Substance P activates osteoclast formation and osteoclastic bene resorption through the neurokinin-1 receptor

Axons containing substance P (SP) serve bone tissue, however, the role of SP in bone metabolism, particularly on osteoclastic bone resorption, is unknown. Therefore, we examined the distribution of neurokinin 1-receptors (NK₁-R), which have a high affinity to SP, in rat osteoclasts in vivo, and investigated the effects of SP on osteoclast formation and osteoclastic bone resorption in vitro. Using electron microscopy, immunoreactive products of NK₁-R were seen in the plasma membrane and cytoplasm of osteoclasts, and were also observed in preosteoclast-like cells. Cell suspensions containing osteoclasts were prepared from neonatal rats and cultured on ivory slices. The addition of 10^-10-10^-6 M SP caused the number of mono- and multi-nucleated tartrate-resistant acid phosphatase positive (TRAP(+)) cells to increase. The increase in TRAP(+) cells with the addition of 10^-8 M SP was inhibited by treatment with the SP receptor antagonist. In cultures on glass coverslips, time-lapse studies show that SP induced cell spreading within 5 min and maintained the spreading. The number of resorption pits excavated by the osteoclasts and the resorption area per osteoclast increased in a 48-hour incubation with 10^-8 M SP. These results suggest that SP stimulates osteoclast formation and activates osteoclastic bone resorption through NK₁-R.