DISTRIBUTION OF FREE D-SERINE IN VERTEBRATE BRAINS

Free D-serine distribution in vertebrate brains was investigated. In various brain regions of the lower vertebrate species, carp, frog and chick, free D-serine levels were low. On the contrary, in the mammals, mouse, rat and bull, the contents of free D-serine were high in the forebrain (around 400 nmol/g weight, and the ratio of D-serine to L-serine, was D/L = 0.4), and low in the hindbrain. In developing mice, D-serine levels in the cerebrum increased with age and attained the adult level (D/L = 0.40) 8 weeks after birth. In the cerebellum and brain stem, the free D-serine levels increased with age until 2 weeks, followed by a decrease to the adult levels: the D/L ratios remained constant until 2 weeks of age, then decreased to 0.03 in the cerebellum and 0.12 in the brain stem. Free D-serine was shown not to be of microbial origin using germ-free mice. In the rat forebrain, D-serine was evenly distributed in two cerebral regions, namely frontal and occipital lobes. The D/L ratios in other regions of forebrain, hippocampus and hypothalamus, were comparable to the cerebrum (D/L = 0.4), while that in the olfactory bulb was lower (D/L = 0.12). In the rat cerebrum, the D-serine content in the grey matter was significantly higher than that in the white matter. The contents of free D-serine in bovine cerebrum and cerebellum were similar to those in other mammalian brains, but the D/L ratio for bovine cerebral grey matter was lower than that for the cerebral white matter. The D-serine level was discussed in terms of D-amino-acid oxidase activity.