Christopher J. Schmidt

In vitro and in vivo neurochemical effects of methylenedioxymethamphetamine on striatal monoaminergic systems in the rat brain

A single high dose of methylenedioxymethamphetamine, a psychedelic agent, produced a rapid and persistent depletion of striatal indoles similar to that observed following administration of the serotonergic neurotoxin p-chloroamphetamine. The drug had little effect on dopaminergic variables. Like p-chloroamphetamine, methylenedioxymethamphetamine was found to be a relatively selective agent for inducing [3H]serotonin release in vitro. The serotonin uptake inhibitor, citalopram, blocked both [3H]serotonin release in vitro and striatal serotonin depletion in vivo, indicating that both processes were carrier dependent. In vivo comparisons of the stereoisomers of methylenedioxymethamphetamine indicated two phases of serotonin depletion similar to those reported for p-chloroamphetamine. Although both the (+)- and (-)-stereoisomers produced an acute (3 hr) decrease in striatal indoles, the long-term effects of the drug showed stereoselectivity in that the (+)-enantiomer produced the most dramatic serotonin depletion. Comparison of the effects of the stereoisomers of methylenedioxymethamphetamine and its n-desmethyl analog, methylenedioxyamphetamine, on [3H]serotonin and [3H]dopamine release in vitro showed the (+)-enantiomer of both drugs to be the more potent releasing agent. In spite of its reported lack of hallucinogenic activity, (+)methylenedioxyamphetamine was found to be of a potency similar to that of (+)methylenedioxymethamphetamine in inducing [3H]serotonin release in vitro. The results are discussed in terms of the neurochemical similarities between methylenedioxymethamphetamine and p-chloroamphetamine as well as the proposed role of serotonin release in the behavioral effects of methylenedioxymethamphetamine.

Methylenedioxymethamphetamine: A potentially neurotoxic amphetamine analogue

The amphetamine analogue, methylenedioxymethamphetamine (MDMA) has received considerable attention recently as a novel and increasingly popular psychoactive agent. When administered acutely to rats in high doses, MDMA caused a selective and dramatic decrease in brain concentrations of serotonin and its metabolite, 5-hydroxyindoleacetic acid. The depletion of serotonin and its metabolite persisted for up to at least one week after a single injection of MDMA at approximately four to five times the acute dose reported for humans. These results are discussed in terms of the possible neurotoxic effects of MDMA.

Depression of rat brain tryptophan hydroxylase activity following the acute administration of methylenedioxymethamphetamine

The psychotomimetic agent, methylenedioxymethamphetamine, produced a rapid, persistent and dose-dependent reduction in cortical tryptophan hydroxylase activity when administered acutely to rats. This effect did not occur in vitro and did not require N-demethylase activity in the whole animal. Kinetic analysis revealed the loss of enzyme activity to be due to an alteration in Vmax with no change in the affinity of the enzyme for either its cofactor or substrate. Coadministration of the serotonin (5-HT) uptake inhibitor, citalopram, only partially antagonized the loss of tryptophan hydroxylase activity 3 hr after methylenedioxymethamphetamine, but completely prevented the loss of cortical 5-HT. Recovery of enzyme activity did occur by 1 week if the neurotoxic effect of methylenedioxymethamphetamine was blocked by fluoxetine. The effect of methylenedioxymethamphetamine on 5-HT synthesis was not affected by pretreatment with alpha-methyl-p-tyrosine, reserpine or yohimbine. Ketanserine and methiothepin, 5-HT receptor antagonists, did partially block the methylenedioxymethamphetamine-induced loss of tryptophan hydroxylase activity, suggesting a possible role for neurotransmitter release in the acute effects of the drug on enzyme activity.

5-HT2 receptors exert a state-dependent regulation of dopaminergic function: studies with MDL 100,907 and the amphetamine analogue, 3,4-methylenedioxymethamphetamine

The highly selective 5-HT2 receptor antagonist, MDL 100,907, was used to explore the role of serotonin in the stimulation of dopaminergic function produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). MDL 100,907 blocked MDMA-stimulated dopamine synthesis in vivo without affecting basal synthesis. The long-term deficits in 5-HT concentrations believed to be a consequence of MDMA-induced dopamine release were also blocked by MDL 100,907 over the same dose range. In vivo microdialysis confirmed that 5-HT2 receptor blockade with MDL 100,907 attenuated MDMA-induced increases in extracellular concentrations of striatal dopamine. In contrast to its effect on MDMA-induced synthesis, MDL 100,907 did not alter dopamine synthesis stimulated by haloperidol or reserpine. In vivo dopamine release produced by haloperidol was also unaffected by MDL 100,907. The results suggest a permissive role for 5-HT2 receptors in the activation of the dopamine system which occurs during states of high serotonergic activity or during conditions of elevated dopamine efflux with high D2 receptor occupancy.

Blockade of Striatal 5‐Hydroxytryptmine2 Receptors Reduces the Increase in Extracellullar Concentrations of Dopamine Produced by the Amphhetamine analogue 3,4‐Methylenedioxymethamphetamine

Abstract: 5‐Hydroxytryptamine2 (5‐HT2) receptor antagonists have been shown to interfere with the stimulation of striatal dopamine synthesis and release produced by the amphetamine analogue 3,4‐methylenedioxymethamphetamine (MDMA). To localize the receptors responsible for the attenuation of MDMA‐induced release, 5‐HT2 receptor antagonists were infused via the microdialysis probe directly into the brains of awake, freely moving rats before the systemic administration of MDMA. Intrastriatal infusions of the selective 5‐HT2 antagonist MDL 100, 907 produced a concentration‐dependent inhibition of MDMA‐induced dopamine release. Similar results were observed with intrastriatal infusions of the 5‐HT2 antagonist amperozide. In contrast, infusion of MDL 100, 907 into the midbrain region near the dopaminergic cell bodies was with out effect on the MDMA‐induced elevation of extracellular dopamine in the ipsilateral striatum. Neither antagonist attenuated basal transmitter efflux nor the MDMA‐stimulated release of [3H]dopamine from striatal slices in vitro indicating that the in vivo effect of the antagonists was not due to inhibition of the dopamine uptake carrier. Intrastriatal infusion of tetrodotoxin reduced both basal and MDMA‐stimulated dopamine efflux and eliminated the effect of intrastriatal MDL 100, 907. The results indicate that 5‐HT2 receptors located in the striatum augment the release of dopamine produced by high doses of MDMA. Furthermore, these 5‐HT2 receptors appear to be located on nondopaminergic elements of the striatum.

Methylenedioxymethamphetamine-induced hyperthermia and neurotoxicity are independently mediated by 5-HT2 receptors

Methylenedioxymethamphetamine (MDMA) produced a significant hyperthermia in rats which was antagonized in a competitive manner by the selective 5-HT2 antagonist, MDL 11,939. The 5-HT antagonist also blocked MDMA-induced neurotoxicity as assessed by the decline in regional 5-HT concentrations observed 1 week later. These two effects of MDL 11,939 were dissociated at higher doses of MDMA where the antagonist still provided virtually complete protection against the neurochemical deficits but only partially attenuated the hyperthermic response. In contrast to the effect of the 5-HT2 antagonist, haloperidol did not alter MDMA-induced hyperthermia but did antagonize its long-term neurochemical effects. Similarly, coadministration of the selective 5-HT uptake inhibitor, MDL 27,777, did not affect the hyperthermia produced by a high dose of MDMA but completely prevented the depletion of 5-HT. When the MDMA-induced hyperthermia was prevented by temporarily maintaining animals at reduced ambient temperature, the neurochemical changes normally observed 1 week later were also blocked. Although these results demonstrate that the drugs tested do not antagonize MDMA-induced neurotoxicity by interfering with its effect on body temperature, they do indicate that MDMA-induced hyperthermia may contribute to the development of the drugs long-term neurochemical effects.

Electrophysiological, biochemical and behavioral evidence for 5-HT2 and 5-HT3 mediated control of dopaminergic function

Several lines of evidence have suggested a link between serotonergic and dopaminergic systems in the brain. The interpretation of much of these early data needs careful reevaluation in light of the recent understanding of the plethora of serotonin receptor subtypes, their distribution in the brain and the new findings with more selective serotonin antagonists. Electrophysiological, biochemical and behavioral evidence obtained using highly selective antagonists of the 5-HT2 or 5-HT3 receptor subtypes, MDL 100,907 or MDL 73,147EF, respectively, supports the thesis that serotonin modulates the dopaminergic system. This modulation is most evident when the dopaminergic system has been activated.

The selective 5-HT2A receptor antagonist, MDL 100,907, increases dopamine efflux in the prefrontal cortex of the rat

Diminished function within the mesocortical dopamine system has been to hypothesized to contribute directly to the negative and indirectly to the positive symptoms of schizophrenia. Based on the proposed role of 5-HT2 receptor blockade in the antipsychotic profile of clozapine and its preferential augmentation of prefrontal dopamine release, we have examined the effects of the selective 5-HT2A receptor antagonist, R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidi ne- methanol (MDL 100,907), on dopamine release in the rat medial prefrontal cortex using in vivo microdialysis. The results indicate that local 5-HT2A receptors exert a tonic inhibitory influence on dopamine efflux in the medial prefrontal cortex. These observations are consistent with the hypothesis that 5-HT2A receptor blockade contributes to the unique antipsychotic profile of clozapine and that MDL 100,907 may have antipsychotic activity.

Direct central effects of acute methylenedioxymethamphetamine on serotonergic neurons

Acute peripheral administration of either the (+) or (-) stereoisomer of methylenedioxymethamphetamine (MDMA) to rats results in a rapid loss of tryptophan hydroxylase (TPH) activity in several brain regions. This decline in enzyme activity precedes a decrease in serotonin (5-HT) concentrations in the same areas. An initial rise in the concentration of 5-hydroxyindole acetic acid after drug administration suggests that an increase in the turnover of 5-HT is an early event in the development of these changes. Unsuccessful attempts to reproduce the in vivo effects of MDMA on TPH activity using in vitro preparations such as cortical slices or the mouse mastocytoma cell line, P-815, suggested a requirement for an intact neuronal system or metabolism of the drug. Injection of MDMA directly into several brain regions also had no effect on TPH activity or 5-HT concentrations. However, when brain concentrations of MDMA were maintained using a constant i.c.v. infusion, TPH activity declined as observed following peripheral administration. The results, therefore, indicate that the acute effect of MDMA on 5-HT synthesis is a direct central effect of the drug which may be triggered by a sustained increase in transmitter turnover.

Antagonism of the neurotoxicity due to a single administration of methylenedioxymethamphetamine

The role of transmitter release in the serotonergic neurotoxicity of methylenedioxymethamphetamine (MDMA) was examined using treatments altering MDMA-induced release or its consequences. The long-term decrease in 5-HT concentrations and tryptophan hydroxylase activity produced by MDMA was antagonized by depletion of vesicular monoamines with reserpine or interruption of monoamine synthesis with the decarboxylase inhibitor, monofluoromethyl DOPA (dihydroxyphenylalanine). Similar results were achieved by selectively inhibiting dopamine synthesis with alpha-methyl-p-tyrosine or through bilateral lesions of the substantia nigra with 6-hydroxydopamine. The dopamine receptor antagonist haloperidol was also effective in this regard. Although these results strongly implicate dopamine release in the long-term neurochemical effects of MDMA, protection was also provided by selective 5-HT2 antagonists indicating that the neurotoxicity is dependent upon the release of both dopamine and 5-HT.