Mechanism of parasitism-induced elevation of dopamine levels in host insect larvae

Abstract

Parasitization with the parasitoid wasp, Cotesia kariyai, causes elevation of dopamine (DA) levels in the haemolymph and nerve cord of the last larval instar of the armyworm, Pseudaletia separata. Injection of DA or DA aganist [(−)-Quinpirole] during the early stages of the last larval instar retards normal growth and delays pupation. This suggests the possibility that DA plays an important role in the retardation of larval development by parasitism. The present study demonstrated that the DA level in the integument was also increased through the elevation of dopa decarboxylase (DDC) activity during parasitism. The total DA content in the integument was calculated to be approximately 50 times greater than in the haemolymph of parasitized Day 1 last instar larvae, thereby suggesting that parasitism-induced elevation of haemolymphal DA could be from DA in the integument. This hypothesis was partly confirmed by evidence that radiolabelled DA in the integument was secreted into in vitro incubation medium. Furthermore, DA elevation due to an increase in DDC activity in the integument was also observed in nonparasitized larvae in their last instar after injection of an insect biogenic peptide, growth-blocking peptide (GBP), whose concentration is elevated by parasitism. Therefore it is proposed that the plasma GBP fiter elevated by parasitism causes the rise in integumental DDC activity and this induces an increase in the DA level in the integument. This integument secretes DA into the haemolymph, raising DA levels in the haemolymph of parasitized larvae.