Ultrastructural changes in the dorsal motor nucleus of monkey following bilateral cervical vagotomy

Summary The neurons of the dorsal motor nucleas (DMN) of the monkey (Macaca fascicularis) were of two main types: small (13 × 8 m) and medium-sized (20 × 13 m). The latter, which were the predominant form, contained a pale oval nucleus surrounded by organelle-rich cytoplasm. Between one and three long principal dendrites per section profile arose from eac¹ of the somata. Both axosomatic and axodendritic synapses were seen on these cells although the latter were more common.No structural changes were noted in the DMN 1–3 days after bilateral cervical vagotomy. Some of the dendrites of the medium-sized axotomized vagal neurons appeared darkened 5–10 days after the operation. With longer surviving intervals, i.e. 21 and 28 days after operation, darkened dendrites were more commonly seen and the cytoplasmic density of these dendrites was dramatically enhanced. Their mitochondria were pale and some of them also showed vesiculation. Both normal and degenerating axon terminals were seen to form synaptic contacts with the darkened dendrites. The degenerating axon terminals were characterized by the clumping of their round agranular vesicles. Both darkened dendrites and degenerating axon terminals were phagocytosed by hypertrophied astrocytes and activated microglial cells. Blood elements infiltrating into the DMN were a possible source for some of the neural macrophages.It was concluded from the present study that the dendrites of the vagal neurons were the first structures to degenerate in axotomy and these were subsequently removed by glial elements. Degenerating axon terminals on the darkened dendrites could represent endings of the central processes of peripheral vagal ganglion cells that had undergonetransganglionic degeneration after damage to their peripheral processes.     

A glutamate-sensitive neuronal system originating from the area postrema terminates in and transports acetylcholinesterase to the nucleus of the solitary tract

Summary The possible cellular mechanism of action of systemically administered monosodium-l-glutamate and the projections of glutamate-sensitive area postrema neurons have been studied in rats. Parenteral administration of monosodium-l-glutamate induced a selective degeneration of a particular population of AChE-containing area postrema neurons. Electron microscopic cytochemistry and X-ray microanalysis revealed the presence of calcium-containing electron-dense deposits in the mitochondria of degenerating area postrema neurons indicating the possible pathogenetic role of an enhanced intracellular calcium level in the mechanism of monosodium-l-glutamate-induced nerve cell degeneration. Degeneration of area postrema neurons was followed by the appearance of degenerating axon terminals in a well-defined region of the nucleus of the solitary tract, the area subpostrema. Degenerating area postrema neurons and axon terminals were rapidly engulfed by phagocytes predominantly of microglial character. AChE activity, localized to the basal lamina of the capillaries of the area subpostrema under normal conditions, could no longer be detected in rats treated with monosodium-l-glutamate 3–4 weeks previously.These findings provide evidence for the existence of a particular population of glutamate-sensitive, AChE-containing area postrema neurons which project and transport AChE to the nucleus of the solitary tract. This specific neuronal pathway connecting the area postrema with the nucleus of the solitary tract may play an important role in some of the functions attributed to the area postrema. The results also strengthen the hypothesis that brain capillary AChE activity may be of neuronal origin.     

The termination of thalamo-cortical fibres in the visual cortex of the cat

Summary An electron microscopic study has been made of the site and mode of termination of thalamo-cortical fibres in area 17 of the visual cortex of the cat. Thalamo-cortical fibres had been selectively interrupted 4–5 days before perfusion of the brain. In agreement with previous studies, degenerating axon terminals were found in layer I, in the deep part of layer III, and in layer IV of the cortex. In addition, a few degenerating thalamo-cortical axon terminals were found making synapses upon spines and small dendrites in layer VI. Two examples were seen of degenerating axon terminals making axo-dendritic synapses upon dendrites in continuity with their cell bodies. These neurons were not large stellate cells and were probably pyramidal.     

弓状核内神经紧张素能神经元与神经肤Y能神经元关系的免疫电镜双标记研究

用包埋前免疫电镜PAP双标记技术,对大鼠下丘脑弓状核内的神经紧张素(NT)和神经肽Y(NPY)的分布进行了超微结构研究。先用DAB法显示NPY免疫反应。然后用钼酸铵-TMB法显示NT免疫反应,再经DAB-氯化钴稳定后作免疫电镜包埋。结果:在弓状核内,NPY免疫反应产物呈电子密度高的颗粒状或絮状沉淀,弥漫分布于核周质的细胞器和基质、树突的微管周围、轴突的小透亮囊泡周围。NT免疫反应产物则为致密的针状或块状,散在分布于核周质、树突和轴突内。两者极易分辨。NPY能和NT能神经结构在弓状核内呈交错分布,彼此关系密切。含NPY的树突和轴突与免疫反应阴性轴突形成突触连接;含NT的胞体和树突与免疫反应阴性轴突形成突触连接。此外,NPY阳性轴突末梢还与NT阳性树突形成对称性轴-树突触。本研究结果为下丘脑NT能神经元的肽能突触调节提供了又一新的超微结构依据。     

Effects of intraneural injection of Ricinus communis agglutinin-60 into rat vagus nerve

Summary The dorsal motor nucleus (DMN) of the rat was studied at various survival periods following an intraneural injection ofRicinus communis agglutinin-60 (RCA-60) into the vagus nerve at the mid-cervical region. No obvious structural changes were noted in the DMN 2 and 4 days after the injection of RCA-60. At 5 and 6 days after the RCA-60 injection, the larger neurons (measuring 19 × 12 m) in the DMN underwent chromatolytic degeneration whereas the smaller ones (measuring 10 × 6 m), characterized by their infolded nuclei, remained unaffected. The majority of the degenerating DMN neurons became pale and crenated in outline. Other structural changes included swollen mitochondria with disrupted cristae and profiles of rough endoplasmic reticulum denuded of ribosome particles. A few of the degenerating neurons became extremely condensed and darkened. Axon terminals which showed synaptic contacts with these cells remained normal. Both pale and darkened degenerating dendrites, derived from the degenerating neurons, were present in the neuropil. In addition to these, degenerating axon terminals with clumping or swelling of synaptic vesicles were also present. They were presynaptic to dendrites of various sizes. Massive infiltration of mononuclear cells occurred in the DMN. These cells reached the DMN by diapedesis and were actively engaged in the phagocytosis of degenerating neuronal elements. While most of the invading cells transformed into active neuronal macrophages, some of them eventually died in the neuropil of the DMN.Light microscopic study by Fink-Heimer's method for degenerating fibres and terminals revealed their distribution to the DMN, nucleus of the tractus solitarius, nucleus commissuralis, dorsolateral and lateral part of the hypoglossal nucleus and the area postrema.It was concluded from this study that RCA-60, when injected into the cervical vagus was retrogradely transported to the cell body of the DMN neurons of the larger category. The selective destruction of the DMN neurons by RCA-60 elicited a massive infiltration of mononuclear cells which gave rise to the neural macrophages. The RCA-60 injected also killed the vagal sensory neurons as demonstrated by the numerous degenerating fibres and axon terminals in the DMN which would represent their central processes.     

Quantitative and ultrastructural study of ascending projections to the medial mammillary nucleus in the rat

Summary We analyzed the termination pattern of axons from the superior central nucleus and the ventral tegmental nucleus of Gudden within the medial mammillary nucleus (MM) in the rat. The neuropil of the MM consists of two classes of terminals, that is, terminals containing round synaptic vesicles and forming asymmetric synaptic contact, and terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact. The number of axodendritic terminals with round vesicles is almost equal to that of terminals with pleomorphic vesicles. Almost all axosomatic terminals contain pleomorphic vesicles with symmetric synaptic contact. Injection of WGA-HRP into the central part of the superior central nucleus permitted ultrastructural recognition of many anterogradely labeled terminals within the median region of MM. The labeled terminals contacted mainly intermediate (1–2 m diameter) and proximal dendrites (more than 2 m diameter) as well as the neuronal somata. Serial ultrathin sections of neurons of the median region of the MM revealed that 37% of the axosomatic terminals were labeled anterogradely. The pars compacta of the superior central nucleus had reciprocal connections with the median region of MM. The axon terminals from this nucleus occupied 53% of axosomatic terminals, and contacted mainly intermediate dendrites. Following injection of WGA-HRP into the ventral tegmental nucleus, many labeled terminals were found in the medial and lateral regions of MM. They contacted mainly intermediate dendrites as well as neuronal somata. In the medial region, 78% of axosomatic terminals contacting retrogradely labeled neurons were labeled anterogradely. All labeled terminals from these nuclei contained pleomorphic vesicles, and made symmetric synaptic contact.     

Neural elements containing glutamic acid decarboxylase (GAD) in the dorsal lateral geniculate nucleus of the rat; immunohistochemical studies by light and electron microscopy

Immunoreactive constituents of the dorsal lateral geniculate nucleus of adult albino rats were examined by light- and electron-microscopy, using the unlabelled antibody enzyme method, following treatment of brain slices with a purified antibody to glutamic acid decarboxylase. The neuropil of the dorsal lateral geniculate nucleus displayed a conspicuous granular immunoreactivity. In addition, the antibody was bound to a class of small neurons of characteristic morphology. These cells possessed few (commonly 2-4) sparsely branched, long dendrites from some of which immunoreactive appendages were traced. Many cells were bipolar in form, and the dendrites of some appeared to be preferentially orientated. The immunoreactive cells closely resembled intrinsic interneurons characterized in previous Golgi studies of this nucleus. By electron-microscopy, immunoreactive presynaptic elements were present both in the extraglomerular neuropil and in the synaptic glomeruli. The former were axon terminals containing flattened synaptic vesicles and making Gray type II axo-dendritic synaptic contact; they appeared to correspond to axon terminals whose origin in the thalamic reticular nucleus has been established in previous studies, but it is possible that some were axon terminals of intrinsic interneurons. The immunoreactive glomerular components also contained flattened vesicles, were presynaptic to presumptive projection cell dendrites, postsynaptic to retinal axon terminals, and participated in triplet (triadic) and other complex synaptic arrangements. They corresponded in all respects to the synaptic portions of the complex dendritic appendages of intrinsic interneurons, identified and characterized in previous studies. The finding that there are high levels of glutamic acid decarboxylase in the cell bodies, dendritic shafts and dendritic appendages of intrinsic interneurons in the dorsal lateral geniculate nucleus of the rat, and in the axon terminals of fibres projecting to this site from the thalamic reticular nucleus, allows us to conclude that the inhibitory inputs to the geniculo-cortical projection cells from both of these sources are probably mediated by gamma-aminobutyric acid.     

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.     

Fine structural survey of tyrosine hydroxylase immunoreactive terminals in the myenteric ganglion of the rat duodenum

We have examined whether the noradrenergic neurons have direct synaptic projections to the myenteric ganglion neurons of the duodenum and the ultrastructure of their terminals by using immunogold–silver labeling for tyrosine hydroxylase. In the neuropil of the myenteric ganglia, about half of the axon terminals contained round clear vesicles and the rest of them contained pleomorphic clear vesicles. The sizes of axon terminals contacting the dendrites as a whole were 1.62 ± 0.07 μm. All axon terminals formed asymmetric synaptic contacts with dendrites or somata. Immunohistochemical study revealed that the tyrosine hydroxylase-immunoreactive nerve terminals were distributed throughout the ganglia and contained exclusively pleomorphic clear synaptic vesicles (about 20–80 nm long). The tyrosine hydroxylase-immunoreactive terminals were generally large (1.99 ± 0.07 μm). A considerable number of the tyrosine hydroxylase-immunoreactive terminals made asymmetric synaptic contacts with small dendrites, spines or somata of the myenteric ganglion neurons. Serial ultrathin sections through the myenteric neurons revealed that about 16% of the total number of axosomatic terminals showed tyrosine hydroxylase immunoreactivity. These results indicated that the myenteric ganglion neurons of the duodenum receive direct synaptic projection of sympathetic noradrenergic neurons and that their terminals contain pleomorphic vesicles and form asymmetric synaptic contacts.     

The ultrastructure of the olivary pretectal nucleus in rats. A tracing and GABA immunohistochemical study

 The olivary pretectal nucleus is a primary visual centre, involved in the pupillary light reflex. In the present study an ultrastructural analysis was made of the olivary pretectal nucleus by means of separate, anterograde and retrograde tracing techniques and immunohistochemistry of gamma-aminobutyric acid. Large-projection neurons and two types of gamma-aminobutyric acid-immunoreactive (GABA-ir) neurons are observed in the olivary pretectal nucleus. The primary dendrites of the projection neurons have a dichotomous appearance, the secondary dendrites a multipolar appearance. At the ultrastructural level the projection neurons have well-developed Golgi fields, abundant rough endoplasmic reticulum and the nucleus is always heavily indented. Numerous small GABA-ir neurons and a few medium-sized GABA-ir neurons are found. The small GABA-ir neurons contain a few stacks of rough endoplasmic reticulum and the nucleus is oval-shaped. The medium-sized GABA-ir neurons have well-developed Golgi fields, a moderate number of rough endoplasmic reticulum stacks and an indented nucleus. GABA-positive dendritic profiles containing vesicles also are observed. In the neuropil of the olivary pretectal nucleus, retinal terminals are found that contain round clear vesicles and electron-lucent mitochondria. They make asymmetric synaptic contacts (Gray type I) with dendritic profiles and with profiles containing vesicles. Terminals originating from the contralateral olivary pretectal nucleus exhibit small, round clear vesicles, electron-dense mitochondria and make asymmetric synaptic contacts (Gray type I) mainly with dendritic profiles. Two types of GABA-ir terminals were found. One type is incorporated in glomerulus-like arrangements, whereas the other type is not. GABA-ir terminals contain pleomorphic vesicles, electron-dense mitochondria and make symmetric synaptic contacts (Gray type II). Retinal terminals, terminals originating from the contralateral olivary pretectal nucleus and GABA-ir terminals are organized in glomerulus-like structures, in which dendrites of the large projection neurons form the central elements. Triadic arrangements are observed in these structures; a retinal terminal contacts a dendrite and a GABA-ir terminal and the GABA-ir terminal also contacts the dendrite. The complexity of the synaptic organization and the abundancy of inhibitory elements in the olivary pretectal nucleus suggest that the olivary pretectal nucleus is strongly involved in processing visual information in the pupillary light reflex arc. Received: 17 July 1996 / Accepted: 24 September 1996     

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。     

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster

Summary The synaptic organization of the sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase.Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains (i) exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; (ii) pre- and postsynaptic pleomorphic-vesiclecontaining P-boutons (interpreted as appendages of the dendrites of interneurons); (iii) large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; (iv) F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synatpic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus.No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.     

Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer

Summary Following section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as largeen passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6–30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component.After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2–3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.     

The synaptic organization of the cerebello-oiivary circuit

Summary Section of the superior cerebellar peduncle just rostral to the deep cerebellar nuclei results in degenerating axon terminals within the contralateral inferior olive. The nuclear origin of this fiber system and its distribution within the subdivisions of the inferior olive were described in a companion study (Martin et al., 1976). Precise localization of these degenerating terminals within the nucleus was accomplished by the examination of 1 plastic sections cut from each tissue block prior to thin sectioning. Degenerating axon terminals are present in all the nuclear subdivisions and when seen with the electron microscope they frequently are localized in the previously described synaptic clusters (King, 1976). These terminals demonstrate an electron dense reaction at survival times of 2 and 3 days. By day 4, they are shrunken and irregular in shape, and typically are surrounded by astrocyte processes. Cerebello-olivary axon terminals measure 1–3 , contain spherical, clear synaptic vesicles and typically contact spiny appendages within the synaptic clusters (glomeruli). Thus, we have demonstrated that one of the primary axon systems which terminates within the synaptic clusters is from the cerebellar nuclei. We have yet to determine the origins of the remaining terminals within the synaptic clusters which include endings with either smaller spherical, pleomorphic or numerous dense core vesicles.This investigation was supported by United States Public Health Service Grant NS 08798.     

皮质—小脑前核—小脑通路—HRP法结合溃变电镜法研究

目的：观察大鼠小脑前核（桥核,下橄榄,外侧网状核,桥被盖网状核和旁正中网状核）中皮质纤维终末的溃变型及其与各核中的小脑投射神经元的突触联系。材料和方法：８只大鼠,用ＨＲＰ逆行示记法和溃变电镜技术。结果：（１）溃变型有三种,电子致密型,微丝增生型以及电子透明型。（２）电子致密型溃变有三种不同形态,即含圆形清亮型小泡,含多形清亮型小泡和混合型小泡的三种终末,其中绝大多数溃变终末含圆形亮型小泡。（４）溃     

The thalamic reticular nucleus of the adult rat: experimental anatomical studies

Summary The thalamic reticular nucleus (TRN) is a sheet-like nucleus partially enclosing the dorsolateral and anterior aspects of the thalamus and traversed by the thalamo-cortical and cortico-thalamic fibre systems. This paper describes the cellular and synaptic organization of the TRN in adult albino rats on the basis of LM and EM studies of normal animals and experimental animals with injections of horseradish peroxidase (HRP) and/or lesions in various parts of the brain. Particular attention was paid to the dorso-caudal part of the TRN, which establishes connections with visual centres.LM-HRP preparations show that the neurons of TRN project only to ipsilateral dorsal thalamus; no labelled cell bodies were found in TRN after injections into the cortex or any part of the brain stem caudal to the thalamus. Small injections into dorsal thalamus result in a small cluster of labelled neurons and an associated patch of terminal label in TRN. The dorso-caudal part of the nucleus projects to the dorsal lateral geniculate nucleus, the ventro-caudal part to the medial geniculate nucleus and a large part of the nucleus anterior to the areas associated with the geniculate nuclei projects to the ventrobasal nucleus. No evidence was found for a widespread distribution of reticulo-thalamic axons and the connections between TRN and the dorsal lateral geniculate nucleus and between TRN and the ventrobasal nucleus show a fine-grain topographical organization with more rostral and dorsal parts of TRN projecting to more rostral and dorsal parts of the dorsal lateral geniculate and ventrobasal nuclei.The neurons of TRN are variable in size (range of somal diametersc. 10–20 m), shape (cell bodies are most commonly ellipsoidal) and dendritic morphology (bitufted and bipolar arrangements most common), but no basis for subdividing them into more than one class was found with any of the techniques used. The cell body and dendrites are commonly aligned parallel to the surface of TRN and at right angles to the traversing fibre bundles. The dendrites do not branch extensively and are only moderately spinous. Long, hair-like spines corresponding to those described by Scheibel & Scheibel (1966) were not found: nor were dendritic bundles found to be as prominent in EM material as reported by these authors in LM-Golgi material. Plasma membranes of dendrites in small bundles and of contiguous somata were commonly in direct contact over large areas, but gap junctions between them were not seen.The neuropil of TRN is simple with three major axon terminal types.D-type terminals (about 56% of all terminals in visual TRN) have closely packed spherical synaptic vesicles (42 nm diameter);L-type terminals (about 31%) are paler, slightly larger and have less densely packed synaptic vesicles (46 nm diameter); both terminal types make Gray type 1 synaptic contacts on dendritic spines and dendritic shafts and rarely also on cell bodies and axon hillocks.F-type terminals (about 8%) contain flattened synaptic vesicles in a dark matrix and make Gray type 2 contacts with dendrites, cell bodies and axon hillocks. In visual TRN, D-type terminals (but not all) degenerate after ablation of ipsilateral visual cortex and L-type terminals (but not all) degenerate after lesion of ipsilateral dorsal lateral geniculate nucleus; the density of degenerating terminals is higher after cortical than after geniculate lesions. Indirect evidence suggests that F-type terminals may be (or may include) collaterals of reticulo-thalamic projection cells, but no evidence was found for a widespread or dense plexus of such collaterals.After injection of HRP into the dorsal lateral geniculate nucleus, labelled axon terminals in visual TRN (many clearly L-type) were found in synaptic contact with retrogradely labelled dendrites of reticulo-geniculate projection cells. When HRP injection was combined with ablation of ipsilateral visual cortex, degenerating axon terminals (most of them identifiable as D-type) were also found in synaptic contact with retrogradely-labelled dendrites of reticulo-geniculate projection cells.Thus, neurons of visual TRN in the rat receive monosynaptic, presumptively excitatory input from collaterals of cortico-geniculate and geniculo-cortical axons, and project in a topographically-organized manner to the ipsilateral dorsal lateral geniculate nucleus (where they make Gray type 2 GABAergic and presumptively inhibitory synaptic contacts chiefly with the dendrites of geniculo-cortical projection cells). A similar pattern of organization is seen in other parts of the TRN and these data are compatible with the view that the TRN (and the perigeniculate nucleus of the cat thalamus, which is similar in several respects to visual TRN) forms part of a negative feed-back system by which the activity of thalamo-cortical projection neurons is regulated.     

Ultrastructure of vestibular commissural neurons related to velocity storage in the monkey.

The angular vestibulo-ocular reflex maintains gaze during head movements. It is thought to be mediated by two components: direct and velocity storage pathways. The direct angular vestibulo-ocular reflex is conveyed by a three neuron chain from the labyrinth to the ocular motoneurons. The indirect pathway involves a more complex neural network that utilizes a portion of the vestibular commissure. The purpose of the present study was to identify the ultrastructural characteristics of commissural neurons in the medial vestibular nucleus that are related to the velocity storage component of the angular vestibulo-ocular reflex. Ultrastructural studies of degenerating medial vestibular nucleus neurons were conducted in monkeys following midline section of rostral medullary commissural fibers with subsequent behavioral testing. After this lesion, oculomotor and vestibular functions attributable to velocity storage were abolished, whereas the direct angular vestibulo-ocular reflex pathway remained intact. Since this damage was functionally discrete, degenerating neurons were interpreted as potential participants in the velocity storage network. Ultrastructural observations indicate that commissural neurons related to velocity storage are small and medium sized cells having large nuclei with deep indentations and relatively little cytoplasm, which are located in the lateral crescents of rostral medial vestibular nucleus. The morphology of degenerating dendritic profiles varied. Some contained numerous round or tubular mitochondria in a pale cytoplasmic matrix with few other organelles, while others had few mitochondria but many cisterns and vacuoles in dense granular cytoplasm. The commissural nature of these cells was further suggested by the presence of two different types of degenerating axon terminals in the rostral medial vestibular nucleus: those with a moderate density of large spherical synaptic vesicles, and those with pleomorphic, primarily ellipsoid synaptic vesicles. The recognition of two types of degenerating terminals further supports our interpretation that at least two morphological types of commissural neurons participate in the velocity storage network. The degenerating boutons formed contacts with a variety of postsynaptic partners. In particular, synapses were observed between degenerating boutons and non-degenerating dendrites, and between intact terminals and degenerating dendrites. However, degenerating pre- and postsynaptic elements were rarely observed in direct contact, suggesting that additional neurons are interposed in the indirect pathway commissural system. On the basis of these ultrastructural observations, it is concluded that vestibular commissural neurons involved in the mediation of velocity storage have distinguishing ultrastructural features and synaptology, that are different from those of direct pathway neurons.     

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.     

Ultrastructure and immunocytochemical distribution of GABA in layer III of the rat medial entorhinal cortex following aminooxyacetic acid-induced seizures

 Layer III of the entorhinal cortex (EC) is lesioned in patients with temporal lobe epilepsy (TLE). A similar neuropathology is also present in different animal models of TLE. For example, injection of the ”indirect” excitotoxin aminooxyacetic acid (AOAA) into the EC of rats causes behavioral seizures and preferential loss of neurons in layer III of the medial EC. The animals also develop hyperexcitability of the EC and the hippocampal region CA1. To further explore the neuropathological changes within the EC, the ultrastructure and distribution of GABA-like immunoreactivity were assessed in layer III, 28 days after an intraentorhinal AOAA injection. At this time point, light microscopic preparations revealed that a large proportion of pyramidal (putative excitatory) neurons in layer III of the medial EC had degenerated, whereas GABA-immunoreactive neurons had survived. In immunogold-labeled ultrathin sections, the lesioned neuropil was found to contain morphologically intact GABA-containing neurons and nerve terminals. Pathologically swollen dendrites and electron-dense neuronal profiles were present in the lesioned sector as well. The majority of the electron-dense profiles was identified as degenerating dendritic spines that were closely apposed to strongly glutamate-immunopositive axon terminals. Thus, the entorhinal chemoarchitecture is dramatically altered following an episode of AOAA-induced epileptic seizures. One possible consequence of this pathology is a reduced ”drive” of the surviving layer III GABA neurons, which in turn may cause hyperexcitability of the EC and the hippocampus. These findings may be of relevance for the genesis and spread of temporal lobe seizures. Received: 8 September 1998 / Accepted: 1 December 1998     

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex, IV terminations upon spiny dendrites

Summary The forms of the spiny dendrites in layer IV receiving degenerating thalamocortical axon terminals have been examined in serial thin sections. Reconstructions of segments of these dendrites show that the axon terminals synapse with both the dendritic spines and the dendritic shafts. No main shafts of apical dendrites of pyramidal neurons were found to synapse with the thalamic afferents, which are received mainly by spiny dendrites 1–2 m in diameter, at least some of which appear to be the oblique branches of apical dendrites. The forms of these postsynaptic dendrites are so variable that is is concluded they arise from more than one morphological type of neuron. The conclusion based on this and previous articles in the series is that most neuronal elements in layer IV which form asymmetric synaptic junctions are potential recipients of the thalamocortical afferents.