Symmetric Axosomatic Synapses Research Articles

Differential distribution of neurons in the gyral white matter of the human cerebral cortex

The neurons in the cortical white matter (WM neurons) originate from the first set of postmitotic neurons that migrates from the ventricular zone. In particular, they arise in the subplate that contains the earliest cells generated in the telencephalon, prior to the appearance of neurons in gray matter cortical layers. These cortical WM neurons are very numerous during development, when they are thought to participate in transient synaptic networks, although many of these cells later die, and relatively few cells survive as WM neurons in the adult. We used light and electron microscopy to analyze the distribution and density of WM neurons in various areas of the adult human cerebral cortex. Furthermore, we examined the perisomatic innervation of these neurons and estimated the density of synapses in the white matter. Finally, we examined the distribution and neurochemical nature of interneurons that putatively innervate the somata of WM neurons. From the data obtained, we can draw three main conclusions: first, the density of WM neurons varies depending on the cortical areas; second, calretinin-immunoreactive neurons represent the major subpopulation of GABAergic WM neurons; and, third, the somata of WM neurons are surrounded by both glutamatergic and GABAergic axon terminals, although only symmetric axosomatic synapses were found. By contrast, both symmetric and asymmetric axodendritic synapses were observed in the neuropil. We discuss the possible functional implications of these findings in terms of cortical circuits.

Electron microscopic data on the neurons of nuclei subpretectalis and posterior-ventralis thalami. A combined immunohistochemical study.

The nucleus rotundus receives GABA-like immunoreactive fibres from the nuclei subpretectalis and postero-ventralis thalami. This result was confirmed by Phaseolus vulgaris leucoagglutinin (PhA-L) anterograde tracer and with electron microscopic (EM) gamma-aminobutiric acid (GABA)-immunogold staining. The detailed electron microscopic analysis of the structure of the neurons in these nuclei revealed that the neurons in the nucleus subpretectalis displayed GABA-like immunoreactivity. In the postero-ventral thalamic nucleus a group of neurons was GABA-positive. The surface of the neurons was covered both with numerous GABA-negative and GABA-like immunoreactive terminals that established asymmetrical and symmetrical synapses, respectively, with the GABA-positive neurons. The GABA-like immunonegative terminals are supposed to be the axon terminals of the collaterals of tecto-rotundal fibres in the subpretectal nucleus and the collateral terminal branches of contralateral tecto-rotundal fibres in the postero-ventralis thalami. In both nuclei, the GABA-like immunoreactive terminals may be developed by the collaterals of local neurons that establish symmetrical synapses. In the Phaseolus lectin-stained preparations these terminals may be labelled. The morphological characteristics of the neurons in the subpretectal and partly, in the posteroventral nuclei are similar to those of interneurons (local circuit neurons) and the numerous asymmetrical and symmetrical axo-somatic synapses, respectively. But these neurons locate outside of their target nucleus, and exert their modulatory effect on rotundo-ectostriatal transmission. Also, a contralateral influence is present in the nucleus rotundus that may interact in the cooperation of the eyes. The neurons of the subpretectal and posteroventral nuclei, similarly to the neurons of isthmic nuclei, are a special group of modulatory neurons with effects at a distance.

Freeze-etching study of the axosomatic synapses in the rat sensorimotor cortex.

The intramembranous structure of axosomatic synapses in the rat sensorimotor cortex was studied by means of thin sections and freeze-etched replicas. In thin sections, symmetric synaptic junctions were located on perikarya of pyramidal and non-pyramidal neurons. In freeze-etched preparations, the postsynaptic neuronal membrane exhibited the same structure as the surrounding nonjunctional membrane on both fracture faces. Probably this is the freeze-etching equivalent of symmetric axosomatic synapses. Asymmetric synapses were found on perikarya of non-pyramidal neurons. In freeze-etched replicas, the postsynaptic non-pyramidal neuronal membrane contains aggregates of particles on the extracellular half (E-face) as an intramembranous equivalent of asymmetric synapses. The particle packing density in these aggregates was measured and compared with that at the E-face of dendritic shaft and dendritic spine membranes. Intramembranous characteristics of the axosomatic synapses in the rat sensorimotor cortex are discussed in terms of plasticity of the synaptic contact zone.

Neurotensin immunoreactivity in the colliculus inferior of cat: Light and electron microscopic investigations

Neuronal perikarya with neurotensin-like immunoreactivity (NT-LI) are found in the pericentral and external nuclei and the dorsomedial part of the central nucleus of the inferior colliculus. On electron-microscopic examination, these neurons are characterized by a relatively large nucleus, a small amount of cytoplasm and paucity of organelles. NT-LI is observed in large and small dendrites and dendritic spines as well as in axon terminals which contain small round or elongated clear vesicles and/or dense core vesicles. The immunoreactive terminal boutons make asymmetrical axodendritic and symmetrical axo-somatic synapses mainly on non-immunoreactive elements.

Effect of retinal impulse blockage on cytochrome oxidase-rich zones in the macaque striate cortex: I. Quantitative electron-microscopic (EM) analysis of neurons

Our previous light-microscopic study indicates that unilateral retinal impulse blockage with tetrodotoxin (TTX) causes a reversible decrease of cytochrome oxidase (CO) in alternating rows of metabolically active zones (puffs) in the adult macaque striate cortex (Wong-Riley & Carroll, 1984b). The goal of the present study was to determine if TTX blockade adversely affects all neurons or only a subpopulation of neurons within the puffs. Three major neuronal types were identified based on mitochondrial CO activities and morphological characteristics. Type A neurons were the most prevalent, consisting of small pyramidal and nonpyramidal neurons that received only symmetrical axosomatic synapses. They had little cytoplasm and relatively low levels of CO activity, and showed the least change with TTX treatment. Type B cells were medium-to-large pyramidal neurons that received exclusively symmetrical axosomatic synapses and were moderately reactive for CO. Impulse blockage caused a decrease in mitochondrial size and packing density, but somal size remained within the control range. Type C cells were medium-sized nonpyramidal neurons contacted by both asymmetrical and symmetrical axosomatic synapses. They contained abundant darkly reactive mitochondria and presumably are metabolically the most active. This cell type suffered the greatest decrease in somal size and packing density of mitochondria, particularly the darkly reactive ones. A rare fourth cell type, type D, was a small, darkly reactive nonpyramidal variety that gave rise to somatodendritic synapses. Their low occurrence prevented statistical analysis under normal and TTX-treated conditions. These data indicate that retinal impulse blockade is most detrimental to the metabolically most active neurons in the adult primate cortical puffs. The alterations are not permanent, because the effects of TTX are fully reversible (Carroll & Wong-Riley, 1987).

Light and electron microscopic studies of calcitonin gene-related peptide-like immunoreactive terminals in the central nucleus of the amygdala and the bed nucleus of the stria terminalis of the rat

Light and electron microscopic analysis of calcitonin gene-related peptide (CGRP)-like immunoreactive (LI) terminals in the bed nucleus of the stria terminalis (BST) and the central nucleus of the amygdala (Ce) was carried out using the peroxidase-antiperoxidase method. CGRP-LI fibers were densely distributed in the dorsal subdivision of the lateral BST (BSTL) and the lateral and lateral capsular subdivisions of the Ce, where the CGRP-LI terminals formed symmetrical and asymmetrical axo-dendritic, and symmetrical axosomatic synapses. One of the most characteristic features of the CGRP-LI terminals was the presence of large, long boutons, each of which surrounded a cell soma and made many synaptic contacts. These findings suggest that CGRP exerts a significant influence on neurons in the BSTL and Ce.

Corticotropin-Releasing Factor Neurons Innervate Dopamine Neurons in the Periventricular Hypothalamus of Juvenile Macaques

Corticotropin-releasing factor (CRF) and dopamine (DA) are important integrators of the endocrine and autonomic response to stress. CRF neurons in the anterior portions of the periventricular nucleus (PV) and parvocellular paraventricular nucleus (pvPVN) occur close to A14 DA neurons in these same locations. Since CRF has been shown to act as an excitatory neurotransmitter, possible CRF interactions with the DA system were investigated using double-label immunocytochemistry. Coronal vibratome sections through the PV and pvPVN were obtained from colchicine-treated and nontreated juvenile female cynomolgus macaques. They were sequentially immunostained for tyrosine hydroxylase (TH) (to identify DA neurons) with PAP and DAB, and for CRF using 15 nm colloidal gold. By light microscopy, areas of coincidence of TH- and CRF-immunoreactive cell bodies in the PV and pvPVN were obvious, but double-stained elements were not observed. By electron microscopy, asymmetrical synapses frequently occurred between CRF axons and TH dendrites or somata. Symmetrical axosomatic synapses sometimes appeared adjacent to these CRF/TH synapses, while symmetrical axoaxonic synapses were rare. We conclude that CRF neuronal efferents synaptically activate A14 DA neurons in the primate PV and pvPVN. Parallel CRF/DA symmetrical synapses also suggest coexistence of a companion transmitter within some of these same CRF neurons. Our own previous work and recent independent studies indicate that this transmitter is probably GABA. Thus the CRF neuronal system, which is known to alter secretion of several pituitary hormones, may also act through hypothalamic periventricular DA neurons to mediate other responses to stress.

Maturation of rat visual cortex: IV. The generation, migration, morphogenesis, and connectivity of atypically oriented pyramidal neurons

The generation, migration, and morphogenesis of atypically oriented pyramidal neurons in the rat visual cortex were examined. In the mature cortex, these neurons were distributed through layers II-VI. Moreover, the atypically oriented pyramidal neurons in a particular layer tended to be oriented in a specific way; atypically oriented pyramidal neurons in layer II, layers III-VIa, and layer VIb were obliquely, radially, and obliquely oriented, respectively. Ultrastructurally, the somata of atypically oriented pyramidal neurons contained large euchromatic ovoid nuclei and cytoplasm that was replete with rough endoplasmic reticulum and Golgi apparatus. These somata formed only symmetric axosomatic synapses. Many atypically oriented pyramidal neurons projected axons into the white matter as demonstrated by a Golgi method and by a retrograde tract-tracing technique; however, some of these pyramidal neurons in layers III-V had axons that ascended to layer I. By using a technique which combined retrograde tract tracing with [3H]thymidine autoradiography, it was determined that most atypically oriented pyramidal neurons in layers V and VIa, layer IV, and layer II were generated on gestational days (GD) 15-17, GD 17-19, and GD 20-21, respectively. Atypically oreinted pyramidal neurons were identified during the period from postnatal day 0 (day of birth) to day 30. On day 0, obliquely oriented pyramidal neurons were distributed in the deep cortical plate, i.e., immature layer VI. On day 3, the youngest atypically oriented pyramidal neurons were radially oriented and were located in layer IV. Some obliquely oriented pyramidal neurons were present in layer II on day 6, but the greatest number and the most severely canted pyramidal neurons in layer II were evident on day 9. The orientations of the cell body and the apical dendrite did not change appreciably after migration was complete, except for those in layers V and VI with obliquely oriented cell bodies and radially oriented apical dendrites. The second and third postnatal weeks were marked by substantial morphological differentiation of all pyramidal neurons as noted by the lengthening and branching of dendrites and by the appearance of dendritic spines. By the fourth postnatal week, atypically oriented pyramidal neurons achieved their mature morphology. The generation, migration, and morphogenesis of atypically oriented pyramidal neurons proceed by an inside-to-outside sequence. This development is similar and concurrent with that of typically oriented pyramidal neurons.

An electron microscopic study of GABAergic neurons and terminals in the central nucleus of the inferior colliculus of the rat

Neurons and terminals in the ventral lateral portion of the central nucleus of the inferior colliculus (ICCN) of the rat were labelled immunocytochemically with antisera to GABA or to its synthesizing enzyme, glutamic acid decarboxylase. Four types of GABAergic neuron are described: small, medium-sized and large multipolar neurons, as well as medium-sized bipolar neurons. All sizes of GABAergic multipolar neurons are characterized by highly infolded nuclei, many mitochondria and both asymmetric and symmetric axosomatic synapses. A dense plexus of terminals occurs on the proximal dendrites of GABAergic neurons, and most of these terminals form asymmetric axodendritic contacts. Small GABAergic neurons (diameter less than 15 microns) are multipolar, and have a large nucleus to cytoplasm ratio, prominent nucleoli and usually two to five axosomatic synapses per thin section, with the majority of these contacts being symmetric. Medium-sized GABAergic neurons (15-25 microns in diameter) display both multipolar and fusiform shaped somata, have a more abundant cytoplasm than the small neurons and show about ten axosomatic contacts per thin section. Large GABAergic neurons (diameters greater than 25 microns) have eccentrically located, highly infolded nuclei, abundant cytoplasm and a denser plexus of terminals that form axosomatic synapses than the other cell types. These results indicate that four of the six major cell types in the ICCN are probably GABAergic inhibitory neurons. The axon initial segments of GABAergic neurons in the ICCN all have similar features in that they are contacted by only one or two terminals that form symmetric synapses on their proximal portions and are invested by a glial sheath from 3 to 20 microns from the cell body. Many immunoreactive myelinated axons (approximately 0.5 micron in diameter) are observed and some terminals that arise from these axons form synapses with small neuronal somata. Both these and other labelled terminals are shown to form symmetric synapses. These data suggest a complex circuitry for the GABAergic neurons within the ICCN.

The synaptic connections between substance P-positive nerve terminals and principal neurons in the superior cervical ganglion of the rat

Using immunoelectron microscopy to observe the distribution of substance P-positive nerve terminals in the superior cervical ganglion of the rat, the authors found that the substance P-containing nerve terminals appear in the neuropil or around the perikarya of the principal neurons and form symmetrical axo-somatic synapses with the body of the principal neurons. Morphological features described suggest that substance P might play a certain role in regulating sympathetic function.

Laminar and cellular localization of cytochrome oxidase in the cat striate cortex

Cytochrome oxidase (C.O.) was histochemically localized in the cat striate cortex at the light and electron microscopic levels. The results indicate that the oxidative metabolic activity within the cat striate cortex may vary between (1) different laminae, (2) neurons and glia, (3) different neuron types, (4) dendrite and soma of the same cell, (5) different types of dendrites, (6) different segments of the same dendrite, and (7) different classes of symmetric and asymmetric axon terminals. Maximal laminar C.O. staining was localized within geniculoreceptive layer IV. Darkly reactive neurons include the large (presumed corticotectal) pyramids of layer V, and various classes of large and medium-sized presumed GABAergic nonpyramidal cells sparsely distributed throughout layers II-VI. The small and medium-sized pyramids of layers II, III, V, and VI, as well as many of the smaller presumed GABAergic neurons, were only lightly or moderately reactive. The darkly reactive neurons tended to be those that received convergent or proximally localized asymmetric axosomatic synapses, implying that they are strongly driven by excitatory synaptic input. The darkly reactive nonpyramids resembled those that form GAD+, symmetric axosomatic synapses with pyramidal cells. The dark reactivity of the symmetric synaptic terminals indicates that they mediate strong inhibition of neuronal discharge. The dark reactivity of a class of large asymmetric terminals in layer IV is likely to represent highly active geniculocortical terminals. The predominant distribution of elevated C.O. reactivity in dendrites is correlated with reported sites of (1) convergent excitatory synaptic input, (2) maximal field potentials, (3) highly active ion transport, and (4) Na+, K+-ATPase.

Ultrastructural localization of phenylethanolamine N-methyltransferase in sensory and motor nuclei of the vagus nerve.

The ultrastructural localization of phenylethanolamine N-methyltransferase (PNMT), the enzyme used in the final step in the synthesis of adrenaline, was examined in the medial nuclei of the solitary tracts (m-NTS) and in the dorsal motor nuclei of the vagus. Adult rats were anesthetized with Nembutal (50 mg/kg intraperitoneally), and the brains were fixed by vascular perfusion with a solution containing 3.75% acrolein and 2% paraformaldehyde in 0.1 M phosphate buffer. Coronal Vibratome sections were collected through the intermediate portions of the m-NTS at the level of the area postrema. These sections were immunocytochemically labeled employing a rabbit polyclonal antiserum against PNMT and the peroxidase-antiperoxidase method. Immunoreactivity was detected in perikarya, dendrites, and axon terminals in the intermediate portion of the m-NTS. The labeled perikarya were either small (10-15 microns diameter) and oval or large 20-30 microns) with two or more proximal processes. The PNMT-containing dendrites received synaptic input from unlabeled, small (0.5-1.0 microns) and large (2-3 microns) vagal-like afferents as well as from a few terminals, which also showed PNMT immunoreactivity. Axons and axon terminals containing immunoreactive PNMT were more frequently observed than the perikarya or dendrites in the m-NTS and were the only labeled profiles in the dorsal motor nuclei. In both regions the PNMT-labeled terminals formed principally symmetric synapses with unlabeled dendrites. However, a few asymmetric axodendritic and symmetric axosomatic synapses also were detected. These findings indicate that the adrenergic neurons may have multiple, but principally inhibitory, actions on other neurons within cardiovagal portions of baroreflex pathways.

The neuronal composition of area 17 of rat visual cortex. I. The pyramidal cells.

The pyramidal cells in area 17 of rat visual cortex have been examined by light microscopy using Golgi preparations and semithin plastic sections, and by electron microscopy. Pyramidal cells have cell bodies in layers II-VIa. The pyramidal cells in the lower portion of layer II/III are typical examples of this neuronal type in that they have pyramidal-shaped cell bodies, apical dendrites which ascend to layer I, and a skirt of basal dendrites. The pyramidal cells in upper layer II/III are similar in form but have shorter apical dendrites, while the most superficial pyramidal cells lack apical dendrites and instead have two or more primary dendrites that emanate from the upper surface of their somata. In layer V the pyramidal cells are of two sizes, medium and large, and both have a typical morphology, although the larger neurons have thicker apical dendrites and better-developed axon hillocks than the medium-sized pyramids. The medium-sized pyramidal cells of layer V outnumber the large ones to a ratio of 2.5:1. In layer IV a few typical medium-sized pyramidal cells are present, but the majority are small and can be regarded as star pyramids for they have dendrites radiating in all directions. No clearly identified spiny stellate cells have been encountered in layer IV. The pyramidal cells of layer VIa are also small, and most of them have apical dendrites which only ascend as far as layer IV. In addition to these varieties, both inverted and horizontally inclined pyramidal cells have been encountered. In electron micrographs it is apparent that although all of the pyramidal cells have symmetric axosomatic synapses, the frequency with which these synapses occur varies. The cell bodies of the various forms of pyramidal cells do not show a standard cytology. The medium-sized pyramidal cells of layer II/III usually have rounded nuclei, while the nuclei of the small pyramidal cells of layers IV and VIa are somewhat more irregular, and the large pyramidal cells of layer V have deeply indented nuclear envelopes. The appearance of the perikaryal cytoplasm also varies. The larger pyramidal cells have numerous mitochondria and well-developed Nissl bodies in their perikaryal cytoplasm, but the smaller cells have much-less-pronounced mitochondria and their rough endoplasmic reticulum is only organized into stacks at the bases of dendrites. Pyramidal cells account for about 87% of profiles of neuronal cell bodies with nuclei in layer II/III, 90% in layer IV, 89% in layer V, and 97% in layer VIa.

The neuronal composition of area 17 of rat visual cortex. II. The nonpyramidal cells.

In the preceding article the characteristics of the various types of pyramidal cells present in area 17 of rat visual cortex were described (Peters and Kara, '85). In the present article the nonpyramidal cell population of this cortex is considered. It is known from Golgi preparations that in layers II-VIa there are bipolar cells, smooth or sparsely spinous multipolar and bitufted cells with either unmyelinated local plexus or myelinated axons, and chandelier cells. Each of these cell types has been previously examined in Golgi-electron microscopic preparations. The question now being asked is whether the information about the characteristics of these different types of nonpyramidal cells derived from the Golgi-electron microscopic studies can be used to identify the cell bodies of nonpyramidal cells in tissue prepared for conventional electron microscopy. If this can be done then the neuronal composition of area 17 can be determined. It has been found that the cell bodies of bipolar cells can be readily identified because they are elongate and have nuclei with a vertical infolding and few axosomatic synapses, which are of both the symmetric and asymmetric varieties. Evidence is presented to show that there are two types of bipolar cells, small ones and large ones, the large ones being distinguished by their well-developed endoplasmic reticulum in which the cisternae are arranged parallel to the cell surface. Bipolar cells account for 6% of the neuronal profiles in layer II/III, 3% in layer IV, 5% in layer V, and 2% in layer VIa. The cell bodies of other types of nonpyramidal cells in layers II-VIa cannot be distinguished from each other in thin sections, because recognition of the different cell types depends upon the characteristics and distribution of their dendrites and axons. However, it is evident that in this group of neurons there are some with small cell bodies and others with large cell bodies, and in both size groups there are varieties of neurons which can be recognized from the characteristics of their perikaryal cytoplasm. All of these neurons have both symmetric and asymmetric axosomatic synapses. The greatest number of these nonpyramidal cells which are not bipolar in form is found within layer II/III, where they account for 7% of all neuronal profiles. These neurons comprise 4% of all neuronal profiles in layer IV, 6% in layer V, and 2% in layer VIa. Layers I and VIb contain only nonpyramidal cells, but these are different from the ones in layers II-VIa.(ABSTRACT TRUNCATED AT 400 WORDS)

Maturation of rat visual cortex. II. A combined Golgi-electron microscope study of pyramidal neurons.

Tissue removed from 3, 6, 9, 12, 15, and 21-day-old rats has been prepared for correlative light and electron microscopy to examine the maturation of cell bodies, dendrites, and axons of pyramidal neurons in layer V of rat visual cortex. As the size of the cell body increases steadily during the first 3 postnatal weeks there is an equivalent growth in nuclear volume. By day 15, there are infoldings in the nuclear envelope which may be induced by eye opening on day 14. Nucleoli increase in size until day 9, after which they appear to condense. Within the perikarya, the most conspicuous change is the amount and organization of the rough endoplasmic reticulum. Symmetric axosomatic synapses are evident by day 6. The ultrastructure of dendrites does not change substantially with age. Dendrites form synapses with symmetric densities as early as day 3 and asymmetric ones by day 9. It seems that dendritic spines begin as low, broad protrusions having symmetric junctions with smaller diameter axonal processes. With time they become taller stumps, before acquiring their mature lollipop shape and participating in asymmetric synapses with axonal varicosities. Other dendritic appendages, filopodia, and growth cones are transient structures, being conspicuous only between days 3 and 12. "Terminal" growth cones are essential for extension of dendritic processes, whereas "en passant" growth cones and filopodia seem important for dendritic branching. Boutons of mature pyramidal cell axons form asymmetric synapses with dendritic shafts and spines, but the developing synapses formed by these axons have more symmetric junctions. The maturation of pyramidal cell features progresses in concert with such extrinsic determinants as afferent input and is probably influenced by the competency of synaptic connections.