Stereological study of the synaptic profiles belonging to interneurons in the dorsal lateral geniculate nucleus of the rabbit.

This study is concerned with some characteristics of the interneurons belonging to the dLGN (dorsal Lateral Geniculate Nucleus) of the rabbit. The work deals with the distribution of such cells in the alpha E sector of the nucleus and their F1 and F2 presynaptic contacts. The F1 and the F2 profiles are present in all three of the alpha E zones studied. The F1 profiles are significantly more numerous in the upper zone (57 +/- 2 profiles per 10(4) microns2 of section) and the middle zone (59 +/- 3 profiles per 10(4) microns2 of section) than in the lower one (41 +/- 2 profiles per 10(4) microns2 of section). The F2 profiles are more abundant in the alpha E sector than the F1 ones are, particularly in the lower zone, where F2 profiles (104 +/- 4 profiles per 10(4) microns2 of section) are not only significantly more numerous than F1 profiles but also more abundant than the F2 profiles in the middle zone (84 +/- 3 profiles per 10(4) microns2 of section) and upper zone (88 +/- 2 profiles per 10(4) microns2 of section). These results and their comments reveal diverse density of the element distribution from the dorsal to the ventral part of the alpha E sector as well as the possible relationship or independence from the extranuclear afferent inputs.     

Control of dendritic outputs of inhibitory interneurons in the lateral geniculate nucleus

The thalamic relay to neocortex is dynamically gated. The inhibitory interneuron, which we have studied in the lateral geniculate nucleus, is important to this process. In addition to axonal outputs, these cells have dendritic terminals that are both presynaptic and postsynaptic. Even with action potentials blocked, activation of ionotropic and metabotropic glutamate receptors on these terminals increases their output, whereas activation of metabotropic (M2 muscarinic) but not nicotinic cholinergic receptors decreases their output. These actions can strongly affect retinogeniculate transmission.     

Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey

Summary Synaptic junctions are found in all parts of the nucleus, being almost as densely distributed between cell laminae as within these laminae.In addition to the six classical cell laminae, two thin intercalated laminae have been found which lie on each side of lamina 1. These laminae contain small neurons embedded in a zone of small neural processes and many axo-axonal synapses occur there.Three types of axon form synapses in all cell laminae and have been called RLP, RSD and F axons. RLP axons have large terminals which contain loosely packed round synaptic vesicles, RSD axons have small terminals which contain closely packed round vesicles and F axons have terminals intermediate in size containing many flattened vesicles.RLP axons are identified as retinogeniculate fibers. Their terminals are confined to the cell laminae, where they form filamentous contacts upon large dendrites and asymmetrical regular synaptic contacts (with a thin postsynaptic opacity) upon large dendrites and F axons. RSD axons terminate within the cellular laminae and also between them. They form asymmetrical regular synaptic contacts on small dendrites and on F axons. F axons, which also occur throughout the nucleus, form symmetrical regular contacts upon all portions of the geniculate neurons and with other F axons. At axo-axonal junctions the F axon is always postsynaptic.Supported by Grant R 01 NB 06662 from the USPHS and by funds of the Neurological Sciences Group of the Medical Research Council of Canada. Most of the observations were made while R. W. Guillery was a visiting professor in the Department of Physiology at the University of Montreal. We thank the Department of Physiology for their support and Mr. K. Watkins, Mrs. E. Langer and Mrs. B. Yelk for their skillful technical assistance.     

The beta sector of the rabbit's dorsal lateral geniculate nucleus

The beta sector of the rabbit's dorsal lateral geniculate nucleus is a small region of nerve cells scattered among the fibres of the geniculocortical pathway. In its topographical relations it resembles the perigeniculate nucleus of carnivores, which contains neurons driven by geniculate and visual cortical neurons and which sends inhibitory fibres back into the geniculate relay. We have traced retinogeniculate, geniculocortical and corticogeniculate pathways in rabbits by using horseradish peroxidase or radioactively labelled proline and have found that the beta sector resembles the perigeniculate nucleus in receiving no direct retinal afferents, sending no efferents to the visual cortex (V-I), and receiving afferents from the visual cortex. The corticogeniculate afferents are organized so that the visual field map in the beta sector and the main part of the lateral geniculate relays are aligned, as are the maps in the cat's perigeniculate nucleus and the main part of the geniculate relay of carnivores. Electron microscopical studies show similar types of axon terminals in the rabbit and the cat for the main part of the geniculate relay on the one hand and for the beta sector and the perigeniculate nucleus on the other. Earlier observations that the proportion of putative inhibitory terminals (F-type terminals) is lower in the rabbit's than the cat's geniculate region are confirmed. A major difference between the beta sector and the perigeniculate nucleus has been revealed by immunohistochemical staining for GABA. Whereas almost all of the cat's perigeniculate cells appear to be GABAergic, the proportion in the beta sector is much lower, and not significantly different from that found in the main part of the rabbit's geniculate relay. It is concluded that the beta sector shares many of the organizational features of the perigeniculate nucleus. A common developmental origin seems probable, but the functional differences remain to be explored.     

A minimal mechanistic model for temporal signal processing in the lateral geniculate nucleus

The receptive fields of cells in the lateral geniculate nucleus (LGN) are shaped by their diverse set of impinging inputs: feedforward synaptic inputs stemming from retina, and feedback inputs stemming from the visual cortex and the thalamic reticular nucleus. To probe the possible roles of these feedforward and feedback inputs in shaping the temporal receptive-field structure of LGN relay cells, we here present and investigate a minimal mechanistic firing-rate model tailored to elucidate their disparate features. The model for LGN relay ON cells includes feedforward excitation and inhibition (via interneurons) from retinal ON cells and excitatory and inhibitory (via thalamic reticular nucleus cells and interneurons) feedback from cortical ON and OFF cells. From a general firing-rate model formulated in terms of Volterra integral equations, we derive a single delay differential equation with absolute delay governing the dynamics of the system. A freely available and easy-to-use GUI-based MATLAB version of this minimal mechanistic LGN circuit model is provided. We particularly investigate the LGN relay-cell impulse response and find through thorough explorations of the model’s parameter space that both purely feedforward models and feedback models with feedforward excitation only, can account quantitatively for previously reported experimental results. We find, however, that the purely feedforward model predicts two impulse response measures, the time to first peak and the biphasic index (measuring the relative weight of the rebound phase) to be anticorrelated. In contrast, the models with feedback predict different correlations between these two measures. This suggests an experimental test assessing the relative importance of feedforward and feedback connections in shaping the impulse response of LGN relay cells.     

A quantitative study of synaptic interconnections in the dorsal lateral geniculate nucleus of the cat

Summary 2,700 synaptic contacts have been classified according to criteria described in an accompanying paper and the results summarized in tabular form. Only about 20% of the synaptic contacts in laminae A and A1 are formed by axons identifiable as retinogeniculate fibers. About 1/4 of these retinogeniculate synapses are axo-axonal. Approximately 45% of the contacts in these laminae are formed by axons tentatively identifiable as corticogeniculate fibers; about 35% by presumed intrageniculate fibers. Close to one half of the synapses occur in encapsulated synaptic zones, where grapelike dendritic appendages are related mainly to intrageniculate and retinogeniculate axons, and about half lie in interstitial zones, where corticogeniculate and some intrageniculate axons contact distal dendritic segments.Regions of the nucleus receiving from peripheral parts of the retina have relatively more corticogeniculate synapses, and have fewer intrageniculate synapses in the encapsulated zones than do regions receiving from the central parts of the retina.Most of the tissue in lamina B resembles the interstitial zones of laminae A and A1 and over 2/3 of the contacts in lamina B may prove to be corticogeniculate. The retinogeniculate fibers in this lamina are associated with relatively few other axons in simple, small encapsulated zones.Supported by Grant NB 06662 from the USPHS. The skillful technical assistance given by Mrs. E. Langer during the course of this work is gratefully acknowledged.     

Dense cored vesicles in presynaptic profiles of the rabbit dorsal lateral geniculate nucleus

We are carrying out a study about the synaptic relations between identified synaptic profiles in the dorsal lateral geniculate nucleus (dLGN) of the rabbit. Here, the types of synaptic vesicle containing profiles of the dLGN are described. There are presynaptic large profiles containing round vesicles and pale mitochondria (RLP terminals) and small profiles that contain round vesicles and dark mitochondria (RSD terminals) which respectively arise from the retina and the visual cortex. Another type of presynaptic profile contains elliptical vesicles (F-boutons) which can be subdivided according to their cytoplasmic content. These F-boutons arise from dLGN interneurons. We have found different sized vesicles that have a dense core within RLP, and F terminals and a possible RSD terminal. The significance of the coexistance of pale and dense cored vesicles in the presynaptic profiles of the rabbit dLGN is discussed.     

Cytoplasmic organelles in neurons of the dorsal lateral geniculate nucleus of aged rats

The ultrastructural characteristics of the neurons containing complex convolutions have been studied in the dorsal lateral geniculate nucleus of the 31-month-old rat. Neurons were seen to contain oval or round dense bodies which were surrounded by a nuclear membrane and granular endoplasmic reticulum. Their perikarya showed rarely clusters of pleomorphic and small clear vesicles intermingled with a few larger vesicles of dense material. Dendrites occasionally exhibited intermediate forms between laminated bodies and complex convolutions. The significance of these features has been discussed.     

Role of calcineurin in activity-dependent pattern formation in the dorsal lateral geniculate nucleus of the ferret

In the retinogeniculate pathway of the ferret, in addition to the separation of the inputs from the two eyes to form eye-specific layers, there is also an anatomical segregation of the terminal arbors of on-center retinal ganglion cells from the terminal arbors of off-center retinal ganglion cell axons to form on/off sublaminae. Sublamination normally occurs during postnatal weeks 3-4 and requires the activity of retinal afferents, N-methyl-D-aspartate receptors, nitric oxide synthase, and a target of nitric oxide, cyclic guanosine monophosphate. Calcineurin is a calcium/calmodulin dependent serine, threonine protein phosphatase suggested to mediate NMDA-receptor dependent synaptic plasticity in the hippocampus. We have examined whether calcineurin plays a role during on/off sublamination in the dorsal lateral geniculate nucleus (dLGN) of the ferret. Immunohistochemistry showed that calcineurin expression is transiently up-regulated in dLGN cells and neuropil during the period of on/off sublamination. A functional role for calcineurin during sublamination was investigated by blocking the enzyme locally via intracranial infusion of FK506. Treatment with FK506 during postnatal weeks 3-4 disrupted the appearance of sublaminae. These results suggest that calcineurin may play a role during this process of activity-dependent pattern formation in the visual pathway.     

The organization of synaptic interconnections in the laminae of the dorsal lateral geniculate nucleus of the cat

Summary Four axon types occur in the lateral geniculate nucleus. Two contain vesicles with mainly round profiles and these are distinguished from each other by their size, the appearance of their contents and by the types of contact they make. The larger RLP axons are interpreted as retinogeniculate and the smaller RSD axons as corticogeniculate fibers. The other two axon types contain many irregular or flattened vesicles and these F axons are regarded as two types of intrageniculate fiber.In laminae A and A 1 encapsulated synaptic zones form around grape-like dendritic appendages. These zones contain all axon types, but RSD axons are rare. Interstitial zones lie between the encapsulated zones and contain synapses formed by many RSD axons, some F and few RLP axons. The interstitial zones continue into the central interlaminar nucleus which forms a narrow band containing no encapsulated zones and few RLP axons. Lamina B contains relatively small RLP axons, very many RSD axons and only a few small encapsulated zones.Axosomatic junctions are rare throughout the nucleus. Axo-axonal junctions occur in all laminae but mostly in the encapsulated zones; the postsynaptic element is always an F axon, RLP or RSD axons generally form the presynaptic element.Supported by Grant NB 06662 from the USPHS. The skillful technical assistance given by Mrs. E. Langer during the course of this work is gratefully acknowledged.     

Neuronal types in the lateral geniculate nucleus of man

Summary Nerve cell types of the lateral geniculate body of man were investigated with the use of a transparent Golgi technique that allows study of not only the cell processes but also the pigment deposits. Three types of neurons have been distinguished:Type-I neurons are medium-to large-sized multipolar nerve cells with radiating dendrites. Dendritic excrescences can often be encountered close to the main branching points. Type-I neurons comprise a variety of forms and have a wide range of dendritic features. Since all intermediate forms can be encountered as well, it appears inadequate to subdivide this neuronal type. One pole of the cell body contains numerous large vacuolated lipofuscin granules, which stain weakly with aldehyde fuchsin.Type-II and type-III neurons are small cells with few, sparsely branching and extended dendrites devoid of spines. In Golgi preparations they cannot be distinguished from each other. Pigment preparations reveal that the majority of these cells contains small and intensely stained lipofuscin granules within their cell bodies (type II), whereas a small number of them remains devoid of any pigment (type III). Intermediate forms do not occur.     

Dendritic development in the dorsal lateral geniculate nucleus of ferrets in the postnatal absence of retinal input: A golgi study

In order to determine the ongoing role of retinal fibers in the development of dorsal lateral geniculate nucleus (dLGN) neurons during postnatal development, the development of dLGN neurons in the postnatal absence of retinal input was studied in pigmented ferrets using the Golgi-Hortega technique. The development of four dLGN cell classes, defined on the basis of somatic and dendritic morphology, was described previously in normal ferrets (Sutton and Brunso-Bechtold, 1991, J. Comp. Neurol. 309 : 71–85). The present results indicate that the morphological development of dLGN neurons is strikingly similar in normal and experimental ferrets. The exuberant dendritic appendages that appear after eye opening in normal ferrets are overproduced and eliminated in the postnatal absence of retinal input; however, the final reduction of these transient appendages is delayed. Because exuberant appendages develop in the absence of retinal input, their production cannot depend upon visual experience. Differences in cell body size between normal and experimental ferrets are apparent only after neurons can be classified at the end of the first postnatal month. Cell body size is markedly reduced for class 1 neurons; class 2 cells also are reduced in size but to a far lesser extent. As there is a general trend for class 1 neurons to have the functional properties of Y-cells, it is likely that the dLGN neurons most affected by the absence of retinal input also are Y-cells. © 1993 John Wiley & Sons, Inc.     

Extended difference-of-Gaussians model incorporating cortical feedback for relay cells in the lateral geniculate nucleus of cat

A striking feature of the organization of the early visual pathway is the significant feedback from primary visual cortex to cells in the dorsal lateral geniculate nucleus (LGN). Despite numerous experimental and modeling studies, the functional role for this feedback remains elusive. We present a new firing-rate-based model for LGN relay cells in cat, explicitly accounting for thalamocortical loop effects. The established DOG model, here assumed to account for the spatial aspects of the feedforward processing of visual stimuli, is extended to incorporate the influence of thalamocortical loops including a full set of orientation-selective cortical cell populations. Assuming a phase-reversed push-pull arrangement of ON and OFF cortical feedback as seen experimentally, this extended DOG (eDOG) model exhibits linear firing properties despite non-linear firing characteristics of the corticothalamic cells. The spatiotemporal receptive field of the eDOG model has a simple algebraic structure in Fourier space, while the real-space receptive field, as well as responses to visual stimuli, are found by evaluation of an integral. As an example application we use the eDOG model to study effects of cortical feedback on responses to flashing circular spots and patch-grating stimuli and find that the eDOG model can qualitatively account for experimental findings.