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Schnee, M. E., & Ricci, A. J. (2003). Biophysical and pharmacological characterization of voltage-gated calcium currents in turtle auditory hair cells. Journal of Physiology, 549(3), 697–717.
Added by: Sarina Wunderlich (18 Nov 2012 17:47:01 UTC) |
| Resource type: Journal Article DOI: 10.1113/jphysiol.2002.037481 BibTeX citation key: Schnee2003 View all bibliographic details  |
Categories: General Keywords: akustische Kommunikation = acoustic communication, Emydidae, Physiologie = physiology, Schildkröten = turtles + tortoises, Trachemys, Trachemys scripta Creators: Ricci, Schnee Collection: Journal of Physiology |
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| URLs http://onlinelibra ... l.2002.037481/full |
| Abstract |
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Trachemys scripta elegans 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 α1D (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. Calcium channels are fundamental to signal processing in auditory sensory hair cells, regulating both the membrane excitability and neurotransmitter release (Roberts et al. 1990). Electrical resonance, the ability of the hair cell's membrane potential to oscillate at a particular frequency, is the primary tuning mechanism of auditory hair cells in lower vertebrates (Crawford & Fettiplace, 1978; Ashmore, 1983; Lewis & Hudspeth, 1983; Fuchs et al. 1988). Electrical resonance is driven by the interaction between calcium channels and calcium-activated potassium (BK) channels (Art et al. 1986; Hudspeth, 1986; Art & Fettiplace, 1987). Tonotopic variations in the magnitude of both channel types as well as kinetic and calcium sensitivity differences in the BK channels underlie the tonotopic distribution of resonant properties (Art & Fettiplace, 1987; Fuchs et al. 1988; Hudspeth & Lewis, 1988b; Fuchs & Sokolowski, 1990; Art et al. 1995; Wu et al. 1995). Whether similar variations in kinetics or steady-state properties of the hair cell calcium channel occur tonotopically is unknown, and is one of the questions addressed by this work. Synaptic transmission is driven by calcium entering hair cells through calcium channels. Calcium channels are clustered, presumably at synaptic release sites (Roberts et al. 1990; Issa & Hudspeth, 1994; Tucker & Fettiplace, 1995). The number of calcium channels and the number of release sites, but not the density of channels, increases with characteristic frequency (Sneary, 1988; Wu et al. 1996; Ricci et al. 2000). Whether calcium channels linked to neurotransmitter release are different from those linked to electrical resonance remains to be elucidated. Calcium channels have been classified biophysically, pharmacologically and molecularly (see Hille, 2001 for review). L-type calcium channels typically activate at depolarized potentials, are sensitive to dihydropyridines and show slow inactivation (Tsien et al. 1988). Calcium channels are multimeric, containing α, β, α2δ and sometimes γ subunits. The α subunits make up the pore-forming region and are mandatory for channel functioning. L-type calcium channels have two main subtypes based on α subunits, the α1C and the α1D. The first identified and characterized L-type channel was the α1C type, which is found largely in skeletal muscle and heart, while the α1D is found in neuronal cells and some epithelial cells. The α1D channels have several unusual properties including a hyperpolarized activation curve, fast (submillisecond) activation rise times and an insensitivity to the L-type dihydropyridine antagonists (Koschak et al. 2001). Recently the α1D channels have been linked to synaptic release proteins and are thought to regulate some forms of synaptic transmission (Yang et al. 1999). In addition, these accessory proteins can modulate channel electrical properties (Yang et al. 1999). The α1D channel type has been identified in the chick auditory papilla (Kollmar et al. 1997a, b), frog saccule (Rodriguez-Contreras & Yamoah, 2001), trout saccule and mammalian cochlea (Zhang et al. 1999; Platzer et al. 2000; Koschak et al. 2001). Hair cell calcium channels are somewhat different from the α1D channels that are expressed heterologously in vitro, in particular with regard to inactivation. Another purpose of the present work was to compare the properties of hair cell calcium channels to those reported for expressed α1D channels. Several different types of calcium channel have been identified in hair cells. L-type channels have been identified in a variety of hair cell organs including the frog saccule (Hudspeth & Lewis, 1988a; Roberts et al. 1990), turtle papilla (Art et al. 1986; Art & Fettiplace, 1987), chick papilla (Fuchs et al. 1990; Zidanic & Fuchs, 1995; Spassova et al. 2001), guinea-pig cochlear hair cells (Bobbin et al. 1990; Nakagawa et al. 1991; Oshima et al. 1996; Zhang et al. 1999) and the frog semicircular canal (Prigioni et al. 1992; Martini et al. 2000). N-, R- and T-type channels have also been described in vestibular hair cells (Rennie & Ashmore, 1991; Martini et al. 2000; Rispoli et al. 2000). In particular, N-type channels have been identified recently in frog saccule hair cells, an end-organ traditionally thought to have only L-type channels (Su et al. 1995; Rodriguez-Contreras & Yamoah, 2001). R-type currents have been identified in frog semicircular canal hair cells (Martini et al. 2000; Rispoli et al. 2000). Whether different channel types are responsible for different aspects of signal processing in turtle auditory hair cells is unknown, and is also a focus of the present work. The data presented here will demonstrate that only L-type channels are present in turtle auditory hair cells. Tonotopic differences in the activation properties of L-type channels are also described. Activation properties and pharmacological sensitivities support the hypothesis that the calcium channels are of the α1D variety. An unexpected novel finding was the identification of calcium-dependent inactivation.
Added by: Sarina Wunderlich |