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

Abstract

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 (H1 R), H2 R, H3 R, and H4 R. 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 H2 R, H3 R, histidine decarboxylase, a histamine synthase, and histamine N-methyltransferase, a histamine metabolizing enzyme. Both forskolin-induced cAMP accumulation and ATP-induced intracellular Ca(2+) transients were reduced by the H3 R agonist imetit but not the H2 R agonist amthamine. H3 R activation on two ubiquitous second messenger signalling pathways suggests that H3 R 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 H3 R 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.

Histamine regulation of microglia: Gene-environment interaction in the regulation of central nervous system inflammation

“…These data suggest an overly exuberant inflammatory response to an immune challenge. Such a model is consistent with in vitro findings in an immortalized microglia-like cell line, in which HA was shown to activate N9 cells (by a chemotaxis assay) but to antagonize the activation produced by LPS stimulation (40); similar findings have recently been reported in primary microglial cultures (42). Our in vivo findings contrast, however, with other studies in acutely isolated microglia from neonates, which suggest that HA induces a pro-inflammatory phenotype, characterized by production of such cytokines as IL-6, IL-1β, and TNF-α, and oxidative enzymes such as iNOS (43-45).…”

“…This leads to the hypothesis that HA might oppose the pro-inflammatory activation of microglia – a role that has also been suggested for other aminergic neurotransmitters (41). Similar findings have recently been reported in microglial cultures (42). …”

“…We speculated that the reduction in IGF-1-expressing microglia in Hdc -KO mice would lead to a dysregulated response to pro-inflammatory stimulation. Such an effect would be consistent with the inhibitory effect that HA has on the response of N9 cells (40) and neonatal microglial cultures (42) to lipopolysaccharide (LPS) in some studies in vitro (though not in all).…”

“…Our finding of microglial abnormalities in the Hdc knockout mouse, a validated model of the pathophysiology of TS (Figure 1) (3), further emphasizes the importance of understanding HA’s role in regulating microglial function. Previous examination of HA regulation of microglia has been limited to in vitro studies of immortalized N9 cells (40) or primary microglial cultures from neonates (42-44). These studies have found HA to regulate microglia but have produced conflicting data as to the nature of that regulation – specifically, whether HA-stimulated microglia have a pro-inflammatory or neuroprotective phenotype (57) (40).…”

“…Different protocols for microglial isolation, culture, and study may produce markedly different cellular phenotypes and thus divergent molecular and functional results (62); this may explain the divergent reports of the effects of HA on microglia studied in vitro (42-44). This reinforces the importance of studying them in vivo .…”