Ginkgo biloba protects against intermittent hypoxia-induced memory deficits and hippocampal DNA damage in rats

Abstract

The aim of the present study was to explore the potential protective effect of Ginkgo biloba extract (EGb 761) on intermittent hypoxia (IH)-induced memory deficits and oxidative stress in rats.

Methods

The passive avoidance reflex (PAR) test was employed to assess the effect of concurrent EGb 761 treatment in different dose levels on the memory deficits that were induced by concurrent long-term exposure to IH (21 days). The levels of hippocampal malondialdehyde (MDA), nitric oxide (NO), and intracellular glutathione (GSH) and the activity of glutathione peroxidase (GSH-Px) were estimated. In addition, serum and hippocampal 8-hydroxy-2′-deoxyguanosine (8-OHdG) levels were assessed to study the effect of EGb 761 on hippocampal oxidative DNA damage induced by IH.

Results

Exposure to long-term IH in rats induced marked memory impairment that was indicated by a significant decrease in the retention latency in the PAR test. This effect was accompanied by a significant increase in hippocampal oxidative stress and DNA damage. EGb 761 that was administered in either 50- or 100-mg/kg doses per day reversed IH-induced memory deficits, an effect that was accompanied by a significant decrease in hippocampal MDA and NO levels. The antioxidant defence (GSH and GSH-Px) that was depressed by IH was significantly reactivated by EGb 761. Furthermore, serum and hippocampal levels of 8-OHdG that were elevated by IH were significantly reduced.

Conclusions

EGb 761 can protect against IH-induced memory impairment, oxidative stress and neuronal DNA damage, possibly through multiple mechanisms involving its potential anti-oxidative effect.

Introduction

Sleep-disordered breathing (SDB), a clinical syndrome characterised by repeated episodes of upper airway obstruction during sleep, is now recognised as a significant and highly prevalent public health problem that imposes substantial cardiovascular and neurocognitive morbidities in all age groups (Chang and Chae 2010). Neuropsychological impairments are consistently observed in patients suffering from SDB, and increased systemic markers of oxidative stress and inflammation have been reported in SDB patients (Godoy et al. 2009). Furthermore, grey matter loss within brain regions known for their role in cognitive function have also been found (Morrell and Twigg 2006), suggesting that alterations in oxygen homeostasis during sleep lead to neural cell losses and consequent neurobehavioural morbidities in SDB patients.

In last few years, rodent models have revealed that chronic exposure to intermittent hypoxia (IH) in the absence of significant sleep fragmentation leads to regional neuronal cell losses. This occurs via induction of pro-apoptotic mechanisms, such as increased oxidant stress and inflammatory responses in neural tissue, which ultimately cause impaired memory and learning in the experimental models of testing the cognitive functions (Li et al. 2003).

The use of Ginkgo biloba leaf extract as a therapeutic agent in treatment of many diseases has been reported for thousands of years. At present, G. biloba is one of the most extensively researched medicinal plants in the world and is used by medical professionals to aid the treatment of problems typically associated with aging, such as poor circulation, mental confusion and memory loss (Gertz and Kiefer 2004).

Numerous studies have shown that G. biloba has antioxidant (Arushanian and Beĭer 2008), free radical scavenging (Louajri et al. 2001) and neuroprotective effects (Saleem et al. 2008). Beneficial actions of the extract against ischemia/reperfusion injury (Erbil et al. 2008), hypoxia (Özdemir et al. 2011), cognitive deficits and dementia (Lovera et al. 2007) have also been described.

The cellular mechanisms underlying the multiple effects of G. biloba can be attributed to the different constituents of the extract, which may act independently or synergistically. The most important constituents of G. biloba extract that contribute to its pharmacological effects include flavone glycosides (quercetin, kaempferol, and isorhamnetin) and terpene lactones (ginkgolides and bilobalide) (Mahadevan and Park 2008). The study of the underlying principle behind the therapeutic action of G. biloba on chronic ailments, such as neurodegenerative diseases, cardiovascular diseases and cancer, has focussed on its antioxidant properties. The two proposed mechanisms of the antioxidant action are (1) directly scavenging free radicals and (2) indirectly inhibiting formation of free radicals. G. biloba can scavenge reactive oxygen and nitrogen species (ROS and RNS), such as hydroxyl radicals (OH), peroxyl radical (ROO), superoxide anion radical (O²⁻), nitric oxide radical (NO), hydrogen peroxide (H₂O₂), and ferryl ion species (Louajri et al. 2001). G. biloba can also enhance activities of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase, and/or heme-oxygenase-1, thereby indirectly contributing as an antioxidant (Atmaca et al. 2005).

Although there is a body of evidence showing the potentially beneficial effects of G. biloba extract on neurodegenerative diseases, it is still not conclusive whether its supplementation can improve cognitive functions impaired by exposure to hypoxia in humans. Therefore, we hypothesised that G. biloba may affect the susceptibility to the oxidant mechanisms underlying the neurobehavioural deficits associated with IH that characterises SDB. Hence, the aim of this study was to investigate the effectiveness of standardised G. biloba extract (EGb 761) in protecting animals against memory dysfunctions induced by long-term exposure to IH and to determine its effects on the IH-induced oxidative and nitrosactive (O and NS) stress and DNA damage in the hippocampus of the tested animals.