Actions of novel antipsychotic agents on apomorphine-induced PPI disruption: influence of combined serotonin 5-HT1A receptor activation and dopamine D2 receptor blockade

DSP-6745, a novel 5-hydroxytryptamine modulator with rapid antidepressant, anxiolytic, antipsychotic and procognitive effects

BACKGROUND

Current treatment of major depressive disorder is facing challenges, including a low remission rate, late onset of efficacy, and worsening severity due to comorbid symptoms such as psychosis and cognitive dysfunction. Serotonin (5-HT) neurotransmission is involved in a wide variety of psychiatric diseases and its potential as a drug target continues to attract attention.

OBJECTIVES

The present study elucidates the effects of a novel 5-HT modulator, DSP-6745, on depression and its comorbid symptoms.

RESULTS

In vitro radioligand binding and functional assays showed that DSP-6745 is a potent inhibitor of 5-HT transporter and 5-HT2A, 5-HT2C, and 5-HT7 receptors. In vivo, DSP-6745 (6.4 and 19.1 mg/kg as free base, p.o.) increased the release of not only 5-HT, norepinephrine, and dopamine, but also glutamate in the medial prefrontal cortex. The results of in vivo mouse phenotypic screening by SmartCube® suggested that DSP-6745 has a behavioral signature combined with antidepressant-, anxiolytic-, and antipsychotic-like signals. A single oral dose of DSP-6745 (6.4 and 19.1 mg/kg) showed rapid antidepressant-like efficacy in the rat forced swim test, even at 24 h post-dosing, and anxiolytic activity in the rat social interaction test. Moreover, DSP-6745 (12.7 mg/kg, p.o.) led to an improvement in the apomorphine-induced prepulse inhibition deficit in rats. In the marmoset object retrieval with detour task, which is used to assess cognitive functions such as attention and behavioral inhibition, DSP-6745 (7.8 mg/kg, p.o.) enhanced cognition.

CONCLUSIONS

These data suggest that DSP-6745 is a multimodal 5-HT receptor antagonist and a 5-HT transporter inhibitor and has the potential to be a rapid acting antidepressant with efficacies in mitigating the comorbid symptoms of depression.

A Computational Model for the Simulation of Prepulse Inhibition and Its Modulation by Cortical and Subcortical Units

The sensorimotor gating is a nervous system function that modulates the acoustic startle response (ASR). Prepulse inhibition (PPI) phenomenon is an operational measure of sensorimotor gating, defined as the reduction of ASR when a high intensity sound (pulse) is preceded in milliseconds by a weaker stimulus (prepulse). Brainstem nuclei are associated with the mediation of ASR and PPI, whereas cortical and subcortical regions are associated with their modulation. However, it is still unclear how the modulatory units can influence PPI. In the present work, we developed a computational model of a neural circuit involved in the mediation (brainstem units) and modulation (cortical and subcortical units) of ASR and PPI. The activities of all units were modeled by the leaky-integrator formalism for neural population. The model reproduces basic features of PPI observed in experiments, such as the effects of changes in interstimulus interval, prepulse intensity, and habituation of ASR. The simulation of GABAergic and dopaminergic drugs impaired PPI by their effects over subcortical units activity. The results show that subcortical units constitute a central hub for PPI modulation. The presented computational model offers a valuable tool to investigate the neurobiology associated with disorder-related impairments in PPI.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

The novel dehydroepiandrosterone (DHEA) derivative BNN27 counteracts cognitive deficits induced by the D1/D2 dopaminergic receptor agonist apomorphine in rats

RATIONALE

Schizophrenia is a devastating mental disease that affects nearly 1% of the population worldwide. It is well documented that the dopaminergic (DAergic) system is compromised in schizophrenia. It is of note that the mixed dopamine (DA) D₁/D₂ receptor agonist apomorphine induces schizophrenia-like symptoms in rodents, including disruption of memory abilities. Neuroactive steroids, comprising dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulphate (DHEAS), were shown to affect brain DAergic system and to be involved in schizophrenia. BNN27 is a novel DHEA derivative, which is devoid of steroidogenic activity. It has recently been reported that BNN27 counteracted schizophrenia-like behavioural deficits produced by glutamate hypofunction in rats.

OBJECTIVES

The aim of the present study was to investigate the ability of BNN27 to attenuate non-spatial, spatial recognition and discrete memory deficits induced by apomorphine in rats.

METHODS

To this end, the object recognition task (ORT), the object location task (OLT) and the step-through passive avoidance test (STPAT) were used.

RESULTS

BNN27 (3 and 6 mg/kg, i.p.) attenuated apomorphine (0.5 mg/kg, i.p.)-induced non-spatial, spatial recognition and discrete memory deficits. Interestingly, the effects of compounds on memory cannot be ascribed to changes in locomotor activity.

CONCLUSIONS

Our findings suggest that BNN27 is effective to DA dysfunction caused by apomorphine, attenuating cognitive impairments induced by this D1/D2 receptor agonist in rats. Additionally, our findings illustrate a functional interaction between BNN27 and the DAergic system that may be of relevance for schizophrenia-like behavioural symptoms.

A Natural Product with High Affinity to Sigma and 5-HT7 Receptors as Novel Therapeutic Drug for Negative and Cognitive Symptoms of Schizophrenia

Dehydrocorybulbine (DHCB), an alkaloid from Corydalis yanhusuo. W.T, has been identified as a dopamine receptor antagonist. We extended our assessment of its pharmacological profile and found that DHCB exhibits high to moderate binding affinities to sigma 1 and 2 receptors, serotonin 5-HT7 receptor, and histamine H2 receptors. This led us to evaluate DHCB properties in pharmacological (apomorphine and MK-801) animal models of schizophrenia in mice. The pharmacological profile of DHCB was screened through radioligand receptor binding assays. Single dose of DHCB reversed the locomotor hyperactivity, stereotypy, and prepulse inhibition deficits induced by the dopaminergic agonist apomorphine. DHCB also reversed the depressive-like behavior and memory deficit induced by the glutamatergic antagonist MK-801 in the forced swim and the novel object recognition assays, respectively. These results indicate that DHCB effectively improves schizophrenia-like behavioral deficits that are induced by the disruption of dopaminergic and glutamatergic systems. The effectiveness of DHCB in reversing responses that mimic negative and cognitive deficits of schizophrenia might suggest that its anti-schizophrenia effects are mediated through modulating the activities of several receptor particularly sigma 1, sigma 2, 5-HT7 and dopamine receptors. Our study casts DHCB as a promising lead for therapeutic treatment of schizophrenia.

LASSBio-1422: a new molecular scaffold with efficacy in animal models of schizophrenia and disorders of attention and cognition

Aiming to identify new antipsychotic lead-compounds, our group has been working on the design and synthesis of new N-phenylpiperazine derivatives. Here, we characterized LASSBio-1422 as a pharmacological prototype of this chemical series. Adult male Wistar rats and CF1 mice were used for in-vitro and in-vivo assays, respectively. LASSBio-1422 [1 and 5 mg/kg, postoperatively (p.o.)] inhibited apomorphine-induced climbing as well as ketamine-induced hyperlocomotion (1 and 5 mg/kg, p.o.), animal models predictive of efficacy on positive symptoms. Furthermore, LASSBio-1422 (5 mg/kg, p.o.) prevented the prepulse impairment induced by apomorphine, (±)-2,5-dimethoxy-4-iodoamphetamine, and ketamine, as well as the memory impairment induced by ketamine in the novel object-recognition task at the acquisition, consolidation, and retrieval phases of memory formation. Potential extrapyramidal side-effects and sedation were assessed by catatonia, rota-rod, locomotion, and barbiturate sleeping time, and LASSBio-1422 (15 mg/kg, p.o.) did not affect any of the parameters observed. Binding assays showed that LASSBio-1422 has a binding profile different from the known atypical antipsychotic drugs: it does not bind to AMPA, kainate, N-methyl-D-aspartate, glicine, and mGluR2 receptors and has low or negligible affinity for D1, D2, and 5-HT2A/C receptors, but high affinity for D4 receptors (Ki=0.076 µmol/l) and, to a lesser extent, for 5-HT1A receptors (Ki=0.493 µmol/l). The antagonist action of LASSBio-1422 at D4 receptors was assessed through the classical GTP-shift assay. In conclusion, LASSBio-1422 is effective in rodent models of positive and cognitive symptoms of schizophrenia and its ability to bind to D4 and 5-HT1A receptors may at least in part explain its effects in these animal models.

Caution When Diagnosing Your Mouse With Schizophrenia: The Use and Misuse of Model Animals for Understanding Psychiatric Disorders

Animal models are widely used in biomedical research, but their applicability to psychiatric disorders is less clear. There are several reasons for this, including 1) emergent features of psychiatric illness that are not captured by the sum of individual symptoms, 2) a lack of equivalency between model animal behavior and human psychiatric symptoms, and 3) the possibility that model organisms do not have (and may not be capable of having) the same illnesses as humans. Here, we discuss the effective use, and inherent limitations, of model animals for psychiatric research. As disrupted-in-schizophrenia 1 (DISC1) is a genetic risk factor across a spectrum of psychiatric disorders, we focus on the results of studies using mice with various mutations of DISC1. The data from a broad range of studies show remarkable consistency with the effects of DISC1 mutation on developmental/anatomical endophenotypes. However, when one expands the phenotype to include behavioral correlates of human psychiatric diseases, much of this consistency ends. Despite these challenges, model animals remain valuable for understanding the basic brain processes that underlie psychiatric diseases. We argue that model animals have great potential to help us understand the core neurobiological dysfunction underlying psychiatric disorders and that marrying genetics and brain circuits with behavior is a good way forward.

Gender and estrous cycle influences on behavioral and neurochemical alterations in adult rats neonatally administered ketamine

Neonatal N-methyl-D-aspartate (NMDA) receptor blockade in rodents triggers schizophrenia (SCZ)-like alterations during adult life. SCZ is influenced by gender in age of onset, premorbid functioning, and course. Estrogen, the hormone potentially driving the gender differences in SCZ, is known to present neuroprotective effects such as regulate oxidative pathways and the expression of brain-derived neurotrophic factor (BDNF). Thus, the aim of this study was to verify if differences in gender and/or estrous cycle phase during adulthood would influence the development of behavioral and neurochemical alterations in animals neonatally administered ketamine. The results showed that ketamine-treated male (KT-male) and female-in-diestrus (KTF-diestrus, the low estrogen phase) presented significant deficits in prepulse inhibition of the startle reflex and spatial working memory, two behavioral SCZ endophenotypes. On the contrary, female ketamine-treated rats during proestrus (KTF-proestrus, the high estradiol phase) had no behavioral alterations. This correlated with an oxidative imbalance in the hippocampus (HC) of both male and KTF-diestrus female rats, that is, decreased levels of GSH and increased levels of lipid peroxidation and nitrite. Similarly, BDNF was decreased in the KTF-diestrus rats while no alterations were observed in KTF-proestrus and male animals. The changes in the HC were in contrast to those in the prefrontal cortex in which only increased levels of nitrite in all groups studied were observed. Thus, there is a gender difference in the adult rat HC in response to ketamine neonatal administration, which is based on the estrous cycle. This is discussed in relation to neuropsychiatric conditions and in particular SCZ.

1-aminocyclopropanecarboxylic acid (ACPC) produces procognitive but not antipsychotic-like effects in rats

RATIONALE

In addition to the negative and positive symptoms of schizophrenia, cognitive deficits, including prefrontal cortical dysfunction, are now recognized as core features of this disorder. Compounds increasing the NMDA receptor function via the strychnine-insensitive glycine receptors have been proposed as potential antipsychotics. Depending on the ambient concentrations of glutamate and glycine, 1-aminocyclopropanecarboxylic acid (ACPC) behaves as either a partial agonist or a functional antagonist at the strychnine-insensitive glycine receptors.

OBJECTIVES

We investigated the procognitive and antipsychotic-like effects of ACPC in rats treated with phencyclidine (PCP) or ketamine (KET), compounds that produce psychotic-like symptoms in humans and laboratory animals.

METHODS

Cognitive effects were investigated in the novel object recognition (NOR) and attentional set-shifting tests (ASST). In addition, the effects of ACPC were investigated in PCP-induced hyperactivity, conditioned avoidance response (CAR), and prepulse inhibition (PPI) tests. The effects on attention and impulsivity were measured in the five-choice serial reaction time task (5-CSRTT).

RESULTS

ACPC (200-400 mg/kg) inhibited memory fading in naive rats and like clozapine prevented PCP- and KET-induced amnesia in the NOR. In naive animals, ACPC at 400 but not 200 mg/kg enhanced cognitive flexibility in the ASST, as the animals required fewer trials to reach the criteria during the extra-dimensional phase. In contrast, ACPC did not affect PCP-induced hyperactivity, CAR, and PPI as well as attention and impulsivity in the 5-CSRTT.

CONCLUSION

The present study demonstrates that ACPC enhanced both object recognition memory and cognitive flexibility dependent on the prefrontal cortex, but did not affect impulsivity nor exhibit an antipsychotic-like profile.

Co-administration of 5-HT6 receptor antagonists with clozapine, risperidone, and a 5-HT2A receptor antagonist: effects on prepulse inhibition in rats

RATIONALE

Some novel antipsychotics manifest antagonistic activity at serotonin-6 receptors; however, little is known about the role of 5-HT6 receptors in ameliorating sensory gating deficits.

OBJECTIVE

We evaluated the effects of the combined administration of the 5-HT6 receptor antagonist SB 271046 with clozapine and haloperidol, as well as the co-administration of SB 271046 or SB 399885 with risperidone and the 5-HT2A antagonist M100907, to overcome the deficits induced by MK-801 in the prepulse inhibition (PPI) test.

RESULTS

MK-801 (0.1 mg/kg) produced reliable PPI deficits. Administration of SB 271046 (6 and 9 mg/kg), SB 399885 (3 and 6 mg/kg), clozapine (2.5 mg/kg), haloperidol (0.1 and 0.2 mg/kg), risperidone (0.25-1 mg/kg), and M100907 (0.5 and 1 mg/kg) did not affect the MK-801-induced deficits, but the administration of clozapine (5 mg/kg) did reverse the effects of MK-801. In MK-801-treated rats, the co-administration of inactive doses of clozapine (2.5 mg/kg) and SB 271046 (6 mg/kg) reversed the PPI impairments compared to animals that were administered inactive doses of either clozapine or SB 271046 alone. Co-administration of risperidone (1 mg/kg) or M100907 (0.5 mg/kg) with SB 271046 (6 mg/kg) or SB 399885 (3 mg/kg) also attenuated the MK-801-induced PPI deficits. In contrast, joint administration of haloperidol and SB 271046 had no effect on the PPI deficit.

CONCLUSION

The present results suggest that the 5-HT6 receptors may play adjunctive roles in antipsychotic drug action, and that the combination of 5-HT2A and 5-HT6 antagonism may represent an important element in the pharmacological profile of antipsychotic drugs.

5-HT1A receptor pharmacophores to screen for off-target activity of α1-adrenoceptor antagonists

The α1-adrenoceptors (α1-ARs), in particular the α1A-AR subtype, are current therapeutic targets of choice for the treatment of urogenital conditions, such as benign prostatic hyperplasia (BPH). Due to the similarity between the transmembrane domains of the α1-AR subtypes, and the serotonin receptor subtype 1A (5-HT1A-R), currently used α1-AR subtype-selective drugs to treat BPH display considerable off-target affinity for the 5-HT1A-R, leading to side effects. We describe the construction and validation of pharmacophores for 5-HT1A-R agonists and antagonists. Through the structural diversity of the training sets used in their development, these pharmacophores define the properties of a compound needed to bind to 5-HT1A receptors. Using these and previously published pharmacophores in virtual screening and profiling, we have identified unique chemical compounds (hits) that fit the requirements to bind to our target, the α1A-AR, selectively over the off-target, the 5-HT1A-R. Selected hits have been obtained and their affinities for α1A-AR, α1B-AR and 5-HT1A-R determined in radioligand binding assays, using membrane preparations which contain human receptors expressed individually. Three of the tested hits demonstrate statistically significant selectivity for α1A-AR over 5-HT1A-R. All seven tested hits bind to α1A-AR, with two compounds displaying K i values below 1 μM, and a further two K i values of around 10 μM. The insights and knowledge gained through the development of the new 5-HT1A-R pharmacophores will greatly aid in the design and synthesis of derivatives of our lead compound, and allow the generation of more efficacious and selective ligands.

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.

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.

cAMP response element binding protein phosphorylation in nucleus accumbens underlies sustained recovery of sensorimotor gating following repeated D₂-like receptor agonist treatment in rats

BACKGROUND

Prepulse inhibition (PPI) is a cross-species measure of sensorimotor gating. PPI deficits are observed in humans and rats upon acute treatment with dopamine D₂-like receptor agonists and in patients with schizophrenia. Repeated treatment with a D₂-like agonist, however, reverses PPI deficits and increases cyclic adenosine monophosphate (cAMP) signaling in the nucleus accumbens (NAc). This study examined the short- and long-term effects on PPI of treatment with quinpirole and ropinirole, dopamine D₂/D₃ receptor agonists, and the molecular mechanism by which they occur.

METHODS

PPI was assessed in adult male Sprague-Dawley rats following acute and chronic treatment with quinpirole or ropinirole and 1, 2, 3, and 4 weeks after termination of repeated ropinirole treatment. Finally, the effect of dominant negative mutant cAMP response element binding protein (CREB) overexpression in the NAc on PPI following chronic quinpirole treatment was assessed.

RESULTS

Acute quinpirole produced dose-dependent PPI deficits, whereas ropinirole caused consistent PPI reduction at all but the highest dose. Repeated ropinirole treatment significantly increased PPI compared with acute treatment, and increased CREB phosphorylation in NAc neurons. Subsequent ropinirole challenge had no effect as long as 28 days later, at which time NAc CREB phosphorylation had normalized. Overexpression of dominant negative mutant CREB prevented PPI recovery induced by chronic quinpirole treatment.

CONCLUSIONS

Chronic quinpirole or ropinirole treatment produces sustained PPI recovery; CREB activity in the NAc is required to induce PPI recovery but not to maintain it. The results suggest that transcriptional regulation by CREB mediates long-lasting changes occurring within NAc circuits to promote recovery of sensorimotor gating.

F15599, a preferential post-synaptic 5-HT1A receptor agonist: activity in models of cognition in comparison with reference 5-HT1A receptor agonists

We assessed the activity of F15599, a selective and high efficacy 5-HT(1A) agonist that preferentially activates post- versus pre-synaptic receptors, in rat cognition/memory models. F15599 (0.16 mg/kg i.p.) partially alleviated detrimental effects of phencyclidine on working and reference memory deficit in a hole-board model. It also attenuated phencyclidine-induced deficit of cognitive flexibility in a reversal learning task, without effects of its own. F13714 (0.04 mg/kg) a chemical congener of F15599, and 8-OH-DPAT (0.01 or 0.16), were inactive against these phencyclidine-induced deficits, and/or even worsened basal performances. F15599 (0.04-2.5) was less disruptive than F13714 (0.005-0.16) or 8-OH-DPAT (0.01-0.63), on basal performance in models of attention (5-choice serial reaction time task) and working memory (delayed non-matching to position). Finally, unlike either comparator, F15599 reduced PPI with modest potency and only partially. To conclude, F15599, in models of memory/cognition, has a more favourable profile than F13714 and 8-OH-DPAT. This suggests that preferential activation of post-synaptic 5-HT(1A) receptors could prove useful in pathologies characterized by cognitive/memory deficiencies, such as schizophrenia and depression.

Modulation of prepulse inhibition and stereotypies in rodents: no evidence for antipsychotic-like properties of histamine H3-receptor inverse agonists

RATIONALE

H(3)-receptor inverse agonists raise a great interest as innovative therapeutics in several central disorders. Whereas their procognitive properties are well established, their antipsychotic-like properties are still debated.

OBJECTIVES

We further explored the effect of maximal doses (3-10 mg/kg) of ciproxifan, BF2.649, and ABT-239, three selective H(3)-receptor inverse agonists, on deficits of prepulse inhibition (PPI) induced by apomorphine, MK-801, and phencyclidine (PCP). Their effect was also investigated on stereotypies induced by apomorphine and methamphetamine.

RESULTS

Ciproxifan, BF2.649, and ABT-239 did not reverse the PPI impairment produced by apomorphine (0.5 mg/kg, subcutaneous) in rats. Ciproxifan and BF2.649 did not reverse the impairment induced in mice by MK-801 (0.3 mg/kg). Ciproxifan and BF2.649 also failed to reverse the disruption induced in mice by PCP (5-10 mg/kg). Low to moderate doses of haloperidol (0.1-0.4 mg/kg, intraperitoneal), alone or co-administered with BF2.649, did not reverse MK-801-induced PPI disruption. A high dose (1 mg/kg) of haloperidol partially reversed the MK-801-induced deficit and BF2.649 tended to increase this effect, although nonsignificantly. Whereas stereotypies induced in mice by apomorphine and methamphetamine were totally suppressed by haloperidol, the decrease induced by ciproxifan was partial against apomorphine and very low, if any, against methamphetamine.

CONCLUSIONS

Their total absence of effect in several validated animal models of the disease does not support antipsychotic properties of H(3)-receptor inverse agonists. However, their positive effects previously reported in behavioral tasks addressing learning, attention, and memory maintain the interest of H(3)-receptor inverse agonists for the treatment of cognitive symptoms of schizophrenia as adjunctive medications.

Agmatine disrupts prepulse inhibition of acoustic startle reflex in rats

Agmatine is a guanidine-amine formed by the enzymatic decarboxylation of arginine. Agmatine has been proposed to be a neuromodulator and its downstream derivatives, the polyamines, have been suggested to be responsible for sensory gating deficits seen in schizophrenia. In this study, male Wistar rats underwent treatments with agmatine, vehicle or other agents known to alter sensory gating in an experimental paradigm of prepulse inhibition (PPI) of the acoustic startle response. Apomorphine (1 mg/kg s.c.), a nonselective dopamine agonist known to disrupt PPI responses, was injected as the positive reference. Neither apomorphine nor agmatine (40-160 mg/kg i.p.) induced effects on the intensity of startle reflex without a prepulse. However, apomorphine or agmatine (160 mg/kg i.p.) disrupted the PPI of acoustic startle reflex. Furthermore, when given 30 min prior, agmatine acted additively with apomorphine's effect on PPI. In an attempt to gain more insight, haloperidol (1 and 2 mg/kg i.p.), clozapine (2.5-7.5 mg/kg i.p.) or quetiapine (2.5 and 7.5 mg/kg i.p.) was also injected prior to agmatine (160 mg/kg i.p.). Haloperidol (1 mg/kg) and clozapine (2.5 and 5 mg/kg) were able to prevent the PPI-disrupting effects of apomorphine. However, none of these antipsychotics prevent the PPI-disrupting effects of agmatine. These results suggest that agmatine disrupts the PPI of acoustic startle reflex of rats in a fundamentally different manner than apomorphine does. It may also have a critical role in the pathogenesis of sensorimotor gating-related dysfunctions.

Effect of aripiprazole, risperidone, and olanzapine on the acoustic startle response in Japanese chronic schizophrenia

BACKGROUND

Studies have also shown that differences in the kind of the antipsychotics influenced disruption of the sensorimotor gating system, including prepulse inhibition (PPI), acoustic startle reflex (ASR), and habituation (HAB). We investigated the influence on startle response in chronic schizophrenia in 20 patients with schizophrenia taking risperidone, 21 patients with schizophrenia taking olanzapine, and 20 patients with schizophrenia taking aripiprazole.

METHOD

The patients who participated in this study were on maintenance therapy with only one antipsychotic drug for 4 months. We performed the test for the association between all PPI measures (ASR, HAB, and PPI at prepulse sound pressure intensities of 82, 86, and 90 dB) and each the risperidene, olanzapine, and aripiprazole groups, with analysis of covariance (ANCOVA; using age, duration of illness, and daily dose of the antipsychotic as covariates). Also, when significant difference was detected in ANCOVA, the differences of PPI measures between every pairs of two drug groups were tested as a post hoc analysis with the use of t test and Bonferroni's correction of multiple tests.

RESULT

We found that PPI90 showed significant differences with ANCOVA among patients with schizophrenia taking each of the antipsychotics. When we performed a post hoc analysis for PPI90, the value was higher in the aripiprazole group than in the olanzapine group and higher in the risperidone group than in the olanzapine group.

CONCLUSION

Aripiprazole and risperidone may improve PPI90. ASR, HAB, PPI82, and PPI86 were no different among the Japanese schizophrenic patient groups with different antipsychotics.

Chronic NT69L potently prevents drug-induced disruption of prepulse inhibition without causing tolerance

NT69L is a neurotensin receptor agonist with antipsychotic-like activity. NT69L blocks apomorphine-induced climbing in rats with no effect on stereotypic behavior, attenuates d-amphetamine-induced hyperactivity, and blocks pharmacologically induced disruption of prepulse inhibition (PPI) of the startle response. Repeated administration of NT69L results in tolerance to some, but not to all of its effects. Because schizophrenic patients require long-term treatment, chronic (21-day) administration of NT69L was tested in PPI with comparisons to chronic haloperidol and clozapine treatment. Sprague-Dawley rats received acute or 21 daily, subcutaneous injections of NT69L (1.0mg/kg). On days 1 and 21 the NT69L injection was followed 30 min later by treatment with either saline; the dopamine agonist, d-amphetamine (5.0mg/kg); or the serotonin 5-HT(2A) psychotomimetic receptor agonist [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] DOI (0.5mg/kg). Experiments were repeated with either haloperidol (1mg/kg) or clozapine (20mg/kg) in place of NT69L. Acute injection of NT69L significantly blocked d-amphetamine and DOI disruption of PPI. As with the acute injection, 21 daily administrations of NT69L also blocked d-amphetamine- and DOI-induced disruption of PPI. The data show that animals do not develop tolerance to the antipsychotic-like effects of NT69L when tested in the PPI of the startle response. The persistent efficacy of NT69L with chronic treatment provides further support for the therapeutic use of neurotensin (NT) agonists to treat schizophrenia and possibly other disorders that are characterized by PPI deficits. The modulatory role of NT69L on the dopaminergic and serotonergic neurotransmission systems both of which are implicated in the pathophysiology of schizophrenia is discussed.

Preclinical behavioral models for predicting antipsychotic activity

Schizophrenia is a major psychiatric disease that is characterized by three distinct symptom domains: positive symptoms, negative symptoms, and cognitive impairment. Additionally, treatment with classical antipsychotic medication can be accompanied by important side effects that involve extrapyramidal symptoms (EPS). The discovery of clozapine in the 1970s, which is efficacious in all three symptom domains and has a reduced propensity to induce EPS, has driven research for new antipsychotic agents with a wider spectrum of activity and a lower propensity to induce EPS. The following chapter reviews existing behavioral procedures in animals for their ability to predict compound efficacy against schizophrenia symptoms and liability to induce EPS. Rodent models of positive symptoms include procedures related to hyperfunction in central dopamine and serotonin (5-hydroxytryptamine) systems and hypofunction of central glutamatergic (N-methyl-d-aspartate) neurotransmission. Procedures for evaluating negative symptoms include rodent models of anhedonia, affective flattening, and diminished social interaction. Cognitive deficits can be assessed in rodent models of attention (prepulse inhibition (PPI), latent inhibition) and of learning and memory (passive avoidance, object and social recognition, Morris water maze, and operant-delayed alternation). The relevance of the conditioned avoidance response (CAR) is also discussed. A final section reviews animal procedures for assessing EPS liability, in particular parkinsonism (catalepsy), acute dystonia (purposeless chewing in rodents, dystonia in monkeys), akathisia (defecation in rodents), and tardive dyskinesia (long-term antipsychotic treatment in rodents and monkeys).

Asenapine effects in animal models of psychosis and cognitive function

RATIONALE

Asenapine, a novel psychopharmacologic agent in the development for schizophrenia and bipolar disorder, has high affinity for serotonergic, alpha-adrenergic, and dopaminergic receptors, suggesting potential for antipsychotic and cognitive-enhancing properties.

OBJECTIVES

The effects of asenapine in rat models of antipsychotic efficacy and cognition were examined and compared with those of olanzapine and risperidone.

MATERIALS AND METHODS

Amphetamine-stimulated locomotor activity (Amp-LMA; 1.0 or 3.0 mg/kg s.c.) and apomorphine-disrupted prepulse inhibition (Apo-PPI; 0.5 mg/kg s.c.) were used as tests for antipsychotic activity. Delayed non-match to place (DNMTP) and five-choice serial reaction (5-CSR) tasks were used to assess short-term spatial memory and attention, respectively. Asenapine doses varied across tasks: Amp-LMA (0.01-0.3 mg/kg s.c.), Apo-PPI (0.001-0.3 mg/kg s.c.), DNMTP (0.01-0.1 mg/kg s.c.), and 5-CSR (0.003-0.3 mg/kg s.c.).

RESULTS

Asenapine was highly potent (active at 0.03 mg/kg) in the Amp-LMA and Apo-PPI assays. DNMTP or 5-CSR performance was not improved by asenapine, olanzapine, or risperidone. All agents (P < 0.01) reduced DNMTP accuracy at short delays; post hoc analyses revealed that only 0.1 mg/kg asenapine and 0.3 mg/kg risperidone differed from vehicle. All active agents (asenapine, 0.3 mg/kg; olanzapine, 0.03-0.3 mg/kg; and risperidone, 0.01-0.1 mg/kg) significantly impaired 5-CSR accuracy (P < 0.05).

CONCLUSIONS

Asenapine has potent antidopaminergic properties that are predictive of antipsychotic efficacy. Asenapine, like risperidone and olanzapine, did not improve cognition in normal rats. Rather, at doses greater than those required for antipsychotic activity, asenapine impaired cognitive performance due to disturbance of motor function, an effect also observed with olanzapine and risperidone.

NT79: A novel neurotensin analog with selective behavioral effects

Neurotensin, a tridecapeptide, is widely distributed in the brain and gastrointestinal tract. It possesses analgesic, hypothermic, and antipsychotic-like properties. Neurotensin's effects are mediated mainly through two receptor subtypes, NTS1 and NTS2. Activation of NTS1 has been implicated in most of the pharmacological effects of neurotensin but is associated with hypothermia and hypotension. We report on a novel neurotensin analog with higher selectivity to NTS2, namely, NT79, which exhibits selective behavioral effects. NT79 was tested in animal models for pain (thermal-hot plate test; visceral-acetic acid-induced writhing test), and in animal models that are predictive of antipsychotic-like effects (apomorphine-induced climbing; d-amphetamine-induced hyperactivity; disruption of prepulse inhibition). Its effects on body temperature and on blood pressure were also determined. Neurochemical changes in extracellular neurotransmitters were measured using in vivo microdialysis while the rats were simultaneously evaluated for acetic acid-induced writhing with and without pretreatment with NT79. Binding data at molecularly cloned hNTS1 and hNTS2 suggest selectivity for hNTS2. NT79 blocked the acetic acid-induced writhing with an ED(50) of 0.14 microg/kg while having no effect on thermal nociception. The writhing was paralleled by an increase in 5-HT which was attenuated by NT79. NT79 demonstrated antipsychotic-like effects by blocking apomorphine-induced climbing, d-amphetamine-induced hyperactivity, and reducing d-amphetamine- and DOI-induced disruption of prepulse inhibition. Uniquely, it caused no significant hypothermia and was without effect on blood pressure. NT79, with its higher selectivity to NTS2, may be potentially useful to treat visceral pain, and psychosis without concomitant side effects of hypothermia or hypotension.

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

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

Realistic expectations of prepulse inhibition in translational models for schizophrenia research

INTRODUCTION

Under specific conditions, a weak lead stimulus, or "prepulse", can inhibit the startling effects of a subsequent intense abrupt stimulus. This startle-inhibiting effect of the prepulse, termed "prepulse inhibition" (PPI), is widely used in translational models to understand the biology of brainbased inhibitory mechanisms and their deficiency in neuropsychiatric disorders. In 1981, four published reports with "prepulse inhibition" as an index term were listed on Medline; over the past 5 years, new published Medline reports with "prepulse inhibition" as an index term have appeared at a rate exceeding once every 2.7 days (n=678). Most of these reports focus on the use of PPI in translational models of impaired sensorimotor gating in schizophrenia. This rapid expansion and broad application of PPI as a tool for understanding schizophrenia has, at times, outpaced critical thinking and falsifiable hypotheses about the relative strengths vs. limitations of this measure.

OBJECTIVES

This review enumerates the realistic expectations for PPI in translational models for schizophrenia research, and provides cautionary notes for the future applications of this important research tool.

CONCLUSION

In humans, PPI is not "diagnostic"; levels of PPI do not predict clinical course, specific symptoms, or individual medication responses. In preclinical studies, PPI is valuable for evaluating models or model organisms relevant to schizophrenia, "mapping" neural substrates of deficient PPI in schizophrenia, and advancing the discovery and development of novel therapeutics. Across species, PPI is a reliable, robust quantitative phenotype that is useful for probing the neurobiology and genetics of gating deficits in schizophrenia.

The antipsychotics clozapine and olanzapine increase plasma glucose and corticosterone levels in rats: comparison with aripiprazole, ziprasidone, bifeprunox and F15063

Several novel antipsychotics activate serotonin 5-HT1A receptors as well as antagonising dopamine D2/3 receptors. Such a pharmacological profile is associated with a lowered liability to produce extrapyramidal side effects and enhanced efficacy in treating negative and cognitive symptoms of schizophrenia. However, 5-HT1A receptor agonists increase plasma corticosterone and many antipsychotics disturb the regulation of glucose. Here, we compared the influence on plasma glucose and corticosterone of acute treatments with 'new generation' antipsychotics which target dopamine D2/3 receptors and 5-HT1A receptors, with that of atypical antipsychotics, and with haloperidol. Olanzapine and clozapine, antipsychotics that are known to produce weight gain and diabetes in humans, both at 10 mg/kg p.o., substantially increased plasma glucose (from 0.8 to 1.7 g/l) at 1 h after administration, an effect that returned to control levels after 4 h. In comparison, F15063 (40 mg/kg p.o.) was without effect at any time point. Olanzapine and clozapine dose-dependently increased plasma glucose concentrations as did SLV313 and SSR181507. Haloperidol and risperidone had modest effects whereas aripiprazole, ziprasidone and bifeprunox, antipsychotics that are not associated with metabolic dysfunction in humans, and F15063 had little or no influence on plasma glucose. The same general pattern of response was found for plasma corticosterone levels. The present data provide the first comparative study of conventional, atypical and 'new generation' antipsychotics on glucose and corticosterone levels in rats. A variety of mechanisms likely underlie the hyperglycemia and corticosterone release observed with clozapine and olanzapine, whilst the balance of dopamine D2/3/5-HT1A interaction may contribute to the less favourable impact of SLV313 and SSR181507 compared with that of bifeprunox and F15063.

Aripiprazole blocks acute self-administration of cocaine and is not self-administered in mice

RATIONALE

The novel antipsychotic aripiprazole in use for treatment of schizophrenia is a partial agonist at dopamine D2 receptors with actions at a variety of other receptors as well. Cocaine is believed to exert an important part of its rewarding effect by increasing extracellular levels of dopamine that subsequently act at dopamine D2 receptors.

OBJECTIVES

As a partial agonist, aripiprazole may antagonize effects at D2 receptors and we accordingly tested whether aripiprazole could antagonize self-administration of cocaine. Because D2-like receptor agonists are self-administered, a D2 receptor partial agonist like aripiprazole might itself be reinforcing. Thus, we also assessed whether mice would acquire self-administration of aripiprazole.

MATERIALS AND METHODS

A single session, mouse self-administration procedure was used.

RESULTS

Oral pretreatment with aripiprazole dose-dependently decreased cocaine self-administration under a fixed ratio 1 schedule at the peak cocaine dose (0.03 mg/kg/infusion), reaching significance at 0.2 and 0.4 mg/kg of aripiprazole. Using 0.4 mg/kg, aripiprazole decreased rates of cocaine self-administration without shifting the peak of the dose-response function. There was no effect of aripiprazole per se, suggesting that its inhibitory action was due to effects on cocaine self-administration rather than non-specific motor effects. Aripiprazole was not found to be self-administered in the tested dose range (0.0003-0.3 mg/kg/infusion). The three highest doses (0.03, 0.1, and 0.3 mg/kg/infusion) even caused significant decreases in nose-poking activity, possibly due to extrapyramidal side effects.

CONCLUSIONS

These data are consistent with a potential role for aripiprazole in treatment of cocaine addiction without abuse potential per se.

Monoaminergic, Ion Channel and Enzyme Inhibitory Activities of Natural Aporphines, their Analogues and Derivatives

The aporphine alkaloids constitute the second-largest group of isoquinoline alkaloids. Nevertheless, only a relatively small number of natural aporphines and their derivatives have been studied from a pharmacological viewpoint. Here we review the pharmacological data available for these compounds as related to their dopaminergic, noradrenergic and serotonergic activities, and also some results pertaining to their effects on ion channels and enzymes.

Haloperidol and clozapine have dissociable effects in a model of attentional performance deficits induced by blockade of NMDA receptors in the mPFC

RATIONALE

Cognitive impairment in schizophrenia is particularly evident in the domains of attention and executive functions. Atypical antipsychotics are somewhat more effective than conventional antipsychotics in improving cognitive functioning in these patients.

OBJECTIVE

The aim of this study was to compare the effects of conventional and atypical antipsychotics in a model of attentional performance deficit of schizophrenia induced by blockade of N-methyl-D: -aspartate (NMDA) receptors in the medial prefrontal cortex.

MATERIALS AND METHODS

Attentional performance was assessed using the five-choice serial reaction time task. The task provides indices of attentional functioning (% correct responses), executive control (measured by anticipatory and perseverative responding), decision time (measured by correct response latency), and omissions. Haloperidol and clozapine were given intraperitoneally (IP) to animals that had received vehicle or a competitive NMDA receptor antagonist, 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP), directly into the medial prefrontal cortex.

RESULTS

Fifty nanograms/side of CPP reduced accuracy (% correct responses) and increased anticipatory and perseverative responding. Haloperidol (0.03 mg/kg IP) reduced the CPP-induced anticipatory and perseverative overresponding but not the impairment in accuracy. In contrast, clozapine (2.5 mg/kg IP) reversed the decrease in accuracy and impulsivity (anticipatory responding) but not perseverative overresponding. CPP increased decision time and omissions, but these effects were not affected by either haloperidol or clozapine.

CONCLUSIONS

The effects on "impulsivity" and "compulsive perseveration" in a rat model of attentional and executive deficit of schizophrenia might differentiate conventional and atypical antipsychotics. Antagonistic activity at 5-HT(2A) receptors may best explain the facilitatory effects of clozapine on cognition.

Aripiprazole, an atypical antipsychotic, prevents the motor hyperactivity induced by psychotomimetics and psychostimulants in mice

Aripiprazole is an atypical antipsychotic that acts as a partial agonist at the dopamine D(2) receptor. It has been mainly investigated in dopamine-based models of schizophrenia, while its effects on glutamate-based paradigms have remained to be further characterized. Due to its unique mechanism of action, aripiprazole has also been considered as a replacement medication for psychostimulant abuse. Thus, in the present study we tested the hypothesis that aripiprazole would prevent the motor hyperactivity induced by psychostimulant and psychotomimetic drugs that act either by dopaminergic or glutamatergic mechanisms. Male Swiss mice received injections of aripiprazole (0.1-1 mg/kg) followed by drugs that enhance the dopamine-mediated neurotransmission, amphetamine (3 mg/kg) or cocaine (5 mg/kg), or by glutamate NMDA-receptor antagonists, ketamine (60 mg/kg) or MK-801 (0.4 mg/kg). Independent groups also received aripiprazole (0.1-1 mg/kg) or haloperidol (0.5 mg/kg) and were tested for catalepsy. All doses of aripiprazole were effective in preventing the motor stimulant effects of amphetamine and cocaine. Moreover, the higher dose also prevented the effects of ketamine and MK-801. The present study reports the effects of aripiprazole in dopaminergic and glutamatergic models predictive of antipsychotic activity, suggesting that both may be useful for screening novel partial agonists with antipsychotic activity. It also shows that aripiprazole may prevent the acute effects of psychostimulant drugs without significant motor impairment.

5-HT6 receptor antagonist reversal of emotional learning and prepulse inhibition deficits induced by apomorphine or scopolamine

5-HT6 receptors have been implicated in consolidation of visuospatial and reward-based learning tasks. Since 5-HT6 receptors may be important in modulation of sensory gating which is often affected in schizophrenic patients, we tested whether Ro 4368554, a 5-HT6 selective antagonist at a dose of 10 mg/kg, could reverse the loss of prepulse inhibition from apomorphine or scopolamine. In addition, we also tested whether Ro 4368554 altered fear conditioning using fear potentiated startle, a model for emotional learning. Prepulse inhibition of startle was disrupted by apomorphine (0.5 mg/kg) when prepulse emissions were 5 dB above background but not above 15 dB, while scopolamine (0.5 mg/kg) caused disruption at both prepulse levels. Scopolamine-mediated disruption was not reversed by Ro 4368854 but apomorphine-mediated disruption was significantly ameliorated by 5-HT6 inhibition. For fear potentiated startle, scopolamine and/or Ro 4368554 were administered before two daily fear conditioning sessions; rats were tested on the following day. Rats that received scopolamine displayed no fear potentiated startle but Ro 4368554 reversed this scopolamine deficit. Additionally, we mapped Fos induction in rats treated with scopolamine and/or Ro 4368554; scopolamine increased Fos expression in the central nucleus of the amygdala and this was attenuated by Ro 4368554. In summary, we have demonstrated the efficacy of 5-HT6 antagonists in modulating sensory gating and fear conditioning, and thus may be of therapeutic use for schizophrenia-related disorders.

Putative antipsychotics with pronounced agonism at serotonin 5-HT1A and partial agonist activity at dopamine D2 receptors disrupt basal PPI of the startle reflex in rats

INTRODUCTION

Prepulse inhibition (PPI) of the startle reflex has been extensively studied because it is disrupted in several psychiatric diseases, most notably schizophrenia. In rats, and to a lesser extent, in humans, PPI can be diminished by dopamine (DA) D(2)/D(3) and serotonin 5-HT(1A) receptor agonists. A novel class of potential antipsychotics (SSR181507, bifeprunox, and SLV313) possess partial agonist/antagonist properties at D(2) receptors and various levels of 5-HT(1A) activation.

MATERIALS AND METHODS

It thus appeared warranted to assess, in Sprague-Dawley rats, the effects of these antipsychotics on basal PPI.

RESULTS

SSR181507, sarizotan, and bifeprunox decreased PPI, with a near-complete abolition at 2.5-10 mg/kg; SLV313 had a significant effect at 0.16 mg/kg only. Co-treatment with the 5-HT(1A) receptor antagonist WAY100,635 (0.63 mg/kg) showed that the 5-HT(1A) agonist activity of SSR181507 was responsible for its effect. By contrast, antipsychotics with low affinity and/or efficacy at 5-HT(1A) receptors, such as aripiprazole (another DA D(2)/D(3) and 5-HT(1A) ligand), and established typical and atypical antipsychotics (haloperidol, clozapine, risperidone, olanzapine, quetiapine, and ziprasidone) had no effect on basal PPI (0.01-2.5 to 2.5-40 mg/kg).

DISCUSSION

The present data demonstrate that some putative antipsychotics with pronounced 5-HT(1A) agonist activity, coupled with partial agonist activity at DA D(2) receptors, markedly diminish PPI of the startle reflex in rats.

CONCLUSIONS

These data raise the issue of the influence of such compounds on sensorimotor gating in humans.

F15063, a potential antipsychotic with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. I. In vitro receptor affinity and efficacy profile

BACKGROUND AND PURPOSE

Combining 5-HT(1A) receptor activation with dopamine D(2)/D(3) receptor blockade should improve negative symptoms and cognitive deficits in schizophrenia. We describe the in vitro profile of F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine).

EXPERIMENTAL APPROACH

F15063 was characterised in tests of binding affinity and in cellular models of signal transduction at monoamine receptors.

KEY RESULTS

Affinities (receptor and pK(i) values) of F15063 were: rD(2) 9.38; hD(2L) 9.44; hD(2S) 9.25; hD(3) 8.95; hD(4) 8.81; h5-HT(1A) 8.37. F15063 had little affinity (40-fold lower than D(2)) at other targets. F15063 antagonised dopamine-activated G-protein activation at hD(2), rD(2) and hD(3) receptors with potency (pK (b) values 9.19, 8.29 and 8.74 in [(35)S]GTP gamma S binding experiments) similar to haloperidol. F15063 did not exhibit any hD(2) receptor agonism, even in tests of ERK1/2 phosphorylation and G-protein activation in cells with high receptor expression. In contrast, like (+/-)8-OH-DPAT, F15063 efficaciously activated h5-HT(1A) (E(max) 70%, pEC(50) 7.57) and r5-HT(1A) receptors (52%, 7.95) in tests of [(35)S]GTP gamma S binding, cAMP accumulation (90%, 7.12) and ERK1/2 phosphorylation (93%, 7.13). F15063 acted as a partial agonist for [(35)S]GTP gamma S binding at hD(4) (29%, 8.15) and h5-HT(1D) receptors (35%, 7.68). In [(35)S]GTP gamma S autoradiography, F15063 activated G-proteins in hippocampus, cortex and septum (regions enriched in 5-HT(1A) receptors), but antagonised quinelorane-induced activation of D(2)/D(3) receptors in striatum.

CONCLUSIONS AND IMPLICATIONS

F15063 antagonised dopamine D(2)/D(3) receptors, a property underlying its antipsychotic-like activity, whereas activation of 5-HT(1A) and D(4) receptors mediated its actions in models of negative symptoms and cognitive deficits of schizophrenia (see companion papers).

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms

BACKGROUND AND PURPOSE

The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia.

EXPERIMENTAL APPROACH

F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems.

KEY RESULTS

Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating.

CONCLUSIONS AND IMPLICATIONS

The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. II. Activity in models of positive symptoms of schizophrenia

BACKGROUND AND PURPOSE

F15063 is a high affinity D(2)/D(3) antagonist, D(4) partial agonist, and high efficacy 5-HT(1A) agonist, with little affinity (40-fold lower than for D(2) receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects.

EXPERIMENTAL APPROACH

Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and 'serotonin syndrome'.

KEY RESULTS

F15063 potently (ED(50)s: 0.23 to 1.10 mg kg(-1) i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED(50)=0.30 mg kg(-1) i.p.). F15063, owing to its 5-HT(1A) agonism, did not produce (ED(50)>40 mg kg(-1) i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg(-1) p.o. Furthermore, F15063 did not induce the 'serotonin syndrome' in rats (flat body posture and forepaw treading: ED(50) >32 mg kg(-1) i.p.).

CONCLUSIONS AND IMPLICATIONS

F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper).

Bifeprunox: a novel antipsychotic agent with partial agonist properties at dopamine D2 and serotonin 5-HT1A receptors

Most second-generation, atypical, dopamine (DA) D2/5-HT2 blocking antipsychotics still induce extrapyramidal side effects (EPS) in higher doses. Weight gain and metabolic disturbances are also a problem, and negative and cognitive symptoms have not been sufficiently addressed. The current brain DA mesolimbic hyperactive/mesocortical hypoactive hypothesis of schizophrenia suggests that DA D2/5-HT1A receptor partial agonist properties may be more efficacious with less side effects. DA D2 receptor partial agonists may stabilize a hyperactive/hypoactive DA condition. Additional 5-HT1A stimulation may enhance therapeutic efficacy and also improve EPS liability profile. In clinical trials in schizophrenic patients, the novel DA D2/5-HT1A partial agonist bifeprunox indeed demonstrates therapeutic efficacy, a safe EPS profile and appears beneficial regarding weight gain, prolactin, blood lipid and glucose levels and cardiac rhythm. The data on bifeprunox are promising and suggest that combined DA D2/5-HT1A partial agonism may well be important properties for future-generation antipsychotics.

Towards a New Generation of Potential Antipsychotic Agents Combining D2 and 5-HT1A Receptor Activities

We report the discovery and the synthesis of novel, potential antipsychotic compounds combining potent dopamine D2 receptor antagonist and serotonin 5-HT1A receptor agonist properties in the same molecule. We describe the structure-activity relationship that lead us to the promising derivative: N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine 16. The latter has high affinity for D2 and 5-HT1A receptors, whereas it possesses only a weak affinity for 5-HT2A sites. In cellular models of signal transduction, 16 behaves as a silent antagonist at rD2 receptors while activating h5-HT1A receptors with an efficacy at least equivalent to that of the prototypical 5-HT1A agonist (+/-)-8-OH-DPAT. These dual actions confer a unique pharmacological profile to the product. In a behavioral model predictive of positive symptoms, 16 has an activity comparable to that of the typical antipsychotic haloperidol, while it is devoid of cataleptogenic effects. Although it produces behaviors characteristic of 5-HT1A receptor activation in rats, these occur at doses 100 times higher than those with (+/-)-8-OH-DPAT. We believe that the relative balance of D2 and 5-HT1A actions in 16 is appropriate, possibly optimal, to ensure superior efficacy and tolerability over existing antipychotic drugs.

Differential Effects of Antipsychotic Drugs on Serotonin-1A Receptor-Mediated Disruption of Prepulse Inhibition

Serotonin-1A (5-HT(1A)) receptors have been implicated in the symptoms of schizophrenia. However, there is limited in vivo evidence for an interaction of antipsychotic drugs with 5-HT(1A) receptor-mediated behavioral effects. We therefore investigated in rats the action of several antipsychotic drugs on prepulse inhibition (PPI), a measure of sensorimotor gating that is deficient in schizophrenia. Disruption of PPI at the 100-ms interstimulus interval (ISI), but not the 30-ms ISI, was induced by treatment with 0.5 mg/kg 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), the 5-HT(1A) receptor agonist. In rats pretreated with 0.25 mg/kg haloperidol (4-[-4-(p-chlorophenyl)-4-hydroxypiperidino]-4'-fluoro butyrophenone) or raclopride [3,5-dichloro-N-(1-ethylpyrrolidin-2-ylmethyl)-2-hydroxy-6-methoxybenzamide tartrate], the disruption of PPI was no longer significant. Of the atypical antipsychotic drugs clozapine (8-chloro-11-(4-methyl-1-piperazinyl)-5H-dibenzo[b,e][1,4]-diazepine), olanzapine (2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine), risperidone [3-[2-[-4-(6-fluoro-1,2-benzisoxazol-3-yl) piperidino] ethyl-6,7,8,9-tetrahydro-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one)], amisulpride (4-amino-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(ethylsulfonyl)-o-anisamide), and aripiprazole (7-[4-[-4[-(2,3-dichlorophenyl)-1-piperazinyl]butoxy]-3,4-dihydrocarbostyrilor 7-[4-[4-(2,3-dichlorophenyl) piperazin-1-yl]butoxy]-1,2,3,4,-tetrahydroquinolin-2-one), only aripiprazole significantly reduced the effect of 8-OH-DPAT on PPI. This effect was mimicked by pretreatment with the 5-HT(1A) receptor partial agonist, buspirone [N-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride]. On the other hand, some of the antipsychotic drugs and other pretreatments showed complex, prepulse-dependent effects on their own. These data show little in vivo interaction of several atypical antipsychotic drugs with the disruption of PPI mediated by 5-HT(1A) receptor stimulation. The action of haloperidol and raclopride suggests a major involvement of dopamine D(2) receptors in this effect, possibly downstream from the initial serotonergic stimulation. The action of aripiprazole could be mediated by its partial agonist properties at 5-HT(1A) receptors or its dopamine D(2)-blocking properties.