Dendrites Of Projection Neurons Research Articles

Mu-opioid receptors in the ventral tegmental area are targeted to presynaptically and directly modulate mesocortical projection neurons.

Mesocorticolimbic projections originating from dopaminergic and GABAergic neurons in the ventral tegmental area (VTA) play a critical role in opiate addiction. Activation of mu-opioid receptors (MOR), which are located mainly within inhibitory neurons in the VTA, results in enhanced dopaminergic transmission in target regions, including the medial prefrontal cortex (mPFC). We combined retrograde tract-tracing and electron microscopic immunocytochemistry to determine if neurons in the VTA that project to the mPFC contain MOR or receive input from MOR-containing terminals. Rats received unilateral injections of the retrograde tracer Fluoro-Gold (FG) into the mPFC. Tissue sections throughout the VTA were then processed for electron microscopic examination of FG and MOR. Immunoperoxidase labeling for FG was present in VTA cell bodies that contained immunogold-silver particles for MOR that often were contacted by profiles exclusively immunoreactive for MOR, including somata and axon terminals. The majority of dually labeled profiles were dendrites that received convergent input from unlabeled axon terminals forming either symmetric or asymmetric type synapses. Within retrogradely labeled cell bodies and proximal dendrites, MOR immunoreactivity was mainly sequestered within the cytoplasm. In contrast, distal retrogradely labeled dendrites contained MOR gold particles located along the plasma membranes. These data suggest that opiates active at MOR in the VTA modulate cortical activity through 1) presynaptic actions on MOR in terminals contacting mesocortical cell bodies, and 2) direct activation of MOR in distal dendrites of projection neurons.

Distribution of the major gamma-aminobutyric acid(A) receptor subunits in the basal ganglia and associated limbic brain areas of the adult rat.

Within the basal ganglia, gamma-aminobutyric acid (GABA) exerts a fundamental role as neurotransmitter of local circuit and projection neurons. Its fast hyperpolarizing action is mediated through GABA(A) receptors. These ligand-gated chloride channels are assembled from five subunits, which derive from multiple genes. Using immunocytochemistry, we investigated the distribution of 12 major GABA(A) receptor subunits (alpha1-5, beta1-3, gamma1-3, and delta) in the basal ganglia and associated limbic brain areas of the rat. Immunoreactivity for an additional subunit (subunit alpha6) was not observed. The striatum, the nucleus accumbens, and the olfactory tubercle displayed strong, diffuse staining for the subunits alpha2, alpha4, beta3, and delta presumably located on dendrites of the principal medium spiny neurons. Subunit alpha1-, beta2-, and gamma2-immunoreactivities were apparently mostly restricted to interneurons of these areas. In contrast, the globus pallidus, the entopeduncular nucleus, the ventral pallidum, the subthalamic nucleus, and the substantia nigra pars reticulata revealed dense networks of presumable dendrites of resident projection neurons, which were darkly labeled for subunit alpha1-, beta2-, and gamma2-immunoreactivities. The globus pallidus, ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, all areas receiving innervations from the striatum, displayed strong subunit gamma1-immunoreactivity compared to other brain areas. In the substantia nigra pars compacta and in the ventral tegmental area, numerous presumptive dopaminergic neurons were labeled for subunits alpha3, gamma3, and/or delta. This highly heterogeneous distribution of individual GABA(A) receptor subunits suggests the existence of differently assembled, and presumably also functionally different, GABA(A) receptors within individual nuclei of the basal ganglia and associated limbic brain areas.

Vesicular acetylcholine transporter in the rat nucleus accumbens shell: subcellular distribution and association with mu-opioid receptors.

Cholinergic interneurons in the nucleus accumbens shell (AcbSh) are implicated in the reinforcing behaviors that develop in response to opiates active at mu-opioid receptors (MOR). We examined the electron microscopic immunocytochemical localization of the vesicular acetylcholine transporter (VAChT) and MOR to determine the functional sites for storage and release of acetylcholine (ACh), and potential interactions involving MOR in this region of rat brain. VAChT was primarily localized to membranes of small synaptic vesicles in axon terminals. Less than 10% of the VAChT-labeled terminals were MOR-immunoreactive. In contrast, 35% of the cholinergic terminals formed symmetric or punctate synapses with dendrites showing an extrasynaptic plasmalemmal distribution of MOR. Membranes of tubulovesicles in other selective dendrites were also VAChT-labeled, and almost half of these dendrites displayed plasmalemmal MOR immunoreactivity. The VAChT-labeled dendritic tubulovesicles often apposed unlabeled axon terminals that formed symmetric synapses. Our results indicate that in the AcbSh MOR agonists can modulate the release of ACh from vesicular storage sites in axon terminals as well as in dendrites where the released ACh may serve an autoregulatory function involving inhibitory afferents. These results also suggest, however, that many of the dendrites of spiny projection neurons in the AcbSh are dually influenced by ACh and opiates active at MOR, thus providing a cellular substrate for ACh in the reinforcement of opiates.

Distribution of terminals from pedunculopontine tegmental nucleus and synaptic organization in lateralis medialis-suprageniculate nucleus of cat's thalamus: anterograde tracing, immunohistochemical studies, and quantitative analysis.

The cat's lateralis medialis-suprageniculate nuclear complex (LM-Sg) in the thalamus receives input from various brain regions such as the superior colliculus, brain stem, and spinal cord, as well as from visual association cortex. In a previous study, we demonstrated that LM-Sg receives cholinergic fibers from the pedunculopontine tegmental nucleus (PPT) and that cholinergic terminals make synaptic contacts with the dendrites of glutamatergic projection neurons and of GABAergic interneurons (Hoshino et al., 1997). In this study, we investigate the distribution and the organization of PPT terminals by means of a combined anterograde tracer (biotinylated dextran amine, BDA) and immunohistochemical methods. When stained by acetylcholinesterase (AChE), the LM-Sg is not uniformly immunoreactive, but rather is patchily labeled and shows a streaming type of reactivity. The tissue content appears high in enzyme activity in AChE-positive zones and is much lighter in activity in AChE-negative zones. We compared the synaptic organization between AChE-positive and AChE-negative portions of the LM-Sg in separate groups of electron-microscopic material: four types of vesicle containing profiles (RS, RL, F1, and PSD) as well as synaptic glomeruli were observed in this nucleus. Among these, the PSD profiles were observed more frequently in AChE-positive portions than in AChE-negative zones. Furthermore, the number of glomeruli was significantly higher in AChE-positive than in AChE-negative zones. Following the injection of BDA into PPT, labeled terminals within LM-Sg were rather more concentrated in the AChE-positive portion. Although the majority of PPT terminals made synaptic contacts with dendrites in the neuropil, a few terminals were involved in the synaptic glomeruli. The present results show that the synaptic organization is distinctly different between the AChE-positive and AChE-negative portions of LM-Sg. These results suggest that the AChE-positive portions of LM-Sg are relatively more involved in integrating information arising from a diverse set of inputs and processing that information within glomeruli in a complex manner than occurs in the AChE-negative portion of LM-Sg.

Dendritic organization of neurons of the superior colliculus in animals with different visual capability

The aim of the study was to compare several morphological characteristics of neurons in the superficial layers of the superior colliculus in diurnal and nocturnal mammals with different visual specialization. Thus, we investigated the rat ( Rattus norvegicus), a nocturnal animal; the tree shrew ( Tupaia glis), a diurnal animal, and the Mongolian rodents, Microtus brandti (nocturnal) and Alticola barakshin (diurnal). The investigation was focused on the study of the organization and extent of dendrites of Golgi-impregnated projection neurons, which were divided in two classes: narrow-field and wide-field cells. We determined that the ratios between the volumes of dendritic fields of the investigated neuronal types and the total volume of the superior colliculus differed to a great extent between the different species. The tree shrew had the largest superior colliculus and the smallest wide-field neurons, while the rat had the largest wide-field neurons. As for the Mongolian rodents, we provided the first description of superior colliculus neurons. The day-active animal Alticola barakshin was found to have a 50% larger volume of the superior colliculus than that of the night-active animal Microtus brandti, and the size of the dendritic field of both wide-field neurons and narrow-field neurons was smaller than that of Microtus brandti. Electron microscopic investigation of wide-field neurons performed in the rat revealed only a few symmetric synaptic contacts on the arborizations of distal and terminal dendrites and numerous asymmetric synapses on the dendritic stem. Our findings support the hypothesis that whereas the narrow-field neurons are relay neurons in the retino-tecto-thalamic pathway of the visual system, the wide-field neurons may play additional roles in the retino-tecto-reticulo-spinal system.

Physiological mismatching between neurons innervating olfactory glomeruli in a moth

The primary olfactory centres of most vertebrates and most neopteran insects are characterized by the presence of spherical neuropils, glomeruli, where synaptic interactions between olfactory receptor neurons and second–order neurons take place. In the neopteran insect taxa investigated so far, receptor neurons of a specific physiological identity target one glomerulus and thus bestow a functional identity on the glomerulus. In moths, input from pheromone–specific receptor neurons is received in a male–specific structure of the antennal lobe, called the macroglomerular complex (MGC), which consists of a number of specialized glomeruli. Each glomerulus of the complex receives a set of peripheral sensory afferents that encode one of several compounds involved in sexual communication. The complex is also innervated by dendritic branches of antennal lobe output neurons called projection neurons, which transfer information from the antennal lobe to higher centres of the brain. A hypothesis stemming from earlier work on moths claims that the receptor neuron innervation pattern of the MGC should be reflected in the pattern of dendrites of projection neurons invading the different MGC glomeruli. In this study we show that in the noctuid moth Trichoplusia ni , as in several other noctuid moth species, this hypothesis does not hold. The degree of matching between axon terminals of receptor neurons and the dendritic branches of identified projection neurons that express similar physiological specificity is very low.

Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

We have carried out an ultrastructural study to determine the characteristics and distribution of glutamate-containing constituents of the anterodorsal (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a polyclonal antibody to glutamate and a postembedding immunogold detection method in animals in which the neurons of AD/AV projecting to the cortex had been retrogradely labelled and the terminals of corticothalamic afferents anterogradely labelled by injection of cholera toxin-horseradish peroxidase (HRP) into the retrosplenial granular cortex. The heaviest immunogold labelling was over axon terminals 0.42 to 2.2 μm in diameter containing round synaptic vesicles and establishing Gray type 1 (asymmetric) synaptic contact (type 1 terminals) on HRP-labelled or non-labelled dendrites. Mean gold particle densities over such terminals were 3–4 times higher than the densities over the dendrites to which they were presynaptic and 5–6 times higher than over terminals establishing Gray type 2 (symmetric) synaptic contacts (type 2 terminals). Gold particle densities over neuronal cell bodies and dendrites and over a subpopulation of myelinated axons were intermediate between the densities over type 1 and type 2 terminals. In adjacent serial sections immunoreacted for gamma aminobutyric acid, type 2 terminals were heavily immunolabelled whereas type 1 terminals and other profiles with moderate gold particle densities after glutamate immunoreaction displayed very low labelling. A subpopulation of small type 1 axon terminals (up to 1 μm diameter) contained HRP reaction product identifying them as cortical in origin; they contacted small dendritic profiles (most <1 μm diameter) many of which also contained HRP reaction product. We conclude that terminals of the corticothalamic projection from retrosplenial granular cortex to AD/AV are glutamatergic and innervate predominantly distal dendrites of thalamocortical projection neurons. All rights reserved

Ultrastructural study of nitric oxide synthase-containing striatal neurons and their relationship with parvalbumin-containing neurons in rats

Single- and double-label electron microscopic immunocytochemistry was used to examine the ultrastructure of striatal neurons containing nitric oxide synthase (NOS +) and evaluate the synaptic relationship of NOS + striatal neurons with those containing parvalbumin (PV +). In both the single-label and double-label studies, NOS + perikarya were observed to possess polylobulated nuclei. In the single-label studies, NOS + terminals were seen forming synaptic contacts with dendritic shafts and dendritic spines that did not contain NOS, but not with NOS + perikarya or dendrites. In the double-label studies (using diaminobenzidine and silver intensified immunogold as markers), nitric oxide synthase and parvalbumin immunoreactions were found in two different populations of medium-sized aspiny striatal neurons. The PV + axon terminals were seen forming symmetric synapses on the dendritic spines of neurons devoid of PV or NOS labeling, on PV + dendrites, and on NOS + soma and dendrites. In contrast, NOS + terminals were not observed to form synaptic contacts with the dendrites or soma of either PV + or NOS + neurons. These findings suggest that NOS + striatal interneurons form synaptic contact with the spines and presumably the dendrites of striatal projection neurons, but not with the dendrites or soma of PV + or NOS + striatal interneurons. NOS + neurons do, however, receive synaptic input from PV + neurons.

Mode of termination of pallidal afferents to the thalamus: a light and electron microscopic study with anterograde tracers and immunocytochemistry in Macaca mulatta.

The mode of termination of individual pallidothalamic fibers in the densicellular subdivision of the ventral anterior thalamic nucleus (VAdc) of Macaca mulatta was analyzed with light and electron microscopy after injections of anterograde tracers in the medial globus pallidus. Three tracers were utilized: tritiated leucine, biotinylated dextran amine, and wheat germ agglutinin conjugated to horseradish peroxidase in combination with postembedding immunocytochemsitry for gamma-aminobutyric acid (GABA). Pallidothalamic fibers, upon entering the VAdc, gave off several collaterals that formed plexuses of varicose terminal branches within different cell clusters. The varicosities were aligned along somata and proximal dendrites of projection neurons providing dense input to each individual cell. At the electron microscopic level, labeled boutons displayed a predominantly flat and elongated shape. They contained a moderate number of pleomorphic synaptic vesicles and very large amounts of mitochondria, displayed symmetric synaptic contacts, and were immunoreactive for GABA. In the total sample of 128 autoradiographically labeled terminals, 64% were in synaptic contact with somata and primary dendrites of projection neurons, 14% formed synapses on proximal dendrites of undefined order, and only 7% established synaptic contacts on distal dendrites. Fifteen percent of the labeled boutons established synapses on distal dendrites of GABAergic local circuit neurons (LCN). Pallidal boutons were also found in complex synaptic arrangements: triads with three GABAergic synapses, and serial synapses with LCN dendrites that in turn established synaptic contacts on projection neuron somata or dendrites. These anatomical results suggest a dual effect of pallidal afferents to projection neurons: direct inhibition and disinhibition mediated by LCN. The findings indicate that the fine structure of pallidothalamic terminals in the monkey is similar to that described earlier in the cat. There are, however, interspecies differences in the distribution of pallidal input on postsynaptic targets and its participation in complex synaptic arrangements.

Pallidal afferent territory of the Macaca mulatta thalamus: neuronal and synaptic organization of the VAdc.

Ventral anterior thalamic nucleus pars densicellularis (VAdc) as delineated earlier (Ilinsky and Kultas-Ilinsky [1987] J. Comp. Neurol. 262:331-364) was analyzed by using qualitative and quantitative neuroanatomical techniques. Projection neurons (PN), retrogradely labeled with wheat germ agglutinin conjugated horseradish peroxidase from the cortex, were small to medium in size (mean area, 312 microm2) with numerous primary dendrites displaying a tufted branching pattern. Local circuit neurons (LCN), immunoreactive for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase, were small (mean area, 110 microm2), and gave off few dendrites. Two subpopulations of GABA positive boutons (F1 type) were distinguished: large (mean area, 2.6 microm2) terminals with symmetric synapses containing few pleomorphic vesicles and numerous mitochondria densely covered proximal PN sites; smaller F1 boutons with a slightly different morphology contacted mostly distal PN dendrites. Two subpopulations of terminals containing round vesicles and forming asymmetric synapses were distinguished by bouton size (mean areas, 0.4 microm2 and 1.6 microm2, respectively). These targeted mainly distal PN dendrites, but some synapsed proximally next to large F1 boutons. On distal dendrites, representatives of both types were labeled from the cortex. The density of boutons with symmetric and asymmetric synapses (the number of boutons per 100 microm of PN membrane length) was 3.3:0.2 on primary, 2.5:1.2 on secondary, and 0.8:12 on distal dendrites. The numerical density of synapses formed by presynaptic LCN dendrites on all PN levels was 20 to 40 times less than that of axon terminals at the same sites. Afferent input to LCN from boutons of all types, including that from 50% of labeled cortical boutons, mainly targeted distal dendrites. Overall, the findings suggest that PN in VAdc receive massive inhibitory input proximally intermingled with some presumably excitatory input, and that LCN contribution to PN inhibition is modest.

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.

Synaptic connections between identified neuron types in the antennal lobe glomeruli of the cockroach,Periplaneta americana: I. uniglomerular projection neurons

A combination of three different labels was used to demonstrate synapses between three types of neurons within the glomeruli: 1) antennal receptor cells, 2) gamma-aminobutyric acid (GABA)-immunoreactive neurons, and 3) uniglomerular projection neurons. Receptor cell axons were experimentally severed and caused to degenerate; uniglomerular projection neurons, a subgroup of glomerular output neurons, were labeled by intracellular horseradish peroxidase (HRP) injection and GABA-containing neurons by postembedding immunogold staining. The following synaptic connections were identified: 1) Receptor cell axons form monosynaptic contacts in a dyadic fashion onto a dendritic process of a uniglomerular projection neuron and in addition onto a GABA-immunoreactive neuron. 2) Receptor cell axons form polysynaptic connections with dendrites of uniglomerular projection neurons via GABA-immunoreactive neurons. 3) GABA-immunoreactive neurons form dyadic output synapses onto receptor cell axons and in addition onto projection neuron dendrites. These findings provide further evidence that signal transfer from receptor cells onto uniglomerular projection neurons is mediated by two different paths: first, a monosynaptic and presumably excitatory route and, second, an inhibitory polysynaptic route via GABAergic, most likely multiglomerular interneurons. The output synapses of GABA-immunoreactive neurons onto both receptor cells and uniglomerular projection neurons are assumed to exert control functions in regulating the neuronal activity within the glomeruli.

Synaptic connections between identified neuron types in the antennal lobe glomeruli of the cockroach, Periplaneta americana: I. uniglomerular projection neurons

A combination of three different labels was used to demonstrate synapses between three types of neurons within the glomeruli: 1) antennal receptor cells, 2) γ-aminobutyric acid (GABA)-immunoreactive neurons, and 3) uniglomerular projection neurons. Receptor cell axons were experimentally severed and caused to degenerate; uniglomerular projection neurons, a subgroup of glomerular output neurons, were labeled by intracellular horseradish peroxidase (HRP) injection and GABA-containing neurons by postembedding immunogold staining. The following synaptic connections were identified: 1) Receptor cell axons form monosynaptic contacts in a dyadic fashion onto a dendritic process of a uniglomerular projection neuron and in addition onto a GABA-immunoreactive neuron. 2) Receptor cell axons form polysynaptic connections with dendrites of uniglomerular projection neurons via GABA-immunoreactive neurons. 3) GABA-immunoreactive neurons form dyadic output synapses onto receptor cell axons and in addition onto projection neuron dendrites. These findings provide further evidence that signal transfer from receptor cells onto uniglomerular projection neurons is mediated by two different paths: first, a monosynaptic and presumably excitatory route and, second, an inhibitory polysynaptic route via GABAergic, most likely multiglomerular interneurons. The output synapses of GABA-immunoreactive neurons onto both receptor cells and uniglomerular projection neurons are assumed to exert control functions in regulating the neuronal activity within the glomeruli. J. Comp. Neurol. 378:307–319, 1997. © 1997 Wiley-Liss, Inc.

Quantitative analyses of synaptic contacts of interneurons in the dorsal lateral geniculate nucleus of the squirrel monkey.

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 microns 2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.

Three-dimensional analysis of cerebellar terminals and their postsynaptic components in the ventral lateral nucleus of the cat thalamus.

Relationships among cerebellar terminals (CTs), dendrites of thalamocortical projection neurons (TCNs), and dendrites of local circuit neurons in the ventral lateral nucleus of the cat thalamus were analyzed quantitatively by observing several series of serial ultrathin sections and by using a computer-assisted program for the three-dimensional reconstruction from serial ultrathin sections. In pentobarbital-anesthetized cats, CTs were labeled either by injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the cerebellar nuclei or by intra-axonal injection of HRP after electrophysiological identification. By using two series of 133 and 73 serial sections, mutual relationships between 43 WGA-HRP-labeled CTs and their postsynaptic structures were analyzed based on their synaptic specializations and shapes of synaptic vesicles. Thirty-nine of these CTs formed a synapse with one TCN dendrite, whereas only four CTs formed synapses with two TCN dendrites. These CTs also synapsed on dendrites containing pleomorphic synaptic vesicles (presynaptic dendrites). Single CTs synapsed on 0-6 presynaptic dendrites (2.2 +/- 1.5, N = 43) through their whole extents, and about 40% of these presynaptic dendrites that were contacted by CTs established synaptic contacts with the same TCN dendrites on which the CTs synapsed. Thus, a CT, a presynaptic dendrite, and a TCN dendrite formed a triadic arrangement. Triadic arrangements were identified in approximately 60% of these 43 CTs. However, they rarely had a glomerulus-like appearance, as described previously in the ventral lateral nucleus and other main thalamic relay nuclei. In another series of 83 and 43 serial sections along dendrites of TCNs, observations were focused on the triadic arrangement. Triadic arrangements were located evenly on the primary and secondary dendrites of TCNs. Computer-assisted three-dimensional reconstructions were made on one WGA-HRP-labeled CT and two intra-axonally labeled CTs (a bouton en passant and a bouton terminal) with their surrounding neuronal elements, and complex spatial arrangement of neuronal processes became obvious. These results provide the quantitative assessment of synaptic arrangements among CTs, presynaptic dendrites, and TCN dendrites and reveal their spatial interrelations in the cat ventral lateral nucleus.

Reevaluation of synaptic relationships of cerebellar terminals in the ventral lateral nucleus of the rhesus monkey thalamus based on serial section analysis and three-dimensional reconstruction.

Terminals of cerebellar afferents (CB) to different regions of the ventral lateral nucleus (VL) of the rhesus monkey thalamus were labeled with wheat germ agglutinin-horseradish peroxidase following injections into the dentate nucleus. Synaptic relationships of 17 CB with projection neuron dendrites (PNd) and local circuit neuron dendrites (LCNd) were analyzed in serial ultrathin sections from dorsal and ventral VL regions, which are known to differ cytoarchitecturally and functionally. Three terminals were reconstructed using three-dimensional (3D) computer image analysis techniques to obtain volumetric and planar measurements. CB in the ventral VL were often flat and elongated with synaptic vesicles arranged in clusters. Each CB was engaged with one PNd and one to four LCNd. A single bouton formed 8-50 synaptic contacts, with those on PNd outnumbering the ones on LCNd 4.1:1. Only some CB in the ventral VL were engaged in complex synaptic arrangements such as triads and serial synapses. Most CB in the dorsal VL displayed a roundish shape and numerous uniformly distributed synaptic vesicles. They formed 5-25 synaptic contacts with a 3:1 ratio of contacts on PNd compared with those on LCNd. CB in the dorsal VL participated in a variety of complex synaptic arrangements. Two types of triads were found: classic with CB, PNd and LCNd, and unconventional with CB and two LCNd. CB were also involved in serial synapses with two LCNd or LCNd and another PNd, and serial sequential synapses with two LCNd and a PNd. Three glomerulus-like structures were encountered in the dorsal VL. 3D reconstruction and volumetric measurements revealed that synaptic contacts formed by CB on PNd had varying shapes and sizes (0.022-0.274 microns2). Synapses formed on LCNd were larger (0.09-0.407 microns2). The total area of all active zones of a single CB on LCNd was either equal to or about 40% smaller than that of synapses on PNd. The entire active zone area comprised 1-1.6% of the total CB surface area and did not seem to correlate with the volume. Synaptic contacts formed by associated LCNd on PNd in complex arrangements were usually small (0.021-0.044 micron2). The results suggest that: synapses formed by CB on PNd and LCNd, and synapses formed by LCNd on PNd may differ in strength; a variety of different circuits participate in the processing of cerebellar afferent information in the primate VL; and these circuits differ in functionally different VL subdivisions.

Neuronal and synaptic organization of the centromedian nucleus of the monkey thalamus: a quantitative ultrastructural study, with tract tracing and immunohistochemical observations.

The ultrastructure of the centromedian nucleus of the monkey thalamus was analysed qualitatively and quantitatively and projection neurons, local circuit neurons, and synaptic bouton populations identified. Projection neurons were mostly medium-sized, with oval-fusiform or polygonal perikarya, few primary dendrites, and frequent somatic spines; local circuit neurons were smaller. Four basic types of synaptic boutons were distinguished: (1) Small- to medium-sized boutons containing round vesicles (SR) and forming asymmetric contacts, identified as corticothalamic terminals. (2) Heterogeneous medium-sized boutons with asymmetric contacts and round vesicles, similar to the so-called large round (LR) boutons, which were in part of cortical origin. (3) Heterogeneous GAD-positive small- to medium-sized boutons, containing pleomorphic vesicles and forming symmetric contacts (F1 type), which included pallidothalamic terminals. (4) Presynaptic profiles represented by GAD-positive vesicle-containing dendrites of local circuit neurons. Complex synaptic arrangements, serial synapses and triads with LR and SR boutons engaging all parts of projection neuron dendrites and somata, were seen consistently, whereas classical glomeruli were infrequent. LR and SR boutons also established synapses on dendrites of local circuit neurons. F1 boutons established synapses on projection neuron somata, dendrites and initial axon segments. Compared to other previously studied motor-related thalamic nuclei, differences in synaptic coverage between proximal and distal projection neuron dendrites were less pronounced, and the density of synapses formed by local circuit dendrites on projection neuron dendrites was lower. Thus, compared to other thalamic nuclei, the overlap of different inputs was higher on monkey centromedian cells, and centromedian inhibitory circuits displayed a different organization.

Nucleus reticularis thalami input to the anterior thalamic nuclei in the monkey: a light and electron microscopic study

Three anterograde tracers (PHA-L, WGA-HRP, BDA) placed in the most dorsomedial segment of the anterior part of the nucleus reticularis thalami (NRT) in the Rhesus monkey combined with postembedding immunoctochemistry with anti-GABA antibody resulted in double labeling of a distinct population of synaptic boutons in the thalamic nuclei of the anterior group as well as laterodorsal and mediodorsal nuclei. EM analysis of the labeled terminals in the anteroventral and anteromedial parts of the anterior nuclear complex showed that the NRT input is distributed diffusely and in equal proportion to the dendrites of projection and local circuit neurons.

Uniglomerular projection neurons participate in early development of olfactory glomeruli in the moth Manduca sexta.

Glomerular organization of the antennal (olfactory) lobe is initiated by the arrival of sensory axons from the antenna. Bundles of axon terminals coalesce into spheroidal knots of neuropil called protoglomeruli. Previous studies have suggested that the protoglomeruli form a template for the mature glomerular array, but an early role for projection neurons in establishing the template has not been excluded. We examined with the confocal laser scanning microscope the morphological development of the uniglomerular projection neurons during the stages in which glomeruli are constructed. Groups of projection neurons were stained with the lipophilic dye DiI to assess the development of the population as a whole; individual neurons were filled intracellularly with Lucifer Yellow to examine in detail the development of shape. In some preparations, sensory axons and glial cells also were labeled by using different fluorescent dyes to reveal possible interactions between projection neuron dendrites and sensory axons or glial cells. Protoglomeruli form in a wave beginning at the entry point of the antennal nerve and proceeding across the lobe to the opposite pole. A second wave follows in which projection neurons become tufted and innervate the newly formed glomeruli, sometimes extending into the glial border surrounding the protoglomeruli. In animals deprived of sensory axons, some projection neurons still form tufted dendritic trees and, in one region of the neuropil, a glomerulus-like structure. The early presence of projection neuron processes in the protoglomeruli and the formation of at least one glomerulus-like structure in unafferented lobes suggest that uniglomerular projection neurons play an active role in the construction of olfactory glomeruli.

Medial lemniscal and spinal projections to the macaque thalamus: an electron microscopic study of differing GABAergic circuitry serving thalamic somatosensory mechanisms

The synaptic relationships formed by medial lemniscal (ML) or spinothalamic tract (STT) axon terminals with neurons of the somatosensory ventroposterolateral thalamic nucleus of the macaque monkey have been examined quantitatively by electron microscopy. ML and STT axons were labeled by the anterograde axon transport of WGA-HRP following injection of the tracer into the contralateral dorsal column nuclei, or the dorsal horn of the spinal cord, respectively. Thalamic tissue was histochemically reacted for the presence of HRP. Serial thin sections were stained with a gold-labeled antibody to GABA, to determine which neuronal elements exhibited GABA immunoreactivity (GABA-ir). Serially sectioned thalamic structures were recorded in electron micrographs and reconstructed in three dimensions by computer. Individual ML axon terminals form multiple synaptic contacts with segments of the proximal dendritic trees of thalamocortical relay neurons and also synapse upon the dendritic appendages of GABA-ir interneurons (local circuit neurons). These GABA-ir dendritic appendages contain synaptic vesicles and are presynaptic (presynaptic dendrites) to the same segments of relay neuron dendrites that receive ML contacts. When analyzed in serial sections and reconstructed by computer, the ML terminals form triadic relationships (ML, GABA appendage, and relay neuron dendrite) or more complex glomerular arrangements involving multiple appendages, all of which then contact the relay neuron dendritic segment. In contrast, multiple STT terminals make synaptic contacts along segments of projection neuron dendrites and are usually the only type of profile to contact that segment of dendrite. More than 85% of the spinal afferents form simple axodendritic synapses with relay cells and do not contact GABA-ir appendages. The thalamic synaptic relationships of ML terminals are fundamentally different from those formed by the STT. Because STT neurons predominatly transmit information about noxious stimuli, the simple axodendritic circuitry of the majority of these spinal afferents suggests that the transmission of noxious information is probably not subject to GABAergic modulation by thalamic interneurons, in contrast to the GABAergic processing of non-noxious information carried by the ML afferents. The differences in the GABAergic circuits of the thalamus that mediate ML and STT afferent information are believed to underlie differential somatosensory processing in the forebrain. We suggest that changes in thalamic GABAergic dendritic appendages and GABA receptors following CNS injury may play a role in the genesis of some central pain states.