- 1. The involvement of N-methyl-D-aspartate (NMDA) receptors in thalamocortical transmission has been demonstrated in early postnatal development, but could not be determined so far in adult animals. We used thalamocortical slices from brains of mature mice to examine whether NMDA receptors exist in adult thalamocortical synapses, and what is their potential contribution to thalamocortical synaptic responses. 2. Thalamocortical fibers to the primary somatosensory area of the cortex were stimulated by an electrode placed in the ventrobasal (VB) nucleus of the thalamus. Horizontal intracortical axons within layer 5 were stimulated for comparison: a cut was made in the slice between layers 4 and 5 lateral to the recording area, and a second stimulating electrode was placed under the cut to activate these fibers. 3. Mg(2+)-free perfusing solution was used to enhance NMDA-receptor-mediated excitatory postsynaptic potentials (EPSPs). It led to synchronized population events that could be evoked by stimulating either in the thalamus or in layer 6. A short-latency, monosynaptic EPSP preceded the synchronous events in layer 5 cells. 4. Bath application of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) abolished only synchronized events triggered from the thalamus, but not intracortically. CNQX reduced, but did not abolish, the monosynaptic thalamocortical EPSP, and had almost no effect on intracortically evoked EPSPs. Focal application of the gamma-aminobutyric acid-A receptor antagonist bicuculline methiodide (10 microM) to layer 4 reestablished the appearance of synchronous events on thalamic stimulation. 5. Monosynaptic thalamocortical and intracortical EPSPs on the same cells were studied in the presence of normal Mg2+ concentration to mimic better the physiological state. Spikes were abolished by intracellular injection of the quarternary lidocaine derivative (QX-314) sodium channel blocker. EPSPs from both sources were reduced compared with control, but not blocked, after bath application of CNQX (10 microM). CNQX-resistant EPSPs of both synaptic tracts were evident at membrane potentials close to rest, exhibited strong voltage dependency, and were blocked by the NMDA antagonist DL-2-amino-5-phosphonovaleric acid (APV), suggesting that they were NMDA receptor dependent. 6. To confirm that NMDA receptors were indeed activated at thalamocortical synapses and not by antidromic activation of intracortical pathways, we used pressure microapplications of glutamate (10 mM) to the VB nucleus of the thalamus. The perfusing solution contained CNQX (10 microM) and cells were recorded with micropipettes containing QX-314. All cells that showed a monosynaptic response to electrical thalamic stimulation also exhibited a barrage of mixed synaptic responses to thalamic glutamate application. The amplitude of these synaptic events was dependent strongly on the membrane voltage, and the application of APV to the cortex abolished the events completely. 7. Our results demonstrate that, in adult animals, both thalamocortical and intracortical synaptic pathways utilize NMDA as well as non-NMDA receptors.