"Greengard's research focused on events inside the neuron caused by neurotransmitters. Specifically, Greengard and his fellow researchers 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 AMP inside the cell. This increase of cyclic AMP, in turn activates a protein called protein kinase A, which turns other proteins on or off by adding phosphate groups in a reaction known as phosphorylation. 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)" (Wikipedia article on Paul Greengard). 
Greengard's work focused on the central regulatory protein DARPP-32. The above paper was the culmination of decades of research. There were 22 co-authors.

In 2000 Paul Greengard shared the Nobel Prize in Physiology or Medicine with Arvid Carlsson and Eric R. Kandel "for their discoveries concerning signal transduction in the nervous system."