In Search of Memory: The Emergence of a New Science of Mind

by Eric R. Kandel

The discoveries in Aplysia and Drosophila also reinforced an important biological principle: evolution does not require new, specialized molecules to produce a new adaptive mechanism. The cyclic AMP pathway is not unique to memory storage. As Sutherland had shown, it is not even unique to neurons: the gut, the kidney, and the liver all make use of the cyclic AMP pathway to produce persistent metabolic changes. In fact, of all the known second messengers, the cyclic AMP system is probably the most primitive. It is the most important, and in some cases the only second-messenger system found in single-celled organisms such as the bacterium E. coli, in which it signals hunger. Thus the biochemical actions underlying memory did not arise specifically to support memory. Rather, neurons simply recruited an efficient signaling system employed for other purposes in other cells and used it to produce the changes in synaptic strength required for memory storage.