Age-related changes in afferent responses in sensory neurons to mechanical stimulation of osteoblasts in coculture...

Bone homeostasis is regulated by mechanical stimulation (MS). The sensory mechanism of bone tissue for MS remains unknown in the maintenance of bone homeostasis. We aimed to investigate the sensory mechanism from osteoblasts to sensory neurons in a coculture system by MS of osteoblasts. Primary sensory neurons isolated from dorsal root ganglia (DRG) of neonatal, juvenile, and adult mice and osteoblasts isolated from calvaria of neonatal mice were cocultured for 24 h. The responses in DRG neurons elicited by MS of osteoblasts with a glass micropipette were detected by increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) with fluo 3-AM. In all developmental stages mice, [Ca(2+)](i)-increasing responses in osteoblasts were promptly elicited by MS. After a short delay, [Ca(2+)](i)-increasing responses were observed in neurites of DRG neurons. The osteoblastic response to second MS was largely attenuated by a stretch-activated Ca(2+) channel blocker, gadolinium. The increases of [Ca(2+)](i) in DRG neurons were abolished by a P2 receptor antagonist; suramin, a P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate; and an ATP-hydrolyzing enzyme, apyrase. Satellite cells were found around DRG neurons in cocultured cells of only neonatal and juvenile mice. After satellite cells were removed, excessive abnormal responses to MS of osteoblasts were observed in neonatal neurites with unchanged osteoblast responses. The present study indicated that MS of bone tissue elicited afferent P2X receptor-mediated purinergic transmission to sensory neurons in all stages mice. This transmission is modulated by satellite cells, which may have protective actions on sensory neurons.