BF2.649 [1-{3-[3-(4-Chlorophenyl)propoxy]propyl}piperidine, Hydrochloride], a Nonimidazole Inverse Agonist/Antagonist at the Human Histamine H3 Receptor: Preclinical Pharmacology

The histamine H₃-receptor inverse agonist pitolisant improves fear memory in mice

Numerous studies have demonstrated that brain histamine plays a crucial role in learning and memory and histamine H3 receptor inverse agonists (H3R inverse agonists) have been proposed to treat cognitive disorders. Pitolisant (BF2.649, 1-{3-[3-(4-chlorophenyl)propoxy]propyl}piperidine, hydrochloride) was the first H3R inverse agonist that has been tested in human trials and is well tolerated. The present study investigated whether Pitolisant (0.625-20mg/kg, i.p.) improves consolidation and reconsolidation processes in the fear conditioning task in female C57BL/6J mice. We also tested whether Pitolisant reverses memory deficits induced by the non-competitive N-methyl-d-aspartate (NMDA) antagonist dizocilpine (MK-801). Our results indicate that post-training systemic injections of Pitolisant facilitated consolidation of contextual fear memory and reversed amnesia induced by an i.p. injection of 0.12 mg/kg dizocilpine. In addition, none of the doses of Pitolisant we have tested after reactivation (reexposure to the context in which training took place 48 h earlier) affected reconsolidation, whereas dizocilpine disrupted it. However, Pitolisant was able to reverse the deficit in reconsolidation induced by 0.12 mg/kg dizocilpine. The present results are the first demonstration that Pitolisant is effective in improving consolidation processes in the fear condition task and add further evidence to its potential for treating cognitive disorders.

Neuropharmacology of Sleep and Wakefulness: 2012 Update

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Neurobiological background for the development of new drugs in schizophrenia

Psychopharmacology of schizophrenia has remained static for many years because the mechanisms explored have been basically monoaminergics, primarily focused toward the modification of dopaminergic function and, later on, serotonergic. In fact, most of the antipsychotics introduced in clinical practice in the last years have been antagonists or selective agonists of these receptors (D(2)/5-HT(2)). The exploration of other receptor pathways, and in particular those additionally involved in the action of the paradigmatic "atypical" antipsychotic clozapine (ie, cholinergic and noradrenergic), has not been very significant. Besides, research in the antipsychotics field has developed also by exploring pathways that are beyond the spectrum of clozapine. Among the most promising mechanisms are those based on the glutamatergic hypothesis of schizophrenia (agonists at the glycine-binding modulatory site of the N-methyl-D-aspartate receptor, glycine transporter inhibitors, modulators of the AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid] receptor and selective agonists of the metabotropic receptor Glu(2)). Other less classic pathways are also under study and have led to some agents that are found in very early stages of development such as those acting on sigma receptors, cholecystokinin antagonists, neurotensin agonists, neurokinin receptor antagonists, GABAergic (+-aminobutyric acid [GABA]) enhancers, and cannabinoid(gamma-aminobutiric) receptor modulators.

Pharmacological properties and procognitive effects of ABT-288, a potent and selective histamine H3 receptor antagonist

Blockade of the histamine H(3) receptor (H(3)R) enhances central neurotransmitter release, making it an attractive target for the treatment of cognitive disorders. Here, we present in vitro and in vivo pharmacological profiles for the H(3)R antagonist 2-[4'-((3aR,6aR)-5-methyl-hexahydro-pyrrolo[3,4-b]pyrrol-1-yl)-biphenyl-4-yl]-2H-pyridazin-3-one (ABT-288). ABT-288 is a competitive antagonist with high affinity and selectivity for human and rat H(3)Rs (K(i) = 1.9 and 8.2 nM, respectively) that enhances the release of acetylcholine and dopamine in rat prefrontal cortex. In rat behavioral tests, ABT-288 improved acquisition of a five-trial inhibitory avoidance test in rat pups (0.001-0.03 mg/kg), social recognition memory in adult rats (0.03-0.1 mg/kg), and spatial learning and reference memory in a rat water maze test (0.1-1.0 mg/kg). ABT-288 attenuated methamphetamine-induced hyperactivity in mice. In vivo rat brain H(3)R occupancy of ABT-288 was assessed in relation to rodent doses and exposure levels in behavioral tests. ABT-288 demonstrated a number of other favorable attributes, including good pharmacokinetics and oral bioavailability of 37 to 66%, with a wide central nervous system and cardiovascular safety margin. Thus, ABT-288 is a selective H(3)R antagonist with broad procognitive efficacy in rodents and excellent drug-like properties that support its advancement to the clinical area.

A pharmacogenetic study of risperidone on histamine H3 receptor gene (HRH3) in Chinese Han schizophrenia patients

Evidence suggests that the human histamine H3 receptor (HRH3) may be involved in the pharmacodynamics of risperidone and influence clinical efficacy. More information on the pharmacogenetics of this receptor may therefore be useful in developing individualized therapy. However, to our knowledge, no study has been reported in this area. The aim of this investigation was to clarify whether H3 receptor polymorphism could affect risperidone efficacy. We genotyped tag single nucleotide polymorphisms (SNPs) of the HRH3 gene (rs3787429 and rs3787430) and analyzed their association with the reduction of Brief Psychiatric Rating Scale (BPRS) score in Chinese Han schizophrenia patients (N = 129), following an eight-week period of risperidone monotherapy. The confounding effects of non-genetic factors were estimated, and then the significant one was included as the covariate for adjustment in statistical analysis. Baseline symptom score was the only significant confounding effect and thus the covariate. After adjustment, significant association of HRH3 with antipsychotic efficacy was detected (for rs3787429, p = 0.013, 0.087 after 4 weeks and 8 weeks of treatment, respectively; for rs3787430, p = 0.024, 0.010 after 4 weeks and 8 weeks of treatment, respectively) and stood up to conservative Bonferroni correction. Our results demonstrate that polymorphism of the HRH3 gene may be a potential genetic marker for predicting the therapeutic effect of risperidone, and suggest novel pharmacological links between HRH3 and risperidone. Further studies with larger samples and different ethnic populations are warranted to confirm our results.

Histamine h3 receptor: a novel therapeutic target in alcohol dependence?

The brain histaminergic system is one of the diffuse modulatory neurotransmitter systems which regulate neuronal activity in many brain areas. Studies on both rats and mice indicate that histamine H3 receptor antagonists decrease alcohol drinking in several models, like operant alcohol administration and drinking in the dark paradigm. Alcohol-induced place preference is also affected by these drugs. Moreover, mice lacking H3R do not drink alcohol like their wild type littermates, and they do not show alcohol-induced place preference. Although the mechanisms of these behaviors are still being investigated, we propose that H3R antagonists are promising candidates for use in human alcoholics, as these drugs are already tested for treatment of other disorders like narcolepsy and sleep disorders.

SAR110894, a potent histamine H₃-receptor antagonist, displays procognitive effects in rodents

SAR110894 is a novel histamine H₃-R ligand, displaying high and selective affinity for human, rat or mouse H₃-Rs. SAR110894 is a potent H₃-R antagonist at native receptors, reversing R-α-methylhistamine-induced inhibition of electrical field stimulation contraction in the guinea-pig ileum. Additionally, SAR110894 inhibited constitutive GTPγS binding at human H₃-Rs demonstrating inverse agonist properties. In behavioral models addressing certain aspects of cognitive impairment associated with schizophrenia (CIAS) and attention deficit/hyperactivity disorder (ADHD), SAR110894 improved memory performances in several variants of the object recognition task in mice (0.3-3 mg/kg, p.o.) or rats (0.3-1 mg/kg, p.o.). Moreover, SAR110894 (1 mg/kg, p.o.) reversed a deficit in working memory in the Y-maze test, following an acute low dose of phencyclidine (PCP) (0.5 mg/kg, i.p.) in mice sensitized by repeated treatment with a high dose of PCP (10 mg/kg, i.p.). In the latent inhibition (LI) model, SAR110894 potentiated LI in saline-treated rats (1 and 3 mg/kg, i.p.) and reversed abnormally persistent LI induced by neonatal nitric oxide synthase (NOS) inhibition in rodents (0.3-3 mg/kg, i.p.). In a social novelty discrimination task in rats, SAR110894 attenuated selective attention deficit induced by neonatal PCP treatment (3 and 10 mg/kg, p.o.) or a parametric modification of the procedure (3 and 10 mg/kg, p.o.). SAR110894 showed efficacy in several animal models related to the cognitive deficits in Alzheimer's disease (AD). It prevented the occurrence of episodic memory deficit induced by scopolamine in rats (0.01-10 mg/kg, p.o.) or by the central infusion of the toxic amyloid fragment β₂₅₋₃₅ in the object recognition test in mice (1 and 3 mg/kg, p.o.). Altogether, these findings suggest that SAR110894 may be of therapeutic interest for the treatment of the cognitive symptoms of AD, schizophrenia and certain aspects of ADHD.

Structure-based prediction of subtype selectivity of histamine H3 receptor selective antagonists in clinical trials

Histamine receptors (HRs) are excellent drug targets for the treatment of diseases, such as schizophrenia, psychosis, depression, migraine, allergies, asthma, ulcers, and hypertension. Among them, the human H(3) histamine receptor (hH(3)HR) antagonists have been proposed for specific therapeutic applications, including treatment of Alzheimer's disease, attention deficit hyperactivity disorder (ADHD), epilepsy, and obesity. However, many of these drug candidates cause undesired side effects through the cross-reactivity with other histamine receptor subtypes. In order to develop improved selectivity and activity for such treatments, it would be useful to have the three-dimensional structures for all four HRs. We report here the predicted structures of four HR subtypes (H(1), H(2), H(3), and H(4)) using the GEnSeMBLE (GPCR ensemble of structures in membrane bilayer environment) Monte Carlo protocol, sampling ∼35 million combinations of helix packings to predict the 10 most stable packings for each of the four subtypes. Then we used these 10 best protein structures with the DarwinDock Monte Carlo protocol to sample ∼50 000 × 10(20) poses to predict the optimum ligand-protein structures for various agonists and antagonists. We find that E206(5.46) contributes most in binding H(3) selective agonists (5, 6, 7) in agreement with experimental mutation studies. We also find that conserved E5.46/S5.43 in both of hH(3)HR and hH(4)HR are involved in H(3)/ H(4) subtype selectivity. In addition, we find that M378(6.55) in hH(3)HR provides additional hydrophobic interactions different from hH(4)HR (the corresponding amino acid of T323(6.55) in hH(4)HR) to provide additional subtype bias. From these studies, we developed a pharmacophore model based on our predictions for known hH(3)HR selective antagonists in clinical study [ABT-239 1, GSK-189,254 2, PF-3654746 3, and BF2.649 (tiprolisant) 4] that suggests critical selectivity directing elements are: the basic proton interacting with D114(3.32), the spacer, the aromatic ring substituted with the hydrophilic or lipophilic groups interacting with lipophilic pockets in transmembranes (TMs) 3-5-6 and the aliphatic ring located in TMs 2-3-7. These 3D structures for all four HRs should help guide the rational design of novel drugs for the subtype selective antagonists and agonists with reduced side effects.

Several down, a few to go: histamine H3 receptor ligands making the final push towards the market?

INTRODUCTION

The histamine H(3) receptor (H(3)R) plays a pivotal role in a plethora of therapeutic areas. Blocking the H(3)R with antagonists/inverse agonists has been postulated to be of broad therapeutic use. Indeed, H(3)R antagonists/inverse agonists have been extensively evaluated in the clinic.

AREAS COVERED

Here, we address new developments, insights obtained and challenges encountered in the clinical evaluations. For recent H(3)R clinical candidates, the status and results of the corresponding clinical trial(s) will be discussed along with preclinical data.

MAIN FINDINGS

In all, it becomes evident that clinical evaluation of H(3)R antagonists/inverse agonists is characterized by mixed results. On one hand, Pitolisant has successfully passed several Phase II trials and seems to be the most advanced compound in the clinic now, being in Phase III. On the other hand, some compounds (e.g., PF-03654647 and MK-0249) failed at Phase II clinical level for several indications.

EXPERT OPINION

A challenging feature in H(3)R research is the multifaceted role of the receptor at a molecular/biochemical level, which can complicate targeting by small molecules at several (pre)clinical levels. Accordingly, H(3)R antagonists/inverse agonists require further testing to pinpoint the determinants for clinical efficacy and to aid in the final push towards the market.

The histamine H3 receptor: from discovery to clinical trials with pitolisant

The third histamine receptor was discovered in 1983 by a traditional pharmacological approach, consisting of assessing the inhibitory effect of histamine on its own release from depolarized rat brain slices. The same in vitro test was used to design, in 1987, the first highly selective and potent H3-autoreceptor ligands, the antagonist thioperamide and the agonist (R)alphamethylhistamine which enhances and inhibits, respectively, the activity of histaminergic neurons in brain. The use of these research tools was instrumental in establishing the main functions of cerebral histaminergic neurons, namely their role in maintenance of wakefulness, attention, learning and other cognitive processes. In 1990, the cloning of the gene of the H3-receptor, a member of the superfamily of heptahelical receptors coupled to G proteins, paved the way to the demonstration of the high constitutive activity of the receptor, including its native form, and its participation in the tonic control of histamine release; it also facilitated the development of H3-receptor inverse agonist programs in many drug companies. Pitolisant (BF2.649, 1-{3-[3-(4-chlorophenyl)propoxy]propyl}piperidine, hydrochloride) is the first inverse agonist to be introduced in the clinics. Its wake-promotion activity was evidenced in excessive diurnal sleepiness of patients with narcolepsy, Parkinson's disease or Obstructive Sleep Apnea/Hypopnea, in which this activity is characterized by a mean decrease of the Epworth Sleepiness Scale by about five units. The procognitive activity of this novel class of drugs may also find therapeutic applications in dementias, schizophrenia or attention deficit hyperactivity disorder.

Histaminergic mechanisms for modulation of memory systems

Encoding for several memory types requires neural changes and the activity of distinct regions across the brain. These areas receive broad projections originating in nuclei located in the brainstem which are capable of modulating the activity of a particular area. The histaminergic system is one of the major modulatory systems, and it regulates basic homeostatic and higher functions including arousal, circadian, and feeding rhythms, and cognition. There is now evidence that histamine can modulate learning in different types of behavioral tasks, but the exact course of modulation and its mechanisms are controversial. In the present paper we review the involvement of the histaminergic system and the effects histaminergic receptor agonists/antagonists have on the performance of tasks associated with the main memory types as well as evidence provided by studies with knockout models. Thus, we aim to summarize the possible effects histamine has on modulation of circuits involved in memory formation.

Sleep neurobiology from a clinical perspective

Many neurochemical systems interact to generate wakefulness and sleep. Wakefulness is promoted by neurons in the pons, midbrain, and posterior hypothalamus that produce acetylcholine, norepinephrine, dopamine, serotonin, histamine, and orexin/hypocretin. Most of these ascending arousal systems diffusely activate the cortex and other forebrain targets. NREM sleep is mainly driven by neurons in the preoptic area that inhibit the ascending arousal systems, while REM sleep is regulated primarily by neurons in the pons, with additional influence arising in the hypothalamus. Mutual inhibition between these wake- and sleep-regulating regions likely helps generate full wakefulness and sleep with rapid transitions between states. This up-to-date review of these systems should allow clinicians and researchers to better understand the effects of drugs, lesions, and neurologic disease on sleep and wakefulness.

Histamine H1, H3 and H4 receptors are involved in pruritus

Histamine has long been recognised as a classical inducer of pruritus. However, the specific mechanism of histamine-induced itch has still not been fully understood. The H1 and H4 receptor appear to be key components in the induction of itch. The specific role of the H3 receptor in histamine-induced itch remains unclear. The aim of our study was to investigate the role of the four known histamine receptors (H1-4) in acute itch in mice. Intradermal injection of the selective H3R inverse agonist pitolisant induced strong itch in mice. Pitolisant (50 nmol/injection)-induced pruritus could be completely blocked by a combined treatment with the H1R antagonist cetirizine (15 mg/kg) and the H4R antagonist JNJ 7777120 (15 mg/kg), whereas the H2R antagonist ranitidine (15 mg/kg) failed to inhibit the scratch response. Next, expression and function of histamine receptors on sensory neurons isolated from dorsal root ganglia of mice were investigated. As the itch sensation results from the excitation of sensory nerves in the skin, we further focused on skin specific sensory neurons. Therefore, neurons were retrograde labelled from the skin by means of a fluorescent tracer. Expression of H1R, H3R and H4R on skin innervating sensory neurons was detected. By single-cell calcium imaging, it was demonstrated that histamine induces a calcium increase in a subset of (skin-specific) sensory neurons via activation of the H1R and H4R as well as inhibition of the H3R. It is assumed that the decreased threshold in response to H3R antagonism activates H1R and H4R on sensory neurons, which in turn results in the excitation of histamine-sensitive afferents and therefore elicits the sensation of itch.

Can small molecules provide truly effective enhancement of cognition? Current achievements and future directions

INTRODUCTION

The prevalence of age-related diseases that implicate a deterioration of cognitive abilities is increasing. Moreover, cognitive decline occurs in numerous CNS disorders affecting patients at younger ages as well, resulting in reduced functional ability and quality of life. Despite the existence of few medications treating cognition, the need for efficacious treatment options to alleviate, halt or even prevent cognitive decline is generally unmet to date. Consequently, extensive research efforts are undertaken to identify medications that can effectively enhance cognition.

AREAS COVERED

This review covers ongoing clinical trials for cognition and reflects on efforts undertaken to increase the success rates of procognitive drug treatment. The review discusses ways to optimize the drug development process for cognition enhancing agents at the preclinical to clinical interface and provides concrete examples.

EXPERT OPINION

The existing efficacy readouts addressing cognition in preclinical research offer little translational validity to the clinical situation. In order to identify truly efficacious drug candidates, biomarkers need to be developed that directly address conserved mechanisms underlying cognitive performances. To this end, technologies such as neuroimaging or electroencephalography constitute promising entry points for identifying both the cognitive domain and the patient population most responsive to drug treatment.

Ciproxifan, a histamine H₃-receptor antagonist / inverse agonist, modulates methamphetamine-induced sensitization in mice

The role of histamine neurons in schizophrenia and psychostimulant abuse remains unclear. Behavioural sensitization to psychostimulants is a cardinal feature of these disorders. Here, we have explored the ability of imetit and ciproxifan (CPX), a reference H₃-receptor agonist and inverse agonist, respectively, to modulate locomotor sensitization induced in mice by methamphetamine (MET). Mice received saline, CPX (3 mg/kg) or imetit (3 mg/kg) 2 h before MET (2 mg/kg), once daily for 12 days, and were killed after a 2-day wash out. Imetit had no effect, but CPX induced a decrease of MET-induced locomotor activity, which became significant at Day 5, and even more at Day 10. Quantitative polymerase chain reaction was used in the sensitized mice to quantify brain-derived neurotrophic factor (BDNF) and N-methyl-D-aspartate (NMDA)-receptor subunit 1 (NR1) mRNAs, two factors that are altered in both schizophrenia and drug abuse. Imetit and CPX used alone had no effect on any marker. Sensitization by MET decreased BDNF mRNAs by 40% in the hippocampus. This decrease was reversed by CPX. Sensitization by MET also induced strong decreases of NR1 mRNAs in the cerebral cortex, hippocampus and striatum, but not hypothalamus. These decreases were also reversed by CPX. The strong modulator effect of CPX in mice sensitized to MET may result from its modulator effect on NR1 mRNAs in the cerebral cortex and striatum. The reversal by CPX of BDNF and NR1 mRNAs in the hippocampus of sensitized animals further strengthens the interest of H₃-receptor inverse agonists for the long-term treatment of cognitive deficits of patients with schizophrenia.

Alzheimer's disease and age-related memory decline (preclinical)

An unfortunate result of the rapid rise in geriatric populations worldwide is the increasing prevalence of age-related cognitive disorders such as Alzheimer's disease (AD). AD is a devastating neurodegenerative illness that is characterized by a profound impairment of cognitive function, marked physical disability, and an enormous economic burden on the afflicted individual, caregivers, and society in general. The rise in elderly populations is also resulting in an increase in individuals with related (potentially treatable) conditions such as "Mild Cognitive Impairment" (MCI) which is characterized by a less severe (but abnormal) level of cognitive impairment and a high-risk for developing dementia. Even in the absence of a diagnosable disorder of cognition (e.g., AD and MCI), the perception of increased forgetfulness and declining mental function is a clear source of apprehension in the elderly. This is a valid concern given that even a modest impairment of cognitive function is likely to be associated with significant disability in a rapidly evolving, technology-based society. Unfortunately, the currently available therapies designed to improve cognition (i.e., for AD and other forms of dementia) are limited by modest efficacy and adverse side effects, and their effects on cognitive function are not sustained over time. Accordingly, it is incumbent on the scientific community to develop safer and more effective therapies that improve and/or sustain cognitive function in the elderly allowing them to remain mentally active and productive for as long as possible. As diagnostic criteria for memory disorders evolve, the demand for pro-cognitive therapeutic agents is likely to surpass AD and dementia to include MCI and potentially even less severe forms of memory decline. The purpose of this review is to provide an overview of the contemporary therapeutic targets and preclinical pharmacologic approaches (with representative drug examples) designed to enhance memory function.

Histamine receptors in the CNS as targets for therapeutic intervention

Histamine has long been known to trigger allergic reactions and gastric acid secretion. However, it was later discovered that, in the brain, histamine regulates basic homeostatic and higher functions, including cognition, arousal, circadian and feeding rhythms. The sole source of brain histamine is neurons localized in the hypothalamic tuberomammillary nuclei. These neurons project axons to the whole brain, are organized into functionally distinct circuits influencing different brain regions and display selective control mechanisms. Although all histamine receptors (H1R, H2R, H3R and H4R) are expressed in the brain, only the H3R has become a drug target for the treatment of neurologic and psychiatric disorders, such as sleep disturbances and cognitive deficits. In this review, we discuss recent developments in the pharmacological manipulation of H3Rs and the implications for H3R-related therapies for neurological and psychiatric disorders. The legacy of Sir James Black.

Kojic acid derivatives as histamine H(3) receptor ligands

The histamine H(3) receptor (H(3)R) is a promising target in the development of new compounds for the treatment of mainly centrally occurring diseases. However, emerging novel therapeutic concepts have been introduced and some indications in the H(3)R field, e.g. migraine, pain or allergic rhinitis, might take advantage of peripherally acting ligands. In this work, kojic acid-derived structural elements were inserted into a well established H(3)R antagonist/inverse agonist scaffold to investigate the bioisosteric potential of γ-pyranones with respect to the different moieties of the H(3)R pharmacophore. The most affine compounds showed receptor binding in the low nanomolar concentration range. Evaluation and comparison of kojic acid-containing ligands and their corresponding phenyl analogues (3-7) revealed that the newly integrated scaffold greatly influences chemical properties (S Log P, topological polar surface area (tPSA)) and hence, potentially modifies the pharmacokinetic profile of the different derivatives. Benzyl-1-(4-(3-(piperidin-1-yl)propoxy)phenyl)methanamine ligands 3 and 4 belong to the centrally acting diamine-based class of H(3)R antagonist/inverse agonist, whereas kojic acid analogues 6 and 7 might act peripherally. The latter compounds state promising lead structures in the development of H(3)R ligands with a modified profile of action.

Neuropharmacology of Sleep and Wakefulness

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Ciproxifan, an H3 receptor antagonist, alleviates hyperactivity and cognitive deficits in the APP Tg2576 mouse model of Alzheimer's disease

Previous research has indicated that the blockade of H(3)-type histamine receptors may improve attention and memory in normal rodents. The purpose of this study was to determine if ciproxifan, an H(3) receptor antagonist, could alleviate the hyperactivity and cognitive deficits observed in a transgenic mouse model (APP(Tg2576)) of Alzheimer's disease. APP(Tg2576) mice displayed significantly greater locomotor activity than wild-type mice, but APP(Tg2576) mice provided with daily ciproxifan treatment showed activity levels that did not differ from wild-type mice. In the swim maze, APP(Tg2576) mice exhibited significantly longer escape latencies, but the APP(Tg2576) mice treated daily with ciproxifan had latencies that were indistinguishable from controls. In probe trials conducted one hour after the last training trial, ciproxifan-treated APP(Tg2576) mice spent more time near the previous platform location and made more crossings of this area than did saline-treated APP(Tg2576) mice. APP(Tg2576) mice also demonstrated a significant impairment in the object recognition task that was reversed by acute treatment with ciproxifan (3.0mg/kg). These data support the idea that modulation of H(3) receptors represents a novel and viable therapeutic strategy in the treatment of Alzheimer's disease.

Histamine and H3 receptor in alcohol-related behaviors

Data from rat models for alcohol preference and histidine decarboxylase knockout (HDC KO) mice suggest that brain histamine regulates alcohol-related behaviors. Histamine levels are higher in alcohol-preferring than in alcohol-nonpreferring rat brains, and expression of histamine H(3) receptor (H(3)R) is different in key areas for addictive behavior. H(3)R inverse agonists decrease alcohol responding in one alcohol-preferring rat line. Conditioned place preference induced by alcohol is stronger in HDC KO mice than in control mice. The HDC KO mice display a weaker stimulatory response to acute alcohol than the wild-type (WT) mice. In male inbred C57BL/6 mice the H(3)R antagonist ciproxifan inhibits ethanol-evoked stimulation of locomotor activity. Ciproxifan also potentiates the ethanol reward, but does not alone result in the development of place preference. At least in one rat model developed to study alcohol sensitivity high histamine levels are characteristic of the alcohol-insensitive rat line, and lowering brain histamine with a HDC inhibitor increases alcohol sensitivity in the tilting plane test. However, the motor skills of HDC KO mice do not seem to differ from those of the WT mice. Current evidence suggests that the histaminergic system is involved in the regulation of place preference behavior triggered by alcohol, possibly through an interaction with the mesolimbic dopamine system. Histamine may also interact with dopamine in the regulation of the cortico-striato-pallido-thalamo-cortical motor pathway and cerebellar mechanisms, which may be important in different motor behaviors beyond alcohol-induced motor disturbances. H(3)R ligands may have significant effects on alcohol addiction.

Histamine H3 receptors and sleep-wake regulation

The histaminergic system fulfills a major role in the maintenance of waking. Histaminergic neurons are located exclusively in the posterior hypothalamus from where they project to most areas of the central nervous system. The histamine H(3) receptors are autoreceptors damping histamine synthesis, the firing frequency of histamine neurons, and the release of histamine from axonal varicosities. It is noteworthy that this action also extends to heteroreceptors on the axons of most other neurotransmitter systems, allowing a powerful control over multiple homeostatic functions. The particular properties and locations of histamine H(3) receptors provide quite favorable attributes to make this a most promising target for pharmacological interventions of sleep and waking disorders associated with narcolepsy, Parkinson's disease, and other neuropsychiatric indications.

[Non-dopaminergic treatments for Parkinson's disease]

It has become increasingly evident over the past years that the pathological process in Parkinson's disease extends well beyond the substantia nigra and involves non-dopaminergic neurotransmitter systems that mediate motor and non-motor symptoms. In this article, both the pathophysiology and the pharmacological management of these non-dopaminergic symptoms are discussed.

The H3 antagonist, ciproxifan, alleviates the memory impairment but enhances the motor effects of MK-801 (dizocilpine) in rats

Antagonists of H(3)-type histamine receptors exhibit cognitive-enhancing properties in various memory paradigms as well as evidence of antipsychotic activity in normal animals. The present study determined if a prototypical H(3) antagonist, ciproxifan, could reverse the behavioral effects of MK-801, a drug used in animals to mimic the hypoglutamatergic state suspected to exist in schizophrenia. Four behaviors were chosen for study, locomotor activity, ataxia, prepulse inhibition (PPI), and delayed spatial alternation, since their modification by dizocilpine (MK-801) has been well characterized. Adult male Long-Evans rats were tested after receiving a subcutaneous injection of ciproxifan or vehicle followed 20 min later by a subcutaneous injection of MK-801 or vehicle. Three doses of MK-801 (0.05, 0.1, & 0.3 mg/kg) increased locomotor activity. Each dose of ciproxifan (1.0 & 3.0 mg/kg) enhanced the effect of the moderate dose of MK-801, but suppressed the effect of the high dose. Ciproxifan (3.0 mg/kg) enhanced the effects of MK-801 (0.1 & 0.3 mg/kg) on fine movements and ataxia. Deficits in PPI were observed after treatment with MK-801 (0.05 & 0.1 mg/kg), but ciproxifan did not alter these effects. Delayed spatial alternation was significantly impaired by MK-801 (0.1 mg/kg) at a longer delay, and ciproxifan (3.0 mg/kg) alleviated this impairment. These results indicate that some H(3) antagonists can alleviate the impact of NMDA receptor hypofunction on some forms of memory, but may exacerbate its effect on other behaviors.

Histamine neuronal system as a therapeutic target for the treatment of cognitive disorders

Much has been learned over the past 20 years about the role of histamine as a neurotransmitter. This brief article attempts to evaluate the progress accomplished in this field, and discusses the therapeutic potential of the H3 receptor (H3R). All histaminergic neurons are localized in the tuberomammillary nucleus of the posterior hypothalamus and project to almost all regions of the CNS. Histamine exerts its effect via interaction with specific receptors (H1R, H2R, H3R and H4R). Antagonists of both H1R and H2R have been successful as blockbuster drugs for treating allergic conditions and gastric ulcers. H4R is still awaiting better functional characterization, but the H3R is an attractive target for potential therapies of CNS disorders. Indeed, considerable interest was raised by reports that pharmacological blockade of H3Rs exerted procognitive effects in a variety of animal tasks analyzing different types of memory. In addition, blockade of H3Rs increased wakefulness and reduced bodyweight in animal models. Such findings hint at the potential use of H3R antagonists/inverse agonists for the treatment of Alzheimer’s disease and other dementias, attention-deficit hyperactivity disorder, obesity and sleep disorders. As a result, an increasing number of H3R antagonists/inverse agonists progress through the clinic for the treatment of a variety of conditions, including attention-deficit hyperactivity disorder, cognitive disorders, narcolepsy and schizophrenia. Moreover, the use of H3R antagonists/inverse agonists that weaken traumatic memories may alleviate disorders such as post-traumatic stress syndrome, panic attacks, specific phobias and generalized anxiety. The use of H3R ligands for the treatment of neurodegenerative disorders is demonstrated in several studies, indicating a role of the histamine neurons and H3Rs in neuroprotection. Recently, direct evidence demonstrated that histaminergic neurons are organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections. The possibility that H3Rs control only some of those functions implies that H3R-directed therapies may achieve selective effects, with minimal side effects, and this may increase the interest regarding this class of drugs.

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.

Acidic elements in histamine H(3) receptor antagonists

Antagonists of the human histamine H(3) receptor (hH(3)R) often contain a second basic moiety, which is well known to boost affinity on this histamine receptor subtype. Here, we prepared compounds with acidic moieties of different pK(a) values to figure out that the hH(3)R tolerates these functionalities when added to a common pharmacophore blueprint. Depending on the acidic, electronic and steric features the designed ligands showed hH(3)R affinities in the nanomolar concentration range. Additionally, selected ligands were tested but failed as dual acting hH(3)R/hPPAR (human peroxisome proliferator-activated receptor) ligands.

Histamine in neurotransmission and brain diseases

Apart from its central role in the mediation of allergic reactions, gastric acid secretion and inflammation in the periphery, histamine serves an important function as a neurotransitter in the central nervous system. The histaminergic neurons originate from the tuberomamillary nucleus of the posterior hypothalamus and send projections to most parts of the brain. The central histamine system is involved in many brain functions such as arousal, control of pituitary hormone secretion, suppression ofeating and cognitive functions. The effects of neuronal histamine are mediated via G-protein-coupled H1-H4 receptors. The prominent role of histamine as a wake-promoting substance has drawn interest to treat sleep-wake disorders, especially narcolepsy, via modulation of H3 receptor function. Post mortem studies have revealed alterations in histaminergic system in neurological and psychiatric diseases. Brain histamine levels are decreased in Alzheimer's disease patients whereas abnormally high histamine concentrations are found in the brains of Parkinson's disease and schizophrenic patients. Low histamine levels are associated with convulsions and seizures. The release of histamine is altered in response to different types of brain injury: e.g. increased release of histamine in an ischemic brain trauma might have a role in the recovery from neuronal damage. Neuronal histamine is also involved in the pain perception. Drugs that increase brain and spinal histamine concentrations have antinociceptive properties. Histaminergic drugs, most importantly histamine H3 receptors ligands, have shown efficacy in many animal models of the above-mentioned disorders. Ongoing clinical trials will reveal the efficacy and safety of these drugs in the treatment of human patients.

Orexin/hypocretin and histamine: distinct roles in the control of wakefulness demonstrated using knock-out mouse models

To determine the respective role played by orexin/hypocretin and histamine (HA) neurons in maintaining wakefulness (W), we characterized the behavioral and sleep-wake phenotypes of orexin (Ox) knock-out (-/-) mice and compared them with those of histidine-decarboxylase (HDC, HA-synthesizing enzyme)-/- mice. While both mouse strains displayed sleep fragmentation and increased paradoxical sleep (PS), they presented a number of marked differences: (1) the PS increase in HDC(-/-) mice was seen during lightness, whereas that in Ox(-/-) mice occurred during darkness; (2) contrary to HDC(-/-), Ox(-/-) mice had no W deficiency around lights-off, nor an abnormal EEG and responded to a new environment with increased W; (3) only Ox(-/-), but not HDC(-/-) mice, displayed narcolepsy and deficient W when faced with motor challenge. Thus, when placed on a wheel, wild-type (WT), but not littermate Ox(-/-) mice, voluntarily spent their time in turning it and as a result, remained highly awake; this was accompanied by dense c-fos expression in many areas of their brains, including Ox neurons in the dorsolateral hypothalamus. The W and motor deficiency of Ox(-/-) mice was due to the absence of Ox because intraventricular dosing of orexin-A restored their W amount and motor performance whereas SB-334867 (Ox1-receptor antagonist, i.p.) impaired W and locomotion of WT mice during the test. These data indicate that Ox, but not HA, promotes W through enhanced locomotion and suggest that HA and Ox neurons exert a distinct, but complementary and synergistic control of W: the neuropeptide being more involved in its behavioral aspects, whereas the amine is mainly responsible for its qualitative cognitive aspects and cortical EEG activation.