Characterization of the mechanisms underlying food deprivation-induced memory deficits in the marine mollusk Aplysia
Malnutrition can cause many severe consequences to brain functions, including memory deficits. The invertebrate Aplysia was chosen as a model system to study prolonged food deprivation-induced memory impairments for its ability to sustain up to 14 days of food deprivation (14DFD) without health deterioration, nervous system damage or behavioral alterations. Here, a well-known learning paradigm in Aplysia was utilized. When presented with aversive stimuli (i.e., electrical shocks to the body wall that mimic attacks of a predator), Aplysia concurrently enhance their defensive responses (an elementary form of learned fear known as sensitization), including the tail-induced siphon withdrawal reflex (TSWR), and suppress feeding. The duration of these behavioral changes depends on the amount of aversive training. Precisely, short-term sensitization (STS) and short-term feeding suppression (STFS) are induced by a single trial of training and last for at least 15 min, while long-term sensitization (LTS) and long-term feeding suppression (LTFS) are induced by 4 consecutive trials and last for at least 24 h. Cellular correlates of sensitization and feeding suppression include increased excitability of the tail sensory neurons (TSNs) controlling TSWR, and decreased excitability of feeding decision-making neuron B51, respectively. A reduced in vitro preparation containing the neural circuits of TSWR, and feeding can express the above cellular correlates of sensitization and feeding suppression following the delivery of an aversive training protocol in vitro. Serotonin (5- HT) is known to mediate sensitization, but not feeding suppression. Recently, 14DFD was reported to cause memory deficits in Aplysia, exhibited as completely blocked sensitization, and attenuated feeding suppression. However, neuronal expressions of TSNs and B51, or the role of 5-HT in 14DFD-induced memory deficits, remain uninvestigated. The goal of this project was to characterize the mechanisms underlying memory deficits caused by prolonged food deprivation in Aplysia. Two feeding regimens were utilized: 2 days of food deprivation (2DFD), which is the standard feeding protocol and control for prolonged food deprived regimens, and 14DFD, which is the maximum period that Aplysia can sustain without health deterioration. Three specific aims were developed. In Aim 1, the expression of the learned neuronal modifications associated with STS and STFS under 14DFD was investigated. The aversive training was delivered in vitro in the reduced preparation described above and membrane properties of TSNs and B51were recorded before and after the in vitro training. In Aim 2, the effects of 14DFD on 5-HT levels were examined. 5-HT concentrations in the hemolymph and ganglia were measured and compared between 14DFD and 2DFD animals. Aim 3 studied whether 5-HT alone induced sensitization and its cellular correlates under 14DFD. In this aim, the ability of 5-HT to induce sensitization (Aim 3.1) and neuronal modifications in TSNs and B51(Aim 3.2) in 14DFD Aplysia were investigated, respectively. Results indicate that 14DFD prevented training-induced increased excitability of TSNs and decreased excitability of B51. These findings suggest that prevented STS may result, at least partly, from the lack of increased excitability of TSNs, while the reduced STFS may be contributed by additional neurons in the feeding neural circuits. 5-HT concentration in the hemolymph was reduced by 14DFD, which may be one of the causes that prevented sensitization under 14DFD. In the ganglia, 5-HT level was not affected by 14DFD. 5-HT was able to induce LTS in 14DFD Aplysia at 24 h post-test, while the amount of LTS induced was significantly smaller compared to 2DFD Aplysia. This indicates that 5-HT alone cannot fully induce LTS under 14DFD. 5-HT alone was capable of inducing increased TSN excitability in 14DFD preparations. Increased TSN excitability in 14DFD preparations induced by 5-HT was comparable to that observed in 2DFD preparations. Conversely, 5-HT failed to induce decreased excitability of B51 under either 2DFD or 14DFD, which indicates that 5-HT was not involved in decreased excitability of B51. These data revealed the cellular mechanisms and role of 5-HT in 14DFD-induced memory deficits. This study lays the foundations for the future use of Aplysia as a valuable model system to investigate mechanisms underlying memory impairments caused by prolonged food deprivation.