Cyclic AMP (cAMP) is a protein messenger present in many different types of cells which activates cAMP-dependent protein kinases, causing them to transfer phosphate groups from free molecules of ATP to various proteins in the cell. In this case, protein kinase A (PKA) transfers these phosphate groups to proteins on ion channels on the cell membrane. Phosphorylating these channel proteins increases the chances of the channels opening, thus increasing the chances of depolarizing the postsynaptic neuron at the axon hillock and setting off an action potential. Such events are called excitatory postsynaptic potentials (EPSPs). In this case, when dopamine receptors are activated by dopamine, the linked G-protein dissociates and stimulates the production of cAMP, which then activates the protein kinase, which in turn phosphorylates the ion channel, allowing sodium (Na+) ions to rush into the postsynaptic call and depolarize it. If enough depolarization occurs, an action potential will be generated and a signal sent down the axon of the postsynaptic cell.