Changes in dopa decarboxylase mRNA but not tyrosine hydroxylase mRNA levels in rat brain following antipsychotic treatment

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Considerations for optimal use of postmortem human brains for molecular psychiatry: lessons from schizophrenia

Schizophrenia is a disabling disease impacting millions of people around the world, for which there is no known cure. Current antipsychotic treatments for schizophrenia mainly target psychotic symptoms, do little to ameliorate social or cognitive deficits, have side-effects that cause weight gain, and diabetes and 30% of people do not respond. Thus, better therapeutics for schizophrenia aimed at the route biologic changes are needed and discovering the underlying neurobiology is key to this quest. Postmortem brain studies provide the most direct and detailed way to determine the pathophysiology of schizophrenia. This chapter outlines steps that can be taken to ensure the best-quality molecular data from postmortem brain tissue are obtained. In this chapter, we also discuss targeted and high-throughput methods for examining gene and protein expression and some of the strengths and limitations of each method. We briefly consider why gene and protein expression changes may not always concur within brain tissue. We conclude that postmortem brain research that investigates gene and protein expression in well-characterized and matched brain cohorts provides an important foundation to be considered when interpreting data obtained from studies of living schizophrenia patients.

Putative presynaptic dopamine dysregulation in schizophrenia is supported by molecular evidence from post-mortem human midbrain

The dopamine hypothesis of schizophrenia posits that increased subcortical dopamine underpins psychosis. In vivo imaging studies indicate an increased presynaptic dopamine synthesis capacity in striatal terminals and cell bodies in the midbrain in schizophrenia; however, measures of the dopamine-synthesising enzyme, tyrosine hydroxylase (TH), have not identified consistent changes. We hypothesise that dopamine dysregulation in schizophrenia could result from changes in expression of dopamine synthesis enzymes, receptors, transporters or catabolic enzymes. Gene expression of 12 dopamine-related molecules was examined in post-mortem midbrain (28 antipsychotic-treated schizophrenia cases/29 controls) using quantitative PCR. TH and the synaptic dopamine transporter (DAT) proteins were examined in post-mortem midbrain (26 antipsychotic-treated schizophrenia cases per 27 controls) using immunoblotting. TH and aromatic acid decarboxylase (AADC) mRNA and TH protein were unchanged in the midbrain in schizophrenia compared with controls. Dopamine receptor D2 short, vesicular monoamine transporter (VMAT2) and DAT mRNAs were significantly decreased in schizophrenia, with no change in DRD3 mRNA, DRD3nf mRNA and DAT protein between diagnostic groups. However, DAT protein was significantly increased in putatively treatment-resistant cases of schizophrenia compared to putatively treatment-responsive cases. Midbrain monoamine oxidase A (MAOA) mRNA was increased, whereas MAOB and catechol-O-methyl transferase mRNAs were unchanged in schizophrenia. We conclude that, whereas some mRNA changes are consistent with increased dopamine action (decreased DAT mRNA), others suggest reduced dopamine action (increased MAOA mRNA) in the midbrain in schizophrenia. Here, we identify a molecular signature of dopamine dysregulation in the midbrain in schizophrenia that mainly includes gene expression changes of molecules involved in dopamine synthesis and in regulating the time course of dopamine action.

A new perspective on the treatment of aromatic L-amino acid decarboxylase deficiency

The final step in production of the neurotransmitters dopamine and serotonin is catalyzed by aromatic l-amino acid decarboxylase (AADC). AADC deficiency is a debilitating genetic condition that results in a deficit in these neurotransmitters, and manifests in infancy as a severe movement disorder with developmental delay. Response to current treatments is often disappointing. We have reviewed the literature to look for improvements to the current treatment strategy and also for new directions for AADC deficiency treatment. There may be differences in the mode of action, side-effect risk and effectiveness between different dopamine agonists and monoamine oxidase inhibitors currently used for AADC deficiency treatment. The range of these drugs used requires re-evaluation as some may have greater efficacy than others. Pyridoxal 5'-phosphate, the AADC cofactor may stabilize AADC and could increase AADC activity. Pyridoxal 5'-phosphate could have advantages as a treatment instead of pyridoxine. Atypical neuroleptics and peripheral AADC inhibitors both increase AADC activity in vivo and could be a future direction for AADC deficiency treatment and related conditions. Parkinson's disease gene therapy to deliver and express the human AADC gene in striatum is being tested in humans. Consequently gene therapy for AADC deficiency could be a realistic aim however an animal model of AADC deficiency is important for further progression.

Enhancing aromatic L-amino acid decarboxylase activity: implications for L-DOPA treatment in Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is an essential enzyme for the formation of catecholamines, indolamines, and trace amines. Moreover, it is a required enzyme for converting L-DOPA to dopamine when treating patients with Parkinson's disease (PD). There is now substantial evidence that the activity of AAAD in striatum is regulated by activation and induction, and second messengers play a role. Enzyme activity can be modulated by drugs acting on a number of neurotransmitter receptors including dopamine (D1-4), glutamate (NMDA), serotonin (5-HT(1A), 5-HT(2A)) and nicotinic acetylcholine receptors. Generally, antagonists enhance AAAD activity; while, agonists may diminish it. Enhancement of AAAD activity is functional, as the formation of dopamine from exogenous L-DOPA mirrors activity. Following a lesion of nigrostriatal dopaminergic neurons, AAAD in striatum responds more robustly to pharmacological manipulations, and this is true for the decarboxylation of exogenous L-DOPA as well. We review the evidence for parallel modulation of AAAD activity and L-DOPA decarboxylation and propose that this knowledge can be exploited to optimize the formation of dopamine from exogenous L-DOPA. This information can be used as a blue print for the design of novel L-DOPA treatment adjuvants to benefit patients with PD.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

Regulatory roles of bone morphogenetic proteins and glucocorticoids in catecholamine production by rat pheochromocytoma cells

We here report a new physiological system that governs catecholamine synthesis involving bone morphogenetic proteins (BMPs) and activin in the rat pheochromocytoma cell line, PC12. BMP type I receptors, including activin receptor-like kinase-2 (ALK-2) (also referred to as ActRIA) and ALK-3 (BMPRIA), both type II receptors, ActRII and BMPRII, as well as the ligands BMP-2, -4, and -7 and inhibin/activin subunits were expressed in PC12 cells. PC12 cells predominantly secrete dopamine, whereas noradrenaline and adrenaline production is negligible. BMP-2, -4, -6, and -7 and activin A each suppressed dopamine and cAMP synthesis in a dose-dependent fashion. The BMP ligands also decreased 3,4-dihydroxyphenylalanine decarboxylase mRNA expression, whereas activin suppressed tyrosine hydroxylase expression. BMPs induced both Smad1/5/8 phosphorylation and Tlx2-Luc activation, whereas activin stimulated 3TP-Luc activity and p38 MAPK phosphorylation. ERK signaling was not affected by BMPs or activin. Dexamethasone enhanced catecholamine synthesis, accompanying increases in tyrosine hydroxylase and 3,4-dihydroxyphenylalanine decarboxylase transcription without cAMP accumulation. In the presence of dexamethasone, BMPs and activin failed to reduce dopamine as well as cAMP production. In addition, dexamethasone modulated mitotic suppression of PC12 induced by BMPs in a ligand-dependent manner. Furthermore, intracellular BMP signaling was markedly suppressed by dexamethasone treatment and the expression of ALK-2, ALK-3, and BMPRII was significantly inhibited by dexamethasone. Collectively, the endogenous BMP/activin system plays a key role in the regulation of catecholamine production. Controlling activity of the BMP system may be critical for glucocorticoid-induced catecholamine synthesis by adrenomedullar cells.

ALDH1 mRNA: presence in human dopamine neurons and decreases in substantia nigra in Parkinson's disease and in the ventral tegmental area in schizophrenia

Dopamine (DA) neurons degenerate in Parkinson's disease and dopamine neurotransmission may be affected in psychotic states seen in schizophrenia. Understanding the regulation of enzymes involved in DA metabolism may therefore lead to new treatment strategies for these severe conditions. We investigated mRNA expression of the cytosolic aldehyde dehydrogenase (ALDH1), presumably involved in DA degradation, by in situ hybridization in DA neurons of human postmortem material. Parallel labeling for GAPDH, neuron-specific enolase, tyrosine hydroxylase, dopamine transporter, and dopamine beta-hydroxylase was used to ensure suitability of tissue specimen and to identify all dopamine neurons. ALDH1 was found to be expressed highly and specifically in DA cells of both substantia nigra (SN) and the ventral tegmental area (VTA) of controls. A marked reduction of ALDH1 expression was seen in surviving neurons of SN pars compacta but not of those in the VTA in Parkinson's disease. In patients suffering from schizophrenia we found ALDH1 expression at normal levels in DA cells of SN but at significantly reduced levels in those of the VTA. We conclude that ALDH1 is strongly and specifically expressed in human mesencephalic dopamine neurons and that low levels of ALDH1 expression correlate with DA neuron dysfunction in the two investigated human conditions.

Dopamine D2 Receptor Modulates Sodium Handling via Local Production of Dopamine in the Kidney

We have recently demonstrated that a deletion of the dopamine D2 receptor gene caused suppression of urinary sodium excretion and salt-sensitive elevation of blood pressure in mice. In order to understand the mechanisms underlying this impaired sodium excretion, we studied renal dopamine production and dopamine-induced sodium excretion in 20- to 30-week-old male D2-receptor knockout (D2KO) mice and age- and sex-matched wildtype (WT) mice. Renal local dopamine synthesis, examined by 24-h urine free dopamine excretion (UDAV), was significantly (p < 0.05) reduced in D2KO mice compared to that in WT mice (D2KO versus WT: 1.06 +/- 0.2 versus 1.5 +/- 0.3 ng/mg creatinine). Such a difference between D2KO and WT mice was also observed after oral administration of 3,4-dihydroxyphenylalanine (L-DOPA), a precursor of dopamine, at 5 mg/kg per day for 24 h. Furthermore, activity of aromatic 1-amino acid decarboxylase, a dopamine synthetase, was significantly suppressed in D2KO mice. Next, we examined changes in 24-h urine flow (UV) and 24-h sodium excretion (UNaV) during chronic infusion of dopamine at sub-pressor doses (3-4 microg/kg per min, sq.) or a vehicle via an osmotic pump. Urine flow in 24 h and UNaV were significantly (p < 0.05) smaller in D2KO mice infused with vehicle than in WT mice infused with vehicle (UV: 210 +/- 43 versus 650 +/- 163 microl/day; UNaV: 20.6 +/- 13.2 versus 44.4 +/- 21.6 microEq/day). After administration of dopamine, UV and UNaV in D2KO mice were restored to a level similar to that in WT mice. These results indicate that D2-dopamine receptors play a significant role in renal local dopamine synthesis and that a shortage of dopamine was, at least in part, responsible for the suppression of UV and UNaV in D2KO mice. However, we could not conclude from the present study whether renal tubular sodium reabsorption is intact in D2KO mice because the baseline dopamine contents in kidneys of D2KO mice and WT mice may be different.

Regulation of tyrosine hydroxylase expression in tottering mouse Purkinje cells

Tottering (tg) mice inherit a missense mutation in the Alpha1A subunit of P/Q-type calcium channels. This mutation results in an increased density of L-type calcium channels in the cerebellum and abnormal regulation of tyrosine hydroxylase (TH) gene expression in a subset of cerebellar Purkinje cells, a cell type that does not normally express TH. The behavioral phenotype includes attacks of dyskinesia, which can be blocked by L-type calcium channel antagonists. To test the hypothesis that cerebellar TH mRNA expression can be manipulated in vivo by L-type calcium channel blockade, control and tottering mice were chronically treated with the L-type calcium channel antagonist nimodipine. Chronic nimodipine treatment significantly reduced the expression of TH mRNA in tottering mouse Purkinje cells. This effect was observed without altering the increased density of L-type calcium channels in tottering mouse cerebella. Chronic nimodipine treatment had no effect on TH mRNA expression in tottering mouse catecholaminergic neurons, including those of the locus coeruleus and substantia nigra. However, a small reduction in TH mRNA expression in the substantia nigra of control mice was observed after drug treatment. These data suggest that the abnormal expression of TH in tottering mouse Purkinje cells is regulated by Purkinje cell excitability.

Differential regulation of hippocampal BDNF mRNA by typical and atypical antipsychotic administration

Apart from their differential propensities to block dopamine D2 and serotonin 5-HT2 receptors, the molecular mechanisms underlying the clinical efficacy of typical and atypical antipsychotics in schizophrenia are largely unknown. Given recent interest in the effects of antipsychotics on neurotrophic and other growth related factors, the effects of antipsychotics on brain-derived neurotrophic factor (BDNF), a neurotrophin crucial to the structural integrity of adult neurons, were investigated in male Wistar rats. Chronic (19 day) but not acute (45 min) antipsychotic administration significantly altered levels of hippocampal BDNF mRNA. In addition, whereas chronic treatment with the strong D2 receptor-blocker haloperidol significantly downregulated hippocampal BDNF mRNA, the selective 5-HT2 receptor-blocker ritanserin significantly upregulated CA1 hippocampal BDNF mRNA in comparison to controls. Since high doses of risperidone and clozapine produce potent inhibition of both 5-HT2 and D2 receptors, while lower doses produce significantly greater 5-HT2 vs. D2 receptor blockade, a dose-response study was employed to determine whether low doses of these atypical antipsychotics would also upregulate hippocampal BDNF mRNA in the absence of significant D2 receptor blockade. Whereas chronic haloperidol and high-dose risperidone significantly downregulated hippocampal BDNF mRNA, intermediate and lower doses of risperidone and clozapine were, unlike ritanserin, without effect when compared to controls. Thus, although the long-term downregulation of hippocampal BDNF mRNA may underlie the different clinical profiles of certain antipsychotics, this effect seems to be associated with antipsychotic doses that not only cause significant D2 receptor inhibition, but are usually associated with side effects rather than therapeutic efficacies.

Antioxidant strategy to counteract the side effects of antipsychotic therapy: an in vivo study in rats

The effects of long-term administration of the dopamine D(2) receptor antagonist haloperidol on Parkinsonian symptoms have been shown to persist after cessation of the drug treatment. In order to determine whether the level of tyrosine hydroxylase could be affected by subchronic administration of haloperidol, we examined tyrosine hydroxylase-positive immunoreactive cells in the substantia nigra after blockade of dopaminergic receptors with this antipsychotic. Three weeks of injections with haloperidol (1.5 mg/kg, i.p.) caused a significant decrease in tyrosine hydroxylase-positive cell counts at 24 h (27%), 5 days (21%) and 2 weeks (10%) after the last administration, an effect that was blocked by concurrent administration of the antioxidant, vitamin C. The level of tyrosine hydroxylase returned to baseline after 4 weeks withdrawal, no change being observed at later time-points. Nissl staining demonstrated that no damage to the cell bodies was observed, suggesting that the decrease in tyrosine hydroxylase-positive cells was not due to dopaminergic cell loss. These results demonstrate a depleting action of a short course of haloperidol on nigral tyrosine hydroxylase that outlasts the period of application by 2-4 weeks. Moreover, the current study has shown the effect of the antioxidant vitamin C in protecting haloperidol effects on tyrosine hydroxylase-immunostaining.

Comparative sequencing and association studies of aromatic L-amino acid decarboxylase in schizophrenia and bipolar disorder

Aromatic L-amino acid decarboxylase (AADC) is a relatively non specific enzyme involved in the biosynthesis of several classical neurotransmitters including dopamine and 5-hydroxytryptamine (5HT; serotonin). AADC does not catalyse the rate limiting step in either pathway, but is rate limiting in the synthesis of 2-phenylethylamine (2PE) which is a positive modulator of dopaminergic transmission and a candidate natural psychotogenic compound.1 We and others have proposed that polymorphism in AADC resulting in altered 2PE activity might contribute to the pathogenesis of psychosis. In order to test this hypothesis, we have used denaturing high performance liquid chromatography (DHPLC)3 to screen 3943 bases of the AADC gene and its promoter regions for variants that might affect protein structure or expression in 15 unrelated people with schizophrenia, and 15 unrelated people with bipolar disorder. Three polymorphisms were identified by DHPLC: a insertion/deletion polymorphism in the 5' UTR of the neuronal specific mRNA (g.-33-30delAGAG, bases 586-589 of GenBank M77828), a T>A variant in the non-neuronal exon 1 (g. -67T>A, GenBank M88070), and a G>A polymorphism within intron 8 (g. IVS8 +75G>A, GenBank M84598). Case-control analysis did not suggest that genetic polymorphism in the AADC gene is associated with liability for developing schizophrenia or bipolar disorder.

Tyrosine hydroxylase, aromatic L-amino acid decarboxylase and dopamine metabolism after chronic treatment with dopaminergic drugs

Mice were treated with dopamine (DA) receptor agonist and antagonist drugs: Agonists: (+/-)-SKF 38393 ((+/-)-1-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol) [DA D1-like]; bromocriptine, [DA D2 selective]; quinpirole, [DA D2/D3 preferring]; (+/-)-7-hydroxy-dipropylamino-tetralin (7-OH-DPAT), [DA D3/D2 preferring], Antagonists: R(+)-SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine), [DA D1-like]; and haloperidol, [DA D2-like]. All drugs were administered intraperitoneally, two injections daily 8 h apart for 30 days. Aromatic L-amino acid decarboxylase (AAAD) and tyrosine hydroxylase (TH) activity, protein and mRNA, as well as DA metabolism were followed with time thereafter in the nigrostriatal neurons. We observed that chronic administration of D1-like agonists had no effect on TH or AAAD activity, while D2-like agonists decreased AAAD, but not TH activity. Additionally, chronic blockade of DA D2-like receptors resulted in prolonged induction of TH and AAAD, while chronic blockade of DA D1-like receptors induced changes of AAAD only. Compared to TH the induction of AAAD was longer lasting. DA metabolism was altered by chronic administration of drugs acting on DA D2-like, but not DA D1-like receptors, and in general the patterns of change did not follow those for TH or AAAD. When studied 48 h after the last dose of the chronic haloperidol schedule TH displayed tolerance to acute drug challenge. At the same time interval, there was tolerance to the enhancing effects of haloperidol and SCH 23390 on DA metabolism. The induction of AAAD by haloperidol or SCH 23990 did not appear to develop tolerance after chronic administration. These observations complement existing knowledge, and provide novel information about AAAD that may have practical importance for Parkinson's patients on L-DOPA therapy.

Effects of glutamate antagonists on the activity of aromatic L-amino acid decarboxylase

This study examines the hypothesis that glutamate tonically suppresses the activity of the enzyme aromatic L-amino acid decarboxylase (AADC), and hence the biosynthesis of dopamine, to explain how antagonists of glutamate receptors might potentiale the motor actions of L-DOPA in animal models of Parkinson's disease. A variety of glutamate antagonists were therefore administered acutely to normal rats, which were sacrificed 30-60 min later and AADC activity assayed in the substantia nigra pars reticulata (SNr) and corpus striatum (CS). The NMDA receptor-ion channel antagonists MK 801, budipine, amantadine, memantine and dextromethorphan all caused a pronounced in creased in AADC activity, more especially in the SNr than CS. The NMDA glycine site antagonist (R)-HA 966 produced a modest increase in AADC activity in the CS but not SNr, whilst the NMDA polyamine site antagonist eliprodil, the NMDA competitive antagonist CGP 40116 and the AMPA antagonist NBQX were without effect. The results suggest that an increase in dopamine synthesis might contribute to the L-DOPA-facilitating actions of some glutamate antagonists.

Compartmental analysis of dopa decarboxylation in living brain from dynamic positron emission tomograms

The trapping of decarboxylation products of radiolabelled dopa analogs in living human brain occurs as a function of the activity of dopa decarboxylase. This enzyme is now understood to regulate, with tyrosine hydroxylase, cerebral dopamine synthesis. Influx into brain of dopa decarboxylase substrates such as 6-[18F]fluorodopa and beta-[11C]dopa measured by positron emission tomography can be analyzed by solution of linear differential equations, assuming irreversible trapping of the decarboxylated products in brain. The isolation of specific physiological steps in the pathway for catecholamine synthesis requires compartmental modelling of the observed dynamic time-activity curves in plasma and in brain. The several approaches to the compartmental modelling of the kinetics of labelled substrates of dopa decarboxylase are now systematically and critically reviewed. Labelled catechols are extensively metabolized by hepatic catechol-O-methyltransferase yielding brain-penetrating metabolites. The assumption of a fixed blood-brain permeability ratio for O-methyl-6-[18F]fluorodopa or O-methyl-beta-[11C]dopa to the parent compounds eliminates several parameters from compartmental models. However, catechol-O-methyltransferase activity within brain remains a possible factor in underestimation of cerebral dopa decarboxylase activity. The O-methylation of labelled catechols is blocked with specific enzyme inhibitors, but dopa decarboxylase substrates derived from m-tyrosine may supplant the catechol tracers. The elimination from brain of decarboxylated tracer metabolites can be neglected without great prejudice to the estimation of dopa decarboxylase activity when tracer circulation is less than 60 minutes. However, elimination of dopamine metabolites from brain occurs at a rate close to that observed previously for metabolites of glucose labelled in the 6-position. This phenomenon can cause systematic underestimation of the rate of dopa decarboxylation in brain. The spillover of radioactivity due to the limited spatial resolution of tomographs also results in underestimation of dopa decarboxylase activity, but correction for partial volume effects is now possible. Estimates of dopa decarboxylase activity in human brain are increased several-fold by this correction. Abnormally low influx of dopa decarboxylase tracers in the basal ganglia is characteristic of Parkinson's disease and other movement disorders. Consistent with postmortem results, the impaired retention of labelled dopa is more pronounced in the putamen than in the caudate nucleus of patients with Parkinson's disease; this heterogeneity persists after correction for spillover. Current in vivo assays of dopa decarboxylase activity fail to discriminate clinically distinct stages in the progression of Parkinson's disease and are, by themselves, insufficient for differential diagnosis of Parkinson's disease and other subcortical movement disorders. However, potential new avenues for therapeutics can be tested by quantifying the rate of metabolism of exogenous dopa in living human brain.

In vivo regulation of DOPA decarboxylase by dopamine receptors in rat brain

To test the hypothesis that dopamine (DA) receptors influence cerebral DOPA-decarboxylase (DDC) activity in vivo, we used HPLC to measure the kinetics of the cerebral uptake and metabolism of [3H]DOPA in carbidopa-treated rats, and in rats also treated acutely with a DA receptor antagonist (flupenthixol, 2 mg/kg, intraperitoneally) or a DA receptor agonist (apomorphine, 200 microg/g, subcutaneously). The unidirectional blood-brain clearance of [3H]DOPA (K1DOPA, 0.030 mL g(-1) min(-1)) increased by 50% after flupenthixol. The magnitudes of the relative DDC activity (k3DOPA) in striatum (0.20 min(-1)), olfactory tubercle (0.11 min(-1)), and hypothalamus (0.15 min(-1)) of carbidopa-treated rats were doubled with flupenthixol, but cortical DDC activity was unaffected (0.02 min(-1)). Apomorphine reduced the magnitude of k3DOPA in striatum by 20%. The rate constant for catabolism of [3H]DA formed in brain (k7', monoamine oxidase [MAO] activity), which ranged from 0.025 min(-1) in striatum to 0.08 min(-1) in hypothalamus of carbidopa-treated rats, globally increased 2- to 4-fold after flupenthixol, and decreased to 0.003 min(-1) in striatum after apomorphine. These in vivo results confirm the claim that acute blockade of DA receptors with flupenthixol stimulates the synthesis of [3H]DA from [3H]DOPA, and that this [3H]DA is subject to accelerated catabolism. Conversely, activation of the DA receptors with apomorphine inhibits DDC activity and DA catabolism.

Does phenylethylamine have a role in schizophrenia?: LSD and PCP up-regulate aromatic L-amino acid decarboxylase mRNA levels

Aromatic L-amino acid decarboxylase (AADC) is rate limiting in the production of 2-phenylethylamine (2PE). AADC activity and 2PE serum concentrations have been found to be increased in schizophrenic patients. Both antipsychotic and psychotogenic drugs, including amphetamine, affect the activity and encoding mRNA levels of AADC. Amphetamine is an analogue of 2PE and has a similar physiological effect. We have looked at the effects of chronic (32 day) treatment of rats with LSD (0.12 microg/kg/day) and phencyclidine (PCP; 10 mg/kg/day) on AADC mRNA levels. Both drugs up-regulated AADC mRNA levels in striatum, nucleus accumbens, hippocampus and cerebellum by between 50% and 150%. A splicing variant of AADC, present in human brain, which lacks the 3rd exon does not appear to be present in rat brain. These results are consistent with the hypothesis that over activity of AADC leading to increased production of 2PE is involved in endogenous psychosis such as schizophrenia.

Aromatic L-amino acid decarboxylase: a neglected and misunderstood enzyme

Classically, aromatic L-amino acid decarboxylase (AADC) has been regarded as an unregulated, rather uninteresting enzyme. In this review, we describe advances made during the past 10 years, demonstrating that AADC is regulated both pre- and post-translation. The significance of such regulatory mechanisms is poorly understood at present, but the presence of tissue specific control of expression raises the real possibility of AADC being involved in processes other than neuro-transmitter synthesis. We further discuss clinical and physiological situations in which such regulatory mechanisms may be important, including the intriguing possibility of AADC gene regulation being linked to that of factors thought to have a role in apoptosis and its prevention.

Tissue-specific expression of the nonneuronal promoter of the aromatic L-amino acid decarboxylase gene is regulated by hepatocyte nuclear factor 1

The rat aromatic l-amino acid decarboxylase (AADC) gene contains alternative promoters which direct expression of neuronal and nonneuronal mRNAs that differ only in their 5'-untranslated regions (UTRs). We have analyzed the expression of the nonneuronal promoter of the rat AADC gene in the kidney epithelial cell line LLC-PK1 and in cells which do not express the nonneuronal form of AADC by transient transfection. These studies revealed that the first 1.1 kilobases of the nonneuronal promoter, including the nonneuronal-specific 5'-UTR (Exon 1), contains sufficient information to direct tissue-specific expression. Serial deletions of this promoter localized the cis-active element to a region between -52 and -28 base pairs upstream of the nonneuronal transcription start site. An A/T-rich sequence, within this region which we have termed KL-1, was found to bind a kidney and liver-specific factor by DNase footprint analysis and was capable of directing tissue-specific expression from a heterologous promoter. Moreover, when the KL-1 sequence was mutated in the context of the entire promoter sequence, all transcriptional activity was abolished. DNA sequence comparison revealed that the KL-1 fragment is highly homologous to the binding site for hepatocyte nuclear factor-1 (HNF-1). Mobility shift studies utilizing an antibody to HNF-1 demonstrated binding of HNF-1 to the KL-1 fragment and cotransfection of HNF-1 cDNA into cells which do not express the nonneuronal form of AADC resulted in activation of transfected AADC nonneuronal promoter constructs. These results strongly suggest that the transcription factor which regulates the tissue-specific expression of the nonneuronal form of AADC mRNA is HNF-1.

Aromatic L-amino acid decarboxylase: biological characterization and functional role

1. Aromatic L-amino acid decarboxylase is the enzyme responsible for the decarboxylation step in both the catecholamine and the indolamine synthetic pathways. Immunological and molecular biological studies suggest that it is a single enzyme with one catalytic site but with different locations for attachment of the substrates. The enzyme is widely distributed in the brain and in peripheral tissues. 2. Recent investigations have shown that the enzyme is regulated by short term mechanisms that may involve activation of adenyl cyclase or protein kinase C. In addition, a long-term mechanism of activation by altered gene expression has also been suggested.

Aromatic L-amino acid decarboxylase modulation and Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is the second enzyme in the sequence leading to the synthesis of the catecholamines and serotonin, and it is the rate-limiting enzyme for the synthesis of the trace amines. In the striatum AAAD activity is increased by neuronal firing and diminished or enhanced by activation or blocking dopamine (DA) D1 or D2 receptors, respectively. At least two biochemical mechanisms appear responsible for modulation, short-term involving second messengers and possible phosphorylation, and long-term involving protein synthesis. In Parkinson's disease AAAD is the rate-controlling enzyme for the synthesis of DA when L-DOPA is administered and any change of AAAD activity could have clinical consequences. Indeed, the "on-off phenomenon" where there are fluctuations between off-periods of marked akinesia over several hours with on-periods of improved motility may be related to oscillating or poorly modulated AAAD activity and conversion of L-DOPA to DA. Studies are presented demonstrating how AAAD activity can be enhanced in an animal model of Parkinson's disease and how rapid fluctuations of AAAD can be provoked via second messenger system activation.

Striatal dihydroxyphenylalanine decarboxylase and tyrosine hydroxylase protein in idiopathic Parkinson's disease and dominantly inherited olivopontocerebellar atrophy

We measured the levels of dopamine, tyrosine hydroxylase (TH) protein, and dihydroxyphenylalanine (DOPA) decarboxylase (DDC) protein in the striatum of 10 patients with idiopathic Parkinson's disease (PD) and 23 patients with dominantly inherited olivopontocerebellar atrophy (OPCA). The levels of dopamine were markedly reduced (2% of control) in the striatum of the patients with PD, whereas striatal dopamine in the patients with OPCA ranged from normal (> 60% of control) to moderately reduced (20-60% of control) to severely depleted (< 20% of control). Both TH and DDC protein levels were significantly lower than those of the controls in the striatum of all of the patients with PD and in the subgroup of patients with OPCA having severely depleted dopamine. In contradistinction, TH but not DDC protein levels were reduced in those patients with OPCA having moderately reduced dopamine levels. This suggests that in the early stage of nigrostriatal dopamine neurone degeneration, DDC levels may be less susceptible to neurodegenerative influences than is TH synthesis or, alternatively, DDC synthesis may be more aggressively upregulated. Unexpectedly, from the blot immunolabeling analysis an additional DDC-immunoreactive band of slightly lower apparent molecular mass was detected in two of the patients with PD and in 12 of the patients with OPCA. This additional DDC band, which was not present in any of the control subjects, may reflect posttranslational modification(s) of DDC related to the neurodegenerative process.

Elevated dopa decarboxylase activity in living brain of patients with psychosis

The hypofrontality theory of the pathogenesis of schizophrenia predicts that cortical lesions cause psychosis. During a search for abnormalities of catecholaminergic neurotransmission in patients with complex partial seizures of the mesial temporal lobe, we discovered an increase of the rate of metabolism of an exogenous dopa tracer (6-[18F]fluoro-L-dopa) in the neostriatum of a subgroup of patients with a history of psychosis. When specifically assayed for this abnormality, patients with schizophrenia revealed the same significant increase of the rate of metabolism in the striatum. The finding is consistent with the theory that a state of psychosis arises when episodic dopamine excess is superimposed on a trait of basic dopamine deficiency in the striatum. The finding is explained by the hypothesis that cortical insufficiency, a proposed pathogenetic mechanism of both disorders, causes an up-regulation of the enzymes responsible for dopa turnover in the neostriatum as well as the receptors mediating dopaminergic neurotransmission.

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 DI and D2 receptors have a synergistic effect in this regulation.

Lack of effect of antidepressant drugs on the levels of mRNAs encoding serotonergic receptors, synthetic enzymes and 5HT transporter

Serotonergic transmission is thought to be central to the aetiology of depression and the therapeutic actions of antidepressant drugs, and the latters' delayed effect has given rise to the hypothesis that an adaptive change may be involved, possibly at the level of gene expression. We have examined this hypothesis by treating rats over a time course of up to 32 days with either imipramine, mianserin, fluvoxamine, citalopram, amoxapine or saline and measuring the levels of mRNAs encoding the 5HT1A, 5HT1B, 5HT1C and 5HT2 receptors, the enzymes tryptophan hydroxylase and aromatic amino acid decarboxylase, and the 5HT transporter. None of the treatments gave rise to significant changes in any of the mRNA levels at any time point. These results suggest that the reported changes in 5HT receptor numbers do not occur as a result of changes in the abundance of their encoding mRNAs, and that changes to the latter is not central to the therapeutic effects of antidepressant drugs.

Striatal L-dopa decarboxylase activity in Parkinson's disease in vivo: implications for the regulation of dopamine synthesis

L-DOPA is a large neutral amino acid subject to transport out of, as well as into, brain tissue. Competition between dopamine synthesis and L-DOPA egress from striatum must favor L-DOPA egress if decarboxylation declines relatively more than transport in Parkinson's disease. To test this hypothesis, we injected patients with Parkinson's disease with a radiolabeled analogue of L-DOPA and recorded regional brain radioactivity as a function of time by means of positron emission tomography. We simultaneously estimated the activity of the decarboxylating enzyme and the amino acid transport. In the striatum of patients, we found the L-DOPA decarboxylase activity to be reduced in the head of the caudate nucleus and the putamen. However, the rate of egress of the DOPA analogue was unaffected by the disease and thus inhibited dopamine synthesis more than predicted in the absence of L-DOPA egress.

Clozapine and sulpiride up-regulate dopamine D3 receptor mRNA levels

Chronic treatment (32 days) with sulpiride (100 mg/kg/day) up-regulated rat brain dopamine D3 receptor mRNA levels by 4-fold but had no effect on the mRNA levels encoding the dopamine D1A, D1B or D2 receptors or the enzymes tyrosine hydroxylase and aromatic amino acid decarboxylase as measured by multiprobe oligonucleotide solution hybridisation. Clozapine (30 mg/kg/day) increased D3 receptor mRNA levels by 5-fold after 4 days, the level dropping to basal after 32 days and also increased D1B mRNA levels by 0.5-fold in a similar pattern. Clozapine did not affect any other dopamine receptors or the synthesising enzyme mRNA levels. We have previously shown that the typical antipsychotics haloperidol and loxapine also increased the mRNA levels of the dopamine D3 receptor and these results suggest that up-regulation of dopamine D3 receptor mRNA may be associated with the therapeutic action of antipsychotic drugs.

NSD-1015 alters the gene expression of aromatic L-amino acid decarboxylase in rat PC12 pheochromocytoma cells

Aromatic L-amino acid decarboxylase (AADC) is involved in the synthesis of the putative neurotransmitters dopamine (DA), 5-hydroxytryptamine (5-HT), and trace amines some of which have been proposed as neuromodulators, such as 2-phenylethylamine and tryptamine. We report here that the gene expression of AADC can be regulated by the AADC inhibitor NSD-1015 in PC12 cells. The cells were treated with different doses of NSD-1015 (0.01-10 microM) for 3 days. Slot blot hybridization was performed to detect AADC mRNA and Western immunoblot to detect AADC protein. 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 (PCR) method. The results demonstrated that NSD-1015 produced a concentration-dependent up-regulation in AADC mRNA levels which is followed by a stable increase in AADC protein. The results suggest that AADC is an enzyme that can be regulated at the level of gene expression. The finding may be of importance in the study of DA transmission and for an improved understanding of this enzyme.

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.

Aromatic L-amino acid decarboxylase activity of mouse striatum is modulated via dopamine receptors

Aromatic L-amino acid decarboxylase (AAAD) activity is enhanced in the striatum of control and MPTP-treated mice after administration of a single dose of the dopamine receptor antagonists haloperidol, sulpiride, and SCH 23390. MPTP-treated mice appear more sensitive to the antagonists, i.e., respond earlier and to lower doses of antagonists than control mice. The rise of AAAD activity induced by the antagonists is prevented by pretreatment with cycloheximide. The apparent Km values for L-3,4-dihydroxyphenylalanine (L-DOPA) and pyridoxal 5-phosphate appear unchanged after treatment with the antagonists. Increased AAAD activity was observed also after subchronic administration of dopamine receptor antagonists or treatment with reserpine. A single dose of a selective dopamine receptor agonists had no effect on AAAD activity. In contrast, administration of L-DOPA, quinpirole, or SKF 23390 for 7 days lowers AAAD activity in the striatum. We conclude that AAAD is modulated in striatum via dopaminergic receptors.