Primary innervation of the avian and mammalian cochlear nucleus

Development of GPCR modulation of GABAergic transmission in chicken nucleus laminaris neurons

Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABA_B receptors (GABA_BRs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABA_BRs and mGluRs; and (3) the physiological conditions under which GABA_BRs and mGluRs are activated by endogenous transmitters. We found that (1) GABA_AR-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABA_BR-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABA_BRs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.

Developmental profiling of spiral ganglion neurons reveals insights into auditory circuit assembly

The sense of hearing depends on the faithful transmission of sound information from the ear to the brain by spiral ganglion (SG) neurons. However, how SG neurons develop the connections and properties that underlie auditory processing is largely unknown. We catalogued gene expression in mouse SG neurons from embryonic day 12, when SG neurons first extend projections, up until postnatal day 15, after the onset of hearing. For comparison, we also analyzed the closely related vestibular ganglion (VG). Gene ontology analysis confirmed enriched expression of genes associated with gene regulation and neurite outgrowth at early stages, with the SG and VG often expressing different members of the same gene family. At later stages, the neurons transcribe more genes related to mature function, and exhibit a dramatic increase in immune gene expression. Comparisons of the two populations revealed enhanced expression of TGFβ pathway components in SG neurons and established new markers that consistently distinguish auditory and vestibular neurons. Unexpectedly, we found that Gata3, a transcription factor commonly associated with auditory development, is also expressed in VG neurons at early stages. We therefore defined new cohorts of transcription factors and axon guidance molecules that are uniquely expressed in SG neurons and may drive auditory-specific aspects of their differentiation and wiring. We show that one of these molecules, the receptor guanylyl cyclase Npr2, is required for bifurcation of the SG central axon. Hence, our dataset provides a useful resource for uncovering the molecular basis of specific auditory circuit assembly events.

Synaptic morphology and the influence of auditory experience

The auditory experience is crucial for the normal development and maturation of brain structure and the maintenance of the auditory pathways. The specific aims of this review are (i) to provide a brief background of the synaptic morphology of the endbulb of Held in hearing and deaf animals; (ii) to argue the importance of this large synaptic ending in linking neural activity along ascending pathways to environmental acoustic events; (iii) to describe how the re-introduction of electrical activity changes this synapse; and (iv) to examine how changes at the endbulb synapse initiate trans-synaptic changes in ascending auditory projections to the superior olivary complex, the inferior complex, and the auditory cortex.

The spiral ganglion: connecting the peripheral and central auditory systems

In mammals, the initial bridge between the physical world of sound and perception of that sound is established by neurons of the spiral ganglion. The cell bodies of these neurons give rise to peripheral processes that contact acoustic receptors in the organ of Corti, and the central processes collect together to form the auditory nerve that projects into the brain. In order to better understand hearing at this initial stage, we need to know the following about spiral ganglion neurons: (1) their cell biology including cytoplasmic, cytoskeletal, and membrane properties, (2) their peripheral and central connections including synaptic structure; (3) the nature of their neural signaling; and (4) their capacity for plasticity and rehabilitation. In this report, we will update the progress on these topics and indicate important issues still awaiting resolution.

Short-term synaptic plasticity and intensity coding

Alterations in synaptic strength over short time scales, termed short-term synaptic plasticity, can gate the flow of information through neural circuits. Different information can be extracted from the same presynaptic spike train depending on the activity- and time-dependent properties of the plasticity at a given synapse. The parallel processing in the brain stem auditory pathways provides an excellent model system for investigating the functional implications of short-term plasticity in neural coding. We review recent evidence that short-term plasticity differs in different pathways with a special emphasis on the 'intensity' pathway. While short-term depression dominates the 'timing' pathway, the intensity pathway is characterized by a balance of short-term depression and facilitation that allows linear transmission of rate-coded intensity information. Target-specific regulation of presynaptic plasticity mechanisms underlies the differential expression of depression and facilitation. The potential contribution of short-term plasticity to different aspects of 'intensity'-related information processing, such as interaural level/intensity difference coding, amplitude modulation coding, and intensity-dependent gain control coding, is discussed.

Connecting the ear to the brain: Molecular mechanisms of auditory circuit assembly

Our sense of hearing depends on precisely organized circuits that allow us to sense, perceive, and respond to complex sounds in our environment, from music and language to simple warning signals. Auditory processing begins in the cochlea of the inner ear, where sounds are detected by sensory hair cells and then transmitted to the central nervous system by spiral ganglion neurons, which faithfully preserve the frequency, intensity, and timing of each stimulus. During the assembly of auditory circuits, spiral ganglion neurons establish precise connections that link hair cells in the cochlea to target neurons in the auditory brainstem, develop specific firing properties, and elaborate unusual synapses both in the periphery and in the CNS. Understanding how spiral ganglion neurons acquire these unique properties is a key goal in auditory neuroscience, as these neurons represent the sole input of auditory information to the brain. In addition, the best currently available treatment for many forms of deafness is the cochlear implant, which compensates for lost hair cell function by directly stimulating the auditory nerve. Historically, studies of the auditory system have lagged behind other sensory systems due to the small size and inaccessibility of the inner ear. With the advent of new molecular genetic tools, this gap is narrowing. Here, we summarize recent insights into the cellular and molecular cues that guide the development of spiral ganglion neurons, from their origin in the proneurosensory domain of the otic vesicle to the formation of specialized synapses that ensure rapid and reliable transmission of sound information from the ear to the brain.

A spiking neural network model of the medial superior olive using spike timing dependent plasticity for sound localization

Sound localization can be defined as the ability to identify the position of an input sound source and is considered a powerful aspect of mammalian perception. For low frequency sounds, i.e., in the range 270 Hz–1.5 KHz, the mammalian auditory pathway achieves this by extracting the Interaural Time Difference between sound signals being received by the left and right ear. This processing is performed in a region of the brain known as the Medial Superior Olive (MSO). This paper presents a Spiking Neural Network (SNN) based model of the MSO. The network model is trained using the Spike Timing Dependent Plasticity learning rule using experimentally observed Head Related Transfer Function data in an adult domestic cat. The results presented demonstrate how the proposed SNN model is able to perform sound localization with an accuracy of 91.82% when an error tolerance of ±10° is used. For angular resolutions down to 2.5°, it will be demonstrated how software based simulations of the model incur significant computation times. The paper thus also addresses preliminary implementation on a Field Programmable Gate Array based hardware platform to accelerate system performance.

Mechanisms of Sound Localization in Mammals

The ability to determine the location of a sound source is fundamental to hearing. However, auditory space is not represented in any systematic manner on the basilar membrane of the cochlea, the sensory surface of the receptor organ for hearing. Understanding the means by which sensitivity to spatial cues is computed in central neurons can therefore contribute to our understanding of the basic nature of complex neural representations. We review recent evidence concerning the nature of the neural representation of auditory space in the mammalian brain and elaborate on recent advances in the understanding of mammalian subcortical processing of auditory spatial cues that challenge the “textbook” version of sound localization, in particular brain mechanisms contributing to binaural hearing.

Development of the delay lines in the nucleus laminaris of the chicken embryo revealed by optical imaging

One strategy in localizing a sound source in the azimuthal plane is the comparison of arrival times of sound stimuli at the two ears. The processing of interaural time differences (ITDs) in the auditory brainstem was suggested by the Jeffress model in 1948. In chicks, binaural neurons in the nucleus laminaris (NL) receive input from both ipsilateral and contralateral nucleus magnocellularis (NM) neurons, with the axons of the latter acting as delay lines. A given neuron in the NL responds maximally to coinciding input from both NM neurons. To achieve maximum resolution of sound localization in the NL, the conduction velocity along these delay lines must be precisely tuned. Here, we examined the development of this velocity between embryonic days (E)12 and E18. Our optical imaging approach visualizes the contralateral delay lines along almost the complete NL of the chicken embryo. Optical imaging with the voltage-sensitive dye RH 795 showed no significant differences in the velocity between E12 and E15, but a significant increase from E15 to E18, at both 21 degrees C and 35 degrees C. Surprisingly, at 21 degrees C the conduction velocity in the dorso-lateral part of the NL was significantly higher compared to the situation in the ventro-medial part. The observed development in contralateral conduction velocity may be due to a developmental increase in myelination of the NM axons. Indeed, antibody staining against myelin-associated glycoprotein (alpha-MAG) showed no myelination of the NM axon branches within the NL at E12 and E15. On the other hand, a clear alpha-MAG immunoreactivity occurred at E18. Our results therefore describe the developmental physiological properties of the delay line in the chicken embryo.

A biophysical model for modulation frequency encoding in the cochlear nucleus

Encoding of amplitude modulated (AM) acoustical signals is one of the most compelling tasks for the mammalian auditory system: environmental sounds, after being filtered and transduced by the cochlea, become narrowband AM signals. Despite much experimental work dedicated to the comprehension of auditory system extraction and encoding of AM information, the neural mechanisms underlying this remarkable feature are far from being understood (Joris et al., 2004). One of the most accepted theories for this processing is the existence of a periodotopic organization (based on temporal information) across the more studied tonotopic axis (Frisina et al., 1990b). In this work, we will review some recent advances in the study of the mechanisms involved in neural processing of AM sounds, and propose an integrated model that runs from the external ear, through the cochlea and the auditory nerve, up to a sub-circuit of the cochlear nucleus (the first processing unit in the central auditory system). We will show that varying the amount of inhibition in our model we can obtain a range of best modulation frequencies (BMF) in some principal cells of the cochlear nucleus. This could be a basis for a synchronicity based, low-level periodotopic organization.

Pitfalls in behavioral estimates of basilar-membrane compression in humans

Psychoacoustic estimates of basilar-membrane compression often compare on- and off-frequency forward masking. Such estimates involve assuming that the recovery from forward masking for a given signal frequency is independent of masker frequency. To test this assumption, thresholds for a brief 4-kHz signal were measured as a function of masker-signal delay. Comparisons were made between on-frequency (4 kHz) and off-frequency (either 2.4 or 4.4 kHz) maskers, adjusted in level to produce the same amount of masking at a 0-ms delay between masker offset and signal onset. Consistent with the assumption, forward-masking recovery from a moderate-level (83 dB SPL) 2.4-kHz masker and a high-level (92 dB SPL) 4.4-kHz masker was the same as from the equivalent on-frequency maskers. In contrast, recovery from a high-level (92 dB SPL) 2.4-kHz forward masker was slower than from the equivalent on-frequency masker. The results were used to simulate temporal masking curves, taking into account the differences in on- and off-frequency masking recoveries at high levels. The predictions suggest that compression estimates assuming frequency-independent masking recovery may overestimate compression by as much as a factor of 2. The results suggest caution in interpreting forward-masking data in terms of basilar-membrane compression, particularly when high-level maskers are involved.

Espin actin-cytoskeletal proteins are in rat type I spiral ganglion neurons and include splice-isoforms with a functional nuclear localization signal

The espins are Ca(2+)-resistant actin-bundling proteins that are enriched in hair cell stereocilia and sensory cell microvilli. Here, we report a novel localization of espins to a large proportion of rat type I spiral ganglion neurons (SGNs) and their projections to the cochlear nucleus (CN). Moreover, we show that a fraction of these espins is in the nucleus of SGNs owing to the presence of splice-isoforms that contain a functional nuclear localization signal (NLS). Espin antibody labeled approximately 83% of type I SGNs, and the labeling intensity increased dramatically during early postnatal development. Type II SGNs and vestibular ganglion neurons were unlabeled. In the CN, espin-positive auditory nerve fibers showed a projection pattern typical of type I SGNs, with intense labeling in the nerve root region and posteroventral CN (PVCN). The anteroventral CN (AVCN) showed moderate labeling, whereas the dorsal CN showed weak labeling that was restricted to the deep layer. Espin-positive synaptic terminals were enriched around nerve root neurons and octopus cells in the PVCN and were also found on globular bushy cells and multipolar neurons in the PVCN and AVCN. SGNs expressed multiple espin transcripts and proteins, including splice-isoforms that contain a nonapeptide, which is rich in positively charged amino acids and creates a bipartite NLS. The nonapeptide was necessary to target espin isoforms to the nucleus and was sufficient to target an unrelated protein to the nucleus when joined with the upstream di-arginine-containing octapeptide. The presence of cytoplasmic and nuclear espins in SGNs suggests additional roles for espins in auditory neuroscience.

Short-term synaptic depression and recovery at the mature mammalian endbulb of Held synapse in mice

The endbulb of Held synapses between the auditory nerve fibers (ANF) and cochlear nucleus bushy neurons convey fine temporal information embedded in the incoming acoustic signal. The dynamics of synaptic depression and recovery is a key in regulating synaptic transmission at the endbulb synapse. We studied short-term synaptic depression and recovery in mature (P22-38) CBA mice with stimulation rates that were comparable to sound-driven activities recorded in vivo. Synaptic depression in mature mice is less severe ( approximately 40% at 100 Hz) than reported for immature animals and the depression is predominately due to depletion of releasable vesicles. Recovery from depression depends on the rate of activity and accumulation of intracellular Ca2+ at the presynaptic terminal. With a regular stimulus train at 100 Hz in 2 mM external [Ca2+], the recovery from depletion was slow (tauslow, approximately 2 s). In contrast, a fast (taufast, approximately 25 ms), Ca2+-dependent recovery followed by a slower recovery (tauslow, approximately 2 s) was seen when stimulus rates or external [Ca2+] increased. In normal [Ca2+], recovery from a 100-Hz Poisson-like train is rapid, suggesting that Poisson-like trains produce a higher internal [Ca2+] than regular trains. Moreover, the fast recovery was slowed by approximately twofold in the presence of calmidazolium, a Ca2+/calmodulin inhibitor. Our results suggest that endbulb synapses from high spontaneous firing rate auditory nerve fibers normally operate in a depressed state. The accelerated synaptic recovery during high rates of activity is likely to ensure that reliable synaptic transmission can be achieved at the endbulb synapse.

Improving the dynamics of responses to amplitude modulated stimuli by modeling inhibitory interneurons in cochlear nucleus

Amplitude modulation is an important feature of communication sounds. A phenomenological model of the auditory pathway that reproduces amplitude modulation coding from the outer ear to the inferior colliculus is presented. It is based on Hewitt and Meddis' work. To improve the temporal coding for high level stimuli, high spontaneous rate and low spontaneous rate auditory nerve fibers innervate chopper cells of the cochlear nucleus. Wideband inhibitory interneurons which limit high spontaneous rate fibers connected to chopper units are added in this nucleus. The realistic structure we propose gives results closer to physiological data in terms of synchronization.

Distribution and phenotypes of unipolar brush cells in relation to the granule cell system of the rat cochlear nucleus

In most mammals the cochlear nuclear complex (CN) contains a distributed system of granule cells (GCS), whose parallel fiber axons innervate the dorsal cochlear nucleus (DCN). Like their counterpart in cerebellum, CN granules are innervated by mossy fibers of various origins. The GCS is complemented by unipolar brush (UBCs) and Golgi cells, and by stellate and cartwheel cells of the DCN. This cerebellum-like microcircuit modulates the activity of the DCN's main projection neurons, the pyramidal, giant and tuberculoventral neurons, and is thought to improve auditory performance by integrating acoustic and proprioceptive information. In this paper, we focus on the rat UBCs, a chemically heterogeneous neuronal population, using antibodies to calretinin, metabotropic glutamate receptor 1alpha (mGluR1alpha), epidermal growth factor substrate 8 (Eps8) and the transcription factor T-box gene Tbr2 (Tbr2). Eps8 and Tbr2 labeled most of the CN's UBCs, if not the entire population, while calretinin and mGluR1alpha distinguished two largely separate subsets with overlapping distributions. By double labeling with antibodies to Tbr2 and the alpha6 GABA receptor A (GABAA) subunit, we found that UBCs populate all regions of the GCS and occur at remarkably high densities in the DCN and subpeduncular corner, but rarely in the lamina. Although GCS subregions likely share the same microcircuitry, their dissimilar UBC densities suggest they may be functionally distinct. UBCs and granules are also present in regions previously not included in the GCS, namely the rostrodorsal magnocellular portions of ventral cochlear nucleus, vestibular nerve root, trapezoid body, spinal tract and sensory and principal nuclei of the trigeminal nerve, and cerebellar peduncles. The UBC's dendritic brush receives AMPA- and NMDA-mediated input from an individual mossy fiber, favoring singularity of input, and its axon most likely forms several mossy fiber-like endings that target numerous granule cells and other UBCs, as in the cerebellum. The UBCs therefore, may amplify afferent signals temporally and spatially, synchronizing pools of target neurons.

Projections of low spontaneous rate, high threshold auditory nerve fibers to the small cell cap of the cochlear nucleus in cats

The marginal shell of the anteroventral cochlear nucleus houses small cells that are distinct from the overlying microneurons of the granule cell domain and the underlying projection neurons of the magnocellular core. This thin shell of small cells and associated neuropil receives auditory nerve input from only the low (<18 spikes/s) spontaneous rate (SR), high threshold auditory nerve fibers; high SR, low threshold fibers do not project there. It should be noted, that most of these auditory nerve terminations reside in the neuropil and intermix with dendrites that originate outside the shell. Consequently, electron microscopy is necessary to determine the synaptic targets. For this report, the terminations of intracellularly labeled low SR auditory nerve fibers in the small cell of cats cap were mapped through serial sections using a light microscope. The terminals were then examined with an electron microscope and found to form synapses with the somata and dendrites of small cells. Moreover, the small cell dendrites were identifiable by an abundance of microtubules and the presence of polyribosomes that were free or associated with membranous cisterns. These data contribute to the concept of a high threshold feedback circuit to the inner ear, and reveal translational machinery for local control of activity-dependent synaptic modification.

Visualization of two distinct classes of neurons by gad2 and zic1 promoter/enhancer elements in the dorsal hindbrain of developing zebrafish reveals neuronal connectivity related to the auditory and lateral line systems

The dorsal hindbrain includes distinct classes of neurons for processing various sensory stimuli, but the developmental aspects of these neurons remain largely unknown. We identify here two distinct classes of neurons in the dorsal hindbrain of developing zebrafish: (1) neurons that express the inhibitory neuronal marker Gad1/2, and (2) neurons that express the zn-5 antigen and Lhx2/9 and require the basic helix-loop-helix transcription factor Atoh1a for development. Neurons were traced to their axon terminals by expressing green fluorescent protein using the Gal4VP16-UAS (UAS, upstream activating sequences) system in combination with the promoter/enhancer regions of gad2 for the Gad1/2(+) neurons and zic1 for the zn-5(+)Lhx2/9(+) neurons. The Gad1/2(+) neurons projected to the contralateral hindbrain, while the zn-5(+)Lhx2/9(+) neurons projected to the contralateral midbrain torus semicircularis, suggesting a role in auditory and lateral line sensory processing. Comparison of these projections with those from the cochlear nuclei to the inferior colliculus in mammals suggests similarities across vertebrate species.

Afferent regulation of neuron number in the cochlear nucleus: cellular and molecular analyses of a critical period

The neurons of the cochlear nucleus are dependent on input from the auditory nerve for survival during a critical period of development in a variety of vertebrate species. The molecules that underlie this age-dependent vulnerability to deafferentation are for the most part unknown, although recent studies have begun to yield interesting candidate genes. Here, we review the studies that originally described the presence of afferent dependent neuron survival in the cochlear nucleus and the age-dependency of this effect, as well as more recent work that seeks to understand the mechanisms underlying the neuron loss that occurs and the basis of this critical period. While much of the past work on cochlear nucleus neuronal susceptibility has been conducted looking at one or two genes at a time, recent advances in genomics make it possible to screen tens of thousands of genes while looking for candidate genes that are determinants of the critical period response to afferent deprivation.

Changes in Sef levels influence auditory brainstem development and function

During development of the CNS, secreted morphogens of the fibroblast growth factor (FGF) family have multiple effects on cell division, migration, and survival depending on where, when, and how much FGF signal is received. The consequences of misregulating the FGF pathway were studied in a mouse with decreased levels of the FGF antagonist Sef. To uncover effects in the nervous system, we focused on the auditory system, which is accessible to physiological analysis. We found that the mitogen-activated protein kinase pathway is active in the rhombic lip, a germinal zone that generates diverse types of neurons, including the cochlear nucleus complex of the auditory system. Sef is expressed immediately adjacent to the rhombic lip, overlapping with FGF15 and FGFR1, which is also present in the lip itself. This pattern suggests that Sef may normally function in non-rhombic lip cells and prevent them from responding to FGF ligand in the vicinity. Consistent with this idea, overexpression of Sef in chicks decreased the size of the auditory nuclei. Cochlear nucleus defects were also apparent in mice with reduced levels of Sef, with 13% exhibiting grossly dysmorphic cochlear nuclei and 26% showing decreased amounts of GFAP in the cochlear nucleus. Additional evidence for cochlear nucleus defects was obtained by electrophysiological analysis of Sef mutant mice, which have normal auditory thresholds but abnormal auditory brainstem responses. These results show both increases and decreases in Sef levels affect the assembly and function of the auditory brainstem.

Beyond timing in the auditory brainstem: intensity coding in the avian cochlear nucleus angularis

Many of the computational principles for sound localization have emerged from the study of avian brains, especially for the construction of codes for interaural timing differences. Our understanding of the neural codes for interaural level differences, and other intensity-related, non-localization sound processing, has lagged behind. In birds, cochlear nucleus angularis (NA) is an obligatory relay for intensity processing. We present our current knowledge of the cell types found in NA, their responses to auditory stimuli, and their likely coding roles. On a cellular level, our recent experimental and modeling studies have shown that short-term synaptic plasticity in NA is a major player in the division of intensity and timing information into parallel pathways. NA projects to at least four brain stem and midbrain targets, suggesting diverse involvement in a range of different sound processing circuits. Further studies comparing processing in NA and analogous neurons in the mammalian cochlear nucleus will highlight which features are conserved and perhaps may be computationally advantageous, and which are species- or clade-specific details demonstrating either disparate environmental requirements or different solutions to similar problems.

Endogenous mGluR activity suppresses GABAergic transmission in avian cochlear nucleus magnocellularis neurons

GABAergic transmission in the avian cochlear nucleus magnocellularis (NM) of the chick is subject to modulation by gamma-aminobutyric acid type B (GABA(B)) autoreceptors. Here, I investigated modulation of GABAergic transmission in NM by metabotropic glutamate receptors (mGluRs) with whole cell recordings in brain slice preparations. I found that tACPD, a nonspecific mGluR agonist, exerted dose-dependent suppression on evoked inhibitory postsynaptic currents (eIPSCs) in NM neurons. At concentrations of 100 or 200 microM, tACPD increased the failure rate of GABAergic transmission. Agonists for group I (3,5-DHPG, 200 microM), group II (DCG-IV, 2 microM), and group III (L-AP4, 10 microM) mGluRs produced a significant reduction in the amplitude of eIPSCs and a significant increase in failure rate, indicating the involvement of multiple mGluRs in this modulation. The frequency, but not the amplitude, of miniature IPSCs (mIPSCs) was decreased significantly by 3,5-DHPG or DCG-IV. Neither frequency nor amplitude of mIPSCs was affected by L-AP4. mGluR antagonists LY341495 (20 microM) plus CPPG (10 microM) significantly increased the amplitude of eIPSCs, indicating that endogenous mGluR activity suppresses GABA release to NM neurons. Furthermore, blockage of mGluRs increased GABA-evoked discharges recorded under physiological Cl(-) concentrations, whereas tACPD (100 microM) eliminated them. The results indicate that mGluRs play important roles in achieving balanced excitation and inhibition in NM and preserving fidelity of temporal information encoded by NM neurons.

Formation of the avian nucleus magnocellularis from the auditory anlage

In the avian auditory system, the neural network for computing the localization of sound in space begins with bilateral innervation of nucleus laminaris (NL) by nucleus magnocellularis (NM) neurons. We used antibodies against the neural specific markers Hu C/D, neurofilament, and SV2 together with retrograde fluorescent dextran labeling from the contralateral hindbrain to identify NM neurons within the anlage and follow their development. NM neurons could be identified by retrograde labeling as early as embryonic day (E) 6. While the auditory anlage organized itself into NM and NL in a rostral-to-caudal fashion between E6 and E8, labeled NM neurons were visible throughout the extent of the anlage at E6. By observing the pattern of neuronal rearrangements together with the pattern of contralaterally projecting NM fibers, we could identify NL in the ventral anlage. Ipsilateral NM fibers contacted the developing NL at E8, well after NM collaterals had projected contralaterally. Furthermore, the formation of ipsilateral connections between NM and NL neurons appeared to coincide with the arrival of VIIIth nerve fibers in NM. By E10, immunoreactivity for SV2 was heavily concentrated in the dorsal and ventral neuropils of NL. Thus, extensive pathfinding and morphological rearrangement of central auditory nuclei occurs well before the arrival of cochlear afferents. Our results suggest that NM neurons may play a central role in formation of tonotopic connections in the auditory system.

Development of Functional Synaptic Connections in the Auditory System Visualized With Optical Recording: Afferent-Evoked Activity Is Present From Early Stages

A comprehensive survey of auditory network formation was performed in the brain stem of the chicken embryo using voltage-sensitive dye recording. Intact medulla/brain stem preparations with the auditory branch of the eighth nerve attached were dissected from 5.5- to 8-day chicken embryos, and responses evoked by nerve stimulation were recorded optically. In the medulla of 7- and 8-day embryos, we identified four response areas, corresponding to ipsilateral Nucleus magnocellularis (NM) and Nucleus angularis (NA), which receive the auditory afferents, and ipsi- and contralateral Nucleus laminaris (NL), which receive projections from NM. The optical responses consisted of a fast spikelike signal followed by a long-lasting slow signal, which reflected the sodium-dependent action potential and glutamatergic excitatory postsynaptic potential (EPSP), respectively. In NM, NA, and NL, the EPSP-related slow optical signals were detected from some 6-day and all 7- and 8-day preparations, indicating that functional synaptic connectivity in these nuclei arises by the 7-day stage. In the pons of 7- and 8-day embryos, we identified two additional response areas, which evidently correspond to ipsi- and contralateral Nucleus lemnisci lateralis (NLL), the higher-order nuclei of the auditory pathway. Furthermore, we detected optical responses from the contralateral cerebellum, which possibly correspond to transient projections observed only during embryogenesis. The present study demonstrates that functional auditory circuits are established in the chicken embryo at stages earlier than previously reported. We discuss the possible role of afferent-evoked activity with reference to auditory neural network formation.

Assembly of the Brainstem Cochlear Nuclear Complex Is Revealed by Intersectional and Subtractive Genetic Fate Maps

The cochlear nuclear complex (CN) is the entry point for central auditory processing. Although constituent neurons have been studied physiologically, their embryological origins and molecular profiles remain obscure. Applying intersectional and subtractive genetic fate mapping approaches, we show that this complex develops modularly from genetically separable progenitor populations arrayed as rostrocaudal microdomains within and outside the hindbrain (lower) rhombic lip (LRL). The dorsal CN subdivision, structurally and topographically similar to the cerebellum, arises from microdomains unexpectedly caudal and noncontiguous to cerebellar primordium; ventral CN subdivisions arise from more rostral LRL. Magnocellular regions receive contributions from LRL and coaxial non-lip progenitors; contrastingly, ensheathing granule cells derive principally from LRL. Also LRL-derived and molecularly similar to CN granule cells are precerebellar mossy fiber neurons; surprisingly, these ostensibly intertwined populations have separable origins and adjacent but segregated migratory streams. Together, these findings provide new platforms for investigating the development and evolution of auditory and cerebellar systems.

A modified Golgi staining protocol for use in the human brain stem and cerebellum

The Golgi silver-impregnation method established itself as an important technique for distinguishing morphology at the individual neuron level. This technique has been especially useful for studying human neuroanatomy because it works on postmortem tissue but it is also unreliable and capricious. In this report, we describe a simple technique that was applied to human autopsy and tissue-bank material yielding useful results for the study of neuronal morphology in the brain stem and cerebellum. Human adult brain stems had been immersion-fixed in formalin for a period of time ranging from weeks to months. Brain stem tissue was cross-sectioned into 3-5mm thick slabs, centered about the cochlear nucleus. Slabs were processed under continuous vacuum (22-26 in. of Hg), a procedure that promoted penetration of reagents into the tissue. Tissue was sectioned using a Vibratome and mounted for light microscopy. The results demonstrated improved staining of neurons in the brain stem. Staining of the large synaptic endings of auditory nerve fibers called end bulbs of Held in the cochlear nucleus was especially evident. These results suggest that an age-graded series could be conducted to describe the development of these large auditory endings in humans.

Quantitative changes in calretinin immunostaining in the cochlear nuclei after unilateral cochlear removal in young ferrets

Neurons of the cochlear nuclei receive axosomatic endings from primary afferent fibers from the cochlea and have projections that diverge to form parallel ascending auditory pathways. These cells are characterized by neurochemical phenotypes such as levels of calretinin. To test whether or not early deafferentation results in changes in calretinin immunostaining in the cochlear nucleus, unilateral cochlear ablations were performed in ferrets soon after hearing onset (postnatal day [P]30-P40). Two months later, changes in calretinin immunostaining as well as cell size, volume, and synaptophysin immunostaining were assessed in the anteroventral (AVCN), posteroventral (PVCN), and dorsal cochlear nucleus (DCN). A decrease in calretinin immunostaining was evident ipsilaterally within the AVCN and PVCN but not in the DCN. Further analysis revealed a decrease both in the calretinin-immunostained neuropil and in the calretinin-immunostained area within AVCN and PVCN neurons. These declines were accompanied by significant ipsilateral decreases in volume as well as neuron area in the AVCN and PVCN compared with the contralateral cochlear nucleus and unoperated animals, but not compared with the DCN. In addition, there was a significant contralateral increase in calretinin-immunostained area within AVCN and PVCN neurons compared with control animals. Finally, a decrease in area of synaptophysin immunostaining in both the ipsilateral AVCN and PVCN without changes in the number of boutons was found. The present data demonstrate that unilateral cochlear ablation leads to 1) decreased immunostaining of the neuropil in the AVCN and PVCN ipsilaterally, 2) decreased calretinin immunostaining within AVCN and PVCN neurons ipsilaterally, 3) synaptogenesis in the AVCN and PVCN ipsilaterally, and 4) increased calretinin immunostaining within AVCN and PVCN neurons contralaterally.

A phenomenological model of peripheral and central neural responses to amplitude-modulated tones

A phenomenological model with time-varying excitation and inhibition was developed to study possible neural mechanisms underlying changes in the representation of temporal envelopes along the auditory pathway. A modified version of an existing auditory-nerve model [Zhang et al., J. Acoust. Soc. Am. 109, 648-670 (2001)] was used to provide inputs to higher hypothetical processing centers. Model responses were compared directly to published physiological data at three levels: the auditory nerve, ventral cochlear nucleus, and inferior colliculus. Trends and absolute values of both average firing rate and synchrony to the modulation period were accurately predicted at each level for a wide range of stimulus modulation depths and modulation frequencies. The diversity of central physiological responses was accounted for with realistic variations of model parameters. Specifically, enhanced synchrony in the cochlear nucleus and rate-tuning to modulation frequency in the inferior colliculus were predicted by choosing appropriate relative strengths and time courses of excitatory and inhibitory inputs to postsynaptic model cells. The proposed model is fundamentally different than others that have been used to explain the representation of envelopes in the mammalian midbrain, and it provides a computational tool for testing hypothesized relationships between physiology and psychophysics.

NT-3 replacement with brain-derived neurotrophic factor redirects vestibular nerve fibers to the cochlea

Survival of inner ear sensory neurons depends on two neurotrophins, BDNF and NT-3, and their respective receptors, TrkB and TrkC. Because both receptors are present in the same neuron, it has been suggested that BDNF and NT-3 are functionally redundant in promoting neuronal survival. Knock-in of one ligand into the locus of the other one confirmed this hypothesis for the cochlea, leaving open the question of why two neurotrophins are required for proper innervation of the mammalian ear. Here, we show that the precise spatiotemporal pattern of expression of the two neurotrophins is essential for proper patterning of the inner ear innervation. Mice expressing BDNF under the control of the NT-3 promoter develop exuberant projections of vestibular sensory neurons to the basal turn of the cochlea. This projection can be enhanced by combining the transgene with a null mutation of BDNF. However, vestibular fibers rerouted into the cochlea do not reach hair cells and remain outside the organ of Corti, suggesting a chemotactic role for neurotrophins on these fibers. Our data provide genetic evidence that neurotrophins in the ear exert both survival and axon guidance roles.