Tuberculoventral neurons project to the multipolar cell area but not to the octopus cell area of the posteroventral cochlear nucleus.

Tuberculoventral neurons in the deep layer of the dorsal cochlear nucleus (DCN) provide frequency-specific inhibition to neurons in the anteroventral cochlear nucleus (AVCN) of the mouse (Wickesberg and Oertel, '88, '90). The present experiments examine the projection from the deep DCN to the posteroventral cochlear nucleus (PVCN). Horseradish peroxidase (HRP) injections into the PVCN reveal that the multipolar cell area, but not the octopus cell area, is innervated by neurons in the deep layer of the DCN. Injections into the multipolar cell area, in the rostral and ventral PVCN, labeled neurons across the entire rostrocaudal extent of the deep DCN. The labeled tuberculoventral neurons generally lay within the band of labeled auditory nerve terminals in the DCN. Injections of HRP into the octopus cell area, in the dorsal caudal PVCN, labeled almost no cells within the band of auditory nerve fiber terminals that were labeled by the same injection. The inhibition from tuberculoventral neurons onto ventral cochlear nucleus (VCN) neurons is likely to be mediated by glycine (Wickesberg and Oertel, '90). Slices of the cochlear nuclear complex were immunolabeled by an antibody against glycine conjugated with glutaraldehyde to bovine serum albumin (Wenthold et al., '87). Glycine-like immunoreactivity was found throughout the DCN, the AVCN and the multipolar cell area, but there was little labeling in the octopus cell area. This finding provides independent evidence that tuberculoventral neurons do not innervate the octopus cell area and indicates that the octopus cell area is anatomically and functionally distinct.     

Intrinsic connections within and between cochlear nucleus subdivisions in cat

The cat cochlear nuclear complex (CNC) is divided into three major subdivisions: the anteroventral, the posteroventral, and the dorsal cochlear nuclei (AVCN, PVCN, and DCN, respectively). Each of these subdivisions receives a topographic projection from the cochlea and each consists of a number of different cell types. The interconnections between these subdivisions and the cell types which give rise to them were studied by means of small injections of horseradish peroxidase (HRP) made at physiologically identified locations. DCN injections resulted in few labeled cells in the DCN, suggesting that its internal connections are very limited. In contrast, these same DCN injections resulted in numerous labeled cells in the PVCN and AVCN. Labeled PVCN cells, consisting of multipolar, octopus, and small spindle-shaped cells, were located in spatially restricted laminae stretching the entire rostrocaudal length of the nucleus, while labeled AVCN cells consisting of multipolar, globular, small spindle-shaped and small spherical cells were broadly distributed over the posterior half of the nucleus. Similar injections placed in the PVCN resulted in numerous labeled cells in all three subdivisions. The PVCN and AVCN cells labeled after PVCN injections were widely distributed across the isofrequency representations in both nuclei, while the labeled DCN cells were restricted to locations over the injection sites. Injections placed in the posterior half of the AVCN resulted in only very few labeled cells in the DCN. No cells were labeled following injections in the rostral AVCN.     

Topographic organization of the central projections of the spiral ganglion in cats

The morphological organization of inputs from restricted sectors of the cat cochlear spiral ganglion into the cochlear nucleus was studied by making focal extracellular injections of horseradish peroxidase (HRP) into the spiral ganglion. Injections resulted in Golgi-like labeling of a small cluster of spiral ganglion cells and their peripheral and central axons. Large injections involved most of the cells within Rosenthal's canal in sectors of the spiral ganglion innervating greater than or equal to 1 mm of the basilar membrane and resulted in narrow, complete laminae of labeled axons and preterminal fields within each cochlear nucleus subdivision. The positions of these bands were consistent with the "isofrequency laminae" appropriate for the frequencies represented at the injection sites, with high frequency laminae situated more dorsally, and lower frequencies progressively more ventral. A discrete projection to the small cell cap area was observed that was discontinuous with the main projection laminae in the ventral cochlear nuclei (VCN). In the dorsal cochlear nucleus, projecting fibers and terminals were excluded from the molecular cell layer. No labeled fibers entered the granule cell areas. In contrast to larger injections, very small HRP deposits labeled only part of an isofrequency lamina. Specifically, injections restricted to the scala tympani aspect of the spiral ganglion labeled only the lateral part of VCN isofrequency laminae, whereas injections limited to the scala vestibuli aspect of the ganglion labeled the medial aspect of the isofrequency planes. Thus these data indicate a previously unrecognized topographic representation of the vertical dimension of the spiral ganglion across VCN isofrequency laminae. Some possible functional implications of this projection organization are discussed.     

Delayed, frequency-specific inhibition in the cochlear nuclei of mice: a mechanism for monaural echo suppression

To understand how auditory information is processed in the cochlear nuclei, it is crucial to know what circuitry exists and how it functions. Previous anatomical experiments have shown that neurons in the deep layer of the dorsal cochlear nucleus (DCN) project topographically to the anteroventral cochlear nucleus (AVCN) (Wickesberg and Oertel, 1988). Because interneurons in the DCN and their targets in AVCN are excited by the same group of auditory nerve fibers, the projection is frequency-specific. Here we report that microinjections of glutamate in the DCN evoke trains of IPSPs in individual, impaled AVCN neurons in brain slices of the cochlear nuclear complex. Only injections along a rostrocaudal band in the DCN, matching the anatomical projection of tuberculoventral neurons, evoke IPSPs; elsewhere, there were no responses to the glutamate. The inhibition is blocked by 0.5 microM strychnine. Both bushy and stellate cells are targets of the inhibitory projection. Inhibition in the AVCN is delayed by an additional synaptic delay with respect to the excitation. Delayed, frequency-specific inhibition allows the first wavefront to be transmitted to higher auditory centers by bushy and stellate cells, while following inputs encoding signals of similar frequencies are attenuated at least for the duration of an IPSP. These findings are consistent with results from psychoacoustic experiments and suggest that this circuit provides a source of monaural echo suppression.     

Morphology and physiology of cells in slice preparations of the dorsal cochlear nucleus of mice

Horseradish peroxidase (HRP) was injected into cells from which intracellular recordings were made in slices of the dorsal cochlear nucleus (DCN) in order to correlate physiology with morphology. In general, the morphology of cells labeled intracellularly with HRP corresponded to those made with Golgi impregnations in mice and other mammals. The following cells were labeled: one granule cell, four cartwheel cells, eight fusiform cells, two other cells in the fusiform cell layer, and two tuberculoventral association cells in the deep layers of the DCN. The axon of the granule cell runs parallel to isofrequency laminae with collaterals branching perpendicularly and running along the tonotopic axis. The cartwheel cells have dendrites in the molecular layer that are densely covered with spines. The axon of one cell terminates just dorsally to the cell body. Fusiform cells have the characteristic spiny, apical and smooth, basal dendrites. The basal dendrites are conspicuously oriented parallel to isofrequency laminae. Axons of the fusiform cells exit through the dorsal acoustic stria without branching. The two tuberculoventral association cells in the deep DCN have axons that terminate both in the deep DCN, within the same isofrequency lamina that contains the cell body, and in the ventral cochlear nucleus (VCN). Intracellular recordings from 11 of these cells show that they cannot be distinguished on the basis of their responses to intracellularly injected current. All cell types fired large action potentials that were followed by a fast and a slower undershoot, distinguishing them from cells of the VCN but not from one another. Most cells responded to shocks of the auditory nerve root with early EPSPs and later IPSPs. The latencies of EPSPs show that some were monosynaptic and others polysynaptic. That there was no systematic relationship between the latencies of EPSPs and the cell types from which they were recorded shows that shocks to the nerve root may have activated more than just the large, myelinated, auditory nerve fibers.     

Tonotopic organization of vertical cells in the dorsal cochlear nucleus of the CBA/J mouse

The systematic and topographic representation of frequency is a first principle of organization throughout the auditory system. The dorsal cochlear nucleus (DCN) receives direct tonotopic projections from the auditory nerve (AN) as well as secondary and descending projections from other sources. Among the recipients of AN input in the DCN are vertical cells (also called tuberculoventral cells), glycinergic interneurons thought to provide on‐ or near‐best‐frequency feed‐forward inhibition to principal cells in the DCN and various cells in the anteroventral cochlear nucleus (AVCN). Differing lines of physiological and anatomical evidence suggest that vertical cells and their projections are organized with respect to frequency, but this has not been conclusively demonstrated in the intact mammalian brain. To address this issue, we retrogradely labeled vertical cells via physiologically targeted injections in the AVCN of the CBA/J mouse. Results from multiple cases were merged with a normalized 3D template of the cochlear nucleus (Muniak et al. [ 2013 ] J. Comp. Neurol. 521:1510–1532) to demonstrate quantitatively that the arrangement of vertical cells is tonotopic and aligned to the innervation pattern of the AN. These results suggest that vertical cells are well positioned for providing immediate, frequency‐specific inhibition onto cells of the DCN and AVCN to facilitate spectral processing. J. Comp. Neurol. 522:937–949, 2014. © 2013 Wiley Periodicals, Inc.     

Functional organization of the cochlear nucleus of rufous horseshoe bats (Rhinolophus rouxi): frequencies and internal connections are arranged in slabs

The functional organization of the cochlear nucleus (CN) was studied with physiological recording and anatomical tracing techniques. Recordings were made from single CN neurons to examine their temporal firing patterns to tone burst stimuli and their frequency tuning characteristics. Recording loci of individual neurons were carefully monitored in order to understand how the functional properties of a cell relate to its location within the CN. We found that tonal frequencies were systematically represented in each of the three CN divisions (anteroventral, AVCN; posteroventral, PVCN; dorsal, DCN). Eight temporal response patterns were observed in CN neurons when stimulated at units' best excitatory frequencies (BF). With a few exceptions, neurons in each CN division could generate all eight firing patterns with different distributions for the three division. A focal injection of horseradish peroxidase (HRP), at the end of the physiological study, to a group of neurons possessing a similar BF in one CN division resulted in anterograde labeling of nerve terminals in the other two divisions at precisely the areas where the same frequency band was processed in these divisions. Labeled terminals in each division were closely congregated in the form of a thin slab. The slab orientation was division specific whereas its location was frequency specific, which could be predicted on the basis of physiological data. HRP injections into the DCN also resulted in retrograde labeling of somata in the AVCN and PVCN. On the other hand, only DCN neurons were retrogradely labeled when HRP was injected into the AVCN or the PVCN. These data showed how the three CN divisions are internally connected. Furthermore, retrogradely labeled cells occupied the same slabs where we found anterogradely labeled nerve terminals. Additionally, in a group of bats, HRP was injected into various functionally (i.e., BF) identified regions of the central nucleus of the inferior coliculus (IC) to clarify the type and location of CN projecting neurons. Retrogradely labeled cells in individual CN divisions likewise were arranged in slabs whose locations in the CN nuclei depended on the BFs of neurons at the injection site in the IC. These results show that slabs represent units of functional organization (i.e., tonal frequency, local connection and central projection) in the CN.     

Projections from the ventral cochlear nucleus to the dorsal cochlear nucleus in rats

Local circuit interactions between the dorsal and ventral divisions of the cochlear nucleus are known to influence the evoked responses of the resident neurons to sound. In the present study, we examined the projections of neurons in the ventral cochlear nucleus to the dorsal cochlear nucleus by using retrograde transport of biotinylated dextran amine injected into restricted but different regions of the dorsal cochlear nucleus. In all cases, we found retrogradely labeled granule, unipolar brush, and chestnut cells in the granule cell domain, and retrogradely labeled multipolar cells in the magnocellular core of the ventral cochlear nucleus. A small number of the labeled multipolar cells were found along the margins of the ventral cochlear nucleus, usually near the boundaries of the granule cell domain. Spherical bushy, globular bushy, and octopus cells were not labeled. Retrogradely-labeled auditory nerve fibers and the majority of labeled multipolar neurons formed a narrow sheet extending across the medial-to-lateral extent of the ventral cochlear nucleus whose dorsoventral position was topographically related to the injection site. Labeled multipolar cells within the core of the ventral cochlear nucleus could be divided into at least two distinct groups. Planar neurons were most numerous, their somata found within the associated band of labeled fibers, and their dendrites oriented within this band. This arrangement mimics the organization of isofrequency contours and implies that planar neurons respond best to a narrow range of frequencies. In contrast, radiate neurons were infrequent, found scattered throughout the ventral cochlear nucleus, and had long dendrites oriented perpendicular to the isofrequency contours. This dendritic orientation suggests that radiate neurons are sensitive to a broad range of frequencies. These structural differences between planar and radiate neurons suggest that they subserve separate functions in acoustic processing. J. Comp. Neurol. 385:245–264, 1997. © 1997 Wiley-Liss, Inc.     

Anatomy of the auditory thalamocortical system of the guinea pig

We investigated the projection from the medial geniculate body (MG) to the tonotopic fields (the anterior field A, the dorsocaudal field DC, the small field S) and to the nontonotopic ventrocaudal belt in the auditory cortex of the guinea pig. The auditory fields were first delimited in electrophysiological experiments with microelectrode mapping techniques. Then, small quantities of horseradish peroxidase (HRP) and/or fluorescent retrograde tracers were injected into the sites of interest, and the thalamus was checked for labeled cells. The anterior field A receives its main thalamic input from the ventral nucleus of the MG (MGv). The projection is topographically organized. Roughly, the caudal part of the MGv innervates the rostral part of field A and vice versa. After injection of tracer into low or medium best-frequency sites in A, we also found a topographic gradient along the isofrequency contours: the dorsal (ventral) part of a cortical isofrequency strip receives afferents from the rostral (caudal) portions of the corresponding thalamic isofrequency band. However, it is not so obvious whether such a gradient exists also in the high-frequency part of the projection. A second, weaker projection to field A originates in a magnocellular nucleus that is situated caudomedially in the MG and was therefore named the caudomedial nucleus. The dorsocaudal field DC receives input from the same nuclei as the anterior field, but the location of the labeled cells in the MGv is different. This was demonstrated by injection of different tracers into sites with like best frequencies in fields A and DC, respectively. After injection of HRP into the 1-2-kHz isofrequency strip in field A and injection of Nuclear Yellow (NY) into the 1-2-kHz site in field DC, the labeled cells in the MGv form one continuous array that runs from caudal to rostral over the whole extent of the MGv. The anterior part of this array consists of NY-labeled cells; i.e., it projects to field DC. The caudal part is formed by HRP-labeled cells; i.e., it innervates field A. These findings indicate that there is only one continuous tonotopic map in the MGv. This map is split when projected onto the cortex so that two adjacent tonotopic fields (A and DC) result. The cortical maps are rotated relative to the thalamic map in that rostral portions of the MGv project to caudal parts of the tonotopic cortex and vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)     

Relationships between neuronal birthdates and tonotopic positions in the mouse cochlear nucleus

Tonotopy is a key anatomical feature of the vertebrate auditory system, but little is known about the mechanisms underlying its development. Since date of birth of a neuron correlates with tonotopic position in the cochlea, we investigated if it also correlates with tonotopic position in the cochlear nucleus (CN). In the cochlea, spiral ganglion neurons are organized in a basal to apical progression along the length of the cochlea based on birthdates, with neurons in the base (responding to high-frequency sounds) born early around mouse embryonic day (E) 9.5–10.5, and those in the apex (responding to low-frequency sounds) born late around E12.5‑13.5. Using a low-dose thymidine analog incorporation assay, we examine whether CN neurons are arranged in a spatial gradient according to their birthdates. Most CN neurons are born between E10.5 ānd E13.5, with a peak at E12.5. A second wave of neuron birth was observed in the dorsal cochlear nucleus (DCN) beginning on E14.5 and lasts until E18.5. Large excitatory neurons were born in the first wave, and small local circuit neurons were born in the second. No spatial gradient of cell birth was observed in the DCN. In contrast, neurons in the anteroventral cochlear nucleus (AVCN) were found to be arranged in a dorsal to ventral progression according to their birthdates, which are aligned with the tonotopic axis. Most of these AVCN neurons are endbulb-innervated bushy cells. The correlation between birthdate and tonotopic position suggests testable mechanisms for specification of tonotopic position.     

Functional organization of ascending and descending connections of the cochlear nucleus of horseshoe bats.

The ascending projections of the cochlear nucleus (CN) and the sources of descending inputs to the CN were investigated in horseshoe bats (Rhinolophus rouxi) by tracing the anterograde and retrograde transport of horseradish peroxidase (HRP or WGA-HRP) injected into the CN. The tracer was iontophoretically deposited into physiologically characterized regions of the cochlear nucleus (Feng and Vater, '85). We report the course and termination of pathways arising from the anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) cochlear nucleus. The projection fields within the auditory brainstem centers (superior olivary complex [SOC]; lateral lemniscus complex [LLC]; and inferior colliculus [IC]) and their tonotopic organization according to the frequency representations at the injection sites are described. While the projection pattern is generally in accordance with other mammals, several species-characteristic features are noted: i) the lateral superior olive (LSO) receives tonotopically organized input from both the AVCN and PVCN; ii) the CN-projections to medial nuclear groups of the SOC located between the LSO and the medial nucleus of the trapezoid body do not support previously suggested homologies; iii) the ventral nucleus of the LLC can be subdivided into two divisions with distinct input patterns from the AVCN and PVCN, respectively.     

Quantitative analysis of spiral ganglion projections to the cat cochlear nucleus

A quantitative examination of the tonotopic organization of primary afferent projections to the cochlear nucleus (CN) in adult cats wasconducted by using focal extracellular injections of Neurobiotin (NB) into the spiral ganglion of the basal cochlea. One to three injections separated by intervals of at least 2 mm were positioned along the basal one-third of the cochlea. Each injection produced discrete projection laminae that appeared as parallel horizontal sheets of labeled axons and terminals distributed sequentially dorsally to ventrally across each major CN subdivision: the anteroventral, posteroventral, and dorsal cochlear nucleus (AVCN, PVCN, and DCN, respectively). The length (rostrocaudal dimension), width (mediolateral dimension), thickness (dorsoventral dimension), and relative placement of 18 “frequency-band” laminae were measured in 10 adult cochlear nuclei. The average AVCN projection thickness was approximately twice that of the PVCN and DCN projections. In double injection cases, the center-to-center separation between AVCN laminae was also approximately twice that in the PVCN and equal to that in the DCN. Lamina thickness did not differ significantly as a function of frequency representation. However, in both width and length, mid-frequency laminae were up to two times larger than high-frequency laminae. Thus, the results indicate that DCN projections are the most discrete (i.e., are the thinnest and have the least overlap between adjacent frequency projections), whereas the AVCN projections are the largest but are as discrete as PVCN projections. In addition, high-frequency projections are smaller and more discrete than mid-frequency projections, which are larger and have greater overlap with adjacent frequency projections. J. Comp. Neurol. 379:133-149, 1997. © 1997 Wiley-Liss, Inc.     

Afferent projections to the central nucleus of the inferior colliculus in the rat

The afferent projections to the inferior colliculus of the rat were studied using the method of retrograde transport of horseradish peroxidase (HRP).Following large injections of HRP into the central nucleus, cells within the cochlear nuclei, superior olivary complex and auditory cortex were stained. Within the contralateral dorsal cochlear nucleus, fusiform cells were heavily labeled. Giant cells were also labeled in deeper layers. In the contralateral ventral cochlear nucleus, virtually all major cell types were labeled, with some types being labeled in greater numbers than others. Octopus cells of posteroventral division of ventral cochlear nucleus (PVCN) were never labeled. HRP-positive cells were found in ipsilateral and contralateral lateral superior olivary nucleus (LSO), ipsilateral medial superior olivary nucleus (MSO), ipsilateral and contralateral lateral nucleus of the trapezoid body (LTB), ipsilateral ventral nucleus of the trapezoid body (VTB), and ipsilateral superior paraolivary nucleus (SPN). Pyramidal cells of layer V of auditory cortex were heavily labeled.Small injections of HRP into the central nucleus resulted in labeled cells within restricted regions of the cochlear nuclei, superior olivary complex and auditory cortex. Injections into dorsal regions of the central nucleus resulted in cells labeled in ventral regions of the dorsal and ventral cochlear nuclei, and in lateral regions of LSO. These regions contain neurons which are considered to have low best frequencies. Injections placed in more ventral regions of the central nucleus led to labeling of cells in more dorsal regions of the cochlear nuclei and more medial regions of LSO in agreement with the tonotopical progressions within these structures.     

Connectional basis for frequency representation in the nuclei of the lateral lemniscus of the bat Eptesicus fuscus

To study the role of the lateral lemniscus as a link in the ascending auditory pathway, injections of neuronal tracers were placed in the anteroventral cochlear nucleus (AVCN) and in the inferior colliculus of the bat Eptesicus fuscus. To correlate the anatomical results with tonotopic organization, the characteristic frequency of cells at each injection site was determined electrophysiologically. Pathways from AVCN diverge to 3 major targets in the lateral lemniscus, the intermediate nucleus and 2 divisions of the ventral nucleus (VNLL). Projections from these 3 nuclei then converge at the inferior colliculus. One cell group is particularly notable for its cytoarchitectural appearance. It is referred to here as the columnar area of VNLL because its cells are organized as a tightly packed matrix of columns and rows. The connections of the columnar area are organized in sheets that are precisely related to the tonotopic organization of both AVCN and the inferior colliculus. Sheets of cells in the dorsal part of the columnar area receive projections from low-frequency parts of AVCN and project to low-frequency parts of the inferior colliculus. These sheets of connections occupy successively more ventral locations as the tonotopic focus of the injection site increases in frequency. The entire range of frequencies audible to the bat is systematically represented along the dorsal-ventral dimension of the columnar area. Because each column is only 20-30 cells in height, frequency representation must be compressed in this dimension. Within the columnar area there is an overrepresentation of frequencies between 25 and 50 kHz, which corresponds roughly to the range of the FM echo-location call in Eptesicus. The connections of the other nuclei of the lateral lemniscus are not as precisely related to the tonotopic organization of the system as are those of the columnar area.     

Dorsal nucleus of the lateral lemniscus in the rat: Concentric organization and tonotopic projection to the inferior colliculus

A basic principle of organization in auditory centers is the topographic-tonotopic order. Whether this applies to the dorsal nucleus of the lateral lemniscus (DNLL), however, is still debated. To clarify this problem, we have utilized the neuroanatomical tracers horseradish peroxidase (HRP) and biotinylated dextran (BD) injected into different regions of the central nucleus of the inferior colliculus (CNIC) in the rat. After large injections of HRP that included most of the CNIC, retrogradely labelled neurons were found all across the ipsi- and contralateral DNLL, showing that all parts of this nucleus innervate the CNIC bilaterally. More neurons were seen consistently on the side contralateral to the injection site. Labelled fibers, however, were abundant ipsilaterally, but scarce in the contralateral DNLL. Single, small injections of HRP or BDinto the CNIC resulted in labelling in restricted areas of the ipsi- and contralateral DNLL. In coronal sections, the neurons and fibers labelled in the ipsilateral DNLL formed a well-defined, ring-shaped structure made of dendrites and axons oriented parallel to each other, which we termed “annular band.” The observation of serial sections revealed that the annular band seen in any individual section represents a slice through a more or less complete three-dimensional, hollow, ovoid structure oriented rostrocau-dally. The position and diameter of the annular band changed as the injection site was shifted along the tonotopic axis of the CNIC. Single injections placed in the ventromedial, high-frequency region of the CNIC produced a large annular band along the periphery of the DNLL. After injections placed in progressively more dorsolateral, lower-frequency regions of the CNIC, the annular band became smaller in diameter and occupied a successively more central position in the DNLL. Double injections along the tonotopic axis of the CNIC resulted in two roughly concentric annular bands. The labelled neurons and fibers in the contralateral DNLL systematically occupied a position symmetric to the annular band seen ipsilaterally. These findings indicate that the rat DNLL is primarily composed of neurons with flattened dendritic arbors and flattened fields of terminal fibers. These two elements intermingle, forming concentric layers around the geometric center of the nucleus. The axons of neurons within corresponding layers on the two sides converge onto the CNIC of both sides in a strict topographic fashion: the peripheral layers project to the ventromedial, high-frequency region of the CNIC, and the central layers project to the dorsolateral, low-frequency region. These results suggest that the concentric arrangement of the DNLL is the substrate of its tonotopic organization. © 1994 Wiley-Liss, Inc.     

The cytoarchitecture of the nucleus angularis of the barn owl (Tyto alba)

The cochlear nucleus angularis (NA) of the barn owl (Tyto alba) was analyzed using Golgi, Nissl, and tract tracing techniques. NA forms a column of cells in the dorsolateral brainstem that partly overlaps with, and is rostral and lateral to, the cochlear nucleus magnocellularis (NM). Highest best frequencies are mapped in lateral NA (NAl), intermediate in medial NA (NAm), and lowest in the foot region (NAf). Cell density followed the tonotopic axis and decreased with decreasing best frequency. NA contained four major cell classes: planar, radiate, vertical, and stubby. Planar and radiate classes were further subdivided into bipolar and multipolar types according to their number of primary dendrites. Planar neurons were confined to an isofrequency band, whereas radiate neurons had dendrites that could extend across an isofrequency band. Vertical cells had long dendrites oriented perpendicularly to isofrequency bands. Stubby cells were the most numerous and were confined to an isofrequency band because of their short dendrites. Neurons in each of these four classes projected to the inferior colliculus and dorsal nucleus of the lateral lemniscus.     

Morphology and physiology of cells in slice preparations of the posteroventral cochlear nucleus of mice

In an effort to understand what integrative tasks are performed in the cochlear nuclei, the present study was undertaken to describe neuronal circuits in the posteroventral cochlear nucleus (PVCN) anatomically and physiologically. The cochlear nuclear complex receives auditory information from the cochlea through the auditory nerve. Within the cochlear nuclei, signals travel along several parallel and interconnected pathways. From the cochlear nuclei, transformed versions of the signals are passed to higher auditory centers in the brainstem. We have recorded electrophysiological responses from cells that were subsequently visualized with horseradish peroxidase (HRP). Responses to shocks to the auditory nerve root and to intracellularly injected current pulses were recorded and correlated with morphology. Two types of stellate cells and octopus cells were distinguished. T stellate cells project out of the cochlear nuclei through the Trapezoid body; D stellate cells do not. The axons of D stellate cells extend Dorsalward to the dorsal cochlear nucleus (DCN) but have not been traced out of the nucleus. Both T and D stellate cells have terminal collaterals in the multipolar cell region of the PVCN and in the DCN. The endings of one T stellate cell formed a narrow band rostrocaudally in the fusiform cell layer of the DCN that resembled an isofrequency band. The endings of one D stellate cell lay closely apposed to multipolar cells in the deep layer of the DCN. The dendrites of T stellate cells are often aligned along the path of auditory nerve fibers and end in tufts, whereas those of D stellate cells extend radially in the plane of the lateral surface of the PVCN toward granule cell areas and branch sparingly. Octopus cells have dendrites oriented perpendicularly to the path of auditory nerve fibers. Their axons were cut medially in the slices; none had collateral branches. Both T and D stellate cells were monosynaptically excited to threshold by shocks to the nerve root, indicating that they could participate in local circuits that we measure physiologically. T stellate cells have action potentials that peak at about 0 mV and are followed by single undershoots. The D stellate cell that was best impaled fired overshooting action potentials that were followed by double undershoots. Octopus cells were monosynaptically excited to threshold by shocks to the auditory nerve.     

Intrinsic and commissural connections of the rat inferior colliculus.

This study analyzes the distribution of the intrinsic and commissural fiber plexuses originating in the central nucleus of the inferior colliculus in the rat. The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected iontophoretically at different places along the tonotopic axis of the central nucleus and visualized immunohistochemically. In coronal sections the terminal fields of axons originating at each injection site are seen to create four well-defined bands across the rostrocaudal extent of the inferior colliculus, two in the ipsilateral and two in the contralateral side. The "ipsilateral main band" extends dorsomedially and ventrolaterally from the injection site, in register with the known isofrequency contours of the central nucleus, spanning this nucleus and extending into the dorsal cortex of the inferior colliculus. The "ipsilateral external band" is located in the external cortex, where it is oriented dorsoventrally, slightly oblique to the pial surface. In caudal sections, the ventral portion of these two bands appear to join. The two bands in the contralateral inferior colliculus occupy a symmetric position to those of the ipsilateral side, forming a mirror-like image. The position of the four bands changes as the position of the injection site is varied along the frequency gradient axis of the central nucleus. After ventromedial (high frequency area) injections, the main band is ventral and medial, and the external band ventral and lateral. After more dorsolateral (lower frequency) injections, the main band is more dorsal and lateral, whereas the external band is more dorsal but more medial. Thus, the change in the position of the external band is separate and opposite to that of the main band. We suggest that the main bands represent isofrequency contours. Since the projection from the central nucleus to the external cortex of the inferior colliculus also appears to be tonotopic, we also propose a tonotopic organization for the external cortex. The main bands overlap the terminal field of the lemniscal fibers in the central nucleus; thus, it is concluded that the intracollicular fibers contribute to the formation of the known fibrodendritic laminae of the central nucleus. A possible role in preservation of frequency information and integration of other different acoustic parameters is proposed for the main bands. The external bands could participate in polysensory integration, and the commissural connections could be involved in hitherto unknown stages of binaural processing of sound. Based on our results, several modifications are proposed for delineating the subdivisions of the inferior colliculus.     

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.     

Dorsal cochlear nucleus projections to the inferior colliculus in the cat: A light and electron microscopic study

In the Present report retrograde and anterograde labeling techniques are used to study the Projections of the dorsal cochlear nucleus (DCN) to the inferior colliculus in the cat. Horseradish peroxidase (HRP) or wheat germ agglutinin (WGA-HRP) injections into the inferior colliculus produce large numbers of labeled neurons in the DCN on the opposite side. Labeled cells with projections to the colliculus are identified as fusiform and giant cells and are organized into rostrocaudal bands. The axons of these DCN neurons are labeled by anterograde transport of ³H-leucine and/or proline and studied in light and electron microscopic autoradiographs. Axons from the DCN terminate within the central nucleus of the inferior colliculus in densely labeled, rostrocaudally oriented bands. Less heavily labeled extensions of these bands are found in the deepest layer of the dorsal cortex, and light labeling is found adjacent to the bands in the central nucleus and in the ventrolateral nucleus. Cells in the dorsomedial DCN project to the most ventromedial part of the central nucleus while progressively more ventrolateral cells in the DCN project to more dorsolateral parts of the central nucleus. This present evidence suggests that the DCN sends afferents to only two of the four subdivisions of the central nucleus. Within these regions, the axons from the DCN form terminal boutons or boutons de passage characterized by medium-sized, round synaptic vesicles. The labeled endings nearly always make asymmetric synaptic contacts on the dendrites of disc-shaped and stellate cells in the central nucleus. A few axosomatic contacts are found on one particular cell type, possibly the stellate variety. The results support the hypothesis that each subdivision of the central nucleus receives afferents from a different set of cell types in the auditory nuclei of the lower brainstem. The banding patterns of the efferent cells in the cochlear nucleus and the axons within the central nucleus suggest that these inputs are congruent to the fibrodendritic layers of the central nucleus and may contribute to tonotopic organization in the central nucleus. Finally, the results suggest that each of the two major classes of cells in the central nucleus receives different patterns of inputs from the DCN. These morphological differences could contribute to different electrophysiological responses to the sound stimuli by these cells.