Distribution of GABA-like immunoreactive neurons in the optic lobes of Periplaneta americana

Summary Specific antisera against protein-conjugated -aminobutyric acid (GABA) were used in immunocytochemical staining procedures to study the distribution of the putative GABA-like immunoreactive neurons in the optic lobes of Periplaneta. GABA-like immunoreactive structures are evident in all three optic neuropil regions. Six different populations of GABAergic neurons, whose perikarya are grouped around the medulla, are found within the optic lobe. The number of these immunoreactive cells varies greatly and corresponds to the number of ommatidia of the eye. In the proximal part of the lamina, a coarse network of GABA-positive fibres is recognizable. These are the processes of large field tangential cells whose fibres pass through the distal surface of the medulla. A second fibre population of the lamina is made up of the processes of the centrifugal columnar neurons whose perikarya lie proximally to the medulla. The medulla contains 9 layers with GABAergic elements of variable immunoreactivity. Layers 1, 3, 5, 7 and 9 exhibit strong labelling, as a result of partial overlapping of the processes of centrifugal and centripetal columnar neurons, tangential fibres and/or lateral processes of perpendicular fibres and (possibly) processes of amacrines. A strong immunoreactivity is found in the proximal and distal layers of the lobula.     

Postembryonic development of serotonin-immunoreactive neurons in the central nervous system of the blowfly,Calliphora erythrocephala

Summary The postembryonic development of serotonin-immunoreactive (5-HTi) neurons was studied in the optic lobe of the blowfly. In the adult fly there are 24 5-HTi neurons invading each optic lobe. The perikarya of two of these neurons are situated in the dorso-caudal part of the protocerebrum (LBO-5HT neurons; large bilateral optic lobe 5-HTi neurons). The cell bodies of the remaining 22 neurons are located anteriorly at the medial base of the medulla (2 innervating the lobula, LO-5HT neurons; and 20 neurons innervating the medulla, ME-5HT neurons). The two central neurons (LBO-5HT neurons) are derived from metamorphosing larval neurons, while the ME- and LO-5HT neurons are imaginai optic lobe neurons differentiating during pupal development.The 5-HTi neurons of the optic lobe seem to have different ancestors. The LBO-5HT neurons are probably derived from segmental protocerebral neuroblasts, whereas the ME-and LO-5HT neurons are most likely derived from the inner optic anlage. The first 5-HTi fibers to reach the imaginal optic lobes are seen in the late third instar larva and are derived from the LBO-5HT neurons. The first ME- and LO-5HT neurons become immunoreactive at 24 h (10%) pupal development. At about 96 h (40%) of pupal development all the 5-HTi neurons of the optic lobes have differentiated and attained their basic adult morphology. The further development mainly entails increase in volume of arborizations and number of finer processes. The differentiation and outgrowth of 5-HTi processes follows that of, e.g., columnar neurons in the optic lobe neuropils. Hence, 5-HTi processes invade neuropil relatively late in the differentiation of the optic lobe.     

The synaptic organization of visual interneurons in the lobula complex of flies

Summary The synaptic organization of three classes of cobalt-filled and silver-intensified visual interneurons in the lobula complex of the blowfly Calliphora (Col A cells, horizontal cells and vertical cells) was studied electron microscopically. The Col A cells are regularly spaced, columnar, small field neurons of the lobula, which constitute a plexus of arborizations at the posterior surface of the neuropil and the axons of which terminate in the ventrolateral protocerebrum. They show postsynaptic specializations in the distal layer of their lobula-arborizations and additional presynaptic sites in a more proximal layer; their axon terminals are presynaptic to large descending neurons projecting into the thoracic ganglion. The horizontal and vertical cells are giant tangential neurons, the arborizations of which cover the anterior and posterior surface of the lobula plate, respectively, and which terminate in the perioesophageal region of the protocerebrum. Both classes of these giant neurons were found to be postsynaptic in the lobula plate and pre- and postsynaptic at their axon terminals and axon collaterals. The significance of these findings with respect to the functional properties of the neurons investigated is discussed.     

A Golgi study of the isthmic nuclei in the pigeon (Columba Iivia)

Summary The isthmic nuclei of the pigeon were studied with the use of three different Golgi techniques. The nucleus isthmo-opticus (IO) consists of a single cell type in which all dendrites of one neuron take the same direction and ramify at identical distances from the perikaryon to form dense dendritic arborizations. The cell bodies of the IO neurons form two parallel layers. The dendrites of these neurons always extend to the area between the two layers so that the dendritic arborizations of opposite neurons overlap. A model of the cellular organization of the IO was constructed based upon these morphological characteristics. The neurons of the n. isthmi/pars parvocellularis (Ipc) have oval perikarya and long, smooth, infrequently branching dendrites. All neurons except those at the borders of the nucleus show the same dorsoventral orientation in their dendritic arborizations and together with their afferents seem to have a columnar organization. The dendrites of the neurons located at the margin of the nucleus ramify within the Ipc along its border. The n. semilunaris (Slu) consists of neurons with round somata that have on an average three dendrites with small spines. The axons leave the nucleus from the medial side and join the lemniscus lateralis. The neurons of the n. isthmi/pars magnocellularis (Imc) comprise a generalized isodendritic type resembling the cells of the reticular formation. Axons from the tectum penetrate the nucleus, making numerous en-passant contacts with several neurons.     

The optic lobes of Lepidoptera

Variants of the Golgi-Colonnier (1964) selective silver procedure have been used to show up neurons in insect brains. Neural elements are particularly clearly impregnated in the optic lobes. Three classes of nerve cells can be distinguished; perpendicular (class I), tangential (class II) and amacrine cells (class III). There are many types of neurons in each class which together have a very wide variety of form. Their components are related to specific strata in the optic lobe regions. Short visual cells from the retina terminate in the lamina in discrete groups of endings (optic cartridges). Pairs of long visual fibres from ommatidia pass through the lamina and end in the medulla. Class I cells link these two regions in parallel with the long visual fibres and groups of these elements define columns in the medulla. These in turn give rise to small-field fibres that project to the lobula complex. Tangential processes intersect the parallel arrays of class I cells at characteristic levels. Some are complex in form and may invade up to three regions. Another type provides a direct link between the ipsi- and contralateral optic lobe. Amacrine cells are intrinsic to single lobe regions and have processes situated at the same levels as those of classes I and II cells. A fifth optic lobe region, the optic tubercle, is connected to the medulla and lobula and also receives a set of processes from the mid-brain. There are at least six separate types of small-field relays which could represent the retina mosaic arrangement in the lobula.     

Insect optic lobe neurons identifiable with monoclonal antibodies to GABA

Five monoclonal antibodies against GABA were tested on glutaraldehyde fixed sections of optic lobes of three insect species, blowflies, houseflies and worker bees. The specificity of these antibodies was analyzed in several tests and compared with commercially available anti-GABA antiserum. A very large number of GABA-like immunoreactive neurons innervate all the neuropil regions of these optic lobes. Immunoreactive processes are found in different layers of the neuropils. The immunoreactive neurons are amacrines and columnar or noncolumnar neurons connecting the optic lobe neuropils. In addition some large immunoreactive neurons connect the optic lobes with centers of the brain. Some neuron types could be matched with neurons previously identified with other methods. The connections of a few of these neuron types are partly known from electron microscopy or electrophysiology and a possible role of GABA in certain neural circuits can be discussed.     

Anatomical and physiological observations on the organization of mechanoreceptors and local interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In the wandering spider Cupiennius salei, the functional neuroanatomy of leg mechanosensory receptor neurons and interneurons associated with a single leg neumere was investigated by combined intracellular recording and Lucifer yellow ionophoresis. Trichobothria axons that selectively respond to air currents and to low-frequency airborne vibrations have arborizations restricted to ventral regions of the appropriate leg neuromere. Receptor afferents that respond selectively to substrateborne vibrations are distributed ventrally in the corresponding leg neuromere and extend into certain interganglionic tract neuropiles. Golgi impregnation and intracellular dye filling show that local interneurons originate in ventral sensory neuropiles of leg neuromeres and ascend dorsally to terminate amongst dendrites of motor neurons. Local interneurons generally show higher thresholds for vibration stimuli than do receptors. Local interneurons typically receive inputs from one or several types of receptors. Some respond to stimulation of a single leg, others respond to stimulation of several legs on the same side of the body. The functional morphology of the receptor afferents is correlated with known physiological characteristics of slit sensilla and trichobothria. Structure and activity of the local interneurons are compared with analogous interneurons in other arthropods.     

Insect optic lobe neurons identifiable with monoclonal antibodies to GABA

Summary Five monoclonal antibodies aginst GABA were tested on glutaraldehyde fixed sections of optic lobes of three insect species, blowflies, houseflies and worker bees. The specificity of these antibodies was analyzed in several tests and compared with commercially available anti-GABA antiserum.A very large number of GABA-like immunoreactive neurons inncrvate all the neuropil regions of these optic lobes. Immunoreactive processes are found in different layers of the neuropils. The immunoreactive neurons are amacrines and columnar or noncolumnar neurons connecting the optic lobe neuropils. In addition some large immunoreactive neurons connect the optic lobes with centers of the brain.Some neuron types could be matched with neurons previously identified with other methods. The connections of a few of these neuron types are partly known from electron microscopy or electrophysiology and a possible role of GABA in certain neural circuits can be discussed.     

A novel wide-field neuron with branches in the lamina of the <Emphasis Type="Italic">Drosophila</Emphasis> visual system expresses myoinhibitory peptide and may be associated with the clock

Although neuropeptides are widespread throughout the central nervous system of the fruifly Drosophila, no records exist of peptidergic neurons in the first synaptic region of the visual system, the lamina. Here, we describe a novel type of neuron that has wide-field tangential arborizations just distal to the lamina neuropil and that expresses myoinhibitory peptide (MIP). The cell bodies of these neurons, designated lateral MIP-immunoreactive optic lobe (LMIo) neurons, lie anteriorly at the base of the medulla of the optic lobe. The LMIo neurons also arborize in several layers of the medulla and in the dorso-lateral and lateral protocerebrum. Since the LMIo resemble LN_v clock neurons, we have investigated the relationships between these two sets of neurons by combining MIP-immunolabeling with markers for two of the clock genes, viz., Cryptochrome and Timeless, or with antisera to two peptides expressed in clock neurons, viz., pigment-dispersing factor and ion transport peptide. LMIo neurons do not co-express any of these clock neuron markers. However, branches of LMIo and clock neurons overlap in several regions. Furthermore, the varicose lamina branches of LMIo neurons superimpose those of two large bilateral serotonergic neurons. The close apposition of the terminations of MIP- and serotonin-producing neurons distal to the lamina suggests that they have the same peripheral targets. Our data indicate that the LMIo neurons are not bona fide clock neurons, but they may be associated with the clock system and regulate signaling peripherally in the visual system.     

A catecholaminergic neuron connecting the first two optic neuropiles (Lamina ganglionaris and Medulla externa) of the crayfish Pacifastacus leniusculus

Summary The crustacean optic neuropiles, the lamina ganglionaris and especially the medulla externa, show a specific pattern of green fluorescence with the fluorescence histochemical method of Falck-Hillarp. Normally, only the terminals and the cell bodies fluoresce, but in reserpine-treated animals exogenous catecholamines are taken up by the whole adrenergic neuron and are thus visualized as a whole. Incubating crayfish optic neuropiles in dopamine or -methylnoradrenaline after reserpine treatment demonstrated a tangential neuron connecting the lamina and the medulla externa. The morphology of this tangential neuron differs from the two types of tangential neurons, Tan₁ and Tan₂, previously characterized with Golgi techniques. The catecholaminergic neuron thus constitutes a third tangential neuron type. Acknowledgement. The present study has been supported by the Swedish Natural Science Research Council, grant B 2760-009, the Magnus Bergvall foundation, and the Swedish Medical Research Council, grant 04X-712, the latter to Prof. Bengt Falck to whom we extend our gratitude. We are also indebted to Mrs. Rita Wallén and Miss Maria Walles for their skilled technical assistance. Reserpine (Serpasil^®) was generously given to us by Hässle-Ciba-Geigy AB     

Neural action fields for optic flow based navigation: a simulation study of the fly lobula plate network

Optic flow based navigation is a fundamental way of visual course control described in many different species including man. In the fly, an essential part of optic flow analysis is performed in the lobula plate, a retinotopic map of motion in the environment. There, the so-called lobula plate tangential cells possess large receptive fields with different preferred directions in different parts of the visual field. Previous studies demonstrated an extensive connectivity between different tangential cells, providing, in principle, the structural basis for their large and complex receptive fields. We present a network simulation of the tangential cells, comprising most of the neurons studied so far (22 on each hemisphere) with all the known connectivity between them. On their dendrite, model neurons receive input from a retinotopic array of Reichardt-type motion detectors. Model neurons exhibit receptive fields much like their natural counterparts, demonstrating that the connectivity between the lobula plate tangential cells indeed can account for their complex receptive field structure. We describe the tuning of a model neuron to particular types of ego-motion (rotation as well as translation around/along a given body axis) by its 'action field'. As we show for model neurons of the vertical system (VS-cells), each of them displays a different type of action field, i.e., responds maximally when the fly is rotating around a particular body axis. However, the tuning width of the rotational action fields is relatively broad, comparable to the one with dendritic input only. The additional intra-lobula-plate connectivity mainly reduces their translational action field amplitude, i.e., their sensitivity to translational movements along any body axis of the fly.     

Comparative anatomy of pigment-dispersing hormone-immunoreactive neurons in the brain of orthopteroid insects

Summary In a comparative study, the anatomy of neurons immunoreactive with an antiserum against the crustacean -pigment-dispersing hormone was investigated in the brain of several orthopteroid insects including locusts, crickets, a cockroach, and a phasmid. In all species studied, three groups of neurons with somata in the optic lobes show pigment-dispersing hormone-like immunoreactivity. Additionally, in most species, the tritocerebrum exhibits weak immunoreactive staining originating from ascending fibers, tritocerebral cells, or neurons in the inferior protocerebrum. Two of the three cell groups in the optic lobe have somata at the dorsal and ventral posterior edge of the lamina. These neurons have dense ramifications in the lamina with processes extending into the first optic chiasma and into distal layers of the medulla. Pigment-dispersing hormone-immunoreactive neurons of the third group have somata near the anterior proximal margin of the medulla. These neurons were reconstructed in Schistocerca gregaria, Locusta migratoria, Teleogryllus commodus, Periplaneta americana, and Extatosoma tiaratum. The neurons have wide and divergent arborizations in the medulla, in the lamina, and in several regions of the midbrain, including the superior and inferior lateral protocerebrum and areas between the pedunculi and -lobes of the mushroom bodies. Species-specific differences were found in this third cell group with regard to the number of immunoreactive cells, midbrain arborizations, and contralateral projections, which are especially prominent in the cockroach and virtually absent in crickets. The unusual branching patterns and the special neurochemical phenotype suggest a particular physiological role of these neurons. Their possible function as circadian pacemakers is discussed.     

Tangential cell migration during layer formation of chick optic tectum

The laminated structure of the optic tectum is formed by radial and tangential cell migration during development. Studies of developing chick optic tectum have revealed two streams of tangential cell migration in the middle and superficial layers, which have distinctive origins, migratory paths, modes of migration, and destinations. We will review the process of the two types of tangential migrations, in order to elucidate their roles in the formation of the optic tectum layers.     

Building a projection map for photoreceptor neurons in the Drosophila optic lobes

The sensory tasks performed by the eye are diverse and complex. In Drosophila, the eye performs motion detection for navigation as well as detection of the quality of light (color and polarized light). Both types of inputs are processed separately, as different photoreceptors are specialized in these tasks and contact different target cell layers in the optic lobe. However, their respective outputs are likely to be integrated in higher brain centers. Here, we discuss the cell diversity and potential role of the several ganglia that form the fly optic lobe. We also discuss the power of modern genetic tools to provide the potential to trace the visual neural networks.     

The ultrastructure of the non-neurosecretory components in the brain of the midge,Chironomus riparius Mg. (Diptera: Nematocera)

The ultrastruct of the neural sheath, glial cells and neurons in the brain of the neoimaginal male Chironomus riparius is described. The neural sheath comprises a neural lamella and underlying perineurium. The neural lamella consists of an amorphous matrix in which fine fibrils occur. The perineurium is composed of two cell types forming a continuous layer around the brain. The subjacent cortical layer, composed of the cell bodies of neurons and glial cells, varies considerably in thickness and surrounds the centrally located neuropiles. Three types of glial cells are distinguished on the basis of their positions and appearances. Five types of neurons are described which differ in size and relative frequency of organelles. Four types of axons, including those of neurosecretory cells, are distinguished by their size and content.     

Golgi studies of the first optic ganglion of the ant,Cataglyphis bicolor

The neurons of the first optic ganglion (the lamina) in the desert ant, Cataglyphis bicolor, have been studied with the light microscope after Golgi silver impregnation. The different types of retinal and laminal fibres and their configuration are compared with the results obtained in the bee. The first synaptic region in the visual system of the ant lies proximally to the fenestrated layer below the basement membrane and the layer containing the monopolar cell bodies. The synaptic region can be separated into three morphologically different zones: (1) The most distal layer where the short visual fibres end at two different levels. The short visual fibres and some laminal fibres (monopolar cell fibres) also show lateral elements in this region. (2) The second layer appears almost free of branches of retinal and laminal fibres. (3) The most proximal layer, which has a characteristically dense horizontal structure resulting from the lateral elements of long visual, centrifugal, monopolar and tangential fibres. Nine cell axons arising from each ommatidium leave the retina. Six of these are short visual fibres and end at two different levels in the lamina. Three different types of short visual fibres can be distinguished by their different terminal depths and lateral branching pattern. The remaining three fibres, the long visual fibres, terminate in the medulla. They can be distinguished from each other by their lateral elements in the lamina neuropile. The five morphologically different laminal fibre types (axons of the monopolar cells in the lamina) have different shapes and different arborizations at different levels. Tangential, centrifugal and incerta sedis-fibres, which originate either from cell bodies in the cell body layer at the periphery of the outer chiasma or more centrally, terminate in the synaptic region of the lamina. Consideration is given to the clearly demarkated arrangement and length of the branching pattern of retinal and laminal fibres at different levels of the synaptic region of the lamina. In addition, a hypothetical connectivity pattern is discussed.     

Cell degeneration in the developing optic lobes of the sine oculis and small-optic-lobes mutants of Drosophila melanogaster

In the small-optic-lobes (sol) and sine oculis (so) mutants of Drosophila melanogaster extensive cell death occurs in the optic lobes during the first half of pupal development. Gynandromorph flies show that the sol mutation acts primarily on cells of the medulla cortex. Degeneration of medullar ganglion cells occurs at an early stage of cellular differentiation, when their axons have not yet participated in the formation of the second optic chiasma. The so gene, on the other hand, acts on the eye anlagen. The analysis of chimeric flies demonstrates that degeneration in the optic lobes of so flies is a consequence of eye reduction. At the level of the second optic chiasma extensive axonal degeneration can be observed in the mutant. Neurons seem to die after their failure to establish a sufficient number of functional contacts. In sol;so double mutants, the mutational effects are cumulative causing complete degeneration of columnar cell types in pupae without any eye anlage. The tiny rudiments of the optic lobes in eyeless double mutants still contain tangential neurons of the medulla and of the lobula complex. The central brain is reduced in size due to the missing visual fibers, however, its overall appearance is surprisingly normal.