The effects of repeated oral administration of an extract of Ginkgo Biloba (EGB 761) on various behavioral models of stress in rodents were investigated. The models in rats included "learned helplessness," shock-suppressed licking (Vogel conflict test) and forced swimming-induced immobility ("behavioral despair"). The models in mice included shock-suppressed exploration (four plates test), spontaneous exploration (staircase test) and food consumption in a novel situation (emotional hypophagia). Further tests in rats examined the effects of EGB 761 on memory (passive avoidance test) and responsiveness to shock to determine whether the preventive effects observed with EGB 761 in the learned helplessness procedure were due either to drug-induced impairment of memory or to reduced shock sensitivity. In all experiments EGB 761 was administered over 5 days at daily doses of 50 and 100 mg/kg PO. In some experiments (Vogel test, four plates test, staircase test, emotional hypophagia) the effects of acute administration were also investigated. The results showed that repeated administration of EGB 761 (50 and 100 mg/kg/day) before exposure to unavoidable shock (preventive treatment) clearly reduced the subsequent avoidance deficits in the learned helplessness procedure but was less effective when first administered after "helplessness" induction (curative treatment). EGB 761 did not affect performance in the passive avoidance task or alter the animals' response to electric shock, suggesting that the effects observed in the learned helplessness procedure were not due to impaired memory or reduced shock sensitivity. Anxiolytic-like activity was also seen in the emotional hypophagia test in mice where repeated administration of EGB 761 increased the amount of food consumed. EGB 761 after acute or repeated administration had no marked effects in the other models tested. The results suggest that EGB 761 reduces the consequences of stress in some experimental situations but that this effect cannot be assimilated to either classical antidepressant or anxiolytic activity. © 1990.
