Axonal Regeneration of Cat Retinal Ganglion Cells may be Promoted by Nitric Oxide Provided by Nipradilol

Abstract

Abstract: : Purpose: Nipradilol is a donor of nitric oxide (NO) and blocker of alpha and beta adrenoreceptors. It protects death of retinal ganglion cells (RGCs) induced by transection of the optic nerve (OpN) as well as promotes axonal regeneration of axotomized cat RGCs (ARVO '03). We examined either its NO supply or blocking the receptors promotes axonal regeneration of cat and rat RGCs. Methods: Cats were anesthetized with a gas mixture of 1% halothane, 0.5 L/min nitrous oxide, and 1 L/min oxygen. The left OpN was totally cut 40–80 min after nipradilol– or blocker–injection. A segment of the common peroneal nerve was sutured to the cut end. After six weeks, RGCs extending axons to >10 mm and those to >20 mm were labeled with different fluorescent dyes injected into the graft at 10 mm or 20 mm separately. Numbers of regenerated RGCs were estimated with counting dye–labeled cells in 0.23 mm2 squares separated 1 mm each. Results: Promotion of Axonal Regeneration with Nipradilol: An injection of nipradilol, 10(–7) mol/eye, increased numbers of regenerated RGCs (R–RGCs) into 13,884 (average of 4) 6 week after the transplantation, comparing 3,470 (N=8) in no injected (control) retinas. Ratio of R–RGCs with >20 mm axon in R–RGCs with 10–20 mm axon implies axonal growth rate. Average of the 20R/10R ratios in injected retinas was 83% at 6 week, higher than in control retinas, 65%. Nipradilol injection 10 min prior to OpN–cut did not increase numbers of R–RGCs. Effect of Sodium Nitroprusside: Sodium nitroprusside (10(–5) mol) injection increased numbers of R–RGCs into 6,585, 1.90 fold, but caused many pathological R–RGCs. Blocking Adrenoreceptors: None of adrenoreceptor blockers, plazosin (alpha–1), timolol (beta–1), or ICI–118,551 (beta–2) increased the number of R–RGCs nor 20R/10R ratios. Supporting this, R–RGCs numbers were not increased when carboxy–PTIO, NO–scavenger, was injected 30 min prior to nipradilol. Addition of Neurotrophic Factors and cAMP: No additive effect was detected when a combination of BDNF, CNTF and forskolin was injected 30 min after nipradilol, although the combination itself increases numbers of R–RGCs (Watanabe et al., '03). Effect on Rat RGC Regeneration: Injection of nipradilol (5x10 (–7)mol) into the rat eye increased R–RGCs by 1.76 fold (4 week) and 1.46 (5 week), indicating the drug has smaller effect on axonal regeneration of rat RGCs. Conclusions: Axonal regeneration of cat RGCs is promoted mainly by NO provided by nipradilol at very low concentration. Smaller effect of nipradilol on axonal regeneration of rat RGCs may be attributed to few population of beta cells in the rat retina.

Keywords: regeneration • nitric oxide • ganglion cells