Distribution of Ca2+ channels on cochlear outer hair cells revealed by fluorescent dihydropyridines

Cellular localization of voltage-gated calcium channels and synaptic vesicle-associated proteins in the guinea pig cochlea

The cellular localization of voltage-gated calcium channels (VGCCs) and synaptic vesicle-associated proteins, SV2, synapsin I, and vesicle-associated membrane protein (VAMP) (synaptobrevin), was investigated in the guinea pig cochlea using immunocytochemistry and confocal laser scanning microscopy. Reactivity, in guinea pig, of antibodies to the alpha1 subunits of L-type, alpha1C [Cav1.2] and alpha 1D [Cav1.3]; P/Q-type, alpha1A [Cav2.1]; and R-type, a1E [Cav2.3] high voltage-activated calcium channels, was determined by Western blotting and immunolabeling of cerebellum. In the cochlea the sensory inner hair cells of the organ of Corti displayed strong intracellular staining, predominantly localized to their basolateral poles, with an antibody directed against the alpha1C subunit. Some alpha1C labeling was also observed in the inner pillar cells, in cell bodies of afferent neurons in the spiral ganglion, and in the inferior region of the spiral ligament. The supporting pillar cells were strongly immunoreactive throughout for alpha1D, but no alpha1D labeling of the inner hair cells was seen. The alpha1A subunit showed a cytoplasmic distribution in all three rows of outer hair cells. alpha1E labeling localized to the outer hair cells, predominantly in the subcuticular plate region, and also to nerve fiber bundles beneath these hair cells. Strong immunoreactivity was consistently seen with antibodies directed against SV2 and synapsin I in neuronal structures surrounding the basolateral surfaces of both the inner and outer hair cells but was absent from the sensory cells themselves. VAMP labeling was found throughout the cytoplasm of the inner hair cells and in neuronal structures beneath the hair cells. These results reveal a differential distribution of VGCC-types in the sensory and nonsensory elements of the guinea pig cochlea, with the inner hair cells expressing alpha1C L-type channels and VAMP but not synapsin I or SV2.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Taicatoxin inhibits the calcium-dependent slow motility of mammalian outer hair cells

The effects of taicatoxin on the slow motility of isolated outer hair cells of guinea pig were studied in the experiments. Pretreatment with taicatoxin (0.19 microM) was able to prevent both the cell shortening induced by high K(+) (50mM), and the cell elongation induced by ionomycin (10 microM). These effects of taicatoxin can be mimicked by pretreatment of cells with Ca(2+)-free medium on the slow motility in response to ionomycin or high K(+). Pretreatment with neither calcium channel blockers such as nifedipine (L-type blocker), omega-conotoxin GVIA (N-type blocker), and omega-agatoxin IVA (P-type blocker); nor potassium channel blockers, such as tetraethylammonium chloride (TEA) and 3,4-diaminopyridine (3,4-DAP) can antagonize the cell shortening effect induced by high K(+) and cell elongation induced by ionomycin. The calcium-imaging experiment indicated that taicatoxin, but not nifedipine, did prevent an increase of intracellular Ca(2+) level significantly induced by high K(+). These results demonstrate that the effect of taicatoxin was to block the calcium entry through calcium channels of cell membrane, without relative to its properties of potassium channel blockers. We conclude that taicatoxin-sensitive-calcium channels at least impart, play a significant role in the slow motility of outer hair cell.

Biophysical and pharmacological characterization of voltage-gated calcium currents in turtle auditory hair cells

Hair cell calcium channels regulate membrane excitability and control synaptic transmission. The present investigations focused on determining whether calcium channels vary between hair cells of different characteristic frequencies or if multiple channel types exist within a hair cell, each serving a different function. To this end, turtle auditory hair cells from high- (317 +/- 27 Hz) and low-frequency (115 +/- 6 Hz) positions were voltage clamped using the whole-cell recording technique, and calcium currents were characterized based on activation, inactivation and pharmacological properties. Pharmacological sensitivity to dihydropyridines (nimodipine, Bay K 8644), benzothiazepines (diltiazem) and acetonitrile derivatives (verapamil, D600) and the insensitivity to non-L-type calcium channel antagonists support the conclusion that only L-type calcium channels were present. Fast activation rise times (< 0.5 ms), hyperpolarized half-activation potentials and a relative insensitivity to nimodipine suggest the channels were of the alpha1D (CaV1.3) variety. Although no pharmacological differences were found between calcium currents obtained from high- and low-frequency cells, low-frequency cells activated slightly faster and at hyperpolarized potentials, with half-activating voltages of -43 +/- 1 mV compared to -35 +/- 1 mV. Inactivation was observed in both high- and low-frequency cells. The time course of inactivation required three time constants for a fit. Long depolarizations could result in complete inactivation. The voltage of half-inactivation was -40 +/- 2 mV for high-frequency cells and -46 +/- 2 mV for low-frequency cells. Calcium channel inactivation did not significantly alter hair cell electrical resonant properties elicited from protocols where the membrane potential was hyperpolarized or depolarized prior to characterizing the resonance. A bell-shaped voltage dependence and modest sensitivities to intracellular calcium chelators and external barium ions suggest that inactivation was calcium dependent.

Efferent function of vestibular afferent endings? Similar localization of N-type calcium channels, synaptic vesicle and synaptic membrane-associated proteins

We investigated the distribution of N-type voltage-dependent calcium channels that mediate Ca(2+) entry initiating transmitter release in the rat vestibular sensory epithelium. We used confocal microscopy to assess the in vitro labeling by fluorescent specific ligand binding, omega-conotoxin-GVIA and also the immunolabeling of presynaptic soluble N-ethylmaleimide-sensitive fusion factor attachment protein receptor (SNARE) proteins, syntaxin, 25,000 mol. wt synaptosome-associated protein and synaptotagmin: components of the neurotransmitter exocytosis machinery. We found that there was a close anatomical association between the voltage-gated calcium channels, the synaptic vesicle and synaptic membrane-associated proteins on the afferent nerve calyces and probably afferent boutons, which are postsynaptic compartments. Our data suggest that these peripheral afferent endings possess the presynaptic Ca(2+) channels and the components of the presynaptic SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. They provide additional evidence for a secretory function and efferent role of these endings in hair cell neurotransmission.

Simultaneous monitoring of slow cell motility and calcium signals of the guinea pig outer hair cells

'Slow' motility (shape changes over seconds to minutes) of the mammalian cochlear outer hair cell (OHC) could play a protection role from intense sound pressure and is associated with elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). In the present work, a new approach was elaborated using fluorescent imaging for continuous monitoring of both [Ca(2+)](i) changes and slow motility of OHCs employing the Ca(2+) fluorescent indicator Fura-2. Whole OHC fluorescence and that of cell segments were analyzed to discriminate between fluorescence changes caused by [Ca(2+)](i) rise and those related to change of the cell shape. The reliability of the method was examined by simultaneous monitoring of [Ca(2+)](i) and OHC length changes induced by change of buffer osmolarity or by increase of KCl concentration. The method revealed that the time course of [Ca(2+)](i) increase and rate of cell shortening often do not coincide. It was also observed that [Ca(2+)](i) increased in 70 mM KCl more slowly than the rate of KCl delivery to OHCs. The comparison of the time courses of [Ca(2+)](i) elevation, induced by increase of K(+)/Na(+) ratio and by substitution of Na(+) with N-methyl-D-glucamine(+), indicated that the relatively slow kinetics of [Ca(2+)](i) increase in the OHC is partially attributed to regulation of Ca(2+) homeostasis by the Na(+)/Ca(2+) exchanger.

In vitro effects of hydrogen peroxide on the cochlear neurosensory epithelium of the guinea pig

Reactive oxygen species (ROS) have been postulated to be involved in drug ototoxicity and noise-induced hearing loss. Hydrogen peroxide (H(2)O(2))-induced cell damage in the inner ear was investigated using the neurosensory epithelium of a guinea pig cochlea. Hair cells and supporting cells of the epithelium incubated in Hanks' balanced salt solution were viable up to 6 h. After 2 h of treatment with 0.2 mM H(2)O(2) about 85% of the outer hair cells lost their viability. In contrast inner hair cells slowly began to die after 2 h of H(2)O(2) treatment. The Deiters cells and Hensen cells did not show any signs of damage in the presence of H(2)O(2). Nifedipine, a calcium channel blocker, Quin-2 AM, an intracellular calcium chelator, and 2,2'-dipyridyl, a membrane-permeable iron chelator, all provided partial protection against H(2)O(2)-induced outer hair cell death. The combination of both chelators showed an additional protective effect. The antioxidants N-acetylcysteine and glutathione-monoethyl ester completely protected against H(2)O(2) damage. These results suggest that calcium, iron, and thiol homeostasis play a crucial role in hair cell death caused by H(2)O(2).

Distribution of L-type calcium channels in rat thalamic neurones

One major pathway for calcium entry into neurones is through voltage-activated calcium channels. The distribution of calcium channels over the membrane surface is important for their contribution to neuronal function. Electrophysiological recordings from thalamic cells in situ and after acute isolation demonstrated the presence of high-voltage activated calcium currents. The use of specific L-type calcium channel agonists and antagonists of the dihydropyridine type revealed an about 40% contribution of L-type channels to the total high-voltage-activated calcium current. In order to localize L-type calcium channels in thalamic neurones, fluorescent dihydropyridines were used. They were combined with the fluorescent dye RH414, which allowed the use of a ratio technique and thereby the determination of channel density. The distribution of L-type channels was analysed in the three main thalamic cell types: thalamocortical relay cells, local interneurones and reticular thalamic neurones. While channel density was highest in the soma and decreased significantly in the dendritic region, channels appeared to be clustered differentially in the three types of cells. In thalamocortical cells, L-type channels were clustered in high density around the base of dendrites, while they were more evenly distributed on the soma of interneurones. Reticular thalamic neurones exhibited high density of L-type channels in more central somatic regions. The differential localization of L-type calcium channels found in this study implies their predominate involvement in the regulation of somatic and proximal dendritic calcium-dependent processes, which may be of importance for specific thalamic functions, such as those mediating the transition from rhythmic burst activity during sleep to single spike activity during wakefulness or regulating the relay of visual information.