Tadaho Nakamura

Molecular mechanism of histamine clearance by primary human astrocytes

Histamine clearance is an essential process for avoiding excessive histaminergic neuronal activity. Previous studies using rodents revealed the predominant role of astrocytes in brain histamine clearance. However, the molecular mechanism of histamine clearance has remained unclear. We detected histamine N‐methyltransferase (HNMT), a histamine‐metabolizing enzyme, in primary human astrocytes and the astrocytes of human brain specimens. Immunocytochemical analysis and subcellular fractionation assays revealed that active HNMT localized to the cytosol, suggesting that histamine transport into the cytosol is crucial for histamine inactivation. We showed that primary human astrocytes transported histamine in a time‐dependent manner. Kinetics analysis showed that two low‐affinity transporters were involved in histamine transport. Histamine uptake by primary human astrocytes was not dependent on the extracellular Na+/Cl− concentration. Histamine is reported to be a substrate for three low‐affinity and Na+/Cl−‐independent transporters: organic cation transporter 2 (OCT2), OCT3, and plasma membrane monoamine transporter (PMAT). RT‐PCR analysis revealed that OCT3 and PMAT were expressed in primary human astrocytes. Immunohistochemistry confirmed OCT3 and PMAT expression in the astrocytes of human brain specimens. Drug inhibition assays and gene knockdown assays revealed the major contribution of PMAT and the minor contribution of OCT3 to histamine transport. The present study demonstrates for the first time that the molecular mechanism of histamine clearance is by primary human astrocytes. These findings might indicate that PMAT, OCT3 and HNMT in human astrocytes play a role in the regulation of extraneuronal histamine concentration and the activities of histaminergic neurons.

Positron emission tomography evaluation of sedative properties of antihistamines

Introduction: H1 antihistamines are often used in the medication for allergic diseases, coughs and colds, and insomnia, with or without prescription, even though their sedative properties are a potentially dangerous unwanted side effect that is not properly recognized. These sedative properties have been evaluated using the incidence of subjective sleepiness, objective cognitive and psychomotor functions, and positron emission tomography (PET) measurement of H1 receptor occupancy. Areas covered: This article reviews the current updated literature on the sedative properties of antihistamines examined by PET measurement of H1 receptor occupancy. Expert opinion: The use of PET to examine antihistamine penetration in the human brain in relation to psychometric and other functional measures of CNS effects is a major breakthrough and provides a new standard by which the functional CNS effects of antihistamines can be related directly to H1 receptor occupancy. Therapy with antihistamines can be better guided by considering histamine H1 receptor occupancy from the view of their sedative properties.

Correlations of 18F-THK5351 PET with Postmortem Burden of Tau and Astrogliosis in Alzheimer Disease

Clinical PET studies using 18F-THK5351 have demonstrated significant tracer retention in sites susceptible to tau burden in Alzheimer disease (AD). However, the in vivo PET signal to reflect tau aggregates remains controversial. Methods: We examined the spatial pattern of tracer binding, amyloid-β, tau, and gliosis in an autopsy-confirmed AD patient who underwent 18F-THK5351 and 11C-Pittsburgh compound B PET before death. Results: Regional in vivo 18F-THK5351 retention was significantly correlated with the density of tau aggregates in the neocortex and monoamine oxidase-B in the whole brain, but not correlated with that of insoluble amyloid-β. Furthermore, significant association was observed between the density of tau aggregates, monoamine oxidase-B, and glial fibrillary acidic protein, suggesting that neocortical tau would strongly influence the formation of reactive astrocytes. Conclusion: 18F-THK5351 PET may have limited utility as a biomarker of tau pathology in AD; however, it could be used to monitor the neuroinflammatory processes in the living brain.

Predominant role of plasma membrane monoamine transporters in monoamine transport in 1321N1, a human astrocytoma‐derived cell line

Monoamine neurotransmitters should be immediately removed from the synaptic cleft to avoid excessive neuronal activity. Recent studies have shown that astrocytes and neurons are involved in monoamine removal. However, the mechanism of monoamine transport by astrocytes is not entirely clear. We aimed to elucidate the transporters responsible for monoamine transport in 1321N1, a human astrocytoma‐derived cell line. First, we confirmed that 1321N1 cells transported dopamine, serotonin, norepinephrine, and histamine in a time‐ and dose‐dependent manner. Kinetics analysis suggested the involvement of low‐affinity monoamine transporters, such as organic cation transporter (OCT) 2 and 3 and plasma membrane monoamine transporter (PMAT). Monoamine transport in 1321N1 cells was not Na+/Cl− dependent but was inhibited by decynium‐22, an inhibitor of low‐affinity monoamine transporters, which supported the importance of low‐affinity transporters. RT‐PCR assays revealed that 1321N1 cells expressed OCT3 and PMAT but no other neurotransmitter transporters. Another human astrocytoma‐derived cell line, U251MG, and primary human astrocytes also exhibited the same gene expression pattern. Gene‐knockdown assays revealed that 1321N1 and primary human astrocytes could transport monoamines predominantly through PMAT and partly through OCT3. These results might indicate that PMAT and OCT3 in human astrocytes are involved in monoamine clearance.

Histamine H3 receptor in primary mouse microglia inhibits chemotaxis, phagocytosis, and cytokine secretion.

Histamine is a physiological amine which initiates a multitude of physiological responses by binding to four known G‐protein coupled histamine receptor subtypes as follows: histamine H1 receptor (H1R), H2R, H3R, and H4R. Brain histamine elicits neuronal excitation and regulates a variety of physiological processes such as learning and memory, sleep–awake cycle and appetite regulation. Microglia, the resident macrophages in the brain, express histamine receptors; however, the effects of histamine on critical microglial functions such as chemotaxis, phagocytosis, and cytokine secretion have not been examined in primary cells. We demonstrated that mouse primary microglia express H2R, H3R, histidine decarboxylase, a histamine synthase, and histamine N‐methyltransferase, a histamine metabolizing enzyme. Both forskolin‐induced cAMP accumulation and ATP‐induced intracellular Ca2+ transients were reduced by the H3R agonist imetit but not the H2R agonist amthamine. H3R activation on two ubiquitous second messenger signalling pathways suggests that H3R can regulate various microglial functions. In fact, histamine and imetit dose‐dependently inhibited microglial chemotaxis, phagocytosis, and lipopolysaccharide (LPS)‐induced cytokine production. Furthermore, we confirmed that microglia produced histamine in the presence of LPS, suggesting that H3R activation regulate microglial function by autocrine and/or paracrine signalling. In conclusion, we demonstrate the involvement of histamine in primary microglial functions, providing the novel insight into physiological roles of brain histamine. GLIA 2015;63:1213–1225

Mechanism of the histamine H3 receptor-mediated increase in exploratory locomotor activity and anxiety-like behaviours in mice

Histaminergic neurons are activated by histamine H(3) receptor (H(3)R) antagonists, increasing histamine and other neurotransmitters in the brain. The prototype H(3)R antagonist thioperamide increases locomotor activity and anxiety-like behaviours; however, the mechanisms underlying these effects have not been fully elucidated. This study aimed to determine the mechanism underlying H(3)R-mediated behavioural changes using a specific H(3)R antagonist, JNJ-10181457 (JNJ). First, we examined the effect of JNJ injection to mice on the concentrations of brain monoamines and their metabolites. JNJ exclusively increased N(τ)-methylhistamine, the metabolite of brain histamine used as an indicator of histamine release, suggesting that JNJ dominantly stimulates the release of histamine release but not of other monoamines. Next, we examined the mechanism underlying JNJ-induced behavioural changes using open-field tests and elevated zero maze tests. JNJ-induced increase in locomotor activity was inhibited by α-fluoromethyl histidine, an inhibitor of histamine synthesis, supporting that H(3)R exerted its effect through histamine neurotransmission. The JNJ-induced increase in locomotor activity in wild-type mice was preserved in H(1)R gene knockout mice but not in histamine H2 receptor (H(2)R) gene knockout mice. JNJ-induced anxiety-like behaviours were partially reduced by diphenhydramine, an H(1)R antagonist, and dominantly by zolantidine, an H(2)R antagonist. These results suggest that H(3)R blockade induces histamine release, activates H(2)R and elicits exploratory locomotor activity and anxiety-like behaviours.

The expression and function of histamine H3 receptors in pancreatic beta cells

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

Insufficient Intake of L-histidine Reduces Brain Histamine and Causes Anxiety-Like Behaviors in Male Mice

L-histidine is one of the essential amino acids for humans, and it plays a critical role as a component of proteins. L-histidine is also important as a precursor of histamine. Brain histamine is synthesized from L-histidine in the presence of histidine decarboxylase, which is expressed in histamine neurons. In the present study, we aimed to elucidate the importance of dietary L-histidine as a precursor of brain histamine and the histaminergic nervous system. C57BL/6J male mice at 8 wk of age were assigned to 2 different diets for at least 2 wk: the control (Con) diet (5.08 g L-histidine/kg diet) or the low L-histidine diet (LHD) (1.28 g L-histidine/kg diet). We measured the histamine concentration in the brain areas of Con diet-fed mice (Con group) and LHD-fed mice (LHD group). The histamine concentration was significantly lower in the LHD group [Con group vs. LHD group: histamine in cortex (means ± SEs): 13.9 ± 1.25 vs. 9.36 ± 0.549 ng/g tissue; P = 0.002]. Our in vivo microdialysis assays revealed that histamine release stimulated by high K(+) from the hypothalamus in the LHD group was 60% of that in the Con group (P = 0.012). However, the concentrations of other monoamines and their metabolites were not changed by the LHD. The open-field tests showed that the LHD group spent a shorter amount of time in the central zone (87.6 ± 14.1 vs. 50.0 ± 6.03 s/10 min; P = 0.019), and the light/dark box tests demonstrated that the LHD group spent a shorter amount of time in the light box (198 ± 8.19 vs. 162 ± 14.1 s/10 min; P = 0.048), suggesting that the LHD induced anxiety-like behaviors. However, locomotor activity, memory functions, and social interaction did not differ between the 2 groups. The results of the present study demonstrated that insufficient intake of histidine reduced the brain histamine content, leading to anxiety-like behaviors in the mice.

The clinical pharmacology of non-sedating antihistamines

&NA; We previously reported on brain H1 receptor occupancy measurements of antihistamines in human brain using [11C]doxepin and positron emission tomography (PET). We proposed the use of brain H1 receptor occupancy to classify antihistamines objectively into three categories of sedating, less‐sedating, and non‐sedating antihistamines according to their sedative effects. Non‐sedating antihistamines are recommended for the treatment of allergies such as pollinosis and atopic dermatitis because of their low penetration into the central nervous system. Physicians and pharmacists are responsible for fully educating patients about the risks of sedating antihistamines from pharmacological points of view. If a sedating antihistamine must be prescribed, its sedative effects should be thoroughly considered before choosing the drug. Non‐sedating antihistamines should be preferentially used whenever possible as most antihistamines are equally efficacious, while adverse effects of sedating antihistamines can be serious. This review summarizes the pharmacological properties of clinically useful non‐sedating antihistamines from the perspective of histamine function in the CNS.

Involvement of the histamine H1 receptor in the regulation of sympathetic nerve activity.

The histamine system is involved in the regulation of the autonomic nervous system. We used gene-targeted mice to investigate the role of histamine receptors in the regulation of the sympathetic nervous system. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed histamine H1, H2, and H3 receptor expression in the superior cervical ganglion, which contains sympathetic nerve cell bodies. We measured the heart rate variability (HRV), the changes in the beat-to-beat heart rate, which is widely used to assess autonomic activity in the heart. H1 blockade attenuated the baroreflex-mediated changes in heart rate in wild-type (WT) mice, whereas the heart rate response to H2- and H3-specific blockers was unaffected. l-Histidine decarboxylase (HDC) expression in the superior cervical ganglion of H1R-null mice was higher than that in WT controls, whereas the enzyme levels in H2R- and H3R-null mice were not significantly different from those in the WT. All mutant mice (H1R-, H2R-, and H3R-null mice) showed normal electrocardiogram (ECG) patterns with little modification in ECG parameters and the expected response to the β-adrenergic blocker propranolol. Similar to our findings in WT mice, H1 blockade attenuated the baroreflex-mediated heart rate change in H1R-null mice, whereas the heart rate response was unaffected in H2R- and H3R-null mice. The HRV analysis revealed relatively unstable RR intervals, an increased standard deviation of the interbeat interval (SDNN), and low-frequency (LF) component in H1R-null mice compared with the other groups, suggesting that sympathetic nerve activity was altered in H1R-null mice. Taken together, our findings indicate that H1 receptors play a major role in the regulation of sympathetic nerve activity.