Altered immunity in crowded Mythimna separata is mediated by octopamine and dopamine

Abstract

Similar to pathogenic infection, high population density alters insects' prophylactic immunity. Density-dependent prophylaxis has been reported in many polyphenic insects, but the regulatory mechanism underlying this phenomenon remains unclear. The biogenic monoamines are known to play critical roles in mediating insect immune responses. In the current study, the immune capacity and the levels of three biogenic monoamines were investigated in the polyphenic larvae of Mythimna separata, reared at the densities of 1, 2, 5, 10, and 30 larvae per 650-mL jar. Concomitant with the increased phenoloxidase (PO) activity and total haemocyte count in the larvae at high densities (5, 10, 30 larvae/jar), the octopamine level was also increased. In contrast, the dopamine level was decreased, and the 5-hydroxytryptamine level was not significantly affected. Injection of octopamine induced significant increases in the total haemocyte count and PO activity. Conversely, epinastine, a specific antagonist of octopamine, decreased the total haemocyte count and PO activity. Another octopamine antagonist, phentolamine, inhibited the activity of PO and lysozymes. In addition, injection of dopamine induced a significant increase in PO activity and decreased the total haemocyte count and lysozyme activity. These results suggested that both octopamine and dopamine mediate the increases in total haemocyte count and PO activity in the crowded larvae.

Figure 1

Effect of larval density on three measures of larval immune function in M. separata. (A) Phenoloxidase activity. (B) Total haemocyte count. (C) Lysozyme activity. Values are means ± SE. Different letters indicate significant differences among the densities of 1, 2, 5, 10 and 30 larvae/jar (P < 0.05).

Figure 2

Effects of larval density on the levels of biogenic amines in larvae of M. separata. (A) Octopamine. (B) Dopamine. (C) 5-hydroxytryptamine. Values are means ± SE. Different letters indicate significant differences among the densities of 1, 2, 5, 10 and 30 larvae/jar (P < 0.05).

Figure 3

The three measures of immune function of M. separata larvae injected with different concentrations (0.2, 2, 20 µg/mL) of octopamine solution. (A) Phenoloxidase activity. (B) Total haemocyte count. (C) Lysozyme activity. Values are means ± SE. Different letters indicate significant differences at P < 0.05.

Figure 4

The three measures of immune function in M. separata larvae injected with different concentrations (0.01, 0.1, 1 µg/mL) of the octopamine antagonist epinastine. (A) Phenoloxidase activity. (B) Total haemocyte count. (C) Lysozyme activity. Values are means ± SE. Different letters indicate significant differences at P < 0.05.

Figure 5

The three measures of immune function in M. separata larvae injected with different concentrations (0.01, 0.1, 1 µg/mL) of the octopamine antagonist phentolamine. (A) Phenoloxidase activity. (B) Total haemocyte count. (C) Lysozyme activity. Values are means ± SE. Different letters indicate significant differences at P < 0.05.

Figure 6

The three measures of immune function of M. separata larvae injected with different concentrations (0.2, 2, 20 µg/mL) of dopamine solution. (A) Phenoloxidase activity. (B) Total haemocyte count. (C) Lysozyme activity. Values are means ± SE. Different letters indicate significant differences at P < 0.05.