The increase in the degree of neural forward masking of cochlea following salicylate application

Evidence That Ultrafast Nonquantal Transmission Underlies Synchronized Vestibular Action Potential Generation

Amniotes evolved a unique postsynaptic terminal in the inner ear vestibular organs called the calyx that receives both quantal and nonquantal (NQ) synaptic inputs from Type I sensory hair cells. The nonquantal synaptic current includes an ultrafast component that has been hypothesized to underlie the exceptionally high synchronization index (vector strength) of vestibular afferent neurons in response to sound and vibration. Here, we present three lines of evidence supporting the hypothesis that nonquantal transmission is responsible for synchronized vestibular action potentials of short latency in the guinea pig utricle of either sex. First, synchronized vestibular nerve responses are unchanged after administration of the AMPA receptor antagonist CNQX, while auditory nerve responses are completely abolished. Second, stimulus evoked vestibular nerve compound action potentials (vCAP) are shown to occur without measurable synaptic delay and three times shorter than the latency of auditory nerve compound action potentials (cCAP), relative to the generation of extracellular receptor potentials. Third, paired-pulse stimuli designed to deplete the readily releasable pool (RRP) of synaptic vesicles in hair cells reveal forward masking in guinea pig auditory cCAPs, but a complete lack of forward masking in vestibular vCAPs. Results support the conclusion that the fast component of nonquantal transmission at calyceal synapses is indefatigable and responsible for ultrafast responses of vestibular organs evoked by transient stimuli.SIGNIFICANCE STATEMENT The mammalian vestibular system drives some of the fastest reflex pathways in the nervous system, ensuring stable gaze and postural control for locomotion on land. To achieve this, terrestrial amniotes evolved a large, unique calyx afferent terminal which completely envelopes one or more presynaptic vestibular hair cells, which transmits mechanosensory signals mediated by quantal and nonquantal (NQ) synaptic transmission. We present several lines of evidence in the guinea pig which reveals the most sensitive vestibular afferents are remarkably fast, much faster than their auditory nerve counterparts. Here, we present neurophysiological and pharmacological evidence that demonstrates this vestibular speed advantage arises from ultrafast NQ electrical synaptic transmission from Type I hair cells to their calyx partners.

Effect of antiepileptic drug levetiracetam on cochlear function

BACKGROUND

Levetiracetam (LEV, 5-100 mg/kg) has been shown to prevent audiogenic seizures in a dose-dependent manner. This chemical is known to bind to synaptic vesicle protein 2A and inhibit l-type calcium channels, affecting neurotransmitter release. We hypothesize that the drug prevents audiogenic seizures partially by affecting cochlear neural response.

METHODS

To test this hypothesis, rats were given 1000, 500, 50, or 0 mg/kg (saline control) LEV-injection. Distortion product otoacoustic emissions (DPOAE), reflecting outer hair cell (OHC) function, and cochlear compound action potentials (CAP), reflecting cochlear neural output, were recorded and compared pre- and post-LEV.

RESULTS

1000 mg/kg LEV-injection did not significantly affect DPOAE. The high dose LEV-injection, however, significantly reduced CAP amplitude resulting threshold shift (TS), prolonged CAP latency, and enhanced CAP forward masking. CAP latency and forward masking were significantly affected at the 500 mg/kg dose, but CAP-TS remained unchanged after LEV-injection. Interestingly, CAP latency wassignificantly prolonged, at least at the low stimulation levels, although the amplitude of CAP remained constant after a clinical dose of LEV-injection (50 mg/kg).

DISCUSSION

Since the clinical dose of LEV-injection does not reduce CAP amplitude, the reduction of cochlear neural output is unlikely to be the underlying mechanism of LEV in the treatment of audiogenic seizure. The delayed cochlear neural response may be partially related to the prevention of audiogenic seizure. However, neuropharmacological changes in the central nervous system must play a major role in the treatment of audiogenic seizure, as it does in the treatment of focal epilepsy.