Glaucoma and the Nitric Oxide System

Glaucoma is a disease characterized, not only by an increase in pressure within the eye (ocular hypertension), but also by the injury and death of nerve cells (ganglion neurons) that carry visual information to the brain. Most current drug therapies for glaucoma are directed toward decreasing ocular hypertension but do little to improve the survival of already damaged ganglion cells. Indeed, such drugs are not always effective in decreasing pressure and may compromise nerve cell survival by constricting arteries and decreasing blood supply. In contrast, the current grant proposes to investigate a new type of natural cell regulator, (nitric oxide or NO), which may not only decrease pressure in the eye but also improve blood circulation to ganglion cells in the retina. The proposed studies will investigate the exciting possibility that certain forms of glaucoma may be due to a natural defect in the production of this substance. If so, this would represent the first identification in humans of a specific biochemical defect that can lead to glaucoma. The data gathered from these studies will also supply important data for developing drugs based upon the nitric oxide principle. Such drugs should be of use whether or not individuals have an endogenous defect in the NO system. There are several facts which have led to the our hypothesis concerning a possible defect in the NO system. The first has to do with the action of a new class of agents (the nitrovasodilators) found by the Principal Investigator’s (PI’s) Laboratory to lower ocular pressure in animals. These agents (which mimic the action of nitric oxide) turn out to be much more effective in animals with elevated pressure and in humans with glaucoma than in humans or animals with normal ocular pressure. This observation, plus other data, suggest that, with aging, or in certain forms of glaucoma, there could be a defect in the eye’s ability to make nitric oxide. In the presence of such a defect, the nitrovasodilators would be expected to be more effective. A second clue has come from clinical studies showing that many glaucoma patients show abnormal reactivity of systemic blood vessels, with a tendency for their vessels to undergo constriction. Such a condition would be explained if there were a biochemical defect, in these individuals, in the production of nitric oxide. Because nitric oxide normally regulates dilation of vessels, this defect would make their vessels much more likely to constrict. In the eye, such a defect would increase the resistance of aqueous humor leaving the eye, raising intraocular pressure. It would also tend to compromise the viability of damaged axons, compounding (or even underlying) the visual defect in glaucoma. Finally, the most intriguing item of evidence supporting our hypothesis is the recent discovery, in the gastrointestinal system, of the first reported disease due to a defect in the nitric oxide system. In this condition – pyloric stenosis of infants – the muscle controlling movement of food from the stomach to the intestine shows too much contraction, which effectively causes an obstruction and increase in pressure in the stomach. Interestingly, it turns out that these patients have a defect in the enzyme which normally makes nitric oxide (which, in turn, normally causes the pyloric muscle to relax). What this report does is to supply a precedent showing that a disease can be caused by a defect in the production of nitric oxide. Given the evidence described above, it becomes an exciting possibility to consider the existence of a similar defect in nitric oxide synthesis in the eyes of aging patients or in patients with glaucoma. It is possible that the cause of glaucoma in certains groups of glaucoma patients results from a defect in the enzyme (NO Synthase) which makes nitric oxide. In this grant, we will use a special chemical stain for NO Synthase to examine the presence, amount, and distribution of NO Synthase in eyes of deceased patients who have donated their eyes for research. We will examine what happens to the enzyme during the aging process in normotensive patients and what happens in patients known to have had various forms of glaucoma. We will also use the techniques of molecular biology (ie., in situ hybridization) to determine if the genes which control the synthesis of the enzyme are disordered. Finally, we will perform test tube enzymatic assays of nitric oxide synthesis using samples of donated human tissue. With these techniques, we will determine whether the enzyme is missing or decreased in certain types of glaucoma. We will investigate whether its activity normally decreases with aging, a condition which could place our elderly population at increased risk for glaucoma. We will determine if certain nerves that normally make nitric oxide are missing or decreased in number. Currently, nothing is known about how nitric oxide is made and distributed in the human eye. (The few studies that have been done have been carried out in the rat, which has a different pressure releasing mechanism than do humans.) Therefore, in addition to determining whether there is a defect in nitric oxide with aging or in glaucoma, the proposed studies will also supply important data about how nitric oxide works in the normal human eye. In addition to effects on vascular and other types of smooth muscle, this regulator exerts important actions on nerve cell communication. Therefore, the present studies will supply important information relevant to many aspects of ocular functioning, including the retina (forming visual images), iris (regulating pupil size), and ciliary muscle (regulating near and far vision), in addition to the fluid outflow system that regulates pressure.