Monitoring the Effect of a Tryptophan Load on Brain Indole Metabolism in Freely Moving Rats by Simultaneous Cerebrospinal Fluid Sampling and Brain Dialysis

Rats were given L-tryptophan, 50 mg/kg i.p., and its concentration in the CNS was monitored in individual freely moving animals using repeated sampling of cisternal CSF and concurrent striatal dialysis. The 5-hydroxytryptamine metabolite 5-hydroxyindoleacetic acid (5-HIAA) was also measured. Results were compared with changes of central tryptophan and 5-HIAA concentrations in brains of rats killed at various times after administration of L-tryptophan, 50 mg/kg i.p. Tryptophan changes in CSF were proportionate to those in whole brain and followed essentially identical time courses. Results for the striatal dialysate and whole striatum also paralleled each other. Similarly, results for 5-HIAA showed proportionality between CSF and brain and between dialysate and striatum. The data obtained were used to determine pharmacokinetic data for individual rats, i.e., areas under curves for both tryptophan and 5-HIAA and half-lives for the decline of tryptophan. Kinetic parameters varied considerably from rat to rat. However, mean half-lives for tryptophan in CSF, brain, dialysate, and striatum were all comparable. Results in general show the value of repeated CSF sampling and intracerebral dialysis for concurrent monitoring of changes of indole metabolism in the whole brain and a specific brain region, respectively. The methods should be suitable for the continuous monitoring of changes of central transmitter metabolism in parallel with observation of behavior following environmental or dietary changes or drug administration. They also should be of use in the investigation of drug kinetics in the CNS.     

MK-801 Increases Extracellular 5-Hydroxytryptamine in Rat Hippocampus and Striatum In Vivo

The effect of MK-801 (0.25 or 0.5 mg/kg) on the extracellular concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat hippocampus and striatum was studied using intracerebral dialysis. The dialysate 5-HT concentration was dose-dependently increased by MK-801 in both regions. In the hippocampus, at the higher drug dose a slow increase in the 5-HIAA level was observed, and this became significant 3 h after treatment. In contrast to this, the extracellular 5-HIAA content in the striatum was significantly decreased 150 min after administration of both doses of MK-801. The data are discussed in the light of the known behavioural effects of MK-801 and possible N-methyl-D-aspartic acid receptor regulation of 5-HT release.     

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.     

Postmortem and Regional Changes of Serotonin, 5-Hydroxyindoleacetic Acid, and Tryptophan in Brain

Using a specific and sensitive high pressure liquid chromatographic technique for the measurement of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and tryptophan (TRP), we found that there were no changes in 5-HT or 5-HIAA in the rat cortex when left in situ for 6 h at room temperature or 24 h at 4 degrees C. Only a minimal 14% increase in 5-HT was observed after 24 h at 4 degrees C in the striatum of the same animals. Concentrations of TRP, however, were increased significantly in both brain regions by these postmortem delay procedures. A second study revealed that there were significant regional 5-HT and 5-HIAA concentration differences within the cerebral cortex. The frontal cortex was shown to have the highest concentrations of 5-HT and 5-HIAA. Further, within the frontal cortex, 5-HIAA levels varied, showing apparent progressive rostral to caudal increases. 5-HT concentrations, however, remained constant within the frontal cortex. These results are discussed in reference to the conflicting reports of the previous human suicide and postmortem studies.     

Reduction of Serotonergic Function in Rat Brain by Tryptophan Depletion: Effects in Control and Fluvoxamine-Treated Rats

Abstract: The administration of tryptophan (Trp)-free amino acid mixtures to depressed patients responding to serotonin [5-hydroxytryptamine (5-HT)] uptake inhibitors (SSRIs) worsens their clinical state. This procedure reduces Trp availability to brain and thus impairs 5-HT synthesis. We have examined the influence of Trp depletion on extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the rat brain using in vivo microdialysis. The treatment with the SSRI fluvoxamine significantly increased 5-HT content in dialysates from frontal cortex, as compared with control rats (10.2 ± 2.7 vs. 3.1 ± 0.4 fmol per fraction), whereas 5-HIAA was unaffected. Food deprivation for 20 h reduced dialysate 5-HT content to almost control values in fluvoxamine-treated rats (10.2 ± 2.7 vs. 4.3 ± 0.6 fmol per fraction) but did not alter dialysate 5-HIAA content (7.8 ± 0.4 vs. 7.2 ± 0.5 pmol per fraction). The administration of Trp-free amino acid mixtures to fluvoxamine-treated rats significantly attenuated the release of 5-HT in frontal cortex (～50%) and, to a lesser extent, in the midbrain raphe nuclei. This effect was more marked in rats not deprived from food before the experiments (67% reduction of dialysate 5-HT content in frontal cortex) and was absent in control rats (treated with saline). In contrast, dialysate 5-HIAA was markedly affected by Trp depletion in all groups, including controls (65–75% reductions). These data show that the administration of an amino acid mixture with the same composition and dose (in milligrams per kilogram of body weight) as those inducing a severe mood impairment in depressed patients reduces 5-HT and 5-HIAA concentrations in brain dialysates. The reduction of 5-HT release, however, occurs only in animals previously treated with the antidepressant fluvoxamine for 2 weeks, which would be consistent with a marked reduction of 5-HT-mediated transmission in treated depressed patients but not in healthy controls.     

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.     

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.     

Intracerebrally administered (6r)-l-erythro-tetrahydrobiopterin does not affect extracellular levels of dopamine and serotonin metabolites in rat striatum in vivo during measurement by brain micro-dialysis system

By the use of the brain micro-dialysis technique combined with HPLC, the changes in the extracellular levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and a serotonin(5-HT) metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were examined in the rat striatum before and after intracerebral injection of a vehicle or (6R)-l-erythro-tetrahydrobiopterin (6R-BH₄), the natural form of the cofactor for the tryrosine hydroxylase and tryptophan hydroxylase. No apparent change after the 6R-BH, treatment was found in the levels of DA, DOPAC, HVA and 5-HIAA in the striatal dialysate. In contrast, the levels of total biopterin in both the operated (dialysis probe-implanted) and unoperated striatum of 6R-BH₄-treated rats increased by 23- and 93-fold, respectively, when compared with those of the control, vehicle-treated rats. The results indicate that increased levels of the tetrahydrobiopterin cofactor may not affect the release of DA and the extracellular level of DA and 5-HT metabolites in the physiologically normal brain.     

Content of Quinolinic Acid and of Other Tryptophan Metabolites Increases in Brain Regions of Rats Used as Experimental Models of Hepatic Encephalopathy

The content of the tryptophan metabolites quinolinic acid (QUIN), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) was measured in various brain areas of rats bearing a portocaval anastomosis (PCA) for 4 weeks, using mass fragmentography or HPLC. In these animals, the content of the excitotoxic compound QUIN increased by 75% in the cortex and 125% in the cerebellum. The content of 5-HT increased by 27% in the brainstem. No changes occurred in other brain areas. On the other hand, the content of 5-HIAA increased by 66% in the cortex, 65% in the caudate, 64% in the hippocampus, 120% in the diencephalon, and 185% in the brainstem. Probenecid administration caused a larger increase of 5-HIAA accumulation in various brain areas of PCA-bearing rats than in those of sham-operated controls. The cortical content of QUIN and 5-HIAA increased after administration of ammonium acetate (7 mmol/kg), whereas an equimolar amount of sodium acetate was inactive. These results confirm that profound changes in the disposition of tryptophan occur in the brains of experimental animals used as models of hepatic encephalopathy. Furthermore, this study adds the excitotoxic compound QUIN to the list of molecules possibly involved in the pathogenesis of this brain disorder.     

[3H]Paroxetine Binding and Serotonin Content of Rat Cortical Areas, Hippocampus, Neostriatum, Ventral Mesencephalic Tegmentum, and Midbrain Raphe Nuclei Region Following p-Chlorophenylalanine and p-Chloroamphetamine Treatment

The agents p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) deplete brain serotonin (5-HT) levels by two different mechanisms; PCPA inhibits the enzyme tryptophan hydroxylase, whereas PCA has a neurotoxic action on certain 5-HT neurons. The parameters of [3H]paroxetine binding to homogenates prepared from the cerebral cortex of rats treated with PCPA, PCA, or saline; vehicle were investigated. The tissue concentrations of 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) were also determined by HPLC in the same brain samples. After PCPA treatment, neither the maximum binding capacity (Bmax) nor the dissociation constant (KD) of [3H]paroxetine for the 5-HT uptake recognition site differed from controls despite a substantial reduction in the concentration of 5-HT and 5-HIAA. In contrast, significant changes in both the Bmax and KD values were observed in the cerebral cortex of rats treated with PCA. Furthermore, [3H]paroxetine binding and tissue concentrations of 5-HT and 5-HIAA were measured in the following different regions of the rat brain: cingulate, parietal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum; ventral mesencephalic tegmentum; and midbrain raphe nuclei region after administration of PCPA, PCA, or saline vehicle. There was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding in control and PCA-treated rats although this correlation was lost after PCPA treatment. Under these conditions, the 5-HT innervation remains unchanged whereas the concentration of 5-HT and 5-HIAA is greatly reduced. Thus, [3H]paroxetine binding appears to provide a reliable marker of 5-HT innervation density within the mammalian CNS.     

Effect of Age on Variables Influencing the Supply of Tryptophan to the Brain

Abstract: The relations of plasma concentrations of substances claimed to influence brain tryptophan concentration (total tryptophan, free tryptophan, large neutral amino acids) with the concentrations of tryptophan, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the forebrain were investigated in rats of different ages (from 8 days to 16 months after birth). In brain, tryptophan fell by 46%, whereas 5-HT rose by 20% between 8 and 40/42 days after birth. Thereafter, the levels of both tryptophan and 5-HT remained essentially constant. Brain 5-HIAA showed a more complex pattern, rising by 63% between 8 and 19 days, falling between 19 and 40/42 days, and then gradually rising until values at 16 months were significantly higher than those at 40/42 days. In plasma, the concentrations of free fatty acids, free and total tryptophan, and large neutral amino acids all decreased between 8 and 19 days and thereafter either remained constant or increased slowly, the exception being total tryptophan values, which showed large increases between 28/30 and 60/70 days. Also, the unidirectional uptake of tryptophan from blood to brain was determined using a carotid artery injection technique. Uptake values obtained using a tracer concentration of tryptophan in the injection solution decreased progressively with age. Kinetic analysis of the data in terms of the Michaelis-Menten equation for carrier-mediated transport indicated significantly lower values for V_max and K_D (a component for nonsaturable transport) in 6-month-old rats as compared to 19-day-old suckling rats, whereas K_m values were the same at both ages. Detailed analysis of these results indicated that the age-related changes in brain tryptophan were largely explicable in terms of plasma free tryptophan in association with blood-brain transport characteristics; moderate differences in concentration of amino acids competing for transport were without apparent effect between 19 days and 16 months. The larger differences between 8 and 19 days after birth could be important.     

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.     

The Effect of Endurance Training on Regional Serotonin Metabolism in the Brain During Early Stage of Detraining Period in the Female Rat

1. The effect was examined of a single bout of nonexhaustive endurance exercise on tryptophan (Try), serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA), and tryptophan hydroxylase (TpH) levels in different parts of rat brain (brain cortex, cerebellum, hypothalamus, midbrain striatum, medulla) on the last day of endurance training and 48 h later (detraining period).2. Female rats were subjected to a 6-week endurance training programme. The effectiveness of the training was evaluated by measuring anaerobic threshold (AT). High performance liquid chromatography (HPLC) was used to determine regional Try, 5-HT, and 5-HIAA contents in the brain, and thin layer chromatography followed by gas-liquid chromatography was used to determine blood levels of free fatty acids. Regional TpH levels were measured by Western blot analysis.3. In the two rat groups subjected to endurance training, in all brain regions studied but cerebellum, 5-HT content was significantly lower after the last bout of nonexhaustive endurance exercise than in resting control rats that were not subjected to the training. Similarly, the cortical and striatal, but not cerebellar, 5-HT/Try ratios were significantly lower in the trained rats at the end of the last training session and at the end of a single bout of nonexhaustive exercise administered after a 48-h detraining period than in the controls. TpH protein level was decreased by 15–25% after the last bout of exercise either during the training process or after the and 1 h bout of endurance exercise performed 48 h after cessation of endurance training in brain cortex and striatum but not cerebellar.4. These results indicate that the reduction in 5-HT level was the adaptive response to endurance training. The lowered 5-HT/Try ratio and lowered TpH protein level attained after the training process suggests and that this change may be, at least partially, attributed to downregulation of TpH activity.     

Peripheral interleukin-6 administration increases extracellular concentrations of serotonin and the evoked release of serotonin in the rat striatum

Previous studies have indicated that peripheral administration of interleukin-6 (IL-6) increases brain concentrations of tryptophan and 5-hydroxyindoleacetic acid (5-HIAA), the major catabolite of serotonin (5-HT). To determine whether these changes were related to increased synaptic release of 5-HT, we studied the responses to peripheral administration of IL-6 by in vivo microdialysis and in vivo amperometry. Intraperitoneal injection of recombinant IL-6 resulted in an elevation of microdialysate concentrations of 5-HT in the rat striatum. Also, amperometric measurements indicated that i.p. IL-6 enhanced the 5-HT-like signal obtained from the striatum following electrical stimulation of the dorsal raphe nucleus. These results indicate that the increases in brain concentrations of 5-HIAA observed in earlier studies indeed reflect increased synaptic release of 5-HT.     

Changes in plasma and brain tryptophan and brain serotonin and 5-hydroxyindoleacetic acid after footshock stress

Brain concentrations of tryptophan, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) and plasma amino acids were measured after 15 or 30 minutes of intermittent footshock. Footshock treatment significantly decreased the content of 5-HT in prefrontal cortex and hypothalamus, but not brainstem at 15 min, but the decreases were reversed by 30 min. 5-HIAA, the major catabolite of 5-HT, increased in prefrontal cortex after 15 min, and in prefrontal cortex and hypothalamus after 30 min footshock. 5-HIAA:5-HT ratios were increased at both timepoints in all three brain regions. Concomitant changes in the ratios of 3,4-dihydroxyphenylacetic acid (DOPAC) to dopamine and 3-methoxy,-4-hydroxyphenylethyleneglycol (MHPG) to norepinephrine were also observed. Brain concentrations of tryptophan increased progressively during the footshock in all three brain regions. Plasma concentrations of both tryptophan and tyrosine were also significantly increased, while those of histidine and lysine were decreased. It is possible that the stress-related changes in 5-HT metabolism are due to increased plasma tryptophan, in turn causing increased brain tryptophan and 5-HT synthesis. However, the transient decreases in 5-HT suggest a footshock-induced increase of 5-HT release, depleting existing stores of 5-HT, that are replenished by the increased systemic availability of tryptophan.     

Effects of Long-Term Low Dietary Tryptophan Intake on Determinants of 5-Hydroxytryptamine Metabolisn in the Brains of Young Rats

Abstract: The relationship between plasma and brain tryptophan (TRP) concentrations and brain 5-hydroxytryptamine (5-HT) metabolism was studied in weanling rats fed diets containing either 0.4 g or 1.45 g TRP/ 100 g casein hydrolysate. Both groups gained weight comparably though food intakes were generally higher in the low-TRP group. Severe depletion of plasma total and free TRP and of brain TRP, 5-HT, and 5-hydrox-yindoleacetic acid (5-HIAA) occurred within 1 day of feeding the 0.4% TRP diet. Levels became stable after 7 days. The decreased brain TRP concentration of the rats on the 0.4% TRP diet did not cause a compensatory rise of the tryptophan hydroxylase (TRP OHase) activity in vitro. In the low-TRP group, neither plasma free TRP nor total TRP correlated significantly with brain TRP and although plasma TRP/large neutral amino acid (NAA) ratios (TRP/NAA) correlated significantly ( P < 0.05) with the time course of brain TRP, this statistical relationship depended almost completely on the variation of the TRP values alone. In the higher TRP group none of these correlations were significant. A plot of mean plasma free TRP versus brain TRP gave two distinct regression lines with similar slopes and corresponding to values before and after 7 days on the diet. The time course of brain 5-hydroxyindole concentrations did not parallel those of brain TRP and suggested that changes of TRP OHase activity also had an influence on 5-HT synthesis.     

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.     

Rapid repletion of brain serotonin in malnourished corn-fed rats following L-tryptophan injection.

The concentration of tryptophan in serum, and the levels of tryptophan, serotonin (5-HT), and 5-hydroxyindole-acetic acid (5-HIAA) in brain are substantially reduced in rats that consume for 6 weeks a diet in which corn is the only source of protein. Single injections of L-tryptophan (25, 50, or 100 mg/kg) cause dose-related increases in brain tryptophan, 5-HT, and 5-HIAA in corn-fed animals. At each dose, brain tryptophan content rises to a proportionately greater extent in corn-fed rats than in well-nourished controls, even though serum tryptophan concentrations attain higher levels in controls. This difference may reflect the greatly reduced serum concentrations in corn-fed rats of other large neutral amino acids that compete with tryptophan for uptake into the brain (tyrosine, phenylalanine, leucine, isoleucine, and valine). However, the substantial decrease in serum albumin levels also diminishes the binding of tryptophan to serum albumin; thus it is not yet possible to state which of these changes is responsible for the much greater increments in brain tryptophan observed in corn-fed rats after tryptophan injection. The fact that tryptophan administration rapidly restores brain 5-hydroxyindole levels in corn-fed animals suggests that the reductions in 5-HT and 5-HIAA levels associated with this type of malnutrition may be largely caused by inadequate availability of substrate.     

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.     

COMPARISON OF 5-HYDROXYINDOLES IN VARIOUS REGIONS OF DOG BRAIN AND CEREBROSPINAL FLUID

Abstract— —Normal values for the concentration of 5-HT, 5-HIAA and tryptophan are established in various regions of the dog brain. After administration of tryptophan by intravenous injection the rise and fall of 5-HT and 5-HIAA were estimated at 1, 2 and 4 hr. Best fit quadratic regression curves obtained by computer programme were fitted to the data. Similar tryptophan doses were given to dogs and the 5-HIAA concentration estimated in the cisternal CSF. Quadratic regression curves fitted to these values show that the concentration of 5-HIAA in CSF reflects the changes of 5-HIAA in the brain and in particular in the brain stem. a-Methyl dopa pretreatment blocked the rise of 5-hydroxyindoles in brain and CSF and appeared to inhibit tryptophan hydroxylase as well as decarboxylase.