Abstract
The purpose of the present experiment was to identify the period when the ability of place learning begins to be ontogenically functional in rats. We gave Morris water-maze task for 3 days to 4 groups of rats : 2 conditions that differ in the starting day of the experiment (18 days old and 21 days old) × 2 stimulus conditions (open and closed). Under the open condition, external stimuli could be utilized, but not under the closed condition. It was not until 20 days old that the search time in the training quadrant (where the platform had been placed during the training phase) was over the expected value in the open condition. But during this period, the escape time to the platform was same as that of the closed condition. On the 21 days and after, the escape time under the open condition was shorter than under closed condition. Moreover, in the open condition for the 22 and 23 days old rats, the number of crossings on the annulus where the platform had been located during the training phase was significantly increased, and the search time in the training quadrant was longer than the expected value. These results demonstrate that 20 days and older rats have the ability of place learning. We discussed that the development of the ability of place learning mainly depends on the functional maturity of neural systems, especially hippocampal system.
