IP3 receptor in the hair cells of frog semicircular canal and its possible functional role

The presence and functional role of inositol trisphosphate receptors (IP3R) have been investigated by electrophysiology and immunohistochemistry in hair cells from the frog semicircular canal. Intracellular recordings have been performed from single fibres of the posterior canal in the isolated, intact frog labyrinth, at rest and during rotation, in the presence of IP3 receptor inhibitors and drugs known to produce Ca2+ release from the internal stores or to increase IP3 production. Hair cell immunolabelling for IP3 receptor has been performed by standard procedures. The drug 2-aminoethoxydiphenyl borate (2-APB), an IP3 receptor inhibitor, produced a marked decrease of mEPSP and spike frequency at low concentration (0.1 mM), without affecting mEPSP size or time course. At high concentration (1 mM), 2-APB is reported to block the sarcoplasmic-endoplasmic reticulum Ca2+- ATPase (SERCA pump) and increase [Ca2+]i; at the labyrinthine cytoneural junction, it greatly enhanced the resting and mechanically evoked sensory discharge frequency. The selective agonist of group I metabotropic glutamate receptors, (RS)-3,5-dihidroxy¬phenylglycine (DHPG, 0.6 mM), produced a transient increase in resting mEPSP and spike frequency at the cytoneural junction, with no effects on mEPSP shape or amplitude. Pre-treatment with cyclopiazonic acid (CPA, 0.1 mM), a SERCA pump inhibitor, prevented the facilitatory effect of both 2APB and DHPG, suggesting a link between Ca2+ release from intracellular stores and quantal emission. Consistently, diffuse immunoreactivity for IP3 receptors was observed in posterior canal hair cells. Our results indicate the presence and a possibly relevant functional role of IP3-sensitive stores in controlling [Ca2+]i and modulating the vestibular discharge.