Frequency selectivity is one of the most important functions of the mammalian hearing organ – the cochlea. The interaction of fluid mass and organ of Corti compliance sets a traveling wave along the basilar membrane (BM), which is longitudinally tuned to different frequencies. Beyond this passive tuning process, cochlear amplification locally enhances the vibration of the best frequency peak by factors of hundreds to boost the frequency selectivity and sensitivity of the cochlea. This amplification is achieved by a positive feedback loop between BM motion and outer hair cell (OHC) electrical-mechanical response. However, this active mechanism is vulnerable to damage and cannot be fully recovered in vivo. As the instruments of cochlear amplification, the frequency response of BM and OHCs are of great importance to understand cochlear tuning process. This thesis used animal models, aimed to understand cochlear tuning and investigate possibilities to manipulate the cochlear amplifier, by testing the cochlear amplifier’s sensitivity to operating conditions. The first project tested whether the cochlear amplification can adjust to a lower endocochlear potential (EP), which controls OHC electromechanical force by providing part of the voltage source to drive OHC transduction current. To investigate this possibility, we use intraperitoneal (IP) and intravenous (IV) injection of furosemide to reversibly reduce EP, while monitoring the EP and cochlear amplification simultaneously. Cochlear amplification was monitored by measuring the local cochlear microphonic (LCM) and distortion product emission (DPOAE). With IV injection, the cochlear amplification observed in LCM could attain nearly full or even full recovery with reduced EP. This showed the cochlea has an ability to adjust to diminished operating condition. Furthermore, the cochlear amplifier and EP recovered with different time courses: cochlear amplification just started to recover after the EP was nearly fully recovered and stabilized. Using a Boltzmann model and the 2nd harmonic of the LCM to estimate the mechanoelectric transducer channel operating point, we found that the recovery of cochlear amplification occurred with re-centering of the operating point. The second project studied the physiological and anatomical effects of perfusing the cochlea with a viscous fluid, for better understanding cochlear fluid mechanics. Perilymphatic perfusion was applied with artificial perilymph and viscous sodium hyaluronate (Healon, HA) in four different concentrations. Using compound action potential (CAP) thresholds as an indicator of cochlear condition, our results and analysis indicated that the cochlea can sustain, without a significant CAP threshold shift, up to a 1.5 Pa shear stress. Histology of the cochleae perfused with higher shear stress showed the Reissner's membrane was torn. These data also indicated that the cochlea mechanics remains normal within increased perilymphatic fluid viscosity up to an increase of a factor of 50. Beside these findings, a temporary CAP threshold shift was observed, perhaps due to the presence and then clearance of viscous fluid within the cochlea, or to a temporary position shift of the organ of Corti. The last project was to test the effect of OHC intracellular Cl- concentration on cochlear amplification. Chloride is known to enable the electromotility of the OHC by binding its motor protein, prestin. By locally perfusing high chloride perilymph and the chloride ionophore tributyltin, this study investigated whether increasing intracellular chloride concentration can restore cochlear sensitivity in a cochlea that was slightly damaged. This had been shown by others in guinea pig. However, we did not observe recovery in several attempts in gerbil.

The cochlea is an organ which extracts frequency information from the input sound wave. It also produces nerve signals, which are further analysed by the brain and ultimately lead to perception of the sound. An existing model of the cochlea by Fragni`ere is first analysed by simulation. This passive model is found to have the properties that the living cochlea does in terms of the frequency response. An analog VLSI circuit implementation of this cochlear model in CMOS weak inversion is proposed, using log-domain filters in current domain. It is fabricated on a chip and a measurement of a basilar membrane section is performed. The measurement shows a reasonable agreement to the model. However, the circuit is found to have a problem related to transistor mismatch, causing different behaviour in identical circuit blocks. An active cochlear model is proposed to overcome this problem. The model incorporates the effect of the outer hair cells in the living cochlea, which controls the quality factor of the basilar membrane filters. The outer hair cells are incorporated as an extra voltage source in series with the basilar membrane resonator. Its value saturates as the input signal becomes larger, making the behaviour rather closer to that of a passive model. The simulation results show this nonlinear phenomenon, which is also seen in the living cochlea. The contribution of this thesis is summarised as follows: a) the first CMOS weak inversion current domain basilar membrane resonator is designed and fabricated, and b) the first active two-dimensional cochlear model for analog VLSI implementation is developed.

Objectives: To verify the necessity for special surgical techniques or clips for fixation of the electrode cable of a cochlea implant against dislocation, and to test the stability of postoperative biologic cicatrization as the sole and solid anchoring of the cable. Material: Temporal bone experiments with a simulated connective tissue sheath around conventional (Med El Combi 40+) and prototype (profiled surface) electrode cables. Results and Conclusions: The electrode cable is anchored securely in a sheath of scar tissue, since unphysiologic loads are needed for pulling it out of its anchorage. The drag during one extraction trial with a profiled cable even resulted in the rupture of the cable. These results confirm our confidence in this biologic fixation of the electrode cable inside its postoperative cicatric tissue sheath. More than 80 cochlea implantations with the electrode simply imbedded in a drop of fibrin glue in the posterior tympanotomy never demonstrated a shift of the electrodes in the last 8 years. Therefore, special fixation of the electrode cable with clips or surgical techniques is not necessary. / Fragestellung: Muss das Elektrodenkabel eines Kochlearimplantats durch spezielle operative Techniken oder Halterungen gegen ein Herausrutschen aus der Kochlea gesichert werden, oder genügt die Einscheidung in dem postoperativ sich ausbildenden Narbengewebe für eine ausreichend stabile Fixierung? Material: Felsenbeinexperimente mit einer Simulation der narbigen Einbettung konventioneller und modifizierter (geriffelter Oberfläche) Elektrodenkabel eines Kochlearimplantats (Med El Combi 40+). Ergebnisse und Schlussfolgerungen: Ein Herausziehen eines in simuliertem Narbengewebe eingescheideten Elektrodenkabels gelang erst bei erheblichen, unphysiologischen Kräften; eine Rifflung der Oberfläche des Silikonkabels erhöhte den Reibungswiderstand über die Reissfestigkeit des Kabels. Das Vertrauen in die biologische Fixierung des Elektrodenkabels durch die Verankerung im Narbengewebe ist somit gerechtfertigt, und wird auch durch unsere klinische Erfahrung bestätigt: in über 80 Operationen, bei denen das Kabel des Kochlearimplantats nur durch Einbettung in Fibrinkleber am Rahmen der posterioren Tympanotomie gesichert worden war, liess sich in den letzten 8 Jahren in keinem Fall eine Elektrodenverlagerung nachweisen. Eine gezielte Fixation des Elektrodenkabels durch künstliche Halterungen oder spezielle OP-Techniken erscheint somit nicht erforderlich. / Problématique: Afin d’éviter tout glissement de l’électrode de l’implant cochléaire, est-il préférable d’en assurer la fixation par une technique opératoire particulière ou bien est-il suffisant de gainer l’électrode dans les tissus cicatriciels qui se forment après l’opération? Méthode: Nous avons procédé à des expériences sur le rocher en simulant une inclusion cicatricielle d’un câble-électrode classique (Med El Combi 40+), d’une part, et modifié (surface cannelée), d’autre part. Résultats et conclusion: Une force dépassant les réalités physiologiques a été nécessaire pour retirer le câble-électrode gainé dans les tissus cicatriciels simulés. Le câble avec la surface cannelée s’avérait encore plus résistant: il déchirait même lorsqu’on a essayé de le retirer. Ceci vient donc conforter la confiance que nous avons dans la fixation biologique, c’est-à-dire dans l’ancrage de l’électrode dans les tissus cicatriciels. Une confiance qui est d’ailleurs confirmée par les expériences que nous avons pu faire dans notre clinique. Ainsi, en 8 ans, sur 80 opérations, au cours desquelles le câble de l’implant a été fixé en étant simplement placé dans de la fibrine, dans le cadre d’une tympanotomie postérieure, aucun déplacement de l’électrode n’a été constaté. Par conséquent, il ne s’avère pas nécessaire d’avoir recours à une fixation artificielle ou à une technique opératoire particulière pour assurer le bon maintien de l’électrode / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

[Truncated abstract] The mammalian cochlea receives an efferent innervation which originates in the superior olive. Known collectively as the olivocochlear system, this efferent innervation can be divided into lateral and medial systems. While the function of the lateral efferent system in the cochlea is still unknown, previous studies have identified both excitatory and inhibitory changes in sound-evoked and spontaneous cochlear responses attributable to the lateral efferent system. One of the neurotransmitters in the lateral efferents is the catecholamine dopamine, which in the central nervous system is known to exert inhibitory and excitatory effects by activating different receptor subtypes. The first experiments in this thesis were designed to determine if activation or blockade of different dopamine receptor subtypes in the cochlea could lead to both excitatory and inhibitory changes in sound-evoked and spontaneous cochlear responses. Adult guinea pigs were anaesthetised (Nembutal and Hypnorm) and highly specific D1/5 (SKF 38393, SKF 81297, SCH 23390), D2 ((+) PHNO, L 741, 626) and D3 (PD 128907, U 99194A) receptor agonists and antagonists were perfused through the cochlea for 15 minutes. Sound-evoked (compound action potential, summating potential, cochlear microphonic) and spontaneous cochlear responses were recorded before and after perfusion. Remarkably, activating or blocking D1/5 or D2 receptor subtypes resulted in the suppression of CAP amplitudes. These findings are paradoxical as the agonist data suggest that the D1/5 and D2 receptor subtypes are inhibitory, but the antagonist perfusions suggest that these receptors are excitatory. We propose that the presence of an agonist induces a process of receptor desensitisation which would elicit changes akin to receptor antagonism. If this is indeed the case then our agonist findings are spurious and require further interpretation. ... The suppression of the cochlear microphonic suggests that dopamine receptor influence is not confined to the primary afferent dendrite may also include the active process of the outer hair cells. The D1/5 and D2 antagonist data also suggests that dopamine receptors are activated by intrinsic dopamine. Therefore, we attempted to investigate the effects of putative dopamine depletion of the cochlea and found that application of the dopaminergic neurotoxin MPTP causes changes in both neural and hair cell responses which have not been reported before. However, we also demonstrated that tyrosine hydroxylase positive nerve fibres are still present in MPTP treated cochleae which suggests that dopamine is still present in these cochleae. Furthermore, we observed significant electrophysiological changes in these same cochleae when these were exposed to a D2 receptor antagonist which again supports the presence of intrinsic dopamine in these 'depleted cochleae'. These data suggest that the currently accepted method of acute dopamine depletion using MPTP is insufficient and different methods must be developed in the future.

The cochlea is an organ which extracts frequency information from the input sound wave. It also produces nerve signals, which are further analysed by the brain and ultimately lead to perception of the sound. An existing model of the cochlea by Fragni`ere is first analysed by simulation. This passive model is found to have the properties that the living cochlea does in terms of the frequency response. An analog VLSI circuit implementation of this cochlear model in CMOS weak inversion is proposed, using log-domain filters in current domain. It is fabricated on a chip and a measurement of a basilar membrane section is performed. The measurement shows a reasonable agreement to the model. However, the circuit is found to have a problem related to transistor mismatch, causing different behaviour in identical circuit blocks. An active cochlear model is proposed to overcome this problem. The model incorporates the effect of the outer hair cells in the living cochlea, which controls the quality factor of the basilar membrane filters. The outer hair cells are incorporated as an extra voltage source in series with the basilar membrane resonator. Its value saturates as the input signal becomes larger, making the behaviour rather closer to that of a passive model. The simulation results show this nonlinear phenomenon, which is also seen in the living cochlea. The contribution of this thesis is summarised as follows: a) the first CMOS weak inversion current domain basilar membrane resonator is designed and fabricated, and b) the first active two-dimensional cochlear model for analog VLSI implementation is developed.

Diss. (sammanfattning) Uppsala : Univ., 2002. / Härtill 5 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2004. / Härtill 5 uppsatser.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 4 uppsatser.

Lic.-avh. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 2 uppsatser.

Introduction: Thyroid hormones (TH) during gestation are critical for fetal growth and development of hearing organ. The lack of maternal TH leads an inadequate development of Organ of Corti with malformation of internal sulcus, tectorial membrane, cochlear ductus and a hair cells differentiation. It can adversely affect the auditory system, which can cause a hearing loss. Experimental studies are performed with postnatal period hypothyroidism induction, but there are no investigations that induce at gestational period. The impact of thyroid hypofunction restricted to the embryonic period remains unknown, which makes this study unpublished. Objective: To evaluate the effect of gestational hypothyroidism on auditory function of adult offspring in rats. Methods: The research was composed by Wistar rats and it was approved by the Ethics Committee on Animal Research of Federal University of Sergipe (Protocol #21/15). Pregnant Wistar rats were given the antithyroid drug methimazole (0.02% - 1-methylimidazole-2-thiol – MMI, in drinking water, ad libitum) from gestational day (GD) 9 to delivery day (GD 21-22), and comprises a offspring from gestational MMI-treated dams group (OGMTD). To lactation hypothyroidism group [offspring from perinatal MMI-treated dams group (OPMTD)] the drug was given from 9ºGD to the 15th postnatal day (PND). Part of the OGMTD and OPMTD groups received replacement of HT with levothyroxine at the concentration of 50 μg / 100 mL in drinking water. All animals were evaluated by tympanometry, distortion product otoacoustic emission (DPOAE) and auditory evoked brainstem response (ABR) at 30, 60, 90 and 120 PND. Results: Our data demonstrated no middle ear dysfunction; regardless of hypothyroidism groups, compliance was lower than the control group (p<0,005). DPOAE was lower in OGMTD from 4 up to 12 kilohertz (kHz) and absent in OPMTD (p<0,001). On the other hand, ABR revealed normal integrity of neural auditory pathways up to brainstem level in the central nervous system, with no latency modification. Additionally, hypothyroidism groups presented a higher electrophysiological threshold (i.e., hearing loss), worse repercussion in OPMTD (p<0,001). Groups treated with levothyroxine did not reveal difference on hearing behavior compared to hypothyroidism groups. Our data suggest that gestational hypothyroidism leads to a cochlear damage function in offspring, with normality of the auditory pathways to the brainsten with moderate to profound sensorineural hearing loss. / Introdução: Os hormônios tireoidianos (HT) durante a gestação são críticos para o desenvolvimento do órgão da audição. A hipofunção da tireoide neste período provoca má formação do órgão de Corti, caracterizada por alteração no sulco interno, membrana tectorial, ducto colcear, além de dificuldade na diferenciação das células ciliadas. Estas alterações repercutem negativamente no sistema auditivo, que pode culminar em perda auditiva. Estudos experimentais são realizados com a indução do hipotireoidismo até o período pós-natal, porém não há pesquisas que induzam apenas no período gestacional. O impacto da hipofunção tireoidiana restrita ao período embrionário permanece desconhecida, o que torna este estudo inédito.Objetivo: Avaliar o efeito do hipotireoidismo gestacional experimental na função auditiva da prole adulta em ratos. Material e Método: A pesquisa foi realizada com ratos Wistar e foi aprovada pelo Comite de ética em pesquisa com animais da UFS, sob o número 21/2015. Foi administrado às ratas Wistar prenhes o fármaco antitireoidiano metimazol (0,02% - 1-metilimidazol-2-tiol, em água potável, ad libitum.) do nono dia gestacional (DG) até o dia do parto (21-22DG), e formaram o grupo da prole de mães induzidas ao hipotireodismo gestacional (PMHG). No grupo até a lactação [prole de mães induzidas ao hipotireodismo perinatal (PMHPN)], o fármaco metimazol foi administrado do 9ºDG ao 15º dia pós natal (DPN). Parte dos grupos PMHG e PMHPN receberam reposição dos HT com Levotiroxina na concentração de 50 μg/100 mL na água de beber. Todos os animais foram submetidos aos seguintes procedimentos: exames de timpanometria, emissão otoacústica por produto de distorção (EOAPD) e potencial evocado auditivo de tronco encefálico (PEATE) nas idades de 30, 60, 90 e 120 DPN. Resultados: Os dados não demonstraram disfunção da orelha média; porém os grupos induzidos ao hipotireidismo apresentaram menores valores de compliância que o grupo prole de mães eutiroidianas (p<0,05). EOAPD foi menor no PMHG de 4 a 12 kilohertz (kHz), com ausência de respostas no PMHPN (p<0,001). Por outro lado, o PEATE revelou integridade das vias auditivas neurais até o nível do tronco encefálico no sistema nervoso central, sem modificação de latência. Além disso, os grupos com hipofunção tireoidiana apresentaram maiores limiares eletrofisiológicos (isto é, perda auditiva), com pior repercussão no grupo PMHPN (p<0,001). Não foi observada reversão da hipofunção tireoidiana nos grupos que receberam o reposição dos HT, uma vez que apresentaram o mesmo comportamento auditivo funcional que os grupos sem o tratamento com levotiroxina. Conclusão: O hipotireoidismo gestacional altera a função coclear da prole, com normalidade da integridade das vias auditivas até tronco encefálico e presença de perda auditiva sensório neural de grau moderado a profundo. / Aracaju, SE