Memory restoration or memory block: differential effects of physostigmine depend on the age of the memory trace.

The authors investigated the effect of physostigmine (Ph) on short-term memory underlying the response-to-change test in rats. In the first test trial (acquisition) the rat could observe the white-black T-maze arms through transparent partitions which prevented the entrance to the arms. In the subsequent trial the rat was allowed a free choice between two identical maze arms, one of them being changed from white to black or vice versa. Physostigmine or saline (NaCl) was injected i.p. 1 min after the acquisition trial. When the retention interval between the two trials was short (10 min), NaCl rats showed a significant preponderance of the changed arm choices, whereas after a longer retention interval (20 min) they selected the changed arm in low percentage. This indicated a rapid decline of memory traces underlying the response to the change of brightness. Physostigmine in a dose of 0.1 mg/kg blocked responding to change after 10 min retention interval, when the memory was strong. However, when tested after a 20 min interval, Ph injected rats showed restoration of memory. After a 15 min interval, retention in both NaCl and Ph injected rats was on a similar, intermediate level. A higher dose of Ph (0.3 mg/kg) had no effect on responding to change after any retention interval examined in this experiment.