Ecdysteroid receptors in the neuroendocrine-endocrine axis of a moth

Ecdysteroids regulate the levels of Molt-Inhibiting Hormone (MIH) expression in the blue crab, Callinectes sapidus

Arthropod molt is coordinated through the interplay between ecdysteroids and neuropeptide hormones. In crustaceans, changes in the activity of Y-organs during the molt cycle have been regulated by molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH). Little has been known of the mode of direct effects of ecdysteroids on the levels of MIH and CHH in the eyestalk ganglia during the molt cycle. This study focused on a putative feedback of ecdysteroids on the expression levels of MIH transcripts using in vitro incubation study with ecdysteroids and in vivo RNAi in the blue crab, Callinectes sapidus. Our results show a specific expression of ecdysone receptor (EcR) in which EcR1 is the major isoform in eyestalk ganglia. The initial elevation of MIH expression at the early premolt stages is replicated by in vitro incubations of eyestalk ganglia with ecdysteroids that mimic the intrinsic conditions of D₀ stage: the concentration (75 ng/ml) and composition (ponasterone A and 20-hydroxyecdysone at a 3:1 (w:w) ratio). Additionally, multiple injections of EcR1-dsRNA reduce MIH expression by 67%, compared to the controls. Our data provide evidence on a putative feedback mechanism of hormonal regulation during molting cycle, specifically how the molt cycle is repeated during the life cycle of crustaceans. The elevated concentrations of ecdysteroids at early premolt stage may act positively on the levels of MIH expression in the eyestalk ganglia. Subsequently, the increased MIH titers in the hemolymph at postmolt would inhibit the synthesis and release of ecdysteroids by Y-organs, resulting in re-setting the subsequent molt cycle.

Edysteroid receptor (EcR) shows marked differences in temporal patterns between tissues during larval-adult development in Rhodnius prolixus: correlations with haemolymph ecdysteroid titres

The presence of ecdysteroid receptor (EcR) in various tissues was studied throughout larval-adult development of the blood-sucking bug, Rhodnius prolixus, using an antibody to EcR that recognizes all isoforms. On Western blots, the antibody recognizes three peptides of approximate molecular masses of 70, 68 and 64 kDa, from epidermis and fat body of developing larvae, which contain high levels of haemolymph ecdysteroids. These peptides are absent from both unfed larvae and adults, which are devoid of ecdysteroids. In vitro treatment of epidermis and fat body from unfed larvae with 20E induces the appearance of all three EcR immunoreactive peptides. The stage-specific appearance and 20E inducibility of the peptides implies that they represent the native EcR(s) of Rhodnius. Confocal fluorescence analysis using this antibody revealed a great diversity of temporal profiles of EcR in various tissues during development. Developmental profiles of EcR were examined in abdominal epidermis, fat body, spermatocytes, brain (including the medial neurosecretory cells), prothoracic glands (PGs), rectal epithelium and Malpighian tubules. EcR fluorescence was confined to the nuclei in close association with chromatin. EcR was absent from tissues of unfed larvae or adults, supporting the results from Western blots. Different tissues develop EcR at different developmental times and in the presence of radically different concentrations of haemolymph ecdysteroids, retain EcR for different lengths of time and lose EcR at different concentrations of ecdysteroids. These results suggest that each tissue possesses a distinctive response mechanism to ecdysteroids. An exception to this, are the PGs, which exhibited no EcR fluorescence at any time during development.

Restriction of nutrient intake results in the increase of a specific Manduca sexta allatotropin (Manse-AT) mRNA in the larval nerve cord

The Manduca allatotropin (Manse-AT) gene is expressed as three mRNAs that differ from each other by alternative splicing. The level of one of these mRNAs (RNA-3) is specifically increased in the nerve cord of last instar larvae that were starved, parasitized, or fed the ecdysteroid agonist RH-5992. Each of these treatments results in reduction of feeding and increased levels of juvenile hormone (JH). The normal decline in JH biosynthesis by the corpora allata does not occur in starved or RH-5992-fed larvae. The increase in RNA-3 levels has the capacity to increase the production of Manse-AT and two related peptides that may be part of the complex response of larvae to nutrient deprivation.

Dopaminergic control of corpora allata activity in the larval tobacco hornworm, Manduca sexta

The corpora allata (CA) of insects are innervated by axons of non-neurosecretory cerebral neurons, and of the various known neurotransmitters in the brain of the tobacco hornworm, Manduca sexta, only dopamine is detected in the CA by electrochemical detection HPLC. This neurotransmitter stimulates the biosynthetic activity of the CA in vitro for the first 2 days of the last larval stadium, but inhibits CA from day 3 through day 6, the beginning of the prepupal period. Stimulation of JH synthesis has previously been linked with an increase in the production of cyclic AMP (cAMP) in the CA, and dopamine stimulates the adenylyl cyclase system of CA from larvae early in the fifth stadium, while on day 6, its effect is inhibitory. These results suggest: (1) the existence in the CA of both D1- and D2-like dopamine receptors, which in vertebrates stimulate and inhibit, respectively, adenylyl cyclase; and (2) the developmental control of their expression. A potent D1 agonist, (+/-)-SKF 82958-HBr, did not stimulate JH biosynthesis by day 0 CA as expected, but appeared to inhibit it at a concentration of 10(-5)M. Thus the apparent D1-like receptor in Manduca CA may be pharmacologically distinct from vertebrate D1 receptors. The existence of D2-like receptors is supported by the finding that a vertebrate D2 receptor agonist, (+/-) PPHT-HCl, and an antagonist, eticlopride, have the predicted effects on JH acid biosynthesis and cAMP production by day 6 Manduca CA. However, the D1 agonist also significantly reduces JH acid biosynthesis and cAMP production, indicating that while the Manduca D2-like receptor is pharmacologically similar to the vertebrate D2, it shares some characteristics with D1 receptors. The developmental regulation of these receptors by ecdysteroids is suggested by the fact that when day 0 larvae are treated in vivo with exogenous ecdysone:20-hydroxyecdysone, the biosynthetic activity of the CA in vitro 24 h later is no longer stimulated by dopamine.

Ecdysteroid binding sites in gastrolith forming tissue and stomach during the molting cycle of crayfish Procambarus clarkii

The distribution of ecdysteroid binding sites in the stomach and gastrolith disc tissue of crayfish (Procambarus clarkii) was examined in relation to the molting stage by thaw-mount autoradiography. The radiolabeled hormone analogue ponasterone A (25-deoxy-20-hydroxyecdysone) was used. Ecdysteroid binding sites were demonstrated only in certain molting stages, the small gastrolith period and the aftermolt stage. In gastrolith epithelium, ponasterone A binding sites first appeared in the cytoplasm, and then in the nuclei and cytoplasm. In the stomach epithelium, many nuclear binding sites were detectable during the period of gastrolith secretion. These periodical changes in specific ponasterone A binding when correlated with the molting stages clearly show that ecdysteroids may function as an initiator for gastrolith formation and reabsorption. The findings also suggest that ecdysteroids control calcium transport in the stomach epithelium. The time-related and functional differences of cytoplasmic and nuclear concentration of ecdysteroid receptors indicate the presence of cytoplasmic and nuclear receptors associated with specific actions.