Wakefulness rather than sleep benefits extinction of an inhibitory operant conditioning memory in Aplysia

Abstract

Sleep enhances memory consolidation which has been shown in mammals as well as in invertebrates, like bees and Drosophila. The current study is part of a series of experiments examining whether this memory function of sleep is preserved in Aplysia with an even simpler nervous system. Previous work showed that Aplysia sleep and that sleep after training supports memory on an inhibitory conditioning task ('learning that food is inedible', LFI). Here, we tested whether sleep in Aplysia would also support memory for an extinction learning on the LFI task. Following Acquisition in which animals learned that netted food is inedible, two groups of animals, a Sleep group (n = 15) and a Wake group (n = 16) underwent extinction training. After a 17-hour Retention interval which contained either regular nocturnal sleep or daytime wakefulness (supported by sleep deprivation) animals were retested on the LFI task. Contrary to our hypothesis, the Wake animals showed significantly prolonged food intake behavior on the LFI, indicating that extinction memory in these animals was better than in the Sleep animals. Performance of a control group not subjected to extinction training, ruled out that the superior extinction performance of Wake animals merely reflected forgetting over time of the LFI memory, and also excluding a possible circadian confound. We speculate that wakefulness mainly acts by accelerating active forgetting of the LFI memory after it was labialized through extinction training, thereby facilitating the re-emergence of the original innate behavior of food intake.

Keywords: Aplysia; Extinction learning; Forgetting; Invertebrate; Memory consolidation; Sleep.