Cyclic Nucleotides in the Central Nervous System

Abstract

Cells are responsive to signals of the changing environment, but only a few types of cells outside the central nervous system (CNS) receive signals with the same temporal and spatial density as the neurons in the brain. Several features of the neurons, such as an excitable membrane, synaptic contacts, and neurotransmitter systems, form the basis of the efficient information transfer. The intracellular signals at chemical synapses are mediated by neurotransmitters that are synthesized, stored, and released in the synaptic region. Binding of neurotransmitters to specific receptor sites in the membrane of the receptive cell influences membrane permeability via reactions that are restricted to the synaptic membrane or via a cascade of reactions that involve intracellular second messengers. Cyclic adenosine monophosphate (cAMP), which was identified as the second messenger of the information carried by epinephrine and glucagon on the periphery, should be involved in information transfer also in the CNS. It has been found that intracellular levels of cAMP and also of cyclic guanosine monophosphate (cGMP) are rather high in the CNS and that these levels change in response to exposure to several neurotransmitters and neuromodulators.