Abstract
Abstract: :
Purpose:We have examined acute and chronic changes in the optic nerves of rats using an experimental model of anterior ischemic optic neuropathy (AION). This method was validated only by electrophysiological measurements and gene expression methods previously. This study represents the first quantification of the short and long term effects of this injury model on number of axons in the optic using histological analysis. The goals of current study were to determine:1. a minimal laser power for producing a noticeable change in optic nerve structure; 2. an optimal time for tissue harvesting that demonstrates optic nerve injury and loss and 3. quantification of long–term optic nerve loss. Methods: Adult female Long Evans rats were anesthetized with tribromoethanol. After injection of Rose–Bengal dye into the tail vein, an argon green laser beam was directed at the optic disc area of the right eye with intensity of 8, 10, 12, 16, or 18 pulses, where one pulse is 100 microWatt. The left eye served as a control. Animals were sacrificed at 1, 3, 7, 21 days and 3, 6, or 8 months. Harvested tissue was embedded in resin, sectioned at 1 micron and stained for quantitative analysis. Two random cross–sections of each optic nerve were chosen for axon counting. Results:There was no immediate change in optic nerve microstructure detected regardless of laser intensity applied to optic nerve vessels. In optic nerves harvested 1 through 7 days after laser injury, the axon count was not different from control. Only in tissue with application of 18 laser pulses and analyzed after 1 month was a 20% loss in optic nerve number observed. However, there was quantifiable axon loss in optic nerves harvested 3 through 8 months after laser injury. Specifically after 6 pulses, examined at 6 months, the nerve loss was 18%, with 8 pulses after 8 months, 30% and with 12 pulses after 3 months loss was 42%. Even though a higher laser power application of 18 pulses generated detectable axon loss, it is likely that it damages the retina as well. Conclusions:This model of anterior optic nerve ischemia results in a loss of optic axons of up to 42% compared to control by 3 months after application of laser at a power of 1200 microWatt. The loss of optic axons was not significantly different from control when the optic nerves were examined after less than one month after receiving laser treatment. Histological assessment of axon loss using this laser injury model gives us a baseline and time course for optic axon loss and will allow us to examine the effectiveness of various neuroprotective agents on the long term maintenance of optic axons after ischemic injury.
Keywords: neuro–ophthalmology: optic nerve • ischemia • neuroprotection