Substance P, not Calcitonin Gene-Related Peptide, Induces Interleukin-8 Production by Human Pulpal Cells

Neurogenic inflammation is considered to play a key role in pulpal inflammation. Released neuropeptides from the sensory nerve fibers in response to stimulation can cause the first sign of inflammation—hyperemia, a result of vasodilatation. The richly innervated dental pulps contain neurons expressing substance P (SP) and calcitonin gene-related peptide (CGRP). Following vasodilatation, influx of leukocytes from blood into tissue ensues, leading to accumulation of inflammatory infiltrates in pulpal tissue. Whether these neuropeptides also play roles in recruiting leukocytes during pulpal inflammation is not clear. Objectives: to investigate the response of human pulpal cells after the stimulation with SP or CGRP by detecting the release of interleukin-8 (IL-8), a potent proinflammatory chemokine that attracts leukocytes, from pulpal cells. Methods: primary pulpal cells grown from human pulpal tissue in DMEM with 10% fetal bovine serum (FBS) between passages 3-5 were used for experiments. Cells were grown in alpha MEM with 1% FBS 24 h before they were stimulated with various doses of SP or CGRP. Results: there was a significant increase of IL-8 secretion from pulpal cells stimulated with SP, whereas no increase of IL-8 production was observed with CGRP stimulation. The dose response studies showed 2- to 7-fold increase of IL-8 secretion at 10^-8 to 10^-4 M of SP stimulation. More than 100-fold of IL-8 induction was observed when SP concentration reached 10^-3 M. The time course studies showed that IL-8 secretion reached the maximal level at 4 h after SP stimulation and sustained at this level up to 16 h. Conclusions: our data indicate that pulpal cells can up-regulate IL-8 secretion in response to SP stimulation, suggesting that released SP in dental pulps may be involved in regulating the initial influx of leukocytes, attracted by IL-8, into the pulpal tissue. (Supported by UCLA School of Dentistry Research Grants)