Purpose: To investigate whether nitric oxide (NO) and/or cGMP protects the retina from chemical ischemia and underlie somatostatin's neuroprotective effects.
Methods: Eyecups of female Sprague-Dawley rats were incubated with PBS or the chemical ischemia mixture [iodoacetic acid (5 mM)/sodium cyanate (25 mM)] in the absence or presence of (1) arginine (0.05-2.0 mM), the substrate of nitric oxide synthase (NOS); (2) the NO donors sodium nitroprusside (SNP; 0.25-4.0 mM), 3-morpholinosydnonimine (SIN-1; 0.1, 0.3, 1.0 mM), SIN-1 (0.1 mM)/L-cysteine (5 mM, peroxynitrite scavenger), and NONOate (1, 5, 10 microM, slow NO releaser); (3) 8-Br-cGMP (0.1, 0.5, 1.0 mM); (4) BIM23014 (sst(2) receptor agonist; 1 microM), alone or in the presence of (5) the NOS inhibitor N(gamma)-monomethyl-L-arginine (NMMA; 0.5 mM); or (6) the guanylyl cyclase inhibitors 1H-[1,2,4]oxadiazolol [4,3-a]quinoxalin-1-one (ODQ;100 microM) and NS2028 (50 microM) for 60 minutes, at 5%CO(2)/air in 37 degrees C. The effect of SIN-1 (0.1, 0.3, 1.0, or 3.0 mM) on the retina was also examined. Subsequently, the eyecups were fixed and sectioned for choline acetyltransferase (ChAT) immunoreactivity and TUNEL staining.
Results: Arginine and SNP had no effect on the chemical ischemia-induced toxicity. SIN-1, NONOate, and 8-Br-cGMP produced a concentration-dependent protective effect, as shown by ChAT immunoreactivity. TUNEL staining also confirmed the neuroprotective effect of these agents. L-cysteine partially reduced the SIN-1-induced protective effect. SIN-1 alone was toxic only at the highest concentration used (3 mM). NMMA, ODQ, and NS2028 reversed the protective effect of BIM23014.
Conclusions: The results suggest that a NO/peroxynitrite/cGMP mechanism may be important in the protection of the retina from ischemic insult. Furthermore, the NO/sGC/cGMP pathway is involved in the neuroprotective effects of sst(2) ligands against retinal ischemia.