Showing papers by "F. Ivy Carroll published in 1998"

Studies of the biogenic amine transporters. VII. Characterization of a novel cocaine binding site identified with [125I]RTI‐55 in membranes prepared from human, monkey and guinea pig caudate

Abstract: [125I]RTI-55 is a cocaine analog with high affinity for dopamine (DA) and serotonin (5-HT) transporters. Quantitative ligand binding studies revealed a novel high affinity [125I]RTI-55 binding site assayed under 5-HT transporter (SERT) conditions which has low affinity for almost all classic biogenic amine transporter ligands, including high affinity 5-HT transporter inhibitors such as paroxetine, but which retains high affinity for cocaine analogs. This site, termed SERTsite2 for its detection under 5-HT transporter conditions (not for an association with the SERT) occurs in monkey caudate, human caudate, and guinea pig caudate membranes, but not in rat caudate membranes. SERTsite2 is distinguished from the DA transporter (DAT) and SERT by several criteria, including a distinct ligand-selectivity profile, the inability to detect SERTsite2 in cells stably expressing the cloned human DAT, and insensitivity to irreversible ligands which inhibit [125I]RTI-55 binding to the DAT and SERT. Perhaps the most striking finding about SERTsite2 is that a wide range of representative antidepressant agents have very low affinity for SERTsite2. The affinity of cocaine for this site is not very different from the concentration cocaine achieves in the brain at pharmacological doses. Viewed collectively with the observation that ligands with high affinity for SERTsite2 are mostly cocaine analogs, these data lead us to speculate that actions of cocaine which differ from those of classic biogenic amine uptake inhibitors may be mediated in part via SERTsite2. Synapse 28:322–338, 1998. Published 1998 Wiley-Liss, Inc.

Striatal [125I]RTI-55 binding sites in cocaine-abusing humans

Abstract: 1. 1. Previous experiments in this laboratory found that striatal [3H]WIN 35428 binding was increased in post mortem specimens from human cocaine users (Little et al, 1993a). Although structurally similar, preliminary studies have suggested that [3H]WIN 35428 and the related cocaine congener [125I]RTI-55 differ in some respects pharmacologically. 2. 2. The present experiments tested the hypothesis that striatal [125I]RTI-55 binding would be increased, as was [3H]WIN 35428 binding, in post mortem specimens from cocaine users compared to matched controls. 3. 3. However, computer-generated parameters derived from saturation experiments found only trends toward increased Bmax and decreased affinity (increased KD) in the cocaine users. The magnitude of the increases were notably smaller than the statistically significant increases previously found in high affinity [3H]WIN 35428 binding in these same subjects. 4. 4. Evidence from the present and earlier experiments suggests that cocaine exposure may induce conformational changes in the dopamine transporter.

Opioid peptide receptor studies. 7. The methylfentanyl congener RTI-4614-4 and its four enantiomers bind to different domains of the rat mu opioid receptor.

Abstract: Mutational analysis of opioid receptors supports the hypothesis that dissimilar receptor domains contribute to the binding affinity of different ligands. To determine whether enantiomeric ligands can serve to distinguish between different binding pockets (which focuses the analysis on asymmetric structural factors while avoiding confounding changes in physiochemical characteristics), we analyzed the binding of the 3-methylfentanyl congeners RTI-4614-4 [(+/-)-cis-N-[1-(2-hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N- phenylpropanamide HCl)], its four stereoisomers [(2S,3R,4S)-1a, (2R,3R,4S)-1b, (2R,3S,4R)-1c, and (2S,3S,4R)-1d], and other mu agonists with cloned rat mu opioid receptors stably expressed in HEK-293 cells and mu/kappa receptor chimeras. Chimera III (kappa[aminoacids 1-141]/mu[aminoacids 151-398]), chimera IV (mu[aminoacids 1-150]/kappa[aminoacids 142-380]), and chimera XII (kappa[aminoacids 1-262]/mu[aminoacids 269-398]) bound [(125)I]IOXY (6beta-iodo-3,14-dihydoxy-17-cyclopropylmethyl-4,5alpha++ +-epoxymorphinan) with high affinities. The Ki values of 1a, 1b, 1c, and 1d at the wild-type mu receptor were 0.55 nM, 0.66 nM, 124 nM, and 59.2 nM, respectively. When the region from the N terminal to the start of the transmembrane helix 3 (TMH3) of the mu receptor was substituted by that of the kappa receptor (chimera III), the Ki value of 1b was increased (relative to the mu receptor) 590-fold compared to a 73-fold increase for 1a. When this portion of the kappa receptor was replaced by that of the mu receptor (chimera IV), the loss of affinity was not as great: 11.7-fold for 1a and 58.5-fold for 1b. Replacement of the middle of the third intracellular loop and third extracellular loop (e3) of the kappa receptor with that of the mu receptor (chimera XII) lowered (relative to their Ki values at the kappa receptor) the Ki values of [D-Ala2,D-Leu5]enkephalin and [D-Ala2-MePhe4,Gly-ol5]enkephalin to a much greater extent than the Ki values of the isomers. The kappa/chimera XII shift was greater for isomers 1c and 1d than for 1b and 1a. Viewed collectively, these data suggest that the region from the N terminal to the start of the TMH3 of the mu opioid receptor determines the binding affinity of RTI-4614-4 and its isomers and that the e3 loop also plays a major role in determining the binding affinity of mu agonist peptides. These data also show that the stereoisomers of RTI-4614-4 probably bind to different domains of the mu receptor and suggest that manipulation of stereochemistry may be a useful tool for designing domain-specific ligands.

Protection of the allylic alcohol double bond from catalytic reduction in the preparation of [1-3H]morphine and [1-3H]codeine

Abstract: The t-butyldimethylsilylation of the allylic alcohol of 1-iodomorphine and 1-iodocodeine protects the double bond of these molecules from catalytic reduction while allowing the reduction of the aryl iodide. This selectivity has been applied to the preparation of tritiated morphine and codeine without complicating over reduction to the dihydromorphine and dihydrocodeine. © 1998 John Wiley & Sons, Ltd.