Transient Isquemia of the Retina Results Three Months Later in Altered Erg, Degeneration of Inner Retinal Layers, and Deafferentation of the Contralateral Superior Colliculus: Long–Term Neuroprotection With Brimonidine

Abstract

Abstract: : Purpose: To investigate the effects of transient ligature of the ophthalmic vessels (LOV) with and without brimonidine (BMD) treatment on inner and outer retina, and on the retinotectal projection. Methods: Adult Sprague–Dawley rats (220g) received in their left eyes, 1 hour prior to ischemia, two 5 µl drops of saline or 0.5% BMD. Retinal ischemia was induced by transient ligature of the left ophthalmic vessels for 90 minutes. At 1, 8 and 12 weeks, ERG b–wave amplitudes were recorded from both eyes. Light stimuli were presented for 5 msec at 5 incresing light intensities, ranging from 10^–3–10¹ cd/m². For each intensity, 4–8 consecutive recordings were averaged. After the last ERG recordings, one half of the animals were processed in paraffin cross sections of the retina to determine the thickness of the inner plexiform and nuclear layers (INP&INL) and of the outer nuclear layer (ONL). The other half of the animals were used to investigate the retinotectal projection by injecting cholera toxin subunit B (CTB), 5 days prior to sacrifice in the vitreous of the left eye. Serial coronal sections (40 µm thick) of the midbrain were analyzed with an image analysis system to determine areas densely innervated by CTB–immunoreactive RGC terminals in the visual layers of the contralateral superior colliculus (SC). Results: ERG b–wave amplitudes in vehicle– or BMD–treated animals at 1, 8 and 12 weeks represented 70 (n=7) or 100% (n=8), 46 (n=7) or 71% (n=8) and 32 (n=19) or 79% (n=20), respectively, of their contralateral control values. Three months after LOV mean thickness, expressed as percentages of their contralateral eyes, were 97% (n=7) and 88%(n=7) for IPL&INL and ONL, respectively, in the BMD–treated group and 69%(n=6) and 97%(n=6) for IPL&INL and ONL, respectively, in the vehicle–treated group. The volume of the retino–tectal projection was 2.30±0.12 (n=11), 1.26±0.33 (n=13), or 1.92±0.22 (n=14) for the unlesioned, vehicle– or BMD–treated groups of rats, respectively. Conclusions: Retinal ischemia results 3 months later in alterations of the ERG and degeneration of inner retinal layers as well as degeneration of retino–tectal afferents. BMD has a strong neuroprotective effect in preserving retinal function, as observed with the ERG, cytoarchitecture of inner retinal layers and the retinotectal projection.

Keywords: neuroprotection • ganglion cells • ischemia