Modulation of memory processes induced by stimulation of the entorhinal cortex

This study investigated the effects of posttrial electrical stimulation of the entorhinal cortex in rats (3 mA or 300 microA; 60 sec, 3 hr or 6 hr after training) on retention of inhibitory or active avoidance responses. In the inhibitory avoidance task 3 mA, but not 300 microA, 60 sec after training produced retrograde amnesia. The 300 microA stimulation produced amnesia when given 3 hr after training, but 3 mA was without effect. Neither level of stimulation affected retention if given 6 hr after training. In the active avoidance task 300 microA given 60 sec and 3 hr after training significantly facilitated acquisition in comparison with unoperated, but not operated control rats. The 3 mA stimulation had no effect in the active avoidance task. Cortical afterdischarge activity occurred 64%, 23%, and 77% of the time when the 300 microA stimulation was given 60 sec, 3 hr, or 6 hr after inhibitory avoidance training, and 54%, 69%, and 92% for 3 mA. In the two conditions where amnesia was observed (3 mA at 60 sec and 300 microA at 3 hr) the lowest proportion of animals exhibited afterdischarge activity. Afterdischarge activity was not related to active avoidance performance.