Effects of diazinon and diazoxon on the lymphoproliferation rate of splenocytes from Nile tilapia (Oreochromis niloticus): the immunosuppresive effect could involve an increase in acetylcholine levels.

The lymphoproliferation rate of spleen cells from Nile tilapia (Oreochromis niloticus) exposed to the organophosphorus pesticide diazinon, to its metabolite diazoxon and to the neurotransmitter acetylcholine, was evaluated in order to explore the immunotoxic mechanism of action of this widely used insecticide. The lymphoproliferative response of spleen cells to mitogenic stimulus was not affected by either diazinon or diazoxon, indicating that these xenobiotic substances do not have direct immunotoxic properties. Conversely, ex vivo assays showed that spleen from fish exposed to diazinon presented a lower acetylcholinesterase activity and a higher acetylcholine concentration than non-exposed controls. Lymphoproliferation assays also indicated that pre-exposure to acetylcholine depleted the proliferative function of spleen cells. Thus the combined information from in vitro and ex vivo experiments suggest that the immunotoxic properties of diazinon in Nile tilapia are indirect and could involve the cholinergic system of lymphocytes.