Serotonergic Neurotoxin Research Articles

A serotonin neurotoxin attenuates the phase-shifting effects of triazolam on the circadian clock in hamsters

Several lines of evidence suggest the potential involvement of serotonergic pathways in mediating the effects of activity-inducing stimuli on the circadian clock in rodents. The aim of the present 3 experiments was to examine the effects of the serotonergic neurotoxin, p-hloroamphetamine (PCA, 10 mg/kg) on: (1) the monoamine levels of the hypothalamus, frontal cortex and hippocampus in the hamster; (2) the phase shifts in the circadian rhythm of locomotor activity of hamsters in response to treatment with the short-acting benzodiazepine, triazolam (7.5 mg/kg); and (3) the magnitude of the acute increase in locomotor activity associated with triazolam administration in this species. The administration of PCA to hamsters caused changes of specific monoaminergic systems in the hypothalamus, that were limited to a selective decrease in serotonin levels 7 days post-treatment. The phase shifts of the circadian clock in response to triazolam treatment at CT 6 were considerably attenuated following the administration of the 5-HT neurotoxin. The total amount and the profiles of triazolam-induced wheel-running and general cage activity between CT 6 and CT 12 were not significantly affected by the PCA treatment. The finding that a 5-HT neurotoxin can attenuate the phase-shifting effects of triazolam in hamsters, without interfering with its activity-inducing properties, suggests that serotonergic afferents might be involved in the mechanism for non-photic phase-shifting of the circadian system.

Lesions of central serotonin systems affect responding on a progressive ratio schedule reinforced either by intravenous cocaine or by food

The effect of intraventricular infusions of the serotonergic neurotoxin, 5,7-dihydroxytrytamine (5,7-DHT), was examined in rats trained on a progressive ratio schedule for either IV cocaine or food reinforcement. Animals in the 5,7-DHT treatment group responded to significantly higher breaking points than vehicle-injected control animals, regardless of whether food or cocaine was used as the reinforcing stimulus. Analysis of the regional brain amines indicated that depletions of mesencephalic 5-HT correlated with postsurgical alterations in responding. These findings suggest that depletion of forebrain 5-HT produces a general effect on responding rather than a specific alteration in the reinforcing effects of psychomotor stimulant drugs.

Possible involvement of the septo-hippocampal cholinergic and raphe-hippocampal serotonergic activations in the penile erection induced by fenfluramine in rats

Fenfluramine (0.1–10 mg/kg, i.p.) induced penile erection in naive rats with a bell-shaped dose response curve. The response to fenfluramine (1 mg/kg) was antagonized by scopolamine (0.032–1 mg/kg) but not by methyl-scopolamine (0.032–1 mg/kg) pre-treatment. The septo-hippocampal cholinergic deafferentations by medial septum lesion or fimbria-fornix transection also attenuated the penile erection induced by fenfluramine, whereas dopaminergic blockade by sulpiride (3.2–100 mg/kg) had hardly any effect. Pindolol (0.1–3.2 mg/kg), a 5-HT 1 antagonist, and ICS205-930, a 5-HT 3 antagonist, but not ketanserin, a 5-HT 2 antagonist, inhibited the penile erection induced by fenfluramine. Depletion of 5-HT not only by systemic injections ofp-chlorophenylalanine (150 mg/kg i.p. at 72, 48 and 24 h before the test) but also by the injections of 5,7-dihydroxytryptamine (5,7-DHT), a serotonergic neurotoxin, into the median- and dorsal-raphe nuclei significantly attenuated the fenfluramine-induced penile erections. Neurochemical analyses revealed that the raphe-lesion significantly reduced the contents of serotonin (5-HT) and its major metabolite, 5-hydroxyindole acetic acid (5-HIAA) but not choline acetyltransferase (ChAT) activities in all the cortical and subcortical regions examined. The results suggest that fenfluramine facilitates the expression of penile erection in rats through an activation of the septo-hippocampal cholinergic pathway as a consequence of excitation of the raphe-hippocampal serotonergic pathway in which 5-HT 1 and/or 5-HT 3 receptors appear to play a regulatory role.

Role of the dopaminergic, serotonergic and cholinergic link in the expression of penile erection in rats.

The neural mechanisms underlying the penile erection induced by serotonergic, cholinergic and dopaminergic stimulants were comparatively investigated. Fenfluramine (0.1-10 mg/kg, i.p.), pilocarpine (0.032-3.2 mg/kg) and apomorphine (0.01-1 mg/kg) induced penile erection in rats with bell-shaped dose-response curves. The penile erection induced by fenfluramine (1 mg/kg) was dose-dependently antagonized by pindolol (0.1-3.2 mg/kg), a 5-HT1 antagonist, or scopolamine (0.032-1 mg/kg), a muscarinic antagonist, but not by sulpiride (1-32 mg/kg), a dopaminergic antagonist. The penile erection induced by pilocarpine (0.32 mg/kg) was countered by pindolol or scopolamine but not by sulpiride, while that induced by apomorphine (0.032 mg/kg) was countered by all three antagonists. Septo-hippocampal cholinergic deafferentations by medial septum lesioning or fimbria-fornix transection also significantly attenuated the penile erection induced by fenfluramine or apomorphine, but scarcely affected that induced by pilocarpine. Raphe lesion by injections of 5,7-dihydroxytryptamine, a serotonergic neurotoxin, into the median- and dorsal-raphe nuclei significantly attenuated the penile erections induced by fenfluramine and apomorphine but not that by pilocarpine. These results suggest that a neuronal link between the dopaminergic, serotonergic and cholinergic systems plays a crucial role in the expression of penile erection; dopaminergic stimulation causes an activation of the raphe serotonergic neurons which in turn enhances the septo-hippocampal cholinergic pathway and results in expression of penile erection.

Further insights into the molecular mechanisms of action of the serotonergic neurotoxin 5,7-dihydroxytryptamine.

Autoxidation and various enzyme-mediated oxidations of the serotonergic neurotoxin 5,7-dihydroxytryptamine (1) give 5-hydroxytryptamine-4,7-dione (2) and 6,6'-bi(5-hydroxytryptamine-4,7-dione) (3) as the major products. When administered into the ventricular system of mice 2 and 3 are general toxins. The LD50 values for 2 (29.6 +/- 0.04 micrograms) and 3 (25.4 +/- 0.30 micrograms) are lower than that for 1 (51.8 +/- 0.28 micrograms). In the presence of cellular reductants (glutathione, cysteine, ascorbate) and molecular oxygen, or when incubated with rat brain homogenate, 2 and 3 redox cycle and form superoxide radical anion, O2.-, as a byproduct. The lethal effects of 2 and 3 when introduced into the brain may in part be due to such redox cycling reactions which deplete oxygen levels and, as a result of Haber-Weiss chemistry deriving from O2.-, form the cytotoxic hydroxyl radical (HO.). Intraventricular administration of 2 and 3 to mice causes only relatively minor and transient (ca. 1 h to 1 day) changes in whole brain levels of dopamine, 5-hydroxytryptamine (from both 2 and 3), acetylcholine, and choline (from 2 only). These changes differ from the profound and long-lasting serotonergic deficit evoked by 1. On the basis of these results a hypothesis has been formulated which proposes that the selective neurotoxicity of 1 derives from its rapid uptake into serotonergic neurons where it is oxidatively converted to 2 and 3. Redox cycling reactions of 2 and 3 then result in the depletion of intraneuronal oxygen and concomitant formation of O2.-. Dismutation of O2.- gives H2O2 which, as a result of transition metal ion-catalyzed Haber-Weiss chemistry, yields HO.. Thus, neuronal damage and death might result from the combined effects of hypoxia and HO. formation.

Effect of serotonin axon injury on the somatostatinergic system in rat frontoparietal cortex

In order to investigate the possibility that, in the rat, some cerebral cortex somatostatin (SS) receptors may be localized presynaptically on the terminals of serotonergic neurons, serotonin [5-hydroxytryptamine, (5-HT)] neurons in the central nervous system were damaged with a local intracerebral injection of the serotonergic neurotoxin, 5, 7-dihydroxytryptamine (5, 7-DHT). The injection of 5, 7-DHT (11 μg free base dissolved in 10 μl of isotonic saline containing 0.01% ascorbic acid) in rats produced an reduction by about 74% in frontoparietal cortical 5-HT content at 1 and 3 weeks after injection. These changes were associated with a significant decrease by about 30% in the total number of specific SS receptors in the frontoparietal cortex at both times studied without influencing the apparent affinity of the receptors. Together, these results suggest that a portion of the frontoparietal cortex SS receptors may be localized presynaptically on the serotonergic nerve terminals. The 5, 7-DHT did not affect SS-like immunoreactivity (SSLI) levels suggesting that SS and 5-HT are not colocalized within the same neuronal elements in the rat frontoparietal cortex.

Effects of paroxetine on 5-HT-induced head twitches and phosphatidylinositol hydrolysis in mice.

Head-twitch response and phosphatidylinositol (PIP) hydrolysis in cortex and spinal cord were measured after single or chronic (21 days) administration of paroxetine to normal mice or to mice neonatally treated with 5,7-dihydroxytryptamine, a serotonergic neurotoxin. In normal animals, a down-regulation of 5-HT-receptor numbers after chronic paroxetine was suggested by the attenuation of head-twitch responses compared with a single dose. There was a concomitant decrease in PIP hydrolysis. In DHT-treated animals, although changes in behavioural responses were comparable to those in normals, PIP hydrolysis in cortex and spinal cord after chronic paroxetine increased significantly. These results demonstrate that head-twitch responses and PIP hydrolysis may not be mediated by the same receptor and that the effects of chronic administration of paroxetine depend on the functional state of the serotonergic pathways.

Distribution of Calcitonin Gene‐Related Peptide in Rat and Rabbit Spinal Cords: Effect of Intrathecal 5,7‐Dihydroxytryptamine

Calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) was measured in selected regions of the cervical, thoracic, and lumbar spinal cord of untreated rabbits and, following intrathecal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), in the thoracolumbar cord in rats using a sheep antiserum raised against tyrosine0 calcitonin gene-related peptide28-37. In the cervical, thoracic, and lumbar segments of the rabbit spinal cord, CGRP-LI levels were 15-50-fold higher in the dorsal than in the ventral grey region in the same segment. The only segmental variation in CGRP-LI levels was in the dorsal white region, where levels in the thoracic cord were lower than those in cervical or lumbar segments. Within individual spinal segments, the pattern of distribution of CGRP-LI in the rabbit spinal cord was analogous to that in other species previously examined, including rat, human, and cat spinal cord. Intrathecal injection of 5,7-DHT, which caused 85-91% depletion of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid from the thoracolumbar ventral spinal cord, did not affect choline acetyltransferase activity, which is colocalized with CGRP in motoneurones in this spinal cord region. In contrast, intrathecal 5,7-DHT produced a threefold increase in CGRP-LI in the ventral thoracolumbar cord, suggesting that spinal motoneurones selectively increase production of CGRP 10 days after neurotoxin-induced denervation of bulbospinal raphe neuronal input.

Chemical and enzyme-mediated oxidation of the serotonergic neurotoxin 5,7-dihydroxytryptamine: mechanistic insights.

The oxidation chemistry and biochemistry of the serotonergic neurotoxin 5,7-dihydroxytryptamine (1) has been studied under anaerobic and aerobic conditions in aqueous solution at physiological pH. Under anaerobic conditions, one-electron oxidants (ferricytochrome c, peroxidase/H2O2, ceruloplasmin, Cu2+) generate a radical intermediate. Dimerization of the C(6)-centered resonance form of this radical followed by secondary oxidations yields 3-(2-aminoethyl)-6-[3-(2-aminoethyl)-1,7-dihydro- 5-hydroxy-7-oxo-6H-indol-6-ylidene]-1-H-indole-5,7(4H,6H)-dione. Under aerobic conditions, molecular O2 attacks the C(4)-centered 1 radical to yield a hydroperoxy radical which decomposes to 5-hydroxytryptamine-4,7-dione (2). Autoxidation of 1 proceeds by primary attack by molecular O2 on a C(4)-centered carbanion to form a superoxide-radical complex. This rearranges to a C(4)-centered hydroperoxide which decomposes to 2. A C(6)-centered carbanion of 1 combines with 2 to give, ultimately, 6,6'-bi-5-hydroxytryptamine-4,7-dione (3). Trace concentrations of transition metal ions (Fe3+, Fe2+, Cu2+, Mn2+) catalyze the autoxidation of 1 by catalytic cycles in which a hydroperoxide intermediate plays key roles. A byproduct of the transition metal-catalyzed oxidation of 1 is superoxide, O2-. Because of its enormous basicity O2- facilitates deprotonation of 1. The C(4)-centered carbanion so produced is oxidized by molecular O2 or by the hydroperoxy radical (HO2) to give radical intermediates and thence 2 and 3. Mechanistic pathways leading to the various products of oxidation of 1 are proposed and the potential roles of oxidation reactions of the indolamine are related to its neurodegenerative properties.

Synthesis and neurotoxicological evaluation of putative metabolites of the serotonergic neurotoxin 2-(methylamino)-1-[3,4-(methylenedioxy)phenyl]propane [(methylenedioxy)methamphetamine

Theoretical considerations and recent experimental data have prompted an investigation of the neurotoxicological properties of the 6-hydroxydopamine analogue 2-(methylamino)-1-(2,4,5-trihydroxyphenyl)propane (5) and its possible precursor 1-[2-hydroxy-4,5-(methylenedioxy)phenyl]-2- (methylamino)propane (4), potential metabolites of the serotonergic neurotoxin MDMA. Systemic, intracerebroventricular, and intraparenchymal (intrastriatal and intracortical) administration of 4 led to no detectable alterations of hippocampal or cortical serotonin or striatal dopamine levels in the rat under conditions that caused significant biogenic amine depletions by established neurotoxins. By contrast, intraparenchymal administration of 5 caused profound depletions of dopamine and serotonin, with the former being more severely depleted than the latter. Although not conclusive, these data suggest a possible role for 5 in the mediation of MDMA's neurotoxic actions.

Peroxidase-promoted oxidation and peroxidation of the serotonergic neurotoxin 5,7-dihydroxytryptamine. A new pathway for its metabolic degradation.

Spectral data provide the first evidence that lactoperoxidase, a model enzyme for most mammalian peroxidases, catalyzed the one-electron oxidation and/or peroxidation of 5,7-dihydroxytryptamine. This process correlates with the production of superoxide radicals as is evident from the observed inhibitory effect of superoxide dismutase on product formation. 5,7-Dihydroxytryptamine is a classical peroxidase-oxidase substrate acting as a one-electron donor for enzyme compounds I, II and III. The one-electron peroxidatic oxidation of this serotonergic neurotoxin, responsible for the selective degeneration of central (5-hydroxytryptamine) neurons, is a fast process requiring measurement on the ms time scale. Attention is drawn to the biochemical and toxicological implications, because this fast reaction results in formation of known cell damaging species: free radicals, superoxide radicals and quinoidal products probably involved in the toxic action of 5,7-dihydroxytryptamine.

Interactions between 5,6-dihydroxytryptamine and cysteine

Autoxidation and enzyme-mediated oxidations of the serotonergic neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) generate the corresponding ortho-quinone ( 1) as the initial reactive intermediate. Autoxidation of 5,6-DHT in the presence of an excess of the cellular reductant/nucleophile cysteine results in a reaction where, initially, quinone 1 oxidizes cysteine to cystine with concomitant formation of 5,6-DHT. When the concentration of cysteine is sufficiently reduced 1 attacks, 5,6-DHT to give 2,7′-bi(5,6-dihydroxytryptamine) and, ultimately, an insoluble indolic melanin. When the oxidation of 5,6-DHT is catalyzed by tyrosinase, o-quinone 1 is attacked by cysteine to give 4- S-cysteinyl-5,6-dihydroxytryptamine ( A). Methods have been devised to isolate A and evidence for its structure is presented. Conjugate A should be useful as a marker of enzyme-catalyzed oxidation of 5,6-DHT in the central nervous system.

Muscimol injections into the median raphe nucleus increase serum ACTH and corticosterone concentrations via a nonserotonergic mechanism

Midbrain raphe serotonin (5-HT) neurons can influence the pituitary-adrenal axis. The midbrain raphe nuclei also contain a number of non-5-HT neurons, including gamma-aminobutyric acid (GABA) interneurons which can modulate 5-HT neuronal activity. We investigated the effects of intraraphe injections of the GAB A agonist, muscimol, on serum adrenocorticotropin hormone (ACTH) and corticosterone concentrations. Rats were infused with muscimol (0, 25, 50, and 100 ng in 0.5 μl saline) into the median raphe nucleus (MR). The animals were killed 30 min later, and trunk blood was collected for measurement of serum concentrations of ACTH and corticosterone by radioimmunoassay. Muscimol dose dependently increased plasma concentrations of these two pituitary-adrenal hormones. In order to determine the role of MR 5-HT neurons in these effects, separate groups of implanted animals were infused with either the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) or ascorbic acid vehicle into the MR. Two weeks later, the animals were infused with muscimol (100 ng in 0.5 μl) and sacrificed as above. Treatment with 5,7-DHT, which markedly reduced hippocampal concentrations of 5-HT (−83%) and 5-HIAA (−73%), did not block intra-MR muscimol-induced elevations in ACTH and corticosterone. Thus, 5-HT neurons within the MR apparently do not mediate the increased activity of the pituitary-adrenal axis produced by stimulation of MR GABA A receptors.

Role of presynaptic serotonergic receptors on the mechanism of action of 5-HT1A and 5-HT1B agonists on masculine sexual behaviour: physiological and pharmacological implications

In order to establish whether the 5-HT1A or the 5HT1B agonists, 8-OH-DPAT or TFMPP, produce their facilitatory or inhibitory actions on masculine sexual behaviour via a mechanism involving: (a) the serotonin synthesis or release; (b) the stimulation of presynaptic receptors, or (c) the stimulation of somatodendritic receptors, three series of experiments were performed. The administration of the serotonin synthesis inhibitor, p-chlorophenylalanine (p-CPA, 300 mg/kg x 3 days), facilitated sexual behaviour but does not interfere neither with the inhibitory nor with the facilitatory effects of TFMPP (0.5 mg/kg) or 8-OH-DPAT (0.5 mg/kg), respectively. The icv or the intraraphé administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), slightly stimulated masculine sexual behaviour and produced a decrease in serotonin and its metabolite levels. In lesioned animals TFMPP (0.5 mg/kg) resulted in an inhibitory effect reflected as a prolongation of the ejaculation latency. The inhibitory effect of this drug on mounting behaviour was not observed in 5,7-DHT treated rats. In lesioned animals 8-OH-DPAT (0.5 mg/kg) produced the same facilitatory effect. Present data indicate that serotonergic postsynaptic receptors mediate both the inhibitory and the facilitatory actions of TFMPP or 8-OH-DPAT in copulation. All data further support the idea that endogenous serotonin acts via the stimulation of 5-HT1B receptors to induce its inhibitory effects on masculine sexual behaviour.

Differential role of serotonin and noradrenaline on anxiety reduction after ejaculation in the rat

As previously reported, a reduction in anxiety after ejaculation was observed. In a previous report it was demonstrated that the GABA-benzodiazepine system is involved in the mediation of this reduction in anxiety. The anxiety levels were measured using a defensive burying model. This work was performed to elucidate the serotonin and noradrenaline participation in the mediation of this phenomenon. Two experiments were made. In the first experiment the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 10 μg/10 μl) was intracerebroventricularly injected. Five days after its administration the behavioral tests were performed. In the second experiment, the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4, 50 mg/kg × 2) was IP administered. The neurochemical data reveal a drastic reduction in various brain areas respective monoamine levels after these treatments. The lesion produced by 5,7-DHT was able to reverse the reduction in anxiety in copulating males, but produced no changes in noncopulating animals. This finding supports the idea that the serotonergic system in involved in the reduction of anxiety observed after ejaculation. The results of the DSP4 experiment suggest that there is not a direct participation of the noradrenergic system in the anxiety reduction observed after ejaculation.

Reactions of the serotonergic neurotoxin 5, 6-dihydroxytryptamine with glutathione

Reactions of the serotonergic neurotoxin 5, 6-dihydroxytryptamine with glutathione

The effect of selective serotonergic neurotoxin treatment on tachykinin levels in the rat ventral spinal cord

The levels of 5-hydroxytryptamine and tachykinin neuropeptides substance P, neurokinin A, neurokinin B and neuropeptide K were measured in the spinal cord of rats treated by intraventricular injection of the selective serotonergic neurotoxin 5,7-dihydroxytryptamine. The spinal cord levels of 5-hydroxytryptamine as measured by high performance liquid chromatography with electrochemical detection decreased by more than 90% in the ventral and dorsal cord compared to controls. The levels of substance P as measured by radioimmunoassay were significantly reduced (66%, P < 0.01) in the ventral lumbar cord only. In this region, neurokinin A, neurokinin B and neuropeptide K levels were determined by combined high performance liquid chromatography and radioimmunoassay. The neurotoxin treatment also caused a significant reduction of neurokinin A (72% reduction, P < 0.01) and a non-significant reduction of neuropeptide K, but virtually no change in the neurokinin B level. Immunohistochemical studies of the ventral lumbar cord of sham-operated animals showed immunoreactivity for 5-hydroxytryptamine as well as for substance P and neurokinin A in nerve fibres around motor neurons. In neurotoxin-treated rats this region was devoid of immunohistochemically detectable substance P- and neurokinin A-positive fibres and showed very sparse or no 5-hydroxytryptamine immunoreactivity. We conclude that among the tachykinins both neurokinin A and substance P, but probably not neurokinin B, co-exist with 5-hydroxytryptamine in nerve terminals in the rat ventral spinal cord.

Evidence for a possible role for serotonergic systems in the control of cerebral blood flow

Cerebral blood flow and glucose utilisation were measured in rats using [ 14C]iodoantipyrine and [ 14C]2-deoxyglucose quantitative autoradiography respectively following repeated exposure to the serotonergic neurotoxin methylenedioxyamphetamine. In both control and treated animals blood flow and glucose use were similarly correlated but the ratio was increased following lesion. Some focal increases in flow were greatly in excess of metabolic demand. These results are consistent with a cerebrovascular vasoconstrictor role for serotonin.

Further insights into the oxidation chemistry and biochemistry of the serotonergic neurotoxin 5,6-dihydroxytryptamine

The neurodegenerative properties of the serotonergic neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) are widely believed to result from its autoxidation in the central nervous system. The autoxidation chemistry of 5,6-DHT has been studied in aqueous solution at pH 7.2. The reaction is initiated by direct oxidation of the indolamine by molecular oxygen with resultant formation of the corresponding o-quinone 1 and H2O2. A rapid nucleophilic attack by 5,6-DHT on 1 leads to 2,7'-bis(5,6-dihydroxytryptamine) (6) which is more rapidly autoxidized than 5,6-DHT to give the corresponding diquinone 7 along with 2 mol of H2O2. The accumulation of 6 in the reaction solution during the autoxidation of 5,6-DHT despite its more rapid autoxidation indicates that diquinone 7 chemically oxidizes 5,6-DHT (2 mol) to quinone 1 so that an autocatalytic cycle is established. The H2O2 formed as a byproduct of these autoxidation reactions can undergo Fenton chemistry catalyzed by trace transition metal ion contaminants with resultant formation of the hydroxyl radical, HO., which directly oxidizes 5,6-DHT to a radical intermediate (9a/9b). This radical is directly attacked by O2 to yield quinone 1 and superoxide radical anion, O2.-, which further facilitates Fenton chemistry by reducing, inter alia, Fe3+ to Fe2+. A minor side reaction of 1 with water leads to formation of at least two trihydroxytryptamines. Diquinone 7 ultimately reacts with 6, 5,6-DHT, and perhaps trihydroxytryptamines, leading via a sequence of coupling and oxidation reactions to a black indolic melanin polymer. Enzymes such as tyrosinase, ceruloplasmin, and peroxidase and rat brain mitochondria catalyze the oxidation of 5,6-DHT to form dimer 7 and, ultimately, indolic melanin. The role of the autoxidation and the enzyme-mediated and mitochondria-promoted oxidations of 5,6-DHT in expressing the neurodegenerative properties of the indolamine are discussed.

Paroxetine as an in vivo indicator of 3,4-methylenedioxymethamphetamine neurotoxicity: a presynaptic serotonergic positron emission tomography ligand?

The present study sought to determine whether [ 3H]paroxetine, a potent and selective inhibitor of serotonin uptake in vitro, could be used to label the serotonin transporter in the rat brain in vivo such that it might be employed to develop a presynaptic serotonergic positron emission tomography ligand. Tritium labeled paroxetine was administered intravenously to rats by means of tail vein injection. Four hours later, specific [ 3H]paroxetine binding was determined by subtracting non-specific binding in the cerebellum from total binding in other brain regions of interest. The distribution of specific [ 3H]paroxetine binding paralleled the distribution of serotonin uptake sites in all brain regions examined. Pretreatment with serotonin re-uptake inhibitors (citalopram or sertraline) reduced in vivo specific [ 3H]paroxetine binding by as much as 99%. Specific in vivo [ 3H]paroxetine binding was further characterized through the use of 5,7-dihydroxytryptamine (5,7-DHT), a known serotonergic neurotoxin. 5,7-DHT (200 μg, i.c.v.) caused a marked reduction in specific [ 3H]paroxetine binding, and induced a prolonged depletion of regional brain serotonin. In a final study, the feasibility of using in vivo [ 3H]paroxetine binding as an indicator of serotonergic damage induced by another neurotoxin (3,4-methylenedioxymethamphetamine, MDMA) was tested. MDMA-treated rats showed a profound reduction in in vivo [ 3H]paroxetine binding, along with a lasting depletion of regional brain serotonin. These results demonstrate that [ 3H]paroxetine can be used to label serotonin uptake sites in rat rat brain in vivo, and that the damage induced by serotonergic neurotoxins can be detected using in vivo [ 3H]paroxetine binding as an indicator. Paroxetine (or one of its derivatives) therefore holds promise as a PET ligand for studying serotonergic neurons in the living human brain in health as well as after neurotoxic injury.