Glutamatergic components of the retrosplenial granular cortex in the rat

The ultrastructural characteristics, distribution and synaptic relationships of identified, glutamate-enriched thalamocortical axon terminals and cell bodies in the retrosplenial granular cortex of adult rats is described and compared with GABA-containing terminals and cell bodies, using postembedding immunogold immunohistochemistry and transmission electron microscopy in animals with injections of cholera toxin- horseradish peroxidase (CT-HRP) into the anterior thalamic nuclei. Anterogradely labelled terminals, identified by semi-crystalline deposits of HRP reaction product, were approximately 1 microm in diameter, contained round, clear synaptic vesicles, and established asymmetric (Gray type I) synaptic contacts with dendritic spines and small dendrites, some containing HRP reaction product, identifying them as dendrites of corticothalamic projection neurons. The highest densities of immunogold particles following glutamate immunostaining were found over such axon terminals and over similar axon terminals devoid of HRP reaction product. In serial sections immunoreacted for GABA, these axon terminals were unlabelled, whereas other axon terminals, establishing symmetric (Gray type II) synapses were heavily labelled. Cell bodies of putative pyramidal neurons, containing retrograde HRP label, were numerous in layers V-VI; some were also present in layers I-III. Most were overlain by high densities of gold particles in glutamate but not in GABA immunoreacted sections. These findings provide evidence that the terminals of projection neurons make synaptic contact with dendrites and dendritic spines in the ipsilateral retrosplenial granular cortex and that their targets include the dendrites of presumptive glutamatergic corticothalamic projection neurons.     

Glutamatergic components of the retrosplenial granular cortex in the rat

The ultrastructural characteristics, distribution and synaptic relationships of identified, glutamate-enriched thalamocortical axon terminals and cell bodies in the retrosplenial granular cortex of adult rats is described and compared with GABA-containing terminals and cell bodies, using postembedding immunogold immunohistochemistry and transmission electron microscopy in animals with injections of cholera toxin- horseradish peroxidase (CT-HRP) into the anterior thalamic nuclei. Anterogradely labelled terminals, identified by semi-crystalline deposits of HRP reaction product, were approximately 1 m in diameter, contained round, clear synaptic vesicles, and established asymmetric (Gray type I) synaptic contacts with dendritic spines and small dendrites, some containing HRP reaction product, identifying them as dendrites of corticothalamic projection neurons. The highest densities of immunogold particles following glutamate immunostaining were found over such axon terminals and over similar axon terminals devoid of HRP reaction product. In serial sections immunoreacted for GABA, these axon terminals were unlabelled, whereas other axon terminals, establishing symmetric (Gray type II) synapses were heavily labelled. Cell bodies of putative pyramidal neurons, containing retrograde HRP label, were numerous in layers V–VI; some were also present in layers I–III. Most were overlain by high densities of gold particles in glutamate but not in GABA immunoreacted sections. These findings provide evidence that the terminals of projection neurons make synaptic contact with dendrites and dendritic spines in the ipsilateral retrosplenial granular cortex and that their targets include the dendrites of presumptive glutamatergic corticothalamic projection neurons.     

Parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat: localization, morphology, connectivity and ultrastructure

Summary We studied the distribution, morphology, ultrastructure and connectivity of parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat. Immunoreactive cell bodies were found in all layers of the entorhinal cortex except layer I. The highest numbers were observed in layers II and III of the dorsal division of the lateral entorhinal area whereas the lowest numbers occurred in the ventral division of the lateral entorhinal area, Most such neurons displayed multipolar configurations with smooth dendrites. We distinguished a type with long dendrites and a type with short dendrites. We also observed pyramidal immunoreactive neurons. A dense plexus of immunoreactive dendrites and axons was prominent in layers II and III of the dorsal division of the lateral entorhinal area and the medial entorhinal area. None of the parvalbuminimmunoreactive cells became retrogradely labelled after injection of horseradish peroxidase into the hippocampal formation. By electron microscopy, immunoreactivity was observed in cell bodies, dendrites, myelinated and unmyelinated axons and axon terminals. Immunoreactive dendrites and axons occurred in all cortical layers. We noted many myelinated immunoreactive axons. Immunoreactive axon terminals were medium sized, contained pleomorphic synaptic vesicles, and established symmetrical synapses. Both horseradish peroxidase labelled and unlabelled immunonegative cell bodies often received synapses from immunopositive axon terminals arranged in baskets. Synapses between immunoreactive axon terminals and unlabelled dendritic shafts and spines were abundant. Synapses with initial axon segments occurred less frequently. In addition, synaptic contacts were present between immunopositive axon terminals and cell bodies and dendrites. Thus, the several types of parvalbumin-containing neuron in the entorhinal cortex are interneurons, connected to one another and to immunonegative neurons through a network of synaptic contacts. Immunonegative cells projecting to the hippocampal formation receive axo-somatic basket synapses from immunopositive terminals. This connectivity may form the morphological substrate underlying the reported strong inhibition of cells in layers II and III of the entorhinal cortex projecting to the hippocampal formation.     

Vasoactive intestinal polypeptide immunoreactive neurons in the primary visual cortex of the cat

Summary When cat visual cortex (area 17) is reacted with an antibody to vasoactive intestinal polypeptide (VIP) a variety of neuronal types is labelled. Many of the labelled neurons are bipolar in form and are most common in layers II and III, although significant numbers of bipolar neurons are also encountered in layer V. Multipolar cells are also labelled. These are most frequent in layer IV and have a variety of shapes. In layer I, the labelled cells are of three varieties, i.e. horizontal bipolar cells, horizontal bitufted cells and multipolar neurons, while in layer VI the few VIP-positive neurons are horizontal bipolar cells. This suggests that all of the VIP-labelled neurons in cat area 17 are non-pyramidal in form, and this has been confirmed by electron microscopy. In these preparations, axon terminals are also labelled and under the light microscope it can be seen that these terminals occur both within the neuropil and around the cell bodies of some neurons, particularly neurons in layers II and III. Electron microscopy has shown that all of the labelled axon terminals form symmetric synapses and that those in the neuropil synapse with the shafts of smooth dendrites. These axodendritic synapses account for about 90% of the synapses formed by the labelled axon terminals. The remainder of the labelled axon terminals synapse with the cell bodies of pyramidal neurons. Parallels are drawn between these results and those previously obtained by examining those neuronal elements labelled with VIP antibodies in rat visual cortex.     

Synaptic connections, axonal and dendritic patterns of neurons immunoreactive for cholecystokinin in the visual cortex of the cat

A subpopulation of gamma-aminobutyric acid (GABA) containing neurons was reported to contain cholecystokinin-immunoreactive material in the visual cortex of cat [Somogyi et al., J. Neurosci. (1984) 4, 2590-2603]. In the present study pre-embedding immunocytochemistry was used to identify which of the several types of presumed GABAergic nonpyramidal cells in areas 17 and 18 contain cholecystokinin immunoreactivity. Most of the cholecystokinin-immunoreactive somata were found in layers II-III, they were less frequent in layers I and VI, and relatively rare in layers IV and V. The distribution and density of the axon terminals resembled that of the cell bodies. Two well defined types of cholecystokinin-immunoreactive neuron were distinguished: (1) double bouquet cells in layers II-III with vertically projecting axons, and (2) small basket cells with local axons either restricted to layers II-III, or descending to layer V. Additional cholecystokinin-positive cells showed features of bitufted or multipolar neurons in layers II-VI and horizontal cells in layer I, but these cells could be defined less well due to partial staining. Cholecystokinin-immunoreactive dendrites were found to run horizontally in layer I for several hundred micrometers. Some of the cholecystokinin-immunoreactive cells in layer VI had very long dendrites ascending radially up to layer III, as did their axons. A few cholecystokinin-immunoreactive cells appeared to have two axons and this was confirmed by electron microscopy. All cholecystokinin-immunoreactive neurons and terminals were separated from the basal lamina of blood vessels by glial endfeet. Random samples of boutons from each layer as well as identified terminals traced to their origin from local neurons were examined in the electron microscope. All of the boutons established symmetrical (type II) synaptic contacts with dendritic shafts, spines or somata. Quantitative electron microscopy of the postsynaptic targets of double bouquet cells and small basket cells demonstrated clear differences between these two types of neuron; basket cells having a higher proportion of their terminals in synaptic contact with somata. The findings that several distinct types of cortical neurons, as defined by their synaptic connections, contain cholecystokinin-immunoreactive material and that identified axons of all examined neurons form type II synaptic contacts suggests that the majority, if not all cholecystokinin-positive boutons forming type II contacts originate from local cortical cells. The distribution of targets postsynaptic to cholecystokinin-positive neurons is compared to those of cells labelled by other methods.     

Light and electron microscopic immunocytochemical analysis of the serotonin innervation of the rat visual cortex

Summary The serotonin afferents of the rat visual cortex were examined immunocytochemically at the light and electron microscopic levels. Immunoreactive fibres were typically thin, tortuous and varicose. Occasionally, some thicker fibres were found. The orientation of labelled axons varied according to laminar position, with fibres running parallel to the pial surface present mainly in layers I and VI, and radially oriented fibres prominent in layers II and III. Branches arising from horizontal or radially oriented fibres were seen to form irregularly shaped loops particularly in layers IV and V. The density of innervation and the prevailing axonal orientation in each cortical layer were similar in both coronal and parasagittal planes.The ultrastructural features of serotonin-labelled axon terminals were examined in single and serial ultrathin sections. While in single sections the majority did not exhibit synaptic specializations, extensive serial section analysis showed that virtually all of these terminals were engaged in junctional complexes. Postsynaptic elements were spines and dendritic shafts, including pyramidal cell apical dendrites, with both symmetrical and asymmetrical membrane specializations. In axospinous synapses, the labelled terminals were usually adjacent to unstained axon terminals contacting the same postsynaptic element.     

Synaptic organization of the cat entopeduncular nucleus with special reference to the relationship between the afferents to entopedunculothalamic projection neurons: An electron microscope study by a combined degeneration and horseradish peroxidase tracing technique

The synaptic organization of the feline entopeduncular nucleus was studied electron microscopically. After horseradish peroxidase injections into the ventral anterior and ventral lateral nuclear complex of the thalamus, normal axon terminals synapsing with entopedunculothalamic projection neurons were classified into four types on the basis of the size and shape of synaptic vesicles in them, and types of the postsynaptic membrane differentiation. Type I and type II axon terminals were characterized by symmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. Type II axon terminals were further classified into two subtypes as to their sizes: one was small (IIa), the other large (IIb). Type III and type IV axon terminals were characterized by asymmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively.

To determine the origin of each type of terminal, electrolytic lesions of the caudate nucleus or the subthalamic nuclear region were combined with horseradish peroxidase injections into the thalamus or the subthalamic nuclear region. After electrolytic lesions of the caudate nucleus, degeneration was seen in type I axon terminals contacting entopedunculothalamic projection neurons. Following electrolysis or horseradish peroxidase injection into the subthalamic nuclear region, type IIa and type IV axon terminals showed degenerations or horseradish peroxidase labelling. Such terminals also synapsed with entopedunculothalamic projection neurons. It was demonstrated that these projection neurons relay the striatal or subthalamic inputs directly to the thalamus. After horseradish peroxidase injection into the thalamus, many labelled type II axon terminals were observed to synapse with entopedunculothalamic projection neurons. Type III axon terminals were left unchanged throughout these experiments. In addition, the entopeduncular neuron was observed to receive convergent inputs from both the caudate nucleus and probably the subthalamic nucleus. Axoaxonal synapses were also found to be involved in the synaptic triad.

These results indicate that type I axon terminals originate from the caudate nucleus, part of type IIa and type IV axon terminals originate from the subthalamic nucleus or caudal to the subthalamic nuclear region, and part of type IIa and type IIb terminals come from intrinsic axon collaterals.     

Anterograde tracer study on the nucleus geniculatus dorsalis and its internal synaptic structure in chick brain

In the present study the terminals of retinal fibres and those of internal layer cells in ventral geniculate nucleus of chicks were labelled with the anterograde tracer biotinylated dextran amine. The tracer showed the connections from the internal cell layers of ventral geniculate nucleus to the medial part of the dorsal lateral geniculate nucleus. The labelled retinal terminals were located exactly in the lateral part of nucleus. The labelled terminals in the two parts of the nucleus were analysed with the electron microscope and showed a different synaptic organisation in the two parts of the dorsal lateral geniculate nucleus. In the lateral part, two kinds of synaptic glomeruli were found mostly in the vicinity of large dendrites, which are proximal dendrites of projection neurons. One type is a simple glomerulus containing a large dendrite, a large optic terminal and a large and/or series of asymmetrical synapses surrounded by glial processes. The other type is a complex synaptic unit with several pre- and postsynaptic components, among them synapses of GABA-positive axon terminals and/or dendraxons. No glomeruli were found in the medial part of the nucleus. In the medial part of the lateral geniculate nucleus, the terminals of internal layer cell axons established asymmetrical synapses with dendrites. Often, a large terminals and large dendritic profiles established serial asymmetrical synapses. GABA-positive myelinated fibres entered and ramified in both parts of the dorsal lateral geniculate nucleus, and GABA-positive terminals were seen to form synapses on the same dendrite near to the asymmetrical contacts. To our knowledge, this is the first report of the connection from ventral geniculate internal layer cells to the dorsal lateral geniculate nucleus in the chick.     

Thalamic inputs to identified commissural neurons in the monkey somatic sensory cortex

Summary Commissurally projecting neurons were identified in the monkey first somatic sensory area (SI) by the retrograde axonal transport of horseradish peroxidase (HRP) injected into the contralateral cortex. Neurons identified in this way have large pyramidal somata primarily in layer IIIB of the SI area. Their basal dendrites lie within the terminal plexus of thalamocortical afferents.Electron microscopy was used to examine the synaptic relations of the labelled commissural cells, in particular to determine whether they receive monosynaptic thalamic connections. To do this, retrogradely labelled commissural cells and Golgi-impregnated large pyramidal neurons from layer IIIB were examined ultrastructurally in material in which thalamocortical terminals were degenerating due to a prior lesion of the thalamus. In a significant number of cases degenerating terminals were found to make synapses on the spines or shafts of labelled dendrites.Injections of HRP into SI or into the white matter adjacent to the corpus callosum labelled callosal axons and terminals in the opposite SI. These axons terminated mainly near the somata of the layer IIIB pyramidal cells. Some of their terminals were found to synapse with dendrites receiving synaptic contacts from thalamocortical axon terminals.     

皮质丘脑投射神经元的超微结构及突触联系

采用ＨＲＰ逆行追踪技术与电镜相结合的方法，对猫大脑皮质体感Ⅰ区内皮质丘脑投射神经元超微结构及突触联系进行了研究。结果证明，皮质丘脑投射神经元超微结构的特点为锥体形的胞体，胞浆丰富，含有多量的粗面内质网，游离核糖体及线粒体。ＨＲＰ标记的皮质丘脑投射神经元作为突触后成份与轴突和树突分别形成轴－树突触，轴－体突触和树－体实触。这些结果提示：皮质丘脑投射神经元接受广泛的传入联系和皮质间的联系。     

An immunohistochemical study of neuronal populations containing neuropeptides or gamma-aminobutyrate within the superficial layers of the rat dorsal horn

Immunohistochemical techniques have been used to characterise a number of interneuronal types found within the superficial layers of the rat dorsal horn. Substance P, methionine-enkephalin, neurotensin, avian pancreatic polypeptide, somatostatin and glutamic acid decarboxylase-like immunoreactivities were found within discrete populations of cell bodies within layers I to III of the spinal cord after colchicine pretreatment. Each immunoreactive cell type was distinct in terms of the distribution of terminals, perikaryal size and position within the dorsal horn. Cell bodies containing substance P were predominantly within layers I, II and III, while surgical isolation of the dorsal horn to remove terminals of extrinsic origin indicated that the terminals of the local substance P-containing interneurones were present throughout layers I and II₀. Enkephalin-immunoreactive neurones were most numerous within layer II, with less frequently observed larger enkephalin immunoreactive neurones in layer I. Neurotensin-containing neurones were found in a fairly narrow layer along the II/III border, with dendritic arborisations extending into both layers II and III. Neurotensin-immunoreactive terminals occurred in two bands, a dense band within layer II, and a less dense one within layers I and II₀. Somatostatin-like immunoreactivity was found in occasional large neurones within layer II_i. Avian pancreatic polypeptide immunoreactivity was localized to neurones within layers I, II and III, as was terminal immunoreactivity. Glutamic acid decarboxylase-containing neurones were large and were found predominantly within layers I, II ₀ and on the II/III border, where terminal immunoreactivity was also concentrated. Hemisection combined with dorsal root section indicated that the majority of these immunohistochemically identifiable neurones were interneurones.The interconnections and relations of these neurones to dorsal horn projection neurones are considered in the light of the reported analgesic properties of a number of these substances when applied directly to the spinal cord.     

Cholinergic neurons and fibres in the rat visual cortex

Summary Choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was localized immunocytochemically in neurons and fibres in the rat visual cortex using a monoclonal antibody. ChAT-labelled cells were non-pyramidal neurons, primarily of the bipolar form, distributed in layers II through VI but concentrated in layers II & III. Their perikarya contained a large nucleus and a small amount of perinuclear cytoplasm. The somata and dendrites of all labelled cells received Gray's type I and type II synapses.ChAT-stained axons formed a dense and diffuse network throughout the visual cortex and particularly in layer V. Electron microscopy revealed that the great majority formed type II synaptic contacts with dendrites of various sizes, unlabelled non-pyramidal somata and, on a few occasions, with ChAT-labelled cells. However, a very small number of terminals appeared to form type I synaptic contacts. This study describes the morphological organization of the cholinergic system in the visual cortex, the function of which has been under extensive investigation.     

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. I. General description.

Lesions were made in the lateral geniculate nucleus of the rat and the consequent degeneration in area 17 of the cerebral cortex was studied by light and electron microscopy. These lesions produced prominent degeneration of axon terminals in layer IV extending into layer III and a much lesser amount in layers I and VI. The darkened degenerating axon terminals forming asymmetric synaptic junctions and were frequently surrounded by hypertrophied astrocytic processes. These terminals appeared to be disposed randomly, forming no discernible patterns. In layer IV 83% of the synapsing, degenerating terminals formed junctions with dendritic spines, 15% with dendritic shafts, and 2% with neuronal perikarya. The dendritic shafts and neuronal perikarya appeared to belong to spine-free stellate cells. The dendrites giving rise to the spines receiving degenerating axon terminals could not be identified, for most of the spines appeared as isolated profiles that could not be traced back to their dendritic shafts. One example of a degenerating axon terminal synapsing with an axon initial segment was encountered. Small, degenerating myelinated axons were prevalent in layers VI, V and IV, but were only infrequent in the supragranular layers. These results are compared with those obtained in other studies of thalamocortical projections.     

Ultrastructural analysis of the central terminals of primary sensory neurones labelled by transganglionic transport of bandeiraea simplicifolia I-isolectin B4

In this study the ultrastructural appearance of primary sensory neurones labelled by the injection of the plant lectin Bandeiraea simplicifolia I-isolectin B(4) (BSI-B(4)) into a peripheral nerve has been examined in the rat. Electron microscopy of the somata of retrogradely labelled neurones showed the lectin to be associated with the inner surface of cytoplasmic vesicles, supporting the premise that the uptake of BSI-B(4) into sensory neurones is by the process of receptor-mediated endocytosis. Light and electron microscopic analysis of the spinal cord revealed transganglionically transported lectin in unmyelinated axons in the dorsolateral funiculus and axon terminals concentrated mainly within lamina II of the dorsal horn. Detailed analysis of 1377 of these axon terminals revealed that the majority were glomerular in shape and surrounded by up to 14 other unlabelled profiles. These findings suggest that primary sensory neurones which transganglionically transport BSI-B(4) have a synaptic ultrastructure similar to that which has been previously reported for unmyelinated primary sensory neurones. Moreover, it appears that the axon terminals of these neurones are subjected to extensive modulation. Examination of the vesicle content of lectin labelled axon terminals revealed that the majority contained small agranular vesicles while large granular vesicles were observed only occasionally. These findings support the suggestion that the populations of neurones expressing binding sites for BSI-B(4) are fairly distinct from those containing neuroactive peptides. In conclusion, the results of the current study suggest that the lectin BSI-B(4) can be used as a histological marker for a subpopulation of small diameter primary sensory neurones and provide further evidence for the potential of this lectin as a useful tool in the study of pain.     

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.     

An electron microscopic study of the mediodorsal cerebral cortex in the lizard Agama agama

Summary An electron microscopic analysis was made of the small-celled part of the mediodorsal cortex of the lizard Agama agama. This cortex consists of four layers: Superficial plexiform layer, cellular layer, deep plexiform layer and fiber layer. In the superficial plexiform layer one type of solitary neuron with smooth dendrites is present.Three types of axon terminals can be observed: terminals with a moderately electron dense matrix packed with spherical vesicles (S1 type), axon terminals with an electron lucent matrix containing fewer spherical synaptic vesicles than the S1 type (S2 type) and axon terminals with an electron lucent matrix and scattered pleomorphic synaptic vesicles (F type). F type axon terminals are larger than S terminals. At the pial surface endfeet of tanycytic processes form a limiting glial layer, contacting one another by means of gap junctions. In the cellular layer perikarya of pyramidal neurons are densely packed. The karyoplasm of these neurons shows either evenly dispersed or discretely clumped chromatin. Spiny dendrites arise from the perikarya and extend into both the superficial and deep plexiform layers. The structure of the deep plexiform layer is roughly similar to that of the superficial plexiform layer. The fiber layer contains the majority of the afferent and efferent axons of the mediodorsal cortex. The axons are myelinated and unmyelinated. Between the fibers, scattered solitary neurons are present, often accompanied by glial cells.The lateral ventricle beneath the fiber layer is lined by a single row of ependymal tanycytes. Tanycytic processes traverse the cortical layers and may form endfeet at the pial surface. Protoplasmic excresenses from some ependymal cells protrude into the ventricle.     

Distribution of GABA immunoreactivity in the retino-recipient layer of the viper optic tectum. A light and electron microscope quantitative study

The distribution of GABA-immunoreactivity was investigated in the principal retino-recipient layer of the optic tectum in the snake Vipera aspis. This layer, the stratum griseum et fibrosum superficiale, contained an important proportion (approximately 50%) of small GABA-immunoreactive interneurons, characterized by a voluminous invaginated nucleus surrounded by a thin rim of cytoplasm poor in organelles and occasionally showing pleiomorphic synaptic vesicles, which could also be observed in some of the dendrites that contained synaptic vesicles. In the neuropile, the GABA-immunoreactive profiles containing synaptic vesicles could be subdivided into dendrites containing synaptic vesicles and axon terminals with pleiomorphic synaptic vesicles. The dendrites containing synaptic vesicles (23.4% of all profiles containing synaptic vesicles) were postsynaptic either to optic terminals (39.2%), GABA-immunoreactive axon terminals with pleiomorphic synaptic vesicles (48.2%) or to immunonegative (S₁) boutons with round synaptic vesicles (12.6%). These dendrites were presynaptic to GABA-immunoreactive (18%) neurons or immunonegative (82%) neurons. The axon terminals with pleiomorphic synaptic vesicles, which represented 47.4% of all profiles, were predominantly (99%) GABA-immunoreactive and four types could be distinguished according to cytological criteria. These axon terminals made synaptic contacts for the most part (78%) with immunonegative profiles, and more rarely (22%) with immunoreactive neurons. These data are compared to those previously obtained in the homologous structure of other vertebrate species, birds and mammals in particular.     

Dual axonal terminations from the retrosplenial and visual association cortices in the laterodorsal thalamic nucleus of the rat

Light and electron microscopic tracing studies were conducted to assess the synaptic organization in the laterodorsal thalamic nucleus (LD) of the rat and the laminar origins of corticothalamic terminals from the retrosplenial and visual association cortices to LD. A survey of the general ultrastructure of LD revealed at least three types of presynaptic terminals identified on the basis of size, synaptic vesicle morphology, and synaptic membrane specializations: (1) small axon terminals with round synaptic vesicles (SR), which accounted for the majority of terminal profiles and made asymmetric synaptic contacts predominantly with small dendritic shafts and spines; (2) large axon terminals with round synaptic vesicles (LR), which formed asymmetric synaptic contacts mainly with large dendritic shafts; and (3) small to medium-size axon terminals with pleomorphic synaptic vesicles (SMP), which symmetrically synapsed with a wide range of postsynaptic structures from cell bodies to small dendrites. Synaptic glomeruli were identified, whereas no presynaptic dendrites were found. To characterize and identify corticothalamic terminals arising from the retrosplenial and visual association cortices that project to LD, wheat germ agglutinin conjugated to horseradish peroxidase (WGA–HRP) was injected into these cortices. Axons anterogradely labeled with WGA–HRP ended in both SR and LR terminals. On the other hand, dextran-tetramethylrhodamine injected into LD as a retrograde fluorescent tracer labeled large pyramidal cells of layer V as well as small round or multiform cells of layer VI in the retrosplenial and visual association cortices. These findings provide the possibility that corticothalamic terminations from cortical neurons in layer V end as LR terminals, while those from neurons in layer VI end as SR boutons.     

Nitric oxide-containing pyramidal neurons of the subiculum innervate the CA1 area

Neurons and axon terminals containing neuron-specific nitric oxide synthase (nNOS) were examined in the rat subiculum and CA1 area of Ammon's horn. In the subiculum, a large subpopulation of the pyramidal neurons and non-pyramidal cells are immunoreactive for nNOS, whereas in the neighbouring CA1 area of Ammon's horn only non-pyramidal neurons are labelled with the antibody against nNOS. In the pyramidal layer of the subiculum, nNOS-positive axon terminals form both asymmetric and symmetric synapses. In the adjacent CA1 area the nNOS-positive terminals that form symmetric synapses are found in all layers, whereas those terminals that form asymmetric synapses are only in strata radiatum and oriens, but not in stratum lacunosum-moleculare. In both the subiculum and CA1 area, labelled terminals make symmetric synapses only on dendritic shafts, whereas asymmetric synapses are exclusively on dendritic spines. Previous observations demonstrated that all nNOS-positive non-pyramidal cells are GABAergic local circuit neurons, which form exclusively symmetric synapses. We suggest that nNOS-immunoreactive pyramidal cells of the subiculum may innervate neighbouring subicular pyramidal cells and, to a smaller extent, pyramidal cells of the adjacent CA1 area, forming a backward projection between the subicular and hippocampal principal neurons. Electronic Publication     

Ultrastructural study of cholecystokinin-like immunoreactive nerve terminals in the rat nucleus accumbens

The cholecystokinin (CCK)-like immunoreactive nerve terminals were studied in the caudal and medial parts of the rat nucleus accumbens (NA), using the indirect immunoperoxidase technique, at the electron microscopic level. In the labelled axon terminals the immunoprecipitate is localized inside large dense-cored vesicles which are occasionally present, and surrounds small and medium-sized, round, clear synaptic vesicles. The immunoreactive nerve terminals participate in synapses of both asymmetrical and symmetrical types containing mostly small synaptic vesicles. The asymmetrical synapses are much more numerous and mainly axo-spinous. The symmetrical synapses are less frequent and are axo-dendritic or axo-somatic.