Antidepressant effects of Soyo-san on immobilization stress in ovariectomized female rats

Soyo-san is a traditional oriental medicinal formula, a mixture of 9 crude drugs, and it has been clinically used for treating mild depressive disorders. The purpose of the study was to examine the effect of Soyo-san on repeated stress-induced alterations of learning and memory on a Morris water maze (MWM) task and also the anxiety-related behavior on the elevated pulse maze (EPM) in ovariectomized female rats. We assessed the changes in the reactivity of the cholinergic system by measuring the immunoreactive neurons of choline acetyltransferase (ChAT) and reactivity of acetylcholinesterase (AChE) in the hippocampus, and the serum levels of corticosterone were assessed after behavioral testing. The female rats were randomly divided into three groups: the nonoperated and nonstressed group (normal), the ovariectomized and stressed group (control), and the ovariectomized, stressed and Soyo-san treated group (SOY). The rats were exposed to immobilization stress (IMO) for 14 d (2 h/d), and Soyo-san (400 mg/kg, i.p.) was administered 30 min before IMO stress. Treatments with SOY caused significant reversals of the stress-induced deficits in learning and memory on a spatial memory task, and it also produced an anxiolytic-like effect on the EPM, and increased the ChAT and AChE reactivities (p<0.05, respectively). The serum level of corticosterone in the SOY group was significantly lower than that in the control group (p<0.05). These results suggest that Soyo-san might prove to be an effective antidepressant agent.