Preclinical evaluation of non-imidazole histamine H3 receptor antagonists in comparison to atypical antipsychotics for the treatment of cognitive deficits associated with schizophrenia

Prenatal MAM exposure raises kynurenic acid levels in the prefrontal cortex of adult rats

BACKGROUND

Elevated brain levels of kynurenic acid (KYNA), a metabolite in the kynurenine pathway, are associated with cognitive dysfunctions, which are nowadays often considered as fundamental characteristics of several psychopathologies; however, the role of KYNA in mental illnesses, such as schizophrenia, is not fully elucidated. This study aimed to assess KYNA levels in the prefrontal cortex (PFC) of rats prenatally treated with methylazoxymethanol (MAM) acetate, i.e., a well-validated neurodevelopmental animal model of schizophrenia. The effects of an early pharmacological modulation of the endogenous cannabinoid system were also evaluated.

METHODS

Pregnant Sprague-Dawley rats were treated with MAM (22 mg/kg, ip) or its vehicle at gestational day 17. Male offspring were treated with the cannabinoid CB1 receptor antagonist/inverse agonist AM251 (0.5 mg/kg/day, ip) or with the typical antipsychotic haloperidol (0.6 mg/kg/day, ip) from postnatal day (PND) 19 to PND39. The locomotor activity and cognitive performance were assessed in the novel object recognition test and the open field test in adulthood. KYNA levels in the PFC of prenatally MAM-treated rats were also assessed.

RESULTS

A significant cognitive impairment was observed in prenatally MAM-treated rats (p < 0.01), which was associated with enhanced PFC KYNA levels (p < 0.05). The peripubertal AM251, but not haloperidol, treatment ameliorated the cognitive deficit (p < 0.05), by normalizing the PFC KYNA content in MAM rats.

CONCLUSIONS

The present findings suggest that the cognitive deficit observed in MAM rats may be related to enhanced PFC KYNA levels which could be, in turn, mediated by the activation of cannabinoid CB1 receptor. These results further support the modulation of brain KYNA levels as a potential therapeutic strategy to ameliorate the cognitive dysfunctions in schizophrenia.

Effects of ortho-eugenol on anxiety, working memory and oxidative stress in mice

Ortho-eugenol is a synthetic derivative from eugenol, the major compound of clove essential oil, which has demonstrated antidepressant and antinociceptive effects in pioneering studies. Additionally, its effects appear to be dependent on the noradrenergic and dopaminergic systems. Depression and anxiety disorders are known to share a great overlap in their pathophysiology, and many drugs are effective in the treatment of both diseases. Furthermore, high levels of anxiety are related to working memory deficits and increased oxidative stress. Thus, in this study we investigated the effects of acute treatment of ortho-eugenol, at 50, 75 and 100 mg/kg, on anxiety, working memory and oxidative stress in male Swiss mice. Our results show that the 100 mg/kg dose increased the number of head-dips and reduced the latency in the hole-board test. The 50 mg/kg dose reduced malondialdehyde levels in the prefrontal cortex and the number of Y-maze entries compared to the MK-801-induced hyperlocomotion group. All doses reduced nitrite levels in the hippocampus. It was also possible to assess a statistical correlation between the reduction of oxidative stress and hyperlocomotion after the administration of ortho-eugenol. However, acute treatment was not able to prevent working memory deficits. Therefore, the present study shows that ortho-eugenol has an anxiolytic and antioxidant effect, and was able to prevent substance-induced hyperlocomotion. Our results contribute to the elucidation of the pharmacological profile of ortho-eugenol, as well as to direct further studies that seek to investigate its possible clinical applications.

Revisiting Preclinical Observations of Several Histamine H3 Receptor Antagonists/Inverse Agonists in Cognitive Impairment, Anxiety, Depression, and Sleep-Wake Cycle Disorder

A relationship appears to exist between dysfunction of brain histamine (HA) and various neuropsychiatric brain disorders. The possible involvement of brain HA in neuropathology has gained attention recently, and its role in many (patho)physiological brain functions including memory, cognition, and sleep-wake cycle paved the way for further research on the etiology of several brain disorders. Histamine H3 receptor (H3R) evidenced in the brains of rodents and humans remains of special interest, given its unique position as a pre- and postsynaptic receptor, controlling the synthesis and release of HA as well as different other neurotransmitters in different brain regions, respectively. Despite several disappointing outcomes for several H3R antagonists/inverse agonists in clinical studies addressing their effectiveness in Alzheimer's disease (AD), Parkinson's disease (PD), and schizophrenia (SCH), numerous H3R antagonists/inverse agonists showed great potentials in modulating memory and cognition, mood, and sleep-wake cycle, thus suggesting its potential role in neurocognitive and neurodegenerative diseases such as AD, PD, SCH, narcolepsy, and major depression in preclinical rodent models. In this review, we present preclinical applications of selected H3R antagonists/inverse agonists and their pharmacological effects on cognitive impairment, anxiety, depression, and sleep-wake cycle disorders. Collectively, the current review highlights the behavioral impact of developments of H3R antagonists/inverse agonists, aiming to further encourage researchers in the preclinical drug development field to profile the potential therapeutic role of novel antagonists/inverse agonists targeting histamine H3Rs.

Role of Brain Modulators in Neurodevelopment: Focus on Autism Spectrum Disorder and Associated Comorbidities

Autism spectrum disorder (ASD) and associated neurodevelopmental disorders share similar pathogenesis and clinical features. Pathophysiological changes in these diseases are rooted in early neuronal stem cells in the uterus. Several genetic and environmental factors potentially perturb neurogenesis and synaptogenesis processes causing incomplete or altered maturation of the brain that precedes the symptomology later in life. In this review, the impact of several endogenous neuromodulators and pharmacological agents on the foetus during pregnancy, manifested on numerous aspects of neurodevelopment is discussed. Within this context, some possible insults that may alter these modulators and therefore alter their role in neurodevelopment are high-lighted. Sometimes, a particular insult could influence several neuromodulator systems as is supported by recent research in the field of ASD and associated disorders. Dopaminergic hy-pothesis prevailed on the table for discussion of the pathogenesis of schizophrenia (SCH), atten-tion-deficit hyperactivity disorder (ADHD) and ASD for a long time. However, recent cumulative evidence suggests otherwise. Indeed, the neuromodulators that are dysregulated in ASD and comorbid disorders are as diverse as the causes and symptoms of this disease. Additionally, these neuromodulators have roles in brain development, further complicating their involvement in comorbidity. This review will survey the current understanding of the neuromodulating systems to serve the pharmacological field during pregnancy and to minimize drug-related insults in pa-tients with ASD and associated comorbidity disorders, e.g., SCH or ADHD.

The Histaminergic System in Neuropsychiatric Disorders

Histamine does not only modulate the immune response and inflammation, but also acts as a neurotransmitter in the mammalian brain. The histaminergic system plays a significant role in the maintenance of wakefulness, appetite regulation, cognition and arousal, which are severely affected in neuropsychiatric disorders. In this review, we first briefly describe the distribution of histaminergic neurons, histamine receptors and their intracellular pathways. Next, we comprehensively summarize recent experimental and clinical findings on the precise role of histaminergic system in neuropsychiatric disorders, including cell-type role and its circuit bases in narcolepsy, schizophrenia, Alzheimer's disease, Tourette's syndrome and Parkinson's disease. Finally, we provide some perspectives on future research to illustrate the curative role of the histaminergic system in neuropsychiatric disorders.

Histamine, Neuroinflammation and Neurodevelopment: A Review

The biogenic amine, histamine, has been shown to critically modulate inflammatory processes as well as the properties of neurons and synapses in the brain, and is also implicated in the emergence of neurodevelopmental disorders. Indeed, a reduction in the synthesis of this neuromodulator has been associated with the disorders Tourette's syndrome and obsessive-compulsive disorder, with evidence that this may be through the disruption of the corticostriatal circuitry during development. Furthermore, neuroinflammation has been associated with alterations in brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there are suggestions that histamine deficiency may leave the developing brain more vulnerable to proinflammatory insults. While most studies have focused on neuronal sources of histamine it remains unclear to what extent other (non-neuronal) sources of histamine, e.g., from mast cells and other sources, can impact brain development. The few studies that have started exploring this in vitro, and more limited in vivo, would indicate that non-neuronal released histamine and other preformed mediators can influence microglial-mediated neuroinflammation which can impact brain development. In this Review we will summarize the state of the field with regard to non-neuronal sources of histamine and its impact on both neuroinflammation and brain development in key neural circuits that underpin neurodevelopmental disorders. We will also discuss whether histamine receptor modulators have been efficacious in the treatment of neurodevelopmental disorders in both preclinical and clinical studies. This could represent an important area of future research as early modulation of histamine from neuronal as well as non-neuronal sources may provide novel therapeutic targets in these disorders.

Histamine 3 receptor inverse agonist Samelisant (SUVN-G3031): Pharmacological characterization of an investigational agent for the treatment of cognitive disorders

BACKGROUND

Central histamine H3 receptors are a family of presynaptic auto and heteroreceptors. Blockade of the presynaptic H3 receptors activates the downstream pathway(s) involved in the processes of learning and memory, making it a potential therapeutic option for ameliorating cognitive dysfunction. Samelisant (SUVN-G3031) is a potent and selective inverse agonist at the H3 receptors.

AIM

The aim of this research is to study the effects of Samelisant in diverse animal models of cognitive functions.

METHODS

The effects of Samelisant on cognitive functions were studied using social recognition, object recognition and Morris water maze tasks. Neurochemical and electrophysiological effects of Samelisant were monitored using microdialysis and electroencephalography techniques.

RESULTS

Samelisant showed procognitive effects in diverse animal models of cognition at doses ranging from 0.3 to 3 mg/kg, per os (p.o.) (social recognition and object recognition task). Samelisant significantly increased the brain acetylcholine levels in the cortex at doses of 10 and 20 mg/kg, p.o. In the Morris water maze task, combined administration of suboptimal doses of Samelisant and donepezil resulted in procognitive effects significantly larger than the either treatment. Similarly, Samelisant significantly potentiated the effects of donepezil on pharmacodynamic biomarkers of cognition i.e. acetylcholine levels in brain and neuronal theta oscillations.

CONCLUSION

Samelisant may have potential utility in the treatment of cognitive deficits associated with hypocholinergic state.

Discovery of a new class of multi-target heterocycle piperidine derivatives as potential antipsychotics with pro-cognitive effect

A series of benzoisoxazoleylpiperidine derivatives were synthesized by using the multi-target strategies and their potent affinities for dopamine (DA), serotonin (5-HT) and human histamine H₃ receptors have been evaluated. Of these compounds, the promising candidate 4w displayed high affinities for D₂, D₃, 5-HT_1A, 5-HT_2A and H₃, a moderate affinity for 5-HT₆, negligible effects on the human ether-a-go-go-related gene (hERG) channel, low affinities for off-target receptors (5-HT_2C, adrenergic α₁ and H₁). In addition, the animal behavioral study revealed that, compared to risperidone, compound 4w significantly inhibited apomorphine-induced climbing and MK-801-induced movement behaviors with a high threshold for catalepsy and low liabilities for weight gain and hyperprolactinemia. Results from the conditioned avoidance response test and novel object recognition task demonstrated that 4w had pro-cognitive effects. Thus, the antipsychotic drug-like activities of 4w indicate that it may be a potential polypharmacological antipsychotic candidate drug.

Selective H3 Antagonist (ABT-239) Differentially Modifies Cognitive Function Under the Impact of Restraint Stress

Background: A considerable number of competitive antagonists/inverse agonists of histamine H3 receptor (H3R) have progressed to clinical assessment, with pitolisant approved for the treatment of narcolepsy. H3R, highly expressed in the CNS, is regarded as a relevant target in CNS disorders. At the same time, new compounds including ABT-239 H3R antagonist (ABT; benzonitrile, 4-[2-[2-[(2R)-2-methyl-1-pyrrolidinyl]ethyl]-5-benzofuranyl]-) are continually being tested. The study aimed to test ABT-239 as a prophylactic agent in stress-induced memory impairments.

Methods: Stressed and non-stressed rats were pre-treated with ABT-239 and subsequently subjected to several behavioral tests aimed at assessing the animals’ working and spatial reference memory [Morris water maze (MWM), Barnes maze (BM)], assessing the locomotor function and anxiety-like behavior [Open field (OF), elevated “plus” maze—EPM].

Results: Chronically stressed rats displayed a significant decline in spatial (working and reference) memory. In the MWM test, we observed an improvement in spatial reference memory in stressed animals and a positive after ABT-239 pre-treatment. In the BM test, the effect of ABT-239 administration on spatial memory changed in successive attempts, from negative initially to favorable in subsequent attempts, and negative in the last trial of the test in the control group of rats. However, a beneficial effect is noted in the group of stressed animals, which remained throughout the entire testing period.

Conclusions: Presented findings demonstrate that ABT-239 shows the potential to abolish or prevent restraint stress-induced spatial memory impairments and cognitive deficits. However, in conditions of appetitive modulation, it could increase damage to memory (unstressed animals).

Experimental Models for the Discovery of Novel Anticonvulsant Drugs: Focus on Pentylenetetrazole-Induced Seizures and Associated Memory Deficits

Epilepsy is a chronic neurological disorder characterized by irregular, excessive neuronal excitability, and recurrent seizures that affect millions of patients worldwide. Currently, accessible antiepileptic drugs (AEDs) do not adequately support all epilepsy patients, with around 30% patients not responding to the existing therapies. As lifelong epilepsy treatment is essential, the search for new and more effective AEDs with an enhanced safety profile is a significant therapeutic goal. Seizures are a combination of electrical and behavioral events that can induce biochemical, molecular, and anatomic changes. Therefore, appropriate animal models are required to evaluate novel potential AEDs. Among the large number of available animal models of seizures, the acute pentylenetetrazole (PTZ)-induced myoclonic seizure model is the most widely used model assessing the anticonvulsant effect of prospective AEDs, whereas chronic PTZ-kindled seizure models represent chronic models in which the repeated administration of PTZ at subconvulsive doses leads to the intensification of seizure activity or enhanced seizure susceptibility similar to that in human epilepsy. In this review, we summarized the memory deficits accompanying acute or chronic PTZ seizure models and how these deficits were evaluated applying several behavioral animal models. Furthermore, major advantages and limitations of the PTZ seizure models in the discovery of new AEDs were highlighted. With a focus on PTZ seizures, the major biochemicals, as well as morphological alterations and the modulated brain neurotransmitter levels associated with memory deficits have been illustrated. Moreover, numerous medicinal compounds with concurrent anticonvulsant, procognitive, antioxidant effects, modulating effects on several brain neurotransmitters in rodents, and several newly developed classes of compounds applying computer-aided drug design (CADD) have been under development as potential AEDs. The article details the in-silico approach following CADD, which can be utilized for generating libraries of novel compounds for AED discovery. Additionally, in vivo studies could be useful in demonstrating efficacy, safety, and novel mode of action of AEDs for further clinical development.

Region-specific regulation of central histaminergic H3 receptor expression in a mouse model of cow's milk allergy

Central histaminergic H3 receptor (H3R) has been extensively investigated as a potential therapeutic target for various neurological and neurodegenerative disorders. Despite promising results in preclinical rodent models, clinical trials have not provided conclusive evidence for the benefit of H3R antagonists to alleviate cognitive and behavioral symptoms of these disorders. Inconsistent pharmacological efficacies may arise from aberrant changes in H3R over time during disease development. Because H3R is involved in feedback inhibition of histamine synthesis and secretion, the expression of the autoreceptor may also be reciprocally regulated by altered histamine levels in a pathological condition. Thus, we investigated H3R expression in a mouse model of cow's milk allergy, a condition associated with increased histamine levels. Mice were sensitized to bovine whey proteins (WP) over 5 weeks and H3R protein and transcript levels were examined in the brain. Substantially increased H3R immunoreactivity was observed in various brain regions of WP-sensitized mice compared to sham mice. Elevated H3R expression was also found in the thalamic/hypothalamic region. The expression of histaminergic H1, but not H2, receptor subtype was also increased in this and the midbrain regions. Unlike the brain, all three histaminergic receptors were increased in the small intestine. These results indicated that the central histaminergic receptors were altered in WP-sensitized mice in a subtype- and region-specific manner, which likely contributed to behavioral changes we observed in these mice. Our study also suggests that altered levels of H3R could be considered during a pharmacological intervention of a neurological disease.

Role of Neuroinflammation in Autism Spectrum Disorder and the Emergence of Brain Histaminergic System. Lessons Also for BPSD?

Many behavioral and psychological symptoms of dementia (BPSD) share similarities in executive functioning and communication deficits with those described in several neuropsychiatric disorders, including Alzheimer's disease (AD), epilepsy, schizophrenia (SCH), and autism spectrum disorder (ASD). Numerous studies over the last four decades have documented altered neuroinflammation among individuals diagnosed with ASD. The purpose of this review is to examine the hypothesis that central histamine (HA) plays a significant role in the regulation of neuroinflammatory processes of microglia functions in numerous neuropsychiatric diseases, i.e., ASD, AD, SCH, and BPSD. In addition, this review summarizes the latest preclinical and clinical results that support the relevance of histamine H1-, H2-, and H3-receptor antagonists for the potential clinical use in ASD, SCH, AD, epilepsy, and BPSD, based on the substantial symptomatic overlap between these disorders with regards to cognitive dysfunction. The review focuses on the histaminergic neurotransmission as relevant in these brain disorders, as well as the effects of a variety of H3R antagonists in animal models and in clinical studies.

Cognitive performance of the MAM-E17 schizophrenia model rats in different age-periods

Cognitive disturbances are among the most important features of schizophrenia, and have a significant role in the outcome of the disease. However, the treatment of cognitive symptoms is poorly effective. In order to develop new therapeutic opportunities, the MAM-E17 rat model of schizophrenia can be an appropriate implement. In the present study we investigated several cognitive capabilities of MAM-treated rats using radial arm maze (RAM) task, which corresponds to the recent research directives. Because of the diachronic appearance of schizophrenia symptoms and the early appearance of cognitive deficiencies, we carried out our experiments in three different age-periods of rats, i.e. in prepuberty, late puberty and adulthood. The performance of MAM-E17 rats was similar to control rats in the acquisition phase of RAM task, except for puberty. However, after rearrangement of reward positions (in the reverse paradigm) the number of errors of MAM-treated rats was higher in each age-period. In the reverse paradigm MAM-treated groups visited more frequently those non-rewarding arms, which were previously rewarding. Our results suggest that working memory of MAM-E17 rats is impaired. This deficit depends on the difficulty of the task and on the age-period. MAM-E17 rats seem to be more sensitive in puberty in comparison to controls. Diminished behavioral flexibility was shown as well. These behavioral results observed in MAM-E17 rats were similar to those of cognitive deficiencies in schizophrenia patients. Therefore, MAM-E17 model can be a useful implement for further research aiming to improve cognition in schizophrenia.

H3 antagonists and postoperative cognitive dysfunction

Since histamine (HA) was first synthesized in 1907 and isolated as a bacterial contaminant of an extract of ergot in 1910, its role in health and disease and its molecular mechanism of action have been unraveled, leading to the formulation of an array of drugs with immense therapeutic value. HA is produced by decarboxylation of histidine, and its biological actions are mediated through four HA receptors, namely, H₁, H₂, H₃, and H₄ based on their sequence, their link to differential intracellular signaling mechanisms, and their unique pharmacological properties. H₁ and H₂ receptors have been targeted for treating allergic conditions and peptic ulcers, respectively. The discovery of a third HA receptor subtype (H₃R) by molecular biologists in 1983, structurally a member of the G-protein-coupled receptor family, has led to the development of many potent and selective H₃ receptor antagonists having the potential to treat a wide spectrum of neurological diseases including postoperative cognitive dysfunction.

7-Pyrrolidinethoxy-4'-Methoxyisoflavone Prevents Amyloid β-Induced Injury by Regulating Histamine H3 Receptor-Mediated cAMP/CREB and AKT/GSK3β Pathways

In studies on the treatment of Alzheimer’s disease (AD), in which cognition is enhanced even modestly or selectively, it has been considered that the histamine H3 receptor (H3R) may be a potential target. In this study, we aimed at evaluating the ability of 7-pyrrolidinethoxy-4′-methoxyisoflavone (indicated as LC1405), a novel potential H3R antagonist identified from our H3R antagonist screening system, to ameliorate amyloid β (Aβ)-induced cognitive deficits, and to explore the underlying mechanisms that are related to H3R-modulated signaling. Our results demonstrated that LC1405 effectively reduced the progression of Aβ-associated disorders, such as improved learning and memory capabilities, preserved tissues from suffering neurodegeneration and ultrastructural abnormalities, and ameliorated cholinergic dysfunction in an APP/PS1 double transgenic mouse model of AD. In an in vitro model, LC1405 protected neuronal cells against copper-induced Aβ toxicity, as demonstrated by the improvement in cell viability and decrease in neuronal apoptotic ratio. In addition, treatment with LC1405 resulted in the up-regulation of acetylcholine (ACh) or histamine release and provided neuroprotection through cellular signaling cascades involving H3R-mediated cAMP/CREB and AKT/GSK3β pathways. Furthermore, the beneficial effects of LC1405 on Aβ-mediated toxicity and H3R-mediated cAMP/CREB and AKT/GSK3β axes were reversed after pharmacological activation of H3R. In conclusion, our results demonstrated that LC1405 blocked Aβ-induced toxicity through H3R-modulated signaling transduction both in vitro and in vivo. The results also suggested that LC1405 might have translational potential as a complementary therapy to control disease progression in AD patients who developed cognitive deficits with H3R-related ACh neurotransmission abnormality.

Histamine H3 receptor antagonist E177 attenuates amnesia induced by dizocilpine without modulation of anxiety-like behaviors in rats

BACKGROUND

Alzheimer disease (AD) is the main cause of dementia in elderly people. The potential of histamine H3 receptor (H3R) antagonists as a pharmacological treatment of several neuropsychiatric diseases is well established.

METHODS

The novel non-imidazole-based H3R antagonist E177 was screened for its pro-cognitive effects on the inhibitory avoidance paradigm (IAP) and novel object recognition (NOR) task in a dizocilpine (DIZ)-induced model of amnesia in male Wistar rats. Donepezil, an acetylcholine esterase inhibitor, was used as the reference drug.

RESULTS

Acute systemic treatment with E177 (1.25, 2.5, 5, and 10 mg/kg intraperitoneally [i.p.]) significantly attenuated the cognitive impairments induced by DIZ in the IAP (all P-values <0.05, n=7), and the protective effect of the most promising dose of E177 (5 mg/kg) was abrogated when H3R agonist R-(α)-methylhistamine (RAMH; 10 mg/kg i.p.) was co-administered (P=0.281 for DIZ-amnesia group vs DIZ + E177 + RAMH group, n=7). The discrimination index calculated for E177 (5 mg/kg, i.p.) showed a significant memory-enhancing effect on DIZ-induced short-term memory impairment in the NOR task (P<0.05, n=6), with the enhancement nullified when animals were co-administered RAMH (10 mg/kg). Moreover, the results revealed that E177 (5 and 10 mg/kg, i.p.) did not alter the anxiety levels and locomotor activity of animals naïve to the open-field test (all P-values >0.05, n=8) or the elevated plus maze test (all P-values >0.05, n=6-8), which indicated that the E177-induced enhancement of memory performance in the IAP or NOR task was unrelated to changes in emotional response or in spontaneous locomotor activity.

CONCLUSION

The observed results suggested a possible contribution of H3Rs in the alteration of brain neurotransmitters that accompany neurodegenerative diseases, such as AD.

Current Enlightenment About Etiology and Pharmacological Treatment of Autism Spectrum Disorder

Autistic Spectrum Disorder (ASD) is a complex neurodevelopmental brain disorder characterized by two core behavioral symptoms, namely impairments in social communication and restricted/repetitive behavior. The molecular mechanisms underlying ASD are not well understood. Recent genetic as well as non-genetic animal models contributed significantly in understanding the pathophysiology of ASD, as they establish autism-like behavior in mice and rats. Among the genetic causes, several chromosomal mutations including duplications or deletions could be possible causative factors of ASD. In addition, the biochemical basis suggests that several brain neurotransmitters, e.g., dopamine (DA), serotonin (5-HT), gamma-amino butyric acid (GABA), acetylcholine (ACh), glutamate (Glu) and histamine (HA) participate in the onset and progression of ASD. Despite of convincible understanding, risperidone and aripiprazole are the only two drugs available clinically for improving behavioral symptoms of ASD following approval by Food and Drug Administration (FDA). Till date, up to our knowledge there is no other drug approved for clinical usage specifically for ASD symptoms. However, many novel drug candidates and classes of compounds are underway for ASD at different phases of preclinical and clinical drug development. In this review, the diversity of numerous aetiological factors and the alterations in variety of neurotransmitter generation, release and function linked to ASD are discussed with focus on drugs currently used to manage neuropsychiatric symptoms related to ASD. The review also highlights the clinical development of drugs with emphasis on their pharmacological targets aiming at improving core symptoms in ASD.

The Histamine H3 Receptor Antagonist DL77 Ameliorates MK801-Induced Memory Deficits in Rats

The role of Histamine H3 receptors (H3Rs) in memory, and the prospective of H3R antagonists in pharmacological control of neurodegenerative disorders, e.g., Alzheimer disease (AD) is well-accepted. For that reason, the procognitive effects of the H3R antagonist DL77 on cognitive impairments induced with MK801 were tested in an inhibitory passive avoidance paradigm (PAP) and novel object recognition (NOR) task in adult male rats, using donepezil (DOZ) as a standard drug. Acute systemic pretreatment with DL77 (2.5, 5, and 10 mg/kg, i.p.) significantly ameliorated memory deficits induced with MK801 in PAP (all P < 0.05, n = 7). The ameliorative effect of most promising dose of DL77 (5 mg/kg, i.p.) was reversed when rats were co-injected with the H3R agonist R-(α)-methylhistamine (RAMH, 10 mg/kg, i.p.) (p = 0.701 for MK801-amnesic group vs. MK801+DL77+RAMH group, n = 6). In the NOR paradigm, DL77 (5 mg/kg, i.p.) counteracted long-term memory (LTM) deficits induced with MK801 (P < 0.05, n = 6-8), and the DL77-provided effect was similar to that of DOZ (p = 0.788, n = 6-8), and was reversed when rats were co-injected with RAMH (10 mg/kg, i.p.) (p = 0.877, n = 6, as compared to the (MK801)-amnesic group). However, DL77 (5 mg/kg, i.p.) did not alter short-term memory (STM) impairment in NOR test (p = 0.772, n = 6-8, as compared to (MK801)-amnesic group). Moreover, DL77 (5 mg/kg) failed to modify anxiety and locomotor behaviors of animals innate to elevated-plus maze (EPM) (p = 0.67 for percentage of time spent exploring the open arms, p = 0.52 for number of entries into the open arms, p = 0.76 for percentage of entries into the open arms, and p = 0.73 number of closed arm entries as compared to saline-treated groups, all n = 6), demonstrating that the procognitive effects observed in PAP or NOR tests were unconnected to alterations in emotions or in natural locomotion of tested animals. These results signify the potential involvement of H3Rs in modulating neurotransmitters related to neurodegenerative disorders, e.g., AD.

The Histamine H3 Receptor Antagonist E159 Reverses Memory Deficits Induced by Dizocilpine in Passive Avoidance and Novel Object Recognition Paradigm in Rats

The involvement of histamine H3 receptors (H3Rs) in memory is well known, and the potential of H3R antagonists in therapeutic management of neuropsychiatric diseases, e.g., Alzheimer disease (AD) is well established. Therefore, the effects of histamine H3 receptor (H3R) antagonist E159 (2.5–10 mg/kg, i.p.) in adult male rats on dizocilpine (DIZ)-induced memory deficits were studied in passive avoidance paradigm (PAP) and in novel object recognition (NOR) using pitolisant (PIT) and donepezil (DOZ) as standard drugs. Upon acute systemic pretreatment of E159 at three different doses, namely 2.5, 5, and 10 mg/kg, i.p., 2.5 and 5 but not 10 mg/kg of E159 counteracted the DIZ (0.1 mg)-induced memory deficits, and this E159 (2.5 mg)-elicited memory-improving effects in DIZ-induced amnesic model were moderately abrogated after acute systemic administration of scopolamine (SCO), H2R antagonist zolantidine (ZOL), but not with H1R antagonist pyrilamine to the animals. Moreover, the observed memory-enhancing effects of E159 (2.5 mg/kg, i.p.) were strongly abrogated when animals were administered with a combination of SCO and ZOL. Furthermore, the E159 (2.5 mg)-provided significant memory-improving effect of in DIZ-induced short-term memory (STM) impairment in NOR was comparable to the DOZ-provided memory-enhancing effect, and was abolished when animals were injected with the CNS-penetrant histamine H3R agonist R-(α)-methylhistamine (RAMH). However, E159 at a dose of 2.5 mg/kg failed to exhibit procognitive effect on DIZ-induced long-term memory (LTM) in NOR. Furthermore, the results observed revealed that E159 (2.5 mg/kg) did not alter anxiety levels and locomotor activity of animals naive to elevated-plus maze (EPM), demonstrating that improved performances with E159 (2.5 mg/kg) in PAP or NOR are unrelated to changes in emotional responding or in spontaneous locomotor activity. These results provide evidence for the potential of drugs targeting H3Rs for the treatment of neuropsychiatric disorders, e.g., AD.

Histamine H3 receptor antagonists display antischizophrenic activities in rats treated with MK-801

BACKGROUND

Animal models based on N-methyl-d-aspartate receptor blockade have been extensively used for schizophrenia. Ketamine and MK-801 produce behaviors related to schizophrenia and exacerbated symptoms in patients with schizophrenia, which led to the use of PCP (phencyclidine)- and MK-801 (dizocilpine)-treated animals as models for schizophrenia.

METHODS

The study investigated the effect of subchronic dosing (once daily, 7 days) of histamine H3 receptor (H3R) antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p.) on MK-801 (0.2 mg/kg, i.p.)-induced locomotor activity and also measured dopamine and histamine levels in rat's brain homogenates. The study also included clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively.

RESULTS

Atypical and typical antipsychotic was used to serve as clinically relevant reference agents to compare the effects of the H3R antagonists. MK-801 significantly increased horizontal locomotor activity, which was reduced with CPX and CBP. MK-801-induced locomotor hyperactivity attenuated by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised striatal dopamine level, which was reduced in rats pretreated with CPX and CBP. CPZ also significantly lowered striatal dopamine levels, although the decrease was less robust compared to CLZ, CPX, and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increased histamine levels in the hypothalamus compared to MK-801 treatment alone. Histamine H3 receptor agonist, R-α methylhistamine (10 mg/kg, i.p.), counteracted the effect of CPX and CBP.

CONCLUSIONS

The present study shows the positive effects of CPX and CBP on MK-801-induced schizophrenia-like behaviors in rodents.

Histamine H3 receptor as a potential target for cognitive symptoms in neuropsychiatric diseases

The potential contributions of the brain histaminergic system in neurodegenerative diseases, and the possiblity of histamine-targeting treatments is attracting considerable interests. The histamine H3 receptor (H3R) is expressed mainly in the central nervous system, and is, consequently, an attractive pharmacological target. Although recently described clinical trials have been disappointing in attention deficit hyperactivity disorder (ADHD) and schizophrenia (SCH), numerous H3R antagonists, including pitolisant, demonstrate potential in the treatment of narcolepsy, excessive daytime sleepiness associated with cognitive impairment, epilepsy, and Alzheimer's disease (AD). This review focuses on the recent preclinical as well as clinical results that support the relevance of H3R antagonists for the treatment of cognitive symptoms in neuropsychiatric diseases, namely AD, epilepsy and SCH. The review summarizes the role of histaminergic neurotransmission with focus on these brain disorders, as well as the effects of numerous H3R antagonists on animal models and humans.

The Effect of Subchronic Dosing of Ciproxifan and Clobenpropit on Dopamine and Histamine Levels in Rats

The present study was designed to investigate the effect of once daily for 7-day (subchronic treatment) dosing of histamine H₃ receptor antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p), including clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively, on MK-801(0.2 mg/kg, i.p.)-induced locomotor activity, and dopamine and histamine levels in rats. Dopamine and histamine levels were measured in striatum and hypothalamus, respectively, of rat brain. Atypical and typical antipsychotics were used to serve as clinically relevant reference agents to compare the effects of the H3 receptor antagonists. MK-801-induced increase of horizontal activity was reduced with CPX and CBP. The attenuation of MK-801-induced locomotor hyperactivity produced by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised dopamine levels in the striatum, which was reduced in rats pretreated with CPX and CBP. CPZ also lowered striatal dopamine levels, though the decrease was less robust compared to CLZ, CPX and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increase in histamine levels in the hypothalamus compared to the MK-801 treatment alone. Histamine H₃ receptor agonist, R-α methylhistamine (10 mg/kg, i.p.) counteracted the effects of CPX and CBP. In conclusion, the subchronic dosing of CPX/CBP suggests some antipsychotic-like activities as CPX/CBP counteracts the modulatory effects of MK-801 on dopamine and histamine levels and prevents MK-801-induced hyperlocomotor behaviors.

Neuronal histamine and cognitive symptoms in Alzheimer's disease

Alzheimer's disease is a neurodegenerative disorder characterized by extracellular amyloid plaque deposits, mainly composed of amyloid-beta peptide and intracellular neurofibrillary tangles consisting of aggregated hyperphosphorylated tau protein. Amyloid-beta represents a neurotoxic proteolytic cleavage product of amyloid precursor protein. The progressive cognitive decline that is associated with Alzheimer's disease has been mainly attributed to a deficit in cholinergic neurotransmission due to the continuous degeneration of cholinergic neurons e.g. in the basal forebrain. There is evidence suggesting that other neurotransmitter systems including neuronal histamine also contribute to the development and maintenance of Alzheimer's disease-related cognitive deficits. Pathological changes in the neuronal histaminergic system of such patients are highly predictive of ensuing cognitive deficits. Furthermore, histamine-related drugs, including histamine 3 receptor antagonists, have been demonstrated to alleviate cognitive symptoms in Alzheimer's disease. This review summarizes findings from animal and clinical research on the relationship between the neuronal histaminergic system and cognitive deterioration in Alzheimer's disease. The significance of the neuronal histaminergic system as a promising target for the development of more effective drugs for the treatment of cognitive symptoms is discussed. Furthermore, the option to use histamine-related agents as neurogenesis-stimulating therapy that counteracts progressive brain atrophy in Alzheimer's disease is considered. This article is part of a Special Issue entitled 'Histamine Receptors'.

Effects of an H3R antagonist on the animal model of autism induced by prenatal exposure to valproic acid

Autism spectrum disorders (ASD) are a group of neurodevelopmental disorders primarily characterized by impaired social interaction and communication, and by restricted repetitive behaviors and interests. Ligands of histamine receptor 3 (H3R) are considered potential therapeutic agents for the treatment of different brain disorders and cognitive impairments. Considering this, the aim of the present study is to evaluate the actions of ciproxifan (CPX), an H3R antagonist, on the animal model of autism induced by prenatal exposure to valproic acid (VPA). Swiss mice were prenatally exposed to VPA on embryonic day 11 and assessed for social behavior, nociceptive threshold and repetitive behavior at 50 days of life. The treatment with CPX (3 mg/kg) or saline was administered 30 minutes before each behavioral test. The VPA group presented lower sociability index compared to VPA animals that were treated with CPX. Compared to the Control group, VPA animals presented a significantly higher nociceptive threshold, and treatment with CPX was not able to modify this parameter. In the marble burying test, the number of marbles buried by VPA animals was consistent with markedly repetitive behavior. VPA animals that received CPX buried a reduced amount of marbles. In summary, we report that an acute dose of CPX is able to attenuate sociability deficits and stereotypies present in the VPA model of autism. Our findings have the potential to help the investigations of both the molecular underpinnings of ASD and of possible treatments to ameliorate the ASD symptomatology, although more research is still necessary to corroborate and expand this initial data.

Predictive validity of a MK-801-induced cognitive impairment model in mice: implications on the potential limitations and challenges of modeling cognitive impairment associated with schizophrenia preclinically

Cognitive impairment associated with schizophrenia (CIAS) is a major and disabling symptom domain of the disease that is generally unresponsive to current pharmacotherapies. Critically important to the discovery of novel therapeutics for CIAS is the utilization of preclinical models with robust predictive validity. We investigated the predictive validity of MK-801-induced memory impairments in mouse inhibitory avoidance (MK-IA) as a preclinical model for CIAS by investigating compounds that have been tested in humans, including antipsychotics, sodium channel blocker mood stabilizers, and putative cognitive enhancers. The atypical antipsychotic clozapine, as well as risperidone and olanzapine (see Brown et al., 2013), had no effect on MK-801-induced memory impairments. For sodium channel blockers, carbamazepine significantly attenuated memory impairments induced by MK-801, whereas lamotrigine had no effect. Nicotine, donepezil, modafinil, and xanomeline all significantly attenuated MK-801-induced memory impairments, but the magnitude of effects and the dose-responses observed varied across compounds. Clinically, only acute administration of nicotine has demonstrated consistent positive effects on CIAS, while inconsistent results have been reported for lamotrigine, donepezil, and modafinil; atypical antipsychotics produce only moderate improvements at best. A positive clinical signal has been observed with xanomeline, but only in a small pilot trial. The results presented here suggest that the MK-IA model lacks robust predictive validity for CIAS as the model is likely permissive and may indicate false positive signals for compounds and mechanisms that lack clear clinical efficacy for CIAS. Our findings also highlight the potential limitations and challenges of using NMDA receptor antagonists in rodents to model CIAS.

Histaminergic system in brain disorders: lessons from the translational approach and future perspectives

Histamine and its receptors were first described as part of immune and gastrointestinal systems, but their presence in the central nervous system and importance in behavior are gaining more attention. The histaminergic system modulates different processes including wakefulness, feeding, and learning and memory consolidation. Histamine receptors (H1R, H2R, H3R, and H4R) belong to the rhodopsin-like family of G protein-coupled receptors, present constitutive activity, and are subjected to inverse agonist action. The involvement of the histaminergic system in brain disorders, such as Alzheimer's disease, schizophrenia, sleep disorders, drug dependence, and Parkinson's disease, is largely studied. Data obtained from preclinical studies point antagonists of histamine receptors as promising alternatives to treat brain disorders. Thus, clinical trials are currently ongoing to assess the effects of these drugs on humans. This review summarizes the role of histaminergic system in brain disorders, as well as the effects of different histamine antagonists on animal models and humans.