Role of Histaminergic System in Blood–Brain Barrier Dysfunction Associated with Neurological Disorders

Blood–brain barrier (BBB) disruption has been associated with several acute and chronic brain disorders such as Alzheimer's disease, Parkinson's disease and epilepsy. This represents a critical situation because damaged integrity of the BBB is related to the influx of immune mediators, plasma proteins and other outside elements from blood to the central nervous system (CNS) that may trigger a cascade of events that leads to neuroinflammation. In this review, evidence that mast cells and the release of factors such as histamine play an important role in the neuroinflammatory process associated with brain disorders such as Alzheimer's disease, Parkinson's disease and epilepsy is presented.

Introduction

Inflammation is the result of a chain of events that are triggered in response to the presence of different types of agents harmful to the organism. Initially, inflammation is considered a defensive event that occurs to isolate and destroy the noxious agent and repair any damage caused. However, changes associated with inflammation can seriously damage the body and lead to death in case of multiorgan failure.

In the nervous system, neuroinflammation is a critical process associated with neurological disorders such as Alzheimer's disease, Parkinson's disease and epilepsy, among others. Neuroinflammation involves the release of various factors that cause changes in the brain parenchyma, breakdown of the blood–brain barrier (BBB), neuronal hyperexcitability and cell damage. In this review, the possible involvement of cerebral histamine in BBB dysfunction during neuroinflammation will be analyzed as well as the therapeutic significance of drugs that block this amine.

Histamine (2-[4-imidazole] ethylamine) is a biogenic amine with an imidazole ring and two nitrogen atoms. Histamine is synthesized from the amino acid L-histidine via the enzyme histidine decarboxylase (HDC) in the presence of the cofactor pyridoxal phosphate. L-histidine is carried to the histaminergic nerve terminals by the L-amino acid transporter.

Histamine was discovered by Dale and Laidlaw in 1910 who described that it produces vasodilation and smooth muscle contraction in the gut (1). Subsequently, stimulation of the secretion of stomach acid by histamine was recognized (2). It was not until 1941 that Kwiatkowski et al. (3) described the presence of histamine in the brain and in 1959 White et al. (4) demonstrated histamine's formation and catabolism in this organ.

In the brain, histamine is found in different cell types. At the neuronal level and once synthesized, histamine is internalized into vesicles by vesicular monoamine transporter 2 (5) and is released into the extracellular space by exocytosis in response to the depolarization of axon terminals (6). It is proposed that the inactivation of histamine in the central nervous system (CNS) requires the enzyme histamine N-methyltransferase (HNMT), which degrades histamine to tele-methylhistamine. However, HNMT is a soluble enzyme located in the cytosol of neurons, glia and endothelial cells and it is not known how histamine is drawn into the intracellular space to be inactivated (7) (Figure 1).

Using antibodies against HDC, which is involved in histamine synthesis, Panula et al. (8) identified histaminergic neurons whose cell bodies range from 25–30 μM in diameter, exclusively located in the tuberomammillary nucleus of the posterior hypothalamus and that give rise to an extensive network of projections throughout the CNS. These histaminergic neurons send axons to almost the entire brain and part of the spinal cord 8, 9. The firing of histaminergic neurons can be influenced by the presence of certain substances in the cerebrospinal fluid (CSF) such as cytokines, as some of the dendrites of these neurons are in contact with the third ventricle and subarachnoid space 10, 11.

Histamine is also found in mast cells, which originate from stem cells in the bone marrow and are significantly involved in inflammatory processes. In the CNS, mast cells are found in the leptomeninges, choroid plexus and brain parenchyma (12). In the latter, 97% of mast cells are found in the perivascular zone of the microvasculature, and it has been suggested that they form part of the neurovascular unit 13, 14.

Different molecules associated with immune responses including cytokines, chemokines, eicosanoids, basic fibroblast growth factor and angiogenic factors such as vascular endothelial growth factor (VEGF) facilitate the migration and degranulation of mast cells 15, 16, 17.

In addition to mast cells and histaminergic neurons, microglia, the resident immune cells in the brain, can express the enzymatic machinery to produce histamine (18). It has also been reported that endothelial cells in the cerebral microvasculature of rats express HDC mRNA and that they are capable of capturing L-histidine to convert it to histamine, which suggests that the latter plays an important role in the BBB 19, 20.

Brain histamine is involved in various processes such as circadian rhythms, the sleep-wake cycle, thermoregulation, neuroendocrine regulation, water and food intake, locomotion, sexual behavior, memory and learning, among others 21, 22.

Evidence indicates that brain histamine is immunomodulatory. For instance, in microglia activated through the histamine receptor subtypes H₁ and H₄, histamine promotes the release of proinflammatory mediators such as the cytokines tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6 and IL-1β 23, 24. Histamine also favors the release of nitric oxide from microglia, likely by inducing the expression of inducible nitric oxide synthase (iNOS) (25).

Histamine exerts its physiological effects through four different subtypes of G protein-coupled receptors (H₁–H₄) (26). In the CNS, receptors H₁ and H₂, which couple to G_αq and G_αs, respectively, are expressed in neurons, glia and vascular endothelial cells. These receptors are located postsynaptically on neurons and are expressed in the cortex, hippocampus, striatum and hypothalamus 27, 28, 29. The H₃ receptor, originally characterized as a presynaptic autoreceptor located in the soma, dendrites and axon varicosities of histaminergic neurons, is also found as a heteroreceptor in different neuronal populations 30, 31. H₃ autoreceptors modulate the synthesis and release of histamine, whereas H₃ heteroreceptors modulate the release of other neurotransmitters 32, 33. Furthermore, autoradiography studies revealed the presence of postsynaptic H₃ receptors on the soma, dendrites and projections of many neuronal populations (34). However, the postsynaptic physiological role of H₃ receptors remains unknown (35). The H₃ receptor is predominantly expressed in neurons, has the highest affinity for histamine and couples to the proteins G_αi/o. Finally, the H₄ receptor, which is coupled to a G_αi/o, is expressed in mast cells, leukocytes, eosinophils, basophils and T cells. This relationship indicates that the H₄ receptor plays an important role in the immune system by regulating cell migration and peripheral inflammation 36, 37, 38. The H₄ receptor is functionally expressed in neurons (39) and, together with the H₃ receptor, it is suggested that they control BBB permeability and the migration of immune cells into the CNS (40) (Figure 1, Table 1).

The notion of BBB was derived from observations made by Ehrlich in 1885 and later by Goldman in 1913 41, 42 who established that the brain parenchyma is isolated from the rest of the body. More than a decade later (1927), Stern and Peyrot established the concept of BBB (43).

The BBB is composed of vascular endothelial cells surrounded by basal membrane, pericytes, end-feet of astroglial cells and microglia (44). The basal membrane is located between the plasma membranes of endothelial cells and astrocytic end-feet and consists of a mixture of proteins including fibronectin, heparin sulfate proteoglycans and collagen IV (45). The assembly of cells comprising perycites, glia, neurons and endothelial cells of the BBB, constitutes the neurovascular unit (Figure 2) in which these components are tightly and reciprocally related to each other, establishing an anatomic and functional entity (46).

The BBB maintains an optimum ionic composition of CSF, thus providing a stable environment for proper brain function (47). It regulates the flow of amino acids, including glutamate, from the plasma to the brain, avoiding neurotoxicity 48, 49. The BBB prevents the penetration of many macromolecules and proteins such as albumin, prothrombin and plasminogen to CNS that damage nervous tissue and can lead to neuroinflammation and cell death 50, 51. In addition, the endothelium of the BBB expresses transporters that allow the entry of water-soluble compounds that are essential for the CNS 45, 49. The tight junctions between the plasma membranes of endothelial cells block the flux of macromolecules, ions and polar solutes across the BBB (52) (Figure 2). Some of the major components of the tight junctions of the BBB that have been identified and characterized are claudins, occludins, and the zonula occludens (ZO) proteins ZO-1, -2 and -3. The last three proteins are members of a family of membrane-associated guanylate kinases containing important domains for signal transduction and anchoring the transmembrane proteins that form tight junctions to the cytoskeleton (53). ZO-1 protein expression in particular plays an important role in the physiology of tight junctions as indicated by the effects that various agents such as glucocorticoids, cytokines and transcription factors exert on its expression (54).

It has also been suggested that mast cells form part of the neurovascular unit. During the early stages of development, immature mast cells from the leptomeninges enter the brain by penetrating across blood vessels and remain attached to the vessels (55). Brain mast cells are not numerous and are mainly of the tryptase-chymase phenotype (56). One study demonstrated that mast cells are constituents of the brain blood vessel wall and are surrounded by the basal lamina of the endothelia and/or smooth muscle cells. By using quantitative stereological measures, it was also shown that brain mast cells reside in large, mature blood vessels and are preferentially associated with sites of angiogenesis (14).

Some evidence suggests that the activation of mast cells plays a role in neuroinflammation. In this regard, it is postulated that hypoxia and cerebral ischemia initially activate mast cells which, in turn, release vasoactive mediators such as serotonin, histamine, TNF-α, heparin and proteases 57, 58, 59, 60, 61. These factors stimulate inflammatory cascades that ultimately result in brain and BBB damage (62).

It is possible that the migration and degranulation of mast cells is facilitated during cerebral ischemia, resulting in acute brain disorders as well as the subsequent breakdown of the BBB. This process can result in histamine release which, in turn, binds to H₁ and H₂ receptors on endothelial cells, increasing selective BBB permeability (63) and contributing to the formation of ischemic cerebral edema (64). Mast cells also contain proteolytic gelatinase enzymes (matrix metalloproteinase (MMP)-2 and MMP-9) that can cause the degradation of claudin-5 (65) and of the basal lamina of the BBB upon release, leading to consequent rupture (66).

Angiogenesis and vascular remodeling are involved in the pathophysiology of several chronic inflammatory diseases 67, 68. It has been suggested that the inflammatory state can promote angiogenesis and that angiogenesis can facilitate chronic inflammation. In chronic inflammatory conditions, new blood vessels are used to transport inflammatory cells, nutrients and oxygen to the site of inflammation. The increased endothelial surface area resulting from angiogenesis promotes the production of cytokines, adhesion molecules and other inflammatory stimuli (68). Furthermore, some inflammatory mediators act directly or indirectly to promote angiogenesis, increasing the expression of angiogenic factors such as VEGF (69). In neuroinflammatory disorders such as multiple sclerosis, there is an increase in vascularity and in VEGF expression in the CNS (70). These findings suggest that angiogenesis plays a significant role in the progression of disorders associated with chronic inflammation.

Mast cells contain and release factors that promote angiogenesis, such as VEGF, cytokines, chemokines and proteases (71). The results from patients and experimental models suggest that VEGF is involved in vascular remodeling through its receptor VEGFR-2 (72), and it alters the permeability of blood vessels and facilitates the extravasation of plasma proteins 73, 74, 75. Mast cells represent the only reservoir of preformed TNF-α, which is a cytokine involved in the acute phase of the inflammatory reaction (76). In animal models, it has been observed that TNF-α activation induces edema and damages the endothelial cells of the BBB 77, 78. Additionally, previous reports indicate that TNF-α has the ability to recruit α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors lacking the GluR2 subunit at the neuronal membrane, favoring the influx of Ca²⁺ into neurons 79, 80. Moreover, TNF-α can induce the endocytosis of γ-aminobutyric acid (GABA_A) receptors (81), a situation that favors resistance to GABAergic agonists 82, 83.

Damage to the BBB facilitates the entry of blood components through the microvasculature, perivascular accumulation of immunoglobulin G (IgG), extravasation of albumin 84, 85, ionic changes, modulation of ion channels 86, 87 and neurotransmitters such as glutamate and adenosine and metabolic disturbances (changes in glucose levels, pH and CO₂, among others) (85) (Figure 2).

Neuroinflammation and BBB damage contribute significantly to the pathogenesis of several neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, multiple sclerosis and epilepsy 88, 89, 90, 91, 92. Because mast cells and the mediators they secrete, especially histamine, modulate inflammation and regulate the permeability of the BBB in several disorders of the central nervous system 93, 94, 95, 96, pharmacological inhibition of the activation of these cells has been considered therapeutically significant (Figure 2).

Several studies support the hypothesis that mast cells play an important role in the pathogenesis of Alzheimer's disease. There is evidence that fibrillar β-amyloid peptides, which are increased in Alzheimer's disease, induce the exocytosis of histamine from mast cells (97). The infiltration of large numbers of mast cells has also been found in the brains of Alzheimer's patients (98). Moreover, administration of mastinib, a mast cell stabilizer, decreases some of the neurological signs associated with Alzheimer's disease (99).

The activation of mast cells, without undergoing degranulation, is associated with the selective release of mediators such as nerve growth factor, which can exert a neuroprotective role in experimental models of Parkinson's disease 100, 101. Additionally, some beneficial effects of drugs associated with the dopaminergic system used in the early stages of Parkinson's disease are associated with blocking the degranulation of mast cells (102).

Experimental evidence suggests that mast cells are involved in the pathophysiology of cerebral ischemia, whereas their stabilization with agents such as sodium cromoglycate could represent a new therapeutic strategy in the prevention of brain damage induced by ischemic stroke (103). In addition, it was demonstrated that histamine receptor blocking agents prevent brain edema formation and neuronal death in animal models of cerebral ischemia 104, 105.

It has been documented that patients with multiple sclerosis have increased expression of tryptases and IgE in their brain and systemic mast cells 106, 107 as well as increased histamine levels in the CSF (108). Furthermore, stabilization of mast cells can reduce the development of experimental autoimmune encephalomyelitis (109), the most commonly used experimental model of human multiple sclerosis.

The participation of mast cells in the formation and progression of brain tumors is complex because their activation can promote or suppress tumor progression (110). It has further been suggested that mast cells contribute to the progression of gliomas because the degree of infiltration correlates positively with the degree of malignancy (111).

It is known that mast cells accumulate in the stroma surrounding tumors and can release mediators such as histamine, IL-8, tryptase and VEGF which, in turn, alter the BBB and favor metastasis (112). Blocking such events is important because it is known that metastasis is a major cause of mortality in patients suffering from the most common forms of cancer, including brain cancer (113).

Certain factors secreted by mast cells are involved in the modulation of behavior as well as in psychiatric disorders such as depression. For instance, patients with mastocytosis, a disease characterized by mast cell accumulation, present an elevated prevalence of depression, a symptom that is reduced with the pharmacological stabilization of mast cells (114). Recently, it has been shown that several antidepressants attenuate the neuroinflammatory response through stabilizing mast cells and blocking their degranulation (115). In addition, the stimulated release of histamine from mast cells induces behavioral changes resembling anxiety in an animal model and the activation of H1 receptors has an anxiogenic effect (116). Furthermore, it has also been observed that the inhibition of CNS mast cells attenuates anxiety in mice 117, 118.

The relationship between the brain inflammatory response, the damaged integrity of BBB, epileptogenesis and seizures is complex. At present, it is known that neuroinflammation plays a significant role in epileptogenesis (119). Furthermore, recurrent seizures promote an increase in the activation of cytokines which, in turn, contributes to neuronal damage (120). Neuronal damage facilitates long-term brain inflammation 121, 122, 123, 124, thus creating a favorable condition for both situations. Another important factor is that brain inflammation can persist for several days after the termination of seizures 121, 122, 123, 124, 125, a condition that may predispose to cell damage and changes in neuronal excitability, thus facilitating epileptogenesis (126).

It has been suggested that the activation of proinflammatory molecules is initiated at the level of the astrocytes and microglia in the brain parenchyma as a result of seizure activity 127, 128, 129. Moreover, BBB dysfunction triggers a change of events including extravasation of blood constituents, inflammation, glial activation and increases in proinflammatory cytokine levels that ultimately may lead to epilepsy 130, 131, 132. However, it is unknown how the inflammatory process spreads to the cerebral microvasculature, activating the perivascular inflammatory processes that result in damage to the BBB and extravasation of leukocytes and serum proteins to the brain 128, 133, 134. Activation of mast cells and release of histamine represent candidates for explaining the disruption of the BBB secondary to neuroinflammation and epileptic activity. In this regard there are studies suggesting that seizure activity is associated with increased serum histamine (135). In addition, it is known that hyperthermia facilitates seizure activity and status epilepticus (136) and that the latter increases brain expression of substance P (137). Mast cells can migrate and degranulate by hyperthermia or changes in the levels of substance P 61, 135, 138. Mast cells activate vascular and intracellular adhesion molecules such as vascular cell adhesion molecule (VCAM)-1 and intercellular adhesion molecule (ICAM-1), which regulate the adhesion and extravasation of blood cells through the blood microvasculature 139, 140. Experimental data indicate that status epilepticus and epileptiform activity increase the brain expression of VCAM-1 141, 142, primarily in areas most affected by such events (143).

The involvement of neuroinflammation in epilepsy and epileptogenesis has prompted the search for drugs with anti-inflammatory and anti-epileptogenic properties. Pharmacological blockage of pro-inflammatory pathways by the administration of interleukin antagonists or steroids reduces seizure activity and epileptogenesis in experimental models 128, 142, 144, 145, 146, 147, 148, 149. For example, application of FTY720 (a potent immunosuppressant), NS-398 and celecoxib (both COX-2 inhibitors) induced anti-inflammatory effects and reduced epileptogenesis in experimental models of epilepsy of the temporal lobe secondary to pilocarpine 150, 151. Experimental and clinical data show that steroids such as hydrocortisone or dexamethasone improve BBB integrity in epileptic conditions 148, 152. With respect to dexamethasone, it has been observed to induce a pro- or anticonvulsant effect, a result that may depend on the experimental model used (153). On the other hand, recent evidence indicates that antiepileptic drugs by themselves may modulate the neuroinflammatory response. For instance, it was found that some of the most effective antiepileptic drugs could induce a brain anti-inflammatory effect 154, 155, 156. Although to our knowledge there is no systematic study addressing the effect of anti-seizure drugs on mast cell activation, a report showed that the local anesthetic lidocaine, which is a sodium channel blocker, inhibited histamine released induced by mast cell activation (157). In contrast, the benzodiazepine diazepam, a drug usually used to treat status epilepticus, induced a small release of histamine from mast cells 158, 159. On the basis of these two studies, the possibility that the antiepileptic drugs with a mechanism of action involving sodium channel blockade could decrease mast cell activation may be worth investigating in future studies.