Histamine promotes angiogenesis through a histamine H1 receptor-PKC-VEGF-mediated pathway in human endothelial cells.

Histamine is a well-known inflammatory mediator, but how histamine induces angiogenesis remains poorly understood. In the present study, we demonstrated a dose-dependent dynamic tube formation in the human endothelial cell line EA.hy926 in the presence of histamine that was completely blocked by histamine H1 receptor (H1R) and protein kinase C (PKC) inhibitors. However, histamine H2, H3, and H4 receptor inhibitors did not inhibit tube formation, suggesting that H1R-PKC signaling is involved in histamine-induced tube formation. Moreover, we found an H1-specific induction of vascular endothelial growth factor (VEGF) expression. Inhibition of VEGF receptor 2 (VEGFR2) suppressed the histamine-induced tube formation, indicating that VEGF is downstream of histamine signaling. Additionally, we demonstrated that histamine stimulation induces the expression of critical regulators of angiogenesis such as matrix metalloproteinase (MMP)-9 and MMP-14 metalloproteases, as histamine-induced tube formation is blocked by MMP inhibitors. In summary, our study indicates that histamine can activate the H1R in human endothelial cells and thereby promote tube formation through the PKC, MMP, and VEGF signaling pathways.

Histamine Signaling Is Essential for Tissue Macrophage Differentiation and Suppression of Bacterial Overgrowth in the Stomach.

BACKGROUND & AIMS: Histamine in the stomach traditionally is considered to regulate acid secretion but also has been reported to participate in macrophage differentiation, which plays an important role in tissue homeostasis. Therefore, this study aimed to uncover the precise role of histamine in mediating macrophage differentiation and in maintaining stomach homeostasis. METHODS: Here, we expand on this role using histidine decarboxylase knockout (Hdc-/-) mice with hypertrophic gastropathy. In-depth in vivo studies were performed in Hdc-/- mice, germ-free Hdc-/- mice, and bone-marrow-transplanted Hdc-/- mice. The stomach macrophage populations and function were characterized by flow cytometry. To identify stomach macrophages and find the new macrophage population, we performed single-cell RNA sequencing analysis on Hdc+/+ and Hdc-/- stomach tissues. RESULTS: Single-cell RNA sequencing and flow cytometry of the stomach cells of Hdc-/- mice showed alterations in the ratios of 3 distinct tissue macrophage populations (F4/80+Il1bhigh, F4/80+CD93+, and F4/80-MHC class IIhighCD74high). Tissue macrophages of the stomachs of Hdc-/- mice showed impaired phagocytic activity, increasing the bacterial burden of the stomach and attenuating hypertrophic gastropathy in germ-free Hdc-/- mice. The transplantation of bone marrow cells of Hdc+/+ mice to Hdc-/- mice recovered the normal differentiation of stomach macrophages and relieved the hypertrophic gastropathy of Hdc-/- mice. CONCLUSIONS: This study showed the importance of histamine signaling in tissue macrophage differentiation and maintenance of gastric homeostasis through the suppression of bacterial overgrowth in the stomach.

New Insight in Histamine Functions.

The first properties of histamine (HA) that were elucidated were vasodilation and contraction of smooth muscles in the gut after stimulating gastric acid secretion and constriction of the bronchial area during anaphylaxis [...].

Histamine as an Alert Signal in the Brain.

Sleep-wake behavior is a well-studied physiology in central histamine studies. Classical histamine H1 receptor antagonists, such as diphenhydramine and chlorpheniramine, promote sleep in animals and humans. Further, neuronal histamine release shows a clear circadian rhythm in parallel with wake behavior. However, the early stages of histamine-associated knockout mouse studies showed relatively small defects in normal sleep-wake control. To reassess the role of histamine in behavioral state control, this review summarizes the progress in sleep-wake studies of histamine-associated genetic mouse models and discusses the significance of histamine for characteristic aspects of wake behavior. Based on analysis of recent mouse models, we propose that neuronal histamine may serve as an alert signal in the brain, when high attention or a strong wake-drive is needed, such as during exploration, self-defense, learning, or to counteract hypersomnolent diseases. Enhanced histaminergic neurotransmission may help performance or sense of signals concerning internal or environmental dangers, like peripheral histamine from mast cells in response to allergic stimuli and inflammatory signals.

Targeting Histamine and Histamine Receptors for the Precise Regulation of Feeding.

Histamine has long been accepted as an anorexigenic agent. However, lines of evidence have suggested that the roles of histamine in feeding behaviors are much more complex than previously thought, being involved in satiety, satiation, feeding motivation, feeding circadian rhythm, and taste perception and memory. The functional diversity of histamine makes it a viable target for clinical management of obesity and other feeding-related disorders. Here, we update the current knowledge about the functions of histamine in feeding and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on weight control and discuss insights into future research on the roles of histamine in feeding. Despite the recent progress in histamine research, the histaminergic feeding circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

The Role of the Central Histaminergic System in Behavioral State Control.

Histamine is a small monoamine signaling molecule that plays a role in many peripheral and central physiological processes, including the regulation of wakefulness. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and histamine neurons are thought to promote wakefulness and vigilance maintenance - under certain environmental and/or behavioral contexts - through their diffuse innervation of the cortex and other wake-promoting brain circuits. Histamine neurons also contain a number of other putative neurotransmitters, although the functional role of these co-transmitters remains incompletely understood. Within the brain histamine operates through three receptor subtypes that are located on pre- and post-synaptic membranes. Some histamine receptors exhibit constitutive activity, and hence exist in an activated state even in the absence of histamine. Newer medications used to reduce sleepiness in narcolepsy patients in fact enhance histamine signaling by blunting the constitutive activity of these histamine receptors. In this chapter, we provide an overview of the central histamine system with an emphasis on its role in behavioral state regulation and how drugs targeting histamine receptors are used clinically to treat a wide range of sleep-wake disorders.

Novel Insight of Histamine and Its Receptor Ligands in Glaucoma and Retina Neuroprotection.

Glaucoma is a multifactorial neuropathy characterized by increased intraocular pressure (IOP), and it is the second leading cause of blindness worldwide after cataracts. Glaucoma combines a group of optic neuropathies characterized by the progressive degeneration of retinal ganglionic cells (RGCs). Increased IOP and short-term IOP fluctuation are two of the most critical risk factors in glaucoma progression. Histamine is a well-characterized neuromodulator that follows a circadian rhythm, regulates IOP and modulates retinal circuits and vision. This review summarizes findings from animal models on the role of histamine and its receptors in the eye, focusing on the effects of histamine H3 receptor antagonists for the future treatment of glaucomatous patients.

The Implications of Pruritogens in the Pathogenesis of Atopic Dermatitis.

Atopic dermatitis (AD) is a prototypic inflammatory disease that presents with intense itching. The pathophysiology of AD is multifactorial, involving environmental factors, genetic susceptibility, skin barrier function, and immune responses. A recent understanding of pruritus transmission provides more information about the role of pruritogens in the pathogenesis of AD. There is evidence that pruritogens are not only responsible for eliciting pruritus, but also interact with immune cells and act as inflammatory mediators, which exacerbate the severity of AD. In this review, we discuss the interaction between pruritogens and inflammatory molecules and summarize the targeted therapies for AD.

Histamine H1 and H2 receptors are essential transducers of the integrative exercise training response in humans.

Exercise training is a powerful strategy to prevent and combat cardiovascular and metabolic diseases, although the integrative nature of the training-induced adaptations is not completely understood. We show that chronic blockade of histamine H1/H2 receptors led to marked impairments of microvascular and mitochondrial adaptations to interval training in humans. Consequently, functional adaptations in exercise capacity, whole-body glycemic control, and vascular function were blunted. Furthermore, the sustained elevation of muscle perfusion after acute interval exercise was severely reduced when H1/H2 receptors were pharmaceutically blocked. Our work suggests that histamine H1/H2 receptors are important transducers of the integrative exercise training response in humans, potentially related to regulation of optimal post-exercise muscle perfusion. These findings add to our understanding of how skeletal muscle and the cardiovascular system adapt to exercise training, knowledge that will help us further unravel and develop the exercise-is-medicine concept.

Angioedema without wheals: a clinical update.

Angioedema without wheals (urticaria) represents a heterogeneous group of clinically indistinguishable diseases of hereditary or acquired etiology. Hereditary angioedema is a rare inherited condition leading to recurrent, sometimes life-threatening angioedema attacks in subcutaneous tissues and gastrointestinal and oropharyngeal mucosa dating back to childhood or adolescence. Most of these patients have mutations in the SERPING1 gene, causing either low C1 inhibitor production (hereditary angioedema with C1 inhibitor deficiency type I) or the production of dysfunctional C1 inhibitor (hereditary angioedema with C1 inhibitor deficiency type II). Hereditary angioedema with normal C1 inhibitor has been defined later. Although C1 inhibitor concentration and function are in the normal range, it leads to typical hereditary angioedema symptoms owing to mutations in FXII, PLG, ANGPT1, KNG1, and MYOF genes. Patients who exhibit none of these genetic mutations despite having a similar clinical presentation are classified as having unknown hereditary angioedema. Fewer than 1 in 10 patients with C1 inhibitor deficiency have acquired angioedema with C1 inhibitor deficiency. The clinical presentation is very similar to that of hereditary angioedema, making it difficult to distinguish these 2 conditions clinically. Unlike hereditary angioedema, there are no genetic mutations, and family history and symptoms tend to appear later in life. Acquired angioedema with C1 inhibitor deficiency is commonly associated with lymphoproliferative and autoimmune diseases. Angioedema attacks might start 1 year before the underlying disease in acquired angioedema with C1 inhibitor deficiency. Approximately half of the patients admitted to the hospital for acute angioedema are patients receiving angiotensin-converting enzyme (ACE) inhibitor therapy. Angioedema typically occurs on the lips, tongue, mouth, pharynx, and subglottic regions. Patients may require hospitalization and intensive care monitoring owing to airway involvement. Idiopathic histaminergic acquired angioedema may be diagnosed only when any possible causes of histaminergic angioedema are excluded (foods, drugs, animal dander, aeroallergens, insect stings, latex, and others), and the symptoms respond well to antihistamine treatment. Idiopathic nonhistaminergic acquired angioedema should be considered when all other types of recurrent angioedema have been ruled out and patients do not respond to high-dose antihistamines. The lack of a standard biochemical laboratory test for patients with idiopathic histaminergic acquired angioedema, idiopathic nonhistaminergic acquired angioedema, angiotensin-converting enzyme inhibitor-induced acquired angioedema, and hereditary angioedema with normal C1 inhibitor makes the diagnosis more challenging. Future efforts should focus on increasing awareness of all the rare types of angioedema among physicians and developing more straightforward and more accessible diagnostic methods.

[Effects of Histamine and Related Compounds on the Central Nervous System].

There has been little information about the role of histamine on the central nervous system (CNS), different from dopamine and serotonin. In the present study, therefore, the effects of histamine and related compounds on the CNS were studied using rats. Intracerebroventricular (i.c.v.) injection of histamine and 2-methylhistamine ameliorated memory deficit after long interrution of learning in active avoidance response. First generation H1-antagonists inhibited active avoidance response, whereas newly develpoed H1-antagonists showed little effect. α-Fluoromethylhistidine, an histidine decarboxylase inhibitor, also inhibited active avoidance response. In radial maze performance, almost the same findings were obtained. I.c.v. injection of histamine and H1-agonists inhibited amygdaloid kindled seizures. First generation H1-antagonists attenuated histamine-induced inhibition of amygdaloid kindled seizures. Both i.c.v. and intraperitoneal injections of H3-antagonist, thioperamide, resulted in a dose-related inhibition of amygdaloid kindled seizures. The effect of thioperamide was inhibited by an H3-agonists and H1-antagonists. Similar to nitrazepam, diphenhydramine and chlorpheniramine caused a shortening of sleep latency. On the other hand, no significant effects were observed with second generation H1-antagonists. These findings suggest that histamine plays an important role in learning and memory via H1-receptors, an inhibition of amygdaloid kindled seizures induced by histamine occurred through not only H1-receptors but also H3-receptors, and that classic H1-antagonists can be useful as a effective hypnotic for difficulty in falling asleep.

Histamine-dependent interactions between mast cells, glia, and neurons are altered following early-life adversity in mice and humans.

Early-life adversity contributes to the development of functional bowel disorders later in life through unresolved mechanisms. Here, we tested the hypothesis that early-life adversity alters anatomical and functional interactions between mast cells and enteric glia. The effects of early-life stress were studied using the neonatal maternal separation (NMS) stress mouse model. Anatomical relationships between mast cells and enteric glia were assessed using immunohistochemistry and mast cell reporter mice (Mcpt5Cre;GCaMP5g-tdT). Immunohistochemistry was used to assess the expression of histamine, histamine 1 (H1) receptors, and glial fibrillary acidic protein. Functional responses of glia to mast cell mediators were assessed in calcium imaging experiments using Sox10CreERT2;GCaMP5g-tdT mice and cultured human enteric glial cells. NMS increases mast cell numbers at the level of the myenteric plexus and their proximity to myenteric ganglia. Myenteric glia respond to mediators released by activated mast cells that are blocked by H1 receptor antagonists in mice and humans and by blocking neuronal activity with tetrodotoxin in mouse tissue. Histamine replicates the effects of mast cell supernatants on enteric glia, and NMS increases histamine production by mast cells. NMS reduces glial responses to mast cell mediators in mouse tissue, while potentiating responses in cultured human enteric glia. NMS increases myenteric glial fibrillary acidic protein expression and reduces glial process length but does not cause neurodegeneration. Histamine receptor expression is not altered by NMS and is localized to neurons in mice, but glia in humans. Early-life stress increases the potential for interactions between enteric glia and mast cells, and histamine is a potential mediator of mast cell-glial interactions through H1 receptors. We propose that glial-mast cell signaling is a mechanism that contributes to enteric neuroplasticity driven by early-life adversity.NEW & NOTEWORTHY Early-life adversity places an individual at risk for developing functional gastrointestinal disorders later in life through unknown mechanisms. Here, we show that interactions between mast cells and glia are disrupted by early-life stress in mice and that histamine is a potential mediator of mast cell-glial interactions.

Effects of histamine on human periodontal ligament fibroblasts under simulated orthodontic pressure.

As type-I-allergies show an increasing prevalence in the general populace, orthodontic patients may also be affected by histamine release during treatment. Human periodontal ligament fibroblasts (PDLF) are regulators of orthodontic tooth movement. However, the impact of histamine on PDLF in this regard is unknown. Therefore PDLF were incubated without or with an orthodontic compressive force of 2g/cm2 with and without additional histamine. To assess the role of histamine-1-receptor (H1R) H1R-antagonist cetirizine was used. Expression of histamine receptors and important mediators of orthodontic tooth movement were investigated. PDLF expressed histamine receptors H1R, H2R and H4R, but not H3R. Histamine increased the expression of H1R, H2R and H4R as well as of interleukin-6, cyclooxygenase-2, and prostaglandin-E2 secretion even without pressure application and induced receptor activator of NF-kB ligand (RANKL) protein expression with unchanged osteoprotegerin secretion. These effects were not observed in presence of H1R antagonist cetirizine. By expressing histamine receptors, PDLF seem to be able to respond to fluctuating histamine levels in the periodontal tissue. Increased histamine concentration was associated with enhanced expression of proinflammatory mediators and RANKL, suggesting an inductive effect of histamine on PDLF-mediated osteoclastogenesis and orthodontic tooth movement. Since cetirizine inhibited these effects, they seem to be mainly mediated via histamine receptor H1R.

Histaminergic Control of Corticostriatal Synaptic Plasticity during Early Postnatal Development.

A reduction in the synthesis of the neuromodulator histamine has been associated with Tourette's syndrome and obsessive-compulsive disorder. Symptoms of these disorders are thought to arise from a dysfunction or aberrant development ofcorticostriatal circuits. Here, we investigated how histamine affects developing corticostriatal circuits, both acutely and longer-term, during the first postnatal weeks, using patch-clamp and field recordings in mouse brain slices (C57Bl/6, male and female). Immunohistochemistry for histamine-containing axons reveals striatal histaminergic innervation by the second postnatal week, and qRT-PCR shows transcripts for H1, H2, and H3 histamine receptors in striatum from the first postnatal week onwards, with pronounced developmental increases in H3 receptor expression. Whole-cell patch-clamp recordings of striatal spiny projection neurons and histamine superfusion demonstrates expression of functional histamine receptors from the first postnatal week onwards, with histamine having diverse effects on their electrical properties, including depolarization of the membrane potential while simultaneously decreasing action potential output. Striatal field recordings and electrical stimulation of corticostriatal afferents revealed that histamine, acting at H3 receptors, negatively modulates corticostriatal synaptic transmission from the first postnatal week onwards. Last, we investigated effects of histamine on longer-term changes at developing corticostriatal synapses and show that histamine facilitates NMDA receptor-dependent LTP via H3 receptors during the second postnatal week, but inhibits synaptic plasticity at later developmental stages. Together, these results show that histamine acutely modulates developing striatal neurons and synapses and controls longer-term changes in developing corticostriatal circuits, thus providing insight into the possible etiology underlying neurodevelopmental disorders resulting from histamine dysregulation.SIGNIFICANCE STATEMENT Monogenic causes of neurologic disorders, although rare, can provide opportunities to both study and understand the brain. For example, a nonsense mutation in the coding gene for the histamine-synthesizing enzyme has been associated with Tourette's syndrome and obsessive-compulsive disorder, and dysfunction of corticostriatal circuits. Nevertheless, the etiology of these neurodevelopmental disorders and histamine's role in the development of corticostriatal circuits have remained understudied. Here we show that histamine is an active neuromodulator during the earliest periods of postnatal life and acts at developing striatal neurons and synapses. Crucially, we show that histamine permits NMDA receptor-dependent corticostriatal synaptic plasticity during an early critical period of postnatal development, which suggests that genetic or environmental perturbations of histamine levels can impact striatal development.

Chronic brain histamine depletion in adult mice induced depression-like behaviours and impaired sleep-wake cycle.

Histamine acts as a neurotransmitter to regulate various physiological processes. Brain histamine is synthesized from an essential amino acid histidine in a reaction catalysed by histidine decarboxylase (Hdc). Hdc-positive neurons exist mainly in the tuberomammillary nucleus (TMN) of the posterior hypothalamus and project their axons to the entire brain. Recent studies have reported that a chronic decrease in histamine levels in the adult human brain was observed in several neurological disorders. However, it is poorly understood whether lower histamine levels play a causative role in those disorders. In the present study, we induced chronic histamine deficiency in the brains of adult mice to allow direct interpretation of the relationship between an impaired histaminergic nervous system and the resultant phenotype. To induce chronic brain histamine deficiency starting in adulthood, adeno-associated virus expressing Cre recombinase was microinjected into the TMN of Hdc flox mice (cKO mice) at the age of 8 weeks. Immunohistochemical analysis showed expression of Cre recombinase in the TMN of cKO mice. The reduction of histamine contents with the decreased Hdc expression in cKO brain was also confirmed. Behavioural studies revealed that chronic histamine depletion in cKO mice induced depression-like behaviour, decreased locomotor activity in the home cage, and impaired aversive memory. Sleep analysis showed that cKO mice exhibited a decrease in wakefulness and increase in non-rapid eye movement sleep throughout the day. Taken together, this study clearly demonstrates that chronic histamine depletion in the adult mouse brain plays a causative role in brain dysfunction.

Histamine and histidine decarboxylase: Immunomodulatory functions and regulatory mechanisms.

Histamine is a bioactive monoamine that is synthesized by the enzymatic activity of histidine decarboxylase (HDC) in basophils, mast cells, gastric enterochromaffin-like (ECL) cells and histaminergic neuronal cells. Upon a series of cellular stimuli, these cells release stored histamine, which elicits allergies, inflammation, and gastric acid secretion and regulates neuronal activity. Recent studies have shown that certain other types of myeloid lineage cells also produce histamine with HDC induction under various pathogenic stimuli. Histamine has been shown to play a series of pathophysiological roles by modulating immune and inflammatory responses in a number of disease conditions, whereas the mechanistic aspects underlying induced HDC expression remain elusive. In the present review, we summarize the current understanding of the regulatory mechanism of Hdc gene expression and the roles played by histamine in physiological contexts as well as pathogenic processes. We also introduce a newly developed histaminergic cell-monitoring transgenic mouse line (Hdc-BAC-GFP) that serves as a valuable experimental tool to identify the source of histamine and dissect upstream regulatory signals.

A natural mouse model reveals genetic determinants of systemic capillary leak syndrome (Clarkson disease).

The systemic capillary leak syndrome (SCLS, Clarkson disease) is a disorder of unknown etiology characterized by recurrent episodes of vascular leakage of proteins and fluids into peripheral tissues, resulting in whole-body edema and hypotensive shock. The pathologic mechanisms and genetic basis for SCLS remain elusive. Here we identify an inbred mouse strain, SJL, which recapitulates cardinal features of SCLS, including susceptibility to histamine- and infection-triggered vascular leak. We named this trait "Histamine hypersensitivity" (Hhs/Hhs) and mapped it to Chromosome 6. Hhs is syntenic to the genomic locus most strongly associated with SCLS in humans (3p25.3), revealing that the predisposition to develop vascular hyperpermeability has a strong genetic component conserved between humans and mice and providing a naturally occurring animal model for SCLS. Genetic analysis of Hhs may reveal orthologous candidate genes that contribute not only to SCLS, but also to normal and dysregulated mechanisms underlying vascular barrier function more generally.

Activation of orexinergic and histaminergic pathway involved in therapeutic effect of histamine H4 receptor antagonist against cisplatin-induced anorexia in mice.

We previously reported that hypothalamic tumor necrosis factor-alpha (TNF-α) mRNA expression via histamine H4 receptors contributes to the development of cisplatin-induced anorexia; however, its precise mechanisms remain unclear. It has been reported that chemotherapeutic agents induce the suppression of orexin neuron activity, and the administration of orexin inhibits chemotherapeutic agent-induced gastric discomfort. Other studies demonstrated that the central administration of TNF-α impairs the orexinergic system, and that orexin excites the histaminergic system. We investigated the involvement of orexinergic and histaminergic systems in the therapeutic effect of an H4 receptor antagonist against cisplatin-induced anorexia. Cisplatin decreased the expression of prepro-orexin mRNA, which encodes precursors of orexin, in the hypothalamus of mice. The period of expression decreased in parallel with the onset of anorexia, and treatment with an H4 receptor antagonist (JNJ7777120, 10 mg/kg) inhibited the decrease in expression. The effect of the H4 receptor antagonist on cisplatin-induced anorexia in mice was antagonized by an orexin OX2 receptor antagonist (JNJ10397049, 5 mg/kg) rather than an orexin OX1 receptor antagonist (SB408124, 30 mg/kg). Although an OX2 receptor agonist (YNT-185, 20 mg/kg) or a histamine H3 receptor inverse agonist (ciproxifan, 1 mg/kg) inhibited the cisplatin-induced anorexia, the inhibitory effect of the OX2 receptor agonist was antagonized by an H3 receptor silent antagonist (VUF5681, 5 mg/kg). The combination of JNJ7777120 (10 mg/kg) and ciproxifan (0.5 mg/kg) completely resolved the cisplatin-induced anorexia. These results suggest that activation of the orexinergic and histaminergic pathway is involved in the therapeutic effect of an H4 receptor antagonist against cisplatin-induced anorexia.

Compound 48/80 reduces the crop-emptying rate, likely through a histamine-associated pathway in chicks.

Infectious conditions are associated with reduced food passage through the digestive tract in both mammals and chicks; however, the precise mechanism mediating this response in chicks remains unclear. The purpose of the present study was to determine if mast cells, a blood cell type which plays an important role in the immune system, might affect food passage through the digestive tract in chicks. Specifically, we performed intraperitoneal (IP) injections of compound 48/80, an inducer of mast cell degranulation, and measured crop emptying. The IP injection of compound 48/80 significantly reduced the crop-emptying rate, but it did not affect the proventriculus to small intestine transit rate or the number of defecations. We also found that IP-injected histamine, which is secreted by mast cells, also reduced the crop-emptying rate. In addition, IP injection of 2-pyridylethylamine (histamine H1 receptor agonist), but not dimaprit, (R)-(-)-α-methylhistamine, and VUF8430 (histamine H2, H3, and H4 receptor agonists, respectively), reduced the crop-emptying rate, implying that histamine reduces the crop emptying rate via the histamine H1 receptor. Finally, we found that IP injection of compound 48/80 reduced mRNA expression of histidine decarboxylase, a rate-limiting enzyme for histamine synthesis, in the esophagus and proventriculus at 1 h and the proventriculus and duodenum at 3 h after the injection. In sum, the present study suggests that the degranulation of mast cells causes a reduction in the crop-emptying rate, possibly via the histamine pathway in chicks.