Extracellular ATP-induced Ca2+Mobilization of Type I Spiral Ganglion Cells from the Guinea Pig Cochlea

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Abstract
Intracellular calcium concentrations ([Ca2 +],) in type I cochlear spiral ganglion cells (SGCs) of the guinea pig were measured by digital imaging microscopy and the Ca2 +-sensitive dye Fura-2. Extracellular ATP induced elevation of [Ca2 +]i in type I SGCs in a concentration-dependent manner. The ATP-induced elevation of [Ca2 +]i in SGC was even evident in the Ca2 + -free solution, thereby suggesting that ATP induces a Ca2 + -release from intracellular stores in SGCs. Suramin and reactive blue 2, both antagonists for the P2-purinergic receptor, inhibited the [Ca2 +]i increase in SGCs induced by extracellular ATP in a dose-dependent manner. Adenosine did not induce any changes of [Ca2 +]i in SGC. These results suggest that type I SGCs may possess P2-purinergic receptor but not P1-purinergic receptor. Extracellular ATP induced a [Ca2 +]i increase in type I SGCs, with and without neuritic processes, while L-glutamate increased [Ca2 +]i in type 1 SGCs with neuritic processes, but not SGCs without neuritic processes. The ATP-induced [Ca2 +]i increase was almost equal in both soma and processes. Therefore, the distribution of P2-purinergic receptor in type I SGCs may be homogeneous in soma and processes. Based on these observations, we suggest that extracellular ATP may act as a neurotransmitter or neuromodulator of the hair cell-afferent nerve synapse in the guinea pig cochlea.