N-Methyl-d-Aspartate-Induced Excitotoxicity and Its Impact on the Renin-Angiotensin System in Retinal Tissue.

Purpose: Renin-angiotensin system (RAS) is expressed in neuronal tissue and plays a role in neurodegenerative diseases involving excitotoxicity as a pathophysiological mechanism. In retina, excessive excitatory neurotransmission via N-methyl-d-aspartate (NMDA) receptors underlies neuronal apoptosis in conditions like glaucoma. However, it is not known if NMDA-mediated excitotoxicity alters retinal RAS expression. Hence, this study investigated the effect of NMDA exposure on the expression of RAS in rat retinas. Methods: Two groups of Sprague-Dawley rats received either phosphate buffer saline or NMDA (160 nmol). On day 7 posttreatment, retinal expression of RAS components including renin, angiotensinogen, angiotensin-converting enzyme (ACE), angiotensin II (Ang II), Ang 1-7, Ang 1-9, MAS receptor, angiotensin II type 1 receptor (AT1R), ACE2, and aldosterone was measured using enzyme-linked immunosorbent assay and polymerase chain reaction. Morphometric studies were done to assess morphological alterations. Results: Following the exposure to NMDA, an upregulation of ACE expression was noted at both the protein (2.03-folds; P < 0.001) and mRNA (1.86-folds; P < 0.01) levels in rat retinas. AT1R protein and mRNA expression were greater by 1.73 (P < 0.0001) and 2.28-folds (P < 0.0001), respectively. However, mRNA expression for ACE2, Ang 1-7, and Ang 1-9, showed a 1.51-(P < 0.05), 2.41-(P < 0.001), and 2.37-(P < 0.0001) fold decrease. Ganglion cell layer (GCL) thickness and linear cell density in GCL were significantly lower in the NMDA-treated group (P < 0.05). Conclusions: NMDA exposure increases expression of the classical RAS and suppresses that of alternate RAS in rat retinas. These alterations are associated with retinal morphological changes indicating significant loss of neuronal cells in the GCL of rat retinas.