Histaminergic neurons are involved in the orexigenic effect of orexin-A

The Diverse Network of Brain Histamine in Feeding: Dissect its Functions in a Circuit-Specific Way

Feeding is an intrinsic and important behavior regulated by complex molecular, cellular and circuit-level mechanisms, one of which is the brain histaminergic network. In the past decades, many studies have provided a foundation of knowledge about the relationship between feeding and histamine receptors, which are deemed to have therapeutic potential but are not successful in treating feeding- related diseases. Indeed, the histaminergic circuits underlying feeding are poorly understood and characterized. This review describes current knowledge of histamine in feeding at the receptor level. Further, we provide insight into putative histamine-involved feeding circuits based on the classic feeding circuits. Understanding the histaminergic network in a circuit-specific way may be therapeutically relevant for increasing the drug specificity and precise treatment in feeding-related diseases.

Orexins (hypocretins): The intersection between homeostatic and hedonic feeding

Orexins are hypothalamic neuropeptides originally discovered to play a role in the regulation of feeding behaviour. The broad connections of orexin neurons to mesocorticolimbic circuitry suggest they may play a role in mediating reward-related behaviour beyond homeostatic feeding. Here, we review the role of orexin in a variety of eating-related behaviour, with a focus on reward and motivation, and the neural circuits driving these effects. One emerging finding is the involvement of orexins in hedonic and appetitive behaviour towards palatable food, in addition to their role in homeostatic feeding. This review discusses the brain circuitry and possible mechanisms underlying the role of orexins in these behaviours. Overall, there is a marked bias in the literature towards studies involving male subjects. As such, future work needs to be done to involve female subjects. In summary, orexins play an important role in driving motivation for high salient rewards such as highly palatable food and may serve as the intersection between homeostatic and hedonic feeding.

Role of the Histamine H3 Receptor in the Central Nervous System

The G_i/o protein-coupled histamine H₃ receptor is distributed throughout the central nervous system including areas like cerebral cortex, hippocampus and striatum with the density being highest in the posterior hypothalamus, i.e. the area in which the histaminergic cell bodies are located. In contrast to the other histamine receptor subtypes (H₁, H₂ and H₄), the H₃ receptor is located presynaptically and shows a constitutive activity. In detail, H₃ receptors are involved in the inhibition of histamine release (presynaptic autoreceptor), impulse flow along the histaminergic neurones (somadendritic autoreceptor) and histamine synthesis. Moreover, they occur as inhibitory presynaptic heteroreceptors on serotoninergic, noradrenergic, dopaminergic, glutamatergic, GABAergic and perhaps cholinergic neurones. This review shows for four functions of the brain that the H₃ receptor represents a brake against the wake-promoting, anticonvulsant and anorectic effect of histamine (via postsynaptic H₁ receptors) and its procognitive activity (via postsynaptic H₁ and H₂ receptors). Indeed, H₁ agonists and H₃ inverse agonists elicit essentially the same effects, at least in rodents; these effects are opposite in direction to those elicited by brain-penetrating H₁ receptor antagonists in humans. Although the benefit for H₃ inverse agonists for the symptomatic treatment of dementias is inconclusive, several members of this group have shown a marked potential for the treatment of disorders associated with excessive daytime sleepiness. In March 2016, the European Commission granted a marketing authorisation for pitolisant (Wakix^R) (as the first representative of the H₃ inverse agonists) for the treatment of narcolepsy.

The histaminergic system as a target for the prevention of obesity and metabolic syndrome

The control of food intake and body weight is very complex. Key factors driving eating behavior are hunger and satiety that are controlled by an interplay of several central and peripheral neuroendocrine systems, environmental factors, the behavioral state and circadian rhythm, which all concur to alter homeostatic aspects of appetite and energy expenditure. Brain histamine plays a fundamental role in eating behavior as it induces loss of appetite and has long been considered a satiety signal that is released during food intake (Sakata et al., 1997). Animal studies have shown that brain histamine is released during the appetitive phase to provide a high level of arousal preparatory to feeding, but also mediates satiety. Furthermore, histamine regulates peripheral mechanisms such as glucose uptake and insulin function. Preclinical research indicates that activation of H1 and H3 receptors is crucial for the regulation of the diurnal rhythm of food consumption; furthermore, these receptors have been specifically recognized as mediators of energy intake and expenditure. Despite encouraging preclinical data, though, no brain penetrating H1 receptor agonists have been identified that would have anti-obesity effects. The potential role of the H3 receptor as a target of anti-obesity therapeutics was explored in clinical trials that did not meet up to the expectations or were interrupted (clinicaltrials.gov). Nonetheless, interesting results are emerging from clinical trials that evaluated the attenuating effect of betahistine (an H1 agonist/H3 antagonist) on metabolic side effects associated with chronic antipsychotics treatment. Aim of this review is to summarize recent results that suggest the clinical relevance of the histaminergic system for the treatment of feeding disorders and provide an up-to-date summary of preclinical research. This article is part of the Special Issue entitled 'Histamine Receptors'.

Betahistine ameliorates olanzapine-induced weight gain through modulation of histaminergic, NPY and AMPK pathways

Olanzapine is widely used to treat schizophrenia and other disorders, but causes adverse obesity and other metabolic side-effects. Both animal and clinical studies have shown that co-treatment with betahistine (a histaminergic H1 receptor agonist and H3 receptor antagonist) is effective for ameliorating olanzapine-induced weight gain/obesity. To reveal the mechanisms underlying these effects, this study investigated the effects of co-treatment of olanzapine and betahistine (O+B) on expressions of histaminergic H1 receptor (H1R), AMP-activated protein kinase (AMPK), neuropeptide Y (NPY), and proopiomelanocortin (POMC) in the hypothalamus associated with reducing olanzapine-induced weight gain. Olanzapine significantly upregulated the mRNA and protein expressions of H1R, while O+B co-treatment significantly downregulated the H1R levels, compared to the olanzapine-only treatment group. The NPY mRNA expression was significantly enhanced by olanzapine, but it was significantly reversed by O+B co-treatment. The hypothalamic H1R expression was positively correlated with total food intake, and NPY expression. Olanzapine also increased AMPKα activation measured by the AMPKα phosphorylation (pAMPKα)/AMPKα ratio compared with controls, whereas O+B co-treatment decreased the pAMPKα/AMPKα ratio, compared with olanzapine only treatment. The pAMPKα/AMPKα ratio was positively correlated with total food intake and H1R expression. Although olanzapine administration decreased the POMC mRNA level, this level was not affected by O+B co-treatment. Therefore, these results suggested that co-treatment with betahistine may reverse olanzapine-induced body weight gain via the H1R-NPY and H1R-pAMPKα pathways.

Effects of loratadine and cetirizine on serum levels of neuropeptides in patients with chronic urticaria

H1-receptor inhibiting drugs, namely loratadine and cetirizine, were frequently used in treatment of chronic urticaria. Urticarial weal and flare reactions, a neurogenic reflex due to neuropeptides, were reported to be more effectively inhibited by cetirizine than loratadine. The aim of this study was to determine and compare the effects of systemic loratadine and cetirizine treatments on serum levels of selected neuropeptides in chronic urticaria. Treatment groups of either systemic loratadine or cetirizine (10 mg/d), consisting of 16 and 22 patients, respectively, were included. Serum levels of stem cell factor (SCF), neuropeptide Y (NPY), calcitonin gene-related peptide (CGRP), nerve growth factor (NGF), vasoactive intestinal peptide (VIP), and substance P (SP) were detected before and after one week of treatment with antihistamines. Serum NPY and VIP levels were significantly decreased when compared before and after treatment with antihistamines (P < 0.001 and P < 0.01, respectively). SCF and NGF values were also decreased after antihistamine treatment (P < 0.05). Post-treatment levels of CGRP were significantly higher compared with pretreatment values, while no significant difference was detected between pre and post treatment levels of SP. Cetirizine was significantly more effective than loratadine on lowering serum levels of SCF among the other neuropeptides. Systemic loratadine and cetirizine treatments in patients with chronic urticaria precisely caused variations in serum levels of neuropeptides. The predominant effect of cetirizine compared to loratadine on reducing serum SCF levels might be explained with anti-inflammatory properties of cetirizine.

Hypothalamic histamine H1 receptor-AMPK signaling time-dependently mediates olanzapine-induced hyperphagia and weight gain in female rats

Although second-generation antipsychotics induce severe weight gain and obesity, there is a lack of detailed knowledge about the progressive development of antipsychotic-induced obesity. This study examined the hypothalamic histamine H1 receptor and AMP-activated protein kinase (H1R-AMPK) signaling at three distinctive stages of olanzapine-induced weight gain (day 1-12: early acceleration, day 13-28: middle new equilibrium, and day 29-36: late heavy weight maintenance). At the early acceleration stage, the rats were hyperphagic with an underlying mechanism of olanzapine-increased H1R mRNA expression and AMPK phosphorylation (pAMPK), in which pAMPK levels positively correlated with H1R mRNA expression and food intake. At the middle stage, when the rats were no longer hyperphagic, the changes in H1R-AMPK signaling vanished. At the late stage, olanzapine increased H1R mRNA expression but decreased pAMPK which were positively and negatively correlated with weight gain, respectively. These data suggest a time-dependent change of H1R-AMPK signaling, where olanzapine activates AMPK by blocking the H1Rs and causing hyperphagia in the acute phase. The chronic blockade of H1R may contribute to the late stage of olanzapine-induced heavy weight maintenance. However, pAMPK was no longer elevated and actually decreased. This indicates that AMPK acts as an energy sensor and negatively responds to the positive energy balance induced by olanzapine. Furthermore, we showed that an H1R agonist, 2-(3-trifluoromethylphenyl) histamine, can significantly inhibit olanzapine-induced hyperphagia and AMPK activation in the mediobasal hypothalamus in a dose dependent manner. Therefore, lowering H1R-AMPK signaling is an effective treatment for the olanzapine-induced hyperphagia associated with the development of obesity.

The role of NPY in hypothalamic mediated food intake

Neuropeptide Y (NPY) is a highly conserved neuropeptide with orexigenic actions in discrete hypothalamic nuclei that plays a role in regulating energy homeostasis. NPY signals via a family of high affinity receptors that mediate the widespread actions of NPY in all hypothalamic nuclei. These actions are also subject to tight, intricate regulation by numerous peripheral and central energy balance signals. The NPY system is embedded within a densely-redundant network designed to ensure stable energy homeostasis. This redundancy may underlie compensation for the loss of NPY or its receptors in germline knockouts, explaining why conventional knockouts of NPY or its receptors rarely yield a marked phenotypic change. We discuss insights into the hypothalamic role of NPY from studies of its physiological actions, responses to genetic manipulations and interactions with other energy balance signals. We conclude that numerous approaches must be employed to effectively study different aspects of NPY action.

The histamine H3 receptor and eating behavior

Interest in the histaminergic system as a potential target for the treatment of feeding disorders is driven by the unsatisfactory history of the pharmacotherapy of obesity. Eating behavior is regulated by a complex interplay of central neurotransmitter systems, peripheral endocrine stimuli, the circadian rhythm, and environmental cues, all factors that change the behavioral state and alter homeostatic aspects of appetite and energy expenditure. Key factors driving eating behavior are appetite and satiety that are regulated through different mechanisms. Brain histamine has long been considered a satiety signal in the nervous system. Recent observations, however, indicate that histamine does not meet the criteria for being a satiety signal, because augmented histamine release accompanies the appetitive phase of feeding behavior rather than food consumption and satiety. The appetitive phase requires a high and yet optimal arousal state, and the histaminergic system is crucial for sustaining a high degree of arousal during motivated behavior. Histamine H(1) receptors in the brain are crucial for the regulation of the diurnal rhythm of food intake and the regulation of obesity; however, from a therapeutic standpoint, no brain-penetrating H(1) receptor agonists have been identified that would have antiobesity effects. Despite conflicting preclinical data, insights are emerging into the potential role of histamine H(3) receptors as a target of antiobesity therapeutics. The aim of this review is to outline the relevance of the histaminergic system in controlling feeding behavior and evaluate the potential therapeutic use of histaminergic ligands for the treatment of eating disorders.

Histamine in the nervous system

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

Identification of zebrafish histamine H1, H2 and H3 receptors and effects of histaminergic ligands on behavior

Neuronal histamine regulates several functions in the vertebrate brain. The zebrafish brain contains a widespread histaminergic system and H(3) receptor ligand binding has been reported. In this study we provide evidence for the existence of histamine H(1), H(2) and H(3) receptor genes in zebrafish. Single copies of putative histamine H(1), H(2) and H(3) receptors were identified and cloned from the zebrafish brain. Expression analysis suggested that they are expressed in the brain and a few other tissues. Widespread distribution of zebrafish H(2) receptor binding sites was detected with [(125)I]iodoaminopotentidine in brain sections. Zebrafish larvae were exposed to 1, 10 or 100 microM of the H(1) ligand pyrilamine, the H(2) ligand cimetidine and the H(3) ligands thioperamide and immepip for 5 days. Significant decreases in swimming distance were observed with the highest dose of all ligands, whereas cimetidine gave a significant decrease also with 1 and 10 microM doses. These results provide the first molecular biological evidence for the presence of histamine receptors in zebrafish. These histamine receptors resemble those of higher vertebrates and they provide a useful model for pharmacological and behavioral studies for characterizing the functions of histamine in more detail.

Histamine and the regulation of body weight

Energy intake and expenditure is regulated by a complex interplay between peripheral and central factors. An exhaustive list of peptides and neurotransmitters taking part in this complex regulation of body weight exists. Among these is histamine, which acts as a central neurotransmitter. In the present article we review current evidence pointing at an important role of histamine in the regulation of appetite and metabolism. Studies using both knockout mouse models as well as pharmacological studies have revealed that histamine acts as an anorexigenic agent via stimulation of histamine H(1) receptors. One effect of histamine in the regulation of appetite is to act as a mediator of the inhibitory effect of leptin on appetite. It seems that histamine may attenuate and delay the development of leptin resistance in high-fat-diet-induced obesity. Furthermore, histamine may also act to accelerate lipolysis. Based on the current evidence of the involvement of histamine in the regulation of body weight, the histaminergic system is an obvious target for the development of pharmacological agents to control obesity. At present, H(3) receptor antagonists that stimulate the histaminergic system may be the most promising histaminergic drugs for antiobesity therapy.

Increased susceptibility to diet-induced obesity in histamine-deficient mice

BACKGROUND AND AIM

The neurotransmitter histamine is involved in the regulation of appetite and in the development of age-related obesity in mice. Furthermore, histamine is a mediator of the anorexigenic action of leptin. The aim of the present study was to investigate a possible role of histamine in the development of high-fat diet (HFD)-induced obesity.

METHODS

Histamine-deficient histidine decarboxylase knock-out (HDC-KO) mice and C57BL/6J wild-type (WT) mice were given either a standard diet (STD) or HFD for 8 weeks. Body weight, 24-hour caloric intake, epididymal adipose tissue size, plasma leptin concentration and quantitative expression of leptin receptor (Ob-R) mRNA were measured.

RESULTS

Both HDC-KO and WT mice fed an HFD for 8 weeks increased their body weight significantly more than STD-fed mice. A significant difference in body weight gain between HDC-KO mice fed an HFD or an STD was seen after 2 weeks, whereas a significant difference in body weight gain was first observed after 5 weeks in WT mice. After 8 weeks 24-hour caloric intake was significantly lower in HFD- than in STD-fed WT mice. In HDC-KO mice no difference in caloric intake was observed between HFD- and STD-fed mice. After 8 weeks epididymal adipose tissue size and plasma leptin concentration had increased significantly in HFD-fed WT and HDC-KO mice compared to their STD-fed controls. Epididymal adipose tissue size was higher in HDC-KO than WT mice, both in STD- and HFD-fed mice. A significant decrease in Ob-R mRNA in HFD-fed HDC-KO mice compared to STD-fed HDC-KO mice was observed, while no such difference was observed in WT mice.

CONCLUSION

Based on our results, we conclude that histamine plays a role in the development of HFD-induced obesity.