Strain Specific Responses in a Microbead Rat Model of Experimental Glaucoma

Sigma-1 receptor agonist, (+)-pentazocine, is neuroprotective in a Brown Norway rat microbead model of glaucoma

PURPOSE

Glaucoma is a worldwide leading cause of irreversible blindness. Standard treatments lower intraocular pressure (IOP). Novel treatments to prevent optic nerve (ON) degeneration are needed. Here, we investigate the hypothesis that sigma-1 receptor (S1R) agonist (+)-pentazocine (PTZ) is neuroprotective in a Brown Norway (BN) rat, microbead model of glaucoma.

METHODS

BN rats (9-11 weeks, male and female) were treated by intraperitoneal injection, 3 times per week with (+)-PTZ (2 mg/kg) or vehicle (VEH) alone. Treatment started 1 week prior to intraocular injection of polystyrene microbeads to elevate IOP. IOP was measured 2-3 times per week. Five weeks post microbead injection, rats were euthanized. ONs were removed, then fixed and processed for 63x oil, light microscope imaging of toluidine blue stained ON cross sections. To facilitate comparison of ON morphology from VEH and (+)-PTZ treated rats with similar ocular hypertensive insults, rats were assigned to low (IOP ≤15.8 mmHg), moderate (15.8 < IOP <28.0 mmHg), and high (IOP ≥28.0 mmHg) groups based on average IOP in the microbead injected eye. Axon numbers, axon density, axonal and glial areas, axon loss, and axon size distributions of naïve, bead, and contralateral ONs were assessed using QuPath program for automated image analysis.

RESULTS

(+)-PTZ treatment of BN rats protected ONs from damage caused by moderate IOP elevation. Treatment with (+)-PTZ significantly reduced axon loss and glial areas, and increased axon density and axonal areas compared to ONs from VEH treated rats with moderate IOP. (+)-PTZ-mediated neuroprotection was independent of IOP lowering effects. At average IOP ≥28.0 mmHg, (+)-PTZ treatment did not provide measurable neuroprotection. ONs from contralateral eyes exhibited subtle, complex changes in response to conditions in the bead eyes.

CONCLUSIONS

S1R agonist (+)-PTZ shows promise as a neuroprotective treatment for glaucoma. Future studies to understand the complex molecular mechanisms by which (+)-PTZ provides this neuroprotection are needed.

Differential Retinal Ganglion Cell Vulnerability, A Critical Clue for the Identification of Neuroprotective Genes in Glaucoma

Retinal ganglion cells (RGCs) are the neurons in the retina which directly project to the brain and transmit visual information along the optic nerve. Glaucoma, one of the leading causes of blindness, is characterized by elevated intraocular pressure (IOP) and degeneration of the optic nerve, which is followed by RGC death. Currently, there are no clinical therapeutic drugs or molecular interventions that prevent RGC death outside of IOP reduction. In order to overcome these major barriers, an increased number of studies have utilized the following combined analytical methods: well-established rodent models of glaucoma including optic nerve injury models and transcriptomic gene expression profiling, resulting in the successful identification of molecules and signaling pathways relevant to RGC protection. In this review, we present a comprehensive overview of pathological features in a variety of animal models of glaucoma and top differentially expressed genes (DEGs) depending on disease progression, RGC subtypes, retinal regions or animal species. By comparing top DEGs among those different transcriptome profiles, we discuss whether commonly listed DEGs could be defined as potential novel therapeutic targets in glaucoma, which will facilitate development of future therapeutic neuroprotective strategies for treatments of human patients in glaucoma.