Search

Bradykinin (BK) and its receptors, B1 and B2, in trigeminal ganglion (TG) neurons are involved in the regulation of pain. Recent studies have revealed that B1 receptors are expressed in neonatal rat TG neurons; however, the intracellular signaling pathway following B1 receptor activation remains to be elucidated. To investigate the mechanism by which B1 receptor activation leads to intracellular Ca2+ mobilization, we measured the intracellular free Ca2+ concentration ([Ca2+]i) in primary-cultured TG neurons. The application of Lys-[Des-Arg9]BK (B1 receptor agonist) increased the [Ca2+]i in these TG neurons even in the absence of extracellular Ca2+. Pretreatment with inhibitors of ryanodine receptors or sarco/endoplasmic reticulum Ca2+-ATPase suppressed the increase in Lys-[Des-Arg9]BK-induced [Ca2+]i. The Lys-[Des-Arg9]BK-induced [Ca2+]i increase was unaffected by phospholipase-C inhibitor. B1 receptor activation-induced [Ca2+]i increase was suppressed by phosphodiesterase inhibitor and enhanced by adenylyl cyclase inhibitor. These results suggest that B1 receptor activation suppresses intracellular cAMP production via adenylyl cyclase inhibition and mobilizes intracellular Ca2+ via ryanodine receptors that access intracellular Ca2+ stores. [ABSTRACT FROM AUTHOR]

Various stimuli induce pain when applied to the surface of exposed dentin. However, the mechanisms underlying dentinal pain remain unclear. We investigated intercellular signal transduction between odontoblasts and trigeminal ganglion (TG) neurons following direct mechanical stimulation of odontoblasts. Mechanical stimulation of single odontoblasts increased the intracellular free calcium concentration ([Ca]) by activating the mechanosensitive-transient receptor potential (TRP) channels TRPV1, TRPV2, TRPV4, and TRPA1, but not TRPM8 channels. In cocultures of odontoblasts and TG neurons, increases in [Ca] were observed not only in mechanically stimulated odontoblasts, but also in neighboring odontoblasts and TG neurons. These increases in [Ca] were abolished in the absence of extracellular Ca and in the presence of mechanosensitive TRP channel antagonists. A pannexin-1 (ATP-permeable channel) inhibitor and ATP-degrading enzyme abolished the increases in [Ca] in neighboring odontoblasts and TG neurons, but not in the stimulated odontoblasts. G-protein-coupled P2Y nucleotide receptor antagonists also inhibited the increases in [Ca]. An ionotropic ATP (P2X) receptor antagonist inhibited the increase in [Ca] in neighboring TG neurons, but not in stimulated or neighboring odontoblasts. During mechanical stimulation of single odontoblasts, a connexin-43 blocker did not have any effects on the [Ca] responses observed in any of the cells. These results indicate that ATP, released from mechanically stimulated odontoblasts via pannexin-1 in response to TRP channel activation, transmits a signal to P2X receptors on TG neurons. We suggest that odontoblasts are sensory receptor cells and that ATP released from odontoblasts functions as a neurotransmitter in the sensory transduction sequence for dentinal pain. [ABSTRACT FROM AUTHOR]