GFAP and epilepsy: Because previous studies have reported that epilepsy induces astrocyte activation and that L-serine (the precursor of D-serine) is primarily synthesized by astrocytes, we first examined the expression of D-serine after seizure induction by double immunostaining for GFAP and SR, a key enzyme that transforms L-serine into D-serine (Wolosker et al., 1999).