Abstracts

Rationale: Tryptophan is presumed to play homeostatic and pathophysiological roles in the brain through the serotonin and kynurenine pathways. Despite a long history of nonclinical and clinical research on the significance of tryptophan in epilepsy, the mechanistic implication of this amino acid in the pathogenesis of epilepsy is still unclear. In this study, we assessed tryptophan metabolisms in EL mice modeling epilepsy by in vivo positron emission tomography (PET) with a radiolabeled tryptophan analog and mass spectrometry of brain tissues to pursue the dynamics and metabolism of tryptophan. Mechanistic associations of tryptophan with the epileptogenesis were also examined by treating these mice with tryptophan. Methods: Male EL and age-matched control ddY mice were used. EL mice develop generalized seizures with aging by vestibular stimulations. The ddY mouse is the maternal line of the EL mouse without seizures. PET scans with [11C]1-methyl-L-tryptophan([11C]1-MT) were conducted in EL mice in an interictal state (n=14) and control ddY mice (n=6) along the course of aging from 5 weeks of age (prior to the seizure onset) to 6 months of age (long after the seizure onset)We also performed mass spectrometry to measure concentrations of tryptophan, serotonin and related metabolites in the hippocampus of EL and ddY mice at 9 weeks of age antecedent to the seizure onset (n=8 for each group). Furthermore, we treated EL mice with either the standard diet containing 0.1% tryptophan or the diet containing 0.5% tryptophan from 5 weeks of age prior to the seizure onset. They were stimulated by tail suspension once a week, and we determined the latency to the first seizure and frequency of seizures in these mice. Results: The uptake of [11C]1-MT in the EL mouse brain significantly increased (by 70%) compared to control ddY mice from 5 weeks to 6 months of age. The content of tryptophan in the EL mouse hippocampus was significantly higher than the control level (18±1.5nmol/g verus 13±1.8nmol/g). Increased levels of serotonin and other related metabolites were also observed in EL mice. The 0.5% tryptophan diet induced seizures in EL mice at an earlier age than the standard diet. There was no significant difference in the frequency of seizures between the two diet groups after 9 weeks of age. Conclusions: Our results indicate that an enhanced uptake of tryptophan in the brain provoke the initiation of the epileptogenesis in EL mice. PET imaging with [11C]1-MT is potentially useful for assessing a change in the tryptophan kinetics and metabolism as a key process linked to the emergence of seizures in animal models and humans, possibly offering a tool for diagnosing and subtyping epilepsy on the mechanistic basis. Funding: none