Multiple dopamine receptors: the D3 receptor and actions of substances of abuse.

Our knowledge of dopamine receptor diversity has markedly increased during the past few years as a result of discovery of five distinct genes, splice variants and polymorphic receptors. The genes can be classified in two subfamilies: the intronless genes that encode the D1 and D5 receptors positively linked to adenylyl cyclase and genes with introns that encode the two isoforms of the D2 receptor and the D3 and D4 receptors. The various dopamine receptor subtypes can be distinguished by their sequence, intracellular signalling systems, pharmacology and localisation. The localisation of the D3 receptor in the shell of nucleus accumbens suggests its participation in brain reward circuits and actions of substances of abuse.