Calcium waves in gray matter are due to voltage-sensitive glial membrane channels

The retina is the most accessible piece of central gray matter in the vertebrate brain. Its wide dynamic operational range makes it the ideal neuronal network to study its excitability. Spreading depression waves in the retina are accompanied by strong intrinsic optical signals (IOS) and thus can be measured non-invasively with optical methods. Additionally, incubation with fluorescent dyes allows to follow calcium fluxes in parallel. The IOS can be divided into red and green scatter of light. We show that during spreading depression the red scatter signal precedes the green scatter signal and that the calcium signal matches the red scatter signal. Incubation of the retina with barium chloride leads to a reversible depression of red scatter and calcium signal whereas the green scatter signal is hardly effected. The wave propagation velocity is reduced, too. This supports the idea that the early red scatter signal is a direct visualisation of glial membrane potential and that glia cells in the chicken retina are involved in the control of extracellular calcium.