Mechanisms involved in activity-dependent synapse formation in mammalian central nervous system cell cultures

Differences in neuronal activity produced by electrical stimulation lead to competition between synapses from sensory afferents converging on a common spinal cord neuron. Studies were performed on neurons dissociated from the mouse spinal cord and grown in culture dishes with three compartments. Synaptic efficacy from stimulated afferents was increased compared with unstimulated convergents, and the number of functional connections was increased by stimulation compared with control cultures. Blocking NMDA channel activation with 100 microM APV in medium containing 1.8 mM calcium inhibited this synaptic plasticity, but plasticity was not blocked by APV in medium in which the calcium concentration was elevated to 3 mM. These experiments support the view that electrical activity differentially influences processes that cause a persistent decrease in synaptic efficacy or lead to synapse elimination and those that increase synaptic strength or lead to synapse augmentation. We interpret our results in terms of a model in which these antagonistic mechanisms are both regulated via changes in calcium levels and second messengers that are modulated by electrical activity. A significant portion of the activity-related calcium influx relevant to synaptic plasticity passes through the NMDA channel, but other sources of calcium are involved. In particular, competitive elimination of synapses appears to occur during blockade of NMDA channels if the extracellular concentration of calcium is elevated moderately. The outcome of competition between the two calcium-dependent but antagonistic processes may depend either on their differential sensitivity to intracellular calcium concentration or separate specificities to NMDA and non-NMDA receptor-linked mechanisms.