Paul Greengard (1925- present) was awarded one-third of the 2000 Nobel Prize in Physiology or Medicine for his discoveries concerning signal transduction in the nervous system. Greengard and colleagues studied the behavior of second messenger cascades that transform the docking of a neurotransmitter with a receptor into permanent changes in the neuron. In a series of experiments, Greengard and his colleagues showed that when dopamine interacts with a receptor on the cell membrane of a neuron, it causes an increase in cyclic adenosine monophosphate inside the cell. This in turn activates protein kinase A, which turns other proteins on or off with the addition of phosphate groups. The proteins activated by phosphorylation can then perform a number of changes in the cell: transcribing DNA to make new proteins, moving more receptors to the synapse -- and thus increasing the neuron's sensitivity -- or moving ion channels to the cell surface -- and thus increasing the cell's excitability.