I. A. Paterson

Regulation of Aromatic l‐Amino Acid Decarboxylase by Dopamine Receptors in the Rat Brain

Abstract: Decarboxylation of phenylalanine by aromatic l‐amino acid decarboxylase (AADC) is the rate‐limiting step in the synthesis of 2‐phenylethylamine (PE), a putative modulator of dopamine transmission. Because neuroleptics increase the rate of accumulation of striatal PE, these studies were performed to determine whether this effect may be mediated by a change in AADC activity. Administration of the D1 antagonist SCH 23390 at doses of 0.01–1 mg/kg significantly increased rat striatal AADC activity in an in vitro assay (by 16–33%). Pimozide, a D2‐receptor antagonist, when given at doses of 0.01–3 mg/kg, also increased AADC activity in the rat striatum (by 25–41%). In addition, pimozide at doses of 0.3 and 1 mg/kg increased AADC activity in the nucleus accumbens (by 33% and 45%) and at doses of 0.1, 0.3, and 1 mg/kg increased AADC activity in the olfactory tubercles (by 23%, 30%, and 28%, respectively). Analysis of the enzyme kinetics indicated that the Vmax increased with little change in the Km with l‐3,4‐dihydroxyphenylalanine as substrate. The AADC activity in the striatum showed a time‐dependent response after the administration of SCH 23390 and pimozide: the activity was increased within 30 min and the increases lasted 2–4 h. Inhibition of protein synthesis by cycloheximide (10 mg/kg, 0.5 h) had no effect on the striatal AADC activity or on the increases in striatal AADC activity produced by pimozide or SCH 23390. The results indicate that the increases in AADC activity induced by dopamine‐receptor blockers are not due to de novo synthesis of the enzyme. These results show that AADC activity in the striatum is regulated by D1 and D2 receptors and that the activities in the nucleus accumbens and olfactory tubercles are regulated by D2 receptors. The observation that dopamine‐receptor antagonists stimulate the synthesis of PE may be explained by the increase in AADC activity.

R-Deprenyl and R-2-Heptyl-N-Methylpropargylamine Prevent Apoptosis in Cerebellar Granule Neurons Induced by Cytosine Arabinoside but Not Low Extracellular Potassium

Abstract: R‐Deprenyl and R‐2‐heptyl‐N‐methylpropargylamine (R‐2‐HMP) are compounds that have been shown to reduce neuronal death in various in vitro and in vivo models involving apoptosis but do not always prevent apoptosis. In the present study we have examined the effects of these compounds and their S enantiomers on cytosine arabinoside (ara C)‐induced apoptosis and low K+‐induced apoptosis in cerebellar granule cells in primary culture. It was found that R‐deprenyl and R‐2‐HMP could prevent ara C‐induced apoptosis with an EC50 around 10−9M but could not prevent low K+‐induced apoptosis. S‐Deprenyl and S‐2‐HMP did not prevent apoptosis under any conditions but were found to antagonize the antiapoptotic actions of R‐deprenyl and R‐2‐HMP. Using the fluorescent mitochondrial dye chloromethyltetramethylrhodamine methyl ester it was found that there was a loss of mitochondrial function in cerebellar granule cells exposed to ara C but not low K+ medium. R‐Deprenyl and R‐2‐HMP prevented the ara C‐induced loss of mitochondrial function. It is concluded that R‐deprenyl and R‐2‐HMP prevent apoptosis of cerebellar granule cells by a mechanism that is independent of monoamine oxidase inhibition and that they act on the same site to prevent specifically apoptosis involving a loss of mitochondrial membrane potential, possibly p53‐dependent apoptosis.

The effects of administration of monoamine oxidase-B inhibitors on rat striatal neurone responses to dopamine

1 (−)−Deprenyl has been shown to potentiate rat striatal neurone responses to dopamine agonists at doses not altering dopamine metabolism. Since there are a number of effects of (−)−deprenyl which could result in this phenomenon, we have investigated the effects of MDL 72,145 and Ro 19–6327, whose only common effect with (−)−deprenyl is an inhibition of monoamine oxidase‐B (MAO‐B), on rat striatal neurone responses to dopamine and on striatal dopamine metabolism. 2 Using in vivo electrophysiology, i.p. injection of either MDL 72,145 or Ro 19–6327 was found to produce a dose‐dependent potentiation of striatal neurone responses to dopamine but not γ aminobutyric acid. 3 Neurochemical investigations revealed that this occurred at doses (0.25‐1 mg kg−1) which, while not affecting levels of dopamine or its metabolites, 3,4‐dihydroxyphenylacetic acid or homo vanillic acid, did cause a significant, dose‐dependent, elevation in striatal levels of the putative neuromodulator, 2‐phenylethylamine (PE). 4 Inhibition of PE synthesis by i.p. injection of the aromatic 1‐amino acid decarboxylase inhibitor, NSD 1015, produced a reversal of the effects of MDL 72,145 and Ro 19–6327. 5 Neurochemical analysis revealed this to occur at a dose of NSD 1015 (10 mg kg−1) selective for reduction of elevated PE levels. 6 These results suggest that PE can act as a neuromodulator of dopaminergic responses and that MAO‐B inhibitors may potentiate neuronal responses to dopamine via the indirect mechanism of elevation of PE following MAO‐B inhibition.

The effect of L-deprenyl on behavior, cognitive function, and biogenic amines in the dog

Behavioral and pharmacological effects of oral administration ofl-deprenyl in the dog are described. Spontaneous behavior is unaffected at doses below 3 mg/kg while at higher doses there was stereotypical responding. There was evidence of improved cognitive function in animals chronically treated with a 1 mg/kg dose but the effectiveness varied considerably between subjects. Chronic administration produced a dose dependent inhibition in brain, kidney and liver monoamine oxidase B, and had no effect on monoamine oxidase A. There were also dose dependent increases in brain phenylethylamine and in plasma levels of amphetamine. Dog platelets did not have significant levels of MAO-B. Brain dopamine and serotonin metabolism were unaffected byl-deprenyl at doses up to 1 mg/kg. It appears that for the dog, deamination of catecholamines is controlled by MAO-A. Nevertheless, it is suggested thatl-deprenyl serves as a dopaminergic agonist, and there is also evidence that it affects adrenergic transmission. These catecholaminergic actions may account for the effects ofl-deprenyl on behavior and cognitive function.

Regulation of aromatic L-amino acid decarboxylase in rat striatal synaptosomes: effects of dopamine receptor agonists and antagonists.

1 In this study we investigated the effects of dopamine receptor agonists and antagonists on rat striatal synaptosomal aromatic l‐amino acid decarboxylase (AADC) activity. 2 The results show that 10−5–10−7 m cis‐flupenthixol increased the striatal synaptosomal AADC activity (by 25% to 57%) in a time‐dependent manner. SCH 23390 and remoxipride alone had little or no effect on striatal synaptosomal AADC activity, but in combination they increased AADC activity by 20%, suggesting that the increases in striatal synaptosomal AADC activity occurred only after blockade of both dopamine D1 and D2 receptors. 3 Treatment with (+)‐amphetamine and (±)‐2‐(N‐phenylethyl‐N‐propyl)amino‐5‐hydroxytetralin hydrochloride ((±)‐PPHT) produced a reduction of striatal synaptosomal AADC activity in a concentration‐ and time‐dependent manner. SKF 38393 and (−)‐quinpirole, however, exhibited no effect on striatal synaptosomal AADC activity, suggesting that only the mixed dopamine receptor agonists can reduce the AADC activity. Incubation with apomorphine at a concentration of 10−4 m inhibited the AADC activity by 74% and this inhibition cannot be antagonized by SCH 23390, remoxipride or cis‐flupenthixol, suggesting that apomorphine‐induced inhibition of striatal synaptosomal AADC activity was not mediated by dopamine receptors. 4 cis‐Flupenthixol can reverse the reduction of AADC activity induced by (+)‐amphetamine and (±)‐PPHT. The inhibition of AADC activity elicited by (±)‐PPHT also can be reversed by SCH 23390 and remoxipride. 5 The inhibition of striatal synaptosomal AADC activity induced by (±)‐PPHT is calcium‐dependent and protein kinase C may play a role in the regulation of striatal AADC activity. 6 These studies show that striatal synaptosomal AADC activity is regulated by dopamine receptors and indicate that in vitro dopamine D1 and D2 receptors have a synergistic effect in this regulation.

Electrical Stimulation of the Substantia Nigra and Changes of 2‐Phenylethylamine Synthesis in the Rat Striatum

Abstract: In rats pretreated with deprenyl (2 mg/kg), electrical stimulation of the left substantia nigra produced an increase in the concentrations of 3,4‐dihydroxyphenylacetic acid and homovanillic acid in the left striatum by 57 and 45%, but the levels of 2‐phenylethylamine and p‐tyramine decreased by 22 and 41%, respectively, as compared with those in the right striatum. The administration of α‐methyl‐p‐tyrosine (1.25 mg/kg, i.p.), a tyrosine hydroxylase inhibitor, 1 h before nigral stimulation, did not affect the concentration of 2‐phenylethylamine in unstimulated striata but prevented the stimulation‐induced decrease in the concentration of 2‐phenylethylamine. Neither stimulation nor α‐methyl‐p‐tyrosine affected the activity of monoamine oxidase A or B, and stimulation did not produce any change in striatal blood flow, a finding demonstrating that the changes in the rate of accumulation of 2‐phenylethylamine were not due to changes in catabolism or removal of 2‐phenylethylamine from the brain. These experiments demonstrate that the rate of synthesis of striatal 2‐phenylethylamine is decreased following nigral stimulation and that this effect is blocked after partial inhibition of tyrosine hydroxylase. This suggests that 2‐phenylethylamine is present in tyrosine hydroxylase‐containing neurons and therefore supports the coexistence of 2‐phenylethylamine and dopamine in the nigrostriatal pathway.

The effects of some neuroleptics and d-amphetamine on striatal 2-phenylethylamine in the mouse

1. Mouse striatal 2-phenylethylamine was not changed at 2 hr following the administration of chlorpromazine, fluphenazine or spiperone. 2. In contrast, when the mice were first given pargyline (2 mg kg-1), treated with chlorpromazine, fluphenazine or spiperone 2 hr later and killed at 4 hr, a significant increase (to 130-170%) in the accumulation of 2-phenylethylamine was observed with respect to the pargyline controls. 3. The effect of chlorpromazine was consistently observed after pretreatment with either deprenyl (2 mg kg -1) or high doses (200 mg kg-1) of pargyline that produced different degrees of MAO inhibition. 4. Following pretreatment with pargyline (2 mg kg-1), d-amphetamine (5 mg kg-1) produced a significant reduction in striatal 2-phenylethylamine concentrations (to 39% of pargyline-treated controls). 5. The findings show that inhibition of dopamine transmission by neuroleptics increases the rate of 2-phenylethylamine accumulation. 6. Conversely, a stimulation of dopamine transmission by d-amphetamine results in a reduction in the rate of accumulation of 2-phenylethylamine and supports the concept of 2-phenylethylamine may be a neuromodulator of dopamine transmission.

The effects of monoamine oxidase B inhibition on dopamine metabolism in rats with nigro-striatal lesions

The purpose of this study was to examine whether monoamine oxidase type B (MAO-B) has a role in striatal dopamine metabolism in animals with a unilateral lesion of the medial forebrain bundle, and whether 2-phenylethylamine (PE) could have a role in amplification of dopamine (DA) responses in DA depleted striatum. Inhibition of MAO-B did not alter DA metabolism in lesioned striata. PE accumulation decreased with loss of DA as long as there was no DA dysfunction. In lesioned striata with dysfunction of DA transmission at the synaptic level, PE accumulation increased,suggesting a compensatory increase in PE synthesis. This increase in PE levels does not appear to be mediated by an increase in the total striatal aromaticl-amino acid decarboxylase (AADC) activity. We conclude that inhibition of MAO-B has no effect on DA metabolism in the hemi-parkinsonian rat striatum and that PE could be involved in the antiparkinsonian action of MAO-B inhibitors.

Inhibition of MAO-B by (−)-deprenyl alters dopamine metabolism in the macaque (Macaca facicularis) brain

The present study has examined whether MAO-B has a role in DA metabolism in the primate CNS in situ. Eleven macaques (macaca facicularis) were used in this study to examine the effects of (-)-deprenyl (1 mg/kg, i.v., 2 and 24 hours). (-)-Deprenyl administration completely and selectively blocked MAO-B activity and blocked DA metabolism in the caudate nucleus and frontal cortex. DA metabolism in the substantia nigra was not affected by MAO-B inhibition. Changes in DA metabolism were accompanied by changes in 5-hydroxytryptamine (5HT) turnover: 5-hydroxyindole acetic acid (5HIAA) levels increased in the caudate and decreased in the frontal cortex. Levels of 2-phenylethylamine (PE), a putative modulator of dopaminergic transmission, were increased by MAO-B inhibition in all three brain regions examined. It is concluded that in some regions of the primate brain, in contrast to the rat, MAO-B has an important role in DA metabolism.

The potentiation of cortical neuron responses to noradrenaline by 2-phenylethylamine is independent of endogenous noradrenaline

Abstract2-Phenylethylamine (PE) is an endogenous brain amine which produces sympathomimetic responses and potentiates cortical neuron responses to noradrenaline (NA). In order to examine further the mechanism of action of PE, extracellular recordings were made of the activity of single neurones in the cerebral cortex in urethane-anesthetized rats. Sympathomimetic responses to PE were blocked by pretreatment with reserpine, reserpine plus α-methyl-p-tyrosine and desipramine. It is concluded that the sympathomimetic responses to PE are indirect. 2-Phenylethylamine potentiated cortical neuron responses to electrical stimulation of the locus coeruleus in a dose-dependent manner. This was seen when PE was given systemically (with as little as 1 μg/kg) and iontophoretically. The effects of PE were not reproduced by its metabolite phenylacetic acid or its putative metabolite phenylethanolamine. Iontophoretic applications of PE (0–6 nA, 2–5 minutes) potentiated cortical neuron responses to iontophoretically applied NA, without affecting the spontaneous firing rate, or the responses to iontophoretically applied GABA or acetylcholine. This effect of PE was not blocked by pretreatment with α-methyl-p-tyrosine or desipramine, and was potentiated by pretreatment with reserpine and reserpine plus α-methyl-p-tyrosine. It is probable that the ability of PE to modulate neuronal responses to NA does not involve the presynaptic NA terminal or endogenous NA and it is likely that PE acts directly to increase the efficacy of NA. These findings are consistent with the hypothesis that the physiological role of PE is to modulate catecholaminergic transmission within the central nervous system.