Thalamic NMDA receptors and nociceptive sensory synaptic transmission

The responses of single thalamic neurones to noxious thermal stimulation were recorded in anaesthetized rats. The selective N-methyl-D-aspartate (NMDA) receptor antagonist, 3-((+-)-2-carboxypiperazin-4-yl)propyl-l-phosphonate (CPP), antagonised nociceptive responses when ejected iontophoretically with currents which produced selective antagonism at NMDA receptors. In contrast, the non-NMDA excitatory amino acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNOX) had little or no effect on nociceptive responses, although it was able to reduce responses to non-nociceptive mechanoreceptor input. These results show that there is substantial NMDA receptor involvement in thalamic nociceptive responses, and that the contribution of CNQX-sensitive non-NMDA receptors to these responses is not extensive. Furthermore, it appears that nociceptive and non-nociceptive input to the thalamus have distinct synaptic pharmacologies.