Contribution of astrocytic histamine N-methyltransferase to histamine clearance and brain function in mice.

Brain histamine acts as a neurotransmitter in the regulation of various brain activities. Previous studies have shown that histamine N-methyltransferase (HNMT), a histamine-metabolizing enzyme, controls brain histamine concentration and brain function. However, the relative contribution of astrocytic or neuronal HNMT to the regulation of the histaminergic system is still inconclusive. Here, we phenotyped astrocytes-specific HNMT knockout (cKO) mice to clarify the involvement of astrocytic HNMT in histamine clearance and brain function. First, we performed histological examinations using HNMT reporter mice and showed a wide distribution of HNMT in the brain and astrocytic HNMT expression. Then, we created cKO mice by Cre-loxP system and confirmed that HNMT expression in cKO primary astrocytes was robustly decreased. Although total HNMT level in the cortex was not substantially different between control and cKO brains, histamine concentration after histamine release was elevated in cKO cortex. In behavioral tests, impaired motor coordination and lower locomotor activity were observed in the cKO mice. However, anxiety-like behaviors, depression-like behaviors, and memory functions were not altered by astrocytic HNMT disruption. Although sleep analysis demonstrated that the quantity of wakefulness and sleep did not change, the increased power density of delta frequency during wakefulness indicated lower cortical activation in cKO mice. These results demonstrate that astrocytic HNMT contributes to histamine clearance after histamine release in the cortex and plays a role in the regulation of motor coordination, locomotor activity, and vigilance state.

Histamine H1 receptor on astrocytes and neurons controls distinct aspects of mouse behaviour.

Histamine is an important neurotransmitter that contributes to various processes, including the sleep-wake cycle, learning, memory, and stress responses. Its actions are mediated through histamine H1-H4 receptors. Gene knockout and pharmacological studies have revealed the importance of H1 receptors in learning and memory, regulation of aggression, and wakefulness. H1 receptors are abundantly expressed on neurons and astrocytes. However, to date, studies selectively investigating the roles of neuronal and astrocytic H1 receptors in behaviour are lacking. We generated novel astrocyte- and neuron-specific conditional knockout (cKO) mice to address this gap in knowledge. cKO mice showed cell-specific reduction of H1 receptor gene expression. Behavioural assessment revealed significant changes and highlighted the importance of H1 receptors on both astrocytes and neurons. H1 receptors on both cell types played a significant role in anxiety. Astrocytic H1 receptors were involved in regulating aggressive behaviour, circadian rhythms, and quality of wakefulness, but not sleep behaviour. Our results emphasise the roles of neuronal H1 receptors in recognition memory. In conclusion, this study highlights the novel roles of H1 receptors on astrocytes and neurons in various brain functions.

Optically active iodohelicene derivatives exhibit histamine N-methyl transferase inhibitory activity.

Optically active helicene derivatives inhibit the activity on histamine N-methyl transferase (HNMT). Specifically, methyl (P)-1,12-dimethylbenzo[c]phenanthrene-8-carboxylate with 6-iodo and 5-trifluoromethanesulfonyloxy groups inhibits HNMT activity on the µM order of IC50. Chirality is important, and (M)-isomers exhibits substantially reduced activity. The 6-iodo group is also essential, which suggests the involvement of halogen bonds in protein binding. Substituents on the sulfonate moiety also affect the inhibitory activity.

Histamine elicits glutamate release from cultured astrocytes.

Astrocytes play key roles in regulating brain homeostasis and neuronal activity. This is, in part, accomplished by the ability of neurotransmitters in the synaptic cleft to bind astrocyte membrane receptors, activating signalling cascades that regulate concentration of intracellular Ca2+ ([Ca2+]i) and gliotransmitter release, including ATP and glutamate. Gliotransmitters contribute to dendrite formation and synaptic plasticity, and in some cases, exacerbate neurodegeneration. The neurotransmitter histamine participates in several physiological processes, such as the sleep-wake cycle and learning and memory. Previous studies have demonstrated the expression of histamine receptors on astrocytes, but until now, only a few studies have examined the effects of histamine on astrocyte intracellular signalling and gliotransmitter release. Here, we used the human astrocytoma cell line 1321N1 to study the role of histamine in astrocyte intracellular signalling and gliotransmitter release. We found that histamine activated astrocyte signalling through histamine H1 and H2 receptors, leading to distinct cellular responses. Activation of histamine H1 receptors caused concentration-dependent release of [Ca2+]i from internal stores and concentration-dependent increase in glutamate release. Histamine H2 receptor activation increased cyclic adenosine monophosphate (cAMP) levels and phosphorylation of transcription factor cAMP response-element binding protein. Taken together, these data emphasize a role for histamine in neuron-glia communication.

Heparan sulfate in pancreatic ß-cells contributes to normal glucose homeostasis by regulating insulin secretion.

Heparan sulfate (HS), a linear polysaccharide, is involved in diverse biological functions of various tissues. HS is expressed in pancreatic ß-cells and may be involved in ß-cell functions. However, the importance of HS for ß-cell function remains unknown. Here, we generated mice with ß-cell-specific deletion of Ext1 (ßExt1CKO), which encodes an enzyme essential for HS synthesis, to investigate the detailed roles of HS in ß-cell function. ßExt1CKO mice decreased body weights compared with control mice, despite increased food intake. Additionally, ßExt1CKO mice showed impaired glucose tolerance associated with decreased insulin secretion upon glucose challenge. Glucose-induced insulin secretion (GIIS) from isolated ßExt1CKO islets was also significantly reduced, highlighting the contribution of HS to insulin secretion and glucose homeostasis. The gene expression essential for GIIS was decreased in ßExt1CKO islets. Pdx1 and MafA were downregulated in ßExt1CKO islets, indicating that HS promoted ß-cell development and maturation. BrdU- or Ki67-positive ß-cells were reduced in ßExt1CKO pancreatic sections, suggesting the involvement of HS in the proliferation of ß-cells. Moreover, insufficient vascularization in ßExt1CKO islets may contribute to central distribution of α-cells. These data demonstrate HS plays diverse roles in ß-cells, and that loss of HS leads to insufficient insulin secretion and dysregulation of glucose homeostasis.

JNJ10181457, a histamine H3 receptor inverse agonist, regulates in vivo microglial functions and improves depression-like behaviours in mice.

Brain histamine acts as a neurotransmitter and regulates various physiological functions, such as learning and memory, sleep-wake cycles, and appetite regulation. We have recently shown that histamine H3 receptor (H3R) is expressed in primary mouse microglia and has a strong influence on critical functions in microglia, including chemotaxis, phagocytosis, and cytokine secretion in vitro. However, the importance of H3R in microglial activity in vivo remains unknown. Here, we examined the effects of JNJ10181457 (JNJ), a selective and potent H3R inverse agonist, on microglial functions ex vivo and in vivo. First, we injected ATP, which is a typical chemoattractant, into hippocampal slices to investigate the effect of JNJ on chemotaxis. ATP-induced microglial migration toward the injected site was significantly suppressed by JNJ treatment. Next, we examined whether JNJ affected microglial phagocytosis in hippocampal slices and in the prefrontal cortex. Microglial engulfment of dead neurons induced by N-methyl-d-aspartate was inhibited in the presence of JNJ. The increase in zymosan particle uptake by activated microglia in the prefrontal cortex was prevented by JNJ administration. Finally, we determined the importance of JNJ in a lipopolysaccharide (LPS)-induced depression model. JNJ reduced the LPS-induced upregulation of microglial pro-inflammatory cytokines and improved depression-like behaviour in the tail-suspension test. These results demonstrate the inhibitory effects of JNJ on chemotaxis, phagocytosis, and cytokine production in microglia inside the brain, and highlight the importance of microglial H3R for brain homeostasis.