Octopamine and 5-hydroxytryptamine enhance the phagocytic and nodule formation activities of cockroach ( Periplaneta americana) haemocytes

The effects of 5-hydroxytryptamine, octopamine and dopamine on the phagocytic and nodule formation activities of cockroach haemocytes have been investigated. Octopamine and 5-hydroxytryptamine (5-HT) increase phagocytosis in vitro with a maximal effect at 10 −6 and 10 −8 M, respectively. Dopamine also increases phagocytosis, but only at high concentrations (10 −4 M). Antagonist and agonist studies for phagocytosis show that both 5-HT- and octopamine-sensitive receptors are present in cockroach haemocytes. Survival of cockroaches exposed to a LD 50 dose of Staphylococcus aureus is enhanced in the presence of 5-HT and octopamine while dopamine has no effect. Antagonist and agonist stuides of cockroach survival support the results obtained in vitro, suggesting that both octopamine- and 5-HT-sensitive receptors are found in cockroach haemocytes and these are involved in the recovery of cockroaches from bacterial infection. Examination of the haemocoel and haemocytes of treated cockroaches showed that at least two mechanisms by which 5-HT and octopamine are able to increase survival is through the elevation of phagocytosis and nodule formation.