Serotonin is a monoamine neurotransmitter implicated in memory storage, mood, and other cognitive processes.  Its receptor is metabotropic, a type of G protein-linked receptor.  The second messenger cAMP is often associated with metabotropic receptor signal transduction, as is the case with the serotonin receptor.

Previous research had implicated serotonin release in a simple form of learning in the invertebrate <em>Aplysia</em>.  In vertebrates, including the mouse model that is the focus of Kandel's current research, the neurotransmitter dopamine plays a role parallel to that of serotonin in memory storage.