Effects of training protocols that do not induce long-term sensitization on the expression of long-term feeding suppression in Aplysia

Research using various animal models has revealed that memory expression and persistence are governed by critical parameters including amount, intensity, and pattern of the stimuli employed during training. In addition, memory formation depends on the organism’s circadian clock, which dictates the time of day when training is conducive to inducing lasting memory. In the diurnal invertebrate Aplysia, repeated exposure to aversive stimuli during the day using long inter-trial intervals induces long-term sensitization (LTS) of defensive responses. Aversive protocols that cause LTS also induce long-term feeding suppression (LTFS). However, unlike LTS, LTFS appears to be less sensitive to the amount of training and is observed in conditions of food deprivation that prevent the occurrence of LTS. These observations led to the hypothesis that LTFS might not depend on some of the factors that regulate LTS expression. To further investigate the relation between LTS and LTFS, in this study, we analyzed whether LTFS was expressed following two aversive protocols that are known to not induce LTS: training delivered with a compressed timeframe (i.e., massed training), and training conducted at night (i.e., nocturnal training). Results indicate that massed training was conducive for the induction of LTFS even in the absence of LTS, whereas nocturnal training failed to induce both LTS and LTFS. These findings indicate that LTS and LTFS are both regulated by the circadian clock. The expression of LTFS in the absence of LTS in massed-trained animals indicates that LTFS differs from LTS in its sensitivity to the pattern of stimulation.