Subdivisions Of Cochlear Nucleus Research Articles

Sensitivity to Pulse Rate and Amplitude Modulation in an Animal Model of the Auditory Brainstem Implant (ABI).

The auditory brainstem implant (ABI) is an auditory neuroprosthesis that provides hearing by electrically stimulating the cochlear nucleus (CN) of the brainstem. Our previous study (McInturff et al., 2022) showed that single-pulse stimulation of the dorsal (D)CN subdivision with low levels of current evokes responses that have early latencies, different than the late response patterns observed from stimulation of the ventral (V)CN. How these differing responses encode more complex stimuli, such as pulse trains and amplitude modulated (AM) pulses, has not been explored. Here, we compare responses to pulse train stimulation of the DCN and VCN, and show that VCN responses, measured in the inferior colliculus (IC), have less adaption, higher synchrony, and higher cross-correlation. However, with high-level DCN stimulation, responses become like those to VCN stimulation, supporting our earlier hypothesis that current spreads from electrodes on the DCN to excite neurons located in the VCN. To AM pulses, stimulation of the VCN elicits responses with larger vector strengths and gain values especially in the high-CF portion of the IC. Additional analysis using neural measures of modulation thresholds indicate that these measures are lowest for VCN. Human ABI users with low modulation thresholds, who score best on comprehension tests, may thus have electrode arrays that stimulate the VCN. Overall, the results show that the VCN has superior response characteristics and suggest that it should be the preferred target for ABI electrode arrays in humans.

Using Extracochlear Multichannel Electrical Stimulation to Relieve Tinnitus and Reverse Tinnitus-Related Auditory-Somatosensory Plasticity in the Cochlear Nucleus

ObjectivesTinnitus has no reliable cure but may be significantly relieved by the usage of cochlear implants. However, not all tinnitus patients necessitate cochlear implantation that can impair hearing. This study was to investigate whether a novel extracochlear electrical stimulation (EES) strategy could relieve tinnitus of guinea pigs without hearing impairment, and the roles of auditory-somatosensory plasticity in the cochlear nucleus in the tinnitus relief. Materials and MethodsWe used a novel four-electrode extracochlear implant to electrically stimulate the cochlea of tinnitus guinea pigs. Tinnitus was assessed by the gap-prepulse inhibition of the acoustic startle reflex (GPIAS) ratios and the tinnitus index. The plasticity of auditory and somatosensory innervation in the different subdivisions of cochlear nucleus was evaluated by immunostaining of vesicular glutamate transporter 1 (VGLUT1) and VGLUT2, respectively. ResultsThe EES induced significant decreases of GPIAS ratios and the tinnitus index of tinnitus guinea pigs, indicating reductions of tinnitus behavioral manifestations. Meanwhile, the EES reversed the abnormal auditory-somatosensory innervation in the cochlear nucleus of tinnitus animals but did not change the hearing and the numbers of inner hair cell synapses. ConclusionsThis study demonstrated that the novel EES strategy could effectively relieve tinnitus without impairment to hearing and cochlear structure of tinnitus animals. The reversal of tinnitus-related auditory-somatosensory plasticity in the cochlear nucleus was correlated with the tinnitus relief induced by the EES.

Effects of brainstem lesions on amino acid levels in the rat cochlear nucleus

There is evidence for glutamate, γ-amino butyric acid (GABA), and glycine as neurotransmitters of centrifugal pathways to the cochlear nucleus, but the quantitative extent of their contributions to amino acid neurotransmission in cochlear nucleus regions has not been known. We used microdissection of freeze-dried tissue sections of rat cochlear nucleus, with mapping of sample locations, combined with a high performance liquid chromatography (HPLC) assay, to measure amino acid levels in cochlear nucleus subregions of rats with unilateral lesions of centrifugal pathways to the cochlear nucleus. In rats with lesions transecting all or almost all pathways to the cochlear nucleus from brain stem regions, GABA, aspartate, and glutamate levels were reduced, compared to contralateral values, in almost all ipsilateral cochlear nucleus regions. The largest reductions, in dorsal (DCN), anteroventral (AVCN), and posteroventral (PVCN) cochlear nucleus regions, approached 50% for GABA, 40% for aspartate, and 30% for glutamate. In contrast, glutamine and taurine levels were typically higher in lesioned-side cochlear nucleus regions than contralaterally. Effects on glycine levels were mixed but usually included increased lesioned-side values compared to contralateral, probably reflecting a balance between increases during protein breakdown and decreases of free glycine in transected pathways. More limited lesions transecting just dorsal pathways showed much less effect on amino acid levels. Lesion of the ipsilateral trapezoid body connection plus ipsilateral superior olivary nuclei resulted in decreases of GABA, aspartate, and glutamate levels especially in ventral cochlear nucleus regions. No clear contralateral effects of this lesion could be shown. The results most strongly support centrifugal GABAergic pathways to the cochlear nucleus, providing almost half of GABAergic neurotransmission in most regions. Our results support and extend previously published measurements of lesion effects on GABA uptake and release in cochlear nucleus subdivisions.

The effects of unilateral cochlear ablation on the expression of vesicular glutamate transporter 1 in the lower auditory pathway of neonatal rats

ObjectivesUnilateral cochlear damage has profound effects on the central auditory pathways in the brain. MethodsWe examined the effects of unilateral cochlear ablation on VGLUT1 expression in the cochlear nucleus (CN) and the superior olivary complex (SOC) in neonatal rats. ResultsVGLUT1 expression in the CN subdivisions (the AVCN, the PVCN and the DCN-deep layers) and the SOC (the MnTB, the LSO and the MSO) ipsilateral to the ablated side was significantly suppressed by unilateral cochlear ablation. Interestingly, VGLUT1 expression in the PVCN and the DCN-deep layers contralateral to the ablated side was also reduced. ConclusionOur findings indicate that unilateral cochlear ablation affects VGLUT1 expression in the central auditory pathways not only ipsilateral but also contralateral to the ablated side.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16–18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14–23 weeks of age.Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Reactive Neurogenesis and Down-Regulation of the Potassium-Chloride Cotransporter KCC2 in the Cochlear Nuclei after Cochlear Deafferentation.

While many studies have been devoted to investigating the homeostatic plasticity triggered by cochlear hearing loss, the cellular and molecular mechanisms involved in these central changes remain elusive. In the present study, we investigated the possibility of reactive neurogenesis after unilateral cochlear nerve section in the cochlear nucleus (CN) of cats. We found a strong cell proliferation in all the CN sub-divisions ipsilateral to the lesion. Most of the newly generated cells survive up to 1 month after cochlear deafferentation in all cochlear nuclei (except the dorsal CN) and give rise to a variety of cell types, i.e., microglial cells, astrocytes, and neurons. Interestingly, many of the newborn neurons had an inhibitory (GABAergic) phenotype. This result is intriguing since sensory deafferentation is usually accompanied by enhanced excitation, consistent with a reduction in central inhibition. The membrane potential effect of GABA depends, however, on the intra-cellular chloride concentration, which is maintained at low levels in adults by the potassium chloride co-transporter KCC2. The KCC2 density on the plasma membrane of neurons was then assessed after cochlear deafferentation in the cochlear nuclei ipsilateral and contralateral to the lesion. Cochlear deafferentation is accompanied by a strong down-regulation of KCC2 ipsilateral to the lesion at 3 and 30 days post-lesion. This study suggests that reactive neurogenesis and down-regulation of KCC2 is part of the vast repertoire involved in homeostatic plasticity triggered by hearing loss. These central changes may also play a role in the generation of tinnitus and hyperacusis.

Effects of acoustic trauma on the auditory system of the rat: The role of microglia

Exposure to loud, prolonged sounds (acoustic trauma, AT) leads to the death of both inner and outer hair cells (IHCs and OHCs), death of neurons of the spiral ganglion and degeneration of the auditory nerve. The auditory nerve (8cn) projects to the three subdivisions of the cochlear nuclei (CN), the dorsal cochlear nucleus (DC) and the anterior (VCA) and posterior (VCP) subdivisions of the ventral cochlear nucleus (VCN). There is both anatomical and physiological evidence for plastic reorganization in the denervated CN after AT. Anatomical findings show axonal sprouting and synaptogenesis; physiologically there is an increase in spontaneous activity suggesting reorganization of circuitry. The mechanisms underlying this plasticity are not understood. Recent data suggest that activated microglia may have a role in facilitating plastic reorganization in addition to removing trauma-induced debris. In order to investigate the roles of activated microglia in the CN subsequent to AT we exposed animals to bilateral noise sufficient to cause massive hair cell death. We studied four groups of animals at different survival times: 30days, 60days, 6months and 9months. We used silver staining to examine the time course and pattern of auditory nerve degeneration, and immunohistochemistry to label activated microglia in the denervated CN. We found both degenerating auditory nerve fibers and activated microglia in the CN at 30 and 60days and 6months after AT. There was close geographic overlap between the degenerating fibers and activated microglia, consistent with a scavenger role for activated microglia. At the longest survival time, there were still silver-stained fibers but very little staining of activated microglia in overlapping regions. There were, however, activated microglia in the surrounding brainstem and cerebellar white matter.

Effects of cochlear ablation on amino acid levels in the rat cochlear nucleus and superior olive

Amino acids have important roles in the chemistry of the auditory system, including communication among neurons. There is much evidence for glutamate as a neurotransmitter from auditory nerve fibers to cochlear nucleus neurons. Previous studies in rodents have examined effects of removal of auditory nerve input by cochlear ablation on levels, uptake and release of glutamate in cochlear nucleus subdivisions, as well as on glutamate receptors. Effects have also been reported on uptake and release of γ-aminobutyrate (GABA) and glycine, two other amino acids strongly implicated in cochlear nucleus synaptic transmission. We mapped the effects of cochlear ablation on the levels of amino acids, including glutamate, GABA, glycine, aspartate, glutamine, taurine, serine, threonine, and arginine, in microscopic subregions of the rat cochlear nucleus. Submicrogram-size samples microdissected from freeze-dried brainstem sections were assayed for amino acid levels by high performance liquid chromatography. After cochlear ablation, glutamate and aspartate levels decreased by 2 days in regions receiving relatively dense innervation from the auditory nerve, whereas the levels of most other amino acids increased. The results are consistent with a close association of glutamate and aspartate with auditory nerve fibers and of other amino acids with other neurons and glia in the cochlear nucleus. A consistent decrease of GABA level in the lateral superior olive could be consistent with a role in some lateral olivocochlear neurons. The results are compared with those obtained with the same methods for the rat vestibular nerve root and nuclei after vestibular ganglionectomy.

Hyperactivity in the Ventral Cochlear Nucleus after Cochlear Trauma

The emergence of hyperactivity in the form of elevated spontaneous firing rates after cochlear trauma has been well documented in a number of central auditory structures, including the auditory cortex, inferior colliculus, and dorsal subdivision of the cochlear nucleus. This hyperactivity is of interest as a possible neural substrate of tinnitus. Whether the ventral subdivision of the cochlear nucleus shows hyperactivity has never been investigated despite the fact that, like the dorsal division, it also receives direct input from the damaged cochlea and supplies major ascending inputs to brainstem and midbrain auditory centers. We investigated spontaneous neuronal firing rates in the ventral cochlear nucleus in a guinea pig model of cochlear trauma in which we have shown that hyperactivity consistently develops in the inferior colliculus (Mulders and Robertson, 2009). The mean spontaneous firing rates of ventral cochlear nucleus neurons was significantly elevated compared to sham controls. This hyperactivity was more evident in primary-like and onset categories of neurons. Hyperactivity in the ventral subdivision of cochlear nucleus therefore needs to be considered in relation to neural models of the genesis of tinnitus.

Three-dimensional tonotopic organization of the C57 mouse cochlear nucleus

The cochlear nucleus (CN) is the first sound processing center in the central auditory system that receives the almost unprocessed auditory information from the auditory periphery. The functional organization of the CN has been studied to a great extent in many mammals, including the cat, rat and bat. Yet, despite the general usefulness of the mouse, including the availability of various inbred strains and gene-manipulated lines, our current understanding of the mouse CN remains limited. The purpose of this study was to illustrate the functional organization of the CN in C57 mice, using an electrophysiological approach. Our results showed that the auditory response properties of CN neurons were similar in all three of the CN subdivisions. Sound frequency was systematically represented in each of the three CN subdivisions, i.e., the anteroventral, posteroventral and the dorsal divisions. The best frequency of CN neurons decreased along the dorsomedial-to-ventrolateral axis in an orderly progression whereas the tonotopic axes were relatively indistinct in the rostrocaudal plane. There was no disruption of the tonotopic map within each subdivision of the CN. The findings indicate that the CN tonotopic organization in the C57 mouse is similar to that in the cat and other mammals.

Identification of a novel Vamp1 splice variant in the cochlear nucleus

Cochlear nucleus neurons propagate auditory impulses to higher brain stem centers at rapid firing rates with high fidelity. Intrinsic to synaptic transmission are the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins engaged in vesicle fusion, release and recycling. Herein we report a novel splice variant of the SNARE protein Vamp1 (vesicle-associated membrane protein 1) within the cochlear nucleus. We previously demonstrated, through serial analysis of gene expression and microarray studies, that Vamp1 is differentially expressed among the subdivisions of the rat cochlear nucleus. The 3′ end of this transcript, however, was poorly characterized and we could not initially confirm our findings. In this study, we designed RT-PCR primers using conserved 5′ regions and the mouse 3′ domain to validate the expression of Vamp1. Several species of Vamp1 were subsequently amplified from a rat brain cDNA library including a full length clone of Vamp1as and a novel splice variant we termed Vamp1nv. Using regional brain libraries Vamp1nv showed expression in the medulla and lack of expression in the cortex, cerebellum and thalamus. Expression of Vamp1nv was further confirmed and characterized by RT-PCR and real-time PCR in each of the cochlear nucleus subdivisions. The predicted protein sequence for Vamp1nv demonstrates a unique modification of the carboxy-terminal end of the protein as compared to known variants. This includes the appearance of two intra-vesicular serine residues with high predicted potential as kinase phosphorylation sites. Such splice variants of Vamp1 may alter the kinetics of SNARE complex formation and vesicle release and impart unique features to expressing neurons. This may be important for central auditory function and contribute to the distinct physiological properties observed in auditory neurons.

Effects of age at onset of deafness and electrical stimulation on the developing cochlear nucleus in cats

This study examined the effects of deafness and intracochlear electrical stimulation on the anatomy of the cochlear nucleus (CN) after a brief period of normal auditory development early in life. Kittens were deafened by systemic ototoxic drug injections either as neonates or starting at postnatal day 30. Total CN volume, individual CN subdivision volumes, and cross-sectional areas of spherical cell somata in the anteroventral CN (AVCN) were compared in neonatally deafened and 30-day deafened groups at 8 weeks of age and in young adults after ∼6 months of electrical stimulation initiated at 8 weeks of age. Both neonatal and early acquired hearing loss resulted in a reduction in CN volume as compared to normal hearing cats. Comparison of 8- and 32-week old groups indicated that the CN continued to grow in both deafened groups despite the absence of auditory input. Preserving normal auditory input for 30 days resulted in a significant increase in both total CN volume and cross-sectional areas of spherical cell somata, as compared to neonatally deafened animals. Restoring auditory input in these developing animals by unilateral intracochlear electrical stimulation did not elicit any difference in CN volume between the two sides, but resulted in 7% larger spherical cell size on the stimulated side. Overall, the brief period of normal auditory development and subsequent electrical stimulation maintained CN volume at 80% of normal and spherical cell size at 86% of normal ipsilateral to the implant as compared to 67% and 74%, respectively, in the neonatally deafened group.

Synaptic influences of pontine nuclei on cochlear nucleus cells

Using the in vitro isolated whole brain preparation of the guinea pig, we tested the synaptic effects induced by the stimulation of pontine nuclei (PN) in intracellularly recorded and stained principal cells of the cochlear nucleus (CN). Twenty percent of the recorded cells in all CN subdivisions responded to stimulation of either ipsilateral or contralateral PN, and 12% of the cells exhibited convergence of inputs from both sides. The responses were recorded only in stellate cells of the ventral CN and in the pyramidal cells of the dorsal CN, whereas no responses were observed in bushy, octopus, and giant cells. PN stimulation produced excitatory and inhibitory postsynaptic potentials as well as mixed responses. The heterogeneous nature and the wide latency range (3.2-18 ms) of observed responses suggest significant variability in the underlying synaptic mechanisms and the implicated pathways. We propose that PN projections to the CN, terminating mainly in the granule cell domain (GCD), together with other non-auditory and auditory inputs contribute to multimodal convergence in the GCD leading ultimately to modulatory actions on the output activity of CN principal cells.

Auditory Responses in the Cochlear Nucleus of Awake Mustached Bats: Precursors to Spectral Integration in the Auditory Midbrain

In the cochlear nucleus (CN) of awake mustached bats, single- and two-tone stimuli were used to examine how responses in major CN subdivisions contribute to spectrotemporal integrative features in the inferior colliculus (IC). Across CN subdivisions, the proportional representation of frequencies differed. A striking result was the substantial number of units tuned to frequencies <23 kHz. Across frequency bands, temporal response patterns, latency, and spontaneous discharge differed. For example, the 23- to 30-kHz representation, which comprises the fundamental of the sonar call, had an unusually high proportion of units with onset responses (39%) and low spontaneous rates (53%). Units tuned to 58-59 kHz, corresponding to the sharply tuned cochlear resonance, had slightly but significantly longer latencies than other bands. In units tuned to frequencies >30 kHz, 31% displayed a secondary excitatory peak, usually between 10 and 22 kHz. The secondary peak may originate in cochlear mechanisms for some units, but in others it may result from convergent input onto CN neurons. In 20% of units tested with two-tone stimuli, suppression of best frequency (BF) responses was tuned at least an octave below BF. These properties may underlie similar IC responses. However, other forms of spectral interaction present in IC were absent in CN: we found no facilitatory combination-sensitive interactions and very few combination-sensitive inhibitory interactions of the dominant IC type in which inhibition was tuned to 23-30 kHz. Such interactions arise above CN. Distinct forms of spectral integration thus originate at different levels of the ascending auditory pathway.

The projection from auditory cortex to cochlear nucleus in guinea pigs: an in vivo anatomical and in vitro electrophysiological study.

Previous anatomical experiments have demonstrated the existence of a direct, bilateral projection from the auditory cortex (AC) to the cochlear nucleus (CN). However, the precise relationship between the origin of the projection in the AC and the distribution of axon terminals in the CN is not known. Moreover, the influence of this projection on CN principal cells has not been studied before. The aim of the present study was two-fold. First, to extend the anatomical data by tracing anterogradely the distribution of cortical axons in the CN by means of restricted injections of biotinylated dextran amine (BDA) in physiologically characterized sites in the AC. Second, in an in vitro isolated whole brain preparation (IWB), to assess the effect of electrical stimulation of the AC on CN principal cells from which intracellular recordings were derived. BDA injections in the tonotopically organized primary auditory cortex and dorsocaudal auditory field at high and low best frequency (BF) sites resulted in a consistent axonal labeling in the ipsilateral CN of all injected animals. In addition, fewer labeled terminals were observed in the contralateral CN, but only in the animals subjected to injections in low BF region. The axon terminal fields consisting of boutons en passant or terminaux were found in the superficial granule cell layer and, to a smaller extent, in the three CN subdivisions. No axonal labeling was seen in the CN as result of BDA injection in the secondary auditory area (dorsocaudal belt). In the IWB, the effects of ipsilateral AC stimulation were tested in a population of 52 intracellulary recorded and stained CN principal neurons, distributed in the three CN subdivisions. Stimulation of the AC evoked slow late excitatory postsynaptic potentials (EPSPs) in only two cells located in the dorsal CN. The EPSPs were induced in a giant and a pyramidal cell at latencies of 20 ms and 33 ms, respectively, suggesting involvement of polysynaptic circuits. These findings are consistent with anatomical data showing sparse projections from the AC to the CN and indicate a limited modulatory action of the AC on CN principal cells.

Protein Kinase C Regulates [3H]d-Aspartate Release in Auditory Brain Stem Nuclei

We previously found that unilateral cochlear ablation altered transmitter release from glutamatergic synaptic endings in several brain stem auditory nuclei. To determine if this release activity could be regulated by protein kinase C (PKC), which has been associated with regulation of transmitter release, the electrically evoked release of [3H]d-aspartate ([3H]d-Asp) was quantified in vitro as an index of exocytosis from glutamatergic presynaptic endings in the major subdivisions of the cochlear nucleus (CN) and in the main nuclei of the superior olivary complex (SOC). Treating dissected tissues with a PKC activator, such as phorbol 12,13-diacetate (PDA) or phorbol 12,13-dibutyrate (PDBu) (3 μM), elevated the evoked release of [3H]d-Asp by 1.5- to 3.3-fold. The PKC inhibitor Ro31-8220 (50 nM) did not alter the evoked release but blocked the stimulatory effects of PDA and PDBu. These findings suggested that PKC could positively regulate transmitter release from glutamatergic presynaptic endings in brain stem auditory pathways. Seven days after unilateral cochlear ablation, when cochlear nerve endings had degenerated in the ipsilateral CN, PDBu elevated the evoked release bilaterally in each CN subdivision and SOC nucleus, implying that PKC could regulate glutamatergic release in the noncochlear pathways remaining in the ipsilateral CN and in the other pathways after unilateral hearing loss. After 145 postlesion days, Ro31-8220 blocked endogenous elevations in the evoked release in the ipsilateral SOC but did not alter the elevated or upregulated release in the other tissues. This suggested that the elevations of glutamatergic release activity in the ipsilateral SOC that appeared after unilateral cochlear ablation depended on endogenous activation of PKC.

Commissural glycinergic inhibition of bushy and stellate cells in the anteroventral cochlear nucleus

Synaptic inputs from one cochlear nucleus (CN) to the other can play an important role in modulating the activity of CN neurons. Using the isolated whole brain preparation of the guinea pig, we tested the effects of electrical stimulation of the contralateral auditory nerve (AN) on intracellularly recorded and stained neurons of the anteroventral cochlear nucleus. Stimulation of the contralateral AN evoked only inhibitory postsynaptic potentials (IPSPs) in 63% of recorded neurons, including bushy and stellate cells. The latency of most IPSPs (88%) was in the range 3.3-7.6 ms, consistent with mono- and disynaptic transmission from the contralateral CN. The IPSPs had an average amplitude of 2.6 +/- 1.9 mV and were blocked by strychnine suggesting their glycinergic nature. These data, together with our similar findings in other CN subdivisions, indicate that principal cells of the CN contribute to binaural interactions at earliest stages of acoustic processing.

Serotonin projection patterns to the cochlear nucleus

The cochlear nucleus is well known as an obligatory relay center for primary auditory nerve fibers. Perhaps not so well known is the neural input to the cochlear nucleus from cells containing serotonin that reside near the midline in the midbrain raphe region. Although the specific locations of the main, if not sole, sources of serotonin within the dorsal cochlear nucleus subdivision are known to be the dorsal and median raphe nuclei, sources of serotonin located within other cochlear nucleus subdivisions are not currently known. Anterograde tract tracing was used to label fibers originating from the dorsal and median raphe nuclei while fluorescence immunohistochemistry was used to simultaneously label specific serotonin fibers in cat. Biotinylated dextran amine was injected into the dorsal and median raphe nuclei and was visualized with Texas Red, while serotonin was visualized with fluorescein. Thus, double-labeled fibers were unequivocally identified as serotoninergic and originating from one of the labeled neurons within the dorsal and median raphe nuclei. Double-labeled fiber segments, typically of fine caliber with oval varicosities, were observed in many areas of the cochlear nucleus. They were found in the molecular layer of the dorsal cochlear nucleus, in the small cell cap region, and in the granule cell and external regions of the cochlear nuclei, bilaterally, of all cats. However, the density of these double-labeled fiber segments varied considerably depending upon the exact region in which they were found. Fiber segments were most dense in the dorsal cochlear nucleus (especially in the molecular layer) and the large spherical cell area of the anteroventral cochlear nucleus; they were moderately dense in the small cell cap region; and fiber segments were least dense in the octopus and multipolar cell regions of the posteroventral cochlear nucleus. Because of the presence of labeled fiber segments in subdivisions of the cochlear nucleus other than the dorsal cochlear nucleus, we concluded that the serotoninergic projection pattern to the cochlear nucleus is divergent and non-specific. Double-labeled fiber segments were also present, but sparse, in the superior olive, localized mainly in periolivary regions; this indicated that the divergence of dorsal and median raphe neurons that extends throughout regions of the cochlear nucleus also extended well beyond the cochlear nucleus to include at least the superior olivary complex as well.

The effect of ethanol on the number of cells in the cochlear nucleus of the male adult rat: A stereological study

AbstractThe effects of ethanol consumption on the means of total neuron numbers in the cochlear nucleus (CN) of male adult rats were investigated by using the disector/fractionator technique to make a precise and unbiased estimate of the total number of neurons in the CN. The rats were randomly divided into eight groups: control, 10%, 25%, 35% ethanol‐consuming groups and four parallel groups given vitamin E in addition to the ethanol. The total number of neurons was significantly decreased by 11.9%, 26.4%, 46.3% in left VCN, 12.7%, 33.6%, 46.5% in right VCN, 8.5%, 32.3%, 49.1% in left DCN, 11.0%, 32.9%, 51.0% in right DCN in rats consuming 10%, 25%, and 35% ethanol, respectively. The administration of vitamin E (100 mg/kg/day i.p) to ethanol consuming rats for 20 days partially prevented ethanol‐induced cell death in both subdivisions of CN. The results of the present study imply that the CN of adult rats is susceptible to the neurotoxic effects of ethanol and that these effects can be partially blocked by vitamin E.

Expression of Shal potassium channel subunits in the adult and developing cochlear nucleus of the mouse

The pattern of expression of potassium (K +) channel subunits is thought to contribute to the establishment of the unique discharge characteristics exhibited by cochlear nucleus (CN) neurons. This study describes the developmental distribution of mRNA for the three Shal channel subunits Kv4.1, Kv4.2 and Kv4.3 within the mouse CN, as assessed with in situ hybridization and RT-PCR techniques. Kv4.1 was not present in CN at any age. Kv4.2 mRNA was detectable as early as postnatal day 2 (P2) in all CN subdivisions, and continued to be constitutively expressed throughout development. Kv4.2 was abundantly expressed in a variety of CN cell types, including all of the major projection neuron classes (i.e., octopus, bushy, stellate, fusiform, and giant cells). In contrast, Kv4.3 was expressed at lower levels and by fewer cell types. Kv4.3-labeled cells were more prevalent in ventral subdivisions than in the dorsal CN. Kv4.3 expression was significantly delayed developmentally in comparison to Kv4.2, as it was detectable only after P14. Although the techniques employed in this study detect mRNA and not protein, it can be inferred from the differential distribution of Kv4 transcripts that CN neurons selectively regulate the expression of Shal K + channels among individual neurons throughout development.