We propose to investigate D2 and D3 dopamine receptors using newly developed selective ligands. The information to be generated by this project is essential for the understanding of pharmacology as well as the relevance of D2 and D3 dopamine receptors on the mechanism of action of neuroleptics for treatment and management of patients with mental illness. The dopaminergic system is very important for normal brain function, and it is also a possible primary action site for various neuroleptic drugs for treatment of schizophrenia and other mental disorders. Application of molecular biology techniques in expressing dopamine receptor subtypes: D1, D2L, D2S, D3, D4 and D5, in cloned cells have been reported. Among the subtypes, the D3 receptor has been postulated as a potential target site for neuroleptics and it may mediate dopaminergic control of cognitive, emotional, and endocrine functions, extremely important mechanisms for antipsychotic actions. Regional distribution of mRNA of D3 receptors in rat brain showed predominantly high density in the limbic system, including the olfactory tubercle, nucleus accumbens, island of calleja and hypothalamus. The neuroleptics, such as haloperidol and sulpiride, showed high D3 receptor binding as well as a high degree of D2 receptor binding. Recently, [3H]7-OH-DPAT was reported as a selective D3 dopamine receptor ligand. Based on this structure we have successfully developed the first iodinated D3 dopamine ligand, [125I]Trans-7-OH-PIPAT(Trans-7-hydroxy-2- (N-n-propyl-N-3'-iodo-2'-propenyl)aminotetralin). This iodinated analog is the first novel ligand demonstrating unique high affinity and selectivity toward the D3 receptor (Kd=0.48 nM in rat striatal membrane homogenates). Preliminary autoradiography of rat brain sections showed that this new ligand displayed a regional distribution pattern consistent with that measured with mRNA hybridization. Using in vitro homogenate binding assay and autoradiography techniques, measurements of D3 receptor density, regional distribution as well as functional linkage of drug- ligand interaction of this receptor in normal and neuroleptic-treated rat brain and proposed. In addition, parallel studies of D2 dopamine receptor with [125I]FIDA2, (S)-(-)-2-Fluoroethoxy-5-iodo-3-methoxy-N-[(1-ethyl-2- pyrrolidinyl)methyl] benzamide, a potent D2 dopamine receptor ligand (Kd=0.02 nM), in normal and treated rats will also be performed. Due to the high binding affinity (in nM) of endogenous dopamine to the D3 dopamine receptor, it is known that D3 receptors are mostly saturated in normal states. If the lack of, or the depressed level of endogenous dopamine is the key event which precipitates the neurological diseases, then D3 ligands may potentially provide a very useful tool to clearly define the role of the dopaminergic system in these diseases.
