Exacerbation of noise-induced hearing loss in mice lacking the glutamate transporter GLAST.
Acoustic overstimulation is one of the major causes of hearing loss. Glutamate is the most likely candidate neurotransmitter for afferent synapses in the peripheral auditory system, so it was proposed that glutamate excitotoxicity may be involved in noise trauma. However, there has been no direct evidence that noise trauma is caused by excessive release of glutamate from the inner hair cells (IHCs) during sound exposure because studies have been hampered by powerful glutamate uptake systems in the cochlea. GLAST is a glutamate transporter highly expressed in the cochlea. Here we show that after acoustic overstimulation, GLAST-deficient mice show increased accumulation of glutamate in perilymphs, resulting in exacerbation of hearing loss. These results suggest that GLAST plays an important role in keeping the concentration of glutamate in the perilymph at a nontoxic level during acoustic overstimulation. These findings also provide further support for the hypothesis that IHCs use glutamate as a neurotransmitter. (+info)
Biophysical basis for inner ear decompression sickness.
Isolated inner ear decompression sickness (DCS) is recognized in deep diving involving breathing of helium-oxygen mixtures, particularly when breathing gas is switched to a nitrogen-rich mixture during decompression. The biophysical basis for this selective vulnerability of the inner ear to DCS has not been established. A compartmental model of inert gas kinetics in the human inner ear was constructed from anatomical and physiological parameters described in the literature and used to simulate inert gas tensions in the inner ear during deep dives and breathing-gas substitutions that have been reported to cause inner ear DCS. The model predicts considerable supersaturation, and therefore possible bubble formation, during the initial phase of a conventional decompression. Counterdiffusion of helium and nitrogen from the perilymph may produce supersaturation in the membranous labyrinth and endolymph after switching to a nitrogen-rich breathing mixture even without decompression. Conventional decompression algorithms may result in inadequate decompression for the inner ear for deep dives. Breathing-gas switches should be scheduled deep or shallow to avoid the period of maximum supersaturation resulting from decompression. (+info)
Prophylactic effect of Ca2+ -deficient artificial perilymph perfusion on noise-induced hearing loss.
OBJECTIVE: To investigate the prophylactic effect of low calcium concentration perilymph on noise-induced hearing loss. METHODS: Forty guinea pigs with normal hearing weighing 250-350 g were assigned to five groups (8 in each group): (1) Ca(2+)-deficient perilymph perfusion (CDP) for 2 h; (2) white noise (120 dB SPL) exposure (WNE) only for 1 h, (3) combination of calcium-deficient perilymph perfusion and white noise (120 dB SPL) exposure (WNE + CDP); (4) normal artificial perilymph (NAP) perfusion for 2 h; and (5) white noise exposure + normal artificial perilymph perfusion (WNE + NAP) for 2 h. Compound action potentials (CAP) evoked by click was recorded from round window every 15 min. The cochleae from 5 animals in each group were examined with scanning electron microscope. RESULTS: The CAP for group 1 experienced a threshold shift (TS) of 15-26 dB, while group 2 yielded a 46-59 dB TS and group 3 a 37-45 dB TS; no threshold shift occurred in group 4. The CAP TS in group 5 was 33-64 dB. The CAP TS of group 3 was less than that of group 2. After one hour of noise exposure, the CAP TS of group 3 were 45.92 +/- 2.90 dB and 59.30 +/- 3.95 dB in group 2. There were significant differences (P < 0.05) between groups 3 and 2. The CAP TS of group 3 was less than that of group 5 at the points of 1, 1.5 and 2 h after noise exposure. There was a significant difference between groups 3 and 5 (P < 0.01). Stereocilia of 89 OHC(3) were in disarray in five cochleae after noise exposure in group 2. The cuticular plates of 8 OHC(2),3 sank and the stereocilia became fused in only one animal cochlea after noise exposure in group 3 combined with low calcium perilymph perfusion. CONCLUSIONS: Low calcium concentration appears to participate in preventing noise-induced hearing loss and the rising of calcium concentrations in inner hair cells after noise exposure, which may have been due to the opening of calcium channels in inner hair cells during noise exposure. The mechanism of the prophylactic effect might be caused by a lower calcium concentration in inner hair cells in the cochlea attenuating the influence of noise exposure on hearing loss; calcium deficient perilymph perfusion prevented calcium accumulation in inner hair cells of the cochlea. The motility of the OHCs might be partially inhibited by low calcium concentration that reduced noise-induced hearing loss in turn. (+info)
X-linked stapes gusher: CT findings in one patient.
A 31-year-old male patient presented with progressive mixed hearing loss since birth. A stapedectomy was attempted and was unsuccessful because of perilymph gushing. CT of the temporal bones showed bulbous dilatation of the fundi of the internal auditory canals and absence of the bone plates separating them from the base of the cochleas. This unusual abnormality was found after the attempted stapedectomy and explains the clinical findings. The findings in male patients are fairly typical X-linked congenital deafness. (+info)
Pneumolabyrinth associated with perilymph fistula.
Pneumolabyrinth associated with perilymph fistula, especially with the presence of air in the cochlea, has rarely been identified using imaging study. We present a 24-year-old woman who experienced right fluctuating sensorineural hearing impairment and dis-equilibrium after sustaining an open-handed slap on the right side of her face. The diagnosis of pneumolabyrinth was established through the detection of air bubbles in the right basal turn of cochlea using high-resolution computerized tomography. Right side exploratory tympanotomy confirmed the presence of a perilymph fistula. The patient was free of symptoms after surgery. Early fistula repair was beneficial in this case. (+info)
Traumatic perilymph fistula in infants: a case report.
Traumatic perilymph fistula is reported to be rare in infants because of the small size of the infant external meatus. We treated an infant with a traumatic perilymph fistula in the right ear. A metallic wire had penetrated the tympanic membrane. Horizontal-rotatory nystagmus was also observed. Computed tomographic images revealed dislocation of the ossicles. The perilymph fistula was closed under general anesthesia. The incus-stapes joint was separated and the footplate of the stapes was dislocated. Leakage of the perilymph fluid was apparent from the oval window and this fistula was closed with connective tissue. The perforation of the tympanic membrane was closed with temporal fascia. After surgery, the spontaneous nystagmus disappeared. The patient is under observation as an outpatient and is growing normally. (+info)
Gentamicin pharmacokinetics in the chicken inner ear.
Avians have the unique ability to regenerate cochlear hair cells that are lost due to ototoxins or excessive noise. Many methodological techniques are available to damage the hair cells for subsequent scientific study. A recent method utilizes topical application of an ototoxic drug to the round window membrane. The current study examines the pharmacokinetics of gentamicin in the inner ear of chickens following topical application to the round window membrane or a single systemic high dose given intraperitoneally. Chickens were given gentamicin topically or systemically and survived for 1, 4, 12, 24, or 120 h (controls at 4 and 120 h). Serum and perilymph samples were obtained prior to sacrifice and measured for gentamicin levels. Results revealed higher levels of gentamicin in the perilymph of topically treated chickens than systemically treated chickens, with significant amounts of gentamicin still present in both at the latest survival time of 5 days. As expected, systemically treated chickens had much higher levels of gentamicin in the serum than topically treated chickens. Advantages and disadvantages to each method of drug administration are discussed. (+info)
Rapid and reversible astrocytic reaction to afferent activity blockade in chick cochlear nucleus.
We describe here rapid proliferation of astrocytic processes immunoreactive for glial fibrillary acidic protein (GFAP) in the chick cochlear nucleus following blockade of action potentials in the afferent nerve. Unilateral eighth nerve activity blockade was achieved through intralabyrinthine injection of TTX. Within 1 hr of activity blockade, a 56% increase in area density of GFAP-immunoreactive processes was found in the ipsilateral cochlear nucleus as compared to the contralateral side of the brain. This increase reached 152% by 3 hr. When eighth nerve activity was allowed to recover and animals were studied 1 week after TTX injection, no difference was found in GFAP immunoreactivity between the ipsilateral and contralateral cochlear nuclei. This is the first report of a glial reaction to documented neuronal inactivity in the absence of neuropathology. These results indicate that neuronal activity may regulate the structure of astrocytic processes. (+info)