No abstract

The role of classical neurotransmitters in the transfer and processing of olfactory information is well established in many organisms. Neuropeptide action, however, is largely unexplored in any peripheral olfactory system. A subpopulation of local interneurons (LNs) in the Drosophila antannal lobe is peptidergic, expressing Drosophila tachykinins (DTKs). We show here that olfactory receptor neurons (ORNs) express the DTK receptor (DTKR). Using twophoton microscopy, we found that DTK applied to the antennal lobe suppresses presynaptic calcium and synaptic transmission in the ORNs. Furthermore, reduction of DTKR expression in ORNs by targeted RNA interference eliminates presynaptic suppression and alters olfactory behaviors. We detect opposite behavioral phenotypes after reduction and over expression of DTKR in ORNs. Our findings suggest a presynaptic inhibitory feedback to ORNs from peptidergic LNs in the antennal lobe.olfactory behavior ͉ presynaptic inhibition ͉ tachykinin ͉ two-photon imaging I n Drosophila, odor detection begins when odor molecules activate olfactory receptor neurons (ORNs) in the antennae and maxillary palps. Each of the ORNs expresses only 1 or a few members of a large family of odorant receptor genes (1-4). These ORNs propagate activity to neurons with dendrites in the glomerular compartments of the antennal lobe; each glomerulus receives inputs from ORNs that express the same odorant receptor (1, 3, 5, 6). In the glomeruli, the activity is read by second-order neurons, designated projection neurons (PNs), which relay information to higher olfactory centers in the brain (7).Inhibitory circuits in the glomeruli, mediated by local interneurons (LNs), play a key role in modulating glomerular signal activity. Presynaptic GABAergic inhibition of the ORNs has been shown in both Drosophila (8, 9) and in mammals (10-12). Conversely, cholinergic LNs in the Drosophila antennal lobe have been suggested to increase and redistribute odor-evoked activity at low odor concentrations (13,14).In addition to GABA and acetylcholine it is likely that certain neuropeptides are used as neuromodulators in the antennal lobe circuitry of insects (15, 16), as also suggested in the olfactory bulb in mammals (17,18). One neuropeptide gene that has been implicated in olfactory processing is dtk (19), a gene encoding 5 tachykinin-related peptides, DTKs (20). The DTKs are expressed in Ϸ150 neurons in the Drosophila brain, and in the antennal lobe glomeruli, there are extensive DTK-immunoreactive arborizations derived from a subset of antennal lobe LNs (21). Two DTK receptors, DTKR and NKD, have been identified in Drosophila (22, 23) and 1 of these, DTKR, is strongly expressed in antennal lobe glomeruli (24). Behavioral evidence for a role of DTKs in olfaction was obtained from analysis of flies where dtk expression was knocked down globally using RNA interference (RNAi); these flies displayed diminished odor sensitivity (19).To gain insight into the neuromodulation provided by the DTK signaling system in the antennal lob...

Peptides structurally related to mammalian tachykinins have recently been isolated from the brain and intestine of several insect species, where they are believed to function as both neuromodulators and hormones. Further evidence for the signaling role of insect tachykinin-related peptides was provided by the cloning and characterization of cDNAs for two tachykinin receptors from Drosophila melanogaster. However, no endogenous ligand has been isolated for the Drosophila tachykinin receptors to date. Analysis of the Drosophila genome allowed us to identify a putative tachykininrelated peptide prohormone (prepro-DTK) gene. A 1.5-kilobase pair cDNA amplified from a Drosophila head cDNA library contained an 870-base pair open reading frame, which encodes five novel Drosophila tachykininrelated peptides (called DTK peptides) with conserved C-terminal FXGXR-amide motifs common to other insect tachykinin-related peptides. The tachykinin-related peptide prohormone gene (Dtk) is both expressed and post-translationally processed in larval and adult midgut endocrine cells and in the central nervous system, with midgut expression starting at stage 17 of embryogenesis. The predicted Drosophila tachykinin peptides have potent stimulatory effects on the contractions of insect gut. These data provide additional evidence for the conservation of both the structure and function of the tachykinin peptides in the brain and gut during the course of evolution.Substance P was the first peptide signaling molecule to be identified by virtue of its effects upon blood pressure and smooth muscle contraction (1) and is the archetypal member of the tachykinin family of peptides. Vertebrate tachykinins represent a large family of peptides that elicit a wide range of both central and peripheral responses (2-5). Although these peptides are structurally diverse, all contain a conserved C-terminal FXGLM-amide motif. Like other biologically active peptides, substance P is derived from a larger prohormone polypeptide (preprotachykinin A (PPT-A) 1 ) that also allows the production of several other biologically active peptides (neurokinin A, neuropeptide K, and neuropeptide ␥) (6). Three different isoforms of preprotachykinin can be produced as a result of alternative splicing of the PPT-A mRNA, which, in conjunction with alternative post-translational processing of the prohormone, allows the production of these peptides in a tissue-specific manner (7-9). A fifth mammalian tachykinin, neurokinin B, is derived from a separate gene product, preprotachykinin B (10).The tachykinin family is not confined to vertebrates, and a large number of tachykinins have now been isolated from a variety of invertebrate species such as the cockroach Leucophaea maderae (11, 12), the mosquito Culex salinarius (13), and the echiuroid worm Urechis unicinctus (14). In contrast to the vertebrate tachykinins, almost all of the invertebrate tachykinins contain a conserved C-terminal FXGXR-amide motif and, for this reason, have been termed tachykinin-related peptides (TRPs). Not...

In Drosophila, neurosecretory cells that release peptide hormones play a prominent role in the regulation of development, growth, metabolism, and reproduction. Several types of peptidergic neurosecretory cells have been identified in the brain of Drosophila with release sites in the corpora cardiaca and anterior aorta. We show here that in adult flies the products of three neuropeptide precursors are colocalized in five pairs of large protocerebral neurosecretory cells in two clusters (designated ipc-1 and ipc-2a): Drosophila tachykinin (DTK), short neuropeptide F (sNPF) and ion transport peptide (ITP). These peptides were detected by immunocytochemistry in combination with GFP expression driven by the enhancer trap Gal4 lines c929 and Kurs-6, both of which are expressed in ipc-1 and 2a cells. This mix of colocalized peptides with seemingly unrelated functions is intriguing and prompted us to initiate analysis of the function of the ten neurosecretory cells. We investigated the role of peptide signaling from large ipc-1 and 2a cells in stress responses by monitoring the effect of starvation and desiccation in flies with levels of DTK or sNPF diminished by RNA interference. Using the Gal4-UAS system we targeted the peptide knockdown specifically to ipc-1 and 2a cells with the c929 and Kurs-6 drivers. Flies with reduced DTK or sNPF levels in these cells displayed decreased survival time at desiccation and starvation, as well as increased water loss at desiccation. Our data suggest that homeostasis during metabolic stress requires intact peptide signaling by ipc-1 and 2a neurosecretory cells.

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