The high morbidity, high socioeconomic costs and lack of specific treatments are key factors that define the relevance of brain pathology for human health and the importance of research on neuronal protective agents. Epidemiological studies have shown beneficial effects of flavonoids on arteriosclerosis-related pathology in general and neurodegeneration in particular. Flavonoids can protect the brain by their ability to modulate intracellular signals promoting cellular survival. Quercetin and structurally related flavonoids (myricetin, fisetin, luteolin) showed a marked cytoprotective capacity in in vitro experimental conditions in models of predominantly apoptotic death such as that induced by medium concentrations (200 µM) of H 2 O 2 added to PC12 cells in culture. Nevertheless, quercetin did not protect substantia nigra neurons in vivo from an oxidative insult (6-hydroxydopamine), probably due to difficulties in crossing the blood-brain barrier. On the other hand, treatment of permanent focal ischemia with a lecithin/quercetin preparation decreased lesion volume, showing that preparations that help to cross the blood-brain barrier may be critical for the expression of the effects of flavonoids on the brain. The hypothesis is advanced that a group of quercetin-related flavonoids could become lead molecules for the development of neuroprotective compounds with multitarget anti-ischemic effects. Key words Brain vascular pathology and oxidative stressIt is known that brain pathology in the form of cerebrovascular and neurodegenerative disease is a leading cause of death all over the world, with an incidence of about 2/1000 and an 8% total death rate (1-3). Moreover, stroke and dementia are a source of high individual and family suffering mainly because of the lack of efficient therapeutic alternatives. The latter motivates research efforts to identify the mechanisms of neuronal death and to discover new compounds to control them.Neuronal death in stroke is a complex event involving failure of metabolic processes, excitotoxicity, loss of calcium homeostasis and oxidative stress, among other factors (4). During ischemic stroke, a decrease in metabolic energy in the form of ATP affecting membrane ionic pumps leads to an increase in intracellular Ca 2+ and Na + concentrations and to increased glutamate

Flavonoids are an important group of recognized antioxidants ubiquitous in fruits, vegetables and herbs. There are epidemiological evidences for the stroke-protecting capacity of flavonoids and while the neuroprotective power of complex extracts rich in flavonoids like those of Ginkgo biloba, green tea or lyophilized red wine have been demonstrated in several studies, neuroprotection by individual flavonoids has been poorly studied in vivo. The neuroprotective capacity of individual flavonoids was studied in PC12 cells in culture and in a model of permanent focal ischemia (permanent Middle Cerebral Artery Occlusion - pMCAO). In the in vivo experiments, flavonoids were administered in lecithin preparations to facilitate the crossing of the blood brain barrier. The simultaneous incubation of PC12 cells with 200 micro M hydrogen peroxide (H2O2) and different flavonoids for 30 min resulted in a conspicuous profile: quercetin, fisetin, luteolin and myricetin significantly increased cell survival while catechin, kaempherol and taxifolin did not. Quercetin was detected in brain tissue 30 min and 1 h after intraperitoneal administration. When one of the protective flavonoids (quercetin) and one of those that failed to increase PC12 cell survival (catechin) were assessed for their protective capacity in the pMCAO model, administered i.p. 30 min after vessel occlusion, quercetin significantly decreased the brain ischemic lesion while catechin did not. It is concluded that when administered in liposomal preparations, flavonoids structurally related to quercetin could become leads for the development of a new generation of molecules to be clinically effective in human brain ischemia.

On the basis of previous work showing that flavonoids structurally related to quercetin are neuroprotective for cells in culture, this work was directed towards determining if several flavonoids (quercetin, fisetin and catechin) could acutely and by an intraperitoneal (IP) route reach significant cerebral concentrations and either prevent or facilitate recovery from a brain lesion induced by focal ischemia in rats. Aqueous and liposomal preparations of quercetin, fisetin and catechin were administered IP in a single dose and assessed in the brain by HPLC at 30 min, 1 h, 2 h and 4 h. Ischemic damage from focal middle cerebral artery occlusion was assessed spectrophotometrically with 2,3,5,-triphenylltetrazolium chloride (TTC). Infarct volume was assessed by an image analysis system following perfusion with TTC. The status of the cerebral tissue was evaluated by hematoxylin-eosin. Flavonoids administered in aqueous preparations were undetected in the brain. Cerebral concentrations of catechin (10.5 ng/g), fisetin (8.23 ng/g) and quercetin (509 ng/g) were detected in the brain only after IP injection of the liposomal preparations. Spectrophotometric analysis of brain tissue with the TTC-technique showed that liposomal quercetin reduced ischemic damage and infarct volume after permanent occlusion of the middle cerebral artery (ischemic: 41.3 mm3 vs liposomal quercetin: 17 mm3). In liposomal quercetin-treated animals there was also recovery of the cytoarchitecture in ischemic areas of striatum and cortex. Although a liposomal preparation of fisetin had similar effects, catechin failed to protect brain tissue. In conclusion, early administration of liposomal preparations of quercetin and structurally related flavonoids are beneficial and neuroprotective in experimental focal ischemia.

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The origin and/or the evolution of brain diseases of high morbidity and mortality such as cerebrovascular diseases, Alzheimer's Dementia or Parkinson´s disease have been linked to oxidative stress. Epidemiological or clinical descriptive studies have shown that diets with predominant vegetarian composition or popular beverages like green tea or red wine have beneficial effects on general pathological markers of oxidative stress. The flavonoids, ubiquitous polyphenols in plants and vegetables, have been identified as mainly by responsible for these actions. In apparent agreement with these data, numerous in vitro and in vivo studies have demonstrated a neuroprotective capacity of flavonoids against oxidative or excitotoxic aggressions. Oral flavonoids undergo several metabolic steps and circulate in the blood mainly as metabolites. These metabolites by themselves or after cleavage into non-metabolised flavonoids in the brain would be the active molecules. The intracellular targets of flavonoids are multiple, from chelation of iron to homeostasis of calcium as well as scavenging of free radicals and involvement through kinase modulation, with key intracellular signalling cascades. As key regulators of cell reactivity against oxidative aggressions, the flavonoid molecule can become an ideal template for compounds therapeutically active in stroke, dementia or aging.