Organization of ventrolateral periolivary cells of the cat superior olive as revealed by PEP-19 immunocytochemistry and Nissl stain

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.

Structural arrangement of auditory brainstem nuclei in the bats Phyllostomus discolor and Carollia perspicillata

The structure of the mammalian auditory brainstem is evolutionarily highly plastic, and distinct nuclei arrange in a species-dependent manner. Such anatomical variability is present in the superior olivary complex (SOC) and the nuclei of the lateral lemniscus (LL). Due to the structure-function relationship in the auditory brainstem, the identification of individual nuclei supports the understanding of sound processing. Here, we comparatively describe the nucleus arrangement and the expression of functional markers in the auditory brainstem of the two bat species Phyllostomus discolor and Carollia perspicillata. Using immunofluorescent labeling, we describe the arrangement and identity of the SOC and LL nuclei based on the expression of synaptic markers (vesicular glutamate transporter 1 and glycine transporter 2), calcium-binding proteins, as well as the voltage-gated ion channel subunits Kv1.1 and HCN1. The distribution of excitatory and inhibitory synaptic labeling appears similar between both species and matches with that of other mammals. The detection of calcium-binding proteins indicates species-dependent differences and deviations from other mammals. Kv1.1 and HCN1 show largely the same expression pattern in both species, which diverges from other mammals, indicating functional adaptations in the cellular physiology of bat neurons.

Distribution of Glutamatergic and Glycinergic Inputs onto Human Auditory Coincidence Detector Neurons

Localization of sound sources in the environment requires neurons that extract interaural timing differences (ITD) in low-frequency hearing animals from fast and precisely timed converging inputs from both ears. In mammals, this is accomplished by neurons in the medial superior olive (MSO). MSO neurons receive converging excitatory input from both the ipsilateral and contralateral cochlear nuclei and glycinergic, inhibitory input by way of interneurons in the medial and lateral nuclei of the trapezoid body (MNTB and LNTB, respectively). Key features of the ITD circuit are MSO neurons with symmetric dendrites that segregate inputs from the ipsilateral and contralateral ears and preferential distribution of glycinergic inputs on MSO cell bodies. This circuit for ITD is well characterized in gerbils, a mammal with a prominent MSO and a low-frequency hearing range similar to humans. However, the organization of this circuit in the human MSO has not been characterized. This is further complicated by limited understanding of the human LNTB. Nonetheless, we hypothesized that the ITD circuit characterized in laboratory animals is similarly arranged in the human MSO. Herein, we utilized neuron reconstructions and immunohistochemistry to investigate the distribution of glutamatergic and glycinergic inputs onto human MSO neurons. Our results indicate that human MSO neurons have simple, symmetric dendrites and that glycinergic inputs outnumber glutamatergic inputs on MSO cell bodies and proximal dendrites. Together these results suggest that the human MSO utilizes similar circuitry to other mammals with excellent low-frequency hearing.

Early Binaural Hearing: The Comparison of Temporal Differences at the Two Ears

Many mammals, including humans, are exquisitely sensitive to tiny time differences between sounds at the two ears. These interaural time differences are an important source of information for sound detection, for sound localization in space, and for environmental awareness. Two brainstem circuits are involved in the initial temporal comparisons between the ears, centered on the medial and lateral superior olive. Cells in these nuclei, as well as their afferents, display a large number of striking physiological and anatomical specializations to enable submillisecond sensitivity. As such, they provide an important model system to study temporal processing in the central nervous system. We review the progress that has been made in characterizing these primary binaural circuits as well as the variety of mechanisms that have been proposed to underlie their function.

Principal cells of the brainstem's interaural sound level detector are temporal differentiators rather than integrators

The brainstem’s lateral superior olive (LSO) is thought to be crucial for localizing high-frequency sounds by coding interaural sound level differences (ILD). Its neurons weigh contralateral inhibition against ipsilateral excitation, making their firing rate a function of the azimuthal position of a sound source. Since the very first in vivo recordings, LSO principal neurons have been reported to give sustained and temporally integrating ‘chopper’ responses to sustained sounds. Neurons with transient responses were observed but largely ignored and even considered a sign of pathology. Using the Mongolian gerbil as a model system, we have obtained the first in vivo patch clamp recordings from labeled LSO neurons and find that principal LSO neurons, the most numerous projection neurons of this nucleus, only respond at sound onset and show fast membrane features suggesting an importance for timing. These results provide a new framework to interpret previously puzzling features of this circuit.

Enhancement of phase-locking in rodents. I. An axonal recording study in gerbil

The trapezoid body (TB) contains axons of neurons in the anteroventral cochlear nucleus projecting to monaural and binaural nuclei in the superior olivary complex (SOC). Characterization of these monaural inputs is important for the interpretation of response properties of SOC neurons. In particular, understanding of the sensitivity to interaural time differences (ITDs) in neurons of the medial and lateral superior olive requires knowledge of the temporal firing properties of the monaural excitatory and inhibitory inputs to these neurons. In recent years, studies of ITD sensitivity of SOC neurons have made increasing use of small animal models with good low-frequency hearing, particularly the gerbil. We presented stimuli as used in binaural studies to monaural neurons in the TB and studied their temporal coding. We found that general trends as have been described in the cat are present in gerbil, but with some important differences. Phase-locking to pure tones tends to be higher in TB axons and in neurons of the medial nucleus of the TB (MNTB) than in the auditory nerve for neurons with characteristic frequencies (CFs) below 1 kHz, but this enhancement is quantitatively more modest than in cat. Stronger enhancement is common when TB neurons are stimulated at low frequencies below CF. It is rare for TB neurons in gerbil to entrain to low-frequency stimuli, i.e., to discharge a well-timed spike on every stimulus cycle. Also, complex phase-locking behavior, with multiple modes of increased firing probability per stimulus cycle, is common in response to low frequencies below CF.NEW & NOTEWORTHY Phase-locking is an important property of neurons in the early auditory pathway: it is critical for the sensitivity to time differences between the two ears enabling spatial hearing. Studies in cat have shown an improvement in phase-locking from the peripheral to the central auditory nervous system. We recorded from axons in an output tract of the cochlear nucleus and show that a similar but more limited form of temporal enhancement is present in gerbil.

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.

In vivo Whole-Cell Recordings Combined with Electron Microscopy Reveal Unexpected Morphological and Physiological Properties in the Lateral Nucleus of the Trapezoid Body in the Auditory Brainstem

The lateral nucleus of the trapezoid body (LNTB) is a prominent nucleus in the superior olivary complex in mammals including humans. Its physiology in vivo is poorly understood due to a paucity of recordings. It is thought to provide a glycinergic projection to the medial superior olive (MSO) with an important role in binaural processing and sound localization. We combined in vivo patch clamp recordings with labeling of individual neurons in the Mongolian gerbil. Labeling of the recorded neurons allowed us to relate physiological properties to anatomy at the light and electron microscopic level. We identified a population of quite dorsally located neurons with surprisingly large dendritic trees on which most of the synaptic input impinges. In most neurons, one or more of these dendrites run through and are then medial to the MSO. These neurons were often binaural and could even show sensitivity to interaural time differences (ITDs) of stimulus fine structure or envelope. Moreover, a subpopulation showed enhanced phase-locking to tones delivered in the tuning curve tail. We propose that these neurons constitute the gerbil main LNTB (mLNTB). In contrast, a smaller sample of neurons was identified that was located more ventrally and that we designate to be in posteroventral LNTB (pvLNTB). These cells receive large somatic excitatory terminals from globular bushy cells. We also identified previously undescribed synaptic inputs from the lateral superior olive. pvLNTB neurons are usually monaural, display a primary-like-with-notch response to ipsilateral short tones at CF and can phase-lock to low frequency tones. We conclude that mLNTB contains a population of neurons with extended dendritic trees where most of the synaptic input is found, that can show enhanced phase-locking and sensitivity to ITD. pvLNTB cells, presumed to provide glycinergic input to the MSO, get large somatic globular bushy synaptic inputs and are typically monaural with short tone responses similar to their primary input from the cochlear nucleus.

Entracking as a Brain Stem Code for Pitch: The Butte Hypothesis

The basic nature of pitch is much debated. A robust code for pitch exists in the auditory nerve in the form of an across-fiber pooled interspike interval (ISI) distribution, which resembles the stimulus autocorrelation. An unsolved question is how this representation can be "read out" by the brain. A new view is proposed in which a known brain-stem property plays a key role in the coding of periodicity, which I refer to as "entracking", a contraction of "entrained phase-locking". It is proposed that a scalar rather than vector code of periodicity exists by virtue of coincidence detectors that code the dominant ISI directly into spike rate through entracking. Perfect entracking means that a neuron fires one spike per stimulus-waveform repetition period, so that firing rate equals the repetition frequency. Key properties are invariance with SPL and generalization across stimuli. The main limitation in this code is the upper limit of firing (~ 500 Hz). It is proposed that entracking provides a periodicity tag which is superimposed on a tonotopic analysis: at low SPLs and fundamental frequencies > 500 Hz, a spectral or place mechanism codes for pitch. With increasing SPL the place code degrades but entracking improves and first occurs in neurons with low thresholds for the spectral components present. The prediction is that populations of entracking neurons, extended across characteristic frequency, form plateaus ("buttes") of firing rate tied to periodicity.

The relative contributions of MNTB and LNTB neurons to inhibition in the medial superior olive assessed through single and paired recordings

The medial superior olive (MSO) senses microsecond differences in the coincidence of binaural signals, a critical cue for detecting sound location along the azimuth. An important component of this circuit is provided by inhibitory neurons of the medial and lateral nuclei of the trapezoid body (MNTB and LNTB, respectively). While MNTB neurons are fairly well described, little is known about the physiology of LNTB neurons. Using whole cell recordings from gerbil brainstem slices, we found that LNTB and MNTB neurons have similar membrane time constants and input resistances and fire brief action potentials, but only LNTB neurons fire repetitively in response to current steps. We observed that LNTB neurons receive graded excitatory and inhibitory synaptic inputs, with at least some of the latter arriving from other LNTB neurons. To address the relative timing of inhibition to the MSO from the LNTB versus the MNTB, we examined inhibitory responses to auditory nerve stimulation using a slice preparation that retains the circuitry from the auditory nerve to the MSO intact. Despite the longer physical path length of excitatory inputs driving contralateral inhibition, inhibition from both pathways arrived with similar latency and jitter. An analysis of paired whole cell recordings between MSO and MNTB neurons revealed a short and reliable delay between the action potential peak in MNTB neurons and the onset of the resulting IPSP (0.55 ± 0.01 ms, n = 4, mean ± SEM). Reconstructions of biocytin-labeled neurons showed that MNTB axons ranged from 580 to 858 μm in length (n = 4). We conclude that while both LNTB and MNTB neurons provide similarly timed inhibition to MSO neurons, the reliability of inhibition from the LNTB at higher frequencies is more constrained relative to that from the MNTB due to differences in intrinsic properties, the strength of excitatory inputs, and the presence of feedforward inhibition.

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.

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.

Presynaptic Na+ channels: locus, development, and recovery from inactivation at a high-fidelity synapse

Na+ channel recovery from inactivation limits the maximal rate of neuronal firing. However, the properties of presynaptic Na+ channels are not well established because of the small size of most CNS boutons. Here we study the Na+ currents of the rat calyx of Held terminal and compare them with those of postsynaptic cells. We find that presynaptic Na+ currents recover from inactivation with a fast, single-exponential time constant (24 degrees C, tau of 1.4-1.8 ms; 35 degrees C, tau of 0.5 ms), and their inactivation rate accelerates twofold during development, which may contribute to the shortening of the action potential as the terminal matures. In contrast, recordings from postsynaptic cells in brainstem slices, and acutely dissociated, reveal that their Na+ currents recover from inactivation with a double-exponential time course (tau(fast) of 1.2-1.6 ms; tau(slow) of 80-125 ms; 24 degrees C). Surprisingly, confocal immunofluorescence revealed that Na+ channels are mostly absent from the calyx terminal but are instead highly concentrated in an unusually long (approximately 20-40 microm) unmyelinated axonal heminode. Outside-out patch recordings confirmed this segregation. Expression of Na(v)1.6 alpha-subunit increased during development, whereas the Na(v)1.2alpha-subunit was not present. Serial EM reconstructions also revealed a long pre-calyx heminode, and biophysical modeling showed that exclusion of Na+ channels from the calyx terminal produces an action potential waveform with a shorter half-width. We propose that the high density and polarized locus of Na+ channels on a long heminode are critical design features that allow the mature calyx of Held terminal to fire reliably at frequencies near 1 kHz.

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

Specialized synapse-associated structures within the calyx of Held

The calyx of Held exhibits fast glutamatergic neurotransmission at high rates with low temporal jitter and has adapted specialized synaptic mechanisms to support its functional demands. We report the presence in calyces of an atypical arrangement of subcellular organelles, called the mitochondria-associated adherens complex (MAC). We demonstrate that MACs are located adjacent to synapses and contain membranous elements linked with coated and uncoated vesicles. Mitochondria that form MACs have more complex geometries than other mitochondria within the calyx and can extend between clusters of synapses. We estimate that the calyx contains 1600 MACs, 2400 synapses, and 6200 readily releasable vesicles. We also identify synaptic vesicle endocytotic regions close to MACs and synapses and hypothesize that calyces are composed of multiple activity modules, each containing machinery for vesicle release and recycling.

The evolution of temporal processing in the medial superior olive, an auditory brainstem structure

A basic concept in neuroscience is to correlate specific functions with specific neuronal structures. By discussing a specific example, an alternative concept is proposed: structures may be linked to rules of processing and these rules may serve different functions in different species or at different stages of evolution. The medial superior olive (MSO), a mammalian auditory brainstem structure, has been thought to solely process interaural time differences (ITD), the main cue for localizing low frequency sounds. Recent findings, however, indicate that this is not its only function since mammals that do not hear low frequencies and do not use ITDs for sound localization also possess a MSO. Recordings from the bat MSO indicate that it processes temporal cues in the milli- and submillisecond range, based on monaural or binaural inputs. In bats, and most likely in other small mammals, this temporal processing is related to pattern recognition and echo suppression rather than sound localization. However, the underlying mechanism, coincidence detection of several inputs, creates an epiphenomenal ITD sensitivity that is of no use for small mammals like bats or ancestral mammals. Such an epiphenomenal ITD sensitivity would have been a pre-adaptation which, when mammals grew larger during evolution and when localization of low frequency sounds became a question of survival, suddenly gained relevance. This way the MSO became involved in a new function without changing its basic rules of processing.

Ultrastructure of neurons and large synaptic terminals in the lateral nucleus of the trapezoid body of the cat

Neurons of the lateral nucleus of the trapezoid body (LNTB), the most prominent periolivary nucleus of the cat superior olivary complex, form an important component of the descending auditory pathways and also innervate the medial superior olive. Cells forming the posteroventral subnucleus (pvLNTB), when investigated by light microscopy, exhibit morphological similarities with globular bushy cells of the cochlear nucleus and principal cells of the medial nucleus of the trapezoid body. These latter two cell types are integral components of brainstem circuitry mediating the early stages of sound localization. In this report, ultrastructural features of LNTB neurons are described. pvLNTB cell bodies are characterized by a round to oval shape, smooth nuclear membrane, and the relative paucity of stacks of rough endoplasmic reticulum. In addition, pvLNTB cell bodies and proximal dendrites are contacted by large synaptic terminals which contain round synaptic vesicles and form multiple asymmetric synaptic junctions. These ultrastructural characteristics are similar to those previously described for globular and principal cells and distinguish pvLNTB cells from cells of the main subnucleus. Large terminals contacting pvLNTB cells contain a specialized organelle assembly, including an adherens plaque associated by filamentous strands with a mitochondrion. We name this organelle assembly the mitochondria-associated adherens complex (MAC) and note its proximity to synaptic junctions. Because high activity rates are characteristic of large terminals in the lower auditory system, the MAC may play a specialized role in membrane stabilization at synapses which generate high rates of vesicle membrane turnover.

PEP-19 immunoreactivity in the cochlear nucleus and superior olive of the cat

We applied antiserum to PEP-19, a presumptive calcium-binding polypeptide, to the auditory brainstem of cats to determine whether this antiserum would selectively reveal cochlear nucleus neurons and their projections. We report that the entire populations of ventral cochlear nucleus bushy and multipolar cells, but not octopus cells, express this peptide in their somata and dendrites. Presumed axons of spherical bushy cells located dorsally and thicker globular bushy cell fibers located ventrally in the trapezoid body are immunostained, as are thin fibers presumed to represent the axons of multipolar cells. Large calyceal endings in the medial nucleus of the trapezoid body are densely immunoreactive as are smaller punctate profiles that outline immunonegative neuronal profiles in the medial and lateral superior olives. These features of immunolabeling indicate that PEP-19 is expressed in all neuronal compartments. Within the entire superior olivary complex, relatively few neurons are immunolabeled, and the vast majority of these are found in the periolivary nuclei. There are many more immunostained neurons in lateral than in medial periolivary cell groups, but their combined numbers are dwarfed by the numbers of immunolabeled cells in the ventral cochlear nucleus. The borders of the principal nuclei and some of the periolivary cell groups are well defined by the distribution of PEP-19-immunoreactive fibers and puncta. Since ventral cochlear nucleus bushy cells comprise the predominant input to principal nuclei of the superior olive, and the entire bushy cell population is immunolabeled by PEP-19 antiserum, the numbers and distribution of their inputs can be quantified. In this study we report that immunoreactive puncta apposed to the cell bodies and proximal dendrites of neurons in the medial superior olive occur at a density of 20/100 microns2. Moreover, we demonstrate by pre-embedding immunoelectron microscopy that the PEP-19-immunoreactive punctate profiles observed in the medial superior olive by light microscopy represent presynaptic terminal boutons that contain round synaptic vesicles and form asymmetric synaptic junctions, features traditionally associated with excitatory synapses. Thus, this antiserum represents a useful tool for investigating the distribution of ventral cochlear nucleus fibers and synaptic terminals within their target nuclei in the superior olive.

Heterogeneous projections of the cat posteroventral cochlear nucleus

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to identify the projections of the posteroventral cochlear nucleus in cats. After labeling predominately cells of the core and multipolar regions, varicose fibers were observed in a variety of auditory nuclei. Ipsilaterally, most varicose fibers were located in periolivary regions situated lateral to the medial superior olive of the superior olivary complex. Contralaterally, the majority of labeled fibers were located in the ventral nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. Labeled varicose fibers were also observed in regions not commonly identified as receiving input from the posteroventral cochlear nucleus. These regions included bilaterally the principal nuclei of the superior olivary complex, some periolivary regions, and the sagulum, as well as the ipsilateral intermediate and dorsal nucleus of the lateral lemniscus, inferior colliculus, and lateral pontine nucleus. Both similarities and differences were observed in the projections of the core and multipolar regions. With the exception of calyceal-type endings in the contralateral ventral nucleus of the lateral lemniscus, the varicose fibers in all regions, including the contralateral medial nucleus of the trapezoid body, were beaded, en passant type terminal varicosities.

Glycine immunoreactivity in the lateral nucleus of the trapezoid body of the cat

The central auditory system contains several predominantly glycine-immunoreactive nuclei, and one of these, the lateral nucleus of the trapezoid body, contains cell bodies exhibiting a spectrum of labeling intensity. By using post-embedding glycine immunocytochemistry on thin sections, and toluidine blue staining of adjacent sections, we established that darkly glycine-immunoreactive neurons constituted a distinct morphological class and form one of three subnuclei of the lateral nucleus of the trapezoid body, called the posteroventral subnucleus. These neurons resemble, in both labeling intensity and cell body morphology, the principal cells of the medial nucleus of the trapezoid body. The other two subnuclei of the lateral nucleus of the trapezoid body, its main and hilus subnuclei, contained predominantly glycine-immunoreactive and glycine-immunonegative neurons, respectively. Glycine immunoreactivity was compared with gamma-aminobutyric acid (GABA) immunoreactivity in order to identify other organizational features of the lateral nucleus of the trapezoid body. Cell bodies that displayed either dark glycine-immunoreactivity or which were glycine-immunonegative were GABA-immunonegative. Cell bodies that displayed GABA immunoreactivity were preferentially located in the main subnucleus. Patterns of distribution of axosomatic innervation in the lateral nucleus of the trapezoid body were revealed in which glycine-immunoreactive puncta were (1) more numerous than GABA-immunoreactive puncta on glycine-immunonegative cell bodies and (2) equal to or less numerous than GABA-immunoreactive puncta on glycine-immunoreactive cell bodies. The characteristics of neural circuitry revealed by glycine and GABA immunoreactivity in the lateral nucleus of the trapezoid body may be generalizable to other populations of neurons of the superior olivary complex and to other regions of the central nervous system containing glycinergic neurons, such as the retina.