A supplemental measure of stay-times in the light box on step-through passive avoidance

The avoidance behavior of rats in a typical step-through passive avoidance task was evaluated by using two types of time measurements that differed from traditional step-through latency (STL). The total stay-time in the light box (TL) was calculated as an index of the dark box avoidance throughout the retention trial, counting the time before and after the first step-through. The total stay-time in the far area of the light box (TF) was calculated as an index of the tendency to keep away from the dark box. TL, TF and the STL increased with electrical shock intensity at acquisition trials and gradually decreased through an extinction procedure. The discrepancy between our new measures and the STL was observed when the effects of a new drug, RS-8359, which was reported to ameliorate ischemic brain damage, were examined in ischemic animals. TL, TF and STL decreased in rats receiving brain ischemia, and we found that after treatment with RS-8359, TL and TF increased, but no effect was observed on STL. The discrepancy observed suggests that a short STL does not necessarily imply a loss of avoidance behaviour. Passive avoidance tests can be more revealing about a drug's effects when stay-time measures are used.