Transient enhancement of immune resistance functions in Litopenaeus vannamei through a low-dose octopamine injection

Octopamine (OA) is known to play an important role in regulating invertebrate immune responses. In this study, we determined the effects of OA on immunity and physiological regulation in the white shrimp Litopenaeus vannamei. The total haemocyte count (THC), differential haemocyte count (DHC), phenoloxidase (PO) activity, respiratory bursts (RBs), superoxide dismutase (SOD) activity, and lysozyme, glucose, and lactate levels in plasma, and phagocytic activity and clearance efficiency in response to the pathogen, Vibrio alginolyticus, were measured when shrimp (11.1-13.0 g) were individually injected with saline or OA at 100 and 1000 pmol shrimp^-1. Results showed significant increases in THC, semigranular cells (SGCs), and PO activity per 50 μL of haemolymph at 0.5-4 h; granular cells at 0.5-2 h; respiratory bursts (RBs) at 0.5-1 h; phagocytic activity at 2-4 h; and clearance efficiency at 2-8 h, but PO activity per granulocyte at 0.5-2 h significantly decreased after the OA injection. All of the immune parameters had returned to control values by 8 h after receiving OA except granular cells at 4 h, RBs at 2 h, clearance efficiency at 16 h, and PO activity per granulocyte at 4 h. However, no significant differences were observed in hyaline cells, RBs per haemocyte, lysozyme and SOD activities, glucose, or lactate during the experimental period. An injection of OA also significantly decreased the mortality of shrimp challenged with V. alginolyticus. In another experiment, the immune-related genes of transglutaminase-I, lipopolysaccharide- and β-1,3-glucan-binding protein, prophenoloxidase-II, and peroxidase of shrimp that received 1000 pmol OA shrimp^-1 for 1 h were significantly higher than those of shrimp that received the saline control. These results suggest that OA administration at ≤1000 pmol shrimp^-1 mediates transient upregulation of immunity, which in turn promotes the resistance of L. vannamei to V. alginolyticus.