Competition controls the growth of an identified axonal arborization

Development of connectivity in a motoneuronal network in Drosophila larvae

Background

Much of our understanding of how neural networks develop is based on studies of sensory systems, revealing often highly stereotyped patterns of connections, particularly as these diverge from the presynaptic terminals of sensory neurons. We know considerably less about the wiring strategies of motor networks, where connections converge onto the dendrites of motoneurons. Here, we investigated patterns of synaptic connections between identified motoneurons with sensory neurons and interneurons in the motor network of the Drosophila larva and how these change as it develops.

Results

We find that as animals grow, motoneurons increase the number of synapses with existing presynaptic partners. Different motoneurons form characteristic cell-type-specific patterns of connections. At the same time, there is considerable variability in the number of synapses formed on motoneuron dendrites, which contrasts with the stereotypy reported for presynaptic terminals of sensory neurons. Where two motoneurons of the same cell type contact a common interneuron partner, each postsynaptic cell can arrive at a different connectivity outcome. Experimentally changing the positioning of motoneuron dendrites shows that the geography of dendritic arbors in relation to presynaptic partner terminals is an important determinant in shaping patterns of connectivity.

Conclusions

In the Drosophila larval motor network, the sets of connections that form between identified neurons manifest an unexpected level of variability. Synapse number and the likelihood of forming connections appear to be regulated on a cell-by-cell basis, determined primarily by the postsynaptic dendrites of motoneuron terminals.

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Growing motoneurons consolidate synapses with existing presynaptic partners

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Motoneuron dendritic arbors are active parties in setting connectivity patterns

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Cell-type-specific features coexist with variations at the individual cell level

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Motoneuron wiring strategies may contrast with those of sensory neurons

Chorda tympani nerve terminal field maturation and maintenance is severely altered following changes to gustatory nerve input to the nucleus of the solitary tract

Neural competition among multiple inputs can affect the refinement and maintenance of terminal fields in sensory systems. In the rat gustatory system, the chorda tympani, greater superficial petrosal, and glossopharyngeal nerves have distinct but overlapping terminal fields in the first central relay, the nucleus of the solitary tract. This overlap is largest at early postnatal ages followed by a significant refinement and pruning of the fields over a 3 week period, suggesting that competitive mechanisms underlie the pruning. Here, we manipulated the putative competitive interactions among the three nerves by sectioning the greater superficial petrosal and glossopharyngeal nerves at postnatal day 15 (P15), P25, or at adulthood, while leaving the chorda tympani nerve intact. The terminal field of the chorda tympani nerve was assessed 35 d following nerve sections, a period before the sectioned nerves functionally regenerated. Regardless of the age when the nerves were cut, the chorda tympani nerve terminal field expanded to a volume four times larger than sham controls. Terminal field density measurements revealed that the expanded terminal field was similar to P15 control rats. Thus, it appears that the chorda tympani nerve terminal field defaults to its early postnatal field size and shape when the nerves with overlapping fields are cut, and this anatomical plasticity is retained into adulthood. These findings not only demonstrate the dramatic and lifelong plasticity in the central gustatory system, but also suggest that corresponding changes in functional and taste-related behaviors will accompany injury-induced changes in brainstem circuits.

Visualization of ensemble activity patterns of mechanosensory afferents in the cricket cercal sensory system with calcium imaging

The cercal sensory system of the cricket mediates the detection and analysis of low velocity air currents in the animal's immediate environment, and is implemented around an internal representation of air current direction that demonstrates the essential features of a continuous neural map. Previous neurophysiological and anatomical studies have yielded predictions of the global spatio-temporal patterns of activity that should be evoked in the sensory afferent map by air current stimuli of different directions. We tested those predictions by direct visualization of ensemble afferent activity patterns using Ca2+ -sensitive indicators. The AM ester of the fluorescent Ca2+ indicator (Oregon Green 488 BAPTA-1 AM) was injected under the sheath of a cercal sensory nerve containing all of the mechanosensory afferent axons from one cercus. Optical signals were recorded with a digital intensified CCD camera. Control experiments using direct electrical stimulation of stained and unstained nerves demonstrated that the observed Ca2+ signals within the terminal abdominal ganglion (TAG) were due to activation of the dye-loaded sensory afferent neurons. To visualize the spatial patterns of air-current-evoked ensemble activity, unidirectional air currents were applied repeatedly from eight different directions, and the optically recorded responses from each direction were averaged. The dispersion of the optical signals by the ganglion limited the spatial resolution with which these ensemble afferent activity patterns could be observed. However, resolution was adequate to demonstrate that different directional stimuli induced different spatial patterns of Ca2+ elevation in the terminal arbors of afferents within the TAG. These coarsely- resolved, optically-recorded patterns were consistent with the anatomy-based predictions.

Differences in sensory projections between macro- and microchaetes in Drosophilid flies

From examination of the central axonal projections of sensory bristles on the notum of several species of Drosophilidae, we demonstrate different features that may indicate different functions for macro- and microchaetes. The large macrochaetes have conserved arborizations that correlate with their conserved position. Nevertheless, we find evidence for only two discrete projection patterns for bristles in the dorsocentral (DC) row, even when there may be four or five bristles present. We show that the small microchaetes of Drosophila melanogaster display regional specificity and subsets of contiguous bristles project to a common region in the thoracic ganglion. Interestingly, the axons of each of these subsets also form a specific fasciculation group on the scutum before joining the axon of a particular macrochaete. The positions of microchaetes on the scutum and the shape of the fasciculation groups vary between closely related species. There is no correlation between body size, bristle patterns, and fasciculation patterns. Furthermore, none of these traits correlate with the phylogenetic relationships between the species studied. We discuss the possibility that macro- and microchaetes may have different functions and that these have implications for evolutionary constraints on bristle patterns.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Synaptic reorganization induced by selective photoablation of an identified neuron

The maintenance of synaptic strength and specificity in the CNS may depend on interactions among postsynaptic dendrites. We examined the effect of removing a neuron on synaptic organization. A single identified postsynaptic neuron in the adult cercal system of the cockroach was removed with photoablation. After a 30 d recovery period, the synaptic connectivity and morphology of the intact presynaptic and postsynaptic neurons were analyzed. The synaptic connectivity was reorganized in a manner that was consistent with functional plasticity. To associate anatomical changes with this reorganization, we analyzed the morphology of the presynaptic and postsynaptic neurons by quantitative morphometry. Both presynaptic and intact postsynaptic neurons maintained a stable morphology after removal of a neighboring postsynaptic neuron. Using the Hausdorff Match method (HM) (Mizrahi et al. 2000), we found that the spatial organization of the intact dendritic and axonal trees after ablation of a postsynaptic neuron remained stable. Thus, interactions with neighboring neurons were not necessary for maintaining dendritic morphology in the adult nervous system. However, adult central synapses were capable of adjusting to maintain normal function.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Growth cones are not required for initial establishment of polarity or differential axon branch growth in cultured hippocampal neurons

Hippocampal neurons developing in culture exhibit two types of differential, seemingly competitive, process outgrowth in the absence of external cues. During the initial acquisition of polarity, one of several equivalent undifferentiated minor neurites preferentially grows to become the axon. Once the axon has formed, it typically branches, and the branches grow differentially rather than concurrently. In axons with only two branches, growth alternates between branches. In both axon establishment and branch growth alternation, growth among sibling processes or branches must be differentially regulated. We found that elaborate and dynamic growth cones were associated with growth, whereas diminished growth cones were associated with nongrowing processes or branches. To test whether growth cones were necessary for differential growth, growth cone motility was eliminated by application of cytochalasin E. Although cytochalasin treatment before axon formation yielded longer processes overall, a similar percentage of both treated and untreated neurons had one process that grew more rapidly and became much longer than its sibling processes. Immunostaining to visualize dephospho-tau, an axonal marker, demonstrated that these single dominant processes were axons. Axons that formed in cytochalasin were thicker and showed more intense anti-tubulin staining than their sibling processes. Branched axons deprived of growth cones retained a pattern of differential growth and often included alternation. These results indicate that neither formation of a single axon nor differential growth of branches are dependent on growth cone motility and suggest that the neuron can regulate neurite elongation at sites other than at the growth cone.

The kakapo mutation affects terminal arborization and central dendritic sprouting of Drosophila motorneurons

The lethal mutation l(2)CA4 causes specific defects in local growth of neuronal processes. We uncovered four alleles of l(2)CA4 and mapped it to bands 50A-C on the polytene chromosomes and found it to be allelic to kakapo (. Genetics. 146:275- 285). In embryos carrying our kakapo mutant alleles, motorneurons form correct nerve branches, showing that long distance growth of neuronal processes is unaffected. However, neuromuscular junctions (NMJs) fail to form normal local arbors on their target muscles and are significantly reduced in size. In agreement with this finding, antibodies against kakapo (Gregory and Brown. 1998. J. Cell Biol. 143:1271-1282) detect a specific epitope at all or most Drosophila NMJs. Within the central nervous system of kakapo mutant embryos, neuronal dendrites of the RP3 motorneuron form at correct positions, but are significantly reduced in size. At the subcellular level we demonstrate two phenotypes potentially responsible for the defects in neuronal branching: first, transmembrane proteins, which can play important roles in neuronal growth regulation, are incorrectly localized along neuronal processes. Second, microtubules play an important role in neuronal growth, and kakapo appears to be required for their organization in certain ectodermal cells: On the one hand, kakapo mutant embryos exhibit impaired microtubule organization within epidermal cells leading to detachment of muscles from the cuticle. On the other, a specific type of sensory neuron (scolopidial neurons) shows defects in microtubule organization and detaches from its support cells.

Axonal outgrowth of cultured neurons is not limited by growth cone competition

We have examined the question of scarcity-driven competition for outgrowth among growth cones of a single neuron. We measured spontaneous neurite elongation rates from 85 hours of videotape of the arbors of 31 chick sensory neurons in culture. These rate measurements were analyzed in ten minute periods that allowed cell bodies to be classified as to the number of their growth cones and the elongation to be analyzed as a series of discrete events. Comparing periods in which neurons maintained simple bipolar morphology we find no temporal competition between the two growth cones. That is, periods of above-average growth by one growth cone are not compensated by below-average growth during the same period by its sibling growth cone. Analyzing all outgrowth from a neuron based on its number of growth cones shows that net elongation rate from a single cell body is a linear function of the number of growth cones from 1 to 11. These observations suggest that growth cones behave independently and are not limited by availability of structural precursors. A surplus pool of structural precursors available for normal growth is also indicated by the high capacity for growth from single neurites when experimentally stimulated by mechanical tension. In addition, towing one or more neurites at above average rates does not cause any decline in simultaneous growth cone-mediated outgrowth from a single neuron compared to the 2–3 hour period prior to experimentally induced elongation. This high capacity for growth combined with the often observed, intermittant growth behavior of individual growth cones suggests that neurite outgrowth is intrinsically limited primarily by poor growth cone ‘performance,’ not scarcity-driven competition. We postulate that growth cones are poor ‘tractors,’ exerting too little tension to exploit the available capacity for axonal elongation.

Removing antennae and maxillae has little effect on feeding on normal host plants by two species of caterpillar

Models of feeding regulation postulate that chemosensory information from available food both initiates and maintains feeding. However, we find that removing antennae and maxillae (AM) from Manduca sexta and Diacrisia virginica larvae has little effect on amounts eaten, patterns of feeding, and the microstructure (each bite and pause) of feeding, as quickly as two days after the operation. However, there was a small change in the microstructure of feeding of AM D. virginica. Bite frequency of AM D. virginica was significantly lower than for controls when larvae began meals without exploring the food first. Exploring was also followed by longer chewing bouts. Acute effects of the ablation were tested using only Manduca. Six hours after the operation larvae ate less than most controls by eating fewer meals and by biting more slowly. Unilateral ablates also ate less 6h post-operatively, by eating fewer meals; their bite frequency was not low. These observations suggest that chemosensory input affects bite frequency but not other aspects of feeding. As quickly as 24h post-operatively other sensory input to the CNS of AM larvae may compensate for the reduction in chemosensory information, but overall, chemosensory input appears relatively unimportant in non-choice situations.

Plasticity of synaptic connections in sensory-motor pathways of the adult locust flight system

We investigated possible roles of retrograde signals and competitive interactions in the lesion-induced reorganization of synaptic contacts in the locust CNS. Neuronal plasticity is elicited in the adult flight system by removal of afferents from the tegula, a mechanoreceptor organ at the base of the wing. We severed one hindwing organ and studied the resulting rearrangement of synaptic contacts between flight interneurons and afferent neurons from the remaining three tegulae (2 forewing, 1 hindwing). This was done by electric stimulation of afferents and intracellular recording from interneurons (and occasionally motoneurons). Two to three weeks after unilateral tegula lesion, connections between tegula afferents and flight interneurons were altered in the following way. 1) Axons from the forewing tegula on the operated side had established new synaptic contacts with metathoracic elevator interneurons. In addition, the amplitude of compound excitatory postsynaptic potentials elicited by electric stimulation was increased, indicating that a larger number of afferents connected to any given interneuron. 2) On the side contralateral to the lesion, connectivity between axons from the forewing tegula and elevator interneurons was decreased. 3) The efficacy of the (remaining) hindwing afferents appeared to be increased with regard to both synaptic transmission to interneurons and impact on flight motor pattern. 4) Flight motoneurons, which are normally restricted to the ipsilateral hemiganglion, sprouted across the ganglion midline after unilateral tegula removal and apparently established new synaptic contacts with tegula afferents on that side. The changes on the operated side are interpreted as occupation of synaptic space vacated on the interneurons by the severed hindwing afferents. On the contralateral side, the changes in synaptic contact must be elicited by retrograde signals from bilaterally arborizing flight interneurons, because tegula projections remain strictly ipsilateral. The pattern of changes suggests competitive interactions between forewing and hindwing afferents. The present investigation thus presents evidence that the CNS of the mature locust is capable of extensive synaptic rearrangement in response to injury and indicates for the first time the action of retrograde signals from interneurons.

Competition among the axonal projections of an identified neuron contributes to the retraction of some of those projections

AP neurons in the embryonic leech CNS extend lateral projections to peripheral targets through the ganglionic nerve roots and longitudinal projections toward neighboring ganglia through the connective nerves. The lateral projections grow extensively in the periphery; in contrast, the longitudinal projections achieve relatively little growth and eventually retract, the majority having essentially disappeared by the end of embryogenesis. Cutting both nerve roots, which eliminates both lateral projections, however, induces the longitudinal projections of the AP neuron to begin to grow rapidly toward adjacent ganglia within 14 hr after the axotomy. By using a laser microbeam to cut just the lateral projections of the AP cells, we further show that it is indeed the loss of its lateral projections, and not a secondary response to the cutting of other components of the root nerves, that induces the longitudinal projections of the AP cell to grow extensively. In addition, we report that reducing the outgrowth of the lateral projections by: (1) cutting only one lateral projection, or (2) ablating pioneer neurons required by the AP neuron to establish its peripheral arbor, also results in a significant increase in the growth of the longitudinal projections. Finally, we demonstrate that increasing the outgrowth of the longitudinal projections by ablating the AP cells in adjacent ganglia results in a significant reduction in the outgrowth of the lateral projections. Taken together, these results indicate, first, that the longitudinal and lateral projections usually grow at the expense of each other, and second, that normally the extensive outgrowth of its lateral projections is a necessary condition for a developing AP neuron to retract its longitudinal projections.

Rapid activity-dependent sprouting of optic fibers into a local area denervated by application of beta-bungarotoxin in goldfish tectum

The retinotectal projection is known to be capable of extensive long-term expansion of connections, but it is not known how fast such changes can occur or what triggers sprouting of terminals. We studied sprouting of optic fibers into an area denervated by local microinjection of beta-bungarotoxin (beta-BTX), a specific presynaptic neurotoxin with phospholipase A2 activity that destroys nerve terminals at the neuromuscular junction. After injection of 0.1 pmol of beta-BTX, the optic terminals fired spontaneously with decreasing amplitude and became silent within 1 to 2 h. Outside the injection zone, the retinotectal map was normal, so the silent zone was associated with a scotoma in the visual field. Horseradish peroxidase (HRP) staining of the entire optic nerve showed a denervated region at the injection site with beaded, degenerating fibers at its edge. Between 3 and 9 days later, optic units were recorded within the injection zone whose receptive fields lay just outside the scotoma in the visual field, indicating that intact surrounding terminals had sprouted into the area. These sprouts made functional connections, as indicated by field potential recordings and current source-density analysis. At this time, HRP staining also demonstrated retinal innervation within the injection zone. By 12 days, normal maps with no scotoma were recorded and HRP staining was normal at the injection site, indicating that the beta-BTX-damaged fibers had regenerated to reclaim their tectal sites. The results show that the retinotectal projection of goldfish is very dynamic, since intact optic fibers can sprout into adjacent vacant postsynaptic territory within 2 to 3 days, much faster than previously reported. In a final experiment, we showed that this sprouting is activity-dependent, since it could be prevented by blocking retinal activity with intraocular tetrodotoxin (TTX) during the first 2 days postinjection, even though TTX block of activity does not block regeneration in this system. One possible mechanism for this rapidly triggered sprouting is that arachidonic acid liberated by beta-BTX acts as a sprouting factor to attract surrounding healthy fibers into the denervated region but requires activity at the terminals to be effective.

Cell-cell interactions define the innervation patterns of central leech neurons during development

In the last 20 years, the nervous system of the developing leech has been used to great advantage to study the processes by which neurons seek and finally innervate their targets. This review summarizes what is presently known about how neurons of the CNS interact with each other and with their targets during embryogenesis.

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.

Development and maintenance of a simple reflex circuit in small-patch mosaics of Drosophila: effects of altered neuronal function and developmental arrest

A combined genetic, anatomical, and behavioral approach has been undertaken to study the developmental and functional plasticity of identified bristle mechanosensory neurons in Drosophila. A stereotyped grooming reflex in decapitated flies enabled simple but reliable assessments of the functional output of individual bristle sensory cells to correlate with their axonal projections and terminal arbors revealed by the cobalt backfill technique. Construction of small-patch mosaics that contain only a single mutant bristle allowed functional perturbation of individual neurons within an otherwise normal environment. Mutations that affect nerve excitability and membrane recycling have been used to examine their effects on neuronal pathfinding, arborization, and the initiation and maintenance of functional connections. Previous studies (Burg and Wu, 1986, J. Neurosci. 6:2968-2976; 1989, Dev. Biol. 131:505-514) have demonstrated that para(ts)nap(ts) double-mutant sensory neurons, in which action potentials are unconditionally blocked by defects in sodium currents, and eag Sh double-mutant sensory cells, in which membrane excitability is increased by alterations in potassium currents, can establish and maintain central projections that are indistinguishable from their functionally normal counterparts. Mutations of the shi(ts) gene cause a temperature-sensitive, reversible block of the membrane recycling process, resulting in arrest of neuronal growth in culture (Kim and Wu, 1987, J. Neurosci. 7:3245-3255) and depletion of synaptic vesicles that leads to transmission blockade at established synapses (Ikeda, Ozawa, and Hagiwara, 1976, Nature 259:489-491; Koenig and Ikeda, 1983, J. Neurobiol. 14:411-419; 1989, J. Neurosci. 9:3844-3860). Prolonged heat treatments (up to 16% of total development time) of small-patch shi(ts) mosaics at different pupal stages did not prevent the establishment of central projections characteristic of the various sensory cell types. However, none of the shi(ts) sensory neurons heat-pulsed during the initial or the final 16% of pupal development were able to initiate the reflex behavior, although a proportion of those treated in other periods apparently established functional contacts with appropriate targets to support the characteristic cleaning reflex. The possibility exists that the membrane recycling process blocked in shi(ts) cells provides a crucial mechanism for cell-cell interactions taking place during initial differentiation and final synaptic stabilization, and possibly competition, in the developing sensory neuron. Heat treatments of adult shi(ts) mosaics blocked the reflex initiated by the mutant (but not the surrounding normal) bristles, as expected from the effect of synaptic block.(ABSTRACT TRUNCATED AT 400 WORDS)

Dendritic reorganization of an identified neuron during metamorphosis of the moth Manduca sexta: the influence of interactions with the periphery

During metamorphosis of the moth, Manduca sexta, an identified leg motor neuron, the femoral extensor motor neuron (FeExt MN) undergoes dramatic reorganization. Larval dendrites occupy two distinct regions of neuropil, one in the lateral leg neuropil and a second in dorsomedial neuropil. Adult dendrites occupy a greater volume of lateral leg neuropil but do not extend to the dorsomedial region of the ganglion. The adult dendritic morphology is acquired by extreme dendritic regression followed by extensive dendritic growth. Towards the end of larval life, MN dendrites begin to regress, but the most dramatic loss of dendrites occurs in the 3 days following pupation, such that only a few sparse dendrites are retained in the lateral region of leg neuropil. Extensive dendritic growth occurs over the subsequent days such that the MN acquires an adult-like morphology between 12 and 14 days after pupation. This basic process of dendritic remodeling is not dependent upon the presence of the adult leg, suggesting that neither contact with the new target muscle nor inputs from new leg sensory neurons are necessary for triggering dendritic changes. The final distribution of MN dendrites in the adult, however, is altered when the adult leg is absent, suggesting that cues from the adult leg are involved in directing or shaping the growth of MN dendrites to specific regions of neuropil.

SSB, an antigen that selectively labels morphologically distinct synaptic boutons at the Drosophila larval neuromuscular junction

In this report we describe the expression of Small Synaptic Bouton (SSB), an antigen that is selectively expressed in a specific subset of neuromuscular junction terminals in the body wall of Drosophila larva. The expression of SSB was studied with a polyclonal antibody raised against the cAMP phosphodiesterase of the Drosophila learning mutant dunce (Nighorn et al., 1991, Neuron 6:455-467); however, immunoreactivity was not abolished by the dunce (dnc) alleles dncM14 and dncM11 or deficiencies of the dnc gene, indicating that the antigen labelled could not be the dnc gene product, but another antigen that we termed SSB. Immunoreactivity was localized in the body wall muscles to a specific subset of neuromuscular junction terminals that have been implicated in activity-dependent plasticity. This demonstrates that these morphologically distinct terminals can be immunocytochemically distinguished and that they probably represent innervation by a distinct neuronal population. Confocal and electron microscopic examination demonstrated that staining was restricted to the synaptic boutons themselves, not to neurites or motor axons. Ultrastructural analysis showed label close to synaptic vesicles in the presynaptic terminal and in the surrounding subsynaptic reticulum. Central nervous system (CNS) staining was restricted to a segmentally repeated pattern of cell bodies in the ventral ganglion and to a few small groups of cells in the brain lobes.

Target-dependent morphological segregation of Aplysia sensory outgrowth in vitro

The adult nervous system is characterized by partial or complete morphological segregation of terminals from different afferent neurons innervating the same postsynaptic target. This segregation is thought to result, in part, from competition between the afferent terminals. To explore the role of the target cell in the spatial distribution of presynaptic inputs, the sensory neurons of Aplysia were cultured either with or without a common target motor neuron. In the presence of a common target, the outgrowth from two different sensory neurons tends to occupy separate postsynaptic regions. When cultured without a target motor neuron, processes from different sensory neurons do not segregate, but rather grow freely along one another. Thus, morphological segregation of sensory outgrowth requires interaction with a target neuron and may reflect competition between presynaptic terminals for a limited number of synaptic sites on the motor neuron, or for a postsynaptic trophic factor.

Anatomical relationships between sensory afferent arborizations in the cricket cercal system

The anatomical relationships between sensory afferents within a topographic map in the cricket cercal sensory system were studied using a computer-based reconstruction system developed in our laboratory. Individual identified mechanosensory afferents were characterized physiologically, stained with cobalt, silver intensified, and reconstructed in three dimensions. All reconstructions were scaled to a common standard. The results indicate that there is very little variability in the position or extent of the terminal arborization of identified mechanosensory afferents. The topographic map was divided relatively equally into four regions representing each of the four classes of afferents studied. These regions were discrete but not completely segregated. Approximately 30% of the topographic map contained regions of overlap between two or more classes or afferents.

Developmental study of afferented and deafferented bee antennal lobes

The role of antennal sensory projections on the ontogeny of the bee antennal lobe was analyzed using both light and transmission electron microscopy. Normal and deafferented developing antennal lobes were examined. The results obtained show that (1) initiation of synaptogenesis in the antennal lobe is independent of the arrival of sensory inputs; (2) sensory inputs are necessary for setting up the glomerular antennal lobe organization; (3) regressive events, such as the reduction of synapse density, occur during the development of the antennal lobe; and (4) glomeruli formation appears as related to glia development.

Morphology of a sensory neuron in Drosophila is abnormal in memory mutants and changes during aging

Several mutations in Drosophila impair learning and the cAMP cascade. We report here that the fine morphology of an identified mechanosensory neuron is abnormal in two of these mutants, dunce (dnc) and rutabaga (rut). The neuron innervating the antero-notopleural bristle was filled with horseradish peroxidase and studied at the light- and electron-microscopy level. In the mutants dnc and rut, this neuron has an abnormally large number of side branches and varicosities in a defined segment of the axon. In wild-type flies, age tends to decrease the number of side branches and variacosities in the same axonal segment that is affected by the mutations. Ultrastructural studies are compatible with the interpretation that the varicosities are potential synaptic sites. The results suggest that the cAMP cascade plays a role in shaping neuronal connectivity.

Regeneration of the projection and synaptic connections of tympanic receptor fibers of Locusta migratoria (Orthoptera) after axotomy

The tergite nerve N6 of the first abdominal segment of the locust Locusta migratoria contains receptor fibers, from the tympanic organ, and hair sensilla as well as motoric axons. The nerve was axotomized in nymphal instars or adults, and the regeneration of nerve fibers was studied. The sensory fibers regrow and regenerate their projection pattern within the central nervous system. They recognize their specific neuropile areas even after entering the ganglion through different pathways. The receptor fibers of the tympanic organ reestablish synaptic connections to auditory interneurons, even though the physiological characteristics of the interneurons are not fully restored. This regenerative capability contrasts with the lack of regeneration of peripheral structures in locusts, but supports the described plasticity in the auditory system of monaural locusts (Lakes, Kalmring, and Engelhard, 1990). The motor fibers do not regenerate nerves innervating muscles of the body wall.

Reorganization of persistent motoneurons in a metamorphosing insect (Tenebrio molitor L., Coleoptera)

The present analysis outlines how the shape of motoneurons which persist through metamorphosis in the beetle Tenebrio molitor is regulated by cellular interactions. This study focused on the structural changes of prothoracic leg motoneurons. The fate of these neurons is described in normal metamorphic development, so as to provide a basis for experimental analysis. In a first experiment the prothoracic leg imaginal discs or part of these were extirpated in the prepupa or early pupa. In a second experiment the leg imaginal discs were rotated by 180 degrees in early larval instars of Tenebrio; the procedure caused a threefold leg anlage. Thereafter, the treated individuals continued to develop. In both experiments the effect of the operation on the structure of the dendritic trees of the persisting motoneurons was analyzed at the imaginal stage. In the first experiment the dendritic tree of the motoneurons is locally deleted. In the second experiment the branching index (quantitative description of dendritic arborization pattern) of the dendritic tree of the persisting motoneurons increased. Both experiments provided evidence that the shape of persistent leg motoneurons is stabilized and even regulated by cellular interactions during metamorphosis. Evidence is presented that sensory neurons are effective both in stabilization and regulation of the shape of persistent motoneurons.

Patterns of appearance of serotonin and proctolin immunoreactivities in the developing nervous system of the American lobster

Serotonin (5-HT) and proctolin, neurohormones widely distributed in the lobster nervous system, have been implicated in a variety of behaviors and also are known to coexist in large pairs of identified neurons in the fifth thoracic (T5) and first abdominal ganglia (A1) of adults (Siwicki, Beltz, and Kravitz, 1987). Earlier studies also have shown that these paired neurons already contain 5-HT in embryos approximately halfway through development, whereas proctolin immunoreactivity does not appear in these cells until near the time of hatching (Beltz and Kravitz, 1987a). In the current studies, the brain and ventral nerve cord have been screened for the appearance of serotonin and proctolin immunoreactivities using immunocytochemical and biochemical methods, in order to determine whether the late appearance of proctolin in the paired T5 and A1 cells is a general feature of development in other neurons as well. In embryos approximately halfway through development, the adult complement of 5-HT-staining cells is already present. In several cases, embryonic serotonin cells are proportionally very large and prominent, suggesting possible developmental roles. In contrast to serotonin, fewer than 10% of the proctolin-staining neurons of juvenile animals are seen in embryos halfway through development. The number of immunoreactive cells gradually increases, but even by the sixth larval stage only half the number of cells that will eventually stain for proctolin are observed. Therefore, the developmental appearance of proctolin in lobster neurons, assayed using immunocytochemical methods, is relatively late and protracted compared to the appearance of serotonin. Quantitative measurements for 5-HT in lobster larvae were performed using high pressure liquid chromatography (HPLC) with dual electrochemical detection and for proctolin using radioimmunoassay. A gradual, probably growth-related increase in the amounts of serotonin and proctolin were seen during larval development. The implications of the biochemical data, in light of the immunocytochemical studies, are discussed.

Modular construction of nervous systems: a basic principle of design for invertebrates and vertebrates

The modular construction of brain tissue is not solely a feature of vertebrate nervous tissue, but is characteristic of many invertebrate nervous systems as well. Modern vertebrate and invertebrate modules vary over several orders of magnitude in volume but vary less in diameter. Although the physiological and anatomical differences between the modules discussed herein are overpowering, their importance to nervous system functions are similar. Modules are the serial and parallel processing units that have allowed large-brained animals to evolve. Many invertebrate modules are discrete, hemispherical lobes, visible on the surface of the brain or nerve cord, whereas most mammalian modules are columnar or ellipsoidal tissue compartments that can only be visualized with specific anatomical methods. Lobes from the largest invertebrates can be more voluminous than any neocortical compartments, but these large lobes are usually not single modules. Large invertebrate lobes contain internal compartments that are single modules and of similar size to their vertebrate analogs. However, vertebrate cortical modules or columns, are far more numerous than the compartments in invertebrate brains and in several cases are known to be adjoined laterally into slabs of tissue that extend for several millimeters. Physiological data support the idea that neural modules are not just anatomical entities, but are active local circuits. The specific activities within each type of module will depend upon its neuronal components, both intrinsic and extrinsic, its functional roles and phylogenetic history. Many cellular and intercellular phenomena common to vertebrates and invertebrates underlie the development of modules. Neuronal and glial interactions and their interplay with the extracellular environment depend upon families of molecules with broad phyletic occurrences. The commonalities of growth mechanisms may to a large degree account for the widespread incidence of neuronal processing units. The strategy of enlarging a nervous system through the replication of the basic units is thought to be advantageous for several reasons. This plan allows nervous systems to economize on the branch sizes and lengths needed for interconnections, to ensure that appropriate targets are reached during development and to modulate specific circuits within a larger network.

Assembly of the cricket cercal sensory system: genetic and epigenetic control

The cercal sensory system of the cricket is being examined using anatomical, physiological, and computer simulation techniques in order to better understand the assembly of sensory systems. This particular sensory system is of interest because it functions like numerically more complex vertebrate sensory systems but offers, to the neuroscientist, the technical advantages of a small number of large identified neurons. Two aspects of sensory processing are being examined in this system; the spatial aspects of a stimulus that tell an animal where a target is in its environment, and the qualities of a stimulus that help the animal to identify the stimulus. The spatial aspects of a stimulus are analyzed by a topographic mapping of the animal's sensory environment. The feature extraction machinery for other aspects of the stimulus lacks any obvious anatomical order and is embedded within the topographic map. We are attempting to tease apart the genetic and the epigenetic components of the assembly process for this sensory system. Here we review our progress with emphasis on the epigenetic aspects of its assembly. We describe previously published work on plasticity as well as new experiments focussed on the role of neuronal activity in the assembly of this neural circuit. Finally, we briefly describe simulation experiments that are helping us understand the role of various forms of synaptic plasticity in the determination of receptive fields.

Localization of the enhanced input to cockroach giant interneurons after partial deafferentation

The ventral giant interneurons (GIs) in the cockroach have two distinct dendritic fields: a small one ipsilateral to the soma, and a larger, contralateral field from which the axon arises. The major input to these GIs is from the cercus on the axon side; when this cercus is ablated in the last instar before the adult stage, input from the other cercus becomes more effective within 30 days (Vardi and Camhi, 1982b). I wished to determine if the input from the intact, soma-ipsilateral cercus contacted the GIs purely ipsilaterally and if EPSPs at this site were larger in deafferented animals. Consistent with earlier anatomical findings, intracellular recordings from the GI somata showed that the majority of cercal inputs synapse on their own side of the ganglion in normal animals. This was evidenced by differences in the size and shape of the synaptic potentials evoked from the two cerci and by the presence of large EPSPs after a ganglion had been split along the midline. Unitary EPSPs produced by stimulation of single, identified cercal afferents, ipsilateral to the soma, were compared between normal and deafferented animals. Column "h" afferents were chosen because they make a large contribution to the receptive fields of GIs 1 and 2 after ablation of the contralateral cercus. In addition, the arbors of these afferents, when stained with cobalt, did not cross the ganglionic midline in normal animals. Unitary EPSPs recorded in GI 2 were significantly larger in the deafferented animals. There was, however, no significant change in the size of EPSPs in GI 1. Nevertheless, the results from GI 2 suggest that partial deafferentation in the central nervous system can increase the efficacy of synapses distant from the locus of denervation.

The refinement of invertebrate synapses during development

Evidence is provided that during invertebrate development synapses undergo a period of refinement during which there are changes in synaptic connectivity and specific synaptic properties. It appears that extrinsic cues such as competition and neural activity are involved in guiding these synaptic changes in invertebrates. Comparisons are made with findings in the vertebrate literature.

Central projections of peripheral mechanosensory cells with increased excitability in Drosophila mosaics

The formation and maintenance of the central projections of identified bristle mechanosensory neurons with altered excitability were examined in Drosophila mosaics. Two mutants, eag (ether à go-go) and Sh (Shaker), are known to increase excitability of both nerve and muscle cells and enhance synaptic transmission by affecting different types of K+ currents. The eag Sh double mutant produces a synergistic effect, resulting in a greatly increased level of spontaneous neuronal activity and extreme behavioral phenotypes. By constructing mosaic flies containing small patches of doubly mutant cuticle, it was possible to alter the excitability of only one or two identified sensory cells without affecting the surrounding tissue. In these mosaic flies, the doubly mutant sensory cells were more responsive to tactile stimulation. A CoCl2 backfilling technique was utilized in staining the sensory cell projections. Both qualitative and quantitative comparisons were made between projections of cells having normal and increased levels of excitability. The length, branching characteristics, and number of terminal varicosities were analyzed for each sensory cell projection. Results indicate that, at the light microscopy level, these characteristics were not obviously altered by an increased level of excitability.

Formation of the transverse nerve in moth embryos. I. A scaffold of nonneuronal cells prefigures the nerve

We have studied the embryonic development of the transverse nerve (TN), an unpaired segmental nerve of the moth Manduca sexta. Two identified motor neurons and 16 identified neuroendocrine neurons project axons within the larval TN; therefore, the TN is both a peripheral nerve and a neurohaemal organ. At 33% of embryogenesis, and prior to the arrival of any neuronal growth cones, the position, shape, and trajectory of the TN are anticipated by two groups of nonneuronal cells that we call the strap and the bridge. At this time the strap and the bridge together consist of approximately 100 cells, all of which express a cell surface epitope recognized by the monoclonal antibody TN-1. As development proceeds, both the number of nonneuronal cells within the strap and the bridge and the fraction that expresses the TN-1 antigen(s) decrease. Moreover, individual cells within the strap become morphologically identifiable before the arrival of the neuronal growth cones. Most of the axons that project to the TN also express the TN-1 antigen(s) during their period of outgrowth. The two motor neuron growth cones are the first to reach the environment of the strap and the bridge, doing so at approximately 37%; having encountered these cellular structures, the growth cones restrict their navigation to this preexisting scaffolding, until they reach their muscle target. The neuroendocrine growth cones arrive later and also grow within the confines of the strap and the bridge (J.N. Carr and P.H. Taghert, 1988, Dev. Biol, 130, 500-512). In this first paper we describe the development of the strap and the bridge, and the interactions of the motor neuron growth cones with these structures. The observations are novel in documenting the extent and precision to which a peripheral nerve pathway is prefigured by a contiguous assemblage of nonneuronal cells.

Formation of the transverse nerve in moth embryos. II. Stereotyped growth by the axons of identified neuroendocrine neurons

We are interested in the cellular mechanisms that guide neuroendocrine axons to their neurohaemal target regions and that regulate the extent and positioning of their terminal arbor. The neurohaemal organ we have studied is the segmentally repeated transverse nerve of the moth Manduca. In the mature animal, two motor neurons and a heterogeneous set of identified neuroendocrine neurons project to this nerve; the latter release hormonal peptides from along its length. In the preceding report, we demonstrated that during embryogenesis, the position, trajectory and extent of the transverse nerve are anticipated by two sets of nonneuronal cells, the strap and the bridge. In this paper we show that four identified neuroendocrine neurons (L1 and B1-3), like the identified motor neurons before them, elaborate growth cones that use this preexisting scaffolding as a substrate for axonal elongation. Moreover, growth cone navigation by these neuroendocrine neurons is as precise and invariant as that displayed by the motor neurons. One feature that differentiates the behavior of the developing neuroendocrine cells from that of the motor neurons is a stereotyped interaction that the L1 and B1-3 axons undergo with an identified syncytial cell that lies in close proximity to the strap. Each neuroendocrine neuron specifically adheres to the syncytium by extending numerous filopodia, and an occasional large lamellopodium, over its surface. These contacts are maintained by the neuroendocrine axons after their growth cones have left the vicinity of the syncytium and proceeded into the strap/bridge complex. Adhesion to the syncytium is transient and specific to the neuroendocrine neurons: although motor neuron axons are present at this same time and place, they display no affinity for the syncytium. This distinction correlates with the fact that the neuroendocrine neurons go on to elaborate arbor within the confines of the transverse nerve, while the motor neurons do not. We suggest that the syncytium may act as a "fictive target" for these neurons to aid in the differentiation of features that are specific to their cellular phenotype.

Growth cone guidance in insects: fasciclin II is a member of the immunoglobulin superfamily

The cellular cues that guide neuronal growth cones toward their targets are highly conserved in such diverse organisms as insects and vertebrates. Evidence presented here suggests that the molecular mechanisms underlying these events may be equally conserved. This article describes the structure and function of fasciclin II, a glycoprotein expressed on a subset of fasciculating axons in the grasshopper embryo. Antibody perturbation experiments suggest that fasciclin II functions in mediating one form of neuronal recognition: selective fasciculation. Fasciclin II is a member of the immunoglobulin gene superfamily and is homologous in structure and function to the neural cell adhesion molecule N-CAM and to several other vertebrate cell adhesion molecules.

Axon extension and retraction by leech neurons: severing early projections to peripheral targets prevents normal retraction of other projections

During leech embryogenesis, interactions between homologous neurons in neighboring segments lead to the selective retraction of longitudinal axonal projections by midbody AP and AE neurons, which maintain lateral axonal projections to the periphery. Results of experiments reported here show that disconnecting the lateral projections from the periphery rescues the projections normally fated to retract. We propose that these neurons normally progress through two states during early development, one in which they are insensitive to interactions with their homologs (state A) and a second in which they are sensitive (state B). Establishment of lateral connections with their targets triggers the switch from state A to state B; cutting these projections puts neurons back to state A.

Remodeling of nerves during claw reversal in adult snapping shrimps

Adult snapping shrimps, Alpheus heterochelis, undergo a reversal of their claw laterality following removal of the major claw, a process in which the existing minor claw transforms into a major and a new minor regenerates at the old major site. During such reversals the nerves to the ganglion are remodeled from one claw type to the other. Conversion of the nerves from the minor to the major type occurs within several days after removal of the contralateral major claw and involves the rapid addition of large numbers of sensory axons together with deletion of a few. Thus modeling of the nerves is essentially complete within the first intermolt in tandem with changes in the motoneurons but well ahead of changes in the muscle and external morphology. Conversion of the major nerves to the minor type is via massive degeneration of sensory axons during the first and second intermolts because of the loss of their peripherally located cell bodies. This is followed by proliferation of largely unmyelinated axons in the third intermolt, some of which become myelinated in the subsequent intermolts. Thus remodeling of the major nerves to minor, which is associated with the loss of a claw and the regeneration of a new minor claw, is a more traumatic and prolonged process compared to the remodeling of the minor nerves to major which is associated with the transformation of an existing claw.

Effect of auditory deafferentation on the synaptic connectivity of a pair of identified interneurons in adult field crickets

In adult crickets, Teleogryllus oceanicus, unilateral auditory deafferentation causes the medial dendrites of an afferent-deprived, identified auditory interneuron (Int-1) in the prothoracic ganglion to sprout and form new functional connections in the contralateral auditory neuropil. The establishment of these new functional connections by the deafferented Int-1, however, does not appear to affect the physiological responses of Int-1's homolog on the intact side of the prothoracic ganglion which also innervates this auditory neuropil. Thus it appears that the sprouting dendrites of the deafferented Int-1 are not functionally competing with those of the intact Int-1 for synaptic connections in the remaining auditory neuropil following unilateral deafferentation in adult crickets. Moreover, we demonstrate that auditory function is restored to the afferent-deprived Int-1 within 4-6 days following deafferentation, when few branches of Int-1's medial dendrites can be seen to have sprouted. The strength of the physiological responses and extent of dendritic sprouting in the deafferented Int-1 progressively increase with time following deafferentation. By 28 days following deafferentation, most of the normal physiological responses of Int-1 to auditory stimuli have been restored in the deafferented Int-1, and the medial dendrites of the deafferented Int-1 have clearly sprouted and grown across into the contralateral auditory afferent field. The strength of the physiological responses of the deafferented Int-1 to auditory stimuli and extent of dendritic sprouting in the deafferented Int-1 are greater in crickets deafferented as juveniles than as adults. Thus, neuronal plasticity persists in Int-1 following sensory deprivation from the earliest juvenile stages through adulthood.

Augmented synaptic release by one excitatory axon in regions in which a synergistic axon was removed in lobster muscle

1. In the lobster, every fibre of the lateral abdominal extensor muscle is innervated by two excitatory axons. When one of the excitatory axons (the common excitor) was removed chronically by intracellular injection of pronase, terminals of the remaining axon (the specific L1 excitor) showed augmented transmitter release. 2. Evidence as to the mechanism of this strengthening can be obtained taking advantage of the peculiar innervation pattern of the abdominal extensors. The L1 excitor axon of one segment sends a branch to part of the next posterior segment. The common excitor axon innervates only muscle fibres of its own segment. 3. 10-20 days after removing the common excitor axon of segment II, the quantum content of release of terminals of the L1 excitor axon was measured in segments I, II and III. Terminals of the L1 excitor axon of segment I which innervate segment II released much more transmitter than controls, while the terminals of the same axon innervating segment I remained normal. Similarly, terminals of the L1 excitor axon of segment II became 'stronger' in segment II but remained normal in segment III. 4. It is concluded that only those terminals of one axon which innervate targets with reduced innervation increased the average release rate. It seems that the signal for synaptic strengthening, after removal of a synergistic axon, is generated and acts locally in partially denervated muscle fibres.

The sensory projections of Drosophila mutants which show abnormal wing formation or flying behavior

The central projection pattern of sensory neurones from the wing of Drosophila melanogaster arise from a small number of differentiated cell types which are localized in a precise and reproducible spatial pattern. We report here the distribution of the nerve pathways revealed by filling with peroxidase and the structure and number of the wing sensory receptors, in flies which have wings but which nevertheless are flightless, including one which presents abnormal wing formation. Our results indicate that in dumpy and taxi mutants, there are no detectable modifications in the neural projections or in the number and morphology of sensory receptors. Vestigial mutants however, present several alterations including the absence of the ovoid projection, a fact consistent with the existence of very few marginal bristles. The projection of the large companiform sensilla branch profusely along the ventromedial tract and more sensory fibers cross the midline in the mesothorax. Wings of vestigial mutants show a decrease in the number of large campaniform sensilla, and more importantly some receptors present abnormally oriented trichomes in the neighboring cells.

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. II. The AP and AE neurons

To assess the generality of our previous finding (Gao and Macagno, 1987) that segmental homologues play a role in the establishment of the pattern of axonal projections of the heart accessory HA neurons, we have extended our studies to two other identified leech neurons: the anterior pagoda (AP) neurons and the annulus erector (AE) motor neurons. Bilateral pairs of AP neurons are found in the first through the twentieth segmental ganglia (SG1 through SG20) of the leech ventral nerve cord. All AP neurons initially extend axonal projections to the contralateral periphery as well as longitudinal projections along the contralateral interganglionic connective nerves toward anterior and posterior neighboring ganglia. Although the peripheral projections are maintained by all AP neurons throughout the life of the animal, the longitudinal projections disappear in all but two segments: the AP neurons in SG1 maintain their anterior projections and extend them into the head ganglion, and those in SG20 maintain their posterior projections and extend them into SG21 and the tail ganglion. When single AP neurons are deleted anywhere along the nerve cord before processes begin to atrophy, however, the longitudinal projections are retained by their ipsilateral homologues in adjacent ganglia. The rescued processes appear to take over the projections of the deleted neurons. In cases where two or more AP neurons on the same side of the nerve cord are deleted from adjacent ganglia, a contralateral homologue sometimes extends projections to the periphery ipsilaterally or on both sides. We obtained similar results when we deleted single AE neurons from midbody ganglia. Thus, our experiments with three different identified neurons consistently show that the initial pattern of projections is the same in all ganglia, but that the existence of homologues in adjacent ganglia leads to the pruning of some of the initial projections. A consequence of this homologue-dependent process retraction is that neurons normally lacking neighboring homologues will have patterns of projections different from those neurons that do have such neighbors. Process loss by the HA, AP, and AE neurons may be the result either of competition for targets, inputs, or growth factors or of direct interactions among homologous cells.

A reflex behavior mediated by monosynaptic connections between hair afferents and motoneurons in the larval tobacco hornworm, Manduca sexta

In the tobacco hornworm caterpillar, tactile stimulation of sensory hairs located on the tip of a proleg (the planta) evokes ipsilateral or bilateral retraction of the prolegs in that segment. We have used electrophysiological and anatomical methods to investigate the excitatory neural pathways linking the planta hair afferents and the proleg retractor motoneurons (MNs). An important technical innovation was the development of an isolated proleg and desheathed ganglion preparation that permits rapid and reversible ionic manipulations and drug applications. Action potentials (spikes) in individual planta hair afferents produce time-locked excitatory postsynaptic potentials (EPSPs) in ipsilateral proleg MNs which appear to be chemically-mediated and monosynaptic: the EPSPs have a short and constant latency, they follow afferent spikes without failure, they are reversibly abolished in elevated Mg++ saline, and they persist in saline with elevated Mg++ and Ca++ levels. Planta hair afferents also excite ipsilateral MNs by polysynaptic pathways, and their excitation of contralateral proleg MNs is exclusively polysynaptic. Cobalt-staining of the proleg MNs and planta hair afferents shows that the afferents terminate in ventral neuropil, and the proleg MNs have an unusual ventral projection into this region. The ventral projection is on the ipsilateral side, which is consistent with the electrophysiological finding that time-locked EPSPs are found only from ipsilateral hairs. Two factors that contribute to the strong monosynaptic excitation of proleg MNs by ipsilateral planta hairs are the convergence of many hair afferents onto each MN, and the facilitation shown at each afferent-MN synapse. At least 6 afferents converge on each MN, and at short interspike intervals the afferent-evoked EPSPs are enhanced by as much as 400% by homosynaptic facilitation. The EPSP is abolished reversibly by the cholinergic antagonists curare and atropine, suggesting that the neurotransmitter at the synapse is acetylcholine (ACh). This is of particular interest because the ACh receptors of tobacco-feeding Manduca larvae are reported to be less nicotine-sensitive than those of other insects.

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. I. The HA cells

The role of homologues in the establishment of the pattern of axonal projections of identified segmentally homologous neurons was investigated by means of selective cell ablation and dye injection. The cells studied were the bilateral pairs of heart accessory (HA) neurons found in the fifth and sixth segmental ganglia of the leech ventral nerve cord. Homologues start their morphological differentiation with identical axonal projections, and segmental differences are manifested later, when specific branches stop growing and disappear. The deletion of single HA cells at early stages, however, permits these branches to survive in their ipsilateral homologues and to grow and take over the projections of the deleted neurons. In addition, if both HA homologues on the same side of the nerve cord, or three of the four HA cells, are deleted in an animal, the remaining HA cells often extend novel projections. These observations suggest that either competition for targets, inputs or growth factors, or direct interactions among homologous cells may play a role in the differentiation of segment specific patterns of axonal projections.

The myth of the inflexible invertebrate: competition and synaptic remodelling in the development of invertebrate nervous systems

As this quote and the article it was taken from reflect, it is a commonly held belief that the development of invertebrate nervous systems is so rigidly programmed that it is qualitatively different from that of vertebrates. The facts do not support this assertion, and I will show, using examples from among the arthropods, that appropriate experiments often reveal competition, feedback, and prolonged periods of malleability much as they do for the vertebrates. Indeed, given the well known advantages of studying the invertebrate nervous system, I think it likely that invertebrates can serve as important and useful preparations for studying, at the cellular level, the same biological phenomena that Easter et al. (1985) are trying to understand in vertebrates.

Radial nerve cross-finger flap differential nerve contribution in thumb reconstruction

Six patients with extensive palmar thumb pulp and soft tissue loss were treated with a staged transfer of radial nerve innervated flap tissues from the dorsum of the index finger. They were followed for a mean period of 3 years. This transfer of radial nerve innervated dorsal index finger tissues to the palmar surface of the thumb has been a successful method of innervation restoration. The ultimate sensibility of the reconstructed thumb seems to be a mixture of median and radial nerve sensibility. Sensory testing shows that after transfer, the ulnar side of the flap is more innervated from the radial nerve than the radial side. Our findings suggest that the presence of active sensory axons in the radial nerve innervated flap may play an inhibitory role in direct and/or functional neurotization of the transferred flap.

Competition and the dynamics of axon arbor growth in the cricket

The growth of an identified axonal arborization in the cricket cercal sensory system was studied under conditions that vary the number of neighboring axonal arborizations. The cell studied is one of a small number of neurons that arborize bilaterally and is called the X-neuron. Normally the axonal arborization of X is distributed roughly symmetrically about the midline. Seven days after the birth of this neuron, as soon after its birth as it can be stained, the axonal arbor is more than half the normal size and it exhibits the normal degree of bilaterality. During the remaining 50 days of postembryonic development, the arbor grows to its adult size, maintaining the bilateral distribution. The relationship between the growth of this axonal arbor and the presence or absence of its neighbors was studied by the removal of one of the sensory appendages. The removal of a cercus removes those neighbors near one half of the X-neuron's arbor and causes a dramatic shift in the bilateral distribution of X's axonal processes; after treatment nearly all of the varicosities were found in the deafferented region (Figs. 2, 6). Thus, neuron X responds to the loss of some to its neighbors by removing synaptic material from the area with normal neighbors and inserting additional material in the region with fewer neighbors. This effect is age dependent. Removal of neighbors early in life, during the initial period of synaptogenesis, causes a very rapid change, while similar treatment later in life causes a much slower response (Fig. 7). Thus the flexibility of the neuron is correlated with its growth rate; it is very flexible during its early, rapid growth phase and less flexible when the growth rate slows later in life. This continuing flexibility of the axonal arbor could also be demonstrated by allowing the amputated cercus to regenerate (Figs. 8, 9). Under these conditions the growth of X's arbor returned toward the normal bilateral distribution. However, regeneration of the neighboring arbors never restored the arbor to complete normality because a bias in the structure had already been imposed. In effect, regeneration arrested the change but could not reverse it. In brief, an identified sensory neuron's growth has been demonstrated to depend in part on the presence or absence of neighbors. This dependence extends throughout the life of the animal and fits the definition of a competitive interaction.

Regeneration of normal afferent input does not eliminate aberrant synaptic connections of an identified auditory interneuron in the cricket, Teleogryllus oceanicus

In the cricket, Teleogryllus oceanicus, the dendritic arborizations of an identified auditory interneuron (Int-1) are normally restricted to the ipsilateral auditory neuropile; unilateral deafferentation causes the medial portion of the dendritic field to sprout across the midline and make functional connections with the contralateral auditory neuropile (Hoy et al., '78: Soc. Neurosci. Abstr. 4:115, '85: Proc. Natl. Acad. Sci. USA 82:7772-7786; Hoy and Moiseff, '79: Soc. Neurosci. Abstr. 5:163). We have found that regeneration of the auditory afferents also results in an aberrant pattern of innervation of Int-1. Crickets were unilaterally deafferented during postembryonic development by crushing or cutting the auditory nerve. Regeneration of afferent-to-Int-1 connections was tested behaviorally. Of 86 nerve-crushed crickets tested as adults in the behavioral assay, 66% showed functional regeneration of the afferents. Similar results were obtained from the nerve-cut group. However, morphological investigations demonstrated that most of the regenerates still retained the aberrant contralateral dendritic projection. Electrophysiological recordings from these Int-1s showed that not only are some of them driven by their regenerated auditory afferents (the normal pathway) but that they retain their excitability via their contralateral dendrites (the aberrant pathway). This demonstrates that reinnervation of Int-1 by its normal afferent pool neither causes retraction nor prevents the formation of connections made with foreign, contralateral afferents. When the contralateral afferent pool was removed after Int-1 had sprouted, the sprouts remained present, but preliminary results suggest that if the contralateral afferents are removed before Int-1 is deafferented, sprouts are not formed. The results are discussed in relation to the roles of competition and conservation of membrane area in controlling synapse formation.