Cell Tissue Res (2006) 326: 187–199 DOI 10.1007/s00441-006-0211-7 REGULAR ARTICLE Jonathan G. Santos . Edit Pollák . Karl-Heinz Rexer . László Molnár . Christian Wegener Morphology and metamorphosis of the peptidergic Va neurons and the median nerve system of the fruit fly, Drosophila melanogaster Received: 9 January 2006 / Accepted: 29 March 2006 / Published online: 10 May 2006 # Springer-Verlag 2006 Abstract Metamorphosis is a fundamental developmental process and has been intensively studied for various neuron types of Drosophila melanogaster . However, detailed accounts of the fate of identified peptidergic neurons are rare. We have performed a detailed study of the larval morphology and pupal remodelling of identified peptider- gic neurons, the CAPA-expressing Va neurons of D. melanogaster . In the larva, Va neurons innervate abdom- inal median and transverse nerves that are typically associated with perisympathetic organs (PSOs), major neurohaemal release sites in insects. Since median and transverse nerves are lacking in the adult, Va neurites have to undergo substantial remodelling during metamorphosis. We have examined the hitherto uncharacterised gross morphology of the thoracic PSOs and the abdominal median and transverse nerves by scanning electron microscopy and found that the complete reduction of these structures during metamorphosis starts around pupal stage P7 and is completed at P9. Concomitantly, neurite pruning of the Va neurons begins at P6 and is preceded by the high expression of the ecdysone receptor (EcR) subtype B1 in late L3 larvae and the first pupal stages. New neuritic outgrowth mainly occurs from P7-P9 and coincides with the expression of EcR-A, indicating that the remodelling of the Va neurons is under ecdysteroid control. Immunogold- labelling has located the CAPA peptides to large translu- cent vesicles, which are released from the transverse nerves, as suggested by fusion profiles. Hence, the transverse nerves may serve a neurohaemal function in D. melanogaster . Keywords Neuropeptide . Perisympathetic organ . Neurohemal organ . Periviscerokinin-pyrokinin . CAPA . Drosophila melanogaster (Insecta) Introduction Neuropeptide hormones play vital roles in the regulation of body functions in all animals. They are stored and released from specialised neurohaemal zones or neurohaemal organs. The major neurohaemal organs associated with the insect central nervous system (CNS) are the retro- cerebral complex in the head region and the perisympa- thetic organs of the thorax (tPSOs) and abdomen (aPSOs) linked to the median nerves (MeNs, also known as as unpaired ventral nerves) and transverse nerves (TVNs) of the ventral ganglia (see Grillot 1983; Baudry-Partiaoglou 1983). In cyclorrhaphan flies, the morphology of these major neurohaemal organs changes dramatically during pupal metamorphosis, as has been well described for the retrocerebral complex of Drosophila melanogaster (Dai and Gilbert 1991) and the tPSOs of Calliphora erythro- cephala (Nässel et al. 1988). In larval D. melanogaster, each of the first three abdominal MeNs/TVNs is innervated by a pair of neurons that express the CAPA peptides CAPA-1 and -2, which belong to the periviscerokinin peptide family, and PK-1, which is a member of the pyrokinin peptide family (Kean et al. 2002; Predel et al. 2004; Wegener et al. 2006). These CAPA peptides share the C-terminal sequence PRXa and are also present in the aPSO/TVN system of other insects (see Wegener et al. 2002) including further cyclorrhaphan fly species (Predel et al. 2003a). In D. melanogaster and other Diptera, CAPA-1 and -2 are potent diuretic factors This work was supported by the German Science Foundation (Deutsche Forschungsgemeinschaft, DFG), grant We 2652/2-1. J. G. Santos . C. Wegener (*) Emmy Noether Neuropeptide Group, Department of Biology, Animal Physiology, Philipps University, Karl-von-Frisch-Strasse, 35032 Marburg, Germany e-mail: wegener@staff.uni-marburg.de Tel.: +49-6421-2823411 Fax: +49-6421-2828941 E. Pollák . L. Molnár Department of General Zoology, University of Pécs, H-7624 Pécs, Hungary K.-H. Rexer Department of Biology, Mycology, Philipps University, 35032 Marburg, Germany