Retinal Degeneration in a Hypoglycemic Mouse

Abstract

Abstract: : Purpose: Determine if retinal degeneration occurs in mice made chronically hypoglycemic by a null mutation of the glucagon receptor (Gcgr–/–). Methods:Retinal sensitivity (ERG b–wave) and anatomy were assessed in mice (C57/BL6J background) with a null mutation of the glucagon receptor (Gcgpr–/–) and littermate controls. Dark–adapted mice were anesthetized with either Ketamine (90mg/ml)/ Xylazine(9mg/ml) or Nembutal (5mg/ml: 60mg/kg), and their pupils were dilated with Tropicamide, corneas were moistened with methylcellulose (Moisture Eyes, Bausch Lomb. ERGs were recorded (100gain, 0.3–1000Hz) with a unipolar Burian Allen ERG electrode placed on the cornea and reference in the mouth. 10–ms flashes were delivered under computer control (P–Clamp, Axon Instruments) from a cluster of three 520nm LEDs placed 1cm from the cornea (Maximum intensity of 230µWcm^–2). The animal was maintained near 38⁰C with a heating pad. Using a a OneTouch Ultra glucose meter blood glucose (BG) levels were measured from tail blood before anesthesia, 15min and 60min after injection of anesthetic. Eyes and brains were removed, fixed, embedded and sectioned for light and electron microscopy. Results: At 6 months of age Gcgr+/+, Gcgr+/– and Gcgr–/– mice have normal retinal sensitivity and anatomy. At 12 months only Gcgr+/+, Gcgr+/– have normal sensitivity and anatomy. The Gcgr–/– mice exhibit small b–waves (>3log unit increase in threshold) and highly degenerate retinas (1/3 normal thickness without ganglion cells). Blood glucose levels after12h fast: Gcgr+/+, 109mg/dl; Gcgr+/–, 82 mg/dl; Gcgr–/–, 60mg/dl. Conclusion: Late onset retinal degeneration occurs in chronically hypoglycemic mice. Such long–term, metabolically stressed mice may provide a useful model for studying possible causes of some types of retinal diseases as age–related macular degeneration.

Keywords: age–related macular degeneration • metabolism • photoreceptors