Resumo

TRPV1 channels are involved in retinal signaling. However, their overactivation seems to induce neuronal death and microglial activation. In this study, we analized the possible participation of TRPV1 receptors in the acute cell loss observed after complete optic nerve transection. We also studied the effects of TRPV1 activation and blockade upon retinal peroxidation and nitration in lesioned rat retinas as well as in normal retinal tissue. TRPV1 total protein expression decreased progressively after optic nerve transection, reaching 62.7% of control values 21 days after axotomy. Immunohistochemical experiments demonstrated that overall TRPV1 expression decreased after optic nerve transection, but increased specifically in the optic nerve layer and in the ganglion cell layer. Fluoro Jade B staining demonstrated that the activation of TRPV1 elicited more intense neuronal death in acutely-lesioned eyes than in control retinas. In lesioned eyes, TRPV1 blockade with capsazepine decreased the number of GFAP-expressing Müller cells (72.5 % of control values) and also decreased protein nitration in the retinal vitreal margin (75.7% of control values), but did not affect lipid peroxidation and retinal ganglion cell survival. Furthermore, retinal explants were treated in vitro with capsaicin, and remarkable protein nitration and TUNEL-positive cells were present. In this case, protein nitration was reduced with high doses of 7-NI and aminoguanidine, which are blockers of the constitutive and inducible nitric oxide synthases, respectively. Apoptosis caused by capsaicin was reduced by AP-5 (54.7 % of control values), an specific NMDA antagonist. We also demonstrated that TRPV1 activation augmented GFAP expression, which was reverted by both TRPV1 antagonism and by 7-NI. Moreover, SNAP, a nitric oxide donor, also positively shifted GFAP expression. Since Müller cells do not express TRPV1, we hypothesize that the increased GFAP expression in these cells could have been induced indirectly by nitric oxide overproduction and peroxinitrite formation caused by TRPV1 activation in neuronal and/or endothelial cells. We conclude that TRPV1 receptor expression decreased after optic nerve injury due to death of TRPV1-containing cells. Furthermore, these data indicate that TRPV1 might be involved in intrinsic protein nitration observed after optic nerve injury, and that neuronal death caused by TRPV1 activation could be partially due to glutamate release and NMDA excitotoxicity.