Evidence for a hyperglycemic effect of methionine-enkephalin in the prawns Penaeus indicus and Metapenaeus monocerus

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

Melatonergic regulation of hemolymph sugar levels in the freshwater edible crab, Oziotelphusa senex senex

In this study, the hyperglycemic effect of melatonin in the freshwater edible crab, Oziotelphusa senex senex, is investigated. Injection of melatonin induced hyperglycemia in a dose-dependent manner. Administration of melatonin produced hyperglycemia in both intact and eyestalk-ablated crabs. Bilateral eyestalk ablation resulted in significant increase in the total carbohydrates and glycogen levels with a significant decrease in phosphorylase activity in the hepatopancreas and muscle of the crabs. Injection of melatonin resulted in significant decrease in the total carbohydrate and glycogen levels, with an increase in phosphorylase activity in hepatopancreas and muscle of both intact and eyestalk-ablated crabs. From the results, it is hypothesized that melatonin-induced hyperglycemia in the crab, O. senex senex, is not mediated by eyestalk hyperglycemic hormone.

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.

Species-specific effects on hemolymph glucose control by serotonin, dopamine, and L-enkephalin and their inhibitors inSquilla mantis andAstacus leptodactylus (crustacea)

Hemolymph glucose level is controlled by crustacean Hyperglycemic Hormone (cHH) released from the eyestalk neuroendocrine centers under conditions of both physiological and environmental stress. Biogenic amines and enkephalin have been found to mediate the release of several neurohormones from crustacean neuroendocrine tissue. We investigated the effect of serotonin, dopamine, and Leucine-enkephalin in vivo--injected into the stomatopod Squilla mantis and the decapod Astacus leptodactylus--whether increasing or depressing glycemia. Serotonin had a marked effect in elevating glucose level compared with initial values in both species. 5-HT1-like receptors are more involved in mediating serotonin action as co-injected cyproheptadine was a more effective antagonist than ketanserin (5-HT2-like receptor inhibitor). Dopamine injection in intact animals produced a decrease below initial levels of hemolymph glucose. This effect was significantly antagonized by domperidone. No significant effect of both amines occurred in eyestalkless animals. L-enkephalin shows a differential effect: in S. mantis it induced hypoglycemia while in A. leptodactylus it caused an increase of glucose level. Co-injected antagonist naloxone affected the direction of the response. Serotonin appears to provide a major control on glucose mobilization, whereas dopamine and L-enkephalin act as modulators whose plasticity in use or action varies among species.