The ultrastructure of GABA-immunoreactive vestibular commissural neurons related to velocity storage in the monkey.

The purpose of the present study was to visualize the synaptic interactions of GABAergic neurons involved in the mediation of velocity storage. In the previous report, ultrastructural studies of degenerating neurons were conducted following midline section of rostral medullary commissural fibers with subsequent behavioral testing. The midline lesion caused functionally discrete damage to the velocity storage component, but not to the direct pathway, of the angular vestibulo-ocular reflex, and the degenerating neurons were interpreted as potential participants in the velocity storage network. We concluded that at least some of the commissural axons mediating velocity storage originate from clusters of neurons in the lateral crescents of the rostral medial vestibular nucleus. In the present report, immunocytochemical evidence is presented that many vestibular commissural neurons, putatively involved in mediating velocity storage, are GABAergic. These cells have large nuclei, small round or narrow tubular mitochondria, occasional cisterns and vacuoles, but few other organelles. Their axons are thinly-myelinated, and terminate in boutons containing mitochondria of similar ultrastructural appearance and a moderate density of round/pleomorphic synaptic vesicles. Such terminals often form axoaxonic synapses, and less frequently axodendritic contacts, with non-GABAergic elements. On the basis of the present results, we conclude that a portion of the commissural neurons of the velocity storage pathway is GABAergic. The observation of GABAergic axoaxonic synapses in this pathway is interpreted as a structural basis for presynaptic inhibition of medial vestibular nucleus circuits by velocity storage-related commissural neurons. Conversely, substantial ultrastructural evidence for postsynaptic inhibition of non-GABAergic commissural cells argues for a dual role for GABAergic terminals mediating velocity storage: presynaptic inhibition of non-GABAergic vestibular cells by GABAergic velocity storage commissural axons, and postsynaptic inhibition of non-GABAergic velocity storage cells by GABAergic axons. Both pre- and postsynaptic inhibitory arrangements could provide the morphologic basis for disinhibitory activation of the velocity storage network within local neuronal circuits.     

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.     

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 study of vestibular and reticular projections to the abducens nucleus

Summary The origin of the synaptic boutons in the abducens nucleus was studied following lesions of the contralateral medial vestibular nucleus, the ipsilateral paramedian pontine reticular formation and the contralateral dorsomedial part of the reticular formation caudal to the abducens nucleus.Lesions in the rostral part of the contralateral medial vestibular nucleus resulted in degeneration of boutons located mainly on dendritic processes. On the other hand, lesions in both ipsilateral and contralateral reticular formations provoked degenerating terminals on the somata of the abducens neurones and on proximal dendrites in the abducens nucleus beneath the genu of the facial nerve.This work was supported by INSERM (U6), CNRS (GR 45), and grant DGRST 78.7.3017     

Two groups of secondary vestibular neurons mediating horizontal canal signals, probably to the ipsilateral medial rectus muscle, under inhibitory influences from the cerebellar flocculus in rabbits

Vestibular nuclear neurons that mediate horizontal canal signals to the ipsilateral medial rectus motoneurons were explored in anesthetized and decerebrate rabbits. These neurons were identified by four criteria: (1) they were activated monosynaptically by ipsilateral vestibular nerve stimulation and (2) antidromically from the oculomotor nucleus region, while they were inhibited by (3) direct floccular stimulation and (4) ipsilateral retinal stimulation that activated floccular Purkinje cells via a climbing fiber afferent pathway. Neurons fulfilling these criteria were found in two anatomically different regions, i.e. the rostrolateral part of the medial vestibular nucleus and in the ventral part of the lateral vestibular nucleus. In decerebrate rabbits, neurons in both loci responded to horizontal rotation of the whole body with the type I pattern (excited by ipsilateral rotation). These results suggest that horizontal canal signals are conveyed to ipsilateral medial rectus motoneurons by two separate groups of vestibular nuclear neurons which may play different roles in the vestibulo-ocular reflex.     

Projections of the temporo-parietal cortex on vestibular complex in the macaque monkey (Macaca fascicularis)

Summary Connections of the posterior parietal cortex (area 7) with the vestibular complex have been studied in 4 macaque monkeys by anterograde axonal transport methods. WGA-HRP and tritiated amino-acids have been injected in the posterior part of area 7 including the caudal end of the superior bank of superior temporal sulcus and the lateral sulcus. Labeled terminals were observed in the vestibular nuclei complex and distributed bilaterally with a greater ipsilateral contribution. Two main groups of area 7 efferences were found to project to vestibular complex: a) A first group terminates on vestibular nuclei (the inferior vestibular nucleus and the caudal part of the medial nucleus) mainly connected with cerebello-spinal system, b) A second group terminates on vestibular nuclei (the medial and the superior vestibular nuclei and the y group) mainly involved in vestibulo-ocular mechanisms. The prepositus hypoglossi nucleus has also been found to receive area 7 projections. It is concluded that the possible control played by area 7 on the vestibulo-ocular reflex might be exerted through these direct cortico-vestibular projections.     

Commissural and intrinsic connections of the vestibular nuclei in the rabbit: a retrograde labeling study

Summary The intrinsic and commissural projection of the vestibular nuclei were investigated by means of retrograde transport of normal (HRP) and wheatgerm-agglutinated horseradish peroxidase (WGA-HRP). It was found that within each vestibular complex, the superior (SV), medial (MV) and descending (DV) vestibular nuclei are reciprocally connected. A rostrocaudally oriented column of medium-sized and large neurons, comprising the central SV and the magnocellular MV (MVmc) receives input from the surrounding neurons and does not reciprocate this projection. Efferents from group y terminate in the SV, MV and DV. The infracerebellar nucleus (INF) as well as the interstitial nucleus of the VIII the nerve (IN) supply fibers to the MV and DV. The neurons that participate in the commissural projection are distributed throughout the vestibular complex with the exception of the lateral vestibular nucleus (LV) and group x. The largest number of cells was found in the MV. The HRP labeled cells show a tendency to cluster into rostrocaudally oriented groups. Each nucleus projects to more than one contralateral nucleus. Group y shows a more extensive contralateral projection than the bordering INF. It was concluded that quantitative differences in connectivity were present between a core region in the vestibular complex and peripheral parts. This core region comprises the central SV, the LV, the MVmc and extends into the rostral DV. It receives predominantly intrinsic input from the surrounding vestibular neurons and is in contrast to these latter neurons only minimally involved in the commissural projection.Abbreviations AChE acetylcholinesterase - bc brachium conjunctivum - bp brachium pontis - CE nucleus cuneatus externus - CO nuclei cochlearis - cr corpus restiforme - DV nucleus vestibularis descendens - DX nucleus dorsalis vagi - F nucleus fastigii - flm fasciculus longitudinalis medialis - gVII genu of the nervus facialis - group x, y, f groups x, y and f of Brodal - HRP horseradish peroxidase - IA nucleus interpositus anterior - IN nucleus interstitialis of nVIII - INF nucleus infracerebellaris - L nucleus lateralis - LV nucleus vestibularis lateralis - flm fasciculus longitudinalis medialis - MV nucleus vestibularis medialis - MVc caudal MV - MVmc magnocellular MV - MVpc parvocellular MV - nV nervus trigeminus - nVI nervus abducens - nVII nervus facialis - NV par nucleus vestibularis parabrachialis - PH nucleus prepositus hypoglossi - rV ramus descendens of nV - S nucleus and tractus solitarius - sad stria acustica dorsalis - SV nucleus vestibularis superior - tu tractus uncinatus - VI nucleus abducens - VM nucleus masticatorius - VOR vestibulo-ocular reflex - VP nucleus princeps trigemini - WGA-HRP wheatgerm-agglutinated HRP - XII nucleus hypoglossus     

Putative commissural and collicular axo-somatic terminals on neurons of the rat ventral cochlear nucleus

The type of synaptic terminals from the cochlear nucleus and inferior colliculus that terminate in the contralateral ventral cochlear nucleus are not known. These terminals were studied with the electron microscope and immunogold after injection of wheat germ agglutinin conjugated to horseradish peroxidase into the inferior colliculus or into the cochlear nucleus. The tracer anterogradely labelled boutons onto the main neurons of the contralateral ventral cochlear nucleus. Most of these cells (95%) were glycine immuno-negative and represent excitatory neurons. After injection of the tracer into the contralateral inferior colliculus few anterogradely labelled boutons were seen on spherical and multipolar cells of type II in the anteroventral cochlear nucleus. Rare labelled boutons were present on multipolar cells of type I and II, globular neurons and octopus cells in the posteroventral cochlear nucleus. After injection into the contralateral dorsal and ventral cochlear nucleus labelled boutons were seen more frequently than after injection into the inferior colliculus. These terminals contacted most of large neurons, especially multipolar cells of type II and less frequently of type I. Also globular and spherical cells were contacted by commissural terminals. Octopus cells received less frequently putative commissural terminals. Most boutons contained pleomorphic vesicles and stored GABA. A lower number of boutons with pleomorphic and flat vesicles contained glycine and sometimes GABA, both inhibitory neurotransmitters. Few boutons containing round vesicles were immuno-negative for both glycine and GABA, and were considered putative commissural excitatory terminals. The latter often contacted glycinergic neurons of type II so that also these terminals might elicit an inhibition with at least a disynaptic mechanism after contralateral stimulation.     

Role of GABA in the extraocular motor nuclei of the cat: a postembedding immunocytochemical study.

The GABAergic innervation of the extraocular motor nuclei in the cat was evaluated using postembedding immunocytochemical techniques. The characterization of GABA-immunoreactive terminals in the oculomotor nucleus was carried out at the light and electron microscopic levels. GABA-immunopositive puncta suggestive of boutons were abundant in semithin sections throughout the oculomotor nucleus, and were found in close apposition to somata and dendrites. Ultrathin sections revealed an extensive and dense distribution of GABA-immunoreactive synaptic endings that established contacts with the perikarya and proximal dendrites of motoneurons and were also abundant in the surrounding neuropil. GABAergic boutons were characterized by the presence of numerous mitochondria, pleiomorphic vesicles and multiple small symmetrical synaptic contacts. The trochlear nucleus exhibited the highest density of GABAergic terminations. In contrast, scarce GABA immunostaining was associated with the motoneurons and internuclear neurons of the abducens nucleus. In order to further elucidate the role of this neurotransmitter in the oculomotor system, retrograde tracing of horseradish peroxidase was used in combination with the GABA immunostaining. First, medial rectus motoneurons were identified following horseradish peroxidase injection into the corresponding muscle. This was carried out because of the peculiar afferent organization of medial rectus motoneurons that contrasts with the remaining extraocular motoneurons, especially their lack of direct vestibular inhibition. Semithin sections of the oculomotor nucleus containing retrogradely labeled medial rectus motoneurons and immunostained for GABA revealed numerous immunoreactive puncta in close apposition to horseradish peroxidase-labeled somata and in the surrounding neuropil. At the ultrastructural level, GABAergic terminals established synaptic contacts with the somata and proximal dendrites of medial rectus motoneurons. Their features and density were similar to those found in the remaining motoneuronal subgroups of the oculomotor nucleus. Second, oculomotor internuclear neurons were identified following the injection of horseradish peroxidase into the abducens nucleus to determine whether they could give rise to GABAergic terminations in the abducens nucleus. About 20% of the oculomotor internuclear neurons were doubly labeled by retrograde horseradish peroxidase and GABA immunostaining. A high percentage (80%) of the oculomotor internuclear neurons projecting to the abducens nucleus showed immunonegative perikarya. It was concluded that the oculomotor internuclear pathway to the abducens nucleus comprises both GABAergic and non-GABAergic neurons and, at least in part, the GABA input to the abducens nucleus originates from this source. It is suggested that this pathway might carry excitatory and inhibitory influences on abducens neurons arising bilaterally.     

Cellular basis of vestibular compensation: analysis and modelling of the role of the commissural inhibitory system

In this study we used a cellular network model of the brainstem vestibulo-ocular reflex (VOR) pathways to investigate the role of the vestibular commissural system in "vestibular compensation", the behavioural recovery that takes place after unilateral labyrinthectomy (UL). The network was initialized on the basis of mathematical analysis and trial simulations to generate a VOR response with a physiologically realistic gain and time constant. The effects of a selective decrease in the strength of commissural inhibitory input to the ipsi-lesional medial vestibular nucleus (MVN) neurones, without changes in other parts of the network, were investigated. Thus we simulated the marked down-regulation of GABA receptor efficacy that our recent experimental results have demonstrated in these cells after UL. The main outcome of this study is the delineation, for the first time, of a specific region of parameter space within which an adaptive change in commissural inhibitory gain is appropriate and sufficient to bring about a re-balancing of bilateral vestibular nucleus activity after UL. For this to be achieved, the relative contribution of the intrinsic, pacemaker-like membrane properties of the ipsi-lesional MVN cells must be equal to or greater than the synaptic input from the primary vestibular afferents in determining the in vivo resting discharge rate of these cells. Recent experimental evidence supports the view that the intrinsic properties of the MVN cells do contribute substantially to their resting discharge in vivo. Previous modelling studies that have excluded a role for the commissural system in vestibular compensation have arrived at this conclusion, because their models operated outside this region of parameter space. A second finding of this study is that, in a network that compensates through a selective change in commissural gain, the time constant of the VOR response is significantly reduced, mimicking the loss of velocity storage after UL in vivo. By contrast, the time constant is unchanged in a network that compensates through changes involving other non-vestibular inputs. These findings indicate that adaptive changes in commissural gain, through the dynamic regulation of GABA receptor efficacy in the vestibular nucleus neurones, may play an important role in vestibular plasticity. Electronic Publication     

Ipsilateral and contralateral projections from upper cervical segments to the vestibular nuclei in the rat

The present study examined uncrossed and crossed projections from upper cervical segments to the vestibular nuclei, and the relationship between the afferents and vestibulospinal neurons in the rat. Afferent axons were labeled following unilateral injections of biotinylated dextran into the C2 and C3 segments, while vestibulospinal neurons were labeled following unilateral injections of cholera toxin subunit B into the same segments. The terminals of uncrossed afferents were distributed in the entire area of the rostrocaudal extent of the lateral vestibular nucleus (LV). In the magnocellular part (MVmc) of the medial vestibular nucleus (MV), they were seen near the parvocellular part (MVpc) of the MV at caudal levels. In the MVpc, terminals were seen laterally and ventromedially, close to the border of the MVmc. At caudal levels of the caudal part (MVc) of the MV, they were distributed within the groups of labeled neurons in the middle and lateral areas. In the descending vestibular nucleus (DV), terminals were abundant dorsally and laterally in the rostral two-thirds. The distribution of contralateral cervical afferents was similar to that of ipsilateral afferents. The terminals of ipsilateral and contralateral cervical afferents were seen in contact with vestibulospinal neurons in the DV. The present study demonstrates bilateral input from upper cervical segments to the LV, DV, and all subdivisions of the MV. The input to the LV would contribute to the tonic neck reflex or cervicovestibulospinal reflex.     

Properties of utricular-activated vestibular neurons that project to the contralateral vestibular nuclei in the cat

The properties of utricular (UT)-activated vestibular neurons that send axons to the contralateral vestibular nuclei (commissural neurons) were investigated intracellularly or extracellularly in decerebrate cats. A total of 27 vestibular neurons were orthodromically activated by stimulation of UT nerves and antidromically activated by stimulation of the contralateral vestibular nuclei. All neurons tested were classified as vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons (V) after antidromic stimulation of the spinal cord and oculomotor/trochlear nuclei. Most UT-activated commissural neurons (20/27) received monosynaptic inputs. Twelve of 27 commissural neurons were located in the medial vestibular nucleus, 5 were in the lateral vestibular nucleus, 10 were in the descending vestibular nucleus, and no commissural neurons were recorded in the superior vestibular nucleus. Seven of 27 neurons were commissural VS neurons, 9 of 27 were commissural VOS neurons, and 11 of 27 were commissural V neurons. No commissural VO neurons were found. All VOS neurons and 3 VS neurons issued descending axons via the medial vestibulospinal tract. We also studied convergent inputs from the posterior semicircular canal (PC) nerve onto UT-activated commissural neurons. Five of 27 UT-activated commissural neurons received converging inputs from the PC nerves. Electronic Publication     

Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit

Summary Intra- and extra-cellular responses were recorded with glass microelectrodes from motoneurons in the VIth cranial nuclei of anesthesized rabbits. VIth nucleus motoneurons were identified by their antidromic activation from the VIth nerve. In these motoneurons stimulation of the ipsilateral VIIIth nerve produced IPSPs with disynaptic latencies (mean and S.D., 1.08 ± 0.1 msec) while stimulation of the contralateral VIIIth nerve produced EPSPs with disynaptic latencies (mean and S.D., 1.20 ± 0.18 msec). Correspondingly, direct stimulation of the ipsilateral medial vestibular nucleus (MV), produced IPSPs with monosynaptic latencies (mean and S.D., 0.61±0.15 msec) while direct stimulation of the contralateral MV produced EPSPs with monosynaptic latencies (mean and S.D., 0.61±0.09 msec). Further, with the recording electrode placed within the VIth nucleus to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla was systematically tracked with a monopolar stimulating electrode. It was demonstrated that the inhibitory relay cells could be effectively stimulated in the rostral half of the ipsilateral MV and the excitatory relay cells in the rostral half of the contralateral MV.Pharmacological investigation suggested that the inhibitory transmitter involved in the vestibular inhibition is gamma amino-butyric acid or a related substance.Electric stimulation of the flocculus produced a prominant depression in the inhibitory vestibulo-ocular reflex pathway to the VIth nucleus, while the excitatory pathway was free of any similar flocculus inhibition.     

A projection from nucleus reticularis tegmenti pontis of Bechterew to the medial vestibular nucleus in rabbits

This study documents a bilateral projection from nucleus reticularis tegmenti pontis (NRTP) to the rostral aspect of the medial vestibular nucleus (MVN) in rabbits. Horseradish peroxidase injections in rostral MVN produced retrogradely labeled neurons in the caudal half of NRTP; caudal MVN injections produced negative results. This supports the hypothesis that NRTP relays visual input to the vestibular nuclei via an extracerebellar pathway (Precht and Strata 1980), and indicates the importance of examining the contributions of both direct and cerebellar-mediated visual pathways to oculomotor physiology.     

Cerebellar inhibitory control of the vestibulo-ocular reflex investigated in rabbit IIIrd nucleus

Summary In anaesthetized rabbits, the vestibulo-ocular reflex was evoked by electric stimulation of VIIIth nerve and was observed by recording postsynaptic potentials and relevant field potentials in Illrd nucleus. The electric stimulation of flocculus produced a prominent inhibition of the vestibulo-ocular reflex in both the inhibitory component relayed by the superior vestibular nucleus and the excitatory component mediated by the brachium conjunctivum. The excitatory component mediated by the medial vestibular nucleus appeared to be free of the flocculus inhibition. The flocculus inhibition was blocked very effectively by systemic injection of picrotoxin. That the flocculus inhibitory action is due to monosynaptic postsynaptic inhibition of secondary vestibular neurones was demonstrated by direct stimulation of, and also by recording from, the superior nucleus. Recording from the superior nucleus was also performed in anaesthetized cats. All of these above results indicate that Purkinje cells in flocculus projecting to vestibular and cerebellar nuclei cells have inhibitory synaptic action. Flocculus stimulation produced also an excitatory effect upon vestibular nuclei neurones. However, this effect could be attributed to intracerebellar activation of the primary vestibular fibers which pass into the flocculus.     

A note on the development of the vestibulo-ocular pathway in the chicken

Summary The vestibulo-ocular pathways have been examined in embryonic chicks using horseradish peroxidase or dil as retrograde and anterograde tracers. The vestibular neurons project to the rostral, external eye motor nuclei over one or the other of three separate pathways; the ipsilateral and controlateral medial longitudinal fascicle and the contralateral brachium conjunctivum. The brachium conjunctivum component originates dorsally in the superior vestibular region and projects to the contralateral inferior oblique and superior rectus motor nuclei. An ipsilateral component of the medial longitudinal fascicle is labeled from more ventral sites in the vestibulo-cerebellar process and terminates in the ipsilateral superior oblique and inferior rectus nuclei. The contralateral medial longitudinal fascicle component originates still more ventrally and terminates in the contralateral superior oblique and inferior rectus motor nuclei. Accordingly, the vestibulo-ocular pathways in chickens operate predominantly on synergistic pairs of external eye muscles. These selective terminal fields are established within a day or two after the first terminals invade the eye motor nuclei during embryo-genesis.Abbreviations Br.C. brachium conjunctivum - EW Edinger Westfahl nucleus - MLF medial longitudinal fascicle - IO inferior oblique muscle - RI inferior rectus muscle - RM medial rectus muscle - RS superior rectus muscle - SO superior oblique muscle - V-O vestibular-ocular - i ipsilateral - x contralateral This paper is dedicated to Professor Fred Walberg on the occasion of his 70th birthday     

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.     

Ultrastructure of degenerating cerebellothalamic terminals in the ventral medial nucleus of the cat

Summary Terminal degeneration of cerebellar afferents in the ventral medial thalamic nucleus (VM) was studied in cats at the ultrastructural level after uni- or bilateral lesions in the brachium conjunctivum (BC). To achieve discrete lesions within the BC, a new very accurate stereotaxic technique was used. Numerous large terminals belonging to a population of so-called LR boutons were observed degenerating in the VM. The boutons displayed a wide variety of degenerative changes. Some revealed the features of the classical neurofilamentous type of degeneration. Others, although containing a slightly increased number of neurofilaments, featured much more prominently large numbers of coated vesicle shells and heavy accumulations of a flocculent electrondense material. Degeneration in a third group of boutons similar to some extent to the light type of degeneration was characterized by tight clumping of enormously swollen or distorted synaptic vesicles within a light matrix. At later stages, however, all these boutons were believed to become shrunken and electron-dense since intermediate stages between the light- and dark-appearing boutons were observed. The degenerating cerebellar boutons formed asymmetrical synaptic contacts. Groups of 3 or 4 boutons terminated upon dendrites of projection neurons synapsing more frequently on spines than on dendritic stems. The synaptic contacts between cerebellar boutons and the vesicle-containing dendrites of local circuit neurons were encountered as often if not more than the contacts on projection neuron dendrites. Triads consisting of cerebellar boutons and dendrites of both types of neurons were observed very regularly. This synaptic arrangement provides the anatomical basis for the modification of cerebellar input in the VM by interneurons.     

Anterograde tracing of projections from the dorsal raphe nucleus to the vestibular nuclei

This study used the anterograde transport of biotinylated dextran amine (BDA) to identify the course and terminal distribution of projections from the dorsal raphe nucleus (DRN) to the vestibular nuclei in rats. After iontophoretic injection of BDA into the medial and lateral regions of DRN, anterogradely labeled fibers descend within the medial longitudinal fasciculus and the ventricular fiber plexus to terminate within two discrete regions of the vestibular nuclear complex. One terminal field was located primarily ipsilateral to the injection site and involved rostrodorsal aspects of the vestibular nuclei, including superior vestibular nucleus and rostral portions of the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN). The other terminal field involved caudoventral aspects of both ipsilateral and contralateral MVN and LVN and was less heavily innervated. These findings confirm that the vestibular nuclei are targeted by a regionally-selective projection from the DRN. The segregation of DRN terminals into anatomically distinct fields indicates that the DRN-vestibular nucleus projections are organized to selectively modulate processing within specific functional domains of the vestibular nuclear complex. In particular, these terminal fields may be organized to modulate vestibular regions involved in eye movement-related velocity storage, coordination of vestibular and affective responses, and the bilateral coordination of horizontal eye movement reflexes.     

GABAergic innervation of corticotropin-releasing hormone (CRH)-secreting parvocellular neurons and its plasticity as demonstrated by quantitative immunoelectron microscopy

GABA has been identified as an important neurotransmitter in stress-related circuitry mediating inhibitory effects on neurosecretory neurons that comprise the central limb of the hypothalamo-pituitary-adrenocortical axis. Using combinations of pre-embedding immunostaining and postembedding immunogold methods at the ultrastructural level, direct synaptic contacts were revealed between GABA-containing terminals and neurosecretory cells that were immunoreactive for corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN). The vast majority of axo-dendritic GABA synapses was symmetric (inhibitory) type, and 46% of all synaptic boutons in the medial parvocellular subdivision of the PVN were immunoreactive to GABA. Using the disector method, an unbiased stereological method on serial ultrathin sections, the total calculated number of synaptic contacts within the medial parvocellular subdivision of the PVN was 55.4 x 10(6)/mm(3). On CRH-positive profiles 20.1 x 10(6) GABAergic synaptic boutons were detected per mm(3) in control, colchicine-treated rats. In the medial parvocellular subdivision, 79% of GABAergic boutons terminated on CRH neurons. Following adrenalectomy, which increases the synthetic and secretory activities of CRH neurons, the number of GABAergic synapses that terminate on CRH-positive profiles was increased by 55%. GABA-containing boutons appeared to be swollen, while the contact surfaces of cellular membranes between GABAergic boutons and CRH-positive profiles were shorter in adrenalectomized animals than in controls.Our data provide ultrastructural evidence for direct inhibitory GABAergic control of stress-related CRH neurons and suggest a pivotal role of GABA-containing inputs in the functional plasticity of parvocellular neurosecretory neurons seen in response to adrenalectomy.