cDNA cloning and nuclear localization of the circadian neuropeptide designated as pigment-dispersing factor PDF in the cricket Gryllus bimaculatus

Pigment-dispersing factor (PDF) was recently reported to be a principal circadian neuromodulator involved in transmitting circadian rhythms of daily locomotion in insects. In Drosophila, PDF functions in some of the neurons expressing the clock genes period, timeless, Clock, and cycle, and those clock genes in turn regulate pdf gene expression. In the present study, we cloned a cDNA encoding PDF in the brain of a nocturnal insect, the cricket Gryllus bimaculatus, and found that an isolated clone (310 bp) codes for an extraordinarily short precursor protein with no definite signal sequence, but a nuclear localization signal (NLS)-like sequence instead. The cricket PDF exhibits high sequence identity (78-94%) and similarity (89-100%) to insect PDFs and also to crustacean beta-PDH peptides. In the optic lobes of G. bimaculatus there are PDF-immunoreactive neurons in both the medulla and lamina neuropiles. Among the strongly immunoreactive lamina PDF neurons, on electron microscopy we identified cells exhibiting distinct staining that is not only cytoplasmic but also nuclear. When GFP-fused PDF precursor proteins were expressed in COS-7 cells, distinct translocation of the fusion protein into the nucleus was observed. This is the first finding of PDF peptide in the nucleus, which suggests a fundamental role of PDF peptide per se in the circadian clock system.

Synaptic organization of the mushroom body calyx in Drosophila melanogaster

The calyx neuropil of the mushroom body in adult Drosophila melanogaster contains three major neuronal elements: extrinsic projection neurons, presumed cholinergic, immunoreactive to choline acetyltransferase (ChAT-ir) and vesicular acetylcholine transporter (VAChT-ir) antisera; presumed gamma-aminobutyric acid (GABA)ergic extrinsic neurons with GABA-like immunoreactivity; and local intrinsic Kenyon cells. The projection neurons connecting the calyx with the antennal lobe via the antennocerebral tract are the only source of cholinergic elements in the calyces. Their terminals establish an array of large boutons 2-7 microm in diameter throughout all calycal subdivisions. The GABA-ir extrinsic neurons, different in origin, form a network of fine fibers and boutons codistributed in all calycal regions with the cholinergic terminals and with tiny profiles, mainly Kenyon cell dendrites. We have investigated the synaptic circuits of these three neuron types using preembedding immuno-electron microscopy. All ChAT/VAChT-ir boutons form divergent synapses upon multitudinous surrounding Kenyon cell dendrites. GABA-ir elements also regularly contribute divergent synaptic input onto these dendrites, as well as occasional inputs to boutons of projection neurons. The same synaptic microcircuits involving these three neuron types are repeatedly established in glomeruli in all calycal regions. Each glomerulus comprises a large cholinergic bouton at its core, encircled by tiny vesicle-free Kenyon cell dendrites as well as by a number of GABAergic terminals. A single dendritic profile may thereby receive synaptic input from both cholinergic and GABAergic elements in close vicinity at presynaptic sites with T-bars typical of fly synapses. ChAT-ir boutons regularly have large extensions of the active zones. Thus, Kenyon cells may receive major excitatory input from cholinergic boutons and considerable postsynaptic inhibition from GABAergic terminals, as well as, more rarely, presynaptic inhibitory signaling. The calycal glomeruli of Drosophila are compared with the cerebellar glomeruli of vertebrates. The cholinergic boutons are the largest identified cholinergic synapses in the Drosophila brain and an eligible prospect for studying the genetic regulation of excitatory presynaptic function.

Endophilin mutations block clathrin-mediated endocytosis but not neurotransmitter release

We have identified mutations in Drosophila endophilin to study its function in vivo. Endophilin is required presynaptically at the neuromuscular junction, and absence of Endophilin dramatically impairs endocytosis in vivo. Mutant larvae that lack Endophilin fail to take up FM1-43 dye in synaptic boutons, indicating an inability to retrieve synaptic membrane. This defect is accompanied by an expansion of the presynaptic membrane, and a depletion of vesicles from the bouton lumen. Interestingly, mutant larvae are still able to sustain release at 15%-20% of the normal rate during high-frequency stimulation. We propose that kiss-and-run maintains neurotransmission at active zones of the larval NMJ in endophilin animals.

Digital in-line holography for biological applications

Digital in-line holography with numerical reconstruction has been developed into a new tool, specifically for biological applications, that routinely achieves both lateral and depth resolution, at least at the micron level, in three-dimensional imaging. The experimental and numerical procedures have been incorporated into a program package with a very fast reconstruction algorithm that is now capable of real-time reconstruction. This capability is demonstrated for diverse objects, such as suspension of microspheres and biological samples (diatom, the head of Drosophila melanogaster), and the advantages are discussed by comparing holographic reconstructions with images taken by using conventional compound light microscopy.

The larval ascidian nervous system: the chordate brain from its small beginnings

The body plan of the tadpole larva of ascidians, or sea-squirts, is widely presumed to be close to that of the hypothetical ancestor of all chordate animal groups, including vertebrates. This is nowhere more obvious than in the organization and development of the dorsal tubular nervous system. Several recent developments advocate this model neural system for studies on neurobiology and neurogenesis. These include advances in our understanding of development in ascidian embryos and of differentiation among the cellular progeny of its neural plate; the application of transgenic and mutant approaches to studies on ascidian larval neurones; and the prospect of advances in genomic analyses. In addition to providing ways to study a working chordate brain in miniature, all these offer insights into the ancestral condition of the developing vertebrate brain.

Neuronal form in the central nervous system of the tadpole larva of the ascidian Ciona intestinalis

The dorsal tubular central nervous system (CNS) of the ascidian tadpole larva is a diagnostic feature by which the chordate affinities of this group, as a whole, are recognized. We have used two methods to identify larval neurons of Ciona intestinalis. The first is serial electron microscopy (EM), as part of a dedicated study of the visceral ganglion (1), and the second is the transient transfection of neural plate progeny with green fluorescent protein (GFP) (2), to visualize the soma and its neurites of individual neurons in whole-mounted larvae of C. intestinalis. Our observations reveal that ascidian larval neurons are simple inform, with a single axonal neurite arising from a soma that is either monopolar or has only very few, relatively simple neurites arising from it, as part of a presumed dendritic arbor. Somata in the visceral ganglion giving rise to axons descending in the caudal nerve cord are presumed to be those of motor neurons.

The central nervous system, its cellular organisation and development, in the tadpole larva of the ascidian Ciona intestinalis

From its numerical composition, the central nervous system (CNS) of the ascidian larva is one of the simplest known nervous systems having a chordate plan. Fewer than 350 cells together constitute a caudal nerve cord, an interposed visceral ganglion containing motor circuits for swimming and, rostrally, an expanded sensory vesicle containing major sensory and interneuron regions of the CNS. Some cells are ependymal, with ciliated surfaces lining the neural canal, while others are clearly either sensory receptors or motoneurons, but most are distinguishable only on cytological grounds. Although reassignments between categories are still being made, there is evidence for determinancy of total cell number. We have made three-dimensional cell maps either from serial semithin sections, or from confocal image stacks of whole-mounted embryos and larvae stained with nuclear markers. Comparisons between the maps of neural tubes in embryos of successive ages, that is, between cells in one map and their progeny in older maps, enable us to follow the line of mitotic descent through successive maps, at least for the caudal neural tube. Details are clear for the lateral cell rows in the neural tube, at least until the latter contains approximately 320 cells, and somewhat for the dorsal cell row, but the ventral row is more complex. In the hatched larva, serial-EM reconstructions of the visceral ganglion reveal two ventrolateral fibre bundles at the caudalmost end, each of 10-12 axons. These tracts include at least five pairs of presumed motor axons running into the caudal nerve cord. Two pairs of axons decussate. Complementing this vertebrate feature in the CNS of the larval form of Ciona, we confirm that synapses form upon the somata and dendrites of its neurons, and that its motor tracts are ventral.

Visualization of synaptic markers in the optic neuropils of Drosophila using a new constrained deconvolution method

The fruitfly Drosophila melanogaster offers compelling genetic advantages for the analysis of its nervous system, but cell size precludes immunocytochemical analysis of wild-type structure and mutant phenotypes beyond the level of neuronal arborizations. For many antibodies, especially when immunoelectron microscopy is not feasible, it would therefore be desirable to extend the resolution limit of confocal microscopy as far as possible. Because high-resolution confocal microscopy suffers from considerable blurring, so-called deconvolution algorithms are needed to remove, at least partially, the blur introduced by the microscope and by the specimen itself. Here, we present the establishment and application of a new deconvolution method to visualize synaptic markers in Drosophila optic neuropils at the resolution limit of light. We ascertained all necessary parameters experimentally and verified them by deconvolving injected fluorescent microspheres in immunostained optic lobe tissue. The resulting deconvolution method was used to analyze colocalization between the synaptic vesicle marker neuronal synaptobrevin and synaptic and putative synaptic markers in photoreceptor terminals. We report differential localization of these near the resolution limit of light, which could not be distinguished without deconvolution.

The determination of histamine in the Drosophila head

Histamine is a neurotransmitter at arthropod photoreceptors. Even though the fruit fly, Drosophila melanogaster, is a widely used model in neuroscience research, the histamine content of its nervous system has not so far been reported. We have developed a high performance liquid chromatography (HPLC) method with pre-column o-phtaldialdehyde-mercaptoethanol (OPA-ME) derivatization and electrochemical detection, to determine this amine in Drosophila. The histamine content of the fly's head averages about 2.0 ng per head. In heads of the mutant hdc(JK910), a presumed null for the gene encoding the enzyme that synthesizes histamine, histamine was not detected in measurable amounts. In heads of the mutant sine oculis, which lacks compound eyes, only 28% of this amine was found compared with wild type flies, so histamine is mainly present in the compound eye photoreceptors. Also observed in histamine-deficient mutants was a decrease in the peak which contains a substance having the same retention time as carcinine (beta-alanyl-histamine). Our method was not able to detect compounds previously reported as histamine metabolites in insects. In spite of this, the method we have developed enables the fast and accurate measurement of histamine in the heads of Drosophila, suitable for screening mutants.

Organization of efferent peripheral synapses at mechanosensory neurons in spiders

The mechanosensory neurons of arachnids receive diverse synaptic inputs in the periphery. The function of most of these synapses, however, is unknown. We have carried out detailed electron microscopic investigations of the peripheral synapses at sensory neurons in the compound slit sense organ VS-3 of the spider Cupiennius salei. Based on the localization of discrete presynaptic vesicle populations, it is possible to discriminate at least four different synapse types, containing either: (1) small round, electron-lucent vesicles 32 nm in diameter; (2) large round, clear 42-nm vesicles; (3) a mixture of small and large clear, round vesicles, similar in size to those in Type 1 and Type 2 synapses, respectively, and granular and dense-core vesicles; or (4) clear, round 37- to 65-nm vesicles. Combined immunocytochemical labeling at the light and the electron microscopic level suggests that gamma-aminobutyric acid (GABA) is the transmitter in many of the 32-nm vesicle synapses, and glutamate in many of the 42-nm ones. Based on vesicle type and particular synaptic configuration, various forms of presumed efferent synaptic contacts are distinguishable with the sensory neurons, the surrounding glia, and between the putative efferent fibers themselves. These include simple unidirectional synapses, reciprocal synapses, serial synapses, and convergent as well as divergent dyads. These various synaptic microcircuits are suited to serve a variety of functions. Among these are direct postsynaptic inhibition or excitation of the mechanosensory neurons, and disinhibition or sensitization via presynaptic inhibition or excitation. The observed synaptic configurations are compared with those at the crustacean muscle receptor organ. They reveal a remarkable complexity of synaptic microcircuits at spider sensilla and suggest manifold possibilities for subtle, efferent control of sensory activity.

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.

Circadian rhythms in light-evoked responses of the fly's compound eye, and the effects of neuromodulators 5-HT and the peptide PDF

Two sets of wide-field neurons extend neurites into the fly's optic lamina, where monopolar cells receive photoreceptor input. They exhibit immunoreactivity to antibodies raised against either 5-hydroxytryptamine or the crustacean peptide PDH, respectively. Both are proposed whole-field neuromodulators of vision, apparently regulating a circadian rhythm of monopolar cell size. Seeking functional correlates, we have re-examined the electroretinogram for circadian rhythmicity, and for responses to locally injected 5-hydroxytryptamine and peptide. Long-term electroretinogram recordings from Calliphora entrained to a light/dark cycle and then transferred to constant darkness, uncovered a gradual, modest increase during the subjective night in the electroretinogram's ON- and OFF-transients, from the lamina's monopolar cells. Five to twenty nl of 5-hydroxytryptamine (10(-3) mol.1(-1)) injected into the head haemolymph strongly enhanced the electroretinogram transients, an action reversed by 5-hydroxytryptamine antagonists. Injected into the eye, 5-hydroxytryptamine (10(-4) mol.1(-1)) had the opposite effect; the rapid onset there suggests direct action, whilst the opposing effect from haemolymph injection suggests a different receptor site. Pigment-dispersing hormone (2.2 x 10(-5) mol.1(-1)) injected into the haemolymph increased the electroretinogram transients along a biphasic course, with a slow partial recovery; injected into the eye, it lacked effect.

Extraretinal photoreceptors at the compound eye's posterior margin in Drosophila melanogaster

Many invertebrates have supplementary extraocular photoreceptors that often are implicated in circadian rhythms. An extraretinal group of candidate photoreceptors in the fruit fly, Drosophila melanogaster, has been revealed previously at the posterior margin of the compound eye by using a photoreceptor-specific monoclonal antibody (Hofbauer and Buchner [1989] Naturwissen 76:335-336), but it never has been characterized. Here, we report the fine structure of this cell cluster reported by Hofbauer and Buchner, which is called "eyelet," as well as the further candidacy of their visual pigment and neurotransmitter. Eyelet forms a specialized, pigmented organ with cells that have numerous microvilli arranged into coherent rhabdomeres. The presence of rhabdomeric microvilli is a defining feature of a photoreceptor, reported here for the first time in eyelet. The rhabdomeres exhibit Rh6 opsin-like immunoreactivity, which provides evidence that the photoreceptors are functional: they fail to immunostain with antibodies against NINAE (Rh1), Rh4, or Rh5. The photoreceptors have been shown previously to exhibit histamine-like immunoreactivity, but they also stain with a monoclonal antiserum raised against Drosophila choline acetyltransferase (ChAT), suggesting that the photoreceptors not only may contain histamine but also can synthesize acetylcholine. A ChAT-immunoreactive axon bundle originating from eyelet terminates in the cortex of the anterior medulla. This bundle also is seen with reduced silver stains. Electron microscopic examination revealed four axon profiles of similar size in this bundle, indicating that eyelet contains at least four photoreceptors. The pathway of eyelet's axon bundle coincides with the precocious pathway of Bolwig's nerve that arises from the larval organ of sight. The origin and possible function of eyelet are discussed.

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.

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.

Peripheral synapses at identified mechanosensory neurons in spiders: three-dimensional reconstruction and GABA immunocytochemistry

The mechanosensory organs of arachnids receive diverse peripheral inputs. Little is known about the origin, distribution, and function of these chemical synapses, which we examined in lyriform slit sense organ VS-3 of the spider Cupiennius salei. The cuticular slits of this organ are each associated with two large bipolar mechanosensory neurons with different adaptation rates. With intracellular recording, we have now been able to correlate directly the staining intensity of a neuron for acetylcholinesterase with its adaptation rate, thus allowing us simply to stain a neuron to identify its functional type. All rapidly adapting neurons stain more heavily than slowly adapting neurons. Immunostaining of whole-mount preparations reveals GABA-like immunoreactive fibers forming numerous varicosities at the surface of all sensory neurons in VS-3; peripheral GABA-like immunoreactive somata are lacking. Sectioning the leg nerve procures rapid degeneration of most fiber profiles, confirming that the fibers are efferent. Punctate synapsin-like immunoreactivity colocalizes to these varicosities, although some synapsin-like immunoreactive puncta are GABA-immunonegative. Fibers with similar immunoreactivities are also associated with trichobothria, tactile hairs, internal joint receptors, i.e. other types of spider mechanosensory organs. In organ VS-3, immunoreactivity is most dense across the initial axon segment. The exact distribution of peripheral synapses was reconstructed from a 10-microm-long electron micrograph series of the dendritic, somatic, and initial axon regions of acetylcholinesterase-stained VS-3 neurons. These reveal a pattern similar to that of the synapsin-like immunoreactivity. Two different types of synapse were distinguished on the basis of their presynaptic vesicle populations. Many peripheral synapses thus appear to derive from efferent GABA-like immunoreactive fibers and probably provide centrifugal inhibitory control of primary mechanosensory activities.

Developmental anatomy of the Drosophila brain: neuroanatomy is gene expression

On-line databases of anatomical information are being compiled for a number of genetically manipulable organisms, including the fruit fly, Drosophila melanogaster. Based on the success of the molecular databases that preceded them, they face formidable problems in data cataloguing, storage, and retrieval. The prospect for such databases, which is apparent already, is to alter permanently the approach to neuroanatomy in such species. Experience with Drosophila indicates the possibility to arbitrate controversies over and, in some cases, to redefine the borders drawn in the brain by conventional neuroanatomical methods. Two publications in this issue of Journal of Comparative Neurology by Nassif et al. and by Hartenstein et al. highlight one of the first demonstrations of a further opportunity in Drosophila. In some cases, it is already possible to suggest how individual cellular elements--neurons, tracts, and neuropil regions--might be traced from the time when they first express a precocious marker, such as the product of the fasciclin-II gene, through the metamorphic pupal stage, and into the adult. In this way, it becomes possible to identify the structures of the adult brain from the time of their first emergence in the embryo and to follow their transitional positions throughout the course of development. Critical in this process is the neuroanatomical organization of the larval brain, which contains not only the fully functional central nervous system of the larva but also the developing elements of the adult brain, because this holds the key to understanding both the cellular elements that are inherited from the embryo and the elements that are in the process of forming the adult nervous system.

Neurotransmitters alter the numbers of synapses and organelles in photoreceptor terminals in the lamina of the housefly, Musca domestica

Various organelles in the lamina terminals of housefly photoreceptors exhibit daily rhythms having a circadian basis. These include changes in the numbers of photoreceptor tetrad and L2 feedback synapses, and longitudinal movements of screening pigment. Circadian information has previously been suggested to spread from the clock to the lamina via widefield cells expressing either 5-hydroxytryptamine or pigment-dispersing hormone-like immunoreactivity. We examined the action of these neuromodulators, and other candidate neurotransmitters, 4 h after injecting either the transmitter or a control into the medulla. We counted electron microscope profiles of organelles that normally exhibit circadian changes, and two types of invagination into photoreceptor terminals, capitate projections and inter-receptor invaginations. No single substance mediated the changes observed. Injected pigment-dispersing hormone peptide decreased the number of pigment granules, implicating this peptide in screening pigment migration, but produced no changes in synapse-related organelles. alpha-Aminobutyric acid exclusively decreased the number of L2 feedback synapses. Responses to other transmitters were specific, and often large, but generally not statistically significant. Histamine, for example, may decrease the number of tetrads, possibly by direct autoregulation. The results suggest that there is likely to be more than one effector in the circadian pathways to the lamina.

Regulated spacing of synapses and presynaptic active zones at larval neuromuscular junctions in different genotypes of the flies Drosophila and Sarcophaga

Synapses at larval neuromuscular junctions of the flies Drosophila melanogaster and Sarcophaga bullata are not distributed randomly. They have been studied in serial electron micrographs of two identified axons (axons 1 and 2) that innervate ventral longitudinal muscles 6 and 7 of the larval body wall. The following fly larvae were examined: axon 1--wild-type Sarcophaga and Drosophila and Drosophila mutants dunce(m14) and fasII(e76), a hypomorphic allele of the fasciclin II gene; and axon 2--drosophila wild-type, dunce(m14), and fasII(e76). These lines were selected to provide a wide range of nerve terminal phenotypes in which to study the distribution and spacing of synapses. Each terminal varicosity is applied closely to the underlying subsynaptic reticulum of the muscle fiber and has 15-40 synapses. Each synapse usually bears one or more active zones, characterized by dense bodies that are T-shaped in cross section; they are located at the presumed sites of transmitter release. The distribution of synapses was characterized from the center-to-center distance of each synapse to its nearest neighbor. The mean spacing between nearest-neighbor pairs ranged from 0.84 microm to 1.05 microm for axon 1, showing no significant difference regardless of genotype. The corresponding values for axon 2, 0.58 microm to 0.75 microm, were also statistically indistinguishable from one another in terminals of different genotype but differed significantly from the values for axon 1. Thus, the functional class of the axon provides a clear prediction of the spacing of its synapses, suggesting that spacing may be determined by the functional properties of transmission at the two types of terminals. Individual dense bodies were situated mostly at least 0.4 microm away from one another, suggesting that an interaction between neighboring active zones could prevent their final positions from being located more closely.

The shaking B gene in Drosophila regulates the number of gap junctions between photoreceptor terminals in the lamina

The molecular structure of insect gap junctions differs from that in vertebrates, and in Drosophila is possibly encoded by the shaking B (= Passover) locus. shaking B2 is a null allele that acts in the nervous system. In the shakB2 mutant, one site of action are gap junctions between photoreceptor terminals in the cartridges of the lamina, beneath the compound eye, which we assayed from the number of close-apposition profiles in thin-section EM. The number of profiles in the Canton-S (C-S) wild type is about 0.5 per cartridge per section in distal and mid-lamina depths, and significantly less, about one quarter this value, closer to the brain, in the proximal lamina. In shakB2, there are fewer profiles, approximately one quarter the number of appositions in distal and mid-lamina depths as in C-S, and their number does not differ significantly from those at the proximal depth in either the mutant or wild type. Thus mutant action is associated with a reduced number of appositions at distal and mid-lamina depths. We propose that R1-R6 gap junctions are partitioned into at least two strata, proximal and distal, and that two populations of gap junctions exist, one extending throughout the lamina that does not require shakB, and a second at distal and mid-depth levels, which does. The number of gap junctions is reduced in mutant shakB2, and surviving appositions at distal and middle lamina depths possibly have wider clefts than in C-S. Gap junctions are reduced equally between all R1-R6 terminals, so the two different types of junction proposed, shakB2- and non-shakB2-dependent, can apparently express in a single receptor terminal.

A rapid method for combined laser scanning confocal microscopic and electron microscopic visualization of biocytin or neurobiotin-labeled neurons

Intracellular recording and dye filling are widely used to correlate the morphology of a neuron with its physiology. With laser scanning confocal microscopy, the complex shapes of labeled neurons in three dimensions can be reconstructed rapidly, but this requires fluorescent dyes. These dyes are neither permanent nor electron dense and therefore do not allow investigation by electron microscopy. Here we report a technique that quickly and easily converts a fluorescent label into a more stable and electron-dense stain. With this technique, a neuron is filled with Neurobiotin or biocytin, reacted with fluorophore-conjugated avidin, and imaged by confocal microscopy. To permit long-term storage or EM study, the fluorescent label is then converted to a stable electron-dense material by a single-step conversion using a commercially available ABC kit. We find that the method, which apparently relies on recognition of avidin's excess biotin binding sites by the biotin-peroxidase conjugate, is both faster and less labor intensive than photo-oxidation procedures in common use. The technique is readily adaptable to immunocytochemistry with biotinylated probes, as we demonstrate using anti-serotonin as an example.