Activation of retinal and optic nerve microglia following optic nerve crush in rats

Background

Traumatic optic neuropathy (TON) is an acute injury of the optic nerve secondary to trauma.As an important immune cell, microglia is involved in a variety of pathologic changes in central nervous system diseases and eye diseases.However, the role of microglia in the lesion development and neural restoration in TON is still unclear.

Objective

This study was to compare the differences of microglial activation between optic nerve axon and retina after optic nerve crush injury, including the morphological changes, number, and distribution of microglia.

Methods

Thirty-five SPF female Sprague-Dawley (SD) rats were numbered from 1 to 35 and randomized into normal control group, sham operation group and modeling 6-hour group, modeling 3-day group, modeling 7-day group, modeling 14-day group and modeling 30-day group according to the random number table.Optic nerve injury models were established by crushing optic nerve at retrobulbar 2 mm for 10 seconds.The same operation was carried out but not clamping optic nerve in the sham operation group, and normal control group without any treatment.The morphological changes, number and distribution of microglia in retina and optic nerve axon were assayed and compared by immunofluorescence staining with lectin-FITC fluorescence labeled antibody detection.The use and care of the experimental animals complied with ARVO.

Results

Microglia were displayed mainly in inner plexiform layer (IPL), and a few of microglia were in inner nuclear layer (INL) and ganglion cell layer (GCL), not any microglia were seen in outer nuclear layer (ONL) and outer plexiform layer (OPL) in the normal control group and sham operation group.Thining synapsis and secondary branches were seen with the dendroid in shape in normal control group.In the model groups, the microglia were more in GCL compared with sham operation group and showed amoeboid microglial cells.The number of microglia in retinas was 6.40±1.52, 7.20±2.05, 12.00±3.54, 14.00±4.06, 18.00±4.36, 18.40±3.13 and 10.80±1.92 in the normal control group, sham operation group, modeling 6-hour group, modeling 3-day group, modeling 7-day group, modeling 14-day group and modeling 30-day group, respectively, and the microglia number were significantly increased in modeling 6-hour group, modeling 3-day group, modeling 7-day group, modeling 14-day group and modeling 30-day group in comparison with the normal control group, and in the modeling 30-day group, the microglial number in retinas was significantly reduced in comparison with the modeling 7-day group and modeling 14-day group (all at P<0.05). The activited microglia in the retinas were nuch more in the modeling 3-day group, modeling 7-day group and modeling 14-day group than that in the sham operation group (P=0.024, 0.009, 0.023). In the optic nerve, the microglial cells were small in size and sparse in arrangement in the normal control group and sham operation group.The cells were enlarged with the amoeboid-like in shape in the model groups and distributed in injury area.The cell number was significantly higher in the modeling 6-hour group, modeling 3-day group, modeling 7-day group and modeling 14-day group than that in the normal control group (P=0.007, 0.001, 0.003, 0.014). The cell number in modeling 30-day group was significnatly reduced in comparison with the modeling 3-day group, modeling 7-day group and modeling 14-day group (all at P<0.05). The activated microglial number were significantly elevated in the modeling 6-hour group, modeling 3-day group, modeling 7-day group compared with the sham operation group (P=0.005, 0.004, 0.030), and the cell number in the modeling 30-day group was evidently lower than that in the modeling 3-day group (P=0.021, 0.004). The cells of activated state in the optic nerve were significantly increased in modeling 6-hour group and modeling 14-day group.

Conclusions

Microglia are activated and keep increasing number in retina and optic nerve at a certain period after optic nerve injury, and these changes are earlier and more distinct in optic nerve axon than those in retina.