Purified crustacean enkephalin inhibits release of hyperglycemic hormone in the crab Carcinus maenas L

The CHH-superfamily of multifunctional peptide hormones controlling crustacean metabolism, osmoregulation, moulting, and reproduction

Apart from providing an up-to-date review of the literature, considerable emphasis was placed in this article on the historical development of the field of "crustacean eyestalk hormones". A role of the neurosecretory eyestalk structures of crustaceans in endocrine regulation was recognized about 80 years ago, but it took another half a century until the first peptide hormones were identified. Following the identification of crustacean hyperglycaemic hormone (CHH) and moult-inhibiting hormone (MIH), a large number of homologous peptides have been identified to this date. They comprise a family of multifunctional peptides which can be divided, according to sequences and precursor structure, into two subfamilies, type-I and -II. Recent results on peptide sequences, structure of genes and precursors are described here. The best studied biological activities include metabolic control, moulting, gonad maturation, ionic and osmotic regulation and methyl farnesoate synthesis in mandibular glands. Accordingly, the names CHH, MIH, and GIH/VIH (gonad/vitellogenesis-inhibiting hormone), MOIH (mandibular organ-inhibiting hormone) were coined. The identification of ITP (ion transport peptide) in insects showed, for the first time, that CHH-family peptides are not restricted to crustaceans, and data mining has recently inferred their occurrence in other ecdysozoan clades as well. The long-held tenet of exclusive association with the eyestalk X-organ-sinus gland tract has been challenged by the finding of several extra nervous system sites of expression of CHH-family peptides. Concerning mode of action and the question of target tissues, second messenger mechanisms are discussed, as well as binding sites and receptors. Future challenges are highlighted.

Role of methionine-enkephalin on the regulation of carbohydrate metabolism in the rice field crab Oziotelphusa senex senex

In the present study, the role of eyestalks and involvement of methionine-enkephalin in the regulation of haemolymph sugar level was studied. Bilateral eyestalk ablation significantly decreased the haemolymph sugar levels, whereas injection of eyestalk extract into ablated crabs significantly increased the haemolymph sugar levels. Total carbohydrate (TCHO) and glycogen levels were significantly increased in hepatopancreas and muscle of eyestalk-ablated crabs, with a decrease in phosphorylase activity. Injection of eyestalk extract into ablated crabs resulted in partial/complete reversal of these changes. Injection of methionine-enkephalin into intact crabs significantly increased the haemolymph sugar level in a dose-dependent manner. Total tissue carbohydrate and glycogen levels were significantly decreased, with an increase in phosphorylase activity in hepatopancreas and muscle tissues of intact crabs after methionine-enkephalin injection. Methionine-enkephalin injection did not cause any changes in haemolymph sugar, tissue total carbohydrate and glycogen levels and activity levels of phosphorylase in eyestalk-ablated crabs. These results suggest that the eyestalks are the main source of hyperglycaemic hormone and methionine-enkephalin induces hyperglycaemia through eyestalks.

Invertebrate opioid precursors: evolutionary conservation and the significance of enzymatic processing

Invertebrate tissues contain mammalian-like proenkephalin, prodynorphin, and proopiomelanocortin. Amino acid sequence determination of these opioid gene products reveals the presence of various opioid peptides exhibiting high sequence identity with their mammalian counterparts. These associated peptides are flanked by dibasic amino acid residues, indicating cleavage sites. Together with the presence of various processing enzymes, i.e., neutral endopeptidase 24.11 and angiotensin-converting enzymes, this suggests that opioid precursor processing is also similar to that described in mammals. It is noted that the levels and/or activity of invertebrate neutral endopeptidase 24.11 can be upregulated by signaling molecules shown to perform the same function in mammals, i.e., morphine. Critical to opioid precursor processing are immunocytes that contain the precursors and transport processing enzymes to sites of inflammation, in part, to cleave these peptide precursors, thus liberating immune-stimulating molecules. Furthermore, in response to lipopolysaccharides, Met-enkephalin levels peak immediately and hours after the exposure, revealing a release and induction process. It appears that the opioid precursors and their processing enzymes first evolved in "simple" animals and the have been maintained and embellished during the course of evolution guided by conformational matching.

Delta and kappa opioid receptors in eyestalk ganglia of a crustacean

Crustacean eyestalk ganglia are part of the protocerebrum and have been demonstrated to produce numerous neurohormones. 3H(2-D-Pen, 5-D-Pen)-enkephalin, 3H-(-)-ethylketocyclazocine and 3H(D-Ala2-NMePhe-Glyol5)-enkephalin were used as ligands for opioid receptors on neuronal membrane preparations of eyestalk ganglia under consideration of their stereospecific binding properties. In context with saturation binding isotherms, association and dissociation plots, we demonstrate here two opioid receptors; a delta-type receptor with high affinity (Bmax 68.5 fmol/mg protein, Kd = 4.0 nM) and low affinity (Bmax 493 fmol/mg, Kd = 83.6 nM) and a second receptor of kappa-type with Bmax of 3.1 pmol/mg protein and Kd = 68.6 nM.

Opioid receptor types for endogenous enkephalin in the thoracic ganglion of the crab, Carcinus maenas

In crustaceans, the endogenous opioid peptides, enkephalins, are known to be concentrated in the thoracic ganglion, although they have been demonstrated in all parts of the nervous system. Bmax and Kd measurements have been obtained for the binding of ligands used to characterize delta- and kappa-type opioid receptors in vertebrates. High- and low affinity binding of [3H] [2-D-Pen5-D Pen] enkephalin ([3H]DPDPE) has been measured with a Kd = 9.2 +/- 2.4 nM, Bmax = 153 fmol/mg, and Kd = 243 +/- 27 nM, Bmax = 1.785 pmol/mg, respectively. In addition a kappa-type receptor with Kd 85.5 +/- 12.6 nM and Bmax = 21.138 pmol/mg protein has been recorded. Binding characteristics of several ligands were monitored. Electrophoretic studies of affinity chromatographically purified receptor fractions revealed a molecular mass of 60 kDa. Isoelectric focusing showed a specific binding of [3H]DPDPE to thoracic ganglion membranes at a pl of 5.5.

Autoradiographic localization of opioid binding sites combined with immunogold detection of Leu-enkephalin, crustacean hyperglycaemic hormone and moult inhibiting hormone at the electron microscopic level in the sinus gland of the shore crab, Carcinus maenas

Double labelling experiments were performed on the same tissue section at the electron microscopic level, in order to show the involvement of the opioid leucine-enkephalin (Leu-enk) in the modulation of crustacean hyperglycaemic hormone (CHH) mobilization. Both neuropeptides were stored in distinct axon terminals of the sinus gland of Carcinus maenas. A post-embedding immunogold cytochemical technique for Leu-enk, CHH and the CHH neurohormone related moult inhibiting hormone (MIH) was combined with a scintillator intensified autoradiographic method to demonstrate binding of the opioid antagonist [3H] naloxone. Ultrathin sections were successively incubated with antisera against Leu-enk, CHH or MIH, and the corresponding colloidal gold labelled antisera, followed by autoradiographic processing. At the ultrastructural level [3H] naloxone binding sites were easily recognized by their silver tracks after development. Opioid binding sites for [3H] naloxone were visualized only at membranes of CHH-containing axon terminals. These results provide morphological evidence for direct enkephalinergic control of CHH containing neurons in the sinus gland of C. maenas and are furthermore the first autoradiographic demonstration of opioid binding sites in the nervous system of invertebrates.