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At least 487 records · Page 27Linked to original sources
PubMed2000

Calcium channels functional roles in the frog semicircular canal.

Different types of voltage-operated calcium channels have been described in hair cells; however, no clear functional role has been assigned to them. As a first functional characterization of vestibular calcium channels, we studied the effect of several calcium channel agonists and antagonists on whole nerve firing rate in an isolated frog semicircular canal preparation. Resting activity was affected by all dihydropyridines tested and by omegaconotoxin GVIA, whereas only nimodipine was able to reduce the mechanically evoked activity. These results indicate that nimodipine-sensitive channels play a major role in afferent transmitter release, and omega-conotoxin GVIA sensitive channels regulate the afferent firing (possibly on the postsynaptic side) but with a less important role.

Animals↗
PubMed2005

Otolith organ or semicircular canal stimulation induces c-fos expression in unipolar brush cells and granule cells of cat and squirrel monkey.

Immediate early gene expression in the cerebellar vermis of cats and squirrel monkeys was stimulated by prolonged whole body rotations. Continuous, earth-horizontal axis rotations that excited only otoliths or high velocity vertical axis rotations that excited only semicircular canals resulted in c-fos immunoreactive nuclei concentrated in the granular layer of lobules X and ventral IX (the nodulus and ventral uvula), which represent the medial parts of the vestibulo-cerebellum. Large clusters of labeled nuclei consisting mainly of granule cells and calretinin-positive unipolar brush cells were present in the granular layer, whereas Purkinje cell nuclei were unlabeled, and labeled basket and stellate cell nuclei were scattered in the molecular layer. In other vermal lobules there was a significant but less dense label than in the nodulus and ventral uvula. Generally, the extent of c-fos labeling of molecular layer interneurons was in relation to nuclear labeling of granular layer neurons: labeling of both basket and stellate cells accompanied nuclear labeling of neurons throughout the depth of the granular layer, whereas only stellate cells were labeled when nuclear labeling was restricted to the superficial granular layer. Yaw horizontal or roll vertical rotations each stimulated c-fos expression in the cat medial vestibulo-cerebellum to approximately the same extent. Low-velocity rotations resulted in much less c-fos expression. Similar, albeit less intense, patterns of c-fos activation were observed in monkeys. Concentrated c-fos expression in the medial vestibulo-cerebellum after exposure to a strong head velocity signal that could originate from either otolith or canal excitation suggests that granule and unipolar brush cells participate in a neuronal network for estimating head velocity, irrespective of the signal source.

Animals↗
PubMed1996

Histochemical demonstration of 11 beta-hydroxysteroid dehydrogenase in ampullae of semicircular canals.

The enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) plays a major role in the protection of the mineralocorticoid (type I) receptor. The cellular mechanism of aldosterone selectivity relies on the coexpression of mineralocorticoid receptors and 11 beta HSD in the same cells. An 11 beta HSD activity has been localized in the ampullae of the semicircular canal by histochemical technique which links steroid metabolism with the deposition of formazan deposition. Oxidation of 11 beta-hydroxyandrostenedione was observed in the dark cell region. Oxidation of 11 beta-hydroxyandrostenedione was nicotine-adenine dinucleotide (NAD) dependent. These results demonstrate the presence of a dehydrogenase activity separate from the nicotineamide-adenine dinucleotide phosphate (NADP) dependent 11 beta HSD.

11-beta-Hydroxysteroid Dehydrogenases↗
PubMedDate not supplied

Vestibularly evoked potentials (VESTEPs) of the horizontal semicircular canals under different body positions in space.

We recorded vestibularly evoked potentials (VESTEPs) using natural vestibular stimuli simulating the form of a normal head movement ("raised cosine"). Its smooth course was designed to minimize all possible sources of artifacts of mechanical, electrical, and physiological origin. This motion profile was applied to subjects (Ss) sitting upright, thus stimulating the horizontal semicircular canals alone, as well as when they were tilted in different positions in steps of 30 degrees about the interaural y-axis, which had the effect of modulating the otolithic contribution to the horizontal VESTEP. The transient bell-shaped VESTEP recorded in the upright sitting S was more and more superimposed or replaced by a sustained negativity the more the S was tilted toward a horizontal position. Thus, the VESTEP gradually changes from a velocity response in the upright S (canals only) to a position response in the lying S (canals plus otoliths). Differences in the VESTEPs obtained for clockwise (cw) and counterclockwise (ccw) turns in the tilted S were found. The reason for this is not clear at present and requires further experimentation. However, the VESTEPs obtained under conditions involving backward tilts and cw turns corresponded to those obtained with forward tilts and ccw turns. A similar relationship was found for the VESTEPs obtained with backward tilts and ccw turns and those obtained with forward tilts and cw turns. The physiological basis of this is discussed.

Adolescent↗
PubMed1994

Changes of endolymphatic pressure in the semicircular canal of pigeon by caloric stimulation.

It gets into difficult to explain the mechanism of caloric nystagmus only by convection theory from results of microgravity experiments. One of the other theories is an occurrence of a relative volume change due to a temperature change. Since the volume change must lead to a pressure change after caloric stimulation, we tried to measure the ampulla pressure of the horizontal semicircular canal in pigeons (Columba livia) using an improved servo micropipette system. The main result was that the ampulla pressure increased by cooling and decreased by heating. The changes of the ampulla pressure depended on the temperature change but were not influenced by the pigeon's head position.

Animals↗
PubMed1993

Effects of modifications of extracellular and intracellular calcium concentrations on the bioelectrical activity of the isolated frog semicircular canal.

In the inner ear, calcium appears to play a major role in different processes including mechanoelectrical transduction, adaptation during prolonged stimulation and electrical resonance. The aim of the present study was to investigate the effect of an increase of the cytosolic calcium content and a reduction of the perilymphatic concentration of calcium, on the bioelectrical activity of the isolated frog semicircular canal. Under resting conditions, the spontaneous activity of the afferent fibers and the difference of potential between the endolymphatic and perilymphatic compartments, called endolymphatic potential, were recorded. When the sensory epithelium was mechanically stimulated three additional parameters were investigated: the variations of the endolymphatic potential (ampullar direct current), the variations of the ampullar nerve potential (nerve direct current) and the frequency of the evoked afferent spikes. Increase of the intracellular calcium concentration by administration of the calcium ionophore A23187 (3 x 10(-6) M, 20 min) into the perilymphatic compartment, caused a biphasic effect on the spontaneous activity of the ampullar nerve which increased rapidly, reaching a maximum within 15 min, and then gradually declined to stabilize at 74% of the control 1 h after withdrawal of A23187. A23187 did not induce any modifications of the endolymphatic potential, the ampullar direct current or the frequency of the evoked afferent spikes. In contrast, A23187 induced a significant reduction of the nerve direct current which decreased by 31% of the control 1 h after withdrawal of the ionophore. Gradual reduction of the perilymphatic concentration of calcium (from 2 to 1 mM) induced a dose-dependent increase of the spontaneous activity of the ampullar nerve and the frequency of the evoked afferent spikes. Reduction of the perilymphatic calcium concentration from 1.6 to 1.2 mM caused a transient increase of the endolymphatic potential, while 1 mM Ca2+ induced a decrease to 88% of the control. The nerve direct current slightly increased for calcium concentrations ranging from 1.8 to 1.4 mM and decreased in the presence of 1.2 mM CaCl2. These data suggest that an increase of calcium into the cytosol induces an alteration of the mechanisms responsible for the spontaneous release of the afferent neurotransmitter and the electrogenic spreading of the postsynaptic potentials. In contrast, an excess of calcium does not impair the mechanisms involved in the generation of the action potentials. Our results also suggest that reduction of the perilymphatic calcium concentration may lead to modifications of the physical and electrical properties of the cell membranes of the labyrinthine epithelium and/or the ampullar afferent fibers.

Animals↗
PubMed1993

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal.

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

Animals↗
PubMed1998

[Postural stabilization after otolithic repositioning for posterior semicircular canal canalization].

Today the canal/cupulolithiasic pathogenesis of benign paroxysmal positional vertigo (BPPV) appears well defined; what is less clear is the origin of the postural "instability" often associated with rotatory vertigo. This form is less marked but still resists all know treatments. The purpose of the present study was to determine the actual postural arrangement of subjects suffering from canalolithiasis (CL) of the posterior semicircular canal (PSC) both before and after the positioning maneuver. In this case a personal method of spectral frequency analysis was used in an attempt to clarify the origin of the residual "instability" following otolithic repositioning. Static posturographic testing was performed on 20 patients with CL of the PSC and another 20 normal subjects of comparable age. In the present study the repositioning maneuver leads to a reduction in latero-lateral oscillation without any anterior-posterior variation. Such stabilization covers all the frequencies if a visual input is also present while it is limited to the lower frequencies when the visual content is lacking. In fact, in the latter case the subject maintains his posture using only vestibular and proprioceptive input. On the other hand, the anterior-posterior oscillations increase in all frequency ranges and this increase remains essentially unchanged after the repositioning maneuver. The authors feel that the anterior-posterior destabilization seen in CL of the PSC may stem from a proprioceptive-macular alteration linked to otolitic detachment rather than abnormal ampullar stimulation. The persistence and residual postural alterations found could, therefore, explain the persistent sense of instability that CL patients often complain of, even after the maneuvers which have encountered such success in controlling BPPV.

Adult↗
PubMed1976

Relations of single semicircular canals to the pontine reticular formation.

The effects of afferent vestibular impulses on single pontine reticular formation units and on a small filament of the IIIrd cranial nerve were recorded with tungsten microelectrodes in 40 curarized guinea pigs. Single-shock and repetitive electrical stimulations were applied by means of stimulating electrodes inserted bilaterally into the perilymphatic space of single ampullae of the anterior and lateral semicircular canals. The reticular unitary response consisted mainly in excitation of the resting discharge rate: most units showed vestibular convergence being affected by separate stimulation of the single four ampullae. the reticular evoked field and unitary potentials accounted for latency values ranging from 0.3 to 2.5 msec. As for the early latencies they can be interpreted as responses mediated by direct vestibulo-reticular fibres. A delimited vestibular projection field in the parameidan pontine reticular formation was not identified.

Animals↗
PubMed1993

Horizontal semicircular canal variant of benign positional vertigo.

We report the clinical features and results of quantitative eye-movement testing in 13 patients with episodic positional vertigo and nonfatiguing direction-changing horizontal positional nystagmus (beating to the right with the head turned to the right and beating to the left with the head turned to the left). The benign history and lack of associated neurologic findings support a peripheral localization of the lesion. This syndrome probably represents a horizontal semicircular canal variant of benign positional vertigo. Free-floating debris in one horizontal canal may explain many of the clinical and oculographic findings.

Adult↗
PubMed2003

Calcium current in type I hair cells isolated from the semicircular canal crista ampullaris of the rat.

The low voltage gain in type I hair cells implies that neurotransmitter release at their afferent synapse should be mediated by low voltage activated calcium channels, or that some peculiar mechanism should be operating in this synapse. With the patch clamp technique, we studied the characteristics of the Ca(2+) current in type I hair cells enzymatically dissociated from rat semicircular canal crista ampullaris. Calcium current in type I hair cells exhibited a slow inactivation (during 2-s depolarizing steps), was sensitive to nimodipine and was blocked by Cd(2+) and Ni(2+). This current was activated at potentials above -60 mV, had a mean half maximal activation of -36 mV, and exhibited no steady-state inactivation at holding potentials between -100 and -60 mV. This data led us to conclude that hair cell Ca(2+) current is most likely of the L type. Thus, other mechanisms participating in neurotransmitter release such as K(+) accumulation in the synaptic cleft, modulation of K(+) currents by nitric oxide, participation of a Na(+) current and possible metabotropic cascades activated by depolarization should be considered.

Animals↗
PubMed1984

Epithelial fusion during early semicircular canal formation in the embryonic zebrafish, Brachydanio rerio.

The developing inner ear of the teleost, Brachydanio rerio, provides an opportunity for observing an epithelial fusion between the apical surfaces of apposed epithelia in a vertebrate embryo in vivo. The developing otocyst was filmed for periods up to 4 days in unanesthetized embryos, and specimens were fixed at intervals and processed for light microscopy, TEM, and SEM. The semicircular canals are formed as a consequence of the union between the tips of three cylindrical projections from the wall of the otocyst, which grow toward corresponding bulges of a projection from the lateral wall. The epithelial cells covering the projections contain extensive rough endopasmic reticulum, exhibit apical junctional complexes, and are not underlain by a basal lamina. The core of each projection contains large amounts of flocculent and fibrillar extracellular material. After a period of growth and elongation, the tip of each projection contacts, and adheres to, the appropriate bulge to create a circular, flattened, bilayered, epithelial plate. Small, focal junctions form between the apposed apical cell surfaces within the plate during this period, but they are not numerous. Junctional complexes do develop, however, between apposed cells at the periphery of the plate. After 1-2 hours, the basal surface of the plate exhibit considerable alteration in contour. Adjacent cells within the plate then separate to allow continuity of the connective tissue components of the two structures. The observations of this study indicate that following an initial period of contact and adhesion, cellular reorientation and changes in junctional contacts between adjacent cells within the epithelial plate, rather than cell degeneration, are responsible for perforation of the plate.

Animals↗
PubMed2000

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

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 (</=10 mV) voltage responses. The cells' small voltage responses and negative resting potentials (V(R) = -81.3 +/- 0.2 mV, n = 144) pose a problem for afferent neurotransmission: how does the receptor potential depolarize the cell into the activation range of Ca(2+) channels (positive to -60 mV) that mediate transmitter release? One possibility, suggested by spontaneous positive shifts in the activation range of I(K,L) during whole cell recording, is that the activation range might be modulated in vivo. Any factor that reduces the number of I(K,L) channels open at V(R) will increase R(m) and depolarize V(R). Nitric oxide (NO) is an ion channel modulator that is present in vestibular epithelia. Four different NO donors, applied externally, inhibited the I(K,L) conductance at -67 mV, with mean effects ranging from 33 to 76%. The NO donor sodium nitroprusside inhibited channel activity in patches when they were cell-attached but not excised, suggesting an intracellular cascade. Consistent with an NO-cGMP cascade, 8-bromo-cGMP also inhibited whole cell I(K,L). Ca(2+)-dependent NO synthase is reported to be in hair cells and nerve terminals in the vestibular epithelium. Excitatory input to vestibular organs may lead, through Ca(2+) influx, to NO production and inhibition of I(K,L). The resulting increase in R(m) would augment the receptor potential, a form of positive feedback.

4-Aminopyridine↗
PubMed1996

The effect of clofilium, a K-channel blocker, on the electrogenic K secretion and the sensory discharge at the frog semicircular canal.

Potassium transport by dark cells produces marked K-concentration differences between endo- and perilymphatic fluids in labyrinthine organs and generates the transepithelial potential. The ensuing electrochemical potential for K sustains the transduction current which regulates activity at the cytoneural junction. Clofilium, a compound which is known to block cardiac K channels and to decrease the endocochlear potential, was applied to the endolymphatic side of the isolated frog semicircular canal. The drug abolished the transepithelial potential and increased K outflux from the lumen to the dark cells (or the basolateral perilymph) with no apparent interference with active K secretion. When applied to the perilymphatic side in the intact labyrinth, clofilium reduced the rate of occurrence of miniature excitatory postsynaptic potentials (mEPSPs), both at rest and in response to mechanical stimulation (sinusoidal rotation at 0.1 Hz, 12.5 deg/s2 peak acceleration). This effect may be related to a reduced K-electrochemical unbalance and a decreased transduction current. The drug consistently reduced mEPSP size, although amplitude distributions remained log-normal and time intervals between successive mEPSPs remained exponentially distributed; this suggests a direct effect of clofilium on the postsynaptic membrane, in addition to any possible presynaptic effects. Spike discharge by the afferent fibre was almost completely abolished at rest and responses to mechanical stimulation were reduced by 85-90%. These effects cannot be accounted for by the mild reduction of mEPSP rates and confirm a direct action of clofilium on the afferent postsynaptic terminal.

Animals↗