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Biomedical subjects

W E Brownell

Publications and source records attributed to W E Brownell.

At least 19 recordsLinked to original sources
PubMed2001

Chlorpromazine alters outer hair cell electromotility.

OBJECTIVE: Outer hair cells (OHCs) of the inner ear rapidly convert electrical gradients into mechanical force, enhancing cochlear sensitivity and frequency selectivity. We investigated the effect of chlorpromazine, an antipsychotic medication that alters membrane biomechanics, on OHC electromotility. STUDY DESIGN: Isolated guinea pig outer hair cells were perfused with chlorpromazine under whole-cell patch-pipette recording conditions. Electromotile responses were measured. RESULTS: A dramatic, reversible, dose-dependent depolarization of voltage at peak capacitance (V(pkCm)) was seen with chlorpromazine treatment. The gain of the electromotile response was maximal near V(pkCm) both before and after chlorpromazine application. Unlike the 3 other agents that alter electromotility (salicylate, lanthanides, membrane tension), chlorpromazine did not change peak capacitance (Cm(pk)), which varies directly with maximal electromotile gain. CONCLUSION: Chlorpromazine changes the membrane voltage at which OHCs exhibit maximal electromotile gain, without changing the magnitude of electromotile responses. SIGNIFICANCE: Chlorpromazine may diminish hearing thresholds or otoacoustic emissions in large doses.

Animals↗
PubMed2001

Micro- and nanomechanics of the cochlear outer hair cell.

Outer hair cell electromotility is crucial for the amplification, sharp frequency selectivity, and nonlinearities of the mammalian cochlea. Current modeling efforts based on morphological, physiological, and biophysical observations reveal transmembrane potential gradients and membrane tension as key independent variables controlling the passive and active mechanics of the cell. The cell's mechanics has been modeled on the microscale using a continuum approach formulated in terms of effective (cellular level) mechanical and electric properties. Another modeling approach is nanostructural and is based on the molecular organization of the cell's membranes and cytoskeleton. It considers interactions between the components of the composite cell wall and the molecular elements within each of its components. The methods and techniques utilized to increase our understanding of the central role outer hair cell mechanics plays in hearing are also relevant to broader research questions in cell mechanics, cell motility, and cell transduction.

Animals↗
PubMed2000

Essential role of BETA2/NeuroD1 in development of the vestibular and auditory systems.

BETA2/NeuroD1 is a bHLH transcription factor that is expressed during development in the mammalian pancreas and in many locations in the central and peripheral nervous systems. During inner ear ontogenesis, it is present in both sensory ganglion neurons and sensory epithelia. Although studies have shown that BETA2/NeuroD1 is important in the development of the hippocampal dentate gyrus and the cerebellum, its functions in the peripheral nervous system and in particular in the inner ear are unclear. Mice carrying a BETA2/NeuroD1 null mutation exhibit behavioral abnormalities suggestive of an inner ear defect, including lack of responsiveness to sound, hyperactivity, head tilting, and circling. Here we show that these defects can be explained by a severe reduction of sensory neurons in the cochlear-vestibular ganglion (CVG). A developmental study of CVG formation in the null demonstrates that BETA2/NeuroD1 does not play a primary role in the proliferation of neuroblast precursors or in their decision to become neuroblasts. Instead, the reduction in CVG neuron number is caused by a combination both of delayed or defective delamination of CVG neuroblast precursors from the otic vesicle epithelium and of enhanced apoptosis both in the otic epithelium and among those neurons that do delaminate to form the CVG. There are also defects in differentiation and patterning of the cochlear duct and sensory epithelium and loss of the dorsal cochlear nucleus. BETA2/NeuroD1 is, thus, the first gene to be shown to regulate neuronal and sensory cell development in both the cochlear and vestibular systems.

Acoustic Stimulation↗
PubMed2000

Voltage- and tension-dependent lipid mobility in the outer hair cell plasma membrane.

The mechanism responsible for electromotility of outer hair cells in the ear is unknown but is thought to reside within the plasma membrane. Lipid lateral diffusion in the outer hair cell plasma membrane is a sigmoidal function of transmembrane potential and bathing media osmolality. Cell depolarization or hyposmotic challenge shorten the cell and reduce membrane fluidity by half. Changing the membrane tension with amphipathic drugs results in similar reductions. These dynamic changes in membrane fluidity represent the modulation of membrane tension by lipid-protein interactions. The voltage dependence may be associated with the force-generating motors that contribute to the exquisite sensitivity of mammalian hearing.

Animals↗
PubMed2000

An analysis of the hydraulic conductivity of the extracisternal space of the cochlear outer hair cell.

The cylindrically shaped cochlear outer hair cell (OHC) plays an important role in the transduction of acoustic energy into electrical energy in the cochlea. The extracisternal space (ECiS) of the lateral wall of the OHC is the fluid-filled space between the plasma membrane (PM) and the intracellular subsurface cisterna (SSC). In the ECiS, an array of cylindrical micropillars extends from the SSC to the PM. We obtain equations for the pressure, osmotic concentration and fluid velocity in the ECiS from the Brinkman-Stokes equations for steady incompressible flow in a plane channel that encloses an array of cylinders and whose upper wall, i.e. the plasma membrane, has a hydraulic conductivity of P(PM). From these equations we obtain an estimate for the hydraulic conductivity of the ECiS, P(ECiS). We show that the ECiS geometry accounts for P(ECiS) being several orders of magnitude larger than P(PM) and that P(ECiS) increases with the width of the ECiS and decreases with the length of the ECiS.

Cochlea↗
PubMed2000

A membrane bending model of outer hair cell electromotility.

We propose a new mechanism for outer hair cell electromotility based on electrically induced localized changes in the curvature of the plasma membrane (flexoelectricity). Electromechanical coupling in the cell's lateral wall is modeled in terms of linear constitutive equations for a flexoelectric membrane and then extended to nonlinear coupling based on the Langevin function. The Langevin function, which describes the fraction of dipoles aligned with an applied electric field, is shown to be capable of predicting the electromotility voltage displacement function. We calculate the electrical and mechanical contributions to the force balance and show that the model is consistent with experimentally measured values for electromechanical properties. The model rationalizes several experimental observations associated with outer hair cell electromotility and provides for constant surface area of the plasma membrane. The model accounts for the isometric force generated by the cell and explains the observation that the disruption of spectrin by diamide reduces force generation in the cell. We discuss the relation of this mechanism to other proposed models of outer hair cell electromotility. Our analysis suggests that rotation of membrane dipoles and the accompanying mechanical deformation may be the molecular mechanism of electromotility.

Animals↗
PubMed2000

Harvesting human hair cells.

The sensory hair cells of the inner ear are responsible for converting balance and hearing stimuli into electrical signals. Until recently, all previous studies of hair cell physiology had been performed on tissue obtained from non-mammals and rodents. In primates, hair cells are difficult to access, because they rest within the densest structure of the body, the otic capsule of the temporal bone. In this report, we describe a technique that we have used in physiological studies to harvest living human hair cells. We collected vestibular and cochlear tissue specimens from adult humans undergoing translabyrinthine and transotic surgical approaches for resection of lateral skull base tumors. Viable hair cells were identified and visualized with light microscopy. The ability to study normal hair cells from humans may further the study of normal and pathological human sensation, hair cell regeneration, and genetic causes of balance and hearing disorders.

Adult↗
PubMed1999

Mechanical and electromotile characteristics of auditory outer hair cells.

The passive and active properties of the cochlear outer hair cell are studied. The outer hair cell is currently considered the major candidate for the active component of mammalian hearing. Understanding of its properties may explain the amplification and sharp frequency selectivity of the ear. To analyse the cell behaviour, a model of a nonlinear anisotropic electro-elastic shell is used. Using the data from three independent experiments, where the mechanical strains of the cell are measured, estimates of the cell wall in-plane Young's moduli and Poisson's ratios are given, as well as estimates of three modes of bending stiffness. Based on these estimates and data from the microchamber experiment, where the cell is under the action of transmembrane potential changes, the characteristics of the outer hair cell active behaviour are found. These characteristics include the coefficients of the active force production per unit of the transmembrane potential change and limiting parameters of the electromotile response for extreme hyperpolarisation and depolarisation of the cell. The obtained estimates provide important information for the modelling of organ-level cochlear mechanics.

Biomechanical Phenomena↗
PubMed1999

Effect of temperature on lateral wall mechanics of the guinea pig outer hair cell.

The outer hair cell is thought to enhance the sensitivity of mammalian hearing. Its lateral wall consists of 3 concentric layers: an outermost plasma membrane, a cortical lattice, and an innermost collection of flattened membranes called the subsurface cisternae. The cytoplasm requires positive pressure for full expression of the outer hair cell's electromotility. Using micropipette aspiration, we investigated the mechanics of the guinea pig's outer hair cell lateral wall at room temperature (22 degrees C) and at the guinea pig's body temperature (39 degrees C). Although there was a 10% decrease in stiffness parameter with an increase from room to body temperature, the difference was not statistically significant; values ranged from 0.45 to 0.65 dyne/cm. With sufficient negative pressure, the cytoplasmic membrane is separated from the rest of the outer hair cell's lateral wall, a process that leads to vesiculation of the plasma membrane. Vesiculation occurs at a lower pressure than at body temperature. Our results demonstrate that the stiffness parameter of the outer hair cell lateral wall at body temperature is similar to that at room temperature. However, the plasma membrane's attachment to the cortical lattice is greatly altered by temperature. The decrease in strength of membrane attachment at body temperature may result from a change in membrane fluidity, making it more easy for membrane attachment sites to break free and permit vesiculation. Whether the tethering of the plasma membrane to the cortical lattice is lost under clinically pathologic conditions deserves future study.

Animals↗
PubMed1999

Transverse and lateral mobility in outer hair cell lateral wall membranes.

Cochlear outer hair cell (OHC) electromotility is associated with the cell's lateral wall. The lateral wall contains two distinct membranes: the plasma membrane (PM) and the subsurface cisternae (SSC). We explored biophysical characteristics of these lipid structures using membrane-specific fluorescent dyes. We have previously demonstrated that di-8-ANEPPS stains the PM while NBD-C6-ceramide partitions to the SSC. In this report we show that NBD-cholesterol also partitions to the SSC. Transmigration of the SSC dyes across the PM was visualized under confocal microscopy, after separating the two membranes using the micropipette aspiration technique. The transverse mobility of NBD-cholesterol was faster than that of NBD-C6-ceramide. We then measured the lateral mobility of the dyes within both the PM and the SSC using fluorescence recovery after photobleaching (FRAP). The diffusion coefficients at 12 37 degrees C and the activation energies for diffusion were found to be similar to those of other biological membranes. These data indicate that both the PM and the SSC are membranes in the fluid phase, with no evidence of temperature-dependent phase transitions. Our observations are consistent with a fluid-mosaic model of the lateral wall membranes.

4-Chloro-7-nitrobenzofurazan↗
PubMed1999

Salicylate induced changes in outer hair cell lateral wall stiffness.

Micropipette aspiration was used to study the lateral wall stiffness of isolated guinea pig outer hair cells (OHCs) perfused with a sodium salicylate solution. Salicylate treatment significantly decreased lateral wall stiffness as measured by a stiffness parameter (S) compared to cells perfused with a standard bathing solution (S = 0.68 +/- 0.26 vs. S= 1.09 +/- 0.25, P < 0.05). The effect was reversible cells treated with salicylate and then with bathing solution exhibited a lateral wall stiffness similar to control cells (S = 1.10 +/- 0.40. P=0.94). Salicylate perfusion diminishes electromotile responses in isolated OHCs and physiologic doses of salicylate produce hearing loss and tinnitus in human subjects. The OHC lateral wall, the locus of electromotility, consists of three concentric layers: (1) an outermost plasma membrane, (2) a cytoskeletal network of actin and spectrin called the cortical lattice and (3) an innermost collection of flattened membranes called the subsurface cisternae (SSC). Ultrastructural studies have shown that salicylate treatment dilates and vesiculates the lateral wall subsurface cisternae (SSC) in guinea pig OHCs. In addition, salicylate causes an outward curvature of plasma membranes in human erythrocytes. The reversible, salicylate induced increase in lateral wall compliance may result from a direct action on the SSC and/or the plasma membrane.

Animals↗
PubMed1999

Nonlinear active force generation by cochlear outer hair cell.

We analyze the nonlinear behavior of the longitudinal and circumferential components of the active force generated by the outer hair cell wall in response to changes of its transmembrane potential. We treat the material of the wall as electroelastic, linear orthotropic in terms of strains and as nonlinear in terms of the transmembrane potential. To describe the nonlinear behavior of the active force versus the transmembrane potential, we use two (Boltzmann and simple exponential) types of approximation. We estimate free parameters of these approximations by combining the previously reported passive stiffnesses with the active strains measured in the microchamber experiment. We analyze the sensitivity of the estimated parameters corresponding to changes of the cell axial stiffness, a characteristic independently measured by several groups. We also study the effect of combining the active strains measured in the microchamber experiment with those measured in the whole cell recording experiment. We show agreement between our prediction of the active force and measurements in the whole cochlea and in isolated cells.

Cell Movement↗
PubMed1998

Fluorescence-imaged microdeformation of the outer hair cell lateral wall.

Outer hair cell (OHC) electromotility appears to be central to mammalian hearing and originates within its lateral wall. The OHC lateral wall is a unique trilaminate structure consisting of the plasma membrane (PM), the cortical lattice (CL), and the subsurface cisternae (SSC). We selectively labeled and imaged the lateral wall components in the isolated guinea pig OHC under confocal microscopy. The PM was labeled with a voltage-sensitive dye, di-8-ANEPPS; the SSC was labeled with the sphingomyelin precursor, NBD-C6-ceramide; and F-actin in the CL was labeled with conjugates of phalloidin. Interactions among the three layers were evaluated with the micropipette aspiration technique. The PM was tethered to the CL and SSC until, at a critical deformation pressure, the PM separated, allowing visualization of the extracisternal space, and ultimately formed a vesicle. After detaching, the stiffness parameter of the PM was 22% of that of the intact lateral wall. We conclude that the lateral wall PM is more compliant than the CL/SSC complex. The data clarify the structural basis for electromotile force coupling in the OHC lateral wall.

Animals↗
PubMed1998

Contribution of membrane cholesterol to outer hair cell lateral wall stiffness.

The outer hair cell can be divided into three domains: the apex, the base, and the lateral wall. With the use of filipin, a polyene fluorescent antibiotic that binds to cholesterol, we found under fluorescence microscopy that the lateral wall membranes were less intensely stained than the apical and basal membranes. This difference in filipin fluorescence between the lateral walls and the ends diminished when cells were incubated in water-soluble cholesterol before staining, suggesting that exogenous cholesterol enters the lateral wall. Under confocal microscopy, we studied the incorporation pattern of a fluorescent cholesterol analogue, NBD-cholesterol. NBD-cholesterol did not stain the apical membranes whereas it intensely labeled the lateral wall. The micropipette aspiration technique was used to assess the effect of cholesterol on lateral wall stiffness. The lateral wall stiffness parameter of cells treated with water-soluble cholesterol (n = 23) was significantly higher than that of controls (n = 27): 0.76+/-0.24 (mean +/- SD) versus 0.46+/-0.10 (Student's t-test, p < 0.001). In conclusion, cholesterol has different distributions among outer hair cell membranes, and when water-soluble cholesterol is incorporated into the cells, the outer hair cell lateral wall stiffness parameter increases.

Affinity Labels↗
PubMed1998

Analysis of the micropipet experiment with the anisotropic outer hair cell wall.

The in-plane and bending stiffnesses of the outer hair cell wall are characteristics crucial to the understanding of force and energy transmission between cochlear elements. A mathematical interpretation is given of the micropipet experiment directed at determining the outer hair cell wall stiffness. On the basis of the microstructural observation of the wall showing that it comprises protein networks with different elastic properties, the model of an orthotropic cylindrical shell is used. The boundary-value problem is analyzed corresponding to the stress-strain of the wall (shell) caused by the action of the micropipet. The solution is given in terms of Fourier series with respect to the circumferential coordinate. An asymptotic analysis of the solution is developed and an approximate formula for the length of the tongue aspirated within the pipet is derived. This leads to an analytical expression for the stiffness parameter measured in the micropipet experiment in terms of Young's moduli and Poisson's ratios of the wall. This expression is an important part of the estimation of the elastic constants of the wall.

Anisotropy↗
PubMed1998

Estimation of elastic moduli and bending stiffness of the anisotropic outer hair cell wall.

The outer hair cell makes both passive and active contributions to basilar membrane mechanics. The outer hair cell mechanics is strongly coupled to the elastic properties of the cell lateral wall. The lateral wall experiences both in-plane deformations and bending under physiological and experimental conditions. To characterize the outer hair cell wall, the model of an orthotropic cylindrical shell is used. The elastic constants of the wall are estimated by solving a set of three equations based on the analyses of three independent experiments. The first equation is derived from a new interpretation of the micropipet experiment; the other two are obtained from the axial loading and the osmotic challenge experiments. The two Young's moduli corresponding to the longitudinal and circumferential directions and two Poisson's ratios are estimated. The longitudinal, circumferential, and mixed modes of the bending stiffness are also estimated. The sensitivity of the derived constants to the variation of the cell axial stiffness, which has been measured by several independent groups, is examined. The new estimates are also compared with results obtained by using the assumption of the wall isotropy.

Anisotropy↗
PubMed1998

Ionic currents and electromotility in inner ear hair cells from humans.

The upright posture and rich vocalizations of primates place demands on their senses of balance and hearing that differ from those of other animals. There is a wealth of behavioral, psychophysical, and CNS measures characterizing these senses in primates, but no prior recordings from their inner ear sensory receptor cells. We harvested human hair cells from patients undergoing surgical removal of life-threatening brain stem tumors and measured their ionic currents and electromotile responses. The hair cells were either isolated or left in situ in their sensory epithelium and investigated using the tight-seal, whole cell technique. We recorded from both type I and type II vestibular hair cells under voltage clamp and found four voltage-dependent currents, each of which has been reported in hair cells of other animals. Cochlear outer hair cells demonstrated electromotility in response to voltage steps like that seen in rodent animal models. Our results reveal many qualitative similarities to hair cells obtained from other animals and justify continued investigations to explore quantitative differences that may be associated with normal or pathological human sensation.

Adult↗