The blockade of H1 receptors attenuates the suppression of feeding and diuresis induced by inhibition of histamine catabolism

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.

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

Simultaneous determination of 10 first-generation histamine h1 receptor blockers in feeds by ultra-high-performance liquid chromatography triple quadrupole ion trap hybrid mass spectrometer

A method for simultaneous determination of 10 first-generation histamine H1 receptor blockers in feeds by ultra-high-performance liquid chromatography triple quadrupole mass spectrometry combined with solid phase extraction. Instrument conditions, extraction solvents, and purification methods have been optimized. Under the optimum conditions, these analytes were separated effectively at 6 min. These feeds have been extracted by acid acetonitrile and purified by mixed cation exchange solid-phase extraction. The performance of this method meets the requirements of veterinary residue detection in feeds in China. It is appropriate for the confirmatory monitoring and quantitative analysis of 105 feed samples, five kinds of histamine H1 receptor blockers have been detected in 10 samples.

An H2R-dependent medial septum histaminergic circuit mediates feeding behavior

Novel targets for treating feeding-related diseases are of great importance, and histamine has long been considered an anorexigenic agent. However, understanding its functions in feeding in a circuit-specific way is still limited. Here, we report a medial septum (MS)-projecting histaminergic circuit mediating feeding behavior. This MS-projecting histaminergic circuit is functionally inhibited during food consumption, and bidirectionally modulates feeding behavior via downstream H2, but not H1, receptors on MS glutamatergic neurons. Further, we observed a pathological decrease of histamine 2 receptors (H2Rs) expression in MS glutamatergic neurons in diet-induced obesity (DIO) mice. Genetically, down-regulation of H2Rs expression in MS glutamatergic neurons accelerates body-weight gain. Importantly, chronic activation of H2Rs in MS glutamatergic neurons (with its clinical agonist amthamine) significantly slowed down the body-weight gain in DIO mice, providing a possible clinical utility to treat obesity. Together, our results demonstrate that this MS-projecting histaminergic circuit is critically involved in feeding, and H2Rs in MS glutamatergic neurons is a promising target for treating body-weight problems.

Clozapine reliably increases the motivation for food: parsing the role of the 5-HT2c and H1 receptors

RATIONALE AND OBJECTIVES

Although clozapine is effective in treating schizophrenia, it is associated with adverse side effects including weight gain and metabolic syndrome. Despite this, the role of clozapine on feeding behaviour and food intake has not been thoroughly characterised. Clozapine has a broad pharmacological profile, with affinities for several neurotransmitter receptors, including serotonin (5-hydroxytriptamine, 5-HT) and histamine. Given that the serotonin 5-HT_2C receptor and histaminergic H₁ receptor are involved in aspects of feeding behaviour, the effect of clozapine on feeding may be linked to its action at these receptors.

METHODS

We assessed, in rats, the effect of acute and subchronic administration of clozapine on responding for food under a progressive ratio (PR) schedule under conditions of food restriction and satiety. We also examined the effect of antagonists of the serotonin 5-HT_2C and histaminergic H₁ receptors on the same schedule. Clozapine reliably increased responding for food, even when rats had ad libitum access to food. The effect of clozapine on responding for food was reproduced by combined (but not individual) antagonism of the serotonin 5-HT_2C and histaminergic H₁ receptors.

CONCLUSION

These findings show that clozapine enhances the motivation to work for food, that this effect is stable over repeated testing, and is independent of hunger state of the animal. This effect may relate to a combined action of clozapine at the serotonin 5-HT_2C and histaminergic H₁ receptors.

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'.

Satiety factor oleoylethanolamide recruits the brain histaminergic system to inhibit food intake

Key factors driving eating behavior are hunger and satiety, which are controlled by a complex interplay of central neurotransmitter systems and peripheral stimuli. The lipid-derived messenger oleoylethanolamide (OEA) is released by enterocytes in response to fat intake and indirectly signals satiety to hypothalamic nuclei. Brain histamine is released during the appetitive phase to provide a high level of arousal in anticipation of feeding, and mediates satiety. However, despite the possible functional overlap of satiety signals, it is not known whether histamine participates in OEA-induced hypophagia. Using different experimental settings and diets, we report that the anorexiant effect of OEA is significantly attenuated in mice deficient in the histamine-synthesizing enzyme histidine decarboxylase (HDC-KO) or acutely depleted of histamine via interocerebroventricular infusion of the HDC blocker α-fluoromethylhistidine (α-FMH). α-FMH abolished OEA-induced early occurrence of satiety onset while increasing histamine release in the CNS with an H3 receptor antagonist-increased hypophagia. OEA augmented histamine release in the cortex of fasted mice within a time window compatible to its anorexic effects. OEA also increased c-Fos expression in the oxytocin neurons of the paraventricular nuclei of WT but not HDC-KO mice. The density of c-Fos immunoreactive neurons in other brain regions that receive histaminergic innervation and participate in the expression of feeding behavior was comparable in OEA-treated WT and HDC-KO mice. Our results demonstrate that OEA requires the integrity of the brain histamine system to fully exert its hypophagic effect and that the oxytocin neuron-rich nuclei are the likely hypothalamic area where brain histamine influences the central effects of OEA.

Proxyfan acts as a neutral antagonist of histamine H3 receptors in the feeding-related hypothalamic ventromedial nucleus

BACKGROUND AND PURPOSE

Centrally acting histamine H(3) receptor ligands are proposed as potential treatments for obesity, although the value of inverse agonists at these receptors is still debated. Functional inhibition of H(3) autoreceptors activates neurones in a hypothalamic 'satiety' centre. The H(3) receptor antagonist, proxyfan was used as a tool to assess the action of histaminergic compounds in this model.

EXPERIMENTAL APPROACH

We compared the actions of histamine on feeding with those of an H(3) receptor agonist (imetit) and inverse agonist (thioperamide) in rats and mice. Sites of action were identified by immunohistochemistry and the hypothalamic ventromedial nucleus (VMN) was investigated using electrophysiological techniques.

KEY RESULTS

Central histamine or thioperamide decreased fast-induced feeding, whereas imetit increased feeding. Systemic thioperamide entered the brain to activate hypothalamic feeding centres and to reduce feeding without causing any adverse behaviours. Thioperamide activated neurones in the VMN through an action on histamine autoreceptors, whilst imetit had the opposite effect. Proxyfan administered alone did not affect either feeding or electrical activity. However, it blocked the actions of both thioperamide and imetit, acting as a neutral antagonist in this system.

CONCLUSIONS AND IMPLICATIONS

The H(3) receptor inverse agonist, thioperamide, potently reduced appetite without adverse behavioural effects. This action was blocked by proxyfan, acting as a neutral antagonist in this model and, therefore, this compound is useful in determining the selectivity of H(3) receptor-directed drugs. A major action of thioperamide is through presynaptic autoreceptors, inducing stimulation by endogenous histamine of postsynaptic H(1 ) receptors on anorectic hypothalamic neurones.

Effects of auricular acupressure on body weight parameters in patients with chronic schizophrenia

Auricular acupressure is widely used in complementary and alternative medicine to reduce body weight, but little is known about the effects of auricular acupressure on body weight parameters in patients with chronic schizophrenia. The purpose of this study was to evaluate the effects of auricular acupressure on body weight parameters in patients with chronic schizophrenia. Eighty-six inpatients with schizophrenia were recruited from chronic wards in a psychiatric center. The participants were randomly divided into experimental (acupressure at 4 acupuncture sites: hunger, stomach, shenmen and endocrine) and control groups, and body weight parameters were determined weekly for 8 weeks. There was no significant difference between the experimental and control groups in mean body weight, waist circumference, or body fat percentage at the pretest or during the entire 8-week study period. Therefore, auricular acupressure did not cause body weight reduction in patients with chronic schizophrenia.

Integrative role of the histaminergic system in feeding and taste perception

Feeding behavior is regulated by a complex interplay of many endogenous substances, such as peptides and neurotransmitters in the central nervous system. Histamine is a neurotransmitter which expresses an anorectic effect on food intake via histamine H(1) receptors. The histaminergic system exists downstream of leptin, a satiety factor secreted from white adipose tissue. Because direct stimulation of the histaminergic system by histamine H(3)-inverse agonists or antagonists can normalize the obese phenotype in which animal models with exogenous leptin resistance, which resembles human obesity, the potential roles of histamine H(3) receptors as a therapeutic target now draw attention. Histaminergic activity is enhanced during feeding, and an oral somatic sensation is thought to affect histaminergic activity while blood glucose levels do not. In addition, gustatory information can modulate histaminergic activity by two mechanisms: by physiological excitation of the chorda tympani nerve, one of the taste nerves and by emotions elicited by taste perception, i.e., taste palatability. Particularly, aversive and hazardous taste stimuli tonically facilitate histaminergic activity, suggesting that the histaminergic system is involved in the response to harmful stimuli. Together with recent findings, it is postulated that the histaminergic system responds to both mechanical and chemical sensory input from the oral cavity during feeding and is exerted as a part of the danger response system.

Pharmacogenetics of leptin in antipsychotic-associated weight gain and obesity-related complications

Second-generation antipsychotics can greatly improve symptoms of psychosis-spectrum disorders. Unfortunately, these drugs are associated with weight gain, which increases a patient's risk for developing chronic diseases including Type 2 diabetes, cardiovascular diseases or other obesity-related complications. There are interindividual differences in weight gain resulting from antipsychotic drug use that may be explained by pharmacodynamic characteristics of these agents as well as clinical factors. In addition, genetic variations in pathways associated with satiety are increasingly recognized as potential contributors to antipsychotic-associated weight gain. Polymorphisms in the leptin gene, as well as the leptin receptor gene, are potential pharmacogenetic markers associated with these outcomes. This article summarizes evidence for the associations of the leptin gene and the leptin receptor gene polymorphisms with antipsychotic-induced weight gain, potential mechanisms underlying these relationships, and discusses areas for future pharmacogenetic investigation.

Olanzapine-induced weight gain in the rat: role of 5-HT2C and histamine H1 receptors

INTRODUCTION

Substantial increases in body weight can be induced by several antipsychotic drugs, most notably olanzapine and clozapine. Antagonism at certain receptors, particularly 5-HT2C and histamine H1 receptors, is implicated in this effect.

MATERIALS AND METHODS

We have investigated the contribution of effects at these receptors to olanzapine-induced weight gain occurring over 5 days following daily intraperitoneal drug injections in groups of eight female rats.

RESULTS

Olanzapine (2 mg/kg) and the 5-HT2C antagonist SB 243213 (1 mg/kg), but not the histamine H1 antagonist mepyramine (1 mg/kg), produced significant increases in percentage body weight above vehicle; olanzapine showed a significantly greater effect than SB 243213. Haloperidol (0.1 mg/kg) alone or with mepyramine had no significant effects on weight gain, while with SB 243213 and with both SB 243213 and mepyramine, it showed olanzapine-like increases in weight.

CONCLUSION

These results suggest that 5-HT2C receptor antagonism or inverse agonism, in the presence of D2 receptor antagonism, may contribute to olanzapine-induced weight gain.

Acute central administration of immepip, a histamine H3 receptor agonist, suppresses hypothalamic histamine release and elicits feeding behavior in rats

Histamine suppresses feeding behavior via histamine H1 receptors in the hypothalamus. This study was performed to examine whether the acute reduction of histamine release in the hypothalamus caused by immepip, a histamine H3 agonist, modulates the feeding behavior of rats. Rats had a catheter implanted in the third cerebral ventricle (i3v) and were given central injections of phosphate-buffered-saline or immepip (100-300 pmol/rat). Following the i3v administration of immepip, the rats developed dose-dependent hypokinesia within 10 min of administration. Next to hypokinesia, the rats showed significant dose-dependent feeding behavior. High-performance liquid chromatography (HPLC) confirmed the reduction in histamine release in the hypothalamus of rats following i3v administration of immepip. These results suggest that i3v administration of immepip, an H3 receptor agonist, suppresses hypothalamic histamine release and elicits feeding behavior in rats.

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.

Nα-Imidazolylalkyl and Pyridylalkyl Derivatives of Histaprodifen: Synthesis and in Vitro Evaluation of Highly Potent Histamine H1-Receptor Agonists

A novel series of N(alpha)()-imidazolylalkyl and pyridylalkyl derivatives of histaprodifen (6, 2-[2-(3,3-diphenylpropyl)imidazol-4-yl]ethanamine) was synthesized and evaluated as histamine H(1)-receptor agonists. The title compounds displayed partial agonism at contractile H(1)-receptors of guinea pig ileum and were at least equipotent with histamine. Agonist effects of the new derivatives were susceptible to blockade by the H(1)-receptor antagonist mepyramine (2-100 nM). In the imidazole series, suprahistaprodifen (51, [2-[2-(3,3-diphenylpropyl)-1H-imidazol-4-yl]ethyl]-[2-(1H-imidazol-4-yl)ethyl]amine, N(alpha)-2-[(1H-imidazol-4-yl)ethyl]histaprodifen) showed the highest H(1)-receptor agonist potency ever reported in the literature (pEC(50) 8.26, efficacy E(max) 96%). Elongation of the alkyl spacer from ethyl to butyl decreased activity from 3630% (ethyl, 51) to 163% (butyl, 53) of histamine potency. The exchange of the terminal imidazole nucleus for a pyridine ring resulted in compounds with comparably high potency. A decrease in agonist potency and efficacy was observed when the attachment of the alkyl spacer was consecutively changed from the ortho to the meta and the para position, respectively, of the pyridine ring. The pyridine series that contained a butyl chain possessed the highest potency and affinity. N(alpha)-[4-(2-pyridyl)butyl]histaprodifen (56) emerged as a strong partial agonist, being almost equipotent with 51 (pEC(50) 8.16, E(max) 89%). Compounds 51 and 56 also showed potent partial agonism at contractile H(1) receptors in guinea pig aorta and potently activated H(1)-receptor-mediated endothelium-dependent relaxation in the rat aorta. Compounds 51-65 displayed low to moderate affinity at H(2), H(3), and M(3) receptors in functional models of guinea pig. Collectively, N(alpha)-imidazolylalkyl- and N(alpha)-pyridylalkyl-substituted histaprodifens represent a novel class of potent H(1)-receptor agonists. These compounds may be useful to define the (patho)physiological role of the H(1)-receptor and refine molecular models of H(1)-receptor activation.

Effect of morphine, naloxone and histamine system on water intake in adult male rats

The present study investigated the interaction between histamine and opioid systems on water intake in adult male rats. Intracerebroventricular (i.c.v.) injections were carried out in all experiments. Water intake was measured 1 h after drug injections. Administration of histamine (40-80 microg/rat) and naloxone (0.5-1 microg/rat) increased, while morphine (2.5 microg/rat), pyrilamine (25-50 microg/rat), the histamine H1 receptor antagonist, and ranitidine (10-20 microg/rat), the histamine H2 receptor antagonist, decreased water intake in isolated rats. Blockade of histamine H1 and H2 receptors attenuated the histamine-induced response. Pyrilamine, but not ranitidine, increased the inhibitory effect induced by morphine. Also, pharmacological blockade of histamine H1 and H2 receptors decreased the naloxone-induced effect on water intake. It is concluded that the histaminergic system may have a close interaction with morphine and naloxone on drinking behavior.

Serotonergic and histaminergic mechanisms involved in intralipid drinking?

Some newer antipsychotic agents are associated with weight gain in humans and a hyperphagic response to intralipid solutions in rodents. To examine the possible contribution of serotonin (5-HT) and histamine (H) receptor blockade in antipsychotic-associated hyperphagia, rats were trained to drink a palatable, high-calorie fat emulsion (10% intralipid) during 30-min sessions and were tested following pretreatment with mepyramine (H1 receptor antagonist), metergoline (5-HT(1/2) receptor antagonist), cyproheptadine (H1 and 5-HT(2A/2B/2C) and muscarinic receptor antagonist), SB 242084 (5-HT2C receptor antagonist) and an SB 242084-mepyramine combination. Total intake and ingestive behaviour microstructure were measured. Mepyramine (10 mg/kg) reduced intake, as did metergoline (3.0 mg/kg). Cyproheptadine (0.1-1.0 mg/kg) increased intake and microstructural analysis suggests that this was due to increased numbers of clusters of licking. SB 242084 (3 mg/kg) reduced intake, either when administered alone, or in combination with mepyramine (1 mg/kg). In conclusion, simple antagonism of either H1 (mepyramine) or 5-HT(1/2) receptors (metergoline) alone was not sufficient to increase intake. Furthermore, combined blockade of H1 and 5-HT2C receptors (SB 242084 and mepyramine) was also insufficient to produce hyperphagia. Conversely, simultaneous blockade of H1, 5-HT(2A/2C) and muscarinic receptors (cyproheptadine) led to a substantial hyperphagia and pattern of ingestive behaviour that was similar to that previously observed with some newer antipsychotic agents.

The role of hypothalamic histamine in leptin-induced suppression of short-term food intake in fasted rats

OBJECTIVE

Leptin suppresses food intake; however, the precise mechanism is not fully understood. Histamine (HA), which acts as a neurotransmitter in the central nervous system, has also been shown to be involved in feeding and exerts an inhibitory effect through activation of H(1) receptors. Therefore, we studied the possible role of HA in short-term leptin-induced suppression of food intake.

METHODS

We studied the 6-h feeding response of overnight-fasted adult (200 g) male Wistar rats to leptin and the HA synthesis inhibitor alpha-fluoromethylhistidine (alpha-FMH). Levels of transcription for neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH), as well as hypothalamic content of HA and the HA metabolite telemethyl-HA were investigated.

RESULTS

Central administration of leptin (3, 5 and 10 microg at 09:00 h) in fasted rats caused a decrease in food intake. In contrast, central administration of alpha-FMH (11, 22 and 112 microg at 09:00 h) increased food intake. Prior administration of alpha-FMH prevented the leptin-induced decrease in food intake. Leptin decreased hypothalamic histamine content, while increasing the ratio between telemethyl-HA and HA, indicating that leptin reduces HA metabolism. Finally, alpha-FMH suppressed basal and leptin-induced CRH expression while stimulating NPY expression in fasted rats.

CONCLUSION

Histamine is involved in leptin-induced inhibition of food intake. The role of histamine may be mediating, i.e. leptin may directly activate and/or change the metabolism of the histaminergic system. Alternatively, the histaminergic system may be involved in a permissive manner.

Targeted disruption of H3 receptors results in changes in brain histamine tone leading to an obese phenotype

Histamine is an aminergic neurotransmitter that is localized in the CNS and in peripheral tissues. To date, four histamine receptors have been identified, and the H3 receptor, which was recently cloned, is predominantly expressed in the CNS. The peripheral functions of histamine have been investigated intensively using available molecular and pharmacological tools, and the molecular identification of the H3 receptor opens up new possibilities for investigating the role of histamine in central tissues. To understand the biological function of the histamine presynaptic autoreceptor H3, we inactivated the receptor through homologous recombination. H3(-/-) mice manifest mild obese phenotypes that are characterized by increases in body weight, food intake, and adiposity and by reductions in energy expenditure. Consistent with these observations, homozygous null mice have insulin and leptin resistance, increased levels of plasma leptin and insulin, and decreased levels of histamine in the hypothalamic/thalamic region of their brains coupled with increased histamine turnover. The expression of UCP1 in brown adipose tissue and of UCP3 in brown adipose tissue, white adipose tissue, and skeletal muscle is decreased in H3(-/-) mutants, and the anorexigenic activity of thioperamide is not observed. These results suggest that neuronal histamine is a mediator of body-weight homeostasis and that neuronal histamine functions through H3 receptors in mice.

Targeted disruption of H3 receptors results in changes in brain histamine tone leading to an obese phenotype

Histamine is an aminergic neurotransmitter that is localized in the CNS and in peripheral tissues. To date, four histamine receptors have been identified, and the H3 receptor, which was recently cloned, is predominantly expressed in the CNS. The peripheral functions of histamine have been investigated intensively using available molecular and pharmacological tools, and the molecular identification of the H3 receptor opens up new possibilities for investigating the role of histamine in central tissues. To understand the biological function of the histamine presynaptic autoreceptor H3, we inactivated the receptor through homologous recombination. H3(-/-) mice manifest mild obese phenotypes that are characterized by increases in body weight, food intake, and adiposity and by reductions in energy expenditure. Consistent with these observations, homozygous null mice have insulin and leptin resistance, increased levels of plasma leptin and insulin, and decreased levels of histamine in the hypothalamic/thalamic region of their brains coupled with increased histamine turnover. The expression of UCP1 in brown adipose tissue and of UCP3 in brown adipose tissue, white adipose tissue, and skeletal muscle is decreased in H3(-/-) mutants, and the anorexigenic activity of thioperamide is not observed. These results suggest that neuronal histamine is a mediator of body-weight homeostasis and that neuronal histamine functions through H3 receptors in mice.

An acute i.c.v. infusion of leptin has no effect on hypothalamic histamine and tele-methylhistamine contents in Wistar rats

The actions of intracerebroventricularly (i.c.v.) infused leptin on food intake, body weight and hypothalamic contents of histamine and tele-methylhistamine, the main histamine metabolite in the mammalian brain, were studied in male Wistar rats. The effect of the histamine H(1) receptor blockade on leptin-induced anorexia was also examined. It was found that leptin at the dose of 10 microg i.c.v. reduced 24-h food intake by 48% as compared with the controls (P<0.01). This leptin dose reduced feeding during 2-4 consecutive days. In spite of the marked changes in food consumption and body weight gain, leptin did not alter the hypothalamic contents of histamine and tele-methylhistamine. Furthermore, the blockade of histamine H(1) receptors by mepyramine did not attenuate the effect of leptin on feeding and body weight. The findings indicate that centrally administered leptin suppresses feeding and promotes weight loss through mechanisms that do not require the direct participation of the brain histaminergic neuron system.

Involvement of the histaminergic system in leptin-induced suppression of food intake

The ob gene product leptin is secreted from white adipose tissue, and may regulate food intake by acting on the hypothalamus in the central nervous system. But the mechanism of this effect is still unclear. The central histaminergic system has been suggested to participate in the control of various physiological functions, particularly in feeding behavior, as it mediates anorectic signals like leptin. Thus, we hypothesized that the central histaminergic system is a target for leptin in its control of feeding. To prove this, we first examined the effect of i.p. administration of alpha-fluoromethylhistidine (FMH), a specific and irreversible inhibitor of histidine decarboxylase, on leptin-induced suppression of food intake in normal C57BL strain mice. Leptin treatment (1.3 mg/kg, i.p.) significantly reduced food intake by 60% of that of control at 6 h and by 84% at 24 h compared with control. When mice were injected with FMH (100 mg/kg, i.p.) before being given leptin, leptin-induced suppression of food intake was abolished and there was no significant difference compared with that of control. Additionally, we further examined the effects of leptin on food intake in mutant mice lacking histamine H, receptors (H1R-KO mice). Leptin injection significantly reduced food intake by 56% of that of control at 6 h and by 79% at 24 h in wild-type mice (WT mice), but not in H1R-KO mice. This finding suggests that leptin affects the feeding behavior through activation of the central histaminergic system via histamine H1 receptors.

Endotoxin- and interleukin-1-induced hypophagia are not affected by adrenergic, dopaminergic, histaminergic, or muscarinic antagonists

Endotoxin (lipopolysaccharide, LPS) and interleukin-1 (IL-1) administration induce hypophagia in rodents. Both IL-1 and LPS are known to activate cerebral norepinephrine and serotonin metabolism, and IL-1 affects that of acetylcholine and histamine. Each of these neurotransmitters has been implicated in feeding behavior. Therefore, the ability of specific antagonists of the above neurotransmitter systems to counteract feeding responses to peripherally injected mIL-1beta and LPS was studied. Feeding was assessed in nondeprived mice by measuring the intake of sweetened milk in a 30-min period, as well as daily food pellet intake. LPS and mIL-1beta reliably reduced milk intake, and often reduced food pellet intake and body weight. Treatment of the mice with peripherally administered alpha-adrenergic (phentolamine or prazosin) or 3-adrenergic antagonists (propranolol), either alone or in combination, did not significantly alter the hypophagic responses to mIL-1beta or LPS. Mice in which cerebral norepinephrine was depleted with DSP-4 or 6-hydroxydopamine also displayed the usual hypophagia in response to mIL-1beta and LPS. The hypophagic responses to mIL-1beta and LPS were not affected by the histaminergic antagonists, pyrilamine (H1), cimetidine (H2), thioperamide (H3), or the histamine-depleting agent, alpha-fluoromethylhistidine, nor by the muscarinic cholinergic antagonist, scopolamine. The responses to mIL-l1 were also unaffected by the dopamine receptor antagonist, haloperidol, the opioid receptor antagonist, naloxone, and the NO synthase inhibitor, L-NAME. These results suggest that adrenergic, dopaminergic, histaminergic, cholinergic, opioid or nitric oxide systems are not essential for the hypophagia induced by IL-1, and that multiple redundant pathways may be involved in illness-related hypophagia.

Effects of intracerebroventricularly infused histamine and selective H1, H2 and H3 agonists on food and water intake and urine flow in Wistar rats

The actions of intracerebroventricularly-infused histamine and selective histamine H1, H2 and H3 receptor agonists on food and water intake and urine flow were studied in rats. It was found that 100-800 nmoles of histamine significantly suppressed feeding. The H1 agonist 2-(3- trifluoromethylphenyl)histamine (FMPH) decreased food intake, whereas the H2 agonist dimaprit was without effect. Histamine- and FMPH-induced suppressions of feeding were attenuated by blockade of H1 but not by H2 receptors. The results clearly demonstrate that activation of brain H1 receptors decreases food intake. In subsequent studies, we found that both metoprine and thioperamide, which increase histaminergic activity through different mechanisms, also reduced food intake. This finding indicates that the brain histaminergic system is associated with feeding behavior. The same is true with body water homeostasis. Histamine caused a long-lasting diuresis. Also dimaprit and metoprine increased urine flow and the blockade of H2 receptors abolished the diuretic responses to histamine and dimaprit. On the other hand, the H3 agonist (R)-alpha-methylhistamine elicited drinking and this effect could be prevented by thioperamide pretreatment. The results imply that activation of H3 receptors predominantly provokes drinking, whereas central H2 receptors mediate the diuretic effect of histamine.