Danuta Marona-Lewicka

Reinforcing effects of certain serotonin-releasing amphetamine derivatives

The present study was designed to characterize further the rewarding and aversive properties of 3,4-methylenedioxymethamphetamine (MDMA), the alpha-ethyl homologue of MDMA (MBDB), fenfluramine, and the selective serotonin releasing agent 5-methoxy-6-methyl-2-aminoindan (MMAI) using the conditioned place preference paradigm (CPP). Extracellular dopamine (DA) and its metabolite DOPAC were also measured in the nucleus accumbens after systemic drug administration, using in vivo microdialysis in freely moving rats. MDMA produced a positive dose-dependent effect in the CPP test, which was maximal at doses of 5 and 10 mg/kg. MBDB also induced a positive CPP, with a maximum effect at 10 mg/kg. The conditioning effect of MBDB was more than 2.5-fold weaker compared with MDMA. Fenfluramine evoked place aversion at doses of 4, 6, and 10 mg/kg. This effect of fenfluramine was independent of dose. MMAI at doses of 1.25, 2.5, and 5 mg/kg produced no significant effect on place conditioning. At doses of 10 and 20 mg/kg, MMAI produced an effect similar to fenfluramine: Place aversion was independent of dose. In the microdialysis experiments, MDMA significantly elevated extracellular DA and induced a decrease of DOPAC in the nucleus accumbens. Thus, activation of dopaminergic systems may be responsible for the rewarding properties of MDMA-like drugs. In contrast to the effects seen with MDMA, no difference in extracellular DA or DOPAC was seen after injection of MBDB, fenfluramine, or MMAI, even though MBDB weakly induced a place preference. The mechanism responsible for the development of place aversion by fenfluramine or MMAI is unknown at this time and requires further study.

p-Methylthioamphetamine is a potent new non-neurotoxic serotonin-releasing agent

p-Methylthioamphetamine (MTA), was compared to p-chloroamphetamine (PCA) in a number of pharmacological assays. MTA was about 2-fold more potent than PCA at inhibiting synaptosomal uptake of [3H]5-hydroxytryptamine ([3H]5-HT), and about 7-fold and 10-fold less potent than PCA at inhibiting synaptosomal uptake of [3H]dopamine and [3H]norepinephrine, respectively. In drug discrimination assays, MTA was nearly equipotent to PCA in animals trained to discriminate saline from 3,4-methylenedioxymethamphetamine (MDMA), or two related analogues S-(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (S-MBDB) or 5-methoxy-6-methyl-2-aminoindan (MMAI). MTA caused dose-dependent increases of tritium efflux from superfused rat frontal cortex slices preloaded with [3H]5-HT, comparable to that induced by an equal molar concentration of PCA. The potential neurotoxicity of MTA was examined by measuring monoamine and metabolite levels at one week following an acute dose. A 10 mg/kg dose of PCA caused a 70-90% decrease of cortical, hippocampal and striatal 5-HT and 5-hydoxyindoleacetic acid (5-HIAA) levels, while twice the molar dose of MTA (21.3 mg/kg) had no effect. Thus, MTA is a potent, selective, serotonin releaser, apparently devoid of serotonin neurotoxic effects. This work also supports the idea that catecholamine systems may play a critical role in the neurotoxicity of PCA-like compounds.

The Effect of Selective Serotonin Releasing Agents in the Chronic Mild Stress Model of Depression in Rats

Chronic exposure to mild unpredictable stress has been found to depress the consumption of, and preference for, highly palatable sucrose solution in rats. Stress-induced behavioral deficits may be maintained for a long time, however chronic administration of clinically effective antidepressants can restore normal behavior. This is the first report showing that Sprague-Dawley rats can be used in this model. A preference deficit in this strain of rats took at least 7 weeks to develop; about twice the time required when hooded Lister or Wistar rats are used in this model. Water consumption was not effected by chronic exposure to the mild stress regime and/or by chronic administration of the selective serotonin (5-HT) releasing agent MMAI (5-methoxy-6-methyl-2-aminoindan). The stress-induced deficit in sucrose intake was completely reversed by chronic treatment with MMAI (5 mg/kg, 2 x day) over 3 weeks in the two-bottle tests. In single-bottle tests, chronic treatment with the selective 5-HT releasers, MMAI (5 mg/kg, 2 x day) or MTA (p-methylthioamphetamine; 5 mg/kg, 2 x day), reversed the deficit in rewarded behavior (anhedonia) measured as a decrease in the consumption of 1% sucrose solution in the chronic mild stress model of depression in rats. With the experimental procedure employed, and at a dose of 10 mg/kg/day of 5-HT releasers, the magnitude and onset of this effect were greater than observed following similar administration of the selective 5-HT reuptake inhibitor (SSRI) sertraline (10 mg/kg/day), used as a standard anti-depressant drug.

Behavioral effects of the highly selective serotonin releasing agent 5-methoxy-6-methyl-2-aminoindan

The behavioral effects of 5-methoxy-6-methyl-2-aminoindan (MMAI) were examined using the drug discrimination procedure and direct observation for classification of the characteristic syndrome induced by MMAI. The stimulus effects of MMAI were studied in 5 different groups of rats trained to discriminate MMAI (1.71 mg/kg; 8 microM/kg), MDMA (3,4-methylenedioxymethamphetamine; 1.75 mg/kg; 7.6 microM/kg), (+)-MBDB ((+)-N-methylamino-(1,3-benzodioxo-5-yl)-2-butanamine; 1.75 mg/kg; 7.18 microM/kg), (+)-amphetamine (1 mg/kg; 5.4 microM/kg), or LSD ((+)-lysergic acid diethylamide tartrate; 0.08 mg/kg; 186 nM/kg) from saline. In substitution tests in rats trained to discriminate MMAI from saline, all the compounds which fully mimicked MMAI were serotonin (5-hydroxytryptamine, 5-HT) releasing agents. This substitution is symmetrical for MDMA and (+)-MBDB. Nevertheless, the dose-response curve of MMAI is parallel to those of (+)-fenfluramine (m-trifluoromethyl-N-ethylamphetamine) and p-chloroamphetamine. The results also show that MMAI lacks amphetamine-like and LSD-like discriminative stimulus effects, suggesting that MMAI is neither a psychostimulant nor a hallucinogen. Tests of the discriminability of MMAI after 5-HT depletion with the selective serotonin synthesis inhibitor, p-chlorophenylalanine (2 x 200 mg/kg i.p., pretreatment 72 h before test), showed only saline appropriate responding. Prolonged block (ca. 1 week) of the MMAI cue by p-chlorophenylalanine further supports the conclusion that endogenous 5-HT is essential for MMAI discrimination. Fluoxetine (10 mg/kg) or paroxetine (2.5 mg/kg), both selective 5-HT uptake inhibitors, reduced the discriminability of MMAI to 40% and 50%, respectively. None of the antagonists (ketanserin, methiothepin, pindolol, yohimbine, haloperidol) used in antagonism tests inhibited the stimulus properties of MMAI. These results and data from radioligand binding studies support the conclusion that direct activation or inhibition of known neurotransmitter receptors did not play a significant role in the discriminative cue of MMAI. The administration of 5, 10, or 20 mg/kg of MMAI to rats induced a behavioral syndrome consisting of hypolocomotion with accompanying catalepsy-like posture, turning, Straub tail, flat body posture, and suppressed sleeping time. In general, this is qualitatively similar to what is seen after administration of 5-HT precursors or 5-HT receptor agonists. In conclusion, the data from the drug discrimination study and the behavioral syndrome induced by MMAI suggest that MMAI is a potential selective releaser of serotonin.

Re-evaluation of lisuride pharmacology: 5-hydroxytryptamine1A receptor-mediated behavioral effects overlap its other properties in rats.

Abstract Rationale. There is substantial evidence that lisuride can produce effects linked to 5-HT1A receptor occupancy. Nevertheless, this action has generally been ignored in the mechanism of action of lisuride, in favor of an exclusive role for dopamine receptors in considering its antiparkinsonian effects, or an exclusive role of 5-HT2A/2C receptor activation in hallucinogenesis. These conclusions are surprising when one considers that the potent interaction of lisuride with 5-HT1A receptors has been demonstrated in several different laboratories and that activation of 5-HT1A and 5-HT1B receptors can modulate dopaminergically mediated responses. Objective. The lack of full substitution of lisuride for lysergic acid diethylamide (LSD) in drug discrimination experiments and induction of a pronounced 5-HT syndrome by this compound at relatively low doses convinced us to execute two series of experiments that might explain the primary mechanism responsible for lisuride-mediated biological effects and its paradoxical classification as a dopamine agonist in the literature. Results. In drug discrimination studies, lisuride fully mimicked the 5-HT1A agonist LY 293284, only partially substituted for LSD and DOI, and failed to substitute for (+)-amphetamine. Lisuride produced a significant dose-related increase in flat body posture, forepaw treading, and lower-lip retraction which reflect a modulation of behavior by action at central 5-HT1A receptors. Only pMPPI [4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridynyl-benzamide hydrochloride], a selective 5-HT1A antagonist, was effective in inhibiting all 5-HT syndrome behaviors produced by lisuride, whereas pMPPI was without effect on any behavior induced by LSD. Lisuride dose dependently decreased body temperature in rats with a potency similar to that of the selective 5-HT1A agonist LY 293284. The hypothermic effect of lisuride was prevented by pre-injection of pMPPI, but not by ketanserin or haloperidol. Conclusion. We have demonstrated that the behavioral effects of low doses of lisuride are clearly mediated by stimulation of 5-HT1A receptors.

Further evidence that the delayed temporal dopaminergic effects of LSD are mediated by a mechanism different than the first temporal phase of action.

Activation of 5-HT(2A) receptors is thought to mediate the hallucinogenic effects of LSD. Nevertheless, in a previous report we provided evidence that a delayed temporal phase of the behavioral pharmacology of LSD is mediated by D(2)-like dopamine receptor stimulation. In this study rats were trained to discriminate LSD with either a 30 min preinjection time (LSD-30, N=12) or a 90 min preinjection time (LSD-90, N=13) from saline, using a two-lever, food-reinforced operant conditioning task. We then tested a large number of agonists and antagonists belonging to distinct pharmacological classes in these animals. As anticipated, classical hallucinogens such as psilocin and mescaline substituted only in LSD-30 rats, and not in LSD-90 rats. The dopamine receptor agonists ABT-724, aripiprazole, dihydrexidine, WAY 100635, and SKF 38393, fully or partially mimicked LSD-90, but not LSD-30. The results reported here support and extend our previous conclusion that the delayed temporal effects of LSD are mediated by activation of a dopaminergic system.

WAY 100635 produces discriminative stimulus effects in rats mediated by dopamine D4 receptor activation

In-vitro studies have shown that WAY 100635 is not only a potent 5-HT1A antagonist, but also has high affinity and efficacy at the dopamine D4 receptor. Nevertheless, the behavioral effects of this compound have not been investigated. This study sought to characterize the discriminative stimulus effects produced by WAY 100635. Male Sprague–Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with WAY 100635 (10 μmol/kg) as a discriminative stimulus. Substitution tests with different doses of WAY 100635 and (−)-pindolol, and combination tests with two 5-HT1A agonists, 8-OH-DPAT and LY 293284; and two dopamine D4 antagonists, sonepiprazole and A-381393, were performed. Rats trained with a low dose (0.74 μmol/kg) of WAY 100635 could not learn the discrimination task after more than 3 months of sessions. Rats trained to discriminate 10 μmol/kg of WAY 100635 from saline achieved the criterion of accuracy after approximately 35 training sessions. WAY 100635 (2.5–10 μmol/kg) produced a dose-dependent increase in WAY 100635-appropriate responding, with a mean effective dose of 3.44 μmol/kg, whereas saline or pindolol (5–25 μmol/kg) administration resulted in 0% drug lever responding. Pretreatment with the 5-HT1A agonists 8-OH-DPAT or LY 293284 did not modify the WAY 100635 curve, but pretreatment with the selective dopamine D4 antagonists sonepiprazole or A-381393 completely blocked the cue. These results indicate that the discriminative stimulus effect produced by WAY 100635 is mediated by activation of dopamine D4 receptors.

Complex stimulus properties of LSD: a drug discrimination study with α2-adrenoceptor agonists and antagonists

The influence of several α2-adrenergic agents on the discriminative stimulus (DS) properties of lysergic acid diethylamide (LSD) was studied in rats trained to discriminate 0.08 mg/kg (186 nmol/kg) of LSD from saline in a two-lever operant paradigm. Only yohimbine fully mimicked LSD with an ED50 of 2.05 mg/kg (5.24 µmol/kg). Yohimbines 5-HT1A agonist properties may be responsible for this substitution. Other α2-adrenoceptor antagonists, idazoxan with an agonist/antagonist profile at 5-HT1A receptors and RS 26026-197, a highly selective α2-adrenoceptor antagonist, failed to produce substitution. Clonidine, an α2-adrenoceptor agonist, did not substitute for LSD but the response rate was dose-dependently reduced. None of the α2-adrenergic agents used for pretreatment before LSD inhibited the response to the LSD training dose. Coadministration of clonidine with LSD produced a leftward shift of the dose-response relationship of LSD without a significant change in the slope of the dose-response line. Simultaneous administration of α2-adrenergic agents with LSD shifted the dose-response curve to the left only when the adrenergic agent also possessed at least moderate affinity for the 5-HT1A receptor. In addition, radioligand competition experiments were performed that showed LSD to have relatively high affinity (Ki=37 nM) for [3H]clonidine-labeled sites in rat cortex with lower affinity for [3H]yohimbine labeled sites. While previous studies have suggested that the nature of the LSD cue may be essentially expressed by 5-HT2 receptor activation, the present data show that this cue can be modulated by effects of LSD at 5-HT1A and at other monoamine neurotransmitter receptors.

Extensive Rigid Analogue Design Maps the Binding Conformation of Potent N-Benzylphenethylamine 5-HT2A Serotonin Receptor Agonist Ligands

Based on the structure of the superpotent 5-HT(2A) agonist 2-(4-bromo-2,5-dimethoxyphenyl)-N-[(2-methoxyphenyl)methyl]ethanamine, which consists of a ring-substituted phenethylamine skeleton modified with an N-benzyl group, we designed and synthesized a small library of constrained analogues to identify the optimal arrangement of the pharmacophoric elements of the ligand. Structures consisted of diversely substituted tetrahydroisoquinolines, piperidines, and one benzazepine. Based on the structure of (S,S)-9b, which showed the highest affinity of the series, we propose an optimal binding conformation. (S,S)-9b also displayed 124-fold selectivity for the 5-HT(2A) over the 5-HT(2C) receptor, making it the most selective 5-HT(2A) receptor agonist ligand currently known.

5-HT2A/2C receptor agonists potentiate the discriminative cue of (+)-amphetamine in the rat.

The possible effect of 5-HT2A/2C receptor agonists on an amphetamine-induced behavioral response was examined using the two-lever drug discrimination paradigm. The experiments were designed to investigate an interaction of the hallucinogenic 5-HT2A/2C agonists lysergic acid diethylamide (LSD) and 2,5-dimethoxy-4-iodoamphetamine (DOI), with the discriminative stimulus elicited by a relatively low dose of (+)-amphetamine (1.35 micromol/kg, 0.25 mg/kg, which produced approximately 50% selection of the drug lever). DOI and LSD did not produce amphetamine-like responding at any dose tested or time of administration. However, LSD alone was able to induce a drug-appropriate response in two of nine amphetamine-trained rats. Simultaneous administration of DOI or LSD with amphetamine was not significantly different from the response produced by amphetamine alone. Pre-administration of DOI (3 hr) or of LSD (2 hr) before amphetamine, however, evoked significant enhancement of the amphetamine cue. The results suggest that the enhanced behavioral response to amphetamine may be due either to an increased sensitivity of dopaminergic neurons in the mesolimbic area, or to an enhanced release of dopamine by amphetamine.