Histaminergic modulation of hippocampal acetylcholine release in vivo

4-Oxypiperidine Ethers as Multiple Targeting Ligands at Histamine H3 Receptors and Cholinesterases

This study examines the properties of a novel series of 4-oxypiperidines designed and synthesized as histamine H3R antagonists/inverse agonists based on the structural modification of two lead compounds, viz., ADS003 and ADS009. The products are intended to maintain a high affinity for H3R while simultaneously inhibiting AChE or/and BuChE enzymes. Selected compounds were subjected to hH3R radioligand displacement and gpH3R functional assays. Some of the compounds showed nanomolar affinity. The most promising compound in the naphthalene series was ADS031, which contained a benzyl moiety at position 1 of the piperidine ring and displayed 12.5 nM affinity at the hH3R and the highest inhibitory activity against AChE (IC50 = 1.537 μM). Eight compounds showed over 60% eqBuChE inhibition and hence were qualified for the determination of the IC50 value at eqBuChE; their values ranged from 0.559 to 2.655 μM. Therapy based on a multitarget-directed ligand combining H3R antagonism with additional AChE/BuChE inhibitory properties might improve cognitive functions in multifactorial Alzheimer's disease.

Histamine H3 receptor antagonists - Roles in neurological and endocrine diseases and diabetes mellitus

Human histamine H3 receptor (H3R) was initially described in the brain of rat in 1983 and cloned in 1999. It can be found in the human brain and functions as a regulator of histamine synthesis and release. H3 receptors are predominantly resident in the presynaptic region of neurons containing histamine, where they modulate the synthesis and release of histamine (autoreceptor) or other neurotransmitters such as dopamine, norepinephrine, gamma-aminobutyric acid (GABA), glutamate, acetylcholine and serotonin (all heteroreceptors). The human histamine H3 receptor has twenty isoforms of which eight are functional. H3 receptor expression is seen in the cerebral cortex, neurons of the basal ganglia and hippocampus, which are important for process of cognition, sleep and homoeostatic regulation. In addition, histamine H3R antagonists stimulate insulin release, through inducing the release of acetylcholine and cause significant reduction in total body weight and triglycerides in obese subjects by causing a feeling of satiety in the hypothalamus. The ability of histamine H3R antagonist to reduce diabetes-induced hyperglycaemia is comparable to that of metformin. It is reasonable therefore, to claim that H3 receptor antagonists may play an important role in the therapy of disorders of cognition, the ability to sleep, oxidative stress, inflammation and anomaly of glucose homoeostasis. A large number of H3R antagonists are being developed by pharmaceutical companies and university research centres. As examples of these new drugs, this review will discuss a number of drugs, including the first histamine H3R receptor antagonist produced.

Convergent cross-species pro-cognitive effects of RGH-235, a new potent and selective histamine H3 receptor antagonist/inverse agonist

The histamine H₃ receptor is a favourable target for the treatment of cognitive deficits. Here we report the in vitro and in vivo profile of RGH-235, a new potent, selective, and orally active H₃ receptor antagonist/inverse agonist developed by Gedeon Richter Plc. Radioligand binding and functional assays were used for in vitro profiling. Procognitive efficacy was investigated in rodent cognitive tests, in models of attention deficit hyperactive disorder (ADHD) and in cognitive tests of high translational value (rat touch screen visual discrimination test, primate fixed-foreperiod visual reaction time task). Results were supported by pharmacokinetic studies, neurotransmitter release, sleep EEG and dipsogenia. RGH-235 displayed high affinity to H₃ receptors (K_i = 3.0-9.2 nM, depending on species), without affinity to H₁, H₂ or H₄ receptors and >100 other targets. RGH-235 was an inverse agonist ([³⁵S] GTPγS binding) and antagonist (pERK1/2 ELISA), showing favourable kinetics, inhibition of the imetit-induced dipsogenia and moderate effects on sleep-wake EEG. RGH-235 stimulated neurotransmitter release both in vitro and in vivo. RGH-235 was active in spontaneously hypertensive rats (SHR), generally considered as a model of ADHD, and revealed a robust pro-cognitive profile both in rodent and primate tests (in 0.3-1 mg/kg) and in models of high translational value (e.g. in a rodent touch screen test and in non-human primates). The multiple and convergent procognitive effects of RGH-235 support the view that beneficial cognitive effects can be linked to antagonism/inverse agonism of H₃ receptors.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Antagonism of Histamine H3 receptors Alleviates Pentylenetetrazole-Induced Kindling and Associated Memory Deficits by Mitigating Oxidative Stress, Central Neurotransmitters, and c-Fos Protein Expression in Rats

Histamine H3 receptors (H3Rs) are involved in several neuropsychiatric diseases including epilepsy. Therefore, the effects of H3R antagonist E177 (5 and 10 mg/kg, intraperitoneal (i.p.)) were evaluated on the course of kindling development, kindling-induced memory deficit, oxidative stress levels (glutathione (GSH), malondialdehyde (MDA), catalase (CAT), and superoxide dismutase (SOD)), various brain neurotransmitters (histamine (HA), acetylcholine (ACh), γ-aminobutyric acid (GABA)), and glutamate (GLU), acetylcholine esterase (AChE) activity, and c-Fos protein expression in pentylenetetrazole (PTZ, 40 mg/kg) kindled rats. E177 (5 and 10 mg/kg, i.p.) significantly decreased seizure score, increased step-through latency (STL) time in inhibitory avoidance paradigm, and decreased transfer latency time (TLT) in elevated plus maze (all P < 0.05). Moreover, E177 mitigated oxidative stress by significantly increasing GSH, CAT, and SOD, and decreasing the abnormal level of MDA (all P < 0.05). Furthermore, E177 attenuated elevated levels of hippocampal AChE, GLU, and c-Fos protein expression, whereas the decreased hippocampal levels of HA and ACh were modulated in PTZ-kindled animals (all P < 0.05). The findings suggest the potential of H3R antagonist E177 as adjuvant to antiepileptic drugs with an added advantage of preventing cognitive impairment, highlighting the H3Rs as a potential target for the therapeutic management of epilepsy with accompanied memory deficits.

Multi-target-directed-ligands acting as enzyme inhibitors and receptor ligands

In this review, we present the latest advances in the field of multi-target-directed ligand (MTDL) design for the treatment of various complex pathologies of multifactorial origin. In particular, latest findings in the field of MTDL design targeting both an enzyme and a receptor are presented for different diseases such as Alzheimer's disease (AD), depression, addiction, glaucoma, non-alcoholic steatohepatitis and pain and inflammation. The ethology of the diseases is briefly described, with special emphasis on how the MTDL can evolve into novel therapies that replace the classic pharmacological dogma "one target one disease". Considering the current needs for therapy adherence improvement, it is exposed as from the medicinal chemistry, different molecular scaffolds are studied. With the use of structure activity relationship studies and molecular optimization, new hybrid molecules are generated with improved biological properties acting at two biologically very distinct targets.

Association between Mastication, the Hippocampus, and the HPA Axis: A Comprehensive Review

Mastication is mainly involved in food intake and nutrient digestion with the aid of teeth. Mastication is also important for preserving and promoting general health, including hippocampus-dependent cognition. Both animal and human studies indicate that mastication influences hippocampal functions through the end product of the hypothalamic-pituitary-adrenal (HPA) axis, glucocorticoid (GC). Epidemiologic studies suggest that masticatory dysfunction in aged individuals, such as that resulting from tooth loss and periodontitis, acting as a source of chronic stress, activates the HPA axis, leading to increases in circulating GCs and eventually inducing various physical and psychological diseases, such as cognitive impairment, cardiovascular disorders, and osteoporosis. Recent studies demonstrated that masticatory stimulation or chewing during stressful conditions suppresses the hyperactivity of the HPA axis via GCs and GC receptors within the hippocampus, and ameliorates chronic stress-induced hippocampus-dependent cognitive deficits. Here, we provide a comprehensive overview of current research regarding the association between mastication, the hippocampus, and HPA axis activity. We also discuss several potential molecular mechanisms involved in the interactions between mastication, hippocampal function, and HPA axis activity.

Histamine induces KCNQ channel-dependent gamma oscillations in rat hippocampus via activation of the H1 receptor

Histamine is an aminergic neurotransmitter, which regulates wakefulness, arousal and attention in the central nervous system. Histamine receptors have been the target of efforts to develop pro-cognitive drugs to treat disorders such as Alzheimer's disease and schizophrenia. Cognitive functions including attention are closely associated with gamma oscillations, a rhythmical electrical activity pattern in the 30-80 Hz range, which depends on the synchronized activity of excitatory pyramidal cells and inhibitory fast-spiking interneurons. We set out to explore whether histamine has a role in promoting gamma oscillations in the hippocampus. Using in-situ hybridization we demonstrate that histamine receptor subtypes 1, 2 and 3 are expressed in stratum pyramidale of area CA3 in rats. We show that both pyramidal cells and fast-spiking interneurons depolarize and increase action potential firing in response to histamine in vitro. The activation of histamine receptors generates dose-dependent, transient gamma oscillations in area CA3 of the hippocampus - the locus of the gamma rhythm generator. We also demonstrate that this histamine effect is independent of muscarinic receptors. Using specific antagonists we provide evidence that histamine gamma rhythmogenesis specifically depends on the H1 receptor. Histamine also depolarized both pyramidal cells and fast-spiking interneurons and increased membrane resistance in pyramidal cells. The increased membrane resistance is potentially mediated by the inhibition of potassium channels because application of the KCNQ channel opener ICA110381 abolished the oscillations. Taken together our data demonstrate a novel and physiological mechanism for generating gamma oscillations in hippocampus and suggest a role for KCNQ channels in this cognition-relevant brain activity.

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 dual-acting H3 receptor antagonist and AChE inhibitor UW-MD-71 dose-dependently enhances memory retrieval and reverses dizocilpine-induced memory impairment in rats

Both the histamine H3 receptor (H3R) and acetylcholine esterase (AChE) are involved in the regulation of release and metabolism of acetylcholine and several other central neurotransmitters. Therefore, dual-active H3R antagonists and AChE inhibitors (AChEIs) have shown in several studies to hold promise to treat cognitive disorders like Alzheimer's disease (AD). The novel dual-acting H3R antagonist and AChEI 7-(3-(piperidin-1-yl)propoxy)-1,2,3,9-tetrahydropyrrolo[2,1-b]quinazoline (UW-MD-71) with excellent selectivity profiles over both the three other HRs as well as the AChE's isoenzyme butyrylcholinesterase (BChE) shows high and balanced in vitro affinities at both H3R and AChE with IC50 of 33.9nM and hH3R antagonism with Ki of 76.2nM, respectively. In the present study, the effects of UW-MD-71 (1.25-5mg/kg, i.p.) on acquisition, consolidation, and retrieval in a one-trial inhibitory avoidance task in male rats were investigated applying donepezil (DOZ) and pitolisant (PIT) as reference drugs. Furthermore, the effects of UW-MD-71 on memory deficits induced by the non-competitive N-methyl-d-aspartate (NMDA) antagonist dizocilpine (DIZ) were tested. Our results indicate that administration of UW-MD-71 before the test session dose-dependently increased performance and enhanced procognitive effect on retrieval. However neither pre- nor post-training acute systemic administration of UW-MD-71 facilitated acquisition or consolidation. More importantly, UW-MD-71 (2.5mg/kg, i.p.) ameliorated the DIZ-induced amnesic effects. Furthermore, the procognitive activity of UW-MD-71 in retrieval was completely reversed and partly abrogated in DIZ-induced amnesia when rats were pretreated with the centrally-acting H2R antagonist zolantidine (ZOL), but not with the CNS penetrant H1R antagonist pyrilamine (PYR). These results demonstrate the procognitive effects of UW-MD-71 in two in vivo memory models, and are to our knowledge the first demonstration in vivo that a potent dual-acting H3R antagonist and AChEI is effective in improving retrieval processes in the one-trial inhibitory avoidance task and provide evidence to such compounds to treat cognitive disorders.

Mastication as a Stress-Coping Behavior

Exposure to chronic stress induces various physical and mental effects that may ultimately lead to disease. Stress-related disease has become a global health problem. Mastication (chewing) is an effective behavior for coping with stress, likely due to the alterations chewing causes in the activity of the hypothalamic-pituitary-adrenal axis and autonomic nervous system. Mastication under stressful conditions attenuates stress-induced increases in plasma corticosterone and catecholamines, as well as the expression of stress-related substances, such as neurotrophic factors and nitric oxide. Further, chewing reduces stress-induced changes in central nervous system morphology, especially in the hippocampus and hypothalamus. In rodents, chewing or biting on wooden sticks during exposure to various stressors reduces stress-induced gastric ulcer formation and attenuates spatial cognitive dysfunction, anxiety-like behavior, and bone loss. In humans, some studies demonstrate that chewing gum during exposure to stress decreases plasma and salivary cortisol levels and reduces mental stress, although other studies report no such effect. Here, we discuss the neuronal mechanisms that underline the interactions between masticatory function and stress-coping behaviors in animals and humans.

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.

Masticatory function and cognitive function

Recent studies have suggest that masticatory (chewing) function is useful for maintaining neurocognitive function in the elderly. For example, a reduced ability to masticate, such as that resulting from toothlessness or soft-diet feeding, causes learning and memory deficits in aged animals and pathologic changes in the hippocampus. In addition, occlusal disharmony impairs hippocampal memory processes via chronic stress, and induces similar hippocampal pathology. Chewing, however, rescues stress-induced suppression of long-term potentiation in the hippocampus and the stress-induced impairment of hippocampal-dependent learning. These findings strongly suggest a link between mastication and neurocognitive function.

Endogenous histamine facilitates long-term potentiation in the hippocampus during walking

Long-term potentiation (LTP) in hippocampal CA1 depends on the behavioral state of LTP induction. We hypothesize that histaminergic activity in the septohippocampal system, which is active during walking compared with other behavioral states, is responsible for the behavioral dependence of LTP. Field basal-dendritic EPSPs of CA1 pyramidal cells were recorded in freely behaving rats, and LTP was induced by a single 200 Hz stimulation train (0.5 s duration). Basal-dendritic LTP was facilitated when induced during walking compared with awake immobility (IMM) or rapid-eye-movement sleep. The facilitation of basal-dendritic LTP during walking was abolished by lesion of tuberomammillary nucleus (TMN) neurons with orexin-saporin or by intramedial-septal infusion of the H(1) histaminergic blocker triprolidine but not the H(2) histaminergic blocker cimetidine. Conversely, histamine infusion in the medial septum enhanced the basal-dendritic LTP induced during IMM to a magnitude similar to that induced during walking. Basal-dendritic LTP induced during walking was not further enhanced by intraseptal histamine infusion. Combined with the previous result that behavior-dependent LTP is mediated by cholinergic septohippocampal neurons, we conclude that the facilitation of basal-dendritic LTP in CA1 during walking was mediated by TMN histaminergic afferents acting on H(1) receptors in the medial septum, which may then enhance cholinergic and noncholinergic inputs to the hippocampus.

Occlusion and brain function: mastication as a prevention of cognitive dysfunction

Research in animals and humans has shown that mastication maintains cognitive function in the hippocampus, a brain area important for learning and memory. Reduced mastication, an epidemiological risk factor for the development of dementia in humans, attenuates spatial memory and causes hippocampal neurons to deteriorate morphologically and functionally, especially in aged animals. Active mastication rescues the stress-attenuated hippocampal memory process in animals and attenuates the perception of stress in humans by suppressing endocrinological and autonomic stress responses. Active mastication further improves the performance of sustained cognitive tasks by increasing the activation of the hippocampus and the prefrontal cortex, the brain regions that are essential for cognitive processing. Abnormal mastication caused by experimental occlusal disharmony in animals produces chronic stress, which in turn suppresses spatial learning ability. The negative correlation between mastication and corticosteroids has raised the hypothesis that the suppression of the hypothalamic-pituitary-adrenal (HPA) axis by masticatory stimulation contributes, in part, to preserving cognitive functions associated with mastication. In the present review, we examine research pertaining to the mastication-induced amelioration of deficits in cognitive function, its possible relationship with the HPA axis, and the neuronal mechanisms that may be involved in this process in the hippocampus.

Effects of mepyramine and famotidine on the physostigmine-induced antinociception in the formalin test in rats

In this study, the effects of mepyramine (H1-receptor antagonist), famotidine (H2-receptor antagonist), physostigmine (a cholinesterase inhibitor) and atropine (muscarinic-receptor antagonist) have investigated on the formalin-induced nociception in rats. The effects of mepyramine and famotidine have also examined on nociceptive changes induced by physostigmine and atropine. Nociception was induced by intraplantar injection of formalin (50 microL, 1%) into the right hind paw and the time spent licking and biting of the injected paw, was taken as a measure of pain. Formalin induced a marked biphasic (first phase: 0-5 min and second phase: 15-45 min) pain response. The used drugs did not change the first phase of formalin-induced pain. Subcutaneous injection of physostigmine significantly (p<0.05) suppressed pain. Subcutaneous injection of atropine alone did not change the intensity of pain, but pretreatment with atropine significantly (p<0.05) prevented physostigmine-induced antinociception. Intraperitoneal injections of mepyramine and famotidine significantly (p<0.05) decreased pain response. Mepyramine did not significantly change, but famotidine significantly (p<0.05) prevented analgesic effect of physostigmine on pain. Atropine did not inhibit the antinociceptive effects of both mepyramine and famotidine on formalin-induced nociception. These results indicate that physostigmine through muscarinic cholinergic receptors suppresses the pain induced by formalin. Both H1 and H2 receptor antagonists produce antinociception. Histamine H2, but no H1 antagonists may be involved in physostigmine-induced antinociception.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

The histamine H3 receptor: an attractive target for the treatment of cognitive disorders

The histamine H3 receptor, first described in 1983 as a histamine autoreceptor and later shown to also function as a heteroreceptor that regulates the release of other neurotransmitters, has been the focus of research by numerous laboratories as it represents an attractive drug target for a number of indications including cognition. The purpose of this review is to acquaint the reader with the current understanding of H3 receptor localization and function as a modulator of neurotransmitter release and its effects on cognitive processes, as well as to provide an update on selected H3 antagonists in various states of preclinical and clinical advancement. Blockade of centrally localized H3 receptors by selective H3 receptor antagonists has been shown to enhance the release of neurotransmitters such as histamine, ACh, dopamine and norepinephrine, among others, which play important roles in cognitive processes. The cognitive-enhancing effects of H3 antagonists across multiple cognitive domains in a wide number of preclinical cognition models also bolster confidence in this therapeutic approach for the treatment of attention deficit hyperactivity disorder, Alzheimer's disease and schizophrenia. However, although a number of clinical studies examining the efficacy of H3 receptor antagonists for a variety of cognitive disorders are currently underway, no clinical proof of concept for an H3 receptor antagonist has been reported to date. The discovery of effective H3 antagonists as therapeutic agents for the novel treatment of cognitive disorders will only be accomplished through continued research efforts that further our insights into the functions of the H3 receptor.

Effect of endogenous histamine in the ventral hippocampus on fear memory deficits induced by scopolamine as evaluated by step-through avoidance response in rats

In the present study, the effects of endogenous histamine in the ventral hippocampus on fear memory deficits induced by scopolamine were investigated as evaluated by step-through avoidance response in adult male rats. Bilateral ventral hippocampal injection of scopolamine (i.h., 2, 5 microg/site) significantly produced depressant effects on the active avoidance response in a dose-dependent manner. Histamine H(3)-antagonist clobenpropit (5, 10 microg/site) significantly ameliorated the fear memory deficits induced by scopolamine in a dose-dependent manner. Its procognitive effect was completely antagonized by immepip (10 microg/site), a selective histamine H(3)-agonist. Both histamine H(1)-antagonist pyrilamine and H(2)-antagonist cimetidine, also inhibited the procognitive effects of clobenpropit. Additionally, the procognitive effects of clobenpropit on the fear memory deficits induced by scopolamine were significantly potentiated by intraperitoneal (i.p.) injection of histidine, a precursor of histamine, but markedly reversed by i.h. injection of alpha-fluoromethylhistidine (FMH, 10 microg/site), a selective and potent histidine decarboxylase inhibitor. It is concluded that the increased endogenous histamine release in the ventral hippocampus ameliorates the scopolamine-induced fear memory deficits, and its action is mainly mediated by histamine presynaptic H(3)-receptors and postsynaptic H(1)- and H(2)-receptors.

The effects of vestibular lesions on hippocampal function in rats

Interest in interaction between the vestibular system and the hippocampus was stimulated by evidence that peripheral vestibular lesions could impair performance in learning and memory tasks requiring spatial information processing. By the 1990s, electrophysiological data were emerging that the brainstem vestibular nucleus complex (VNC) and the hippocampus were connected polysynaptically and that hippocampal place cells could respond to vestibular stimulation. The aim of this review is to summarise and critically evaluate research published in the last 5 years that has seen major progress in understanding the effects of vestibular damage on the hippocampus. In addition to new behavioural studies demonstrating that animals with vestibular lesions exhibit impairments in spatial memory tasks, electrophysiological studies have confirmed long-latency, polysynaptic pathways between the VNC and the hippocampus. Peripheral vestibular lesions have been shown to cause long-term changes in place cell function, hippocampal EEG activity and even CA1 field potentials in brain slices maintained in vitro. During the same period, neurochemical investigations have shown that some hippocampal subregions exhibit long-term changes in the expression of neuronal nitric oxide synthase, arginase I and II, and the NR1 and NR2A N-methyl-D-aspartate (NMDA) receptor subunits following peripheral vestibular damage. Despite the progress, a number of important issues remain to be resolved, such as the possible contribution of auditory damage associated with vestibular lesions, to the hippocampal effects observed. Furthermore, although these studies demonstrate that damage to the vestibular system does have a long-term impact on the electrophysiological and neurochemical function of the hippocampus, they do not indicate precisely how vestibular information might be used in hippocampal functions such as developing spatial representations of the environment. Understanding this will require detailed electrical stimulation and lesion studies to elucidate the way in which different kinds of vestibular information are transmitted to various hippocampal subregions.

Effects of histamine H3 receptor antagonists in two models of spatial learning

Despite the well-described attention and short-term memory enhancing effects of H3 receptor antagonists, and evidence to suggest a close relationship between central histaminergic and cholinergic systems, there is a paucity of evidence for a role for H3 receptor blockade in spatial learning. To address this, we investigated two H3 receptor antagonists in a visual discrimination water maze in rats, and in a Barnes circular maze in mice. Thioperamide and ciproxifan significantly attenuated a scopolamine-induced deficit in the water maze task, while only ciproxifan showed a modest attenuation in the Barnes maze. Taken together, these data suggest a role for H3 receptors in spatial learning that appears to be task-dependent.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Histamine innervation and activation of septohippocampal GABAergic neurones: involvement of local ACh release

Recent studies indicate that the histaminergic system, which is critical for wakefulness, also influences learning and memory by interacting with cholinergic systems in the brain. Histamine-containing neurones of the tuberomammillary nucleus densely innervate the cholinergic and GABAergic nucleus of the medial septum/diagonal band of Broca (MSDB) which projects to the hippocampus and sustains hippocampal theta rhythm and associated learning and memory functions. Here we demonstrate that histamine, acting via H(1) and/or H(2) receptor subtypes, utilizes direct and indirect mechanisms to excite septohippocampal GABA-type neurones in a reversible, reproducible and concentration-dependent manner. The indirect mechanism involves local ACh release, is potentiated by acetylcholinesterase inhibitors and blocked by atropine methylbromide and 4-DAMP mustard, an M(3) muscarinic receptor selective antagonist. This indirect effect, presumably, results from a direct histamine-induced activation of septohippocampal cholinergic neurones and a subsequent indirect activation of the septohippocampal GABAergic neurones. In double-immunolabelling studies, histamine fibres were found in the vicinity of both septohippocampal cholinergic and GABAergic cell types. These findings have significance for Alzheimer's disease and other neurodegenerative disorders involving a loss of septohippocampal cholinergic neurones as such a loss would also obtund histamine effects on septohippocampal cholinergic and GABAergic functions and further compromise hippocampal arousal and associated cognitive functions.

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.

Long-term changes in hippocampal n-methyl-D-aspartate receptor subunits following unilateral vestibular damage in rat

Previous studies have indicated that damage to the peripheral vestibular system results in dysfunction of hippocampal place cells and an impairment of spatial learning and memory. The aim of this study was to determine whether lesions of one vestibular labyrinth (unilateral vestibular deafferentation, UVD) result in changes in the expression of the NR1 and NR2A subunits of the N-methyl-D-aspartate (NMDA) receptor, and the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor, in subregions of the rat hippocampus (CA1, CA2/3 and the dentate gyrus) at 10 h or 2 weeks following UVD. Compared with sham surgery controls and anaesthetic controls, the expression of the NR1 subunit was significantly reduced in the ipsilateral CA2/3 region at 2 weeks post-UVD. The expression of the NR2A subunit was also significantly reduced in the ipsilateral CA2/3 and, to a smaller extent, in the contralateral CA2/3 region, at 2 weeks post-UVD. The only other change in NR2A expression was an increase in the ipsilateral CA1 at 10 h post-UVD. No other changes in NR1, NR2A or GluR2 expression were observed in any hippocampal subregion, at any time point, or in cortical tissue at any time point. These results suggest that UVD may result in long-term changes in NMDA receptor subunit expression in the rat hippocampus.

Potentiation by saiboku-to of diazepam-induced decreases in hippocampal and striatal acetylcholine release in rats

Effects of saiboku-to, a traditional oriental herbal medicine, on diazepam-induced changes in cerebral acetylcholine (ACh) were investigated in rat striatum and hippocampus. Diazepam (10 mg/kg, i.p.) increased tissue concentrations of the ACh in both regions. The increase was enhanced in rats subacutely treated with saiboku-to (2.0 g/kg, p.o., once a day) for 7 days. Diazepam also decreased release levels of ACh in both regions. The release levels were further decreased in saiboku-to-treated rats. On the other hand, no significant changes in ACh synthesizing and the hydrolyzing enzyme activities in either brain region were observed in saiboku-to-, diazepam- and combination-treated rats. These results suggest that not only is the diazepam-induced increase in tissue ACh due to the inhibition of ACh release but also that saiboku-to potentiates diazepam-induced inhibition of ACh release.

Endogenous histamine in the medial septum-diagonal band complex increases the release of acetylcholine from the hippocampus: a dual-probe microdialysis study in the freely moving rat

The effects of histaminergic ligands on both ACh spontaneous release from the hippocampus and the expression of c-fos in the medial septum-diagonal band (MSA-DB) of freely moving rats were investigated. Because the majority of cholinergic innervation to the hippocampus is provided by MSA-DB neurons, we used the dual-probe microdialysis technique to apply drugs to the MSA-DB and record the induced effects in the projection area. Perfusion of MSA-DB with high-KCl medium strongly stimulated hippocampal ACh release which, conversely, was significantly reduced by intra-MSA-DB administration of tetrodotoxin. Histamine or the H2 receptor agonist dimaprit, applied directly to the hippocampus, failed to alter ACh release. Conversely, perfusion of MSA-DB with these two compounds increased ACh release from the hippocampus. Also, thioperamide and ciproxifan, two H3 receptor antagonists, administered into MSA-DB, increased the release of hippocampal ACh, whereas R-alpha-methylhistamine, an H3 receptor agonist, produced the opposite effect. The blockade of MSA-DB H2 receptors, caused by local perfusion with the H2 receptor antagonist cimetidine, moderated the spontaneous release of hippocampal ACh and antagonized the facilitation produced by H3 receptor antagonists. Triprolidine, an H1 receptor antagonist, was without effect. Moreover, cells expressing c-fos immunoreactivity were significantly more numerous in ciproxifan- or thioperamide-treated rats than in controls, although no colocalization of anti-c-fos and anti-ChAT immunoreactivity was observed. These results indicate a role for endogenous histamine in modulating the cholinergic tone in the hippocampus.

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.

Modification of acetylcholine release by nociceptin in conscious rat striatum

Nociceptin (NOC), an endogenous ligand for the orphan opioid receptor ORL1 (ORL1), has recently been recognized as a neuropeptide. We used brain microdialysis and on-line high performance liquid chromatography (HPLC) to examine the effect of NOC on the basal outflow of acetylcholine (ACh) in the freely moving rat striatum in vivo. ACh release was reduced by nociceptin at a concentration of 10(-5) M to 79% of control release. This effect of NOC was attenuated by [Phe1Psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2 (PhePsi), suggesting that NOC activates the ORL1 receptor and (PhePsi) acts as an antagonist on ORL1 in rat striatum in vivo. These findings indicate that NOC may act as a neuropeptide which inhibits ACh release in the striatum via ORL1.

Noradrenaline and serotonin levels in the guinea pig hippocampus following unilateral vestibular deafferentation

Recent evidence indicates that the hippocampus uses input from the vestibular system in order to accomplish its spatial computational functions. At present, there are few data on the neurochemical basis of the interactions between the vestibular system and the hippocampus. The aim of this study was to determine levels of noradrenaline (NA), serotonin (5-HT) and the 5-HT metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in the CA1, CA2 and dentate gyrus (DG) regions of the ipsilateral and contralateral hippocampi, at 10 h following deafferentation of the peripheral vestibular nerve (UVD) in guinea pig, using high-performance liquid chromatography (HPLC) with electrochemical detection (ECD). There were no significant differences in NA levels in the ipsilateral or contralateral CA1 following UVD. However, there was a significant increase in NA levels in the contralateral CA2 following UVD, compared to both the sham and intact anesthetic control conditions (p<0.05). No such change was seen in the ipsilateral CA2. In the contralateral DG, there was a significant increase in NA levels in both the UVD and sham conditions, compared to the intact anesthetic controls (p<0.05). No significant changes in 5-HT or 5-HIAA levels were seen in the ipsilateral or contralateral CA1, CA2 or DG following UVD. This study provides the first evidence that UVD may cause an increase in NA levels in the CA2 region of the contralateral hippocampus.

Pharmacological characterization of nicotine-induced acetylcholine release in the rat hippocampus in vivo: evidence for a permissive dopamine synapse

In this study, the mechanism of nicotine-induced hippocampal acetylcholine (ACh) release in awake, freely moving rats was examined using in vivo microdialysis. Systemic administration of nicotine (0.4 mg kg(-1), s.c.) increased the levels of ACh in hippocampal dialysates. The nicotine-induced hippocampal ACh release was sensitive to the pretreatment of neuronal nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine (3.0 mg kg(-1), s.c.) and dihydro-beta-erythrodine (DHbetaE; 4.0 mg kg(-1), s.c.) as well as systemic administration of the dopamine (DA) D1 receptor antagonist SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzaz epine; 0.3 mg kg(-1), s.c.). Local perfusion of mecamylamine (100 microM), DHbetaE (100 microM) or SCH-23390 (10 microM) through microdialysis probe did not increase basal hippocampal ACh release. Hippocampal ACh release elicited by systemic administration of nicotine (0.4 mg kg(-1), s.c.) was antagonized by local perfusion of SCH-23390 (10 microM), but not by MEC (100 microM) or DHbetaE (100 microM). Direct perfusion of nicotine (1 mM, but not 0.1 mM) increased hippocampal ACh levels; however, this effect was relatively insensitive to blockade by co-perfusion of either mecamylamine (100 microM) or SCH-23390 (10 microM). These results suggest that nicotine-induced hippocampal ACh release occurs by two distinct mechanisms: (1) activation of nAChRs outside the hippocampus leading to DA release and subsequent ACh release involving a permissive DA synapse, and (2) direct action of nicotine within the hippocampus leading to ACh release via non-DA-ergic mechanism.

Thioperamide, a histamine H3 receptor antagonist, powerfully suppresses peptide YY-induced food intake in rats

BACKGROUND

Whether or not peptide YY (PYY)-induced hyperphagia is modified by the histaminergic system in the brain is not yet known.

METHODS

We investigated the effect on feeding of intracerebroventricular (ICV) administration of a specific histamine H3 receptor antagonist prior to ICV administration of PYY in rats.

RESULTS

PYY (1, 3, and 10 micrograms/10 microL) strongly induced feeding behavior in a dose-dependent manner in sated rats. The 4-hour food intake induced by 3 micrograms/10 microL of PYY was equal to that induced by a 16-hour fast. The ICV administration of thioperamide (40.8, 122.4, and 408.5 micrograms/10 microL) did not suppress the 4-hour food intake induced by 16-hour fasting; however, thioperamide produced dose-dependent and strong inhibition of hyperphagia induced by a 3-microgram dose of PYY.

CONCLUSIONS

These results suggest that the effect of PYY on appetite is different than that induced by fasting and may involve a histaminergic mechanism.

Temporal bone surgery causes reduced nitric oxide synthase activity in the ipsilateral guinea pig hippocampus

There is a lack of data on the neurochemical basis for the interaction between the vestibular system and the hippocampus. The aim of the present study was to determine levels of nitric oxide synthase (NOS) activity in the ipsilateral and contralateral hippocampi at 10 h following unilateral deafferentation of the peripheral vestibular nerve (UVD) in guinea pig, using a radio-enzymatic technique. The levels of NOS activity were similar in the contralateral hippocampus following either a sham temporal bone operation or the UVD. However, NOS activity was significantly lower in the ipsilateral hippocampus following both the UVD and the sham temporal bone surgery (P<0.05 for both comparisons). These results suggest that even sham temporal bone surgery results in a reduction in NOS activity in the ipsilateral hippocampus.

In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application

We investigated the effect of the new potent and selective histamine H3 receptor agonist, immepip, and the histamine H3 receptor antagonist, clobenpropit, on in vivo neuronal histamine release from the anterior hypothalamic area of urethane-anesthetized rats, using microdialysis. Intrahypothalamic perfusion with immepip at concentrations of 1 and 10 nM reduced histamine release to 75% and 35% of its basal level, respectively. Peripheral injection of immepip (5 mg/kg) caused a sustained decrease in histamine release of 50%. Clobenpropit potently increased histamine release after intrahypothalamic perfusion. The maximal increase in histamine release was 2-fold, observed at a concentration of 10 nM clobenpropit. Peripheral injection of clobenpropit (5-15 mg/kg) increased histamine release to about 150% of the basal value. A more marked increase in histamine release was found after injection of the histamine H3 receptor antagonist, thioperamide (5 mg/kg). In conclusion, intrahypothalamic perfusion of the histamine H3 receptor agonist, immepip and the histamine H3 receptor antagonist, clobenpropit, potently and oppositely modulated in vivo histamine release from the anterior hypothalamic area. The decreased histamine release after peripheral injection of immepip indicates that this novel agonist readily crosses the blood-brain barrier, making it a potential candidate for in vivo histamine H3 receptor studies. The differential increase in histamine release after peripheral injection of clobenpropit and thioperamide is discussed.

Histamine H3 receptor-mediated inhibition of excitatory synaptic transmission in the rat dentate gyrus in vivo

We investigated the effects of histamine H3-receptor ligands on hippocampal synaptic transmission by using anesthetized rats in vivo. The medial perforant path was stimulated, and the population excitatory postsynaptic potential (pEPSP) and population spike were recorded from the granule cell layer of the dentate gyrus. Intracerebroventricular injection of the H3-receptor agonist (R)-alpha-methylhistamine decreased both the pEPSP and population spike, while H3-receptor antagonists, clobenpropit and thioperamide, increased both the pEPSP and population spike. These results suggest that the histaminergic system plays a role in inhibition of hippocampal synaptic excitation via the H3 receptor.

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.

Pharmacological characterisation of the histamine H3 receptor in the rat hippocampus

The purpose of this report was to pharmacologically characterise the histamine H3 in the rat hippocampus using radioligand binding studies with the H3 receptor antagonist [125I]iodophenpropit and the H3 receptor mediated inhibition of [3H]noradrenaline release. A dissociation constant of 0.33 nM and a maximal number of binding sites of 125 fmol/mg protein were found for [125I]iodophenpropit. Competition studies showed stereoselectivity for the (R) and (S) enantiomers of alpha-methylhistamine and 10 microM of GTPgammaS shifted the curve of (R)-alpha-methylhistamine rightwards. Up to 1 microM, (R)-alpha-methylhistamine displaced only 30% whereas the tested H3-antagonists displaced 50-60% of the total [125I]iodophenpropit bound. This indicates the presence of an additional non-H3 receptor binding site(s) for [125I]iodophenpropit in the rat hippocampus. This secondary site shows low affinity for H3 agonists, but high affinity for the tested H3 antagonists. Electrically evoked [3H]acetylcholine release was shown in slices of rat hippocampus. No H3 receptor modulation of [3H]acetylcholine release from hippocampal slices was detectable. However, H3 receptor activation inhibited 42% of the electrically-evoked [3H]noradrenaline release in rat hippocampal slices. The inhibition of [3H]noradrenaline release was effectively antagonized by the H3 antagonists thioperamide and burimamide. We describe the pharmacological identification of the histamine H3 receptor in the rat hippocampus and its similarities and differences from the cortical H3 receptor. These studies enable us to investigate changes in density and functionality of the hippocampal H3 receptor under (patho)physiological conditions.