News & Media
What can a mollusk tell us about memory?
The Sossin lab works on the molecular changes that occur when a memory is formed, what we term the molecular memory trace. How memories are formed is one of the great, unsolved mysteries in science and we take a reductionist approach, studying memory formation in a very simple animal, the mollusk Aplysia. The major advantage of Aplysia is that the synaptic connections that are changed during memory formation have been identified and these changes can be recapitulated in cultured neurons under controlled conditions. Using this preparation we can not only identify specific molecules involved in learning, but by using modern imaging techniques, we can also watch the changes as memories are formed.
A molecular memory trace is a specific biochemical mechanism underlying a change in synaptic connectivity that underlies a memory. For example, one trace may be due to an increase in the level of a critical protein at the synapse, and another may be due to triggering changes in the shape of the synapse through modification of a protein by phosphorylation. Even in this simple system of the Aplysia, we have found that distinct memory traces retain memories in different time frames. Remarkably, some of these molecular changes are independent of each other, for example, traces that retain memory for long periods of time can be formed in the absence of traces that are important for shorter duration memories. Thus, through the appropriate molecular activity, long-term memories can be made in the absence of short-term memories. Another question that we can ask is how different memory traces are formed from distinct experiences.
By comparing our Aplysia model with vertebrate brains we are also learning about the evolution of memory formation. Recently we have characterized a particular molecular trace, where we have found that an ancient mechanism used in Aplysia to generate a form of memory that lasts a few hours has been modified in vertebrates to generate a memory that lasts for days.
We have become interested in how the ability to make memories changes during aging, and have found a remarkable loss of the ability to make some kinds of memory traces, but not others as the animal ages. This is not due to a ‘run down’ of the system, but to a specific switch in a signaling pathway. By identifying this pathway, we will be able to understand how these switches are induced during the life of the animal. We now think that the loss of this memory trace occurs because the needs for memory change; for example, when an Aplysia grows in size, it does not have to worry about being eaten but about whether it can reproduce. We may find that as some switches and signals change, other molecular traces, perhaps those important for breeding, may increase.
