N-methyl-D-aspartate (NMDA) receptors (NMDA-Rs) have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study investigated the subunit composition of the presynaptic NMDA-Rs in layer II/III pyramidal neurons of the rat visual cortex. We recorded evoked a-amino-3-hydroxy- 5-methyl-4-isoxazolepropionic acid (AMPA) receptor – mediated excitatory postsynaptic currents (eEPSCs) using wholecell voltage clamp with the open-channel NMDA receptor (NMDA-R) blocker, (+)-5-Methyl-10,11-dihydro-5Hdibenzo( a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801), in the recording pipette. We found that the paired-pulse ratio (PPR) by two successive stimuli with inter-pulse intervals of 50 ms was significantly increased by D-APV, a selective NMDA-R antagonist. Using a specific antagonist for NR2B-NMDA-Rs, (αR,βS)-α-(4-hydroxyphenyl)-β- methyl-4-(phenylmethyl)-1-piperidinepropanol hydrochloride (Ro 25-6981), instead of d-2-amino-5-phosphonovalerate (D-APV), we found that the PPR of eEPSCs was also significantly increased. Moreover, Zn2+, an NR2A-NMDA-R antagonist, did not influence on the PPR. These results suggest that presynaptic NR2B-containing NMDA-Rs are located in layer II/III pyramidal neurons of the rat visual cortex, and that presynaptic NR2B-containing NMDA autoreceptors but not NR2A-containing NMDA autoreceptors mediate glutamate release in the rat visual cortex. Moreover, these findings may be clinically relevant to schizophrenia, where enhancing NMDA-R function is considered to be a promising strategy for treatment of the disease.
Keywords: NMDA receptors, NR2A,, NR2B, eEPSCs paired-pulse ratio, PPR, synaptic transmission, visual cortex