Two biogenic monoamines, octopamine and 5-hydroxytryptamine (5-HT), are known to play immune mediators in insects. They induce hemocyte behaviors by stimulating cytoskeleton rearrangement. However,it is not clear how they activate the hemocytes in terms of intracellular signal transduction. This study analyzed their interactions with signal pathways implicated in nodule formation via eicosanoids or hemocyte locomotory behavior via a small GTPase. Both octopamine and 5-HT increased hemocytic nodule formation in response to bacterial challenge in the beet armyworm, Spodoptera exigua. However, their immune mediation was inhibited by a treatment of dexamethasone (a specific inhibitor to phospholipase A2). In the presence of phentolamine (a specific antoganist to octopamine) or ketanserin (a specific antoganist to 5-HT), the inhibitory activity of dexamethasone was rescued by adding arachidonic acid (a precursor of eicosanoid biosynthesis). These results suggest that the mediation of nodule formation by the two monoamines is followed by eicosanoid signaling. Two monoamines also induced up-regulation of circulating hemocyte counts in S. exigua. This increase of hemocyte counts was not explained by de novo production of hemopoietic organ because even ligation between thorax and abdomenin order to block hemolymph circulation did not inhibit the increase of circulating hemocyte counts by octopamine.A small GTPase, Rac1, appeared to be involved in this hemocyte mobilization from a sessile compartment in S. exigua. Inhibition of Rac1 activity significantly suppressed hemocyte spreading behavior and the hemocyte mobilization. In summary, octopamine and 5-HT mediate cellular immune responses of S. exigua via eicosanoid signal or independently by activating Rac1 following increase of cAMP in the hemocytes.

Monoamines have been known to mediate cellular immune responses such as hemocyte nodulation and phagocytosis in some insects. Eicosanoids also mediate the cellular immune reactions responsible for clearing bacterial infection from hemolymph circulation in various insects. In this study, we investigated a functional relationship of both mediators in expressing hemocyte nodulation and phagocytosis of fifth instar larvae of the beet armyworm, Spodoptera exigua. Addition of octopamine or serotonin along with bacteria enhanced hemocyte nodulation and phagocytosis of S. exigua larvae. Phentolamine (octopamine inhibitor) and ketanserin (serotonin inhibitor) suppressed hemocyte nodulation and phagocytosis of S. exigua in response to a bacterial challenge. Each inhibitory effect of both monoamine inhibitors was recovered by addition of either octopamine or serotonin, suggesting their independent signaling pathways. Both inhibitor effects were also recovered by addition of arachidonic acid (a precursor of eicosanoid biosynthesis). Furthermore, the stimulatory effects of monoamines on the cellular immune responses were prevented by addition of dexamethasone (a specific phospholipase A2). However, the inhibitory effect of dexamethasone was not recovered by excess amounts of monoamines. These results suggest that monoamines mediate hemocyte nodulation and phagocytosis through a downstream eicosanoid signal pathway in S. exigua.