Long-term memory rules within the feeding neural circuit of the marine snail Aplysia Californica

Long-term memory (LTM), which is memory that lasts for at least 24 h, is known to follow specific rules for formation and retention such as: 1) spaced training (S-T) protocols induce more persistent memory compared to massed training (M-T) protocols, 2) diurnal training (D-T) is more effective in inducing LTM than nocturnal (N-T) training, 3) LTM requires transcription of DNA into mRNA and translation of mRNA into new proteins. This project utilized the marine snail Aplysia californica to explore these LTM rules across two neural circuits that mediate both defensive and appetitive behaviors. Previous research in Aplysia revealed that repeated exposure to aversive stimuli induces an enhancement of defensive responses, known as long-term sensitization (LTS), as well as a decrease in feeding motivation, known as long-term feeding suppression (LTFS). These behavioral modifications are mediated at least in part by long-term increased excitability (LTIE) of sensory neurons, and long-term decreased excitability (LTDE) of decision-making neuron B51 (Shields-Johnson et al. 2013; Byrne and Hawkins, 2015). This project explored whether the feeding neural circuit in Aplysia follows the above LTM rules. Behavior results indicate that spaced training successfully induced both LTS and LTFS. However, massed training induced the expression of feeding suppression in the absence of sensitization, uncoupling these two normally co-expressed behavioral modifications. Results also revealed expected LTS and LTFS in the D-T group compared to D-UT group. However, nocturnal training did not induce either LTS or LTFS compared to the N-UT group. These findings indicate that nocturnal training is not conducive for LTM formation in both defensive and appetitive behaviors. Current electrophysiology experiments show a trend suggesting that B51 LTDE requires both transcription and translation for LTM formation. If the current trend continues, this result would indicate that learning-induced long-term plasticity within the feeding neural circuit of Aplysia requires transcription and translation mechanisms analogous to those necessary for long-term plasticity within the defensive neural circuit. Collectively, these findings indicate the complex nature of LTM formation within the feeding neural circuit of Aplysia by revealing that feeding follows some of the above LTM rules, but not all of them. Specifically, massed training induced LTFS contradicts the LTM rule that massed training is not conducive for LTM formation. Therefore, this study filled a previous gap in knowledge in how feeding in Aplysia is mediated by a subset of general LTM rules. Future directions from this study will reveal further mechanisms for the formation and retention of sustained memories in Aplysia and the universality of memory across many different organisms.