Interaction between nitric oxide and serotonin in the formation of long-term sensitization in the mollusk Aplysia

Learning is a process by which all animals modify their behaviors to ensure increased fitness (Nordell 2021). Long-term sensitization (LTS) in the mollusk Aplysia is a well-known example of learning in which defensive responses, such as the tail-siphon withdrawal reflex (TSWR), are heightened for prolonged periods following repeated exposure to noxious stimuli (i.e., aversive training; Byrne and Hawkins 2015). The neurotransmitter serotonin (5-HT) mediates LTS formation in Aplysia (Byrne and Hawkins 2015). In vivo, 5-HT exposure alone can induce LTS (Levenson et al. 1999). However, recent findings indicate that the neurotransmitter nitric oxide (NO) is also necessary for LTS formation (Farruggella et al. 2019). Therefore, in this study, an experiment was designed to position NO on the 5-HT-mediated biochemical pathway.