Fine structure and neurotransmitter cytochemistry of neurons in the rat ventral cochlear nucleus projecting to the ipsilateral dorsal cochlear nucleus

The neural tracer wheat germ agglutinin conjugated to horse radish peroxidase was injected into the rat dorsal cochlear nucleus and acoustic stria. Some labelled neurons in the ipsilateral ventral cochlear nucleus were found as a result. These neurons were studied at the ultrastructural level, and their axo-somatic synaptic profile and glycine immunoreactivity were determined. Most neurons were glycine negative and classified as type I multipolar neurons. The latter showed a different synaptic profile from that of neurons projecting to the contralateral inferior colliculus or cochlear nucleus. This suggests the presence of differing populations of multipolar cells based on their synaptic profile. Few labelled multipolar neurons of type II were found, which appeared glycine negative and, rarely, glycine positive. The latter show an ultrastructure and axo-somatic profile similar to that of glycinergic commissural neurons in the dorsal and ventral cochlear nucleus. In particular, about one-third of boutons contained round synaptic vesicles, which are believed to contain an excitatory neurotransmitter. The ultrastructural analysis of the synaptic boutons in the cochlear nucleus confirms the presence of numerous cases of colocalization of glycine and GABA where flat and pleomorphic synaptic vesicles are mixed. The present study is in accordance with previous tract-tracing light microscopic studies which have indicated that large glycinergic neurons in the ventral cochlear nucleus act as broad-band inhibitory neurons in microcircuits of the dorsal cochlear nucleus and contralateral cochlear nucleus.     

Cytology, synaptology and immunocytochemistry of commissural neurons and their putative axonal terminals in the dorsal cochlear nucleus of the rat

The first binaural integration within the auditory system responsible for sound localization depends upon commissural neurons that connect the two symmetrical cochlear nuclei. These cells in the deep polymorphic layer of the rat dorsal cochlear nucleus were identified with the electron microscope after injection of the retrograde tracer, Wheat Germ Agglutinin conjugated to Horseradish Peroxydase, into the contralateral cochlear nucleus. Commissural neurons are multipolar or bipolar with an oval to fusiform shape. Few commissural neurons, most inhibitory but also excitatory, connect most of the divisions of the rat cochlear nuclei. The most common type is a glycinergic, sometimes GABAergic, moderately large cell. Its ergastoplasm is organized into peripheral stacks of cisternae, and few axo-somatic synaptic boutons are present. Another type of commissural neuron is a medium-sized, spindle-shaped cell, glycine and GABA-negative, with sparse ergastoplasm and synaptic coverage. A giant, rare type of commissural neuron is glycine-positive and GABA-negative, with short peripheral stacks of ergastoplasmic cisternae. It is covered with synaptic boutons, many of which contain round synaptic vesicles. Another rare type of commissural neuron is a moderately large cell, oval to fusiform in shape, immunonegative for both glycine and GABA, and contacted by many axo-somatic boutons. It contains large dense mitochondria and numerous dense core vesicles of peptidergic type. Some labelled boutons, mostly inhibitory and probably derived from commissural neurons, contact pyramidal, cartwheel, giant and tuberculo-ventral neurons. The prevalent inhibition of electrical activity in a cochlear nucleus observed after stimulation of the contralateral cochlear nucleus may be due to commissural inhibitory terminals which contact excitatory neurons such as pyramidal and giant cells. Other inhibitory commissural terminals which contact inhibitory neurons such as cartwheel and tuberculo-ventral neurons, may explain the stimulation of electrical activity in the DCN after contralateral stimulation.     

Ultrastructural immunocytochemistry for glycine in neurons of the dorsal cochlear nucleus of the guinea pig

Neurons in the dorsal cochlear nucleus of the guinea pig were classified according to their positivity to the inhibitory neurotransmitter glycine, ultrastructure and projections to the inferior colliculus as indicated by tract-tracing and ultrastructural immunocytochemistry. Only some pyramidal and few giant cells, surrounded by glycinergic boutons containing flat and pleomorphic vesicles, projected to the inferior colliculus as glycine-negative excitatory cells. Smaller neurons in superficial layers of the dorsal cochlear nucleus did not project to the inferior colliculus, and were recognized as glycine-negative granule and unipolar brush cells. Few glycinergic, inhibitory neurons among granule cells were indicated as Golgi-stellate neurons. All small neurons associated to the granule cell areas received few, mainly glycinergic synapses, and their dendrites contacted large boutons (mossy fibers). Other medium-large glycine positive neurons in the superficial (cartwheel) and deep layers (tuberculo-ventral and large-giant) of the dorsal cochlear nucleus did not project to the inferior colliculus. Giant-large glycinergic neurons surrounded by sparse axo-somatic, mostly glycinergic synapses, probably represent commissural neurons projecting to the contralateral cochlear nucleus. Rare boutons, possibly descending from the inferior colliculus, were seen onto pyramidal cells or their dendrites, and these boutons mainly stored glycine positive pleomorphic vesicles or glycine negative round vesicles. No descending mossy fibers storing round vesicles were labelled from the central nucleus of the inferior colliculus. These observations suggest that very few terminals in the dorsal cochlear nucleus of the guinea pig are derived from the inferior colliculus.     

Putative inhibitory collicular boutons contact large neurons and their dendrites in the dorsal cochlear nucleus of the rat

Within the circuits of the acoustic nuclei, the inferior colliculus sends descending (collicular) terminals to control with a feedback mechanism, part of the activity of the dorsal cochlear nucleus (DCN). It is not known whether this descending projection is prevalently excitatory or inhibitory. Using the neuronal tracer Wheat Germ Agglutinin conjugated to Horse Radish Peroxidase (WGA-HRP) the connections between the inferior colliculus and the DCN of the rat have been investigated. By far most retrograde labelled large neurons were glycine and GABA negative (pyramidal and giant neurons) and rare medium-size cells were glycine positive. The ultrastructural immunocytochemical analysis for glycine and GABA shows that mainly large, excitatory, neurons innervate the inferior colliculus. Rare medium-size glycine-positive cells with intermediate characteristics between pyramidal and cartwheel cells, seem also to project to the colliculus. Few WGA-HRP labelled boutons contact the large cells or their dendrites, have symmetric pre- and post-synaptic thickenings, contain pleomorphic and/or flat vesicles, and are labelled for GABA or glycine. Since no GABA labelled cells in both the dorsal and ventral cochlear nucleus were retrograde labelled from the colliculus, the source of these intrinsic anterograde labelled boutons must be external to the cochlear nucleus. GABA positive neurons are both present in the inferior colliculus (injected with the tracer) and superior olivary complex (not injected with the tracer). This suggests that the double labelled boutons (WGA-HRP and GABA) are inhibitory GABA-ergic collicular terminals contacting the excitatory neurons of the DCN. Other few boutons or mossy fibers containing round vesicles and immunonegative for both glycine and GABA, were also seen contacting the large neurons and their dendrites in the DCN. As the round vesicles boutons may be derived from other retrograde cells of the cochlear nucleus (pyramidal and stellate cells) and those glycine positive from the glycinergic neurons in paraolivary nuclei, it is more likely that only the WGA-HRP and GABA labelled boutons are true collicular terminals.     

Ultrastructure and immunocytochemical characteristics of cells in the octopus cell area of the rat cochlear nucleus: comparison with multipolar cells

Cells in the octopus cell area of the rat ventral cochlear nucleus have been connected to the monaural interpretation of spectral patterns of sound such as those derived from speech. This is possible by their fast onset of firing after each octopus cell and its dendrites have been contacted by many auditory fibres carrying different frequencies. The cytological characteristics that make these large cells able to perform such a function have been studied with ultrastructural immunocytochemistry for glycine, GABA and glutamate, and compared to that of other multipolar neurons of other regions of the ventral cochlear nucleus. Cells in the octopus cell area have an ultrastructure similar to large-giant D-multipolar neurons present in other areas of the cochlear nucleus, from which they differ by the presence of a larger excitatory axo-somatic synaptic input and larger mitochondria. Octopus cells are glycine and GABA negative, and glutamate positive with different degree. Large octopus cells receive more axo-somatic boutons than smaller octopus cells. Fusiform octopus cells are found sparsely within the intermediate acoustic striae. These cells are large to giant excitatory neurons (23-35 microm) with 62-85% of their irregular perimeter covered with large axo-somatic synaptic boutons. Most boutons contain round vesicles and are glycine and GABA negative but glutamate positive. The latter excitatory boutons represent about 70% of the input to octopus cells. Glycine positive boutons with flat and pleomorphic vesicles account for 9-10% of the input while GABA-ergic boutons with pleomorphic vesicles represent about 20% of the synaptic input. Other few, multipolar cells within the rat octopus cell area are surrounded by more inhibitory than excitatory terminals which contain flat and pleomorphic vesicles, a feature distinctive from that of true octopus cells. The latter resemble multipolar cells seen outside the octopus cell area that project to the contralateral inferior colliculus and cochlear nucleus. Based on this study, two types of large multipolar cells are present in the octopus cell area: 1) those that receive about 70% of axo-somatic R boutons and stain more intensely for glutamate may correspond to pure onset neurons (Oi); 2) those with less than 33% of R axosomatic boutons, with less immunoreactivity to glutamate and sometimes glycine positive may represent the onset chopper neurons (Oc). In the octopus cell area the first type appears more prevalent. The present study suggests that octopus cells are a special type of excitatory D-multipolar neuron confined to the octopus cell area and mainly innervated by glutamatergic cochlear nerve terminals.     

Mossy fibers in granule cell areas of the rat dorsal cochlear nucleus from intrinsic and extrinsic origin innervate unipolar brush cell glomeruli

Non tonotopic transmission between cochlear nuclei and other auditory and non-auditory nuclei in the brain is probably due to large axonal terminals (mossy fibers) innervating granule cell areas of cochlear nuclei. The origin of mossy fibers in the dorsal cochlear nucleus (DCN) is multiple, from other auditory or non-auditory nuclei but possibly also from intrinsic neurons. The present ultrastructural immunocytochemical study reports for the first time the presence of anterograde-labeled mossy fibers in the DCN of the rat after injection of the neural tracer WGA-HRP into 3 different nuclei. Labeled mossy fibers were seen in 9.0% of mossy fibers detected after tracer injection into the ipsilateral anteroventral cochlear nucleus, in 7.3% of mossy fibers after contralateral collicular injection, and 13.2% after contralateral cochlear nucleus injection. Most (over 95%) mossy fibers contained round vesicles, both large and small, and were likely excitatory terminals, but few showed flat-pleomorphic vesicles that contained the inhibitory neurotransmitters GABA and glycine. Most of the anterograde-labeled ipsilateral mossy fibers containing small round synaptic vesicles, are probably derived from multipolar neurons within the ipsilateral anteroventral cochlear nucleus. After injections into the contralateral inferior colliculus, it was not possible to distinguish putative descending collicular mossy fibers from intrinsic mossy fibers. The latter would suggest the presence of an amplification pathway within the DCN, from collateral axons of pyramidal or stellate cells of the ipsilateral ventral cochlear nucleus to form glomeruli with granule-unipolar brush cells. After injection into the contralateral cochlear nucleus, it was not possible to distinguish between commissural mossy fibers and those derived from ipsilateral recurrent axon-terminals of commissural neurons within the DCN or the ventral cochlear nucleus. Despite this limitation, the present observations show that extrinsic or intrinsic mossy fibers reach granule cell areas in layers 2 and 3 of the DCN and form glomeruli of large or small dimension (1.5-4 microm) with unipolar brush and granule cells. These mossy fibers probably carry a fast excitatory non-tonotopic input which may influence the electrical response of granule cell areas.     

Multipolar cells in the ventral cochlear nucleus project to the dorsal cochlear nucleus and the inferior colliculus

When retrograde markers are placed in the dorsal cochlear nucleus two classes of labeled cells are found in the ventral cochlear nucleus. These are multipolar cells and granule cells. The structure and distribution of labeled multipolar cells greatly resemble those seen following injection of retrograde markers into the contralateral inferior colliculus. When one retrograde marker is placed in the dorsal cochlear nucleus and another simultaneously placed into the contralateral inferior colliculus, large numbers of multipolar cells containing both markers are found in the ventral cochlear nucleus. These findings show that all or most cells in the ventral cochlear nucleus that project to the inferior colliculus also send collaterals to the ipsilateral dorsal cochlear nucleus.     

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations.

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neuron morphology and synaptic architecture in the medial superior olivary nucleus

Summary Dendritic arborization pattern, spatial and synaptic relations of various neuron types and the terminal distribution of afferent axons of various origin were studied in the medial superior olivary nucleus of the cat using Golgi, degeneration, electron microscope and horseradish peroxidase techniques. Three types of neurons clearly different in morphological features, distribution, neighbourhood relations, input and output characteristics were distinguished: (1) fusiform cells having specific dendritic orientations and arborization patterns and synaptic relations to various types of terminal axon arborizations (2) multipolar neurons with wavy dendrites bearing spine-like appendages, receiving relatively few synaptic contacts and having a locally arborizing axon, and (3) elongated marginal cells, largely restricted to the fibrous capsule of the nucleus. The fusiform and marginal neurons were identified by retrograde peroxidase labeling as the olivo-collicular projection cells.Ultrastructural analysis of normal and experimental material revealed the presence of four distinct kinds of axon terminals differing in size, synaptic vesicles type, relation to postsynaptic targets and in origin: (i) large terminals with multiple extended asymmetric synaptic membrane specializations and containing round, clear vesicles arise from the spherical cells of the ipsilateral anteroventral cochlear nucleus, (ii) most of the small axon terminal profiles — engaged in asymmetric synaptic contacts — originated from the trapezoid nucleus, (iii) terminal boutons containing pleomorphic vesicles belong to fibers descending from the ipsilateral multipolar neurons in the central nucleus of the inferior colliculus and from the nuclei of the lateral lemniscus while (iv) boutons containing exclusively ovoid vesicles and remaining intact after complete deafferentation of the nucleus were considered to be of local origin.     

Desending and intrinsic inputs to dorsal cochlear nucleus of cats: A horseradish peroxidase study

Horseradish peroxidase was injected unilaterally into the dorsal cochlear nucleus of adult cats in efforts to find neurons innervating the dorsal cochlear nucleus from (1) higher auditory nuclei or (2) other subdivisions of the cochlear nucleus. Following horseradish peroxidase injections and short survival periods, reactive neurons were most common in the dorsal and ventral nuclei of the lateral lemniscus and in the superior olivary complex of both sides of the brain stem. In the superior olivary complex, most neurons of the medial segment and border cells of the lateral segment reacted as did periolivary cells of the ventrolateral, dorsomedial, and preolivary areas, but not in the medial nucleus of the trapezoid body. Hilus neurons of the lateral superior olive reacted contralateral to the injection site. Although inferior colliculus neurons contained lightly stained granules bilaterally, more reactive neurons (including unusually large tripolar neurons) contained heavily stained granules in the contralateral colliculus. Intrinsic reactive neurons mainly included ipsilateral octopus cells, multipolar neurons of the nerve root regions, and stellate cells of the more rostra] anteroventral cochlear nucleus. All findings were confirmed by comparison to control animals.Our findings of specific neuronal types projecting to the cat dorsal cochlear nucleus suggest a relatively greater input from the nuclei of the lateral lemnisci of both sides than previously believed. Also, our results showed an unusually heavy input from the nearby superior olivary complex to the dorsal cochlear nucleus as well as inputs from specific cell types of the ipsilateral antero- and postero-ventral cochlear nucleus. By correlating these findings with those of other types of studies, we concluded that (1) too much emphasis has been placed upon inputs to the dorsal cochlear nucleus from the inferior colliculus relative to the descending pontine inputs and that (2) a new circuit involving the ventral cochlear nucleus, the dorsal cochlear nucleus and the medial superior olive may provide binaural information to large dorsal cochlear nucleus cells that terminate in their own unique areas of higher auditory nuclei.     

Laminar inputs from dorsal cochlear nucleus and ventral cochlear nucleus to the central nucleus of the inferior colliculus: two patterns of convergence

The central nucleus of the inferior colliculus is a laminated structure composed of oriented dendrites and similarly oriented afferent fibers that provide a substrate for tonotopic organization. Although inputs from many sources converge in the inferior colliculus, how axons from these sources contribute to the laminar pattern has remained unclear. Here, we investigated the axons from the cochlear nuclei that terminate in the central nucleus of the cat and rat. After characterization of the best frequency of the neurons at the injection sites in the cochlear nucleus, the neurons were labeled with dextran in order to visualize their axons and synaptic boutons in the central nucleus. Quantitative methods were used to determine the size and distribution of the boutons within the laminar organization. Two components in the laminae were identified: (1) a narrow axonal lamina that included the largest fibers and largest boutons; (2) a wide axonal lamina, surrounding the narrow lamina, composed of thin fibers and only small boutons. The wide lamina was approximately 30-40% wider than the narrow lamina, and it often extended more than 100 microm beyond the larger boutons on each side. The presence of both thick and thin fibers within the acoustic striae following these injections suggests that large and small fibers/boutons within these bands may originate from different neuronal types in the dorsal and ventral cochlear nucleus. We conclude that the narrow laminae that contain large fibers and boutons originate from larger cell types in the cochlear nucleus. In contrast, the wide lamina composed exclusively of small boutons may represent an input from other, perhaps smaller neurons in the cochlear nucleus. Thus, two types of inferior colliculus laminar structures may originate from the cochlear nucleus, and the small boutons in the wide laminae may contribute a functionally distinct input to the neurons of the inferior colliculus.     

Uptake and retrograde transport of [3H]GABA from the cochlear nucleus to the superior olive in the guinea pig.

The purpose of the present study is to determine which descending projections to the cochlear nucleus may use gamma-aminobutyric acid (GABA) as a neurotransmitter. [3H]GABA (120 microM) was injected into the cochlear nucleus of albino and pigmented guinea pigs. After survival times between 0.25 and 16 h, the brain stems were prepared for light microscopic autoradiography. After 2 h survival there was a pulse of label, which progressed through the fibres from the cochlear nucleus to the ipsilateral superior olive. After 5 h, retrogradely labelled neuronal cell bodies and fibres were located in the superior olivary complex bilaterally. In the trapezoid body, clusters of labelled cells were seen in the lateral nucleus, ipsilaterally, and in the ventral nucleus, bilaterally. Also there were labelled cells in the ipsilateral dorsal and anterolateral periolivary nucleus. Large and small cells of several types were labelled. Survival times of 10 h or more resulted in very light, diffuse labelling. Projections to the cochlear nucleus labelled by retrograde transport of horseradish peroxidase that did not take up [3H]GABA included the inferior colliculus, bilaterally, and the cochlear nucleus and periolivary nuclei (other than ventral trapezoid nucleus), contralaterally. The selective labelling of cell groups in the superior olive with the moderately low concentration of [3H]GABA used is consistent with the high-affinity uptake of [3H]GABA by synaptic endings in the cochlear nucleus and its retrograde by transport GABA-ergic neurons. This provides evidence for a descending projection system for inhibitory feedback from the superior olive to the cochlear nucleus.     

Quantitative and ultrastructural study of ascending projections to the medial mammillary nucleus in the rat

Summary We analyzed the termination pattern of axons from the superior central nucleus and the ventral tegmental nucleus of Gudden within the medial mammillary nucleus (MM) in the rat. The neuropil of the MM consists of two classes of terminals, that is, terminals containing round synaptic vesicles and forming asymmetric synaptic contact, and terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact. The number of axodendritic terminals with round vesicles is almost equal to that of terminals with pleomorphic vesicles. Almost all axosomatic terminals contain pleomorphic vesicles with symmetric synaptic contact. Injection of WGA-HRP into the central part of the superior central nucleus permitted ultrastructural recognition of many anterogradely labeled terminals within the median region of MM. The labeled terminals contacted mainly intermediate (1–2 m diameter) and proximal dendrites (more than 2 m diameter) as well as the neuronal somata. Serial ultrathin sections of neurons of the median region of the MM revealed that 37% of the axosomatic terminals were labeled anterogradely. The pars compacta of the superior central nucleus had reciprocal connections with the median region of MM. The axon terminals from this nucleus occupied 53% of axosomatic terminals, and contacted mainly intermediate dendrites. Following injection of WGA-HRP into the ventral tegmental nucleus, many labeled terminals were found in the medial and lateral regions of MM. They contacted mainly intermediate dendrites as well as neuronal somata. In the medial region, 78% of axosomatic terminals contacting retrogradely labeled neurons were labeled anterogradely. All labeled terminals from these nuclei contained pleomorphic vesicles, and made symmetric synaptic contact.     

Structural and functional classes of multipolar cells in the ventral cochlear nucleus

Multipolar cells in the ventral cochlear nucleus (VCN) are a structurally and functionally diverse group of projection neurons. Understanding their role in the ascending pathway involves partitioning multipolar cells into distinct populations and determining where in the brain each sends its coded messages. In this study, we used retrograde labeling techniques in rats to identify multipolar neurons that project their axons to the ipsilateral dorsal cochlear nucleus (DCN), the contralateral CN, or both structures. Three rats received injections of biotinylated dextran amine in the ipsilateral DCN and diamidino yellow in the contralateral CN. Several radiate multipolar neurons (defined by their axonal projections to the ipsilateral DCN and their dendrites that traverse VCN isofrequency sheets) were double-labeled but over 70% were not. This result suggests two distinct populations: (1) radiate-commissural (RC) multipolar cells that project to the ipsilateral DCN and the contralateral CN, and (2) radiate multipolar cells that project exclusively (in this context) to the ipsilateral DCN. In a different group of animals, we retrogradely labeled multipolar neurons that project their axons to the contralateral CN and measured the size of their cell bodies. The mean size of this population (266 +/- 156 microm2) was significantly smaller than those of RC-multipolar cells (418 +/- 140 microm2). We conclude that the CN commissural pathway is composed of at least two components: (1) RC multipolar cells and (2) commissural multipolar cells that are small- and medium-sized neurons that project exclusively (in this context) to the contralateral CN. These results identify separate structural groups of multipolar cells that may correspond to physiological unit types described in the literature. They also provide protocols for isolating and studying different populations of multipolar cells to determine the neural mechanisms that govern their responses to sound.     

Time of origin of neurons of the rat inferior colliculus and the relations between cytogenesis and tonotopic order in the auditory pathway

Summary Groups of pregnant rats were injected with two successive daily doses of ³H-thymidine from gestational day 12 and 13 (E12+13) until the day before parturition (E21+22) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific embryonic days was determined quantitatively in six vertical strips of the inferior colliculus. It was established that the neurons of the inferior colliculus are produced between days E14 and the perinatal period in an orderly sequence: the earliest generated cells are situated rostrally, laterally and ventrally in the principal nucleus, the latest generated cells are situated caudally, medially and dorsally in the pericentral nucleus. This cytogenetic gradient suggested that the cells are produced dorsally in the caudal recess of the embryonic aqueduct and are deployed in an outsidein pattern.This study has brought to a conclusion our datings of neuron production in the central auditory pathway of the rat. The results revealed that in those structures in which a cytogenetic gradient could be recognized, the orientation of this gradient and the regional tonotopic order (demonstrated mostly in species other than the rat) tended to be aligned. Moreover, with the exception of the medial trapezoid nucleus and the dorsal nucleus of the lateral lemniscus (which receive contralateral input from the cochlear nuclei), sites with early-produced neurons correlated with units responding preferentially to high frequency tones and vice versa. This suggested that the orderly production of neurons within different components of the auditory system is a factor in their subsequent topographic organization. A comparison of the temporal order of neuron production in different components of the auditory pathway suggested that the establishment of orderly topographic relations between some of the structures (e.g., the medial geniculate body and the primary auditory cortex) takes place before this spatial relationship could be specified as a cochleotopic order.Abbreviations ab cochlear nerve, ascending branch - Ai aqueduct, inferior collicular recess - AI primary auditory cortex - bi brachium of inferior colliculus - c caudal - CE cerebellum - CI central nucleus, inferior colliculus - CNa anteroventral cochlear nucleus - CNd dorsal cochlear nucleus - CNp posteroventral cochlear nucleus - d dorsal - db cochlear nerve, descending branch - DI diencephalon - ds dorsal acoustic stria (stria of Monakow) - DM dorsomedial nucleus, inferior colliculus - EX external nucleus, inferior colliculus - IC inferior colliculus - is intermediate acoustic stria (stria of Held) - l lateral - LD dorsal nucleus of lateral lemniscus - ll lateral lemniscus - LV ventral nucleus of lateral lemniscus - m medial - ME medulla - MG medial geniculate body - MS mesencephalon - PC pericentral nucleus, inferior colliculus - PR principal nucleus, inferior colliculus - py pyramidal cells, dorsal cochlear nucleus - r rostral - SOl lateral superior olivary nucleus - SOm medial superior olivary nucleus - TRl lateral trapezoid nucleus - TRm medial trapezoid nucleus - v ventral - VL lateral ventricle - vs ventral acoustic stria (trapezoid body) - V3 third ventricle - VIIIn cochlear nerve     

Descending projections from the inferior colliculus to the dorsal cochlear nucleus in the cat: An autoradiographic study

Descending auditory projections from different subdivisions of the inferior colliculus to the dorsal cochlear nucleus were investigated in experiments using the autoradiographic technique. Tritiated leucine injections confined to the pericentral nucleus of the inferior colliculus resulted in the appearance of dense grain clusters distributed over the outer fusiform cell and molecular layers of the ipsilateral dorsal cochlear nucleus. The pattern and distribution of dense grain clusters strongly resembled the central terminals of glomeruli described previously in the dorsal cochlear nucleus. Injections into the dorsal region of the central nucleus of the inferior colliculus led to a more diffuse distribution of grains over the middle and outer fusiform cell layer and over the innermost molecular layer of the dorsal cochlear nucleus on both sides. Dense grain clusters were also evident after these injections but they appeared to result from the concomitant injection into the overlying pericentral nucleus. Finally, injections of tritiated leucine into the ventral region of the central nucleus of the inferior colliculus (which included some cells of the dorsal nucleus of the lateral lemniscus) resulted in the heaviest labelling of the dorsal cochlear nucleus. Grains were distributed perisomatically and peridendritically around fusiform cells of the fusiform cell layer and giant cells of the deep dorsal cochlear nucleus on both sides.The results indicate that the pericentral nucleus and the more dorsal region of the central nucleus of the inferior colliculus establish overlapping connections with the outermost fusiform cell and molecular layers of the dorsal cochlear nucleus. Both sets of connections seem to be made principally with interneurons through glomerular and other inputs to scattered small cells which exist in these laminae. Since cortical and thalamic descending fibers directly innervate only the most dorsal regions of the inferior colliculus, it may be that this region of the tectum selectively mediates activity from these higher auditory centers. Such centers may indirectly influence fusiform cell response properties through collicular inputs to small cells of the dorsal cochlear nucleus that contact fusiform cells. A more substantial and direct projection was shown to arise from the ventral region of the inferior colliculus to innervate both the fusiform and giant cells. As such, the descending connections from the ventral inferior colliculus may be more likely to influence directly the output of both the fusiform and giant cells and, therefore, the projection of auditory information from the dorsal cochlear nucleus to higher levels.     

Effect of monaural and binaural stimulation on cytoplasmic RNA content in cells of the central nucleus of the cat inferior colliculus

A cytophotometric study of sections stained with gallocyanin and chrome alum showed that monaural stimulation for 2 h and binaural stimulation for 1.5 h with rhythmic noise signals led to a marked increase in the cytoplasmic RNA content per cell in the principal and large multipolar neurons of the dorsal and ventral parts of the ventrolateral region of the central nucleus of the inferior colliculus. The increase in cytoplasmic RNA content in the principal cells of the ipsiand contralateral parts of this nucleus relative to the stimulated ear in the case of monaural stimulation and the increase in RNA content in response to binaural stimulation suggests a uniform distribution of bilaterally converging connections from the lower nuclei of the auditory system on the principal cells. The increase in cytoplasmic RNA in the large multipolar cells of the contralateral central nucleus in response to monaural stimulation is evidence of the predominantly contralateral projection to these cells. The results are evidence of convergence of binaural influences on the principal and large multipolar cells of the central nucleus of the inferior colliculus.Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 598–605, November–December, 1978.     

Electron microscopic investigation of the vestibular projection to the cat trochlear nuclei

Ultrastructural degeneration studies were carried out on the cat trochlear nucleus following lesion of the vestibulo-trochlear pathway in order to characterize the location and type of presynaptic endings involved in this pathway. Four types of boutons are found in the normal trochlear nucleus. Types I and II are large and demonstrate typical en passant profiles with small diameter synaptic vesicles (35 and 40 nm). These terminals are characterized by the absence of neurofilaments in the Type II endings. Types III and IV are smaller boutons, located more axondendritically, and contain larger diameter synaptic vesicles (45 nm). Type V terminals contain large, granulated vesicles and occur only rarely. Following the interruption of the ascending projection from the ipsilateral superior and medial vestibular nuclei by parasagittal medullary lesions, degeneration of Type II boutons was commonly encountered in the ipsilateral trochlear nucleus. Predominantly Type III degeneration was found in the contralateral trochlear nucleus. Electrical stimulation of the vestibular nerve showed that these lesions resulted in (1) a complete loss of inhibition in the ipsilateral trochlear nucleus and (2) a significant (75-90%) reduction in the contralateral excitatory pathway to the trochlear nucleus. Midline sagittal lesions in the floor of the fourth ventricle interrupting the decussating fiber projection from the bilateral medial vestibular nuclei resulted in selective degeneration of only Type III boutons in both trochlear nuclei. We conclude that inhibitory vestibular neurons eminating from the superior vestibular nucleus terminate on trochlear motoneurons with Type II boutons and excitatory vestibular neurons from the contralateral medial vestibular nucleus end on trochlear motoneurons with Type III boutons.     

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey.

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.