D-, L- and DL-fenfluramine cause long-lasting depletions of serotonin in rat brain

Evidence for a role of transporter-mediated currents in the depletion of brain serotonin induced by serotonin transporter substrates

Serotonin (5-HT) transporter (SERT) substrates like fenfluramine and 3,4-methylenedioxymethamphetamine cause long-term depletion of brain 5-HT, while certain other substrates do not. The 5-HT deficits produced by SERT substrates are dependent upon transporter proteins, but the exact mechanisms responsible are unclear. Here, we compared the pharmacology of several SERT substrates: fenfluramine, d-fenfluramine, 1-(m-chlorophenyl)piperazine (mCPP) and 1-(m-trifluoromethylphenyl)piperainze (TFMPP), to establish relationships between acute drug mechanisms and the propensity for long-term 5-HT depletions. In vivo microdialysis was carried out in rat nucleus accumbens to examine acute 5-HT release and long-term depletion in the same subjects. In vitro assays were performed to measure efflux of [(3)H]5-HT in rat brain synaptosomes and transporter-mediated ionic currents in SERT-expressing Xenopus oocytes. When administered repeatedly to rats (6 mg/kg, i.p., four doses), all drugs produce large sustained elevations in extracellular 5-HT (>5-fold) with minimal effects on dopamine. Importantly, 2 weeks after dosing, only rats exposed to fenfluramine and d-fenfluramine display depletion of brain 5-HT. All test drugs evoke fluoxetine-sensitive efflux of [(3)H]5-HT from synaptosomes, but d-fenfluramine and its bioactive metabolite d-norfenfluramine induce significantly greater SERT-mediated currents than phenylpiperazines. Our data confirm that drug-induced 5-HT release probably does not mediate 5-HT depletion. However, the magnitude of transporter-mediated inward current may be a critical factor in the cascade of events leading to 5-HT deficits. This hypothesis warrants further study, especially given the growing popularity of designer drugs that target SERT.

Serotonin release in the caudal nidopallium of adult laying hens genetically selected for high and low feather pecking behavior: an in vivo microdialysis study

Severe feather pecking (FP) is a detrimental behavior causing welfare problems in laying hens. Divergent genetic selection for FP in White Leghorns resulted in strong differences in FP incidences between lines. More recently, it was shown that the high FP (HFP) birds have increased locomotor activity as compared to hens of the low FP (LFP) line, but whether these lines differ in central serotonin (5-hydroxytryptamine, 5-HT) release is unknown. We compared baseline release levels of central 5-HT, and the metabolite 5-HIAA in the limbic and prefrontal subcomponents of the caudal nidopallium by in vivo microdialysis in adult HFP and LFP laying hens from the ninth generation of selection. A single subcutaneous d-fenfluramine injection (0.5 mg/kg) was given to release neuronal serotonin in order to investigate presynaptic storage capacity. The present study shows that HFP hens had higher baseline levels of 5-HT in the caudal nidopallium as compared to LFP laying hens. Remarkably, no differences in plasma tryptophan levels (precursor of 5-HT) between the lines were observed. d-fenfluramine increased 5-HT levels in both lines similarly indirectly suggesting that presynaptic storage capacity was the same. The present study shows that HFP hens release more 5-HT under baseline conditions in the caudal nidopallium as compared to the LFP birds. This suggests that HFP hens are characterized by a higher tonic 5-HT release.

Quantitative Positron Emission Tomography Studies of the Serotonin Transporter in Humans Previously Treated with the Appetite Suppressants Fenfluramine or Dexfenfluramine

PURPOSE

The appetite suppressants fenfluramine and dexfenfluramine were widely prescribed before being withdrawn from the market in 1997. Both drugs are known to have the potential to damage brain serotonin (5-HT) axons and axon terminals in animals, including nonhuman primates. This study used quantitative positron emission tomography (PET) with [(11)C] McN5652, a serotonin transporter (SERT) ligand to determine whether humans previously exposed to fenfluramines showed reductions in SERT binding parameters.

PROCEDURES

Subjects previously treated with fenfluramines for weight loss (N = 15) and age-matched controls (N = 17) underwent PET studies with [(11)C] McN5652. Global and regional distribution volumes (DVs) of [(11)C] McN5652 were compared in the two subject groups using parametric statistical analyses.

RESULTS

Compared to controls, subjects previously exposed to fenfluramines had significant reductions in [(11)C]McN5652 binding in 14 of 15 regions of interest, more than four years after drug discontinuation.

CONCLUSIONS

These results are the first to provide direct evidence for fenfluramine-induced 5-HT neurotoxicity in humans.

Emetic action of the prostanoid TP receptor agonist, U46619, in Suncus murinus (house musk shrew)

The emetic action of the prostanoid TP receptor agonist, 11alpha,9alpha-epoxymethano-15S-hydroxyprosta-5Z,13E-dienoic acid (U46619; 300 microg/kg, i.p.), was investigated in Suncus murinus. The emetic response was reduced by 76% following bilateral abdominal vagotomy (P<0.001) and by reserpine (5 mg/kg, i.p., 24 h pretreatment; P<0.05) but U46619 administered i.c.v. (30-300 ng) was not emetic, suggesting a peripheral mechanism involving monoamines. However, fenfluramine (5 mg/kg, repeated treatment) and para-chlorophenylalanine (100-400 mg/kg) and ondansetron (0.3-3 mg/kg) were inactive (P>0.05) to reduce U46619-induced emesis precluding a role of 5-HT and 5-HT(3) receptors in the mechanism. Similarly, phentolamine (0.3-3 mg/kg), propranolol (3 mg/kg), and their combination, and metoclopramide (0.3-3 mg/kg), domperidone (0.3-3 mg/kg), droperidol (0.3-3 mg/kg), scopolamine (0.3-3 mg/kg) and promethazine (0.3-3 mg/kg) were inactive (P>0.05) to reduce the retching and vomiting response. However, the tachykinin NK(1) receptor antagonist, (+)-2S,3S(-3-(2-methoxy-5-trifluoromethoxybenzyl)amino-2-phenylpiperidine) (CP-122,721; 1-10 mg/kg) antagonized emesis (P<0.01). In conclusion, U46619-induced emesis appears to be mediated via a predominant peripheral mechanism sensitive to reserpine and is not likely to involve adrenoceptors, dopamine, 5-HT(3), muscarinic or histamine (H(1)) receptors. The action of CP-122,721 to reduce U46619-induced emesis extends the spectrum of anti-emetic action tachykinin NK(1) receptor antagonists to mechanisms involving TP receptors.

Fenfluramine-induced serotonergic neurotoxicity in mice: lack of neuroprotection by inhibition/ablation of nNOS

Previous studies have implicated a role for nitric oxide (NO) and peroxynitrite in methamphetamine-induced dopaminergic neurotoxicity. The present study was undertaken to investigate whether NO is involved in serotonergic neurotoxicity caused by fenfluramine. In the first experiment, the effect of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI; 25 mg/kg x 4) on fenfluramine (25 mg/kg x 4)-induced serotonergic neurotoxicity in Swiss Webster mice was investigated. In the second experiment, the effect of fenfluramine (25 mg/kg x 4) on nNOS (-/-) and wild-type (WT) mice was investigated. Fenfluramine induced hypothermia in all three mouse strains, and 7-NI had no thermoregulatory effect. Selective depletion of 5-HT and 5-HT transporter binding sites in the striatum, frontal cortex and hippocampus in all three mouse strains was observed, with no evidence of dopaminergic neurotoxicity. In the first experiment, 7-NI did not attenuate serotonergic neurotoxicity in Swiss Webster mice. In the second experiment, nNOS(-/-) and WT mice were equally sensitive to serotonergic neurotoxicity. These findings suggest that NO and peroxynitrite do not mediate fenfluramine-induced serotonergic neurotoxicity, and that NO is a selective mediator of amphetamines-induced dopaminergic neurotoxicity.

Liquid chromatography studies on the pharmacokinetics of phentermine and fenfluramine in brain and blood microdialysates after intraperitoneal administration to rats

A highly sensitive and simple HPLC method with fluorescence detection for the determination of phentermine (Phen), fenfluramine (Fen) and norfenfluramine (Norf, the active metabolite of Fen) in rat brain and blood microdialysates has been developed. The brain and blood microdialysates were directly subjected to derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride (DIB-Cl) in the presence of carbonate buffer (0.1 M, pH 9.0) at room temperature. The chromatographic conditions consisted of an ODS column and mobile phase composition of acetonitrile and water (65:35, v/v) with flow rate set at 1.0 ml/min. The detection was performed at excitation and emission wavelengths of 325 and 430 nm, respectively. Under these conditions, the DIB-derivatives of Phen, Fen and Norf were well separated and showed good linearities in the studied ranges (5-2000 nM for Phen and 10-2000 nM for Norf and Fen) with correlation coefficients greater than 0.999. The obtained detection limits were less than 23 fmol on column (for the three compounds) in both brain and blood microdialysates at a signal-to-noise ratio of 3 (S/N=3). The intra- and the inter-assay precisions were lower than 10%. The method coupled with microdialysis was applied for a pharmacokinetic drug-drug interaction study of Phen and Fen following individual and combined intraperitoneal administration to rats. In addition, since the role of protein binding in drug interactions can be quite involved, the method was applied for the determination of total and free Phen and Fen in rat plasma and ultrafiltrate, respectively. The results showed that Fen and/or Norf significantly altered the pharmacokinetic parameters of Phen in both blood and brain but did not alter its protein binding. On the other hand, there was no significant difference in the pharmacokinetics of Fen when administered with Phen.

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.

Altered prolactin response to M-chlorophenylpiperazine in monkeys previously treated with 3,4-methylenedioxymethamphetamine (MDMA) or fenfluramine

3,4-Methylenedioxymethamphetamine ("Ecstasy," MDMA) and fenfluramine, widely used by humans, are potent brain serotonin (5-HT) neurotoxins in animals. Thus, there is concern that humans previously exposed to these amphetamine derivatives may have incurred brain 5-HT neurotoxicity. However, assessing the status of brain 5-HT neurons in the living organism is challenging. To determine whether MDMA- and/or fenfluramine-induced 5-HT neurotoxicity can be detected during life using neuroendocrine methods, groups of monkeys previously treated with neurotoxic regimens of MDMA or fenfluramine, along with saline-treated controls, underwent neuroendocrine challenge with the direct 5-HT agonist and 5-HT-releasing drug, m-chlorophenylpiperazine (m-CPP). Animals treated 2 weeks previously with MDMA exhibited a nonsignificant reduction in the prolactin response to m-CPP. In contrast, monkeys treated 3 1/2 years previously with MDMA or 2 years previously with fenfluramine exhibited significantly increased prolactin responses to m-CPP. No significant differences in cortisol concentrations were noted between groups at any time point. These data indicate that neuroendocrine challenge with m-CPP is capable of detecting substituted amphetamine-induced 5-HT neurotoxicity in living primates, but that the recency of drug exposure is an important consideration. Changes in the neuroendocrine response to m-CPP over time in animals with substituted amphetamine-induced neurotoxicity may be related to aberrant 5-HT reinnervation of the basal forebrain that occurs over time in monkeys previously treated with neurotoxic doses of MDMA or fenfluramine.

Serotonin releasing agents. Neurochemical, therapeutic and adverse effects

This review summarizes the neurochemical, therapeutic and adverse effects of serotonin (5-HT) releasing agents. The 5-HT releaser (plus minus)-fenfluramine is composed of two stereoisomers, (+)-fenfluramine and (minus sign)-fenfluramine, which are N-de-ethylated to yield the metabolites, (+)-norfenfluramine and (minus sign)-norfenfluramine. Fenfluramines and norfenfluramines are 5-HT transporter substrates and potent 5-HT releasers. Other 5-HT releasing agents include m-chlorophenylpiperazine (mCPP), a major metabolite of the antidepressant drug trazodone. Findings from in vitro and in vivo studies support the hypothesis that fenfluramines and mCPP release neuronal 5-HT via a non-exocytotic carrier-mediated exchange mechanism involving 5-HT transporters. (+)-Norfenfluramine is a potent 5-HT(2B) and 5-HT(2C) receptor agonist. The former activity may increase the risk of developing valvular heart disease (VHD), whereas the latter activity is implicated in the anorectic effect of systemic fenfluramine. Anorectic agents that increase the risk of developing primary pulmonary hypertension (PPH) share the common property of being 5-HT transporter substrates. However, these drugs vary considerably in their propensity to increase the risk of PPH. In this regard, neither trazodone nor mCPP is associated with PPH. Similarly, although some 5-HT substrates can deplete brain 5-HT (fenfluramine), others do not (mCPP). In addition to the established indication of obesity, 5-HT releasers may be helpful in treating psychiatric problems such as drug and alcohol dependence, depression and premenstrual syndrome. Viewed collectively, it seems possible to develop new medications that selectively release 5-HT without the adverse effects of PPH, VHD or neurotoxicity. Such agents may have utility in treating a variety of psychiatric disorders.

Intermittent, chronic fenfluramine administration to rats repeatedly suppresses food intake despite substantial brain serotonin reductions

The mechanisms by which fenfluramine suppresses food intake and body weight have been linked to its ability to enhance transmission across serotonin synapses in brain. This drug initially lowers body weight and suppresses food intake, yet after repeated administration food intake soon returns to normal and body weight no longer decreases. Fenfluramine also causes rapid and prolonged reductions in brain serotonin concentrations, which might account for its loss of appetite suppression. This possibility has been evaluated in rats by assessing if intermittent, chronic fenfluramine administration could suppress food intake during each treatment period, and if so, whether such an effect occurs in the presence of reduced brain serotonin levels. Rats were injected once daily with 10 mg/kg D,L-fenfluramine for 5 days, and then received no injections for the next 5 days. Control rats received only vehicle injections. This 10-day sequence was repeated five more times. During each period of fenfluramine administration, daily food intake dropped markedly the first 1-2 days of treatment, but returned to pretreatment values by day 5. Daily food intake was normal or slightly above normal during non-injection periods. Body weight dropped modestly during each period of fenfluramine administration, and rose during each subsequent period when injections had ceased. Serotonin concentrations and synthesis rates in several brain regions were markedly reduced at early, middle, and late periods of the experiment. Despite the long-term reduction in brain serotonin pools produced by fenfluramine, the drug continues to reduce food intake and body weight. Several possible interpretations of these findings are considered, based on the multiple mechanisms through which this drug has been proposed to modify synaptic serotonin transmission.

Long-term impairment of anterograde axonal transport along fiber projections originating in the rostral raphe nuclei after treatment with fenfluramine or methylenedioxymethamphetamine

To further evaluate the serotonin (5-HT) neurotoxic potential of substituted amphetamines, we used tritiated proline to examine anterograde transport along ascending axonal projections originating in the rostral raphe nuclei of animals treated 3 weeks previously with (+/-)fenfluramine (FEN, 10 mg/kg, every 2 h x 4 injections; i.p.) or (+/-)3,4-methylenedioxymethamphetamine (MDMA, 20 mg/kg, twice daily for 4 days; s.c.). The documented 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 75 microg; ICV; 30 min after pretreatment with pargyline, 50 mg/kg; i.p., and desipramine 25 mg/kg; i.p.), served as a positive control. Along with anterograde axonal transport, we measured two 5-HT axonal markers, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). Prior treatment with FEN or MDMA led to marked reductions in anterograde transport of labeled material to various forebrain regions known to receive 5-HT innervation. These reductions were associated with lasting decrements in 5-HT axonal markers. In general, decreases in axonal transport were less pronounced than those in 5-HT and 5-HIAA. However, identical changes were observed after 5,7-DHT. These results further indicate that FEN and MDMA, like 5,7-DHT, are 5-HT neurotoxins.

Sibutramine alters the central mechanisms regulating the defended body weight in diet-induced obese rats

Chronic administration of sibutramine lowers body weight, presumably by altering brain monoamine metabolism. Here the effect of sibutramine on sympathoadrenal function (24-h urine norepinephrine and epinephrine levels) and arcuate nucleus (ARC) neuropeptide Y (NPY) and proopiomelanocortin (POMC) expression was assessed in diet-induced obese rats fed a low-fat diet. Chronic (10 wk) sibutramine [5 mg. kg(-1). day(-1) ip; rats fed ad libitum and injected with sibutramine (AS)] lowered body weight by 15% but only transiently (3-4 wk) reduced intake compared with vehicle-treated controls [rats fed chow ad libitum and injected with vehicle daily (AV)]. Other rats food restricted (RS) to 90% of the weight of AS rats and then given sibutramine restored their body weights to the level of AS rats when allowed libitum food intake. After reequilibration, RS rats were again energy restricted to reduce their weight to 90% of AS rats, and additional vehicle-treated rats (RV) were restricted to keep their body weights at the level of AS rats for 3 wk more. Terminally, total adipose depot weights and leptin levels paralleled body weights (AV > AS = RV > RS), although AS rats had heavier abdominal and lighter peripheral depots than RV rats of comparable body weights. Sibutramine treatment increased sympathetic activity, attenuated the increased ARC NPY, and decreased POMC mRNA levels induced by energy restriction in RV rats. Thus sibutramine lowered the defended body weight in association with compensatory changes in those central pathways involved in energy homeostasis.

Neuroendocrine correlates of depression in abstinent heroin-dependent subjects

The functions of the central alpha-adrenergic, serotoninergic and dopaminergic systems were investigated in 28 heroin-dependent subjects 6-8 weeks after detoxification, and in 22 healthy control subjects (group C). Fourteen heroin-dependent subjects with depressive comorbidity (group A), and 14 heroin-dependent subjects without other Axis I and II pathologies (group B) were included among abstinent substance abusers. Norepinephrine (NE) function was evaluated by growth hormone (GH) responses to acute stimulation with clonidine (clon); serotonin (5-HT) function by prolactin (PRL) and cortisol (CORT) responses to acute stimulation with D-fenfluramine (D-fen) and dopamine (DA) function by GH and PRL responses to acute administration of bromocriptine (brom). Central NE activity, as measured by the GH-clon test, seems to be well preserved both in A and B subjects. PRL and CORT responses to D-fen were significantly blunted both in A subjects and in B subjects, in comparison with control subjects (C); the PRL response in A subjects was significantly lower than in B subjects. The DA system of B subjects was found unimpaired; in contrast, a significantly higher GH response to brom in A subjects (depressed) could express D2 post-synaptic receptor hypersensitivity and, therefore, decreased pre-synaptic DA release. In sum, the study of central monoamine function revealed an alteration only of the 5-HT system in detoxified heroin-dependent subjects without psychiatric comorbidity, which might be a trait character of these subjects, possibly involved in the pathogenesis of the disorder. A more significant impairment of 5-HT function and the hypersensitivity of post-synaptic DA receptors in A subjects suggests that specific biological correlates of psychiatric comorbidity may characterize substance abuser subtypes.

Serotonin transporters, serotonin release, and the mechanism of fenfluramine neurotoxicity

Administration of d,l-fenfluramine (FEN), or the more active isomer d-fenfluramine (dFEN), causes long-term depletion of forebrain serotonin (5-HT) in animals. The mechanism underlying FEN-induced 5-HT depletion is not known, but appears to involve 5-HT transporters (SERTs) in the brain. Some investigators have postulated that 5-HT release evoked by FEN is responsible for the deleterious effects of the drug. In the present work, we sought to examine the relationship between drug-induced 5-HT release and long-term 5-HT depletion. The acute 5-HT-releasing effects of dFEN and the non-amphetamine 5-HT agonist 1-(m-chlorophenyl)piperazine (mCPP) were evaluated using in vivo microdialysis in rat nucleus accumbens. The ability of dFEN and mCPP to interact with SERTs was assessed using in vitro assays for [3H]-transmitter uptake and release in rat forebrain synaptosomes. Drugs were subsequently tested for potential long-lasting effects on brain tissue 5-HT after repeated dosing (2.7 or 8.1 mg/kg, ip x 4). dFEN and mCPP were essentially equipotent in their ability to stimulate acute 5-HT release in vivo and in vitro. Both drugs produced very selective effects on 5-HT with minimal effects on dopamine. Interestingly, when dFEN or mCPP was administered repeatedly, only dFEN caused long-term 5-HT depletion in the forebrain at 2 weeks later. These data suggest that acute 5-HT release per se does not mediate the long-term 5-HT depletion associated with dFEN. We hypothesize that dFEN and other amphetamine-type releasers gain entrance into 5-HT neurons via interaction with SERTs. Once internalized in nerve terminals, drugs accumulate to high concentrations, causing damage to cells. The relevance of this hypothesis for explaining clinical side effects of FEN and dFEN, such as cardiac valvulopathy and primary pulmonary hypertension, warrants further study.

Comparative study of fluoxetine, sibutramine, sertraline and dexfenfluramine on the morphology of serotonergic nerve terminals using serotonin immunohistochemistry

We compared the effects of treatment with high doses of fluoxetine, sibutramine, sertraline, and dexfenfluramine for 4 days on brain serotonergic nerve terminals in rats. Methylenedioxymethamphetamine (MDMA) and 5,7-dihydroxytryptamine (5,7-DHT) were used as positive controls because both compounds deplete brain serotonin. Food intake and body weight changes were also monitored and yoked, pair-fed animals were used to control for possible changes in morphology due to nutritional deficits. Fluoxetine, sibutramine, sertraline and dexfenfluramine all produced a significant reduction in body weight. Fluoxetine, sibutramine and sertraline treatment resulted in no depletion of brain serotonin but produced morphological abnormalities in the serotonergic immunoreactive nerve network. In contrast, dexfenfluramine and MDMA depleted brain serotonin and produced morphological changes in the serotonin nerve network. These results indicate that even though fluoxetine, sibutramine and sertraline do not deplete brain serotonin, they do produce morphological changes in several brain regions (as identified by serotonin immunohistochemistry). Dexfenfluramine and MDMA, on the other hand, markedly deplete brain serotonin and also produce morphological changes. Collectively, these results lend support to the concept that all compounds acting on brain serotonin systems, whether capable of producing serotonin depletion or not, could produce similar effects on the morphology of cerebral serotonin systems.

Acute and chronic D-fenfluramine treatments have different effects on serotonin synthesis rates in the rat brain: an autoradiographic study

The effects of acute and chronic treatments with D-fenfluramine on the regional rates of serotonin (5-hydroxy-tryptamine; 5-HT) synthesis were investigated using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. In the first series of experiments, acute D-fenfluramine treatment (5 mg/kg; i.p.) given 20 min before the tracer injection significantly (p<0.05) decreased 5-HT synthesis in the dorsal raphe, and significantly (p<0.05) increased the rates in the cerebral cortices and caudate nucleus, when compared to the rates in the control rats (saline treated). In a second series of experiments, following a 7-day treatment with D-fenfluramine (5 mg/kg/day; i.p.), a significant (p<0.05) decrease of 5-HT synthesis, in the dorsal raphe was observed, and significant (p<0.05) increases were observed in the hypothalamus, the dorsal thalamus, the medial and lateral geniculate body and some brain stem regions (locus ceruleus, inferior and superior colliculus). No significant changes were observed in the cerebral cortices.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

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.

Intraregional variation in expression of serotonin transporter messenger RNA by 5-hydroxytryptamine neurons

The distribution of the messenger RNA encoding the 5-hydroxytryptamine transporter was investigated in rat brain. 5-Hydroxytryptamine transporter messenger RNA was found exclusively in the B1-B9 cell groups containing the cell bodies of 5-hydroxytryptamine neurons. Combined in situ hybridization and 5-hydroxytryptamine immunocytochemistry demonstrated 5-hydroxytryptamine transporter gene expression in the majority of and exclusively in 5-hydroxytryptamine neurons. Cells differed in their levels of expression of 5-hydroxytryptamine transporter messenger RNA and 5-hydroxytryptamine immunofluorescence, but with a tight correlation between the two parameters. Image analysis of cells from B7, the dorsal raphe nucleus, and B8, the median raphe nucleus, revealed significant differences between groups in the mean cellular level of 5-hydroxytryptamine transporter gene expression. Cells in the ventromedial subdivision of B7 displayed higher levels of expression than cells in B8 or cells in the lateral wings of B7. There was also heterogeneity in the distribution of the cellular levels of expression for two other genes expressed by 5-hydroxytryptamine neurons: l-aromatic amino acid decarboxylase messenger RNA and tryptophan hydroxylase messenger RNA. However, the relative levels of expression of these two genes within the four regions studied differed from that of 5-hydroxytryptamine transporter messenger RNA. These results indicate intraregional differences between 5-hydroxytryptamine neurons with respect to 5-hydroxytryptamine transporter messenger RNA levels. Such differences may account for the differential sensitivity of 5-hydroxytryptamine neurons to cytotoxins.

Neurotoxic effects of +/-fenfluramine and phenteramine, alone and in combination, on monoamine neurons in the mouse brain

Until recently, (+/-)fenfluramine (FEN) was widely prescribed as an appetite suppressant. In animals, FEN is a potent and selective brain serotonin neurotoxin. The present studies assessed the effects of phentermine (PHEN), an appetite suppressant frequently used clinically in combination with FEN, on FEN-induced serotonin neurotoxicity. Groups (n = 6/group) of mice were treated with FEN (10 mg/kg), PHEN (20 mg/kg or 40 mg/kg), FEN (10 mg/kg) plus PHEN (20 mg/kg or 40 mg/kg), or vehicle twice daily for four days. Food intake and body weight were measured during and after drug treatment. Brains were evaluated for regional brain serotonin and dopamine axonal markers two weeks after drug treatment. PHEN enhanced the anorectic and weight-reducing effects of FEN. PHEN also significantly enhanced FEN's long-term toxic effects on 5-HT axons. This effect was evident in some (hypothalamus, striatum) but not all (hippocampus, cortex) brain regions examined. PHEN alone produced no long-term effects on 5-HT axonal markers. However, whether given alone or in combination with FEN, PHEN produced significant, dose-related decreases in striatal DA axonal markers. These results, coupled with those from previous studies, suggest that PHEN has the potential to exacerbate FEN-induced serotonin neurotoxicity, if utilized in certain doses. Further, the present results indicate that PHEN possesses dopamine (DA) neurotoxic potential. The relevance of these data to humans previously treated with FEN/PHEN is discussed.

Adverse neuropsychiatric events associated with dexfenfluramine and fenfluramine

1. There is a large body of evidence indicating that fenfluramines damage brain serotonin neurons in animals. 2. Little is known about potential adverse neuropsychiatric consequences in humans associated with use of fenfluramines that could potentially be related to serotonergic dysfunction. 3. The authors now report numerous cases of severe and, sometimes persistent, neuropsychiatric syndromes associated with fenfluramine use. 4. Thirty one representative cases are presented and summarized in table form. 5. Several of the cases presented suggest long-lasting deleterious effects of fenfluramines on brain serotonin function. 6. Clinicians should be vigilant for disorders of mood, anxiety, cognitive function and impulse control in patients previously exposed to fenfluramines.

The effect of the spin trapping agent α-phenyl-n-tert-butyl nitrone on dexfenfluramine-induced serotonin depletion in rat brain

Oxygen-free radical formation from either the parent compound amphetamine, its metabolites or drug-released serotonin (5-HT) has been implicated in the reduction of serotoninergic markers caused by amphetamine derivatives. Therefore, the present study investigated the effects of the spin-trapping agent α-phenyl-tert-butyl nitrone (PBN) on the 5-HT-lowering action of dexfenfluramine (DF) in rats, compared with p-chloroamphetamine (PCA). PBN (150 mg/kg, i.p, divided in two doses) almost totally prevented the reduction of 5-HT in particularly sensitive regions of the rat brain (cortex and striatum) 1 and 7 days after DF (10 mg/kg, i.p.). It also provided complete protection against the acute 5-HT-depleting action of PCA (5 mg/kg, i.p.), reducing it at 7 days in striatum, although with the higher dose (300 mg/kg, divided in two doses) there was a tendency to antagonize the long-term effects in both regions. With DF, however, the antagonistic effect of PBN was associated with a marked reduction of the plasma and brain concentrations of the parent drug, but particularly its active metabolite dexnorfenfluramine (DNF). Thus, reduced brain availability of the total active drug (DF+DNF) may explain why PBN prevents the neurochemical effects of DF (but not PCA), including the long-term one which possibly depends on the extent of the initial 5-HT lowering.

The effect of D-fenfluramine on brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in male and female rats

Brain regional 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined in freely feeding male and female rats 7 days after giving a single dose of D-fenfluramine (3.8 mg/kg, p.o.) or vehicle. Males showed negligible effects except for a significant decrease of 5-HT in the rest of the cortex, whereas females showed significant decreases of 5-HT and 5-HIAA in the frontal cortex, the rest of the cortex, hippocampus and hypothalamus; 5-HT was also decreased in female midbrain. Females had substantially higher plasma and brain concentrations of fenfluramine and moderately but significantly lower concentrations of norfenfluramine than the males. Plasma fenfluramine + norfenfluramine concentrations of the females were significantly higher than those of the males. Corresponding brain values showed smaller but significant differences. Female brain and plasma areas under the curve for fenfluramine + norfenfluramine (0-24 h after administration of D-fenfluramine) were 20 and 35% higher than male values. However, results suggest that the sex difference in the effect of D-fenfluramine on brain 5-HT metabolism is not due to differences in the metabolism of the drug.

Neuropharmacological effects of low and high doses of repeated oral dexfenfluramine in rats: a comparison with fluoxetine

The neuropharmacological effects of repeated oral doses of dexfenfluramine (DF; 1.25-10 mg/kg, twice daily for 21 days) were examined in rats and related to the drug brain levels. Results were compared with fluoxetine (FL) given at similar doses relative to its anorectic ED50. Both drugs dose-dependently slowed body weight gain and reduced brain serotonin (5-HT). However, at 1.25 mg/kg DF caused only a slight and transient decrease in cortical 5-HT. Comparable doses of FL (6.25-12.5 mg/kg) lowered 5-HT more than DF, besides slightly reducing striatal dopamine. At higher doses DF markedly reduced 5-HT in all regions, and to a lesser extent noradrenaline in hippocampus. There was a negative relationship between 5-HT and log total active drug levels and the indole was approximately halved at drug levels about 50 times lower with DF than FL. However, the ratio between drug levels causing marked 5-HT reductions and those considered anorectic was similar for DF and FL because brain levels at the anorectic ED50 were higher with FL than DF. Long-lasting reductions of 5-HT were also observed but recovery was only consistently slow beginning from 5 mg/ kg DF. Comparable doses of FL could not be used because its general toxicity leads to the death of rats after only 2-4 multiples of its anorectic ED50.

Combined phentermine/fenfluramine administration enhances depletion of serotonin from central terminal fields

Administration of phentermine (Phen) together with (+/-) fenfluramine (Fen) enhances the weight reduction that is observed with either drug alone; consequently, these anorectic agents are commonly prescribed together for weight reduction. Repeated administration of Fen is known to cause long-term depletion of axonal serotonin (5-HT) and loss of 5-HT transporters, and is therefore considered neurotoxic. We now report that combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg, and 20 mg/kg/3.125 mg/kg) can enhance the neurotoxic effect of Fen (3.125 mg/kg) and Phen (5 mg/kg and 20 mg/kg) on central 5-HT systems. Rats were repeatedly treated once each hour for a total of four injections with saline, Phen (5 mg/kg and 20 mg/kg), Fen (3.125 mg/kg and 12.5 mg/kg), or combined Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg), and sacrificed either 7 or 28 days after cessation of treatment. Combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg) caused significantly greater reductions of 5-HT levels in the striatum, nucleus accumbens/olfactory tubercle, hypothalamus, amygdala, frontal parietal cortex, and hippocampus than either drug alone. Combined Phen/Fen at the higher drug-dose combination (20 mg/kg/3.125 mg/kg) was observed to reduce the density of 5-HT transporters in rat striatum at both 7 and 28 days after cessation of treatment. In addition, combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg) caused greater weight loss than that observed with either compound alone. Collectively, the present data demonstrate that combined Phen/Fen administration enhances the neurotoxicity of Phen or Fen on 5-HT neurons.

Effects of repeated oral doses of dexnorfenfluramine on 5-HT levels and 5-HT uptake sites in rat brain

The effects of oral dexnorfenfluramine (DNF; 1-4 mg/kg, twice daily for 4 days), the active metabolite of dexfenfluramine, were examined on rat regional brain indole contents and [3H]citalopram binding. Two hours after the last dose, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were dose-dependently lowered at doses above 1.5 mg/kg, with slight regional differences. Cortical 5-HT uptake sites were reduced only at the highest dose. Above 2 mg/kg DNF also caused a more lasting reduction (4 weeks) of regional indoles and cortical 5-HT uptake sites. At this longer time while the decrease in hippocampal 5-HT levels and cortical 5-HT uptake sites remained essentially constant, cortical and striatal 5-HT levels were lowered less than at 2 h, suggesting a return toward control values.

Microdialysis monitoring of variations in extracellular levels of serotonin, GABA and excitatory amino acids in the frontal cortex of awake rats in response to a single peripheral or central administration of dexfenfluramine

The effects of a single dexfenfluramine (D-fen) administration on the release of endogenous serotonin (5-hydroxytryptamine, 5-HT), excitatory (glutamate, Glu, aspartate, Asp) and inhibitory (gamma-aminobutyric acid, GABA) amino acids from the frontal cortex were studied by using in vivo microdialysis in freely-moving rats. Extracellular levels of these neurotransmitters were measured with HPLC coupled to electrochemical detection or with capillary electrophoresis coupled to laser-induced fluoresence detection (CE-LIFD). In a first study, single intraperitoneal administration of D-fen (0.5, 1.3, 5 and 10 mg/kg) increased extracellular 5-HT levels in a dose-dependent manner (maximal increase by 982% over baseline for the highest dose) while changes in Glu, Asp or GABA never reached statistical significance. In a second study, 73 nM of D-fen applied locally through the frontocortical dialysis probe, at a flow rate of 1.5 microliters/min in 30 microliters of perfusion fluid for 20 min, increased extracellular 5-HT and Asp levels [the maximal increases were to 1804% and 280% of the respective basal values (100%)] without altering extracellular levels of Glu and GABA. Thus, the order of magnitude of the changes induced by systemic administration or local infusion of D-fen on frontocortical extracellular levels of several neurotransmitters (5-HT > > Asp > GABA = Glu) demonstrate that D-fen, an indirect serotoninergic agonist, mainly increases 5-HT release while producing slight (Asp) or no (Glu, GABA) short-term in vivo variations in amino acid extracellular levels in the rat frontal cortex.

Down-regulation of rat brain 5-HT uptake carriers after treatment with high doses of D-fenfluramine

Male rats were treated with 10 mg/kg D-fenfluramine (DF) i.p., twice a day for 4 days. Five days later there was a strong reduction (70-100%) in the Bmax of [3H]citalopram binding and the Vmax of [3H]5-HT uptake in cortical and hippocampal synaptosomes; 2 months after the treatment these parameters were reduced by 40-70%. The effect of treatment was also evaluated in synaptosomes preloaded with [3H]5-HT, superfused and exposed for 3 min to a releasing stimulus (15 mM K+ or 0.5 microM DF). In our experimental conditions, the stimulated [3H]5-HT release is Ca(2+)-dependent and takes place only from 5-HT nerve endings. The K(+)-stimulated release was not consistently altered by the DF treatment whereas DF-stimulated [3H]5-HT release was markedly reduced, either 5 days and 2 months after the treatment. The effect of chronic DF was different from the effect of i.c.v. 5,7-DHT, a specific 5-HT neurotoxin which completely abolished the K(+)-induced release. Since the decrease of synaptosomal [3H]5-HT uptake induced by 5,7-DHT (82%) was similar to that found after chronic DF (70-80%), these data suggest that the decrease of 5-HT uptake sites induced by chronic DF is not (only) due to neurodegeneration. That chronic DF could induce a functional down-regulation of 5-HT uptake sites (i.e. decreased density per intact nerve ending) was suggested by the decrease of DF-induced release, since the releasing activity of DF is dependent on functional 5-HT uptake sites. However, due to the characteristics of our model, our results are compatible with either the absence or the presence of a concomitant, partial neurodegeneration of 5-HT nerve endings in DF-treated rats. In summary, our data indicate that after treatment with high doses of DF, the 5-HT uptake carriers undergo a long-lasting down-regulation, thus totally or partly explaining the lower [3H]citalopram binding and the lower synaptosomal [3H]5-HT uptake.

In vitro and in vivo effects of the anorectic agent dexfenfluramine on the central serotoninergic neuronal systems of non-human primates. A comparison with the rat

The effects of repeated subcutaneous (s.c) injections of dexfenfluramine (d-F; 10 mg/kg, twice daily, for 4 days) on the contents of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the brain were assessed in primates (cynomolgus and rhesus monkeys) and compared with the regional brain concentrations of unchanged drug and its active metabolite, dexnorfenfluramine (d-NF). This four-day, high-dose, regimen caused a large depletion of 5-HT (more than 95%) and of 5-HIAA (80-90%) in all brain areas studied (cortex, hippocampus, putamen, caudate nucleus and hypothalamus) 2 h after the last injection of d-F. Analysis of the plasma and brain contents of d-F and d-NF confirmed that both compounds were concentrated as in other species, in regions of the primate brain. However, d-NF was concentrated to a greater extent than d-F, and there were differences between the two primate species. Unlike in the rat brain, concentrations of d-NF greatly exceeded those of d-F in the primate brain suggesting that in these primates the d-NF may play a major role in the overall neurochemical response. The effects of d-F and d-NF on different in vitro parameters of serotoninergic neuronal function did not show appreciable differences between cynomolgus or rhesus monkeys when compared to rats, the ability of the two compounds to inhibit 5-HT reuptake, to enhance its release, and to affect the binding of [3H] -d-F or of [3H] -mesulergine (a ligand for 5-HT2C receptors) being similar. Kinetic differences in the disposition of d-F appear to have more relevance than biochemical effects in providing an explanation for the more marked brain depletion induced by d-F in primates than in rodents.

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.

Depletion and time-course of recovery of brain serotonin after repeated subcutaneous dexfenfluramine in the mouse. A comparison with the rat

The indole-depleting effects of repeated subcutaneous doses of dexfenfluramine (D-F) (2.5, 5, 10, 20 and 40 mg/kg/day, for four days) in mice were examined with regard to the initial response and time-course of recovery and related to the pharmacokinetics of D-F and its active metabolite dexnorfenfluramine (D-NF). Steady-state plasma and brain concentrations of D-F rose dose-dependently with a metabolite-to-drug ratio averaging 0.4 in brain. This confirmed that in mice D-NF contributes less than in other species to the effects of D-F. Regional serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents were decreased dose-dependently 4 hr after the last injection of D-F. However, two weeks after D-F (2.5-10 mg/kg/day) brain indoles had almost totally recovered, and the long-term effects of the 20 mg/kg/day dose were completely reversed by six weeks, when significant effects are still observable in rats. Although substantial recovery was evident even at 40 mg/kg/day, 5-HT but not 5-HIAA was still slightly reduced nine weeks later. Comparative studies in rats given 2.5-20 mg/kg/day D-F indicated much more severe initial indole depletions than in mice. Brain levels of D-F and D-NF were much higher in rats than in mice. The total active drug brain concentration (D-F + D-NF) was significantly correlated with 5-HT content in both species, with approx 20 nmol/g of total drug causing 50% reduction. These findings point to species differences in D-F kinetics as a main reason for differences in the neurochemical response, supporting the view that the recovery of indoles over time is related to the extent of initial depletion, which in turn depends on critical drug brain concentrations. In view of the qualitative and quantitative species differences in the pharmacodynamics and pharmacokinetics of D-F neither of these rodent species is a suitable model for predicting potential drug toxicity in humans.

Oral kinetics of dexfenfluramine and dexnorfenfluramine in non-human primates

1. Large doses of dexfenfluramine in animals cause a decrease of serotoninergic markers but none of the species so far investigated shows sufficient kinetic and metabolic similarity with man to be a valid model for safety studies. The plasma kinetics of dexfenfluramine and its active metabolite dexnorfenfluramine were therefore studied in baboon, rhesus and cynomolgus monkeys given dexfenfluramine hydrochloride orally (2 mg/kg) in order to investigate whether any of these primates have a biodisposition particularly similar to man. 2. The drug was rapidly N-deethylated to dexnorfenfluramine achieving comparatively low mean maximum plasma levels (Cmax) of 12-14 ng/ml in all primates, and rapidly disappeared thereafter with half-lives (t1/2) ranging from 2 to 3 h in the baboon and rhesus monkey to 6 h in the cynomolgus monkey. Its normetabolite reached higher mean Cmax (52-97 ng/ml) and the t1/2's were longer, varying from about 11 h in the rhesus monkey to 22 h in the cynomolgus monkey. The metabolite-to-parent drug ratio (14-37), in terms of plasma area under curve (AUC), greatly exceeded that in man (< 1), being higher than in all species investigated so far. 3. Comparative repeat dose simulation in monkey and man indicated that the dosage in primates would need to be increased 10-fold to achieve comparable dexfenfluramine steady-state plasma Cmax, producing nor-metabolite levels several times those in man, whilst for comparable metabolite Cmax, those of the parent drug would be correspondingly too low. 4. In view of the different mechanism of action of dexfenfluramine and dexnorfenfluramine within the serotoninergic system none of these primates is therefore a suitable model for safety assessment in terms of exposure of the active moieties in comparison with man.

The role of temperature, stress, and other factors in the neurotoxicity of the substituted amphetamines 3,4-methylenedioxymethamphetamine and fenfluramine

Amphetamines (AMPs) can cause long-term depletions in striatal dopamine (DA) and serotonin (5-HT), and these decrements are often accepted as prima facie evidence of AMP-induced damage to the dopaminergic and serotonergic projections to striatum. Rarely are indices linked to neural damage used to evaluate the neurotoxicity of the AMPs. Here, we determined the potential neurotoxic effects of two substituted AMPs, d-methylenedioxymethamphetamine (d-MDMA) and d-fenfluramine (d-FEN) in group-housed female C57BL6/J mice. Astrogliosis, assessed by quantification of glial fibrillary acidic protein (GFAP), was the main indicator of d-MDMA-induced neural damage. Assays of tyrosine hydroxylase (TH), DA, and 5-HT were used to determine effects on DA and 5-HT systems. Since AMPs are noted for both their stimulatory and hyperthermia-inducing properties, activity, as well as core temperature, was monitored in several experiments. To extend the generality of our findings, these same end points were examined in singly housed female C57bL6/J mice and in group-housed male C57BL6/J or female B6C3F1 mice after treatment with d-MDMA. Mice received either d-MDMA (20 mg/kg) (singly housed mice received dosages of 20, 30, or 40 mg/kg) or d-FEN (25 mg/kg) every 2 h for a total of four sc injections. d-MDMA caused hyperthermia, whereas d-FEN induced hypothermia. d-MDMA cause a large (300%) increase in striatal GFAP that resolved by 3 wk and a 50-75% decrease in TH and DA that was still apparent at 3 wk, d-FEN did not affect any parameters in striatum. d-MDMA is a striatal dopaminergic neurotoxicant in both male and female C57BL6/mice, as evidenced by astrogliosis and depletions of DA in this area in both sexes. The greater lethality to males suggests they may be more sensitive, at least to the general toxicity of d-MDMA, that females. d-MDMA (20 mg/kg) induced the same degree of damage whether mice were housed singly or in groups. Higher dosages in singly housed mice induced greater lethality, but not greater neurotoxicity. d-MDMA was also effective in inducing striatal damage in mice of the B6C3F1 strain. Significant increases in activity were induced by d-MDMA, and these increases were not blocked by pretreatment with MK-801, despite the profound lowering of body temperature induced by this combination. A lowering of body temperature, whether by a 15 degree C ambient temperature (approx 2 degree drop), pretreatment with MK-801 (1.0 mg/kg prior to the first and third d-MDMA injections; approx 5-6 degrees C drop) or restraint (approx 5-6 degrees C drop) was effective in blocking the neurotoxicity of d-MDMA in both C57BL6/J and B6C3F1. The stimulatory effects of d-MDMA appeared to have little impact on the neurotoxicity induced by d-MDMA or the protection conferred by MK-801. These data suggest that in the mouse, the neurotoxic effects of d-MDMA, and most likly other AMPs, are linked to an effect on body temperature.

Examination of developmental neurotoxicity by the use of tissue culture model systems

1. The rotation-mediated three-dimensional reaggregate culture system is uniquely suited for studies on developmental neurotoxicity. In this system, it is possible to reconstruct central neuronal pathways and follow their development. 2. Exposure to drugs of abuse including methamphetamine and methylenedioxyamphetamine or the appetite suppressant, fenfluramine, reduces monoamines in the cultures in a dose-dependent manner and interrupts normal monoaminergic development. 3. While the monoaminergic neurones may attain normal rates of development following drug removal, the affected neurones are not capable of overcoming the drug-induced insults and a deficiency in monoamines persists throughout development. 4. In addition, the production of immortalized monoclonal hybrid cells obtained by fusion of fetal mesencephalic neurones with a neuroblastoma has yielded cell lines expressing a dopaminergic phenotype. 5. Such cells have been useful in establishing the relationship of neurotoxicity to cell lineage and can serve as models for the study of the cellular and molecular mechanisms of neurotoxicity.

Effect of d-fenfluramine on the indole contents of the rat brain after treatment with different inducers of cytochrome P450 isoenzymes

The effects of pretreatment with inducers of hepatic cytochrome P450 isoenzymes (phenobarbital, dexamethasone and beta-naphthoflavone) on the metabolism of d-fenfluramine (d-F) and its acute and long-lasting indole-depleting effects were studied in rats, in an effort to obtain further information on the importance of hepatic drug metabolism in relation to its neurochemical actions. Twenty-four hours after the last dose of each inducer, rats were injected with d-F hydrochloride (5 mg/kg, IP) and killed at various times thereafter for parallel determination of indoles and drug concentrations in plasma and brain. Additional rats were treated as above and killed 1 week after d-F hydrochloride (5 and 10 mg/kg) to study the recovery of indole in the cortex, a particularly sensitive brain area. Phenobarbital and beta-naphthoflavone and, to a lesser degree, dexamethasone, stimulated the metabolism of d-F, as evidenced by a decrease in plasma and brain areas under the curve (AUC) compared to vehicle-treated rats. This indicated that multiple isoenzymes are capable of mediating the drug's metabolism, primarily by N-dealkylation to d-norfenfluramine (d-NF). None of the inducers raised plasma and brain AUC of the nor-derivative, and in fact phenobarbital and particularly beta-naphthoflavone reduced it. These different effects were even apparent in rats given d-NF (2.5 mg/kg), indicating that both phenobarbital and beta-naphthoflavone also stimulate the sequential metabolism of the nor-metabolite (by N-deamintaion) which, however, is apparently enhanced most actively by beta-naphthoflavone-inducible forms of P-450.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative immunocytochemical evaluation of serotonergic innervation in alcoholic rat brain

Neurotoxicity can be divided into three levels: depletion, degeneration and denervation. The first level is determined by the transmitter content, and the second and third levels, which require anatomical evaluation, can be analyzed by quantitative immunocytochemistry on a specific neurotransmitter system. An antibody specific to serotonin (5-HT) can reveal detailed normal as well as degenerative morphology of 5-HT neurons. Quantitative alterations of 5-HT fibers in a particular brain region indicate degenerative or plastic changes. This study demonstrates quantitative immunocytochemistry by using image analysis of immunostained 5-HT fibers in selectively-bred, alcohol-preferring and nonpreferring rats, which are known to have divergent drinking behaviors, and 5-HT contents in specific brain regions. The method of the image analysis is described in detail and the advantages and disadvantages of using this method to detect the degeneration of a particular fiber system are discussed. The 'area density' traditionally measured in image analysis was converted (with Zhou-Tam formula) into 'volume density' to correct the mismeasurement of fibers through the optical depth. Quantitative immunostaining shows that the difference in 5-HT fiber density in particular brain regions between the two rat lines is consistent with changes in content of the 5-HT/5-HIAA and hypersensitivity of 5-HT1 a receptor. This result indicates either (a) the 5-HT content is too low to be detected in the nerve terminals; (b) degeneration of 5-HT fibers occurs in P rats sometime during development; or (c) a smaller number of 5-HT fibers was preprogrammed in the brain regions of P than NP rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine agonists prevent or counteract the suppression of brain stimulation reward by fenfluramine

The interaction between the serotonin (5-HT) and the dopamine (DA) systems in the modulation of intracranial self-stimulation (ICSS), a DA-dependent behavior, was investigated. Chronically implanted rats for ICSS in the medial forebrain bundle were tested for the effects of fenfluramine at a dose of 20 mg/kg, and then for the effects of 10 mg/kg piribedil plus 2 mg/kg amphetamine, injected 30 min before fenfluramine or 60 min after fenfluramine. Our aim was to determine whether the action of fenfluramine at the DA binding site could be blocked by prior occupation, or whether if it were occupied by fenfluramine it could be reversed. Fenfluramine, 20 mg/kg, injected alone, suppressed ICSS for 5-7 h. The suppression was followed by a prolonged recovery during which ICSS was profounded depressed. Repeating the treatment 7 days later produced the same response, except that the suppression was of shorter duration. In another group of animals, pretreatment with piribedil plus amphetamine 30 min before fenfluramine prevented the suppression of ICSS. Instead, ICSS was briefly attenuated, then restored to baseline levels, and then facilitated. Repeating the treatment 7 days after the first treatment potentiated this response. The attenuation was now even briefer, the recovery more rapid, and the facilitation more robust. In still another group of animals, fenfluramine was given just before the ICSS session began. Predictably, the effect was a total cessation of ICSS. At 60 min into the session, piribedil plus amphetamine was injected. The response showed a rapid recovery of ICSS followed by facilitation.(ABSTRACT TRUNCATED AT 250 WORDS)

p-Chloroamphetamine (PCA), 3,4-methylenedioxy-methamphetamine (MDMA) and d-fenfluramine pretreatment attenuates d-fenfluramine-evoked release of 5-HT in vivo

Previous work has suggested that repeated treatment with substituted amphetamines including PCA, MDMA and d-fenfluramine produces a persistent neurodegeneration which is relatively selective for the fine serotoninergic terminals arising from the dorsal raphe nucleus. The aim of the present study was to investigate whether the acute releasing effect of d-fenfluramine might also be sensitive to lesions produced by PCA, MDMA and d-fenfluramine itself. Basal and 5-HT release evoked by d-fenfluramine or 100 mM KCl was measured by microdialysis in frontal or parietal cortex of rats 2 weeks after they had been treated with a neurodegenerative regime of PCA, MDMA, d-fenfluramine, or vehicle. In frontal cortex of vehicle controls, d-fenfluramine (10 mg/kg IP) and KCl (100 mM via microdialysis probe) evoked an increase in 5-HT of 1740% and 779% of basal, respectively. PCA pretreatment reduced d-fenfluramine-evoked 5-HT release by 90.9% while potassium-evoked release was reduced by only 66.8%. Similar results were obtained in parietal cortex. MDMA (20 mg/kg x 8) and d-fenfluramine (1.25 mg/kg x 8) pretreatment reduced d-fenfluramine-evoked release of 5-HT in frontal cortex by 45.2% and 72.0%, respectively. Overall, the present data are consistent with the hypothesis that the acute release of 5-HT evoked by d-fenfluramine occurs via those terminals destroyed by pretreatment with PCA, MDMA and d-fenfluramine, while KCl evokes release from both PCA-sensitive and PCA-insensitive terminals. The significance of these results for the interpretation of neuroendocrine data from d-fenfluramine challenge tests is discussed.

Chronic D-fenfluramine decreases serotonin transporter messenger RNA expression in dorsal raphe nucleus

In situ hybridization was used to measure the effects of chronic fenfluramine administration on serotonin transporter messenger RNA expression in cells of the dorsal raphe nucleus complex. Fenfluramine produced a significant, but transient, down-regulation of serotonin transporter mRNA in cells which lie in the ventral portion of the dorsal raphe nucleus, but not in the dorsal part of the dorsal raphe nucleus. Our findings suggest that cells which lie in the ventral part of the dorsal raphe nucleus are more sensitive to the effects of chronic fenfluramine administration, but that fenfluramine does not cause long-term changes in gene expression in serotonin cell bodies.

Microglial response to degeneration of serotonergic axon terminals

The neurotoxic drug p-chloramphetamine (PCA) causes widespread degeneration of fine, unmyelinated serotonergic (5-HT) axons in the forebrain. PCA toxicity is selective for 5-HT axon terminals; preterminal axons and cell bodies are spared. Degeneration is followed by slowly progressive axonal sprouting and partial reinnervation. PCA is injected subcutaneously; this route of administration avoids mechanical disruption of the blood brain barrier. The present study analyzed the response of microglia and astrocytes in rat brain to selective ablation of 5-HT axons by PCA. Several microglial markers were analyzed with immunocytochemical methods. An increase in the number of microglial processes and in immunoreactive staining was observed with antibodies directed against CR-3, MHC-I, CD4, and rat LCA. The microglial response was maximal 3 weeks after PCA treatment, became less evident 6 weeks after treatment, and by 9 weeks no difference was observed between treated and control rats. No change was detected in MHC-II or the macrophage marker ED1, nor in expression of GFAP by astrocytes. Thus, degeneration of 5-HT axon terminals affects only a subset of the microglial markers examined; in comparison, retrograde reaction to facial nerve transection causes a robust increase in all of these markers and in GFAP. The microglial response to PCA-induced axon loss is slow in onset and small in magnitude. These findings indicate that CNS microglia are activated by degeneration of fine, unmyelinated 5-HT axon terminals; furthermore, sensitive microglial markers can detect a subtle axonal lesion that provokes no detectable increase in GFAP expression by astrocytes.

Opposite effects of PCA and chlorimipramine on ICSS and on its facilitation by amphetamine

The long-term effects of chloramphetamine (PCA) and chlorimipramine (CHLOR) on intracranial self-stimulation (ICSS) were investigated in sessions lasting 13 h. PCA, 5 mg/kg given IP, led first to an attenuation of ICSS lasting 3 h, then to a slow recovery to baseline rates, and then to a facilitation of ICSS lasting 6 h. Repeating the treatment 7 days later resulted in less attenuation of ICSS, more rapid recovery, and longer-lasting facilitation. Again, repeating the treatment with PCA 7 days later but injecting simultaneously amphetamine (AMPH) 2 mg/kg IP, altered the response seen with PCA alone. The attenuation phase was missing but the facilitatory phase remained except that it occurred early and was of shorter duration than after PCA given alone. Pretreatment with haloperidol (HALO) 0.5 or 1.0 mg/kg IP before PCA blocked the facilitatory phase of the response. CHLOR injected at a dose of 15 mg/kg IP attenuated ICSS. The combined administration of CHLOR and AMPH led to the CHLOR-attenuation of ICSS being replaced by a modest facilitation. These results are discussed in terms of the biochemical actions of PCA and CHLOR on the serotonin and dopamine systems.

Effects of d-fenfluramine on feeding and hypothalamic 5-hydroxytryptamine and dopamine in male and female rats

Male and female rats were given d-fenfluramine and its effects on feeding and on hypothalamic concentrations of the drug, its metabolite norfenfluramine and 5-hydroxytryptamine (5-HT) and dopamine determined. ID50 values (i.p.) for the hypophagic effect of the drug on 30-, 42- and 100-day-old rats measured over 2 h during the light phase after 24 h food deprivation did not vary significantly with sex but tended to decrease with age approximately in parallel with daily percentage increases and (after deprivation) of decreases in body weight. However, male but not female 30-day-old rats showed a rebound of feeding during the subsequent 2 h. ID50 values of 42-day-old rats on a palatable diet or measured during the dark phase when freely feeding also did not vary with sex. Male 30-day-old rats killed at 2-10 h after an ID75 (p.o.) dose of d-fenfluramine had substantially lower hypothalamic concentrations of the drug and comparable or slightly lower concentrations of its metabolite norfenfluramine than 30-day-old females. Similarly treated 100-day-old males also had lower concentrations of fenfluramine but significantly higher norfenfluramine levels than females so that drug plus metabolite concentrations were essentially independent of sex. 100-day-old females killed 2 h, 24 h and 7 days after d-fenfluramine (3.8 mg/kg p.o. = ID75) had larger percentage decreases of hypothalamic 5-HT than identically treated males. Percentage decreases of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) tended to become less marked with time after injection in males but not females.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of d-fenfluramine and 5,7-dihydroxytryptamine on the levels of tryptophan hydroxylase and its mRNA in rat brain

Repeated high doses of d-fenfluramine (dF; 10 mg/kg, i.p. twice daily for 4 days) markedly reduced serotonin (5-HT) concentrations in the hippocampus and striatum of rat brain up to 1 month after treatment, while tryptophan hydroxylase (TPH) levels were reduced only in the hippocampus 5 days after injection. Unlike dF, an intracerebroventricular (i.c.v.) injection of 5,7-dihydroxytryptamine (5,7-DHT 150 micrograms/20 microliters) induced a marked and long-lasting reduction of 5-HT and TPH in both brain regions. Thirty days after injection, 5,7-DHT, but not dF, markedly reduced the number of labelled neurons in the dorsal and ventral regions of the nucleus raphe dorsalis (NRD) and raised the levels of TPH mRNA in the spared neurons at all times examined. TPH mRNA levels were raised 5 and 15 days after dF treatment in the NDR suggesting that changes in the TPH gene expression or transcript stability result following 5-HT depletion. These data are in agreement with the suggestion that 5,7-DHT damages 5-HT nerve terminals and perikarya, but leave unanswered the question of the mechanism of the long-lasting reduction of 5-HT levels caused by high, repeated doses of dF.

The role of d-norfenfluramine in the indole-depleting effect of d-fenfluramine in the rat

The importance of d-norfenfluramine in regard to the indole-depleting action of d-fenfluramine has not been well studied in sensitive animal species. The present study therefore examined the intensity and time course of the neurochemical effects of i.p. injected d-fenfluramine (2.5 and 5 mg/kg) and d-norfenfluramine (2.5 mg/kg) in vehicle- and SKF-525A-pretreated rats, relating the effects to the brain concentration-time profiles of the drug and its active metabolite. At the lower dose d-fenfluramine caused only a small, short-lasting decrease in brain serotonin (5-HT) without affecting the 5-hydroxyindoleacetic acid (5-HIAA). Higher doses affected both 5-HT and 5-HIAA (50-60 and 30-40% reductions, respectively), the effect being maximal for at least 8 h. d-Norfenfluramine reduced the brain content of 5-HT and 5-HIAA less (by about 30%) than 5 mg/kg d-fenfluramine did. Brain concentrations of d-norfenfluramine at the time of the maximal depletion of indoles were close to those of the metabolite after 5 mg/kg d-fenfluramine, indicating that the acute indole-depleting effects did not depend solely on the brain concentrations of its nor-metabolite. SKF-525A changed the metabolite-to-parent drug ratios in brain without appreciably influencing the action of d-fenfluramine. However, the maximum decrease in indole content caused by 2.5 mg/kg d-fenfluramine in SKF-525A-pretreated rats was only 12% of the control level, although the brain concentration of unchanged drug was comparable to that after 5 mg/kg d-fenfluramine in vehicle-pretreated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The presence of a serotonin uptake inhibitor alters pharmacological manipulations of serotonin release

The present study investigated the effects of the presence of the serotonin uptake inhibitor citalopram in the perfusion medium on pharmacological manipulations which increased and decreased striatal serotonin release using in vivo microdialysis. A high performance liquid chromatography detection system equipped with a microbore column was used which reduced the detection limit to 0.5 fmol serotonin/5 microliters sample and enabled basal striatal serotonin release to be measured without the addition of a serotonin uptake inhibitor to the perfusion medium. Although serotonin uptake inhibitors have frequently been used to enhance the serotonin content of dialysate samples, the effects of the presence of serotonin uptake inhibitors on pharmacological manipulations which increased and decreased the release of serotonin have not yet been characterized. Serotonin release was reduced by the systemic administration of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Although 5-HT release was reduced by 8-OH-DPAT after the addition of citalopram, the 5-HT1A receptor agonist did not reduce absolute levels of extracellular serotonin below basal values of serotonin measured in the absence of citalopram. In addition, citalopram dramatically prevented the four-fold increase in the release of serotonin produced by the systemic administration of the serotonin-releasing agent fenfluramine. The blockade of fenfluramine's effects by citalopram supports the hypothesis that transport of fenfluramine into serotonergic neurons is necessary to increase serotonin release. This study demonstrates that the use of an HPLC detection system equipped with a microbore column can reliably measure basal serotonin release using in vivo microdialysis.(ABSTRACT TRUNCATED AT 250 WORDS)