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D P Corey

Publications and source records attributed to D P Corey.

At least 55 records · Page 3Linked to original sources
PubMed1993

Loss of Na+ channel inactivation by anemone toxin (ATX II) mimics the myotonic state in hyperkalaemic periodic paralysis.

1. Mutations that impair inactivation of the sodium channel in skeletal muscle have recently been postulated to cause several heritable forms of myotonia in man. A peptide toxin from Anemonia sulcata (ATX II) selectively disrupts the inactivation mechanism of sodium channels in a way that mimics these mutations. We applied ATX II to rat skeletal muscle to test the hypothesis that myotonia is inducible by altered sodium channel function. 2. Single-channel sodium currents were measured in blebs of surface membrane that arose from the mechanically disrupted fibres. ATX II impaired inactivation as demonstrated by persistent reopenings of sodium channels at strongly depolarized test potentials. A channel failed to inactivate, however, in only a small proportion of the depolarizing steps. With micromolar amounts of ATX II, the ensemble average open probability at the steady state was 0.01-0.02. 3. Ten micromolar ATX II slowed the relaxation of tension after a single twitch by an order of magnitude. Delayed relaxation is the in vitro analogue of the stiffness experienced by patients with myotonia. However, peak twitch force was not affected within the range of 0-10 microM ATX II. 4. Intracellular injection of a long-duration, constant current pulse elicited a train of action potentials in ATX II-treated fibres. After-depolarizations and repetitive firing often persisted beyond the duration of the stimulus. Trains of action potentials varied spontaneously in amplitude and firing frequency in a similar way to the electromyogram of a myotonic muscle. Both the after-depolarization and the post-stimulus firing were abolished by detubulating the fibres with glycerol. 5. We conclude that a loss of sodium channel inactivation alone, without changes in resting membrane conductance, is sufficient to produce the electrical and mechanical features of myotonia. Furthermore, in support of previous studies on myotonic muscle from patients, this model provides direct evidence that only a small proportion of sodium channels needs to function abnormally to cause myotonia.

Action Potentials↗
PubMed1992

Mechanoelectrical transduction by hair cells.

Hair cells of the inner ear are one of nature's great success stories, appearing early in vertebrate evolution and having a similar form in all vertebrate classes. They are specialized columnar epithelial cells, with an array of modified microvilli or stereocilia on their apical surface, interconnected by a series of linkages. The mechanical stimulus causes deflection of the stereocilia, stretching linkages between them, and opening the mechanotransducer channels. On a slower timescale, hair cells adapt in order to maintain optimum sensitivity, with an adaptation motor within the stereocilia acting to keep the resting tension on channels constant.

Animals↗
PubMed1992

Sensational science. Sensory Transduction: 45th Annual Symposium of the Society of General Physiologists, Marine Biological Laboratory, Woods Hole, MA, USA, September 5-8, 1991.

In the course of several days of formal and informal talks, in the idyllic setting of Woods Hole, the impression grew among many of the participants that useful common themes have emerged for comparison among sensory transduction systems. Many of these were made explicit in a talk on biophysical principles of sensory transduction by Steven Block (Cambridge, MA, USA). In one hour, Block summarized the rest of the symposium and much more, in a dazzling tour through the senses. One of his points was that all sensory transducers must fulfill common goals: detection of the signal, which involves the functions of collecting, selecting or tuning, and capture of the stimulus; amplification, to raise the signal energy (without adding noise) for transmission to other parts of the organism; adaptation or feedback, to extract behaviorally useful parts of the signal; termination, to re-prime the system for the next signal; and encoding, which puts the information in a useful form for downstream processing or effector elements. Another useful comparison was between quantum-detecting systems, such as photoreception and olfaction, where the energy of the stimulus quantum (photon or odor ligand) is large and a uniform response is desired, and noise-limited systems, such as auditory transduction or magnetoreception, where thermal noise is larger than the smallest stimuli and time-averaging helps pull the signal out of the noise. A third observation from Block was that sensory transduction systems--while often performing at physical limits--have not necessarily been perfected by the process of evolution.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

An active motor model for adaptation by vertebrate hair cells.

Bullfrog saccular hair cells adapt to maintained displacements of their stereociliary bundles by shifting their sensitive range, suggesting an adjustment in the tension felt by the transduction channels. It has been suggested that steady-state tension is regulated by the balance of two calcium-sensitive processes: passive "slipping" and active "tensioning." Here we propose a mathematical model for an adaptation motor that regulates tension, and describe some quantitative tests of the model. Slipping and tensioning rates were determined at membrane potentials of -80 and +80 mV. With these, the model predicts that the I(X) curve (relating bundle displacement and channel open probability) should shift negatively by 124 nm when the cell is depolarized, with an exponential time course that is slower on depolarization from -80 to +80 mV than on repolarization. This was observed: on depolarization, the I(X) curve shifted by an average of 139 nm, and displayed the expected difference in rates at the two potentials. Because the negative shift of the I(X) curve on depolarization represents an increase in the tension on transduction channels, the model also predicts this tension should cause an unrestrained bundle to pivot negatively by 99 nm on depolarization. Such movement was observed using high-resolution video microscopy; its amplitude was variable but ranged up to about 100 nm, and its time course was asymmetric in the same way as that of the I(X) curve shift. In additional comparisons, the active bundle movements and I(X) curve shift exhibited a similar steady-state voltage dependence, and were both reversibly abolished by reduced bath Ca2+ or by the transduction channel blocker streptomycin. Lastly, among different cells, the amplitude of the movement increased with the size of the transduction current. Thus, a quantitative mechanical model for adaptation also accounts for the observed mechanical behavior of the bundle, suggesting that the same mechanism is responsible for both, and that adaptation is mediated by an active, force-producing mechanism.

Animals↗
PubMed1991

A sodium channel defect in hyperkalemic periodic paralysis: potassium-induced failure of inactivation.

Hyperkalemic periodic analysis (HPP) is an autosomal dominant disorder characterized by episodic weakness lasting minutes to days in association with a mild elevation in serum K+. In vitro measurements of whole-cell currents in HPP muscle have demonstrated a persistent, tetrodotoxin-sensitive Na+ current, and we have recently shown by linkage analysis that the Na+ channel alpha subunit gene may contain the HPP mutation. In this study, we have made patch-clamp recordings from cultured HPP myotubes and found a defect in the normal voltage-dependent inactivation of Na+ channels. Moderate elevation of extracellular K+ favors an aberrant gating mode in a small fraction of the channels that is characterized by persistent reopenings and prolonged dwell times in the open state. The Na+ current, through noninactivating channels, may cause the skeletal muscle weakness in HPP by depolarizing the cell, thereby inactivating normal Na+ channels, which are then unable to generate an action potential. Thus the dominant expression of HPP is manifest by inactivation of the wild-type Na+ channel through the influence of the mutant gene product on membrane voltage.

Electrophysiology↗
PubMed1991

Tip-link integrity and mechanical transduction in vertebrate hair cells.

An attractive hypothesis for hair-cell transduction is that fine, filamentous "tip links" pull directly on mechanically sensitive ion channels located at the tips of the stereocilia. We tested the involvement of tip links in the transduction process by treating bundles with a BAPTA-buffered, low-Ca2+ saline (10(-9) M). BAPTA abolished the transduction current in a few hundred milliseconds. BAPTA treatment for a few seconds eliminated the tip links observed by either scanning or transmission electron microscopy. BAPTA also eliminated the voltage-dependent movement and caused a positive bundle displacement of 133 nm, in quantitative agreement with a model for regulation of tension. We conclude that tip links convey tension to the transduction channels of hair cells.

Animals↗
PubMed1990

Ion channel expression by white matter glia: the type-1 astrocyte.

We describe the electrophysiological properties of acutely isolated type-1 astrocytes using a new "tissue print" dissociation procedure. Because the enzymes used did not destroy or modify the ion channels, and the cells retained many processes, the properties may reflect those in vivo. The types of ion channels in type-1 astrocytes changed rapidly during the first 10 postnatal days, when they attained their adult phenotype. This change was dependent on the presence of neurons. In culture, most of these channel types were not expressed, but a phenotype more typical of that in vivo could be induced by co-culture with neurons. The electrophysiological properties of astrocytes make some existing hypotheses of astrocyte function less likely.

Aging↗
PubMed1990

Ion channel expression by white matter glia: the O-2A glial progenitor cell.

We describe electrophysiological properties of the O-2A glial progenitor cell in a new serum-free culture system. O-2A progenitors have many properties characteristic of neurons: they have glutamate-activated ion channels, express the neuronal form of the sodium channel, fire single regenerative potentials, and synthesize the neurotransmitter GABA by an alternative synthetic pathway. Nearly identical properties were observed in acutely isolated O-2A progenitors, indicating that this phenotype is not an artifact of culture. The O-2A did not express a simple subset of channel types found in its descendant cells, the type-2 astrocyte and oligodendrocyte, studied in the same culture system. During development, these electrophysiological properties may contribute to O-2A function in vivo.

Action Potentials↗
PubMed1990

Actin cores of hair-cell stereocilia support myosin motility.

The actin cores of hair-cell stereocilia were tested as a substrate for the movement of myosin-coated beads in an in vitro assay. Large numbers of stereocilia from bullfrog sacculi and semicircular canals were isolated by blotting onto coverglasses and were demembranated to expose the polar actin tracks of their cytoskeletal cores. Silica or polystyrene beads, coated with thick filaments of chicken skeletal muscle myosin, were added to this core preparation in the presence of ATP. Myosin-coated beads could reach some of the cores by diffusion alone, but the efficiency and precision of the assay were improved considerably by the use of "optical tweezers" (a gradient-force optical trap) to deposit the beads directly on the cores. Beads applied in this fashion bound and moved unidirectionally at 1-2 microns/s, escaping the retarding force of the trap. Actin filaments within the stereocilia are cross-linked by fimbrin, but this did not appear to interfere with the motility of myosin. Beads coated with optic-lobe kinesin were also tested for movement; these bound and moved unidirectionally at 0.1-0.2 microns/s when applied to microtubule-based kinociliary cores, but not when applied to actin-based stereociliary cores. Our results are consistent with, and lend support to, a model for hair cell adaptation in which a molecular motor such as myosin maintains tension on the mechanically gated transduction channels. Optical tweezers and video-enhanced differential interference contrast optics provide high efficiency and improved optical resolution for the in vitro analysis of myosin motility.

Actins↗
PubMed1989

Glial and neuronal forms of the voltage-dependent sodium channel: characteristics and cell-type distribution.

Two functionally different forms of the voltage-dependent sodium channel were observed in glia and in neurons of the mammalian nervous system. Both forms had identical conductance and tetrodotoxin sensitivity and displayed steady-state inactivation, a strongly voltage-dependent rate of activation, and a faster but weakly voltage-sensitive rate of inactivation. However, the glial form had significantly slower kinetics and a more negative voltage dependence, suggesting that it was functionally specialized for glia. This form was found in most glial types studied, while the neuronal form was observed in retinal ganglion cells, cortical motor neurons, and O2A glial progenitor cells. Both forms occurred in type-2 astrocytes. The presence of the glial form correlated with the RAN-2 surface antigen.

Action Potentials↗
PubMed1989

"Bundle blot" purification and initial protein characterization of hair cell stereocilia.

Stereocilia were isolated from bullfrog (Rana catesbeiana) saccular hair cells by nitrocellulose adhesion. The high purity and high yield of the preparation were demonstrated by microscopy. SDS/PAGE of stereociliary proteins resolved 12-15 major bands. Actin, previously identified as a component of the stereociliary core, was identified in purified stereocilia as a band comigrating with authentic actin and by phalloidin labeling of intact isolated stereocilia. Fimbrin was identified in immunoblots of purified stereocilia. The most abundant other proteins migrated at 11, 14, 16-19, 27, and 36 kDa. Demembranated stereociliary cores consisted primarily of protein bands corresponding to actin and fimbrin and several proteins ranging from 43 to 63 kDa. Because the adaptation mechanism in hair cells is calcium-sensitive and seems localized to stereocilia, we sought evidence for calcium-binding proteins in stereocilia. Calmodulin and calbindin antibodies labeled stereocilia in intact cells. A protein band in purified stereocilia exhibited a Ca2+-dependent shift in electrophoretic mobility identical to that of authentic calmodulin, and the 27-kDa band may represent calbindin. These biochemical data demonstrate that stereocilia consist of a relatively small set of proteins. Most of these, including those involved in transduction and adaptation, are as yet uncharacterized. The availability of purified stereocilia should prove useful in further studies of structure-function relationships in these mechanically sensitive organelles.

Actins↗
PubMed1989

Voltage dependence of adaptation and active bundle movement in bullfrog saccular hair cells.

Hair cells of the bullfrog sacculus adapt to maintained displacement stimuli in a manner that suggests an active regulation of the tension stimulus reaching transduction channels. We have examined adaptation in dissociated hair cells by whole-cell patch-clamp recording and video microscopy. Adaptation was present in these cells, and it depended on extracellular calcium. The adaptation rate--as well as the position of the resting current-displacement curve--also depended on membrane potential, suggesting that calcium passes into the cytoplasm to reach its site of action. After abrupt hyperpolarization, the adaptation rate increased within milliseconds, suggesting that the calcium site is within a few micrometers of the ion channels through which calcium enters. The voltage dependence of the resting current-displacement curve, together with the "gating springs" hypothesis for transduction, predicts movement of the bundle away from the kinocilium when the cell is depolarized. This was observed.

Adaptation, Physiological↗
PubMed1989

Regulation of tension on hair-cell transduction channels: displacement and calcium dependence.

An epithelial preparation of the bullfrog sacculus was used to characterize the initial rate of the adaptation mechanism in hair cells and its dependence on displacement and calcium. The I(X) curve relating transduction current and bundle displacement shifted along the X-axis without substantial change in slope, as previously observed, suggesting that adaptation involves a change in the attachment point of the elastic element connected to ion channels. If the "tip links" model of transduction is correct, this implies that one end of the link moves along the side of the stereocilium. The rates were highly asymmetric: in the tensioning direction the rate was roughly constant at 1-2 microns/sec (calculated as motion along a stereocilium); this is similar to that of myosin on actin. In the relaxing direction it appeared linearly dependent on tension. Calcium preferentially potentiated the relaxation, and apparently reduced the resting tension in the elastic element. The calcium site appears specific for calcium, as other divalent cations inhibited its action. Dihydrostreptomycin inhibited the positive rate, but its effect could not be explained by a simple channel block, and it seems inconsistent with screening of negative charge in the mouth of the transduction channel.

Animals↗
PubMed1989

Calcium current in cortical astrocytes: induction by cAMP and neurotransmitters and permissive effect of serum factors.

Voltage-dependent L-type calcium channels were induced in highly purified, cultured, cortical astrocytes by exposure to substances known to increase their intracellular cAMP: 8-bromo-cAMP, forskolin, isoproterenol, and vasoactive intestinal peptide. In untreated control cultures, L-type calcium currents were entirely absent. The induction of this calcium current was specific to cortical astrocytes and a closely related astrocyte type in white matter and did not occur in meningeal cells or oligodendrocytes. The ability of forskolin to induce L-type calcium current in astrocytes depended on previous culture of these cells in a permissive lot of serum for at least 48 hr. In addition, certain lots of sera caused expression of a voltage-dependent T-type calcium channel in untreated control cultures. Several possible mechanisms of calcium channel induction by cAMP are consistent with our data: modification of a silent channel already present in the membrane, indirect effect of cytoskeletal alteration, or insertion of channels from submembrane stores. If the environment in vivo is permissive, regulation of glial ion channels may be under neuronal control.

Animals↗
PubMed1988

Ion channel expression by white matter glia: I. Type 2 astrocytes and oligodendrocytes.

White matter is a compact structure consisting primarily of neuronal axons and glial cells. As in other parts of the nervous system, the function of glial cells in white matter is poorly understood. We have explored the electrophysiological properties of two types of glial cells found predominantly in white matter: type 2 astrocytes and oligodendrocytes. Whole-cells and single-channel patch-clamp techniques were used to study these cell types in postnatal rat optic nerve cultures prepared according to the procedures of Raff et al. (Nature, 303:390-396, 1983b). Type 2 astrocytes in culture exhibit a "neuronal" channel phenotype, expressing at least six distinct ion channel types. With whole-cell recording we observed three inward currents: a voltage-sensitive sodium current qualitatively similar to that found in neurons and both transient and sustained calcium currents. In addition, type 2 astrocytes had two components of outward current: a delayed potassium current which activated at 0 mV and an inactivating calcium-dependent potassium current which activated at -30 mV. Type 2 astrocytes in culture could be induced to fire single regenerative potentials in response to injections of depolarizing current. Single-channel recording demonstrated the presence of an outwardly rectifying chloride channel in both type 2 astrocytes and oligodendrocytes, but this channel could only be observed in excised patches. Oligodendrocytes expressed only one other current: an inwardly rectifying potassium current that is mediated by 30- and 120-pS channels. Because these channels preferentially conduct potassium from outside to inside the cell, and because they are open at the resting potential of the cell, they would be appropriate for removing potassium from the extracellular space; thus it is proposed that oligodendrocytes, besides myelinating axons, play an important role in potassium regulation in white matter. The conductances present in oligodendrocytes suggest a "modulated Boyle and Conway mechanism" of potassium accumulation.

Animals↗
PubMed1988

Immunological, morphological, and electrophysiological variation among retinal ganglion cells purified by panning.

Two different monoclonal antibodies to the Thy-1 antigen, T11D7 and 2G12, were used to purify and characterize retinal ganglion cells from postnatal rat retina. Although Thy-1 has been reported to be a specific marker for ganglion cells in retina, retinal cell suspensions contained several other types of Thy-1-positive cells as well. Nevertheless, a simple two-step "panning" procedure allowed isolation of ganglion cells to nearly 100% purity. We found that postnatal ganglion cells differed in antigenic, morphological, and intrinsic electrophysiological characteristics, and that these properties were correlated with one another. Minor variations of this panning protocol should allow rapid, high yield purification to homogeneity of many other neuronal and glial cell types.

Action Potentials↗