The number and structure of giant vertical cells (VS) in the lobula plate of the blowflyCalliphora erythrocephala

Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons

Most of our knowledge about insect calcium currents is derived from studies on cultured or dissociated somata. So far, only little data on calcium currents are available for neurons including their dendritic and presynaptic structures. Here we combined the switched-electrode voltage-clamp technique with optical recording using calcium-sensitive dyes in identified fly visual interneurons in vivo to characterize the voltage dependence and dynamics of calcium currents quantitatively and in a spatially resolved way. For all three cell types considered, i.e., centrifugal horizontal (CH), horizontal system (HS), and vertical system (VS) cells, the activation curve is rather flat and covers a voltage range from -60 to -20 mV in dendritic as well as presynaptic areas of the cells. The calcium increase is fastest for CH cells with a time constant of approximately 70 ms. In HS and VS cells, the time constant amounts to 400-700 ms. The calcium dynamics as determined in different regions of the cells are similar except for a small segment between the axon and the dendrite in HS and VS cells, where the calcium increase is significantly faster. In summary, the results show the existence of a low-voltage-activated calcium current with little or no inactivation in dendritic as well as presynaptic regions of fly lobula plate tangential cells.

Neurotransmitter regulation of circadian structural changes in the fly's visual system

The visual system of the fly's compound eye undergoes a number of cyclical day/night changes that have a circadian basis. Such responses are seen in the synaptic terminals of the photoreceptors and in their large monopolar-cell interneurons in the first optic neuropile, or lamina. These changes include, in the photoreceptor terminals, rhythms in the numbers of synapses and the vertical migration of screening pigment; and, in the monopolar cells L1 and L2, a rhythm in the transients of the electroretinogram and in the cyclical swelling of L1 and L2 lamina axons, as well as of the epithelial glia that surround these. Some of these changes are seen in both the housefly and the fruit fly, but the time-course of such changes differs between the two species. Many of the changes are influenced by the injection of various transmitter candidates, in a direction that can be reconciled with the possibility of normal endogenous release of two substances, 5HT from the neurites of 5HT-immunoreactive neurons, and pigment dispersing factor peptide from the neurites of PDH cells. Consistent with this interpretation, the immunoreactive varicosities of PDH cells exhibit size changes attributable to their cyclical release of peptide, or to its cyclical synthesis and/or transport from the PDH cell somata. Thus, neurotransmitter substances not only have rapid electrophysiological actions in the optic lobe, but also longer-lasting, presumably indirect, neuromodulatory actions, which are manifest as structural changes among the lamina's neurons and synapses. These actions involve an interplay between aminergic and peptidergic systems, but the exact role and especially the site of action of each has still to be elucidated.

Dendritic integration and its role in computing image velocity

The mechanisms underlying visual motion detection can be studied simultaneously in different cell compartments in vivo by using calcium as a reporter of the spatiotemporal activity distribution in single motion-sensitive cells of the fly. As predicted by the Reichardt model, local dendritic calcium signals are found to indicate the direction and velocity of pattern motion but are corrupted by spatial pattern properties. The latter are canceled out by spatial integration, thus leading to a purely directional selective output signal in the axon. These findings attribute a specific computational task to the dendrites of visual interneurons and imply a functional interpretation of dendritic morphology.

Dendritic structure and receptive-field organization of optic flow processing interneurons in the fly

The third visual neuropil (lobula plate) of the blowfly Calliphora erythrocephala is a center for processing motion information. It contains, among others, 10 individually identifiable "vertical system" (VS) neurons responding to visual wide-field motions of arbitrary patterns. We demonstrate that each VS neuron is tuned to sense a particular aspect of optic flow that is generated during self-motion. Thus the VS neurons in the fly supply visual information for the control of head orientation, body posture, and flight steering. To reveal the functional organization of the receptive fields of the 10 VS neurons, we determined with a new method the distributions of local motion sensitivities and local preferred directions at 52 positions in the fly's visual field. Each neuron was identified by intracellular staining with Lucifer yellow and three-dimensional reconstructions from 10-micron serial sections. Thereby the receptive-field organization of each recorded neuron could be correlated with the location and extent of its dendritic arborization in the retinotopically organized neuropil of the lobula plate. The response fields of the VS neurons, i.e., the distributions of local preferred directions and local motion sensitivities, are not uniform but resemble rotatory optic flow fields that would be induced by the fly during rotations around various horizontal axes. Theoretical considerations and quantitative analyses of the data, which will be presented in a subsequent paper, show that VS neurons are highly specialized neural filters for optic flow processing and thus for the visual sensation of self-motions in the fly.

A fast stimulus procedure to determine local receptive field properties of motion-sensitive visual interneurons

We present a method to determine, within a few seconds, the local preferred direction (LPD) and local motion sensitivity (LMS) in small patches of the receptive fields of wide-field motion-sensitive neurons. This allows us to map, even during intracellular recordings, the distribution of LPD and LMS over the huge receptive fields of neurons sensing self-motions of the animal. Comparisons of the response field of a given neuron with the optic flow fields caused by different movements in space, allows us to specify the particular motion of the animal sensed by that neuron.

Estimation of self-motion by optic flow processing in single visual interneurons

Humans, animals and some mobile robots use visual motion cues for object detection and navigation in structured surroundings. Motion is commonly sensed by large arrays of small field movement detectors, each preferring motion in a particular direction. Self-motion generates distinct 'optic flow fields' in the eyes that depend on the type and direction of the momentary locomotion (rotation, translation). To investigate how the optic flow is processed at the neuronal level, we recorded intracellularly from identified interneurons in the third visual neuropile of the blowfly. The distribution of local motion tuning over their huge receptive fields was mapped in detail. The global structure of the resulting 'motion response fields' is remarkably similar to optic flow fields. Thus, the organization of the receptive fields of the so-called VS neurons strongly suggests that each of these neurons specifically extracts the rotatory component of the optic flow around a particular horizontal axis. Other neurons are probably adapted to extract translatory flow components. This study shows how complex visual discrimination can be achieved by task-oriented preprocessing in single neurons.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: I. Passive membrane properties

The passive membrane properties of the tangential cells in the fly lobula plate (CH, HS, and VS cells, Fig. 1) were determined by combining compartmental modeling and current injection experiments. As a prerequisite, we built a digital base of the cells by 3D-reconstructing individual tangential cells from cobalt-stained material including both CH cells (VCH and DCH cells), all three HS cells (HSN, HSE, and HSS cells) and most members of the VS cell family (Figs. 2, 3). In a first series of experiments, hyperpolarizing and depolarizing currents were injected to determine steady-state I-V curves (Fig. 4). At potentials more negative than resting, a linear relationship holds, whereas at potentials more positive than resting, an outward rectification is observed. Therefore, in all subsequent experiments, when a sinusoidal current of variable frequency was injected, a negative DC current was superimposed to keep the neurons in a hyperpolarized state. The resulting amplitude and phase spectra revealed an average steady-state input resistance of 4 to 5 M omega and a cut-off frequency between 40 and 80 Hz (Fig. 5). To determine the passive membrane parameters Rm (specific membrane resistance), Ri (specific internal resistivity), and Cm (specific membrane capacitance), the experiments were repeated in computer simulations on compartmental models of the cells (Fig. 6). Good fits between experimental and simulation data were obtained for the following values: Rm = 2.5 k omega cm2, Ri = 60 omega cm, and Cm = 1.5 microF/cm2 for CH cells; Rm = 2.0 k omega cm2, Ri = 40 omega cm, and Cm = 0.9 microF/cm2 for HS cells; Rm = 2.0 k omega cm2, Ri = 40 omega cm, and Cm = 0.8 microF/cm2 for VS cells. An error analysis of the fitting procedure revealed an area of confidence in the Rm-Ri plane within which the Rm-Ri value pairs are still compatible with the experimental data given the statistical fluctuations inherent in the experiments (Figs. 7, 8). We also investigated whether there exist characteristic differences between different members of the same cell class and how much the exact placement of the electrode (within +/-100 microns along the axon) influences the result of the simulation (Fig. 9). The membrane parameters were further examined by injection of a hyperpolarizing current pulse (Fig. 10). The resulting compartmental models (Fig. 11) based on the passive membrane parameters determined in this way form the basis of forthcoming studies on dendritic integration and signal propagation in the fly tangential cells (Haag et al., 1997; Haag and Borst, 1997).

Oculomotor control in calliphorid flies: GABAergic organization in heterolateral inhibitory pathways

In calliphorid Diptera, motor neurons mediating visually evoked head movements can be excited or inhibited by visual stimuli, depending on the directionality of the stimulus and whether it is in the ipsi- or contralateral visual field. The level at which inhibition occurs is of special interest because binocular activation of homolateral tangential neurons in the lobula plate demonstrates that excitatory interaction must occur between the left and right optic lobes. Recordings and dye fillings demonstrate a variety of motion-sensitive heterolateral pathways between the lobula plates, or between them and contralateral deutocerebral neuropil, which provides descending pathways to neck motor centers. The profiles of heterolateral tangential cells correspond to neurons stained by an antibody against gamma-aminobutyric acid (GABA). Other GABA-immunoreactive interneurons linking each side of the brain correspond to uniquely identified motion-sensitive neurons linking the deutocerebral. Additional inhibitory pathways include heterolateral GABAergic descending and ascending neurons, as well as heterolateral GABAergic neurons in the thoracic ganglia. The functional significance of heterolateral GABAergic pathways was tested surgically by making selective microlesions and monitoring the oculomotor output. The results demonstrate an important new attribute of the insect visual system. Although lesions can initially abolish an excitatory or inhibitory response, this response is reestablished through alternative pathways that provide inhibitory and excitatory information to the same motor neurons.

A preparation of the blowfly (Calliphora erythrocephala) brain for in vitro electrophysiological and pharmacological studies

We describe a method for the preparation and maintenance of the blowfly (Calliphora erythrocephala) brain in a recording chamber under in vitro conditions in a semi-slice configuration. Large identification neurones in the posterior part of the 3rd optic lobe (lobula plate) can be penetrated easily with microelectrodes. The so-called vertical system (VS) cells which respond to vertical image motion in vivo could be encountered best because their axons are escorted individually by specific tracheae. Fluorescent stained cells show their natural shape as being in vivo. Electrophysiological properties of the cells investigated so far, i.e., resting potential (about -40 mV) and firing properties (single rebound spikes), are comparable to recordings in intact flies. Initial pharmacological experiments on VS cells in this preparation reveal that iontophoretical application of acetylcholine and carbamylcholine results in depolarization. VS cells also respond to bath-applied nicotine (1 microM) with a slow depolarization of their membrane potential in normal fly saline as well as in a Ca(2+)-free saline, suggesting direct cholinergic input via nicotinic receptors. The suitability of the preparation for a wide range of electrophysiological and pharmacological studies is discussed.

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.

Expression of the Drosophila optomotor-blind gene transcript in neuronal and glial cells of the developing nervous system

Mutations in the complex gene locus optomotor-blind (omb) can lead to defects in the development of both the optic lobes and external features of the adult fly. We describe here the expression of omb in the developing and adult nervous system using in situ hybridization. During embryogenesis, omb expression is first observed in the optic lobe anlagen. It later expands to a larger part of the developing larval brain and to the gnathal lobes. Cells in the ventral and peripheral nervous systems begin to express omb after completion of germ band extension. Later in embryonic development, expression declines and only persists in the antennomaxillary complex and in part of the brain hemispheres. During the larval and pupal stages, omb expression in the brain is confined to the developing optic lobes and contiguous regions of the central brain. At these stages, only a few cells show expression in the ventral ganglion. In the eye imaginal disc, transcript accumulation is most conspicuous in a group of presumptive glia precursor cells posterior to the morphogenetic furrow and in the optic stalk. In the adult brain, expression is prominent in several regions of the optic lobe cortex and along the border between central brain and optic lobes. In the mutation In(1)ombH31, 40 kb of regulatory DNA, downstream from the transcription unit, are removed from the omb gene. In(1)ombH31 is characterized by the lack of a set of giant interneurons from the lobula plate of the adult optic lobes. We find that, already during embryogenesis, there is a drastic difference between wild type and In(1)ombH31 in the level of the omb transcript in the optic lobe primordia. The adult mutant phenotype may thus be caused by omb misexpression during embryonic development.

Two visual systems in one brain: neuropils serving the secondary eyes of the spider Cupiennius salei

Like other araneans, the wandering spider Cupiennius salei is equipped with one pair of principal eyes and three pairs of secondary eyes. Primary and secondary eyes serve two distinct sets of visual neuropils in the brain. This paper describes cellular organization in neuropils supplied by the secondary eyes, which individually send axons into three laminas resembling their namesakes serving insect superposition eyes. Secondary eye photoreceptors send axons to small-field projection neurons (L-cells) which extend from each lamina to supply three separate medullas. Each medulla is a vault of neuropil comprising only a few morphological types of neurons. These can be compared to a subset of retinotopic neurons in the medullas of calliphorid Diptera supplying giant motion-sensitive neurons in the lobula plate. In Cupiennius, neurons from secondary eye medullas converge at a single target neuropil called the "mushroom body." This region contains giant output neurons which, like their counterparts in the calliphorid lobula plate, lead to descending pathways that supply thoracic motor circuits. It is suggested that the cellular arrangements serving Cupiennius's secondary eyes are color independent pathways specialized for detecting horizontal motion. The present results do not support the classical view that the spider "mushroom body" is phylogenetically homologous or functionally analogous to its namesake in insects.

Dendritic integration of motion information in visual interneurons of the blowfly

Dendritic integration plays a key role in the way information is processed by nerve cells. The large motion-sensitive interneurons of the fly appear to be most appropriate for an investigation of this process. These cells are known to receive input from numerous local motion-sensitive elements and to control visually-guided optomotor responses (e.g., Trends Neurosci., 11 (1988) 351-358; Stavenga and Hardie, Facets of Vision, Springer, 1989). The retinotopic input organization of these cells allows for in vivo stimulation of selected parts of their dendritic tree with their natural excitatory and inhibitory synaptic input signals. By displaying motion in either the cells' preferred or null direction in different regions of the receptive field we found: (i) Responses to combinations of excitatory and inhibitory motion stimuli can be described as the sum of the two response components. (ii) Responses to combination of excitatory stimuli show saturation effects. The deviation from linear superposition depends on the distance and relative position of the activated synaptic sites on the dendrite and makes the responses almost insensitive to the number of activated input channels. (iii) The saturation level depends on different stimulus parameters, e.g. the velocity of the moving pattern. The cell still encodes velocity under conditions of spatial saturation. The results can be understood on the basis of passive dendritic integration of the signals of retinotopically organized local motion-detecting elements with opposite polarity.

Small-field neurons associated with oculomotor and optomotor control in muscoid flies: functional organization

In fleshflies, Sarcophaga bullata, intracellular recording and Lucifer yellow dye-filling have revealed small-field elements of sexually isomorphic retinotopic arrays in the lobula and lobula plate, the axons of which project to premotor channels in the deutocerebrum that supply head-turning and flight-steering motor neurons. The dendrites of the small-field elements visit very restricted oval areas of the retinotopic mosaic, comprising fields that are typically 6-8 input columns wide and 12-20 high. Their physiologically determined receptive fields are also small, typically 20 degrees or less in diameter. The neurons are hyperpolarized in stationary illumination and are transiently depolarized by light OFF and to a lesser degree by light ON. Motion of a striped grating elicits a periodic excitation at the fundamental or second harmonic of the stimulus temporal contrast frequency. The arrangement of these elements in retinotopic arrays with their small receptive fields and flicker-sensitive dynamic properties make these neurons well suited for the position-dependent, direction-insensitive detection of small objects in the fly's visual field, which is known to drive fixation and tracking.

Small-field neurons associated with oculomotor control in muscoid flies: cellular organization in the lobula plate

In muscoid flies, the lobula plate is the last station in the optic lobes for processing spectrally independent information from retinotopic afferents. Until recently, it was thought that most lobula plate neurons were color-insensitive wide-field tangential neurons that respond to direction-specific motion. It has been suggested that certain of these supply inputs to premotor descending neurons involved in the control of flight and head movements. The present account describes a Golgi and cobalt-silver analysis that reports evidence for additional lobula plate outputs, which are numerically complex and structurally elaborate. Beneath a retina with approximately 4,000 ommatidia, each of at least 15 populations of morphologically distinct small-field neurons comprises approximately 110-440 elements that contribute to an isomorphic neural assembly subtending the whole retina. Morphologically small-field efferents form three classes according to the origin of their axons and their arborization in the lobula plate and lobula. Neurons arising from the lobula plate, or shared by it and the lobula, visit dorsal descending neurons supplying the neck and flight motor in contrast to output neurons from the lobula, which project to ventral descending neurons supplying leg motor neuropils. The possible functional significance of small-field lobula plate outputs onto descending neurons in the dorsal deutocerebrum is discussed.

How to use Heidenhains azan staining in insects

A newly modified, histochemical Azan staining procedure which was originally designed by Heidenhain was adapted for insects. This led to new insights in the structure of the central nervous system of flies. The somata are weakly stained with a red colored nucleus. Neuroglial regions are stained intensively red. Some undefined portions of the neuronal tracts seem to be uncolored but the unstained profiles are still visible. The other portions contain blue colored cells. These differences may be due to the cells' different monoamine content. Using this method new morphological substructures could be identified in both the mushroom bodies and the central complex of the central nervous system that do not seem to be described earlier.

Visual interneurons in the lobula complex of the fleshfly, Boettcherisca peregrina

The physiology and morphology of visual interneurons in the lobula complex of the fleshfly, Boettcherisca peregrina, were studied using intracellular recordings and intracellular cobalt stainings, respectively. Using responses to movements of a spot of light and on-off stimuli at single positions, we classified the interneurons into five physiological groups ON, OFF, ON-OFF, non-directional motion sensitive (NDM) and directional motion sensitive (DM) neurons. They could be further divided into four morphological types, depending on the location and extent of their dendrites and terminal branches.

Cobalt-coupled neurons of a giant fibre system in Diptera

Certain intact nerve cells in flies can be filled with cobalt from presynaptic or postsynaptic neurons. This cobalt coupling is best demonstrated in giant fibre systems where the phenomenon was originally termed 'transsynaptic staining'. Fine structural analysis of silver-intensified, cobalt-coupled neurons indicates that the passage of cobalt ions occurs at gap junctions that are accompanied by conventional chemical synapses. Cobalt-coupled systems in dipterous insects are uniquely identifiable and can always be detected between the same kinds of neurons. The visualization of cobalt-coupled neurons allows the identification of functional pathways linking the brain to motor neuropils.