In vivo effects of some histamine H1-receptor antagonists on monoamine metabolism in the mouse brain

Summary The in vivo effects of four Hr-antagonists, diphenhydramine, chlorpheniramine, mepyramine, and promethazine, on the metabolism of noradrenaline (NA), dopamine (DA), and 5-hydroxytryptamine (5-HT) were investigated in the whole mouse brain. Diphenhydramine and chlorpheniramine had no significant effect on levels of NA, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), DA, and 5-HT, but they significantly decreased levels of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). In particular chlorpheniramine markedly decreased 5-HIAA levels at doses as low as 1 mg/kg, i. p. Mepyramine significantly decreased 5-HIAA levels but not those of other substances. High doses of promethazine significantly decreased NA levels but markedly increased those of MHPG, DOPAC, HVA, 5-HT, and 5-HIAA. The DA reduction induced by -methyl-p-tyrosine (-MT) was significantly inhibited by diphenhydramine, chlorpheniramine, and promethazine, but the -MT-induced NA decrease was significantly enhanced by promethazine. The 5-HIAA accumulations induced by probenecid were significantly inhibited by chlorpheniramine and mepyramine. These results suggest: (1) Diphenhydramine and chlorpheniramine inhibit the turnover of both DA and 5-HT by blocking their neuronal uptake. (2) Promethazine and mepyramine inhibit DA and 5-HT turnover, respectively, as a result of the inhibition of the uptake mechanism. (3) Promethazine increases NA turnover by enhancing NA release. The discriminative effects of these drugs on the monoamine systems may be related to some differences in their CNS actions. Send offprint requests to K. Saeki at the above address     

Determinations of 5-hydroxyindoleacetic acid and homovanillic acid in human CSF with monitoring of probenecid levels in CSF and plasma

The accumulation of 5-HIAA and HVA in cerebrospinal fluid (CSF) was studied in eight healthy volunteers after oral administration of probenecid. Simulation indicated that a dose of 4.5 g probenecid should be used to achieve probenecid plasma concentrations between 200 and 400 g/ml. Almost complete inhibition of the active transport of the acidic metabolites was assumed to be obtained at these concentrations. Probenecid 4.5 g was administered in two doses (2.5 g and 2 g), separated by 4 h. Plasma samples were drawn at varying intervals over a period of 46 h and lumbar puncture (LP) was performed at either 14 h or 20 h after the first administration of probenecid. The concentration of probenecid, 5-HIAA and HVA in CSF was estimated and the probenecid-induced accumulation of 5-HIAA and HVA was compared with their baseline values. There were no statistically significant differences (P>0.05) in the accumulation of the monoamine metabolites between the two LP (14 h and 20 h), neither were there any differences in CSF concentrations of probenecid at the time of LP. There were only small differences in probenecid plasma concentrations, although statistically significant. Due to maximum blockade of the active transport system no correlation was observed between the CSF concentration of probenecid and the induced accumulation of 5-HIAA and HVA, respectively. The range of probenecid-induced accumulation for 5-HIAA and HVA in these volunteers was 156–429% and 183–600%, respectively. The suggested monitoring of probenecid plasma levels is proposed as a suitable model to investigate central neuronal activity of dopamine and serotonin in the central nervous system.     

Effect of the antiepileptic di-n-propylacetamide on 5-hydroxytryptamine turnover in the brain and 5-hydroxyindoleacetic acid level in the cerebrospinal fluid

The effect of antiepileptic drug di-n-propylacetamide (DPM) on 5-hydroxytryptamine (5-HT) turnover in rat brain and 5-hydroxyindoleacetic acid (5-HIAA) in cat cerebrospinal fluid (CSF) was investigated. DPM (200 mg/kg) increased brain 5-HIAA without altering the 5-HT level. DPM augmented the accumulation of 5-HT induced by monoamine oxidase inhibition with pargyline (80 mg/kg) and enhanced the accumulation of 5-HIAA in the brain following blockade of transport of this metabolite by probenecid (200 mg/kg). Prior inhibition of 5-HT synthesis by p-chlorophenylalanine (300 mg/kg) abolished the DPM-induced increase in cerebral 5-HIAA. DPM (100 mg/kg) given daily for 5 days considerably elevated 5-HIAA in the CSF of cat during the treatment period. We conclude that DPM increases the turnover of 5-HT in brain and that this can be observed by monitoring the 5-HIAA content of CSF.     

Increase in the plasma concentration of free tryptophan caused by probenecid in humans

Probenecid was administered orally in a dose of 1 g twice daily for 3 days to eight patients nutriated through a gastric tube with a standarized diet containing a known amount of tryptophan. Probenecid caused an increase by 52% (P<0.01) in the free (non protein-bound) concentration of tryptophan in plasma (from 1.22±0.16 to 1.86±0,28 g/ml; mean±SEM). The total (free + protein-bound) plasma tryptophan concentration was not significantly changed by the present dose of probenecid. There was a positive correlation (Spearmans rank correlation coefficient =0.74; P<0.05) between the increase in percentage free tryptophan and the achieved plasma concentration of probenecid. The cerebrospinal fluid (CSF) concentration of tryptophan was not significantly changed by probenecid (2 g/day during 21/2 days) given to another group of five patients.It is concluded from the present study, that the increase in the CSF concentration of 5-hydroxyindoleacetic acid (5-HIAA) caused by probenecid, in addition to blockade of the 5-HIAA transport out of the CSF, might be explained by an increased rate of synthesis of brain serotonin since the availability of its precursor is increased due to the probenecid-induced increase in the plasma concentration of free tryptophan.     

The influence of probenecid on the metabolism of serotonin,dopamine and their precursors in man

Probenecid inhibits the efflux of acid monoamine (MA) metabolites from the CNS. The probenecid-induced accumulation of these metabolites in the CSF in man supplies data on the central turnover of the corresponding amines. In this study an attempt was made to establish whether probenecid also exerts an influence on the metabolism of the MA precursors: tryptophan and tyrosine. The principal conclusions were the following.

1. Whereas in rats probenecid lowers the serum tryptophan level and elevates that in the brain (probably as a result of interference with the binding of tryptophan to serum albumins), the serum tryptophan level in human individuals is also lowered but the CSF concentration remains unchanged. After an oral tryptophan load, however, the CSF tryptophan concentration does increase.It is suggested that in man, endogenously released and exogenous tryptophan enter different metabolic pools in the brain.
2. After an oral load of 1-tryptophan, for the most part depressive patients showed a more rapid increase in CSF tryptophan concentration and a less marked rise of the CSF 5-HIAA level than did the psychologically undisturbed test subjects examined by Eccleston et al. It seems possible that, in the depressive patient, the tryptophan-metabolizing capacity is disturbed.
3. After an oral load of 1-tryptophan the CSF HVA concentration increased. Possibly, tryptophan is transformed to 5-HT in dopaminergic neurons, and this 5-HT may supersede the DA from the storage sites. This effect may well be of significance in the therapeutic application of 5-HT precursors.
4. The CSF concentrations of 5-HIAA, HVA, tryptophan and tyrosine show no systematic variations during the day.

     

Endogenous depressions with and without disturbances in the 5-hydroxytryptamine metabolism: A biochemical classification?

The influence of i.v. administered probenecid on the 5-HIAA concentration in the CSF was studied in a group of patients with vital (endogenous) depression and a non-depressive control group.The average increase of the 5-HIAA concentration in the CSF after probenecid administration was smaller in the depressive group than in the control group. Moreover, the depressive group included patients with a normal as well as patients with a subnormal probenecid effect. This could mean, that the group of v tal depressions encompasses two biochemically different categories: patients with and patients without demonstrable disturbances in the metabolism of 5-HT in the brain.This study was supported by grants from the Netherlands Organization for Pure Research (Z.W.O.).     

Accelerated metabolism of probenecid during long-term treatment of rats with anticonvulsant drugs: effect on central serotonin turnover studies

Studies were carried out in rats to determine the effects of long-term administration (once daily for 9 days) of phenytoin (50 mg kg^?1), sodium phenobarbital (25 mg kg^?1), primidone (50 mg kg^?1), and l-5-hydroxytryptophan (l-5-HTP; 50 mg kg^?1) on probenecid metabolism and on serotonin turnover rates as estimated by probenecid-induced accumulation of 5-hydroxyindoleacetic acid (5-HIAA) in brain. Without probenecid, mean brain 5-HIAA levels were similar in control and drugtreated rats, suggesting that the turnover rate of brain serotonin was not affected by the chronic anticonvulsant drug pretreatment. But, in the rats treated with phenobarbital, the rate of accumulation of 5-HIAA in brain during the first 90 min after probenecid (200 mg kg^?1) was significantly lower than the rate of accumulation in the control rats. Also, at 6 hr after probenecid, brain 5-HIAA levels were similar to pre-probenecid values in the rats pretreated with phenobarbital or primidone, while 5-HIAA levels were still increased in the rats treated with phenytoin, l-5-HTP, or vehicle. Examination of serum revealed that the concentration of probenecid in serum decreased more rapidly in rats pretreated with either primidone or phenobarbital than in rats given vehicle, l-5-HTP, or phenytoin. It is likely, therefore, that the decreased 5-HIAA accumulation in the brains from these animals were due to decreased inhibition of 5-HIAA efflux and not to a decreased rate of serotonin turnover in brain. Since a sustained inhibition of acid transport by probenecid is required, drug interactions with probenecid may be important in clinical studies using probenecid-induced accumulations of 5-HIAA in cerebrospinal fluid to estimate central serotonin turnover rates.     

Cerebrospinal fluid 5-hydroxyindoleacetic acid (5HIAA) and homovanillic acid (HVA) following probenecid in unipolar depressives treated with amitriptyline

Lumbar cerebrospinal fluid 5-HIAA, HVA, and the ratio 5-HIAA/HVA were measured followed probenecid administration in eleven patints with unipolar depression before and during treatment with amitriptyline (AMI). Control values were obtained from a group of inmate volunteers. Prior to treatment CSF 5HIAA formation in the depressives was not different from controls. During treatment with AMI, CSF 5-HIAA formation decreased. One patient with psychotic symptoms prior to AMI and two patients who developed psychotic reactions on AMI showed relatively low CSF 5HIAA formation prior to antidepressant therapy. Compared to controls CSF HVA values were higher in the depressives prior to AMI therapy.     

Estimation of the rate of 5-HT synthesis in the mouse brain by varius methods

Summary Non-isotopic and isotopic methods were used to estimate the rate of 5-HT synthesis in the mouse brain. 5-HT and 5_HIAA levels were measured in tissues up to 10 min after the i. p. injection of pargyline (75 or 150 mg/kg) or pheniprazine (5 or 10 mg/kg) (5-HT and 5-HIAA, MAO-inhibitor methods). 5-HIAA levels were also estimated at various times after probenecid administration (200 or 400 mg/kg i. p.) (5-HIAA-probenecid method). 5-HTP levels were estimated at various times after the blockade of 5-HTP decarboxylase by Ro4-4602. (800, 1000 or 1200 mg/kg i. p.) (5-HTP method). Finally the rate of conversion of tryptophan into 5-HT has been estimated by measuring the initial accumulation of ³H-5-HT and ³H-5-HIAA in tissues following the intravenous injection of ³H-tryptophan (isotopic method). Rates of 5-HT synthesis obtained with the two MAO inhibitor methods were much higher (about 3 to 4 times) than those calculated with the 5-HTP and 5-HIAA-probenecid methods. An intermediate rate of 5-HT synthesis was found with the isotopic technique. The high rate of 5-HT synthesis observed with the 5-HT, MAO inhibitor method (about 8 nmoles/g/h) was not related to a stimulation of 5-HT synthesis. Differences seen with all methods are discussed with respect to results obtained by various groups of workers.Research Fellow from the Laboratoire de Physiologie Acoustique CNRZ-INRA, Domaine de Vilvert, 78 Jouy-en-Josas.     

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-hydroxyindoleacetic 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.     

Influence of convulsive therapy on 5-hydroxyindoleacetic acid and homovanillic acid in cerebrospinal fluid in endogenous depression

The cerebrospinal fluid (CSF) levels of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) were determined in 20 patients with endogenous depression before and 2–6 days after a full course of convulsive therapy, the depressive symptoms being simultaneously rated.

1. The level of 5-HIAA was similar to that in previous series of healthy controls while the level of HVA appeared somewhat low.
2. The levels did not change after therapy in spite of considerable clinical improvement.
3. There was no relation between the levels and the severity of the depressive state, nor between changes of the levels and degree of clinical improvement.
4. There was no relation between the levels and the severity of the depressive state, nor between changes of the levels and degree of clinical improvement.

The validity of determination of acid monoamine metabolites in CSF relative to the cerebral turn-over of the amines may be increased, if the elimination of metabolites from CSF is blocked with probenecid.     

Differential effects of electrical stimulation,blockade of neuronal amine uptake and activation of α2-adrenoceptors on the release of endogenous noradrenaline and 5-hydroxytryptamine from the isolated rat pineal gland

Summary Isolated rat pineal glands were incubated in vitro and the release of endogenous noradrenaline or 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection. In the absence of test drugs, the spontaneous outflow of noradrenaline was about 10 fmol/10 min and electrical stimulation (5 Hz, 1500 pulses) evoked the release of about 70 fmol noradrenaline. Nomifensine enhanced the spontaneous outflow of noradrenaline about threefold and the electrically evoked release of noradrenaline about sixfold. In the presence of nomifensine, the ₂-adrenoceptor antagonist yohimbine markedly increased the electrically evoked release of noradrenaline, whereas the ₁-adrenoceptor antagonist prazosin had no effect. Clonidine inhibited the electrically evoked release of noradrenaline by about 65%, and this was antagonized by yohimbine in a competitive manner. In the absence of drugs, the initial spontaneous outflow of 5-HT was (compared with noradrenaline) very high 64 mol/10 min. It declined by 80% within 1 h of incubation in vitro. The outflow of 5-HIAA amounted initially to 38 mol/10 min and declined by 40% within 1 h of incubation. Addition of l-tryptophan (10 mol/1) after 1 h of incubation in vitro largely enhanced the outflow of 5-HT and 5-HIAA within 30 min of incubation (about ten- and fourfold, respectively). When l-tryptophan was present from the onset of incubation the initial outflow of 5-HT and 5-HIAA was only slightly elevated, but the decline was largely attenuated. Neither omission of calcium nor addition of nomifensine, clonidine or yohimbine significantly affected the spontaneous outflow of 5-HT or 5-HIAA. Likewise, neither electrical stimulation in the absence or presence of nomifensine and yohimbine nor stimulation by high potassium (45 mmol/1) significantly affected the outflow of 5-HT or 5-HIAA.In conclusion, the release of endogenous noradrenaline from the sympathetic nerves terminating in the pin eal gland is inhibited by presynaptic ₂-adrenoceptors. The outflow of 5-HT from the pineal gland originates almost exclusively from non-neuronal cells, most probably the pinealocytes, and depends largely on a continuous de novo synthesis. Catabolism of 5-HT to 5-HIAA in the pineal gland occurs mainly in an extraneuronal compartment, probably the pinealocytes and/or the interstitial cells of the pineal gland. Send offprint requests to K. Racké at the above address     

5-Hydroxyindole acetic acid and homovanillic acid in cerebrospinal fluid in manic-depressive psychosis and the effect of probenecid treatment

Summary The increased concentrations of 5-hydroxyindole acetic acid and homovanillic acid produced in cerebrospinal fluid by probenecid has been investigated in 15 manic-depressive patients and 21 psychiatric control patients, and has been related to the concentrations of probenecid in the CSF. The pharmacokinetics of probenecid were the same in the manic-depressive patients and the controls, as judged by its concentrations in plasma (bound and free) and CSF after a standard oral dose p.o., and by measurements of half-life and volume of distribution after intravenous injection. — The manic-depressive patients had lower concentrations of 5-HIAA and HVA than controls at similar CSF concentrations of probenecid; this was concluded from results with pairs of patients matched with regard to probenecid in CSF, and from differences between the patients and controls in the slopes of the regression lines for probenecid in CSF against 5-HIAA/HVA. The differences in 5-HIAA/HVA between the diagnostic groups were greater with increasing concentrations of probenecid in CSF; and, with concentrations of probenecid in CSF>1.0 µg/ml, by using the 5HIAA concentrations it was possible to classify the patients correctly into their diagnostic groups in 92% of cases.     

Absence of effect of para-chlorophenylalanine on 5-hydroxyindoleacetic acid in cerebrospinal fluid in man

The effect of para-chlorophenylalanine (PCPA) on the 5-hydroxy-indoleacetic acid (5-HIAA) content in human cerebrospinal fluid (CSF) has been studied. There was no effect on the CSF-content of 5-HIAA 12, 24 or 48 h after a single dose of PCPA 1 g p.o. Neither was there any effect after 1 g/day during 4 days. The increase of 5-HIAA after administration of probenecid was reduced during treatment with PCPA 1 g/day. This reduction, however, is not due to a diminished production of 5-HIAA by PCPA but probably caused by a pharmacological interaction between probenecid and PCPA since the concentrations of probenecid in CSF were lower during administration of PCPA than without that drug.It is concluded that PCPA in the doses used in this study gives no effects on the CSF-concentrations of 5-HIAA.     

The effect of CO2 on monoamine metabolism in rat brain

Summary White male albino rats were exposed to artificial atmosphere containing 20% of O₂ and 5, 10 and 15% of CO₂, for periods of time ranging from 30 min to 2 h. Monoamine synthesis in different dissected brain areas was measured by estimating Dopa and 5-hydroxytryptophan (5-HTP) accumulation after inhibition of their decarboxylation with the L-aromatic aminoacid decarboxylase inhibitor NSD 1015 (3-hydroxybenzylhydrazine). Endogenous levels of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), and NA and DA levels after inhibition of catecholamine synthesis with -methyltyrosine (-MT) were measured in dissected areas and whole brain.The synthesis of all three monoamines was found to be increased in all the cerebral regions. Endogenous dopamine levels with or without -MT treatment were increased while NA levels decreased. 5-HT decreased in at least one area and 5-HIAA increased in several brain areas. These observations indicate that the utilization of NA and 5-HT is increased and that of DA is diminished in acidosis, presumably as a consequence of a corresponding change in nerve impulse flow.The similarity between the monoamine changes induced by hypercarbia and by GABA are pointed out. It is suggested that GABA-ergic activity is elevated in acidosis and that GABA may be involved in the pH dependence of seizure thresholds, as well as in the increased tendency to respiratory depression induced by central nervous system depressant drugs in patients with chronic respiratory failure.     

Central effects of acetylcholine

The influence of acetylcholine (Ach) injected into the lateral ventricle of the brain on the behaviour of white, male Wistar rats was investigated, together with the effect on the levels of noradrenaline (NA), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) in discrete brain areas. Ach in doses of 1–100 g caused a decrease of locomotor activity of rats. A dose of 0.1 g produced the same effect in medium and high activity rats, but did not affect the behaviour of low activity rats. Atropine injected i.p. in a dose of 2.5 mg/kg abolished the behavioural effect of Ach. 50 g of Ach caused various changes of NA, 5-HT and 5-HIAA level in discrete brain areas. No correlation between biochemical and behavioural changes were seen.     

Characterization of 5-hydroxytryptamine release from isolated rabbit and rat trachea: the role of neuroendocrine epithelial cells and mast cells

Rabbit or rat isolated tracheae were incubated in vitro, and the release of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection.Release of 5-HT from rabbit tracheae could be evoked by the calcium ionophore A 23187 and, in a calcium-dependent manner, by depolarizing concentrations of potassium (45 mmol/1), but not by the mast cell degranulating drug compound 48/80. High potassium-and A 23187-evoked release of 5-HT was markedly higher from tracheae of newborn compared to adult rabbits. In rabbit tracheae, mechanical removal of the mucosa resulted in 80–90% reduction in tissue 5-HT and in a similar reduction in high potassium-evoked 5-HT release. 5-Hydroxytryptophan, but not tryptophan, caused a marked increase in the spontaneous outflow of 5-HT and 5-HIAA from tracheae of newborn rabbits, and the effect on 5-HT, but not that on 5-HIAA, required an intact mucosa. Furthermore, treatment with 5-hydroxytryptophan caused an increase in tissue 5-HT and 5-HIAA, and these effects required an intact mucosa. In tracheae of adult rabbits 5-hydroxytryptophan caused similar, although less profound, effects. Adrenaline (I mol/l) enhanced the release of 5-HT from newborn rabbit tracheae, and this effect was inhibited by 1 mol/l phentolamine or 1 mol/l prazosin, but not affected by 100 nmol/1 propranolol. In rat tracheae, compound 48/80 evoked a large release of 5-HT, whereas depolarizing concentrations of potassium (45 mmol/1) had only a very minor effect. In rat tracheae, 5-hydroxytryptophan had small effects on the outflow and tissue contents of 5-HT and 5-HIAA in comparison to the effects on rabbit tracheae; and removal of the mucosa resulted in only a minor reduction in tissue 5-HT.In conclusion, neuroendocrine epithelial (NEE) cells and mast cells are the major source of 5-HT in tracheae of the rabbit and rat, respectively. Isolated tracheae of newborn rabbits appear to be a useful model to study 5-HT secretion from NEE cells. 5-HT secretion from NEE cells is activated by a rise in intracellular calcium, and calcium influx through voltage-regulated channels appears to be one activating pathway. 5-HT secretion from NEE cells can be stimulated via -adrenoceptors.Dedicated to Prof. E. Mutschler on occasion of his 65th birthday     

Effect of a transverse cerebral hemisection on 5-hydroxytryptamine metabolism in the rat brain

Summary Hemisection of the brains of rats was performed at a level slightly rostral to the mesencephalo-diencephalic junction. In chronic experiments (7 to 14 days after the lesion) a considerable decrease in 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels was found in the ipsilateral hemiforebrain. In acute experiments (3 to 6 h after the lesion) the effect of the lesion on synthesis and metabolism of 5-HT was investigated by estimating tryptophan, 5-hydroxytryptophan (5-HTP), 5-HT and 5-HIAA levels in different parts of the brain. In order to get an insight into the different steps in 5-HT metabolism inhibitors of enzymes or transport were used (Ro 4-4602 [N-(dl-seryl)-N¹-(2,3,4-trihydroxybenzyl) hydrazine], pargyline, and probenecid). The effect of l-tryptophan loading was also investigated.The accumulation of 5-HTP and 5-HT following inhibition of 5-HTP decarboxylase and monoamine oxidase, respectively, was not significantly influenced by the hemisection. On the other hand, the accumulation of 5-HIAA following probenecid treatment, and the disappearance of 5-HT and 5-HIAA following inhibition of 5-HTP decarboxylase and monoamine oxidase, respectively, were slightly or moderately retarded.—The tryptophan level was moderately increased on the side of the lesion.The data indicate that a considerable synthesis and metabolism of 5-HT occurs intraneuronally even in the absence of an impulse flow. The nerve impulses induce an increased release and extraneuronal metabolism, primarily at the expense of stored 5-HT. In addition, the nerve impulses possibly cause a certain increase in 5-HT synthesis.     

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 a 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.Is 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.     

Cholinergic modulation of the release of 5-hydroxytryptamine from the guinea pig ileum

Summary Isolated segements of the guinea pig ileum were vascularly perfused and the release of 5-HT and its metabolite 5-HIAA into the portal venous effluent determined by HPLC with electrochemical detection. Test substances were applied via the arterial perfusion medium. Oxotremorine inhibited concentration-dependently the release of 5-HT and 5-HIAA (by 47% at 1 mol/l). Scopolamine (0.1 mol/1) did not affect the release of 5-HT and 5-HIAA, but antagonized the effect of oxotremorine. In the presence of TTX (1 mol/1) oxotremorine (1 pmol/1) increased the release of 5-HT by 150% and that of 5-HIAA by 220%. This increase was completely blocked by scopolamine. Hexamethonium (100 pmol/1) and TTX (1 pmol/1) reduced the release of 5-HT by 32 and 40%, respectively. DMPP (10 pmol/1) increased the release of 5-HT by 57%, and this effect was prevented by hexamethonium. Neither DMPP nor hexamethonium significantly affected the release of 5-HIAA. The enhancing effect of DMPP on 5-HT release was increased and prolonged in the presence of TTX or scopolamine. Nicotine (1, 10 or 30 mol/l) alone did not cause a consistent increase in the release of 5-HT. However, in the presence of scopolamine nicotine increased the release of 5-HT by 57%. In conclusion, the release of intestinal 5-HT is facilitated via muscarine and nicotine receptors located on the enterochromaffin cells. Indirect evidence suggests that the release of 5-HT is additionally modulated by an as yet unknown inhibitory neurotransmitter released by muscarine receptor activation.Abbreviations DMPP 1,1-dimethyl-4-phenylpiperazinium - 5-HT 5-hydroxytryptamine - 5-HIAA 5-hydroxyindoleacetic acid - TTX tetrodotoxin Send offprint requests to H. S. at the above address