A time course analysis of systemic administration of aqueous licorice extract on spatial memory retention in rats.

In the present study, the time course of the effects of Glycyrrhiza glabra L. (Leguminosae) aqueous extract (GE), administered systemically to rats, on the spatial memory retention in the Morris water maze was investigated. The dose of glycyrrhizin (GL), i. e., 0.5, 2.5 and 5 mg/mL in daily water intake of GE was administered to three groups of rats. The first, second and third groups received GE for 1, 2 and 4 weeks, respectively (each group included 3 subgroups). Three additional control groups of animals received only tap water during the same periods of time. After terminating the treatments, all animals were trained for four days; each day included one block and each block contained four trials. Test trials were conducted 48 h after the completion of the training period. Nicotine (1 microg/side) was infused into the CA1 region of the hippocampus as a positive drug control. GE treatment decreased both escape latency and traveled distance, but not swimming speed, compared with control, suggesting significant spatial memory retention enhancement by GE. Statistical analysis did not show any significant difference between GE-treated animals and the nicotine group in escape latency and traveled distance. At the end of the testing trials plasma samples were collected and the concentrations of glycyrrhetinic acid (GA) as a major metabolite of GL were measured in the different groups of treated rats. The maximum concentration was observed after four weeks of GE administration at 5 mg/mL of GL. These results showed that the enhancement effect of GE on spatial memory retention does not correlate with GA blood levels.