Histamine Influences Depolarization-Induced Calcium Ion Influx in Sympathetic Neurons
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The superior mesenteric ganglion (SMG) provides sympathetic input to areas of the small intestine, colon, spleen, and mesenteric lymph nodes. Interactions between the nervous and immune systems in the SMG influence sympathetic regulation of gastrointestinal (GI) and immune function. Previous work in our laboratory has demonstrated changes in SMG neuron activity resulting from exposure to inflammatory mediators such as tumour necrosis factor α (TNFα). The current project focused on interactions between mast cells and sympathetic neurons. Mast cells within the SMG release mediators, including histamine, that can act on neurons and alter their activity. We tested the hypothesis that histamine influences signaling in SMG neurons by inhibiting calcium ion influx during cell depolarization using immunohistochemistry and calcium imaging. Immunohistochemistry revealed H3R on the majority of tyrosine hydroxylase-positive sympathetic neurons in the ganglia. Dissociated neurons were incubated in the ratiometric fluorescent calcium indicator dye Fura-2 acetoxymethyl ester, then superfused with extracellular solution containing histamine receptor agonists (histamine, HTMT, imetit) and antagonists (thioperamide) before being depolarized with a KCL solution (70 mM). Application of both histamine (10 μM) and the H3 receptor agonist imetit (100 nM) caused a decrease in depolarization-induced calcium ion influx. However, the inhibition of calcium ion influx became smaller as the concentration of histamine was increased (100 μM, 1 mM) until the inhibition was no longer statistically significant. Application of H3R antagonist thioperamine (300nM) reversed the inhibition of calcium ion influx caused by histamine (10 μM). Application of H1R & H2R agonist histamine trifluoromethyl toluidide (HTMT) (10 μM) caused an increase in calcium ion influx during depolarization. We conclude that activation of H3R decreases calcium ion influx through voltage-gated calcium ion channels, while activation of H1R / H2R increases calcium ion influx. H3R has a higher affinity for histamine, and therefore is preferentially activated at lower concentrations. Increases in histamine receptor activation may alter SMG input to the spleen, mesenteric lymph nodes, small intestine, and colon, resulting in changes in immune and gut function, such as those described in irritable bowel syndrome.