Memory consolidation: Further evidence for the four-phase model from the time-courses of diethyldithiocarbamate and ethacrinic acid amnesias

Abstract

Thirty min prior to training rats in an active-avoidance task they were injected intracisternally (IC) with 3 mg diethyldithiocarbamate (DDC), a norepinephrine (NE) synthesis inhibitor. They showed complete retention of memory for about 10 min after training. Subsequently, memory decayed to the naive level over the next 80 min but reappeared later. The brain NE level fell to 50% of its normal level 30 min after the injection. 90 min later it recovered to 85% of its normal level. The dopamine level did not change. We have shown previously that brief-hypoxia causes a delayed transient amnesia. The time-course of the DDC-induced amnesia was identical. Both treatments are interpreted to interfere with Medium-Term-Memory (MTM) formation. The DDC-induced transient amnesia could be prevented by injecting 10 μg NE IC 30 min prior to training. It was concluded that NE is needed shortly after training for MTM formation. Rats were injected IC with 4 μg ethacrinic acid 30 min prior to training. They showed complete retention of memory 2 min after training, amnesia 10 min after training, complete retention 15–90 min after training and partial amnesia 3.5 hr later. Thus, ethacrinic acid appears to interfere with short-term-memory formation. These and earlier results obtained with other treatments are incorporated into a four-phase model of memory formation with parallel processing.