Regulation of synaptic frequency: comparison of the effects of hypoinnervation with those of hyperinnervation in the fly's compound eye

The evolution and development of neural superposition

Visual systems have a rich history as model systems for the discovery and understanding of basic principles underlying neuronal connectivity. The compound eyes of insects consist of up to thousands of small unit eyes that are connected by photoreceptor axons to set up a visual map in the brain. The photoreceptor axon terminals thereby represent neighboring points seen in the environment in neighboring synaptic units in the brain. Neural superposition is a special case of such a wiring principle, where photoreceptors from different unit eyes that receive the same input converge upon the same synaptic units in the brain. This wiring principle is remarkable, because each photoreceptor in a single unit eye receives different input and each individual axon, among thousands others in the brain, must be sorted together with those few axons that have the same input. Key aspects of neural superposition have been described as early as 1907. Since then neuroscientists, evolutionary and developmental biologists have been fascinated by how such a complicated wiring principle could evolve, how it is genetically encoded, and how it is developmentally realized. In this review article, we will discuss current ideas about the evolutionary origin and developmental program of neural superposition. Our goal is to identify in what way the special case of neural superposition can help us answer more general questions about the evolution and development of genetically “hard-wired” synaptic connectivity in the brain.

Optic lobe development

The optic lobes comprise approximately half of the fly's brain. In four major synaptic ganglia, or neuropils, the visual input from the compound eyes is received and processed for higher order visual functions like motion detection and color vision. A common characteristic of vertebrate and invertebrate visual systems is the point-to-point mapping of the visual world to synaptic layers in the brain, referred to as visuotopy. Vision requires the parallel extraction of numerous parameters in a visuotopic manner. Consequently, the optic neuropils are arranged in columns and perpendicularly oriented synaptic layers that allow for the selective establishment of synapses between columnar neurons. How this exquisite synaptic specificity is established during approximately 100 hours of brain development is still poorly understood. However, the optic lobe contains one of the best characterized brain structures in any organism-both anatomically and developmentally. Moreover, numerous molecules and their function illuminate some of the basic mechanisms involved in brain wiring. The emerging picture is that the development of the visual system of Drosophila is (epi-)genetically hard-wired; it supplies the emerging fly with vision without requiring neuronal activity for fine tuning of neuronal connectivity. Elucidating the genetic and cellular principles by which gene activity directs the assembly of the optic lobe is therefore a fascinating task and the focus of this chapter.

Visual circuit development in Drosophila

Fly visual circuits are organized into lattice-like arrays and layers. Recent genetic studies have provided insights into how these reiterated structures are assembled through stepwise processes and how precise connections are established during development. Afferent-derived morphogens, such as Hedgehog, play a key role in organizing the overall structure by inducing and recruiting target neurons and glia. In turn, the target-derived ligand DWnt4 guides Frizzled2-expressing photoreceptor afferents to their proper destination. Photoreceptor afferents select specific synaptic targets by forming adhesive interactions and regulating actin cytoskeleton in growth cones. Target specificity is probably achieved by restricting the expression of adhesive molecules, such as Capricious, to appropriate presynaptic and postsynaptic partners, and by differentially regulating the function of broadly expressed adhesive molecules such as N-cadherin.

Activity-independent prespecification of synaptic partners in the visual map of Drosophila

Specifying synaptic partners and regulating synaptic numbers are at least partly activity-dependent processes during visual map formation in all systems investigated to date . In Drosophila, six photoreceptors that view the same point in visual space have to be sorted into synaptic modules called cartridges in order to form a visuotopically correct map . Synapse numbers per photoreceptor terminal and cartridge are both precisely regulated . However, it is unknown whether an activity-dependent mechanism or a genetically encoded developmental program regulates synapse numbers. We performed a large-scale quantitative ultrastructural analysis of photoreceptor synapses in mutants affecting the generation of electrical potentials (norpA, trp;trpl), neurotransmitter release (hdc, syt), vesicle endocytosis (synj), the trafficking of specific guidance molecules during photoreceptor targeting (sec15), a specific guidance receptor required for visual map formation (Dlar), and 57 other novel synaptic mutants affecting 43 genes. Remarkably, in all these mutants, individual photoreceptors form the correct number of synapses per presynaptic terminal independently of cartridge composition. Hence, our data show that each photoreceptor forms a precise and constant number of afferent synapses independently of neuronal activity and partner accuracy. Our data suggest cell-autonomous control of synapse numbers as part of a developmental program of activity-independent steps that lead to a "hard-wired" visual map in the fly brain.

Development and structure of synaptic contacts in Drosophila

Structural synapses are key regulators of information flow in neuronal networks. To understand the function and formation of neuronal circuits, the development and function of synapses have therefore been intensely studied in both vertebrate and invertebrate species. Precise descriptions of synapses and their amenability to genetic analysis in the model organism Drosophila provide an efficient platform from which to explore mechanisms and principles of synapse formation, which find many counterparts in other animals. Here we summarise our knowledge of the structure of Drosophila synapses. Focussing on neuromuscular junctions and photoreceptor synapses, we provide an overview of mechanisms underlying the development of synaptic structure in Drosophila.

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.

Reactive synaptogenesis following degeneration of photoreceptor terminals in the fly’s optic lobe: a quantitative electron microscopic study

Following photo-ablation of receptor cells in the retina of the housefly's compound eye, their synaptic terminals degenerate with a timecourse which we have followed over 8 d. Degeneration deprives the monopolar interneurons in the first optic neuropile, the lamina, of their main synaptic input. Simultaneously it deprives one monopolar interneuron (L2) of one of its synaptic targets, as L2 makes numerous feedback synaptic contacts at which it is pre-synaptic upon receptor terminals. Because the feedback synapses are dyadic, input still remains available to the second element post-synaptic at the dyad, which does not degenerate. This element is T1, a higher-order interneuron from the next most proximal neuropile (the medulla). Some of the original feedback synaptic sites soon disappear as a consequence of the photo-ablation, but their loss is partly offset by the production of new synaptic contacts. The new pre-synaptic ribbons resemble those at the original sites except for being smaller. The sites are, moreover, monadic, with T1 now the sole post-synaptic partner. These results show that interneurons in the fly's lamina retain a dynamic capacity for synaptogenesis throughout much of adult life, normally a few weeks in Musca, and that during this synaptogenesis they re-enact the same cell preferences expressed earlier in development.

Evidence for site selection during synaptogenesis: the surface distribution of synaptic sites in photoreceptor terminals of the files Musca and Drosophila

1. Photoreceptor terminals in the flies Musca domestica and Drosophila melanogaster have been reconstructed in three dimensions from serial EM to reveal the surface distributions of afferent tetrad synapses. 2. The terminals are cylindrical and surround two target cells; they have synaptic sites distributed along their length and around their circumference, except for a strip along the face that lies furthest away from the target cells. 3. Over their inner faces, the terminals have presynaptic sites that are distributed evenly. 4. The distribution of sites in maps plotted from reconstructed membrane surfaces was examined by quadrat analyses. The frequency of sites per quadrat division was not Poissonian, i.e. was non-random. Thus, some form of site selection must exist during synaptogenesis. 5. The sites were shown by variance ratio analysis to be regular (evenly dispersed, not clustered). This suggests that some form of interaction exists, so as to reduce the probability that a synapse will form close to an already existing synaptic site. 6. Distances between nearest-neighbour pairs of synapses had a closest minimum spacing of about 0.8 micron in Musca that was violated by about 5% of pairs, whereas the corresponding distances were about 0.2 micron shorter in Drosophila, which had 13% of pairs situated closer together than 0.8 micron. 7. During synaptogenesis, either initially in the pupa or later in the adult, the probability that a synapse will form is therefore effectively zero within these distances from an existing synaptic site, perhaps through an inhibitory influence exerted by the latter. The nearest-neighbour distances are normally distributed. 8. Unlike the distribution of presynaptic sites, the distribution of postsynaptic sites over the surfaces of the dendrites of the target cells is not even. Although not studied in detail, the corresponding nearest-neighbour distances are much smaller, as little as 0.1 micron. Thus the wider spacing seen between sites over the receptor terminals is a function of the presynaptic cells, and not of their postsynaptic partners, and implies the existence of interactions between synaptic sites.

Ultrastructure and quantification of synapses in the insect nervous system

Standard EM methods can be successfully used to reveal the various organelles of synaptic junctions in different insect species. The individual junctions of a synaptic class exhibit a high level of morphological stereotypy, but the study of serial sections is generally necessary to understand the different appearances of a junction's profiles when it is cut in different planes. Most synaptic profiles seen in single sections may then be attributed to one or a few morphological classes, not to many. Probably most central synapses are of the multiple-contact type, containing a number of postsynaptic elements, with the diversity of the combinations of these providing the major difference between particular synaptic junctions. The different profiles of a synapse when cut serially in oblique, non-canonical section planes provide the investigator with search images, prior knowledge of which is needed for a comprehensive identification of synaptic sites in single sections. The latter can be used to describe the synaptic organization of an unknown neuropile from the variety of synaptic contacts that form between different neurons. This requires that continuity be established between a postsynaptic dendrite and its parent axon, and that the position of the axon can then be used to identify the neuron of origin. Tracing between dendrite and axon can be undertaken either systematically in serial sections of a restricted region or by protracted searches of single sections. The number of synaptic profiles in a single section can be used to estimate the number of synaptic contacts, either in relative terms, as the number of profiles per section in different cells, or as the absolute number of synapses per cell. The latter requires use of correction formulae, taking into account the influence of section thickness and of the mean size of the synaptic junction on the number of synaptic profiles recorded in a particular section.

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.

The early causal influence of cell size upon synaptic number: the mutant gigas of Drosophila

The number of synaptic contacts formed by a neuron is known to vary with its surface area. This could be because large neurons are able to establish more synaptic sites, or because those neurons that are able to establish more sites are subsequently able to enlarge. To test between these two possibilities clones of enlarged ommatidia were generated in the retina of the Drosophila mutant gigas, by mitotic recombination following gamma-irradiation in the third-instar larva. The numbers of afferent synaptic contacts formed by the photoreceptor terminals in the first optic neuropil, or lamina, were then counted in the adult. The terminals of mutant photoreceptors were also enlarged, but by varying degrees. The sizes of their profiles in single sections merged with the size distribution of terminals having a wild-type phenotype, lying outside the clone in the same lamina. A perimeter of 6.0 microns for the profiles of receptor terminal in cross section was established as a criterion for distinguishing between normal and mutant phenotypes. The mutant terminals had more presynaptic sites. Because only the gigas terminals are mutant and because they enlarged at a time long before synapse formation occurred in the lamina we may conclude that cell enlargement preceded elevated synaptic number. The increase in synaptic number roughly matched the increased membrane surface of the terminals, so as nearly to preserve a constant areal density of synaptic sites over a 5-fold range in synaptic frequency.

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.

mip causes hyperinnervation of a retinotopic map in Drosophila by excessive recruitment of R7 photoreceptor cells

The two central photoreceptor neurons of the Drosophila eye, R7 and R8, form a retinotopic map in the optic lobe of the fly brain. We have developed a technique that allows us to visualize the projections of these neurons with high resolution. Using this technique, we have identified a new mutant, mip (more inner photoreceptors), in which this map shows a striking hyperinnervation. The extra terminals in the brain derive from an excessive recruitment of sevenless-independent R7 photoreceptor cells during eye development. The original R7, however, remains sevenless responsive. The behavior of this gene suggests that recruitment to the R7 pathway, and possibly to multiple programs in ommatidial assembly, is partially regulated by inhibition.

The optic lobe of Drosophila melanogaster. II. Sorting of retinotopic pathways in the medulla

We present a quantitative evaluation of Golgi-impregnated columnar neurons in the optic lobe of wild-type Drosophila melanogaster. This analysis reveals the overall connectivity pattern between the 10 neuropil layers of the medulla and demonstrates the existence of at least three major visual pathways. Pathway 1 connects medulla layer M10 to the lobula plate. Input layers of this pathway are M1 and M5. Pathway 2 connects M9 to shallow layers of the lobula, which in turn are tightly linked to the lobula plate. This pathway gets major input via M2. Pathways 1 and 2 receive input from retinula cells R1-6, either via the lamina monopolar cell L1 (terminating in M1 and M5) or via L2 and T1 (terminating in M2). Neurons of these pathways typically have small dendritic fields. We discuss evidence that pathways 1 and 2 may play a major role in motion detection. Pathway 3 connects M8 to deep layers of the lobula. In M8 information converges that is derived either from M3 (pathway 3a) or from M4 and M6 (pathway 3b), layers that get their major input from L3 and R8 or L4 and R7, respectively. Some neurons of pathway 3 have large dendritic fields. We suggest that they may be involved in the computation of form and colour. Possible analogies to the organization of pathways in the visual system of vertebrates are discussed.

Fly photoreceptor synapses: their development, evolution, and plasticity

Recent studies are reviewed on the synapses of photoreceptor terminals in the first optic neuropile of the flies, Musca and Drosophila. Afferent synaptic contacts are of uniform dimensions; they have a postsynaptic tetrad with a membrane organization of P-face particles, resembling other inhibitory synapses. A distributed population of such contact sites forms progressively during synaptogenesis by the selective, sequential accretion of identified postsynaptic elements at the receptor terminal. The comparative anatomy of this synapse indicates that elements have also been added during its phylogeny from an ancestral dyad. All cells are homologs of those in other species of Diptera. The number of synaptic sites is regulated by both pre- and postsynaptic cells, in proportion to their cell surfaces; an independent size increase in the receptor terminals (procured in the Drosophila mutant gigas) produces an increase in their synaptic population. The number of sites declines with age, however, accompanied by an increase in size of those synaptic sites remaining; this occurs for both afferent and feedback photoreceptor synapses. Lastly, the number of sites changes with visual experience; the frequency of feedback synapses is larger following dark rearing during early adult life than following visual experience.