjecture about the utility of these atypical drugs to facilitate traditional antidepressant therapy, either in combination (from the initiation of therapy) or as adjunctive therapy (in the case of partial/incomplete response). Nevertheless, at present, available evidence from randomized, placebo-controlled trials is sparse, and a formal risk/benefit assessment of the use of these agents in a nonpsychotic patient population is not yet possible. As a representative agent from the atypical antipsychotic class with a novel mechanism of action and a relatively low adverse effect burden, aripiprazole represents an interesting potential treatment for depressive and anxiety disorders. In this review, we focus on the rationale for the use of aripiprazole in these disorders. Preclinical data suggests that aripiprazole has a number of possible mechanisms of action that may be important in the treatment of depressive and anxiety disorders. Such mechanisms include aripiprazole action at serotonin (5-HT) receptors as a 5-HT1A partial receptor agonist, a 5-HT2C partial receptor agonist and a 5-HT2A receptor antagonist. Aripiprazole also acts as a dopamine D2 partial receptor agonist, and has a possible action at adrenergic receptors. Furthermore, aripiprazole may have possible neuroprotective effects. Clinical studies demonstrate that aripiprazole may be useful in the treatment of bipolar depression, major depressive disorder, treatment-resistant depression and possibly anxiety disorders. Clinical data also suggest that aripiprazole may have a lower adverse effect burden than the other atypical drugs. Future research may confirm the potential utility of aripiprazole in the treatment of depressive and anxiety disorders.

Treatment of cognitive deficits associated with schizophrenia: potential role of catechol-O-methyltransferase inhibitors

In the last two decades, understanding of the dynamics of dopamine function in the prefrontal cortex and its role in prefrontal cortex physiology has opened up new avenues for therapeutic interventions in conditions in which prefrontal cortex function is compromised. Neuropsychological and imaging studies of prefrontal information processing have confirmed specific cognitive and neurophysiological abnormalities in individuals with schizophrenia. Because such findings are also observed in the healthy siblings of patients with schizophrenia, they may represent intermediate phenotypes related to schizophrenia susceptibility genes.Catechol-O-methyltransferase (COMT) represents an important candidate as a susceptibility gene for cognitive dysfunction in schizophrenia because of the unique role this enzyme plays in regulating prefrontal dopaminergic function. A functional COMT polymorphism (Val158Met) predicts performance in tasks of prefrontal executive function and the neurophysiological response measured with electroencephalography and functional magnetic resonance imaging in tasks assessing working memory. In fact, individuals with the Val/Val genotype, which encodes for the high-activity enzyme resulting in lower dopamine concentrations in the prefrontal cortex, perform less well and are less efficient physiologically than Met/Met individuals. These findings raise the possibility of new pharmacological interventions for the treatment of prefrontal cortex dysfunction and of predicting outcome based on COMT genotype. One strategy consists of the use of CNS-penetrant COMT inhibitors such as tolcapone. A second strategy is to increase extracellular dopamine concentrations in the frontal cortex by blocking the noradrenaline (norepinephrine) reuptake system, a secondary mechanism responsible for the disposal of dopamine from synaptic clefts in the prefrontal cortex. A third possibility involves the use of modafinil, a drug with an unclear mechanism of action but with positive effects on working memory in rodents. The potential of these drugs to improve executive cognitive function by selectively increasing dopamine load in the frontal cortex but not in subcortical territories, and the possibility that response to them may be modified by a COMT polymorphism, provides a novel genotype-based targeted pharmacological approach without abuse potential for the treatment of cognitive disorder in schizophrenia and in other conditions involving prefrontal cortex dysfunction.

In vivo actions of aripiprazole on serotonergic and dopaminergic systems in rodent brain

RATIONALE

Aripiprazole is an atypical antipsychotic drug with high in vitro affinity for 5-HT(1A), 5-HT(2A) and dopamine (DA) D2 receptors. However, its in vivo actions in the brain are still poorly characterized.

OBJECTIVE

The aim was to study the in vivo actions of aripiprazole in the rat and mouse brain.

METHODS

Brain microdialysis and single-unit extracellular recordings were performed.

RESULTS

The systemic administration of aripiprazole reduced 5-HT output in the medial prefrontal cortex (mPFC) and dorsal raphe nucleus of the rat. Aripiprazole also reduced extracellular 5-HT in the mPFC of wild-type (WT) but not of 5-HT(1A) (-/-) knockout (KO) mice. Aripiprazole reversed the elevation in extracellular 5-HT output produced by the local application of the 5-HT(2A/2C) receptor agonist DOI in mPFC. Aripiprazole also increased the DA output in mPFC of WT but not of 5-HT(1A) KO mice, as observed for atypical antipsychotic drugs, in contrast to haloperidol. Contrary to haloperidol, which increases the firing rate of DA neurons in the ventral tegmental area (VTA), aripiprazole induced a very moderate reduction in dopaminergic activity. Haloperidol fully reversed the inhibition in dopaminergic firing rate induced by apomorphine, whereas aripiprazole evoked a partial reversal that was significantly different from that evoked by haloperidol and from the spontaneous reversal of dopaminergic activity in rats treated with apomorphine.

CONCLUSIONS

These results indicate that aripiprazole modulates the in vivo 5-HT and DA release in mPFC through the activation of 5-HT(1A) receptors. Moreover, aripiprazole behaves as a partial agonist at DA D2 autoreceptors in vivo, an action which clearly distinguishes it from haloperidol.

Aripiprazole blocks reinstatement of cocaine seeking in an animal model of relapse

BACKGROUND

Aripiprazole (Abilify) is an atypical antipsychotic drug primarily characterized by partial agonist activity at dopamine (DA) D2 receptors and low side effects. Based on pharmacologic properties that include a stabilization of mesocorticolimbic DA activity, a pathway implicated in addiction, aripiprazole was tested for its ability to prevent relapse to cocaine seeking in rats.

METHODS

We assessed the dose-dependent effects of aripiprazole on conditioned cue-induced and cocaine-primed reinstatement of drug-seeking behavior following chronic intravenous cocaine self-administration in an animal model of relapse.

RESULTS

Aripiprazole potently and dose-dependently attenuated responding on the previously cocaine-paired lever during both reinstatement conditions, with slightly greater efficacy at reducing conditioned-cued reinstatement. Aripiprazole was effective at doses that failed to alter cocaine self-administration, food self-administration, reinstatement of food-seeking behavior, or basal locomotor activity, suggesting selective effects of aripiprazole on motivated drug-seeking behavior.

CONCLUSIONS

These results in a relapse model show that aripiprazole can block cocaine seeking without affecting other behaviors. The D2 partial agonist properties of aripiprazole likely account for the blockade of reinstatement of cocaine-seeking behavior. Given its established efficacy and tolerability as a treatment for psychosis, aripiprazole may be an excellent therapeutic choice for reducing craving and preventing relapse in people with cocaine dependency.