Toxicology of ryegrass endophyte in livestock

The ryegrass endophyte/plant interaction produces a number of toxic metabolites responsible for a range of toxicoses including ryegrass staggers. Although lolitrem B has long been considered the toxin responsible for ryegrass staggers in farm animals, it is only recently that we have demonstrated that oral administration of the pure toxin causes tremors in mice consistent with the symptoms of ryegrass staggers. Various levels of the toxin were incorporated into the diet of mice and tremor response measured regularly. Mice were very susceptible to the tremorgenic action of lolitrem B with a dose of only 1.1 mg/kg per day being sufficient to induce a tremor response. Analysis of faeces from a dosed mouse showed that approximately 40% of the toxin ingested was excreted unchanged. The profile of tremor response shows that tremors build up over a 24-hour period and then reach a dose dependent plateau suggesting that toxin turnover reaches a steady state. In experiments designed to test the possibility that tremorgens bind to specific receptors in the brain, mice were injected with the 14C-labelled tremorgen, paxilline. Paxilline was chosen as it is similar in structure and possible action to lolitrem B and could be more easily prepared with an isotopic label. Following injection of 14C-paxilline to mice their brains and spinal cord were sectioned and analysed for 14C content. Localised binding was not detected but rather an even distribution of isotope was observed. This is probably due to the lipophilicity of the tremorgens, which distribute randomly throughout the fatty brain matrix, masking any binding to specific regions or receptors. This study showed, however, that only a minute proportion of the administered tremorgen reached the brain and spinal cord, indicating that, if this is the site of action, the receptors involved in the initiation of staggers are extremely sensitive to the tremorgens. Keywords: endophyte, lolitrem, Lolium perenne, mycotoxin, Neotyphodium lolii, neurotoxin, oral dosing, ryegrass staggers, tremor, tremorgen