Stronger efferent suppression of cochlear neural potentials by contralateral acoustic stimulation in awake than in anesthetized chinchilla

Abstract

There are two types of sensory cells in the mammalian cochlea, inner hair cells, which make synaptic contact with auditory-nerve afferent fibers, and outer hair cells that are innervated by crossed and uncrossed medial olivocochlear (MOC) efferent fibers. Contralateral acoustic stimulation activates the uncrossed efferent MOC fibers reducing cochlear neural responses, thus modifying the input to the central auditory system. The chinchilla, among all studied mammals, displays the lowest percentage of uncrossed MOC fibers raising questions about the strength and frequency distribution of the contralateral-sound effect in this species. On the other hand, MOC effects on cochlear sensitivity have been mainly studied in anesthetized animals and since the MOC-neuron activity depends on the level of anesthesia, it is important to assess the influence of anesthesia in the strength of efferent effects. Seven adult chinchillas (Chinchilla laniger) were chronically implanted with round-window electrodes in both cochleae. We compared the effect of contralateral sound in awake and anesthetized condition. Compound action potentials (CAP) and cochlear microphonics (CM) were measured in the ipsilateral cochlea in response to tones in absence and presence of contralateral sound. Control measurements performed after middle-ear muscles section in one animal discarded any possible middle-ear reflex activation. Contralateral sound produced CAP amplitude reductions in all chinchillas, with suppression effects greater by about 1–3 dB in awake than in anesthetized animals. In contrast, CM amplitude increases of up to 1.9 dB were found in only three awake chinchillas. In both conditions the strongest efferent effects were produced by contralateral tones at frequencies equal or close to those of ipsilateral tones. Contralateral CAP suppressions for 1–6 kHz ipsilateral tones corresponded to a span of uncrossed MOC fiber innervation reaching at least the central third of the chinchilla cochlea.

Efferent modulation of pre-neural and neural distortion products

“…Given the periodic nature of our probe stimuli, it is also possible that our FFRs included contributions from cochlear microphonics, especially at higher frequencies. Our few observations of FFR enhancement with CAS at higher probe frequencies are consistent with reports that the cochlear microphonic amplitude grows with MOC reflex activation (e.g., Fex, 1959; Aedo et al, 2015). In pilot work for this experiment, we observed that forward masking obliterated F1-FFR, F2-FFR, and CDP-FFR components in four participants, suggesting a neural origin.…”

Noise-induced cochlear synaptopathy in rhesus monkeys ( Macaca mulatta )

“…It is unlikely that this resilience is mediated by the middle-ear muscle reflex (MEMR) or medial olivocochlear reflex (MOCR), based on the observations that central anesthetics attenuate the strength of both reflexes ( MEMR : Borg et al, 1975; Valero et al, 2017; MOCR : Chambers et al, 2013; Aedo et al, 2015). Such resistance might arise from the mechanical strength of the reticular lamina and the tight junctions that also provide the diffusion barrier between endolymph and perilymphatic scalae.…”

Reflex Modification Audiometry Reveals Dual Roles for Olivocochlear Neurotransmission

“…Interestingly, Longenecker and Lobarinas have both found PPI thresholds (assessed in quiet) were similar to ABR wave 1 amplitude losses when animals were noise-exposed; yet found PPI thresholds remained stable, whereas ABR thresholds recovered (Longenecker and Galazyuk, 2016 ; Longenecker et al, 2016 ; Lobarinas et al, 2017 ). Moreover, as PPI testing does not involve anesthesia, efferent feedback is maximized (Maison et al, 2012 ; Aedo et al, 2015 ).…”