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

Electroacupuncture ameliorates gastrointestinal dysfunction by modulating DMV cholinergic efferent signals to drive the vagus nerve in p-MCAO rats

Background

The use of proton pump inhibitors in the acute phase of cerebral infarction may lead to adverse long-term outcomes, this study aims to explore the potential of electroacupuncture (EA) in replacing omeprazole in exerting post-stroke gastrointestinal protection.

Methods

A permanent middle cerebral artery infarction model was established using the modified Longa thread occlusion technique. Gastrointestinal motility, gastrointestinal mucosal damage, cerebral infarct volume, and alterations in choline acetyltransferase (ChAT)-positive neurons within the dorsal motor nucleus of the vagus nerve (DMV) were assessed after 7 days of EA at Zusanli (ST36) or omeprazole intervention. To evaluate the role of the vagal nerve in mitigating post-stroke gastrointestinal dysfunction, we employed subdiaphragmatic vagotomy and the ChAT-specific inhibitor α-NETA. Additionally, we utilized methyllycaconitine (MLA), a selective inhibitor of the α7-type nicotinic acetylcholine receptor (α7nAChR), and PNU282987, an agonist, to identify the target of EA.

Results

EA restored ChAT neurons lost in the DMV, activated the vagus nerve and conferred cerebroprotection while ameliorating gastrointestinal mucosal injury and gastrointestinal motility disorders. In addition, following the administration of the α7nAChR antagonist, the attenuation of gastric mucosal injury and inflammatory factors induced by EA was hindered, although gastrointestinal motility still exhibited improvement.

Conclusion

EA at ST36 promotes the restoration of cholinergic signaling in the DMV of stroke-afflicted rats, and its excitation of the vagal nerve inhibits gastrointestinal inflammation after stroke via α7nAChR, while improvement in gastrointestinal motility could be mediated by other acetylcholine receptors.

Dose-related effects of ciproxifan on brain tissue in rats with cerebral ischemia-reperfusion

PURPOSE

Cerebral ischemia is the result of decreased or interrupted blood flow to the brain. It is the third leading cause of death after cardiovascular disease and cancer. Cerebral ischemia is reversible or irreversible in neurons in the affected area, and subsequent free radical damage can be exacerbated if reperfusion occurs. Ciproxifan is used to study the involvement of histaminergic neurons in different phases such as wakefulness and cognition. We wanted to find out whether ciproxifan has a protective effect on the brain of rats with cerebral ischemia-reperfusion injury.

MATERIALS AND METHODS

A total of 64 adult rats (32 male and 32 female) were used for the experiment. Eight cages were formed with randomly selected rats. No substance was administered to the rats in Group 1 and no surgical procedure was performed. The cerebral ischemia-reperfusion model (clamping of the left common carotid artery for 15 min followed by reperfusion for 24 h) was applied to rats in Group 2, Group 3, and Group 4 after 7 days/single dose of saline and ciproxifan (10 mg/kg, 30 mg/kg). After that, the activitymeter, forced swim test (FST), and Morris water maze (MWM) were performed on all animals.

RESULTS

Rats treated with ciproxifan exhibit neurons and glial cells with histologic structures similar to those of the control group, and interestingly, these differences became more pronounced with increasing dose. Rats administered ciproxifan improved motor coordination, decreased total distance behavior, and improved learning ability. However, when the groups were compared by sex, no significant difference was found in the parameters.

CONCLUSION

Thus, we could conclude that ciproxifan has a protective effect on the brain to a certain extent, regardless of the dose.

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.

Guanidine Derivatives: How Simple Structural Modification of Histamine H3R Antagonists Has Led to the Discovery of Potent Muscarinic M2R/M4R Antagonists

This article describes the discovery of novel potent muscarinic receptor antagonists identified during a search for more active histamine H₃ receptor (H₃R) ligands. The idea was to replace the flexible seven methylene linker with a semirigid 1,4-cyclohexylene or p-phenylene substituted group of the previously described histamine H₃R antagonists ADS1017 and ADS1020. These simple structural modifications of the histamine H₃R antagonist led to the emergence of additional pharmacological effects, some of which unexpectedly showed strong antagonist potency at muscarinic receptors. This paper reports the routes of synthesis and pharmacological characterization of guanidine derivatives, a novel chemotype of muscarinic receptor antagonists binding to the human muscarinic M₂ and M₄ receptors (hM₂R and hM₄R, respectively) in nanomolar concentration ranges. The affinities of the newly synthesized ADS10227 (1-{4-{4-{[4-(phenoxymethyl)cyclohexyl]methyl}piperazin-1-yl}but-1-yl}-1-(benzyl)guanidine) at hM₂R and hM₄R were 2.8 nM and 5.1 nM, respectively.

Histamine H3 receptor and cholinesterases as synergistic targets for cognitive decline: Strategies to the rational design of multitarget ligands

The role of histamine and acetylcholine in cognitive functions suggests that compounds able to increase both histaminergic and cholinergic neurotransmissions in the brain should be considered as promising therapeutic options. For this purpose, dual inhibitors of histamine H₃ receptors (H₃ R) and cholinesterases (ChEs) have been designed and assessed. In this context, this paper reviews the strategies used to obtain dual H₃ R/ChEs ligands using multitarget design approaches. Hybrid compounds designed by linking tacrine or flavonoid motifs to H₃ R antagonists were obtained with high affinity for both targets, and compounds designed by merging the H₃ R antagonist pharmacophore with known anticholinesterase molecules were also reported. These reports strongly suggest that key modifications in the lipophilic region (including a second basic group) seem to be a strategy to reach novel compounds, allied with longer linker groups to a basic region. Some compounds have already demonstrated efficacy in memory models, although the pharmacokinetic and toxicity profile should be considered when designing further compounds. In conclusion, the key features to be considered when designing novel H₃ R/ChEs inhibitors with improved pharmacological profile were herein summarized.

Neuronal histamine and the memory of emotionally salient events

In this review, we describe the experimental paradigms used in preclinical studies to unravel the histaminergic brain circuits that modulate the formation and retrieval of memories associated with aversive events. Emotionally arousing events, especially bad ones, are remembered more accurately, clearly and for longer periods of time than neutral ones. Maladaptive elaborations of these memories may eventually constitute the basis of psychiatric disorders such as generalized anxiety, obsessive-compulsive disorders and post-traumatic stress disorder. A better understanding of the role of the histaminergic system in learning and memory has not only a theoretical significance but also a translational value. Ligands of histamine receptors are among the most used drugs worldwide; hence, understanding the impact of these compounds on learning and memory may help improve their pharmacological profile and unravel unexplored therapeutic applications. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

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

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

Histamine modulates spinal motoneurons and locomotor circuits

Spinal motoneurons and locomotor networks are regulated by monoamines, among which, the contribution of histamine has yet to be fully addressed. The present study investigates histaminergic regulation of spinal activity, combining intra- and extracellular electrophysiological recordings from neonatal rat spinal cord in vitro preparations. Histamine dose-dependently and reversibly generated motoneuron depolarization and action potential firing. Histamine (20 µM) halved the area of dorsal root reflexes and always depolarized motoneurons. The majority of cells showed a transitory repolarization, while 37% showed a sustained depolarization maintained with intense firing. Extracellularly, histamine depolarized ventral roots (VRs), regardless of blockage of ionotropic glutamate receptors. Initial, transient glutamate-mediated bursting was synchronous among VRs, with some bouts of locomotor activity in a subgroup of preparations. After washout, the amplitude of spontaneous tonic discharges increased. No desensitization or tachyphylaxis appeared after long perfusion or serial applications of histamine. On the other hand, histamine induced single motoneuron and VR depolarization, even in the presence of tetrodotoxin (TTX). During chemically induced fictive locomotion (FL), histamine depolarized VRs. Histamine dose-dependently increased rhythm periodicity and reduced cycle amplitude until near suppression. This study demonstrates that histamine induces direct motoneuron membrane depolarization and modulation of locomotor output, indicating new potential targets for locomotor neurorehabilitation.

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 Antagonist Prevents Memory Deficits and Synaptic Plasticity Disruption Following Isoflurane Exposure

BACKGROUND

Exposure to pharmacological concentration of inhaled anesthetics such as isoflurane can cause short- or long-term cognitive impairments in preclinical studies. The selective antagonists of the histamine H3 receptors are considered as a promising group of novel therapeutic agents for the treatment of cognitive disorders. In this study, we investigated whether ciproxifan, a nonimidazole antagonist of H3 histamine receptors, could overcome the functional and electrophysiological sequela associated with isoflurane anesthesia.

METHODS

Adult male Sprague Dawley rats were exposed to 1.4% isoflurane or vehicle gas for 2 h. The memory tests (novel object recognition and passive avoidance) as well as in vivo hippocampal excitatory synaptic potentials were recorded 24 h postanesthesia. Locomotor activity, anxiety, and nociception 24 h after isoflurane were also examined. The drugs (ciproxifan 3 mg/kg or saline) were intraperitoneally injected 30 min prior to the behavioral tests or long-term potentiation induction.

RESULTS

Animals that were previously (24 h) exposed to 1.4% isoflurane for 2 h displayed no preference for novel objects and had impaired retention of a passive avoidance response at 1 h after sample phase. Treating isoflurane-exposed rats with ciproxifan significantly improved the memory performance, as evidenced by an increased discrimination ratio in objects recognition and prolonged retention time in passive avoidance test. Accordingly, hippocampus long-term potentiation was reduced in animals that received isoflurane, while administration of ciproxifan completely abolished the effect of isoflurane exposure on synaptic plasticity. Neither isoflurane nor ciproxifan altered motor performance, anxiety, and nociceptive responses.

CONCLUSION

These results suggest that H3R in the CNS, probably in the hippocampus, may serve as therapeutic target for improvement of anesthesia-associated cognitive deficits.

Ligand autoradiographical quantification of histamine H3 receptor in human dementia with Lewy bodies

Dementia with Lewy bodies (DLB) is a serious age-dependent human neurodegenerative disease, with multiple debilitating symptoms, including dementia, psychosis and significant motor deficits, but with little or no effective treatments. This comparative ligand autoradiographical study has quantified histamine H₃ receptors (H₃R) in a series of major cortical and basal ganglia structures in human DLB and Alzheimer’s (AD) post-mortem cases using the highly selective radioligand, [³H] GSK189254.

In the main, the levels of H₃ receptor were largely preserved in DLB cases when compared with aged-matched controls. However, we provide new evidence showing variable levels in the globus pallidus, and, moreover, raised levels of Pallidum H₃ correlated with positive psychotic symptoms, in particular delusions and visual hallucinations, but not symptoms associated with depression. Furthermore, no correlation was detected for H₃ receptor levels to MMSE or IUPRS symptom severity.

This study suggests that H₃R antagonists have scope for treating the psychotic symptomologies in DLB and other human brain disorders.

The Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H₁-H₄) identified as mediators of the biologic effects of histamine, the H₃ receptor (H₃R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H₃Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H₃R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H₃R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H₃R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.

Donepezil, an acetylcholine esterase inhibitor, and ABT-239, a histamine H3 receptor antagonist/inverse agonist, require the integrity of brain histamine system to exert biochemical and procognitive effects in the mouse

Histaminergic H3 receptors (H3R) antagonists enhance cognition in preclinical models and modulate neurotransmission, in particular acetylcholine (ACh) release in the cortex and hippocampus, two brain areas involved in memory processing. The cognitive deficits seen in aging and Alzheimer's disease have been associated with brain cholinergic deficits. Donepezil is one of the acetylcholinesterase (AChE) inhibitor approved for use across the full spectrum of these cognitive disorders. We addressed the question if H3R antagonists and donepezil require an intact histamine neuronal system to exert their procognitive effects. The effect of the H3R antagonist ABT-239 and donepezil were evaluated in the object recognition test (ORT), and on the level of glycogen synthase kinase 3 beta (GSK-3β) phosphorylation in normal and histamine-depleted mice. Systemic administration of ABT-239 or donepezil ameliorated the cognitive performance in the ORT. However, these compounds were ineffective in either genetically (histidine decarboxylase knock-out, HDC-KO) or pharmacologically, by means of intracerebroventricular (i.c.v.) injections of the HDC irreversible inhibitor a-fluoromethylhistidine (a-FMHis), histamine-deficient mice. Western blot analysis revealed that ABT-239 or donepezil systemic treatments increased GSK-3β phosphorylation in cortical and hippocampal homogenates of normal, but not of histamine-depleted mice. Furthermore, administration of the PI3K inhibitor LY294002 that blocks GSK-3β phosphorylation, prevented the procognitive effects of both drugs in normal mice. Our results indicate that both donepezil and ABT-239 require the integrity of the brain histaminergic system to exert their procognitive effects and strongly suggest that impairments of PI3K/AKT/GSK-3β intracellular pathway activation is responsible for the inefficacy of both drugs in histamine-deficient animals.

Ventral hippocampal histamine increases the frequency of evoked theta rhythm but produces anxiolytic-like effects in the elevated plus maze

The neurobiological underpinnings of anxiety are of paramount importance to the development of effective therapeutic treatments. To date, there is considerable pharmacological evidence suggesting that the suppression of hippocampal theta frequency is a robust and predictive assay of anxiolytic drug action. Recently, this idea has been challenged using histamine (2-(4-imidazolyl)ethanamine), an endogenous neurotransmitter involved in a number of brain and behavioral functions. Here, we systematically evaluate the effects of dorsal and ventral hippocampal histamine infusions on evoked theta frequency and behavioral anxiety. Given the complex pharmacological profile of histamine and its receptors in the hippocampus, we reasoned that local intra-hippocampal infusions would be a powerful test of the theta suppression model. While dorsal hippocampal infusions of histamine produced neither significant changes in anxious-like behavior in the elevated plus maze nor changes of evoked theta, ventral infusions of histamine produced potent behavioral anxiolysis which corresponded to an increase, and not a decrease, in evoked theta frequency. As a positive neurophysiological control, we demonstrated that diazepam, a proven anxiolytic drug, decreased the frequency of hippocampal theta following both dorsal and ventral hippocampal infusions. Our results further challenge the hippocampal theta frequency suppression model as a measure of anxiolytic drug action. This article is part of the Special Issue entitled 'Histamine Receptors'.

Theta-rhythmic drive between medial septum and hippocampus in slow-wave sleep and microarousal: a Granger causality analysis

Medial septum (MS) plays a critical role in controlling the electrical activity of the hippocampus (HIPP). In particular, theta-rhythmic burst firing of MS neurons is thought to drive lasting HIPP theta oscillations in rats during waking motor activity and REM sleep. Less is known about MS-HIPP interactions in nontheta states such as non-REM sleep, in which HIPP theta oscillations are absent but theta-rhythmic burst firing in subsets of MS neurons is preserved. The present study used Granger causality (GC) to examine the interaction patterns between MS and HIPP in slow-wave sleep (SWS, a nontheta state) and during its short interruptions called microarousals (a transient theta state). We found that during SWS, while GC revealed a unidirectional MS→HIPP influence over a wide frequency band (2-12 Hz, maximum: ∼8 Hz), there was no theta peak in the hippocampal power spectra, indicating a lack of theta activity in HIPP. In contrast, during microarousals, theta peaks were seen in both MS and HIPP power spectra and were accompanied by bidirectional GC with MS→HIPP and HIPP→MS theta drives being of equal magnitude. Thus GC in a nontheta state (SWS) vs. a theta state (microarousal) primarily differed in the level of HIPP→MS. The present findings suggest a modification of our understanding of the role of MS as the theta generator in two regards. First, a MS→HIPP theta drive does not necessarily induce theta field oscillations in the hippocampus, as found in SWS. Second, HIPP theta oscillations entail bidirectional theta-rhythmic interactions between MS and HIPP.

Superficial Layer-Specific Histaminergic Modulation of Medial Entorhinal Cortex Required for Spatial Learning

The medial entorhinal cortex (MEC) plays a crucial role in spatial learning and memory. Whereas the MEC receives a dense histaminergic innervation from the tuberomamillary nucleus of the hypothalamus, the functions of histamine in this brain region remain unclear. Here, we show that histamine acts via H1Rs to directly depolarize the principal neurons in the superficial, but not deep, layers of the MEC when recording at somata. Moreover, histamine decreases the spontaneous GABA, but not glutamate, release onto principal neurons in the superficial layers by acting at presynaptic H3Rs without effect on synaptic release in the deep layers. Histamine-induced depolarization is mediated via inhibition of Kir channels and requires the activation of protein kinase C, whereas the inhibition of spontaneous GABA release by histamine depends on voltage-gated Ca(2+) channels and extracellular Ca(2+). Furthermore, microinjection of the H1R or H3R, but not H2R, antagonist respectively into the superficial, but not deep, layers of MEC impairs rat spatial learning as assessed by water maze tasks but does not affect the motor function and exploratory activity in an open field. Together, our study indicates that histamine plays an essential role in spatial learning by selectively regulating neuronal excitability and synaptic transmission in the superficial layers of the MEC.

Differential effects of the histamine H(3) receptor agonist methimepip on dentate granule cell excitability, paired-pulse plasticity and long-term potentiation in prenatal alcohol-exposed rats

BACKGROUND

We previously reported that prenatal alcohol-induced deficits in dentate gyrus (DG) long-term potentiation (LTP) are ameliorated by the histamine H3 receptor inverse agonist ABT-239. ABT-239 did not enhance LTP in control rats, suggesting that the possibility of a heightened H3 receptor-mediated inhibition of LTP in prenatal alcohol-exposed (PAE) offspring.

METHODS

To further investigate this mechanism, we examined the effect of methimepip, a selective histamine H3 receptor agonist, on DG granule cell responses and LTP in saccharin control and PAE rats. Long-Evans rat dams voluntarily consumed either a 0 or 5% ethanol solution 4 hours each day throughout gestation. Adult male offspring from these dams were anesthetized with urethane and electrodes implanted into the entorhinal cortical perforant path and the DG.

RESULTS

In control offspring, methimepip reduced the coupling of fast excitatory postsynaptic field potentials to population spikes (E-S coupling), the probability of glutamate release, as measured by paired-pulse ratio (PPR) and diminished DG LTP. Similar reductions in E-S coupling and LTP were observed in saline-treated PAE offspring. In contrast to the control group, methimepip did not exacerbate PAE-induced reductions in E-S coupling or LTP.

CONCLUSIONS

While the effects of methimepip in control offspring were consistent with speculation of a PAE-induced enhancement of H3 receptor-mediated inhibition of E-S coupling and LTP, the absence of an added effect of methimepip in PAE offspring could indicate either an inability to further inhibit these responses with methimepip in PAE rats or the presence of more complex regulatory neural interactions with in vivo recordings in PAE rats. Follow-up studies of H3 receptor-mediated responses in DG slice preparations are under way to provide clearer insights into the role of the H3 receptor regulation of excitatory transmission in PAE rats.

Histaminergic modulation of cholinergic release from the nucleus basalis magnocellularis into insular cortex during taste aversive memory formation

The ability of acetylcholine (ACh) to alter specific functional properties of the cortex endows the cholinergic system with an important modulatory role in memory formation. For example, an increase in ACh release occurs during novel stimulus processing, indicating that ACh activity is critical during early stages of memory processing. During novel taste presentation, there is an increase in ACh release in the insular cortex (IC), a major structure for taste memory recognition. There is extensive evidence implicating the cholinergic efferents of the nucleus basalis magnocellularis (NBM) in cortical activity changes during learning processes, and new evidence suggests that the histaminergic system may interact with the cholinergic system in important ways. However, there is little information as to whether changes in cholinergic activity in the IC are modulated during taste memory formation. Therefore, in the present study, we evaluated the influence of two histamine receptor subtypes, H1 in the NBM and H3 in the IC, on ACh release in the IC during conditioned taste aversion (CTA). Injection of the H3 receptor agonist R-α-methylhistamine (RAMH) into the IC or of the H1 receptor antagonist pyrilamine into the NBM during CTA training impaired subsequent CTA memory, and simultaneously resulted in a reduction of ACh release in the IC. This study demonstrated that basal and cortical cholinergic pathways are finely tuned by histaminergic activity during CTA, since dual actions of histamine receptor subtypes on ACh modulation release each have a significant impact during taste memory formation.

Single dose of H3 receptor antagonist--ciproxifan--abolishes negative effects of chronic stress on cognitive processes in rats

RATIONALE

The role of histamine neurons in stress evoked cognitive impairments remains unclear. Previous research has indicated that the blockade of H(3)-type histamine receptors may improve attention and memory in naïve rodents.

OBJECTIVES

The purpose of this study was to determine if ciproxifan, (cyclopropyl-(4-(3-1H-imidazol-4-yl) propyloxy) phenyl) ketone, an H(3) receptor antagonist, could alleviate cognitive deficits observed in chronically stressed rats.

METHODS

Specifically, we attempted to characterize the preventive action of single dose of ciproxifan (3 mg/kg, i.p.) against an impairment caused by chronic restraint stress (2 h daily for 21 days) on recognition memory tested in an object recognition task and on the long-term memory tested in a passive avoidance test.

RESULTS

We found that administration of ciproxifan potently prevented deleterious effects of chronic restraint stress, when administered prior to learning, or immediately after learning, or before retrieval on both the recognition (p<0.001) and the passive avoidance behavior (p<0.001).

CONCLUSIONS

These data support the idea that modulation of H(3) receptors represents a novel and viable therapeutic strategy in the treatment of stress evoked cognitive impairments.

Involvement of the dorsal hippocampal GABA-A receptors in histamine-induced facilitation of memory in the Morris water maze

Several types of learning and memory processes are regulated by the hippocampus which is an important subcortical structure in the mammalians' brain. Previous investigations have shown that different receptor systems in the CA1 region of hippocampus are involved in learning and memory functions. Investigating the possible influence of dorsal hippocampal GABA-A receptors on histamine-induced spatial facilitation in adult male Wistar rats was the focus of the current study. Rats were bilaterally implanted with dorsal hippocampal (CA1) cannulae, recovered from surgery and then trained in Morris water maze (MWM) for 4 consecutive days. A block of four trials was given each day. All drugs were injected into CA1 regions, 5min before training. Pre-training intra-CA1 microinjection of muscimol, a GABA-A receptor agonist, at the dose of 0.01 or 0.02μg/rat, increased the traveled distance or the escape latency and traveled distance to the hidden platform, respectively, indicating a water maze spatial acquisition impairment. Intra-CA1 administration of bicuculline, a GABA-A receptor antagonist however, significantly decreased the escape latency and traveled distance to the hidden platform, suggesting a spatial learning facilitation. On the other hand, pre-training intra-CA1 microinjection of the subthreshold dose of muscimol plus different doses of histamine (0.025, 0.05 and 0.1μg/rat) did not alter the histamine response. Meanwhile, the co-administration of the ineffective dose of bicuculline together with histamine potentiated the spatial learning. Moreover, bilateral infusion of histamine (0.025, 0.05 and 0.1μg/rat) by itself, facilitated the spatial learning. Notably, the drug injections had no effect on swimming speed during the MWM training sessions. Our results suggest that the dorsal hippocampal (CA1) GABA-A mechanism(s) may influence the histamine-induced facilitation of spatial acquisition.

The 5-hydroxytryptamine4 receptor agonists prucalopride and PRX-03140 increase acetylcholine and histamine levels in the rat prefrontal cortex and the power of stimulated hippocampal θ oscillations

5-Hydroxytryptamine (5-HT)(4) receptor agonists reportedly stimulate brain acetylcholine (ACh) release, a property that might provide a new pharmacological approach for treating cognitive deficits associated with Alzheimer's disease. The purpose of this study was to compare the binding affinities, functional activities, and effects on neuropharmacological responses associated with cognition of two highly selective 5-HT(4) receptor agonists, prucalopride and 6,7-dihydro-4-hydroxy-7-isopropyl-6-oxo-N-[3-(piperidin-1-yl)propyl]thieno[2,3-b]pyridine-5-carboxamide (PRX-03140). In vitro, prucalopride and PRX-03140 bound to native rat brain 5-HT(4) receptors with K(i) values of 30 nM and 110 nM, respectively, and increased cAMP production in human embryonic kidney-293 cells expressing recombinant rat 5-HT(4) receptors. In vivo receptor occupancy studies established that prucalopride and PRX-03140 were able to penetrate the brain and bound to 5-HT(4) receptors in rat brain, achieving 50% receptor occupancy at free brain exposures of 330 nM and 130 nM, respectively. Rat microdialysis studies revealed that prucalopride maximally increased ACh and histamine levels in the prefrontal cortex at 5 and 10 mg/kg, whereas PRX-03140 significantly increased cortical histamine levels at 50 mg/kg, failing to affect ACh release at doses lower than 150 mg/kg. In combination studies, donepezil-induced increases in cortical ACh levels were potentiated by prucalopride and PRX-03140. Electrophysiological studies in rats demonstrated that both compounds increased the power of brainstem-stimulated hippocampal θ oscillations at 5.6 mg/kg. These findings show for the first time that the 5-HT(4) receptor agonists prucalopride and PRX-03140 can increase cortical ACh and histamine levels, augment donepezil-induced ACh increases, and increase stimulated-hippocampal θ power, all neuropharmacological parameters consistent with potential positive effects on cognitive processes.

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.

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.

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.

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.

Classifying antipsychotic agents : need for new terminology

Converging data from multiple lines of research provide growing understanding of the pharmacological basis of the efficacy and tolerability of antipsychotic agents. This review highlights some of the drawbacks of the current practice of classifying antipsychotic agents into first- and second-generation agents, and argues that much of what is known about an antipsychotic agent in terms of its efficacy and tolerability can be predicted from its binding affinity at different receptors. This makes a case for a new system of classification that reflects the receptor binding affinity profiles of individual antipsychotic agents. In its quest to make a compelling case, the review provides detailed explanations for the pharmacological basis of antipsychotic efficacy, antipsychotic-induced weight gain and diabetes mellitus, cognitive effects and other adverse effects.

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.

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 histamine on muscimol-induced working memory deficits in radial maze performance

We investigated the participation of gamma-aminobutyric acid (GABA) neurons of the medial septal area in eight-arm radial maze performance in rats. The intra-septal injection of muscimol, a GABA(A) agonist, caused an increase in total error and working memory error. On the other hand, no significant effect was observed with reference memory error. Donepezil and tacrine (cholinesterase inhibitors) antagonized the muscimol-induced spatial memory deficits. Histidine (1500 mg/kg, i.p.) also improved the total error and working memory error induced by muscimol. At this dose, histidine caused a significant increase in the histamine content of the cortex, hippocampus, and hypothalamus in rats. In addition, the intra-hippocampal injection of histamine also antagonized muscimol-induced spatial memory deficits. The practical conclusion is that the GABA(A) receptor of the medial septal area plays an important role in working memory, and also, the disturbance of working memory induced by muscimol is mediated not only by cholinergic but also by histaminergic systems in the spatial memory of rats.

Histamine in the brain: Beyond sleep and memory

A few decades elapsed between the attribution of unwanted side effects of classic antihistamine compounds to the blockade of central H(1) receptors, and the acceptance of the concept that the histaminergic system commands general states of metabolism and consciousness. In the early 80s, two laboratories discovered independently that histaminergic neurons are located in the posterior hypothalamus and project to the whole CNS [Panula P, Yang HY, Costa E. Histamine-containing neurons in the rat hypothalamus. Proc Natl Acad Sci 1984;81:2572-76, Watanabe T, Taguchi Y, Hayashi H, Tanaka J, Shiosaka S, Tohyama M, Kubota H, Terano Y, Wada H. Evidence for the presence of a histaminergic neuron system in the rat brain: an immunohistochemical analysis. Neurosci Lett 1983;39:249-54], suggesting a global nature of histamine regulatory effects. Recently, functional studies demonstrated that activation of the central histaminergic system alters CNS functions in both behavioral and homeostatic contexts, which include sleep and wakefulness, learning and memory, anxiety, locomotion, feeding and drinking, and neuroendocrine regulation. These actions are achieved through interactions with other neurotransmitter systems, and the interplay between histaminergic neurons and other neurotransmitter systems are becoming clear. Hence, numerous laboratories are pursuing novel compounds targeting the three known histamine receptors found in the brain for various therapeutic indications. Preclinical studies are focusing on three major areas of interest and intense research is mainly oriented towards providing drugs for the treatment of sleep, cognitive and feeding disorders. This commentary is intended to summarize some of the latest findings that suggest functional roles for the interplay between histamine and other neurotransmitter systems, and to propose novel interactions as physiological substrates that may partially underlie some of the behavioral changes observed following manipulation of the histaminergic system.

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.

Altered sleep-wake characteristics and lack of arousal response to H3 receptor antagonist in histamine H1 receptor knockout mice

Histaminergic neurons play an important role in the regulation of sleep-wake behavior through histamine H(1) receptors (H(1)R). Blockade of the histamine H(3) receptor (H(3)R) is proposed to induce wakefulness by regulating the release of various wake-related transmitters, not only histamine. In the present study, we characterized sleep-wake cycles of H(1)R knockout (KO) mice and their arousal responses to an H(3)R antagonist. Under baseline conditions, H(1)R KO mice showed sleep-wake cycles essentially identical to those of WT mice but with fewer incidents of brief awakening (<16-sec epoch), prolonged durations of non-rapid eye movement (NREM) sleep episodes, a decreased number of state transitions between NREM sleep and wakefulness, and a shorter latency for initiating NREM sleep after an i.p. injection of saline. The H(1)R antagonist pyrilamine mimicked these effects in WT mice. When an H(3)R antagonist, ciproxifan, was administered i.p., wakefulness increased in WT mice in a dose-dependent manner but did not increase at all in H(1)R KO mice. In vivo microdialysis revealed that the i.p. application of ciproxifan increased histamine release from the frontal cortex in both genotypes of mice. These results indicate that H(1)R is involved in the regulation of behavioral state transitions from NREM sleep to wakefulness and that the arousal effect of the H(3)R antagonist completely depends on the activation of histaminergic systems through H(1)R.

The histamine H3 receptor as a novel therapeutic target for cognitive and sleep disorders

Histamine H3 receptor pharmacology, functions and biochemistry are far from being fully understood; however, progress is being made. Activation of this Gi/GO-protein-coupled receptor affects cognition, the sleep-wake cycle, obesity and epilepsy, which are physiological and pathological conditions that are the main focus of research into the therapeutic potential of selective H3 receptor ligands. This heterogeneity of targets can be reconciled partially by the fact that the histamine system constitutes one of the most important brain-activating systems and that H3 receptors regulate the activity of histamine and other neurotransmitter systems. Furthermore, the H3 receptor shows functional constitutive activity, polymorphisms in humans and rodents with a differential distribution of splice variants in the CNS, and potential coupling to different intracellular signal transduction mechanisms. In light of the genetic, pharmacological and functional complexity of the H3 receptor, the importance of the histamine system as a therapeutic target to control the sleep-wake cycle and cognitive disorders will be discussed.

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.

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.