Proton- and ammonium-sensing by histaminergic neurons controlling wakefulness

Deciphering Post-Stroke Sleep Disorders: Unveiling Neurological Mechanisms in the Realm of Brain Science

Sleep disorders are the most widespread mental disorders after stroke and hurt survivors' functional prognosis, response to restoration, and quality of life. This review will address an overview of the progress of research on the biological mechanisms associated with stroke-complicating sleep disorders. Extensive research has investigated the negative impact of stroke on sleep. However, a bidirectional association between sleep disorders and stroke exists; while stroke elevates the risk of sleep disorders, these disorders also independently contribute as a risk factor for stroke. This review aims to elucidate the mechanisms of stroke-induced sleep disorders. Possible influences were examined, including functional changes in brain regions, cerebrovascular hemodynamics, neurological deficits, sleep ion regulation, neurotransmitters, and inflammation. The results provide valuable insights into the mechanisms of stroke complicating sleep disorders.

OLHA (Nα-oleoylhistamine) modulates activity of mouse brain histaminergic neurons

Histaminergic (HA) neurons are located in the tuberomamillary nucleus (TMN) of the posterior hypothalamus, from where they project throughout the whole brain to control wakefulness. We examined the effects of N^α-oleoylhistamine (OLHA), a non-enzymatic condensation product of oleic acid (OLA) and histamine, on activity of mouse HA neurons in brain slices. OLHA bidirectionally modulated the firing of HA neurons. At 10 nM OLHA inhibited or had no action, whereas at 1 μM it evoked excitatory and inhibitory responses. Inhibition was not seen in presence of the histamine receptor H3 (H₃R) antagonist clobenpropit and in calcium-free medium. Pre-incubation with a histamine-reuptake blocker prevented the decrease in firing by OLHA. OLHA-evoked increase in firing (EC₅₀ ∼44 nM) was insensitive to blockers of cannabinoid 1 and 2 receptors and of the capsaicin receptor, but was significantly impaired by the peroxisome proliferator-activated receptor-alpha (PPAR-alpha) antagonist MK886, which suppressed also the rise in intracellular calcium level caused by OLHA. The OLHA-evoked excitation was mimicked by synthetic PPAR-alpha agonists (gemfibrozil and GW7647) and was abolished by the PKA inhibitor H-89. The H₃R affinity (Ki) for histamine, measured in HEK293 cells with stable expression of human H₃R, was higher than for OLHA (Ki: 42 vs 310 nM, respectively). Expression of PPAR-alpha was not different between TMN regions of males and females, responses to OLHA did not differ. Molecular modelling of PPAR-alpha bound to either OLHA or OEA showed similar binding energies. These findings shed light on a novel biotransformation product of histamine which may play a role in health and disease.

Histamine/H1 receptor signaling in the parafacial region increases activity of chemosensitive neurons and respiratory activity in rats

Histaminergic neurons of the tuberomammillary nucleus (TMN) are pH-sensitive and contribute to CO₂/H⁺-dependent behaviors including arousal and respiratory activity. TMN neurons project to several respiratory centers including the ventral parafacial region (pF) where chemosensitive retrotrapezoid (RTN) neurons are located, and since RTN neurons are an important source of CO₂/H⁺-dependent respiratory drive, we wondered whether histamine contributes to RTN chemoreception. To test this, we characterized effects of histamine on mean arterial pressure (MAP) and diaphragm muscle activity (DIA_EMG) in urethane-anaesthetized, vagotomized and artificially ventilated male Wistar rats. Unilateral injection of histamine (25 mM) in the pF increased DIA_EMG amplitude without changing DIA_EMG frequency and MAP. Bilateral pF injections of the H1 receptor antagonist diphenhydramine hydrochloride (DPH; 0.5 mM) decreased baseline DIA_EMG amplitude and frequency and MAP. Despite the strong inhibitory effect of DPH on baseline breathing, the hypercapnic ventilatory response was preserved under these experimental conditions. At the cellular level, chemosensitive RTN neurons showed a dose-dependent excitatory response to histamine that was blunted by DPH and mimicked by the H1 receptor agonist 2-pyridylethylamine dihydrochloride (2PYEA) under both control conditions and when fast neurotransmitter receptors are blocked. We also tested effects of 2PYEA in the presence of serotonin, another wake-on neurotransmitter that activates RTN chemoreceptors partly by activation of Gq-coupled receptors. We found the response to 2PYEA was diminished in serotonin, suggesting RTN neurons have a limited capacity to respond to multiple Gq-coupled modulators. These results suggest histamine can modulate breathing at the pF level by a mechanism involving H1 receptors.

A Study on the Mechanism of the Sedative-hypnotic Effect of Cinnamomum camphora chvar. Borneol Essential Oil Based on Network Pharmacology

In this paper, we investigated the sedative-hypnotic effect of Cinnamomum camphora chvar. Borneol essential oil (BEO, 16.4% borneol), a by-product of steam distillation of Cinnamomum camphora chvar. Borneol, from which natural crystalline borneol (NCB, 98.4% borneol) is obtained. Using locomotor activity tests and pentobarbital sodium-induced sleep test, it was found that BEO significantly reduced locomotor activity (p < 0.05), shortened sleep latency (p < 0.0001), prolonged sleep duration (p < 0.05), and had a sedative-hypnotic effect. We constructed the "components-targets-signaling pathways" and "protein-protein interaction" (PPI) network of BEO using network pharmacology. The results show that the 24 active components of BEO acted on 17 targets, mainly through response to alkaloid and catecholamine transport, and neuroactive ligand-receptor interaction. The PPI network identified 12 key proteins, mainly dopamine receptor (DR)D2, opioid receptor mu 1 (OPRM1), and opioid receptor kappa 1 (OPRK1), and we further analyzed the active components and targets of BEO through molecular docking. The results showed that the active components and targets obtained by network pharmacology analyses had good binding activity, which reflected their multi-component, multi-target, multi-pathway action characteristics. This paper provides a theoretical basis for further study of the mechanism of action of BEO in the treatment of insomnia.

Tachykinins amplify the action of capsaicin on central histaminergic neurons

Substance P (SP), a product of the tachykinin 1 (Tac1) gene, is expressed in many hypothalamic neurons. Its wake-promoting potential could be mediated through histaminergic (HA) neurons of the tuberomamillary nucleus (TMN), where functional expression of neurokinin receptors (NKRs) waits to be characterized. As in the process of nociception in the peripheral nervous system (PNS) capsaicin-receptor (transient potential vanilloid 1: TRPV1) signalling is amplified by local release of histamine and SP, we tested the involvement of tachykinins in the capsaicin-induced long-lasting enhancement (LLEcaps) of HA neurons firing by investigating selective neurokinin receptor ligands in the hypothalamic mouse brain slice preparation using patch-clamp recordings in cell-attached mode combined with single-cell RT-PCR. We report that the majority of HA neurons respond to SP (EC50 3 nM), express the SP precursor tachykinin 1 (Tac1) gene and at least one of the neurokinin receptors. Responses to selective agonists of three known neurokinin receptors were sensitive to corresponding antagonists. LLEcaps was significantly impaired by the neurokinin receptor antagonists, indicating that in hypothalamus, as in the PNS, release of tachykinins downstream to TRPV1 activation is able to boost the release of histamine. The excitatory action of SP on histaminergic neurons adds another pathway to the noradrenergic and orexinergic ones to synergistically enhance cortical arousal. We show NK1R to play a prominent role on HA neurons and thus the control of wakefulness.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

Histaminergic neurons in the tuberomammillary nucleus as a control centre for wakefulness

Histamine plays pleiotropic roles as a neurotransmitter in the physiology of brain function, this includes the maintenance of wakefulness, appetite regulation and memory retrieval. Since numerous studies have revealed an association between histaminergic dysfunction and diverse neuropsychiatric disorders, such as Alzheimer's disease and schizophrenia, a large number of compounds acting on the brain histamine system have been developed to treat neurological disorders. In 2016, pitolisant, which was developed as a histamine H₃ receptor inverse agonist by Schwartz and colleagues, was launched for the treatment of narcolepsy, emphasising the prominent role of brain histamine on wakefulness. Recent advances in neuroscientific techniques such as chemogenetic and optogenetic approaches have led to remarkable progress in the understanding of histaminergic neural circuits essential for the control of wakefulness. In this review article, we summarise the basic knowledge about the histaminergic nervous system and the mechanisms underlying sleep/wake regulation that are controlled by the brain histamine system. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

Instant activation of TRP channels by NH4 + promotes neuronal bursting and glutamate spikes in CA1 neurons

Ammonia ( NH 4 + ) is a by-product of cell metabolism and may elicit subcellular effects with specific physiological responses. Chronic effects have been implicated in several neurological diseases and attributed to persistent elevation in blood ammonia levels transferred to the brain. In previous studies the activities of neurons and astrocytes have been examined at ammonia concentrations an order of magnitude higher than measured in the blood. The effects developed within several minutes. Here we focused upon acute responses of neurons to ammonia and whether they may occur at much lower doses. To this end, we combined patch-clamp in CA1 neurons with glutamate imaging in hippocampal slices. Particular attention was paid to the Rett syndrome that has been originally attributed to hyperammonemia. We compared the responses in the wild-type (WT) and model Rett mice (MECP2-null, RTT) to ammonia doses from 0.3 mM on. In both preparations NH 4 + promptly depolarized neurons and increased the ambient glutamate. The bursting activity emerged in WT and it was augmented in RTT. Searching for subcellular mechanisms we examined possible modulation of ion channels by ammonia. We did not find any changes in HCN- and Ca²⁺ currents, which substantially contribute to the bursting activity. The non-selective cation channels were markedly potentiated by ammonia. ASIC channels had a major contribution to the augmentation of neuronal activity by ammonia. Interestingly, their general blocker amiloride (100 μM) moderately excited CA1 cells akin to NH 4 + . In its presence subsequent ammonia effects were markedly compromised. Blockade of TRPC1 channels partially occluded NH 4 + effects. ASIC and TRPC1 blockers decreased the amplitude of excitatory postsynaptic currents (EPSC) and neuronal bursts, congruent with a postsynaptic location of the channels. Inhibition of TRPV1 channels potentiated the responses to NH 4 + . EPSC amplitudes did not change, but the frequency decreased, indicating presynaptic effects. All extracellular NH 4 + actions were observed at concentrations as low as 0.3 mM and the neurons reacted immediately after ammonia arrived the slice. We propose that a brief augmentation of neuronal activity by NH 4 + may occur either spontaneously during arousal or induced by inhalation of smelling salts.

Electrophysiological Properties of Genetically Identified Histaminergic Neurons

Histaminergic neurons of the tuberomammillary nucleus (TMN) are important regulators of behavioral and homeostatic processes. Previous work suggested that histaminergic neurons exhibit a characteristic electrophysiological signature, allowing for their identification in brain slice preparations. However, these previous investigations focused on neurons in the ventral subregion of the TMN of rats. Consequently, it remains unclear whether such electrophysiological properties extend to mice, including other subregions of the TMN, and the potential for differences between males and females. To further characterize the electrophysiological properties of histaminergic neurons, we performed whole-cell patch-clamp recordings on transgenic mice expressing Cre recombinase in histidine decarboxylase (HDC)-expressing cells; the sole enzyme for histamine synthesis (Hdc-cre::tdTomato). Despite similarities with the electrophysiological properties reported in rats, we observed considerable variability in mouse HDC neuron passive membrane properties, action potential firing, and intrinsic subthreshold active membrane properties. Overall, the electrophysiological properties of HDC neurons appeared similar across subregions of the TMN, consistent with a lack of topographical organization in this nucleus. Moreover, we found no obvious sex differences in the electrical excitability of HDC neurons. However, our data reveal a diversity in the electrophysiological properties of genetically identified histaminergic neurons from mice not previously appreciated from rat studies. Thus, these data highlight the utility of mouse genetics to target the widespread histaminergic neuronal population within the TMN and support the idea that histaminergic neurons are a heterogeneous neuronal population.

Melanocortin regulation of histaminergic neurons via perifornical lateral hypothalamic melanocortin 4 receptors

OBJECTIVE

Histaminergic neurons of the tuberomammillary nucleus (TMN) are wake-promoting and contribute to the regulation of energy homeostasis. Evidence indicates that melanocortin 4 receptors (MC4R) are expressed within the TMN. However, whether the melanocortin system influences the activity and function of TMN neurons expressing histidine decarboxylase (HDC), the enzyme required for histamine synthesis, remains undefined.

METHODS

We utilized Hdc-Cre mice in combination with whole-cell patch-clamp electrophysiology and in vivo chemogenetic techniques to determine whether HDC neurons receive metabolically relevant information via the melanocortin system.

RESULTS

We found that subsets of HDC-expressing neurons were excited by melanotan II (MTII), a non-selective melanocortin receptor agonist. Use of melanocortin receptor selective agonists (THIQ, [D-Trp8]-γ-MSH) and inhibitors of synaptic transmission (TTX, CNQX, AP5) indicated that the effect was mediated specifically by MC4Rs and involved a glutamatergic dependent presynaptic mechanism. MTII enhanced evoked excitatory post-synaptic currents (EPSCs) originating from electrical stimulation of the perifornical lateral hypothalamic area (PeFLH), supportive of melanocortin effects on the glutamatergic PeFLH projection to the TMN. Finally, in vivo chemogenetic inhibition of HDC neurons strikingly enhanced the anorexigenic effects of intracerebroventricular administration of MTII, suggesting that MC4R activation of histaminergic neurons may restrain the anorexigenic effects of melanocortin system activation.

CONCLUSIONS

These experiments identify a functional interaction between the melanocortin and histaminergic systems and suggest that HDC neurons act naturally to restrain the anorexigenic effect of melanocortin system activation. These findings may have implications for the control of arousal and metabolic homeostasis, especially in the context of obesity, in which both processes are subjected to alterations.

Is Neuronal Histamine Signaling Involved in Cancer Cachexia? Implications and Perspectives

In this paper, we present evidence in support of our hypothesis that the neuronal histaminergic system might be involved in cancer cachexia¹. To build our premise, we present the research and the reasonable inferences that can be drawn from it in a section by section approach starting from one of the key issues related to cachexia, increased resting energy expenditure (REE), and progressing to the other, anorexia. Based on an extensive survey of the literature and our own deliberations on the abovementioned topics, we investigate whether histamine signaling might be the mechanism used by a tumor to hijack the body's thermogenic machinery. Our hypothesis in short is that hypothalamic histaminergic neurons are stimulated by inputs from the parasympathetic nervous system (PSNS), which senses tumor traits early in cancer development. Histamine release in the preoptic area of the hypothalamus primarily activates brown adipose tissue (BAT), triggering a highly energy demanding mechanism. Chronic activation of BAT, which, in this context, refers to intermittent and/or low grade activation by the sympathetic nervous system, leads to browning of white adipose tissue and further enhances thermogenic potential. Aberrant histamine signaling not only triggers energy-consuming processes, but also anorexia. Moreover, since functions such as taste, smell, and sleep are governed by discrete structures of the brain, which are targeted by distinct histaminergic neuron populations even relatively minor symptoms of cachexia, such as sleep disturbances and taste and smell distortions, also might be ascribed to aberrant histamine signaling. In late stage cachexia, the sympathetic tone in skeletal muscle breaks down, which we hypothesize might be caused by a reduction in histamine signaling or by the interference of other cachexia related mechanisms. Histamine signaling thus might delineate distinct stages of cachexia progression, with the early phase marked by a PSNS-mediated increase in histamine signaling, increased sympathetic tone and symptomatic adipose tissue depletion, and the late phase characterized by reduced histamine signaling, decreased sympathetic tone and symptomatic muscle wasting. To support our hypothesis, we review the literature from across disciplines and highlight the many commonalities between the mechanisms underlying cancer cachexia and current research findings on the regulation of energy homeostasis (particularly as it relates to hypothalamic histamine signaling). Extrapolating from the current body of knowledge, we develop our hypothetical framework (based on experimentally falsifiable assumptions) about the role of a distinct neuron population in the pathophysiology of cancer cachexia. Our hope is that presenting our ideas will spark discussion about the pathophysiology of cachexia, cancer's devastating and intractable syndrome.

Histamine-induced plasticity and gene expression in corticostriatal pathway under hyperammonemia

Aims

Histamine H3 receptor (H3R) antagonists/inverse agonists increase vigilance. We studied brain histaminergic pathways under hyperammonemia and the transcriptome of receptors and their signaling cascades to provide a rationale for wake‐promoting therapies.

Methods

We analyzed histamine‐induced long‐lasting depression of corticostriatal synaptic transmission (LLDhist). As the expression of dopamine 1 receptors (D1R) is upregulated in LGS‐KO striatum where D1R‐H3R dimers may exist, we investigated actions of H3R and D1R agonists and antagonists. We analyzed transcription of selected genes in cortex and dorsal striatum in a mouse model of inborn hyperammonemia (liver‐specific glutamine synthetase knockout: LGS‐KO) and compared it with human hepatic encephalopathy.

Results

LGS‐KO mice showed significant reduction of the direct depression (DD) but not the long‐lasting depression (LLD) by histamine. Neither pharmacological activation nor inhibition of D1R significantly affected DDhist and LLDhist in WT striatum, while in LGS‐KO mice D1R activation suppressed LLDhist. Histaminergic signaling was found unchanged at the transcriptional level except for the H2R. A study of cAMP‐regulated genes indicated a significant reduction in the molecular signature of wakefulness in the diseased cortex.

Conclusions

Our findings provide a rationale for the development of aminergic wake‐promoting therapeutics in hyperammonemic disorders.

Reassessing the Role of Histaminergic Tuberomammillary Neurons in Arousal Control

The histaminergic neurons of the tuberomammillary nucleus (TMN^HDC) of the posterior hypothalamus have long been implicated in promoting arousal. More recently, a role for GABAergic signaling by the TMN^HDC neurons in arousal control has been proposed. Here, we investigated the effects of selective chronic disruption of GABA synthesis (via genetic deletion of the GABA synthesis enzyme, glutamic acid decarboxylase 67) or GABAergic transmission (via genetic deletion of the vesicular GABA transporter (VGAT)) in the TMN^HDC neurons on sleep-wake in male mice. We also examined the effects of acute chemogenetic activation and optogenetic inhibition of TMN^HDC neurons upon arousal in male mice. Unexpectedly, we found that neither disruption of GABA synthesis nor GABAergic transmission altered hourly sleep-wake quantities, perhaps because very few TMN^HDC neurons coexpressed VGAT. Acute chemogenetic activation of TMN^HDC neurons did not increase arousal levels above baseline but did enhance vigilance when the mice were exposed to a behavioral cage change challenge. Similarly, acute optogenetic inhibition had little effect upon baseline levels of arousal. In conclusion, we could not identify a role for GABA release by TMN^HDC neurons in arousal control. Further, if TMN^HDC neurons do release GABA, the mechanism by which they do so remains unclear. Our findings support the view that TMN^HDC neurons may be important for enhancing arousal under certain conditions, such as exposure to a novel environment, but play only a minor role in behavioral and EEG arousal under baseline conditions.SIGNIFICANCE STATEMENT The histaminergic neurons of the tuberomammillary nucleus of the hypothalamus (TMN^HDC) have long been thought to promote arousal. Additionally, TMN^HDC neurons may counter-regulate the wake-promoting effects of histamine through co-release of the inhibitory neurotransmitter, GABA. Here, we show that impairing GABA signaling from TMN^HDC neurons does not impact sleep-wake amounts and that few TMN^HDC neurons contain the vesicular GABA transporter, which is presumably required to release GABA. We further show that acute activation or inhibition of TMN^HDC neurons has limited effects upon baseline arousal levels and that activation enhances vigilance during a behavioral challenge. Counter to general belief, our findings support the view that TMN^HDC neurons are neither necessary nor sufficient for the initiation and maintenance of arousal under baseline conditions.

Hypothalamic Tuberomammillary Nucleus Neurons: Electrophysiological Diversity and Essential Role in Arousal Stability

Histaminergic (HA) neurons, found in the posterior hypothalamic tuberomammillary nucleus (TMN), extend fibers throughout the brain and exert modulatory influence over numerous physiological systems. Multiple lines of evidence suggest that the activity of HA neurons is important in the regulation of vigilance despite the lack of direct, causal evidence demonstrating its requirement for the maintenance of arousal during wakefulness. Given the strong correlation between HA neuron excitability and behavioral arousal, we investigated both the electrophysiological diversity of HA neurons in brain slices and the effect of their acute silencing in vivo in male mice. For this purpose, we first validated a transgenic mouse line expressing cre recombinase in histidine decarboxylase-expressing neurons (Hdc-Cre) followed by a systematic census of the membrane properties of both HA and non-HA neurons in the ventral TMN (TMNv) region. Through unsupervised hierarchical cluster analysis, we found electrophysiological diversity both between TMNv HA and non-HA neurons, and among HA neurons. To directly determine the impact of acute cessation of HA neuron activity on sleep-wake states in awake and behaving mice, we examined the effects of optogenetic silencing of TMNv HA neurons in vivo We found that acute silencing of HA neurons during wakefulness promotes slow-wave sleep, but not rapid eye movement sleep, during a period of low sleep pressure. Together, these data suggest that the tonic firing of HA neurons is necessary for the maintenance of wakefulness, and their silencing not only impairs arousal but is sufficient to rapidly and selectively induce slow-wave sleep.SIGNIFICANCE STATEMENT The function of monoaminergic systems and circuits that regulate sleep and wakefulness is often disrupted as part of the pathophysiology of many neuropsychiatric disorders. One such circuit is the posterior hypothalamic histamine (HA) system, implicated in supporting wakefulness and higher brain function, but has been difficult to selectively manipulate owing to cellular heterogeneity in this region. Here we use a transgenic mouse to interrogate both the characteristic firing properties of HA neurons and their specific role in maintaining wakefulness. Our results demonstrate that the acute, cell type-specific silencing of HA neurons during wakefulness is sufficient to not only impair arousal but to rapidly and selectively induce slow-wave sleep. This work furthers our understanding of HA-mediated mechanisms that regulate behavioral arousal.

Histamine and the striatum

The neuromodulator histamine is released throughout the brain during periods of wakefulness. Combined with an abundant expression of histamine receptors, this suggests potential widespread histaminergic control of neural circuit activity. However, the effect of histamine on many of these circuits is unknown. In this review we will discuss recent evidence for histaminergic modulation of the basal ganglia circuitry, and specifically its main input nucleus; the striatum. Furthermore, we will discuss recent findings of histaminergic dysfunction in several basal ganglia disorders, including in Parkinson's disease and most prominently, in Tourette's syndrome, which has led to a resurgence of interest in this neuromodulator. Combined, these recent observations not only suggest a central role for histamine in modulating basal ganglia activity and behaviour, but also as a possible target in treating basal ganglia disorders. This article is part of the Special Issue entitled 'Histamine Receptors'.

Identification of histaminergic neurons through histamine 3 receptor-mediated autoinhibition

Using a reporter mouse model with expression of the tomato fluorescent protein under the dopamine transporter promoter (Tmt-DAT) we discovered a new group of neurons in the histaminergic tuberomamillary nucleus (TMN), which, in contrast to tuberoinfundibular dopaminergic neurons of the dorsomedial arcuate nucleus, do not express tyrosine hydroxylase but can synthesize and store dopamine. Tmt-DAT neurons located within TMN share electrophysiological properties with histaminergic neurons: spontaneous firing at a membrane potential around -50 mV and presence of hyperpolarization-activated cyclic nucleotide-gated ion channels. Histamine (30 μM) depolarizes and excites Tmt-DAT neurons through H1R activation but inhibits histaminergic neurons through H3R activation thus allowing a pharmacological identification of the different neurons. Single-cell RT-PCR revealed that all histaminergic neurons expressing histidine decarboxylase (HDC) also express H3R. This includes neurons retrogradely traced from the striatum whose inhibition by a selective H3R agonist was indistinguishable from the whole population. Prolonged depolarization reduces the autoinhibition. The potency of histamine at H3R depends on membrane potential and on extracellular and intracellular calcium. Autoinhibition can be impaired by preincubation with capsaicin, a ligand of the calcium-permeable TRPV1 channel or by blockade of Ca(2+)-ATPase with thapsigargin. The pharmacology of autoinhibition is revisited and physiological conditions for its functionality are determined. Usage of reporter mouse models for the safe identification of aminergic neurons under pathophysiological conditions is recommended. This article is part of the Special Issue entitled 'Histamine Receptors'.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Benzodiazepine-site pharmacology on GABAA receptors in histaminergic neurons

BACKGROUND AND PURPOSE

The histaminergic tuberomamillary nucleus (TMN) of the posterior hypothalamus controls the cognitive aspects of vigilance which is reduced by common sedatives and anxiolytics. The receptors targeted by these drugs in histaminergic neurons are unknown. TMN neurons express nine different subunits of the GABAA receptor (GABAA R) with three α- (α1, α2 and α5) and two γ- (γ1, γ 2) subunits, which confer different pharmacologies of the benzodiazepine-binding site.

EXPERIMENTAL APPROACH

We investigated the actions of zolpidem, midazolam, diazepam, chlordiazepoxide, flumazenil (Ro15-1788) and methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) in TMN neurons using mouse genetics, electrophysiological and molecular biological methods.

KEY RESULTS

We find the sensitivity of GABAA R to zolpidem, midazolam and DMCM significantly reduced in TMN neurons from γ2F77I mice, but modulatory activities of diazepam, chlordiazepoxide and flumazenil not affected. Potencies and efficacies of these compounds are in line with the dominance of α2- and α1-subunit containing receptors associated with γ2- or γ1-subunits. Functional expression of the γ1-subunit is supported by siRNA-based knock-down experiments in γ2F77I mice.

CONCLUSIONS AND IMPLICATIONS

GABAA R of TMN neurons respond to a variety of common sedatives with a high affinity binding site (γ2F77I) involved. The γ1-subunit likely contributes to the action of common sedatives in TMN neurons. This study is relevant for understanding the role of neuronal histamine and benzodiazepines in disorders of sleep and metabolism.

Optogenetic-mediated release of histamine reveals distal and autoregulatory mechanisms for controlling arousal

Histaminergic neurons in the tuberomammillary nucleus (TMN) are an important component of the ascending arousal system and may form part of a "flip-flop switch" hypothesized to regulate sleep and wakefulness. Anatomical studies have shown that the wake-active TMN and sleep-active ventrolateral preoptic nucleus (VLPO) are reciprocally connected, suggesting that each region can inhibit its counterpart when active. In this study, we determined how histamine affects the two branches of this circuit. We selectively expressed channelrhodopsin-2 (ChR2) in TMN neurons and used patch-clamp recordings in mouse brain slices to examine the effects of photo-evoked histamine release in the ventrolateral TMN and VLPO. Photostimulation decreased inhibitory GABAergic inputs to the ventrolateral TMN neurons but produced a membrane hyperpolarization and increased inhibitory synaptic input to the VLPO neurons. We found that in VLPO the response to histamine was indirect, most likely via a GABAergic interneuron. Our experiments demonstrate that release of histamine from TMN neurons can disinhibit the TMN and suppresses the activity of sleep-active VLPO neurons to promote TMN neuronal firing. This further supports the sleep-wake "flip-flop switch" hypothesis and a role for histamine in stabilizing the switch to favor wake states.

Acid-sensing hypothalamic neurons controlling arousal

Breathing and vigilance are regulated by pH and CO2 levels in the central nervous system. The hypocretin/orexin (Hcrt/Orx)- and histamine (HA)-containing hypothalamic neurons synergistically control different aspects of the waking state. Acidification inhibits firing of most neurons but these two groups in the caudal hypothalamus are excited by hypercapnia and protons, similar to the chemosensory neurons in the brain stem. Activation of hypothalamic wake-on neurons in response to hypercapnia, seen with the c-Fos assay, is supported by patch-clamp recordings in rodent brain slices: Hcrt/Orx and HA neurons are excited by acidification in the physiological range (pH from 7.4 to 7.0). Multiple molecular mechanisms mediate wake-promoting effects of protons in HA neurons in the tuberomamillary nucleus (TMN): among them are acid-sensing ion channels, Na(+),K(+)-ATPase, group I metabotropic glutamate receptors (mGluRI). HA neurons are remarkably sensitive to the mGluRI agonist DHPG (threshold concentration 0.5 µM) and mGluRI antagonists abolish proton-induced excitation of HA neurons. Hcrt/Orx neurons are excited through block of a potassium conductance and release glutamate with their peptides in TMN. The two hypothalamic nuclei and the serotonergic dorsal raphe cooperate toward CO2/acid-induced arousal. Their interactions and molecular mechanisms of H(+)/CO2-induced activation are relevant for the understanding and treatment of respiratory and metabolic disorders related to sleep-waking such as obstructive sleep apnea and sudden infant death syndrome.

Seven transmembrane G protein-coupled receptor repertoire of gastric ghrelin cells

The molecular mechanisms regulating secretion of the orexigenic-glucoregulatory hormone ghrelin remain unclear. Based on qPCR analysis of FACS-purified gastric ghrelin cells, highly expressed and enriched 7TM receptors were comprehensively identified and functionally characterized using in vitro, ex vivo and in vivo methods. Five Gαs-coupled receptors efficiently stimulated ghrelin secretion: as expected the β1-adrenergic, the GIP and the secretin receptors but surprisingly also the composite receptor for the sensory neuropeptide CGRP and the melanocortin 4 receptor. A number of Gαi/o-coupled receptors inhibited ghrelin secretion including somatostatin receptors SSTR1, SSTR2 and SSTR3 and unexpectedly the highly enriched lactate receptor, GPR81. Three other metabolite receptors known to be both Gαi/o- and Gαq/11-coupled all inhibited ghrelin secretion through a pertussis toxin-sensitive Gαi/o pathway: FFAR2 (short chain fatty acid receptor; GPR43), FFAR4 (long chain fatty acid receptor; GPR120) and CasR (calcium sensing receptor). In addition to the common Gα subunits three non-common Gαi/o subunits were highly enriched in ghrelin cells: GαoA, GαoB and Gαz. Inhibition of Gαi/o signaling via ghrelin cell-selective pertussis toxin expression markedly enhanced circulating ghrelin. These 7TM receptors and associated Gα subunits constitute a major part of the molecular machinery directly mediating neuronal and endocrine stimulation versus metabolite and somatostatin inhibition of ghrelin secretion including a series of novel receptor targets not previously identified on the ghrelin cell.

Characterization of gastric and neuronal histaminergic populations using a transgenic mouse model

Histamine is a potent biogenic amine that mediates numerous physiological processes throughout the body, including digestion, sleep, and immunity. It is synthesized by gastric enterochromaffin-like cells, a specific set of hypothalamic neurons, as well as a subset of white blood cells, including mast cells. Much remains to be learned about these varied histamine-producing cell populations. Here, we report the validation of a transgenic mouse line in which Cre recombinase expression has been targeted to cells expressing histidine decarboxylase (HDC), which catalyzes the rate-limiting step in the synthesis of histamine. This was achieved by crossing the HDC-Cre mouse line with Rosa26-tdTomato reporter mice, thus resulting in the expression of the fluorescent Tomato (Tmt) signal in cells containing Cre recombinase activity. As expected, the Tmt signal co-localized with HDC-immunoreactivity within the gastric mucosa and gastric submucosa and also within the tuberomamillary nucleus of the brain. HDC expression within Tmt-positive gastric cells was further confirmed by quantitative PCR analysis of mRNA isolated from highly purified populations of Tmt-positive cells obtained by fluorescent activated cell sorting (FACS). HDC expression within these FACS-separated cells was found to coincide with other markers of both ECL cells and mast cells. Gastrin expression was co-localized with HDC expression in a subset of histaminergic gastric mucosal cells. We suggest that these transgenic mice will facilitate future studies aimed at investigating the function of histamine-producing cells.

Advances in diagnosis and treatment of minimal hepatic encephalopathy

Minimal hepatic encephalopathy (MHE), also known as subclinical hepatic encephalopathy (SHE), is a common complex and potentially reversible neuropsychiatric syndrome secondary to chronic liver disease or cirrhosis. Although a clear pathogenesis is yet to be determined, elevated ammonia in serum and the central nervous system is the mainstay for pathogenesis and treatment of MHE. Patients with MHE, regardless of its cause, show a number of quantifiable neuropsychological defects, yet have a normal mental and neurological status on global clinical examination. Although ammonia is the main toxic substance involved in the pathogenesis of hepatic encephalopathy, other mechanisms, such as modifications of the blood-brain barrier, disruptions in neurotransmission and abnormalities in GABAergic and benzodiazepine pathways, may also play a role. Current treatments are based on reducing intestinal ammonia load by agents such as antibiotics, disaccharides and probiotics whose efficacy is yet to be clearly established. This paper summarizes the latest advances in the diagnosis and treatment of MHE.

Histamine neurons in the tuberomamillary nucleus: a whole center or distinct subpopulations?

Histamine axons originate from a single source, the tuberomamillary nucleus (TMN) of the posterior hypothalamus, to innervate almost all central nervous system (CNS) regions. This feature, a compact cell group with widely distributed fibers, resembles that of other amine systems, such as noradrenaline or serotonin, and is consistent with a function for histamine over a host of physiological processes, including the regulation of the sleep-wake cycle, appetite, endocrine homeostasis, body temperature, pain perception, learning, memory, and emotion. An important question is whether these diverse physiological roles are served by different histamine neuronal subpopulation. While the histamine system is generally regarded as one single functional unit that provides histamine throughout the brain, evidence is beginning to accumulate in favor of heterogeneity of histamine neurons. The aim of this review is to summarize experimental evidence demonstrating that histamine neurons are heterogeneous, organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histamine neurons according to their respective origin and terminal projections.