P2RX7 and retinal degeneration: ATP-mediated cell death is thought to play a role in inherited retinal degenerations (Puthussery and Fletcher, 2009; Fletcher, 2010), and although the process likely involves direct phototoxic activation of the P2X7 purinergic receptor (Puthussery and Fletcher, 2004; Puthussery et al., 2006; Vessey et al., 2012, 2014), purinergic receptors are also found throughout the retina on neural, glial and endothelial cells (Puthussery and Fletcher, 2004; Puthussery et al., 2006; Mitchell and Reigada, 2008).