The histamine H3 receptor (H3R) is abundantly expressed in the Central Nervous System where it regulates several functions pre and postsynaptically. H3Rs couple to Gαi/o proteins and trigger or modulate several intracellular signaling pathways, including the cAMP/PKA pathway and the opening of N- and P/Q-type voltage-gated Ca2+ channels. In transfected cells, activation of the human H3R of 445 amino acids (hH3R445) results in phospholipase C (PLC) stimulation and release of Ca2+ from intracellular stores. In this work we have studied whether H3R activation induces Ca2+ mobilization from intracellular stores in native systems, either isolated nerve terminals (synaptosomes) or neurons in primary culture. In rat striatal synaptosomes H3R activation induced inositol 1,4,5-trisphosphate (IP3) formation but failed to increase the intracellular calcium concentration ([Ca2+]i). In striatal primary cultures H3R activation resulted in IP3 formation and increased the [Ca2+]i in 18 out of 70 cells that responded with an elevation in the [Ca2+]i to membrane depolarization with KCl (100 mM) as evaluated by microfluorometry. Confocal microscopy studies corroborated the increase in [Ca2+]i induced by H3R activation in a fraction of those cells that were responsive to membrane depolarization. These results indicate that H3R activation stimulates the PLC/IP3/Ca2+ pathway but only in a subpopulation of striatal neurons.
Keywords: Calcium mobilization; H(3) receptor; Histamine; Primary culture; Striatum; Synaptosomes.
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