Effects of axotomy and intraocular administration of NT-4, NT-3, and brain-derived neurotrophic factor on the survival of adult rat retinal ganglion cells - A quantitative in vivo study

Purpose. To investigate in vivo the survival of retinal ganglion cells (RGC) 4 to 14 days after optic nerve (ON) transection alone or in combination with a single intraocular injection of neurotrophin-4 (NT-4), neurotrophin-3 (NT-3), or brain-derived neurotrophic factor (BDNF). Methods. In adult rats, RGCs were labeled with fluorogold (FG) applied to their main targets in the brain. Seven days later, the left ON was intraorbitally transected, and, in several groups of animals, the left eye received a sham injection or was injected with 5 mu l of 1% bovine serum albumin-phosphate-buffered saline alone or containing 5 mu g of NT-4, NT-3, or BDNF. Four, 5, 7, 9, 12, and 14 days after ON transection, the retinas were examined under fluorescence microscopy to estimate RGC survival. Results. In control retinas, the mean densities (cells/mm(2) +/- SEM) of FG-labeled RGCs were 2421 +/- 55 (n = 20). Four days after axotomy, the densities of RGCs were similar to those observed in control retinas, but 5 and 7 days after axotomy, the mean densities had decreased to 2028 +/- 63 (n = 6) and 1568 +/- 50 (n = 6), respectively. In the group of retinas with sham injection, with vehicle alone or with NT-3, RGC densities also decreased by 7 days to 1261 +/- 71 (n = 5), 1506 +/- 98 (n = 10), and 1474 +/- 125 (n = 4), respectively. However, similar densities to those observed in control retinas were observed 7 days after ON transection in the groups of retinas treated with NT-4 (2505 +/- 91; n = 7) or BDNF (2380 +/- 74; n = 7). Fourteen days after axotomy, RGC densities decreased to 521 +/- 39 (n = 10). Comparable densities were found in the groups that underwent axotomy and either sham injection (533 +/- 51; n = 5), injection of vehicle (588 +/- 19; n = 10), or NT-3 treatment (634 +/- 62; n = 6). However, at this time, higher densities were observed in the groups treated with NT-4 839 +/- 39 (n = 8) or BDNF 1321 +/- 120 (n = 7). Conclusions. Axotomy-induced RGC death first appears by day 5 and reaches 80% of the original RGC population by day 12. NT-4 and BDNF administered intraocularly at the time of axotomy exert a neuroprotective effect on axotomy-induced RGC death, thus increasing the population of surviving RGCs and delaying the onset of axotomy-induced RGC death by approximately 3 days. Intraocular administration of NT-3 did not modify the survival of RGCs after injury.