Isoforms of nitric oxide synthase in the optic nerves of rat eyes with chronic moderately elevated intraocular pressure

PURPOSE. To investigate the hypothesis that nitric oxide (NO) in the optic nerve heads of mts with chronic moderately elevated intraocular pressure (I OP) contributes to neurotoxicity of the retinal ganglion cells, the presenc e of the three isoforms of nitric oxide synthase (NOS) was determined in th e tissue. METHODS. Unilateral chronic moderately elevated IOP was produced in rats by cautery of three episcleral vessels. Histologic sections of optic nerves f ront eyes with normal IOP and with chronic moderately elevated IOP were stu died by immunohistochemistry and by immunoblot analysis. Polyclonal antibod ies to NOS-1, NOS-2, NOS-3, and glial fibrillary acidic protein (GFAP) were localized with immunoperoxidase. RESULTS. In the optic nerve of rat eyes with normal IOP, NOS-1 was constitu tively present in astrocytes, pericytes and nerve terminals in the walls of the central artery, NOS-2 was not present in eyes with normal IOP. In thes e eyes, NOS-3 was constitutively present in the vascular endothelia of larg e and small vessels. Rat eyes treated with three-vessel cautery had sustain ed elevated IOP (1.6 fold) for at least 3 months. In these eyes, no obvious changes in NOS-1 or NOS-3 were noted. However, at time points as early as 4 days of chronic moderately elevated IOP, NOS-2 appeared in astrocytes in the optic nerve heads of these eyes and persisted for up to 3 months. Immun oblot analysis did not detect differences in NOS isoforms. CONCLUSIONS. The cellular distributions of constitutive NOS isoforms in the rat optic nerve suggest physiological roles for NO in this tissue. NOS-1 i n astrocytes may produce NO as a mediator between neighboring cells. NO, pr oduced by NOS-1 in pericytes and nitrergic nerve terminals and by NOS-3 in vascular endothelia, is probably a physiological vasodilator in this tissue . In eyes with chronic moderately elevated IOP, NOS-2 is apparently induced in astrocytes. The excessive NO production that is associated with this is oform may contribute to the neurotoxicity of the retinal ganglion cells in eyes with chronic moderately elevated IOP.