Local vs systemic effects of acoustic trauma on cochlear structure and transport.

Whether certain effects of acoustic trauma on cochlear structure and function were due to local or systemic agents was investigated in the chinchilla. After unilateral ossicular disarticulation, the animals were given a noise exposure known to cause cochlear damage and vessel transport changes in the stria vascularis. The cochleas with disarticulated ossicular chains received an effective exposure well below one that causes pathologic damage. Only the cochleas with intact ossicular chains showed an increase in the permeability of the strial vessels to horseradish peroxidase tracer and manifested strial edema and organ of Corti damage. These results indicate that the effects of acoustic trauma on cochlear structure and strial vessel transport are induced locally, not systemically. In addition, the minimum time of exposure to 120-dB, 700- to 2800-Hz noise that causes massive cochlear damage in the chinchilla was shown to be between five and ten minutes.

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.

Strial dysfunction in mice with cochleo-saccular abnormalities.

Most viable dominant spotting (Wv/Wv) mutant mice, which show cochleo-saccular degeneration, were found to have an endocochlear potential (EP) around zero together with a structurally abnormal stria vascularis. Inner hair cells were well preserved, but outer hair cells in the basal half of the cochlea were degenerating, possibly as a result of primary strial dysfunction. Thresholds for the detection of a compound action potential were raised to around 100 dB SPL in the mutants with no EP, and there was little if any cochlear microphonic at the round window. Of the 20 Wv/Wv mice studied, five partially escaped the effects of the mutation and had measurable positive potentials (15-86 mV) in scala media in the basal turn; responses in these animals were intermediate between control responses and those of mutants with no EP. These findings confirm that the pathological processes in this mutant, with cochleo-saccular abnormalities, are fundamentally different from the pathological processes in animals with neuroepithelial abnormalities reported previously [see Steel and Bock (1983) Arch. Otolaryngol. 109, 22-29, for references].

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.

The site of involvement of hypertension within the cochlea. A comparative study of normotensive and spontaneously hypertensive rats.

The function and morphology of the cochlea of the spontaneously hypertensive rat (SHR) were examined and compared with the age-paired normotensive Wistar Kyoto rat (WKY). Electro- cochleographic study revealed that the function of the cochlea in the SHR declined with increasing age to a greater extent than that of WKY. Electronmicroscopic study revealed that the primary site of the cochlear deterioration of the SHR was the vascular stria, followed by the organ of Corti. Some hypotheses to explain this phenomenon are proposed for further study.

Ultrastructural study of the effect of acute hypertension on the stria vascularis and spiral ligament.

The effect of acute hypertension, induced in rats by intravenous injection of methoxamine chloride (Mexan), on the stria vascularis and spiral ligament was studied electronmicroscopically with the tracer method of horseradish peroxidase (HRP). Considerable extravasation of HRP occurred in the stria vascularis, due to the increased vesicular transport. The leaked HRP spread into intercellular spaces, but was prevented from spreading towards the endolymph by zonulae occludentes between marginal cells and towards the perilymph by zonulae occludentes between basal cells. The reaction product was occasionally found between basal cells. No leakage of HRP from capillaries was observed in the spiral ligament, although some labelled micropinocytotic vesicles were present in the endothelium. It is suggested that, under acute hypertensive conditions, areas of zonulae occludentes bordering the stria vascularis play an important role as a barrier to HRP, whereas capillaries in the spiral ligament themselves act as a barrier to it.

The relationship of stria vascularis volume and cochlear duct volume.

To determine whether stria vascularis volume correlated with cochlear duct volume, 1,180 human temporal bones were examined. Fifty-eight ears in 44 persons showed the presence of endolymphatic hydrops. Strial atrophy was found in 143 ears in 83 persons, in five of which it was severe. Eight cases of endolymphatic hydrops occurred in the presence of mild to moderate strial atrophy. A decrease in cochlear duct volume usually accompanied severe strial atrophy. However, a case in which cochlear duct volume was normal in the presence of severe strial atrophy was seen. Cochlear duct volume does not always correlate with stria vascularis volume.

[Kanamycin-induced lesions of the stria vascularis: preliminary results of an electrophysiological and ultrastructural study (author's transl)]. / Lésions de la strie vasculaire par la kanamycine: étude électro-physiologique et ultra-structurale. Notes préliminaires.

Ten guinea-pigs were treated with kanamycin 400 mg/kg for a 10-day period. Electrophysiological investigations were carried out, and the animals were killed one month after the end of the treatment for electron microscope study of the second turn of the cochlear spiral. The microphonic potential was considerably reduced, and the stria vascularis was the seat of lysosomal inclusions in the marginal and intermediate cells associated with myeloid bodies in the vascular endothelium. There was a gradient of lysosomal catabolic residues, probably lipoproteins, from the vascular sector to the cochlear canal; some of these products passed into the cochlear canal. Alterations of the hair cells in the second spire appeared to be minimal. It is hypothesized that damage to the stria vascularis precedes damage to the hair cells.

Changes of the stria vascularis induced by ethacrynic acid.

Histological changes in the stria vascularis of guinea pigs after the injection of ethacrynic acid (50 mg/kg) were investigated using a transmission electron microscope and a scanning electron microscope. The changes in permeability in the stria vascularis was also observed using horseradish peroxidase as a tracer. The pathological changes originating in endothelial cells of stria blood vessels and intermediate cells spread into the marginal cells and basal cells. The most remarkable changes were observed one hour after the injection of ethacrynic acid. Subsequently, recovery began in the cells around the blood capillaries and was most complete 3--4 hours after the injection. It is presumed that the strial damages by the ethacrynic acid are caused not only by the direct effect of this drug on the strial cells but also by the disturbances of blood flow and permeability of the capillaries.