Abstract
Pharmacologic preconditioning with morphine has been shown to protect several kinds of tissues against ischemia-reperfusion injury. The aim of the present study was to investigate whether intravitreal administration of morphine induces structural protection against ischemic damage in a rabbit model of ischemic retinopathy. Twenty-eight male white New Zealand rabbits were used. Animals in saline control group received 0.1 mL of phosphate-buffered saline (PBS) intravitreally with no postinjection ischemia. In the saline-control ischemia group, 15 minutes after injection of PBS, retinal ischemia was induced by raising intraocular pressure to 150 mmHg for 60 minutes. In three treatment-ischemia groups, morphine (1, 5, and 10 micromol/L) was administered intravitreally 15 minutes before induction of ischemia. In another experiment, naloxone (40 micromol/L) was administered 5 minutes before intravitreal administration of morphine (10 micromol/L) followed by 60 minutes of ischemia to investigate the role of opioid receptors in mediating the possible protective effect of morphine. Toxicity controls were performed with morphine (10 micromol/L) and naloxone (40 micromol/L) without ischemia. Histologic evaluation was performed for all groups on the seventh postoperative day. Sixty minutes of ischemia led to severe cell loss in ganglion cell layer and thinning of the inner nuclear layer in saline-control ischemia compared to that of the nonischemia control group (P < 0.001). Thickness of the inner plexiform layer to the inner limiting membrane was significantly increased due to edema (P < 0.001). Administration of morphine in higher doses (5 and 10 micromol/L) significantly improved all of the above mentioned indices (P < 0.05). Administration of naloxone 15 minutes before morphine reversed most of the morphine protective effects. Morphine pretreatment provides significant histologic protection against ischemic injury in rabbit retina. Pharmacologic evidence suggests that this protective phenomenon may be mediated in part by opiate receptors.
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