Transient appearance of calbindin-D28k-positive neurons in the superior olivary complex of developing rats

Impact of repeated intranasal gentamicin irrigation on auditory brainstem evoked potentials in rats

Gentamicin is a bactericidal aminoglycoside antibiotic that broadly targets Gram-negative microbes. Both human and animal studies have shown that administration of gentamicin is ototoxic by several routes of administration and results in sensorineural hearing loss due to damaged hair cell at the base of the cochlea. However, gentamicin is also administered intranasally to treat sinusitis in humans, but no animal studies have examined ototoxicity of gentamicin administered via this route. We hypothesized that intranasal irrigation of gentamicin will result in ototoxicity and impaired auditory function similar to systemic delivery. We investigated this hypothesis in Sprague-Dawley rats that received intranasal irrigations of gentamicin or saline from postnatal day (P) 21-31. We examined auditory function by assessing brainstem auditory evoked potentials in response to both broadband clicks and pure tone-pips (4, 8, 16, 24 and 32 kHz) on P41. We found significant changes in auditory function in gentamicin-exposed animals. Specifically, gentamicin-exposed animals had significantly higher thresholds in response to both clicks and tone-pips. In response to broadband clicks, there were no changes in latency for waves I through IV. However, we found significantly longer wave and interwave latencies for all waves in response to the 24 kHz tone-pip. Together, these findings suggest that intranasal administration of gentamicin results in impaired auditory function consistent with other routes of delivery.

Cortical dysmorphology and reduced cortico-collicular projections in an animal model of autism spectrum disorder

Autism spectrum disorder is a neurodevelopmental disability that includes sensory disturbances. Hearing is frequently affected and ranges from deafness to hypersensitivity. In utero exposure to the antiepileptic valproic acid is associated with increased risk of autism spectrum disorder in humans and timed valproic acid exposure is a biologically relevant and validated animal model of autism spectrum disorder. Valproic acid-exposed rats have fewer neurons in their auditory brainstem and thalamus, fewer calbindin-positive neurons, reduced ascending projections to the midbrain and thalamus, elevated thresholds, and delayed auditory brainstem responses. Additionally, in the auditory cortex, valproic acid exposure results in abnormal responses, decreased phase-locking, elevated thresholds, and abnormal tonotopic maps. We therefore hypothesized that in utero, valproic acid exposure would result in fewer neurons in auditory cortex, neuronal dysmorphology, fewer calbindin-positive neurons, and reduced connectivity. We approached this hypothesis using morphometric analyses, immunohistochemistry, and retrograde tract tracing. We found thinner cortical layers but no changes in the density of neurons, smaller pyramidal and non-pyramidal neurons in several regions, fewer neurons immunoreactive for calbindin-positive, and fewer cortical neurons projecting to the inferior colliculus. These results support the widespread impact of the auditory system in autism spectrum disorder and valproic acid-exposed animals and emphasize the utility of simple, noninvasive auditory screening for autism spectrum disorder.

The nuclei of the lateral lemniscus: unexpected players in the descending auditory pathway

Introduction

In the mammalian auditory pathway, the nuclei of the lateral lemniscus (NLL) are thought to be exclusively involved in the bottom-up transmission of auditory information. However, our repeated observation of numerous NLL neurons labeled after injection of retrograde tracers into the superior olivary complex (SOC) led us to systematically investigate with retrograde tracers the descending projections from the NLL to the SOC of the rat.

Methods

We performed large injections of FluoroGold into the SOC to determine NLL contributions to descending projections, and focal injections of biotinylated dextran amine (BDA) to pinpoint the specific nuclei of the SOC innervated by each NLL.

Results

The SOC is innervated by thousands of neurons distributed across four nuclei or regions associated with the lateral lemniscus: the ipsilateral ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL); the medial paralemniscal region (PL) of both sides; and the ipsilateral semilunar nucleus (SLN), a previously unrecognized nucleus that wraps around the INLL dorsally, medially, and caudally and consists of small, flat neurons. In some experiments, at least 30% of neurons in the VNLL and INLL were retrogradely labeled. All nuclei of the SOC, except the medial and lateral superior olives, are innervated by abundant lemniscal neurons, and each SOC nucleus receives a unique combination of lemniscal inputs. The primary target of the projections from the VNLL is the ventral nucleus of the trapezoid body (VNTB), followed by the superior paraolivary nucleus (SPON), and the medial nucleus of the trapezoid body (MNTB). The INLL selectively innervates the VNTB. The PL innervates dorsal periolivary regions bilaterally. The SLN preferentially innervates the MNTB and may provide the first identified non-calyceal excitatory input to MNTB neurons.

Discussion

Our novel findings have strong implications for understanding acoustic information processing in the initial stages of the auditory pathway. Based on the proportion of lemniscal neurons involved in all the projections described, the NLL should be considered major players in the descending auditory pathway.

Characterization of the superior olivary complex of chimpanzees (Pan troglodytes) in comparison to humans

The superior olivary complex (SOC) is a collection of nuclei in the hindbrain of mammals with numerous roles in hearing, including localization of sound sources in the environment, encoding temporal and spectral elements of sound, and descending modulation of the cochlea. While there have been several investigations of the SOC in primates, there are discrepancies in the descriptions of nuclear borders and even the presence of certain cell groups among studies and species. Herein, we aimed to clarify some of these issues by characterizing the SOC from chimpanzees using Nissl staining, quantitative morphometry and immunohistochemistry. We found the medial superior olive (MSO) to be the largest of the SOC nuclei and the arrangement of its neurons and peri-MSO to be very similar to humans. Additionally, we found neurons in the medial nucleus of the trapezoid body (MNTB) to be immunopositive for the calcium binding protein calbindin. Further, most neurons in the MNTB, and some neurons in the lateral nucleus of the trapezoid body were associated with large, calretinin-immunoreactive calyx terminals. Together, these findings indicate the organization of the SOC of chimpanzees is organized very similar to the SOC in humans and suggests modifications to this region among species consistent with differences in head/body size, restricted hearing range and sensitivity to low frequency sounds.

Shared and organ-specific gene-expression programs during the development of the cochlea and the superior olivary complex

The peripheral and central auditory subsystems together form a complex sensory network that allows an organism to hear. The genetic programs of the two subsystems must therefore be tightly coordinated during development. Yet, their interactions and common expression pathways have never been systematically explored. MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression and are essential for normal development of the auditory system. We performed mRNA and small-RNA sequencing of organs from both auditory subsystems at three critical developmental timepoints (E16, P0, P16) to obtain a comprehensive and unbiased insight of their expression profiles. Our analysis reveals common and organ-specific expression patterns for differentially regulated mRNAs and miRNAs, which could be clustered with a particular selection of functions such as inner ear development, Wnt signalling, K+ transport, and axon guidance, based on gene ontology. Bioinformatics detected enrichment of predicted targets of specific miRNAs in the clusters and predicted regulatory interactions by monitoring opposite trends of expression of miRNAs and their targets. This approach identified six miRNAs as strong regulatory candidates for both subsystems. Among them was miR-96, an established critical factor for proper development in both subsystems, demonstrating the strength of our approach. We suggest that other miRNAs identified by this analysis are also common effectors of proper hearing acquirement. This first combined comprehensive analysis of the developmental program of the peripheral and central auditory systems provides important data and bioinformatics insights into the shared genetic program of the two sensory subsystems and their regulation by miRNAs.

Leveling up: a long-range olivary projection to the medial geniculate without collaterals to the central nucleus of the inferior colliculus in rats

The medial nucleus of the trapezoid body (MNTB) is one of the monaural cell groups situated within the superior olivary complex (SOC), a constellation of brainstem nuclei with numerous roles in hearing. Principal MNTB neurons are glycinergic and express the calcium-binding protein, calbindin (CB). The MNTB receives its main glutamatergic, excitatory input from the contralateral cochlear nucleus via the calyx of Held and converts this into glycinergic inhibition directed toward nuclei in the SOC and the ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL). Through this inhibition, the MNTB plays essential roles in localization of sound sources and encoding spectral and temporal features of sound. In rats, very few MNTB neurons project to the inferior colliculus. However, our recent study of SOC projections to the auditory thalamus revealed a substantial number of retrogradely labeled MNTB neurons. This observation led us to examine whether the rat MNTB provides a long-range projection to the medial geniculate body (MGB). We examined this possible projection using retrograde and anterograde tract tracing and immunohistochemistry for CB and the glycine receptor. Our results demonstrate a significant projection to the MGB from the ipsilateral MNTB that does not involve a collateral projection to the inferior colliculus.

Transcriptional profiling reveals roles of intercellular Fgf9 signaling in astrocyte maturation and synaptic refinement during brainstem development

Neural tissue maturation is a coordinated process under tight transcriptional control. We previously analyzed the kinetics of gene expression in the medial nucleus of the trapezoid body (MNTB) in the brainstem during the critical postnatal phase of its development. While this work revealed timed execution of transcriptional programs, it was blind to the specific cells where gene expression changes occurred. Here, we utilized single-cell RNA-Seq to determine transcriptional profiles of each major MNTB cell type. We discerned directional signaling patterns between neuronal, glial, and vascular-associated cells for VEGF, TGFβ, and Delta-Notch pathways during a robust period of vascular remodeling in the MNTB. Furthermore, we describe functional outcomes of the disruption of neuron-astrocyte fibroblast growth factor 9 (Fgf9) signaling. We used a conditional KO (cKO) approach to genetically delete Fgf9 from principal neurons in the MNTB, which led to an early onset of glial fibrillary acidic protein (Gfap) expression in astrocytes. In turn, Fgf9 cKO mice show increased levels of astrocyte-enriched brevican (Bcan), a component of the perineuronal net matrix that ensheaths principal neurons in the MNTB and the large calyx of Held terminal, while levels of the neuron-enriched hyaluronan and proteoglycan link protein 1 (Hapln1) were unchanged. Finally, volumetric analysis of vesicular glutamate transporters 1 and 2 (Vglut1/2), which serves as a proxy for terminal size, revealed an increase in calyx of Held volume in the Fgf9 cKO. Overall, we demonstrate a coordinated neuron-astrocyte Fgf9 signaling network that functions to regulate astrocyte maturation, perineuronal net structure, and synaptic refinement.

Multiscale modeling of presynaptic dynamics from molecular to mesoscale

Chemical synapses exhibit a diverse array of internal mechanisms that affect the dynamics of transmission efficacy. Many of these processes, such as release of neurotransmitter and vesicle recycling, depend strongly on activity-dependent influx and accumulation of Ca²⁺. To model how each of these processes may affect the processing of information in neural circuits, and how their dysfunction may lead to disease states, requires a computationally efficient modelling framework, capable of generating accurate phenomenology without incurring a heavy computational cost per synapse. Constructing a phenomenologically realistic model requires the precise characterization of the timing and probability of neurotransmitter release. Difficulties arise in that functional forms of instantaneous release rate can be difficult to extract from noisy data without running many thousands of trials, and in biophysical synapses, facilitation of per-vesicle release probability is confounded by depletion. To overcome this, we obtained traces of free Ca²⁺ concentration in response to various action potential stimulus trains from a molecular MCell model of a hippocampal Schaffer collateral axon. Ca²⁺ sensors were placed at varying distance from a voltage-dependent calcium channel (VDCC) cluster, and Ca²⁺ was buffered by calbindin. Then, using the calcium traces to drive deterministic state vector models of synaptotagmin 1 and 7 (Syt-1/7), which respectively mediate synchronous and asynchronous release in excitatory hippocampal synapses, we obtained high-resolution profiles of instantaneous release rate, to which we applied functional fits. Synchronous vesicle release occurred predominantly within half a micron of the source of spike-evoked Ca²⁺ influx, while asynchronous release occurred more consistently at all distances. Both fast and slow mechanisms exhibited multi-exponential release rate curves, whose magnitudes decayed exponentially with distance from the Ca²⁺ source. Profile parameters facilitate on different time scales according to a single, general facilitation function. These functional descriptions lay the groundwork for efficient mesoscale modelling of vesicular release dynamics.

Most information transmission between neurons in the brain occurs via release of neurotransmitter from synaptic vesicles. In response to a presynaptic spike, calcium influx at the active zone of a synapse can trigger the release of neurotransmitter with a certain probability. These stochastic release events may occur immediately after a spike or with some delay. As calcium accumulates from one spike to the next, the probability of release may increase (facilitate) for subsequent spikes. This process, known as short-term plasticity, transforms the spiking code to a release code, underlying much of the brain’s information processing. In this paper, we use an accurate, detailed model of presynaptic molecular physiology to characterize these processes at high precision in response to various spike trains. We then apply model reduction to the results to obtain a phenomenological model of release timing, probability, and facilitation, which can perform as accurately as the molecular model but with far less computational cost. This mesoscale model of spike-evoked release and facilitation helps to bridge the gap between microscale molecular dynamics and macroscale information processing in neural circuits. It can thus benefit large scale modelling of neural circuits, biologically inspired machine learning models, and the design of neuromorphic chips.

Impact of cochlear ablation on calbindin and synaptophysin in the gerbil medial nucleus of the trapezoid body before hearing onset

Spontaneous bursting activity is already generated in the cochlea before hearing onset and represents an important condition of the functional and anatomical organization of auditory brainstem nuclei. In the present study, cochlea ablation induced changes were characterized in auditory brainstem nuclei indirectly innervated by auditory nerve fibers before hearing onset. In Meriones unguiculatus immunohistochemical labeling of calbindin-D28k (CB) and synaptophysin (SYN) were performed. The influence of cochlea-ablation on CB or SYN was analyzed by considering their differential immunoreaction during development. During the normal postnatal development, CB was first detected in somata of the medial nucleus of the trapezoid body (MNTB) at postnatal day (P)4. The immunoreaction increased gradually in parallel to the appearance of CB-immunoreactive terminal fields in distinct superior olivary complex (SOC) nuclei. Cochlear removal at P5 or P9 in animals with 24 and 48 h survival times resulted in an increase in somatic CB-labeling in the lesioned MNTB including terminal fields compared to the non-lesioned MNTB. SYN-immunolabeling was first detected at P0 and began to strongly encircle the MNTB neurons at P4. A further progression was observed with age. Cochlear ablation resulted in a significant reduction of SYN-labeled MNTB areas of P5-cochlea-ablated gerbils after 48 h post-lesion. In P9 cochlea-ablated gerbils, a redistribution of SYN-positive terminals was seen after 24 and 48 h. Taken together, the destruction of cochlea differentially influences CB- and SYN-labeling in the MNTB, which should be considered in association with different critical periods before hearing onset.

Secretagogin expression in the vertebrate brainstem with focus on the noradrenergic system and implications for Alzheimer's disease

Calcium-binding proteins are widely used to distinguish neuronal subsets in the brain. This study focuses on secretagogin, an EF-hand calcium sensor, to identify distinct neuronal populations in the brainstem of several vertebrate species. By using neural tube whole mounts of mouse embryos, we show that secretagogin is already expressed during the early ontogeny of brainstem noradrenaline cells. In adults, secretagogin-expressing neurons typically populate relay centres of special senses and vegetative regulatory centres of the medulla oblongata, pons and midbrain. Notably, secretagogin expression overlapped with the brainstem column of noradrenergic cell bodies, including the locus coeruleus (A6) and the A1, A5 and A7 fields. Secretagogin expression in avian, mouse, rat and human samples showed quasi-equivalent patterns, suggesting conservation throughout vertebrate phylogeny. We found reduced secretagogin expression in locus coeruleus from subjects with Alzheimer’s disease, and this reduction paralleled the loss of tyrosine hydroxylase, the enzyme rate limiting noradrenaline synthesis. Residual secretagogin immunoreactivity was confined to small submembrane domains associated with initial aberrant tau phosphorylation. In conclusion, we provide evidence that secretagogin is a useful marker to distinguish neuronal subsets in the brainstem, conserved throughout several species, and its altered expression may reflect cellular dysfunction of locus coeruleus neurons in Alzheimer’s disease.

Characterization of the superior olivary complex of Canis lupus domesticus

The superior olivary complex (SOC) is a collection of brainstem auditory nuclei which play essential roles in the localization of sound sources, temporal coding of vocalizations and descending modulation of the cochlea. Notwithstanding, the SOC nuclei vary considerably between species in accordance with the auditory needs of the animal. The canine SOC was subjected to anatomical and physiological examination nearly 50 years ago and was then virtually forgotten. Herein, we aimed to characterize the nuclei of the canine SOC using quantitative morphometrics, estimation of neuronal number, histochemistry for perineuronal nets and immunofluorescence for the calcium binding proteins calbindin and calretinin. We found the principal nuclei to be extremely well developed: the lateral superior olive (LSO) contained over 20,000 neurons and the medial superior olive (MSO) contained over 15,000 neurons. In nearly all non-chiropterian terrestrial mammals, the MSO exists as a thin, vertical column of neurons. The canine MSO was folded into a U-shaped contour and had associated with the ventromedial tip a small, round collection of neurons we termed the tail nucleus of the MSO. Further, we found evidence within the LSO, MSO and medial nucleus of the trapezoid body (MNTB) for significant morphological variations along the mediolateral or rostrocaudal axes. Finally, the majority of MNTB neurons were calbindin-immunopositive and associated with calretinin-immunopositive calyceal terminals. Together, these observations suggest the canine SOC complies with the basic plan of the mammalian SOC but possesses a number of unique anatomical features.

An organotypic slice culture to study the formation of calyx of Held synapses in-vitro

The calyx of Held, a large axo-somatic relay synapse containing hundreds of presynaptic active zones, is possibly the largest nerve terminal in the mammalian CNS. Studying its initial growth in-vitro might provide insights into the specification of synaptic connection size in the developing brain. However, attempts to maintain calyces of Held in organotypic cultures have not been fruitful in past studies. Here, we describe an organotypic slice culture method in which calyces of Held form in-vitro. We made coronal brainstem slices with an optimized slice angle using newborn mice in which calyces have not yet formed; the presynaptic bushy cells were genetically labeled using the Math5 promoter. After six to nine days of culturing, we readily observed large Math5—positive nerve terminals in the medial nucleus of the trapezoid body (MNTB), but not in the neighboring lateral superior olive nucleus (LSO). These calyx—like synapses expressed the Ca²⁺- sensor Synaptotagmin-2 (Syt-2) and the Ca²⁺ binding protein Parvalbumin (PV), two markers of developing calyces of Held in vivo. Application of the BMP inhibitor LDN-193189 significantly inhibited the growth of calyx synapses, demonstrating the feasibility of long-term pharmacological manipulation using this organotypic culture method. These experiments provide a method for organotypic culturing of calyces of Held, and show that the formation of calyx—like synapses onto MNTB neurons can be preserved in-vitro. Furthermore, our study adds pharmacological evidence for a role of BMP-signaling in the formation of large calyx of Held synapses.

Mechanism of auditory hypersensitivity in human autism using autism model rats

BACKGROUND

Auditory hypersensitivity is one of the major complications in autism spectrum disorder. The aim of this study was to investigate whether the auditory brain center is affected in autism model rats.

METHODS

Autism model rats were prepared by prenatal exposure to thalidomide on embryonic day 9 and 10 in pregnant rats. The superior olivary complex (SOC), a complex of auditory nuclei, was immunostained with anti-calbindin d28k antibody at postnatal day 50.

RESULTS

In autism model rats, SOC immunoreactivity was markedly decreased. Strength of immunostaining of SOC auditory fibers was also weak in autism model rats. Surprisingly, the size of the medial nucleus of trapezoid body, a nucleus exerting inhibitory function in SOC, was significantly decreased in autism model rats.

CONCLUSIONS

Auditory hypersensitivity may be, in part, due to impairment of inhibitory processing by the auditory brain center.

Evolution of mammalian sound localization circuits: A developmental perspective

Localization of sound sources is a central aspect of auditory processing. A unique feature of mammals is the smooth, tonotopically organized extension of the hearing range to high frequencies (HF) above 10kHz, which likely induced positive selection for novel mechanisms of sound localization. How this change in the auditory periphery is accompanied by changes in the central auditory system is unresolved. I will argue that the major VGlut2(+) excitatory projection neurons of sound localization circuits (dorsal cochlear nucleus (DCN), lateral and medial superior olive (LSO and MSO)) represent serial homologs with modifications, thus being paramorphs. This assumption is based on common embryonic origin from an Atoh1(+)/Wnt1(+) cell lineage in the rhombic lip of r5, same cell birth, a fusiform cell morphology, shared genetic components such as Lhx2 and Lhx9 transcription factors, and similar projection patterns. Such a parsimonious evolutionary mechanism likely accelerated the emergence of neurons for sound localization in all three dimensions. Genetic analyses indicate that auditory nuclei in fish, birds, and mammals receive contributions from the same progenitor lineages. Anatomical and physiological differences and the independent evolution of tympanic ears in vertebrate groups, however, argue for convergent evolution of sound localization circuits in tetrapods (amphibians, reptiles, birds, and mammals). These disparate findings are discussed in the context of the genetic architecture of the developing hindbrain, which facilitates convergent evolution. Yet, it will be critical to decipher the gene regulatory networks underlying development of auditory neurons across vertebrates to explore the possibility of homologous neuronal populations.

Differential morphology of the superior olivary complex of Meriones unguiculatus and Monodelphis domestica revealed by calcium-binding proteins

In mammals, the superior olivary complex (SOC) of the brainstem is composed of nuclei that integrate afferent auditory originating from both ears. Here, the expression of different calcium-binding proteins in subnuclei of the SOC was studied in distantly related mammals, the Mongolian gerbil (Meriones unguiculatus) and the gray short-tailed opossum (Monodelphis domestica) to get a better understanding of the basal nuclear organization of the SOC. Combined immunofluorescence labeling of the calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k, and calretinin as well as pan-neuronal markers displayed characteristic distribution patterns highlighting details of neuronal architecture of SOC nuclei. Parvalbumin was found in almost all neurons of SOC nuclei in both species, while calbindin and calretinin were restricted to specific cell types and axonal terminal fields. In both species, calbindin displayed a ubiquitous and mostly selective distribution in neurons of the medial nucleus of trapezoid body (MNTB) including their terminal axonal fields in different SOC targets. In Meriones, calretinin and calbindin showed non-overlapping expression patterns in neuron somata and terminal fields throughout the SOC. In Monodelphis, co-expression of calbindin and calretinin was observed in the MNTB, and hence both CaBPs were also co-localized in terminal fields within the adjacent SOC nuclei. The distribution patterns of CaBPs in both species are discussed with respect to the intrinsic neuronal SOC circuits as part of the auditory brainstem system that underlie the binaural integrative processing of acoustic signals as the basis for localization and discrimination of auditory objects.

Different tonotopic regions of the lateral superior olive receive a similar combination of afferent inputs

The mammalian lateral superior olive (LSO) codes disparities in the intensity of the sound that reaches the two ears by integrating ipsilateral excitation and contralateral inhibition, but it remains unclear what particular neuron types convey acoustic information to the nucleus. It is also uncertain whether the known conspicuous morphofunctional differences and gradients along the tonotopic axis of the LSO relate to qualitative and/or quantitative regional differences in its afferents. To clarify these issues, we made small, single injections of the neuroanatomical tracer biotinylated dextran amine (BDA) into different tonotopic regions of the LSO of albino rats and analyzed the neurons labeled retrogradely in brainstem auditory nuclei. We demonstrate that the LSO is innervated tonotopically by four brainstem neuron types: spherical bushy cells and planar multipolar neurons of the ipsilateral ventral cochlear nucleus, principal neurons of the ipsilateral medial nucleus of the trapezoid body, and small multipolar neurons of the contralateral ventral nucleus of the trapezoid body. Unexpectedly, the proportion of labeled neurons of each type was virtually identical in all cases, thus indicating that all tonotopic regions of the LSO receive a similar combination of inputs. Even more surprisingly, our data also suggest that the representation of frequencies in the LSO differs from that of the nuclei that innervate it: compared to the latter nuclei, the LSO seems to possess a relatively larger portion of its volume devoted to processing frequencies in the lower-middle part of the spectrum, and a relative smaller portion devoted to higher frequencies. J. Comp. Neurol. 524:2230-2250, 2016. © 2015 Wiley Periodicals, Inc.

Heterogeneous calretinin expression in the avian cochlear nucleus angularis

Multiple calcium-binding proteins (CaBPs) are expressed at high levels and in complementary patterns in the auditory pathways of birds, mammals, and other vertebrates, but whether specific members of the CaBP family can be used to identify neuronal subpopulations is unclear. We used double immunofluorescence labeling of calretinin (CR) in combination with neuronal markers to investigate the distribution of CR-expressing neurons in brainstem sections of the cochlear nucleus in the chicken (Gallus gallus domesticus). While CR was homogeneously expressed in cochlear nucleus magnocellularis, CR expression was highly heterogeneous in cochlear nucleus angularis (NA), a nucleus with diverse cell types analogous in function to neurons in the mammalian ventral cochlear nucleus. To quantify the distribution of CR in the total NA cell population, we used antibodies against neuronal nuclear protein (NeuN), a postmitotic neuron-specific nuclear marker. In NA neurons, NeuN label was variably localized to the cell nucleus and the cytoplasm, and the intensity of NeuN immunoreactivity was inversely correlated with the intensity of CR immunoreactivity. The percentage of CR + neurons in NA increased from 31 % in embryonic (E)17/18 chicks, to 44 % around hatching (E21), to 51 % in postnatal day (P) 8 chicks. By P8, the distribution of CR + neurons was uniform, both rostrocaudal and in the tonotopic (dorsoventral) axis. Immunoreactivity for the voltage-gated potassium ion channel Kv1.1, used as a marker for physiological type, showed broad and heterogeneous postsynaptic expression in NA, but did not correlate with CR expression. These results suggest that CR may define a subpopulation of neurons within nucleus angularis.

Time-dependent gene expression analysis of the developing superior olivary complex

The superior olivary complex (SOC) is an essential auditory brainstem relay involved in sound localization. To identify the genetic program underlying its maturation, we profiled the rat SOC transcriptome at postnatal days 0, 4, 16, and 25 (P0, P4, P16, and P25, respectively), using genome-wide microarrays (41,012 oligonucleotides (oligos)). Differences in gene expression between two consecutive stages were highest between P4 and P16 (3.6%) and dropped to 0.06% between P16 and P25. To identify SOC-related genetic programs, we also profiled the entire brain at P4 and P25. The number of differentially expressed oligonucleotides between SOC and brain almost doubled from P4 to P25 (4.4% versus 7.6%). These data demonstrate considerable molecular specification around hearing onset, which is rapidly finalized. Prior to hearing onset, several transcription factors associated with the peripheral auditory system were up-regulated, probably coordinating the development of the auditory system. Additionally, crystallin-γ subunits and serotonin-related genes were highly expressed. The molecular repertoire of mature neurons was sculpted by SOC-related up- and down-regulation of voltage-gated channels and G-proteins. Comparison with the brain revealed a significant enrichment of hearing impairment-related oligos in the SOC (26 in the SOC, only 11 in the brain). Furthermore, 29 of 453 SOC-related oligos mapped within 19 genetic intervals associated with hearing impairment. Together, we identified sequential genetic programs in the SOC, thereby pinpointing candidates that may guide its development and ensure proper function. The enrichment of hearing impairment-related genes in the SOC may have implications for restoring hearing because central auditory structures might be more severely affected than previously appreciated.

Embryonic origins of the mouse superior olivary complex

Many areas of the central nervous system are organized into clusters of cell groups, with component cell groups exhibiting diverse but related functions. One such cluster, the superior olivary complex (SOC), is located in the ventral auditory brainstem in mammals. The SOC is an obligatory contact point for most projection neurons of the ventral cochlear nucleus and plays central roles in many aspects of monaural and binaural information processing. Despite their important interrelated functions, little is known about the embryonic origins of SOC nuclei, due in part to a paucity of developmental markers to distinguish individual cell groups. In this report, we present a collection of novel markers for the developing SOC nuclei in mice, including the transcription factors FoxP1, MafB, and Sox2, and the lineage-marking transgenic line En1-Cre. We use these definitive markers to examine the rhombic lip and rhombomeric origins of SOC nuclei and demonstrate that they can serve to uniquely identify SOC nuclei and subnuclei in newborn pups. The markers are also useful in identifying distinct nuclear domains within the presumptive SOC as early as embryonic day (E) 14.5, well before morphological distinction of individual nuclei is evident. These findings indicate that the mediolateral and dorsoventral position of SOC nuclei characteristic of the adult brainstem is established during early neurogenesis.

Differential distribution of glycine receptor subtypes at the rat calyx of Held synapse

The properties of glycine receptors (GlyRs) depend upon their subunit composition. While the prevalent adult forms of GlyRs are heteromers, previous reports suggested functional α homomeric receptors in mature nervous tissues. Here we show two functionally different GlyRs populations in the rat medial nucleus of trapezoid body (MNTB). Postsynaptic receptors formed α1/β-containing clusters on somatodendritic domains of MNTB principal neurons, colocalizing with glycinergic nerve endings to mediate fast, phasic IPSCs. In contrast, presynaptic receptors on glutamatergic calyx of Held terminals were composed of dispersed, homomeric α1 receptors. Interestingly, the parent cell bodies of the calyces of Held, the globular bushy cells of the cochlear nucleus, expressed somatodendritic receptors (α1/β heteromers) and showed similar clustering and pharmacological profile as GlyRs on MNTB principal cells. These results suggest that specific targeting of GlyR β-subunit produces segregation of GlyR subtypes involved in two different mechanisms of modulation of synaptic strength.

Developmental changes of the protein repertoire in the rat auditory brainstem: a comparative proteomics approach in the superior olivary complex and the inferior colliculus with DIGE and iTRAQ

Protein profiles of developing neural circuits undergo manifold changes. The aim of this proteomic analysis was to quantify postnatal changes in two auditory brainstem areas in a comparative approach. Protein samples from the inferior colliculus (IC) and the superior olivary complex (SOC) were obtained from neonatal (P4) and young adult (P60) rats. The cytosolic fractions of both areas were examined by 2-D DIGE, and the plasma membrane-enriched fraction of the IC was analyzed via iTRAQ. iTRAQ showed a regulation in 34% of the quantified proteins. DIGE revealed 12% regulated spots in both the SOC and IC and, thus, numeric congruency. Although regulation in KEGG pathways displayed a similar pattern in both areas, only 13 of 71 regulated DIGE proteins were regulated in common, implying major area-specific differences. 89% of regulated glycolysis/gluconeogenesis and citrate cycle proteins were up-regulated in the SOC or IC, suggesting a higher energy demand in adulthood. Seventeen cytoskeleton proteins were regulated, consistent with complex morphological reorganization between P4 and P60. Fourteen were uniquely regulated in the SOC, providing further evidence for area-specific differences. Altogether, we provide the first elaborate catalog of proteins involved in auditory brainstem development, several of them possibly of particular developmental relevance.

Egr2::cre mediated conditional ablation of dicer disrupts histogenesis of mammalian central auditory nuclei

Histogenesis of the auditory system requires extensive molecular orchestration. Recently, Dicer1, an essential gene for generation of microRNAs, and miR-96 were shown to be important for development of the peripheral auditory system. Here, we investigated their role for the formation of the auditory brainstem. Egr2::Cre-mediated early embryonic ablation of Dicer1 caused severe disruption of auditory brainstem structures. In adult animals, the volume of the cochlear nucleus complex (CNC) was reduced by 73.5%. This decrease is in part attributed to the lack of the microneuronal shell. In contrast, fusiform cells, which similar to the granular cells of the microneural shell are derived from Egr2 positive cells, were still present. The volume reduction of the CNC was already present at birth (67.2% decrease). The superior olivary complex was also drastically affected in these mice. Nissl staining as well as Vglut1 and Calbindin 1 immunolabeling revealed that principal SOC nuclei such as the medial nucleus of the trapezoid body and the lateral superior olive were absent. Only choline acetyltransferase positive neurons of the olivocochlear bundle were observed as a densely packed cell group in the ventrolateral area of the SOC. Mid-embryonic ablation of Dicer1 in the ventral cochlear nucleus by Atoh7::Cre-mediated recombination resulted in normal formation of the cochlear nucleus complex, indicating an early embryonic requirement of Dicer1. Quantitative RT-PCR analysis of miR-96 demonstrated low expression in the embryonic brainstem and up-regulation thereafter, suggesting that other microRNAs are required for proper histogenesis of the auditory brainstem. Together our data identify a critical role of Dicer activity during embryonic development of the auditory brainstem.

Pre- and postsynaptic properties of glutamatergic transmission in the immature inhibitory MNTB-LSO pathway

The lateral superior olive (LSO) integrates excitatory inputs driven by sound arriving at the ipsilateral ear with inhibitory inputs driven by sound arriving at the contralateral ear in order to compute interaural intensity differences needed for localizing high-frequency sound sources. Specific mechanisms necessary for developmental refinement of the inhibitory projection, which arises from the medial nucleus of the trapezoid body (MNTB), have only been partially deciphered. The demonstration that immature MNTB-LSO synapses release glutamate has led to a model in which early glutamate neurotransmission plays a major role in inhibitory plasticity. We used whole cell electrophysiology in acute auditory brain stem slices of neonatal rats to examine glutamatergic transmission in the developing MNTB-LSO pathway. Unexpectedly, AMPA receptor (AMPAR)-mediated responses were prevalent at the earliest ages. We found a salient developmental profile for NMDA receptor (NMDAR) activation, described both by the proportion of total glutamate current and by current durations, and we found evidence for distinct release probabilities for GABA/glycine and glutamate in the MNTB-LSO pathway. The developmental profile of NMDAR is consistent with the possibility that the inhibitory MNTB-LSO pathway experiences a sensitive period, driven by cochlear activity and mediated by GluN2B-containing NMDARs, between postnatal days 3 and 9. Differing neurotransmitter release probabilities could allow the synapse to switch between GABA/glycinergic transmission and mixed glutamate/GABA/glycinergic transmission in response to changing patterns of spiking activity.

Cav1.3 calcium channels are required for normal development of the auditory brainstem

Within the Ca(v)1 family of voltage-gated calcium channels, Ca(v)1.2 and Ca(v)1.3 channels are the predominant subtypes in the brain. Whereas specific functions for each subtype were described in the adult brain, their role in brain development is poorly understood. Here we assess the role of Ca(v)1.3 subunits in the activity-dependent development of the auditory brainstem. We used Ca(v)1.3-deficient (Ca(v)1.3(-/-)) mice because these mice lack cochlea-driven activity that deprives the auditory centers from peripheral input. We found a drastically reduced volume in all auditory brainstem centers (range 25-59%, total 35%), which was manifest before hearing onset. A reduction was not obvious outside the auditory system. The lateral superior olive (LSO) was strikingly malformed in Ca(v)1.3(-/-) mice and had fewer neurons (1/3 less). The remaining LSO neurons displayed normal dendritic trees and received functional glutamatergic input, yet they fired action potentials predominantly with a multiple pattern upon depolarization, in contrast to the single firing pattern prevalent in controls. The latter finding appears to be due to a reduction of dendrototoxin-sensitive potassium conductances, presumably mediated through the K(v)1.2 subtype. Fura2 imaging provided evidence for functional Ca(v)1.3 channels in the LSO of wild-type mice. Our results imply that Ca(v)1.3 channels are indispensable for the development of the central auditory system. We propose that the unique LSO phenotype in Ca(v)1.3(-/-) mice, which hitherto was not described in other hereditary deafness models, is caused by the synergistic contribution of two factors: on-site loss of Ca(v)1.3 channels in the neurons plus lack of peripheral input.

NMDAR-Mediated Calcium Transients Elicited by Glutamate Co-Release at Developing Inhibitory Synapses

Before hearing onset, the topographic organization of the inhibitory sound localization pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) is refined by means of synaptic silencing and strengthening. During this refinement period MNTB-LSO synapses not only release GABA and glycine but also release glutamate. This co-released glutamate can elicit postsynaptic currents that are predominantly mediated by NMDA receptors (NMDARs). To gain a better understanding of how glutamate contributes to synaptic signaling at developing MNTB-LSO inhibitory synapses, we investigated to what degree and under what conditions NMDARs contribute to postsynaptic calcium responses. Our results demonstrate that MNTB-LSO synapses can elicit compartmentalized calcium responses along aspiny LSO dendrites. These responses are significantly attenuated by the NMDAR antagonist APV. APV, however, had no effect on somatically recorded electrical postsynaptic responses, indicating little, if any, contribution of NMDARs to spike generation. NMDAR-mediated calcium responses were decreased when increasing extracellular magnesium concentrations to physiological levels indicating that MNTB-LSO synapses activate magnesium sensitive NMDAR on immature LSO dendrites. In Fura-2 AM loaded neurons, blocking GABA_A and glycine receptors increased NMDAR contribution to somatic calcium responses suggesting that GABA and glycine, perhaps by shunting backpropagating action potentials, decrease the level of NMDAR activation under strong stimulus conditions.

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.

Characterization of cochlear nucleus principal cells of Meriones unguiculatus and Monodelphis domestica by use of calcium-binding protein immunolabeling

Antibodies directed against calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k and calretinin were used as neuronal markers to identify and characterize different principal cell types in the mammalian cochlear nucleus. For this purpose, double immunofluorescence labeling and the combination of CaBP-labeling with pan-neuronal markers were applied to analyze the CaBPs distribution in neurons of the cochlear nucleus (CN) of the Mongolian gerbil (Meriones unguiculatus) and the gray short-tailed opossum (Monodelphis domestica). Despite of the fact, that these two mammalian species are not closely related, principal cell types in the CN of the two species showed many corresponding morphological features and similarities in immunolabeling of the CaBPs. Parvalbumin seems not to be suited as a differential neuronal marker in the CN since it is expressed by almost all neurons. In contrast, calbindin and calretinin were more restricted to specific cell types and showed a mostly complementary labeling pattern. As one of the most interesting findings, calbindin and calretinin were predominantly found in subpopulations of globular bushy cells and octopus cells in the ventral CN. Such a neuron-specific CaBP-expression in subpopulations of morphologically defined cell types argues for a more refined classification of CN cell types in Meriones and Monodelphis. Additionally, other cell types (cartwheel cells, unipolar brush cells, fusiform cells) were marked with calbindin or calretinin as well. Calretinin staining was predominantly observed in auditory nerve fibers and their endings including endbulbs of Held in Meriones. Spherical bushy cells showed a different calretinin-immunolabeling in Meriones and Monodelphis. This species-specific difference may be related to adaptive differences in auditory function.

Immediate early gene expression invoked by electrical intracochlear stimulation in some but not all types of neurons in the rat auditory brainstem

Specific patterns of sensory activity may induce plastic remodeling of neurons and the communication network they form in the adult mammalian brain. Among the indicators for the initiation of neuronal remodeling is the expression of immediate early genes (IEGs). The IEGs c-fos and egr-1 encode transcription factors. Following spectrally and temporally precisely defined unilateral electrical intracochlear stimulation (EIS) that corresponded in strength to physiological acoustic stimuli and lasted for 2 h under anesthesia, we characterized those neuronal cell types in ventral (VCN) and dorsal cochlear nucleus (DCN), lateral superior olive (LSO) and central nucleus of the inferior colliculus (CIC) of the rat brain that expressed IEGs. We found that EIS affected only specific types of neurons. Whereas sub-populations of glutamatergic and glycinergic cells responded in all four regions, GABAergic neurons failed to do so except in DCN. Combining immunocytochemistry with axonal tracing, neurons participating in major ascending pathways, commissural cells of VCN and certain types of neurons of the descending auditory system were seen to respond to EIS with IEG expression. By contrast, principal LSO cells projecting to the contralateral CIC as well as collicular efferents of the DCN did not. In total, less than 50% of the identified neurons turned up expression of the IEGs studied. The pattern of IEG expression caused by unilateral EIS led us to suggest that dominant sensory activity may quickly initiate a facilitation of central pathways serving the active ear at the expense of those serving the unstimulated ear.

Characterization of the rhesus monkey superior olivary complex by calcium binding proteins and synaptophysin

This study was performed in order to characterize the main nuclei of the rhesus monkey superior olivary complex by means of antibodies against the calcium binding proteins parvalbumin, calbindin and calretinin and the synaptic vesicle protein synaptophysin. These markers revealed the neuronal morphology and organization of nuclei located within the rhesus monkey superior olivary complex. The architectural details included the distribution of axonal terminals on neurons. The medial superior olivary nucleus was present as a column of neurons. No clear segregation of calretinin-positive terminals was noticed on the medial and lateral dendritic fields of these neurons. The lateral superior olivary nucleus was characterized by a distinct nuclear shape. Calretinin-, parvalbumin- or calbindin-positive terminals contacted somata and dendrites. The medial nucleus of trapezoid body could be clearly differentiated as a distinct region in the rhesus monkey superior olivary complex. Somata of that nucleus showed calbindin- and parvalbumin-labelling whereas somatic calyces of Held were reavealed by calretinin and synaptophysin labelling. The results are discussed with respect to the processing of acoustic information in primate species and their ability to hear high and low frequencies, which is reflected by anatomical correlates.

Gene expression profiling of the rat superior olivary complex using serial analysis of gene expression

The superior olivary complex (SOC) is an auditory brainstem region that represents a favourable system to study rapid neurotransmission and the maturation of neuronal circuits. Here we performed serial analysis of gene expression (SAGE) on the SOC in 60-day-old Sprague-Dawley rats to identify genes specifically important for its function and to create a transcriptome reference for the subsequent identification of age-related or disease-related changes. Sequencing of 31 035 tags identified 10 473 different transcripts. Fifty-seven per cent of the unique tags with a count greater than four were statistically more highly represented in the SOC than in the hippocampus. Among them were genes encoding proteins involved in energy supply, the glutamate/glutamine shuttle, and myelination. Approximately 80 plasma membrane transporters, receptors, channels, and vesicular transporters were identified, and 25% of them displayed a significantly higher expression level in the SOC than in the hippocampus. Some of the plasma membrane proteins were not previously characterized in the SOC, e.g. the purinergic receptor subunit P2X(6) and the metabotropic GABA receptor Gpr51. Differential gene expression between SOC and hippocampus was confirmed using RNA in situ hybridization or immunohistochemistry. The extensive gene inventory presented here will alleviate the dissection of the molecular mechanisms underlying specific SOC functions and the comparison with other SAGE libraries from brain will ease the identification of promoters to generate region-specific transgenic animals. The analysis will be part of the publicly available database ID-GRAB.

Developmental expression of the Ca2+-binding proteins calretinin and parvalbumin at the calyx of Held of rats and mice

Ca(2+)-binding proteins of the EF-hand family are widely expressed in the CNS, and contribute to intracellular Ca(2+) buffering in neurons. In nerve terminals, Ca(2+)-binding proteins are likely to regulate transmitter release probability and synaptic short-term-plasticity. Here, we investigated the developmental expression pattern of calretinin and parvalbumin at a large excitatory synapse, the calyx of Held in the medial nucleus of the trapezoid body (MNTB) of rats and mice. We used two-colour immunofluorescence imaging with primary antibodies detecting one of the Ca(2+)-binding proteins, and a presynaptic marker protein, Rab-3A. Calretinin was found in nerve terminals of the calyx of Held, but not in postsynaptic principal cells. The presynaptic density of Calretinin staining, and the degree of colocalization with Rab-3A increased during postnatal development (P6-P31). Surprisingly, not all calyces of Held expressed calretinin. In rats, calretinin-containing calyces were irregularly interspersed with calretinin-negative calyces, whereas in mice, calretinin-positive calyces were preferentially located in the lateral portion of the MNTB. The percentage of calretinin-positive calyces increased during development, to about 75% and 20% at P30 in rats and in mice, respectively. Parvalbumin was present in the presynaptic calyces of Held and in the nerve fibres entering the MNTB, as well as in the somata of the MNTB principal neurons. An up-regulation of calretinin and parvalbumin in calyces of Held probably increases the presynaptic Ca(2+) buffering strength during postnatal development, but the unexpected heterogeneity of calretinin expression might cause differences in Ca(2+) signalling and transmitter release probability between calyces of Held.

Effects of calcium buffers and calbindin-D28k upon histamine-induced calcium oscillations and calcium waves in HeLa cells

The effects of the artificial Ca(2+) buffers EGTA and BAPTA upon histamine-induced Ca(2+) oscillations and calcium waves were studied in HeLa cells. These events were also examined in HeLa cell lines transfected with the intracellular calcium-binding protein calbindin-D28k (CaBP; HeLa-CaBP) or the pCINeo vector alone (HeLa-pCINeo). High concentrations of the Ca(2+) indicators fluo-3 and fura-2 significantly influenced the oscillatory pattern of intracellular Ca(2+) in HeLa-pCINeo cells exposed to 1 microM histamine. Loading cells with low concentrations of the cell-permeant esters of the artificial Ca(2+)-buffers EGTA or BAPTA, resulted in fewer cells with a distinct "baseline" oscillatory pattern, and loading with higher concentrations of BAPTA almost completely abolished them. In HeLa-CaBP cells, stimulation with 1 microM histamine resulted in individual Ca(2+) spikes that had a flattened profile when compared to control cells; peak [Ca(2+)](i) was lowered, the rate of increase in [Ca(2+)](i) was slower and transients were prolonged. When compared to HeLa-pCINeo cells, loading with EGTA or BAPTA, or transfection of CaBP, significantly reduced the propagation velocity (by up to 60%) of Ca(2+) waves induced by exposure to 100 microM histamine. We conclude that intracellular Ca(2+) buffering exerts a significant influence on global Ca(2+) responses in HeLa cells and the propagation of Ca(2+) waves that underlie them. The relative effectiveness of different Ca(2+) buffers, including CaBP, appears to be particularly dependent upon the rapidity of their binding kinetics, with BAPTA being the most effective.

Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI-32)

This study provides a morphologic characterization of the human superior olivary complex as revealed by immunohistochemistry by using antibodies against the calcium binding proteins parvalbumin, calbindin, calretinin, and the nonphosphorylated neurofilament H SMI-32. By combining these markers, it was possible to establish the neuronal architecture and details of the morphologic organization (including axonal terminals) of the different nuclei. The medial superior olivary nucleus is formed by a sheet of parallel-oriented cells. A clear segregation of axon terminals was noticed on the medially and laterally oriented dendrites of the mostly bipolar neurons. The lateral superior olivary nucleus lacked a distinct nuclear shape but was formed by several patches of rather irregularly arranged neurons. Calretinin or parvalbumin immunoreactive afferent terminals were observed which contacted somata or dendrites of these neurons. The immunolabeling also revealed the boundaries of the dorsal periolivary nucleus and morphologic detail of its neurons. A coherent nuclear structure that could be addressed as the medial nucleus of the trapezoid body was not identified by any single one or by combinations of the markers used. The data were also used to establish a three-dimensional-reconstruction of the three major subnuclei of the superior olivary complex. The results are discussed with respect to the possible role of the superior olivary complex in the processing of spatial acoustic information in the azimuthal plane.

Immunohistochemical localization of calcium-binding proteins in the brainstem vestibular nuclei of the jaundiced Gunn rat

Vestibular gaze and postural abnormalities are major sequelae of neonatal hyperbilirubinemia. The sites and cellular effects of bilirubin toxicity in the brainstem vestibular pathway are not easily detected. Since altered intracellular calcium homeostasis may play a role in neuronal cell death, we hypothesized that altered expression of calcium-binding proteins may occur in brainstem vestibular nuclei of the classic animal model of bilirubin neurotoxicity. The expression of the calcium-binding proteins calbindin-D28k and parvalbumin in the brainstem vestibular pathways and cerebellum of homozygous recessive jaundiced (jj) Gunn rats was examined by light microscopy and immunohistochemistry at 18 days postnatally and compared to the findings obtained from age-matched non-jaundiced heterozygous (Nj) littermate controls. Jaundiced animals exhibited decreased parvalbumin immunoreactivity specifically in synaptic inputs to superior, medial, and inferior vestibular nuclei, and to oculomotor and trochlear nuclei, whereas the neurons retained their normal immunoreactivity. Jaundiced animals also demonstrated a decrease in calbindin expression in the lateral vestibular nuclei and a paucity of calbindin-immunoreactive synaptic endings on the somata of Deiters' neurons. The involved regions are related to the control of the vestibulo-ocular and vestibulospinal reflexes. Decreased expression of calcium-binding proteins in brainstem vestibular neurons may relate to the vestibulo-ocular and vestibulospinal dysfunction seen with clinical kernicterus, and may provide a sensitive new way to assess bilirubin toxicity in the vestibular system.

Changes in calcium-binding protein expression in the auditory brainstem nuclei of the jaundiced Gunn rat

Sensorineural hearing loss and auditory dysfunction are major sequelae of neonatal hyperbilirubinemia. The sites and cellular effects of bilirubin toxicity in the auditory brainstem pathway are not easily detected. Since altered intracellular calcium homeostasis may play a role in neuronal cell death, we hypothesized that the expression of calcium-binding proteins may be altered in the classic animal model of bilirubin neurotoxicity. The expression of the calcium-binding proteins, calbindin-D28k and parvalbumin, in the brainstem auditory pathway of homozygous recessive jaundiced (jj) Gunn rats was examined by light and electron microscopic immunohistochemistry at 18 days postnatally and compared to the findings obtained from age-matched non-jaundiced heterozygous (Nj) littermate control rats. Immunoreactive staining for both calbindin and parvalbumin was reduced in the cochlear nuclei and the superior olivary complex in jj rats. The extent of the reduction in immunoreactivity was related to the severity of the clinical symptoms. By contrast, immunoreactive staining in other brainstem areas (e.g., dorsal and ventral nuclei of the lateral lemniscus, inferior colliculus), thalamic (medial geniculate body) auditory areas, and neighboring non-auditory structures was similar in jaundiced and control rats. Calbindin-immunoreactive staining in the superior paraolivary and medial superior olivary nuclei in Nj rats was associated with myelinated axons, whereas parvalbumin-immunoreactive staining was localized postsynaptically in neuronal somata and dendrites. Immunoreactive staining for the calcium-binding proteins calbindin and parvalbumin in lower brainstem auditory nuclei shows abnormalities in areas susceptible to the effects of hyperbilirubinemia and provides a sensitive new way to assess bilirubin toxicity in the auditory system.

Role of Ca(2+) in the synchronization of transmitter release at calyceal synapses in the auditory system of rat

The synchronization of transmitter release in the synapse of the medial nucleus of the trapezoid body (MNTB) is achieved during early postnatal development as a consequence of elimination of delayed asynchronous releases and appears to reflect changes in the dynamics of Ca(2+) entry and clearance. To examine the role of Ca(2+) in regulating synchronization of transmitter release in the mature synapse (after postnatal day 9, P9), we perturbed Ca(2+) dynamics systematically. Replacement of external Ca(2+) (2 mM) with Sr(2+) induced delayed asynchronous release following the major EPSC. We tried to reproduce asynchronous releases without using Sr(2+) and instead by manipulating the time course and the size of Ca(2+) transient in the presynaptic terminal, under the assumption that replacement of external Na(+) with Li(+) or application of eosin-Y would prolong the lifetime of Ca(2+) transient by reducing the rate of Ca(2+) extrusion from the terminal. With application of Li(+), Ca(2+) transient in the terminal was prolonged, the EPSC decay time course was prolonged, and the EPSC amplitude increased. However, these EPSCs were not followed by delayed asynchronous release. When Ca(2+) influx was reduced, either by partial Ca(2+) channel blockade with a low concentration of Cd(2+) or omega-agatoxin IVA, a marked asynchronous release resulted. This was further enhanced by the combined application of Li(+) or eosin-Y. These results suggest that cooperative increases of both Ca(2+) influx and Ca(2+) clearance capacities leading to a sharper Ca(2+) spike in the presynaptic terminal underlie synchronized transmitter release in the presynaptic terminal of the MNTB.

Differential development of Ca2+ dynamics in presynaptic terminal and postsynaptic neuron of the rat auditory synapse

Postnatal development of Ca2+ influx and Ca2+ clearance capacity were investigated in the synapse of medial nucleus of the trapezoid body (MNTB) of rat with fura-2 fluorimetry. In contrast to the presynaptic terminal, Ca2+ dynamics does not basically change in the postsynaptic principal neuron developmentally. This differential development of Ca2+ dynamics between pre- and postsynaptic neurons might be crucial for the organized formation and functional maturation of this synapse.

Perineuronal nets in the rat medial nucleus of the trapezoid body surround neurons immunoreactive for various amino acids, calcium-binding proteins and the potassium channel subunit Kv3.1b

Perineuronal nets (PNs) are known as chondroitin sulfate-rich, lattice-like coatings of the extracellular matrix ensheathing mainly GABAergic, parvalbumin-containing neurons especially in the cerebral cortex. PNs have also been detected around GABA-immunonegative cells which were shown to be not aminergic, cholinergic, nitrinergic or peptidergic in various brain regions of some mammalian species. To find out whether glycine and aspartate may occur in net-bearing neurons the present study was focused on the rat medial nucleus of the trapezoid body (MNTB) which contains a large portion of cells immunoreactive for these amino acids, but appears to be devoid of GABA-immunoreactive cell bodies. PNs were detected around many glycine- and aspartate-immunopositive neurons in the MNTB by carbocyanine double labeling and confocal laser scanning microscopy. An additional finding was that the lectin-cytochemically stained extracellular matrix surrounds the calretinin-immunoreactive calyces of Held known as giant glutamatergic endbulbs which cover glycinergic principal cells in the MNTB. As elucidated by triple fluorescence labeling, the vast majority of somata co-expressed the calcium-binding proteins parvalbumin and calbindin, but not calretinin. The observed co-localization of PNs and immunoreactivity for the voltage-dependent potassium channel Kv3.1b - as an established marker of fast-firing parvalbumin-containing neurons - supports the assumed function of PNs as a cation exchanger ensuring rapid ion transport as required by highly active nerve cells.

Synchronisation of neurotransmitter release during postnatal development in a calyceal presynaptic terminal of rat

Mechanisms contributing to the synchronisation of transmitter release during development were studied in synapses of the medial nucleus of the trapezoid body (MNTB) using patch recording and Ca2+ imaging techniques in a rat brainstem slice preparation. Excitatory postsynaptic currents (EPSCs) were generated in an all-or-none manner at immature synapses (postnatal days earlier than P6). Many delayed miniature EPSC (mEPSC)-like currents followed EPSCs at immature synapses, while observations of delayed mEPSC-like currents were rare at mature synapses (later than P9). At immature synapses bath application of either omega-conotoxin GVIA or omega-agatoxin-IVA reduced EPSCs (both to 40% of control), and Ca2+ currents in the presynaptic terminal (both to 70% of control). The frequency of delayed mEPSC-like currents was reduced by omega-conotoxin GVIA, but not by omega-agatoxin IVA. At immature synapses delayed mEPSC-like currents were rare after incubation of the slice with extrinsic Ca2+ buffers (EGTA AM). At mature synapses many mEPSC-like currents followed evoked EPSCs after partial block of Ca2+ channels by bath application of a low concentration of Cd2+ (3 microM) or omega-agatoxin IVA (50 nM) but not by low [Ca2+]o (0.5-1 mM). Ca2+ transients induced by action potentials in presynaptic terminals were monitored by adding a high concentration of fura-2 (200 microM) to the pipette. Their decay time course was slower at immature presynaptic terminals than at mature terminals. Both the Ca2+ extrusion rate and the endogenous Ca2+ binding capacity were estimated to be smaller at immature terminals than at mature terminals. These results suggest that the maturation of synaptic transmission in MNTB progresses with the capacity for Ca2+ clearance from the presynaptic terminal. The possible importance of developmental increases in both Ca2+ clearance capacity and Ca2+ currents is discussed in relation to the synchronisation of transmitter release.

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.

Development and influence of inhibition in the lateral superior olivary nucleus

While studies of neuronal development and plasticity have focused on excitatory pathways, the inhibitory projection from the MNTB to the LSO provides a favorable model for studies of synaptic inhibition. This review covers recent studies from our laboratories indicating that inhibitory connections are quite dynamic during development. These findings suggest that there are two phases inhibitory transmission. During an initial depolarizing phase is growth and branching of pre- and postsynaptic elements in the LSO. During a second hyperpolarizing phase there is refinement of inhibitory afferent arborizations and the LSO dendrites that they innervate.

Plasticity of the auditory brainstem: cochleotomy-induced changes of calbindin-D28k expression in the rat

Calbindin is a calcium binding protein that is characteristically expressed in several auditory brainstem nuclei during ontogeny and is thought to serve as a buffer, protecting cells against toxic levels of calcium. Upon maturation, calbindin is drastically reduced or entirely lost in many auditory nuclei. We made cochleotomies in mature rats to study effects of deafening and deafferentation on the expression of calbindin in the auditory brainstem. Following unilateral cochleotomy, we observed a substantial increase in the number of calbindin-immunoreactive fibers and boutons in the ventral subdivisions of the ipsilateral cochlear nucleus. At the same time, calbindin-positive astrocytes emerged in the dorsal and ventral cochlear nucleus. Beyond the immediately affected ipsilateral cochlear nucleus, we found calbindin-positive neurons in the lateral superior olive and in the central inferior colliculus, both contralateral to the operation. The loss of one cochlea reduces auditory input and puts the flow of neuronal activity originating in the two ears out of balance. Our findings indicate that the need for the neuronal networks in the auditory brainstem to adjust to this drastically changed pattern of sensory signals invokes the expression of calbindin in glial cells as well as in directly and indirectly affected neuronal cell populations.

Development of calretinin immunoreactivity in the brainstem auditory nuclei of the barn owl (Tyto alba)

The early development of calretinin immunoreactivity (CR-IR) was described in the auditory nuclei of the brainstem of the barn owl. CR-IR was first observed in the auditory hindbrain at embryonic day (E17) and a day later (E18) in the inferior colliculus. In each of the auditory nuclei studied, CR-IR did not develop homogeneously, but began in the regions that map high best frequencies in the adult barn owl. In the hindbrain, CR-IR was first observed in the rostromedial regions of the cochlear nucleus magnocellularis and the nucleus laminaris, and in the dorsal regions of the nucleus angularis and in the nucleus of the lateral lemniscus. In the inferior colliculus, CR-IR began in the ventral region of the central core. The edge of these gradients moved along the future tonotopic axes during the development of all nuclei studied, until adult patterns of CR-IR were achieved about a week after hatching.