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In Vivo Protection against Retinal Neurodegeneration by Sigma Receptor 1 Ligand (+)-Pentazocine

¹From the Departments of Cellular Biology and Anatomy, ²Ophthalmology, and ³Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia.

PURPOSE. To evaluate the neuroprotective properties of the sigma receptor 1 (R1) ligand, (+)-pentazocine in an in vivo model of retinal neurodegeneration.

METHODS. Spontaneously diabetic Ins2^Akita/+ and wild-type mice received intraperitoneal injections of (+)-pentazocine for 22 weeks beginning at diabetes onset. Retinal mRNA and protein were analyzed by RT-PCR and Western blot analysis. Retinal histologic sections were measured to determine total retinal thickness, thicknesses of inner–outer nuclear and plexiform layers (INL, ONL, IPL, INL), and the number of cell bodies in the ganglion cell layer (GCL). Immunolabeling experiments were performed using antibodies specific for 4-hydroxynonenal and nitrotyrosine, markers of lipid peroxidation, and reactive nitrogen species, respectively, and an antibody specific for vimentin to view radial Müller fibers.

RESULTS. R1 mRNA and protein levels in the Ins2^Akita/+ retina were comparable to those in the wild-type, indicating that R1 is an available target during the disease process. Histologic evaluation of eyes of Ins2^Akita/+ mice showed disruption of retinal architecture. By 17 to 25 weeks after birth, Ins2^Akita/+ mice demonstrated 30% and 25% decreases in IPL and INL thicknesses, respectively, and a 30% reduction in ganglion cells. In the (+)-pentazocine-treated group, retinas of Ins2^Akita/+ mice showed remarkable preservation of retinal architecture; IPL and INL thicknesses of (+)-pentazocine-treated Ins2^Akita/+ mouse retinas were within normal limits. The number of ganglion cells was 15.6 ± 1.5 versus 10.4 ± 1.2 cells/100 µm retinal length in (+)-pentazocine-treated versus nontreated mutant mice. Levels of nitrotyrosine and 4-hydroxynonenal increased in Ins2^Akita/+ retinas, but were reduced in (+)-pentazocine-treated mice. Retinas of Ins2^Akita/+ mice showed loss of the uniform organization of radial Müller fibers. Retinas of (+)-pentazocine-treated mice maintained the radial organization of glial processes.

CONCLUSION. Sustained (+)-pentazocine treatment in an in vivo model of retinal degeneration conferred significant neuroprotection, reduced evidence of oxidative stress, and preserved retinal architecture, suggesting that R1 ligands are promising therapeutic agents for intervention in neurodegenerative diseases of the retina.