Metarhodopsin control by arrestin, light-filtering screening pigments, and visual pigment turnover in invertebrate microvillar photoreceptors

The visual pigments of most invertebrate photoreceptors have two thermostable photo-interconvertible states, the ground state rhodopsin and photo-activated metarhodopsin, which triggers the phototransduction cascade until it binds arrestin. The ratio of the two states in photoequilibrium is determined by their absorbance spectra and the effective spectral distribution of illumination. Calculations indicate that metarhodopsin levels in fly photoreceptors are maintained below ~35% in normal diurnal environments, due to the combination of a blue-green rhodopsin, an orange-absorbing metarhodopsin and red transparent screening pigments. Slow metarhodopsin degradation and rhodopsin regeneration processes further subserve visual pigment maintenance. In most insect eyes, where the majority of photoreceptors have green-absorbing rhodopsins and blue-absorbing metarhodopsins, natural illuminants are predicted to create metarhodopsin levels greater than 60% at high intensities. However, fast metarhodopsin decay and rhodopsin regeneration also play an important role in controlling metarhodopsin in green receptors, resulting in a high rhodopsin content at low light intensities and a reduced overall visual pigment content in bright light. A simple model for the visual pigment–arrestin cycle is used to illustrate the dependence of the visual pigment population states on light intensity, arrestin levels and pigment turnover

Neural circuits underlying colour vision and visual memory in Drosophila melanogaster

Focusing at the fly visual system I am addressing the identity and function of neurons accomplishing two fundamental processing steps required for survival of most animals: neurons of peripheral circuits underlying colour vision as well neurons of higher order circuits underlying visual memory. Colour vision is commonly assumed to rely on photoreceptors tuned to narrow spectral ranges. In the ommatidium of Drosophila, the four types of so-called inner photoreceptors express different narrow-band opsins. In contrast, the outer photoreceptors have a broadband spectral sensitivity and are thought to exclusively mediate achromatic vision. Using computational models and behavioural experiments, I here demonstrate that the broadband outer photoreceptors contribute to colour vision in Drosophila. A model of opponent processing that includes the opsin of the outer photoreceptors scores the best fit to wavelength discrimination behaviour of flies. To experimentally uncover the contribution of individual photoreceptor types, I used blind flies with disrupted phototransduction (norpA-) and rescued norpA function in genetically targeted photoreceptors and receptor combinations. Surprisingly, dichromatic flies with only broadband photoreceptors and one additional receptor type can discriminate different colours, indicating the existence of a specific output comparison of outer and inner photoreceptors. Furthermore, blocking interneurons postsynaptic to the outer photoreceptors specifically impairs colour but not intensity discrimination. These findings show that outer receptors with a complex and broad spectral sensitivity do contribute to colour vision and reveal that chromatic and achromatic circuits in the fly share common photoreceptors. Higher brain areas integrate sensory input from different modalities including vision and associate these neural representations with good or bad experiences. It is unclear, however, how distinct sensory memories are processed in the Drosophila brain. Furthermore, the neural circuit underlying colour/intensity memory in Drosophila remained so far unknown. In order to address these questions, I established appetitive and aversive visual learning assays for Drosophila. These allow contrasting appetitive and aversive visual memories using neurogenetic methods for circuit analysis. Furthermore, the visual assays are similar to the widely used olfactory learning assays and share reinforcing stimuli (sugar reward and electric shock punishment), conditioning regimes and methods for memory assessment. Thus, a direct comparison of the cellular requirements for visual and olfactory memories becomes feasible. I found that the same subsets of dopamine neurons innervating the mushroom body are necessary and sufficient for formation of both sensory memories. Furthermore, expression of D1-like Dopamine Receptor (DopR) in the mushroom body is sufficient to restore the memory defect of a DopR null mutant (dumb-). These findings and the requirement of the mushroom body for visual memory in the used assay suggest that the mushroom body is a site of convergence, where representations of different sensory modalities may undergo associative modulation.Mit Fokus auf das visuelle System von Fliegen behandle ich in meiner Dissertation die Identität und Funktion von Neuronen, welche zwei fundamentale Verarbeitungsschritte ausführen, die für das Überleben der meisten Tiere notwendig sind. Zum einen sind dies dem Farbensehen zugrunde liegende Neuronen und zum anderen solche, die essentiel für visuelles Gedächtnis sind. Allgemein wird angenommen, dass Farbensehen auf Photorezeptoren mit Sensitivitäten für schmale Spektralbereiche aufbaut. Im Ommatidium von Drosophila exprimieren die sogenannten inneren Photorezeptoren verschiedene spektral schmalbandige Opsine. Im Gegensatz dazu haben die äußeren Photorezeptoren eine breitbandige spektrale Sensitivität und man nimmt an, dass diese ausschließlich achromatisches Sehen ermöglichen. Mit Hilfe von computergestützten Modellen und Verhaltensexperimenten zeige ich hier, dass die breitbandigen äußeren Photorezeptoren zum Farbensehen in Drosophila beitragen. Ein Modell mit opponenter Verarbeitung von Photorezeptorsignalen, welches das Opsin der äußeren Photorezeptoren beinhaltet, passt am besten zum spektralen Unterscheidungsverhalten von Fliegen. Um experimentell den Beitrag der einzelnen Photorezeptortypen zu ermitteln verwendete ich blinde Fliegen mit einem Defekt in der Phototransduktion (norpA-) und rettete die norpA Funktion gezielt in einzelnen oder verschiedenen Kombinationen von Photorezeptortypen mit Hilfe des GAL4/UAS Genexpressionssystems. Erstaunlicherweise können dichromatische Fliegen mit nur äußeren Photorezeptoren und einem weiteren Rezeptortyp Farben unterscheiden, was auf die Existenz eines spezifischen Vergleichs der Signale von äußeren und inneren Photorezeptoren hindeutet. Außerdem beeinträchtigt der Block von Interneuronen, welche postsynaptisch von den äußeren Photorezeptoren sind, spezifisch das Farbensehen aber nicht die Intensitätsunterscheidung. Diese Ergebnisse zeigen zum einen, dass die äußeren Photorezeptoren mit einer komplexen und breitbandigen spektralen Sensitivität zum Farbensehen beitragen und zum anderen, dass chromatische und achromatische neuronale Netzwerke in der Fliege gemeinsame Photorezeptoren teilen. Höher geordnete Gehirnbereiche integrieren sensorische Information verschiedener Modalitäten insbesondere visueller Natur und assoziieren deren neuronale Representation mit guten und schlechten Erfahrungen. Es ist jedoch unklar, wie unterschiedliche sensorische Gedächtnisse im Gehirn von Drosophila verarbeitet werden. Außerdem ist das neuronale Netzwerk, welches Farb- und Intensitätsgedächtnis zugrunde liegt völlig unbekannt. Um diese Fragen zu beantworten etablierte ich appetitive und aversive Verhaltensassays für Drosophila. Diese erlauben die Gegenüberstellung von appetitivem und aversivem visuellen Gedächtnis unter Verwendung von neurogenetischen Methoden zur Netzwerkanalyse. Desweiteren sind die visuellen Verhaltensassays sehr ähnlich zu den verbreiteten olfaktorischen Lernsassays, da diese verstärkende Stimuli (Zuckerbelohnung und Elektroschockbestrafung), Konditionierungsablauf und Methoden zur Gedächtnismessung gemein haben. Dadurch wird ein direkter Vergleich der zellulären Grundlagen von visuellem und olfaktorischem Gedächtnis möglich. Ich fand, dass die gleichen Gruppen von Dopaminneuronen, welche den Pilzkörper innervieren, sowohl notwendig als auch ausreichend für die Bildung beider sensorischer Gedächtnisse sind. Außerdem ist die Expression des D1-ähnlichen Dopaminrezeptors (DopR) im Pilzkörper ausreichend um den Gedächtnisdefekt einer DopR Nullmutante (dumb-) zu retten. Diese Ergebnisse sowie die Notwendigkeit des Pilzkörpers für visuelles Gedächtnis in dem benutzen Assay deuten darauf hin, dass der Pilzkörper ein Konvergenzareal ist, in welchem Repräsentationen von verschiedenen sensorischen Modalitäten assoziativer Modulation unterliegen

Neurons against Noise : Neural adaptations for dim light vision in hawkmoths

All animals perceive the world through their senses, which form the basis for their decisions and motor actions. However, when these all-important senses reach their limit and cease to provide reliable information, the animal’s survival is threatened. Among the senses, vision is brought to its limits on a daily basis, because its signal strength is diminished as night falls, and increases again as the sun rises. In this thesis, I investigated adaptations that enable the visual system of hawkmoths, a group of insects, to cope with the low light intensities they face at night. I have focused on neural adaptations, manifested in the processing of visual neurons, in contrast to anatomical adaptations, such as modifications of the eye. I showed that neural adaptations exist in the motion vision system of hawkmoths, in the form of integration of visual information in space and time. Furthermore, I demonstrated that a combination of such spatial and temporal summation increased sensitivity and information content in dim light (Paper I). The amount of spatial and temporal summation matched the ecological needs of different hawkmoth species, as well as their anatomical adaptations for visual sensitivity: night active species, and species with less sensitive eyes had more extensive spatial and temporal summation than day-active species and species with very sensitive optics (Paper II). Furthermore, I identified and characterised candidate neurons that carry out spatial and temporal summation in the brain of hawkmoths (Paper III). Finally, I quantified the effects of temporal summation on the ability of hawkmoths to track flowers in hovering flight at different light levels, and showed that a subset of the observed behavioural phenomena could be explained by temporal processing in the nervous system (Paper IV). Taken together, this work has provided detailed insight into how neural processing can increase visual reliability in dim light. The results presented are not only relevant to hawkmoths, since neural summation is also expected to increase visual sensitivity in other species of nocturnal insects, and can be compared to similar mechanisms in vertebrates. Furthermore, this work is instructive for the development of artificial visual systems, for which insect brains have proven to be a successful biomimetic model

Spectral and ocellar inputs to honeybee motion-sensitive descending neurons

Optomotor reflexes have been observed in many insects and in some cases the neural pathways that mediate these reflexes have been identified physiologically and anatomically. In honeybees Kaiser (1975) established that the spectral sensitivity of optomotor responses in bees almost exactly matched that of the green photoreceptors, suggesting an exclusive input from green photoreceptors. However, physiological studies showed that the motion detectors in the optic lobes have a secondary response peak in the UV region of the spectrum suggesting that there may be more than one type of photoreceptor involved in the optomotor response. Thus in this thesis, I investigate the neural basis of motion and spectral wavelength processing in motion-sensitive descending neurons, which are on the optomotor response pathway, to reveal the neural contributions from other spectral receptor types. In this study, intracellular recording techniques were utilised. The stimuli consisted of a wide-field LED (light emitting diode) display in which green (peak 530 nm) and short-wavelength (peak 380 nm) LEDs were mounted in pairs across a wide visual area. Six types of motion-sensitive descending neurons were recorded and anatomically identified, including two pitch-sensitive neurons (Locth3, DNII2), two roll-sensitive neurons (DNIV2 and DNIV3) and two yaw-sensitive neurons (DNVII1 and DNVII2). The results show that for the vertical sensitive (pitch and roll) neurons, the cells have equal-sized excitatory responses to motion when using short-wavelength or green motion stimulation. However, for the horizontal sensitive (yaw-sensitive) neurons excitatory responses only occurred for the green stimulus in the preferred direction. The short-wavelength stimulus induced clear inhibitory responses for all tested motion directions. The results suggest that besides green photoreceptors, the motion-sensitive descending neurons also receive inputs from the short-wavelength photoreceptors, but only for motion detectors tuned for vertical motion. Honeybees, like most flying insects, have three ocelli (simple eyes) located on the top of the head, in addition to the compound eyes. However, the exact function of the bee ocelli and the information computation between the ocelli, compound eyes and central brain remain unclear. In this thesis, I investigate the ocellar properties morphologically, anatomically and physiologically. Semi-thin sections and focal length measurements were performed on both median and lateral ocelli, a 3-dimensional reconstruction model of the honeybee ocellar lenses and retinas was developed to understand the visual fields of the ocelli. Intracellular electrophysiology experiments were carried out on descending neurons to understand the information processing between the ocelli and compound eyes. Cell responses to different stimuli were recorded with and without the ocelli covered. It is shown that the ocellar input provides a faster response to motion stimuli than with compound eye stimulation alone, and also increases the amplitude of responses to flashed stimuli. In the case of the DNII2 neuron, it is also shown that the ocelli provide a directional contribution to the responses