Effects of clobenpropit (VUF-9153), a histamine H3-receptor antagonist, on learning and memory, and on cholinergic and monoaminergic systems in mice

Histamine may contribute to vortioxetine's procognitive effects; possibly through an orexigenic mechanism

Vortioxetine is a novel multimodal antidepressant that acts as a serotonin (5-HT)3, 5-HT7, and 5-HT1D receptor antagonist; 5-HT1B receptor partial agonist; 5-HT1A receptor agonist; and 5-HT transporter inhibitor in vitro. In preclinical and clinical studies vortioxetine demonstrates positive effects on cognitive dysfunction. Vortioxetine's effect on cognitive function likely involves the modulation of several neurotransmitter systems. Acute and chronic administration of vortioxetine resulted in changes in histamine concentrations in microdialysates collected from the rat prefrontal cortex and ventral hippocampus. Based on these results and a literature review of the current understanding of the interaction between the histaminergic and serotonergic systems and the role of histamine on cognitive function, we hypothesize that vortioxetine through an activation of the orexinergic system stimulates the tuberomammilary nucleus and enhances histaminergic neurotransmission, which contributes to vortioxetine's positive effects on cognitive function.

Multiple Targeting Approaches on Histamine H3 Receptor Antagonists

With the very recent market approval of pitolisant (Wakix®), the interest in clinical applications of novel multifunctional histamine H₃ receptor antagonists has clearly increased. Since histamine H₃ receptor antagonists in clinical development have been tested for a variety of different indications, the combination of pharmacological properties in one molecule for improved pharmacological effects and reduced unwanted side-effects is rationally based on the increasing knowledge on the complex neurotransmitter regulations. The polypharmacological approaches on histamine H₃ receptor antagonists on different G-protein coupled receptors, transporters, enzymes as well as on NO-signaling mechanism are described, supported with some lead structures.

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.

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.

Pretreatment with l-histidine produces a shift from methamphetamine-induced stereotypical biting to persistent locomotion in mice

The administration of methamphetamine (METH; 10mg/kg, i.p.) to male ICR mice induced bizarre behaviors including persistent locomotion and stereotypical behaviors, which were classified into four categories: stereotypical head-bobbing, circling, sniffing, and biting. Pretreatment with l-histidine (750 mg/kg, i.p.) significantly decreased the stereotypical biting induced by METH and significantly increased persistent locomotion. This effect of l-histidine on behavior was completely abolished by simultaneous administration of pyrilamine or ketotifen (brain-penetrating histamine H(1) receptor antagonists; 10mg/kg each, i.p.), but not by the administration of fexofenadine (a non-sedating histamine H(1) receptor antagonist that does not cross the blood-brain barrier; 20mg/kg), zolantidine (a brain-penetrating histamine H(2) receptor antagonist; 10mg/kg), thioperamide, or clobenpropit (brain-penetrating histamine H(3) receptor antagonists; 10mg/kg each). The histamine content of the hypothalamus was significantly increased by l-histidine treatment. These data suggest that l-histidine modifies the effects of METH through central histamine H(1) receptors.

H3 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing retrieval

RATIONALE

Accumulated evidence suggests a role for histamine in cognition and the use of H3 receptor antagonists in the treatment of learning and memory disorders.

OBJECTIVES

The aim of the current study was to investigate the cognition enhancing properties of ciproxifan, an H3 receptor antagonist, after natural forgetting in normal adult rats.

MATERIALS AND METHODS

The novel object discrimination task, a recognition memory test based on spontaneous exploratory behaviour, was used. Briefly, rats exposed to two identical objects during an acquisition trial can discriminate between a novel object and a familiar one during a subsequent choice trial after a short delay but not after a 24-h inter-trial interval.

RESULTS

The scopolamine (0.5 mg/kg, i.p.)-induced impairment after a short delay was abolished by ciproxifan (p < 0.001). Natural forgetting was prevented by a single administration of ciproxifan (3 mg/kg) prior to the retention test (p < 0.001) but not when administered before or immediately after the acquisition trial (schedule effect p < 0.05), demonstrating a specific activity on memory retrieval. Pretreatment with either pyrilamine (10 mg/kg), an H1 antagonist, or zolantidine (10 mg/kg), an H2 antagonist, prevented the retrieval enhancement effect of ciproxifan (p < 0.05 and p < 0.001, respectively).

CONCLUSIONS

Histamine H3 receptor antagonists restore the performance of rats impaired by scopolamine and enhance recognition memory after acute administration before the retrieval phase via a mechanism dependent on H1 and H2 receptor activation.

Histamine and Schizophrenia

With the availability of an increased number of experimental tools, for example potent and brain-penetrating H1-, H2-, and H3-receptor ligands and mutant mice lacking the histamine synthesis enzyme or the histamine receptors, the functional roles of histaminergic neurons in the brain have been considerably clarified during the recent years, particularly their major role in the control of arousal, cognition, and energy balance. Various approaches tend to establish the implication of histaminergic neurons in schizophrenia. A strong hyperactivity of histamine neurons is induced in rodent brain by administration of methamphetamine or NMDA-receptor antagonists. Histamine neuron activity is modulated by typical and atypical neuroleptics. H3-receptor antagonists/inverse agonists display antipsychotic-like properties in animal models of the disease. Because of the limited predictability value of most animal models and the paucity of drugs affecting histaminergic transmission that were tried so far in human, the evidence remains therefore largely indirect, but supports a role of histamine neurons in schizophrenia.

Involvement of histamine receptors in the acquisition of inhibitory avoidance in Carassius auratus

This study investigated the involvement of H(1) and H(2) histaminegic receptors on the acquisition of a new task in Carassius auratus by using an inhibitory avoidance paradigm in which the animals had to learn to avoid an aversive stimulus. Before training, the fish received injections of H(2) antagonist zolantidine at a dose of 20 mg/kg, or H(1) antagonist chlorpheniramine at a dose of 4 or 16 mg/kg. Control animals were injected with distilled water. A facilitatory effect of chlorpheniramine was observed at the dose of 16 mg/kg. On the other hand, the administration of 20 mg/kg of zolantidine inhibited acquisition. Place preference conditioning was used to observe the aversive or reinforcing effects of the drugs, which could interfere with the inhibitory avoidance procedure; however, no effects were observed. Thus, it can be suggested that both receptors, H(1) and H(2), are involved in the acquisition of a new task in this species.

Keynote review: histamine H3 receptor antagonists reach out for the clinic

Antagonists of the histamine H(1) and H(2) receptors have been successful as blockbuster drugs for treating allergic conditions and gastric ulcers, respectively. As such, histamine receptors have made a significant contribution to establishing G-Protein-coupled receptors as the favored drug targets of the industry. In this light, it can easily be understood that the discovery of a third histamine receptor subtype (H(3)R) in 1983 was greeted with considerable excitement. However, characterization of the H(3)R turned out to be far from trivial. In the past five years, molecular biology approaches have given fresh impetus to the H(3)R research field. As a result, H(3)R ligands are where they were anticipated to be 20 years ago: at the center of attention and on the verge of an anticipated breakthrough as the next generation of histaminergic blockbuster drugs. Here, we assess the status of the H(3)R medicinal chemistry programs of the various players in the field, as far as can be deduced from patent applications and scientific literature.

Mouse behavioural analysis in systems biology

Molecular techniques allowing in vivo modulation of gene expression have provided unique opportunities and challenges for behavioural studies aimed at understanding the function of particular genes or biological systems under physiological or pathological conditions. Although various animal models are available, the laboratory mouse (Mus musculus) has unique features and is therefore a preferred animal model. The mouse shares a remarkable genetic resemblance and aspects of behaviour with humans. In this review, first we describe common mouse models for behavioural analyses. As both genetic and environmental factors influence behavioural performance and need to be carefully evaluated in behavioural experiments, considerations for designing and interpretations of these experiments are subsequently discussed. Finally, common behavioural tests used to assess brain function are reviewed, and it is illustrated how behavioural tests are used to increase our understanding of the role of histaminergic neurotransmission in brain function.

Histamine at high dilution reduces spectral density in delta band in sleeping rats

Histamine is a central neurotransmitter, it increases arousal via H1 receptors. This study examines the effect of ultra-diluted histamine on arousal through changes in the sleep pattern of Wistar rats. The spectral density in delta (0.5-2.5 Hz) band, one of the three major spectral components of the sleep-electroencephalogram, was analyzed against time. Rats were randomized to receive histamine 30c (histamine 30c, 0.05 ml every 20 min during the first 2 h orally), histamine intraperitoneal pre-treatment/histamine 30c (histamine 6mg/kg i.p., followed by histamine 30c) or solvent control. The mean delta band spectral density was lower in the histamine 30c and histamine pretreatment/histamine 30c groups than the control group. Significant differences between histamine 30c and baseline during the first 2 h imply an immediate effect. These results also suggest a dynamic process in which the system spontaneously evolves between two locally stationary states according to a power law. From the time perspective, the system approaches, asymptotically, an equifinal state.

Histamine H3 antagonist thioperamide dose-dependently enhances memory consolidation and reverses amnesia induced by dizocilpine or scopolamine in a one-trial inhibitory avoidance task in mice

In the literature, there is some evidence indicating that H3 histamine receptor antagonists, in particular thioperamide, can facilitate learning and memory retrieval in laboratory rodents. The present study aimed at verifying whether this also holds for memory consolidation, a phase of memory for which there is scarcity of convincing data on the effects of H3 receptor antagonists given systemically. To that end, memory consolidation was assessed in C57BL/6J mice using the one-trial step-through inhibitory avoidance task, the compounds being injected immediately after training (foot-shock) and performance measured 24 h later. More specifically, the following effects of thioperamide (1.25-20 mg/kg) were dose-dependently analysed: (1) its potential direct effects on memory consolidation; (2) its potential reversing effects on retrograde amnesia induced by the NMDA antagonist dizocilpine (MK-801, 0.5 mg/kg) and (3) its potential reversing effects on the well-known amnesia induced by the muscarinic antagonist scopolamine (0.25 mg/kg). We found that thioperamide exerted a dose-dependent facilitative effect on memory consolidation. Furthermore, the H3 receptor antagonist reversed scopolamine- and especially dizocilpine-induced amnesia. The results strongly support the view that the brain mechanisms of memory consolidation involve a functional interaction between the NMDA and the H3 sites.

Selective histamine H3 receptor antagonists for treatment of cognitive deficiencies and other disorders of the central nervous system

Evidence exists to implicate the monoamine histamine in the control of arousal and cognitive functions. Antagonists of H(3) receptors are postsynaptic and presynaptic modulators of neural transmission in a variety of neuronal circuits relevant to cognition. Accumulating neuroanatomical, neurochemical, pharmacological, and behavioral data support the idea that H(3) receptor antagonists may function to improve cognitive performances in disease states (e.g., Alzheimer's disease and mild cognitive impairment states). Thus, H(3) receptor antagonists have been shown to increase performance in attention and memory tests in nonhuman experiments and prevent the degradation in performances produced by scopolamine, MK-801, or age. In contrast, agonists of the H(3) receptor generally produce cognitive impairing effects in animal models. The role of H(3) receptors in these behavioral effects is substantiated by data indicating a central origin for their effects, the selectivity of some of the H(3) receptor antagonists studied, and the pharmacological modification of effects of H(3) receptor antagonists by selective H(3) receptor agonists. Data and issues that challenge the potential role for H(3) receptor antagonists in cognitive processes are also critically reviewed. H(3) receptor antagonists may also have therapeutic value in the management of obesity, pain, sleep disorders, schizophrenia, and attention deficit hyperactivity disorder.

Role of H3-receptor-mediated signaling in anxiety and cognition in wild-type and Apoe-/- mice

Increasing evidence supports a role for histamine as a neurotransmitter and neuromodulator in emotion and cognition. The H(3) receptor was first characterized as an autoreceptor that modulates histamine release and synthesis via negative feedback. Mice deficient in apoE (Apoe(-/-)) have been used to define the role of apoE in brain function. In the present study, we investigated the possible role of histamine H(3)-receptor-mediated signaling in anxiety and cognition in mice Apoe(-/-) and wild-type mice. H(3) antagonists increased measures of anxiety in wild-type, but not Apoe(-/-), mice. In contrast, H(3) antagonists similarly impaired object recognition in wild-type and Apoe(-/-) mice. In Apoe(-/-) mice, reduced negative feedback via H(3) receptors could contribute to increased signaling of H(1) receptors. Apoe(-/-) mice showed higher sensitivity to the anxiety-reducing effects of the H(1) receptor antagonist mepyramine than wild-type mice. These effects were dissociated from effects of mepyramine on the HPA axis. Compared to saline controls, mepyramine reduced plasma ACTH and corticosterone levels in wild-type, but not Apoe(-/-), mice. These data support a role for apoE in H(3) receptor signaling. H(3) antagonists were proposed as a treatment for cognitive disorders such as Alzheimer's disease, which is associated with increased anxiety and cognitive impairments. As H(3) antagonists increase measures of anxiety and impair object recognition in wild-type mice, the use of H(3) antagonists in cognitive disorders may be counterproductive and should be carefully evaluated.

Medicinal chemistry approaches for the treatment and prevention of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, which is characterised by progressive deterioration of memory and higher cortical functions that ultimately result in total degradation of intellectual and mental activities. Modern strategies in the search of new therapeutic approaches are based on the morphological and biochemical characteristics of AD, and focused on following directions: agents that compensate the hypofunction of cholinergic system, agents that interfere with the metabolism of beta-amyloid peptide, agents that protect nerve cells from toxic metabolites formed in neurodegenerative processes, agents that activate other neurotransmitter systems that indirectly compensate for the deficit of cholinergic functions, agents that affect the process of the formation of neurofibrillary tangles, anti-inflammatory agents that prevent the negative response of nerve cells to the pathological process. The goal of the present review is the validation and an analysis from the point of view of medicinal chemistry of the principles of the directed search of drugs for the treatment and prevention of AD and related neurodegenerative disorders. It is based on systematization of the data on biochemical and structural similarities in the interaction between physiologically active compounds and their biological targets related to the development of such pathologies. The main emphasis is on cholinomimetic, anti-amyloid and anti-metabolic agents, using the data that were published during the last 3 to 4 years, as well as the results of clinical trials presented on corresponding websites.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, modulates the effects of methamphetamine on neuropeptide mRNA expression in rat striatum

We have explored the effect of histamine H3-receptor ligands on the regulation of neuropeptide mRNA expression in the striatum by using in situ hybridization performed with proenkephalin, prodynorphin, substance P and proneurotensin riboprobes. Acute administration of ciproxifan, an H3-receptor antagonist/inverse agonist, or (R)-alpha-methylhistamine, an H3-receptor agonist, did not modify the striatal expression of the neuropeptides by itself. However, ciproxifan strongly and differentially modulated the effect of a single administration of 3 mg/kg methamphetamine on neuropeptide mRNA expression. This modulation was suppressed by the administration of (R)-alpha-methylhistamine and occurred in both the caudate-putamen and nucleus accumbens. Ciproxifan strongly potentiated the decrease of proenkephalin mRNA expression induced by methamphetamine. In contrast, it suppressed the increase in prodynorphin and substance P mRNA expression induced by methamphetamine. Methamphetamine alone or with ciproxifan did not modify proneurotensin mRNA expression. These neurochemical findings indicate that ciproxifan differentially regulates the effect of methamphetamine on the neuropeptides contained in striatonigral and striatopallidal neurons. They suggest that endogenous histamine and dopamine cooperate to modulate the activity of striatal projection neurons and strengthen the interest of H3-receptors as new targets for the treatment of psychotic disorders and drug abuse.

Effects of alpha-adrenoceptor agonists and antagonists on histamine-induced impairment of memory retention of passive avoidance learning in rats

The effect of alpha-adrenoceptor agents on the impairment induced by histamine was measured for memory retention of passive avoidance learning in rats. Post-training intracerebroventricular (i.c.v.) injection was carried out in all the experiments. Histamine (5, 10 and 20 microg/rat) reduced, while a histamine H(1) receptor antagonist, chlorpheniramine (0.1, 1 and 10 microg/rat), increased memory retention. The histamine H(2) receptor antagonist, ranitidine (0.1, 1, 10 and 20 microg/rat), did not elicit any response in this respect. Different doses of chlorpheniramine but not ranitidine reversed the histamine-induced impairment of memory. Clonidine and prazosin decreased, but yohimbine and phenylephrine increased, memory retention. Yohimbine decreased the inhibitory response to histamine. Phenylephrine, clonidine and prazosin did not alter the histamine effect. It is concluded that a histamine-induced impairment of memory retention through histamine H(1) receptors and an alpha(2)-adrenoceptor mechanism may be involved in the histamine response.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat

By using double in situ hybridization performed with proenkephalin and H3-receptor riboprobes on the same sections from rat brain, we show that histamine H3 receptors are expressed within striatopallidal neurons of the indirect movement pathway. The majority ( approximately 70%) of striatal enkephalin neurons express H3-receptor mRNAs. This important degree of coexpression of proenkephalin and H3-receptor mRNAs prompted us to explore the effect of H3-receptor ligands on the regulation of enkephalin mRNA expression in the striatum. Acute administration of ciproxifan, a H3-receptor antagonist/inverse agonist, did not modify the expression of the neuropeptide by itself but strongly increased the upregulation of its expression induced by haloperidol. This potentiation (1) was suppressed by the administration of (R)-alpha-methylhistamine, a H3-receptor agonist, (2) occurred both in the caudate-putamen and nucleus accumbens, and (3) was also observed with a similar pattern on c-fos and neurotensin mRNA expression. Similarly, whereas it was devoid of any motor effect when used alone, ciproxifan strongly potentiated haloperidol-induced locomotor hypoactivity and catalepsy, two behaviors in which striatal neurons are involved. The strong H3-receptor mRNA expression in enkephalin neurons suggests that the synergistic neurochemical and motor effects of ciproxifan and haloperidol result from direct H3/D2-receptor interactions, leading to an enhanced activation of striatopallidal neurons of the indirect movement pathway. The potentiation of the effects of haloperidol by ciproxifan strengthens the potential interest of H3-receptor antagonists/inverse agonists to improve the symptomatic treatment of schizophrenia.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Antiamnesic effect of metoprine and of selective histamine H(1) receptor agonists in a modified mouse passive avoidance test

The aim of this study was to elucidate the effect caused by the inhibition of histamine catabolism by means of metoprine and the activation of histamine H(1) receptors by selective agonists on learning and memory processes, using a modified method of the mouse passive avoidance test. The administration of scopolamine 1 mg/kg (i. p.) immediately after the training session caused statistically-significant amnesia during the retention trial performed 24 h later. Piracetam (30 mg/kg (i.p.)), used as a positive control, and administered 20 min before the training session, prevented scopolamine-induced memory impairment. The histamine-N-methyltransferase inhibitor, metoprine, (2 and 5 mg/kg (s.c.)) had effects similar to those of this nootropic drug. The highly-selective H(1) receptor agonist, 2-(3-trifluoromethylphenyl)histamine (FMPH) (2.65 and 6.5 microg/mouse (i.c.v.)) and the less selective agonist, 2-thiazolylethylamine (2-TEA) (0.1 and 0.3 microg/mouse (i.c.v.)) both antagonized the scopolamine-induced amnesia significantly and in a dose-related manner. The selective H(1) receptor antagonist, pyrilamine (20 mg/kg (i.p.)), revealed no effect by itself, but significantly prevented the antiamnesic action both that of the H(1) receptor agonists, and that of endogenous histamine, released by metoprine, thus suggesting a cognitive improvement via the activation of H(1) receptors.

Structure-property relationships on histamine H3-antagonists: binding of phenyl-substituted alkylthioimidazole derivatives to rat plasma proteins

The binding of a series of H3-antagonists to rat plasma proteins was investigated by dialysis experiments, with RP-HPLC measurement of the free ligand. The series was composed of 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles having, on the phenyl ring, meta- and para-substituents, with different physico-chemical characteristics. As high protein binding had been proposed as being one of the features limiting brain access for the reference H3-antagonist thioperamide, the title series was employed to test the possibility of achieving lower protein binding by modulation of lipophilicity, while maintaining good receptor affinity. The compounds tested showed quotas of bound drug ranging from 60 to 97.5%, while for thioperamide a 78% bound drug quota was observed at high total concentrations, with a steep increase in bound percentage at lower concentrations. Two of the tested compounds, having a carboxamide substituent, showed lower protein binding compared to thioperamide over a wide range of total concentration, without a significant loss in affinity with respect to the parent compound. A strict dependence of protein binding on lipophilicity was observed, and a QSPR model was derived which could also account for the protein binding observed for thioperamide, while receptor affinity had been reported to be quite insensitive to phenyl ring substitution. It is therefore possible to modulate protein binding of these H3-antagonists, through lipophilicity adjustment, without losing receptor affinity; this finding could help in the design of new compounds with improved brain access.

Central histaminergic system and cognition

The neurotransmitter histamine is contained within neurons clustered in the tuberomammillary nuclei of the hypothalamus. These cells give rise to widespread projections extending through the basal forebrain to the cerebral cortex, as well as to the thalamus and pontomesencephalic tegmentum. These morphological features suggest that the histaminergic system acts as a regulatory center for whole-brain activity. Indeed, this amine is involved in the regulation of numerous physiological functions and behaviors, including learning and memory, as indicated by extensive research reviewed in this paper. Histamine effects on cognition might be explained by the modulation of the cholinergic system. However, interactions of histamine with any transmitter system, and/or a putative intrinsic procognitive role cannot be excluded. Furthermore, although experimental evidence indicates that attention-deficit hyperactivity disorder symptoms arise from impaired dopaminergic and noradrenergic transmission, recent research suggests that histamine is also involved. The possible relevance of histamine in disorders such as age-related memory deficits, Alzheimer's disease and attention-deficit hyperactivity disorder is worth of consideration, and awaits validation with clinical trials that will prove the beneficial effects of histaminergic drugs in the treatment of these diseases.

Synthesis and in vitro pharmacology of a series of new chiral histamine H3-receptor ligands: 2-(R and S)-Amino-3-(1H-imidazol-4(5)-yl)propyl ether derivatives

To investigate stereospecificity and the mechanism of activation of the histamine H3-receptor, a series of 2-(R and S)-amino-3-(1H-imidazol-4(5)-yl)propyl ether derivatives were synthesized. In these compounds, the structures of the well-known antagonist iodoproxyfan and the full agonists R- or S-(alpha)-methylhistamine were combined in one molecule. The obtained "hybrid" molecules were tested for H3-receptor affinity on rat cerebral cortex. Some selected compounds were further screened for H3-receptor functional activity with GTPgamma[35S] autoradiography studies using rat brain tissue sections. The affinity of all the synthesized compounds (-log Ki = 5.9-7.9) was lower than that found for iodoproxyfan or two of its analogues; however, the compounds showed stereospecificity. The S-configuration of the series of 2-amino-3-(1H-imidazol-4(5)-yl)propyl ether derivatives, which resembles the stereochemistry of R-(alpha)-methylhistamine, was more favorable. Incorporation of an amino group in the propyl chain of iodoproxyfan and analogues did not alter the antagonistic behavior for compounds with an aromatic side chain. However, when also the aromatic moiety was replaced by a cyclohexyl group, the compounds behaved as agonists. This indicates that an interaction between the side chain amino group and the H3-receptor protein is involved in H3-receptor activation. The 2-(S)-amino-3-(1H-imidazol-4(5)-yl)propyl cyclohexylmethyl ether (23) has H3-receptor agonistic properties with high affinity for the histamine H3-receptor (-log Ki = 7.9 +/- 0.2) and might serve as a useful tool for further studies concerning drug design and receptor-ligand interactions.

Therapeutic potential of histamine H3 receptor agonists and antagonists

The histamine H3 receptor was discovered 15 years ago, and many potent and selective H3 receptor agonists and antagonists have since been developed. Currently, much attention is being focused on the therapeutic potential of H3 receptor ligands. In this review, Rob Leurs, Patrizio Blandina, Clark Tedford and Henk Timmerman describe the available H3 receptor agonists and antagonists and their effects in a variety of pharmacological models in vitro and in vivo. The possible therapeutic applications of the various compounds are discussed.