Lateral Olivocochlear System Research Articles

Immunoreactivity to calcitonin gene-related peptide in the superior olivary complex and cochlea of cat and rat

In both cat and rat, the cells of origin, axons, and terminals of the lateral olivocochlear system exhibit immunoreactivity to antisera to calcitonin gene-related peptide (CGRP). In the cat, immunoreactive neurons in the brainstem are located in the hilus of the lateral superior olivary nucleus and around its margins. In the rat, immunoreactive neurons are located within the lateral superior olivary nucleus proper. In both species, immunoreactivity in the cochlear duct is limited to the region beneath the inner hair cells. Immunoreactive axons in the cochlear nucleus could not be traced to their source but may arise as collaterals of the lateral olivocochlear system. No other components of the brainstem auditory system react to any extent with the antisera.

Effect of noise level on the Met-enkephalin content of the guinea pig cochlea

Using a highly sensitive and specific radioimmunoassay for Met-enkephalin, we have monitored in two series of experiments the changes of the Met-enkephalin content of guinea pig cochleas following a 60 min exposure to different intensities of white noise (70 dB SPL, 90 dB SPL, 110 dB SPL). Our results indicate that the Met-enkephalin content was significantly lower after noise exposures than after exposure to the silence of a sound attenuated chamber. After a stimulation at 70 dB SPL, the levels of Met-enkephalin were 70% (series I) and 61% (series II) of those obtained after a period of silence. After a 110 dB SPL stimulation, these values fell to 41% (series I) and 55% (series II) of those in silence. These results strengthen the hypothesis that enkephalins are olivocochlear neuroactive substances. They suggest that the enkephalin-containing lateral olivocochlear system discharges with noise stimuli of moderate intensity.

Selective retrograde labeling of lateral olivocochlear neurons in the brainstem based on preferential uptake of 3H-D-aspartic acid in the cochlea.

We have previously shown that perfusion of the gerbil cochlea with probe concentrations of 3H-D-aspartic acid (D-ASP) results in immediate, selective labeling of 50-60% of the efferent terminals under the inner hair cells, presumably by high-affinity uptake. The present study was undertaken to determine the origin of these endings. Twenty-four hours after cochlear perfusion with D-ASP, labeled neurons were observed in the ipsilateral, and to a much lesser extent in the contralateral, lateral superior olivary nucleus (LSO). The cells were small, primarily fusiform, and showed fewer synaptic contacts than other LSO cells. Combined transport of D-ASP and horseradish peroxidase indicated that all olivocochlear neurons within the LSO that projected to the injected cochlea were labeled by D-ASP. Labeled fibers coursed dorsally from the LSO, joined contralateral fibers that had passed under the floor of the fourth ventricle, and entered the VIIIth nerve root at its ventromedial edge. Adjacent to the ventral cochlear nucleus (VCN), densely labeled collateral fibers crossed the nerve root to enter the VCN. Labeled fibers and terminals were prominent in the central VCN. Neither retrograde transport of D-ASP by medial olivocochlear and vestibular efferents nor anterograde transport by VIIIth nerve afferents was observed. The D-ASP-labeled cells and fibers are clearly lateral olivocochlear efferents. Retrograde transport of D-ASP thus allows the cells, axons, and collaterals of the lateral olivocochlear system to be studied, morphologically, in isolation from other cells that project to the cochlea. Since the olivocochlear neurons are almost certainly cholinergic, retrograde amino acid transport does not necessarily identify the primary neurotransmitter of a neuron. Rather, it indicates the presence of selective uptake by the processes of that neuron at the site of amino acid injection. Retrograde labeling appears to be markedly enhanced by the use of metabolically inert compounds such as d-isomer amino acids.

Coexistence of putative neuroactive substances in lateral olivocochlear neurons of rat and guinea pig

We have used the retrograde axonal transport of Fast Blue, injected intra-cochlearly, to identify in the rat lateral superior olive (LSO) neurons which belong to the lateral olivocochlear system (LOCS). Using immunohistofluorescence technique, we have localized within Fast Blue-labeled neurons immunostainings for enkephalins (Met-enkephalin, Met-enkephalin-Arg 6-Gly 7-Leu 8), dynorphins ( α-neo-endorphin, dynorphin 1-17) or choline acetyltransferase (ChAT). Many Fast Blue-labeled neurons did not show any immunostaining, but all the immunostained neurons found in the LSO were Fast Blue-labeled. In immunohistofluorescence colocalization experiments of two antigens, we could colocalize within the same neurons of the rat LSO immunostainings for ChAT and enkephalins and for ChAT and dynorphins. In each case, neurons only immunostained for ChAT, enkephalins or dynorphins could also be observed. A colocalization of the immunostainings for Met-enkephalin and dynorphins within neurons of the guinea pig and rat LSO was also found. However, in this case, neurons which did not show colocalization were only Met-enkephalin-immunoreactive, thus suggesting that all the dynorphins immunoreactive LSO neurons also contain enkephalins. These findings support the idea that the neurons of the LSO which contain ChAT-, enkephalin- or dynorphin-immunostainings project to the cochlea and belong to the LOCS. It can also be concluded that acetylcholine, enkephalins and dynorphins coexist within a same population of neurons of the LOCS, although other patterns of co-containment of neuroactive substances within LOCS neurons may also exist.

Glutamic acid decarboxylase immunoreactivity of olivocochlear neurons in the organ of Corti of guinea pig and rat.

The distribution of glutamic acid decarboxylase (GAD)-like immunoreactivity in the organ of Corti of guinea pig and rat was studied under the light microscope. Indirect immunohistochemical techniques were used. Cochleae were first incubated with a specific antiserum to rat brain GAD and then stained through an avidin-biotin-horseradish peroxidase (HRP) procedure. GAD-like immunoreactivity was visualized as staining with HRP reaction product. Surface preparations were prepared from the immunoreacted cochleae. GAD-like immunoreactivity was found in the inner spiral bundle, tunnel spiral bundle, tunnel crossing fibers, outer hair cell synaptic regions and outer spiral bundles. Little staining was seen in the basal turn. Most of the immunoreactivity was seen in the third and lower fourth turn of the guinea pig cochlea, but even there many efferent fibers and endings were unstained. It is concluded that GAD-like immunoreactivity is present in a subpopulation of cochlear efferents that contains elements from both the medial and the lateral olivocochlear system. Future studies are needed to determine whether this subpopulation is GABA-ergic (i.e. uses gamma-aminobutyric acid as a neurotransmitter) and/or cholinergic.

Topographic organization of the olivocochlear projections from the lateral and medial zones of the superior olivary complex.

By anterograde tracing using autoradiography, we have found topographic organizations in the projections of both medial and lateral olivocochlear (OC) systems in the cat. Lateral-zone injections show an ipsilateral cochleotopic projection pattern with more medial injections projecting more basally in the cochlea. In the contralateral cochlea, in contrast, the projections from all of the lateral-zone injections were predominantly to the apex. However, detailed analysis suggests the possibility that the contralateral lateral-zone projections may have the same cochleotopic organization as the ipsilateral projections but with a heavy bias toward the apex. Medial-zone injections show a pattern in which more dorsal regions project more basally in both cochleas. The ipsilateral projections of lateral OC neurons appear to connect regions with similar best frequencies but the projections of medial OC neurons do not. Summation of data from all of the injections in each zone indicates that lateral OC projections are relatively evenly distributed throughout the ipsilateral cochlea but are predominantly to the apex in the contralateral cochlea. Medial OC projections are predominantly to the middle and basal parts of the cochlea on both sides with contralateral projections somewhat more basal than ipsilateral projections.

Differential olivocochlear projections from lateral versus medial zones of the superior olivary complex.

An anterograde tracer (35S-methionine) was injected unilaterally in the superior olivary complex (SOC) at regions previously demonstrated by retrograde labeling to contain olivocochlear (OC) cell bodies. Quantitative analysis of cochlear autoradiographs from these cats demonstrates that there are two OC systems. The lateral OC system has cell bodies lateral to the medial superior olivary nucleus (MSO) and projects to the inner hair cell (IHC) region bilaterally (mostly ipsilaterally). The medial OC system has cell bodies medial, ventral, and anterior to the MSO and projects to the outer hair cell (OHC) region bilaterally (mostly contralaterally). A single medial OC neuron innervates many small patches of OHCs with substantial gaps between the patches. Medial OC neurons also appear to project to the IHC region to a small extent. A review of the literature with the medial-lateral division of OC efferents in mind reveals many differences between these two systems. In particular, lateral OC axons are unmyelinated and innervate the dendrites of radial afferent fibers under IHCs, whereas medial OC axons are myelinated and directly innervate OHCs. Although both systems appear to be cholinergic, the lateral OC system also shows met-enkephalin-like immunoreactivity. The synapses of the medial OC system are formed in development before those of the lateral OC system and they degenerate more slowly after the OC axons are cut. The many differences between these two OC systems suggest that they are functionally separate systems.