P2RX5 and Alzheimer disease: For example, there is evidence of the involvement of purinergic receptors in Alzheimer’s disease (AD), and A1 receptor expression is lost or reduced in the outer layers of hippocampal dentate gyrus in human brain tissue from AD patients (Jansen et al., 1990;Ulas et al., 1993), while the expression of A1 and A2A receptors appears to be increased in the frontal cortex (Albasanz et al., 2008).