Extracellular Adenosine 5'-ATP-induced Calcium Signaling in Isolated Vestibular Ganglion Cells of the Guinea Pig

Abstract

Extracellular adenosine 5'-triphosphate (ATP)-induced intracellular calcium concentration ([Ca2+]i) changes in acutely isolated vestibular ganglion cells (VGCs) of the guinea pig were investigated using the Ca2+ -sensitive dye Fura-2. Extracellular ATP induced an increase in [Ca2+]i in VGCs in a dose-dependent manner. ATP induced an increase in [Ca2+]i even in the absence of extracellular Ca2+ (1 mM Ethylene Glycol-bis (beta-aminoethyl Ether) N,N,N',N'-Tetraacetic Acid (EGTA)), thus suggesting that ATP induces Ca2+ release from the intracellular stores. The P2-receptor antagonists suramin and reactive blue 2 inhibited the ATP-induced [Ca2+]i increase in a dose-dependent manner. The P1-receptor agonist adenosine did not induce any changes in [Ca2+]i. These results suggest that VGCs may possess a P2-purinergic receptor but not a P1-purinergic receptor. La3+, a receptor-mediated calcium channel blocker, inhibited the ATP-induced [Ca2+]i increase but, in contrast, nifedipine, a L-type calcium channel blocker, did not. These results suggest that ATP induces both a Ca2+ -release from the intracellular stores and a Ca2+ influx from the extracellular space through La3+ -sensitive and nifedipine-insensitive Ca2+ channels in VGCs. Our results also suggest that extracellular ATP may act as a neurotransmitter or neuromodulator of the vestibular peripheral system in the guinea pig.

Purinergic signaling in cochleovestibular hair cells and afferent neurons

“…In adult rat VGNs, immunohistochemistry demonstrated P2X 1 -P2X 6 subtypes as in SGNs [14]. [39]. Moreover, we recently characterized electrophysiological properties of ionotropic (P2X) conductance in VGNs (Table 4) [40].…”

Functional ligand-gated purinergic receptors (P2X) in rat vestibular ganglion neurons

“…Examples of the receptor demonstration course (P2XR in spiral ganglion neurons: SGNs, the counterpart of VGNs in the 8th cranial nerve) are: immunohistochemistry (Xiang et al, 1999), calcium responses (Cho et al, 1997), and then electrophysiology (Ito and Dulon, 2002;Salih et al, 2002). For VGNs, immunohistochemistry (Xiang et al, 1999) and calcium responses (Nagata et al, 2000) have already been reported. The present study achieved the next stage, electrophysiological proof of functional receptors.…”

“…P2X purinergic receptors on VGNs have already been detected immunohistochemically (Xiang et al, 1999) and by in situ hybridization (Kreindler et al, 2001). In terms of function, calcium signaling induced by ATP, with the possible involvement of P2X receptors, was shown (Nagata et al, 2000). However, the electrophysiological characteristics of P2X receptors, whether they can induce significant membrane currents in VGNs or not, have not been investigated.…”

Purinergic signaling in the peripheral vestibular system

“…The ATP-induced current was detected using patch-clamp experiments in the study, but the type of P2X receptor was not functionally identified. Another study also detected ionotropic and metabotropic P2 receptor activity in the adult guinea pig vestibular ganglion by performing intracellular [Ca 2+ ] imaging using Fura-2 AM but did not precisely identify the types of receptors [34]. A detailed discussion of the functional evidence and its implications for P2 receptors in the vestibular compartment will be described in the next sections.…”

“…Two functional studies of P2 receptors have been reported to date. A study measuring intracellular [Ca 2+ ] using Fura-2 AM in the isolated adult guinea pig vestibular ganglion identified that ATP increased the [Ca 2+ ] in a dosedependent manner when administered in concentrations ranging from 0.1 to 1000 μM [34]. The increase in [Ca 2+ ] was even detectable without extracellular Ca 2+ , indicating that ATP induced Ca 2+ release from the intracellular stores, which was likely to be accomplished by P2Y receptors.…”