Nitric oxide (NO) is an important signaling molecule in the CNS, regulating neuronal survival, proliferation and differentiation. Here, we explored the mechanism by which NO, produced from the NO donor S-nitroso-acetyl-d-l-
penicillamine (SNAP), exerts its
neuroprotective effect in purified cultures of chick retinal neurons. Cultures prepared from 8-day-old chick embryo retinas and incubated for 24 h (1 day in culture, C1) were treated or not with SNAP, incubated for a further 72 h (up to 4 days in culture, C4), fixed, and the number of cells estimated, or processed for cell death estimation, by measuring the reduction of the metabolic
dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Experimental cultures were run in parallel but were re-fed with fresh medium in the absence or presence of SNAP at culture day 3 (C3), incubated for a further 24 h up to C4, then fixed or processed for the MTT assay. Previous studies showed that the re-feeding procedure promotes extensive cell death. SNAP prevented this death in a concentration- and time-dependent manner through the activation of
soluble guanylate cyclase; this protection was significantly reversed by the
enzyme inhibitors 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) or
LY83583, and mimicked by 8-bromo cyclic
guanosine 5'-phosphate (8Br-cGMP) (GMP) or 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl
indazole (YC-1),
guanylate cyclase activators. The effect was blocked by the NO scavenger
2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (
PTIO). The effect of NO was also suppressed by
LY294002,
Wortmannin,
PD98059,
KN93 or
H89, indicating the involvement, respectively, of phosphatidylinositol-3
kinase, extracellular-regulated
kinases,
calmodulin-dependent
kinases and
protein kinase A signaling pathways. NO also induced a significant increase of neurite outgrowth, indicative of neuronal differentiation, and blocked cell death induced by
hydrogen peroxide.
Cyclosporin A, an inhibitor of the
mitochondrial permeability transition pore considered an important mediator of apoptosis and
necrosis, as well as
boc-aspartyl (OMe) fluoromethylketone (BAF), a
caspase inhibitor, also blocked cell death induced by re-feeding the cultures. These findings demonstrate that NO inhibits apoptosis of retinal neurons in a cGMP/
protein kinase G (PKG)-dependent way, and strengthens the notion that NO plays an important role during CNS development.