Benzodiazepines alter cochleo-cochlear loop in humans

Functional role of GABAergic innervation of the cochlea: phenotypic analysis of mice lacking GABA(A) receptor subunits alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta

The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABA(A) receptor subunit (alpha1, alpha2, alpha5, alpha6, beta2, beta3, or delta). Four of the lines (alpha1, alpha2, alpha6, and delta) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (alpha5, beta2, and beta3) showed threshold elevations indicative of outer hair cell dysfunction. Alpha5 and beta2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in alpha5 (>6 weeks) than beta2 (<6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (beta2 and beta3) showed signs of neuropathy, either decreased density of afferent innervation (beta3) or decreased neural responses without concomitant attenuation of hair cell responses (beta2). One of the lines (beta2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.

Spectrum of neural electrical activity in guinea pig cochlea: Effects of anaesthesia regimen, body temperature and ambient noise

Spectral analysis of electric noise recorded from the round window of the cochlea is thought to represent the summed spontaneous activity of the auditory nerve. It has been postulated that it could provide a possible tinnitus index. Because experimental conditions could change this neural activity, the effect of anaesthesia regimen, body temperature and ambient noise on the spectrum of spontaneous neural noise (SNN) were investigated in guinea pig cochlea. SNN was studied in awake guinea pigs and after anaesthesia with pentobarbital (P), xylazine/ketamine (XK) or xylazine/tiletamine-zolazepam (XTZ). Body temperature varied gradually from 33 to 41 degrees C under XK regimen. In awake animals, broadband noise was generated with intensity varying from 0 to 50 dB. The SNN consisted in a broad peak at approximately 900 Hz. With ambient broadband noise, it increased exponentially with the sound level with no shift in frequency. Soon after anaesthetic induction, the lowest frequencies were constantly decreased, and gradually the 900 Hz peak either increased moderately (P) or dropped steeply (XTZ) or remained unchanged (XK). Peak frequency increased linearly with body temperature whereas the amplitude reached a maximum at around 39.5 degrees C. In conclusion, these data indicate that experimental conditions such as anaesthesia regimen, body temperature and ambient noise modify the spontaneous neural outflow of the cochlea and must be taken into account when studying SNN.

Lateralization of the effects of the benzodiazepine drug oxazepam on medial olivocochlear system activity in humans

Benzodiazepines (Bzd) are known to interact with GABAergic inhibitory neurotransmission. Previous research on their effect on human auditory efferent pathways--through evoked otoacoustic emissions suppression by contralateral acoustic stimulation (CAS)--indicated a decrease in medial olivocochlear (MOC) efferent system inhibitory activity, after oral intake of oxazepam--representative of the Bzd drug class. To date, this pharmacological effect was only assessed in the right ear. Since a leftward asymmetry of Bzd receptors localization in human auditory cortex has been described recently, we explored in this study the hypothesis of an asymmetrical action of Bzd on MOC efferent functioning. The results revealed a significant difference of Bzd effect probing the right ear versus the left ear, with CAS-induced suppression being less effective in the right than left ear after oxazepam intake. This finding raises the question of possible neurochemical left-right asymmetry in the descending auditory pathways. The potential localization of this asymmetry is discussed.

Isoflurane increases amplitude and incidence of evoked and spontaneous otoacoustic emissions

The volatile anesthetic isoflurane was tested for its effect on cochlear function by means of measuring distortion product otoacoustic emissions (DPOAE) and spontaneous otoacoustic emissions (SOAE) in the mustached bat (Pteronotus parnellii parnellii). Averaged growth functions of DPOAE and spontaneous otoacoustic emissions were assessed and compared between the control group (no isoflurane application) and the isoflurane group (application of isoflurane at vaporizer settings sof about 1.5-2%). Isoflurane significantly increases the DPOAE amplitude, e.g. at a primary tone level l2 of 40 dB SPL by 10.7 dB. Additionally, the incidence of SOAEs was highly increased during application of isoflurane. The sound-evoked efferent effect on the generation of otoacoustic emissions was significantly reduced in the isoflurane group. We suggest that isoflurane might affect the postsynaptic action of acetylcholine (ACh) released by the efferent terminals of outer hair cells (OHCs). This could lead to the observed decrease of efferent suppression and to a disinhibition of cochlear amplification.

Medial olivocochlear efferent activity in awake guinea pigs

Cochlear outer hair cells receive numerous connections from the medial olivocochlear efferent neurons. Medial olivocochlear efferent activity is highly dependent on the level of anesthesia. The present study was thus designed to investigate the efficiency of contralateral white noise stimulation on the distortion product otoacoustic emissions (DPOAEs) in a large number of awake guinea pigs, and to compare in the same animals the effect of urethane- and pentobarbitone-anesthesia. The monitoring of DPOAEs during contralateral white noise stimulation in awake animals requires the development of a soft restraining box, together with a conditioning technique for the animals to accept the contralateral sound and DPOAEs monitoring device. This technique allows us to demonstrate that contralateral sound suppression is much stronger in awake than in anesthetized animals. In all the cases, the contralateral sound suppression was abolished 3 h after i.m. injection of gentamicin, an aminoglycoside antibiotic which blocks the medial olivocochlear efferents. These results suggest that future studies have to explore the function of medial olivocochlear efferents in awake animals.

Influence of benzodiazepines on auditory perception

The aim of this study was to test for an influence of benzodiazepine (BZD) on various perceptual and/or cognitive auditory processes. Loudness, auditory selective attention, and the ability of subjects to form perceptual streams out of alternating tone sequences were tested. Nine subjects were tested before, 1, 3, 7, and 24 h after a single-dose oxazepam vs placebo administration in a crossover design. A sample of blood allows us to measure plasma oxazepam concentration. The results revealed a significant reduction in stream segregation expressed as d' scores 1 h after oxazepam intake in the test subjects. No significant change occurred across time in the same subjects when they were administrated a placebo in another session. Furthermore, oxazepam had no substantial and systematic influence either on auditory selective attention or on loudness perception. Altogether, these results suggest that the perceptual organization of sound sequences involves inhibitory neural mechanisms, which can be affected by BZDs. This outcome is consistent with existing models of auditory stream segregation and may be paralleled with earlier findings on the effect of BZDs on perceptual binding in the visual modality.

The influence of hypothermia on outer hair cells of the cochlea and its efferents

Transient evoked otoacoustic emissions (TEOAE) were recorded in 21 guinea-pigs undergoing hypothermia. The minimal average body temperature during cooling was 26 degrees C/24.9 degrees C measured orally or rectally, respectively. The animals were subsequently warmed to normal body temperature. A clear influence of body temperature on TEOAE could be documented. During cooling the amplitude and reproducibilities decreased, disappearing completely at a mean temperature below 28.5 degrees C (oral) and 27.3 degrees C (rectal). The emissions reappeared during rewarming at a mean temperature of 30.1 degrees C (oral) and 30.8 degrees C (rectal). Contralateral auditory stimulation (CAS) led to a decrease of the amplitudes of TEOAE during cooling down to a mean of 33 degrees C/32 degrees C (oral/rectal temperature). During rewarming, influences of the CAS could be recognized, again at an oral temperature above 35 degrees C. The changes to the TEOAE observed in these experiments suggest that hypothermia affects not only the outer hair cells (OHC) of the cochlea but also the efferent supply to the cochlea.