Change in serotonin metabolism in the rat brain in response to the repeated stimulus presentation

The content of serotonin (5-HT), its metabolite 5-hydroxyindoleacetic acid (5-HIAA), monoamine oxidase (MAO) activity and kinetic parameters (K(m) and Vmax) for the reaction of 5-HT deamination, were examined in various regions of the rat brain after repeated presentation of a contextual stimulus. Habituation to the stimulus was accompanied by an increase of 5-HT metabolism and active transport of 5-HIAA in the amygdala, striatum and midbrain, while these changes were not found in the prefrontal cortex and hippocampus. Kinetic studies have revealed that the enhancement of 5-HT deamination by MAO in the brain structures was mediated by different catalytic mechanisms. A significant decrease in K(m) value for 5-HT deamination in the amygdala indicated an increase in the affinity of enzyme towards 5-HT. In the striatum the enhanced MAO activity was provided by increasing maximal rate of 5-HT deamination. It is concluded that an activation of presynaptic mechanisms of the serotonergic transmission in the amygdala and striatum is involved in the inhibition of biological significance and attention to repeated presentation of stimulus.     

Plasma 5-Hydroxyindoleacetic Acid as an Indicator of Monoamine Oxidase-A Inhibition in Rat Brain and Peripheral Tissues

We have examined the changes induced by the monoamine oxidase (MAO; EC 1.4.3.4) inhibitors tranylcypromine, clorgyline, and deprenyl on MAO activity and 5-hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in rat brain and blood (plasma and whole blood). The decreases of MAO-A activity observed in the liver and lungs after different doses of clorgyline or tranylcypromine correlated significantly (r > 0.80 in all cases) with the decline of plasma 5-HIAA. This was unaffected by 0.25 and 5 mg kg^?1 of deprenyl, indicating that 5-HT was deaminated exclusively in the periphery by MAO-A. It is interesting that very potent and significant correlations (r > 0.75) were found between plasma 5-HIAA and MAO-A activity, 5-HIAA and 5-HT content in brain tissue. These results suggest that plasma 5-HIAA can be used confidently as a peripheral indicator of the inhibition of MAO-A in brain. This may represent a favorable alternative to the analysis of 5-HIAA in CSF in psychiatric patients undergoing antidepressant treatment with nonspecific MAO inhibitors or with the new selective MAO-A inhibitors.     

Distinctions in metabolism of serotonin and dopamine in rat brain during latent inhibition

Latent inhibition (LI) is a behavioral phenomenon, in which repeated presenting of a non-reinforced stimulus retards conditioning to this stimulus when it is coupled with a reinforcer. In order to find specific serotonin (5-HT- and dopamine (DA) changes mediating the LI, the 5-HT and DA metabolism was investigated in certain brain regions. Oxidative deamination of 5-HT and DA by monoamine oxidase (MAO) was determined in the prefrontal cortex, striatim, amygdala, and hippocampus at preexposure and testing stages of the LI using the passive avoidance procedure in rats. Preexposed animals demonstrated high MAO activity for 5-HT deamination in the amygdala and striatum and lower MAO activity for DA deamination in the amygdala and hippocampus. After testing the LI, a high level of 5-HT deamination by MAO was revealed in the amygdala, white the lower level of 5-HT deamination by MAO was shown in the prefrontal cortex. At the same time, no changes in DA metabolism were found in all the brain regions studied. Thus, the role of dopaminergic system in the LI effect may be limited by the preexposure stage. The obtained evidence suggests that the enhanced 5-HT activity in the amygdala and striatum induced by the preexposed stimulus is a principal biochemical mechanism underlying the LI.     

5-HYDROXYTRYPTAMINE CATABOLISM IN THE RAT BRAIN DURING ONTOGENESIS

Although the serotoninergic innervation is immature in the brains of young rats, the 5-HIAA content is similar to that found in adults. As indicated by the ratio of 5-HIAA to 5-HT levels in the brain stem and the forebrain, the catabolism of the indolamine was more rapid during the first 3 postnatal weeks than in adults. This was contirmed by measuring the total formation of [³H]5-HIAA from [³H]5-HT newly synthesized from L-[³H]tryptophan in brain stem slices of young and adult rats. Electrolytic lesions of midbrain raphe nuclei (B7 and B8) performed on the 5th postnatal day resulted in parallel decreases in brain 5-HT and 5-HIAA levels; this ruled out the possibility that 5-HIAA might be formed from 5-HT synthesized outside serotoninergic neurons, using peripheral 5-hydroxytryptophan. Inhibition of 5-HT storage by reserpine pretreatment did not alter the higher capacity of newborn tissues to catabolize exogenous [³H]5-HT. Therefore, possible differences in 5-HT binding in serotoninergic neurons between newborn and adult rats were not likely to account for the differences in 5-HT catabolism. Estimation of the rate of 5-HIAA efflux from the brain after MAO inhibition did not reveal marked changes with age. The activity of MAO type A, the enzyme involved in 5-HT catabolism, was higher during early life than later on. This could be shown by using 5-HT as substrate and clorgyline as a selective inhibitor. An opposite pattern of development was seen for MAO B, measured with benzylamine as substrate and deprenyl as selective inhibitor. These results suggest that the high 5-HIAA levels found in the brains of young rats can be attributed mainly to the presence of high MAO A activity during early life.     

Role of monoamine oxidase-B in medroxyprogesterone acetate (17-acetoxy-6 alpha-methyl-4-pregnene-3, 20-dione) induced changes in brain dopamine levels of rats

The effect of medroxyprogesterone acetate (MPA) on brain monoamine levels and monoamine oxidase (MAO) activity was studied in adult, healthy, non-pregnant female rats. MpA was injected in a single dose of 100 mg/kg i.m. Dopamine (DA), noradrenaline (NA), 5-hydroxytryptamine (5-HT) levels and MAO activity were estimated fluorometrically in rat brian. No change in DA, NA, 5-HT or MAO activity was observed after 7 days of MPA treatment while a significant decrease in DA levels along with a significant increase in MAO activity was observed after 21 days of MPA treatment. However, there was no change in NA and 5-HT levels after 21 days of MPA administration. The selective reduction of DA by MPA could be due to an increase in MAO-B activity. MPA does not appear to increase MAO-A activity because neither of the specific substrates (NA and 5-HT) of MAO-A was found to be decreased inspite of the increase in MAO activity as estimated by the kynuramine method. These findings suggest the importance of MAO-B also in DA metabolism in rat brain.     

Seasonal and Daily Variations in Hypothalamic Monoamine Levels and Monoamine Oxidase Activity in the Teleost Channa Punctatus (Bloch)

In Channa punctatus, a significant daily variation in hypothalamic S-HT level and monoamine oxidase (MAO) activity was noticed in preparatory phase (February), but not in prespawning (May) or postspawning (November) phases. Hypothalamic dopamine (DA) and noradrenaline (NA), on the other hand, showed marked daily variation in their levels during all the three seasons with peak values in the photophase. The overall activity of MAO (mean ± SEM on 24-hr period) increased from November to May through February, whereas the 5-HT content which was high in November decreased during February and May. The NA and DA levels were low in November and Februry and high in May. The catecholamine (CA) content and MAO activity increased with increasing photoperiod and temperature which is indicative of an enhanced CA metabolism.     

Seasonal and Daily Variations in Hypothalamic Monoamine Levels and Monoamine Oxidase Activity in the Teleost Channa Punctatus (Bloch)

In Channa punctatus, a significant daily variation in hypothalamic S-HT level and monoamine oxidase (MAO) activity was noticed in preparatory phase (February), but not in prespawning (May) or postspawning (November) phases. Hypothalamic dopamine (DA) and noradrenaline (NA), on the other hand, showed marked daily variation in their levels during all the three seasons with peak values in the photophase. The overall activity of MAO (mean ± SEM on 24-hr period) increased from November to May through February, whereas the 5-HT content which was high in November decreased during February and May. The NA and DA levels were low in November and Februry and high in May. The catecholamine (CA) content and MAO activity increased with increasing photoperiod and temperature which is indicative of an enhanced CA metabolism.     

Inhibitory effects of cis- and trans-resveratrol on noradrenaline and 5-hydroxytryptamine uptake and on monoamine oxidase activity

This study investigated for the first time the potential effects of cis- and trans-resveratrol (c-RESV and t-RESV) on noradrenaline (NA) and 5-hydroxytryptamine (5-HT) uptake by synaptosomes from rat brain, on 5-HT uptake by human platelets, and on monoamine oxidase (MAO) isoform activity. Both c-RESV and t-RESV (5-200 microM) concentration-dependently inhibited the uptake of [3H]NA and [3H]5-HT by synaptosomes from rat brain and the uptake of [3H]5-HT by human platelets. In both experimental models, t-RESV was slightly more efficient than c-RESV. Furthermore, in synaptosomes from rat brain, the RESV isomers were less selective against [3H]5-HT uptake than the reference drug fluoxetine (0.1-30 microM). On the other hand, both c-RESV and t-RESV (5-200 microM) concentration-dependently inhibited the enzymatic activity of commercial (human recombinant) MAO isoform (MAO-A and MAO-B) activity, c-RESV being slightly less effective than t-RESV. In addition, both RESV isomers were slight but significantly more selective against MAO-A than against MAO-B. Since the principal groups of drugs used in the treatment of depressive disorders are NA/5-HT uptake or MAO inhibitors, under the assumption that the RESV isomers exhibit a similar behaviour in humans in vivo, our results suggest that these natural polyphenols may be of value as structural templates for the design and development of new antidepressant drugs with two important biochemical activities combined in the same chemical structure: NA/5-HT uptake and MAO inhibitory activity.     

Changes in brain monoamine oxidase activity in conditioned passive avoidance reaction in rats]

Activity of the enzyme monoamine oxidase (MAO) and kinetic parameters (Km, Vmax) for the 5-hydroxytryptamine (5-HT) and dopamine deamination were examined in the brain of rats with conditioned passive avoidance recall. Changes of the 5-HT and dopamine deamination were found in amygdala, striatum and frontal cortex. MAO activity was not changed in hippocampus. In amygdala the rate of 5-HT deamination was significantly increased and kinetic studies revealed increased affinity of the enzyme for 5-HT. The metabolism of dopamine in amygdala was unchanged. In frontal cortex the deamination of 5-HT was not changed, but the dopamine deamination significantly decreased. This decrease was due to lowering of MAO affinity for dopamine. In striatum the metabolism of both 5-HT and dopamine was reduced, and kinetic studies showed the lowering of Vmax for 5-HT and dopamine deamination.     

Hyperserotonergic phenotype after monoamine oxidase inhibition in larval zebrafish

Serotonin (or 5-hydroxytryptamine; 5-HT) and monoamine oxidase (MAO) are involved in several physiological functions and pathological conditions. We show that the serotonergic system and its development in zebrafish are similar to those of other vertebrates rendering zebrafish a good model to study them. Development of MAO expression followed a similar time course as the 5-HT system. MAO expression and activity were located in or adjacent to serotonergic nuclei and their targets, especially in the ventral hypothalamus. MAO mRNA was detected in the brain from 24 h post-fertilization and histochemical enzyme activity from 42 h post-fertilization. Deprenyl (100 μM) decreased MAO activity 34–74% depending on the age. Inhibition of MAO by deprenyl strongly increased 5-HT but not dopamine and noradrenaline levels. Deprenyl decreased 5-HT-immunoreactivity in serotonergic neurons and induced novel ectopic 5-HT-immunoreactivity neurons in the diencephalon in a manner dependent on 5-HT uptake. Deprenyl administration decreased locomotion, altered vertical positioning and increased heart rate. Treatment with p -chlorophenylalanine normalized 5-HT levels and rescued the behavioral alteration, indicating that the symptoms were 5-HT dependent. These findings suggest that zebrafish MAO resembles mammalian MAO A more than MAO B, metabolizing mainly 5-HT. Applications of this model of hyperserotonergism include pharmacological and genetic screenings.     

SUBCELLULAR DISTRIBUTION OF 5-HYDROXYTRYPTAMINE IN RAT BRAIN DURING DEVELOPMENT: EFFECT OF DRUGS AND FASTING

Abstract— Distribution of brain 5-HT content between the high-speed supernatant and particulate fractions under normal and experimental conditions was studied in postnatal and adult rats. In adult and 35-day-old rats the 5-HT content of the supernatant fraction was about 25% of that of the total homogenate and significantly higher than that in 1, 7 and 21-day-old rats. In 1-day-old rats fasting caused an increase of 100% in the turnover, 50% in the content and no alteration in the subcellular distribution of brain 5-HT, which suggests that under normal conditions 5-HT stores may be filled near to capacity. After 5-hydroxytryptophan administration, the 5-HT content of the adult rat brain increased 3-fold and that of the supernatant fraction to 35% of 5-HT content of the total homogenate. In postnatal rats, the brain 5-HT content rose to an adult level and the supernatant 5-HT percentage to a markedly higher than adult level, probably because of the known higher than adult 5-hydroxytryptophan decarboxylase activity of brain capillaries. Administration of tranylcypromine to adult rats caused a 2.6-fold increase of brain 5-HT content and a slight increase of the supernatant 5-HT percentage. At various times after the administration of the MAO inhibitors (tranylcypromine or pargyline) and fasting to the 1-day-old rats, brain 5-HT content increased 4, 5 and 7-fold, respectively, and the supernatant 5-HT rose consistently and, as in the adult, to about 30% of the 5-HT content of the total homogenate. After pargyline following reserpine pretreatment, the 5-HT content of the adult and 1-day-old rat brain increased to 2–3 times the control level and that of the supernatant fraction to about 40% of the 5-HT content of the total homogenate. The adult values for 5-HT in the particulate fraction of the 1-day-old rats after the drug treatments are in sharp contradiction to the low endogenous 5-HT content and known lack of nerve terminals and synaptic vesicles in their brains, and suggest that after MAO inhibition brain 5-HT neurons may bind the amine by some other mechanism than the Mg²⁺-ATP-dependent, reserpine-sensitive granular storage.     

Qualitative and quantitative changes in monoamine oxidase activity after acute third ventricle treatment of GABA, muscimol, and picrotoxin in rat

The effect of acute IIIrd ventricle injection of GABA, muscimol, and picrotoxin on the activity of monoamine oxidase (MAO) has been investigated in serum and a few hypothalamic nuclei of the rat brain using biochemical, histochemical, and cytophotometric techniques. Biochemical estimation demonstrated a significant reduction in MAO enzyme activity after GABA and muscimol injection, whereas picrotoxin produced pronounced increase in the enzyme activity. Histochemical and cytophotometric studies confirmed the biochemical findings. Even in brain, GABA and muscimol inhibited and picrotoxin stimulated the MAO activity. From the above findings, it may be concluded that GABA, muscimol, and picrotoxin regulate the MAO activity, possible mechanisms for which are being discussed.     

Effects of Methiothepin and Lysergic Acid Diethylamide on Serotonin Release In Vitro and Serotonin Synthesis In Vivo: Possible Relation to Serotonin Autoreceptor Function

Abstract: An in vitro system characterizing the presyn- aptic serotonin (5-HT) autoreceptor which controls the release of 5-HT from rat brain slices is described. Using this system, methiothepin (1–10 μ M) demonstrated 5-HT autoreceptor antagonist activity -by enhancing 5-HT release, while several recognized postsynaptic 5-HT receptor antagonists were inactive: mianserin, cinanserin, cyproheptadine, methysergide. The activity of methiothepin was highest in hypothalamic slices and lowest in striatal slices and was inhibited by the autoreceptor agonists lysergic acid diethylamide (LSD) and 5-methoxy- tryptamine (5-MT). The reversal of the methiothepin-enhanced 5-HT release from hypothalamic slices by LSD was not influenced by 0.3 μ M tetrodotoxin. The peripheral administration of LSD to rats has been shown to reduce 5-HT synthesis and release by a mechanism thought to involve, in part, an autoreceptor-mediated reduction in impulse flow of 5-HT neurons. In the present experiments, intraperitoneal injection of methiothepin antagonized the LSD-induced reduction in hypothalamic 5-HT synthesis (5-hydroxytryptophan accumulation) while exerting no influence by itself. Conversely, compounds which were not active as 5-HT autoreceptor antagonists in vitro (i.e., cyproheptadine, methysergide, cinanserin) did not influence the effect of LSD on 5-HT synthesis. Further, the reduction in 5-hydroxytryptophan (5-HTP) accumulation by LSD showed regional differences in inhibition by methiothepin (hypothalamus > cortex > striatum) which paralleled the autoreceptor antagonist activity of methiothepin in vitro. These data suggest that similar autoreceptor mechanisms control 5-HT release and synthesis in terminal 5-HT projection areas and that the reduction in 5-HT accumulation by LSD and the antagonism by methiothepin may represent a useful biochemical measure of 5-HT autoreceptor activity in vivo.     

Phospholipid dependency of carp brain and liver mitochondrial monoamine oxidase

1. The effects of lipid-protein interactions on carp brain and liver mitochondrial MAO with respect to substrate and inhibitor preference, thermostability and Arrhenius parameters were studied and compared.2. Treatment with phospholipase A₂, C or D decreased MAO activities towards 5-hydroxytryptamine (5-HT), β-phenylethylamine and tyramine similarly, accompanied by great changes in their apparent affinities for MAO, but not by changes in V_max values.3. Minimum phospholipid binding to mitochondria might be essential for enzyme activity.4. Among these activities, 5-HT deamination was the most sensitive to the changes in mitochondrial phospholipids and bulk lipid phase transition (fluidity).5. Sensitivity of MAO to clorgyline or l-deprenyl was not affected by these phospholipase treatments.6. Of the phospholipids tested, only phosphatidylinositol significantly activated MAO activity towards 5-HT in both intact and phospholipase-treated mitochondria.     

Monoaminoxidase activity and serotonin metabolism in the brain of rats during latent inhibition]

The monoamine oxidase (MAO) activities and the concentrations of 5-HT and 5-hydroxyindoleacetic acid were investigated in four brain regions in rats during the acquisition of latent inhibition in one-trial passive avoidance task. 5-HT metabolism was not altered in the hippocampus. Changes of 5-HT metabolism were found in the frontal cortex during testing of latent inhibition and were accompanied by lowering of MAO activity. No change of 5-HT metabolism was observed in this structure at the stage of pre-exposition to conditioned stimulus. 5-HT metabolism was activated at the stage of pre-exposition to conditioned stimulus in the amygdala and striatum and was maintained on high level, in these structures during testing of latent inhibition. The data presented here indicate that serotoninergic system in various brains regions is specifically involved in the formation of different stages of latent inhibition.     

Selective inhibition of MAO-A in serotonergic synaptosomes by two amphetamine metabolites, p-hydroxyamphetamine and p-hydroxynorephedrine

The present study was carried out mainly to clarify whether the two amphetamine metabolites, p-hydroxyamphetamine (P-OHA) and p-hydroxynorephedrine (p-OHN) are taken up by mouse brain 5-hydroxytryptamine (5-HT) nerve terminals to inhibit type A monoamine oxidase (MAO-A) and then potentiate the abnormal behavior, head-twitch. Of the two metabolites, only intracerebroventricular p-OHA, at 80 μg/mouse, sufficient to cause a head-twitch response (HTR), appreciably inhibited MAO-A activity without affecting MAO-B activity in homogenates of the mouse striatum, hypothalamus and the rest of the forebrain; and p-OHN did not inhibit either type of MAO at the dose tested. Estimation of intra- and extrasynaptosomal MAO-A activity showed that both metabolites significantly inhibited only the intrasynaptosomal deamination of 5-HT by MAO-A with p-OHA being more potent. Taken together with our previous findings, these present results clearly indicate that p-OHA may accumulate in the 5-HT nerve terminals through the uptake system, and concomitantly inhibit MAO-A activity. These actions of p-OHA may increase intraneuronal 5-HT levels and then potentiate 5-HT release to cause interaction with the post-synaptic 5-HT receptors.     

TRYPTOPHAN AND 5-HYDROXYTRYPTAMINE METABOLISM IN THE IMMATURE RAT BRAIN DURING RECOVERY FROM ASPHYXIA

Abstract— The turnover of cerebral 5-HT in the 5-day-old rat during recovery from asphyxia was assessed in the brain by determining 5-hydroxyindoleacetic acid concentration, and by following the accumulation of 5-HT after inhibition of MAO. Marked increases in turnover were found to persist for up to 3 h after resuscitation and this increase was accompanied by increases in brain tryptophan, plasma total tryptophan and the fraction of the plasma total tryptophan that was not bound to albumin. None of these parameters were different from those in control animals 24 h after resuscitation. Brain and plasma tyrosine levels remained unaltered by the treatment at all times investigated.     

The involvement of brain 5-HT(1A)-receptors in genetically determined aggressive behavior

The hypothesis was tested that one of the critical mechanisms underlying genetically determined aggressiveness involves brain serotonin 5-HT(1A)-receptors. The expression of 5-HT(1A)-receptor mRNA in brain structures and functional correlate for 5-HT(1A)-receptors identified as 8-OH-DPAT-induced hypothermia were studied in Norway rats bred over the course of 59 generations for the low and high affective (defensive) aggressiveness with respect to man and in highly aggressive (offensive) MAO A-knockout mice (Tg8 strain). Considerable differences between the aggressive and the nonaggressive animals were shown. Agonist of 5-HT(1A)-receptor 8-OH-DPAT (0.5 mg/kg for rats and 2.0 mg/kg for mice, i.p.) produced a distinct hypothermic reaction in nonaggressive rats and mice and did not affect significantly the body temperature in aggressive animals. In aggressive rats, a significant reduction of the expression of 5-HT(1A)-receptor mRNA was found in the midbrain. In Tg8 mice, 5-HT(1A)-receptor mRNA level was increased in the frontal cortex and amygdala and not changed in the hypothalamus and the midbrain. The results provide support for the idea that brain 5-HT(1A)-receptors contribute to the genetically determined individual differences in aggressiveness.