Monoamine Synthesis and Concentration in Neonatal Rat Brain During Hypercapnia and Recovery

One-day-old rats were exposed to a gas mixture of 15% CO2-21% O2-64% N2 for a 30-min period. Monoamine synthesis in whole brain was measured during, and at various intervals after, hypercapnia by estimating the accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after inhibition of aromatic L-amino-acid decarboxylase with NSD 1015. Endogenous concentrations of tyrosine, dopamine (DA), noradrenaline (NA), tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured at the same intervals. Exposure to CO2 induced an increased synthesis of catecholamines and 5-HT. Further, an increase in DA concentration was seen during hypercapnia, while NA and 5-HT were unchanged. After the CO2 exposure the increased in vivo synthesis rates of catecholamines and 5-HT were rapidly normalized, as was the endogenous DA concentration. A slight increase in 5-HT and 5-HIAA concentrations was seen immediately after CO2 exposure. These results indicate that in neonatal animals, hypercapnia induces changes in central monoamine neurons, primarily an increased synthesis. These alterations may be relevant to some physiological changes seen during CO2 exposure, such as the alteration in central respiratory performance.     

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.     

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.     

Effect of orally administered l-tryptophan on serotonin,melatonin, and the innate immune response in the rat

To assess the effects of external administration of L-tryptophan on the synthesis of serotonin and melatonin as well as on the immune function of Wistar rats, 300 mg of the amino acid were administered through an oral cannula either during daylight (08:00) or at night (20:00) for 5 days. Brain, plasma, and peritoneal macrophage samples were collected 4 h after the administration. The accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used to measure the rate of tryptophan hydroxylation in vivo. Circulating melatonin levels were determined by radioimmunoassay, and the phagocytic activity of macrophages was measured by counting, under oil-immersion phase-contrast microscopy, the number of particles ingested. The results showed a diurnal increase (p < 0.05) in the brain 5-HTP, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindolacetic acid (5-HIAA) of the animals which had received tryptophan at 08:00 and were killed 4 h later. In the animals which received tryptophan during the dark period, the 5-HT declined but the 5-HT/5-HIAA ratio remained unchanged. There was also a significant increase (p < 0.05) in nocturnal circulating melatonin levels and in the innate immune response of the peritoneal macrophages in the animals which had received tryptophan at 20:00. The results indicated that the synthesis of serotonin and melatonin, as well as the innate immune response, can be modulated by oral ingestion of tryptophan.     

DECARBOXYLATION OF EXOGENOUS l-5-HYDROXYTRYPTOPHAN AFTER DESTRUCTION OF THE CEREBRAL RAPHE SYSTEM

Abstract— l -5-Hydroxytryptophan ( l -5-HTP) was administered intravenously to rats (12 mg/kg) after inhibition of the peripheral aromatic l -amino acid decarboxylase with l -α-hydrazino-α-methyl dopa (MK 486). The accumulation of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid in the cerebral cortex was measured 1, 2 and 4 h after injection of 5-HTP with automated assay techniques. Besides controls two groups of rats were studied: rats after inhibition of tryptophan-5-hydroxylase with p -chlorophenylalanine (pcpa) and subjects with a chronic lesion in the area of the raphe nucleus. The net accumulation of both measured 5-hydroxyindoles was diminished in rat cerebral cortex after degeneration of 5-HT containing nerve endings, compared with control animals and pcpa-treated rats. These results indicate that the formation of 5-HT in the cerebral cortex from exogenous l -5-HTP, after inhibition of the peripheral aromatic amino acid decarboxylase, occurs predominantly in 5-HT containing nerve endings possibly by a specific 5-HTP-decarboxylating enzyme.     

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.     

TRYPTOPHAN LOADING: CONSEQUENT EFFECTS ON THE SYNTHESIS OF KYNURENINE AND 5-HYDROXYINDOLES IN RAT BRAIN

Abstract— Tryptophan loading of rats resulted in a continuous non-linear uptake of l -tryptophan from plasma into the brain. The optimum tryptophan load for increasing cerebral 5-hydroxytryptamine (5-HT) level was 25 mg/kg. Above this, there was a gradual decrease both in the levels and synthesis of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) as assessed from simultaneous intraperitoneal or intraventricular injections of l [¹⁴C]tryptophan. A 5–10 fold increase in cerebral tryptophan produced a limited stimulation of 5-HT synthesis. When the cerebral tryptophan level reached 1 ± 10 -4 , substrate inhibition in vivo of the tryptophan monooxygenase (tryptophan-5-hydroxylase) but not of the indoleamine-2,3-dioxygenase occurred. Cerebral synthesis of kynurenine increased linearly with increasing tryptophan load. At a plasma ratio of 50:1 tryptophan to kynurenine, tryptophan loading interfered with the entry of peripheral kynurenine. Tryptophan loading also increased the efflux of 5-hydroxyindoles from the brain. One hour after intraperitoneal injection of l -kynurenine sulfate (5 mg/kg) into rats, there was a shift in the plasma ratio of l -tryptophan to l -kynurenine to 4:1. In these rats, a 20% reduction of cerebral tryptophan was noted.     

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.     

Evidence for Differing Origins of the Serotonergic Innervation of Major Cerebral Arteries and Small Pial Vessels in the Rat

We have studied the nature and origin of the serotonergic innervation of two distinct anatomical cerebrovascular compartments, namely, small pial vessels and major cerebral arteries, in the rat. To this end, the levels of serotonin [5-hydroxytryptamine (5-HT)] and 5-hydroxyindoleacetic acid (5-HIAA) were measured by HPLC in both cerebrovascular compartments after either bilateral sympathectomy or destruction of the ascending serotonergic pathways, which originate from the raphe nuclei. We first showed that the small pial vessel samples were not contaminated by underlying cortical tissues through the use of an immunohistochemical approach that revealed the glia limitans, the most superficial cortical layer. Superior cervical ganglionectomy caused a marked decrease in noradrenaline concentrations in major cerebral arteries (-77%), although the reduction was less pronounced (-34%) in small pial vessels. Sympathectomy decreased by 33% 5-HT concentrations in the major cerebral arteries but was without effect on 5-HT levels in the small pial vessels. Destruction of the ascending serotonergic pathways (via local administration of 5,7-dihydroxytryptamine into the ventral tegmental area) produced a dramatic fall in 5-HT and 5-HIAA concentrations in both vascular compartments. To establish the authenticity of the serotonergic innervation, the synthesis of 5-HT [as assessed by measuring the accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition] was measured in the two vascular beds under control conditions and after destruction of the ascending serotonergic pathways. The rate of accumulation of 5-HTP was higher in the small pial vessels than in major cerebral arteries, an observation that indicates an important de novo synthesis of 5-HT in small pial vessels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polyunsaturated fatty acids (PUFA) regulate neurotransmitter contents in rat brain

The effects of feeding of 6-propylthiouracil (6-PTU) and polyunsaturated fatty acids (PUFA) independently and in combination and administration (ip) of a single dose of triiodothyronine (T3) (2.5 microg/100 g body wt) along with feeding of 6-PTU and PUFA were studied in rat brain. Dopamine (DA), 5-hydroxytryptophan (5-HTP), serotonin (5-HT), 5-hydroxy indole acetic acid (5-HIAA), norepinephrine (NE) and epinephrine (EPI) contents were assayed in the hypothalamus and cerebral cortex regions. It was found that 6-PTU feeding resulted in decrease in dopamine, 5-HT, 5-HTP and 5-HIAA in both regions. In animals fed with PUFA followed by administration of T3, the DA level was found normal.     

Influence of Repeated Levodopa Administration on Rabbit Striatal Serotonin Metabolism,and Comparison Between Striatal and CSF Alterations

Parkinson's disease (PD) is characterized by decreased striatal dopamine, but serotonin (5-HT) is also reduced. Because 5-HT decreases following a single levodopa injection, levodopa has been suggested to contribute to PD's serotonergic deficits. However, in a recent study, rat striatal serotonin levels were reported to increase following 15-day levodopa administration. To address this issue, we administered levodopa (50 mg/kg) to rabbits for 5 days, then measured serotonin, its precursors tryptophan and 5-hydroxytryptophan (5-HTP), and its major metabolite 5-hydroxyindole-acetic acid (5-HIAA) in striatum and CSF. Striatal serotonin and tryptophan were unchanged, while 5-HTP and 5-HIAA increased 4- and 7-fold, respectively. CSF 5-HTP and 5-HIAA were also significantly increased. In levodopa-treated animals, 5-HTP concentrations were moderately correlated (r = 0.679) between striatum and CSF, while weak correlations were present between striatal and CSF concentrations of both serotonin and 5-HIAA. These results suggest that repeated levodopa treatment increases striatal serotonin turnover without changing serotonin content. However, levodopa-induced alterations in striatal serotonin metabolism may not be accurately reflected by measurement of serotonin and 5-HIAA in CSF.     

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 injections of D,L-5-hydroxytryptophan on the efflux of endogenous serotonin and 5-hydroxyindoleacetic acid from a synaptosomal fraction.

Abstract— The effects of i.p. injections of SO mg/kg d,l-5-hydroxytryptophan (5-HTP) and saline alone on the in uitro release of endogenous serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were studied using preparations of axon terminals (P₂ isolated from the telencephalon of rats. The level of 5-HT was 2-fold greater and the level of 5-HIAA was 5-fold greater in the P₂ fraction isolated from rats given the d,l-5-HTP injection than from rats given saline injections. At 37°C the in vitro efflux of 5-HT and 5-HIAA from the P₂ fractions of animals injected with 5-HTP 30min before killing was approx 3 times higher than the saline control group. The amount of 5-HT and 5-HIAA released at 37°C was 3–5 times higher than the amount released at 0°C for both the 5-HTP and saline injected rats. Increasing the concentration of potassium ions in the media to 55 mm significantly increased the release of 5-HT but not 5-HIAA in both groups of animals. The amount of 5-HT released by 55mm-K⁺ was about 2-fold higher from the P₂ fraction isolated from rats given 5-HTP injections with respect to those given saline injections. The potassium stimulated release of 5-HT was calcium dependent. The data thus indicate that injection of 50 mg/kg d,l-5-HTP in rats can cause an increase in the level of 5-HT and 5-HIAA in a crude synaptosomal fraction and that as a result of this increase, there is a temperature dependent increased release of 5-HT and 5-HIAA under normal resting membrane conditions. There is also an increased release of 5-HT as a result of membrane depolarizing conditions induced by elevated potassium levels which is calcium dependent.     

Effects of Injury on the Indoleamines in Cerebral Cortex

It was shown previously that focal cortical freezing lesions in rats cause widespread depression of local cerebral glucose utilization (LCGU) in cortical areas of the lesioned hemisphere. This was interpreted as reflecting functional depression. The underlying mechanisms were postulated to involve alterations of biogenic amine systems. Accordingly, levels of serotonin (5-HT), its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and its precursor tryptophan were determined by an HPLC method with electrochemical detection in frontoparietal cortical areas of both hemispheres at 4 h and 1, 3, 6, 8, and 10 days after a unilateral cortical freezing lesion. The 5-HT content was significantly lower than normal in the lesioned hemisphere only at 24 h, whereas the 5-HIAA level peaked at 24 h but was significantly elevated above normal values between 4 h and 6 days after lesioning. No changes were noted in 5-HT and 5-HIAA contents in the hemisphere contralateral to the lesion. These results indicate that cortical 5-HT metabolism is increased throughout the lesioned hemisphere of a focally injured brain. The increase in tryptophan content of the lesioned brain appeared to have a time course more closely related to previously demonstrated changes in cortical LCGU than to the increase in 5-HIAA content.     

Effect of food restriction on serotonin metabolism in rat brain

The brain concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) increased in rats maintained on restricted volume of low-protein or normal-protein diet, whereas these two agents decreased in rats fed low-protein diet ad libitum. In these two food-restricted groups brain 5-HT and 5-HIAA concentrations were not correlated with brain tryptophan hydroxylase activity, but the concentrations correlated closely with cerebral tryptophan concentrations. The cerebral tryptophan concentration in the two food-restricted groups was not consistent with the total or free tryptophan concentration in plasma. In these restricted rats cerebral tryptophan concentration was elevated, and, unlike the plasma tryptophan, it showed no diurnal variation. These results suggested that tryptophan uptake into the brain from plasma was enhanced by limiting food volume intake. Tryptophan uptake was increased by glucagon injection without changing the plasma tryptophan level, but injection of hydrocortisone or insulin had little or no effect on tryptophan concentration in either the plasma or brain.d-Glucose injection elevated plasma tryptophan concentration but decreased brain tryptophan concentration.     

SEROTONIN DEFICIENCY IN EXPERIMENTAL HYPERPHENYLALANINEMIA

Abstract— The mechanism of serotonin depletion was studied in the preweanling rat in which a chemical simulation of phenylketonuria had been induced by injections of p-CPA and l -PA. Experimental conditions were selected to effectively minimize the contribution by deficient tryptophan hydroxylation and 5-HTP transport. Excessive degradation of 5-HT in the hyperphenylalaninemic brain could be eliminated as a possible mechanism. The observed levels of cerebral 5-HTP, 5-HT, 5-HIAA before and 1 h after 5-HTP loading, with and without pargyline pretreatment, clearly demonstrate greatly diminished in vivo synthesis of 5-HT in the hyperphenylalaninemic animal. This deficient synthesis could largely be accounted for by decreased activity of aromatic l -amino acid decarboxylase measured in the high speed soluble supernatant extracts of whole brain. Decreased storage of 5-HT in the particulate subcellular fraction of whole brain was also noted in the hyperphenylalaninemic animal. Significant lowering of bound serotonin levels in the brain occurred with injections of PEA into normal animals.     

A rapid and simple method to determine the specific activities of serotonin, 5-hydroxyindoleacetic acid,and 5-hydroxytryptophan in brain by HPLC with electrochemical detection

The specific activities of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) have been determined in the brain of rats by HPLC using electrochemical detection. The method allows, from a single sample, the simultaneous measurement of all three compounds and collection of each peak for radioactivity determinations. Five male Wistar rats were injected i.v. with 2.0 mCi/kg ofDl-5-hydroxy-[G-³H]tryptophan (2.6 Ci/mmol) and 30 min later the animals were killed by near freezing. Whole brains were removed and homogenized in an acid medium. The content of 5-HTP, 5-HT, and 5-HIAA were determined by HPLC. Each peak of interest was immediately collected after detection in scintillation vials by use of a small dead space detector (TL-9A, Bioanalytical Systems, Inc.). The amounts of radioactivity were determined and specific activities calculated from the results. A second chromatography system (TLC) was used to check the authenticity and purity of compounds separated by the HPLC.     

Effect of Tryptophan on Extracellular Concentrations of Tryptophan and 5-Hydroxyindoleacetic Acid in the Striatum and Cerebellum

The effects of L-tryptophan (50 mg/kg i.p.) on extracellular concentrations of tryptophan and the 5-hydroxytryptamine (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in the rat striatum and cerebellum, regions with rich and poor 5-HT innervation, respectively. Determinations were on perfusates from dialysis probes in the brains of conscious, freely moving rats. The pharmacokinetic profiles of dialysate tryptophan after tryptophan load (peak concentration, time to peak concentration, area under curve, and half-life) in the two regions did not differ significantly. The dialysate 5-HIAA concentration in the striatum rose two- to threefold after the administration of tryptophan. Therefore, as 5-HIAA was undetectable in the cerebellum either before or after the administration of tryptophan, the increase of 5-HIAA in the striatum is unlikely to depend appreciably on its production within the cerebral vasculature or outside the brain or on its entering the striatum through a blood-brain barrier damaged by placement of the dialysis probe. Overall, the findings strengthen previous evidence that extracellular 5-HIAA concentrations determined by cerebral dialysis are a valid measure of the metabolism of 5-HT of brain neuronal origin.     

Effect of immobilization stress on serotonin content and turnover in regions of the rat brain.

Sprague-Dawley rats were stressed by immobilization from 30 to 300 minutes and the effects on serotonin (5-HT) and 5-hydroxy-indoleacetic acid (5-HIAA) content were determined in the cerebral cortex, diencephalon, striatum, hippocampus and the brain stem. In a subsequent study 5-HT turnover rate in these brain areas was estimated by measuring 5-HIAA accumulation 0, 30, 60 and 90 minutes after probenecid. The content of 5-HIAA and the turnover rate of 5-HT were significantly increased in the cerebral cortex shortly after the onset of immobilization. The content of 5-HIAA in the brainstem was increased by immobilization although 5-HT turnover rate was not increased. Short term increases in 5-HIAA content were observed in the striatum and hippocampus. However, no significant changes in 5-HT turnover rate were observed in either of these 2 brain areas. Immobilization did not affect 5-HIAA content or 5-HT turnover in the diencephalon. The sensitivity of the serotonergic system in the cerebral cortex to immobilization stress suggests that this brain region could be used in future studies of the interrelationships between stress and the brain serotonergic system.     

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.