Extending the duration of long-term memories: Interactions between environmental darkness and retinoid signaling

- Retinoid receptor agonists extend long-term memory duration in Lymnaea.
- 48 h of constant darkness extend long-term memory duration in Lymnaea.
- This memory extension was not affected by a DNA methyltransferase inhibitor.
- Constant darkness increases expression of retinoid receptors in the Lymnaea CNS.
- Constant darkness reduces firing in a key respiratory network neuron.

Retinoid signaling plays an important role in hippocampal-dependent vertebrate memories. However, we have previously demonstrated that retinoids are also involved in the formation of long-term implicit memory following operant conditioning of the invertebrate mollusc Lymnaea stagnalis. Furthermore, we have discovered an interaction between environmental light/dark conditions and retinoid signaling and the ability of both to convert intermediate-term memory into long-term memory. In this study, we extend these findings to show that retinoid receptor agonists and environmental darkness can both also extend the duration of long-term memory. Interestingly, exposure to constant environmental darkness significantly increased the expression of retinoid receptors in the adult central nervous system, as well as induced specific changes in a key neuron mediating the conditioned behaviour. These studies not only shed more light on how retinoids influence memory formation, but also further link environmental light conditions to the retinoid signaling pathway.