Biochemical and histological changes in the brain of the cricket Nemobius sylvestris infected by the manipulative parasite Paragordius tricuspidatus (Nematomorpha).

Hairworms (nematomorpha) alter the behaviour of their insect hosts, making them commit 'suicide' by jumping into an aquatic environment required by the adult parasite for the continuation of its life cycle. To explore the physiological and neuronal basis of this behavioural manipulation, we first performed a biochemical study to quantify different neurotransmitters or neuromodulators (monoamines and amino acids) in the brain of crickets (Nemobius sylvestris) uninfected and infected by the hairworm Paragordius tricuspidatus. We also analysed several polyamines and amino-acids having no known neuromodulatory function. The presence/absence of the parasite explained the largest part of the variation in compound concentrations, with infected individuals displaying on average lower concentrations than uninfected individuals. However, for three amino acids (taurine, valine and tyrosine), a significant part of the variation was also correlated with the manipulative process. In order to compare neurogenesis between infected and uninfected crickets, we also performed a histological study on mushroom bodies in the cricket's brain. The mitotic index exhibited a two-fold increase in infected crickets as compared with uninfected crickets. This is the first study to document changes in the brain of insects infected by nematomorphs.