Effects of Calcitonin-Gene-Related-Peptide on Auditory Nerve Activity.

Calcitonin-gene-related peptide (CGRP) is a lateral olivocochlear (LOC) efferent neurotransmitter. Depression of sound-driven auditory brainstem response amplitude in CGRP-null mice suggests the potential for endogenous CGRP release to upregulate spontaneous and/or sound-driven auditory nerve (AN) activity. We chronically infused CGRP into the guinea pig cochlea and evaluated changes in AN activity as well as outer hair cell (OHC) function. The amplitude of both round window noise (a measure of ensemble spontaneous activity) and the synchronous whole-nerve response to sound (compound action potential, CAP) were enhanced. Lack of change in both onset adaptation and steady state amplitude of sound-evoked distortion product otoacoustic emission (DPOAE) responses indicated CGRP had no effect on OHCs, suggesting the origin of the observed changes was neural. Combined with results from the CGRP-null mice, these results appear to confirm that endogenous CGRP enhances auditory nerve activity when released by the LOC neurons. However, infusion of the CGRP receptor antagonist CGRP (8-37) did not reliably influence spontaneous or sound-driven AN activity, or OHC function, results that contrast with the decreased ABR amplitude measured in CGRP-null mice.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity.

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

Dynorphin release by the lateral olivocochlear efferents may inhibit auditory nerve activity: a cochlear drug delivery study.

Dynorphin (dyn) is suggested to excite the auditory nerve (AN) when released by the lateral olivocochlear (LOC) efferents. However, previous studies evaluated either intravenously delivered dyn-like agents, raising the potential for systemic (central) effects, or agent concentrations unlikely to be achieved via endogenous cochlear release. This study tested the hypothesis that biologically relevant increases in dyn levels in the cochlea achieved via diffusion of the drug of (-)pentazocine across the round window membrane enhances AN firing. In general, amplitude of the cochlear whole-nerve action potential (CAP) was depressed following drug application. These results suggest that dyn released by the LOC neurons would likely act as an inhibitory transmitter substance in the LOC system; neurotransmission is one of the LOC system's vast unknowns.

Ventral cochlear nucleus responses to contralateral sound are mediated by commissural and olivocochlear pathways.

In the normal guinea pig, contralateral sound inhibits more than a third of ventral cochlear nucleus (VCN) neurons but excites <4% of these neurons. However, unilateral conductive hearing loss (CHL) and cochlear ablation (CA) result in a major enhancement of contralateral excitation. The response properties of the contralateral excitation produced by CHL and CA are similar, suggesting similar pathways are involved for both types of hearing loss. Here we used the neurotoxin melittin to test the hypothesis that this "compensatory" contralateral excitation is mediated either by direct glutamatergic CN-commissural projections or by cholinergic neurons of the olivocochlear bundle (OCB) that send collaterals to the VCN. Unit responses were recorded from the left VCN of anesthetized, unilaterally deafened guinea pigs (CHL via ossicular disruption, or CA via mechanical destruction). Neural responses were obtained with 16-channel electrodes to enable simultaneous data collection from a large number of single- and multiunits in response to ipsi- and contralateral tone burst and noise stimuli. Lesions of each pathway had differential effects on the contralateral excitation. We conclude that contralateral excitation has a fast and a slow component. The fast excitation is likely mediated by glutamatergic neurons located in medial regions of VCN that send their commissural axons to the other CN via the dorsal/intermediate acoustic striae. The slow component is likely mediated by the OCB collateral projections to the CN. Commissural neurons that leave the CN via the trapezoid body are an additional source of fast, contralateral excitation.

Asphyxia and depolarization increase adenosine levels in perilymph.

Extracellular adenosine has been suggested as a modulator of cochlear function. To date the release of adenosine into the extracellular spaces of the cochlea has not been demonstrated. Therefore, experiments were designed to examine whether adenosine release into perilymph could be detected in response to depolarization by high potassium concentrations or in response to asphyxia. For this purpose, the perilymph compartment of guinea pigs was perfused with an artificial perilymph and the effluent assayed for ATP, ADP, AMP and adenosine. Results indicate that potassium induced a slight, significant increase and asphyxia induced a very large, significant increase in adenosine levels in perilymph effluent. No changes in the levels of the other compounds were measured. It is concluded that depolarization and asphyxia can induce the release of adenosine into perilymph.

Disruption of lateral olivocochlear neurons via a dopaminergic neurotoxin depresses sound-evoked auditory nerve activity.

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.

Chronic excitotoxicity in the guinea pig cochlea induces temporary functional deficits without disrupting otoacoustic emissions.

Brief cochlear excitotoxicity produces temporary neural swelling and transient deficits in auditory sensitivity; however, the consequences of long-lasting excitotoxic insult have not been tested. Chronic intra-cochlear infusion of the glutamate agonist AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) resulted in functional deficits in the sound-evoked auditory brainstem response, as well as in behavioral measures of hearing. The electrophysiological deficits were similar to those observed following acute infusion of AMPA into the cochlea; however, the concentration-response curve was significantly shifted as a consequence of the slower infusion rate used with chronic cochlear administration. As observed following acute excitotoxic insult, complete functional recovery was evident within 7 days of discontinuing the AMPA infusion. Distortion product otoacoustic emissions were not affected by chronic AMPA infusion, suggesting that trauma to outer hair cells did not contribute to AMPA-induced deficits in acoustic sensitivity. Results from the current experiment address the permanence of deficits induced by chronic (14 day) excitotoxic insult as well as deficits in psychophysical detection of longer duration acoustic signals.

Characterization of implantable microfabricated fluid delivery devices.

The formal characterization of the performance of microfluidic delivery devices is crucial for reliable in vivo application. A comprehensive laboratory technique was developed and used to optimize, calibrate and validate microfabricated fluid delivery devices. In vivo experiments were carried out to verify the accuracy and reliability of the pressure driven devices. Acute guinea pig experiments were conducted to measure the response to alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid, an excitatory neurotransmitter, at multiple locations in the inferior colliculus. A nondimensional parameter, Q, was successfully used to classify devices in terms of geometry alone (i.e., independent of fluid properties). Functional devices exhibited long-term linearity and reliability in delivering single phase, Newtonian fluids, in discrete volumes with a resolution of 500 picoliters at less than 0.45 lbf/in2 (30 mbar) pressure drop. Results for non-Newtonian fluids are not presented here. The acute results showed a proportional increase in the firing rate for delivered volumes of 2 nL up to 10 nL (at rates of between 0.1 and 1 nL/s). Flow characteristics are maintained during acute experiments and post-implant. A control experiment conducted with Ringer solution produced negligible effects, suggesting the results to be truly pharmacological. The experimental techniques employed have proven to be reliable and will be used for future calibration and testing of next generation chronic microfluidic delivery devices.

Disruption of lateral efferent pathways: functional changes in auditory evoked responses.

The functional consequences of selectively lesioning the lateral olivocochlear efferent system in guinea pigs were studied. The lateral superior olive (LSO) contains the cell bodies of lateral olivocochlear neurons. Melittin, a cytotoxic chemical, was injected into the brain stem using stereotaxic coordinates and near-field evoked potentials to target the LSO. Brain stem histology revealed discrete damage to the LSO following the injections. Functional consequences of this damage were reflected in depressed amplitude of the compound action potential of the eighth nerve (CAP) following the lesion. Threshold sensitivity and N1 latencies were relatively unchanged. Onset adaptation of the cubic distortion product otoacoustic emission (DPOAE) was evident, suggesting a reasonably intact medial efferent system. The present results provide the first report of functional changes induced by isolated manipulation of the lateral efferent pathway. They also confirm the suggestion that changes in single-unit auditory nerve activity after cutting the olivocochlear bundle are probably a consequence of disrupting the more lateral of the two olivocochlear efferent pathways.