Evidence for Glutamate–Mediated Excito–Toxic Mechanisms During Photoreceptor Degeneration in the rd1 Mouse Retina

Abstract

Abstract: : Purpose: Kinetic study of photoreceptor cell death in the rd1 mouse retina suggests that photoreceptor degeneration could result from cumulative damages (Clarke et al., Nature, 2000). Since alterations in glutamate metabolism during photoreceptor degeneration have been described in different models of retinitis pigmentosa, we studied glutamate turnover during rod degeneration in rd1 retinas and evaluated whether an increase in glutamate could accelerate rod cell death. Methods: First, free amino acid levels were quantified using an amino acid analyzer selecting times corresponding to early, intermediate and late stages of rod degeneration. Secondly, reverse–transcription polymerase chain reaction (RT–PCR) and Western blot analyses were used to compare expression levels of the glial glutamate transporter GLAST and of glutamine synthetase (GS) between rd1 and control mouse retinas. Finally, glutamate receptor antagonists were daily administered to rd1 mice while control rd1 mice received only physiological saline solution (vehicle). Number of surviving rods in drug– and saline–treated mice were estimated using an unbiased stereological approach and compared. Results: Gradual increases in free glutamate and glutamine levels were observed during photoreceptor degeneration and were associated with increases in GLAST and GS expression levels. Administration of glutamate receptor antagonists induced a moderate but statistically significant morphological rescue of rods. Conclusions: Our results demonstrate that, in the rd1 mouse retina, photoreceptor degeneration is associated with excessive free glutamate levels and with an up–regulation of glutamate turnover (i.e. increases in GLAST, in GS and in free glutamine levels). Excessive glutamate levels further contribute to rod cell degeneration, thus implying the occurrence of non–cell autonomous mechanisms in the degenerative process in the rd1 retina.

Keywords: cell death/apoptosis • degenerations/dystrophies • retinitis