The inhibition of A and B forms of MAO in the production of a characteristic behabioural syndrome in rats after 1-tryptophan loading

Immunodetection of the serotonin transporter protein is a more valid marker for serotonergic fibers than serotonin

Tracking serotonergic pathways in the brain through immunodetection of serotonin has widely been used for the anatomical characterization of the serotonergic system. Immunostaining for serotonin is also frequently applied for the visualization of individual serotonin containing fibers and quantification of serotonin positive fibers has been widely used to detect changes in the serotonergic innervation. However, particularly in conditions with enhanced serotonin metabolism the detection level of serotonin may lead to an underestimation of the true number of serotonergic fibers. The serotonin transporter (SERT) protein, on the other hand, is less liable to metabolism and for that reason we hypothetized that SERT immunostaining is a more stable marker of serotonergic fibers. Rats were pretreated with monoamine oxidase (MAO) inhibitor and compared with placebo treated rats. Brains were double immunostained for serotonin and SERT protein and colocalization was quantified in several brain areas by confocal microscopy. In comparison with untreated rats, MAO inhibitor treated rats had a significantly higher number (almost 200% increase) of serotonin immunopositive fibers whereas no difference was observed in the number of the SERT positive fibers. Colocalization between serotonin and SERT positive fibers was close to 100% in MAO inhibitor treated animals but only 30% in untreated rats. We conclude that the rapid metabolism of serotonin leads to an underestimation of immunodetected serotonergic fibers and that in many instances, SERT immunostaining may be a better indicator of serotonergic fibers.

Cellular expression of mRNAs encoding monoamine oxidases A and B in the rat central nervous system

Monoamine oxidases A and B (MAO-A and MAO-B) oxidatively deaminate neurotransmitter and xenobiotic amines. The cellular localization of these isoenzymes in the central nervous system (CNS) differs markedly and only partly reflects the distribution of their presumed natural substrates. In the present study, by using in situ hybridization with 35S-labelled oligonucleotide probes, we examined the distribution of mRNAs encoding MAO-A and MAO-B in the rat CNS. Probes for tyrosine hydroxylase, histidine decarboxylase, and tryptophan hydroxylase mRNAs were used to demonstrate the catecholaminergic, histaminergic, or serotoninergic nature of some cell populations in adjacent sections. The radioligands [3H]-Ro 41-1049 and [3H]lazabemide (reversible and selective inhibitors of MAO-A and MAO-B, respectively) were used to reveal the protein distribution by enzyme radioautography. The distribution and abundance of transcripts for both isoenzymes in the tissues investigated differed markedly but, in general, correlated with the protein distribution. MAO-A mRNA and protein were most abundant in noradrenergic neurons. However, moderate levels of transcript expression and protein were also detected in the serotoninergic neurons, and low but significant levels were detected in the dopaminergic neurons. An unexpectedly remarkable degree of hybridization signal was apparent in nonaminergic cell populations, e.g., in the cerebral cortices, the hippocampal formation (CA1-3, dentate gyrus), the cerebellar granule cell layer, and the spinal cord motoneurons. In contrast, MAO-B mRNA and protein were most abundant in serotoninergic and histaminergic neurons, Bergmann glial cells, and circumventricular organs, including the ependyma. MAO-B transcripts were also weakly expressed in nonaminergic cells, e.g., in the hippocampal formation (CA1-2). A further nonneuronal localization of MAO-B transcripts was also resolved, e.g., in the glia limitans, the olfactory nerve layer, and the cerebellar peduncle. These findings reveal further the potential of various cell populations to synthesize the isoenzymes, and homologous (aminergic) and heterologous (nonaminergic) patterns of expression as well as coexpression of MAO mRNAs are described.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

Relationship between extracellular 5-hydroxytryptamine and behaviour following monoamine oxidase inhibition and L-tryptophan

1. The present study investigates the effects of selective and a non-selective monoamine oxidase (MAO) inhibitors combined with L-tryptophan on MAO-A and -B activity, hypothalamic extracellular 5-hydroxytryptamine (5-HT) in vivo and the occurrence of the 5-HT behavioural syndrome. 2. Selective inhibition of intraneuronal MAO-A with MDL 72394 (0.5 mg kg-1, i.p.) had no effect on extracellular 5-HT and following administration of L-tryptophan (50 mg kg-1, i.p.) the 5-HT behavioural syndrome was not induced. 3. Selective inhibition of MAO-A at all sites with clorgyline (5 mg kg-1, i.p.) increased extracellular 5-HT but did not induce the 5-HT behavioural syndrome when combined with L-tryptophan administration. 4. Selective inhibition of MAO-B with selegiline (10 mg kg-1, i.p.) had no effect on extracellular 5-HT and the 5-HT behavioural syndrome was not observed after L-tryptophan administration. 5. Inhibition of MAO-A and -B with a higher and therefore non-selective, dose of MDL 72394 (2 mg kg-1) markedly increased extracellular 5-HT but failed to induce the 5-HT behavioural syndrome after L-tryptophan administration. 6. Inhibition of MAO-A and -B at all sites in the brain (tranylcypromine 20 mg kg-1, i.p. or clorgyline 5 mg kg-1 plus selegiline 10 mg kg-1) increased extracellular 5-HT and induced the behavioural syndrome on administration of L-tryptophan. 7. The results demonstrate that inhibition of MAO-A and -B both within amine neurones and elsewhere in the brain is essential for the development of the 5-HT behavioural syndrome. Whilst the syndrome is associated with increased extracellular 5-HT this does not appear necessarily to result in the syndrome and may indicate that increased extracellular 5-HT is not solely involved in the induction of the '5-HT behavioural syndrome'. Whilst the syndrome is associated with increased extracellular 5-HT this does not appear necessarily to result in the syndrome and may indicate that increased extracellular 5-HT is not solely involved in the induction of the '5-HT behavioural syndrome'.

Effects of antidepressant drug combinations on cortical 5-HT2 receptors and wet-dog shakes in rats

Rats pretreated with the monoamine oxidase inhibitor, phenelzine 18 h (46.8 mg/kg) and 90 min (11.7 mg/kg) previously or only 90 min (46.8 mg/kg) previously developed a 5-HT dependent syndrome (including wet-dog shakes, WDS) when given the 5-HT uptake inhibitor, paroxetine (11.6 mg/kg). After 2 h, but only in rats pretreated with 2 injections of phenelzine, there was a gradual reduction in the number of cortical 5-HT2 receptors, determined in vitro with [3H]ketanserin, and this was temporally related to a reduction in the frequency of WDS. Both effects (down-regulation and WDS) were prevented by the 5-HT2 receptor antagonist, pirenperone. A second injection of paroxetine at 3 h evoked additional WDS in rats pretreated with 1 injection of phenelzine but not in rats pretreated with 2 injections, suggesting that spinal 5-HT2 receptors might also have been down-regulated at the same time. Similar results were obtained when rats were pretreated instead with the selective MAO A inhibitor, clorgyline or when given either citalopram or fenfluramine instead of paroxetine. 5-HTP also evoked WDS in phenelzine-treated rats and markedly increased brain 5-HT concentration but only slowly down-regulated 5-HT2 receptors; in carbidopa-treated animals, 5-HTP was without effect on receptor numbers despite production of frequent WDS. It thus appears that drugs which increase synaptic 5-HT (as indicated by production of WDS) by interference with the release or reuptake of 5-HT more readily down-regulate 5-HT2 receptors than 5-HTP which does not directly affect these mechanisms.

Combined inhibition of serotonin uptake and oxidative deamination attenuates audiogenic seizures in DBA/2J mice

We previously reported that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) attenuated audiogenic seizures (AGS) in 21-day-old DBA/2J mice and also inhibited brain monoamine oxidase-A (MAO-A) and serotonin (5-hydroxytryptamine, 5-HT) uptake leading to increased brain 5-HT concentration. In this study, the sensitivity of AGS to 5-HT manipulation was evaluated by utilizing drug combinations which paralleled the actions of 6-MeO-THBC and which also have been associated with the production of a serotonergic motor syndrome in rats. Combination of a specific 5-HT uptake inhibitor (fluoxetine or citalopram) with the MAO-A inhibitor clorgyline inhibited AGS more effectively than the individual drugs but combination with the MAO-B inhibitor deprenyl did not. Combined administration of clorgyline plus deprenyl also suppressed AGS. Inhibition of AGS by tryptophan was potentiated by combination with either of the mixed MAO inhibitors nialamide or tranylcypromine. The effects of these drugs individually and in combination on brain MAO-A and MAO-B activity and 5-HT uptake were also determined ex vivo and were consistent with expected mechanisms of action. These results suggest, first of all, that the inhibition of AGS produced by 6-MeO-THBC is a consequence of its combined MAO-A and 5-HT uptake inhibition properties. Secondly, the similarity of results of pharmacological manipulations of the 5-HT system which produce the rat motor syndrome and which inhibit AGS in the mouse suggests that AGS in 21-day-old DBA/2J mice may be a useful system for assessing functional consequences of these serotonergic manipulations.

Interactions between relatively selective monoamine oxidase inhibitors and an inhibitor of 5-hydroxytryptamine re-uptake, clomipramine

Features of interactions with combined antidepressants in man were evoked by clomipramine in rats pretreated with both the relatively selective monoamine oxidase (MAO) inhibitors clorgyline and deprenyl, but not when clomipramine was given to rats pretreated with deprenyl or clorgyline alone, i.e. inhibition of both MAO A and B was a likely prerequisite for clomipramine to elicit the syndrome (with the larger dose of clorgyline and deprenyl, MAO A and B inhibition exceeded 95%). The features evoked were myoclonic--forelimb flexor-extensor movements, wet dog shakes and head and body twitches; hyperthermia and ECG anomalies also developed, and locomotor activity was augmented. Myoclonic phenomena were prevented when the above pretreatment also included p-chlorophenyl-alanine, but were unaffected or even intensified when pretreatment instead included alpha-methyl-p-tyrosine; these phenomena were attenuated or abolished by pirenperone, a 5HT2 antagonist. Relevance of these findings to safer combinations of antidepressants is discussed.

Effect of nonselective and selective inhibitors of monoamine oxidases A and B on pethidine toxicity in mice

The LD50 of pethidine was determined in mice pretreated (4 h) either with the nonselective monoamine oxidase (MAO) inhibitor, phenelzine or with clorgyline, a selective inhibitor of MAO A or deprenyl, a selective inhibitor of MAO B. Phenelzine or combined clorgyline plus deprenyl pretreatments decreased pethidine LD50. Clorgyline or deprenyl alone did not affect pethidine toxicity. Whole brain 5-hydroxytryptamine (5-HT) concentrations were measured in the pretreated mice. 5-HT levels were approximately doubled (P less than 0.001) after phenelzine or clorgyline plus deprenyl treatment, but not after clorgyline or deprenyl given alone. These results indicate that both MAO A and MAO B need to be inhibited to increase pethidine toxicity and brain 5-HT levels. They support the involvement of 5-HT in the toxic interaction between pethidine and MAO inhibitors.

Clinical and experimental aspects of interactions between amine oxidase inhibitors and amine re-uptake inhibitors

Dangerous and even fatal interactions can occur in man following combinations of antidepressants which include non-selective MAO inhibitors. To ascertain the causation, interactions reproducing the clinical phenomena have been elicited in animals with these combinations, and the mechanisms involved have been explored by various pharmacological strategies; 5-HT re-uptake inhibitors proved especially hazardous in combination. Interactions could, however, be avoided even with the 5-HT re-uptake inhibitors, by combination with relatively selective MAO A or B inhibitors, an approach with potential clinical value.

Behavioural hyperactivity in rats treated with selective monoamine oxidase inhibitors and LM 5008, a selective 5-hydroxytryptamine uptake blocker

The administration of 4-[2-(3-indolyl)ethyl]piperidine (LM 5008), a selective 5-hydroxytryptamine (5-HT) uptake blocker to rats pretreated with tranylcypromine (Tcp) resulted in a behavioural syndrome of locomotor hyperactivity which is indistinguishable from that following combined treatment with Tcp and L-tryptophan. A similar behavioural response was elicited by the administration of LM 5008 to rats pretreated with 5-hydroxytryptophan. The response to LM 5008 after monoamine oxidase (MAO) inhibition was abolished by pretreatment with p-chlorophenylalanine, indicating the involvement of 5-HT in producing the hyperactivity syndrome. The administration of imipramine and chlorimipramine in combination with Tcp also resulted in hyperactivity, but these drugs were much less potent than LM 5008 in producing the syndrome. In contrast to L-tryptophan, which can produce hyperactivity only after the inhibition of both type A and type B MAO, LM 5008 can elicit the syndrome after selective inhibition of MAO type A only but not after inhibition of MAO type B. The behavioural results indicate that when MAO type A is inhibited, LM 5008 treatment elicits hyperactivity by preventing the availability of 5-HT to be metabolized by MAO-B component.

Stereoselective effect of tranylcypromine enantiomers on brain serotonin

Brain levels of 5-HT, 5-HIAA and L-tryptophan were measured in reserpinized rats given the enantiomers of tranylcypromine (TCP). (+)-TCP (1.5-30 mg/kg) increased levels of 5-HT, whereas (-)-TCP failed to affect 5-HT levels. (+)-TCP at high dosage (30 mg/kg) decreased brain levels of 5-HIAA, whereas (-)-TCP failed to affect 5-HIAA levels. Neither (+)-TCP nor (-)-TCP influenced brain levels of L-tryptophan. The effects of (+)-TCP on behavior resembled those of drugs which influence 5-HT processes, whereas (-)-TCP failed to produce 5-HT-dependent behaviors. The findings support the notion that TCP enantiomers have stereoselective effects on serotonergic mechanisms.

Peripherally administered serotonin decreases food intake in rats

We report that intraperitoneal injection of serotonin produces a dose-related decrease in the food intake of hungry rats. The efficacy of serotonin was increased by prior treatment with clorgyline, a type A monoamine oxidase inhibitor. Doses of serotonin which were anorectic did not significantly impair locomotor activity or sensorimotor performance. Further, 2 mg/kg serotonin (ED50 on food intake) did not produce a conditioned taste aversion when paired repeatedly with sucrose ingestion. We conclude that the anorectic effects of serotonin are not secondary to nonspecific effects of the agent, and suggest that peripheral serotonin may play a role in normal satiation.

Inhibition of monoamine oxidase activity after combined action of chlordimeform with the antidepressant nialamide

The 40-day oral administration of 5, 10, and 50 mg/kg chlordimeform to male rats moderately decreased monoamine oxidase activity (MAO) in the brain, liver, and serum, determined with the substrates kynuramine, tyramine, tryptamine, serotonin, dopamine, and benzylamine. The enzyme was inhibited predominantly in the liver. The study of MAO inhibition after 10 days application of 2, 6, 10, 15, and 20 mg/kg nialamide revealed that 2 mg/kg had a threshold effect. A dose of 6 mg/kg resulted in a pronounced decrease in the enzyme activity. Experiments with a 30-day application of 5, 10 and 50 mg/kg chlordimeform were made, followed by treatment with 6 mg/kg nialamide in combination for another 10 days. The statistical evaluation by the Student's t-test demonstrated increased MAO inhibitory action. Dose-effect relationship was established with kynuramine in liver, brain, and serum, and with benzylamine in liver.

The effects of 5-HT uptake- and MAO-inhibitors on L-5-HTP-induced excitation in rats

The behavioural syndrome caused by L-5-HTP in rats was used for the study of effects of selective 5-HT uptake inhibitors and inhibitors of MAO on central 5-HT receptors. A good correlation was found between the relative potencies of drugs in inhibiting the 5-HT uptake in the rat brain and in intensifying L-5-HTP-induced behavioural stimulation. The potentiation of the L-5-HTP syndrome by the MAO inhibitors correlated with the inhibition of the A- but not of the B-form of the brain monoamine oxidase. In rats treated with the maximally inhibiting dose of a 5-HT uptake inhibitor, MAO inhibitors were still able to increase the intensity of the L-5-HTP syndrome, while the combination of maximal doses of two 5-HT uptake inhibitors did not produce a more intense syndrome than that produced by one 5-HT uptake inhibitor alone. The L-5-HTP-induced behavioural syndrome in rats seems to afford an experimental model allowing the quantification and characterization of the interaction of drugs with serotonin metabolism in the brain.

Effects of 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THbetaC) on audiogenic seizures in DBA/2J mice

It was found previously that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THbetaC) increased brain concentration of the neurotransmitter serotonin (5-HT) and decreased the concentration of its metabolite 5-hydroxyindole acetic acid (5-HIAA) at the same time the compound attenuated audiogenic seizures (AGS) in DBA/2J mice. In the present study we determined the time-course and dose-response effects of 6-MeO-THbetaC for blockade of AGS. Drugs sharing common effects with 6-MeO-THbetaC were also tested. At a dose of 100 mg/kg, 6-MeO-THbetaC blocked AGS between 10 min and 12 hr after injection, with maximal inhibition at 1 hr at which time a dose-related decrease in AGS was also demonstrated. All of the drugs tested which blocked AGS, including 6-MeO-THbetaC, THbetaC, 5-Hydroxytryptophan, chlorimipramine and pargyline, have biochemical similarities suggesting that facilitating serotonin function may be responsible for seizure-attenuating effects.

Irritable aggression and open field behavior in rats given 2-phenylcyclopropylamines

Adult male Wistar rats given an i.p. injection (3 ml/kg) of a 2-phenylcyclopropylamine (59 mumol/kg) were tested 3--5 h later for shock-induced aggression, jump threshold, stereotypic movement, rearing and ambulation. Shock-induced aggression was facilitated by trans-1-2-phenylcyclopropylamine (1-TPCP) and was suppressed by trans-d-2-phenylcyclopropylamine (d-TPCP). Neither drug affected jump threshold. The effects of 1-TPCP and d-TPCP on irritable aggression are discussed in terms of the effects of the drugs on monoaminergic mechanisms. A behavioral syndrome of increased stereotypic behavior of the serotonin-dependent type as well as decreased rearing and ambulation occurred in rats given d-TPCP, trans-dl-2-phenylcyclopropylamine, 2-(4-chlorophenyl)-cyclopropylamine and 2-(3,4-dichlorophenyl)-cyclopropylamine, but not in rats given 1-TPCP, cis-dl-2-phenylcyclopropylamine, 2-(2-chlorophenyl)-cyclopropylamine or 2-(2,5-dichlorophenyl)-cyclopropylamine. The findings suggest that the behavioral syndrome depended in part on the absence of a chlorine substitution in the ortho position of the phenyl ring.

Effects of tranylcypromine stereoisomers, clorgyline and deprenyl on open field activity during long term lithium administration in rats

Locomotor activity of male rats was studied in an open field after an i.p. injection (15 mg/kg) of the d- or l-isomers of tranylcypromine (d-Tc and l-Tc, respectively) or after s.c. injection of either clorgyline (0.5, 1 or 5 mg/kg) which selectively inhibits Type A MAO or deprenyl (0.5, 5 or 10 mg/kg) which selectively inhibits Type B MAO. The rats were fed a diet containing either no lithium (control group) or lithium chloride (lithium group) for at least 28 days prior to tests. In the control group, d-Tc increased ambulation while l-Tc, deprenyl and clorgyline failed to affect activity. In the lithium group, d-Tc and deprenyl increased ambulation, l-Tc increased ambulation and rearing, while clorgyline failed to affect activity. Lithium appeared to potentiate the behavioral effects of deprenyl and l-Tc. Symptoms of serotonin-dependent hyperactivity appeared in the control group and lithium group given d-Tc. The role of biogenic amines in the effects of the drugs on open field activity is discussed.

Effects of monoamine oxidase inhibition by clorgyline, deprenil or tranylcypromine on 5-hydroxytryptamine concentrations in rat brain and hyperactivity following subsequent tryptophan administration

1 The effect of various doses of tranylcypromine on the degree of inhibition of rat brain monoamine oxidase (MAO) using 5-hydroxytryptamine (5-HT), dopamine and phenylethylamine as substrates has been examined 120 min after injection of the inhibitor. The concentration of brain 5-HT was also examined both after tranylcypromine alone and also when L-tryptophan (100 mg/kg) had been given 30 min after the tranylcypromine. 2 All doses of tranylcypromine greater than 2.5 mg/kg totally inhibited MAO oxidation of 5-HT, phenylethylamine and dopamine as measured in vitro and produced a similar rise of brain 5-HT in vivo. When tryptophan was also given, there was a further rise of brain 5-HT, which was comparable after all doses of tranylcypromine above 2.5 mg/kg and the characteristic syndrome of hyperactivity made is appearance. 3 Clorgyline (a "Type A" MAO inhibitor), in doses up to 10 mg/kg, did not totally inhibit MAO activity towards phenylethylamine although it did inhibit 5-HT oxidation by 100%. Deprenil (a "Type B" MAO inhibitor) at doses up to 10 mg/kg did not fully inhibit 5-HT oxidation although phenylethylamine oxidation was inhibited almost completely. Administration of either compound alone did not produce as great an accumulation of brain 5-HT as that seen after tranylcypromine (2.5 mg/kg) and subsequent administration of tryptophan did not cause hyperactivity or the rise of brain 5-HT seen after tranylcypromine (2.5 mg/kg) plus tryptophan. 4 Administration of clorgyline plus deprenil (2.5 mg/kg of each) almost totally inhibited oxidation of both 5-HT and phenylethylamine; subsequent tryptophan administration resulted in a rise of brain 5-HT nearly as great as that seen following tranylcypromine (2.5 mg/kg) plus tryptophan and the animals became hyperactive. 5 No evidence was found pointing to the formation of any other 5-substituted indole in the brain following tranylcypromine plus L-tryptophan administration as suggested by others. 6 It is concluded that while 5-HT may normally be metabolized in the brain by "Tye A" MAO in vivo, when this form is inhibited, 5-HT can still be metabolized by "Type B" enzyme. It is only when both forms are almost totally inhibited that the largest rise of brain 5-HT is seen and subsequent tryptophan administration produces the hyperactivity syndrome.

The monoamine oxidase B inhibitor deprenyl potentiates phenylethylamine behaviour in rats without inhibition of catecholamine metabolite formation

The drug l-deprenyl has been reported to have antidepressant properties, and in the present study three possible mechanisms of action were investigated in animal experiments. l-Deprenyl, which is a type B monoamine oxidase (MAO) inhibitor, was compared to clorgyline, an MAO A inhibitor with regard to its inhibitory effect on the formation of three major catecholamine metabolites, homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxyphenylglycol (MOPEG) in the rat brain in vivo. Apart from a difference in dose levels the two drugs showed no difference in the dose--response pattern of all three metabolites. Clorgyline inhibited the formation of HVA, DOPAC and MOPEG with an ED50 of about 0.2 mg/kg s.c. and l-deprenyldopamine and noradrenaline are formed by the same type of monoamine oxidase(s), probably type A, in the rat brain in vivo. Antidepressant properties of l-deprenyl therefore seem to be independent of catecholamine deamination. l-Deprenyl but not clorgyline (2 or 8 mg/kg s.c.) potentiated the stereotyped sniffing behaviour induced by beta-phenylethylamine, a specific substrate for type B monoamine oxidase. This result is discussed in relation to a new hypothesis of phenylethylamine and dopamine involvement in depression. l-Deprenyl was 10,000 times less potent than DMI as inhibitor of noradrenaline uptake in crude synaptosomes from the occipital cortex of rat brain. Inhibition of noradrenaline uptake was therefore excluded as a possible mechanism for the antidepressant action of l-deprenyl.