Effects of the inhibition of miRNA biogenesis in the central ring ganglia of a widely used invertebrate model species, Lymnaea stagnalis

MicroRNAs (miRNAs) are key regulators of gene expression, shaping neuroplasticity, stress responses, and neuromodulation. In the pond snail Lymnaea stagnalis, inhibition of the miRNA-processing enzyme Dicer with Poly-L-Lysine (PLL) has been shown to impair long-term memory (LTM) formation, yet the molecular pathways affected remain unclear. Here, we examined PLL injection's transcriptional and cellular/neurochemical effects in untrained (i.e., non-associatively conditioned) snails. We focused on genes involved in neuroplasticity (LymGRIN1, LymCREB1), stress response (LymHSP70), and serotonergic/dopaminergic signaling (LymTPH, LymSERT, LymDDC), along with measurements of the levels of serotonin, dopamine, and HSP70 protein. We found that PLL did not alter the expression of memory-related genes in the non-associatively conditioned snails. However, we observed a marked downregulation of LymTPH and LymSERT, which was accompanied by a significant reduction in the level of serotonin. In contrast, the expression of LymDDC and the level of dopamine remained unchanged. Although we detected a significant upregulation in the expression of LymHSP70, indicating a strong stress response, the level of HSP70 protein did not change significantly. These findings suggest that PLL-induced Dicer inhibition primarily disrupts serotonin homeostasis, potentially altering motivation and feeding behaviour rather than directly impairing memory processes. This study highlights the broader role of miRNA pathways in regulating stress resilience and neuromodulation. By linking miRNA activity to serotonergic signaling, our findings suggest that miRNAs influence behavioural states beyond synaptic plasticity, with potential implications for understanding how miRNA dysregulation affects mood, motivation, and cognitive function across species.