The fine structure of the perigeniculate nucleus in the cat

The fine structure of the cat's perigeniculate nucleus has been analyzed and compared to that of dorsal thalamic relay nuclei. Golgi preparations and electron micrographs of perigeniculate cells commonly show somatic spines. The most common presynaptic elements for these spines and for the adjacent perikaryal surfaces are relatively large axon terminals containing round synaptic vesicles and making multiple asymmetric contacts. These "RLD" terminals (so termed for their round vesicles, large average size of the terminals, and dark mitochondria) are also presynaptic to dendritic spines and shafts of proximal and secondary dendrites. Comparisons with adjacent parts of the dorsal lateral geniculate nucleus show that these RLD terminals are cytologically distinct from retinogeniculate terminals and that small numbers of RLD terminals also occur in the geniculate A laminae. Three other major classes of perigeniculate synaptic terminals, resemble major classes of terminals in the dorsal lateral geniculate nucleus. These include two types of terminal with flat or ovoid synaptic vesicles and dark mitochondria, "FD1" and "FD2" terminals, and a class of small terminal with densely clustered round vesicles and dark mitochondria, "RSD" terminals. RSD terminals, which resemble corticogeniculate axon terminals, represent the only class of perigeniculate terminal that does not contact perikarya. FD2 terminals resemble lateral geniculate presynaptic dendrites and participate in serial and triadic synaptic contacts, being both pre- and postsynaptic; however, in contrast to the arrangement characteristic of thalamic relay nuclei, these contacts do not occur within synaptic glomeruli. A fifth major class of perigeniculate presynaptic terminal has large flat or polymorphic synaptic vesicles and pale mitochondria. These "FP" terminals are seen infrequently in the lateral geniculate A laminae. Similarities between perigeniculate and lateral geniculate fine structure may relate in part to common sources of afferent input to the two nuclei.     

Connectional studies of the primate lateral geniculate nucleus: distribution of axons arising from the thalamic reticular nucleus of Galago crassicaudatus.

Anterograde and retrograde transport methods have been used to explore the interconnections between the thalamic reticular nucleus (TRN) and the dorsal lateral geniculate nucleus of Galago crassicaudatus. We first defined the region of the TRN, which is connected to the lateral geniculate nucleus, by examining the distribution of geniculo-TRN axons, cortico-TRN axons arising from area 17, and the location of TRN-geniculate neurons. Following an intraocular injection of 3H-proline/3 H-leucine, trans-synaptically transported protein is present bilaterally within the lateral portion of the caudal TRN. This same caudal and lateral region is also targeted by cortico-TRN axons and contains neurons which project upon the lateral geniculate nucleus. Light microscopic anterograde transport methods were used to analyze the distribution of TRN-geniculate axons. Our data reveal that all layers and interlaminar zones of the dorsal lateral geniculate nucleus contain TRN axons. Electron microscopic-autoradiographic data support and extend our light microscopic findings by revealing labeled TRN terminals within all geniculate layers. These TRN profiles are the same size throughout the geniculate and exhibit morphological characteristics similar to F1 terminals described by others. That is, they possess predominantly pleomorphic vesicles, a dark cytoplasmic matrix, dark mitochondria, and symmetrical synaptic contacts. Two additional features of TRN terminals have been observed in some profiles. These include dense-core vesicles and a dense, punctate cytoplasmic matrix, which is sometimes associated with the postsynaptic specialization. In addition to their morphology and size, the postsynaptic targets of TRN terminals are similar within the three sets (parvi-, magno-, and koniocellular) of geniculate layers. TRN profiles terminate upon dendrites of all sizes and somata. These findings suggest that the TRN modulates the retino-geniculocortical pathway and that this modulation is occurring in all three streams.     

An EM-autoradiographic analysis of the projection from cortical areas 17, 18, and 19 to the superior colliculus in the cat

The electron microscopic autoradiographic method was used to identify terminals of axons from cortical areas 17, 18, and 19 in the superficial layers of the superior colliculus. The results show that terminals of area 17 neurons contain round vesicles and made asymmetrical synaptic contacts predominantly onto one or more dendrites or dendritic appendages. Some profiles postsynaptic to labeled terminals contain vesicles and presumably are involved in serial synaptic arrangements. Terminals of area 18 and 19 neurons in the superficial collicular layers appear to comprise two populations, one similar in most respects to area 17 terminals, containing round vesicles and making asymmetrical contacts. The other contains pleomorphic vesicles and makes symmetrical contacts upon dendrites and dendritic appendages. These terminals rarely contact more than one postsynaptic profile, and rarely do the postsynaptic profiles contain vesicles. The two populations of area 18 and 19 terminals containing round and pleomorphic vesicles, respectively, are present in the ratio of approximately 3:1, although this ratio varies throughout the sublaminae of the superficial collicular layers. The presence of two distinct types of cortical terminals in the colliculus suggests that cortical modulation of collicular processing is more complex than was previously conceived.     

Ultrastructural studies of the primate parabigeminal nucleus: electron microscopic autoradiographic analysis of the tectoparabigeminal projection in Galago crassicaudatus

The normal ultrastructure of the parabigeminal nucleus and the morphology and synaptic relationships of tectoparabigeminal terminals have been examined. Five different morphological types of terminals have been observed within the parabigeminal nucleus. Three of these profiles contain round vesicles and make asymmetrical synapses, while two contain pleomorphic vesicles and make symmetrical synapses. Electron microscopic autoradiographic data indicate that labeled tectoparabigeminal terminals represent only one of the three profiles containing round vesicles. Such terminals are primarily presynaptic to dendritic shafts, and several labeled profiles have been observed presynaptic to the same dendrite.     

Ultrastructural studies of retinal, visual cortical (area 17), and parabigeminal terminals within the superior colliculus of Galago crassicaudatus.

The morphology and synaptic relationships of anterogradely labeled retinal, visual cortical (area 17), and parabigeminal terminals have been analyzed within the superficial gray (stratum griseum superficiale) of Galago crassicaudatus. Our data regarding the retinocollicular projection reveal two populations of terminals based upon size. The population of smaller terminals are found in clusters, while the larger occur in isolation. Both populations of retinocollicular terminals form synapses primarily with dendritic spines, but synapses upon pale vesicle filled (PVF) profiles and dendritic shafts also occur. Corticotectal terminals contain round vesicles and make asymmetrical synapses, primarily onto dendritic spines; few form synapses with PVF profiles. Our findings suggest the possibility that there are two populations of corticotectal terminals based upon differences in size and morphology. Parabigeminotectal profiles contain densely packed round vesicles and make asymmetrical synapses. These terminals, which are exclusively cholinergic in Galago, are presynaptic to dendrites of various sizes. Convergence of retinal and cortical terminals has been observed. This convergence occurs on distinctly separate regions of the postsynaptic membrane. In contrast, convergence of retinal and parabigeminal terminals occurs on the same region of the postsynaptic cell(s).     

Synaptic circuitry of physiologically identified W-cells in the cat's dorsal lateral geniculate nucleus

The cat's retinogeniculocortical system is comprised of at least 3 parallel pathways, the W-, X-, and Y-cell pathways. Prior studies, particularly at the level of the lateral geniculate nucleus, have focused on X- and Y-cells. In the present study, we describe the synaptic inputs for 2 geniculate W-cells from the parvocellular C-laminae after these neurons were physiologically identified and intracellularly labeled with HRP. For each of the W-cells, we examined electron micrographs taken from over 500 consecutive thin sections; we reconstructed the entire soma plus roughly 15% of the dendritic arbor and determined the pattern of synaptic inputs to these reconstructed regions of each neuron. In several ways, each W-cell exhibits a similar pattern of synaptic inputs. First, we estimate that each W-cell receives approximately 3000-4000 synaptic contacts, which occur most densely on dendrites 50-150 microns from each soma. Second, axosomatic contacts are extremely rare, and most derive from terminals with flattened or pleomorphic vesicles (F terminals). Third, terminals with round vesicles, large profiles, and pale mitochondria (RLP terminals), which are presumed to be retinal terminals, form only about 2-4% of all synapses onto these W-cells; these synapses occur on proximal dendrites. Fourth, F terminals, which provide roughly 15-20% of all synaptic input to these cells, occupy the same region of proximal dendritic arbor as do the RLP terminals. Fifth, and finally, terminals with round vesicles, small profiles, and dark mitochondria (RSD terminals) provide the majority of synapses along all portions of the dendritic arbor. Compared with geniculate X- and Y-cells of the A-laminae (Wilson et al., 1984), these W-cells are innervated by fewer synapses overall and, in particular, by dramatically fewer synapses from RLP (or retinal) terminals. This paucity of direct retinal input to geniculate W-cells might explain the remarkably poor responsiveness of these neurons to visual stimuli and to electrical activation of the optic chiasm.     

Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

Ultrastructure and GABA Immunoreactivity in Layers 8 and 9 of the Optic Tectum of Xenopus laevis

This study presents an ultrastructural analysis of layers 8 and 9 in the optic tectum of Xenopus laevis. Retinotectal axons were labelled with horseradish peroxidase and tectal cells were labelled with antibody to GABA. Four distinct axonal and dendritic structures were identified. GABA-negative axon terminals formed asymmetric synapses and were categorized as type a-1 (which included retinotectal axons), characterized by medium size synaptic vesicles and pale mitochondria, and type a-2 (non-retinotectal) with large vesicles and dense mitochondria. GABA-negative dendrites (type d) contained dense mitochondria, microtubules in the dendritic shafts, and dendritic spines devoid of microtubules. GABA-positive structures contained small synaptic vesicles and dense mitochondria. Some dendrites (type D) were not only postsynaptic but were also presynaptic elements, as defined by the presence of vesicles and distinct synaptic clefts with symmetric specializations. GABA-positive presynaptic structures were mostly located in vesicle-filled, bulbous extensions of dendritic shafts and usually terminated onto dendritic spines. Some type D dendrites were the middle element in serial synapses, with input from either GABA-positive or GABA-negative structures and output to GABA-negative structures. Retinotectal terminals were identified as one of the synaptic inputs to GABA-positive processes. Glia were characterized by granular cytoplasm and large mitochondria, often displaying a crystalline matrix structure. These results indicate that GABA-positive neurons are a prominent component of circuitry in the superficial layers of the tectum of Xenopus and that, as in mammals, they participate in serial synaptic arrangements in which retinotectal axons are the first element. These arrangements are consistent with complex processing of visual input to the tectum and a central role for inhibitory processes in the shaping of tectal responses.     

Ultrastructure of synapses from the A-laminae of the lateral geniculate nucleus in layer IV of the cat striate cortex

The morphology of synapses in layer IV of the cat striate cortex was studied by electron microscope (EM) autoradiography of serial sections following injection of tritiated amino acids into the lateral geniculate nucleus. Of the terminals in the neuropil, 22% had 2 or more silver grains in 10 successive sections and were labeled at 8-80 times the background level. These terminals were considered to be specifically labeled and to be derived from the lateral geniculate. Two forms of geniculate synapse were observed. One had medium-size, round vesicles and a modest postsynaptic asymmetry (RA); the other had smaller, pleomorphic vesicles and hardly any postsynaptic opacity; that is, it appeared symmetrical (PS). The geniculate RA terminals were presynaptic to dendritic spines, fine processes, and cell bodies; the geniculate PS terminals were presynaptic to dendrites and cell bodies but not to spines. The possible sources of geniculate PS terminals are discussed.     

Postembedding immunocytochemistry demonstrates directly that both retinal and cortical terminals in the cat superior colliculus are glutamate immunoreactive

Although the excitatory neurotransmitter glutamate is known to be present in the cat superior colliculus (SC), the types of synapses that contain glutamate have not been examined. We, therefore, studied the ultrastructure of synaptic profiles labeled by a glutamate antibody by using electron microscopic postembedding immunocytochemistry. In addition, unilateral aspiration lesions of areas 17–18 were made at 5–28 days before death in order to determine whether degenerating terminals from visual cortex were glutamate immunoreactive (Glu-ir). Three types of axon terminal were glu-ir: 1) those containing large, round synaptic vesicles and pale mitochondria, characteristic of retinal terminals (RT profiles); 2) those containing small, round synaptic vesicles and dark mitochondria (RSD profiles); and 3) those containing large, round synaptic vesicles and dark mitochondria (RLD profiles). Measures of mean gold particle density revealed that RT, RSD, and RLD profiles had similar average grain densities (11.3–12.7 particles/unit area). Other labeled profile types included cell bodies, large-calibre dendrites, and myelinated axons. Axon terminals containing flattened synaptic vesicles and vesicle-containing presynaptic dendrites, both of which contain γ-aminobutyric acid (GABA), had many fewer gold particles (3.6 and 4.8 mean particles/unit area, respectively). Following unilateral removal of visual cortex, normal RSD terminals were observed infrequently in the SC ipsilateral to the lesion. Synaptic terminals in the initial stages of degeneration were heavily labeled by the glutamate antibody, as were axon terminals and myelinated axons undergoing hypertrophied or neurofilamentous degeneration. These results show that both major sensory afferents to the superficial layers of cat SC contain glutamate—RT terminals from the retina and RSD terminals from visual cortex. The origin of RLD terminals is unknown. © 1996 Wiley-Liss, Inc.     

Ultrastructure of the mammillotegmental projections to the ventral tegmental nucleus of Gudden in the rat.

This study examines the termination pattern of axons from the medial mammillary nucleus within the ventral tegmental nucleus of Gudden (TV) in rats by using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The neuropil of TV contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic synaptic vesicles. These types make up 55.6%, 26.1%, and 18.3%, respectively, of all normal axodendritic terminals. Injection of WGA-HRP into the medial mammillary nucleus permits ultrastructural recognition of anterogradely labeled terminals within the TV. More than 80% of the labeled terminals contain round synaptic vesicles and form asymmetric synaptic contacts, whereas about 16% contain flat synaptic vesicles with symmetric synaptic contacts. There are a few labeled terminals with pleomorphic vesicles and only a few axosomatic terminals. Almost all labeled terminals are small, having diameters of less than 1.5 microns. Compared with the distributions of normal and labeled terminals with round vesicles, there is an increase of the percentage of labeled terminals with round vesicles on the intermediate dendrites (1-2 microns diameter) and a decrease on the distal dendrites (less than 1 micron diameter). Anterogradely labeled axon terminals often contact retrogradely labeled dendrites. These results suggest that the medial mammillary neurons send mainly excitatory as well as a few inhibitory inputs to the dendrites of TV and have direct reciprocal contacts with the TV neurons.     

Fine structure of the lateral mammillary projection to the dorsal tegmental nucleus of Gudden in the rat.

The synaptic organization of projections from the lateral mammillary neurons within the dorsal tegmental nucleus of Gudden is studied in the rat with the aid of anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The dorsal tegmental nucleus consists of the pars ventralis (TDV) and the pars dorsalis (TDD). The normal neuropil of the dorsal tegmental nucleus contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic vesicles. They make up 44%, 5%, and 51%, respectively, of all axodendritic terminals in the TDV, and 62%, 1%, and 37% in the TDD. Injection of WGA-HRP into the lateral mammillary nucleus permits ultrastructural recognition of many anterograde labeled terminals within both the TDV and TDD. In the TDV, 81% of the labeled terminals contain round synaptic vesicles and make asymmetric synaptic contacts. A few of the labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts. More than 50% of the labeled terminals contact intermediate dendrites (1-2 microns diameter). In the TDD, almost all labeled terminals are small, contain round vesicles, and make asymmetric synaptic contacts. These terminals mainly contact intermediate as well as distal (less than 1 micron diameter) dendrites. There are only a few labeled terminals with pleomorphic vesicles and no terminals with flat vesicles. The termination pattern of the lateral mammillary neurons in the TDV is similar to that in the TDD. Anterograde labeled axon terminals often contact retrograde labeled dendrites in the TDV. No reciprocal connections are present in the TDD. These results suggest that the TDV and the TDD receive mainly excitatory and a few inhibitory inputs from the lateral mammillary nucleus. The TDV neurons also have direct reciprocal connections with the lateral mammillary neurons.     

Synaptic organization of the tectorecipient zone of the rat lateral posterior nucleus

Dorsal thalamic nuclei have been categorized as either “first‐order” nuclei that gate the transfer of relatively unaltered signals from the periphery to the cortex or “higher order” nuclei that transfer signals from one cortical area to another. To classify the tectorecipient lateral posterior (LPN), we examined the synaptic organization of tracer‐labeled cortical and tectal terminals and terminals labeled with antibodies against the type 1 and type 2 vesicular glutamate transporters (vGLUT1 and vGLUT2) within the caudal/lateral LPN of the rat. For this zone, we found that all tracer‐labeled cortical terminals, as well as vGLUT1 antibody‐labeled terminals, are small profiles with round vesicles (RS profiles) that innervate small‐caliber dendrites. Tracer‐labeled tecto‐LPN terminals, as well as vGLUT2 antibody‐labeled terminals, were medium‐sized profiles with round vesicles (RM profiles). Tecto‐LPN terminals were significantly larger than cortico‐LPN terminals and contacted significantly larger dendrites. These results indicate that, within the tectorecipient zone of the rat LPN, cortical terminals are located distal to tectal terminals and that vGLUT1 and vGLUT2 antibodies may be used as markers for cortical and tectal terminals, respectively. Finally, comparisons of the synaptic patterns formed by tracer‐labeled terminals with those of vGLUT antibody‐labeled terminals suggest that individual LPN neurons receive input from multiple cortical and tectal axons. We suggest that the tectorecipient LPN constitutes a third category of thalamic nucleus (“second‐order”) that integrates convergent tectal and cortical inputs. This organization may function to signal the movement of novel or threatening objects moving across the visual field. J. Comp. Neurol. 515:647–663, 2009. © 2009 Wiley‐Liss, Inc.     

A Golgi and ultrastructural analysis of the centromedian nucleus of the cat

The morphology of neurons in the centromedian nucleus (CM) was studied in rapid Golgi preparations of the adult cat. The ultrastructure of the nucleus, particularly its synaptic organization, was also studied with electron microscopy. The CM contains three types of neurons referred to as principal neurons, Golgi type II neurons, and bushy neurons. Principal neurons are the most numerous, have long dendrites, which branch infrequently, and are divided into two subgroups: principal-A neurons with dendrites that arborize radially, whereas principal-B neurons display horizontal orientations. Both subgroups show a frontal orientation in their dendritic organization and give rise to myelinated axons. Golgi type II neurons with their characteristic sinuous dendrites and unmyelinated axons are thought to be interneurons. The occurrence of bushy neurons in the cat's CM is a new finding. These bushy neurons resemble those of thalamic specific relay nuclei and give rise to myelinated axons. In addition to these three cell types, neurons with intermediate features between these three neuronal types are also described. The ultrastructure of CM neurons resembles, in general, typical central nervous system neurons. Presynaptic profiles are classified into four main categories. SR (small round) boutons are small in size, contain clear, round vesicles, and form asymmetrical synaptic contacts with predominantly small-diameter dendrites. LR (large round) boutons are relatively large and contain both clear and dense-cored vesicles. They interdigitate and form multiple, moderately asymmetrical synapses with their postsynaptic targets. Pale profiles are identified by their relatively electron-light appearance. They contain round vesicles and are thought to be dendritic in origin. The last category of presynaptic profiles is pleomorphic boutons. They contain vesicles of different shapes and are further subdivided into two subtypes: pleomorphic-I ends on soma and dendritic trunks, whereas pleomorphic-II contacts small-diameter dendrites. Both subtypes form symmetrical synapses. The glomeruli of specific thalamic relay nuclei generally contain dendrites, LR boutons, and pale profiles. In addition to these, pleomorphic-II boutons also participate in the formation of the glomerulus of the cat's CM.     

Laminar and sublaminar ultracytochemical localization of cytochrome oxidase in the optic tectum of normal goldfish

Distinct laminae and sublaminae in the goldfish optic tectum exhibit substantial differences in cytochrome oxidase (C.O.) reactivity. To determine whether these differences are associated with differential reactivity of different neuronal profiles, each tectal sublamina was examined at the ultrastructural level following C.O. treatment. The greatest abundance of darkly reactive mitochondria was found in the optically innervated layers within both pre- and postsynaptic profiles in correspondence with the most intense staining of these layers at the light microscopic level. Many reactive mitochondria were localized within terminals that were presumed to be optic on the basis of cytological criteria or were shown to be optic by filling optic fibers with HRP and processing so as to simultaneously demonstrate both mitochondrial C.O. reactivity and HRP labeling. These optic terminals tended to differ from each other in size and level of reactivity. The largest terminals were located within sublamina d of the stratum fibrosum et griseum superficials (SFGSd), and these were the most intensely reactive and contained the greatest number of darkly reactive mitochondria. Medium-sized terminals were found within sublaminae SFGSa, SFGSb, and a and c of the stratum album centrale (SACa,c). These were also darkly reactive but contained fewer mitochondria. Other medium-to-small optic terminals were found in stratum opticum a and b (60a,b), SFGSb, SFGSc, and stratum griseum centrale c (SGCc). These typically contained fewer mitochondria that also tended to be relatively less reactive, although darkly reactive mitochondria were also present. We suggest that the metalbolic differences within optic terminals of different size and sublaminar stratification arise from different ganglion cell classes and that the different optic layers of tectum are functionally substratified. As expected, darkly reactive mitochondria were most abundant in th intensely stained sublaminae, which included the optic lamina SFGS and nonoptic sublamina SGCa, and they were found not only within optic terminals but also within dendrites, presynaptic dendrites, and nonoptic terminals as well. Glial processes tended to contain less reactive mitochondria. The most prominent of the nonoptic terminals were the large-diameter P1 terminals, which contained pleomorphic vesicles and formed symmetric (presumed inhibitory) synapses. In stratum marginale most of the darkly reactive mitochondria were localized within dendrites. In the rest of the tectal layers most of the darkly reactive mitochondria were found in both presynaptic terminals and dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)     

Calbindin 28kD and parvalbumin immunoreactive neurons receive different patterns of synaptic input in the cat superior colliculus.

Recent evidence suggests that neurons containing the calcium binding proteins calbindin 28kD (CB) and parvalbumin (PV) have differing distributions which match respectively the distribution of W and Y retinal ganglion cell inputs to the cat superior colliculus (SC). In this study we have used electron microscope immunocytochemistry to study directly the synaptic inputs to neurons containing CB and PV. Aspiration lesions of areas 17-18 of visual cortex were made 4 days prior to sacrifice in order to identify degenerating cortical terminals (CT). Retinal terminals (RTs) were identified by their characteristic morphology including large round synaptic vesicles and pale mitochondria. We photographed RTs and CTs that were in contact with immunoreactive profiles sampled in both the superficial gray and optic layers (ol) of SC. CB immunoreactive (ir) dendrites were usually of small to medium caliber and were found to receive synaptic input from RTs. These RTs were all small profiles forming a single synaptic contact with asymmetric densifications. CBir profiles also received other synaptic input, including from terminals with dark mitochondria that contained flattened synaptic vesicles (F profiles). No CBir dendrites were found to receive CT input even though degenerating CTs were found in the vicinity of CBir profiles. By contrast, both RT and CT were found to contact PVir dendrites. RT terminals contacting PVir dendrites were both small and larger profiles with round synaptic vesicles and asymmetric synaptic densifications. CT were undergoing electron dense degeneration but still sometimes formed asymmetric synaptic densifications with PV neurons. PV cells also received F profile synaptic input. We conclude that CB neurons receive small RT synapses that are probably of W origin, while PV neurons receive both RT and CT synapses which are likely related to the Y pathway.     

Projections from auditory cortex to the cochlear nucleus in rats: Synapses on granule cell dendrites

Previous work has demonstrated that layer V pyramidal cells of primary auditory cortex project directly to the cochlear nucleus. The postsynaptic targets of these centrifugal projections, however, are not known. For the present study, biotinylated dextran amine, an anterograde tracer, was injected into the auditory cortex of rats, and labeled terminals were examined with light and electron microscopy. Labeled corticobulbar axons and terminals in the cochlear nucleus are found almost exclusively in the granule cell domain, and the terminals appear as boutons (1–2 μm in diameter) or as small mossy fiber endings (2–5 μm in diameter). These cortical endings contain round synaptic vesicles and form asymmetric synapses on hairy dendritic profiles, from which thin (0.1 μm in diameter), nonsynaptic “hairs” protrude deep into the labeled endings. These postsynaptic dendrites, which are typical of granule cells, surround and receive synapses from large, unlabeled mossy fiber endings containing round synaptic vesicles and are also postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles. No labeled fibers were observed synapsing on profiles that did not fit the characteristics of granule cell dendrites. We describe a circuit in the auditory system by which ascending information in the cochlear nucleus can be modified directly by descending cortical influences. © 1996 Wiley-Liss, Inc.     

Electron-microscopic analysis of synaptic input from the perigeniculate nucleus to the A-laminae of the lateral geniculate nucleus in cats.

The perigeniculate nucleus of carnivores is thought to be a part of the thalamic reticular nucleus related to visual centers of the thalamus. Physiological studies show that perigeniculate neurons, which are primarily GABAergic, provide feedback inhibition onto neurons in the lateral geniculate nucleus. However, little is known about the anatomical organization of this feedback pathway. To address this, we used two complementary tracing methods to label perigeniculate axons for electron microscopic study in the geniculate A-laminae: intracellular injection of horseradish peroxidase (HRP) to fill an individual perigeniculate cell and its axon; and anterograde transport of Phaseolus vulgaris leucoagglutinin to label a population of perigeniculate axons. Labeled perigeniculate terminals display features of F1 terminals in the geniculate neuropil: they are small, contain dark mitochondria, and form symmetric synaptic contacts. We found that most of the perigeniculate terminals (greater than 90%) contact geniculate cell dendrites in regions that also receive a rich innervation from terminals deriving from visual cortex (e.g., "cortico-recipient" dendrites). The remainder of the perigeniculate synapses (10%) contacted dendrites in regions that also received direct retinal input (e.g., "retino-recipient" dendrites). Serial reconstruction of segments of dendrites postsynaptic to perigeniculate terminals suggests that these terminals contact both classes of relay cell in the A-laminae (X and Y), although our preliminary conclusion is that an individual perigeniculate cell contacts only one class. Finally, our quantitative comparison between labeled perigeniculate terminals and unlabeled F1 terminals indicates that these perigeniculate terminals form a distinct subset of F1 terminals. We quantitatively compared the labeled perigeniculate terminals to unlabeled F1 terminals. Although the parameters of the perigeniculate terminals fell entirely within the range of those for the unlabeled F1 terminals, as populations, we found consistent differences between these two groups. We thus conclude that, as populations, other sources of F1 terminals are morphologically distinct from perigeniculate terminals and innervate different targets.     

Comparison of the ultrastructure of cortical and retinal terminals in the rat dorsal lateral geniculate and lateral posterior nuclei

We compared the ultrastructure and synaptic targets of terminals of cortical or retinal origin in the rat dorsal lateral geniculate nucleus (LGN) and lateral posterior nucleus (LPN). Following injections of biotinylated dextran amine (BDA) into cortical area 17, two types of corticothalamic terminals were labeled by anterograde transport. Type I terminals, found throughout the LGN and LPN, were small, drumstick-shaped terminals that extended from thin axons. At the ultrastructural level in both the LGN and LPN, labeled type I corticothalamic terminals were observed to be small profiles that contained densely packed round vesicles (RS profiles) and contacted small-caliber dendrites. In tissue stained for gamma amino butyric acid (GABA) using postembedding immunocytochemical techniques, most dendrites postsynaptic to type I corticothalamic terminals did not contain GABA (97%). Type II corticothalamic terminals, found only in the LPN, were large terminals that sometimes formed clusters. At the ultrastructural level, type II terminals were large profiles that contained round vesicles (RL profiles) and contacted large-caliber dendrites, most of which did not contain GABA (98%). Retinogeniculate terminals, identified by their distinctive pale mitochondria, were similar to type II corticothalamic terminals except that 26% of their postsynaptic targets were vesicle-containing profiles that contained GABA (F2 profiles). Our results suggest that type I corticothalamic terminals are very similar across nuclei but that the postsynaptic targets of RL profiles vary. Comparison of the responses to retinal inputs in the LGN and to layer V cortical inputs in the LPN may provide a unique opportunity to determine the function of interneurons in the modulation of retinal signals and, in addition, may provide insight into the signals relayed by cortical layer V.