Recovery of the vestibulocolic reflex after aminoglycoside ototoxicity in domestic chickens. (1/169)

Avian auditory and vestibular hair cells regenerate after damage by ototoxic drugs, but until recently there was little evidence that regenerated vestibular hair cells function normally. In an earlier study we showed that the vestibuloocular reflex (VOR) is eliminated with aminoglycoside antibiotic treatment and recovers as hair cells regenerate. The VOR, which stabilizes the eye in the head, is an open-loop system that is thought to depend largely on regularly firing afferents. Recovery of the VOR is highly correlated with the regeneration of type I hair cells. In contrast, the vestibulocolic reflex (VCR), which stabilizes the head in space, is a closed-loop, negative-feedback system that seems to depend more on irregularly firing afferent input and is thought to be subserved by different circuitry than the VOR. We examined whether this different reflex also of vestibular origin would show similar recovery after hair cell regeneration. Lesions of the vestibular hair cells of 10-day-old chicks were created by a 5-day course of streptomycin sulfate. One day after completion of streptomycin treatment there was no measurable VCR gain, and total hair cell density was approximately 35% of that in untreated, age-matched controls. At 2 wk postlesion there was significant recovery of the VCR; at this time two subjects showed VCR gains within the range of control chicks. At 3 wk postlesion all subjects showed VCR gains and phase shifts within the normal range. These data show that the VCR recovers before the VOR. Unlike VOR gain, recovering VCR gain correlates equally well with the density of regenerating type I and type II vestibular hair cells, except at high frequencies. Several factors other than hair cell regeneration, such as length of stereocilia, reafferentation of hair cells, and compensation involving central neural pathways, may be involved in behavioral recovery. Our data suggest that one or more of these factors differentially affect the recovery of these two vestibular reflexes.  (+info)

Differentiation of mammalian vestibular hair cells from conditionally immortal, postnatal supporting cells. (2/169)

We provide evidence from a newly established, conditionally immortal cell line (UB/UE-1) that vestibular supporting cells from the mammalian inner ear can differentiate postnatally into more than one variant of hair cell. A clonal supporting cell line was established from pure utricular sensory epithelia of H2k(b)tsA58 transgenic mice 2 d after birth. Cell proliferation was dependent on conditional expression of the immortalizing gene, the "T" antigen from the SV40 virus. Proliferating cells expressed cytokeratins, and patch-clamp recordings revealed that they all expressed small membrane currents with little time-dependence. They stopped dividing within 2 d of being transferred to differentiating conditions, and within a week they formed three defined populations expressing membrane currents characteristic of supporting cells and two kinds of neonatal hair cell. The cells expressed several characteristic features of normal hair cells, including the transcription factor Brn3.1, a functional acetylcholine receptor composed of alpha9 subunits, and the cytoskeletal proteins myosin VI, myosin VIIa, and fimbrin. Immunofluorescence labeling and electron microscopy showed that the cells formed complex cytoskeletal arrays on their upper surfaces with structural features resembling those at the apices of normal hair cells. The cell line UB/UE-1 provides a valuable in vitro preparation in which the expression of numerous structural and physiological components can be initiated or upregulated during early stages of mammalian hair cell commitment and differentiation.  (+info)

Regional distribution of ionic currents and membrane voltage responses of type II hair cells in the vestibular neuroepithelium. (3/169)

Basolateral ionic currents and membrane voltage responses were studied in pigeon vestibular type II hair cells using a thin slice through either the semicircular canal (SCC) crista or utricular macular epithelium. Whole cell tight-seal patch-clamp recording techniques were used. Current-clamp and voltage-clamp studies were carried out on the same cell. One hundred ten cells were studied in the peripheral (Zone I) and central (Zone III) zones of the SCC crista, and 162 cells were studied in the striolar (S Zone) and extrastriolar (ES Zone) zones of the utricular macula. One of the major findings of this paper is that hair cells with fast activation kinetics of their outward currents are found primarily in one region of the SCC crista and utricular macula, whereas hair cells with slow activation kinetics are found in a different region. In Zone I of the crista, 95% of the cells have fast activation kinetics ("fast" cells) and in Zone III of the crista, 86% of the cells have slow activation kinetics ("slow" cells). In the utricular macula slice, 100% of the cells from the S Zone are slow cells, whereas 86% of the cells from the ES Zones are fast cells. Oscillation frequency (f) and quality factor (Q) of the damped oscillations of the membrane potential during extrinsic current injections were studied in hair cells in the different regions. The slow cells in Zone III and in the S Zone have a statistically significantly lower f, as a function of the amplitude of injected current, when compared with the fast cells in Zone I and the ES Zone. Although Q varied over a small range and was <2.6 for all cells tested, there was a statistically significant difference between Q for the membrane oscillations of the slow cells and fast cells in response to a range of current injections.  (+info)

Calcium currents in hair cells isolated from semicircular canals of the frog. (4/169)

L-type and R-type Ca(2+) currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 microM); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 microM), omega-conotoxin MVIIC (5 microM), and omega-agatoxin IVA (0.4 microM), but was sensitive to mibefradil (10 microM). Both currents were sensitive to Ni(2+) and Cd(2+) (>10 microM). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected. Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages > or =0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba(2+) was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs(+) current flowing through the same Ca(2+) channel. The physiological role of these currents on hair cell function is discussed.  (+info)

Responses to efferent activation and excitatory response-intensity relations of turtle posterior-crista afferents. (5/169)

Multivariate statistical formulas were used to infer the morphological type and longitudinal position of extracellularly recorded afferents. Efferent fibers were stimulated electrically in the nerve branch interconnecting the anterior and posterior VIIIth nerves. Responses of bouton (B) units depended on their inferred position: BP units (near the planum semilunatum) showed small excitatory responses; BT units (near the torus) were inhibited; BM units (in an intermediate position) had a mixed response, including an initial inhibition and a delayed excitation. Calyx-bearing (CD-high) units with an appreciable background discharge showed large per-train excitatory responses followed by smaller post-train responses that could outlast the shock train by 100 s. Excitatory responses were smaller in calyx-bearing (CD-low) units having little or no background activity than in CD-high units. Excitatory response-intensity functions, derived from the discharge during 2-s angular-velocity ramps varying in intensity, were fit by empirical functions that gave estimates of the maximal response (r(MAX)), a threshold velocity (v(T)), and the velocity producing a half-maximal response (v(1/2)). Linear gain is equal to r(MAX)/v(S), v(S) = v(1/2) - v(T). v(S) provides a measure of the velocity range over which the response is nearly linear. For B units, r(MAX) declines by as much as twofold over the 2-s ramp, whereas for CD units, r(MAX) increases by 15% during the same time period. At the end of the ramp, r(MAX) is on average twice as high in CD as in B units. Thresholds are negligible in most spontaneously active units, including almost all B and CD-high units. Silent CD-low units typically have thresholds of 10-100 deg/s. BT units have very high linear gains and v(S) < 10 deg/s. Linear gains are considerably lower in BP units and v(S) > 150 deg/s. CD-high units have intermediate gains and near 100 deg/s v(S) values. CD-low units have low gains and v(S) values ranging from 150 to more than 300 deg/s. The results suggest that BT units are designed to measure the small head movements involved in postural control, whereas BP and CD units are more appropriate for monitoring large volitional head movements. The former units are silenced by efferent activation, whereas the latter units are excited. This suggests that the efferent system switches the turtle posterior crista from a "postural" to a "volitional" mode.  (+info)

Effect of transgenic GDNF expression on gentamicin-induced cochlear and vestibular toxicity. (6/169)

Gentamicin administration often results in cochlear and/or vestibular hair cell loss and hearing and balance impairment. It has been demonstrated that adenovirus-mediated overexpression of glial cell line-derived neurotrophic factor (GDNF) can protect cochlear hair cells against ototoxic injury. In this study, we evaluated the protective effects of adenovirus-mediated overexpression of GDNF against gentamicin ototoxicity. An adenovirus vector expressing the human GDNF gene (Ad.GDNF) was administered into the scala vestibuli as a rescue agent at the same time as gentamicin, or as a protective agent, 7 days before gentamicin administration. Animals in the Rescue group displayed hearing thresholds that were significantly better than those measured in the Gentamicin or Ad.LacZ/Gentamicin groups. In the Protection group, Ad.GDNF afforded significant preservation of utricular hair cells. The data demonstrated protection of the inner ear structure, and rescue of the inner ear structure and function against ototoxic insults. These experiments suggest that inner ear gene therapy may be developed as a clinical tool for protecting the ear against environmentally induced insults.  (+info)

Major potassium conductance in type I hair cells from rat semicircular canals: characterization and modulation by nitric oxide. (7/169)

Mammalian vestibular organs have two types of hair cell, type I and type II, which differ morphologically and electrophysiologically. Type I hair cells alone express an outwardly rectifying current, I(K, L), which activates at relatively negative voltages. We used whole cell and patch configurations to study I(K,L) in hair cells isolated from the sensory epithelia of rat semicircular canals. I(K,L) was potassium selective, blocked by 4-aminopyridine, and permeable to internal cesium. It activated with sigmoidal kinetics and was half-maximally activated at -74.5 +/- 1.6 mV (n = 35; range -91 to -50 mV). It was a very large conductance (91 +/- 8 nS at -37 mV; 35 nS/pF for a cell of average size). Patch recordings from type I cells revealed a candidate ion channel with a conductance of 20-30 pS. Because I(K,L) was activated at the resting potential, the cells had low input resistances (R(m)): median 25 MOmega at -67 mV versus 1.3 GOmega for type II cells. Consequently, injected currents comparable to large transduction currents (300 pA) evoked small (+info)

The motor and tail regions of myosin XV are critical for normal structure and function of auditory and vestibular hair cells. (8/169)

Recessive mutations in myosin 15, a class XV unconventional myosin, cause profound congenital deafness in humans and both deafness and vestibular dysfunction in mice homozygous for the shaker 2 and shaker 2(J) alleles. The shaker 2 allele is a previously described missense mutation of a highly conserved residue in the motor domain of myosin XV. The shaker 2(J) lesion, in contrast, is a 14.7 kb deletion that removes the last six exons from the 3"-terminus of the Myo15 transcript. These exons encode a FERM (F, ezrin, radixin and moesin) domain that may interact with integral membrane proteins. Despite the deletion of six exons, Myo15 mRNA transcripts and protein are present in the post-natal day 1 shaker 2(J) inner ear, which suggests that the FERM domain is critical for the development of normal hearing and balance. Myo15 transcripts are first detectable at embryonic day 13.5 in wild-type mice. Myo15 transcripts in the mouse inner ear are restricted to the sensory epithelium of the developing cristae ampularis, macula utriculi and macula sacculi of the vestibular system as well as to the developing organ of Corti. Both the shaker 2 and shaker 2(J) alleles result in abnormally short hair cell stereocilia in the cochlear and vestibular systems. This suggests that Myo15 may be important for both the structure and function of these sensory epithelia.  (+info)