Serotonin-immunoreactivity in the monkey lateral geniculate nucleus

Summary Serotonin-immunoreactivity in the monkey lateral geniculate nucleus appears as a plexus of fine, beaded fibers decreasing in density from magnocellular to parvocellular laminae. Ultrastructurally, these fibers show strictures and dilations, and are filled with dense round particles as well as granular material attached to outer mitochondrial membranes and microtubules. Most of the profiles followed in serial sections lack morphologically defined synapses. The few synapses observed are asymmetric, some with subjunctional dense bodies. This appearance suggests a possible excitatory effect mainly on interneurons which in turn would inhibit principal cells. Serotonin released non-synaptically may block the delivery of transmitters from retinal terminals and/or the receptors for such transmitters, thereby exerting a modulatory depressing action on principal cells.     

Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey

Horseradish peroxidase was deposited in the optic nerve to retrogradely label and reveal the dendritic form of all classes of ganglion cell, or it was injected into the dorsal lateral geniculate nucleus to reveal only those classes projecting to the thalamus. The results were compared with those of the accompanying paper in which the ganglion cells projecting to the midbrain are selectively revealed. Two major classes of ganglion cells are described, the P alpha and P beta cells. For both classes dendritic field size increases with eccentricity from the fovea and there is no overlap in the two classes at any given eccentricity. Cell body size shows a similar mean difference but with a slight overlap. Both cell bodies and dendritic fields are larger along the temporal horizontal meridian than the nasal horizontal meridian, for P alpha and for P beta cells, but these differences are reduced when naso-temporal differences in ganglion cell density are taken into account, that is, size correlates closely with density. Injections restricted to the parvocellular layers of the lateral geniculate nucleus labelled almost exclusively P beta cells, whereas injections confined to the magnocellular layers labelled almost exclusively P alpha cells. As midbrain injections label no P beta cells and few P alpha cells it can be shown that about 80% of ganglion cells are P beta cells projecting to parvocellular lateral geniculate nucleus, and that about 10% are P alpha cells projecting to magnocellular layers. The coverage factor, that is the number of cells covering each point on the retina, varied from 1.9-2.3 for P beta cells, and from 2-7 for P alpha cells. Comparing the results with those of comparable investigations on cats and rabbits shows a much clearer segregation of the terminal targets of different classes of ganglion cell in monkeys, the greatest difference being the absence in the monkey of a projection to the geniculate from gamma- and epsilon-like cells. Further, axons which branch and innervate both thalamus and midbrain are rare in monkeys but common in other mammals. Comparing the results with those from physiological investigations suggests that the P beta cells correspond to colour-opponent cells, whereas P alpha cells correspond to the achromatic broad-band magnocellular cells.     

Synaptogenesis in the dorsal lateral geniculate nucleus of the rat

Summary Synapse formation and maturation were examined in the rat dorsal lateral geniculate nucleus (dLGN) from birth to adulthood. Examination of animals, whose ages were closely spaced in time, showed that the maturation of the synaptic organization of the nucleus takes place chiefly during the first 3 weeks of postnatal life. This period of maturation may be divided into 3 broad stages. During the first stage, which spans the first 4 days of life, there are only a few immature synapses scattered throughout the nucleus; occasionally aggregates of 3 or 4 synapses are encountered. Dendrodendritic synapses first appear at the end of this stage. The second stage, which lasts from the end of the first stage through day 8, is characterized by intensive synaptogenesis as well as extensive growth and degeneration. For the first time, large boutons resembling retinal terminals form multiple synaptic contacts with dendrites and dendritic protrusions; these synaptic arrangements are partially covered by glial processes.A feature characteristic of the developing dLGN during the first 2 postnatal weeks, and particularly during the second stage, is the presence of membrane specializations that resemble vacant postsynaptic densities. These specializations, which may be unapposed or opposite another neuronal process, decrease in frequency as the number of synapses increases. It is not known whether these densities are converted to synapses or whether they result from loss of presynaptic elements.The third stage in the process of synaptogenesis, which spans a period between days 10 and 20, is characterized by myelination and by the diminution of growth cones, degenerating profiles and vacant postsynaptic densities. There is also a very significant increase in the number and maturation of synapses including synaptic glomeruli. However, it is not until the end of this stage that synapses appear qualitatively indistinguishable from synaptic arrangements identified in adult animals.     

Orientation of dendrites in the lateral geniculate nucleus of the monkey

Summary Quantitative analysis of dendritic orientation in rapid Golgi sections of the lateral geniculate nucleus of Old World monkeys was performed with the help of a computer-microscope and a tree-analysing program. The orientation of intermediate and terminal dendritic segments of parvocellular and magnocellular multipolar neurons was measured at different ages in relationship to a reference line defined by the major direction of afferent fibres visualized in Golgi preparations and by silver staining. The direction of fibre bundles crossing the nucleus is approximately perpendicular to the layers near their apex, but more oblique laterally and medially. There is a clear tendency for terminal dendritic segments of both parvocellular and magnocellular neurons to be oriented preferentially along these fibres and fewer are perpendicular to them. The orientation of intermediate segments is much less clear, showing a more random orientation with regard to the fibres. There is no obvious influence of the order of branching or of the age of the animal (between late gestation and maturity) on the orientation of terminal and intermediate dendritic segments. A parallel study (Leuba and Garey 1984) suggests that there is greater plasticity in the terminal part of the dendritic tree, and it is possible that the maturation of afferent fibres causes these terminal segments to orient along them very early during normal development.     

Morphology of neurons in the lateral geniculate nucleus of the monkey

Summary Rapid Golgi preparations of the lateral geniculate nucleus of old- and new-world monkeys were analysed in an attempt to classify the neuronal types. Four main types are described. The commonest, the multipolar neurons, are found in all laminae. Their somata can be large, medium or small and bear dendrites with sparse spines. Some have a radiate dendritic arbor and others have dendrites grouped in tufts. The next most frequent class is of bipolar neurons with two thick dendrites arising from opposite poles of the soma, which is usually large. Otherwise the dendrites are similar to those of multipolar neurons. Relatively rare is a class of medium-sized neurons with beaded dendrites, found here only in magnocellular laminae. There is a fourth class of small neurons distinguished by fine axonlike dendritic processes. They are in all laminae and form two subgroups, one with very long, cylindrical dendrites and few axon-like processes, the other with shorter dendritic arbors and many axon-like processes. In addition, a class of capsular neurons is found in the circumgeniculate capsule between layer 6 and the pregeniculate nucleus. They are large neurons with umbrella-like dendritic arbors sending ramifications into layer 6. The interlaminar zones contain scattered somata of all types except beaded and capsular neurons.The work described in this paper forms part of a study for a doctoral dissertation in the University of Lausanne by K.D. Saini     

Dynamic properties of corticothalamic excitatory postsynaptic potentials and thalamic reticular inhibitory postsynaptic potentials in thalamocortical neurons of the guinea-pig dorsal lateral geniculate nucleus

The properties of postsynaptic potentials evoked by stimulation of cortical, retinal and GABAergic thalamic afferents were examined in vitro in thalamocortical neurons of the guinea-pig dorsal lateral geniculate nucleus. Brief trains of stimulation (2–10 stimuli) delivered to corticothalamic fibers led to a frequency-dependent increase in excitatory postsynaptic potential amplitude associated with an increase in activation of both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In addition, repetitive stimulation of corticothalamic fibers also gave rise to a slow excitatory postsynaptic potential that was blocked by local application of the glutamate metabotropic receptor antagonist α-methyl-4-carboxyphenylglycine. In contrast, repetitive stimulation of optic tract fibers resulted in monosynaptic excitatory postsynaptic potentials that did not potentiate and were not followed by the generation of a slow excitatory postsynaptic potential.Repetitive activation of the optic radiation also evoked both GABA_A and GABA_B receptor-mediated inhibitory postsynaptic potentials. These inhibitory postsynaptic potentials exhibited frequency-dependent depression during repetitive activation. The presence of frequency-dependent facilitation of corticothalamic excitatory postsynaptic potentials and frequency-dependent decrement of inhibitory postsynaptic potentials, as well as the ability of corticothalamic fibers to activate glutamate metabotropic receptors, suggests that sustained activation of corticothalamic afferents in vivo may result in postsynaptic responses in thalamocortical cells that are initially dominated by GABAergic inhibitory postsynaptic potentials followed by prominent monosynaptic excitatory postsynaptic potentials as well as a slow depolarization of the membrane potential.Therefore, the corticothalamic system may inhibit or enhance the excitability and responsiveness of thalamocortical neurons, based both on the spatial and temporal features of thalamocortical interactions.     

Binocular interaction in the dorsal lateral geniculate nucleus of the cat

Summary We have investigated binocular interaction in the dorsal lateral geniculate nucleus (LGN) of the cat. Neurons were recorded extracellularly during visual stimulation with sinusoidal gratings which were presented at different interocular phases (disparities). The large majority of cells (91%) exhibited some type of binocular interaction. For 75% and 16% of the total number of cells, the binocular interaction was inhibitory or facilitatory, respectively. For the remaining 9% of cells, no interaction was evident. In marked distinction from visual cortex, the facilitatory and inhibitory interactions in the LGN are independent of the relative interocular phase of the patterns. Neurons in the LGN are therefore insensitive to the stereoscopic depth cue, retinal disparity.     

Stereological evaluation of neurons and glia in the monkey dorsal lateral geniculate nucleus following an early cerebral hemispherectomy

The effects of an early, unilateral cerebral hemispherectomy on the cytoarchitecture of the dorsal lateral geniculate nucleus (dLGN) were quantitatively evaluated in the green monkey. The dLGN ipsilateral to the lesion showed a 73% reduction in size, more than 99% neuronal cell loss, 50% increase in glial cell density, but a 50% reduction in the total number of glial cells. The total number of neural and glial cells estimated for the dLGN contralateral to the ablation did not differ from control values. Despite evidence for substantial degeneration of the ipsilateral dLGN, cytochrome oxidase histochemistry revealed a small population of surviving cells that exhibited features of neuronal cells. More surviving cells were found in the parvocellular than in the magnocellular layers, and surviving parvocellular cells had the same size-frequency distribution as Nissl-stained neurons in an intact animal. These findings suggest that the intrinsic geniculate circuitry may be able to sustain the residual interneurons that can, in turn, contribute to maintaining retinal and brainstem afferents. The remaining neurons in the dLGN following hemispherectomy appear to be insufficient in number to be importantly implicated in the residual visual functions that have been reported in some hemispherectomized patients.     

On the ontogenetic development of the rat dorsal lateral geniculate nucleus

Summary The morphology and synaptic input of four geniculo-cortical relay neurons of albino rats of postnatal day (PD) 7 has been studied with light and electron microscopy, utilizing the combined Golgi-EM technique.Although the nerve cells investigated did show a characteristic relay-cell appearance of the adult, certain dendritic structures exhibited immature character. These were the dendritic swellings, or growth buds occurring throughout the whole length of the dendrites, but particularly along the segments and at bifurcations of thin distal dendritic portions. These dendritic thickenings together with the transient spine-like hairy processes were seen to receive many synaptic endings of various developmental stages. The majority of synaptic junctions both on the dendritic swellings and on dendritic shafts were of the asymmetric type. Contrary to our expectation, no proximo-distal gradient could be demonstrated in the maturation of axodendritic synapses, at least not during the first synaptogenetic period, i.e., at PD 7. Also, all axosomatic synapses in the PD7 rat LGNd were asymmetric, indicating that the replacement of asymmetric synapses by symmetric ones (characteristic for the adult stage) occurs only after the first synaptogenetic period.The rarity of Gray II type, symmetric contacts during the early synaptogenesis (i.e., at PD 7) explains the absence of triadic, or serial synapses which will develop only later, during the second synaptogenetic period.     

Postnatal growth of the dorsal lateral geniculate nucleus of the cat

Summary The postnatal growth of the dorsal part of the lateral geniculate nucleus (LGNd) is studied in paraffin sections through the brains of 32 cats of known age. The changes in shape and position of the LGNd are described and it is shown that its volume increases from about 3.4 mm³ at birth to about 26.4 mm³ in the adult cat. When this value is corrected for shrinkage, the volume of the LGNd in the adult cat turns out to be about 44 mm³. The detailed measurements reveal that during the second and third week of postnatal life there is a particularly steep increase in volume and that the final values are already reached at around the 40th day. Concomitant with the increase in volume there is a decrease of the number of cells per unit volume of grey matter. In the binocular segment of lamina A the number of cells decreases from about 470 per (0.1 mm)³ at birth to between 95 and 130 per (0.1 mm)³ in the adult cat. Separate measurements of nerve cells and neuroglial cells indicate that the absolute number of nerve cells remains fairly constant during postnatal life, whereas between the second and sixth week a great number of neuroglial cells are newly formed.Dedicated to Professor W. Bargmann, Kiel, on the occasion of his seventieth birthday     

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster

Summary The synaptic organization of the sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase.Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains (i) exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; (ii) pre- and postsynaptic pleomorphic-vesiclecontaining P-boutons (interpreted as appendages of the dendrites of interneurons); (iii) large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; (iv) F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synatpic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus.No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.     

Sources of subcortical afferents to the macaque's dorsal lateral geniculate nucleus

Background: The dorsal lateral geniculate nucleus (dLGN) is the thalamic region responsible for transmitting retina signals to cortex. Brainstem pathways to this nucleus have been described in several species and are believed to control the retinocortical pathway depending on the state of the animal (awake, asleep, drowsy, etc.). The purpose of this study was to determine all of the subcortical sources of afferents to the dLGN in a higher primate, the macaque monkey, whose visual system is similar to that of humans. Methods: Injections of horseradish peroxidase (HRP), with or without conjugation to wheat germ agglutinin, were made into the dLGNs of seven macaque monkeys, followed by perfusion, brain sectioning, and analyses of neurons in the brainstem, thalamus, and hypothalamus that contained the retrogradely transported marker. Results: The reticular nucleus of the thalamus, pedunculopontine nucleus, parabigeminal nucleus, pretectal nucleus of the optic tract, superior colliculus, dorsal raphe nucleus, and tuberomammillary region of the hypothalamus contained many retrogradely labeled neurons ipsilateral to the injections. In the contralateral brainstem, HRP-labeled cells were found only in the pedunculopontine nucleus, nucleus of the optic tract, and dorsal raphe nucleus. The number of labeled neurons on the contralateral side was about one-half of that in corresponding ipsilateral nuclei. The locus coeruleus contained no labeled neurons in four of the macaques that had injections limited to the dLGN. Conclusion: There are seven subcortical regions that send afferents to the dLGNs of macaque monkeys. Except for the locus coeruleus, these are the same as observed for other species, such as the cat and rat, and indicate the possible sources of subcortical control over the dLGNs of humans. © 1995 Wiley-Liss, Inc.     

Correlation analysis of units recorded in the cat dorsal lateral geniculate nucleus

Spike activity was simultaneously recorded from pairs of units consisting of both optic tract fibers and relay cells in the cat dLGN. The unit pairs were classified with regard to the types of receptive fields involved, the relative location of receptive fields, and their occular drive. Statistical dependencies between the discharge patterns of the simultaneously observed units were assessed by computing their auto- and crosscorrelogram during a period of maintained discharge. Two distinctly different types of statistical dependence were observed. Unit pairs having like and overlapping receptive field centers exhibited statistical dependencies manifested in their crosscorrelograms by a peak near the origin. Conversely, unit pairs having opposite and overlapping receptive field centers were characterized by a prominent valley in their crosscorrelograms near the origin. These statistical dependencies are inferred to represent functional interaction most probably occurring in the retina and characterized by a common element exerting an influence on both units.     

A ventral lateral geniculate nucleus projection to the dorsal thalamus and the midbrain in the cat

Summary Retrograde tracing experiments using horseradish peroxidase (HRP) have been utilized for demonstrating the origin of efferent projections of the ventral lateral geniculate nucleus (LGNv) in the cat. HRP-positive cells identifiable as origins of thalamic projections were found in LGNv after injections of HRP into the lateral central intralaminar nucleus. The labeled cells appeared concentrated in the medial part of the internal division of LGNv, consisting of medium-sized multipolar cells. Contralaterally, fewer labeled cells were present in the corresponding part of LGNv. In the case of injections of HRP into the midbrain (pretectum and superior colliculus), labeled cells in LGNv were distributed almost exclusively in its external division, composed of mainly small cells. Little overlap of the distribution of HRP-positive cells was seen in LGNv between the thalamic and midbrain injection cases.Abbreviations Ad Dorsal anterior nucleus - Am Medial anterior nucleus - Av Ventral anterior nucleus - BSC Brachium of superior colliculus - Cg Central gray - Cl Lateral central nucleus - Ld Dorsal lateral nucleus - LGNd Dorsal lateral geniculate nucleus - LGNv Ventral lateral geniculate nucleus - Lp Posterior lateral nucleus - Md Dorsal medial nucleus - NIII Oculomotor complex - NOT Nucleus of the optio tract - NPC Nucleus of posterior commissure - OT Optic tract - P Posterior nucleus (Rioch 1929) - Pc Paracentral nucleus - Po Posterior group of thalamic nuclei - Pt Parataenial nucleus - PTa Anterior pretectal nucleus - PTm Medial pretectal nucleus - PTp Posterior pretectal nucleus - Pul Pulvinar - R Red nucleus - Rt Thalamic reticular nucleus - Sg Suprageniculate nucleus - Va Anterior ventral nucleus - VI Lateral ventral nucleus - Vm Medial ventral nucleus - Vpl Posterolateral ventral nucleus - Vpm Posteromedial ventral nucleus - Zi Zona incerta - II Layer of superior colliculus - III Layer of superior colliculus - IV (Kanaseki and Sprague, 1974)     

The projection from the dorsal lateral geniculate nucleus of the thalamus to extrastriate visual association cortex in the macaque monkey

Both autoradiographic and horseradish peroxidase tracing techniques were used to characterize a projection from the dorsal lateral geniculate (DLG) nucleus of the thalamus to visual association cortex (extrastriate cortex) in the macaque monkey. The results show that medium to large caliber DLG axons end to discontinuous terminal "patches" in layers V and lower IV of extrastriate cortex. There is a topographical organization to these projections which are mainly to area 19 and anterior 18 located on the lateral and medial surfaces of the hemisphere. Apparently there are no DLG projections to any of the cortical subdivision located within the lunate and superior temporal sulci.     

Stereological changes in the capillary network of the aging dorsal lateral geniculate nucleus

In this work we studied the effect of aging on the capillaries of the dorsal lateral geniculate nucleus in 3-, 18-, 24-, and 28-month-old rats. The parameters analyzed were the capillary profile density, capillary volume fraction, length and surface area per unit volume, and capillary average diameter. The quantitative analysis showed in all parameters an increase between 3 and 18 months, and a significant decrease in capillary volume fraction (-18.75%) and diameter (-5.5%) between 18 and 24 months. No changes from 24 months onwards were observed. The increase observed in capillary profile density and capillary volume fraction between 3 and 18 months may indicate an increase of the capillary network. Furthermore, we observed an increase in the length and surface area per unit volume, which we interpret as an expansion of the exchange surface between blood and nerve tissue. The reduction in the capillary parameters that takes place between 18 and 24 months is slight, and may indicate the onset of decline characteristic of aging.