Cyclic nucleotides are integral to a communication system that acts within cells.[1] They act as "second messengers" by relaying the signals of many first messengers, such as hormones and neurotransmitters, to their physiological destinations. Cyclic nucleotides participate in many physiological responses,[2] including receptor-effector coupling, down-regulation of drug responsiveness, protein-kinase cascades, and transmembrane signal transduction.[1]