Fornix afferents to the anteroventral thalamic nucleus: An EM study in the rat

LENN, N. J. Fornix afferents to the anteroventral thalamic nucleus: An EM study in the rat. BRAIN RES. BULL. 3(6) 589–593, 1978.—Three types of synapses occur in the anteroventral thalamic nucleus (AVN). Type 1 consists of small (0.5–0.8 μm) axonal endings densely packed with spherical synaptic vesicles. They form markedly asymmetrical synaptic contacts with distal portions of dendrites. Degenerative changes in these axons following destruction of the fornix identify them as the endings of the subicular projection to AVN. Type 2 synapses consist of large (1.0–1.5 μm) axonal processes containing spherical vesicles which form asymmetrical synapses on more proximal dendrites. Type 3 endings consist of large unidentified processes containing spherical, and occasionally flattened, synaptic vesicles forming symmetrical contacts with the largest stem dendrites. Neither of these synaptic types were modified by fornix lesions. The synaptic arrangements within AVN are simpler than other thalamic nuclei in that serial synapses and synaptic glomeruli are not present.     

Marginal neurons of the spinal cord: types, afferent synaptology and functional considerations

Electron microscopic studies of the marginal zone of the dorsal horn in the cat spinal cord revealed 4 different types of synaptic endings contacting marginal neurons. These included boutons with clear spherical vesicles, boutons with clear flattened vesicles, boutons with a mixture of clear spherical and flattened vesicles and boutons with a mixture of clear spherical and dense-cored vesicles.The origin of the synaptic input to marginal neurons was determined by means of degeneration studies at the ultrastructural level following various surgical lesions. It was found that primary afferents contribute approximately 40% of the boutons on the distal dendrites, 20% on the proximal dendrites and 10% on the soma of marginal neurons, while the axons of the tract of Lissauer, which stem from gelatinosa neurons and are inhibitory in function, contribute approximately 20% of the boutons on the distal dendrites, 60% of those on the proximal dendrites and 50% of the somatic endings on marginal neurons. Cranial nerve afferents contribute approximately 10% of the boutons on each area of the marginal neurons within the first cervical segment; this converging afferent input suggests that these cells function as non-specific central nociceptive relay neurons.Fluorescence histochemical studies for the demonstration of biogenic amines revealed a fairly dense array of fluorescent fibers and terminals in the marginal zone. There was no change in this fluorescence following the surgical lesions and their origin remains unknown.Although the observed synaptology of the marginal zone is more complex than that upon which the central inhibitory balance theory of pain was originally proposed, our data provide good anatomic support for that theory.     

Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: A study using electron microscopic autoradiography

The superior olivary complex is the first site in the central auditory system where binaural interactions occur. The output of these nuclei is direct to the central nucleus of the inferior colliculus, where binaural inputs synapse with monaural afferents such as those from the cochlear nuclei. Despite the importance of the olivary pathways for binaural information processing, little is known about their synaptic organization ir the colliculus. The present study investigates the structure of the projections from the lateral and medial superior olivary nuclei to the inferior colliculus at the electron microscopic level. Stereotaxic placement and electrophysi ological responses to binaural sounds were used to locate the superior olive. Anterograde axonal transport of 3H-leucine was combined with light and electron microscopic autoradiography to reveal the location and morphology of the olivary axonal endings. The results show that the superior olivary complex contributes different patterns of synaptic input to the central nucleus of the inferior colliculus. Each projection from the superior olivary complex to the colliculus differs in the number and combinations of endings. Axonal endings from the ipsilateral medial superior olive were exclusively the round (R) type that contain round synaptic vesicles and make asymmetrical synaptic junctions. This morpholo is usually associated with excitatory synapses and neurotransmitters such as glutamate. Endings from medial superior olive terminate densely in the central nucleus. The projection from the contralateral lateral superior olive also terminates primarily as R endings. This projection also includes small numbers of pleomorphic (PL) endings that contain pleomorphic synaptic vesicles and usually make symmetrical synaptic junctions. The PL morpholo is associated with inhibitory synapses and transmitters such as gamma-aminobutyric acid and glycine. All endings from the contralateral lateral superior olive terminate much less densely than endings from the medial olive. In contrast, the projection from the ipsilateral lateral superior olive contributes both R and PL endings in roughly equal proportions. These ipsilateral afferents are heterogeneous in density and can terminate in lower or higher concentrations than endings from the contralateral side. These data show that the superior olive is a major contributor to the synaptic organization of the centr nucleus of the inferior colliculus. The ipsilateral projections of the medial and lateral superior olive may produce higher concentrations of R endings than other inputs to the central nucleus. Such endings may participate in excitatory synapses. The highest concentra tions of PL endings come from the ipsilateral lateral superior olive. In combination with inputs from the contralateral dorsal nucleus of the lateral lemniscus, PL endings from the superior olive may participate in many inhibitory synapses found in the central nucleus. These different patterns of synaptic input from the superior olivary complex will influence how binaural information is transmitted to the inferior colliculus. © 1995 Wiley-Liss, Inc.     

An electron microscopic study of synaptic organization in the medial superior olive of normal and experimental chinchillas

The medial superior olive (MSO) was studied in normal animals to determine the types of synaptic endings and their distribution over the surface of MSO neurons. Unilateral lesions were made in the anteroventral cochlear nucleus (AVCN) of experimental animals to determine the source of at least one synaptic type in the MSO. The surfaces of MSO neurons in normal animals were studded with three distinct types of synaptic endings distinguished mainly by the size of their synaptic vesicles. There were endings with large vesicles, 510 Å in mean diameter; endings with small vesicles, 380 Å; and endings with vesicles intermediate in size. 435 Å. The large vesicle ending typically was greater than 2 μm in maximum diameter. It appeared as the termination of a myelinated axon or as a swollen portion of a node and made multiple asymmetrical synapses. Large vesicle endings occurred exclusively on dendrites where they formed 85% of the synaptic endings. Small vesicle endings typically were less than 2 μm in diameter. They appeared as the termination of a fine unmyelinated axon and made only one symmetrical synapse. Small vesicle boutons occurred infrequently over the entire neuronal surface. Intermediate vesicle synaptic endings were similar to large vesicle endings except that they were present only on the cell body, axon hillock, and proximal portions of the dendrites where they formed most of the synapses. In AVCN lesioned animals degenerating myelinated axons and large vesicle synaptic endings were distributed to the lateral dendrites of the ipsilateral MSO and medial dendrites of the contralateral one. In addition, a few degenerating axons and large vesicle endings were found among the ipsilateral medial dendrites. The changes in the degenerating endings were characterized by an early proliferation of neurofilaments and swelling of the endings followed by collapse of the endings and increase in electron density, disappearance of filaments and synaptic vesicles, and phagocytosis of the degenerated endings by reactive glial cells. No degenerative changes were observed in the small and intermediate vesicle endings. The results of this study indicate that the more numerous large vesicle endings presynaptic to the MSO dendrites are the axon terminals of neurons in the AVCN. The persistence after lesions of the small and intermediate vesicle endings suggests that they arise from as yet unidentified sources.     

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.     

Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

In order to classify the presynaptic elements contacting the principle class of globus pallidus neurons, electron microscopic examination of serial sections made from a medially located large globus pallidus neuron, labeled with intracellular horseradish peroxidase, was undertaken. In addition, the use of labeled and light microscopically reconstructed material allowed us to quantitatively determine the distribution of each bouton type along the soma and dendrites. Six types of presynaptic terminals contacting the labeled cell have been recognized. Type 1 endings, the most numerous (84%), make symmetrical contacts on all portions of the cell, except spines, contain large pleomorphic, and a few large dense-core vesicles. Type 2 endings are filled with small spherical-to-ellipsoidal synaptic vesicles. They make asymmetrical contacts only with higher-order dendrites and account for 12% of synaptic contacts onto the labeled neuron. Type 3 endings are large, contain sparsely distributed large pleomorphic vesicles, and make two symmetrical synapses per bouton, one onto a spine head and the other onto the underlying dendritic shaft. They are infrequent (0.2%), being found only in association with dendritic spines. Type 4 endings contain large pleomorphic synaptic vesicles and no dense-core vesicles. They make symmetrical contacts with the short primary dendrites. Type 5 endings contain a mixture of small clear pleomorphic vesicles and numerous large dense-core vesicles. They contact only the cell body and the short primary dendrites, making up 20% of somatic synaptic contacts but less than 1% of contacts onto dendrites. Type 6 boutons contain oval and flattened synaptic vesicles and establish symmetrical contacts with higher-order dendritic branches and the cell body.     

EM autoradiographic study of the projections from the dorsal nucleus of the lateral lemniscus: a possible source of inhibitory inputs to the inferior colliculus

The fine structure of the projection from the dorsal nucleus of the lateral lemniscus (DNLL) to the inferior colliculus is examined in the cat. Anterograde axonal transport of 3H-leucine and EM autoradiographic techniques are used to label axonal endings from DNLL. The primary finding is that axonal endings from DNLL contain pleomorphic synaptic vesicles and make symmetrical synaptic contacts. This morphology is associated with inhibitory synapses. The projection from DNLL is the source of approximately one-third of the axonal endings with pleomorphic vesicles in the central nucleus of the inferior colliculus. In the contralateral central nucleus, only labeled endings with pleomorphic vesicles are found. By comparison, on the ipsilateral side, both endings with pleomorphic vesicles and, to a lesser degree, endings with round vesicles are labeled. Endings from DNLL are more numerous per unit area on the contralateral side. About half of the labeled axonal endings from DNLL terminate upon small dendrites, and another third terminate upon more proximal dendrites and several types of cell bodies. Many axonal endings form multiple synaptic contacts, sometimes on more than one postsynaptic structure. Sites of termination for axonal endings include dendritic spines and branch points of dendrites. These data support the hypothesis that the DNLL pathway to the inferior colliculus may have an inhibitory function. Previous studies show that DNLL neurons exhibit immunoreactivity to GAD and GABA antibodies. The crossed projection of DNLL to the inferior colliculus forms tonotopically organized bands that terminate as endings with pleomorphic vesicles. These endings may supply GABAergic inputs to the inferior colliculus. Thus, bands from DNLL could provide inhibitory inputs and overlap with bands from other sources that provide excitatory inputs. Overlapping bands may form unique synaptic domains in the inferior colliculus. The uncrossed projections from DNLL may provide the inferior colliculus with a more diffusely organized projection that could include excitatory and inhibitory inputs. Since the DNLL on one side may inhibit the opposite DNLL and the inferior colliculus, the DNLL pathway may regulate ascending inhibition to the midbrain. Presumed inhibitory inputs from DNLL to the inferior colliculus could be involved in binaural information processing and contralateral dominance.     

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.     

Quantitative demonstration of somatic synapse sprouting following dendritic deafferentation in neonatal rat interpeduncular nucleus

Neonatal habenular lesions deafferent the dendrites of interpeduncular nucleus (IPN) neurons by preventing formation of S and crest synapses. The small number of synapses normally contacting IPN neuronal perikarya are of unknown, but non-habenular, origin. The number of synapses on IPN perikarya is significantly increased (p less than 0.001) when 10 control animals are compared to 7 animals with unilateral habenular lesions and 7 animals with bilateral lesions. The increases with bilateral lesions are approximately twice those resulting from unilateral lesions (p less than 0.01). This phenomenon involves sprouting of both the somatic synapses which contain spherical vesicles and those with flattened vesicle endings, and is greater when the latter are considered alone (p less than 0.001). Only the small group of somatic synapses with asymmetrical contacts failed to show a change. The increases suggest a postsynaptic control mechanism which acts in the direction of preserving synaptic input, but permits displacement of the site of input from dendrites to soma. Factors which may be important in determining this outcome are the production of the lesions prior to synaptogenesis, and the presumed shrinkage of the dendrites of the IPN neurons.     

The synaptic organization of the motor nucleus of the trigeminal nerve in the opossum

The motor nucleus of the opossum trigeminal nerve consists of a main body and a small dorsomedial cell cluster. The cell bodies form a unimodal population with areas that range from 150–2700 μm². Golgi impregnations reveal that each neuron has three to six primary dendrites which radiate in all planes from the cell body. Within 300 μm from the soma, the primary dendrites divide into secondary branches and these, in turn, bifurcate into thinner distal dendrites. The overall diameter of the dendritic tree often extends as much as 1 mm, with a rare branch leaving the confines of the nucleus to enter the neighboring reticular formation. Somatic and dendritic spines are often present and are either sessile or complex appendage forms. The perikarya and initial dendritic trunks of trigeminal neurons are contacted by four types of presynaptic terminals which cover more than 40% of the membrane. Most endings are 1–3 μm long and contain either spherical (S) or pleomorphic (P) synaptic vesicles. Another, less common, type of bouton is marked by large dense-core (DC) vesicles. Approximately 8% of the terminals on trigeminal cell bodies are large (2–5 μm) with spherical synaptic vesicles and are always associated with a subsynaptic cistern (C-boutons). These terminals very often interdigitate with adjacent synaptic endings. S-, P-, and C-boutons synapse on the dendritic tree of trigeminal neurons in the following characteristic pattern: proximal dendrites (greater than 5 μm in diameter) are contacted by all three types of terminals; intermediate-sized dendrites (between 2.5 and 5.0 μm in diameter) are most often contacted by S-boutons although P-boutons are also present; and small, distal dendrites (less than 2.5 μm in diameter) are almost always contacted by S- boutons. Both S- and P-boutons contact spines. In order to determine the ultrastructural identity of some of the major afferent systems to the trigeminal motor nucleus, adult opossums were subjected to two different types of lesions. Three and 5 days subsequent to lesions which destroyed most of the trigeminal mesencephalic nucleus, degenerating terminals containing spherical vesicles were found. These endings were S-boutons on more distal parts of the dendritic tree while on the cell body and proximal dendrites they were C-boutons. Seven days after a mesencephalic lesion, expanded glial processes approximated the trigeminal cell membrane. Two days subsequent to lesions which transected commissural fibers from the contralateral trigeminal complex, degenerating S- and P-boutons were found in contact with intermediate and distal parts of the trigeminal dendritic tree.     

Three classes of inhibitory amino acid terminals in the cochlear nucleus of the guinea pig

Electron microscopic postembedding immunocytochemistry was used to analyze and assess the synaptic distribution of glycine (GLY) and γ-amino butyric acid (GABA) immunoreactivities in the guinea pig cochlear nucleus (CN). Three classes of endings were identified containing immunolabeling for glycine, GABA, or both glycine and GABA (GLY/GABA). All classes were similar in that the terminals contained pleomorphic vesicles and formed symmetric synapses with their postsynaptic targets. A fourth class, which labeled with neither antibody, contained round vesicles and formed asymmetric synapses. Glycine endings predominated in the ventral CN, while GLY/GABA endings were prevalent in the dorsal CN. GABA endings were the least common and smallest in size. Glycine, GLY/GABA, and GABA endings differed in their proportions and patterns of distribution on the different classes of projection neurons in the CN, including spherical bushy, type I stellate/multipolar, and octopus cells in the ventral CN and fusiform cells in the dorsal CN. The vast majority of anatomically-defined, putative inhibitory endings contain GLY, GABA, or both, suggesting that most of the inhibition in the cochlear nucleus is mediated by these three cytochemically and, probably, functionally distinct classes of endings. The results of this study also suggest that a large proportion of the GABA available for inhibition in the CN coexists in terminals with glycine. © 1996 Wiley-Liss, Inc.     

Fine structure and synaptic connections of the common spiny neuron of the rat neostriatum: A study employing intracellular injection of horseradish peroxidase

Medium-sized spiny neurons of the rat neostriatum, identified by intracellular injection of horseradish peroxidase, were examined at both light and electron microscopic levels. These neurons were characterized by their heavy investment of dendritic spines, beginning about 20 μm from the soma and continuing to the tips of the dendrites. Their axons arose from the soma or from a large dendritic trunk very near the soma, and tapered rapidly to form a main axonal branch from which arose several smaller initial collaterals. These arborized extensively throughout an area of about the same size as, and highly overlapping with, the dendritic field of the cell, while the main axon could be followed for distances of up to 1 mm in the direction of the globus pallidus. Three major synaptic types were seen in contact with spiny neurons. Boutons containing small round synaptic vesicles formed synapses exclusively with spiny regions of the dendrites, and most of these were axo-spinous. Small, very pleomorphic synaptic vesicles characterized a second bouton type of unknown origin, which made contacts with somata, initial segments, and dendrites, but not dendritic spines. Boutons containing large pleomorphic synaptic vesicles had the most widespread distribution, contacting all regions including dendritic spines. Spines receiving these contacts also were postsynaptic to boutons containing small round vesicles. Axon collaterals of spiny cells formed synapses with large pleomorphic vesicles and made synapses with somata, initial segments of axons, dendrites, and dendritic spines of striatal neurons, including other spiny cells.     

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 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...     

Direct observations of synapses between GABA-immunoreactive boutons and identified spinocervical tract neurons in the cat's spinal cord.

Three spinocervical tract neurons in adult cats were physiologically characterized and intracellularly labelled with horseradish peroxidase. The neurons were reconstructed and examined with the light microscope and were prepared for postembedding immunochemical analysis by using an antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Semithin sections were tested and examined with the light microscope. Somata, proximal, and distal dendrites of all three cells were associated with numerous punctate GABA-immunoreactive structures. Immunoreactive perikarya of small neurons in the vicinity of spinocervical tract cells were also observed. Ultrastructural analysis, with the immunogold technique, revealed that somata and proximal dendrites of all three neurons received synaptic contacts (about 37% of total synapses) from GABA-immunoreactive boutons and that distal dendrites were also associated with substantial numbers of immunoreactive structures (about 27% of synapses). Immunoreactive boutons were small (about 1 micron in diameter), contained irregularly shaped agranular vesicles, and formed symmetrical synaptic junctions with identified neurons. An additional group of immunoreactive boutons was observed to be associated with one of the cells only; these contained many large dense-core vesicles in addition to small agranular vesicles. Boutons containing round agranular vesicles and flattened agranular vesicles were not observed to be immunoreactive. The evidence supports the idea that much of the postsynaptic inhibition observed in spinocervical tract neurons is mediated by GABA and that even the most distal dendrites of these neurons receive inhibitory inputs.     

Spatial distribution of synapses on tyrosine hydroxylase–expressing juxtaglomerular cells in the mouse olfactory glomerulus

Olfactory sensory axons converge in specific glomeruli where they form excitatory synapses onto dendrites of mitral/tufted (M/T) and juxtaglomerular (JG) cells, including periglomerular (PG), external tufted (ET), and superficial‐short axon cells. JG cells consist of heterogeneous subpopulations with different neurochemical, physiological, and morphological properties. Among JG cells, previous electron microscopic (EM) studies have shown that the majority of synaptic inputs to tyrosine hydroxylase (TH)‐immunoreactive neurons were asymmetrical synapses from olfactory nerve (ON) terminals. However, recent physiological results revealed that 70% of dopaminergic/γ‐aminobutyric acid (GABA)ergic neurons received polysynaptic inputs via ET cells, whereas the remaining 30% received monosynaptic ON inputs. To understand the discrepancies between EM and physiological data, we used serial EM analysis combined with confocal laser scanning microscope images to examine the spatial distribution of synapses on dendrites using mice expressing enhanced green fluorescent protein under the control of the TH promoter. The majority of synaptic inputs to TH‐expressing JG cells were from ON terminals, and they preferentially targeted distal dendrites from the soma. On the other hand, the numbers of non‐ON inputs were fewer and targeted proximal dendrites. Furthermore, individual TH‐expressing JG cells formed serial synapses, such as M/T→TH→another presumed M/T or ON→TH→presumed M/T, but not reciprocal synapses. Serotonergic fibers also associated with somatic regions of TH neurons, displaying non‐ON profiles. Thus, fewer proximal non‐ON synapses provide more effective inputs than large numbers of distal ON synapses and may occur on the physiologically characterized population of dopaminergic‐GABAergic neurons (70%) that receive their most effective inputs indirectly via an ON→ET→TH circuit. J. Comp. Neurol. 525:1059–1074, 2017. © 2017 Wiley Periodicals, Inc.     

Morphology of afferent synapses in the Mauthner cell of larval Xenopus laevis

The fine structure of the afferent synapses on the Mauthner cell of larval Xenopus laevis has been studied as a first step toward comparing the fine structure of the afferent synaptic apparatus before and after metamorphosis. There are various types of afferent endings on this cell, some of which are confined to specific cellular regions, while others are distributed over most of the large surface of the neuron. Four different main types of endings have been observed: club endings, round-vesicle end bulbs, flattened-vesicle end bulbs and spiral fibers endings. While the myelinated club endings and the spiral fibers endings are located at the distal end of the lateral dendrite and in the axon cap, respectively, the end bulbs are widely distributed over the whole cell. A further type of ending has been observed, although rarely, on the Mauthner cell soma and dendrites: end bulbs characterized by an unusually dense presynaptic substance. Results obtained in the present research suggest that, as in fish, different endings on the anuran Mauthner neuron correspond to different synaptic inputs. The possible origin of some of these inputs is discussed.     

Projection of olfactory bulb efferents to layer I GABAergic neurons in the entorhinal area. Combination of anterograde degeneration and immunoelectron microscopy in rat

Glutamic acid decarboxylase (GAD) immunoelectron microscopy in combination with anterograde degeneration was applied in rats to study the synaptic targets of olfactory bulb afferents to the lateral subdivision (LEA) of the entorhinal area (EA). Immunoreactive neurons and terminals are scattered throughout all layers of LEA. After olfactory bulb resection, terminal degeneration occurs in layer Ia of EA. Using the electron microscope we examined serial thin sections of 12 and 14 immunoreactive neurons sampled from layer Ia of the dorsal (DLEA) and ventral (VLEA) subdivisions of LEA, respectively. The morphology of all these neurons is similar: they are small (short axis 5-9 micron, long axis 7-12 micron) and possess eccentrically located, indented nuclei provided with filamentous nuclear rodlets. The immunoreactive neurons have thin, smooth dendrites which usually emerge abruptly from the somata. We observed a single cilium on 5 of the immunoreactive neurons. In layer Ia of both DLEA and VLEA, the somata of the immunoreactive neurons are contacted by degenerating, non-immunoreactive boutons showing asymmetric synaptic junctions. In addition to these boutons, 4 other categories of axo-somatic terminals can be distinguished: normal, non-immunoreactive boutons forming asymmetric synapses and containing spherical synaptic vesicles; normal, non-immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles; normal, non-immunoreactive boutons with asymmetric synapses, containing dense-cored vesicles in addition to spherical synaptic vesicles; and normal, immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles. It is suggested that the GAD-immunoreactive neurons which receive olfactory bulb input correspond to local circuit neurons with intralaminar axons which innervate each other as well as the distal segments of the apical dendrites of projection neurons with cell bodies in layers II and III. Thus, the olfactory input in EA seems to be wired not only for excitation of layers II and III pyramidal neurons but also for feed-forward inhibition using GABAergic intermediary neurons, strategically located in the area of termination of olfactory bulb fibers.     

Serotonergic synaptic input to cholinergic neurons in the rat mesopontine tegmentum

Serotonergic synaptic inputs to cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei were examined with pre-embedding dual-label immunoelectron microscopy. Numerous serotonin-immunoreactive axon terminals visualized with a silver-enhanced immunogold method were present in both of these tegmental nuclei. Serotonergic terminals occasionally made synaptic contacts with the soma and proximal dendrites of cholinergic tegmental neurons labelled with a choline acetyltransferase-immunoreactive peroxidase-anti-peroxidase diaminobenzidine reaction product. In the rostralmost region of the laterodorsal tegmental nucleus, a few serotonergic neurons of the dorsal raphe nucleus were interspersed among cholinergic neurons. Some dendrites of these serotonergic neurons appeared to contain synaptic vesicles. Both myelinated and unmyelinated serotonergic axons were present in the mesopontine tegmentum. The presence of serotonergic synapses onto tegmental cholinergic neurons is consistent with previous behavioral and electrophysiological findings suggesting an inhibitory role of serotonin in the induction of rapid eye movement sleep and its phenomenology through an action on cholinergic neurons in the mesopontine tegmentum.     

An electron microscopic analysis of the trans-synaptic effects of peripheral nerve injury subsequent to tooth pulp extirpations on neurons in laminae I and II of the medullary dorsal horn

To examine the effects of peripheral nerve injury on second-order neurons in laminae I and II of the medullary dorsal horn, tooth pulps of all mandibular teeth in adult cats on one side were extirpated. This procedure severed and removed the receptors and terminal branches of the primary trigeminal neurons which innervate the tooth pulps of these teeth. The empty pulp chambers were then filled with dental cement to prevent regeneration. At 30 and 60 days postoperatively, membrane-lined cavities had formed inside many of the small-caliber dendrites of second-order neurons in laminae I and II. Cavity formation occurred mainly in dendritic shafts less than 2 micron in diameter and involved dendrites with synaptic vesicles as well as those without synaptic vesicles. The cavities extensively hollowed out these dendrites, often occupying more than half the cross-sectional diameter of the shafts and extending for appreciable distances in the long axis of the shaft. The process of cavitation ultimately resulted in the destruction of the affected dendrites. Many cavities became patent to the intercellular space with the cavity membrane establishing continuity with the dendritic membrane. Many cavities often formed in a single dendrite, and such severely cavitated dendrites became reduced to a trabeculated shell which ultimately fragmented into several small pieces. The presence of synaptic connections from a number of different kinds of axonal endings, including scalloped and dome-shaped endings, was not sufficient to prevent cavitation. The actual severing of synaptic connections on the cavitated dendrite appeared to be a relatively late event in the process since small pieces of dendritic debris could still be found clinging to their axodendritic synapses. Evidence that dendrites were being lost from the neuropil was most readily apparent in many of the disrupted glomeruli in lamina II in which many of the scalloped depressions in the central axonal endings that normally contained small dendrites were empty. Many central axonal endings remained in synaptic contact with only a single dendrite which often showed signs of cavitation. Such central endings showed only subtle remaining traces of their normal scalloped contours. This study demonstrates that injury to the distal branches of primary trigeminal neurons which innervate tooth pulps resulted in trans-synaptic degenerative changes in the dendritic arbors of second-order neurons which destroyed fine-caliber higher order dendrites.