Types Of Synaptic Boutons Research Articles

Light and electron-microscopic study of leucine enkephalin immunoreactivity in the cat claustrum

The claustrum is a complex telencephalic structure owing to its reciprocal connectivity with most--if not all--cortical areas. However, there is a paucity of data in the literature concerning its histochemical components, including opioid peptide neurotransmitters. The aim of the present study was to examine the morphology, distribution and ultrastructure of leucine-enkephalin-immunoreactive (Leu-enk-ir) neurons and fibers in the dorsal claustrum (DC) of the cat. Seven healthy, adult male and female cats were used in our study. All animals received humane care. They were irreversibly anesthetized and transcardially perfused with fixative. Brains were removed, postfixed, blocked and sectioned. Sections were incubated with polyclonal anti-Leu-enk antibodies using the Avidin-Biotin-Peroxidase Complex method. Leu-enk-ir neurons and fibers were distributed throughout the DC. Some of the neurons were lightly-stained, while others were darkly-stained. Light-microscopically, they varied in shape: oval, fusiform, multipolar and irregular. With regard to size, they were categorized as small (15 μm or less in diameter), medium (16-20 μm in diameter) and large (21 μm or more in diameter). No specific pattern of regional distribution was found. On the electron microscope level, immunoproduct was observed in neurons, dendrites and terminal boutons. Different types of Leu-enk-ir neurons differ in their ultrastructural features, including two types of synaptic boutons. No gender-specific features were observed. In conclusion, it is our hope that our study will serve to contribute to a better understanding of the functional neuroanatomy of the DC in the cat, and that it can be extrapolated and applied to other mammals, including humans.

Efficient synaptic vesicle recycling after intense exocytosis concomitant with the accumulation of non-releasable endosomes at early developmental stages

Following the exocytosis of neurotransmitter-containing synaptic vesicles, endocytosis is fundamental to re-establishing conditions for synaptic transmission. As there are distinct endocytotic pathways that each differ in their efficiency to generate releasable synaptic vesicles, we used the dye FM1-43 to track vesicle recycling, and to determine whether nerve terminals use multiple pathways of endocytosis. We identified two types of synaptic boutons in cultured cerebellar granule cells that were characterized by weak or strong FM1-43-unloading profiles. Decreasing the extent of exocytosis dramatically increased the proportion of synaptic boutons that exhibited strong FM1-43-unloading and dramatically reduced the number of endosome-like structures. Hence, we concluded that efficient recycling of synaptic vesicles is concomitant with the formation of non-releasable endosomes in both types of synaptic boutons, although to different extents. Furthermore, cell maturation in culture increased the proportion of synaptic boutons that were capable of an intense release response, whereas the chronic blockage of synaptic activity diminished the capacity of boutons to release dye.

Parvalbumin in the cat claustrum: Ultrastructure, distribution and functional implications

The presence of the calcium-binding protein (CaBP) parvalbumin (PV) in the neuronal elements of the cat's dorsal claustrum was studied by immunohistochemistry at the light- and electron-microscopic level. PV-immunoreactive neurons and fibers were detected in all parts of the claustrum. The PV-immunoreactive neurons were divided into several subtypes according to their size and shape. Approximately 7% of all PV-immunoreactive neurons were classified as large, while approximately half of the labeled neurons were medium-sized. The small PV-immunoreactive neurons were 45% of the total PV-immunoreactive neuronal population. Ultrastructurally, many spiny and aspiny dendrites were heavily immunolabeled, and the reaction product was present in dendritic spines as well. Several types of synaptic boutons containing reaction product were also found. These boutons terminated on both labeled and unlabeled postsynaptic targets (soma, dendrites, etc.), forming asymmetric or symmetric synapses. Approximately 70% of all PV-immunoreactive terminals contained round synaptic vesicles and formed asymmetric synapses. The majority of these boutons were of the ''large round'' type. A lesser percentage were of the ''small round'' type. This paper represents the first study demonstrating the existence of PV, a CaBP, in the cat claustrum, and its distribution at the light and electron microscope level. Beyond the relevance of this research from the standpoint of adding to the paucity of literature on PV immunoreactivity in the claustrum of various other mammals (e.g. monkey, rabbit, rat, mouse), it is of particular significance that the cat claustrum is more similar to the rabbit claustrum than to any other mammalian species studied thus far, noted by the existence of four distinct morphologic subtypes. We also demonstrate a lack of intrinsic, and possibly functional, heterogeneity as evidenced by the uniform distribution of PV throughout the cat claustrum, across the four cell subtypes (i.e. inhibitory interneurons as well as projection neurons). Indeed, the association with, and influence of, the cat claustrum on diverse multisensory mechanisms may have more to do with its afferent than efferent relationships, which speaks strongly for its importance in the sensory hierarchy. Exactly what role PV plays in the claustrum is subject to discussion, but it can be postulated that, since CaBP is associated with GABAergic interneurons, synaptogenesis and neuronal maturation, it may also serve as a neuroprotectant, particularly with regard to pathologies associated with the aging process, such as in Alzheimer's disease.

Parvalbumin-like immunostaining in the cat inferior colliculus. Light and electron microscopic investigation

The presence of the calcium-binding protein parvalbumin (PV) was studied in neuronal elements of the cat's inferior colliculus (IC) by means of light and electron microscopic immunocytochemistry. Immunostaining of PV was detected in all three main parts of the IC. Several subtypes of large neurons that differed in size and shape were immunostained, comprising approx. 15% of the total number of PV-containing neurons. Approx. half of the labeled neurons were medium sized. Two types of small neurons were found to be PV synthesizing, and comprised approx. 35% of the total PV-containing population. Ultrastructurally, many dendrites were heavily immunolabeled, and the reaction product was present in dendritic spines as well. Several types of synaptic boutons contained reaction product, and terminated on both labeled and unlabeled postsynaptic targets forming asymmetric and symmetric synapses. Approx. 70% of all PV-immunolabeled terminals contained round synaptic vesicles and formed asymmetric synapses. The majority of these boutons were of the “large round” type and corresponded to the terminals of cochlear nuclei. A lower number were of the “small round” type, and were probably corticotectal terminals. The remaining 30% of PV-containing terminals contained pleomorphic or elongated vesicles and formed symmetric synapses. These terminals corresponded with “P” and “F1” bouton types. Part of these boutons appeared to arise from nuclei of the lateral lemniscus and the superior olive, and a certain percentage likely represented endings of inhibitory interneurons.

Alpha-motoneurons of the injured cervical spinal cord of the adult rat can reinnervate the biceps brachii muscle by regenerating axons through peripheral nerve bridges: combined ultrastructural and retrograde axonal tracing study.

Following our previous studies related to brachial plexus injury and repair, the present experimentation was designed to examine the ultrastructural features of those motoneurons of the locally injured cervical spinal cord of adult rats that were seen to regenerate into peripheral nerve (PN) bridges and to reinnervate nearby skeletal muscles. Here, the peripheral connection of the PN bridge was made with the biceps brachii (BB) muscle. Three months postsurgery, the spinal motoneurons labelled by retrograde axonal transport of horseradish peroxidase (HRP), after its injection into the BB, were selected on thick sections, using light microscopy, for the presence of dark amorphous granules of the HRP reaction product. Serial ultrathin sections were then made from the selected material. For the 10 labelled neurons studied, we examined the synaptic boutons present on the membrane of the neuronal soma. For five of them, we could observe three of the six types of synaptic boutons described for the alpha-motoneurons of the cat (S-type with spherical vesicles, F-types with flattened vesicles, and C-type with subsynaptic cistern). The largest boutons (type C) are specific to alpha-motoneurons. In comparison to normal material, we noticed a decrease in the number of boutons and an increase in the number of glial processes. After a transient phase of trophic changes, the reinnervated BB muscles showed a return of their fibers to nearly normal diameters as well as evidence of fiber type grouping. Simultaneous staining with silver and cholinesterase also revealed the presence of new motor endplates frequently contacted by several motoneurons. The present study indicates that, after a local spinal injury, typical alpha-motoneurons can reinnervate a skeletal muscle by regenerating axons into the permissive microenvironment provided by a PN graft. These data offer prospects for clinical reconstruction of the brachial plexus after avulsion of one or several nerve roots.

18 Patch-Clamp Recordings from Drosophila Presynaptic Terminals

Publisher Summary This chapter focuses on patch-clamp recordings from drosophila presynaptic terminals. In general, the events that take place in synaptic terminals have had to be inferred from recordings performed in the soma. However, it is generally admitted that the synaptic membrane is endowed with a wide and specialized repertoire of ion channels whose activity determines the modulation of neural signaling. Also, mounting evidence from the in situ expression of synaptic proteins and, most relevant, localization of specific protein isoforms indicates that synapses are very diverse. The necessity of approaching synaptic studies from a multidisciplinary perspective justifies the effort to develop suitable methods in organisms where genetic and behavioral studies are also feasible. To that end, the chapter has tried to develop a new preparation for synaptic transmission in Drosophila. The Drosophila larval body-wall muscle fibers are innervated by several axons that produce morphologically distinct synaptic contacts on specific fibers. The chapter describes several types of synaptic boutons, classed as types I to III, which differ in their size, morphology, and vesicle phenotype.

Ultrastructure of the Rat Mesencephalic Trigeminal Nucleus

The subcellular morphology of the mesencephalic trigeminal (Me5) nucleus in the rat was studied by transmission electron microscopy. Most neurons in the thin rostral as well as in the major caudal part of Me5 appeared as large (40-50 microns), round- to ovoid-shaped unipolar cells. A few neurons (estimated 5%) appeared to be multipolar, usually bipolar. The Me5 neurons had a large, round, centrally located nucleus, and their cytoplasm was characterized by a dense network of lamellar granular endoplasmic reticulum, an abundant Golgi apparatus, many mitochondria and neurofilaments suggesting very active cells with a high rate of synthesis and axoplasmatic transport. Numerous small spinous processes covered the surface of the Me5 neurons. Clustering of 2 or 3 cells was accomplished by maculae, i.e. zones of gap junctions and close cell appositions. Boutons contacting the soma of Me5 neurons and boutons contacting large and small dendrites were defined as axosomatic and axodendritic synapses, respectively. Four types of synaptic boutons were distinguished: (1) S boutons, with round vesicles and asymmetrical as well as symmetrical synapses, (2) F boutons, with pleomorphic admixture of flattened and spherical vesicles and asymmetrical synapses, (3) P boutons, which resembled the F-type boutons but contained predominantly spherical vesicles and symmetrical synapses, and (4) G boutons, characterized by a heterogeneous population of vesicles. This description of the Me5 nucleus is particularly useful for future studies that attempt to correlate the structure of a particular synapse with its function.

Ultrastructural characterization of the central component of the medial preoptic nucleus

The fine structure of the sexually dimorphic central part of the medial preoptic nucleus was studied in male and female rats. Neurons of this subnucleus are characterized by having convoluted nuclear membranes and fibrillar nuclear inclusions. Within the perinuclear domain there is a well developed neurosecretory apparatus, i.e., Golgi complexes, assorted types of secretory granules, and lysosome-like bodies, whereas the marginal cytosol contains stacked cisterns of rough endoplasmic reticulum, nucleolus-like inclusions, and few mitochondria. The neuropil is made up of thick dendrites, fascicles of thin unmyelinated axons, and few small myelinated axons. Additionally four types of synaptic boutons were found according to the morphology of their vesicular contents. No apparent qualitative sex differences were noted. These cytologic features which suggest secretory activity, coupled with reports that have shown the presence of several neurohormones and peptides within the central part of the medial preoptic nucleus lend further support to the neuroendocrine nature of these neurons.

Synapse formation and other ultrastructural features of postnatal development in the basilar pontine nuclei of the rat.

The postnatal development of the basilar pontine nuclei (BPN) was studied in the rat from birth through postnatal day 21. Although 3 maturational phases could be discerned within this time span, the events taking place within each phase overlap to some degree and the postnatal maturation of this region is best characterized as a continuum comprised of several simultaneously occurring processes. The early postnatal period (birth to postnatal day 5) is best described as a period of relative immaturity. Although generally similar to the adult BPN cytoarchitectonically, cell density in the early neonate is substantially higher than in the adult and this obscures to some degree the various subdivisions of the pontine gray. Neuronal somata are quite small in cross-sectional area relative to the adult and the neuropil is similarly immature and exhibits wide expanses of extracellular space and few synaptic boutons. These features are in marked contrast to the second developmental period which extends from postnatal days 6 through 13 and is characterized by an intense wave of synaptic proliferation. It is during this period that each of the various types of synaptic boutons that have been described in the adult make their appearance. Curiously, during this same time period, as well as in the early postnatal period, there also appears to be an ongoing process of spontaneous axon terminal elimination. The third developmental period begins on postnatal day 14 and extends through day 21 at which time the cytoarchitectonics, cytology and synaptic organization of the BPN are adult-like. During this period, no additional types of synaptic boutons appear, although the number of boutons continues to increase in a fairly uniform manner across all the various categories of synaptic boutons.

Identification des terminaisons vestibulaires dans les noyaux oculomoteurs communs chez le chat par radioautographie en microscopie electronique

The projection of vestibular pathways to the oculomotor nuclei was investigated by electron microscopic radioautography. Unilateral injection of tritiated amino acids into the rostral vestibular complex was used in order to characterize the location and to identify the different types of labeled synaptic terminals involved in these pathways. In the normal oculomotor nuclei, 4 types of synaptic boutons were identified. Following the labeling of the vestibular synapses, in the ipsilateral oculomotor nucleus, types I and II boutons are the most prominent group and make up 75% of the total number of this labeled synaptic population. Large boutons with small diameter synaptic vesicles, they are distributed on the cellular soma and the large dendrites of the oculomotor neurons. In contrast, in the contralateral oculomotor nucleus, type III boutons which are smaller and have larger diameter synaptic vesicles were predominant; they are prevalent on the distal part of the dendritic tree. From the results obtained, a relationship between the present anatomical findings and previously published physiological studies is established. The following conclusion is suggested: the inhibitory vestibular inputs probably terminate on the oculomotor neurons by these large types I and II boutons and the excitatory vestibular inputs by the smaller type III boutons. Also discussed is the complexity of the pattern of afferentation and the functional arrangement of the oculomotor nuclei.

Ultrastructural observations in the frog spinal cord in relation to the generation of primary afferent depolarization

The spatial distribution of different types of synaptic boutons over the motoneuron cell body and dendrites was studied both in the normal and in the chronically deafferented frog spinal cord. In both preparations, endings with S-type (spherical) vesicles were more numerous than those with F (flat) vesicles, the S F synapse ratio being approximately constant when measured at the soma, the proximal dendrites or the fine dendrites. However, the proportion of S boutons, as well as the proportion of degenerated boutons of afferent fibers in close apposition with other boutons, was higher on the fine dendrites in the lateral neuropile than on the motoneuron cell body and proximal dendrites, and in all cases significantly higher than the proportion of F boutons showing close appositions with other boutons. Degenerated boutons from descending fibers were found to synapse over the motoneuron cell body and proximal dendrites, but these boutons were rarely seen to be closely apposed to other boutons. The possible relation of these findings to the mechanisms generating primary afferent depolarization is discussed.

The ultrastructure of the nucleus of the oculomotor nerve (somatic efferent portion) of the cat.

The ultrastructure of the somatic efferent portion of the nucleus of the oculomotor nerve was studied in four adult cats. The neuronal population is composed of neurons of variable size. A continuous pattern of morphological aspects is evident between the large neurons, which show abundant cytoplasm with well developed organelles, and the small neurons which have a reduced amount of cytoplasm. The dendrites are generally smooth, with few short spines. Axo-dendritic synapses are numerous. Synaptic boutons are also present on the axon hillock. The neuropil is characterized by the occurrence of small groups of dendrites which may be in direct touch with their membranes. Direct membrane appositions may also occur between neighbouring neurons and between the cell somata and tangentially running dendrites. Generally beneath the site of apposition there is accumulation of mitochondria, multivesicular bodies, coated vesicles and moderately dense amorphous material. The morphological features suggest the possibility of cellular interchanges at the sites of direct membrane apposition. Five types of synaptic boutons were recognized on the basis of their vesicular content, the presence of abundant filaments in the pre-synaptic bag, the occurrence of post-synaptic specializations. The different synaptic types and their distribution are similar to those reported in the spinal motor nuclei. Many of the synapses make synaptic contacts with two or more post-synaptic elements. Axo-axonic synapses were also observed.