Reversible and irreversible phases of serotonin depletion by 4-chloroamphetamine

5-hydroxytryptamine- and dopamine-releasing effects of ring-substituted amphetamines on rat brain: a comparative study using in vivo microdialysis

Using in vivo microdialysis, a comparative study was conducted to examine the effects of amphetamine-related compounds (methamphetamine, MAP; 3,4-methylenedioxymethamphetamine, MDMA; p-methoxyamphetamine, PMA; p-methoxymethamphetamine, PMMA; 4-methylthioamphetamine, 4-MTA; 3,4,5-trimethoxyamphetamine, TMA; 2,5-dimethoxy-4-iodoamphetamine, DOI) on extracellular levels of serotonin (5-HT) and dopamine (DA). Dialysates were assayed using HPLC equipped with electrochemical detector following i.p. administration with each drug at a dose of 5 mg/kg. MAP was found to drastically and rapidly increase 5-HT and DA levels (870% and 1460%, respectively). PMA, PMMA, and 4-MTA slightly increased DA levels (150-290%) but remarkably increased 5-HT levels (540-900%). In contrast, TMA and DOI caused no detectable changes in levels of both monoamines. We observed that the potent DA-releasing action of MAP was remarkably decreased by introduction of methoxy or methylthio group at the para position (MAP vs. PMMA or 4-MTA), but introduction of two additional adjacent methoxy groups into PMA totally abolished its 5-HT-/DA-releasing action (PMA vs. TMA). In addition, para-mono-substituted compounds inhibited both monoamine oxidase (MAO) enzymes more strongly than other compounds; PMA and 4-MTA exhibited submicromolar IC50 values for MAO-A. On the other hand, TMA scarcely affected the activity of both MAO enzymes as well as extracellular levels of 5-HT and DA. In this comparative study, MDMA, PMA, and 4-MTA functioned similar to PMMA, a typical empathogen; these findings therefore could be helpful in clarifying the psychopharmacological properties of amphetamine-related, empathogenic designer drugs.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part I. General background and discovery of serotonin as a basis for 5-HT receptor identification

This review contains background information on the serotonin system, furthermore the suggestion to introduce the term Contemporary Witness Report (CWR) for a novel type of review and, as the main part, an overview over the history of serotonin discovery as a basis for the identification of its receptor heterogeneity and the increase in complexity by genetic and allosteric variation. The present article conforms to CWRs in historical and autobiographical elements, in more emphasis on the author's work than in conventional reviews and in aspects neglected in previous reviews, but not in the main feature namely the work of a scientist with comprehensive expertise in a field in which, over long time, he/she continuously performed research and published. A scientist complying with these requirements is a contemporary witness in that field. His report on the scientific achievements in that period, a CWR, comprises confirmation and putative re-interpretation of data from a superior viewpoint. Identification of serotonin's vascular properties (publication year: 1912) as an "adrenaline mimicking substance" (without attempt to isolate it) by O'Connor preceded the discovery of serotonin in the gastrointestinal tract by Erspamer [1937] and in blood by Rapport [1948, 1949], who identified its structure as 5-hydroxytryptamine [1949]. Detection as a neurotransmitter in invertebrates suggested its occurrence in vertebrate CNS as well. This was verified by finding it in dog, rat and rabbit brain [1953]. The Falck-Hillarp technique [1962] visualized serotonin neurones as fluorescent structures. The neurotoxin 5,7-dihydroxytryptamine [1972] indirectly proved the involvement of 5-HT in multiple CNS functions.

On the mechanism of the acute decrease of rat brain tryptophan hydroxylase activity by 4-chloroamphetamine

The decrease of rat brain soluble tryptophan hydroxylase activity 4 h after the injection of 20 mg/kg i.p. of 4-chloroamphetamine was antagonized by 20 mg/kg i.p. of zimelidine, a selective inhibitor of membrane serotonin (5-HT) uptake, by 2.5 mg/kg i.p. of chlorpromazine and by 5 mg/kg i.p. of methergoline, a proposed antagonist of central 5-HT receptors, when these compounds were injected 15 min prior to 4-chloroamphetamine. Zimelidine partially reversed the fall in tryptophan hydroxylase when injected 4 h after 4-chloroamphetamine and the enzyme activity was recorded after further 4 h. Reserpine, 5 mg/kg i.p., 18 h before 4-chloroamphetamine prevented the decrease of the enzyme activity but had no effect when injected 1 h beforehand. It is suggested that the decrease of the tryptophan hydroxylase activity is secondary to the release of 5-HT and mediated by 5-HT receptor stimulation.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Identification and characterization of metallothionein-1 and -2 gene expression in the context of (+/-)3,4-methylenedioxymethamphetamine-induced toxicity to brain dopaminergic neurons

In mice, the recreational drug (+/-)3,4-methylenedioxymethamphetamine [MDMA ("ecstasy")] produces a selective toxic effect on brain dopamine (DA) neurons. Using cDNA microarray technology in combination with an approach designed to facilitate recognition of relevant changes in gene expression, the present studies sought to identify genes potentially involved in murine MDMA-induced toxicity to DA neurons. Of 15,000 mouse cDNA fragments studied, metallothionein (Mt)-1 and Mt2 emerged as candidate genes possibly involved in MDMA-induced toxicity to DA neurons. Northern blot analysis confirmed the microarray findings and revealed a dynamic upregulation of Mt1 and Mt2 mRNA in the ventral midbrain within 4-12 hr after MDMA treatment. Western blot analysis showed a similar increase in MT protein levels, with peak times occurring subsequent to increases in mRNA levels. Mt1-2 double knock-out mice were more vulnerable to MDMA-induced toxicity to DA neurons than corresponding wild-type mice. Stimulation of endogenous expression of MT protein with zinc acetate conferred complete protection against MDMA-induced toxicity to DA neurons, and administration of exogenous MT protein afforded partial protection. Collectively, these results indicate that MDMA-induced toxicity to DA neurons is associated with increased Mt1 and Mt2 gene transcription and translation, possibly as part of a neuroprotective mechanism. The present findings may have therapeutic implications for neuropathological conditions involving DA neurons.

Therapeutic and adverse actions of serotonin transporter substrates

A variety of drugs release serotonin (5-HT, 5-hydroxytryptamine) from neurons by acting as substrates for 5-HT transporter (SERT) proteins. This review summarizes the neurochemical, therapeutic, and adverse actions of substrate-type 5-HT-releasing agents. The appetite suppressant (+/-)-fenfluramine is composed of (+) and (-) isomers, which are N-de-ethylated in the liver to yield the metabolites (+)- and (-)-norfenfluramine. Fenfluramines and norfenfluramines are potent 5-HT releasers. (+/-)-3,4-Methylenedioxymethamphetamine ((+/-)-MDMA, "ecstasy") and m-chlorophenylpiperazine (mCPP) are substrate-type 5-HT releasers. Fenfluramines, (+/-)-MDMA, and mCPP release neuronal 5-HT by a common non-exocytotic diffusion-exchange mechanism involving SERTs. (+)-Norfenfluramine is a potent 5-HT(2B) and 5-HT(2C) receptor agonist. The former activity may increase the risk of valvular heart disease, whereas the latter activity is implicated in the anorexic effect of systemic fenfluramine. Appetite suppressants that increase the risk for developing primary pulmonary hypertension (PPH) are all SERT substrates, but these drugs vary considerably in their propensity to increase this risk. For example, fenfluramine and aminorex are clearly linked to the occurrence of PPH, whereas other anorectics are not. Similarly, some SERT substrates deplete brain tissue 5-HT in animals (e.g., fenfluramine), while others do not (e.g., mCPP). In addition to the established indication of obesity, 5-HT releasers may help treat psychiatric disorders, such as drug and alcohol dependence, depression, and premenstrual syndrome. Viewed collectively, we believe new medications can be developed that selectively release 5-HT without increasing the risk for adverse effects of valvular heart disease, PPH, and neurotoxicity. Such agents may be useful for treating a variety of psychiatric disorders.

Dual serotonin (5-HT) projections to the nucleus accumbens core and shell: relation of the 5-HT transporter to amphetamine-induced neurotoxicity

Dopamine release in the nucleus accumbens (NAc) has been implicated as mediating the rewarding effects of stimulant drugs; however, recent studies suggest that 5-HT release may also contribute. In an effort to assess the role of 5-HT in drug-mediated reward, this study analyzed the serotonergic innervation of NAc using immunocytochemistry for 5-HT and the 5-HT transporter (SERT). We report that in control rats the NAc receives two distinct types of 5-HT axons that differ in regional distribution, morphology, and SERT expression. Most regions of the NAc are innervated by thin 5-HT axons that express SERT, but in the caudal NAc shell nearly all 5-HT axons lack SERT and have large spherical varicosities. Two weeks after methamphetamine or p-chloroamphetamine (PCA) treatment, most 5-HT axons in dorsal striatum and NAc have degenerated; however, the varicose axons in the shell appear intact. These drug-resistant 5-HT axons that lack SERT densely innervate the caudal one-third of the accumbens shell, the same location where dopamine axons are spared after methamphetamine. Moreover, 4 hr after PCA, the varicose axons in the caudal shell retain prominent stores of 5-HT, whereas 5-HT axons in the rest of the NAc are depleted of neurotransmitter. The results demonstrate that two functionally different 5-HT projections innervate separate regions of the NAc and that selective vulnerability to amphetamines may result from differential expression of SERT. We postulate that action potentials conducted from the raphe nuclei can release 5-HT throughout the NAc, whereas transporter-mediated release induced by stimulant drugs is more restricted and unlikely to occur in the caudal NAc shell.

Effects of antihistamines on fenfluramine-induced depletion of indoles in the brain of rats

Various effects of chlorpheniramine (CPA), diphenhydramine (DIPH), tripelennamine (TRIP), and pyrilamine (PYRI) on fenfluramine (FEN)-induced depletion of serotonin in the brain of rats were observed to be dependent on body temperature. Levels of 5-HT and 5-HIAA in the frontal cortex, hippocampus, and striatum of rats treated with FEN (10 mg/kg, once or twice daily x 4 days) decreased to approximately 30% (P < 0.01) that of controls with no significant changes after CPA, DIPH, TRIP, and PYRI. Treatment with FEN plus CPA (5, 10, 20 mg/kg) and FEN plus DIPH (20 mg/kg), but not FEN plus TRIP (20 mg/kg) and FEN plus PYRI (20 mg/kg), increased brain serotonin levels 2- to 3-fold more than those treated with FEN plus saline. Treatment with FEN plus CPA and FEN plus DIPH, but not FEN plus TRIP and FEN plus PYRI, decreased rectal temperature with no significant change after FEN. The antihistamines alone decreased temperature at a 1-hour period and enhanced FEN-induced reduction in body weight. Possible mechanisms of the different effects of antihistamines on FEN-induced depletion of serotonin are discussed.

The effect of N-methylation on fenfluramine's neurotoxic and pharmacologic actions

N-Methylation separates methamphetamine's neurotoxic and pharmacologic effects. In particular, N-methylation eliminates methamphetamine's neurotoxic activity while preserving its behavioral pharmacologic activity. The purpose of the present studies was to determine whether N-methylation could also be used to separate fenfluramine's neurotoxic and pharmacologic effects. Fenfluramine-induced serotonin neurotoxicity was assessed by measuring serotonin axonal markers 2 weeks after fenfluramine administration. Pharmacologic effects of fenfluramine were assessed by measuring fenfluramine-induced anorexia and fenfluramine discrimination. Both fenfluramine and its N-methylated analog, N-methylfenfluramine, produced dose-related effects in food intake, drug-discrimination and neurotoxicity studies. Although N-methylation reduced the neurotoxic potency of fenfluramine, it also reduced its pharmacologic activity. Neurotoxic potency was reduced 4- to 8-fold (depending on brain region), while pharmacologic potency was reduced 4- to 10-fold (depending on paradigm). Notably, N-methylation did not change the efficacy of fenfluramine as a serotonin neurotoxin, anorectic agent or discrimination stimulus. These results indicate that fenfluramine's behavioral and neurotoxic effects, unlike those of methamphetamine, are not dissociated by N-methylation. Further, the present results suggest that the effectiveness of side-chain nitrogen substitution in separating the behavioral and neurotoxic effects of amphetamine derivatives is strongly influenced by ring substitutions.

In vivo evidence for free radical involvement in the degeneration of rat brain 5-HT following administration of MDMA ('ecstasy') and p-chloroamphetamine but not the degeneration following fenfluramine

1. Administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') to several species results in a long lasting neurotoxic degeneration of 5-hydroxytryptaminergic neurones in several regions of the brain. We have now investigated whether this degeneration is likely to be the result of free radical-induced damage. 2. Free radical formation can be assessed by measuring the formation of 2,3- and 2,5-dihydroxybenzoic acid (2,3-DHBA and 2,5-DHBA) from salicylic acid. An existing method involving implantation of a probe into the hippocampus and in vivo microdialysis was modified and validated. 3. Administration of MDMA (15 mg kg-1, i.p.) to Dark Agouti (DA) rats increased the formation of 2,3-DHBA (but not 2,5-DHBA) for at least 6 h. Seven days after this dose of MDMA, the concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was reduced by over 50% in hippocampus, cortex and striatum, reflecting neurotoxic damage. There was no change in the concentration of dopamine or 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. 4. p-Chloroamphetamine (PCA), another compound which produces a neurotoxic loss of cerebral 5-HT content, when given at a dose of 5 mg kg-1 also significantly increased the formation of 2.3-DHBA (but not 2,5-DHBA) in the dialysate for over 4.5 h. post-injection starting 2 h after treatment. 5. In contrast, fenfluramine administration (15 mg kg-1, i.p.) failed to increase the 2,3-DHBA or 2,5-DHBA concentration in the dialysate. A single fenfluramine injection nevertheless also markedly decreased the concentration of 5-HT and 5-HIAA in the hippocampus, cortex and striatum seven days later. 6. When rats pretreated with fenfluramine (15 mg kg-1, i.p.) seven days earlier were given MDMA (15 mg kg-1, i.p.) no increase in 2,3-DHBA was seen in the dialysate from the hippocampal probe. This indicates that the increase in free radical formation following MDMA is occurring in 5-HT neurones which have been damaged by the prior fenfluramine injection. 7. Administration of the free radical scavenging agent alpha-phenyl-N-tert-butyl nitrone (PBN; 120 mg kg-1, i.p.) 10 min before and 120 min after an MDMA (15 mg kg-1, i.p.) injection prevented the acute rise in the 2,3-DHBA concentration in the dialysate and attenuated by 30% the long term damage to hippocampal 5-HT neurones (as indicated by a smaller MDMA-induced decrease in both the concentration of 5-HT and 5-HIAA and also the binding of [3H]-paroxetine). 8. These data indicate that a major mechanism by which MDMA and PCA induce damage to 5-hydroxytryptaminergic neurones in rat brain is by increasing the formation of free radicals. These probably result from the degradation of catechol and quinone metabolites of these substituted amphetamines. In contrast, fenfluramine induces damage by another mechanism not involving free radicals; a proposal supported by some of our earlier indirect studies. 9. We suggest that these different modes of action render untenable the recent suggestion that MDMA will not be neurotoxic in humans because fenfluramine appears safe at clinical doses.

Studies on the mechanism of p-chloroamphetamine neurotoxicity

Studies were conducted to investigate the sensitivity of p-chloroamphetamine (PCA)-induced neurochemical changes to various pharmacological manipulations known to block the neurochemical effects of 3,4-methylenedioxymethamphetamine (MDMA). The monoamine oxidase-B (MAO-B) inhibitor L-deprenyl (2 mg/kg) given 4 hr before a nonneurotoxic dose of PCA (2 mg/kg) was shown not to alter the amount of [3H]paroxetine bound to serotonin (5-HT) uptake sites 7 days after treatment. L-Deprenyl 4 hr before a neurotoxic dose of PCA (10 mg/kg) did not change the acute hyperthermia. Further, neither L-deprenyl nor another selective MAO-B inhibitor, MDL-72,974 (1.25 mg/kg), given 30 min before or daily for 4 days before a single dose of PCA attenuated or potentiated the decrease in the number of [3H]paroxetine binding sites measured 7 days after PCA treatment. The combination of the MAO-A inhibitor clorgyline (2.5 mg/kg) or a nonspecific dose of L-deprenyl (10 mg/kg) with the selective 5-HT releasing agent 5,6-methylenedioxy-2-aminoindan did not lead to changes in the levels of 5-HT, 5-hydroxyindoleacetic acid or dopamine 7 days after treatment. Finally, the 5-HT2A receptor antagonist MDL-11,939 (5 mg/kg) did not protect against the neurotoxicity of PCA. By comparing the present work with previous studies of MDMA, these results can be interpreted to suggest that the mechanism of the neurotoxicity induced by PCA is not identical to that induced by MDMA. The relationship of these results to the neurotoxicity induced by MDMA is also discussed.

Effect of p-chlorophenylalanine at moderate dosage on 5-HT and 5-HIAA concentrations in brain regions of control and p-chloroamphetamine treated rats

The effects of p-chlorophenylalanine (PCPA, 100-150 mg/kg x 1. i.p.), doses which decrease brain 5-hydroxytryptamine (5-HT) by 30-50%, were investigated in both intact rats and 14 days after giving p-chloroamphetamine (PCA, 10 mg/kg/day x 2, i.p.). The PCPA dose-dependently decreased brain regional 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) 24 hr later. As per cent decreases of 5-HIAA were greater than those of 5-HT in cortex, striatum and hippocampus 5-HIAA/5-HT ratios fell, suggesting that partial inhibition of 5-HT synthesis by PCPA increases 5-HT conservation in these terminal regions. In the hypothalamus and brain stem, decreases of the ratio were small or absent. The PCA given without subsequent PCPA treatment decreased 5-HT and 5-HIAA so that 5-HT fell by about 70% in the cortex, striatum and hippocampus, 55% in the brain stem but only by 27% in the hypothalamus. The PCPA given after PCA decreased 5-HT and 5-HIAA further but not the 5-HIAA/5-HT ratios and increased the ratio in the brain stem. The 5-HIAA/5-HT findings imply that the increase of 5-HT conservation after PCPA treatment does not occur after partial depletion of 5-HT by PCA. The increase of the 5-HIAA/5-HT ratio in the brain stem is explicable by the resistance to both PCA and PCPA of 5-HT in cell bodies where the ratio is high. Results are discussed in relation to the question of whether the PCA treatment used destroys axon terminals projecting from the dorsal but not from the median raphe.

5-HT modulation of auditory and visual sensorimotor gating: I. Effects of 5-HT releasers on sound and light prepulse inhibition in Wistar rats

Increasing evidence suggests an important role for serotonin (5-HT) neurons in the etiology and treatment of schizophrenia. The prepulse inhibition paradigm is used as a model for sensorimotor gating processes that are disrupted in schizophrenia. The present study assessed the general role of 5-HT in modulating auditory and visual prepulse inhibition in Wistar rats. A general overactivation of central serotonerigic pathways was produced pharmacologically by four different agents which all shared the common property of releasing 5-HT, i.e., p-chloroamphetamine, 3,4-methylenedioxymethamphetamine, N-ethyl-3,4-methylenedioxymethamphetamine, or fenfluramine. Within each test session, both sound and light prepulses were used to obtain a cross-modal assessment of auditory and visual sensory gating processes. All four 5-HT releasing agents produced dose-related disruptions of auditory and visual prepulse inhibition, with p-chloroamphetamine being the most potent. The releasers depressed baseline to varying degrees. The alpha 2-adrenergic agonist clonidine decreased baseline startle without substantially disrupting prepulse inhibition, demonstrating that the two effects were dissociable. Using fenfluramine as the most selective 5-HT releaser, two approaches were used to demonstrate 5-HT mediation of its disruptive effect on prepulse inhibition. In the first approach, the selective 5-HT uptake blocker MDL 28,618A was used to prevent fenfluramine-induced 5-HT release. In the second approach, prior exposure to a neurotoxic dose of p-chloroamphetamine (10 mg/kg) was used to produce a substantial, sustained depletion of cortical 5-HT, presumably reflecting the loss of 5-HT terminals. Both approaches reduced the disruptive effect of fenfluramine on auditory and visual prepulse inhibition, thereby demonstrating 5-HT mediation of these effects. Neither manipulation significantly affected the depressant effect of fenfluramine on startle baseline, demonstrating that the baseline-reducing and prepulse inhibition-reducing effects of fenfluramine could be dissociated. MDL 28,618A alone did not affect prepulse inhibition or basal startle levels, demonstrating an important functional difference between pharmacologically induced 5-HT uptake blockade and 5-HT release. In summary, these data indicate that serotonergic overactivation can disrupt auditory and visual sensorimotor gating as measured using sound and light prepulse inhibition in rats. These data support a potential role of excessive 5-HT activity as a contributing factor to disrupted sensory gating processes seen in schizophrenia and possibly other neuropsychiatric disorders.

The spin trap reagent PBN attenuates degeneration of 5-HT neurones in rat brain induced by p-chloroamphetamine but not fenfluramine

Dark Agouti rats injected with either p-chloroamphetamine (PCA; 2.5 mg/kg i.p.) or fenfluramine (15 mg/kg i.p.) had substantial decreases (approximately 50%) in the concentration of 5-HT and 5-HIAA and binding of [3H]paroxetine in the cerebral cortex 7 days later. This indicates that both compounds had produced neurodegeneration of 5-HT axon terminals. Two doses of alpha-phenyl-N-tert-butyl nitrone (PBN; 150 mg/kg i.p.) 130 min apart had no effect on cortical 5-HT content or [3H]paroxetine binding. However, when PBN (150 mg/ kg) was given 10 min before and 120 min after PCA (2.5 mg/kg) it attenuated the PCA-induced neurodegeneration. In contrast, PBN was without significant effect on the fenfluramine-induced damage. Changes in rectal temperature following either the neurotoxins or neurotoxins+ PBN were no more than +/-1 degree C of saline-injected control rats. These data indicate that PCA, like MDMA, probably induces neurotoxic degeneration because of the formation of catechol or quinone metabolites and subsequent reactive tree radical formation. Such a mechanism does not appear to explain fenfluramine-induced damage to 5-HT neurones.

Psychostimulant-like effects of p-fluoroamphetamine in the rat

The present study was undertaken to compare the pharmacological properties of p-fluoroamphetamine with those of amphetamine and of other halogenated amphetamines, using several in vivo and in vitro tests. These included substitution testing in (+)-amphetamine (1 mg/kg, 5.4 mu mol/kg, i.p.)-, (+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine [(+)-MBDB] (1.75 mg/kg, 7.8 mu mol/kg, i.p.)-, and 5-methoxy-6-methyl-2-aminoindan (MMAI) (1.71 mg/kg, 8 mu mol/kg, i.p.)-trained rats, [3H]5-HT and [3H]dopamine uptake inhibition in whole brain synaptosomes, and changes in striatal extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) as measured by in vivo microdialysis in freely moving rats. In drug discrimination substitution tests, p-fluoroamphetamine fully mimicked (+)-amphetamine (ED50 0.43 mg/kg, 2.11 mu mol/kg), whereas 'no substitution' was observed in rats trained to discriminate the serotonin (5-hydroxytryptamine, 5-HT)-releasing agents (+)-MBDB or MMAI from saline. p-Chloroamphetamine did not substitute for amphetamine but fully substituted for the (+)-MBDB and MMAI cues (ED50 0.17 mg/kg, 0.82 mu mol/kg, and 0.14 mg/kg, 0.69 mu mol/kg, respectively). p-Fluoroamphetamine, in comparison with p-chloroamphetamine and p-iodoamphetamine, showed much stronger inhibition of [3H]dopamine than [3H]5-HT uptake into rat brain synaptosomes but was less selective than amphetamine. p-Fluoroamphetamine (7.0 mg/kg, i.p.), 1 h after administration, strongly elevated (849% of baseline) extracellular dopamine in rat striatum measured using in vivo microdialysis. Amphetamine (2 mg/kg, i.p.) increased extracellular dopamine in rat striatum with a maximum at the same time as did p-fluoroamphetamine, but the latter gave a smaller increase. The data presented suggest that p-fluoroamphetamine resembles amphetamie more than it does the 5-HT-releasing type amphetamines.

Short-term and long-term effects of p-chloroamphetamine on hippocampal serotonin and corticosteroid receptor levels

Hippocampal corticosteroid receptors are regulated by corticosterone as well as by neurotransmitters, such as serotonin (5-HT). Studies have demonstrated that long-term changes in 5-HT levels are associated with alterations in hippocampal glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) number. However, the effect of short-term manipulations of 5-HT levels on hippocampal corticosteroid receptor levels has not been thoroughly investigated. The present set of studies examined the effect of para-chloroamphetamine (PCA) administration on both short-term and long-term regulation of hippocampal 5-HT and corticosteroid receptor levels. PCA is a selective serotonergic neurotoxin which initially releases 5-HT to cause a short-term depletion of 5-HT stores, followed by a long-term decrease in 5-HT levels which presumably reflects the destruction of 5-HT nerve terminals. In the initial study rats were adrenalectomized and 24 h later injected with PCA (20 mg/kg) and sacrificed 3 h later. PCA produced a large decrease in hippocampal 5-HT (-79%) and 5-hydroxyindoleacetic acid (5-HIAA) (-40%) concentrations. In addition, PCA significantly decreased both hippocampal GR (-28%) and MR (-35%) levels. Pretreatment with fluoxetine (20 mg/kg), which presumably blocks the uptake of PCA into 5-HT nerve terminals, completely blocked the PCA-induced decreases in both 5-HT and corticosteroid receptor concentrations. In a final experiment, the long-term (7 days) effect of PCA administration on hippocampal 5-HT and corticosteroid receptor levels was examined. PCA (10 mg/kg given on 2 consecutive days) was administered to adrenal-intact rats which were adrenalectomized 6 days later and subsequently sacrificed following a 24 h interval. PCA produced an 87% decrease in hippocampal 5-HT and 5-HIAA levels, but did not alter hippocampal GR or MR levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Review of the pharmacology and clinical pharmacology of 3,4-methylenedioxymethamphetamine (MDMA or "Ecstasy")

3,4-Methylenedioxymethamphetamine (MDMA or "Ecstasy") was first synthesised 80 years ago, but has recently received prominence as an illegally synthesised recreational drug of abuse. There is a widely held belief among misusers that it is safe. In the last 2-3 years there have been a number of reports of the drug producing severe acute toxicity and death and there are concerns that it may cause long term toxic damage to 5-hydroxytryptamine (5-HT) nerve terminals. There is a considerable literature on the acute pharmacological effects of MDMA in experimental animals, and this is reviewed. The drug produces both hyperthermia and the "serotonin syndrome", a series of behavioural changes which result from increased 5-HT function. Acute clinical toxicity problems following MDMA ingestion also include hyperthermia and the appearance of the serotonin syndrome. The hyperthermia appears to precipitate other severe clinical problems and the outcome can be fatal. In agreement with others, we suggest that the recent increase in the number of reports of MDMA toxicity probably results from the widespread use of the drug at all night dance parties or "raves". The phenomenon of amphetamine aggregation toxicity in mice was reported 40 years ago. If applicable to MDMA-induced toxicity in humans, all the conditions necessary to induce or enhance toxicity are present at raves: crowded conditions (aggregation), high ambient temperature, loud noise and dehydrated subjects. Administration of MDMA to rodents and non-human primates results in a long term neurotoxic decrease in 5-HT content in several brain regions and there is clear biochemical and histological evidence that this reflects neurodegeneration of 5-HT terminals. Unequivocal data demonstrating that similar changes occur in human brain do not exist, but limited and indirect clinical evidence gives grounds for concern. There are also data suggesting that long term psychiatric changes can occur, although there are problems of interpretation and these are reviewed. Suggestions for the rational treatment of the acute toxicity are made on the basis of both pharmacological studies in animals and current clinical practice. Cases presenting clinically are usually emergencies and unlikely to allow carefully controlled studies. Proposals include decreasing body temperature (possibly with ice), the use of dantrolene and anticonvulsant and sedative medication, particularly benzodiazepines. The use of neuroleptics requires care because of the theoretical risk of producing the neuroleptic malignant syndrome and the possibility of precipitating seizures. In rats, chlormethiazole antagonises the hyperthermia produced by MDMA and has been shown clinically to block MDMA-induced convulsive activity.

Tryptophan pretreatment augmentation of p-chloroamphetamine-induced serotonin and dopamine release and reduction of long-term neurotoxicity

The impact of tryptophan (TRP) pretreatment on the neurochemical effects of p-chloroamphetamine (PCA) was investigated. The neurotoxic effects of PCA on serotonin (5-HT) neurons, the acute effects of PCA on extracellular 5-HT and dopamine (DA), and the displacement by PCA of whole blood 5-HT were examined. TRP pretreatment (400 mg/kg of the methyl ester) significantly reduced the long-term (1 week) decrease in tissue 5-HT resulting from PCA (2 mg/kg, i.p., of the hydrochloride salt) in the prefrontal cortex and striatum, but not in the dorsal hippocampus. Microdialysis studies in awake animals showed that this pretreatment regimen resulted in augmented PCA-induced increases in extracellular 5-HT (4-fold) and DA (2-fold). TRP pretreatment also resulted in increased displacement of 5-HT from whole blood. The implications of these results toward possible mechanisms of action of PCA-induced neurotoxicity are discussed.

A study of the mechanism of MDMA ('ecstasy')-induced neurotoxicity of 5-HT neurones using chlormethiazole, dizocilpine and other protective compounds

1. An investigation has been made in rats into the neurotoxic effect of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxin, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy') using chlormethiazole and dizocilpine, both known neuroprotective compounds and also gamma-butyrolactone, ondansetron and pentobarbitone. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in a 50% loss of cortical and hippocampal 5-HT and 5-hydroxyindole acetic acid (5-HIAA) 4 days later. This reflects the long term neurotoxic loss of 5-HT that occurs. Injection of gamma-butyrolactone (GBL; 400 mg kg-1, i.p.) 5 min before and 55 min after the MDMA provided substantial protection. Pentobarbitone (25 mg kg-1, i.p.) using the same dose regime was also protective, but ondansetron (0.5 mg kg-1 or 0.1 mg kg-1, i.p.) was without effect. 3. MDMA (20 mg kg-1) had no significant effect on striatal dopamine concentration 4 days later but did produce a small decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) content. There were few significant changes in rats given MDMA plus GBL, ondansetron or pentobarbitone. 4. A single injection of MDMA (20 mg kg-1, i.p.) resulted in a greater than 80% depletion of 5-HT in hippocampus and cortex 4 h later, reflecting the initial rapid release that had occurred. None of the neuroprotective compounds (chlormethiazole, 50 mg kg-1; dizocilpine, 1 mg kg-1; GBL, 400 mg kg-1; pentobarbitone, 25 mg kg-1) given 5 min before and 55 min after the MDMA injection, altered the degree of 5-HT loss. 5. Acute MDMA injection increased striatal dopamine content (28%) and decreased the DOPAC content. In general, administration of the drugs under investigation did not significantly alter these MDMA-induced changes. Both chlormethiazole and GBL produced a greater increase in dopamine than MDMA alone, but this was apparently an additive effect to the action of either drug alone. 6. The 5-HT loss 4 h following administration of the neurotoxin p-chloroamphetamine (2.5 mg kg-1,i.p.) was not affected by chlormethiazole or dizocilpine. p-Chloroamphetamine did not appear to alter striatal dopamine metabolism.7. None of the protective drugs inhibited the initial 5-HT loss following MDMA, rendering unlikely any proposal that they are protective because they inhibit 5-HT release and the subsequent formation ofa toxic indole derivative. All the protective compounds (unlike ondansetron) probably inhibit dopamine release in the striatum. Since the neurotoxic action of some substituted amphetamines is dependent on the integrity of nigro-striatal neurones, this fact may go some way to explain the protective action of this diverse group of compounds.

Effect of D-amphetamine on the extracellular concentrations of glutamate and dopamine in iprindole-treated rats

A single administration of D-amphetamine and iprindole has been reported to produce selective, long-lasting decreases in brain dopamine (DA) content because of axon terminal degeneration. It has been found that the noncompetitive glutamate (GLU) antagonist, MK 801, blocks D-amphetamine-induced DA depletion in iprindole-treated rats. In the present study, the effect of D-amphetamine (9.2 mg/kg) and iprindole (10 mg/kg) on the extracellular concentrations of DA and GLU was determined in the striatum of awake, freely moving rats by the use of in vivo microdialysis. D-Amphetamine significantly increased DA and GLU efflux in the striatum of iprindole-treated rats as compared to the vehicle-treated group. The increase in the extracellular concentration of GLU occurred 4-6 hr following drug administration. The concentration of DA was decreased significantly in the striatum of D-amphetamine and iprindole-treated rats 7 days following administration as compared to the vehicle-treated group. Inhibition of tyrosine hydroxylase after alpha-methylparatyrosine (150 mg/kg) administration attenuated D-amphetamine-induced DA and GLU release. The DA antagonist, haloperidol (1 mg/kg), blocked D-amphetamine-induced GLU release without affecting the increase in the extracellular concentration of DA produced by the combination of D-amphetamine and iprindole. Both alpha-methylparatyrosine and haloperidol blocked the depletion of DA in the striatum 7 days after D-amphetamine and iprindole as compared to the vehicle group. In addition, administration of MK-801 (2 mg/kg) 2 hr after D-amphetamine significantly attenuated the long-term (7 day) decrease in striatal DA content produced by the combination of D-amphetamine and iprindole.2+

Effect of acute monoamine depletion on 3,4-methylenedioxymethamphetamine-induced neurotoxicity

The effect of acute, reversible depletion of either serotonin [5-hydroxytryptamine (5-HT)] or dopamine (DA) on the long-term (7-day) decrease of brain 5-HT content produced after 3,4-methylenedioxymethamphetamine (MDMA) administration was investigated. The tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (alpha-MPT) significantly attenuated the acute increase in DA efflux produced by MDMA in the striatum as measured by in vivo microdialysis. Treatment with alpha-MPT had no effect on MDMA-induced 5-HT release. alpha-MPT treatment blocked the long-term (7-day) depletion of striatal 5-HT content after MDMA administration. The tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA) completely blocked the acute increase in the extracellular concentration of 5-HT produced by MDMA. Although PCPA significantly attenuated the increase in DA efflux produced by MDMA, the effect was small in magnitude. More importantly, treatment with PCPA had no effect on MDMA-induced decrease of 5-HT uptake sites in the frontal cortex. These data are suggestive that acute depletion of DA but not 5-HT protects against the long-term neurotoxic effects of MDMA on 5-HT axon terminals. In addition, these data are supportive of the hypothesis that DA plays a major role in the neurotoxic effects of MDMA.

Integrative transporter-mediated release from cytoplasmic and vesicular 5-hydroxytryptamine stores in cultured neurons

The direct effects of 3,4-methylenedioxymethamphetamine (MDMA) and p-chloroamphetamine (PCA) were studied in microculture of fetal 5-hydroxytryptamine (5-HT) neurons. Both MDMA and PCA released 5-HT with the potency of PCA > MDMA by a mechanism inhibited by fluoxetine, and inhibitor of the 5-HT transporter. The transporter-mediated release by MDMA and PCA reduced intracellular stores of 5-HT. Both MDMA and PCA inhibit MAO-A activities, which also contributes to the increase of extracellular 5-HT levels. Deprenyl (10(-7) M) increased the amount of intracellular 5-HT and potentiated the MDMA- or PCA-induced release of 5-HT. Conversely, reserpine (10(-9) M) reduced the intracellular 5-HT levels and attenuated the transporter-mediated release. In addition, MDMA- or PCA-mediated release was attenuated by nimodipine (10(-8) M), an L-type Ca2+ channel antagonist. Our results indicate that MDMA- or PCA-induced release of 5-HT occurs from the cytoplasm to the media through the 5-HT transporter, and that the release may incorporate 5-HT from the vesicular stores.

5-HT loss in rat brain following 3,4-methylenedioxymethamphetamine (MDMA), p-chloroamphetamine and fenfluramine administration and effects of chlormethiazole and dizocilpine

1. The present study has investigated whether the neurotoxic effects of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxins, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy'), p-chloroamphetamine (PCA) and fenfluramine on hippocampal and cortical 5-HT terminals in rat brain could be prevented by administration of either chlormethiazole or dizocilpine. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in an approximate 30% loss of cortical and hippocampal 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) content 4 days later. Injection of chlormethiazole (50 mg kg-1) 5 min before and 55 min after the MDMA provided complete protection in both regions, while dizocilpine (1 mg kg-1, i.p.) protected only the hippocampus. 3. Administration of a single dose of chlormethiazole (100 mg kg-1) 20 min after the MDMA also provided complete protection to the hippocampus but not the cortex. This regime also attenuated the sustained hyperthermia (approx +2.5 degrees C) induced by the MDMA injection. 4. Injection of PCA (5 mg kg-1, i.p.) resulted in a 70% loss of 5-HT and 5-HIAA content in hippocampus and cortex 4 days later. Injection of chlormethiazole (100 mg kg-1, i.p.) or dizocilpine (1 mg kg-1, i.p.) 5 min before and 55 min after the PCA failed to protect against the neurotoxicity, nor was protection afforded by chlormethiazole when a lower dose of PCA (2.5 mg kg-1, i.p.) was given which produced only a 30% loss of 5-HT content. Chlormethiazole did prevent the hyperthermia induced by PCA (5 mg kg-1), while the lower dose of PCA (2.5 mg kg-1) did not produce a change in body temperature.5. Neither chlormethiazole nor dizocilpine prevented the neurotoxic loss of hippocampal or cortical 5-HT neurones measured 4 days following administration of fenfluramine (25 mg kg-1, i.p.).6. In general, chlormethiazole and dizocilpine were effective antagonists of the 5-HT-mediated behaviours of head weaving and forepaw treading which appeared following injection of all three neurotoxins.7. Both chlormethiazole and dizocilpine have previously been shown to prevent the neurotoxic effects ofa high dose of methamphetamine on cerebral 5-HT and dopamine pathways. These drugs also prevent MDMA-induced neurotoxicity of 5-HT pathways, but not that induced by injection of PCA or fenfluramine. This suggests that the mechanisms of neurotoxic damage to 5-HT pathways produced by substituted amphetamines cannot be identical. The monoamine loss does not appear to result from the hyperthermia produced by the neurotoxic compounds.

Dextromethorphan antagonizes the acute depletion of brain serotonin by p-chloroamphetamine and H75/12 in rats

A role for calcium in p-chloroamphetamine-induced neurotoxicity has been inferred previously from protective effects of dextromethorphan. We found that dextromethorphan reduces rat brain concentrations of 5-hydroxyindoleacetic acid and blocks the acute, non-neurotoxic depletion of brain serotonin by p-chloroamphetamine and by H75/12. Inhibition of the membrane transporter on brain serotonin neurons by dextromethorphan in vivo might explain its protective effect against p-chloroamphetamine neurotoxicity.

p-Chloroamphetamine induces serotonin release through serotonin transporters

p-Chloroamphetamine (PCA) interacts with serotonin transporters in two membrane vesicle model systems by competing with serotonin for transport and stimulating efflux of accumulated serotonin. In plasma membrane vesicles isolated from human platelets, PCA competes with [3H]imipramine for binding to the serotonin transporter with a KD of 310 nM and competitively inhibits serotonin transport with a KI of 4.8 nM. [3H]Serotonin efflux from plasma membrane vesicles is stimulated by PCA in a Na(+)-dependent and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, PCA competitively inhibits ATP-dependent [3H]serotonin accumulation with a KI of 1.7 microM and, at higher concentrations, stimulates efflux of accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due in part to dissipation of the transmembrane pH difference (delta pH) generated by ATP hydrolysis. Part of PCA's ability to stimulate efflux may be due to its transport by the vesicular amine transporter. Flow dialysis experiments demonstrated uptake of [3H]PCA into chromaffin granule membrane vesicles in response to the delta pH generated in the presence of Mg2+ and ATP. In plasma membrane vesicles, no accumulation was observed using an NaCl gradient as the driving force. We conclude that rapid nonmediated efflux of transported PCA prevents accumulation unless PCA is trapped inside by a low internal pH.

Neurotoxic amphetamine analogues: effects in monkeys and implications for humans

A wealth of evidence has accrued over the last 20 years indicating that certain amphetamine analogues have the potential to damage central monoaminergic neurons. For example, amphetamine has been shown to be toxic to dopamine neurons, MDMA to serotonin neurons, and methamphetamine to both (Table 1). In rodents, the toxic effects of amphetamines appear to be limited to axon terminals, and regenerative sprouting tends to be the rule. By contrast, in primates, nerve cell bodies appear to be affected, and the deleterious effects of amphetamine derivatives tend to be longer lasting, and possibly permanent (Fig. 2). Although findings in animals are compelling, observations in humans are less clear. In particular, it remains to be determined whether amphetamine analogues damage central monoaminergic neurons in humans and, if they do, whether functional consequences ensue. Also, the mechanism by which amphetamines damage monoaminergic neurons remains to be defined. Further insight into these basic and clinical aspects of amphetamine neurotoxicity should enhance our understanding of central monoaminergic systems in normal brain function, and their role in the pathophysiology of neuropsychiatric disorders.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

The substituted amphetamines 3,4-methylenedioxymethamphetamine, methamphetamine, p-chloroamphetamine and fenfluramine induce 5-hydroxytryptamine release via a common mechanism blocked by fluoxetine and cocaine

The abilities of the substituted amphetamines 3,4-methylenedioxymethamphetamine (MDMA), methamphetamine, p-chloroamphetamine (PCA) and fenfluramine to induce synaptosomal [3H]serotonin (5-HT) release were compared using a novel microassay system. The rank order of release potencies was found to be (+/-)PCA congruent to (+)-fenfluramine greater than (+)-MDMA much greater than (+)-methamphetamine. Combination of two drugs at their EC50 did not cause more release than either drug alone at an equivalent concentration. In addition, the 5-HT uptake blockers fluoxetine and cocaine inhibited the release induced by MDMA, methamphetamine, PCA and fenfluramine to the same percentage. However, threshold concentrations of the substituted amphetamines known to inhibit uptake did not attenuate the release caused by higher concentrations of these compounds. These results suggests that MDMA, methamphetamine, PCA and fenfluramine cause 5-HT release via a common mechanism. Furthermore, these results indicate that the 5-HT uptake blockade induced by these substituted amphetamines in vitro is different from that induced by either fluoxetine or cocaine.

The neurotoxic effects of p-chloroamphetamine in rat brain are blocked by prior depletion of serotonin

Systemic administration of p-chloroamphetamine (PCA) causes degeneration of serotonergic (5-HT) axons, but recent data indicate that this drug itself is not neurotoxic when applied directly to 5-HT axons. The present study was designed to test whether the toxic effects of PCA in the brain are dependent on release of endogenous 5-HT and to identify which stores of 5-HT are involved. The long-term effects of PCA on brain levels of 5-HT and on central 5-HT axons were determined in rats that had been initially depleted of 5-HT by administration of p-chlorophenylalanine and reserpine. The results show that transient depletion of 5-HT provides substantial protection against subsequent PCA-induced degeneration of 5-HT axon terminals; the neurotoxicity induced by PCA thus appears to be dependent on the presence of endogenous stores of 5-HT. In addition, the protective effect of 5-HT depletion is found only after pretreatment regimens that deplete peripheral as well as central stores of 5-HT. We interpret this finding as evidence that release of 5-HT from peripheral storage sites may be necessary for the expression of PCA-induced toxicity. Based on these results, we propose that central neurotoxicity is not induced by a direct action of PCA alone but may require or be augmented by a toxic metabolite of 5-HT.

The molecular mechanism of "ecstasy" [3,4-methylenedioxy-methamphetamine (MDMA)]: serotonin transporters are targets for MDMA-induced serotonin release

MDMA ("ecstasy") has been widely reported as a drug of abuse and as a neurotoxin. This report describes the mechanism of MDMA action at serotonin transporters from plasma membranes and secretory vesicles. MDMA stimulates serotonin efflux from both types of membrane vesicle. In plasma membrane vesicles isolated from human platelets, MDMA inhibits serotonin transport and [3H]imipramine binding by direct interaction with the Na(+)-dependent serotonin transporter. MDMA stimulates radiolabel efflux from plasma membrane vesicles preloaded with [3H]serotonin in a stereo-specific, Na(+)-dependent, and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, which contain the vesicular biogenic amine transporter, MDMA inhibits ATP-dependent [3H]serotonin accumulation and stimulates efflux of previously accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due to dissipation of the transmembrane pH difference generated by ATP hydrolysis and to direct interaction with the vesicular amine transporter.

Evidence for various tryptamines and related compounds acting as substrates of the platelet 5-hydroxytryptamine transporter

The aim of the present study was to answer the question whether amines other than 5-hydroxytryptamine (5-HT) and tryptamine act as substrates of the platelet 5-HT transporter. To this end, a large number of tryptamines, 5-HT receptor agonists and phenethylamines (which had IC50 values for 3H-5-HT uptake inhibition of 145-24,500 nmol l-1) was examined in rabbit platelets in order to determine their ability to induce an outward transport of 3H-5-HT. Platelets (the MAO of which was blocked) from reserpine-pretreated animals were loaded with 3H-5-HT and then exposed for 5 min to various concentrations (ranging from 0.25 to 40 times the IC50) of each compound. The concentration-effect curves for the drug-induced increase in 3H-5-HT efflux served to determine values of Emax (maximum increase in efflux expressed in % of the 3H-5-HT content of cells) and EC50 (drug concentration producing Emax/2). For the 24 compounds studied here (which included the 5-HT uptake inhibitors imipramine, citalopram, fluoxetine and cocaine) a linear correlation between EC50 and IC50 (r = 0.975) and a mean ratio of EC50/IC50 of 2.4 was found. Most of the compounds +ADe.g., (+/-)8-hydroxy-2-(N,N-dipropylamino)tetralin, S(+)alpha-methyl-5-HT, 5-carboxamidotryptamine and 5-methoxytryptamine+BD gave rise to Emax values (15.8-32.5%) that exceeded that brought about by imipramine (6.6%), indicating that they act as substrates of the 5-HT transporter; the 3H-5-HT outward transport observed in response to these substances was abolished in the presence of imipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual serotoninergic projections to forebrain in the rat: morphologically distinct 5-HT axon terminals exhibit differential vulnerability to neurotoxic amphetamine derivatives

The cerebral cortex of the rat and other mammals is innervated by two morphologically distinct classes of serotoninergic (5-HT) axon terminals: fine axons with minute varicosities and beaded axons characterized by large, spherical varicosities. Fine and beaded 5-HT axons exhibit different regional and laminar distributions in forebrain and arise from separate brainstem nuclei, the dorsal and median raphe nuclei, respectively. The present neuroanatomic study, based on immunocytochemical methods to visualize 5-HT axons, demonstrates that the two axon types differ markedly in their vulnerability to the neurotoxic amphetamine derivatives, methylenedioxyamphetamine (MDA), and p-chloroamphetamine (PCA). While both drugs cause extensive degeneration of fine 5-HT axons throughout forebrain, beaded 5-HT axons are consistently spared. Fine 5-HT axons, which richly innervate most regions of dorsal forebrain in control rats, are rarely seen 2 weeks after treatment with MDA or PCA; this loss of fine axons reflects a marked denervation that persists for months after drug administration. The serotoninergic axon terminals remaining after MDA or PCA administration are almost entirely of the beaded type and appear to be unaffected by both drugs. Over a wide range of doses (2.5-40 mg/kg PCA) and survival times (2 weeks to 2 months), these spared 5-HT axons with large, spherical varicosities cannot be distinguished from the normal, beaded 5-HT axons in control rats by morphologic criteria. Moreover, beaded 5-HT axons exhibit a highly characteristic regional distribution which is the same in control as in MDA- and PCA-treated rats: these axons innervate specific zones or layers within parietal and occipital cortex, hippocampus, cingulate cortex, entorhinal cortex, and the olfactory bulb, among other forebrain areas, and they form a dense plexus lining the ventricular system. Taken together, the results of this study demonstrate that fine 5-HT axons are highly vulnerable to the neurotoxic effects of the amphetamine derivatives MDA and PCA, while beaded 5-HT axons are markedly resistant. These findings are consistent with the hypothesis that there are two anatomically and functionally distinct sets of serotoninergic neurons projecting to forebrain. While both of these neuronal systems utilize 5-HT as a neurotransmitter, they differ in several features: 1) origin from separate nuclei in the brainstem (the dorsal and median raphe), 2) two types of morphologically distinct axon terminals, 3) markedly different distribution and innervation patterns in forebrain, and 4) dissimilar pharmacological properties. The results further suggest that psychotropic amphetamine derivatives have a selective action upon fine serotoninergic axons that arise from the dorsal raphe nucleus.

L-DOPA potentiation of the serotonergic deficits due to a single administration of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine or methamphetamine to rats

The role of dopamine in the serotonergic neurotoxicity of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine, methamphetamine, N-ethyl-3,4-methylenedioxyamphetamine and fenfluramine was assessed by determining the long-term effect of their coadministration with the dopamine precursor, L-DOPA (L-2,4-dihydroxyphenylalanine). L-DOPA administration potentiated the regional deficits in brain concentrations of serotonin measured one week after a single high dose of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine or methamphetamine but did not alter the neurochemical response to N-ethyl-3,4-methylenedioxyamphetamine nor to fenfluramine. Consistent with this, in vitro release studies found the latter two agents to be the weakest of the five at increasing [3H]dopamine efflux from preloaded rat striatal slices. As an estimate of in vivo release, the effect of each agent on striatal dopamine concentrations was determined. Only those agents showing a synergism with L-DOPA in the long-term studies also produced changes in striatal dopamine consistent with an increase in transmitter release and synthesis. These results provide additional support for the hypothesis that dopamine release plays a role in the neurotoxicity of methylenedioxymethamphetamine, p-chloroamphetamine and methamphetamine. The lack of effect of L-DOPA on the neurotoxicity of fenfluramine as well as the modest effects of fenfluramine on dopamine release indicate this drug may produce its long-term effects on the serotonergic system through a unique mechanism not involving dopamine.

Parachloroamphetamine selectively alters regional cerebral metabolic responses to the serotonergic agonist metachlorophenylpiperazine in rats

To determine if reported reductions of regional cerebral metabolic rates for glucose (rCMRglc) induced by the 5-HT agent metachlorophenylpiperazine (MCPP) (2.5 mg/kg) are due to a presynaptic action, 3-month old Fischer-344 rats were given parachloroamphetamine (PCA), a serotonin neurotoxin, and rCMRglc was measured 1 or 3 weeks later with the quantitative autoradiographic [14C]2-deoxyglucose procedure in 74 brain regions after administering saline, MCPP or other drugs. PCA alone increased rCMRglc significantly only in the raphe nuclei and in visual structures (visual cortex, lateral geniculate, superior colliculus). MCPP alone reduced rCMRglc in 75% of the regions studied. In PCA-lesioned rats, metabolic responses to MCPP 2.5 mg/kg were virtually abolished and rCMRglc was increased in interanteromedial and centrolateral thalamic nuclei. rCMRglc responses to quipazine, a postsynaptic serotonin agonist, and to arecoline and bromocriptine, cholinergic and dopaminergic agonists, were unchanged by PCA-pretreatment. Selective abolition by PCA of the metabolic response to MCPP confirms that MCPP, at the dose studied, reduces rCMRglc in the forebrain via a presynaptic mechanism and that postsynaptic serotonergic function is not altered by PCA.

3,4-Methylenedioxyamphetamine (MDA) analogues exhibit differential effects on synaptosomal release of 3H-dopamine and 3H-5-hydroxytryptamine

The effect of various analogues of the neurotoxic amphetamine derivative, MDA (3,4-methylenedioxyamphetamine) on carrier-mediated, calcium-independent release of 3H-5-HT and 3H-DA from rat brain synaptosomes was investigated. Both enantiomers of the neurotoxic analogues MDA and MDMA (3,4-methylenedioxymethamphetamine) induce synaptosomal release of 3H-5-HT and 3H-DA in vitro. The release of 3H-5-HT induced by MDMA is partially blocked by 10(-6) M fluoxetine. The (+) enantiomers of both MDA and MDMA are more potent than the (-) enantiomers as releasers of both 3H-5-HT and 3H-DA. Eleven analogues, differing from MDA with respect to the nature and number of ring and/or side chain substituents, also show some activity in the release experiments, and are more potent as releasers of 3H-5-HT than of 3H-DA. The amphetamine derivatives (+/-)fenfluramine, (+/-)norfenfluramine, (+/-)MDE, (+/-)PCA, and d-methamphetamine are all potent releasers of 3H-5-HT and show varying degrees of activity as 3H-DA releasers. The hallucinogen DOM does not cause significant release of either 3H-monoamine. Possible long-term serotonergic neurotoxicity was assessed by quantifying the density of 5-HT uptake sites in rats treated with multiple doses of selected analogues using 3H-paroxetine to label 5-HT uptake sites. In the neurotoxicity study of the compounds investigated, only (+)MDA caused a significant loss of 5-HT uptake sites in comparison to saline-treated controls. These results are discussed in terms of the apparent structure-activity properties affecting 3H-monoamine release and their possible relevance to neurotoxicity in this series of MDA congeners.

Assessment of the role of alpha-methylepinine in the neurotoxicity of MDMA

To assess the potential involvement of metabolism of 3,4-methylenedioxymethamphetamine (MDMA) to the catechol alpha-methylepinine in producing serotonergic neurotoxicity, we attempted to correlate aspects of this reaction with the neurotoxicity profile of MDMA. In contrast to the stereoselectivity of S-(+)-MDMA in causing persistent declines in rat brain 5-hydroxyindole levels, no stereochemical component to the metabolic reaction was apparent. Rat liver microsomes generated a significantly greater amount of alpha-methylepinine than did mouse microsomes, but similar amounts of metabolite were produced by brain microsomes from the two species. Formation of alpha-methylepinine by hepatic, but not brain, microsomes was inhibited by SKF 525A and induced by phenobarbital, possibly indicating a tissue specificity in cytochrome P-450-dependent metabolism of MDMA. To directly assess whether alpha-methylepine is a likely mediator of MDMA neurotoxicity, the compound was administered intracerebroventricularly. No persistent declines in biogenic amines or their metabolites were observed one week following treatment. These data suggest that alpha-methylepinine alone is not responsible for the neurotoxic effects of MDMA.

Persistent blockade of potassium-evoked serotonin release from rat frontocortical terminals after fluoxetine administration

We examined 5-HT and 5-HIAA release from frontal cortex evoked by high potassium chloride concentrations in rats pretreated for 3 days with high doses of the 5-HT uptake blocker fluoxetine or of dexfenfluramine, which both releases 5-HT and blocks its reuptake. The standard fluoxetine dose (30 mg/kg i.p.) was about 4 times the drug's ED50 in producing a serotonin-related behavioral effect, anorexia, while the dexfenfluramine dose (7.5 mg/kg i.p.) was about 6 times its ED50. These high doses were chosen in order to elucidate the mechanism by which similar doses of fluoxetine and dexfenfluramine had been found to produce long-term changes in serotonin dynamics. Fluoxetine decreased the basal release of both compounds; dexfenfluramine decreased basal 5-HIAA efflux without affecting the release of 5-HT release. Potassium-evoked 5-HT release was unchanged after dexfenfluramine pretreatment but was suppressed by fluoxetine doses as low as 7.5 mg per kg per day. Basal release of 5-HT and 5-HIAA returned to normal after 7 days of fluoxetine pretreatment, but evoked release continued to be suppressed. These data suggest that long-term changes in brain serotonin dynamics after high doses of dexfenfluramine or fluoxetine are related to the drug's mechanisms of action, specifically their blockade of 5-HT reuptake.

5-Iodo-2-aminoindan, a nonneurotoxic analogue of p-iodoamphetamine

A rigid analogue, 5-iodo-2-aminoindan (5-IAI), of the serotonin neurotoxic halogenated amphetamine p-iodoamphetamine (PIA) was pharmacologically evaluated for production of serotonin neurotoxicity. A comparison was also made between 5-IAI and PIA in the two-lever drug discrimination paradigm in rats trained to discriminate saline from 3,4-methylenedioxymethamphetamine (MDMA) or saline from the alpha-ethyl homologue of MDMA, MBDB. PIA and 5-IAI were both behaviorally active, and fully substituted in both groups of animals, but were considerably less potent than p-chloroamphetamine (PCA). PIA had about twice the potency of PCA as an inhibitor of [3H]-5-HT uptake in rat brain cortical synaptosomes, while 5-IAI was only about 75% as potent as PCA in this assay. A single 40 mg/kg dose of PIA resulted in a 40% reduction of 5-HT and 5-HIAA levels and in the number of 5-HT uptake sites in rat cortex at one week sacrifice. The same dose of 5-IAI with one week sacrifice led to about a 15% decrease in 5-HIAA levels and number of 5-HT uptake sites, but only the latter was statistically significant. In rat hippocampus, PIA gave significant decreases in all serotonin markers examined, while 5-IAI slightly but significantly decreased only 5-HT levels. Neither compound produced any change in catecholamine or catecholamine metabolite levels. The results confirm earlier reports of the selective serotonin neurotoxicity of PIA, which is less severe than that of PCA, and also demonstrate that its rigid analogue 5-IAI does not appear to cause significant serotonin deficits in the rat.

Neurotoxicity of MDMA and related compounds: anatomic studies

The cytotoxic effects of amphetamine derivatives were studied by immunocytochemistry to identify the cellular compartments affected by these drugs, to obtain morphologic evidence of neuronal degeneration, and to assess the potential for regeneration. The substituted amphetamines, MDA, MDMA, PCA, and fenfluramine, all release serotonin and cause acute depletion of 5-HT from most axon terminals in forebrain. (1) Unequivocal signs of axon degeneration were seen at 36-48 hour survivals: 5-HT axons exhibited increased caliber, huge, swollen varicosities, fragmentation, and dilated proximal axon stumps. (2) Fine 5-HT axon terminals were persistently lost after drug administration, while beaded axons and raphe cell bodies were spared. These two types of 5-HT axons, which arise from separate raphe nuclei and form distinct ascending projections, are differentially vulnerable to psychotropic drugs. (3) From 2-8 months after treatment, there was progressive serotonergic re-innervation of neocortex along a fronto-occipital gradient. Longitudinal 5-HT axons grew into layers I and VI from rostral to caudal, before sprouting into middle cortical layers; this bilaminar pattern of growth simulates perinatal development of 5-HT innervation. This study demonstrates differential vulnerability of 5-HT projections, evidence for axonal degeneration, and sprouting of 5-HT axons leading to re-innervation of forebrain. While the sprouting axons are anatomically similar to the type that was damaged, it is not known whether a normal pattern of innervation is re-established.

Unlike systemic administration of p-chloroamphetamine, direct intracerebral injection does not produce degeneration of 5-HT axons

Systemic administration of the amphetamine derivative p-chloroamphetamine (PCA) causes degeneration of 5-HT axon terminals in rat brain. The present study was designed to determine whether PCA induces neurotoxic effects by a direct action on 5-HT axon terminals. PCA was administered by microinjection directly into the cerebral cortex of rats. Continuous intracerebral infusions were made over extended time periods (10 min-48 h) to explore whether the induction of neurotoxicity requires a prolonged exposure of axon terminals to the drug. Two weeks after drug administration, brain sections that passed through the injection site were processed for 5-HT immunohistochemistry. The 5-HT innervation of cerebral cortex in PCA-injected animals was compared with that after intracortical injection of saline or of 5,7-dihydroxytryptamine. The results demonstrate that, in the concentrations used, direct application of PCA into the neocortex does not elicit axonal degeneration, even after a continuous infusion for 2 days. This finding suggests that PCA itself is not directly toxic to 5-HT axons.

Comparative serotonin neurotoxicity of the stereoisomers of fenfluramine and norfenfluramine

The optical isomers of fenfluramine and norfenfluramine were administered to rats to examine their relative potency for destruction of serotonin neurons. Rats were sacrificed one week following a single 10 mg/kg SC injection of one of the four compounds and monoamine and metabolite levels in the frontal cortex and hippocampus brain regions were examined by HPLC-EC techniques. In addition, [3H]-paroxetine binding to homogenates of these brain regions was determined. With the exception of hippocampal 5-HT levels following d-fenfluramine treatment, there was a decrease in all the serotonergic markers assayed, following treatment with the d-enantiomers of fenfluramine and norfenfluramine. No decrease in any serotonergic marker was seen at this dose following treatment with the l-enantiomers of fenfluramine or norfenfluramine. Also, none of the drug treatments resulted in a significant decrease in catecholamines or their metabolites. With all the serotonergic markers examined, d-norfenfluramine was found to cause a significantly greater decrease than d-fenfluramine. The significance of these results is discussed in terms of the hypothesis that the long-term serotonergic deficits observed with d-fenfluramine may result from its metabolite, d-norfenfluramine.

Anatomic evidence for a neurotoxic effect of (+/-)-fenfluramine upon serotonergic projections in the rat

Immunocytochemistry was used to determine whether (+/-)-fenfluramine causes structural damage to serotonergic (5-HT) neurons. Sections from rat forebrain were examined 4 h, 36 h and 2 weeks after various dose regimens of fenfluramine. At all time points there was a reduction of fine 5-HT axon terminals in the forebrain, while beaded axons were spared. The presence of markedly swollen, fragmented 5-HT axons 36 h after injection is indicative of axonal degeneration, and provides morphologic evidence for a neurotoxic effect of (+/-)-fenfluramine upon 5-HT axon terminals.

Effect of 3,4-methylenedioxymethamphetamine on 3,4-dihydroxyphenylalanine accumulation in the striatum and nucleus accumbens

The effect of the racemic mixture of 3,4-methylenedioxymethamphetamine (MDMA) on the synthesis of dopamine in the terminals of nigrostriatal and mesolimbic neurons was estimated by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the striatum and nucleus accumbens 30 min following the administration of the L-aromatic amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine. MDMA produced an increase in DOPA accumulation in the striatum which was greater in magnitude and longer in duration than that in the nucleus accumbens. Although the concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in both the striatum and nucleus accumbens were reduced 3 h following an injection of MDMA (20 mg/kg), 5-HT and 5-HIAA concentrations were significantly reduced only in the striatum 7 days after the administration of MDMA. Pretreatment with a 5-HT2 antagonist, ketanserin, significantly attenuated the reduction in 5-HT concentration in the striatum 3 h following MDMA administration and completely blocked 5-HT depletion at 7 days post administration. Moreover, ketanserin completely blocked MDMA-induced DOPA accumulation in the striatum. The results obtained in these studies suggest that MDMA activates nigrostriatal dopaminergic pathways via 5-HT2 receptors. In addition, these data are supportive of the hypothesis that dopamine plays a role in MDMA-induced 5-HT depletion.

Characterization of acute N-ethyl-3,4-methylenedioxyamphetamine (MDE) action on the central serotonergic system

The effect of N-ethyl-3,4-methylenedioxyamphetamine (MDE) on the central serotonergic system was studied. Within 1 hr after administration of MDE (10 mg/kg), the concentration of 5-hydroxytryptamine (5-HT) and the activity of tryptophan hydroxylase (TPH) had declined significantly in the hippocampus but returned to control within 12 hr. Hippocampal 5-hydroxyindoleacetic acid (5-HIAA) content decreased within 2 hr, rebounded to 22% above control by 12 hr, and returned to control by 24 hr. Blockade of the 5-HT uptake carrier with fluoxetine (10 mg/kg) prevented or attenuated the MDE-induced changes in 5-HT content and TPH activity, except for neostriatal TPH activity which remained unresponsive to the fluoxetine treatment. The MDE-induced decline in TPH activity could be reversed by incubating the TPH preparation with dithiothreitol and Fe2+ under nitrogen for 24 hr. This suggests that the loss in TPH activity induced by MDE results from an alteration of the oxidation-reduction state of a sulfhydryl group located on the enzyme. The inhibition of monoamine oxidase (MAO) by the administration of pargyline (75 mg/kg) failed to protect the neostriatal TPH activity from the MDE-induced decline while potentiating the MDE-induced decrease in cortical TPH activity. This suggests that H2O2 generated by MAO in vivo is not responsible for oxidation of the sulfhydryl site located on TPH during the MDE treatment.

Glucocorticoids and 3,4-methylenedioxymethamphetamine (MDMA)-induced neurotoxicity

The present study was carried out in order to explore the role of glucocorticoids in 3,4-methylenedio-xymethamphetamine (MDMA)-induced neurotoxicity of the central serotonergic system. The activity of tryptophan hydroxylase (TPH) was used as an index of this drug-induced neuronal degeneration. One week after a single high dose of MDMA (20 mg/kg), a significant decrease in the enzyme activity was measured in both the frontal cortex and hippocampus. Adrenalectomy (ADX) attenuated or blocked this decrease in TPH activity in the hippocampus but not in the frontal cortex. This protective effect of ADX on hippocampal serotonergic neurons disappeared with concurrent administration of corticosterone (CORT) and MDMA administration. The long-term MDMA-induced decreases in hippocampal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were similarly affected by CORT replacement. However, ADX did not alter the short-term decline in hippocampal TPH activity and 5-HT concentrations measured 3 h after a single dose of MDMA (10 mg/kg s.c.). This study suggests that CORT play a role in the development of neurotoxicity induced by MDMA in the hippocampal serotonergic system, but may be less important in other brain structures.

Depletion of serotonin using p-chlorophenylalanine (PCPA) and reserpine protects against the neurotoxic effects of p-chloroamphetamine (PCA) in the brain

The present study attempts to determine whether the neurotoxicity of p-chloroamphetamine (PCA) is dependent on a releasable pool of serotonin (5-HT). Rats treated with PCA alone or with reserpine and PCA exhibit a profound loss of 5-HT innervation in cerebral cortex after a 2-week survival period. However, depletion of 5-HT by combined treatment with p-chlorophenylalanine (PCPA) and reserpine provides substantial protection against the neurotoxic effects of PCA. These results indicate that release of 5-HT is a necessary step in the neurotoxicity of PCA and that a peripheral source of 5-HT is involved. We suggest that 5-HT release from platelets into the peripheral circulation may result in the formation of a neurotoxic 5-HT metabolite.