Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. III. Origins and frequency of occurrence of the terminals

Summary The cell bodies of the layer II/III pyramidal cells in rat visual cortex receive three morphologically distinct types of axon terminals. These axon terminals all form symmetric synapses and have been termed large, medium-sized, and dense axon terminals. The present study shows that each of these different kinds of axon terminals contains gamma-aminobutyric acid (GABA) which suggests that they are inhibitory. From an analysis of the profiles of 50 cell bodies it is calculated that the average layer II/III pyramidal cell has 65 axosomatic synapses, of which 43 are formed by medium-sized terminals, 10 by large terminals/and 12 by dense terminals. Comparison of these different kinds of axon terminals with labelled axon terminals of known origin suggests that the medium-sized terminals are derived from smooth multipolar cells with unmyelinated axons, and that at least some of the dense terminals originate from bipolar cells that contain vasoactive intestinal polypeptides. The source of the large axon terminals is not known, but it is suggested that they originate from multipolar non-pyramidal cells with myelinated axons.Since the initial axon segments of these same neurons receive GABAergic axon terminals from chandelier cells, at least four different types of neurons provide inhibition to the cell bodies and axons of layer II/III pyramidal cells. This serves as an illustration; of the complexity of the neuronal circuits in which pyramidal cells are involved.     

The axon terminals of vasoactive intestinal polypeptide (VIP)-containing bipolar cells in rat visual cortex

Summary In vasoactive intestinal polypeptide (VIP)-immunoreacted preparations, bipolar neurons are the cells most commonly labelled. The VIP-positive axon terminals form symmetrical synapses, and their most common postsynaptic targets are small and medium sized dendrites. These are of both smooth and spiny types. Additionally, there is a concentration of VIP-positive axon terminals around the cell bodies of pyramidal neurons, and it is suggested that an important function of VIP-labelled bipolar cells is to inhibit vertically oriented groups of pyramidal cells.In order to further examine the features of axon terminals that label with VIP antibodies, conventionally prepared material was examined by electron microscopy. Those terminals which label with VIP antibody are characterized by irregular profiles of varying sizes and shapes, and by containing closely packed pleomorphic vesicles. Such terminals form symmetrical synapses. The junctions are not well marked by associated cytoplasmic densities, but there is an inherent density within the synaptic cleft. It is suggested that these features characterize all axon terminals in which GABA coexists with peptides in cerebral cortex.     

Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. I. Morphometric analysis

Summary An examination of material prepared for conventional electron microscopy has indicated that there are at least four different types of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells in the rat visual cortex. One type of terminal synapses with the initial axon segment and it is derived from the chandelier cell. Because the location and features of these terminals allow them to be readily recognized, chandelier cell terminals were used to determine the extent of morphometric variability that can exist among terminals originating from one cell type. It was found that there is a wide range of mean synaptic vesicle size among chandelier terminals, so that calculated mean vesicle profile diameters for individual terminals can be between 32 and 39 run. Similar ranges of mean synaptic vesicle sizes also exist among populations of the other three axon terminal types. These terminal types are referred to as large, medium-sized, and dense terminals. The large terminals synapse with the cell bodies of layer II/III pyramids and their profiles often measure 1.5 × 0.8 m. The large terminals contain rather loosely packed pleomorphic vesicles and they frequently synapse with a second neuronal element. The medium-sized terminals are smaller, being 1.0 × 0.6–0.8 m in size, and their synaptic vesicles are usually more closely packed than those within the large terminals. The medium-sized terminals are the ones encountered most frequently on the cell bodies of pyramidal cells and they can also occur on the axon hillock and initial axon segment. The dense terminals are usually flattened against the cell body, and they contain rather rounded and closely packed synaptic vesicles, which often seem to be enmeshed in a rather dark cytoplasmic matrix. This matrix and the close packing of the vesicles makes these terminals appear to be more dense than the others. It is now necessary to determine the origins of the large, medium and dense terminals, and to ascertain if they all use GABA as their neurotransmitter.     

Ultrastructure of symmetric interneuronal junctions after staining with ethanolic phosphotungstic acid.

1. The symmetric interneuronal junctions (attachment plaques) of large axon terminals in the dorsal lateral geniculate body of the rat were studied, applying besides the conventional staining the block staining method with ethanolic phosphotungstic acid (E-PTA). 2. After staining these contact zones with E-PTA 3 main components become visible: the interior presynaptic layer, the intermembranous material and the postsynaptic layer. 3. As opposed to the asymmetric synaptic junctions the interior presynaptic layer is not divided into dense projections. 4. The intermembranous material consists of 2 median situated intracleft lines, which are more or less fused with each other. 5. The staining intensity of the paramembranous material of the symmetric junctions stained with E-PTA corresponds to the intensity of the material of the asymmetric synaptic junctions.     

Synaptic targets of calretinin-containing axon terminals in macaque monkey prefrontal cortex

The coordinated activity of specific populations of pyramidal cells and GABA-containing, local circuit neurons in the primate prefrontal cortex (PFC) appears to be critical for working memory. Different subclasses of GABA-containing neurons can be distinguished by their content of the calcium-binding proteins parvalbumin (PV) and calretinin (CR). The postsynaptic targets of PV-containing cells have been well characterized in the primate PFC, but the postsynaptic targets of CR-containing neurons in this cortical region remain unknown. In the present study, we used immuno-electron microscopy to examine the synaptic type and postsynaptic targets of CR-immunoreactive (IR) axon terminals in the superficial and deep layers of macaque monkey PFC. Labeled axon terminals formed both symmetric and asymmetric synapses. Within the superficial layers, 93% of the synapses formed by CR-IR were symmetric, whereas in the deep layers the labeled axon terminals forming synapses were more evenly divided between symmetric (57%) and asymmetric (43%). The primary postsynaptic target of these two populations of CR-IR axon terminals also differed; unlabeled dendritic shafts were the predominant target of the symmetric synapses, whereas dendritic spines were the most common target of the asymmetric synapses. In addition, the mean cross-sectional area of the terminals forming asymmetric synapses was significantly larger than that of the terminals forming symmetric synapses. The presence of CR-IR asymmetric synapses suggested that they might arise from neurons that do not utilize GABA; indeed, dual-label fluorescent immunocytochemistry revealed that a subpopulation (23%) of CR-containing neurons in monkey PFC were not GABA-IR. These findings indicate that the synaptology of CR-containing neurons is more heterogeneous than that of PV-containing cells and suggests that the contributions of CR-containing neurons to cognitive processes mediated by the PFC may be more diverse.     

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. II. Terminations upon neuronal perikarya and dendritic shafts

Summary The forms of dendrites in layer IV receiving degenerating thalamocortical axon terminals directly on their shafts were examined in serial thin sections. Reconstructions showed these dendrites varied in thickness between 2.5 and 0.5 m. They had essentially smooth contours and rarely showed evidence of protrusions or spines. They were further characterized by the presence of many synapses along their shafts. Only about one in 12 of these synapses was formed by degenerating thalamocortical axon terminals.These smooth dendrites emerged from neuronal perikarya that also received degenerating axon terminals which formed asymmetric synaptic junctions. Such cell bodies bore both symmetric and asymmetric synaptic junctions, and not all of the latter were caused to degenerate after a thalamic lesion. These postsynaptic neurons appeared to be of two kinds, ones with thin dendrites that often contained closely packed microtubules, and others with thicker dendrites that emerged from the poles of oval perikarya.     

Morphology of synapses formed by cholecystokinin-immunoreactive axon terminals in regio superior of rat hippocampus

Immunocytochemical and electron microscopic methods were used to examine neurons in regio superior of rat hippocampus displaying cholecystokinin octapeptide-like immunoreactivity. Cholecystokinin-immunoreactive synaptic terminals and somata are found in all layers of regio superior but are most numerous in stratum pyramidale. The vast majority of terminals form symmetric synaptic contacts onto the somata and proximal dendrites of hippocampal pyramidal cells and onto smaller dendrites which may also arise from pyramidal cells. A very small number of Cholecystokinin-immunoreactive terminals form synapses that appear asymmetric and contact dendritic shafts or spines. The somata of some pyramidal cells receive symmetric synapses from Cholecystokinin-immunoreactive terminals that are joined by cytoplasmic bridges to form parts of pericellular baskets. These and adjacent pyramidal cell somata are also contacted by terminals that are not immunoreactive for cholecystokinin. No cholecystokinin-positive terminals contacted the initial segments of pyramidal cell axons. Cholecystokinin-immunoreactive cells are found in all layers of regio superior. Their somata receive a few symmetric synapses, most of which are formed by terminals not immunoreactive for cholecystokinin. Their dendrites receive a greater number of both symmetric and asymmetric contacts, some of which are immunoreactive for cholecystokinin.We conclude the following: (1) The localization of cholecystokinin immunoreactivity in synaptic terminals contacting the somata and dendrites of hippocampal pyramidal cells is consistent with the suggestion that cholecystokinin acts as a neurotransmitter at these sites and at sites in other parts of the cerebral cortex. (2) Results from the present and previous studies suggest that cholecystokinin-like immunoreactivity may co-exist with γ-aminobutyrate in some non-pyramidal neurons of regio superior. (3) Cholecystokinin-immunoreactive terminals arise mainly from non-pyramidal cells intrinsic to the hippocampus, one class of which appears to be a type of basket cell.     

Reciprocal synapses between cholinergic axons and small granule-containing cells in the rat cardiac ganglion

Electron microscopy of the rat cardiac ganglion shows occurence of small granule-containing cells that form reciprocal synaptic junctions with cholinergic terminals. At the synaptic junctions which are from axon to granule-containing cell, the intraaxonal vesicles are clustered against the junctional axolemma, but dense-cored vesicles in the postynaptic cell do not cluster towards the membrane densities in these synapses. By contrast, the synaptic zone polarized in the opposite direction shows an absence of axonal vesicles in close proximity to the postsynaptic axolemma, but there is a marked aggregation of dense-cored vesicles towards the presynaptic specializations of granule-containing cells. The synaptic zones are multifocal rather than bifocal, and the minimal distance separating each synaptic zone is about 0.3 μ. These findings may indicate that cholinergic excitation of some or all granule-containing cells causes a reciprocal inhibition of one or more cholinergic terminals.     

Catecholamine-containing axon terminals in the hypoglossal nucleus of the rat: an immuno-electronmicroscopic study

Summary A correlative light and electron microscopic investigation was undertaken to determine the morphology and distribution of catecholamine (CA)-containing axon terminals in the hypoglossal nucleus (XII) of the rat. This was accomplished immunocytochemically with antibody to tyrosine hydroxylase (TH). The major findings in this study were the following: 1) Immunoreactive profiles were found throughout XII and included unmyelinated axons, varicosities, axon terminals and dendrites; 2) Nonsynaptic immunoreactive profiles (preterminal axons, varicosities) were more frequently observed (55.2%) than synaptic profiles (43.5%); 3) CA-containing axon terminals ending on dendrites were more numerous (71.8%) than those synapsing on somata (25.4%) or nonlabeled axon terminals (2.7%); 4) The morphology of labeled axon terminals was variable. Axodendritic terminals typically contained numerous small, round agranular vesicles, a few large dense-core vesicles and were associated with either a symmetric or no synaptic specialization, axosomatic terminals were often associated with a presynaptic membrane thickening or a symmetric synaptic specialization and contained small, round and a few elliptical-shaped vesicles, while axoaxonic synapses formed asymmetric postsynaptic specializations; and 5) CA-positive dendritic processes were identified in XII. These findings confirm the CA innervation of XII, and suggest a complex, multifunctional role for CA in controlling oro-lingual motor behavior.     

Synaptic junctions between sympathetic axon terminals and pinealocytes in the monkey Macaca fascicularis

Summary The distribution of axon terminals in the pineal gland of monkeys was studied by electron microscopy. Numerous terminals bearing small pleomorphic agranular and dense-cored vesicles were localized in the perivascular space and among the pinealocytes in the parenchyma in normal monkeys. Following bilateral superior cervical ganglionectomy, they underwent degenerative changes, including the accumulation of glycogen masses, appearance of dense residual bodies and the displacement of synaptic vesicles. Some of these degenerating terminals showed synaptic contacts with the cell bodies of pinealocytes. At the synaptic junction the postsynaptic membrane was thickened asymmetrically. Examples of synaptic contacts were most frequently observed in 5 and 7 days postoperative animals. In the longer surviving (30 days) monkey, most of the axon terminals showed round agranular vesicles, and they were mainly presynaptic to the intrapineal ganglion cells with some of the pinealocytes. They remained structurally unchanged following the resection of both the superior cervical ganglia. A few axon terminals containing small dense-cored vesicles appeared to have survived the initial insult, but some of their vesicles appeared swollen 30 days after the operation. It is concluded from this study that some of the pinealocytes are under the influence by the postganglionic neurons in the superior cervical ganglia through direct synaptic contacts. The intrapineal ganglion cells are postsynaptic to fibres originating exclusively from the central nervous system. Some of these fibres, however, may be presynaptic directly to pinealocytes.     

Compensation in the number of presynaptic dense projections and synaptic vesicles in remaining parallel fibres following cerebellar lesions

Summary Our previous investigations demonstrated an increase in the size of remaining synaptic sites as an intermediate or possible alternative to sprouting plasticity. The total amount of postsynaptic contact area remained relatively constant for each target neuron even though there was a marked decrease in the number of sites on these neurons. In addition, enlarged boutons containing numerous synaptic vesicles were positioned adjacent to enlarged postsynaptic sites.The question posed by this study was to determine whether dense projections, parts of the presynaptic grids of the remaining parallel fibres, spread to cover the enlarged postsynaptic sites, or if the number of these densities increased on each site to maintain the structural organization of the presynaptic grid. In addition, the number of synaptic vesicles per bouton was quantitated to determine whether they compensated by increasing their number in relationship to the increased area of the presynaptic grid.The number of parallel fibre synapses on Purkinje cells was reduced by transection of a narrow bundle of parallel fibres accompanied by a small lesion undercutting the molecular layer to destroy granule cells contributing to this bundle. The number of presynaptic dense projections was quantitated in control and lesioned preparations (using ethanolic phosphotungstic acid staining) in order to determine their correlation to the area of each site. In addition, the average number of synaptic vesicles in boutons was compared to the average size of boutons and the average contact area of the synaptic sites. At 3 to 7 days following partial deafferentation of Purkinje cells in adult rats, the density of dense projections of parallel fibre synapses on Purkinje cell spines remained uniform. This occurred throughout a range of reduction in the number of synapses in conjunction with a reciprocal increase in the size of sites. The finding of a uniform density of these projections and an increase in the size of sites implies that each granule cell axon must gain dense projections. In addition, the remaining presynaptic boutons had a uniform density of synaptic vesicles even though the volume of the boutons and the area of the synaptic contact doubled. Thus, the number of synaptic vesicles gained in proportion to the total enlargement of the contact site and the bouton size.These results strongly suggest that deficits or losses in synaptic connections of parallel fibre on Purkinje cell spines produces a compensation in the total number of synaptic vesicles and presynaptic dense projections of the remaining boutons. An enlargement of the presynaptic grid occurs in concert with redistribution of the constant total area of membrane occupied by macromolecules (or insertion of new ones) on remaining postsynaptic sites. These compensations could be facilitating efficacy of neuronal connections after lesions or neuronal attrition by re-establishing available transmitter and release sites in proportion to the constant amount of receptor area.     

Relationship of cannabinoid CB1 receptor and cholecystokinin immunoreactivity in monkey dorsolateral prefrontal cortex

Exposure to cannabis impairs cognitive functions reliant on the circuitry of the dorsolateral prefrontal cortex (DLPFC) and increases the risk of schizophrenia. The actions of cannabis are mediated via the brain cannabinoid 1 receptor (CB1R), which in rodents is heavily localized to the axon terminals of cortical GABA basket neurons that contain cholecystokinin (CCK). Differences in the laminar distribution of CB1R-immunoreactive (IR) axons have been reported between rodent and monkey neocortex, suggesting that the cell type(s) containing CB1Rs, and the synaptic targets of CB1R-IR axon terminals, may differ across species; however, neither the relationship of CB1Rs to CCK-containing interneurons, nor the postsynaptic targets of CB1R and CCK axon terminals, have been examined in primate DLPFC. Consequently, we compared the distribution patterns of CB1R- and CCK-IR structures, determined the proportions of CB1R and CCK neurons that were dual-labeled, and identified the synaptic types and postsynaptic targets of CB1R- and CCK-IR axon terminals in macaque monkey DLPFC. By light microscopy, CB1R- and CCK-IR axons exhibited a similar laminar distribution, with their greatest densities in layer 4. Dual-label fluorescence experiments demonstrated that 91% of CB1R-IR neurons were immunopositive for CCK, whereas only 51% of CCK-IR neurons were immunopositive for CB1R. By electron microscopy, all synapses formed by CB1R-IR axon terminals were symmetric, whereas CCK-IR axon terminals formed both symmetric (88%) and asymmetric (12%) synapses. The primary postsynaptic target of both CB1R- and CCK-IR axon terminals forming symmetric synapses was dendritic shafts (81–88%), with the remainder targeting cell bodies or dendritic spines. Thus, despite species differences in laminar distribution, CB1Rs are principally localized to CCK basket neuron axons in both rodent neocortex and monkey DLPFC. These axons target the perisomatic region of pyramidal neurons, providing a potential anatomical substrate for the impaired function of the DLPFC associated with cannabis use and schizophrenia.     

Ultrastructure and synaptic connections of vasoactive intestinal polypeptide-like immunoreactive non-pyramidal neurons and axon terminals in the rat hippocampus

In the hippocampus, antibody raised against vasoactive intestinal polypeptide (VIP) labeled perikarya and processes of non-pyramidal neurons whereas these structures remained unlabeled in pyramidal cells and granule cells. In the present study, VIP-immunostaining was used to investigate the fine structure and synaptic connections of identified non-pyramidal neurons and of imrnunoreactive axon terminals in the CA1 region of the rat hippocampus by means of electron microscopic immunocytochemistry.From a number of cells studied, two VIP-like imrnunoreactive non-pyramidal neurons in the regio superior were selected for an electron microscopic analysis of serial thin sections. These cells were different with regard to the location of their cell bodies and the orientation of their dendrites. One cell was located in the stratum lacunosum-moleculare with dendritic processes oriented parallel to the hippocampal fissure. The second neuron was found in the inner one-third of the stratum radiatum. The dendrites of this cell ran nearly parallel to the ascending apical dendrites of the pyramidal cells. Both cells had a round or ovoid perikaryon and an infolded nucleus. The aspinous dendrites of both neurons were densely covered with synaptic boutons. These terminals were small, filled with spherical vesicles and established asymmetric synaptic contacts. No variations in the fine structure of the presynaptic boutons were found along the course of the labeled dendrites through the various hippocampal layers, although different afferents are known to terminate in these layers.Vasoactive intestinal polypeptide-like immunopositive axon terminals course through all layers of the hippocampus. In the stratum pyramidale they established symmetric synaptic contacts with the perikarya of pyramidal cells. In the stratum radiatum they made symmetric contacts with the shafts of apical dendrites of pyramidal cells but never contacted dendritic spines.The symmetric contacts with pyramidal cell perikarya suggest an involvement of the VIP-like immunoreactive axon terminals in pyramidal cell inhibition.     

Presynaptic dendrites and serial synapses in the intermediate and deep layers of the cat superior coliculus.

Presynaptic dendrites (PSDs) which participate in the serial synapses have frequently been found in the intermediate and deep layers of the cat superior colliculus. The PSDs are presynaptic to small dendritic shafts or spines with symmetrical membrane thikening, and postsynaptic to axon terminals with asymmetrical synaptic contact. Two types of the axon terminals are observed, both of which contain pleomorphic vesicles.     

家兔脊髓腰骶部中间外侧核区的超微结构——神经纤维网和突触

本实验用家兔7只,取腰髓2～4和骶髓2～4节中间外侧核区,做超薄切片,电镜观察。此区的神经纤维网内含树突、轴突、轴突终末、终端树突、突触和突触球。胶质细胞的突起穿行其间。树突散在,形态和大小多变。轴突则常成束分布。突触连接以轴树和轴体突轴为多见,偶见轴轴突触。多数突触单独存在,部分形成以树突或轴突为中心的突触球。突触内的突触小泡有清亮的圆形、椭圆形、扁平形和不规则形,还有相当多见的大致密核心小泡和少数有衣小泡。依终末囊内突触小泡的形态和突触前后膜的对称与不对称,所见突触可分为三类:1.圆形小泡不对称型;2.扁平小泡对称型;3.其它中间类型。     

The mormyrid brainstem--III. Ultrastructure and synaptic organization of the medullary "pacemaker" nucleus

The ultrastructure and synaptic organization of the presumed medullary pacemaker nucleus, nucleus c of the weakly electric mormyrid fish, Gnathonemus petersii has been investigated. Nucleus c consists of about 12-15 small (20-25 micron) neurones (P-cells), which form a group situated ventrally to the medullary relay nucleus and embedded in a neuropil of myelinated fibres and dendritic processes. The P-cells often exhibit an enhanced electron density of their cytoplasm and dendroplasm. They possess several dendrites of different diameter, a short, thin axon initial segment and a thickly myelinated axon running in dorsal direction. The pacemaker neurons are interconnected by complex electronic coupling, established by somatosomatic, dendrosomatic and dendrodendritic gap junctions. Perikarya and dendrites are frequently interconnected serially by gap junctions; dendrites showed sometimes triadic gap-junction arrangement. It is suggested that this high degree of electrotonic coupling amongst the pacemaker cells represents the first level of the highly ordered synchronization processes which characterize the electric discharge command system of Gnathonemus. Pacemaker cells receive synaptic input from club endings with mixed synapses and from bouton-like terminals with chemical synapses, both of them originating from medium-sized myelinated fibres and contacting mainly neuronal perikarya and dendritic processes. The axon initial segment receives only few synaptic inputs. Bouton-like terminals were found to be of two types according to their vesicle content, namely, boutons with ovoid, clear synaptic vesicles forming Gray type-1 synapses and boutons with pleomorphic clear synaptic vesicles forming Gray type-2 synapses. Different functional roles for the two types of boutons in modulating pacemaker cell activity are suggested.     

Synaptic plasticity in the oedematous human cerebral cortex

Synaptic plastic changes are fundamental events which occur spontaneously during development, maturity and aging processes or can be induced by injury or trauma. To examine lesion-induced synaptic plasticity, cortical biopsies were taken from the frontal, parietal, temporal and occipital cortex of living patients during neurosurgical treatment of brain trauma, brain tumours and vascular malformations, and processed for transmission electron microscopy. Enlargement of both pre- and postsynaptic endings, irregularly shaped, lobulated, stellate and bifurcated presynaptic endings and conformational changes of dendritic spines were observed. Numerous flat, curved and invaginated axodendritic and axospinous asymmetric synapses were distinguished and a smaller proportion of axodendritic and axosomatic symmetric synapses. Activated or sensitized synapses showed numerous frontline spheroid synaptic vesicles, prominent dense presynaptic dense projections and increased length of synaptic membrane complex. Perforated synapses, multiple synapses and serial synapses were also found evincing synaptic splitting and formation of new synaptic connections. The overall images suggest increased number of excitatory circuits, which were correlated with the tonico-clonic convulsion or post-traumatic seizures observed in some patients. Numerous coated vesicles were observed in pre- and postsynaptic structures. Increased number of polyribosomes were found in the dendritic shafts. The dilated spine apparatus, the coated vesicles and the increased number of polyribosomes seem to represent a system for synthesis, transport and storage of synaptic proteins for the formation of new synapses. Coexisting synaptic plasticity and synaptic degeneration were observed in the patients under study. Dendritic and astrocyte synapse-like junctions were also characterized.     

Membrane structure of parallel-fibre synaptic terminals in the cerebellum of the jaundiced Gunn rat: freeze-fracture and E-PTA study

Summary Parallel-fibre synaptic membranes were examined by freeze-fracture and ethanolic-phosphotungstic acid methods in the cerebellum of homozygous (j/j) Gunn rats with hereditary jaundice. Parallel-fibre synapses with dendritic spines of Purkinje cell were severely affected since many Purkinje cells degenerated during the early postnatal period. Some parallel-fibre synaptic terminals lacked their postsynaptic partners and faced astrocytic processes from 18 days of age to the adult stage. These parallel-fibre terminals contained clusters of synaptic vesicles adjacent to synaptic membranes, and synaptic membranes and synaptic cleft materials were identical to those of parallel fibres with postsynaptic partners, as visualized by conventional electron microscopy with osmium tetroxide postfixation and staining of sections with uranyl acetate and lead citrate. In freeze-fractured specimens, the presynaptic membrane of parallel fibres had diffusely distributed large particles and tiny pits on the P-face and protuberances on the E-face, together representing synaptic vesicle attachment sites. Such vesicle attachment sites were present on the presynaptic membranes of parallel fibres without postsynaptic partners from day 18 to the adult stage. After ethanolic-phosphotungstic acid staining, parallel-fibre terminals displayed presynaptic dense projections, intercleft materials and postsynaptic thickening, but some parallel fibres lacked postsynaptic thickening. These observations suggest that the presynaptic membrane structure of the parallel fibre is preserved, even in the absence of a postsynaptic partner, in j/j cerebella. A mechanism for persistence of presynaptic membrane structures without postsynaptic partners in j/j cerebella is discussed.     

Vasoactive intestinal polypeptide-immunoreactive neurons in rat visual cortex

An antibody to vasoactive intestinal polypeptide (VIP) was used to examine the forms of VIP-positive neurons and the synapses made by VIP-positive axon terminals. Vasoactive intestinal polypeptide-positive cells are most common in layers II and III and the majority of them are typical bipolar neurons, with two primary dendrites which emanate from the upper and lower poles of the cell body. Their somata, which have only a few symmetric and asymmetric synapses, generally have a fusiform or "tear-drop" shape and contain nuclei with a vertically oriented cleft. The dendritic trees are arranged vertically and often extend through five cortical layers. The axons are thin and extend either from the soma or from one of the primary dendrites. The axons also follow a vertical trajectory. Other VIP-positive neurons are modified bipolar cells and a few of them are multipolar cells. The synapses formed by the VIP-positive axon terminals in the neuropil are symmetric in form, and although the synaptic clefts are narrow, the junctions are usually long and continuous, rather like those described for asymmetric synapses. Most of the VIP-positive axon terminals synpase with small dendritic shafts, but a few synapse with neuronal cell bodies. Since the majority of the VIP-positive neurons are bipolar cells it is concluded that these are the source of most of the VIP-positive axon terminals. If this is so, then the VIP-positive bipolar cells form symmetric synapses. This is in contrast to the observations of Peters and Kimerer (1981. J. Neurocytol. 10, 921-946) for the bipolar cells they examined in a Golgi-electron microscopic study had axon terminals forming asymmetric synapses. It is suggested that this disparity can be reconciled if it is assumed that the bipolar cell population consists of subgroups which have different biochemical characteristics and different synaptic relationships.     

Corticotropin-releasing factor is preferentially colocalized with excitatory rather than inhibitory amino acids in axon terminals in the peri-locus coeruleus region

Corticotropin-releasing factor(CRF)-immunoreactive terminals form synaptic specializations with locus coeruleus (LC) dendrites in rat brain. Within these terminals, CRF-immunoreactive dense core vesicles are colocalized with non-labeled dense core vesicles and clear vesicles, implicating other neuromodulators in the actions of CRF on LC neurons. Excitatory (glutamate) and inhibitory (GABA) amino acid afferents to the LC, have been identified which regulate noradrenergic responses to sensory stimuli. This study was designed to determine whether these amino acid neurotransmitters are colocalized with CRF in terminals within the LC/peri-LC region in the rat. Sections through the LC region that were dually labeled using immunohistochemical techniques to visualize either CRF and glutamate or CRF and GABA were examined using electron microscopy. Numerous terminals that contained immunolabeling for both CRF and glutamate (e.g. 30% of 106 CRF-immunoreactive terminals and 13% of 232 glutamate-immunolabeled terminals) were observed in the peri-LC. Additionally, single labeled CRF and glutamate terminals were often apposed to one another or found to converge on common dendritic targets. In contrast, relatively few terminals exhibited immunolabeling for both GABA and CRF (5% of 317 CRF-immunoreactive terminals). However, evidence for a postsynaptic effect of CRF on GABA-containing profiles included synapses between CRF axon terminals and GABA-labeled dendrites (10% of 317 CRF-labeled terminals), as well as appositions between CRF- and GABA-labeled terminals.These results indicate that CRF is preferentially colocalized with glutamate in the rostrolateral LC region and may impact on glutamate neurotransmission in the LC via presynaptic or postsynaptic actions. They argue against colocalization of CRF with GABA, although CRF may modulate GABA release via postsynaptic effects in the peri-LC region.