Explore the vast range of titles available.

Key Points There are four known receptors for histamine. Antagonists for H 1 and H 2 receptors are currently in clinical use for the treatment of allergies, insomnia and gastric acid disorders. However, there are many conditions such as asthma and chronic pruritus in which histamine has been implicated in the disease pathology, but for which the currently marketed antihistamines have little, if any, effect. The most recently discovered histamine receptor, the H 4 receptor, has been shown to modulate the activity of numerous cell types associated with inflammation and has been shown to drive inflammatory processes in vivo . As indicated, while evidence exists for a role of histamine in driving asthmatic responses, there is little evidence for clinical efficacy of H 1 receptor antagonists for the disease. However, their use at high doses or early in the disease history is supported by recent animal data. Furthermore, preclinical models suggest that H 4 receptors may account for other functions of histamine that are not blocked by the H 1 receptor. Histamine is also the best characterized pruritogen in humans and is known be involved in driving pruritic responses induced by mast-cell degranulation, such as acute urticaria. H 1 receptor antagonists appear to be clinically effective in these conditions, but not so in more chronic pruritic conditions such as those associated with atopic dermatitis or psoriasis. Pruritus in these conditions may be mediated by the H 4 receptor as preclinical models have shown that it is also involved in driving pruritic responses. The recent data on the novel functions of H 1 and H 4 receptors have opened the possibility for new breakthrough therapies for the treatment of conditions such as asthma and pruritus. In addition, there seems to be some overlap in function between the receptors, suggesting that combining H 1 and H 4 receptor antagonism might bring added benefit over monotherapy. Traditional antihistamines that antagonize the histamine H 1 receptor have long been used to treat allergic inflammatory conditions, but so far have proved largely ineffective in other disorders exhibiting elevated histamine levels, including asthma and pruritus. This Review discusses how the identification and understanding of the roles of the histamine H 4 receptor has revealed the potential of targeting H 4 receptors or combined H 1 and H 4 receptor blockade to treat such disorders. Histamine has a key role in allergic inflammatory conditions. The inflammatory responses resulting from the liberation of histamine have long been thought to be mediated by the histamine H 1 receptor, and H 1 -receptor antagonists — commonly known as antihistamines — have been used to treat allergies for many years. However, the importance of histamine in the pathology of conditions such as asthma and chronic pruritus may have been underestimated. Here, we review accumulating evidence suggesting that histamine indeed has roles in inflammation and immune function modulation in such diseases. In particular, the discovery of a fourth histamine receptor (H 4 ) and its expression on numerous immune and inflammatory cells has prompted a re-evaluation of the actions of histamine, suggesting a new potential for H 4 -receptor antagonists and a possible synergy between H 1 and H 4 -receptor antagonists in targeting various inflammatory conditions.

This article reviews the development of our knowledge of the actions of histamine which have taken place during the course of the 20th century. Histamine has been shown to have a key physiological role in the control of gastric acid secretion and a pathophysiological role in a range of allergic disorders. The synthesis of, and pharmacological studies on, selective agonists and antagonists has established the existence of four types of histamine receptor and histamine receptor antagonists have found very important therapeutic applications. Thus, in the 1940s, H1‐receptor antagonists (‘the antihistamines’) yielded and still provide valuable treatment for allergic conditions such as hay fever and rhinitis. In the late 1970s and 1980s, H2‐receptor antagonists (in the discovery of which the two authors were personally involved) revolutionised the treatment of peptic ulcer and other gastric acid‐related diseases. The H3‐receptor antagonists, although available since 1987, have been slower to find a therapeutic role. However, the discovery of nonimidazole derivatives such as brain‐penetrating H3 antagonists has provided drugs that are in early‐phase clinical trials, possibly for application in obesity, and a variety of central nervous system disorders, such as memory, learning deficits and epilepsy. Finally, the most recently (1999) discovered H4 receptor promises the potential to provide drugs acting on the immunological system with possible applications in asthma and inflammation. British Journal of Pharmacology (2006) 147, S127–S135. doi:10.1038/sj.bjp.0706440

Background Histamine is commonly acknowledged as an inflammatory mediator in peripheral tissues, leaving its role in brain immune responses scarcely studied. Therefore, our aim was to uncover the cellular and molecular mechanisms elicited by this molecule and its receptors in microglia-induced inflammation by evaluating cell migration and inflammatory mediator release. Methods Firstly, we detected the expression of all known histamine receptor subtypes (H 1 R, H 2 R, H 3 R and H 4 R), using a murine microglial cell line and primary microglia cell cultures from rat cortex, by real-time PCR analysis, immunocytochemistry and Western blotting. Then, we evaluated the role of histamine in microglial cell motility by performing scratch wound assays. Results were further confirmed using murine cortex explants. Finally, interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) levels were evaluated by ELISA measurements to determine the role of histamine on the release of these inflammatory mediators. Results After 12 h of treatment, 100 μM histamine and 10 μg/ml histamine-loaded poly (lactic- co -glycolic acid) microparticles significantly stimulated microglia motility via H 4 R activation. In addition, migration involves α5β1 integrins, and p38 and Akt signaling pathways. Migration of microglial cells was also enhanced in the presence of lipopolysaccharide (LPS, 100 ng/ml), used as a positive control. Importantly, histamine inhibited LPS-stimulated migration via H 4 R activation. Histamine or H 4 R agonist also inhibited LPS-induced IL-1β release in both N9 microglia cell line and hippocampal organotypic slice cultures. Conclusions To our knowledge, we are the first to show a dual role of histamine in the modulation of microglial inflammatory responses. Altogether, our data suggest that histamine per se triggers microglia motility, whereas histamine impedes LPS-induced microglia migration and IL-1β release. This last datum assigns a new putative anti-inflammatory role for histamine, acting via H 4 R to restrain exacerbated microglial responses under inflammatory challenge, which could have strong repercussions in the treatment of CNS disorders accompanied by microglia-derived inflammation.

Non-reinforced retrieval induces memory extinction, a phenomenon characterized by a decrease in the intensity of the learned response. This attribute has been used to develop extinction-based therapies to treat anxiety and post-traumatic stress disorders. Histamine modulates memory and anxiety but its role on fear extinction has not yet been evaluated. Therefore, using male Wistar rats, we determined the effect of the intra-hippocampal administration of different histaminergic agents on the extinction of step-down inhibitory avoidance (IA), a form of aversive learning. We found that intra-CA1 infusion of histamine immediately after non-reinforced retrieval facilitated consolidation of IA extinction in a dose-dependent manner. This facilitation was mimicked by the histamine N-methyltransferase inhibitor SKF91488 and the H2 receptor agonist dimaprit, reversed by the H2 receptor antagonist ranitidine, and unaffected by the H1 antagonist pyrilamine, the H3 antagonist thioperamide and the antagonist at the NMDA receptor (NMDAR) polyamine-binding site ifenprodil. Neither the H1 agonist 2-2-pyridylethylamine nor the NMDAR polyamine-binding site agonist spermidine affected the consolidation of extinction while the H3 receptor agonist imetit hampered it. Extinction induced the phosphorylation of ERK1 in dorsal CA1 while intra-CA1 infusion of the MEK inhibitor U0126 blocked extinction of the avoidance response. The extinction-induced phosphorylation of ERK1 was enhanced by histamine and dimaprit and blocked by ranitidine administered to dorsal CA1 after non-reinforced retrieval. Taken together, our data indicate that the hippocampal histaminergic system modulates the consolidation of fear extinction through a mechanism involving the H2-dependent activation of ERK signalling.

Orexin neurons are exclusively localized in the lateral hypothalamic area and project their fibers to the entire central nervous system, including the histaminergic tuberomammillary nucleus (TMN). Dysfunction of the orexin system results in the sleep disorder narcolepsy, but the role of orexin in physiological sleep-wake regulation and the mechanisms involved remain to be elucidated. Here we provide several lines of evidence that orexin A induces wakefulness by means of the TMN and histamine H1receptor (H1R). Perfusion of orexin A (5 and 25 pmol/min) for 1 hr into the TMN of rats through a microdialysis probe promptly increased wakefulness for 2 hr after starting the perfusion by 2.5- and 4-fold, respectively, concomitant with a reduction in rapid eye movement (REM) and non-REM sleep. Microdialysis studies showed that application of orexin A to the TMN increased histamine release from both the medial preoptic area and the frontal cortex by ≈2-fold over the baseline for 80 to 160 min in a dose-dependent manner. Furthermore, infusion of orexin A (1.5 pmol/min) for 6 hr into the lateral ventricle of mice produced a significant increase in wakefulness during the 8 hr after starting infusion to the same level as the wakefulness observed during the active period in wild-type mice, but not at all in H1R gene knockout mice. These findings strongly indicate that the arousal effect of orexin A depends on the activation of histaminergic neurotransmission mediated by H1R.

Mast cells trigger inflammation that is associated with local pain, but the mechanisms mediating pain are unclear. Interstitial cystitis (IC) is a bladder disease that causes debilitating pelvic pain of unknown origin and without consistent inflammation, but IC symptoms correlate with elevated bladder lamina propria mast cell counts. We hypothesized that mast cells mediate pelvic pain directly and examined pain behavior using a murine model that recapitulates key aspects of IC. Infection of mice with pseudorabies virus (PRV) induces a neurogenic cystitis associated with lamina propria mast cell accumulation dependent upon tumor necrosis factor alpha (TNF), TNF-mediated bladder barrier dysfunction, and pelvic pain behavior, but the molecular basis for pelvic pain is unknown. In this study, both PRV-induced pelvic pain and bladder pathophysiology were abrogated in mast cell-deficient mice but were restored by reconstitution with wild type bone marrow. Pelvic pain developed normally in TNF- and TNF receptor-deficient mice, while bladder pathophysiology was abrogated. Conversely, genetic or pharmacologic disruption of histamine receptor H1R or H2R attenuated pelvic pain without altering pathophysiology. These data demonstrate that mast cells promote cystitis pain and bladder pathophysiology through the separable actions of histamine and TNF, respectively. Therefore, pain is independent of pathology and inflammation, and histamine receptors represent direct therapeutic targets for pain in IC and other chronic pain conditions.

Pitolisant (Wakix™) is an inverse agonist of the histamine H 3 receptor that is being developed by Bioproject. Oral pitolisant is approved in the EU for the treatment of narcolepsy with or without cataplexy in adults. Pitolisant has received a Temporary Authorization of Use in France for this indication in case of treatment failure, intolerance or contraindication to currently available treatment. Pitolisant has orphan drug designation in the EU and the USA. In the pivotal HARMONY I trial, pitolisant significantly decreased excessive daytime sleepiness versus placebo in adults with narcolepsy with or without cataplexy (primary endpoint). Pitolisant also significantly decreased cataplexy rate versus placebo in these patients. This article summarizes the milestones in the development of pitolisant leading to this first approval for narcolepsy.

How do fluctuations in the level of generalized arousal of the brain affect the performance of specific motivated behaviors, such as sexual behaviors that depend on sexual arousal? A great deal of previous work has provided us with two important starting points in answering this question: (i) that histamine (HA) serves generalized CNS arousal and (ii) that heightened electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMN) is necessary and sufficient for facilitating the primary female sex behavior in laboratory animals, lordosis behavior. Here we used patch clamp recording technology to analyze HA effects on VMN neuronal activity. The results show that HA acting through H1 receptors (H1R) depolarizes these neurons. Further, acute administration of estradiol, an estrogen necessary for lordosis behavior to occur, heightens this effect. Hyperpolarization, which tends to decrease excitability and enhance inhibition, was not affected by acute estradiol or mediated by H1R but was mediated by other HA receptor subtypes, H2 and H3. Sampling of mRNA from individual VMN neurons showed colocalization of expression of H1 receptor mRNA with estrogen receptor (ER)-α mRNA but also revealed ER colocalization with the other HA receptor subtypes and colocalization of different subtypes with each other. The latter finding provides the molecular basis for complex \"push-pull\" regulation of VMN neuronal excitability by HA. Thus, in the simplest causal route, HA, acting on VMN neurons through H1R provides a mechanism by which elevated states of generalized CNS arousal can foster a specific estrogen-dependent, aroused behavior, sexual behavior.

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 histamine H 4 receptor is expressed preferentially on immune cells, indicating a possible role of the H 4 receptor in inflammation. Studies of inflammation in several animal models point to a pro-inflammatory function of the H 4 receptor. However, studies on experimental murine bronchial asthma yielded conflicting results, a fact which is neglected in most H 4 receptor publications. Therefore, the present review critically analyzes available data on the role of the H 4 receptor in the murine bronchial asthma model.