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

Abstract

The presence and functional role of inositol trisphosphate receptors (IP3R) was investigated by electrophysiology and immunohistochemistry in hair cells from the frog semicircular canal. Intracellular recordings were 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 was performed by standard procedures. The drug 2-aminoethoxydiphenyl borate (2APB), 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), 2APB 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-dihydroxyphenylglycine (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. Pretreatment 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.

Juxtaglomerular cell CaSR stimulation decreases renin release via activation of the PLC/IP3pathway and the ryanodine receptor

“…While in our model we are unable to directly measure intracellular calcium we believe the primary source of increased intracellular calcium is from the endoplasmic reticulum. Calcium release from the endoplasmic reticulum is known to be mediated by PLC-IP 3 activation, as well as a ryanodine activation (3,23,38,49,56). Because we have previously shown that the target adenylyl cyclase (AC-V) is localized on the renin-containing granules, it suggests the compartment into which ionized free calcium is being released is the cytoplasm surrounding the granules.…”

“…In the parathyroid glands, the CaSR plays an important role in calcium homeostasis, monitoring serum calcium and signaling for suppression of parathyroid hormone (48) secretion in response to elevated serum calcium. Activation of this Gq protein-coupled receptor by extracellular calcium results in inducing PLC, leading to the generation of IP 3 which binds to an inositol-P 3 receptor on the membrane of the endoplasmic reticulum, opening a calcium channel and releasing calcium from intracellular stores in this organelle (3,23,38,49,56).…”

Exposure to reduced gravity impairs junctional transmission at the semicircular canal in the frog labyrinth

“…In some experiments, tricaine methane sulfonate was added to the Ringer solution (final concentration 10 Ϫ4 M, buffered to 7.2 pH). For further details of experimental and recording procedures, see previous papers (7,(15)(16)(17)(18). The afferent discharge recorded from single fibers of the posterior nerve was examined both in terms of spike frequency and of frequency and waveform of miniature excitatory postsynaptic potentials (mEPSPs; see…”

“…To analyze quantal transmitter release at the cytoneural junction, we developed in the past an original procedure, capable to measure mEPSP rates of occurrence above 100/s (15,16). The procedure, which was previously used to analyze cytoneural transmission in various experimental conditions (7,17,18), was based on the following steps: 1) identification of the general waveform of the mEPSP by means of fits to the spectral composition and autocorrelation of synaptic recordings: 2nd-to 5th-order autoregressive fits of the autocorrelation are used to estimate the ␥-distribution function best-fitting the "average" event waveform:…”

Effects of Phenylalanine and its Metabolites on Cytoplasmic Free Calcium in Cortical Neurons

“…Neurons adjust calcium signals by regulating the entry of calcium into the cell through the plasma membrane and controlling its release from the intracellular Sarco/endoplasmic reticulum [7]. Calcium influx is regulated by voltage-dependent, ligand-gated Ca 2+ channels, inositol triphosphate (IP 3 ) receptor and ryanodine (Rye) receptor [9,10]. Calcium efflux is regulated by plasma membrane Ca 2+ -ATPase (PMCA), Na/Ca exchanger (NCX) and Sarco/Endoplasmic reticulum Ca 2+ -ATPase (SERCA) [11,12,13].…”