Synaptology and origin of somatostatin fibers in the rat lateral septal area: convergent somatostatinergic and hippocampal inputs of somatospiny neurons.

This study deals with the synaptology, morphologically identified postsynaptic targets, and origin of somatostatin (SOM) fibers in the rat lateral septal area (LSA) with special reference to those forming pericellular baskets. Septal vibratome sections were immunostained for SOM-14 in 3 experimental groups: control animals, rats subjected to a chronic transection of the ascending afferents to the septum, and animals with acute fimbria-fornix lesion. Light microscopy revealed that the SOM-immunoreactive fibers form pericellular baskets predominantly in the intermediate and ventral parts of the caudal half of the LSA. Electron microscopic analysis showed that the somatospiny neurons are postsynaptic targets of these pericellular baskets. Eight days after a unilateral cut placed at the ventral border of the septum, virtually all SOM-immunoreactive axon terminals disappeared from the ipsilateral intermediate and ventral LSA, and they were substantially reduced in the dorsal LSA. However, in these rats SOM-positive neurons could be observed in the LSA on the lesioned, but not on the contralateral side. Furthermore, on the lesion side of the anterior periventricular hypothalamus an increase was detected both in the number and the intensity of immunostaining of SOM-positive neurons. Thirty-six h following a unilateral transection of the fimbria-fornix, the SOM-immunoreactive axon terminals in the LSA remained intact; only immunonegative degenerated hippocamposeptal boutons were detected forming synaptic contacts with somatospiny neurons. Axosomatic synapses of SOM-positive boutons regularly appeared at the neck of somatic spines which were postsynaptic to degenerated hippocamposeptal fibers. The results indicate that the septal SOM fibers are of multiple origin. Those forming pericellular baskets in the LSA originate in ventral extraseptal, probably periventricular hypothalamic areas. SOM fibers scattered in the dorsal LSA are most likely processes of local SOM neurons. The accumulation of immunoreactive SOM in some cells of the undercut septum is a sign of axonal lesion, indicating that these neurons project outside the septum. The SOM innervation of somatospiny neurons which also receive hippocampal input and have been reported to contain gamma-aminobutyric acid (GABA) may be a morphological substrate of the SOM-related disinhibition in the LSA.     

Catecholaminergic, GABAergic, and hippocamposeptal innervation of GABAergic "somatospiny" neurons in the rat lateral septal area

This study deals with the neurochemical characterization of the rat lateral septal area (LSA) somatospiny neurons and their innervation by hippocamposeptal, catecholaminergic, and GABAergic fibers. Electron microscopic single and double immunostaining methods were used to label catecholaminergic fibers and GABAergic cells and boutons. Axon terminals originating in the hippocampus were labeled by acute anterograde axon degeneration induced by fimbria-fornix transection 36 hours before sacrifice. Three types of experiments were performed. The convergent catecholaminergic and hippocamposeptal innervation of LSA somatospiny neurons was studied by combining immunostaining for tyrosine hydroxylase (TH) with fimbria-fornix transection. GABAergic neurons and their hippocamposeptal afferents were identified and characterized in colchicine pretreated animals immunostained for glutamic acid decarboxylase (GAD) combined with fimbria-fornix transection. The third experiment aimed at simultaneously visualizing the relationships between catecholaminergic boutons, hippocamposeptal excitatory amino acid containing axon terminals and GABAergic profiles by double immunostaining for TH (the PAP technique) and GAD (the immunogold method) combined with fimbria-fornix transection. The results are summarized as follows: 1) The same LSA somatospiny neurons receive synaptic inputs from the hippocampus and TH immunoreactive fibers which form pericellular baskets around these cells. 2) LSA somatospiny neurons are GABAergic and are postsynaptic targets of GABAergic boutons with unknown origin and hippocamposeptal axon terminals. 3) The double immunostaining experiment, finally, provided direct evidence that the same GABAergic somatospiny neurons are postsynaptic targets of both catecholaminergic and hippocamposeptal afferents. The synaptic interconnections described in this study provide anatomical basis for a better understanding of the action of catecholamines, excitatory amino acids, and GABA on the activity of LSA neurons.     

Organization of the septal region in the rat brain: cholinergic-GABAergic interconnections and the termination of hippocampo-septal fibers

This study deals with two characteristic cell types in the rat septal complex i.e., cholinergic and GABAergic neurons, and their synaptic connections. Cholinergic elements were labeled with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Antiserum against glutamate decarboxylase (GAD), the GABA synthesizing enzyme, was employed to identify GABAergic perikarya and terminals, by using either the peroxidase-antiperoxidase (PAP) technique or a biotinylated second antiserum and avidinated gold or ferritin. With these contrasting immunolabels we have studied the cholinergic-GABAergic interconnections in double-labeled sections of intact septal regions and the GABAergic innervation of medial septal area cholinergic neurons in sections taken from animals 1 week following lateral septal area lesion. In other electron microscopic experiments we have studied cholinergic and GABAergic neurons in the septal complex for synaptic contacts with hippocamposeptal fibers, which were identified by anterograde degeneration following fimbria-fornix transection. Our results are summarized as follows: (1) GAD-positive terminals form synaptic contacts on ChAT-immunoreactive dendrites in the medial septum/diagonal band complex (MSDB), (2) surgical lesion of the lateral septal area resulted in a dramatic decrease of the number of GABAergic boutons on MSDB cholinergic neurons, (3) cholinergic terminals establish synaptic contacts with GAD immunoreactive cell bodies and proximal dendrites in the MSDB as well as in the lateral septum (LS), (4) degenerated terminals of hippocampo-septal fibers were mainly observed in the LS, where they formed asymmetric synaptic contacts on dendrites of GABAergic neurons and on nonimmunoreactive spines. We did not observe degenerated boutons in contact with ChAT-positive dendrites or cell bodies in the MSDB. From these results and from data in the literature we conclude that excitatory hippocampo-septal fibers activate GABAergic cells, and as yet unidentified spiny neurons in the LS, which may control the discharge of medial septal cholinergic neurons known to project back to the hippocampal formation.     

Hypothalamo-septal enkephalinergic fibers terminate on AMPA receptor-containing neurons in the rat lateral septal area

A large number of septal neurons express α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA)-type excitatory glutamate receptors. It has been demonstrated that in the mediolateral part of the rat lateral septum, calbindin-containing neurons are heavily innervated by hypothalamic, enkephalinergic fibers forming exclusively asymmetric synaptic contacts. This connection was suggested to be excitatory. In order to further elucidate this hypothesis, the aim of the present study was to determine whether these enkephalinoceptive neurons express GluR1 and GluR2/3 AMPA receptor subunits. Correlated light and electron microscopic analysis, using single immunostaining for GluR1 and GluR2/3, and double immunostaining for Leu-enkephalin and GluR1 or GluR2/3, was performed on vibratome sections of the rat lateral septal area. The studies revealed that while GluR1 is mainly associated with dendritic and somatic spines, GluR2/3 is mostly present in the perisomatic area. Leu-enkephalin boutons establish asymmetric synaptic contacts at the level of the soma and initial dendrites of both of these cells. A semiquantitative analysis showed that these enkephalin-targeted cells represent 50% of the total number of both GluR1 and GluR2/3-containing lateral septal neurons. These results suggest that: (1) AMPA receptor-expressing neurons appear to be the exclusive recipient of hypothalamic Leu-enkephalin boutons; (2) these enkephalinoceptive neurons contain both GluR1 and GluR2/3 AMPA receptor subunits; however, (3) only the GluR2/3 subtype, located in the perisomatic area, may be associated with Leu-enkephalin-containing inputs. Synapse 25:263–271, 1997. © 1997 Wiley-Liss, Inc.     

Identification of synaptic terminals of thalamic or cortical origin in contact with distinct medium-size spiny neurons in the rat neostriatum

In order to determine what types of neurons in the striatum receive direct synaptic input from corticostriatal and thalamostriatal fibres and whether these afferents converge on individual striatal neurons, double anterograde labelling of axon terminals was combined with Golgi impregnation at both the light and electron microscopic levels. The area of the central neostriatum that receives input from both the parafascicular nucleus of the thalamus and the somatosensory cortex was identified by retrograde transport of a conjugate of horseradish peroxidase and wheat germ agglutinin (HRP-WGA). The same region of the neostriatum was studied in rats that had received multiple electrolytic lesions in the somatosensory cortex and also an injection of HRP-WGA in different parts of the parafascicular nucleus. Sections of this part of the neostriatum were impregnated by the single-section Golgi procedure after revealing anterogradely transported HRP-WGA. Twelve Golgi-impregnated spiny neurons were recovered and examined in the light and electron microscope after gold-toning. Ten of these neurons were typical very densely spiny medium-size neurons and they were all found to receive asymmetric synaptic input on dendritic spines from degenerating corticostriatal boutons. However, even though numerous boutons labelled anterogradely by HRP-WGA from the parafascicular nucleus were found within the dendritic fields of neurons that received cortical input, none of the terminals from the thalamus made synaptic contact with these neurons. Instead, all 96 thalamostriatal boutons studied were found in asymmetric synaptic contact with dendritic shafts of other neurons. Two such neurons that received input from the parafascicular nucleus were Golgi-impregnated and appeared to be medium-size spiny neurons, but they had a lower density of spines than the typical very densely spiny neurons. An independent confirmation that the targets of thalamostriatal neurons originating in the parafascicular nucleus are dendritic shafts was provided by studying the boutons labelled following electrolytic lesioning or injection of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) into this nucleus: these boutons were also found to form asymmetric synaptic contacts with dendritic shafts within the neostriatum. It is concluded that although afferents from the somatosensory cortex and from the parafascicular nucleus converge upon the same part of the neostriatum, they probably do not converge upon the same spiny neurons. The afferents from the neocortex form synapses on the spines of densely spiny medium-size neurons, whereas terminals originating from neurons in the parafascicular nucleus are in synaptic contact with the dendritic shafts of what appears to be a morphologically distinct type of medium-size spiny neuron. The integration of the inputs from these two areas probably involves the local axon collaterals of these two types of spiny neuron.     

Comparative distribution of calbindin and Met-enkephalin immunoreactivities in the guinea-pig lateral septum, with reference to electrophysiologically characterized neurons in the mediolateral part

The distribution both of Met-enkephalinergic nerve terminals and of calbindin-containing neurons was investigated in the guinea-pig lateral septum, using a double-immunostaining technique. The findings show that the two immunoreactivities overlapped for neurons located in areas of the dorsal part and of the mediolateral part of the lateral septum. Nine cells of the mediolateral part which were electrophysiologically characterized and intracellularly labelled were subjected to the double-immunostaining protocol. All these cells displayed characteristic discharges due to the activation of high-threshold Ca²⁺ conductances. Two of them contained calbindin and were the target of enkephalinergic inputs; they possessed somatic spines. These data demonstrate: (1) that the guinea-pig lateral septum contains subpopulations of calbindin neurons which are postsynaptic to enkephalinergic inputs; (2) that Ca²⁺ conductances are not related to the presence of calbindin; (3) that somatospiny neurons, which are involved in the regulation of the hippocampo-septo-hypothalamic complex, contain calbindin and are the target of enkephalinergic endings.     

Synaptic organization of septal projections in the rat medial habenula: a wheat germ agglutinin-horseradish peroxidase and immunohistochemical study.

The synaptic organization of septal inputs to the rat habenular complex of the dorsal diencephalon was examined employing the anterograde tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The cellular distribution of substance P (SP) and choline acetyltransferase (ChAT) immunoreactivity was also studied at the light and electron microscopic level. Following placements of tracer within the entire septum, labeled axons were observed in the stria medullaris and in the medial and lateral subnuclei of the habenula. Following injections of tracer in the nuclei triangularis and septofimbrialis of the posterior septum, the medial subnucleus was heavily labeled, whereas the lateral subnucleus was devoid of peroxidase activity. The medial subnucleus possessed labeled myelinated axons and terminals that contained clear, spherical vesicles and formed asymmetric contacts with dendritic spines and shafts. Terminals possessing WGA-HRP activity also formed non-synaptic junctions with other labeled or unlabeled terminals. SP and ChAT immunoreactivity in normal and colchicine-treated animals was confined to dendrites and somata within the medial habenula. Terminals containing clear spherical vesicles formed asymmetric synaptic contacts with these immunoreactive somatic and dendritic profiles. Based on the combined anterograde tracing and immunohistochemical data, it is proposed that septal projections provide a direct innervation to habenular neurons that contain ChAT or SP activity. These septal inputs may play an important role in the facilitation of the ChAT- and SP-positive habenular neurons, both of which provide prominent afferent inputs to the interpeduncular nucleus. Thus, neurons of the habenula and interpeduncular nucleus are under the direct and indirect influence of septal neurons within the limbic forebrain circuit.     

Distribution and targets of the relaxin-3 innervation of the septal area in the rat

Neural tracing studies have revealed that the rat medial and lateral septum are targeted by ascending projections from the nucleus incertus, a population of tegmental GABA neurons. These neurons express the relaxin-family peptide, relaxin-3, and pharmacological modulation of relaxin-3 receptors in medial septum alters hippocampal theta rhythm and spatial memory. In an effort to better understand the basis of these interactions, we have characterized the distribution of relaxin-3 fibers/terminals in relation to different septal neuron populations identified using established protein markers. Dense relaxin-3 fiber plexuses were observed in regions of medial septum containing hippocampal-projecting choline acetyltransferase (ChAT)-, neuronal nitric oxide synthase (nNOS)-, and parvalbumin (PV)-positive neurons. In lateral septum (LS), relaxin-3 fibers were concentrated in the ventrolateral nucleus of rostral LS and the ventral nucleus of caudal LS, with sparse labeling in the dorsolateral and medial nuclei of rostral LS, dorsal nucleus of caudal LS, and ventral portion nuclei. Relaxin-3 fibers were also observed in the septofimbrial and triangular septal nuclei. In the medial septum, we observed relaxin-3-immunoreactive contacts with ChAT-, PV-, and glutamate decarboxylase-67-positive neurons that projected to hippocampus, and contacts between relaxin-3 terminals and calbindin- and calretinin-positive neurons. Relaxin-3 colocalized with synaptophysin in nerve terminals in all septal areas, and ultrastructural analysis revealed these terminals were symmetrical and contacted spines, somata, dendritic shafts, and occasionally other axonal terminals. These data predict that this GABA/peptidergic projection modulates septohippocampal activity and hippocampal theta rhythm related to exploratory navigation, defensive and ingestive behaviors, and responses to neurogenic stressors.     

7 types of neurons in the substantia nigra of the rat. Golgi rapid-impregnation study

The substantia nigra of the adult rat was investigated by means of the Golgi rapid impregnation technique. Seven neuron types could be distinguished which differ with respect to morphological features (size and shape of pericaryon and dendritic pattern), behaviour of axons and prevalent localization within the nuclear area: 1. Polygonal neurons with somatic spines 2. Polygonal neurons without somatic spines 3. Fusiform neurons 4. Trigonal neurons 5. Small spherical neurons 6. Small spindle-shaped neurons 7. Neuroglioform neurons. The neuron types 1-4 are considered to represent nigral projection neurons; the types 5-7 might be interneurons. The findings presented here are discussed in view of recent physiological and biochemical as well as behavioral-pharmacological results.     

Nucleus of the accessory optic tract. Light and electron microscopic study in normal monkeys and after eye enucleation

The fine morphology and synaptic organization of the accessory optic tract nucleus was investigated in monkeys (Macaca mulatta) by light and electron microscopic methods. Nissl stains delineated the wedge-shaped nucleus between the brachium of the inferior colliculus and the medial lemniscus. Most of the neurons are of medium size with dark coarse Nissl bodies. Smaller and paler cells are also present. Golgi material revealed the two classic type I and II neurons, the former with early bifurcating dendrites exhibiting numerous spines. Electron microscopy showed mostly medium size neurons with relatively frequent occurrence of somatic spines. The dendritic profiles have a thick smooth initial protion followed by the appearance of increasing number of spines. The dendrites bifurcate close to the soma and finally end in brushlike fashion. Axon terminals make synaptic contacts with the soma, dendrites (spines and trunk) and dendritic endings. The latter articulations appear as “glomeruli”. At this level, profiles of ambiguous nature, possibly dendrites of Golgi type II cells, containing synaptic vesicles are frequently seen. Following eye enucleation, both the contralateral and ipsilateral nuclei showed terminal boutons in different stages of degeneration depending on the survival time. The optic fibers terminate on the soma as well as on the proximal spineless part of the dendrites and at the dendritic bifurcations. There are no optic terminals participating in the “glomeruli”. This nucleus receives direct retinal fibers and the system is both crossed and uncrossed. The presence of abundant axosomatic contacts of optic terminals as well as the absence of such endings in the glomeruli suggest a very different functional mechanism than that of the geniculate pathway. The nucleus receives many more afferents of unknown origin.     

Comparative characterization of the basic forebrain cortical zones in Emys orbicularis (Linnaeus) and Testudo horsfieldi (Gray)

The neuronal and synaptic organization of forebrain basic cortical zones in Testudo horsfieldi and Emys orbicularis and their dendritic spines have been studied using Nissle, Golgi and electron microscopic methods. It has been shown by comparison of the results that the two spicies have marked differences in the structural organization of the forebrain cortical zones. The cortical formation in Testudo horsfieldi is different from that of Emys orbicularis in a greater diversity of neuronal types, smaller size of neurons, smaller cell density in each cortical zones, the presence of horizontal dendritic terminals in dorsomedial dorsal cortex, the absence of the large neurons in dorsomedial medial cortex, ect. Moreover, in both species the dorsomedial dorsal cortex in comparison with medial and lateral cortex is characterized by a marked complexity of the structural organization (diverse neuronal composition, the presence of the stellate cells, the highest cell density, the smallest neuronal size). The investigation of spines in the different dendritic levels (proximal, middle and distal) of neurons in three cortical basic zones has been shown that in both species it has been observed the tendency to increasing of the spine density from proximal to distal part of dendrite. at all dendritic levels noninvaginated forms of spines predominated. Invaginated spines were recorded at the proximal and middle levels of dendrites and contain more organelles and inclusions than noninvaginated spines. In Testudo horsfieldi and Emys orbicularis differences of spine thin structure and spine density in the cortical basic zones were revealed. Moreover, in both species the spines of dorsomedial dorsal cortex were more numerous, more variable in shape, more abundant in organelles. It was there that a bush-like distribution of spines was found which in evidently a special form of synaptic organization of cortical neurons. The above features of this cortical zone indicates a higher degree of differentiation, suggesting that the dorsomedial dorsal cortex is phylogenetic youth as compared with hippocampus and lobus pyriformis.     

Organization of the septal region in the rat brain: a Golgi/EM study of lateral septal neurons

The combined Golgi/electron microscope (EM) technique was used to analyze the fine structure and synaptic organization of the various types of neurons in the rat lateral septum (LS), i.e., in the dorsolateral (LSd), intermediolateral (LSi), and ventrolateral (LSv) nuclei of the septal complex. Two characteristic cell types were observed in the LSd: type I with thick, short dendrites densely covered with short spines, and type II with longer and thinner dendrites exhibiting fewer but longer spines. This latter type was by far the most frequently impregnated cell type in the LSd and was also present in the LSi. Synaptic contacts on spines of either cell type were asymmetric; the majority of the presynaptic boutons contained clear round synaptic vesicles. Occasionally terminals were found that contained both clear and dense-core vesicles. Typical fusiform neurons with a low number of spines and rather long dendrites, sometimes invading other LS nuclei, were found in the LSi. The LSv contained numerous small neurons with small dendritic fields. A relatively large number of terminals with dense-core vesicles were found to establish synaptic contacts with identified LSv neurons. The morphological heterogeneity of LS neurons is discussed with regard to other studies on afferent and efferent fiber systems as well as immunohistochemical studies of this particular region of the septal complex.     

Structural development of the lateral geniculate nucleus and visual cortex in monkey and man

This study concerns the development of the primary visual pathway of the primate. The lateral geniculate nucleus (LGN) is the principal thalamic relay to the visual cortex (area 17), and its neurons have similar morphological characteristics in both monkey and man, as identified by Golgi impregnation. The commonest neuron is the multipolar with a radiate or tufted dendritic tree; next is the bipolar neuron with two or three diametrically opposed dendritic trunks. Less frequent are neurons with beaded dendrites and others with fine, axon-like dendritic processes, possibly interneurons. The dendritic tree of all neurons remains generally within a lamina, but some dendrites cross interlaminar zones. LGN neurons are identifiable before birth and differ from their adult form by the presence of immature features, especially numerous dendritic and somatic spines, most frequent at birth in monkeys and at about 4 months postnatally in man. They disappear almost completely by 3 months in monkeys and 9 months in man. The human LGN has reached its ‘adult’ volume by this age.Two stages in the development of the human area 17 can be defined. The first is marked by a rapid growth to its ‘adult’ volume by about 4 months, and by intense synaptogenesis beginning in the foetus and reaching a maximum around 8 months. The second stage is one of stabilization in the volume of area 17 and loss of synapses to reach ‘adult’ synaptic density around 11 years, at about 60% of the maximum values.The formation of transitory morphological features in the first weeks or months of life coincides with a period of visual plasticity in infant monkeys and humans. Our observations can be correlated with experimental evidence for visual development in monkeys and with clinical evaluation of visual activity during the human preverbal stage, a period of great importance in the establishment of visual acuity, of stereopsis and of oculomotor function, all very sensitive to the numerous forms of visual deprivation.     

Cholinergic synaptic input to different parts of spiny striatonigral neurons in the rat

The postsynaptic targets of cholinergic boutons in the rat neostriatum were assessed by examination in the electron microscope of boutons that were immunoreactive for choline acetyltransferase, the synthetic enzyme for acetylcholine. These boutons formed symmetrical synaptic specializations with neostriatal neurons. Of 209 immunoreactive synaptic boutons observed in random searches of the neostriatum, 45% made contact with dendritic shafts, 34% with dendritic spines, and 20% with neuronal perikarya. Many of the postsynaptic structures had ultrastructural characteristics of the most common type of striatal neuron, the medium-size densely spiny neuron. This was confirmed by the examination in the electron microscope of Golgi-impregnated medium-size spiny neurons from sections that had also been immunostained for choline acetyltransferase. Immunoreactive boutons formed symmetrical synaptic specializations with all parts of the neurons examined, i.e., perikarya, proximal and distal dendritic shafts, and dendritic spines. Two of the Golgi-impregnated medium-size spiny neurons that received input from the cholinergic boutons were also retrogradely labelled with horseradish peroxidase that had been injected into the substantia nigra, they were thus further characterized as striatonigral neurons. Similarly, seven retrogradely labelled perikarya of striatonigral neurons were found to receive input from the cholinergic boutons. It is concluded that cholinergic boutons in the neostriatum form synaptic specializations and that one of their major targets is the medium-size densely spiny neuron that projects to the substantia nigra. The topography of the cholinergic afferents of these cells is distinctly different from that of other boutons derived from local neurons and from boutons that form asymmetrical synaptic specializations, but it is similar to that of the dopaminergic boutons originating from neurons in the substantia nigra.     

The development of synaptic contacts in the cerebellum of Macaca mulatta

The maturation of various cerebellar cortical cells, the appearance of afferent fibers to the cerebellum, and the development of synaptic contacts in the cerebellar cortex and deep nuclei was investigated in the fetal macaque. Ultrastructural studies were done on cerebellum obtained from fetuses at 75, 100, 125 and 150 days after conception to interrelate the temporal development of these three systems. At 75 days, synaptic contacts were seen on somas and axons of neurons in the deep cerebellar nuclei, and climbing fibers formed pericellular baskets around Purkinje cells. By 100 days the climbing fibers synapsed with somatic spines of the Purkinje cells, and mossy fiber endings were present in the internal granule cell layer. Synaptic contacts were also seen on dendritic processes of neurons in the deep cerebellar nuclei at this time. In the 125 day cerebellum, Golgi cells were identified for the first time and climbing fibers and parallel fibers made synaptic contact with both Purkinje and Golgi cells. At 150 days parallel fibers made synaptic contact with superficial stellate cells and mature cerebellar glomeruli had appeared. At this stage, axosomatic contacts of climbing fibers on the soma of Purkinje cells had disappeared. The relationship of these anatomical observations to possible functional activity is discussed.     

Light microscopic analysis of Golgi-impregnated rat subthalamic neurons

The neuronal morphology of the rat subthalamic nucleus (STH) was studied using Golgi techniques and Nissl stain. The results show that the somatic shapes of STH neurons vary from fusiform to oval or polygonal. Somatic cross-sectional areas vary between 140 microns2 and 440 microns2. Some of the cells have a few somatic spines. Two to six primary dendrites gave rise to tapering daughter dendrites which extend up to 500 microns. These dendrites are sparsely covered with spines. Some distal dendrites and primary dendrites of the STH also bear filiform appendages. Neurons located in the deep portion of the STH have oval dendritic fields whose long axis is parallel to the long axis of the nucleus in frontal or sagittal planes. Some of these neurons have one or two dendrites which cross the borders of the STH into the zona incerta, the lateral hypothalamus, or the cerebral peduncle. Generally, neurons located at the borders of the STH have their dendritic fields extending parallel to the borders and are confined to the nucleus. However, some neurons adjacent to the ventrolateral border of the nucleus have some dendrites extending into the cerebral peduncle. Quantitative analysis of the STH neurons showed a unimodal distribution of somatic sizes as well as the number of primary dendrites. No neurons with obvious Golgi type II characteristics were found. Two types of afferent fibers were observed entering the STH. One type consists of axon collaterals arising from the cerebral peduncle ventrolaterally, or the internal capsule rostrally, while the other enters the nucleus after crossing the internal capsule rostrally. These results suggest that the rat STH is an open nucleus in contrast to other species such as man, monkey, and cat, where it is closed, and that the rat STH may contain only one type of neuron.     

The laminar distribution and ultrastructure of fibers projecting from three thalamic nuclei to the somatic sensory-motor cortex of the opossum

The projections of the ventrobasal complex (VB), the ventrolateral complex (VL), and the central intralaminar nucleus (CIN) to the somatic sensory-motor (SSM) cortex of the Virginia opossum were studied with light and electron microscopic autoradiographic methods. VB, VL, and CIN have overlapping projections to SSM cortex and each one also projects to an additional cortical area. Unit responses to somatic sensory stimulation and the areal and laminar distribution of axons in cortex is different for VB, VL, and CIN, but the axons from each form similar round asymmetrical synapses, predominantly with dendritic spines. As in other mammals, VB units in the opossum have discrete, contralateral cutaneous receptive fields. VB projects somatotopically to SSM cortex and also projects to the second somatic sensory representation. Within the cortex, VB axons terminate densely in layer IV and the adjacent part of layer III. A few axons also terminate in the outermost part of layer I and the upper part of layer VI. Most VB axon terminate upon dendritic spines (86.6%), but they also contact dendritic shafts (10%) and neuronal cell bodies (3%). Neurons in VL have no reliable response to somatic stimulation under our recording conditions. VL projects to the SSM cortex and to the posterior parietal area. Throughout this entire projection field VL fibers terminate in layers I, III, and IV most densely, and sparsely in the other cortical layers. The density of termination in the mid-cortical laminae is quite sparse compared to VB, but the projection to layer I is considerably greater. Nearly all (93%) of VL axons contact dendritic spines, the remainder (7%) end dendritic shafts. CIN is a thalamic target of ascending medial lemniscal, cerebellar, spinal, and reticular formation axons. Neurons in CIN respond to stimulation restricted to a particular body part, but typically responses may be evoked from larger areas and at longer latencies than neurons in VB that are related to the same body part. CIN neurons require a firm tap or electrical stimulation within their receptive field to elicit a response in the anesthetized preparation. CIN axons terminate throughout the entire parietal cortex, but unlike VB and VL, CIN fibers end almost exclusively in the outer part of layer I. Approximately 21% of CIN fibers contact dendritic shafts in layer I, which is twice the percentage of shafts contacted by VL or VB axons. All of the other CIN synapses are formed with dendritic spines. These experiments demonstrate three different pathways to SSM cortex. The results suggest that each projection has a unique role in controlling the patterns of activity of neurons within the SSM cortex.     

Light and electron microscopic characterization of dopamine-immunoreactive axons in human cerebral cortex.

The distribution and synaptic connections of dopamine axons were studied by light and electron microscopy in human cerebral cortex. For this purpose, dopamine immunoreactivity was characterized in apparently normal anteriolateral temporal cortex, which was removed to gain access to the medial temporal lobe during tumor excision or treatment of epilepsy. Nissl sections showed this to be granular neocortex. Dopamine fibers were distributed throughout this cortex, although there were relatively more fibers in layers I-II and in layers V-VIa, compared to layers III-IV and VIb, resulting in a bilaminar pattern of labeling. In all layers, fibers were seen to form numerous varicosities, and to vary in size from thick to very fine. Fibers were relatively straight, sparsely branched and were oriented in various planes within the cortex. However, in layer I, they often ran parallel to the pial surface. In order to analyze the functional interactions of dopamine fibers, individual cortical layers were surveyed for dopamine synapses. These were usually symmetrical (Gray's type II), although 13% of them were asymmetrical. Approximately 60% of dopamine synapses were made with dendritic spines, and 40% with dendritic shafts, and this ratio was similar in all layers. On both spines and shafts, it was common to see dopamine synapses closely apposed to an unlabeled asymmetric input, suggesting a dopamine modulation of excitatory input. Some postsynaptic dendritic shafts had features of pyramidal cells, including formation of spines. Since pyramidal cells are the major type of cortical spiny neuron, they probably represent the main target of dopamine synapses in this cortex. There were also dopamine profiles apposed to membrane densities on unlabeled axon terminals, suggesting another type of synaptic interaction. These findings provide the first documentation of dopamine synapses in the human cortex, and show that they form classical synaptic junctions. The location of these synapses on spines and distal dendrites, and their proximity to asymmetric synapses, suggest a modulatory role on excitatory input to pyramidal cells.     

The neurons in the centromedian-parafascicular complex of the monkey (Macaca mulatta): A Golgi Study

The neurons of the nucleus centrum medianum and the neurons of the nucleus parafascicularis were studied in Golgi preparations of the adult monkey(Macaca mulatta) The cell bodies of the principal neurons in the nucleus centrum medianum have a few somatic spines and vary in shape: some are cubical with protruding angles; some are egg-shaped;some are elongated and sausage-shaped. Four to six slightly branched dendrites of unequal thickness radiate from the cell body. Some dendrites extend for nearly 500 microns; all have dendritic spines. In the nucleus parafascicularis there are two varieties of principal neurons:(1) neurons with somatic spines and(2) neurons without somatic spines. The neurons with somatic spines are most numerous. They have polygonal-shaped cell bodies, prominent somatic spines and processes, larger than spines but considerably smaller than dendrites. These processes bear spines and are designated here “microdendrites.” Spines and occasionally a “microdendrite” are found on the axon-hillocks. Five to six dendrites of unequal thickness emerge from the cell bodies. Some extend for more than 500 microns; all have prominent dendritic spines. The neurons without somatic spines are relatively few. Usually three exceptionally long, slightly branched dendrites, one apical and two basal, emerge from their elongated, slim cell bodies. Some dendrites extend for more than 800 microns; all have a few scattered spines. The Golgi type II neurons found in both of these intralaminar nuclei have small cell bodies and a few, relatively long, undulating dendrites, which bear bulbous dendritic appendages and beaded axon-like processes. Distally on these dendrites, where the appendages and processes are more numerous, the dendritic appendages and axon-like processes form complex entanglements. Beaded axons are found on some but not all of the cell bodies. Morphologically these neurons resemble the local interneurons that have been described in various thalamic nuclei.     

The lateral reticular nucleus of the opossum (Didelphis virginiana). I. Conformation,cytology and synaptology

When viewed in Nissl preparations, the lateral reticular nucleus (LRN) of the opossum can be divided into three subgroups: a medial internal portion, a lateral external portion and a rostral trigeminal division. Neurons within the internal division measure 13-45 μ in their greatest dimension whereas those within the external and trigeminal portions measure 11-32 μ and 14-27 μ respectively. Golgi impregnations reveal that many neurons in all three subdivisions display a radial dendritic pattern although some of the nerve cells within the external division have dendrites which orient mainly in a ventromedial to dorsolateral direction. The cell bodies of LRN neurons are relatively spine-free. However, a small percentage of neurons exhibit clusters of sessile spines on proximal and more distal dendritic segments. No locally ramifying axons or axon collaterals were found within the LRN. Synaptic terminals within the LRN were divided into four categories: (1) small terminals measuring 2.5 μ or less containing agranular spherical vesicles; (2) small terminals (2.5 μ or less) with agranular pleomorphic synaptic vesicles, i.e., a mixture of spherical and elliptical synaptic vesicles; (3) small terminals (2.5 μ or less) containing agranular spherical or pleomorphic vesicles with a variable number (4-27) of dense core vesicles; and (4) large terminals (greater than 2.5 μ) which contain agranular spherical synaptic vesicles and a variable number of dense core vesicles (1-17). Dendritic diameters were measured from Golgi impregnations and correlated with cross-sectioned profiles in electron micrographs to help determine the post-synaptic distribution of synaptic endings. Small terminals containing agranular spherical or pleomorphic synaptic vesicles contact the soma and entire dendritic tree in each portion of the nucleus, whereas the small terminals containing dense core vesicles are usually located on distal dendrites or spines. Some large terminals make multiple synaptic contacts with a cluster of spines, others contact groups of small (distal) dendrites. In order to identify two of the major afferent systems to the LRN, 15 adult opossums were subjected to either a cervical spinal cord hemisection or a stereotaxic lesion of the red nucleus. Two days subsequent to spinal hemisection, large terminals in the caudal part of the ipsilateral LRN exhibit either an electron dense or filamentous reaction. Their postsynaptic loci are spines and shafts of proximal dendrites or a number of distal dendrites and spines. In addition, small terminals containing spherical agranular synaptic vesicles undergo an electron dense reaction in the same areas. Their postsynaptic loci are proximal or distal dendrites. Two days subsequent to rubral lesions, small terminals containing agranular spherical synaptic vesicles undergo a dark reaction in rostral portions of the contralateral nucleus. They contact intermediate or distal dendrites and occasionally spines.