Abstracts

Rationale: Tryptophan (TRP) is an essential amino acid and the significance of TRP metabolism has received broad attention as a treatment target of epilepsy. However, the mechanistic implication of TRP in the pathogenesis of epilepsy is still unclear. TRP is known to be metabolized in astrocytes, and the astrocyte function can be mediated by intracellular glycogen through osmotic control. In the present study, examining the relationship between tryptophan metabolism, astrocyte functional change and glycogen metabolism by in vivo imaging of EL mice modeling epilepsy, we aimed to reveal the role of TRP metabolism in epilepsy. Methods: Male EL and age-matched control ddY mice (n = 28 for each group) were used. EL mice develop generalized seizures at 12 weeks of age in natural course. The difference of TRP metabolism between EL mice and ddY mice was examined by 1) PET scans with [11C]1-methyl-L-tryptophan ([11C]1-MT), which is a radiolabeled TRP analog, in EL mice and ddY mice from 5 weeks to 6 months and mass spectrometry  measuring the concentrations of TRP and related metabolites in the hippocampus of EL and ddY mice at 9 weeks of age. The causal involvement of TRP metabolism for epileptic seizure was investigated by 3)tail suspension test comparing the standard diet EL mice with the TRP loading diet EL mice from 5 weeks of age. The participation of astrocytes in the pathology of epilepsy was assessed by 4)GFAP immunostaining between EL mice and ddY mice and 5)two photon imaging targeting astrocytes and neurons of the sensory cortex comparing the TRP loading diet ddY mice and the standard diet ddY mice at 10 weeks of age. The facilitation of glycogen functional changes of astrocytes induced by abnormal TRP metabolism was analyzed by 6)glycogen assay of the hippocampus of EL and ddY mice treated the standard diet and TRP loading diet at 8 weeks of age was conducted. Results: 1) The uptake of [11C]1-MT in the EL mouse brain and 2) the content of TRP, serotonin and related metabolites in the EL mouse hippocampus was significantly higher than the control level. 3) The TRP loading diet induced seizures in EL mice at an earlier age than the standard diet. 4) The immunostaining of GFAP was significantly increased in EL mice compared to ddY mice in the cortex. 5) The astrocyte volume changes as reaction to stimulation was decreased in TRP loading diet ddY mice relative to the standard diet ddY mice. The neuronal oscillation was increased by TRP loading. 6) Glycogen was significantly decreased in EL mice compared ddY mice. Glycogen was significantly decreased by TRP loading both in EL mice and ddY mice. Additionally, increased metabolites of glycogen in EL mice than ddY mice was revealed by mass spectrometry. Conclusions: We revealed that abnormally accelerated TRP metabolism induced glycogenolysis and increase of glycogen metabolites in astrocytes which caused the osmotic control failure in astrocytes and neuronal excitation leading to epilepsy. This mechanism can be a new target of treatment. Funding: None