Evidence for glutamate-mediated excitotoxic mechanisms during photoreceptor degeneration in the rd1 mouse retina.

PURPOSE: Kinetic studies of photoreceptor cell death in the retinal degeneration (rd1) mouse model suggest that photoreceptor degeneration could result from cumulative damage. Since alterations in glutamate metabolism have been described in different models of retinitis pigmentosa, we investigated in the present work whether changes in glutamate turnover occur in the degenerating rd1 retina and whether glutamate-mediated excitotoxic mechanisms may contribute to rod photoreceptor death in this model.
METHODS: Free amino acid levels were quantified in rd1 and wild-type retinas using an amino acid analyzer selecting times corresponding to early, intermediate, and terminal phases of rod photoreceptor degeneration. Reverse transcription-polymerase chain reaction (RT-PCR) was used to compare the mRNA expression levels of the glial L-glutamate/L-aspartate transporter GLAST, glutamine synthetase (GS), and vimentin, a marker for retinal glia, between rd1 and wild-type mouse retinas. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of both AMPA and kainate subtypes of ionotropic glutamate receptors, was then daily administered from postnatal day 3 (PN3) to PN21 to rd1 mice while control rd1 mice received only physiological saline solution (7 per treatment). At PN22, the respective numbers of surviving rods in CNQX- and saline-treated mice were estimated using an unbiased stereological approach.
RESULTS: Gradual increases in free glutamate and glutamine levels were observed during photoreceptor degeneration in rd1 retinas and were associated with increases in GLAST and GS expression levels. Administration of CNQX induced a statistically significant morphological rescue of rods (>25%, p<0.05).
CONCLUSIONS: Our data demonstrated that, in the rd1 mouse retina, photoreceptor degeneration was associated with excessive free glutamate levels and with an upregulation of glutamate turnover (i.e., increases in GLAST, GS, and free glutamine levels). This may indicate that 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.