 All of them function as metabotropic, G protein-coupled receptors, meaning that they exert their effects via a complex second messenger system.These receptors can be divided into two families, known as D1-like and D2-like.For receptors located on neurons in the nervous system, the ultimate effect of D1-like activation (D1 and D5) can be excitation (via opening of sodium channels) or inhibition (via opening of potassium channels); the ultimate effect of D2-like activation (D2, D3, and D4) is usually inhibition of the target neuron.