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.     

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.     

A multi-compartment model for interneurons in the dorsal lateral geniculate nucleus

GABAergic interneurons (INs) in the dorsal lateral geniculate nucleus (dLGN) shape the information flow from retina to cortex, presumably by controlling the number of visually evoked spikes in geniculate thalamocortical (TC) neurons, and refining their receptive field. The INs exhibit a rich variety of firing patterns: Depolarizing current injections to the soma may induce tonic firing, periodic bursting or an initial burst followed by tonic spiking, sometimes with prominent spike-time adaptation. When released from hyperpolarization, some INs elicit rebound bursts, while others return more passively to the resting potential. A full mechanistic understanding that explains the function of the dLGN on the basis of neuronal morphology, physiology and circuitry is currently lacking. One way to approach such an understanding is by developing a detailed mathematical model of the involved cells and their interactions. Limitations of the previous models for the INs of the dLGN region prevent an accurate representation of the conceptual framework needed to understand the computational properties of this region. We here present a detailed compartmental model of INs using, for the first time, a morphological reconstruction and a set of active dendritic conductances constrained by experimental somatic recordings from INs under several different current-clamp conditions. The model makes a number of experimentally testable predictions about the role of specific mechanisms for the firing properties observed in these neurons. In addition to accounting for the significant features of all experimental traces, it quantitatively reproduces the experimental recordings of the action-potential- firing frequency as a function of injected current. We show how and why relative differences in conductance values, rather than differences in ion channel composition, could account for the distinct differences between the responses observed in two different neurons, suggesting that INs may be individually tuned to optimize network operation under different input conditions.     

Morphometric characteristics of synaptic apparatus in the dorsal nucleus of the medial geniculate body of the cat

The synaptic apparatus in the dorsal nucleus of the medial geniculate body, MGB(d), of the cat was examined using electron microscopy. Within 2166 µm² of studied sections obtained from five regions of MGB(d) tissue, 455 presynaptic terminal (PST) profiles were found, which corresponds, on average, to (210.0±28.4) · 10³ PST per 1 mm² of section surface. In accordance with their ultrastructural pattern (dimension of PST profile, shape of synaptic vesicles, SV, pattern of their arrangement within the terminal, and type of synaptic contact, SC), PST were classified into five main groups:RL, RS, F, P, andUT. The relative amount of PST of these groups constituted 8.1% (RL group), 50.5% (RS), 26.0% (F), 9.2% (P), and 6.2% (UT). According to the dimension of profile, number of SV, and pattern of their arrangement within the terminal,RS-PST were additionally divided into four subgroups:RS1, RS2, RS3, andRS4, whileF-PST were divided into three subgroups:F1, F2, andF3. Thus, MGB(d) possesses five various forms of PST with round SV and asymmetric SC, three PST forms with flattened SV and symmetric SC, one with a mixture of flattened and round SV and symmetric SC, and one with round SV and symmetric SC. It can be supposed that the MGB(d) neurons are supplied with afferent inputs from numerous different sources.Neirofiziologiya/Neurophysiology, Vol. 28, No. 4/5, pp. 197–206, July–October, 1996.     

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.     

Tailoring of variability in the lateral geniculate nucleus of the cat

 Variability is usually considered an unwanted component in a sensory signal, yet the visual system does not seem to filter out the noise. On the contrary, noise is ‘tailored’ to scale with the signal size. We show that this tailoring occurs in the lateral geniculate nucleus, preferentially in X-cells, which are the cells most likely to transmit pattern information. Tailoring the variability to the signal size may be the visual system’s way of providing the right amount of variability for a signal of any magnitude at all times during the computation. Received: 13 November 1995/Accepted in revised form: 20 May 1996     

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.     

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.     

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.     

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.     

Quantitative parameters of synaptic apparatus in the ventral nucleus of the medial geniculate body of the cat

The synaptic apparatus in the ventral nucleus of the medial geniculate body (MGBv) of the cat was examined using electron microscopy and stereological methods, which made it possible to measure the synaptic density. Within 7015 µm² of examined sections, 1586 presynaptic terminal (PST) profiles were found, which corresponds to 226.0·10³ PST per 1 mm² of section surface. The PSP were classified into five groups:RL,RS,F,P, andUT, in accordance with their ultrastructural pattern (dimension of PST profile, dimension and shape of synaptic vesicles, and type of synaptic contact, SC) [18–22]. On the above surface, there were 1012 SC formed by PST of different groups, which corresponds to 144.0·10³ SC per 1 mm² of section surface. TheRL-,RS-,F-,P-, andUT-type PST formed 14.8%, 50.1%, 13.1%, 16.8%, and 5.2% of analyzed SC, respectively. The calculated mean SC numerical density equalled (260.8±54.8)·10⁶ SC per 1 mm³ of fixed MGBv tissue. Among them, 40.2·10⁶ (15.4%) belonged toRL-PST, i.e., to axonal terminals of thecolliculus inferior neurons; 130.2·10⁶ (49.9%) toRS-PST, i.e., mostly to axonal terminals of the auditory cortex neurons; and 33.9·10⁶ (13.0%) toF-PST, i.e., to axons of the GABA-ergic interneurons and neurons of the perigeniculate division of the reticular thalamic nucleus. Group-P PST, i.e., terminal structures of the dendritic arborizations of interneurons, formed 42.7·10⁶ (16.4%) SC per 1 mm³, and 13.8·10⁶ (5.3%) SC belonged toUT-PST, i.e., to terminals of unidentified nature. Among 260.8·10⁶ SC in 1 mm³ of tissue, only 23.8·10⁶ (9% of total number)RL-SC, localized on the relay neurons, are directly involved in the MGBv relay function. All other SC transmit control influences from various structures of the nervous system, and provide adjustment of relay function to the constantly changing environmental conditions and varying status of an orgamism. The mean number of SC, localized on an averaged MGBv relay neuron, was calculated as 9100. Among them, about 1200 SC belong toRL-PST, 5200 SC toRS-PST, 1200 SC toF-PST, 1100 SC toP-PST, and 400 SC toUT-PST.Neirofiziologiya/Neurophysiology, Vol. 27, No. 3, pp. 208–219, May–June, 1995.     

A quantitative electron-microscopical study of the postnatal development of the lateral geniculate nucleus in normal kittens and in kittens with eyelid suture

The postnatal development of the lateral geniculate nucleus has been studied quantitatively with the electron microscope in normal kittens and in kittens with eyelid closure. The maturation of the synaptic organization of glomeruli in the normal kitten occurs during the period of susceptibility to eyelid closure and is due predominantly to a logarithmic increase in the number of symmetric presynaptic dendritic synapses. In contrast, the proportion of symmetric synapses falls with age in non-glomerular neuropil over this period. Unilateral and bilateral eyelid suture do not interfere with the normal development of the lateral geniculate nucleus.