Divergent projections of physiologically characterized rat ventral cochlear nucleus neurons as shown by intra-axonal injection of horseradish peroxidase

Summary An attempt was made to correlate electrophysiological and morphological characteristics of rat ventral cochlear nucleus neurons. Their axonal course and their soma morphology were investigated using the intra-axonal horseradish peroxidase method. Prior to labeling, neurons were characterized by recording their response patterns to acoustic stimulation with pure tones. Three types of cells were found: Category I (37 neurons) exhibited primarylike responses and a spontaneous firing rate below 10 spikes/s. Category II (21 neurons) showed on responses and little spontaneous activity. Category III (9 neurons) had primarylike responses like neurons in category I. However, the spontaneous activity rate of these neurons was significantly higher (mean: 95 spikes/s). Among the response categories, the morphological characteristics differed in some prominent aspects. Within each category, however, the morphological properties were rather similar. All neurons in category I were globular/bushy cells located in the area of the entrance of the cochlear nerve. The axon of each cell coursed along the ventral acoustic stria and consistently innervated the lateral superior olive ipsilaterally, and the nucleus of the trapezoid body and the nucleus of the lateral lemniscus contralaterally. Some neurons also projected to periolivary nuclei ipsilaterally and contralaterally. Neurons in category II were located in the posteroventral cochlear nucleus and were presumably multipolar/stellate cells. Their axons coursed via the intermediate acoustic stria and innervated mainly contralateral periolivary regions as well as the contralateral nucleus of the lateral lemniscus. Ipsilaterally, the lateral superior olive and the superior periolivary nucleus were innervated by some of the category II neurons. Somata types of neurons in category III could not be identified morphologically, but somata were located in caudal parts of the posteroventral cochlear nucleus that correspond to the octopus cell area. Their axons coursed via the intermediate acoustic stria and innervated periolivary regions and the contralateral nucleus of the lateral lemniscus. Thus, their axonal distribution differed only slightly from neurons in category II. These data confirm and extend previous findings regarding the efferent connections of ventral cochlear neurons. They emphasize the complexity of the axonal projection patterns of single cochlear nucleus cells. Since two types of response patterns and three types of axonal projection patterns have been observed, there remains an ambiguous relation between response pattern and axonal projection site. It is concluded that the response pattern to pure tone stimulation alone is not sufficient to describe physiological characteristics allowing for the establishment of structure-function relations. Only if one considers additional physiological properties like the spontaneous activity rate and the shape of tuning curves, does a correspondence between structure and function become apparent.Abbreviations AVCN anteroventral cochlear nucleus - c contralateral - CF characteristic frequency - CN cochlear nucleus - DCN dorsal cochlear nucleus - dor dorsal - DPO dorsal periolivary region - HRP horseradish peroxidase - i ipsilateral - IAS intermediate acoustic stria - IC inferior colliculus - lat lateral - LL lateral lemniscus - LNTB lateral nucleus of the trapezoid body - LSO lateral superior olive - LVPO lateroventral periolivary nucleus - med medial - MNTB medial nucleus of the trapezoid body - MSO medial superior olive - MVPO medioventral periolivary nucleus - NLL nucleus of the lateral lemniscus - NTB nucleus of the trapezoid body - PSTH peristimulus time histogram - PVCN posteroventral cochlear nucleus - RF reticular formation - rost rostral - RPO rostral periloivary region - SOC superior olivary complex - SPO superior paraolivary nucleus - VAS ventral acoustic stria - VCN ventral cochlear nucleus - VNLL ventral nucleus of the lateral lemniscus - VNTB ventral nucleus of the trapezoid body - 8n cochlear nerve     

Aging in the rat medial nucleus of the trapezoid body. III. Alterations in capillaries

The medial nucleus of the trapezoid body (MTB), a large cell group in the rat brainstem auditory pathway, undergoes significant cell loss and loss of synapses with advancing age [5,6]. The purpose of the present study was to examine the microvasculature of the MTB in rats of the following ages: 3 months (MO), 6 MO, 24 MO, 27 MO, and 33 MO. In rats aged 24 to 33 MO, the following ultrastructural changes were observed in MTB capillaries: (1) large cavitations or spaces within capillary basal laminae, and (2) membranous debris, indicative of cellular degeneration within leaflets of capillary basal lamina. The volume density ratio (VDR) of capillaries decreases significantly between 6 and 33 MO of age.     

Encoding of temporal features of auditory stimuli in the medial nucleus of the trapezoid body and superior paraolivary nucleus of the rat

Neurons in the superior paraolivary nucleus (SPON) of the rat respond to the offset of pure tones with a brief burst of spikes. Medial nucleus of the trapezoid body (MNTB) neurons, which inhibit the SPON, produce a sustained pure tone response followed by an offset response characterized by a period of suppressed spontaneous activity. This MNTB offset response is duration dependent and critical to the formation of SPON offset spikes [Kadner A, Kulesza RJ Jr, Berrebi AS (2006) Neurons in the medial nucleus of the trapezoid body and superior paraolivary nucleus of the rat may play a role in sound duration coding. J Neurophysiol. 95:1499-1508; Kulesza RJ Jr, Kadner A, Berrebi AS (2007) Distinct roles for glycine and GABA in shaping the response properties of neurons in the superior paraolivary nucleus of the rat. J Neurophysiol 97:1610-1620]. Here we examine the temporal resolution of the rat's MNTB/SPON circuit by assessing its capability to i) detect gaps in tones, and ii) synchronize to sinusoidally amplitude modulated (SAM) tones. Gap detection was tested by presenting two identical pure tone markers interrupted by gaps ranging from 0 to 25 ms duration. SPON neurons responded to the offset of the leading marker even when the two markers were separated only by their ramps (i.e. a 0 ms gap); longer gap durations elicited progressively larger responses. MNTB neurons produced an offset response at gap durations of 2 ms or longer, with a subset of neurons responding to 0 ms gaps. SAM tone stimuli used the unit's characteristic frequency as a carrier, and modulation rates ranged from 40 to 1160 Hz. MNTB neurons synchronized to modulation rates up to approximately 1 kHz, whereas spiking of SPON neurons decreased sharply at modulation rates >or=400 Hz. Modulation transfer functions based on spike count were all-pass for MNTB neurons and low-pass for SPON neurons; the modulation transfer functions based on vector strength were low-pass for both nuclei, with a steeper cutoff for SPON neurons. Thus, the MNTB/SPON circuit encodes episodes of low stimulus energy, such as gaps in pure tones and troughs in amplitude modulated tones. The output of this circuit consists of brief SPON spiking episodes; their potential effects on the auditory midbrain and forebrain are discussed.     

Connections of the superior paraolivary nucleus of the rat: projections to the inferior colliculus

GABAergic neurotransmission contributes to shaping the response properties of inferior colliculus (IC) neurons. In rodents, the superior paraolivary nucleus (SPON) is a prominent and well-defined cell group of the superior olivary complex that sends significant but often neglected GABAergic projections to the IC. To investigate the trajectory, distribution and morphology of these projections, we injected the neuroanatomical tracer biotinylated dextran amine into the SPON of albino rats. Our results demonstrate that: (1) the SPON innervates densely all three subdivisions of the ipsilateral IC: central nucleus (CNIC), dorsal cortex (DCIC) and external cortex (ECIC). The SPON also sends a sparse projection to the contralateral DCIC via the commissure of the IC. (2) SPON axons are relatively thick (diameter >1.2 μm), ascend to the midbrain tectum in the medial aspect of the lateral lemniscus, and, for the most part, do not innervate the nuclei of the lateral lemniscus. (3) SPON fibers ramify profusely within the IC and bear abundant en passant and terminal boutons. (4) The axons of neurons in discrete regions of the SPON form two laminar terminal plexuses in the ipsilateral IC: a medial plexus that spans the CNIC and DCIC parallel to the known fibrodendritic laminae of the CNIC, and a lateral plexus located in the ECIC and oriented more or less parallel to the surface of the IC. (5) The projection from SPON to the ipsilateral IC is topographic: medial SPON neurons innervate the ventromedial region of the CNIC and DCIC and the ventrolateral region of the ECIC, whereas more laterally situated SPON neurons innervate more dorsolateral regions of the CNIC and DCIC and more dorsomedial regions of the ECIC. Thus, SPON fibers follow a pattern of distribution within the IC similar to that previously reported for intracollicular and corticocollicular projections.     

Aging in the rat medial nucleus of the trapezoid body. I. Light microscopy

The medial nucleus of the trapezoid body (MTB), a cell group in the superior olivary complex, was examined in an age-graded series of rats for neuron loss, changes in the giant synaptic endings (chalices of Held) on MTB neurons, and accumulation of age pigment. Neuron counts were done on protargol-stained paraffin sections of MTBs from a series of 17 rats aged 2-3, 6, 18, and 24 months. Between 2-3 and 24 months a 34% decrease in the mean number of MTB neurons was observed. Significant loss (p less than 0.05) was first evident in the early portion of the life span, between 2-3 and 6 months. In thionin-stained sections, there was no change with aging in the proportions of three MTB neuron types: principal cells (approximately 82%), elongate cells (approximately 15%), and stellate cells (approximately 3%). In young adult rats, 25-26% of all MTB neurons were associated with identifiable chalices of Held in protargol stained sections. This ratio did not vary significantly with aging. Age pigment accumulation in the MTB was examined in 2 micrometers Araldite sections stained with toluidine blue. Age pigment deposits were larger and more numerous in the MTBs of old animals, but not as extensive as has been described previously in many other parts of the nervous system. This study is the first to report neuron loss in an animal brainstem nucleus.     

The medial nucleus of the trapezoid body: comparative physiology

Principal cells of the medial nucleus of the trapezoid body (MNTB) receive their excitatory input through large somatic terminals, the calyces of Held, which arise from axons of globular bushy cells located in the contralateral ventral cochlear nucleus. Discharges of MNTB neurons are characterized by high stimulus evoked firing rates, temporally precise onset responses, and a high degree of phase-locking to either pure tones or stimulus envelopes. Since the calyx of Held synapse is accessible to in vitro and to in vivo recordings, it serves as one of the most elaborate models for studying synaptic transmission in the mammalian brain. Although in such studies, the major emphasis is on synaptic physiology, the interpretation of the data will benefit from an understanding of the MNTB's contribution to auditory signal processing, including possible functional differences in different species. This implies the consideration of possible functional differences in different species. Here, we compare single unit recordings from MNTB principal cells in vivo in three different rodent species: gerbil, mouse and rat. Because of their good low-frequency hearing gerbils are often used in in vivo preparations, while mice and rats are predominantly used in slice preparations. We show that MNTB units in all three species exhibit high firing rates and precise onset-timing. Still there are species-specific specializations that might suggest the preferential use of one species over the others, depending on the scope of the respective investigation.     

An electrophysiological study of the medial prefrontal cortical projection to the nucleus of the solitary tract in rat

The medial prefrontal cortex (MPFC) has been described as a visceromotor cortical area, since autonomic effects such as depressor responses may be elicited from this area. The central circuitry which mediates these depressor responses may include a projection from the MPFC to the nucleus of the solitary tract (NTS). Neurones were recorded extracellularly in the MPFC and were tested for antidromic (AD) activation from the NTS. These were all tested for (1) constant spike latency, (2) ability to follow high-frequency stimulation to more than 200 Hz, and (3) where possible, collision of stimulation-evoked spike with spontaneous spike or spikes evoked by iontophoretic application of glutamate. Of the 34 cells studied, all had constant AD latency (30±1 ms, range 16–46 ms); they followed high-frequency stimulation up to 354±19 Hz, and only seven cells were spontaneously active (range 1–19 spikes/s). The threshold stimulation intensity for AD activation was 102±9 A (n=34, range 8–200 A). Depth-threshold curves (n=7) showed minimum-threshold AD activation currents that corresponded to the dorsal and ventral sub-divisions of the NTS. Small shifts in AD latency were found in the depth-threshold curves, suggesting axonal branching. Analysis of recording sites showed that NTS-projecting MPFC neurones were predominantly found in the infralimbic and ventral prelimbic regions of the MPFC. These findings indicate that there is a population of neurones in the MPFC that projects to, and probably terminates within, the NTS. It is possible that this projection may, in part, mediate the cardiovascular response to MPFC stimulation.     

Dendritic arrangements in the cat medial superior olive

Examination of material prepared for electron microscopy and with Golgi and reduced silver methods for light microscopy, reveals the existence of both rostro-caudal and dorso-ventral components of the dendritic arrangement within the cell plate of the medial superior olivary nucleus. Cell types in addition to the central bipolar and marginal cells classically associated with the medial superior olive were found. They include multipolar cells, cells which are rostro-caudally elongated and those whose dendritic treees are restricted to one half of the nucleus. Many cells that appear bipolar in transverse section also have rostro-caudal dendritic shafts that can be seen in sagittal sections to form an overlapping mesh. The dendritic terminal arbors curve and intertwine at all medio-lateral levels across the nucleus. Beyond the glial sheets that cover the cell surface and its single layer of synaptic terminals are fascicles of myelinated axons running either perpendicularly to the cell plate or rostro-caudally. This pattern of soma, terminal, glia and axons is interrupted by occasional sites of direct contact between dendrites that are often characterized by attachment plaques.Multiple cell types, varying dendritic arrangements and the presence of dendro-dendritic appositions provide an anatomical substrate for a greater complexity of connections and interactions than previously associated with the medial superior olive.     

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

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

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.     

Relation of the parabigeminal nucleus to the superior colliculus and dorsal lateral geniculate nucleus in the hooded rat

Summary The retino-recipient layers of the superior colliculus project predominantly to the dorsal and ventral divisions of the ipsilateral parabigeminal nucleus, while receiving an input chiefly from the medial division of the contralateral nucleus. A variety of retrograde tracing techniques was used to confirm that there is a projection from the medial division of the parabigeminal nucleus to the contralateral dorsal lateral geniculate nucleus in normal adult hooded rats. Some parabigeminal cells branch to supply both dorsal lateral geniculate nucleus and retino-recipient layers of the superior colliculus.     

Development of banded afferent compartments in the inferior colliculus before onset of hearing in ferrets

Axonal projections from the lateral superior olivary nuclei (LSO), as well as from the dorsal cochlear nucleus (DCN) and dorsal nucleus of the lateral lemniscus (DNLL), converge in frequency-ordered layers in the central nucleus of the inferior colliculus (IC) where they distribute among different synaptic compartments. A carbocyanine dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), was used as a tracer to study the postnatal development of axonal projections in the ferret IC. The results indicated that projections from all three nuclei are present at birth, but are not segregated into bands. During the postnatal week between approximately postnatal days 4 and 12 (P4-P12), axons from LSO proliferate in IC, become more branched, and segregate into a series of bands composed of densely packed fibers and endings. LSO projections in these afferent bands course parallel to IC layers and are separated by intervening regions with few endings. A modest fit of a sine curve (R2>0.15) to the pattern of spacing of LSO projections in IC indicated that regularly spaced bands are forming by P7. Similarly, banded patterns of DCN and DNLL projections to IC have developed by the end of the first postnatal week. Thus, well before hearing onset in ferret (P28-30), three different afferent projections have segregated into banded compartments along layers in the central nucleus of the ferret IC. Possible mechanisms in circuit development are discussed.     

Colocalization of calbindin and GABA in medial nucleus of the trapezoid body of the rat

Using immunocytochemical methods, both calbindin and GABA were found to be colocalized in the somas of all the cells of the medial nucleus of the trapezoid body (NMTB) of the rat auditory system. In the lateral superior olive (LSO), calbindin was also found in the terminals but not in the cells. Some terminal labelling was found in the medial superior olive (MSO). GABA was also found in the somas of some cells in both LSO and MSO, but most of the labelling was in terminals. In the rat, calbindin appears to be more involved in a pathway that detects interaural intensity differences.     

An ultrastructural study of the lateral reticular nucleus in the rat

Summary A systematic study of the normal synaptic patterns within the lateral reticular nucleus (LRN) of the rat revealed various synaptic relationships. Two types of axon terminals were identified according to the morphology of the synaptic vesicles contained within them. Axon terminals with round vesicles established asymmetrical synaptic contacts with the somata and all areas of the dendritic trees including somatic and dendritic appendages. Pleomorphic-vesicle terminals established symmetrical synaptic contacts on somata and their appendages and on all sizes of dendrites and their appendages. Both round and pleomorphicvesicle terminals were infrequently seen to synapse upon the somata and proximal dendrites. The round-vesicle terminals outnumbered the pleomorphic-vesicle terminals on the dendritic trees. Terminals of the en passant type were also common throughout the LRN. Both round and pleomorphic-vesicle terminals were observed simultaneously contacting the soma and one or more dendritic profiles, or two different dendritic profiles. Synaptic configurations (glomeruli) were also observed in all three divisions of the nucleus. They consisted of a large, central, round-vesicle terminal contacting a number of small-calibre dendritic processes. This arrangement was surrounded by one or more sheets of glial lamellae. Puncta adherentia were observed on the apposed membranes of adjacent cells, adjacent dendrites and adjacent axon terminals.     

A Golgi study of the lateral reticular nucleus in the rat

Summary The organization of the lateral reticular nucleus (LRN) of the rat was investigated by using the Golgi technique. Golgi-Cox preparations revealed neurons with shapes similar to those observed in Nissl-stained preparations. Fusiform cells possess rectilinear dendrites with secondary dendrites which are longer than the parent stem. The remaining cell types have short dendrites which branch for three or four generations and follow a tortuous course. These two types of neurons are similar to the isodendritic and allodendritic neurons which have been reported in the reticular formation. The neurons throughout the LRN form cell clusters. In Golgi preparations five to ten cells are seen in each cluster but counterstaining reveals that the clusters are made up of many more cells than the Golgi preparations suggest. Many cells lie in close apposition and the dendrites of the cells in each cluster intertwine to form dendritic plexuses. Dendritic input from both neighbouring and distant cell clusters also contributes to the plexus formations within each cell cluster. Under high magnification, the dendrites show irregularities in their contours, including warty excrescenses, bumps and an array of spines, some of which are pedunculated. The appendages are confined primarily to distal portions of the dendrites, with few spines observed on the somata and proximal dendrites. Varicosed dendrites are also in common occurrence throughout the nucleus.     

Supraoptic neurosecretory cells: Synaptic inputs from the nucleus accumbens in the rat

Summary Synaptic inputs from the nucleus accumbens (ACB) to neurosecretory cells of the supraoptic nucleus (SON) were studied in the rat. One hundred and twenty SON neurones responded antidromically to pituitary stalk stimulation and were identified as neurosecretory cells. Sixty-three of these cells were identified as vasopressin-secreting cells and 45 as oxytocin-secreting cells by their spontaneous firing patterns. About one half of the vasopressin-cells and two thirds of the oxytocin-cells were responsive to stimulation of the basal forebrain including the ACB. More vasopressin-cells were excited than were inhibited, and oxytocin-cells were mainly inhibited. Depth profile of effective stimulation sites in the basal forebrain revealed that ACB stimulation selectively produced the responses. Most of those SON neurones responsive to ACB stimulation also responded to septal stimulation. A positive correlation was observed between responses to ACB and septal stimulation in each unit. After septal lesion, the number of SON neurones which were responsive to ACB stimulation was significantly decreased. In two rats, a single SON unit was tested for ACB stimulation both before and after septal lesion, and the previously observed synaptic inputs were not seen after the lesion. Fifty septal neurones projecting to the area including the SON were antidromically identified after SON stimulation. About one half of these neurones were excited by ACB stimulation. These results demonstrate the existence of a neural pathway from the ACB to the SON and suggest that the pathway is mediated by septal neurones.Supported by grant no. 57770111 from the Ministry of Education, Science and Culture, Japan     

Correlation of electrophysiology, morphology, and functions in corticotectal and corticopretectal projection neurons in rat visual cortex

In most mammals the superior colliculus (SC) and the pretectal nucleus of the optic tract (NOT) receive direct input from the ipsilateral visual cortex via projection neurons from infragranular layer V. We examined whether these projection neurons belong to different populations and, if so, whether it is possible to correlate the electrophysiological features with the suggested function of these neurons. Projection cells were retrogradely labeled in vivo by rhodamine-coupled latex beads or fast blue injections into the SC or the NOT 2–5 days prior to the electrophysiological experiment. Intracellular recordings of prelabeled neurons were made from standard slice preparations and cells were filled with biocytin in order to reveal their morphology. Both cell populations consist of layer V pyramids with long apical dendrites that form terminal tufts in layer I. In electrophysiological terms, 12 of the corticotectal cells could be classified as intrinsically bursting (IB), while two neurons showed a doublet firing characteristic and one neuron was classified as regular-spiking (RS). Intracortical microstimulation of cortical layer II/III revealed that SC-projecting neurons responded optimally to stimulation sites up to a distance of 1000 μm from the recorded cell. The morphological features of the SC-projecting cells reveal an apical dendritic tuft in layer I with a lateral extension of 300 μm, a mean spine density of 65 spines per 40 μm on the apical dendrites located in layer II/III, and a bouton density of 13 boutons per 100 μm on the intracortical axons. Sixteen NOT-projecting neurons exhibited an IB and five cells an RS characteristic. Intracortical microstimulation of cortical layer II/III showed that NOT-projecting neurons responded optimally to stimulation sites up to a distance of 1500 μm. Their morphological features consist of an apical dendritic tuft with a lateral extension of 500 μm, a mean spine density of 25 spines per 40 μm on the apical dendrites located in layer II/III, and a bouton density of 6 boutons per 100 μm on the intracortical axons. When the passive membrane parameters, responses to intracortical microstimulation in layer V, the extension of the basal dendritic field, and spine densities in layers I or V were compared between SC- and NOT-projecting cells, no differences were revealed. Differences were only consistently found in the supragranular layers, either for morphological parameters or for intracortical microstimulation. The results suggest that NOT-projecting and SC-projecting neurons, although biophysically similar, could integrate and transmit different spatial aspects of cortical visual information to their target structures. Received: 22 November 1996 / Accepted: 8 October 1997     

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.     

Membrane specializations and their relation to HRP transport in the medial habenular nucleus of the rat

Summary In the medial habenular nucleus of the rat, ependymal and endothelial membrane specializations were studied with TEM and freeze-fracturing. They comprise ependymal adherent junctions — not manifest in freeze-fracture replicas-, gap junctions, and membrane-associated orthogonal particle complexes (assemblies) — not identifiable in thin-sectioned material. Ependymal tight junctions being absent, no brain-liquor barrier exists. The capillary endothelium is provided with tight junctions only.Intraventricularly injected HRP was transported in large amounts through the ependyma, mainly through the intercellular spaces and additionally by way of massive pinocytosis through the cytoplasm of particular ependymal cells only, and finally through the parenchymal intercellular compartments towards habenular capillaries. Following intravenous injection of HRP, considerable transport of the enzyme took place by means of transendothelial pinocytosis, followed by some pinocytotic transport through diverse parenchymal elements and markedly profuse incorporation and lysis within pericytes. The habenular blood-brain barrier appeared to be considerably leaky with respect to HRP.     

The differentiation of cerebral dendrites: A study of the post-migratory neuroblast in the medial nucleus of the trapezoid body

Summary The differentiation of dendrites in the medial trapezoid nucleus of the opossum and cat has been traced from the stage of the post-migratory neuroblast in developmental series prepared with the rapid Golgi technique. The post-migratory neuroblast is an elongated cell. Its perikaryon is located initially at the outer limiting layer of the medulla. Its primitive internal process grows into the primordial medial trapezoid nucleus and gives rise to an axon. On the part of the neuroblast adjacent to the axon's origin the endings of the afferent axons beging to differentiate. The perikaryon moves to the same part of the neuroblast through the primitive internal process. Subsequently the dendrites differentiate. Dendrites and their branches form from budding growth cones. The cell body and dendritic processes of the young growing neuron are covered with transitory filopodia. Sprouting growth cones and filopodia appear at the tips and along the shafts of the elongating and enlarging dendrites. The locomotor and synthetic activities of the growth cones establish the stereotyped dendritic branching patterns of each kind of neuron. The development of the dendritic branches accompanies the elaboration of the particular type of axonal plexus that will become synaptically related. This suggests that the patterns of the dendritic trees and of the afferent axonal end-branches derive from mutual interactions of the growing dendritic and axonal branches. These interactions may be mediated by physical contacts as well as chemotactic factors. The filopodia are implicated in the formation of dendritic appendages. Filopodia could participate in membrane synthesis, locomotion, and synaptogenesis. There is an indication that the afferent axons can induce the differentiation of the post-synaptic parts of the neuroblast. The findings imply that the influence of physical and chemical factors in the differentiation of the synaptic organization of the brain depends on their temporal and spatial sequences.Supported in part by U.S. Public Health Service Research Grant NB 06115 to Harvard University.With the technical aid of Mrs. R. R. Morest and of Miss P. E. Palmer.