Effects of intrastriatal kainic acid injection on [3H]dopamine metabolism in rat striatal slices: evidence for postsynaptic glial cell metabolism by both the type A and B forms of monoamine oxidase

Monoamine oxidase inhibition causes a long-term prolongation of the dopamine-induced responses in rat midbrain dopaminergic cells

The way monoamine oxidase (MAO) modulates the depression of the firing rate and the hyperpolarization of the membrane caused by dopamine (DA) on rat midbrain dopaminergic cells was investigated by means of intracellular recordings in vitro. The cellular responses to DA, attributable to the activation of somatodendritic D2/3 autoreceptors, were prolonged and did not completely wash out after pharmacological blockade of both types (A and B) of MAO. On the contrary, depression of the firing rate and membrane hyperpolarization induced by quinpirole (a direct D2 receptor agonist) were not affected by MAO inhibition. Furthermore, although the inhibition of DA reuptake by cocaine and nomifensine caused a short-term prolongation of DA responses, the combined inhibition of MAO A and B enzymes caused a long-term prolongation of DA effects. Moreover, the effects of DA were not largely prolonged during the simultaneous inhibition of MAO and the DA reuptake system. Interestingly, the actions of amphetamine were not clearly augmented by MAO inhibition. From the present data it is concluded that the termination of DA action in the brain is controlled mainly by MAO enzymes. This long-term prolongation of the dopaminergic responses suggests a substitutive therapeutic approach that uses MAO inhibitors and DA precursors in DA-deficient disorders in which continuous stimulation of the dopaminergic receptors is preferable.

Electrophysiological effects of monoamine oxidase inhibition on rat midbrain dopaminergic neurones: an in vitro study

1 The effects of the inhibition of monoamine oxidase (MAO) type A and B have been evaluated on the spontaneous firing activity of the dopaminergic (principal) neurones of the rat midbrain intracellularly recorded from a slice preparation. 2 The non-specific MAO inhibitor, pargyline, superfused at a concentration of 10-100 microM, decreased or abolished the spontaneous firing discharge of the principal neurons in the subtantia nigra pars compacta and ventral tegmental area. This effect had a slow onset and appeared to be sustained. 3 The administration of the dopamine D2/3 receptor antagonist, sulpiride (100-300 nM), antagonized the pargyline-induced effect, while the superfusion of the dopamine D1 receptor antagonist, SCH 23390 (1-3 microM) did not counteract the induced inhibition of the firing rate. 4 The inhibitor for the MAO A, clorgyline (30-100 microM), reduced the firing rate of the dopaminergic neurones. A similar depressant effect was also observed when a MAO B inhibitor, deprenyl (30-100 microM), was applied. Lower concentrations of both drugs (300 nM-10 microM) did not produce consistent effects on neuronal discharge. 5 Our data suggest that only the blockade of both types of MAO enzymes favours the inhibitory action of endogenous dopamine on somato-dendritic D2/3 autoreceptors.

Selective protection against AMPA- and kainate-evoked neurotoxicity by (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahyd roisoquinoline- 3-carboxylic acid (LY293558) and its racemate (LY215490)

Glutamate receptor-mediated excitotoxicity is linked to the activation of multiple receptors including those activated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), N-methyl-D-aspartate (NMDA), and kainate. In this study, the novel glutamate receptor antagonist, as its active isomer (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]-decahyd roisoquinoline-3- carboxylic acid ((-)LY293558) and it's +/- racemate (LY215490), was examined for neuroprotectant effects against excitotoxic injury in vitro and in vivo. This agent selectively protected against AMPA and kainate injury in cultured primary rat hippocampal neurons, an in vivo rat striatal neurotoxicity model, and against agonist-evoked seizures in mice. Thus, (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahydr -oisguino-line-3-carboxylic acid represents a novel receptor selective and potent systemically active AMPA/kainate receptor antagonist for exploring neuroprotection via non-NMDA receptor mechanisms.

The NMDA receptor agonist DL-(tetrazol-5-yl)glycine is a highly potent excitotoxin

DL-(Tetrazol-5-yl)glycine is a highly selective N-methyl-D-aspartate (NMDA) receptor agonist with nanomolar in vitro potency. Previous work showed that DL-(tetrazol-5-yl)glycine has greater affinity and efficacy at NMDA receptors than other NMDA receptor agonists such as cis-methanoglutamate and NMDA. In this study, the in vivo excitotoxic potency of DL-(tetrazol-5-yl)glycine was compared to cis-methanoglutamate and NMDA. Adult (250-300 g) and neonatal (7-day-old) rats were anesthetized and compounds were unilaterally injected into the striatum. In adult rats DL-(tetrazol-5-yl)glycine (0.3-1.0 nmol/microliters) produced highly significant losses of striatal gamma-aminobutyric acid neurons (as indexed by glutamic acid decarboxylase activity) and cholinergic neurons (as indexed by choline acetyltransferase activity). Dose-response showed that DL-(tetrazol-5-yl)glycine was about 100 and 500 times more potent than cis-methanoglutamate and NMDA, respectively. In neonatal rats, DL-(tetrazol-5-yl)glycine (0.1-0.3 nmol/microliters) produced significant brain damage as indicated by brain weight losses 5 days later. DL-(Tetrazol-5-yl)glycine was about 50 and 150 times more potent than cis-methanoglutamate and NMDA, respectively, in the neonate. The excitotoxic potency of DL-(tetrazol-5-yl)glycine is likely due to its greater efficacy and potency at the NMDA receptor, when compared to other NMDA receptor agonists. The remarkable in vivo potency of DL-(tetrazol-5-yl)glycine in producing excitotoxic lesions makes it a useful agent to further probe NMDA receptor mediated excitotoxicity in brain pathologies.

Reactive gliosis and monoamine oxidase B

A double-staining method was applied to cryosections of human spinal cord from patients who died with amyotrophic lateral sclerosis (ALS) and corresponding controls in order to investigate cellular content of monoamine oxidase B (MAO-B). 3H-L-Deprenyl emulsion autoradiography was used in combination with histochemical methods for the detection of astrocytes and monocytes/microglia. In the ALS spinal cords an increased number of astrocytes as well as an increased content of MAO-B in reactive species of astrocytes was demonstrated. No significant 3H-L-deprenyl binding was observed in cells derived from the mesoderm, e.g. monocytes or microglia. Furthermore, a sub-population of reactive astrocytes that contained low levels of MAO-B was observed in spinal sections. These findings were further substantiated by studies performed on primary astrocyte cultures.

Pharmacological modulation of endogenous dopamine and DOPAC outflow from nucleus accumbens

The release of endogenous DA and DOPAC from nucleus accumbens slices were studied measuring net outflow of DA and DOPAC in the superfusate of static chambers, to analyze the correlation between DA and DOPAC outflows and identify which DA stores may serve as possible sources for DOPAC formation. Under resting conditions, or following stimulation with low (< 15 mM) KCl concentration, DOPAC outflow was greater than DA. When DA release was stimulated by higher (> 25 mM) KCl concentrations, DA outflow increased, proportionally more than DOPAC. In the virtual absence of Ca2+ in the Krebs solution DA outflow, induced by 25 mM KCl, was reduced to about 10%, while DOPAC outflow was only reduced to 45%. When the synthesis of DA was inhibited with alpha-MPT, DA and DOPAC outflow were unchanged during the first stimulation period. During a second stimulation period, however, their outflow were significantly reduced. Nomifensine, a DA uptake inhibitor, increased the basal DA outflow by about 100%, but only blocked DOPAC basal outflow by about 25%. The 25 mM KCl stimulated DA outflow was not affected by Nomifensine, while the stimulated DOPAC outflow was reduced by about 50%. These results demonstrate that there is a weak correlation between the outflows of DA and DOPAC, suggesting a complex relationship between the mobilization of the different DA pools and DOPAC outflow. The formation of DOPAC from some of these pools, appear to be dependent on the stimulation levels and on the pharmacological manipulation of the tissue.

Brain gamma-aminobutyrate aminotransferase (GABA-T) and monoamine oxidase (MAO) in patients with Alzheimer's disease

Activities of Gamma-aminobutyrate aminotransferase (GABA-T) and Monoamine oxidase (MAO)-A and -B were estimated in postmortem brains from 6 control subjects without psychiatric or neurologic disorders and 8 histopathologically verified cases of patients with Alzheimer's disease and senile dementia of Alzheimer type (AD/SDAT). The enzyme activities were examined in four cortical brain regions, three nuclei in the basal ganglia, thalamus and white matter. GABA-T activities in the cortical regions (frontal, parietal, occipital and temporal cortices) and nucleus caudatus were significantly lowered in the AD/SDAT patients. The MAO-A activities were significantly increased in the occipital cortex, caudate nucleus, thalamus and white matter in the AD/SDAT patients. No significant differences were found in the other regions (frontal cortex, parietal cortex, temporal cortex, putamen and globus pallidus). The MAO-B activities in three cortical regions (frontal, parietal and occipital cortices), thalamus and white matter were significantly increased in the AD/SDAT patients, whereas no difference was apparent in the other regions. The changed activities could not be correlated with age or postmortem time. The present results are the first describing decreased GABA-T activities as well as increased MAO-A activities in brain from patients with AD/SDAT, while the results with MAO-B support previous findings. A possible connection was found between the order of magnitude of the changes in enzyme activities and the severity of the disease.

Effects of COMT inhibitors on striatal dopamine metabolism: a microdialysis study

In vivo microdialysis was used to examine the effect of two new catechol-O-methyltransferase (COMT) inhibitors, Ro 40-7592 and OR-611, on extracellular levels of dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in rat striatum. The interactions of the COMT inhibitors with nomifensine, clorgyline and deprenyl were also studied. Ro 40-7592 (3-30 mg/kg, i.p.) decreased dose-dependently the efflux of HVA, increased that of DOPAC, and tended to increase that of dopamine. Higher doses of OR-611 (30-100 mg/kg, i.p.) also decreased the dialysate level of HVA, increased that of DOPAC, and tended to increase that of dopamine. Ro 40-7592 was about ten-fold as potent as OR-611. Neither of the COMT inhibitors changed dialysate levels of 5-HIAA. An OR-611 dose of 10 mg/kg i.p. had no significant effect, in contrast to Ro 40-7592, on any of the parameters studied; this dose was thus used to differentiate between the effects of central and peripheral COMT inhibition. Both nomifensine (15 mg/kg, i.p.) and clorgyline (4 mg/kg, i.p.) alone elevated extracellular dopamine levels, and lowered those of DOPAC and HVA, though there were quantitative and temporal differences between the drugs. L-Deprenyl (1 mg/kg, i.p.) alone had no significant effect on any of the compounds measured. Ro 40-7592 (10 mg/kg, i.p.) potentiated the effect of nomifensine on dopamine efflux, and it tended to increase clorgyline-induced dopamine efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoamine oxidase, dopamine and Parkinson's disease

Four aspects about monoamine oxidase (MAO; E.C. 1.4.3.4) are of obvious interest in relation to Parkinson's disease and its treatment with the irreversible and selective MAO-B inhibitor L-deprenyl and are discussed in this review: 1) To what extent the two forms of MAO are of importance for the deamination of dopamine and to what degree MAO localised inside and outside of dopaminergic nerve terminals contributes 2) The kinetics of the MAO-protein, i.e. the rate of recovery of MAO after irreversible inhibition. 3) To what extent MAO may be changed as a consequence of the pathophysiological processes. 4) To what extent MAO may be involved as a force in the pathophysiological processes.

Neuroprotectant effects of LY274614, a structurally novel systemically active competitive NMDA receptor antagonist

Antagonists for the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor may have therapeutic potential as neuroprotectant agents in conditions of neuronal degeneration that include brain ischemia, Huntington's chorea, and Alzheimer's disease. Here we have investigated the pharmacological actions of LY274614, a structurally novel competitive NMDA receptor antagonist, for pharmacological selectivity and neuroprotectant effects following systemic administration. LY274614 potently displaced NMDA receptor ([3H]CGS19755) binding (IC50 = 58.8 +/- 10.3 nM), but had no appreciable affinity at [3H]AMPA or [3H]kainate receptor sites at up to 10,000 nM. NMDA-induced convulsions in neonatal rats or NMDA-induced lethality in mice are potently and selectively antagonized by i.p. or p.o. LY274614. Oral doses showed a delayed but prolonged duration of effect. In adult rats, the neurodegenerative effects (loss of choline acetyltransferase activity) following the intrastriatal infusions of NMDA or quinolinate, but not kainate, were prevented by LY274614 (2.5 to 20 mg/kg i.p.). LY274614 is an effective neuroprotectant agent against NMDA receptor-induced toxicity when administered systemically and is a promising therapeutic agent for conditions where glutamate plays a role in the pathology of neuronal degeneration.

Monoamine oxidase inhibitors prevent striatal neuronal necrosis induced by transient forebrain ischemia

Seven days after 30 min of ischemia, neuronal necrosis was observed in the striatum. Pretreatment with type A monoamine oxidase (MAO-A) inhibitors, clorgyline and RS-8359 ((+)-4-(4-cyanoanilino)-7-hydroxycyclopenta (3,2-e) pyrimidine) decreased significantly the number of necrotic neurons and inhibited changes in the dopamine metabolite contents during and after transient ischemia. An MAO-B inhibitor, deprenyl also decreased the neuronal necrosis, but it inhibited only the changes in 3,4-dihydroxyphenylacetic acid (DOPAC) content after reperfusion. The results suggest that the activation of dopamine metabolism after transient ischemia was mainly mediated by MAO-A and partly by MAO-B and suggest a possible role of dopamine deamination by MAO in the development of ischemic neuronal necrosis.

Monoamine oxidase B in brains from patients with Alzheimer's disease: a biochemical and autoradiographical study

In vitro quantitative autoradiography using [3H]L-deprenyl, an irreversible and preferential inhibitor of monoamine oxidase B, was performed to investigate the localization of the enzyme in brains from senile dementia of Alzheimer type and control cases. Brains from three male patients with the clinical diagnosis of senile dementia of Alzheimer type and from three male control patients, without any known clinical history of neurological disorder, were obtained at autopsy. Cryosections of 100 microns thickness were mounted on gelatinized glass plates and dried over desiccant for one week at -20 degrees C. The sections were incubated with 10 nM [3H]L-deprenyl for 1 h and then exposed to film for four weeks. The autoradiographs were analysed by computer-assisted densitometry. Monoamine oxidase-B activities were also estimated in 1% homogenates from 10 different regions, using 10 microM beta-[ethyl-14C]phenylethylamine, in order to study the consonance between the autoradiographical and biochemical techniques. Both [3H]L-deprenyl binding and monoamine oxidase-B activities in senile dementia of Alzheimer type were higher than in the controls in all brain regions studied. The increase was highest in the white matter (about 70%) and in the order of 20-50% in the various gray matter regions. A high correlation coefficient (r approximately 0.9) was obtained between [3H]L-deprenyl binding and monoamine oxidase-B activity, both in the senile dementia of Alzheimer type and in the control brains.

Serotonin metabolism by monoamine oxidase in rat primary astrocyte cultures

The oxidative deamination of serotonin (5-HT) to 5-hydroxyindoleacetic acid (5-HIAA) by rat primary astrocyte cultures was investigated in intact cells using HPLC. All detectable 5-HIAA accumulated in the extracellular medium, and its rate of production was proportional to the 5-HT concentration over the tested range of 5 x 10(-7) to 10(-4) M. At 5 x 10(-7) M 5-HT, intracellular 5-HT was detectable only in astrocytes treated with monoamine oxidase (MAO) inhibitors. These findings are consistent with the idea that 5-HT taken up into astrocytes is not stored for re-release, but is rapidly metabolized to 5-HIAA, which is then extruded from the cell. At 5 x 10(-7) M 5-HT, 5-HIAA formation in intact cells was blocked 63% by the selective high-affinity 5-HT uptake inhibitor fluoxetine. 5-HT oxidation to 5-HIAA is carried out principally by MAO-A, because clorgyline was more effective at inhibiting the production of 5-HIAA than was pargyline. Radioenzymatic determinations of MAO activity in cell homogenates supported these findings, because under these conditions clorgyline was 1,000-fold more effective than pargyline at inhibiting MAO activity toward 14C-labelled 5-HT. However, the relatively selective MAO-B substrate beta-phenylethylamine (PEA) was also oxidized, showing that these cultures also contained MAO-B activity; the Km values for MAO-A oxidation of 5-HT and MAO-B oxidation of PEA were 135 and 45 microM, and Vmax values were 88 and 91 nmol/mg of total cell protein/h, respectively. Higher concentrations of PEA (greater than 20 microM) were oxidized by both MAO-A and MAO-B isozymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of selective monoamine oxidase inhibitors on the in vivo release and metabolism of dopamine in the rat striatum

Brain microdialysis was used to examine the in vivo efflux and metabolism of dopamine (DA) in the rat striatum following monoamine oxidase (MAO) inhibition. Relevant catecholamines and indoleamines were quantified by HPLC coupled with a electrochemical detection system. The MAO-B inhibitor selegiline only affected DA deamination at a dose shown to inhibit partially type A MAO. Alterations in DA and metabolite efflux were not observed when using the MAO-B-selective dose of 1 mg/kg of selegiline. At 10 mg/kg, selegiline reduced the efflux of DA metabolites to approximately 70% of basal values without affecting DA efflux. K(+)- and veratrine-stimulated DA efflux was not affected by selegiline. Experiments using amphetamine and the DA uptake inhibitor nomifensine demonstrated that the effect of selegiline on DA metabolism was unlikely to be mediated either by inhibition of DA uptake or by an indirect effect of its metabolite amphetamine. The possibility that the effect of selegiline is mediated via a nonspecific inhibition of MAO is discussed. In contrast, the MAO-A inhibitor clorgyline inhibited basal DA metabolism and increased basal and depolarisation-induced DA efflux. A 1 mg/kg dose of clorgyline reduced basal DA metabolite efflux (40-60% of control values) without affecting DA efflux. At 10 mg/kg of clorgyline, DA efflux increased to 253 +/- 19% of basal values, whereas efflux of DA metabolites was reduced to between 15 and 26% of control values. The release of DA induced by K+ and veratrine was not affected by 1 mg/kg of clorgyline but was increased by approximately 200% following pretreatment with 10 mg/kg of clorgyline. The nonselective MAO inhibitor pargyline caused similar but more pronounced alterations in these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrahippocampal kainic acid reduces glutamine synthetase

Kainic acid was injected into the hippocampus of rats and glutamine synthetase was measured to determine whether astrocytes are involved in the early effects of this neurotoxic agent. Glutamine synthetase was reduced by 38%, 24 h after the stereotaxic application of 4 nmol of kainic acid to this region. The reduction in glutamine synthetase by kainic acid was not due to direct inhibition of the brain enzyme. This effect also was not due to seizure activity since rats peripherally injected with a convulsant dose of kainic acid were found to have normal hippocampal glutamine-synthetase activity. Exposure of astrocyte cultures to kainic acid for 24 h produced no evidence of gliotoxicity and no change in glutamine synthetase activity. The effect of intrahippocampal kainic acid on glutamine synthetase appears to be indirect, most likely produced secondarily to its neuronal effects. Several studies have shown that endogenous glutamate is involved in kainate neurotoxicity. A reduction in glutamine synthetase by kainic acid may impair the capacity for astrocytes to metabolize glutamate. Such an impairment could contribute to the glutamate-mediated cell death following kainic acid exposure.

CGS-19755 and MK-801 selectively prevent rat striatal cholinergic and gabaergic neuronal degeneration induced by N-methyl-D-aspartate and ibotenate in vivo

The in vivo efficacies and potencies of various excitatory amino acid agonists in inducing cholinergic neuronal degeneration were compared following unilateral injections into the rat striatum. Kainic acid (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), ibotenic acid (IBO), and N-methyl-D-aspartic acid (NMDA) all produced dose-related decreases in choline acetyltransferase (ChAT) activity. The relative order of potency was KA greater than AMPA greater than IBO greater than NMDA. Quisqualic acid (QUIS) was about as potent as NMDA, but the maximal decrease in ChAT activity was less (36%). N-acetylaspartyl-L-glutamate (NAAG) did not significantly decrease ChAT activity when up to 1,000 nmoles was injected. Approximate equitoxic doses of agonists were then used to examine the ability of i.p. administered CGS-19755 and MK-801 to prevent in vivo excitatory amino acid-induced cholinergic and GABAergic neuronal degeneration. NMDA-induced decreases in ChAT and glutamic acid decarboxylase (GAD) activities were prevented by CGS-19755 (10-40 mg/kg) and MK-801 (1-10 mg/kg). CGS-19755 (40 mg/kg) and MK-801 (10 mg/kg) did not prevent loss of ChAT or GAD induced by KA or AMPA, but did prevent the degenerative effects of IBO. This study shows that CGS-19755 and MK-801, two NMDA receptor antagonists that act by different mechanisms, are completely selective following systemic administration. Moreover, the in vivo excitotoxic effects of IBO are mediated at NMDA receptor sites that are blocked by these compounds.

Studies of the in vivo catabolism of exogenous dopamine as infused through a push-pull cannula implanted in the rat caudate nucleus

In this report, using a push-pull perfusion technique, we examined in vivo the effects of exogenous dopamine (DA) on the output of neurochemical substances from the caudate nucleus (CN) of freely behaving rats. Exogenous DA, at concentrations of 10(-5) M, 5 x 10(-5) M and 10(-4) M, in modified Krebs-Ringer phosphate medium (KRP) was infused directly into the CN for 15 min each. Exogenous DA at the doses tested elicited increases in 3,4-dihydroxyphenylacetic acid (DOPAC) output in a dose-dependent manner. In addition, the higher two doses of exogenous DA also induced increases in homovanillic acid (HVA) output from the rat CN. The increases in DOPAC output by 5 x 10(-5) M DA was partially blocked by the inclusion of 10(-3) M nomifensine in KRP. Interestingly, exogenous DA-induced increases in HVA output were little affected by the nomifensine treatment. However, the catabolism of exogenous DA was almost completely eliminated by a prior 6-hydroxydopamine lesion in the ipsilateral substantia nigra. Furthermore, infusions of exogenous DA did not change 5-hydroxyindoleacetic acid output from the CN. In conclusion, our results confirm in vivo that (a) DA catabolic pathway via DOPAC intermediate predominates over the alternative pathway via 3-methoxytyramine, (b) increases in extracellular DA will lead to increases in DOPAC and HVA levels in extracellular space and (c) the majority of the DA is oxidized by intradopaminergic monoamine oxidase.

Localization of monoamine oxidase B in human brain by autoradiographical use of 11C-labelled L-deprenyl

11C-labelled L-deprenyl in vitro autoradiography was used to study the regional distribution of MAO-B in human brain. 80 microns thick cryosections from two human brains, a 67 years old female and a 58 years old male, were taken on tape/paper and transferred on to a gelatinized glass plate. The sections were then incubated with 34 and 54 nM 11C-L-deprenyl for 15 min and exposed to a film sensitive to high energy radiation for 2 hours. The autoradiograms obtained were analyzed by computerized densiotometry. High 11C-deprenyl binding was found in the caudate nucleus, putamen, thalamus, substantia nigra, medial and lateral geniculate bodies, hippocampus and periaqueductal gray. Moderate to low binding was observed in cerebral cortex. Cerebral cortex and white matter showed the lowest binding. The autoradiographic technique described proved to be a fast and reliable method to investigate the topographic localization of MAO-B in large cryosections of human brain.

The cholinergic neurotoxin ethylcholine mustard aziridinium (AF64A) induces an increase in MAO-B activity in the rat brain

Recently it was reported that there is an increase in monoamine oxidase B (MAO-B) activity in post-mortem brains of patients with Alzheimer's disease. It was postulated that this increase in MAO-B activity was due to gliosis associated with neuronal degeneration. The aim of the present investigation was to evaluate the effect on MAO of neuronal degeneration primarily affecting the cholinergic system. The specific cholinergic toxin AF64A (3 and 4.5 nmol) was injected bilaterally into the cerebral ventricles of rats. We then estimated MAO-A, MAO-B, dopamine (DA) uptake rates and choline acetyltransferase (ChAT) activities in hippocampus, striatum and cortex, 1, 2.5 and 4.5 weeks after the injection. Marked long-lasting reduction in ChAT activities appeared only in hippocampus, consistent with previous reports. The MAO-A activity was unchanged as were DA uptake rates. Neither was there any change in MAO-B activity found 1 week after the injection. However, a significant increase in MAO-B activity appeared after 2.5 weeks and persisted after 4.5 weeks in all 3 brain regions investigated. This result is likely to reflect progressive gliosis after cholinergic neuronal degeneration. Previous results have shown an increased MAO-B activity with age and a further accelerated increase in Alzheimer's disease. Experimentally, hemitransection and injection of kainic acid have been shown to cause a similar increase. The present results show that changes in MAO-B activity also reflect degenerative processes in brain mainly affecting the cholinergic system.

Histochemical localization of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine oxidation in the mouse brain

The sites in the mouse brain where 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine can be oxidized to the toxic metabolite 1-methyl-4-phenylpyridine were determined using a histochemical technique. The method involved the demonstration of monoamine oxidase activity using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine as the substrate by means of a sensitive coupled peroxidase technique. The distribution of neurons displaying the ability to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine via a monoamine oxidase catalysed reaction was compared to that of various amine systems identified with immunohistochemistry. Dopamine neurons, and in particular the nigrostriatal dopamine cells, did not display the capacity to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Noradrenergic neurons showed intense monoamine oxidase activity when 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was used as substrate, and this activity was blocked by the monoamine oxidase-A inhibitor clorgyline. Serotonin neurons and histamine neurons were also able to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The reaction in these neurons was blocked by deprenyl, an inhibitor of monoamine oxidase-B. Pretreatment with inhibitors of monoamine oxidase-B has been previously shown to prevent the neurotoxic action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on dopaminergic neurons. Therefore, since serotonin and histamine neurons are able to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by means of monoamine oxidase-B, these neurons may be involved in the production of the toxic metabolites of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in vivo.

Excitatory amino acid treatment of the ventromedial globus pallidus enhances dopamine utilization in the prefrontal cortex of the rat via the thalamic mediodorsal nucleus

Infusion of a low dose (5 microM) of the cell-selective chemical excitant quisqualic acid (QUIS) into rostral ventromedial globus pallidus (GP) had no immediate effect on DA utilization (assessed as [DOPAC]:[DA] and [HVA]:[DA] ratios) in either the medial bank of the prefrontal cortex (FCx) or the agranular insular cortex (AgCx). In contrast, a larger dose (630 microM) of another excitant sodium ibotenate (IBO) produced an immediate bilaterally symmetrical increase in both indices of DA utilization in FCx. There was also a marked trend towards a bilateral increase in these indices of DA utilization in AgCx. In order to determine whether these effects on cortical DA utilization are mediated by a direct cortical route or via the thalamic mediodorsal nucleus (lateral division, MDL), infusions of IBO into GP were repeated in animals with a 1-week-old N-methyl-D-aspartate lesion of MDL. The increase in DA utilization of FCx following infusion of IBO into GP was abolished, although the trend towards increased DA utilization in AgCx was still maintained. Since MDL innervates FCx but not AgCx and since we have previously shown that MDL lesions alone have no effect on DA utilization in either cortical region, the present results suggest that the changes in cortical DA utilization are probably mediated via MD. Thus in addition to the well-documented control exerted by the thalamus over brain DA function, this has now been extended in the present study to include GP, which projects both directly and indirectly to the thalamus.

Amine oxidases and their endogenous substrates (with special reference to monoamine oxidase and the brain)

The roles of MAO, BzO, DAO and PAO in the metabolism of endogenous substrates and the functional implications of their action and inhibition is reviewed, the emphasis being on MAO on one hand and on brain on the other. The major issues are the following: There is no discrete subdivision into substrates selective for MAO-A, MAO-B, or mixed ones, but rather a continuum. Tissue differences in substrate specificity are not likely to be due to molecular variability of MAO. For the deamination of DA, 5-HT and PEA at least, the relative participation of either MAO form in a given tissue is primarily determined by the relative abundance of the two forms; only at 10(-5) M and above, substrate concentration begins to matter also. In vivo, compartmentation is of paramount importance: since there seems to be more MAO-A than B inside monoaminergic neurons, DA, 5-HT and NA are predominantly metabolized by MAO-A if metabolism occurs mainly intraneuronally. Conversely, since MAO-B is more abundant extraneuronally, e.g. in glia cells, the relative participation of this form increases if a significant portion of these amines is deaminated outside monoaminergic neurons. In vivo, monoamine deamination is reduced concomitantly with the degree of MAO inhibition, whereas signs of increased transmitter function are only observed if enzyme inhibition is at least 80%. This is likely to be the result of the action of compensatory mechanisms such as feedback inhibition of transmitter release and synthesis. BzO is particularly abundant in vascular tissue, lung and bone. Low levels are found in brain. Endogenous substrates and physiological function are not known. DAO also occurs only in minimal amount in brain, if at all. Its principal substrates are histamine and the polyamines, and the disposal of these amines is probably its main function. Of the PAO's, the type of enzyme found in the rat liver attacks the secondary amino groups and may have a more prominent role in the metabolism of polyamines in the brain than in the periphery. Bovine plasma PAO, which attacks primary amino groups, is only found in the serum of ruminants, but not other species. Its function in the metabolism of polyamines is not known.

In vivo labelling and axonal transport of monoamine oxidase in the rat basal ganglia using radioactive pargyline

The enzyme monoamine oxidase was labelled in the rat striatum or substantia nigra with locally injected radioactive pargyline. The binding was prevented by a pretreatment with non-radioactive pargyline, or with a combination of clorgyline and deprenyl. Most of the MAO labelled with 3H-pargyline was of the B-type, but also some MAO-A was labelled, as shown in rats pretreated with clorgyline or deprenyl separately. Seven days after the injection of (3H)-pargyline into the striatum a significant labelling was observed in the substantia nigra. This labelling was clorgyline sensitive, indicating type A MAO, and was not present when striatal neurons were destroyed with kainic acid. Labelling of the striatum following 3H-pargyline injection into the substantia nigra was also less in kainate intoxicated striata. Damage of nigral dopamine neurons with 6-hydroxydopamine did not influence the distribution of the label. Thus by using 3H-pargyline, specific labelling and axonal transport of type A MAO in striatal neurons projecting to the substantia nigra was demonstrated.

Autoradiography of high affinity uptake of catecholamines by primary astrocyte cultures

Uptake of D.L-[3H]norepinephrine ([3H]NE) and [3H]dopamine ([3H]DA) by primary astrocyte cultures prepared from neonatal rat brains, which are greater than or equal to 95% glial fibrillary acidic protein (GFAP(+)), was studied by measuring accumulation of tritium label, and localizing such uptake at the cellular level by autoradiography. Uptake of [3H]NE was 95% Na+ dependent at 10(-7) M and 80% Na+ dependent at 7.5 X 10(-7) M [3H]NE. Uptake of [3H]DA at 7.5 X 10(-7) M was 58% Na+ dependent, but total uptake of [3H]DA was greater than uptake of [3H]NE. Autoradiography of cells incubated with 7.5 X 10(-7) M [3H]NE or [3H]DA showed that a high proportion of all the cells in these cultures had a grain density which was clearly above background. When Na+ was omitted from the medium, the temperature was lowered to 4 degrees C, or 10(-7) M desmethylimipramine or 10(-7) M amitryptyline were present, cellular grain density after exposure to both [3H]NE and [3H]DA was greatly reduced, to close to background levels. It also appeared necessary to have inhibitors of both monoamine oxidase (pargyline) and catecholamine-O-methyltransferase (tropolone) present to see clear cellular localization for [3H]DA. In the case of [3H]NE the presence of tropolone alone was adequate to observe cellular localization. These results confirm our previous findings of the existence of a high affinity uptake process for catecholamines in primary astrocyte cultures based on uptake properties, and in the present study also localizes such uptake to the major, astrocytic cell type.

Effects of intrahypothalamic kainic acid injection on taurine levels, binding and uptake

The neurochemical effects of unilateral intrahypothalamic injection of kainic acid on taurine levels and synaptosomal uptake and binding of taurine were investigated. Seven days after the kainic acid injections, there were no changes in either taurine uptake or binding. However, taurine levels were significantly increased by 54% over the control contralateral side. These data are consistent with the hypothesis that taurine is localized in glial cells; the increased levels being a result of gliosis after kainic acid injections.

Further studies on the nature of postsynaptic dopamine uptake and metabolism in rat striatum: sodium dependency and investigation of a possible role for carrier-mediated uptake into serotonin neurons

The nature of postsynaptic sites involved in the uptake and metabolism of striatal 3,4-dihydroxyphenylethylamine (dopamine, DA) was investigated. The accumulation of [3H]DA (10(-7) M) into slices of rat striatum was found to be greatly dependent (greater than 99%) on the presence of sodium ion in the incubation medium. However, the formation of the [3H]dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) was only partially reduced in the absence of sodium (DOPAC, 27% of control; HVA, 47% of control). Inhibition of carrier-mediated DA neuronal uptake with nomifensine (10(-5) M) significantly decreased DA accumulation (18% of control) and [3H]DOPAC formation (62% of control), but enhanced [3H]HVA production (143% of control). Inhibition of the 5-hydroxytryptamine (5-HT, serotonin) neuronal uptake system with fluoxetine (10(-6) M) or selective 5-HT neuronal lesions with 5,7-dihydroxytryptamine (5,7-DHT) had no effect on [3H]DOPAC or [3H]HVA formed from [3H]DA in the presence or absence of nomifensine. These results demonstrate that the uptake and subsequent metabolism of striatal DA to DOPAC and HVA is only partially dependent on carrier-mediated uptake mechanism(s) requiring sodium ion. These data support our previous findings suggesting a significant role for synaptic glial cell deamination and O-methylation of striatal DA. Further, experiments with fluoxetine or 5,7-DHT suggest that 5-HT neurons do not significantly contribute in the synaptic uptake and metabolism of striatal DA.

Cellular localization of MAO A and B in brain: evidence from kainic acid lesions in striatum

The cellular localization of the two forms of monoamine oxidase (MAO A and MAO B) was studied by measuring their activities in rat striatum following unilateral stereotaxic injection of kainic acid to produce selective degeneration of striatal neurons and subsequent proliferation of astrocytes. The results demonstrated a persistent loss of 15-20% in MAO A activity, whereas MAO B activity decreased initially by 25% and then increased to more than twice the control value by 54 days after lesions. The changes in activity were compared to parallel estimates of the postsynaptic neuronal enzyme markers glutamic acid decarboxylase (GAD) and neuron-specific enolase (NSE), astroglial enzyme markers glutamine synthetase (GS) and non-neuronal enolase (NNE), and the presynaptic enzyme marker DOPA decarboxylase (DDC). The results suggest that a small amount of striatal MAO A is present in kainic acid-sensitive postsynaptic striatal neurons and that MAO B is probably localized in both neurons and astrocytes.

Brain peptidases: their possible neuronal and glial localization

Neuronal and glial localization of brain peptidases was investigated by means of the kainic acid (KA) lesion technique. Activities of 6 different peptidases were measured in the rat caudate-putamen (CP) and substantia nigra (SN) 2, 7 and 21 days after unilateral intra-CP injection with 2.5 micrograms of KA. As an indicator of KA lesion in CP, substance P content in both CP and SN was also determined. In addition, activities of the same peptidases in the primary and secondary glial cell cultures of fetal rats were measured and compared to those in CP homogenate. After the KA injection, prolyl endopeptidase (Pro-EP) activity was decreased in the lesioned CP and, to a lesser extent, in the ipsilateral SN. The activity of angiotensin-converting enzyme (ACE) in the lesioned CP was decreased with a complex time course, whereas a slow and progressive reduction was observed in the SN. Alanyl and leucyl aminopeptidase (Ala-AP and Leu-AP respectively) activities gave only small changes after the lesion; Ala-AP was decreased and Leu-AP was increased in the lesioned CP, while both were decreased in the SN. Dipeptidyl aminopeptidase (DAP) and arginyl endopeptidase (Arg-EP) activities were increased 5-fold in the CP 7 days after the KA injection. Their increases paralleled that of beta-glucuronidase, the lysosomal marker enzyme. Cultured glial cells contained only a trace amount of ACE activity. Ala-AP and Pro-EP activities were considerably lower in the glial culture cells than in the CP homogenate. In contrast, DAP and Arg-EP as well as lysosomal marker enzymes showed much higher activity in the former than in the latter. These results suggest that (1) Ala-AP and Pro-EP have large neuronal components, (2) ACE is preferencially localized in neurons and (3) DAP and Arg-EP are associated with glial lysosomal function. It is, therefore, concluded that at least a part of the brain peptidases are differentially localized in neurons and glia, and may be involved in specific neuronal or glial function.

Negative symptoms and platelet monoamine oxidase activity in male schizophrenic patients

A significant positive correlation was found between negative symptoms and platelet monoamine oxidase (MAO) activity in unmedicated male, but not female, schizophrenic patients. This correlation was significant in split halves of the male patients. There was no indication that the correlation was due to either outliers or medication effects. Male schizophrenic patients with high negative symptom scores had significantly higher mean platelet MAO activity than either male normal control subjects or male schizophrenic patients with low negative symptom scores. This finding suggests that the extent of negative symptoms in a population of males could affect whether the schizophrenic subjects will be found to have platelet MAO activity which differs from that of normal control subjects. The implications of the correlation between platelet MAO activity and negative symptoms for the role of brain MAO activity and two of its substrates, dopamine and serotonin, in the etiology of negative symptoms in male schizophrenic patients are discussed.

Sulfate conjugation of dopamine in rat brain: regional distribution of activity and evidence for neuronal localization

Brain tissue contains at least two forms of phenolsulfotransferase that are involved in the sulfate conjugation of biogenic amines and their metabolites. Two apparent Km values were obtained for p-nitrophenol at pH 7.4 (0.6 microM and 0.3 mM) but only one enzyme had the capacity to conjugate dopamine (Km = 130 microM). Dopamine sulfotransferase activity was found to vary 17-fold in different brain regions, with the highest levels in diencephalon, hippocampus, and striatum. To determine the cellular localization of the enzymes, phenolsulfotransferase activity was measured in striatum following selective destruction of striatal neurons by stereotaxic injection of 2 micrograms kainic acid. Fourteen days after injection the catecholamine sulfotransferase activity in the lesioned striatum was reduced to approximately 40-50% of that in the control contralateral striatum. There was a statistically significant correlation between the ratio of lesioned to control activity for the sulfotransferase and the neuronal marker enzymes glutamate decarboxylase and neuron-specific enolase. p-Nitrophenol sulfotransferase activity was also decreased in the lesioned striatum. These results suggest that PST activity is present within the kainic acid-sensitive neurons of the striatum. The regional variation in activity, together with the results of the kainic acid studies, suggest that sulfate conjugation of biogenic amines and their metabolites in brain may take place within specific types of neurons.