Guy Rebillard

Cochlear origin of 2f1-f2 distortion products assessed by using 2 types of mutant mice

Mutant mice with a particular type of cochlear pathology are excellent models to study the functional role of various structures in the cochlea. In order to assess the contribution of inner and outer hair cells to the generation of distortion product emissions (DPEs) we have recorded the 2f1-f2 DPE in a control group of CBA mice, which have normal numbers of inner and outer hair cells and two different types of mutant mice: the Bronx-waltzer mice and the Wv/Wv mice. In the Bronx waltzer mutant mice, 70% of inner hair cells are missing whereas the outer hair cells are present in normal number. The distortion product emissions 2f1-f2 is clearly recordable with a 10-20 dB lower magnitude as compared to normal CBA control mice. The homozygous Wv/Wv mutant mice on the other hand present a selective outer hair cell loss as a constant defect with no progressive degeneration of the organ of Corti and an essentially normal inner hair cell population. The cubic distortion products 2f1-f2 could not be detected in all but one animal. Therefore, the present study strongly suggests, that the outer hair cells are critically involved in the production of DPEs.

Identification of preferentially expressed cochlear genes by systematic sequencing of a rat cochlea cDNA library.

107 expressed sequence tags (ESTs) from a rat cochlea cDNA library were identified by systematic sequencing coupled to database selection and RT-PCR analysis of novel sequences. This approach led us to select a clone, pCO8, showing no significant homology with any database sequence, that corresponds to a mRNA whose expression is restricted to the cochlea, except for traces detected in brain. Additional clones with novel sequences enriched in the cochlea were also found. ESTs bearing significant homologies with database sequences (63 out of 107) were classified according to the putatively encoded protein. They include tissue-specific genes not previously described in the cochlea as well as known genes from other species. We performed in situ hybridization in cochlear tissues to localize the pCO8 mRNA and that of clone pCO6 which is 100% homologous to the delayed rectifier potassium channel drk1. We found that both mRNAs were exclusively expressed in the cellular body of the primary auditory neurons from the spiral ganglion of the cochlea. These results indicate that this approach is an efficient way to identify novel genes that could be of importance in cochlear function.

Dopamine transporter is essential for the maintenance of spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation

Dopamine, a neurotransmitter released by the lateral olivocochlear efferents, has been shown tonically to inhibit the spontaneous and sound‐evoked activity of auditory nerve fibres. This permanent inhibition probably requires the presence of an efficient transporter to remove dopamine from the synaptic cleft. Here, we report that the dopamine transporter is located in the lateral efferent fibres both below the inner hair cells and in the inner spiral bundle. Perilymphatic perfusion of the dopamine transporter inhibitors nomifensine and N‐[1‐(2‐benzo[b]thiophenyl)cyclohexyl]piperidine into the cochlea reduced the spontaneous neural noise and the sound‐evoked compound action potential of the auditory nerve in a dose‐dependent manner, leading to both neural responses being completely abolished. We observed no significant change in cochlear responses generated by sensory hair cells (cochlear microphonic, summating potential, distortion products otoacoustic emissions) or in the endocochlear potential reflecting the functional state of the stria vascularis. This is consistent with a selective action of dopamine transporter inhibitors on auditory nerve activity. Capillary electrophoresis with laser‐induced fluorescence (EC‐LIF) measurements showed that nomifensine‐induced inhibition of auditory nerve responses was due to increased extracellular dopamine levels in the cochlea. Altogether, these results show that the dopamine transporter is essential for maintaining the spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation.

Changes in 2f1-f2 distortion product otoacoustic emissions following alterations of cochlear metabolism

This paper summarizes the results obtained from investigations in which distortion product otoacoustic emissions (DPOAEs) were studied together with other cochlear physiological parameters. The cochlear metabolism was subjected to three different experimental conditions: guinea pigs were either submitted to hypoxia, to an intra-cochlear perfusion of ouabain or to an intra-cochlear perfusion of naloxone. The data show that DPOAEs remain affected for a certain time after the metabolic perturbations were removed. The comparison of the behaviour of DPOAEs and of other cochlear parameters gives good indications on the way these different experimental procedures affect the functioning of the cochlea during and after their application.

Effects of in vivo perfusion of glutamate dehydrogenase in the guinea pig cochlea on the VIIIth nerve compound action potential

Glutamate is considered as the best candidate for the neurotransmission between the inner hair cell and the primary efferent neurons in the mammalian cochlea. In order to test its presence in the synapse, a degradative enzyme for glutamate, glutamate dehydrogenase (GDH) was perfused in the cochlea of guinea pigs. The intensity function of the VIIIth nerve compound action potential was recorded as a physiological test. The results show that the GDH induces a decrease in the auditory nerve responsiveness. The threshold elevation observed is dependent upon the enzyme concentration.

Wayfinding through an Unfamiliar Environment

Strategies for finding ones way through an unfamiliar environment may be helped by 2D maps, 3D virtual environments, or other navigation aids. The relative effectiveness of aids was investigated. Experiments were conducted in a large, park-like environment. 24 participants (12 men, 12 women; age range = 22–50 years; M = 32, SD = 7.4) were divided into three groups of four individuals, who explored a 2D map of a given route prior to navigation, received a silent guided tour by means of an interactive virtual representation, or acquired direct experience of the real route through a silent guided tour. Participants then had to find the same route again on their own. 12 observers were given a “simple” route with only one critical turn, and the other 12 a “complex” route with six critical turns. Compared to three people familiar with the routes, among the naïve participants, those who had a direct experience prior to navigation all found their way again on the simple and complex routes. Those who had explored the interactive virtual environment were unable to find their way on the complex route. The relative scale representation in the virtual environment may have given incorrect impressions of relative distances between objects along the itinerary, rendering important landmark information useless.

Activation of muscarinic cholinergic receptors stimulates inositol phosphates synthesis in the developing avian cochlear duct

We previously reported that the inositol phosphates (IPs) synthesis is induced by muscarinic agonists in the rat cochlea and that this stimulation is maximal at postnatal day 12. This peak response is concomitant with the onset of the efferent synaptogenesis at the outer hair cell level. Whether the correlation between this neuronal plasticity and the enhanced IPs formation is unique to the rat or a general feature of the developing vertebrate cochlea is not known. To examine this question, we measured, in the presence of LiCl, the accumulation of (3H)‐IPs induced by carbachol, in the developing chick cochlear duct during a period ranging from embryonic day (E) 8 to post‐hatching day (P) 20. Carbachol (1 mM) causes a significant increase of IPs formation relative to basal values at all ages. This IPs accumulation is maximal at E8 (1854% of the basal level), then, rapidly decreases until P13 when it reaches a steady‐state level of 294% of the basal level. Strikingly, this gradual decline in IPs formation is interrupted between E15 and E19, by a transient increase in IPs synthesis. This rise peaks at E16 with a stimulation value of 757% of the control level. This maximal stimulation is inhibited by atropine in a dose‐dependent manner, as is the case at E9, suggesting the involvement of muscarinic receptors. Interestingly, the occurrence of the peak response is concomitant with the plastic events associated with the maturation of the efferent innervation of the cochlear duct. Thus, these results suggest that there may be a correlation between cochlear plasticity and enhanced IPs synthesis, which is not species‐specific. The possible significance of the overall decrease in IPs formation, occurring during embryonic development, is discussed.