Plasticity of synaptic endings in the cochlear nucleus following noise-induced hearing loss is facilitated in the adult FGF2 overexpressor mouse

In adult mammals a single exposure to loud noise can damage cochlear hair cells and initiate subsequent episodes of degeneration of axonal endings in the cochlear nucleus (CN). Possible mechanisms are loss of trophic support and/or excitotoxicity. Fibroblast growth factor 2 (FGF2), important for development, might be involved in either mechanism. To test this hypothesis, we noise-exposed FGF2 overexpressor mice and observed the effects on synaptic endings by immunolabelling for SV2, a synaptic vesicle protein, at 1, 2, 4, and 8 weeks after noise exposure. SV2 staining was observed in two major locations; perisomatic, representing axo-somatic terminals, and neuropil, representing axo-dendritic terminals. The wildtype (WT) lost both perisomatic and neuropil clusters with an intervening period of modest recovery for the perisomatic. In contrast, in the overexpressor, the perisomatic clusters remained unchanged after intervening periods of increase. The neuropil clusters underwent a period of initial decline, followed by a transient recovery and ultimate decline. Changes in SV2 immunostaining correlated with changes in vesicular glutamate and GABA transporters at synapses and, in the overexpressor, with staining changes for FGF2 and FGF receptor 1. These molecules may contribute to the synaptic reorganization after noise damage; they may protect and/or aid recovery of synapses after overstimulation.     

Fine structure of long-term changes in the cochlear nucleus after acoustic overstimulation: chronic degeneration and new growth of synaptic endings

The companion study showed that acoustic overstimulation of adult chinchillas, with a noise level sufficient to damage the cochlea, led to cytological changes and degeneration of synaptic endings in the cochlear nucleus within 1-16 weeks. In the present study, the same stimulus was used to study the long-term effects on the fine structure of synaptic endings in the cochlear nucleus. For periods of 6 and 8 months after a single exposure to a damaging noise level, there ensued a chronic, continuing process of neurodegeneration involving excitatory and inhibitory synaptic endings. Electron microscopic observations demonstrated freshly occurring degeneration even as late as 8 months. Degeneration was widespread in the neuropil and included the synapses on the globular bushy cell, which forms part of the main ascending auditory pathway. Neurodegeneration was accompanied by newly formed synaptic endings, which repopulated some of the sites vacated previously by axosomatic endings on globular bushy cells. Many of these synaptic endings must arise from central interneurons. The findings suggest that overstimulation can induce a self-sustaining condition of progressive neurodegeneration accompanied by a new growth of synaptic endings. Noise-induced hearing loss thus may progress as a neurodegenerative disease with the capacity for synaptic reorganization within the cochlear nucleus.     

Synaptophysin immunoreactivity in the cochlear nucleus after unilateral cochlear or ossicular removal

This study determined if unilateral cochlear removal in adult guinea pigs led to synaptic loss followed by synaptogenesis in the cochlear nucleus (CN) and if unilateral middle ear ossicle removal led to synaptic loss in the CN. Synaptic endings were identified immunohistochemically, using a monoclonal antibody to synaptophysin. Immunolabeling was quantified densitometrically in the CN 4–161 days after cochlear removal and 161 days after ossicle removal. Fiber degeneration was visualized with the Nauta-Rasmussen silver method. Tissue shrinkage was measured from drawings of CN sections. Compared to the contralateral side, immunolabeling density ipsilaterally was reduced by 4 days in the anterior division of the anteroventral CN (a-AVCN) and by 7 days in the anterior part of the posteroventral CN (a-PVCN). At 7 days, preterminal fiber degeneration was abundant in both areas. These findings were consistent with the loss of cochlear nerve endings and fibers. At later times, immunolabeling density recovered. In the a-AVCN, tissue shrinkage explained approximately half the recovery of staining density; the rest was attributed to synaptogenesis. In the a-PVCN, the entire recovery was attributed to tissue shrinkage. In the polymorphic layer of the dorsal CN, immunostaining density increased transiently at 4 days, while at 7 days preterminal fiber degeneration was abundant. A net loss of synaptic endings was not detected immunohistochemically. The increased immunostaining density may reflect a transient growth of immature processes or presynaptic endings. Ossicle removal produced a deficit in immunolabeling density only in the ipsilateral a-PVCN, without fiber degeneration, suggesting a loss of presynaptic endings or of synaptophysin expression. Synapse 25:243–257, 1997. © 1997 Wiley-Liss, Inc.     

Ascending connections of the anterior ventral cochlear nucleus in the rat

The purpose of the experiment was to determine the connections of the three regions of the anterior ventral cochlear nucleus in the rat. Lesions were placed in the regions of the nucleus in 11 rats. After 25 days each animal was sacrificed and its brain impregnated by the protargol method of Bodian. The superior olivary complex, the lateral lemniscus and the dorsal acoustic stria were examined for diminution of synaptic endings, thinning of neuropil and debris of degeneration. Complete destruction of region III of the anterior ventral cochlear nucleus produced degeneration in the ipsilateral lateral superior olive and in the medial superior olive of both sides. Destruction of other regions of the anterior ventral cochlear nucleus, region III remaining intact, had no effect upon the lateral or medial superior olive. Destruction of region II of the anterior ventral cochlear nucleus produced degeneration in the anterior halves of the ipsilateral lateral nucleus of the trapezoid body and the contralateral medial nucleus of the trapezoid body. Complete destruction of other regions of the anterior ventral cochlear nucleus, region II remaining intact, was without effect upon the lateral and medial nuclei of the trapezoid body. No degeneration was detected following destruction of region I of the anterior ventral cochlear nucleus.     

Quantitative study of degeneration and new growth of axons and synaptic endings in the chinchilla cochlear nucleus after acoustic overstimulation

To determine if acoustic overstimulation altered synaptic connections in the cochlear nucleus, anesthetized adult chinchillas, with one ear protected by a silicone plug, were exposed for 3 hr to a 108-dB octave-band noise, centered at 4 kHz, and allowed to survive for periods up to 32 weeks. This exposure led to cochlear damage in the unprotected ear, mainly in the basal regions of the organ of Corti. The anterior part of the ipsilateral posteroventral cochlear nucleus consistently contained a band of degenerating axons and terminals, in which electron microscopic analysis revealed substantial losses of axons and synaptic terminals with excitatory and inhibitory cytology. The losses were significant after 1 week's survival and progressed for 16-24 weeks after exposure. By 24-32 weeks, a new growth of these structures produced a resurgence in the number of axons and terminals. The net number of excitatory endings fully recovered, but the quantity with inhibitory cytology was only partially recouped. Neuronal somata lost both excitatory and inhibitory endings at first and later recovered a full complement of excitatory but not inhibitory terminals. Dendrites suffered a net loss of both excitatory and inhibitory endings. Excitatory and inhibitory terminals with unidentified postsynaptic targets in the neuropil declined, then increased in number, with excitatory terminals exhibiting a greater recovery. These findings are consistent with a loss and regrowth of synaptic endings and with a reorganization of synaptic connections that favors excitation.     

Fine structure of degeneration in the cochlear nucleus of the chinchilla after acoustic overstimulation

To study plastic changes in the cochlear nucleus after acoustic stimulation, adult chinchillas were exposed once to a 4-kHz octave-band noise at 108 dB SPL for 3 hr. After survival times of 1, 2, 4, 8, and 16 weeks, samples were taken for electron microscopy from a part of the cochlear nucleus, where cochlear nerve fibers degenerated after the noise exposure. Progressive changes in fine structure were characterized as early, intermediate, and late stages of degeneration. Freshly occurring synaptic degeneration appeared in each period from 1-16 weeks. Endings with large round vesicles, putative excitatory synapses of the cochlear nerve, displayed progressive increases in neurofilaments and enlarged synaptic vesicles. Compared to controls, synaptic vesicles seemed fewer, often in small clusters in the interior of endings, and smaller in the synaptic zone. These early changes progressed to mitochondrial disintegration and overt "watery" degeneration. Some surviving endings, however, were shrunken and displaced partially by enlarged spaces in the synaptic complex. Dense-cored vesicles gathered in these endings. In terminals with pleomorphic and flattened vesicles, presumed inhibitory endings, cytological changes appeared within 1 week and persisted for months. The synaptic endings darkened, some vesicles disintegrated, and many smaller flatter vesicles collapsed into heaps. Especially at the presynaptic membrane, vesicles were shriveled, but a few mitochondria were preserved. Without overt signs of synaptic degeneration, some of these cytological changes presumably reflect reduced synaptic activity in the inhibitory endings. These changes may contribute to a continuing process associated with abnormal auditory functions, including hyperacusis and tinnitus.     

Noise trauma alters D-[3H]aspartate release and AMPA binding in chinchilla cochlear nucleus

Exposure of adults to loud noise can overstimulate the auditory system, damage the cochlea, and destroy cochlear nerve axons and their synaptic endings in the brain. Cochlear nerve loss probably results from the death of cochlear inner hair cells (IHC). Additional degeneration in the cochlear nucleus (CN) is hypothesized to stem from overstimulation of the system, which may produce excitotoxicity. This study tested these predictions by exposing one ear of anesthetized adult chinchillas to a loud noise, which damaged the ipsilateral cochlea and induced degeneration in the glutamatergic cochlear nerve. During the first postexposure week, before cochlear nerve axons degenerated, glutamatergic synaptic release in the ipsilateral CN was elevated and uptake was depressed, consistent with hyperactivity of glutamatergic transmission and perhaps with the operation of an excitotoxic mechanism. By 14 days, when cochlear nerve fibers degenerated, glutamatergic synaptic release and uptake in the CN became deficient. By 90 days, a resurgence of transmitter release and an elevation of AMPA receptor binding suggested transmission upregulation through plasticity that resembled changes after mechanical cochlear damage. These changes may contribute to tinnitus and other pathologic symptoms that precede and accompany hearing loss. In contrast, the other ear, protected with a silicone plug during the noise exposure, exhibited virtually no damage in the cochlea or the cochlear nerve. Altered glutamatergic release and AMPA receptor binding activity in the CN suggested upregulatory plasticity driven by signals emanating from the CN on the noise-exposed side.     

Projections of the trapezoid body and the superior olivary complex of the Kangaroo rat (Dipodomys merriami).

Glass micropipettes filled with 2 M sodium cyanide were used to physiologically locate and iontophoretically damage the nucleus of the trapezoid body (NTB), the medial superior olive (MSO), and the lateral superior olive (LSO). Mechanical lesions were made in the trapezoid body as it leaves the cochlear nuclei. After a 3- to 10-day survival time the projections and terminal degeneration were traced with the Fink-Heimer and Nauta-Gygax stains. The ventral cochlear nucleus (VCN) projects via the trapezoid body to ipsilateral LSO, ipsilateral preolivary nuclei, ipsilateral lateral and a contralateral medial dendritic fields of MSO, and contralateral NTB; there is also a small ipsilateral projection to the ventral nucleus of the lateral lemniscus (VNLL) and the central nucleus of the inferior colliculus (CNIC). Some trapezoid body fibers ascend via the contralateral lateral lemniscus to VNLL, DNLL (dorsal nucleus of the lateral lemniscus), and CNIC. There is no projection from the ventral cochlear nucleus to the ipsilateral NTB and contralateral preolivary nuclei. All portions of NTB project ipsilaterally to LSO (ventral NTB to dorsomedial LSO, dorsal NTB to ventral LSO) and to the retro-olivary nucleus. In two animals with NTB lesions there is also degeneration in the ventromedial portion of the ipsilateral facial nucleus. NTB projects contralaterally by way of the stria of Monakow to the pyramidal and molecular cell layers of the dorsal cochlear nucleus (DCN). The NTB does not project ipsilaterally to MSO, preolivary nuclei, VNLL, DNLL and CNIC. Contralaterally there are no projections to any of the nuclei of the auditory pathway except the DCN. Most MSO projections are ipsilateral. The densest goes by way of the lateral lemniscus to the lateral aspect of the ipsilateral CNIC, terminating throughout its dorsoventral axis. MSO also projects bilaterally to the pyramidal and molecular cell layers of dorsal cochlear nucleus (DCN), and ipsilaterally to the ventral portion of the motor nucleus of V and to the facial nucleus. MSO does not project ipsilaterally to the LSO, NTB, preolivary, VCN and retro-olivary nuclei. On the contralateral side, all structures except the DCN are free of projection patterns from axons originating in the MSO. LSO projects bilaterally to the central and ventral portions of CNIC and to the nuclei of the lateral lemnisci, and ipsilaterally to the large and small spherical cell areas of anterior ventral cochlear nucleus (AVCN) and to all portions of DCN. The LSO does not project ipsilaterally to the NTB, MSO, preolivary and retro-olivary nuclei. On the side opposite, this nucleus does not project to NTB, MSO, retro-olive, VCN, preolivary and LSO. For all lesions regardless of the site, there is no degeneration found rostral to the CNIC. The medial geniculate body or other structures in the diencephalon or cortex are free of any fields of terminal degeneration.     

Chemical anatomy of excitatory endings in the dorsal cochlear nucleus of the rat: Differential synaptic distribution of aspartate aminotransferase,glutamate, and vesicular zinc

In order to identify cytochemical traits relevant to understanding excitatory neurotransmission in brainstem auditory nuclei, we have analyzed in the dorsal cochlear nucleus the synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc, three molecules probably involved in different steps of excitatory glutamatergic signaling. High levels of glutamate immunolabeling were found in three classes of synaptic endings in the dorsal cochlear nucleus, as determined by quantitation of immunogold labeling. The first type included auditory nerve endings, the second were granule cell endings in the molecular layer, and the third very large endings, better described as “mossy.” This finding points to a neurotransmitter role for glutamate in at least three synaptic populations in the dorsal cochlear nucleus. The same three types of endings enriched in glutamate immunoreactivity also contained histochemically detectable levels of aspartate aminotransferase activity, suggesting that this enzyme may be involved in the synaptic handling of glutamate in excitatory endings in the dorsal cochlear nucleus. There was also extrasynaptic localization of the enzyme. Zinc ions were localized exclusively in granule cell endings, as determined by a Danscher-selenite method, suggesting that this ion is involved in the operation of granule cell synapses in the dorsal cochlear nucleus. J. Comp. Neurol. 399:341–358, 1998. © 1998 Wiley-Liss, Inc.     

Selective labeling of spiral ganglion and granule cells with D-aspartate in the auditory system of cat and guinea pig

The present study sought to locate putative glutamatergic or aspartatergic pathways in the auditory system of cats and guinea pigs. We injected 0.06 to 3 mM D-[3H] aspartate (D-Asp) in the cochlear nucleus before preparation for light microscopic autoradiography. At short survival times (15 and 40 min) there was heavy labeling of astrocytic somata. Labeling patterns typical of cochlear nerve endings decorated neurons in the cochlear nucleus, e.g., cell bodies and dendritic trunks of octopus cells. Labeling patterns consistent with retrograde axonal transport by the parallel fibers of granule cells appeared in the molecular layer of the dorsal cochlear nucleus and in the external granular layer. Retrograde labeling of the cochlear nerve root fibers also occurred. Consistent with these results are companion biochemical findings on the rapidly dissected cochlear nuclei of guinea pigs. The dorsal, anteroventral, and posteroventral cochlear nuclei, each, evinced uptake of D-Asp. Subsequently, electrical stimulation of each nucleus released a portion of the accumulated amino acid. Most of this release probably came from synaptic endings. Another group of experiments compared autoradiographic localization of 0.06 to 3 mM D-Asp to that of horseradish peroxidase (HRP) 6 hr to 2 d after injections in the cochlear nucleus. Astroglial cell bodies were no longer labeled by D-Asp, but spiral ganglion cell bodies in the cochlea and granule cell bodies in the cochlear nucleus were. Perikarya of the periolivary and ventral cochlear nuclei projecting to the dorsal cochlear nucleus were labeled by HRP and not by D-Asp. Thus, comparisons with the HRP findings indicate that D-Asp labeling resulted from a selective retrograde transport. There was no evidence for a selective anterograde axonal transport. The present observations support the hypothesis that cochlear nerve fibers and granule cells may use L-glutamate and/or L-aspartate as a transmitter in the cochlear nucleus.     

Cochlear damage induces GAP-43 expression in cholinergic synapses of the cochlear nucleus in the adult rat: a light and electron microscopic study

Recent studies suggest a potential for activity-dependent reconstruction in the adult mammalian brainstem that exceeds previous expectations. We found that a unilateral cochlear lesion led within 1 week to a rise of choline acetyltransferase (ChAT) immunoreactivity in the ventral cochlear nucleus of the affected side, matching the lesion-induced expression of growth-associated protein 43 (GAP-43) previously described. The rise of both ChAT and GAP-43 immunoreactivity was reflected in the average density of the staining. Moreover, the number of light-microscopically identifiable boutons increased in both stains. GAP-43-positive boutons could, by distinct ultrastructural features, regularly be identified as presynaptic endings. However, GAP-43 immunoreactivity was not only found in presynaptic endings with a classical morphology, but also in profiles that suggest morphological dynamic structures by showing filopodia, assemblages of pleomorphic vesicles, large vesicles (diameter up to 200 nm) fusing with the presynaptic plasma membrane close to synaptic contacts, small dense-core vesicles (diameter about 80 nm) and presynaptic ribosomes. Moreover, we observed perforated synapses as well as GAP-43 immunoreactivity condensed in rafts, both indicative of growing or changing neuronal connections. Classical and untypical ultrastructural profiles that contained GAP-43 also contained ChAT. We conclude that there is extensive deafness-induced GAP-43-mediated synaptic plasticity in the cochlear nucleus, and that this plasticity is predominantly, if not exclusively, based on cholinergic afferents.     

Cytology of periolivary cells and the organization of their projections in the cat

Projections of cells located near principal nuclei of the superior olive, periolivary cells, were studied by injecting horseradish peroxidase or fluorescent tracers into the cochlea, cochlear nucleus, and inferior colliculus. At least two distinct cytological classes of periolivary cells were found to project to each of these structures. "Large" and "small" olivocochlear cells were labelled. Their cytology and locations were found to be as had been previously described. Some olivocochlear cells also project to the cochlear nucleus. Other major periolivary cell classes that project to the cochlear nucleus include a lateral group of multipolar cells whose members are located around the ipsilateral lateral superior olive and have coarse, darkly staining Nissl substance. The other major periolivary cell class that projects to the cochlear nucleus is the small cell of the ventral nucleus of the trapezoid body. This cell is characterized by its size and by only one or two intensely staining clumps of Nissl substance. Projections of these cells to the cochlear nucleus is from both sides. Periolivary cells that project to the inferior colliculus include medial and lateral groups. Cells of the lateral group project from both sides. These cells are multipolar in shape and contain lightly staining, flocculent Nissl substance. They are predominantly located immediately ventral to the lateral superior olive. Projections from the medial group are predominantly ipsilateral and arise from the region medial to the medial superior olive. The cells are multipolar and contain clumped Nissl substance. They often lie near "large" olivocochlear cells, which they resemble in Nissl material, but are distinguished from the latter in Protargol material by having ring-type axosomatic endings. The appearance and locations of these six classes of periolivary cells make it possible to recognize them in nonexperimental material and to infer with confidence what their projections are. These results show considerable organization of these previously little understood structures.     

An electron microscopic study of synaptic organization in the medial superior olive of normal and experimental chinchillas

The medial superior olive (MSO) was studied in normal animals to determine the types of synaptic endings and their distribution over the surface of MSO neurons. Unilateral lesions were made in the anteroventral cochlear nucleus (AVCN) of experimental animals to determine the source of at least one synaptic type in the MSO. The surfaces of MSO neurons in normal animals were studded with three distinct types of synaptic endings distinguished mainly by the size of their synaptic vesicles. There were endings with large vesicles, 510 Å in mean diameter; endings with small vesicles, 380 Å; and endings with vesicles intermediate in size. 435 Å. The large vesicle ending typically was greater than 2 μm in maximum diameter. It appeared as the termination of a myelinated axon or as a swollen portion of a node and made multiple asymmetrical synapses. Large vesicle endings occurred exclusively on dendrites where they formed 85% of the synaptic endings. Small vesicle endings typically were less than 2 μm in diameter. They appeared as the termination of a fine unmyelinated axon and made only one symmetrical synapse. Small vesicle boutons occurred infrequently over the entire neuronal surface. Intermediate vesicle synaptic endings were similar to large vesicle endings except that they were present only on the cell body, axon hillock, and proximal portions of the dendrites where they formed most of the synapses. In AVCN lesioned animals degenerating myelinated axons and large vesicle synaptic endings were distributed to the lateral dendrites of the ipsilateral MSO and medial dendrites of the contralateral one. In addition, a few degenerating axons and large vesicle endings were found among the ipsilateral medial dendrites. The changes in the degenerating endings were characterized by an early proliferation of neurofilaments and swelling of the endings followed by collapse of the endings and increase in electron density, disappearance of filaments and synaptic vesicles, and phagocytosis of the degenerated endings by reactive glial cells. No degenerative changes were observed in the small and intermediate vesicle endings. The results of this study indicate that the more numerous large vesicle endings presynaptic to the MSO dendrites are the axon terminals of neurons in the AVCN. The persistence after lesions of the small and intermediate vesicle endings suggests that they arise from as yet unidentified sources.     

Patterns of glutamate decarboxylase immunostaining in the feline cochlear nuclear complex studied with silver enhancement and electron microscopy

The cochlear nuclear complex of the cat was immunostained with an antiserum to glutamate decarboxylase (GAD), the biosynthetic enzyme for the inhibitory neurotransmitter GABA, and studied with different procedures, including silver intensification, topical colchicine injections, semithin sections, and immunoelectron microscopy. Immunostaining was found in all portions of the nucleus. Relatively few immunostained cell bodies were observed: most of these were in the dorsal cochlear nucleus and included stellate cells, cartwheel cells, Golgi cells, and unidentified cells in the deep layers. An accumulation of immunoreactive cells was also found within the small cell cap and along the medial border of the ventral cochlear nucleus. Immunostained cells were sparse in magnocellular portions of the ventral nucleus. Most staining within the nucleus was of nerve terminals. These included small boutons that were prominent in the neuropil of the dorsal cochlear nucleus, the granule cell domain, in a region beneath the superficial granule cell layer within the small cell cap region, and along the medial border of the ventral nucleus. Octopus cells showed small, GAD-positive terminals distributed at moderate density on both cell bodies and dendrites. Larger, more distinctive terminals were identified on the large cells in the ventral nucleus, in particular on spherical cells and globular cells. There was a striking positive correlation of the size, location, and complexity of GAD-positive terminals with the size, location, and complexity of primary fiber endings on the same cells. This correlation did not hold in the dorsal nucleus, where pyramidal cells receive many large GAD-positive somatic terminals despite the paucity of primary endings on their cell bodies. The GAD-positive terminals contained pleomorphic synaptic vesicles and formed symmetric synaptic junctions that occupied a substantial portion of the appositional surface to cell bodies, dendrites, axon hillocks, and the beginning portion of the initial axon segments. Thus, the cells provided with large terminals can be subjected to considerable inhibition that may be activated indirectly through primary fibers and interneurons or by descending inputs from the auditory brainstem.     

Degeneration in the ventral cochlear nucleus after severe noise damage in mice

To study the mechanisms of noise-induced hearing loss and the phantom noise, or tinnitus, often associated with it, we used a mouse model of noise damage designed for reproducible and quantitative structural analyses. We selected the posteroventral cochlear nucleus, which has shown considerable plasticity in past studies, and correlated its changes with the distribution of neurotrophin 3 (NT3). We used volume change, optical density analysis, and microscopic cluster analysis to measure the degeneration after noise exposure. There was a fluctuation pattern in the reorganization of nerve terminals. The data suggest that the source and size of the nerve terminals affect their capacity for regeneration. We hypothesize that the deafferentation of ventral cochlear nucleus is the structural basis of noise-induced tinnitus. In addition, the immunofluorescent data show a possible connection between NT3 and astrocytes. There appears to be a compensatory process in the supporting glial cells during this degeneration. Glia may play a role in the mechanisms of noise-induced hearing loss.     

Projections to the inferior colliculus from the anteroventral cochlear nucleus in the cat: possible substrates for binaural interaction

The projections to the inferior colliculus of the cat are shown in autoradiographs after injections of 3H-amino acids into the anteroventral cochlear nucleus and anterograde axonal transport. Labeled bands of axons are seen in the central nucleus of the inferior colliculus, parallel to the fibrodendritic laminae, and in layers 3 and 4 of the dorsal cortex. A bilateral projection to the lateral, low-frequency part of the inferior colliculus is observed. In contrast, the more ventromedial, mid- and high-frequency parts receive only a contralateral input. The projections from the cochlear nucleus to both the contralateral midbrain and bilaterally to the superior olivary complex appear to be tonotopically organized. After HRP injections in the inferior colliculus, small numbers of stellate neurons are labeled in the lateral and ventral low-frequency parts of the anteroventral cochlear nucleus on the ipsilateral side. EM autoradiographs show labeled axonal endings from both sides of the anteroventral cochlear nuclei are present in the same proportion in pars lateralis. Axonal endings from either cochlear nucleus have small, round synaptic vesicles and make asymmetric synaptic contacts on dendrites. Axons from the contralateral side also make axosomatic contacts on large disc-shaped or stellate cells. Neurons from the ipsilateral anteroventral cochlear nucleus apparently make more synaptic endings per cell as compared to neurons from the contralateral side. Together, bilateral inputs from the anteroventral cochlear nucleus can account for a third of the endings with round synaptic vesicles in pars lateralis of the central nucleus. Morphological similarities among the ascending inputs to the inferior colliculus are discussed. Both direct circuits from the cochlear nucleus to the inferior colliculus and indirect circuits via the superior olivary complex or lateral lemniscus may display banding patterns, nucleotopic organization, or differential synaptic organization. The direct inputs from the anteroventral cochlear nucleus to the colliculus may influence binaural interactions which occur in the superior olivary complex. In addition, direct inputs may create new binaural responses in the inferior colliculus that are independent of lower centers.     

Projections of the pontine nuclei to the cochlear nucleus in rats.

In the cochlear nucleus, there is a magnocellular core of neurons whose axons form the ascending auditory pathways. Surrounding this core is a thin shell of microneurons called the granule cell domain (GCD). The GCD receives auditory and nonauditory inputs and projects in turn to the dorsal cochlear nucleus, thus appearing to serve as a central locus for integrating polysensory information and descending feedback. Nevertheless, the source of many of these inputs and the nature of the synaptic connections are relatively unknown. We used the retrograde tracer Fast Blue to demonstrate that a major projection arises from the contralateral pontine nuclei (PN) to the GCD. The projecting cells are more densely located in the ventral and rostral parts of the PN. They also are clustered into a lateral and a medial group. Injections of anterograde tracers into the PN labeled mossy fibers in the contralateral GCD. The terminals are confined to those parts of the GCD immediately surrounding the ventral cochlear nucleus. There is no PN projection to the dorsal cochlear nucleus. These endings have the form of bouton and mossy fiber endings as revealed by light and electron microscopy. The PN represent a key station between the cerebral and cerebellar cortices, so the pontocochlear nucleus projection emerges as a significant source of highly processed information that is introduced into the early stages of the auditory pathway. The cerebropontocerebellar pathway may impart coordination and timing cues to the motor system. In an analogous way, perhaps the cerebropontocochlear nucleus projection endows the auditory system with a timing mechanism for extracting temporal information.     

Distribution of BDNF, NT-3 and NT-4 in the developing auditory brainstem

This study describes the developmental expression of three neurotrophins, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin (NT-4) in the rat auditory brain-stem using immunohistochemistry. At postnatal day 0 (PND 0), neurotrophins expression was virtually absent from all auditory nuclei in the brainstem, even though some positive neurons were observed in the mesencephalic trigeminal nucleus at this age. However, BDNF, NT-3 and NT-4 positive neurons were observed in most brainstem auditory nuclei by PND 6. At the following stages, there was a general increase in the intensity of the neurotrophins immunoreactivity and BDNF labeling was particularly prominent in most cochlear nucleus neurons. A differential pattern of staining emerged in cochlear nucleus subdivisions, with more intense staining present in the ventral part. The superior olivary complex nuclei followed a similar pattern of BDNF staining compared to the cochlear nucleus. In the adult, BDNF heavily labeled most neurons of the superior olivary nuclei and moderately labeled neurons of the inferior colliculus (IC). NT-3 and NT-4 showed a similar pattern of staining in most auditory brainstem nuclei. The first staining was observed by PND 6 in some neuronal cell bodies. NT-3 and NT-4 immunoreactivity increased in the following stages and in the adult moderate labelings were observed in most neurons of the cochlear nucleus, the superior olivary nuclei and the IC. These results show that neurotrophins are expressed 1 week before the onset of hearing and the increase of their expressions correlate with the appearance of sound-evoked activity in the system. The temporal distribution of neurotrophins does not correlate with neuronal birth, axonal outgrowth or the formation of connection in the auditory structures, suggesting a role primarily in the maintenance and/ or modulation of postnatal and adult functions.     

The course and termination of the striae of Monakow and held in the cat

The course and termination of the stria of Monakow and stria of Held were studied with Nauta-Gygax technique following a localized lesion of the dorsal cochlear nucleus in the cat. A dense preterminal degeneration within all homolateral primary cochlear nuclei and a tract of degenerated fibers crossing the restiform body dorsally were found. This tract divides in two branches forming the stria of Held and stria of Monakow. The stria of Held followed the course described by classical anatomists. In the homolateral side, the stria gave terminals to the medial and lateral preolivary nuclei, lateral superior olive nucleus and retro-olivary groups. In the contralateral side, preterminal degeneration was found in the medial preolivary nucleus and medial retro-olivary group. The stria of Monakow is essentially crossed pathway given numerous terminals to the contralateral dorsal retro-olivary group, rostral part of the lateral preolivary nucleus, the ventral and dorsal nuclei of the lateral lemniscus, and nucleus of the inferior colliculus.     

Descending inputs to the caudal cochlear nucleus of the cat: degeneration and autoradiographic studies.

Descending paths from the inferior colliculus (IC) and dorsal (DNLL) and ventral (VNLL) nuclei of the lateral lemniscus and their terminations in the caudal cochlear nucleus in cats were studied both in silver degeneration material following central lesions and in autoradiographs following central injections of tritiated leucine (³H-leu). Eight adult cats received electrolytic lesions of the IC, the IC and DNLL, or the IC, DNLL and VNLL. Control cats were subjected either to surgery alone or to a lesion in a non-auditory nucleus. After survival periods of 1 to 16 days, all cats were killed and their brain stems prepared by modified Nauta methods. Twenty-two other cats received injections of ³H-leu into the dorsal IC, the ventral IC and DNLL, or the DNLL and VNLL. Their brains were prepared for LM autoradiography. After lesions of the IC, Nauta material contained moderate degeneration of fine fibers that coursed predominantly in the lateral lemniscus, and partially decussated in the caudal trapezoid body to enter the cochlear nuclei of both sides. Sparse, mainly peridendritic degeneration of fine fibers occurred in the PVCN, while moderate, peridendritic and perisomatic degeneration occurred in the deep and innermost fusiform cell layers of the dorsal cochlear nucleus (DCN). Lesions of both the ventral IC and the DNLL showed additional bilateral degeneration of medium-caliber axons in the descending tracts as well as preterminal degeneration of these larger axons sparsely distributed in the deep layers of the DCN. The density of medium-caliber axonal degeneration (particularly in the deep DCN) increased when lesions also included the VNLL. After ³H-leu injections into the tectum and/or upper pons, autoradiographs confirmed the presence of sparse bilateral inputs from both the IC and from the DNLL and VNLL to ventral cochlear nucleus neurons, including some octopus cell dendrites. The autoradiographic studies also clearly revealed a predominant input to the outer layer of the DCN from the dorsal IC, a predominant input to the middle layer from the ventral IC, and a predominant input to the deeper DCN from the lateral lemniscal nuclei. Limited evidence of a topographic, perhaps tonotopic, order within the descending connections was best seen in Nauta material. The differences shown here in fiber types, trajectories, and target sites from the IC, the DNLL, and the VNLL are discussed in terms of possible functional differences among the descending influences on neurons of the caudal cochlear nucleus.