Isolation of Guinea pig inner hair cells using manual microsurgical dissection

Calcium action on the membrane currents possessing the properties of mechano-electric transducer currents in inner hair cells of the Guinea-pig cochlea

In free-standing hair bundle, depolarization to +80 mV evoked a stable outward current and repolarization to -80 mV evoked a transient inward current attributable to the opening of mechano-electric transducer channels. The study investigated the Ca2+ dependence of this transducer-like membrane current in isolated inner hair cells of guinea-pig cochlea. The amplitude of outward currents increased and the rate of inward current decay, corresponding to adaptation kinetics, decreasing as the extracellular Ca2+ concentrations lessened, whereas the amplitude of outward current decreased and an adaptation accelerated as the extracellular Ca2+ elevated. Treatment with the cAMP agonist, 8-bromo-cAMP, induced an effect similar to that caused by elevating the extracellular Ca2+.

Involvement of the nitric oxide-cyclic GMP pathway and neuronal nitric oxide synthase in ATP-induced Ca2+ signalling in cochlear inner hair cells

We recently demonstrated that extracellular adenosine 5'-triphosphate (ATP) induced nitric oxide (NO) production in the inner hair cells (IHCs) of the guinea pig cochlea, which inhibited the ATP-induced increase in the intracellular Ca(2+) concentrations ([Ca(2+)](i)) by a feedback mechanism [Shen, J., Harada, N. & Yamashita, T. (2003) Neurosci. Lett., 337, 135-138]. We herein investigated the role of the NO-cGMP pathway and neuronal NO synthase (nNOS) in the ATP-induced Ca(2+) signalling in IHCs using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye DAF-2. Fura-2 fluorescence-quenching experiments with Mn(2+) showed that ATP triggered a Mn(2+) influx. L-N(G)-nitroarginine methyl ester (L-NAME), a nonspecific NOS inhibitor, accelerated the ATP-induced Mn(2+) influx while S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, suppressed it. 1H-[1,2,4]oxadiazole[4,3-a] quinoxalin-1-one, an inhibitor of guanylate cyclase, and KT5823, an inhibitor of cGMP-dependent protein kinase, enhanced the ATP-induced [Ca(2+)](i) increase. 8-Bromoguanosine-cGMP, a membrane-permeant analogue of cGMP mimicked the effects of SNAP. Moreover, the effects of 7-nitroindazole, a selective nNOS inhibitor, mimicked the effects of L-NAME regarding both the enhancement of the ATP-induced Ca(2+) response and the attenuation of NO production. Immunofluorescent staining of nNOS using a single IHC revealed that nNOS was distributed throughout the IHCs, but enriched in the apical region of the IHCs as shown by intense staining. In conclusion, the ATP-induced Ca(2+) influx may be the principal source for nNOS activity, which may interact with P2X receptors in the apical region of IHCs. Thereafter, NO can be produced and conversely inhibits the Ca(2+) influx via the NO-cGMP-PKG pathway by a feedback mechanism.

Nitric oxide inhibits adenosine 5'-triphosphate-induced Ca2+ response in inner hair cells of the guinea pig cochlea

To investigate the interaction between Ca(2+) and nitric oxide (NO) in inner hair cells of the guinea pig cochlea (IHCs), the extracellular adenosine 5'-triphosphate (ATP)-induced NO production and the effects of NO on ATP-induced increase of intracellular Ca(2+) concentrations ([Ca(2+)](i)) were investigated in IHCs using the NO-sensitive dye DAF-2 and the Ca(2+)-sensitive dye Fura-2. Extracellular ATP induced an increase in DAF-2 fluorescence, which thus indicates NO production in IHCs. The ATP-induced NO production was mainly due to Ca(2+) influx through the activation of P2 receptor. L-N(G)-nitroarginine methyl ester, a NO synthesis inhibitor, enhanced the ATP-induced [Ca(2+)](i) increase in IHCs while S-nitroso-N-acetylpenicillamine, a NO donor, inhibited it. We conclude that NO inhibits the ATP-induced [Ca(2+)](i) increase in IHCs by a negative-feedback mechanism.