Pablo Gil-Loyzaga

Neurotransmitters of the Olivocochlear Lateral Efferent System: With an Emphasis on Dopamine

The olivocochlear lateral efferent system (OLES) of the adult mammalian cochlea uses variety of neuroactive substances, such as acetyl choline, GABA, dopamine (DA), enkephalins, dynorphins and CGRP. These neuroactive substances have been located within the efferent, small and dense matrix, fibers and terminals of the inner spiral and tunnel bundles. However, some of these neuroactive substances have also been found outside the OLES. Acetyl choline and CGRP, for instance, appear within the olivocochlear medial efferent fibers, and DA and CGRP may also be present in the perivascular innervation. A special case is GABA innervation at the apical coil, where nerve fibers containing GABA also make synapses with OHCs bodies. All these substances of the OLES could be involved in a highly selective filter modulating the activity of primary afferent fibers. For instance, sound stimulation results in an increase of cochlear DA turnover, indicating the release of DA from OLES fibers. DA probably acts on D-2 receptors since the administration of piribedil, a D-2 agonist, results in blocking of noise effects, while D-1 receptor stimulation does not modify cochlear DA turnover induced by noise. Therefore, DA could play an important role in the modulation and noise-protection of cochlear primary afferents. During cochlear development, all the aforementioned neuroactive substances appear a long time before the onset of hearing (evidenced by the recording of cochlear compound action potential and microphonic potentials). Thus, they may act during development on the late reorganization and plasticity on the afferent and efferent fibers. Moreover, the positive neurotrophic effect observed in cultured cochlear neurons, with GABA or glutamate, add new support to that hypothesis.

HPLC detection of dopamine and noradrenaline in the cochlea of adult and developing rats

The presence and postnatal development of dopamine (DA) and noradrenaline (NA) in the rat cochlea were detected by high-performance liquid chromatography coupled with electrochemical detection. Cochlear DA content rose gradually after birth. Conversely, NA concentrations rose rapidly between postnatal day 1 and 8; then, up to day 30, it increased more slowly. On day 30, both DA and NA levels were around 5 times higher than on day 1. In the adult rat cochlea, NA mean content was 234 +/- 22.2 pg/mg protein, while DA mean content was 23.6 +/- 1.1 pg/mg protein. Adrenaline was always undetectable. The present study is the first report describing directly the presence of DA in the rat cochlea. DA might serve as one of the lateral efferent neurotransmitters, whereas NA probably acts as a neurotransmitter of the sympathetic cochlear innervation. Nevertheless, their influences on the cochlear physiology, either in adulthood or during development are still a matter of discussion.

Serotonergic innervation of the organ of corti of the cat cochlea

THE presence and distribution of serotonin-containing fibers within the adult cat cochlea is reported here for the first time. The 5-HT-like immunoreactive fibers were mainly found in the middle coils of the cat cochlea, following a peripheral distribution similar to that of the olivocochlear lateral efferent system (OLES). Thus, 5-HT-like immunoreactive varicose fibers were found within the intraganglionic spiral bundle, the inner and the tunnel bundles, although, some variation with the distribution of other neuroactive substances of the OLES were observed. In particular, some isolated fibers from the tunnel bundle branched towards the first row of the outer hair cells. Cochlear serotonergic fibers might be involved in the auditory processing within the cochlea as efferent modulating fibers.

Serotonergic innervation of the organ of Corti.

The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.

Choline-acetyltransferase-like immunoreactivitity in the organ of Corti of the rat during postnatal development

The mammalian cochlea receives efferent innervation from neurons located in the superior olivary complex. This efferent olivocochlear innervation is divided in two separate systems, lateral and medial, which mainly innervate afferent dendrites connected to inner hair cells and the cell body of outer hair cells, respectively. Besides other substances, lateral and medial efferent terminals of the adult cochlea use acetylcholine (ACh) as a neurotransmitter. In this study, we have used immunocytochemistry to detect the presence of choline acetyltransferase (ChAT), the synthesizing enzyme of ACh, in efferent olivocochlear terminals during the development of the rat. The appearance and distribution of immunoreactivity to ChAT has been studied in developing rat cochleas from birth (postnatal day 1, P1) to adulthood. Attention was paid to the temporal relationships between the expression of ChAT, the presence of other putative neuroactive substances, the onset of hearing and other developmental phenomena. Our results indicate that ChAT-like immunoreactivity is already present at birth (P1) in the region of inner hair cells, that it appears at P3 in the outer hair cell area and that it reaches an adult pattern of distribution by P15. ACh may thus be present early in the developing cochlea, before the onset of hearing, as it also occurs with other putative transmitters/modulators such as enkephalins, CGRP or GABA. It is suggested that ACh could be involved in the modulation of sound-evoked potentials as soon as they appear, and in the regulation of other developmental phenomena such as neurite outgrowth or synaptogenesis.

Immunocytochemical detection of calcitonin gene-related peptide in the postnatal developing rat cochlea

Using the immunofluorescence technique, three populations of fibers staining for calcitonin gene‐related peptide are identified in postnatal developing cochleas and in adult rat cochleas. During the maturation of the cochlea, the immunostaining first appears in the basal turn and then extends toward the apex of the cochlea. The first population of immunostained fibers belongs to the lateral olivocochlear innervation. It is observed at postnatal day 4 within the inner spiral bundle of the organ of Corti. The second population of calcitonin gene‐related peptide‐like immunostained fibers belongs to the medial olivocochlear innervation. It is first identified at postnatal day 6 under the outer hair cells, generally in the first row. In the older stages, this population of fibers progressively extends toward the external row of outer hair cells. Finally, the third population of immunostained fibers belongs to the sympathetic supply of the cochlea. They can be identified at postnatal day 6 around the cochlear artery and its branches, and also within the entire modiolus.

Effects of noise stimulation on cochlear dopamine metabolism

Dopamine (DA) appears to be one of the putative neurotransmitters of the lateral efferent olivocochlear fibers. However, its role in the cochlear physiology remains unknown. In this study, animals were exposed for 1 h to white noise at 70, 90 or 110 dB SPL or were kept in silence conditions. Afterwards, the cochlear content of DA and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were analyzed using HPLC coupled to electrochemical detection. Cochlear DA concentration decreased with the noise intensity, while cochlear DOPAC and HVA concentrations increased. Males presented higher cochlear DOPAC contents and lower HVA contents than females. This sexual dimorphism could be related to the link between DA and gonadal steroids. Present results show that DA, as other lateral efferent neurotransmitters, is released and metabolized in relationship with the noise stimulation, and suggest that DA could be involved in the modulation of the type I afferent fiber activity.

Piribedil affects dopamine turnover in cochleas stimulated by white noise

The presence of dopamine (DA) within the cochlea has been previously reported, indicating that its turnover increases under noise stimulation. In the present report, piribedil, a dopaminergic D2 agonist, was used in order to provide evidence of the activity of D2 receptors in the turnover of DA under noise stimulation. Long-Evans rats were intraperitoneally injected with distilled water or with a solution of piribedil one hour previously to either noise or silence exposure. Noise stimulation was performed in an anechoic chamber at 70, 90 or 110 dB SPL for one hour. The animals were then sacrificed and the cochlear contents of DA and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were quantified by HPLC with electrochemical detection. The administration of piribedil to animals kept in silence did not modify the cochlear DA, DOPAC and HVA content. Noise stimulation resulted in a decrease of the cochlear DA content and an increase of the cochlear DOPAC and HVA contents in vehicle treated animals. The administration of piribedil resulted in a blockade of this noise induced cochlear DA turnover. These results suggest that piribedil stimulates cochlear D2 receptors controlling the cochlear DA release. Piribedil action on D2 receptors could explain the improvement observed in some cochleo-vestibular diseases signs after piribedil treatment.

Ontogeny of γ-aminobutyric acid in efferent fibers to the rat cochlea

Abstract Cochlear efferent innervation originates in two different groups of neurons located in the superior olivary complex. A first group of olivocochlear neurons (lateral efferent neurons) lies in the lateral superior olive. They send axons to the organ of Corti, where they synapse with radial afferent dendrites of primary auditory neurons, postsynaptic to the inner hair cells. The second group of neurons (medial efferent neurons) is found in medial subnuclei of the superior olivary complex and sends axons to synapse with outer hair cells. Subpopulations of both medial and lateral olivocochlear neurons probably use γ-aminobutyric acid (GABA) as a neurotransmitter. We have used an immunoperoxidase technique to detect GABA-like immunoreactivity (GABA-LI) in postnatal maturing rat cochleas. The GABA-LI appeared in the inner hair cell region by P3 (P1 = birth) and reached a mature appearance by P15–P16. In the outer hair cell region, GABA-like immunoreactive fibers and terminals could not be identified until P9 and they were only found in the apical end of the cochlea. There was a dual gradient of maturation of GABA-LI in the cochlea. The GABA-LI appeared first at the cochlear base and then extended towards the apex. It also appeared earlier (about a week) in the inner hair cell region than in the outer hair cell region. This dual gradient of maturation is in close agreement with previous data concerning the maturation of the cochlea.

Immunocytochemical detection of glutamate decarboxylase in the postnatal developing rat organ of corti

Using a fluorescent avidin‐biotin technique, we have immunolocalized the GABA synthesizing enzyme, glutamate decarboxylase, in postnatal developing and adult rat organs of Corti. At birth, the glutamate decarboxylase‐like immunoreactivity is already present in the basal turn below the inner hair cells, i.e. within lateral olivocochlear efferent fibers of the inner spiral bundle. In the apical turn, the inner spiral bundle displays an immunoreactivity as early as postnatal day 3. Only the outer hair cells of the upper second turn and apex receive fibers immunostained for glutamate decarboxylase that most probably belong to the medial olivocochlear efferent innervation. They first appear at this level at postnatal day 15. Within these two regions of the organ of Corti, the glutamate decarboxylase‐like immunoreactivity reaches an adult‐like pattern at postnatal days 17–18. These results strengthen the hypothesis that GABA is a putative neurotransmitter that could be used by subpopulations of the two olivocochlear innervations. They also suggest that GABA either plays a neurotrophic function or participates in the regulation of the first cochlear potentials at the level of lateral efferent synapses.