Fourth Turn Research Articles

Modulation of cochlear blood flow by extracellular purines

Humoral adenosine 5′-triphosphate (ATP), adenosine and uridine 5′-triphosphate (UTP) have been shown to have a role in controlling local blood flow in a variety of tissues. The presence of P1 and P2 receptors in the cochlea, and particularly the highly vascular region, the stria vascularis, implies a vasoactive role for these compounds in the inner ear. To test the effect of extracellular purines and pyrimidines on cochlear blood flow, cochleae from anaesthetised guinea-pigs were perfused with ATP (1 μM–10 mM), adenosine (1 μM–10 mM) and UTP (1 mM) in artificial perilymph while blood flow through the cochlea was measured. An acute perilymphatic perfusion technique was established via tubing placed through a hole in the bone overlying scala tympani of the first cochlear turn, with an outlet hole in scala vestibuli of the fourth turn. Blood flow was measured by placing the probe of a laser Doppler blood perfusion monitor on the bone overlying the stria vascularis in the third cochlear turn. ATP and adenosine produced a significant dose dependent increase in cochlear blood flow (28.8–229.0% and 35.8–258.1%, respectively). The effect of ATP (100 μM) on cochlear blood flow was reduced in the presence of reactive blue 2 (1 mM) and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (1 mM). The blood flow response to adenosine (10 μM) was reduced in the presence of 1,3-dimethylxanthine (theophylline, 100 μM), but not with either 3,7-dimethyl-1-propargylxanthine (10 μM) or 8-cyclopentyl-1,3-dipropylxanthine (10 μM). UTP did not produce any change in the cochlear blood flow. To determine if the ATP effect was also mediated by adenosine derived from ectonucleotidase activity, the perilymphatic compartment was perfused with either ATP plus theophylline (100 μM) or with the non-metabolisable form of ATP, adenosine 5′- O-(3-thiophosphate) (ATPγS, 100 μM). The effect of ATP on cochlear blood flow was unaffected with the inclusion of theophylline while ATPγS produced an increase in cochlear blood flow similar to the one observed with ATP. These findings indicate that extracellular ATP and its metabolite adenosine have a modulatory role in cochlear blood flow possibly mediated by both P1 and P2 receptors.

GABA A receptor modulates the activity of inner hair cell afferents in guinea pig cochlea

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is mediated by two main categories of receptors: the GABA A and GABA B receptor. Recent immunocytochemical and electron microscopical studies revealed the existence of GABA at the efferent olivocochlear innervation of the guinea pig cochlea. In this microiontophoretic study we examined the effect of GABA on spontaneous and glutamate or acetylcholine induced activity of afferent fibres in the dendritic region of inner hair cells. Furthermore, the receptor subtypes being responsible for this GABA action were analysed using specific agonists and antagonists on α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl- d-aspartic acid (NMDA) induced activity. The spike activities of the subsynaptic area were recorded in the third or fourth turn of the cochlea of anaesthetised guinea pigs. Application of GABA had little effect on spontaneous activity whereas the glutamate or acetylcholine induced firing rate could be depressed by GABA. AMPA and NMDA induced activity was reduced by the GABA A agonist muscimol but not by the GABA B agonist baclofen. The GABA A antagonist blocked the inhibition of both GABA and the GABA A agonist. In contrast, the GABA B antagonist saclofen was without effect. These results reveal that GABA reduces the activated firing rate of inner hair cell afferents mediated by the GABA A receptor subtype.

Spreading of Labrador Sea Water in the eastern North Atlantic

The spreading of Labrador Sea Water (LSW) in the eastern North Atlantic south of 55°N is described on the basis of recent, high‐resolution hydrographic lines and using an inverse model of that basin general circulation. The isopycnic “salinity anomaly” relative to a standard θ/S curve is used to detect the main branches and limits of dominant influence of the intermediate water masses. The LSW crosses the Mid‐Atlantic Ridge at latitudes around 50°N with parts spreading farther eastward and southward. One southward pathway along the eastern side of the Mid‐Atlantic‐Ridge stands out as an eastern basin counterpart of the western Atlantic “Deep Western Boundary Current.” An entry into the Bay of Biscay in its northern part is also evidenced with striking similitudes between two data sets from 1974 and 1989–1990. At the southern limit of the LSW extension, in the vicinity of the Azores‐Biscay Rise, a marked front exists basinwide around 1800 m with the more saline Deep Mediterranean Water (DMW). The inverse model velocity field is consistent with most features of the tracer distributions and allows estimation of LSW transports: 6.3±1.2 Sv of LSW with salinities lower than 34.94 cross the 35°W meridian eastward between 44°N and 54°N, one fourth of which recirculates southwestward in the western basin, while another fourth turns northward across 54°N and the remaining half proceeds southward in the eastern basin. The model circulation suggests that DMW results from mixing between eastern basin LSW and the overlying Mediterranean Outflow Water, most probably through double diffusion. We finally describe several mesoscale structures of anomalously high LSW influence south of the LSW‐DMW front, notably one sampled by a RAFOS float. The velocity and potential vorticity fields suggest that these “LSW‐eddies” are formed through baroclinic instability at the LSW‐DMW front.

Dopamine regulates the glutamatergic inner hair cell activity in guinea pigs

Recent immunocytochemical and biochemical studies support a possible neurotransmitter function of dopamine (DA) in the efferent olivocochlear innervation of the guinea pig cochlea. However, the physiological role of DA in cochlear neurotransmission remains unknown. In the present study microiontophoretic techniques were used for testing the action of DA as well as D1- and D2-agonists and -antagonists on spontaneous and N-methyl-d-aspartic acid (NMDA)-, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-, kainic acid- or glutamate-induced firing of afferent fibres in the dendritic region of inner hair cells. Subsynaptic spike activities of anaesthetised guinea pigs were recorded after exposing the third or fourth turn of the cochlea for electrode penetration. Application of DA alone had very little effect on the spontaneous afferent firing rate. In contrast, firing induced by NMDA or AMPA could be depressed by additional administration of DA in a dose-dependent manner. A similar reduction of the induced spike activity was seen after co-administration of D1- or D2-agonists. The action of DA on glutamate agonist-induced firing could be blocked by D1- as well as D2-antagonists. These results show that DA can depress the activated firing rate of the afferent fibres and that this action is mediated by both D1- and D2-receptor subtypes.

Effects of Local Anaesthetics on the Gross Receptor Potentials in the Guinea Pig Cochlea

Local anaesthetics have been used intravenously and intratympanally to reduce tinnitus. In order to clarify its action in the periphery, we applied 0.5 mM tetracaine in the scala tympani in 18 cochleae and studied the effects on the receptor potentials. We used a temporal bone preparation of the guinea pig ear in vitro exposing the fourth cochlear turn where the cochlear microphonics (CM) and the summating potential (SP) were recorded. The perfusion was kept at a rate of 50 μl/min. The frequency response of the cochlea was determined at the beginning of each experiment and the responses were recorded at the best frequency of the preparation. In another five cochleae an accumulated dose-response relationship was determined by increasing the tetracaine concentration in steps (50, 100, 300, 500, 1000 and 2000 μM), measuring the difference in amplitude of the receptor potentials. The CM decreased significantly (p < 0.001; mean 0.37 mV; SD 0.29). In 12 cochleae the SP was initially positive and did not increase significantly (p =0.16; mean 0.07 mV; SD 0.16). In six cochleae the SP was initially negative and all changed polarity to positive and increased significantly (p < 0.05; mean 0.36 mV; SD 0.28). The effects on both the CM and the SP were reversible. Owing to the inter-individual variation between the cochleae the SP/CM ratio was determined and it increased significantly (p < 0.001; mean 0.18; SD 0.11). In the accumulated dose-response experiments the CM decreased significantly (p < 0.05) in a dose-dependent way, whereas the SP did not increase significantly. The SP/CM ratio increased significantly (p < 0.05) at 300 μM and 500 μM. We hypothesize that the peripheral tinnitus-reducing action of local anaesthetics is in part due to a reversal of the SP, but also to a reduction of the CM. The difference in effect of tetracaine on the receptor potentials, the CM and the SP, suggests that the SP is not dependent on the CM.

Electromotile responses and frequency tuning of isolated outer hair cells of the guinea pig cochlea

Isolated outer hair cells (OHC) of the guinea pig cochlea were exposed to external alternating electric fields parallel to the longitudinal axis of the cells. This resulted in oscillations of the cells' length that were measured photoelectrically using a ratiometric light amplifier. At 5 Hz and elongations up to 300 nm, amplitude of the cell length during oscillation was a linear function of the amplitude of the sinusoidal electric field. When increasing the stimulus frequency up to 32 kHz, OHC length changes followed the stimulus cycle-by-cycle. Oscillations at frequencies above 32 kHz escaped the experimental approach by their small amplitudes and could not be excluded. The frequency dependence of the motile response measured at 5-12,000 Hz had low-pass filter characteristics in cells of the second, third and fourth turns of the cochlea. However, frequency tuning of the motile response was absent in each OHC and systematic differences between different turns were not observed.

Endolymph calcium increases with time after surgical induction of hydrops in guinea-pigs

The ionized Ca 2+ concentration in cochlear endolymph is normally extremely low. Previous studies have shown that endolymph Ca 2+ levels become elevated when measured at long intervals after endolymphatic hydrops is surgically induced. The present study was designed to investigate how rapidly endolymph Ca 2+ increases following endolymphatic duct ablation. Hydropic animals were tested at either 4 days, 4 weeks, 8 weeks or 16 weeks after surgery. In each animal endolymph Ca 2+ and endocochlear potentials were measured in all four cochlear turns using double-barreled Ca 2+-sensitive electrodes. Cochlear sensitivity was assessed using compound action potential thresholds. Our results confirm that hydropic animals show an elevation of endolymph Ca 2+ and a reduction of EP which is initially small, but becomes more pronounced at longer times after surgery. At 16 weeks endolymph Ca 2+ was increased by an average factor of 2.0 in the basal turn and 7.5 in the fourth turn. These findings suggest that endolymph Ca 2+ changes may not be the primary factor responsible for hydrops generation, but probably contribute to cochlear dysfunction in later phases of hydrops. For some experimental groups, the elevation of AP threshold was more closely correlated with endolymph Ca 2+ level than it was with endolymph volume. Endolymph Ca 2+ changes must therefore be considered in order to account for dysfunction in the hydropic cochlea.

Methionyl-tRNA Synthetase Zinc Binding Domain: Three-dimensional Structure and Homology with Rubredoxin and gag Retroviral Proteins

Methionyl-tRNA synthetase from Escherichia coli contains one tightly bound zinc atom per subunit. The region encompassing residues 138 to 163 of this enzyme is responsible for the metal binding. A 28-mer peptide corresponding to these residues was expressed in vivo and shown to contain approximately 1 mol of tightly bound Zn/mol of peptide. In this study, the three-dimensional solution structure of this peptide was solved by means of two-dimensional proton NMR spectroscopy. A total of 133 nuclear Overhauser effect distance constraints and 22 dihedral angle restraints were used for the calculations, using a hybrid distance-geometry-simulated annealing strategy. Excluding the first four residues, the resulting structure is well-defined (r.m.s.d. 0·71 Å for backbone atoms) and composed of a series of four tight turns. The second and the fourth turns are composed of CXXC sequences which are structurally homologous to the NH-S tuns found in the metal binding sites of gag retroviral proteins and rubredoxin. The solution structure of the zinc binding peptide shows significant discrepancies with the crystal structure of methionyl-tRNA synthetase.

Turn-specific differences in the endocochlear potential between albino and pigmented guinea pigs

Recent findings indicate that structural differences exist in the stria vascularis (SV) between albino and pigmented guinea pigs. In the higher cochlear turns, volume density for marginal cells in the albino SV is abnormally large, while that for intermediate cells (melanocytes) is abnormally small. These anatomical variations suggest that functional differences between albino and pigmented inner ears also may be found. To examine this possibility, four strains of guinea pigs were studied, consisting of Hartley albino ( N = 9) and NIH outbred pigmented ( N = 15) guinea pigs, as well as albino ( N = 11) and pigmented ( N = 15) guinea pig siblings born to mixed litters. Tracheotomy and carotid artery cannulation were performed. Animals were mechanically ventilated, with periodic samples drawn for arterial blood gas analysis. Blood pressure, heart rate and rectal temperature were monitored. Compound action potentials were measured first to assess cochlear viability. Positive endocochlear potentials (+EP) then were recorded, beginning with the fourth turn, followed by the first, second and third turns. Results showed that the +EP in albinos remained relatively constant across cochlear turns, but decreased significantly from base to apex in the pigmented inner ears. Across all animals, mean +EPs (mV ± S.E.M.) for turns 1–4 in albinos were: 72.5 (2.5), 68.7 (2.3), 59.2 (2.7), 68.1 (3.3); pigmented values were: 72.9 (2.9), 66.9 (2.6), 53.8 (3.0), 57.0 (2.7). One-way ANOVAs did not show a significant difference in albino +EPs between any of the cochlear turns, but did indicate a highly significant difference between turns in the pigmented inner ears ( P < 0.000004). Post hoc comparisons demonstrated +EPs in turns 3 and 4 were smaller than in turn 1. Since turn 3 was recorded last in these experiments, and was reduced in value relative to turn 4 in both groups, it is likely that cochlear deterioration contributed to this result more than any other factor. These results, combined with previous anatomical data, indicate that a diminution of melanocyte cell volume in the albino SV is accompanied by an increase in marginal cell volume density and larger +EPs in the higher cochlear turns, at least at resting levels.

Sound induced displacement response of the guinea pig hearing organ and its relation to the cochlear potentials

The sound induced motion of the cells within the fourth turn of the guinea pig organ of Corti was studied in an in vitro preparation (Ulfendahl et al. 1989). The cells were visualised by relief microscopy, achieved by an oblique illumination technique. The motion of the sensory cells was observed during the recording of the extracellular receptor potentials; the cochlear microphonics (CM) and the summating potential (SP). Our results show that the temporal bone preparation sustains an endocochlear potential and maintains the receptor potentials for 3–4 h. During the tone stimulus the outer hair cells were seen to elongate and the surface of the organ of Corti was displaced in the direction of scala vestibuli. The displacement response showed two frequency maxima, one at 150 and one at 300 Hz. The mechanical tuning of the sensory organ coincided with the tuning of the receptor potentials. Both the mechanical and the electrical responses at the 300 Hz peak were vulnerable to the administration of methylene blue suggesting cyclic GMP dependence, whereas the 150 Hz peak was unaffected. We conclude that the outer hair cells provide active tuning in the organ of Corti.

Gamma-Aminobutyric acid receptor activation of outer hair cells in the guinea pig cochlea.

Gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter of the central nervous system, may also be released from olivocochlear efferent nerves reaching the outer hair cells of the cochlea. In the present study the cell potential of isolated outer hair cells of the third and fourth turns of the guinea pig cochlea was measured with patch-clamp electrodes. GABA-receptor agonists and antagonists were applied extracellularly. The cell membrane hyperpolarized in a reversible manner with increasing concentrations of extracellular GABA. Half-maximal hyperpolarization (2 mV) was achieved with approximately 10(-7) M GABA. Desensitization was not observed. The hyperpolarizing effect of GABA was potentiated by the benzodiazepine clorazepate and was blocked by picrotoxin.

Surgical anatomy of the sphenoid sinus--relationship to the internal carotid artery

HRCT images in axial sections obtained from various otologic lesions were used for morphological studies of the sphenoid sinus. 412 cases including 224 males and 188 females, with an average age of 47.5 years were included in the study. None of the cases showed any evidence of sinus lesions. The internal carotid artery, at its close connection with the sphenoid sinus, was classified into five portions: I, the second turn; II, between the second and third turns; III, the third turn, IV, between the third and fourth turns, and V, ascending portion of the fourth turn. Protrusion into the sphenoid sinus was seen at I in 30.4% of the cases, at II in 39.8%, at III in 34.3%, at IV in 22.3%, and at V in 11.2%. The second turn, where the internal carotid artery emerges from the carotid canal in the petrous bone and turns upward into the cavernous sinus, showed the highest incidence of the carotid eminence. The incidence of protrusion increases as the sinus grows larger. The average distance between the natural orifice and the carotid canal was 19.3mm on the right and 18.9mm on the left. The distance between the posterior end of the nasal septum and the natural orifice was, on average, 3.6mm on the right and 3.6mm on the left.

Effects of Frequency and Intensity of Sound on Cochlear Blood Flow

Laser Doppler flowmetry demonstrates that loud sound induces a decrease of blood flow in the cochlea of the guinea pig. In this experiment, we observed the effects of frequency and intensity of sound on cochlear blood flow using 15 guinea pigs. In the first 5 guinea pigs, a Doppler probe was attached to the basal turn of the cochlea and sounds of 6, 7, 8, 9 and 10 kHz were delivered to the ear serially from lower to higher frequency, i.e. from 6 kHz to 10 kHz. In the next 5 guinea pigs, the sound was delivered from higher to lower frequency, i.e. from 10 kHz to 6 kHz. The sound intensity delivered to the ear was changed from lower to higher intensity (80 to 120 dB SPL by 10 dB width) at each frequency. In the last 5 guinea pigs, the blood flow in the basal, second, third, and fourth turns of the cochlea was measured at 120 dB SPL of 10 kHz. No change of blood flow was seen in the cochlear basal turn at 6 and 7 kHz up to 120 dB SPL, but a decrease of blood flow was found at 110 and 120 dB SPL at 8, 9, and 10 kHz. On the other hand, the sound of 120 dB SPL at 10 kHz induced a decrease of blood flow only in the basal turn of the cochlea. Our results suggest that there is a corresponding blood flow area which is sensitive to specific frequency in the cochlea.

Further Observations on Anaphylatoxin Activity in the Cochlear Organ of Guinea Pigs

Complement is known to be related to many inflammatory reactions, and C4a, C3a and C5a, known as anaphylatoxins, cause strong inflammatory reaction although they are unrelated to anaphylaxis caused by immunoglobulin E.We made anaphylatoxins from guinea pig serum plus zymosan. Anaphylatoxins were inoculated into the carotid artery. We have already reported that the stria vascularis (S.V.) in the various cochlear turns, except the cochlear basal turns, was extremely atrophied after 72 hours.In this study, anaphylatoxins were inoculated into the carotid artery and after 7, 10 and 15 days, the S.V. was examined. It was extremely atrophied, similar to its appearance after 3 days. After 7 days, atrophy was observed in the third turn in 5 out of 12 S.V. (42%), in the fourth turn in 12 out of 12 S.V. (100%) and in the apical turn in 3 out of 3 S.V. (100%). After 10 days, atrophy was observed in the third turn in 2 out of 12 S.V. (17%), in the fourth turn in 9 out of 12 S.V. (75%) and in the apical turn in 3 out of 3 S.V. (100%). After 15 days, atrophy was observed in the third turn in 2 out of 12 S.V. (17%), in the fourth turn in 6 out of 12 S.V. (50%), and in the apical turn in 2 out of S.V. (67%).Thus atrophy in the S.V. due to anaphylatoxin improved with time.

Guinea pig tectorial membrane profile in an in vitro cochlear preparation

The guinea pig cochlea was examined under high-magnification light microscopy in an in vitro preparation. After extraction of the otic capsule, the bulla was opened widely and a small hole made into the fourth turn of the scala vestibuli. The organ of Corti was visualized under artificial endolymph at 600 X magnification. Added 1-micron titanium dioxide particles settled on the upper surface of the transparent tectorial membrane. Particle positions showed that much of this upper surface lay in a flat sheet that extended centrifugally almost to the Hensen's cells, giving the impression it was attached there. The sheet extended at least to the level of the inner hair cells, where a tectorial membrane thickness of about 40 micron was reached. Titanium dioxide particles were seen regularly in immediate proximity to the hair cell cilia, indicating that scala media is continuous with the subtectorial space. Upon mechanical manipulation, Hensen's cells proved to be extremely cohesive and elastic. It is suggested that hair cell stereocilia provide major mechanical connections for the tectorial membrane.

GABA visualized by immunocytochemistry in the guinea pig cochlea in axons and endings of efferent neurons

Antiserum raised against GABA coupled with glutaraldehyde to bovine serum albumin was applied to the guinea pig cochlea. Immunoreactivity was visualized as horseradish peroxidase reaction product in surface preparations of the organ of Corti using immunocytochemical techniques. Bright-field, differential interference contrast and video-enhanced contrast light microscopy were used. GABA-like immunoreactivity was found in axons and endings of efferent neurons in all turns of the cochlear spiral, but predominantly in the third turn and first half of the fourth turn. In these apical turns, immunoreactivity was seen in the efferent components: inner spiral bundle, tunnel spiral bundle, tunnel-crossing fibers, large nerve endings synapsing on outer hair cell bases, nerve endings high up on outer hair cells, nerve endings or varicosities close to outer hair cells, and outer spiral fibers. Some immunoreactive large nerve endings at outer hair cells were found in the apical half of the fourth turn. This study shows that axons and endings of efferent neurons in the organ of Corti of guinea pig contain GABA-like immunoreactivity with a distribution similar to that of GAD-like immunoreactivity as shown in a previous study. In both studies, many efferent nerve axons and endings were unstained, even in regions of maximal density of immunoreactivity in the apical turns. The evidence indicates that a subpopulation of efferent neurons projecting to the organ of Corti is GABAergic and very likely different from the lateral and the medial olivocochlear efferent systems.

Neurobiology of cochlear inner and outer hair cells: intracellular recordings.

Recordings were made in the low-frequency region (third and fourth turns) of the guinea pig cochlea from both inner (IHC) and outer (OHC) hair cells. Certain electrical characteristics of these cells that have been described before (P. Dallos (1985) J. Neurosci. 5, 1591-1608) are reviewed. These are resting membrane potentials, response magnitude and saturation, and response phase. A comparison of sensitivity and best frequency is provided for IHCs and OHCs that are located in the same cochlear region. New data are presented for the level-dependence of response phase and for the properties of response asymmetry and the resulting dc component. The effects of intracellular polarizing current upon ac response magnitude are shown for both cell types.

Intracellular recordings from supporting cells in the mammalian cochlea

The mammalian organ of Corti is constituted of a variety of different cell types. We have utilized horseradish peroxidase marking, along with intracellular recording techniques, to examine whether this heterogeneity is also reflected in the cells' electrical properties. Recordings from third and fourth turn supporting cells, hair cells, and fluid spaces have been made from guinea pig cochleas. A fast Fourier analysis was performed on responses to tone bursts. In all supporting cell types examined, the magnitude of the ac response component recorded from within the supporting cells was less than or equal to that recorded from adjacent fluid spaces. In contrast, differences were observed with respect to the dc response component. Dc components greater than those recorded from adjacent fluid spaces were frequently seen in supporting cell types located in the inner hair cell region. A slowly increasing dc component reminiscent of the slow electrical potential changes that occur in glial cells was also seen. These slow, depolarizing potentials are similar to those recorded from goldfish saccular supporting cells [T. Furukawa, J. Physiol. in press]. [Work supported by NINCDS Grant NS‐08635.]

Glutamic acid decarboxylase immunoreactivity of olivocochlear neurons in the organ of Corti of guinea pig and rat.

The distribution of glutamic acid decarboxylase (GAD)-like immunoreactivity in the organ of Corti of guinea pig and rat was studied under the light microscope. Indirect immunohistochemical techniques were used. Cochleae were first incubated with a specific antiserum to rat brain GAD and then stained through an avidin-biotin-horseradish peroxidase (HRP) procedure. GAD-like immunoreactivity was visualized as staining with HRP reaction product. Surface preparations were prepared from the immunoreacted cochleae. GAD-like immunoreactivity was found in the inner spiral bundle, tunnel spiral bundle, tunnel crossing fibers, outer hair cell synaptic regions and outer spiral bundles. Little staining was seen in the basal turn. Most of the immunoreactivity was seen in the third and lower fourth turn of the guinea pig cochlea, but even there many efferent fibers and endings were unstained. It is concluded that GAD-like immunoreactivity is present in a subpopulation of cochlear efferents that contains elements from both the medial and the lateral olivocochlear system. Future studies are needed to determine whether this subpopulation is GABA-ergic (i.e. uses gamma-aminobutyric acid as a neurotransmitter) and/or cholinergic.

Longitudinal distribution of cochlear potentials and the K + concentration in the endolymph after acoustic trauma

Guinea pigs were exposed to 142 dB third-octave band of noise centred at 1 kHz for 1 h. At different times after exposure the endocochlear potential (EP), the anoxic negative endocochlear potential (-EP), the concentration of K + (K e +) and microphonic potentials were recorded in scala media in four cochlear turns. The remaining hair cells were counted in each animal. Immediately after the exposure, the EP and K e + decreased evenly in all four cochlear turns and gradually returned to normal physiological values in 5–20 days. When measured 20 days after the exposure, essentially normal EP and K e + values were observed, with an apicalwards decline, which was similar to that found along the cochlea in nonexposed animals. Abnormal increased EP was observed in some animals 20 days after the exposure in the first and second turns. In contrast to positive EP and K e + values, the anoxic negative EP attained less negative values in the second turn of exposed animals i.e., in the turn where the narrow band noise exerted the major destructive effect. An almost normal distribution of hair cells and most negative EP values were found in the fourth turn. The distribution of persistent hair cells correlated positively with the values of the anoxic negative EP and amplitudes of the microphonic potentials. It is assumed that, in addition to the difference in K + concentration between endolymph and perilymph, the anoxic negative EP is dependent upon the functional state of the organ of Corti.