Radioprotective effect of 5-hydroxytryptamine and 5-hydroxytryptophan on mammalian cells irradiated in vitro.

The radioprotective effect in vitro of 5-hydroxytryptamine (5-HT) and 5-hydroxytryptophan (k-HTP) was studied with cultured mammalian cells of three cell lines: 5-HT-synthesizing FMA and 5-HT-non-synthesizing FM3A and B16-C2W. In these cells, the addition of 5-HT to the suspending medium induced only a weak protection or no protection at all. The increase in the 5-HT content of these cells at the time of irradiation was negligible. Pre-incubation of cells for 40 min in a 5-HTP-containing medium resulted in an elevation of the 5-HT content concomitantly with an increase in the radioresistance of FMA cells, where the DRF at 1 per cent was 1 x 8. In FM3A and B16-C2W cells such an effect was not observed. The same relationship between 5-HT content and radioresistance was also observed in FMA cells which were cultured in different densities or with reserpine. These results strongly suggest that the substance playing the main role in the induction of radioresistance in cells in vitro is the 5-HT that exists in the cells.     

Site of action of 5-hydroxytryptophan and 5-hydroxytryptamine on the hypothalamo-pituitary-adrenal system in vitro.

5-hydroxytryptamine at a concentration of 0.04 microgram/ml was able to block the ACTH release caused by hypothalamic tissue (CRF) while it was ineffective when a hypothalamic extract containing CRF was used. 5-hydroxytryptophan added to rat adrenal tissue caused a dose-dependent increase in corticosterone production. In a dose of 0.04 microgram/ml, 0.4 microgram/ml and 4.0 microgram/ml, 5-hydroxytryptophan was able to inhibit the ACTH release caused by hypothalamic tissue in vitro. However 0.04 microgram/ml was ineffective on the increase in ACTH secretion elicited by hypothalamic extract CRF. The data suggest that the inhibitory action of 5-hydroxytryptophan and 5-hydroxytryptamine is exerted at the hypothalamic level by inhibiting the release and/or synthesis of the corticotrophic releasing factor.     

Precursor utilization of 5-hydroxytryptophan for 5-hydroxytryptamine biosynthesis in isolated and perfused rabbit and rat lungs

The present study was designed to investigate whether lungs can utilize 5-hydroxytryptophan (5-HTP), formed elsewhere and transported, for the synthesis of 5-hydroxytryptamine (5-HT). [14C]5-HTP uptake was 7.7 +/- 1.1 and 3.9 +/- 0.2% by rabbit and rat lungs, respectively, after 1 h of perfusion with 10 microM [14C]5-HTP. There was an increase in the lung uptake of [14C]5-HTP when the lungs were preperfused with 0.5 mM chlorphentermine (CP) and the uptake was low when the lungs were preperfused with 0.1 mM hydroxybenzylhydrazine dihydrochloride (HBH). The perfusate concentration of 5-hydroxyindole acetic acid (5-HIAA) increased significantly (3-4 micrograms/100 mL) during rabbit lung perfusion with 10 microM [14C]5-HTP and this did not change significantly when the lungs were preperfused with 0.5 mM CP. However, 5-HT increased with time in the perfusate. 5-HT, but not 5-HIAA, was detected in the perfusate and increased with time of perfusion when the rat lungs were perfused either with 10 microM 5-HTP or with 0.5 mM CP and 10 microM 5-HTP. However, no metabolites were detected in either the rabbit lung or rat lung perfusates when they were preperfused with 0.1 mM HBH. Lung contents of 5-HT and 5-HIAA were significantly higher in the rat lungs and only 5-HIAA increased in rabbit lungs after 1 h of perfusion with 10 microM 5-HTP. Preperfusion with 0.5 mM CP resulted in a greater increase in the 5-HT content of both rabbit and rat lungs.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of 5-hydroxytryptophan and 5-hydroxytryptamine on dopamine synthesis and release in rat brain striatal synaptosomes.

The effects of 5-hydroxytryptophan (5-HTP) and serotonin (5-HT) on dopamine synthesis and release in rat brain striatal synaptosomes have been examined and compared to the effects of tyramine and dopamine. Serotonin inhibited dopamine synthesis from tyrosine, with 25% inhibition occurring at 3 μM-5-HT and 60% inhibition at 200 μM. Dopamine synthesis from DOPA was also inhibited by 5-HT, with 30% inhibition occurring at 200 μ. At 200 μM-5-HTP, dopamine synthesis from both tyrosine and DOPA was inhibited about 70%. When just the tyrosine hydroxylation step was measured in the intact synaptosome, 5-HT, 5-HTP, tyramine and dopamine all caused significant inhibition, but only dopamine inhibited soluble tyrosine hydroxylase [L-tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2] prepared from lysed synaptosomes. Particulate tyrosine hydroxylase was not inhibited by 10 μM-5-HT, but was about 20% inhibited by 200 μM-5-HT and 5-HTP. At 200 μM both 5-HT and 5-HTP stimulated endogenous dopamine release. These experiments suggest that exposure of dopaminergic neurons to 5-HT or 5-HTP leads to an inhibition of dopamine synthesis, mediated in part by an intraneuronal displacement of dopamine from vesicle storage sites, leading to an increase in dopamine-induced feedback inhibition of tyrosine hydroxylase, and in part by a direct inhibition of DOPA decarboxylation.     

On the uptake and storage of 5-hydroxytryptamine, 5-hydroxytryptophan and catecholamines by adrenal chromaffin cells and nerve endings

Summary Light-microscopic autoradiographs of the adrenal medulla at various intervals after the intravenous injection of [³H] 5-HTP, [³H] 5-HT, [³H] noradrenaline and [³H] adrenaline have been studied. The distribution of silver grains following [³H] 5-HTP uptake was found to be uniform over each of the two main cell populations, adrenaline-storing (A) cells and noradrenaline-storing (NA) cells in the adrenal medulla, but A cells were twice as active as NA cells in incorporating the isotope, a situation very similar to that found after [³H] dopa uptake. 5-HT administration resulted in a pattern resembling the distribution of [³H] noradrenaline uptake, with A cells being 4 or 5 times more active than NA cells and a gradient of activity from the periphery of the medulla inwards. However, the time-course for the loss of radioactivity was not the same for both amines: levels of 5-HT activity were not significantly reduced after one week whereas the degree of [³H] noradrenaline labelling after one week was less than 10% of that at one hour. Thus 5-HT may be bound to sites in the adrenal medulla normally occupied by noradrenaline but it would appear that the release mechanism is different. There was no evidence of 5-HT uptake by adrenal nerve endings.     

Comparison of the cardiovascular responses to intracerebroventricular administration of tryptamine, 5-hydroxytryptamine, tryptophan and 5-hydroxytryptophan in rats

General characteristics of the cardiovascular responses to intracerebroventricular (i.c.v.) injection of tryptamine, 5-hydroxytryptamine (5-HT), tryptophan and 5-hydroxytryptophan (5-HTP) were compared. Relatively small doses of tryptamine and 5-HT (0.005-0.1 microM) produced considerable, long-lasting and dose-dependent pressor effects, which sometimes were followed by prolonged depressor effects. Tryptophan (0.02-0.5 microM) and 5-HTP (0.02-0.2 microM) caused variable and usually slight, but long-lasting, vascular responses or no vascular response A large dose of tryptamine (0.5 microM) evoked variable vascular effects, while the same dose of 5-HT and 5-HTP evoked marked and prolonged depressor effects. The vascular responses to the drugs were accompanied by variable changes in heart rate. Tryptamine, 5-HT and 5-HTP, in the majority of rats, produced a bradycardia. The present study provides evidence that the cardiovascular response to i.c.v. administration of tryptamine is similar to that of 5-HT, supporting the idea that tryptamine, in addition to 5-HT, participates in the central physiological regulation of the rat cardiovascular system. The role of tryptophan and 5-HTP by themselves in this regulation, if any is of secondary importance.     

Determination of 5-hydroxytryptophan, 5-hydroxytryptamine, and 5-hydroxyindoleacetic acid in 20 rat brain nuclei using liquid chromatography with electrochemical detection

High-performance liquid chromatography with electrochemical detection is utilized for the simultaneous determination of serotonin, its precursor 5-hydroxytryptophan, and its major metabolite 5-hydroxyindoleacetic acid in nervous tissue samples. Tissue preparation required only homogenization in acidic solution and centrifugation prior to application to the chromatograph. Detection limits in the low picogram range were obtained for those indoles separated. This assay was used in combination with a micropunch dissection technique of 20 discrete rat brain nuclei to measure serotonin, its precursor, and major metabolite. The specificity of the assay was checked with pharmacological experiments aimed to increase or decrease serotonin levels. Pargyline, a monoamine oxidase inhibitor, led to a marked increase in serotonin and a decrease of 5-hydroxyindoleacetic acid while p-chlorophenylalanine, by blocking the conversion of tryptophan to 5-hydroxytryptophan, selectively depleted 5-hydroxytryptophan, serotonin, and 5-hydroxyindoleacetic acid.     

The occurrence, synthesis and metabolism of 5-hydroxytryptamine and 5-hydroxytryptophan in the cestode Hymenolepis diminuta: a high performance liquid chromatographic study

The biosynthesis and metabolism of 5-hydroxytryptamine (serotonin; 5-HT) in the cestode Hymenolepis diminuta was investigated by High Performance Liquid Chromatography (HPLC). Incubation of intact H. diminuta in [3H]tryptophan resulted in substantial radioactivity recovered in 5-HT, 5-hydroxytryptophan (5-HTP), and 5-hydroxyindoleacetic acid (5-HIAA). Furthermore, the tissue levels of 5-HT and 5-HTP, as determined by HPLC with electrochemical detection, were significantly depressed when the animals were deprived of tryptophan. On the other hand, the tissue levels of 5-HTP were significantly increased following incubation with the 5-HTP decarboxylase inhibitor m-hydroxybenzylhydrazine. The synthesis and metabolism of 5-HT are discussed in the light of 5-HT as a physiological transmitter in H. diminuta.