Hydroxytryptamine turnover decreased by the antidepressant drug chlorimipramine

The tryptophan hydroxylase activation inhibitor, AGN-2979, decreases regional 5-HT synthesis in the rat brain measured with alpha-[14C]methyl-L-tryptophan: an autoradiographic study

Many experimental conditions are stressful for animals. It is well known that stress induces tryptophan hydroxylase (TPH) activation, resulting in increased serotonin (5-HT) synthesis. In our experimental procedure to measure 5-HT synthesis using alpha-[(14)C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method, the hind limbs of animals are restrained using a loose-fitted plaster cast such that the forelimbs of the animal remain free. The objective of the present investigation was to evaluate the changes, if any, in 5-HT synthesis, after injecting these restrained rats with the TPH activation inhibitor AGN-2979. The effect on regional 5-HT synthesis was studied using the alpha-MTrp autoradiographic method. The hypothesis was that the TPH activation inhibitor would reduce 5-HT synthesis, if TPH activation was induced by this restraint. The rats received injection of AGN-2979 (10 mg/kg, i.p.) or distilled water vehicle (1 mL/kg, i.p.) 1 h prior to tracer administration. The free- and total tryptophan concentrations were not significantly different between the treatment and control groups. The results demonstrate that 5-HT synthesis in AGN-2979 treated rats is significantly decreased (-12 to -35%) in both the raphe nuclei and their terminal areas when compared to the control rats. These findings suggest that restrained conditions, such as those used in our experimental protocol, induce TPH activation resulting in an increased 5-HT synthesis throughout the brain. The reduction in 5-HT synthesis in the AGN-2979 group is not related to a change in the plasma tryptophan. Because there was no activation in the pineal body, the structure having a different isoform of TPH, we can propose that it is only the brain TPH that becomes activated with this specific restraint.

Psychoactive drugs and human sexual behavior: the role of serotonergic activity

A wide range of both prescription and nonprescription drugs has been reported to affect human sexual functioning. While the sexual side effects resulting from drug use have often been attributed to adrenergic, anticholinergic or dopaminergic activity, the present review considers the potential role of serotonin. Based on animal studies, serotonin has been shown to either facilitate or inhibit sexual activity depending on which serotonin receptor subtype is activated. However, few studies have been done in the human that assess the effects of drugs that bind selectively to serotonin receptors. Consequently, little is known about the role of serotonin in human sexual functioning. In this review, a wide range of drugs that affect both brain serotonergic systems and human sexual behavior is examined in an effort to determine the possible role of serotonin in human sexual behavior. A review of the literature is consistent with the hypothesis that the 5-HT1A and the 5-HT2 receptor subtypes play a facilitatory role in human sexual behavior. The evidence suggests that drugs that act as agonists on these receptor sites enhance sexual functioning in the human, while those that act as antagonists inhibit sexual functioning.

Dopaminergic hypothesis for retarded depression: a symptom profile for predicting therapeutical responses

We assessed the therapeutical efficacy of various antidepressants (amineptine, minaprine and clomipramine) in patients affected by retarded depression. All patients exhibited symptoms of retardation, including hypokinesia, anergia, reduction of speech, increased salivation, hypersomnia, Parinaud's syndrome, reduced sexual activity, slowness, hypomimia, orthostatic hypotension, dysphagia and drowsiness. Antidepressant drugs were administered for a 6-week period in a randomized double-blind vs placebo design. The rank order of clinical effectiveness (amineptine much greater than minaprine greater than clomipramine greater than placebo) paralleled the specificity of antidepressants as dopaminomimetic agents. These results support the view that a reduced dopaminergic transmission contributes to the pathophysiology of retarded depression.

Changes of serotonin and catecholamines are related to pharmacokinetic alterations of clomipramine in rat brain

When rats received a single i.p. injection of clomipramine (20 mg/kg), clomipramine and desmethylclomipramine were rapidly distributed into the brain and their concentrations were markedly higher in the brain than in the serum, while the concentration of the metabolite in the brain was much lower than that of clomipramine. Chronic administration of clomipramine gradually increased the concentrations of the metabolite in both serum and brain, but did not significantly change those of clomipramine. The levels of serotonin (5-HT), norepinephrine (NE), dopamine (DA) and their metabolites in the brain were compared with the concentrations of clomipramine and desmethylclomipramine at different times. The levels of 5-HT, NE, DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain did not appear to be affected by changes in the concentrations of the drug and metabolite. The level of 5-hydroxyindole acetic acid (5-HIAA) was reduced following clomipramine injection and the reduced level was maintained during chronic treatment. Chronic treatment for more than 7 days increased the 3-methoxy-4-hydroxyphenylglycol (MHPG) level with no alteration of NE level. This elevation appeared to be induced by desmethylclomipramine.

A relationship between clomipramine brain concentration and its effect on serotonin metabolism

The relationship between clomipramine (CMP) brain concentration and its inhibitory effect on serotonin (5HT) turnover was investigated in rats treated with a single dose and multiple doses of CMP. CMP reduced the 5-hydroxyindole acetic acid (5HIAA) brain levels in two groups of rats. Concerning the 5HT turnover measured by the probenecid (PBC) technique, there were significant correlations between the CMP brain level (x) and 5HIAA brain level (y) after PBC injections in both acute and chronic experiments. The regression lines for the respective groups were y = 764 - 117 log x (r = 0.84, P less than 0.001) and y = 770 - 97.7 log x (r = 0.68, P less than 0.001). The regression coefficient for the acute experiment was greater than that for the chronic one (p less than 0.001), indicating a less dose-response in the latter. From these findings, the acute dosing with CMP seemed to reduce the 5HT turnover without reference to the dose but a prolonged administration seemed to vary in its effect. This may be due to a compensatory mechanism in the 5HT system induced by chronic CMP-treatment.

Changes in mouse brain serotonin turnover following chronic imipramine administration

Mice were injected daily for 2 weeks with saline, tryptophan or p-chlorophenylalanine, alone or in combination with the tricyclic antidepressant imipramine. Serotonin turnover in several brain regions was determined by the accumulation of serotonin after pargyline and 5-hydroxyindoleacetic acid after probenecid. Both methods agreed closely. Imipramine tended to depress serotonin turnover. This effect was more marked in hypothalamus, which has high serotonin activity, and was much less in cerebellum, which has low serotonin activity. Chronic imipramine treatment completely abolished the increase in serotonin turnover induced by tryptophan, but had no effect on serotonin turnover in mice treated with p-chlorophenylalanine, which itself reduced serotonin activity.

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Measurement of 5-hydroxytryptamine synthesis and metabolism in selected discrete regions of the rat brain using high performance liquid chromatography and electrochemical detection: pharmacological manipulations

High performance liquid chromatography coupled with electrochemical detection (LCEC) was employed to measure 5-hydroxytryptamine (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophan (5-HTP) in the suprachiasmatic (SCN), medial preoptic (MPO) and arcuate (AN) nuclei as well as the median eminence (ME) and striatum (ST) of individual rat brains. Biochemical estimations of changes in 5-HT neuronal activity were made by measuring: (1) concentrations of 5-HT and 5-HIAA and (2) the rate of 5-HT synthesis (5-HTP accumulation following the administration of NSD 1015, an inhibitor of aromatic L-amino acid decarboxylase) after the administration of pharmacological agents known to influence these neurons. Pargyline increased the concentration of 5-HT and decreased the concentration of 5-HIAA while probenecid increased the concentration of 5-HIAA in all 5 brain regions. At both 2 and 24 hours after reserpine the concentration of 5-HT decreased, 5-HIAA increased or did not change, and the rate of 5-HT synthesis increased. In most of the brain regions blockers of 5-HT neuronal uptake (fluoxetine, chlorimipramine) did not influence 5-HT or 5-HIAA concentrations dramatically, but increased the rate of 5-HT synthesis. L-tryptophan generally increased the concentrations of 5-HT and 5-HIAA as well as the rate of accumulation of 5-HTP in all regions except the ME where 5-HIAA and 5-HTP concentrations both were unaffected. These results reveal that the method using LCEC is sensitive enough to detect pharmacologically-induced changes in 5-HT metabolism and synthesis in discrete regions of rat brain. The drugs examined in the present study generally caused similar changes in 5-HT dynamics in all 5 brain regions examined.

Use of a single compartment LCEC cell in the determinations of biogenic amine content and turnover

Content and specific radioactivity of the biogenic monoamines and content of their precursors and metabolites were simultaneously determined in CNS tissue extracts with a high pressure liquid chromatography system (HPLC). The content of dopamine, norepinephrine, serotonin. 5-hydroxyindoleacetic acid, homovanillic acid, 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylethyleneglycol and the turnover rate of dopamine, norepinephrine and serotonin were measured in discrete rat brain regions using a one compartment electrochemical detector cell coupled to a C18-reverse phase HPLC column. The small fluid volume dead space of the cell allows the direct and precise collection of individual peaks for determining specific radioactivities. This method is especially suitable for central nervous system tissue samples from 8-20 mg wet weight and the sensitivity of the system in its routine configuration is approximately 2 pmol. This method for determinations of turnover is appropriate for investigations of animals in sensitive behavioral paradigms.U

The effect of niaprazine on the turnover of 5-hydroxytryptamine in the rat brain

Niaprazine (60 mg/kg i.p.) increased rat brain 5-hydroxyindole acetic acid (5-HIAA) concentrations 30 min after treatment, and reduced them at 3-8 hr after treatment. Rat brain 5-hydroxytryptamine (5-HT) levels were unchanged. Niaprazine also produced a short-lasting depletion of rat brain noradrenaline (NA) and dopamine (DA). Pretreatment with alpha-phenyl-alpha-propyl-benzeneacetic acid, 2-(diethylamino) ethyl ester hydrochloride (SKF 525A) (75 mg/kg i.p.) potentiated the increase in 5-HIAA and depletion of catecholamines produced 1 hr after niaprazine, but abolished the reduction in 5-HIAA produced 8 hr after the drug. This suggested that a metabolite might be responsible for the delayed reduction in 5-HIAA levels. A potential metabolite, p-fluoro-phenylpiperazine (FPP) (5-40 mg/kg i.p.) reduced rat brain 5-HIAA and 3,4-dihydroxyphenyl acetic acid (DOPAC), and inhibited 5-HT and NA uptake in vitro. Unlike niaprazine, FPP produced no behavioural sedation, but in large doses produced a behavioural syndrome indicative of serotonergic stimulation. Studies of the metabolism of 14C-niaprazine in rats indicated the presence of a urinary metabolite with the same chromatographic characteristics as FPP. These results suggest that niaprazine itself depletes brain catecholamines and increases 5-HT turnover, while a metabolite, FPP, subsequently reduces the turnover of 5-HT and DA.

Effect of caroxazone, a new antidepressant drug, on monoamine oxidases in healthy volunteers

1 Caroxazone is a new antidepressant drug with a reversible inhibitory effect on monoamine oxidases (MAO) as previously shown experimentally in animals.

2 The effect of caroxazone on MAO was explored in healthy volunteers and compared with that of tranylcypromine and imipramine. Daily urinary excretion of tryptamine and MAO activity in platelets were assayed at various times during and after treatment and the differences from basal values were statistically analysed. In addition, caroxazone plasma levels were determined.

3 Caroxazone administered orally at doses of 300 or 600 mg/day for 12 days induced a significant, dose-dependent increase in urinary tryptamine excretion. Tranylcypromine (20 mg/day for 8 days) was even more active in this respect; imipramine (50 mg/day for 12 days) was completely inactive.

4 MAO activity in platelets was not affected by caroxazone or imipramine, but was completely inhibited by tranylcypromine.

5 Mean steady-state plasma levels of caroxazone were about 6 μg/ml and 11-12 μg/ml with the dose of 300 mg/day and 600 mg/day respectively.

6 It can be concluded that caroxazone is a MAO inhibitor in man too, at a clinically effective dose such as 600 mg/day. The fact that its MAO inhibition is not apparent in platelet preparation may be explained by its reversibility.

7 Tranylcypromine confirmed its potent irreversible MAO inhibitory effect, while imipramine lacked any effect at the tested dose.

Extrapyramidal side effects and increased serum prolactin following fluoxetine, a new antidepressant

Fluoxetine (Lilly 110140) is a potent, specific serotonin (5-HT) uptake blocker which is being tested in man for antidepressant activity. One of 9 depressed patients receiving this drug developed a dystonic reaction, parkinsonian rigidity, and increased serum prolactin levels, all signs of decreased dopaminergic activity. Homovanillic acid levels also decreased in the cerebrospinal fluid of this subject. We postulate that fluoxetine, via the increase in 5-HT activity resulting from 5-HT uptake blockade, inhibited both the nigro-striatal and tubero-infundibular dopaminergic neurons. These results provide additional evidence for a linkage between serotonergic and dopaminergic neurons in man.

Probenecid-induced increase of 5-hydroxytryptamine synthesis in rat brain, as measured by formation of 5-hydroxytryptophan

Probenecid blocks the efflux of 5-hydroxyindole acetic acid (5-HIAA) from the central nervous system, and has therefore been used for turnover measurements of central 5-hydroxytryptamine (5-HT). This substance also elevates tryptophan (TP) levels in rat brain. In this investigation, the time courses of probenecid and TP levels in rat serum and brain after administration of probenecid were studied. Maximal levels of probenecid were reached within 15 min, followed by 50% decrease of serum TP and a 40% increase of brain TP. Brain levels of probenecid were about ten times lower than those in serum. Because TP level in brain is an important factor in the control of cerebral 5-HT synthesis, the effects of probenecid on 5-HT formation in rat brain were investigated. By means of the aromatic L-amino acid decarboxylase inhibitors Ro 4-4602 and NSD 1015, an enhancement of TP hydroxylation of about 35% was demonstrated. It was concluded that penetration of probenecid into the brain is very limited and that probenecid, in addition to blocking egress of 5-HIAA from the CNS, stimulates 5-HT synthesis.

The effects of antidepressants on serotonin turnover in discrete regions of rat brain

Serotonin (5-HT) turnover was measured in hypothalamus, hippocampus, cortex, septum and nucleus caudatus of rats after acute or chronic treatment with antidepressants. Acute chlorimipramine (1.8 -- 16.2 mg/kg i.p.) decreased 5-HT turnover in all the areas tested as measured by the rate of accumulation of 5-hydroxyindoleacetic acid after probenecid, or the rate of accumulation of 5-hydroxytryptophan after decarboxylase inhibition. However, chlorimipramine failed to reduce the rate of 5-HT accumulation after monoamine oxidase inhibition. Chronic chlorimipramine treatment (3 times daily for 2 weeks) did not change the 5-HT turnover. Fluoxetine, which like chlorimipramine specifically blocks 5-HT uptake also decreased 5-HT synthesis. In contrast, no change in 5-HT turnover was observed after desmethylimipramine, amitriptyline, iprindole or amphetamine which affect the catecholaminergic, but not serotoninergic systems.

Serotonergic reduction of dorsal central gray area stimulation-produced aversion

Stimulating electrodes were implanted into the dorsal central gray area (DCG) of rats. The animals were trained to bar press to decrement the aversive DCG stimulation current. Rats treated with 5-hydroxytryptophan (5-HTP), 75 mg/kg or 150 mg/kg, showed a dose-dependent reduction in decremental bar pressing. In a second study, animals received either chlorimipramine, 15 mg/kg, protriptyline, 15 mg/kg, or 5-HTP, 150 mg/kg. Chlorimipramine, a strong blocker of serotonin reuptake, and 5-HTP produced significant reductions in decremental bar pressing. Protriptyline, a weak serotonin reuptake blocker, produced no significant effect. These results suggest that serotonin reduces aversive neural mechanisms associated with the dorsal central gray area.

Effects of antidepressant agents on the synthesis of brain monoamines

The effect of seventeen established or possible antidepressant agents on the synthesis of 5-HT, noradrenaline and dopamine in rat brain has been investigated by measuring the accumulation of 5-hydroxytryptophan and DOPA induced by an inhibitor of the aromatic L-aminoacid decarboxylase (3-hydroxybenzylhydrazine hydrochloride, 100 mg/kg i.p.). All the established inhibitors of 5-HT, noradrenaline and dopamine uptake were found to inhibit the synthesis of the respective monoamines, presumably by influencing a receptor-mediated feedback mechanism. A close correlation appears to exist between blockade of transmitter uptake and inhibition of transmitter synthesis. The results support current ideas on the mode of action of antidepressant agents.

Metabolism of 5-hydroxytryptamine and levels of tricyclic antidepressant drugs in rat brain after acute and chronic treatment

Because tricyclic antidepressants (TAD) are usually given chronically to patients, both their acute and their chronic effects on 5-hydroxytryptamine (5-HT) metabolism were studied. The probenecid method was used and, in addition to 5-hydroxy-indoleacetic acid (5-HIAA), some other indole compounds in brain were measured. Simultaneously, TAD levels in brain and plasma were determined. Dimethylated as well as monomethylated TADs were administered, both at 10 and 25 mg/kg i.p. Treatment with either 10 mg/kg during 14 days or 25 mg/kg given acutely resulted in a similar brain level of TAD, so any differences found could be attributed to differences in administration schedule. Drug levels in brain and plasma differed considerably after chronic and acute treatments but no major differences in the effect on 5-HIAA level in the brain were found, although accumulation of 5-HIAA following probenecid treatment was mostly lowered after treatment with dimethylated TAD. The TAD level in rat brain was not decisive for the effect on central 5-HT turnover. The monomethylated TAD affected the 5-HT turnover very little, not only acutely but also chronically.

Effects of mazindol, fenfluramine and chlorimipramine on the 5-hydroxytryptamine uptake and storage mechanisms in rat brain: similarities and differences

Mazindol and fenfluramine inhibited in vitro the uptake of 5-HT into rat forebrain synaptosomes, whether the synpatosomes were incubated in vitro with the drugs or obtained from animals pretreated in vitro. Chlorimipramine was also effective in this latter preparation. Dose-response relationships and time course of this effect for the various drugs were determined. Fenfluramine also caused release of 5-HT from preloaded synaptosomes in in vitro incubations. Mazindol did not. Brain 5-HT levels were measured after acute and chronic administration of mazindol, fenfluramine and chlorimipramine. Mazindol had no effect, fenfluramine was active in reducing brain 5-HT concentrations acutely and chlorimipramine only after chronic administration. Therefore, it seems that even a long lasting inhibition of the uptake, such as that induced by mazindol, is not sufficient, per se, to cause depletion of brain 5-HT.

Effects of pyrroxan and chlordiazepoxide on biogenic amine metabolism in the rat brain

Pyrroxan (20 mg/kg, i.p.), a new potential antianxiety agent, increased brain norepinephrine (NE) turnover in rats, reflecting a possible central alpha-adrenergic receptor blocking activity. In contrast, chlordiazepoxide (20 mg/kg, i.p.), a widely used antianxiety agent, did not alter the NE turnover. Pyrroxan did not affect overall DA turnover although it did appear to accelerate DA turnover initially. The initial potentiation of DA turnover may indicate a short-lasting blocking action on DA receptors. In comparison, chlordiazepoxide (20 mg/kg, i.p.) decreased the turnover rate of DA. Effects of both drugs on 5-HT indicate a decrease in turnover with no significant monoamine oxidase activity or blockade of the 5-HT reuptake mechanism. Both drugs antagonized the decline in intraventicularly-injected 14C-5-HT. Neither drug caused consistent changes in endogenous 5-HT, 5-hydroxyindoleacetic acid, or tryptophan levels. Neither drug potentiated the behavioral effects of L-Dopa nor increased the 5-HTP behavoiral syndrome in the mouse. Pyrroxan may be expected to exhibit a spectrum of activity between that of minor and major tranquilizers, characterized by antianxiety action together with sedative or tranquilizing activity.

Tryptophan hydroxylase in discrete brain nuclei: comparison of activity in vitro and in vivo

The increase of 5-hydroxytryptophan after treatment with the decarboxylase inhibitor Ro 4/4602 was 30 times greater in the raphe nuclei (which contain serotonergic cell bodies) than the hippocampus or caudate (which contain serotonergic terminals). Pretreatment with chlorimipramine reduced the 5-hydroxytryptophan accumulation in the median but not dorsal raphe. The large amount of tryptophan hydroxylase in cell bodies appears to be active in vivo. The serotonin rapidly turning over there may have a functional role.

Electroconvulsive shock increases the behavioural responses of rats to brain 5-hydroxytryptamine accumulation and central nervous system stimulant drugs

1 A single electroconvulsive shock (ECS) of 150 V for 1 s increased the concentration of rat brain 5-hydroxyindoleacetic acid (5-HIAA) but did not alter brain 5-hydroxytryptamine (5-HT) or tryptophan concentrations 3 h later. 2 A single ECS decreased 5-HT synthesis 3 h and 6 h later. Synthesis was back to normal after 24 hours. The ECS-treated rats did not show greater hyperactivity produced by the increased brain 5-HT accumulation following administration of L-tryptophan and tranylcypromine at any time up to 24 h later. This suggests that a single electroshock does not alter 5-HT functional activity. 3 Twenty-four hours after the final ECS of a series of 10 shocks given once daily, the rats were given tranylcypromine and L-tryptophan. They displayed greater hyperactivity than control rats not treated with ECS, suggesting that ECS increases 5-HT functional activity. Brain concentrations of 5-HT, 5-HIAA and tryptophan were then unchanged by ECS. 5-HT synthesis and accumulation of 5-HT following tranylcypromine and L-tryptophan were not altered by ECS. 4 The hyperactivity following administration of the 5-HT agonist 5-methoxy N,N-dimethyltryptamine was enhanced by repeated (10 day) ECS, suggesting altered post-synaptic responses to 5-HT receptor stimulation. 5 Repeated ECS enhanced locomotor activity following tranylcypromine and L-DOPA. It did not alter brain noradrenaline or dopamine concentrations. 6 The latent period before a pentylenetetrazol-induced convulsion was shortened by repeated ECS. 7 Following repeated ECS there appears to be increased neuronal sensitivity to certain stimuli producing centrally mediated behavioural stimulation. This is discussed in relation to the mechanism by which electroconvulsive therapy (ECT) produces its therapeutic effect.

Dextromethorphan inhibits 5-hydroxytryptamine uptake by human blood platelets and decreases 5-hydroxyindoleacetic acid content in rat brain

The effect of dextromethorphan on the uptake and metabolism of 5-hydroxytryptamine (5-HT) was studied in human blood platelets and in rat brain. In the concentration of 120 nM dextromethorphan inhibited the uptake of 5-HT (1 mu-M) into platelets by 50%. The corresponding concentrations of imipramine and methadone under similar conditions were 22 and 590 nM, respectively. Dextromethorphan (20 to 40 mg kg-1) decreased the concentration of brain 5-hydroxyindoleacetic acid (5-HIAA) and the probenecid-induced accumulation of 5-HIAA time- and dose-dependently. However, dextromethorphan did not alter the pargyline-induced changes in brain 5-HT metabolism. Dextromethorphan-induced changes in brain 5-HT metabolism could arise from the inhibition of the re-uptake of 5-HT into neurons.

Failure of decreased serotonin uptake or monoamine oxidase inhibition to block the acceleration in brain 5-hydroxyindole synthesis that follows food consumption

The acceleration in brain serotonin synthesis produced by injecting rats with tryptophan or allowing them to consume a carbohydrate diet was not blocked by the prior elevation of brain serotonin levels (by administration of a MAO inhibitor: Lilly 51641) or by a treatment (chlorimipramine administration) that decreases impulse flow along serotoninergic neurons.