Non-N-methyl-D-aspartate receptors may mediate ipsilateral excitation at lateral superior olivary synapses

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.

Chemical Effects of Kainic Acid Injection into the Rat Superior Olivary Region

Kainic acid injections have been used to destroy neuron somata in particular regions without damaging fiber tracts. We injected a solution of kainic acid into the region of the rat superior olivary complex in an effort to destroy its cholinergic projections to the cochlea and cochlear nucleus, which derive especially from the lateral superior olivary nucleus and ventral nucleus of the trapezoid body. In the lateral superior olivary nucleus, there were relatively small but fairly consistent decreases of choline acetyltransferase (ChAT) activity, larger decreases of acetylcholinesterase (AChE) activity, and consistent decreases of malate dehydrogenase activity, as a marker for oxidative metabolism. Other superior olivary regions were less affected by the kainic acid injections, but most showed overall significant decreases of AChE activity. Our results suggest that the cholinergic neurons giving rise to the centrifugal pathways to the cochlea and cochlear nucleus are more resistant to the effects of kainic acid than are those that receive major ascending input from the cochlear nucleus and project to higher levels of the auditory system. Comparison with published anatomical studies suggests that this resistance to the effects of kainic acid is related to relatively little glutamatergic input to the somata and proximal dendrites of these neurons. We also found a consistent approximately 16 % decrease of ChAT activity in the injected-side facial nerve root, which is most easily explained as a small effect of kainic acid on the facial nerve fibers passing through the injection site.

Effects of ketamine on response properties of neurons in the superior paraolivary nucleus of the mouse

The superior paraolivary nucleus (SPON; alternative abbreviation: SPN for the same nucleus in certain species) is a prominent brainstem structure that provides strong inhibitory input to the auditory midbrain. Previous studies established that SPON neurons encode temporal sound features with high precision. These earlier characterizations of SPON responses were recorded under the influence of ketamine, a dissociative anesthetic agent and known antagonist of N-methyl-d-aspartate glutamate (NMDA) receptors. Because NMDA alters neural responses from the auditory brainstem, single unit extracellular recordings of SPON neurons were performed in the presence and absence of ketamine. In doing so, this study represents the first in vivo examination of the SPON of the mouse. Herein, independent data sets of SPON neurons are characterized that did or did not receive ketamine, as well as neurons that were recorded both prior to and following ketamine administration. In all conditions, SPON neurons exhibited contralaterally driven spikes triggered by the offset of pure tone stimuli. Ketamine lowered both evoked and spontaneous spiking, decreased the sharpness of frequency tuning, and increased auditory thresholds and first-spike latencies. In addition, ketamine limited the range of modulation frequencies to which neurons phase-locked to sinusoidally amplitude-modulated tones.

Bilirubin enhances neuronal excitability by increasing glutamatergic transmission in the rat lateral superior olive

Hyperbilirubinemia is one of the most common clinical phenomena observed in human newborns. To achieve effective therapeutic treatment, numerous studies have been done to determine the molecular mechanisms of bilirubin-induced neuronal excitotoxicity. However, there is no conclusive evidence for the involvement of glutamatergic synaptic transmission in bilirubin-induced neuronal hyperexcitation and excitotoxicity. In the present study, using gramicidin-perforated patch-clamp techniques, spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from lateral superior olive (LSO) neurons isolated from postnatal 11-14-day-old (P11-14) rats. The application of 3 μM bilirubin increased the frequency, but not the amplitude, of sEPSCs. The action of bilirubin was tetrodotoxin (TTX)-insensitive, as bilirubin also increased the frequency, but not the amplitude, of mEPSCs. The amplitudes of GABA-activated (I(GABA)) and glutamate-activated (I(glu)) currents were not affected by bilirubin. Under current-clamp conditions, no spontaneous action potentials were observed in control solution. However, the application of 3 μM bilirubin for 4-6 min evoked a considerable rate of action-potential firing. The evoked firing was partially occluded by D,L-2-amino-5-phosphonovaleric acid (APV), an NMDA receptor antagonist, but completely inhibited by a combination of APV and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX), an AMPA receptor antagonist. These results indicate that bilirubin facilitates presynaptic glutamate release, enhances glutamatergic synaptic transmission by activating postsynaptic AMPA and NMDA receptors, and leads to neuronal hyperexcitation. This study provides a better understanding of the mechanism of bilirubin-induced excitotoxicity and determines for the first time that both AMPA and NMDA receptors are likely involved in the excitotoxicity produced by bilirubin.

The potassium channel KCNQ5/Kv7.5 is localized in synaptic endings of auditory brainstem nuclei of the rat

KCNQ, also called Kv7, is a family of voltage-dependent potassium channels with important roles in excitability regulation. Of its five known subunits, KCNQ5/Kv7.5 is extensively expressed in the central nervous system and it contributes to the generation of M-currents. The distribution of KCNQ5 was analyzed in auditory nuclei of the rat brainstem by high-resolution immunocytochemistry. Double labeling with anti-KCNQ5 antibodies and anti-synaptophysin or anti-syntaxin, which mark synaptic endings, or anti-microtubule-associated protein 2 (MAP2) antibodies, which mark dendrites, were used to analyze the subcellular distribution of KCNQ5 in neurons in the cochlear nucleus, superior olivary complex, nuclei of the lateral lemniscus, and inferior colliculus. An abundance of KCNQ5 labeling in punctate structures throughout auditory brainstem nuclei along with colocalization with such synaptic markers suggests that a preferred localization of KCNQ5 is in synaptic endings in these auditory nuclei. Punctate KCNQ5 immunoreactivity virtually disappeared from the cochlear nucleus after cochlea removal, which strongly supports localization of this channel in excitatory endings of the auditory nerve. Actually, neither glycinergic endings, labeled with an anti-glycine transporter 2 (GlyT2) antibody, nor gamma-aminobutyric acid (GABA)ergic endings, labeled with an anti-glutamic acid decarboxylase (GAD65) antibody, contained KCNQ5 immunoreactivity, suggesting that KCNQ5 is mostly in excitatory endings throughout the auditory brainstem. Overlap of KCNQ5 and MAP2 labeling indicates that KCNQ5 is also targeted to dendritic compartments. These findings predict pre- and postsynaptic roles for KCNQ5 in excitability regulation in auditory brainstem nuclei, at the level of glutamatergic excitatory endings and in dendrites.

Expression of functional kainate and AMPA receptors in developing lateral superior olive neurons of the rat

A functional analysis of AMPA and kainate receptors (AMPARs and KARs) in the lateral superior olive (LSO), a major nucleus in the auditory brainstem, has not been performed so far, to our knowledge. Here we investigated the presence and characteristics of such receptors in the rat LSO by means of whole-cell patch-clamp recordings in combination with pharmacology. Current responses evoked by 200 microM AMPA were completely blocked by the specific AMPAR antagonist GYKI 52466 (100 microM). Properties of the AMPAR-mediated currents (latency, activation time constant, and peak amplitude) remained constant between postnatal day 3 (P3) and P10. Current responses evoked by 100 microM KA were not completely blocked by 100 microM GYKI 52466, indicating that the residual component was mediated by KARs. Throughout development, two groups of KAR-mediated currents (fast I(KA) and slow I(KA)) were distinguished because they had significantly different mean activation time constants. Moreover, the mean peak amplitude of fast I(KA) was significantly higher than that of slow I(KA). The differentiation into fast I(KA) and slow I(KA) can be explained by the existence of two groups of LSO neurons displaying different KAR densities, distributions, and/or diverse types with differences in conductance. Application of the specific KAR subunit agonists SYM 2081 (10 microM), ATPA (10 microM), or iodowillardiine (1 microM) evoked currents in almost all cells tested, showing that GluR5 subunits are a component of functional KARs in LSO neurons. Electrical stimulation of ipsilateral input fibers in the presence of KAR antagonists (NS-102 and GAMS), modulators (WGA), or GYKI 52466 revealed the presence of synaptic KARs in LSO neurons.

Glutamatergic calcium responses in the developing lateral superior olive: receptor types and their specific activation by synaptic activity patterns

The lateral superior olive (LSO) is a binaural auditory brain stem nucleus that plays a central role in sound localization. Survival and maturation of developing LSO neurons critically depend on intracellular calcium signaling. Here we investigated the mechanisms by which glutamatergic afferents from the cochlear nucleus increase intracellular calcium concentration in LSO neurons. Using fura-2 calcium imaging in slices prepared from neonatal mice, we found that cochlear nucleus afferents can activate all major classes of ionotropic and metabotropic glutamate receptors, each of which contributes to an increase in intracellular calcium. The specific activation of different glutamate receptor classes was dependent on response amplitudes and afferent stimulus patterns. Low-amplitude responses elicited by single stimuli were entirely mediated by calcium-impermeable AMPA/kainate receptors that activated voltage-gated calcium channels. Larger-amplitude responses elicited by either single stimuli or stimulus trains resulted in additional calcium influx through N-methyl-d-aspartate receptors. Finally, high-frequency stimulation also recruited group I and group II metabotropic glutamate receptors, both of which mobilized intracellular calcium. This calcium release in turn activated a strong influx of extracellular calcium through a membrane calcium channel that is distinct from voltage-gated calcium channels. Together, these results indicate that before hearing onset, distinct patterns of afferent activity generate qualitatively distinct types of calcium responses, which likely serve in guiding different aspects of LSO development.

The lateral superior olive: a functional role in sound source localization

Sound location in azimuth is signaled by differences in the times of arrival (interaural time difference, ITDs) and the amplitudes (interaural level differences, ILDs) of the stimuli at the ears. Psychophysical studies have shown that low- and high-frequency sounds are localized based on ITDs and ILDs, respectively, suggesting that dual mechanisms mediate localization. The anatomical and physiological bases for this "duplex theory" of localization are found in the medial (MSO) and lateral (LSO) superior olives, two of the most peripheral sites in the ascending auditory pathway receiving inputs from both ears. The MSO and LSO are believed to be responsible for the initial encoding of ITDs and ILDs, respectively. Here the author focuses on ILDs as a cue to location and the role of the LSO in encoding ILDs. Evidence from disparate fields of study supports the hypothesis that the LSO is the initial ILD processor in the mammalian auditory system.

Amino acid concentrations in rat cochlear nucleus and superior olive

Distributions of 10 amino acids were mapped in the cochlear nucleus and superior olive of rats by microdissection of freeze-dried sections combined with high performance liquid chromatography. Glutamate concentrations were relatively high in regions containing granule cell bodies, axons and terminals, whereas aspartate concentrations were higher in the rest of the cochlear nucleus. The distribution of glutamine, a metabolic precursor of glutamate, correlated highly with that of glutamate. In the superior olive, glutamate concentrations were similar among the nuclei, whereas aspartate concentrations were higher in the more dorsal nuclei. Glycine concentrations were relatively high in dorsal portions of the cochlear nucleus and superior olive and were much higher in all regions than those of gamma-aminobutyrate (GABA). Both GABA and taurine showed decreasing gradients from superficial to deep layers of the dorsal cochlear nucleus. Concentrations of serine, threonine, arginine and alanine were generally lower than those of the other six amino acids. The results support other evidence for prominent roles of glutamate and glycine as neurotransmitters in the cochlear nucleus and superior olive. They support a neurotransmitter role also for GABA, especially in the superficial layers of the dorsal cochlear nucleus, but less in the superior olive. The literature related to our results is reviewed.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.

The AMPA receptors of auditory neurons

The ionotropic glutamate receptor (GluR) subtype known as the AMPA receptor, which mediates rapid excitatory synaptic transmission in many regions of the nervous system, is composed of four different protein subunits, termed GluRs 1-4. The functional properties of each AMPA receptor are determined by the relative levels of GluRs 1-4 and by post-transcriptional modifications of these proteins through mRNA editing and alternative exon splicing. The present paper reviews the published evidence for (1) localization of mRNAs and immunoreactivity for GluRs 1-4 in the cochlea and subcortical central nervous system auditory pathways of mammals and birds, and (2) involvement of AMPA receptors in synaptic transmission in the auditory system. Recent biochemical and electrophysiological evidence concerning the specialized properties of AMPA receptors on brainstem auditory neurons is also reviewed, along with data concerning how these properties emerge during normal development.

Glutamate receptor subunits in neuronal populations of the gerbil lateral superior olive

The distribution of AMPA-preferring ionotropic glutamate receptors (GluR) within the gerbil lateral superior olive (LSO) was investigated immunocytochemically using antibodies to GluR1, 2, 2/3 and 4. Light microscopy showed GluR1 antibody preferentially labeling a population of small neurons located in the dorsal hilus and a population mainly at or near the margins of the LSO. GluR4 antibody strongly stained most large LSO neuronal somata and proximal dendrites including all principal cells. GluR2/3 antibody showed very modest staining and appeared in most cell types. GluR2 showed less intense neuronal staining than GluR2/3 and was observed as a punctate accumulation at the surface of some neuronal profiles. GluR1, 2, 2/3 and 4 immunoreactivity was found along dendrites of most large LSO neurons and in their somata. Postsynaptic specializations positive for GluR2 were rare on LSO somata compared to the high frequency of GluR4 and 1 specializations. Double labeling studies showed that different portions of the distal dendrites showed a preponderance of GluR1 or GluR4 subunits. Electron microscopic observations confirm similarities in the localization of immunoreactivity for the antibodies tested in the cytoplasm of somata and dendrites, but reveal differences at the plasmalemma, at synaptic appositions and appositions with glial processes. Receptor composition varied with cell type and location on cells.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

Glutamate receptors underlying excitatory synaptic transmission in the rat's lateral superior olive studied in vitro

Glutamate receptors underlying synaptic excitation in the rat's lateral superior olive were studied by whole-cell patch clamp recordings in a brain slice preparation. Recordings from two morphological types of cells, bipolar and multipolar, identified by intracellular labeling with biocytin, showed that there were no obvious differences in responses mediated or modulated by ionotropic and metabotropic receptors between these two types of neurons. The excitatory postsynaptic potentials (EPSPs) elicited by ipsilateral stimulation of the trapezoid body consisted of two components. An earlier component, which had faster rise time constant and decay time constant, was mediated by non-NMDA receptors. A later component, which had slower rise time and decay time constants, was mediated by NMDA receptors. Suprathreshold responses (action potentials), which arose from the early component, were always abolished by the non-NMDA antagonist, CNQX, but not by the NMDA antagonist, APV. These results suggest that both non-NMDA and NMDA receptors are present in LSO neurons, and that fast excitatory transmission in LSO is primarily mediated by non-NMDA receptors. The metabotropic glutamate receptor agonists, t-ACPD and L-AP4, reduced the size of EPSPs evoked by stimulation of the ipsilateral trapezoid body in LSO neurons; the reductive action of t-ACPD was reversed by the antagonist, MCPG, indicating that metabotropic glutamate receptors, probably group II and III subtypes, can modulate excitatory synaptic transmission in LSO.

Afferent regulation of glycine receptor distribution in the gerbil LSO

Synaptic activity plays an important role in many aspects ofneuronal development, particularly the expression of proteins. In this study, the influence of inhibitory and excitatory afferents on the development of glycine receptor density in the lateral superior olive (LSO) of Mongolian gerbils was investigated. Afferent activity was manipulated by removing one or both cochleas at postnatal day 7, prior to the onset of sound-evoked responses. Due to the anatomy of the LSO, these manipulations result in either excitatory denervation, inhibitory denervation, or both. The density of glycine receptors in the LSO was determined at 21 days postnatal. Glycine receptors were either labeled with tritiated strychnine (3H-SN) or with an antibody directed against gephyrin, a protein closely associated with the receptor complex. Antibody binding was used to quantify the differential glycine receptor density between the medial limb (high frequency area) and the lateral limb (low frequency area) of the LSO. 3H-SN was used to quantify the amount of glycine receptors in each part of the LSO in control and experimental animals. In addition, changes in neuron density and neuron cross-sectional area were quantified following cochlear ablations. In control animals, the amount of glycine receptors is about 2- to 3-fold higher in the high-frequency than in the low-frequency region. In bilaterally ablated animals, the same density of glycine receptors was measured in the high- and low-frequency region. Unilateral ablations had no significant effect on glycine receptor distribution, either ipsi- or contralateral to the ablation. The neuron cross-sectional area decreased about 30% in the ipsilateral LSO of unilaterally ablated animals and in bilaterally ablated animals. However, alterations of soma density and cross-sectional area were similar in the high- and low-frequency projection region. These results suggest that the distribution of glycine receptors is only changed when excitatory and inhibitory afferents have been denervated.

Regulation of D-aspartate release and uptake in adult brain stem auditory nuclei after unilateral middle ear ossicle removal and cochlear ablation

In young adult guinea pigs, the effects of unilateral ossicle removal and cochlear ablation were determined on transmitter release from glutamatergic presynaptic endings and glutamate inactivation via uptake. (i) D-[3H]Aspartate release and uptake were measured in subdivisions of the cochlear nucleus (CN) and in nuclei of the superior olive (SOC) and auditory midbrain (MB) up to 145 days after placing the lesions. Activities were compared to those from age-matched unlesioned controls. Fiber degeneration was visualized histologically. (ii) In the ipsilateral CN, changes in release and uptake were governed by the type of lesion. Ossicle removal produced sparse pruning of fibers only after 112 days and decreased release and uptake at 145 days, consistent with regulatory weakening of excitatory glutamatergic transmission. Cochlear ablation deafferented the CN, producing deficient release and uptake at 2 days and abundant fiber degeneration at 7 days. Subsequently, the residual release and uptake increased in magnitude, consistent with strengthening of excitatory glutamatergic transmission. (iii) In the contralateral CN, after either lesion, changes in release and uptake usually matched those in the ipsilateral CN. Thus, the auditory pathway associated with the lesioned ear probably provided cues for the regulation of synaptic strength in the contralateral CN. (iv) Both lesions increased release in the SOC and MB, and uptake in the SOC, consistent with strengthening of excitatory glutamatergic transmission. Sparse fiber degeneration, suggesting axonal pruning, appeared in the SOC and MB after cochlear ablation. (v) The strengthening of excitatory glutamatergic transmission may facilitate and maintain symptoms such as loudness recruitment and tinnitus which often accompany hearing loss.

Decreased inhibition to lateral superior olive neurons in young and aged Sprague-Dawley rats

Lateral superior olive (LSO) neurons in young and aged Sprague-Dawley rats have functional properties consistent with a limited contralateral inhibition, which is markedly different from other animals. An unusually low proportion of LSO cells (36/113) exhibited contralateral inhibition (and ipsilateral excitation, IE), while over 25% of LSO units exhibited excitatory responses to contralateral stimuli. Inhibition of most IE LSO neurons was evident only when the contralateral intensity was greater than the ipsilateral intensity, resulting in a marked shift in sensitivity to interaural intensity differences (IID). The firing rate of IE neurons was also affected more by a change in intensity of ipsilateral compared to contralateral stimuli. The shift in the IID sensitivity and the relative decrease in effectiveness of contralaterally driven inhibition in Sprague-Dawley rat LSO neurons could be due to decreased inhibitory inputs from the MNTB principal cells, increased contralateral excitatory effects and/or increased ipsilateral excitatory effects. Age-related decreases in the numbers of MNTB neurons observed anatomically is not reflected in a change in LSO function. The Sprague-Dawley rat may be a useful model for the effect of reduced inhibition in the superior olivary complex on auditory behavior.

Physiological evidence for ipsilateral inhibition in the lateral superior olive: synaptic responses in mouse brain slice

The incidence of ipsilateral inhibition in the lateral superior olive (LSO) was examined in a brain slice preparation of the mouse superior olivary complex. A 400 microns brain slice was taken in the frontal plane and maintained in a warm, oxygenated saline solution. Intracellular recordings were made from the LSO with micropipettes filled with 4 M potassium acetate. Synaptic responses were elicited by electrical stimulation of the trapezoid body in different slices at various locations between the cochlear nucleus and the ipsilateral superior olivary complex (SOC). The results show that ipsilateral stimulation can evoke inhibitory as well as excitatory postsynaptic potentials. The ipsilateral IPSPs have short latencies and are elicited by stimulation of the trapezoid body at any point along its course between cochlear nucleus and LSO. Short-latency IPSPs can also be produced by direct stimulation of the ventral cochlear nucleus itself. Ipsilateral IPSPs are blocked by low concentrations of the glycine antagonist, strychnine. In addition, bath application of sodium pentobarbital in one case eliminated ipsilateral IPSPs without eliminating EPSPs. The results suggest that there is a rapidly conducting, glycinergic pathway from cochlear nucleus through the trapezoid body to the LSO on the same side of the brain. This pathway is probably served by either a direct projection from the ventral cochlear nucleus to the LSO or an indirect one from cochlear nucleus to LSO through the lateral nucleus of the trapezoid body (LNTB).

Response properties in young and old Fischer-344 rat lateral superior olive neurons: a quantitative approach

Significant age-related functional deficits may result from a selective loss of inhibitory processing in the lateral superior olivary (LSO) nucleus. To test this hypothesis, methods were developed for quantitative comparisons of the excitation evoked by ipsilateral acoustic stimuli and the inhibition evoked by contralateral acoustic stimuli in neurons recorded from this binaural structure. Data were obtained from 103 LSO neurons from 22 young adult (3-6 month) and 70 LSO neurons from 14 old (20-23 month) Fischer-344 (F-344) rats. Age-related increases in the thresholds of auditory brainstem responses as well as in the inhibitory and excitatory responses of LSO neurons were observed. Spike-discharge rates were analyzed using multiple regression analysis for interaural intensity data and by calculating correlation coefficients between excitatory and inhibitory isointensity curves. Most LSO principal cells exhibited ipsilateral excitation and contralateral inhibition of similar strength for similar stimuli. Inhibitory and excitatory response areas of these neurons were similar based on visual inspection and correlation coefficients. No statistically significant age-related changes were observed for (a) rate-level functions generated by ipsilateral or contralateral stimuli; (b) maximal discharge rate; (c) conduction latencies; or (d) measures of binaural function. However, a small percentage of LSO neurons both in young and old rats displayed "unmatched" inhibitory and excitatory response areas. Quantitative methods developed in this study are used to examine age-related changes in binaural function in the inferior colliculus of F-344 rats and in the LSO of the Sprague-Dawley rat. Although no asymmetrical aging changes were observed for the F-344 rat LSO, there appear to be significant differences between the aging auditory system in the F-344 and Sprague-Dawley rats. Age-related changes have been previously described for the F-344 in other brainstem auditory structures.

Patterns of glutamate, glycine, and GABA immunolabeling in four synaptic terminal classes in the lateral superior olive of the guinea pig

The goal of this study was to correlate synaptic ultrastructure with transmitter specificity and function in the lateral superior olive (LSO), a nucleus that is thought to play a major role in sound localization. This was accomplished by means of postembedding immunogold immunocytochemistry. Four classes of synaptic terminals were identified in the LSO. They were distinguishable from one another both morphologically and on the basis of their different patterns of immunolabeling for glutamate, glycine, and gamma-aminobutyric acid (GABA). The highest level of glutamate immunoreactivity was found in terminals that contained round vesicles (R) and formed synaptic contacts with asymmetric synaptic junctions. Round-vesicle terminals predominated on small caliber dendrites by a ratio of at least 2:1 over the other classes combined. The thinnest dendrites were typically contacted by R terminals only. The ratio of R terminals to the other types decreased as the caliber of the dendritic profiles they apposed increased so that on the soma, R terminals were outnumbered by at least 2:1 by the other types. Terminals containing flattened vesicles (F) exhibited intense immunoreactivity for both glycine and glutamate, although the glutamate immunolabeling was not as high as that in the R terminals. Flattened-vesicle terminals formed symmetric synaptic contacts with their targets and their distribution was the reverse of that described for R terminals; i.e., they were most abundant on LSO perikarya and fewest on small caliber dendrites. Two terminal types, both containing pleomorphic vesicles and forming symmetric synaptic junctions, were found in far fewer numbers. One group contained large pleomorphic vesicles (LP) and was immunoreactive for both glycine and GABA. The other group contained small pleomorphic vesicles (SP) along with a few dense-core vesicles and labeled for GABA only. The LP terminals were preferentially distributed on somata and large-caliber dendrites, while the SP terminals most often contacted smaller dendrites. Previous work suggests that a large percentage of the R terminals arise from spherical cells in the ipsilateral cochlear nucleus and are excitatory in action. This pathway may use glutamate as a transmitter. Many of the F terminals are thought to originate from the ipsilateral medial nucleus of the trapezoid body and appear to be the inhibitory (glycinergic) terminals from a pathway that originates from the contralateral ear. The origins and functions of LP and SP terminals are unknown, but a few possibilities are discussed along with the significance of cocontainment of neuroactive substances in specific terminal types.

Acoustic chiasm V: inhibition and excitation in the ipsilateral and contralateral projections of LSO

When this series of experiments was begun in 1984, the activity of each lateral superior olive (LSO) in the mammalian hindbrain was known to encode the hemifield of acoustic space containing a sound source. However, the almost random bilaterality of its ascending projections seemed to jumble that identification before reaching the midbrain. At the same time, electrophysiological studies of LSO and its efferent target in the inferior colliculus, along with the strictly contralateral deficits in sound localization resulting from unilateral lesions above the level of the superior olives, indicated that hemifield allegiance was largely maintained (though reversed) at the midbrain. Here we present seven lines of biochemical evidence, some combined with prior ablations, supporting the notion that the anatomical segregation of the ipsilateral and contralateral fibers ascending from the LSO is accompanied by a corresponding segregation of their neurotransmitters: most of the ascending ipsilateral projection is probably glycinergic and, hence, inhibitory in effect, while most of the contralateral projection is probably glutamatergic/aspartergic and, hence, excitatory in effect. Taken together, the inhibitory ipsilateral projections and the excitatory contralateral projections serve to amplify functional contralaterality at the higher levels of the auditory system.

NMDA, non-NMDA and glycine receptors mediate binaural interaction in the lateral superior olive: Physiological evidence from mouse brain slice

Binaural interaction was investigated in a 400 microns brain slice taken through the mouse lateral superior olive (LSO). Ipsilateral excitatory and contralateral inhibitory inputs to LSO neurons were examined by recording physiological responses to electrical stimulation of the trapezoid body. Bath application of non-N-methyl-D-aspartate (non-NMDA) antagonists blocked ipsilateral excitation and strychnine blocked contralateral inhibition. N-methyl-D-aspartate (NMDA) had little effect on ipsilateral responses but completely blocked contralateral inhibition. These results suggest that ipsilateral excitation is mediated by non-NMDA receptors and contralateral inhibition by strychnine dependent glycine receptors. NMDA receptors may play a role by modulating contralateral inhibition in LSO.

Effects of excitant amino acids on acoustic responses of inferior colliculus neurons

Iontophoretic application of the excitant amino acids (EAAs), glutamate, aspartate and N-methyl-D-aspartate (NMDA) resulted in increased acoustically evoked and spontaneous firing of most neurons in the central nucleus of inferior colliculus (ICC). The excitatory effects of these EAAs were blocked by simultaneous application of EAA antagonists which selectively block the NMDA receptor subtype, 2-amino-5-phosphonovalerate or D-alpha-aminoadipate and to a lesser extent with non-selective EAA antagonists, such as glutamic acid diethylester. Application of NMDA receptor-selective EAA antagonists alone greatly reduced the firing of most ICC neurons examined, but non-selective EAA antagonists either increased or produced little change in firing of most ICC neurons examined. In this and previous studies cholinergic agonists were found to increase the firing of ICC neurons, but the cholinergic agonists were less effective in exciting ICC neurons than EAA agonists. Cholinergic antagonists in a previous study were considerably less effective in inhibiting the discharge of ICC neurons than were the EAA antagonists in the present study. These results, in conjunction with previous neurochemical and anatomical localization studies, support a possible role of an EAA as a candidate for afferent excitatory transmitter in neurons of the inferior colliculus.