Calcium signal communication between glial and vascular brain cells

The brain is composed of neurons that communicate electrical signals over neurites and chemical signals across synapses, and non-neuronal cells like glial and vascular cells that communicate calcium signals among each other. Calcium ions have an important signaling function in the cytoplasm that depends on their amplitude, time course of change and subcellular localisation. Work over the last decade has added an additional dimension to this rich repertoire by including the possibility that calcium signals can be communicated between cells. In astrocytes and endothelial cells, connexins appear to be at the crossroad of calcium signal communication pathways, because they are the building blocks of gap junction channels that functionally connect cells, and because they can arrange as hemichannels that act as a conduit for cellular ATP release, thus initiating paracrine purinergic signaling. The two pathways appear to be operational in astrocytes and endothelial cells and we review in this paper possible functions of astrocyte-to-blood vessel calcium signaling at the level of arterioles where blood flow is controlled, at the level of capillaries where the blood-brain barrier is located and at the level of blood immune cells.