Alpha2-adrenergic receptors and their core involvement in the process of axonal growth in retinal explants.

PURPOSE: To determine the patterns of α2-adrenergic receptor (α2-AR) subtype expression in normal and degenerated retinas and to analyze the response of these receptors to the α2-AR agonist brimonidine tartrate (BT).

METHODS: The binding characteristics of α2-ARs in the retina were evaluated in experimental and matching sham groups by in vitro quantitative autoradiographic saturation with [(3)H]-clonidine. Retinal explants from juvenile and adult rats with either elevated intraocular pressure or after optic nerve crush (ONC) were cultured with BT over 96 hours in vitro to analyze the effects of BT on axonal growth by videomicroscopy and axon counting. Changes in retinal protein expression by BT were monitored by two-dimensional gel electrophoresis (2D-PAGE) and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS).

RESULTS: The total number of α2-ARs in the retina increased significantly after ONC compared with the sham group. BT supported axonal growth in the juvenile, glaucomatous, and injured retinas (P < 0.004) most effectively at a concentration of 0.001 mg/mL, without influencing the axonal growth rate. Immediate supplementation of BT was more effective than delayed supplementation (P < 0.001). Proteomic analysis revealed treatment-specific expression patterns of glial fibrillary acidic protein (GFAP), glucose-related protein (GRP)58, platelet-activating factor (PAF), and laminin-binding protein (LBP).

CONCLUSIONS: These data are the first to show differences in α2-AR expression in normal and degenerated retinas. BT supports neuronal growth in cultured retinal pieces, suggesting that α2-ARs play a role in retinal metabolism.