Induction of LTM Following an Insulin Injection

Abstract

The pond snail Lymnaea stagnalis learns conditioned taste aversion (CTA) and consolidates it into long-term memory (LTM). One-day food-deprived snails (Day 1 snails) show the best CTA learning and memory, whereas more severely food-deprived snails (5 days) do not express good memory. However, previous studies showed that CTA-LTM was indeed formed in 5-day food-deprived snails (Day 5 snails), but its recall was prevented by the effects of food deprivation. CTA-LTM recall in Day 5 snails was expressed following 7 days of feeding and then 1 day of food deprivation (Day 13 snails). In the present study, we thus hypothesized that memory recall occurs because Day13 snails are in an optimal internal state. One day of food deprivation before the memory test in Day 13 snails increased the mRNA level of molluscan insulin-related peptide (MIP) in the CNS. Thus, we further hypothesized that an injection of insulin into Day 5 snails following 7 additional days with access to food (Day 12 snails) activates CTA neurons and mimics the food deprivation state before the memory test in Day 13 snails. Day 12 snails injected with insulin could recall the memory. In addition, the simultaneous injection of an anti-insulin receptor antibody and insulin into Day 12 snails did not allow memory recall. Insulin injection also decreased the hemolymph glucose concentration. Together, the results suggest that an optimal internal state (i.e., a spike in insulin release and specific glucose levels) are necessary for LTM recall following CTA training in snails.

Significance Statement When snails are trained for conditioned taste aversion (CTA) in a relatively long food-deprived state, they express long-term memory (LTM) if food is deprived again before the memory test. We hypothesized that there is an optimal state, such as increased insulin levels and reduced glucose levels in the hemolymph, in snails that allows memory recall. An insulin injection instead of food deprivation before the memory test caused the optimal internal state to occur in the snails that initially did not express CTA-LTM. Insulin directly modulates synaptic transmission in CNS neurons, and alters learning and memory.