Biomedical subjects

J Syka

Publications and source records attributed to J Syka.

At least 37 records · Page 2Linked to original sources

PubMed1994

Enhancement of the auditory cortex evoked responses in awake guinea pigs after noise exposure.

In a previous paper [Popelár et al., Hear. Res. 26, 239-247 (1987)] we have shown that amplitudes of the auditory cortex evoked responses (AC-ER) in awake guinea pigs were enhanced for several hours after 1 h of noise exposure whereas amplitudes of the compound potential of the auditory nerve (CAP) and of the inferior colliculus evoked responses (IC-ER) declined. The present study demonstrates that the duration of the AC-ER amplitude increase is related to the intensity of the noise exposure (white noise, for 30 min or 1 h, intensity range 105-125 dB). The AC-ER amplitude as well as the threshold shift increased linearly with increasing intensity of the noise. The maximum AC-ER increase occurred when clicks served as stimuli; amplitude enhancement was smaller for 1 kHz tone pips and was absent when 20 kHz tone pips were used. The amplitude enhancement was specific for the auditory cortex since the amplitude of visually evoked responses, recorded in the occipital cortex, was unchanged after noise exposure. It is suggested that the postexposure amplitude enhancement of the AC-ER is produced by temporary exhaustion of inhibitory processes in the auditory cortex.

Acoustic Stimulation↗

PubMed1994

Modulation of thresholds to acoustical and electrical stimulation of the intact ear in guinea pig by furosemide and noise.

Middle latency responses (MLR) to acoustical stimulation (AS) and to electrical stimulation (ES) of the intact inner ear were recorded in guinea pigs. ES threshold curve decreased in the frequency range 2-16 kHz with a slope 5.4 dB/octave. Immediately after 50 mg/kg intravenous injection of the furosemide, which resulted in a temporary suppression of the cochlear function, the ES thresholds increased and resembled thresholds found in gentamicin-treated animals. Whereas temporary threshold shift (TTS) at 1 kHz ES was negligible at this time, maximum TTS at 8 kHz and 20 kHz ES was limited to 27 dB and 37 dB resp. TTS to acoustical stimulation was larger than TTS to ES (in some cases exceeded 50 dB) and it was similar at all frequencies. Amplitude-intensity functions (AIF) to high-frequency ES stimuli (20 kHz) consisted of two parts--a flat part at low intensities and a steep part at high intensities of the ES. High-frequency noise exposure (third-octave band noise, centered at 16 kHz, intensity 105 dB for 1 h) reduced or abolished only the flat part of the AIF, the steep part, as well as the responses to low-frequency ES, were not substantially changed. TTS at high frequencies, elicited by the noise exposure, were similar for ES and AS. However, amplitudes of acoustically evoked MLR significantly increased after the noise exposure while MLR amplitudes to ES did not change. The results characterize the frequency-intensity domain of the electrophonic effect in the guinea pig and its changes after influencing the inner ear function by furosemide and noise.

Acoustic Stimulation↗

PubMed1993

Hair cell mediated responses of the auditory nerve to sinusoidal electrical stimulation of the cochlea in the cat.

Electrical stimulation of the cochlea elicits discharges of auditory nerve fibres which are mediated by the electrical or mechanical stimulation of inner hair cells (electrophonic responses). In order to isolate hair-cell mediated responses from those elicited by electrical stimulation of the nerve, the compound action potential (CAP) evoked by an acoustic probe was forward-masked by sinusoidal monopolar, or localized bipolar electrical stimulation of the base of the cochlea. The degree of masking of a given probe estimated the synaptically mediated response to the masker of the population of auditory nerve fibres innervating the cochlear location tuned to the probe. There was a peak of masking for probes close to the frequency of the electrical stimulus, suggesting a spatial tuning of the hair cell mediated response along the cochlea. This is consistent with excitation of the inner hair cells by a propagating mechanical response which is generated within the electrical field at the base of the cochlea. Furthermore, tuning curves for masking of a given probe were sharply tuned to electrical stimulation close to the probe frequency. This masking was not dependent upon the presence of functional outer hair cells close to the electrodes, suggesting an alternate transduction of electrical to mechanical energy.

Action Potentials↗

PubMed1993

Middle latency responses to electrical stimulation of the auditory nerve in unanaesthetized guinea pigs.

Middle latency responses (MLR) to sinusoidal and pulsatile electrical stimulation (ES) of the cochlea and to acoustical stimulation (AS) were evaluated in awake guinea pigs with chronically implanted electrodes. The ear, which was later electrically stimulated, was deafened by local intracochlear application of gentamicin, the opposite ear was left intact. Waveforms and P1-P2 interpeak intervals of the electrically evoked MLR (ES-MLR) were similar to those evoked by acoustical stimulation of the intact ear (AS-MLR) and the latencies of the ES-MLR were shorter by about 1-3 ms. Thresholds of ES-MLR in the frequency range 0.5-32 kHz increased with increasing ES frequency (slope 3.2 dB/octave), thresholds were 3.5-9.5 dB lower for intracochlear than for extracochlear ES. Dynamic ranges for ES-MLR varied between 6-20 dB. MLR amplitude-intensity functions for ES were steeper (slope 2-12 microV/dB) than those for AS (slope 0.2-2 microV/dB). Maximal ES-MLR amplitudes exceeded usually 1.5-3 times the amplitudes of the acoustically evoked MLR. Both types of stimulations evoked larger MLR amplitudes to contralateral stimulation than to ipsilateral stimulation (average ratio = 4.1 +/- 2.2 for AS and 3.3 +/- 2.2 for ES). Because of the relatively long latency and therefore insensitivity to electrical artifact, the ES-MLR can be used for the evaluation of different strategies of the electrical stimulation of the cochlea in awake guinea pig.

Acoustic Stimulation↗

PubMed1993

NADPH-diaphorase activity in the central auditory structures of the rat.

Distribution of NADPH-diaphorase (NADPH-d) positive neurones was investigated in the central auditory structures of the rat. NADPH-d positive perikarya were found in the dorsal and external cortices of the inferior colliculus and in the intercollicular commissure. Within the medial geniculate body (MGB), stained cell bodies were observed in the dorsal part of the suprageniculate nucleus and in the basal part of the medial division of the MGB. Occurrence of the NADPH-d positive perikarya in the MGB is limited to its caudal third. NADPH-d positive cells are dispersed in the auditory temporal cortex within layers II-VI, with a slight prevalence in the infragranular layers. NADPH-d positive neurones have smooth dendrites without stained dendritic spines.

Animals↗

PubMed1992

Electrophysiological responses of the cochlea to transient asphyxia are influenced by arachidonate metabolites.

The influence of a transient asphyxia on cochlear potentials, i.e. endolymphatic potential (EP), summating potential (SP) and cochlear microphonics (CM) was investigated in guinea pigs when pretreating the animals with various substances interacting with the arachidonic acid (AA) cascade at the level of thromboxane. The controls showed the well-known decline of the EP, CM and the SP increase. When infusing a thromboxane synthetase inhibitor (dazoxiben) or two different thromboxane receptor blockers (daltroban or sulotraban) before the 3-minutes' period of asphyxia was started, the electrophysiological responses of the inner ear (cochlea) could significantly be influenced. The results indicate that a shift of the thromboxane (TXA2)/prostacyclin (PGI2)-balance in favour of the last improve the metabolic conditions for a survival of the cochlea when it is challenged.

Analysis of Variance↗

PubMed1992

Carboplatin and cisplatin ototoxicity in guinea pigs.

Ototoxic effects of cisplatin (1.5 mg/kg), cisplatin with mannitol (1.5 mg/kg cisplatin with 15 mg/kg mannitol) and carboplatin (6 mg/kg) were compared in guinea pigs treated 5 days a week for 5 consecutive weeks. Auditory thresholds were measured in awake animals by means of the compound action potential recorded from electrodes implanted near the round window. The endocochlear potential (EP) was measured in the first turn through the round window, and hair cell counts were estimated by the surface specimen technique. The most pronounced ototoxicity (threshold shifts and hair cell loss) was observed with pure cisplatin. Cisplatin with mannitol appeared to be less toxic than pure cisplatin, and carboplatin the least toxic. Even carboplatin produced significant threshold shifts, particularly at high frequencies, but they were not accompanied by hair cell loss. Values of EP were not different from those in normal control animals.

Acoustic Stimulation↗

PubMed1990

Deterioration of hearing function in mice with neural crest defect.

Cochlear potentials were recorded in black-eyed white-mutant mice (strains W/Wv and S1/S1d) during the first months of postnatal life. In these animals melanocytes from the neural crest do not reach the stria vascularis. Hearing thresholds indicated by the compound action potential of the auditory nerve progressively increased. Hearing loss in comparison with the control CBA mice amounted 40 dB six weeks after birth, at eight months practically all mutants were deaf. Endocochlear potential was near zero already in the majority of 6 week-old animals. Abnormalities were observed also in intracellular potentials of the stria vascularis cells which were recorded in vitro and dye-marked. While in marginal cells of CBA mice positive potentials of about 10 mV were recorded, potentials of marginal cells in mutants were near zero. Basal cell potentials were negative in CBA mice (approximately -25 mV) and positive in young mutants (approximately +12 mV). However, in mutants older than 3 months the intracellular potentials of basal cells changed to negativity (approximately -10 mV). The results demonstrate gradual deterioration of hearing function observed in both W/Wv and S1/S1d mutants during first months of the postnatal life. In addition to the dysfunction of stria vascularis, where intermediate cells are missing, also the function of receptors progressively deteriorates.

Action Potentials↗

PubMed1989

Arachidonate metabolites change furosemide-induced cochlear potentials.

Furosemide-induced changes of cochlear potentials were used as a model to study the influence of arachidonic acid metabolites on ion movements within the cochlea. No influence was exerted by the drugs Esculetin - blocking the synthesis of lipoxygenase products - and Dazoxiben - suppressing thromboxane A2 levels within the cochlea. A weakening of the furosemide-induced changes of the endocochlear potential was found when infusing the thromboxane (TX) receptor antagonists BM 13,505 and BM 13,177 before furosemide was given. This effect was also observed when pre-treating the guinea pig with a specific platelet-activating factor receptor antagonist, BN 52,021, before the diuretics was given. Summating potential and cochlear microphonics remained insignificantly changed against controls. The results suggest that a TX receptor contributes to the control of ion movements within the cochlea. A possible involvement of loop diuretics' receptors is discussed.

Acoustic Stimulation↗

PubMed1989

[Electric stimulation of auditory nerve fibers. Experimental study].

The authors tested in animal experiments the effectiveness of electrical stimulation of acoustic nerve fibres. To guinea pigs platinum-iridium electrodes were implanted in the round window of the cochlea. The receptors of the inner ear were functionally eliminated by the local application of the ototoxic antibiotic gentamicin. Cortical responses produced by electric stimulation of the contralateral ear and acoustic stimulation of the ipsilateral intact ear were recorded. The threshold for producing a cortical response by the electric stimulus was lowest at the frequency of 0.5 kHz and rose with the rising frequency. This relationship was the same for extracochlear and intracochlear stimulation, the absolute threshold values of the current in the same animal were, however, as a rule lower on intracochlear stimulation. The dynamic range of responses to electrical stimulation (difference between threshold intensity of the current and the intensity which produces a painful response of the animal) was 6-20 dB, as compared with 80-100 dB of the dynamic range for acoustic stimulation. The results of the experiments were used in the preparation of the Czechoslovak cochlear neuroprosthesis and serve as a basis for the further development of the implant.

Animals↗

PubMed1989

The effect of PAF in the cochlea of guinea pigs.

The influence of 10(-10) and 10(-9) M PAF/animal given into the jugular vein over 30 sec on inner ear potentials, i.e. endolymphatic potential (EP), summating potential (SP) and cochlear microphonics (CM) was investigated. The EP showed the most pronounced changes. When infusing a specific PAF receptor antagonist, ginkgolide B, or the TXA2 receptor antagonist, sulotraban, before the the infusion of PAF, the changes in cochlear potentials could be completely prevented. A second TXA2 receptor antagonist, daltroban, did not effectively prevent PAF actions. It is hypothesized that these PAF effects are due to an interference with ion transport in the non-sensory structures of the inner ear.

Acoustic Stimulation↗

PubMed1989

Stimulus-related potassium changes in the organ of Corti of guinea-pig.

1. Potassium concentration was measured with double-barrelled K+-selective microelectrodes within the organ of Corti in the first turn of the guinea-pig cochlea. 2. Penetration of the electrode from scala tympani through the basilar membrane was accompanied by an increase in K+ resting level from 3.0 mmol/l in perilymph to 3.4 mmol/l in cortilymph (n = 8). K+ resting level was not significantly different in various extracellular regions of the organ of Corti. On penetration of the cuticular plate, the K+ level reached 140 mmol/l simultaneously with the occurrence of a +80 mV endocochlear potential. Impalement of hair cells and supporting cells was accompanied by an increase in K+ level, but intracellular K+ level was not systematically measured. 3. Stimulation with pure tones over the frequency range 500 Hz to 25 kHz produced changes in the K+ level in the organ of Corti. The magnitude of these changes was dependent on stimulus frequency and intensity. At high sound intensities the K+ level in the tunnel of Corti could increase by typically 1 mmol/l, while a maximum increase of 3 mmol/l with respect to the resting level was observed immediately adjacent to inner hair cells. 4. During brief exposures to moderate intensity, pure tone acoustic stimulation (10 s, less than 80 dB SPL (sound pressure level] of frequency 4 kHz or greater the K+ level in the extracellular fluid of the organ of Corti rose monotonically to a steady peak level. On cessation of the stimulus the K+ level fell monotonically with a time constant of about 2 s to a level close to the pre-stimulus level. In some cases this level was slightly above the pre-stimulus level. 5. For brief exposures to moderate intensity sound (10 s, less than 80 dB SPL) the extracellular potential in the organ of Corti became more positive. The amplitude of this sound-evoked change adapted during stimulation to a level approximately one-fifth of its initial value. Upon cessation of the stimulus the potential fell transiently below its pre-stimulus level, before recovering to that level. The time constant of these changes was between 2 and 3 s. 6. Iso-response tuning curves for the sound-evoked elevation in K+ level in the organ of Corti in animals in good condition were similar to iso-rate tuning curves for primary afferent fibres reported previously.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1987

Longitudinal flow of endolymph measured by distribution of tetraethylammonium and choline in scala media.

Longitudinal endolymph flow rate in the guinea pig cochlea was measured by determining the rate of migration of extrinsic ions, tetraethylammonium chloride (TEA) or choline, with a potassium sensitive ion-selective microelectrode (ISM). Low concentrations of iontophoretically injected TEA were detected with the ISM at various distances from the injection electrode. The results were variable when the ISM was used to record spread of TEA from turn II to turn I and vice versa. However, consistent data were obtained when the TEA spread was measured at different electrode separations (0.2, 0.5, 0.7 mm) within turn II. Electrode locations were systematically exchanged without changing their distance, i.e. the ISM electrode was placed basally or apically with respect to the TEA electrode. Comparison of data with a model, which combines the bulk diffusion of TEA and the flow of endolymph, is consistent with a rate of endolymph flow in turn II of about 0.2 mm/min, apex to base. A similar value was also obtained with the iontophoretic injection of choline. The endolymph flow rate may be different in turn I as indicated by measurements of compound action potential (CAP) changes. However, the results of experiments when TEA spread is measured at large distances must be interpreted cautiously because TEA may enter cellular walls of the cochlear duct and alternative routes of transport may be involved.

Action Potentials↗

PubMed1987

Effect of noise on auditory evoked responses in awake guinea pigs.

Changes in the auditory nerve action potential (AP), evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) were assessed after exposure to white noise of 120 dB SPL for 1 h in awake guinea pigs. Auditory thresholds were estimated with the aid of averaged AP, IC-ER and AC-ER, besides the threshold shifts also the changes in amplitude-intensity functions were evaluated. Auditory thresholds for tone pips and clicks increased by 20-30 dB 1 h after exposure and were similar in all the three investigated structures. The maximum threshold shifts for tone pips were observed at 8 kHz and were 33.2 +/- 12.9 dB for AP, 30.4 +/- 12.7 dB for IC-ER and 30.8 +/- 13.0 dB for AC-ER (n = 20). The thresholds recovered to preexposure levels within one week. Reduction in AP and IC-ER amplitudes 1 h after exposure was similar, the amplitude-intensity functions were shifted by 20-40 dB. In contrast, the amplitude-intensity functions in the auditory cortex 1 h after exposure were steeper than before exposure and this amplitude enhancement was present for 24 h after exposure. The enhancement of the AC-ER which resembles recruitment and which may be a sign of hypersensitivity of the animal to auditory stimuli was present only when the animals exposed to noise were awake. The noise exposure in animals anaesthetized with urethane reduced the amplitude-intensity functions of all three recorded potentials.

Anesthesia↗

PubMed1987

DC potentials in different cells of the stria vascularis measured in vitro.

Fine glass microelectrodes were inserted into the stria vascularis (SV) from the endolymphatic side under conditions in vitro. DC potentials were recorded in individual cells of the SV. The electrophoretic dye marking technique was used for identification of cells with different DC potentials. Penetration of the luminal membrane of marginal cells (MC) was accompanied by an unstable transient negative potential of about -15 mV. When the electrode penetrated further into the MC, a positive potential of about +10 mV was recorded. In the basal cells (BC) of the SV a negative DC potential of approx. -45 mV was always measured. Addition of ouabain into the perfusion solution (10(-3) mol/l) abolished the positive potential of MC, whereas the negative potential of BC decreased to approximately one half of the original value. A higher positive potential (+17 mV) was found in the MC of animals which were exposed to noise twenty days before the recording. Results are discussed in the light of the knowledge about localization of different transporting systems within the SV cells.

Animals↗

PubMed1987

Electrophysiological measurements of the stria vascularis potentials in vivo.

Glass microelectrodes were introduced into the stria vascularis (SV) to measure the DC potential profile of the SV in vivo from its apical as well as basal side. The K+ concentration gradient in the lateral cochlear wall was measured by means of double barrel K+-selective microelectrodes. A positive potential 10.4 +/- 6.3 mV higher than the endocochlear potential (EP) was found in the SV. In noise-exposed animals the positive potential found in the SV was 14.6 +/- 7.2 mV higher than the EP. K+ concentration observed during penetration into the SV was in the range of 50-100 mmol/l. Simultaneous measurement of the DC positive potential in the SV and EP showed a nearly parallel time course during anoxia with a tendency to increase the difference between the SV potential and the EP. The difference reached approximately 30 mV 20 min after the beginning of anoxia. It may be assumed that the electrogenic pump localized at the basolateral membrane of the marginal cell (MC) is a source of the positive potential inside the MC and is hence a source of the potential drop across the luminal side of the MC membrane.

Animals↗

PubMed1985

The effect of loop diuretics upon summating potentials in the guinea pig.

Ototoxic diuretics, ethacrynic acid (50 mg/kg) and furosemide (80 mg/kg) were injected intravenously in guinea pigs. Cochlear microphonics (CM), summating potentials (SPs) and endocochlear potential (EP) were recorded with a microelectrode in scala media of the second turn. CM changes after the injection were comparable with changes in the EP: an initial decrease was followed by an increase which was significantly slower in the case of the ethacrynic acid. SP changes following either diuretic were different from the CM and EP changes. In the first phase, roughly corresponding with the EP decrease to negative values, all SPs irrespective of the original polarity, attained high positive values. The high positive SPs then decreased and 12-18 min after injection reversed polarity. In the late phase all sounds evoked high negative SPs. Approximately 90 min after injection of furosemide normal SPs were again recorded. The return of SPs to control values was very slow after ethacrynic acid; even 140 min after injection the SPs were abnormal. The observed changes in the SPs were compared with those found during asphyxia and anoxia and are considered to result from diuretic effects on the inner and outer hair cells.

Animals↗

PubMed1984

Inferior colliculus in the rat: neuronal responses to stimulation of the auditory cortex.

Responses to electrical stimulation of the auditory cortex (silver ball bipolar electrodes, single pulses, duration 0.2 ms, current 0.1-1.5 mA) were recorded in neurones in the inferior colliculus of rats anaesthetized with pentobarbital. Excitatory or inhibitory effects were obtained in 84 out of 162 recorded neurones. The majority of neurones responded with a short excitatory burst (with a latency from 3 to 15 ms); in some of them the initial excitation was followed by inhibition lasting from 30 to 150 ms. Few neurones only reacted to electrical stimulation by inhibition, which occurred 3-10 ms after the stimulus and lasted up to 300 ms. The inhibition either suppressed the spontaneous activity or the acoustically evoked response. Neurones reacting to stimulation of the auditory cortex were found mainly in the caudal and dorsal parts of the inferior colliculus.

Animals↗