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

The influence of methionine-enkephalin on carbohydrate metabolism of the pr awns Penaeus indicus and Metapenaeus monocerus was studied. Injection of th e opioid methionine-enkephalin into intact prawns induced significant hyper glycemia in a dose-dependent manner. Total tissue (midgut gland and muscle) carbohydrate and glycogen levels decreased following methionine-enkephalin injection, with a significant activation of phosphorylase in intact prawns , indicating glycogenolysis leading to hyperglycemia. In contrast, injectio n of methionine- enkephalin into eyestalk-ablated crabs did not affect the levels of hemolymph glucose, total tissue carbohydrates and glycogen, and a ctivity of phosphorylase. These results support an earlier hypothesis for c rabs which proposed that methionine-enkephalin acts as a neurotransmitter i n crustaceans and stimulates the release of hyperglycemic hormone in induci ng hyperglycemia. (C) 2001 Academic Press.