Synaptic organization in the dorsal cochlear nucleus of the cat: a light and electron microscopic study

This report describes some observations of the synaptic organization of one region of the cat dorsal cochlear nucleus (DCN). The large “fusiform cell” and its innervation from the cochlea are emphasized. The morphology of the mature fusiform cell and its postnatal development are described in rapid Golgi impregnations of perfusion-fixed littermate cats. The mature features are correlated with profiles of fusiform cell bodies, apical dendrites, and basal dendritic trunks in electron micrographs from adult cat brains. Small neurons and granule cells are also identified in electron micrographs. In Golgi impregnations, axons of small cells and granule cells may terminate upon fusiform cells. Six classes of axons can be distinguished in rapid Golgi impregnations of the DCN. Two classes are of cochlear origin. One axonal class arises from small cells. The sources of the remaining axonal classes have not been identified in this study. Primary afferents can terminate as large, mossy endings in the DCN neuropil. They can also participate in axonal nests along with axons and dendrites of small cells. In electron micrographs, four synaptic endings can be distinguished. Primary cochlear fibers end in large terminals with asymmetrical synaptic complexes and round, clear vesicles. Primary axons can end in glomeruli, resembling those of the cerebellum, or in synaptic nests which are conglomerates of neuronal processes including other types of endings. The origins of the other synaptic types are not yet known. According to this study, primary afferent input could influence fusiform cells directly or indirectly, via small cells and granule cells.     

Fine structure of rat locus coeruleus

Locus coeruleus of the rat was studied in material prepared by aldehyde-osmium fixation. Cell bodies of locus coeruleus neurons possess large nuclei with a prominent nucleolus, a homogeneous karyoplasm of moderate density, and occasional indentations of the nuclear membrane. The cytoplasm is rich in organelles, including an extensive network of endoplasmic reticulum which forms well organized Nissl bodies. The highly developed Golgi apparatus surrounds the nucleus and extends into large dendritic trunks. In coronal section, cell bodies appear elongated along an approximate dorso-ventral axis, and most dendrites as well as axons appear in cross-section. In parasagittal sections the cells are very elongate, with dendrites and axons in the neuropil mostly cut longitudinally. Thus, locus coeruleus neurons possess disc-shaped dendritic fields parallel to the anterior-posterior axis of the brainstem, with predominantly longitudinal axo-dendritic synaptic configurations. Presynaptic profiles in locus coeruleus neuropil were classified according to the characteristics of their vesicle populations and other features. The most frequently encountered synaptic ending was characterized by small, round, densely packed synaptic vesicles, and comprised approximately 41% of the total sample of 775 synapses. Another group having large, rounded synaptic vesicles, which could be traced in a number of instances to large myelinated axons, accounted for 20% of the sample. Synaptic endings having large, flattened vesicles were also numerous, comprising 23% of the total. Another category of presynaptic endings was identified as those possessing numerous, small, flattened vesicles and comprising about 11% of the sample. Presynaptic endings having many vesicles of mixed sizes accounted for 2% of the total, and another group of the same proportion having small, rounded synaptic vesicles but also an unusually large number of larger, dense-cored vesicles was also present. Two other categories of synaptic endings were encountered, each comprising less than 1% of the total. One of these was derived from small, unmyelinated axons and contained clusters of pleomorphic synaptic vesicles. The other consisted of dendro-dendritic synapses between locus coeruleus neurons and also displayed small clusters of pleomorphic synaptic vesicles near the zone of synaptic apposition. Quantitative analysis revealed that most afferents to the nucleus synapse onto dendrites ranging between 0.5 and 2.5 micrometers in diameter and onto spine-like appendages derived from somata and dendrites. There were no significant differences between different categories of afferent terminals and their spatial distribution onto various postsynaptic targets of locus coeruleus neurons.     

Anatomy of the gustatory system in the hamster: central projections of the chorda tympani and the lingual nerve

The sensory modalities of taste and touch, for the anterior tongue, are relegated to separate cranial nerves. The lingual branch of the trigeminal nerve mediates touch: the chorda tympani branch of the facial nerve mediates taste. The chorda tympani also contains efferent axons which originate in the superior salivatory nucleus. The central projections of these two nerves have been visualized in the hamster by anterograde labelling with horseradish peroxidase (HRP). Afferent fibers of the chorda tympani distribute to all rostral-caudal levels of the solitary nucleus. They synapse heavily in the dorsal half of the nucleus at its rostral extreme; synaptic endings are sparser and located laterally in caudal regions. These taste afferents travel caudally in the solitary tract and reach different levels by a series of collateral branches which extend medially in the the solitary nucleus, where they exhibit preterminal and terminal swellings. Taste afferent axons range in diameter from 0.2 micrometer to 1.5 micrometers. The thickest axons project exclusively to the rostral and intermediate subdivisions of the solitary nucleus; the find ones may distribute predominantly to the caudal subdivision. Afferent fibers of the lingual nerve terminate heavily in the dorsal one-third of the spinal nucleus of the trigeminal nerve and also as a dense patch in the lateral solitary nucleus at the midpoint between its rostral and caudal poles. This latter projection overlaps that of the chorda tympani. Thus the two sensory nerves which subserve taste and touch from coincident peripheral fields on the tongue converge centrally on the intermediate subdivision of the solitary nucleus. Efferent neurons of the superior salivatory nucleus were labelled retrogradely following application of HRP to the chorda tympani. These cells are located ipsilaterally in the medullary reticular formation ventral to the rostral pole of the solitary nucleus; their dendrites are oriented dorsoventrally. The efferent axons course dorsally, form a genu lateral to the facial somatomotor genu, and course ventrolaterally through the spinal nucleus of the trigeminal nerve to exit the brain ventral to the entering facial afferents.     

Distribution and ultrastructure of primary afferent axons in Lissauer's tract in the northern leopard frog (Rana pipiens)

Dorsal roots were transected and filled with horseradish peroxidase to identify the primary afferent axons in Lissauer's tract (LT) of the spinal cord in a ranid frog with light and electron microscopy. Axons in LT could be traced at least 2-3 segments rostral and caudal to the level of the filled root. Some axons in LT entered a nucleus in the dorsal lateral funiculus at brachial and lumbar levels, while other axons terminated in widespread regions of the dorsal horn. Electron microscopy revealed unlabelled terminals with numerous large, dense-cored vesicles in LT. However, large vesicles were rarely observed and were never abundant in labelled primary afferent axons. S type synaptic contacts between labelled axons and unlabelled profiles were observed. The presynaptic axons in these synapses contained many small, spherical vesicles. The cross-sectional area of LT was related to the spinal cord level, with the largest area at the brachial level, and intermediate area at the lumbar level, and the smallest area at the thoracic level. No difference in number of labelled axons was observed in the medial and lateral parts of LT.     

Projections from auditory cortex to the cochlear nucleus in rats: Synapses on granule cell dendrites

Previous work has demonstrated that layer V pyramidal cells of primary auditory cortex project directly to the cochlear nucleus. The postsynaptic targets of these centrifugal projections, however, are not known. For the present study, biotinylated dextran amine, an anterograde tracer, was injected into the auditory cortex of rats, and labeled terminals were examined with light and electron microscopy. Labeled corticobulbar axons and terminals in the cochlear nucleus are found almost exclusively in the granule cell domain, and the terminals appear as boutons (1–2 μm in diameter) or as small mossy fiber endings (2–5 μm in diameter). These cortical endings contain round synaptic vesicles and form asymmetric synapses on hairy dendritic profiles, from which thin (0.1 μm in diameter), nonsynaptic “hairs” protrude deep into the labeled endings. These postsynaptic dendrites, which are typical of granule cells, surround and receive synapses from large, unlabeled mossy fiber endings containing round synaptic vesicles and are also postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles. No labeled fibers were observed synapsing on profiles that did not fit the characteristics of granule cell dendrites. We describe a circuit in the auditory system by which ascending information in the cochlear nucleus can be modified directly by descending cortical influences. © 1996 Wiley-Liss, Inc.     

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat.

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.     

Electron microscopic analysis of the synaptic organization of the globus pallidus in the cat

The synaptic organization of the feline globus pallidus (GP) was studied electron microscopically. The axon terminals were classified into five types on the basis of the size and shape of synaptic vesicles and the type of postsynaptic differentiations. Type I and II axon terminals were characterized by large, pleomorphic vesicles and by a symmetric and an asymmetric synaptic contact, respectively. Type III and IV axon terminals were characterized by small, pleomorphic vesicles and by a symmetric and an asymmetric synaptic contact, respectively. Type V axon terminals were characterized by elongated and large round vesicles and by a symmetric synaptic contact. The origins of these terminals were determined by a combined degeneration and HRP tracing technique. Following injections of HRP into the caudate nucleus or electrolytic lesions in this nucleus, type I terminals were anterogradely labeled with HRP or degenerated, respectively. Although type III, IV, and V terminals were labeled with HRP after HRP injections into the subthalamic nuclear region, only type IV and V terminals degenerated after lesions in that area. Type II terminals did not show any alterations following such treatment. These results suggest that type I terminals originate from the caudate nucleus, that type IV and V terminals come from the subthalamic nucleus or caudal to it, and that type III terminals are the terminals of intrinsic axon collaterals of GP neurons which send axons to the subthalamic nucleus. Occasionally convergence of different kinds of axon terminals on the same GP neuron was also observed. These terminals originated from the caudate nucleus and the subthalamic nucleus or caudal to it.(ABSTRACT TRUNCATED AT 250 WORDS)     

The ultrastructure of the subnucleus gelatinosus of the nucleus of the tractus solitarius in the cat

The dorsomedial region of the nucleus of the tractus solitarius termed the subnucleus gelatinosus (SNG) was studied at the light and electron microscopic level in the cat. In cresyl violet and luxol fast blue stained sections the SNG contained small neuronal somata that were scattered throughout a pale-staining neuropil containing few myelinated fibers. These neurons were difficult to impregnate with Golgi staining techniques, but in successful impregnations the somata were observed to be 10--19 micrometers in diameter and bore few sparsely branching primary dendrites. Spines were present on the dendrites of some neurons and were more numerous on distal portions of the dendritic tree. Ultrastructural examination of the SNG revealed that the neuronal complement consisted of round, oval, or spindle shaped neurons with little or no organized Nissl substance. Rare myelin-like ensheathments of neuronal perikarya were also observed. Bundles of fine unmyelinated axons that coursed mainly longitudinally were a prominent feature of the area. The most common type of axon terminal observed contained mainly round clear vesicles, approximately 31 nm in diameter, and made asymmetrical synaptic contact with a dendritic profile. Pleomorphic vesicle-containing terminals involved in symmetrical synaptic contact were also commonly seen. Axodendritic and axosomatic synapses were associated with terminals containing either round clear vesicles or pleomorphic vesicles. Less commonly, dendrodendritic and dendrosomatic synapses were seen, the presynaptic elements of which contained pleomorphic vesicles. Following removal of a nodose ganglion, degenerating terminals of vagal afferent fibers were observed throughout the neuropil. Such terminals contained round, clear vesicles with an occasional large, dense-cored vesicle, and made axodendritic and axosomatic synaptic contacts.     

Termination in trigeminal nucleus oralis of ascending intratrigeminal axons originating from neurons in the medullary dorsal horn: an HRP study in the rat employing light and electron microscopy

The anterograde horseradish peroxidase (HRP) technique was used to identify ascending intratrigeminal axons originating from neurons in the medullary dorsal horn (MDH) which terminate in trigeminal nucleus oralis (Vo). HRP injections into the MDH labeled two populations of axons ascending ipsilaterally within the spinal trigeminal nucleus. The first population was composed of parent branches which each gave off a single branching collateral strand to Vo as they ascended. These collaterals were characterized by boutons filled with small, round synaptic vesicles and forming asymmetrical synaptic contacts with large diameter dendritic shafts. The second axonal population was made up of parent branches which terminated directly in Vo. Their short terminal strands were distinguished by axonal endings containing pleomorphic synaptic vesicles and forming symmetrical synaptic junctions with small diameter dendritic shafts and spines.     

Cytology and immunocytochemistry of the nucleus of the lateral line lobe in the electric fish Gnathonemus petersii (Mormyridae): evidence suggesting that GABAergic synapses mediate an inhibitory corollary discharge

Knollenorgans, low-threshold electroreceptors found in mormyrid fish, are involved primarily, if not exclusively, in communication. The rhombencephalic nucleus of the lateral line lobe (nLLL) is the target nucleus of knollenorgan afferents. Cells in the nLLL receive a few medium size to large endings with round synaptic vesicles (classified as spoon; large club; small club-, and rodlet-shaped endings) with which they form nexus (gap junction) and asymmetrical chemical synapses associated with the round synaptic vesicles. In addition these endings emit thin collaterals which terminate as small boutons on nLLL neurons; these boutons also have round vesicles and make mixed (electrotonic and chemical) synapses. In addition, cells in the nLLL receive synaptic input from numerous small boutons containing pleomorphic vesicles and making symmetric synapses. We have not found any interneurons within nLLL. Our ultrastructural analysis suggests that boutons synapsing on nLLL neurons belong to only two afferent fiber systems and that the wiring diagram of nLLL is extremely simple. We have studied the immunolocalization in nLLL of glutamic acid decarboxylase (GAD), the enzyme essential for the synthesis of GABA that is also a useful marker for this widely distributed inhibitory neurotransmitter. GAD immunoreactivity was confined to the small boutons with pleomorphic vesicles. GAD was also found in a nucleus projecting to the nLLL, here named the sublemniscal nucleus (SL), which probably conveys corollary discharge signals to the nLLL. This GABAergic projection may be responsible for the potent inhibition associated with the electric organ discharge command that has been described in these cells.     

Synaptic relationships of Golgi type II cells in the medial geniculate body of the cat.

A combined analysis with the Golgi and silver-degeneration methods and electron microscopy in the ventral nucleus of the medial geniculate body has confirmed that the Golgi type II neuron forms dendro-dendritic synapses with the principal neuron in terminal aggregates called synaptic nests. Both types of neurons receive synaptic contacts from the afferent axons that ascend from the posterior colliculus and from those that descend from the auditory cortex. Only the principal neuron projects to the auditory cortex. The Golgi type II cells that receive endings from afferent axons send presynaptic processes to principal cells that are also contacted by the very same afferent axons. The axons of Golgi type II cells project to synaptic nests other than those supplied by the dendrites of the parent cell and link the Golgi type II cells with each other. On the surface of the Golgi type II cell there is a segregation of the different types of synaptic endings and a consistent sequence in their synaptic relationships. The endings of colliculogeniculate and Golgi type II axons predominate on the distal dendrites in the synaptic nests. Corticogeniculate endings congregate more on the soma and proximal dendrites. In the synaptic nests the Golgi type II dendrites are presynaptic to the principal cell dendrites, whereas both kinds of dendrites are postsynaptic to the very same axons, which project either from the posterior colliculus or from Golgi II cells...     

Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nucleus of the cat

The glomeruli in the substantia gelatinosa layer of the spinal trigeminal nucleus of the cat contain three kinds of dendritic processes. One of these, the type 2 dendrite, contains large synaptic vesicles in its spine heads and in its shaft. The type 2 dendrite receives axodendritic synapses from primary trigeminal afferent (C) axons and an occasional axodendritic synapse from small axonal (P) endings with small synaptic vesicles. The type 2 dendrites in turn form dendroaxonic synapses on the C endings. The dendroaxonic synapse and the axodendritic synapse of the C ending typically occur in reciprocal pairs. The axodendritic synapse usually lies in the depths of scalloped depressions in the surface of the C ending while the dendroaxonic synapse is found on the rim of the depression. Type 1 spines, i.e., dendritic spines receiving axodendritic synapses from the primary ending and lacking synaptic vesicles, also receive dendrodendritic synapses from type 2 dendrites. The types 2 dendrite with its large, rounded synaptic vesicles is considered to be excitatory at its dendroaxonic and dendrodendritic synapses. The type 2 dendrites course from glomerulus to glomerulus receiving their excitatory input through the axodendritic synapses of C axons. A type 2 dendrite, in response to C axon excitation would activate type 1 spines directly through their dendrodendritic synapses (C→2→1) and indirectly by increasing transmitter release at the axodendritic synapses of the C axonal endings through their dendroaxonic synapses (2→C→1). The type 2 dendrites could serve two functions. First, they may prolong transmitter release from the axodendritic synapses of C axonal endings beyond the time of arrival of incoming action potentials because of the reciprocal pairing of dendroaxonic and axodendritic synapses (C?2). Second, they may extend the spatial range of the excitatory output of active primary afferent axons to type 1 spines of glomeruli whose primary afferent axons may be inactive (C→2→1).     

Ultrastructure of supraspinal dorsal root projections in the toads. I. The obex region

The ultrastructure of the dorsal column nucleus (DCN) has been investigated at the level of the obex region in normal and experimental toads. Large 'isolated' neurons (greater than 20 micrometer) and clusters of small neurons (less than 20 micrometer) have been identified in this region. Synaptic profiles have been classified into three types: large 'en passant' LR boutons, containing round synaptic vesicles and neurofilaments, small R boutons with round vesicles and F boutons with pleomorphic vesicles. The axon terminals exhibited synaptic contacts with cell somata, with dendrites of varying calibers and with other axons. The terminals involved in the axo-axonic contact were the F boutons which were presynaptic to the LR boutons, thus representing the morphological basis for presynaptic inhibition. Transection of the second dorsal root was performed in order to identify the terminals of the primary afferents to the DCN, after different survival periods (16 h--50 days). Only the LR boutons underwent degeneration, thus representing the central endings of the primary dorsal root afferents. The functional significance of these findings was discussed.     

Hippocampal and midline thalamic fibers and terminals in relation to the choline acetyltransferase-immunoreactive neurons in nucleus accumbens of the rat: a light and electron microscopic study

The synaptic interactions between terminals of allocorticostriatal and thalamostriatal fibers and the cholinergic neurons in the nucleus accumbens were investigated using degeneration and dual labelling immunocytochemistry in Wistar rats. The presumptive cholinergic neurons were labelled with antibodies directed against choline acetyltransferase and the afferent fibers were labelled anterogradely with Phaseolus vulgaris-leucoagglutinin. Fibers from the subiculum of the hippocampal formation and from the midline and intralaminar thalamus project densely into the medial nucleus accumbens where they overlap a relatively dense population of choline acetyltransferase-immunoreactive neurons. Varicosities containing Phaseolus vulgaris-leucoagglutinin juxtapose the immunoreactive neurons. To study the possibility that the cholinergic neurons could be the synaptic targets of these incoming fibers, the subiculum, the fornix, and the midline/intralaminar thalamus were lesioned in separate animals and brain sections were immunoprocessed for choline acetyltransferase and studied with the electron microscope. In addition, dual-labelling electron microscopic immunocytochemistry was employed. In total, 164 synaptic terminals from the subiculum/hippocampus and 130 from the midline/intralaminar thalamus were examined; all formed asymmetrical synaptic specializations. No hippocampal endings were seen to contact the somata or primary dendrites of the choline acetyltransferase-immunoreactive neurons; however, three were found in synaptic contact with distal, immunolabelled dendritic shafts. Most hippocampal terminals established contacts with unlabelled spines. Fifteen percent of the thalamic endings were found to synapse on the somata and the primary and distal dendrites of the choline acetyltransferase-immunoreactive neurons. The remaining thalamic terminals established synaptic junctions with small unlabelled dendrites or spines. These findings have important implications not only for our understanding of the synaptic organization of the hippocampal and thalamic projections to the nucleus accumbens, but also for the contribution of the cholinergic neurons to the circuitry of this nucleus.     

Changes in the dorsal lateral geniculate nucleus of the squirrel monkey after unilateral ablation of the visual cortex

Young adult squirrel monkeys were subjected to unilateral occipital lobectomies, and were allowed to survive for 3 to 42 days. Both light and electron microscopic techniques were employed to examine orthograde as well as retrograde changes in the ipsilateral dorsal lateral geniculate nucleus. Three stages of degenerative change were described. During the early stage (third to fourth post-operative day) there were early signs of corticogeniculate terminal degeneration. During the intermediate stage (fifth to eighth day) most of the RS (small axonal terminals with round vesicles) or corticofugal endings were seen to undergo a sequence of degenerative change: swelling or irregularity of the synaptic vesicles; electron dense disintegration; and finally the engulfment by glial elements. The small myelinated axons also become electron dense at this stage, and there were early signs of retrograde degeneration. The late stage was characterized by severe changes and final disappearance of the geniculocortical relay neurons as well as their axons and dendrites. Concurrently, most of the RS terminals were also removed from the LGN, and there was considerable re-organization of the rest of the synaptic population.     

Morphology and synaptic connections of myelinated primary axons in the ventrolateral region of rat trigeminal nucleus oralis

Neurons in the ventrolateral (VL) subdivision of rat trigeminal nucleus oralis (Vo) have most of their dendritic arbors confined within this region. This study examines the morphology and synaptic connections of a population of myelinated primary trigeminal axons that arborize within VL and are in a position to provide input directly to VL neurons. Primary axons were visualized for light and electron microscopic analysis by injecting 30% horseradish peroxidase (HRP) in 2% dimethylsulfoxide (DMSO) into the sensory root of the trigeminal nerve and allowing 24-36 hours for the anterograde transport of HRP into the terminal axonal arbors. This population is characterized by its cone-shaped terminal arbors, which generate many axonal endings (2-8 micron in diameter) along unmyelinated terminal strands. These arbors arise from collaterals emanating from thinly myelinated (2-5 micron in diameter) parent branches descending in the spinal V tract, which, on the basis of their size, are considered to be small myelinated (A sigma) primary trigeminal axons. HRP-labeled P endings belonging to this population of primary axons are scalloped, filled with spherical to ovoid (40-70 nm in diameter) synaptic vesicles, and lie centrally in glomeruli where they make asymmetrical axodendritic synapses on dendritic shafts and spine heads. It is at these synapses that this population of primary trigeminal axons is probably transferring its input directly to the dendritic arbors of VL neurons. The dendritic shafts and spine heads also receive symmetrical to intermediate axodendritic synapses from endings containing flattened (70 X 29 nm) synaptic vesicles. These terminals also establish axo-axonic synapses on the P ending. Other synaptic components found less often in the glomeruli include small terminals containing oval (14-23 nm) synaptic vesicles that establish symmetrical to intermediate synapses on the P ending, boutons containing pleomorphic (35-80 nm) synaptic vesicles that form symmetrical to intermediate synapses on the P ending as well as on dendritic shafts, and small peripheral endings containing round (20-40 nm) synaptic vesicles that establish asymmetrical synapses on dendritic shafts.     

Organization and synaptic connections of cholinergic fibers in the cat superior colliculus

The cat superior colliculus (SC) receives a dense cholinergic input from three brainstem nuclei, the pedunculopontine tegmental nucleus, the lateral dorsal tegmental nucleus, and the parabigeminal nucleus (PBG). The tegmental inputs project densely to the intermediate gray layer (IGL) and sparsely to the superficial layers. The PBG input probably projects only to the superficial layers. In the present study, the morphology of choline acetyltransferase (ChAT)-immunoreactive axons and synaptic endings in the superficial and deep layers of the SC was examined by light and electron microscopy to determine whether these cholinergic afferents form different types of synapses in the superifical and deep layers. Two types of fibers were found within the zonal (ZL) and upper superficial gray layers (SGL): small diameter fibers with few varicosities and larger diameter fibers with numerous varicosities. Quantitative analysis demonstrated a bimodal distribution of axon diameters, with one peak at approximately 0.3–0.5 μm and the other at 0.9–1.0 μm. On the other hand, ChAT-immunoreactive fibers in the IGL were almost all small and formed discrete patches within the IGL. Two types of ChAT-immunoreactive synaptic profiles were observed within the ZL and upper SGL using the electron microscope. The first type consisted of small terminals containing predominantly round synaptic vesicles and forming asymmetric synaptic contacts, mostly on dendrites. The second type was comprised of varicose profiles that also contained round synaptic vesicles. Their synaptic contacts were always symmetric in profile. ChAT-immunoreactive terminals in the IGL patches contained round or pleomorphic synaptic vescles, and the postsynaptic densities varied from symmetric to asymmetric, including intermediate forms. However, no large varicose profiles were observed. This study suggests that cholinergic fibers include at least two differnt synaptic morphologies: small terminals with asymmetric thickenings and large varicose profiles with symmetric terminals. The large varicose profile in the superficial layers is absent in the IGL. This result suggests that the cholinergic inputs that innervate the superficial layers and the patches in the IGL of the cat SC differ in their synaptic organization and possibly also in their physiological actions. © 1993 Wiley-Liss, Inc.     

Projections to the inferior colliculus from the anteroventral cochlear nucleus in the cat: possible substrates for binaural interaction

The projections to the inferior colliculus of the cat are shown in autoradiographs after injections of 3H-amino acids into the anteroventral cochlear nucleus and anterograde axonal transport. Labeled bands of axons are seen in the central nucleus of the inferior colliculus, parallel to the fibrodendritic laminae, and in layers 3 and 4 of the dorsal cortex. A bilateral projection to the lateral, low-frequency part of the inferior colliculus is observed. In contrast, the more ventromedial, mid- and high-frequency parts receive only a contralateral input. The projections from the cochlear nucleus to both the contralateral midbrain and bilaterally to the superior olivary complex appear to be tonotopically organized. After HRP injections in the inferior colliculus, small numbers of stellate neurons are labeled in the lateral and ventral low-frequency parts of the anteroventral cochlear nucleus on the ipsilateral side. EM autoradiographs show labeled axonal endings from both sides of the anteroventral cochlear nuclei are present in the same proportion in pars lateralis. Axonal endings from either cochlear nucleus have small, round synaptic vesicles and make asymmetric synaptic contacts on dendrites. Axons from the contralateral side also make axosomatic contacts on large disc-shaped or stellate cells. Neurons from the ipsilateral anteroventral cochlear nucleus apparently make more synaptic endings per cell as compared to neurons from the contralateral side. Together, bilateral inputs from the anteroventral cochlear nucleus can account for a third of the endings with round synaptic vesicles in pars lateralis of the central nucleus. Morphological similarities among the ascending inputs to the inferior colliculus are discussed. Both direct circuits from the cochlear nucleus to the inferior colliculus and indirect circuits via the superior olivary complex or lateral lemniscus may display banding patterns, nucleotopic organization, or differential synaptic organization. The direct inputs from the anteroventral cochlear nucleus to the colliculus may influence binaural interactions which occur in the superior olivary complex. In addition, direct inputs may create new binaural responses in the inferior colliculus that are independent of lower centers.     

The lateral reticular nucleus of the opossum (Didelphis virginiana). I. Conformation,cytology and synaptology

When viewed in Nissl preparations, the lateral reticular nucleus (LRN) of the opossum can be divided into three subgroups: a medial internal portion, a lateral external portion and a rostral trigeminal division. Neurons within the internal division measure 13-45 μ in their greatest dimension whereas those within the external and trigeminal portions measure 11-32 μ and 14-27 μ respectively. Golgi impregnations reveal that many neurons in all three subdivisions display a radial dendritic pattern although some of the nerve cells within the external division have dendrites which orient mainly in a ventromedial to dorsolateral direction. The cell bodies of LRN neurons are relatively spine-free. However, a small percentage of neurons exhibit clusters of sessile spines on proximal and more distal dendritic segments. No locally ramifying axons or axon collaterals were found within the LRN. Synaptic terminals within the LRN were divided into four categories: (1) small terminals measuring 2.5 μ or less containing agranular spherical vesicles; (2) small terminals (2.5 μ or less) with agranular pleomorphic synaptic vesicles, i.e., a mixture of spherical and elliptical synaptic vesicles; (3) small terminals (2.5 μ or less) containing agranular spherical or pleomorphic vesicles with a variable number (4-27) of dense core vesicles; and (4) large terminals (greater than 2.5 μ) which contain agranular spherical synaptic vesicles and a variable number of dense core vesicles (1-17). Dendritic diameters were measured from Golgi impregnations and correlated with cross-sectioned profiles in electron micrographs to help determine the post-synaptic distribution of synaptic endings. Small terminals containing agranular spherical or pleomorphic synaptic vesicles contact the soma and entire dendritic tree in each portion of the nucleus, whereas the small terminals containing dense core vesicles are usually located on distal dendrites or spines. Some large terminals make multiple synaptic contacts with a cluster of spines, others contact groups of small (distal) dendrites. In order to identify two of the major afferent systems to the LRN, 15 adult opossums were subjected to either a cervical spinal cord hemisection or a stereotaxic lesion of the red nucleus. Two days subsequent to spinal hemisection, large terminals in the caudal part of the ipsilateral LRN exhibit either an electron dense or filamentous reaction. Their postsynaptic loci are spines and shafts of proximal dendrites or a number of distal dendrites and spines. In addition, small terminals containing spherical agranular synaptic vesicles undergo an electron dense reaction in the same areas. Their postsynaptic loci are proximal or distal dendrites. Two days subsequent to rubral lesions, small terminals containing agranular spherical synaptic vesicles undergo a dark reaction in rostral portions of the contralateral nucleus. They contact intermediate or distal dendrites and occasionally spines.