I. Alick Paterson

Regulation of striatal aromatic L-amino acid decarboxylase: effects of blockade or activation of dopamine receptors.

Previous experiments have shown that blockade of dopamine D1 or D2 receptors by SCH 23390 or pimozide increases aromatic L-amino acid decarboxylase (AADC) activity in the rat striatum and the mesolimbic system. This study examined whether other dopamine receptor antagonists affect AADC activity and if there is an interaction between dopamine D1 and D2 receptor blockade on AADC activity. The possible effect of dopamine receptor agonists on AADC activity has been investigated as well. Administration of cis-flupenthixol (0.5 and 1 mg/kg) increased striatal AADC activity (by 25 and 26% above controls) and similar effects were observed with remoxipride (0.5-4 mg/kg) (by 18-27% above controls). Pretreatment with cycloheximide (10 mg/kg) did not change the increases produced by cis-flupenthixol (0.5 mg/kg). The administration of non-neuroleptic trans-flupenthixol did not change AADC activity. Combined treatment with SCH 23390 (0.1 mg/kg) and remoxipride (0.5 mg/kg), but not combination of SCH 23390 (0.1 mg/kg) and pimozide (0.3 mg/kg), showed higher increases of AADC activity than by the individual treatments, suggesting an interaction between the effects of the two drugs. Bromocriptine, but not (-)-quinpirole and d-amphetamine, significantly reduced the striatal AADC activity by 23% at the dose of 10 mg/kg. The results further demonstrate that AADC is a regulated enzyme in the rat brain.

Aliphatic propargylamines: New antiapoptotic drugs

Two series of drugs, the aliphatic‐N‐methyl propargylamines and the aliphatic propargylamines, have been synthesised and shown to be specific, irreversible, and potent monoamine oxidase B inhibitors and neural rescue agents. In the latter case, an absolute stereochemical requirement for the R isomer exists. Both series of compounds have been shown, in numerous in vitro and in vivo experimental paradigms, to be effective neuronal rescue agents. Candidates from both series exhibit excellent bioavailability and pharmacokinetics and offer opportunities for treating neurodegenerative disorders and stroke and cognitive decline in companion animals. Drug Dev. Res. 42:150–156, 1997.

Gene Expression of Aromatic l-Amino Acid Decarboxylase in Cultured Rat Glial Cells

Abstract: Northern blot hybridization was performed to detect aromatic L‐amino acid decarboxylase (AADC) mRNA in primary cultures of astrocytes and C6 glioma cells. The cDNA probe for rat AADC was generated by reverse transcription from rat adrenal gland total RNA and was amplified by the polymerase chain reaction method. AADC mRNA from cultured astrocytes and C6 glioma cells was present as a single band, 2.2 kbp in size, that comigrated with the RNA from rat kidney. Western immunoblot showed a single protein band at 52 kDa for AADC enzyme protein. These findings demonstrate that AADC is expressed in rat glial cells.

Decarboxylation ofl-Dopa by cultured mouse astrocytes

Cultured astrocytes contain immunologically specific aromatic L-amino acid decarboxylase (AADC) protein and express the AADC gene. Following incubation with L-Dopa, glial cultures synthesize and metabolize dopamine. The addition of pyridoxal 5-phosphate did not change the rate of dopamine synthesis. The formation of dopamine was blocked by NSD-1015. These experiments show that mouse cultured astrocytes are capable to convert L-Dopa into dopamine in a dose-dependent fashion.

Specific irreversible monoamine oxidase B inhibitors stimulate gene expression of aromatic L-amino acid decarboxylase in PC12 cells

Abstract: The effect of some selective monoamine oxidase (MAO) inhibitors on aromatic L‐amino acid decarboxylase (AADC) gene expression in PC12 cells has been examined. Irreversible MAO B inhibitors [(–)‐deprenyl, pargyline, and MDL 72,974A] stimulated AADC gene expression, whereas a selective irreversible MAO A inhibitor (clorgyline) and a reversible MAO B inhibitor (Ro 19‐6327) had no effect. Because there is no apparent MAO B activity in PC12 cells, it is postulated that there is a novel site of action for these MAO B inhibitors and that the pharmacological profile of this site matches that of neuroprotective MAO B inhibitors. Finally, it is suggested that the stimulation of AADC gene expression may be relevant to the antiparkinsonian effects of MAO B inhibitors.

Tryptamine receptors: neurochemical and electrophysiological evidence for postsynaptic and functional binding sites

[3H]Tryptamine binding characteristics and responsiveness of spontaneously active caudate nucleus neurons to intravenous application of drugs were assessed 6 weeks following unilateral application of 6-hydroxydopamine (6-OHDA, 8 micrograms) to the substantia nigra of male Wistar rats. The effects of this lesion procedure on caudatal levels of dopamine, 5-hydroxytryptamine (5-HT) and their acid metabolites, and on pargyline-induced (200 mg/kg, 2 h, s.c.) accumulation of tryptamine in the caudate nucleus were also assessed. Levels of caudatal dopamine and metabolites were reduced ipsilateral to the lesion. Concurrently there was a reduction in the extent of pargyline-induced tryptamine accumulation. Caudatal [3H]tryptamine binding was increased ipsilateral to the lesion, indicating a postsynaptic localization of this binding site. Bmax values in the caudatal samples ipsilateral to the lesion were increased by an average of 34% relative to the contralateral side. Contralateral Bmax values were equivalent to those routinely obtained in control animals. The affinity (Kd) of these binding sites for [3H]tryptamine was unchanged by the lesion procedure. The firing rate of caudate neurons was inhibited by intravenous application of tryptamine, apomorphine and 5-MeODMT. The lesion procedure did not affect these responses to 5-MeODMT. Responses to tryptamine and to apomorphine were enhanced ipsilateral to the lesion by 10- and 3-fold respectively. Haloperidol (0.5 mg/kg, i.v.) reversed apomorphine-induced inhibition of caudatal neuronal firing rate. The effects of tryptamine were not reversed by haloperidol. These data indicate a classical adaptive increase in [3H]tryptamine binding in caudate following 6-OHDA lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal Sparing and Behavioral Effects of the Antiapoptotic Drug, (−)Deprenyl, Following Kainic Acid Administration ☆

(-)Deprenyl is an irreversible inhibitor of monoamine oxidase B (MAO-B) frequently used as an adjunct therapy in the treatment of Parkinsons Disease. Recent evidence, however, has found that deprenyls metabolites are associated with an antiapoptotic action within certain neuronal populations. Interestingly, deprenyls antiapoptotic actions appear not to depend upon the inhibition of MAO-B. Due to a paucity of information surrounding (-)deprenyls ability to spare neurons in vivo, a series of studies was conducted to further investigate this phenomenon within an apoptotic neuronal death model: kainic acid induced excitotoxicity. Results indicated that (-)deprenyl increased hippocampal neuronal survival compared to saline-matched controls following kainic acid insult. Furthermore, it was discovered that (-)deprenyl treatment could be stopped 14 days following CNS insult by kainate, with evidence of neuronal sparing still present by day 28. In open-field locomotor activity testing of kainate-treated animals, those given subsequent (-)deprenyl treatment showed habituation curves similar to control subjects, while saline-treated animals did not. Given deprenyls antiapoptotic actions, it is proposed that (-)deprenyl may be beneficial in the treatment of a variety of neurodegenerative diseases where evidence of apoptosis exists, such as Parkinsons and Alzheimers Disease, by slowing the disease process itself.

Dopamine metabolism in the guinea pig striatum: role of monoamine oxidase A and B

These studies were carried out to determine whether the greater abundance of monoamine oxidase B in the guinea pig affects the actions of (-)-deprenyl on dopamine metabolism in whole tissue or in extracellular fluid. The administration of (-)-deprenyl in doses that do not affect monoamine oxidase A activity (1-4 mg kg-1, 2 h) increases striatal 2-phenylethylamine and dopamine concentrations and reduces 3,4-dihydroxyphenylacetic acid. No effects were observed on striatal homovanillic acid, 5-HT and 5-hydroxyindole acetic acid. Inhibition of monoamine oxidase A with clorgyline with doses up to 8 mg kg-1 (2 h) does not affect striatal 2-phenylethylamine but increases dopamine and 5-HT concentrations and reduces 3,4-dihydroxyphenyl-acetic acid and 5-hydroxyindole acetic acid. (-)-Deprenyl (2-4 mg kg-1) did not change the extracellular concentrations of dopamine but the higher dose produced a limited reduction in extracellular 3,4-dihydroxyphenylacetic acid. Inhibition of monoamine oxidase A and monoamine oxidase B with pargyline (75 mg kg-1, 2 h) significantly increased the levels of extracellular dopamine and reduced those of their acid metabolites. These results show that in the guinea pig striatum inhibition of monoamine oxidase B by (-)-deprenyl impairs the metabolism of dopamine in the whole tissue but does not produce a marked increase in extracellular dopamine.

Absence of 2-phenylethylamine binding after monoamine oxidase inhibition in rat brain

Earlier work has suggested the existence of saturable and highly specific binding sites for [3H]2-phenylethylamine in rat forebrain membranes. Since monoamine oxidase (MAO) was not inhibited during the assay, the [3H]2-phenylethylamine binding may have been affected by an interaction between 2-phenylethylamine and the enzyme. This is an investigation of [3H]2-phenylethylamine binding to rat forebrain membranes in the presence of two MAO inhibitors, (-)-deprenyl and pargyline. The results show that the high affinity specific binding of [3H]2-phenylethylamine to rat forebrain membranes is inhibited by pretreatment of the membrane with the MAO inhibitors and in vivo injection of the MAO inhibitors in a concentration-dependent manner. In the presence of higher concentrations of MAO inhibitors, the specific binding of [3H]2-phenylethylamine is completely blocked, suggesting that the binding sites reported earlier represent binding to MAO-B.

Effect of dietary phenylalanine on the plasma concentrations of phenylalanine, phenylethylamine and phenylacetic acid in healthy volunteers

1. Phenylethylamine has been proposed as a neuromodulator in several psychiatric and other brain disorders, and its concentration and that of its major metabolite, phenylacetic acid, in plasma may prove useful as state or trait markers in diagnosis, treatment or in the elucidation of biochemical mechanisms of these disorders. 2. The effect of dietary phenylalanine intake and changes in dietary phenylalanine intake on the plasma concentrations and changes in plasma concentrations, respectively, of phenylalanine, phenylethylamine and unconjugated and conjugated phenylacetic acid have been investigated. 3. Dietary phenylalanine affects the concentration of plasma phenylalanine on the following day, but has no effect on phenylethylamine or phenylacetic acid concentrations. Thus single measurements per subject of phenylethylamine or phenylacetic acid do not need to take dietary factors into account. 4. Changes in dietary phenylalanine (whether in absolute amount or in the proportion of phenylalanine in the diet) are significantly correlated with changes in unconjugated phenylacetic acid. Therefore, in longitudinal studies, dietary factors should be taken into account.