Preferential localization of monoamine oxidase type A activity in neurons of the locus coeruleus and type B activity in neurons of the dorsal raphe nucleus of the rat: a detailed enzyme histochemical study

Alternative splicing in serotonergic system: Implications in neuropsychiatric disorders

BACKGROUND

The serotonergic system is a key component of physiological brain function and is essential for proper neurological activity. Numerous neuropsychiatric disorders are associated with deregulation of the serotonergic system. Accordingly, many pharmacological treatments are focused on modulation of this system. While providing a promising line of therapeutic moderation, these approaches may be complicated due to the presence of alternative splicing events for key genes in this pathway. Alternative splicing is a co-transcriptional process by which different mRNA transcripts can be produced from the same gene. These different isoforms may have diverse activities and functions, and their relative balance is often critical for the maintenance of homeostasis. Alternative splicing greatly increases the production of proteins, augmenting cell plasticity, and provides an important control point for regulation of gene expression.

AIM

The objective of this narrative review is to discuss the potential impact of alternative splicing of different components of the serotonergic system and speculate on their involvement in several neuropsychiatric disorders.

CONCLUSIONS

The specific role of each isoform in disease and their relative activities in the signalling pathways involved are yet to be determined. We need to gain a better understanding of the basis of alternative isoforms of the serotonergic system in order to fully understand their impact and be able to develop new effective pharmacological isoform-specific targets.

Phrenic long-term depression evoked by intermittent hypercapnia is modulated by serotonergic and adrenergic receptors in raphe nuclei

Intermittent hypercapnia evokes prolonged depression of phrenic nerve activity (phrenic long-term depression, pLTD). This study was undertaken to investigate the role of 5-HT and α2-adrenergic receptors in the initiation of pLTD. Adult male urethane-anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats were exposed to a protocol of acute intermittent hypercapnia (AIHc; 5 episodes of 15% CO2in air, each episode lasting 3 min). The experimental group received microinjection of the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), the broad-spectrum 5-HT antagonist methysergide, or the α2-adrenergic antagonist yohimbine, whereas the control group received microinjection of 0.9% saline into the caudal raphe region. Peak phrenic nerve activity (pPNA) and burst frequency ( f) were analyzed during baseline (T0), during 5 hypercapnic episodes (THc1–THc5), and at 15, 30, and 60 min after the end of the last hypercapnic episode. In the control group, pPNA decreased 60 min after the end of the last hypercapnic episode compared with baseline values, i.e., pLTD developed ( P = 0.023). In the 8-OH-DPAT group, pPNA significantly decreased at T15, T30, and T60 compared with baseline values, i.e., pLTD developed ( P = 0.01). In the methysergide and yohimbine groups, AIHc did not evoke significant changes of the pPNA at T15, T30, and T60 compared with baseline values. In conclusion, activation of 5-HT1Areceptors accentuated induction of pLTD, whereas blockade of α2-adrenergic receptors prevented development of pLTD following AIHc in anesthetized rats. These results suggest that chemical modulation of 5-HT and α2-adrenergic receptors in raphe nuclei affects hypercapnia-induced pLTD, offering important insights in understanding the mechanisms involved in development of respiratory plasticity.

NEW & NOTEWORTHY Hypercapnia is a concomitant feature of many breathing disorders, including obstructive sleep apnea. In this study, acute intermittent hypercapnia evoked development of phrenic long-term depression (pLTD) 60 min after the last hypercapnic episode that was preserved if the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide was microinjected in the caudal raphe region before the hypercapnic stimulus. This study highlights that both 5-HT and adrenergic receptor activation is needed for induction of pLTD in urethane-anesthetized rats following intermittent hypercapnia exposure.

Monoamine Oxidase A (MAOA) Gene and Personality Traits from Late Adolescence through Early Adulthood: A Latent Variable Investigation

Very few molecular genetic studies of personality traits have used longitudinal phenotypic data, therefore molecular basis for developmental change and stability of personality remains to be explored. We examined the role of the monoamine oxidase A gene (MAOA) on extraversion and neuroticism from adolescence to adulthood, using modern latent variable methods. A sample of 1,160 male and 1,180 female participants with complete genotyping data was drawn from a British national birth cohort, the MRC National Survey of Health and Development (NSHD). The predictor variable was based on a latent variable representing genetic variations of the MAOA gene measured by three SNPs (rs3788862, rs5906957, and rs979606). Latent phenotype variables were constructed using psychometric methods to represent cross-sectional and longitudinal phenotypes of extraversion and neuroticism measured at ages 16 and 26. In males, the MAOA genetic latent variable (AAG) was associated with lower extraversion score at age 16 (β = −0.167; CI: −0.289, −0.045; p = 0.007, FDRp = 0.042), as well as greater increase in extraversion score from 16 to 26 years (β = 0.197; CI: 0.067, 0.328; p = 0.003, FDRp = 0.036). No genetic association was found for neuroticism after adjustment for multiple testing. Although, we did not find statistically significant associations after multiple testing correction in females, this result needs to be interpreted with caution due to issues related to x-inactivation in females. The latent variable method is an effective way of modeling phenotype- and genetic-based variances and may therefore improve the methodology of molecular genetic studies of complex psychological traits.

Monoamine oxidases as sources of oxidants in the heart

Oxidative stress can be generated at several sites within the mitochondria. Among these, monoamine oxidase (MAO) has been described as a prominent source. MAOs are mitochondrial flavoenzymes responsible for the oxidative deamination of catecholamines, serotonin and biogenic amines, and during this process they generate H2O2 and aldehyde intermediates. The role of MAO in cardiovascular pathophysiology has only recently gathered some attention since it has been demonstrated that both H2O2 and aldehydes may target mitochondrial function and consequently affect function and viability of the myocardium. In the present review, we will discuss the role of MAO in catecholamine and serotonin clearance and cycling in relation to cardiac structure and function. The relevant contribution of each MAO isoform (MAO-A or -B) will be discussed in relation to mitochondrial dysfunction and myocardial injury. Finally, we will examine both beneficial effects of their pharmacological or genetic inhibition along with potential adverse effects observed at baseline in MAO knockout mice, as well as the deleterious effects following their over-expression specifically at cardiomyocyte level. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Notch-Rbpj signaling is required for the development of noradrenergic neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the main source of noradrenaline in the brain and is implicated in a broad spectrum of physiological and behavioral processes. However, genetic pathways controlling the development of noradrenergic neurons in the mammalian brain are largely unknown. We report here that Rbpj, a key nuclear effector in the Notch signaling pathway, plays an essential role in LC neuron development in the mouse. Conditional inactivation of Rbpj in the dorsal rhombomere (r) 1, where LC neurons are born, resulted in a dramatic increase in the number of Phox2a- and Phox2b-expressing early-differentiating LC neurons, and dopamine-β-hydroxylase- and tyrosine-hydroxylase-expressing late-differentiating LC neurons. In contrast, other neuronal populations derived from the dorsal r1 were either reduced or unchanged. In addition, a drastic upregulation of Ascl1, an essential factor for noradrenergic neurogenesis, was observed in dorsal r1 of conditional knockout mice. Through genomic sequence analysis and EMSA and ChIP assays, a conserved Rbpj-binding motif was identified within the Ascl1 promoter. A luciferase reporter assay revealed that Rbpj per se could induce Ascl1 transactivation but this effect was counteracted by its downstream-targeted gene Hes1. Moreover, our in vitro gene transfection and in ovo electroporation assays showed that Rbpj upregulated Ascl1 expression when Hes1 expression was knocked down, although it also exerted a repressive effect on Ascl1 expression in the presence of Hes1. Thus, our results provide the first evidence that Rbpj functions as a key modulator of LC neuron development by regulating Ascl1 expression directly, and indirectly through its target gene Hes1.

Monoamine oxidases (MAO) in the pathogenesis of heart failure and ischemia/reperfusion injury

Recent evidence highlights monoamine oxidases (MAO) as another prominent source of oxidative stress. MAO are a class of enzymes located in the outer mitochondrial membrane, deputed to the oxidative breakdown of key neurotransmitters such as norepinephrine, epinephrine and dopamine, and in the process generate H(2)O(2). All these monoamines are endowed with potent modulatory effects on myocardial function. Thus, when the heart is subjected to chronic neuro-hormonal and/or peripheral hemodynamic stress, the abundance of circulating/tissue monoamines can make MAO-derived H(2)O(2) production particularly prominent. This is the case of acute cardiac damage due to ischemia/reperfusion injury or, on a more chronic stand, of the transition from compensated hypertrophy to overt ventricular dilation/pump failure. Here, we will first briefly discuss mitochondrial status and contribution to acute and chronic cardiac disorders. We will illustrate possible mechanisms by which MAO activity affects cardiac biology and function, along with a discussion as to their role as a prominent source of reactive oxygen species. Finally, we will speculate on why MAO inhibition might have a therapeutic value for treating cardiac affections of ischemic and non-ischemic origin. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Interaction between a functional MAOA locus and childhood sexual abuse predicts alcoholism and antisocial personality disorder in adult women

Women who have experienced childhood sexual abuse (CSA) have an increased risk of alcoholism and antisocial personality disorder (ASPD). Among male subjects, a functional polymorphism (MAOA-LPR, monoamine oxidase A linked polymorphic region) in the promoter region of the monoamine oxidase A gene (MAOA) appears to moderate the effect of childhood maltreatment on antisocial behavior. Our aim was to test whether MAOA-LPR influences the impact of CSA on alcoholism and ASPD in a sample of 291 women, 50% of whom have experienced CSA; we also tested whether haplotypes covering the region where both MAOA and monoamine oxidase B (MAOB) genes are located predict risk of alcoholism and ASPD better than the MAOA-LPR locus alone. Participants included 168 alcoholics (39 with ASPD (antisocial alcoholics) and 123 controls (no alcoholics, no ASPD). Antisocial behavior was also modeled as a continuous trait: ASPD symptoms count. The MAOA-LPR low activity allele was associated with alcoholism (P=0.005), particularly antisocial alcoholism (P=0.00009), only among sexually abused subjects. Sexually abused women who were homozygous for the low activity allele had higher rates of alcoholism and ASPD, and more ASPD symptoms, than abused women homozygous for the high activity allele. Heterozygous women displayed an intermediate risk pattern. In contrast, there was no relationship between alcoholism/antisocial behavior and MAOA-LPR genotype among non-abused women. The MAOA-LPR low activity allele was found on three different haplotypes. The most abundant MAOA haplotype containing the MAOA-LPR low activity allele was found in excess among alcoholics (P=0.008) and antisocial alcoholics (P=0.001). Finally, a MAOB haplotype, which we termed haplotype C, was significantly associated with alcoholism (P=0.006), and to a lesser extent with antisocial alcoholism (P=0.03). In conclusions, MAOA seems to moderate the impact of childhood trauma on adult psychopathology in female subjects in the same way as previously shown among male subjects. The MAOA-LPR low activity allele appears to confer increased vulnerability to the adverse psychosocial consequences of CSA. Haplotype-based analysis of the MAOA gene appeared to strengthen the association, as compared to the MAOA-LPR locus alone. A MAOB haplotype was associated with alcoholism independently from ASPD.

Molecular mechanism of the relation of monoamine oxidase B and its inhibitors to Parkinson's disease: possible implications of glial cells

Monoamine oxidases A and B (MAO A and MAO B) are the major enzymes that catalyze the oxidative deamination of monoamine neurotaransmitters such as dopamine (DA), noradrenaline, and serotonin in the central and peripheral nervous systems. MAO B is mainly localized in glial cells. MAO B also oxidizes the xenobiotic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to a parkinsonism-producing neurotoxin, 1-methyl-4-phenyl-pyridinium (MPP+). MAO B may be closely related to the pathogenesis of Parkinson's disease (PD), in which neuromelanin-containing DA neurons in the substantia nigra projecting to the striatum in the brain selectively degenerate. MAO B degrades the neurotransmitter DA that is deficient in the nigro-striatal region in PD, and forms H2O2 and toxic aldehyde metabolites of DA. H2O2 produces highly toxic reactive oxygen species (ROS) by Fenton reaction that is catalyzed by iron and neuromelanin. MAO B inhibitors such as L-(-)-deprenyl (selegiline) and rasagiline are effective for the treatment of PD. Concerning the mechanism of the clinical efficacy of MAO B inhibitors in PD, the inhibition of DA degradation (a symptomatic effect) and also the prevention of the formation of neurotoxic DA metabolites, i.e., ROS and dopamine derived aldehydes have been speculated. As another mechanism of clinical efficacy, MAO B inhibitors such as selegiline are speculated to have neuroprotective effects to prevent progress of PD. The possible mechanism of neuroprotection of MAO B inhibitors may be related not only to MAO B inhibition but also to induction and activation of multiple factors for anti-oxidative stress and anti-apoptosis: i.e., catalase, superoxide dismutase 1 and 2, thioredoxin, Bcl-2, the cellular poly(ADP-ribosyl)ation, and binding to glyceraldehydes-3-phosphate dehydrogenase (GAPDH). Furthermore, it should be noted that selegiline increases production of neurotrophins such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrphic factor (GDNF), possibly from glial cells, to protect neurons from inflammatory process.

MAOA haplotypes associated with thrombocyte-MAO activity

Background

The aim was to ascertain whether thrombocyte MAO (trbc-MAO) activity and depressed state are genetically associated with the MAO locus on chromosome X (Xp11.3 – 11.4). We performed novel sequencing of the MAO locus and validated genetic variants found in public databases prior to constructing haplotypes of the MAO locus in a Swedish sample (N = 573 individuals).

Results

Our results reveal a profound SNP desert in the MAOB gene. Both the MAOA and MAOB genes segregate as two distinct LD blocks. We found a significant association between two MAOA gene haplotypes and reduced trbc-MAO activity, but no association with depressed state.

Conclusion

The MAO locus seems to have an effect on trbc-MAO activity in the study population. The findings suggest incomplete X-chromosome inactivation at this locus. It is plausible that a gene-dosage effect can provide some insight into the greater prevalence of depressed state in females than males.

Presence of monoamine oxidase type B protein but absence of associated enzyme activity in neurons within the inferior olive nucleus of the rat

A previous study demonstrated that monoamine oxidase type B (MAOB) mRNA is located in the inferior olive complex (IO). The purpose of the present study was to examine whether neuronal cell bodies within the IO also express MAOB protein and whether they exhibit associated MAOB enzyme activity. Using immunohistochemistry and enzyme histochemistry, we demonstrated that IO neuronal cell bodies were positive for MAOB immunohistochemistry but negative for MAOB enzyme histochemistry. These findings indicate that IO neuronal cell bodies express MAOB mRNA and produce MAOB protein but curiously do not exhibit MAOB enzyme activity, as might be expected. The mechanism responsible for the failure of MAOB protein to result in enzymatic activity in IO neuronal cell bodies is clearly of significance in terms of functionality but remains to be elucidated.

Effects of ovarian steroids and raloxifene on proteins that synthesize, transport, and degrade serotonin in the raphe region of macaques

In the monkey dorsal raphe, we reported that 1-month (mo) of estrogen replacement, with or without progesterone supplementation for 14 days, significantly increased tryptophan hydroxylase-1 (TPH-1) mRNA; decreased serotonin reuptake transporter (SERT) mRNA and decreased monoamine oxidase (MAO)-A mRNA, but had no effect on MAO-B mRNA. Here, we questioned what effect would 1 or 5 mo of ovarian hormones or the selective estrogen receptor modulator (SERM), raloxifene, have on TPH protein and phosphorylation, and on protein expression of SERT, MAO-A or MAO-B? Raloxifene antagonizes estrogen in breast or uterus, but estrogen-like activities in the brain have been reported. Cytoplasmic and membrane extracts of the dorsal raphe region were processed for Western blotting. TPH, phosphoserine, SERT, MAO-A, and MAO-B were detected with specific antibodies. The optical densities of the signals were measured with NIH image and analyzed by ANOVA. Both 1 and 5 mo of estrogen, with or without progesterone, and 5 mo of raloxifene significantly increased TPH protein. Administration for 5 mo of estrogen plus progesterone and raloxifene also increased TPH phosphorylation. Estrogen, with or without progesterone, for 1 mo had no effect on SERT protein. However, 5 mo of estrogen and 5 mo of raloxifene increased SERT protein. Estrogen alone or combined with progesterone for 1 mo caused a significant reduction in MAO-A. Yet, after 5 mo of the same treatments, MAO-A was not different from spayed controls. Estrogen alone had no effect on MAO-B. However, the addition of progesterone significantly increased MAO-B. Raloxifene for 5 mo had no effect on MAO-A or MAO-B. Thus, to various extents, estrogen, progesterone, and raloxifene may increase serotonin production and transport. The expression of the degradative enzymes suggests a complex combination of gene transcription, post-transcriptional processing, and substrate feedback mechanisms.

MAO activity in serotonergic endings of rat major cerebral arteries

The present work studies the existence of monoamine oxidase (MAO) activity in serotonergic endings present in rat major cerebral arteries. Enzymatic activity was appraised in vivo by serotonin (5-HT) accumulation or 5-hydroxyindole acetic acid (5-HIAA) disappearance with time after systemic administration of MAO inhibitors. Pargyline (75 mg/Kg, ip) brought about significant 5-HT increase and 5-HIAA decrease in major cerebral arteries 30 and 60 min after its administration. Clorgyline (75 mg/Kg, ip) also induced 5-HT enhancement and 5-HIAA decline in these arteries 30 and 60 min after its injection. However, treatment with deprenyl (75 mg/Kg, ip) only evoked a significant 5-HT increase at 60 min. When either clorgyline (5 mg/Kg, ip) or deprenyl (5 mg/Kg, ip) were administered 5-HT and 5-HIAA levels remained unaffected. Two weeks after performing electrolytical lesion of dorsal raphe nucleus and 60 min after clorgyline (75 mg/Kg, ip) injection 5-HT and 5-HIAA levels appeared significantly reduced in cerebral arteries and striatum when compared to sham-lesioned controls. These results suggest that MAO-A isoform acting on endogenous 5-HT is present in rat major cerebral arteries and is located in nerve endings of fibers arising from dorsal raphe nucleus.

Immunohistochemical Localization of Monoamine Oxidase Type B in the Taste Bud of the Rat

We have used immunohistochemistry to examine the subcellular localization of monoamine oxidase type B (MAO-B) in the taste bud of the rat circumvallate papilla. Electron microscopy showed that MAO-B was localized to the outer membranes of mitochondria in nerve terminals of afferent and efferent fibers, as well as in taste bud cells. MAO-B also existed on the mitochondrial outer membranes within myelinated and unmyelinated axons in the lamina propria beneath the taste bud. It is suggested that MAO-B-containing mitochondria are localized in peripheral branches and their terminals of sensory neurons for taste. The present study is the first to reveal the localization of MAO-B in sensory organs.

Differential subcellular location of mitochondria in rat serotonergic neurons depends on the presence and the absence of monoamine oxidase type B

Monoamine oxidase type A and type B are major neurotransmitter-degrading enzymes in the CNS. The type A is present on mitochondrial outer membranes in the whole extent of noradrenergic and dopaminergic neurons, including their axon terminals. The type B is present in serotonergic neurons, but its subcellular localization has not been elucidated. In the present study, we used both a double-labeling immunofluorescence method and electron microscopic immunohistochemistry to examine the subcellular localization of monoamine oxidase type B in serotonergic neurons projecting from the dorsal raphe nucleus to the suprachiasmatic nucleus in the rat brain. In the dorsal raphe nucleus, serotonin-positive neuronal cell bodies were clustered, and virtually all of these cell bodies were also positive for monoamine oxidase type B. By contrast, serotonin-negative neuronal cell bodies were mostly free of this enzyme. Within the neuronal cell bodies and dendrites that were positive for monoamine oxidase type B, most mitochondria contained this enzyme on their outer membranes, but a substantial proportion of mitochondria lacked this enzyme. In the suprachiasmatic nucleus, serotonin-positive varicosities were concentrated, but none of these varicosities exhibited monoamine oxidase type B. In this nucleus, mitochondria were found in almost all serotonin-positive axon terminals, but monoamine oxidase type B was not observed in any axon terminal that contained mitochondria. Our results show that there are two kinds of mitochondria in serotonergic neuronal cell bodies and dendrites: one containing monoamine oxidase type B on their outer membranes, and the other lacking this enzyme. In addition, mitochondria in serotonergic axon terminals do not possess monoamine oxidase type B. It is suggested in serotonergic neurons that only mitochondria lacking monoamine oxidase type B are transported by axonal flow up to axon terminals. It is also probable that mitochondria containing monoamine oxidase type B are transported along the axons, but that this enzyme undergoes a change, for example, conformational change, decomposition or removal from the membranes.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Altered presynaptic function in monoaminergic neurons of monoamine oxidase-A knockout mice

Monoamine oxidase-A knockout (MAO-A KO) mice have elevated brain serotonin (5-HT) and noradrenaline (NA) levels, and one would therefore anticipate increased monoamine release and compensatory changes in other aspects of presynaptic monoamine function. In this study we used voltammetry in brain slices from the locus coeruleus (LC), dorsal raphe (DRN) and striatum (CPu) in 7-week-old MAO-A KO and C3H control mice to measure stimulated monoamine efflux and its control by amine transporters and autoreceptors. In LC, peak NA efflux on stimulation (99 pulses, 100 Hz) was higher in MAO-A KO than C3H mice (938 +/- 58 nm cf. 511 +/- 42 nm; P < 0.001). The NA uptake half time (t(1/2)) was longer in MAO-A KO than in C3H mice (6.0 +/- 0.9 s cf. 1.9 +/- 0.3 s; P < 0.001) and the selective NA reuptake inhibitor desipramine (50 nm) had a smaller effect in MAO-A KO mice. NA transporter binding was significantly lower in the LC of MAO-A KO mice compared to C3H controls (P < 0.01) but not in the DRN. The alpha 2 agonist dexmedetomidine (10 nm) decreased stimulated NA efflux more in C3H than in MAO-A KO mice (73.3% cf. 29.6% inhibition, P < 0.001). In DRN, peak 5-HT efflux on stimulation (99 pulses, 100 Hz) was greater (P < 0.01) in MAO-A KO (262 +/- 44 nm) than C3H mice (157 +/- 16 nm). Moreover, 5-HT uptake t(1/2) was longer (P < 0.05) in MAO-A KO than in C3H mice (8.8 +/- 1.1 s cf. 4.9 +/- 0.6 s, P < 0.05) and the effect of citalopram (75 nm) was attenuated in MAO-A KOs. Serotonin transporter binding was also lower in both the DRN and LC of MAO-A KO mice. The 5-HT(1A) agonist 8-OH-DPAT (1 microm) decreased 5-HT efflux more in C3H than in MAO-A KO mice (38.3% inhibition cf. 21.6%, P < 0.001). In contrast, there were no significant differences between MAO-A KO and C3H mice in CPu dopamine efflux and uptake and the effect of the D(2/3) agonist quinpirole was similar in the two strains. In summary, MAO-A KO mice show major dysregulation of monoaminergic presynaptic mechanisms such as autoreceptor control and transporter kinetics.

Catecholamine degradation by monoamine oxidase in locus coeruleus neurons of the rat. An immunohistochemical study

We examined by immunohistochemistry the effects of monoamine oxidase (MAO) inhibition on the content of dopamine (DA) and noradrenaline (NA) in locus coeruleus (LC) neurons of the rat. In normal rats, clusters of DA- and NA-immunopositive neurons were identified in the LC. Rats treated with intraperitoneal injections of pargyline, an MAO inhibitor, showed significantly stronger DA- and NA-staining intensities in LC neurons compared to normal rats. In LC noradrenergic neurons, it is believed that DA is formed in the cytoplasm and then transported into the storage vesicles where it is converted to NA, and the secreted NA is recycled by a reuptake mechanism and transported back into storage vesicles via the cytoplasm. Furthermore, LC neurons of the rat have been shown to contain DA- and NA-degrading MAO activities on the outer membranes of the mitochondria. Therefore, our findings suggest that endogenous MAO degrades not only part of the DA formed in the cytoplasm of LC neurons, but also part of the secreted NA that has been transported back into the cytoplasm.

Monoamine oxidase in the intermediolateral nucleus of the thoracic spinal cord of the rat. A histochemical study

We examined monoamine oxidase (MAO) activity in the intermediolateral nucleus (IML) of the rat thoracic spinal cord by histochemistry with tyramine as a common substrate for both MAO types A and B. Light microscopy showed MAO activity in neuronal cell bodies, processes, and varicosities. Electron microscopic examination showed both MAO-positive and -negative neuronal cell bodies. In the stained cell bodies, histochemical reaction products were localized in the cytoplasm showing a selective association with mitochondrial outer membranes. MAO-positive axon terminals were often found in contact with MAO-negative neurons but only occasionally with MAO-positive neurons. MAO histochemistry in the IML was also performed using serotonin (a MAO type A preferential substrate) and beta-phenylethylamine (a MAO type B preferential substrate). Light microscopy identified MAO activity for serotonin in a plexus of varicosities but not in any neuronal cell bodies. The activity for beta-phenylethylamine was detected frequently in neuronal cell bodies but rarely in varicosities. Our findings indicate that two groups of IML neurons can be chemically distinguished, one contains MAO type B while the other lacks both MAO types A and B. In addition, many axon terminals contain MAO type A but only a few fibers include MAO type B in the IML.

Noradrenaline-degrading activity of monoamine oxidase is localized in noradrenergic neurons of the locus coeruleus of the rat

We found intense monoamine oxidase (MAO) activity in rat locus coeruleus (LC) neurons by means of a histochemical method using noradrenaline as a substrate. This MAO activity was abolished by clorgyline, a specific inhibitor of MAO type A. Fluorescence immunohistochemistry for tyrosine hydroxylase (TH) combined with MAO histochemistry revealed intense MAO activity in virtually all TH-immunoreactive LC neurons (i.e. noradrenergic neurons). The results indicate that noradrenaline produced in LC neurons might be degraded by MAO type A activity.

Self-injurious behavior is decreased by cyproterone acetate in adult male rhesus (Macaca mulatta)

Self-injurious behavior (SIB) presents a serious problem in laboratory macaques that cannot be socially housed for scientific reasons and among institutionalized children and adults where it is often associated with different forms of brain dysfunction. We have experienced limited success in reducing SIB in macaques by enhancing their environment with enrichment devices. Psychotropic drugs also help, but problems are associated with their use. Because sexual and aggressive behavioral problems in men have been treated with progestational drugs, we tested the efficacy of cyproterone acetate (CA, 5-10 mg/kg/week) on reducing SIB in 8 singly housed, adult male rhesus macaques. The main findings were: (1) SIB and other atypical behaviors were significantly reduced during CA treatment; (2) serum testosterone was significantly reduced during CA treatment; (3) cerebral spinal fluid (CSF) levels of 5HIAA and HVA, metabolites of serotonin and dopamine, respectively, declined significantly during CA treatment; (4) the duration of SIB positively correlated with levels of 5HIAA in CSF; but (5) sperm counts were not reduced during treatment. Thus, CA was a partially effective treatment (3 months) for adult male macaques whose behavioral problems include SIB. In summary, CA reduced SIB, overall aggression, serum testosterone, CSF 5HIAA, and CSF HVA. We hypothesized that the progestin activity of CA represses the hypothalamic gonadal axis and decreases testosterone, which in turn decreases SIB. In addition, we speculate that the decrease in 5HIAA and HVA in CSF may have been caused by progestins decreasing the activity of MAO. Therefore, the reduction of SIB may also be related to an increase in the availability of active monoamines in the CNS.

Monoamine oxidase activity in noradrenaline neurons of the locus coeruleus of the rat. A double-labeling histochemical study

The aim of the present study is to examine whether noradrenergic neurons of the locus coeruleus (LC) of the rat contain monoamine oxidase (MAO) activity. Sections were processed initially for MAO enzyme histochemistry using tyramine as a substrate, followed by fluorescence immunohistochemistry for tyrosine hydroxylase (TH). In the LC, virtually all TH-immunoreactive neurons (i.e., noradrenergic neurons) were also positive for MAO. No MAO activity was found in any TH-negative neurons. Neurons in the LC have previously been shown to form dopamine during noradrenaline biosynthesis and to produce serotonin from exogenously administered l-5-hydroxytryptophan. Moreover, dopamine- and serotonin-degrading MAO activity has also been found in LC neurons. Therefore, our results indicate that MAO activity is localized within noradrenergic neurons in the LC and is likely involved in the degradation of dopamine that is endogenously synthesized, and also in the elimination of serotonin that is produced from exogenous precursors.

Dopamine-degrading activity of monoamine oxidase is not detected by histochemistry in neurons of the substantia nigra pars compacta of the rat

Monoamine oxidase (MAO) activity was examined in neurons of the substantia nigra pars compacta (SNC) of the rat using a histochemical method, and compared to MAO activity in neurons of the locus coeruleus (LC) and dorsal raphe nucleus (DR). Using dopamine as a substrate, dopamine-degrading MAO activity was not detected in any SNC neurons, although LC and DR neurons were intensely stained for this activity. We further examined MAO activity in these neurons using other substrates, including serotonin (an MAO type A preferential substrate), beta-phenylethylamine (an MAO type B preferential substrate), and tyramine (a substrate common to both MAO types A and B). As for dopamine, no SNC neurons were stained for MAO activity using any of these other substrates. In contrast, LC neurons were intensely stained when either serotonin or tyramine was used, and DR neurons were darkly stained when either beta-phenylethylamine or tyramine was used. The lack of evidence of MAO activity in the SNC is surprising given that there are densely packed tyrosine hydroxylase (TH)-immunoreactive neurons in the SNC (i.e., dopaminergic neurons). By comparison, in the LC and DR the distribution patterns of the MAO-stained neurons were similar to those of TH-immunolabeled neurons (i.e., noradrenergic neurons) and serotonin-immunoreactive neurons, respectively. Our results suggest that dopamine-degrading MAO activity and MAO types A and B activities in SNC dopamine neurons are very low compared to MAO activity in LC noradrenaline neurons and in DR serotonin neurons.

Dopamine-degrading activity of monoamine oxidase in locus coeruleus and dorsal raphe nucleus neurons. A histochemical study in the rat

Dopamine-degrading activity of monoamine oxidase (MAO) was detected in the rat using a new histochemical method, with dopamine as the substrate. Our new method, designed to minimise the non-enzymatic oxidation of dopamine, was applied in combination with tyrosine hydroxylase (TH) and serotonin immunohistochemistry. We showed that the distribution pattern of MAO neurons was similar to that of TH-immunoreactive neurons (i.e. noradrenergic neurons) in the locus coeruleus (LC) and to that of serotonergic neurons in the dorsal raphe nucleus (DR). Since LC neurons form dopamine during noradrenaline biosynthesis, and DR neurons produce dopamine from exogenously administered L-dopa, our results indicate that dopamine produced in LC and DR neurons may be degraded, at least in part, by MAO.

Monoamine oxidase B (MAOB)-containing structures in MAOA-deficient transgenic mice

Monoamine oxidase (MAO)-containing structures were studied for the first time in type A MAO (MAOA)-deficient transgenic mice (Tg8) derived from C3H strain, using MAO enzyme histochemistry. In this mutant line, MAOA activity was not detected in neurons of the locus coeruleus. In contrast, in their dorsal raphe neurons, we noted an intense activity of type B MAO (MAOB). Based on pharmacological MAOA suppression experiments employing a specific inhibitor (clorgyline), we confirmed that the localization of MAOB-positive structures are not different between Tg8 mutant and normal C3H line. Many of MAOB-positive structures which have not been described previously in the rat, cat and primates were described in this study. In the forebrain, MAOB-containing neurons were discriminated in the striatum, septal nuclei, major island of Calleja, diagonal band, medial forebrain bundle, ventral pallidum and amygdaloid nucleus. Stained neurons in the thalamus and hypothalamus were much more extensively distributed in the mouse than the rat. Pontine laterodorsal tegmental neurons showed MAOB activity. The present data suggest that serotonin, a preferential substrate for MAOA, can be oxidized by MAOB in MAOA-deficient Tg8 mice.