Ibogaine, a noncompetitive inhibitor of serotonin transport, acts by stabilizing the cytoplasm-facing state of the transporter

Ibogaine, a hallucinogenic alkaloid with purported anti-addiction properties, inhibited serotonin transporter (SERT) noncompetitively by decreasing V(max) with little change in the K(m) for serotonin (5-HT). Ibogaine also inhibited binding to SERT of the cocaine analog 2beta-2-carbomethoxy-3-(4-[(125)I]iodophenyl)tropane. However, inhibition of binding was competitive, increasing the apparent K(D) without much change in B(max). Ibogaine increased the reactivity of cysteine residues positioned in the proposed cytoplasmic permeation pathway of SERT but not at nearby positions out of that pathway. In contrast, cysteines placed at positions in the extracellular permeation pathway reacted at slower rates in the presence of ibogaine. These results are consistent with the proposal that ibogaine binds to and stabilizes the state of SERT from which 5-HT dissociates to the cytoplasm, in contrast with cocaine, which stabilizes the state that binds extracellular 5-HT.     

Long-Lasting Ibogaine Protection against NMDA-Induced Convulsions in Mice

Ibogaine, a putative antiaddictive drug, is remarkable in its apparent ability to downgrade withdrawal symptoms and drug craving for extended periods of time after a single dose. Ibogaine acts as a non-competitive NMDA receptor antagonist, while NMDA has been implicated in long lasting changes in neuronal function and in the physiological basis of drug addiction. The purpose of this study was to verify if persistent changes in NMDA receptors could be shown in vivo and in vitro after a single administration of ibogaine. The time course of ibogaine effects were examined on NMDA-induced seizures and [³H] MK-801 binding to cortical membranes in mice 30min, 24, 48, and 72h post treatment. Ibogaine (80 mg/kg, ip) was effective in inhibiting convulsions induced by NMDA at 24 and 72 hours post administration. Likewise, [³H] MK-801 binding was significantly decreased at 24 and 72 h post ibogaine. No significant differences from controls were found at 30min or 48h post ibogaine. This long lasting and complex pattern of modulation of NMDA receptors prompted by a single dose of ibogaine may be associated to its antiaddictive properties.     

The mechanistic basis for noncompetitive ibogaine inhibition of serotonin and dopamine transporters

Ibogaine, a hallucinogenic alkaloid proposed as a treatment for opiate withdrawal, has been shown to inhibit serotonin transporter (SERT) noncompetitively, in contrast to all other known inhibitors, which are competitive with substrate. Ibogaine binding to SERT increases accessibility in the permeation pathway connecting the substrate-binding site with the cytoplasm. Because of the structural similarity between ibogaine and serotonin, it had been suggested that ibogaine binds to the substrate site of SERT. The results presented here show that ibogaine binds to a distinct site, accessible from the cell exterior, to inhibit both serotonin transport and serotonin-induced ionic currents. Ibogaine noncompetitively inhibited transport by both SERT and the homologous dopamine transporter (DAT). Ibogaine blocked substrate-induced currents also in DAT and increased accessibility of the DAT cytoplasmic permeation pathway. When present on the cell exterior, ibogaine inhibited SERT substrate-induced currents, but not when it was introduced into the cytoplasm through the patch electrode. Similar to noncompetitive transport inhibition, the current block was not reversed by increasing substrate concentration. The kinetics of inhibitor binding and dissociation, as determined by their effect on SERT currents, indicated that ibogaine does not inhibit by forming a long-lived complex with SERT, but rather binds directly to the transporter in an inward-open conformation. A kinetic model for transport describing the noncompetitive action of ibogaine and the competitive action of cocaine accounts well for the results of the present study.     

Regional Effects of Sodium Valproate on Extracellular Concentrations of 5-Hydroxytryptamine, Dopamine, and Their Metabolites in the Rat Brain: An In Vivo Microdialysis Study

The effects of sodium valproate (VPA; 100, 200, and 400 mg/kg, i.p.) on ventral hippocampal and anterior caudate putamen extracellular levels of dopamine (DA) and 5-hydroxytryptamine (5-HT) were examined using in vivo microdialysis. VPA induced dose-related increases in dialysate DA, 3,4-dihydroxyphenylacetic acid, and 5-HT in the ventral hippocampus. Anterior caudate putamen dialysate 5-HT was also dose dependently elevated by the drug, whereas DA levels tended to decrease with increasing VPA dose. In contrast, VPA (200, 400, and 800 mg/kg, i.p.) produced no significant elevation of DA in posterior caudate putamen dialysates, although 5-HT levels were significantly elevated at the 400- and 800-mg/kg doses. In all three regions studied, dialysate concentrations of 5-hydroxyindoleacetic acid and homovanillic acid remained at basal levels following VPA treatments. The results are discussed with regard to the possible anticonvulsant mode of action of VPA.     

SK3 K+ channel-deficient mice have enhanced dopamine and serotonin release and altered emotional behaviors

SK3 K(+) channels influence neuronal excitability and are present in 5-hydroxytryptamine (5-HT) and dopamine (DA) nuclei in the brain stem. We therefore hypothesized that SK3 channels affect 5-HT and DA neurotransmission and associated behaviors. To explore this, we used doxycycline-induced conditional SK3-deficient (T/T) mice. In microdialysis, T/T mice had elevated baseline levels of striatal extracellular DA and the metabolites dihydroxyphenylacetic acid and homovanillic acid. While baseline hippocampal extracellular 5-HT was unchanged in T/T mice, the 5-HT response to the 5-HT transporter inhibitor citalopram was enhanced. Furthermore, baseline levels of the 5-HT metabolite 5-hydroxyindoleacetic acid were elevated in T/T mice. T/T mice performed equally to wild type (WT) in most sensory and motor tests, indicating that SK3 deficiency does not lead to gross impairments. In the forced swim and tail suspension tests, the T/T mice displayed reduced immobility compared with WT, indicative of an antidepressant-like phenotype. Female T/T mice were more anxious in the zero maze. In contrast, anxiety-like behaviors in the open-field and four-plate tests were unchanged in T/T mice of both sexes. Home cage diurnal activity was also unchanged in T/T mice. However, SK3 deficiency had a complex effect on activity responses to novelty: T/T mice showed decreased, increased or unchanged activity responses to novelty, depending on sex and context. In summary, we report that SK3 deficiency leads to enhanced DA and 5-HT neurotransmission accompanied by distinct alterations in emotional behaviors.     

Alterations in Extracellular and Tissue Levels of Biogenic Amines in Rat Brain Induced by the Serotonin2 Receptor Antagonist, Ritanserin

Systemic administration of ritanserin elicited rapid changes in dopamine (DA) and serotonin (5-HT) levels in both dialysate and neuronal tissue extracts. These effects occurred in both a site-selective and a dose-related manner. Increases in extracellular levels of DA and 5-HT in the nucleus accumbens were maximal at 120-140 min after treatment. A dose of 0.63 mg/kg of ritanserin elicited larger and more prolonged increases in extracellular DA and 5-HT levels than did the 0.3 mg/kg dose. By contrast, 0.63 mg/kg of ritanserin elicited no changes in either DA or 5-HT levels with dialysate collected from the striatum. Ritanserin also induced dose-related decreases in tissue levels of DA and 5-HT from the nucleus accumbens. The site specificity of action was again noted in that there were no dose-dependent decreases in tissue levels of DA or 5-HT measured from the striatum. Ritanserin exerted little effect on metabolite levels from either dialysate or tissue extracts. Taken together, these findings show that selective 5-HT2 receptor antagonism modulates DA and 5-HT neurotransmission in a specific manner. These actions appear to involve increased release of DA and 5-HT rather than significant changes in metabolism. These findings add further weight to the importance of 5-HT2 receptor interactions as an important component of antipsychotic activity.     

Characterization of a serotonin transporter and an adenylate cyclase-linked serotonin receptor in the cestode Hymenolepis diminuta

[3H]5-HT exhibited specific binding in membrane preparations of Hymenolepis diminuta. The specific binding was saturable, reversible and temperature dependent. A non-linear Scatchard plot was obtained in a concentration range of 11 nM - 1000 nM [3H]5-HT, which could be resolved into sites having apparent dissociation constants (KD) of 0.10 microM and 6.25 microM for the high-affinity and low-affinity components, respectively. The latter could be selectively eliminated by binding [3H]5-HT to H. diminuta membranes in the presence of 10(-3) M nitroimipramine. Drug displacement studies, using 0.20 microM and 2.0 microM [3H]5-HT, revealed that while low-affinity [3H]5-HT binding was displaced by unlabelled 5-HT and inhibitors of 5-HT uptake, high affinity [3H]5-HT binding was affected only by tryptamine derivatives and, to a lesser extent, methysergide. In addition, high-affinity binding was stimulated by MgCl2 while low-affinity binding showed sodium-dependency. The data implicate the low-affinity site as a putative 5-HT transporter and the high-affinity site as a putative 5-HT 1 receptor. Exposure of H. diminuta membranes to 5-HT resulted in a 3-4 fold stimulation of cAMP levels. The EC 50 for the 5-HT-induced activation of adenylate cyclase (0.76 microM) was of the same order of magnitude as the apparent KD for high-affinity binding. Furthermore, the order of drug potency for the elevation of cAMP levels by 5-HT agonists and reversal by 5-HT antagonists was identical to the order of drug potency for the inhibition of high-affinity binding, suggesting linkage of the putative 5-HT 1 receptor to adenylate cyclase in H. diminuta.     

Hypotensive activity of serotonin antagonists; correlation with alpha 1-adrenoceptor and serotonin receptor blockade

For a series of 12 serotonin antagonists, largely varying in potency, the decrease in diastolic pressure was determined after intravenous injection into pentobarbitone-anaesthetized normotensive rats. The hypotensive activity of these antagonists was correlated with their affinity for alpha 1-adrenoceptors, established by [3H]prazosin radioligand displacement, and the 5-HT2 serotonergic receptor, determined by inhibition of specific [3H]mianserin binding. The radioligand binding assays were performed since they correspond to the in vivo antagonistic potencies of the antagonists at alpha 1--and 5-HT2-receptors, respectively. A close correlation (r = 0.963) was found between the affinity for alpha 1-adrenoceptors and hypotensive activity. On the other hand, a negative correlation of lower statistical quality (r = -0.808) existed between the affinity for 5-HT2-receptors and the depressor potency. In this series of 12 compounds, the new antihypertensive drug ketanserin is included for which it has been speculated that it lowers blood pressure by virtue of its serotonin antagonistic activity. The results of the present study, however, point towards alpha 1-adrenolytic potency as an important mechanism in the hypotensive action of the drug.     

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D and 5-HT receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.     

Intracerebrally administered (6r)-l-erythro-tetrahydrobiopterin does not affect extracellular levels of dopamine and serotonin metabolites in rat striatum in vivo during measurement by brain micro-dialysis system

By the use of the brain micro-dialysis technique combined with HPLC, the changes in the extracellular levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and a serotonin(5-HT) metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were examined in the rat striatum before and after intracerebral injection of a vehicle or (6R)-l-erythro-tetrahydrobiopterin (6R-BH₄), the natural form of the cofactor for the tryrosine hydroxylase and tryptophan hydroxylase. No apparent change after the 6R-BH, treatment was found in the levels of DA, DOPAC, HVA and 5-HIAA in the striatal dialysate. In contrast, the levels of total biopterin in both the operated (dialysis probe-implanted) and unoperated striatum of 6R-BH₄-treated rats increased by 23- and 93-fold, respectively, when compared with those of the control, vehicle-treated rats. The results indicate that increased levels of the tetrahydrobiopterin cofactor may not affect the release of DA and the extracellular level of DA and 5-HT metabolites in the physiologically normal brain.     

Selective serotonin reuptake blockade increases extracellular dopamine in noradrenaline-rich isocortical but not prefrontal areas: dependence on serotonin-1A receptors and independence from noradrenergic innervation

The present study investigated the effects of two serotonin (5-HT) uptake inhibitors, citalopram and paroxetine, and of a non-selective noradrenaline (NA) and 5-HT uptake blocker, imipramine, on extracellular NA and dopamine (DA) in the prefrontal cortex (PfCX), parietal cortex (ParCX) and occipital cortex (OccCX). Citalopram, the most selective 5-HT uptake blocker, increased dialysate DA in the OccCX and ParCX but not in the PfCX and this effect was prevented in the OccCX by WAY-100635, an antagonist of serotonin-1A (5-HT(1A)) receptors, but not by dorsal noradrenergic bundle (DNAB) lesions that reduced to unmeasurable levels basal dialysate NA but did not affect dialysate DA. Paroxetine, a less selective 5-HT uptake inhibitor than citalopram, at the dose of 5 mg/kg, increased DA in the OccCX but not in the PfCX; however, at doses of 10 mg/kg, which increase PfCX NA, paroxetine increased DA also in this area. Imipramine increased dialysate DA and NA both in the PfCX and in the OccCX and this effect was abolished by DNAB lesions and was reduced but not abolished by WAY-100635. Administration of doses of reboxetine and citalopram that do not increase DA release in the OccCX if given separately, markedly increased DA when combined. These results indicate that endogenous 5-HT, raised by selective blockade of the 5-HT carrier, can increase extracellular DA in the OccCX and in the ParCX by stimulating 5-HT(1A) receptors independently from the presence of NA terminals, although blockade of 5-HT and NA carrier can strongly interact to raise extracellular DA in this area. These observations are consistent with the existence of DA neurons separate from the NA ones contributing to extracellular DA even in NA-rich/DA poor isocortical areas.     

The cytoplasmic substrate permeation pathway of serotonin transporter

Serotonin transporter (SERT) catalyzes reuptake of the neurotransmitter serotonin (5-HT) and is a target for antidepressant drugs and psychostimulants. It is a member of a large family of neurotransmitter and amino acid transporters. A recent study using site-directed cysteine modification identified a helical region of the transporter with high accessibility to the cytoplasm. Subsequently, the high resolution structure of LeuT, a prokaryotic homologue, showed that the residues corresponding to this helical region are part of the fifth transmembrane domain. The accessibility of these positions is now shown to depend on conformational changes corresponding to interconversion of SERT between two forms that face the extracellular medium and the cytoplasm, respectively. Binding of the extracellular inhibitor cocaine decreased accessibility at these positions, whereas 5-HT, the transported substrate, increased it. The effect of 5-HT required the simultaneous presence of Na+ and Cl-, which are transported into the cell together (symported) with 5-HT. In light of the LeuT structure, these results begin to define the pathway through which 5-HT diffuses between its binding site and the cytoplasm. They also confirm a prediction of the alternating access model for transport, namely, that all symported substrates must bind together before translocation.     

The role of dopamine and serotonin in regulating bone mass and strength: studies on dopamine and serotonin transporter null mice

Neurotransmitter regulation of bone metabolism has been a subject of increasing interest and investigation. Dopamine (DA) has been reported to have effects on calcium and phosphorus metabolism. The dopamine transporter (DAT) is believed to control the temporal and spatial activity of released DA by rapid uptake of the neurotransmitter into presynaptic terminals. We have evaluated the histologic and biomechanical properties of the skeleton in mice homozygous for deletion of the DA transporter gene (DAT (-/-)) to help delineate the role of DA in bone biology. We have demonstrated that DAT (-/-) mice have reduced bone mass and strength. DAT (-/-) animals have shorter femur length and dry weight, and lower ash calcium content. Cancellous bone volume in the DAT (-/-) proximal tibial metaphysis is significantly decreased with reduced trabecular thickness. DAT (-/-) vertebrae have lower cancellous bone volume as a consequence of increased trabecular spacing and reduced trabecular number, and cortical thickness and bone area in the femoral diaphysis are reduced. The ultimate bending load (femoral strength) for the DAT (-/-) mice is 30% lower than the wild-type mice. Thus, deletion of the DAT gene results in deficiencies in skeletal structure and integrity. Since serotonin (5-HT) plays a role as a regulator of craniofacial morphogenesis, we explored the expression and function of 5-HT receptors and the 5-HT transporter (5-HTT) in bone. Primary cultures of rat osteoblasts (rOB) and a variety of clonal osteoblastic cell lines including ROS 17/2.8, UMR 106-H5 and Py1a show mRNA expression for the 5-HTT, and the 5-HT(1A), 5-HT(1D), 5-HT(2A) and 5-HT(2B) receptors by RT-PCR analysis and immunoblot. A relatively high density of nanomolar affinity 5-HTT binding sites is present in ROS 17/2.8 and UMR 106-H5 cells. The maximal [(3)H]5-HT uptake rate in ROS cells was 110 pmol/10 min/well, with a K(m) value of 1.13 microM. In normal differentiating rOB cultures, 5-HTT functional activity was observed initially at day 25, and activity increased by almost eight-fold at day 31. In mature rOB cultures, the estimated density of [(125)I]RTI-55 binding sites was 600 fmol/mg protein. PMA treatment caused a significant 40% reduction in the maximal uptake rate of [(3)H]5-HT, an effect prevented by pretreatment with staurosporine. 5-HT potentiates the PTH-induced increase in AP-1 activity in UMR 106-H5 cells. In 5-HTT (-/-) animals, cancellous bone volume (BV/TV) in the lumbar vertebrae is reduced, with a trend toward decreased trabecular thickness and trabecular number. These results demonstrate that osteoblastic cells express a functional serotonin system, with mechanisms for responding to and regulating uptake of 5-HT, and disruption of the 5-HTT gene may cause osteopenia.     

五羟色胺和多巴胺与攻击行为的关联研究进展

五羟色胺（5-HT）和多巴胺（DA）是影响攻击行为的重要神经递质。参与这两种神经递质合成和分解、运输及信号转导等过程的物质均可能影响攻击行为,如影响5-HT作用的色氨酸、色氨酸羟化酶、单胺氧化酶、5-羟吲哚乙酸及5-HT转运体和5-HT受体;影响DA作用的多巴胺β羟化酶和儿茶酚胺邻位甲基转移酶以及DA转运体。未来攻击行为研究,应考虑色氨酸自身代谢、受体亚型及其他单胺类和儿茶酚胺类神经递质的影响。将肠道微生物纳入攻击行为研究也是未来研究的新方向。     

Biochemical and behavioral effects of long-term citalopram administration and discontinuation in rats: role of serotonin synthesis

We have investigated effects of continuous SSRI administration and abrupt discontinuation on biochemical and behavioral indices of rat brain serotonin function, and attempted to identify underlying mechanisms. Biochemistry of serotonin was assessed with brain tissue assays and microdialysis; behavior was assessed as the acoustic startle reflex. Long-term SSRI administration to rats reduced the content of 5-HT and its main metabolite shortly after inhibition of 5-HT synthesis in many brain areas with more than 50%. Turnover was not appreciably decreased, but significantly increased within 48h of drug discontinuation. The microdialysis experiments indicate that neuronal release of 5-HT depends strongly on new synthesis and emphasize the role of 5-HT(1B) receptors in the regulation of these processes. Discontinuation of the SSRI rapidly increased behavioral reactivity to the external stimulus. Additional startle experiments suggest that the increased reactivity is more likely related to the reduced extracellular 5-HT levels than to impaired synthesis. The combination of the marked reduction of serotonin content and limited synthesis may destabilize brain serotonin transmission during long-term SSRI treatment. These combined effects may compromise the efficacy of an SSRI therapy and facilitate behavioral changes following non-compliance.     

Nongenomic steroidal modulation of high-affinity serotonin transport

The ability of steroids to modulate high-affinity 5-HT transport was investigated using cell-based models which stably manifest all known properties of this transport system. beta-Estradiol (E2) exhibited noncompetitive, and possibly allosteric, inhibition of both radiolabeled serotonin ([3H]5-HT) transport by, and radiolabeled cocaine congener ([3H]CFT) binding to, this system. Such inhibitory effects were observed within short time courses and unlikely to result from genomic effects normally ascribed to estrogen action. Rather, such nongenomic effects on 5-HT uptake were more akin to modulatory effects of select steroid metabolites on other plasma membrane systems such as neurotransmitter receptors and ionic channels. Beyond E2, preliminary examination of other steroid metabolites and synthetic steroid receptor agonists/antagonists revealed that inhibition of 5-HT transport is additionally attributable only to estriol (E3, an E2 metabolite) and tamoxifen (a nonsteroidal, E2 receptor antagonist). These findings indicate that the present form of transport modulation is only rendered by select compounds and not a general property of steroidal and related agents. Assessments of covalent conjugates of E2 suggested that E2 interacts with the transporter protein at allosteric site(s) inaccessible from the extracellular domain. These findings collectively suggest that steroid-mediated regulation of 5-HT transport may be a physiologically relevant mechanism, and that antidepressant as well as psychostimulant effects in vivo may contain a steroidal component.     

Effect of Global Ischaemia, Under Simulated Penumbral Conditions, on Brain Monoamine Neurochemistry and Subsequent Neurological and Histological Deficits

Abstract— We have measured changes in the levels of do-pamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites in striatal dialysates during 30 min of global ischaemia under simulated penumbral conditions, and compared these with neurological assessments over the following 7 days and histological damage at the end of this period. On the basis of dialysate DA levels during ischaemia, the animals fell into two subgroups; group I, with little or no DA increase (less than three times basal); and group II, with a much larger increase (greater than 30 times basal). Changes in 5-HT, though of lesser magnitude, showed a similar pattern. These findings may indicate that the amine changes depend on a critical reduction of blood flow within the range obtained by our experimental procedure. Levels of deaminated metabolites fell in all ischaemic animals, with comparable decreases of 3, 4-dihydroxyphenylacetic acid plus homovanillic acid in both groups. Decreases of 5-hydroxyindoleacetic acid were greater in group II than in group I, but the relative differences between the groups were much less marked than those of 5-HT. These neuro-chemical findings suggest that moderate ischaemia affects extracellular amine and deaminated metabolite levels by different mechanisms. Only one of the ischaemic rats (a member of group II) showed a marked neurological deficit, but histological damage, as indicated by neuronal loss and gliosis in vulnerable structures, was apparent in all ischaemic animals. Although damage tended to be greater in animals with marked increases in extracellular monoamines, differences were not significant. These findings suggest that the large increases of extracellular DA and 5-HT that sometimes occur in ischaemia may play a relatively small part in the genesis of neuronal damage, though these transmitters may well have a permissive role.     

Ibogaine reduces amphetamine-induced locomotor stimulation in C57BL/6By mice, but stimulates locomotor activity in rats.

The effect of ibogaine hydrochloride on locomotor stimulation induced by d-amphetamine sulfate was tested in male C57BL/6By mice and in female Sprague-Dawley rats. In mice, locomotor stimulation induced by d-amphetamine at 1 or 5 mg/kg s.c. was reduced by prior administration of one or two injections of ibogaine (40 mg/kg), given 2 or 18 hours earlier. This reduction in locomotor activity persisted for two days. Locomotor stimulation induced by a higher dose (10 mg/kg) of d-amphetamine was not reduced by such prior administration of ibogaine. A lower dose of ibogaine (20 mg/kg) did not reduce the subsequent locomotor activity induced by d-amphetamine. Ibogaine decreased striatal dopamine levels, while d-amphetamine increased them. Ibogaine treatment (2 x 40 mg/kg, 18 hours apart) induced a decrease by 30% in the level of striatal dopamine and its metabolites measured in tissue extracts 3 hours after the second ibogaine injection. One hour after d-amphetamine (5 mg/kg) administration, the level of striatal dopamine increased by 26%. Although the level of striatal dopamine was initially lower in the ibogaine-pretreated mice, d-amphetamine (5 mg/kg) administration induced an increase in striatal dopamine and its metabolites. The effect of ibogaine seems to be species specific, since in rats pretreated with ibogaine 18 hours before d-amphetamine, locomotor stimulation induced by d-amphetamine was further increased. In addition, the in vitro electrical-evoked release of [3H]dopamine from striatal tissue was either unchanged or inhibited in the presence of d-amphetamine, and after ibogaine pretreatment in vivo, the release of tritium in the presence of d-amphetamine was inhibited or stimulated in mice and rats, respectively.     

D-fenfluramine,but not d-norfenfluramine,uses calcium to increase extracellular serotonin

Microdialysis in the hippocampus of freely moving rats was used to assess extracellular serotonin (5-HT) in response to local infusion of d-fenfluramine and its metabolite d-norfenfluramine with and without local calcium depletion. Verapamil (1 mM) in calcium-free Ringer infused via the microdialysis probe increased extracellular 5-HT and prevented the full increase in extracellular 5-HT normally caused by 1 mM d-fenfluramine. The results suggest d-fenfluramine might act in part as a calcium channel agonist favoring a calcium influx that in turn would trigger the exocytotic process in 5-HT terminals, d-norfenfluramine, on the other hand, was capable of releasing 5-HT, in vivo, in spite of depleted Ca levels.