Does GFAP mRNA and mitochondrial benzodiazepine receptor binding detect serotonergic neuronal degeneration in rat?

Methylenedioxymethamphetamine (MDMA, 'Ecstasy'): Neurodegeneration versus Neuromodulation

The amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA, ‘ecstasy’) is widely abused as a recreational drug due to its unique psychological effects. Of interest, MDMA causes long-lasting deficits in neurochemical and histological markers of the serotonergic neurons in the brain of different animal species. Such deficits include the decline in the activity of tryptophan hydroxylase in parallel with the loss of 5-HT and its main metabolite 5-hydoxyindoleacetic acid (5-HIAA) along with a lower binding of specific ligands to the 5-HT transporters (SERT). Of concern, reduced 5-HIAA levels in the CSF and SERT density have also been reported in human ecstasy users, what has been interpreted to reflect the loss of serotonergic fibers and terminals. The neurotoxic potential of MDMA has been questioned in recent years based on studies that failed to show the loss of the SERT protein by western blot or the lack of reactive astrogliosis after MDMA exposure. In addition, MDMA produces a long-lasting down-regulation of SERT gene expression; which, on the whole, has been used to invoke neuromodulatory mechanisms as an explanation to MDMA-induced 5-HT deficits. While decreased protein levels do not necessarily reflect neurodegeneration, the opposite is also true, that is, neuroregulatory mechanisms do not preclude the existence of 5-HT terminal degeneration.

Ultrastructural characterization of tryptophan hydroxylase 2-specific cortical serotonergic fibers and dorsal raphe neuronal cell bodies after MDMA treatment in rat

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is a widely used recreational drug known to cause selective long-term serotonergic damage.

OBJECTIVES

The aim of this study was to characterize the ultrastructure of serotonergic pericarya and proximal neurites in the dorsal raphe nucleus as well as the ultrastructure of serotonergic axons in the frontal cortex of adolescent Dark Agouti rats 3 days after treatment with 15 mg/kg i.p. MDMA.

METHODS

Light microscopic immunohistochemistry and pre-embedding immunoelectron microscopy with a novel tryptophan hydroxylase-2 (Tph2) specific antibody, as a marker of serotonergic structures.

RESULTS

Light microscopic analysis showed reduced serotonergic axon density and aberrant swollen varicosities in the frontal cortex of MDMA-treated animals. According to the electron microscopic analysis, Tph2 exhibited diffuse cytoplasmic immunolocalization in dorsal raphe neuronal cell bodies. The ultrastructural-morphometric analysis of these cell bodies did not indicate pathological changes or significant alteration in the cross-sectional areal density of any examined organelles. Proximal serotonergic neurites in the dorsal raphe exhibited no ultrastructural alteration. However, in the frontal cortex among intact fibers, numerous serotonergic axons with destructed microtubules were found. Most of their mitochondria were intact, albeit some injured axons also contained degenerating mitochondria; moreover, a few of them comprised confluent membrane whorls only.

CONCLUSIONS

Our treatment protocol does not lead to ultrastructural alteration in the serotonergic dorsal raphe cell bodies and in their proximal neurites but causes impairment in cortical serotonergic axons. In these, the main ultrastructural alteration is the destruction of microtubules although a smaller portion of these axons probably undergo an irreversible damage.

Loss of serotonin transporter protein after MDMA and other ring-substituted amphetamines

We studied in vivo expression of the serotonin transporter (SERT) protein after 3,4-methylenedioxymethamphetamine (MDMA), p-chloroamphetamine (PCA), or fenfluramine (FEN) treatments, and compared the effects of substituted amphetamines to those of 5,7-dihydroxytryptamine (5,7-DHT), an established serotonin (5-HT) neurotoxin. All drug treatments produced lasting reductions in 5-HT, 5-HIAA, and [(3)H]paroxetine binding, but no significant change in the density of a 70 kDa band initially thought to correspond to the SERT protein. Additional Western blot studies, however, showed that the 70 kDa band did not correspond to the SERT protein, and that a diffuse band at 63-68 kDa, one that had the anticipated regional brain distribution of SERT protein (midbrain>striatum>neocortex>cerebellum), was reduced after 5,7-DHT and was absent in SERT-null animals, was decreased after MDMA, PCA, or FEN treatments. In situ immunocytochemical (ICC) studies with the same two SERT antisera used in Western blot studies showed loss of SERT-immunoreactive (IR) axons after 5,7-DHT and MDMA treatments. In the same animals, tryptophan hydroxylase (TPH)-IR axon density was comparably reduced, indicating that serotonergic deficits after substituted amphetamines differ from those in SERT-null animals, which have normal TPH levels but, in the absence of SERT, develop apparent neuroadaptive changes in 5-HT metabolism. Together, these results suggest that lasting serotonergic deficits after MDMA and related drugs are unlikely to represent neuroadaptive metabolic responses to changes in SERT trafficking, and favor the view that substituted amphetamines have the potential to produce a distal axotomy of brain 5-HT neurons.

The effect of amphetamine analogs on cleaved microtubule-associated protein-tau formation in the rat brain

The present study quantified the cleaved form of the microtubule-associated protein tau (cleaved MAP-tau, C-tau), a previously demonstrated marker of CNS toxicity, following the administration of monoamine-depleting regimens of the psychostimulant drugs amphetamine (AMPH), methamphetamine (METH), +/-3,4-methylenedioxymethamphetamine (MDMA), or para-methoxyamphetamine (PMA) in an attempt to further characterize psychostimulant-induced toxicity. A dopamine (DA)-depleting regimen of AMPH produced an increase in C-tau immunoreactivity in the striatum, while a DA- and serotonin (5-HT)-depleting regimen of METH produced an increase in the number of C-tau immunoreactive cells in the striatum and CA2/CA3 and dentate gyrus regions of the hippocampus. MDMA and PMA, two psychostimulant drugs that produce selective 5-HT depletion in the striatum, had no effect on C-tau immunoreactivity in the striatum or hippocampus. Furthermore, 5,7-dihydroxytryptamine (5,7-DHT), an established 5-HT selective neurotoxin, did not produce an increase in C-tau immunoreactivity. Dual fluorescent immunocytochemistry with antibodies to glial fibrillary acidic protein (GFAP) and C-tau indicated that C-tau immunoreactivity was present in astrocytes, not neurons, suggesting that increased C-tau may be an alternative indicator of reactive gliosis. The present results are consistent with previous findings that the DA-depleting psychostimulants AMPH and METH produce reactive gliosis whereas the 5-HT-depleting drugs MDMA and PMA, as well as the known 5-HT selective neurotoxin 5,7-DHT, do not produce an appreciable glial response.

Pharmacokinetic profile of single and repeated oral doses of MDMA in squirrel monkeys: relationship to lasting effects on brain serotonin neurons

A large body of data indicates that (+/-)3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') can damage brain serotonin neurons in animals. However, the relevance of these preclinical data to humans is uncertain, because doses and routes of administration used in animals have generally differed from those used by humans. Here, we examined the pharmacokinetic profile of MDMA in squirrel monkeys after different routes of administration, and explored the relationship between acute plasma MDMA concentrations after repeated oral dosing and subsequent brain serotonin deficits. Oral MDMA administration engendered a plasma profile of MDMA in squirrel monkeys resembling that seen in humans, although the half-life of MDMA in monkeys is shorter (3 vs 6-9 h). MDMA was biotransformed into MDA, and the plasma ratio of MDA to MDMA was 3-5 / 100, similar to that in humans. MDMA accumulation in squirrel monkeys was nonlinear, and plasma levels were highly correlated with regional brain serotonin deficits observed 2 weeks later. The present results indicate that plasma concentrations of MDMA shown here to produce lasting serotonergic deficits in squirrel monkeys overlap those reported by other laboratories in some recreational 'ecstasy' consumers, and are two to three times higher than those found in humans administered a single 100-150 mg dose of MDMA in a controlled setting. Additional studies are needed on the relative sensitivity of brain serotonin neurons to MDMA toxicity in humans and non-human primates, the pharmacokinetic parameter(s) of MDMA most closely linked to the neurotoxic process, and metabolites other than MDA that may play a role.

(±)-3,4-Methylenedioxymethamphetamine Administration to Rats Does Not Decrease Levels of the Serotonin Transporter Protein or Alter Its Distribution between Endosomes and the Plasma Membrane

We showed that the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) reduces brain tissue 5-HT, decreases expression of 5-HT transporter (SERT) protein, and increases expression of glial fibrillary acidic protein (GFAP). In contrast, doses of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) that decrease brain tissue 5-HT fail to alter expression of SERT or GFAP. Using a new and highly sensitive anti-SERT antibody, we determined whether MDMA alters the subcellular distribution of SERT protein by measuring SERT expression in endosomes and plasma membranes 2 weeks after MDMA administration. Rat brain tissues (caudate, cortex, and hippocampus) were collected 3 days and 2 weeks after MDMA (7.5 mg/kg i.p., every 2 h x 3 doses) or 5,7-DHT (150 microg/rat i.c.v.) administration. Representative results from cortex are as follows. At both 3 days and 2 weeks postinjection, MDMA decreased tissue 5-HT (65%) and had no effect on GFAP expression. MDMA increased heat shock protein 32 (HSP32; a marker for microglial activation) expression (30%) at 3 days, but not 2 weeks. MDMA did not alter SERT expression at either time point and did not alter SERT levels in either endosomes or plasma membranes (2 weeks). 5,7-DHT decreased tissue 5-HT (80%), increased HSP32 expression at both time points (about 50%), and increased GFAP expression at 2 weeks (40%). 5,7-DHT decreased SERT expression (33%) at 2 weeks, but not at 3 days. These findings indicate that a dosing regimen of MDMA that depletes brain 5-HT does not alter SERT protein expression or the distribution of SERT between endosomes and the plasma membrane and does not produce detectable evidence for neurotoxicity.

Microglial activation is a pharmacologically specific marker for the neurotoxic amphetamines

Neurotoxic amphetamines cause damage to monoamine nerve terminals of the striatum by unknown mechanisms. Microglial activation contributes to the neuronal damage that accompanies injury, disease, and inflammation, but a role for these cells in amphetamine-induced neurotoxicity has received little attention. We show presently that D-methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA), D-amphetamine, and p-chloroamphetamine, each of which has been linked to dopamine (DA) or serotonin nerve terminal damage, result in microglial activation in the striatum. The non-neurotoxic amphetamines l-methamphetamine, fenfluramine, and DOI do not have this effect. All drugs that cause microglial activation also increase expression of glial fibrillary acidic protein (GFAP). At a minimum, microglial activation serves as a pharmacologically specific marker for striatal nerve terminal damage resulting only from those amphetamines that exert neurotoxicity. Because microglia are known to produce many of the reactive species (e.g., nitric oxide, superoxide, cytokines) that mediate the neurotoxicity of the amphetamine-class of drugs, their activation could represent an early and essential event in the neurotoxic cascade associated with high-dose amphetamine intoxication.

3,4-methylenedioxymethamphetamine (MDMA) administration to rats decreases brain tissue serotonin but not serotonin transporter protein and glial fibrillary acidic protein

Previous experiments conducted in this laboratory showed that administration of high-dose D-fenfluramine (D-FEN) and p-chloroamphetamine (PCA) decreased 5-HT transporter (SERT) binding and tissue 5-HT by 30-60% in caudate and whole brain tissue 2 days and 2 weeks after drug administration. However, protein expression as determined by Western blot analysis did not change in either tissue or time point, except for a 30% decrease in the caudate 2 days after PCA administration. In the present study, we studied the effect of MDMA and 5,7-dihydroxytryptamine (5,7-DHT) on tissue 5-HT levels and the protein expression level of SERT and glial fibrillary acidic protein (GFAP), a validated neurotoxicity marker.

HYPOTHESIS

MDMA administration decreases SERT expression.

METHODS

Two weeks after MDMA administration (7.5 mg/kg i.p., q 2 h x 3 doses) or 2 weeks after i.c.v. administration of 5,7,-DHT (150 microg/rat), male Sprague-Dawley rats were sacrificed and the caudate, cortex, and hippocampal tissue collected. Western blots for SERT and GFAP were generated using published methods. Tissue 5-HT levels were determined by HPLC coupled to electrochemical detection.

RESULTS

MDMA treatment decreased tissue 5-HT in cortex, hippocampus, and caudate by about 50%. However, MDMA treatment had no significant effect on expression level of SERT and GFAP in any brain region. In contrast, 5,7-DHT reduced tissue 5-HT by more than 90%, decreased SERT protein expression by 20-35%, and increased GFAP by 30-39%.

CONCLUSION

These data suggest the MDMA treatment regimen used here does not cause degeneration of 5-HT nerve terminals. Viewed collectively with our previous results and other published data, these data indicate that MDMA-induced persistent 5-HT depletion may occur in the absence of axotomy.

The pharmacology and clinical pharmacology of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy")

The amphetamine derivative (+/-)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a popular recreational drug among young people, particularly those involved in the dance culture. MDMA produces an acute, rapid enhancement in the release of both serotonin (5-HT) and dopamine from nerve endings in the brains of experimental animals. It produces increased locomotor activity and the serotonin behavioral syndrome in rats. Crucially, it produces dose-dependent hyperthermia that is potentially fatal in rodents, primates, and humans. Some recovery of 5-HT stores can be seen within 24 h of MDMA administration. However, cerebral 5-HT concentrations then decline due to specific neurotoxic damage to 5-HT nerve endings in the forebrain. This neurodegeneration, which has been demonstrated both biochemically and histologically, lasts for months in rats and years in primates. In general, other neurotransmitters appear unaffected. In contrast, MDMA produces a selective long-term loss of dopamine nerve endings in mice. Studies on the mechanisms involved in the neurotoxicity in both rats and mice implicate the formation of tissue-damaging free radicals. Increased free radical formation may result from the further breakdown of MDMA metabolic products. Evidence for the occurrence of MDMA-induced neurotoxic damage in human users remains equivocal, although some biochemical and functional data suggest that damage may occur in the brains of heavy users. There is also some evidence for long-term physiological and psychological changes occurring in human recreational users. However, such evidence is complicated by the lack of knowledge of doses ingested and the fact that many subjects studied are or have been poly-drug users.

Increased susceptibility of striatal mitochondria to calcium-induced permeability transition

Mitochondria were simultaneously isolated from striatum and cortex of adult rats and compared in functional assays for their sensitivity to calcium activation of the permeability transition. Striatal mitochondria showed an increased dose-dependent sensitivity to Ca2+ compared with cortical mitochondria, as measured by mitochondrial depolarization, swelling, Ca2+ uptake, reactive oxygen species production, and respiration. Ratios of ATP to ADP were lower in striatal mitochondria exposed to calcium despite equal amounts of ADP and ATP under respiring and nonrespiring conditions. The Ca2+-induced changes were inhibited by cyclosporin A or ADP. These responses are consistent with Ca2+ activation of both low and high permeability pathways constituting the mitochondrial permeability transition. In addition to the striatal supersensitivity to induction of the permeability transition, cyclosporin A inhibition was less potent in striatal mitochondria. Immunoblots indicated that striatal mitochondria contained more cyclophilin D than cortical mitochondria. Thus striatal mitochondria may be selectively vulnerable to the permeability transition. Subsequent mitochondrial dysfunction could contribute to the initial toxicity of striatal neurons in Huntington's disease.

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

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

Serotonergic reinnervation reverses lesion-induced decreases in PSA-NCAM labeling and proliferation of hippocampal cells in adult rats

Serotonin (5-HT) is believed to play a role in structural plasticity in the adult brain, and cell adhesion molecules may be involved in such adaptive processes. The present study sought to determine the effects of 5-HT denervation and reinnervation of the hippocampal formation on the expression of glial and neuronal markers and neurogenesis in adult rats. Injections of 5,7-dihydroxytryptamine (5,7-DHT) in the dorsal and medial raphe nuclei, producing a partial loss of 5-HT neurons, induced rapid and transient increases in glial fibrillary acidic protein immunoreactivity indicative of a reactive gliosis, but no changes in the S100beta or tenascin-C normally secreted by astroglial cells. In contrast, as long as the hippocampal formation was deprived of 5-HT innervation, significant decreases were observed in the number of granule cells expressing the highly polysialylated form of the neural cell adhesion molecule (PSA-NCAM) as well as the PSA-NCAM staining of the hilus in the dentate gyrus. Similar decreases in the number of newly generated granule cells labeled with bromodeoxyuridine were also detected during this time. All these effects were reversed later, when the hippocampal formation was reinnervated by 5-HT fibers. These results indicate that 5-HT is one factor which may regulate the number of granule cells proliferating in the adult dentate gyrus and thereafter expressing PSA-NCAM immunoreactive at the level of cell bodies, dendrites, and axonal paths (mossy fibers). They emphasize the critical role played by 5-HT in the neuronal organization of the hippocampus.

Serotonin depletion in the adult rat produces differential changes in highly polysialylated form of neural cell adhesion molecule and tenascin-C immunoreactivity

Levels of immunoreactivity for highly polysialylated neural cell adhesion molecule (PSA-NCAM), NCAM, and tenascin-C (TN-C), were examined in the basal ganglia regions and hypothalamic nuclei of adult rats after serotonergic (5-HT) lesions induced by 5,7-dihydroxytryptamine injections in the dorsal and medial raphe nuclei. Decreases in the density of serotonin fibers were associated with no changes in NCAM and general decreases in PSA-NCAM staining, the time-course of changes being selective for each region. Taken that the confocal analysis indicated that serotonin neurons do not express PSA-NCAM and that similar decreases in PSA-NCAM staining were observed after inhibition of 5-HT synthesis induced by parachlorophenylalanine administration, these results suggest that 5-HT may reduce adhesion by acting on PSA-NCAM expression in its environment, and thus facilitate plasticity in adult brain. Two months after the neurotoxin lesions, a normalization of PSA-NCAM staining was associated with a partial restoration in 5-HT fiber density in the nucleus accumbens and the supraoptic nucleus, suggesting that PSA-NCAM may facilitate sprouting of 5-HT fibers. Since a similar normalization was also detected in the suprachiasmatic nucleus, which remained deprived of serotonin fibers, negative factors are likely to be involved in regeneration processes. Indeed, increases in glial fibrillary acidic protein (GFAP) followed by increases in TN-C were observed in these areas, suggesting that the secretion of TN-C by astrocytes may have negative consequences on the sprouting of 5-HT fibers. Finally, the lack of changes in striatal PSA-NCAM or TN-C staining observed after selective lesions of the dopaminergic pathway induced by intranigral injections of 6-hydroxydopamine indicates that 5-HT has a selective and critical role in adult brain plasticity.

Immunohistochemical localization and quantification of glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25000) in the ageing hippocampus following administration of 5,7-dihydroxytryptamine

Responses to injury in the ageing hippocampus were assessed utilizing the synaptic markers glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25,000) following administration of the neurotoxin, 5,7-dihydroxytryptamine, into the fimbria-fornix and cingulum bundle to denervate serotonergic afferent input to the dorsal hippocampus. Age-dependent alterations in hippocampal immunohistochemical localization of glial fibrillary acidic protein and synaptosomal-associated protein were evaluated in female Fischer 344 rats following serotonergic deafferentation with 5,7-dihydroxytryptamine. Across the lifespan, as indicated by measurements taken at three, 18, 21 and 29 months, marked increases in glial fibrillary acidic protein, but not synaptosomal-associated protein immunoreactivity, occurred throughout the hippocampus at 21 and 29 months compared to three and 18 months. Following three weeks pretreatment with 5,7-dihydroxytryptamine (20 microg total dose) or vehicle (0.1% ascorbic saline; 2 microl total volume) infused in the fimbria-fornix/cingulum bundle, immunohistochemical analysis demonstrated marked increases of glial fibrillary acidic protein, but not synaptosomal-associated protein, in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and 3-month 5,7-dihydroxytryptamine groups. Additionally, a significant increase in glial fibrillary acidic protein concentration was found by enzyme-linked immunosorbent assay in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and three-month 5,7-dihydroxytryptamine groups. These results demonstrate that selective neurotoxicant damage of the hippocampal serotonergic system differentially alters the expression of glial fibrillary acidic protein. This approach may provide a valuable tool to determine the ability of the hippocampus to respond to age-related neurodegenerative injury.

Differential effects of 5,7-dihydroxytryptamine-induced serotoninergic degeneration of 5-HT1A receptors and 5-HT uptake sites in the rat brain

The time-course of 5,7-dihydroxytryptamine-induced lesions (2, 5 and 14 days after i.c.v. injection of 150 micrograms) and the effects of acute reserpine treatment (10 mg/kg, i.p., one or 5 days before scheduled death), were evaluated by autoradiography of [3H]paroxetine binding sites in the rat brain. Reserpine had no significant effect on [3H]paroxetine binding, indicating that the depletion of serotonin is not sufficient per se to alter the serotonin uptake sites in any region. Two days after the 5,7-dihydroxytryptamine lesion, [3H]paroxetine binding was already decreased in the majority of brain regions. In the caudate putamen these binding sites were significantly decreased only 14 days after the lesion, whereas the ventral tegmental area (or the enclosed median forebrain bundle), the dorsal raphe (mainly the ventral portion) and the median raphe maintained their high density of serotonin uptake sites even after 14 days. Results were similar using [3H]citalopram as ligand for the serotonin uptake sites, in the brains of rats lesioned 5 days before death; an exception was the ventral portion of the dorsal raphe, where there was a significant increase with [3H]paroxetine and a decrease with [3H]citalopram binding. In adjacent sections of the same brains we also measured [3H]8-OH-DPAT binding, confirming that it completely disappears in the dorsal raphe after the lesion. Thus, considering the extent of serotonin cell body degeneration, there appears to be a paradoxical mismatch between the excessive loss of [3H]8-OH-DPAT binding and the resistance of [3H]citalopram or [3H]paroxetine binding in the dorsal raphe, suggesting that the two binding sites may undergo adaptive regulation in surviving neurons.