Step-down passive avoidance in the rat ontogeny.

Ontogeny of step-down passive avoidance (PA) was studied in 174 Wistar rats at the age of 2, 3, 4, 6, 8 and 13 weeks. Latency to the first descent from the safe bench (DL), number of descents (DN), time to criterion meeting (TC) and time during which animals were shocked (TS), were evaluated within learning (L) and two consecutive retention tests (R1 and R2); R1 took place at short to intermediate intervals (5 min-6 h), R2 24 h after learning. None of 2-week pups mastered the task. The most efficient learning was in 6-week pups, and also three indices of R1 (DN, TC and TS) were best at this age. However, the longest DL (most commonly used memory index in PA) was found in R1 and R2 of 3-week pups, due probably to high fear motivation. In 3-month animals DL was zero in R1. It is emphasized that the development of learning and memory is non-monotonous, which was also seen in our previous studies. There were differences between DL recalled from memory and other three indices (DN, TC and TS) in which further reinforcement within the retention test facilitated the retrieval.