Ethanol-induced increase in brain concentrations of administered neutral amino acids

Summary Ethanol 2 g kg^–1 i.p. to rat increased the concentrations in the brain of administered large neutral amino acids (tyrosine, tryptophan, 5-hydroxytryptophan and -methyldopa). We have previously found a similar effect of ethanol on administered l-Dopa, resulting in increased brain/plasma ratios of dopa. Since large neutral amino acids are known to compete with each other for the carrier-mediated transport into the brain we suggest that the increased concentrations of the administered amino acids in the brain are at least partly the consequence of the ethanol-induced decrease in plasma amino acids observed previously.     

Effects of l-tryptophan on motor activity in mice

The effects of l-tryptophan (50–800 mg/kg i.p.) on motor activity in mice were studied. l-tryptophan, 800 mg/kg, caused a reduction of motor activity while lower doses had no significant effect. The role of different metabolites of l-tryptophan in behavioural depression was tested by pretreating groups of animals with inhibitors of tryptophan hydroxylase (p-chlorophenylalanine), peripheral aromatic amino acid decarboxylase (DC) (MK-486), peripheral and central DC (NSD 1015), or tryptophan pyrrolase (allopurinol). None of these pretreatments antagonized the l-tryptophan induced depression of motor activity. Pretreatment with MK-486 or NSD-1015 potentiated the depression. Pretreatment with allopurinol potentiated as well as prolonged the depressive effect. Pretreatment with chlorimipramine had no significant effect on the l-tryptophan induced depression. The elevations of brain tryptophan and 5-hydroxytryptamine (5-HT) concentrations after l-tryptophan (800 mg/kg i.p.) were prolonged by pretreatment with allopurinol. The l-tryptophan induced increases in brain concentrations of 5-HT and 5-hydroxyindoleacetic acid were more pronounced after pretreatment with allopurinol. It is suggested that the l-tryptophan induced reduction of motor activity in mice is mediated via the amino acid itself and not via its metabolites.     

Biochemical and pharmacological properties of a peripherally acting catechol-O-methyltransferase inhibitor entacapone

Summary Entacapone, OR-611, was found to be a potent peripherally acting inhibitor of catechol-O-methyl-transferase (COMT). IC₅₀ values of 10 nmol/1 and 160 nmol/1 were obtained for rat duodenum and liver-soluble COMT, respectively. There were no effects on other catecholamine metabolizing enzymes. Entacapone showed reversible, tight-binding type of inhibition of soluble rat liver COMT with a K; value of 14 nmol/1 and it also caused 50% inhibition of rat duodenal, erythrocyte, liver and striatal COMT activity 1 h after oral dosing with 1.1, 5.4, 6.7 and 24.2 mg/kg, respectively. However, penetration of entacapone into the brain was poor, since the formation of homovanillic acid (HVA), the O-methyl metabolite of dopamine in the striatum, was not reduced, even after the highest dose of 30 mg/kg.In rat blood serum, the concentration of 3-0-methyldopa (3OMD), the O-methylated product of l-dopa, was reduced in a dose-dependent manner, and the concentration of l-dopa was increased after the administration of entacapone (3 - 30 mg/kg p. o.) together with l-dopa + carbidopa. These changes were reflected, in the striatum, by a significant rise in the dopamine concentration and a reduction in the 30MD concentration.Consequently, when entacapone was added to the treatment with l-dopa + carbidopa, the dose of l-dopa could be lowered from 50 mg/kg to 15 mg/kg in order to produce the same striatal dopamine concentrations as with 50 + 50 mg/kg of l-dopa + carbidopa alone.Correspondence to E. Nissinen at the above address     

The effect of tryptophan and a tryptophan/5-hydroxytryptophan combination on indoles in the brains of rats fed a tryptophan deficient diet

Rats maintained on a tryptophan deficient diet had reduced brain and serum tryptophan and brain 5-hydroxy-indolacetic acid levels compared to controls. 5-Hydroxytryptophan and l-tryptophan administered to these deficient rats in a combination (595) produced a greater elevation of indolamines and tryptophan in the brain than either amino acid alone. In rats maintained on a normal diet the urinary output of 3-hydroxykynurenine was considerably reduced by treatment with the combination of amino acids as com pared to tryptophan treatment. 5-Hydroxytryptophan reduced the induction of kynurenine synthesis in the liver produced by tryptophan, implying that it is capable of inhibiting the enzyme tryptophan pyrrolase in vivo. It is suggested that the possession by 5-hydroxytryptophan of tryptophan pyrrolase inhibitory properties may make the administration of the combination a better treatment of depressed patients exhibiting an indolamine deficit than either amino acid alone.     

Biotransformation of locally applied l-dopa in the corpus striatum of the hemi-Parkinsonian rat studied with microdialysis

Microdialysis was used to study the biotransformation of l-dopa in intact and denervated striata of rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the substantia nigra. Microdialysis probes were placed in the intact and in the denervated striatum. Observations were then made on freely moving rats. Extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid; HVA) were monitored before, during and after the local administration of l-dopa via the microdialysis probe for 20 min.A dose-dependent increase in extracellular dopamine levels was seen in intact striatum after application of l-dopa in concentrations ranging between 100 nmol/l and 10 mol/l. In the denervated striatum, the severity of the lesion influenced dopamine formation, so that no dose-effect relation could be established.The effects of the continuous intra striatal infusion of nomifensine, tetrodotoxin or benserazide on the l-dopa-induced dopamine outflow revealed that in the intact striatum this dopamine release is mainly voltage dependent. It was concluded that in the denervated striatum other cells of non-neuronal origin and containing aromatic l-amino acid decarboxylase make a major contribution to the increase in extracellular dopamine levels. Furthermore, l-dopa itself shows no dopamine-releasing properties, at least under the present experimental conditions. Correspondence to: S. Sarre at the above address     

Decarboxylation of orally administered l-dopa in the human digestive tract

Summary Carboxyl labelled l-dopa was administered orally and intravenously to Parkinsonian patients and control subjects. Total radioactivity was measured in the expired air, urine and plasma. The activity in plasma was also measured before and after removal of carbon dioxide-dicarbonate. After intravenous administration about four times more radioactivity was recovered in the urine than after oral administration with a roughly corresponding decrease in the recovery from expired air, whereas a considerably smaller proportion of the total radioactivity in plasma was derived from carbon dioxide-bicarbonate. It is concluded that the major fraction of the orally administered l-dopa undergoes decarboxylation in the digestive tract before reaching the general circulation. No difference was observed between the Parkinsonian and the non-Parkinsonian group.     

Evidence that the large neutral amino acidl-valine decreases electrically-evoked release of 5-HT in rat hippocampus in vivo

l-Valine competes with tryptophan for transport into the brain and has previously been shown to decrease brain 5-HT synthesis. In the present study, the effect ofl-valine on electrically evoked hippocampal 5-HT release was determined in the anaesthetized rat using microdialysis. In control animals two electrical stimulations of the dorsal raphe nucleus 120 min apart (S1 and S2, respectively) released similar amounts of 5-HT. In contrast, in animals which receivedl-valine (200 mg/kg) between stimulations, S2 released a significantly smaller amount of 5-HT than did S1, although basal 5-HT release was unchanged. The data demonstrate thatl-valine decreases the electrically-evoked release of 5-HT in hippocampus in vivo.     

Striatal dopamine turnover and l-dopa treatment after short-term exposure of rats to manganese

The turnover rate of striatal dopamine (DA) and the effect of l-dopa treatment was investigated in rats after the daily oral administration of MnCl₂ · 4H₂O for a period of 30 days. The turnover rate of striatal DA, as determined by the administration of -methyl-p-tyrosine, increased significantly in manganese-exposed rats. l-Dopa administration resulted in a significant elevation in the levels of DA and its metabolite, homovanillic acid, in manganese-exposed rats, but these neurochemical changes could not be correlated with the concentration of manganese in the striatum. We therefore advise that l-dopa therapy should not be tried in early manganese intoxication, as it may aggravate manic symptoms due to marked increase in brain DA.     

Effects of d-amphetamine,maprotiline, l-dopa,and haloperidol on the components of the predatory behavior of the ferret,Putorius furo l.

Ferret predation on rats was examined in an arena. One hour before the test one of the following drugs was administered: d-Amphetamine (0.8 and 1.4 mg/kg IM), maprotiline (10 and 40 mg/kg orally), l-dopa (30 and 60 mg/kg orally), or haloperidol (0.14 and 0.6 mg/kg IM). Provided that capture was successful, the sequence of the behavioral components was not changed by these drugs. With the exceptions of paw movements and rolling over, which were not affected by the drugs, the components of predatory behavior were influenced differently. This leads to the assumption that a drug affects different mechanisms which control behavior. It is assumed that dopamine is involved in the control of capture elicitation as well as in the control of pursuit and biting. Capture elicitation was inhibited by d-amphetamine and l-dopa, but not by maprotiline, and was even facilitated by haloperidol. The orientation of pursuit movements and biting was impaired by l-dopa and improved by haloperidol, whereas maprotiline did not influence these components.     

Influence of l-3-methoxytyrosine on monoamine synthesis in rat brain

Summary Rats were injected i.p. with l-3-methoxytyrosine, 100 or 300 mg/kg. One h later brain, liver, heart and blood plama were analyzed for catecholamines and their precursors. In brain Dopa as well as dopamine and noradrenaline levels were unchanged, while demethylation of l-3-methoxytyrosine might have occurred in peripheral organs since Dopa levels in liver and dopamine in heart were elevated. 3-Methoxytyramine could not be detected in brain and liver after treatment with l-3-methoxytyrosine.Monoamine synthesis in vivo was measured in whole brain by determining the accumulation of Dopa and 5-hydroxytryptophan 30 min after the i.p. injection of NSD 1015 (3-hydroxybenzylhydrazine HCl, 100 mg/kg), an inhibitor of the aromatic amino acid decarboxylase. l-3-Methoxytyrosine attenuated the formation of Dopa and 5-hydroxytryptophan by about 25% in brain tissue. The effect was paralleled by a decrease in the brain concentration of tryptophan.     

Pavlovian conditioning ofl-dopa induced movement

Using the unilateral 6-hydroxydopamine (6-OHDA) substantia nigra pars compacta (SNC) lesion rat model and a Pavlovian conditioning protocol, the present investigation determined that the contralateral rotation response induced by the antiparkinsonian dopaminergic drugl-dopa can become conditioned to exteroceptive test environment stimuli. Two non-drug conditioning tests indicated that contralateral rotation was elicited by the test environment without the presence ofl-dopa. This conditioned response had a rotation diameter profile that was qualitatively the same as thel-dopa induced contralateral rotation response. Additionally, drug tests with the combined dopaminergic receptor antagonists, SCH 23390 (0.1 mg/kg) and haloperidol (0.5 mg/kg), at doses sufficient to block spontaneous behavior andl-dopa (20 mg/kg)-induced rotation, revealed that the conditioned contralateral rotation response, unlikel-dopa-induced contralateral rotation, is not affected by D₁/D₂ receptor blockade. Thus, the conditoned stimuli of the test environment can elicit the contralateral rotation response even in animals rendered akinetic by D₁/D₂ antagonists. This activation of a conditioned dopaminergic drug response by the situational stimuli, independent of dopaminergic mechanisms, may, therefore, contribute to the untoward overstimulation clinical effects ofl-dopa through summation of conditioned and drug-induced effects. Furthermore, the use of conditioning procedures to elicit movement in akinetic animals may provide a new research methodology to investigate the phenomenon of paradoxical kinesia.     

The amino acid <Emphasis Type="SmallCaps">l</Emphasis>-lysine blocks the disruptive effect of phencyclidine on prepulse inhibition in mice

Rationale The cognitive and attentional deficits observed in schizophrenic patients are now considered central to the pathophysiology of the disorder. These deficits include an inability to filter sensory input as measured by, e.g., prepulse inhibition (PPI) reflex. Administration of phencyclidine (PCP), a drug that can induce a schizophrenia-like psychosis in humans, disrupts PPI in experimental animals. In rodents, this PCP-induced deficit can be blocked by pretreatment with nitric oxide (NO) synthase inhibitors. This suggests that some of the behavioral effects of PCP are mediated via NO. The substrate for in vivo NO production is l-arginine, and active transport of l-arginine via the cationic amino acid transporter may serve as a regulatory mechanism in NO production. Objectives The aim of the present study was to study the effects of l-arginine transport inhibition, using acute and repeated l-lysine treatment, on PCP-induced disruption of PPI in mice. Results Subchronic, and to some extent acute, pretreatment with l-lysine blocked a PCP-induced deficit in PPI without affecting basal PPI. Conclusions l-lysine has been shown to block l-arginine transport in vitro, most likely via a competitive blockade and down regulation of cationic amino acid transporters. However, the importance of l-arginine transport as a regulatory mechanism in NO production in vivo is still not clear. The present results lend further support to the notion that some of the effects of PCP in the central nervous system are mediated via NO and that l-arginine transport may play a role in the regulation of NO production in the brain.     

Behavioral evidence for supersensitivity after chronic bromocriptine administration

Behavioral evidence for tolerance and supersensitivity during and after chronic (30 day) administration of bromocriptine (BRC) or bromocriptine + L-dopa in mice was assessed by measuring wheel running (WR) behavior during and after chronic drug administration, and apomorphine- and methylphenidate-(MP-) induced stereotyped gnawing after termination of chronic injections. In both BRC and BRC +L-dopa groups, tolerance developed fairly quickly to the depressing effect of BRC on WR seen on day 1 of drug administration. Mice receiving BRC showed significant increases in WR by week 2 of chronic drug administration, which persisted for at least two days after the termination of chronic injections. During the first week after termination of chronic injections, low doses of both apomorphine and MP induced significantly more stereotyped gnawing in BRC and BRC + L-dopa mice than in the control mice or the mice treated with L-dopa alone. This behavioral evidence for dopaminergic supersensitivity after chronic BRC administration may have relevance for the clinical use of BRC in combination with L-dopa or other dopamine agonists.     

The effect of l-dopa and propranolol on human CSF cyclic nucleotides

Human CSF cyclic AMP and cyclic GMP have been measured as possible indicators of activity of central neurotransmitter-sensitive adenylate or guanylate cyclase. In an attempt to help to identify the specific neurotransmitter systems of origin of human CSF cyclic AMP and GMP, we studied Parkinson patients with and without l-dopa therapy and schizophrenic patients before and after propranolol therapy. No effect of l-dopa or propranolol was found on CSF cyclic nucleotides. However, Parkinson patients had a 40–50% reduction of CSF cyclic AMP and a 80–90% reduction of CSF cyclic GMP compared with the schizophrenic patients. Implications of this finding are discussed.     

Demethylation of l-3-methoxytyrosine in vivo

Summary After administration of purified l-¹⁴C-3-methoxytyrosine (l-¹⁴C-3-MTO) to rats, the fractions of labelled amino acids, catecholamines and phenolcarboxylic acids of urine and brain have been separated by column chromatography. Prior to performing the quantitative determinations, the individual metabolites of each urinary fraction and of the cerebral catecholamine fraction were isolated by paper chromatography using different systems. Susbtantial amounts of ¹⁴C-3,4-dihydroxyphenylacetic acid (¹⁴C-DOPAC) as well as some ¹⁴C-3,4-dihydroxyphenylalanine (¹⁴C-DOPA) and traces of dopamine (DA) appeared in the urine. Furthermore, small amounts of ¹⁴C-DA and ¹⁴C-norepinephrine were found in the brain with two different chromatographic systems. The urinary excretion of ¹⁴C-DOPAC and ¹⁴C-DA was increased by pretreatment with dopacetamide, an inhibitor of catechol-3-O-methyl-transferase. A possible contamination of the l-¹⁴C-3-MTO with traces of l-¹⁴C-DOPA as a major source of the dihydroxylated metabolites has been ruled out. It is concluded that part of l-3-MTO undergoes demethylation in vivo and that the finding of DA in brain and urine after administration of l-3-MTO is not an artifact.     

Characterisation of dyskinesias induced byl-dopa in MPTP-treated squirrel monkeys

Intermittent treatment withl-dopa over a 2-year period induced abnormal involuntary movements in MPTP-treated squirrel monkeys. Dyskinesias included a choreic and dystonic component. Dose-response curves for chorea and dystonia revealed that the same dose ofl-dopa (30 mg/kg) induced the highest score for both dyskinesias; however, the severity was much greater for chorea. Choreic movements were always most prevalent at the time of peak effect, whereas dystonia was apparent at the time of peak effect and at end-of-dose, and was occasionally observed spontaneously. Our findings indicate that squirrel monkeys treated with MPTP developl-dopa-induced dyskinesias which closely resemble those observed in Parkinson's disease. This species provides a valuable animal model to develop improved therapeutic agents.     

The metabolic fate of infusedl-tryptophan in men: possible clinical implications of the accumulation of circulating tryptophan and tryptophan metabolites

l-Tryptophan (Trp) was widely used as a natural tool for the support of serotonin-mediated brain functions and as a challenge probe for the assessment of serotonin-mediated neuroendocrine responses. The metabolic fate of the administered Trp and the kinetics of the accumulation of Trp metabolites in the circulation, however, have never thoroughly been investigated.This study describes the time- and dose-dependent alterations in the plasma levels of various Trp metabolites and large neutral amino acids after the infusion of Trp to healthy young men (1, 3 and 5 g; placebo-controlled, double-blind, cross-over study during day- and night-time).The major Trp metabolites (kynurenine, indole acetic acid and indole lactic acid) in plasma increased dose-dependently but rather slowly after Trp administration to reach their maximal plasma levels (up to 10-fold after the 5 g dose) at about 3 h p.i., and remained at an elevated level (about 5-fold) for up to 8 h. N-acetyl-Trp and 5-hydroxy-Trp rose rapidly and massively after Trp infusions, at the 5 g dose more than 200- and 20-fold, respectively, and declined rapidly to about 5-fold baseline levels within 2 h. Whole blood serotonin levels were almost unaffected by the Trp infusions. A rather slow increase of 5-hydroxyindole acetic acid was seen, reaching maximum values (3-fold at the 5 g dose) at about 2 h after the infusion of Trp. Additionally, a dose-dependent rise of circulating melatonin was observed afterl-Trp infusions. The administration ofl-Trp caused a depletion of the concentrations of the other large neutral amino acids and a dose dependent decrease of the ratio between plasma tyrosine and the sum of the plasma concentrations of the other large neutral amino acids. Apparently, none of the existing pathways of peripheral Trp metabolism is saturated by its substrate, Trp in men. At least some of the central effects reported afterl-Trp administration may be mediated by the Trp-stimulated formation of neuroactive metabolites or by the decreased availability of tyrosine for catecholamine synthesis.Part of this work was presented at the International Symposium on Future Prospects ofl-Tryptophan in Medicine, Heidelberg, Germany, November 22–23, 1991     

Tryptophan tolerance and metabolism in endogenous depression

The concentration of free and total tryptophan and kynurenine in plasma from 49 female depressives and 26 female controls was measured following oral loading with l-tryptophan, 100 mg/kg body weight. There was no significant difference between five depressives and six controls in the area under curve for free or total tryptophan or kynurenine in plasma. The peak concentration of kynurenine occured 4 h after loading and it correlated significantly with the area under curve for kynurenine. There was no significant correlation between the l-tryptophan dose (g) and the plasma concentration of kynurenine at 4 h in the 49 depressives or 26 controls. The mean plasma levels of tryptophan and kynurenine at 4 h in the depressives were not significantly different from control levels. There was no clear relationship between the plasma levels of tryptophan or kynurenine at 4 h and the therapeutic response in 13 depressives treated with l-tryptophan for 14 days.It is concluded that the absorption, the plasma clearance, and the degradation to kynurenine of loading doses of l-tryptophan are normal in depressed patients. Results further-more suggest that the plasma levels of tryptophan and kynurenine at 4 h are poor predictors of the response to l-tryptophan treatment in depressives.     

Failure of MIF-I to affect behavioral responses in patients with parkinson's disease under l-dopa therapy

In eight subjects with Parkinson's disease under an optimal daily dose of l-dopa, acute administration of MIF-I (200 mg i.v.) did not ameliorate either the total disability score or the intellectual test PM 38 when evaluated in comparison with the effect induced by acute administration of a placebo. Also concomitant evaluation of the effect of MIF-I on the secretion of anterior pituitary hormones which are under dopaminergic control i.e., growth hormone and prolactin, did not reveal any potentiation of the l-dopa-induced stimulus.     

Benzodiazepines: Specific competitors for the binding of l-tryptophan to human serum albumin

Summary By means of the gel filtration technique, the effect of nine benzo-diazepine derivates on the binding of l-tryptophan to human serum albumin was investigated. Using equimolar tryptophan and benzodiazepine concentrations, all benzodiazepines with binding constants higher than 10⁴ (M^–1), displace l-tryptophan from its binding site to a high degree. The mechanism of the displacement was characterized as a competition for a common binding site. Some of the benzodiazepines displace l-tryptophan to a greater extent than salicylic acid. The benzodiazepines and tryptophan are the only substances known with a high degree of stereospecific binding to human serum albumin. This study shows that there is only one binding site on the human serum albumin molecule, which binds tryptophan and the benzodiazepines in a highly stereospecific manner. Therefore it is concluded that the benzodiazepines and l-tryptophan must have similarities in their molecular structure, so that both can bind to the common binding site in such specific manner. These considerations are discussed in regard to the known influence of benzodiazepine derivatives on the l-tryptophan metabolism in brain. A direct involvement of the reported displacement in the pharmacological actions of the drugs seems not to be relevant because of their small therapeutical plasma levels.