Intra- and inter-individual relationships between central and peripheral serotonergic activity in humans: a serial cerebrospinal fluid sampling study

Data are lacking concerning the longitudinal covariability and cross-sectional balance between central and peripheral 5-HIAA concentrations in humans and on the possible associations between tobacco smoking or post-traumatic stress disorder (PTSD) and CSF and plasma 5-HIAA concentrations. Using serial cerebrospinal fluid (CSF) and blood sampling, we determined the concentrations of 5-HIAA in CSF and plasma over 6 h, and examined their relationships in healthy volunteers and patients with PTSD-both smokers and nonsmokers. Patients with PTSD and healthy volunteers had very similar CSF 5-HIAA concentrations. Significant and positive correlations between CSF and plasma 5-HIAA levels were observed within individuals, but this CNS-peripheral 5-HIAA relationship was significantly reduced in smokers (nonsmokers: mean r = 0.559 +/- 0.072; smokers: mean r = 0.329 +/- 0.064 p < 0.038). No significant cross-sectional, interindividual correlation of mean CSF and mean plasma 5-HIAA was seen (r = 0.094). These data show that changes in CSF 5-HIAA levels within an individual over time are largely reflected in plasma 5-HIAA, albeit significantly less so in smokers. The present results therefore suggest that clinically, longitudinal determination of plasma 5-HIAA concentrations within an individual patient can be used to make inferences about relative changes in integrated CSF 5-HIAA concentrations. However, plasma 5-HIAA concentrations provide no significant information about absolute levels of the serotonin metabolite in the CSF.     

CSF 5-HIAA concentration as an early screening tool for predicting significant life history outcomes in female specific-pathogen-free (SPF) rhesus macaques (Macaca mulatta) maintained in captive breeding groups

We examined relationships among cerebrospinal fluid (CSF) concentrations of the major serotonin metabolite (5-HIAA, 5-hydroxyindoleacetic acid) and significant medical and behavioral outcomes for female rhesus macaques. Based on earlier findings with males we predicted that low CSF 5-HIAA concentrations would be associated with a range of negative life history outcomes in our captive specific-pathogen-free (SPF) breeding colony. We found that the mean CSF 5-HIAA concentration among animals that died over the course of the study period was significantly lower than among animals that survived. Further examination indicated an inverse relationship between CSF 5-HIAA concentration and number of treatments for illness, further suggesting a link between serotonergic functioning and overall animal health. Examination of behavioral data indicated that individuals with low CSF 5-HIAA concentrations were more often the targets of aggressive bouts than were individuals with high CSF 5-HIAA concentrations. Finally, we found a positive relationship between CSF 5-HIAA concentration and infant survivorship. These results suggest negative life history consequences of impaired serotonergic functioning in captive female rhesus macaques, and indicate that CSF 5-HIAA concentration sampled early in life may provide a useful tool in facilitating colony management decisions concerning utilization of scarce and increasingly valuable non-human primate resources.     

Serotonin metabolism in the rat brain during water deprivation and hydration

Water deprivation (WD) decreased the serotonin (5-HT) level and significantly increased the 5-hydroxyindoleacetic acid (5-HIAA) in the rat midbrain and hypothalamus, the catabolic 5-HIAA/5-HT ratio increasing three-fold. Hydration (H) produced a moderate increase in the 5-HT and 5-HIAA levels in the hypothalamus with no changes in the 5-HIAA/5-HT ratio. Hydration exerted no significant effect upon the 5-HT level and metabolism in the midbrain. A two-fold increase of corticosterone concentration in water deprivation and its decrease in hydration were shown to occur in peripheral blood plasma.     

The effects of the anticonvulsant valproic acid on cerebral indole amine metabolism.

The effects of valproic acid (500 mg/kg, ip, 1 h prior to testing) on indole amine metabolism were studied in rats by measurement of the contents of tryptophan, 5-hydroxytryptophan (5-HTP), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the cerebral hemisphere. Tryptophan and 5-HIAA levels were increased, whereas 5-HTP and 5-HT remained unchanged. Furthermore, valproic acid failed to alter the levels of 5-HTP and DOPA, 5-HT and DA, and 5-HIAA in animals pretreated, respectively, with 3-hydroxybenzyl hydrazine (a decarboxylase inhibitor), pargyline (a monoamine oxidase inhibitor), or probenecid (a compound which blocks 5-HIAA transport out of the brain and cerebrospinal fluid). These results militate against the possibility that valproic acid alters the rate of tryptophan hydroxylation or the synthesis of 5-HT. However they do support the concept that valproic acid increases brain 5-HIAA by inhibition of the transport mechanism which removes 5-HIAA from the brain.     

Plasma 5-Hydroxyindoleacetic Acid as an Indicator of Monoamine Oxidase-A Inhibition in Rat Brain and Peripheral Tissues

We have examined the changes induced by the monoamine oxidase (MAO; EC 1.4.3.4) inhibitors tranylcypromine, clorgyline, and deprenyl on MAO activity and 5-hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in rat brain and blood (plasma and whole blood). The decreases of MAO-A activity observed in the liver and lungs after different doses of clorgyline or tranylcypromine correlated significantly (r > 0.80 in all cases) with the decline of plasma 5-HIAA. This was unaffected by 0.25 and 5 mg kg^?1 of deprenyl, indicating that 5-HT was deaminated exclusively in the periphery by MAO-A. It is interesting that very potent and significant correlations (r > 0.75) were found between plasma 5-HIAA and MAO-A activity, 5-HIAA and 5-HT content in brain tissue. These results suggest that plasma 5-HIAA can be used confidently as a peripheral indicator of the inhibition of MAO-A in brain. This may represent a favorable alternative to the analysis of 5-HIAA in CSF in psychiatric patients undergoing antidepressant treatment with nonspecific MAO inhibitors or with the new selective MAO-A inhibitors.     

ON THE ELIMINATION OF CENTRALLY FORMED 5-HYDROXYINDOLEACETIC ACID BY CEREBROSPINAL FLUID AND URINE

Abstract— In rats, the release of centrally formed 5-hydroxyindoleacetic acid (5-HIAA) into brain and spinal cord perfusates and urine was measured. Data from spinal cord perfusion of anaesthetized rats indicate that more than about 36% of the spinal production (122ng/h) of 5-HIAA is eliminated via the cerebrospinal fluid (CSF). More than 30% of cerebrally formed 5-HIAA (265.0 ng/h) was calculated to be released into ventricular-cisternal perfusates. Of the total amount of 5-HIAA found in the urine we estimated that about 8% originates in the central nervous system (CNS).
In probenecid treated animals there was a substantial increase in the outflow of 5-HIAA in both perfusion systems. In the combined perfusion experiments no proportional increase of the cerebral contribution to the cisternal outflow was found after probenecid. Our data indicate that a significant proportion of centrally formed 5-HIAA is eliminated by the CSF. No evidence was found for an increased contribution of cerebral 5-HIAA to lumbar CSF after application of the probenecid test. Urinary levels of 5-HIAA do not reflect quantitatively central 5-hydroxytryptamine metabolism.     

Urinary excretion of 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol after oral loading with serotonin.

The urinary excretion patterns of the serotonin (5-hydroxytryptamine; 5-HT) metabolites 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophol (5-HTOL) were examined after ingestion of bananas, a food rich in 5-HT. The bananas contained on an average 25 micrograms 5-HT/g pulp. Both urinary 5-HIAA and 5-HTOL increased markedly (15- to 30-fold) shortly after eating 3-4 bananas, with the highest concentrations found in urine specimens collected after 2-4 h, and did not return to normal until after 8-10 h. The excretion of 5-HIAA increased from a control mean value of 3.9 mg/24 h to 12.7 mg/24 h, when conventional diets were supplemented with 3-4 bananas. The corresponding results for 5-HTOL were 16.8 micrograms/24 h and 60.7 micrograms/24 h, respectively. Of the banana-derived 5-HT ingested, 60-80% was recovered in the urine as 5-HIAA and only 0.3-0.5% as 5-HTOL. However, since both the time-course and relative increase in 5-HTOL was similar to that of 5-HIAA, there was no effect on the urinary 5-HTOL to 5-HIAA ratio. By contrast, acute alcohol consumption produced a considerable elevation of this ratio.     

Correlations between aminergic metabolites simultaneously obtained from human CSF and brain

Brain and cerebrospinal fluid (CSF) levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were simultaneously measured in 48 individuals at autopsy. Concentrations of 5-HIAA and HVA in the cerebral cortex were positively correlated with their levels in the CSF for the same individual. Additionally a positive correlation was observed between postmortem CSF levels of 5-HIAA and HVA and a significant concentration gradient for both metabolites was observed in serial fractions of CSF. These findings suggest that determinations of 5-HIAA and HVA in CSF from living individuals may reflect brain metabolite levels as well as the functional activity of these specific neuronal systems.     

Measurement of Serotonin Turnover Rate in Rat Dorsal Raphe Nucleus by In Vivo Electrochemistry

5-Hydroxytryptamine (5-HT; serotonin) turnover rate in dorsal raphe nucleus of the urethane-anesthetized rat was estimated by using the in vivo electrochemical detector to measure the decay of extraneuronal 5-hydroxyindole acetic acid (5-HIAA) after monoamine oxidase inhibition. Carbon paste electrodes were scanned by semiderivative voltammetry and revealed two peaks: one at +0.15 V and the other at +0.25 V. The higher potential peak is composed primarily of the 5-HT metabolite 5-HIAA. After administration of pargyline, 75 mg/kg i.p., this peak declined exponentially. Regression analysis of these data by an exponential decay model yielded the fractional rate constant 0.82 +/- 0.06 h-1 (mean +/- SEM). This rate constant of 5-HIAA disappearance measured by in vivo electrochemistry is identical to the rate constant found by others measuring 5-HIAA disappearance by direct tissue assay methods. In animals not treated with pargyline, tissue 5-HIAA concentrations in the dorsal raphe nucleus were measured by HPLC with electrochemical detection. The average 5-HT turnover rate calculated as the product of the fractional rate constant and steady-state tissue 5-HIAA concentration was 12.6 nmol/g/h. These results demonstrate that electrochemical detection of extraneuronal 5-HIAA combined with monoamine oxidase inhibition can be used to measure neurotransmitter turnover in vivo in a discrete brain region.     

Simultaneous Effects of p-Chloroamphetamine, d-Fenfluramine, and Reserpine on Free and Stored 5-Hydroxytryptamine in Brain and Blood

The effects of acute treatment with p-chloramphetamine, d-fenfluramine, and reserpine on intracellular (brain tissue and whole blood) and extracellular (CSF and platelet-free plasma) compartments of 5-hydroxytryptamine (5-HT) in the brain and blood of the same rats have been examined. These treatments affected 5-HT in brain tissue and whole blood similarly (r = 0.823). Reserpine significantly reduced both intracellular pools at 2 and 24 h. p-Chloroamphetamine and d-fenfluramine were more effective on brain tissue 5-HT. The concentration of 5-HT in CSF was significantly increased by all treatments. p-Chloroamphetamine induced a dramatic 70-fold increase of CSF 5-HT, paralleling a 42% decrease in brain tissue. d-Fenfluramine significantly increased CSF 5-HT to 212% of controls and reduced whole brain 5-HT (-23%). The effects of p-chloroamphetamine and d-fenfluramine on 5-HIAA in brain, CSF, and plasma were nonsignificant. Individual values of 5-hydroxyindoleacetic acid (5-HIAA) in CSF and brain were highly correlated (r = 0.855), indicating that CSF 5-HIAA reflects well the concentration of 5-HIAA in brain tissue. Yet the intra- and extracellular concentrations of 5-HIAA were unrelated to the 5-HT changes. This indicates that CSF 5-HIAA does not reflect the active (extracellular) compartment of 5-HT in brain.     

Regionally specific effects of diazepam on brain serotonin metabolism in rats: Sustained effects following repeated administration

The effects of single (1mg/kg) and repeated (1mg/kg 2¹ daily for 4 days) diazepam administration are investigated on brain regional 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxy indoleacetic acid (5-HIAA) concentration in rats. Daily treatment decreased food intakes but body weights did not decrease. Administration of diazepam (1mg/kg) to 4 day sahne injected rats on the 5th day decreased 5-HT levels in the hippocampus and increased it in the hypothalamus. 5-HIAA levels were increased in the striatum and decreased in the hypothalamus. 4 day diazepam injected rats injected with sahne on the 5th day also exhibited silmilar changes of 5-HT and 5-HIAA. Cortical levels of 5-HIAA were also smaller in these rats. Administration of diazepam to 4 day diazepam injected rats again decreased 5-HT in the hippocampus and 5-HIAA in the hypothalamus. 5-HT and 5-HIAA were both decreased in the striatum. Regionally specific effects of diazepam on brain serotonin metabolism are discussed in relation to their possible functions.     

Calcium-Dependent, Tetrodotoxin-Sensitive Stimulation of Cortical Serotonin Release After a Tryptophan Load

The effect of intraperitoneal administration of tryptophan (50, 100, or 200 mg/kg) on extracellular concentrations of tryptophan, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) was studied in the cortex of freely moving rats by transcerebral dialysis. Rats were implanted with dialysis probes in the frontal cortex, and experiments were performed 24 h later. Tryptophan, 5-HT, and 5-HIAA were quantified in 20-min samples of dialysate by HPLC with electrochemical detection after separation on reverse-phase columns. Tryptophan administration resulted in a significant increase of tryptophan, 5-HT, and 5-HIAA levels in dialysates. The maximal increase of 5-HT and 5-HIAA output was approximately 150% over basal values. Perfusion with Ringer's solution containing tetrodotoxin (1 microM) reduced 5-HT output by 90% and prevented the increase of 5-HT and 5-HIAA content after 100 mg/kg of tryptophan. Similar results were obtained after perfusion with Ringer's solution without Ca2+. The results indicate that a tryptophan load stimulates the physiological release of 5-HT.     

Simultaneous determination of 5-hydroxyindoleacetic acid and 5-hydroxytryptamine in urine samples from patients with acute appendicitis by liquid chromatography using poly(bromophenol blue) film modified electrode

The fabrication and application of a novel electrochemical detection (ED) system with a poly(bromophenol blue) (PBPB) film chemically modified electrode (CME) for high performance liquid chromatography (HPLC) were described. The electrochemical behaviors of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) at this CME were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the PBPB CME efficiently exhibited electrocatalytic effect on the current responses of 5-HT and 5-HIAA with relatively high sensitivity, stability and long life of activity. In HPLC-ED, the two analytes had good and stable current responses at the CME and their linear ranges were over four orders of magnitude (R> or =0.9992) with the detection limits being 0.25 nmol L(-1) for 5-HT and 0.50 nmol L(-1) for 5-HIAA. The application of this method for the determination of 5-HT and 5-HIAA in urine samples from patients with acute appendicitis (AA) was satisfactory.     

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.     

Regional Distribution of Extracellular 5-Hydroxytryptamine and 5-Hydroxyindoleacetic Acid in the Brain of Freely Moving Rats

The extracellular concentrations of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been determined in six brain areas of awake rats (frontal cortex, striatum, hypothalamus, hippocampus, inferior colliculus, and raphe nuclei) using intracerebral microdialysis. The extracellular levels of 5-HT showed no significant differences among the brain regions studied. The tissue levels of 5-HT and 5-HIAA as well as the extracellular concentration of 5-HIAA were significantly higher in raphe nuclei. The regional distribution of tissue and extracellular 5-HIAA were very similar, suggesting that extracellular 5-HIAA depends mainly on the output from the intracellular compartment. On the other hand, extracellular 5-HT and tissue 5-HT showed a different distribution pattern. The tissue/extracellular ratio for 5-HT ranged from 739 in frontal cortex to 2,882 in raphe, whereas it only amounted to 1.8-3.6 for 5-HIAA. The relationship between the present results and the density of 5-HT uptake sites in these areas is discussed.     

Additive effects of apomorphine and clonidine on serotonin neurons in the dorsal raphe

Effects of apomorphine (APO) and clonidine (CLON) on the mesostriatal and mesolimbic serotonergic systems were examined in the present study. Both drugs selectively elevated serotonin (5-HT) concentrations in the dorsal raphe and the striatum without significantly altering 5-HT measures in the median raphe and the hippocampus. Apomorphine also increased tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) levels in the dorsal raphe and 5-HIAA level in the striatum. Clonidine did not markedly alter tryptophan and 5-HIAA measures, while it decreased 5-HT turnover rate in both region, as indicated by the ratio of 5-HIAA/5-HT levels. Co-administration of APO and CLON, at doses of each drug exerted maximum effects on 5-HT alone, produced an additive effect on 5-HT in the dorsal raphe, while their effects on 5-HT and 5-HIAA in the striatum were counteracting each other. Effects of APO on 5-HT and 5-HIAA were attributed to the elevation of 5-HT precursor tryptophan, while effects of CLON on 5-HT and 5-HIAA were due to a decreased rate of 5-HT turnover. Therefore, the present results support the hypothesis that the additive effects of APO and CLON on dorsal raphe 5-HT are mediated through different receptors and neuropharmacological mechanisms.     

Effects of β-endorphin on brain serotonin metabolism

Human β-endorphin (15 μg) administered intracisternally increased concentrations of serotonin (5HT) and its metabolite, 5-hydroxyindoleacetic. acid (5-HIAA), in brain stem and hypothalamus and decreased 5-HIAA concentrations in hippocampus. These data are compatible with the hypothesis that β-endorphin increases 5HT turnover in brain stem and hypothalamus and decreases 5HT turnover in hippocampus. β-endorphin increased in brain stem and hypothalamus and decreased in hippocampus the rate of pargyline-induced decline of 5-HIAA. β-endorphin decreased the rate of pargyline-induced accumulation of 5HT in all these brain regions. The probenecid-induced accumulation of 5-HIAA in brain stem was decreased by β-endorphin. These data are compatible with the hypothesis that β-endorphin increases release of 5HT from neurons in brain stem and hypothalamus and decreases release of 5HT from neurons in hippocampus. The data require further a hypothesis that β-endorphin either decreases 5HT reuptake in these three brain regions or increases 5-HIAA egress from brain.     

Production of 5-hydroxyindoleacetic acid from serotonin by cultured endothelial cells

Endothelial cells cells from bovine aorta and human umbilical vein and fibroblasts from human foreskin were cultured and subsequently evaluated for ability to metabolize serotonin (5-HT) to 5-hydroxyindoleacetic acid (5-HIAA). Cells were incubated for three hours with 4 X 10(-6) M [14C] 5-HT creatinine sulfate. [14C] 5-HIAA was separated from labeled 5-HT by column chromatography and measured for scintillation counting. Production of 5-HIAA by bovine aorta cells was 39.0+/-7.5 (S.E.M., n=6) nmoles per 10(9) cells per hour. Production of 5-HIAA was markedly inhibited by the presence of 10(-4) M iproniazid (an inhibitor of monoamine oxidase) or 10(-4) M imipramine (an inhibitor of amine transport). 5-HIAA was the only product of 5-HT metabolism detected by thin layer chromatography. Production of 5-HIAA by human umbilical vein endothelial cells was 5.4+/-2.0 nmoles per 10(9) cells per hour (n=5) and by human foreskin fibroblasts was 3.9+/-1.4 nmoles per 10(9) cells per hour (n=5). The results obtained during incubation in the presence and absence of inhibitors indicate that bovine aorta endothelial cells maintained in tissue culture are able to transport serotonin with subsequent production of 5-HIAA. By contrast, human umbilical vein endothelial cells and fibroblasts exhibited relatively low rates of 5-HT uptake and metabolism.     

Serotonergic influences on life-history outcomes in free-ranging male rhesus macaques

Several studies have demonstrated that nonhuman primate males with low cerebrospinal fluid (CSF) levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) exhibit antisocial behavior patterns. Included in these deleterious patterns are impulse control deficits associated with violence and premature death. No studies to date have longitudinally studied the long-term outcome of young subjects with low CSF 5-HIAA concentrations as they mature into adults. In this study we examined longitudinal relations among serotonergic and dopaminergic functioning, as reflected in CSF metabolite concentrations, aggression, age at emigration, dominance rank, and mortality in free-ranging rhesus macaque (Macaca mulatta) males. Our results indicate long-term consistency of individual differences in levels of 5-HIAA in CSF in the subject population from the juvenile period of development through adulthood. We found a significant negative correlation between 5-HIAA concentrations measured in juveniles and rates of high-intensity aggression in the same animals as adults. Further, CSF 5-HIAA concentrations were lower in juveniles that died than in animals that survived. For the young animals that migrated there was a positive correlation between CSF 5-HIAA concentration and age at emigration, whereas for the animals that remained in their troop until later in sexual maturity there was a negative correlation between CSF 5-HIAA concentration and age of emigration. After animals emigrated to a new troop, social dominance rank in the new troop was positively correlated with early family social dominance rank, but inversely correlated with juvenile CSF 5-HIAA concentrations. Taken together, our findings suggest that males with low central serotonin levels early in life delay migration and show high levels of violence and premature death, but the males that survive achieve high rank. These findings indicate that longitudinal measures of serotonergic and dopaminergic functioning are predictive of major life-history outcomes in nonhuman primate males. Low concentrations of CSF 5-HIAA are associated with negative life-history patterns characterized by social instability and excessive aggression, and positive life-history patterns characterized by higher dominance rank.     

Evidence for involvement of 5-hydroxytryptamine(1B) autoreceptors in the enhancement of serotonin turnover in the mouse brain following repeated treatment with fluoxetine

The effect of repeated treatment with the selective serotonin reuptake inhibitor fluoxetine on synthesis and turnover of 5-hydroxytryptamine (5-HT) was studied in the mouse brain in vivo. The concentration of 5-hydroxytryptophan (5-HTP), 5-hydroxyindoleacetic acid (5-HIAA) and 5-HT was measured in hypothalamus, hippocampus and frontal cortex after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD 1015). Fluoxetine 6.9 mg/kg s.c. was injected once daily for three weeks. Three days after the final daily injection of fluoxetine 5-HT synthesis (5-HTP accumulation) and turnover (5-HIAA/5-HT ratio) were significantly enhanced compared with saline-treated mice. The 5-HIAA/5-HT ratio was already significantly elevated after 3 days of fluoxetine treatment and continued to increase during treatment for 2-3 weeks. The increase in 5-HIAA/5-HT ratio was considerably larger (150-200% of controls) than the increase in 5-HTP accumulation (110-120%), which reached significance only after 3 weeks of treatment. The increase in 5-HT synthesis may be secondary to that of the turnover. The 5-HIAA/5-HT ratio returned to control values after a 14 days washout period. Simultaneous treatment with the long-acting 5-HT(1B)-receptor antagonist, SB 224289 for 14 days counteracted the fluoxetine-induced increase in 5-HIAA/5-HT ratio that indicates involvement of 5-HT(1B) autoreceptors in the development of this increase. It is proposed that the fluoxetine-induced enhancement of 5-HT turnover was evoked by the long-lasting stimulation of 5-HT(1B) autoreceptors that resulted in an intraneuronal compensatory adaptation of the basal 5-HT release.