Alteraciones en el sistema nervioso durante la biabetes experimentaltratamiento con antioxidantes

Resumen

Diabetes mellitus is an endocrine disorder of carbohydrate metabolism resulting primarily from, either inadequate insulin release (Type 1 insulin-dependent diabetes mellitus), or insulin insensitivity which is coupled with inadequate compensatory insulin release (Type 2 non-insulin-dependent diabetes mellitus). Diabetes mellitus is manifested by a misregulation of plasma glucose levels, with elevated glucose being the primary observation. Although strict glycemic control is a desirable way to prevent diabetes complications, this is seldom achievable and, with time, this misregulation of plasma glucose often results in diseases related to the visual and circulatory systems. Clearly, adjuvant therapies are needed to help in preventing or delaying the onset of diabetic complications. It has been repeatedly suggested that oxidative stress, an imbalance situation between pro-oxidant agents and antioxidants in the body (Sies, 1991), may play an important role in the pathogenesis of late diabetes complications (Baynes et al., 1996). It is not clearly evident whether increased oxidative stress has a primary role in the pathogenesis of diabetic complications, or if it is simply the consequence of these complications (Baynes et al., 1999), but it is clear that the elevated glucose levels observed in diabetes and the existence of oxidative stress are inseparable (Packer et al., 2001). Hyperglycemia has the effect of reducing antioxidant levels and concomitantly increasing the production of free radicals. These effects contribute to the tissue damage associated with diabetes by, among other mechanisms, modifying or inactivating proteins, altering nuclear structures and peroxidation of lipid membranes. In addition, this increase in free radicals leads to alterations in the redox potential of the cell with subsequent activation of redox-sensitive genes (Bonnefont-Rousselot, 2002). Apoptosis is a type of programmed cell death in which the dying cell shows highly conserved morphological changes including chromatin compaction, membrane blebbing and cell shrinkage. Multiple lines of evidence support the fact that oxidative stress can cause cell death via apoptosis, and this process may prevent by the up-regulation of an antioxidant pathway, and is considered to act as a free radical scavenger (Jang et al., 2004). Programmed cell death is important for sculpting tissues and destroying harmful cells, but apoptosis in excess can also be harmful and contribute to various diseases, such as diabetic complications (Asnaghi et al., 2003, Li et al., 2002). Diabetes mellitus has long been considered a risk factor for the development of vascular diseases. Diabetic retinopathy is a frequent complication of diabetes that has been included, together with nephropathy and neuropathy, in the microangiopathy triad. However, there is more and more evidence that retinal microangiopathy is just a facet of a more global retinal dysfunction occurring in diabetes and, that neurodegeneration is an important component of diabetic retinopathy. There are studies reporting that both, experimental diabetes in rats and diabetes mellitus in humans, are accompanied by increased apoptosis of retinal neural cells and that neuronal cells begin to die soon after the onset of experimental diabetes in rats (Barber et al., 1998, Asnaghi et al., 2003). An increase in neural cell apoptosis could also contribute to microangiopathy because the apoptotic cells may include glia. Glial cells induce barrier properties in brain and tight junction protein expression in retinal vascular endothelial cells, so retinal glial cell death, could lead to a loss of endothelial cell barrier properties (Gardner et al., 1997). Furthermore, epidemiologic evidence has suggested that diabetes mellitus significantly increases risk for the development of Alzheimer's disease, in a manner independent on vascular risk factors (Grossman, 2003). In this way, in the recent years, evidence is emerging that diabetes affects the central nervous system by impairing the functional and structural integrity in the brain of diabetic patients. Long-term effects of diabetes on the brain are manifested at the structural, neurophysiological and neuropsychological level, and the emerging view is that the diabetic brain features many symptoms that are best described as accelerated brain ageing (Biessels et al., 2002). Decreased peripheral glucose regulation has been shown to be associated with decreased general cognitive performance, memory impairment, and atrophy of the hippocampus in humans, a brain area that is central for learning and memory (Convit et al., 2003). The causes why cognitive disturbances progress to dementia are speculative (Lupien et al., 2003), however, one of the mechanisms by which hyperglycemia is known to cause neural degeneration is via the increased oxidative stress that accompanies diabetes. This is further evidenced by the observation that metabolic and oxidative insults often cause rapid changes in glial cells (Baydas et al., 2003). Other experimental studies (Li et al., 2004) suggested that neuronal apoptosis, which is related to caspase-3 activation and precence of positive-TUNEL nuclei, may play an important role in neuronal loss and impaired cognitive function. Additionally, in the hippocampus of streptozotocin-treated rats, not only a strong increase in oxygen reactive species is observed, but also a persistent activation of caspase-3 (Aragno et al., 2002). Lutein belong to the xanthophyll family of carotenoids that are not synthesized within the body and therefore have to be provided by dietary intake. Lutein is found in high amounts in human serum, and is abundantly present in dark green, leafy vegetables (spinach, kale, collard greens, and others), corn, and egg yolks. Lutein and zeaxanthin are the two major components of the macular pigment of the retina. Lutein appears to have affinity for the peripheral retina and rods, while zeaxanthin seems to be preferentially taken up by the cones of the macula (Sommerbur et al., 1999). Even so, lutein can be found in almost all ocular tissues (Ahmed et al., 2005). Known mostly because its importance for eye health, lutein is also important for the prevention of cardiovascular disease, stroke or lung cancer (Alves-Rodrigues et al., 2004). Lutein and zeaxanthin differ from other carotenoids in that each one of them has two hydroxyl groups, one on each side of the molecule. The hydroxyl groups appear to control the biological function of these two xanthophylls, that act as powerful antioxidants and filter high-energy blue light (Young et al., 2001). In addition lutein, has properties as peroxynitrite scavenger, being also useful in a nitrosative stress situations (Panasenko et al., 1995). Docosahexaenoic acid (DHA), a dietary essential omega-3 fatty acid, concentrates in membrane phospholipids at synapses and in retinal photoreceptor (Bazan, 2005). Recently studies demonstrate that a decreased DHA serum content correlates with cognitive impairment (Gamoh et al., 2001). Moreover, epidemiological studies suggest neuroprotective consequences of diets enriched in omega-3 fatty acids (Morris et al., 2003). A recent study reported a correlation between carotenoids and DHA levels and severity of Alzheimer disease (Wang et al., 2008). However, the effect of lutein and DHA on cognitive impairment associated with type 2 diabetes mellitus is not well known. In this study, we try to address the cellular mechanisms of the diabetes-induced damage and examined whether DHA and lutein could attenuate the degenerative changes in the diabetic hippocampus in a rodent model of diabetes.