Christina M. Dersch

Amphetamine‐type central nervous system stimulants release norepinephrine more potently than they release dopamine and serotonin

A large body of evidence supports the hypothesis that mesolimbic dopamine (DA) mediates, in animal models, the reinforcing effects of central nervous system stimulants such as cocaine and amphetamine. The role DA plays in mediating amphetamine‐type subjective effects of stimulants in humans remains to be established. Both amphetamine and cocaine increase norepinephrine (NE) via stimulation of release and inhibition of reuptake, respectively. If increases in NE mediate amphetamine‐type subjective effects of stimulants in humans, then one would predict that stimulant medications that produce amphetamine‐type subjective effects in humans should share the ability to increase NE. To test this hypothesis, we determined, using in vitro methods, the neurochemical mechanism of action of amphetamine, 3,4‐methylenedioxymethamphetamine (MDMA), (+)‐methamphetamine, ephedrine, phentermine, and aminorex. As expected, their rank order of potency for DA release was similar to their rank order of potency in published self‐administration studies. Interestingly, the results demonstrated that the most potent effect of these stimulants is to release NE. Importantly, the oral dose of these stimulants, which produce amphetamine‐type subjective effects in humans, correlated with the their potency in releasing NE, not DA, and did not decrease plasma prolactin, an effect mediated by DA release. These results suggest that NE may contribute to the amphetamine‐type subjective effects of stimulants in humans. Synapse 39:32–41, 2001. Published 2001 Wiley‐Liss, Inc.

Aminorex, Fenfluramine, and Chlorphentermine Are Serotonin Transporter Substrates Implications for Primary Pulmonary Hypertension

BACKGROUND Coadministration of phentermine and fenfluramine (phen/fen) effectively treats obesity and possibly addictive disorders. The association of fenfluramine and certain other anorexic agents with serious side effects, such as cardiac valvulopathy and primary pulmonary hypertension (PPH), limits the clinical utility of these drugs. Development of new medications that produce neurochemical effects like phen/fen without causing unwanted side effects would be a significant therapeutic breakthrough. METHODS AND RESULTS We tested the hypothesis that fenfluramine (and other anorexic agents) might increase the risk of PPH through interactions with serotonin (5-HT) transporters. Because 5-HT transporter proteins in the lung and brain are identical, we examined, in rat brain, the effects of selected drugs on 5-HT efflux in vivo and monoamine transporters in vitro as a generalized index of transporter function. Our data show that drugs known or suspected to increase the risk of PPH (eg, aminorex, fenfluramine, and chlorphentermine) are 5-HT transporter substrates, whereas drugs that have not been shown to increase the risk of PPH are less potent in this regard. CONCLUSIONS We speculate that medications that are 5-HT transporter substrates get translocated into pulmonary cells where, depending on the degree of drug retention, their intrinsic drug toxicity, and individual susceptibility, PPH could develop as a response to high levels of these drugs or metabolites. Emerging evidence suggests that it is possible to develop transporter substrates devoid of adverse side effects. Such medications could have therapeutic application in the management of obesity, drug dependence, depression, and other disorders.

Neurochemical neutralization of methamphetamine with high-affinity nonselective inhibitors of biogenic amine transporters: a pharmacological strategy for treating stimulant abuse.

The abuse of methamphetamine (METH) and other amphetamine‐like stimulants is a growing problem in the United States. METH is a substrate for the 12‐transmembrane proteins which function as transporters for the biogenic amines dopamine (DA), serotonin (5‐HT), and norepinephrine (NE). Increased release of CNS DA is thought to mediate the addictive effects of METH, whereas increased release of NE in both the peripheral and CNS is thought to mediate its cardiovascular effects. The neurotoxic effects of METH on both dopaminergic and serotonergic nerves requires the transport of METH into the nerve terminals. Thus, transport of METH into nerve terminals is the crucial first step in the production of METH‐associated pharmacological and toxicological effects. A single molecular entity which would block the transport of METH at all three biogenic amine transporters might function to neurochemically neutralize METH. This agent would ideally be a high‐affinity slowly dissociating agent at all three transporters, and also be amenable to formulation as a long‐acting depot medication, such as has been accomplished with an analog of GBR12909. As a first step towards developing such an agent, we established an in vitro assay which selectively detects transporter substrates and used this assay to profile the ability of a lead compound, indatraline, to block the releasing effects of METH and MDMA at the DA, 5‐HT, and NE transporters. The major finding reported here is that indatraline blocks the ability of METH and MDMA to release these neurotransmitters. Synapse 35:222–227, 2000. Published 2000 Wiley‐Liss, Inc.

1-(m-Chlorophenyl)piperazine (mCPP) Dissociates In Vivo Serotonin Release from Long-Term Serotonin Depletion in Rat Brain

Serotonin (5-HT) releasing agents such as d-fenfluramine are known to cause long-term depletion of forebrain 5-HT in animals, but the mechanism of this effect is unknown. In the present study, we examined the relationship between drug-induced 5-HT release and long-term 5-HT depletion in rat brain. The 5-HT-releasing actions of d-fenfluramine and a non-amphetamine 5-HT drug, 1-(m-chlorophenyl)piperazine (mCPP), were compared using in vivo microdialysis in the nucleus accumbens. The ability of d-fenfluramine and mCPP to interact with 5-HT transporters was tested using in vitro assays for [3H]5-HT uptake and radioligand binding. Local infusion of d-fenfluramine or mCPP (1–100 μM) increased extracellular 5-HT, with elevations in dopamine occurring at high doses. Intravenous injection of either drug (1–10 μmol/kg) produced dose-related increases in 5-HT without affecting dopamine. d-Fenfluramine and mCPP exhibited similar potency in their ability to stimulate 5-HT efflux in vivo and interact with 5-HT transporters in vitro. When rats received high-dose d-fenfluramine or mCPP (10 or 30 μmol/kg, i.p., every 2 h, 4 doses), only d-fenfluramine-treated rats displayed long-term 5-HT depletions. Thus, mCPP is a 5-HT releaser that does not appear to cause 5-HT depletion. Our data support the notion that 5-HT release per se may not be sufficient to produce the long-term 5-HT deficits associated with d-fenfluramine and other amphetamines.

Salvinorin A: Allosteric Interactions at the μ-Opioid Receptor

Salvinorin A [(2S,4aR,6aR,7R,9S,10aS,10bR)-9-(acetyloxy)-2-(3-furanyl)-dodecahydro-6a,10b-dimethyl-4,10-dioxo-2h-naphtho[2,1-c]pyran-7-carboxylic acid methyl ester] is a hallucinogenic κ-opioid receptor agonist that lacks the usual basic nitrogen atom present in other known opioid ligands. Our first published studies indicated that Salvinorin A weakly inhibited μ-receptor binding, and subsequent experiments revealed that Salvinorin A partially inhibited μ-receptor binding. Therefore, we hypothesized that Salvinorin A allosterically modulates μ-receptor binding. To test this hypothesis, we used Chinese hamster ovary cells expressing the cloned human opioid receptor. Salvinorin A partially inhibited [3H]Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO) (0.5, 2.0, and 8.0 nM) binding with EMAX values of 78.6, 72.1, and 45.7%, respectively, and EC50 values of 955, 1124, and 4527 nM, respectively. Salvinorin A also partially inhibited [3H]diprenorphine (0.02, 0.1, and 0.5 nM) binding with EMAX values of 86.2, 64, and 33.6%, respectively, and EC50 values of 1231, 866, and 3078 nM, respectively. Saturation binding studies with [3H]DAMGO showed that Salvinorin A (10 and 30 μM) decreased the μ-receptor Bmax and increased the Kd in a dose-dependent nonlinear manner. Saturation binding studies with [3H]diprenorphine showed that Salvinorin A (10 and 40 μM) decreased the μ-receptor Bmax and increased the Kd in a dose-dependent nonlinear manner. Similar findings were observed in rat brain with [3H]DAMGO. Kinetic experiments demonstrated that Salvinorin A altered the dissociation kinetics of both [3H]DAMGO and [3H]diprenorphine binding to μ receptors. Furthermore, Salvinorin A acted as an uncompetitive inhibitor of DAMGO-stimulated guanosine 5′-O-(3-[35S]thio)-triphosphate binding. Viewed collectively, these data support the hypothesis that Salvinorin A allosterically modulates the μ-opioid receptor.

Herkinorin Analogues with Differential Beta-Arrestin-2 Interactions

Salvinorin A is a psychoactive natural product that has been found to be a potent and selective kappa opioid receptor agonist in vitro and in vivo. The activity of salvinorin A is unusual compared to other opioids such as morphine in that it mediates potent kappa opioid receptor signaling yet leads to less receptor downregulation than observed with other kappa agonists. Our initial chemical modifications of salvinorin A have yielded one analogue, herkinorin ( 1c), with high affinity at the microOR. We recently reported that 1c does not promote the recruitment of beta-arrestin-2 to the microOR or receptor internalization. Here we describe three new derivatives of 1c ( 3c, 3f, and 3i) with similar properties and one, benzamide 7b, that promotes recruitment of beta-arrestin-2 to the microOR and receptor internalization. When the important role micro opioid receptor regulation plays in determining physiological responsiveness to opioid narcotics is considered, micro opioids derived from salvinorin A may offer a unique template for the development of functionally selective mu opioid receptor-ligands with the ability to produce analgesia while limiting adverse side effects.

Opioid receptor probes derived from cycloaddition of the hallucinogen natural product salvinorin A.

As part of our continuing efforts toward more fully understanding the structure-activity relationships of the neoclerodane diterpene salvinorin A, we report the synthesis and biological characterization of unique cycloadducts through [4+2] Diels-Alder cycloaddition. Microwave-assisted methods were developed and successfully employed, aiding in functionalizing the chemically sensitive salvinorin A scaffold. This demonstrates the first reported results for both cycloaddition of the furan ring and functionalization via microwave-assisted methodology of the salvinorin A skeleton. The cycloadducts yielded herein introduce electron-withdrawing substituents and bulky aromatic groups into the C-12 position. Kappa opioid (KOP) receptor space was explored through aromatization of the bent oxanorbornadiene system possessed by the cycloadducts to a planar phenyl ring system. Although dimethyl- and diethylcarboxylate analogues 5 and 6 retain some affinity and selectivity for KOP receptors and are full agonists, their aromatized counterparts 13 and 14 have reduced affinity for KOP receptors. The methods developed herein signify a novel approach toward rapidly probing the structure-activity relationships of furan-containing natural products.

Uptake and release effects of diethylpropion and its metabolites with biogenic amine transporters.

Three metabolites of diethylpropion (1), (+/-)-2-ethylamino-1-phenyl-propan-1-one (2), (1R,2S)-(-)-N,N-diethylnorephedrine (3a) and (1S,2R)-(-)-N,N-diethylnorephedrine (3b) were synthesized. Their uptake and release effects with biogenic amine transporters were evaluated. A major finding of this study is that the in vivo activity of diethylpropion on biogenic amine transporters is most likely due to metabolite 2 as diethylpropion (1) and the metabolites 3a and 3b showed little or no effect in the assays studied. These studies also revealed that 2 acted as a substrate at the norepinephrine (IC50 = 99 nM) and serotonin transporters (IC50 = 2118 nM) and an uptake inhibitor at the dopamine transporter (IC50 = 1014 nM). The potent action of 2 at the NE transporter supports the hypothesis that amphetamine-type subjective effects may be mediated in part by brain norepinephrine.

Studies of the biogenic amine transporters. 1. Dopamine reuptake blockers inhibit [3H]mazindol binding to the dopamine transporter by a competitive mechanism: Preliminary evidence for different binding domains

The present study addressed the hypothesis that the DA transporter ligand, [3H]mazindol, labels multiple sites/states associated with the dopamine (DA) transporter in striatal membranes. Incubations with [3H]mazindol proceeded for 18–24 hr at 4δC in 55.2 mM sodium phosphate buffer, pH 7.4, with a protease inhibitor cocktail. In order to obtain data suitable for quantitative curve fitting, it was necessary to repurify the [3H]mazindol by HPLC before a series of experiments. Under these conditions, we observed greater than 80% specific binding. The method of binding surface analysis was used to characterize the interaction of GBR12935, BTCP, mazindol, and CFT with binding site/sites labeled by [3H]mazindol. A one site model fit the data as well as the two site model: Bmax=16911 fmol/mg protein, Kd of [3H]mazindol=75 nM, Ki of GBR12935 =8.1 nM, Ki of CFT=50 nM and Ki of BTCP=44 nM. The inhibitory mechanism (competitive or noncompetitive) of several drugs (GBR12935, CFT, BTCP, cocaine, cis-flupentixol, nomifensine, WIN35,065-2, bupropion, PCP, and benztropine) was determined. All drugs inhibited [3H]mazindol binding by a competitive mechanism. Although the ligand-selectivity of the [3H]mazindol binding site indicates that it is the uptake inhibitor recognition site of the classic DA transporter, the quantitative differences among the ligand-selectivities of different radioligands for the same site suggest that each radioligand labels different overlapping domains of the DA uptake inhibitor recognition site. It is likely that development of domain-selective drugs may further our under-standing of the DA transporter.

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

[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.