Nonselective cation conductance activated by muscarinic and purinergic receptors in rat spiral ganglion neurons

Abstract

Ito, Ken, and Didier Dulon. Nonselective cation conductance activated by muscarinic and purinergic receptors in rat spiral ganglion neurons. Am J Physiol Cell Physiol 282: C1121-C1135, 2002. First published December 19, 2001 10.1152/ajpcell.00364.2001.-The present study characterizes the ionic conductances activated by acetylcholine (ACh) and ATP, two candidate neuromodulators, in isolated spiral ganglion neurons (SGNs). Brief application (1 s) of ACh evoked in a dose-dependent manner (EC50 ϭ 4.1 M) a reversible inward current with a long latency (average 1.3 s), at holding potential (Vh) ϭ Ϫ50 mV. This current was reversibly blocked by atropine and mimicked by muscarine. Application of ATP also evoked a reversible inward current at V h ϭ Ϫ50 mV, but the current showed two components. A fast component with a short latency was largely reduced when N-methyl-D-glucamine (NMDG) replaced extracellular sodium, implying a P2X-like ionotropic conductance. The second component had a longer latency (average 1.1 s) and was presumably activated by metabotropic P2Y-like receptors. The second component of ATP-evoked current shared similar characteristics with the responses evoked by ACh: the current reversed near 0 mV, displayed inward rectification, could be carried by NMDG, and was insensitive to extracellular and intracellular calcium. This ACh-/ATP-evoked conductance was reversibly inhibited by preapplication of ionomycin. These results suggest that muscarinic receptors and purinergic metabotropic receptors activate a similar large nonselective cation conductance via a common intracellular pathway in SGNs, a candidate mechanism to regulate neuronal excitability of SGNs.acetylcholine; adenosine 5Ј-triphosphate; cochlear neuron SPIRAL GANGLION NEURONS (SGNs) are peripheral bipolar neurons located in the cochlea that convey to the brain stem the acoustic information arising from the mechanoelectrical transduction of the inner hair cells (IHCs). The dendrites of SGNs below IHCs are contacted by efferent fibers that originate from small central neurons in the lateral superior olive. This lateral efferent innervation from the brain stem has long been suggested as a central control of the auditory nerve activity at the periphery [for review, see Warr (70)]. However, efferent regulation of SGN excitability still remains largely unexplored.Several immunocytochemical studies have suggested that acetylcholine (ACh) could be one of the main neurotransmitters present at these lateral efferent synapses [for review, see Eybalin (15)]. On the other hand, microiontophoresis of ACh in the subsynaptic area below IHCs in vivo has been shown to increase the subsynaptic spiking activity of the afferent nerve fibers (18), thus suggesting an excitatory regulation by ACh. However, the type of cholinergic receptors involved at these lateral efferent synapses is still debated, because mRNA expression of both muscarinic (12, 52) and nicotinic (44) receptors has been reported in SGNs. Recently, our laboratory demonstrated (50) the presence of functi...

Extracellular Nucleotides Elicit Cytosolic Free Calcium Oscillations in Arabidopsis

“…In mammals, these two activities show tissue specificity (Burnstock, 2007); occasionally, either or both ionotropic and metabotropic receptors exist together even in the same cell type (e.g. in rat spiral ganglion neurons; Ito and Dulon, 2002). We observed biphasic kinetics of [Ca 2+ ] cyt induced by ATP using whole seedlings of Arabidopsis (Fig.…”

Differential expression of P2Y receptors in the rat cochlea during development

“…Earlier electrophysiological investigations of purinergic signaling in spiral ganglion neurons had confirmed that both P2X and P2Y receptors mediate inward currents. The SGN P2X receptors exhibit desensitizing currents in response to micromolar ATP [59][60][61], while the P2Y receptors are activated by nanomolar ATP and UTP concentrations, and via intracellular second messenger signaling, engage a slowly developing non-selective cation conductance [62]. The identity of this cation conductance is thought to be a TRP channel which provides cross-talk with other metabotropic receptors involving the GPCR-PLC -DAG pathway, including the muscarinic acetylcholine receptor and the substance P receptor [17,63].…”

Purinergic signaling in cochleovestibular hair cells and afferent neurons

“…This slowly activating nonselective conductance is likely driven by a pore channel that can pass large cations such as NMDG + and TEA + [33]. Intriguingly, this large pore channel can also be activated by other metabotropic receptors, i.e., tachykinin receptors (NKR, ligand: substance P) and muscarinic acetylcholine (ACh) receptors (mAChR) as confirmed by mutual desensitization and nonadditive nature of responses.…”

“…As in IHCs, Ca 2+ responses showed functional P2X and P2Y receptors [31], and electrophysiology demonstrated ionotropic (P2X) cation nonselective conductance [32,33]. Moreover, a large conductance evoked by metabotropic (P2Y) receptors was reported [33].…”

“…Moreover, a large conductance evoked by metabotropic (P2Y) receptors was reported [33]. A more detailed review of ATP-evoked conductances in SGNs will appear in the following section.…”