Cover picture: Release of ATP from retinal glial cells. This pseudocolor image of luciferin-luciferase chemiluminescence shows the release of ATP after stimulation of glial cells on the surface of the rat retina. ATP released from glial cells in the retina is metabolized to adenosine, which, in turn, activates neuronal A₁ adenosine receptors and inhibits retinal neurons. The image was taken 12 sec after glial cells were stimulated and shows a region of the retinal surface 480 µm wide. For details, see the article by Newman in this issue (pages 1659–1666).