The internal structure of the nucleus geniculatus lateralis ventralis in the avian brain: a Golgi study and electron microscopic investigation.

Different types of neurons in the ventral geniculate nucleus of the thalamus of chicks were visualised by Golgi impregnation. The dendritic tree of projection neurons branched in a sphere-like territory in both the ventral and middle areas of the lamina externa. The dendrites of projection neurons in the lamina interna descended into the lamina externa and entered both the ventral and middle dendritic areas. One or two dendrites of the lamina interna neurons also emitted branches that developed a dorsal sphere-like dendritic territory. Optic terminals labelled by Golgi impregnation or injection of biotinylated dextran amine were found in these dendritic territories gathered into groups. They established synapses in these areas (synaptic islands or fields without a glial sheath) with different dendritic profiles, and a few gamma-aminobutyric acid (GABA)-positive terminals synapsed with them. No glomerulus-like synaptic complexes ensheathed by glial processes were found. Optic terminals also contacted the stem dendrites of projection neurons and GABA-positive neuron cell bodies and dendrites. Numerous synapses established by both optic and GABA-positive terminals were found on the proximal dendritic stems of the lamina interna projection neurons.

Anterograde tracer study on the nucleus geniculatus dorsalis and its internal synaptic structure in chick brain.

In the present study the terminals of retinal fibres and those of internal layer cells in ventral geniculate nucleus of chicks were labelled with the anterograde tracer biotinylated dextran amine. The tracer showed the connections from the internal cell layers of ventral geniculate nucleus to the medial part of the dorsal lateral geniculate nucleus. The labelled retinal terminals were located exactly in the lateral part of nucleus. The labelled terminals in the two parts of the nucleus were analysed with the electron microscope and showed a different synaptic organisation in the two parts of the dorsal lateral geniculate nucleus. In the lateral part, two kinds of synaptic glomeruli were found mostly in the vicinity of large dendrites, which are proximal dendrites of projection neurons. One type is a simple glomerulus containing a large dendrite, a large optic terminal and a large and/or series of asymmetrical synapses surrounded by glial processes. The other type is a complex synaptic unit with several pre- and postsynaptic components, among them synapses of GABA-positive axon terminals and/or dendraxons. No glomeruli were found in the medial part of the nucleus. In the medial part of the lateral geniculate nucleus, the terminals of internal layer cell axons established asymmetrical synapses with dendrites. Often, a large terminals and large dendritic profiles established serial asymmetrical synapses. GABA-positive myelinated fibres entered and ramified in both parts of the dorsal lateral geniculate nucleus, and GABA-positive terminals were seen to form synapses on the same dendrite near to the asymmetrical contacts. To our knowledge, this is the first report of the connection from ventral geniculate internal layer cells to the dorsal lateral geniculate nucleus in the chick.

The intrinsic organization of the nucleus lentiformis mesencephali magnocellularis: a light- and electron-microscopic examination.

The nucleus lentiformis mesencephali magnocellularis (nLMmc) is an essential part of the accessory optic nuclei and is responsible for stabilization of the horizontal eye movement. The morphology of this nucleus and its intrinsic structural connectivity were studied with Golgi, biotinylated dextran amine anterograde immunotracer and GABA immunostaining methods by light and electron microscopy. In the Golgi preparations neurons of large, medium-large, medium and small sizes were distinguished. The small neurons are GABA-immunopositive local circuit neurons, the others are proposed to be partly projection, partly local circuit neurons. The large and medium-large projection neurons are located in a tight topographical relationship observed in the Golgi preparations. The dendrites of the large and medium-large cells are also observed to be in close proximity with each other, and also with retinal fibre terminals. The morphological arrangement suggests that the retinal fibres make synaptic contacts with dendrites from both types of cell, and this is confirmed by the examination of retinal fibre terminals using electron microscopy. The optic fibre terminals establish synaptic contacts with small dendritic branches, dendritic processes and dendritic spines of large and medium-large neurons in the nLMmc. This arrangement allows the two types of nLMmc neuron access to very similar, if not identical, inputs, which may facilitate some of the different aspects of visual processing. Optic transmission by these cells may be modulated by the GABA-immunopositive terminals from various local circuit neurons, and very probably from GABAergic myelinated fibres as well, which may originate from the contralateral nLMmc and/or the visual Wulst.

The ramifications and terminals of optic fibres in layers 2 and 3 of the avian optic tectum: a golgi and light and electron microscopic anterograde tracer study.

The ramification patterns and terminals of optic fibres in layers 2 and 3 of the optic tectum were studied in Golgi-stained and immunolabelled preparations made from the brains of chicks and pigeons. The different neuronal structures of layers 2 and 3 were also examined. In Golgi preparations, two types of optic fibre were found both in chick and pigeon tectum according to their thickness and terminal branching patterns. The same types of optic fibres were also found to be present in the anterograde tracer experiments after iontophoresis of biotinylated dextran amine into the optic nerve. The varicose terminals of thin fibres mostly terminated on terminal dendritic sections of radiate and pyramidal-like neurons, contacting them on their apical dendrites. The medium-thick fibre terminals in layer 2 mainly established synapses with horizontally extending dendrites, which may therefore be contacts with inhibitory local circuit neurons. The medium-thick optic fibre bushy-like arborisation in layer 3 established synapses with larger dendrites and also stem dendrites. Their terminals formed groups with different dendritic profiles, some of which were partly covered by glial processes, and/or were located among converging dendrites. The presence of these glomerular-like synapses in layer 3 proves that the optic terminals in layer 3 also take part in the transmission of optic impulses to the nucleus rotundus.

The ramification and connections of retinal fibres in layer 7 of the domestic chick optic tectum: a golgi impregnation, anterograde tracer and GABA-immunogold study.

Layer 7 is one of the retinorecipient layers of the avian optic tectum. However, little information is available about the neuronal organization of this layer and its implications for visual function. Golgi impregnation was used to investigate the retinal input to and the neuronal architecture of layer 7 of the chick optic tectum, which forms a narrow band between the two cell-dense layers 6 and 8. Anterograde tracers were also used to investigate the afferent and efferent connections of layer 7, in both the light and the electron microscope, together with GABA immunogold labelling. Three types of radial neuron were defined according to the origin and course of their axons. The perikarya of these neurons were situated in tectal layers 10-11. The principal dendrites of these radial neurons ascended to the tectal surface and gave rise to dendritic side-branches in layer 7. These dendritic side-branches received asymmetric synapses from the terminations of retinal fibre arborisations. Type 2 radial neurons, whose axons arose from the deep pole of the perikaryon or occasionally from a basal dendrite, were shown to project to the nucleus isthmi pars magnocellularis, which has previously been demonstrated to be GABAergic and to project to glomerulus-like complexes in tectal layers 4-5. In these layers, the dendritic branches of layer 13 neurons that project to the nucleus rotundus have previously been shown to receive retinal fibre input. Therefore, the retinal input to layer 7 may be able to modulate the transmission of information to the visual thalamus, by way of a feed-back loop to layers 4-5 of the tectum involving the nucleus isthmi pars magnocellularis.

The intrinsic neuronal organisation of the nucleus of the basal optic root in the domestic chicken; a light and electron microscopic study using anterograde tracers and postembedding GABA-immunostaining.

The intrinsic neuronal organisation in the nucleus of the basal optic root of chickens was investigated. The divergent connections with various areas and the functional complexity of the nucleus require a complex intrinsic structural arrangement. Therefore, an analysis of Golgi impregnated material, ultrastructure, GABA-immunocytochemistry and biotinylated dextran-amine anterograde tracer analysis of the nucleus was carried out. In the Golgi analysis, a characteristic dendritic ramification pattern of two types of putative projection neurons was observed. These neurons form dendritic nests with their overlapping dendritic terminal sections, that develop synaptic fields with the optic fibre terminals. These synaptic fields were confirmed by electron microscopy. GABA-immunopositive terminals synapse with distinct loci of the dendritic trees of projection neurons; they may therefore play an important role in the inhibitory-modulatory system of the nucleus of the basal optic root. The GABA-immunopositive terminals derive from small and/or elongated local circuit neurons which receive retinal afferents, and from myelinated fibres afferents to the nucleus of unknown origin.