Low Extracellular Magnesium Unmasks N-methyl-D-aspartate-mediated Graft-Host Connections in Rat Neocortex Slice Preparation.

The main purpose of this study was to investigate the role of N-methyl-D-aspartate receptors in host-graft synaptic transmission in the neocortex. The effects of low extracellular magnesium, the glutamate agonist N-methyl-D-aspartate and N-methyl-D-aspartate antagonists on the synaptic activation of connections between embryonic neocortical graft tissue and the surrounding host tissue were studied in 17 perfused slices of rat neocortex. In standard artificial cerebrospinal fluid, stimulation of the host white matter evoked field potentials in four of 17 grafts. However, in Mg(2+)-free medium, the same stimulation evoked field potentials in an additional six grafts, with significant increases in the mean duration of the evoked responses in the 10 responsive grafts. In five of these slices stimulation of the graft also evoked field potentials in the host tissue, suggesting reciprocal interaction between graft and host. Simultaneous extracellular recordings from graft and host tissues in Mg(2+)-free medium showed that spontaneous epileptiform discharges developed in the graft and host tissue synchronously. In Mg(2+)-free medium, application of N-methyl-D-aspartate induced a shift of the baseline with superimposed epileptiform discharges in both graft and host. Application of the non-competitive N-methyl-D-aspartate antagonist ketamine and the competitive antagonist D,L-2-amino-5-phosphonovaleric acid attenuated or reversibly blocked both the spontaneous epileptiform discharges and the evoked field potentials. Our data provides evidence that N-methyl-D-aspartate receptors are present at synapses created between fetal graft and host neocortex, and that the N-methyl-D-aspartate-activated receptor-channel complex plays an active role in mediating excitatory synaptic transmission in host-graft circuitry.