Richard B. Rothman

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.

Salvinorin A: A potent naturally occurring nonnitrogenous κ opioid selective agonist

Salvia divinorum, whose main active ingredient is the neoclerodane diterpene Salvinorin A, is a hallucinogenic plant in the mint family that has been used in traditional spiritual practices for its psychoactive properties by the Mazatecs of Oaxaca, Mexico. More recently, S. divinorum extracts and Salvinorin A have become more widely used in the U.S. as legal hallucinogens. We discovered that Salvinorin A potently and selectively inhibited 3H-bremazocine binding to cloned κ opioid receptors. Salvinorin A had no significant activity against a battery of 50 receptors, transporters, and ion channels and showed a distinctive profile compared with the prototypic hallucinogen lysergic acid diethylamide. Functional studies demonstrated that Salvinorin A is a potent κ opioid agonist at cloned κ opioid receptors expressed in human embryonic kidney-293 cells and at native κ opioid receptors expressed in guinea pig brain. Importantly, Salvinorin A had no actions at the 5-HT2A serotonin receptor, the principal molecular target responsible for the actions of classical hallucinogens. Salvinorin A thus represents, to our knowledge, the first naturally occurring nonnitrogenous opioid-receptor subtype-selective agonist. Because Salvinorin A is a psychotomimetic selective for κ opioid receptors, κ opioid-selective antagonists may represent novel psychotherapeutic compounds for diseases manifested by perceptual distortions (e.g., schizophrenia, dementia, and bipolar disorders). Additionally, these results suggest that κ opioid receptors play a prominent role in the modulation of human perception.

Evidence for possible involvement of 5-HT(2B) receptors in the cardiac valvulopathy associated with fenfluramine and other serotonergic medications

Background—Serotonergic medications with various mechanisms of action are used to treat psychiatric disorders and are being investigated as treatments for drug dependence. The occurrence of fenfluramine-associated valvular heart disease (VHD) has raised concerns that other serotonergic medications might also increase the risk of developing VHD. We hypothesized that fenfluramine or its metabolite norfenfluramine and other medications known to produce VHD have preferentially high affinities for a particular serotonin receptor subtype capable of stimulating mitogenesis. Methods and Results—Medications known or suspected to cause VHD (positive controls) and medications not associated with VHD (negative controls) were screened for activity at 11 cloned serotonin receptor subtypes by use of ligand-binding methods and functional assays. The positive control drugs were (±)-fenfluramine; (+)-fenfluramine; (−)-fenfluramine; its metabolites (±)-norfenfluramine, (+)-norfenfluramine, and (−)-norfenfluramine; ergotamine; and methysergide and its metabolite methylergonovine. The negative control drugs were phentermine, fluoxetine, its metabolite norfluoxetine, and trazodone and its active metabolite m-chlorophenylpiperazine. (±)-, (+)-, and (−)-Norfenfluramine, ergotamine, and methylergonovine all had preferentially high affinities for the cloned human serotonin 5-HT2B receptor and were partial to full agonists at the 5-HT2B receptor. Conclusions—Our data imply that activation of 5-HT2B receptors is necessary to produce VHD and that serotonergic medications that do not activate 5-HT2B receptors are unlikely to produce VHD. We suggest that all clinically available medications with serotonergic activity and their active metabolites be screened for agonist activity at 5-HT2B receptors and that clinicians should consider suspending their use of medications with significant activity at 5-HT2B receptors.

The Designer Methcathinone Analogs, Mephedrone and Methylone, are Substrates for Monoamine Transporters in Brain Tissue

The nonmedical use of ‘designer’ cathinone analogs, such as 4-methylmethcathinone (mephedrone) and 3,4-methylenedioxymethcathinone (methylone), is increasing worldwide, yet little information is available regarding the mechanism of action for these drugs. Here, we employed in vitro and in vivo methods to compare neurobiological effects of mephedrone and methylone with those produced by the structurally related compounds, 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine. In vitro release assays using rat brain synaptosomes revealed that mephedrone and methylone are nonselective substrates for plasma membrane monoamine transporters, similar to MDMA in potency and selectivity. In vivo microdialysis in rat nucleus accumbens showed that i.v. administration of 0.3 and 1.0 mg/kg of mephedrone or methylone produces dose-related increases in extracellular dopamine and serotonin (5-HT), with the magnitude of effect on 5-HT being greater. Both methcathinone analogs were weak motor stimulants when compared with methamphetamine. Repeated administrations of mephedrone or methylone (3.0 and 10.0 mg/kg, s.c., 3 doses) caused hyperthermia but no long-term change in cortical or striatal amines, whereas similar treatment with MDMA (2.5 and 7.5 mg/kg, s.c., 3 doses) evoked robust hyperthermia and persistent depletion of cortical and striatal 5-HT. Our data demonstrate that designer methcathinone analogs are substrates for monoamine transporters, with a profile of transmitter-releasing activity comparable to MDMA. Dopaminergic effects of mephedrone and methylone may contribute to their addictive potential, but this hypothesis awaits confirmation. Given the widespread use of mephedrone and methylone, determining the consequences of repeated drug exposure warrants further study.

Rapid Communication: Attenuation of Methamphetamine‐Induced Neurotoxicity in Copper/Zinc Superoxide Dismutase Transgenic Mice

Abstract: Administration of methamphetamine (METH) to rats and nonhuman primates causes loss of terminals in the nigrostriatal dopaminergic system. The mechanism by which METH causes its neurotoxicity is not known. To evaluate further the role of oxyradicals in METH‐induced neurotoxicity, we have tested its effects in CuZn superoxide dismutase (SOD) transgenic (Tg) mice, which express the human CuZnSOD gene. In non‐Tg mice, acute METH administration causes significant decreases in levels of dopamine (DA) and 3, 4‐dihydroxyphenylacetic acid (DOPAC) in the striata and cortices of non‐Tg mice. In contrast, there were no significant decreases in cortical or striatal DA in the SOD‐Tg mice. The effects of METH on DOPAC were also attenuated in both structures of these SOD‐Tg mice. Chronic METH administration caused decreases in levels of striatal DA and DOPAC in the non‐ Tg mice, whereas the SOD‐Tg mice were not affected. These results suggest that METH‐induced dopaminergic toxicity in mice may be secondary to increased production of reactive oxygen species such as the superoxide radical.

Powerful Cocaine-Like Actions of 3,4-Methylenedioxypyrovalerone (MDPV), a Principal Constituent of Psychoactive ‘Bath Salts’ Products

The abuse of psychoactive ‘bath salts’ containing cathinones such as 3,4-methylenedioxypyrovalerone (MDPV) is a growing public health concern, yet little is known about their pharmacology. Here, we evaluated the effects of MDPV and related drugs using molecular, cellular, and whole-animal methods. In vitro transporter assays were performed in rat brain synaptosomes and in cells expressing human transporters, while clearance of endogenous dopamine was measured by fast-scan cyclic voltammetry in mouse striatal slices. Assessments of in vivo neurochemistry, locomotor activity, and cardiovascular parameters were carried out in rats. We found that MDPV blocks uptake of [3H]dopamine (IC50=4.1 nM) and [3H]norepinephrine (IC50=26 nM) with high potency but has weak effects on uptake of [3H]serotonin (IC50=3349 nM). In contrast to other psychoactive cathinones (eg, mephedrone), MDPV is not a transporter substrate. The clearance of endogenous dopamine is inhibited by MDPV and cocaine in a similar manner, but MDPV displays greater potency and efficacy. Consistent with in vitro findings, MDPV (0.1–0.3 mg/kg, intravenous) increases extracellular concentrations of dopamine in the nucleus accumbens. Additionally, MDPV (0.1–3.0 mg/kg, subcutaneous) is at least 10 times more potent than cocaine at producing locomotor activation, tachycardia, and hypertension in rats. Our data show that MDPV is a monoamine transporter blocker with increased potency and selectivity for catecholamines when compared with cocaine. The robust stimulation of dopamine transmission by MDPV predicts serious potential for abuse and may provide a mechanism to explain the adverse effects observed in humans taking high doses of ‘bath salts’ preparations.

N-Desalkylquetiapine, a potent norepinephrine reuptake inhibitor and partial 5-HT1A agonist, as a putative mediator of quetiapine's antidepressant activity

Quetiapine is an atypical antipsychotic drug that is also US FDA approved for treating bipolar depression, albeit by an unknown mechanism. To discover the potential mechanism for this apparently unique action, we screened quetiapine, its metabolite N-Desalkylquetiapine, and dibenzo[b,f][1,4]thiazepine-11(10-H)-one (DBTO) against a large panel of G-protein–coupled receptors, ion channels, and neurotransmitter transporters. DBTO was inactive at all tested molecular targets. N-Desalkylquetiapine had a high affinity (3.4 nM) for the histamine H1 receptor and moderate affinities (10–100 nM) for the norepinephrine reuptake transporter (NET), the serotonin 5-HT1A, 5-HT1E, 5-HT2A, 5-HT2B, 5-HT7 receptors, the α1B-adrenergic receptor, and the M1, M3, and M5 muscarinic receptors. The compound had low affinities (100–1000 nM) for the 5-HT1D, 5-HT2C, 5-HT3, 5-HT5, 5-HT6, α1A, α2A, α2B, α2C, H2, M2, M4, and dopamine D1, D2, D3, and D4 receptors. N-Desalkylquetiapine potently inhibited human NE transporter with a Ki of 12 nM, about 100-fold more potent than quetiapine itself. N-Desalkylquetiapine was also 10-fold more potent and more efficacious than quetiapine at the 5-HT1A receptor. N-Desalkylquetiapine was an antagonist at 5-HT2A, 5-HT2B, 5-HT2C, α1A, α1D, α2A, α2C, H1, M1, M3, and M5 receptors. In the mouse tail suspension test, N-Desalkylquetiapine displayed potent antidepressant-like activity in VMAT2 heterozygous mice at doses as low as 0.1 mg/kg. These data strongly suggest that the antidepressant activity of quetiapine is mediated, at least in part, by its metabolite N-Desalkylquetiapine through NET inhibition and partial 5-HT1A agonism. Possible contributions of this metabolite to the side effects of quetiapine are discussed.

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.

N-substituted piperazines abused by humans mimic the molecular mechanism of 3,4-methylenedioxymethamphetamine (MDMA, or 'Ecstasy').

3,4-Methylenedioxymethamphetamine (MDMA, or ‘Ecstasy’) is an illicit drug that stimulates the release of serotonin (5-HT) and dopamine (DA) from neurons. Recent evidence reveals that drug users are ingesting piperazine analogs, like 1-benzylpiperazine (BZP, or ‘A2’) and 1-(m-trifluoromethylphenyl)piperazine (TFMPP, or ‘Molly’), to mimic psychoactive effects of MDMA. In the present study, we compared the neurochemistry of MDMA, BZP, and TFMPP in rats. The effects of MDMA, BZP, and TFMPP on transporter-mediated efflux of [3H]5-HT and [3H]MPP+ (DA transporter substrate) were determined in synaptosomes. The effects of drugs on extracellular levels of 5-HT and DA were examined using in vivo microdialysis in conscious rats. MDMA evoked transporter-mediated release of [3H]5-HT and [3H]MPP+. BZP released [3H]MPP+, whereas TFMPP was a selective releaser of [3H]5-HT. MDMA (1–3 mg/kg, i.v.) increased dialysate 5-HT and DA in a dose-related fashion, with actions on 5-HT being predominant. BZP (3–10 mg/kg, i.v.) elevated dialysate DA and 5-HT, while TFMPP (3–10 mg/kg, i.v.) elevated 5-HT. Administration of BZP plus TFMPP at a 1:1 ratio (BZP/TFMPP) produced parallel increases in dialysate 5-HT and DA; a 3 mg/kg dose of BZP/TFMPP mirrored the effects of MDMA. At a 10 mg/kg dose, BZP/TFMPP increased dialysate DA more than the summed effects of each drug alone, and some rats developed seizures. Our results show that BZP/TFMPP and MDMA share the ability to evoke monoamine release, but dangerous drug–drug synergism may occur when piperazines are coadministered at high doses.

Autoradiographic localization of μ- and δ-opiate receptors in the forebrain of the rat

The autoradiographic distributions of μ opiate receptors, labeled in vitro by [125I] d -Ala2-MePhe4-Met(o)5-ol-enkephalin (FK), and δ-opiate receptors, labeled by [3H] d -Ala2- d -Leu5-enkephalin (DADLE) in the presence of oxymorphone to block high affinity binding to the μ site, were examined and compared in the forebrain of the rat. The μ- and δ-receptors were differentially distributed in most structures. μ Binding sites were found in nearly all gray matter structures and showed heterogeneous patterns of density that were correlated with cytoarchitecture and neuronal connections. Laminar density profiles were seen in laminated structures such as olfactory bulb, cerebral cortex and hippocampus. Highest μ binding densities were in striatal patches and the habenular streak. δ Sites had distinct laminar patterns in the main olfactory bulb and cortex which differed from the μ patterns. The external plexiform layer of the main olfactory bulb had the greatest density of δ binding sites; cortex and striatum were also densely labeled. The septum, globus pallidus, preoptic area and hypothalamus were lightly labeled by both ligands. The magnocellular hypothalamic nuclei had negligible μ and δ labeling. The thalamus had dense μ but sparse δ sites. μ And δ binding sites were both present in the amygdala but had different distributions. Two fibers tracts — optic tract and fasciculus retroflexus — had FK labeling. In contrast, a portion of the corpus callosum was labeled by DADLE and not by FK. The results suggest an association of μ-opiate receptors with sensory, especially olfactory, and limbic projections in the forebrain, and δ-opiate receptors with intrinsic and commissural forebrain pathways.