No abstract

D-serine is thought to be a glia-derived transmitter that activates N-methyl D-aspartate receptors (NMDARs) in the brain. Here, we investigate the pathways for D-serine release using primary cultures, brain slices, and in vivo microdialysis. In contrast with the notion that D-serine is exclusively released from astrocytes, we found that D-serine is released by neuronal depolarization both in vitro and in vivo. Veratridine (50 microM) or depolarization by 40 mM KCl elicits a significant release of endogenous D-serine from primary neuronal cultures. Controls with astrocyte cultures indicate that glial cells are insensitive to veratridine, but release D-serine mainly by the opening of volume-regulated anion channels. In cortical slices perfused with veratridine, endogenous D-serine release is 10-fold higher than glutamate receptor-evoked release. Release of D-serine from slices does not require internal or external Ca(2+), suggesting a nonvesicular release mechanism. To confirm the neuronal origin of D-serine, we selectively loaded neurons in cortical slices with D-[(3)H]serine or applied D-alanine, which specifically releases D-serine from neurons. Depolarization with veratridine promotes D-serine release in vivo monitored by high temporal resolution microdialysis of the striatum. Our data indicate that the neuronal pool of D-serine plays a major role in D-serine dynamics, with implications for the regulation of NMDAR transmission.

No abstract

No abstract

d-Serine is a co-agonist of NMDA receptors (NMDARs) whose activity is potentially regulated by Asc-1 (SLC7A10), a transporter that displays high affinity for d-serine and glycine. Asc-1 operates as a facilitative transporter and as an antiporter, though the preferred direction of d-serine transport is uncertain. We developed a selective Asc-1 blocker, Lu AE00527, that blocks d-serine release mediated by all the transport modes of Asc-1 in primary cultures and neocortical slices. Furthermore, d-serine release is reduced in slices from Asc-1 knockout (KO) mice, indicating that d-serine efflux is the preferred direction of Asc-1. The selectivity of Lu AE00527 is assured by the lack of effect on slices from Asc-1-KO mice, and the lack of interaction with the co-agonist site of NMDARs. Moreover, in vivo injection of Lu AE00527 in P-glycoprotein-deficient mice recapitulates a hyperekplexia-like phenotype similar to that in Asc-1-KO mice. In slices, Lu AE00527 decreases the long-term potentiation at the Schaffer collateral-CA1 synapses, but does not affect the long-term depression. Lu AE00527 blocks NMDAR synaptic potentials when typical Asc-1 extracellular substrates are present, but it does not affect AMPAR transmission. Our data demonstrate that Asc-1 mediates tonic co-agonist release, which is required for optimal NMDAR activation and synaptic plasticity.