Neuron-derived D-Serine Release Provides a Novel Means to Activate N-Methyl-D-aspartate Receptors

d-Serine is a coagonist of N-methyl-d-aspartate (NMDA) receptors that occurs at high levels in the brain. Biosynthesis of d-serine is carried out by serine racemase, which converts l-to d-serine. d-Serine has been demonstrated to occur in glial cells, leading to the proposal that astrocytes are the only source of d-serine. We now report significant amounts of serine racemase and d-serine in primary neuronal cultures and neurons in vivo. Several neuronal culture types expressed serine racemase, and d-serine synthesis was comparable with that in glial cultures. Immunohistochemical staining of brain sections with new antibodies revealed the presence of serine racemase and d-serine in neurons. Cortical neurons expressing serine racemase also expressed the NR2a subunit in situ. Neuron-derived d-serine contributes to NMDA receptor activation in cortical neuronal cultures. Degradation of endogenous d-serine by addition of the recombinant enzyme d-serine deaminase diminished NMDA-elicited excitotoxicity. Release of neuronal d-serine was mediated by ionotropic glutamate receptor agonists such as NMDA, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate. Removal of either external Ca2+ or Na+ blocked d-serine release. Release of d-serine was mostly through a cytosolic route because it was insensitive to bafilomycin A1, a potent inhibitor of vesicular neurotransmitter uptake. d-Serine was also not transported into purified synaptic vesicles under conditions optimal for the uptake of known transmitters. Our results suggest that neurons are a major source of d-serine. Glutamate-induced neuronal d-serine release provides a novel mechanism for activating NMDA receptors by an autocrine or paracrine way.