The dopamine receptor, shown in violet, is adjacent to a G-protein shown here in yellow. In the inactive state, the G protein binds the nucleotide GDP. The G-protein has three subunits, alpha, beta and gamma. Activation of the receptor by the neurotransmitter dopamine causes the alpha subunit to exchange its GDP for a GTP. The G protein then disassociates. The alpha subunit, with GTP, pulls away leaving behind the beta and gamma subunits.