Metabotropic glutamate receptor agonists evoke calcium waves in isolated muller cells

Glutamate is the most prominent excitatory neurotransmitter in the retina and brain. It has become clear that the physiology of many glial cells, including retinal Muller cells, is modified by a host of neurotransmitters, including glutamate. The experiments presented here demonstrate that Muller cells isolated from the tiger salamander retina have metabotropic glutamate receptors that, when activated, lead to the release of calcium ions (Ca²⁺) from intracellular stores. The Ca²⁺-sensitive fluorescent dye, Fura-2, and video imaging microscopy were used to monitor changes in cytosolic calcium ion concentration ([Ca²⁺](i)) evoked by glutamate (30-50 μM), (1S,3R)-ACPD (50-200 μM), quisqualate (10-50 μM), and L-AP4 (5-100 μM). Bath application of each of these metabotropic receptor agonists in the absence of extracellular Ca²⁺ resulted in an increase in [Ca²⁺](i) that often began in the distal end of the cell and occurred later in the endfoot. This wavelike increase in [Ca²⁺](i) is reminiscent of the Ca²⁺ waves evoked in these cells by other Ca²⁺ releasing agents such as ryanodine and caffeine. Extracellular application of ATP also evoked increases in [Ca²⁺] in Muller cells. The presence on Muller cells of receptors for retinal neurotransmitters, such as glutamate and ATP, demonstrates that these glial cells can respond to changes in the retinal extracellular environment and hence neuronal activity. Since Muller cells span almost all layers of the retina, they are likely to be exposed to most retinal neurotransmitters. The Ca²⁺ waves evoked in Muller cells by neurotransmitters could represent a form of signaling from the outer retinal layers to the inner ones.