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.

The effects of 20-hydroxyecdysone on the differentiation in vitro of cells from the eye imaginal disc from Drosophila melanogaster

We have examined the effects of the insect ecdysteroid, 20-hydroxyecdysone, on the differentiation of neuronal and non-neuronal elements in the developing adult visual system, using in vitro methods in Drosophila. We examined the differentiation of early neuronal markers in the presence and absence of 1 microgram/ml 20-hydroxyecdysone. Immunoreactivity to 22C10, a marker of an early neuronal antigen, as well as to the photoreceptor-specific antibody 24B10, suggests that differentiation of neuronal and photoreceptor antigens does not require 20-hydroxyecdysone. In eye-discs cultured from animals 5 hours after the white prepupa (P + 5), ommochrome pigmentation first appeared after 2 days in 1 microgram/ml 20-hydroxyecdysone, but cultures lacked pigmentation without 20-hydroxyecdysone. Our culture conditions failed to support the formation of the second screening pigment, drosopterins, even with 20-hydroxyecdysone. Eye discs from P + 5 also formed lenses and interommatidial bristles in culture when 20-hydroxyecdysone was added but not in cultures devoid of the hormone. The differentiation of synaptotagmin and the elongation of extending photoreceptor neurites from eye disc fragments both occur in the absence of 20-hydroxyecdysone in cultures, but adding the hormone increased average neurite length. The threshold for enhanced neurite length was less than 125 ng/ml 20-hydroxyecdysone. Eye-disc cultures also developed immunoreactivity to histamine, the photoreceptor transmitter, from synthesis not re-uptake, in both the presence and in the absence of 20-hydroxyecdysone. These findings suggest that photoreceptor axons may be able to release transmitter in vivo both when they grow into the optic lobe and during the subsequent events in synapse formation.

Circadian rhythms in screening pigment and invaginating organelles in photoreceptor terminals of the housefly's first optic neuropile

Screening pigment granules occur in the synaptic terminals of photoreceptors in the fly's (Musca domestica, L.) compound eye. The granules resemble ommochrome granules in the overlying photoreceptor cell body. There are also two types of invagination into receptor terminals: capitate projections (from glial cells) and invaginations from neighboring receptor terminals. The number of profiles of these organelles in the first optic neuropile, the lamina, have been counted using single-section quantitative electron microscopic methods. Pigment granules are concentrated proximally in the terminal, toward the brain. The numbers change, increasing during the night (1 h after lights off) up to values more than twice the number 1 h after lights on, apparently by longitudinal migration of granules from the cell body into the terminal. Files entrained to day/night conditions and then held under constant darkness continue to exhibit changes in the numbers of profiles. Even though overall there were 80-90% fewer granule profiles than under day/night conditions, the numbers attained a peak many times higher at the end of the subjective day. Thus, the changes are endogenous, showing circadian rhythmicity. Although their significance is unknown, these changes parallel previously described circadian rhythms in the receptor terminals and their lamina monopolar-cell targets. The invaginations from receptor terminals were more numerous under day/night conditions than under constant darkness, and cycled in constant darkness, peaking at the end of subjective night. Capitate projections, by contrast, failed to change significantly under the experimental conditions analyzed, a lack of responsiveness they share with photoreceptor tetrad synapses.

Experience-dependent developmental plasticity in the optic lobe of Drosophila melanogaster

Early experience can affect nervous system development in both vertebrate and invertebrate animals. We have now demonstrated that visual stimulation modifies the size of the optic lobes in the laboratory fruitfly Drosophila melanogaster. Monocular deprivation (painting over one eye) decreases the aggregate volume of the lamina, medulla, and lobula plate by up to 6%. The laminae of control flies kept in complete darkness showed a more robust volume difference that could be as much as 30%. An electron microscopy study revealed that the changes in the lamina are largely attributable to an increase in the terminals of the photoreceptor cell axons. The volume of the lamina increases during the first 24 hr after emergence, and it grows more in the light than in darkness. When flies are kept in the dark for the first 12 hr of their adult life and are then brought back to light for the next 3.5 days, the lamina is almost as small as in flies raised for 4 d in constant darkness. Twelve hour dark shifts at a later time are less effective. This finding suggests a critical period for lamina development during day 1 of the adult. The lamina depends on visual stimulation to maintain its size during the first 5 d after emergence. Dark-rearing for 1 d or more at any stage during that period decreases its volume to the level of flies raised in constant darkness. A lamina that is once reduced in size seems not to recover.

The effects of light reversals on photoreceptor synaptogenesis in the fly Musca domestica

The short-term influence of visual experience was studied in the first-order tetrad synapses of the first optic neuropil, or lamina, underlying the compound eye in the housefly (Musca domestica). We report the effects of single light reversals, in which flies reared in constant darkness were exposed to light or those in constant light were exposed to darkness, on this defined population of synapses. The synaptic population was characterized by quantitative electron microscopic methods using three parameters: (i) the number of synaptic contacts per photoreceptor terminal; (ii) the size of these synapses, given by the mean platform width of their presynaptic ribbons; and (iii) the proportion (up to approximately 20%) of small synapses-those having a ribbon platform less than 0.17 micron wide. The effects of light exposures in flies reared in constant darkness include decreased mean synaptic size and increased numbers of synapses. These effects are seen in flies of all ages (to 10 days). Long light exposures (2-6 h) are much less effective than short exposures (down to 20 s), with maximum effects obtained at 15 min (up to 45% more synapses). Small synapses are reasoned to be new junctions formed only recently; the decreased mean synaptic size and the increased number of synapses seen after short light exposures are both interpreted to result from a recent burst of synaptogenesis in the adult lamina. The effects of dark exposure in flies reared in constant light are the reciprocal of those seen in dark-reared flies exposed to light, but they are less pronounced. Although the function of such changes is not yet known, they may form part of the light adaptation mechanism of the photoreceptor, and occur along with a redistribution of other organelles involving membrane invaginations into its terminal. These changes occur against a background trend for control flies reared under constant conditions to have fewer, larger synapses with increasing age up to 10 days, an effect that is most pronounced in constant darkness, when synaptic number decreases by 21% and size increases by 13%.

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.

Daily rhythms in cells of the fly's optic lobe: taking time out from the circadian clock

Considerable progress has recently been reported in locating the cellular basis and molecular mechanisms of the circadian clock in the fruitfly, Drosophila melanogaster. To advance beyond the clock, towards the outputs that lie between the clock itself and the circadian rhythms in behaviour that it regulates, will present new challenges. This is because most behaviours are generated by complex neuronal circuits, which are themselves difficult to unravel. Recently described anatomical changes in the optic lobe of the related housefly, Musca domestica, exhibit a circadian rhythm that is, by contrast, relatively easy to assay. This rhythm is apparently controlled by at least two sets of diffuse modulatory neurones. One of these, immunoreactive to the peptide pigment-dispersing hormone, also expresses in Drosophila the product of the period (per) gene, the most widely studied of the so-called clock genes that are essential for the correct expression of circadian rhythmicity. The second, called LBO5HT, is immunoreactive to 5-HT, a widely invoked transmitter system in insect circadian rhythms. The identification of these elements, and a widening cascade of events which their actions apparently trigger, opens up new opportunities to examine old problems in the regulation of circadian rhythms in the nervous system.

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.

Conditions for the primary culture of eye imaginal discs from Drosophila melanogaster

We have established a primary culture system for Drosophila eye imaginal discs. With this system, we were able to obtain neurite outgrowth from intact eye discs, eye disc fragments, and dissociated eye imaginal disc cells. Immunoreactivity to antibody 24B10 indicates that these extending neurites are photoreceptor axons. Three culture media were tested for their ability to support the survival of and neurite extension from eye disc fragments in vitro at 23 degrees C. These, with supplements, were: five parts of Schneider's Drosophila medium with four parts of basal Eagle's medium ("4 + 5"); Leibovitz's L-15 medium (L-15); and Shields and Sang's M3 modified medium (MM3). We obtained the best results with MM3 supplemented with 2% fetal bovine serum (FBS). Eye disc fragments survived in this medium for at least 20 days. Pigmentation in the nonphotoreceptor pigment cells in cultures from the prepupa required the presence of 20-hydroxyecdysone (20-HE) (1 micrograms/ml), whereas neurite outgrowth was seen in the absence of 20-HE. Donor animals had to fall within a range of ages to obtain appropriate eye disc differentiation in vitro. Eye disc from 5-h pupae (P + 5) or older commenced ommachrome synthesis in vitro in a temporal sequence close to that found in vivo, whereas the in vitro synthesis of this pigment was delayed in eye discs from younger flies. Average neurite length was not affected by age among pupae younger than P + 5; but neurite outgrowth from P + 24 was scarce, probably because by this time photoreceptor axons had already grown in vivo and were severed and unable to regenerate in vitro. Eye discs taken from third instar larvae or white prepupae continued their mitotic activity in vitro. Together with the advance of the morphogenetic furrow at the leading edge of retinal development, this observation is consistent with the evidence that pattern formation continues in vitro. Morphogenetic changes were manifested in cultures. Viability tests with calcein AM and ethidium bromide revealed few dead cells in living cultures.

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.

Monopolar cell axons in the first optic neuropil of the housefly, Musca domestica L., undergo daily fluctuations in diameter that have a circadian basis

Two types of monopolar cell interneurons, each with a single representative in every unit cartridge of the first optic neuropil, or lamina, of the housefly's optic lobe, have axons that undergo cyclical changes in diameter. The axons are largest during the beginning of day in a normal LD light cycle and smallest during the middle of the night, changes that were however significant only for one of the cells (L2). The axon cross-sectional area and its cyclical change for both L1 and L2 were both larger in the proximal lamina. The changes are not a simple consequence of relative osmotic change. Dehydration paradoxically increases axon size, and also fails to alter the day/night rhythm of axon size changes. Under conditions of constant darkness, both axons decrease in size, and one of the cells (L2) retains its cyclical size changes, being larger in the subjective day than in the subjective night. Under conditions of constant light, both axons increase in size, and L2 again shows a cyclical size change, just as under conditions of constant darkness. These changes seen under constant conditions are, by definition, circadian in origin. The effects of exposure to light or darkness can partially reset these circadian changes. One extra hour of light during the day increases the size of L1 and L2, whereas 1 hr of extra dark during the night does not decrease their size. It takes 13 hr of light to reverse the rhythm in size. The mechanism for all these changes is unclear but may involve ionic fluxes, possibly that are secondary to osmotic shifts and probably that involve at least two independent processes.

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.

Cell recognition during synaptogenesis is revealed after temperature-shock-induced perturbations in the developing fly's optic lamina

Houseflies (Musca domestica) were exposed to pulses of heat (1 h) or cold (several hours) during early pupal life, and the effects were investigated on the development of the first optic neuropile, or lamina, of the visual system. The treatments were designed to perturb the cellular organization of the cartridges, the unit synaptic structures of the lamina, so as to provide novel synaptic opportunities among the normally fixed composition of these modules, thereby testing the preferences of their component cells during synaptogenesis. Various abnormalities were identified, but these were not always consistent between flies: retinal abnormalities included the loss and fusion of rhabdomeres, especially of the central cells of the ommatidium, whereas in the lamina low frequencies of abnormal cartridges were found. These included seven that were studied with serial sections, which instead of the normal pair of L1 and L2 monopolar interneurons had supernumerary cells of this type. The normal pairing of L1 and L2 at postsynaptic sites of receptor terminal tetrad synapses was preserved in these cases, the cells eschewing pairings of homologous L1/L2 or L2/L2 partners. This meant that more than one L1 could pair with a single L2 and vice versa, even at the same terminal, and appeared to do so opportunistically on the basis of proximity, with cells closer to each other pairing more frequently. Thus the cells behave during synaptogenesis as if they recognize other cells only as cell types (receptor, L1 or L2) and not as individual cells.

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 Patterns of Bromodeoxyuridine Incorporation in the Nervous System of a Larval Ascidian, Ciona intestinalis

The fates of cells from the anterior region of the ascidian neural plate are described either as neural or as mixed neural and non-neural. In Ciona intestinalis, all cellular progeny are accounted for until a time 60% between the onset of embryonic development and larval To resolve the issue of their fates in this species, we have examined the later mitotic history of neural-plate cells. Because cessation of cell division in the neural plate has been claimed to occur at 70% of embryonic development, we need to account for cell production from 60% onward, to determine whether more cells are produced than populate the larval CNS, allowing some to adopt non-neural fates. The embryonic incorporation of bromodeoxyuridine (BrdU), 500 {mu}M in seawater, was monitored in 1 -h larvae by anti-BrdU immunocytochemistry. The pattern of incorporations indicates that all larval neurons are born before 70% of embryonic development, but that cell division unexpectedly continues to generate ependymal cells until at least 95%. Divisions in the neurohypophysis continue throughout embryonic development. The total number of cells produced appears sufficient only to complete the complement of larval CNS cells, denying non-neural fates for anteriorly migrating neural plate cells, and indicating a general absence of cell death. Consistent numbers of incorporations after the same exposure in different larvae provide evidence for determinacy of neural plate lineages. The last three conclusions confirm those reached previously (Nicol and Meinertzhagen, 1988b).

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.

Invagination of presynaptic ribbons in the fly's optic lobe following loss of their target neuron

In the first optic neuropile of the housefly Musca, photoreceptor terminals innervate fixed clusters of interneurons, one of which is the monopolar cell L2; L2's synapses in turn feed back upon the terminals. We examined the ultrastructure of these feedback synapses following degeneration of their normal targets, the receptor terminals; this was accomplished by photo-ablating the receptor cells after intraretinal injections of sulforhodamine. Even when all the terminals degenerated, their deafferentated target cells, including L2, remained structurally intact for at least 14 d. Despite this lack of obvious trans-synaptic degeneration, L2's synaptic connections did alter. Presynaptic organelles of the feedback synapses, synaptic ribbons and associated synaptic vesicles, soon appeared in L2's cytoplasm, separating from their site of attachment at the presynaptic membrane by invagination. Similar free-floating organelles and vesicles also occurred in another monopolar cell, L4. They were also occasionally encountered in L2, in normal, newly emerged flies at a time when a naturally occurring loss of feedback synapses is greatest. We interpret the process of internalization that forms these floating ribbons to be the first step in synaptic loss which occurs spontaneously, and that the rate is enhanced in L2 when its main synaptic targets, the receptor terminals, degenerate.

Cell counts and maps in the larval central nervous system of the ascidian Ciona intestinalis (L.)

Although the ascidian tadpole larva harbors a prospectively valuable prototype of the chordate nervous system, with extensively characterized neural plate cell lineages, the simple cellular composition of the resultant central nervous system (CNS) is not documented in detail. The average total number of cells in the larval CNS of Ciona intestinalis is 335 (range +/- 4, n = 3), 65 or 66 of which reside in the nerve cord of the tail. The estimates were made by tracing and counting the number of nuclei in serial semithin (1 micron) sections cut longitudinally through three larvae, fixed no later than 2 hours after hatching. Within a single fourth larva, L4, 266 cells constituted the CNS in the trunk region of the larva, 45 of which occurred within the visceral ganglion, 215 in the sensory vesicle, and 6 in the neck between the two. Each cell was assigned to one of thirteen categories. Most (182, roughly 68%) are classified as ependymal, a specialized non-neural cell peculiar to embryonic and larval chordates, from their position lining the cavities of the neural tube's elaborations or from clear similarities in the cytological appearance to those that do. Five cells are accessory cells of the sensory structures: three lens cells and a pigment-cup cell in the ocellus, and a single pigment cell in the otolith. Of the remaining 79 cells, 36 are sensory, 17 receptors in the ocellus and 19 presumed hydrostatic pressure receptors; these lie on the right and left sides of the sensory vesicle, respectively. Eighteen of the visceral ganglion cells have been tentatively classified as neurons, as have the remaining 25 cells which form two clusters in the posterior region of the sensory vesicle.

Terminal degeneration and synaptic disassembly following receptor photoablation in the retina of the fly's compound eye

A long-term objective of our studies on the first optic neuropil (or lamina) underlying the fly's compound eye is to explore how afferent photoreceptor synapses disappear during normal adult experience. To increase the frequency of this loss and the chances for its detection artificially, we have examined in this study the synapses during the degeneration of their presynaptic elements, the synaptic terminals of the receptor cells. This may be reliably procured by illuminating for 12 min with strong green light eyes that have received an injection of the dye sulforhodamine 101 (Picaud et al., 1988). The lesion is local and develops rapidly. Degeneration among terminals is progressive but asynchronous. There are several different types of degeneration, most interpretable as stages in a temporal progression after illumination-induced injury. Degenerative changes include shrinkage and darkening of terminals and mitochondrial swelling. Synaptic sites are lost in a defined sequence: (1) the T-shaped presynaptic ribbon disappears first; (2) the members of what is normally a tetrad of postsynaptic elements withdraw as an ensemble from the receptor terminal's membrane, and the surrounding epithelial glial cells extend between former pre- and postsynaptic partners; and (3) the postsynaptic elements then separate from each other. In the most rapidly affected terminals, the frequencies for those synaptic sites at which both presynaptic ribbons and postsynaptic elements remain intact decline by 85%, even in the first 8 hr postillumination.

Synaptic organization of columnar elements in the lamina of the wild type in Drosophila melanogaster

The synaptic connections within the lamina, the first of the optic neuropiles underlying the insect's compound eye, have been little studied in Drosophila melanogaster until now, despite the genetic advantages of this animal. Here we report the reconstruction through its entire depth of one of the lamina modules, or cartridges, of a female wild-type Drosophila, for which a series of EM cross sections was analysed at levels extending from the retinal basement membrane to the first optic chiasma. A complete, comprehensive catalogue of the synaptic connections of all columnar elements has been compiled from this single series, confirmed from comparisons with less completely photographed cartridges. Combinations of the 12 types of cartridge neurons form divergent multiple-contact synapses (dyads, triads, and tetrads) throughout the lamina's depth. These 12 neuron types include 11 narrow-field elements (one class of receptor terminal, R1-R6, providing input to the cartridge; two types of long visual fiber from the ommatidium, R7 and R8; five types of monopolar cell, L1-5; and three types of medulla cell--two centrifugal neurons C2 and C3, and a third, T1) as well as a wide-field intrinsic or amacrine cell. Connections within the lamina formed by L4 from two adjacent cartridges (posterodorsal and posteroventral) contribute to the matrix of connections. In addition, connections of at least one other wide-field element have also been incorporated.

Direct connections between the R7/8 and R1-6 photoreceptor subsystems in the dipteran visual system

Musca and related flies have three main photoreceptor subsystems. The R1-6 group has short axons that terminate in the cartridges of the first optic neuropile, the lamina. The cartridges are bypassed by the longer axons of R7 and R8, which run together to terminate at different levels in the underlying medulla neuropile. The present account describes a shallow, previously unidentified zone in the lamina within which R7/8 make glancing contact with R1-6. At the distal border of the cartridge over no more than 3-4 microns depth, the tangentially directed short axon of R6 squeezes between the pair from R7 and R8, forming quite large areas of mutual contact (approximately 7 microns2). Less frequently, R1 is contacted. At least some of these sites contain smaller membrane specialisations indistinguishable from the more numerous gap junctions found more proximally that interconnect the terminals of R1-6. The R7/8 junctions with R6 are of comparable size (0.15 micron 2) and likewise possess symmetrical membrane densities. They provide proposed pathways for direct electrical interaction to account for observed electrical input from R7/8 to the R1-6 subsystem. In two cases R7/8 was possibly postsynaptic to R1-6 at a multiple-contact synapse, but even if functional, these sites were so rare that they are unlikely to have much operational significance.