Cyclic AMP (cAMP) is a second messenger molecule involved in may different signaling cascades, including both short- and long-term facilitation.  Upon ligand binding to a metabotropic receptor, such as the 5-HT receptor, an enzyme called adenylyl cyclase catalyzes the formation of cAMP from ATP.  cAMP goes on to bind cAMP-dependent kinase, known as protein kinase A (PKA), which this paper implicates in the activation of CREB and the transcription of genes necessary for long-term facilitation.  Kandel decided to explore the role of cAMP in memory storage because previous research had suggested that it regulates signaling in muscle and fat (7).