Clinical chemistry of serotonin and metabolites

Analyses of serotonin and other 5-hydroxyindoles, such as its precursor 5-hydroxytryptophan and major metabolite 5-hydroxyindoleacetic acid (5-HIAA), are indispensable for the elucidation of their (patho)physiological roles. In clinical chemistry attention is mainly focused on the diagnosis and follow-up of carcinoid tumours. For this most laboratories routinely measure urinary 5-HIAA. More recently, measurements of serotonin in platelets and urine have been advocated. Platelet serotonin may be the most sensitive indole marker for the detection of carcinoid tumours that secrete only small amounts of serotonin and/or its precursor 5-hydroxytryptophan. Although several chromatographic techniques have emerged for the analysis of tryptophan-related indoles, HPLC with either electrochemical or fluorometric detection have become the methods of choice for their quantification. HPLC-based methods combine selectivity, sensitivity and high precision, and enable the simultaneous investigation of several metabolically related indoles. This review aims to place the analysis of indoles in biological matrices in a biochemical, physiological and clinical perspective and highlights several important steps in their chromatographic analysis and quantification.     

A method for concurrent measurement of picomole quantities of acetylcholine, choline, dopamine, norepinephrine, serotonin, 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, tryptophan, tyrosine, glycine, aspartate, glutamate, alanine, and gamma-aminobutyric acid in single tissue samples from different areas of rat central nervous system.

A rapid and sensitive method for separation and concurrent assay of 14 compounds at the picomole level in individual rat brain parts is described. The putative amino acid neurotransmitters (aspartate, γ-aminobutyric acid, glutamate, and glycine) are extracted from a 20–30 mg portion of the tissue with 5% TCA and assayed as their respective DNP-amino acid methyl ester derivatives by glc. Four other putative neurotransmitters (acetylcholine, dopamine, norepinephrine, and serotonin) and some of their precursors and metabolites (choline, tryptophan, 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, and tyrosine) are extracted in 1n formic acid-acetone (v/v:15/85) from the remaining tissue. The lipids are removed with a heptane-chloroform (v/v:8/1) wash and the aqueous phase is dried at 37°C under N₂. The dried extract is dissolved in water (pH 4). With one portion of this solution, acetylcholine and choline are assayed using a radioenzymatic method whereas with the rest, dopamine, norepinephrine, serotonin, 5-hydroxyindoleacetic acid, 5-hydroxytryptophan, tryptophan, and tyrosine are separated with three ion exchange resins arranged in tandem. These compounds are assayed fluorometrically with modified microadaptations of previously reported methods.     

Microbore liquid chromatography with dual electrochemical detection for the determination of serotonin and 5-hydroxyindoleacetic acid in rat brain dialysates

A microbore liquid chromatographic assay with dual electrochemical detection is described for the determination of serotonin and its metabolite 5-hydroxyindoleacetic acid in rat brain dialysates. The concentration of serotonin in these samples is usually in the low nanomolar range (fmol per 20 μl range). To optimize separation and detection, several adaptations were made to the system with respect to the injection valve, flow-rate of the pump, connections between injector, column and detector, and cell volume of the detector. These aspects are discussed, as well as the procedure developed for optimal peak identification of serotonin and correct estimation of 5-hydroxyindoleacetic acid. The assay allows the measurement of basal serotonin release without the use of a re-uptake inhibitor added to the perfusion fluid.     

Changes in the serotonin system of the brains of rats genetically predisposed to catalepsy

Some changes in the brain serotonergic system were found in rats bred for predisposition to catalepsy, and in those bred for its absence. The genetic predisposition for catalepsy was found to be characterized by an increased tryptophan hydroxylase activity in the striatum, and an increased serotonin content in the midbrain. No changes in 5-hydroxyindoleacetic acid level were found. A selection for predisposition to catalepsy turned out to entail a decrease in the sensitivity of postsynaptic serotonin receptors as estimated by the "head twitch" test after 5-hydroxytryptophan administration, while a selection for the absence of catalepsy increased the sensitivity of serotonin receptors.     

Influence of Griffonia simplicifolia on male sexual behavior in rats: Behavioral and neurochemical study

The seeds of Griffonia simplicifolia Baill. are rich in 5-HTP (5-hydroxytryptophan), a direct precursor of the neurotransmitter serotonin. In the present study we investigated the influence of the plant extract on male sexual behavior. The seed extract was orally administered to Sprague-Dawley male rats at three dose levels (25, 50 and 100 mg/kg) both acutely and subchronically (daily for 9 days). Mating test with receptive female rats was performed 60 min after the acute treatment or the last dose when repetitively administered. Mount, intromission and ejaculation latencies and post-ejaculatory interval were recorded. Food intake and body weight were measured over the 9-day period of treatment. Microdialysis technique was used to detect the extracellular levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in rat brain following the acute administration of the extract dosed at 100 mg/kg. The acute treatment significantly increased mount latency (at any dosage), intromission and ejaculation latencies (at 100 mg/kg) and post-ejaculatory interval (at 50 and 100 mg/kg). On the contrary the subchronic treatment failed to exert a significant influence on copulatory behavior. The daily administration of the extract dosed at 50 and 100 mg/kg for 9 days significantly reduced food intake and body weight. Finally in the microdialysis experiments we found a dramatic increase in 5-HT and its metabolite 5-HIAA.     

A simple, sensitive and reliable assay for serotonin and 5-HIAA in brain tissue using liquid chromatography with electrochemical detection

An extremely sensitive and simple method for simultaneously measuring both serotonin and 5-hydroxyindoleacetic acid (5-HIAA) has been developed using liquid chromatography (LC) and electrochemical detection. Assay conditions are described which resolve and measure as little as 22 picograms of serotonin and its deaminated metabolite in deproteinated brain samples.     

The correlation between urinary 5-hydroxyindoleacetic acid and sperm quality in infertile men and rotating shift workers

Background  

Serotonin is a neurotransmitter that modulates a wide range of neuroendocrine functions. However, excessive circulating serotonin levels may induce harmful effects in the male reproductive system. The objective of this study was to evaluate whether the levels of urinary 5-hydroxyindoleacetic acid (5-HIIA), a major serotonin metabolite, correlate with different classical seminal parameters.     

Effect of lithium oxybutyrate on the serotonin and 5-hydroxyindoleacetic acid content in the brain of rabbits

Single administration of lithium hydroxybutyrate (10 mg/kg) to rabbits decreased serotonin and 5-hydroxyindoleacetic acid (5-HIAA) content in the caudate nucleus. The drug administration for 8 days is accompanied by mediator accumulation in the cortex, caudate nucleus, tonsils, hypothalamus, thalamus, and midbrain with parallel reduction in 5-HIAA level in these structures. 15 days of lithium hydroxybutyrate administration lead to the increase of serotonin and 5-HIAA concentration, while 28 days of administration reduced the content of mediator and its metabolite.     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

A Comparison of Biochemical Indices of 5-Hydroxytryptaminergic Neuronal Activity Following Electrical Stimulation of the Dorsal Raphe Nucleus

The activity of 5-hydroxytryptaminergic neurons has been estimated from measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of accumulation of 5-hydroxytryptophan following the administration of an aromatic L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid following the administration of a monoamine oxidase inhibitor (e.g., pargyline). The purpose of the present study was to compare these different methods under conditions of changing neuronal impulse traffic produced by electrical stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus were measured using HPLC coupled to an electrochemical detector. In each brain region, stimulation elicited an increase in the concentration of 5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic acid in pargyline-treated rats. The results o f this study indicate that although the first three methods serve as valid indices of 5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.     

Quipazine-metoclopramide inhibition of CB-154-induced prolactin suppression in rats: neurotransmitter-metabolite correlations

The ability of quipazine and metoclopramide to protect rats from CB-154-induced suppression of serum prolactin concentrations was studied. These drugs affect whole brain concentrations of dopamine and serotonin, and their major metabolites dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid. Serum prolactin concentrations have been correlated with the concentrations of the neurotransmitters and their respective metabolites. Differences in the metabolite/precursor ratios have been used to compare turnover rates of the neurotransmitters dopamine and serotonin. Increased turnover of dopamine and decreased turnover of serotonin correlate with elevated prolactin concentrations for quipazine and metoclopramide administered together. The combination of quipazine and metoclopramide protects rats against CB-154-induced prolactin suppression better than either of the drugs given alone. This study suggests that a quipazine-metoclopramide regimen may have therapeutic potential for combating ergotlike fescue and other similar toxicities observed in cattle grazing on endophyte-infected pasture grasses.     

Effect of the genotype on the serotonin and 5-hydroxyindoleacetic acid content in different sections of the mouse brain

The levels of serotonin and 5-hydroxyindoleacetic acid contents were estimated in hypothalamus, hyppocampus and midbrain of inbred mice of 12 strains. The levels of serotonin and its metabolite in various parts of brain representing different links of its serotoninergic system were shown to be genetically determined. The correlation analysis revealed that there were two, relatively autonomous genetic systems controlling biosynthesis and catabolism of serotonin in brain.     

Changes in serotonin metabolism during food satiation in the mink and its effect on predatory aggression

Effect of single and protracted alimentary satiation on predatory aggression and content of serotonin and its metabolite 5-hydroxyindoleacetic acid in the amygdalar complex and hypothalamus was studied in mink--a representative of predators. A single alimentary satiation was not accompanied by any marked changes in serotonin metabolism and predatory behaviour. A long-term alimentary satiation significantly heightened the content of the 5-hydroxyindoleacetic acid in the lateral hypothalamus and amygdala without any changes in serotonin level, testifying to a high synthesis of serotonin with its simultaneous intensive destruction. Long-term satiation also greatly increased the latencies of aggression and killing the victim. It is suggested that serotonin is one of endogenous factors controlling predatory behaviour in predators, and this control is realized in interrelation with feeding behaviour.     

Serotonin metabolism in the brain during sexual motivation and activation of the murine hypothalamo-hypophyseal-testicular system]

Serotonin metabolism was studied in male CBA mice during sexual arousal. It was shown that placement of a receptive female into a cage department separated from a male with a perforated partition, which prevented from the physical contact but allowed a male to see and smell a female, caused an elevation of serotonin metabolism. It was originally shown that 10-min female exposure produced in a male an increase in the level of the main serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the midbrain and its decrease in the hypothalamus. The catabolism coefficient (5-HIAA/serotonin ratio) also increased in the midbrain and decreased in the hypothalamus, while the serotonin content was unchanged. Longer sexual activation of male rats (for 20 min) resulted in an increase in the content of serotonin and 5-HIAA in the amygdala and olfactory bubs, while in the hippocampus only the level of 5-HIAA increased. Thus, for the first time, two stages were distinguished in male sexual arousal. They were determined according to the response of the pituitary-testicular system and involvement of serotonin in different brain regions.     

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.     

Effect of probenecid on 5-hydroxyindoleacetic acid in cisternal cerebrospinal fluid of rats with portacaval anastomosis

Portal-systemic encephalopathy (PSE) is characterized by a neuropsychiatric disorder progressing through personality changes, to stupor and coma. Previous studies have revealed alterations of serotonin and of its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in brain tissue and CSF in experimental (rat) and human PSE. Increased brain 5-HIAA concentrations could result from its decreased removal rather than to increased serotonin metabolism. In order to evaluate this possibility, CSF 5-HIAA concentrations were measured using an indwelling cisterna magna catheter technique at various times following end-to-side portacaval anastomosis in rats (the most widely used animal model of PSE) treated with probenecid, a competitive inhibitor that blocks the active transport of acid metabolites out of the brain and CSF. Following portacaval anastomosis and probenecid treatment, CSF concentrations of 5-HIAA were increased to a greater extent than in sham-operated controls. When data were expressed as per-cent baseline values, the relative increase of CSF 5-HIAA in portacaval shunted rats following probenecid treatment was not significantly different from sham-operated controls. These findings confirm that increased 5-HIAA in the CNS in experimental PSE results from increased 5HT metabolism or turnover and that the probenecid-sensitive acid metabolite carrier is intact in PSE.     

Dietary tryptophan does not alter the function of brain serotonin neurons

The hypothesis that alterations in dietary tryptophan modify the functional activity of brain serotonin-containing neurons was tested by recording the electrophysiological activity of single serotonergic cells in awake, behaving cats after meal ingestion of diets containing varying proportions of tryptophan and the neutral amino acids that compete with tryptophan for uptake into the brain. The data revealed that while the various diets produced significant changes in brain serotonin and its major metabolite, 5-hydroxyindoleacetic acid, there was no change in the activity of serotonin-containing dorsal raphe cells following meal ingestion. Furthermore, a pulse injection of tritiated labeled tryptophan following the various diets produced no significant change in the release of tritiated serotonin into the lateral ventricles, while tritiated 5-hydroxyindoleacetic acid was significantly increased. These data suggest that dietary tryptophan does not alter the functional activity of central serotonergic neurons, in contrast with current popular beliefs that such dietary manipulations alter brain function.     

Simultaneous Determination of 3,4-Dihydroxyphenylalanine, 5-Hydroxytryptophan, Dopamine, 4-Hydroxy-3-Methoxyphenylalanine, Norepinephrine, 3,4-Dihydroxyphenylacetic Acid, Homovanillic Acid, Serotonin, and 5-Hydroxyindoleacetic Acid in Rat Cerebrospinal Fluid and Brain by High-Performance Liquid Chromatography with Electrochemical Detection

A method using reversed-phase ion-pair high-performance liquid chromatography with electrochemical detection for the simultaneous determination of tryptophan (TRP), 3,4-dihydroxyphenylalanine (DOPA), and their metabolites in whole brain, small-brain parts, and cerebrospinal fluid of rats has been developed. The sample preparation requires only homogenization in perchloric acid and centrifugation before injection onto the column. With a LiChrosorb RP-18 (10 micrometer) column and a mobile phase consisting of a phosphate (NaH2PO4, 0.1 M)-methanol mixture with octylsulfonate (2.6 x 10(-3) M) at pH 3.35 and 26 degrees C, the separation of DOPA, dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 4-hydroxy-3-methoxyphenylalanine, TRP, 5-hydroxytryptophan (5-HTP), serotonin, and 5-hydroxyindoleacetic acid was achieved. The method has been applied to study the effect of alpha-monofluoromethyldopa alone and in combination with L-DOPA or L-5-HTP, on the catechol and 5-OH indole levels in brain and CSF of the rat.     

A comparative study of serotonin metabolism in helminths and their hosts]

Studies have been made on the content of the main metabolite of serotonin, namely 5-hydroxyindoleacetic acid in parasitic worms from various classes. It was shown that 5-hydroxyindoleacetic acid level is lower than that of serotonin, which is taken as an indication of low catabolism of serotonin in worms. This tendency was observed in helminths from different taxonomic, ecological and age groups invading media with both low and high levels of serotonin metabolism.     

Streptozotocin-Induced Diabetes Reduces Brain Serotonin Synthesis in Rats

The rate of brain 5-hydroxytryptamine (serotonin) synthesis and turnover in streptozotocin-diabetic rats was assessed using three separate methods: the rate of 5-hydroxytryptophan accumulation following decarboxylase inhibition with Ro 4-4602; the decline in 5-hydroxyindoleacetic acid levels following monoamine oxidase inhibition with pargyline; and the rate of 5-hydroxyindoleacetic acid accumulation following blockade of acid transport with probenecid. Each of the three methods revealed that 5-hydroxytryptamine synthesis and turnover is decreased by 44-71% in diabetic rats with plasma glucose levels of between 500 and 600 mg%. In addition, the levels of free and bound plasma tryptophan were measured and the levels of the free amino acid were found to be the same in control and diabetic rats. Since diabetic rats exhibit a 40% decrease in brain tryptophan, the free tryptophan level in plasma does not predict brain tryptophan levels in diabetic rats. These data are discussed within the context of psychiatric disturbances experienced by diabetic patients.