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POSTEMBRYONIC NEURONAL
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PLASTICITY AND ITS HORMONAL CONTROL DURING INSECT METAMORPHOSIS Janis C. Weeks
Institute of Neuroscience, University of Oregon, Eugene, Oregon 97403 Richard B. Levine
Arizona Research Laboratories, Division of Neurobiology, University of Arizona, Tucson, Arizona 85721 Introduction
Much attention has been focused recently upon postembryonic neuronal plasticity, especially as evidence accumulates for the continued modi fication of neuronal form in adults (e.g. DeVoogd & Nottebohm 1981, Purves & Hadley 1985, Kurz et aI1986). The behavioral correlates of such changes, as well as the cellular and molecular mechanisms by which they are induced and regulated, are of great interest. A striking natural example of such neuronal plasticity is offered by insect metamorphosis. Meta morphosis entails substantial changes in body form and behavior, and the nervous system must be reorganized to accommodate these changes. Notably, there is a continuation into postembryonic life of many neuro developmental processes that are normally viewed as being restricted to embryonic life, such as neuronal birth and death, and changes in neuronal structure and synaptic connectivity. The relative simplicity of the insect nervous system offers an excellent opportunity to study these phenomena in individually identified neurons. Furthermore, many aspects of nervous system metamorphosis are controlled by steroid hormones, just as these hormones exert profound effects on the developing vertebrate nervous 183 0147--006X/90/0301--0183$02.00
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system (Arnold & Gorski 1984). As we hope to document in this review, studies of the reorganization of the nervous system during metamorphosis and its endocrine control yield insights relevant to numerous areas of contemporary neurobiology. The moth, Manduca sexta, has provided a particularly attractive system in which to study metamorphosis, as indicated by our heavy emphasis in this review on this species. It is large enough to allow standard elec trophysiological and neuroanatomical techniques to be used at all post embryonic stages, and its endocrinology is well characterized. Important contributions have also been provided from recent studies of meta morphosis in the fruitfly Drosophila, in which genetic and molecular tech niques offer information that complements that obtained in Manduca. Our purpose in this review is to provide an overview of recent advances, and to identify promising new approaches to understanding neural events during metamorphosis. The Endocrinology of Metamorphosis
Metamorphosis is controlled by the relative blood levels of ecdysteroids (ecdysone and 20-hydroxyecdysone) and juvenile hormone, which influ ence target cells via changes in gene expression (reviewed in Riddiford 1985). The fluctuations in blood levels of ecdysteroids and juvenile hormone during metamorphosis are known in more detail in Manduca than in any other insect. Every molt is triggered by an ecdysteroid surge, with the direction of the molt determined by the juvenile hormone titer. Fo1\owing embryonic development and hatching from the egg, the caterpillar under goes four molts over a period of about 2 wk, each triggered by an elevation of blood ecdysteroids in the presence of juvenile hormone. ! Several key endocrinc events occur during the nine days of the fifth (final) larval instar. Early in the instar the juvenile hormone titer drops, followed by a small "commitment pulse" of ecdysteroids in the absence of juvenile hormone, which reprograms tissues for pupal development; e.g. in epidermal cells that secrete the cuticular exoskeleton, larval-specific genes become inac tivated. The commitment pulse also triggers wandering behavior, during which the larva burrows underground to pupate. This is followed by a larger, molt-inducing "prepupal peak" of ecdysteroids, that activates pupal-specific genes in epidermal cells and culminates in ecdysis to the pupa. The prepupal peak of ecdysteroids is accompanied by a low level of juvenile hormone, to which most tissues are indifferent due to their pre vious exposure to the commitment pulse. Development of the adult moth
I
This description is based on insects reared under typical laboratory conditions.
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within the pupal case takes 18 days and is triggered by a prolonged rise and fall of ecdysteroids in the absence of juvenile hormone. Most studies of the relationship between hormonal fluctuations and nervous system metamorphosis involve experimental manipulation of hor mone titers (e.g. Weeks & Truman 1986a). Ecdysteroids originate from the prothoracic glands located in the first thoracic segment, whereas juvenile hormone is released from brain-associated endocrine organs. It is therefore particularly convenient to study the effects of hormones on the abdominal nervous system, because ecdysteroids and juvenile hormone can be elimin ated from the abdomen by ligating the body at the abdominal-thoracic junction and discarding the anterior fragment. Ligated abdomens typically survive for weeks, and the missing hormones can be replaced by direct infusion into the blood, or by topical application to the body surface. Postembryonic Neurogenesis
During embryonic development of the Manduca central nervous system (eNS), neurons are produced from a stereotyped array of neuroblasts in a manner similar to that of Drosophila and the grasshopper (Thomas et al 1984). In the insect brain, neurogenesis typically continues post embryonically (Edwards 1969, Nordlander & Edwards 1969a,b, White & Kankel 1978); the new neurons contribute to the extensive elaboration of the visual (Bate 1978) and olfactory (Tolbert et al 1983) neuropils. In addition, it has been established recently that neuroblasts in the thoracic and abdominal ganglia of Manduca produce large numbers of neurons, termed imaginal nest celis, throughout larval life (Booker & Truman 1987a). These cells differentiate as interneurons during adult development (Booker & Truman 1987b, Witten & Truman 1988). Based on studies in Drosophila, Truman & Bate (1988) suggest that the neuroblasts that divide postembryonically may be retained embryonic neuroblasts that delay their final divisions until the larval stage. The postembryonic mitotic activity of the Manduca neuroblasts and the subsequent differentiation of their progeny are controlled by ecdysteroids and juvenile hormone (Booker & Truman 1987b). Interestingly, chemical ablation of specific subpopulations of the imaginal nest cells produces only minor behavioral deficits in adult moths (Truman & Booker 1986). It remains to be determined what functional role the imaginal nest cells play in the nervous system, and how these late-differentiating neurons are integrated into pre-existing neural circuits. Programmed Neuron Death
The death of identified neurons during metamorphosis and its hormonal control have been studied extensively in Manduca (reviewed by Truman
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1987). A wave of programmed neuron death follows each metamorphic molt, to eliminate neurons that are not needed for the next life stage. Most studies have concerned the deaths of motoneurons that innervate abdominal muscles (Taylor & Truman 1974, Truman 1983, Truman & Schwartz 1984, GiebuItowicz & Truman 1984, Levine & Truman 1985, Weeks & Truman 1985, 1986b, Weeks 1987). Motoneurons present in the larval abdomen can have several fates. Some innervate the same target muscle until adulthood. At the larval pupal transformation, many motoneurons are rendered targetless by the degeneration of larval abdominal muscles. Some of these motoneurons persist and later innervate newly generated adult muscles (see below), but many die. The best-studied of thcse dying motoneurons is PPR, which innervates a retractor muscle of the larval abdominal proleg (Weeks & Truman 1984). The target muscle of PPR degenerates before pupal ecdysis, and PPR dies two days after pupation. By combining endocrine and surgical manipulations, Weeks & Truman (1985, 1986b) have shown that the rise in blood ecdysteroids during the prepupal peak triggers PPR's death. This response requires previous exposure to ecdysteroids in the absence of juvenile hormone (i.e. the commitment pulse), but PPR is indifferent to juvenile hormone during the prepupal period. Furthermore, interactions with its target muscle do not play a role in PPR's death. The independence of the fates of motoneurons from that of their muscles during metamorphosis has been demonstrated repeatedly (Truman & Schwartz 1984, Bennett & Truman 1985, Weeks 1987, Kent & Levine 1988c and in preparation). After emergence of the adult moth, approxi mately 50% of the persistent larval motoneurons die in response to the decline in blood ecdysteroids at the end of adult development (Truman 1983, Truman & Schwartz 1984). The elimination of larval motoneurons in the pupal and adult stages reflects the progressive simplification of the abdominal musculature that accompanies metamorphosis. The finding that a rise or fall in ecdysteroid levels can trigger neuronal death (in the pupa and adult, respectively) emphasizes the general finding that the interpretation of a particular hormonal signal is gated by the cell's previous history of endocrine exposure (discussed in Weeks & Truman 1986a). Although motoneuron death is clearly regulated by ecdysteroid action on the nervous system, it has not yet been possible technically to identify specific motoneurons as direct ecdysteroid targets. All of the data to date are consistent with the dying motoneurons' being direct targets, but this assumption has yet to be demonstrated experimentally. An important step in this direction has been the autoradiographic studies of Fahrbach & Truman (1989), who showed that subsets of Manduca motoneurons accumulate radio-labeled ecdysteroid analogs at specific times during
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metamorphosis. It remains to be established which of the radioactively labeled motoneurons are those known from physiological studies. As has been found in other systems, the programmed death of Manduca neurons may involve de novo mRNA and protein synthesis; for instance, cyclo heximide treatment can prevent the death of motoneurons (Fahrbach & Truman 1988; J. C. Weeks & B. H. G. Debu, unpublished data). mRNAs associated with the ecdysteroid-triggered degeneration of Manduca muscle have been identified (Schwartz & Kay 1987), and probes developed in muscle may also prove useful in the nervous system. Once neurons that are direct targets for ecdysteroids and juvenile hormone have been identified, it should be possible to approach the hormonal regulation of neuron death at the molecular level (see below). Although ecdysteroids play a key role in controlling neuron death, other factors may intervene. For instance, Fahrbach & Truman ( 1987) showed that sectioning the nerve cord to interrupt descending neural input prevents the death of an identified Manduca motoneuron, MN-ll , after adult emergence. Another important influence is segmental location, as illus trated by the finding that a proleg motoneuron, APR, dies in only a subset of abdominal ganglia after pupation (Weeks & Ernst-Utzschneider 1989). Finally, the sexually-dimorphic motoneuron death that occurs in the ter minal ganglion of Manduca (Giebultowicz & Truman 1984) suggests, since there are no known differences in the hormone titers of male and female larvae, that the genes involved in sex determination can also influence motoneuron death. An important goal for the future is to elucidate how transsynaptic influences, segmental location, and genetic factors may inter act with endocrine cues to control motoneuron survival. Changes in Neuronal Arbors
One of the more remarkable aspects of metamorphosis is that individual neurons can exhibit markedly different structures and functions in the different life stages (reviewed by Truman et al 1986, Levine 1987, Levine & Weeks 1989). The most detailed studies have involved motoneurons, although some information is also available for persistent sensory neurons and interneurons. A combination of electrophysiological and anatomical techniques are used to demonstrate the persistence of individual neurons. For instance, motoneurons can be re-identified in different stages by back filling the nerve containing their axons (Taylor & Truman 1974, Truman & Reiss 1976, Thorn & Truman 1989), or by intracellular recording and dye injection at different stages (Levine & Truman 1982, 1985). A more definitive technique, which has been useful for following motoneurons innervating targets that are reorganized dramatically (such as the larval and adult thoracic legs of Manduca) involves retrogradely labeling a larval
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motoneuron by applying a fluorescent dye to its target muscle, and then visualizing the labeled motoneuron in the adult for intracellular recording (Kent & Levine 1988b,c). Another approach to following neuronal fates during metamorphosis is to use antibodies to neurotransmitters or peptides. This technique has been used to follow a persistent serotonin immunoreactive neuron in the Manduca brain (Kent et al 1987), and to document metamorphic changes in populations of neurons staining for a variety of substances in flies (e.g. White et a1 1986, Cantera & Niisse1 1987, Niissel et a1 1988, Valles & White 1988, Budnik & White 1988). Unlike motoneurons that innervate the same muscle throughout meta morphosis, persistent Manduca motoneurons that change their target muscles typically show morphological changes. Many motoneurons whose larval muscles degenerate exhibit significant regression around the time of pupation. In the case of the motoneurons innervating proleg retractor muscles, dendritic regression is triggered by the prepupal peak of ecdy steroids and is independent of the degeneration of the motoneurons' target muscles (Weeks & Truman 1985, 1986b). Furthermore, although some motoneurons that regress may subsequently die, the two phenomena are not necessarily linked; many motoneurons that regress at pupation survive and later regrow their dendritic fields during adult development (Truman & Reiss 1988, Kent & Levine 1988a--c, Wceks & Ernst-Utzschneider 1989). Definitive proof that regression and death are separate developmental options was provided by showing that the two events can be uncoupled within an individual motoneuron by manipulating ecdysterojd levels (Weeks 1987). Larval abdominal motoneurons that innervate new muscles in the adult undergo extensive dendritic growth, with a typical pattern being the expan sion of a larval unilateral arbor to a bilateral arbor in the adult (Levine & Truman 1985, Thorn & Truman 1989, Weeks & Ernst-Utzschneider 1989). Dendritic outgrowth depends on the elevation of ecdysteroids that nor mally accompanies adult development, and can be blocked if ecdysteroid release is delayed (such as during pupal diapause), or by treating with juvenile hormone at specific times (Levine & Truman 1985, Truman & Reiss 1988). As is discussed below, the regression or expansion of moto neuron arbors is correlated with developmental changes in synaptic con nections. The best-studied persistent sensory neurons innervate mechanosensory hairs located on a region of the larval abdomen that in the pupa develops into a specialized sensory structure called the gin trap. The larval afferents have characteristic axonal arbors within the eNS that expand significantly at pupation in response to the prepupal peak of ecdysteroids (Levine et al 1985, 1986, Levine 1989). Due to the location of their cell bodies in the
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periphery, these sensory neurons have proved to be ideal for studies of the endocrine dependence of neuronal outgrowth. Local application of juvenile hormone to the presumptive gin trap region during the com mitment pulse blocks the sensory neurons' reprogramming, so that at pupation a heterochronic mosaic pupa is formed that bears a small patch of larval epidermis and hairs. The arbors of the treated sensory neurons fail to expand despite their location in a pupal CNS, thus indicating that the endocrine environment of the cell body is of paramount importance in directing changes in neuronal shape (Levine et al 1986). Similarly, local application of 20-hydroxyecdysone to presumptive gin trap sensory neurons in abdomens that were ligated to prevent the prepupal peak causes pupal growth of the sensory neurons arbors within an otherwise larval CNS (Levine 1989). These data provide the strongest cvidcncc to date of a direct action of ecdysteroids and juvenile hormone in controling neuronal structure. The hormonally directed changes in sensory neuron arbors have demonstrable behavioral consequences (see below). Developmental studies of persistent interneurons are in their infancy, but at least some larval interneurons can be re-identified in pupae (Sand strom & Weeks 1988, Waldrop & Levine 1988, Levine & Weeks 1989). To ensure reliable re-identification, these studies have concentrated on interneurons that do not undergo major structural modifications, but it is likely that other interneurons do change morphologically. Although hormonally triggered changes in neuronal structure are now well documented in Manduca, a challenge for the future is to elucidate the mechanisms involved. For instance, it is desirable to determine whcthcr the neurons whose structures change are direct hormone targets or whether transsynaptic interactions with neurons or muscles are involved (e.g. Murphey et al 1975, Schneiderman et al 1982, Murphey 1986). A direct hormone action is consistent with the observed inability to prevent morphological changes in motoneurons by manipulating presynaptic afferents or target muscles (Weeks & Truman 1985, Kent & Levine 1988c, Jacobs & Weeks 1990). Although the evidence is good that hormones act directly on sensory neurons to trigger morphological changes, and the circumstantial evidence suggests that the same is true of motoneurons, more direct approaches are desirable. For instance, it would be of great interest to identify gene products involved in the hormonal responses of Manduca neurons (see below). Changes in Synaptic Connections and Behavior
The relationship between structural and functional changes in Manduca neurons during metamorphosis has been reviewed recently by Levine (1987), Weeks et al (1989), and Levine & Weeks (1989). In several cases it
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has been possible to correlate the hormonally mediated regression or growth of neuronal arbors with changes in synaptic connections and behavior. For instance, the contralateral growth of abdominal moto neuron arbors during adult development is associated with the acquisition of new, monosynaptic excitatory inputs from stretch receptor sensory neurons that are important for postural reflexes of the moth (Levine & Truman 1982). During the larval-pupal transformation, a proleg with drawal reflex mediated by monosynaptic excitatory connections between mechanosensory proleg sensory neurons and the proleg retractor moto neurons (Weeks & Jacobs 1987, Peterson & Weeks 1988) is lost as the motoneurons regress (Weeks et al 1989). In heterochronic mosaics that have larval sensory neurons and regressed pupal motoneurons, the syn aptic coupling between the sensory and motoneurons is reduced to the same extent as in normal pupae (Weeks & Jacobs 1988, Jacobs & Weeks 1990). Thus, the loss of the reflex behavior during metamorphosis appears to be due to regression of the motoneuron dendrites. Heterochronic mosaics have also been used to demonstrate that expansion of the axonal arbors of gin trap sensory neuron arbors within the CNS is necessary but not sufficient for the sensory neurons' ability to evoke the pupal-specific gin trap reflex (Levine et al 1986, 1989, Levine 1989). Thus, during meta morphosis, the remodeling of neuronal arbors is involved in the acquisition of new behaviors needed for the next life stage, and the elimination of outmoded behaviors leftover from the previous stage. These studies have all concerned relatively simple reflexes involving sensory and motoneurons, but most of the behavioral changes during metamorphosis undoubtedly involve changes in interneuron connections (e.g. Levine & Truman 1982, 1983, Mesce & Truman 1988, Miles & Weeks 1988 and in preparation, Waldrop & Levine 1989, Weeks et al 1989). Therefore, it will be especially illuminating to follow structural and func tional changes in interneurons during metamorphosis. Initial studies indi cate that the responses of abdominal interneurons to sensory input may differ in the larval and pupal stages (B. Waldrop and R. B. Levine, unpub lished observations), and that the size of interneuron-evoked synaptic potentials in motoneurons may change markedly (D. J. Sandstrom and J. C. Weeks, unpublished observations). Future Directions
We have attempted to convey the substantial progress that has taken place in understanding the cellular changes that accompany insect meta morphosis. Several approaches will be important for future progress. Con tinued electrophysiological and anatomical studies of identified neurons
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and their synaptic interactions will provide further insights into how ner vous systems can be modified to produce new behaviors, and how such modifications are regulated hormonally. Insect metamorphosis is also an ideal situation in which to explore the molecular mechanisms underlying neuronal development and differentiation, an area that ought to yield particularly exciting results in the coming years. The molecular biology of ecdysteroid and juvenile hormone action in Manduca epidermis is understood in considerable detail (e.g. Riddiford 1985, 1987). By utilizing the complementary advantages afforded by Drosophila and Manduca, it should be possible to identify neuronal genes that respond to ecdysteroids and juvenile hormone (e.g. Restifo & White 1988), and to determine how their products affect neuronal structure and function. Relevant to this approach are recent demonstrations that many genes involved in initial pattern formation within Drosophila embryos are expressed later during embryonic or postembryonic neural development (e.g. Brower 1987, Doe & Scott 1988, Doe et a11988). These genes, or others known to be involved in early neuronal differentiation in Drosophila (e.g. Campos-Ortega 1988, Thomas et al 1988, Caudy et al 1988), might also be involved in the differentiation of neurons derived from postembryonic neurogenesis or the re-differentiation of persistent neurons. By continued multidisciplinary approaches to the study of insect metamorphosis, we hope to better under stand how hormones influence structural and functional plasticity in all nervous systems. ACKNOWLEDGMENTS Research carried out in the authors' laboratories was supported by National Institute of Health (NIH) grant NS23208, a National Science Foundation (NSF) Presidential Young Investigator Award, and an Alfred P. Sloan Fellowship to J. C. Weeks; and NIH grant NS24822, NIH training grant NS07309, and NSF grant BNS8607066 to R. B. Levine. We thank Dr. K. S. Kent and Dr. B. A. Trimmer for their comments on the manu script. Literature Cited
Arnold, A. P., Gorski, R. A. 1984. Gonadal steroid induction of structural sex differ ences in the CNS. Annu. Rev. Neurosci. 7: 413-42 Bate, C. M. 1978. Development of sensory systems in arthropods. In Handbuuk uf Sensory Physiology, Develupment uf Sen sory Systems, ed. M. Jacobson, 9: 1-53.
Berlin: Springer-Verlag. 469 pp. Bennett, K., Truman, J. W. 1985. Steroid-
dependent survival of identifiable neurons in cultured ganglia of the moth Manduca sexta. Science 229: 58-60 Booker, R., Truman, J. W. 1987a. Post embryonic neurogenesis in the eNS of the tobacco hornworm, Manduca sexta. I. Neurobla�t arrays and the fate of their progeny during metamorphosis. J. Camp. Neural. 255: 548-59 Booker, R., Truman, J. W. 1987b. Post-
Annu. Rev. Neurosci. 1990.13:183-194. Downloaded from www.annualreviews.org Access provided by Mahidol University on 01/22/15. For personal use only.
192
WEEKS & LEVINE
embryonic neurogenesis in the CNS of the tobacco hornworm, Manduca sexta. II: Hormonal control of imaginal nest cell degeneration and differentiation during metamorphosis. J. Neurosci. 7: 4107-14 Brower, D. L. 1987. Ultrabithorax gene expression in Drosophila imaginal discs and larval nervous system. Development 101: 83-92 Budnik, V., White, K. 1988. Catecholamine containing neurons in Drosophila melano gaster: Distribution and development. J. Compo Neurol. 268: 400--13 Campus-Ortega, J. A. 1988. Cellular inter actions during early neurogenesis of Dro sophila melanogaster. Trends Neurosci. 1 1: 400--4 Cantera, R., Nasse!, D. R. 1987. Post embryonic development of serotonin immunoreactive neurons in the blowfly CNS. II. The thoracico-abdominal gan glia. Cell Tissue Res. 250: 449-59 Caudy, M., Viissin, II., Brand, M., Tuma, R., Jan, L. Y., Jan, Y. N. 1988. Daughter less, a Drosophila gene essential for both neurogenesis and sex determination, has sequence similarities to myc and the achaete-scute complex. Cell 55: 1061-67 DeVoogd, T., Nottebohm, F. 1981. Gonadal hormones induce dendritic growth in the adult avian brain. Science 214: 202-4 Doe, C. Q., Scott, M. P. 1988. Segmentation and homeotic gene function in the developing nervous system of Drosophila. Trends Neurosci. 11: 101--6 Doe, C. Q., Smouse, D., Goodman, C. S. 1988. Control of neuronal fate by the Dro sophila segmentation gene even-skipped. Nature 333: 376-78 Edwards, 1. S. 1969. Postemhryonic develop ment and regeneration of the insect ner vous system. Adv. Insect Physiol. 6: 97137 Fahrbach, S. E., Truman, J. W. 1987. Pos sible interactions of a steroid hormone and neural inputs in controlling the death of an identified neuron in the moth Manduca sexta. J. Neurobiol. 18: 497-508 Fahrbach, S. E., Truman, J. W. 1988. Cycloheximide inhibits ecdysteroid-regu lated neuronal death in the moth Manduca sexta. Soc. Neurosci. Abstr. 14: 368 Fahrbach, S. E., Truman, J. W. 1989. Auto radiographic identification of ecdysteroid binding cells in the nervous system of the moth Manduca sexta. J. Neurobiol. I n press Giebultowicz, 1. M., Truman, J. W. 1984. Sexual differentiation in the terminal gan glion of the moth Manduca sexta: Role of sex-specific neuronal death. J. Compo Neurol. 226: 87-95 Jacobs, G. A., Weeks, 1. C. 1990. Post-
synaptic changes at a sensory-to-motor synapse contribute to the developmental loss of a reflex behavior during insect metamorphosis. J. Neurosci. In press Kent, K. S., Hoskins, S. G., Hildebrand, 1. G. 1987. A novel serotonin-immuno reactive neuron in the antennal lobe of the Sphinx moth Manduca sexta persists throughout postembryonic life. J. Neuro bioi. 18: 451-65 Kent, K. S., Levine, R. B. 1988a. Neural control of leg movements in a mcta morphic insect: Sensory and motor elements of the larval thuracic h::gs in Man duca sexta. J. Compo Neurol. 271: 559-76 Kent, K. S., Levine, R. B. 1988b. Neural control of leg movements in a meta morphic insect: Persistence of the larval leg motor neurons to innervate the adult legs of Manduca sex/a. J. Compo Neurol. 276: 30-43 Kent, K. S., Levine, R. B. 1988c. Reor ganization of an identified leg motor neuron during metamurphosis of the moth Manduca sexta. Soc. Neurosci. Abstr. 14: 1004 Kurz, E. M.,Sengelaub, D. R., Arnold, A. P. 1986. Androgens regulate the dendritic length of mammalian motoneurons in adulthood. Science 232: 345-98 Levine, R. B. 1987. Neural reorganization and its endocrine control during meta morphosis. Curr. Top. Dev. Bioi. 2 1: 34165 Levine, R. B. 1989. Expansion of the axonal arborizations of persistent sensory neurons during insect metamorphosis: The role of 20-hydroxyecdysone. J. Neuro sci. 9: 1045-54 Levine, R. B., Pak, C., Linn, D. 19R5. The structure, function, and metamorphic reorganization of somatotopically pro jecting sensory neurons in Manduca sexta larvae. J. Compo Physiol. 157: 1-13 Levine, R. B., Truman, J. W. 1982. Meta morphosis of the nervous system: Changes in the morphology and synaptic inter actions of identified cells. Nature 299: 250-52 Levine, R. B., Truman, J. W. 1983. Peptide activation of a simple neural circuit. Brain Res. 279: 335-38 Levine, R. B., Truman, J. W. 1985. Dendritic reorganization of abdominal moto neurons during metamorphosis of the moth, Manduca sexta. J. Neurosci. 5: 2424-31 Levine, R. B., Truman, J. W., Linn, D., Bate, C. M. 1986. Endocrine regulation of the form and function ofaxonal arbors during insect metamorphosis. J. Neurosci. 6: 29399 Levine, R. B., Waldrop, B., Tamarkin, D.
Annu. Rev. Neurosci. 1990.13:183-194. Downloaded from www.annualreviews.org Access provided by Mahidol University on 01/22/15. For personal use only.
INSECT METAMORPHOSIS 1989. The use of hormonally-induced mosaics to study alterations in the syn aptic connections made by persistent sen sory neurons during insect metamor phosis. J. Neurobiol. 20: 362�38 Levine, R. B., Weeks, J. C. 1989. Reor ganization of neural circuits and behavior during insect metamorphosis. In Per spectives on Neural Systems and Behavior, ed. T. J. Carew, D. Kelley, pp. 195�228. New York: Liss Mesce, K. A., Truman, J. W. 1988. Meta morphosis of the ecdysis motor pattern in the hawkmoth Manduca sexta. J. Compo Physiol. 163: 287�99 Miles, C. I., Weeks, J. C. 1988. Develop mental changes in pre-ecdysis motor pat terns of the moth, Manduca sexta. Soc. Neurosci. Abstr. 14: 998 Murphey, R. K. 1986. Competition and dynamics of axon arbor growth in the cricket. J. Comp. Neurol. 251 : 100-10 Murphey, R. K., Mendenhall, B., Palka, J., Edwards, J. S. 1975. Deafferentation slows the growth of specific dendrites of identi fied giant interneurons. J. Comp. Neurol. 159: 407�18 Nassel, D. R., Ohlsson, L., Cantera, R. 1988. Metamorphosis of identified neurons innervating thoracic neurohemal organs in the blowfly: Transformation of chole cystokininlike immunoreactive neurons. J. Compo Neurol. 267: 343�56 Nordlander, R. H., Edwards, J. S. I 969a. Postembryonic brain development in the monarch butterfly Danaus plexippus plexippus L. 1. Cellular events during brain morphogenesis. Wilhelm Roux Arch. Entwicklungsmech. Org. 162: 197�217 Nordlander, R. H., Edwards, J. S. 1969b. Postembryonic brain development in the monarch butterfly Danaus plexippus plexippus L. II. The optic lobes. Wilhelm Roux Arch. Entwicklungsmech. Org. 163: 197�220 Peterson, B. A., Weeks, J. C. 1988. Soma totopic mapping of sensory neurons inner vating mechanosensory hairs on the larval prolegs of Manduca sexta. J. Compo Neural. 275: 128-44 Purves, D., Hadley, R. D. 1985. Changes in the dendritic branching of adult mam malian neurons revealed by repeated imaging in situ. Na ture 315: 404-6 Restifo, L. L., Whitc, K. 1988. Expression of a steroid hormone regulated gene in the CNS of Drosophila melanogaster. Soc. Neurosci. Abstr. 14: 733 Riddiford, L. M. 1985. Hormone action at the cellular level. In Comprehensive Insect Physiology, Bi ochemistry, and Pharma cology, ed. G. A. Kerkut, L. I. Gilbert, 8: 37�84. New York: Pergamon. 595 pp.
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Riddiford, L. M. 1987. Hormonal control of sequential gene expression in insect epi dermis. In Molecular Entomology, ed. J. H. Law, pp. 211�22. New York: Liss. 500 pp. Sandstrom, D. J., Weeks, J. C. 1988. Identi fied interneurons in larval and pupal abdominal ganglia of the tobacco hornworm, Manduca sexta. Soc. Neurasci. Abstr. 14: 1003 Schneiderman, A. M., Matsumoto, S. G., Hildebrand, J. G. 1982. Trans-sexually grafted antennae influence the develop ment of sexually dimorphic neurons in the brain of Manduca sexta. Nature 298: 84446 Schwartz, L. M., Kay, B. K. 1987. De novo transcription and translation of new genes is required for the programmed death of the intersegmental muscles of the tobacco hawkmoth, Manduca sexta. Soc. Neurosci. Abstr. 13: 8 Taylor, H. M., Truman, J. W. 1974. Meta morphosis of the abdominal ganglia of the tobacco hornworm, Manduca sexta. J. Compo Physiol. 90: 367�88 Thomas, J. B., Bastiani, M. J., Bate, N., Goodman, C. S. 1984. From grasshopper to Drosophila: A common plan for neuronal development. Nature 310: 203�7 Thomas, J. B., Crews, S. T., Goodman, C. S. 1988. Molecular genetics of the single minded locus: A gene involved in the development of the Drosophila nervous system. Cell 52: 133-41 Thorn, R., Truman, J. W. 1989. Sex-specific neuronal respectification during the meta morphosis of the genital segments of the tobacco hornworm moth, Manduca sexta. J. Comp. Neurol. 284: 489�503 Tolbert, L. P., Matsumoto, S. M., Hilde brand, J. G. 1983. Development of syn apses in the antennal lobe of the moth, Manduca sexta, during metamorphosis. J. Neurosci. 3: 1158�75 Truman, J. W. 1983. Programmed cell death in the nervous system of an adult insect. J. Compo Neurol. 216: 445�52 Truman, J. W. 1987. The insect nervous sys tem as a model system for the study of neuronal death. Curro Top. Dev. B ioi. , 21: 99�1l6 Truman, J. W., Bate, M. 1988. Spatial and temporal patterns of neurogt:nesis in tht: central nervous system of Drosophila melanogaster. Dev. Bioi. 125: 145�57 Truman, J. W., Booker, R. 1986. Adult specific neurons in the nervous system of Manduca sexta: Selective chemical ablation using hydroxyurea. J. Neurobiol. 17: 613�25 Truman, J. W., Reiss, S. E. 1 976 . Dendritic reorganization of an identified moto-
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neuron during metamorphosis of the tobacco hornworm, Manduca sexta. Science 192: 477-79 Truman, J. W., Reiss, S. E. 1988. Hormonal regulation of the shape of identified moto neurons in Manduca sexta. J. Neurosci. 8: 765-75 Truman, J. W., Schwartz, L. M. 1984. Ster oid regulation of neuronal death in the moth nervous systcm. J. Neurosci. 4: 27480 Truman, J. W., Weeks, J. c., Levine, R. B. 1986. Developmental plasticity during metamorphosis of an insect nervous system. In Comparative Neurobiology: Modes 0/ Communication in the Nervous System, ed. M. J. Cohen, F. Strum wass er, pp. 25-44. New York: Wiley. 397 pp. Valles, A. M., White, K. 1988. Serotonin containing neurons in Drosophila melano gaster: Development and distribution. J. Compo Neurol. 268: 414-28 Waldrop, B., Lev ine, R. B. 1988. Inter neurons involved in multisegmental reflexes in larvae and pupae of the moth, Manduca sexta. Soc. Neurosci. Abstr. 14: 1003 Waldrop, B., Levine, R. B. 1989. Develop ment of the gin trap reflex in Manduca sexta a comparison of larval and pupal motor responses. J. Compo Physiol. In press Weeks, J. C. 1987. Time course of hormonal independence for developmental events in neurons and other cell types during insect metamorphosis. Dev. Bioi. 124: 163-76 Weeks, J. c., Ernst-Utzschneider, K. 1989. Respecification of larval proleg moto neurons during metamorphosis of the tobacco hornworm, Manduca sexta: seg mental dependence and hormonal regu lation. J. Neurobiol. 20: 569-92 Weeks, J. c., Jacobs, G. A. 1987. A reflex behavior mediated by monosynaptic con nections between hair afferents and moto neurons in the larval tobacco hornworm, Manduca sexta. J. Compo Physiol. 1 60: 315-29
Weeks, J. c., Jacobs, G. A. 1988. Develop mental changes in postsynaptic neurons cause loss of a monosynaptic reflcx during metamorphosis in Manduca sexta. Soc. Neurosci. Abstr. 14: 998 Weeks, J. c., Truman, J. W. 1984. Neural organization of peptide-activated ecdysis during the metamorphosis of Manduca sexta. II. Retention of the prolcg motor pattern despite loss of the prolegs at pupation. J. Compo Physiol. 155: 42333 Weeks, J. C., Truman, J. W. 1985. Inde pendent steroid control of the fates of motoneurons and their muscles during insect metamorphosis. J. Neurosci. 5: 2290-2300 Weeks, J. c., Truman, J. W. 1986a. Steroid control of neuron and muscle develop ment during the metamorphosis of an insect. J. Neurobiol. 17: 249-67 Weeks, J. c., Truman, J. W. 1986b. Hor monally mediated reprogramming of muscles and motoneurons during the lar val-pupal transformation of the tobacco hornworm, Manduca sexta. J. Exp. Bioi. 125: 1-13 Weeks, J. c., Jacobs, G. A., Miles, C. I. 1989. Hormonally-mediated modifications of neuronal structure, synaptic connectivity, and behavior during metamorphosis of the tobacco hornworm, Manduca sexta. Am. Zool. In press White, K., Hurteau, T., Punsal, P. 1986. Neuropeptide-FRMFamide-like immuno re activity in Drosophila: Development and distribution. J. Compo Neurol. 247: 430-38 White, K., Kankel, D. R. 1978. Patterns of cell division and movement in the for mation of the imaginal nervous system in Drosophila melanogaster. Dev. Bioi. 65: 296-321 Witten, J. L., Truman, J. W. 1988. Regu lation of transmitter choice in an identified lineage in Manduca sexta. Soc. Neurosci. Abstr. 14: 28
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POSTEMBRYONIC NEURONAL
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PLASTICITY AND ITS HORMONAL CONTROL DURING INSECT METAMORPHOSIS Janis C. Weeks
Institute of Neuroscience, University of Oregon, Eugene, Oregon 97403 Richard B. Levine
Arizona Research Laboratories, Division of Neurobiology, University of Arizona, Tucson, Arizona 85721 Introduction
Much attention has been focused recently upon postembryonic neuronal plasticity, especially as evidence accumulates for the continued modi fication of neuronal form in adults (e.g. DeVoogd & Nottebohm 1981, Purves & Hadley 1985, Kurz et aI1986). The behavioral correlates of such changes, as well as the cellular and molecular mechanisms by which they are induced and regulated, are of great interest. A striking natural example of such neuronal plasticity is offered by insect metamorphosis. Meta morphosis entails substantial changes in body form and behavior, and the nervous system must be reorganized to accommodate these changes. Notably, there is a continuation into postembryonic life of many neuro developmental processes that are normally viewed as being restricted to embryonic life, such as neuronal birth and death, and changes in neuronal structure and synaptic connectivity. The relative simplicity of the insect nervous system offers an excellent opportunity to study these phenomena in individually identified neurons. Furthermore, many aspects of nervous system metamorphosis are controlled by steroid hormones, just as these hormones exert profound effects on the developing vertebrate nervous 183 0147--006X/90/0301--0183$02.00
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system (Arnold & Gorski 1984). As we hope to document in this review, studies of the reorganization of the nervous system during metamorphosis and its endocrine control yield insights relevant to numerous areas of contemporary neurobiology. The moth, Manduca sexta, has provided a particularly attractive system in which to study metamorphosis, as indicated by our heavy emphasis in this review on this species. It is large enough to allow standard elec trophysiological and neuroanatomical techniques to be used at all post embryonic stages, and its endocrinology is well characterized. Important contributions have also been provided from recent studies of meta morphosis in the fruitfly Drosophila, in which genetic and molecular tech niques offer information that complements that obtained in Manduca. Our purpose in this review is to provide an overview of recent advances, and to identify promising new approaches to understanding neural events during metamorphosis. The Endocrinology of Metamorphosis
Metamorphosis is controlled by the relative blood levels of ecdysteroids (ecdysone and 20-hydroxyecdysone) and juvenile hormone, which influ ence target cells via changes in gene expression (reviewed in Riddiford 1985). The fluctuations in blood levels of ecdysteroids and juvenile hormone during metamorphosis are known in more detail in Manduca than in any other insect. Every molt is triggered by an ecdysteroid surge, with the direction of the molt determined by the juvenile hormone titer. Fo1\owing embryonic development and hatching from the egg, the caterpillar under goes four molts over a period of about 2 wk, each triggered by an elevation of blood ecdysteroids in the presence of juvenile hormone. ! Several key endocrinc events occur during the nine days of the fifth (final) larval instar. Early in the instar the juvenile hormone titer drops, followed by a small "commitment pulse" of ecdysteroids in the absence of juvenile hormone, which reprograms tissues for pupal development; e.g. in epidermal cells that secrete the cuticular exoskeleton, larval-specific genes become inac tivated. The commitment pulse also triggers wandering behavior, during which the larva burrows underground to pupate. This is followed by a larger, molt-inducing "prepupal peak" of ecdysteroids, that activates pupal-specific genes in epidermal cells and culminates in ecdysis to the pupa. The prepupal peak of ecdysteroids is accompanied by a low level of juvenile hormone, to which most tissues are indifferent due to their pre vious exposure to the commitment pulse. Development of the adult moth
I
This description is based on insects reared under typical laboratory conditions.
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within the pupal case takes 18 days and is triggered by a prolonged rise and fall of ecdysteroids in the absence of juvenile hormone. Most studies of the relationship between hormonal fluctuations and nervous system metamorphosis involve experimental manipulation of hor mone titers (e.g. Weeks & Truman 1986a). Ecdysteroids originate from the prothoracic glands located in the first thoracic segment, whereas juvenile hormone is released from brain-associated endocrine organs. It is therefore particularly convenient to study the effects of hormones on the abdominal nervous system, because ecdysteroids and juvenile hormone can be elimin ated from the abdomen by ligating the body at the abdominal-thoracic junction and discarding the anterior fragment. Ligated abdomens typically survive for weeks, and the missing hormones can be replaced by direct infusion into the blood, or by topical application to the body surface. Postembryonic Neurogenesis
During embryonic development of the Manduca central nervous system (eNS), neurons are produced from a stereotyped array of neuroblasts in a manner similar to that of Drosophila and the grasshopper (Thomas et al 1984). In the insect brain, neurogenesis typically continues post embryonically (Edwards 1969, Nordlander & Edwards 1969a,b, White & Kankel 1978); the new neurons contribute to the extensive elaboration of the visual (Bate 1978) and olfactory (Tolbert et al 1983) neuropils. In addition, it has been established recently that neuroblasts in the thoracic and abdominal ganglia of Manduca produce large numbers of neurons, termed imaginal nest celis, throughout larval life (Booker & Truman 1987a). These cells differentiate as interneurons during adult development (Booker & Truman 1987b, Witten & Truman 1988). Based on studies in Drosophila, Truman & Bate (1988) suggest that the neuroblasts that divide postembryonically may be retained embryonic neuroblasts that delay their final divisions until the larval stage. The postembryonic mitotic activity of the Manduca neuroblasts and the subsequent differentiation of their progeny are controlled by ecdysteroids and juvenile hormone (Booker & Truman 1987b). Interestingly, chemical ablation of specific subpopulations of the imaginal nest cells produces only minor behavioral deficits in adult moths (Truman & Booker 1986). It remains to be determined what functional role the imaginal nest cells play in the nervous system, and how these late-differentiating neurons are integrated into pre-existing neural circuits. Programmed Neuron Death
The death of identified neurons during metamorphosis and its hormonal control have been studied extensively in Manduca (reviewed by Truman
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1987). A wave of programmed neuron death follows each metamorphic molt, to eliminate neurons that are not needed for the next life stage. Most studies have concerned the deaths of motoneurons that innervate abdominal muscles (Taylor & Truman 1974, Truman 1983, Truman & Schwartz 1984, GiebuItowicz & Truman 1984, Levine & Truman 1985, Weeks & Truman 1985, 1986b, Weeks 1987). Motoneurons present in the larval abdomen can have several fates. Some innervate the same target muscle until adulthood. At the larval pupal transformation, many motoneurons are rendered targetless by the degeneration of larval abdominal muscles. Some of these motoneurons persist and later innervate newly generated adult muscles (see below), but many die. The best-studied of thcse dying motoneurons is PPR, which innervates a retractor muscle of the larval abdominal proleg (Weeks & Truman 1984). The target muscle of PPR degenerates before pupal ecdysis, and PPR dies two days after pupation. By combining endocrine and surgical manipulations, Weeks & Truman (1985, 1986b) have shown that the rise in blood ecdysteroids during the prepupal peak triggers PPR's death. This response requires previous exposure to ecdysteroids in the absence of juvenile hormone (i.e. the commitment pulse), but PPR is indifferent to juvenile hormone during the prepupal period. Furthermore, interactions with its target muscle do not play a role in PPR's death. The independence of the fates of motoneurons from that of their muscles during metamorphosis has been demonstrated repeatedly (Truman & Schwartz 1984, Bennett & Truman 1985, Weeks 1987, Kent & Levine 1988c and in preparation). After emergence of the adult moth, approxi mately 50% of the persistent larval motoneurons die in response to the decline in blood ecdysteroids at the end of adult development (Truman 1983, Truman & Schwartz 1984). The elimination of larval motoneurons in the pupal and adult stages reflects the progressive simplification of the abdominal musculature that accompanies metamorphosis. The finding that a rise or fall in ecdysteroid levels can trigger neuronal death (in the pupa and adult, respectively) emphasizes the general finding that the interpretation of a particular hormonal signal is gated by the cell's previous history of endocrine exposure (discussed in Weeks & Truman 1986a). Although motoneuron death is clearly regulated by ecdysteroid action on the nervous system, it has not yet been possible technically to identify specific motoneurons as direct ecdysteroid targets. All of the data to date are consistent with the dying motoneurons' being direct targets, but this assumption has yet to be demonstrated experimentally. An important step in this direction has been the autoradiographic studies of Fahrbach & Truman (1989), who showed that subsets of Manduca motoneurons accumulate radio-labeled ecdysteroid analogs at specific times during
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metamorphosis. It remains to be established which of the radioactively labeled motoneurons are those known from physiological studies. As has been found in other systems, the programmed death of Manduca neurons may involve de novo mRNA and protein synthesis; for instance, cyclo heximide treatment can prevent the death of motoneurons (Fahrbach & Truman 1988; J. C. Weeks & B. H. G. Debu, unpublished data). mRNAs associated with the ecdysteroid-triggered degeneration of Manduca muscle have been identified (Schwartz & Kay 1987), and probes developed in muscle may also prove useful in the nervous system. Once neurons that are direct targets for ecdysteroids and juvenile hormone have been identified, it should be possible to approach the hormonal regulation of neuron death at the molecular level (see below). Although ecdysteroids play a key role in controlling neuron death, other factors may intervene. For instance, Fahrbach & Truman ( 1987) showed that sectioning the nerve cord to interrupt descending neural input prevents the death of an identified Manduca motoneuron, MN-ll , after adult emergence. Another important influence is segmental location, as illus trated by the finding that a proleg motoneuron, APR, dies in only a subset of abdominal ganglia after pupation (Weeks & Ernst-Utzschneider 1989). Finally, the sexually-dimorphic motoneuron death that occurs in the ter minal ganglion of Manduca (Giebultowicz & Truman 1984) suggests, since there are no known differences in the hormone titers of male and female larvae, that the genes involved in sex determination can also influence motoneuron death. An important goal for the future is to elucidate how transsynaptic influences, segmental location, and genetic factors may inter act with endocrine cues to control motoneuron survival. Changes in Neuronal Arbors
One of the more remarkable aspects of metamorphosis is that individual neurons can exhibit markedly different structures and functions in the different life stages (reviewed by Truman et al 1986, Levine 1987, Levine & Weeks 1989). The most detailed studies have involved motoneurons, although some information is also available for persistent sensory neurons and interneurons. A combination of electrophysiological and anatomical techniques are used to demonstrate the persistence of individual neurons. For instance, motoneurons can be re-identified in different stages by back filling the nerve containing their axons (Taylor & Truman 1974, Truman & Reiss 1976, Thorn & Truman 1989), or by intracellular recording and dye injection at different stages (Levine & Truman 1982, 1985). A more definitive technique, which has been useful for following motoneurons innervating targets that are reorganized dramatically (such as the larval and adult thoracic legs of Manduca) involves retrogradely labeling a larval
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motoneuron by applying a fluorescent dye to its target muscle, and then visualizing the labeled motoneuron in the adult for intracellular recording (Kent & Levine 1988b,c). Another approach to following neuronal fates during metamorphosis is to use antibodies to neurotransmitters or peptides. This technique has been used to follow a persistent serotonin immunoreactive neuron in the Manduca brain (Kent et al 1987), and to document metamorphic changes in populations of neurons staining for a variety of substances in flies (e.g. White et a1 1986, Cantera & Niisse1 1987, Niissel et a1 1988, Valles & White 1988, Budnik & White 1988). Unlike motoneurons that innervate the same muscle throughout meta morphosis, persistent Manduca motoneurons that change their target muscles typically show morphological changes. Many motoneurons whose larval muscles degenerate exhibit significant regression around the time of pupation. In the case of the motoneurons innervating proleg retractor muscles, dendritic regression is triggered by the prepupal peak of ecdy steroids and is independent of the degeneration of the motoneurons' target muscles (Weeks & Truman 1985, 1986b). Furthermore, although some motoneurons that regress may subsequently die, the two phenomena are not necessarily linked; many motoneurons that regress at pupation survive and later regrow their dendritic fields during adult development (Truman & Reiss 1988, Kent & Levine 1988a--c, Wceks & Ernst-Utzschneider 1989). Definitive proof that regression and death are separate developmental options was provided by showing that the two events can be uncoupled within an individual motoneuron by manipulating ecdysterojd levels (Weeks 1987). Larval abdominal motoneurons that innervate new muscles in the adult undergo extensive dendritic growth, with a typical pattern being the expan sion of a larval unilateral arbor to a bilateral arbor in the adult (Levine & Truman 1985, Thorn & Truman 1989, Weeks & Ernst-Utzschneider 1989). Dendritic outgrowth depends on the elevation of ecdysteroids that nor mally accompanies adult development, and can be blocked if ecdysteroid release is delayed (such as during pupal diapause), or by treating with juvenile hormone at specific times (Levine & Truman 1985, Truman & Reiss 1988). As is discussed below, the regression or expansion of moto neuron arbors is correlated with developmental changes in synaptic con nections. The best-studied persistent sensory neurons innervate mechanosensory hairs located on a region of the larval abdomen that in the pupa develops into a specialized sensory structure called the gin trap. The larval afferents have characteristic axonal arbors within the eNS that expand significantly at pupation in response to the prepupal peak of ecdysteroids (Levine et al 1985, 1986, Levine 1989). Due to the location of their cell bodies in the
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periphery, these sensory neurons have proved to be ideal for studies of the endocrine dependence of neuronal outgrowth. Local application of juvenile hormone to the presumptive gin trap region during the com mitment pulse blocks the sensory neurons' reprogramming, so that at pupation a heterochronic mosaic pupa is formed that bears a small patch of larval epidermis and hairs. The arbors of the treated sensory neurons fail to expand despite their location in a pupal CNS, thus indicating that the endocrine environment of the cell body is of paramount importance in directing changes in neuronal shape (Levine et al 1986). Similarly, local application of 20-hydroxyecdysone to presumptive gin trap sensory neurons in abdomens that were ligated to prevent the prepupal peak causes pupal growth of the sensory neurons arbors within an otherwise larval CNS (Levine 1989). These data provide the strongest cvidcncc to date of a direct action of ecdysteroids and juvenile hormone in controling neuronal structure. The hormonally directed changes in sensory neuron arbors have demonstrable behavioral consequences (see below). Developmental studies of persistent interneurons are in their infancy, but at least some larval interneurons can be re-identified in pupae (Sand strom & Weeks 1988, Waldrop & Levine 1988, Levine & Weeks 1989). To ensure reliable re-identification, these studies have concentrated on interneurons that do not undergo major structural modifications, but it is likely that other interneurons do change morphologically. Although hormonally triggered changes in neuronal structure are now well documented in Manduca, a challenge for the future is to elucidate the mechanisms involved. For instance, it is desirable to determine whcthcr the neurons whose structures change are direct hormone targets or whether transsynaptic interactions with neurons or muscles are involved (e.g. Murphey et al 1975, Schneiderman et al 1982, Murphey 1986). A direct hormone action is consistent with the observed inability to prevent morphological changes in motoneurons by manipulating presynaptic afferents or target muscles (Weeks & Truman 1985, Kent & Levine 1988c, Jacobs & Weeks 1990). Although the evidence is good that hormones act directly on sensory neurons to trigger morphological changes, and the circumstantial evidence suggests that the same is true of motoneurons, more direct approaches are desirable. For instance, it would be of great interest to identify gene products involved in the hormonal responses of Manduca neurons (see below). Changes in Synaptic Connections and Behavior
The relationship between structural and functional changes in Manduca neurons during metamorphosis has been reviewed recently by Levine (1987), Weeks et al (1989), and Levine & Weeks (1989). In several cases it
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has been possible to correlate the hormonally mediated regression or growth of neuronal arbors with changes in synaptic connections and behavior. For instance, the contralateral growth of abdominal moto neuron arbors during adult development is associated with the acquisition of new, monosynaptic excitatory inputs from stretch receptor sensory neurons that are important for postural reflexes of the moth (Levine & Truman 1982). During the larval-pupal transformation, a proleg with drawal reflex mediated by monosynaptic excitatory connections between mechanosensory proleg sensory neurons and the proleg retractor moto neurons (Weeks & Jacobs 1987, Peterson & Weeks 1988) is lost as the motoneurons regress (Weeks et al 1989). In heterochronic mosaics that have larval sensory neurons and regressed pupal motoneurons, the syn aptic coupling between the sensory and motoneurons is reduced to the same extent as in normal pupae (Weeks & Jacobs 1988, Jacobs & Weeks 1990). Thus, the loss of the reflex behavior during metamorphosis appears to be due to regression of the motoneuron dendrites. Heterochronic mosaics have also been used to demonstrate that expansion of the axonal arbors of gin trap sensory neuron arbors within the CNS is necessary but not sufficient for the sensory neurons' ability to evoke the pupal-specific gin trap reflex (Levine et al 1986, 1989, Levine 1989). Thus, during meta morphosis, the remodeling of neuronal arbors is involved in the acquisition of new behaviors needed for the next life stage, and the elimination of outmoded behaviors leftover from the previous stage. These studies have all concerned relatively simple reflexes involving sensory and motoneurons, but most of the behavioral changes during metamorphosis undoubtedly involve changes in interneuron connections (e.g. Levine & Truman 1982, 1983, Mesce & Truman 1988, Miles & Weeks 1988 and in preparation, Waldrop & Levine 1989, Weeks et al 1989). Therefore, it will be especially illuminating to follow structural and func tional changes in interneurons during metamorphosis. Initial studies indi cate that the responses of abdominal interneurons to sensory input may differ in the larval and pupal stages (B. Waldrop and R. B. Levine, unpub lished observations), and that the size of interneuron-evoked synaptic potentials in motoneurons may change markedly (D. J. Sandstrom and J. C. Weeks, unpublished observations). Future Directions
We have attempted to convey the substantial progress that has taken place in understanding the cellular changes that accompany insect meta morphosis. Several approaches will be important for future progress. Con tinued electrophysiological and anatomical studies of identified neurons
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and their synaptic interactions will provide further insights into how ner vous systems can be modified to produce new behaviors, and how such modifications are regulated hormonally. Insect metamorphosis is also an ideal situation in which to explore the molecular mechanisms underlying neuronal development and differentiation, an area that ought to yield particularly exciting results in the coming years. The molecular biology of ecdysteroid and juvenile hormone action in Manduca epidermis is understood in considerable detail (e.g. Riddiford 1985, 1987). By utilizing the complementary advantages afforded by Drosophila and Manduca, it should be possible to identify neuronal genes that respond to ecdysteroids and juvenile hormone (e.g. Restifo & White 1988), and to determine how their products affect neuronal structure and function. Relevant to this approach are recent demonstrations that many genes involved in initial pattern formation within Drosophila embryos are expressed later during embryonic or postembryonic neural development (e.g. Brower 1987, Doe & Scott 1988, Doe et a11988). These genes, or others known to be involved in early neuronal differentiation in Drosophila (e.g. Campos-Ortega 1988, Thomas et al 1988, Caudy et al 1988), might also be involved in the differentiation of neurons derived from postembryonic neurogenesis or the re-differentiation of persistent neurons. By continued multidisciplinary approaches to the study of insect metamorphosis, we hope to better under stand how hormones influence structural and functional plasticity in all nervous systems. ACKNOWLEDGMENTS Research carried out in the authors' laboratories was supported by National Institute of Health (NIH) grant NS23208, a National Science Foundation (NSF) Presidential Young Investigator Award, and an Alfred P. Sloan Fellowship to J. C. Weeks; and NIH grant NS24822, NIH training grant NS07309, and NSF grant BNS8607066 to R. B. Levine. We thank Dr. K. S. Kent and Dr. B. A. Trimmer for their comments on the manu script. Literature Cited
Arnold, A. P., Gorski, R. A. 1984. Gonadal steroid induction of structural sex differ ences in the CNS. Annu. Rev. Neurosci. 7: 413-42 Bate, C. M. 1978. Development of sensory systems in arthropods. In Handbuuk uf Sensory Physiology, Develupment uf Sen sory Systems, ed. M. Jacobson, 9: 1-53.
Berlin: Springer-Verlag. 469 pp. Bennett, K., Truman, J. W. 1985. Steroid-
dependent survival of identifiable neurons in cultured ganglia of the moth Manduca sexta. Science 229: 58-60 Booker, R., Truman, J. W. 1987a. Post embryonic neurogenesis in the eNS of the tobacco hornworm, Manduca sexta. I. Neurobla�t arrays and the fate of their progeny during metamorphosis. J. Camp. Neural. 255: 548-59 Booker, R., Truman, J. W. 1987b. Post-
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192
WEEKS & LEVINE
embryonic neurogenesis in the CNS of the tobacco hornworm, Manduca sexta. II: Hormonal control of imaginal nest cell degeneration and differentiation during metamorphosis. J. Neurosci. 7: 4107-14 Brower, D. L. 1987. Ultrabithorax gene expression in Drosophila imaginal discs and larval nervous system. Development 101: 83-92 Budnik, V., White, K. 1988. Catecholamine containing neurons in Drosophila melano gaster: Distribution and development. J. Compo Neurol. 268: 400--13 Campus-Ortega, J. A. 1988. Cellular inter actions during early neurogenesis of Dro sophila melanogaster. Trends Neurosci. 1 1: 400--4 Cantera, R., Nasse!, D. R. 1987. Post embryonic development of serotonin immunoreactive neurons in the blowfly CNS. II. The thoracico-abdominal gan glia. Cell Tissue Res. 250: 449-59 Caudy, M., Viissin, II., Brand, M., Tuma, R., Jan, L. Y., Jan, Y. N. 1988. Daughter less, a Drosophila gene essential for both neurogenesis and sex determination, has sequence similarities to myc and the achaete-scute complex. Cell 55: 1061-67 DeVoogd, T., Nottebohm, F. 1981. Gonadal hormones induce dendritic growth in the adult avian brain. Science 214: 202-4 Doe, C. Q., Scott, M. P. 1988. Segmentation and homeotic gene function in the developing nervous system of Drosophila. Trends Neurosci. 11: 101--6 Doe, C. Q., Smouse, D., Goodman, C. S. 1988. Control of neuronal fate by the Dro sophila segmentation gene even-skipped. Nature 333: 376-78 Edwards, 1. S. 1969. Postemhryonic develop ment and regeneration of the insect ner vous system. Adv. Insect Physiol. 6: 97137 Fahrbach, S. E., Truman, J. W. 1987. Pos sible interactions of a steroid hormone and neural inputs in controlling the death of an identified neuron in the moth Manduca sexta. J. Neurobiol. 18: 497-508 Fahrbach, S. E., Truman, J. W. 1988. Cycloheximide inhibits ecdysteroid-regu lated neuronal death in the moth Manduca sexta. Soc. Neurosci. Abstr. 14: 368 Fahrbach, S. E., Truman, J. W. 1989. Auto radiographic identification of ecdysteroid binding cells in the nervous system of the moth Manduca sexta. J. Neurobiol. I n press Giebultowicz, 1. M., Truman, J. W. 1984. Sexual differentiation in the terminal gan glion of the moth Manduca sexta: Role of sex-specific neuronal death. J. Compo Neurol. 226: 87-95 Jacobs, G. A., Weeks, 1. C. 1990. Post-
synaptic changes at a sensory-to-motor synapse contribute to the developmental loss of a reflex behavior during insect metamorphosis. J. Neurosci. In press Kent, K. S., Hoskins, S. G., Hildebrand, 1. G. 1987. A novel serotonin-immuno reactive neuron in the antennal lobe of the Sphinx moth Manduca sexta persists throughout postembryonic life. J. Neuro bioi. 18: 451-65 Kent, K. S., Levine, R. B. 1988a. Neural control of leg movements in a mcta morphic insect: Sensory and motor elements of the larval thuracic h::gs in Man duca sexta. J. Compo Neurol. 271: 559-76 Kent, K. S., Levine, R. B. 1988b. Neural control of leg movements in a meta morphic insect: Persistence of the larval leg motor neurons to innervate the adult legs of Manduca sex/a. J. Compo Neurol. 276: 30-43 Kent, K. S., Levine, R. B. 1988c. Reor ganization of an identified leg motor neuron during metamurphosis of the moth Manduca sexta. Soc. Neurosci. Abstr. 14: 1004 Kurz, E. M.,Sengelaub, D. R., Arnold, A. P. 1986. Androgens regulate the dendritic length of mammalian motoneurons in adulthood. Science 232: 345-98 Levine, R. B. 1987. Neural reorganization and its endocrine control during meta morphosis. Curr. Top. Dev. Bioi. 2 1: 34165 Levine, R. B. 1989. Expansion of the axonal arborizations of persistent sensory neurons during insect metamorphosis: The role of 20-hydroxyecdysone. J. Neuro sci. 9: 1045-54 Levine, R. B., Pak, C., Linn, D. 19R5. The structure, function, and metamorphic reorganization of somatotopically pro jecting sensory neurons in Manduca sexta larvae. J. Compo Physiol. 157: 1-13 Levine, R. B., Truman, J. W. 1982. Meta morphosis of the nervous system: Changes in the morphology and synaptic inter actions of identified cells. Nature 299: 250-52 Levine, R. B., Truman, J. W. 1983. Peptide activation of a simple neural circuit. Brain Res. 279: 335-38 Levine, R. B., Truman, J. W. 1985. Dendritic reorganization of abdominal moto neurons during metamorphosis of the moth, Manduca sexta. J. Neurosci. 5: 2424-31 Levine, R. B., Truman, J. W., Linn, D., Bate, C. M. 1986. Endocrine regulation of the form and function ofaxonal arbors during insect metamorphosis. J. Neurosci. 6: 29399 Levine, R. B., Waldrop, B., Tamarkin, D.
Annu. Rev. Neurosci. 1990.13:183-194. Downloaded from www.annualreviews.org Access provided by Mahidol University on 01/22/15. For personal use only.
INSECT METAMORPHOSIS 1989. The use of hormonally-induced mosaics to study alterations in the syn aptic connections made by persistent sen sory neurons during insect metamor phosis. J. Neurobiol. 20: 362�38 Levine, R. B., Weeks, J. C. 1989. Reor ganization of neural circuits and behavior during insect metamorphosis. In Per spectives on Neural Systems and Behavior, ed. T. J. Carew, D. Kelley, pp. 195�228. New York: Liss Mesce, K. A., Truman, J. W. 1988. Meta morphosis of the ecdysis motor pattern in the hawkmoth Manduca sexta. J. Compo Physiol. 163: 287�99 Miles, C. I., Weeks, J. C. 1988. Develop mental changes in pre-ecdysis motor pat terns of the moth, Manduca sexta. Soc. Neurosci. Abstr. 14: 998 Murphey, R. K. 1986. Competition and dynamics of axon arbor growth in the cricket. J. Comp. Neurol. 251 : 100-10 Murphey, R. K., Mendenhall, B., Palka, J., Edwards, J. S. 1975. Deafferentation slows the growth of specific dendrites of identi fied giant interneurons. J. Comp. Neurol. 159: 407�18 Nassel, D. R., Ohlsson, L., Cantera, R. 1988. Metamorphosis of identified neurons innervating thoracic neurohemal organs in the blowfly: Transformation of chole cystokininlike immunoreactive neurons. J. Compo Neurol. 267: 343�56 Nordlander, R. H., Edwards, J. S. I 969a. Postembryonic brain development in the monarch butterfly Danaus plexippus plexippus L. 1. Cellular events during brain morphogenesis. Wilhelm Roux Arch. Entwicklungsmech. Org. 162: 197�217 Nordlander, R. H., Edwards, J. S. 1969b. Postembryonic brain development in the monarch butterfly Danaus plexippus plexippus L. II. The optic lobes. Wilhelm Roux Arch. Entwicklungsmech. Org. 163: 197�220 Peterson, B. A., Weeks, J. C. 1988. Soma totopic mapping of sensory neurons inner vating mechanosensory hairs on the larval prolegs of Manduca sexta. J. Compo Neural. 275: 128-44 Purves, D., Hadley, R. D. 1985. Changes in the dendritic branching of adult mam malian neurons revealed by repeated imaging in situ. Na ture 315: 404-6 Restifo, L. L., Whitc, K. 1988. Expression of a steroid hormone regulated gene in the CNS of Drosophila melanogaster. Soc. Neurosci. Abstr. 14: 733 Riddiford, L. M. 1985. Hormone action at the cellular level. In Comprehensive Insect Physiology, Bi ochemistry, and Pharma cology, ed. G. A. Kerkut, L. I. Gilbert, 8: 37�84. New York: Pergamon. 595 pp.
193
Riddiford, L. M. 1987. Hormonal control of sequential gene expression in insect epi dermis. In Molecular Entomology, ed. J. H. Law, pp. 211�22. New York: Liss. 500 pp. Sandstrom, D. J., Weeks, J. C. 1988. Identi fied interneurons in larval and pupal abdominal ganglia of the tobacco hornworm, Manduca sexta. Soc. Neurasci. Abstr. 14: 1003 Schneiderman, A. M., Matsumoto, S. G., Hildebrand, J. G. 1982. Trans-sexually grafted antennae influence the develop ment of sexually dimorphic neurons in the brain of Manduca sexta. Nature 298: 84446 Schwartz, L. M., Kay, B. K. 1987. De novo transcription and translation of new genes is required for the programmed death of the intersegmental muscles of the tobacco hawkmoth, Manduca sexta. Soc. Neurosci. Abstr. 13: 8 Taylor, H. M., Truman, J. W. 1974. Meta morphosis of the abdominal ganglia of the tobacco hornworm, Manduca sexta. J. Compo Physiol. 90: 367�88 Thomas, J. B., Bastiani, M. J., Bate, N., Goodman, C. S. 1984. From grasshopper to Drosophila: A common plan for neuronal development. Nature 310: 203�7 Thomas, J. B., Crews, S. T., Goodman, C. S. 1988. Molecular genetics of the single minded locus: A gene involved in the development of the Drosophila nervous system. Cell 52: 133-41 Thorn, R., Truman, J. W. 1989. Sex-specific neuronal respectification during the meta morphosis of the genital segments of the tobacco hornworm moth, Manduca sexta. J. Comp. Neurol. 284: 489�503 Tolbert, L. P., Matsumoto, S. M., Hilde brand, J. G. 1983. Development of syn apses in the antennal lobe of the moth, Manduca sexta, during metamorphosis. J. Neurosci. 3: 1158�75 Truman, J. W. 1983. Programmed cell death in the nervous system of an adult insect. J. Compo Neurol. 216: 445�52 Truman, J. W. 1987. The insect nervous sys tem as a model system for the study of neuronal death. Curro Top. Dev. B ioi. , 21: 99�1l6 Truman, J. W., Bate, M. 1988. Spatial and temporal patterns of neurogt:nesis in tht: central nervous system of Drosophila melanogaster. Dev. Bioi. 125: 145�57 Truman, J. W., Booker, R. 1986. Adult specific neurons in the nervous system of Manduca sexta: Selective chemical ablation using hydroxyurea. J. Neurobiol. 17: 613�25 Truman, J. W., Reiss, S. E. 1 976 . Dendritic reorganization of an identified moto-
Annu. Rev. Neurosci. 1990.13:183-194. Downloaded from www.annualreviews.org Access provided by Mahidol University on 01/22/15. For personal use only.
194
WEEKS & LEVINE
neuron during metamorphosis of the tobacco hornworm, Manduca sexta. Science 192: 477-79 Truman, J. W., Reiss, S. E. 1988. Hormonal regulation of the shape of identified moto neurons in Manduca sexta. J. Neurosci. 8: 765-75 Truman, J. W., Schwartz, L. M. 1984. Ster oid regulation of neuronal death in the moth nervous systcm. J. Neurosci. 4: 27480 Truman, J. W., Weeks, J. c., Levine, R. B. 1986. Developmental plasticity during metamorphosis of an insect nervous system. In Comparative Neurobiology: Modes 0/ Communication in the Nervous System, ed. M. J. Cohen, F. Strum wass er, pp. 25-44. New York: Wiley. 397 pp. Valles, A. M., White, K. 1988. Serotonin containing neurons in Drosophila melano gaster: Development and distribution. J. Compo Neurol. 268: 414-28 Waldrop, B., Lev ine, R. B. 1988. Inter neurons involved in multisegmental reflexes in larvae and pupae of the moth, Manduca sexta. Soc. Neurosci. Abstr. 14: 1003 Waldrop, B., Levine, R. B. 1989. Develop ment of the gin trap reflex in Manduca sexta a comparison of larval and pupal motor responses. J. Compo Physiol. In press Weeks, J. C. 1987. Time course of hormonal independence for developmental events in neurons and other cell types during insect metamorphosis. Dev. Bioi. 124: 163-76 Weeks, J. c., Ernst-Utzschneider, K. 1989. Respecification of larval proleg moto neurons during metamorphosis of the tobacco hornworm, Manduca sexta: seg mental dependence and hormonal regu lation. J. Neurobiol. 20: 569-92 Weeks, J. c., Jacobs, G. A. 1987. A reflex behavior mediated by monosynaptic con nections between hair afferents and moto neurons in the larval tobacco hornworm, Manduca sexta. J. Compo Physiol. 1 60: 315-29
Weeks, J. c., Jacobs, G. A. 1988. Develop mental changes in postsynaptic neurons cause loss of a monosynaptic reflcx during metamorphosis in Manduca sexta. Soc. Neurosci. Abstr. 14: 998 Weeks, J. c., Truman, J. W. 1984. Neural organization of peptide-activated ecdysis during the metamorphosis of Manduca sexta. II. Retention of the prolcg motor pattern despite loss of the prolegs at pupation. J. Compo Physiol. 155: 42333 Weeks, J. C., Truman, J. W. 1985. Inde pendent steroid control of the fates of motoneurons and their muscles during insect metamorphosis. J. Neurosci. 5: 2290-2300 Weeks, J. c., Truman, J. W. 1986a. Steroid control of neuron and muscle develop ment during the metamorphosis of an insect. J. Neurobiol. 17: 249-67 Weeks, J. c., Truman, J. W. 1986b. Hor monally mediated reprogramming of muscles and motoneurons during the lar val-pupal transformation of the tobacco hornworm, Manduca sexta. J. Exp. Bioi. 125: 1-13 Weeks, J. c., Jacobs, G. A., Miles, C. I. 1989. Hormonally-mediated modifications of neuronal structure, synaptic connectivity, and behavior during metamorphosis of the tobacco hornworm, Manduca sexta. Am. Zool. In press White, K., Hurteau, T., Punsal, P. 1986. Neuropeptide-FRMFamide-like immuno re activity in Drosophila: Development and distribution. J. Compo Neurol. 247: 430-38 White, K., Kankel, D. R. 1978. Patterns of cell division and movement in the for mation of the imaginal nervous system in Drosophila melanogaster. Dev. Bioi. 65: 296-321 Witten, J. L., Truman, J. W. 1988. Regu lation of transmitter choice in an identified lineage in Manduca sexta. Soc. Neurosci. Abstr. 14: 28
