D-serine modulates the NMDA receptor/nitric oxide/cGMP pathway in the rat cerebellum during in vivo microdialysis.

Several lines of investigation indicate that D-serine may be an endogenous ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in some CNS regions. We here studied the in vivo effects of D-serine on the NMDA receptor/nitric oxide/cGMP pathway by monitoring extracellular cGMP in the cerebellum of freely-moving rats subjected to transcerebral microdialysis. Local application of NMDA (200, 500 microM) through the dialysis probe for 20 min evoked transient, concentration-dependent cGMP responses which peaked in the fraction of drug administration, the nucleotide levels returning to basal values after 40 min. The NMDA-induced elevation of the extracellular nucleotide was completely inhibited by the selective receptor channel blocker dizocilpine (MK-801) locally co-perfused at the concentration of 10 microM. The non-competitive antagonist had no effect on its own suggesting that endogenous glutamic acid does not tonically activate NMDA receptors. The effect of 200 microM NMDA was largely attenuated by 30 microM 7-chloro-kynurenic acid and completely abrogated when the concentration of the strychnine-insensitive glycine receptor antagonist was raised to 100 microM. D-serine (300 microM), perfused in the presence of 7-chloro-kynurenate (30 microM), was able to fully restore the NMDA (200 microM)-induced increase of cGMP extracellular levels. On the other hand, the D-amino acid directly potentiated in a concentration-dependent manner (0.3, 1 and 10 mM) the NMDA (200 microM)-evoked cGMP production whereas it was inactive on its own. These data show that in vivo the activation of the strychnine-insensitive glycine site is essential for the functioning of the NMDA receptor complex and can be activated by the selective agonist D-serine. They also confirm that cerebellar NMDA receptors do not have their glycine sites saturated.