Dopaminergic Modulation of Short-Term Associative Memory in Caenorhabditis elegans

Abstract

Forgetting, the inability to retrieve previously encoded memories, is an active process involving neurotransmission, second messenger signalling and cytoskeletal modifications. Forgetting is thought to be essential to remove irrelevant memories and to increase the capacity to encode new memories. Therefore, identifying key regulators of active forgetting is crucial to advance our understanding of neuroplasticity. In this study, we utilised the compact and tractable Caenorhabditis elegans model to investigate the role of the neurotransmitter dopamine in forgetting. We conducted butanone associative learning assays based on an established protocol and used mutant strains deficient in dopamine synthesis (tyrosine hydroxylase CAT-2 and dopamine transporter DAT-1) and signalling (G protein-coupled receptors DOP-1, DOP-2 and DOP-3) to assess the impact on learning and memory retention. Learning was measured immediately post-training, and memory retention was evaluated every 0.5 h up to 2 h. Our results show that animals lacking dopamine display a modest enhancement in learning relative to wild-type, with the learned association persisting for at least 2 h after training. We also found that D2-like receptors DOP-2 and DOP-3 act together to modulate the forgetting process, with D1-like receptor DOP-1 functioning redundantly. Furthermore, re-expression of CAT-2 tyrosine hydroxylase in ADE and/or CEP neurons was unable to rescue the memory retention phenotype observed in cat-2 mutants, suggesting that dopamine release from all dopaminergic neurons is required to modulate forgetting. These findings highlight the critical role of dopamine in forgetting, consistent with findings in Drosophila, and suggest potential relevance for understanding memory retention during healthy ageing and in conditions with dopamine imbalances such as Parkinson's disease.