Lateral Olivocochlear Research Articles

Olivocochlear innervation of inner and outer hair cells during postnatal maturation: an immunocytochemical study

Changes in the expression of several neurochemical markers associated with either axonal growth (GAP-43), synaptic vesicles (synaptophysin), or the cholinergic population of lateral olivocochlear (OC) efferents were investigated in the postnatal cochlea of hamsters. Growth-associated protein was expressed in the neonatal cochlea but not in the adult; immunoreactivity was found below inner hair cells (IHCs) from postnatal day (P) 2 through P14 and below outer hair cells (OHCs) from P5 to P14. In contrast, synaptophysin was expressed in both the neonate and adult cochlea; immunoreactivity was found below IHCs around P4 and below OHCs at P5. Both GAP-43 and synaptophysin immunoreactivities occurred first below IHCs in basal regions of the cochlea. Efferent fibers containing calcitonin gene-related peptide (CGRP) immunoreactivity were identified as early as P4 within the cochlear nerve but were not observed underneath IHCs until P7. Similar to GAP-43 and synaptophysin immunoreactivity, CGRP expression followed a basal to apical gradient; however, expression below OHCs appeared restricted to apical regions. These data raise the possibility that efferents expressing growth proteins and efferents expressing synaptic vesicle proteins co-exist during the first postnatal week. Furthermore, it is hypothesized that CGRP-containing lateral OC neurons from part of a later, secondary innervation to the cochlea.

Olivocochlear innervation of inner and outer hair cells during postnatal maturation: an immunocytochemical study

Changes in the expression of several neurochemical markers associated with either axonal growth (GAP-43), synaptic vesicles (synaptophysin), or the cholinergic population of lateral olivocochlear (OC) efferents were investigated in the postnatal cochlea of hamsters. Growth-associated protein was expressed in the neonatal cochlea but not in the adult; immunoreactivity was found below inner hair cells (IHCs) from postnatal day (P) 2 through P14 and below outer hair cells (OHCs) from P5 to P14. In contrast, synaptophysin was expressed in both the neonate and adult cochlea; immunoreactivity was found below IHCs around P4 and below OHCs at P5. Both GAP-43 and synaptophysin immunoreactivities occurred first below IHCs in basal regions of the cochlea. Efferent fibers containing calcitonin gene-related peptide (CGRP) immunoreactivity were identified as early as P4 within the cochlear nerve but were not observed underneath IHCs until P7. Similar to GAP-43 and synaptophysin immunoreactivity, CGRP expression followed a basal to apical gradient; however, expression below OHCs appeared restricted to apical regions. These data raise the possibility that efferents expressing growth proteins and efferents expressing synaptic vesicle proteins co-exist during the first postnatal week. Furthermore, it is hypothesized that CGRP-containing lateral OC neurons from part of a later, secondary innervation to the cochlea.

Morphological subclasses of lateral olivocochlear terminals? Ultrastructural analysis of inner spiral bundle in cat and guinea pig.

The lateral olivocochlear efferent pathway terminates in vesicle-filled swellings in the inner spiral bundles under inner hair cells (IHCs) and has been suggested to include at least two chemically distinct subclasses (see, e.g., Vetter et al. [1991] Synapse 7:21-43). In the present study, the ultrastructure and peripheral targets of vesicle-filled swellings in the IHC area of the cat and guinea pig cochleas were quantitatively analyzed to determine 1) whether morphological subclasses could be defined based on swelling size or on the density, size or shape of clear and dense-cored vesicles and 2) whether swellings with different postsynaptic targets differed morphologically. In both cat and guinea pig, all swellings contained large, round, clear vesicles and a variable number of dense-core vesicles. Although evidence of clear-cut subclasses was not compelling, the smallest swellings tended to be rich in dense-core and poor in clear vesicles and rarely formed synaptic contacts. Most of the larger swellings, which tended to contain few dense-core vesicles and a rich complement of clear round vesicles, formed synapses with radial afferent fibers. However, there were no morphological differences between swellings contacting afferents originating on the modiolar vs. pillar sides of the IHC (the source of afferents with low and high spontaneous discharge rates, respectively). We conclude that 1) if distinct gamma-amino butyric acid (GABA)ergic and cholinergic subclasses of lateral olivocochlear (LOC) fibers exist, then the vesicle morphology of their terminals does not differ as it does in the central nervous system and that 2) if peptide neurotransmitters, such as calcitonin gene-related peptide and enkephalins, are packaged in dense-core vesicles, then the LOC terminals synapsing with IHC afferent fibers are not particularly rich in these peptides.

Enkephalin-like immunoreactivity in the chick brainstem: possible relation to the cochlear efferent system

Mammalian lateral olivocochlear (LOC) neurons that are immunoreactive for choline acetyltransferase (ChAT) are also immunoreactive for enkephalin (Enk). To determine whether cochlear efferent neurons in birds might also contain Enk-like immunoreactivity (Enk-LI), we studied the auditory brainstem of the domestic chicken using antisera to ChAT, leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk). Enk-LI terminals are found around, but not within, the superior olivary nucleus (SO) and the nucleus of the lateral lemniscus, pars intermedia (LLi). A moderate concentration of Enk-LI terminals is found ventromedial to the ventral facial nucleus (VIIv) where the ventrolateral group of ChAT-I cochlear efferent neurons is located. After colchicine injections into the lateral ventricle, a population of intensely stained Enk-LI perikarya was found in the nucleus of the lateral lemniscus, pars ventralis (LLv) with scattered cells in the LLi and the nucleus subceruleus ventralis (SCv). The distribution of Enk-LI and ChAT-I somata, however, never overlapped, even in adjacent sections. Thus, in the chick, Enk-LI perikarya are not distributed in areas where cochlear efferent neurons are found. Instead, a dense concentration of Enk-I terminals can be found in areas containing ChAT-I cochlear efferent neurons. The source of these enkephalinergic terminals may be a population of Enk-LI cells in the LLv.

Fiber pathways and branching patterns of biocytin-labeled olivocochlear neurons in the mouse brainstem.

Olivocochlear neurons have somata in the superior olivary complex in the brainstem and project fibers to the cochlea. The purpose of the present study was to demonstrate the fiber pathways and branching patterns of olivocochlear fibers within the brainstem. Olivocochlear fibers were labeled by extracellular injections of biocytin into the cochlea of mice. The injections labeled two populations of olivocochlear fibers. Thin olivocochlear fibers arose from small somata of the lateral olivocochlear group located ipsilaterally in the lateral superior olive. Thick olivocochlear fibers arose from larger somata of the medial olivocochlear group located bilaterally in the periolivary nuclei. The lateral olivocochlear and medial olivocochlear fibers had similar courses but differed in their branching patterns. Branches from lateral olivocochlear fibers terminated near their somata of origin in the lateral superior olive or in the lateral vestibular nucleus. Branches from medial olivocochlear fibers terminated in the inferior vestibular nucleus or in the cochlear nuclear complex. A few branches from medial olivocochlear fibers projected to the contralateral side. Although they project primarily to the cochlea, olivocochlear neurons also give off branches to a variety of nuclei in the brainstem, thus involving auditory and non-auditory nuclei in the olivocochlear reflex system.

Neurotransmitters and neuromodulators of the mammalian cochlea.

Major topics addressed in this review on neurotransmitters and neuromodulators of the mammalian cochlea are ; 2) neuronal circuity of the organ of Corti ; 2) hair cell neurotransmitter(s) ; efferent neurotransmitters and neuromodulators (acetylcholine, γ-aminobutyric acid, dopamine, enkephalins, dynorphins, calcitonin gene-related peptide, excitatory amino acids ; and 3) other neuroactive substances in the cochlea (adenosine, ATP, taurine, auditory nerve-activating substance, histamine)

Selective retrograde transport of nipecotic acid, a GABA analog, labels a subpopulation of gerbil olivocochlear neurons.

Perfusion of the gerbil cochlea with micromolar quantities of 3H-gamma-aminobutyric acid (GABA) results in rapid, selective labeling of 50-60% of the olivocochlear (OC) efferent terminals on afferent dendrites beneath the inner hair cells, and all of the efferent terminals beneath the outer hair cells. In order to identify the neurons from which these GABA-accumulating terminals originate, the cell bodies were localized by using retrograde transport of 3H-nipecotic acid, a metabolically inert GABA analog. With survival times of 6-30 hours after cochlear injection, myelinated OC efferent fibers and cell bodies were well labeled, with the greatest number being labeled at 12-18 hours. All of the labeled neurons belonged to the medial OC system, and no lateral OC neurons were labeled. It is concluded that the GABA-accumulating endings in the gerbil cochlea arise from medial OC neurons, and therefore that medial OC efferent neurons in this species project to both inner and outer hair cell regions.

Course and distribution of efferent fibers in the cochlea of the mouse

The course, distribution and termination of single efferent fibers to the cochlea has been described in only a few animals and relatively few fibers have been studied with knowledge of their ipsilateral or contralateral origin. In order to examine the efferent fibers in the mouse, the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophoretically injected into one side of the brain stem near the location of known efferent nuclei. Examination of surface preparations of the cochlea revealed detailed information for both the lateral olivocochlear (LOC) and medial olivocochlear (MOC) systems. Many, but not all, fibers entered the cochlea within the intraganglionic spiral bundle (IGSB). The LOC fibers were restricted to the ipsilateral cochlea and rarely branched within the IGSB and osseous spiral lamina (OSL). In the organ of Corti, they traveled either basally or apically in the region of the inner hair cells (IHCs), spanning lengths up to 130 μm (basally) and 890 μm (apically). Terminal swellings of these fibers were ca 3.0 μm in diameter. Numerous en passant swellings were present where the fibers formed a plexus in the area of the IHCs. The MOC fibers followed a similar course in the IGSB and OSL, and within the OSL the fibers had few branches. Within the organ of Corti they traveled apically (up to 70 μm) in the nerve bundles located in the IHC area before they crossed the tunnel of Corti. In the region of the OHCs, 9% of the traceable fibers branched to innervate two to three OHCs while 91% appeared to innervate only one OHC. There was no discernible difference in the distribution of contralateral and ipsilateral MOC projections in terms of cochlear region or outer hair cell rows.

Collaterals from lateral and medial olivocochhlear efferent neurons innervate different regions of the cochlear nucleus and adjacent brainstem

Two populations of superior olivary neurons which project to different sensory cell regions in the cochlea also give off collateral projections to the ventral cochlear nucleus (VCN) and adjacent brainstem. To determine whether these VCN projections also have different targets they were characterized by selective retrograde amino acid transport. Retrograde transport of 3H-d-aspartate (D-ASP) selectively labeled the unmyelinated fibers and neurons of the lateral olivocochlear (OC) system including a dense collateral projection to the central VCN. Retrograde transport of 3H-nipecotic acid (NIP) labeled the myelinated fibers and neurons of the medial OC system, including collateral projections to the peripheral VCN, subpeduncular granule cells, and nucleus Y. Medial and lateral OC efferent collaterals thus innervate different regions of the CN. Lateral system collaterals overlap extensively with Type I spiral ganglion cell afferent input. They are well positioned to play a role in modulating afferent input to the central auditory system, as is the primary projection of these efferents to the cochlea. The medial system collaterals project near the recently described afferent projections of Type II spiral ganglion cells. The medial system collaterals may therefore be related to the function of outer hair cells, as the medial system primary axons appear to be in the cochlea.

Coexistence of Calcitonin gene-related peptide and Enkephalin in Single Neurons of the Lateral Superior Olivary Nucleus of the guinea pig that project to the Cochlea as Lateral Olivocochlear System

Coexistence of Calcitonin gene-related peptide and Enkephalin in Single Neurons of the Lateral Superior Olivary Nucleus of the guinea pig that project to the Cochlea as Lateral Olivocochlear System

Brainstem branches from olivocochlear axons in cats and rodents.

Horseradish peroxidase was used to label axons of olivocochlear (OC) neurons by intracellular injections in cats and extracellular injections in rodents. These axons arise from cell bodies in the superior olivary complex and project to the cochlea. En route to the cochlea, the thick axons (greater than 0.7 micron diam.) of medial olivocochlear (MOC) neurons formed collaterals that terminated in the ventral cochlear nucleus, the interstitial nucleus of the vestibular nerve (in cats), and the inferior vestibular nucleus (in rodents). The thin axons (less than 0.7 micron diam.), presumed to arise from lateral olivocochlear (LOC) neurons, did not branch near the CN. Within the CN, the MOC collaterals tended to ramify in and near regions with high densities of granule cells, regions also associated with the terminals of type II afferent axons (Brown et al.: J. Comp. Neurol. 278:581-590, '88). These results suggest that those fibers associated peripherally with outer hair cells (MOC efferents and type II afferents) are associated centrally with regions containing granule cells, whereas those fibers associated with inner hair cells peripherally (LOC efferents and type I afferents) are not.

Effects of electrical stimulation of medial olivocochlear neurons on ipsilateral and contralateral cochlear responses.

Recent anatomical evidence has cast doubt on the interpretation of the neural elements involved in past experiments in which efferents were electrically stimulated. To separate effects produced by medial olivocochlear (MOC) efferents from effects produced by lateral olivocochlear (LOC) efferents, MOC efferents were selectively stimulated by an electrode in the region of the MOC cell bodies in cats. For comparison, efferents were also stimulated with an electrode in the fourth ventricle (OCB stimulation, previously called COCB stimulation). MOC stimulation and fourth-ventricle OCB stimulation both produced qualitatively similar results bilaterally in that auditory-nerve compound action potential (N1) and endocochlear potential were reduced, and cochlear microphonic (CM) was increased. Both efferent-induced changes were affected in similar ways by changes in shock parameters, and were blocked by strychnine. At low sound levels, the decrease in N1 amplitude was approximately equivalent to a shift (decrease) in sound level but the change in N1 latency was not. The ratio of the CM increase to the N1 sound-level shift was independent of shock level or location. MOC stimulation typically produced an N1 sound-level shift of 11-16 dB in the contralateral ear and 4-7 dB in the ipsilateral ear. The ratio of these shifts almost equals the ratio of MOC neurons which had cell bodies on the stimulating-electrode side. Previous results reported by others with 'UOCB stimulation' now seem attributable to excitation of uncrossed MOC efferents rather than to excitation of uncrossed LOC efferents as previously thought. There is no effect reported in the literature or seen by us which can definitely be attributed to LOC neurons. Fourth-ventricle OCB stimulation typically produced an N1 sound-level shift in both ears of 19-22 dB which is approximately the sum of the crossed and uncrossed MOC shifts. Considering also that many uncrossed-MOC fibers course close to the midline (i.e. near the stimulating electrode), it seems likely that fourth-ventricle OCB stimulation excites both crossed and uncrossed MOC efferents. Referring to such stimulation in the cat as 'COCB stimulation' is therefore inaccurate and may lead to wrong conclusions about the functional role of various components of the olivocochlear fibers.

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.

The dual origins of the olivocochlear bundle in the albino rat.

Recent studies of the origins and terminations of the olivocochlear bundle (OCB) in the cat provide evidence that separate efferent systems differentially innervate the two types of hair cells in the organ of Corti. To begin to test the generality of these separate olivocochlear systems, the cells of origin of the OCB were determined in the albino rat by using axonal transport of horseradish peroxidase. Our findings are that, as in the cat, two classes of olivocochlear (OC) neurons project to the cochlea. These neurons could be dichotomized according to their location in the superior olivary complex (lateral or medial), their size (small or large), and their preferred side of projection (ipsilateral or contralateral). All labeled OC neurons also exhibited a positive reaction for acetylcholinesterase. In the rat, however, lateral and medial OC neurons are each restricted to a single nucleus, and, furthermore, the lateral OC neurons project only ipsilaterally. The rat also has significantly fewer mean totals of both lateral (240 vs. 710) and medial (240 vs. 520) OC neurons than the cat. The present methods also demonstrated the course of axons of the OCB and axon collaterals entering the cochlear nuclear complex. Vestibular efferent neurons were also labeled bilaterally medial and lateral to the facial genu. Our results suggest that the general organizational plan of OC neurons in the rat may offer advantages over the situation in the cat for studies of connectional neuroanatomy, neurophysiology, and behavioral functions of the two olivocochlear systems.