Extracellular ATP-induced Ca2+ mobilization of type I spiral ganglion cells from the guinea pig cochlea

Intracellular calcium concentrations ([Ca2+](i)) 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 T 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 P-2-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 P-2-purinergic receptor but not P-1-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 I 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 P-2-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.