Induction of LTM following an Insulin Injection

Area of Science:

• Neuroscience
• Animal Behavior
• Molecular Biology

Background:

• Pond snails (Lymnaea stagnalis) exhibit conditioned taste aversion (CTA) learning and long-term memory (LTM) consolidation.
• While CTA-LTM forms in food-deprived snails, recall is impaired by severe deprivation but can be restored by subsequent feeding and re-deprivation.
• This suggests an optimal internal state is required for memory recall.

Purpose of the Study:

• To investigate the role of an optimal internal state, specifically insulin and glucose levels, in the recall of CTA-LTM in Lymnaea stagnalis.
• To determine if insulin injection can mimic the conditions necessary for memory recall in food-deprived snails.

Main Methods:

• Snails were subjected to varying food deprivation schedules (1-day vs. 5-day).
• Molluscan insulin-related peptide (MIP) mRNA levels were measured in the central nervous system (CNS).
• Insulin was injected into snails to assess its effect on memory recall, with and without an anti-insulin receptor antibody. Hemolymph glucose concentrations were measured.

Main Results:

• Memory recall was observed in snails that experienced a period of feeding followed by brief food deprivation (day 13 snails).
• Food deprivation before testing increased molluscan insulin-related peptide (MIP) mRNA levels in the CNS.
• Insulin injection into day 12 snails (5-day deprivation followed by 7 days feeding) restored memory recall, an effect blocked by an anti-insulin receptor antibody. Insulin also lowered hemolymph glucose.

Conclusions:

• An optimal internal state, characterized by elevated insulin and specific glucose levels, is necessary for the recall of CTA-LTM in pond snails.
• Insulin signaling through its receptor is critical for activating CTA neurons and enabling memory recall.
• These findings highlight the intricate link between metabolic state and memory retrieval in invertebrates.