In vitro reactivation of rat cortical tryptophan hydroxylase following in vivo inactivation by methylenedioxymethamphetamine

Activation of Antioxidant and Proteolytic Pathways in the Nigrostriatal Dopaminergic System After 3,4-Methylenedioxymethamphetamine Administration: Sex-Related Differences

3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is an amphetamine-related drug that may damage the dopaminergic nigrostriatal system. To investigate the mechanisms that sustain this toxic effect and ascertain their sex-dependence, we evaluated in the nigrostriatal system of MDMA-treated (4 × 20 mg/kg, 2 h apart) male and female mice the activity of superoxide dismutase (SOD), the gene expression of SOD type 1 and 2, together with SOD1/2 co-localization with tyrosine hydroxylase (TH)-positive neurons. In the same mice and brain areas, activity of glutathione peroxidase (GPx) and of β2/β5 subunits of the ubiquitin-proteasome system (UPS) were also evaluated. After MDMA, SOD1 increased in striatal TH-positive terminals, but not nigral neurons, of males and females, while SOD2 increased in striatal TH-positive terminals and nigral neurons of males only. Moreover, after MDMA, SOD1 gene expression increased in the midbrain of males and females, whereas SOD2 increased only in males. Finally, MDMA increased the SOD activity in the midbrain of females, without affecting GPx activity, decreased the β2/β5 activities in the striatum of males and the β2 activity in the midbrain of females. These results suggest that the mechanisms of MDMA-induced neurotoxic effects are sex-dependent and dopaminergic neurons of males could be more sensitive to SOD2- and UPS-mediated toxic effects.

In utero exposure to dexamethasone causes a persistent and age-dependent exacerbation of the neurotoxic effects and glia activation induced by MDMA in dopaminergic brain regions of C57BL/6J mice

Clinical and preclinical evidence indicates that prenatal exposure to glucocorticoids may induce detrimental effects in the offspring, including reduction in fetal growth and alterations in the CNS. On this basis, the present study investigated whether in utero exposure to high levels of glucocorticoids is a risk factor that may lead to an exacerbation of the central noxious effects induced by psychoactive drugs consumed later in life. To this end, pregnant C57BL6/J dams were treated with dexamethasone (DEX, 0.05 mg/kg per day) from gestational day 14 until delivery. Thereafter, the male offspring were evaluated to ascertain the magnitude of dopaminergic damage, astrogliosis and microgliosis elicited in the nigrostriatal tract by the amphetamine-related drug 3,4--methylenedioxymethamphetamine (MDMA, 4 × 20 mg/kg, 2 h apart, sacrificed 48 h later) administered at either adolescence or adulthood. Immunohistochemistry was performed in the substantia nigra pars compacta (SNc) and striatum, to evaluate dopaminergic degeneration by measuring tyrosine hydroxylase (TH), as well as astrogliosis and microgliosis by measuring glial fibrillary acidic protein (GFAP) and ionized calcium-binding adapter molecule 1 (IBA-1), respectively. Moreover, immunohistochemistry was used to ascertain the co-localization of IBA-1 with either the pro-inflammatory interleukin (IL) IL-1β or the anti-inflammatory IL IL-10, in order to determine the microglial phenotype. In utero administration of DEX induced dopaminergic damage by decreasing the density of TH-positive fibers in the striatum, although only in adult mice. MDMA administration induced dopaminergic damage and glia activation in the nigrostriatal tract of adolescent and adult mice. Mice exposed to DEX in utero and treated with MDMA later in life showed a more pronounced loss of dopaminergic neurons (adolescent mice) and astrogliosis (adolescent and adult mice) in the SNc, compared with control mice. These results suggest that prenatal exposure to glucocorticoids may induce an age-dependent and persistent increase in the susceptibility to central toxicity of amphetamine-related drugs used later in life.

The Mediating Role of A2A Adenosine Receptors in the Mitochondrial Pathway of Apoptotic Hippocampal Cell Death, Following the Administration of MDMA in Rat

Introduction:

The 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a popular recreational drug and a major source of substance abuse, which ultimately leads to sensations of well-being, elation and euphoria, moderate derealization/depersonalization, and cognitive disruptions, as well as intense sensory awareness. The mechanisms involved in memory impairment induced by MDMA are not completely understood.

Methods:

The current study used 40 Sprague-Dawley rats, weighted 200 to 250 g. Experiments were performed in four groups, each containing 10 rats. The first group of rats was used as the control, treated with dimethyl sulfoxide (DMSO). The second group was treated with MDMA. The third group was treated with MDMA and CGS (the adenosine A_2A receptor agonist, 2-[p-(2-carboxyethyl) phenethylamino]-5′-N-ethylcarboxamidoadenosine) (CGS 21680) and the fourth group was treated with MDMA and SCH (the A_2A receptor antagonist [7-(2-phenylethyl)-5-amino-2-(2-furyl-) pyrazolo-[4, 3-e]-1, 2, 4 triazolo [1,5-] pyrimidine]) (SCH 58261). The drugs in all groups were administrated intraperitoneally (i.p.) once a day for 7 days. In 5 rats of each group, following perfusion, samples were taken from hippocampi to investigate apoptosis. Accordingly, the samples were stained using the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay kit, and studied by light microscopy. In other rats, fresh tissue was also removed to study the expression of bax and bcl-2 by Western blotting technique.

Results:

It was observed that the coadministration of MDMA with CGS reduced bax expression and prevented apoptosis of hippocampal cells. The coadministration of MDMA and SCH increased bax expression, and also increased the frequency of hippocampal cell apoptosis.

Conclusion:

The results of the current study showed that administration of CGS with MDMA decreased the common side effects associated with MDMA.

Progression and Persistence of Neurotoxicity Induced by MDMA in Dopaminergic Regions of the Mouse Brain and Association with Noradrenergic, GABAergic, and Serotonergic Damage

The amphetamine-related drug 3,4-methylenedioxymethamphetamine (MDMA) is known to induce neurotoxic damage in dopaminergic regions of the mouse brain. In order to characterize how the number of administrations influenced the severity of MDMA-induced dopaminergic damage and to describe the localization and persistence of this damage, we evaluated the changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) in different regions of the mouse brain. Moreover, we investigated whether dopaminergic damage was associated with noradrenergic, GABAergic, and serotonergic damage, by evaluating the changes in noradrenaline transporter (NET), glutamic acid decarboxylase-67 (GAD-67), and serotonin transporter (SERT). Mice received 14, 28, or 36 MDMA administrations (10 mg/kg twice a week) and were sacrificed at different time points (postnatal days 85, 110, 138, or 214) for immunohistochemical evaluation. Mice receiving 28 administrations showed reduced levels of DAT-positive fibers in caudate-putamen (CPu) and medial prefrontal cortex (mPFC) and reduced levels of TH-positive nigral neurons. These mice also displayed increased NET-positive hippocampal fibers, reduced GAD-67-positive neurons in CPu and hippocampus, and reduced GAD-67-positive fibers in mPFC. Similar effects of MDMA on DAT, TH, and GAD-67 were found in mice receiving 36 administrations, which also displayed reduced levels of striatal, cortical, and hippocampal TH-immunoreactive fibers. The reductions in dopaminergic markers and GAD-67 persisted at 3 months after MDMA discontinuation. Finally, MDMA never modified the levels of SERT. These results provide further insight into the localization and persistence of MDMA-induced dopaminergic damage and show that this effect may associate with GABAergic but not noradrenergic or serotonergic damage.

Effects of repeated exposure to MDMA on 5HT1a autoreceptor function: behavioral and neurochemical responses to 8-OHDPAT

A consistent effect of repeated exposure to 3,4 methylenedioxymethamphetamine (MDMA) is a decrease in the tissue levels of serotonin (5-HT). A variety of behavioural and neurochemical tests were conducted to determine whether the tissue deficits were accompanied by an increased sensitivity of the 5-HT1a autoreceptor. Tests were conducted 2 weeks following MDMA exposure (four injections of 10.0 mg/kg, IP, administered at 2-h intervals in a single day). The response to the 5-HT1a agonist, 8-OHDPAT (0.003-0.5 mg/kg, SC), was assessed using lower lip retraction (LLR), hypoactivity, and 5-hydroxytryptophan (5-HTP) accumulation following decarboxylase inhibition. The 8-OHDPAT produced a dose-dependent increase in LLR and hypoactivity, but these effects were comparable for MDMA and saline pretreated groups. MDMA decreased tissue levels of 5-HT and the accumulation of 5-HTP, but these effects were not reflected in the changes in autoreceptor sensitivity. The data suggest that the decrease in tissue levels of 5-HT produced by MDMA is accompanied by a decrease in tryptophan hydroxylase activity but cannot be explained by supersensitivity of the 5-HT1a autoreceptor.

Initial deficit and recovery of function after MDMA preexposure in rats

RATIONALE

3,4-methylenedioxymethamphetamine (MDMA) exposure was reported to result in deficits in serotonergic neurotransmission with concomitant behavioral suppression and tolerance to MDMA. Some data have also suggested that the neurochemical deficits recover over time, raising the question as to whether behavioral suppression would show a similar recovery.

OBJECTIVES

The possibility of recovery of behavioral deficits was examined in the present study. Rats were administered an MDMA pretreatment regimen that was shown to produce numerous serotonergic deficits and behavioral suppression 2 weeks thereafter. The full expression of MDMA-produced hyperactivity was dependent upon serotonergic integrity, therefore, the present study aimed to determine whether MDMA pretreated rats were tolerant to MDMA 2 weeks after exposure. Further, because serotonergic deficits have shown recovery over time, similar behavioral tests were conducted at a later time point to determine whether functional recovery was evident.

METHODS

MDMA-produced hyperactivity was measured at different withdrawal periods (2 and 12 weeks) to determine initial effects and the possibility of recovery of function.

RESULTS

In saline-pretreated control rats, +/-MDMA (0.0-10.0 mg/kg) produced a dose-dependent increase in locomotor activity. Rats that had received prior exposure to MDMA (4 x 10 mg/kg MDMA injections administered at 2 h intervals) demonstrated tolerance when the activity was measured 2 weeks after pretreatment. For these rats, there was a downward shift in the dose-effect curve for MDMA-produced hyperactivity. MDMA-produced hyperactivity in rats that were tested 12 weeks after pretreatment was, however, comparable to controls, suggesting recovery of function.

CONCLUSION

These data are consistent with the idea that high dose MDMA exposure produces neuroadaptations that exhibit recovery with extended abstinence from the drug.

Distinctive iron requirement of tryptophan 5-monooxygenase: TPH1 requires dissociable ferrous iron

A peripheral type of tryptophan 5-monooxygenase (EC 1.14.16.4), TPH1, is very unstable in vitro, but the inactivation was reversible and full reactivation occurs upon anaerobic incubation with a high concentration of dithiothreitol (DTT, 15 mM). In this study, distinctive iron requirement of TPH1 was revealed through analysis of the enzyme's inactivation and activation by DTT. For this purpose, all the glasswares, plastics, Sephadex G-25 gels, and reagents including protein solutions had been treated with metal chelators, and apo-TPH was prepared by treatment with EDTA. Apo-TPH thus prepared exclusively required free Fe2+ for its catalytic activity; 10(-8) M was enough under the strict absence of Fe3+ but 10(-12) M was too low. No other metal ions including Fe3+ were effective. It appeared that Fe3+ bound to the enzyme with a higher affinity than Fe2+, resulting in the inactivation. Ascorbate, a non-thiol reducing agent, did not substitute DTT in the activation of TPH1, but enhanced the Fe2+-dependent activity of apo-TPH as effectively as DTT. Thus, the DTT-activation was essentially substituted by preparation of apo-TPH by the EDTA treatment and the assay of apo-TPH in the presence of Fe2+ and ascorbate. The activation of TPH1 by incubation with DTT was accompanied by exposure of 9 sulfhydryls out of the total 10 cysteine residues, but the cleavage of disulfide bonds seemed not to be crucial, even if it occurred. The effect of DTT was substituted by some other sulfhydryls whose structure was analogous to that of commonly used metal chelators. Based on these observations, the following dual roles of DTT are proposed: (1) in the activation of TPH, DTT removes inappropriate bound iron (Fe3+) as a chelator, keeping Fe3+ away from the enzyme's binding site which needs to bind Fe2+ for the catalytic activity, and (2) in both the activation and reaction processes, DTT prevents oxidation of Fe2+ to Fe3+ as a reducing agent.

Reactions of the putative neurotoxin tryptamine-4,5-dione with L-cysteine and other thiols

Tryptamine-4,5-dione (1) is formed by oxidation of 5-hydroxytryptamine by reactive oxygen and reactive nitrogen species. Dione 1 is a powerful electrophile that can covalently modify cysteinyl residues of proteins and deactivate key enzymes. Thus, 1 has been suggested to play a role in the degeneration of serotonergic neurons in brain disorders such as Alzheimer's disease or evoked by amphetamine drugs. However, if formed in the brain, it is also likely that 1 would react with low molecular weight thiols such as cysteine (CySH) and glutathione (GSH). The resulting metabolites might not only contribute to the degeneration of serotonergic neurons but also, perhaps, serve as biomarkers of such neurodegeneration. In this investigation, it is shown that in oxygenated buffer at pH 7.4 dione 1 reacts with CySH and other low molecular weight sulfhydryls such as GSH, N-acetylcysteine, and cysteamine to form, first, the corresponding 7-S-thioethers of the dione. However, unlike the glutathionyl and N-acetylcysteinyl conjugates of 1, the 7-S-cysteinyl conjugate is very unstable at pH 7.4 forming a number of novel products, the nature of which are dependent on the relative concentrations of 1 and CySH. These products have been isolated, and spectroscopic and other evidence is provided in support of their proposed chemical structures.

Increased CRE-binding activity and tryptophan hydroxylase mRNA expression induced by 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") in the rat frontal cortex but not in the hippocampus

A single administration of either 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") or p-chloroamphetamine (PCA) produced a rapid and marked reduction of serotonin (5-HT) content in rat frontal cortex and hippocampus. In the cortex of MDMA-treated rats, 5-HT levels returned to control values 48 h after drug administration. This recovery was correlated with an induction of CRE-binding activity and an enhanced expression of tryptophan hydroxylase (TPH) mRNA, the rate-limiting enzyme in 5-HT biosynthesis, suggesting that MDMA may up-regulate the TPH gene through a CREB-dependent mechanism. In the cortex of PCA-treated rats, neither a recovery of 5-HT levels nor changes in DNA-binding or TPH mRNA were found at the same time point. In the hippocampus of rats receiving either PCA or MDMA a decrease in TPH mRNA levels was found at all times, along with a reduced CRE-binding at the 8-h time point. The results show region-specific effects of MDMA. In the frontal cortex, the increased TPH expression suggests a compensatory response to MDMA-induced loss of serotonergic function.

Inhibition of the alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase complexes by a putative aberrant metabolite of serotonin, tryptamine-4,5-dione

A transient energy impairment with resultant release and subsequent reuptake of 5-hydroxytryptamine (5-HT) and NMDA receptor activation with consequent cytoplasmic superoxide (O(2)(-)(*)), nitric oxide (NO(*)), and peroxynitrite (ONOO(-)) generation have all been implicated in a neurotoxic cascade which ultimately leads to the degeneration of serotonergic neurons evoked by methamphetamine (MA) and 3,4-methylenedioxymethamphetamine (MDMA). Such observations raise the possibility that the O(2)(-)(*)/NO(*)/ONOO(-)-mediated oxidation of 5-HT, as it returns via the plasma membrane transporter to the cytoplasm of serotonergic neurons when the MA/MDMA-induced energy impairment begins to subside, may generate an endogenous neurotoxin. In vitro the O(2)(-)(*)/NO(*)/ONOO(-)-mediated oxidation of 5-HT forms tryptamine-4,5-dione (T-4,5-D). When incubated with intact rat brain mitochondria, T-4,5-D strongly inhibits state 3 respiration with pyruvate or alpha-ketoglutarate as substrates at concentrations which do not affect succinate-supported (complex II) respiration. Experiments with freeze-thawed rat brain mitochondria reveal that T-4,5-D inhibits the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes. These and other properties of T-4,5-D raise the possibility that it may be an endogenously formed intraneuronal metabolite of 5-HT that contributes to the serotonergic neurotoxicity of MA and MDMA.

A putative metabolite of serotonin, tryptamine-4,5-dione, is an irreversible inhibitor of tryptophan hydroxylase: possible relevance to the serotonergic neurotoxicity of methamphetamine

Tryptamine-4,5-dione (T-4,5-D) is formed as a result of oxidation of 5-hydroxytryptamine by superoxide (O(2)(-)(*), nitric oxide (NO*), and peroxynitrite (ONOO(-)). T-4,5-D rapidly inactivates tryptophan hydroxylase (TPH), derived from rat brain, probably as a result of covalent modification of active site cysteine residues. The activity of TPH exposed to T-4,5-D cannot be restored by anaerobic reduction with dithiothreitol (DTT) and ferrous iron (Fe(2+)) indicating that the inactivation is irreversible. 7-S-Glutathionyl-tryptamine-4,5-dione, formed by the rapid reaction between T-4,5-D and glutathione, also inhibits TPH but in this case the activity is restored by anaerobic reduction with DTT/Fe(2+). The results of this investigation may be relevant to the initial reversible and subsequent irreversible inactivation of TPH evoked by methamphetamine and 3,4-methylenedioxymethamphetamine.

Ascorbic acid prevents 3,4-methylenedioxymethamphetamine (MDMA)-induced hydroxyl radical formation and the behavioral and neurochemical consequences of the depletion of brain 5-HT

MDMA-induced 5-HT neurotoxicity has been proposed to involve oxidative stress due to increased formation of hydroxyl radicals. Recently, MDMA-induced 5-HT neurotoxicity has been shown to be accompanied by a suppression of behavioral and neurochemical responses to a subsequent injection of MDMA. The intent of the present study was to examine whether suppression of the MDMA-induced formation of hydroxyl radicals by an antioxidant, ascorbic acid, attenuates both the MDMA-induced depletion of 5-HT and the functional consequences associated with this depletion. Treatment of rats with ascorbic acid suppressed the generation of hydroxyl radicals, as evidenced by the production of 2,3-dihydroxybenzoic acid from salicylic acid, in the striatum during the administration of a neurotoxic regimen of MDMA. Ascorbic acid also attenuated the MDMA-induced depletion of striatal 5-HT content. In rats treated with a neurotoxic regimen of MDMA, the ability of a subsequent injection of MDMA to increase the extracellular concentration of 5-HT in the striatum, elicit the 5-HT behavioral syndrome, and produce hyperthermia was markedly reduced compared to the responses in control rats. The concomitant administration of ascorbic acid with the neurotoxic regimen of MDMA prevented the diminished neurochemical and behavioral responses to a subsequent injection of MDMA. Finally, a neurotoxic regimen of MDMA produced significant reductions in the concentrations of vitamin E and ascorbic acid in the striatum and hippocampus. Thus, the MDMA-induced depletion of brain 5-HT and the functional consequences thereof appear to involve the induction of oxidative stress resulting from an increased generation of free radicals and diminished antioxidant capacity of the brain.

Age-dependent differential responses of monoaminergic systems to high doses of methamphetamine

Abuse of methamphetamine (METH) by adolescents is a major public health issue in the U.S.A. Because of the neurotoxic potential of METH, we examined the response of CNS monoaminergic systems in young (adolescent) animals [postnatal day (PND) 40] to high-dose treatments (10 mg/kg, four injections, 2-h intervals) of this drug and contrasted these effects to those seen in older (young adult) rats (PND 90). Consistent with previous reports, we observed that PND 40 animals did not manifest the long-term (7-day) deficits in extrapyramidal dopamine (DA) parameters observed in PND 90 rats. In contrast, METH-induced rapid (1-h) reduction in the activity of striatal DA transporters occurred in both age groups. In addition, both persistent (7-day) and rapid (1-h) deficits in serotonergic systems (measured as reductions in tryptophan hydroxylase activity) were observed in PND 40 and 90 rats. Age-related differences in METH-induced hyperthermia did not appear to be a principal cause for our observations; however, age-dependent pharmacokinetics of this drug might have contributed to the differential METH monoaminergic responses by PND 40 and 90 animals.

Tryptophan hydroxylase regulation. Drug-induced modifications that alter serotonin neuronal function

Tryptophan hydroxylase is the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin. A variety of drugs are known to diminish the function of this enzyme, and possibly cause damage to serotonin neurons. These include the substituted amphetamines methamphetamine and 3,4-methylenedioxy-methamphetamine, as well as L-DOPA, the most common therapy for Parkinsons Disease. In view of the important role for dopamine in the effects of these drugs on tryptophan hydroxylase and on serotonin neurons, we tested whether dopamine could alter the activity of this important enzyme. We found that dopamine-derived quinones, but not dopamine, inactivate tryptophan hydroxylase and convert the protein to a redox-cycling quinoprotein. This posttranslational modification of tryptophan hydroxylase could play a role in the drug-induced reduction in serotonin synthesis.

Involvement of the serotonin transporter in the formation of hydroxyl radicals induced by 3,4-methylenedioxymethamphetamine

The mechanism of 3,4-methylenedioxymethamphetamine (MDMA)-induced depletion of brain serotonin (5-hydroxytryptamine, 5-HT) has been proposed to involve the generation of reactive oxygen species. In the present study, quantification of the extracellular concentration of 2,3-dihydroxybenzoic acid (2,3-DHBA) from salicylic acid was used as an index of hydroxyl radical generation. Although both MDMA and D-amphetamine markedly increased the extracellular concentration of dopamine in the striatum, only MDMA increased the extracellular concentration of 2,3-DHBA. Treatment with fluoxetine either 1 h prior to or 4 h following the administration of MDMA reduced the MDMA-induced formation of 2,3-DHBA and also attenuated the MDMA-induced depletion of 5-HT in the striatum. These results are supportive of the view that the MDMA-induced generation of hydroxyl radicals and, ultimately, the long-term depletion of 5-HT, is dependent, in part, on the activation of the 5-HT transporter.

Tyrosine hydroxylase is inactivated by catechol-quinones and converted to a redox-cycling quinoprotein: possible relevance to Parkinson's disease

Quinone derivatives of DOPA, dopamine, and N-acetyldopamine inactivate tyrosine hydroxylase, the initial and rate-limiting enzyme in the biosynthesis of the catecholamine neurotransmitters. The parent catechols are inert in this capacity. The effects of the catecholquinones on tyrosine hydroxylase are prevented by antioxidants and reducing reagents but not by scavengers of hydrogen peroxide, hydroxyl radicals, or superoxide radicals. Quinone modification of tyrosine hydroxylase modifies enzyme sulfhydryl groups and results in the formation of cysteinyl-catechols within the enzyme. Catecholquinones convert tyrosine hydroxylase to a redox-cycling quinoprotein. Quinotyrosine hydroxylase causes the reduction of the transition metals iron and copper and may therefore contribute to Fenton-like reactions and oxidative stress in neurons. The discovery that a phenotypic marker for catecholamine neurons can be converted into a redox-active species is highly relevant for neurodegenerative conditions such as Parkinson's disease.

Mazindol attenuates the 3,4-methylenedioxymethamphetamine-induced formation of hydroxyl radicals and long-term depletion of serotonin in the striatum

The formation of hydroxyl radicals following the systemic administration of 3,4-methylenedioxymethamphetamine (MDMA) was studied in the striatum of the rat by quantifying the stable adducts of salicylic acid and D-phenylalanine, namely, 2,3-dihydroxybenzoic acid (2,3-DHBA) and p-tyrosine, respectively. The repeated administration of MDMA produced a sustained increase in the extracellular concentration of 2,3-DHBA and p-tyrosine, as well as dopamine. The MDMA-induced increase in the extracellular concentration of both dopamine and 2,3-DHBA was suppressed in rats treated with mazindol, a dopamine uptake inhibitor. Mazindol also attenuated the long-term depletion of serotonin (5-HT) in the striatum produced by MDMA without altering the acute hyperthermic response to MDMA. These results are supportive of the view that MDMA produces a dopamine-dependent increase in the formation of hydroxyl radicals in the striatum that may contribute to the mechanism whereby MDMA produces a long-term depletion of brain 5-HT content.

Dopamine inactivates tryptophan hydroxylase and forms a redox-cycling quinoprotein: possible endogenous toxin to serotonin neurons

Exposure of tryptophan hydroxylase (TPH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin, to dopamine under mild oxidizing conditions (iron + H2O2) or in the presence of tyrosinase results in a concentration-dependent inactivation of the enzyme. Dopamine, iron, H2O2, or tyrosinase alone does not alter TPH activity. Similarly, N-acetyldopamine oxidized with one equivalent of sodium periodate causes a concentration-dependent inactivation of TPH as well. TPH is protected from dopamine-induced inactivation by reduced glutathione, ascorbic acid, and dithiothreitol but not by the radical scavengers DMSO, mannitol, or superoxide dismutase. Parallel studies with [3H]dopamine reveal a high negative correlation between inhibition of catalysis and incorporation of tritium into the enzyme. Those reducing agents and antioxidants that protect TPH from inactivation are effective in preventing the labeling of TPH by [3H]dopamine. Acid hydrolysis and HPLC with electrochemical detection (HPLC-EC) analysis of inactivated TPH revealed the formation of cysteinyl-dopamine residues within the enzyme. Exposure of dopamine-modified TPH to redox-cycling staining after SDS-PAGE confirmed the formation of a quinoprotein. These results indicate that dopamine-quinones covalently modify cysteinyl residues in TPH, leading directly to the loss of catalytic activity, and establish that TPH could be a target for dopamine-quinones in vivo after drugs (e.g., neurotoxic amphetamines) that cause dopamine-dependent inactivation of TPH. Redox cycling of a TPH-quinoprotein could also participate in the serotonin neuronal toxicity caused by these same drugs.

Advances in the molecular characterization of tryptophan hydroxylase

The neurotransmitter serotonin has been implicated in numerous physiological functions and pathophysiological disorders. The hydroxylation of the aromatic amino acid tryptophan is rate-limiting in the synthesis of serotonin. Tryptophan hydroxylase (TPH), as the rate-limiting enzyme, determines the concentrations of serotonin in vivo. Relative serotonin concentrations are clearly important in neural transmission, but serotonin has also been reported to function as a local antioxidant. Identification of the mechanisms regulating TPH activity has been hindered by its low levels in tissues and the instability of the enzyme. Several TPH expression systems have been developed to circumvent these problems. In addition, eukaryotic expressions systems are currently being developed and represent a new avenue of research for identifying TPH regulatory mechanisms. Recombinant DNA technology has enabled the synthesis of TPH deletions, chimeras, and point mutations that have served as tools for identifying structural and functional domains within TPH. Notably, the experiments have proven long-held hypotheses that TPH is organized into N-terminal regulatory and C-terminal catalytic domains, that serine-58 is a site for PKA-mediated phosphorylation, and that a C-terminal leucine zipper is involved in formation of the tetrameric holoenzyme. Several new findings have also emerged regarding regulation of TPH activity by posttranslational phosphorylation, kinetic inhibition, and covalent modification. Inhibition of TPH by L-DOPA may have implications for depression in Parkinson's disease (PD) patients. In addition, TPH inactivation by nitric oxide may be involved in amphetamine-induced toxicity. These regulatory concepts, in conjunction with new systems for studying TPH activity, are the focus of this article.

Why tryptophan hydroxylase is difficult to purify: a reactive oxygen-derived species-mediated phenomenon that may be implicated in human pathology

1. Attempts and apparently successful procedures to obtain reasonable quantities of electrophoretically homogenous mammalian brain-derived tryptophan hydroxylase, (TPH), have been described, starting in the early 1970s. This work has been carried out with the primary objective to obtain specific antisera to this enzyme to map out serotonergic pathways in the nervous system. 2. By using a multitude of techniques, antisera have indeed been fabricated and employed. However, it is doubtful if pure, native TPH has ever been produced. Indeed, there is strong evidence that more than one isoform of TPH exists in the rat brain. Thus, these antisera are probably directed against TPH-derived polypeptides and not the holoenzyme(s). 3. The difficulty in the purification of TPH lies not only in its subjectivity to proteolysis, but more importantly in its probable capacity to produce superoxide leading to hydrogen perioxide formation. This, in turn, may undergo Fenton chemistry with iron at the active site of the protein to produce hydroxyl radicals that directly attack and destroy the enzyme molecule. Evidence for such a mechanism is presented together with possible protocols that might be used to produce pure stable holo TPH(s). 4. It is hypothesized that similar oxidative events may take place in vivo under certain conditions leading to pathological results. Strategies to block these events are suggested.

Causes and consequences of the loss of serotonergic presynapses elicited by the consumption of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") and its congeners

The massive and prolonged stimulation of serotonin (5-HT)-release and the increased dopaminergic activity are responsible for the acute psychomimetic and psychostimulatory effects of 3,4-methylenedioxy-methamphetamine (MDMA, "ecstasy") and its congeners. In vulnerable subjects, at high doses or repeated use, and under certain unfavorable conditions (crowding, high ambient temperature), severe, in some cases fatal, averse systemic reactions (hyperthermia, serotonin-syndrome) may occur during the first few hours. Animal experiments revealed the existence of similar differences in vulnerability and similar dose- and context-related influences on a similar sequence of acute responses. The severity of these acute systemic responses is closely related to the severity of the long-term damage to 5-HT axon terminals caused by the administration of substituted amphetamines. Attempts to identify the mechanisms involved in this selective degeneration of 5-HT presynapses brought to light a multitude of different factors and conditions which either attenuate or potentiate the loss of 5-HT terminals caused by MDMA and related amphetamine derivatives. These puzzling observations suggest that the degeneration of 5-HT presynapses represents only the final step in a sequence of events which compromise the ability of 5-HT terminals to maintain their functional and structural integrity. Substituted amphetamines selectively tax energy metabolism in 5-HT presynapses through their ability to exchange with 5-HT and to dissipate transmembrane ion gradients. The active carrier systems in the vesicular and presynaptic membrane operate at a permanently activated state. The resulting energy deficit can no longer adequately restored by the 5-HT presynapses when their availability of substrates for ATP production is additionally reduced by the hyperthermic and other energy consuming reactions which are elicited by the systemic administration of substituted amphetamines. The exhaustion of energy in 5-HT nerve terminals compromised all energy-requiring endogenous mechanisms involved in the regulation of transmembrane-ion exchange, internal Ca(++)-homeostasis, prevention of oxidative stress, detoxification, and repair. Above a critical threshold the failure of these self-protective mechanisms will lead to the degeneration of the 5-HT axon terminals. Based on the role of 5-HT as a global modulatory transmitter-system involved in the stabilization and integration of impulse flow between distributed multifocal neuronal networks, the partial loss of 5-HT presynapses must be expected to impair the ability of these networks to maintain the integrity of signal flow pattern, and increase the likelihood of switching to unstable information processing. Behavioral responding may therefore become more dominated by activities generated in individual networks, and hitherto "buffered" personality traits and predisposition may become manifested as defined psychiatric syndromes in certain predisposed subjects.

Molecular mechanism of the inactivation of tryptophan hydroxylase by nitric oxide: attack on critical sulfhydryls that spare the enzyme iron center

Tryptophan hydroxylase (TPH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin (5-HT), is irreversibly inactivated by nitric oxide (NO). We have expressed brain TPH as a recombinant glutathione-S-transferase fusion protein and delineated the catalytic domain of the enzyme as the region spanning amino acids 99-444. Highly purified TPH catalytic core, like the native enzyme from brain, is inactivated by NO in a concentration-dependent manner. Removal of iron from TPH produces an apoenzyme with low activity that can be reconverted to its highly active holo-form by the addition of ferrous iron. Apo-TPH exposed to NO cannot be reactivated by iron. Treatment of holo-TPH (iron-loaded) with the disulfide 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB) causes an inactivation of TPH that is readily reversed by dithiothreitol (DTT). DTNB-treated TPH [sulfhydryl (SH)-protected] exposed to NO is returned to full activity by thiol reduction with DTT. The inactivation of native TPH by NO cannot be reversed by either iron or DTT. These data indicate that NO inactivates TPH by selective action on critical SH groups (i.e., cysteine residues) while sparing catalytic iron sites within the enzyme. The results are interpreted with reference to the substituted amphetamines, which are neurotoxic to 5-HT neurons, that inactivate TPH in vivo and are now known to produce NO and other reactive oxygen species in vivo.

A rapid and reversible change in dopamine transporters induced by methamphetamine

Because high doses of methamphetamine promote free radical formation, and striatal dopamine transporters are rapidly inactivated by oxidative events, we determined the effect of a single high dose of methamphetamine on dopamine transporter activity in striatal synaptosomes. One hour after methamphetamine administration, dopamine uptake decreased by 48%. This dramatic decline was totally reversed by 24 h after treatment. These findings suggest that methamphetamine reversibly decreases dopamine transporter activity by oxidative mechanisms.

Effect of ascorbate and cysteine on the 3,4-methylenedioxymethamphetamine-induced depletion of brain serotonin

The extent of long-term depletion of serotonin (5-HT) produced by 3,4-methylenedioxymethamphetmaine (MDMA) was assessed in rats treated with the antioxidants sodium ascorbate or L-cysteine. There was a 30-35% reduction in the striatal concentration of 5-HT 7 days following a single injection of MDMA (20 mg/kg, s.c.). MDMA had no significant effect on striatal concentrations of 5-HT in rats that had been treated with ascorbate (250 mg/kg, i.p.) or cysteine (500 mg/kg, i.p.) 30 min prior to and 5 hrs following the administration of MDMA. Treatment with ascorbate or cysteine did not alter the accumulation of MDMA in brain as determined by in vivo microdialysis. Moreover, neither ascorbate nor cysteine altered the stimulation of dopamine release elicited by MDMA. These data are supportive of the view that MDMA-induced toxicity of 5-HT neurons may be related to the production of free radicals and subsequent oxidative damage.

Body temperature effect on methylenedioxymethamphetamine-induced acute decrease in tryptophan hydroxylase activity

Brain tryptophan hydroxylase activity decreases within 15 min after a single administration of 3,4-methylenedioxymethamphetamine. In the present study, the effect of body temperature on this acute decrease of tryptophan hydroxylase activity was examined. 2 h after a single dose of 3,4-methylenedioxymethamphetamine (20 mg/kg, s.c.), rats exhibited hyperthermia (38.7 degrees C) or hypothermia (35.8 degrees C) when maintained at 25 degrees C or 6 degrees C, respectively. The rectal temperature of control animals maintained at 6 degrees C was not altered. Tryptophan hydroxylase activity measured in the hippocampus, striatum and frontal cortex of hyperthermic rats treated with 3,4-methylenedioxymethamphetamine was decreased to 61%, 65%, and 71% of control levels, respectively, 2 h after drug treatment. However, in hypothermic rats, 3,4-methylenedioxymethamphetamine had no effect on tryptophan hydroxylase activity in the hippocampus, striatum or frontal cortex. Non-drug-induced hyperthermia or hypothermia did not affect tryptophan hydroxylase activity. Since hypothermia may prevent the 3,4-methylenedioxymethamphetamine-induced decrease in tryptophan hydroxylase activity by reducing the formation of free radicals, the effect of a free radical scavenging agent, N-tert-butyl-alpha-phenylnitrone, was examined. N-tert-butyl-alpha-phenylnitrone (200 mg/kg, i.p.) alone caused hypothermia but had no direct effect on tryptophan hydroxylase activity. Preadministration of N-tert-butyl-alpha-phenylnitrone prevented 3,4-methylenedioxymethamphetamine from raising the temperature above normal and attenuated the drug-induced decrease in tryptophan hydroxylase activity in hippocampus, striatum and frontal cortex. However, when the rats treated with a combination of N-tert-butyl-alpha-phenylnitrone and 3,4-methylenedioxymethamphetamine were maintained at hyperthermic conditions, N-tert-butyl-alpha-phenylnitrone had no protective effect. These results suggest that body temperature plays a prominent role in the 3,4-methylenedioxymethamphetamine-induced acute decrease in tryptophan hydroxylase activity.

[3H]monoamine releasing and uptake inhibition properties of 3,4-methylenedioxymethamphetamine and p-chloroamphetamine analogues

The ability of several 3,4-methylenedioxymethamphetamine (MDMA) analogues to inhibit the uptake of [3H]serotonin (5-HT), dopamine (DA) and norepinephrine (NE) into synaptosomes was examined. In addition, the ability of the compounds to inhibit the uptake of [3H]5-HT and DA into synaptosomes from rats pretreated with reserpine (5 mg/kg i.p., 16 h pretreatment) was compared to control experiments. All of the test compounds were found to be potent releasers of non-vesicular 5-HT (the reserpine IC50 was significantly smaller than the control IC50). The range of 5-HT inhibitory activity corresponds well to the small range of ED50 values of the test compounds to substitute in drug discrimination experiments with animals trained to discriminate MDMA or S-(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-aminobutane (S-MBDB) from saline. In contrast, there was a wide range of potency for the inhibition of NE and DA uptake. In addition, several of the analogues appeared to be pure uptake inhibitors of DA while others were found to be releasers of non-vesicular DA. Several of the compounds were very selective for 5-HT over DA or NE uptake inhibition, including 3-methoxy-4-methylamphetamine (MMA) and 5-methoxy-6-methyl-2-aminoindan (MMAI). A correlation was noted between the 5-HT neurotoxic potential of some of the test compounds and their relative ability to induce a release of non-vesicular DA. The potential catechol metabolites of the methylenedioxy-substituted compounds also showed potent monoamine releasing properties, suggesting that metabolism may play a role in the neurotoxic actions of some of these drugs. The present data support the hypothesis that drug-stimulated non-vesicular 5-HT release is primarily responsible for the discriminative cue of MDMA.

Isolation and structural characterization of the murine tryptophan hydroxylase gene

The mouse tryptophan hydroxylase gene was isolated and its intron/exon boundaries and putative regulatory sequences identified. To isolate the gene a mouse mastocytoma cDNA clone encoding tryptophan hydroxylase was used to identify and isolate ten overlapping DNA fragments from a mouse genomic library. Restriction mapping and sequence analysis of the clones revealed that the gene contains 11 exons and covers a region of DNA of approximately 21 kb. The transcription initiation site was mapped and the major site of initiation yields an untranslated leader sequence of 124 nucleotides. A minor initiation site is located 9 nucleotides 3' of the major site. The 5' untranslated sequence is interrupted by the first intron. Analysis of the sequence upstream of the initiation site showed the presence of several putative promoter and regulatory sequences. Nine of the ten intron/exon boundaries of tryptophan hydroxylase are conserved with tyrosine hydroxylase and phenylalanine hydroxylase, further delineating the evolutionary relationship of these three genes.

Reversal of the acute effects of 3,4-methylenedioxymethamphetamine by 5-HT uptake inhibitors

Recent evidence suggests that the acute 3,4-methylenedioxymethamphetamine (MDMA)-induced loss of tryptophan hydroxylase activity (TPH) may be due to the oxidation of critical sulf-hydryl groups on the molecule. To determine if TPH activity could be regenerated in vivo we administered a 5-HT uptake inhibitor at various times immediately after MDMA. Although enzyme activity began to decline immediately following MDMA administration, rats receiving the uptake inhibitor 1 h post MDMA showed a rapid recovery of TPH activity. Administration of an uptake inhibitor 3 h post MDMA was without effect on the time course of TPH inactivation. The results suggest that systems exist within the serotonergic neuron for the reductive regeneration of active TPH. Furthermore, these systems are acutely compromised following the administration of MDMA.

Neurochemical effects of an acute treatment with 4-methylaminorex: a new stimulant of abuse

4-Methylaminorex (4-MAX) is an amphetamine analog which has recently gained attention due to its potential as a stimulant of abuse. The present study characterized the acute neurochemical changes elicited after a single dose of 4-MAX. Thus, dose-response and time-response studies were conducted in order to assess the effects of this drug on monoaminergic and neuropeptide systems in extrapyramidal and limbic structures. The most dramatic responses in the dose-effect experiments (animals killed 3 h after treatment) were a 2-fold increase in neostriatal homovanillic acid levels and a decrease in neostriatal tryptophan hydroxylase activity to 33% of control in the 20 mg/kg group. Because all animals in the 20 mg/kg group experienced convulsions, 10 mg/kg was used for the time-response studies. The most striking effects in these studies included a reduction in dopamine concentrations to 71% of control, and an increase to 270% of control in the concentrations of dihydroxyphenylacetic acid 30 min after 4-MAX administration. In addition, neostriatal neurotensin and dynorphin A levels increased to approximately 200 and 400% of control, respectively, 18 h after a 10 mg/kg dose. These data suggest that 4-MAX is a potent dopamine releaser, which decreases tryptophan hydroxylase activity in a manner similar to other amphetamine-related drugs. However, in contrast to other amphetamine analogs, 4-MAX has potent convulsant actions.