Spherical Bushy Cells Research Articles

Effects of congenital deafness in the cochlear nuclei of Shaker-2 mice: an ultrastructural analysis of synapse morphology in the endbulbs of Held.

It is well established that manipulation of the sensory environment can significantly alter central auditory system development. For example, congenitally deaf white cats exhibit synaptic alterations in the cochlear nucleus distinct from age-matched, normal hearing controls. The large, axosomatic endings of auditory nerve fibers, called endbulbs of Held, display reduced size and branching, loss of synaptic vesicles, and a hypertrophy of the associated postsynaptic densities on the target spherical bushy cells. Such alterations, however, could arise from the cat's genetic syndrome rather than from deafness. In order to examine further the role of hearing on synapse development, we have studied endbulbs of Held in the shaker-2 ( sh2 ) mouse. These mice carry a point mutation on chromosome 11, affecting myosin 15 and producing abnormally short stereocilia in hair cells of the inner ear. The homozygous mutant mice are born deaf and develop perpetual circling behavior, although receptor cells and primary neurons remain intact at least for the initial 100 days of postnatal life. Endbulbs of Held in 7-month old, deaf sh2 mice exhibited fewer synaptic vesicles in the presynaptic ending, the loss of intercellular cisternae, and a hypertrophy of associated postsynaptic densities. On average, postsynaptic density area for sh2 endbulbs was 0.23 +/- 0.19 microm(2) compared to 0.07 +/- 0.04 microm(2) ( p < 0.001) for age-matched, hearing littermates. These changes at the endbulb synapse in sh2 mice resemble those of the congenitally deaf white cat and are consistent with the idea that they represent a generalized response to deafness.

Interaction of Excitation and Inhibition in Anteroventral Cochlear Nucleus Neurons That Receive Large Endbulb Synaptic Endings

Spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their main excitatory input from auditory nerve fibers (ANFs) through large synapses, endbulbs of Held. These cells are also the target of inhibitory inputs whose function is not well understood. The present study examines the role of inhibition in the encoding of low-frequency sounds in the gerbil's AVCN. The presynaptic action potentials of endbulb terminals and postsynaptic action potentials of SBCs were monitored simultaneously in extracellular single-unit recordings in vivo. An input-output analysis of presynaptic and postsynaptic activity was performed for both spontaneous and acoustically driven activity. Two-tone stimulation and neuropharmacological experiments allowed the effects of neuronal inhibition and cochlear suppression on SBC activity to be distinguished. Ninety-one percent of SBCs showed significant neuronal inhibition. Inhibitory sidebands enclosed the high- or low-frequency, or both, sides of the excitatory areas of these units; this was reflected as a presynaptic to postsynaptic increase in frequency selectivity of up to one octave. Inhibition also affected the level-dependent responses at the characteristic frequency. Although in all units the presynaptic recordings showed monotonic rate-level functions, this was the case in only half of the postsynaptic recordings. In the other half of SBCs, postsynaptic inhibitory areas overlapped the excitatory areas, resulting in nonmonotonic rate-level functions. The results demonstrate that the sound-evoked spike activity of SBCs reflects the integration of acoustically driven excitatory and inhibitory input. The inhibition specifically affects the processing of the spectral, temporal, and intensity cues of acoustic signals.

An immunogold investigation of the relationship between the amino acids GABA and glycine and their transporters in terminals in the guinea-pig anteroventral cochlear nucleus

The majority of terminals contacting spherical bushy cell bodies in the guinea-pig anteroventral cochlear nucleus contain GABA, glycine or both (colocalizing). Double labeling with antibodies to each amino acid and the plasma membrane transporter for the other was performed using different sizes of gold particles. The transporter for GABA occurs in the plasma membranes of some terminals containing glycine and vice versa suggesting that colocalizing terminals can retrieve both amino acids.

The effects of congenital deafness on auditory nerve synapses and globular bushy cells in cats

It is well known that auditory deprivation affects the structure and function of the central nervous system. Congenital deafness represents one form of deprivation, and in the adult white cat, it has been shown to have a clear effect upon the synaptic interface between endbulbs of Held and spherical bushy cells. It is not known, however, whether all primary synapses are affected and/or whether they are affected in the same way and to the same extent. Thus, we studied a second neuronal circuit in the deaf white cat involving modified (small) endbulbs and globular bushy cells. Compared to normal hearing cats, modified endbulbs of congenitally deaf cats were 52.2% smaller but unchanged in structural complexity. There was also a striking loss of extracellular space between ending and cell body. The somata of postsynaptic globular bushy cells were 13.4% smaller and had enlarged postsynaptic densities. These data reveal that axosomatic synapses demonstrate abnormal structure as a consequence of deafness and that the extent of the abnormalities can vary with respect to the circuits involved. The implication of these observations is that synaptic anomalies would introduce differential delays within separate circuits, thereby desynchronizing neural activity from sound stimuli. This loss of synchronization could in turn disrupt temporal processing and compromise a host of related functions, including language comprehension.

Diversity and plasticity in amino acid receptor subunits in the rat auditory brain stem

Glutamate, gamma aminobutyric acid (GABA) and glycine receptors have different properties depending on the specific subunit combination utilized. The subunit composition of amino acid receptors may help to shape the responses of neurons and can provide a diversity of response properties in different neuronal types and regions. This allows a synaptic fine tuning for an optimization of processing requirements and may also allow for changes in response to changes in input. This article reviews the diversity that has been found in the subunit composition of GABA, glycine, alpha amino-3-hydroxy-5-methyl-4 isoxazole propionic acid and N-Methyl, D-aspartate (NMDA) receptors in the mammalian auditory brain stem and provides new data on how the NMDAR1 glutamate receptor subunit changes as a consequence of deafness. In the latter study, quantitative in situ hybridization was used to assess NMDAR1 mRNA expression in six cell types of the rat cochlear nucleus. A unilateral cochlear ablation was performed and expression determined in the ipsilateral and contralateral cochlear nucleus 5 and 20 days later. Significantly decreased expression, compared to normal, was found 5 days following deafness, in ipsilateral spherical bushy cells, octopus cells and shell neurons, but not in fusiform cells, corn cells or granule cells. At 20 days the expression was not significantly different from normal in any of the six cell types.

Ultrastructural and immunocytochemical characterization of commissural neurons in the ventral cochlear nucleus of the rat

Medium to large-giant multipolar neurons in the rat ventral cochlear nucleus were retrograde labelled after injection of the tracer Wheat Germ Agglutinin conjugated to Horse Radish Peroxidase into the contralateral cochlear nucleus. Light microscopy immunocytochemistry showed that 42.45% of these retrograde labelled neurons, generally strongly labelled with the tracer, were markedly glycine immunopositive, and that 57.55%, usually weakly retrograde labelled neurons, were immunonegative or weakly positive for glycine. These commissural neurons were generally GABA negative and variably immunopositive for glutamate. About 1/3rd of the commissural neurons had variably developed a rough endoplasmic reticulum whilst axo-somatic boutons covered 20-40% of the cell body. These cells were recognized as multipolar neurons of type I. Most of them were weakly glycine positive or even negative and a few appeared glycinergic. A little less than the remaining 2/3rds of the whole commissural population in the postero-ventral cochlear nucleus presented a surface which was 65-85% covered with synaptic boutons, among which some also appeared labelled. These cells were recognized as multipolar neurons of type II. Many microtubules and neurofilaments were present, free ribosomes being more numerous around Nissl bodies with short cisternae. A few low retrograde labelled type II were weakly or non glycinergic. A small number of large to giant neurons type II, strongly retrograde labelled, appeared to be glycine positive, consistently GABA negative and variably glutamate positive. A very small proportion of retrograde labelled neurons appeared having the characteristics of globular bushy neurons. Their weak labelling, however, suggests that they project by collaterals or thin axons to the contralateral cochlear nucleus. Spherical bushy cells in the rat anteroventral cochlear nucleus lack the nuclear capping of rough endoplasmic reticulum observed in the cat, and none was labelled after injection into the contralateral cochlear nucleus. Globular and spherical neurons were variably glutamate positive but glycine and GABA negative. In conclusion, the present study suggests that commissural neurons include a small number of strongly labelled large to giant glycinergic and presumably inhibitory type II and, less frequently type I. A large group of less heavily labelled commissural neurons of type I and II contain low levels or no glycine, which is probably used for metabolic purposes rather than as a neurotransmitter. This suggests that these neurons are presumably excitatory.

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A–C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A–C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed. Microsc. Res. Tech. 41:217–223, 1998. © 1998 Wiley-Liss, Inc.

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus.

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A-C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A-C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed.

Endbulb Synapses in the Anteroventral Cochlear Nucleus Express a Specific Subset of AMPA-Type Glutamate Receptor Subunits

The anteroventral cochlear nucleus (AVCN) acts as the first relay center in the conduction of auditory information from the ear to the brain, and it probably performs a crucial role in sound localization. Auditory nerve input to the principal neurons of the AVCN, the spherical bushy cells, appears to be mediated by an excitatory amino acid such as glutamate, which acts at a specialized, large synaptic ending called an endbulb of Held. Presumably, endbulb synapses contain some specific combination of glutamate receptors to facilitate rapid neurotransmission of auditory signals. AMPA glutamate receptor composition at the endbulb synapses was examined with both light and electron microscope immunocytochemistry. Electron microscope localization of AMPA receptors was examined with two techniques, preembedding immunoperoxidase and postembedding immunogold, which provide maximum sensitivity and greatest accuracy, respectively. Dense and frequent labeling was seen with the AMPA receptor subunit antibodies GluR2/3 and GluR4, which were colocalized at the endbulb synapses. In contrast, immunolabeling with antibody to GluR2 was low. These data indicate that the major glutamate receptor at this synapse is an AMPA receptor made up mainly of GluR3 and GluR4 subunits. Receptors composed of these subunits display properties, such as calcium permeability and rapid desensitization, that facilitate their specialized functions in auditory information processing.

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 20/100μm2. 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.

Firing properties of spherical bushy cells in the anteroventral cochlear nucleus of the gerbil

In gerbils, spherical bushy cells (SBCs) encode low frequency sound signals into a temporal firing pattern. To investigate the support for the timing in this temporal code, we characterized the membrane electrical properties of visually identified SBCs in brainstem slices. A brief depolarizing subthreshold transient potential (TP) triggered, with relatively invariant latency, a single spike at the onset of a response to depolarizing current pulses. The activation of a subthreshold Na+-conductance, sensitive to blockade with tetrodotoxin, and a high threshold Ca 2+-conductance, sensitive to blockade with Co 2+ or Cd 2+, accelerated the rising phase and amplified the TP. A K +-conductance, sensitive to blockade by 4-aminopyridine (4-AP, 50 μM), shaped the decay of the TP. Following a single spike, voltage-gated activation of transient and sustained K +-conductances suppressed any tendency to repetitively discharge. A reduction in either K +-conductance due to application of 4-AP or etraethylammonium (TEA, 10 mM), converted the single spike mode to repetitive firing during the depolarizing pulses. A persistent, tetrodotoxin-sensitive Na +-conductance amplified steady-state depolarizing responses. A hyperpolarization-activated conductance, greatly decreased by extracellular Cs + (3 mM) but resistant to Ba 2+ (up to 1 mM), filtered the responses to hyperpolarizing current inputs. A depolarized membrane potential promoted repetitive firing in SBCs. This state, expected in pathophysiological conditions, would corrupt the temporal code.

Ultrastructural analysis of primary endings in deaf white cats: morphologic alterations in endbulbs of Held.

Changes in structure and function of the auditory system can be produced by experimentally manipulating the sensory environment, and especially dramatic effects result from deprivation procedures. An alternative deprivation strategy utilizes naturally occurring lesions. The congenitally deaf white cat represents an animal model of sensory deprivation because it mimics a form of human deafness called the Scheibe deformity and permits studies of how central neurons react to early-onset cochlear degeneration. We studied the synaptic characteristics of the endbulb of Held, a prominent auditory nerve terminal in the cochlear nucleus. Endbulbs arise from the ascending branch of the auditory nerve fiber and contact the cell body of spherical bushy cells. After 6 months, endbulbs of deaf white cats exhibit alterations in structure that are clearly distinguishable from those of normal hearing cats, including a diminution in terminal branching, a reduction in synaptic vesicle density, structural abnormalities in mitochondria, thickening of the pre- and postsynaptic densities, and enlargement of synapse size. The hypertrophied membrane densities are suggestive of a compensatory response to diminished transmitter release. These data reveal that early-onset, long-term deafness produces unambiguous alterations in synaptic structure and may be relevant to rehabilitation strategies that promote aural/oral communication.

Ventral nucleus of the lateral lemniscus in guinea pigs: cytoarchitecture and inputs from the cochlear nucleus.

Cytoarchitectonic criteria were used to distinguish three subdivisions of the ventral nucleus of the lateral lemniscus in guinea pigs. Axonal tracing techniques were used to examine the projections from the cochlear nucleus to each subdivision. Based on the cell types they contain and their patterns of input, we distinguished ventral, dorsal, and anterior subdivisions of the ventral nucleus of the lateral lemniscus. All three subdivisions receive bilateral inputs from the cochlear nucleus, with contralateral inputs greatly outnumbering ipsilateral inputs. However, the relative density of the inputs varies: the ventral subdivision receives the densest projection, whereas the anterior subdivision receives the sparsest projection. Further differences are apparent in the morphology of the afferent axons. Following an injection of Phaseolus vulgaris-leucoagglutinin into the ventral cochlear nucleus, most of the axons on the contralateral side and all of the axons on the ipsilateral side are thin. Thick axons are present only in the ventral subdivision contralateral to the injection site. The evidence from both anterograde and retrograde tracing studies suggests that the thick axons originate from octopus cells, whereas the thin axons arise from multipolar cells and spherical bushy cells. The differences in constituent cell types and in patterns of inputs suggest that each of the three subdivisions of the ventral nucleus of the lateral lemniscus makes a distinct contribution to the analysis of acoustic signals.

Morphological changes in the cochlear nucleus of congenitally deaf white cats

Investigations in animal models and humans have indicated that congenital deafness produces degenerative changes in the central auditory pathway. The cochlear nucleus is the first central structure that receives cochlear input, and may be considered the origin of ascending auditory pathways. In this context, we studied congenitally deaf white cats, who express early onset cochlear receptor loss, in order to assess the nature of structural changes in cells of the cochlear nucleus. It is conceivable that pathologic alterations in higher auditory structures are transneuronally distributed through this nucleus. The cochlear nuclei of nonwhite cats with normal hearing were compared to those of deaf white cats exhibiting hearing loss in excess of 70 dB SPL. The cochlear nuclei of the deaf white cats were smaller in volume by roughly 50%, with the ventral and dorsal divisions being equally affected. Cell body silhouette area was determined for spherical bushy cells of the anteroventral cochlear nucleus (AVCN), pyramidal cells of the dorsal cochlear nucleus (DCN), sensory neurons from the principal trigeminal nucleus, and motoneurons of the facial nucleus. We found no statistical difference in neuronal cell body size between nonauditory neurons of these two groups of cats, whereas auditory neurons of deaf white cats were 30.8–39.4% smaller than those of normal cats. These data imply that neuronal changes in congenitally deaf cats are specific to the auditory pathway. Although cochlear nucleus volume loss was uniform for both divisions, there was a differential effect on cell density: AVCN cell density increased by 40%, whereas DCN cell density was relatively unaffected (10% increase). Astrocyte density was also greater in the AVCN (52%) compared to that in the DCN (5%). These observations reveal a differential impact on cells in the cochlear nucleus to congenital deafness, suggesting selective processing impairment at this level. If similar patterns of degeneration occur in humans, such pathologies may underlie reduced processing of input from cochlear implants in congenitally deaf adults.

Deafness induced cell size changes in rostral AVCN of the guinea pig

The right cochleae of 250–350 g guinea pigs were lesioned by topical administration of neomycin in the middle ear cavity. Eight weeks after the lesion, the cochleae and cochlear nuclei were analyzed. Cochlear hair cell loss was assessed, and cell areas of spherical bushy cells in the rostral anteroventral cochlear nucleus (AVCN) were compared between the lesioned and normal hearing sides for each animal. In five animals with both inner and outer hair cell loss in the lesioned cochlea, the average area of neuronal somata in the rostral AVCN in the lesioned side was 22% smaller than the average area of these cells in the normal hearing side. In two animals with outer hair cell loss but inner hair cells remaining, there was no difference in cell size between the lesioned and non-lesioned AVCN. These results provide evidence that there is significant shrinkage in AVCN cell size in the mature mammal after hearing loss associated with inner hair cell loss.

Enhancement of neural synchronization in the anteroventral cochlear nucleus. I. Responses to tones at the characteristic frequency.

1. Encoding temporal features of the acoustic waveform is an important attribute of the auditory system. Auditory nerve (AN) fibers synchronize or phase-lock to low-frequency tones and transmit this temporal information to cells in the anteroventral cochlear nucleus (AVCN). Phase-locking in the AVCN is usually reported to be similar to or weaker than in the AN. We studied phase-locking in axons of the trapezoid body (TB), which is the output tract of the AVCN, and found, to our surprise, that most TB axons exhibited enhanced synchronization compared with AN fibers. 2. Responses from axons in the TB of the cat were obtained with horseradish peroxidase (HRP)- or Neurobiotin-filled micropipettes or metal microelectrodes. A series of short tone bursts at increasing sound pressure level (SPL) was presented at the characteristic frequency (CF) of the fiber and phase-locking was quantified with the vector strength R at each SPL. For each fiber the maximum R value (Rmax) was then determined. 3. Low-frequency fibers in the TB showed very precise phase-locking: Rmax values could approach 0.99. For the majority of fibers (33/44, 75%) with CF < 700 Hz, Rmax was > or = 0.9 and therefore higher than is ever observed in the AN. We define such fibers as "high-sync." Most of these fibers also entrained to the stimulus, i.e., they fired a precisely timed action potential to almost every stimulus cycle. Some fibers showed perfect entrainment, with maximum discharge rates equaling the stimulus frequency. 4. To exclude the possibility that stimulus paradigms or acoustic and recording equipment were the source of this enhancement, we obtained additional data on low-frequency AN fibers using the same experimental protocol as in our TB experiments. These AN data agree well with published reports. 5. The morphological class of some of the cells studied was identified on the basis of anatomic features revealed by intra-axonal injection of HRP or Neurobiotin. Labeled low-CF axons (N = 7), which were all high-sync, originated from AVCN bushy cells: five were globular and two were spherical bushy cell axons. 6. Spontaneous rate of high-sync fibers covered a range from 0 to 176 spikes/s but were biased toward low values (mean 16 spikes/s). Responses to broadband clicks and sinusoidally amplitude-modulated signals provided additional evidence of improved timing properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Projections of physiologically characterized spherical bushy cell axons from the cochlear nucleus of the cat: Evidence for delay lines to the medial superior olive

Bushy cells in the anteroventral cochlear nucleus (AVCN) receive their principal excitatory input from the auditory nerve and are the primary source of excitatory input to more centrally located brainstem auditory nuclei. Despite this pivotal position in the auditory pathway, details of the basic physiological information being carried by axons of these cells and their projections to more central auditory nuclei have not been fully explored. In an attempt to clarify these details, we have physiologically characterized and anatomically labeled individual axons of the spherical bushy cell (SBC) class of the cat AVCN. The characteristic frequencies (CFs) of our injected SBC population are low, all less than 12 kHz and primarily (83%) less than 3 kHz, while their spontaneous activity is comparatively high (mean of 59 spikes/sec). In response to short tone bursts at CF, low CF (< 1 kHz) SBC units can phase-lock better than auditory nerve fibers. SBCs with CFs above 1 kHz have primary-like responses at all stimulus levels and can show robust phase-locking to an off-CF, 500 Hz tone. When compared with our previously reported population of labeled globular bushy cells (GBC; Smith et al., 1991, J. Comp. Neurol. 304:387-407), some similarities and differences are apparent in both physiological response properties and axonal projection pattern. GBCs show no low frequency bias in CFs, have lower spontaneous rates, and the high CF units exhibit a primary-like-with-notch response at high stimulus levels as a consequence of a very well timed onset component. Low CF, GBC short tone responses are indistinguishable from those of SBCs. Anatomically, the axons of SBCs cross the midline in the dorsal component of the trapezoid body and typically innervate the medial superior olive (MSO) on both sides, the ipsilateral lateral superior olive (LSO), and the contralateral ventral nucleus of the lateral lemniscus (VNLL). The projections to the contralateral, but not the ipsilateral MSO, show a rostral to caudal delay line configuration, similar to the scheme first proposed by Jeffress (1948, J. Comp. Psychol. 41:35-39). The form of this delay line is consistent with the topographic map of interaural time delays reported by Yin and Chan (1990, J. Neurophysiol. 64:465-488). Projections to the ipsilateral LSO often take an indirect route. In contrast, GBC axons travel in the ventral component of the trapezoid body, never innervate the MSO, rarely innervate the ipsilateral LSO, and always innervate the contralateral medial nucleus of the trapezoid body. The terminal specializations of both SBC and GBC axons contain round vesicles.

Growth cones and structural variation of synaptic end-bulbs in the cochlear nucleus of the adult cat brain.

To explore the potential for structural variation and new growth at the synapse, we studied the morphological patterns of the end-bulbs of cochlear nerve axons in adult cats by using rapid Golgi, reduced silver, and electron microscopic methods. Horseradish peroxidase labeling of these endings in the anterior division of the antroventral cochlear nucleus was produced by anterograde transport following injection into the cochlea. Three types of end-bulbs were distinguished, regardless of method: reticular, coalescent, and ringed forms, all synapsing on spherical bushy cells. The reticular variety corresponds to the classically described end-bulb and constitutes the majority in all regions of the tonotopic map. The ringed end-bulb, described here for the first time, forms an excitatory synaptic cuff around the base of a bushy cell's main dendrite; these endings were localized to the region receiving cochlear input in the 1-6 kHz range, which is used in vocalization. The coalescent ending forms a small fraction of the end-bulb population throughout the region studied. The findings raise the possibility of functional differences between these synaptic types. Growth cones and retraction clubs were present on most, if not all, of the end-bulbs in every adult cat studied. A systematic survey of the end-bulb patterns revealed a continuous gradient of variation, in which each synaptic type forms a distinct mode. These findings lead us to hypothesize that the end-bulbs are in a continual state of structural and functional flux. These endings should prove useful for studies on the modifiable properties of central synapses.

Calbindin‐like immunoreactivity in the central auditory system of the mustached bat, Pteronotus parnellii

With the aid of a polyclonal antibody specific for Calbindin D-28k, we studied the distribution of this calcium-binding protein in the central auditory system of the mustached bat, Pteronotus parnelli. Components of the cochlear nucleus (CN) that were calbindin-positive (cabp(+] included the root of the auditory nerve, multipolar and globular bushy cells in the anteroventral CN, multipolar and octopus cells in the posteroventral CN, and small and medium-size cells in the dorsal CN. Not stained were spherical bushy cells of the anteroventral CN and pyramidal/fusiform cells in the dorsal CN. In the superior olivary complex, labeled cells were found in the lateral and medial nuclei of the trapezoid body, the ventral and ventromedial periolivary nuclei, and the anterolateral periolivary nucleus. No cellular labeling was seen in the lateral superior olive. In the medial superior olive, only marginal cells were cabp(+). Labeled fibers could be seen surrounding the gosts of unlabeled cells in both the latter nuclei. Most cells in the intermediate nucleus and the columnar division of the ventral nucleus of the lateral lemniscus were cabp(+). However, the dorsal nucleus was cabp(-). A group of cabp(+) cells was also seen in the paralemniscal zone. The inferior colliculus had a relatively low density of cabp(+) cells. Labeled cells were more common in the caudal half of the central nucleus, and in the external nucleus and dorsal cortex. In the auditory thalamus, nearly every cell in the medial geniculate body was cabp(+), but those in the suprageniculate nucleus and in the posterior group did not stain. Small cells in the intermediate layer and giant cells in the deep layers of the superior colliculus were densely cabp(+). In the pons, cabp(+) cells and neuropil could be seen in the medial and lateral pontine nuclei (pontine gray). In conclusion, calbindin-like immunoreactivity was found in most of the brainstem auditory system, as well as in regions associated with acoustic orientation or control of vocalization. However, except for a minority of cells of the medial superior olive, it is conspicuously absent from the nuclei receiving binaural input below the level of the inferior colliculus.

Synaptic connections of the auditory nerve in cats: Relationship between endbulbs of held and spherical bushy cells

This report focuses on a class of large synaptic endings, the endbulbs of Held. These endings are located in the anteroventral cochlear nucleus and arise from the axons of type I spiral ganglion neurons. Axons were stained with horseradish peroxidase (HRP) using intracellular injections of single fibers or extracellular injections into the auditory nerve. Individual endbulbs or pairs of endbulbs that converged onto the same spherical bushy cell were examined with the aid of a light microscope and subjected to morphometric analyses. Endbulbs of fibers having low spontaneous discharge rates (SR, less than or equal to 18 spikes/sec) have a more complex shape than those of high SR fibers (greater than 18 s/s), a feature represented by systematic differences in endbulb silhouette perimeter without differences in endbulb silhouette area. Consequently, the ratio, silhouette area divided by silhouette perimeter, yields a "form factor" separating endbulbs of high SR from those of low SR. High SR fibers had ratios greater than 0.52 (mean = 0.63 +/- 0.09), whereas low SR fibers had ratios less than 0.52 (mean = 0.45 +/- 0.06). Pairs of endbulbs with unknown physiological properties had similar form factor values, despite the wide range of values observed in the endbulb population. These data imply that endbulbs converging upon the cell body of a spherical bushy cell arise from fibers of the same SR group. Electron microscopic examination was conducted on the endbulb of one physiologically characterized and intracellularly stained auditory nerve fiber (CF = 1.4 kHz; SR = 55 s/s) and its unstained endbulb mate with the aid of serial ultrathin sections. In addition to the well-known axosomatic synapses, these endbulbs formed axodendritic synapses: 11.7% for the HRP-labeled endbulb and 13.3% for the unlabeled endbulb. The axodendritic synapses appear to occur on dendrites of nearby spherical bushy cells and may represent a mechanism whereby single endbulbs can disperse activity to multiple neurons in the cochlear nucleus. We propose that axosomatic synapses preserve fiber SR groupings, whereas axodendritic synapses may not.