The effect of prolonged cortical spreading depression on consolidation of visual engrams in rats

Abstract

The effect of prolonged cortical spreading depression (PCSD) on the retention of a visual discrimination was examined in 213 hooded rats. PCSD was maintained for 5 h by repeated application of 25% KCl onto the exposed cortical surface. Behavioral tests were performed 24 or more h before and/or after the treatment. A single application of 25% KCl did not affect subsequent acquisition of a horizontal-vertical discrimination, the learning of which took 140% and 125% to-criterion trials 1 and 3 days after PCSD. Whereas a single application of 25% KCl did not impair retention of the pattern discrimination learnt in a single session 24 h earlier, PCSD caused severe amnesia under the above conditions and partial amnesia even for pattern discriminations overlearned in 3 sessions. The amnesic effect nearly disappeared, however, when the habit was overlearned for 14 days, or when the engram established in a single session was allowed to consolidate for 2 weeks. When PCSD was applied 24 h after a single learning session amnesia was more marked on day 3 after treatment, and retrieval hardly improved over 2 weeks. In an experiment in which 14 sessions of black-white discrimination training were followed by a single session of horizontal-vertical training, PCSD evoked 24 h after acquisition of the pattern discrimination caused complete amnesia for the latter habit without affecting retrieval of the former one. The relevance of these findings to the consolidation hypothesis is discussed.