Monoamine oxidase enzymes and oxidative stress in the rat optic nerve: age-related changes

Abstract

In this study, age-related changes in the monoamine oxidases (MAO) were studied in the optic nerve (ON) of both young and aged male rats. The aim of the study was to assess the role of MAO in age-related changes in the rat ON and explain the mechanisms of neuroprotection mediated by MAO-B-specific inhibitors. Fifteen three month old and fifteen 26 month old Sprague-Dawley rats were used. The animals were killed by terminal anaesthesia. Staining of MAO, quantitative analysis of images, biochemical assays and statistical analysis of data were carried out. Samples of the ON were washed in water, fixed in Bowen fluid, dehydrated and embedded in Entellan. Histological sections were stained for MAO-enzymatic activities. The specificity of the reaction was evaluated by incubating control sections in a medium either without substrate or without dye. The quantitative analysis of images was carried out at the same magnification and the same lighting using a Zeiss photomicroscope. The histochemical findings were compared with the biochemical results. After enzymatic staining, MAO could be demonstrated in the ON fibres of both young and aged animals; however, MAO were increased in the nerve fibres of the elderly rats. These morphological findings were confirmed biochemically. The possibility that age-related changes in MAO levels may be attributed to impaired energy production mechanisms and/or represent the consequence of reduced energy needs is discussed.

Figure 1

(a) Transversal section of a young rat optic nerve (ON). The black line indicates the connective sheath that envelops all the three fascicles of the ON (1, central; 2, nasal upper; and 3, temporal lower; magnification 100×). (b) Transversal section of an elderly rat ON. There is an increase in the enzymatic staining (total MAO-A-B-C) in the ON of the elderly rats compared with the young rats (a). The black line indicates atrophic ON fibres. The numbers and diameters of the nerve axons are strongly decreased, and the nerve fibres/meningeal membranes ratio is also decreased. The ON is surrounded by orbital adipose tissue (magnification 100×). (c) MAO activity in transversal section of the ON of a young rat. Centrally, cross sections of the ophthalmic artery and the central artery of the retina are observed. All the nerve structures (fascicles of nerve fibres, connective sheath and septa) are well preserved (magnification 250×). (d) MAO activity in a transversal section of the ON of an elderly rat. The ophthalmic artery and its branches, as well as the central artery of the retina, appear obstructed and strongly positive at the enzymatic staining. The connective septa are increased. All the structures of the ON are strongly positive for the MAO activity (magnification 250×).

Figure 2

(a) Histochemical staining of MAO activity in the optic nerve (ON) of a young rat. A few nerve fibres stain positively for MAO. The whole sample appears black and therefore scarcely positive after the reaction (magnification 800×). (b) Histochemical staining of MAO activity in the ON of an elderly rat. An intense positive MAO reaction is apparent. The whole sample appears dark and therefore strongly positive after the reaction (magnification 800×).