Biomedical subjects

A L Nuttall

Publications and source records attributed to A L Nuttall.

At least 91 records · Page 5Linked to original sources

PubMed1993

Osmotic pump implant for chronic infusion of drugs into the inner ear.

Continuous long-term delivery of experimental drugs to the cochlea of a small animal, such as a young guinea pig, presents several technical problems. A method of placing and securing a cannula-osmotic pump system is described in this paper. Guinea pigs (225-410 g) were unilaterally implanted with an Alzet micro-pump and cannula for delivery of 20 mM tetrodotoxin (TTX) (six animals) or saline (three animals) for three days (1 microliter/h). Auditory brainstem responses (ABRs) were recorded under light anesthesia on post-implant day 1 and day 3 and compared with pre-implant baseline values. In all six cochleas infused with TTX, most frequencies showed a 30-60dB decrease in sensitivity within 24 h. Saline control animals showed little or no change from baseline sensitivity for most frequencies. In three TTX-infused animals, the cannula-pump unit was removed on day 3, and ABRs were followed during recovery. Most frequencies returned to, or near, pre-implant levels after pump removal but recovery times varied. By day 6, all animals had recovered post-surgical weight loss and showed a gain of 10-40 g. Brains and cochleas were removed and processed for sectioning. Assessment of the cochlear nucleus of non-recovery TTX-treated animals showed a deafness-related flattening of auditory nerve active zones on the treated side.

Animals↗

PubMed1993

3-D analysis of F-actin in stereocilia of cochlear hair cells after loud noise exposure.

Fluorescence microscopy can be a useful tool in the early detection of pathological changes in the stereocilia of outer hair cells which have undergone acoustic overstimulation. Fluorescent phalloidin, a highly specific F-actin stain, can be used to label F-actin in stereocilia. In this study, phalloidin label is used to determine quantitative changes of F-actin in the stereocilia of guinea pigs exposed to loud noise (117 dB; octave band noise, centered at 1 kHz; 4 h). Reliably determining three-dimensional (3-D) structural changes in stereocilia is a challenging problem in optical microscopy since stereocilia diameter is close to the optical resolution limit. In order to alleviate the problem, a computational 3-D microscopy technique is used (Avinash et al., 1992). Whole-mounts of the cochlear second and third turns were examined in a Leitz Orthoplan microscope through a Leitz Plan Apo objective lens (100 x; 1.32 N.A.; 170/0.17). Images were acquired with a charge-coupled device camera where the focus was shifted in 0.2 microns steps using a piezoelectric translator. Images were processed with the appropriate point spread function of the optical system. Analysis of control cochleas indicate that our technique can resolve single stereocilia and distinguish between various intensities of label along each stereocilia. In noise-exposed cochleas, our data show length and intensity changes in the phalloidin label. These results suggest that both depolymerization and polymerization of F-actin can occur in stereocilia of outer hair cells after acoustic overstimulation. Our findings demonstrate the applicability of computational 3-D microscopy to quantitative and qualitative analysis of stereocilia.

Actins↗

PubMed1993

Two-tone suppression of inner hair cell and basilar membrane responses in the guinea pig.

Recordings of receptor potentials from inner hair cells (IHCs) and the basilar membrane (BM) motion were made in pigmented guinea pigs. The acoustic stimuli were single tones near best frequency (BF) and two-tone complexes. Single tone input/output (I/O) functions had a saturating growth for the magnitude and their phase shifts were strongly dependent on the tone frequency relative to BF. For IHCs, a BF tone stimulus produced no phase shift in the ac receptor potential response. Phase lag or lead occurred for tones below or above BF, respectively. BM velocity I/O functions were not as compressively saturating as IHC ac I/O curves. BM phase shifts (in relation to BF) were similar to those of the IHCs. Two-tone suppression was observed in both IHC and BM response measures. Suppressor tones on the low-frequency side of BF produced complex suppression results, which were inconsistent with a simple attenuation model for suppression. The growth of suppression was faster than the attenuation from equivalent level reductions of the probe tone, and phase shifts were phase lead. Depending upon experimental conditions, phase change with suppression may be in the opposite direction from phase change observed from pure attenuation of the probe tone. High-frequency suppressors (relative to BF) are consistent with an attenuation model of suppression for the IHCs of the current study. High side suppression of basilar membrane velocity, however, differed from the IHCs in a systematic way. The phase change caused by suppression of BM velocity was always smaller than that of an equivalent reduction in the probe tone level.

Acoustic Stimulation↗

PubMed1993

Intermodulation distortion (F2-F1) in inner hair cell and basilar membrane responses.

Round window (RW) recordings in guinea pigs show a large response at the frequency of the quadratic difference tone (QDT) at F2-F1 for high-frequency (> 8 kHz) tone pairs. The magnitude of the RW response is dependent on frequency and level of the primaries. The QDT cochlear microphonic produced is largest for primaries with a frequency separation of approximately 900 Hz. Its source is probably the local activity of hair cells in basal cochlear locations where the high-frequency tones interact. QDT measurements from locations inside the organ of Corti show that the magnitude of the QDT is largest in the inner hair cell (IHC) region and monotonically increases as the difference in the frequency of the primaries decreases. Measured as IHC intracellular ac receptor potential, the QDT appears to be the result of a mechanical stimulus to the cell rather than as an inherent property of the nonlinear transduction in the IHC. However, QDT of the same form is not evident in the velocity responses of the basilar membrane. These results suggest that the outer hair cells (OHC) produce a strong quadratic distortion product mechanical force to stimulate inner hair cells. This mechanical drive may not be present, or is a weak component, in basilar membrane motion.

Acoustic Stimulation↗

PubMed1993

Voltage-dependent block by neomycin of the ATP-induced whole cell current of guinea-pig outer hair cells.

1. The effects of externally applied ATP and neomycin on whole cell currents of isolated guinea pig cochlear outer hair cells (OHCs) were studied using the whole cell voltage-clamp technique. In OCHs held at -70 mV, ATP activated a large inward current. In the presence of neomycin, the ATP-induced whole cell current activated along a relatively unaltered time course, but the current then decreased to a reduced steady level. The neomycin inhibition of the ATP-induced current was dose dependent. The half-inhibitory concentration (IC50) of neomycin measured at steady state was estimated to be 90 microM. 2. Neomycin inhibition of the ATP response could not be reversed by increasing the concentration of ATP, indicating that the effect was noncompetitive. The inhibition was voltage dependent and was greatly reduced when OHCs were held at positive potentials. 3. Cells treated with 100 microM ATP gave maximal current responses. Addition of neomycin substantially increased membrane current noise of the 100 microM ATP responses. When neomycin concentration was varied from 10 to 500 microM, the current noise level peaked between 50 and 100 microM. The noise increase was observed at negative holding potentials but not at positive potentials. 4. The neomycin-induced whole cell current noise was used to estimate the size of the underlying elementary current. The ATP-induced single channel current of OHCs at -70 mV was estimated to be approximately 0.3 pA. The number of ATP-activated channels in a single OHC was estimated to be in the range of a few thousand. 5. The characteristics of the neomycin inhibition of ATP-induced currents were consistent with an open channel blocking mechanism. Analysis of the voltage dependence of the steady state neomycin inhibition suggested a neomycin binding site at an electrical distance of 0.3 from the extracellular side.

Acoustic Stimulation↗

PubMed1993

Effects of stellate ganglion stimulation on bilateral cochlear blood flow.

The effect of intraneural electrical stimulation of the stellate ganglion (SG) on bilateral cochlear blood flow (CBF) was investigated with laser-Doppler flowmetry. The SG of 15 anesthetized guinea pigs was exposed by a novel surgical approach and stimulated with a specially designed intraneural bipolar platinum-iridium electrode. Bilateral CBF was continuously monitored. Stimulation of 0.25 mA caused a detectable increase of the systemic blood pressure (BP) and a bilateral decrease of the cochlear vascular conductance (R, defined as the ratio CBF/BP). A stimulus of 0.5 mA elicited a statistically significant ipsilateral CBF (CBFi) decrease of 3.6% +/- 5.1% from the baseline and a contralateral CBF (CBFc) decrease of 3.1% +/- 5.5%. That no statistical difference was found between CBFi and CBFc indicates that a unilateral sympathetic stimulation of the SG can cause equal bilateral responses. These responses were accompanied by a significantly increased BP (8.7% +/- 5.2% of baseline) and consequently a greatly decreased R (12.2% +/- 6.5%) of the ipsilateral cochlea. Bilateral sections of the cervical sympathetic trunk below the level of the superior cervical ganglion did not alter the evoked changes in CBF, BP, and R. It is concluded that SG stimulation can decrease the conductivity of the cochlear vessels or the supplying vessels of the cochlea. Additionally, the SG nerve fibers that cause these effects do not pass through the superior cervical ganglion.

Animals↗

PubMed1993

Effects of electrical stimulation of the superior cervical ganglion on cochlear blood flow in guinea pig.

It has been proposed that cochlear blood flow (CBF) is controlled in part by the sympathetic nervous system. In the present study the effect of electrical stimulation of the superior cervical ganglion (SCG) on CBF in guinea pigs was investigated using laser Doppler flowmetry (LDF). Animals were anesthetized with diazepam and fentanyl and the SCG was exposed. A custom-designed bipolar cuff electrode was fixed around the ganglion and 1 ms biphasic current pulses were injected at 0.15 mA to 1.5 mA, 6 Hz. Bilateral CBF was monitored, while the ganglion was stimulated for 3 or 5 min before and after the ascending sympathetic trunk and nerve branches from SCG were sectioned. Electrical stimulation of 0.5 mA caused the ipsilateral CBF (CBFi) to decrease 11.7% +/- 1.3 from the baseline (BL), while the contralateral CBF (CBFc) increased slightly due to the change in systemic blood pressure (BP). A linear relation was observed between the level of current stimulation and evoked reduction in CBF. Cervical sympathetic trunk section (between the SCG and the middle cervical ganglion) did not influence the pattern or the amplitude of CBF change in response to electrical stimulation of SCG. Sectioning the efferent fibers of the medial inferior and medial superior branch of the SCG only minimally reduced the amplitude of the CBF decrease evoked by electrical stimulation. However, sectioning the superior lateral branch abolished this decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Provoked flux motion of cochlear blood flow measured with laser Doppler flowmetry in guinea pig.

Although progress has been made in the study of cochlear blood flow (CBF) regulation since laser Doppler flowmetry (LDF) was introduced, cochlear vasomotion has not been investigated. Therefore, the primary objective of this study was to determine if oscillatory fluctuations of CBF could be provoked. Guinea pigs were anesthetized with diazepam (5 mg/kg) and fentanyl (0.32 mg/kg). Blood pressure (BP) was recorded from a carotid artery cannula. The cochlea and pons cerebellum were ventrally exposed; the bilateral CBF and brain blood flow (BBF) or skin blood flow (SBF) were monitored by LDF. After administration of phentolamine (0.25-0.75 mg/kg, i.v.), ipsilateral CBF in 7 of 16 animals showed a 2-5 min episode of oscillation. During artificial hyperventilation, continuous oscillation of CBF was recorded (the flux motion frequency was 3.5 +/- 0.5 cycles per min and its amplitude 25.8 +/- 5.6% from baseline). The time-dependent flux change (the waveform) was the same throughout a single cochlea but different between cochleae of the same animal. Compared to BBF, CBF vasomotion frequency was lower, and amplitude larger. SBF exhibited no such motion. Flux motion could be eliminated by inhalation of pure oxygen or 5% CO2 in oxygen or by the smooth muscle relaxants, papaverine and hydralazine. Phentolamine-induced vasomotion may be due to a hypotensive perfusion pressure, and hyperventilation-enhanced vasomotion may be caused by changing blood gas concentrations and by hormonal or neuronal activity. Oxygen and CO2 inhalation slightly increased BP and this change in perfusion pressure was probably associated with weakened vasomotion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Betahistine-induced vascular effects in the rat cochlea.

Betahistine (BH) has been used widely to treat cochlear disorders, such as tinnitus and Meniere's disease. The mechanism of action of BH in the cochlea is assumed to be based on its histamine-like effect on H1 receptors in the cochlear vasculature, leading to an increased cochlear blood flow (CBF). Recently it has been shown that BH can strongly affect H3 heteroreceptors (a novel histamine receptor subclass) in the periphery, via an autonomic ligand. This mechanism may also contribute to the BH effects on CBF. This study was to validate BH effects in the cochlear vasculature and to investigate the possible mechanisms of action of this drug in the inner ear vasculature. We assessed the effects of BH on CBF with the laser Doppler flowmeter in 23 rats and concluded that BH affects vascular conductivity in the cochlea in a dose-dependent fashion; betahistine diffuses through the round window, but does not have access to vascular receptors or ligands once in the labyrinthine fluids; and the H1 receptors mediate the systemic and peripheral vascular effects of BH, whereas the cochlear effect involves cholinergic receptors.

Animals↗

PubMed1992

Cochlear blood flow in response to dilating agents.

Reduced cochlear blood flow (CBF) has been implicated in various pathologies of the inner ear, including sudden deafness, noise-induced hearing loss and Meniere's disease. Thus the aim of some current therapeutic regimens to treat these conditions is to increase CBF and thereby improve oxygenation of the inner ear tissues. Most of the vasodilating agents in clinical use, however, do not have specific experimental evidence to support their effects on CBF. The hypotension which can follow systemic administration may limit their local effectiveness and general utility, just as it complicates the interpretation of the data in animal experiments. In the current study we investigated the effect of six agents, known for their systemic cardiovascular actions, on CBF: hydralazine, sodium nitroprusside, papaverine, nicotinic acid, verapamil and histamine. The effect of these drugs was studied after topical applications on the round window membrane (RWM) and systemic intravenous administrations. CBF was monitored with a laser Doppler flowmeter (LDF). Topical administration of sodium nitroprusside was the most effective in increasing CBF, followed, in order, by hydralazine and histamine. No change in CBF was observed for papaverine, verapamil or nicotinic acid. Systemic administrations of all the agents caused a marked decrease in blood pressure and variable effects on CBF. We discuss the CBF changes in relation to the different pharmacological mechanisms of action of each drug. The study demonstrates the effectiveness of topical application of vasodilating agents in increasing CBF.

Administration, Topical↗

PubMed1992

The influence of loud sound on red blood cell velocity and blood vessel diameter in the cochlea.

Using intravital microscopy, we observed both decreases in red blood cell velocity and possible vasoconstriction in stria vascularis capillaries of the rat cochlea in response to loud sound (Quirk et al., 1991). However, our observation of vasoconstriction was subject to error in measurements from the two dimensional images obtained with our silicon intensified (SIT) camera due to the influence of focus causing image blur. The purpose of the current study was to apply an extended focus microscopy technique to obtain quantitative assessment of vessel diameter changes (Avinash et al., 1992), as well as to extend these studies to the guinea pig model. Broad-band sound stimulation at intensities of 84 dB SPL and 110 dB SPL were used. The results show that loud sound induces a sequence of changes in cochlear blood flow. Stimulation with 110 dB SPL resulted in a mean increase (maximum = 27%) in red blood cell velocity for the first 20 min of exposure followed by a gradual decrease below baseline (minimum = -12%) prior to termination of the signal. This velocity decrease and subsequent recovery were associated with significant changes in vessel diameters of selected and measured capillaries. In contrast, the 84 dB SPL stimulus caused an increase in red blood cell velocity (maximum = 20%) and vessel diameter (mean = 7.5) during the stimulation period. No recovery was observed during the 10 min observation period following sound. Several possible mechanisms responsible for these changes are discussed.

Acoustic Stimulation↗

PubMed1992

Cochlear blood flow and microvascular resistance changes in response to hypertonic glycerol, urea, and mannitol infusions.

The effect of hyperosmotic agents on cochlear blood flow (CBF) was tested in normal guinea pigs and in guinea pigs having prior unilateral operations to ablate the endolymphatic duct. Laser-Doppler-measured CBF was normalized to remove apparent changes related directly to systemic blood pressure. Hyperosmotic fluids were given via venous infusion: glycerol (20% and 40% solutions), urea (10%, 30%, and 40% solutions), and mannitol (40% solution). All agents were dissolved in 0.9% saline and the mixtures were given at a rate of 0.3 to 0.6 mL/min for 5 minutes. Control infusions were of 0.9% saline and isotonic dextran 70 (Pharmacia). All hyperosmotic infusions resulted in similar increases in normalized cochlear blood flow (nCBF) that extended to a maximum of 300% of the baseline value in a dose-dependent way during the infusion time period. Within approximately 30 minutes following infusions, nCBF had returned to baseline levels. Saline infusion alone had little effect on nCBF, but isotonic dextran 70 gave a sustained increase to 122% of the baseline levels. There was no difference between the responses of nCBF in hydropic and normal cochleas for either control or hyperosmotic solutions. Measurements of systemic hematocrit at time intervals during and following the infusions showed that transient reductions of up to approximately 8% (for the maximum osmotic challenge) occurred during the infusion. It is concluded that the hyperosmotic treatments tested here are equally effective for short-term enhancements of nCBF in both normal and hydropic cochleas. The basis of the flow increase is partially rheologic and partially due to a local vasodilation.

Animals↗

PubMed1992

The effect of pentoxifylline on cochlear blood flow.

Pentoxifylline, a phosphodiesterase inhibitor and hemorrheologic agent has been found to increase oxygen delivery to ischemic tissue. Intravenous pentoxifylline was administered to normal guinea pigs in order to assess the effect of pentoxifylline on cochlear blood flow and to elucidate its mechanism of action. Intravenous pentoxifylline was found to acutely increase cochlear blood flow in a dose-dependent manner. In normal animals, the effect appeared strongly related to the rheologic properties of this agent rather than a vasodilative action. Normovolemic hemodilution with 75% dextran resulted in no increase in cochlear blood flow during infusion of pentoxifylline, whereas the application of nitroprusside over the round window failed to abolish the effect of pentoxifylline.

Animals↗

PubMed1992

Experimental basis for lidocaine therapy in cochlear disorders.

In order to further our basic understanding of the effects of lidocaine hydrochloride in the inner ear, cochlear potentials and blood flow (CBF) were assessed after intravenous (i.v.), anterior inferior cerebellar artery (AICA), and local round window (RW) lidocaine administrations in guinea pigs and rats. Lidocaine RW applications produced a dose dependent decrease in compound action potentials (CAP) and cochlear microphonics (CM). The sensitivity changes were more pronounced at high frequencies. These findings suggest that lidocaine has specific pharmacological action in the inner ear other than simple anesthesia of the auditory nerve. The basal turn endocochlear potentials (EP) were not altered by topical lidocaine, implicating altered organ of Corti function following local application of lidocaine. RW applications of lidocaine had no effect on CBF or systemic blood pressure (BP). I.v. infusions caused substantial reductions in BP. In the case of systemic infusions the percent changes in CBF were equal to and accountable by the BP changes. The microinfusions (50 mg/ml, 100 nl/min) through AICA produced a 30%, long lasting increase in CBF. However, neither systemic lidocaine nor AICA infusions had an effect on CAP or CM. These findings indicate that systemically given lidocaine may not cross the blood-cochlear barrier and that the cochlear electrophysiological effects due to lidocaine when given locally are partly mediated by direct influence on cochlear hair cell function; they also suggest that lidocaine-induced interference with active ion transport in the lateral wall or an influence on CBF are not contributing factors.

Action Potentials↗

PubMed1991

Laser Doppler velocimetry of basilar membrane vibration.

A method is described for the measurement of basilar membrane (BM) vibration velocimeter (LDV). The instrumentation was coupled to a compound microscope which served to visualize reflective glass microbeads placed on the BM. The laser beam of the LDV was focused in the microscope object plane and positioned over the reflective bead. We show examples of frequency tuning curves and displacement input/output intensity functions obtained with the technique.

Acoustic Stimulation↗

PubMed1991

Cochlear microphonic enhancement in two tone interactions.

Two tone interaction functions of the cochlear microphonic (CM) were obtained from pigmented guinea pigs. First (basal) cochlear turn recording locations show optimally enhanced levels of CM when the interfering tone (F2) was positioned about 4 kHz above probe tones (F1) of 12 kHz and 20 kHz. Maximum enhancement occurred for equal level tones. No enhancement was seen for a probe tone of 4 kHz. When basal turn cochlear sensitivity was compromised, CM enhancement caused by the interfering tone was altered and only CM reduction was then seen. The CM reduction was the typical characteristic described by many earlier studies. Guinea pigs with various changes in cochlear sensitivity were studied, providing evidence in support of earlier reports that CM interference (both reductions and enhancements) depends on far field vector summation of the outputs of hair cells from a restricted area of the basilar membrane. No CM enhancement was seen in micropipette recordings from within the organ of Corti.

Acoustic Stimulation↗

PubMed1991

The effect of noise exposure on the aging ear.

The effect of noise exposure on auditory sensitivity and inner ear morphology was compared in aged and young mature mice. Hearing thresholds were obtained by auditory evoked brain stem responses (ABR) before and after noise exposure, and hair cell loss was quantified. The study was done in two parts: first to assess the effect of noise exposure on subjects with presbycusis, and second to assess its effect on aged subjects without measurable presbycusis. In the first experiment C57BL/6 mice, with an age-related hearing loss, were used as a model for presbycusis. C57BL/6 mice exhibiting presbycusis were more susceptible to noise injury than age-matched CBA/Ca mice. In the second experiment CBA/Ca mice were used. These mice retain normal hearing even with advancing age. The aged CBA/Ca mice had the same susceptibility to noise injury as young CBA/Ca mice.

Aging↗

PubMed1991

The effects of Carbogen, carbon dioxide, and oxygen on noise-induced hearing loss.

An investigation into the effect of Carbogen (95% O2/5% CO2), 5% CO2/air, and 100% oxygen on cochlear threshold shifts caused by noise was undertaken. Five groups of eight pigmented guinea pigs were exposed to 105 dB broad band noise for 6 h per day for five consecutive days with each group receiving the various gaseous mixtures either during noise exposure or for 1 h immediately after noise exposure. A control group received the same noise exposure but respired air. Auditory threshold shifts, as measured by the auditory evoked brainstem response, were measured at 2,4,8,12,16, 20 and 24 kHz. Recordings were taken pre-exposure and at Day 1, 3, 5, and Weeks 2 and 3 after noise exposure. Carbogen, given during noise exposure, resulted in a trend toward less post noise exposure threshold shift (as compared to controls) which reached statistical significance by Week 3 at all frequencies except 2 and 20 kHz. Subjects given Carbogen after exposure also showed a general trend toward decreased noise induced threshold shifts, as compared to controls, but this was not statistically significant. The mixture of 5% CO2/air given during noise exposure yielded no difference in threshold shifts as compared to controls. When 100% oxygen was administered during noise exposure, a marked decrease in noise induced threshold shifts could be seen as compared to controls, with differences reaching statistical significance by day 5 at most frequencies. These results indicate that oxygen (i.e. cochlear-oxygenation) is a more important factor than CO2 (i.e., as a vasodilator) in protection of the cochlea from noise induced damage.

Animals↗