Non-invasive in-vivo autoradiographic method to measure axonal transport in serotoninergic neurons in the rat brain

Difference in magnitude of psychostimulant-induced extracellular norepinephrine in the ventral tegmental area contributes to discrepant prefrontal dopamine outflow

The dopamine (DA) efflux in the medial prefrontal cortex (mPFC) can be modulated by the interaction between afferent norepinephrine (NE) and somatodendritic DA in the ventral tegmental area (VTA). However, it is unclear how locally administered amphetamine (AMPH) or cocaine in the VTA results in discrepant response of DA efflux in the mPFC. In this study, intra-VTA infusion of AMPH (1000 microM) or cocaine (200 microM) in anesthetized rats was employed to study the concurrent profile of extracellular DA level in the VTA and mPFC. In addition, the extracellular NE levels during the intra-VTA infusion of these two psychostimulants were analyzed to compare their effects on prefrontal DA efflux. During the intra-VTA infusion of AMPH, both extracellular DA and NE increased significantly in the VTA (270 +/- 12% and 819 +/- 40%, respectively). Meanwhile, the DA efflux in the mPFC elevated significantly. During the intra-VTA infusion of cocaine, the extracellular DA and NE in the VTA also increased (271 +/- 21% and 150 +/- 15%, respectively). However, the DA efflux decreased significantly in the mPFC. Noteworthy, the increase of extracellular NE in the VTA was much more robust via AMPH infusion, as compared with cocaine. It is suggested that AMPH and cocaine enhance the extracellular NE concentrations in the VTA in different magnitudes, which in turn contribute to discrepant profiles of distal DA efflux in the mPFC.

Neurochemical characterization of the release and uptake of dopamine in ventral tegmental area and serotonin in substantia nigra of the mouse

In the present report, fast-scan cyclic voltammetry was used to identify the monoamines that were released by electrical stimulation in mouse brain slices containing ventral tegmental area (VTA), substantia nigra (SN) -pars compacta (SNc) and -pars reticulata (SNr). We showed that voltammograms obtained in mouse VTA were consistent with detection of a catecholamine, while those in both subregions of the SN were consistent with detection of an indolamine, based on the reduction peak potentials. We used pharmacological blockade and genetic deletion of monoamine transporters to further confirm the identity of released monoamines in mouse midbrain and to assess the control of monoamines by their transporters in each brain region. Inhibition of dopamine and norepinephrine transporters by nomifensine (1 and 10 microm) decreased uptake rates in the VTA, but did not change uptake rates in either subregion of the SN. Serotonin transporter inhibition by fluoxetine (10 microm) decreased uptake rates in the SNc and SNr, but was without effect in the VTA. Selective inhibition of the norepinephrine transporter by desipramine (10 microm) had no effect in any brain region. Using dopamine transporter- and serotonin transporter-knockout mice, we found decreased uptake rates in VTA and SN subregions, respectively. Peak signals recorded in each midbrain region were pulse number dependent and exhibited limited frequency dependence. Thus, dopamine is predominately detected by voltammetry in mouse VTA, while serotonin is predominately detected in mouse SNc and SNr. Furthermore, active uptake occurs in these areas and can be altered only by specific uptake inhibitors, suggesting a lack of heterologous uptake. In addition, somatodendritic dopamine release in VTA was not mediated by monoamine transporters. This work offers an initial characterization of voltammetric signals in the midbrain of the mouse and provides insight into the regulation of monoamine neurotransmission in these areas.

Effects of acute or chronic administration of anti-migraine drugs sumatriptan and zolmitriptan on serotonin synthesis in the rat brain

Triptans are 5-HT1 receptor agonists used as anti-migraine drugs. They act primarily on meningeal blood vessels and on trigeminovascular afferents, but they may also exert central effects. We studied the regional effects of acute and chronic treatment with sumatriptan or zolmitriptan on the rate of serotonin (5-HT) synthesis in the rat brain, using the alpha-14C-methyl-L-tryptophan quantitative autoradiographic method. Sumatriptan at low (300 microg/kg, s.c.) and high (1 mg/kg) doses, as well as zolmitriptan (100 microg/kg), acutely decreased (15-40%, P < 0.05-0.001) 5-HT synthetic rate in many brain regions, including the dorsal raphe nucleus. Chronically, sumatriptan (21 days, approximately 300 microg/kg per day via osmotic minipumps) induced significant increases in the 5-HT synthesis rate in many projection areas but had no effect in the dorsal raphe nucleus. The acute effects on 5-HT synthesis rate would be compatible with activation of 5-HT1 autoreceptors that inhibit serotonin release. In contrast, the increased 5-HT synthesis rate observed after chronic sumatriptan might possibly result from a down-regulation/desensitization of 5-HT1 receptors and/or unmasking of excitatory triptan-sensitive 5-HT receptors. Overall, the present findings indicate that not only zolmitriptan but also sumatriptan affect brain serotonergic neurotransmission.

Study of the brain serotonergic system with labeled alpha-methyl-L-tryptophan

alpha-Methyl-L-tryptophan (alpha-MTrp) is an artificial amino acid and an analog of tryptophan (Trp), the precursor of the neurotransmitter serotonin (5-HT). In this article we have summarized available data, which suggest that the measurement of the unidirectional uptake of alpha-MTrp and its conversion to 5-HT synthesis rates is a valid approach for the determination of brain 5-HT synthesis rates. The main feature on which the model is based is the trapping of labeled alpha-MTrp in brain tissue. An overview of opposing opinions, which suggest that there is a need for a metabolic conversion of tracer, is also presented and discussed critically. As with all biological modeling there is likely to be room for improvements of the proposed biological model. In addition, there are a limited number of clearly defined circumstances in which the method is confounded by the metabolism of labeled alpha-MTrp via the kynurenine pathway. Nonetheless, a significant body of evidence suggests that labeled alpha-MTrp is a useful tracer to study brain 5-HT synthesis in most circumstances. Calculation of 5-HT synthesis rates depends on the plasma-free tryptophan concentration, which, according to the balance of arguments in the literature, is a more appropriate parameter than the total-plasma tryptophan. The method, as proposed by us, can be used in conjunction with autoradiographic measurements in laboratory animals, and with positron emission tomography in large animals and humans. We review studies in animals looking at the normal control of 5-HT synthesis and the way in which it is altered by drugs, as well as initial studies investigating healthy humans and patients with neuropsychiatric disorders.

Pet studies of serotonin synthesis in the human brain

The method for measuring serotonin synthesis in human brain uses [11C]alpha-methyl-L-tryptophan as a tracer and positron emission tomography. The alpha-methyl-L-tryptophan is converted to alpha-methylserotonin, which is not a substrate for monoamine oxidase and therefore accumulates in the brain. In a pilot study published recently, rates of serotonin synthesis were found to be higher in men than in women. This was due to the lower plasma free tryptophan in the women under the experimental conditions used, and does not necessarily reflect the situation in all circumstances. Acute tryptophan depletion lowered brain serotonin synthesis by 90% or more. Patients with borderline personality disorder, who exhibit emotional lability and impulsivity, may have lower brain serotonin synthesis rates than healthy controls.

Positron emission tomography of 5-HT transporter sites in the baboon brain with [11C]McN5652

[11C]McN5652 is a new radioligand specific for 5-hydroxytryptamine (5-HT; serotonin) transporters. In this study we used [11C]McN5652 to image the 5-HT transporter sites in baboon brain by positron emission tomography (PET). Dynamic PET studies were performed in three Papio anubis baboons. The animals were injected intravenously first with 11C-labeled (+)-McN5652([11C](+)McN5652), then with pharmacologically inactive enantiomer 11C-labeled (-)-McN5652 ([11C](-)McN5652); two animals received a third study with [11C](+)McN5652 after pretreatment with the specific 5-HT uptake site inhibitor fluoxetine (5 mg/kg). Initial uptake into the brain was similar for both [11C](+)McN5652 and [11C](-)McN5652. At later times (45-120 min after injection), only [11C](+)McN5652 showed a distribution characteristic for 5-HT uptake sites. In contrast, in studies with [11C](-)McN5652 and in those with [11C](+)McN5652 after 5-HT uptake site blockade with fluoxetine, 11C radioactivity concentrations were significantly lower and the distribution pattern was relatively even. The differences between [11C](+)-and (-)McN5652 were calculated for the time interval 95-125 min postinjection and used to estimate specific binding. Specific binding correlated well (r = 0.95, p < 0.001) with the known density of 5-HT uptake sites in human brain. These results indicate that [11C](+)McN5652 is suitable for PET imaging of 5-HT uptake sites in primate brain.

In vivo-synthesized radioactively labelled alpha-methyl serotonin as a selective tracer for visualization of brain serotonin neurons

To investigate the use of alpha-[3H]methyl tryptophan (alpha-[3H]MTrp) as a tracer for the in vivo study of brain serotonergic neurons, we examined whether alpha-[3H]MTrp and its metabolite alpha-[3H]methyl serotonin (alpha-[3H]M5-HT) selectively label serotonergic neurons and whether once accumulated in these neurons, the radioactive metabolite behaves like endogenous serotonin. Rats received a systemic injection of 1-5 mCi of alpha-[3H]MTrp and 24 h later their brains were immediately removed or fixed by perfusion before removal. Tissue sections in which serotonergic neurons had been immunostained for 5-HT or its synthesizing enzyme, tryptophan hydroxylase, were processed for radioautography at the light and electron microscopic level. In another group of rats, the release of radioactivity from different brain areas was studied both under basal and depolarizing conditions. In the dorsal raphe nucleus, the light microscopic examination revealed almost complete colocalization between serotonergic neurons and those that accumulated radioactivity, with a heterogeneity in the content of alpha-[3H]M5-HT among the various cells. At the ultrastructural level, immunoidentified serotonergic perikarya and dendritic processes in the dorsal raphe nucleus, as well as nerve terminals in the cerebral cortex were also found to contain alpha-[3H]M5-HT. Under basal conditions, radioactivity was released from the brainstem raphe region and from projection areas such as the striatum and hippocampus. The basal output of alpha-[3H]M5-HT increased approximately twofold after a depolarizing 50 mM KCl solution was added to the perfusion fluid. These findings suggest that newly synthesized alpha-[3H]M5-HT can be released both at somatodendritic and terminal sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Asymmetric radiosynthesis of alpha-[11C]methyl-L-tryptophan for PET studies

Asymmetric radiosynthesis of alpha-[11C]methyl-L-tryptophan has been achieved using the enantioselective [11C]methylation of the enolate of either 8-(phenylsulfonyl) or 8-acetyl substituted derivatives of dimethyl (2S.3aR, 8aS)-(+)-hexahydropyrrolo[2,3-b]indole-1,2-dicarboxylate. Reaction of the enolates generated by treatment with LDA at -78 degrees C, with [11C]methyl iodide at -78 degrees C gave in 5 min incorporation of the radiolabel of 86% for 8-phenylsulfonyl derivative and 63% for 8-acetyl derivative. The hexahydropyrrolo[2,3-b]indoles were then decyclized to the fully protected alpha-[11]methyl-L-tryptophan by treatment with trifluoroacetic acid. Removal of all the protecting groups, including the phenylsulfonyl, was achieved by reaction with 10 N NaOH at 210 degrees C in a sealed vial. Neutralization of the alkali with 10 N H2SO4 followed by purification by HPLC gave alpha-[11C]methyl-L-tryptophan with an overall radiochemical yield of 20% (uncorrected for decay) relative to the amounts of [11C]CH3I from 8-phenylsulfonyl derivative, and 9% (relative to [11C]CH3I; uncorrected for decay) from 8-acetyl derivative in a preparation time of 40 min after [11C]methyl iodide was introduced into the reaction mixture.

Effect of hypothalamic 5,7-dihydroxytryptamine lesion on the anterograde transport of serotonin as measured with labeled alpha-methyl serotonin

The anterograde axonal transport of serotonin along the medial forebrain bundle (MFB) 5 days after unilateral dorsal hypothalamic 5,7-dihydroxytryptamine (5,7-DHT) lesion was examined in rat brain. Measurements were done using in vivo synthesized radioactively labeled alpha-methyl serotonin (used here as a serotonin tracer), from i.v. injected labeled alpha-methyl-L-tryptophan, a substitute neurotransmitter and analog of serotonin. Data indicated that after destruction of the presynaptic terminals with 5,7-DHT, the rate of axonal transport of serotonin and/or the rate of serotonin synthesis in the spared and/or damaged neurons was increased. The rate of serotonin anterograde transport on the lesioned side was above 25 mm/d compared to 12.31 +/- 0.49 mm/d on the contralateral side and 12.13 +/- 0.44 mm/d in control (sham injected) rats. The difference in rate between the lesioned and control rats was highly significant; P < 0.005 (two-tailed t-test). The amount of neurotransmitter anterogradely transported along the medial forebrain bundle and/or synthesized in the neurons was 18.8 +/- 4.1 times greater on the side of the 5,7-DHT hypothalamic lesion than that on the contralateral sides or that found in the saline-injected rats. There was no difference in the radioactivity found on the left or right side of the medial forebrain bundle in the rats 5 days after lesion in the rostal midbrain with 5,7-DHT. Five days after the lesion there was also no difference in the anterograde protein transport measured by stereotaxic injection of [3H]proline into the dorsal raphe.

Synthesis of enantiomerically pure alpha-[14C]methyl-L-tryptophan

A practical method for the preparation of large amounts of enantiomerically pure alpha-[14C]methyl-L-tryptophan using the enzymatic resolution of the corresponding D,L-methyl ester is reported. The radiolabelled alpha-methyl group was introduced using the alpha-methylation of the lithium enolate of the Schiff base of L-tryptophan methyl ester. Hydrolysis of the Schiff base with 1N HCl provided the D,L-methyl ester of alpha-[14C]methyl tryptophan. Enantioselective enzymatic hydrolysis of the L-methyl ester by alpha-chymotrypsin gave the enantiomerically pure alpha-[14C]methyl-L-tryptophan. The overall yield of this preparation was 33%.

alpha-Methyl tryptophan as a tracer for in vivo studies of brain serotonin system, from autoradiography to positron emission tomography

The procedure for measuring the brain serotonin synthesis rate in living brain using labelled alpha-methyl-L-tryptophan (alpha-MTrp), a tryptophan analog, is described. This tracer could also be used for the anterograde axonal transport measurement of serotonin. One advantage of this method is that alpha-MTrp is converted in situ into a serotonin tracer, alpha-methyl serotonin, and should be distributed into the same compartments as serotonin.

Involvement of microtubules in modifications associated with cellular aging

Microtubules are ubiquitous cellular components involved in the control of cell structure and functions, such as cell division, regulation of shape and polarity, intracellular transport, etc. Consequently, any alteration affecting them in structure or function has a good chance of affecting the cell and generally leads to cell dysfunctions. This has been shown for instance, after treatment with microtubule-interacting drugs. Cellular aging is also characterized by the appearance of various cell dysfunctions, but the possible involvement of the microtubules in the aging process, although a rather tempting hypothesis, has not yet been extensively investigated. In this paper, I will first rapidly review the different components that build, organize and control the microtubules in normal cells, independently of the aging process. I will then consider the possible involvement of the microtubules in the aging process, more particularly in models of cells aging in vitro and in aging neuronal cells, which have been the most extensively investigated. There is some evidence for alterations in the microtubule organization both in cells aging in vitro and in the aging brain. But the interpretation of these data awaits further experiments, taking into account the latest progress in tubulin genetics and in microtubule biochemistry. Microtubules could also represent one of the cellular targets affected after signal transduction and could thus be involved in the resulting cellular responses. This hypothesis will be discussed, as it offers new insights into the regulation of microtubule organization, dynamics and functions in normal cells, which will be worthwhile to investigate during the aging process.

Alpha-methyltryptophan as a therapeutic agent

1. Alpha-methyltryptophan, on administration to experimental animals gives rise to cerebral alpha-methylserotonin, which substitutes for serotonin in certain behavioral and functional tests. 2. Such results are consistent with the similarity of the properties of the two indoleamines with respect to storage, uptake and release, as well as with the 5HT2-receptor agonist activity of alpha-methylserotonin. 3. It is proposed that alpha-methyltryptophan be regarded as a candidate drug for the provision of alpha-methylserotonin as a substitute for serotonin in disorders where the latter amine is thought to be deficient.