To evaluate posttreatment intervention, animals were fed with LBP from weeks 5 to 15. This time point was chosen after considering the following: (1) the pattern of compartmentalized neuronal degeneration reported in experimental glaucoma models,
55–57 and (2) the rate of structural and functional changes observed with the circumlimbal OHT approach in Sprague-Dawley rats.
44 As RNFL thinning was observable after 4 weeks of OHT induction, this was reasoned to be a good time point for LBP posttreatment intervention. In the LBP posttreatment group, OHT eyes showed RNFL loss at week 4 (−25 ± 12%), which was equivalent to the OHT-vehicle control (week 4: −23 ± 15%). With LBP intervention from week 5, there was no significant improvement in the rate of RNFL loss (week 15, −28 ± 9%). However, the raw data (see
Supplementary Fig. S2) revealed a small increase in RNFL thickness in both OHT and fellow control eyes at week 8, which was sustained up through week 15. As LBP was administered orally, a corresponding increase in RNFLT in the fellow control could be attributed to the systemic effect of LBP or the contralateral eye effect. When calculating the rate of loss with respective to the fellow eyes, an equal increase in the thickness in these control eyes rendered no detectable change. However, the preservation of pSTR responses in OHT eyes until the end of the study corroborated with the increase in RNFL thickness. Also, LBP posttreatment improved the scotopic responses from week 8, which were otherwise decreased in vehicle control. LBP posttreatment also relatively prevented the RGC loss and ORL thickening. At weeks 12 and 15, the posttreatment functional benefits were similar to those of LBP pretreatment. One of the possible contributors for the improvement in RNFL thickness and retinal functions observed in the posttreatment could be attributed to the neuromodulatory effects of LBP on endothelin-1 (ET-1) and its receptors (ET-A, ET-B) located in the retinal neurons and its vasculature.
35 Increased levels of ET-1 in aqueous humor in patients with POAG
58 and ET-B receptor expression in the optic nerve of the postmortem glaucomatous eyes
59 have been reported. The potential role of ET-1 on the RGC loss has been further investigated by intraocular administration of ET-1
60–62 and experimental IOP elevation.
63,64 Under both conditions, there was a concomitant increase of ET-B receptor expression in the inner retinal layers, which was associated with a preferential apoptotic loss of RGCs. On the other hand, RGC loss was reduced when RGC-5 cells were pretreated with ET-B receptor antagonist
62 and in ET-B–deficient rats.
62,64 Furthermore, ET-1 was associated with a decreased ocular blood flow in animals
60 and human
65,66 and such reductions was inhibited by co-administrating ET-B receptor antagonist.
65 Treatment with Sulfisoxazole, a nonselective endothelin antagonist, improved ERG retinal functions in an acute ischemic insult model.
67 Because LBP pretreatment has shown to exert neuroprotective effects in a chronic OHT model by modulating ET-1, ET-A, and ET-B expressions in RGCs and the retinal vasculature,
35 we speculate that the protective effects on RFNL thickness and retinal function observed in the current study were mediated by similar mechanisms.