Effects of perilymphatic perfusion with neomycin on the cochlear microphonic potential in the guinea pig

Selective Neuronal Activation by Cochlear Implant Stimulation in Auditory Cortex of Awake Primate

Despite the success of cochlear implants (CIs) in human populations, most users perform poorly in noisy environments and music and tonal language perception. How CI devices engage the brain at the single neuron level has remained largely unknown, in particular in the primate brain. By comparing neuronal responses with acoustic and CI stimulation in marmoset monkeys unilaterally implanted with a CI electrode array, we discovered that CI stimulation was surprisingly ineffective at activating many neurons in auditory cortex, particularly in the hemisphere ipsilateral to the CI. Further analyses revealed that the CI-nonresponsive neurons were narrowly tuned to frequency and sound level when probed with acoustic stimuli; such neurons likely play a role in perceptual behaviors requiring fine frequency and level discrimination, tasks that CI users find especially challenging. These findings suggest potential deficits in central auditory processing of CI stimulation and provide important insights into factors responsible for poor CI user performance in a wide range of perceptual tasks.

SIGNIFICANCE STATEMENT

The cochlear implant (CI) is the most successful neural prosthetic device to date and has restored hearing in hundreds of thousands of deaf individuals worldwide. However, despite its huge successes, CI users still face many perceptual limitations, and the brain mechanisms involved in hearing through CI devices remain poorly understood. By directly comparing single-neuron responses to acoustic and CI stimulation in auditory cortex of awake marmoset monkeys, we discovered that neurons unresponsive to CI stimulation were sharply tuned to frequency and sound level. Our results point out a major deficit in central auditory processing of CI stimulation and provide important insights into mechanisms underlying the poor CI user performance in a wide range of perceptual tasks.

Representations of Time-Varying Cochlear Implant Stimulation in Auditory Cortex of Awake Marmosets (Callithrix jacchus)

Electrical stimulation of the auditory periphery organ by cochlear implant (CI) generates highly synchronized inputs to the auditory system. It has long been thought such inputs would lead to highly synchronized neural firing along the ascending auditory pathway. However, neurophysiological studies with hearing animals have shown that the central auditory system progressively converts temporal representations of time-varying sounds to firing rate-based representations. It is not clear whether this coding principle also applies to highly synchronized CI inputs. Higher-frequency modulations in CI stimulation have been found to evoke largely transient responses with little sustained firing in previous studies of the primary auditory cortex (A1) in anesthetized animals. Here, we show that, in addition to neurons displaying synchronized firing to CI stimuli, a large population of A1 neurons in awake marmosets (Callithrix jacchus) responded to rapid time-varying CI stimulation with discharges that were not synchronized to CI stimuli, yet reflected changing repetition frequency by increased firing rate. Marmosets of both sexes were included in this study. By comparing directly each neuron's responses to time-varying acoustic and CI signals, we found that individual A1 neurons encode both modalities with similar firing patterns (stimulus-synchronized or nonsynchronized). These findings suggest that A1 neurons use the same basic coding schemes to represent time-varying acoustic or CI stimulation and provide new insights into mechanisms underlying how the brain processes natural sounds via a CI device.SIGNIFICANCE STATEMENT In modern cochlear implant (CI) processors, the temporal information in speech or environmental sounds is delivered through modulated electric pulse trains. How the auditory cortex represents temporally modulated CI stimulation across multiple time scales has remained largely unclear. In this study, we compared directly neuronal responses in primary auditory cortex (A1) to time-varying acoustic and CI signals in awake marmoset monkeys (Callithrix jacchus). We found that A1 neurons encode both modalities using similar coding schemes, but some important differences were identified. Our results provide insights into mechanisms underlying how the brain processes sounds via a CI device and suggest a candidate neural code underlying rate-pitch perception limitations often observed in CI users.

Effects of hearing preservation on psychophysical responses to cochlear implant stimulation

Previous studies have shown that residual acoustic hearing supplements cochlear implant function to improve speech recognition in noise as well as perception of music. The current study had two primary objectives. First, we sought to determine how cochlear implantation and electrical stimulation over a time period of 14 to 21 months influence cochlear structures such as hair cells and spiral ganglion neurons. Second, we sought to investigate whether the structures that provide acoustic hearing also affect the perception of electrical stimulation. We compared psychophysical responses to cochlear implant stimulation in two groups of adult guinea pigs. Group I (11 animals) received a cochlear implant in a previously untreated ear, while group II (ten animals) received a cochlear implant in an ear that had been previously infused with neomycin to destroy hearing. Psychophysical thresholds were measured in response to pulse-train and sinusoidal stimuli. Histological analysis of all group I animals and a subset of group II animals was performed. Nine of the 11 group I animals showed survival of the organ of Corti and spiral ganglion neurons adjacent to the electrode array. All group I animals showed survival of these elements in regions apical to the electrode array. Group II animals that were examined histologically showed complete loss of the organ of Corti in regions adjacent and apical to the electrode array and severe spiral ganglion neuron loss, consistent with previous reports for neomycin-treated ears. Behaviorally, group II animals had significantly lower thresholds than group I animals in response to 100 Hz sinusoidal stimuli. However, group I animals had significantly lower thresholds than group II animals in response to pulse-train stimuli (0.02 ms/phase; 156 to 5,000 pps). Additionally, the two groups showed distinct threshold versus pulse rate functions. We hypothesize that the differences in detection thresholds between groups are caused by the electrical activation of the hair cells in group I animals and/or differences between groups in the condition of the spiral ganglion neurons.

Effects of deafening and cochlear implantation procedures on postimplantation psychophysical electrical detection thresholds

Previous studies have shown large decreases in cochlear implant psychophysical detection thresholds during the weeks following the onset of electrical testing. The current study sought to determine the variables underlying these threshold decreases by examining the effects of four deafening and implantation procedures on detection thresholds and implant impedances. Thirty-two guinea pigs were divided into four matched groups. Group I was deafened and implanted Day 0 and began electrical testing Day 1. Group II was deafened and implanted Day 0 and began electrical testing Day 45. Group III was deafened Day 0, implanted Day 45 and began electrical testing Day 46. Group IV was not predeafened but was implanted Day 0 and began electrical testing Day 1. All groups showed threshold decreases over time but the magnitude of change, time course and final stable threshold levels depended on the type and time course of treatment. Impedances increased over the first two weeks following the onset of electrical testing except in Group II. Results suggest that multiple mechanisms underlie the observed threshold shifts including (1) recovery of the cochlea from a temporary pathology caused by the deafening and/or implantation procedures, (2) effects of electrical stimulation on the auditory pathway, and (3) tissue growth in the implanted cochlea. They also suggest that surviving hair cells influence electrical threshold levels.

Auditory cortex phase locking to amplitude-modulated cochlear implant pulse trains

Cochlear implant speech processors transmit temporal features of sound as amplitude modulation of constant-rate electrical pulse trains. This study evaluated the central representation of amplitude modulation in the form of phase-locked firing of neurons in the auditory cortex. Anesthetized pigmented guinea pigs were implanted with cochlear electrode arrays. Stimuli were 254 pulse/s (pps) trains of biphasic electrical pulses, sinusoidally modulated with frequencies of 10-64 Hz and modulation depths of -40 to -5 dB re 100% (i.e., 1-56.2% modulation). Single- and multiunit activity was recorded from multi-site silicon-substrate probes. The maximum frequency for significant phase locking (limiting modulation frequency) was >or=60 Hz for 42% of recording sites, whereas phase locking to pulses of unmodulated pulse trains rarely exceeded 30 pps. The strength of phase locking to frequencies >or=40 Hz often varied nonmonotonically with modulation depth, commonly peaking at modulation depths around -15 to -10 dB. Cortical phase locking coded modulation frequency reliably, whereas a putative rate code for frequency was confounded by rate changes with modulation depth. Group delay computed from the slope of mean phase versus modulation frequency tended to increase with decreasing limiting modulation frequency. Neurons in cortical extragranular layers had lower limiting modulation frequencies than did neurons in thalamic afferent layers. Those observations suggest that the low-pass characteristic of cortical phase locking results from intracortical filtering mechanisms. The results show that cortical neurons can phase lock to modulated electrical pulse trains across the range of modulation frequencies and depths presented by cochlear implant speech processors.

Auditory prosthesis with a penetrating nerve array

Contemporary auditory prostheses ("cochlear implants") employ arrays of stimulating electrodes implanted in the scala tympani of the cochlea. Such arrays have been implanted in some 100,000 profoundly or severely deaf people worldwide and arguably are the most successful of present-day neural prostheses. Nevertheless, most implant users show poor understanding of speech in noisy backgrounds, poor pitch recognition, and poor spatial hearing, even when using bilateral implants. Many of these limitations can be attributed to the remote location of stimulating electrodes relative to excitable cochlear neural elements. That is, a scala tympani electrode array lies within a bony compartment filled with electrically conductive fluid. Moreover, scala tympani arrays typically do not extend to the apical turn of the cochlea in which low frequencies are represented. In the present study, we have tested in an animal model an alternative to the conventional cochlear implant: a multielectrode array implanted directly into the auditory nerve. We monitored the specificity of stimulation of the auditory pathway by recording extracellular unit activity at 32 sites along the tonotopic axis of the inferior colliculus. The results demonstrate the activation of specific auditory nerve populations throughout essentially the entire frequency range that is represented by characteristic frequencies in the inferior colliculus. Compared to conventional scala tympani stimulation, thresholds for neural excitation are as much as 50-fold lower and interference between electrodes stimulated simultaneously is markedly reduced. The results suggest that if an intraneural stimulating array were incorporated into an auditory prosthesis system for humans, it could offer substantial improvement in hearing replacement compared to contemporary cochlear implants.

Laser-induced collagen remodeling and deposition within the basilar membrane of the mouse cochlea

The cochlea is the mammalian organ of hearing. Its predominant vibratory element, the basilar membrane, is tonotopically tuned, based on the spatial variation of its mass and stiffness. The constituent collagen fibers of the basilar membrane affect its stiffness. Laser irradiation can induce collagen remodeling and deposition in various tissues. We tested whether similar effects could be induced within the basilar membrane. Trypan blue was perfused into the scala tympani of anesthetized mice to stain the basilar membrane. We then irradiated the cochleas with a 694-nm pulsed ruby laser at 15 or 180 Jcm(2). The mice were sacrificed 14 to 16 days later and collagen organization was studied. Polarization microscopy revealed that laser irradiation increased the birefringence within the basilar membrane in a dose-dependent manner. Electron microscopy demonstrated an increase in the density of collagen fibers and the deposition of new fibrils between collagen fibers after laser irradiation. As an assessment of hearing, auditory brainstem response (ABR) thresholds were found to increase moderately after 15 Jcm(2) and substantially after 180 Jcm(2). Our results demonstrate that collagen remodeling and new collagen deposition occurs within the basilar membrane after laser irradiation in a similar fashion to that found in other tissues.

Cochlear microperfusion: experimental evaluation of a potential new therapy for severe hearing loss caused by inflammation

HYPOTHESIS

Cochlear microperfusion will be a useful treatment of severe sensorineural hearing loss caused by inflammation.

BACKGROUND

Viruses, bacteria, and autoimmunity can initiate inflammation in the inner ear. The acute phase is associated with elevations in cytokines, nitrous oxide, and cellular infiltrates and the breakdown of the blood-labyrinthine barrier. The chronic phase leads to irreversible ossification of the labyrinth.

METHODS

The authors developed cochlear microperfusion to facilitate removal of inflammatory cells and their byproducts during the acute phase of inflammation. Using a ventral approach to the guinea pig cochlea, the authors displaced resident perilymph by delivering perfusate into the scala vestibuli and collecting the effluent from the scala tympani. The authors evaluated the benefit of the procedure in an animal model of severe hearing loss caused by inflammation.

RESULTS

Healthy controls undergoing cochlear microperfusion with phosphate-buffered saline incurred a mean hearing loss of 16 dB (n=4). This hearing loss was associated with the creation of two cochleostomies and not the perfusion itself. Sterile labyrinthitis (n=5) generated by perfusion of the cochlea with antigen consistently produced severe hearing loss over the initial 48 hours, and this hearing loss persisted for the subsequent 7 days. Therapeutic cochlear microperfusion, performed within the first 24 hours of developing severe hearing loss (n=9), immediately restored on average 24 dB (p <0.007) of hearing.

CONCLUSION

Cochlear microperfusion is a promising new technique for treating severe deafness caused by inflammation. The benefit may be sustained when combined with local delivery of immunosuppressive agents to the inner ear.

Genetic factors in aminoglycoside toxicity

Ototoxicity is the major irreversible toxicity of aminoglycosides, and it occurs both in a dose-dependent and idiosyncratic fashion. The idiosyncratic pathway is presumably due to genetic predispositions, and an inherited mutation in the mitochondrial 12S ribosomal RNA gene that predisposes carriers to aminoglycoside ototoxicity was identified in 1993. Up to a third of patients with aminoglycoside ototoxicity carry this mutation. Two other mutations in the same mitochondrial gene affect a small minority of additional patients. Thus, the prevention of aminoglycoside-induced ototoxicity through family history and molecular diagnosis is possible in many cases. It is the challenge of genomic medicine to translate this more than a decade-old knowledge into clinical practice.

Effects of cochlear-implant pulse rate and inter-channel timing on channel interactions and thresholds

Interactions among the multiple channels of a cochlear prosthesis limit the number of channels of information that can be transmitted to the brain. This study explored the influence on channel interactions of electrical pulse rates and temporal offsets between channels. Anesthetized guinea pigs were implanted with 2-channel scala-tympani electrode arrays, and spike activity was recorded from the auditory cortex. Channel interactions were quantified as the reduction of the threshold for pulse-train stimulation of the apical channel by sub-threshold stimulation of the basal channel. Pulse rates were 254 or 4069 pulses per second (pps) per channel. Maximum threshold reductions averaged 9.6 dB when channels were stimulated simultaneously. Among nonsimultaneous conditions, threshold reductions at the 254-pps rate were entirely eliminated by a 1966-micros inter-channel offset. When offsets were only 41 to 123 micros, however, maximum threshold shifts averaged 3.1 dB, which was comparable to the dynamic ranges of cortical neurons in this experimental preparation. Threshold reductions at 4069 pps averaged up to 1.3 dB greater than at 254 pps, which raises some concern in regard to high-pulse-rate speech processors. Thresholds for various paired-pulse stimuli, pulse rates, and pulse-train durations were measured to test possible mechanisms of temporal integration.

Cortical responses to cochlear implant stimulation: channel interactions

This study examined the interactions between electrical stimuli presented through two channels of a cochlear implant. Experiments were conducted in anesthetized guinea pigs. Multiunit spike activity recorded from the auditory cortex reflected the cumulative effects of electric field interactions in the cochlea as well as any neural interactions along the ascending auditory pathway. The cochlea was stimulated electrically through a 6-electrode intracochlear array. The stimulus on each channel was a single 80- micro s/phase biphasic pulse. Channel interactions were quantified as changes in the thresholds for elevation of cortical spike rates. Experimental parameters were interchannel temporal offset (0 to +/-2000 micro s), interelectrode cochlear spacing (1.5 or 2.25 mm), electrode configuration (monopolar, bipolar, or tripolar), and relative polarity between channels (same or inverted). In most conditions, presentation of a subthreshold pulse on one channel reduced the threshold for a pulse on a second channel. Threshold shifts were greatest for simultaneous pulses, but appreciable threshold reductions could persist for temporal offsets up to 640 micro s. Channel interactions varied strongly with electrode configuration: threshold shifts increased in magnitude in the order tripolar, bipolar, monopolar. Channel interactions were greater for closer electrode spacing. The results have implications for design of speech processors for cochlear implants.

Cochlear microphonic potentials: a new recording technique

A new instrumentation and a particular method for detecting and recording cochlear microphonic potentials (CMPs) are described here. The CMPs were recorded in rats by means of pure tones (4,000, 2,000, 1,000, 500, and 250 Hz) and intraepidermic electrodes; the electrocochleography technique was avoided. An experimental design that included the use of a glutamatergic agonist (kainic acid [KA]) and an aminoglycoside antibiotic (kanamycin [KANA]) was carried out to demonstrate the origin of the recorded potential. Morphological studies showed that KA selectively eliminated the afferent type I dendrites of the spiral ganglion, while the administration of KANA resulted in the absence of outer hair cells. When CMPs were recorded after KA administration, no alterations were detected. In contrast, KANA administration resulted in the absence of any selective electrophysiological activity corresponding to CMPs. All these results were compared with the recording of the compound action potential of the eighth nerve obtained by electrocochleography. These findings and the great specificity of the reproduction of the sound stimulus confirm that the CMPs can be recorded by the new equipment.

Genetic factors in aminoglycoside toxicity

Ototoxicity is the major irreversible toxicity of aminoglycosides, and occurs both in a dose-dependent and idiosyncratic fashion. The idiosyncratic pathway is presumably due to genetic predispositions, and in 1993 we identified an inherited mutation that predisposes to aminoglycoside ototoxicity, the A1555G mutation in the mitochondrial 12S ribosomal RNA gene. Seventeen-33% of patients with aminoglycoside ototoxicity carry this mutation. In a search for additional susceptibility mutations, a dual strategy of yeast genetics and candidate genes was employed. Through yeast genetics 8 genes that by overexpression prevent aminoglycoside toxicity were identified. The human homologues of these genes may harbor aminoglycoside susceptibility mutations. Another candidate gene is the mitochondrial ribosomal protein S12, which interacts with the ribosomal RNA gene and in bacteria can harbor aminoglycoside resistance mutations. Analysis of this gene in 41 patients with aminoglycoside ototoxicity did not reveal any mutations in the coding regions. However, an Italian family with five maternally related family members who went deaf after aminoglycosides led to the identification of the second susceptibility mutation in the mitochondrial 12S ribosomal RNA gene, delta T961Cn. While these findings have immediate clinical implications for prevention of aminoglycoside-induced ototoxicity, they have not yet led to a clear understanding of the pathophysiology of aminoglycoside toxicity or the development of therapeutic options after the onset of symptoms.

Trigeminal ganglion innervation of the cochlea--a retrograde transport study

Innervation patterns of sensory nerves from the trigeminal ganglion to the cochlear blood vessels were studied using retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Guinea-pigs (n=7) were unilaterally implanted with an osmotic pump and a cannula for cochlear delivery of 2% or 20% wheat germ agglutinin horseradish peroxidase (Group 1), 2% wheat germ agglutinin-horseradish peroxidase followed by 100 micromol capsaicin (Group 2), or vehicle alone. Histological sections of the trigeminal ganglia, the C1 and C2 dorsal ganglia, the superior and inferior ganglia of the glossopharyngeal nerve bilaterally, the midbrain and the brainstem were obtained after 48 h of infusion. In Group 1, a large number of labeled nerve cell bodies were observed in the anteromedial portion of the trigeminal ganglion and at the origin of the ophthalmic nerve. Some labeled cells were also found on the lateral side of the ophthalmic nerve, as well as on the medial side of the maxillary nerve root. Capsaicin pretreatment significantly reduced the density of labeled neurons in the trigeminal ganglion. A few labeled neurons were also found in the trigeminal brainstem nucleus complex and in certain auditory brainstem nuclei. No wheat germ agglutinin horseradish peroxidase-positive cells were observed in the spinal C1 or C2 cervical ganglia or in the superior or inferior glossopharyngeal ganglia. In contrast, wheat germ agglutinin-horseradish peroxidase application to the middle ear resulted in labeled cells in the middle posterolateral portion of the trigeminal ganglia and in the superior ganglia of the glossopharyngeal nerve. These results provide the first direct evidence that the trigeminal ganglion sends projections to the cochlea.

Extracellular adenosine 5'-triphosphate (ATP) in the endolymphatic compartment influences cochlear function

There is strong evidence for the presence of P2 purinoceptors on cochlear tissues, but the role of extracellular ATP in cochlear function is still unclear. Our previous studies have determined the presence of ATP in the cochlear fluids and indicated that the purinoceptors are substantially localized to the tissues lining the endolymphatic compartment. This implies that extracellular ATP may have an humoral role confined to the endolymphatic space. In order to study the influence of extracellular ATP in the endolymphatic space, a series of studies were undertaken in which ATP (10 microM to 10 mM) in artificial endolymph (EL) (test solution: 2-12.5 nl) was injected into the scala media and the effect on the cochlear microphonic (CM) and endocochlear potential (EP) evaluated. A double-barrelled pipette, with one barrel containing the test solution and the other artificial EL (control solution) was inserted into scala media of the third turn of the guinea-pig cochlea. A known volume (2-12.5 nl) of test or control solution was then pressure-injected into the space. ATP had a significant dose-dependent suppressive effect on both EP and CM with a threshold of approximately 2 x 10(-14) mol; the response was readily reversible, also in a dose-dependent fashion. Artificial EL of the same volume had no effect on EP and CM. The ATP effect on EP was blocked by the P2 purinoceptor antagonists suramin and reactive blue 2 (RB2). Neither adenosine (2 x 10(-13) to 2 x 10(-11) mol) nor suramin or RB2 on their own had any effect on EP and CM. This study provides the first evidence for an effect of extracellular ATP in the endolymphatic compartment on cochlear function which is mediated via P2 purinoceptors. This provides supporting evidence for an humoral role for extracellular ATP in the modulation of cochlear function.

Rapid, reversible elimination of medial olivocochlear efferent function following single injections of gentamicin in the guinea pig

Measurements were made of the ability of nonsimultaneous, contralaterally presented broadband noise maskers to suppress ipsilaterally-evoked compound action potentials (CAPs) of the auditory nerve to 8-kHz tone pips in guinea pigs. Contralateral acoustic stimuli have been shown to suppress responses to ipsilateral stimuli through the action of the medial olivocochlear efferent bundle. In this study, maximum suppression across subjects ranged from 2-6 dB. The ability of the contralateral noise to suppress ipsilateral CAPs was rapidly reduced beginning within minutes of a single intramuscular injection of the aminoglycoside antibiotic gentamicin (150 mg/kg), and, in general, was completely abolished by 1.5-2 h. Changes in suppression were presumably produced by gentamicin-induced blockade of presynaptic calcium channels on the medial olivocochlear efferents and closely follow the pharmacokinetics of gentamicin in perilymph. Reductions in efferent-mediated suppression were observed in the absence of any measurable change in either CAP thresholds to different frequencies or CAP input/output. Suppression was partially restored in some animals after 24 h, and was fully recovered in all subjects within 96 h postinjection. The present studies suggest that measures of efferent function might form the rational basis for an objective means to assess aminoglycoside insult on the cochlea in the absence of hair cell damage. These results also indicate that single injections of gentamicin offer a reversible, non-invasive model in which medial efferent function can be studied in behavioral animals.

Neural processes in the dorsal cochlear nucleus of the anaesthetised cat investigated from unit responses to electrical stimulation of the auditory nerve

Extracellular responses of dorsal cochlear nucleus single units were recorded in response to biphasic, bipolar electrical stimulation of spiral ganglion cells and their peripheral processes using a banded electrode array in the scala tympani of the barbiturate anaesthetised cat. The DCN responses to this stimulus were the result of excitatory and suppressive (including inhibitory) processes. The excitatory responses from DCN units were usually within a range of 1.8-2.8 ms and these responses were probably the result of monosynaptic input from the auditory nerve. Latencies > 2.8 ms were most likely due to activation of di- and poly-synaptic pathways from auditory nerve fibres, except that latencies between 3.5-4.75 in hearing animals could have arisen from electrophonic mechanisms. Suppression of spontaneous activity was usually long acting, lasting > 70 ms following each pulse of the pulse train, but short acting suppression with a latency of 3.5-4.75 ms and a duration of < 10 ms was occasionally observed. These suppressive responses probably resulted from synaptic inhibitory input, but neural membrane properties may have contributed. In hearing animals, excitatory latencies within the range 1.8-5.2 ms were similar for units with different response area types or different PSTH patterns in response to acoustic CF tones or noise.

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.

Inhibition of the carbachol-evoked synthesis of inositol phosphates by ototoxic drugs in the rat cochlea

The ability of amikacin, neomycin, ethacrynate, mercuric chloride and cisplatin to alter the inositol phosphate (IP) signalling pathway was assessed in the 12-day-old rat cochlea, where the turnover of IPs is coupled to muscarinic receptors. This study was motivated by: (1) the demonstration of neomycin binding to phosphatidylinositol 4,5-biphosphate, the precursor of IPs, and (2) the fact that ototoxic drugs induce some common symptoms in outer hair cells. At concentrations below 1 mM, none of the compounds changed the control 3H-IP formation. Mercuric chloride, cisplatin and ethacrynate inhibited the carbachol-induced formation of IPs in a dose-dependent manner with IC50 values of 74,340 and 430 microM, respectively. The aminoglycosides were less efficient in reducing the carbachol-stimulated accumulation of IPs, since neither amikacin nor neomycin, both at 1 mM, had any significant effect. However, neomycin applied at 15 and 30 microM induced 29% and 43% of inhibition of the stimulated IP response. Finally, additive effects are obtained between some of the toxic drugs. The results suggest that a block of the IP transduction system, associated with the cholinergic efferent innervation of the organ of Corti, is a feature that may be involved in some types of ototoxicity. The inefficiency of aminoglycosides and the putative targets of the ototoxic agents are discussed.

Selective retrograde transport of nipecotic acid, a GABA analog, labels a subpopulation of gerbil olivocochlear neurons

Perfusion of the gerbil cochlea with micromolar quantities of 3H-gamma-aminobutyric acid (GABA) results in rapid, selective labeling of 50-60% of the olivocochlear (OC) efferent terminals on afferent dendrites beneath the inner hair cells, and all of the efferent terminals beneath the outer hair cells. In order to identify the neurons from which these GABA-accumulating terminals originate, the cell bodies were localized by using retrograde transport of 3H-nipecotic acid, a metabolically inert GABA analog. With survival times of 6-30 hours after cochlear injection, myelinated OC efferent fibers and cell bodies were well labeled, with the greatest number being labeled at 12-18 hours. All of the labeled neurons belonged to the medial OC system, and no lateral OC neurons were labeled. It is concluded that the GABA-accumulating endings in the gerbil cochlea arise from medial OC neurons, and therefore that medial OC efferent neurons in this species project to both inner and outer hair cell regions.

Chronic drug infusion into the scala tympani of the guinea pig cochlea

This research describes a unique, effective and inexpensive delivery system to provide discrete quantities of drugs on a chronic basis to the inner ear. The amount of the drug administered and specific timing of each administration are under investigator control. A micro-injection system mounted atop an animal's head is shown to permit repeated application of agents which effectively block neural responsiveness (tetrodotoxin) on a daily basis for periods up to 2 weeks. Cannulation of the inner ear and chronic delivery of control substances (artificial perilymph) do not affect function. This system may be used to administer drugs to other compartments of the body (e.g., the brain) on a chronic basis for neurophysiologic and neuropharmacologic investigations.

Cholinergic regulation of the phosphoinositide second messenger system in the guinea pig organ of Corti

The effect of cholinergic agents on the phosphoinositide second messenger system was investigated in the cochlea of the adult guinea pig in vivo and in vitro. In vivo, phospholipids were labeled with [32P]-orthophosphate by perilymphatic perfusion and their hydrolysis assayed in 'chase' experiments with non-radioactive orthophosphate. Carbachol (1 mM) reduced the content of 32P-labeled phosphatidylinositol 4,5-bisphosphate in the organ of Corti from 31% to 21% of total 32P-lipids, indicating stimulated hydrolysis. The pharmacology of this effect was studied in detail in vitro via the release of inositol phosphates from phosphoinositides pre-labeled with 3H-inositol. Release was increased 2-fold by 1 mM carbachol, 1.6-fold by 1 mM muscarine, but was unaffected by dimethylphenylpiperazinium; the stimulation was blocked by 1 microM atropine but not mecamylamine. These responses indicate the coupling of phosphoinositides to a muscarinic receptor. Furthermore, stimulated inositol phosphate release was higher in the base of the organ of Corti than in the apex which correlates with the increased cholinergic efferent innervation of outer hair cells in the basal region. These results suggest that muscarinic-stimulated inositol phosphate release occurs at the level of the outer hair cell and thus may have an important modulatory role in auditory transduction.

Cochlear permeability of neomycin and gentamicin: an immunohistochemical study

Aminoglycoside antibiotics (AGAs) target specifically the cochleo-vestibular hair cells, but with varied ototoxicity. Differences in their penetration and clearance rates into the membranous labyrinth may play a role. This in turn may be related to a difference in the number of amine groups, the cationic nature, as well as the molecular weight and size of the AGA molecule. Immunohistochemical labeling techniques were used to study the pathways of gentamicin and neomycin from the perilymph into cochlear tissues and target cells. The more cochleotoxic AGA, neomycin, penetrated into cochlear tissues faster than the less cochleotoxic AGA, gentamicin.

Differential susceptibility to gentamicin ototoxicity between albino and pigmented guinea pigs

The known chemical affinity of melanin pigment for aminoglycoside antibiotics has led to the suggestion that higher concentrations of these drugs will bind to the pigmented inner ear and produce greater ototoxicity compared to the nonpigmented albino cochlea. Although this has provided a compelling hypothesis, results from the few investigations to address this question have been equivocal. In the present study, cochlear microphonic (CM) thresholds were recorded from albino and pigmented guinea pigs both before and two weeks after exposure for 14 consecutive days to 100 mg/Kg gentamicin. Cochleae were dissected and half-turn segments prepared for surface examination of the organ of Corti. After gentamicin exposure, threshold shifts averaged a statistically reliable 33 dB in albinos and 19 dB for the pigmented animals. Anatomical studies revealed a significant 44% mean outer hair cell loss in albinos compared to a 21% loss in the pigmented inner ears. The results showed that albinos display greater ototoxicity from gentamicin than do pigmented guinea pigs. Aminoglycosides are known to exert toxicity through interaction with polyphosphoinositides found in high concentrations in the inner ear. Cochleae in both albino and pigmented animals appear to possess significant phospholipid concentrations and bind toxic levels of these drugs independent of inner ear pigment content. However, evidence showing that melanin can inhibit aminoglycoside activity in vitro suggests that, once these drugs bind to pigmented tissue, they may undergo inactivation in a manner unavailable to the nonpigmented albino cochlea. The present results are consistent with the possibility that cochlear melanin may inhibit gentamicin activity in vivo and decrease the severity of aminoglycoside ototoxicity in the pigmented inner ear.

Morphological features of five neuronal classes in the gerbil lateral superior olive

Five morphologically distinct classes of neurons can be identified within the neuropil of the gerbil lateral superior olivary nucleus (LSO) by using a variety of histological techniques and electron microscopy. The physical features of these five classes resemble those found in the cat LSO and are identified, by using criteria and nomenclature established for the cat, as principal neurons, multiplanar neurons, marginal neurons, small neurons, and class 5 neurons. Principal cells compose approximately 75% of the total LSO neuronal population. They possess a discoid dendritic organization and are oriented rostrocaudally, perpendicular to the transverse curvatures of the LSO. Roughly 8% of the LSO population is composed of multiplanar neurons, whose dendritic fields are not restricted to any single plane of section. Both principal and multiplanar neurons share similar cytoplasmic features, and greater than 65% of their perikaryal surface is in contact with synaptic terminals. Small neurons compose approximately 11% of the LSO neurons, have the lowest percentage of their somal surface contacted by synaptic terminals (approximately 8%), and are found mostly in the middle/medial portions of the LSO. Marginal neurons, which compose approximately 6% of the LSO population, appear similar to principal neurons at the light microscopic level except that they are found along the contours of the LSO, oriented orthogonal to principal neurons. Approximately 28% of the somal surface of marginal neurons is in contact with synaptic terminals. The class 5 neuronal somata receive a similar number of axosomatic synaptic contacts as marginal neurons (approximately 31%) but are found well within the matrix of the LSO, aligned parallel to principal neurons. Class 5 neurons share the same light microscopic features as principal neurons and can be identified electron microscopically based only on the reduced percentage of somal surface occupied by synaptic terminals.

Selective retrograde labeling of lateral olivocochlear neurons in the brainstem based on preferential uptake of 3H-D-aspartic acid in the cochlea

We have previously shown that perfusion of the gerbil cochlea with probe concentrations of 3H-D-aspartic acid (D-ASP) results in immediate, selective labeling of 50-60% of the efferent terminals under the inner hair cells, presumably by high-affinity uptake. The present study was undertaken to determine the origin of these endings. Twenty-four hours after cochlear perfusion with D-ASP, labeled neurons were observed in the ipsilateral, and to a much lesser extent in the contralateral, lateral superior olivary nucleus (LSO). The cells were small, primarily fusiform, and showed fewer synaptic contacts than other LSO cells. Combined transport of D-ASP and horseradish peroxidase indicated that all olivocochlear neurons within the LSO that projected to the injected cochlea were labeled by D-ASP. Labeled fibers coursed dorsally from the LSO, joined contralateral fibers that had passed under the floor of the fourth ventricle, and entered the VIIIth nerve root at its ventromedial edge. Adjacent to the ventral cochlear nucleus (VCN), densely labeled collateral fibers crossed the nerve root to enter the VCN. Labeled fibers and terminals were prominent in the central VCN. Neither retrograde transport of D-ASP by medial olivocochlear and vestibular efferents nor anterograde transport by VIIIth nerve afferents was observed. The D-ASP-labeled cells and fibers are clearly lateral olivocochlear efferents. Retrograde transport of D-ASP thus allows the cells, axons, and collaterals of the lateral olivocochlear system to be studied, morphologically, in isolation from other cells that project to the cochlea. Since the olivocochlear neurons are almost certainly cholinergic, retrograde amino acid transport does not necessarily identify the primary neurotransmitter of a neuron. Rather, it indicates the presence of selective uptake by the processes of that neuron at the site of amino acid injection. Retrograde labeling appears to be markedly enhanced by the use of metabolically inert compounds such as d-isomer amino acids.

Auditory brainstem response in young burn-wound patients treated with ototoxic drugs

Burn-wound patients often require potentially ototoxic doses of aminoglycoside drugs in the treatment of gram-negative sepsis. Cochlear hearing impairment may be an unfortunate consequence of this medical therapy. We evaluated auditory sensitivity with the auditory brainstem response (ABR) in a group of 32 children with acute, severe thermal burns ranging in age from 18 months to 17 years. The mean percent of total body surface area burns was 64%. None of the subjects had a known history of hearing deficits or aminoglycoside therapy, and all yielded a normal baseline ABR upon hospital admission. Eight of the subjects (22%) showed either an abnormal ABR, or no response, at 40 dB prior to hospital discharge. The medical treatment for this group of subjects (gentamicin, amikacin, vancomycin, amphotericin B) was compared to that of a second subgroup of 7 subjects without auditory deficit but with a statistically comparable percentage of burns. The mean dosage of vancomycin was higher for the auditory impairment group than for the unimpaired group. Prediction of ototoxicity in the acute burned patient is extremely difficult as there are numerous factors that may influence the risk of cochlear damage. We conclude, however, that the ABR can be applied in early detection of auditory deficit. Follow-up audiometric assessment is advisable since auditory deficits in this population may be delayed or progressive after discontinuance of drug therapy.

Ototoxicity of neomycin and its penetration through the round window membrane into the perilymph

The ototoxicity of neomycin and its concentration in the perilymph after direct application on the round window membrane were studied. After placing 5 mg of neomycin on the round window membrane of guinea pigs for various time intervals, concentration of the drug in the perilymph was determined by high performance liquid chromatography, and the cochlea was examined by light microscopy. Neomycin penetrated the round window membrane quite easily, and its concentration in the perilymph became extremely high in a short time and then decreased gradually. This indicates that high concentration of neomycin in the perilymph can be attained by application of a small amount of the drug on the round window membrane. Ototoxicity of neomycin was observed after application for 4 hours. Cochlear damage increased as neomycin application time became longer, but no consistent relationship was noted between the concentration of neomycin and the amount of damage. This result is discussed from the point of concentration and persistence of the drug in the inner ear fluids.

Regional differences of brain glucose metabolic compensation after unilateral labyrinthectomy in rats: a [14C]2-deoxyglucose study

A unilateral labyrinthectomy was performed on anesthetized adult albino rats. Brain [14C]2-deoxyglucose (2DG) uptake was measured autoradiographically 3.5 h to 20 days later and compared to sham-operated controls. In the vestibular nuclei (nn.) of labyrinthectomized subjects, large left-right differences of 2DG uptake occurred, which decreased over time. The equalization of vestibular nuclear 2DG uptake paralleled behavioral compensation of body, neck and head postural abnormalities, and known equalization of vestibular nuclear cell firing rates during compensation. There was a small difference of 2DG uptake in medial and lateral vestibular nn. 20 days after lesions when animals had a residual head tilt and tonic eye deviation. In the oculomotor nn., trochlear nn. and interstitial n. of Cajal, large left-right differences of 2DG uptake occurred, which did not change over time. The higher 2DG uptake in these nn. occurred ipsilateral to the labyrinthine lesion and did not correlate with the onset and cessation of nystagmus. The persistent asymmetry did appear to correlate with ipsilateral downward and contralateral upward eye deviation which continued for long periods after the lesion. We hypothesize that the non-compensating metabolic asymmetry in the oculomotor and trochlear nn. could be due to lesioned otolithic input to the vestibular nn. which relays to trochlear and oculomotor nn.

Effects of noise and ototoxic drugs at the cellular level in the cochlea: a review

Currently available information concerning the cellular mechanisms involved in acoustic trauma and aminoglycoside ototoxicity is reviewed to shed some new light on the cellular events that may be related to functional impairment of the auditory organ. Based on the available data, the following postulations can be made concerning the cellular mechanisms involved. 1) The macromolecular disruption of the stereocilia and cuticular plates is the initial cellular event in acoustic trauma. This disruption would affect the micromechanics of the transduction process, leading to temporary threshold shift. Further cellular impairment would involve basic cellular functions such as the protein, lipid, and glucose synthesis needed for cell repair and survival, and such impairment would result in permanent cell injury or cell death, leading to permanent threshold shift. 2) It can be postulated that the cellular mechanisms involved in aminoglycoside ototoxicity include two events. The early event is the reversible blockage of the transduction channels from the endolymph side of the hair cells. The later event is the interference in such cellular functions as protein and/or phospholipid synthesis because of binding of aminoglycoside to the phospholipids and/or protein, leading to cell death. The latter event may be facilitated by penetration or membrane-mediated internalization of the aminoglycoside from the perilymph side of the hair cell.

Distribution of gentamicin by immunofluorescence in the guinea pig inner ear

We studied the distribution of gentamicin in the inner ear, brain and kidney of the guinea pig following intraperitoneal administration or perfusion of gentamicin through the perilymphatic space. The resulting histopathological changes were examined by immunofluorescence using antigentamicin antiserum. After perfusion of gentamicin through the perilymphatic space, specific fluorescence was found in the cochlea, and was especially prominent in the outer hair cells, basilar membrane and basilar crest. Although no fluorescence was observed in the cochlea following intraperitoneal administration of high doses of gentamicin, type I hair cells in the vestibule were seen to be selectively stained with the antibody. Furthermore, some of the vestibular ganglion cells, Purkinje cells and unidentified nuclei in the brain stem were also stained. In particular, fine granules showing relatively intense fluorescence were recognized in the cytoplasm of the stained cells. In the cortex of kidney, only proximal tubular cells were stained with intense fluorescence. Our results suggest that the aminoglycoside antibiotics have two sites of action: one is the cell membrane of the sensory hair cells and the other is the cytoplasm.

Structural abnormalities in the stria vascularis following chronic gentamicin treatment

Freeze-fracture and thin-sectioning have been used to examine the stria vascularis of albino guinea pigs chronically treated with gentamicin. Immediately following the end of treatment, most marginal cells showed lipid bodies in the cell body region and freeze-fracture revealed alterations to the marginal cell plasma membrane. Intermediate cells also showed peculiarities including a dilation of the rough endoplasmic reticulum. A co-incidence was noted in the location in the cochlea in which effects in the stria and in outer hair cells occurred. At 4 weeks post-treatment, the stria was significantly thinner than normal and appeared less structurally complex. A minority of marginal cells degenerated. Some morphological features associated with degeneration resembled those of apoptosis, a process of controlled, cellular self-destruction. There were also indications of turnover of gap-junctions throughout the post-treatment period examined. The results indicate significant ototoxic effects of gentamicin occur in the stria and that changes to plasma membranes are one of the initial alterations.

Tuning characteristics of cochlear nucleus units in response to electrical stimulation of the cochlea

The responses of single units in the ventral cochlear nucleus of acute anesthetized guinea pigs were studied with continuous sinusoidal electrical stimuli presented through a multi-electrode implant in the scala tympani. Implants had two or four electrodes along the axis of the scala with 1 mm separations. Best frequencies were consistently in the 100 Hz range (50-250 Hz) with thresholds of about 0.063 mA peak-to-peak. Tuning curves were usually symmetrical with slopes of 3-4 dB/octave, both below and above the best frequency. The relative sharpness of the tuning curves, as measured by Q10dB, averaged 0.2. Dynamic ranges as determined by the intensity-rate functions for the various frequencies were 2-15 dB. No significant difference was found between tuning characteristics of units in response to stimulation via the apical or basal pair of implant electrodes. The findings suggest some limitations on the applicability of independent stimulating channels in multi-electrode implants.

Effects of perilymphatically perfused gentamicin on microphonic potential, lipid labeling and morphology of cochlear tissues

The perilymphatic space of the guinea pig was perfused in situ with artificial perilymph containing [32P] orthophosphate and 3H-glycerol while cochlear microphonic potentials (CM) were monitored. The perfusion of 10 mM gentamicin suppressed CM by 62% within 30 min. 32P-incorporation into phosphatidylinositol bisphosphate in the organ of Corti was decreased but distribution of 3H-glycerol in neutral lipids and phospholipids was not affected by gentamicin. The morphological evaluation included all structures lining the scala media, the spiral ligament, the basal membrane and the vestibular organs. Hair cells sometimes contained small intracellular 'precipitation' lines but, in general, ultrastructure of all tissues was essentially normal and no overt histopathological changes were evident. The observed biochemical changes should, therefore, represent early and primary actions of the drug. This confirms the hypothesis that an interaction with the acidic phospholipid, phosphatidylinositol bisphosphate, is an important step in the ototoxic mechanism of amino-glycoside antibiotics.

Uptake and transport of horseradish peroxidase by cochlear spiral ganglion neurons

Cochlear spiral ganglion neurons in the cat have been classified as type I and type II. The type II neurons are of special interest since they are reported to provide the afferent innervation to three-quarters of the cochlear hair cells, and also because of a recent assertion that they do not project to the brain. In the present study the neuronal marker horseradish peroxidase (HRP) was utilized to re-examine these issues. HRP was injected into the intranuclear cochlear nerve root of adult cats. 18 to 48 h post-injection HRP reaction product was observed in both type I and type II neurons. The majority of labeled cells were characterized by granular reaction product, while a smaller number of cells were also diffusely filled with HRP, resulting in intense staining of the cell soma and its processes. These data indicate that the central axons of type II neurons are constituents of the cochlear nerve root at the level of the brainstem and thus do project centrally. In the organ of Corti HRP-filled fibers to both inner and outer hair cells were observed and demonstrated in electron microscopy to be afferent neurons. This intra-axonal accumulation of the HRP by outer hair cell afferents constitutes direct evidence of the functional projection of these neurons to the cochlear nucleus.

Acute perilymphatic perfusion of the guinea pig cochlea

A method for the continuous perfusion of the perilymphatic space of the inner ear in the guinea pig is described. Artificial perilymph is supplied to the cochlea and drained away through a tubing system while flow rates from 10 microliters/min to 0.3 ml/min are established by gravity syphon pressure. Techniques are also presented which allow control over the temperature of the perfusate and over the level of dissolved oxygen in the perfusate. Alone with these variables, the pH of the artificial perilymph can be manipulated and various drugs can be added to the perfusate to test their effect on the inner ear. The function of the inner ear is monitored by continuous recording of the sound evoked bioelectric potentials, the cochlear microphonic and the compound action potential. The cochlear perfusion technique has many applications in the study of cochlear physiology and metabolism, and in testing the sensitivity of the inner ear to ototoxic drugs.

Pure-tone and speech intelligibility disturbances in patients with ototoxic disorders

The prevalence and severity of hearing impairment caused by ototoxic drugs are surprisingly high. This emphasizes the importance of the questions arising from the treatment. In 33.7% of the cases, the hearing impairment caused by ototoxic antibiotics was of severe degree and in 25.4%, it was extremely severe. Parenteral, topical or oral administration of aminioglycoside antibiotics is dangerous. Because of the very poor speech intelligibility, most probably not only the spiral organ but the vestibulocochlear nerve and the higher auditory pathways are also affected by these antibiotics. In some cases, the severe distortion in sound perception cannot be compensated even by a hearing aid of the best quality, and lip-reading which was advised occasionally was without any result. To prevent these toxic effects, these drugs should be administered very parsimoniously and then under very strict conditions and close control.

Are aminosugars ototoxic?

The ototoxicity of the kanamycin fragment 3-amino-3- deoxy-D-glucose (3-aminoglucose, kanosamine), was investigated by perilymphatic perfusion in the guinea pig. Concentrations of 10 or 28 mM of this compound had no effect on cochlear microphonic potentials (CM), contrasting with previous observations (Owada 1962). Kanamycin at 1 mM decreased CM significantly under otherwise identical conditions. The action of kanamycin was at least partially reversible.

The sensitivity of cochlear albino melanocytes to horseradish peroxidase

The effects of low concentrations (0.5%--0.0005%) of HRP and varying postperfusion survival times (5.5 hours--15 seconds) on cochlear albino melanocytes were studied by light and electron microscopy. Approximately 50 microliter of differing concentrations of HRP dissolved in artificial perilymph were perfused through the perilymphatic space in 32 albino guinea pigs. Tissues were subsequently treated histochemically to demonstrate HRP reaction product. The results indicate that concentrations of HRP > 0.0005% in combination with postperfusion survival times > 1 hour bring about acute and selective injury to the albino melanocytes; cytoplasmic vacuolization and rupture of cell membranes occurred. The structural integrity of the albino melanocytes appeared unaltered when exposed to 0.0005% concentrations of HRP for less than or equal to 1 hour. An apparently non-toxic 0.0005% concentration of HRP may provide a means of labeling cochlear melanocytes when the effects of other substances on the cells are investigated.

Effects of kanamycin on the auditory evoked responses during postnatal development of the hearing of the rat

The ototoxic effects of kanamycin were studied in rats during the early postnatal period and at an adult age. Brain stem potentials as well as auditory cortical potentials were used for the estimating of ototoxic damage. The auditory potentials decreased promptly and markedly in the animals which were treated daily with 400 mg/kg body weight of kanamycin starting from the 11th day after birth. In these animals, the auditory potentials were almost completely abolished within 10 days after the beginning of the kanamycin treatment. However, when the same amount of kanamycin was applied earlier or later than that, i.e., avoiding the period of the initial appearance and the greatest development of auditory functions (from the 11th to the 15th day after birth in the rat), the auditory potentials were not apparently damaged. In light and scanning electronmicroscopy, marked ototoxic changes were observed which underlay the functional damage. The meaning of these findings is discussed.

Horseradish peroxidase acute ototoxicity and the uptake and movement of the peroxidase in the auditory system of the guinea pig

When guinea pig cochlea was perfused in vivo with a solution of 1% horseradish peroxidase (HRP) in artificial perilymph, the enzyme was found in the basilar membrane, spiral limbus, some outer and inner hair cells and some supporting cells and it was gradually cleared away with time. Acute signs of cell damage included swelling, vacuolization and diffuse labeling of some hair cells, but stereocilia remained normal in configuration. Albino melanocytes of the spiral ligament were also damaged, and vacuolization of Reissner's membrane occurred after 10% HRP. Both concentrations caused a gradual decline in CM, showing that HRP is acutely ototoxic but its mode of action is unknown. No retrograde transport of HRP to spiral ganglion cells or to brain stem neurons occurred, but some brain stem neurons took up HRP from the neuropil following diffusion from the cochlea.

Radioactive labeling of phospholipids and proteins by cochlear perfusion in the guinea pig and the effect of neomycin

Phospholipids and proteins of guinea pig stria vascularis, spiral ligament and organ of Corti were radioactively labeled by perilymphatic perfusion with artificial perilymph containing [32P] orthophosphate or radioactive amino acids. Phospholipids were separated by thin-layer chromatography, proteins by disc gel electrophoresis and quantitated by liquid scintillation counting. The addition of 10-4M to 10-2M neomycin to the perfusion fluid resulted in a dose-dependent increase of tissue permeability to the radioactive precursors, and a specific decrease in the 32P-incorporation into phosphatidylinositol diphosphate in stria vascularis and organ of Corti. No effect of neomycin on protein labeling was observed using a double label approach with [3H]methionine and [35S]-methionine. In vitro, low concentrations of neomycin led to the formation of a complex and polyphosphoinositides. Much higher concentrations of the drug were needed for a comparable reaction with the acid mucopolysaccharide, chondroitin sulfate A. The implications of these findings for the mechanism of neomycin ototoxicity are discussed.