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

The influence of methionine-enkephalin on carbohydrate metabolism of the prawns Penaeus indicus and Metapenaeus monocerus was studied. Injection of the opioid methionine-enkephalin into intact prawns induced significant hyperglycemia 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, injection of methionine-enkephalin into eyestalk-ablated crabs did not affect the levels of hemolymph glucose, total tissue carbohydrates and glycogen, and activity of phosphorylase. These results support an earlier hypothesis for crabs which proposed that methionine-enkephalin acts as a neurotransmitter in crustaceans and stimulates the release of hyperglycemic hormone in inducing hyperglycemia.