Decrease of Opsin Content in the Developing Rat Photoreceptor Cells by Systemic Administration of L-Glutamate.

Abstract

ABSTRACT. i -Gliitamate, a putative photoreceptor cell neurotransmitter, causes thinning of the inner layers of the retina and has been used for preparing biologically fractionated photoreceptor cells. However, it is possible that absence of the inner retinal layers may affect the remaining retina, and/or glutamate may directly affect photoreceptor cells. Weevaluated quantitatively the effects of i -glutamate on the developing photoreceptor cells by measuring the rod photoreceptor cell-specific protein, opsin. Wepurified rat rhodopsin and used it as the standard for measuring opsin content of rat retinas with competitive enzyme-linked immunosorbentassay. Various concentrations of glutamate were injected into 7-day-old rats, and the effects of the amino acid concentration on opsin expression were determined on postnatal day 14. Inner layers of the retina degenerated when10 ju\ or 15 ju\ of 2.4 M glutamate/gram body weight was administered subcutaneously. Opsin content of these glutamate-treated retinas decreased significantly compared with control retinas. Weadministered glutamate to rats at various stages of development and determined the effects by light microscopy on postnatal day 14. The administration of glutamate resulted in no degeneration of the inner retina if injected on postnatal day 1 or 2, degeneration of the inner retina between day 3 to 7, and again, no degeneration after postnatal day 13. Opsin content decreased significantly when glutamate was administered between postnatal day 1 to 7, but not after day 13, the day the blood-retinal barrier seems to reach maturity. Our findings indicate that systemic administration of L-glutamate affects the expression of opsin in the developing rod photoreceptor cells.Glutamate is a leading candidate for the excitatory neurotransmitter of the vertebrate retinal photoreceptors (2, 4, 10) and bipolar cells (8,15,26). Neurotoxicity by endogenous excitatory amino acids such as glutamate has been implicated as a mechanism for central (24) and retinal neuronal cell loss (29) after ischemia.Moreover, it is shown that an abnormality in the glutamate metabolism may cause a retinal dysfunction (1).Since Lucas and Newhouse(13) reported the toxic effects of glutamate on the inner retina, the amino acid has been used to prepare biologically fractionated photoreceptors since the photoreceptors appeared normal in form and concentration. The results using the fractionated photoreceptors provided important clues to amino acid levels (6), electroretinograms (21), and distribution of the enzymes (22) In the present study, to evaluate the effects of neurotoxic glutamate on the developing rat photoreceptor cells quantitatively, an assay of the rod photoreceptorspecific protein, opsin, in glutamate-administered rat retinas was carried out.

Developing Stage-dependent Retinal Toxicity Induced by l-glutamate in Neonatal Rats

“…Blood–retinal barrier segregates the circulating blood from retinal interstitial fluid and regulates the exchange of compounds between the circulating blood and the retina. In addition, vascular endothelial cells in inner retina and Müller cells express glutamate transporters and play a central role in retinal glutamate homeostasis and in preventing glutamate accumulation (Kanno et al 1991; Pow and Barnett 1999; Sakurai et al 2015). We proposed that the milder damage in rats treated on PND 9 than on PND 8 was caused by more effective glutamate homeostasis in older animals, which have developed mechanisms to better regulate glutamate.…”

“…The thinning of inner retina composed of internal limiting membrane to the inner nuclear layer has been recognized in glutamate-induced excitotoxic animal models (Kanno et al 1991), indicating that glutamate injures not only RGC but also other inner retinal cells. However, the excitotoxic effects of glutamate against inner retinal cells other than RGC have not been clarified.…”

“…Glutamate-induced retinal damage in neonatal rats is a well-known animal model of glutamate-induced excitotoxicity in the retina. The retina is not fully developed in newborn rats, taking approximately 3 weeks to mature (Zucker et al 1994), and the degree of retinal damage depends on age at glutamate administration (Kanno et al 1991; Guerin et al 2011). Here, we investigated retinal excitotoxicity in neonatal rats by administering l -glutamate subcutaneously as a single dose on each postnatal day (PND) and evaluated the glutamate-induced retinal damage and target cells on various developing stage by morphological and morphometric examinations.…”

Gene expression profile in retinal excitotoxicity induced by L-glutamate in neonatal rats

“…Glutamate-induced retinal damage in neonatal rats is a well-known animal model of glutamate-induced excitotoxicity. The retina is not fully developed in newborn rats, taking approximately 3 weeks to mature 6 , and the degree of retinal damage depends on the age at glutamate administration 7 , 8 . Recently, the developing stage-dependent retinal thinning induced by L-glutamate in neonatal rats has been reported in detail 9 .…”