Modulation of NMDA receptor-mediated responses by glycine and D-serine in the rat thalamus in vivo.

The effects of iontophoretically applied glycine and D-serine upon responses of rat ventrobasal thalamus neurones to excitatory amino acids and sensory stimulation were investigated. Glycine inhibited excitatory responses to kainate, quisqualate and N-methyl-aspartate (NMA). Strychnine antagonised inhibitory responses to glycine, but in only a small number (2/7) of cases did such an antagonism reveal a facilitatory action of glycine on NMA responses. D-Serine inhibited responses of almost all neurones to either kainate or quisqualate, but enhanced responses to NMA on nearly half of the tested neurones, whilst having a weaker inhibitory action on the remainder. It is thus possible that D-serine has a dual action: (i) a facilitation of N-methyl-D-aspartate (NMDA) receptor-mediated responses and (ii) a non-selective inhibitory action.