Characterization of the nitric oxide and adrenomedullin signaling pathways in normal and diabetic mouse retina

Nitric oxide (NO), a gaseous neuromodulator, has physiological functions in every cell type in the retina. NO often functions through the second messenger cyclic guanosine monophosphate (cGMP). Immunocytochemistry was used to localize and characterize the NO/cGMP signaling pathway in specific regions and cell types within the mouse retina. The NO signaling pathway is activated in eyes of diabetic patients and NO imaging shows increased NO production in diabetic mouse retinas. In contrast, there are no significant changes in neuronal nitric oxide synthase (nNOS) mRNA levels, and nNOS protein is decreased. The signaling peptide adrenomedullin (ADM) can activate calcineurin, a Ca 2+ activated phosphatase that dephosphorylates nNOS at Ser 847 to increase its enzyme activity. ADM levels are elevated in eyes of diabetic patients, and therefore ADM may play a role in the pathology of diabetic retinopathy. The results in this thesis and are the first to show that ADM, and it's receptors were present and functional in the mouse retina. Inhibition of this pathway decreased NO production in high glucose retinal cultures. Treating diabetic mice with the PKC beta inhibitor ruboxistaurin for 5 weeks lowered ADM mRNA levels and ADM-like immunoreactivity, and preserved retinal function as assessed by electroretinography. The results of this thesis indicate that inhibiting the ADM/NO signaling pathway prevents neuronal pathology and functional losses in early diabetic retinopathy.