(58,61). Its concentration in the blood of silkworms was reported to increase during particular stages of metamorphosis, although the physiological role of D-Ser in metamorphosis remains unclear (11). It is also present in the mammalian forebrain, where it persists at high concentrations throughout the life of the animal. DSer is now considered a neuromodulator that binds to the glycinebinding site of the N-methyl-D-Asp (NMDA) receptor, a subtype of the L-glutamate (L-Glu) receptor, and potentiates glutamatergic neurotransmission in the central nervous system (51,60,68). In fact, astroglia-derived D-Ser has been shown to regulate NMDA receptor-dependent long-term potentiation and/or long-term depression, which are basic processes of learning and memory, in the hypothalamic and hippocampal excitatory synapses (25,54). In addition, D-Ser is also found in the cerebellum during the early postnatal period, and it was recently reported that D-Ser derived from the Bergmann glia serves as an endogenous ligand for the ␦2 L-Glu receptor to regulate long-term depression at synapses between parallel fibers and Purkinje cells in the immature cerebellum but not the mature one (31). These lines of evidence suggest the physiological significance of D-Ser in the regulation of higher brain functions through the L-Glu receptors, and indeed, perturbation of D-Ser levels in the central nervous system has now been implicated in the pathophysiology of various neuropsychiatric disorders, including schizophrenia (4, 23, 24, 71), Alzheimer's disease (28, 70), and amyotrophic lateral sclerosis (59).Unlike the tissue-specific expression of D-Ser, substantial amounts of free D-Asp are present in a wide variety of tissues and cells in invertebrates and vertebrates, particularly in the central nervous, neuroendocrine, and endocrine systems. D-Asp is proposed to play an important role in regulating developmental processes, hormone secretion, and steroidogenesis (12,26,35). For instance, it has been reported that in male lizards, intraperitoneally administered D-Asp is taken up rapidly by the testis, which induces a significant increase in testosterone levels and a significant decrease in 17-estradiol levels in the testis and plasma (57). Interestingly, a reverse relationship is found between males and females; specifically, in female lizards, exogenous D-Asp induces a significant decrease in testosterone levels in the ovary and plasma, while it enhances follicular production of 17-estradiol by upregulating the local aromatase activity (2). Similar findings have also been reported in studies of the green frog (16, 56). These observations suggest that D-Asp is an important regulatory molecule of gonads. In addition, it was recently shown that the amount of D-Asp in human seminal plasma and spermatozoa is significantly reduced in oligoasthenoteratospermic and/or azoospermic donors compared with normospermic donors (14). Moreover, in human female patients undergoing in vitro fertilization, the D-Asp content of the preovulatory follicular fluid ...