NMDA receptor-mediated calcium entry in the absence of AMPA receptor activation in rat dorsal horn neurons.

This study tested quantitatively the degree to which selective activation of NMDA receptors increases the intracellular concentration of free Ca2+ ions ([Ca2+]i) in neurons isolated from the rat spinal cord dorsal horn and grown in culture. The results show that activation of NMDA receptors, without additional depolarizing stimuli, produce [Ca2+]i increases in the neuronal cell bodies in the presence of physiological concentrations of Mg2+. Furthermore, this phenomenon occurs during synaptic activation of NMDA receptors. Thus, it cannot be assumed that in physiological extracellular [Mg2+], there is an absolute requirement for additional membrane depolarization before NMDA receptor activation stimulates significant increases in [Ca2+]i.