A dopamine receptor has been characterized which differs in its pharmacology and signalling system from the D1 or D2 receptor and represents both an autoreceptor and a postsynaptic receptor. The D3 receptor is localized to limbic areas of the brain, which are associated with cognitive, emotional and endocrine functions. It seems to mediate some of the effects of antipsychotic drugs and drugs used against Parkinson's disease, that were previously thought to interact only with D2 receptors.

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We have identified ([3H]7-OH-DPAT) as a selective probe for the recently cloned dopamine D3 receptor and used it to assess the presence of this receptor and establish its distribution and properties in brain. In transfected Chinese hamster ovary (CHO) cells, it binds to D3 receptors with subnanomolar affinity, whereas its affinity is approximately 100-, 1000-, and 10,000-fold lower at D2, D4, and D, receptors, respectively. Specific [3H]7-OH-DPAT binding sites, with a Kd of0.8 nM and a pharmacology similar to those at reference D3 receptors of CHO cells, were identified in rat brain. D3 receptors differ from D2 receptors in brain by their lower abundance (2 orders of magnitude) and distribution, restricted to a few mainly phylogenetically ancient areas-e.g., paleostriatum and archicerebellum-as evidenced by membrane binding and autoradiography studies. Native D3 receptors in brain are characterized by an unusually high nanomolar affinity for dopamine and a low modulatory influence of guanyl nucleotides on agonist binding. These various features suggest that D3 receptors are involved in a peculiar mode of neurotransmission in a restricted subpopulation of dopamine neurons.Dopamine is an important neurotransmitter in brain, being involved physiologically in the control of cognitive, motor, and endocrine processes and pathologically in Parkinson and possibly mental diseases.Until recently, it was largely believed that its various actions were mediated by two receptor subtypes, termed D1 and D2 (1, 2). Molecular biology approaches have led, however, to the identification and cloning of the genes corresponding not only to these two receptor subtypes (3-7) but also to additional and less expected ones, termed D3 (8), D4 (9), and D5 (10, 11). For the latter receptors, the information so far available derives from molecular biology approaches; their pharmacology and signaling system have only been studied in transfected cells, generally fibroblasts, and their distribution has been indirectly approached by studies of mRNA localization. Nevertheless, these otherwise valuable approaches suffer from some limitations. When expressed in fibroblasts, receptors might find a membrane environment, which could modify their pharmacological specificity, and a repertoire of cellular components, particularly GTP-binding proteins (G proteins), which may differ from that found in neurons. Regarding distribution, mRNA detection reveals the sites of receptor synthesis rather than receptor localization, which may be different. Therefore, there is an obvious interest in studying the native receptor protein in brain, particularly in the case ofthe D3 receptor, whose pharmacology in Chinese hamster ovary (CHO) cells and cerebral localization of its mRNA suggest that it may represent an important target for antipsychotics (8,12,13).Starting with the idea that designing a selective radioactive probe for the D3 receptor would help to settle these issues, we have screened a series of dopamine agonists and thereby identified 7-hydroxy-N,...