Targeting excitotoxic/free radical signaling pathways for therapeutic intervention in glaucoma

Abstract

Glaucoma is a visual disorder characterized by progressive loss of retinal ganglion cells (RGCs), which is often associated with high intraocular pressure. However, mechanisms of RGC death in glaucoma still remain a mystery. Two theories have been proposed as pathogeneses of glaucoma: mechanical and vascular. We demonstrate that glutamate excitotoxicity triggered by overactivation of the N-methyl-D-aspartate (NMDA)-type glutamate receptors may contribute according to both theories to RGC death in glaucoma and other retinal diseases such as ischemia. From a therapeutic standpoint, NMDA receptors and downstream signaling pathways, triggered by p38 mitogen-activated protein kinase (MAPK) and caspases, are potential targets of intervention to prevent RGC death. Glutamate, however, mediates synaptic transmission essential for normal function of the nervous system. Hence, complete blockade of NMDA receptor activity causes unacceptable side effects. Studies in our laboratory have shown that an open-channel blocker of the NMDA receptors, memantine, blocks only excessive NMDA receptor activity while leaving normal function relatively intact. This characteristic endows memantine with clinical tolerability, as demonstrated by its approval for treatment of Alzheimer's disease and vascular dementia, and clinical trials for glaucoma. In this review, we discuss improved memantine derivatives, p38 MAPK, and caspase inhibitors as plausible therapeutics to prevent RGC death.