Remember the poke: microRNAs are required for long-term memory formation following operant conditioning in Lymnaea.

MicroRNAs (miRNAs) play an important role in learning and memory formation by controlling the expression of genes through epigenetic processes. Although miRNAs unquestionably play a role in memory, past literature focusing on whether miRNAs play key roles in the consolidation of associative long-term memory in Lymnaea contained confounding variables. Using operant conditioning of aerial respiratory behaviour, we investigated long-term memory (LTM) formation after injection of poly-L-lysine (PLL), an inhibitor of Dicer-mediated miRNA biogenesis, in Lymnaea stagnalis. Homeostatic breathing experiments were also performed to test whether PLL affects breathing. Homeostatic breathing was significantly suppressed 45 min but not 24 h after PLL injection. The operant conditioning procedure involved two 30-min training sessions separated by 1 h to cause LTM. Using this operant conditioning procedure, LTM formation was significantly impaired when snails were injected with PLL 15 min after the second training session. In contrast, when snails were injected with PLL 24 h before the first training session, LTM formation was not impaired. These results are consistent with past literature and highlight an important role for miRNAs in LTM formation.