Amino acid concentrations in rat cochlear nucleus and superior olive

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.

Amino acid and acetylcholine chemistry in mountain beaver cochlear nucleus and comparisons to pocket gopher, other rodents, and cat

The mountain beaver and pocket gopher are two rodents that live mostly underground in tunnel systems. Previous studies have suggested that their cochlear nucleus structure, particularly that of the dorsal cochlear nucleus (DCN), differs significantly from that of other mammals, that the hearing ability of the pocket gopher is deficient compared to that of other rodents, and that the DCN of the mountain beaver is more responsive to slow oscillations of air pressure than to sounds. We conducted some electrophysiological recordings from mountain beaver DCN and then used microchemical methods to map in mountain beaver cochlear nuclei the distributions of amino acids, including the major neurotransmitters of the brain, and enzyme activities related to the metabolism of the neurotransmitter acetylcholine, which functions in centrifugal pathways to the cochlear nucleus. Similar measurements were made for a pocket gopher cochlear nucleus. Responses to tonal stimuli were found in mountain beaver DCN. Distributions and magnitudes of neurotransmitter and related amino acids within mountain beaver and pocket gopher cochlear nuclei were not very different from those of other rodents and cat. However, the enzyme of synthesis for acetylcholine, choline acetyltransferase, had only low activities in the DCN of both mountain beaver and pocket gopher. The chemical distributions in the mountain beaver DCN support a conclusion that it corresponds to just the superficial DCN portion of other mammals. High correlations between the concentrations of γ-aminobutyrate (GABA) and glycine were found for both mountain beaver and pocket gopher cochlear nuclei, suggesting that their co-localization in cochlear nucleus synapses may be especially prominent in these animals. Previous evidence suggests convergence of somatosensory and auditory information in the DCN, and this may be especially true in animals spending most of their time underground. Our results suggest that the enlarged DCN of the mountain beaver and that of the pocket gopher are not very different from those of other rodents with respect to involvement of amino acid neurotransmitters, but they appear to have reduced cholinergic innervation.

Anti-inflammatory Effects of Batillaria multiformis Water Extracts via NF-кB and MAPK Signaling Pathways in LPS-Induced RAW 264.7 Cells

Batillaria multiformis (B. multiformis) belong to gastropods. They live generally in the sandpit of the lagoons and the estuaries of the intertidal zone. Most of them are distributed in Korea, Japan and China. In this study, we investigated the anti-inflammatory potential of B. multiformis water extracts (BMW). The results showed that the extracts significantly decreased the production of nitric oxide (NO) and pro-inflammatory cytokines, including tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in LPS-induced RAW 264.7 macrophages. In addition, the extracts suppressed the protein levels of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in a dose dependent manner. Further investigation indicated that BMW suppressed phosphorylated c-Jun N-terminal kinase (JNK), extracellular regulated protein kinase (ERK) and p38 through the MAPK signaling pathway and influenced the NF-κB signaling pathway by suppressing the IκBα degradation in LPS-induced RAW 264.7 macrophages.

Taurine Attenuates Doxorubicin-Induced Toxicity on B16F10 Cells

This study aimed to investigate the effect of doxorubicin co-treatment with taurine on B16F10 melanoma cells. Frequently, Doxorubicin is used in the treatments of many different kinds of cancers, some of which are soft tissue sarcomas, hematological malignancies and carcinomas. However, the clinical application of doxorubicin is compromised by its severe adverse effects, including cardiotoxicity. In the present study, the efficacy of doxorubicin co-treatment with taurine was investigated. B16F10 cell viability was evaluated using MTT assays, trypan blue dye exclusion assays, and fluorescent staining technique. Apoptotic cells were detected by flow cytometry and the proteins associated with apoptosis and cellular differentiations were assessed by immunoblotting. Doxorubicin inhibited cell growth and induced cell death in B16F10 cells. Interestingly, doxorubicin co-treatment with taurine inhibited apoptosis in B16F10 cells. These results indicate that doxorubicin co-treatment with taurine attenuates doxorubicin-induced cytotoxicity and reduces ROS production in B16F10 cells.

Cochlear ablation effects on amino acid levels in the chinchilla cochlear nucleus

Inner ear damage can lead to hearing disorders, including tinnitus, hyperacusis, and hearing loss. We measured the effects of severe inner ear damage, produced by cochlear ablation, on the levels and distributions of amino acids in the first brain center of the auditory system, the cochlear nucleus. Measurements were also made for its projection pathways and the superior olivary nuclei. Cochlear ablation produces complete degeneration of the auditory nerve, which provides a baseline for interpreting the effects of partial damage to the inner ear, such as that from ototoxic drugs or intense sound. Amino acids play a critical role in neural function, including neurotransmission, neuromodulation, cellular metabolism, and protein construction. They include major neurotransmitters of the brain - glutamate, glycine, and γ-aminobutyrate (GABA) - as well as others closely related to their metabolism and/or functions - aspartate, glutamine, and taurine. Since the effects of inner ear damage develop over time, we measured the changes in amino acid levels at various survival times after cochlear ablation. Glutamate and aspartate levels decreased by 2weeks in the ipsilateral ventral cochlear nucleus and deep layer of the dorsal cochlear nucleus, with the largest decreases in the posteroventral cochlear nucleus (PVCN): 66% for glutamate and 63% for aspartate. Aspartate levels also decreased in the lateral part of the ipsilateral trapezoid body, by as much as 50%, suggesting a transneuronal effect. GABA and glycine levels showed some bilateral decreases, especially in the PVCN. These results may represent the state of amino acid metabolism in the cochlear nucleus of humans after removal of eighth nerve tumors, which may adversely result in destruction of the auditory nerve. Measurement of chemical changes following inner ear damage may increase understanding of the pathogenesis of hearing impairments and enable improvements in their diagnosis and treatment.

Distribution of Vesicular Glutamate Transporter 2 and Ionotropic Glutamate Receptors in the Auditory Ganglion and Cochlear Nuclei of Pigeons (Columba livia)

Glutamate is a principal excitatory neurotransmitter in the auditory system. Our previous studies revealed localization of glutamate receptor mRNAs in the pigeon cochlear nuclei, suggesting the existence of glutamatergic input from the auditory nerve to the brainstem. This study demonstrated localization of mRNAs for vesicular glutamate transporter 2 (vGluT2) and ionotropic glutamate receptors (AMPA, kainate and NMDA) in the auditory ganglion (AG) and cochlear nuclei (magnocellular, angular and laminar nuclei). VGluT2 mRNA was intensely expressed in AG and intensely or moderately in the cochlear nuclei. The AG and cochlear nuclei showed intense-to-moderate mRNA signals for GluA2, GluA3, GluA4, GluK4 and GluN1. These results suggest that the pigeon AG neurons receives glutamatergic input from hair cells and in turn projects to the magnocellular and angular nuclei. Glutamate may play a pivotal role in the excitatory synapse transmission in the peripheral auditory pathway of birds.

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.

Taurine protects against bilirubin-induced hyperexcitation in rat anteroventral cochlear nucleus neurons

No effective medication for hyperbilirubinemia yet exists. Taurine is believed to exert a neuroprotective action. The aim of the present study was to determine whether taurine protected neurons of the rat anteroventral cochlear nucleus (AVCN) against bilirubin-induced neuronal hyperexcitation. AVCN neurons were isolated from 13 to 15-day-old Sprague-Dawley rats. The effects of bilirubin on the spontaneous excitatory postsynaptic currents (sEPSCs) and action potential currents were compared with those exerted by bilirubin and taurine together. Bilirubin dramatically increased the frequencies of sEPSCs and action potential currents, but not sEPSC amplitude. Taurine suppressed the enhanced frequency of action potentials induced by bilirubin, in a dose-dependent manner. In addition, taurine decreased the amplitude of voltage-dependent calcium channel currents that were enhanced upon addition of bilirubin. We explored the mechanism of the protective effects exerted by taurine using GABAA and glycine receptor antagonists, bicuculline and strychnine, respectively. Addition of bicuculline and strychnine eliminated the protective effects of taurine. Neither bilirubin nor taurine affected the sensitivity of the glutamate receptor. Our findings thus indicate that taurine protected AVCN neurons against bilirubin-induced neuronal hyperexcitation by activating the GABAA and glycine receptors and inhibiting calcium flow through voltage-gated channels. Thus, taurine may be effective in treatment of neonatal hyperbilirubinemia.

Interaction between taurine and GABA(A)/glycine receptors in neurons of the rat anteroventral cochlear nucleus

Taurine, one of the most abundant endogenous amino acids in the mammalian central nervous system (CNS), is involved in neural development and many physiological functions. In this study, the interaction between taurine and GABA(A)/glycine receptors was investigated in young rat (P13-P15) anteroventral cochlear nucleus (AVCN) neurons using the whole-cell patch-clamp method. We found that taurine at low (0.1mM) and high (1mM) concentrations activated both GABA(A) and glycine receptors, but not AMPA and NMDA receptors. The reversal potentials of taurine-, GABA- or glycine-evoked currents were close to the expected chloride equilibrium potential, indicating that receptors activated by these agonists were mediating chloride conductance. Moreover, our results showed that the currents activated by co-application of GABA and glycine were cross-inhibitive. Sequential application of GABA and glycine or vice versa also reduced the glycine or GABA evoked currents. There was no cross-inhibition when taurine and GABA or taurine and glycine were applied simultaneously, but the response was larger than that evoked by GABA or glycine alone. These results suggest that taurine can serve as a neuromodulator to strengthen GABAergic and glycinergic neurotransmission in the rat AVCN.

Amino acid concentrations in the hamster central auditory system and long-term effects of intense tone exposure

Exposure to intense sounds often leads to loss of hearing of environmental sounds and hearing of a monotonous tonal sound not actually present, a condition known as tinnitus. Chronic physiological effects of exposure to intense tones have been reported for animals and should be accompanied by chemical changes present at long times after the intense sound exposure. By using a microdissection mapping procedure combined with high-performance liquid chromatography (HPLC), we have measured concentrations of nine amino acids, including those used as neurotransmitters, in the cochlear nucleus, inferior colliculus, medial geniculate, and auditory cortex of hamsters 5 months after exposure to an intense tone, compared with control hamsters of the same age. No very large differences in amino acid concentrations were found between exposed and control hamsters. However, increases of glutamate and γ-aminobutyrate (GABA) in some parts of the inferior colliculus of exposed hamsters were statistically significant. The most consistent differences between exposed and control hamsters were higher aspartate and lower taurine concentrations in virtually all regions of exposed hamsters, which reached statistical significance in many cases. Although these amino acids are not considered likely neurotransmitters, they indirectly have roles in excitatory and inhibitory neurotransmission, respectively. Thus, there is evidence for small, widespread, long-term increases in excitatory transmission and decreases in inhibitory transmission after a level of acoustic trauma previously shown to produce hearing loss and tinnitus.

Multiple origins of cholinergic innervation of the cochlear nucleus

Acetylcholine (Ach) affects a variety of cell types in the cochlear nucleus (CN) and is likely to play a role in numerous functions. Previous work in rats suggested that the acetylcholine arises from cells in the superior olivary complex, including cells that have axonal branches that innervate both the CN and the cochlea (i.e. olivocochlear cells) as well as cells that innervate only the CN. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of ACh in the CN of guinea pigs. The results confirm a projection from cholinergic cells in the superior olivary complex to the CN. In addition, we identified a substantial number of cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT) that project to the CN. On average, the PPT and LDT together contained about 26% of the cholinergic cells that project to CN, whereas the superior olivary complex contained about 74%. A small number of additional cholinergic cells were located in other areas, including the parabrachial nuclei.The results highlight a substantial cholinergic projection from the pontomesencephalic tegmentum (PPT and LDT) in addition to a larger projection from the superior olivary complex. These different sources of cholinergic projections to the CN are likely to serve different functions. Projections from the superior olivary complex are likely to serve a feedback role, and may be closely tied to olivocochlear functions. Projections from the pontomesencephalic tegmentum may play a role in such things as arousal and sensory gating. Projections from each of these areas, and perhaps even the smaller sources of cholinergic inputs, may be important in conditions such as tinnitus as well as in normal acoustic processing.

Cholinergic cells of the pontomesencephalic tegmentum: connections with auditory structures from cochlear nucleus to cortex

Acetylcholine (ACh) is a neuromodulator that is likely to play a role in plasticity as well as other phenomena at many sites in the auditory system. The auditory cortex receives cholinergic innervation from the basal forebrain, whereas the cochlea receives cholinergic innervation from the superior olivary complex. Much of the remainder of the auditory pathways receives innervation from the pedunculopontine and laterodorsal tegmental nuclei, two nuclei referred to collectively as the pontomesencephalic tegmentum (PMT). The PMT provides the major source of ACh to the auditory thalamus and the midbrain, and is a substantial source (in addition to the superior olivary complex) of ACh in the cochlear nucleus. Individual cholinergic cells in the PMT often have axon branches that innervate multiple auditory nuclei, including nuclei on both sides of the brain as well as nuclei at multiple levels of the auditory system. The auditory cortex has direct axonal projections to the PMT cells, including cholinergic cells that project to the inferior colliculus or cochlear nucleus. The divergent projections of PMT cholinergic cells suggest widespread effects on the auditory pathways. These effects are likely to include plasticity as well as novelty detection, sensory gating, reward behavior, arousal and attention. Descending projections from the forebrain, including the auditory cortex, are likely to provide a high level of cognitive input to these cholinergic effects. Dysfunction associated with the cholinergic system may play a role in disorders such as tinnitus and schizophrenia.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

Neuronal subtype identity in the rat auditory brainstem as defined by molecular profile and axonal projection

The nuclei of the auditory brainstem harbor a diversity of neuronal cell types and are interconnected by excitatory as well as inhibitory ascending, descending, and commissural pathways. Classically, neurons have been characterized by size and shape of their cell body and by the geometry of their dendrites. Our study is based on the use of axonal tracers in combination with immunocytochemistry to identify and distinguish neuronal subtypes by their molecular signature in dorsal and ventral cochlear nucleus, lateral superior olive, medial superior olive, medial nucleus of the trapezoid body, and inferior colliculus of the adult rat. The presumed neurotransmitters glutamate, glycine, and GABA were used alongside the calcium-binding proteins parvalbumin, calretinin, and calbindin-D28k as molecular markers. Our data provide distinct extensions to previous characterizations of neuronal subtypes and reveal regularities and differences across auditory brainstem nuclei that are discussed for their functional implications.

Neurosteroids Involved in Regulating Inhibition in the Inferior Colliculus

Fast inhibitory neurotransmission in the brain is largely mediated by the γ-aminobutyric acid-type A (GABAA) receptor. The 3α,5α-reduced neurosteroids (e.g., allopregnanolone) are the most potent endogenous modulators of the GABAA receptor. Although it is known that 3α,5α-reduced neurosteroid levels change during stress or depression and over the estrus cycle, a basic physiological role consistent with their pharmacological action remains elusive. We used the unique architecture of the auditory midbrain to reveal a role for 3α,5α-reduced neurosteroids in regulating inhibitory efficacy. After blocking the massive GABAergic projection from the dorsal nucleus of the lateral lemniscus (DNLL) to the contralateral central nucleus of the inferior colliculus (ICC) in anesthetized rats, a reactive increase in the efficacy of other inhibitory circuits in the ICC (separable because of the dominant ear that drives each circuit) was demonstrated with physiological measures—single-neuron activity and a neural-population-evoked response. This effect was prevented by blocking 3α,5α-reduced neurosteroid synthesis with a 5α-reductase inhibitor: finasteride. Immunohistochemistry confirmed that the DNLL blockade induced an increase in 3α,5α-reduced neurosteroids in the contralateral ICC. This study shows that when GABAergic inhibition is reduced, the brain compensates within minutes by locally increasing synthesis of neurosteroids, thereby balancing excitatory and inhibitory inputs in complex neural circuits.

Distinct roles for glycine and GABA in shaping the response properties of neurons in the superior paraolivary nucleus of the rat

The superior paraolivary nucleus (SPON) is a prominent periolivary cell group of the superior olivary complex. SPON neurons use gamma-aminobutyric acid (GABA) as their neurotransmitter and are contacted by large numbers of glycinergic and GABAergic punctate profiles, representing a dense inhibitory innervation from the medial nucleus of the trapezoid body (MNTB) and from collaterals of SPON axons, respectively. SPON neurons have low rates of spontaneous activity, respond preferentially to the offset of pure tones, and phase-lock to amplitude-modulated tones. To determine the roles of glycine and GABA in shaping SPON responses, we recorded from single units in the SPON of anesthetized rats before, during, and after application of the glycine receptor antagonist strychnine, the GABA(A) receptor antagonist bicuculline, or both drugs applied simultaneously. Strychnine caused a major increase in spike counts during the stimulus presentation, followed by the disappearance of offset spikes. In half of the recorded units, bicuculline caused moderately increased firing during the stimulus. However, in 86% of units bicuculline also caused a large increase in the magnitude of the offset response. Application of the drug cocktail caused increased spontaneous activity, dramatically increased spike counts during the stimulus presentation, and eliminated the offset response in most units. We conclude that glycinergic inhibition from the MNTB suppresses SPON spiking during sound stimulation and is essential in generating offset responses. GABAergic inhibition, presumably from intrinsic SPON collaterals, plays a subtler role, contributing in some cells to suppression of firing during the stimulus and in most cells to restrict firing after stimulus offset.

Effects of cochlear ablation on muscarinic acetylcholine receptor binding in the rat cochlear nucleus

Cholinergic synapses in the cochlear nucleus (CN) have been reported to modulate spontaneous activity via muscarinic acetylcholine receptors. In this study, muscarinic receptor binding was measured as specific binding of 1-[N-methyl-(3)H]scopolamine in CN regions of control rats and 7 days, 1 month, and 2 months after unilateral cochlear ablation. In control rats, the strongest binding was found in granular regions, followed in order by fusiform soma, molecular, and deep layers of the dorsal cochlear nucleus (DCN), with much lower binding in the anteroventral CN (AVCN) and posteroventral CN (PVCN). After unilateral cochlear ablation, binding in the AVCN, PVCN, and their associated granular regions on the lesion side became progressively greater than on the control side through 2 months after lesion. A significant asymmetry, with binding higher on the lesion side, was also found in the DCN fusiform soma layer at 7 days, and there and in the DCN deep layer at 1 and 2 months after lesion. There was also evidence of increased binding on the control side in most CN regions. By contrast, binding in the ipsilateral facial nucleus decreased, compared with the control side, by 7 days after the lesion and showed some recovery toward symmetry by 2 months after lesion, and there was no evidence for contralateral changes. These muscarinic receptor binding changes reflect receptor plasticity after loss of auditory nerve innervation. Such plasticity may underlie some of the central auditory functional changes that occur following peripheral lesions, such as tinnitus and hyperacusis.

Amino acid concentrations in chinchilla cochlear nucleus at different times after carboplatin treatment

Amino acid concentrations were measured in the cochlear nucleus for a group of 20 chinchillas: four each of control and 4, 8, 29, and 85 days after treatment with the ototoxic anti-tumor drug carboplatin (100 mg/kg, i.p.). The treated chinchillas showed various extents of inner hair cell loss, generally more complete at longer survival times, but little loss of outer hair cells. Aspartate concentration in rostral anteroventral cochlear nucleus (AVCN) showed a decline to 28% less than the control value at 29 and 85 days after treatment, whereas glutamate concentration showed little change through 29 days, then dropped by 22% at 85 days after treatment. In caudal posteroventral cochlear nucleus (PVCN), the aspartate concentration decreased by 32% at 29 days, in animals with significant inner hair cell loss, and 48% at 85 days after treatment, while the glutamate concentration showed no decrease through 29 days and 40% decrease at 85 days. The concentration of gamma-aminobutyrate (GABA) was about 18% lower than control in caudal PVCN at all survival times. Significant correlations were found between the proportion of inner hair cells remaining and glutamate and aspartate concentrations in PVCN and AVCN, but not GABA or other amino acids.

Deafness-related decreases in glycine-immunoreactive labeling in the rat cochlear nucleus

There is increasing evidence of activity-related plasticity in auditory pathways. The present study examined the effects of decreased activity on immunolocalization of the inhibitory neurotransmitter glycine in the cochlear nucleus of the rat after bilateral cochlear ablation. Specifically, glycine-immunoreactive puncta adjacent to somatic profiles were compared in normal hearing animals and animals deafened for 14 days. The number of glycine-immunoreactive puncta surrounding somatic profiles of spherical and globular bushy cells, glycine-immunoreactive type I stellate multipolar cells, radiate neurons (type II stellate multipolar cells), and fusiform cells decreased significantly. In addition, the number of glycine immunopositive tuberculoventral (vertical or corn) cells in the deep layer of the dorsal cochlear nucleus also decreased significantly. These results suggest that decreased inhibition reported in cochlear nucleus after deafness may be due to decreases in glycine.

Effects of cochlear ablation on choline acetyltransferase activity in the rat cochlear nucleus and superior olive

Using microdissection and quantitative microassay, choline acetyltransferase (ChAT) activity was mapped in the cochlear nucleus (CN) and in the source nuclei of the olivocochlear bundle, the lateral superior olive and ventral nucleus of the trapezoid body. In control rats, gradients of ChAT activity were found within the major subdivisions of the CN and in the lateral superior olive. These gradients correlated with the known tonotopic organizations, with higher activities corresponding to locations representing higher sound frequencies. No gradient was found in the ventral nucleus of the trapezoid body. In rats surviving 7 days or 1 or 2 months after cochlear ablation, ChAT activity was increased 1 month after ablation in the anteroventral CN by 30-50% in most parts of the lesion-side and by 40% in the contralateral ventromedial part. ChAT activity in the lesion-side posteroventral CN was increased by approximately 40-50% at all survival times. Little change was found in the dorsal CN. Decreases of ChAT activity were also found ipsilaterally in the lateral superior olive and bilaterally in the ventral nucleus of the trapezoid body. Our results suggest that cholinergic neurons are involved in plasticity within the CN and superior olive following cochlear lesions.

Differential distribution of synaptic endings containing glutamate, glycine, and GABA in the rat dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) integrates the synaptic information depending on the organization of the excitatory and inhibitory connections. This study provides, qualitatively and quantitatively, analyses of the organization and distribution of excitatory and inhibitory input on projection neurons (fusiform cells), and inhibitory interneurons (vertical and cartwheel cells) in the DCN, using a combination of high-resolution ultrastructural techniques together with postembedding immunogold labeling. The combination of ultrastructural morphometry together with immunogold labeling enables the identification and quantification of four major synaptic inputs according to their neurotransmitter content. Only one category of synaptic ending was immunoreactive for glutamate and three for glycine and/or gamma-aminobutyric-acid (GABA). Among those, nine subtypes of synaptic endings were identified. These differed in their ultrastructural characteristics and distribution in the nucleus and on three cell types analyzed. Four of the subtypes were immunoreactive for glutamate and contained round synaptic vesicles, whereas five were immunoreactive for glycine and/or GABA and contained flattened or pleomorphic synaptic vesicles. The analysis of the distribution of the nine synaptic endings on the cell types revealed that eight distributed on fusiform cells, six on vertical cells and five on cartwheel cells. In addition, postembedding immunogold labeling of the glycine receptor alpha1 subunit showed that it was present at postsynaptic membranes in apposition to synaptic endings containing flattened or pleomorphic synaptic vesicles and immunoreactive for glycine and/or GABA on the three cells analyzed. This information is valuable to our understanding of the response properties of DCN neurons.

Localization of the GABAB receptor 1a/b subunit relative to glutamatergic synapses in the dorsal cochlear nucleus of the rat

Metabotropic gamma-aminobutyric acid receptors (GABA(B)) are involved in pre- and postsynaptic inhibitory effects upon auditory neurons and have been implicated in different aspects of acoustic information processing. To understand better the mechanisms by which GABA(B) receptors mediate their inhibitory effects, we used pre-embedding immunocytochemical techniques combined with quantification of immunogold particles to reveal the precise subcellular distribution of the GABA(B1) subunit in the rat dorsal cochlear nucleus. At the light microscopic level, GABA(B1) was detected in all divisions of the cochlear complex. The most intense immunoreactivity for GABA(B1) was found in the dorsal cochlear nucleus, whereas immunoreactivity in the anteroventral and posteroventral cochlear nuclei was very low. In the dorsal cochlear nucleus, a punctate labeling was observed in the superficial (molecular and fusiform cell) layers. At the electron microscopic level, GABA(B1) was found at both post- and presynaptic locations. Postsynaptically, GABA(B1) was localized mainly in the dendritic spines of presumed fusiform cells. Quantitative immunogold immunocytochemistry revealed that the highest concentration of GABA(B1) in the plasma membrane was in dendritic spines, followed by dendritic shafts and somata. Thus, the most intense immunoreactivity for GABA(B1) was observed in dendritic spines with a high density of immunogold particles at extrasynaptic sites, peaking around 300 nm from glutamatergic synapses. This is in contrast to GABAergic synapses, in which GABA(B1) was only occasionally found. Presynaptically, receptor immunoreactivity was detected primarily in axospinous endings, probably from granule cells, in both the active zone and extrasynaptic sites. The localization of GABA(B1) relative to synaptic sites in the DCN suggests a role for the receptor in the regulation of dendritic excitability and excitatory inputs.

Effects of unilateral vestibular ganglionectomy on glutaminase activity in the vestibular nerve root and vestibular nuclear complex of the rat

The metabolism of glutamate, the most likely neurotransmitter of vestibular ganglion cells, includes synthesis from glutamine by the enzyme glutaminase. We used microdissection combined with a fluorometric assay to measure glutaminase activity in the vestibular nerve root and nuclei of rats with unilateral vestibular ganglionectomy. Glutaminase activity in the lesioned-side vestibular nerve root decreased by 62% at 4 days after ganglionectomy and remained at similar values through 30 days. No change occurred in the contralateral vestibular nerve root. Glutaminase activity changes in the vestibular nuclei were lesser in magnitude and more complex, including contralateral increases as well as ipsilateral decreases. At 4 days after ganglionectomy, glutaminase activity was 10-20% lower in individual lesioned-side nuclei compared with their contralateral counterparts. By 14 and 30 days after ganglionectomy, there were no statistically significant differences between the nuclei on the two sides. This transient asymmetry of glutaminase activities in the vestibular nuclei contrasts with the sustained asymmetry in the vestibular nerve root and suggests that intrinsic, commissural, or descending pathways are involved in the recovery of chemical symmetry. This recovery resembles our previous finding for glutamate concentrations in the vestibular nuclei and may partially underlie central vestibular compensation after peripheral lesions.

Effects of carboplatin on amino acid chemistry in chinchilla cochlear nucleus

Carboplatin, a drug widely used against solid head and neck tumors, selectively destroys cochlear inner hair cells and type I auditory nerve fibers in chinchilla. This should affect neurotransmitter chemistry, involving amino acids, where the type I auditory nerve fibers terminate in the cochlear nucleus. Using microdissection combined with high-performance liquid chromatography, amino acid concentrations were mapped in the cochlear nuclei of chinchillas injected intraperitoneally 6-8 weeks earlier with 100 mg/kg carboplatin and in those of control animals. Glutamate concentrations were 23% lower in the anteroventral cochlear nucleus (AVCN) and 40% lower in the posteroventral cochlear nucleus (PVCN) of carboplatin-injected chinchillas as compared to controls, while aspartate concentrations were 18% lower in AVCN and 27% lower in PVCN. Using a fluorometric assay, activities of glutaminase, an enzyme which catalyzes glutamate synthesis, were 30% lower in AVCN and 38% lower in PVCN of carboplatin-injected chinchillas. Concentrations of glutamine, gamma-aminobutyrate, and glycine were also lower in some ventral and dorsal cochlear nucleus regions of treated animals. These changes probably result mainly from both primary and later effects of reduced type I auditory nerve fiber input to the cochlear nucleus.