Effect of d-fenfluramine and 5,7-dihydroxytryptamine on the levels of tryptophan hydroxylase and its mRNA in rat brain

3,4-Methylenedioxymethamphetamine induces differential regulation of tryptophan hydroxylase 2 protein and mRNA levels in the rat dorsal raphe nucleus

Previous investigations with 3,4-methylenedioxymethamphetamine (MDMA) have suggested that administration of this drug results in a degeneration of 5-HT nerve terminals and subsequent alterations in 5-HT neurotransmission. However, only limited investigations have examined the effects of MDMA on the dorsal raphe nucleus. The present study was designed to assess the effect of MDMA on the rate-limiting enzyme in 5-HT biosynthesis, tryptophan hydroxylase (TPH), by measuring TPH2 protein and mRNA levels in rat dorsal raphe (DR) nucleus. Rats were administered MDMA (20 mg/kg, s.c.) or saline twice daily for 4 days and killed 14 days later. Tissue sections of the DR were processed for quantitative immunoautoradiography and in situ hybridization histochemistry for measurements of the levels of TPH2-immunoreactivity (IR) and TPH2 mRNA. To assess 5-HT axon terminal integrity after MDMA treatment, the density of 5-HT transporter (SERT) binding sites was measured by quantitative autoradiography using [125I]RTI-55 ((-)-2beta-carbomethoxy-3 beta-(4-iodophenyl) tropane) ((125)I-RTI-55) as a ligand. TPH2-IR levels were significantly decreased by 45% in the mid DR and by 40% in the caudal DR in the MDMA-treated rats compared with saline-injected rats. In contrast, TPH2 mRNA levels were significantly increased by 24% in the mid DR and by 12% in the caudal DR. MDMA treatment significantly decreased (125)I-RTI-55 labeled SERT binding sites in the striatum, nucleus accumbens and cingulate cortex demonstrating a loss of 5-HT terminals. The increase in TPH2 mRNA levels in both the mid DR and caudal DR of MDMA-treated rats may reflect a compensatory mechanism in the injured 5-HT neurons to increase TPH2 protein synthesis. Taken together, our results suggest that a serious defect occurs in the biosynthesis of TPH2 in the DR following MDMA administration.

Restoration of 3,4-methylenedioxymethamphetamine-induced 5-HT depletion by the administration of l-5-hydroxytryptophan

BACKGROUND

3,4-Methylenedioxymethamphetamine (MDMA) causes persistent decreases in brain 5-HT content and 5-HT transporter (SERT) binding, with no detectable changes in SERT protein. Such data suggest that MDMA impairs 5-HT transmission but leaves 5-HT nerve terminals intact. To further test this hypothesis, we carried out two types of experiments in rats exposed to high-dose MDMA. First, we examined the effects of MDMA on SERT binding and function using different in vitro assay conditions. Next, we treated rats with the 5-HT precursor, l-5-hydroxytryptophan (5-HTP), in an attempt to restore MDMA-induced depletions of 5-HT.

METHODS

Rats received three i.p. injections of saline or MDMA (7.5 mg/kg), one injection every 2 h. Rats in one group were decapitated, and brain tissue was assayed for SERT binding and [(3)H]5-HT uptake under conditions of normal (100 or 126 mM) and low (20 mM) NaCl concentration. Rats from another group received saline or 5-hydroxytryptophan/benserazide (5-HTP-B), each drug at 50 mg/kg i.p., and were killed 2 h later.

RESULTS

MDMA reduced SERT binding to 10% of control when assayed in 100 mM NaCl, but this reduction was only 55% of control in 20 mM NaCl. MDMA decreased immunoreactive 5-HT in caudate and hippocampus to about 35% of control. Administration of 5-HTP-B to MDMA-pretreated rats significantly increased the 5-HT signal toward normal levels in caudate (85% of control) and hippocampus (66% of control).

CONCLUSION

1) Following high-dose MDMA treatment sufficient to reduce SERT binding by 90%, a significant number of functionally intact 5-HT nerve terminals survive. 2) The degree of MDMA-induced decreases in SERT binding depends on the in vitro assay conditions. 3) 5-HTP-B restores brain 5-HT depleted by MDMA, suggesting that this approach might be clinically useful in abstinent MDMA users.

Enhanced head-twitch response to 5-HT-related agonists in thiamine-deficient mice

While many studies suggest an involvement of brain serotonergic systems in neuro-psychiatric disorders such as schizophrenia and depression, their role in Wernicke-Korsakoff syndrome (WKS) remains unclear. Since dietary thiamine deficiency (TD) in mice is considered as a putative model of WKS, it was used in the present study to investigate the function of serotonergic neurons in this disorder. After 20 days of TD feeding, the intensity of tryptophan hydroxylase immunofluorescence was found to be significantly decreased in the dorsal and medial raphe nuclei. In addition, the head-twitch response (HTR) elicited by the intracerebroventricular administration of the 5-HT(2A) agonist 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) was significantly increased in TD versus control mice, whereas the injection of ketanserin, a 5-HT(2A) receptor antagonist, prevented this enhancement. A single injection of thiamine HCl on the 19th day of TD feeding did not reduce the enhanced DOI-induced HTR. On the other hand, the administration of d-fenfluramine, a 5-HT releaser, did not enhance the HTR in TD mice. Together, our results indicate that TD causes a super-sensitivity of 5-HT(2A) receptors by reducing presynaptic 5-HT synthesis derived from degenerating neurons projecting from the raphe nucleus.

Rat brain serotonin neurones that express neuronal nitric oxide synthase have increased sensitivity to the substituted amphetamine serotonin toxins 3,4-methylenedioxymethamphetamine and p-chloroamphetamine

Substituted amphetamines such as p-chloroamphetamine and the abused drug methylenedioxymethamphetamine cause selective destruction of serotonin axons in rats, by unknown mechanisms. Since some serotonin neurones also express neuronal nitric oxide synthase, which has been implicated in neurotoxicity, the present study was undertaken to determine whether nitric oxide synthase expressing serotonin neurones are selectively vulnerable to methylenedioxymethamphetamine or p-chloroamphetamine. Using double-labeling immunocytochemistry and double in situ hybridization for nitric oxide synthase and the serotonin transporter, it was confirmed that about two thirds of serotonergic cell bodies in the dorsal raphé nucleus expressed nitric oxide synthase, however few if any serotonin transporter immunoreactive axons in striatum expressed nitric oxide synthase at detectable levels. Methylenedioxymethamphetamine (30 mg/kg) or p-chloroamphetamine (2 x 10 mg/kg) was administered to Sprague-Dawley rats, and 7 days after drug administration there were modest decreases in the levels of serotonin transporter protein in frontal cortex, and striatum using Western blotting, even though axonal loss could be clearly seen by immunostaining. p-Chloroamphetamine or methylenedioxymethamphetamine administration did not alter the level of nitric oxide synthase in striatum or frontal cortex, determined by Western blotting. Analysis of serotonin neuronal cell bodies 7 days after p-chloroamphetamine treatment, revealed a net down-regulation of serotonin transporter mRNA levels, and a profound change in expression of nitric oxide synthase, with 33% of serotonin transporter mRNA positive cells containing nitric oxide synthase mRNA, compared with 65% in control animals. Altogether these results support the hypothesis that serotonin neurones which express nitric oxide synthase are most vulnerable to substituted amphetamine toxicity, supporting the concept that the selective vulnerability of serotonin neurones has a molecular basis.

Overexpression of S100beta in transgenic mice does not protect from serotonergic denervation induced by 5,7-dihydroxytryptamine

Transgenic mice overexpressing S100beta were used to examine whether the chronic elevation of this protein alters the response to selective partial serotonergic lesions produced by bilateral intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT). Basal levels of S100beta mRNA examined by in situ hybridization were two- to threefold higher throughout the brain in transgenic than in control mice, whereas 5-HT levels in forebrain were similar in both. After the 5,7-DHT-induced lesions, no differences were found in the S100beta mRNA levels in either normal or transgenic mice. At 5 and 60 days after the lesion, forebrain 5-HT levels were reduced by 56% and 35%, respectively, in control mice and by 51% and 35%, respectively, in the transgenic mice. Analysis of the 5-HT immunostaining showed a marked decrease of the immunoreactivity in various brain regions, which was comparable at the two intervals postlesion. One exception was the medial hypothalamus, where an almost complete disappearance of 5-HT immunoreactivity was observed in the medial region at 5 days after lesion, followed by a marked reinnervation 60 days later. These hypothalamic changes were seen in both controls and S100beta-overexpressing transgenic mice. Quantitative analysis of the density of 5-HT transporter sites using [(3)H]citalopram binding, a marker of serotonergic terminals, showed a marked decrease in different brain regions at both 5 and 60 days after 5,7-DHT injections. No difference in basal and postlesion levels of [(3)H]citalopram binding was seen between transgenic and control mice. In conclusion, this study demonstrates that constitutive overexpression of S100beta in transgenic mice does not modify serotonin levels during development, nor does it protect the serotonergic neurons from selective neurotoxicity or modify the serotonergic sprouting induced by partial lesion.

Intraregional variation in expression of serotonin transporter messenger RNA by 5-hydroxytryptamine neurons

The distribution of the messenger RNA encoding the 5-hydroxytryptamine transporter was investigated in rat brain. 5-Hydroxytryptamine transporter messenger RNA was found exclusively in the B1-B9 cell groups containing the cell bodies of 5-hydroxytryptamine neurons. Combined in situ hybridization and 5-hydroxytryptamine immunocytochemistry demonstrated 5-hydroxytryptamine transporter gene expression in the majority of and exclusively in 5-hydroxytryptamine neurons. Cells differed in their levels of expression of 5-hydroxytryptamine transporter messenger RNA and 5-hydroxytryptamine immunofluorescence, but with a tight correlation between the two parameters. Image analysis of cells from B7, the dorsal raphe nucleus, and B8, the median raphe nucleus, revealed significant differences between groups in the mean cellular level of 5-hydroxytryptamine transporter gene expression. Cells in the ventromedial subdivision of B7 displayed higher levels of expression than cells in B8 or cells in the lateral wings of B7. There was also heterogeneity in the distribution of the cellular levels of expression for two other genes expressed by 5-hydroxytryptamine neurons: l-aromatic amino acid decarboxylase messenger RNA and tryptophan hydroxylase messenger RNA. However, the relative levels of expression of these two genes within the four regions studied differed from that of 5-hydroxytryptamine transporter messenger RNA. These results indicate intraregional differences between 5-hydroxytryptamine neurons with respect to 5-hydroxytryptamine transporter messenger RNA levels. Such differences may account for the differential sensitivity of 5-hydroxytryptamine neurons to cytotoxins.

Microdialysis monitoring of variations in extracellular levels of serotonin, GABA and excitatory amino acids in the frontal cortex of awake rats in response to a single peripheral or central administration of dexfenfluramine

The effects of a single dexfenfluramine (D-fen) administration on the release of endogenous serotonin (5-hydroxytryptamine, 5-HT), excitatory (glutamate, Glu, aspartate, Asp) and inhibitory (gamma-aminobutyric acid, GABA) amino acids from the frontal cortex were studied by using in vivo microdialysis in freely-moving rats. Extracellular levels of these neurotransmitters were measured with HPLC coupled to electrochemical detection or with capillary electrophoresis coupled to laser-induced fluoresence detection (CE-LIFD). In a first study, single intraperitoneal administration of D-fen (0.5, 1.3, 5 and 10 mg/kg) increased extracellular 5-HT levels in a dose-dependent manner (maximal increase by 982% over baseline for the highest dose) while changes in Glu, Asp or GABA never reached statistical significance. In a second study, 73 nM of D-fen applied locally through the frontocortical dialysis probe, at a flow rate of 1.5 microliters/min in 30 microliters of perfusion fluid for 20 min, increased extracellular 5-HT and Asp levels [the maximal increases were to 1804% and 280% of the respective basal values (100%)] without altering extracellular levels of Glu and GABA. Thus, the order of magnitude of the changes induced by systemic administration or local infusion of D-fen on frontocortical extracellular levels of several neurotransmitters (5-HT > > Asp > GABA = Glu) demonstrate that D-fen, an indirect serotoninergic agonist, mainly increases 5-HT release while producing slight (Asp) or no (Glu, GABA) short-term in vivo variations in amino acid extracellular levels in the rat frontal cortex.

Effects of dexfenfluramine or 5,7-dihydroxytryptamine on tryptophan hydroxylase and serotonin transporter mRNAS in rat dorsal raphe

Dexfenfluramine (DF), given in high doses, can produce long-lasting decreases in brain levels of serotonin (5-HT) and 5-HT transporter (5-HTT) protein. The purpose of this study was to determine if DF-induced decreases in 5-HT and 5-HTT in rat forebrain are correlated with compensatory changes in the expression of the genes for tryptophan hydroxylase (TPH) and 5-HTT in the dorsal raphe nucleus. Gene transcripts were measured by quantitative reverse transcription-polymerase chain reaction (RT-PCR). Rats were treated with either one or eight injections of DF at either high (10 mg/kg) or low (2 mg/kg) doses. A positive control group for 5-HT cell loss received a single cerebroventricular injection of 5,7-dihydroxytryptamine (DHT). Rats were killed either 5, 15 or 30 days after their last treatment. Paroxetine binding to the 5-HTT protein in frontal cortex was, as expected, reduced in all of the treated groups relative to vehicle controls. TPH mRNA levels in the dorsal raphe of animals that received DHT were significantly higher than those measured in all other treatment groups 15 days following treatment. By 30 days, the amount of TPH mRNA in DHT-treated rats had fallen to well below control levels. None of the DF regimens significantly affected TPH mRNA levels. Unlike the TPH mRNA changes in DHT-treated rats, the 5-HTT mRNA levels in the dorsal raphe declined progressively throughout the 30 day survival period. None of the DF regimens significantly affected 5-HTT mRNA levels. The significance of these data are discussed in terms of whether loss of forebrain markers for 5-HT reflects either the loss of fine caliber 5-HT axon terminals or a decrease in the expression of these markers in the somata of these cells which are located in the dorsal raphe.

Chronic D-fenfluramine decreases serotonin transporter messenger RNA expression in dorsal raphe nucleus

In situ hybridization was used to measure the effects of chronic fenfluramine administration on serotonin transporter messenger RNA expression in cells of the dorsal raphe nucleus complex. Fenfluramine produced a significant, but transient, down-regulation of serotonin transporter mRNA in cells which lie in the ventral portion of the dorsal raphe nucleus, but not in the dorsal part of the dorsal raphe nucleus. Our findings suggest that cells which lie in the ventral part of the dorsal raphe nucleus are more sensitive to the effects of chronic fenfluramine administration, but that fenfluramine does not cause long-term changes in gene expression in serotonin cell bodies.