The neuroprotective actions of quercetin

Abstract

Trauma-induced spinal cord injury (SCI) is the most prevalent form of spinal cord injury affecting over 80% of the 36,000 Canadians living with this condition. The pathophysiological profile of traumatic SCI consists of an initial stage of direct damage followed by a series of secondary events, including reduced blood flow and increased generation of free radicals that leads to excitotoxicity, oxidative stress, hemorrhagic necrosis, inflammation, and apoptosis. We examined the hypotheses that delayed administration of the flavonoid quercetin inhibits the propagation of secondary events and promotes functional recovery after traumatic SCI by inhibiting inflammatory processes and signaling pathways that promote apoptosis and thereby promoting axon survival. To determine whether delayed quercetin treatment promoted functional recovery following SCI, male Wistar rats were subjected to a spinal cord compression injury by application of a 50 g modified aneurysm clip at the mid thoracic cord level. A treatment regimen of 75 µmol quercetin per kg rat or saline only (controls) was administered for a period of 3 days, 1 week or 2 weeks beginning at 2 weeks post surgery. Delayed quercetin treatment improved locomotion in injured animals although with severe deficit. To determine whether improved functional outcome correlated with improved tissue preservation and reduced scarring, we performed histological examinations at the injury site. In saline treated animals, at 8 weeks post injury we found over 80% of tissue loss with the majority of the remaining cells undergoing apoptosis. However, with 2 weeks delayed quercetin treatment, at least 50% of the tissue was still present at 8 weeks post surgery with a significant reduction of apoptosis. Quercetin treated animals also showed a reduction of reactive gliosis. To determine which intracellular signaling pathways may mediate the protective effects of quercetin, we carried out Western blots and immunocytochemical analyses of a number of potential pro-apoptotic pathways. We found that quercetin reduced the levels of the phosphorylated (activated) forms of the MAPK p38, ERK 1/2 (p42/44) and SAPK/JNK seen after SCI. We conclude that delayed quercetin treatment likely rescues neurons that would otherwise have died between the third and sixth weeks following injury by inhibiting apoptosis of glia cells. Quercetin may be acting via selective inhibition of kinase pathways that have been shown to be involved in apoptosis and cell growth. These findings not only reveal the protective effects of quercetin in reducing secondary damage after chronic SCI but also shed some light on some of the mechanisms underlying its actions.