LAUSR

ABSTRACT Tyramine (TA), a neuroactive chemical, plays various important physiological roles in insects by activating distinct G‐protein‐coupled receptors (GPCRs). In this study, we investigated the effects of by pharmacological injection of TA on immune resistance regulation and its signal pathway in white shrimp Litopenaeus vannamei. Results showed significant increases in the total haemocyte count (THC), semigranular cells (SGCs), granular cells (GCs), phenoloxidase (PO) activity per 50 &mgr;L of haemolymph and respiratory bursts (RBs) at 0.5, 1, 2 and/or 4 h; hyaline cells (HCs) at 0.5 h, as well as phagocytic activity (PA) and clearance efficiency (CE) at 2, 4 and/or 8 h, but significantly decreased PO activity per granulocyte at 0.5–2 h for shrimp injected with TA at 100 and 1000 pmol shrimp−1. Plasma lysozyme activities of TA‐injected shrimp were significantly higher than those of the saline control at 1 h. All of the immune parameters had returned to control levels by 8 h after receiving TA except the clearance efficiency, which had returned to its control value by 16 h. The TA injection also significantly decreased the mortality of shrimp challenged with Vibrio alginolyticus. Furthermore, immune parameters of shrimp that received TA at 1000 pmol shrimp−1 for 1 h were higher than those of shrimp that received the saline. The upregulating effect of TA was blocked by co‐injection with phentolamine (Phe) in terms of the THC, HC, SGCs, PO activity, PA and CE; by co‐injection with prazosin (Pra) in terms of the THC, HC, SGCs, PO activity, PA and CE; by co‐injection with propranolol (Prop) in terms of the PA and CE; and by co‐injection with metoprolol (Meto) in terms of the THC and SGCs. The most potent effect in immunocompetence of tested antagonists was Pra, and except for circulating haemocyte, it was Phe. These results suggest that ≤1000 pmol shrimp−1 of a TA injection mediates transient upregulation of immunity, which in turn promotes the resistance of L. vannamei to V. alginolyticus, and the active effects are mediated via octopamine/tyramine (OA/TA) receptors. HighlightsTA promoted the immune responses of shrimp.The increased resistance to pathogen infection in shrimp was investigated after receiving TA.The effects of TA on immunocompetence was modulated via OA/TA receptors.