Muscarinic receptor-mediated calcium signaling in spiral ganglion neurons of the mammalian cochlea.

Using indo-1 microspectrofluorometry, we examined the effects of cholinergic agonists on the concentration of intracellular Ca(2+) ions ([Ca(2+)](i)) in spiral ganglion neurons, isolated from rat cochleae at different stages of post-natal development (from P3 to P30). Extracellular application of acetylcholine (ACh) or carbamylcholine generated a rapid and transient increase in [Ca(2+)](i). The ACh concentration-response curve indicated an apparent dissociation constant (K(d)) of 8 microM and a Hill coefficient of 1.0. Removing extracellular free Ca(2+) did not suppress the ACh-induced Ca(2+) responses suggesting an intracellular Ca(2+)-release mechanism. When we compared the cholinergic response at different stages of postnatal development, there were no significant differences on the aspect of the Ca(2+) response and the percentage of responsive neurons, which ranged between 50 and 65% per cochlear preparation. The application of muscarine triggered reversible Ca(2+) responses similar to those observed with ACh, with an apparent K(d) of 10 microM and a Hill coefficient of 1.0. The cholinergic-induced Ca(2+)pirenzepine. Nicotine (10 to 100 microM) did not evoke Ca(2+) responses and the nicotinic antagonist curare (10 microM) did not block the ACh-evoked responses. The present study is the first direct demonstration of functional muscarinic receptors (mAChRs) in spiral ganglion neurons. These mAChRs activated by the cholinergic lateral efferent system may participate in the regulation of the electrical activity of the afferent auditory fibers contacting the inner hair cells.