NMDA receptors in cortical development are essential for the generation of coordinated increases in [Ca²⁺]_i in 'neuronal domains'

Spontaneous correlated activity regulates the precision of developing neural circuits. A synchronized elevation of intracellular calcium ion concentration, [Ca²⁺]i, occurred in 5-50 adjacent neurons - known as a 'neuronal domain' - in developing neocortex. This coordinated response of neuronal cells is mediated by the diffusion of inositol trisphosphate (lP³) via gap-junction channels. In this study, we utilized the N-methyl-D-aspartate (NMDA)-type glutamate receptor ε2 (GluRε2/NR2B)-l- mouse, which does not possess any functional NMDA receptors in the developing neocortex, and showed that NMDA receptors are essential for the generation of 'neuronal domains'. First, the frequency of spontaneously occurring neuronal domains in brain slices from GluRε2^-1- mice was significantly reduced compared to that seen in brain slices from wild-type mice. Secondly, IP³ injection into a single neuron in a cortical slice from a GluRε2^-1- brain resulted in very few neuronal domains being observed, but an injection similarly made into a neuron in a wild-type slice promptly resulted in neuronal domains. Even in the GluRε2^-1- brain, the elevation of intracellular [Ca²⁺]i, was observed frequently in single neurons and microinjection of lP3 produced an elevation of [Ca²⁺], in the injected cells. These results suggest that the diffusion of lP3 into the surrounding neurons via gap junctions is almost completely absent in the GluRε2^-1- brain. Our results may reflect the critical role of NMDA receptors in the formation of cortical circuitry, probably via the regulation of gap-junction channels between immature cortical neurons.