The antipsychotics olanzapine, risperidone, clozapine, and haloperidol are D2-selective ex vivo but not in vitro

Dopamine Dynamics and Neurobiology of Non-Response to Antipsychotics, Relevance for Treatment Resistant Schizophrenia: A Systematic Review and Critical Appraisal

Treatment resistant schizophrenia (TRS) is characterized by a lack of, or suboptimal response to, antipsychotic agents. The biological underpinnings of this clinical condition are still scarcely understood. Since all antipsychotics block dopamine D2 receptors (D2R), dopamine-related mechanisms should be considered the main candidates in the neurobiology of antipsychotic non-response, although other neurotransmitter systems play a role. The aims of this review are: (i) to recapitulate and critically appraise the relevant literature on dopamine-related mechanisms of TRS; (ii) to discuss the methodological limitations of the studies so far conducted and delineate a theoretical framework on dopamine mechanisms of TRS; and (iii) to highlight future perspectives of research and unmet needs. Dopamine-related neurobiological mechanisms of TRS may be multiple and putatively subdivided into three biological points: (1) D2R-related, including increased D2R levels; increased density of D2Rs in the high-affinity state; aberrant D2R dimer or heteromer formation; imbalance between D2R short and long variants; extrastriatal D2Rs; (2) presynaptic dopamine, including low or normal dopamine synthesis and/or release compared to responder patients; and (3) exaggerated postsynaptic D2R-mediated neurotransmission. Future points to be addressed are: (i) a more neurobiologically-oriented phenotypic categorization of TRS; (ii) implementation of neurobiological studies by directly comparing treatment resistant vs. treatment responder patients; (iii) development of a reliable animal model of non-response to antipsychotics.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Cariprazine alleviates core behavioral deficits in the prenatal valproic acid exposure model of autism spectrum disorder

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction and restricted, repetitive behaviors. The unmet medical need in ASD is considerable since there is no approved pharmacotherapy for the treatment of these deficits in social communication, interaction, and behavior. Cariprazine, a dopamine D₃-preferring D₃/D₂ receptor partial agonist, is already approved for the treatment of schizophrenia and bipolar I disorder in adults; investigation in patients with ASD is warranted.

OBJECTIVES

The aim of this study was to investigate the effects of cariprazine, compared with risperidone and aripiprazole, in the rat prenatal valporic acid (VPA) exposure model on behavioral endpoints representing the core and associated symptoms of ASD.

METHODS

To induce the ASD model, time-mated Wistar rat dams were treated with VPA during pregnancy. Male offspring were assigned to groups and studied in a behavioral test battery at different ages, employing social play, open field, social approach-avoidance, and social recognition memory tests. Animals were dosed orally, once a day for 8 days, with test compounds (cariprazine, risperidone, aripiprazole) or vehicle before behavioral assessment.

RESULTS

Cariprazine showed dose-dependent efficacy on all behavioral endpoints. In the social play paradigm, only cariprazine was effective. On the remaining behavioral endpoints, including the reversal of hyperactivity, risperidone and aripiprazole displayed similar efficacy to cariprazine.

CONCLUSIONS

In the present study, cariprazine effectively reversed core behavioral deficits and hyperactivity present in juvenile and young adult autistic-like rats. These findings indicate that cariprazine may be useful in the treatment of ASD symptoms.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Evaluation of dopamine D3 receptor occupancy by blonanserin using [11C]-(+)-PHNO in schizophrenia patients

RATIONALE

Unlike other antipsychotics, our previous positron emission tomography (PET) study demonstrated that a single dose of blonanserin occupied dopamine D₃ as well as dopamine D₂ receptors in healthy subjects. However, there has been no study concerning the continued use of blonanserin.

OBJECTIVES

We examined D₂ and D₃ receptor occupancies in patients with schizophrenia who had been treated with blonanserin.

METHODS

Thirteen patients with schizophrenia participated. PET examinations were performed on patients treated with clinical dosage of blonanserin or olanzapine alone. A crossover design was used in which seven patients switched drugs after the first scan, and PET examinations were conducted again. D₂ and D₃ receptor occupancies were evaluated by [¹¹C]-(+)-PHNO. We used nondisplaceable binding potential (BP_ND) of 6 healthy subjects which we previously reported as baseline. To consider the effect of upregulation of D₃ receptor by continued use of antipsychotics, D₃ receptor occupancy by blonanserin in seven subjects who completed 2 PET scans were re-analyzed by using BP_ND of olanzapine condition as baseline.

RESULTS

Average occupancy by olanzapine (10.8 ± 6.0 mg/day) was as follows: caudate 32.8 ± 18.3%, putamen 26.3 ± 18.2%, globus pallidus - 33.7 ± 34.9%, substantia nigra - 112.8 ± 90.7%. Average occupancy by blonanserin (12.8 ± 5.6 mg/day) was as follows: caudate 61.0 ± 8.3%, putamen 55.5 ± 9.5%, globus pallidus 48.9 ± 12.4%, substantia nigra 34.0 ± 20.6%. EC₅₀ was 0.30 ng/mL for D₂ receptor for caudate and putamen (df = 19, p < 0.0001) and 0.70 ng/mL for D₃ receptor for globus pallidus and substantia nigra (df = 19, p < 0.0001). EC₅₀ for D₃ receptor of blonanserin changed to 0.22 ng/mL (df = 13, p = 0.0041) when we used BP_ND of olanzapine condition as baseline.

CONCLUSIONS

Our study confirmed that blonanserin occupied both D₂ and D₃ receptors in patients with schizophrenia.

Cariprazine alleviates core behavioral deficits in the prenatal valproic acid exposure model of autism spectrum disorder

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction and restricted, repetitive behaviors. The unmet medical need in ASD is considerable since there is no approved pharmacotherapy for the treatment of these deficits in social communication, interaction, and behavior. Cariprazine, a dopamine D₃-preferring D₃/D₂ receptor partial agonist, is already approved for the treatment of schizophrenia and bipolar I disorder in adults; investigation in patients with ASD is warranted.

OBJECTIVES

The aim of this study was to investigate the effects of cariprazine, compared with risperidone and aripiprazole, in the rat prenatal valporic acid (VPA) exposure model on behavioral endpoints representing the core and associated symptoms of ASD.

METHODS

To induce the ASD model, time-mated Wistar rat dams were treated with VPA during pregnancy. Male offspring were assigned to groups and studied in a behavioral test battery at different ages, employing social play, open field, social approach-avoidance, and social recognition memory tests. Animals were dosed orally, once a day for 8 days, with test compounds (cariprazine, risperidone, aripiprazole) or vehicle before behavioral assessment.

RESULTS

Cariprazine showed dose-dependent efficacy on all behavioral endpoints. In the social play paradigm, only cariprazine was effective. On the remaining behavioral endpoints, including the reversal of hyperactivity, risperidone and aripiprazole displayed similar efficacy to cariprazine.

CONCLUSIONS

In the present study, cariprazine effectively reversed core behavioral deficits and hyperactivity present in juvenile and young adult autistic-like rats. These findings indicate that cariprazine may be useful in the treatment of ASD symptoms.

Negative Symptoms of Schizophrenia: New Prospects of Cariprazine Treatment

Background

Cariprazine is a new piperazine derivative atypical antipsychotic, like aripiprazole and brexpiprazole. It has been approved for treating schizophrenia in many countries and has recently been included on the List of Essential Medicines in Russia. Unlike most other atypical antipsychotics, it shows high in vivo occupancy of dopamine D2 and D3 receptors at clinically relevant doses. In animal models, cariprazine has demonstrated dopamine D3 receptor-dependent pro-cognitive and anti-anhedonic effects, suggesting its potential for treating negative symptoms. This review summarizes the efficacy of cariprazine in the treatment of negative symptoms of schizophrenia.

Methods

A literature search of databases covering international and Russian journals, for articles published between 1st January 2010 and 1st June 2020.

Results

Cariprazine demonstrated at least comparable efficacy in the treatment of schizophrenia symptoms to active comparators including risperidone, olanzapine or aripiprazole. The drug has a good safety profile. It appeared to be associated with a lower risk of metabolic syndromes and most extrapyramidal symptoms. The positive effect of cariprazine on the negative symptoms of schizophrenia may be associated with the elimination of secondary negative symptoms. However, of all the atypical antipsychotics to date, only cariprazine has a convincingly, methodologically robust proven advantage over risperidone in eliminating the predominant negative symptoms of schizophrenia. Yet only four studies have investigated the effect of cariprazine on the negative symptoms of schizophrenia. There is a lack of research into its direct impact on emotional-volitional disorders, anhedonia, cognitive symptoms and personality changes. However, there is evidence to suggest cariprazine is effective in treatment-resistant cases, but this requires further confirmation.

Conclusion

Cariprazine is an effective and well-tolerated agent for the treatment of schizophrenia and may be effective in cases where other antipsychotics have failed. Cariprazine has been shown to have a positive effect on negative symptoms. Further studies are needed to collect more data on long-term treatment of schizophrenia and especially negative symptoms.

Antipsychotic olanzapine-induced misfolding of proinsulin in the endoplasmic reticulum accounts for atypical development of diabetes

Second-generation antipsychotics are widely used to medicate patients with schizophrenia, but may cause metabolic side effects such as diabetes, which has been considered to result from obesity-associated insulin resistance. Olanzapine is particularly well known for this effect. However, clinical studies have suggested that olanzapine-induced hyperglycemia in certain patients cannot be explained by such a generalized mechanism. Here, we focused on the effects of olanzapine on insulin biosynthesis and secretion by mouse insulinoma MIN6 cells. Olanzapine reduced maturation of proinsulin, and thereby inhibited secretion of insulin; and specifically shifted the primary localization of proinsulin from insulin granules to the endoplasmic reticulum. This was due to olanzapine's impairment of proper disulfide bond formation in proinsulin, although direct targets of olanzapine remain undetermined. Olanzapine-induced proinsulin misfolding and subsequent decrease also occurred at the mouse level. This mechanism of olanzapine-induced β-cell dysfunction should be considered, together with weight gain, when patients are administered olanzapine.

A randomized-controlled trial of blonanserin and olanzapine as adjunct to antipsychotics in the treatment of patients with schizophrenia and dopamine supersensitivity psychosis: The ROADS study

Dopamine supersensitivity psychosis (DSP) is a key factor contributing to the development of antipsychotic treatment-resistant schizophrenia. We examined the efficacy and safety of blonanserin (BNS) and olanzapine (OLZ) as adjuncts to prior antipsychotic treatment in patients with schizophrenia and DSP in a 24-week, multicenter (17 sites), randomized, rater-blinded study with two parallel groups (BNS and OLZ add-on treatments) in patients with schizophrenia and DSP: the ROADS Study. The primary outcome was the change in the Positive and Negative Syndrome Scale (PANSS) total score from baseline to week 24. Secondary outcomes were changes in the PANSS subscale scores, Clinical Global Impressions, and Extrapyramidal Symptom Rating Scale (ESRS), and changes in antipsychotic doses. The 61 assessed patients were allocated into a BNS group (n = 26) and an OLZ group (n = 29). The PANSS total scores were reduced in both groups (mean ± SD: -14.8 ± 24.0, p = 0.0042; -10.5 ± 12.9, p = 0.0003; respectively) with no significant between-group difference (mean, -4.3, 95 %CI 15.1-6.4, p = 0.42). The BNS group showed significant reductions from week 4; the OLZ group showed significant reductions from week 8. The ESRS scores were reduced in the BNS group and the others were reduced in both groups. The antipsychotic monotherapy rates at the endpoint were 26.3 % (n = 6) for BNS and 23.8 % (n = 5) for OLZ. The concomitant antipsychotic doses were reduced in both groups with good tolerability. Our results suggest that augmentations with BNS and OLZ are antipsychotic treatment options for DSP patients, and BNS may be favorable for DSP based on the relatively quick responses to BNS observed herein.

In vivo 5-HT6 and 5-HT2A receptor availability in antipsychotic treated schizophrenia patients vs. unmedicated healthy humans measured with [11C]GSK215083 PET

While 5-HT₆ receptor is a potential therapeutic target for cognitive impairment in schizophrenia (SCZ), in vivo 5-HT₆ receptor availability following antipsychotic treatment has not been examined to-date. We examined the availability of 5-HT₆ and 5-HT_2A receptors following treatment with olanzapine, risperidone, aripiprazole and quetiapine in male patients with SCZ vs unmedicated age-matched healthy male controls (HC) using positron emission tomography (PET) imaging with [¹¹C]GSK215083. [¹¹C]GSK215083 has been shown to have selectivity for 5-HT₆ in the striatum and 5-HT_2A in the cortex. Patients with SCZ (n = 9) were scanned with [¹¹C]GSK215083 on HR+ PET scanner at presumed steady-state trough and peak serum levels following 7 days of confirmed inpatient antipsychotic treatment. Time-activity curves in regions-of-interest were fitted with multilinear analysis-1 (MA1). Regional nondisplaceable binding potential (BP_ND) values were calculated using cerebellum as the reference region and corrected for partial volume effects. Compared to HCs (n = 9), olanzapine was associated with significantly lower BP_ND (range: 53%-95%) in ventral striatum, putamen, caudate and frontal cortex at both trough and peak scans. Risperidone was associated with significantly lower BP_ND in frontal cortex at both trough and peak scans. The study provides preliminary evidence that treatment with different second-generation antipsychotics results in differing profiles of 5-HT_2A and 5-HT₆ availability.

Preclinical pharmacodynamic and pharmacokinetic characterization of the major metabolites of cariprazine

INTRODUCTION

Cariprazine, a dopamine D3-preferring D3/D2 receptor partial agonist and serotonin 5-HT1A receptor partial agonist, has two major human metabolites, desmethyl-cariprazine (DCAR) and didesmethyl-cariprazine (DDCAR). The metabolite pharmacology was profiled to understand the contribution to cariprazine efficacy.

METHODS

In vitro receptor binding and functional assays, electrophysiology, animal models, microdialysis, and kinetic-metabolism approaches were used to characterize the pharmacology of DCAR and DDCAR.

RESULTS

Similar to cariprazine, both metabolites showed high affinity for human D3, D2L, 5-HT1A, 5-HT2A, and 5-HT2B receptors, albeit with higher selectivity than cariprazine for D3 versus D2 receptors. In [35S]GTPγS binding assays, cariprazine and DDCAR were antagonists in membranes from rat striatum and from cells expressing human D2 and D3 receptors, and were partial agonists in membranes from rat hippocampus. In cAMP signaling assays, cariprazine, DCAR, and DDCAR acted as partial agonists at D2 and D3 receptors; cariprazine and DDCAR were full agonists, whereas DCAR was a partial agonist at 5-HT1A receptors. Cariprazine, DCAR, and DDCAR were pure antagonists at human 5-HT2B receptors. Cariprazine and DDCAR increased rat striatal dopamine and reduced cortical serotonin turnover. Cariprazine and DDCAR showed similar in vivo D3 receptor occupancy in rat brain; however, cariprazine was more potent for D2 receptor occupancy. Both cariprazine and DDCAR dose-dependently but partially suppressed the spontaneous activity of midbrain dopaminergic neurons in rats, with the parent compound being more potent but shorter acting than its metabolite. Consistent with the D2 receptor occupancy profile, DDCAR was 3- to 10-fold less potent than cariprazine in rodent models of antipsychotic-like activity. Following acute cariprazine administration, DDCAR was detected in the rodent brain but at much lower levels than cariprazine.

CONCLUSION

Overall, in vitro and in vivo pharmacological profiles of DCAR and DDCAR demonstrated high similarity with cariprazine, suggesting that the major metabolites of cariprazine contribute significantly to its clinical efficacy.

Effects of the T-type calcium channel antagonist Z944 on paired associates learning and locomotor activity in rats treated with the NMDA receptor antagonist MK-801

RATIONALE

Currently available antipsychotics are unsatisfactory given their side effects and limited efficacy for the cognitive symptoms of schizophrenia. Many currently available drugs, such as haloperidol, are T-type calcium channel antagonists in addition to their well-established antagonism of dopamine D2 receptors. Thus, preclinical research into the effects of T-type calcium channel antagonists/blockers in behavioral assays related to schizophrenia may inform novel therapeutic strategies.

OBJECTIVES

We explored the effects of a recently developed highly selective T-type calcium channel antagonist, Z944 (2.5, 5.0, 10.0 mg/kg), on the MK-801 (0.15 mg/kg) model of acute psychosis.

METHODS

To examine the effects of Z944 on behaviors relevant to schizophrenia, we tested touchscreen-based paired associates learning given its relevance to the cognitive symptoms of the disorder and locomotor activity given its relevance to the positive symptoms.

RESULTS

Acute treatment with Z944 failed to reverse the visuospatial associative memory impairments caused by MK-801 in paired associates learning. The highest dose of drug (10.0 mg/kg) given alone produced subtle impairments on paired associates learning. In contrast, Z944 (5.0 mg/kg) blocked the expected increase in locomotion following MK-801 treatment in a locomotor assay.

CONCLUSIONS

These experiments provide support that Z944 may reduce behaviors relevant to positive symptoms of schizophrenia, although additional study of its effects on cognition is required. These findings and other research suggest T-type calcium channel antagonists may be an alternative to currently available antipsychotics with less serious side effects.

The affinity of antipsychotic drugs to dopamine and serotonin 5-HT2 receptors determines their effects on prefrontal-striatal functional connectivity

One of the major challenges of cross-species translation in psychiatry is the identification of quantifiable brain phenotypes linked to drug efficacy and/or side effects. A measure that has received increasing interest is the effect of antipsychotic drugs on resting-state functional connectivity (FC) in magnetic resonance imaging. However, quantitative comparisons of antipsychotic drug-induced alterations of FC patterns are missing. Consideration of receptor binding affinities provides a means for the effects of antipsychotic drugs on extended brain networks to be related directly to their molecular mechanism of action. Therefore, we examined the relationship between the affinities of three second-generation antipsychotics (amisulpride, risperidone and olanzapine) to dopamine and serotonin receptors and FC patterns related to the prefrontal cortex (PFC) and striatum in Sprague-Dawley rats. FC of the relevant regions was quantified by correlation coefficients and local network properties. Each drug group (32 animals per group) was subdivided into three dose groups and a vehicle control group. A linear relationship was discovered for the mid-dose of antipsychotic compounds, with stronger affinity to serotonin 5-HT_2A, 5-HT_2C and 5-HT_1A receptors and decreased affinity to D₃ receptors associated with increased prefrontal-striatal FC (p = 0.0004, r² = 0.46; p = 0.004, r² = 0.33; p = 0.002, r² = 0.37; p = 0.02, r² = 0.22, respectively). Interestingly, no correlation was observed for the low and high dose groups, and for D₂ receptors. Our results indicate that drug-induced FC patterns may be linked to antipsychotic mechanism of action on the molecular level and suggest the technique's value for drug development, especially if our results are extended to a larger number of antipsychotics.

When dysphoria is not a primary mental state: A case report of the role of the aromatic L-aminoacid decarboxylase

RATIONALE

The aromatic L-amino acid decarboxylase (AADC) deficiency (AADCD) is a rare, autosomal recessive neurometabolic disorder caused by a deficit of the AADC that is involved in serotonin and dopamine biosynthesis, causing as a consequence, their deficits, but also a lack of norepinephrine and epinephrine, given that dopamine is their precursor.

PATIENT CONCERNS

We report the case of a Caucasian 43-year-old woman heterozygous for p.Ser250Phe in DDC, encoding for AADC with a positive family history for behavioral problems.

DIAGNOSES

Since adolescence, she manifested behavioral abnormalities. Three months before the admission to our hospital, she presented with a permanent dystonic posture at the 4 limbs with numbness and tingling, diplopia, and low potassium levels. She was treated with muscle relaxants and potassium, but with no results. Olanzapine was administrated, worsening mood problems. Later, after fever, low potassium levels, and increased difficulty to move, she was admitted to the neurology unit where, after bradycardia alternating with atrial and ventricular fibrillation, she had loss of consciousness. She started to complain involuntary parossistic eye and head movements, bilateral ptosis, oculogyric crises with dystonia of the head, muscle hypotrophy, and absent deep tendon reflexes. During the hospital stay, she continued having episodes of untreatable bradycardia and fever.

INTERVENTIONS

Hemocultures were performed, resulting positive for Enterococcus faecalis and Acinetobacter baumanii. Whole exome sequencing was performed evidencing that the patient harbored the heterozygous p.Ser250Phe variant in the gene DDC.

OUTCOMES

A treatment with Pyridoxine and Pramipexole was prescribed, but never started because she died.

LESSONS

The heterozygosity for p.Ser250Phe may have influenced the clinical manifestations, given that the patient presented some overlapping symptoms with those in AADCD, but while AADCD normally is diagnosed during childhood, the fact that the patient carried the mutation in heterozygosity may have alleviated and delayed the clinical manifestations.

Effects of antipsychotic drugs on neurites relevant to schizophrenia treatment

Although antipsychotic drugs are mainly used for treating schizophrenia, they are widely used for treating various psychiatric diseases in adults, the elderly, adolescents and even children. Today, about 1.2% of the worldwide population suffers from psychosis and related disorders, which translates to about 7.5 million subjects potentially targeted by antipsychotic drugs. Neurites project from the cell body of neurons and connect neurons to each other to form neural networks. Deficits in neurite outgrowth and integrity are implicated in psychiatric diseases including schizophrenia. Neurite deficits contribute to altered brain development, neural networking and connectivity as well as symptoms including psychosis and altered cognitive function. This review revealed that (1) antipsychotic drugs could have profound effects on neurites, synaptic spines and synapse, by which they may influence and regulate neural networking and plasticity; (2) antipsychotic drugs target not only neurotransmitter receptors but also intracellular signaling molecules regulating the signaling pathways responsible for neurite outgrowth and maintenance; (3) high doses and chronic administration of antipsychotic drugs may cause some loss of neurites, synaptic spines, or synapsis in the cortical structures. In addition, confounding effects causing neurite deficits may include elevated inflammatory cytokines and antipsychotic drug-induced metabolic side effects in patients on chronic antipsychotic therapy. Unraveling how antipsychotic drugs affect neurites and neural connectivity is essential for improving therapeutic outcomes and preventing aversive effects for patients on antipsychotic drug treatment.

The novel atypical antipsychotic cariprazine demonstrates dopamine D2 receptor-dependent partial agonist actions on rat mesencephalic dopamine neuronal activity

Aim

Cariprazine, a dopamine D₃‐preferring D₃/D₂ receptor partial agonist, is FDA approved for the treatment of schizophrenia and acute manic or mixed episodes of bipolar disorder. This study used in vivo electrophysiological techniques in anesthetized rats to determine cariprazine's effect on dopaminergic cell activity in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc).

Methods

Extracellular recordings of individual dopaminergic neurons were performed after oral or intravenous administration of cariprazine, the D₃ receptor antagonist SB 277011A, the D₂ receptor antagonist L741,626, and/or the D₃ receptor agonist PD 128,907.

Results

Acute oral treatment with cariprazine significantly increased and chronic cariprazine significantly decreased the number of spontaneously firing dopaminergic neurons in the VTA, but not in the SNc. Intravenous administration of cariprazine partially but significantly inhibited dopaminergic neuronal firing in both regions, which was prevented by L741,626 but not SB 277011A. In both VTA and SNc, cariprazine, SB 277011A, and L741,626 significantly antagonized the suppression of dopamine cell firing elicited by PD 128,907.

Conclusions

Cariprazine significantly modulates the number of spontaneously active VTA dopamine neurons and moderately suppresses midbrain dopamine neuronal activity. The contribution of dopamine D₂ receptors to cariprazine's in vivo effects is prevalent and that of D₃ receptors is less apparent.

Liraglutide prevents metabolic side-effects and improves recognition and working memory during antipsychotic treatment in rats

BACKGROUND

Antipsychotic drugs (APDs), olanzapine and clozapine, do not effectively address the cognitive symptoms of schizophrenia and can cause serious metabolic side-effects. Liraglutide is a synthetic glucagon-like peptide-1 (GLP-1) receptor agonist with anti-obesity and neuroprotective properties. The aim of this study was to examine whether liraglutide prevents weight gain/hyperglycaemia side-effects and cognitive deficits when co-administered from the commencement of olanzapine and clozapine treatment.

METHODS

Rats were administered olanzapine (2 mg/kg, three times daily (t.i.d.)), clozapine (12 mg/kg, t.i.d.), liraglutide (0.2 mg/kg, twice daily (b.i.d.)), olanzapine + liraglutide co-treatment, clozapine + liraglutide co-treatment or vehicle (Control) ( n = 12/group, 6 weeks). Recognition and working memory were examined using Novel Object Recognition (NOR) and T-Maze tests. Body weight, food intake, adiposity, locomotor activity and glucose tolerance were examined.

RESULTS

Liraglutide co-treatment prevented olanzapine- and clozapine-induced reductions in the NOR test discrimination ratio ( p < 0.001). Olanzapine, but not clozapine, reduced correct entries in the T-Maze test ( p < 0.05 versus Control) while liraglutide prevented this deficit. Liraglutide reduced olanzapine-induced weight gain and adiposity. Olanzapine significantly decreased voluntary locomotor activity and liraglutide co-treatment partially reversed this effect. Liraglutide improved clozapine-induced glucose intolerance.

CONCLUSION

Liraglutide co-treatment improved aspects of cognition, prevented obesity side-effects of olanzapine, and the hyperglycaemia caused by clozapine, when administered from the start of APD treatment. The results demonstrate a potential treatment for individuals at a high risk of experiencing adverse effects of APDs.

Dichlorophenyl piperazines, including a recently-approved atypical antipsychotic, are potent inhibitors of DHCR7, the last enzyme in cholesterol biosynthesis

While antipsychotic medications provide important relief from debilitating psychotic symptoms, they also have significant adverse side effects, which might have relevant impact on human health. Several research studies, including ours, have shown that commonly used antipsychotics such as haloperidol and aripiprazole affect cholesterol biosynthesis at the conversion of 7-dehydrocholesterol (7-DHC) to cholesterol. This transformation is promoted by the enzyme DHCR7 and its inhibition causes increases in plasma and tissue levels of 7-DHC. The inhibition of this enzymatic step by mutations in the Dhcr7 gene leads to Smith-Lemli-Opitz syndrome, a devastating human condition that can be replicated in rats by small molecule inhibitors of DHCR7. The fact that two compounds, brexpiprazole and cariprazine, that were recently approved by the FDA have substructural elements in common with the DHCR7 inhibitor aripiprazole, prompted us to evaluate the effect of brexpiprazole and cariprazine on cholesterol biosynthesis. We report that cariprazine affects levels of 7-DHC and cholesterol in cell culture incubations at concentrations as low as 5 nM. Furthermore, a common metabolite of cariprazine and aripiprazole, 2,3-(dichlorophenyl) piperazine, inhibits DHCR7 activity at concentrations comparable to those of the potent teratogen AY9944. The cell culture experiments were corroborated in mice in studies showing that treatment with cariprazine elevated 7-DHC in brain and serum. The consequences of sterol inhibition by antipsychotics in the developing nervous system and the safety of their use during pregnancy remains to be established.

Comparison of Dopamine D3 and D2 Receptor Occupancies by a Single Dose of Blonanserin in Healthy Subjects: A Positron Emission Tomography Study With [11C]-(+)-PHNO

BACKGROUND

Blockade of D3 receptor, a member of the dopamine D2-like receptor family, has been suggested as a possible medication for schizophrenia. Blonanserin has high affinity in vitro for D3 as well as D2 receptors. We investigated whether a single dose of 12 mg blonanserin, which was within the daily clinical dose range (i.e., 8-24 mg) for the treatment of schizophrenia, occupies D3 as well as D2 receptors in healthy subjects.

METHODS

Six healthy males (mean 35.7±7.6 years) received 2 positron emission tomography scans, the first prior to taking blonanserin, and the second 2 hours after the administration of a single dose of 12 mg blonanserin. Dopamine receptor occupancies by blonanserin were evaluated by [11C]-(+)-PHNO.

RESULTS

Occupancy of each region by 12 mg blonanserin was: caudate (range 64.3%-81.5%; mean±SD, 74.3±5.6%), putamen (range 60.4%-84.3%; mean±SD, 73.3%±8.2%), ventral striatum (range 40.1%-88.2%; mean±SD, 60.8%±17.1%), globus pallidus (range 65.8%-87.6%; mean±SD, 75.7%±8.6%), and substantia nigra (range 56.0%-88.7%; mean±SD, 72.4%±11.0%). Correlation analysis between plasma concentration of blonanserin and receptor occupancy in D2-rich (caudate and putamen) and D3-rich (globus pallidus and substantia nigra) regions showed that EC50 for D2-rich region was 0.39 ng/mL (r=0.43) and EC50 for D3-rich region was 0.40 ng/mL (r=0.79).

CONCLUSIONS

A single dose of 12 mg blonanserin occupied D3 receptor to the same degree as D2 receptor in vivo. Our results were consistent with previous studies that reported that some of the pharmacological effect of blonanserin is mediated via D3 receptor antagonism.

The effects of buspirone on occupancy of dopamine receptors and the rat gambling task

BACKGROUND

The dopamine D3 receptor (DRD₃) has been proposed as a target for drug development for the treatment of addictive disorders. Recently, the anxiolytic buspirone has been shown to have affinity for DRD₃ and DRD₄, and interest in repurposing it for addictive disorders has grown.

METHODS

Binding of [³H]-(+)-PHNO in the rat cerebellum and striatum was used to measure occupancy by buspirone of DRD₃ or DRD₂, respectively. Effects of buspirone in the rat gambling task (rGT) and the five-choice serial reaction time task (5-CSRTT) were examined.

RESULTS

Buspirone occupied both the DRD₂ and DRD₃ at high doses and the DRD₃, but not the DRD₂, in the narrow dose range of 3 mg/kg. At 10 mg/kg, a disruption of performance on rGT was observed. All measures of performance on the rGT, except for perseverations, were affected at 3 mg/kg. On the 5-CSRTT, omissions were increased. Impairments in the rGT were not mimicked by the effects induced by satiation. Further, buspirone did not impair food-maintained responding under a progressive ratio schedule of reinforcement at any dose, suggesting that the effects of buspirone on the rGT cannot be explained by non-selective actions.

CONCLUSIONS

Although buspirone had effects on the rGT at the dose that selectively occupied the DRD₃, the effects found do not parallel those found in previous studies of the effects of selective DRD₃ antagonists on the rGT. Thus, buspirone may impair performance on the rGT through actions at multiple receptor sites.

Buspirone Counteracts MK-801-Induced Schizophrenia-Like Phenotypes through Dopamine D3 Receptor Blockade

Background: Several efforts have been made to develop effective antipsychotic drugs. Currently, available antipsychotics are effective on positive symptoms, less on negative symptoms, but not on cognitive impairment, a clinically relevant dimension of schizophrenia. Drug repurposing offers great advantages over the long-lasting, risky and expensive, de novo drug discovery strategy. To our knowledge, the possible antipsychotic properties of buspirone, an azapirone anxiolytic drug marketed in 1986 as serotonin 5-HT_1A receptor (5-HT_1AR) partial agonist, have not been extensively investigated despite its intriguing pharmacodynamic profile, which includes dopamine D₃ (D₃R) and D₄ receptor (D₄R) antagonist activity. Multiple lines of evidence point to D₃R as a valid therapeutic target for the treatment of several neuropsychiatric disorders including schizophrenia. In the present study, we tested the hypothesis that buspirone, behaving as dopamine D₃R antagonist, may have antipsychotic-like activity.

Materials and Methods: Effects of acute administration of buspirone was assessed on a wide-range of schizophrenia-relevant abnormalities induced by a single administration of the non-competitive NMDAR antagonist MK-801, in both wild-type mice (WT) and D₃R-null mutant mice (D₃R^-/-).

Results: Buspirone (3 mg⋅kg^-1, i.p.) was devoid of cataleptogenic activity in itself, but resulted effective in counteracting disruption of prepulse inhibition (PPI), hyperlocomotion and deficit of temporal order recognition memory (TOR) induced by MK-801 (0.1 mg⋅kg^-1, i.p.) in WT mice. Conversely, in D₃R^-/- mice, buspirone was ineffective in preventing MK-801-induced TOR deficit and it was only partially effective in blocking MK-801-stimulated hyperlocomotion.

Conclusion: Taken together, these results indicate, for the first time, that buspirone, might be a potential therapeutic medication for the treatment of schizophrenia. In particular, buspirone, through its D₃R antagonist activity, may be a useful tool for improving the treatment of cognitive deficits in schizophrenia that still represents an unmet need of this disease.

Cariprazine Exhibits Anxiolytic and Dopamine D3 Receptor-Dependent Antidepressant Effects in the Chronic Stress Model

Background

Cariprazine, a D3-preferring dopamine D2/D3 receptor partial agonist, is a new antipsychotic drug recently approved in the United States for the treatment of schizophrenia and bipolar mania. We recently demonstrated that cariprazine also has significant antianhedonic-like effects in rats subjected to chronic stress; however, the exact mechanism of action for cariprazine's antidepressant-like properties is not known. Thus, in this study we examined whether the effects of cariprazine are mediated by dopamine D3 receptors.

Methods

Wild-type and D3-knockout mice were exposed to chronic unpredictable stress for up to 26 days, treated daily with vehicle, imipramine (20 mg/kg), aripiprazole (1 and 5 mg/kg), or cariprazine (0.03, 0.1, 0.2, and 0.4 mg/kg), and tested in behavioral assays measuring anhedonia and anxiety-like behaviors.

Results

Results showed that cariprazine significantly attenuated chronic unpredictable stress-induced anhedonic-like behavior in wild-type mice, demonstrating potent antidepressant-like effects comparable with aripiprazole and the tricyclic antidepressant imipramine. This antianhedonic-like effect of cariprazine was not observed in D3-knockout mice, suggesting that the cariprazine antidepressant-like activity is mediated by dopamine D3 receptors. Moreover, cariprazine significantly reduced drinking latency in the novelty-induced hypophagia test in wild-type mice, further confirming its antianhedonic-like effect and showing that it also has anxiolytic-like activity.

Conclusions

In combination with previous studies, these results suggest that cariprazine has a unique pharmacological profile and distinct dopamine D3 receptor-dependent mechanism of action that may be beneficial in the treatment of schizophrenia, bipolar disorder, and major depressive disorder.

Antipsychotics promote GABAergic interneuron genesis in the adult rat brain: Role of heat-shock protein production

Current antipsychotics reduce positive symptoms and reverse negative symptoms in conjunction with cognitive behavioral issues with the goal of restoring impaired occupational and social functioning. However, limited information is available on their influence on gliogenesis or their neurogenic properties in adult schizophrenia brains, particularly on GABAergic interneuron production. In the present study, we used young adult subventricular zone (SVZ)-derived progenitor cells expressing proteoglycan NG2 cultures to examine the oligodendrocyte and GABAergic interneuron genesis effects of several kinds of antipsychotics on changes in differentiation function induced by exposure to the NMDA receptor antagonist MK-801. We herein demonstrated that antipsychotics promoted or restored changes in the oligodendrocyte/GABAergic interneuron differentiation functions of NG2(+) cells induced by the exposure to MK-801, which was considered to be one of the drug-induced schizophrenia model. We also demonstrated that antipsychotics restored heat-shock protein (HSP) production in NG2(+) cells with differentiation impairment. The antipsychotics olanzapine, aripiprazole, and blonanserin, but not haloperidol increased HSP90 levels, which were reduced by the exposure to MK-801. Our results showed that antipsychotics, particularly those recently synthesized, exerted similar GABAergic interneuron genesis effects on NG2(+) neuronal/glial progenitor cells in the adult rat brain by increasing cellular HSP production, and also suggest that HSP90 may play a crucial role in the pathophysiology of schizophrenia and is a key target for next drug development.

Preferential binding to dopamine D3 over D2 receptors by cariprazine in patients with schizophrenia using PET with the D3/D2 receptor ligand [(11)C]-(+)-PHNO

RATIONALE

Second-generation antipsychotics occupy dopamine D2 receptors and act as antagonists or partial agonists at these receptors. While these drugs alleviate positive symptoms in patients with schizophrenia, they are less effective for treating cognitive deficits and negative symptoms. Dopamine D3 receptors are highly expressed in areas of the brain thought to play a role in the regulation of motivation and reward-related behavior. Consequently, the dopamine D3 receptor has become a target for treating negative symptoms in combination with D2 antagonism to treat positive symptoms in patients with schizophrenia.

OBJECTIVE

The purpose of this study was to determine the cariprazine receptor occupancies in brain for D2 and D3 receptors in patients with schizophrenia.

METHODS

Using [(11)C]-(+)-PHNO as a radioligand, positron emission tomography (PET) scans were performed in eight patients at baseline and postdose on days 1, 4, and 15. Plasma and cerebrospinal fluid (CSF) samples were analyzed for cariprazine concentrations.

RESULTS

A monotonic dose-occupancy relationship was observed for both receptor types. After 2 weeks of treatment, near complete (∼100 %) occupancies were observed for both receptors at a dose of 12 mg/day. At the lowest cariprazine dose (1 mg/day), mean D3 and D2 receptor occupancies were 76 and 45 %, respectively, suggesting selectivity for D3 over D2 receptors at low doses. An exposure-response analysis found a ∼3-fold difference in EC50 (D3 = 3.84 nM and D2 = 13.03 nM) in plasma after 2 weeks of dosing.

CONCLUSION

This PET imaging study in patients with schizophrenia demonstrated that cariprazine is a D3-preferring dual D3/D2 receptor partial agonist.

The atypical antipsychotic blonanserin reverses (+)-PD-128907- and ketamine-induced deficit in executive function in common marmosets

Antagonism of the dopamine D3 receptor is considered a promising strategy for the treatment of cognitive impairment associated with schizophrenia. We have previously reported that the atypical antipsychotic blonanserin, a dopamine D2/D3 and serotonin 5-HT2A receptor antagonist, highly occupies dopamine D3 receptors at its antipsychotic dose range in rats. In the present study, we evaluated the effects of blonanserin on executive function in common marmosets using the object retrieval with detour (ORD) task. The dopamine D3 receptor-preferring agonist (+)-PD-128907 at 1mg/kg decreased success rate in the difficult trial, but not in the easy trial. Since the difference between the two trials is only cognitive demand, our findings indicate that excess activation of dopamine D3 receptors impairs executive function in common marmosets. Blonanserin at 0.1mg/kg reversed the decrease in success rate induced by (+)-PD-128907 in the difficult trial. This finding indicates that blonanserin has beneficial effect on executive function deficit induced by activation of the dopamine D3 receptor in common marmosets. Next, and based on the glutamatergic hypothesis of schizophrenia, the common marmosets were treated with the N-methyl-d-aspartate (NMDA) receptor antagonist ketamine. Ketamine at sub-anesthetic doses decreased success rate in the difficult trial, but not in the easy trial. Blonanserin at 0.1mg/kg reversed the decrease in success rate induced by ketamine in the difficult trial. The findings of this study suggest that blonanserin might have beneficial effect on executive dysfunction in patients with schizophrenia.

Examination of clozapine and haloperidol in improving ketamine-induced deficits in an incremental repeated acquisition procedure in BALB/c mice

RATIONALE

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, causes locomotor hyperactivity, aberrant prepulse inhibition and impaired reversal learning among other deficits. There are numerous clinical and pre-clinically uses of NMDAR antagonists and a growing need to characterize their neurobehavioral effects.

OBJECTIVES

The present study was designed to characterize 1) ketamine's effect on incremental repeated acquisition (IRA), a procedure that taps multiple neurobehavioral functions and has performance measures correlated with IQ in humans, and 2) the extent to which clozapine (CLZ) and haloperidol (HAL) block ketamine's detrimental effects.

METHODS AND RESULTS

In experiment 1 (Exp. 1), BALB/c mice nose-poked under an IRA procedure for sucrose pellets. Systemic ketamine (1-30 mg/kg) dose-dependently decreased measures of cognitive and motor function. CLZ pretreatment (CLZ 0.1-4.0 mg/kg) dose-dependently attenuated ketamine-induced (30 mg/kg) deficits; the effective dose range of CLZ was 0.3-1.0 mg/kg. HAL pretreatment (0.01-0.1 mg/kg) did not attenuate any ketamine-induced deficits. In experiment 2 (Exp. 2), BALB/c mice lever-pressed under an IRA procedure for sweetened condensed milk. Ketamine (30 mg/kg) produced a global impairment in the IRA procedure and CLZ pretreatment (0.3-1.0 mg/kg) dose-dependently attenuated that impairment; motor-based performance recovered to a greater extent than cognitive performance. When tested alone, these doses of CLZ had little effect on IRA performance.

CONCLUSIONS

These findings support the notion that CLZ is more effective than HAL at blocking ketamine-induced deficits. The IRA procedure may be beneficial for distinguishing the efficacy of drugs that seek to alleviate deficits in complex behavior that result from acute NMDAR antagonism.

Occupancy of Dopamine D3 and D2 Receptors by Buspirone: A [11C]-(+)-PHNO PET Study in Humans

There is considerable interest in blocking the dopamine D3 receptor (DRD3) versus the D2 receptor (DRD2) to treat drug addiction. However, there are currently no selective DRD3 antagonists available in the clinic. The anxiolytic drug buspirone has been proposed as a potential strategy as findings suggest that this drug has high in vitro affinity for DRD3, binds to DRD3 in brain of living non-human primate, and also disrupts psychostimulant self-administration in preclinical models. No study has explored the occupancy of DRD3 by buspirone in humans. Here, we used positron emission tomography (PET) and the D3-preferring probe, [(11)C]-(+)-PHNO, to test the hypothesis that buspirone will occupy (decreases [(11)C]-(+)-PHNO binding) the DRD3 more readily than the DRD2. Eight healthy participants underwent [(11)C]-(+)-PHNO scans after single oral dose administration of placebo and 30, 60, and 120 mg of buspirone in a single-blind within-subjects design. [(11)C]-(+)-PHNO binding in DRD2- and DRD3-rich areas was decreased by the highest (60-120 mg), but not the lowest (30 mg), doses of buspirone. The maximal occupancy obtained was ~25% in both areas. Plasma levels of prolactin (a DRD2 marker) correlated with percentage occupancy after orally administered buspirone. Self-reported dizziness and drowsiness increased after buspirone but that did not correlate with receptor occupancy in any region. Overall, the modest occupancy of DRD2 and DRD3 even at high acute doses of buspirone, yielding high levels of metabolites, suggests that buspirone may not be a good drug to preferentially block DRD3 in humans.

Differential effects of clozapine, metoclopramide, haloperidol and risperidone on acquisition and performance of operant responding in rats

RATIONALE

Prior research has not systematically investigated the effects of systemic antipsychotic drugs on operant response acquisition, specifically their behavioural microstructure, reinforcement blunting and relative potency in acquisition compared to performance once operant responding has stabilized.

OBJECTIVES

This study aims to systematically investigate the effects of systemically administered clozapine, metoclopramide, haloperidol and risperidone during free operant response acquisition and performance.

METHODS

Following magazine training, food-restricted male Wistar rats lever pressed for food reward in 15 min daily operant conditioning sessions.

RESULTS

All drugs suppressed operant response acquisition and performance. Risperidone and metoclopramide, but not clozapine or haloperidol, suppressed operant responding more potently during acquisition than performance. The dopamine D2-like receptor antagonists haloperidol and metoclopramide that affect the ventral and dorsal striatum blunted reinforcement and decreased inactive lever presses in acquisition. In contrast, the atypical antipsychotics clozapine and risperidone that affect the ventral striatum and prefrontal cortex failed to decrease inactive lever presses during acquisition, suggesting a possible decision-making deficit. Haloperidol decreased active lever pressing over performance days. The drugs did not appear to affect rats' sensitivity to active lever press outcome, even though they suppressed active lever pressing.

CONCLUSIONS

Results suggest that reinforcement impact during operant acquisition is dependent on dopamine D2 receptors while drugs affecting, among other areas, the prefrontal cortex produce a deficit in ability to suppress inactive lever press responses.

Cariprazine in the treatment of acute mania in bipolar I disorder: a double-blind, placebo-controlled, phase III trial

BACKGROUND

This Phase III, randomized, double-blind, placebo-controlled study investigated the efficacy and tolerability of flexibly-dosed cariprazine in patients with acute manic or mixed episodes associated with bipolar I disorder.

METHODS

Patients were randomized to 3 weeks of double-blind treatment with cariprazine 3-12mg/day (n=158) or placebo (n=154). The primary efficacy parameter was change from baseline to Week 3 in Young Mania Rating Scale (YMRS) total score. The secondary efficacy parameter was change from baseline to Week 3 in Clinical Global Impressions-Severity (CGI-S) score.

RESULTS

Mean change from baseline to Week 3 in YMRS total score was significantly greater for patients receiving cariprazine 3-12mg/day versus placebo (P=0.0004). Significant differences between groups in YMRS total score mean change were observed by Day 4 (first postbaseline assessment) and maintained throughout double-blind treatment (all assessments, P<0.01). Cariprazine also demonstrated statistically significant superiority over placebo on YMRS response (≥50% improvement: cariprazine, 58.9%; placebo, 44.1%; P=0.0097) and remission (YMRS total score≤12: cariprazine, 51.9%; placebo, 34.9%; P=0.0025) and mean change in CGI-S (P=0.0027) score and Positive and Negative Syndrome Scale (PANSS) (P=0.0035) total score. The most common cariprazine-related (≥10% and twice placebo) treatment emergent adverse events (TEAEs) were akathisia, extrapyramidal disorder, tremor, dyspepsia, and vomiting. Mean change from baseline in metabolic parameters were generally small and similar between groups.

LIMITATIONS

Lack of active comparator arm; short duration of study.

CONCLUSION

In this study, cariprazine 3-12mg/day was effective and generally well tolerated in the treatment of manic and mixed episodes associated with bipolar I disorder.

Blonanserin extensively occupies rat dopamine D3 receptors at antipsychotic dose range

Antagonism of the dopamine D3 receptor has been hypothesized to be beneficial for schizophrenia cognitive deficits, negative symptoms and extrapyramidal symptoms. However, recent animal and human studies have shown that most antipsychotics do not occupy D3 receptors in vivo, despite their considerable binding affinity for this receptor in vitro. In the present study, we investigated the D3 receptor binding of blonanserin, a dopamine D2/D3 and serotonin 5-HT2A receptors antagonist, in vitro and in vivo. Blonanserin showed the most potent binding affinity for human D3 receptors among the tested atypical antipsychotics (risperidone, olanzapine and aripiprazole). Our GTPγS-binding assay demonstrated that blonanserin acts as a potent full antagonist for human D3 receptors. All test-drugs exhibited antipsychotic-like efficacy in methamphetamine-induced hyperactivity in rats. Treatment with blonanserin at its effective dose blocked the binding of [(3)H]-(+)-PHNO, a D2/D3 receptor radiotracer, both in the D2 receptor-rich region (striatum) and the D3 receptor-rich region (cerebellum lobes 9 and 10). On the other hand, the occupancies of other test-drugs for D3 receptors were relatively low. In conclusion, we have shown that blonanserin, but not other tested antipsychotics, extensively occupies D3 receptors in vivo in rats.

Low dose pramipexole causes D3 receptor-independent reduction of locomotion and responding for a conditioned reinforcer

Pramipexole is a clinically important dopamine receptor agonist with reported selectivity for dopamine D3 receptors over other dopaminergic and non-dopaminergic sites. Many of its behavioural effects are therefore attributed to D3 receptor activity. Here we relate pramipexole's ex vivo D2 and D3 receptor binding (measured using [(3)H]-(+)-PHNO binding experiments) to its effects on locomotion and operant responding for primary and conditioned reinforcers. We show that pramipexole has inhibitory behavioural effects on all three behaviours at doses that occupy D3 but not D2 receptor. However, these effects are 1) not inhibited by a D3 selective dose of the antagonist SB-277011-A, and 2) present in D3 receptor knockout mice. These results suggest that a pharmacological mechanism other than D3 receptor activity must be responsible for these behavioural effects. Finally, our receptor binding results also suggest that these behavioural effects are independent of D2 receptor activity. However, firmer conclusions regarding D2 involvement would be aided by further pharmacological or receptor knock-out experiments. The implications of our findings for the understanding of pramipexole's behavioural and clinical effects are discussed.

Prior haloperidol, but not olanzapine, exposure augments the pursuit of reward cues: implications for substance abuse in schizophrenia

Drug abuse and addiction are excessively common in schizophrenia. Chronic antipsychotic treatment might contribute to this comorbidity by inducing supersensitivity within the brain's dopamine system. Dopamine supersensitivity can enhance the incentive motivational properties of reward cues, and reward cues contribute to the maintenance and severity of drug addiction. We have shown previously that rats withdrawn from continuous haloperidol (HAL) treatment (via subcutaneous minipump) develop dopamine supersensitivity and pursue reward cues more vigorously than HAL-naive rats following an amphetamine (AMPH) challenge. Atypical antipsychotic drugs are thought to be less likely than typicals to produce dopamine supersensitivity. Thus, we compared the effects of HAL and the atypical antipsychotic olanzapine (OLZ) on the pursuit of reward cues. Rats were trained to associate a light-tone cue with water then treated with HAL or OLZ. Following antipsychotic withdrawal, we assessed AMPH-induced enhancement of lever pressing for the cue. Withdrawal from HAL, but not from OLZ, enhanced this effect. HAL, but not OLZ, also enhanced AMPH-induced psychomotor activation and c-fos mRNA expression in the caudate-putamen. Thus, prior HAL, but not OLZ, enhanced conditioned reward following an AMPH challenge, and this was potentially linked to enhanced behavioral sensitivity to AMPH and AMPH-induced engagement of the caudate-putamen. These findings suggest that HAL, but not an atypical like OLZ, modifies reward circuitry in ways that increase responsiveness to reward cues. Because enhanced responsiveness to reward cues can promote drug-seeking behavior, it should be investigated whether atypical antipsychotics might be a preferential option in schizophrenic patients at risk for drug abuse or addiction.

Synthesis and dual D2 and 5-HT1A receptor binding affinities of 5-piperidinyl and 5-piperazinyl-1H-benzo[d]imidazol-2(3H)-ones

A series of new 5-piperidinyl and 5-piperazinyl-1H-benzo[d]imidazol-2(3H)-ones have been synthesized and evaluated for dual D2 and 5-HT1A receptor binding affinities. The synthesized ligands are structurally related to bifeprunox, a potential atypical antipsychotic, having potent D2 receptor antagonist and 5-HT1A receptor agonist properties. The Suzuki-Miyaura reaction of cyclic vinyl boronate with appropriate aryl halide yielded arylpiperidine, which was eventually transformed to piperidinyl-1H-benzo[d]imidazol-2(3H)-one. The reductive amination of the latter with appropriate biarylaldehdyes rendered the synthesis of 5-piperidinyl-1H-benzo[d]imidazol-2(3H)-ones. Likewise, the Buchwald-Hartwig coupling reactions of 1-boc-piperazine with appropriate aryl halide and subsequent removal of the boc group rendered arylpiperazine. The reductive amination of the latter with appropriate biarylaldehdyes accomplished the synthesis of 5-piperazinyl-1H-benzo[d]imidazol-2(3H)-ones. The structure-activity relationship studies showed that cyclopentenylpyridine and cyclopentenylbenzyl groups contribute significantly to the dual D2 and 5-HT1A receptor binding affinities of these compounds.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

The stereotypy-inducing and OCD-like effects of chronic 'binge' cocaine are modulated by distinct subtypes of nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

High rates of cigarette smoking occur in cocaine-dependent individuals, reflecting an involvement of nicotinic acetylcholine receptors (nAChRs) in cocaine-elicited behaviour. This study was designed to assess the contribution of different nAChR subtypes to the behavioural and neurochemical effects of chronic cocaine treatment.

EXPERIMENTAL APPROACH

Cocaine (15 mg·kg(-1) , i.p.) was administered to male C57BL/6J mice in a chronic 'binge' paradigm, with and without the coadministration of the α7 preferring nAChR antagonist methyllycaconitine (MLA; 5 mg·kg(-1) , i.p.) or the β2* nAChR antagonist dihydro-β-erythroidine (DHβE; 2 mg·kg(-1) , i.p.). Quantitative autoradiography was used to examine the effect of cocaine exposure on α7 and α4β2* nAChRs, and on the high-affinity choline transporter.

KEY RESULTS

MLA+cocaine administration induced an intense self-grooming behaviour, indicating a likely role for α7 nAChRs in modulating this anxiogenic, compulsive-like effect of cocaine. In the major island of Calleja, a key area of action for neuroleptics, MLA+cocaine reduced choline transporter binding compared with cocaine (with or without DHβE) administration. DHβE treatment prevented the induction of stereotypy sensitisation to cocaine but prolonged locomotor sensitisation, implicating heteromeric β2* nAChRs in the neuroadaptations mediating cocaine-induced behavioural sensitisation. 'Binge' cocaine treatment region-specifically increased α4β2* nAChR binding in the midbrain dopaminergic regions: ventral tegmental area and substantia nigra pars compacta.

CONCLUSIONS AND IMPLICATIONS

We have shown a differential, subtype-selective, contribution of nAChRs to the behavioural and neurochemical sequelae of chronic cocaine administration. These data support the clinical utility of targeting specific nAChR subtypes for the alleviation of cocaine-abuse symptomatology.

Residual dopamine receptor desensitization following either high- or low-dose sub-chronic prior exposure to the atypical anti-psychotic drug olanzapine

RATIONALE

Anti-psychotic drugs are antagonists of dopamine D2 receptors and repeated administration may lead to the development of dopamine receptor supersensitivity.

OBJECTIVES

The objective of this study is to investigate the effects of sub-chronic olanzapine treatments upon the induction of dopamine receptor supersensitivity.

METHODS

Rats were administered ten daily low or high doses of the atypical anti-psychotic drug olanzapine (0.01 or 1.0 mg/kg). After 5 days of withdrawal, all groups received 2.0 mg/kg apomorphine on five successive days. Five days after the apomorphine sensitization protocol, in separate experiments, either a conditioning test or an apomorphine sensitization test was conducted.

RESULTS

During the anti-psychotic treatment the high dose of olanzapine induced profound locomotion suppression, whereas the low dose had no effect upon locomotion. The apomorphine treatments given to the vehicle control group generated locomotor sensitization. This sensitization effect was attenuated by the same degree for both the low or high dose prior olanzapine treatments. Also, the low and high-dose olanzapine pre-treatments diminished subsequent apomorphine-conditioned and apomorphine-sensitized locomotor responses.

CONCLUSIONS

The equivalent attenuation of the apomorphine sensitization produced by both olanzapine doses indicates that this effect was unrelated to the direct effects of olanzapine upon locomotion. Furthermore, the persistence of the desensitization effects well after the termination of the olanzapine treatments is indicative of a residual desensitization of the dopamine system. These findings are of importance when considering the use of atypical anti-psychotic drugs in the treatment of psychoses and other disorders in which overactivity of the dopamine system is considered a contributory factor.

Clozapine-induced locomotor suppression is mediated by 5-HT2A receptors in the forebrain

The need for safer, more effective therapeutics for the treatment of schizophrenia is widely acknowledged. To optimally target novel pharmacotherapies, in addition to establishing the mechanisms responsible for the beneficial effects of antipsychotics, the pathways underlying the most severe side effects must also be elucidated. Here we investigate the role of serotonin 2A (5-HT(2A)), serotonin 2C (5-HT(2C)), and dopamine 2 receptors (D₂) in mediating adverse effects associated with canonical first- and second-generation antipsychotic drugs in mice. Wild-type (WT) and 5-HT(2A) knockout (KO) mice treated with haloperidol, clozapine, and risperidone were assessed for locomotor activity and catalepsy. WT mice showed a marked reduction in locomotor activity following acute administration of haloperidol and high-dose risperidone, which was most likely secondary to the severe catalepsy caused by these compounds. Clozapine also dramatically reduced locomotor activity, but in the absence of catalepsy. Interestingly, 5-HT(2A) KO mice were cataleptic following haloperidol and risperidone, but did not respond to clozapine's locomotor-suppressing effects. Restoration of 5-HT(2A) expression to cortical glutamatergic neurons re-instated the locomotor-suppressing effects of clozapine in the open field. In sum, we confirm that haloperidol and risperidone caused catalepsy in rodents, driven by strong antagonism of D₂. We also demonstrate that clozapine decreases locomotor activity in a 5-HT(2A)-dependent manner, in the absence of catalepsy. Moreover, we show that it is the cortical population of 5-HT(2A) that mediate the locomotor-suppressing effects of clozapine.

In vivo occupancy of dopamine D3 receptors by antagonists produces neurochemical and behavioral effects of potential relevance to attention-deficit-hyperactivity disorder

Dopamine D(3) receptors have eluded definitive linkage to neurologic and psychiatric disorders since their cloning over 20 years ago. We report a new method that does not employ a radiolabel for simultaneously defining in vivo receptor occupancy of D(3) and D(2) receptors in rat brain after systemic dosing using the tracer epidepride (N-[[(2S)-1-ethylpyrrolidin-2-yl]methyl]-5-iodo-2,3-dimethoxybenzamide). Decreases in epidepride binding in lobule 9 of cerebellum (rich in D(3) receptors) were compared with nonspecific binding in the lateral cerebellum. The in vivo occupancy of the dopamine D(3) receptors was dose dependently increased by SB-277011A (trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolinecarboxamide) and U99194 (2,3-dihydro-5,6-dimethoxy- N,N-dipropyl-1H-inden-2-amine). Both antagonists increased extracellular levels of acetylcholine (ACh) in the medial prefrontal cortex of rats and modified brain-tissue levels of ACh and choline. Consistent with these findings, the D(3) receptor antagonists enhanced the acquisition of learning of rats either alone or in the presence of the norepinephrine uptake blocker reboxetine as with the attention-deficit-hyperactivity disorder (ADHD) drug methylphenidate. Like reboxetine, the D(3) receptor antagonists also prevented deficits induced by scopolamine in object recognition memory of rats. Mice in which the dopamine transporter (DAT) has been deleted exhibit hyperactivity that is normalized by compounds that are effective in the treatment of ADHD. Both D(3) receptor antagonists decreased the hyperactivity of DAT(-/-) mice without affecting the activity of wild type controls. The present findings indicate that dopamine D(3) receptor antagonists engender cognition-enhancing and hyperactivity-dampening effects. Thus, D(3) receptor blockade could be considered as a novel treatment approach for cognitive deficits and hyperactivity syndromes, including those observed in ADHD.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

Opposite effects of low versus high dose haloperidol treatments on spontaneous and apomorphine induced motor behavior: evidence that at a very low dose haloperidol acts as an indirect dopamine agonist

Anti-psychotic drugs are antagonists at the dopamine D2 receptors and repeated administration can lead to the development of dopamine receptor supersensitivity. In two experiments, separate groups of rats were administered 10 daily low or high doses of the typical anti-psychotic drug haloperidol (0.03 or 1.0 mg/kg). The high dose decreased locomotion whereas, the low dose increased locomotion. After 5 days of withdrawal, all groups received 2.0 mg/kg apomorphine on 5 successive days. The apomorphine treatments given to the vehicle group generated a progressive locomotion sensitization effect and this effect was potentiated by pre-exposure to 0.03 mg/kg haloperidol. Initially, the prior high dose of haloperidol exaggerated the apomorphine locomotor stimulant effect but with repeated apomorphine treatments desensitization developed. Following a 5-day withdrawal period an apomorphine challenge test was conducted and apomorphine sensitization was absent in the haloperidol high dose pre-exposure group but potentiated in the low dose pre-exposure group. In the second replication experiment a conditioning test instead of a sensitization challenge test was conducted 5 days after completion of the 5-day apomorphine treatment protocol. The repeated apomorphine treatments induced conditioned hyper- locomotion and this conditioned effect was prevented by the prior high dose haloperidol pre-exposure but enhanced by the prior low dose haloperidol pre-exposure. Two new key findings are (a) that a low dose haloperidol regimen can function as a dopamine agonist and these effects persist after withdrawal and (b) that repeated apomorphine treatments can desensitize D2 receptors previously sensitized by a high dose haloperidol treatment regimen.

Role of dopamine D(2) receptors for antipsychotic activity

This review summarizes the current state of knowledge regarding the proposed mechanisms by which antipsychotic agents reduce the symptoms of schizophrenia while giving rise to adverse side effects. The first part summarizes the contribution of neuroimaging studies to our understanding of the neurochemical substrates of schizophrenia, putting emphasis on direct evidence suggestive of a presynaptic rather than a postsynaptic dysregulation of dopaminergic neurotransmission in this disorder. The second part addresses the role of D(2) and non-D(2) receptor blockade in the treatment of schizophrenia and highlights a preponderant role of D(2) receptors in the mechanism of antipsychotic action. Neuroimaging studies have defined a narrow, but optimal, therapeutic window of 65-78 % D(2) receptor blockade within which most antipsychotics achieve optimal clinical efficacy with minimal side effects. Some antipsychotics though do not conform to that therapeutic window, notably clozapine. The reasons for its unexcelled clinical efficacy despite subthreshold levels of D(2) blockade are unclear and current theories on clozapine's mechanisms of action are discussed, including transiency of its D(2) receptor blocking effects or preferential blockade of limbic D(2) receptors. Evidence is also highlighted to consider the use of extended antipsychotic dosing to achieve transiency of D(2) blockade as a way to optimize functional outcomes in patients. We also present some critical clinical considerations regarding the mechanisms linking dopamine disturbance to the expression of psychosis and its blockade to the progressive resolution of psychosis, keeping in perspective the speed and onset of antipsychotic action. Finally, we discuss potential novel therapeutic strategies for schizophrenia.

Effects of antipsychotics on D3 receptors: a clinical PET study in first episode antipsychotic naive patients with schizophrenia using [11C]-(+)-PHNO

Most antipsychotics are thought to have an effect on D(2) and D(3) receptors, although their D(3), versus D(2) binding has not been clearly established in vivo in humans. However, the development of [(11)C]-(+)-PHNO now permits the differentiation of antipsychotic activity on these two receptor subtypes. In this study we examined the effects of antipsychotics on D(2) and D(3) receptors by comparing [(11)C]-(+)-PHNO in D(2)-rich (caudate, CAU and putamen, PUT), mixed (ventral striatum) and D(3)-rich (globus-pallidus, GP and substantia nigra, SN) regions before and after the initiation of antipsychotic medication. The investigation therefore represents a longitudinal within-subject follow-up design wherein antipsychotic-naive patients with schizophrenia spectrum disorders were first scanned in a drug-naïve state and then again after ~2.5 weeks of antipsychotic treatment (risperidone or olanzapine). Binding potential (non displaceable or BP(ND)) was obtained to derive estimates of drug occupancy in the identified brain regions. Antipsychotic treatment was associated with the expected occupancies in the D(2)-rich regions; unexpectedly though, patients showed a higher, rather than the expected lower, [(11)C]-(+)-PHNO BP(ND) in the GP and SN despite simultaneous evidence for ongoing D(2) blockade in the other regions (CAU and PUT). In conclusion, patients treated with atypical antipsychotics demonstrated no evidence of D(3) receptor occupancy, but instead possible D(3) up-regulation following short-term treatment. The present findings add to a very limited body of evidence related to D(3) binding in vivo. [(11)C]-(+)-PHNO offer new opportunities for exploring the potential therapeutic significance of the D(3) receptor in schizophrenia and the action of antipsychotics.

Modeling chronic olanzapine exposure using osmotic minipumps: pharmacological limitations

Animal models can face unique challenges in mirroring what occurs in humans. This is the case for antipsychotics in rodents, where these drugs are metabolized much more rapidly. One strategy to address this issue has been the use of osmotic minipumps to ensure continuous antipsychotic exposure over prolonged intervals, which is routinely the case when these same drugs are administered to humans. More recently, it has been identified that with olanzapine this approach may be compromised by oxidative degradation, a process that can be observed within days. Further, in vivo evidence has reported progressive decreases in plasma levels over a 1-month interval. To address this issue in vitro, osmotic minipumps (n=4), with olanzapine at a concentration resulting in a dose of 7.5mg/kg/day in vivo, were placed in saline-filled Falcon tubes and immersed in a water bath. Olanzapine concentrations were assessed in the minipumps as well as the surrounding water bath at baseline, 1h, and days 1, 7, 14, 21, and 28. Minipump results indicated a monophasic exponential decay and a half-life of 14.8 days (95% CI=13.1-17.1 days). Results from the water bath demonstrated a linear increase in olanzapine up to and including day 21, followed thereafter by a decrease to day 28. It is concluded that administration of olanzapine via osmotic minipump is viable in animal models to mirror what occurs in humans, although the interval should be confined to 2 weeks. As well, strategies in dissolving olanzapine to diminish oxidation are discussed.

Comparative pharmacology of antipsychotics possessing combined dopamine D2 and serotonin 5-HT1A receptor properties

RATIONALE

There is increasing interest in antipsychotics intended to manage positive symptoms via D(2) receptor blockade and improve negative symptoms and cognitive deficits via 5-HT(1A) activation. Such a strategy reduces side-effects such as the extrapyramidal syndrome (EPS), weight gain, and autonomic disturbance liability.

OBJECTIVE

This study aims to review pharmacological literature on compounds interacting at both 5-HT(1A) and D(2) receptors (as well as at other receptors), including aripiprazole, perospirone, ziprasidone, bifeprunox, lurasidone and cariprazine, PF-217830, adoprazine, SSR181507, and F15063.

METHODS

We examine data on in vitro binding and agonism and in vivo tests related to (1) positive symptoms (e.g., psychostimulant-induced hyperactivity or prepulse inhibition deficit), (2) negative symptoms (e.g., phencyclidine-induced social interaction deficits and cortical dopamine release), and (3) cognitive deficits (e.g., phencyclidine or scopolamine-induced memory deficits). EPS liability is assessed by measuring catalepsy and neuroendocrine impact by determining plasma prolactin, glucose, and corticosterone levels.

RESULTS

Compounds possessing "balanced" 5-HT(1A) receptor agonism and D(2) antagonism (or weak partial agonism) and, in some cases, combined with other beneficial properties, such as 5-HT(2A) receptor antagonism, are efficacious in a broad range of rodent pharmacological models yet have a lower propensity to elicit EPS or metabolic dysfunction.

CONCLUSIONS

Recent compounds exhibiting combined 5-HT(1A)/D(2) properties may be effective in treating a broader range of symptoms of schizophrenia and be better tolerated than existing antipsychotics. Nevertheless, further investigations are necessary to evaluate recent compounds, notably in view of their differing levels of 5-HT(1A) affinity and efficacy, which can markedly influence activity and side-effect profiles.