The dopamine transporter (DAT) terminates dopamine (DA) neurotransmission by reuptake of DA into presynaptic neurons. -(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059) prevented the quinpirole-evoked increase in ASPϩ accumulation, whereas inhibition of PI3K was without effect. Fluorescence flow cytometry and biotinylation studies revealed a rapid increase in DAT cell-surface expression in response to D 2 R stimulation. These experiments demonstrate that D 2S R stimulation increases DAT cell surface expression and therefore enhances substrate clearance. Furthermore, they show that the increase in DAT function is ERK1/2-dependent but PI3K-independent. Our data also suggest the possibility of a direct physical interaction between DAT and D 2 R. Together, these results suggest a novel mechanism by which D 2S R autoreceptors may regulate DAT in the central nervous system. Dopamine (DA) is the predominant catecholamine neurotransmitter in the central nervous system. Dysregulation of DA neurons has been implicated in the pathogenesis of Parkinson's disease, schizophrenia, and drug addiction (Sotnikova et al., 2006). Extracellular DA levels are primarily regulated by the DA transporter (DAT), an integral membrane protein that is a member of the Na ϩ /Cl Ϫ -dependent J.J., A.Z., and T.S.S. contributed equally to this work.

Morphine produces analgesia by activating mu opioid receptors encoded by the MOR-1 gene. Although morphine-6 beta-glucuronide (M6G), heroin and 6-acetylmorphine also are considered mu opioids, recent evidence suggests that they act through a distinct receptor mechanism. We examined this question in knockout mice containing disruptions of either the first or second coding exon of MOR-1. Mice homozygous for either MOR-1 mutation were insensitive to morphine. Heroin, 6-acetylmorphine and M6G still elicited analgesia in the exon-1 MOR-1 mutant, which also showed specific M6G binding, whereas M6G and 6-acetylmorphine were inactive in the exon-2 MOR-1 mutant. These results provide genetic evidence for a unique receptor site for M6G and heroin analgesia.

We have identified four new mu-opiod receptor (MOR)-1 exons, indicating that the gene now contains at least nine exons spanning more than 200 kilobases. Replacement of exon 4 by combinations of the new exons yields three new receptors. When expressed in Chinese hamster ovary cells, all three variants displayed high affinity for mu-opioid ligands, but kappa and delta drugs were inactive. However, there were subtle, but significant, differences in the binding profiles of the three variants among themselves and from MOR-1. Immunohistochemically, the major variant, MOR-1C, displayed a regional distribution quite distinct from that of MOR-1. Region-specific processing also was seen at the mRNA level. Antisense mapping revealed that the four new exons were all involved in morphine analgesia. Together with two other variants generated from alternative splicing of exon 4, there are now six distinct MOR-1 receptors.

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