Cochlear hair cells are thought to be inhibited by the release of ACh from efferent neurons. Several studies have implicated Ca2+ as a postsynaptic intermediary in hair cell inhibition, but its role remains unproven. We have made whole-cell, tight-seal recordings from single short hair cells (the avian analog of outer hair cells in the mammalian cochlea), isolated from the chick's cochlea, to determine the mechanism of cholinergic inhibition. These cells hyperpolarized upon exposure to ACh, although a brief depolarization preceded the much larger, longer-lasting hyperpolarization. In voltage clamp ACh evoked an outward current that reversed in sign near the K+ equilibrium potential. A small, transient inward current preceded the predominant outward current. The ACh-evoked K+ current depended on Ca2+ in the external saline, or could be prevented when the cell was dialyzed with the rapid Ca2+ buffer BAPTA. In BAPTA-loaded cells a residual inward current was seen. This activated with very little delay upon exposure of the cell to ACh and reversed near 0 mV membrane potential. Thus, the hair cell ACh receptor appears to be a nonspecific cation channel through which Ca2+ enters and triggers the opening of nearby Ca(2+)-activated K+ channels. However, the ACh-evoked K+ channels are not the same as the "maxi" K+ channels activated by Ca2+ influx through voltage-gated Ca2+ channels in these same cells.

Tall (inner) hair cells were isolated from specific locations in the chick cochlea. The electrical membrane properties of these cells were recorded using the tight-seal whole-cell technique. Depolarizing current steps elicited damped voltage oscillations that ranged in frequency from 100 to 250 Hz among cells from the middle third of the cochlea (basal cells). The current-voltage relation obtained under voltage clamp was dominated by calcium-activated potassium current in the voltage range over which these oscillations occurred. Tall hair cells isolated from the apical tip of the cochlea (apical cells) exhibited action potentials and lower frequency voltage oscillations (5– 14 Hz) during depolarizing current steps. Outward currents in these cells were 20-fold slower than those found in the basal cells. These results suggest that electrical tuning of hair cells may play a role in determining the frequency selectivity of the chick cochlea.

Inward barium current (IBa) through voltage-gated calcium channels was recorded from chick cochlear hair cells using the whole-cell clamp technique. IBa was sensitive to dihydropyridines and insensitive to the peptide toxins omega-agatoxin IVa, omega-conotoxin GVIa, and omega-conotoxin MVIIC. Changing the holding potential over a -40 to -80 mV range had no effect on the time course or magnitude of IBa nor did it reveal any inactivating inward currents. The activation of IBa was modeled with Hodgkin-Huxley m2 kinetics. The time constant of activation, tau m, was 550 microseconds at -30 mV and gradually decreased to 100 microseconds at +50 mV. A Boltzmann fit to the activation curve, m infinity, yielded a half activation voltage of -15 mV and a steepness factor of 7.8 mV. Opening and closing rate constants, alpha m and beta m, were calculated from tau m and m infinity, then fit with modified exponential functions. The H-H model derived by evaluating the exponential functions for alpha m and beta m not only provided an excellent fit to the time course of IBa activation, but was predictive of the time course and magnitude of the IBa tail current. No differences in kinetics or voltage dependence of activation of IBa were found between tall and short hair cells. We conclude that both tall and short hair cells of the chick cochlea predominantly, if not exclusively, express noninactivating L-type calcium channels. These channels are therefore responsible for processes requiring voltage-dependent calcium entry through the basolateral cell membrane, such as transmitter release and activation of Ca(2+)-dependent K+ channels.

The mechanism for electrical tuning in non‐mammalian hair cells rests within the widely diverse kinetics of functionally distinct, large‐conductance potassium channels (BK), thought to result from alternative splicing of the pore‐forming α subunit and variable co‐expression with an accessory β subunit. Inside‐out patches from hair cells along the chicken basilar papilla revealed ‘tonotopic’ gradations in calcium sensitivity and deactivation kinetics. The resonant frequency for the hair cell from which the patch was taken was estimated from deactivation rates, and this frequency reasonably matched that predicted from the originating cell's tonotopic location. The rates of deactivation for native BK channels were much faster than rates reported for cloned chicken BK channels including both α and β subunits. This result was surprising since patches were pulled from hair cells in the apical half of the papilla where β subunits are most highly expressed. Heterogeneity in the properties of native chicken BK channels implies a high degree of molecular variation and hinders our ability to identify those molecular constituents.

We have used whole-cell patch clamp techniques to record from tall hair cells isolated from the apical half of the alligator cochlea. Some of these cells gave action potentials in response to depolarizing current injections. When the same cells were voltage clamped, large transient inward currents followed by smaller outward currents were seen in response to depolarizing steps. We studied the transient inward current after the outward current had been blocked by external tetraethylammonium (20 mM) or by replacing internal potassium with cesium. It was found to be a sodium current because it was abolished by either replacing external sodium with choline or by external application of tetrodotoxin (100 nM). The sodium current showed voltage-dependent activation and inactivation. Most of the spiking hair cells came from the apex of the cochlea, where they would be subject to low-frequency mechanical stimulation in vivo.