Fly Optic Lobe Research Articles

A glial variant of the vesicular monoamine transporter is required to store histamine in the Drosophila visual system.

Unlike other monoamine neurotransmitters, the mechanism by which the brain's histamine content is regulated remains unclear. In mammals, vesicular monoamine transporters (VMATs) are expressed exclusively in neurons and mediate the storage of histamine and other monoamines. We have studied the visual system of Drosophila melanogaster in which histamine is the primary neurotransmitter released from photoreceptor cells. We report here that a novel mRNA splice variant of Drosophila VMAT (DVMAT-B) is expressed not in neurons but rather in a small subset of glia in the lamina of the fly's optic lobe. Histamine contents are reduced by mutation of dVMAT, but can be partially restored by specifically expressing DVMAT-B in glia. Our results suggest a novel role for a monoamine transporter in glia that may be relevant to histamine homeostasis in other systems.

Conserved Role of the Vsx Genes Supports a Monophyletic Origin for Bilaterian Visual Systems

Components of the genetic network specifying eye development are conserved from flies to humans, but homologies between individual neuronal cell types have been difficult to identify. In the vertebrate retina, the homeodomain-containing transcription factor Chx10 is required for both progenitor cell proliferation and the development of the bipolar interneurons, which transmit visual signals from photoreceptors to ganglion cells. We show that dVsx1 and dVsx2, the two Drosophila homologs of Chx10, play a conserved role in visual-system development. DVSX1 is expressed in optic-lobe progenitor cells, and, in dVsx1 mutants, progenitor cell proliferation is defective, leading to hypocellularity. Subsequently, DVSX1 and DVSX2 are coexpressed in a subset of neurons in the medulla, including the transmedullary neurons that transmit visual information from photoreceptors to deeper layers of the visual system. In dVsx mutant adults, the optic lobe is reduced in size, and the medulla is small or absent. These results suggest that the progenitor cells and photoreceptor target neurons of the vertebrate retina and fly optic lobe are ancestrally related. Genetic and functional homology may extend to the neurons directly downstream of the bipolar and transmedullary neurons, the vertebrate ganglion cells and fly lobula projection neurons. Both cell types project to visual-processing centers in the brain, and both sequentially express the Math5/ATO and Brn3b/ACJ6 transcription factors during their development. Our findings support a monophyletic origin for the bilaterian visual system in which the last common ancestor of flies and vertebrates already contained a primordial visual system with photoreceptors, interneurons, and projection neurons.

Hofbauer-Buchner Eyelet Affects Circadian Photosensitivity and Coordinates TIM and PER Expression in Drosophila Clock Neurons

Extraretinal photoreception is a common input route for light resetting signals into the circadian clock of animals. In Drosophila melanogaster, substantial circadian light inputs are mediated via the blue light photoreceptor CRYPTOCHROME (CRY) expressed in clock neurons within the brain. The current model predicts that, upon light activation, CRY interacts with the clock proteins TIMELESS (TIM) and PERIOD (PER), thereby inducing their degradation, which in turn leads to a resetting of the molecular oscillations within the circadian clock. Here the authors investigate the function of another putative extraretinal circadian photoreceptor, the Hofbauer-Buchner eyelet (H-B eyelet), located between the retina and the medulla in the fly optic lobes. Blocking synaptic transmission between the H-B eyelet and its potential target cells, the ventral circadian pacemaker neurons, impaired the flies' ability to resynchronize their behavior under jet-lag conditions in the context of nonfunctional retinal photoreception and a mutation in the CRY-encoding gene. The same manipulation also affected synchronized expression of the clock proteins TIM and PER in different subsets of the clock neurons. This shows that synaptic communication between the H-B eyelet and clock neurons contributes to synchronization of molecular and behavioral rhythms and confirms that the H-B eyelet functions as a circadian photoreceptor. Blockage of synaptic transmission from the H-B eyelet in the presence of functional compound eyes and the absence of CRY also results in increased numbers of flies that are unable to synchronize to extreme photoperiods, supplying independent proof for the role of the H-B eyelet as a circadian photoreceptor.

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.

Retinotopic pathways providing motion‐selective information to the lobula from peripheral elementary motion‐detecting circuits

Recordings from afferent channels from the medulla supplying deep neuropils of the fly's optic lobes reveal different filter properties among the three classes of afferent neurons: transmedullary cells, T2 neurons, and Y cells. Whereas transmedullary cells respond to local flicker stimuli without discriminating these from directional or oriented motion, the T2 afferent neurons show clear motion orientation selectivity, which corresponds closely with a morphological bias in the orientation of their dendrites and could also be influenced by systems of local recurrent neurons in the medulla. A Y cell having a clearly defined terminal in the lobula, but having dendrite-like processes in the medulla and, possibly, the lobula plate, discriminates the direction of motion and its orientation. These results demonstrate unambiguously that the lobula receives information about motion and that the channels carrying it are distinct from those supplying wide-field motion-selective neurons in the lobula plate. Furthermore, recordings from a newly identified recurrent neuron linking the lobula back to the inner medulla demonstrate that the lobula discriminates nondirectional edge motion from flicker, thereby reflecting a property of this neuropil that is comparable with that of primary visual cortex in cats. The present findings support the proposal that elementary motion detecting circuits supply several parallel channels through the medulla, which segregate to, but are not shared by, the lobula and the lobula plate. The results are discussed in the context of other intracellular recordings from retinotopic neurons and with analogous findings from mammalian visual systems.

Binocular contributions to optic flow processing in the fly visual system.

Integrating binocular motion information tunes wide-field direction-selective neurons in the fly optic lobe to respond preferentially to specific optic flow fields. This is shown by measuring the local preferred directions (LPDs) and local motion sensitivities (LMSs) at many positions within the receptive fields of three types of anatomically identifiable lobula plate tangential neurons: the three horizontal system (HS) neurons, the two centrifugal horizontal (CH) neurons, and three heterolateral connecting elements. The latter impart to two of the HS and to both CH neurons a sensitivity to motion from the contralateral visual field. Thus in two HS neurons and both CH neurons, the response field comprises part of the ipsi- and contralateral visual hemispheres. The distributions of LPDs within the binocular response fields of each neuron show marked similarities to the optic flow fields created by particular types of self-movements of the fly. Based on the characteristic distributions of local preferred directions and motion sensitivities within the response fields, the functional role of the respective neurons in the context of behaviorally relevant processing of visual wide-field motion is discussed.

Neurite morphogenesis of identified visual interneurons and its relationship to photoreceptor synaptogenesis in the flies, Musca domestica and Drosophila melanogaster.

The first neuropile, or lamina, of the fly's optic lobe comprises a model set of identified neurons that are arrayed in cylindrical modules, called cartridges. The cartridge acquires adult form only in the second half of the fly's pupal life. All cells are by then correctly located within each of the lamina's cartridges (Drosophila, Musca), becoming invested by glial cells after 75% of pupal development (P + 75%). In adult cartridges, two lamina cells, L1 and L2, receive input from photoreceptor terminals R1-R6, at so-called tetrad synapses that form in the pupa when these cells' dendrites contact R1-R6. Single-section electron microscopy (EM, Drosophila) and serial-EM reconstructions of L1 and L2 (Musca) reveal relationships between the morphogenesis of L1/L2 dendrites and the formation of tetrads. Neurite outgrowth is initially (P + 55%) random and neurites are unbranched; many neurites invaginate surrounding terminals of R1-R6 but, later, embrace the outer surfaces of these. The maximum profusion of neurites at P + 74% coincides with peak numbers of nascent tetrads; neurites then branch vertically, in the lamina's depth. Later, neurites failing to reach R1-R6's outer surfaces regress. Down the length of their axons, L1 and L2's neurites initially form a random sequence, L1 partnering L1 as often as L2, etc., but beginning at P + 74%, L1 partners L2, and L2 partners L1, with progressive strictness. L1 has more neurites overall than L2. These observations are consistent with the following hypotheses: a neurite only survives if it contacts a presynaptic site; a synapse only survives if it progressively acquires the appropriate number and combination of postsynaptic neurites, culminating in a tetrad; an interaction exists between the neurites of L1 and L2, so that the growth of one respects the pattern of growth of the other.

Daily rhythmic changes of cell size and shape in the first optic neuropil in Drosophila melanogaster.

Daily rhythms of changes in axon size and shape are seen in two types of monopolar cell-L1 and L2-that are unique cells within each of the modules or cartridges of the first optic neuropil or lamina in the fly's optic lobe. In the fruit fly Drosophila, L1 and L2's axons swell at the beginning of both day and night, with larger size increases occurring at the beginning of night. Later, they shrink during the day and night, respectively. Simultaneously, they change shape from an inverted conical form during the day to a cylindrical one at night. This is because the axonal cross section of L1 increases during the night, especially at proximal depths of the lamina, closest to the brain, whereas the axon of L2 increases in size at distal lamina depths. The cross-sectional areas of the L1 cell and of an individual cartridge both change under constant darkness (DD), indicating the circadian origin of changes observed under day/night (LD) conditions. We sought to see whether such changes impart a net change to the entire lamina's volume or shape that is visible by light microscopy, but oscillations in the volume or the curvature of the whole lamina neuropil are found neither in LD nor in DD. These size changes are discussed in relation to previous findings in the housefly Musca, with respect to differences in L1 and L2 between the two species, and to differences in the time course of their circadian changes.

Neurites of period-expressing PDH cells in the fly's optic lobe exhibit circadian oscillations in morphology.

Circadian rhythms have been shown both in the expression of the period (per) gene in 'lateral neurons' and in cells of the outermost neuropil, or lamina, of the fly's optic lobe. Some lateral neurons also exhibit PDH peptide-like immunoreactivity, arborizing widely throughout the optic lobe. Using confocal microscopy in the housefly, we analysed the size and spacing of PDH neurite varicosities, sites of possible peptide release exhibiting circadian rhythmicity. During the subjective day in constant darkness, there were fewer, larger varicosities than during subjective night. The endogenous rhythm was masked by the light exposure that occurred under a day-night cycle and continuous light conditions. Our findings indicate that PDH neurites convey circadian information out from the pacemaker, where they could regulate the circadian rhythms that have been described in the lamina, possibly via cyclical release of their peptide.

Neurotransmitters regulate rhythmic size changes amongst cells in the fly's optic lobe.

Axon calibre in monopolar cells L1 and L2 of the fly's lamina can change dynamically. Swelling by day, L2 exhibits a daily rhythm of changing size apparently mediated by wide-field LBO5HT and PDH cells. L1/L2 axon profiles were measured planimetrically in the housefly, Musca domestica, from 1 microns cross sections. Four hours after injecting 80-100 nl of 1.25 x 10(-4) M 5-HT into the optic lobe, L1's axon swelled but L2's did not, whereas 2.2 x 10(-5) M of PDH enlarged both axons. Similar to 5-HT, 1.63 x 10(-4) M histamine (the photoreceptor transmitter) enlarged L1 but not L2, mimicking light exposure, while 1.7 x 10(-4) M glutamate and 1.94 x 10(-4) M GABA both decreased L1 and L2. 2.5 x 10(-4) M of 5,7-dihydroxytryptamine decreased L2 and, somewhat, L1, an effect attributable to the loss of LBO5HT neurites. Twenty four hours after cutting LBO5HT and PDH commissural pathways, L1 and L2 both shrank. Apparently, L2's size depends on either LBO5HT or sufficient 5-HT, and L1 and L2 have different response ranges to 5-HT. Responses to PDH imply that daytime PDH release drives a circadian rhythm, enlarging L1 and L2.

The rapid assembly of synaptic sites in photoreceptor terminals of the fly's optic lobe recovering from cold shock.

When a housefly, Musca domestica, is subject to cold exposure (0 degrees C for 24 hr), a number of obvious changes are seen in the first optic neuropil, or lamina, beneath the compound eye. In particular, the number of afferent photoreceptor synapses declines by about 30%. This loss is dramatically restored after warm recovery at 23 degrees C for 24 hr. Synapses disappear at an average rate of 2-3/hr during cold exposure and reappear at a maximal rate of more than 20/hr during the first 2 hr of warm recovery. Thereafter their number temporarily overshoots control values, to increase at 6 hr of warm recovery to 60% above their cold-exposed minimum. The number subsequently returns more or less to normal. These changes demonstrate the lability of synaptic sites under these conditions, with individual sites forming and disappearing rapidly. The changes also interrupt the close correlation between synaptic number and the surface area of the receptor terminal, a correlation that normally conserves synaptic spacing density. The density is preserved during cold exposure but increases during warm recovery at a time when the addition of newly formed synapses exceeds the slower increase in receptor terminal size.

Mechanisms of dendritic integration underlying gain control in fly motion-sensitive interneurons.

In the compensatory optomotor response of the fly the interesting phenomenon of gain control has been observed by Reichardt and colleagues (Reichardt et al., 1983): The amplitude of the response tends to saturate with increasing stimulus size, but different saturation plateaus are assumed with different velocities at which the stimulus is moving. This characteristic can already be found in the motion-sensitive large field neurons of the fly optic lobes that play a role in mediating this behavioral response (Hausen, 1982; Reichardt et al., 1983; Egelhaaf, 1985; Haag et al., 1992). To account for gain control a model was proposed involving shunting inhibition of these cells by another cell, the so-called pool cell (Reichardt et al., 1983), both cells sharing common input from an array of local motion detectors. This article describes an alternative model which only requires dendritic integration of the output signals of two types of local motion detectors with opposite polarity. The explanation of gain control relies on recent findings that these input elements are not perfectly directionally selective and that their direction selectivity is a function of pattern velocity. As a consequence, the resulting postsynaptic potential in the dendrite of the integrating cell saturates with increasing pattern size at a level between the excitatory and inhibitory reversal potentials. The exact value of saturation is then set by the activation ratio of excitatory and inhibitory input elements which in turn is a function of other stimulus parameters such as pattern velocity. Thus, the apparently complex phenomenon of gain control can be simply explained by the biophysics of dendritic integration in conjunction with the properties of the motion-sensitive input elements.

Daily and circadian rhythms of synaptic frequency in the first visual neuropile of the housefly’s ( Musca domestica L.) optic lobe

Photoreceptors of the fly's compound eye generally show no very obvious daily or circadian rhythms, a lack which prompted us to examine whether their function might be regulated not in the retina, but at the site of transmission in the first visual neuropile, or lamina. Here, photoreceptor terminals (R1-R6) are reciprocally interconnected with one class of lamina monopolar cell, L2: L2 receives input from R1-R6 at so-called tetrad synapses, and in turn is presynaptic to R1-R6 at feedback synapses. We have calculated the mean frequencies of these synaptic profiles in electron micrographs of single lamina sections. L2 feedback synapses were more numerous at night than during the day, whereas the number of tetrads showed only small modulations between day and night. These changes persisted amongst feedback synapses in flies held in constant darkness, and are thus circadian. In contrast to the slow modulations during a 24 h cycle, the number of L2 feedback synapses after 1 h light pulse in flies held in constant darkness showed no clear change, whereas it increased the number of tetrad profiles. These findings support the occurrence of cyclical daily and circadian changes amongst the two lamina synaptic populations, with tetrads showing rather weak modulations in frequency, but more pronounced responses to the light pulse than feedback synapses.

The effects of the loss of target cells upon photoreceptor inputs in the fly's optic lobe.

The sensitivity of sensory neurons to target cell denervation varies in the CNS. We have examined the effects of surgically interrupting the output axons of the first optic neuropil, or lamina, in the optic lobe of the fly (Musca domestica), upon the receptor terminal inputs to the lamina. Two of the output interneurons are the monopolar cells L1 and L2, which are found as a pair in each of the unit modules or cartridges of the lamina neuropil. The lamina axons of L1 and L2 degenerate rapidly (within 0.5 h) in a retrograde direction from their lesion site, but there is no sign of retrograde transneuronal degeneration to the receptor terminals, across the input synapse. At each of these synaptic sites, L1 and L2 are invariable contributors to two of the four elements of a postsynaptic tetrad. Not only do the receptor terminals persist, but the presynaptic ribbons at the tetrad sites do also, opposite the degenerated spines of L1 and L2, indicating their lack of target dependence at least over the longest period of post-lesion recovery (48 h) examined. The areal density of presynaptic sites was conserved in the face of the degenerative loss of L1 and L2, as were the numbers of capitate projections (glial invaginations into receptor terminals). The stability of both synaptic density and capitate projection number indicates that they are predominantly influenced by the receptor terminals, which are still intact. A reduction in the number of mitochondrial profiles was one of the few observed changes in the receptor terminals. The results reflect the autonomy which the terminals have, during development, from their interneurons; they especially reflect the role of the terminals in the adult, in maintaining the presynaptic site of their afferent synapses, the tetrads.

A test for multiplication in insect directional motion detectors

The H1 neuron is a directionally sensitive motion-detector neuron with a large field that is fed by many high-resolution motion detectors in the fly optic lobe. As a stimulus pattern for it we used a random pattern of 50% bright and 50% dark squares on an oscilloscope screen. When this pattern is jumped by a small increment the HI neuron gives a directional response. When the jump is greater than one pixel on the screen the response falls and becomes non-directional because jump direction can no longer be inferred. When the contrast is reversed at the jump, the response is the same for both directions, and is the same as when the contrast is reversed without motion. For the motion receptors this represents a nondirectional ‘on’ or ‘off’ response. The result is discussed with reference to theories of motion perception.

Descending neurons supplying the neck and flight motor of diptera: Organization and neuroanatomical relationships with visual pathways

In dipterous insects, a volume of behavioral and electrophysiological studies promote the contention that three wide-field motion-sensitive tangential neurons provide a necessary and sufficient input to specific channels that drive the torque motor during flight. The present studies describe the results of neuroanatomical investigations of the relationships between motion-sensitive neuropil in the fly optic lobes and descending neurons that arise from a restricted area of the brain and supply segmental neck and flight motor neuropil. The present observations resolve at least 50 pairs of descending neurons supplying flight motor centers in the thoracic ganglia. The majority of descending neurons receive a distributed output from horizontal motion-sensitive neurons. However, the same descending neurons are also visited by numerous small-field retinotopic neurons from the lobula plate as well as hitherto undescribed small tangential neurons. Neuroanatomical studies, using cobalt, Golgi, and Texas red histology, demonstrate that these smaller inputs onto descending neurons have dendrites that are organized at specific strata in retinotopic neuropil and that these correspond to horizontal and vertical motion sensitivity layers. Conclusions that only a restricted number of wide-field neurons are necessary and sufficient for visually stabilized flight may be premature. Rather, neuroanatomical evidence suggests that descending neurons to the flight motor may each be selectively tuned to specific combinations of wide- and small-field visual cues, so providing a cooperative descending network controlling the rich repertoire of visually evoked flight behavior.

A theory of insect vision: velocity parallax

Many insects show by their behaviour that they detect visually the existence of separate objects. The experimental material to analyse how they perceive objects is provided by an insect that walks to the end of a stick; then, because it has no alternative, it reaches with a foreleg towards a neighbouring object that it perceives to be within range. Some insects make horizontal peering movements as an aid to vision. The peering motion is exactly appropriate for generating an apparent velocity of nearby objects relative to the background. These experiments, when put together with the known properties of optic lobe neurons, suggest that a mechanism based on velocity parallax projected to the horizontal plane accounts for much insect visual behavour. Velocity parallax is defined as the discrepancy seen at the edge of an object against a distant background when the eye moves laterally. On this theory, perception of an object is inseparable from the local detection of velocity differences. The background may not be ‘perceived' at all when an object occurs in the foreground. The postulated mechanism is a two- or three-stage feedback, in which the perceived velocity (or, more accurately, the spatially correlated contrast frequency) in small-field motion-perception units is reduced by the averaged contrast frequency in larger fields, which feed back upon them. Contrast frequency is defined as the frequency of the flicker that is generated by a pattern moving across the eye. An alternative mechanism to the feedback of the velocity signal with lateral spread is adaptation to the local average background velocity, while sensitivity to a smaller local change in velocity is retained. That idea comes from recent work on the H1 neuron in the fly optic lobe, and could be the basis of a primitive form vision that, if present in mediumfield neurons, is adequate for the whole of the normal visual behaviour of a freely moving insect. These speculations invite a variety of experimental tests, ranging from visual discrimination tests with bees that are shown the velocity parallax situation, to appropriate stimulation of optic lobe neurons, to simulation of a visual processing system that relies on velocity parallax cues to detect objects.

Neuronal organization in fly optic lobes altered by laser ablations early in development or by mutations of the eye.

The role of afferent and efferent connections in the differentiation of optic lobe interneurons was investigated by using laser ablations of neuronal precursors in the brain of Musca domestica and analysis of two eye mutants of the same species. The first mutant, split eye, had no connections between the retina and the optic lobes. In this case the optic lobes were drastically reduced in volume and the neural organization within the neuropil regions was altered. The other mutant, spindle, had reduced retinae that innervated reduced optic lobes with a normal-appearing orderly arrangement of neurons. In addition disordered neuropil, composed of identified visual interneurons, was found that had no afferent innervation. Three main types of alterations resulting from laser ablations were analyzed. These ablations removed entire neuropil regions or parts of these: (1) removal of the first optic neuropil region (the lamina) resulting in receptor axons projecting directly to the second neuropil (the medulla) and sprouting of medulla neurons toward the receptor layer; (2) removal of one part of the third optic neuropil (the lobula plate) and severe alteration of the other part (the lobula) resulting in sprouting of lobula neurons into the medulla neuropil; and (3) removal of the entire optic lobe resulting in reduction of the volume of the lateral midbrain and photoreceptor axons forming a tangle beneath the retina. Our findings confirm that afferent retinal input is essential for normal differentiation and maintenance of many optic lobe interneurons. Furthermore, it was seen that a normal columnar organization of the neuropils and the dendritic patterns of visual interneurons are dependent on afferent inputs. A common response to removal of inputs was a reorganization of axonal and dendritic projections.

Responses of visual interneurons in the fly optic lobe during stimulation by motion in depth

A high-order wide-field neuron in the optic lobe of the fly,Phormia terraenovae (Calliphoridae), has been investigated by stimulation with simple or multiple black stripes on a white background moving sinusoidally towards and away from the head. The variation in spike frequency in correlation with the optical stimulation provides evidence that the unit detects angular velocity as well as more complex features of the stimulus. It responds preferentially to movement towards the head. Spike frequency turned out to be a two-valued function of relative angular velocity across the eye. A simple formula predicts the highly reliable responses to visual stimuli in different parts of the visual field.

Intracellular recording and staining of directionally selective motion detecting neurons in fly optic lobe

Intracellular recording and staining of directionally selective motion detecting neurons in fly optic lobe