We have investigated the role of the glycine recognition site of the N-methyl-D-aspartate receptor (the GlyNMDA site) in the facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons that is brought about by neurokinin1 (NK1) receptor agonist and the contribution of protein kinase C (PKC) activation to this phenomenon. Ionophoresis of the selective NMDA receptor agonist 1-aminocyclobutane-cis-1,3-dicarboxylic acid (ACBD) produced a sustained increase in the firing rate of single laminae III-V neurons recorded extracellularly using multibarrelled glass electrodes. The highly selective NK1 receptor agonist acetyl-[Arg6,Sar9,Met(O2)11]-SP6-11 (Sar9-SP) greatly facilitated this response, but under the present conditions had no effect when applied alone or with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonist) at the same current. In the presence of the GLyNMDA site antagonists 2-carboxy-4,6-dichloro-(1H)-indole-3-propanoic acid (MDL 29951), 7-chloro-3-(cyclopropylcarbonyl)-4-hydroxy-2(1H)-quinoline (L701,252), 5,7-dinitroquinaxoline-2,3-dione (MNQX) or 7-chlorothiokynurenic acid (7-CTK), or the PKC inhibitors, chelerythrine or GF109203X, the Sar9-SP-induced facilitation of ACBD-evoked activity was prevented, generally restoring activity to a level similar to that in the presence of ACBD alone, whilst an AMPA receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX) did not inhibit the facilitation. At the same ionophoretic currents these compounds had no effect on ACBD-evoked activity in the absence of Sar9-SP but were inhibitory at significantly greater currents. To further substantiate the importance of the GlyNMDA site in the interaction, the effects of NMDA receptor antagonists selective for alternative recognition sites on the NMDA receptor were investigated. MK-801, a non-competitive NMDA receptor antagonist and arcaine, a competitive inhibitor at the polyamine site, were applied to the facilitated activity seen in the presence of Sar9-SP and ACBD, and to ACBD-evoked activity alone. Unlike the GlyNMDA site antagonists and PKC inhibitors, these compounds reduced both facilitated and ACBD-evoked activity at similar currents. Furthermore, like the NK1 receptor agonist, a selective GlyNMDA site agonist 1-aminocyclopropane carboxylic acid (ACPC) caused facilitation of ACBD-evoked activity which was also blocked by currents of L701,252 that did not alter activity evoked by ACBD alone. These data suggest that activation of the GlyNMDA site (perhaps as a consequence of glycine release or modification of its influence by intracellular signalling cascades) is an essential component of the means by which NK1 receptor activation results in facilitated responsiveness of dorsal horn neurons towards NMDA receptor agonists.