Extracellular calcium depletion as a mechanism of short-term synaptic depression

Recent experiments have demonstrated that normal neural activity can cause significant decrements in external calcium levels, and that these decrement s mediate a form of short-term synaptic depression. These findings raise th e possibility that certain forms of short-term synaptic depression at gluta matergic synapses throughout the mammalian CNS may be influenced by similar changes in external calcium. We use a computational model of the extracell ular space, combined with experimental data on calcium consumption, to show that such short-term depression can be accounted for by changes in calcium just outside active synapses, provided that external calcium diffusion is restricted. Remarkably, the model suggests the novel possibility that synap ses may possess private pools of external calcium that enforce some forms o f short-term depression in a synapse-specific manner.