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At least 433 records · Page 24Linked to original sources
PubMed1994

Autoradiographic measurement of regional brainstem blood flow: occlusion of the anterior inferior cerebellar artery.

Autoradiography was used to measure regional brainstem blood flow in Wistar rats following permanent left anterior inferior cerebellar artery (AICA) occlusion. With the AICA occluded, blood flow to the left vestibular nucleus decreased 31% while flow to the left cochlear nucleus decreased 47% when compared to the right (unobstructed) side. In the rat, the median pontine branch of the basilar artery was found to provide the principal blood supply to the vestibular nucleus. Electrocochleography was also used to measure the action potential (AP), summating potential (SP) and cochlear microphonics (CM) during left AICA occlusion. The AP disappeared completely after at least 7 min, while the SP polarity changed from negative to positive. Findings also showed that CM2 did not disappear completely in pre-mortem animals.

Action Potentials↗
PubMed1994

[Effects of lowering perilymph calcium concentration on various cochlear potentials].

In the present experiment, changes in compound action potentials of auditory nerve (CAP), cochlear microphonics (CM) and endocochlear potentials were observed when the calcium concentration of perilymph was reduced by means of perilymph perfusion, with the aim of analyzing how calcium was involved. Perfusion with Ca(2+)-free artificial perilymph reversibly suppressed both CAP and CM amplitudes, but did not alter the basic nonlinear properties of CAP I/O curve. Furthermore, the perfusion did not alter the EP and negative EP (n-EP) induced by anoxia but eliminated the fast change of EP with respect to turning on and off of intense sound. The mechanisms underlying the effects of calcium are discussed.

Action Potentials↗
PubMed1990

The effect of adenylate cyclase stimulation on endocochlear potential in the guinea pig.

Forskolin, a diterpene extracted from Coleus forskohlii, is potentially an important tool for studying the modulation of ionic currents by cAMP because it stimulates adenylate cyclase in a variety of cells. We studied the effect of forskolin on cochlear potentials and found that its perfusion of the scala vestibuli (SV) to a concentration more than 10(-5) M and the scala tympani (ST) to more than 10(-4) M produced a reversible elevation of the endocochlear potential (EP) in a dose-dependent manner. The cochlear microphonics recorded simultaneously with the EP was not depressed during the EP elevation. A large negative EP was induced by anoxia following the SV perfusion with forskolin (2 X 10(-4) M). The results suggest that the EP elevation produced by forskolin does not result from the decrease in the negative component of EP but from the increase in the positive component of EP.

Adenylyl Cyclases↗
PubMed1980

The Davis theory: a review, and implications of recent electrophysiological evidence.

The Davis theory of mechano-electrical transduction asserts that the endocochlear potential and the hair cell resting potential summate to provide a driving force for current flow through the hair cell. However, while a variety of agents which depress the endocochlear potential simultaneously depress auditory nerve sensitivity and reduce the cochlear microphonic, recent reports suggest that the hair cells may be depolarised without such effects ensuing. The relevant literature is reviewed.

Animals↗
PubMed1989

Development of morphological and physiological changes in the cochlea induced by cytomegalovirus.

The effect of viral infection in the cochlea was investigated by inoculation of live cytomegalovirus or inactivated virus. Auditory thresholds were measured on the day of inoculation and on the terminal day. Two to 8 days following inoculation, the animals were killed and the cochleas were evaluated histologically. The compound nerve potential showed an increase in threshold prior to the cochlear microphonic, indicating the nerve was affected prior to the outer hair cells. All experimental cochleas contained inflammatory and cytomegalic inclusion cells and showed degenerative changes. The number of infected cells was small relative to the histopathology. Control cochleas had normal structure and function. The degeneration, therefore, might be mediated by inflammation as well as by the cytopathic effect of the virus. Viral infections, therefore, might be better managed with anti-inflammatory therapy in addition to antiviral agents.

Animals↗
PubMed1999

Micromechanical effects in the cochlea of tetracaine.

Local anesthetics applied in the tympanic cavity have earlier been shown to affect the gross receptor potentials in reducing the cochlear microphonics and increasing the positive summating potential. To study the effects of this drug on the mechanical responses in the cochlea, vibrations were measured using laser heterodyne interferometry in an isolated in vitro temporal bone preparation from the guinea pig. Measurements were made at a set of frequencies in the fourth cochlear turn from the Hensen's cells and the outer hair cells in response to sound applied to the ear. The tuning curves of the fundamental and the second harmonic components of the vibratory responses were plotted. When 2 mM tetracaine was applied, the high frequency slope of the second harmonic curve shifted down in frequency, this caused the frequency of the maximum of second harmonic tuning to shift down. These changes were reversible when tetracaine was washed out. Observations were also made in the temporal bone preparation in vitro with a confocal microscope. Fluorescent probes were used to label various structures in the organ of Corti. Optical sections were obtained by tilting the organ permitting a view from the side like a radial section through the organ. Images were acquired before, during and after application of tetracaine and were later analyzed with a computer program. Simultaneously, cochlear microphonics and the summating potential were obtained to monitor the electrical response of the preparation. Although the cochlear microphonics and summating potential decreased when 2 mM tetracaine was applied, structural changes were not measurable in the organ of Corti. The decrease was reversible when tetracaine was washed out. It is concluded that tetracaine affected the high frequency part of the non-linear second harmonic component, possibly by lowering the stiffness of the stereocilia bundle or the body of the outer hair cells.

Acoustic Stimulation↗
PubMed1998

[Abnormal augmentation of the evoked potential and morphological changes of guinea pig cochlea induced by cisplatin].

OBJECTIVE: To investigate the characteristic of cochlear microphone(CM), summating potential(SP), compound action potential(CAP) and the morphological changes of hair cells after selective damage to the inner ear by acute cochlear perfusion of cisplatin. METHODS: Dynamic changes of CM, -SP and CAP were recorded by glass electrode from the third turn of the scala media guinea pig's cochlea before and during cisplatin perfusion of the whole cochlea for 2 hours. RESULTS: It indicated that after one hour of the perfusion, the amplitude of CM, -SP and CAP decreased at stimulation intensity < or = 60 dB SPL, while the amplitude increased at stimulation intensity > or = 70 dB SPL, as compared with those before perfusion (the average of CM increased by 3.6 mV at 90 dB SPL, average of -SP increased by 1.6 mV at 120 dB SPL, average of CAP increased by 0.23 mV at 90 dB SPL). After two hours of perfusion, the amplitude of CM, -SP and CAP decreased in all the stimulation intensity. The succinic dehydrgenase (SDH) staining was decreased in OHCs, while that of IHC's remained normal. Transmission electron microscopic examination of organ of Corti showed morphological changes in OHCs, such as disappearance of nuclear chromosome, denature and reduction number, mitochondria while the structure of IHCs remained normal. CONCLUSION: The abnormal augmentation phenomenon of CM and -SP may be due to the abnormal modulation of Ca2+ in IHCs and OHCs. The abnormal augmentation of CAP suggests that the suppression effects of OHCs and efferent neurotransmitter on IHCs and afferent neurotransmitter may be reduced after OHCs damage.

Action Potentials↗
PubMed1980

Potentials of outer hair cells and their membrane properties in cationic environments.

The intracellular potentials of outer hair cells were identified by means of Alcian Blue dye marking in the guinea pig cochlea. The d.c. resting potentials were less negative than in the supporting cells. Mean maximal amplitude of the a.c. component in intracellular responses to middle tone stimulation was about 3.8 mV. The phase of the a.c. component was opposite to that of the subtectorial space. The a.c. component decreased as the potential gradient across the sensory hair surface was reduced. Calcium ions and the other divalent cations introduced into the scala media reduced cochlear microphonics roughly in proportion to their concentration. Calcium reduction produced by chelation in the scala media induced the reduction of the microphonics. The intracellular potentials of outer hair cells are unstable and different from those of inner hair cells. Calcium levels may be important in controlling the receptive function of the sensory hair surface, which is exposed to endolymph with a high potassium concentration.

Animals↗
PubMed1981

Eighth-nerve action potentials evoked by tone bursts in cats before and after inducement of an acute noise trauma.

Properties of eighth-nerve action potentials (AP) evoked by single-frequency tone-bursts (test tone) were studied in cats. Curves representing AP threshold as a function of test-tone frequency have a shape similar to behavioral and single-fiber threshold curves. The absolute level of AP thresholds is higher than that of behavioral and single-fiber thresholds. Cats were exposed to broad-band noise (equal intensities per octave) halfway into the experiments. This exposure resulted in a long-term temporary threshold shift (TTS) which remained fairly steady during the measurements. APs were measured before and during an acute noise trauma in the same animal. After inducement of the trauma the greatest threshold shift is found between 2 and 6 kHz. Curves representing AP amplitude as a function of stimulus SPL are displaced to higher stimulus SPLs. Sometimes the slope of the curve is steeper after the noise exposure than before. AP latency at threshold did not change due to the excessive noise exposure. AP-latency values compared at equal sound pressure levels before and after inducement of the trauma showed higher values during the trauma than before.

Action Potentials↗
PubMed1984

Summating potential (SP) tuning curves.

Two methods were used to extract frequency specific information from the gross d.c. cochlear potential, the summating potential (SP). The first approach was to derive SP tuning curves using a two-tone simultaneous masking procedure; the second to obtain SP iso-response functions. The influence of various parameter changes on the configuration of these functions was also investigated. While SP tuning curves measured using the two-tone paradigm have higher Q10 dB values than SP iso-response functions in the base of the cochlea, the latter have the advantage of avoiding contamination by various nonlinear phenomena which are inherent in the simultaneous tone-on-tone masking procedure. Since SP tuning curves are similar to those for basilar membrane motion (Sellick, P.M., Patuzzi, R. and Johnstone, B.M. (1982): J. Acoust. Soc. Am. 72, 131-141) and the whole nerve action potential (AP) (Cheatham, M.A. and Dallos, P. (1979): J. Acoust. Soc. Am. 65, S13), nearly the same degree of tuning may be reflected at these peripheral recording locations.

Action Potentials↗
PubMed1988

Volume flow rate of perilymph in the guinea-pig cochlea.

The rate of longitudinal flow of perilymph has been measured using an ionic tracer technique. Spread of the tracer trimethylphenylammonium (TMPA) along the perilymphatic scalae was monitored with ion-selective microelectrodes following injection of a minute bolus (approximately 50 nl) of 150 mM TMPAC1 one turn away. This amount of TMPA had virtually no toxic effect on cochlear function. The spread of tracer by longitudinal volume flow and passive diffusion were separated by comparing tracer movements in both apical and basal directions along the scalae in two groups of animals. Experimental findings were compared with a mathematical model which combined diffusion and volume flow. Our results demonstrated that when electrodes were completely sealed into the cochlea, the rate of longitudinal volume flow in scala tympani was extremely slow, approximately 1.6 nl/min in the apical direction. Longitudinal flow was not detectable in scala vestibuli. When the otic capsule was perforated, flow rates of over 1 microliter/min were recorded in scala tympani, probably as a result of cerebrospinal fluid entry through the cochlear aqueduct. When the cochlea was sealed (with recording electrodes in place) and cerebrospinal fluid pressure was released, there was no significant basally-directed flow of perilymph in scala tympani. These findings support the concept that perilymph composition is maintained by local, cochlear mechanisms which do not involve longitudinal volume flow. They provide strong evidence that perilymph is not secreted in one region and resorbed at a spatially distant site.

Action Potentials↗
PubMed1994

A nicotinic-like receptor mediates suppression of distortion product otoacoustic emissions by contralateral sound.

The purpose of this investigation was to provide in vivo pharmacologic characterization of a cholinergic receptor mediating the suppressive effects of medial olivocochlear (MOC) efferent activation. MOC neurons were activated by contralateral sound and the resulting suppression of ipsilateral distortion product otoacoustic emissions (DPOAEs) was monitored before and after intracochlear perfusions of cholinergic antagonists. Results revealed a dose-dependent blockade of contralateral suppression of DPOAEs by a wide variety of nicotinic and muscarinic cholinergic receptor antagonists, as well as by non-traditional antagonists of cholinergic activity. The nicotinic antagonists, alpha-bungarotoxin, curare and kappa-bungarotoxin, and the glycine antagonist, strychnine, blocked contralateral suppression at nanomolar concentrations and demonstrated similar potencies. IC50 values were 2.38 x 10(-7), 2.79 x 10(-7), 3.81 x 10(-7) and 2.96 x 10(-7) M, respectively. These agents were followed in potency by the nicotinic antagonist, trimethaphan (1.75 x 10(-6) M), the M3 muscarinic antagonist, 4-DAMP (1.88 x 10(-6) M) and the GABAA antagonist, bicuculline (2.39 x 10(-6) M). Increasingly greater concentrations of the muscarinic antagonists, atropine (9.52 x 10(-6) M), AF-DX 116 (2.72 x 10(-5) M) and pirenzepine (8.24 x 10(-4) M) were necessary to block contralateral suppression of DPOAEs. The in vivo pharmacology of this putative outer hair cell cholinergic receptor suggests that it may be a member of the nicotinic family of receptors.

Acoustic Stimulation↗
PubMed2002

Partial recovery of cisplatin-induced hearing loss in the albino guinea pig in relation to cisplatin dose.

The objective of the present study was to further characterize cochlear recovery after cisplatin damage. We equipped albino guinea pigs with permanent round window electrodes. Cisplatin was injected i.p. on a daily basis at either 1.5 or 2.0 mg/kg/day. Treatment was stopped when the criterion of > or =40 dB loss in the compound action potential iso-response level at 8 kHz had occurred. Either shortly (1-3 days) or long (4 weeks or more) after this stop, the endocochlear potential (EP) was measured and all animals were sacrificed for histology. At a cisplatin dose of 2.0 mg/kg/day, the time needed to reach the criterion hearing loss varied from 5 to 11 days. With 1.5 mg/kg/day this period lasted longer, the cumulative dose being the first-order predictor. The cochlear potentials gradually recovered in the first 2 weeks after treatment. At the lower frequencies, recovery was often complete. At the higher frequencies complete recovery was never seen. EP was depressed when measured just after treatment but had normal values long after. Basal outer hair cell (OHC) loss was found for both the short and the long post-treatment period. Thus, loss and recovery of cochlear potentials can for a large part be explained by loss and recovery of the EP. Recovery is limited by permanent OHC loss.

Action Potentials↗
PubMed2002

Inputs from the cochlea and the inferior colliculus converge on olivocochlear neurones.

Medial olivocochlear (MOC) neurones, located in the superior olivary complex, can suppress cochlear gain by their action on the cochlear outer hair cells. Inputs from the contralateral cochlea and the inferior colliculus (IC) have been separately shown to increase activity of MOC neurones. In this study we have investigated in guinea-pigs under barbiturate anaesthesia the interactions between these two inputs by combining electrical stimulation of the IC with acoustic stimulation of the contralateral cochlea. Electrical stimulation of the IC resulted in a significant suppression of the amplitude of the compound action potential (CAP) of the auditory nerve to test tones. This suppression was equivalent to an average decrease in sound intensity of 5.7 dB and 3.7 dB for contralateral and ipsilateral stimulation, respectively. Acoustic stimulation of the contralateral cochlea with broadband noise produced no detectable change in the amplitude of the CAP in the test cochlea in all but one animal. However, simultaneous electrical stimulation of the IC and acoustic stimulation of the contralateral cochlea resulted in a reduction in CAP amplitude that was markedly larger than that produced by IC stimulation alone. The suppression with the addition of contralateral noise was equivalent to a mean reduction in sound intensity of 8.7 dB with contralateral and 5.7 dB with ipsilateral IC stimulation. We hypothesise that excitatory input from the contralateral cochlea converges with excitatory input from the IC on the MOC neurones and in this way augments the activity of these neurones, resulting in a larger peripheral effect.

Acoustic Stimulation↗
PubMed1998

Role of nitric oxide in kainic acid-induced elevation of cochlear compound action potential thresholds.

Nitric oxide (NO) has specifically been found to mediate the effects of excitatory amino acids in the central nervous system (CNS). Excitatory amino acids are the primary neurotransmitters at the cochlear hair cell afferent nerve synapse. Recent studies in our laboratory demonstrate that NO synthase is an active enzyme in the spiral ganglion cells of the cochlea. Given our current understanding of neurotransmission in the cochlea, it is reasonable to postulate that the actions of NO in cochlear neuronal tissue are similar to the actions of NO in the CNS, and that NO acts as a neurotransmitter/neuromodulator in the cochlea. In addition, NO is implicated as a mediator of excitotoxicity in the CNS and may therefore play a similar role in excitotoxicity in the cochlea. To further elucidate the role of NO in cochlear excitotoxicity, this study investigated the effects of 7-nitroindazole (7-NI), a competitive inhibitor of neuronal nitric oxide synthase, with regard to kainic acid (KA)-induced elevation of compound action potential (CAP) thresholds. KA is a conformationally restricted analog of glutamate with well-known excitotoxic effects on SGC's and previously described inhibitory actions on cochlear CAP thresholds. In anesthetized gerbils, CAP thresholds were recorded before and after cochlear perfusions with control solutions of artificial perilymph solution and test solutions of KA. 7-NI was administered i.p. prior to KA perfusion in an effort to block its depolarizing and toxic effects. Results showed that cochlear perfusion with KA caused significant elevation (p < 0.05) of the mean CAP threshold. This threshold shift was significantly reduced (p < 0.05) in animals pretreated with 7-NI. These results indicate that NO is involved in the toxic effects on CAP thresholds elicited by KA in the cochlea.

Action Potentials↗
PubMedDate not supplied

Effect of glycerol on the EP decrease caused by furosemide.

A change in endocochlear potential (EP) following furosemide injection was observed in regard to prior glycerol administration in two groups of guinea pigs. In one group, various doses of furosemide (20, 30, 40, 50 mg/kg) were injected, while in the other, glycerol (50 v/v %, 1 ml/kg) was injected prior to the furosemide (20, 30, 40 mg/kg) injection. In the glycerol-furosemide group, the decrease in EP was 40% greater than in the furosemide group. Therefore, glycerol was thought to potentiate the EP lowering action of furosemide. Such an effect of glycerol was assumed to be resulted by facilitating the access of furosemide to the site of action in the stria vascularis.

Action Potentials↗