Local vestibular blood flow and systemic vascular responses to natural vestibular stimulation in the Mongolian gerbil.

HYPOTHESIS: Natural stimulation of the vestibular end organs will produce alterations in the local vestibular microvascular blood flow. BACKGROUND: The vestibular and cardiovascular systems require a coordinated interaction to maintain organ perfusion during rapid positional and postural changes. However, the detailed relationship of these systems is not well understood. There have been no previous descriptions of local vestibular blood flow (VBF) during natural stimulation (NS) conditions. METHODS: In vivo VBF and systemic blood pressure (BP) in the Mongolian gerbil during natural stimulation. Using laser Doppler flowmetry, the authors obtained continuous measures of local VBF in both anesthetized and alert gerbils during sinusoidal rotational stimuli. Simultaneous recordings of systemic BP were collected from the contralateral common carotid artery. RESULTS: The anesthetized gerbils showed stable VBF and BP during all vestibular stimuli. By contrast, alert subjects demonstrated a significant response to natural stimulation. The VBF increased 28% over baseline, and systemic BP increased 8% during a 45-second, 0.133-Hz sinusoid. Decreases in BP of 8% and 5%, respectively, were seen with a 0.10 and 0.20 Hz, 360-second stimulus. A corresponding determination of VBF during the extended stimulus conditions was not technically possible. CONCLUSIONS: To the authors' knowledge, these are the first in vivo descriptions of vestibular blood flow during natural stimulation. In the alert animals, VBF increased in response to NS. This increase in flow does not appear to be directly dependent on systemic blood pressure changes and indicates that the vestibular microvasculature is closely regulated.

Measurement of peripheral vestibular blood flow in a gerbil model of endolymphatic hydrops.

Presently, many investigators believe that the dysfunction of microcirculatory mechanisms may be responsible for vestibular symptoms in Meniere's disease. This study, using intravital microscopy (IVM), a technique that provides in vivo microcirculatory measures, was designed to determine whether impaired vestibular blood flow exists in endolymphatic hydrops. Hydrops was induced in the gerbil model by obliteration of the vestibular aqueduct and was confirmed histologically after IVM. Posthydrops gerbil subjects at 8 weeks as well as control animals were prepared for IVM. With a customized intravital microscope, red blood cell velocity and vessel diameter measurements were calculated for individual arterioles and capillaries from the microvascular bed at the horizontal ampulla. Mean arteriole diameter was significantly larger in the control group than in the hydrops group, whereas mean capillary diameters were similar for both groups. No significant difference was observed for mean red blood cell velocity in capillaries or arterioles between control and hydrops animals.

Effects of kynurenic acid as a glutamate receptor antagonist in the guinea pig.

Glutamate excitotoxicity is implicated in both the genesis of neural injury and noise-induced hearing loss (NIHL). Acoustic overstimulation may result in excessive synaptic glutamate, resulting in excessive binding to post-synaptic receptors and the initiation of a destructive cascade of cellular events, thus leading to neuronal degeneration and NIHL. The purpose of this study was to determine whether this apparent excitotoxicity can be attenuated by kynurenic acid (KYNA), a broad-spectrum glutamate receptor antagonist, and protect against noise-induced temporary threshold shifts (TTS). Guinea pigs were randomly assigned to three separate groups. Base-line compound action potentials (CAP) thresholds and cochlear microphonics (CM) were recorded. Group I was treated with physiologic saline as a vehicle control applied to the round window membrane that was followed by 110 dB SPL wide-band noise for 90 min. Group II received 5 mM KYNA followed by noise exposure, and group III received 5 mM KYNA alone without noise exposure. Post-drug and noise levels of CAP thresholds and CM were then obtained. Noise exposure in the control group caused a significant temporary threshold shift (TTS) of 30-40 dB across the frequencies tested (from 3 kHz to 18 kHz). Animals that received 5 mM KYNA prior to noise exposure (group II) showed statistically significant protection against noise-induced damage and demonstrated a minimal TTS ranging between 5 and 10 dB at the same frequencies. Animals in group III receiving KYNA without noise exposure showed no change in thresholds. Additionally, cochlear microphonics showed no considerable difference in threshold shifts when controls were compared to KYNA-treated animals. These results show that antagonizing glutamate receptors can attenuate noise-induced TTS, suggesting that glutamate excitotoxicity may play a role in acoustic trauma.

Biologic activity of mitochondrial metabolites on aging and age-related hearing loss.

HYPOTHESIS: Compounds that upregulate mitochondrial function in an aging model will improve hearing and reduce some of the effects of aging. BACKGROUND: Reactive oxygen metabolites (ROM) are known products of oxidative metabolism and are continuously generated in vivo. More than 100 human clinical conditions have been associated with ROM, including atherosclerosis, arthritis, autoimmune diseases, cancers, heart disease, cerebrovascular accidents, and aging. The ROM are extremely reactive and cause extensive DNA, cellular, and tissue damage. Specific deletions within the mitochondrial DNA (mtDNA) occur with increasing frequency in age and presbyacusis. These deletions are the result of chronic exposure to ROM. When enough mtDNA damage accrues, the cell becomes bioenergetically deficient. This mechanism is the basis of the mitochondrial clock theory of aging, also known as the membrane hypothesis of aging. Nutritional compounds have been identified that enhance mitochondrial function and reverse several age-related processes. It is the purpose of this article to describe the effects of two mitochondrial metabolites, alpha-lipoic acid and acetyl L-carnitine, on the preservation of age-related hearing loss. METHODS: Twenty-one Fischer rats, aged 24 months, were divided into three groups: acetyl-l-carnitine, alpha-lipoic acid, and control. The subjects were orally supplemented with either a placebo or one of the two nutritional compounds for 6 weeks. Auditory brainstem response testing was used to obtain baseline and posttreatment hearing thresholds. Cochlear, brain, and skeletal muscle tissues were obtained to assess for mtDNA mutations. RESULTS: The control group demonstrated an expected age-associated threshold deterioration of 3 to 7 dB in the 6-week study. The treated subjects experienced a delay in progression of hearing loss. Acetyl-l-carnitine improved auditory thresholds during the same time period (p<0.05). The mtDNA deletions associated with aging and presbyacusis were reduced in the treated groups in comparison with controls. CONCLUSIONS: These results indicate that in the proposed decline in mitochondrial function with age, senescence may be delayed by treatment with mitochondrial metabolites. Acetyl-l-carnitine and alpha-lipoic acid reduce age-associated deterioration in auditory sensitivity and improve cochlear function. This effect appears to be related to the mitochondrial metabolite ability to protect and repair age-induced cochlear mtDNA damage, thereby upregulating mitochondrial function and improving energy-producing capabilities.

Reactive oxygen metabolites, antioxidants and head and neck cancer.

This manuscript will review the probable role of reactive oxygen metabolites (ROM) in the etiopathogenesis of head and neck cancer (HNC). Cancer is a heterogeneous disorder with multiple etiologies including somatic and germ-line mutations, cellular homeostatic disturbances, and environmental triggers. Certain etiologies are characteristic of HNC and include infectious agents such as the Epstein-Barr virus, the use of tobacco, and consumption of alcohol. A large body of evidence implicates ROM in tumor formation and promotion. ROM species are formed in the process of cellular respiration, specifically during oxidative phosphorylation. These ubiquitous molecules are highly toxic in the cellular environment. Of the many effects of ROM, especially important are their effect on DNA. Specifically, ROM cause a variety of DNA damage, including insertions, point mutations, and deletions. Thus, it is hypothesized that ROM may be critically involved in the etiology of malignant disease through their possible impact on protooncogenes and tumor suppressor genes. Additionally, empirical evidence suggests that ROM may also affect the balance between apoptosis and cellular proliferation. If apoptotic mechanisms are overwhelmed, uncontrolled cellular proliferation may follow, potentially leading to tumor formation. Thus, this manuscript will critically review the evidence that supports the role of ROM in tumorigenesis. ROM scavengers and blockers have shown both in vivo and in vitro effects of attenuating the toxicity of ROM. Such compounds include the antioxidant vitamins (A, C, and E), nutrient trace elements (selenium), enzymes (superoxide dismutase, glutathione peroxidase, and catalase), hormones (melatonin), and a host of natural and synthetic compounds (lazaroids, allopurinol, gingko extract). Thus, this paper will also review the possible benefit derived from the use of such scavengers/blockers in the prevention of HNC.

Betahistine increases vestibular blood flow.

Betahistine is used for treatment of several vestibular disorders. Despite the accepted use of this histamine-like substance, its mechanism of action is not well understood. The purpose of this study was to assess the possibility that one of the activities of betahistine is increasing blood flow in the peripheral vestibular end organs. Using a novel surgical approach, we identified the posterior semicircular canal ampulla of guinea pigs and placed a laser Doppler probe in position to obtain blood flow measurements from the posterior semicircular canal ampulla. Blood pressure, heart rate, and vestibular blood flow were continuously recorded. Concentration-response curves were obtained for betahistine (2.5, 5, 7.5, and 10 mg/kg) and control-vehicle (0.15 mol/L NaCl) infusions. A separate group of subjects was pretreated with the competitive selective H3 agonist, thioperimide maleate, before betahistine treatment. Increases in vestibular blood flow and decreases in blood pressure were observed in response to betahistine infusions. Pretreatment with thioperamide maleate abolished these changes at low doses of betahistine and attenuated the responses at higher doses of betahistine. These results show that betahistine administration induces increases in vestibular blood flow. These findings support the potential use of betahistine for treatment of vestibular disorders, which may be caused by compromised circulation.

Mechanisms of alterations in the microcirculation of the cochlea.

Labyrinthine function is tightly coupled to proper homeostasis. This includes appropriate blood flow that is under strict autoregulatory control. Perturbations in labyrinthine microcirculation can lead to significant cochlear and vestibular dysfunction. The etiology of many otologic disorders, including sudden sensorineural hearing loss, presbyacusis, noise-induced hearing loss, and certain vestibulopathies, are suspected of being related to alterations in blood flow. Some of the mechanisms responsible for hypoperfusion and possibly ischemia, within the cochlea, are addressed, with emphasis on the possibility that both noise and age contribute to localized low blood-flow states and stasis. This reduction in blood supply to the cochlea is likely, in part, responsible for reduced auditory sensitivity associated with chronic noise exposure and aging.

The effects of flunarizine and pentoxifylline on vestibular blood flow in the guinea pig.

Several authors have proposed that complications arising from vestibular disorders are the result of compromised circulation. The purpose of the current study was to assess the ability of flunarizine and pentoxifylline to increase peripheral vestibular blood flow (VBF), since flunarizine is a selective calcium-channel entry blocker that inhibits calcium-related contraction of smooth muscle, while pentoxifylline is a xanthine derivative that promotes microcirculation by affecting red blood cell malleability. Both of these treatment strategies have received considerable attention in clinics and laboratory, but their effects on blood flow are unclear. Changes in VBF were evaluated from the posterior semicircular canal ampulla in guinea pigs using a laser Doppler flowmeter. One group of animals was infused with pentoxifylline at concentrations of 10-40 mg/ml, while a second group was treated with 0.3-1.5 mg/kg flunarizine. VBF, blood pressure (BP) and heart rate (HR) were monitored continuously. Findings showed that pentoxifylline induced a concentration-dependent increase in VBF. In contrast, no increase in VBF occurred in response to flunarizine infusions. These studies suggest that the effectiveness of pentoxifylline in the clinical treatment of vestibular disorders may be the result of improved blood flow.

Sarthran preserves cochlear microcirculation and reduces temporary threshold shifts after noise exposure.

The cause of noise-induced hearing loss remains unclear despite years of both epidemiologic and experimental investigation. Among the many possible pathophysiologic mechanisms that may contribute to noise-induced temporary or permanent threshold shifts are insufficiencies in cochlear blood flow. Although the literature is inconsistent, several histologic and physiologic studies demonstrate signs of reduced circulation in the cochlea after noise exposure. Recent studies using computer-enhanced intravital microscopy complement these earlier findings. Evidence suggests that these microcirculatory events are mediated in part by several circulating factors, including the potent vasoactive peptide angiotensin. This study investigated this possibility by pretreating with the angiotensin receptor antagonist sarthran during noise exposure and examining both cochlear microcirculation and auditory sensitivity. The results of these experiments show noise-induced ischemia in the lateral wall of the cochlea and temporary threshold shifts. Treatment with sarthran prevented this noise-induced microcirculatory ischemia and preserved auditory sensitivity at the low frequencies tested. These findings support a role for the angiotensinergic system during noise exposure and suggest that preservation of cochlear blood flow is functionally related to auditory sensitivity.

The vascular mechanism of action of betahistine in the inner ear of the guinea pig.

The aim of this study was to investigate the mechanism and site of action of betahistine dihydrochloride in the inner ear of the guinea pig. Betahistine-evoked increases in cochlear blood flow (CBF) have been presumed to be due to the drug effect on the later wall capillary bed or larger feeding vessels in the cochlea vascular system. As such, the mechanism of action could be due to inhibition of H3 receptors. Betahistine may also have a direct effect on postsynaptic H1/H2 receptors and/or an effect modulated by other autonomic receptors. Betahistine-evoked CBF responses were assessed by laser Doppler flowmetry in the presence of an H3 agonist (alpha N-methyl-histamine dihydrochloride), an H3 antagonist (thioperamide), an H2 antagonist (cimetidine) or an alpha 2 antagonist (idazoxan). The effects of betahistine on circulation in the anterior inferior cerebellar artery (AICA) and ipsilateral stria vascularis (SV) were assessed using intravital microscopy (IVM). Findings showed that betahistine increased CBF and reduced systemic blood pressure (BP). In contrast, alpha N-methylhistamine dihydrochloride had no effect on baseline CBF or BP and did not influence betahistine-induced increases in CBF. Thioperamide reversed the effects of betahistine on CBF, but had no effect on baseline CBF or BP. Cimetidine had no marked effect on baseline CBF or betahistine-induced increases in CBF Idazoxan had no consistent effects on baseline CBF, but abolished the effect of betahistine on CBF. The mean increase of red blood cell velocity in SV capillaries was 15% and occurred without a demonstrable change in capillary diameters. In contrast, the diameter of the AICA increased by 17-20%, indicating that betahistine-evoked increases in CBF resulted primarily from vasodilatation of the AICA. We suggest that this effect may be mediated via presynaptic H3 heteroreceptors and autonomic alpha 2 receptors.

Mitochondrial DNA deletions associated with aging and presbyacusis.

BACKGROUND: The membrane hypothesis of aging proposes an association between reactive oxygen metabolites and aging processes. Reactive oxygen metabolites are a normal by-product of oxidative phosphorylation and are also formed under conditions of ischemia, hypoperfusion, and as a result of environmental contaminants. Among the many detrimental activities of reactive oxygen metabolites, also known as free oxygen radicals, is direct damage to mitochondrial DNA. Progressive accumulation of mitochondrial DNA damage renders cells unable to conduct oxidative phosphorylation reactions effectively, thereby leading to a bioenergetically deficient cell. Over time, mitochondrial DNA damage accumulates and leads to cellular dysfunction with subsequent organ failure, aging, and ultimately, death. This sequence forms the basis of the membrane hypothesis of aging. OBJECTIVE: To determine if the membrane hypothesis of aging may be involved in the development of presbyacusis. DESIGN: Fischer rats from 4 age groups were tested for auditory sensitivity using the auditory brainstem response. Brain, stria vascularis, and auditory nerve tissues were harvested and mitochondrial DNA was amplified to identify the highly conserved cytochrome b and ND1-16S ribosomal RNA segment of the NADH genes, as well as a 4834-base pair (bp) deletion associated with aging. SUBJECTS: Fischer rats (n=28) from 4 age groups were used: young (2-4 months [n=9]), mid-young (9-11 months [n=5]), mid-old (18-20 months [n=5]), and old (30-34 months [n=9]). RESULTS: The results demonstrate a progressive reduction in auditory sensitivity with age. The mitochondrial DNA studies identify a significant increase in the presence of the 4834-bp deletion in the aged subjects compared with the young. CONCLUSIONS: These findings raise the possibility that the 4834-bp deletion may be associated with presbyacusis, as well as with aging.

Calcium channel blockade reduces noise-induced vascular permeability in cochlear stria vascularis.

Exposure to noise results in multiple perturbations of cochlear microcirculation, including increases in vascular permeability. There is evidence that these events are mediated, in part, by calcium channels. The current study examined the effects of calcium channel blockade on the diameter and permeability of cochlear vessels during noise exposure. Subjects were exposed to either noise alone or noise after verapamil pretreatment. Vessels of the cochlear stria vascularis were imaged using intravital microscopy. Animals exposed to noise showed decreases in diameter and increased permeability above baseline levels. Animals pretreated with the calcium channel blocker verapamil and exposed to noise demonstrated increases in vessel diameter and no changes in permeability. These data indicate that calcium channel blockade reduces noise-induced microvascular permeability. Treatment strategies designed to protect from increases in vascular permeability due to noise exposure may reduce temporary threshold shifts.

In vivo vestibular blood flow in the Mongolian gerbil: angiotensin III-provoked changes in systemic and local factors.

The current literature contains little information on vestibular end organ blood flow. The absence of an accepted model, difficulties applying dynamic in vivo measurement techniques and the inaccessibility of the inner ear organs contribute to the shortage of experimental findings. The purpose of the current study is to introduce the gerbil as a viable model for the in vivo study of vestibular blood flow dynamics. The potent vasoactive peptide, angiotensin III (AIII), was used to provoke blood pressure and blood flow changes. The results of this study demonstrate that viable blood flow measures may be obtained from the vestibule of the gerbil. Dose-dependent changes in blood pressure and vestibular blood flow were observed in response to high concentrations of AIII. Pretreatment with the receptor antagonist, sarthran, attenuated both blood pressure and blood flow increases in response to subsequent AIII infusions. The gerbil model offers the advantages of easily accessible and identifiable peripheral vestibular organs, as well as responsive local blood flow. Investigations using this model may provide information on the regulation of blood flow during presentation with a variety of stimulus modalities. Information from such studies may lead to development of strategies for treatment of vestibulopathies suspected to be of vascular origins.

Mitochondrial DNA deletions associated with aging and possibly presbycusis: a human archival temporal bone study.

HYPOTHESIS: We attempted to determine if the common mitochondrial DNA aging deletion is also associated with presbycusis. BACKGROUND: Presbycusis is the most common cause of deafness in adults in the United States, affecting approximately 40% of the population older than 75 years of age. The ability to identify a gene(s) or a specific genetic deficit(s) associated with presbycusis has significant clinical importance. METHODS: The current study examined mitochondrial DNA (mtDNA) from cochlear sections of 34 human temporal bones: 17 with normal hearing and 17 with presbycusis. DNA was extracted from celloidin-embedded temporal bone sections; and specific oligonucleotide primers were designed to amplify the cytochrome b gene and a 4,977 base pair (bp) deletion of the mtDNA. Polymerase chain reaction (PCR) was used to amplify the base pair products that correspond to targeted gene regions, and sequencing was used to verify the products. RESULTS: Fourteen of the 17 patients with hearing loss showed the 4,977 bp deletion and this deletion was present in only eight of the 17 human specimens with normal audiograms. The cytochrome b gene was amplified from all specimens. CONCLUSIONS: The current study demonstrates the presence of a 4,977 bp deletion in human mitochondrial DNA genome that is associated with aging and with some forms of presbycusis. These results, coupled with previous animal studies, suggest that this 4,977 deletion may be associated with presbycusis.

Changes in dorsal cochlear nucleus blood flow during noise exposure.

Changes in dorsal cochlear nucleus (DCN) blood flow during noise exposure were assessed using both intravital microscopy (IVM) and laser Doppler flowmetry (LDF). Mature Syrian golden hamsters were anesthetized, tracheotomized and implanted with a carotid artery catheter for monitoring blood pressure and infusion of fluorescent dye. An occipital craniectomy was performed and the cerebellum partly aspirated for access to the DCN. Fluorescent dye was infused to enhance observations of the DCN surface using a customized IVM. Red blood cell velocity, vessel diameter and blood pressure were continuously monitored. Subjects were presented with 110 dB SPL broad-band noise for 15 min. A second group was presented with the same protocol for laser Doppler assessment of blood-flow changes. Control groups included animals not exposed to noise using both IVM and laser Doppler measures. Additional control measurements were obtained for noise-exposed and control groups with laser Doppler measures obtained from the obex, a brainstem structure with no known auditory function. Arterioles of the DCN showed a low-frequency oscillatory pattern of red blood cell velocity in control animals and in baseline conditions of the noise-exposure group. Presentation of noise abolished these velocity oscillations. Additionally, significant decreases in both red blood cell velocity and vessel diameter were measured during stimulation. These microvascular measures recovered slightly during the post-stimulus period. Laser Doppler measures of the overall blood flow in the nucleus were consistent with IVM findings. Measurements from DCN and obex in groups not exposed to noise, and animals exposed to noise and measured from the obex, showed stable oscillatory flow. These data show exposure to noise eliminates oscillatory patterns of blood flow and induces decreases in vascular perfusion. Furthermore, these changes appear specific to the auditory structure studied.

The tonic sympathetic input to the cochlear vasculature in guinea pig.

Vascular tones is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activation of the sympathetic fibers to the cochlear vasculature, other studies have failed to show evidence of sympathetic contribution to CBF regulation when the cervical sympathetic fibers were unilaterally sectioned. We hypothesized that the bilateral 'sympathectomy of the stellate ganglia' would remove sufficient sympathetic input to the cochlea to yield a change in CBF resting level. To test this hypothesis a new technique was used to expose the stellate ganglia (SG) bilaterally and induce a chemical sympathectomy. We observed that unilateral SG blockade with 2 microliters of 4 mM lidocaine hydrochloride on either side produced a 5-10% increase in CBF, which recovered to baseline during the following 2 min. A subsequent blockade of the contralateral SG produced a rapid 25-35% increase, which then recovered partially during the following 3-4 min, remaining 5-15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Our results provide evidence for the existence of a tonic sympathetic component in the control of vascular tone in guinea pig cochlea. This neural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiological studies on the bilaterality of evoked changes in CBF due to electric stimulation of SG.

Local effects of nitric oxide on vestibular blood flow in the Mongolian gerbil.

There is a paucity of studies regarding the regulation of vestibular blood flow (VBF), despite the possibility that vascular alterations may contribute to specific vestibulopathies. The current experiments used the Mongolian gerbil as an animal model since it provides easy surgical access to the vestibular end-organs and has been previously used for physiologic studies involving inner ear function. VBF changes were measured in the posterior semicircular canal using laser Doppler flowmetry following round window membrane (RWM) application of the nitric oxide donor 1, 3-propanediamine-N-[4-1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazi no] butyl (spermine NONOate; SPNO) as a vasodilator. The specificity of the responses induced was tested via pretreatment with an NO scavenger, 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazonline-1-oxyl-3-oxide (carboxy-PTIO; cPTIO). cPTIO, SPNO, vehicle (control) or cPTIO/SPNO were applied to the RWM, during which blood pressure and VBF were monitored for baseline, treatment, and recovery conditions. Results showed concentration-dependent increases in flow, probably resulting from NO's vasodilatory action on local vasculature. cPTIO pretreatment was found to attenuate SPNO-induced VBF increases. These findings support a role of NO in maintaining the vestibular microcirculation.

Guinea pig vestibular blood flow in response to calcitonin-gene related peptide.

Little is known about the physiologic regulation of the vestibular end organ blood flow. The purpose of the current study was to examine posterior semicircular canal ampulla blood flow in addition to systemic factors during intravenous infusions of calcitonin-gene related peptide (CGRP), a factor involved in the tonic regulation of blood flow. Receptors for this factor are known to be available to the vascular supply of the vestibular organs. Local blood flow using laser Doppler flowmetry and systemic parameters were monitored during infusion of CGRP. CGRP antagonists and control vehicle. The results show relatively stable vestibular blood flow (VBF), concentration-dependent decreases in systemic blood pressure, and elevations in heart rate. Pretreatment with CGRP(8-37), a specific receptor antagonist, attenuated these responses to subsequent CGRP infusions. These findings suggest a rigid regulation of VBF in the presence of a systemically active vasodilator.

Age-related differences in cochlear microcirculation and auditory brain stem response.

OBJECTIVE: To examine possible age-related differences in auditory sensitivity and cochlear vascular properties. DESIGN: This study is designed to provide information regarding cochlear function using physiological and audiological measures. Each animal underwent intravital microscopic evaluation of red blood cell velocity, vessel diameter, and vascular permeability in the second turn of the cochlear lateral wall. Auditory brain stem responses were used to determine hearing sensitivity. SUBJECTS: Four age ranges of male Fischer rats were studied: young, 2 to 4 months (n = 9); mid-young, 9 to 11 months (n = 8); mid-old, 18 to 20 months (n = 6); and old, 30 to 34 months (n = 10). RESULTS: Auditory brain stem response testing showed an age-related decrease in auditory sensitivity. Intravital microscopic analysis showed age-related statistically significant decreases in red blood cell velocity and increased vascular permeability with a trend for reduced capillary diameters. CONCLUSIONS: The process of aging is associated with many biochemical and physiological changes that include decrease in cellular water concentration, ionic changes, and decreased elasticity of cellular membranes. One contributing factor to this process may be altered vascular characteristics, such as reduced flow and vascular plasticity, as well as increased vascular permeability. These age-related changes may result in reductions in oxygen and nutrient delivery, and also waste elimination. Our results suggest that progressive age-associated vascular compromise may be a contributing factor in presbycusis.

Association of mitochondrial DNA deletions and cochlear pathology: a molecular biologic tool.

The purpose of these experiments was to develop a method of isolation, amplification, and identification of cochlear mitochondrial DNA (mtDNA) from minute quantities of tissue. Additionally, studies were designed to detect mtDNA deletions (mtDNA del) from the cochlea that previously have been amplified from other organ systems and tissues. MtDNA del have been associated with many pathologies, including neurological disorders, sensorineural hearing loss, ischemia, cardiomyopathies, and aging. DNA was extracted from rat and human tissues, and polymerase chain reaction was used to amplify mtDNA sequences. A 360 base pair (bp) cytochrome-b gene product and the highly conserved ND1-16S ribosomal ribonucleic acid regions found only in mtDNA were amplified from all tissues. Preliminary studies have identified a 4834 bp mtDNA del in aged rats and a corresponding 4977 bp mtDNA del in aged humans. Additionally, preliminary results in human archival temporal bone studies reveal the presence of the 4977-bp mtDNA deletion in two out of three patients with presbycusis. The deletion was not evident in age-matched control patients without a history of presbycusis. This technique of mtDNA identification makes it possible to investigate specific mtDNA defects from a single cochlea, promoting the study of hereditary hearing loss and presbycusis at a molecular biologic level.