Effect of 8-hydroxy-2-(di-n-propylamino)-tetralin on the tryptophan-induced increase in 5-hydroxytryptophan accumulation in rat brain

Plasma Tryptophan/Large Neutral Amino Acids Ratio in Domestic Dogs Is Affected by a Single Meal with High Carbohydrates Level

Aim of this study was to evaluate the plasma ratio between l-tryptophan (TRP) and five large neutral amino acids (isoleucine + leucine + phenylalanine + tyrosine + valine) (5LNAAs) after a single meal with high carbohydrates level. Five female Labrador Retrievers were involved. Each dog was fed three different meals: M1 (a mix of puffed rice, minced meat and olive oil), M2 (puffed rice and olive oil) and M3 (commercial dry food usually consumed) once in the morning per day for 30 days. Blood was collected right before the first meal (t0) and after 2, 4, 6, 8, 10 and 24 h. Plasma amino acids’ concentrations were measured using an HPLC (High-performance liquid chromatography) method with fluorimetric detection. Plasmatic TRP concentrations showed no significant difference between M1, M2 and M3 samples at any sampling time. M2 led to a decrease in 5LNAAs levels and consequently led to a significant higher TRP/5LNAAs ratios in the 6 h period after the provision of carbohydrates, compared to both M1 and M3. In addition, the mean TRP/5LNAAs ratio was significantly higher in M2 than in M3 at t8 and t10. These results indicate that meal composition affects TRP/5LNAAs ratio and possibly, TRP bioavailability.

Effect of sub chronic tryptophan supplementation on stress-induced cortisol and appetite in subjects differing in 5-HTTLPR genotype and trait neuroticism

Stress or negative effect often increases preference for, and intake of, palatable snack foods and this may be influenced by cognitive and genetic factors related to stress and 5-HT vulnerability. The short (S) compared to the long (L) allele of the 5-HT transporter linked polymorphic region (5-HTTLPR) has been associated (i) with decreased 5-HT transporter function and availability and hence, with 5-HT vulnerability, and (ii) with greater stress-responsiveness. Stress-proneness is furthermore promoted by cognitive stress-vulnerability, a key feature of trait neuroticism. Brain 5-HT function can be manipulated by dietary administration of its amino acid precursor tryptophan (Trp), and the beneficial effects of dietary Trp on stress experience and emotional eating may be greatest following repeated administration in both stress- and 5-HT-vulnerable subjects. The aim was to examine the influence of repeated Trp administration on stress responsiveness and emotional eating in homozygous 5-HTTLPR S-allele (N=60) and L-allele (N=58) carriers with high and low neuroticism. Following seven days of Trp or PLC intake, mood, cortisol and appetite were assessed before and after exposure to acute stress and snack intake and preference were measured post-stress. It was hypothesized that Trp would reduce stress experience and emotional eating particularly in S-allele carriers with high neuroticism. Results revealed Trp treatment caused a clear reduction in stress-induced cortisol levels in S/S-allele carriers exclusively, and prevented a stress-induced increase in appetite only in S/S-allele carriers with high trait neuroticism. The findings reveal an advantageous effect of sub chronic Trp treatment on stress experience and appetite depending on stress and (genetic) serotonergic vulnerability.

Effects and side effects associated with the non-nutritional use of tryptophan by humans

The daily nutritional requirement for L-tryptophan (Trp) is modest (5 mg/kg). However, many adults choose to consume much more, up to 4-5 g/d (60-70 mg/kg), typically to improve mood or sleep. Ingesting L-Trp raises brain tryptophan levels and stimulates its conversion to serotonin in neurons, which is thought to mediate its actions. Are there side effects from Trp supplementation? Some consider drowsiness a side effect, but not those who use it to improve sleep. Though the literature is thin, occasional side effects, seen mainly at higher doses (70-200 mg/kg), include tremor, nausea, and dizziness, and may occur when Trp is taken alone or with a drug that enhances serotonin function (e.g., antidepressants). In rare cases, the "serotonin syndrome" occurs, the result of too much serotonin stimulation when Trp is combined with serotonin drugs. Symptoms include delirium, myoclonus, hyperthermia, and coma. In 1989 a new syndrome appeared, dubbed eosinophilia myalgia syndrome (EMS), and was quickly linked to supplemental Trp use. Key symptoms included debilitating myalgia (muscle pain) and a high peripheral eosinophil count. The cause was shown not to be Trp but a contaminant in certain production batches. This is not surprising, because side effects long associated with Trp use were not those associated with the EMS. Over 5 decades, Trp has been taken as a supplement and as an adjunct to medications with occasional modest, short-lived side effects. Still, the database is small and largely anecdotal. A thorough, dose-related assessment of side effects remains to be conducted.

Large neutral amino acids: dietary effects on brain neurochemistry and function

The ingestion of large neutral amino acids (LNAA), notably tryptophan, tyrosine and the branched-chain amino acids (BCAA), modifies tryptophan and tyrosine uptake into brain and their conversion to serotonin and catecholamines, respectively. The particular effect reflects the competitive nature of the transporter for LNAA at the blood-brain barrier. For example, raising blood tryptophan or tyrosine levels raises their uptake into brain, while raising blood BCAA levels lowers tryptophan and tyrosine uptake; serotonin and catecholamine synthesis in brain parallel the tryptophan and tyrosine changes. By changing blood LNAA levels, the ingestion of particular proteins causes surprisingly large variations in brain tryptophan uptake and serotonin synthesis, with minimal effects on tyrosine uptake and catecholamine synthesis. Such variations elicit predictable effects on mood, cognition and hormone secretion (prolactin, cortisol). The ingestion of mixtures of LNAA, particularly BCAA, lowers brain tryptophan uptake and serotonin synthesis. Though argued to improve physical performance by reducing serotonin function, such effects are generally considered modest at best. However, BCAA ingestion also lowers tyrosine uptake, and dopamine synthesis in brain. Increasing dopamine function in brain improves performance, suggesting that BCAA may fail to increase performance because dopamine is reduced. Conceivably, BCAA administered with tyrosine could prevent the decline in dopamine, while still eliciting a drop in serotonin. Such an LNAA mixture might thus prove an effective enhancer of physical performance. The thoughtful development and application of dietary proteins and LNAA mixtures may thus produce treatments with predictable and useful functional effects.

Tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area

To assess the effects of increased hypothalamic tryptophan (TRP) availability on 5-HT content in preoptic area on thermoregulation and work production during exercise on treadmill, 20.3 microM of L-TRP (n=7) or 0.15M NaCl (n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18 m min(-1) 5% inclination). Exercise time to fatigue (min), and workload (kgm) were analysed. Core temperature was measured by telemetry. At fatigue, brains were quickly removed and preoptic area (POA), hypothalamus (HP), frontal cortex (FC), hippocampi (HC) were rapidly dissected and frozen immediately in dry ice. Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured by HPLC. TRP-exercised rats showed the highest content of 5-HT in the POA and the lowest in the hippocampi compared to the rested and SAL-exercised rats. An inverse relationship between TF and a direct correlation with body temperature changes and POA-5HT levels were observed. A correlation between HC 5-HT content and TF was also found. However, there was no correlation between HC 5-HT content and changes in Tb at fatigue. Finally, our results bring further evidences that increased 5-HT content in POA is involved with an increase in heat production during exercise. In addition, the direct correlation of 5-HT level in hippocampi and TF of TRP-exercised rats suggests that this brain area is also related to motor activity control during exercise. In conclusion, our data indicated that tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area.

Exercise, serum free tryptophan, and central fatigue

Brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis and release increase during running. This increase in 5HT function may promote central fatigue and contribute to suboptimal physical performance. The rise in brain TRP is reputed to result from exercise-induced elevations in serum nonesterified fatty acid (NEFA) concentrations, which dissociate TRP from albumin in blood and increase the serum free TRP pool. But, as discussed in this article, ample evidence exists that the serum free TRP pool does not control brain TRP uptake. The clearest data are dietary, but pharmacologic data in exercising rats also support this conclusion. Changes in the serum levels of amino acids that compete with TRP for brain uptake appear also not to explain the rise in brain TRP. The mechanism is therefore not presently known. The link between the rise in brain TRP and 5HT synthesis/release is not simple: a rise in brain TRP stimulates 5HT synthesis/release in actively firing neurons. Hence, during exercise, only 5HT neurons that are firing should increase 5HT production/release when brain TRP rises. It is not known which 5HT neurons fire during exercise; the 5HT neurons that respond to exercise-induced increases in brain TRP are therefore not known. Hence, it is not possible to conclude which 5HT neurons contribute to the generation of central fatigue. Because some 5HT neurons control specific functions important to physical performance (e.g., respiration), the current understanding of 5HT neuronal function in central fatigue might benefit from the study of specific 5HT pathways during exercise.

Serotonin und Persönlichkeit

Zusammenfassung: Es werden kurz drei wichtige Persönlichkeitstheorien vorgestellt, die die Beteiligung des serotonergen Neurotransmittersystems an der Ausprägung dispositioneller Merkmale auf der Ebene von Temperamentseigenschaften konstatieren. Der vorliegende Beitrag fasst die zentralen Befunde zusammen. Zunächst wird jedoch beschrieben, über welche Charakteristika das serotonerge Neurotransmittersystem verfügt, und welche Schwierigkeiten sich hinsichtlich der Messung von Indikatoren serotonerger Aktivität bzw. Ansprechbarkeit ergeben. Basierend auf einigen ausgewählten Befunden aus dem Bereich der biologischen Psychiatrie wird dann dargestellt, dass sich serotonerge Auffälligkeiten klinischer Populationen durchaus in den Bereich der gesunden Persönlichkeit übertragen lassen. Konkreter wird gezeigt, dass sich Personen mit erhöhter Ausprägung auf den Dimensionen Depressivität, Aggressivität und Impulsivität über eine geringe Ansprechbarkeit des serotonergen Systems im Zuge des so genannten Neurotransmitter-Challenge-Tests charakterisieren lassen. Implikationen für ein Verständnis der zugrundeliegenden Mechanismen interindividueller Differenzen und Implikationen für ggfs. veränderte diagnostische Methoden werden diskutiert.

Cerebrospinal fluid concentrations of tryptophan and 5-hydroxyindoleacetic acid in Macaca mulatta: diurnal variations and response to chronic changes in dietary protein intake

In rats, dietary protein is known to influence brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis. However, few studies have examined this relationship in primates (including humans). We therefore studied the effect in monkeys of changes in chronic protein intake on plasma and cerebrospinal fluid (CSF) concentrations of TRP and 5-hydroxyindoleacetic acid (5HIAA), the principal 5HT metabolite. Juvenile male monkeys (Macacca mulatta) consumed for sequential 4-week periods diets differing in protein content (approximately 23%-->approximately 16%--> approximately 10%-->approximately 6% protein [%-energy/day]). Each day, food was presented as a morning meal of fruit, and an afternoon meal consisting of a pelleted, commercial diet and fruit. During week 4 on each diet, blood and CSF were sampled diurnally via indwelling catheters. Plasma and CSF TRP varied diurnally and with dietary protein content. On all diets, CSF TRP declined modestly in the morning, and increased in the afternoon; the magnitude of the increments varied directly with dietary protein content. Diurnal variations were absent for CSF 5HIAA; however, CSF 5HIAA varied directly with chronic dietary protein content. We conclude that dietary protein content can chronically influence CSF TRP concentrations in monkeys. The variation in CSF 5HIAA suggests chronic protein intake may influence serotonin synthesis and turnover, perhaps via changes in TRP concentrations.

Ex vivo inhibitory effect of the 5-HT uptake blocker citalopram on 5-HT synthesis

Serotonin (5-hydroxytryptamine, 5-HT) synthesis was determined in vivo by measuring the accumulation of 5-hydroxytryptophan (5-HTP) in rat frontal cortex after inhibition of aromatic amino acid decarboxylase by administrative of m-hydroxybenzylhydrazine (NSD 1015) (100 mg/kg, i.p.). The selective 5-HT reuptake inhibitor, citalopram, the 5-HT1A agonists, (+/-) 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), ipsapirone, gepirone and the 5-HT1A/B agonist, 7-trifluoromethyl-4(4-methyl-1-piperazinyl-pyrolo[1,2-a]-quinox ali ne (CGS 12066B), the 5-HT1A/B ligands and beta-adrenoceptor antagonists, (+/-) pindolol and (+/-) alprenolol, and the non-selective 5-HT ligands, m-chlorophenylpiperazine (mCPP) and metergoline, all inhibited the synthesis of 5-HT. The 5-HT1A/5-HT2 antagonist, spiperone, alone, had no effect on basal 5-HT synthesis, however it attenuated the effect of 8-OH-DPAT by 56% and CGS 12066B by 39% but only barely that of citalopram by 17%. The selective 5-HT1A antagonist, WAY 100635, which did not modify by itself 5-HT synthesis, had no effect on citalopram-induced reduction of 5-HT synthesis. Neither the 5-HT2 agonist, (+/-)1-(2,5-dimethoxy-4-indophenyl)-2-aminopropane (DOI) nor the 5-HT2 antagonist, ritanserin, had any effect on the synthesis of 5-HT. In addition, ritanserin did not modify the inhibitory effect of citalopram. Methiothepin was the only compound to increase 5-HT synthesis. These results suggest that the effect of citalopram on the synthesis of 5-HT is not mediated by 5-HT1A or 5-HT2 receptors and that other receptors may be involved.

The alpha 2-adrenoceptor antagonist idazoxan is an agonist at 5-HT1A autoreceptors modulating serotonin synthesis in the rat brain in vivo

The in vivo effects of the alpha 2-adrenoceptor idazoxan, rauwolscine and phentolamine on alpha 2-auto/heteroreceptors and 5-HT1A autoreceptors modulating the synthesis of dopa/noradrenaline and 5-HTP/serotonin were assessed in rats, using the accumulation of dopa and 5-HTP after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation. The acute administration of idazoxan (0.1-40 mg/kg) induced a pronounced dose-dependent increase in the synthesis of dopa in the cerebral cortex (22-86%) and hippocampus (8-80%), as a consequence of the powerful blockade of alpha 2-autoreceptors. However, idazoxan did not increase the synthesis of 5-HTP in these brain regions, as it would have been expected by the concurrent blockade of alpha 2-heteroreceptors on serotonergic terminals. Instead, idazoxan decreased the synthesis of 5-HTP in the cerebral cortex (13-33%) and hippocampus (25-48%), suggesting that these inhibitory effects were mediated through activation of 5-HT1A autoreceptors. Similar results were obtained for rauwolscine. Pre-treatment of rats with the selective 5-HT1A receptor antagonist WAY100135 (10 mg/kg) fully antagonized the inhibitory effects of idazoxan (10 mg/kg) on 5-HTP synthesis, but it did not prevent the stimulatory effects of idazoxan on dopa synthesis. The results indicate that idazoxan is a potent and specific agonist at 5-HT1A autoreceptors modulating brain serotonin synthesis in vivo.

Differential potentiation of L-tryptophan-induced head-twitch response in mice by cocaine and sertraline

Using selective monoamine uptake blockers and appropriate selective monoamine receptor antagonists, we have previously shown that cocaine enhances the frequency of 5-HT2A receptor-mediated 5-hydroxytryptophan (5-HTP)-induced head-twitch response (HTR) in mice via inhibition of serotonin uptake. Concomitantly, cocaine prevented the maximal producible HTR frequency via simultaneous indirect stimulation of the inhibitory presynaptic 5-HT1A and postsynaptic alpha 2 receptors. In the present study, we have investigated the effects of cocaine and the selective 5-HT (sertraline), norepinephrine (nisoxetine) and dopamine (GBR 12935) uptake inhibitors on the L-tryptophan-induced HTR in the presence of a nonselective monoamine oxidase inhibitor, tranylcypromine. We utilized two experimental protocols where cocaine or sertraline were administered either after (protocol 1) or prior to (protocol 2) L-tryptophan injection. Cocaine potentiated the ability of L-tryptophan to induce HTR to a greater extent in protocol 1, whereas sertraline induced a greater effect in protocol 2. However, in our earlier study cocaine (and also sertraline) up to 10 mg/kg produced a similar degree of potentiation in both experimental protocols on the 5-HTP-induced HTR. Furthermore, as in the latter study on the 5-HTP-induced HTR, in the present investigation nisoxetine potently attenuated whereas GBR 12935 did not modulate the induced HTR. The results show that the respective serotonergic and noradrenergic effects of cocaine also operate on the L-tryptophan-induced HTR. The differential effects of cocaine and sertraline in experimental protocols 1 and 2 on the L-tryptophan- versus 5-HTP-induced HTRs suggest that cocaine has additional effects on the conversion of L-tryptophan to 5-HT.

Diet-induced changes in serum cholesterol concentrations do not alter tryptophan hydroxylation rate or serotonin concentrations in gerbil brain

The relationship between serum cholesterol concentrations and serotonin synthesis rate in brain was examined in Mongolian gerbils chronically fed diets containing 20% fat (safflower oil, beef tallow or butterfat) with or without added cholesterol (0.5%, dry weight). After 22 days on these diets, circulating cholesterol concentrations ranged between approximately 1.5 and approximately 20 mumol/ml. Despite this enormous range, in vivo tryptophan hydroxylation rate, and serotonin and 5-hydroxyindoleacetic acid concentrations in cerebral cortex, hypothalamus and brainstem did not differ significantly among the diet groups. Tryptophan concentrations in serum and brain were also unaffected. These results do not support the hypothesis that the link between depression, suicide and violent deaths and below-normal or reduced serum cholesterol concentrations in humans involves an alteration in serotonin synthesis and/or release by brain neurons.

Alpha 2-autoreceptors and alpha 2-heteroreceptors modulating tyrosine and tryptophan hydroxylase activity in the rat brain in vivo: an investigation into the alpha 2-adrenoceptor subtypes

The subtype determination of auto- and hetero-alpha 2-adrenoceptors modulating the synthesis of noradrenaline (NA) and serotonin (5-HT), respectively, was assessed using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. In the cerebral cortex and hippocampus, Org 3770 (non-selective alpha 2-adrenoceptor antagonist, 0.5-10 mg/kg, i.p.) increased (43%-58%) and clonidine (non-selective alpha 2-adrenoceptor agonist, 1 mg/kg) decreased (37%-49%) the synthesis of dopa. Also the antagonist ARC 239 (alpha 2B/C selective, 5-40 mg/kg) increased the synthesis of dopa in cortex (39%-46%) and hippocampus (17%-85%). In contrast, the antagonist BRL 44408 (alpha 2D selective, 1-10 mg/kg) did not increase the synthesis of dopa in cortex, and increased it modestly in hippocampus only. The agonist guanoxabenz (alpha 2B/C selective, 0.03-3 mg/kg) decreased the synthesis of dopa in both brain regions (20%-65%), whereas the agonist oxymetazoline (alpha 2D selective, 0.1-3 mg/kg) failed to do so. These results indicated that the alpha 2-autoreceptors that modulate the synthesis of dopa/NA are probably associated with the alpha 2B/C-subtypes. In cortex and hippocampus, clonidine decreased (35%-53%) the synthesis of 5-HTP but Org 3770 failed to induce the opposite effect (except the 2 mg/kg dose in cortex). BRL 44408 markedly increased the synthesis of 5-HTP in cortex (113%-148%) but not in hippocampus. Similarly, also ARC239 increased the formation of 5-HTP in cortex (36%-48%) but not in hippocampus, where it was decreased (30%-55%). Oxymetazoline decreased the synthesis of 5-HTP in hippocampus (28%-30%) but failed to do so in cortex. Guanoxabenz in the low dose range (0.03-0.3 mg/kg) did not decrease the synthesis of 5-HTP in any brain region. These results indicated that the alpha 2-heteroreceptors that modulate the synthesis of 5-HTP/5-HT may well be different from the proposed alpha 2B/C-autoreceptors modulating the synthesis of dopa/NA. These alpha 2-heteroreceptors appear to be associated with the alpha 2D-subtype.

Acute tyrosine depletion reduces tyrosine hydroxylation rate in rat central nervous system

An amino acid cocktail was devised that would rapidly reduce central nervous system (CNS) tyrosine levels in rats following gastric intubation. The effect of this treatment on in vivo tyrosine hydroxylation rate was examined. Serum tyrosine (TYR) levels, the serum ratio of TYR to the sum of its transport competitors, and CNS TYR concentrations fell substantially within 60 minutes of intubation and remained low for at least 3 hr. In vivo tyrosine hydroxylation rate, evaluated in hypothalamus and retina 2 hr after amino acid intubation, also declined significantly. The results suggest that an amino acid mixture can be devised that will cause an acute reduction in TYR levels and hydroxylation rate in rat CNS. This procedure may ultimately prove applicable to humans to examine functional consequences in particular CNS regions of reducing neuronal catecholamine synthesis.

The effects of a novel and selective inhibitor of tryptophan 2,3-dioxygenase on tryptophan and serotonin metabolism in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3- pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO) (EC 1.13.11.11), were examined on tryptophan catabolism in vitro and in vivo and on brain levels of tryptophan, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA). 680C91 was a potent (Ki = 51 nM) and selective TDO inhibitor with no inhibitory activity against indoleamine 2,3-dioxygenase (EC 1.13.11.17), monoamine oxidase A and B, 5-HT uptake and 5-HT1A,1D,2A and 2C receptors at a concentration of 10 microM. 680C91 had no effect on the binding of tryptophan to serum albumin in plasma and inhibited TDO competitively with respect to its substrate tryptophan. 680C91 inhibited the catabolism of tryptophan by rat liver cells and rat liver perfused in situ. The catabolism of L-[ring-2-14C]-tryptophan and a load dose of tryptophan (100 mg/kg) in vivo were inhibited by prior administration of 680C91. Administration of 680C91 alone produced marked increases in brain tryptophan, 5-HT and 5-HIAA. A load dose of tryptophan (100 mg/kg), producing increases in brain tryptophan 4-fold greater than that seen with 680C91, did not increase brain 5-HT and 5-HIAA to levels greater than those seen with 680C91 and produced a shorter-lasting increase in these parameters. These data therefore demonstrate the importance of TDO as a regulator of whole-body tryptophan catabolism and brain levels of tryptophan and 5-HT and suggest that a greater antidepressant efficacy might be achieved with inhibitors of TDO than tryptophan administration alone.

The effects of an inhibitor of tryptophan 2,3-dioxygenase and a combined inhibitor of tryptophan 2,3-dioxygenase and 5-HT reuptake in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3-pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO), and a novel inhibitor 709W92 ((E)-6-fluoro-3-[2-(4-pyridyl)vinyl]-1H-indole), of both TDO and 5-hydroxytryptamine (5-HT) reuptake, were examined on tryptophan catabolism, cerebrospinal fluid (CSF) concentrations of tryptophan and 5-HT and serotonergic-mediated physiology and behaviour in the rat. The catabolism of L-[ring-2-14C]tryptophan in vivo was completely inhibited by prior administration of 709W92. 709W92, but not 680C91, potentiated head-twitch produced by 5-hydroxytryptophan, prevented head-twitch and whole brain 5-HT depletion produced by p-chloroamphetamine and rapidly decreased dorsal raphe firing. Both 709W92 and 680C91 elevated CSF tryptophan by up to 260% of basal concentration. A maximally effective dose of 680C91 elevated a global measure of brain extracellular 5-HT (CSF 5-HT) to concentrations similar to those seen maximally after exogenous tryptophan administration (approx 170% of basal). Maximally effective doses of 709W92 increased CSF 5-HT to concentrations comparable to those seen after tryptophan and 5-HT reuptake inhibitor coadministration (approx 900% of basal) and to concentrations greater than those achieved maximally with serotonergically active antidepressant monotherapy (approx 500% of basal). 709W92 did not elevate CSF 5-HT to concentrations associated with the serotonin syndrome (approx 3000% of basal). The combined TDO inhibitor/5-HT reuptake inhibitor, 709W92, showed anxiolytic activity in the rat-pup vocalization model of anxiety. These results show that 709W92 (a novel inhibitor of both TDO and 5-HT reuptake), can produce an elevation of CSF 5-HT similar to that achieved with a serotonin reuptake inhibitor/tryptophan combination therapy but with a more sustained timecourse; such compounds may therefore have superior antidepressant efficacy in the clinic.

α2-Adrenoceptor modulation of 5-HT biosynthesis in the rat brain

The purpose of the present study is to clarify the modulation of the biosynthesis of serotonin (5-HT) via the alpha 2-adrenoceptors in the brain. For this purpose, 5-hydroxytryptophan (5-HTP) accumulation was determined using an HPLC-ECD system in the presence of the inhibition of aromatic L-amino acid decarboxylase. Administration of alpha 2-adrenoceptor agonist, clonidine, produced a reduction of the in vivo 5-HTP accumulation in both the rat hippocampus and dorsal raphe nucleus. In addition, alpha 2-adrenoceptor antagonist, idazoxan, increased the 5-HTP accumulation in both the hippocampus and the dorsal raphe nucleus. In rats with catecholaminergic neurons denervated by pretreatment with 6-hydroxydopamine, clonidine failed to produce a reduction of 5-HTP accumulation in the dorsal raphe nucleus. On the other hand, hippocampal 5-HTP accumulation was decreased significantly. Brain tryptophan levels were unaffected by either clonidine or idazoxan. These results suggest that alpha 2-adrenoceptors might modulate serotonin biosynthesis and this modulation might be related to the neuroanatomical differences in the rat brain.

Effect of acute administration of L-tryptophan on the release of 5-HT in rat hippocampus in relation to serotoninergic neuronal activity: an in vivo microdialysis study

Here we have used the brain microdialysis method to test the effect of the 5-HT precursor L-tryptophan on 5-HT release. The release of endogenous 5-HT was measured in ventral hippocampus of the anesthetized rat both under basal conditions and when serotoninergic neuronal activity was raised by electrical stimulation of the dorsal raphe nucleus (DRN). Low frequency electrical stimulation of the DRN evoked a frequency-dependent (2-10 Hz) release of hippocampal 5-HT. The electrically evoked release of 5-HT was markedly enhanced by pretreatment with L-tryptophan (50 and 100 mg/kg i.p.). The effect of L-tryptophan on evoked release of 5-HT was dose-related, detectable at low (2 Hz) stimulation frequencies, and became stronger as the stimulation frequency increased. L-Tryptophan (10, 50 and 100 mg/kg i.p.) had no effect on basal output of 5-HT. We conclude from these findings that elevation of 5-HT precursor availability increases 5-HT release in hippocampus in vivo under conditions of increased serotoninergic neuronal activity.

Aflatoxin B1 alters central and systemic tryptophan and tyrosine metabolism: influence of immunomodulatory drugs

Semi-chronic exposure of ICR male Mice to Aflatoxin B1 (AFB1) in non-toxic doses results in elevated lung tryptophan (TRP) levels without change in serotonin (5-HT) or 5-hydroxyindole-3-acetic acid (5-HIAA) levels. This change is organ specific in that TRP levels are not altered in spleen, duodenum, heart or central nervous system (CNS). Acute (48 hour) flunixin treatment decreases lung TRP levels and reverses the AFB1 mediated increase in lung TRP levels. On the other hand, flunixin treatment decreases CNS TRP levels in control mice but not in AFB1 treated mice. Aflatoxin B1 treated mice have an increase in splenic serotonin (5-HT) content. Acute (48 hour) treatment of mice with E. coli lipopolysaccharide (LPS) also increases splenic 5-HT, and AFB1 treatment followed by LPS have a slightly additive effect on spleen 5-HT content. Treatment of mice with LPS increases heart 5-HT, an effect which is not altered in AFB1 pretreated mice. Both LPS and AFB1 per se increases lung TYR levels although the combination of treatments is not significantly different from the control value. Flunixin treatment increases lung tyrosine (TYR) levels, an effect which is not altered by AFB1 pretreatment. Acute treatment with either LPS or flunixin decreases the CNS TRP/TYR ratio; pretreatment with AFB1 prevents those changes in the CNS TRP/TYR ratio. Central nervous system catecholamines are reduced in AFB1 pretreated mice. However, CNS catecholamine changes in AFB1 treated mice are normalized by vitamin E supplementation during the treatment period.