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Insect allatoregulating neuropeptides: evolutionary trends and multifunctional tasks Klaus H. Hoffmann, Matthias W. Lorenz and Martina Meyering-Vos Department of Animal Ecology I, University of Bayreuth, 95440 Bayreuth,
Germany Neuropeptides control key functions in the life cycle of insects. Factors that stimulate (allatotropins) or inhibit (allatostatins) the activity of the juvenile hormone (JH) producing corpora allata (CA) have been isolated from a variety of insects. The allatoregulating peptides can be classified into four groups: an allatotropin (Manse-AT) and an allatostatin (Manse-AS) from Manduca sexta that both seem to act as allatoregulators only in lepidopterans; the FGL-amide allatostatins, originally isolated from cockroaches which inhibit JH biosynthesis in cockroaches and crickets, and the W2W9-amides where allatostatic functions seems to be restricted to crickets. The finding of allatoregulating peptides in insect species where they do not act on the CA suggests other functions such as neuro- and myomodulatory or ecdysiostatic activity. Considerable progress has been made in the characterization of these neuropeptides and their genes, but also in the structure elucidation of their receptors. Our knowledge on the mechanisms of action of the peptides, however, is still poor. In the fall armyworm, Spodoptera frugiperda, we demonstrated a novel mechanism of a Manse-AS action. JH biosynthesis in the CA of adult moths was inhibited only in those glands which had previously been activated by Manse-AT. Multiple injections of Manse-AT into larvae and adults of S. frugiperda reduced their weight gain, increased mortality, prolonged the larval stages and lowered the number of deposited eggs, respectively. Injections of Manse-AS hardly affected growth, development and fecundity, whereas combined injections of Manse-AS and Manse-AT resulted in effects similar to those obtained with Manse-AT alone. Supported by the Deutsche Forschungsgemeinschaft (Ho 631/15-3).
Neurohormones affecting Arthropod colour changes with special reference to the dark-colour-inducing neurohormone (DCIN) of locusts Meir Paul Pener Department of Cell and Animal Biology, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel Crustacean neurohormones, RPCH (red pigment concentrating hormone) and PDHs (pigment dispersing hormones) evoke rapid colour changes by respectively inducing concentration and dispersionof pigments in specific cells, the chromatophores. Such chromatophore-mediated colour changes are rare in insects and the chemical structure of the responsible neurohormone(s) is not yet known. Usually, insect colour changes are regulated by neurohormones evoking reconstruction of cuticular (melanin) and epidermal pigmentation. MRCH (melanization and reddish colouration hormone) induces darkening in certain moth larvae and the same effect is exerted by several insect neurohormones having the F-X-PRL-NH2 C-terminal. Recently, an additional colour-affecting neurohormone, DCIN (= [His7]corazonin, pGluTFQYSHGWTN-NH2 ), was identified from locusts' corpora cardiaca, by employing a bioassay based on a DCIN-deficient albino mutant of Locusta migratoria (Tawfik et al., 1999, Proc. Natl. Acad. Sci. USA, 96: 7083-7087). DCIN induces darkening in L. migratoria nymphs and adults; it also affects phase-dependent colour polymorphism in locusts. [Arg7 ]corazonin and DCIN are equally effective. DCIN also induces darkening in nymphs and adults of a homochrome non-locust grasshopper, Oedipoda miniata. High doses of certain neuropeptides of the RPCH/AKH family with some structural similarity to DCIN, including RPCH itself, induce limited darkening in L. migratoria albinos, but a lepidopteran MRCH-related neurohormone having the FTPRL-NH2 C-terminal is ineffective. N-terminal shortened and C-terminal shortened DCIN evoke some darkening, but the complete sequence is necessary to obtain maximum effect. Same or similar neurohormones may have different physiological effects in different species and different species may employ different neurohormones for similar physiological effects.
Endocrine control of ecdysis in crustaceans by CHH, CCAP, and a novel brain factor Heinrich Dircksen Institute of Zoophysiology, University of Bonn, Bonn, Germany Crustacean hyperglycaemic hormone (CHH) and crustacean cardioactive peptide (CCAP; PFCNAFTGC-NH2) are involved in the control of ecdysis of crab and crayfish. CHH is accumulated during premoult and released into the haemolymph only during ecdysis from novel foregut and hindgut endocrine cells and initiates water uptake. CCAP shows a dramatic increase during the active phase (ca. 15-20 min) of ecdysis composed of a distinct sequence of behavioural changes leading to the shedding of head appendages, abdomen and pereiopods. By ELISA, we have found during the final phase of the active ecdysis of crayfish a surge in CCAP titres of more than 100 fold (7.4 x 10-9 mol.l-1)compared with intermoult titres (4 x 10-11 mol.l-1) correlated with the initiation of scaphognathite ventilation and peristaltic contractions of abdominal and pereiopod muscles. After exuviation CCAP titres rapidly decline within 30 min due to degradation forming the heptapeptide CNAFTGC-NH2. Isolated! crayfish ventral nerve cords (VNCs) can be provoked by high-K+ saline to release large quantities of CCAP in vitro. Similar experiments in normal saline as bioassay showed that brain extracts from crayfish in late premoult stage D4 only evoked a CCAP release from the VNC of the same animal while brain extracts of stages other than D4 did not. Our results provide first evidence for another novel peptidic brain factor in crayfish specifically promoting the release of CCAP from identified neurones in the VNC during a sensitive phase in late premoult stage D4, which indicates parallels in the neuroendocrine regulation of crustacean and insect ecdysis.
The proctodeal glands and their release of an ecdysterostatic hormone in Manduca sexta. Norman T. Davis Division of Neurobiology, University of Arizona, Tucson, AZ, USA In immature insects the release prothoracicotropic hormone (PTTH) from the brain activates the prothoracic glands to initiate a rapid increase in ecdysone titer which initiates the molting process. For normal development to continue, the ecdysteroid titer must then decline rapidly. Recently, it has been shown that a myoinhibitory peptide, first identified in Manduca sexta, acts as an ecdysterostatic hormone (ESH) to block PTTH-stimulated secretion edysone in Bombyx mori. (65) Using an antisera to Manduca myoinhibitory peptide (MIP), I have found that a pair of large, peripheral neurosecretory cells, the proctodeal glands, that apparently release MIP/ESH into the hemolymph to initiate the decline in ecdysteroid titer. These cells were described in previous studies and shown to be multinucleate and located on the proctodeal nerve at the junction of the hindgut and rectum. Immunostaining demonstrates that the proctodeal glands have a very extensive array of varicose, neurohemal processes extending on the surface of branches of the proctodeal nerve. The proctodeal glands exhibit a distinct secretory cycle that is correlated with the molting cycle; in the early larval instar the glands are in the synthesis phase of secretion, and the glands become depleted at about the time of the ecdysone peak. Similar glands have been found in B. mori and other insects.
Hamid R. Habibi Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4. Gonadotropin-releasing hormone (GnRH) plays a key role in the regulation of pituitary gonadotropin production in vertebrates. Five hundred million years of evolution resulted in divergence of GnRH peptides as well as GnRH receptors leading to multiple forms of GnRH molecules and diverse cellular functions in the brain, pituitary, gonads and other peripheral tissues. The structure of thirteen GnRH variants have been elucidated and there is evidence for the presence of two or more GnRH forms in all classes of vertebrates. We used goldfish as primary experimental model to investigate the role of GnRH in the control of pituitary and ovarian function. Teleosts such as goldfish are seasonal spawners and undergo annual reproductive changes in response to environmental cues. The control of reproduction in teleosts involves production of GnRH as well as a number of other neurohormones leading to synthesis and release of gonadotropins which in turn stimulate maturation of the gonads, steroidogenesis and ovulation. In addition, GnRH plays a critical paracrine role in the maintenance of ovarian synchronicity and control of apoptosis which is the main cause of follicular atresia. Our findings support the postulate that ovarian GnRH which increases during mid to late follicular development in goldfish acts in a paracrine fashion to block pseudomaturation of follicles as well as preventing apoptosis and follicular atresia. This is very important in a species like goldfish which is a batch spawner and maintains synchronous ovary containing large number of preovulatory follicles. Towards the end of follicular development as estrogen level is reduced, gonadotropin levels increase and ovary becomes progressively progestogenic. These conditions favour reduced expression of ovarian GnRH during normal period of gonadotropin-induced ovulation, resulting in production of progestogen metabolites, resumption of meiosis and synchronous ovulation. Funded by a grant from Natural Sciences and Engineering Research Council of Canada.
John P. Chang1 and Hamid R. Habibi2 Departments of Biological Sciences, University of Alberta1 and University of Calgary2, Alberta, CANADA Cellular functions of anterior pituitary cell types are controlled by multiple stimulatory and inhibitory neuroendocrine factors. We have used primary cultures of goldfish somatotropes as a study model to investigate how agonist- and function-specificity can be mediated at the level of intracellular signal transduction. Extracellular Ca2+ entry, mobilization of Ca2+ from TMB8-sensitive intracellular stores, and activation of calmodulin kinase are all involved in the GH release responses to several stimulatory factors, including two endogenous GnRH forms, dopamine and PACAP. In contrast, cAMP/PKA-dependent pathways selectively mediate dopamine and PACAP stimulation of GH release, whereas Na+/H+ antiport and PKC-dependent mechanisms participate in GnRH-induced GH secretion. The differential ability of three endogeneous somatostatin isoforms to inhibit basal and stimulated GH release is also reflected in their selective ability to modulate the GH response to individual signaling pathways. Other pharmacological studies reveal the presence of several intracellular Ca2+ compartments. Selective manipulation of these stores differentially modulates basal GH secretion, cellular contents and mRNA levels, as well as the release responses to the two GnRHs. The multiplicity of signal transduction cascades and the complexity of Ca2+ stores provide the basis upon which functional and ligand specificity of multiple neuroendocrine signals may be integrated at the intracellular level.
C. Jackson and R.T.F. Bernard Department of Zoology and Entomology, Rhodes University, Grahamstown.
6140. It was recently shown that the reproductive activity of male Rhabdomys pumilio can be inhibited by a combination of reduced ambient temperature and food availability. However it also became apparent that this inhibition may be reduced by the presence of body fat reserves. To determine how these external factors influence reproductive activity, we examined the effects of exogenous GnRH and leptin on reproductively inactive male R. pumilio. Thirty mice were exposed to 15°C and a reduced food intake and once all the mice were non-scrotal, they were divided into one of two groups. In the first group mice were either injected with GnRH or saline (n=10 per treatment) for twenty-one days. In the second group, mice were either injected with leptin or saline (n=5 per treatment) and were treated for four days. After the respective treatments, the mice were sacrificed, weighed and dissected. Their reproductive organs were removed and weighed and blood was collected for testosterone, LH and leptin assays. Mice injected with GnRH had significantly larger testes and epididymides compared to their controls,, eventhough they were of similar body mass. however, leptin treatment resulted in no significant change in either body mass or reproductive organ mass compared to the control mice. Hormone assays indicated no significant difference in the levels of testosterone, LH or leptin in either experiment. From this we concluded that exogenous GnRH can assist in overcoming the negative effects of reduced food availability and ambient temperature, while exogenous leptin appears to have little effect on reproductive activity.
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