Cytoarchitecture of the tectum opticum in the Japanese quail

Role of En2 in the tectal laminar formation of chick embryos

The chick optic tectum consists of 16 laminae. Here, we report contribution of En2 to laminar formation in chick optic tecta. En2 is specifically expressed in laminae g-j of stratum griseum et fibrosum superficiale (SGFS). Misexpression of En2 resulted in disappearance of En2-expressing cells from the superficial layers (laminae a-f of SGFS), where endogenous En2 is not expressed. Misexpression of En2 before postmitotic cells had left the ventricular layer indicated that En2-misexpressing cells stopped at the laminae of endogenous En2 expression and that they did not migrate into the superficial layers. Induction of En2 misexpression using a tetracycline-inducible system after the postmitotic cells had reached superficial layers also resulted in disappearance of En2-expressing cells from the superficial layers. Time-lapse analysis showed that En2-misexpressing cells migrated back from the superficial layers towards the middle layers, where En2 is strongly expressed endogenously. Our results suggest a potential role of En2 in regulating cell migration and positioning in the tectal laminar formation.

Morphological properties of tectal neurons that project to the nucleus geniculatus lateralis, pars ventralis (GLv) and the surrounding ventral thalamus in chicks

Layer 10 neurons of the chick tectum were morphologically investigated. The layer 10 neurons displayed heterogeneous immunoreactivities to calcium-binding proteins (CaBPs). Calbindin (CB)-immunoreactive (ir) neurons had pyramidal or round somata, primarily found in layers 5, 9, and 13. Parvalbumin (PV)-ir neurons were of various shapes with small to large somata (109.7±48.6μm(2)) that were located mainly in layers 4 and 10. Calretinin (CR)-ir neurons had small to middle-sized somata (79.3±9.7μm(2)) located primarily in layers 10 and 13, and most of them were similar to typical radial cells in size and shape. Two distinct types of neurons that projected to the nucleus geniculatus lateralis, pars ventralis (GLv) and ventral thalamus were demonstrated in layer 10. Type 1 cells had small to middle-sized somata (74.3±33μm(2)), and each cell had a single apical dendrite that ramified into bush-like branches in layer 7. These cells corresponded to CR-ir neurons and radial cells in size and shape. Type 2 cells had larger somata (124.7±52.6μm(2)), and their shapes were pyramidal, polygonal, or oval. They had multiple obliquely ascending dendrites that ramified into bush-like branches in layer 7. These cells often appeared similar to PV-ir neurons.

Calcium binding protein expression in the optic tectum of Alligator during development

The onset and distribution of the calcium binding proteins, calretinin, calbindin, and parvalbumin, were examined in the optic tectum of Alligator mississipiensis embryos between Stages 18 and 26-28. The immunoreactivity of each calcium binding protein correlated well with the results from the Western blot experiments. In terms of onset and distribution, calretinin expressison was the most widespread of the three calcium binding proteins that were examined, and was also the earliest to be visualized. Calbindin expression occurred next, whereas parvalbumin expression was the most limited and appeared last. For small calretinin (+) neurons, the pattern of immunoreactivity during development was from inside to outside, whereas for the larger cells, it was from outside to inside. For calbindin immunoreactive cells in the superficial zone, the pattern was from outside to inside. The distribution of the parvalbumin immunopositive neurons did not change significantly over the time period examined. Similar data on other amniotes is limited. However, the pattern in Alligator shares some similarities with kittens in regards to the distribution of calbindin and parvalbumin in the developing superior colliculus.

Monoaminergic markers in the optic tectum of the domestic chick

The avian optic tectum has become a reliable model system to study the basic mechanisms that underlie the computation of visual stimuli. Many aspects of its cytoarchitecture, chemoarchitecture, connectivity and development are thoroughly characterized. However, knowledge about its monoaminergic innervation is still incomplete. As a prerequisite to understand a possible functional role of the monoaminergic neurotransmitters, the serotonergic, noradrenergic, and dopaminergic innervation of the optic tectum as well as the distribution of serotonin 2A receptors, the dopamine- and cAMP-regulated phosphoprotein DARPP-32 and calbindin D-28K was studied in domestic chicks by immunohistochemical techniques. Serotonergic, noradrenergic, and tyrosine hydroxylase positive axons and axon terminals were present in all layers of the optic tectum. Generally, the highest densities of serotonergic, noradrenergic, and tyrosine hydroxylase positive fibers were found in the superficial tectal layers 1-8, whereas only moderate densities of serotonergic, noradrenergic, and tyrosine hydroxylase positive fibers became obvious in the deep tectal layers 9-15. Serotonergic fibers were particularly abundant in layers 4, 5a and 7 and serotonin 2A receptors in layer 13. Noradrenergic fibers were densest in layers 4 and 5a, whereas tyrosine hydroxylase positive fibers showed a slightly different distribution pattern with additional dense labeling in layer 7. As revealed by double-labeling immunohistochemistry, serotonergic fibers were closely related to the cell bodies of calbindin-positive horizontal cells in layer 5b and tyrosine hydroxylase positive fibers often contacted DARPP-32+ dendritic shafts in layers 9 and 10. These findings indicate that the catecholaminergic innervation of the optic tectum consists of a noradrenergic and a dopaminergic component and that the noradrenergic, serotonergic, and dopaminergic system may be potentially involved in the modulation of retinal input in the superficial layers of the optic tectum as well as in the modulation of tectal output via the deep tectal layers.

Differential effects of ocular BDNF-injections onto the development of tectal cells characterized by calcium-binding proteins in pigeons

The optic tectum of vertebrates bears a set of visual neurons which can be differentiated by the expression of distinct calcium-binding proteins (CaBPs). Using immunohistochemistry, we mapped the distribution of the CaBPs calbindin (CB) and parvalbumin (PV) in the pigeon's optic tectum and examined if their differentiation is affected by retinal brain-derived neurotrophic factor (BDNF)-injections. CB-immunoreactive (ir) and PV-ir cells displayed a lamination pattern which differed from other birds. While PV-ir cells were present in several retinorecipient tectal laminae, CB-ir cells were confined to layer 3 and 5 and - as a specialization of pigeons - were also detected in a subpopulation of layer 13 neurons. Comparison of saline- and BDNF-injected animals revealed that this general expression pattern was not affected by ocular BDNF-injections. In contrast, the size of tectal cells was differentially modulated. While CB-ir cells in layers 3 and 13 were unaffected by retinal BDNF, cells in layer 5 developed enlarged cell bodies. The PV-ir cells displayed smaller soma sizes within both tectal hemispheres suggesting also an indirect effect of retinal BDNF. These data indicate a differential sensitivity of tectal cell types to retinal BDNF, which might be one mechanism by which retinal input modulates tectal circuitries.

Cytoarchitecture of the avian optic tectum: neuronal substrate for cellular computation

To analyse cellular computation in the vertebrate brain, a thorough knowledge of the underlying anatomy, physiology and connectivity of the neuronal substrate is essential. This review compiles data on one of the best known structures of the vertebrate brain, the optic tectum of birds. The functions of this structure are multifold, but can be attributed largely to orientation and the basic analysis of sensory data in a spatial context. In the tectum, a wealth of data on physiology and anatomy has been gathered over more than a century and provides an excellent background for computational studies. The analysis of the optic tectum is facilitated by several principles of organisation, including the retinotopic input and the highly laminated layout with separated input and output layers. Moreover, the molecular mechanisms guiding the development and connectivity have been analysed in detail. As the avian tectum and the mammalian superior colliculus are partly homologous, the cellular mechanisms unraveled in the tectum can also be transferred to the colliculus and thus contribute to the understanding of the vertebrate visual system in general.

Neuronal differentiation patterns in the optic tectum of the lizard Gallotia galloti

This study examines in detail the sequences of morphological differentiation and deduces mode of migration into specific layers of all types of neurons present in the optic tectum of the lizard Gallotia galloti. It complements previous similar work on tectal histogenesis in the chick. It was found that the neuronal population diversity in the lizard tectum can be reduced by developmental analysis to three neuroblast classes, called Types I, II and III. These classes correspond closely to those present in the developing avian tectum. Neurons belonging to each developmental class were characterized by their initial polarity, mode of translocation into the mantle layer and pattern of sprouting of primary axonal and dendritic processes. Each class produced along time a subset of the cell types distinguished in the mature tectum. Some aspects of sauropsidian tectal histogenesis are also common of other vertebrates, suggesting that fundamental mechanisms of tectal neuronal differentiation are conserved in tetrapods. Analysis of evolutive differences of tectal structure points to changes affecting the layering and perhaps the population size of specific cell types. Whereas tectal cell-type homology can be easily fundamented on embryological evidence and seems to be consistent with hodological and, to some extent, functional homology, the periventricular, central and superficial strata of the tectum are heterogeneous in cellular composition in different species and therefore represent analogous, rather than homologous entities.

Anatomy and physiology of horizontal cells in layer 5b of the chicken optic tectum

In the visual midbrain of birds, a variety of cell types has recently been characterized with both anatomical and physiological techniques to gain insight into the mechanisms of visual information processing. Here we present data from a horizontal cell type located in the retinorecipient layer 5b of the chick optic tectum. Intracellular labeling revealed that these neurons are multipolar, have no axonal structures and arborize completely within the layer 5b where they extend over considerable distances. Immunohistochemistry with an antibody against calbindin labeled a population of horizontal cells in layer 5b; however, double labeling showed that these neurons represent a subpopulation of approximately one third of the neurons in that layer. Whole-cell patch recordings with additional cell filling from horizontal cells revealed that the physiological responses to depolarization changes with maturation, from a comparatively slow oscillatory pattern reminiscent of hair cell physiology at embryonal stages to a damped series of small action potentials at posthatching. In response to electrical stimulation in the vicinity of the neurons, cells responded with either excitatory postsynaptic potentials or small action potentials. Horizontal cell types are found in the visual midbrain of both avian and mammalian species. On the basis of the data presented here and data from the literature, the functional role of these cells is discussed. As in layer 5b of the chick optic tectum specific synaptic glomeruli have been found, the horizontal cells might constitute local inhibitory circuits within the retino-tectal synapses and, in addition, contribute to mechanisms of directional selectivity in these projections.

Development of retino-recipient projection neurons in the optic tectum of the chicken

The dendritic development of a well-characterized retino-recipient neuronal type in the chicken optic tectum has been traced with intracellular labeling. Normal dendritic development can be divided into three phases: extension, differentiation and pruning. During the first phase, cells extend their dendrites, generate large dendritic fields and position their distal endings in a certain retino-recipient tectal layer. In the second phase, these dendritic endings arborize into characteristic bottlebrush-like structures, while the overall morphology of the neurons remains unaltered. After hatching, the number and width of the bottlebrush endings are reduced. The findings are discussed with respect to the innervation of the optic tectum by retinal afferents and possible guidance mechanisms for synapse formation in this system.

Distribution of NADPH diaphorase-containing neurons in the pigeon central nervous system

The aim of the present study was to determine the distribution of nitric oxide-synthesizing neurons in the pigeon brain and spinal cord. Tissue sections were stained for reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d). In the telencephalon, intensely stained neurons with dendrites extending distally were seen in most regions. The ectostriatum was characterized by intensely and diffusely stained neuropil. In the diencephalon, intensely positive neurons were seen in the lateral hypothalamic region and lateral mammillary nucleus. In the mesencephalon, intensely stained, multipolar neurons were abundantly scattered in the central gray, nucleus intercollicularis, reticular formation, nucleus tegmenti pedunculo-pontinus, pars compacta, area ventralis of Tsai, and ansa lenticularis. In the rhombencephalon, positively-stained neurons were found in the pontine nuclei and reticular formation. The cerebellar cortex, except for Purkinje cells, was a preferential region for NADPH-d activity. Positive end-bulbs made contact on somata in the nucleus magnocellularis cochlearis. In the spinal cord, NADPH-d positive neurons were seen in layer II and the marginal nucleus. Our results demonstrated that the distribution of NADPH-d-containing neurons in the pigeon brain and spinal cord is more complex than in other avian species. Our findings indicate that NADPH-d-containing neurons are present in several sensory pathways, including olfactory, visual, auditory, and somatosensory tracts, although some nuclei in each system did not show NADPH-d activity. The wide distribution of NADPH-d activity in the pigeon CNS suggests that nitric oxide modulates sensory transmission in avian central nervous system.

Projection neurons in the superficial layers of the superior colliculus in the rat: a topographic and quantitative morphometric analysis

The present study deals with qualitative and quantitative investigations in the superior colliculus of the rat. Tracer studies were correlated with Nissl staining to calculate the quantitative ratio between projection neurons and interneurons in the upper three layers of the superior colliculus. In order to reveal the projections from the superior colliculus, the first group of rats received injections of the tracer FluoroGold into the nucleus lateralis posterior thalami, the lateral geniculate body, the nucleus parabigeminalis, and the predorsal bundle. Commissural connections between the superior colliculus were traced in a second group of animals, which received Biocytin and FluoroGold injections in the upper layers of the right superior colliculus and small deposits of the carbocyanine tracer DiI in the deeper layers of the left superior colliculus. Additionally, double-labelling with FluoroGold tracing and the histochemical detection of NADPH-diaphorase activity was carried out to distinguish between projection neurons and interneurons. These experiments showed that 66% of the neurons within the superficial layers of the superior colliculus were represented by ascending projection neurons, whereas only 2-3% could be identified as descending neurons. Ascending neurons were scattered throughout the three laminae and descending neurons were localized in a cluster-like pattern. Approximately 2-3% of the neurons in the superficial layers were found to be commissural and interlayer neurons which were represented by an identical cell type, since both transcommissural and interlayer processes were originated from their somata. The somata of these commissural-interlayer neurons were all located in the mediorostral part of the superior colliculus and contained NADPH-diaphorase activity. The axon terminals of the interlayer-commissural neurons formed net-like structures which surrounded neuronal somata within the ipsilateral deep layers and within the contralateral upper layers of the superior colliculus, respectively.