Immunocytochemical demonstration ofγ-amino butyric acid and glutamic acid decarboxylase in R7 photoreceptors and C2 centrifugal fibres in the blowfly visual system

Summary Neurons within the compound eye of the flyCalliphora erythrocephala, suspected of containing gamma-aminobutyric acid were revealed immunocytochemically, using antibodies directed against gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). The GABA content within putative GABAergic neurons was increased by high affinity uptake of GABA and selective blocking of GABA metabolism with Gabaculine. Only neuronal populations which were labelled with the GABA as well as the GAD antibodies were presumed to be GABAergic. The first optic neuropil (lamina) exhibited two distinct GA-BAergic fibre populations amongst a larger population comprised of fourteen cell classes. One fibre population was formed by the axons of the photopic photoreceptors R7 which pass through the lamina and terminate in the second optic neuropil (the medulla). The identity of R7 was established from longitudinal and transverse sections of the retina where R7 can be unequivocally distinguished from the six scotopic photoreceptors R1-6 and the other photopic receptor, R8.The other fibre population matched the profiles in the lamina of terminals of efferent C2 neurons. These neurons project distally from beneath the medulla out to the lamina ganglionaris where each retinotopic unit (cartridge) contains a characteristic hook-like terminal arbor distally. We propose from these data that the photoreceptors R7 and the efferent C2 neurons use GABA as a neurotransmitter.     

Localization of substance P-like,leucine-enkephalin-like,methionine-enkephalin-like,and FMRFamide-like immunoreactivity in the eyestalk of the fiddler crab,Uca pugilator

Summary The occurrence and distribution of substance P (SP)-like, methionine-(Met)- and leucine-(Leu)-enkephalinlike, and FMRFamide-like immunoreactivities were determined in the neuroendocrine complex of the eyestalk of the fiddler crab, Uca pugilator, by immunocytochemistry. SP-like immunoreactivity was found in the optic peduncle, sinus gland, medulla externa, medulla interna, lamina ganglionaris, and retinular cells. Met-enkephalin-like and Leuenkephalin-like immunoreactivity was observed in most of the retinular cells, optic peduncle, sinus gland, medulla terminalis, and lamina ganglionaris. However, Met-enkephalin-like, but no Leu-enkephalin-like, immunoreactivity was seen in the medulla terminalis X-organ. FMRFamide-like immunoreactivity could be seen in all parts of the eyestalk except in the sinus gland, lamina ganglionaris, and retinular cells. FMRF-amide-like activity was especially strong in the three chiasmatic regions connecting the optic ganglia. The possibility that these four peptides may function as neuroregulators in the fiddler crab is discussed.This investigation was supported in part by Grant No. PCM-8300064 from the National Science Foundation to MF and Biomedical Research Support Grant No. 2 SO7RRO5373 SUB from the University of Kansas Medical Center to LLV     

Heterogenic components of a fast electrical potential in Drosophila compound eye and their relation to visual pigment photoconversion

The electroretinogram of the dipteran compound eye in response to an intense flash contains an early, diphasic potential that has been termed the M potential. Both phases of the M potential arise from the photostimulation of metarhodopsin. The early, corneal-negative component, the M1, can be recorded intracellularly in the photoreceptors and has properties similar to the classical early receptor potential (ERP). The M1 is resistant to cold, anaesthesia, and anoxia and has no detectable latency. It depends on flash intensity and metarhodopsin fraction in the manner predicted for a closed, two-state pigment system, and its saturation is shown to correspond to the establishment of a photoequilibrium in the visual pigment. On the other hand, the dominant, corneal-positive component, the M2, does not behave like an ERP. It arises, not in the photoreceptors, but deeper in the retina at the level of the lamina, and resembles the on-transient of the electroretinogram in its reversal depth and sensitivity to cooling or CO2. The on-transient, which is present over a much wider range of stimulus intensity than the M potential, has been shown to arise from neurons in the lamina ganglionaris. Visual mutants in which the on- transient is absent or late are also defective in the M2. It is proposed that the M2 and the on-transient arise from the same or similar groups of second-order neurons, and that the M2 is a fast laminar response to the depolarizing M1 in the photoreceptors, just as the on-transient is a fast laminar response to the depolarizing late receptor potential. Unlike the M1, the M2 is not generally proportional to the amount of metarhodopsin photoconverted, and the M2 amplitude is influenced by factors, such as a steady depolarization of the photoreceptor, which do not affect the M1.     

The optic neuropiles and chiasmata of Crustacea

Summary On the basis of ontogeny and adult morphology, an interpretation of the arrangement of optic neuropiles and fibre connexions of the Crustacean compound eye is presented. In the embryo of phyllopods and decapods, the ommatidia, the lamina ganglionaris, and the medulla externa are developed synchronously from a common medial proliferation zone. As this zone persists in all investigated adult Crustacea that possess compound eyes, such a derivation of the mentioned structures is taken to be universal within the group. The direction of growth of the lamina ganglionaris is parallel with the row of ommatidia, the growth direction of the medulla externa is perpendicular to it and parallel with the long axis of the eyestalk. This arrangement is more or less retained in most adult non-Malacostracan Crustacea, and the axons of fully developed neurons pierce the optic neuropiles and leave and enter on the neuropile side. As a result, there is no chiasma in the non-Malacostracan groups.The Malacostraca have an extra neuropile, the medulla interna, derived from the medulla terminalis. Chiasmata occur between the lamina ganglionaris and the medulla externa, and between the medulla externa and the medulla interna. This difference from the non-Malacostracans depends on the course of the fibres. Those coming from the lamina ganglionaris leave the lamina on the neuropile side and enter medulla externa between the cell bodies in the perikaryon layer of the medulla externa neurons and the neuropile of the medulla. The fibres from the medulla externa to the lamina come from T-shaped neurons and emanate from the perikaryon layer side, entering the lamina on its neuropile side. The fibre relations between the medulla externa and the medulla interna are similar. Thus in both cases, chiasmata are present from the beginning, but they become obvious when the medulla externa rotates through part of a circle.The directed growth of the optic neuropiles and the course of the fibre connexions are consequently crucial to the understanding of the topographic relations between the neuropiles. A pattern with short neurons connecting neighbouring optic neuropiles and long neurons connecting the medulla externa with the central nervous system is common to all crustaceans.In memoriam Bertil Hanström.This work has been supported by a grant from the Swedish Natural Science Research Council 2760-3, 99-35.     

The neurons of the first synaptic region of the optic neuropil of the firefly,Phausis splendidula L. (Coleoptera)

On the basis of Golgi preparations the neuronal elements of the lamina ganglionaris (first synaptic region of the visual system) of the firefly. Phausis splendidula L., are described. Of the set of 8 retinula fibres that originate from each ommatidium of the compound eye, at least 6 terminate in the esternal plexiform layer. At least one, probably two, retinula fibres per ommatidium penetrate this layer to end in the medulla, via the first optic chiasma. Five types (m1-m5) of monopolar cells can be distinguished. Only two of these, m1 and m3 have dendritic fields limited to one column of the lamina mosaic; all other monopolar cells have larger fields of up to 45 mum diameter. m2 and m4 have various field spreads in different strata of the external plexiform layer. m5 has process in only one stratum of the external plexiform layer. Medulla-to-lamina cells with arborisations associated with only a single column of the lamina mosaic were not observed; medulla-to-lamina cells whose fields coincide with the various strata of the external plexiform layer were found, however. The present observations are briefly compared with those made on another beetle, Hoplia farinosa L. Comparisons with other species of insects, and the relationship between structure of the eye and structure of the lamina are also discussed.     

Gap junctions coupling photoreceptor axons in the first optic ganglion of the fly

Cell and Tissue Research - The first optic ganglion of the fly, the lamina ganglionaris, was investigated with the transmission electron microscope for the purpose of demonstrating possible...     

Ultrastructure of the compound eye and first optic neuropile of the photoreceptor mutant ora JK84 of Drosophila

Summary The developmental mutant of Drosophila (ora ^JK84) is characterized by nonfunctional photoreceptor cells (R1–6), while the R7/R8 cells are normal. A fundamental question is: Does the near absence of photosensitive membranes inhibit development of the Rl-6 axons and their synapses at the other end of the cell? The retina and first optic neuropile (lamina ganglionaris) were examined with freeze-fracture technique and high voltage electron microscopy. R1–6 have reduced rhabdomere caps; rhabdomeric microvilli have about 50% of the normal diameter and 20% of the normal length. Affected cells exhibit prominent vacuoles which appear to communicate with some highly convoluted microvillar membranes. Almost no P-face particles (putative rhodopsin molecules) are present in the R1–6 rhabdomeres, and particle densities are lower in R7 than previously reported. Near the rhabdomere caps, microvilli of R1–6 are fairly normal, but at more proximal levels they are greatly diminished in length and changed in orientation, while at still more proximal levels they are lost. R1–6, R7, and R8 axons from each ommatidium are bundled into normal pseudocartridges beneath the basement membrane. No abnormalities are found in the lamina ganglionaris, and all synaptic associations as well as the presumed virgin synapses (of R1–6) appear normal. No glial anomalies are present, and R7/R8 axons project through the lamina in the usual fashion. These fine structural findings are correlated with known electrophysiological, biochemical, and behavioral correlates of both sets of photoreceptors (R1–6, and R7/R8).This study was supported substantially by the UW-HVEM Laboratory, in addition to a Faculty Development Award, a UMC Biomedical Research Support Grant N.I.H. RR07053 to W.S.S., and a Hatch Grant, Project 2100 to S.D.C. Freeze fracture was done at the Wisconsin Regional Primate Research Center, N.I.H. Grant RR00167. We thank Professor Hans Ris, Dr. J. Pawley, Dr. D. Neuberger, and Ms. M. Bushlow, HVEM Laboratory, Dept. of Zoology, UW. We also thank Mrs. K. Srivastava, Mr. M.B. Garment, Mr. G. Gaard, and Mr. D. Liu for technical assistance.     

Nocturnal vision and landmark orientation in a tropical halictid bee

BACKGROUND: Some bees and wasps have evolved nocturnal behavior, presumably to exploit night-flowering plants or avoid predators. Like their day-active relatives, they have apposition compound eyes, a design usually found in diurnal insects. The insensitive optics of apposition eyes are not well suited for nocturnal vision. How well then do nocturnal bees and wasps see? What optical and neural adaptations have they evolved for nocturnal vision? RESULTS: We studied female tropical nocturnal sweat bees (Megalopta genalis) and discovered that they are able to learn landmarks around their nest entrance prior to nocturnal foraging trips and to use them to locate the nest upon return. The morphology and optics of the eye, and the physiological properties of the photoreceptors, have evolved to give Megalopta's eyes almost 30 times greater sensitivity to light than the eyes of diurnal worker honeybees, but this alone does not explain their nocturnal visual behavior. This implies that sensitivity is improved by a strategy of photon summation in time and in space, the latter of which requires the presence of specialized cells that laterally connect ommatidia into groups. First-order interneurons, with significantly wider lateral branching than those found in diurnal bees, have been identified in the first optic ganglion (the lamina ganglionaris) of Megalopta's optic lobe. We believe that these cells have the potential to mediate spatial summation. CONCLUSIONS: Despite the scarcity of photons, Megalopta is able to visually orient to landmarks at night in a dark forest understory, an ability permitted by unusually sensitive apposition eyes and neural photon summation.     

The superposition eye of Cloeon dipterum: The organization of the lamina ganglionaris

Summary The lamina ganglionaris of the superposition eye of Cloeon dipterum is composed of separate optic cartridges arranged in a hexagonal pattern. Each optic cartridge consists of one central, radially branched monopolar cell (Li) surrounded by a crown of seven retinula cell terminals and two more unilaterally branched monopolar cells (La1/La2) situated close together outside the cartridge. Projections to neighbouring cartridges have not been observed.In most cases, synaptic contacts could be seen between a presynaptic retinula cell and more than two other postsynaptic profiles, which belong to monopolar cells or sometimes to glial cells.Seven retinula cell fibers of one ommatidium pass in a bundle through the basement membrane, run into their respective cartridges without changing orientation and terminate at approximately equal levels in the lamina. Long visual fibers with endings in the medulla are not visible in the superposition eye lamina, but are present in the lateral apposition eye. The relationship between the behaviour of the animal, optic mechanisms of the superposition eye and the structure of the lamina is discussed.     

Organization of the lamina ganglionaris of the optic lobe of the butterfly Pararge aegeria (Linné) (Lepidoptera : Satyridae)

The present work reports on a neuroanatomical study of the butterfly Pararge aegeria (Lepidoptera : Satyridae) focusing on the lamina ganglionaris underlying two different regions of the retina of the compound eye: the dorsal rim area and the large dorsal region. No differences between both lamina regions, concerning the structure of the cartridges and the morphology of the identified neurons, could be detected. After passing the basement membrane, the visual cell axons are organized in retinotopic bundles (pseudocartridges), in which the axons of the 9 visual cells (V1 and 5, D2, 4, 6, 8, H3 and 7, B9) are arranged in the same way as in the retina. In the pseudocartridge there are no synaptic contacts. Before entering the lamina cartridge, the bundles rotate 90 °. The cartridges are joined by the fibres of 4 monopolar cells (L1, L2, L3 and L4), which could be identified and located inside the lamina cartridges in serial EM-sections. Golgi impregnations revealed the morphology of these fibres. Thus, the regional specialization of the retina (dorsal rim area and large dorsal region) does not seem to be reflected at the level of the first visual neuropil. Additionally, the cartridges of both lamina regions were investigated qualitatively for synaptic contacts among fibres. In addition to monadic chemical synapses and multiple contact synapses with presynaptic ribbons, cell contacts are also facilitated by invaginations and bridges. These cellular interactions and their functional implications are discussed.     

Übertragungseigenschaften der Sehzelle der SchmeißfliegeCalliphora erythrocephala

Zusammenfassung In der Lamina ganglionaris vonCalliphora erythrocephala läßt sich bei der Ableitung aus dem Axon einer Sehzelle die Summation abgeschwächter Belichtungspotentiale aus den in einem Neuroommatidium vereinten Sehzellen nachweisen. Eine Analyse des Axonpotentials bei sinusförmig moduliertem Licht mit einem Modulationsgrad von 16% ergibt, daß durch die Summation die dem Belichtungspotential überlagerten rezeptoreigenen Störungen reduziert werden. Dies läßt sich nicht mit einem Tiefpaß erklären, sondern beruht auf den verschiedenen Gesetzmäßigkeiten, denen die Summation von Signalen und Störungen folgt, wie abgeleitet wird. Das Signal-Störungs-Verhältnis wird im Axon um den Faktor 6 gegenüber dem Soma der einzelnen Sehzelle verbessert. Bei niedrigen Beleuchtungs-intensitäten findet sich eine geringere Abschwächung der Belichtungspotentiale, so da/sB die summierte Signalamplitude im Axon gegenüber dem Soma vergrößert wird. Das entspricht einer Kontrastverstärkung. Die Verbesserung des Signal-Störungs-Verhältnisses und die Kontrastverstarkung zusammen ergeben eine um das mindestens 10fache höhere Empfindlichkeit für das System aus den Sehzellen 1–6 gegenüber der 7. und 8. Sehzelle (bei einem Vergleich auf der Ebene der Lamina ganglionaris).

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Transfer characteristics of the visual cell ofCalliphora erythrocephala 3. Improvement of the signal-to-noise ratio by presynaptic summation in the lamina ganglionaris

Summary In the lamina ganglionaris ofCalliphora erythrocephala intracellular recordings from the axon of a visual cell show that there is a summation of the attenuated receptor potentials of the visual cells contained within one cartridge. An analysis of the axon potential under sinusoidally modulated light (modulation degree 16%) reveals that the summation reduces the intrinsic noise of a receptor that is superimposed on the receptor potential. This reduction cannot be explained by a frequency response with a low frequency cutt off but by the different laws of summation for the signal and the noise. The signal-to-noise ratio is 6 times better in the axon than in the soma of a single visual cell. The receptor potentials are more weakly attenuated at low intensities, so that the summarized amplitude of the signal in the axon as compared to the soma is increased. This is equivalent to a contrast amplification. The improvement of the signal-to-noise ratio and the contrast-amplification leads to a sensitivity at least 10 times greater for the system of the visual cells 1–6 than for the visual cells 7 and 8 when these are compared on the level of the lamina ganglionaris.

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Die Experimente wurden mit Sachmitteln durchgeführt, die Herrn Prof. Dr. Dr. h. c. H. Autrum von der Deutschen Forschungsgemeinschaft zur Verfügung gestellt wurden.     

Catalase,Bax and p53 expression in the visual system of the crab <Emphasis Type="Italic">Ucides cordatus</Emphasis> following exposure to ultraviolet radiation

In invertebrates, a few studies have suggested apoptosis as the mechanism of choice to protect the retina after exposure to ultraviolet (UV) radiation. We demonstrated previously, by electron microscopy, that the retina and lamina ganglionaris (or lamina) cells of the crab Ucides cordatus displayed subcellular signs of apoptosis after exposure to UVB and UVC. Here, we first ascertained, by the TdT-mediated dUTP-biotin nick end-labeling (TUNEL) technique, that UV irradiation indeed produced the previously reported results. We next tested, in the visual system of U. cordatus, whether the expression (as analyzed by immunohistochemistry and observed with laser scanning microscopy) and levels (as examined by Western blotting) of catalase, Bax, and p53 were affected by the same dose of UV radiation as that used previously. Our data revealed that the intensity of catalase, Bax, and p53 labeling was stronger in irradiated retina and lamina cells than in non-irradiated retina and lamina. However, no significant difference was observed in the concentrations of these proteins isolated from the whole optic lobe. The results thus suggest that UVB and UVC induce apoptosis in the crustacean retina and lamina by increasing catalase expression and activating the Bax- and p53-mediated apoptosis pathways. Nadia Campos de Oliveira Miguel and Inês Júlia R. Wajsenzon contributed equally to this work. This study was supported by the Brazilian Research Council for Science and Technology (CNPq), Rio de Janeiro State Foundation for the Advancement of Science (FAPERJ), José Bonifácio Foundation (FUJB), and Federal University of Rio de Janeiro Research Chamber (SR2/UFRJ).     

Intraocular gene transfer of ciliary neurotrophic factor rescues photoreceptor degeneration in RCS rats

Ciliary neurotrophic factor (CNTF) is known as an important factor in the regulation of retinal cell growth. We used both recombinant CNTF and an adenovirus carrying the CNTF gene to regulate retinal photoreceptor expression in a retinal degenerative animal, Royal College of Surgeons (RCS) rats. Cells in the outer nuclear layer of the retinae from recombinant-CNTF-treated, adenoviral-CNTF-treated, saline-operated, and contralateral untreated preparations were examined for those exhibiting CNTF photoreceptor protective effects. Cell apoptosis in the outer nuclear layer of the retinae was also detected. It was found that CNTF had a potent effect on delaying the photoreceptor degeneration process in RCS rats. Furthermore, adenovirus CNTF gene transfer was proven to be better at rescuing photoreceptors than that when using recombinant CNTF, since adenoviral CNTF prolonged the photoreceptor protection effect. The function of the photoreceptors was also examined by taking electroretinograms of different animals. Adenoviral-CNTF-treated eyes showed better retinal function than did the contralateral control eyes. This study indicates that adenoviral CNTF effectively rescues degenerating photoreceptors in RCS rats.S.-P.H. and P.-K.L. contributed equally to this work.     

Autoradiographic localization of 3H-γ-aminobutyric acid uptake in the lamina ganglionaris of Musca and Drosophila

Summary The uptake of ³H-GABA in the visual system of half-head preparations of Musca and Drosophila was studied by means of light and electron microscope autoradiography. Of all three ganglia, only the first synaptic region, the lamina ganglionaris, showed accumulation of radioactive grains, and there a preferential glial uptake could be found. Under normal light conditions at incubation (constant light flux of 100 Lux) the maximum of radio-activity was found in the marginal glia cells. Increasing the time of incubation produced also an increase in the number of grains per surface unit in the marginal glia cells. After changing the light intensity during incubation, quantitative modifications of the distribution of radio-activity were observed: incubating with stroboscopic illumination, the number of grains diminished in the marginal glia cells and remained constant in the epithelial cells; incubated in darkness, the epithelial cells became more intensely labelled whilst the number of grains decreased in the marginal cells.The possibility is discussed that the receptor axons 1–6 are the neurological elements of the lamina which use GABA as a transmitter. This hypothesis is lent some support from results of similar experiments with neurological mutants of Drosophila. In opm 18 there was a delayed uptake of ³H-GABA whereas in opm 3 and ebony the results were comparable to those found in Musca incubated in darkness. Behavioral studies on these mutants have demonstrated a defect, most probably related to the receptor system 1–6.     

Immunocytochemical demonstration of S-antigen (arrestin) in the brain of the blowfly Calliphora vicina

S-Antigen (arrestin)-immunoreaction can be considered as a marker for retinal and extraretinal photoreceptors in both vertebrate and invertebrate species. The present immunocytochemical study with the blowfly Calliphora vicina revealed S-antigen immunoreaction in retinal photoreceptors and various groups of neurons bilaterally distributed in the optic lobes and in the proto-, deuto- and tritocerebrum. S-Antigen-immunoreactive processes and terminal formations were found in the lower division of the central body complex and in the neuropil of the mushroom body. Also neuropil regions of the optic lobe, the lamina, medulla and lobula displayed S-antigen-immunoreactive fibers which were arranged in different patterns. These immunocytochemical data suggest that extraocular photoreceptors may be located in various parts of the blowfly brain. They provide a structural basis for further experiments which are needed to identify definitely these elements as extraretinal photoreceptors.     

Maintenance of Glia in the Optic Lamina Is Mediated by EGFR Signaling by Photoreceptors in Adult Drosophila

The late onset of neurodegeneration in humans indicates that the survival and function of cells in the nervous system must be maintained throughout adulthood. In the optic lamina of the adult Drosophila, the photoreceptor axons are surrounded by multiple types of glia. We demonstrated that the adult photoreceptors actively contribute to glia maintenance in their target field within the optic lamina. This effect is dependent on the epidermal growth factor receptor (EGFR) ligands produced by the R1-6 photoreceptors and transported to the optic lamina to act on EGFR in the lamina glia. EGFR signaling is necessary and sufficient to act in a cell-autonomous manner in the lamina glia. Our results suggest that EGFR signaling is required for the trafficking of the autophagosome/endosome to the lysosome. The loss of EGFR signaling results in cell degeneration most likely because of the accumulation of autophagosomes. Our findings provide in vivo evidence for the role of adult neurons in the maintenance of glia and a novel role for EGFR signaling in the autophagic flux.     

TRP, TRPL and Cacophony Channels Mediate Ca(2+) Influx and Exocytosis in Photoreceptors Axons in Drosophila

In Drosophila photoreceptors Ca(2+)-permeable channels TRP and TRPL are the targets of phototransduction, occurring in photosensitive microvilli and mediated by a phospholipase C (PLC) pathway. Using a novel Drosophila brain slice preparation, we studied the distribution and physiological properties of TRP and TRPL in the lamina of the visual system. Immunohistochemical images revealed considerable expression in photoreceptors axons at the lamina. Other phototransduction proteins are also present, mainly PLC and protein kinase C, while rhodopsin is absent. The voltage-dependent Ca(2+) channel cacophony is also present there. Measurements in the lamina with the Ca(2+) fluorescent protein G-CaMP ectopically expressed in photoreceptors, revealed depolarization-induced Ca(2+) increments mediated by cacophony. Additional Ca(2+) influx depends on TRP and TRPL, apparently functioning as store-operated channels. Single synaptic boutons resolved in the lamina by FM4-64 fluorescence revealed that vesicle exocytosis depends on cacophony, TRP and TRPL. In the PLC mutant norpA bouton labeling was also impaired, implicating an additional modulation by this enzyme. Internal Ca(2+) also contributes to exocytosis, since this process was reduced after Ca(2+)-store depletion. Therefore, several Ca(2+) pathways participate in photoreceptor neurotransmitter release: one is activated by depolarization and involves cacophony; this is complemented by internal Ca(2+) release and the activation of TRP and TRPL coupled to Ca(2+) depletion of internal reservoirs. PLC may regulate the last two processes. TRP and TRPL would participate in two different functions in distant cellular regions, where they are opened by different mechanisms. This work sheds new light on the mechanism of neurotransmitter release in tonic synapses of non-spiking neurons.     

Nonvisual photoreceptors in arthropods with emphasis on their putative role as receptors of natural Zeitgeber stimuli

In various insect and arachnid species, three different types of photoreceptors that do not serve image processing have been discovered and analyzed by means of neurobiological methods: They can be found for example: (1) as lamina and lobula organs (LaOs and LoOs) next to the optic neuropils in the optic lobes of holo- and hemimetabolous insects; (2) inside the last ganglia of the cord of the scorpion and a marine midge; and (3) as modified visual photoreceptors in metamorphosized larval stemmata and the lateral eyes of scorpions, which have been compound eyes in fossil scorpion relatives. Imnunocytology with various antibodies against proteins of the phototransduction cascade, the rhabdom turnover cycle and neurotransmitters of afferent and efferent pathways, was combined with light- and ultrastructural investigations in well-defined adaptational states, in order to study their photoreceptive function and neuronal wiring. Pilot chronobiological experiments with a newly developed twilight simulating lamp, behavioral studies, and model calculations provide evidence that these photoreceptors may well serve a role in the complex task of detecting time cues out of natural dawn and dusk.     

Giant lens, a gene involved in cell determination and axon guidance in the visual system of Drosophila melanogaster.

Mutations in the Drosophila gene giant lens (gil) affect ommatidial development, photoreceptor axon guidance and optic lobe development. We have cloned the gene using an enhancer trap line. Molecular analysis of gil suggests that it encodes a secreted protein with an epidermal-growth-factor-like motif. We have generated mutations at the gil locus by imprecise excision of the enhancer trap P-element. In the absence of gil, additional photoreceptors develop at the expense of pigment cells, suggesting an involvement of gil in cell determination during eye development. In addition, gil mutants show drastic effects on photoreceptor axon guidance and optic lobe development. In wildtype flies, photoreceptor axons grow from the eye disc through the optic stalk into the larval brain hemisphere, where retinal innervation is required for the normal development of the lamina and distal medulla. The projection pattern of these axons in the developing lamina and medulla is highly regular and reproducible. In gil, photoreceptor axons enter the larval brain but fail to establish proper connections in the lamina or medulla. We propose that gil encodes a new type of signalling molecule involved in the process of axon pathfinding and cell determination in the visual system of Drosophila.