Dendrites Of Projection Neurons Research Articles

Synaptic reorganization in the feline ventral anterior thalamic nucleus induced by lesions in the basal ganglia

This report presents the results of quantitative analysis of synaptic coverage of the dendrites of thalamocortical projection neurons in the adult cat ventral anterior thalamic nucleus (VA) and its changes at short (4 days)- and long-term (1 year) survival times after combined unilateral kainic acid lesions in the substantia nigra pars reticularis and entopeduncular nucleus. The study showed that deafferentation induced an increase in the appositional length of the dendrites of GABAergic local circuit neurons along projection neuron dendrites, accompanied by an increase in the number of dendrodendritic synapses. Dendritic or axonic origin of other unusual presynaptic structures at 1 year postlesion could not be established with certainty. All of them displayed some features of growth cones, such as an abundance of tubular and vesicular structures many with electron-dense contents. Projection neuron dendrites postsynaptic to these profiles contained numerous coated vesicles at the same survival time. The findings suggest that (i) the lesioning of the two basal ganglia output structures induces synaptic reorganization in the VA, (ii) this process appears to be active at 1 year after deafferentation, and (iii) all reactive systems display symmetric contacts and positive GAD immunoreactivity suggesting that homotypic remodeling is taking place. Despite the signs of remodeling, the density of presynaptic terminals with symmetric contacts on primary and secondary projection neuron dendrites in the VA remained below the control level at 1 year postlesion suggesting that an imbalance of excitatory and inhibitory inputs on thalamocortical projection neurons may persist for long periods after deafferentation.

Relationships of the Dendritic Arborizations of Ventral Striatomesencephalic Projection Neurons With Boundaries of Striatal Compartments. An In Vitro Intracellular Labelling Study in the Rat.

We studied the relationships of the dendrites of ventral striatomesencephalic projection neurons with the compartmental structure of the ventral striatum, as revealed by enkephalin immunohistochemistry. Lightly fixed slices were employed in which Lucifer yellow was intracellularly injected into neurons that were retrogradely labelled following Fast Blue injections in the ventral tegmental area. Double immunohistochemical staining was carried out using antisera to Lucifer yellow and Leu-enkephalin. Most of the 226 injected cells were located in the core region of the nucleus accumbens. All these neurons were of the small- to medium-sized spiny type. The dendritic arborizations of over 90% of the cells remained within the compartment in which the parent cell bodies resided. The dendrites of most of these neurons abutted the border of the compartment, whereas a smaller number of neurons had dendrites that were distant from any compartmental boundary. The dendrites of fewer than 10% of the neurons crossed the borders of compartments. Only a few cells were injected in the shell region of the nucleus accumbens. None of these neurons extended its dendrites into the core region of the nucleus or into the territory of the clusters of small cells which characterize the shell. The present results demonstrate that the dendrites of the great majority of ventral striatomesencephalic neurons comply with the boundaries of ventral striatal enkephalin compartments. Together with the results of previous studies showing that such compartments are selectively innervated by thalamic and cortical afferents, and have outputs to different areas in the ventral mesencephalon, the present data suggest the existence of discrete channels through the ventral striatum.

Fine structural organization of the ependymal region of the paraventricular nucleus of the rat thalamus and its relation with projection neurons.

The ependymal lining of the diencephalic third ventricle is known to exhibit significant variation in zonal architecture and the relations of neurites to the ventricular surface in different regions remains obscure. The present study explores the fine structural organization of the ependymal region of the thalamic paraventricular nucleus. Methodology was developed for tracing neurites of cells retrogradely labelled with horseradish peroxidase based on our recent observation that paraventricular neurons projecting to the amygdaloid complex cluster near the ventricle and emit numerous dendrites extending toward the ependymal surface. A relatively uniform population of cuboidal 'pale' ependymocytes dominates the ventricular lining of the thalamic paraventricular nucleus, although a few 'dark' ependymocytes are interspersed. The subependymal region displays a variety of glial elements. Dendrites of thalamic paraventricular projection neurons terminate in proximity to the ependymal layer from which they are generally separated by thin cytoplasmic processes of putative astrocytes, and few indent the basal portion of ependymal cells. Thin 'terminal' (i.e., serially traced) horseradish peroxidase-labelled dendrites filled with lipid and lysosome-like dense bodies were often enveloped by astrocyte membrane whorls. This feature may constitute a reactive glial response in horseradish peroxidase-labelled dendritic terminals. A distinctive arrangement of tortuous astrocyte leaflets was insinuated between the basal portion of ependymocytes in a zone exhibiting numerous caveolae, apparently isolating neurites from direct contact with the cerebrospinal fluid. These findings indicate that the ependymal region of the thalamic paraventricular nucleus is not characterized by those features of the basal third ventricle suspected to confer neuroendocrine interaction between neurons and cerebrospinal fluid. The structural arrangement between ependymocytes and thalamic paraventricular projection cells indicate a specialized relation of these neurons with the ependymal interface, but apparently not directly with the overlying cerebrospinal fluid.

Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (V Amc) of Macaca mulatta: I. Cell types and synaptology

The nucleus ventralis anterior pars magnocellularis (VAmc) is recognized only in primates and is the major recipient of the nigrothalamic projections. The neuronal and synaptic composition of this nucleus in the rhesus monkey was studied with the use of a variety of neuroanatomical techniques that included quantitative morphometry, anterograde and retrograde labeling with WGA-HRP from the prefrontal cortex, and immunocytochemistry for glutamic acid decarboxylase (GAD). Two major cell types were identified in the nucleus: thalamocortical projection neurons (PN) that were multipolar cells of various sizes, and small GAD-immunoreactive cells, apparently local circuit neurons (LCN). The approximate ratio of the two types of cells was 10:1. The major type of bouton encountered in the neuropil was of medium to large-sized (areas from 1.5 to 12 microns 2) and mostly of en passant type. These terminals formed symmetric contacts, contained moderate amounts of pleomorphic or mostly flat synaptic vesicles and large numbers of mitochondria, and displayed numerous puncta adhaerentia. All of these boutons exhibited positive GAD immunoreactivity. These boutons constituted the only synaptic population on somata and primary dendrites of PN and formed an overwhelming majority on the secondary PN dendrites. There were fewer of these axon terminals on tertiary PN and LCN dendrites. Additionally, boutons with similar features formed synapses on axon hillocks or initial axonal segments of PN, and somata or very proximal parts of primary dendrites of LCN. With the exception of the boutons in the last two locations, all of the other boutons in this group were shown to be terminals of the nigrothalamic afferents in the parallel EM autoradiographic study (Kultas-Ilinsky and Ilinsky: J. Comp Neurol. 294:479-489, '90). The second major bouton population in the VAmc was represented by small to medium-sized terminals (areas range from 0.2 to 2.0 microns 2) that formed distinct asymmetric contacts and contained large numbers of round vesicles and few or no mitochondria. These boutons were labeled anterogradely from the cortex and dominated on distal PN and LCN dendrites. Some of them were found on secondary PN dendrites where they formed synapses either directly or indirectly via LCN dendrites and dendro-dendritic contacts. The latter arrangements, i.e., serial synapses, were also formed between the cortical boutons and PN somata or tertiary dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (V Amc) of Macaca mulatta: II. Organization of nigrothalamic afferents as revealed with EM autoradiography

An EM-autoradiographic technique was used to identify the ultrastructural features and synaptic sites of nigral afferents to the ventral anterior nucleus pars magnocellularis (VAmc) of the rhesus monkey thalamus. The findings demonstrate that the nigral boutons are of medium-sized to large, with the majority being of the en passant type. These boutons form symmetric synaptic contacts, and contain pleomorphic or entirely flat vesicles and numerous mitochondria. The nigral input is heavily biased towards thalamocortical projection neurons (PN), whose somata and dendrites represent about 82% of the postsynaptic sites of labeled boutons. The distal dendrites of local circuit neurons (LCN) comprise 13% of the postsynaptic sites. Nigral terminals appear to represent a single extrinsic afferent input to the somata and primary dendrites of thalamocortical projection neurons. A nigral input to LCN somata was not demonstrated but the possibility could not be excluded. Although the basic ultrastructural features of nigral boutons in the monkey are similar to those described earlier in the cat (Kultas-Ilinsky et al.: J. Comp. Neurol. 216:390-405, '83), essential species differences exist in the intensity of the nigral input and its distribution on thalamic neurons.

Substance P-immunoreactive retinal ganglion cells and their central axon terminals in the rabbit

Retinal ganglion cells are the projection neurons that link the retina to the brain. Peptide immunoreactive cells in the ganglion cell layer (GCL) of the mammalian retina have been noted but their identity has not been determined. We now report that, in the rabbit, 25-35% of all retinal ganglion cells contain substance P-like (SP) immunoreactivity. They were identified by either retrograde transport of fluorescent tracers injected into the superior colliculus, or by retrograde degeneration after optic nerve section. SP immunoreactive cells are present in all parts of the retina and have medium to large cell bodies with dendrites that ramify extensively in the proximal inner plexiform layer. Their axons terminate in the dorsal lateral geniculate nucleus, superior colliculus and accessory optic nuclei, and these terminals disappear completely after contralateral optic nerve section and/or eye enucleation. In the dorsal lateral geniculate nucleus large, beaded, immunoreactive axons and varicosities make up a narrow plexus just below the optic tract, where they define a new geniculate lamina. The varicosities make multiple synaptic contacts with dendrites of dorsal lateral geniculate nucleus projection neurons and presumptive interneurons in complex glomerular neuropil. This is direct evidence that some mammalian retinal ganglion cells contain substance P-like peptides and strongly suggests that, in the rabbit, substance P (or related tachykinins) may be a transmitter or modulator in a specific population or populations of retinal ganglion cells.

Morphology and physiology of single neurons in the medial interlaminar nucleus of the cat's lateral geniculate nucleus

Physiological studies have shown that the cat's retinogeniculocortical system is comprised of at least three parallel and independent pathways, the W-, X-, and Y-cell pathways. The morphological correlates of the constituent W-, X-, and Y-cells have been determined both in the retina and in the A and C laminae of the lateral geniculate nucleus. The aim of this study was to extend these structure/function relationships to neurons in laminae 1 and 2 of the medial interlaminar nucleus (MIN), which is a division of the cat's dorsal lateral geniculate nucleus. We used intracellular injection of horseradish peroxidase (HRP) into individual, physiologically identified MIN neurons. Since this procedure may yield an unrepresentative sample of MIN neurons, two controls were performed. First Nissl staining showed that the soma sizes of intracellularly labeled cells were representative of those of all MIN cells. Second, retrograde labeling following HRP injections into the optic radiations or specific visual cortical areas showed that the intracellularly labeled MIN cells were representative of MIN relay neurons. Many of the retrogradely labeled cells were so well filled that their entire dendritic arbors were revealed. Of 70 MIN neurons recorded physiologically, 22 were injected with HRP and successfully recovered. We also completely labeled the somata and dendrites of 114 MIN neurons from HRP injections into the optic radiations and retrogradely labeled 165 MIN neurons by injection of HRP into visual cortical areas. Our sample of intracellularly injected neurons, which were all Y-cells, were morphologically representative of all MIN relay cells. We thus conclude that laminae 1 and 2 of the MIN contain a nearly homogeneous population of Y-cells with properties essentially identical to those of Y-cells in the A and C laminae of the lateral geniculate nucleus. When viewed in the coronal plane, MIN projection neurons typically exhibited oval or elongated somata. In the medial and ventral parts of the MIN, these somata were smaller and more flattened. MIN soma sizes extended over the full range of those seen in the A laminae. Dendritic arbors of most MIN relay neurons radiated in a fairly spherical fashion. In the medial and ventral parts of the MIN, however, dendrites were oriented in a more bipolar fashion, but intermediate forms between spherical and bipolar arbors were also common. Dendrites of MIN projection neurons were typically smooth; most primary dendrites were straight, but secondary dendrites were more variable in structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Projection of olfactory bulb efferents to layer I GABAergic neurons in the entorhinal area. Combination of anterograde degeneration and immunoelectron microscopy in rat

Glutamic acid decarboxylase (GAD) immunoelectron microscopy in combination with anterograde degeneration was applied in rats to study the synaptic targets of olfactory bulb afferents to the lateral subdivision (LEA) of the entorhinal area (EA). Immunoreactive neurons and terminals are scattered throughout all layers of LEA. After olfactory bulb resection, terminal degeneration occurs in layer Ia of EA. Using the electron microscope we examined serial thin sections of 12 and 14 immunoreactive neurons sampled from layer Ia of the dorsal (DLEA) and ventral (VLEA) subdivisions of LEA, respectively. The morphology of all these neurons is similar: they are small (short axis 5-9 micron, long axis 7-12 micron) and possess eccentrically located, indented nuclei provided with filamentous nuclear rodlets. The immunoreactive neurons have thin, smooth dendrites which usually emerge abruptly from the somata. We observed a single cilium on 5 of the immunoreactive neurons. In layer Ia of both DLEA and VLEA, the somata of the immunoreactive neurons are contacted by degenerating, non-immunoreactive boutons showing asymmetric synaptic junctions. In addition to these boutons, 4 other categories of axo-somatic terminals can be distinguished: normal, non-immunoreactive boutons forming asymmetric synapses and containing spherical synaptic vesicles; normal, non-immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles; normal, non-immunoreactive boutons with asymmetric synapses, containing dense-cored vesicles in addition to spherical synaptic vesicles; and normal, immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles. It is suggested that the GAD-immunoreactive neurons which receive olfactory bulb input correspond to local circuit neurons with intralaminar axons which innervate each other as well as the distal segments of the apical dendrites of projection neurons with cell bodies in layers II and III. Thus, the olfactory input in EA seems to be wired not only for excitation of layers II and III pyramidal neurons but also for feed-forward inhibition using GABAergic intermediary neurons, strategically located in the area of termination of olfactory bulb fibers.

A description of the GABAergic neurons and axon terminals in the motor nuclei of the cat thalamus

The GABA neurons and their processes in the cat motor thalamic nuclei were identified and studied with glutamic acid decarboxylase (GAD) immunocytochemistry at both the light and electron microscopic levels. The three nuclei that comprise the motor thalamus, ventral anterior (VA), ventral medial (VM), and ventral lateral (VL), each displayed a characteristic distribution pattern of GAD-positive structures that was consistent with their afferent and intrinsic neuronal organization. All three thalamic nuclei displayed a population of small, GAD-positive cells the dendrites of which contained synaptic vesicles and participated in complex synaptic arrays such as serial synapses, triads, and glomeruli. Based on their ultrastructural features, these GAD-containing cells were identified as local circuit neurons. In contrast, the larger, GAD-negative cells were presumed to be the thalamocortical projection neurons. The axons of GAD-positive local circuit neurons could not be identified in these preparations. The number of GAD-positive dendrites in the neuropil was different for the three thalamic nuclei. In the VA and VM, the GAD-positive dendrites were numerous and formed symmetric synapses with dendrites of GAD-negative cells, mainly in association with corticothalamic boutons. Within VL, the GAD-containing dendrites were more numerous than in VA and VM and formed synapses at influential locations on presumed thalamocortical projection neurons, such as bases of primary dendrites, and bifurcation sites of primary and secondary dendrites. The VA and anterolateral VM nuclei that receive inhibitory GABAergic afferents from the entopeduncular nucleus and substantia nigra contained the highest concentration of large GAD-positive axon terminals. These boutons contained pleomorphic vesicles and numerous mitochondria and formed symmetric synapses and multiple puncta adherentes with dendrites and somata of presumed thalamocortical projection neurons. The size, ultrastructural features, and distribution of these GAD-positive boutons were similar to those features described for basal ganglia terminals in the motor thalamus of the cat. In addition, similar large-size GAD-positive boutons were observed in the medial VM, which receives basal ganglia afferents exclusively from the substantia nigra. The concentration of these terminals in medial VM along the dendrites of thalamocortical projection neurons was much less than that in VA and anterolateral VM. The VL nucleus which lacks basal ganglia input did not contain any large GAD-positive boutons.(ABSTRACT TRUNCATED AT 400 WORDS)

An electron microscope study of the cortico-pretecto-olivary projection in the cat by a combined degeneration and horseradish peroxidase tracing technique

The anterior pretectal nucleus (PTA) of the cat was observed electron microscopically after motor cortical ablation and horseradish peroxidase (HRP) injection into the inferior olive of the same animal. Direct synaptic connections were found between degenerated cortico-pretectal axon terminals and dendrites of pretecto-olivary projection neurons retrogradely labeled with HRP in the ventrolateral part of the PTA. Therefore, this combined method revealed that the PTA is a relay station of the cortico-olivary projection.

A quantitative study of the thalamocortical and other synapses in layer IV of pyramidal cells projecting from mouse SmI cortex to the caudate-putamen nucleus.

The thalamocortical and other synapses of the apical dendrites of corticostriatal projection neurons in mouse primary somatosensory cortex (SmI) were examined by combining anterograde degeneration with the retrograde transport of horseradish peroxidase (HRP). Electrolytic lesions were made in the ventrobasal thalamus, followed 3 days later by injections of 40% HRP into the ipsilateral caudate-putamen nucleus. The next day, the mice were perfused and the SmI cortex ipsilateral to the lesion and injection sites was chopped at 125-micrometer and reacted for HRP using a CoCl2-DAB method. HRP-labeled corticostriatal cells in SmI cortex were medium-sized pyramidal cells, having somata located in the superficial portion of layer V and apical dendrites extending into layer I. Seven corticostriatal cells were serially thin sectioned and the layer IV portions of their apical dendrites were reconstructed. Each apical dendrite formed only one or two thalamocortical synapses (0.3 to 0.9% of their synapses in layer IV) indicating that corticostriatal neurons may be minimally responsive to direct synaptic input from the specific thalamic nuclei. Each apical dendrite formed about 12.6 asymmetrical synapses for every symmetrical synapse, suggesting that the relative numbers of excitatory and inhibitory synapses impinging on apical dendrites belonging to an individual class of neurons may be specified.

Opiates and pain pathways: demonstration of enkephalin synapses on dorsal horn projection neurons : Science, 215 (1982) 1523–1525

The participation of the opiate peptide enkephalin in the neural circuitry of the dorsal horn was examined at the light and ultrastructural level through the use of the combined techniques of immunocytochemistry and retrograde transport of horseradish peroxidase. Enkephalin immunoreactive axonal endings made direct synaptic contact with the soma and proximal dendrites of dorsal horn thalamic projection neurons. This observation demonstrates that one major synaptic site of enkephalin modulation of the transfer of nociceptive information in the dorsla horn is on the projection neurons themselves.U

Opiates and pain pathways: demonstration of enkephalin synapses on dorsal horn projection neurons.

The participation of the opiate peptide enkephalin in the neural circuitry of the dorsal horn was examined at the light and ultrastructural level through the use of the combined techniques of immunocytochemistry and retrograde transport of horseradish peroxidase. Enkephalin immunoreactive axonal endings made direct synaptic contact with the soma and proximal dendrites of dorsal horn thalamic projection neurons. This observation demonstrates that one major synaptic site of enkephalin modulation of the transfer of nociceptive information in the dorsla horn is on the projection neurons themselves.U

A Golgi study on the neuronal organization of the interhemispheric cortex in the mouse. I. Projection neurons.

Projection neurons in the interhemispheric cortex (IHC) of the mouse were studied by the rapid Golgi method. Five layers were discerned in the IHC. Projection neurons in layer I had stellate or piriform cell bodies with dendrites which were distributed in layers I and II. The cell bodies of projection neurons in layer II were fusiform, piriform, triangular or stellate in shape. Fine axons of these neurons sent collaterals mainly to layer IV. Projection neurons in layer III were medium-sized pyramidal, and small spindle cells. Basal dendrites of the former neurons were distributed mainly in layer III, while those of the latter neurons extended into layer IV. Projection neurons in layer IV were largely pyramidal, medium-sized pyramidal, medium-sized fusiform, and small cells. In the large pyramidal cells, the basal dendrites were distributed mainly in layer IV, and the apical dendrites extended into layer I. The axons of these neurons sent collaterals to all cortical layers. In layer V, spindle and small stellate projection neurons were observed. All apical dendrites of projection neurons in layers I-III extended into layer I, whereas some apical dendrites of projection neurons in layers IV and V did not reach layer I.

Ultrastructure of degenerating cerebellothalamic terminals in the ventral medial nucleus of the cat.

Terminal degeneration of cerebellar afferents in the ventral medial thalamic nucleus (VM) was studied in cats at the ultrastructural level after uni- or bilateral lesions in the brachium conjunctivum (BC). To achieve discrete lesions within the BC, a new very accurate stereotaxic technique was used. Numerous large terminals belonging to a population of so-called LR boutons were observed degenerating in the VM. The boutons displayed a wide variety of degenerative changes. Some revealed the features of the classical neurofilamentous type of degeneration. Others, although containing a slightly increased number of neurofilaments, featured much more prominently large numbers of coated vesicle shells and heavy accumulations of a flocculent electron-dense material. Degeneration in a third group of boutons similar to some extent to the light type of degeneration was characterized by tight clumping of enormously swollen or distorted synaptic vesicles within a light matrix. At later stages, however, all these boutons were believed to become shrunken and electron-dense since intermediate stages between the light- and dark-appearing boutons were observed. The degenerating cerebellar boutons formed asymmetrical synaptic contacts. Groups of 3 or 4 boutons terminated upon dendrites of projection neurons synapsing more frequently on spines than on dendritic stems. The synaptic contacts between cerebellar boutons and the vesicle-containing dendrites of local circuit neurons were encountered as often if not more than the contacts on projection neuron dendrites. Triads consisting of cerebellar boutons and dendrites of both types of neurons were observed very regularly. This synaptic arrangement provides the anatomical basis for the modification of cerebellar input in the VM by interneurons.