Abstract: BACKGROUND: Weightlessness can affect intraocular pressure and ocular structure changes. Methazolamide can reduce intraocular pressure, but its effect on intraocular pressure, retinal and choroid thickness in weightlessness is rarely reported.
OBJECTIVE: To investigate the effect of the methazolamide on intraocular pressure and retinal and choroidal thickness in rats with simulated microgravity for 2 weeks. 
METHODS: Sixteen healthy Sprague-Dawley rats were randomly divided into two groups: control group (n=8) and drug group (n=8). A model of simulated microgravity environment was established in each rat by -30° tail suspension method. All the rats were not treated before tail suspension, and intragastrically given normal saline and methazolamide saline solution in the two groups respectively after tail suspension, once a day, for 14 continuous days. iCare tonometer was used to measure the changes of intraocular pressure in tail-suspended rats before tail suspension and 1, 3, 7, and 14 days after tail suspension. EDI SD-OCT was used to measure the changes of retinal and choroidal thickness in tail-suspended rats before tail suspension and 1, 3, 7, and 14 days after tail suspension. The study protocol was approved by the Animal Experiment Ethics Committee of the PLA Strategic Support Force Characteristic Medical Center.
RESULTS AND CONCLUSION: Intraocular pressure in the control group increased after tail suspension, which was significantly higher at 14 days after tail suspension than the baseline (P < 0.05) as well as significantly higher at 7 days than 23 days after tail suspension (P < 0.05). In the drug group, there was no significant difference in the intraocular pressure before and after tail suspension (P > 0.05). Compared with the control group, the intraocular pressure was significantly reduced in the drug group (P < 0.05). Retinal thickness showed no significant changes in each group before and after tail suspension (P > 0.05), and there was also no significant difference between the two groups (P > 0.05). In the control group, choroidal thickness was increased after tail suspension, and became thicker at 3 days than 1 day after tail suspension (P < 0.05). However, there was no significant difference at 7 and 14 days after tail suspension (P > 0.05). In the drug group, choroidal thickness was increased after tail suspension, which was significantly increased at 3, 7, 14 days compared with 1 day after tail suspension (P < 0.05). However, there was no significant difference at 3 days and 7, 14 days after tail suspension (P > 0.05). Compared with the control group, the choroidal thickness was significantly reduced in the drug group (P < 0.05). To conclude, 2-week simulated microgravity has a significant effect on the rat’s intraocular pressure. With the extension of tail suspension time, the intraocular pressure is increased and become significantly elevated at 14 days after tail suspension. The use of methazolamide can effectively inhibit the elevation of intraocular pressure. Moreover, 2-week simulated microgravity has no obvious effect on the retinal thickness, but has a significant effect on the choroidal thickness of rats. With the extension of tail suspension time, the choroidal thickness is increased, while the use of methazolamide can effective reduce the increase in choroidal thickness.

Key words: simulated microgravity, methazolamide, intraocular pressure, retinal thickness, choroidal thickness