Histamine is an important mediator in immune responses, but it is unclear whether periodontal tissues express histamine receptors and are able to respond to histamine. We hypothesized that histamine, inflammatory cytokines, and bacterial components released in inflamed periodontal tissues may be synergistically involved in periodontitis. The present study showed that human gingival fibroblasts mainly express histamine receptor H1R, and responded to histamine to produce interleukin (IL)-8. Stimulation of gingival fibroblasts with tumor necrosis factor-alpha, IL-1alpha, and lipopolysaccharide markedly induced IL-8 production, and the IL-8 production was synergistically augmented in the presence of or pre-treatment with histamine. Selective inhibitors of mitogen-activated protein kinases (MAPKs), nuclear factor (NF)-kappaB, and phospholipase C (PLC) significantly inhibited the synergistic effect. These results indicate that histamine induces IL-8 production from gingival fibroblasts through H1R, and synergistically augments the inflammatory stimuli by amplification of the MAPK and NF-kappaB through H1R-linked PLC. Abbreviations used: HDC, histidine decarboxylase; LPS, lipopolysaccharide; IL, interleukin; TNF, tumor necrosis factor; HR, histamine receptor; PLC, phospholipase C; MAPK, mitogen-activated protein kinase; NF, nuclear factor; ERK, extracellular signal-related kinase; JNK, c-Jun N-terminal kinase; R, receptor; TLR, Toll-like receptor; alpha-MEM, alpha-minimum essential medium; FCS, fetal calf serum; RT-PCR, reverse-transcriptase polymerase chain-reaction; ELISA, enzyme-linked immunosorbent assay; SD, standard deviation; LDH, lactate dehydrogenase.